Study On The Design,Preparation Of The Catalyst And Reaction Mechanism For Methanol To Olefins

Methanol to olefins(MTO)as the most important non-oil route to produce light olefins is a vital technology for the clean and efficient utilization of coal.The MTO catalyst is the key of the further development of MTO technology.It is of great significance to develop efficient MTO catalysts due to China's basic conditions of rich coal and low oil.Multipe MTO catalysts were designed and prepared,and the textuaral properties,morphology,acidity,coke amount and coke speices of the catalysts were investigated by the complementary techniques such as BET,XRD,NH3-TPD,H2-TPR,SEM,TEM,ICP,MAS NMR,TGA,GC-MS,in-situ UV and in-situ IR.The effect of preparation condition on the molecular sieve crystallization,morphology and acdity was investigated and discussed.The catalyst performance was evatluated,and the effects of acidity and mass transfer distance on the coking behavior were studied.The research results were as follows:(1)A core-shell SAPO-34@SAPO-18 molecular sieve was synthesized by epitaxial growth method.The results showed that the growth of the SAPO-18 shell was affected by the grain size of core SAPO-34,surface roughness,and mother liquor silicon content.The reduction of core crystals size and improving surface roughness could create more points for heterogeneous nucleation,and make more SAPO-18 grow onto SAPO-34 external surface.The SAPO-18 shell could weaken the strong acid sites on the outer surface of SAPO-34 and increase the utilization of the internal cage of the catalyst.The MTO reaction life of the core-shell S APO-34@SAPO-18 was twice that of the core catalyst and the selectivity of ethylene and propylene olefins was increased by 9%.(2)Core-shell molecular sieve with multi-type pore structures was successfully prepared.SSZ-13 was selected as the core phase.The surface of SSZ-13 was coated with a mesoporous silica shell layer successfully when cetrimonium bromide(CTAB)or polyethylene oxide-polypropylene oxide-polyethylene oxide(P123)surfactant was used as a template agent,and tetraethoxysilane(TEOS)was used as a silicon source.The shell thickness of mesoporous silica could be controlled by adjusting the ratio of TEOS/SSZ-13.The complete mesoporous silicon shell with low thickness can effectively weaken the strong acid sites of the SSZ-13 catalyst,and prolong the reaction life of the MTO catalyst,and improve the selectivity of light olefins.(3)A series of SSZ-13 molecular sieves with different silicon-aluminum ratios(SAR)were synthesized by changing the SAR of the mother liquid.The SAR variation of the mother liquid on the crystallization of SSZ-13 was investigated.The nucleation and crystallization could not occur spontaneously when the mother liquid did not contain aluminum source.Increasing the SAR reduced the nucleation amount and accelerated the crystallization,and meanwhile enlarged the grain size.Increasing the SAR was beneficial to reducing the acidity of the catalyst and improved its light olefins selectivity.SSZ-13 sythesized from a mother liquid SAR of 35 showed the best MTO performance.(4)The grain size of SSZ-13 could be controlled from 110nm to 1.21?m by adding anionic polyacrylamide(APAM)and adjusting the hydrothermal synthesis temperature.The crystallization process of SSZ-13 was studied and the results show that hydrogel network formed from APAM can effectively suppress the self-assembly of SSZ-13 crystallite.The nano-sized SSZ-13 showed the longest MTO lifetime.In-situ TGA indicates that the rate of catalyst carbon deposition decreases with the grain size reduction.The decrease of SSZ-13 grain size inhibited the formation of phenanthrene,could activate more naphthalene species which catalyst the MTO reaction as an intermediate in the SSZ-13 cage.(5)Submicron SSZ-13 samples with different acidity were synthesized successfully with seeds-assissted method.Compared with micron SSZ-13,submicron SSZ-13 exhibited more stable catalytic performance.Weakening the acidity of the catalyst can increase the selectivity of low-carbon olefins at the beginning of the MTO reaction.TG,GC-MS,and in-situ UV/vis spectra were used to investigate the coking behavior on sub-micron catalysts with different acidity.The results showed that the main active carbon species is methylbenzene over SSZ-13 with the low acid density(125.2 ?mol/g).Over SSZ-13 with the high acid density(330.2?mol/g),it is converted into methylnaphthalene.Meanwhile,the secondary reactions of 1-butene,propylene,and ethylene in different acidic molecular sieves were investigated.The increase in acidity exacerbated the hydrogen transfer reaction and the chain growth of olefins,which make more polycyclic aromatic hydrocarbons such as naphthalene formed in SSZ-13 cages,and changes the evolution of coke species.(6)A new bifunctional catalyst with a combination of CeO2 and SSZ-13 was developed.The formaldehyde formed during the MTO reaction can be adsorbed by CeO2.and decomposed into CO or CO2.The process inhibited the reaction between formaldehyde and coke species in SSZ-13 and the inactive coke species(tetramethylnaphthalene)formation.The lifetime of the bifunctional catalyst is more than four times that of SSZ-13.Nano-rod CeO2 offered abundant external surface and surface oxygen for the adsorption and deposition of formaldehyde.SSZ-13 mixed with rod-like CeO2 exhibited the longest lifetime.