Abstract
The adsorption of methanol in HZSM-5 at low temperatures has long been regarded as an associative process involving hydrogen bonding to the acidic zeolite hydroxyl groups. Recent studies employing inelastic neutron scattering spectroscopy (INS) have reported that complete dissociation to methoxylate the zeolite occurs at 298 K, and infrared evidence for a partial dissociation at 298 K has also been described. Here we investigate the apparent contradictions between different techniques, using a combination of INS, infrared spectroscopy and solid-state NMR spectroscopy, including isotopic substitution experiments. Different possible explanations are proposed and considered; we conclude that at room temperature methanol is very largely associatively adsorbed, although the presence of some small extent (>1%) of methoxylation cannot be ruled out.
Highlights
The interaction between methanol and acidic zeolite catalysts is important because of its relevance to Methanolto-Hydrocarbons (MTH) technology
This paper has shown that observing the OH functionality of methanol when methanol is loaded on ZSM-5 is complex with inelastic neutron scattering spectroscopy (INS) spectroscopy
At room temperature, both infrared and ss-NMR spectroscopies definitively show that the methanol OH functionality is still present and dominates both sets of spectra
Summary
The interaction between methanol and acidic zeolite catalysts is important because of its relevance to Methanolto-Hydrocarbons (MTH) technology. A more recent study of methanol interaction on both ZSM-5 and HY using inelastic neutron scattering (INS) spectroscopy and quasielastic neutron scattering. Scheme 1 Schematic of methoxylation reaction occurring when methanol is introduced into an acidic zeolite catalyst (QENS) reported complete methanol dissociation to methoxy species at room temperature on HZSM-5 [14]. A DRIFTS study by Matam et al has suggested that it is possible to see formation of methoxy groups from methanol in HZSM-5 at room temperature by infrared spectroscopy [17], the methoxy bands are difficult to distinguish due to the intense ABC structure of the hydrogen bonded methanol. In the present study we have attempted to rationalise the apparently contradictory results obtained from the different forms of spectroscopy by applying a combination of techniques (INS, DRIFTS and solid state NMR (ssNMR)) to look at methanol in the same HZSM-5 zeolite
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