M. V. Tsodikov

Study of cumene oxidation over zirconia-, titania- and alumina-based complex oxides obtained by sol-gel methods: activity-structure relationships

Abstract Activity-structure relationships have been studied in the low temperature liquid phase cumene oxidation over Fe O/ZrO 2 , Fe O/TiO 2 and Fe O/Al 2 O 3 complex oxides obtained by sol-gel methods. The study has been carried out by the measurements of overall kinetics, product distribution and by X-ray diffraction and Mossbauer spectroscopic methods. Kinetic study in the presence of homogeneous initiator (azo-bis-diazobutyronitrile) has led to the conclusion that the active surface of complex oxides participates in chain initiation most probably via R H bond rupture. Three groups of complex oxides are described: (1) two-phase polycrystalline zirconia-based catalysts showing high activity at low iron loading; (2) one-phase polycrystalline zirconia- and titania-based catalysts exhibiting either activity drop at some critical iron content (zirconia) or monotonic activity dependence (titania) on iron loading; (3) amorphous alumina-based catalysts containing γ-ferric oxide clusters and showing monotone activity dependence. Electronically excited terminal Fe O groups related to the surface FeO 6 polyhedra and iron-containing species located at the interface have been suggested as active centers acquiring the anion-radical behavior. When comparing thermodynamically stable titania (anatase) with metastable one-phase zirconia (cubic or tetragonal) as host matrices, the suggestion was introduced that greater activity of zirconia-based catalysts is due to more energy transfer from the host matrix to the particular active center.

Low-temperature formation mechanism of double oxides FexZr(Ti)1−0.75xO2−δ prepared from alkoxides and acetylacetonates

As a result of titanium and zirconium alcoholates hydrolysis in the presence of dissolved Fe(acac)3, amorphous iron-containing gels have been synthesized. Their heat treatment has led to polycrystalline double oxides FexZr1−0.75xO2−δ (C) and FexTi1−0.75xO2−δ (T) formation. It has been shown that oxides (C) and (T) are likely to be solid solutions with 0.01<x<0.17 and 0.01<x<0.14, respectively. On the basis of X-ray diffraction and extended X-ray absorption fine structure data, iron-zirconium and iron-titanium crystallite models for gels and oxides have been proposed. It has been found that the crystallization process does not lead to a significant change in interatomic distances typical for local structures detected in gels.

Cocatalytic effect of palladium and zinc in the condensation of alcohol carbon backbones into hydrocarbons

The results of the direct conversion of ethanol and its mixture with glycerol into a C4–C10+ alkane and olefin fraction in the presence of Pd-, Zn-, and Pd-Zn-containing catalysts, which were prepared by supporting homo- and heterometallic acetate complexes onto the surface of γ-Al2O3, are reported. It was found that, in the presence of mono- and bicomponent Pd-ZnO (Pd; ZnO)/γ-Al2O3 systems, selectivity in the formation of alkanes, olefins, or their mixtures in the target fraction can be controlled as a result of the cocatalytic effect of active components that are responsible for the catalyst activity in condensation and hydrogenation reactions. The structures of the active components were studied and the genesis of the catalytic systems was characterized using XAFS, XPS, and XRD analysis. It was found that the addition of glycerol considerably increased the yield of the target hydrocarbon fraction.

Asymmetric Effects in Catalytic Membranes

A catalytic membrane hybrid system based on a cermet membrane with a channel size 〈d〉 of ∼0.12 μm has been produced using sol-gel processing. A layer of a superfine methanol conversion catalyst with the composition Cr2O3 · Al2O3 · ZnO has been formed on the inner surface of the channels, and a thin oxide coating of composition P0.03Ti0.97O2 ± δ with a homogeneous porous structure and 〈d〉 ∼ 2 nm has been formed on the geometric membrane surface. The methanol conversion rate and the gas permeability of the membrane depend considerably on the methanol vapor and gas (H2, He, CO2, Ar, CH4) flow directions. When methanol vapor diffuses toward the mesoporous layer, the catalytic activity is one order of magnitude higher and the gas permeability coefficients are 3–8 times lower than in the case of the reverse flow of the gaseous molecules. The temperature dependence of the gas permeability taking into account the possible types of mass transfer in porous solids suggests that, when the gases move toward the mesoporous coating consisting of phosphorus-modified titanium oxide, surface flow and activated diffusion dominate, whereas the reverse gas motion is dominated by free molecular flow.

Alumina-platinum catalyst in the reductive dehydration of ethanol and diethyl ether to alkanes

The reductive dehydration of ethanol and diethyl ether selectively occurs with the formation of alkanes to C10+ on an AP-64 alumina-platinum catalyst (0.6 wt % Pt/γ-Al2O3) after its reduction with hydrogen at 450°C for 12 h in Ar. It was found that one of the main reaction paths is the insertion of ethylene into substrate intermediates with the predominant formation of normal alkanes. It was found by XAFS spectroscopy that Pt2Al intermetallide particles were formed along with platinum metal clusters after long reduction. The ammonia TPD data indicated a change in the acid properties of the surface after the long reduction of the catalyst: the concentration of medium-strength surface aprotic acid sites increased by a factor of 2. It was found that the interaction of aprotic sites with water vapor resulted in the formation of strong proton acid sites. It is likely that these latter are responsible for the growth of a carbon skeleton in the course of alkane formation from ethanol.

Selectivity and mechanism of cumene liquid-phase oxidation in the presence of powdered mixed iron-aluminum oxides prepared by alkoxy method

Abstract Sol–gel chemistry routes using metallo-organic complexes as precursors were used to prepare iron–aluminum catalysts for low temperature liquid phase cumene oxidation. Catalytic and structural properties of Fe–O/Al 2 O 3 catalysts were studied by kinetic measurements combined with other techniques in order to obtain information about the nature of catalytic active centers and the selectivity of the cumene liquid-phase oxidation in the presence of single and mixed oxides. Our results allow us to expect a bifunctional role of the Fe–O/Al 2 O 3 surface as a combined participation of Lewis acid and terminal oxygen centers by a proposed mechanism which is known for homogeneous catalysis and is non-typical for heterogeneous catalytic oxidation.

Catalytic conversion of rape oil into alkane-aromatic fraction in the presence of Pd-Zn/MFI

Results on the conversion of rape oil into the alkane-aromatic fraction in the presence of a prototype of a MFI-based industrial catalyst (Si/Al = 30) promoted with 0.6 wt % Pd and 1 wt % Zn are presented. It has been shown that an increase in the process temperature from 360 to 420°C leads to a significant increase in the yield of aromatic compounds, and an increase in the substrate space velocity leads to a three-fold increase in the yield of C4–C6 alkanes of primarily the iso-branched structure. The genesis of active Pd and Zn clusters is discussed on the basis of X-ray data.

The nature of permeability anisotropy and catalytic activity

The phenomena of permeability anisotropy and an increase in the rates of catalytic reactions in porous membranes modified with highly dispersed catalytic systems were analyzed. A model of stochastic gas motions was proposed; this model is based on the hypothesis of the specific interaction of molecules with the inner surface of pores resulting in a nonisotropic distribution of molecules over traveling directions. The effects of asymmetric gas transfer in porous and gradient-porous membranes were considered to explain differences in the rates of heterogeneous catalytic reactions in a nanoporous membrane reactor under changes in the direction of supplying a reaction mixture. From the model proposed, it follows that the transversal diffusion of gas molecules is most probable in the porous medium of a ceramic membrane with a pore-size distribution gradient from large to small pores along the flow direction. This diffusion results in an increase in the frequency of molecular collisions with the wall of a microchannel and, correspondingly, in an increase in the contact time. The model proposed explains the intensification of a number of heterogeneous catalytic reactions performed in the porous media of catalytic porous membranes.

Conversion of biomass products to energy sources in the presence of nanocatalysts and membrane-catalyst systems

A particular role in the harmonious exploitation of raw materials is assigned to organic sources of fuels based on renewable biomass. The most promising feedstocks of major energy sources, such as hydrogen and organic components of motor fuels, include ethanol and other bioalcohols, i.e., the primary products of its conversion. In this work, we describe the results for new reactions of conversion of ethanol and a mixture of ethanol and glycerol, which are the major products of biomass, to the C3–C10 alkane-olefin fraction in the presence of nanoscale mono- and bimetal-containing active components supported on γ-Al2O3 (〈d〉 = 5–8 nm) and on the inner surface of microchannels of ceramic membranes (〈d〉 = 15–20 nm). Mono- and bimetallic alkoxide and acetate complexes are used as precursors. It is found that the selectivity for the ethanol conversion to aliphatic hydrocarbons, as well as the content of branched structures, heavily depends on the nuclearity and composition of metal-complex precursors supported on γ-Al2O3. It is found for the first time that glycerol exhibits high reactionary ability in the reaction of cross-condensation of the carbon skeleton of alcohols of different nature. In the presence of a Ta-Re-containing system, a mixture of ethanol and glycerol is converted to 60% of C4–C10+ olefins, which contain up to 50% of branched structures. It is shown that by varying the composition of Pd-Zn-containing active components, it is possible to targetedly convert ethanol to the olefin, alkane, or alkane-olefin fraction. Porous membrane-catalyst systems are designed to produce hydrogen and syngas from biomass products; the systems exhibit high activity in the carbon dioxide and steam reforming of ethanol, a mixture of ethanol and glycerol, and acetic acid. A scheme for the production of a wide range of valuable organic products based on bioalcohols containing no toxic impurities and independent of crude oil is described. According to this scheme, alkanes derived from ethanol and other bioalcohols are the major components of motor fuels; a large number of organic synthesis products can be derived from olefins, hydrogen, and carbon monoxide in the carbonylation/hydrocarbonylation processes.

Aromatization of propane: Techno-economic analysis by multiscale "kinetics-to-process" simulation

Abstract This paper addresses the techno-economic analysis of the propane aromatization process, by adopting a novel kinetics-to-process approach. The recent interest in this technological route derives from the development of new third generation biorefinery concepts, in which, algal oil is subjected to catalytic hydrodeoxygenation processes for the production of (Hydrotreated Renewable Jet) HRJ fuels. Beside biofuels, co-production of large amounts of propane is observed, which can be upgraded by a catalytic conversion to aromatics on zeolites. Kinetic studies of propane aromatization over H-ZSM-5 zeolite in a wide range of conversions are reported in the literature. Based on these results, a general kinetic model of propane aromatization has been developed. The revised kinetic scheme is then embedded in a process simulation, performed with the commercial code SimSci PRO/II by Schneider Electric. Basing on the process simulation and on available price assessments, a techno-economic analysis has been performed to show limits as well as potentialities of the proposed layout.