Conversion of methanol to hydrocarbons on zeolite HZSM‐5 investigated by in situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography

In situ MAS NMR spectroscopy under flow conditions and on‐line gas chromatography have been applied to study the onset of the conversion of methanol on zeolite HZSM‐5 at temperatures between 373 and 573 K. In the steady states of methanol conversion at T ⩾ 523 K, by on‐line gas chromatography mainly the formation of ethene and propene was observed. Simultaneously recorded in situ ¹³C MAS NMR spectra show signals at 12–25 ppm and at ca. 125–131 ppm indicating the presence of adsorbed C₄–C₆ olefins. The observation of these adsorbates on a working catalyst supports the “hydrocarbon pool” mechanism previously proposed for the methanol‐to‐hydrocarbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K and subsequent purging of the catalyst with dry nitrogen at 293 K led to a ¹³C MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presence of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium ions were found by in situ ¹³C MAS NMR spectroscopy under flow conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of 1-benzoylpyrene using silica-supported phosphotungstic heteropoly acid as an efficient and reusable catalyst

A novel one-pot catalytic synthesis of 1-benzoylpyrene through acylation of pyrene with benzoic anhydride catalyzed by several heteropoly acids (HPAs) was investigated. Pure 1-benzoylpyrene was obtained and its structure was identified by GC/MS, FT-IR and ¹H NMR spectra. Silica-supported phosphotungstic heteropoly acid (PW/SiO₂) was found to be the most active catalyst in the acylation. The yield and the selectivity of 1-benzoylpyrene were up to 62.5% and 100%, respectively. The effects of experimental parameters on the catalytic acylation reaction, and the possibility of reusability of PW/SiO₂ catalyst were studied. PW/SiO₂ catalyst is easily separable from the reaction mixture and reusable without loss of its activity.     

Dependence of Synergetic Effect of Palladium–Manganese-Hexaaluminate Combustion Catalyst on Nature of Palladium Precursor

A synergetic effect in the methane oxidation activity of palladium and manganese hexaaluminate was studied over Pd-modified manganese-hexaaluminate catalysts, prepared by incipient wetness impregnation and calcined at 1,200?°C. The magnitude of the synergetic effect is found to be depends on the palladium precursor: it is higher for palladium nitrate and palladium acetate than for tetrachloropalladic acid. The Pd/MnLaAl₁₁O₁₉ catalysts were characterized by X-ray diffraction, X-ray microanalysis, transmission electron microscope and temperature-programmed reduction with hydrogen. These data were compared with the properties of Pd/Al₂O₃ catalysts. At variation of Pd-precursors, a minor trend to the decrease of the Pd particle size was observed at transition from the ex-chloride Pd/MnLaAl₁₁O₁₉ catalyst with uniform Pd-distribution profile to the ex-nitrate and ex-acetate catalysts with egg-shell Pd-distribution. Slightly smaller size of metal palladium particles in the ex-nitrate and ex-acetate catalysts leads to the formation of larger amount of PdO dispersed on their surface during oxygen-pretreatment in H₂-TPR experiments (Pd/PdO atomic ratio was 1/4) and under methane-oxidation mixture in comparison with ex-chloride catalysts (Pd/PdO?=?4/1). The palladium addition to manganese-hexaaluminate changes strongly its redox properties, as result Mn³⁺ reduction to Mn²⁺ take place about 100?°C below that of pure hexaalunimate. The latter indicate probably on the higher oxygen mobility in Pd-modified manganese-hexaaluminate. A higher PdO/Pd ratio formed in the ex-nitrate and ex-acetate Pd-modified manganese-hexaaluminate catalysts together with the high oxygen mobility provide the synergetic effect in methane oxidation activity at light-off temperature region. The high catalytic activity of manganese-hexaaluminate ensures methane combustion efficiency of the Pd-modified manganese-hexaaluminate catalysts at temperature above 700?°C.     

Multinuclear NMR and catalytic investigations of deactivation of zeolite H-ZSM-5

Coke formation and catalyst deactivation by the reaction of methanol over a zeolitic catalyst based on H-ZSM-5 for various stream gases (N₂, H₂ or C₄H₁₀) were studied by¹³C CP MAS NMR,²⁹Si MAS NMR,¹²⁹Xe NMR and catalytic test reactions with different stream gases (N₂, H₂, C₄ H₁₀). The rate of deactivation varies in the order N₂ < H₂ < C₄H₁₀, which can be explained by cracking of paraffinic, olefinic and alkylaromatic coke deposits under the influence of the hydrogenating properties of the stream gas.     

AFM studies of Pd silica supported thin film catalysts

AFM has been used to study the effects of pretreatment gases on Pd/SiO₂ supported thin film catalysts during 1,3-butadiene hydrogenation. The Pd/SiO₂ catalyst, treated with O₂ followed by H₂ at 450°C, has an initial conversion of 85% and a surface morphology of 60 x 65 nm² Pd grains and only deactivates slightly. After a second treatment, the reactivity was fully recovered and the surface morphology exhibits a redispersion of the Pd grains. The catalyst with a similar initial reactivity and morphology but only treated in H₂, shows a decrease in activity and coalescence of Pd grains after repeated treatment. XPS studies have shown that the O₂ and H₂ treated Pd/SiO₂ catalyst has a lower Pd binding energy than the H₂ treated Pd/SiO₂. The effect of the substrate thickness and composition is also reported.     

An enhancement in the photocatalytic activity of TiO2 by the use of Pd: the question of layer sequence in the resulting hierarchical structure

TiO₂ and Pd-modified TiO₂ photocatalysts were prepared by sol–gel method. X-ray diffraction (XRD) data showed that the presence of Pd in TiO₂ catalyst decreases the crystalline size of TiO₂ and stabilizes anatase phase. The study of the photocatalytic activity of the films via Linear Sweep Voltametry (LSV) plots and antibacterial test against Escherichia coli ATCC 25922—a gram negative bacterium—showed that Pd increases the photocatalytic activity of the coatings. Besides, the sequence of layer deposition (TiO₂ and Pd-modified TiO₂) influences the photocatalytic properties. In other words, more photocatalytic activity is obtained when Pd-modified layer is deposited first.     

Allylic substitution catalysed by silica-supported aqueous phase palladium(0) catalysts

The supported aqueous phase methodology was applied to the catalytic system Pd(OAc)₂/5TPPTS, a catalyst precursor of the Trost–Tsuji reaction. The characterization of the solid by ³¹P MAS NMR spectroscopy confirms the reduction by the phosphines of Pd(II) to Pd(0) as Pd(TPPTS)₃. The catalytic properties of this solid were determined for the allylic substitution of (E)-cinnamyl ethyl carbonate by nucleophiles such as ethyl acetoacetate, dimethyl malonate, morpholine, phenol and 2-mercaptopyridine and were compared to those of the same catalytic system in a biphasic water/nitrile medium. Having demonstrated the absence of palladium leaching and having solved the problem of water leaching from the solid into the organic solvent, the SAP Pd catalyst was engaged with success in a continuous flow experiment, demonstrating the possibility to reach a productivity superior to 2200 moles of carbonate selectively converted per mole of palladium within 11 h. This revised version was published online in June 2006 with corrections to the Cover Date.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

EXAFS characterization of bimetallic Pt–Pd/SiO2–Al2O3 catalysts for hydrogenation of aromatics in diesel fuel

Bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts exhibited much higher activities in aromatic hydrogenation of distillates than monometallic Pt/SiO₂–Al₂O₃ and Pd/SiO₂–Al₂O₃ catalysts. The studies of extended X‐ray absorption fine structure (EXAFS) indicated that there was an interaction between Pt and Pd in the Pt–Pd/ SiO₂–Al₂O₃ catalyst. Furthermore, from the EXAFS, it was assumed that the active metal particle on the Pt–Pd/SiO₂–Al₂O₃ catalysts is composed of the “Pd dispersed on Pt particle” structure. Regarding both the activities of aromatic hydrogenation and the EXAFS results, it was concluded that the Pd species dispersed on Pt particles were responsible for the high activity of the bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

A novel SiO2 supported Pd metal catalyst for the Heck reaction

A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO₂ (O)) derived by calcination of Pd(acac)₂/SiO₂ in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd²⁺ is reduced to Pd⁰ on SiO₂ by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd⁰/SiO₂ is highly active for the Heck reaction, the remaining Pd²⁺/SiO₂ is not responsible for the high activity. Pd/SiO₂ (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO₂ (H)) prepared by reduction of Pd(acac)₂/SiO₂ in H₂ as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO₂ (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO₂ (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature.     

Air‐Stable and Highly Active Dendritic Phosphine Oxide‐ Stabilized Palladium Nanoparticles: Preparation,Characterization and Applications in the Carbon‐Carbon Bond Formation and Hydrogenation Reactions

Dendrimer‐stabilized palladium nanoparticles were formed in the reduction of palldium bis(acetylacetonate) [Pd(acac)₂] in the presence of phosphine dendrimer ligands using hydrogen in tetrahydrofuran. The resulting Pd nanoparticles were characterized by TEM, ³¹P NMR and ³¹P MAS NMR. The results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer‐stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the separation and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling reaction, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh₃)₄]. In addition, the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the soluble Pd(0) and/or Pd(II) species leached from the nanoparticle surface.     

Influence of the different dechlorination time on catalytic performances of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

Catalytic dehydrogenation of propane has recently received considerable attention because of the increasing demand for propene. Among several catalysts, PtSnNa/ZSM-5 catalyst is one of the most suitable ones. In this study, PtSnNa/ZSM-5 catalysts with different content of chlorine were prepared by changing the time of catalyst dechlorination. The obtained catalysts were characterized by X-ray fluorescence (XRF), XRD, nitrogen adsorption, ²⁷Al MAS NMR, NH₃-TPD, H₂ chemisorption and TPR. It was found that with the increase of treatment time, more framework aluminum atoms were removed from tetrahedral positions, leading to the loss of Sn species and the decrease of catalyst acidity. Meantime, the porous properties and the interactions between Pt and Sn of the catalysts changed remarkably, which was disadvantageous to the reaction. Compared with the dechlorinated catalysts, the fresh sample with suitable content of chlorine exhibited the best reaction activity and stability. The average yield of propene was about 30.4% over 45 h for the reaction of propane dehydrogenation at 590 °C. Finally, a model was proposed for the influence of dechlorinated treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation.     

Catalytic dehydrogenation of propane on chromia,palladium and platinum supported catalysts

The dehydrogenation of propane to propylene over Cr₂O₃/Al₂O₃, Pd/Al₂O₃ and Pt/SiO₂ has been investigated in the temperature range 580–618°C. Runs were performed on propane, alone or in the presence of nitrogen (as a diluent), with complete analysis of the reaction products. The reaction was carried out in a fixed bed reactor at space velocities from 450–800 h^?1 which are close to industrial values and at pressures from 0.3 to 1 atm. A set of runs was made over a commercial chromia-alumina catalyst (10% Cr₂O₃) and over a promoted catalyst prepared in the laboratory by impregnation (16.8% Cr₂O₃ + 2% K₂O). The latter catalyst showed high selectivity and stability even when subjected to continuous cycles of dehydrogenation, regeneration and purging. Of the two noble metal supported catalysts used, reduced Pd/Al₂O₃ showed higher activity than Pt/SiO₂ at 618°C. The former catalyst gave a propylene yield of around 98% at 20% conversion level.     

Enantioselectivity promotion by achiral surface functionalization on SiO2-supported Cu-bis(oxazoline) catalysts for asymmetric Diels–Alder reactions

Novel SiO₂-supported chiral Cu-bis(oxazoline) (BOX) complexes for asymmetric Diels–Alder reactions were prepared by combining metal-complex immobilization with surface functionalization using achiral silane-coupling reagents on SiO₂. We found that the surface functionalization of a SiO₂-supported Cu-BOX catalyst with achiral 3-methacryloxypropyltrimethoxysilane dramatically increased enantioselectivity in the asymmetric Diels–Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone. The Cu-BOX complexes on bare and functionalized SiO₂ surfaces were characterized by XAFS, ESR, FT-IR, UV/vis, and ²⁹Si solid-state MAS NMR. The large increase in enantioselectivity by achiral surface species without chiral center may be due to a glue effect, creating a new chiral ensemble structure at the surface.     

Environmentally benign catalytic synthesis and hydro‐refining of linear alkylbenzenes

The synthesis technology of linear alkylbenzenes (LABs) was studied through the preparation of an Al‐SBA‐15 catalyst, the optimization of alkylation conditions, and the regeneration of deactivated catalyst. The hydro‐refining over the Pd/Al₂O₃ catalyst was carried out to remove unsaturated hydrocarbon impurities from the LAB. The results of the alkylation reactions over the Al‐SBA‐15 catalyst in a liquid fixed bed reactor showed that the olefin conversion remained above 98 % for time on stream of 3000 h, and the LAB selectivity was above 93 % under the following conditions: temperature of 260 °C; pressure of 5.0 MPa; weight hourly space velocity (WHSV) of 1.0 h^?1; and the molar ratio of benzene to olefin of 25:1. Through the burning coke regeneration, the catalytic performance of the deactivated alkylation catalyst was satisfactorily restored. The quality of the LAB synthesized through alkylation and hydro‐refining was better than that of the industrial LAB produced using the hydrofluoric acid catalytic process.     

Enhanced performance of methane dehydro-aromatization on Mo-based HZSM-5 zeolite pretreated by NH4F

Enhanced performance of methane dehydro-aromatization reaction (MDA) were achieved on a Mo-based HZSM-5 zeolite catalyst in which HZSM-5 were pretreated by a proper amount of NH₄F (Mo/HZ(F)). The results of NH₃-TPD and ²⁷Al MAS NMR demonstrated that the number of Brönsted acid sites decreased on the HZSM-5 zeolite and Mo/HZSM-5 catalyst after NH₄F treatment. TGA and TPO measurements showed that the Mo/HZ(F) catalysts were highly resistant to coke deposition, which resulted mainly from the elimination of the Brönsted acid sites after the pretreatment of the HZSM-5 zeolite with NH₄F.     

Solid phosphoric acid oligomerisation: Manipulating diesel selectivity by controlling catalyst hydration

Solid phosphoric acid (SPA) catalyst is traditionally used in crude oil refineries to produce unhydrogenated motor-gasoline by propene and butene oligomerisation. SPA is also used in High-Temperature Fischer–Tropsch refineries (HTFT) to produce synthetic fuels albeit with a different emphasis. The petrol/diesel ratio of an HTFT refinery is very different from crude refining and it is often necessary to shift this ratio depending on market requirements. The influence of hydration was investigated as a means of improving diesel selectivity. This was achieved by studying SPA over a hydration range of 99–110% H₃PO₄, a temperature range of 140–230 °C and using C₃–C₆ model and synthetic FT-derived olefinic feedstocks. A direct correlation was found between the selectivity towards diesel range products and the distribution of the phosphoric acid species viz. H₃PO₄, H₄P₂O₇ and H₅P₃O₁₀. For various olefinic feedstocks, diesel selectivity increased with decreasing catalyst hydration with a maximum around 108% H₃PO₄ for propene oligomerisation. Commercial tests confirmed the increase in diesel selectivity with lowered catalyst hydration.     

Oxidative Regeneration of Sulfide-fouled Catalysts for Water Treatment

This study tested the stability, activity, and selectivity of an alumina-supported Pd–In bimetallic catalyst during repetitive sulfide fouling and oxidative regeneration conditions. Nitrate reduction with hydrogen was used as the probe reaction in a continuous-flow packed-bed reactor to assess changes in the catalyst structure as a result of the fouling and regeneration processes. Partial regeneration of a severely sulfide-fouled Pd–In catalyst was achieved with a NaOCl/NaHCO₃ solution. However, the regenerated catalyst had a reduced activity for NO₃ ^? reduction and increased selectivity towards NH₃. Analysis of the catalyst bed after regeneration experiments using XPS, ICP-MS, and BET surface area revealed that bulk structural transformations of the Pd–In bimetallic catalyst occurred, as a result of preferential Pd dissolution near the column influent. The dissolved Pd showed limited mobility in the column, and was re-deposited on the catalyst, resulting in Pd enrichment on the catalyst surface and redistribution of Pd towards the end of the column. These changes along with residual sulfur content on the catalyst surface were likely responsible for the increased selectivity towards NH₃. These results indicate the importance of limiting the exposure of reduced sulfur species to Pd-based catalysts, especially when treating contaminants like NO₃ ^?, where product selectivity is a priority.     

The role of surface vanadia species in butane dehydrogenation over VOx/Al2O3

A series of θ-Al₂O₃ supported VO_x catalysts, of different vanadium loadings, have been characterised and employed for the selective dehydrogenation of n-butane. Characterisation of the unreacted catalysts has been carried out by solid-state NMR (⁵¹V MAS NMR, ²⁷Al MAS NMR and ²⁷Al 3Q-MAS NMR), and FT-IR spectroscopies, with reference to previously acquired Raman and UV–vis spectroscopy data. As vanadium loading increases, so does the domain size of the supported vanadate units with significant quantities of V₂O₅ observed at the highest loadings. The influence of calcination, pre-reduction, reaction and regeneration on the structure of the catalysts has been studied by NMR, FT-IR, EPR, microanalysis and TEOM. Calcination disperses crystalline vanadate units, and at high loadings AlVO₄ formation is observed. Pre-reduction reduces the vanadium oxidation state from 5+ to 3+, while regeneration results in the formation of highly crystalline V⁵⁺ species. From these data it is possible to determine structure–activity relationships, with polymeric vanadia clusters favouring the formation of butenes and butadienes, while more isolated species are highly active towards the formation of polynuclear aromatic hydrocarbons retained on the catalyst surface post-reaction. These large polynuclear aromatic hydrocarbons are however not the principle cause of catalyst deactivation in this reaction.     

Impregnated carbon based catalyst for protection against carbon monoxide gas

Activated carbon based palladium impregnated catalyst (Pd/C) was prepared for the reactive removal of carbon monoxide (CO) gas under ambient air conditions. For this, active carbon of 1250 m²/g surface area was impregnated with palladium salt to get Pd/C catalyst, containing palladium from 4.0 to 8.0% (w/w). Catalytic efficiency of the catalyst against CO gas was determined under dynamic conditions by passing CO–air mixture to the fixed bed of the Pd/C catalyst. Results indicated that Pd/C catalyst was continuously adsorbing and actively removing CO gas during the course of the palladium catalyzed reaction, i.e., CO + 1/2 O₂ → CO₂ and was found capable of providing excellent protection against CO gas. Moisture content (humidity) of inlet CO–air mixture indicated it to be an important factor affecting the CO removal efficiency of the catalyst, as an increase in humidity after the CO breakthrough resulted in to the activation of the catalyst due to the generation of hydroxyl groups and enhanced protection by the regeneration of the catalyst. Study indicated that Pd/C catalyst works as a catalytic converter, i.e., the continuous conversion of CO to CO₂ using atmospheric oxygen and moisture. In order to determine the shelf life, the Pd/C catalyst was also evaluated for its performance after accelerated ageing at 70 °C and 50% relative humidity (RH) for 3.75 and 7.5 months. The catalyst was found to be working efficiently for 3.75 months but after 7.5 months it could not provide 100% protection against CO gas, however, the same catalyst started giving 100% protection after regeneration. Hence, studies indicated the Pd/C catalyst to be a promising catalyst for the reactive removal of CO gas in enclosed spaces/compartments, coal mines, fire accidents and for getting the protection for longer duration under ambient air conditions.