Coke precursor formation and zeolite deactivation: mechanistic insights from hexamethylbenzene conversion
Introduction
More than 40 years ago Sullivan et al. described the conversion of hexamethylbenzene (hexaMB) over a nickel sulfide on silica-alumina catalyst and over neat silica-alumina [1]. The reactions were carried out at about 350 °C. Over the nickel sulfide catalysts the reactions were carried out at a quite high hydrogen pressure. Over the silica-alumina catalyst nitrogen was used as carrier gas. HexaMB partial pressure was well above 5 bar. The reaction products over silica-alumina were aliphatics, predominantly isobutane, propane, and isopentane, and lower methylbenzenes. Over the nickel sulfide on silica-alumina catalyst there was also a sizable naphthene formation. The reaction did not give any significant ring rupture, and due to the apparent paring (peeling) of methyl groups from hexaMB, the reaction was named the “paring reaction.” A relatively detailed carbenium ion mechanism involving 5–6 ring shifts and growing side chains was proposed. The aliphatic products were assumed to be formed by breaking the alkyl side chain bonds to the benzene ring.
In isolation, the paring reaction is of limited practical value. However, more generally this reaction has implications on the fundamental mechanistic understanding of product formation and coke formation during arene methylations as well as methanol and olefin reactions in zeolites. Our primary goal was to obtain a deeper understanding of the reactions leading to zeolite deactivation, i.e., ring closure for extension of the aromatic system and hydrogen transfer reactions. The results did, however, also give interesting results relevant to the methanol-to-hydrocarbon (MTH) reaction mechanism.
Since the discovery nearly 30 years ago that methanol reacts over protonated zeolites to give a mixture of hydrocarbons and water, a large amount of work has been carried out to obtain an understanding of the reaction mechanism involved [2], [3]. Initially, attention centered on how two or more C1 entities (e.g., methanol, dimethyl ether, trimethyloxonium ions) could react so that CC bonds are formed [3], [4]. During the past 15 years or so increasing evidence has, however, appeared that the reaction actually mainly proceeds by a mechanism where there is a pool of adsorbed hydrocarbons (initially not further specified) that is all the time adding methanol and splitting off ethene, propene, and possibly even higher homologues [5], [6], [7]. During the past few years it has become clear that methylbenzenes may play a central role in the MTH reaction and actually be the essential part of the catalytic cycle [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. We also suspected that they may be an essential factor in the coking reactions leading to catalyst deactivation, and we therefore found it of importance to study the reactivity of hexamethylbenzene in a wide-pore zeolite like beta zeolite.
Sassi et al. very recently published an extensive study on the hexaMB reactions in beta zeolite [14]. In their case, however, the primary goal was to throw further light on the MTH reaction mechanism. The line of attack on the problems and the experimental conditions employed were very different from those employed in the work reported here. The two works are complementary. Sassi et al. concentrated their work on analysis of the gas-phase products in the effluent at very short times on stream. The influence of Si/Al ratio, reaction temperature, reactivities of various methylbenzenes, etc., were studied, as were experiments where [13C]methanol and alkylbenzenes were coreacted. In contrast to this line of attack, we concentrated on the confined hydrocarbons formed during the reaction, and less attention has been paid to a detailed analysis of the effluent products and the MTH reaction mechanism.
In order to slow down the reactions we chose to work at 325 °C. Sassi et al. worked mostly at 450 °C, but also at 550 °C. The most prominent feature we observed when hexaMB was reacted over H-beta was a rapid buildup of naphthalene derivatives inside the catalyst framework. These naphthalene derivatives are dominated by a hexamethylnaphthalene (hexaMN). Already after 10–20 min on stream, when the amount of hexaMB that has reached the catalyst is only 10–20% w/w, the amount of hexaMN was larger than pentamethylbenzene (pentaMB), the second most prominent constituent. HexaMN is formed from dihydro-trimethylnaphthalene by methylations and hydrogen transfers. Dihydro-trimethylnaphthalene (dihydro-triMN) is the lowest observed naphthalene derivative and is formed from methylated hexaMB (the heptamethylbenzenium ion) by a molecular rearrangement and hydrogen transfers.
If the hexaMB feed is stopped and the catalyst flushed with carrier gas, hydrocarbon formation (C1C5) goes on for several minutes. The likely source for this hydrocarbon formation, after the first minute or so, is hexaMN. The easy formation of naphthalene derivatives inside zeolite beta may therefore account for parts of the formation of small hydrocarbons. At the same time, the easy formation of condensed benzene rings may explain why deactivation takes place so readily in beta zeolite.
Section snippets
Catalyst
A commercially available H-beta (Si/Al = 12) from P.Q. Zeolites B.V. was used in this study. Characterization data of the catalyst are given in Ref. [15].
Catalytic testing
The catalytic reactions were carried out in a fixed bed Pyrex glass microreactor with internal diameter 3 mm. The catalyst was pressed to tablets that were gently crushed and sieved to obtain particles with sizes in the range 0.25 to 0.42 mm. The experiments were carried out at 325 °C using 40 mg catalyst. The temperature was measured using a
Results
The preceding investigations of the gas-phase products that are formed when hexaMB is passed over an acidic catalyst were carried out under conditions which were rather different from our conditions [1], [14]. Despite this, the product spectra in the effluent are broadly speaking similar, but there are also significant differences. We paid particular attention to catalyst deactivation behavior. Simultaneously with monitoring the compounds in the effluent, we studied the compounds that are
Discussion
The product spectrum in the reactor effluent as given in Fig. 1, Fig. 2 is in broad general agreement with the previous reports by Sullivan et al. [1] and Sassi et al. [14]. They did, however, not investigate catalyst deactivation behavior. A discussion of gas-phase products and catalyst deactivation is best postponed until the properties of the organic material retained in the zeolite cavities have been discussed and is therefore given in Section 4.3.
Conclusions
It has been shown that the paring reaction is more complex than hitherto known. Besides having a reaction where hexamethylbenzene is transformed into small aliphatics and less methylated benzenes (in particular pentaMB) there is a quite rapid formation of dihydro-triMN (two isomers dominate), which again are further methylated and split off hydrogen to give hexaMN. Only one of the many possible isomers is observed. HexaMN may also undergo a paring type reaction and produce small aliphatics.
Acknowledgements
M.B. thanks the VISTA program of Statoil for financial support.
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