QENS shows that methanol is immobile in a fresh ZSM-5 catalyst and one used for methanol conversion at 623 K, but undergoes isotropic rotation within the mesopores generated in a catalyst used for methanol conversion at 673 K.
The properties of 1-octene adsorbed in zeolite ZSM-5 at 293 K are studied by means of inelastic and quasielastic neutron scattering (INS and QENS) in order to investigate interactions relevant to the zeolite solid acid catalysis of fluidised catalytic cracking reactions. The INS spectrum is compared to that recorded for the solid alkene and reveals significant changes of bonding on adsorption at ambient temperatures; the changes are attributed to the oligomerization of the adsorbed 1-octene to form a medium chain n-alkane or n-alkane cation. QENS analysis shows that these oligomers are immobilised within the zeolite pore structure but a temperature-dependant fraction is able to rotate around their long axis within the pore channels.
How
the methyl torsion transition energy in unsaturated systems
is affected by its environment is investigated. It is strongly influenced
by both its immediate neighborhood, (the number of methyl groups present
in the molecule) and the intermolecular interactions. It is clear
that the intermolecular interactions have a major influence on the
torsion transition energy, as demonstrated unambiguously previously
for mesitylene and also seen here for other systems. In part, this
may be caused by the fact that the methyl torsion is rarely a pure
mode (unless enforced by symmetry). Where the crystal structure is
available, the assignments have been supported by CASTEP calculations
of the unit cell. The agreement between the observed and calculated
spectra is generally good, although not perfect, toluene being a case
in point, and highlights just how demanding it is to obtain accurate
transition energies for low energy modes. The disagreement between
observed and calculated inelastic neutron scattering spectra for meta-xylene and 9,10 dimethylanthracene is so severe that
it would suggest that there are additional phases to those presently
known. Comparison between the full periodic calculations and those
for the isolated molecule shows that intermolecular interactions raise
the methyl torsion transition energy by at least 8% and in some
cases by more than 50%. The presence of more than one methyl group
in the molecule generally raises the average torsion energy from the
<100 cm–1 seen for single methyl groups to 150–200
cm–1.
Observation of the oligomerization of propene in ZSM-5 at 293 K by neutron vibrational spectroscopy shows that the product species are linear alkyl chains. No evidence is found for the formation of branched products. The selective formation of linear alkyl chains is attributed to a confinement effect within the zeolite pore structure. A role for zeolite crystallite size, a controllable parameter within the catalyst preparative stage, in being able to influence the product composition in technically relevant olefin oligomerization reactions is considered.
The
techniques of quasi-elastic and inelastic neutron scattering
(QENS and INS) are applied to investigate the oligomerization of propene
over a ZSM-5 zeolite. Investigations are performed at low temperatures,
allowing identification of the onset of the oligomerization reaction
and observation of the low-energy spectral changes due to intermediate
formation that are difficult to observe by optical methods. Oligomerization
proceeds via formation of a hydrogen-bonded precursor by an interaction
of the propene with an internal acid site followed by protonation
and chain growth with protonation being the rate-limiting step. The
use of quasi-elastic neutron scattering to observe changes in system
mobility with temperature via the elastic window scan technique allows
identification of the active temperature range where catalyst activity
commences and permits targeting of the more time-consuming INS investigations
to conditions of interest. From examination of the product’s
spectrum, the structure of the resulting oligomer is deduced to be
primarily linear.
The nature of the hydrocarbon pool at different stages of the methanol‐to‐hydrocarbons reaction over ZSM‐5 is examined. A combination of reaction testing, analytical and spectroscopic techniques is employed to investigate changes in the nature and form of the hydrocarbon pool as a function of reaction conditions and reaction time. It is shown that inelastic neutron scattering spectroscopy (INS) complements other spectroscopic methods for observing molecular components in the hydrocarbon pool of working catalysts. INS is uniquely able to spectroscopically identify the form of coke species present in deactivated catalysts.
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