Showing papers in "Topics in Catalysis in 1996"

Catalytic conversion of methane to benzene over Mo/ZSM-5

Abstract: Dehydroaromatization of methane to benzene occurs over a 2 wt% Mo/ZSM-5 catalyst at 700‡C under non-oxidizing conditions. Following an initial induction period, during which CH4 reactant reduces the original Mo6+ ions in the zeolite to Mo2C and deposition of coke occurs, a benzene selectivity of ∼ 70% at a CH4 conversion of 8–10% could be sustained for more than 16 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements indicate that the reduced Mo is highly dispersed in the channels of the zeolite. Initial activation of CH4 reactant occurs on Mo2C sites, leading to the formation of C2H4 as the primary product. The latter then undergoes subsequent oligomerization reactions on acidic sites of the zeolite to form aromatic products.

Synthesis and characterisation by X-ray absorption spectroscopy of a suite of seven mesoporous catalysts containing metal ions in framework sites

Abstract: MCM-41 type mesoporous silicas have been prepared in which one or more of the following elements are accommodated in framework sites: titanium, iron, chromium, vanadium, manganese, boron and aluminium. XRD and FTIR are used as aids to characterisation, which is achieved chiefly — and to a degree that arrives at valence states, bond lengths and coordination numbers of the metal ion — by X-ray absorption spectroscopy (XAS). Ti-containing MCM-41, as well as the Fe-, V- and Cr-containing variants, yield self-consistent, XAS-based, structural data of the respective metal-ion sites. Some of these (especially those containing Ti) are exceptionally good catalysts for the selective oxidation of large organic molecules such as limonene and norbornene.

CO2 reforming and partial oxidation of methane

Abstract: CO2 reforming and partial oxidation of CH4 were investigated on different supported noble metal and Ni catalysts. A detailed thermodynamic analysis was performed for both reactions. The observed reaction behaviour can be predicted by thermodynamics. Product selectivity is catalyst independent, the role of the catalyst is to bring the reactants to approach equilibrium. The partial oxidation is a two-stage process, total oxidation of CH4 is followed by CO2 and H2O reforming of the remaining CH4. A staged addition of O2 to the reactor is tested and recommended. TPSR show that the catalyst surface for CO2 reforming was highly covered with carbonaceous species of four different types; two were identified as reactive intermediates.

Titania-silica aerogels with superior catalytic performance in olefin epoxidation compared to large pore Ti-molecular sieves

Abstract: The paper provides a short overview on the various Ti- and Si-containing epoxidation catalysts, with a focus on the development and characterisation of titania-silica mixed oxides prepared by the sol-gel-aerogel method. Titania-silica aerogels, dried by semicontinuous extraction of the solvent with supercritical CO2 at low temperature, possess high surface area, an amorphous mesoporous structure, and a rather even distribution of Ti in the silica matrix. The catalytic activities of mixed oxides, silica-supported titania and Ti-substituted molecular sieves (TS-1, Ti-Β and Ti-MCM-41) are compared in the epoxidation of 1-hexene, 1-octene, cyclohexene, cyclododecene and norbornene, based on literature data. The activities (i.e. the amount of converted olefin per unit time and catalyst weight) vary in a broad range of several orders of magnitude. It is shown that the low temperature aerogel containing 20 wt% TiO2 is superior to any other Ti- and Si-containing catalyst for the epoxidation of cyclic olefins. In the epoxidation of linear alkenes TS-1 has outstanding activity. Ti-substituted large and ultra-large pore molecular sieves require further development concerning their catalytic performance.

Oxidative dehydrogenation of isobutyric acid: Characterization and modeling of vanadium containing polyoxometalate catalysts

Abstract: The catalyzed oxidative dehydrogenation of isobutyric acid by H4[PMo11VO40] and VOH[PMo12O40] was studied at 593 K. Thermal evolution of H4[PMo11VO40] at 593 K leads to a complex mixture of different species with or without vanadium inside the Keggin structure. After reaction, the one and only heteropolyanion is partly reduced [PMo12O40]3− and vanadium is present as vanadyl cations. Vanadyl cations are bound to the oxygen atoms of the heteropolyanion at 593 K and enhance the selectivity to methacrylic acid with respect to acetone. On the other hand, copper cations improve the reoxidation of the catalyst and increase the IBA conversion. Synergism between copper and vanadyl cations was observed.

On the influence of the acid-base character of catalysts on the oxidative dehydrogenation of alkanes

Abstract: Vanadium oxides supported on metal oxide, i.e. Al2O3, MgO and Mg-Al mixed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5) have been tested in the oxidative dehydrogenation of C2-C4 alkanes. In all cases, tetrahedral vanadium species (isolated and/or associated) were mainly observed from51V-NMR and diffuse reflectance spectroscopies. The reducibility of V5+-species, determined from the onset-reduction temperature, decreases as follows: VOx/AL > VAPO-5 > MgVAPO-5 =VOx/MG > VOx/MG + AL. The acid character of catalysts, determined from the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VOx/AL > VAPO-5 > VOx/MG + AL > VOx/MG. A similar trend between V-reducibility of the catalyst and its catalytic activity for the alkane conversion was observed. However, the selectivity to olefins depends on the acid-base character of catalyst and the alkane fed. In the ODH ofn-butane, the higher the acid character of the catalyst the lower the selectivity to C4-olefins, while in the ODH of ethane an opposite trend between the catalyst acidity and the selectivity to ethene was observed.

Effect of alkali metals additives to V2O5/TiO2 catalyst on physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

Abstract: The effect of alkali metal additives Li, K, and Rb to V2O5/TiO2 catalyst on the rate of catalyst reduction with propane and reoxidation with oxygen, sorption of propene, and the electron work function has been examined The results have been correlated with the catalytic performance in oxidative dehydrogenation, ODH, of propane It has been found that the rates of reduction, reoxidation and the ODH of propane decrease in the order: VTi>LiVTi>KVTi>RbVTi The activation energies of the reduction and reoxidation are not, however, affected by the presence of the alkali metals The same sequence has been observed for the work function values of the catalysts It is argued that alkali metal poisons the centres of the hydrocarbon activation The yield and selectivity to propene in the ODH of propane increase, however, for the promoted catalysts, following the above sequence This effect is ascribed to the decrease in the heat of the propene adsorption, which is due to the increase in the basicity and decrease in acidity on the promoted catalysts

Active sites in methanol oxidation on Cu(110) determined by STM and molecular beam measurements

Abstract: STM has been combined with molecular beam rate measurements to gain an understanding of the oxidative dehydrogenation of methanol to produce formaldehyde, both at the macroscopic and microscopic level. From this a model of the reaction is developed where the methanol initial reacts at very few active oxygen sites located at the short sides of oxygen islands on the Cu(110) surface. Such sites are very much rarer on a surface which is saturated with 0.5 monolayers of oxygen and the reaction rate is initially very low, but shows autocatalytic behaviour, rising in time as vacancies are created in the oxygen layer.

Ammoximation of ketones on titanium silicalite. A study of the reaction byproducts

Abstract: In the new EniChem process cyclohexanone oxime is directly synthesized by ammoximation of cyclohexanone with ammonia and hydrogen peroxide on titanium silicalite catalyst. The ammoximation reaction is suitable for the synthesis of several oximes by reaction of the corresponding ketones with ammonia and hydrogen peroxide on titanium silicalite. New results on ketones ammoximation and on reaction byproducts are reported. The effect of some reaction parameters on the oxime yield and on byproducts formation is discussed.

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Abstract: Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co0.95MoO x and Mg0.95MoO x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.

Adaptation of the microscopic properties of redox catalysts to the type of gas-solid reactor

Abstract: Catalysts of selective oxidation usually work in a simultaneous redox mode in reactant/air cofed reactors. The solid must provide selective lattice oxygen according to a kinetic mechanism depending on operating conditions that differ from one reactor to another. Better catalytic performance can be obtained in a recirculating solids reactor because it allows separate optimization of the reduction and oxidation steps. Among the microscopic properties of the catalyst, the crystal morphology is to be taken into account because it influences its reactivity on stream. These considerations lead to a new approach of the catalyst-reaction-reactor trio.

A density functional study of molecular adsorption in zeolites

Abstract: Non-local density functional theory has been used to investigate the adsorption of water, methanol and ammonia on cluster representations for zeolites. Only in the case of ammonia is the chemisorbed ion pair complex found to be a minimum, while binding energies are close to the lower bounds of experimental values. Large frequency shifts are found for hydroxyl stretching modes, particularly as the loading of adsorbate is increased.

Reaction pathway of benzonitrile formation during toluene ammoxidation on vanadium phosphate catalysts

Abstract: The reaction pathway of the ammoxidation of toluene on (VO)2P2O7 used as catalyst and the interaction of potential intermediates with the pyrophosphate were studied by spectroscopic techniques (FTIR, EPR), temperature-programmed chemisorptions/ reactions (TPD, TPRS) and transient studies such as the temporal analysis of products (TAP) technique. NH3 is chemisorbed on the catalyst surface, forming three different species, i.e., NH4+ ions located on BrOnsted sites, coordinatively bound NH3 on Lewis sites and NH2− groups, presumably P-NH2. Toluene that is probably adsorbed on Lewis sites reacts in a first step to a benzyl radical. A subsequent partial oxidation by interaction of VIV=O groups generates a V...O=CH-C6H5 surface structure. This benzaldehyde-like surface species reacts with adsorbed NH3 according to a Langmuir-Hinshelwood mechanism. TAP experiments on ammonium-containing vanadium phosphates revealed that NH4+ ions could act as potential N-insertion species. No formation of benzylamine as well as the generation of V=NH surface groups as possible intermediates or N-insertion sites were observed.

Oxidation catalysis with well-characterised vanadyl bis-bipyridine complexes encapsulated in NaY zeolite

Abstract: Vanadyl exchanged faujasite (VO2+-NaY) allows bipyridine complexation, giving a heterogeneous“ship-in-a-bottle” catalyst denoted as [VO(bpy)2]2+-NaY. The [VO(bpy)2]2+ complexes associated with the zeolite are characterised with FT-Raman, FT-IR, XPS, and diffuse reflectance spectroscopy (DRS), as well as with electron paramagnetic resonance (EPR). It is established that the cationic complexes are intrazeolitic and homogeneously distributed across the zeolite crystals and both the zeolite and the neutral bipyridine ligands stabilise VIV. The catalytic oxidation of cyclohexane and cyclohexene with different peroxides or mono-oxygen atom donors in presence of several solvents is described. Good epoxide selectivity results from the complexation by bipyridine thus favoring the heterolytic over the homolytic decomposition pathway of V-peroxo-intermediates.

Catalysis by molecular design

Abstract: The possibilities of molecular design in the development of new catalysts and catalytic technologies are discussed with the data obtained recently at the Boreskov Institute of Catalysis as particular examples Examples from the following areas are presented: homogeneous catalysis with metal complexes, heterogeneous catalysis with anchored metal complexes, heterogeneous catalysis with catalysts prepared via anchored metal complexes and organometallics, catalysis of olefin polymerization, catalysis by metals, catalysis by oxides, catalysis by zeolites, catalysis by heteropolyacids, catalysis with nontraditional oxidants and biomimetic catalysis

Solid-state NMR as a probe of porous catalysts and catalytic processes

Abstract: The power of solid-state NMR for the interrogation of porous catalytic materials is illustrated using three examples. First, for the investigation of catalytic processes occurring within the confines of a microporous catalyst NMR is shown to reveal both the details of shape-selectivity and the nature of internal surface species. Second, NMR is shown to be a powerful short-range tool to reveal precise structural information on highly disordered microporous titanosilicates. Despite long-range disorder the short-range order is maintained and can be easily studied. Finally, the same utility of probing short-range chemical phenomena is shown to be crucial for the investigation of novel-ordered-amorphous-mesoporous materials known generically as M41S. This class of material is currently one of the most important with potential catalytic application.

Ammoximation reaction with titanium doped amorphous silica

Abstract: The catalytic behavior and the role of the acidic sites and of the activated forms of molecular oxygen in the ammoximation reaction of cyclohexanone to cyclohexanone oxime were investigated on pure and titanium doped amorphous silicas. The samples were prepared by the sol-gel method under acidic and basic conditions. The introduction of low amounts of titanium on silicas, prepared under basic conditions, strongly increased the oxime yield and conversion. The sample containing 0.25% titanium resulted in a very active and selective catalyst. A further increase of the titanium content decreased the catalytic activity and also the catalyst deactivation, because of a decrease of the tars formation rate. The comparison of the catalytic data and the results of FT-IR and EPR analysis confirmed the bifunctional nature of the amorphous silica catalyst. On the first step of the reaction pathway, the imine formation, the paramount parameter is an intrinsic property of pure silica, related to its textural and structural features. The adsorption experiments showed that the silica prepared under basic conditions, the only active in ammoximation, was able to form surface imine on acidic sites. The nature and the role of the oxidizing sites seem more complex. EPR results showed the presence of radical species on titanium modified silica and also, in small amount, on silica itself; however, the oxime formation did not seem to be a property related only to the presence of activated oxygen species anchored to titanium sites. It has been proposed that the formation of oxime is due to a combined effect of the presence of tars and of the activated oxygen species due to the titanium, which are able to oxidize the imine to oxime.

Alkene oxidation catalyzed by transition metal substituted Keggin-type heteropolyanions

Abstract: Alkene oxidations with various oxidants (tert-butyl hydroperoxide, iodosylbenzene and molecular oxygen in the presence of isobutyraldehyde (IBA)) catalyzed by transition metal monosubstituted heteropolyanions, PW11MO 39 − (PW11M; M=CoII, MnII, CuII, TiIV, RuIV, VV and NbV), have been studied. Orders of catalytic activity of PW11M are different for the oxidants studied. Radical chain mechanisms are proposed fort-BuOOH and O2/IBA. Preliminary coordination of the oxidant to PW11M is not a necessary step of its homolytic activation. Epoxidation with PhIO requires its coordination to the catalyst and most likely includes the formation of active metal-oxo species.

Thermal stability and catalytic properties of the Wells-Dawson K6P2W18O62·10H2O heteropoly compound in the oxidative dehydrogenation of isobutane to isobutene

Abstract: The thermal stability of the Wells-Dawson heteropoly compound K6P2W18O62·10H2 was examined under both reducing and oxidizing conditions, and its structural and morphological evolution was characterized by several complementary solid state techniques. It is shown that the primary structure of this compound remains intact up to 770 K, while at higher temperature structural changes and rearrangements are observed. These modifications depend considerably on the treatment conditions and catalyst composition. Under oxidizing conditions the Wells-Dawson compound rearranges with formation of a mixed phase containing the Keggin-type unit K3PW12O40 and the hexatungstate K2W6O19. Furthermore, the catalytic activity in the oxidation of isobutane is affected considerably by these changes. The best catalytic performance was shown by the rearranged Wells-Dawson compound.

Catalysis and plasma chemistry at solid surfaces

Abstract: Heterogeneous and homogeneous non-equilibrium plasma chemistry offers a range of potential advantages compared to conventional thermal activation and synthesis methods. Some examples of how such low temperature electrical discharges can be effectively utilised for selective chemical reaction pathways are given. These include the fluorination of C-H bonds, the synthesis of polyozonides, selective ring opening polymerisation, and the conversion of alkali metal halides to nitrates using air.

Investigation of the molybdenum-vanadium-niobium (oxidic) system and its phases for their catalytic effects

Abstract: Crystalline phases of a catalyst have been identified and prepared in pure form. Their properties are presented in a comparable way to those of tie catalyst. Activity measurements have been performed for the catalytic accelerated reactions of ethane and air to ethene and acetic acid.

Evidence for the instability of surface oxygen at the Zn(0001)-O-Cu interface from core-level and X-ray induced Auger spectroscopies

Abstract: By monitoring the O(1s) and Zn(LMM) Auger spectra it has been shown that the deposition of copper atoms at a Zn(0001)-O overlayer results in the desorption of oxygen with simultaneous reduction of Zn2+ to Zn0 at 300 K. The surface concentrations of oxygen and copper adatoms are calculated from the O(1s) and Cu(2p) intensities while the X-ray induced Auger Zn(LMM) transition provides evidence for Zn2+ and its reduction to Zn0. The driving force for oxygen desorption is suggested to arise from the formation of a copper-zinc intermetallic brass-like overlayer which has little affinity for oxygen.

Reflections on applications of NMR in heterogeneous catalysis and surface chemistry

Abstract: The use of NMR to characterise heterogeneous catalytic systems and processes is assessed, critically, in overview. The generally wide scope and applicability of NMR is placed in the context of the constraints of NMR experimental requirements and the general goal of studying catalytic systems under conditions relevant to their actual use. In particular, the issues of sensitivity, resolution and dynamic processes in NMR are considered alongside the aims of surface selectivity, sample environment control and the desirability of associated characterisation of the catalytic systems by methods complementary to NMR.

A model of the interaction of ionic tips with ionic surfaces for interpretation of scanning force microscope images

Abstract: Scanning force microscopy (SFM) is an increasingly popular tool in surface studies. With the promise of lateral as well as vertical atomic resolution, its use is sure to become widespread in fields such as catalysis and crystal growth. However, the interpretation of the observed images is still unclear and therefore theoretical models are very important for an understanding of the imaging mechanism. We present a review of our recent calculations on the interaction between ionic tips and ionic surfaces and its effects on the scanning process. Our theoretical model of the SFM experiment combines an atomistic treatment of the interaction between a crystalline sample and the nanoasperity at the end of the tip with a semiempirical treatment of the mesoscopic van der Waals attraction between tip and surface and the macroscopic parameter of cantilever deflection. These static calculations based on total energy minimisation were used to determine the surface and tip geometries and energy as a function of tip height at each point of a scan. Scanlines of the perfect (001) surfaces of LiF and MgO were studied at different constant vertical forces exerted on the tip with and without jump to contact. Although scanlines showing lattice periodicity were obtained in some cases, the calculations demonstrate that the tip-surface interaction is a collective phenomenon with the tip probing several rows of surface ions. The calculations demonstrated a wide range of deformations of the tip and sample during a surface scan, and the (often reversible) interchange of material between the tip and sample. The conditions required for the resolution of point defects as well as the mechanism of friction are discussed.

Creating chiral centres in zeolite Y by the introduction of R-1,3-dithiane 1-oxide as a modifier: Computer simulation of the modifier stability

Abstract: In an earlier paper we have shown that R-1,3-dithiane 1-oxide adsorbed in zeolite Y is able to dehydrate the S enantiomer of butan-2-ol at a rate of up to 19 times greater than the R enantioner. Thus a heterogeneous enantioselective catalyst has been produced by the introduction of a chiral modifier into an achiral zeolite. The modifier remains stable at temperatures of up to at least 180°C. In this paper we present results of computer simulation work on this system which suggest that the observed stability of the R-1,3-dithiane 1-oxide in the zeolite is due to proton transfer between the zeolite and the molecule. We suggest that the molecule is present as a counter cation to the framework. A combination of energy minimisation, Monte Carlo and molecular dynamics techniques are used to study the zeolite Y and R-1,3-dithiane 1-oxide system. Consistent force field potentials are used to describe both the zeolite framework and the adsorbed molecule.

Simulating the structures of crystals and their surfaces

Abstract: We review the application of computer simulation techniques to modelling the bulk and surface structures of complex materials. We show how recent developments have led to an increasingly predictive role for such calculations. We describe the use of simulations in modelling the adsorption of molecules on surfaces in problems of direct relevance to current catalytic studies.

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

Abstract: FTIR spectra are reported of CO2 and CO2/H2 on a silica-supported caesium-doped copper catalyst. Adsorption of CO2 on a “caesium”/silica surface resulted in the formation of CO2− and complexed CO species. Exposure of CO2 to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm−1. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H2, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.

Photoluminescence properties of La2O3 methane coupling catalysts

Abstract: La2O3 catalysts prepared at 923 K (La2O3-LT) and 1073 K (La2O3-HT) exhibit different photoluminescence properties due to notably different concentrations of ions in position of low coordination at the surface or coordinatively unsaturated surface sites (cus). The catalyst which exhibits the higher yields of photoluminescence due to the higher concentration of cus corresponds to the one which gives the higher C2+ selectivity in the oxidative methane coupling reaction.