Application of Inelastic Neutron Scattering to the

Authors: Russell F Howe James McGregor Stewart F Parker Paul Collier David Lennon

Publish Date: 2016/04/15

Volume: 146, Issue: 7, Pages: 1242-1248

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Abstract

Inelastic neutron scattering INS is used to investigate a ZSM5 catalyst that has been exposed to methanol vapour at elevated temperature Inline mass spectrometric analysis of the catalyst exit stream confirms methanoltogasoline 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 ZSM5 during the MTG reactionThe conversion of alcohols to hydrocarbons was first introduced in the Mobil methanoltogasoline MTG process using an HZSM5 catalyst commercialised in New Zealand in 1986 Lurgi’s methanol to olefins MTO process also using HZSM5 UOPStatoil’s MTO process using a SAPO34 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 eg 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 feedstocksThe 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 2 3 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 steadystate working conditions and 3 the catalyst deactivation through socalled coke formation As in any catalytic system understanding the reaction pathway is essential if optimum product selectivity and catalyst performance are to be achievedMost attention in the last 10 years has focussed on the reactions occurring under steadystate working conditions The socalled ‘hydrocarbon pool’ mechanism has found widespread support 2 5 6 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 cyclesThe mechanisms by which the hydrocarbon pool is initially formed from methanol are much less clearcut 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 9 10 11 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 ZSM5 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 SAPO34 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 ie the catalysts become dark in colour One alternative means of accessing the full vibrational spectrum of the catalyst at all stages of the reaction coordinate is to employ the technique of inelastic neutron scattering INS 14The use of INS to obtain vibrational spectra of molecular systems has been reviewed by Parker et al 15 16 highlighting how direct geometry INS spectrometers eg MAPS 17 can provide supplementary and additional information to that achievable with indirect geometry INS spectrometers eg TOSCA 17 the latter being the more typical instrument of choice for molecular spectroscopy investigations by INS That work also outlines an increasing trend of using of INS to examine a variety of heterogeneously catalysed reaction systems 15 For example following on from the work of Silverwood et al who used INS to examine aluminasupported nickel catalysts applied to the dry 18 19 20 21 and steam 22 reforming of methane Warringham and coworkers have used a combination of indirect 23 and direct 24 25 26 geometry INS spectrometers to investigate iron based Fischer–Tropsch synthesis FTS catalysts Moreover Warringham et al 27 recently reported on sample environment details relevant to the acquisition of INS spectra of heterogeneous catalystsAgainst this background it is timely to consider whether INS can be applied to look at MTG and/or MTO chemistry over zeolitic materials such as ZSM5 Given the ability of INS to selectively probe hydrogeneous vibrational modes 15 it is enticing to discover whether the technique can provide new information on the adsorbed hydrocarbon species present at various stages of the methanol to hydrocarbon reaction Very recently O’Malley et al 28 have used a combination of neutron scattering methods matched by ab initio calculations to investigate amongst other things the reaction of methanol with ZSM5 at room temperature That study showed methanol to be immobilized due to methoxylation and that the formation of adsorbed methoxy groups is facile over this material at room temperature In this paper we present a preliminary report on the INS spectra of a commercial grade ZSM5 catalyst exposed to a methanol feedstream at 623 K With mass spectrometry indicating representative MTG chemistry the reaction was stopped quenched by rapid cooling to room temperature in flowing helium and the catalyst sample taken to the INS spectrometer without exposure to air for spectral acquisition The resulting ex situ spectra unambiguously reveal the presence of hydrocarbon species which may be associated with a hydrocarbon pool Whereas O’Malley et al 28 concentrated on methanol exposure to ZSM5 at room temperature this communication describes an investigation performed at reaction temperatures representative of those utilised in MTG unit operations Thus INS is being used here to assess how hydrogen is partitioned within the catalyst matrix during conditions that correspond to the commercially relevant active phase of MTG operation Although further work is necessary to examine spectra at different reaction times and to better correlate catalytic performance with the vibrational spectra this short communication demonstrates the capability of INS to positively contribute to the understanding of this economically relevant but technically challenging reaction systemA commercial grade ZSM5 catalyst ACS Material LLC USA SiO2/Al2O3 molar ratio 381 specific surface area ≥250 m2 g−1 pore volume ≥025 ml g−1 pore size ~5 Å bulk density ~072 kg l−1 supplied in the form of column shaped pellets diameter 2 mm length 210 mm was employed Two Inconel reactors 29 were charged with the catalyst pellets The first reactor was attached to a gas manifold apparatus 27 and the catalyst dried by the following procedure whilst continually maintaining a flow of helium over the catalyst 1000 sccm CK Gas 990  the sample was heated to 623 K at a heating rate of 10 K min−1 and then maintained at that temperature for 2 h The heating was stopped and the sample allowed to cool to ambient temperature in a continuous flow of He The reactor was then isolated and transferred to an argonfilled glove box MBraun UniLab MB20G H2O  1 ppm O2  2 ppm for loading into an aluminium INS cell that is sealed via an indium wire gasket 17 The second sample was similarly dried but the dehydration stage was followed by dosing of the catalyst with methanol vapour by means of a bubbler arrangement whilst the catalyst was maintained at 623 K The eluting stream was analysed by inline mass spectrometry Hiden Analytical HPR20 with the spectrometer connected to the reactor exit line via a differentiallypumped heated quartz capillary The apparatus is equipped with a ‘catchpot’ downstream of the catalyst Material collected there was subsequently analysed by GC–MS After approximately 6 h onstream the methanol flow and heating were stopped and the sample allowed to cool to ambient temperature in a continuous flow of He The reactor was then isolated and transferred to an aluminium INS cell using the procedure outlined above The two samples dried catalyst and reacted catalyst were then transported to the INS spectrometer and the spectrum recorded at 20 K

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