Microwave-assisted liquefaction has shown potential for rapid thermal processing of lignocellulosic biomass. The efficiency of microwave heating depends largely on the dielectric properties of the materials being heated. The objective of this study was to investigate the dynamic interactions between microwave energy and the reaction system during the liquefaction of a woody biomass sample. The dielectric properties of poplar wood particles, model compounds representing the three main chemical components of wood, and individual liquefaction solvent components, along with their mixtures, were measured to evaluate their abilities to convert microwave energy to heat at different frequencies. Dry samples of wood particles, cellulose, xylan, and lignin were all poor microwave energy absorbers as indicated by their low dielectric values relative to the liquefaction solvent components and their mixtures; among the two solvents, polyethylene glycol had lower dielectric values than glycerol, likely due to its larger molecular size. Ionic conduction significantly affected the dielectric loss factor of the liquefaction solvent mixture upon the addition of the sulfuric acid catalyst. During the wood liquefaction reactions, temperature was the main factor that governed the dielectric properties through the preheating stage, and then when the system reached 130 °C, the dielectric properties were governed by changes in chemical composition.Keywords: Biomass; Dynamic dielectric properties; Liquefaction; Microwave energy;

An efficient and robust bimetallic catalyst has been developed for the transfer hydrogenation of biomass derived ethyl levulinate to γ-valerolactone with 2-butanol as the hydrogen donor. Several bimetallic catalysts were prepared and characterized by Brunauer–Emmett–Teller, transmission electron microscopy, X-ray power diffraction and X-ray photoelectron spectrometry. They exhibited different catalytic activities in the catalytic transfer hydrogenation (CTH) reaction. Results showed that 10Cu-5Ni/Al2O3 had the highest activity, providing a 97% yield of GVL product in 12 h at 150 °C. The reaction temperature, reaction time and catalyst loading were also investigated and found to affect the product yield. The catalyst was also successfully applied to the CTH of various levulinate esters with different secondary alcohols. Comparing experiments between Cu–Ni and Cu catalysts and the poisoning experiments revealed that the introduction of Ni to Cu remarkably enhanced the catalyst’s activity and stability, showing an outstanding recycling ability in the 10 runs recycling experiments without notable loss in the activity.Keywords: Bimetallic catalyst; Ethyl levulinate; Stability; Transfer hydrogenation; γ-Valerolactone;

The production of alkyl levulinates from furfuryl alcohol (FAL) was investigated in the presence of metal salt catalysts under microwave irradiation. Various metal salts were tested in the reaction and Al2(SO4)3 showed excellent catalytic activity for the FAL alcoholysis, giving a 80.6% yield of methyl levulinate within 5 minutes. Various alcohols were used to obtain different alkyl levulinates. The dielectric properties of these alcohols were also measured to explain their different performances in the reaction. Microwave heating was proved to dramatically increase the reaction rate of the FAL alcoholysis compared to traditional oil heating. Identification of the reaction intermediates and products provided some insight into the reaction mechanism, where methoxymethylfuran (MMF) and 4,5,5-trimethoxypentan-2-one (TMP) were the key intermediates. Finally, the catalyst was recycled and reused 6 times without much drop in the activity.

A silica-supported peroxycarboxylic acid oxidant, 2-percarboxyethyl silica (SiO2@(CH2)2COOOH), was successfully prepared and used for the epoxidation of fatty acid methyl esters (FAMEs) and vegetable oils. Among the oxidants prepared under different conditions, C-SiO2@(CH2)2COOOH (TEOS:CTES = 2:1) had the highest surface area, pore volume, and peroxide value. It also exhibited the highest activity for the epoxidation reactions. For methyl linoleate, 92.76% yield of epoxidized product was obtained at room temperature. Other unsaturated FAMEs with 1 and 3 double bonds were also efficiently converted to their epoxides. Moreover, the oxidant was also successfully applied to the epoxidation of vegetable oils, olive oil, and linseed oil with high product yield at room temperature. Finally, the used oxidant was regenerated through a simple oxidation process with H2O2 and recycled at least 5 times without much drop in the reactivity.Keywords: Epoxidation; Fatty acid methyl esters; Supported peroxycarboxylic acid; Vegetable oils

The dissolution of cellulose in tetrabutylammonium acetate (TBAA) and dimethyl sulfoxide (DMSO) mixed solvent was studied at room temperature (approximately 25 °C). The ratio of TBAA in the mixed solvent system (WTBAA) was found to have great influence on the solubility of cellulose and the corresponding dissolution time. The mixed solvent of WTBAA = 0.15 possessed the highest cellulose solubility and shortest dissolution time. Various cellulosic materials were well-dissolved in the solvent with a maximum solubility up to 8.17 wt %. A mechanistic study regarding the interaction between the solvent system and the model compound cellobiose was conducted using 1H NMR, 13C NMR, ATR-FTIR, conductivity, and viscosity measurements. The results implied that TBAA existed at two different states in the mixed solvent as the ratio of TBAA varied (i.e., ion-split stage (WTBAA ≤ 0.15) and ion-paired stage (WTBAA ≥ 0.15)). WTBAA = 0.15 was the turning point of these two stages, and the mixed solvent displayed the best dissolving ability at this ratio. This finding suggests that a balance between the ion concentration and ion mobility is crucial to the dissolving ability of a mixed solvent. The solvation effect of the cosolvent DMSO helped to dissociate TBAA into free ions and facilitate the ion mobility. The hydroxyl protons of cellobiose were demonstrated to form strong hydrogen bonds with CH3COO–, which was key to the dissolution of cellulose. Finally, the interaction between cellobiose and DMSO in the TBAA/DMSO/cellobiose system was investigated and was demonstrated to be another important factor for the dissolution of cellulose by stabilizing the dissolved cellulose chain from further formation of inter- and intramolecular hydrogen bonds.Keywords: Cellulose dissolution; Dimethyl sulfoxide; Dissolution mechanism; Room temperature; Tetrabutylammonium acetate;

Torrefaction is an effective pretreatment method to improve the uniformity and quality of lignocellolosic biomass before further thermal processing (e.g., gasification, combustion). The objective of this study was to determine the impacts of torrefaction as a pretreatment before liquefaction. Wood chips were torrefied for 2 h at three different temperatures (230, 260, 290 °C) and then subjected to microwave-assisted liquefaction, as was the untreated wood control. The dielectric properties of liquefaction materials, including the biomass samples and liquefaction reagent, were measured to evaluate their abilities to convert electromagnetic energy to heat. The effects of liquefaction time, temperature, and catalyst concentration on the liquefaction efficiency were also investigated. It showed that torrefaction temperature had significant influence on the liquefaction behavior of wood materials. Wood treated at the lowest torrefaction temperature (230 °C) retained the most structural/compositional characteristics of untreated wood and therefore they both exhibited similar liquefaction behaviors. The higher treatment temperature (290 °C) led to higher liquefaction residue contents, attributed to the increase in carbon content and hydrophobicity from torrefaction.

The esterification of levulinic acid (LA) to alkyl levulinates has been investigated in the presence of various metal salt catalysts under microwave irradiation. The reaction obtained 99.4% yield of methyl levulinate (ML) in the presence of Al2(SO4)3 catalyst in methanol solution under microwave conditions. The optimized reaction conditions were 110 °C and 10 minutes with a 20 mol% catalyst loading. Alcohols with longer carbon chains showed lower reactivities in the microwave electromagnetic field due to their poorer abilities to absorb and transmit microwave energy. Moreover, microwave irradiation provided a significantly higher reaction rate compared to conventional oil bath heating. LA aqueous solution was also converted to ML with high yields. The Al2(SO4)3 catalyst was successfully applied to the esterification of other biomass derived organic acids to their corresponding esters in high yields. Finally, the catalyst was recycled 5 times without much decrease in activity.

An active heterogeneous catalyst, namely a molybdenum acetylacetonate complex immobilized on expanded corn starch (ECS-MoO2(acac)2), was prepared and its catalytic activity for epoxidation of stillingia oil with tert-butyl hydroperoxide (TBHP) was investigated. The heterogeneous catalysts were characterized using inductively coupled plasma optical emission spectrometry, Fourier transform infrared spectroscopy, thermogravimetry and differential thermal analyses, scanning electron microscopy, N2 adsorption–desorption and X-ray photoelectron spectroscopy. By using this catalyst, an environmentally benign process for epoxide production in a heterogeneous manner was developed. The catalyst could be recovered easily and reused without significant degradation in its activity for at least 5 times.

Epoxidation of vegetable oils or fatty acid methyl esters (FAMEs) produce important monomers which are widely used as plasticizers or stabilizers in the polymer industry. However, little attention has been focused on the influence of the alkenyl structure of the fatty acid on the efficiency and selectivity of their epoxidation. In this work, the influence of the alkenyl structure (the number of double bonds) of the FAMEs on the epoxidation reaction has been investigated. Three model FAMEs with 1 to 3 double bonds were epoxidized using both a weak (formic acid) and a strong (sulfuric acid/acetic acid) acid system. It was found that FAMEs with more double bonds have higher reactivities toward the epoxidation reaction. In addition, the electron-donating effect of the double bonds on the fatty acid chain tends to stabilize the resulting epoxide adjacent to it with the weak acid system. Furthermore, FAMEs with more double bonds easily undergo side reactions with the strong acid system (H2SO4). Epoxidation of two vegetable oils with different fatty acid compositions were carried out with the same two acid catalyst systems. And the results were in agreement with those from the FAMEs. The current findings could provide useful guidance for the epoxidation of different vegetable oils with different alkenyl structure compositions.