Synthesis And Modification Of SSZ-13 Molecular Sieve And Its MTO Catalytic Performance

Ethylene,propylene and other light olefins are important raw materials in the chemical industry.The traditional process of producing light olefins is based on the petroleum route,but the current situation of "more coal,less oil and less gas" in China limits the further development of this route.Methanol can be produced from coal or natural gas and has a wide range of sources.Therefore,the process of preparing olefins from methanol(MTO)is widely concerned.The key of MTO process is the preparation of catalyst with high activity and selectivity.SAPO-34 molecular sieve is one of the most widely used MTO catalysts with high methanol conversion and selectivity of ethylene and propylene in MTO reaction,but it has the disadvantages of phosphorus content and high reaction temperature.SSZ-13 has the same CHA topological structure as SAPO-34,and there is no phosphorus in the molecular sieve,and the operating temperature is low.In recent years,SSZ-13 began to emerge in the MTO catalyst.However,at present,the most commonly used template for the synthesis of SSZ-13 is1-adamantyl trimethylammonium hydroxide(TMAda OH),which is expensive and toxic.At the same time,SSZ-13 has stronger acidity than SAPO-34 molecular sieve,so it is very easy to generate coke and deactivate when used MTO process.In this paper,co-templates synthesizing method and metal modification were used,respectively,to modify the crystal size,pore structure and acidity of SSZ-13.The physicochemical properties of the obtained SSZ-13 s were investigated and those catalytic performance in MTO reaction were evaluated.The contents and conclusions are as follows:(1)A series of SSZ-13 molecular sieves were synthesized via co-templates synthesizing method by using TMAda OH and choline chloride as the templates,and the effects of different ratio of the two templates on the physi-cochemical properties of SSZ-13 were investigated.The results show that pure phase SSZ-13 can be synthesized in a certain range of molar ratio of TMAda OH to choline chloride;and within this range,the crystal size and the amount of strong acid of the obtained SSZ-13 decreased with the increasing of the amount of the choline chloride.When used in MTO process,its lifetime has been extended from 42 min to 75 min.(2)SSZ-13 molecular sieves were synthesized by adding sodium chloride,ultrasonic aging and adding the crystal seeds,respectively,and the effects of the three methods on the crystal size of SSZ-13 were compared.The results show that the addition of seed in the crystallization system can effectively reduce the crystal size of the resulted SSZ-13 to about 1 μm,and its relative crystallinity is significantly increased to about 150%;while,the acidity of SSZ-13 is increased considerably,resulting its relatively shorter lifetime(80 min)and slightly lower diene(ethylene and propylene)selectivity when used in MTO process.(3)In order to modify the acidity and pore structure properties of SSZ-13 molecular sieves,two different methods of metal Ce modification and alkali treatment were used,respectively.The results show that with the increasing of the amount of Ce,the amount of strong acid of the modified zeolite decreased,while its mesopore volume increased.When used in MTO reaction,it was found that the lifetime of the zeolite increased first and then decreased with the increase of Ce loading,and the sample obtained by loading 1% Ce had the longest lifetime of 95 min and the highest diene selectivity of 82%.Similarly,alkali treatment can also increase the mesopore volume,thus can improve the lifetime of the molecular sieve.It was found that the lifetime of the SSZ-13 molecular sieve treated by 0.05 mol/L alkali solution can reach to 92 min.