Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion

Abstract

Understanding the mechanisms of thermal transformations of model lignin compounds (MLC) over nanoscale catalysts is important for improving the technologic processes occurring in the pyrolytic conversion of lignocellulose biomass into biofuels and value-added chemicals. Herein, we investigate catalytic pyrolysis of MLC (pyrocatechol (P), guaiacol (G), ferulic (FA), and vanillic acids (VA)) over nanoceria using FT-IR spectroscopy, temperature-programmed desorption mass spectrometry (TPD MS), and thermogravimetric analysis (DTG/DTA/TG). FT-IR spectroscopic studies indicate that the active groups of aromatic rings of P, G, VA, and FA as well as carboxylate groups of VA and FA are involved in the interaction with nanoceria surface. We explore the general transformation mechanisms of different surface complexes and identify their decomposition products. We demonstrate that decomposition of carboxylate acid complexes occurs by decarboxylation. When FA is used as a precursor, this reaction generates 4-vinylguaiacol. Complexes of VA and FA formed through both active groups of the aromatic ring and decompose on the CeO2 surface to generate hydroxybenzene. The formation of alkylated products accompanies catalytic pyrolysis of acids due to processes of transalkylation on the surface.

Decarboxylation of p-Coumaric Acid during Pyrolysis on the Nanoceria Surface

“…In the study put forth by [35], the pCmA absorption in this region was attributed to β(CH) C=C . Although it was known that for phenolic acids (ferulic, vanilla, and caffeic), β(OH) bands could also be found in this part of the spectrum [16,[36][37][38]40]. The detected changes may have been due to the formation of carboxylate complexes on the oxide surface.…”

“…Analysis of our previous data of catalytic pyrolysis of cinnamic acids [16,40] and the results of deconvolution of the TPD curve for ion with m/z 120 (4-vinylphenol) suggested that following peaks II-IV (Figure 6) were due to decarboxylation of different types of surface complexes (Scheme 2). Earlier, we found that coumaric acid is a thermally unstable compound and decomposes via heating.…”

“…The main absorption bands (Table 2) were assigned based on the literature data [16,[34][35][36][37][38]. The symbol "ν" denotes stretching vibrations, "β" denotes in-plane deformations, and "δ" denotes out-of-plane deformations; the band is marked with "as" -asymmetric vibrations, "s" -symmetric, and "ar" -vibrations of the aromatic ring.…”

“…The Δν values (Δν = νas(СОО − )−νs(СОО − ) [39,40]) were 62 and 146 cm −1 , and indicated the formation of bidentate carboxylate complexes with chelated and bridged structures. The ν(С=О) vibrations for monodentate complexes could be located The main absorption bands (Table 2) were assigned based on the literature data [16,[34][35][36][37][38]. The symbol "ν" denotes stretching vibrations, "β" denotes in-plane deformations, and "δ" denotes out-of-plane deformations; the band is marked with "as"-asymmetric vibrations, "s"-symmetric, and "ar"-vibrations of the aromatic ring.…”

“…The use of catalysts is one way to achieve a more selective analysis, as well as biomass conversion [5]. Cerium dioxide, due to its catalytic properties [15], can be used both for processing lignin products [16][17][18] and in the conversion of lignocellulosic raw materials [19,20]. However, its potential in this area is still insufficiently disclosed.…”