Catalysts, Volume 6, Issue 2 (February 2016) – 14 articles

Enormous interest in biocatalysis in non-aqueous phase has recently been triggered due to the merits of good enantioselectivity, reverse thermodynamic equilibrium, and no water-dependent side reactions. It has been demonstrated that enzyme has high activity and stability in non-aqueous media, and the variation of enzyme activity is attributed to its conformational modifications. This review comprehensively addresses the stability and activity of the intact enzymes in various non-aqueous systems, such as organic solvents, ionic liquids, sub-/super-critical fluids and their combined mixtures. It has been revealed that critical factors such as Log P, functional groups and the molecular structures of the solvents define the microenvironment surrounding the enzyme molecule and affect enzyme tertiary and secondary structure, influencing enzyme catalytic properties. Therefore, it is of high importance for biocatalysis in non-aqueous media to elucidate the links between the microenvironment surrounding enzyme surface and its stability and activity. In fact, a better understanding of the correlation between different non-aqueous environments and enzyme structure, stability and activity can contribute to identifying the most suitable reaction medium for a given biotransformation. Full article

Composites of TiO₂ (Hombikat, P25, sol-gel synthesis) and zeolite ZSM-5 (n_Si/n_Al = 55) with mass fractions from 25/75 to 75/25 were prepared by mechanical mixing, solid-state dispersion and sol-gel synthesis. Characterization of the composites by X-ray diffraction (XRD), N₂-sorption, scanning electron microscopy (SEM), and UV-Vis spectroscopy show that mechanical mixing and solid-state dispersion lead to comparable textural properties of the composites. A homogeneous distribution and intimate contact of small TiO₂ particles on the crystal surface of zeolite ZSM-5 were achieved by sol-gel synthesis. The composites were studied in the photocatalytic oxidation (PCO) of NO in a flatbed reactor under continuous flow according to ISO 22197-1. The highest NO conversion of 41% at an NO₂ selectivity as low as 19% stable for 24 h on-stream was reached over the TiO₂/ZSM-5 composite from sol-gel synthesis with equal amounts of the two components after calcination at 523 K. The higher activity and stability for complete NO oxidation than for pure TiO₂ from sol-gel synthesis, Hombikat, or P25 is attributed to the adsorptive properties of the zeolite ZSM-5 in the composite catalyst. Increasing the calcination temperature up to 823 K leads to larger TiO₂ particles and a lower photocatalytic activity. Full article

The present study attempts to provide insights for both the chemical composition (Si/Al) and the crystal morphology of ZSM-5 zeolites while using biomass template compounds in the synthesis. The solution containing biomass-derivative compounds was obtained after treating biomass in a sodium hydroxide aqueous solution under reflux. The latter alkaline solution was used as a solvent for zeolite nuclei ingredients to form the gel phase under hydrothermal conditions (170 °C during 24, 48 or 72 h). This approach allowed for preparing MFI zeolites having a broad range of Si/Al ratio, i.e., from 25 to 150. Likewise, MFI crystals with different morphologies could be obtained, being different from the pristine zeolite formed in the absence of biomass. Full article

The O–H bond breaking in H₂O molecules on metal surfaces covered with pre-adsorbed oxygen atoms is an important topic in heterogeneous catalysis. The adsorption configurations of H₂O and relevant dissociation species on clean and O-pre-adsorbed Fe(100) surfaces were investigated by density functional theory (DFT). The preferential sites for H₂O, HO, O, and H were investigated on both surfaces. Both the first H abstraction from adsorbed H₂O and the subsequent OH dissociation are exothermic on the O-pre-adsorbed Fe(100) surface. However, the pre-adsorbed O significantly reduces the kinetics energy barriers for both reactions. Our results confirmed that the presence of pre-adsorbed oxygen species could significantly promote H₂O dissociation. Full article

Dialkyl hexane-1,6-dicarbamate was synthesized, for the first time, by a one-pot reaction of 1,6-hexanediamine (HDA), urea, and alcohols, including methanol, ethanol, propanol, and butanol, in a self-designed batch reactor, using zinc-incorporated berlinite (ZnAlPO₄) as a catalyst. The yield of dibutyl hexane-1,6-dicarbamate (2) was systematically investigated as a function of Zn/Al molar ratio, reaction temperature, reaction time, catalyst usage and urea/HDA/butanol molar ratio. Based on these studies, the optimized reaction conditions were as follows: molar ratio urea/HDA/butanol = 2.6:1:8.6, catalyst usage = 3.0 g, reaction temperature = 493 K, reaction time = 6 h and reaction pressure = 1.2 MPa; a yield of 2 of 89.7% was achieved over the ZnAlPO₄ (molar ratio Zn/Al = 0.04) catalyst. The catalysts were characterized by X-ray photoelectric spectroscopy (XPS) and scanning electron microscope (SEM). Additionally, based on these experimental results, it was also proposed that the catalysis recycle of the one-pot synthesis of 2 from urea, HDA, and butanol over the ZnAlPO₄ catalyst. Full article

The development of a new facile method for the acetylation of alcohols and carbohydrate-derived polyols is described. This method relies on the nature of the cationic palladium catalyst, Pd(PhCN)₂(OTf)₂ which is generated in situ from Pd(PhCN)₂Cl₂ and AgOTf to catalyze the acetylation reaction. This new acetylation protocol is very rapid and proceeds under mild conditions with only 1 mol% of catalyst loading at room temperature. This new method has been applied to a variety of different alcohols with different levels of steric hindrance, as well as carbohydrate-derived polyols to provide the corresponding fully acetylated products in excellent yields. Full article

A series of Ce_1−xCo_xO_2−y precursors were prepared by homogeneous precipitation using urea as a precipitant. The Co/CeO₂ catalysts obtained from the Ce_1−xCo_xO_2−y precursors were used for the steam reforming of ethanol to produce hydrogen. Co ions could enter the CeO₂ lattices to form Ce_1−xCo_xO_2−y mixed oxides at x ≤ 0.2 using the homogeneous precipitation (hp) method. CeO₂ was an excellent support for Co metal in the steam reforming of ethanol because a strong interaction between support and metal (SISM) exists in the Co/CeO₂ catalysts. Because Co/CeO₂ (hp) prepared by homogeneous precipitation possessed a high BET surface area and small Co metal particles, Co/CeO₂ (hp) showed a higher ethanol conversion than the Co/CeO₂ catalysts prepared using the co-precipitation (cp) method and the impregnation (im) method. The selectivity of CO₂ over Co/CeO₂ (hp) increased with increasing reaction temperature at from 573 to 673 K, and decreased with increasing reaction temperature above 673 K due to the increase of CO formation. The carbonaceous deposits formed on the catalyst surface during the reaction caused a slow deactivation in the steam reforming of ethanol over Co/CeO₂ (hp). The catalytic activity of the used catalysts could be regenerated by an oxidation-reduction treatment, calcined in air at 723 K and then reduced by H₂ at 673 K. Full article

This work tested two V₂O₅-WO₃/TiO₂ catalysts with different vanadium precursors for selective catalytic reduction (SCR) of flue gas NO using NH₃ at 150–450 °C. While catalyst A was prepared using ammonium metavanadate (NH₄VO₃) through incipient impregnation, catalyst B was made according to the solvothermal method using vanadyl acetylacetonate (VO(acac)₂) as the vanadium precursor. The catalytic evaluation for denitration was in a laboratory fixed bed reactor using simulated flue gas under conditions of a gas hourly space velocity (GHSV) of 40,000 h⁻¹ and an NH₃/NO molar ratio of 0.8. Without SO₂ and water vapor in the flue gas at 200 °C, the realized NO conversion was 56% for catalyst A but 80% for B. The presence of 350 ppm SO₂ and 10 vol. % water vapor in the flue gas slightly reduced the NO conversion over catalyst B, and its activity was stable in a 108-h continuous test at temperatures varying from 450 °C to 220 °C. Via fourier transformation infrared spectroscopy (FT-IR) and thermogravimetric (TG) analysis, it was shown that over catalyst B a dynamic balance between the formation and decomposition of ammonium sulfite or sulfate is built possibly at temperatures as low as 220 °C. For this catalyst there was a higher surface atomic concentration of vanadium and a higher ratio of V⁴⁺/(V⁴⁺ + V⁵⁺), while the NH₃ adsorption test revealed more acidic sites on catalyst B. The study discloses a potentially new approach to prepare a V₂O₅-WO₃/TiO₂ catalyst with good performance for SCR of flue gas NO at 220–300 °C. Full article

The aerobic oxidation of hydrocarbon is of great significance from the viewpoints of both fundamental and industry studies as it can transfer the petrochemical feedstock into valuable chemicals. In this work, we investigated the aerobic oxidation of cyclohexene over TiZrCo catalysts, in which 2-cyclohexen-1-one was produced with a high selectivity of 57.6% at a conversion of 92.2%, which are comparable to the best results reported for the aerobic oxidation of cyclohexene over heterogeneous catalysts. The influences of kinds of solvent, substrate concentration and reaction temperature were evaluated. Moreover, the catalytic performance of the TiZrCo catalyst and the main catalytic active species were also discussed. The results of SEM, XRD and XPS suggested that the surface CoO and Co₃O₄ species are the catalytic active species and contribute to the high activity and selectivity in the present cyclohexene oxidation. The present catalytic system should have wide applications in the aerobic oxidation of hydrocarbons. Full article

A high-throughput (HT) screening platform developed at hte with the application focus on automotive catalysis is described. hte HT units are configured for performing steady-state testing, as well as dynamic tests with fast feed switches, such as lean/rich excursions for the evaluation of NO_x storage capacity and efficiency of lean NO_x traps (LNT), ammonia storage capacity for selective catalytic reduction (SCR), evaluation of oxygen storage capacity (OSC), as well as lambda sweep tests for screening of three-way catalysts (TWC). Even though catalysts are screened on a rather small scale (~100 mg powder), experience showed that dosing rather complex gas mixtures in concentrations close to that found in real exhaust for the given application is mandatory to generate relevant data. The objective of this work is to give additional insight into HT technology. In the industrial research laboratory, HT screening has matured to become a reliable approach for rapid screening of both reaction parameter spaces, as well as material properties relevant for exhaust gas catalyst development. Due to the speed of optimized screening involving 48 parallel reactors, automated handling of primary data is an imported requirement. Software for data reduction, like estimation of light-off temperature, needs to be robust and handle results for diverse sample libraries in an unattended fashion. In combination with the statistical design of experiment and multivariate data analysis, HT testing has become a valuable enhancement to automotive catalyst development. Full article

This paper presents an overview of heterogeneous partial (amm)oxidation and oxidative dehydrogenation (ODH) of hydrocarbons. The review has been voluntarily restricted to metal oxide-type catalysts, as the partial oxidation field is very broad and the number of catalysts is quite high. The main factors of solid catalysts for such reactions, designated by Grasselli as the “seven pillars”, and playing a determining role in catalytic properties, are considered to be, namely: isolation of active sites (known to be composed of ensembles of atoms), Me–O bond strength, crystalline structure, redox features, phase cooperation, multi-functionality and the nature of the surface oxygen species. Other important features and physical and chemical properties of solid catalysts, more or less related to the seven pillars, are also emphasized, including reaction sensitivity to metal oxide structure, epitaxial contact between an active phase and a second phase or its support, synergy effect between several phases, acid-base aspects, electron transfer ability, catalyst preparation and activation and reaction atmospheres, etc. Some examples are presented to illustrate the importance of these key factors. They include light alkanes (C₁–C₄) oxidation, ethane oxidation to ethylene and acetic acid on MoVTe(Sb)Nb-O and Nb doped NiO, propene oxidation to acrolein on BiMoCoFe-O systems, propane (amm)oxidation to (acrylonitrile) acrylic acid on MoVTe(Sb)Nb-O mixed oxides, butane oxidation to maleic anhydride on VPO: (VO)₂P₂O₇-based catalyst, and isobutyric acid ODH to methacrylic acid on Fe hydroxyl phosphates. It is shown that active sites are composed of ensembles of atoms whose size and chemical composition depend on the reactants to be transformed (their chemical and size features) and the reaction mechanism, often of Mars and van Krevelen type. An important aspect is the fact that surface composition and surface crystalline structure vary with reaction on stream until reaching steady state, which makes characterisation of active and selective surface sites quite difficult. The use of oxidants other than O₂, such as H₂O₂, N₂O or CO₂, is also briefly discussed. Based on such analysis and recent discoveries and process developments, our perspective is given. Full article

Aiming at an improved mechanistic understanding of the preferential oxidation of CO on supported Au catalysts, we have investigated the competition between CO and H₂ for stable, active oxygen (O_act) species on a Au/TiO₂ catalyst during the simultaneous exposure to CO and H₂ with various CO/H₂ ratios at 80 °C and 400 °C by quantitative temporal analysis of products (TAP) reactor measurements. It is demonstrated that, at both higher and lower temperature, the maximum amount of active oxygen removal is (i) independent of the CO/H₂ ratio and (ii) identical to the amount of active oxygen removal by CO or H₂ alone. Hence, under preferential CO oxidation (PROX) reaction conditions, in the simultaneous presence of CO and H₂, CO and H₂ compete for the same active oxygen species. In addition, also the dependency of the selectivity towards CO oxidation on the CO/H₂ ratio was evaluated from these measurements. Consequences of these findings on the mechanistic understanding of the PROX reaction on Au/TiO₂ will be discussed. Full article

A sulfur transfer agent in catalysts can effectively reduce the emission of SO₂ with minimum adverse effects on the catalytic cracking ability of the primary catalyst. In this paper, the composition and performance of sulfur transfer agents with different oxidative active components (such as Cu, Fe, Ni, Co, Ba, Zn and Cr) were prepared by acid peptization technique and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and N₂ adsorption-desorption technique. The relationship between the composition and performance of the new sulfur transfer agents was investigated and the regeneration and recycling of the agents were performed. The results indicates that copper is a very good desulfurization active component. Moreover, the presence of CO has no significant effect on the absorption ability of SO₂ by the sulfur transfer agent. Full article

High selectively catalytic conversion of lignin-based phenols (m-cresol, p-cresol, and guaiacol) into para-/m-xylene was performed over Pt/HZSM-5 through hydrodeoxygenation and in situ methylation with methanol. It is found that the p-/m-xylene selectivity is uniformly higher than 21%, and even increase up to 33.5% for m-cresol (with phenols/methanol molar ratio of 1/8). The improved p-/m-xylene selectivity in presence of methanol is attributed to the combined reaction pathways: methylation of m-cresol into xylenols followed by HDO into p-/m-xylene, and HDO of m-cresol into toluene followed by methylation into p-/m-xylene. Comparison of the product distribution over a series of catalysts indicates that both metals and supporters have distinct effect on the p-/m-xylene selectivity. Full article