Optimized Synthesis Of Zeolite SSZ-13 And Its Catalysis Performance In Methanol To Olefins Reaction

With the rapid development of the global economy,catalytic science has brought a qualitative leap for modern chemical industry,among which the core is the synthesis and application of catalysts.As an efficient catalyst for environmental friendliness,zeolite molecular sieve synthesis has promoted the development of modern chemical technology.As a kind of zeolite molecular sieve with high silica CHA topological structure,SSZ-13 molecular sieve has the features of order pore structure,good hydrothermal stability and more surface proton acidic centers.Because of these characteristics,SSZ-13 molecular sieve exhibits excellent performance in the fields of removal of nitrogen oxides from automobile exhaust,methanol to olefin?MTO?and gas adsorption and separation,etc.However,SSZ-13 molecular sieve has the disadvantages of poor coking resistance,short catalytic life,low light olefins?ethylene+propylene?selectivity in MTO.Therefore,improving the anti-coking performance of SSZ-13 molecular sieve and synthesizing SSZ-13molecular sieve with low acidity,good hydrothermal stability,excellent pore properties and silicon-controlled aluminum ratio have become a hot research topic.The traditional hydrothermal synthesis of SSZ-13 molecular sieve mainly has the disadvantages of narrow synthesis phase region,slow synthesis rate and low solid yield.Therefore,it has important industrial application value to seek the optimal synthesis route of SSZ-13 molecular sieve.Based on the above research background,the following three aspects of work were mainly carried out in this study:Firstly,the synthesis of SSZ-13 molecular sieve using quaternary ammonium hydroxide as an auxiliary was investigated.The effect of replacement of NaOH with tetramethylammonium hydroxide?TMAOH?on the synthesis of zeolite SSZ-13 was investigated in the presence of the N,N,N-trimethyl-1-1-adamantanammonium hydroxide?TMAdaOH?as a structure-directing agent.The influence of silica-alumina ratios?SARs?of the synthetic gel on the structural,textural and acidic properties of SSZ-13 was also discussed.The obtained SSZ-13 samples were characterized by XRD,SEM,FT-IR,TG-DTG,NH₃-TPD and N₂ physisorption techniques.The catalytic performance of the samples was evaluated by methanol to olefins?MTO?reaction.The results demonstrated that the crystallinity of the product increased significantly from 92.68%to 108.75%when the organic alkali TMAOH completely took place of the inorganic NaOH.Moreover,the range of SARs in the reaction gel can be significantly broadened from 33 to 100 and the SARs of the products were enhanced accordingly.Meanwhile,these materials processed large BET surface area of⁸00 m²·g^-1 and the reduced strong acid amount,and exhibited a longer catalytic life expectancy and a higher light olefins?ethylene+propylene?selectivity in MTO reaction.Secondly,topotactic transformation of low silica LTL to high silica CHA molecular sieve was studied.Direct conversion of LTL zeolites having a low SiO₂/Al₂O₃ ratio?SAR?into high silica CHA molecular has been explored in the presence of N,N,N-trimethyl-1-adamantanammonium hydroxide?TMAdaOH?as a structure-directing agent.The influence of silica-alumina ratios?SARs?,alkali-silica ratios,SDA-silica ratios of the synthetic gel on the structural,textural and acidic properties of SSZ-13 was also discussed.The obtained SSZ-13 samples were characterized by XRD,SEM,FT-IR,NH₃-TPD and N₂ physisorption techniques.The results showed the SAR of the final products can be controlled easily in a wide range only by the amount of aluminosilicate zeolite L added to the synthesis gel.The reconstructed CHA structure zeolites have high relative crystallinity,solid yield and reproducibility.Meanwhile,these materials have BET surface area of⁷60 m²·g^-1 and the reduced strong acid amount.The obtained SSZ-13s with wide SAR ranges?30-181?were applied as catalysts in the methanol to olefins?MTO?reaction to understand the effect of the SARS on the MTO performance,including the catalytic stability and selectivity for ethylene and propylene.It was observed that these SSZ-13s exhibited longer catalytic lifetime and higher light olefins?ethylene+propylene?selectivity in MTO and were better or competitive against conventional SSZ-13s synthesized from amorphous aluminosilicate hydrogel.Therefore,topotactic transformation from LTL structure can be suggested as a competitive method to synthesize high silica SSZ-13zeolites with wide compositional range.Finally,the crystallization kinetics of topotactic transformation of low silica LTL to high silica CHA molecular sieve were discussed.Based on“composite building unit?CBU?hypothesis”,the key of the conversion is the common composite building units d6r between the LTL and CHA-type zeolites.The samples prepared at different crystallization stages were characterized by XRD,SEM,FT-IR and ²⁹Si/²⁷Al MAS NMR to investigate the intermediates in the topotactic transformation from LTL to CHA structure.It has been demonstrated that LTL structure was directly transformed into CHA structure by common composite building units d6r with the combination called‘‘synergism''of TMAdaOH,NaOH,and SiO₂.Additionally,the crystallization rate of CHA type zeolite upon utilization of LTL-type zeolite in the synthesis were notably elevated compared to that in conventional hydrothermal syntheses utilizing amorphous aluminosilicate gel,which could be due to the locally ordered aluminosilicate nanoparts?d6r?can be preserved while in the topotactic transformation of L zeolites in the presence of TMAdaOH and supplementary SiO₂.Nanopart assembly in the presence of a TMAdaOH-OSDA results in rapid crystallization and high selectivity for CHA type zeolite.