ИСТИНА

Propylene is a raw material being produced in a large scale. It is used in the synthesis of cumene, acrylonitrile, acrylic acid, propylene oxide, isopropyl alcohol, polypropylene [1]. Chromium oxide catalysts are widely used in propylene production by propane dehydrogenation. The oxidative propane dehydrogenation is a promising way to enhance this process, but traditional oxidative agents such as O2 or halogens cause the deep oxidation and thus negatively affect the selectivity to olefins. Thus, the mild oxidative agents are preferable. In the case of using CO2 as a mild oxidizing agent it is involved in chemical process and so its utilization is attained. The catalytic behavior of the material highly depends on the dispersion of the active phase on the surface of the catalyst. To obtain the high dispersion, the porous carriers are used. Traditionally, the template synthesis is applied to obtain the high-porous materials, such as SiO2. In this work we used beta-cyclodextrin as a template. Beta-cyclodextrin is a cyclic oligosaccharide comprising seven D-glucopyranoside units linked by 1,4-glycosidic bonds; the molecules have a truncated cone shape. This molecule is interesting because of its ability to form column-like associates in water solutions [2]. This work was aimed at investigation of chromia catalysts obtained with carriers prepared using beta-cyclodextrin and urea as a template and their characterization by different physico-chemical methods to clear the dependencies between synthesis conditions, the structure of the samples and catalytic activity in terms of application of such materials in the oxidative propane dehydrogenation reaction. During the synthesis we varied the used amounts of water as following: mH2O = x * mSiO2 where x was 1.5, 5 and 20. The obtained carriers were denoted as SiO2-1.5, SiO2-5, SiO2-20, respectively. The synthetic procedure consisted of tetraethyl orthosilicate hydrolysis with 0.1M HNO3 water solution, beta-cyclodextrin and urea addition, gelation and gel aging, drying and calcination at 600 oC. This resulted in mesoporous silicas, which were used as carriers. The silicas were wet impregnated with chromium(III) nitrate solution and also dried and calcined at 600 oC under air atmosphere. The amounts of chromium were 3, 5 and 7 wt. %. The textural properties were investigated by low-temperature nitrogen adsorption-desorption method. All the samples demonstrate large specific surface area (410–680 m2g-1), while all the pores are mesopores (3–13 nm). Pore size distribution maxima depend on the synthesis conditions: the larger the water amount, the larger pores are formed in the sample while the surface area decreases. Additionally, the silicas were examined by SAXS and the reflexes at about 2theta = 1o took place for the samples SiO2-1.5 and SiO2-5. This can be interpreted as a presence of order, but generally the samples are amorphous as can be seen by wide-angle XRD. The catalysts were examined by XRD, XPS, DRS UV-Vis, SEM, TEM, ICP AES, TPR-H2. All the catalysts were investigated in propane dehydrogenation with CO2, the selectivity to propylene was up to 80% while propane conversion was 20% on the sample with 7 wt. % on the carrier SiO2-5. The samples with the carrier SiO2-5 demonstrate the largest catalytic activity. It can be explained by the combination of optimal porosity, large surface area and also optimal tri- and hexavalent chromium species ratio on the surface. The catalytic results for our catalysts are similar with the performance of the samples using MCM-41 as a carrier, but the difference takes place: our catalysts do not demonstrate the dramatical decreasing of selectivity to propylene while the temperature increases.