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)