The catalytic performance of PtSn catalysts supported by molecular sieves in propane dehydrogenation will be further improved.

Molecular sieves have become a hotspot in material science and catalysis scientific research due to their large specific surface area, high thermal stability and hydrothermal stability. In the past ten years, PtSn catalysts supported by molecular sieves have been one of the hotspots in the process of propane hydrodehydrogenation. The molecular sieves involved include ZSM-5, SAPO-34, Y-type molecular sieve and L-type molecular sieve. ZSM-5 molecular sieve belongs to the orthorhombic system, and its silicon-alumina ratio can be changed in a large range. The skeleton contains two intersecting channel systems. The ten-membered ring channel parallel to the a-axis is Z-shaped, with a corner of about 150° and a pore diameter of 0.54×0.56nm. The ten-membered ring channel parallel to the c-axis is linear, and the elliptical diameter is 0.51 × 0.55 nm. At present, ZSM-5 molecular sieve has been widely used in catalytic cracking, isomerization, aromatization and other reactions. The PtSn catalyst was supported on ZSM-15, and it was found that the acidity of the support had an important influence on the propylene selectivity of the catalyst. When the Si/Al ratio was 300, the propylene selectivity was the best. In order to further improve the catalytic performance of the catalyst, the catalyst must be further modified. The ZSM-5 catalyst contains some strong acid sites. For this reason, they added sodium ions or potassium ions to the catalyst, and found that an appropriate amount of addition can reduce the protonic acid and moderately strong Lewis acid on the surface of the carrier, and enhance the bond between the carrier and the metal. Therefore, the formation of carbon deposits is inhibited and the catalytic stability of the catalyst is improved. The addition of Na ions is more beneficial to improve the catalytic performance of the catalyst. They found that the addition of alkaline earth metals (Ca, Mg) can stabilize the Sn component in the oxidation state. For this reason, they added the above ions to PtSnNa/ZSM-5, respectively, and found that the catalytic performance was further improved, and more effectively inhibited generation of carbon deposits. Especially after adding 0.5 wt % Mg, when the hydrogen-hydrocarbon ratio is 0.5, the reaction temperature is 590 °C, and the mass space velocity is 3.0 h-1, the selectivity of propylene is still greater than 95% after 70 h of reaction. They also investigated the effect of the addition of rare earth element La on the catalytic performance of PtSnNa/ZSM-5 in propane dehydrogenation. It is found that adding an appropriate amount of La component can not only reduce the acid content of the carrier, but also inhibit the reduction of the oxidized Sn component and stabilize the active component of the carrier, which can further inhibit the formation of carbon deposits and improve the propylene selectivity and catalytic performance of the catalyst. stability. SAPO-34 is a molecular sieve of silicon-phosphorus-aluminum series. The crystal framework is chabazite type. It has an elliptical spherical cage composed of eight-membered rings and a three-dimensional pore structure. The pore size is 0.43-0.5 nm, and the pore volume is 0.42 cm3/g. SAPO-34 molecular sieve is currently mainly used in the field of methanol catalytic production of olefins. The propylene selectivity of PtSn/SAPO-34 catalyst is much higher than that of PtSn/ZSM-5, which may be due to the large specific surface area and lower acidity of the support. They added Na, La, Ca, Zn, Ce and other metal elements to PtSn/SAPO-34 respectively to investigate the effect on the catalytic performance of propane dehydrogenation. The results show that alkali metals, alkaline earth metals and transition elements can significantly improve the catalytic performance of the catalysts. Among the obtained catalysts, the one with the highest catalytic activity and propylene selectivity was PtSnCa/SAPO-34. The working SAPO-34 supported catalysts were treated with steam and found that still good catalytic performance could be obtained. In conclusion, we believe that with the development and application of new molecular sieves and the continuous innovation of preparation technology, the catalytic performance of PtSn catalysts supported by molecular sieves in propane dehydrogenation will be further improved.