Silica-supported Catalysts Research Articles

Efficient hydrogenation of biomass-derived phenol to cyclohexanol over 3D mesoporous silica-supported Ni catalysts in a continuous gas phase conditions

High surface area three-dimensional (3D-cubic SBA-16) and two-dimensional mesoporous silica (2D-hexagonal SBA-15) supported Ni composities were employed as active catalysts for industrially important cyclohexanol synthesis via phenol hydrogenation. Among synthesised porous materials, Ni/SBA-16 catalysts were dramatically displayed a superior activity compared to Ni/SBA-15 catalysts. It was obviously due to three-dimensional Ni/SBA-16 catalysts were exhibited unusual surface textural properties, like adequate surface area, interconnected-cage type pore structure and thermal stability. Moreover, uniformly distributed smaller size Ni nanoparticles results in efficient utilisation of external H2 source for highest phenol reduction on porous SBA-16 material, compared to SBA-15 catalyst. Hence, 20 (wt.%) Ni/SBA-16 catalyst afforded a remarkable activity in terms of 91% phenol conversion and 79% cyclohexanol yield even after 8-h time-on-stream study. In contrast, 20 (wt.%) Ni/SBA-15 catalyst showed low catalytic activity about 28% phenol conversion and 19% cyclohexanol yield during the 8-h time-on-stream. All synthesized catalysts were characterised by X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), scanning electron microscopy (SEM), BET surface area, transmission electron microscopy (TEM) techniques and inductive coupled plasma-optical emission spectroscopy (ICP-OES).

Propane Dehydrogenation on Ga2O3-Based Catalysts: Contrasting Performance with Coordination Environment and Acidity of Surface Sites

α-Ga2O3, β-Ga2O3, and γ-Ga2O3 as well as the silica-supported catalysts γ-Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO2-supported Ga2O3 catalysts were studied using FTIR, 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in γ-Ga2O3 and β-Ga2O3 (the latter obtained from colloidal nanocrystals of γ-Ga2O3 via thermal treatment at 750 °C) are similar, except that β-Ga2O3 contains a larger relative fraction of weak Ga3+ Lewis acid sites. In contrast, α-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: i.e., weak Lewis acid surface sites are more abundant in β-Ga2O3 relative to α-Ga2O3 and γ-Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C3H6 m–2 (Ga2O3) h–1 at 550 °C, for respectively β-, α-, and γ-Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of γ-Ga2O3 or β-Ga2O3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The γ-Ga2O3/SiO2 catalyst was slightly more active than β-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol Ga–1 h–1 (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga–1 h–1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga2O3 surface show that tetra- and pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of β-Ga2O3.

Hydrodeoxygenation of C4–C6 sugar alcohols to diols or mono-alcohols with the retention of the carbon chain over a silica-supported tungsten oxide-modified platinum catalyst

Sugar alcohols can be converted to mono-alcohols in good yields over a Pt–WOx/SiO2 catalyst without C–C dissociation, and 1,4-butanediol and 1,4-anhydroxylitol can also be produced in good yields with related systems.

Chemo- and regioselective cross-dehydrogenative coupling reaction of 3-hydroxycarbazoles with arenols catalyzed by a mesoporous silica-supported oxovanadium.

Cross-dehydrogenative coupling between 3-hydroxycarbazoles and 2-naphthols has been achieved by using a mesoporous silica-supported oxovanadium catalyst.

Early-stage particle fragmentation behavior of a commercial silica-supported metallocene catalyst

Olefin polymerization starts at the outer surface and at the macropores walls. Here fragmentation occurs by peeling off the catalyst in a layer-by-layer mode with, in ideal conditions, the simultaneous formation of bisection-type fractures across the catalyst inner domains.

Influence of Site Pairing in Hydrophobic Silica-Supported Sulfonic Acid Bifunctional Catalysts.

Imparting hydrophobicity to solid acid catalysts is critical to regulating their performances by allowing the creation of a less polar environment and improved partitioning of the reactants. Here we present different approaches for the preparation of silica-based catalysts comprising sulfonic acid (-SO3H) sites and hydrophobic decyl (-C10) chains by either simultaneous or sequential postfunctionalization of an azide-functionalized mesoporous silica platform. This set of hybrid bifunctional catalysts is compared in the model esterification of octanol with acetic acid, and the influence of the preparation methods together with the resulting site spatial distribution is discussed. In parallel, we show that pairing the two functional groups affords a maximum synergistic effect compared to more traditional mixed catalysts with random distributions of acid and hydrophobic functions.

A silica-supported Ni-based catalyst prepared using TEPA for the plasma synthesis of ammonia

Ammonia is a potential hydrogen carrier and can be used as an alternative fuel. In this study, plasma cooperative catalytic synthesis ammonia was used. The catalyst consisted of Ni impregnated into silica, using one of three complexing agents (TEPA, EDTA and GLY) and compared with conventional impregnation methods. The unsaturated coordinated Ni(tepa)2+ will coordinate with the hydroxyl groups on the silica surface. When using the catalyst prepared by TEPA, the ammonia synthesis rate was 3.8 times higher than with no catalyst, and 1.4 times higher than the catalyst made by conventional impregnation method. XRD and TEM show that the catalyst prepared using TEPA exhibited the smaller metal particle size, the higher dispersion and the stronger interaction between metal and support than other catalysts. XPS indicate that the good metal dispersion of the catalyst facilitate form more Ni3N during the reaction, which will greatly improve the activity of the catalyst.

Effects of Hydrogen and Hydrothermal Pretreatments on a Silica-Supported Copper Catalyst for CO Oxidation: Copper Hydroxy Active Species

As an inert support, silica is appropriate to disperse active copper species to study the intrinsic structure–activity relationship for redox reactions including CO oxidation. Herein, a copper–sili...

Water-Resistant Pt Sites in Hydrophobic Mesopores Effective for Low-Temperature Ethylene Oxidation

The structure–activity relationship of silica-supported Pt catalysts in aerobic oxidation of 50 ppm ethylene was studied at 0 °C with a fixed-bed flow reactor and in situ characterization technique...

Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons.

There is a great interest in direct conversion of methane to valuable chemicals. Recently, we reported that silica-supported liquid-metal indium catalysts (In/SiO2) were effective for direct dehydrogenative conversion of methane to higher hydrocarbons. However, the catalytic mechanism of liquid-metal indium has not been clear. Here, we show the catalytic mechanism of the In/SiO2 catalyst in terms of both experiments and calculations in detail. Kinetic studies clearly show that liquid-metal indium activates a C–H bond of methane and converts methane to ethane. The apparent activation energy of the In/SiO2 catalyst is 170 kJ mol–1, which is much lower than that of SiO2, 365 kJ mol–1. Temperature-programmed reactions in CH4, C2H6, and C2H4 and reactivity of C2H6 for the In/SiO2 catalyst indicate that indium selectively activates methane among hydrocarbons. In addition, density functional theory calculations and first-principles molecular dynamics calculations were performed to evaluate activation free energy for methane activation, its reverse reaction, CH3–CH3 coupling via Langmuir–Hinshelwood (LH) and Eley–Rideal mechanisms, and other side reactions. A qualitative level of interpretation is as follows. CH3–In and H–In species form after the activation of methane. The CH3–In species wander on liquid-metal indium surfaces and couple each other with ethane via the LH mechanism. The solubility of H species into the bulk phase of In is important to enhance the coupling of CH3–In species to C2H6 by decreasing the formation of CH4 though the coupling of CH3–In species and H–In species. Results of isotope experiments by combinations of CD4, CH4, D2, and H2 corresponded to the LH mechanism.

Ordered mesoporous silica-supported metal catalysts for hydrodeoxygenation of anisole derivatives

The aim of the study presented was a comprehensive analysis of catalytic performance of palladium and platinum catalysts (1 or 3 wt% relative to the support), on SBA-16, in hydrodeoxygenation (HDO) of chemical compounds generated in biomass processing (anisole, 4-methylanisole and 1,3-dimethoxybenzene). HDO of these compounds was carried out in a high pressure reactor for 4 h in temperatures from 90 to 130 °C and under hydrogen pressure of 25–60 bar. The catalytic performance of palladium and platinum modified materials was found to differ considerably. The SBA-16 supported catalyst modified with palladium permitted getting high conversion of the model molecules studied (often 100%), while those modified with platinum showed higher selectivity to the compounds with no oxygen atom in their structures, which allowed a significant reduction of the O/C ratio. Direct impact of the process conditions (temperature, pressure) on the degree of conversion and selectivity of the HDO process was established and characterized. • Pd and Pt catalysts supported on SBA-16 for HDO of anisole derivatives. • Influence of the type of metal on HDO activity was studied. • Influence on the reaction conditions on HDO activity was studied. • HDO reactions conducted under mild experimental conditions.

Ammonium Salts Catalyzed Acetalization Reactions in Green Ethereal Solvents

Cyclopentyl methyl ether and 2-methyltetrahydrofuran, low impact ethereal solvents forming a positive azeotrope with water, were successfully employed as solvents in the synthesis of a variety of acetals carried out under Dean–Stark conditions in the presence of heterogeneous acidic catalysts. Under these conditions, ammonium salts, either as such or supported on SiO2, performed better or equally well than widely employed homogeneous and heterogeneous acidic catalysts such as p-toluenesulfonic acid, Amberlyst 15®, or Montmorillonite K10. Several examples highlight the advantage of tuning the relative acidities of ammonium salts by appropriately selecting the counterion. Within one of these examples, our protocol clearly outweighs the classic p-toluenesulfonic acid/toluene protocol in terms of chemoselectivity. Silica-supported catalysts were characterized by SEM, TEM, and FTIR spectroscopies, as well as by N2 physisorption. Such a characterization reveals an even distribution of ammonium salts on silica, thus confirming the formation of expected catalytic supports.

Etherification of biomass-derived furanyl alcohols with aliphatic alcohols over silica-supported nickel phosphide catalysts: Effect of surplus P species on the acidity

The acidity of nickel phosphide (Ni2P) catalysts plays a crucial role in producing a desired hydrodeoxygenation molecule from biomass-derived substrates; yet, it has never been explored in acid-catalyzed reactions. Herein, we demonstrated the activity of silica-supported Ni2P catalyst prepared with the nominal P/Ni ratio of 2 (Ni2P/SiO2-2P) in the etherification of furanyl alcohols (particularly, 5-(hydroxymethyl)furfural) with aliphatic alcohols including ethanol. By comparing the characteristics of Ni/SiO2, PxOy/SiO2, and Ni2P/SiO2-xP (x = 0.5 and 1), Ni2P/SiO2-2P was revealed to contain the Brønsted and Lewis acid sites of which both contributed to the etherification reaction. Notably, the Brønsted acidity was associated with the surplus P species added to produce the Ni2P phase. Consequently, supported Ni2P catalysts can work in acid-catalyzed reactions if an adequate ratio of Brønsted to Lewis acid sites is provided by the amount of the surplus P species determined by adjusting the P/Ni ratio.

Electro-assisted sulfite activation by a silica supported cobalt catalyst for the degradation of organic pollutants in near-neutral pH condition

The desulfurization by-product (e.g. sulfite) treatment and water pollution are two widespread concerns, while the sulfite activation by transition metals can be adopted to solve both of them simultaneously. In this study, a silica-supported cobalt catalyst (CoSi) was used to activate sulfite for the degradation of organic pollutants in an electrochemical system (the EC/CoSi/sulfite system), which demonstrated remarkable water treatment efficiencies due to sulfate radical oxidation. The synergy mechanisms of the EC/CoSi/sulfite system were revealed. (i) The as-prepared CoSi played the dual roles as an electro-adsorbent and a catalyst for sulfite activation. (ii) The electrolytic oxygenation supported the conversion of cobalt species and the chain reactions of oxysulfur radicals. (iii) The anodic oxidation of sulfite also generated oxysulfur radicals. The influencing factors and operating parameters were investigated and optimized systematically. Moreover, the efficient degradation of highly concentrated X-3B and non-selective oxidation of various organic compounds confirmed the promising application of the EC/CoSi/sulfite system in practical water treatment. Overall, the EC/CoSi/sulfite system created new value of the industrial waste (e.g. sulfite) for further application in the environmental remediation.

Oxidative Dehydrogenation of Ethane on VMoTeNbО/SiO2 Catalysts and the Effect of the Initial Support Compound on Their Physicochemical and Catalytic Properties

Silica-supported catalysts of cation composition V0,3Mo1Te0,23Nb0,12 are produced using slurry method with the nature of the initial compound for the support (TEOS, silica sol, silica gel, aerosil) being varied. The synthesized catalysts are characterized via nitrogen thermodesorption, chemical and X-ray analysis, and high resolution electron microscopy. Catalytic properties are studied in the reaction of ethane oxidative dehydeogenation at a temperature of 400°С in a reaction mixture with the composition С2Н6 : О2 : N2 = 10 : 10 : 80 (vol %). It is found that samples obtained with silica gel display the greatest activity, due to the formation of nanodomens of active phase M1 in the amorphous support matrix and its partial dispersion.

Selective conversion of CO2 to methanol over intermetallic Ga-Ni catalyst: Microkinetic modeling

The conversion of CO2 from industrial effluent flue gas streams into methanol will not only help in mitigating global warming but also provide a clean source of alternate energy. In this study, the microkinetic model for methanol synthesis from CO2/H2 on supported intermetallic Ga3Ni5 catalyst based on the single-site mechanism is presented. The reaction route (RR) analysis was used to develop a rate expression without any presumption of the most abundant reactive intermediate (MARI) and rate-determining steps (rds). The silica-supported Ga3Ni5 catalyst was prepared through co-precipitation method and characterized by various techniques. The developed kinetic model was tested, and its parameters were estimated with the experimental data obtained over the lab synthesized catalyst. From the results obtained, it was found that the hydrogenation of formate intermediate is the rds and surface adsorbed CO2 and H-atom were MARI of the system. Based on rds and MARI, the final kinetic expression was tested against experimental data for a wide range of temperatures. The calculated data from the kinetic expression was found to fit the experimental results well, thereby validating the kinetic parameters and reaction mechanism proposed in this study. In addition, based on a plug flow reactor model, the kinetic parameters predicted the theoretical CO2 conversion at high temperatures, thus, demonstrating the robustness of the rate expression and accuracy of kinetic parameters proposed in this study.

Mesoporous silica-supported Pt catalysts in enantioselective hydrogenation of ethyl pyruvate

Catalytic properties of Pt catalysts supported on mesoporous silica (Pt/m-SiO2) have been studied in enantioselective hydrogenation of ethyl pyruvate. The influences of pore structure of mesoporous silica (m-SiO2), type of chiral modifier, and H2 pressure on the catalytic performance have been investigated by using various m-SiO2 supports and cinchona alkaloids and by varying H2 pressure. The use of MCM-41, SBA-15, KIT-6, and MCF reveals that characteristic pore structure and size of m-SiO2 supports significantly affect both activity and enantioselectivity. A facile diffusion of chiral modifier through large mesopores of MCF support enables Pt/MCF to exhibit excellent performance. A comparison of the efficiency of cinchona alkaloids-modified Pt catalysts shows that QN and QD lead to higher performance than CD and CN at ambient H2 pressure. The influence of cinchona alkaloids on enantioselectivity noticeably depends on H2 pressure. Cinchona alkaloid-modified Pt/m-SiO2 exhibit superior enantioselectivity to the corresponding Pt/Al2O3 under various H2 pressures. These results imply that m-SiO2 is a promising support and that fine control of pore structure can further improve catalytic performance of Pt/m-SiO2 in heterogeneous enantioselective hydrogenation.

Catalyst Characteristics and Performance of Silica-Supported Zinc for Hydrodeoxygenation of Phenol

The present investigation aimed to study the physicochemical characteristics of supported catalysts comprising various percentages of zinc dispersed over SiO2. The physiochemical properties of these catalysts were surveyed by N2 physisorption (BET), thermogravimetry analysis (TGA), H2 temperature-programmed reduction, field-emission scanning electron microscopy (FESEM), inductively coupled plasma-optical emission spectrometry (ICP-OES), and NH3 temperature-programmed desorption (NH3-TPD). In addition, to examine the activity and performance of the catalysts for the hydrodeoxygenation (HDO) of the bio-oil oxygenated compounds, the experimental reaction runs, as well as stability and durability tests, were performed using 3% Zn/SiO2 as the catalyst. Characterization of silica-supported zinc catalysts revealed an even dispersion of the active site over the support in the various dopings of the zinc. The acidity of the calcinated catalysts elevated clearly up to 0.481 mmol/g. Moreover, characteristic outcomes indicate that elevating the doping of zinc metal led to interaction and substitution of proton sites on the SiO2 surface that finally resulted in an increase in the desorption temperature peak. The experiments were performed at temperature 500 °C, pressure 1 atm; weight hourly space velocity (WHSV) 0.32 (h−1); feed flow rate 0.5 (mL/min); and hydrogen flow rate 150 (mL/min). Based on the results, it was revealed that among all the prepared catalysts, that with 3% of zinc had the highest conversion efficiency up to 80%. However, the selectivity of the major products, analyzed by gas chromatography flame-ionization detection (GC-FID), was not influenced by the variation in the active site doping.

Carbon-coated mesoporous silica-supported Ni nanocomposite catalyst for efficient hydrogen production via steam reforming of toluene

Catalytic steam reforming of biomass tars is a promising technique for H2-rich gas production. However, the undesired deactivation of nickel-based catalysts due to nanoparticle (NP) sintering and carbon deposition remains a challenge for industrial applications. In this study, we prepared a carbon-coated ordered mesoporous silica-supported Ni-based catalyst (Ni-SiO2@C) via an evaporation-induced self-assembly (EISA) and calcination process. Characterizations revealed that the Ni-SiO2@C has a regular mesostructure with graphitic carbon layers protected Ni NPs (average 5.5 nm in diameter) highly dispersed and embedded into the silica wall of mesoporous channels. Compared to the SBA-15 and activated carbon-supported catalysts (NiO/SBA-15 and Ni/AC), the Ni-SiO2@C exhibits a higher toluene (a biomass tar model compound) conversion efficiency of 99.9% at 550 °C with an excellent stability for at least 46 h during steam reforming. The good performance can be attributed to the improved Ni NP dispersion, electron transfer and hydrothermal stability by enhanced metal-support interaction (MSI) as well as carbon coating. This work provides a cost-effective and easy handling approach for the synthesis of anti-sintering and coke-resistant tar reforming catalysts.

Selective Hydrodeoxygenation of γ-Valerolactone over Silica-supported Rh-based Bimetallic Catalysts

2-Methyltetrahydrofuran (2-MTHF) is a promising fuel additive and renewable solvent. Herein, we report a selective hydrodeoxygenation of cellulose-derived γ-valerolactone (GVL) to 2-MTHF under mild...