Development of real-time Kerr-gated Raman Spectroscopy for the interrogation of the role of hydrocarbon pool species in catalytic reaction mechanisms

Raman spectroscopy is a particularly powerful technique for studying catalysts under reaction conditions although can often be compromised by impurities in the sample leading to fluorescence swamping the scattered signal. This can be circumvented by using an alternative excitation wavelength although at the expense of sensitivity to the catalytically important species which often benefit from resonance enhancement with visible excitation. We propose therefore to develop further the application of time-resolved (with time resolution in the order of a few minutes) Raman scattering using the ULTRA@CLF facility (@Harwell) as a picosecond laser source in combination with a Kerr-Gated spectrometer to 'circumvent' fluorescence [1], so to examine the significance of the catalytic hydrocarbon pool species with time for a series of zeolite samples with various topologies used for the conversion of methanol-to-hydrocarbons (MTH, producing light olefins and/or aromatics depending on zeolite topology) and for catalytic upgrading of vapors from fast biomass pyrolysis (FBP).
Previous Raman studies performed on H-ZSM-5 zeolites after the MTH reaction (a recently commercialised alternative fuel/bulk chemical production technology), have demonstrated the presence of 'substituted aromatics', likely responsible for C6 hydrocarbon products. However, recent research has suggested that a 'polyene mechanism' maybe responsible for light hydrocarbon formation. [2] The significance of these two pathways has been under debate simply because it has not been possible to perform the Raman measurements under proper process conditions and to correlate with the evolving catalytic products. We recently performed such proof-of-principle, time resolved Kerr-gate Raman studies on zeolites undergoing MTH which yielded unprecedented and well correlated (with catalytic activity) insight into this relationship [3]; see Fig. 1. Note that when no gating is applied it is not possible to record such signals nor is it with 1064 nm FT-Raman instrument.
Although our initial studies were successful, the linkam reactor cell used suffers from dead volume problems and gradients across the sample length. In addition, the liquid delivery system tended to pulse rather than provide a continuous delivery of liquid. The project aims will therefore be in the first instance to develop and commission a quartz capillary flow reactor and gas delivery system capable of operating at temperatures up to 873 K. Subsequently the system should be configured to deliver vapourised liquids above 468 K using a combination of peristaltic delivery/heated transfer lines. Systems to be studied would include both 1D - 3D zeolite topologies such as ZSM-5, SSZ-13, SAPO-35, -39 both for MTH and the more exploratory FBP, where initial studies have shown a possible route to phenolic compounds via a polyene mechanism and where a better understanding of this will help in further optimising this sustainable technology [4].
Offline tests will be performed at the Research Complex and Johnson Matthey Technology Centres (JMTC). The project is well aligned with the strategies of the Research Complex at Harwell and the STFC and is particularly timely as it builds on some very exploratory but highly successful initial studies. Furthermore, it engages with new stakeholders (JMTC Billingham, CLF@STFC) whom are not currently involved in the sponsorship of PhD students @ UCL Chemistry - they will contribute 50 % each to the cost of the studentship.