The Chemical Engineering Journal and the Biochemical Engineering Journal最新文献

Heteropoly compounds are receiving increasing interest as environmentally compatible solid acid catalysts. Partial Cs⁺ exchange into 12-tungstophosphoric acid converts the water soluble, low surface area (< 5 m² g⁻¹) parent acid into submicron water-insoluble Brønsted acids with surface areas exceeding 100 m² g⁻¹. The insolubility of the Cs acid salt makes preparation of large particle supported catalysts by conventional impregnation procedures impractical, although applications in packed bed or slurry reactors require millimeter-sized pellets to avoid pressure drop or filtration problems. Here, we report on a new but simple technique that produces an eggwhite distribution of active Cs-acid salt of 12-tungstophosphoric acid within a large silica extrudate by carrying out a reaction deposition directly inside the extrudate. We examine these catalysts in aromatic alkylation reactions.

Acidic properties (acid sites' strength and concentration) of three series of mixed oxide catalysts (titania-silica, zirconia-silica and alumina-silica) were analyzed by proton affinity distributions. Catalytic properties under mild dehydration conditions (1-butene isomerization at 423 K) of these materials were also evaluated. A Brønsted-type correlation was found between first-order apparent isomerization rate constants and acid strengths of certain components of the pK spectra of the catalysts. This was explained in terms of proton transfer processes that take place at both wet and dry interfaces, i.e. at the surface hydroxyl layer. The results were discussed in terms of Fourier transform infrared data which indicate the presence of large surface hydroxyl populations on all catalysts under the reaction conditions.

In this paper, The CuCeO₂ system is chosen as a model catalyst and CO oxidation is used as a probe reaction to illustrate the strong interaction and symergism observed when transition metal-promoted fluorite oxides are used as total oxidation catalysts. It was found that only small amounts of copper are sufficient to promote the catalytic activity of CeO₂ by several orders of magnitude, while excessive amounts of copper (Cu/(Cu+Ce) > 0.05) are detrimental to the catalyst thermal and hydrothermal stability. Heating the catalyst to temperatures over 800 °C has a significant impact on the catalytic activity because of crystal growth of cerium oxide, loss of surface oxygen, and copper aggregation.

Recent progress in the understanding of the effect of preparation variables on the properties of oxide materials, either to be used as catalysts or catalyst supports, are summarized. The methods include sol-gel technique, use of surfactant to prepare mesoporous and related materials, equilibrium adsorption technique, preparation of hydrotalcite materials, and use of preformed multicomponent complexes. The control of the properties of the oxide material, such as pore size, surface area, and homogeneity by controlling the preparation variables in different methods, and the chemical processes involved in these methods are discussed.

Nanocrystalline materials are associated with high surface area and reactivity. Gas condensation techniques have been developed to synthesize such systems with a unique control of stoichiometry and dispersion. The high concentrations of oxygen vacancies and surface adsorbed species in nanocrystalline CeO_2−x were found to greatly enhance catalytic activity in SO₂ reduction and CO oxidation. By having an ultrahigh dispersion of Cu on nanocrystalline CeO_2−x, we have further demonstrated the possibility of tailoring intimate synergistic effects between different components in a nanocomposite material. The resulting supported base metal system has an unusually high thermal stability and could selectively catalyze SO₂ reduction and CO oxidation at significantly lower temperatures (420 °C and 80 °C, respectively). The high redox activity of the nanocrystalline CeO_2−x-based system is tied to the microstructure, surface chemistry, and electrical conductivity of this advanced catalyst.

Manipulation of sulfate content, silica content, and activation temperature provides the means for controlling the strength of surface Brønsted acid sites in the zirconia—silica—sulfate system. This capability allows a development of an acid strength hierarchy, based on the adsorption of pyridine and isomerization of 1-butene and n-butane, as a rational basis for acid catalyst design. Introduction of silica into zirconia—sulfate co-gels also provides insight into the activation behavior of this important class of materials. Silica retards sintering upon heat treatment, thereby delaying crystallization of the zirconia component to higher temperatures. Activation of sulfate to a form capable of catalyzing the isomerization of n-butane is also delayed to higher heat treatment temperatures, confirming the role of crystallization in initiating the activation sequence.

A large number of candidate soot oxidation catalysts were screened on their catalytic activity with a model soot. It was found that the intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation rate. Two types of contact were studied: many catalysts increase the rate of soot oxidation considerably when, under model conditions, the contact is intimate (“tight”); whereas under conditions of poor (‘loose”) contact, which can be resemblant of the contact in practice, only some catalysts are able to accelerate this oxidation reaction. It is tenatively suggested that (i) mobility of the catalyst is a major parameter determining the loose contact activity of catalysts, and (ii) this mobility correlates with the melting point or the partial pressure of the catalyst.

A matrix of mesoporous molecular sieves (MCM-41) with various Si/Al ratios and various pore sizes was synthesized. These were structurally characterized by X-ray diffraction, IR and ²⁷Al NMR. The relative acidity of these materials was estimated catalytically based on the isomerization selectivity of 2-methyl-2-pentene. The activity and selectivity of this reaction are affected by both the Si/Al ratio and the pore size at fixed composition. The stability of Al in the wall structure (as tetrahedral Al) is also affected by the pore size, the smallest pore size resulting in the most stable tetrahedral Al.