Calcination catalytic performance

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Hua, J., Wei, K., Zheng, Q., and Lin, X. 2004. Influence of calcination temperature on the structure and catalytic performance of Au/iron oxide catalysts for water-gas shift reaction. Appl. Catal. A Gen. 259 121-30. 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

M(lI)AlSn-LDHs with M(II) being Mg, Ni or Co were synthesized by a coprecipitation method. The influence of Sn on the thermal transformations and redox properties were investigated in detail using XRD, TG/DTA, SEM, TPR, 1 l9Sn-MAS NMR and UV-visible diffuse-reflectance (DR) spectroscopy methods. Some of these samples calcined at 450 °C were tested as catalysts in the partial oxidation of methanol (POM) reaction. In this paper we discuss briefly the effect of Sn-incorporation on the structural features and reducibility of CoAI-LDH. The catalytic performance of Co-spinel microcrystallites derived from CoAl-, and CoAlSn-LDHs was also evaluated. 

Improved catalytic performance, selectivity and resistance to fusion, over bismuth molybdate catalysts was reported by McClellan (90) for catalysts obtained by chemically combining bismuth, molybdenum, phosphorus, and silica. After calcination at 450°C, the bismuth phosphomolybdate-on-silica catalyst showed an X-ray pattern of mainly crystalline Bi2(Mo04)3 which subsequently was converted to a new, substantially amorphous, phase after calcination at 800°C. Substantially morphous meant that the X-ray diffraction lines were broad diffuse bands of low intensity. The pattern of lines for this novel phase indicated a scheelite structure. A special interaction of silica with bismuth molybdate was also suggested by Callahan et al. (91). 

The chosen catalytic test reaction was the oxidation of phenol, which yields a mixture of catechol, hydro-quinonc, and 1,4-benzoquinone (Scheme I). The reaction was conducted at atmospheric pressure by continuously adding aqueous H2O2 to a mixture of catalyst, phenol, water, and a solvent (either methanol or acetone) at the reaction temperature (usually 373 K) reaction times were l-4h. Conversions and product sclectivities depended on the composition of this mixture under the best conditions, H2O2 conversion was 100%, phenol conversion 27%, and phenol hydrox-ylation selectivity 91%. The ratio of o />-substituted products (Scheme 1) was usually about unity. It was concluded that catalytic performance depended critically on calcination conditions, i.e. on the completeness of removal of the template. Many facets of the reaction remain to be investigated. 

Through the use of hexadecane cracking alone, we have been unable in this work to elucidate the role of mesoporosity in the catalytic behavior of calcined or steamed zeolites. Steamed AFS and USY zeolites show differences in mesoporosity but exhibit similar catalytic performance. While mesoporosity may affect diffusion in actual FCC catalysts, larger molecules than hexadecane will be required to determine mesopore effects. 

Catalyst calcination conditions are additional critical preparative variables that have a significant impact on the structure as well as the catalytic performance of Mo V-Te-Nb oxide catalyst. From one precursor, two catalysts of very different crystal phases are obtained under different calcination atmospheres. The catalyst calcined in nitrogen flow exhibits a high catalytic performance. In contrast, catalyst calcined in air is inactive toward propane oxidation. 

Thermal treatments induce dehydration, dehydroxylation and loss of the charge-compensating anions, resulting in mixed oxides with the MgO-type structure. Hydrotalcites are consequently a class of precursors useful for the preparation of catalytically active oxides showing basic properties [94], The acid-base properties of Mg-Al mixed oxides are governed by the Mg Al molar ratio, calcination temperature and preparation conditions. The study of the influence of the acid-base properties and chemical composition on the catalytic performance of calcined hydrotalcites is thus of interest. 

Most works found in the literature studying the catalytic performance of spinel-like materials, used a fixed calcining temperature (in the range 500-700"C). As far as we know, the influence of calcining temperature in the performance of spinels as combustion catalysts has not been studied, although this parameter largely affects the properties of the solids [3]. 

Various reactions have been studied on mixed rare earth and the La and Ce forms. These include ethylation of benzene 18), propylation of toluene 14), o-xylene isomerization 21), butane cracking 14), cracking of n-hexane, n-heptane, and ethylbenzene (8), and isomerization and disproportionation of 1-methy 1-2-ethylbenzene (7). Other reactions are summarized by Venuto and Landis 18). In several reports, an optimum calcination temperature for best catalytic performance has been demonstrated (7, 8, 14, 18, 21). 

Kim et al.32 reported the preparation, characterization, and catalytic performance of a finely dispersed and thermally stable nickel catalyst incorporated into mesopo-rous alumina. Mesoporous alumina catalysts that incorporate Ni (Ni-alumina) with different Ni/Al molar ratios were synthesized by a one-step sol-gel method using lauric acid as a template. The prepared Ni-alumina catalysts showed a relatively high surface area with a narrow pore size distribution after calcination at 700 °C these effects were independent of the Ni/Al molar ratio. The Ni-alumina catalysts were found to be highly active in the POX of methane. The deactivation of catalysts examined in this work was not due to catalyst sintering, but mainly to carbon deposition. 

Influence of Calcination and Reduction Temperatures on Catalytic Performance. 

The unique basic properties of HT make them very useful for catalytic purposes. However, notably, what often is indicated as a hydrotalcite is not really this compound with a layered stmcture. During thermal treatments a HT transforms first into an amorphous oxide and then, at higher temperatures, into a crystalline spinel-like oxide. Therefore, when the sample is calcined above 300 °C it is instead present as an amorphous oxide or as a crystalline spinel-like oxide. HT derived oxides, however, have the peculiar characteristic of reconstmction during catalytic reactions, returning to a stmcture resembling the starting HT stmcture (memory effect), which often is quite relevant in determining the catalytic performance [259-261]. 

---