Mechanistic insight into the methanol-to-hydrocarbons reaction

doi:10.1016/j.cattod.2005.07.135

Catalysis Today

Volume 106, Issues 1–4, 15 October 2005, Pages 108-111

https://doi.org/10.1016/j.cattod.2005.07.135 Get rights and content

Abstract

For nearly 30 years, it has been known that methanol may be converted to a mixture of hydrocarbons and water over protonated zeolites. During this time, a large amount of work has been carried out to obtain an understanding of the reaction mechanism involved [C.D. Chang, Catal. Rev. 25 (1983) 1; M. Stöcker, Micropor. Mesopor. Mater. 29 (1999) 3; C.D. Chang, Shape-selective catalysis: chemicals synthesis and hydrocarbon processing, in: C. Song, J.M. Garcés, Y. Sugi, (Eds.), ACS Symposium Series, vol. 738, Washington, DC, 2000; J.F. Haw, Phys. Chem. Chem. Phys. 4 (2002) 5431; J.F. Haw, W. Song, D.M. Marcus, J.B. Nicholas, Acc. Chem. Res. 36 (5) (2003) 317]. We here aim at presenting some major contributions to today's rather unified view on the MTH reaction mechanism, and, based on this detailed knowledge, to point at possible options for optimizing the performance of MTH catalysts.

Section snippets

The hydrocarbon pool

Initial work on the MTH mechanism focused on how two or more C₁-entities (e.g., methanol, dimethyl ether, trimethyloxonium ions) could react so that C single bond C bonds are formed [1], [2], [3], [4], [5]. Ten years ago evidence did, however, appear that the reaction mainly proceeds by a mechanism where a pool of adsorbed hydrocarbons is all the time adding methanol and splitting off ethene, propene and possibly even higher homologues (Fig. 1) [6], [7], [8]. Today, the importance of the initial C single bond C bond

Product formation

Most detailed studies of the reactivity of polymethylbenzenes have been carried out over H-beta zeolite, not least because the 12-ring pore structure allows direct feeding of polymethylbenzenes. In H-beta zeolite, it has been shown that hexamethylbenzene has, by far, the highest reactivity for product formation, compared to lower polymethylbenzenes [13]. It has further been shown that the amount of hexaMB retained inside the pores during the MTH reaction decreases dramatically when stopping the

Parallel reactions

In the 1980s, Dessau and co-workers suggested that the MTH reaction proceeds via sequential methylation of light olefins into higher olefins, which are easily cracked into lower olefins that are again methylated [23], [24]. Recently, Svelle et al. studied the methylation of ¹²C-ethene, -propene and -butene with ¹³C-labelled methanol at low contact times over a H-ZSM-5 catalyst [25], [26]. It was observed that the methylation reaction is first order in the olefin and zero order in methanol, for

Catalyst deactivation

Coking is a major challenge in MTH processes. The individual influence of catalyst topology, acid strength and acid site density, respectively, on coke formation is not yet fully understood. Dahl et al. compared the stability of SAPO-34 and its zeolite analogue, chabazite, with different Si/Al ratios, for the MTH reaction. They concluded that the acid site density was the most important parameter for the deactivation behaviour, although a smaller influence of acid strength was also observed [27]

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