Methanol to hydrocarbons over large cavity zeolites: Toward a unified description of catalyst deactivation and the reaction mechanism Morten Bjørgen a,* , Sema Akyalcin b , Unni Olsbye c , Sandrine Benard c , Stein Kolboe c , Stian Svelle c,** a Norwegian University of Science and Technology, Department of Chemistry, N-7491 Trondheim, Norway b Anadolu University, Department of Chemical Engineering, 26555 Eskisehir, Turkey c inGAP Center of Research Based Innovation/Center for Materials Science and Nanotechnology (SMN), University of Oslo, Department of Chemistry, N-0315 Oslo, Norway article info Article history: Received 17 March 2010 Revised 21 July 2010 Accepted 1 August 2010 Keywords: MTH MTO MTG H-beta H-MCM-22 H-mordenite Isotopic labeling Reaction mechanism abstract The reaction mechanism for the conversion of methanol to hydrocarbons over three large cavity zeolites, H-beta, H-MCM-22, and H-mordenite, has been investigated. 13 C methanol was co-reacted with 12 C ben- zene to study the buildup and further reactions of the intermediates formed. Co-reaction was required, as these aromatic intermediates will not be formed from pure methanol at temperatures low enough to actually monitor these events. The reactions were followed by dissolving quenched catalysts in HF fol- lowed by extraction of the organic compounds and analysis by GC–MS. The same hydrocarbon com- pounds are formed inside the pores of three zeolites, and it is the most substituted methylbenzenes that function as reaction intermediates in the hydrocarbon pool mechanism for the conversion of meth- anol. The heptamethylbenzenium cation was for the first time detected and shown to serve as a key reac- tion intermediate in zeolite catalysts other than H-beta. The formation of bicyclic coke precursors was also investigated, and progress has been made toward a more complete description of the reactions lead- ing to catalyst deactivation. Quantum chemical calculations have shed light on the processes leading to coke precursors. The profound similarities between H-beta, H-mordenite, and H-MCM-22 shown herein constitute a significant step toward a unified understanding of the MTH reaction over acidic zeolites. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction During the last two decades, substantial advances in the funda- mental understanding of the reaction mechanism of the metha- nol-to-hydrocarbons reaction have been made [1,2]. A major breakthrough was the proposal of the hydrocarbon pool mechanism by Dahl and Kolboe [3,4]. It now seems clear that multiple methyl- ated aromatics play key roles in this mechanism proposal. These species are frequently referred to as hydrocarbon pool species. In particular, the highest substituted congeners, i.e. hexamethylben- zene (hexaMB) and its further methylation product, the heptameth- ylbenzenium cation (heptaMB + ), have been proved to be the main constituents of the hydrocarbon pool in the spacious H-SAPO-34 [5–8] and H-beta [9–11] catalysts, respectively. It is believed that al- kenes in the C 2 –C 4 range may be split off from these species in a ser- ies of complex rearrangement and dealkylation reactions with concomitant formation of less substituted homologs. This monomo- lecular mode of alkene formation from the hydrocarbon pool spe- cies is known as the paring route [12–14]. Alternatively, a cationic methylbenzenium species (e.g. heptaMB + ) may be deprotonated resulting in formation of an exocyclic double bond, such as seen in hexamethylmethylenecyclohexadiene (HMMC), see Scheme 1. Methylations of this exocyclic double bond will lead to alkyl side chains on the benzene ring, which can be eliminated as an alkene. This is referred to as the side-chain methylation route [15–17]. The initial studies of the hydrocarbon pool mechanism and the reactivity of the highly methylated aromatics were conducted on the H-SAPO-34 catalyst. HexaMB, once formed from methanol within the H-SAPO-34, cages were found to be highly reactive, decomposing into alkenes and lower methylbenzenes [5,6,18]. Fur- ther insights were reached when hexaMB was reacted alone over the H-beta zeolite [9,17]. Again, hexaMB was found to be highly reactive, and the products formed were closely similar to those obtained with a methanol feedstock. Another step forward was made using isotopic labeling. 12 C benzene and 13 C methanol were co-reacted at low temperatures over H-beta [10]. At the very low- est reaction temperatures (210 °C), HMMC, which resides in the zeolite in its protonated form as heptaMB + , was unequivocally identified. HMMC/heptaMB + was shown to be unstable upon heat- ing and could hardly be observed above 300 °C. Moreover, using isotopic labeling, the formation of alkenes from HMMC/heptaMB + was found to be in accord with the paring route [10]. Subsequently, the high relevance of heptaMB + as a hydrocarbon pool species was 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.08.001 * Corresponding author. Fax: +47 73 55 08 77. ** Corresponding author. Fax: +47 22 85 54 41. E-mail addresses: morten.bjorgen@chem.ntnu.no (M. Bjørgen), stian.svelle@ kjemi.uio.no (S. Svelle). Journal of Catalysis 275 (2010) 170–180 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat