Base-Catalyzed Reactions in Biomass Conversion: Reaction Mechanisms and Catalyst Deactivation

Abstract

Although less studied than acid-catalyzed reactions, mainly because of their marginal interest in petrochemical processes, base-catalyzed reactions (either homogeneous or heterogeneous) play a key role in the upgrading of biomass-derived platform molecules. As in most of the chemical processes, the replacement of homogeneous catalysts by heterogeneous catalysts is of key interest. This article reviews the state of the art in the most important base-catalyzed reactions in the biofuel manufacture (biodiesel from triglycerides) and the upgrading of biomass-derived platform molecules (such as acetone, ethanol, furfural, 5-hydroxymethylfurfural, or acetic acid). Transesterification reactions, aldol condensation (alone or in combination with other reactions), and ketonization are the main reactions involved in these biorefinery processes. Reviewed are the most common catalysts proposed for these reactions (mainly heterogeneous), as well as the proposed mechanisms for these reactions, and the main factors governing catalyst deactivation when used in each of these reactions.

Toward the Efficient Synthesis of Pseudoionone from Citral in a Continuous-Flow Microreactor

“…Afterward an acidic α-hydrogen of the aldol intermediate is removed under basic conditions, yielding an enolate ion and the latter expels the −OH leaving group in an E1cB reaction leading to PI finally (Scheme ). ,, …”

“…Afterward an acidic α-hydrogen of the aldol intermediate is removed under basic conditions, yielding an enolate ion and the latter expels the −OH leaving group in an E1cB reaction leading to PI finally (Scheme 2). 1,5,40 However, the reaction kinetics of aldol condensation of citral with acetone catalyzed by aqueous base are difficult to be investigated experimentally owing to the following reasons: (a) The reaction is generally operated in the heterogeneous regime and influenced by mass transfer; (b) The PI selectivity is relatively low and the complicated side reactions in the reaction system have to be considered; (c) The main reaction for the synthesis of PI is complicated and constructed by multistep elementary reactions.…”

“…Ionones, including α- and β-ionone, are considered as important chemicals, owing to their widespread use in flavor, fragrance and pharmaceutical industry, especially as important precursor for synthesis of vitamin A. , Commercially, ionones can be prepared in a two-step process, containing condensation of citral with acetone to yield pseudoionone (PI) and then cyclization of PI to produce ionones (Scheme ). , The condensation process is a classic aldol condensation, which is a typical carbon–carbon bond formation reaction between carbonylic compounds with acidic or basic catalyst . Traditionally, the condensation of citral with acetone is carried out in the liquid phase catalyzed by liquid bases, such as aqueous NaOH, LiOH and Ba­(OH) 2 . ,, However, this process is usually operated in a time-consuming batch reactor and PI yield is only about 60%–80%. , The demand for ionones provides impetus for further exploration of the condensation of citral with acetone, thus improving the conventional process.…”

“…3,4 The condensation process is a classic aldol condensation, which is a typical carbon− carbon bond formation reaction between carbonylic compounds with acidic or basic catalyst. 5 Traditionally, the condensation of citral with acetone is carried out in the liquid phase catalyzed by liquid bases, such as aqueous NaOH, LiOH and Ba(OH) 2 . 1,6,7 However, this process is usually operated in a time-consuming batch reactor and PI yield is only about 60%−80%.…”

Catalyst Deactivation and Its Mitigation during Catalytic Conversions of Biomass

“…Numerous reviews have been published on the conversion of lignocellulosic biomass in recent decades, which often focus on the chemistry and catalyst design with sporadic discussion on catalyst stability in long-term operation. ,− ,, Other reviews on catalyst deactivation have been focused on general heterogeneous catalyst deactivation. − Specific reviews on catalyst deactivation for biomass are rare. , This review focuses on literatures from recent decades on catalyst deactivation and mitigation for catalytic processes in biomass conversion. It will provide information on the correlation of characteristics of biomass-derived streams, their potential impact on catalyst lifetime, and possible mitigation approaches, which could guide more rational design of catalysts and processes for biomass conversion.…”