Current Organic Chemistry - Volume 23, Issue 20, 2019

The utilization of biomass in the production of renewable bioenergy and biomaterials has been a popular topic since the past decades as they are rich in carbohydrates. Most biomasses, such as wood, monocotyledons, and agriculture residues, need to be pretreated before the conversion of carbohydrates in order to break down the recalcitrant cell wall structure and increase the fiber accessibility. To date, a variety of pretreatment methods have been developed that vary from physical to chemical and biological methods. Pretreatment processes affect the cell wall physical structure as well as the chemical structure of the cell wall constituents. Comparing to the studies of the cellulose and hemicelluloses structural changes during pretreatment, such studies on lignin are relatively limited. On the other hand, in order to utilize the part of lignin from biorefinery processes, the understanding of the lignin structural changes during the refining process becomes important. In this study, typical pretreatment methods such as hydrothermal pretreatment, alkaline pretreatment, biodegradation, and oxidative pretreatment are introduced and their corresponding impacts on the lignin structures are reviewed.

Lignocellulosic biomass can be converted to significant platform molecule 5- hydroxymethylfurfural (HMF), from which one can envision a number of biofuels and chemicals through either chemical or biological conversions. Chemoselective hydrogenation is one of the important pathways for the upgrading of HMF into furanyl diols consisting of 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF). BHMF and BHMTHF are all-purpose intermediates for the manufacture of chemicals, fuels, and functional materials. In this context, we comprehensively summarized the studies on the chemoselective hydrogenation of HMF into furanyl diols in terms of different H-donors, including molecular H2, alcohols, formic acid, and other alternative H-donors. Through the systematic survey of the previous works, a feasible research direction is discussed for the production of furanyl diols.

With the depletion of fossil energy, liquid biofuels are becoming one of the effective alternatives to replace fossil fuels. The catalytic transfer and hydrogenation of biomass-based furanic compounds into fuels and value-added chemicals has become a spotlight in this field. Gas hydrogen is often used as the H-donor for the hydrogenation reactions. It is a very straightforward and simple method to implement, but sometimes it comes with the danger of operation and the difficulty of regulation. In recent years, diverse liquid hydrogen donor reagents have been employed in the catalytic transfer hydrogenation (CTH) of biomass. Amongst those H-donors, alcohol is a kind of green and benign reagent that has been used in different biomass conversion reactions. This type of reagent is very convenient to use, and the involved operation process is safe, as compared to that of H2. In this review, the application of alcohols as liquid H-donors in the catalytic transfer hydrogenation of biomass-derived furanic compounds is depicted, and the representative reaction mechanisms are discussed. Emphasis is also laid on the selective control of product distribution in the described catalytic systems.

Biomass energy has attracted much attention because of its clean and renewable characteristics. At present, C2–C3 polyols such as glycerol, 1,2-propanediol, and ethylene glycol, widely used as platforms for downstream chemicals or directly used as chemicals in diversified industries, mainly depend on the petrochemical industry. In terms of the feedstock for C2–C3 polyol production, the C3-derived glycerol is a side product during biodiesel synthesis, whereas the C5-derived xylitol and C6-derived sorbitol can be mainly obtained by hydrolysis–hydrogenation of hemicellulose and cellulose from lignocellulosic biomass, respectively. In this review, we summarize the catalysts and catalysis for selective hydrogenolysis of these polyhydric compounds to C2–C3 polyols and introduce the reaction pathways for the target polyol formation based on the C3, C5, and C6 polyhydric alcohol hydrogenolysis. Finally, state-of-the-art technologies are described and the remaining challenges and further prospects are presented in view of the technical aspects.

The major sources of fuels in today's world predominantly come from traditional fossil resources such as coal, petroleum and natural gas, which are limited and nonrenewable. Meanwhile, their consumption releases large undesirable greenhouse gas and noxious gases. Therefore, the development of renewable and sustainable feedstocks to replace traditional fossil resources has attracted great interest. Biodiesel, mainly produced through esterification and transesterification reaction from renewable oil resources using acids and bases as catalysts, is deemed as a green and renewable biofuel that shows enormous potential to replace fossil diesel. Compared to homogeneous catalytic systems, the development of efficient and stable heterogeneous catalysts is vital to synthesizing biodiesel in an efficient and green manner. Among the developed solid catalysts, organic polymer- based catalytic materials are an extremely important topic, wherein distinct advantages of higher concentration of active sites and better stability of active groups are associated with each other. In this review, effective catalytic valorization of sustainable feedstocks into biodiesel via transesterification and esterification reactions mediated by functionalized organic polymer-based catalysts is discussed. Special emphasis has been given to the synthetic routes to the versatile organic polymers-based catalytic materials, and some other interesting catalytic roles derived from physicochemical property, like adjustable hydrophilicity and hydrophobicity along with swelling property in transesterification and esterification, are also illustrated.

This review describes the non-exhaustive scenery of the synthesis of various biologically interesting pyrimidine annulated five-membered heterocyclic ring systems that have been appeared in the literature during the last two decades. During this period, different synthetic routes and various methodologies have been developed for the functionalization of pyrimidine ring towards the construction of five-membered heterocyclic rings. The aim of this review is to give an overview of the assorted methodologies that have been reported about the chemistry of construction of pyrimidines annulated nitrogen, oxygen and sulphur containing five-membered heterocycles.