Daiqi Ye

Conversion of fructose into 5-hydroxymethylfurfural catalyzed by recyclable sulfonic acid-functionalized metal–organic frameworks

A series of sulfonic acid-functionalized metal–organic frameworks (MOF-SO3H) were prepared by postsynthetic modification (PSM) of the organic linkers within the MOF with chlorosulfonic acid. The obtained MOF-SO3H, including sulfonic acid-functionalized MIL-101(Cr) [MIL-101(Cr)-SO3H], UIO-66(Zr) [UIO-66(Zr)-SO3H], and MIL-53(Al) [MIL-53(Al)-SO3H], have been systematically studied as solid acids in fructose transformation to 5-hydroxymethylfurfural (HMF). With MIL-101(Cr)-SO3H as catalyst, a HMF yield of 90% with a full fructose conversion was obtained at 120 °C for 60 min in DMSO. The concentration of –SO3H in MOF-SO3H as well as the contribution of Bronsted acidity of MOF-SO3H parallels its –SO3H grafting rate. Under a lower –SO3H grafting level, a good linear correlation between catalytic activity, in terms of turnover frequency, and sulfonic acid-site density of MOF-SO3H was found. Moreover, the sulfonic acid groups, which function as the catalytic sites, are equivalent in all MOF-SO3H for fructose-to-HMF transformation, regardless of precursor MOFs. Both conversions of fructose and selectivities towards HMF increase with the sulfonic acid-site density of MOF-SO3H at an initial stage of fructose-to-HMF transformation. Kinetics studies reveal that the MIL-101(Cr)-SO3H promoted fructose-to-HMF transformation may follow pseudo-first-order kinetics with observed activation energy of 55 kJ mol−1 under the investigated conditions. Moreover, MIL-101(Cr)-SO3H behaves as a heterogeneous catalyst and can be easily recovered and reused. The research highlights a good prospect for catalytic application of MOF-derived solid acid catalysts for biomass carbohydrate valorization.

Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation

A series of highly active Pt/TiO2 catalysts were prepared by impregnation and deposition precipitation methods with different reduction processes. Their catalytic activities were evaluated by catalytic decomposition of formaldehyde (HCHO) at room temperature. The effects of reduction treatment on structural properties and catalytic activity were studied. Reduced Pt/TiO2 catalysts showed large differences in structural properties (such as particle size, oxidation state, surface content and electronic property of Pt nanoparticles, and surface oxygen) and catalytic activity for HCHO oxidation compared with the unreduced ones. Nearly 100% HCHO conversion was achieved on the former. Especially, sodium borohydride reduced Pt/TiO2 catalysts even with 0.1% Pt loading showed nearly complete oxidation of HCHO. Well-dispersed and negatively charged metallic Pt nanoparticles, and rich chemisorbed oxygen are probably responsible for their high catalytic activities.

Amine-functionalized metal-organic frameworks for the transesterification of triglycerides

For the application of functionalized metal–organic frameworks (MOFs) as a solid base, amine-functionalized MOF materials are achieved by (i) dative modification of unsaturated metal sites located at the secondary building units of MOFs with diamine, and (ii) covalent modification of the amine-tagged organic linkers within the MOF by alkylation with 2-dimethylaminoethyl chloride. The resulting amine-functionalized MOFs exhibit excellent results in the liquid phase transesterification of triglycerides and methanol, with the triglyceride conversions exceeding 99%, which are important model reactions for the production of biodiesel. The relationship between the catalytic activity towards transesterification and the basicity of amine-functionalized MOFs reveals a linear correspondence in terms of turnover frequency and basic site density. The basicity of the MOFs and reaction parameters are shown to significantly affect the catalytic performance. Kinetics studies reveal that the reaction follows first-order kinetics with the calculated activation energy of 48.2 kJ mol−1. This research opens up new perspectives on the postsynthetic modification of MOFs and more generally on the rational design of MOF-derived solid base catalysts.

A computational study on the hydrogenation of CO2 catalyzed by a tetraphos-ligated cobalt complex: monohydride vs. dihydride

Density functional theory (DFT) calculations were used to study the mechanisms of hydrogenation of carbon dioxide catalyzed by the tetraphos-ligated (PP3: P(CH2CH2PPh2)3) cobalt complexes. We investigated the binding modes between CO2 and the cobalt metal center to determine whether the CO2 coordinated to the metal center in the catalytic processes. The monohydride catalytic pathway (Path I) and the dihydride catalytic pathway (Path II) for the hydrogenation of CO2 have been explored. In these two cases, a weak H⋯CO2 interaction leads to cleavage of the Co–H bond and subsequent formation of the formate ligand. Moreover, the transformation from the dihydride to the monohydride is endergonic by 9.2 kcal mol−1 and the relevant free energy barrier is about 20.9 kcal mol−1. However, the largest free energy barrier of 19.1 kcal mol−1 in Path II is significantly lower than that in Path I (22.8 kcal mol−1), which corresponds to the hydride transfer from the cobalt center to CO2. The detailed comparisons of the possible pathways suggest that Path II is much more favoured than Path I and that it is not necessary to form the monohydride species in the whole catalytic cycle. Our results, which unambiguously demonstrate that the active catalyst is the dihydride rather than the monohydride, are consistent with the experimental observations and, most importantly, provide detailed mechanistic insight.

Effect of oxygen mobility in the lattice of Au/TiO2 on formaldehyde oxidation

Two Au catalysts supported on TiO2 were prepared by impregnation method followed by sodium borohydride reduction or calcination in air (Au/TiO2-R and Au/TiO2-C, respectively). The 1 wt % Au/TiO2-R sample was found to be highly efficient for the oxidation of low concentrated formaldehyde at room temperature. A HCHO conversion of 98.5% was achieved with this catalyst, whereas the Au/TiO2-C sample showed almost no activity under the same conditions. Highly dispersed metallic Au nanoparticles with small size (∼3.5 nm) were identified in the 1 wt % Au/TiO2-R catalyst. A significant negative shift of Au4f peak in XPS spectra with respect to bulk metallic Au was observed for the 1 wt % Au/TiO2-R but no similar phenomena was found for the heat-treated catalyst. More Au nanoparticles and higher content of surface active oxygen were identified on the surface of the Au/TiO2-R in comparison with the Au/TiO2-C, suggesting that the Au/TiO2-R catalyst can enhance the amount of active sites and species involved in for HCHO oxidation. The reduction treatment by sodium borohydride promotes the formation of dispersed metallic Au nanoparticles with small size because it facilitates the electron transfer and increases the content of surface Au nanoparticles and activated oxygen. All these factors are responsible for a high activity of this catalyst in the oxidation of HCHO.

Enhancement of the non-thermal plasma-catalytic system with different zeolites for toluene removal

Based on the important effect of catalyst on the plasma-catalytic system, various types of zeolites (5A, HZSM-5, Hβ, HY and Ag/HY) were chosen as catalysts to remove toluene under non-thermal plasma conditions in this work. The results showed that all the zeolites, with or without toluene adsorption abilities, significantly enhanced the toluene removal efficiency in the plasma discharge zone. Moreover, the carbon balance and CO2 selectivity showed the same tendency of Ag/HY > HY > Hβ (HZSM-5) > 5A, which was basically consistent with toluene adsorption ability, while being opposite to the ozone emission. Loading silver on the zeolite greatly decreased organic byproduct emission, and further improved the mineralization of toluene oxidation. At the same time, the intermediates including ring-opening products on the catalyst surface were identified, and the pathways of toluene decomposition were proposed.

Abatement of Toluene in the Plasma-Driven Catalysis: Mechanism and Reaction Kinetics

The mechanism and reaction kinetics of toluene destruction in a plasma-driven catalysis (PDC) system were studied. The results show that the toluene removal efficiency (TRE) is greatly increased while the level of O3 by-product is significantly reduced in PDC as compared with that in nonthermal plasma (NTP). The rate constant of toluene destruction in the PDC is more than twice than that in NTP. Among the multiple reactive species responsible for toluene destruction in the PDC, hydroxyl radicals (·OH) had a small contribution, whereas energetic electrons and atomic oxygen (O) were the most important. The enhanced performance of toluene destruction by PDC was mainly due to greater amounts of O formed during the process. The catalysts improved toluene destruction by catalytic decomposition of O3 and generation of O. Essentially, better toluene abatement can be achieved by focusing on the increased energy density and improved performance of the catalyst for O3 decomposition.

Plasma-Driven Catalysis Process for Toluene Abatement: Effect of Water Vapor

Plasma-driven catalysis (PDC) was used to remove toluene in air. Water vapor is a critical operating parameter in this process. Its effect on toluene removal efficiency, carbon balance, CO2 selectivity, and outlet O3 concentration was systematically investigated. Results showed that water vapor imposed negative effect on toluene decomposition since it depressed the formation and catalytic decomposition of O3. Water vapor deposited on the catalyst would cover the catalytic active sites, resulting in the deactivation of the catalyst. There was an optimum water vapor content for the highest carbon balance and CO2 selectivity. The present paper sheds some insight into the effect of water vapor and provides a valuable basis for the application of the PDC technology.

Toluene decomposition performance and NOx by-product formation during a DBD-catalyst process.

Characteristics of toluene decomposition and formation of nitrogen oxide (NOx) by-products were investigated in a dielectric barrier discharge (DBD) reactor with/without catalyst at room temperature and atmospheric pressure. Four kinds of metal oxides, i.e., manganese oxide (MnOx), iron oxide (FeOx), cobalt oxide (CoOx) and copper oxide (CuO), supported on Al2O3/nickel foam, were used as catalysts. It was found that introducing catalysts could improve toluene removal efficiency, promote decomposition of by-product ozone and enhance CO2 selectivity. In addition, NOx was suppressed with the decrease of specific energy density (SED) and the increase of humidity, gas flow rate and toluene concentration, or catalyst introduction. Among the four kinds of catalysts, the CuO catalyst showed the best performance in NOx suppression. The MnOx catalyst exhibited the lowest concentration of O3 and highest CO2 selectivity but the highest concentration of NOx. A possible pathway for NOx production in DBD was discussed. The contributions of oxygen active species and hydroxyl radicals are dominant in NOx suppression.

Controllable synthesis of 3D hierarchical Co3O4 nanocatalysts with various morphologies for the catalytic oxidation of toluene

Three-dimensional (3D) hierarchical Co3O4 nanocatalysts with different morphologies and various exposed crystal planes were synthesized via a hydrothermal process without the use of a cobalt surfactant precursor and subsequent direct thermal decomposition. The morphologies obtained include 3D hierarchical cube-stacked Co3O4 microspheres (C sample), 3D hierarchical plate-stacked Co3O4 flowers (P sample), 3D hierarchical needle-stacked Co3O4 double-spheres with an urchin-like structure (N sample), and 3D hierarchical sheet-stacked fan-shaped Co3O4 (S sample), which exhibit high efficiency for the total oxidation of volatile organic compounds (VOCs). Among them, the C sample exhibits the best activity with the temperature required for achieving a toluene conversion of 90% (T90%) of approximately 248 °C and the activity energy (Ea) of 80.2 kJ mol−1, which is at least 32 °C lower than that of the S sample with a higher Ea of 114.9 kJ mol−1 at a space velocity (WHSV) of 48 000 mL g−1 h−1. The effects of morphology on the physicochemical properties and catalytic activity of the Co3O4 catalysts are investigated using numerous analytical techniques. It is concluded that the large specific surface area, highly defective structure with abundant surface adsorbed oxygen species and rich high valence Co ions in the C sample are responsible for its excellent catalytic performance. Moreover, no significant decrease in catalytic efficiency is observed over 120 h at 255 °C on the C sample, which indicates that it exhibits excellent stability for toluene oxidation. Therefore, it shows potential as a non-noble catalyst in practical applications.