Study of vanadium based mesoporous silicas for oxidative dehydrogenation of propane and n-butane

The comparative study of catalytic performance of V-containing high-surface mesoporous siliceous materials (HMS, SBA-16, SBA-15 and MCM-48) in oxidative dehydrogenation of propane and n-butane (C₃-ODH and C₄-ODH, respectively) was carried out. The aim of study was to investigate effect of silica support texture on the speciation of vanadium complexes and its impact on catalytic behavior in both above mentioned reactions is reported. Prepared catalysts were characterized by XRF for determination of vanadium content, XRD, SEM and N₂-adsorption for study of morphology and texture, and H₂-TPR and DR UV-vis spectroscopy for determination of vanadium complex speciation. All prepared materials were tested in propane and n-butane ODH reaction at 540 °C and obtained catalytic results were correlated with their structural and surface characteristics. On the basis of obtained data we conclude that the structure of mesoporous silica support plays decisive role in the case of application of catalysts in n-butane ODH reaction, whereas catalytic performance of investigated catalysts in propane ODH reaction is comparable for all investigated structures. Catalytic performance of investigated materials in C₃-ODH and C₄-ODH can be correlated with population of all tetrahedrally coordinated VO_x complexes and only isolated monomeric VO_x complexes, respectively.     

Chromium Supported on Mesocellular Silica Foam (MCF) for Oxidative Dehydrogenation of Propane

A series of chromium-containing mesocellular silica foam (MCF) catalysts have been prepared and characterized in the oxidative dehydrogenation (ODH) of propane between 350 and 600 °C. It is demonstrated that the chromium catalysts supported on MCF exhibit much higher catalytic activity in terms of propane conversion and propylene yield than literature results obtained over chromium-supported mesoporous SBA-15 or MCM-41 catalysts during the ODH of propane. Enhanced catalytic performance of the chromium-containing mesoporous MCF catalysts has been attributed to the unique three-dimensional (3D), continuous, ultralarge mesopore structure of the MCF materials, which allow a much faster internal molecular transport during the ODH of propane.     

Highly Effective Oxidative Dehydrogenation of Propane Over Vanadia Supported on Mesoporous SBA-15 Silica

Vanadia-containing mesoporous SBA-15 catalysts were prepared and characterized for the oxidative dehydrogenation (ODH) of propane. It is demonstrated that the vanadia-supported SBA-15 catalysts exhibit a much higher catalytic activity than those reported in the literature obtained over vanadium-supported mesoporous MCM-41 catalysts in the ODH of propane. The high catalytic performance of the mesoporous SBA-15 catalysts is attributed to the particularly large pore diameters and low surface acidity.     

One-pot Hydrothermal Synthesis of Mesoporous V-SBA-16 with a Function of the pH of the Initial Gel and its Improved Catalytic Performance for Benzene Hydroxylation

Abstract  

V-SBA-16 catalysts with uniform cubic mesoporous structure were prepared by direct hydrothermal method as a function of the pH of the initial gel and characterized by ICP, XRD, TEM, N₂ adsorption–desorption, DRUV—vis and Raman spectra. The pH of the initial gel in synthesis of V-SBA-16 show important effects on the maintenance of well ordered mesoporous structure, introduced vanadium content and the incorporation of vanadium into the network of SBA-16 type mesoporous material. The initial gel system with a pH value of 2.0 was found to be a suitable for incorporation of vanadium and retaining the mesostructure of SBA-16. The catalytic activities of V-SBA-16 catalysts were evaluated for the hydroxylation of benzene using molecular O₂ as the oxidant. The highest phenol yield of 30.4% with a selectivity of 90% and turnover number of 105 were obtained over the VS-2.0 (1.67) sample prepared at the initial gel system with pH value of 2.0, which is attributed to its high V content and uniform framework V species that highly dispersed on the well ordered SBA-16 type mesoporous materials.     

Oxidative Dehydrogenation of Propane over Mesoporous HMS Silica Supported Vanadia

Vanadium-containing mesoporous HMS catalysts have been prepared and characterized for the oxidative dehydrogenation (ODH) of propane. It is demonstrated that the vanadium supported HMS catalysts exhibit a much higher catalytic activity than the literature results obtained over the vanadium supported MCM-41 catalysts in the ODH of propane. The improved catalytic activity of the V-HMS catalysts has been attributed to the presence of high concentration of well-dispersed vanadium species on the surface of the mesoporous HMS materials.     

Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Synthesis,characterization, and catalytic performance of highly dispersed vanadium grafted SBA-15 catalyst

Vanadium oxide grafted on mesoporous silica SBA-15 has been synthesized using a controlled grafting process. Its structure has been thoroughly investigated using different characterization techniques, including N₂-physisorption, X-ray diffraction, transmission electron microscopy (TEM), Raman spectroscopy, H₂ temperature-programmed reduction, X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). The spectroscopic results revealed that under dehydrated conditions, the grafted vanadium domains are highly dispersed on the SBA-15 surface, composed predominately of isolated VO₄ units with distorted tetrahedral coordination. The suggested (SiO)₃VO sites on the silica surface include one short bond (～1.54 Å) and three long bonds (1.74 Å). Methanol oxidation was used as a chemical probe reaction to examine the catalytic properties of these catalysts. At low vanadium loading, the vanadium species grafted on the surface show structural properties similar to those of vanadium-incorporated MCM-41 catalyst. However, the present mesoporous V-SBA-15 catalysts in the oxidation of methanol to formaldehyde show remarkable catalytic performance compared with that of VO_x/SBA-15 catalysts synthesized through a conventional wet impregnation method, which has been attributed to the homogeneous dispersion and uniformity of the catalytic vanadium species achieved on the SBA-15 support with large pore diameter and surface area. The acidic properties of V-SBA-15 was investigated by pyridine temperature-programmed desorption, which indicated the existence of both Lewis and Brönsted acid sites of the surface.     

Ordered Mesoporous Silica (OMS) Supported Vanadium Nitride and Carbide Catalysts

Nanostructured vanadium nitride and carbide catalysts were prepared by the nitridation and carburization of vanadium oxide supported on M41S materials (MCM-41 and SBA-15) and activated carbon. The oxide precursors, V₂O₅/M41S, were obtained in three different synthesis strategies using “in situ” and “ex situ” incorporation of vanadia precursors (V(acac)₃) into the mesoporous host. For the oxide precursors specific surface areas exceeding 1,200 m² g⁻¹ were achieved. After nitridation a slight decrease of surface area was observed. All VN catalysts show a high activity in propane dehydrogenation with a selectivity exceeding 80% towards propene. Impregnation and nitridation conditions have profound influence upon the catalytic activity. The highest activity was observed for VN supported on NORIT A.     

Catalytic performance of Ti-SBA-15 prepared by chemical vapor deposition for propylene epoxidation

Ti-SBA-15C catalysts were prepared by supporting Ti on SBA-15 with chemical vapor deposition (CVD) using TiCl₄ as titanium source, and were characterized by XRD, N₂ adsorption, FT-IR and ICP. The results show that SBA-15E prepared by ethanol solution extracting template has higher concentration of surface Si-OH groups than SBA-15 calcined at 550 °C, resulting in high Ti content on Ti-SBA-15EC prepared by CVD. The temperature and time of TiCl₄ deposition affect the Ti content and catalytic activity of Ti-SBA-15EC for the epoxidation of propylene with cumene hydroperoxide (CHP). Ti-SBA-15EC prepared by CVD at 700 °C for 1.5 h exhibits more excellent performance than Ti-SBA-15C, Ti-SBA-15 prepared hydrothermally and Ti/SBA-15 (impregnation method), and the 87.3% conversion of CHP and 96.4% selectivity to propylene oxide can be obtained at 80 °C for 4 h. The performance of Ti-SBA-15EC is decreased hardly for the epoxidation of propylene after used repeatedly 6 times.     

SBA-15 as Support for MoS2 and Co-MoS2 Catalysts Derived from Thiomolybdate Complexes in the Reaction of HDS of DBT

Molybdenum sulfide and cobalt-molybdenum sulfide catalysts supported on mesoporous SBA-15 were prepared by thermal decomposition of ammonium thiomolybdate (ATM). SBA-15 was synthesized at 353 K and 413 K to obtain pore diameters of about 6 and 9 nm, respectively. The (Co)-MoS₂/SBA-15 catalysts were characterized with X-ray diffraction (XRD), N₂-physisorption and high-resolution transmission electron microscopy (HRTEM). HRTEM images give evidence for the presence of a poorly dispersed MoS₂ phase with long MoS₂ slabs and a pronounced MoS₂ stacking. The catalytic performance in the hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT) was examined at T = 623 K and P = 3.4 MPa. The Co-MoS₂/SBA-15 materials show a relatively high catalytic activity with a strong preference for the direct desulfurization (DDS) pathway. This is an interesting result in view of the significant stacking of MoS₂ particles and the size of the slabs. The generation of the catalytically active CoMoS phase and a large number of coordinately unsaturated sites (CUS) may explain the high performance of Co promoted MoS₂/SBA-15 catalysts in the HDS reaction. A confinement effect of the mesoporous channels of SBA-15 is observed for the unpromoted MoS₂/SBA-15 catalysts. SBA-15 with 9 nm channel diameter with 11 wt.% Mo loading shows a higher selectivity for the hydrogenation pathway than SBA-15 with 6 nm channel and 16 wt.% Mo loading.     

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of V---O in-plane bonds around the reduced V centres.     

Oxidative dehydrogenation of ethane over vanadium-based hexagonal mesoporous silica catalysts

Vanadium-containing hexagonal mesoporous silica catalysts were tested in oxidative dehydrogenation of ethane. V-HMS catalysts (0.3–9.0 wt.% V) were prepared by impregnation with solution of vanadyl acetylacetonate, and by incorporation of vanadium in the synthesis process. The prepared catalysts achieved a different distribution of vanadium species (isolated monomeric units with tetrahedral coordination, oligomeric units connected by VOV bonds up to distorted tetrahedral coordination, two-dimensional polymeric units in octahedral coordination, and bulk vanadium oxides). The contribution deals with the understanding of the relationship between the distribution of vanadium species and their activity in ODH of ethane. It has been found that both monomeric and oligomeric vanadium species play important role in ODH of ethane. The activity correlated with the population of oligomeric tetrahedrally coordinated vanadium species, which were evidenced by the UV–vis band at 315 nm. To analyze this effect, V-HMS catalysts were characterized by means of UV–vis spectroscopy, H₂-TPR and N₂-adsorption.     

Amine modified mesocellular siliceous foam (MCF) as a sorbent for CO2

Adsorption is considered a promising method for carbon capture. CO₂ adsorbents take a variety of forms - but one approach is to fill mesoporous substrates with a polymeric CO₂ selective sorbent. SBA-15 and mesocellular siliceous foam (MCF) are high pore volume, high surface area ordered mesoporous materials for which modification with amine should result in high capacity, highly selective adsorbents. SBA-15 and MCF were separately loaded with approximately one pore volume equivalent of linear polyethyleneimine (PEI) (M_w = 2500) or branched PEI (M_n = 1200). CO₂ adsorption/desorption isotherms under dry CO₂ were obtained at 75, 105 and 115 °C. The CO₂ adsorption/desorption kinetics were improved with temperature, though the CO₂ capacities generally decreased. The adsorption capacity for MCF loaded with branched PEI at 105 and 115 °C were 151 and 133 mg/g adsorbent, respectively (in 50% CO₂/Ar, 20 min adsorption time). These are significantly higher than the adsorption capacity observed for SBA-15 loaded with branched PEI under same conditions, which were 107 and 83 mg/g adsorbent, respectively. Thus the results indicate that, on a unit mass basis, amine modified MCF's are potentially better adsorbents than amine modified SBA-15 for CO₂ capture at modestly elevated temperature in a vacuum swing adsorption process.     

Flame sprayed tri-metallic Pt–Sn–X/Al2O3 catalysts (X = Ce,Zn, and K) for propane dehydration

Trimetallic nanocrystalline Pt–Sn–X/Al₂O₃ catalysts (X = Ce, Zn, and K) consisting of 0.3 wt.% Pt, 1 wt.% Sn, and 0.5 wt.% X have been prepared by one-step flame spray pyrolysis (FSP). As shown by the X-ray diffraction (XRD) and the transmission electron microscopy (TEM) results, the as-synthesized FSP-made catalysts were consisted of single-crystalline γ-alumina particles with average primary particle sizes 8 to 9 nm. The N₂ physisorption results revealed that all the catalysts contained only the macropore structure. The catalytic properties of the FSP-made catalysts were investigated in the dehydration of propane. Addition of Ce during FSP synthesis resulted in higher Pt dispersion as well as improved catalytic activity and stability than the non-promoted Pt–Sn/Al₂O₃. An opposite trend was found with the ones doped with Zn and K in which high surface coverage of Zn and K resulted in a significant loss of Pt active sites. The mechanism for the formation of the trimetallic nanoparticles during one-step FSP synthesis appeared to depend strongly on the differences in the vapor pressure of the metals and the alumina support in flame.     

Improving the electrocapacitive properties of mesoporous CMK-5 carbon with carbon nanotubes and nitrogen doping

Nitrogen-containing carbon composite materials composed of mesoporous carbon CMK-5 and carbon nanotubes (CNTs) were prepared by the chemical vapor deposition method with Fe(NO₃)₃-impregnated SBA-15 as template and pyridine as the carbon precursor. The Fe nanoparticles confined in the channels of SBA-15 induced the formation of mesoporous carbon characteristic of CMK-5, whereas Fe particles homogeneously dispersed on the external surface of SBA-15 served as catalysts for CNTs growth. The contents of CNTs, the N doping level and the microstruture of the carbon composite were closely related to the initial Fe/Si atomic ratio in SBA-15 template. Incorporation of CNTs in the composite was found to substantially reduce the electric resistance, leading to the composite materials exhibiting excellent rate-performance. A maximum specific capacitance of 208 F/g and a power density of 10 kW/kg were achieved in 6.0 mol/L KOH aqueous electrolyte when these carbon composites were applied as supercapacitor electrodes. Moreover, the composite electrode also exhibited good electrochemical stability with no capacitance loss after 1000 cycles of galvanostatic charge-discharge process.     

Preparation of solid acid catalyst packing AAO/SBA-15-SO3H and application for dehydration of xylose to furfural

The solid acid catalyst packing AAO/SBA-15-SO₃H was prepared by the co-condensation and grafting method with porous anodic aluminum oxide (AAO) as support. FT-IR, SEM and TEM were applied to characterize the prepared samples. Results showed that catalysts prepared by two methods both contained active centers, and SBA-15 nanorod arrays grow inside a porous alumina membrane AAO and are perpendicular to the substrate. Their catalytic performances were tested for dehydration of xylose to furfural. The conversion of xylose and selectivity of furfural were 90% and 74% on the AAO/SBA-15-SO₃H(C) catalyst prepared by the co-condensation method, respectively. The deactivation and regeneration of the AAO/SBA-15-SO₃H(C) catalyst for the dehydration of xylose were also investigated, the activity of catalyst treated by 30 wt.% H₂O₂ almost was recovered.     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.     

Hydrotreatment of heavy oil from a direct coal liquefaction process on sulfided Ni-W/SBA-15 catalysts

A series of Ni-W catalysts supported on SBA-15 with different pore sizes were prepared by incipient wetness impregnation method and characterized by N₂ adsorption-desorption and X-ray diffraction. The hydrogenation of heavy oil (distillation temperature: 320-340 °C) derived from the direct coal liquefaction process using Shengli coal in the presence of sulfided Ni-W/SBA-15 catalysts with different pore sizes were evaluated at 400 °C and initial H₂ pressure of 5.0 MPa. The results showed that the catalyst preparation method and the pore size of the support had a significant influence on the Ni/W crystallite size, hydrodenitrogenation (HDN) and hydrodearomatization (HDA) activities of coal-derived heavy oil. The larger pore could cause the Ni-W/SBA-15 to form larger Ni-W crystallite. The catalysts with largest pore in the range studied displayed highest HDN and HDA activities for upgrading of the coal-derived heavy oil.     

Nitrous Oxide Reduction with Ammonia and Methane Over Mesoporous Silica Materials Modified with Transition Metal Oxides

Transition metal oxides (Cu, Cr and Fe) were deposited on various mesoporous silicas (MCM-48, SBA-15, MCF and x-MSU) by an impregnation method. Electron microprobe analysis, BET, UV-VIS-DRS and temperature programmed desorption of NH₃ were used for the characterization of the samples. The modified mesoporous silicas were tested as catalysts of the N₂O decomposition and the N₂O reduction using ammonia and methane. The Cu-containing samples presented the highest catalytic activity in the N₂O decomposition, while the Cr- and Fe-modified materials were more active in the reduction of nitrous oxide with NH₃ and CH₄. The type of the silica support strongly influenced the catalytic performance of the studied materials.     

Pd/SBA-15 nanocomposite: Synthesis, structure and catalytic properties in Heck reactions

Pd nanoparticles supported in mesoporous silica SBA-15 (or Pd/SBA-15 nanocomposites) were prepared by ion-exchange with cationic Pd precursor in an alkaline solution on an uncalcined silica. The high Pd loading in these nanocomposites can be achieved up to 5.21 wt.% by adjusting the pH value of the solution. The surface area and the pore volume decrease with increasing Pd loading. The Pd nanoparticles equal to or smaller than 6 nm in size in the nanocomposites are distributed in the channels of the mesoporous SBA-15. The Pd/SBA-15 nanocomposites exhibit excellent catalytic activities and high reuse ability in air for the Heck carbon-carbon coupling reactions.