Herein, hierarchical bifunctional catalysts of Sn-USY-supported Au nanoparticles were designed for the one-pot conversion of glycerol (GLY) to methyl lactate (MLA). Over Au/Sn-USY catalyst, 79% MLA yield can be obtained with a high selectivity (∼90%). The promotion effect of Sn was investigated, and the interaction between extraframework SnOx and Au was confirmed by TEM, pyridine-FT-IR, CO-FT-IR, and XPS. The interaction promotes the dispersion of Au particles (smaller and much more uniform). This is important for the oxidation of GLY to 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GA), which are the intermediate species for the conversion of GLY to MLA. Meanwhile, introduction of Sn modified the acid properties of the catalyst, which are critical for the selective conversion of DHA and GA to MLA.Keywords: Au; bifunctional catalysts; glycerol; methyl lactate; Sn; zeolite;

An oxidation–hydrolysis strategy was developed for depolymerization of cellulose. Part of the hydroxymethyl groups on glucose units were oxidized to carboxyl groups during the preoxidation treatment, and the generated acid sites act as the catalytic active centers for the following depolymerization of cellulose. After α-cellulose was pretreated by air at 210 °C for 48 h, 23.3% yield of glucose was obtained in water at 150 °C for 8 h without additional catalyst. The ratio of cellulose/water has no obvious influence on the yield of glucose. 17.9% yield of glucose for α-cellulose can still be obtained even at the weight ratio of 0.4, where up to 75 g L−1 of glucose solution is obtained. For microcrystalline cellulose, 25.2% yield of glucose was obtained for hydrolysis at 170 °C for 8 h. It was revealed that the generated carboxyl acid groups in the oxidation step are the catalytic sites for the hydrolysis of cellulose.

•Carbohydrates were converted to methyl lactate (MLA) catalyzed by SnCln.•MLA selectivity was improved by controlling the acidity of reaction solution.•Inorganic base such as NaOH was used to adjust the acidity of reaction solution.•The method can efficiently convert glucose, fructose, and sucrose to MLA.A facile and efficient method to improve the selectivity of methyl lactate (MLA) in the chemical conversion of glucose in methanol catalyzed by homogeneous Lewis acid was established. The yield of MLA was efficiently improved through controlling the acidity of the reaction solution by neutralization of protons generated from the hydrolysis/methanolysis of SnCl4. The mechanism of glucose conversion to MLA catalyzed by SnCl4-NaOH was explored. The effects of the concentration of catalyst and substrate and the reaction temperature and time were systematically studied. The catalyst system of SnCl4-NaOH can efficiently convert glucose, fructose, and sucrose to MLA with yields of 47%, 57%, and 51% at 160 °C for 2.5 h, respectively. The catalyst can be regenerated and reused at least three times in the conversion of glucose to MLA without significant loss of activity and selectivity.

With NHPI/Co(OAc)2 as catalyst and air as oxidant, carboxylic group functionalized cellulose was prepared by oxidation of cellulose in acetic acid. Fourier transform infrared spectroscopy was utilized to detect the generation of carboxylic group and the acid amount was determined by acid–base titration method. The present results revealed that C6 primary hydroxyl groups on glucose units were partly converted to carboxylic groups during the catalytic oxidation process. The degree of polymerization of oxidized cellulose, which was determined by viscosity measurement, decreased slightly as compared with its parent. The structure of cellulose was characterized by X-ray diffraction and scanning electron microscopy, and it was almost unchanged.

•Sulfonated hierarchical H-USY zeolite was prepared by grafting method.•It showed high catalytic activity for the hydrolysis of hemicellulose and cellulose.•Both the acidity and the pore structure determined the activity of zeolites.Sulfonated hierarchical H-USY zeolite was prepared and characterized by X-ray diffraction, N2 physisorption, Fourier transform infrared spectroscopy, inductively coupled plasma atomic emission spectroscopy, temperature-programmed desorption of ammonia, and acid–base titration. It was proved that sulfonic group was successfully anchored onto the hierarchical H-USY zeolite. The acidity of the hierarchical H-USY was remarkably improved. Sulfonated hierarchical H-USY zeolite was efficient for the hydrolysis of hemicellulose and cellulose. The yield of TRS for hydrolysis of hemicellulose reached 78.0% at 140 °C for 9 h. For hydrolysis of α-cellulose, 60.8% conversion with 22.4% yield of glucose was obtained. Even for microcrystalline cellulose, 43.7% conversion with 15.1% yield of glucose can be obtained. These results are much higher than those obtained over hierarchical H-USY zeolite, indicating that both the acidity and the pore structure determine the activity of zeolite as catalyst in the hydrolysis of biomass.

Hierarchical H-USY zeolite prepared by oxalic acid treatment was demonstrated to be an effective catalyst for the hydrolysis of hemicellulose. The yield of total reducing sugars (TRS) increased remarkably to 55.7% over hierarchical H-USY compared with 5.8% yield of TRS over H-USY-parent under the same reaction conditions. Mesopores/macropores created by oxalic acid treatment played a main role in elevation of the yield of TRS, which not only improved the accessibility of the acid sites, but also facilitated the product to diffuse out the catalyst and thus restrained its further conversion to byproducts. Conditions of oxalic acid treatment for preparation of hierarchical H-USY zeolite and hydrolysis of hemicellulose were optimized to obtain high yield of TRS. Furthermore, hierarchical H-USY zeolite showed good activity in the selective hydrolysis of hemicellulose in lignocellulosic biomass.Graphical abstractHighlights► Hierarchical H-USY zeolites were prepared with oxalic acid treatment of H-USY. ► The catalyst showed high activity for the hydrolysis of hemicellulose. ► Mesopores/macropores played a main role in elevation of the yield of sugars. ► The catalyst showed good activity in hydrolysis of hemicellulose of lignocellulose.

Hierarchical molecular sieves possess the characters of both microporous molecular sieves and meso- and/or macro-porous materials, and have potential application in adsorption and separation of macromolecules and diffusion limited catalytic reactions. In this work, hierarchical MeAPO-5 (MeAPO-5-meso, Me = Co, Mn, Fe, Mg and Ti) molecular sieves were synthesized directly using glucose as mesopore template. The synthesized MeAPO-5-meso molecular sieves were characterized by X-ray diffraction, X-ray fluorescence, N2 physisorption, thermogravimetric analysis and scanning electron microscopy. It was proved that mesopores with the pore size distribution of 5–30 nm were introduced to MeAPO-5-meso. The improvement effect of the introduced mesopores on the catalytic performance of MeAPO-5-meso (Me = Mn, Fe and Co) was investigated in the oxidation of various hydrocarbons with different molecular dimensions including cyclohexene, ethylbenzene, indan, tetralin, diphenylenemethane and fluorene. For comparison, these oxidation reactions were also performed over the ordinary MeAPO-5. The results indicated that mesopores benefited the diffusion of the reactants and the products, so the conversion of the reactants was improved while the selectivity of the products was slightly improved or maintained at higher conversion. In addition, the promotion effect is dependent on the dimension of the substrates with respect to the micropore size of molecular sieves.Graphical abstractHighlights► Hierarchical MeAPO-5 molecular sieves (Me = Co, Mn, Fe, Mg and Ti) were synthesized. ► Hierarchical MeAPO-5 (Me = Co, Mn and Fe) was used as catalyst in oxidation of hydrocarbons. ► Mesopores benefited the diffusion of the reactants and the products. ► The promotion effect was dependent on the dimension of substrates with respect to the size of micropore of molecular sieves.

The roles of acidity and micropore structure of zeolite were studied in the hydrolysis of the model oligosaccharide of cellulose–cellobiose. HZSM-5, HY, HMOR and Hβ zeolites were selected as model catalysts for the hydrolysis of cellobiose. The effect of acidity of zeolite, including the strength, type and location, on its catalytic activity was investigated. The strong Brönsted acid sites located in micropores are the active sites for the hydrolysis of cellobiose to glucose. Meanwhile, the catalytic performance of zeolite is also dependent on the micropore size of zeolite.Download high-res image (105KB)Download full-size imageThe roles of micropore structure and acidity of zeolite were studied in the hydrolysis of cellobiose. Strong Brönsted acid sites located in micropores are the active sites. Catalytic performance of zeolite is also dependent on its micropore size.

•Mesopores were formed on USY zeolite by treatment with nitric acid.•Acid in low concentration mainly removes extraframework Al.•Acid sites are primary for the conversion of glucose to MLE.•Mesopores promote the efficiency of acid sites.H-USY was treated with nitric acid solution to remove Al species and thus adjust the mesoporosity and acidity. Effects of mesopore and acidity of H-USY zeolite were studied on the conversion of carbohydrate to methyl levulinate (MLE). Low concentration of nitric acid mainly removed the extraframework aluminum species with the increase of mesoporosity and resulted in a slight decrease of acidity. Both extraframework and framework Al species were removed to some extent under high concentration of nitric acid, associating with the obvious decrease of acidity. H-USY treated with low concentration of nitric acid showed higher yield of MLE than H-USY-parent. Generated mesopores improved the diffusion limitations, facilitated the accessibility of reactant to the acid sites, and thus promoted the formation of MLE.Download full-size image

•Mesoporous TiO2 (meso-TiO2) with sharp pore size distribution was synthesized by hydrothermal method.•Au/meso-TiO2 was efficient for the catalytic oxidation of primary alcohol to acid.•Aliphatic and aromatic alcohols can be converted to the corresponding carboxyl acids.Mesoporous TiO2 (meso-TiO2) with sharp pore size distribution was synthesized by hydrothermal method. The obtained meso-TiO2 is in pure anatase phase and presents spheric aggregates with diameter of 1.0–1.5 μm, which consists of nanoparticles with size of 6–10 nm. Au supported on meso-TiO2 (Au/meso-TiO2) was prepared by urea deposition–precipitation method using HAuCl4 as gold source. The catalyst was characterized by X-ray diffraction, N2 adsorption, transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The catalytic performance of Au/meso-TiO2 was studied in the oxidation of 1-pentanol to n-valeric acid with molecular oxygen as oxidant in water under basic conditions. It was found that meso-TiO2 is a better support for gold catalyst in the oxidation of 1-pentanol than NaY zeolite, hydrotalcite, and nano-TiO2. Deposition–precipitation time, calcination temperature and Au loading affected the catalytic performance of Au/meso-TiO2. The catalyst can also effectively catalyze the oxidation of aliphatic (C3–C10) and aromatic alcohols to the corresponding carboxylic acids.Download high-res image (67KB)Download full-size image