Supports For Heterogeneous Catalysts Research Articles

ChemInform Abstract: Green Chemistry by Nano‐Catalysis

AbstractReview: 48 refs.

ChemInform Abstract: Synthesis, Stability, and Catalytic Properties of High Surface Area Silicon Oxynitride and Silicon Carbide

Abstract The aim of this work is to extend the range of inorganic materials which find application in heterogeneous catalysis, as catalysts or catalyst supports, through use of progress in the field of advanced ceramics. Developments in synthetic routes to high surface area non-oxides (nitrides, carbides, borides) are briefly reviewed. Work on the synthesis, stability and catalytic properties of high surface area silicon oxynitride and silicon carbide is described. For the former compound, three synthetic routes were investigated: the reaction of an amorphous silica with ammonia proved to be the preferred route, giving a silicon oxynitride of ca. 150 m 2 g −1 . The surface composition of the material obtained by the latter method showed good stability, as indicated by retention of surface nitrogen under reducing or oxidising conditions at about 700°C. The oxynitride surface was unreactive towards butene-1, causing no double-bond isomerisation at 400°C. However, the material was also found to be capable of functioning as a solid basic catalyst, catalysing Knoevenagel condensations in solution at 50°C. The use of nitrides as solid basic catalysts appears to be novel, and this result suggests new applications for such materials. In the case of silicon carbide, a mesoporous form was obtained by polymer pyrolysis. The texture shows improved thermal (but not hydrothermal) stability compared to silica. The surface is less reactive than that of the oxynitride, catalysing neither the double-bond isomerisation of butene-1 nor the Knoevenagel condensation. This compound, and the oxynitride, may be useful as inert support materials for reactions involving alkenes.

Synthesis of High Surface Area ZnO(0001) Plates as Novel Oxide Supports for Heterogeneous Catalysts

We demonstrate a technique to prepare high surface area ZnO powders that preferentially favor a plate-like morphology, exposing the $$(0001)/(000\bar{1})$$ facets. A solution-based synthetic route was used to decompose zinc acetate in the presence of amine and citrate ions to block the $$(0001)/(000\bar{1})$$ facets and favor growth from the pyramid planes. The ZnO platelets remained stable upon heating to 250 °C as evidenced by electron diffraction patterns. The high surface area (75 m2/g) and surface energetics of the (0001) plane make these powders suitable as supports for heterogeneous catalysts.

Mesoporous Molecular Sieves as Advanced Supports for Olefin Metathesis Catalysts

AbstractSummary: Siliceous mesoporous molecular sieves MCM‐41, MCM‐48 and SBA‐15 and organised mesoporous alumina represent progressive supports for new heterogeneous catalysts for olefin metathesis and metathesis polymerization. In combination with Mo and Re oxides they provide catalysts of considerably higher activity in comparison with those based on conventional silica and alumina. Immobilization of Mo and Ru alkylidenes on these materials led to the highly active and selective catalysts with negligible leaching of transition metal.

Fly ash from a Mexican mineral coal I: Mineralogical and chemical characterization

The properties of coal fly ash are strongly dependent on the geological origin and the combustion process of the coal. It is important to characterize regional fly ash in detail to ascertain its potential uses as raw material in the production of high value products. The physicochemical properties of fly ash coming from the “Jose Lopez Portillo” coal-fired power plant, Coahuila, Mexico (MFA), are presented in this work. A detailed study of trace elements, the chemical composition of the amorphous phase, thermal stability and the leaching of contaminant elements under different conditions are included. MFA is composed of mullite, quartz, calcite, magnetite and an amorphous phase. This material contains mainly silica (59.6%), alumina (22.8%) and magnetite (5.6%). Its amorphous phase (78.3%) has a high silica (49.4%) and alumina (14.4%) content. According to its mineralogical and chemical composition, MFA is potentially useful as a raw material for making cement, silica, and alumina, as well as low silica/alumina ratio zeolites. Deleterious elements could be removed during the zeolitization process or with an additional acid treatment. Because of its morphological properties and structural and thermal stability, MFA can be used in thermal isolation and refractory materials and as a support for heterogeneous catalysts.

Chitosan as a support for heterogeneous Pd catalysts in liquid phase catalysis

Four different chitosan-supported palladium catalysts were prepared, whereby two of them were modified as Schiff base by reaction with salicylaldehyde and 2-pyridinecarboxaldehyde before complexation with palladium. The remaining differ in their preparation method: co-precipitation or adsorption. The properties of the catalysts were characterized by FTIR, XPS, ICP-MS, and TGA. Comparison of the catalysts activity was assessed in microwave-assisted Suzuki reactions in aqueous media, resulting in good yields and excellent selectivities concerning cross-coupling product. Additionally, the catalysts prove their activity under conductive heating conditions. The study was extended to microwave-assisted Heck and Sonogashira reactions in DMF, confirming the efficiency of chitosan-supported palladium derivatives as catalysts for C–C couplings. Experiments revealed that catalysts prepared by co-precipitation furnished inferior yields concerning the employed C–C coupling reactions. Modification of chitosan with 2-pyridinecarboxaldehyde and subsequent palladium deposition resulted in highly active catalysts affording high product selectivities and yields.

Green chemistry by nano-catalysis

Nano-materials are important in many diverse areas, from basic research to various applications in electronics, biochemical sensors, catalysis and energy. They have emerged as sustainable alternatives to conventional materials, as robust high surface area heterogeneous catalysts and catalyst supports. The nano-sized particles increase the exposed surface area of the active component of the catalyst, thereby enhancing the contact between reactants and catalyst dramatically and mimicking the homogeneous catalysts. This review focuses on the use of nano-catalysis for green chemistry development including the strategy of using microwave heating with nano-catalysis in benign aqueous reaction media which offers an extraordinary synergistic effect with greater potential than these three components in isolation. To illustrate the proof-of-concept of this “green and sustainable” approach, representative examples are discussed in this article.

Preparation and catalytic studies of palladium nanoparticles stabilized by dendritic phosphine ligand-functionalized silica

Silica is a prominently utilized heterogeneous metal catalyst support. Functionalization of the silica with poly(ether imine) based dendritic phosphine ligand was conducted, in order to assess the efficacy of the dendritic phosphine in reactions facilitated by a silica supported metal catalyst. The phosphinated poly(ether imine) (PETIM) dendritic ligand was bound covalently to the functionalized silica. For this purpose, the phosphinated dendritic ligand containing an amine at the focal point was synthesized initially. Complexation of the dendritic phosphine functionalized silica with Pd(COD)Cl 2 yielded Pd(II) complex, which was reduced subsequently to Pd(0), by conditioning with EtOH. The Pd metal nanoparticle thus formed was characterized by physical methods, and the spherical nanoparticles were found to have >85% size distribution between 2 nm and 4 nm. The metal nanoparticle was tested as a hydrogenation catalyst of olefins. The catalyst could be recovered and recycled more than 10 times, without a loss in the catalytic efficiency.

Tuneable porous carbonaceous materials from renewable resources

Porous carbon materials are ubiquitous with a wide range of technologically important applications, including separation science, heterogeneous catalyst supports, water purification filters, stationary phase materials, as well as the developing future areas of energy generation and storage applications. Hard template routes to ordered mesoporous carbons are well established, but whilst offering different mesoscopic textural phases, the surface of the material is difficult to chemically post-modify and processing is energy, resource and step intensive. The production of carbon materials from biomass (i.e. sugars or polysaccharides) is a relatively new but rapidly expanding research area. In this tutorial review, we compare and contrast recently reported routes to the preparation of porous carbon materials derived from renewable resources, with examples of our previously reported mesoporous polysaccharide-derived "Starbon" carbonaceous material technology.

Carbon‐Carbon Double Bond versus Carbonyl Group Hydrogenation: Controlling the Intramolecular Selectivity with Polyaniline‐Supported Platinum Catalysts

AbstractThe use of polyaniline (PANI) as catalyst support for heterogeneous catalysts and their application in chemical catalysis is hitherto rather poorly known. We report the successful synthesis of highly dispersed PANI‐supported platinum catalysts (particle sizes between 1.7 and 3.7 nm as revealed by transmission electron microscopy, TEM) choosing two different approaches, namely (i) deposition‐precipitation of H2PtCl6 onto polyaniline, suspended in basic medium (DP method) and, (ii) immobilization of a preformed nanoscale platinum colloid on polyaniline (sol‐method). The PANI‐supported platinum catalysts were applied in the selective hydrogenation of the α,β‐unsaturated aldehyde citral. In order to benchmark their catalytic performance, citral hydrogenation was also carried out by using platinum supported on the classical support materials silica (SiO2), alumina (Al2O3), active carbon and graphite. The relations of the structural characteristics and surface state of the catalysts with respect to their hydrogenation properties have been probed by EXAFS and XPS. It is found that the DP method yields chemically prepared PtO2 on polyaniline and, thus, produces a highly dispersed and immobilized Adams catalyst (in the β‐PtO2 form) which is able to efficiently hydrogenate the conjugated CC bond of citral (selectivity to citronellal=87%), whereas reduction of the CO group occurs with polyaniline‐supported platinum (selectivity to geraniol/nerol=78%) prepared via the sol‐method. The complete reversal of the selectivity between the preferred hydrogenation of the conjugated CC or CO group is not only particularly useful for the selective hydrogenation of α,β‐unsaturated aldehydes but also unveils the great potential of conducting polymer‐supported precious metals in the field of hitherto barely investigated chemical catalysis.

Facile synthesis of ordered nanocrystalline alumina thin films with tunable mesopore structures

Mesoporous alumina layers have attracted attention for their potential use in ultrafiltration of salts, as a heterogeneous catalyst support, an adsorbent in environmental cleanup, and in petroleum refinement. The ability to control the fast hydrolysis rate of the inorganic precursors using simple and inexpensive routes is important for that potential to be realized. Herein, we introduce a novel and facile route to synthesize mesoporous alumina thin films from the combination of inexpensive and commercially available copolymer with aluminum chloride or nitrate (salts) in an EtOH–surfactant–NH 3 · H 2O–salts (EsNs) system through the evaporation-induced self-assembly (EISA) method. Mesoporous alumina layers obtained utilizing the EsNs system have ordered and tunable pore structures. The ability to easily control the mesophases of the alumina layers within a short time provides distinct advantages over previously reported synthesis procedures. Most importantly, we demonstrate that the binding of surfactant and NH 3 · H 2O for the formation of hydrogen bond between them in the EsNs system controls the fast hydrolysis rate of the inorganic species. This allows for the synthesis of nanocrystalline alumina layers via the aluminum oxo-clusters’ assembly with the surfactant. Such simple route may be applied in the synthesis of other non-silica mesostructured oxides.

Metal Nanoparticles and Related Materials Supported on Carbon Nanotubes: Methods and Applications

Carbon nanotubes are one of the most intensively explored nanostructured materials. In particular, carbon nanotubes are unique and ideal templates onto which to immobilize nanoparticles allowing the construction of designed nanoarchitectures that are extremely attractive as supports for heterogeneous catalysts, for use in fuel cells, and in related technologies that exploit the inherent 'smallness' and hollow characteristics of the nanoparticles. Here we overview the recent developments in this area by exploring the various techniques in which nanotubes can be functionalized with metals and other nanoparticles and explore the diverse applications of the resulting materials.

From Hydrocolloids to High Specific Surface Area Porous Supports for Catalysis

Polysaccharide hydrogels are effective supports for heterogeneous catalysts. Their use in solvents different from water has been hampered by their instability upon drying. While the freeze-drying process or air-drying of hydrocolloid gels led to compact solids with a low surface area, drying the gel in CO2 beyond the critical point provided mesoporous materials with a high specific surface area. Their effectiveness as a support for catalysis was exemplified in the reaction of substitution of an allyl carbonate with morpholine catalyzed by the hydrosoluble Pd(TPPTS)3 complex. The influence of water on the catalytic activity and the properties of the support was evidenced.

Effect of Alkali Ions on the Amorphous to Crystalline Phase Transition of Silica

The effect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural effect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS diffractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identification of the fraction of the different silica polymorphs like quartz, tridymite, and cristobalite. SAXS measurements, using the classical method based on Porod's law, yielded the total surface area of the systems. The calculated areas were compared with the surface areas determined by the nitrogen adsorption technique using the analytical method of Brunauer–Emmett–Teller. The results are explained in terms of sizes of the alkali ions and cell volume of the different crystalline phases.

Activity enhancement by the support in the hydrogenation of C=C bonds over polymer-supported palladium catalysts

Four synthetic ion-exchange resins (AH, BH, CH, DH) of different hydrophilic/hydrophobic properties were used as supports for heterogeneous palladium catalysts (A, B, C, D). The resins contained styrene (STY) and 2-(methacryloxy)ethylsulfonic acid (MESA) as the comonomers. Either divinylbenzene (DVB: CH, DH resins) or N,N'-methylenebisacrylamide (MBAA: AH, BH resins) were used as the cross-linker. AH contained also N,N-dimethylacrylamide (DMAA) as the third comonomer. The catalysts (Pd 0.25-0.45% w/w) were obtained by ion-exchanging the acidic forms of the resins with [Pd(OAc)2] and reducing palladium(II) with excess sodium borohydride. The use of NaBH4 also ensured the neutralization of the acidic sites of the supports. No effect of the hydrophilic/hydrophobic properties of the supports was observed in the hydrogenation of cyclohexene and 2-cyclohexen-1-one in methanol, at 25 degrees C and 0.5, 1, and 1.5 MPa, respectively. However, catalysts A and B, containing amido groups provided by either DMAA or MBAA, proved to be more active than C and D. The observed activity enhancement was directly proportional to the nitrogen/ palladium molar ratio in the catalysts. This finding suggests that amido groups promote palladium through a direct interaction with the metal surface.

Nano‐pits: supports for heterogeneous model catalysts prepared by interference lithograpy

Nanostructured 4 inch Si wafers were prepared as new supports for model catalysts. A single wafer exhibits 109–1010 pits on an otherwise flat silicon oxide surface. To produce these nanostructures, oxidized wafers were covered with photoresist material and exposed to laser interference patterns. Using hydrofluoric acid, wet‐chemical etching of the SiO2 through the structured resist resulted in a pitted surface. The etched pits with diameters of 200–400 nm and depths between 50 and 70 nm were loaded with metal particles by evaporation (palladium, silver) or by means of spin‐coating (copper). Optimizing the methods enabled exact deposition of single metal clusters in the pits. The resulting model catalysts are remarkably stable against sintering, a major problem of conventional model catalysts when exposed to elevated temperatures and oxidizing gas atmosphere. The topography and chemical composition as well as their changes, induced by the reaction conditions applied, including stability and chemical behavior of the nanostructured systems, were investigated by means of AFM, SEM, temperature‐programmed methods and XPS. To determine their usefulness in catalysis, specially designed reactors were developed for catalytic investigations.

Characterization of Iron(III) Tetramesitylporphyrin and Microperoxidase-8 Incorporated into the Molecular Sieve MCM-41.

The hydroxylation of aniline with H2O2 as the oxidant has been utilized to probe the catalytic activity of the water-insoluble model porphyrin in a water-soluble MCM-41 system and to investigate the catalytic activity and operational stability of microperoxidase-8 upon its incorporation into MCM-41. Our results demonstrate that metalloporphyrins supported on MCM-41 can catalyze cytochrome P450-like reactions in aqueous solutions but point out limitations in the use of MCM-41 as a solid support for heterogeneous catalysts.

Influence of the preparation conditions on the synthesis of high surface area SiC for use as a heterogeneous catalyst support

The influence of different parameters (temperature, duration and SiO source) on the synthesis of silicon carbide SiC according to the gas-solid reaction between SiO vapors and activated charcoal was investigated. The material obtained retained the general shape of the activated charcoal, which is an advantage because of the difficulty in post shaping SiC, due to the high strength of the material. High temperature (>1250 °C) and long reaction duration led to a high C* → SiC conversion but with a relatively low surface area (20–25 m2 · g−1) due to sintering via the surface diffusion phenomenon. The combination of a lower reaction temperature (1200 °C), longer reaction duration (15 h) and high (Si + SiO2)/C* weight ratio allowed SiC to be obtained with a surface area of around 50 m2 · g−1, which can be used as a support material for heterogeneous catalysis.

Surface chemistry of materials deposition at atomic layer level

Structures in modern semiconductor devices are getting smaller and smaller, coming close to atomic dimensions. The demand for materials processing at atomic layer level can be approached from extreme process control or from delicate utilization of surface chemistry. The opportunity in the surface chemistry approach is to create conditions for monoatomic layer buildup through saturated surface reactions. Material layer processing through sequentially performed saturated surface reactions is generally referred to as atomic layer epitaxy (ALE). ALE has been successfully applied in commercial manufacturing of thin film electroluminescent displays. Also, extensive scientific work has been done for applying atomic layer controlled growth of epitaxial layers and superlattice structures of III–V and II–VI semiconductors. Surface controlled build-up of molecular structures has recently been applied to porous supports for heterogeneous catalysts. For further progress in atomic layer level controlled materials processing well understood surface chemistry is of major importance.

High Surface Area Silicon Carbide Doped with Zirconium for use as Heterogeneous Catalyst Support

ABSTRACTHigh surface area (> 100 m2 · g−1) SiC doped with zirconium was prepared by the gas-solid reaction. The material was made up of three phases: β-SiC, covered by ZrO2 and an amorphous phase composed of Si, Zr and O. The characterization of the sample was performed by means of powder X-ray diffraction (XRD), surface area and porosity measurements by the BET method, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Preliminary catalytic tests, the standard n-C7 isomerization on supported MoOxCy showed that this new support was at least as effective as pure SiC.