A surface science perspective on TiO2 photocatalysis

Recent developments of zinc oxide based photocatalyst in water treatment technology: A review

Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

https://cyberleninka.org/article/n/638974.pdf

Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: A review

Gas–solid reactions involving porous catalysts are ubiquitous in chemical and petroleum industries, and it is vital to understand the kinetics of heterogeneous catalytic reactions to develop robust reactor models. This chapter begins by providing an insight on effective diffusivity, which accounts for various mechanisms by which the gas-phase species get transported to the active sites inside a porous catalyst. The influence of pore diffusion on reaction rate is described by considering an isothermal first-order reaction occurring inside the pores of catalyst particles of different geometries. The relative importance of reaction and pore diffusion resistance is described using dimensionless Thiele modulus, and the actual rate of reaction is described using effectiveness factor. After elucidating this treatment for reactions occurring under nonisothermal conditions, various examples involving series and parallel reactions are discussed in terms of selectivity of the catalyst toward a specific product. The external mass transfer of the reactant from the fluid stream to the catalyst pore is then integrated with the rate expression to determine the overall rate. The elementary steps, viz., adsorption, surface reaction, and desorption, involved in a catalytic reaction are discussed, and the approach to derive the overall rate expression at steady state in presence of various rate-determining steps is explained. The effect of catalyst deactivation on the reaction rate is also described. Modeling of ideal and nonideal catalytic packed bed reactors operating under both isothermal and nonisothermal conditions is elucidated using examples. Finally, an approach to model the conversion of solid particles in presence of a reactive gas stream involved in noncatalytic reactions is elucidated using the shrinking unreacted core model. Expressions for solid conversion under various controlling resistances are derived, and its extension to shrinking particles and film growth is outlined.

Chapter 3 – Gas–Solid Reactions and Reactors

Lignin is a three-dimensional, amorphous, polyphenolic and aromatic-rich material linked by aryl ether, biphenyl and diaryl ether bonds, and is a primary recalcitrant molecule in lignocellulosic biomass Unlocking valuable chemicals and fuel molecules from lignin requires the cleavage of typical inter-unit linkages like β─O─4, α─O─4 and 4─O─5 in a selective manner Catalytic hydrogenolysis and hydrodeoxygenation are promising techniques to produce valuable chemicals and hydrocarbons in high yields and selectivities Deriving phenolic derivatives such as guaiacol, alkyl guaiacol, guaiacyl alcohols, and syringyl alcohols is of great interest in lignin depolymerization This perspective article will highlight the importance of catalyst structure, active metal, nature of support, solvent, and operating conditions to obtain phenolic compounds from a variety of lignins via catalytic hydrogenolysis Tuning the acidity of the catalyst is shown to influence its activity, and to curb the unwanted repolymerization and char forming reactions Finally, the stability and recyclability of the catalysts are also evaluated to develop a sustainable lignin valorization approach

Catalytic hydrogenolysis of lignin to phenols: Effect of operating conditions on product distribution

Lignin is the second-most abundant compound in lignocellulosic biomass (up to 30% dry weight) and a major by-product of the pulp and paper industries. Even though it is projected as a primary source of renewable phenolic compounds, its complex and highly-condensed structure with phenyl propane monomers, viz., p-hydroxyphenyl, guaiacyl, and syringyl sub-units, makes lignin conversion challenging. The use of lignin as a source of phenolic compounds is also exacerbated by its wide molecular weight distribution and branching. Pyrolysis and catalytic fast pyrolysis have emerged as promising thermochemical conversion technologies to convert lignin into phenols and aromatic hydrocarbons. Pyrolysis of lignin model compounds is valuable to unravel the mechanism of formation of phenols through the cleavage of specific linkages in lignin, and their secondary gas phase decomposition reactions. This chapter focuses on experimental and theoretical studies of free radical and concerted reactions of lignin model compounds for the production of phenolic and other aromatic compounds. The purpose of this chapter is to provide an essence of fast pyrolysis chemistry of lignin and its model compounds, and the associated reaction kinetics. Challenges in obtaining a mechanistic understanding of lignin pyrolysis are highlighted, and the need for a synergistic combination of experimental and computational studies is emphasized.

Production of Valuable Chemicals and Fuel Molecules from Lignin Via Fast Pyrolysis: Experimental and Theoretical Studies Using Model Compounds

Microwave-assisted torrefaction of biomass Kappaphycus alvarezii–based biochar and magnetic biochar for removal of hexavalent chromium [Cr(VI)] from aqueous solution

Ravikrishnan Vinu

Papers

Chapter 3 – Gas–Solid Reactions and Reactors

Catalytic hydrogenolysis of lignin to phenols: Effect of operating conditions on product distribution

Production of Valuable Chemicals and Fuel Molecules from Lignin Via Fast Pyrolysis: Experimental and Theoretical Studies Using Model Compounds

Microwave-assisted torrefaction of biomass Kappaphycus alvarezii–based biochar and magnetic biochar for removal of hexavalent chromium [Cr(VI)] from aqueous solution

Understanding the physico‐chemical and surface discharge properties of epoxy silicon carbide nanocomposites