Abstract
The dynamics of catechol in zeolite Beta was studied using quesielastic neutron scattering (QENS) experiments and molecular dynamics simulations at 393 K, to understand the behaviour of phenolic monomers relevant in the catalytic conversion of lignin via metal nanoparticles supported on zeolites. Compared to previous work studying phenol, both methods observe that the presence of the second OH group in catechol can hinder mobility significantly, as explained by stronger hydrogen-bonding interactions between catechol and the Brønsted sites of the zeolite. The instrumental timescale of the QENS experiment allows us to probe rotational motion, and the catechol motions are best fit to an isotropic rotation model with a D^{rot} of 2.9 × 10^{10} s^{-1}. While this D^{rot} is within error of that measured for phenol, the fraction of molecules immobile on the instrumental timescale is found to be significantly higher for catechol. The MD simulations also exhibit this increased in ‘immobility’, showing that the long-range translational diffusion coefficients of catechol are lower than phenol by a factor of 7 in acidic zeolite Beta, and a factor of sim3 in the siliceous material, further illustrating the significance of Brønsted site H-bonding. Upon reproducing QENS observables from our simulations to probe rotational motions, a combination of two isotropic rotations was found to fit the MD-calculated EISF; one corresponds to the free rotation of catechol in the pore system of the zeolite, while the second rotation is used to approximate a restricted and rapid “rattling”, consistent with molecules anchored to the acid sites through their OH groups, the motion of which is too rapid to be observed by experiment.
Highlights
Lignin is a primary constituent of biomass, with the potential to become a dominant source of fuel and fine chemicals [1]
Lignin is a three-dimensional polymer of phenolic monomers, which has to be catalytically degraded into smaller components in order to maximise the returns of its utilization [2, 3]
A combination of mechano-catalysis and solvent extractions [4,5,6] is usually followed by a hydro-deoxygenation (HDO) process that aims to transform the lower molecularweight phenolics obtained after the de-polymerization of lignin
Summary
Lignin is a primary constituent of biomass, with the potential to become a dominant source of fuel and fine chemicals [1]. In the HDO transformation, the transition metal guides the hydrogenation and further de-polymerization of the soluble derivatives of the lignin degradation, with the resulting products going through additional dehydration, alkylation. The acid sites of the zeolite, together with its topology and pore dimension, play an important role in the selectivity and yield of the overall conversion [10, 11]. To understand the selectivity and any potential rate-limiting steps, it is essential to analyse the dynamical behaviour of relevant molecules in the micropore system of zeolite catalysts. Of recent interest for use in HDO catalysis has been zeolite Beta (framework type BEA) [12,13,14], which features relatively large pore windows of 12 tetrahedral sites (T-sites), allowing the relatively unhindered entry of phenolic monomers into the micropore system [10]. A full understanding of factors governing the activity and selectivity of the catalytic system is hindered by the complexity of both the catalyst and the sorbates involved, and multiple techniques are required to understand the behaviour on a range of scales
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