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Study and modification of organic species formed in HSAPO-34 cages during methanol-to-olefin catalysis by ex situ analysis

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Study and modification of organic species formed in HSAPO-34 cages during methanol-to-olefin catalysis by ex situ analysis

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Content STUDY AND MODIFICATION
OF ORGANIC SPECIES FORMED IN HSAPO-34 CAGES DURING
METHANOL-TO-OLEFIN CATALYSIS BY
EX-SITU ANALYSIS
by
Hui Fu
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfilment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMISTRY)
December 2001
Copyright 2001 Hui Fu
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UNIVERSITY OF SOUTHERN CALIFORNIA
T he G raduate School
U n iversity Park
LOS ANGELES, CALIFORNIA 90089-1695
This th esis, w ritten b y
Hui Fu
U nder th e direction o f h 3 lL Thesis
C om m ittee, and approved b y a ll its m em bers,
has been presen ted to an d a ccep ted b y The
G raduate School in p a rtia l fu lfillm en t o f
requirem ents fo r th e degree o f
Master of Science
Desat o f G raduate S tu dies
December 17. 2001
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Dedication
To m y husband and my daughter
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Acknowledgments
The years that I have spent in USC were certainly fruitful and enjoyable. I
feel so thankful to so many people who are working here.
To choose a research group, I entered Dr. Haw’s office without much
confidence on a January afternoon of 2000. On that evening I told my husband that I
definitely wanted to become a student of Dr. Haw. It turned out to be a wonderful
decision. Under Dr.’s Haw supervision, I became the first or co-author of five
scientific articles. Without Dr. Haw’s encouragement and help and his physical
insight and knowledge, none of these would be possible.
Professors McKenna and Bau are not only members of my M.S committee
but also tutors who have provided their insightful thoughts and encouragement on
many occasions.
I owe a ‘thank you’ to every member of Dr. Haw’s group, especially to Dr.
Weiguo Song, who taught me how to use solid state NMR among many other things,
and Dr. Catherine Heneghen, Dr. Alain Sassi and Dr. Wei Wang for sharing their
knowledge and experience with me.
The thesis is dedicated to my husband. Yi Qin Gao and our daughter Farrah.
Without their love and support, I would have never gone this far and become a
graduate of USC.
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Table of Contents
Dedication ii
Acknowledgments iii
List of Figures vi
Abstract ix
Chapter I Introduction I
LI Methanol to Olefin (MTO) 2
1.2 HSAPO-34 and Organic Reaction Centers 3
L.2.1 HSAPO-34 3
L.2.2 Reaction Centers 5
1.2.3 Modification of the Organic 7
Species in HSAPO-34
1.3 E t Situ Analysis 7
L.3.1 Acid Digestion 8
1.3.2 Cryogenic Grinding 9
1.4 Reactor 11
1.5 References and Notes 15
Chapter 2 Polycyclic Aromatics Formation in HSAPO-34 17
During Methanol-to-Olefin catalysis: Ex Situ
Characterization After Cryogenic Grinding
2.1 Introduction 18
2.2 Experimental Section 21
2.3 Results 24
2.4 Discussion 32
2.5 References and Notes 37
Chapter 3 Selective Synthesis o f Methyinaphthalenes in 39
HSAPO-34 Cages and Their Function as
Reaction Centers in Methanol-to-OIefin Catalysis
3.1 Introduction 40
3.2 Experimental Section 43
3.3 Results 45
3.4 Discussion 53
3.5 References and Notes 59
iv
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Chapter 4 Ship-in-a-Bottle Synthesis o f Methylphenols in 60
HSAPO-34 Cages From Methanol and Air
4.1 Introduction 6 1
4.2 Experimental Section 63
4.3 Results 65
4.4 Discussion 79
4.5 References and Notes 82
Bibliography 84
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List of Figures
Chapter 1
Figure L .L Model of an HSAPO-34 cage. 4
Figure 1.2 A Bronsted acid site of HSAPO-34. 5
Figure L.3 Outlook of SPEX CertiPrep 6750 Freezer/mill. 1 0
Figure L.4 Schematic drawing of freezer/mill grinding action. 1 1
Figure L.5 Schematics of a pulse-quench reactor. 1 2
Figure L . 6 Different kinds of quench experiments. 13
Chapter 2
Figure 2.1 121.4 MHz 31P MAS NMR spectra showing
the effect of cryogenic grinding on HSAPO-34
catalyst.
25
Figure 2.2 GC/MS total ion chromatograms from control
experiments testing whether or not cryogenic
grinding promotes the reaction of p-xylene.
26
Figure 2.3 Gas chromatography (flame-ionization detection)
analyses of the volatile products captured
immediately prior to thermal quench from the
experiments used to prepare the HSAPO-34
samples for NMR analysis and cryogenic grinding.
29
Figure 2.4 75 MHz 1 3 C MAS NMR spectra (Bloch decays) 31
of the HSAPO-34 catalysts prepared in the
experiments described in Figure 2.3.
vi
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Figure 2.5
Figure 2.6
Chapter 3
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Chapter 4
Figure 4.1
GC-MS total ion chromatograms from analyses
of CCI4 extracts of samples prepared from the
catalyst bed that received 4 mL of methanol.
GC-MS total ion chromatograms from analyses
of CCI4 extracts of cryogenically ground samples
from all five catalyst beds (cf.. Figures 2.3 and 2.4).
Preparation and characterization of HSAPO-34
catalysts containing methylnaphthalene reaction
centers.
Preparation and characterization of HSAPO-34
catalysts containing methylbenzene reaction
centers at concentrations comparable to that of the
methylnaphthalene centers in the previous result.
Bar graphs showing ion mass distributions in the
vicinities of molecular ions for methylbenzenes
formed in experiments similar to that in Figure 3.2b.
Bar graphs showing ion mass distributions in the
vicinities of molecular ions for naphthalenes and
phenanthrene formed in an experiment similar to
that in Figure 3.1b.
GC-MS total ion chromatograms from the ex situ
analyses of aromatics formed from methanol in
HSAPO-34 catalyst beds at 350 °C with various
carrier gases.
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Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
75 MHz I3C CP/MAS NMR spectra of aromatic
products formed in HSAPO-34 from methanol- C
at 350 °C.
Study of the aging of methylphenols in HSAPO-34
at various times and temperatures.
GC-MS total ion chromatograms from the analyses
of volatile olefinic products sampled 4.0 s after
13
pulsing 15 pL of methanol- C onto HSAPO-34
catalyst beds containing either methylbenzenes or
methylphenols prepared from natural abundance
methanol.
Bar graphs showing ion mass distributions in the
vicinities of molecular ions for methylbenzenes and
methylphenols from ex situ analyses of the catalyst
beds from the same experiments giving rise to
Figure 4.4.
GC (FID) analyses of the volatile products sampled
2.4 s after injecting 12.5 pi- of solutions of methanol
onto 300 mg catalyst beds of zeolite HZSM-5 at
375 °C.
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Abstract
Silicoaluminophasphate HSAPO-34 (CHA topology) is a promising catalyst for
the methanol-to-olefin (MTO) reaction. The essential features of CHA topology are
cages of ca. 10 by 6.7 A interconected by 3.8 A diameter windows. Methylbenzenes
formed during the MTO reaction are trapped inside the HSAPO-34 cages and act as
organic reaction centers to help making and breaking C-C bonds. The organic
reaction centers are modified by selective synthesis of naphthalene and
methylphenols. respectively. Methylnaphthalenes are reaction centers with higher
ethylene selectivity but one-third the activity of the methylbenzenes reaction centers
at comparable conversion. Methylphenols show almost no activity towards methanol
conversion. One new ex situ analysis method, cryogenic grinding, has been
developed to destroy the HSAPO-34 framework. Total ion chromatograms show
identical results for CCU extracts from acid digestion and cryogenic grinding.
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Chapter 1
Introduction
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1.1 Methanol to Olefin (MTO)
In 1977, Chang and co-workers at Mobil Company discovered that methanol
and other oxygenates can be converted into gasoline range hydrocarbons over the
zeolite catalyst HZSM-5. 1 This was a remarkable discovery, as it provided a low-cost
method to convert coal or natural gas into gasoline. The so-called methanol-to-
gasoline (MTG) technology was developed quickly and commercialized in New
Zealand in 1979.2
The MTG process has not become a worldwide industry however. Crude oil is
still the major resource for gasoline because of its relative low-cost and convenience.
Also, the MTG process produces a high yield of aromatics that are undesirable in
modem gasoline. In addition to gasoline, under optimal MTG reaction conditions,
the product mixture contains approximately 40% light olefins. 3 A great deal of
research has been conducted in an attempt to adjust the reaction conditions and the
catalysts to increase olefin yield. A new technology, the catalytic conversion of
methanol to olefins (MTO) has been developed. '1' 6 MTO chemistry has been
receiving increasing attention because light olefins, especially ethylene and propene.
are key raw materials for the polymer industry.
Traditionally, light olefins are produced by catalytic cracking of crude oil, a
limited resource. The MTO process provides a new synthesis route for light olefins,
creating an important link between two already existing technologies. Abundant coal
and natural gas resources can be converted through the synthesis gas route into
7 S
methanol,' and the conversion of light olefins to polyolefins is a mature technology.
- >
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Early MTO research was based on the aluminosilicate zeolite HZSM-5. a
a
material with interconnected channels ca. 5.5 A in diameter. The product distribution
is broad, ranging from C2 to Cio aliphatics and C6 to C9 aromatics. 9 Modifications,
such as reducing the catalyst acidity and changing pore size, have been made in an
attempt to enhance olefin formation while reducing the formation of aromatics. 10' 11
Different types of zeolites have also been investigated for the MTO reaction. The
silcoaluminophosphate HSAPO-34 is considered to be one of the most promising
catalysts for the MTO reaction because of its high selectivity for ethylene and
propene. This thesis is based on the MTO catalysis of HSAPO-34.
1.2 HSAPO-34 and Organic Reaction Centers
1.2.1 HSAPO-34
The silcoaluminophosphate HSAPO-34 is the most extensively studied
catalyst for the MTO reaction. HSAPO-34 is a small pore size zeolite with chabazite
topology containing 6.7 A X 10 A cages connected by 3.8 A X 3.7 A 8 -member ring
oxygen windows. Figure 1.1 illustrates the cage structure of HSAPO-34. This
structure allows only small molecules, such as methanol, ethylene, propene and
linear C4 hydrocarbons to diffuse through the cages, while larger molecules, such as
branched hydrocarbons and aromatics are trapped inside the cages. The remarkable
product shape selectivity of HSAPO-34 results in alight olefin selectivity over 90%
for the MTO reaction. 12
3
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AI: Black
P: Dark gray
O: Light gray
Si: White
Like all other zeolites, the framework of HSAPO-34 is constructed from TO4
tetrahedra (T = Si, AI. P) connected to each other by oxygen bridges. For
aluminophosphate-34 (AlPO.t-34) with an AI to P ratio of 1:1, the framework is
neutral. In HSAPO-34 silicon atoms incorporate into the AIPO4 framework,
replacing phosphorous atoms and creating negative charges on the framework. These
negative charges are balanced by cations. A Bransted acid site of HSAPO-34, which
is similar to that of an aluminosilicate zeolite, is illustrated in figure 1 .2 .
4
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Figure 1.1. Model of an HSAPO-34 cage.
Figure 1.2. A Brzmsted acid site of HSAPO-34.
1.2.2 Reaction Centers
The mechanism of the MTO reaction is of great interest. Hundreds of papers
related to MTO mechanisms have been published, and more than 20 different
mechanisms for the formation of first C-C bond have been proposed. Most of these
mechanisms can be referred to as “direct routes” because they propose that the first
C-C bond is formed directly from methanol and/or dimethyl ether through some key
intermediates including oxonium ylides. carbenes, and free radicals1 3 .
Mole and co-workers first proposed an indirect route in 1983 after finding that
toluene acted as a “co-catalyst” when co-fed with methanol on HZSM-5. 14 Later.
Kolboe and co-workers discovered that the conversion of methanol might depend on
a “hydrocarbon pool”. The unspecified species of the proposed hydrocarbon pool,
represented as (CHjln, would add methanol and split off small molecules such as
ethylene and propene during the MTO reaction. 15' 16 The nature of the hydrocarbon
pool was investigated by Haw and co-workers. They found that the dimethyl
cyclopentenyl cation (I) is an important and persistent intermediate in the MTO
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reaction over HZSM-5 and proposed that the dimethyl cyclopentenyl cation is the
“reaction center” . 17
On HSAPO-34, methylbenzenes were identified as hydrocarbon pool species
by Haw and Kolboe independenly. 18'25 By a combination of solid state l3C NMR and
gas chromatography. Haw and co-workers used pulse-quench techniques to
introduce identical pulses of methanol-IjC onto HSAPO-34 at 673 K and then
quenched the reaction thermally after different periods of time. Within 4 seconds
after the pulse, only methanol, dimethyl ether and framework methoxonium (II)
were observed by ljC NMR, and gas chromatography indicated low methanol
conversion.
CH3
I
JX O
• V V
/ S \ N
II
Thirty seconds after the pulse, large amounts of methylbenzenes formed and the
freshly activated sample became a working catalyst with 1 0 0 % methanol conversion.
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Further research by the Haw group related the product selectivity to the average
number of methyl groups per benzene ring. 19
1.23 Modification of the Organic Species in HSAPO-34
Product selectivity and catalytic activity are two major concerns in MTO
research. The small pore size of HSAPO-34 provides product shape selectivity by
only allowing small molecules such as ethylene and propene. the major products, to
diffuse through the cages. Enhancement of ethylene formation over propene
formation is desirable. The Haw group has attempted modification of the HSAPO-34
inorganic framework with PH3 to promote ethylene formation by altering transition-
state shape selectivity. The strong evidence that methylbenzenes are the organic
reaction centers of the catalytic MTO reactions on HSAPO-34 raises further
questions: can we modify the organic species and how will different organic species
affect the selectivity and activity of the MTO reaction? In chapters 3 and 4 of this
thesis, syntheses of naphthalene and methylphenols inside the HSAPO-34 cages and
their effects on the MTO reaction are discussed, respectively.
1 3 Ex situ Analysis
In situ spectroscopic techniques such as FL1R. X-ray diffraction and solid state
NMR have been widely applied in studying heterogeneous catalysis. 2 0 *2 2 For
example, carbenium ions have been found and identified on HZSM-5 and HSAPO-
34 by in situ solid state NMR study.1 7 ' 23 However, the limitations of in situ
7
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spectroscopy make it difficult to study the organic species confined within HSAPO-
34 cages. The organic species trapped in HSAPO-34 cages range from
methylbenzenes to polyaromatic hydrocarbons. In solid-state l3C NMR spectra, only
two broad peaks are observed, one in the alkyl group region and the other in the
aromatic ring region. Therefore, the organic species must be released from the cages
for subsequent analysis and identification by ex situ methods. Two methods for
releasing the organic species are acid digestion and cryogenic grinding, both of
which are discussed in chapter 2 .
1.3.1 Acid Digestion
Acid digestion was first utilized to characterize the “coke” formed during
aluminosilicate zeolite deactivation by applying 40% hydrofluoric acid to dissolve
the zeolite framework. 2 4 Liberated organic compounds were extracted with
methylene chloride and analyzed by 'H NMR, GC-MS and HPLC. Control
experiments proved that reactive hydrocarbons adsorbed on an inert solid (SiOj)
could be recovered intact after HF treatment. Low temperature (room temperature)
and separate phases of coke and acid solution allow the organic compounds to
maintain their nature.
For silicoaluminophosphate zeolites like HSAPO-34, dilute HCl (IM) can
be used to dissolve the catalyst framework. 25 This allows for greater ease and safety
while liberating organic species from HSAPO-34 cages. The transformation of
reactive hydrocarbons is also less likely since IM HCl is less acidic than 40% HF.
8
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Recently, in our group, study of H/D exchange on methylbenzenes in HSAPO-34 has
used this method. 28 In the following chapters, most of the research utilizes the acid
digestion method.
1.3.2 Cryogenic grinding
Even though acid digestion is very reliable, alternative methods are still
needed. Most of the aluminosilicate zeolites can only be dissolved by HF (normally
40%), which creates numerous safety issues. In addition, some compounds such as
cyclic hydrocarbons could be acid sensitive. This could create problems for the study
of the MTO mechanism that have not been issues with the study of catalyst
deactivation. Furthermore, isotopic scrambling, which is a sensitive mechanistic
probe, might need a control method with which to compare the results from acid
digestion.
To release organic compounds from inorganic framework without changing
their nature, physical destruction of the framework is an ideal method. The idea of
cryogenic grinding came out after several unsuccessful trials using a standard
laboratory mill (Wig-L-bug SPEX Certiprep 3110) at room temperature. Both the
sample and the container reached very high temperatures, and the organic
compounds could not be recovered. In order to preserve the sample’s key aspects,
grinding at cryogenic temperature is necessary.
Chilling materials in liquid nitrogen has two important consequences for
sample preparation. Flexible samples can be embrittled and ground by impact
9
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milling, and the structural and compositional aspects damaged or lost during room-
temperature grinding can be retained. The applications of cryogenic grinding are
widespread, including DNA extraction, polymer structure study, trace element
analysis, volatile compound recovery and medical research. The first application of
catalysis study by cryogenic grinding is presented in chapter 2 .
The tool that has been chosen for cryogenic grinding is the SPEX Certiprep
6750 Freezer/mill. The grinding mechanism is a magnetic coil assembly immersed in
an insulated tub filled with liquid nitrogen. The sample and a stainless-steel impactor
are placed into a polycarbonate or stainless steel sample vial that is sealed by two
stainless steel end plugs. The vial is then inserted into the coil assembly and lowered
into the liquid nitrogen bath. The magnetic coil rapidly shuttles the impactor back
and forth, pulverizing the sample against the end plugs of the vial. Figures 1.3 and
1.4 illustrate the freezer mill and its grinding action.
Figure 1.3. Outlook of SPEX Certiprep 6750 Freezer/mill
10
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Impactor
End Plug AC Coil
End Plug
Figure 1.4. Schematic drawing of Freezer/Mill grinding action.
1.4 Reactor
The reactor used for the MTO studies of this thesis is a “pulse quench
reactor.” 26 Figure 1.5 details the configuration of this reactor. This reactor is capable
of either pulsing reactant onto the catalyst bed in a short time or flowing reactant at
constant rate. In a pulse experiment, a Valeo injector is switched on. pulsing pre
stored reactant onto the catalyst bed from the sample loop in a fraction of a second.
Meanwhile, a GC sample can be taken a few seconds after the pulse. This method
gives product formation data immediately after the reactant pulse. A mechanical
syringe pump, capable of introducing a constant reactant flow onto the catalyst bed
can be substituted for the injector valve. Controlled amount of organic compounds
can be synthesized inside the catalyst under steady state conditions by this method.
This reactor also contains a thermal-quench system that can lower the temperature of
the catalyst bed to ambient in less than one second. The catalytic experiment can be
quenched in different stages and the organic species retained inside the cages can be
analyzed by ex or in situ methods.
11
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r -
Mass flow
controller
r{
Injector
valve
w
Helium gas
Quench
vent
GQGCMS
3 )
Heat
exchanger
Nitrogen for
cooling
Figure 1.5. Schematics of a puise-quench reactor.
For a typical experiment, approximately 0.3 g of catalyst is activated at 400
°C in the catalyst bed with a constant flow of carrier gas, usually helium. The
temperature of the catalyst bed is monitored by a thermocouple, the tip of which is
placed in the middle of the catalyst bed. The reactant is then introduced by either
pulse or flow methods. The reaction can be rapidly quenched during this stage. A
12
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Single pulse-quench experiment
Pulse Quench
Double pulse experiment
Pulse 1 Pulse 2 Quench
Flow quench experiment
Flow Quench
Flow-pulse experiment
Flow Pulse Quench
Flow-flow experiment
Flow I Flow 2 Quench
Figure 1.6. Different kinds of quench experiments.
13
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second reactant can be introduced by pulse or flow to study the effect of, or modify
the pre-formed organic compounds. Figure 1.6 demonstrates typical quench
experiments.
Single pulse and double pulse experiments have given strong evidence that
methylbenzenes are reaction centers. 18 Ship-in-a-bottle synthesis was performed by
single flow-quench experiments in most cases. In this thesis, flow-pulse and flow-
flow experiments are used to study and modify the organic species.
14
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1.5 References and Notes
(L ) Chang, C. D.; Silvestri. A. J. /. CataL 1977, 47. 249.
(2) Meisel, S. L. Chemtech. 1988,1, 32.
(3) Chang, C. D. Catal. Rev. -Sci. Eng. 1984, 26(3&4), 323.
(4) Chang, C. D.; Lang, W. H.; Silvestri, A. J. U.S. patent, 4,062,095,1978.
(5) Wunder, F. A.; Leupold, E. I. Angew. Chem., Int. Ed. Engl. 1980,19. 126.
(6 ) Singh, B. B.; Liu. F. N.: Anthony, R. G. Chem. Eng. Commun. 1980, 4,
749.
(7) Maj, J. J; Colmenares. C.; Somotjai. G. A. Appl. Catal. 1984.10.313.
(8 ) Thomas, J. M.; Thomas, W. J. Principles and practice o f Heterogeneous
Catalysis; VCH: New York. 1996, p. 515.
(9) Chang, C. D.: Lang, W. H.: Bell, W. K. Catalysis o f Organic Reactions
(Moser, W. R.. ed.), Dekker, New York. 1981. p. 73.
(10) Kaeding, W. W.; Butter. S. A. J. Catal. 1980. 61, 155.
(11) Reckenstein, T.: Litterer. H.: Fetling, F. Chem. -Ing. — Tech. 1980. 52
(10), 816.
(12) Kaiser. S. W. Arab. J. Sci. Engng. 1985.10, 361.
(13) Stocker, M. Microporous and Mesoporous Mater. 1999. 29. 3.
(14) Mole, T.: Whiteside J . A.: Seddon, D J. 7 Catal. 1983,82, 261.
(15) Dahl, I. M.: Kolboe, S. J. Catal. 1994.149.458.
(16) Dahl, I. M.: Kolboe. Catal. Letter. 1993,2 0 ,329.
(17) Haw, J. F.; Nicholas. J. B.: Song, W.; Deng, F.; Xu. T.; Heneghan. C. S. J.
Am. Chem. Soc. 2000,122,4763.
(18) Song, W.; Haw, J. F.; Nicholas, J. B.: Heneghan. C. S. J. Am. Chem. Soc.
2000,122.10726.
15
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(L9) Song, W.; Fu, H.; Haw, J. F. J. Am. Chem. Soc. 2001,123, 4749.
(20) Howe, R. F. In Bibby, D. M.; Chang, C. D.; Howe, R. F.; Yurchak, S.
(Eds.). Methane Conversion. Elsevier. Amsterdam. 1988, p. 157.
(21) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc.
Chem. Res. 1996,29 (6), 259.
(22) Forester, T. R.; Howe, R. F. J. Am. Chem. Soc. 1987,109, 5076.
(23) Song, W.; Nicholas, J. B.; Haw, J. F. J. Phys. Chem. B 2001,105.43 L7.
(24) Magnoux, P.: Roger, P.; Canaff, C.: Fouche, V.: Gnep, N.S.; Guisnet, M.
In Froment, G. F. et al. (Eds.), Studies in Surface, Science and Catalysis,
Vol. 34, Catalyst Deactivation 1987, p.317.
(25) Arstad, B.; Kolboe, S. Catal. Lett. 2001, 71, 209.
(26) Haw, I. F.; Goguen, P. W.; Xu. T.; Skloss, T. W.; Song, W.: Wang, Z.
Angew. Chem. 1988.37,948.
(27) Arstad. B.: Kolboe. S. J. Am. Chem. Soc. 2001.123, 8137.
(28) Song, W.; Ng, L.; Marcus, D. M.; Haw, J. F. Angew, chem. 2001.
(submitted)
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Chapter 2
Polycyclic Aromatics Formation in HSAPO-34
During Methanol-to-olefin Catalysis:
Ex Situ Characterization After Cryogenic Grinding
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2.1 Introduction
In several recent papers1' 3 we have shown that cyclic organic species are part
of the active sites for methanol-to-olefin (MTO) catalysis on microporous solid
acids.6 ' 7 On zeolite HZSM-5 these “reaction centers” are either methyl-substituted
cyclopentenyl carbenium ions or methylbenzenes. 2 On the silico-aluminophosphate
acid catalyst HSAPO-34, 8 the reaction centers are methylbenzene molecules, 3’5 and
we can not rule out the possibility that other types of arenes, e.g.
methylnaphthalenes, could also be reaction centers. HSAPO-34 has the CHA
topology (its I nm cages are interconnected through 0.37 nm windows), and
aromatic molecules, even benzene, once formed can not escape.
The reaction centers work in tandem with a Bronsted acid site to make and
break carbon-carbon bonds without recourse to the higher energy intermediates and
transition states that would be required without an organic “scaffold” to stabilize
these reactions. The most specific evidence for the roles of methylbenzenes in MTO
catalysis on HSAPO-34 is from in situ NMR studies/ -5 but earlier work by Kolboe
and co-workers used isotope scrambling to obtain evidence for a phenomenological
“hydrocarbon pool” of unspecified structure/ 10 The first evidence that aromatics
could accelerate the rate of methanol conversion on any solid acid catalyst was
reported by Mole and co-workers who identified a co-catalytic effect when toluene
was co-fed with methanol on HZSM-5. 11' 12
As a step toward a kinetic analysis of MTO chemistry in HSAPO-34, we
treated each nanocage containing a methylbenzene as an independent supramolecule
18
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capable of unimolecular decomposition to either ethylene, {n}— >{n-2 , e}, or
propene, {n}— »{n-3, p}, where n implies a methylbenzene molecule with n methyl
groups, and {} designates the nanocage and its associated Brpnsted acid site. 3 We
found that when n = 5 or 6 , the unimolecular decomposition to olefins proceeds
rapidly and favors propene, and when n = 3 the supramolecule is more stable but
yields ethylene with enhanced selectivity. Any unique molecule that we may
assemble in an HSAPO-34 cage provides a unique supramolecule that could be
tested for correlations between structure, activity, and selectivity, and we have
embarked on a synthetic program to synthesize and test a variety of supramolecules.
This program necessarily requires the analytical capability to identify and quantify
mixtures of supramolecules (or more specifically their organic constituents).
Unfortunately, the l3C solid state NMR method that first identified roles for
methylbenzenes in HSAPO-34 catalysts3 is. for lack of resolution, ill-suited to the
study of catalysts containing diverse aromatic species. Thus, when we characterized
a deactivated HSAPO-34 catalyst containing 16 wt % C, the CP/MAS NMR
spectrum was consistent with an average structure similar to methylnaphthalene, 3 but
the NMR data said nothing about the distribution of different ring systems, and did
not preclude other interpretations such as a mixture of methylbenzenes and graphitic
coke on the exterior of the catalyst particles.
A standard method for the analysis of organic species in zeolites and similar
catalysts is acid digestion followed by chromatographic analysis. Magnoux and co-
19
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workers found that aluminosilicate zeolites could be digested using 40 % aqueous
HF, liberating organic compounds which could be extracted using methylene
chloride. 13 Control experiments were performed in which reactive hydrocarbons
were first adsorbed onto inert silica and then recovered intact following HF
digestion. As demonstrated by Arstad and Kolboe, SAPO-34 can be digested using I
M HCl, and the organics thus released are conveniently extracted using CCI4 . 14
While we have every reason to believe that acid digestion is generally a
reliable method for ex situ analysis of organics in microporous catalysts, we are
concerned about the possibility that some compounds formed in SAPO-34 might
prove to be acid sensitive, either alone or in combination. Furthermore, l3C label
scrambling within a molecule or between molecules is a sensitive mechanistic probe,
and we need a way to control against scrambling as a result of acid digestion. As an
adjunct method to confirm the results of acid digestion we considered cryogenic
grinding, a mechanical method for liberating organic compounds from inorganic
matrices. Cryogenic grinding using a freezer mill is used to pulverize bone and
dental material to liberate DNA for forensic analysis. 15 Most famously, this method
was used in the conclusive identification of the skeletal remains of Tsar Nicholas II
and his family. 16
Here we report the application of cryogenic grinding to the ex situ study of
aromatic compounds in SAPO-34. 3 lP NMR showed that the long-range crystalline
structure of this catalyst is destroyed after ca. 40 min. of grinding. Control
20
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experiments using physical mixtures of p-xylene and HSAPO-34 or p-xylene
adsorbed onto zeolite HZSM-5 showed that cryogenic grinding does not promote
reactions such as xylene isomerization or disproportionation. We carried out a series
of MTO reactions in which HSAPO-34 converted varying amounts of methanol in a
flow reactor at 400 °C at a fixed space velocity prior to thermal quenches. Activity
and selectivity were measured immediately prior to quench and quenched samples
were characterized by l3C solid state NMR. NMR showed that the amount of
methylated aromatics entrained in the catalyst increased with time on stream and
suggested an increase in fused rings as the catalyst deactivated, but as before the
distribution of aromatics could not be deduced. Following NMR analysis, the
samples were divided for both acid digestion ( I M HCl) and cryogenic grinding. GC
analysis produced essentially identical traces for CCI4 extracts of samples prepared
by the two methods. We confirmed that methylbenzenes are the dominant aromatics
in HSAPO-34 early in the life of the catalyst, but methylnaphthalenes are present at
comparable levels well before deactivation occurs. As the catalyst deactivates it
accumulates appreciable amounts of phenanthrene and pyrene.
2.2 Experimental Section
Materials and reagents. HSAPO-34 was prepared according to a patent
procedure. 17 XRD showed a pure crystalline phase with the CHA structure. The
product was calcined at 873 K for 10 hours to remove the template agent and pressed
21
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into 10-20 mesh pellets. The Bronsted site concentration was determined to be 1.1
mmol/g. We used 10 % l3C enriched methanol (Isotech, Inc) to facilitate NMR
characterization.
Catalysis. Experiments were performed using the pulse quench reactor18 with
a motor-driven syringe pump (Harvard Apparatus model PHD 2000) as described
previously. 3 For each experiment a bed consisting of 0.3 g of catalyst was activated
at 673 K in the reactor under 200 seem He flow for 2 h immediately prior to use.
This carrier gas feed rate was also used during methanol introduction in all
experiments. Methanol flow was abruptly ceased several seconds prior to quench.
Previous studies have shown that the temperature of the catalyst pellets decreases
150 K in the first 170 ms of a quench. After quenching each reacted catalyst sample,
the reactor was sealed off and transferred into a glove box filled with nitrogen. The
catalyst pellets were ground and transferred to a 7.5-mm MAS rotor which was
sealed with a Kel-F end-cap.
Gas Chromatography. A Hewlett-Packard Model 6890 gas chromatograph
with flame-ionization detector was used to analyze gases sampled from the reactor
product streams using a Valeo valve. The column was 150 m dhl50 (Supelco)
operated isothermally at 323 K to permit sampling of the gas stream more frequently
than the total analysis time for any given sample. Extracted aromatic hydrocarbons
were analyzed using the HP 5973 mass selective detector on a second model 6890
gas chromatograph. The column was a HP-1 (cross linked methyl silicone gum, 0.5
22
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nm film), 50 m long, 0.2 nun diameter. The oven temperature was programmed
from an initial temperature of 60 °C to a final temperature of 250 °C, and the He
carrier gas was maintained at a constant flow rate of 0.3 mL/min.
NMR spectroscopy. ,3C solid state NMR experiments were performed with
magic angle spinning (MAS) on a Varian Infinity plus 300 MHz spectrometer
operating at 75.4 MHz for l3C. Hexamethylbenzene (17.4 ppm) was used as an
external chemical shift standard, and 85 % H3 PO4 was used for 3 1 P.
Chemagnetics-style pencil probes spun 7.5 nun zirconia rotors at typically 6.5 kHz
with active spin speed control (± 3 Hz).
Typical ,3C experiments included: cross polarization (CP. contact time = 2
ms, pulse delay = I s. 2 0 0 0 transients): cross polarization with interrupted
decoupling (contact time = 2 ms, pulse delay = I s, 2 0 0 0 transients, dipolar
dephasing time of 50 ps); single pulse excitation with proton decoupling (Bloch
decay, pulse delay = 10 s, 400 transients). CP and Bloch decay spectra gave very
similar values for the average number of methyl groups per aromatic ring carbon.
All spectra reported here were measured using Bloch decay.
Cryogenic grinding. The freezer mill was a SPEX (Metuchen. NJ) CertiPrep
6750 with 6751C polycarbonate tubes. We programmed 12 grinding cycles of 3.5
min. with 4 min. between cycles. The grinding frequency was 6 Hz for the first
cycles and this was progressively increased to 12 Hz for the final cycles. The sample
was bathed in liquid nitrogen for the entire procedure.
23
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2.3 Results
Mechanical grinding must destroy the crystalline structure of a microporous
material in order to expose entrained organic compounds for extraction. We used
3lP MAS NMR to confirm that HSAPO-34 looses long-range order as a result of
freezer milling. Figure 2.1a shows that HSAPO-34 has a sharp 31P resonance at — 28
ppm prior to treatment. Following 24 min. of cryogenic grinding most of the
integrated intensity is in a broad shoulder at - 2 0 ppm. but the sharper feature at -28
ppm is still evident. With longer grinding times (e.g., 60 min.. Figure 2.1c), the
sharp feature is no longer evident. We used 40 min total of grinding for all of the
work reported here.
Figure 2.2 reports GC traces (total ion chromatograms) from control
experiments in which we probed whether or not cryogenic grinding was likely to
promote the reactions of aromatic hydrocarbons in the presence of debris from
microporous solid acids, p-xylene could isomerize to o- or m-xylene, it could
disproportionate to toluene and trimethylbenzene, or it could dimerize to substituted
diphenylmethanes, all familiar reactions on microporous solid acids, p-xylene is too
large to be adsorbed into SAPO-34. so we ground a physical mixture. The GC-MS
trace in Figure 2.2a shows that p-xylene was the only organic recovered by
extraction. An analogous grinding experiment with p-xylene adsorbed onto zeolite
HZSM-5 initially seemed more equivocal: Figure 2.2b shows several minor products
with appreciably higher molecular weights, but these are apparently a consequence
24
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-24
-28
-2 4
-28
o -50 -150 100 50 -100
ppm
Figure 2.1. 121.4 MHz 3tP MAS NMR spectra showing the effect of cryogenic
grinding on HSAPO-34 catalyst, (a) Without grinding HSAPO-34 shows a single,
very narrow resonance characteristic of a crystalline material, (b) After 24 min. of
grinding the catalyst is partially degraded to amorphous material, (c) After 60 min.
of grinding the NMR spectrum shows little evidence of crystallinity.
25
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m /z-168
m /z=170
i 1 - - - - - - - - 1 - - - - - - - - 1 - - - - - - - - 1- - - - - - - - r
15 20 25 30 35 40
Retention time (min.)
Figure 2.2. GC/MS total ion chromatograms from control experiments testing
whether or not cryogenic grinding promotes the reaction of p-xylene. (a) After 40
min. of grinding a mixture of HSAPO-34 and p-xylene, there is no evidence of
reaction, (b) p-xylene was adsorbed onto zeolite HZSM-5 and ground for 40 min.
No isomerization or disproportionation occurred but several higher mass products
were found at low yields, (c) p-xylene was adsorbed onto zeolite HZSM-5 and this
was then extracted with CCI4 without grinding. Some of the higher mass products
were observed here implying that these reactions were due to adsorption in the
zeolite and not grinding.
26
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of adsorption rather than freeze milling. We adsorbed p-xylene onto HZSM-5 and
then extracted the zeolite with CCI4 , omitting grinding. As Figure 2.2c shows, we
were able to recover all but the highest mass species by this simple procedure. We
assume that even the m/z 2 1 0 product is formed upon adsorption, and conclude that
cryogenic grinding alone is not likely to promote the reactions of aromatic
hydrocarbons.
We carried out five experiments in which totals of 0.5 to 8 mL of methanol
was delivered onto fresh HSAPO-34 catalyst beds at 400 °C with a space velocity of
8 h [. The GC traces in Figure 2.3 reflect the product streams from these reactions
immediately prior to thermal quench. A decrease in activity was evident after the
first 4 mL of methanol, and activity was all but lost after 8 mL. As noted previously,
there was an increase in ethylene selectivity as the catalyst began to deactivate.
Figure 2.4 reports the l3C CP/MAS spectra of the catalyst beds obtained by
thermal quenching the experiments in Figure 2.3 immediately after sampling the
product stream for GC analysis. In each case the reactor was opened in a glove box
and the contents were transferred to a magic angle spinning rotor for spectral
acquisition at room temperature. As in our previous NMR studies of organic species
on HSAPO-34, J '5 the spectra do not show many features. The signal at 19 ppm is, in
every case, indicative of methyl substituents on aromatic rings. The aromatic signal
near 134 ppm suggests that most of the ring carbons are either substituted by methyl
groups or at bridgehead positions in fused rings. In each case the ratio of methyl
27
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Figure 2.3. Gas chromatography (flame-ionization detection) analyses of the
volatile products captured immediately prior to thermal quench from the experiments
used to prepare the HSAPO-34 samples for NMR analysis and cryogenic grinding.
Various total volumes of methanol were delivered at a weight-hourly space velocity
of 8 h 1 to form volatile olefinic products and aromatic compounds entrained in the
SAPO-34 cages. The catalyst was almost completely deactivated after 8 mL of
reactant. DME denotes dimethylether.
28
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c h 3o h
Propene
DME
E thylene
12.7% conv.
8 mL
t i r
14 15 16
82.9% conv.
4 mL
100% conv.
2 mL
100% conv.
1 mL
100% conv.
0.5 m L
" I I I T
17 18 19 20
Retention time (min.)
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groups to aromatic carbons, CHyC^r, is reported next to the spectra. This number is
only 0.20 for the nearly deactivated ( 8 mL) catalyst.
The NMR spectra in Figure 2.4 also show the progressive accumulation of
carbon on the catalyst during methanol conversion. We submitted the I mL and 4
mL samples for carbon weight percentage analysis and obtained values of 5.3 and
13.3 %, respectively. We previously found that fully deactivated HSAPO-34
catalyst contains ca. 16 % carbon.
We selected the 4 mL sample for a detailed comparison of acid digestion and
cryogenic grinding. We used Kolboe’s acid digestion procedure14 in which the spent
catalyst is treated with I M HCl to liberate the aromatics. which are then extracted
into CCI4 . Cryogenic grinding was carried out for 40 min. based on the results in
Figure 2.1. GC-MS total ion chromatograms from the analyses of these extracts are
compared in Figure 2.5. The differences between these two results are so slight as to
be considered insignificant. The compounds present at the highest levels include
methylbenzenes from toluene to durene, naphthalene and methylnaphthalenes up to
trimethyl isomers, phenanthrene, and pyrene.
Figure 2.6 presents GC-MS total ion chromatograms for all five samples. The
catalyst that saw only 0.5 mL of methanol contains methylbenznenes and only small
amounts of naphthalenes, while the latter are very significant after 4 mL of methanol.
The deactivated ( 8 mL) catalyst contains significantly reduced amounts of
methylbenznenes, methylnaphthalenes. and a great deal of both phenanthrene and
30
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134 ppm
/
19
8 mL
0.20
4 mL
0.30
2 mL
1 mL
A _ _ _A
0.5 mL 0.71
250 200 150 100 50 -50 300 0
ppm
13,
Figure 2.4. 75 MHz C MAS NMR spectra (Bloch decays) of the HSAPO-34
catalysts prepared in the experiments described in Figure 2.3. The aromatic
hydrocarbons trapped in the HSAPO-34 cages are reflected in the aromatic carbon
signal at ca. 134 ppm and the methyl carbon signal at 19 ppm. While it is clear that
the total content of aromatics increases with time on stream, the identities and
relative compositions of various compounds can not be deduced from these spectra.
* denotes spinning sidebands. Numbers to the right of the spectra report ratios of
methyl carbons to aromatic ring carbons from integrated intensities.
31
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pyrene. No methyl-substituted pyrene was observed and only a very small fraction
of the phenanthrene was substituted. Yet, a large fraction of the benzene and
naphthalene rings were methylated in the deactivated catalyst.
2.4 Discussion
The results of this first application of cryogenic grinding to catalyst samples
are very encouraging. While acid digestion was not detrimental to the samples
studied here, our expectation is that grinding at liquid nitrogen temperature is less
likely to alter more add sensitive compounds than those studied here. Agreement
between the two approaches, as obtained here, builds confidence in the application of
either method.
We used elemental analysis (before and after grinding) to asses the fraction of
the organic compounds liberated. For both samples studied ( I mL and 4 mL) ca. 30
% of the carbon initially present was removed by milling followed by CCU
extraction. We also observe a black, CCI4 insoluble residue following acid
digestion, but we have not sought to quantify the amount of carbon associated with
this. It may be that some of the carbon on spent HSAPO-34 is indeed graphite-like
material on the outside of the crystallites, and this can not be extracted. It also seems
likely that a fraction of the aromatic compounds remains associated with the
inorganic debris after grinding. For example, it may be that milling for 40 min.
produces very small crystallites of SAPO-34 only a few nm in dimension, and some
32
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Figure 2.5. GC-MS total ion chromatograms from analyses of CCI4 extracts of
samples prepared from the catalyst bed that received 4 mL of methanol: (a) a
fraction digested using I M HCI, and (b) a fraction cryogenically ground for 40 min.
The two methods yield essentially identical results for the analysis of aromatic
hydrocarbons in the HSAPO-34 catalyst bed. We were able to identify all major
species from the mass spectra and/or comparison of retention times with authentic
samples. Some of the more prominent peaks are assigned in the figure.
33
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34
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Retention time (min.)
8 m L
J!____
V
jj l l L
4 m L
2 m L
(CHjK
(CHj)2
(CH3 )j
1 m L
(Q h cH jh
0 .5 m L ^
J 1 I J L J L m
j 6 :
r
J i I It n
15 20 25 30 35 40 45 50
R etention tim e (m in.)
Figure 2.6. GC-MS total ion chromatograms from analyses of CCI4 extracts of
cryogenically ground samples from all five catalyst beds (cf., Figures 2.3 and 2.4).
Several peaks are assigned on the figure to provide benchmarks; others can be
identified by reference to Figure 2.5. While methylbenzenes are the major species
present early in the lifetime of the catalyst, methylnaphthalenes are also significant
after one or two mL of methanol. Phenanthrene and pyrene are prominent on the
deactivated catalyst.
35
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of the organic compounds are still included in intact cages within the debris. For the
present we are satisfied with the result that cryogenic grinding and acid digestion
liberate organic species that are equally representative of those in the used catalyst.
A recent paper applied acid digestion to determine the aromatic compounds
confined in HSAPO-34 after short times on stream. 14 That study identified
methylbenzenes and a small amount of naphthalene, but not phenanthrene and
pyrene, which presumably would have formed with longer time on stream. The
earlier study also found that di- and trimethylphenols formed in the catalyst in
moderate amounts. We observed phenols in only one case, but then they were
determined to be an artifact. During cryogenic grinding the sample is contained in a
polycarbonate capsule. In one case this capsule failed while milling a catalyst
sample, and the interior of the capsule was heavily scored. The CCI4 extract of this
sample contained small amount of phenols, but since we had used methanol-I3C for
the experiment, the origin of the phenols was readily attributed to debris from the
capsule. We also did not observe methylphenols in our acid digested samples of
HSAPO-34. We currently have no explanation for the formation of phenols in the
earlier study but not in the present.
36
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2.5 References and Notes
(1) Goguen, P. W.; Xu, T.; Barich, D. H.; Skloss, T. W.; Song, W.; Wang, Z.;
Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1998,120, 2650.
(2) Haw, J. F.: Nicholas, J. B.; Song, WDeng, F.: Wang, Z. and Heneghan,
C. S. J. Am. Chem. Soc. 2000,122,4763.
(3) Song, W.: Haw, J. F.; Nicholas, J. B.; and Heneghan, K. J. Am. Chem.
Soc. 2000,122, 10726.
(4) Song, W.: Nicholas. J. B.: and Haw. J. F. J. Phys. Chem. B 2001,105 (19),
4317.
(5) Song, W.; Fu. H. and Haw. J. F. J. Am. Chem. Soc. 2001,123.4749.
(6) Stocker. M. Microporous Mesoporous Mater. 1999,29.3.
(7) Wilson. S. and Barger, P. Microporous Mesoporous Mater. 1999,29, 117.
(8) Wilson. S. and Barger. P. Microporous Mesoporous Mater. 1999, 29, 117.
(9) Dahl. I. M. and Kolboe. S. J. Catal. 1996,161,304.
(10) Dahl. I. M. and Kolboe. S. J. Catal. 1994,149,458.
( 11) Mole, T.; Whiteside, J. A. and Seddon, D. J. J. Catal. 1983,82, 261.
(12) Mole, T.; Bett, G. and Seddon, D. J. Catal. 1983,84,435.
(13) Magnoux. P.; Roger, P.; Canaff. C.: Fouche, V.: Gnep, N. S. and Guisent,
M. Catalyst Deactivation 1987,34, 317.
(14) Arstad. B. and Kolboe, S. Catal. Lett. 2001, 71, 209.
(15) Sweet, D.: and Hildebrand, D.; J. Forensic Sci. 1998,43, 1199.
(16) Gill, P.: Ivanov, P. L.; Kimpton, C.; Piercy, R.; Benson, N.; TuIIy, G.;
Evett, I.; Hagelberg, E. and Sullivan. K. Nature Genetics 1994, 6, 130.
(17) Lok. B. M.; Messina, C. A.: Patton, R. L.; Gajek. R. T.; Cannan. T. R.;
Flanigen, E. M. U.S. Patent 4.440, 871,1984.
37
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(18) Haw, J. F.; Goguen, P. W.; Xu, T.; Skloss, T. W.; Song, W. and Wang, Z.
Angew. Chem. 1998,37,948.
38
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Chapter 3
Selective Synthesis of Methylnaphthalenes in
HSAPO-34 Cages and Their Function as Reaction
Centers in Methanol-to-OIefin Catalysis
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3.1 Introduction
The conversion of methanol to hydrocarbons on zeolites and other
microporous solid acids is one of the most-studied fundamental problems in
catalysis/ ^ Hundreds of papers have attacked some part of this problem, usually
some part of the reaction mechanism, using catalytic, spectroscopic, or theoretical
approaches. Practical interest in this chemistry is considerable: the petrochemical
industry is investing in integrated natural gas to polyolefins plants that will
incorporate methanol-to-olefin (MTO) catalysis with otherwise mature technologies.
The catalysts most likely to be commercialized are based on silico-
aluminophosphates of the CHA topology, SAPO-34 or derivatives thereof/ Cages of
ca. 1.0 by 0.7 nm interconnected by 8 -ring windows ca. 0.38 nm in diameter are the
essential features of the CHA topology. The eight-ring windows permit the diffusion
of methanol, dimethylether. ethylene, propene, and linear C4 hydrocarbons, but
branched products, even isobutane, are unable to pass from one cage to the next.
The first indications that cyclic organic species can have roles in methanol
conversion catalysis came from the work of Mole who reported that toluene acted as
a “co-catalyst” for the conversion of methanol to hydrocarbons on zeolite HZSM-
Kolboe studied MTO chemistry on both HZSM-5 and HSAPO-34 and
proposed a phenomenological “hydrocarbon pool” mechanism in which olefin
7 8
synthesis occurs on an entrained organic species of unspecified composition. ' We
used solid state NMR analysis of quenched catalyst samples and GC or GC-MS
40
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analysis of the volatile product stream to identify specific organic species formed
during MTO conditions. On zeolite HZSM-5 we found that cyclic carbenium ions
form under MTO reaction conditions and that these function as scaffolds for carbon-
9.10
carbon bond forming and breaking reactions in MTO catalysis. ‘ We found that on
the less strongly acidic catalyst HSAPO-34 the organic species formed under MTO
reaction conditions are primarily methylbenzenes, and that these are reaction centers
for MTO catalysis. 11 HSAPO-34 catalysts without methylbenzenes (or other
aromatics) were not active for MTO catalysis, and fully deactivated catalysts have a
carbon weight percentage and NMR spectrum consistent with an average structure of
one methylnaphthalene molecule per cage. A follow-up study established a
relationship between the number of methyl groups on a methylbenzene reaction
12
center and the ethylene/propene selectivity. We found that cages containing
benzene molecules with four or more methyl groups favored propene (and an
aromatic with three fewer methyl groups) while those with two or three methyl
groups favored ethylene (and xylene or toluene, respectively). Kolboe has also
obtained evidence that methylbenzenes constitute the hydrocarbon pool in HSAPO-
34. 1 3 Acid digestion was used in that work to dissolve the catalyst framework and
liberate the organics for chromatographic analysis.
Every catalyst cage containing a reaction center can be regarded as a
P
supramolecule with distinct properties. As an example we use the notation
41
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{B6 }— »{B4, e} to denote a reaction in which a supramolecule comprised of a
hexamethylbenzene molecule, B6 , in a cage with its associated acid site, {},
undergoes unimolecular decomposition to form tetramethylbenzene, B4,
(equilibration of isomers is assumed to be rapid) and ethylene, e, with a rate constant
ke6 C - The major alternative pathway in HSAPO-34 would be formation of propene
{B6 }— »{B3, p} with a rate constant k{$ 6 P- The relative values of ks 6C and kB6 P
would define the intrinsic product selectivity for {B6 }, and knowledge of the
concentrations of all methylbenznene supramolecules {Bn} and their associated
unimolecular rate constants for ethylene or propene formation would quantitatively
relate product selectivity (in the absence of secondary reactions) to the molecular
structure of the catalyst. The role of methanol (and equilibrated dimethylether)
apparently is to remethylate the reaction centers, e.g., {B4, CH3 OH}— »{B5. H2 O}.
A major objective in MTO catalysts is the rational control of olefin product
selectivity and increases in ethylene are particularly desirable. The realization that
entrained organic species (i.e.. reaction centers or hydrocarbon pool) are key to MTO
catalysis suggests the possibility of many new catalytic materials through the
synthesis of supramolecules distinct from {Bn}. Here we report the selective
synthesis of naphthalene in HSAPO-34, giving rise to the first example of a distinct
catalyst prepared by constructing an organic reaction center other than
methylbenzenes in this catalyst. We demonstrate that the supramolecules based on
methylnaphthalenes {Nn} exhibit higher ethylene selectivity than methylbenzene-
42
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based catalysts tested under very similar conditions, while their activities are reduced
by only a factor of ca. three.
3.2 Experimental Section
Materials and Reagents. HSAPO-34 was prepared according to a patent
14
procedure. XRD showed a pure crystalline phase with the CHA structure. The
product was calcined at 600 °C for 10 hours to remove the template agent and
pressed into 10-20 mesh pellets. The Bronsted site concentration was determined to
be 1.1 mmol/g. Methanol-l3C was obtained from Isotech, Inc.
Catalysis. Experiments were performed using the pulse quench reactor
described elsewhere1 3 with the exception that aliquots of methanol larger than 20 ptL
were delivered using a motor-driven syringe pump (Harvard Apparatus model PHD
2 0 0 0 ) while smaller aliquots were delivered using in pulses using a switchable valve
as before. For each experiment a bed consisting of 300 mg of catalyst was activated
at 400 °C in the reactor under 600 seem He flow for 2 h immediately prior to use.
This carrier gas feed rate was also used during methanol introduction in all
experiments. We treated each catalyst as described below to form either
methylbenzenes or methylnaphthalenes as reaction centers and then evaluated
activity and selectivity at 400 °C. This was done by pulsing 5 pL of methanol or
m ethanol-onto the catalyst and then collecting a gas sample 2.4 s later for GC or
GC/MS analysis. A small volume of methanol was used for this test so as to
43
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minimize alteration of the catalyst by synthesis of additional reaction centers. In
order to provide for the analysis of the organic reaction centers following either
synthesis or subsequent use in catalysis, the catalyst bed was thermally quenched
immediately following gas sampling. Quenched catalyst beds were then transferred
to a nitrogen glove box for subsequent handling.
Gas Chromatography of Olefin Products. A Hewlett-Packard Model 6890
gas chromatograph with flame-ionization detector was used to analyze gases
sampled from the reactor product streams using a Valeo valve. The column was 150
m dhl50 (Supelco) operated isothermally at 323 K.
Ex Situ Analysis o f Organic Reaction Centers. Thermally quenched
catalyst beds were subjected to acid digestion, extraction, and GC-MS analysis to
determine the distributions of entrained organic species and in some cases to identify
carbon label scrambling. The entire catalyst bed was ground and then a 60 mg
representative sample was treated with 2.0 mL of 1.0 M HCI to destroy the inorganic
framework. Entrained organic matter was then extracted into 0.2 mL CCI4 (Aldrich.
99.99 %). 2 pL of this was then injected into a Hewlett-Packard Model 6890 gas
chromatograph with a model number 5973 mass-selective detector. The split ratio
was 0.1:1. The column was a 50 m HP-1, the He gas flow was 0.3 mL per min., and
the temperature program was 60 °C to 250 °C at a 4 °C/min. ramp.
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3.3 Results
Naphthalene catalysts. We explored strategies for the synthesis of
naphthalenes by varying conditions of treatment using ex situ analysis of the
organics formed in cages as a guide. Figure 3.1 reports representative results for a
naphthalene-based catalyst. We flowed 250 pL of methanol onto a 300 mg HSAPO-
34 catalyst bed at 600 °C at a rate of 50 pL/min., then aged the catalyst with He flow
for 15 min. prior to quenching the reactor temperature to ambient. The catalyst bed
was digested using 1 M HCl and the CClq-extracts were analyzed using GC-MS.
The total ion chromatogram of this sample (Figure 3.1a) shows 80 % naphthalene
and methylnaphthalenes, 2 % phenathrene and 9 % toluene. The balance included
pyrene and various oxidation products. Organic-treated HSAPO-34 catalysts
invariably contain small amounts of toluene which is inactive for olefin synthesis.
Standard addition was used to determine that the naphthalene content of the catalyst
corresponded to one molecule per ca. 90 cages.
We repeated the synthesis of naphthalenes at 600 °C and reduced the reactor
temperature to 400 °C for catalyst testing. Since methylbenzenes naturally form on
HSAPO-34 during the conversion of methanol to olefins it was necessary to test the
naphthalene catalyst using the smallest practical amount of methanol so as to avoid
altering catalyst composition. We pulsed 5 pL of methanol onto the catalyst at 400
°C, collected a gas sample for GC analysis of volatile olefin products 2.4 s after
injection, and then thermally quenched the catalyst bed. Ex situ analysis of this
catalyst (Figure 3.1 b) showed only negligible synthesis of methylbenzenes. Most of
45
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Figure 3.1. Preparation and characterization of HSAPO-34 catalysts containing
methylnaphthalene reaction centers, (a) GC-MS total ion chromatogram from the
ex situ analysis of organic centers formed by flowing 250 fiL of methanol onto
SAPO-34 at 600 °C at a rate of 50 |xL per min. Naphthalene is the major product
with a smaller amount of phenanthrene. Small amounts of unreactive toluene are
ubiquitous on treated HSAPO-34 catalysts, (b) As previous except that 5 jiL of
methanol was then pulsed onto the catalyst at 400 °C and allowed to react for 2.4 s
prior to thermal quench. Ex situ analysis revealed extensive methylation of
naphthalene rings, (c) GC (FID) analysis of the volatile products sampled 2.4 s after
the 5 pL methanol pulse delivered at 400 °C. Total conversion was ca. 21 % and
ethylene selectivity was ca. 38 %. DME denotes dimethylether.
46
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DME Propene
\
Ethylene
\
Methanol
10
I i i i i I i i i i | i i i i |
15 20 25
Retention Time (min.)
i
X 4
JUL
(CH3 ),
a
j Q O
i
. 1
15 20 25 30 35 40
Retention Time (min.)
47
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the naphthalene rings had between one and four methyl groups after this procedure.
Figure 3.1c is the GC (FDD) trace of the volatile hydrocarbon products obtained
using the naphthalene catalyst. Conversion was only ca. 21 % due to the low
concentration of reaction centers and reduced activity of methylnaphthalenes (vide
infra). The ethylene selectivity was 38 %, at face value an encouraging result.
Methylbenzene catalysts. In order to interpret the 38 % ethylene selectivity
on the naphthalene-based catalyst, we must make a comparison with a
methylbenzene-based catalyst operated at comparable methanol conversion. Our
usual methylbenzene-based HSAPO-34 catalysts have much higher concentrations of
reaction centers, and they show 100 % methanol conversion. We prepared two
methylbenzene-based catalysts with lower than normal organic content so as to
reduce conversion below 100 %. These catalysts were prepared and tested at 400 °C.
We first pulsed either 10 or 20 pL of methanol to synthesize methylbenzenes at
levels comparable to that of the naphthalene. The catalysts were then held at 400 °C
for 30 min. to match the time needed to cool the naphthalene catalyst prior to testing.
In each case, we then pulsed 5 |iL of methanol, sampled the product stream at 2.4 s
and thermally quenched the catalyst beds for ex situ analysis.
Results for the catalyst prepared with the smaller concentration of
methylbenzenes are shown in Figures 3.2a and 3.2c. This catalyst achieved 20 %
conversion, very similar to that for the naphthalene catalyst, but it showed an
ethylene selectivity of only 30 %. The organics trapped in the catalyst were
48
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Figure 3.2. Preparation and characterization of HSAPO-34 catalysts containing
methylbenzene reaction centers at concentrations comparable to that of the
methylnaphthalene centers in the previous result. (a) GC-MS total ion
chromatogram from the ex sitii analysis of organic centers formed by pulsing 10 pL
of methanol onto SAPO-34 at 400 °C followed 30 min. later by 5 pL of methanol,
(b) As previous except that 20 pL of methanol was delivered in the first pulse. The
total level of methylbenzenes is ca. four-fold higher, (c) GC (FID) analysis of the
volatile products sampled 2.4 s after the 5 pL methanol pulse was delivered to the
catalyst in (a). Total conversion was ca. 20 % and ethylene selectivity was ca. 30 %.
(d) GC (FID) analysis of the volatile products sampled 2.4 s after the 5 pL methanol
pulse was delivered to the catalyst in (b). Total conversion was ca. 56 % and
ethylene selectivity was ca. 29 %.
49
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Ethylene
Propene
DME
Methanol
- *
i i i i I i i i i I i i i i I
10 15 20 25
Retention Time (min.)
_L l l
b
ICH3 )2
. i d
6 x .
j — i i .
i i i i i i i i ■ i
L i
I I I ! 1 I I T
* x 4
S a
i i i I ' T r n n
15 20 25 30 35 40
Retention Time (min.)
50
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primarily tri-, tetra-. and pentamethylbenzenes. The catalyst with the larger
concentration of methylbenzenes (Figures 3.2b and 3.2d) had a total concentration of
methylbenzenes gave 56 % conversion and 29 % ethylene selectivity.
Label Scrambling. Kolboe and co-workers reported l3C label scrambling
between ring and methyl positions under MTO conditions. For example, when they
converted a mixture of methanol-l3C and natural abundance benzene or toluene to
hydrocarbons on zeolite HZSM-5, the recovered methylbenzenes clearly showed 13C
incorporation into the ring. 1 6 We carried out experiments similar to those reported in
Figures 3.1 and 3.2 in order to determine whether or not the methylnaphthalene
reaction centers also show I3C scrambling into the ring positions. Aromatic rings
were synthesized using natural abundance methanol and then l3 C-labeled methanol
was pulsed onto the catalyst at 400 °C and allowed to react for 2.4 s prior to thermal
quench, acid digestion and GC-MS analysis. As a control experiment for the study of
l3C scrambling in methylnaphthalene rings we checked for scrambling in
methylbenzenes using an experiment very similar to that in 3.2b except using R e
labeled methanol for the second pulse. Similar to previous work by Kolboe we see a
modest degree of scrambling for toluene and extensive scrambling for
pentamethylbenzene with the overall extent of scrambling increasing with the
number of methyl groups. Some of the pentamethylbenzene scrambles completely
51
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m-2 m m+12 m-2 m m+12
Figure 3 3 . Bar graphs showing ion mass distributions in the vicinities of molecular
ions for methylbenzenes formed in experiments similar to that in Figure 3.2b. In
each case benzene rings were formed at 400 °C using a 20 pL pulse of natural
abundance methanol followed 30 min. later by a 5 (iL pulse of either natural
abundance methanol (left) or C-methanol (right). Some of the
pentamethylbenzene molecules showed C label exchange into all 11 positions.
52
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as reflected in the observation of a significant m+11 peak. Thus, scrambling into
benzene rings active as reaction centers for MTO chemistry is confirmed in our
studies.
13
Figure 3.4 reports results for C scrambling into methylnaphthalenes from
a
experiment very similar to that in 3.1b, except using C-labeied methanol in the
second pulse. A modest amount of scrambling was seen for unsubstituted
naphthalene recovered from the catalyst, and most of the I3C label incorporation
observed for scrambling for methylnaphthalene can be accounted for by methylation
of naphthalene to methyl- l3 C-naphthalene. However, the di- and
trimethylnaphthalenes readily incorporated l3C into ring positions. Figure 3.4 also
shows that the small amount of phenanthrene that also formed in the cages of
HS APO-34 during the naphthalene synthesis procedure does not exchange l3C with
methanol. Pyrene (not shown) also did not exchange.
3.4 Discussion
The synthesis of naphthalene in HSAPO-34 was remarkably simple.
Methylnaphthalenes. like methylbenzenes are also reaction centers for the
conversion of methanol and dimethylether to olefins. Since methylbenzenes form
spontaneously on HSAPO-34 during methanol conversion, these will quickly
overtake the methylnaphthalene centers, and a methylnaphthalene catalyst will be
53
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■ a l l
(CH3 )4
I I I
(C H 3)3
(CH3)2
m = 184
m = 170
m = 156
m = 142
m = 128
I I I | I I I IITI I I I II I I I I
m-3 m m+16
Figure 3.4. Bar graphs showing ion mass distributions in the vicinities of molecular
ions for naphthalenes and phenanthrene formed in an experiment similar to that in
Figure 3.1b. In each case naphthalene rings were formed at 600 °C using 250 |xL of
natural abundance methanol followed 30 min. later by a 5 pL pulse of C-methanol.
54
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ephemeral under reaction conditions. While no practical catalyst based on
methylnaphthalenes in HSAPO-34 seems feasible, knowledge of the relative activity
of methylnaphthalene and methylbenzene centers in HSAPO-34 is of fundamental
interest. Since the catalysts in figures 3.1b and 3.2a gave almost identical
conversions, we can compare their activities. Comparing these figures, it first must
be acknowledged that methylation of naphthalene rings in HSAPO-34 occurs to a
lesser extent than methylation of benzene rings. We observe quite a bit of
pentamethylbenzene (Figure 3.2a), but only a trace of pentamethylnaphthalenes
(Figure 3.1b). We consider only the relative propensity of methylnaphthalenes to
decompose to ethylene or propene and we use the absolute ion intensitities (not
shown on the Figures) to compare the relative amounts of various species in Figures
3.1b and 3.2a. If we include only those aromatic rings with two or more methyl
groups we count approximately three times as many active (i.e.. di-. tri-. tetra-, or
pentamethyl-) naphthalene rings in Figure 3.1b as active (dimethyl- or higher)
benzene rings in Figure 3.2a. We get the same factor of ca. three using the more
restrictive comparison of considering only those rings with three or more methyl
groups. Thus, we estimate that for the same number of methyl groups a naphthalene
ring in HSAPO-34 is approximately one third as active in the production of olefins
as a benzene-based reaction center in the same catalyst.
We recently reported that penta- and hexamethylbenzene are very active for
olefin production in HSAPO-34 and favor propene, while ethylene selectivity is
1 2
enhanced on less active reaction centers with three or fewer methyl groups. Thus,
55
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ethylene selectivity is enhanced by conditions that reduce the average number of
methyl groups per ring, although these conditions also reduce activity. It seems to us
that the average number of methyl groups per ring may also account for the higher
ethylene selectivity of naphthalenes. Figure 3.1 confirms the presence of
naphthalenes with up to four methyl groups in HSAPO-34 cages (and a trace of
naphthalenes with five), but these are almost certainly distributed over both rings,
and we do not known whether or not methyl migration between ring to ring (i.e.,
from the I position to the 8 position) is facile in HSAPO-34. Thus, 2, 3, 6 , 7-
tetramethylnaphthalene might have an olefin selectivity like tetramethylbenzene,
weakly favoring propene, or like .xylene, which can decompose only to form
ethylene. Steric effects may also be important. For example, a
tetramethylmethylnaphthaIene-HSAPO-34 supramolecule {N4} has much less free
volume than a tetramethyIbenzene-HSAPO-34 supramolecule {B4}, and the former
might favor ethylene as a result of transition state shape selectivity. A third
possibility would be that the naphthalene-based materials better restrict diffusion of
bulky products out the catalysts (product shape selectivity), but with one naphthalene
molecule per ca. 90 cages this seems unlikely.
Aromatic molecules that act as reaction centers for olefin synthesis also
scramble between side-chain and ring positions under reaction conditions. A
1961 paper described the reaction of hexamethylbenzene on a bi-functional catalyst
17
in the presence of H2 to form isobutane. The proposed mechanism of this “pairing
56
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reaction” involves ring contraction (6 — >5) and expansion (5— > 6 ) steps on an acid site
that extend an alkyl chain leading to elimination of isobutene that was subsequently
reduced on the metal site. The pairing reaction necessarily scrambles ring and side
chain carbons in the
course of converting a methylbenzene (or methylnaphthalene) to an olefin and a less
highly substituted aromatic. The observation here and in Kolboe’s work that ring
carbon scrambling occurs under MTO conditions is consistent with but does not
prove a pairing mechanism. An alternative possibility would be scrambling through
a nonproductive benzyl cation * - * ■ tropylium cation ( 6 < -» 7) route operating in
parallel to olefin synthesis. A recent theoretical study predicted a high effective
barrier for this process in the gas phase. 1 8
In experiments of the sort presented in Figures 3.1 and 3.4 every di. tri-, or
tetramethylnaphthalene molecule has on average 17 turnovers resulting in olefin
products, and Figure 3.4 shows, for example, that some of trimethylnaphthalene has
at least six l3C atoms scrambled into the rings. Thus, if ring carbon scrambling is an
artifact of a parallel reaction such as a 6 « - ► 7 route, this parallel reaction fortuitously
occurs at almost exactly the rate of olefin synthesis. While the exact mechanism by
which methylbenzenes and methylnaphthalenes form olefins on HSAPO-34 is not
firmly established by this contribution, the most parsimonious explanation for the
similar rates of olefin synthesis and ring carbon scrambling is a pairing reaction 6 « - > •
57
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5 that extends an alkyl chain while potentially exchanging ring and side chain
carbons.
This investigation is the first report of the synthesis of reaction centers in
HSAPO-34 other than methylbenzenes and their use in catalysis. No doubt others
will be forthcoming, and heteroatom-substituted reaction centers are particularly
attractive synthetic targets. Unfortunately, the methylnaphthalene supramolecules
{Nn} described here are not practical catalysts because they are less active than the
methylbenzene {Bn} sites that spontaneously form on an HSAPO-34 catalyst that is
producing olefins. Thus, a methylnaphthalene catalyst is ephemeral under reaction
conditions and its selectivity advantages are quickly masked by other more active
sites.
58
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3.5 References and Notes
( 1) Chang, C. D. CataL Rev. 1983,25, I-l 18.
(2) Stocker, M. Microporous Mesoporous Mater. 1999, 29, 3-48.
(3) Keil, F. J. Microporous Mesoporous Mater. 1999, 29,49-66.
(4) Wilson, S; Barger, P Microporous Mesoporous Mater. 1999.29, 117-126.
(5) Mole, T.; Whiteside, J. A.; Seddon, D. J. Catal. 1983,82,261-266.
(6 ) Mole, T.: Bett, G.: Seddon, D. J. Catal. 1983,84,435-445.
(7) Dahl, I. M.: Kolboe, S. J. Catal. 1996,161, 304-309.
(8 ) Dahl. I. M.; Kolboe, S. J. Catal. 1994,149,458-464.
(9) Goguen. P. W.; Xu. T.; Barich. D. H.: Skloss, T. W.; Song, W.; Wang, Z.:
Nicholas. J. B.; Haw, J. F. J. Am. Chem. Soc. 1998.120. 2651-2652.
(10) Haw. J. F.: Nicholas. J. B.; Song, W.: Deng, F.: Wang, Z.: Heneghan. C.
S. J. Am. Chem. Soc. 2000,122, 4763-4775
(11) Song, W.; Haw. J. F: Nicholas. J. B.: Heneghan. K. J. Am. Chem. Soc.
2000,122, 10726-10727.
(12) Song, W.; Fu, H.: Haw. J. F. J. Am. Chem. Soc. 20 01 ,123,4749-4754.
(13) Arstad, B.: Kolboe, S. Catal. Lett. 2001. 71. 209-212.
(14) Lok, B. M.; Messina. C. A.; Patton. R. L.; Gajek, R. T.: Cannan. T. R.;
Flanigen. E. M. U.S. Patent 4,440, 871,1984.
(15) Haw. J. F.; Goguen. P. W.; Xu, T.; Skloss, T. W.: Song, W.; Wang, Z.
Angew. Chem. 1998.37.948-949.
(16) Mikkelsen, O.; Ronning, P. O.: Kolboe. S. Microporous Mesoporous
Mater. 2000,40. 95-113.
(17) Sullivan. R. F.; Egan, C. J.; Langlois, G. E.: Sieg, R. P. J. Am. Chem. Soc.
1961,55, 1156-1160.
(18) Ignatyev, I.S.; Sundius, T. Chem. Phys. Lett. 2000,326, 101-108.
59
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Chapter 4
Ship-in-a-Bottle Synthesis of Methylphenols in
HSAPO-34 Cages From Methanol and Air
60
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4.1 Introduction
The conversion of methanol to olefins or other hydrocarbons on microporous
solid acids is an important problem that has generated much research. 1 3 The
catalysts most likely to be commercialized are based on silico-aluminophosphates of
the CHA topology, SAPO-34 or derivatives thereof/ 1 Cages of ca. 1.0 by 0.7 nm
interconnected by 8 -ring windows ca. 0.38 nm in diameter are the essential features
of the CHA topology. The eight-ring windows permit the diffusion of methanol.
dimethylether, ethylene, propene, and linear C4 hydrocarbons, but aromatic
compounds, even benzene, are unable to pass from one cage to the next.
5 6 7 8
Drawing on earlier work by Mole and co-workers * and Kolboe et al. ’ we
demonstrated that methylbenzene molecules formed in the cages of HSAPO-34
function as reaction centers on which carbon-carbon bonds are made and broken
9 10
during methanol-to-olefin (MTO) chemistry. ‘ Without suitable organic molecules
in the cages, HSAPO-34 is not active in methanol conversion. The mechanism by
which methybenzenes decompose on HSAPO-34 to form olefinic products, primarily
ethylene and propene, is presently unknown, but it may be related to the “pairing
reaction” described 40 years ago for the decomposition of hexamethylbenzene to
form isobutane on a bi-functional catalyst in the presence of hydrogen. 1 1 The
mechanism proposed for the pairing reaction involves ring contraction (6<->5)
followed by expansion (5<-*6) that permits extension of an alkyl group that is
61
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eliminated as an olefin from a cationic intermediate. Ring expansion and contraction
are likely to scramble l3C labels between ring and methyl positions, and Kolboe has
1 2
observed scrambling in MTO reactions on various catalysts.
We want to substitute various heteroatoms onto the methylbenzenes in
HSAPO-34 cages and investigate their activity in MTO catalysis. As the first
example we report the selective synthesis of methylphenols in HSAPO-34 cages
which we achieved using 2 0 % air as an oxidant during an otherwise normal
procedure in which methanol was flowed onto the catalyst at 350 °C in a He stream.
Although the literature provides a number of examples of the conversion of benzene
to phenol on aluminosilicate zeolites using N2 O as an oxidant, 1 3 1 7 we found that
even L O O % N2 O was less effective than 20 % air (i.e., 4 % O2 ). We preferentially
formed penta-, tetra-. tri-. and dimethylphenols in the HSAPO-34 cages. No
hydroquinones, catechols, or other products with multiple oxygens were formed.
Methylphenols, unlike methybenzenes, prove to have very low activity for the
formation of olefins in HSAPO-34. Even pentamethylphenol is stable to olefin
elimination at 350 °C. Carbon isotope studies suggest that the limited conversion of
methanol over an HSAPO-34 catalyst containing methylphenols occurs on the small
amounts of methylbenzenes also present on such materials. While methylphenols on
HSAPO-34 exchanged carbon labels only in the methyl positions, tri-, tetra-, and
pentamethylbenzene also scrambled carbon into the ring positions. We also found
that co-feeding p-cresol with methanol into a catalyst bed of HZSM-5 resulted in no
62
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MTO rate enhancement whereas a significant enhancement was seen with toluene as
a co-feed. 5 ’ 5 We interpret the low catalytic activity of phenols in terms of their high
gas phase proton affinities.
4.2 Experimental Section
Materials and Reagents. HSAPO-34 was prepared according to a patent
1 8
procedure. XRD showed a pure crystalline phase with the CHA structure. The
product was calcined at 600 °C for 10 hours to remove the template agent and
pressed into 10-20 mesh pellets. The Bronsted site concentration was determined to
be 1.1 mmol/g. Methanol-tJC was obtained from Isotech. Inc. N2 O (99%) was from
Aldrich. We also used zeolite HZSM-5 (Si/AI = 15) from Zeolyst International.
Catalysis. Experiments were performed using the pulse quench reactor
19-2 L
described elsewhere. Aliquots of methanol larger than 20 pL were delivered
using a motor-driven syringe pump (Harvard Apparatus model PHD 2000) while
smaller aliquots were delivered using in pulses using a switchable valve. For each
experiment a bed consisting of 350 mg of catalyst was activated at 400 °C in the
reactor under 600 seem He flow for 2 h immediately prior to equilibration at 350 °C
for use. In order to provide for the analysis of the organic compounds in HSAPO-34
samples following either synthesis or subsequent use in catalysis, the catalyst bed
was thermally quenched then transferred to a nitrogen glove box.
63
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Gas Chromatography o f Olefin Products. A Hewlett-Packard Model 6890
gas chromatograph with either a flame-ionization detector or a model 5973 mass-
selective detector was used to analyze gases sampled from the reactor product
streams using a Valeo valve. The column was 150 m dhl50 (Supelco) operated
isothermally at 323 K.
NMR Spectroscopy. ,3C solid state NMR experiments were performed with
magic angle spinning (MAS) on a Varian Infinity plus 300 MHz spectrometer
operating at 75.4 MHz for I3C. Hexamethylbenzene (17.4 ppm) was used as an
13
external C chemical shift standard. Chemagnetics-style pencil probes were used.
We measured spectra using cross polarization (CP. contact time = 2 ms, pulse delay
= I s. 4000 transients) and cross polarization with interrupted decoupling (contact
time = 2 ms. pulse delay = I s, 4000 transients, dipolar dephasing time of 50 ps).
Ex Situ Analysis of Organic Reaction Centers. Thermally quenched
T ?
catalyst beds were subjected to acid digestion, extraction, and GC-MS analysis to
determine the distributions of entrained organic species and in some cases to identify
carbon label scrambling. The entire catalyst bed was ground and then a 60 mg
representative sample was treated with 2.0 mL of 1.0 M HCI to destroy the inorganic
framework. Entrained organic matter was then extracted into 0.2 mL CCI4 (Aldrich.
99.99 %). 2 pL of this was then injected into a Hewlett-Packard Model 6890 gas
chromatograph with a model 5973 mass-selective detector. The split ratio was 0.1:1.
64
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The column was a 50 m HP-1, the He gas flow was 0.3 mL per min, and the
temperature program was 60 °C to 250 °C at a 4 °C/min ramp.
4 3 Results
Synthesis of methylphenols in HSAPO-34. Figure 4.1 reports GC-MS total
ion chromatograms that profile the organic compounds retrieved from various
~ r >
catalyst samples through acid digestion followed by extraction. This procedure
has previously been validated for aromatic hydrocarbons in HSAPO-34 using the
alternative method of cryogenic grinding, which does not involve exposure to
aqueous acid. The three results in Figure 4.1 were obtained using otherwise identical
procedures except for the composition of the carrier gas. In each case 100 |xL of
methanol was flowed onto 350 mg of catalyst at a rate of 50 pLmin 1 at a
temperature of 350 °C. This temperature was maintained for an additional 5 min.
then the catalyst temperature was quenched to ambient.
Figure 4.1a shows that methylbenzenes are formed selectively in HSAPO-34
when the carrier gas is 100 % He. Using instead 100 % N2 O (Figure 4.1b) we
observed a modest conversion of the entrained aromatics to phenols, especially tri-
and tetramethylphenols, and the yield of naphthalenes was also elevated. Figure 4.1c
reports the result obtained using 20 % air. Here, the overall yield of organics is
reduced by a factor of ca. two, suggesting some total oxidation, and we do indeed
detect (by GC) a small amount of CCH in the product stream. However, the
65
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CH,
♦
OH
(CH3 )j
6 -
OH
(CH3 )4
J L u l L ^ . J i J L i k
/
(CHj),
r t
r S
CC H 3 )3
J L jlJ
(CH3 )4
r ) >
« i « —
W a> a >
i i I i m i I i i i i I i in p i i m I i i m I m i i i i i i i i i i n i m i i i m i m r i p i i niiT T
14 16 18 20 22 24 26 28 30 32 34 36 38
Retention time (min.)
Figure 4.1. GC-MS total ion chromatograms from the ex sitii analyses of aromatics
formed from methanol in HSAPO-34 catalyst beds at 350 °C with various carrier
gases, (a) Control experiment using 100 % He (200 seem) as the carrier gas.
Methylbenzenes were formed preferentially in the HSAPO-34 cages, (b) Using 100
% NjO (50 seem) as the carrier gas there was a modest yield of methylphenols as
well as some conversion to naphthalenes, (c) Using 20 % air in He (250 seem total)
there was some reduction in total aromatics due to total oxidation, but the products
remaining in the HSAPO-34 ages were primarily methylphenols.
66
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predominant species in the HSAPO-34 cages are methylphenols with two to five
methyl groups. We used this procedure for much of the work reported below. We
also determined that methylphenols could not be extracted from HSAPO-34 without
prior acid digestion to destroy the framework.
We also used l3C solid state NMR to characterize the formation of
methylphenols in HSAPO-34. Samples very similar to those in Figures 4.1a and
4.1c were prepared using methanol-l3 C. Following thermal quench, the catalyst
beds were transferred to magic-angle spinning rotors without exposure to air, and
NMR spectra were measured at room temperature. Inspecting those spectra (Figure
4.2) one first notes that the degree of spectral overlap is such that ex situ analysis is
clearly more informative than an in situ measurement in this case. Yet. even here,
NMR provides complimentary information not found in the ex situ analysis. The
prominent signals at 56 ppm in all spectra in Figure 4.2 are due to framework-bound
methoxy (methoxonium) groups formed by O-methylation of conjugate base sites on
9
the framework of HSAPO-34. These species, plausible intermediates in ring
methylation, are necessarily hydrolyzed by acid digestion, and the liberated methanol
is partitioned into the aqueous acid rather than CCI4 . The cross polarization
spectrum in Figure 4.2a reveals intensity near 150 ppm, and this signal is slightly
clearer in the spectrum obtained with interrupted decoupling(Figure 4.2b). This
signal is not present in corresponding spectrum of methylbenzene material (Figure
67
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Figure 4.2. 75 MHz 1 3 C CP/MAS NMR spectra of aromatic products formed in
HSAPO-34 from methanol-l3C at 350 °C. The signal at 56 ppm in all spectra is due
to framework-bound methoxy species, (a) Cross polarization spectrum of a material
prepared using 20 % air in He as the carrier gas. The broad signal near 150 ppm is
characteristic of phenols. (b) Interrupted decoupling spectrum of the sample
prepared with 20% air confirms that the 150 ppm signal is from a substituted carbon
rather than a C-H group. Most signals from this sample survived interrupted
decoupling because they were either substituted aromatic carbons or methyl groups,
(c) Cross polarization spectrum from a control experiment in which 100 % He was
used as a carrier gas. This spectrum is consistent with methylbenzenes and does not
show the 150 ppm signal assigned to phenols, (d) The interrupted decoupling
spectrum from the control experiment supports the assignments to methylbenzenes
and framework methoxy species.
68
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f n i i | t i 1 1 1 1 1 1 1 1 1 1 1 r y m 1 1 i n 1 1
250 200 150 100 50 0 -50
ppm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.2b and 4.2c). The 150 ppm signal is characteristic of phenols; for example 152
ppm for C l of p-cresol.
In a recent paper we described experiments in which we synthesized
methylbenzenes in HSAPO-34 then abruptly terminated methanol flow and
monitored the decrease in the number of methyl groups per ring as these compounds
eliminated olefins. 1 0 Here we used the acid digestion method to make an analogous
study of the stability of methylphenols in HSAPO-34, and these results are reported
in Figure 4.3. Figure 4.3a is the sample from Figure 4.1c as a control with no aging.
Figure 4.3b shows that after 30 min of aging at 350 °C there was little decrease in the
amounts of methylphenols. In particular, pentamethylphenol was essentially
unchanged, whereas we would expect a substantial decline in pentamethylbenzene as
a result of elimination of propene and ethylene. Using more severe conditions, aging
at 450 °C for 30 min (Figure 4.3c) does show a decline in methylphenols, but with a
corresponding increase
in naphthalenes, indene. and toluene. Some of the more prominent oxygenates
remaining in the catalyst after aging at 450 °C included naphthols and phenalen-l-
one.
Catalyst Testing. We prepared HSAPO-34 materials with methylbenzenes
(very similar to that analyzed in Figure 4.1a) and methylphenols (cf. Figure 4.1c) and
immediately tested their MTO activity using a 5 |xL pulse of methanol-l^C. This
pulse was small enough so as to not form a significant amount of additional
70
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Figure 4.3. Study of the aging of methylphenols in HSAPO-34 at various times and
temperatures. Shown are GC-MS total ion chromatograms from ex situ analyses
(acid digestions). All samples were formed by first feeding methanol into HSAPO-
34 catalyst beds at 350 °C using 20 % air in He as the carrier gas to form phenols and
then holding at the indicated temperature and time while flowing 100 % He (200
seem) to age the sample prior to quench, (a) Control experiment (identical to Figure
Ic) without aging, (b) After aging for 30 min at 350 °C the loss of methyl groups
from pentamethylphenol was negligible, (c) Aging for 30 min at 450 °C resulted in a
significant reduction in phenols, large increases in fused polycyclic species,
especially naphthalene, indene. and naphthols, and the formation of other oxidation
products such as phenalen-l-one.
71
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CH,
o >
Jjl_ -JL J i n i l J L l uj i j j L
(CHj)3
OH
t
1 1 I J i « i .1 1 1 ! a. jLi HW JsL.
(CH3 )2
OH OH
o h n j K C H j h yfar
< C H 3 )j \ \ Z'
\
m kjJU m
ii i n i]iti i p 1111111 rp rTi]i 11111111111111 m 1 1 m 1 1
14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
Retention time (min.)
72
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
methylbenzenes. The volatile products leaving the catalyst bed 4.0 s after injection
were analyzed by GC-MS, and the catalyst bed temperature was quenched to
ambient for acid digestion and ex-situ analysis of aromatic compounds by GC-MS.
Figure 4.4 reports the GC-MS total ion chromatograms from the analyses of volatile
products as well as bar graphs depicting ion mass distributions in the vicinities of
molecular ions from the centers of selected chromatographic peaks.
Figure 4.4a shows that the HSAPO-34 bed with methylbenzenes in the cages
realized ca. 8 6 % conversion of methanol and DME to hydrocarbons. The ion mass
distributions from this experiment show extensive carbon label scrambling for both
ethylene and propene, consistent with olefin synthesis on a hydrocarbon pool.
Unreacted DME and methanol however were fully l3C labeled. Aromatic ring
methylation by these reagents is very exothermic; thus, the reaction is essentially
12
irreversible, and C does not accumulate in the unreacted oxygenates. Figure 4.4b
shows that the sample of HSAPO-34 with methylphenols in the cages was far less
active as an MTO catalyst —conversion was only 9 %. Ethylene and propene were
here also scrambled.
Figure 4.5 reports GC-MS results from ex situ analyses of the aromatics in the
two catalysts used to obtain the results in Figure 4.4. The methylbenzenes
underwent a considerable amount of ring scrambling; a fraction of the p-xylene was
scrambled in all eight positions and much of the hexamethylbenzene scrambled into
ten to twelve positions. A very different result was obtained for l3C scrambling into
73
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Figure 4.4. GC-MS total ion chromatograms from the analyses of volatile olefinic
products sampled 4.0 s after pulsing 15 (iL of methanol-l3C onto HSAPO-34
catalyst beds containing either methylbenzenes or methylphenols prepared from
natural abundance methanol. Ion mass distributions in the vicinity of the molecular
ion are shown for ethylene, propene, dimethylether (DME) and methanol. On both
catalysts the olefins showed extensive label scrambling, but methanol and DME
13 13
maintained their C enrichment. Methanol- C is also slightly enriched in ~H. and
this accounts for the presence of small signals at m/e 34 and 35. (a) On the
methylbenzene material conversion to volatile hydrocarbons was ca. 8 6 %. (b) On
the methylphenol material conversion to volatile hydrocarbons was much lower, only
ca. 9 %.
74
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Propene
42 D M E
Ethylene
Air
■ ■
Ethylene
28
\
A
42 Propene
.1
DME
46
CH3 OH
32
\
r i
C4
a*
J\ A _
1111111111 1111 111 11111111111 M 11 k 1 1 1 1 1 111111 11 11 1111
15 16 17 18 19 20
Retention time (min.)
75
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m=162
m=148
. 1
m=134
&
m=120
6
111111111111111111
m-4 m m+12
x x
m=164
(CH3 )4
m=150
(CH3 )3
m=136
(CH3 )2
m=122
m-4 m m+5
Figure 4.5. Bar graphs showing ion mass distributions in the vicinities of molecular
ions for methylbenzenes and methylphenols from ex situ analyses of the catalyst
beds from the same experiments giving rise to Figure 4.4. (a) The methylbenzenes
originally formed in HSAPO-34 from natural abundance methanol incorporated a
substantial amount of C into ring positions following a small pulse of methanol-
13
C. (b ) In contrast, methylphenols exchanged carbon in methyl positions but not
ring positions.
76
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the methylphenols. The ion mass distribution patterns in Figure 4.5 suggest that only
methyl group positions exchanged for the methylphenols; for example,
pentamethylphenol had an m+5 peak but not an m+ 6 peak. This conclusion was
supported by a detailed interpretation of fragment peaks in the mass spectra (not
shown). We also carried out these experiments on a sample of HSAPO-34 with
similar amounts of both methylbenzenes and methylphenols. Results analogous to
Figure 4.5 confirmed that I3C label exchange occurred preferentially into the rings
of methylbenzenes and not methylphenols in the identical catalyst.
As a final test of the potential of methylphenols as reaction centers
(hydrocarbon pool species) in MTO chemistry we co-injected a solution of p-cresol
in methanol (1:10 molrmol) into the aluminosilicate zeolite HZSM-5 at 375 °C in a
flow reactor using conditions very similar to those for the experiments in Figure 4.4.
HZSM-5 has pore openings large enough to admit methylbenzene derivatives, thus
they may be introduced in the feed stream and need not be synthesized in situ as with
HSAPO-34. The control experiment in Figure 4.6a shows that the first pulse of pure
methanol is almost unreactive on an HSZSM-5 catalyst bed. Consistent with the
earlier reports of Mole and coworkers, we found that toluene in methanol (1:10
molrmol) was very reactive on the HZSM-5 catalyst bed (Figure 4.6b). Substituting
p-cresol for toluene (Figure 4.6c) yielded a conversion much closer to that of
methanol alone, confirming that methylphenols are inactive or at best very weakly
active for MTO chemistry.
77
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DME
Ethylene
Propene
Propane
C4
Air
15 16 17 18 19
Retention time (min.)
Figure 4.6. GC (FID) analyses of the volatile products sampled 2.4 s after injecting
12.5 |iL of solutions of methanol onto 300 mg catalyst beds of zeolite HZSM-5 at
375 °C. (a) Control experiment using methanol alone. Conversion was only ca. 9
% . (b ) Using toluene in methanol (1:10 molrmol) conversion was high. ca. 95 %.
(c) Using p-cresol in methanol (1:10 molrmol) conversion was also ca. 9 %.
78
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4.4 Discussion
Feeding 20 % air into an otherwise inert carrier gas stream during methanol
conversion afforded a high yield of methylphenols trapped in the cages of HSAPO-
34. A number of literature reports1 3 1 5 and patents16' 1 7 describe the use of N2 O for
the oxidation of benzene to phenol on various zeolites, either underivatized or treated
to introduce transition metals. Given that prior art we were surprised that N2 O was
less active for the oxidation of methylbenznenes in HSAPO-34 cages. Never-the-
Iess dilute air was effective, and we were able to prepare HSAPO-34 materials with a
single phenolic function on the majority of entrained benzene rings. Given the high
yields of pentamethylphenol we speculate that the more highly substituted
methylbenzenes are more easily oxidized under these conditions, reflecting the
decrease in ionization energies for phenols as methyl substitutents are added. A
recent study from another laboratory used acid digestion to characterize the
22
methylbenzenes formed in HSAPO-34 during MTO chemistry. That study also
reported the formation of low levels of phenols. We reproduced this finding only
when traces of air were introduced into the reactor.
By every measurement we made, methylphenols are not active as reaction
centers for MTO chemistry in HSAPO-34. Materials with high fractions of
methylphenols show much lower conversions than materials with comparable
methylbenzene contents and the activity of the phenolic catalysts is traceable to
residual methylbenzenes which undergo far more extensive carbon label scrambling
79
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including ring scrambling. Methylphenols in HSAPO-34 undergo methyl label
exchange with methanol but not ring exchange. Futhermore, when we co-fed p-
cresol and methanol into zeolite HZSM-5 there was no MTO rate acceleration
compared to pure methanol whereas toluene co-feed showed a considerable rate
enhancement, as was originally reported by Mole and co-workers.6 ' 7
Methylphenols are not reaction centers on either HSAPO-34 or HZSM-5. The
most active methylbenzenes are those with the most methyl groups. Given that we
easily made pentamethylphenol in HSAPO-34, methylation is not the limiting factor,
yet even this molecule was stable to olefin elimination. The detailed mechanism by
which methylbenzenes form olefins on MTO catalysts is unknown, although ring
carbon scrambling is consistent with the mechanism proposed for the pairing
reaction. 11 This mechanism involves ring contraction (6 «-o) followed by expansion
(5<-*6) that extends of an alkyl group which is eliminated as an olefin from a
positively charged intermediate. This complex sequence of mechanistic steps is ripe
with opportunities for perturbation by a heteroatom substituent. Yet, the explanation
here may be simple. Phenol is a very basic molecule in the gas phase. Its proton
affinity (195.3 kcal/mol) is 16 kcal/mol greater than that of benzene. Furthermore,
phenols may be able to form co-operative (donor and acceptor) hydrogen bonds with
the acid site of HSAPO-34. Thus, a methylphenol in an HSAPO-34 cage may tie up
the acid site in a hydrogen bonding interaction that is too strong to permit entry into
a sequence of intermediates with positive charge on carbon rather than oxygen.
80
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If the above interpretation is correct then it is still possible that other
heteroatom-substituted methylbenznenes may be active as reaction centers for MTO
chemistry. The synthesis of materials with such derivatives is under active
investigation.
81
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4.5 References and Notes
(L) Chang, C. D. Catal. Rev. 1983, 25, l-l 18.
(2) Stocker, M. Microporous Mesoporous Mater. 1999, 29, 3-48.
(3) Keil, F. J. Microporous Mesoporous Mater. 1999,29,49-66.
(4) Wilson, S; Barger, P Microporous Mesoporous Mater. 1999,29, 117-126.
(5) Mole. T.: Whiteside, J. A.: Seddon. D. J. Catal. 1983,82, 261-266.
(6 ) Mole, T.; Bett, G.: Seddon, D. J. Catal. 1983,84,435-445.
(7) Dahl, I. M.; Kolboe, S. J. Catal. 1996,161,304-309.
(8 ) Dahl, I. M.; Kolboe, S. J. Catal. 1994,149,458-464.
(9) Song, W.: Haw, J. F: Nicholas, J. B.; Heneghan. K. J. Am. Chem. Soc.
2000, 722, 10726-10727.
(10) Song, W.; Fu, H.: Haw, J. F. J. Am. Chem. Soc. 2 0 01 .123. 4749-4754.
(11) Sullivan, R. F.; Egan, C. J.; Langlois, G. E.: Sieg, R. P. J. Am. Chem. Soc.
1961,83, 1156-1160.
(12) Mikkelsen, O.; Ronning, P. O.: Kolboe, S. Microporous Mesoporous
Mater. 2000,40,95-113.
(13) Panov. G. I; Kharitonov, A. S.: Sobolev. V. I. Applied Catalysis A:
General, 1993,98. 1-20.
(14) Panov, G. I.: Sheveleva, G. A.; Kharitonov, A. S.; Romannikov, V. N.;
Vostrikova. L. A. Applied Catalysis A: General, 1992,82. 31-36.
(15) Burch R.: Howitt. C. Applied Catalysis A: General. 1993,103, 135-162.
(16) Sobolev. V. I.; Rodkin, M. A.: Panov, G. I. U.S. Patent 6.156,938. 2000.
(17) McGhee, W. D. U.S. Patent 5.977,008, 1999.
(18) Lok, B. M.; Messina, C. A.: Patton, R. L.; Gajek. R. T.; Cannan, T. R.;
Flanigen, E. M. U.S. Patent 4,440, 871. 1984.
82
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(19) Haw, J. F.; Goguen, P. W.; Xu, T.; Skloss, T. W.; Song, W.; Wang, Z.
Angew. Chem. 1998,3 7 ,948-949.
(20) Goguen, P. W.; Xu, T.; Barich, D. H.; Skloss, T. W.; Song, W.; Wang, Z.;
Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1998,120, 2651-2652.
(21) Haw, J. F.; Nicholas, J. B.; Song, W.; Deng, F.; Wang, Z.: Heneghan, C.
S. J. Am. Chem. Soc. 2000,122,4763-4775.
(22) Arstad, B.; Kolboe, S. Catal. Lett. 2001, 71,209-212.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Asset Metadata

Creator Fu, Hui (author)

Core Title Study and modification of organic species formed in HSAPO-34 cages during methanol-to-olefin catalysis by ex situ analysis

School Graduate School

Degree Master of Science

Degree Program Chemistry

Degree Conferral Date 2001-12

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, organic,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-291421

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