To investigate its role as an intermediate in methanol-to-hydrocarbons (MTH) chemistry, the reaction of propene over H-ZSM-5 zeolite at temperatures of 473, 573 and 673 K is studied over a period of 6 hours and the post-reaction catalysts examined by inelastic neutron scattering and ancillary analytical techniques. Low temperatures favour production of gasoline-range alkanes and alkenes, whilst the product distribution shifts to a primarily aromatic product stream as reaction temperature increases, with cyclopentadienyl intermediates from the aromatic formation process being detected spectroscopically in the reacted catalysts. The 473 K reaction deactivates the zeolite due to pore blockage from the growth of large, branched oligomer chains but coke build-up at higher temperatures is minimal and primarily consists of pure carbon. No evidence of immobilised poly-methylated aromatic species is observed at any temperature. A scheme for the full propene reaction series is proposed that involves a dual-cycle hydrocarbon pool mechanism like that found in MTH chemistry and supporting propene's role as an intermediate in that process. Minor differences in the product distribution of the propene-only reactions compared to classical MTH chemistry are identified due to the lack of a significant methylation reaction pathway that results in a more restricted range of substituted products.
Bibliographical noteFunding Information:
Johnson Matthey plc. is thanked for supplying the ZSM-5 zeolite and for financial support through the provision of industrial CASE studentships in partnership with the EPSRC (APH (EP/P510506/1), AZ (EP/N509176/1)). Experiments at the ISIS Neutron and Muon Source were made possible by a beam time allocation from the Science and Technologies Facilities Council. 53 The resources and support provided by the UK Catalysis Hubviamembership of the UK Catalysis Hub consortium and funded by EPSRC grants EP/R026815/1 and EP/R026939/1 are gratefully acknowledged. This research has been performed with the use of facilities and equipment at the Research Complex at Harwell; the authors are grateful to the Research Complex for this access and support.