Effects Of Acidity And Size Of SAPO-34 On The Catalytic Performance In MTO Reaction

Considerating the domestic energy structure and the special role of light olefins in the modern chemical industry, MTO technology, as the most promising alternative to oil route, has attracted extensive attention. Typical catalyst in MTO reaction is the silicoaluminophosphate molecular sieve SAPO-34, whose catalytic performance maily depends on their acidity and size. This thesis systematically investigates the effects of Br?nsted acid density, crystal size and particle size on the catalytic performance in MTO reaction over SAPO-34, on the basis of the measurement of the species and amount of organic deposits in active and deactivated SAPO-34, the deactivation mechanisms involving of acidity and size are investigated.In this thesis, various highly crystalline SAPO-34 molecular sieves are successfully synthesized using Morpholine(M), trimethylamine(TEA) and tetraethylammonium(TEAOH) as the templates, and then applied as the catalysts for MTO reaction to evaluate the catalytic performance in a fixed-bed reactor.The effects of Br?nsted acid density are firstly investigated. Results clearly demonstrate that four SAPO-34-M catalysts all exhibite short catalyst lifetime due to the large crystal size, 100% methanol conversion can be only kept for less than 45 min, and their catalyst lifetime seems to be indenpendent of the Br?nsted acid density. In addition, comparing the four SAPO-34-TEA catalysts, it can be found that the catalyst lifetime and the selectivity towards light olefins(ethylene and propylene) both decrease with Br?nsted acid density, while the selectivity towards light alkans is just the opposite, thereinto, SAPO-34-TEA-0.2 exhibites the longest catalyst lifetime(351 min), the highest selectivity towards light olefins(86.1%) and the minimum paraffinic byproducts(0.25% and 1.05% for ethane and propane, respectively). Therefore, the Br?nsted acid density of SAPO-34 is indeed considerated as a crucial role in determining their MTO catalytic performance, however, when the crystal size increases to a certain value, their catalytic performance seems to be indenpendent of the Br?nsted acid density, the main reason may be the effect of diffusion.The species and amount of organic deposits in active and deactivated SAPO-34-TEA-0.2 and SAPO-34-TEA-0.6 catalysts are determined by GC-MS and NMR. Results indicate that lower Br?nsted acid density could postpone the evolution rate of polymethylbenzens to polycyclic aromatics and reduce the production rate of polycyclic aromatics, leading to a longer catalyst lifetime of SAPO-34-TEA-0.2.The effects of crystal size and particle size are also investigated, it is found that the catalyst lifetime and propylene selectivity are remarkably enhanced over SAPO-34 with small crystal size under study, while the ethylene selectivity is just the opposite, thereinto, SAPO-34-TEAOH-N exhibite the longest catalytic lifetime, 100% methanol conversion can be kept for 470 min. Furthermore, comparing with SAPO-34-TEA-0.2 before squash, the pressed samples show longer catalyst lifetime, significantly, SAPO-34-TEA-0.2 with particle size of 120-360 mesh is the most optimal catalyst, over which 100% methanol conversion can be kept for 457 min. When the particle size exceeds 360 mesh, the catalyst lifetime became shorter, but the time on stream from the beginning to 70% methanol conversion is the longest(780 min), and the time would decrease with the crystal size. The crystal size and particle size of SAPO-34 are closely related to the diffusion, this thesis definitely confirms the importance of diffusion in MTO reaction from the experimental viewpoint.