Catalytic Conversion of Lignin: Studies on Oxidation of Lignin Model Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts

Abstract

INTRODUCTIONLignin is an integral component of plants and is most abundant aromatic biopolymer in earth which can serve as a renewable resource of chemical feedstock. It is suggested that catalytic oxidation of lignin or lignin derived fragments is interesting as it is expected to yield functionalized chemicals which can be employed in fine chemical synthesis [1]. In this subject, several reports have been appeared in literature largely focused on catalytic reactions of lignin model compounds which could provide useful information for the development of selective and high yield processes of lignin oxidation namely the conversion of lignin to aromatic monomers catalyzed by triflic acid [2], depolymerization of oxidized lignin to aromatics by formic acid [3], oxidation of phenolic and non-phenolic compounds in presence of metal chlorides in ionic liquid medium [4] are some of the significant achievements in the area of lignin valorization. Among the lignin model phenolic monomers, apocynol, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol is commonly studied over many catalysts. Recently, much attention is focussed on the catalytic oxidation of lignin derived fragments to fine chemicals. Wozniak et al. [5] have studied the liquid phase oxidation of apocynol over potassium nitrosodisulfonate (Fremy's salt) and corresponding benzo- The present study focussed on investigation of the potentiality of cobalt containing mesoporous material (M41S) as a robust heterogeneous catalyst for the oxidation of lignin model phenolic compound. Cobalt containing MCM-41 and MCM-48 were prepared under hydrothermal condition and characterized by various spectroscopic and analytical techniques. Formation of well ordered hexagonal and cubic mesopore structures of MCMs, containing cobalt in the silicate framework was inferred from XRD, TG-DTA and N2 adsorption-desorption studies. DR UV-visible spectral analysis revealed existence of Co(II) and Co(III) in the tetrahedral framework. The oxidative ability of CoMCMs were studied for the lignin model phenolic monomer, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol, under mild conditions using environmentally benign H2O2 as the oxidant. The catalytic results showed that under optimum reaction conditions, apocynol undergoes selective oxidation yielding 2-methoxybenzoquinone and acetovanillone. CoMCM-41 was found to be more selective towards acetovanillone, on the contrary, CoMCM-48 was better catalyst for 2-methoxybenzoquinone yield.