Introduction
Currently, much attention is paid to the development of new technologies and increasing the efficiency of existing processes for converting natural gas into “light” olefins, of which propylene is of greatest interest.
Methods
Synchrotron X-ray diffraction (s-XRD), Raman scattering, inductively coupled plasma atomic emission spectroscopy, low-temperature nitrogen adsorption, and a catalytic cracking unit were used.
Results
To obtain the catalyst, molybdates Ln₂(MoO₄)₃ were synthesized according to the developed method using rare earth nitrate and sodium molybdate dihydrate Na₂MoO₄×2H₂O. Lanthanum, praseodymium, neodymium, and ytterbium molybdate catalysts for propane dehydrogenation increased feedstock conversion by 40.0% without recycling and the process yield of target propylene by 33.0% while maintaining high propylene selectivity (70.0%) and total selectivity for "light" olefins (83.0%). Over 8 hours of continuous operation, the catalyst activity decreased by less than 1.0%. The reaction activation energy decreased from 114 kJ/mol to 91.0 kJ/mol. A decrease in catalyst activity (about 15.0%) was observed after 200 hours of continuous operation. To regenerate the catalytic systems, it is proposed to treat their surface with an air flow at 573 K for 10 hours.
Conclusions
The formation of the mixed structure of Ln₂(МоO₄)₃ leads to a shift in the degree of conversion to the region of catalytic temperatures of 700-900 K with a predominance of the dehydrogenation reaction of 80%. It was revealed that the steric factor prevails over the energetic one in the process of propane destruction, making the reaction of ethylene formation stereospecific, with the achievement of a maximum in ethylene selectivity of 83%. It was determined that the non-isovalent substitution of K⁺- leads to the deformation of the crystal structure and blocking of the most energetic Lewis centers, which leads to an insignificant decrease in propylene selectivity from 83% to 81% with a decrease in surface carbonization at high cracking temperatures.