Density functional theory study of the zeolite-mediated tautomerization of phenol and catechol

Carlos E Hernandez-Tamargo,Alberto Roldan,Nora H De Leeuw

doi:10.1016/j.mcat.2016.12.020

Carlos E Hernandez-Tamargo, Alberto Roldan + Show 1 more

Open Access

https://doi.org/10.1016/j.mcat.2016.12.020

Copy DOI

Abstract

Because the structure of lignin consists mostly of inter-linked phenolic monomers, its conversion into more valuable chemicals may benefit from isomerization processes that alter the electronic structure of the aromatic rings. The tautomerization of phenolic-type compounds changes the hybridization from sp2 to sp3 of the carbon atom at the ortho position, which disables the aromaticity and facilitates the subsequent hydrogenation process. Here, we have performed a Density Functional Theory study of the tautomerization of phenol and catechol at the external surface of zeolite MFI. The tautomerization starts with the adsorption of the molecule on three-coordinated Lewis acid sites, followed by the dissociation of the phenolic hydroxyl group, with the transfer of the proton to the zeolite framework. The rotation of the deprotonated molecule enables a more favourable orientation for the back-transfer of the proton to the carbon atom at the ortho position. The energy barriers of the process are smaller than 55kJ/mol, suggesting that this transformation is easily accessible under standard reaction conditions.

Highlights

Lignin is one of the most abundant components of biomass, representing 10–35% in mass and 40% in energy [1,2]
We have shown that the tautomerization of phenol and catechol is effectively catalysed by the three-coordinated Lewis acid site T9 at the external surface of zeolite MFI
We have analysed the tautomerization of phenol and catechol mediated by the Lewis acid sites at the external surface of Alsubstituted zeolite MFI by means of Density Functional Theory including dispersions corrections

Summary

Introduction

Lignin is one of the most abundant components of biomass, representing 10–35% in mass and 40% in energy [1,2]. The modification of the C:O and H:C ratios is primarily restricted by the aromatic moieties that constitute the lignin: the C O bond in phenolic species is approximately 84 kJ/mol stronger than the same type of bond in aliphatic compounds [9]. In this regard, mono- and bi-metallic nanoparticles supported on zeolites show promising results towards the hydrodeoxygenation (HDO) of lignin [7,10,11,12,13]. The hydrogenation/hydrogenolysis of the aromatic ring is essentially considered to be due to the metal catalyst, an enhancement of its function is noted in the presence of the zeolite [13]

Methods

Results

Conclusion