Study On Modification Of ZSM-35 Zeolite For Butene Skeleton Isomerization Reaction

Isobutylene is a kind of important petrochemical basic raw material,which is used to produce MTBE,tertiary butyl alcohol,butyl rubber,and other organic chemicals.Because of the cheap price and desirable availability of butylene,the n-butene skeleton isomerization process route is very promising for isobutylene production.At present,because of its proper channel structure and appropriate acid distribution,ZSM-35 molecular sieve is an ideal catalytic material for butene skeleton isomerization.However,there are still some problems of low selectivity and poor stability in the process of butene skeleton isomerization,so the modification of ZSM-35 molecular sieve is propsed to improve its catalytic performance to achieve high selectivity and good stability.In this paper,the HZSM-35 molecular sieve was mainly used as the original catalyst,which had the two-dimensional channel structure with eight-member ring and tenmember ring vertical crossing.The catalytic performance of the n-butene skeleton isomerization reaction was surveyed by adopting a variety of modification methods.The relationship between physicochemical properties and catalytic performance of ZSM-35 molecular before and after modification is systematically investigated in this paper with the help of different catalyst characterization methods.Firstly,ZSM-35 molecular sieve is modified by different ways,including citric acid treatment,sodium hydroxide treatment,isomorphous replacement and alkaline earth metal ion exchange.According to the reaction evaluation,ZSM-35 molecular sieve modified by alkaline earth metal ion exchange has higher selectivity and yield of isobutylene and less by-products compared to other modification methods.The strong B acid centers of ZSM-35 molecular sieve modified by citric acid treatment are significantly increased.The L acid centers have a large increase after sodium hydroxide treatment and isomorphous replacement with gallium.But the strong B acid centers and L acid centers are favorable for bimolecular reaction,which could lead to many side reactions.After isomorphous replacement with ammonium fluosilicate,the strong acid content and L acid content were decreased.But the molecular sieve holes were blocked by amorphous silica generated by ammonium fluosilicate,which led to bad catalytic performance.By contrast,after ion exchange of alkaline earth metal,total acid content and strong B acid content had a small decrease,but moderate B acid content increased.As a result,single molecule reaction mechanism dominated,which led to the increase of the selectivity and yield of isobutylene.Therefore,ion exchange of alkaline earth metal was suitable for molecular sieve modification.Ion exchange of alkaline earth metal was selected as optimal modification method.On this basis,cationic type,exchange frequency and calcination temperature were further investigated to the impact of ion exchange.Through the characterization analysis and the response evaluation,among four kinds of alkaline earth metal ions(Mg,Ca,Sr,Ba),suitable pore structure and acid distribution were generated when employed Mg,which promoted the single molecule reaction because of appropriate ionic radius and polarization effect of Mg;Extra L acid centers were introduced when the times of ion exchange increased,which resulted in the increase of bimolecular reaction and led to the decrease of the selectivity of isobutylene.So the suitable time of magnesium ion exchange is one;The framework of molecular sieve was damaged with the increase of calcination temperature,which made catalytic properties worse.Optimal calcination temperature was 550 ? according to reaction evaluation.The optimum process conditions of n-butene skeleton isomerization were examined on the ZSM-35 molecular sieve modified by magnesium ion exchange,including reaction temperature,space velocity and the ratio of volume between nitrogen and butene.It is found that catalytic performance of n-butene skeleton isomerization is the best when reaction temperature is 350 ?,space velocity is 4 h-1 and the ratio of volume between nitrogen and butene is 2.Under process conditions,selectivity of isobutylene is 72.16 %,and the yield of isobutylene is 39.74 wt%.Finally,in optimal process conditions,the stability of the catalyst was investigated through carrying out a 100-hour long period experiment on the ZSM-35 molecular sieve modified by magnesium ion exchange.It turned out that selectivity of isobutylene basically stay above 70%,and increased to 80.70% in the 100 h.Yields of isobutylene remain at around 40 wt %,and achieve 42.80 wt% in the 100 h.In addition,yields of byproducts propylene and C5 and above components keep in a low level.As a result,ZSM-35 molecular sieve modified by magnesium ion exchange has excellent catalytic performance and stability,and it's a favourable catalytic material for butene skeleton isomerization.