J. Am. Chem. Soc. 2001, 123, 8137-8138
The Reactivity of Molecules Trapped within the
8137
SAPO-34 Cavities in the Methanol-to-Hydrocarbons
Reaction
Bjørnar Arstad and Stein Kolboe*
Department of Chemistry, UniVersity of Oslo
P.O. Box 1033, N-0315 Oslo, Norway
ReceiVed March 13, 2001
Since the initial discovery in 1976 that methanol may be
converted to hydrocarbons over H-ZSM-5 various aspects of the
reaction mechanism have been important issues.1,2 Despite a very
considerable effort no general agreement on the mechanism has
been reached. However, during the past few years evidence for a
mechanism based on a pool of adsorbed hydrocarbons that is all
the time adding methanol and splitting off ethene and propene
has been presented.3-8 This pool mechanism differs markedly
from the prevailing views on the methanol-to-hydrocarbons
(MTH) reaction during the first 20 years or so after its discovery.
The results to be presented here lend further strong support for
the hydrocarbon pool mechanism, and suggest that the reaction
mainly proceeds via penta- and hexamethylbenzene.
The MTH reaction is also catalyzed by other zeotype catalysts.
The product spectrum varies strongly with the pore size of the
catalytic material (shape selectivity), and when the small pore
SAPO-34 (chabasite structure) is used as catalyst the hydrocarbon
products are mostly ethene and propene, the only product
molecules small enough to escape with ease through the narrow
pores. The reaction is in this case most often referred to as a
methanol-to-olefins (MTO) reaction. The fundamental mechanism
is likely to be the same over all catalysts based on protonated
zeotype materials.
Previous work carried out in our group where 13C-methanol
and ordinary toluene were co-reacted over medium and large pore
zeolites pointed to arenes adsorbed within the zeolite cavities as
being important participants in the MTH reaction.7 Guided by
the results in ref 7 we decided to extend the work by studying
the molecules that are retained within the catalyst cavities when
SAPO-34 is used as catalyst in the MTO reaction.9 The rationale
for choosing this catalyst being that only small linear hydrocarbons
may escape the pores, all other hydrocarbons are retained.
Knowledge about which molecules are present inside the working
catalyst might give valuable information about the reaction. In
addition SAPO-34 is easily dissolved in dilute acids, so detailed
analysis of the trapped molecules by gas chromatography may
be carried out. The mixture of organic molecules trapped in the
cavities is very complex, ranging from the C3 molecule propane
to C12-C15 molecules. Polymethylbenzenes are, however, strongly
dominating.9 It was found that if methanol feeding was stopped
and the catalyst flushed with carrier gas, hexamethylbenzene
(HMB), one of the major retained products, was evidently unstable
and disappeared within a couple of minutes. The amount of
pentamethylbenzene (PMB) also decreased rapidly, but not so
fast. Simultaneously, the content of di-, tri-, and tetramethylben-
Figure 1. Isotopic analysis of the trapped organic material in SAPO-
34. The catalyst was first exposed to ordinary methanol for 6 min,
thereafter to 13C-methanol for 60 s. WHSV ) 10 h-1. Reaction
temperature 325 °C.
zenes in the SAPO-34 cavities stayed constant or even appeared
to increase. This observation showed that the aromatics are not
the rather inert pore-filling and deactivating compoundssnot
taking part in the main reactionsthey are often tacitly supposed
to be.10,11 The observation also suggested that PMB and HMB
might split off C2 and C3 alkenes (possibly also C4).
We decided that valuable detailed information about the
reactions taking place might be obtained by using isotopic labeling
and performing the experiments in such a way that at a given
time on stream the feed might be switched from ordinary methanol
to 13C-methanol. By carrying out a series of experiments with
varying times of exposure to 13C-methanol before cutting the
experiment, the rate of incorporation of 13C-atoms might possibly
be monitored. This knowledge might then give quite detailed
information about the processes taking place in the interior of
working catalysts, vital information that until now was unavailable
for this kind of catalytic chemistry.
We used the same catalyst batch as before and the experiments
were carried out largely as described earlier,9 i.e., at 325 °C, with
methanol pressure ) 120 mbar and WHSV ) 10 h-1. The isotopic
compositions of the products were determined by the same pro-
cedure as previously outlined for isotopically mixed molecules.3
The isotopic compositions of the polymethylbenzenes as
obtained in an experiment are displayed in Figure 1. The sum of
(1) Sto¨cker, M. Microporous Mesoporous Mater. 1999, 29, 3-48.
(2) Chang, C. D. In Handbook of Heterogeneous Catalysis; Ertl, G.,
Kno¨zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, Germany, 1997; pp
1894-1908.
(3) Dahl, I. M.; Kolboe, S. Catal. Lett. 1993, 20, 329-336.
(4) Dahl, I. M.; Kolboe, S. J. Catal. 1994, 149, 458-464.
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Z.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1998, 120, 2650-2651.
(7) Mikkelsen, Ø.; Rønning, P. O.; Kolboe, S. Microporous Mesoporous
Mater. 2000, 40, 95-113.
(10) Karge, H. G. In Introduction to Zeolite Science and Practice; van
Bekkum, H., Flanigen E. M., Janssen, J. C., Eds.; Stud. Surf. Sci. Catal. No.
58; Elsevier Science B. V.: Amsterdam, The Netherlands, 1991; pp 531-
570.
(8) Haw, J. F.; Nicholas, J. B.; Song, W.; Deng, F.; Wang, Z.; Xu T.;
Heneghan C. S. J. Am. Chem. Soc. 2000, 122, 4763-4775.
(9) Arstad, B.; Kolboe, S. Catal. Lett. 2001, 71, 209-212.
(11) Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1-29.
10.1021/ja010668t CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/26/2001