Yihang Guo

Advancements in solid acid catalysts for biodiesel production

Biodiesel has emerged as one of the best potential renewable energy sources to replace current petroleum-based diesel. It is a sustainable, biodegradable and non-toxic diesel fuel substitute that can be easily produced through base- or acid-catalyzed esterification and transesterification reactions. The conventional base catalysts, although effective, are limited to use of refined vegetable oils, leading to impractical and uneconomical processes due to high feedstock cost and priority as food resources. Biodiesel production processes based on the use of acid catalysts are good alternatives to conventional processes because of their simplicity and the simultaneous promotion of esterification and transesterification reactions from low-grade, highly-acidic and water-containing oils without soap formation. Highly reactive homogeneous Bronsted acid catalysts are efficient for this process, but they suffer from serious contamination and corrosion problems that require the implementation of good separation and purification steps. More recently, a “green” approach to biodiesel production has stimulated the application of sustainable solid acid catalysts as replacements for such liquid acid catalysts so that the use of harmful substances and generation of toxic wastes are avoided; meanwhile, the ease of catalyst separation after the reactions can be realized. Recent studies have proven the technical feasibility and the environmental and economical benefits of biodiesel production via heterogeneous acid-catalyzed esterification and transesterification. In this perspective, various solid acids including sulfated metal oxides, H-form zeolites, sulfonic ion-exchange resins, sulfonic modified mesostructured silica materials, sulfonated carbon-based catalysts, heteropolyacids and acidic ionic liquids are reviewed as heterogeneous catalysts in esterification and transesterification. Meanwhile, for the purpose of facilitating mass-transport of solid acid-catalyzed biodiesel production processes and improving the catalytic stability of the solid acid catalysts in esterification and transesterification reactions, novel and robust organic–inorganic hybrid acid catalysts with unique advantages including strong Bronsted as well as Lewis acid properties, well-defined mesostructure and enhanced surface hydrophobicity are successfully designed, which have been highlighted in this review.

Fabrication of Z-scheme plasmonic photocatalyst Ag@AgBr/g-C3N4 with enhanced visible-light photocatalytic activity

A series of Ag@AgBr grafted graphitic carbon nitride (Ag@AgBr/g-C3N4) plasmonic photocatalysts are fabricated through photoreducing AgBr/g-C3N4 hybrids prepared by deposition-precipitation method. The phase and chemical structures, electronic and optical properties as well as morphologies of Ag@AgBr/g-C3N4 heterostructures are well-characterized. Subsequently, the photocatalytic activity of Ag@AgBr/g-C3N4 is evaluated by the degradation of methyl orange (MO) and rhodamin B (RB) under visible-light irradiation. The enhanced photocatalytic activity of Ag@AgBr/g-C3N4 compared with g-C3N4 and Ag@AgBr is obtained and explained in terms of the efficient visible-light utilization efficiency as well as the construction of Z-scheme, which keeps photogenerated electrons and holes with high reduction and oxidation capability, evidenced by photoelectrochemical tests and free radical and hole scavenging experiments. Based on the intermediates identified in the reaction system, the photocatalytic degradation pathway of MO is put forward.

Mixed phase titania nanocomposite codoped with metallic silver and vanadium oxide: New efficient photocatalyst for dye degradation

Titania nanocomposite codoped with metallic silver and vanadium oxide was prepared by a one-step sol-gel-solvothermal method in the presence of a triblock copolymer surfactant (P123). The resulting Ag/V-TiO(2) three-component junction system exhibited an anatase/rutile (weight ratio of 73.8:26.2) mixed phase structure, narrower band gap (2.25 eV), and extremely small particle sizes (ca. 12 nm) with metallic Ag particles well distributed on the surface of the composite. The Ag/V-TiO(2) nanocomposite was used as the visible- and UV-light-driven photocatalyst to degrade dyes rhodamine B (RB) and coomassie brilliant blue G-250 (CBB) in an aqueous solution. At 1.8% Ag and 4.9% V doping, the Ag/V-TiO(2) system exhibited the highest visible- as well as UV-light photocatalytic activity; additionally, the activity of the three-component system exceeded that of Degussa P25, pure TiO(2), single-doped TiO(2) system (Ag/TiO(2) or V-TiO(2)) as well as P123-free-Ag/V-TiO(2) codoped system. The reasons for this enhanced photocatalytic activity were revealed.

Preparation of mesoporous polyoxometalate-tantalum pentoxide composite catalyst and its application for biodiesel production by esterification and transesterification

Mesoporous polyoxometalate-tantalum pentoxide composite catalyst, H3PW12O40/Ta2O5, with H3PW12O40 loading from 3.6 to 20.1% was prepared by a one-step sol–gel-hydrothermal route in the presence of a triblock copolymer surfactant. Bulk and surface sensitive probe testing results indicated that the primary Keggin structure remained intact after formation of the composite, and strong interaction between the Keggin unit and Ta2O5 framework existed in the composite. Additionally, the composite exhibited larger and well-distributed three-dimensionally interconnected pores (3.9–5.0 nm), larger BET surface area (106.0–126.9 m2 g−1), high porosity (0.44–1.37 cm3 g−1), and homogeneous dispersion of the Keggin unit throughout the composite. As an environmentally friendly solid acid catalyst, the catalytic performance of the H3PW12O40/Ta2O5 was evaluated in the esterification of lauric acid and myristic acid, the transesterification of tripalmitin as well as the direct use of soybean oil for biodiesel production. Regardless of the presence of free fatty acids, the H3PW12O40/Ta2O5 composite showed high reactivity and selectivity towards simultaneous esterification and transesterification under mild conditions. The catalyst can be recovered, reactivated and reused several times.

Preparation and enhanced visible-light photocatalytic activity of graphitic carbon nitride/bismuth niobate heterojunctions

A series of graphitic carbon nitride/bismuth niobate (g-C3N4/Bi5Nb3O15) heterojunctions with g-C3N4 doping level of 10-90 wt% were prepared by a facile milling-heat treatment method. The phase and chemical structures, surface compositions, electronic and optical properties as well as morphologies of the prepared g-C3N4/Bi5Nb3O15 were well-characterized. Subsequently, the photocatalytic activity and stability of g-C3N4/Bi5Nb3O15 were evaluated by the degradation of aqueous methyl orange (MO) and 4-chlorophenol (4-CP) under the visible-light irradiation. At suitable g-C3N4 doping levels, g-C3N4/Bi5Nb3O15 exhibited enhanced visible-light photocatalytic activity compared with pure g-C3N4 or Bi5Nb3O15. This excellent photocatalytic activity was revealed in terms of the extension of visible-light response and efficient separation and transportation of the photogenerated electrons and holes due to coupling of g-C3N4 and Bi5Nb3O15. Additionally, the active species yielded in the pure g-C3N4- and g-C3N4/Bi5Nb3O15-catalyzed 4-CP photodegradation systems were investigated by the free radical and hole scavenging experiments.

Pore morphology-controlled preparation of ZrO2-based hybrid catalysts functionalized by both organosilica moieties and Keggin-type heteropoly acid for the synthesis of levulinate esters

A series of ZrO2-based hybrid catalysts functionalized by both organosilica groups and Keggin type heteropolyacid, H3PW12O40/ZrO2–Si(Et/Ph)Si and H3PW12O40/ZrO2–Si(Ph), with different structural orderings and pore geometries were controllably prepared by a one-pot template-assisted sol–gel co-condensation–hydrothermal treatment route. The pore morphologies, textural properties and acidity of as-prepared hybrid catalysts were characterized by XRD, TEM, nitrogen gas porosimetry, in situ pyridine-FTIR spectroscopy and titration, while the structural integrity of the incorporated functionalities was studied by FT-IR, 31P MAS NMR, 13C CP-MAS NMR and 29Si MAS NMR. As the novel solid acid catalysts, the catalytic performance of as-prepared materials was evaluated by esterification of levulinic acid with various alcohols under mild conditions. Meanwhile, comparative studies were carried out to find out the influences of acidity, porosity and surface hydrophobicity on the catalytic activity of the catalysts. Finally, the recyclability of the catalysts was tested.

Design of a highly ordered mesoporous H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst for methyl levulinate synthesis

A highly ordered mesoporous ZrO2-based material functionalized by benzene-bridged organosilica groups and the Keggin type heteropolyacid, prepared in a single co-condensation–hydrothermal treatment step, exhibited much higher catalytic activity towards methyl levulinate synthesis compared to alkyl-free H3PW12O40/ZrO2.

Efficient degradation of tetrabromobisphenol A by heterostructured Ag/Bi5Nb3O15 material under the simulated sunlight irradiation.

Heterostructured metallic silver-layered bismuth niobate two-component system (Ag/Bi(5)Nb(3)O(15)) was developed for the first time by a mild hydrothermal method combined with photodeposition. The Ag/Bi(5)Nb(3)O(15) exhibited single-crystalline orthorhombic structure with small particle size (50-200 nm) and octahedral as well as sheet-like shape; additionally, it possessed photoresponse in both UV and visible region. As a novel alternative photocatalysts to TiO(2), the photocatalytic activity of the Ag/Bi(5)Nb(3)O(15) was evaluated by the degradation of tetrabromobisphenol A, a member from the family of the brominated flame retardant, under solar simulating Xe lamp irradiation, and enhanced photocatalytic activity in compared to Bi(5)Nb(3)O(15) itself and Degussa P25 was obtained.

Transesterification of soybean oil to biodiesel catalyzed by mesostructured Ta2O5-based hybrid catalysts functionalized by both alkyl-bridged organosilica moieties and Keggin-type heteropoly acid

A series of Ta2O5-based hybrid catalysts functionalized by both alkyl-bridged organosilica fragments and the Keggin-type heteropoly acid, Ta2O5/Si(R)Si-H3PW12O40 (R = –CH2CH2- or –C6H4–), were prepared via a one-pot co-condensation method in the presence of a triblock copolymer surfactant (P123). The materials were well characterized by spectroscopy methods, X-ray diffraction analysis, transmission electron microscopy, and nitrogen physisorption measurement to confirm the structural integrity of the Keggin unit and alkyl-bridged organosilica units in the hybrid materials, to investigate the interaction between the Ta2O5 matrix and the organic or inorganic functionalities, and to test the mesostructure, morphology, and porosity of the materials. The materials were subsequently utilized as environmentally-friendly solid acid catalysts in the transesterification of soybean oil (containing 20 wt% myristic acid) with methanol to produce fatty acid methyl esters under atmosphere refluxing. Compared with bulk H3PW12O40 and alkyl-free H3PW12O40/Ta2O5, as-prepared Ta2O5/Si(R)Si-H3PW12O40 hybrid materials with suitable concentrations of bridging alkyl groups exhibited higher reactivity toward the target reaction. This enhanced acid-catalytic reactivity after the introduction of both acidic and hydrophobic functionalities within the Ta2O5 matrix is discussed. Finally, the reusability of the hybrid materials was evaluated through three catalytic runs.

Design of polyoxometallate–titania composite film (H3PW12O40/TiO2) for the degradation of an aqueous dye Rhodamine B under the simulated sunlight irradiation

A series of polyoxometallate/titania (H(3)PW(12)O(40)/TiO(2)) composite films with different H(3)PW(12)O(40) loadings (6.3%, 7.7%, 14.7% and 16.7%) were prepared by a modified sol-gel-hydrothermal route followed by a spin-coating method. The smooth films are constructed by the well-distributed H(3)PW(12)O(40)/TiO(2) sphere with particle size in the range from 80 to 100 nm, and the bandgap of the composite films is somewhat narrower compared with as-prepared pure TiO(2) film. As a novel photocatalytic material, the photocatalytic performances of the H(3)PW(12)O(40)/TiO(2) composite films were evaluated by the degradation and mineralization of an aqueous dye Rhodamine B (RB) under solar simulating Xe lamp irradiation (320 nm<λ<780 nm), and the enhanced photocatalytic activity in comparison to pure TiO(2) film as well as the H(3)PW(12)O(40)/TiO(2) and Degussa P25 TiO(2) powder was obtained. Additionally, the composite films can be reused at least for three times without losing their catalytic activity.