Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst

Catalysis Letters, Apr 2016

Inelastic neutron scattering (INS) is used to investigate a ZSM-5 catalyst that has been exposed to methanol vapour at elevated temperature. In-line mass spectrometric analysis of the catalyst exit stream confirms methanol-to-gasoline chemistry, whilst ex situ INS measurements detect hydrocarbon species formed in/on the catalyst during methanol conversion. These preliminary studies demonstrate the capability of INS to complement infrared spectroscopic characterisation of the hydrocarbon pool present in/on ZSM-5 during the MTG reaction. Graphical Abstract Open image in new window

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Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst

Catal Lett Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst Russell F. Howe 0 1 2 3 4 5 6 7 James McGregor 0 1 2 3 4 5 6 7 Stewart F. Parker 0 1 2 3 4 5 6 7 Paul Collier 0 1 2 3 4 5 6 7 David Lennon 0 1 2 3 4 5 6 7 0 Stewart F. Parker 1 James McGregor 2 & David Lennon 3 School of Chemistry, University of Glasgow , Joseph Black Building, Glasgow G12 8QQ , UK 4 Johnson Matthey Technology Centre , Blounts Court, Sonning Common, Reading RG4 9NH , UK 5 ISIS Facility, STFC Rutherford Appleton Laboratory , Chilton, Oxon OX11 0QX , UK 6 Department of Chemical and Biological Engineering, University of Sheffield , Sheffield S1 3JD , UK 7 Department of Chemistry, University of Aberdeen , Aberdeen AB24 3UE , UK Inelastic neutron scattering (INS) is used to investigate a ZSM-5 catalyst that has been exposed to methanol vapour at elevated temperature. In-line mass spectrometric analysis of the catalyst exit stream confirms methanol-to-gasoline chemistry, whilst ex situ INS measurements detect hydrocarbon species formed in/on the catalyst during methanol conversion. These preliminary studies demonstrate the capability of INS to complement infrared spectroscopic characterisation of the hydrocarbon pool present in/on ZSM-5 during the MTG reaction. Keywords scattering The conversion of alcohols to hydrocarbons was first introduced in the Mobil methanol-to-gasoline (MTG) process using an HZSM-5 catalyst, commercialised in New Zealand in 1986. Lurgi’s methanol to olefins (MTO) process, also using HZSM-5, UOP-Statoil’s MTO process using a SAPO-34 catalyst, and Topsoe’s improved gasoline synthesis (TIGAS) using a proprietary zeolite catalyst followed. The availability of cheap methanol derived from natural gas was the initial driver for these technologies. More recently, methanol derived from coal has become the source of transport fuels or olefin feedstocks via MTG or MTO type processes. In the future biomass and other renewable resources are likely to provide a ready supply of methanol, e.g. through gasification and subsequent hydrogenation of CO/CO2, or via direct oxidation of methane produced by aerobic digestion. MTG and MTO processes therefore provide a route to fuels and chemicals from renewable feedstocks. The past 30 years have seen numerous investigations of the reaction pathways and mechanisms by which methanol is converted to hydrocarbons over acid zeotype catalysts, as reviewed recently for example in references [ 1–4 ]. Three different components of the reaction pathway can be distinguished: (1) the initial reaction steps in which methanol reacts with acid sites in the zeolite or SAPO catalysts; (2) the formation of hydrocarbon products during steady-state working conditions; and (3) the catalyst deactivation through so-called coke formation. As in any catalytic system, understanding the reaction pathway is essential if optimum product selectivity and catalyst performance are to be achieved. Most attention in the last 10 years has focussed on the reactions occurring under steady-state working conditions. The so-called ‘hydrocarbon pool’ mechanism has found widespread support [ 2, 5–7 ]. In this mechanism, two catalytic cycles operate in parallel: alkenes are methylated and subsequently cracked in one cycle, while aromatics are methylated and subsequently dealkylated in the other. Experimental support for this mechanism has come from, for example, 13C labelling studies, NMR and UV–VIS identification of polymethyl aromatic species occluded in working catalysts, and post reaction analysis of occluded species liberated from used catalysts by GC–MS. The differences in product distribution found between zeolites with different pore sizes have been rationalised in terms of different contributions from the two different cycles. The mechanisms by which the hydrocarbon pool is initially formed from methanol are much less clear-cut (and this subject has been comprehensively reviewed in [ 1 ]). Infrared spectroscopy has been extensively used to investigate species formed when methanol first contacts the zeolite catalyst, and clear evidence obtained for the formation of reactive methoxy groups from reaction of methanol or dimethylether with Brønsted acid sites [ 8–12 ]. After longer reaction times at higher temperatures more complex infrared spectra develop which have been assigned variously to adsorbed methylaromatics and olefinic species [ 12, 13 ]. Infrared spectroscopy on ZSM-5 is limited to the energy range 1350–4000 cm-1 as a consequence of the intense absorption bands below 1350 cm-1 due to Si–O and Al–O stretching vibrations of the zeolite framework (although Qian et al. [12] were able to observe a small window in the spectrum of SAPO-34 between 800 and 900 cm-1). A second limitation of the FTIR technique is that catalysts at a later stage in the reaction path become difficult to observe because of the presence of strongly absorbing species [ 13 (...truncated)


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Russell F. Howe, James McGregor, Stewart F. Parker, Paul Collier, David Lennon. Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst, Catalysis Letters, 2016, pp. 1242-1248, Volume 146, Issue 7, DOI: 10.1007/s10562-016-1742-5