Current Green Chemistry - Volume 7, Issue 3, 2020

Glycerol is an organic polyol compound, and is an important raw material with extensive applications in daily/petrochemical and pharmaceutical industry. Glycerol is typically obtained by propylene chlorination, while the method used is complicated process and requires high energy consumption. Interestingly, glycerol is recognized as a major by-product of biodiesel production. Approximately 100 kg of glycerol is yielded for 1 tonne of biodiesel production. With the rapid development of the biodiesel industry, glycerol production capacity has been a serious surplus. This review introduces the selective conversion of glycerol into a variety of value-added chemicals such as propylene glycol, propanol, glyceraldehyde, and dihydroxyacetone via selective hydrogenation and oxidation, as well as hydrocarbons and ethers via pyrolysis, gasification and etherification, respectively. The efficiency of different types of catalysts and the influence of reaction parameters on the valorisation of glycerol have been elucidated. Emphasis is also laid on the study of catalytic mechanisms and pathways for some specific reactions.

Decreasing fossil fuel reserves and growing concerns are driving the utilization of renewable feedstocks as green fuel sources. Heteropolyacids (HPAs) are one of the most promising catalytic materials in green biodiesel production. Their unique chemical and physical properties are outstanding and could allow the preparation of composite materials. In this review, esterification /transesterification for biodiesel synthesis using various types of HPAs base catalysts including pristine HPAs, substituted HPAs, supported HPAs and encapsulated HPAs has been described. In addition, esterification/transesterification reaction conditions such as reaction time and temperature, alcohol content and catalyst dosage affect the catalytic activity of the HPAs base catalysts, which are also discussed in detail. Finally, the present drawbacks and future challenges on HPAs based catalytic systems for the production of biodiesel are also addressed in the concluding remarks.

Over the past decade, increasing attention has been paid to the exploration of environmentalfriendly and alternative resources to prepare basic chemicals for relieving the stress of fossil resources and environmental issues. Lactic acid (LA, 2-hydroxypropanoic acid), the biomass-derived platform molecule, has been used intensively in food, pharmaceuticals, and cosmetics. Considering the fermentation method for lactic acid production possesses environmental impact and high-cost issues, chemocatalytic approaches to manufacturing LA from biomass have attracted much attention due to higher selectivities and lower costs. This paper emphasizes a review on the state-of-the-art production of LA from triose, hexose, cellulose and other biomass over heterogeneous acidic and alkaline catalysts.

Carbonaceous materials are proven to be vital in day-to-day life as well as in advanced science and technology applications. Rice straw, a secondary agricultural lignocellulosic biomass, has drawn great attention for the production of value-added carbonaceous material. Because, it can provide an alternative economic, greener and sustainable resource of carbon to non-renewable fossil fuelbased precursors while controlling the worsening situation of environmental pollution due to improper disposal and stubble burning. In this review, recent developments in the production of carbonaceous materials from rice straw are presented. Biochar and activated carbon were reported to be the prime carbonaceous materials prepared from the rice straw. Thus, pyrogenic preparation of biochar and the influence of its pyrolysis temperature to the yield, composition, surface area, porosity and morphology are preliminarily discussed. This is followed by a detailed discussion on the preparation of activated carbon with an emphasis on the influencing reaction factors for improving the characteristic properties of the activated carbons. Additionally, the major characterization techniques dealing with determining the surface area and porosity (BET analyzer) and microstructure (secondary electron microscope (SEM) and transmission electron microscope (TEM)) for both the carbonaceous materials are also discussed. Finally, major applications of both the carbonaceous materials are briefly reviewed. Thus, the present review clearly highlights the usefulness of agricultural lignocellulosic waste rice straw for the conversion of waste to value-added carbonaceous materials.

γ-Valerolactone (GVL) is a kind of significant platform molecules in the modern industry, which can be directly produced from biomass-derivatives, such as sugar, levulinic acid (LA) and ethyl levulinate (EL). In general, GVL could be produced from LA using gas hydrogen as H-donor with heterogeneous or homogeneous catalysts. But this strategy always has the danger of operation and requirement of unique reactors due to explosive hydrogen as well as the acidity of reactant. Over the past decade, researchers in this field have established new processes and strategies to meet the above problems through the CTH process by using alcohol as H-donor and EL as the substrate over different kinds of catalysts. In this review, we collect and discuss the literature on the production of GVL from EL, and applications of LA, EL, and GVL with particular typical mechanisms. The catalyst preparation methods in the mentioned reaction systems are also concerned.

Green chemistry emphasizes designing novel routes to overcome health and environmental problems that occur during a chemical reaction. Green solvents are used in place of conventional solvents that are hazardous to both human and the environment. Solvents like water, ionic liquids, supercritical CO2, biosolvents, organic carbonates, and deep eutectic mixtures can be used as green solvents. The review focuses on the properties, applications, and limitations of these solvents.

Light energy can be considered as an ideal eco-friendly source of energy for green chemistry. This perspective was used to synthesize 5-aryl-1,2,4-thiazolidine-3-thiones derivative as a quick, efficient and highly improved protocol. The present method developed an energy competent and integrated technique by one-pot condensations of aromatic aldehyde and thiosemicarbazide under the catalyst-free condition in aqueous-alcoholic media. The impact of light on reaction along with mechanical stirring provided a prominent yield of the product. The mild reaction conditions, short reaction time and easy workup procedure, avoidance of heavy metal catalyst and harsh reaction conditions make this protocol greener.