The methanol-to-hydrocarbons (MTH) reaction is a flexible alternative step in the upgrading of natural gas, coal or biomass. By tuning the catalyst and process conditions, methanol can be converted into a variety of hydrocarbon products including gasoline and polymer-grade olefins. While the reaction has been known for many years, reaction mechanisms are still not fully understood. Most previous mechanistic studies have been performed on aluminosilicate zeolites, so the aim of the present work was to investigate the effect of a less acidic catalyst on the mechanisms of olefin formation. For this reason, the silicoaluminophosphate H-SAPO-5 was synthesised, characterised and tested for methanol conversion.
During the synthetic effort to produce H-SAPO-5 samples suitable for mechanistic studies it was found that samples with plate-like morphology could be obtained after a short crystallisation time from mixtures containing triethylamine. While the competing growth of other zeolitic phases proved a challenge, several parameters were investigated and conditions favouring growth of the desired phase was found. Characterisation of synthesised materials revealed numerous structural defects, but also that the materials possessed similar acidic strength to the commercial silicoaluminophosphate catalyst H-SAPO-34.
Catalytic studies revealed that MTH over H-SAPO-5 yields primarily light olefins, especially propene and isobutene. A series of catalytic tests at different conditions and use of both labelled methanol and co-feeding of benzene and methanol revealed that methylbenzene intermediates are not as important as what has been observed in aluminosilicates such as H-Beta. While methylbenzenes were found to be active for olefin formation, it was found that isobutene and larger olefins were primarily generated via other intermediates. A mechanism where olefins are produced both by a cycle of successive methylation and cracking, where 2,2,3-trimethylpentene is a key intermediate, and from methylbenzenes was thus proposed. The decreased importance of methylbenzenes may be caused by the less acidic H-SAPO-5 not catalysing hydride transfers as efficiently as aluminosilicate catalysts. The mechanistic proposal shares many similarities with observations in zeolitic catalysts during low-temperature processes and when steric effects hinder the formation of methylbenzenes.