Biomass, which is primarily made of vegetable oil and lignocellulose, has been considered one of the most potentially sustainable sources of raw carbon material for the processing of value-added chemicals and transport fuels.
Representative examples of application of acetalization strategy in biomass valorization. Image Credit: Changzhi Li, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
The catalytic conversion of vegetable oil/lignocellulose and their associated derivatives has gained great focus in biomass valorization. Several sophisticated approaches comprising dehydration, hydrogenolysis, hydrogenation, oxidation, esterification, etherification, amination, Diels–Alder, aldol condensation, Knoevenagel condensation, and acetalization have been produced for the valorization of vegetable oil/lignocellulose derivatives toward value-added biofuels and chemicals.
Specifically, acetalization is advocated as an interesting method in biomass valorization as it acts as both a protection approach to improving product selectivity and a synthesis tool for renewable acetal fuel additives. A group of researchers briefed the recent advances about the application of the acetalization strategy in biomass valorization. Their research is published in Industrial Chemistry & Materials on June 28th, 2023.
The development of efficient and selective strategies is crucial for valorizing lignocellulose/vegetable oil derivatives. In this review, we systematically discussed the recent advancement of the application of acetalization strategy in biomass valorization. The latest progresses in the development of catalytic systems for the acetalization of biobased furanic compounds and biogenic ethylene glycol/glycerol are systematically summarized and discussed, with an emphasis on the reaction pathway, relationship between catalyst structures and their performance, and relevant catalytic mechanism. Moreover, the application of the acetalization strategy for protecting carbonyl groups/diol structure functionalities to improve the target products' selectivity in lignin depolymerization, 5-hydroxymethylfurfural oxidation, sorbitol dehydration, and xylose hydrogenation is highlighted. We also provided an outlook on the remaining challenges to this field.
Acetalization, a well-known reversible reaction between carbonyl compounds and alcohols, usually need excess of one of the reactant to compel reversible acetalization completion. Nevertheless, studies on the recovery of excess reactant after the reaction are scarce. Moreover, the separation and purification of cyclic acetals/ketals deserve much attention. Expectedly, rectification or designing a suitable biphasic reaction system for this transformation may probably realize the recovery of the surplus substrate and/or separation of the acetals product.
A five-membered-ring acetal (i.e., 1,3-dioxolane) and a six-membered-ring acetal (i.e., 1,3-dioxane) are available from the acetalization of furanic compounds and glycerol. However, it is still a significant challenge to achieve the selective synthesis of 1,3-dioxolane or 1,3-dioxane. Designing structurally adjustable catalysts or choosing suitable solvents may provide an opportunity for achieving the selective synthesis of 1,3-dioxolane or 1,3-dioxane. In addition, the use of crude glycerol, stemming from biodiesel production, for acetalization is more economically viable, and the influence of impurities on the acetalization reaction needs to be investigated.
Changzhi Li, Professor, Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Currently, the available studies for getting details of the elaborate catalytic mechanisms for the acetalization of furanic compounds and ethylene glycol/glycerol are very minimal. Therefore, further effort should be dedicated to the basic comprehension of the catalytic mechanism through density functional theory calculation and in situ spectroscopic measurements.
The synthesis of a single product with high selectivity during the valorization of biomass derivatives is very challenging due to the presence of multiple functional groups (e.g., C=O, C=C, and C-O) in biomass molecules. Taking advantages of the fact that acetalization is a reversible reaction and the formed cyclic acetals/ketals are stable/low reactivity in basic media, acetalization as a protection strategy of the carbonyl group is worth further promotion in biomass valorization such as exclusive hydrogenation of C=C in biobased multifunctional compounds while leaving the C=O group unreduced. In this review, our main goal is to provide readers with timely and accurate latest research progress on the application of acetalization strategy in biomass valorization.
Changzhi Li, Professor, Dalian Institute of Chemical Physics, Chinese Academy of Sciences
The study group comprises Jian He, Li Bai, Huazhong Yu, and Shima Liu from Jishou University in China; and Qian Qiang, Wentao Su, and Changzhi Li from Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
This work is financially supported by the National Key R&D Program of China, National Natural Science Foundation of China, and Hunan Provincial Natural Science Foundation of China.
Journal Reference:
He, J., et al. (2023) Acetalization strategy in biomass valorization: a review. Industrial Chemistry and Materials. dx.doi.org/10.1039/D3IM00050H.