Silica-supported Platinum Catalysts Research Articles

Ethylene oxidation activity of silica-supported platinum catalysts for the preservation of perishables

Silica-supported platinum catalysts can remove trace amounts of ethylene from perishables and extend their shelf-lives.

Hydrogen production from decalin over silica-supported platinum catalysts: a kinetic and thermodynamic study

Hydrogen production from decalin over silica-supported platinum catalysts: a kinetic and thermodynamic study

Platinum Catalyzed C–H Activation and the Effect of Metal–Support Interactions

Catalytic C–H bond activation of methane and ethane on a series of silica supported platinum catalysts (Pt/SiO2) was studied by using hydrogen/deuterium (H/D) exchange. Kinetic experiments demonstrate that under the reaction conditions studied, the rate of C–H bond activation shows approximate first order dependence in alkane and inverse first order dependence in D2. The rate of C–H activation is affected by the presence of sodium on the silica support, where sodium-free supports have the fastest rates of C–H activation, as assessed by H/D exchange. CO adsorption and FTIR studies indicate that the Pt particles on the sodium-free support are more electron-deficient, having the most blue-shifted linear CO stretch, while sodium-containing supports are more electron-donating, having the most red-shifted linear CO stretch. It is proposed, based on the results described in this article and previous work in the literature, that more electron-donating supports cause the Pt particles to be more electron-rich and t...

Facile Route for Bio-Phenol Siloxane Synthesis via Heterogeneous Catalytic Method and its Autonomic Antibacterial Property.

Eugenol, used as bio-phenol, was designed to replace the hydrogen atom of hydrogenterminated siloxane by hydrosilylation reaction under the presence of alumina-supported platinum catalyst (Pt-Al2O3), silica-supported platinum catalyst (Pt-SiO2) and carbon nanotube-supported platinum catalyst (Pt-CNT), respectively. The catalytic activities of these three platinum catalysts were measured by nuclear magnetic resonance hydrogen spectrometer (1H NMR). The properties of bio-phenol siloxane were characterized by Fourier transform infrared spectrometer (FT–IR), UV-visible spectrophotometer (UV) and thermogravimeter (TGA), and its antibacterial property against Escherichia coli was also studied. The results showed that the catalytic activity of the catalyst Pt-CNT was preferable. When the catalyst concentration was 100 ppm, the reaction temperature was 80 °C and reaction time was 6 h, the reactant conversion rate reached 97%. After modification with bio-phenol, the thermal stability of the obtained bio-phenol siloxane was improved. For bio-phenol siloxane, when the ratio of weight loss reached 98%, the pyrolysis temperature was raised to 663 °C which was 60 °C higher than hydrogenterminated siloxane. Meanwhile, its autonomic antibacterial property against Escherichia coli was improved significantly.

Engineering of silica-supported platinum catalysts with hierarchical porosity combining latex synthesis, sonochemistry and sol-gel process – II. Catalytic performance

As a follow-up of the paper “Engineering of silica-supported platinum catalysts with hierarchical porosity combining latex synthesis, sonochemistry and sol-gel process – I. Material preparation” (A.F. Sierra-Salazar, et at., Microporous Mesoporous Mater. 234 (2016) 207–214. http://dx.doi.org/10.1016/j.micromeso.2016.07.009), we propose waterborne Pt-based catalysts with hierarchical porosity and controlled Pt nanoparticles (NPs) distribution within the support. The materials exhibit specific surface areas and total pore volumes as high as 490 m2 g−1 and 0.77 cm3 g−1, with ∼5 nm Pt NPs mainly located in the macropores. The Pt NPs were characterised using X-ray photoemission spectroscopy (XPS) and high resolution transmission electron microscopy (HR-TEM). Considering the selectivity challenge of the catalytic hydrogenation of halonitrobenzenes to produce haloanilines, which are important raw materials for several industrial products, we evaluated these hierarchically porous catalysts for the hydrogenation of p-chloronitrobenzene (p-CNB) in batch mode to produce p-chloroaniline (p-CAN). It was possible to obtain up to 100% selectivity at 80% conversion and initial reaction rates up to 34 molCNB min−1 molPt−1. Such selectivity was higher than that exhibited by a commercial Pt/SiO2 catalyst (up to 92%).

Hydrogenation of 2,5-dimethylfuran on hexagonal-boron nitride- and silica-supported platinum catalysts

Hydrogenation of furanic compounds is one of the important reactions for upgrading of bio-oils and production of diesel fuels. Platinum catalysts supported on amorphous silica and hexagonal boron nitride (h-BN) were prepared by incipient wetness impregnation and subsequent reduction, and were used for the vapor-phase hydrogenation of 2,5-dimethylfuran in hydrogen at atmospheric pressure and a temperature range of 150–350°C. For the same amount of Pt loading (1wt%), the particle size of Pt supported on h-BN was larger than that on silica, resulting in a lower amount of CO chemisorption for Pt/BN than that for Pt/SiO2. Using the same amount of active sites, Pt/BN exhibited a 3-fold higher turnover frequency than Pt/SiO2 for the hydrogenation of 2,5-dimethylfuran whereas both catalysts gave similar product distributions with high selectivity to 2-hexanone at any conversion and low selectivity to n-hexane at high conversion. Contact time studies of 2,5-dimethylfuran hydrogenation on Pt/BN suggested that 2,5-dimethylfuran formed the ring-opening product, 2-hexanone and the ring-saturation product, 2,5-dimethyltetrahydrofuran in parallel, with the rate of the direct furan ring-opening being 9-fold higher than that of the furan ring-saturation.

Super-microporous silica-supported platinum catalyst for highly regioselective hydrosilylation

Super-microporous micelle-templated platinum on silica catalysts were successfully prepared by an improved one-pot procedure, using a water/acetonitrile/n-dodecylamine mixture. The catalyst showed high surface area and narrow pore size and was further characterized by powder X-ray diffraction and inductively coupled plasma mass spectrometry. The unique super-microporous materials were proven to be highly active for the microwave-assisted hydrosilylation reaction of terminal alkenes or alkynes using dichloromethylsilane as silicon source. The super-microporous structure was beneficial for the control of hydrosilylation regioselectivity, as the selectivity to the β-adduct reached 99%. Moreover, the catalyst could be easily recovered by simple filtration and re-used seven times without appreciable loss of activity and selectivity.

Engineering of silica-supported platinum catalysts with hierarchical porosity combining latex synthesis, sonochemistry and sol-gel process – I. Material preparation

A novel catalyst preparation procedure is proposed in order to enhance the noble metal distribution at low loadings, while controlling the hierarchical porosity of the support material. Thus, a silica-supported platinum catalyst with hierarchical porosity was prepared using a combination of three processes performed in aqueous medium: emulsion polymerisation, sonochemistry and sol-gel synthesis. First, a polystyrene latex template of ca. 130 nm was synthesised by emulsion polymerisation and subsequently decorated with Pt nanoparticles of ca. 2.1 nm by sonochemical reduction of sodium tetrachloroplatinate. Then, the mesoporous silica support was prepared by a two-step acid-base catalysed sol-gel synthesis around the Pt-loaded latex spheres. Materials with specific surface areas and total pore volumes as high as 615 m2 g−1 and 0.74 cm3 g−1, respectively, were obtained.

Catalytic Removal of Volatile Organic Compounds

The degradation of air quality by the release of volatile organic compounds (VOCs) into the air particularly harms human health and our environment. [...]

Sensitivity of the glycerol oxidation reaction to the size and shape of the platinum nanoparticles in Pt/SiO2 catalysts

The structure sensitivity of the catalytic oxidation of glycerol with atmospheric O2 in neutral pH water solutions was evaluated by using three series of silica-supported platinum catalysts. Transmission electron microscopy and CO titration experiments with infrared absorption spectroscopy detection were used to characterize the catalysts. Higher loadings led to larger average Pt nanoparticle sizes, but comparisons across catalysts made by different methods were unreliable because of differences in particle size distributions. Nevertheless, strong dependences of the selectivity for oxidation at the terminal versus central alcohol groups were identified as a function of both the extent of reaction and Pt particle size, with high conversions and large nanoparticles favoring terminal-carbon oxidation. Additional increases in turnover frequencies were also observed with increasing average metal nanoparticle size and with cuboctahedral versus tetrahedral Pt nanoparticles. On the other hand, no significant trends were seen in terms of further oxidation of glyceraldehyde to glyceric acid. The reported trends were ascribed to the general need for multi-atom surface Pt ensembles for conversion and to a possible unique access to the central carbonyl in glycerol only at low-coordination Pt sites.

Silica Supported Platinum Catalysts for Total Oxidation of the Polyaromatic Hydrocarbon Naphthalene: An Investigation of Metal Loading and Calcination Temperature

A range of catalysts comprising of platinum supported on silica, prepared by an impregnation method, have been studied for the total oxidation of naphthalene, which is a representative Polycyclic Aromatic Hydrocarbon. The influence of platinum loading and calcination temperature on oxidation activity was evaluated. Increasing the platinum loading up to 2.5 wt.% increased the catalyst activity, whilst a 5.0 wt.% catalyst was slightly less active. The catalyst containing the optimum 2.5 wt.% loading was most active after calcination in air at 550 °C. Characterisation by carbon monoxide chemisorption and X-ray photoelectron spectroscopy showed that low platinum dispersion to form large platinum particles, in combination with platinum in metallic and oxidised states was important for high catalyst activity. Catalyst performance improved after initial use in repeat cycles, whilst there was slight deactivation after prolonged time-on-stream.

Impact of Chemical Heterogeneity on the Accuracy of Pore Size Distributions in Disordered Solids

Disordered porous solids, such as heterogeneous catalysts, possess internal void spaces with high degrees of both chemical and geometric heterogeneity. For pore size distributions (PSDs) to be useful void space descriptors for aiding understanding of phenomena such as catalyst activity, their level of accuracy must be known. Due to differences in specific interactions and wetting behavior, the choice of adsorbate can severely impact the accuracy of PSDs derived for disordered materials using gas sorption. Wetting effects in the capillary condensation region have recently been studied extensively by simulation, but there have been no complementary, definitive experimental studies of sorption mechanisms. However, in this work, the novel integrated gas sorption and mercury porosimetry technique has been used to prepare a particular subset of pores, located within the disordered network of a given amorphous material (eg silica or alumina), that had dead-ends with different chemical properties to the walls, and then definitively compare the individual sorption behavior of two different, common adsorbates (nitrogen and argon) within just these pores. The presence of a heavy metal surface was found to lead to a shift to higher pressure in the positions of adsorption and desorption branches and a widening of the hysteresis for argon isotherms for silica pores, but not for alumina pores. In contrast, no such effect was observed for nitrogen. Hence, using argon adsorption rather than nitrogen adsorption was found likely to lead to pore size inaccuracies of at least ∼200% in characterizing silica-supported platinum catalysts, for example. The experimental findings on effects of wetting and pore length were found to be consistent with findings from recent Monte Carlo and MFDFT simulations. Further, analysis of hysteresis widths also suggested that the standard approach using NLDFT would be unsuitable for the type of samples studied here. The results highlight particular issues for the accuracy of PSDs for catalysts consisting of metal nanoparticles supported on oxide materials.

Carboxy-Telechelic Polyolefins in Cross-Linked Elastomers

We demonstrate that ring-opening metathesis copolymerization of 3-hexyl-cis-cyclooctene and cis-cyclooctene in the presence of maleic acid as a chain transfer agent leads to precise control over the molar mass, branching and crystallinity of the resulting carboxy-telechelic polyalkenamers. Subsequent hydrogenation using a silica-supported platinum catalyst had a negligible effect on the carboxy functionality and led to telechelic polyolefins with high thermal resistance, low glass transition temperatures (<−57 °C), little to no crystallinity, and viscosities similar to those of liquid silicon rubber prior to cross-linking. These properties are ideal for easily processable reactive polyolefin prepolymers that can be readily cured to form elastomers. We demonstrate that a fast and controlled curing process can be achieved in the presence of polyfunctional aziridines. The properties of the thermoset elastomer products were evaluated by tensile, differential scanning calorimetry, and dynamic mechanical analyses.

Catalytic oxidation of formaldehyde over different silica supported platinum catalysts

Three kinds of silica materials, including fumed SiO2, porous granular SiO2 and mesoporous SBA-15, were adopted to prepare supported Pt catalysts by impregnation method. The catalytic properties of these silica supported Pt catalysts (Pt/SiO2) were investigated for the complete oxidation of formaldehyde (HCHO). Among them, fumed SiO2 supported Pt catalyst (Pt/f-SiO2) shows very high catalytic activity, which could completely oxidize HCHO even at ambient temperature. According to the results of catalyst characterization, it was proposed that the nature of silica supports could affect the particle size and the chemical states of platinum species and then further influence the redox property of Pt/SiO2 catalysts. Compared with other silica supported Pt catalysts, Pt/f-SiO2 catalyst possesses higher ratio of metallic Pt species, which might be a key factor in improving its capability to activate molecular oxygen and consequently to oxidize HCHO at low temperature.

Hydrodechlorination of tetrachloromethane on alumina- and silica-supported platinum catalysts

Two series of alumina- and silica-supported platinum catalysts, representing a wide range of metal dispersion were characterized by CO chemisorption, XRD and TEM, and investigated in the hydrodechlorination of tetrachloromethane at 343–363K. Catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination shows very strong inverse relationship with metal dispersion; highly dispersed Pt samples exhibit very low turnover frequencies. Similar, but much milder trend, was found for silica-supported Pt catalysts. This significant support effect on the catalytic behavior was attributed to an extensive surface chloriding of small Pt particles interacting with Lewis acid sites of γ-alumina. It is suggested that such interactions lead to formation of electrodeficient Pt sites, which are quickly blocked by produced chloride species. On silica, similar in kind metal-support interactions do not occur, and, in effect, deactivation of Pt/SiO2 catalysts is much less marked. Very strong dependence of catalytic activity of alumina-supported platinum on metal dispersion implies that even very small changes in Pt particle size dispersion during reaction and catalyst regeneration should not be ignored in interpreting variations in the catalytic behavior. Indeed, changes in Pt particle size caused by different regeneration procedures are found to be an important factor in shaping the ultimate hydrodechlorination behavior. Carried out in an oxidative atmosphere for an efficient removal of coke, the regeneration of a highly dispersed Pt/Al2O3 catalyst resulted in a significant growth of metal particles which, in line with the correlation between turnover frequency and metal dispersion established for this catalytic system, exhibited much better performance than that of a freshly reduced catalyst sample. In the case of highly dispersed Pt/SiO2 catalyst the regeneration also leads to decrease of metal dispersion and corresponding improvement of catalytic activity.

Impact of supported ionic liquids on supported Pt catalysts

Silica supported platinum catalysts coated with a thin film of 1-butyl-2,3-dimethyl-imidazolium trifluoromethane sulfonate (BDiMIm) were investigated with respect to the interaction of the ionic liquid with the oxide support and the metal clusters. IR, inelastic neutron scattering and NMR spectroscopy indicate that the vibrations of the imidazolium ring of ionic liquid are less restricted when supported on SiO2, while the viscosity of the supported ionic liquid increased. The presence of Pt particles enhances the electron density of the ionic liquid at the nitrogen atom inducing higher basicity. The coverage of the catalyst surface and the metal particles by the ionic liquid protects the metal against oxidation. The catalysts are active and stable for hydrogenation of ethene.

Study of CO Dissociative Adsorption over Pt and Rh Catalysts by Inverse Gas Chromatography

Carbon monoxide dissociative adsorption was studied over silica-supported platinum, rhodium and Pt-Rh alloy catalysts, by Reversed Flow-Gas Chromatography. Using appropriate mathematical analysis physicochemical quantities such as fractional catalytic conversions of CO to CO2, as well as rate constants for the adsorption, desorption and surface reaction, describing the dissociative adsorption of CO, were determined. From the variation of the above parameters against the nature of the studied catalysts (Rh content) useful conclusions concerning the mechanism of CO dissociative adsorption were extracted.

Proposal for a common reactive adsorbate for ethylbenzene and indenic compounds in the conversion of n-propylbenzene over a precoked silica-supported platinum catalyst

The effect of moderate sintering of the platinum particles of a precoked Pt/SiO 2 catalyst on the transformation of n-PB was investigated under reforming conditions (773 K, 5 bar, and H 2/HC = 5). The results showed that the larger the platinum particles, the greater the formation of cyclic compounds. Furthermore, a concentration balance between the ethylbenzene and the cyclic compounds was revealed, suggesting a common reactive adsorbate for both reaction products. According to a previously proposed reactive adsorbate for ethylbenzene and the results of studies of Pt/SiO 2 and PtSn/SiO 2 catalysts, a reactive adsorbate was proposed for the formation of the cyclic compounds. This latter reactive adsorbate, which results from the reactive adsorbate leading to ethylbenzene, is consistent with either a decrease in the size of the Pt ensembles or an increase in the electron density of the platinum particles of the Pt/SiO 2 catalyst sample with the smaller particles after coke deposition. It is still uncertain as to whether the concentration balance phenomenon is due to a geometric ensemble effect, a change in the electronic density of the platinum clusters, or both. The studies with a tin-promoted catalyst were discussed in relation to this new finding and a detailed reaction network for the transformation of n-PB over metallic sites is proposed.

Study of structure-function relationships in platinum-silica catalysts using hydrocarbon hydrogenation as a probe reaction

In the field of heterogeneous catalysis, the relationship between catalyst synthesis and hence structure, and the resulting catalyst activity and selectivity are not well understood. The focus of this work has been to study the structural and catalytic characteristics of eight silica-supported platinum catalysts containing 0.5 wt% platinum. In particular, we wish to identify the specific characteristics of these materials which determine their catalytic behaviour. A further aim is to relate these material properties to the method of catalyst synthesis used. This paper highlights the application of 29 Si nuclear magnetic resonance (NMR) and Raman spectroscopies to the characterisation of the hydroxyl and siloxane ring structure environments on the silica surface. We then investigate how these structures are influenced by catalyst synthesis and the role, if any, they play in the activity and selectivity characteristics of the resulting catalysts. Reactor data are reported for ethene and ethyne hydrogenation reactions. The results suggest that while catalytic activity is determined by metal surface area, which is controlled by the method of metal deposition, for both the ethene and ethyne hydrogenations, the selectivity of the ethyne hydrogenation to ethane production appears to correlate with the structure of the silica surface.

An Infrared Spectroscopic Study of the Hydrogenation and Dehydrogenation of the Complexes of Aromatic Compounds and of Fullerene C60 with Silica-Supported Platinum

An infrared spectroscopic study has been made of the surface complexes from the chemisorption of the aromatic compounds benzene, toluene, para-xylene (1,4-dimethylbenzene), mesitylene (1,3,5-trimethylbenzene), naphthalene, and anthracene, and also of fullerene C60, on silica-supported platinum catalysts, and of their subsequent hydrogenation and dehydrogenation reactions. The complexes of the initial adsorbates with platinum can be hydrogenated under relatively low pressures of hydrogen (<1 atm) and then dehydrogenated to considerable degrees by evacuation even at ambient temperatures. In the case of benzene the hydrogenated cyclohexane-like complex gives very extensive reversion to adsorbed benzene on evacuation. The surface complexes gradually diminish in amount during successive cycles of hydrogenation and dehydrogenation, with the generation of gas-phase cyclohexane. For the other compounds, except for C60, the hydrogenation is also virtually (or actually) complete but the dehydrogenation is only partial and the surface complexes remain present during multiple cycles of hydrogenation and dehydrogenation. The room temperature spectra from the methyl-substituted benzenes under vacuum support chemisorption via methyl-group dissociation. For naphthalene and anthracene the spectra of stable partially dehydrogenated species are consistent with the rearomatization of only one end ring. For fullerene C60 the spectrum of the hydrogenated species suggests that a five-membered ring is in selective contact with the surface but that some adjacent six-membered rings are also hydrogenated. The hydrogenation and dehydrogenation processes are both slow but the rates, together with the extent of hydrogenation, increase with temperature. The hydrogenated spectra of the fullerene-based complex differ from those reported earlier for C60H18 and C60H36.