Conversion Behavior Of Olefins And Its Inhibitation Mechanism In SAPO-34 Catalyzed MTO Process

Methanol to olefin（MTO）,which is industrially catalyzed by SAPO-34,has become an important process to product light olefins in China.As products of MTO process,ethene and propene also further convert via reactions such as oligomerization,alkylation,cracking,cyclization,hydrogen transfer,and dehydro-aromatization.These reactions lower down the selectivity of olefins,lead to the formation of coke,and accelerate the deactivation of catalyst.This work investigated the reactions and the coking of methanol and olefins on SAPO-34zeolite in fixed bed reactor.The coke is analyzed with thermogravimetry（TG）,gas chromatography-mass spectrum（GC-MS）,gas chromatography（GC）,direct sampling mass spectrum（DSMS）,and scanning electron microscopy（SEM）.Reaction network and coking mechanism of methanol and olefins was proposed.Effects of water on olefins’conversion and coking were also studied.The inhibition mechanism of water in MTO process was discussed.Main results and conclusions are as follows.（1）Olefins,which are products of methanol conversion,tend to further conversion and lead to the formation of parafins and aromatics（coke）.The further conversion of olefins lowers the selectivity of olefin and accelerates the deactivation of catalyst.The catalyst bed can be spatially divided into methanol reaction section,olefins reaction section and deactivation section.In the initiation period,methanol convert into olefins in upper section of catalyst bed,then olefins further convert in lower section of catalyst bed.With the progress of reaction,coke is formed and accumulated in upper section and lead to the deactivation of catalyst,methanol reaction section moves downwards.Ascribed to the cage size and molecule sizes,confined species are mainly methylbenzenes and methylnaphthalenes in olefin reaction section;phenathrenes and pyrenes are mainly formed in methanol reaction section.It was found that insoluble coke can be formed in SAPO-34 cages.Insoluble coke with reticulate structure is composed of carbon-chain bridged cycloalkanes and aromatics.This reticulate structure located in cages near the surface of SAPO-34 crystal.（2）The reaction rate of ethene is low on SAPO-34 and the conversion is lower than 15%.Propene shows medium reaction rate and the conversions reach 63.9%and 27.9%at initiatial stage and 200 min,respectively.Conversion of 1-butene is higher than 97%in 200 min.Both propene and 1-butene promote the conversion of ethene.Tendencies of coking rate are similar to that of conversion rate.It is supposed that the reaction and coking of olefin begins with the formation of carbocation via the protonation of olefin with acidic site.The carbonium ionization of ethene is difficult,thus ethene shows low reaction and coking rate.Propene and1-butene are easy to undergo carbonium ionization,thus they show high reaction and coking rate.Structures of coke molecules are related to reactant.Coke of 1-butene is mainly composed of single ring（benzenes）and dual ring（naphthalenes）components.More three ring components are observed in coke of propene.While for ethene coke,there are certain amount of pyrene.It is supposed that long olefin alkylate with confined aromatics across the cage window,and then cyclize and aromatize in adjacent cages to form bridged species,which at last leads the formation of insoluble coke.（3）Remarkable reduction effects of water on the conversion and coking of ethene,ethene+propene and ethene+1-butene were observed.The conversion and coking of propene and 1-butene were also inhibited,though less noticeably.The co-feeding of water strongly suppressed the formation of insoluble coke and had little effect on the formation of soluble coke.The coking rate increases with the increase of C₄ and C₄₊component in flow.It is supposed that the conversion and coking of olefins is initiated with the oligomerization and alkylation of olefins which lead to the formation of long intermediates.Then these long molecules undergo cracking to form small species or cyclization and aromatization to form confined cokes.Water competes with olefins in accessing acidic sites and leads to the desorption of olefins,which reduces the formation of long intermediates.Ethene can be easily desorbed when water is co-fed.The oligomerization and alkylation of ethene is suppressed greatly by water which reduces intermediates for the following cracking,cyclization and dehydro-aromatization,thus the conversion and coking of ethene-rich streams were remarkably reduced.The effect of water on the adsorption of propene and 1-butene is much weaker than that on the adsorption of ethene.Therefore,some inhibition effects of water on the conversion and coking of propene and 1-butene was observed,but much less significant than that of ethene.This work indicates that the key factor which leads the deactivation of catalyst in MTO process should be the further reaction of olefins.Inhibiting olefins’further reaction should be a practical feasible method to improve olefin selctivity and prolong catalytic life.