Hexagonal Mesoporous Silica Catalysts Research Articles

Green synthesis of α-methylcinnamaldehyde via Claisen-Schmidt condensation of benzaldehyde with propanal over Mg–Zr mixed oxide supported on HMS

α-Methylcinnamaldehyde (2-methyl-3-phenyl-propen-2-al) is used in a variety of products such as balsamic compounds, cosmetics, flavors, perfumes, chewing gums, spice blends, soaps, mouth wash, detergents, shampoos and also as an intermediate for making the drug epalrestat. In this work, it was synthesized by Claisen-Schmidt condensation of benzaldehyde with propanal using supported solid bases as catalysts. Different mole ratios of magnesium to zirconium (1:1, 2:1 and 3:1) were used for Mg-Zr mixed oxide and loaded on hexagonal mesoporous silica (HMS). The catalysts were thoroughly characterized using FT-IR, TG–DSC, TPD, XRD, SEM-EDX, TEM and pore size analysis. 20% (w/w) Mg-Zr (2:1)/HMS catalyst was the best among all studied materials. The catalyst was thermally stable, active and selective. Benzaldehyde to propanal mole ratio of 1:1.5 at 443 K leads to 71% of benzaldehyde conversion and 92% selectivity of α-methylcinnamaldehyde on 20% (w/w) Mg-Zr (2:1)/HMS which exhibits bi-functional character. A mechanism was proposed and kinetics derived using experimental data. The reaction obeys second order kinetics with apparent activation energy of 9.54 kcal/mol.

Synthesis and characterization of novel Ti doped hexagonal mesoporous silica catalyst for nonenzymatic hydrogen peroxide oxidation

A new electrocatalyst, Ti-HMS was successfully synthesized by sol-gel method employing Ti source tetrabutyl orthotitanate (TBOT) and silica source tetraethyl orthosilicate (TEOS, 99% purity). Ti-HMS catalysts were prepared at varying Ti:Si ratios by employing sol-gel method. Physicochemical properties of these nanoparticles were characterized and confirmed by BET, TG-DTA, XPS, and XRD. Electrochemical properties of Ti-HMS catalysts prepared at varying Ti:Si ratios were examined by cyclic voltammetry (CV) and chronoamperometry (CA) for determination of the oxidation and reduction activity of H2O2. It was observed that the Ti-HMS (Ti:Si = 0.02) electrode exhibits significant oxidation and reduction of H2O2 at applied potentials of 0.65 V and −0.30 V with the addition of H2O2, respectively. The excellent electrocatalytic response to H2O2 is mainly attributed to varying electronic properties with the incorporation of Ti to HMS.

Efficient syntheses of substituted (±)-3-oxoisoindoline-1-carbonitriles and carboxamides using OSU-6

A tunable synthesis of substituted oxoisoindolines is reported using OSU-6, a modified MCM-41 type hexagonal mesoporous silica catalyst.

Hydroformylation of propene heterogeneously catalyzed by HRh(CO)(PPh 3) 3 encapsulated in to hexagonal mesoporous silica—Parametric variation and mass transfer study

An in situ encapsulated HRh(CO)(PPh 3) 3 in to the pores of hexagonal mesoporous silica (HMS) acting as nanophase reactors, was investigated for the catalytic hydroformylation of propene. The encapsulated catalyst (Rh–HMS) was synthesized and characterized by PXRD, FT-IR, surface area measurements and TEM. The catalyst was effectively active with 99% conversion of propene and 100% selectivity to aldehydes. The effects of reaction parameters: temperature, partial pressure of CO and H 2, amount of catalyst and HRh(CO)(PPh 3) 3 to TEOS ratio on conversion, selectivity and rates were investigated in detail. The rates determined in term of the formation of aldehydes were found to be first order with respect to hydrogen pressure. Catalyst showed first order dependence towards its lower amount. CO pressure variation showed positive order towards lower pressure and inhibition at higher pressures. The investigated mass transfer effects on the kinetics indicated that the reaction runs with negligible mass transfer limitations. The heterogenized Rh–HMS catalyst was effectively recycled for six times.

Effect of preparation method on nature and distribution of vanadium species in vanadium-based hexagonal mesoporous silica catalysts: Impact on catalytic behavior in propane ODH

This work reports comparative study of catalytic performance of VO x -HMS catalysts prepared by ethanolic impregnation method and direct synthesis. In order to detailed investigation of vanadium speciation depending on vanadium content and its impact on catalytic behavior in C3–ODH, number of samples in wide range of vanadium content (1–16 wt.%) was prepared by both types of preparation. Prepared catalysts were tested in the oxidative dehydrogenation of propane at 540 °C under different contact times and catalytic results were correlated with physicochemical characteristics of the catalysts examined by XRD, XRF, SEM, N 2 adsorption/desorption isotherms, H 2-TPR and DR-UV–visible spectroscopy. Study led to conclusion, that (i) both monomeric VO x complexes and oligomeric VO x complexes with tetrahedral coordination containing V O V bonds are active and selective catalytic sites for C3–ODH, (ii) active VO x species are characterized by distinct reduction peak in H 2-TPR pattern centered at 570–590 °C, (iii) VO x -HMS vanadosilicates can be directly synthesized under ambient conditions without need for autoclave in wide range of vanadia loading with preservation of good catalytic performance and (iv) the catalytic results in C3-ODH were significantly better for synthesized catalysts compared to impregnated catalysts resulting in three times higher propene productivity of the best synthesized catalyst compared to the best impregnated one under the same reaction conditions. The higher is vanadium loading the more distinct is the difference in catalytic performance of both types of catalysts.

N-Butane oxidative dehydrogenation over VO X-HMS catalyst

The demand for olefins increases in recent years. Oxidative dehydrogenation (ODH) of n-butane is possible alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes. The role of particular VO X species supported on hexagonal mesoporous silica (HMS) in oxidative dehydrogenation (ODH) of n-butane was investigated on two sets of VO X -HMS catalysts prepared by wet impregnation and direct synthesis differing in amount and distribution of VO X species. The materials were characterized by XRF, N 2-BET isotherms, XRD, SEM, H 2-TPR, O 2-TPO and DR UV–vis spectroscopy and tested in ODH of n-butane in the range of temperature from 460 to 540 °C. The highest activity and selectivity to olefins were reached on materials with high content of isolated monomeric VO X units with tetrahedral coordination which are generated up to 4–5 wt.% of vanadium. The species with high degree of polymerization participate mainly on total oxidation reactions and those species are formed especially by wet impregnation.

Quantitative LIBS analysis of vanadium in samples of hexagonal mesoporous silica catalysts

The method for the analysis of vanadium in hexagonal mesoporous silica (V-HMS) catalysts using Laser Induced Breakdown Spectrometry (LIBS) was suggested. Commercially available LIBS spectrometer was calibrated with the aid of authentic V-HMS samples previously analyzed by ICP OES after microwave digestion. Deposition of the sample on the surface of adhesive tape was adopted as a sample preparation method. Strong matrix effect connected with the catalyst preparation technique (1st vanadium added in the process of HMS synthesis, 2nd already synthesised silica matrix was impregnated by vanadium) was observed. The concentration range of V in the set of nine calibration standards was 1.3–4.5% (w/w). Limit of detection was 0.13% (w/w) and it was calculated as a triple standard deviation from five replicated determinations of vanadium in the real sample with a very low vanadium concentration. Comparable results of LIBS and ED XRF were obtained if the same set of standards was used for calibration of both methods and vanadium was measured in the same type of real samples. LIBS calibration constructed using V-HMS-impregnated samples failed for measuring of V-HMS-synthesized samples. LIBS measurements seem to be strongly influenced with different chemical forms of vanadium in impregnated and synthesised samples. The combination of LIBS and ED XRF is able to provide new information about measured samples (in our case for example about procedure of catalyst preparation).

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 V O V 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, H 2-TPR and N 2-adsorption.

The effects of promoters over PdCl2-CuCl2/HMS catalysts for the synthesis of diethyl carbonate by oxidative carbonylation of ethanol

Diethyl carbonate (DEC) has been produced by the oxidative carbonylation of ethanol in the gas phase over a heterogeneous PdCl2-CuCl2 catalyst supported on hexagonal mesoporous silica (HMS). The various quaternary ammonium salt (QAS) promoters have a great effect on the performance of the catalyst, which was due to the high dispersal of the active species on the film composed of nonpolar groups. The catalytic performance of the PdCl2-CuCl2/HMS catalyst was greatly improved by the addition of promoter tetraethyl ammonium bromide (TEAB). Under the PdCl2-CuCl2-TEAB/HMS catalyst system, space time yield (STY) of DEC was 210.9 g DEC/(L-cat h), and conversion of EtOH was 13.6%. An optimized TEAB/Pd mole ratio existed for catalytic activity, which was about 12/1. From X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low-temperature electron spin resonance (ESR) spectroscopy, and infrared resonance (IR) spectroscopy, it could be concluded that the essential role of TEAB not only promoted the formation of surface-stabilized copper species but also improved a high dispersal of the active species, which enhanced catalytic performance in the gas phase DEC synthesis.

HMS mesoporous silica as cobalt support for the Fischer–Tropsch Synthesis: Pretreatment, cobalt loading and particle size effects

The use of mesostructurated materials as cobalt support for the Fischer–Tropsch Synthesis (FTS) has taken increasing importance in the last decade. However, little use has been made of the hexagonal mesoporous silica (HMS), as most of the work has been reported for Co/MCM-41 and Co/SBA-15 catalytic systems. The effect of cobalt loading, impregnation method, particle size and pretreatment, on the catalytic properties of cobalt supported on HMS catalysts for the Fischer–Tropsch Synthesis have been studied. Catalysts were synthesized by incipient wetness impregnation, using an aqueous solution of hexahydrated cobalt nitrate as precursor salt. Additionally, a series of catalysts was prepared using an ethylene diamine cobalt complex which enabled cobalt nanoparticles formation. All catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), chemical analysis and nitrogen adsorption. Calcination pretreatment has great influence in the catalytic behavior of the cobalt catalysts. All the catalysts prepared showed high C 5 + fraction selectivity, specifically towards the diesel fraction. The use of HMS as cobalt catalyst support for the Fischer–Tropsch Synthesis seems to be very promising. When SiO 2 is used as support, C 5 + hydrocarbons distribution shows a marked tendency towards C 19 + fraction, while when HMS is used, the diesel fraction is favored. This result leads to the conclusion that the HMS pore structure produced a chain growth hindrance, tailoring the product distribution towards the diesel fraction.

High hydrogenation performance of the mesoporous NiMo/Al(Ti, Zr)–HMS catalysts

Hexagonal mesoporous silica (HMS) material with and without Al, Ti and Zr ions (Si/X, X = Al, Ti or Zr atomic ratio of 40) were used as support for the preparation of the hydrotreating NiMo catalysts. The samples were characterized by 29Si NMR, S BET, XRD, HRTEM, Raman spectroscopy, FT-IR spectroscopy of adsorbed NO on freshly sulfided samples, XPS of the used samples and H 2 chemisorption. The catalytic activity of the samples was evaluated in the reaction of hydrogenation of naphthalene. All samples showed high hydrogenation activity in the first step of the reaction when tetraline is formed. However, in the second step of the reaction leading to the formation of cis-decaline and trans-decaline, the presence of Al, Ti and Zr ions increased the hydrogenation ability of the samples. The NiMo/HMS and a reference (NiMo/ γ-Al 2O 3) catalysts were good for mild hydrogenation (tetraline formation), while in the NiMo/(Al, Ti, Zr)–HMS catalysts were superior when deep hydrogenation is necessary ( cis-decaline and trans-decaline formation). We found three interesting correlations, (i) oxidized Ni species in the sulfided samples with the deep (complete) hydrogenation (tetraline to decalines), (ii) the dispersion of the sulfided Ni and Mo species with the rate constant of the naphthalene consumption and, (iii) the population of Mo O T species (T terminal bonding) with the second hydrogenation step (tetraline transformation to decaline).

Characterization of silylated Ti-grafted HMS catalyst and its excellent epoxidation performance

Silylated Ti-grafted hexagonal mesoporous silica (HMS) catalyst was prepared by the chemical vapor deposition (CVD) using TiCl 4 as titanium source and hexamethyldisilazane (HMDSZ) as silylating agent. The samples were characterized by XRD, N 2-adsorption, FTIR, 29Si NMR, DR UV–vis, and evaluated by epoxidation of styrene, propylene, cyclohexene, and 1-hexene with cumene hydroperoxide (CHP) as oxidant, respectively. It is revealed that the catalyst possesses typical mesoporous structure, high hydrophobicity and highly dispersed tetracoordinated titanium sites and hence exhibits excellent performance in epoxidation of olefins.

Catalytic degradation of polyolefins over hexagonal mesoporous silica: Effect of aluminum addition

Degradations of polypropylene (PP) and polyethylene (PE) over pure hexagonal mesoporous silica and aluminum-containing hexagonal mesoporous silica catalysts were studied in a fixed bed catalytic reactor at 380 and 430 °C, respectively. The thermal and catalytic degradations of both PP and PE in liquid-phase-contact and vapor-phase-contact modes over pure hexagonal mesoporous silica had no significant effect on the product yields. The liquid products were widely distributed in hydrocarbons with boiling point ranges of 36–405 °C. By adding a small amount of aluminum to the hexagonal mesoporous material, aluminium-containing hexagonal mesoporous silica exhibited good performance in cracking heavy molecular weight hydrocarbons into light hydrocarbons. High liquid yields and less coke deposits were obtained in liquid-phase-contact reaction with increasing aluminum content. The liquid products were mainly composed of C 5–C 10 hydrocarbons with boiling points of 36–174 °C, and propene, butene, and butane were main components in gaseous products. The effect of degradation temperature was not observed on product yields though degradation rate of polyolefin into liquid products was faster. Conversely, in vapor-phase-contact reaction, an increase in gaseous yield was observed when increasing the amount of aluminum and temperature of the cracking reactor, while the residue yield remained constant.

Effect of Ti on the catalytic properties of CoMo/Ti( x)-HMS catalysts in the reaction of hydrodesulfurization of 4-ethyl-6-methyl dibenzothiophene

The effect of the presence of varying titanium concentrations introduced into hexagonal mesoporous silica (HMS) support on the catalytic activity of the catalysts was evaluated in the hydrodesulfurization of 4-ethyl-6-methyl dibenzothiophene (4E6MDBT) in a fixed-bed reactor at T = 598 K , P = 5.0 MPa , and WHSV = 46.4 h −1. The catalysts were characterized by S BET , XRD, DRS, FTIR spectroscopy of the framework vibration, DRIFTS in the OH region, H 1 - NMR , FT-IR spectroscopy of adsorbed NO and pyridine, XPS, and TGA. All Ti-containing catalysts showed higher activity than the Ti-free CoMo/HMS sample. The catalyst with a Si/Ti molar ratio of 40 is the most active. The HDS of 4E6MDBT over the Ti-free catalyst proceeds mainly via the dealkylation (DA) route, and then HDS of the more reactive DBT occurs. After Ti-incorporation into the HMS material, additional acid-catalyzed isomerization occurs, followed by direct HDS and HYD. Based on the catalyst activity–structure correlation, the increase in HDS activity over the Ti-containing catalysts with respect to the Ti-free sample was attributed to increased CoS 2 and MoS 2 surface exposure, as well as increased Lewis and Brønsted acidity.

Characterization and hydrodesulfurization catalysis on WS2 supported on mesoporous Al–HMS material

Hexagonal mesoporous silica (HMS) and Al–HMS materials with high surface area and mesoporous structure were synthesized following the neutral templating path way. These materials were used as a support for the first time in preparing W/HMS, and W/Al–HMS catalysts and their Co, and Ni promoted analogues. On Al–HMS support (Si/Al=35) the tungsten loading was varied from 10 to 25 wt.%. On a catalyst containing 21 wt.% W the promoter concentration was varied from 1 to 5 wt.%. Oxygen uptakes and crystallite sizes of WS2 derived from it indicated that WS2 is well dispersed at all the loadings studied. The hydrodesulfurization and hydrogenation activities varied in a similar manner to that of oxygen uptakes as a function of W loading indicating that there exists a correlation between oxygen chemisorption and catalytic sites. The HMS and Al–HMS mesoporous material supported WS2 catalysts showed outstanding activities in comparison with WS2 supported on γ-Al2O3.

Highly active methanol decomposition catalyst derived from Pd-hydrotalcite dispersed on mesoporous silica

Pd‐hydrotalcite (abbreviated as Pd(HT)) was dispersed on HMS (hexagonal mesoporous silica) by synthesizing Pd(HT) in an HMS suspension, and the resultant product (Pd(HT)/HMS) was used as a catalyst precursor for methanol decomposition to synthesis gas. The IR spectra of Pd(HT)/HMS showed all the bands of Pd(HT) and HMS with little shift, which indicated that Pd(HT) was synthesized in the Pd(HT)/HMS. Pd(HT)/HMS did not show the XRD pattern of Pd(HT) when the mass ratio of Pd(HT) to HMS was from 2/1 to 1/2. This indicated that Pd(HT) was formed in very small particles in the Pd(HT)/HMS after dispersion. Two endothermic peaks of Pd(HT) in the DTA curve shifted to lower temperatures in the Pd(HT)/HMS because the small Pd(HT) particles formed in the Pd(HT)/HMS were easily collapsed by heat treatment. Pd(HT)/HMS was thermally decomposed and reduced to form a supported Pd catalyst (abbreviated Pd(Mg(Al)O)/HMS) for methanol decomposition. Pd(Mg(Al)O)/HMS at 3.6 wt% showed a 52.5% conversion which was much higher than those over 3.6 wt% Pd(Mg(Al)O) (34.7%) and 3.6 wt% Pd/HMS (13.7%) for methanol decomposition at 523 K. The conversions of methanol over Pd(Mg(Al)O) and Pd/HMS increased with the increase in Pd loadings from 3.6 to 15 wt% and decreased when the Pd loadings were over 15 wt%. In contrast, the conversion over Pd(Mg(Al)O)/HMS increased with the increase in Pd loading even when the Pd loading was up to 30%. 30 wt% Pd(Mg(Al)O)/HMS showed a 91.7% conversion which was about twice that over 15 wt% Pd(Mg(Al)O) (47.1%) at 523 K. The Pd(Mg(Al)O)/HMS catalyst showed a larger BET surface area and Pd metal surface area than those of Pd(Mg(Al)O). By characterization using XPS analyses, the metal–support interaction between small Pd and small Mg(Al)O became stronger in the Pd(Mg(Al)O)/HMS catalyst. Large surface area, high Pd dispersion and strong metal–support interaction caused the high catalytic activity for methanol decomposition to synthesis gas over the Pd(Mg(Al)O)/HMS catalyst.