Catalytic Study On The Preparation Of Aromatics/olefins From Methane Via Oxybromination Pathway

Aromatics?especially benzene,toluene and xylene?and light olefins?especially ethylene and propylene?are basic feedstocks in chemical industries.Aromatics are mainly produced catalytically through the reforming of heavy naphtha and olefins from steam cracking as well as fluid catalytic cracking of naphtha.Owing to the growing demand for light olefins and the limited oil reserve,it is highly desirable to develop new processes for efficient aromatics and olefins production.Considering the abundant reserve and increasing availability of natural gas,we developed a two-step process for the conversion of methane to aromatics/olefins,involving the conversion of methane into CH₃Br,followed by the catalytic transformation of CH₃Br to aromatics/olefins.The two-step process can be achieved under relatively mild conditions and is a new process for the utilization of natural gas and the efficient synthesis of aromatics/olefins.In this dissertation,a series of highly efficient catalysts for methane oxybromination and CH₃Br conversion were synthesized by means of noble metal loading,carrier optimization and surface property regulation of zeolite catalysts.The physical and chemical properties of the as-prepared catalysts were characterized by XRD,N₂physisorption,SEM,TEM,NH₃-TPD,XRF,²⁹Si MAS NMR and so on.The relationship between structures and performance of catalysts was also discussed carefully.The main innovative results and conclusions were summarized as follows.?1?A series of supported noble metal catalysts were prepared via incipient wetness impregnation.The effects of active components?Rh,Ru,Pd and Pt?and supports?SiO₂,MgO and Al₂O₃?on the catalysis of MOB were systematically studied.It was observed that Rh/SiO₂ and Ru/SiO₂ are comparable in catalytic performance and exhibit activity superior to that of Pd/SiO₂ and Pt/SiO₂.In addition,support type also has a significant influence on catalytic performance of rhodium,especially on product distribution.Due to low propensity for CH₃Br oxidation,Rh/SiO₂ shows much higher selectivity to CH₃Br than Rh/Al₂O₃.In addition,it is found that Rh particles of smaller size show higher MOB activity and CH₃Br selectivity.Under the optimum conditions,Rh/SiO₂ is the best in performance?CH₄ conversion of ca.20%and CH₃Br selectivity exceeding 70%?.The slight decrease in catalytic activity of Rh/SiO₂ as a function of on-stream time may be related to the sintering of Rh particles.?2?The effects of steam treatment on the physicochemical properties of HZSM-5and its catalytic performance for the conversion of CH₃Br to aromatics were investigated.The results show that due to steam treatment,there is a decrease of total amount of acid sites,a decline of acid strength,and a lower ratio of Br?nsted and Lewis acid sites?B/L?.Furthermore,the treatment results in the generation of mesopore structure.HZSM-5steamed at 600°C shows the best performance?an aromatics yield of about 27%and a lifetime of over 50 h?.The improved stability is attributed to the reduced acidity and the newly created mesopores,and the enhanced BTX yield is related to the synergism between the moderate Br?nsted acid sites and Lewis acid sites.?3?SSZ-13 zeolite was synthesized rapidly in 6 h via interzeolite conversion of NaA zeolite by solid-state crystallization method.The results demonstrated that basicity?NaOH/SiO₂ molar ratio?and Si/Al ratio of the synthetic materials have significant influence on synthesis of SSZ-13.SSZ-13 with regular morphology and high crystallinity can be synthesized when the basicity and Si/Al ratio are 0.3 and 30,respectively.Catalytic test of CH₃Br conversion to olefins demonstrates that the sample SSZ?30,6?shows the best performance.Conversion of CH₃Br is not less than 90%and ethylene and propylene selectivity reaches 70%within 75 min of evaluation.The good catalytic performance of SSZ?30,6?is attributed to its low acidity and high crystallinity.?4?Hierarchical SAPO-34 zeolites were synthesized via polyvinyl alcohol-assisted method under hydrothermal conditions and evaluated in CH₃Br conversion to olefins.The results show that the addition of PVA is beneficial to the synthesis of SAPO-34 with mesoporous structure and low acidity.Under the reaction conditions of 450°C and 6.11h^-1 WHSV,the SAPO-34?3.5?exhibits the best catalytic performance?conversion of 85%,ethylene and propylene selectivity up to 90%and a lifetime up to 100 min?The superior catalytic performance of SAPO-34?3.5?is attributed to its hierarchical structure and low acidity.?5?Submicron-sized SAPO-34 zeolites were successfully synthesized by seed-assisted method using DEA as template.The effect of seed property and seed dosage on the particle size and acidity of SAPO-34 products was investigated.The results demonstrate that compared with micron-sized SAPO-34 seed of high crystallinity,the low-crystallinity SAPO-34 seed in the form of nano-sheet is more conducive to generate small-sized SAPO-34.Submicron-sized SAPO-34?ca.700 nm?can be obtained by employing only 2 wt%seed.Further increasing the amount of seed has little influence on the particle size but leads to increase of SAPO-34 acidity.Catalytic test of CH₃Br conversion to olefins demonstrates that catalyst lifetime and olefins selectivity are highly dependent on particle size and acidity of SAPO-34 zeolites.The results demonstrate that the decrease of particle size and acidity are beneficial to alleviating carbon deposition,resulting in improved catalytic stability.Also,the reduced particle size and decreased acidity could diminish second hydrogen transformation of olefins,hence contributing to the improved selectivity to light olefins.Among the tested catalysts,S-0.3-2%shows the best performance?conversion of 95%,ethylene and propylene selectivity up to 94%and a lifetime over 125 min?.