The Synthesis And Modification Of SAPO-34 Zeolite And Its Catalytic Performance On The Conversion Of CH₃Br Into Light Olefins

Light olefins are extensively used as important feedstocks to synthesize a variety of products such as polymers,solvents,drugs,cosmetics,and detergentsin the chemical industry.More than 90%of light olefins are produced from light oil cracking.With the continuing shortage of global oil resources,the technologies for the synthesis of light olefins from natural gas rather than petroleum attracted a wide spread attention.Methane by brominating to create light olefins is a simple and mild process.In this technology process,SAPO-34 zeolite is one of the hottest topic catalysts of research,it has feasible acidity,special ducts and good adsorption property,which endored it better catalytic performance.However,the low availability of active site and the quick deactivation rate are major defects which confined its utiliaztion.Thence,on the basis of the established work in our team,in this paper,we optimized the grain size,open-framework structure and acid property of SAPO-34 to enhance its diffusion properties and catalytic performance.At the same time,the occurrence of carbon deposition was delayed,thus both the conversion of bromomethane and selectivity to light olefins were enhanced.First,SAPO-34 molecular sieveswith different grain size were successfully prepared by adding crystal growth inhibitors N-Octyl-D-glucamine?NODG?in the preparation process.The as-prepared SAPO-34 molecular sieves with different grain size showed different catalytic activities in the conversion of bromomethane into light olefins.When the content of N-Octyl-D-glucamine increased,crystal particle of SAPO-34 molecular sieve decreased from 10 ?m to 1.35 ?m.Due to the largest amount of acid sites of the SAPO-34 samples with additive amount of N-Octyl-D-glucamine is 0.75 g,it showed the highest CH3Br conversion?89.6%?and yield?60%?.Owing to the smallest particle size and shortest diffusion path,the SAPO-34 catalyst achieved with additive amount of N-Octyl-D-glucamine is 1 g showed the highest selectivity of light olefins.Second,SAPO-34 molecular sieves synthesized by traditional hydrothermal method were treated by means of steam disposition at different temperatrue and acdic treatment?HF/NH₄F acid solution?.It was found both hydro-thermal and acid treatment have an impact on catalytic performance in bromomethane to light olefins process.By means of hydro-thermal treatment,acid site distributionof SAPO-34 molecular sieves was transformed by Al etching,therefore,the conversion of bromomethane and selectivity of light olefins increased.We also found,the optimized stream treated temperature is 500 ?.On the other hand,treated by HF/NH₄F can also resulted in the decrease of Al content andmeanwhile producing mesoporous structure of SAPO-34 molecular sieves,thus enhanced the conversion of bromomethane to 90%.