Catalysts, Volume 6, Issue 1 (January 2016) – 18 articles

The bis(pyrazole)zinc(II) benzoate complexes bis(3,5-diphenylpyrazole)zinc(II) benzoate (1), bis(3,5-diphenylpyrazole)zinc(II) 3,5-dinitrobenzoate (2), bis(3,5-diphenylpyrazole)zinc(II) 4-hydroxybenzoate (3), and bis(3,5-di-tert-butylpyrazole)zinc(II) 2-chlorobenzoate (4) were synthesized from the reaction of 3,5-diphenylpyrazole (L1) or 3,5-di-tert-butylpyrazole (L2), zinc(II) acetate and the appropriate benzene carboxylic acid. The molecular structure of complex 2 confirmed that these zinc(II) benzoate complexes adopt a 4-coordinate tetrahedral geometry. All four complexes were screened as catalysts for the copolymerization of carbon dioxide (CO₂) and cyclohexene oxide (CHO) and were found to be active for the formation of poly(cyclohexene carbonate) (PCHC) at CO₂ pressures as low as 1.0 MPa under solvent-free conditions and without the use of a co-catalyst. At some reaction condition, most of the catalysts produced PCHC with high carbonate content of up to 98% and a good amount of cyclic cyclohexene carbonate (CCHC). The copolymers produced have low to moderate molecular weights (5200–12300 g/mol) and with polydispersity indices that vary from 1.19 to 2.50. Matrix Assisted Laser Desorption/Ionization-Time of Flight Mass Spectra (MALDI-TOF MS) of these copolymers showed they have benzoate and hydroxyl end groups. Full article

Electrochemical Promotion of Catalysis (EPOC) with alkali ionic conductors has been widely studied in literature due to its operational advantages vs. alkali classical promotion. This phenomenon allows to electrochemically control the alkali promoter coverage on a catalyst surface in the course of the catalytic reaction. Along the study of this phenomenon, a large variety of in situ and ex situ surface analysis techniques have been used to investigate the origin and mechanism of this kind of promotion. In this review, we analyze the most important contributions made on this field which have clearly evidenced the presence of adsorbed alkali surface species on the catalyst films deposited on alkaline solid electrolyte materials during EPOC experiments. Hence, the use of different surface analysis techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning photoelectron microscopy (SPEM), or scanning tunneling microscopy (STM), led to a better understanding of the alkali promoting effect, and served to confirm the theory of electrochemical promotion on this kind of catalytic systems. Given the functional similarities between alkali electrochemical and chemical promotion, this review aims to bring closer this phenomenon to the catalysis scientific community. Full article

A series of metal cation modified layered-double hydroxides (LDHs) and mixed oxides were prepared and used to be the selective oxidation of toluene with O₂. The results revealed that the modified LDHs exhibited much higher catalytic performance than their parent LDH and the modified mixed oxides. Moreover, the metal cations were also found to play important roles in the catalytic performance and stabilities of modified catalysts. Under the optimal reaction conditions, the highest toluene conversion reached 8.7% with 97.5% of the selectivity to benzyldehyde; moreover, the catalytic performance remained after nine catalytic runs. In addition, the reaction probably involved a free-radical mechanism. Full article

In this study, the dehydrogenation component of Ga₂O₃ was introduced into ZSM-5 nanocrystals to prepare Ga₂O₃/ZSM-5 hollow fiber-based bifunctional catalysts. The physicochemical features of as-prepared catalysts were characterized by means of XRD, BET, SEM, STEM, NH₃-TPD, etc., and their performances for the catalytic conversion of n-butane to produce light olefins and aromatics were investigated. The results indicated that a very small amount of gallium can cause a marked enhancement in the catalytic activity of ZSM-5 because of the synergistic effect of the dehydrogenation and aromatization properties of Ga₂O₃ and the cracking function of ZSM-5. Compared with Ga₂O₃/ZSM-5 nanoparticles, the unique hierarchical macro-meso-microporosity of the as-prepared hollow fibers can effectively enlarge the bifunctionality by enhancing the accessibility of active sites and the diffusion. Consequently, Ga₂O₃/ZSM-5 hollow fibers show excellent catalytic conversion of n-butane, with the highest yield of light olefins plus aromatics at 600 °C by 87.6%, which is 56.3%, 24.6%, and 13.3% higher than that of ZSM-5, ZSM-5 zeolite fibers, and Ga₂O₃/ZSM-5, respectively. Full article

The catalytic hydrogenation of lignin-derived monophenols with high efficiency and selectivity is important for the sustainable production of chemicals and fuels. Here, Pd/γ-Al₂O₃ was prepared via impregnation and used as catalyst for the hydrogenation of phenols to cyclohexanols under mild conditions in aqueous solution. 3 wt. % Pd/γ-Al₂O₃ exhibited good catalytic activity for the selective hydrogenation of 4-ethylphenol into 4-ethylcyclohexanol, and a conversion of 100% with selectivity of 98.9% was achieved at 60 °C for 12 h. Other lignin-derived monophenolic model compounds such as 4-methyl phenol and 4-propyl phenol could be hydrogenated into cyclohexanols selectively under optimal conditions. Moreover, the Pd/γ-Al₂O₃ catalyst displayed good activity for the hydrogenation of the mixture of monophenols directly derived from raw biomass system to cyclohexanols as the main products, and was favorable for the depolymerization of lignin oligomers under milder conditions. Pd/γ-Al₂O₃ catalyst showed good water resistance and stability after recycling four times. This result might provide a promising approach to selectively producing cyclohexanol directly from raw biomass material under mild conditions in aqueous solutions. Full article

Nickel catalysts are the most popular for steam reforming, however, they have a number of drawbacks, such as high propensity toward coke formation and intolerance to sulfur. In an effort to improve their behavior, a series of Ni-catalysts supported on pure and La-, Ba-, (La+Ba)- and Ce-doped γ-alumina has been prepared. The doped supports and the catalysts have been extensively characterized. The catalysts performance was evaluated for steam reforming of n-hexadecane pure or doped with dibenzothiophene as surrogate for sulphur-free or commercial diesel, respectively. The undoped catalyst lost its activity after 1.5 h on stream. Doping of the support with La improved the initial catalyst activity. However, this catalyst was completely deactivated after 2 h on stream. Doping with Ba or La+Ba improved the stability of the catalysts. This improvement is attributed to the increase of the dispersion of the nickel phase, the decrease of the support acidity and the increase of Ni-phase reducibility. The best catalyst of the series doped with La+Ba proved to be sulphur tolerant and stable for more than 160 h on stream. Doping of the support with Ce also improved the catalytic performance of the corresponding catalyst, but more work is needed to explain this behavior. Full article

The influence of reaction conditions on the catalytic performance of lanthanum vanadate (La_0.1V_0.9O_x) catalyst in the ammoxidation of 2-methylpyrazine (MP) to 2-cyanopyarazine (CP) has been investigated. This novel catalytic material exhibited remarkably good performance with very high space-time-yields (STY) of CP. The reaction parameters such as the effect of temperature, gas hourly space velocity (GHSV) and all other reaction variables (e.g., NH₃, air, and MP feed rates) on the catalytic performance were explored and optimized. For example, an increase in MP feed rate from 2 to >16 mmol/h led to decreased conversion of MP but increased the STY of CP significantly. Optimal performance was achieved when the reaction temperature was 420 °C and the molar ratio of 2-MP, ammonia, air, H₂O and N₂ in the feed gas was set to 1:7:26:13:22. Under these optimal reaction conditions, the catalyst showed a MP conversion of ~100%, CP selectivity of 86%, and STY of >500 g_CP/(kg_cat∙h). On the other hand, the formation of pyrazine (Py) as a by-product was found to be high when the NH₃:MP ratio was lower at increased contact time. This suggests possible differences in the reaction mechanism pathways with respect to feed composition over La_0.1V_0.9O_x catalysts. Full article

A series of Mn-Co/TiO₂ catalysts were prepared by wet impregnation method and evaluated for the oxidation of NO to NO₂. The effects of Co amounts and calcination temperature on NO oxidation were investigated in detail. The catalytic oxidation ability in the temperature range of 403–473 K was obviously improved by doping cobalt into Mn/TiO₂. These samples were characterized by nitrogen adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and hydrogen temperature programmed reduction (H₂-TPR). The results indicated that the formation of dispersed Co₃O₄·CoMnO₃ mixed oxides through synergistic interaction between Mn-O and Co-O was directly responsible for the enhanced activities towards NO oxidation at low temperatures. Doping of Co enhanced Mn⁴⁺ formation and increased chemical adsorbed oxygen amounts, which also accelerated NO oxidation. Full article

Catalytic cracking of n-hexane to selectively produce propylene on Beta zeolite was carried out. The H-Beta (HB) (Si/Al = 77) zeolite showed higher catalytic stability and propylene selectivity than the Al-rich HB (Si/Al = 12), due to its smaller number of acid sites, especially Lewis acid sites (LAS). However, catalytic stability and propylene selectivity in high n-hexane conversions were still not satisfactory. After dealumination with HNO₃ treatment, catalytic stability was improved and propylene selectivity during high n-hexane conversions was increased. On the other hand, catalytic stability was not improved after desilication with NaOH treatment, although mesopores were formed. This may be related to the partially destroyed structure. However, propylene selectivity in high n-hexane conversions was increased after alkali treatment. We successfully found that the catalytic stability was improved and the propylene selectivity in high n-hexane conversions was further increased after the NaOH treatment followed by HNO₃ treatment. This is due to the decrease in the number of acid sites and the increase in mesopores which are beneficial to the diffusion of coke precursor. Full article

Four types of nickel catalysts supported on aluminum oxide (Ni/Al₂O₃) with different nickel loadings were synthesized using the co-precipitation method and were used for the hydrogenation of levulinic acid (LA) to prepare γ-valerolactone (GVL). The synthesized Ni/Al₂O₃ catalysts exhibited excellent catalytic activity in dioxane, and the activity of the catalysts was excellent even after being used four times in dioxane. The catalytic activity in dioxane as a solvent was found to be superior to the activity in water. Nitrogen physisorption, X-ray diffraction, and transmission electron microscopy were employed to characterize the fresh and used catalysts. The effects of the nickel loading, temperature, hydrogen pressure, and substrate/catalyst ratio on the catalytic activity were investigated. Full article

The development of high efficient and low energy consumption approaches for the transformation of cellulose is of high significance for a sustainable production of high value-added feedstocks. Herein, electrocatalytic oxidation technique was employed for the selective conversion of cellulose to gluconate in alkaline medium by using concentrated HNO₃ pretreated carbon aerogel (CA) supported Au nanoparticles as anode. Results show that a high gluconate yield of 67.8% and sum salts yield of 88.9% can be obtained after 18 h of electrolysis. The high conversion of cellulose and high selectivity to gluconate could be attributed to the good dissolution of cellulose in NaOH solution which promotes its hydrolysis, the surface oxidized CA support and Au nanoparticles catalyst which possesses high amount of active sites. Moreover, the bubbled air also plays important role in the enhancement of cellulose electrocatalytic conversion efficiency. Lastly, a probable mechanism for electrocatalytic oxidation of cellulose to gluconate in alkaline medium was also proposed. Full article

Hyperbranched core-shell structure can be constructed by the modification of hyperbranched polyethylenimine (HPEI) with different amide shells. Functionalized HPEI with acetic amide (HPEI-ACAm), propionic amide (HPEI-PRAm), butyric amide (HPEI-BUAm) and isobutyric amide (HPEI-IBAm) shells have been successfully prepared and used as protectors for gold nanoparticles (AuNPs). Novel AuNP composites were obtained through the non-covalent interaction between HPEI-XXAm and gold nanoparticles (XXAm represents ACAm, PRAm, BUAm or IBAm). The resulted AuNP composites can catalyze the reduction reaction of 4-nitrophenol by NaBH₄. Interestingly, the catalytic activity of the AuNPs mainly depends on the structure of protectors and the degree of carbon chain arrangement denseness, which should affect the diffusivity of the reactants. In addition, the order of reaction rate is HPEI10K-IBAm_0.80 > HPEI10K-ACAm_0.80 > HPEI10K-PRAm_0.82 > HPEI10K-BUAm_0.83. It was found that the increase of the concentrations of the capping HPEI-XXAm polymers can enhance both the reaction rate and the turnover frequency (TOF) values. Furthermore, the reaction rate was accelerated with increasing the reaction temperature for AuNPs-HPEI10K-ACAm_0.80 and AuNPs-HPEI10K-PRAm_0.82 systems. Interestingly, the reaction rate was accelerated with elevating reaction temperature at the beginning but reached a plateau or decreased sharply for AuNPs-HPEI10K-IBAm_0.80 and AuNPs-HPEI10K-BUAm_0.82 systems, owing to the thermoresponsivity of the corresponding AuNP composites. As a consequence, the catalytic activity could be controlled by adjusting the different shells of the hyperbranched polyethylenimine. Full article

An inorganic-organic porous silica network catalyst was prepared by linking silica nanoparticles using ionic liquid and followed by anion-exchange with phosphotungstate. Characterization methods of FT-IR, TG, SEM, TEM, BET, etc., were carried out to have a comprehensive insight into the catalyst. The catalyst was used for catalyzing cyclooctene epoxidation with high surface area, high catalytic activity, and convenient recovery. The conversion and selectivity of epoxy-cyclooctene could both reach over 99% at 70 °C for 8 h using hydrogen peroxide (H₂O₂) as an oxidant, and acetonitrile as a solvent when the catalyst was 10 wt. % of cyclooctene. Full article

A series of Lewis acidic metal salts were used for glucose dehydration to 5-hydroymethylfurfural (HMF) in water. Effect of valence state, ionic radii of Lewis acidic cation, and the type of anions on the catalytic performance have been studied systematically. The experimental results showed that the valence state played an important role in determining catalytic activity and selectivity. It was found that a higher glucose conversion rate and HMF selectivity could be obtained over high valent Lewis acid salts, where the ionic radii of these Lewis acidic metal salts are usually relatively small. Analysis on the effect of the anions of Lewis acid salts on the catalytic activity and the selectivity suggested that a higher glucose conversion and HMF selectivity could be readily obtained with Cl⁻. Furthermore, the recyclability of high valence state Lewis acid salt was also studied, however, inferior catalytic performance was observed. The deactivation mechanism was speculated to be the fact that high valence state Lewis acid salt was comparatively easier to undergo hydrolysis to yield complicated metal aqua ions with less catalytic activity. The Lewis acidic activity could be recovered by introducing a stoichiometric amount of hydrochloric acid (HCl) to the catalytic before the reaction. Full article