Synthesis Of Chiral Polymorph A Highly Enriched Zeolite Beta

The history of zeolites began in 1756 and during the recent 50 years,the booming developpment of zeolitic materials make it become one of the most important catalysts.Zeolitic materials,which have the unique pore structure,periodic one-to-three dimensional frameworks and fine thermal stability,have found widespread application in catalysis,adsorption and ion exchange.Chiral zeolitic materials are of particular interest because they can combine both enantioselectivity and shape-selectivity,which are desirable for asymmetric adsorption,asymmetric catalysis and enantioseparation.However,the synthesis of chiral zeolitic materials is still a significant challenge.To date,there are 235 different zeolitic framework types have been synthesized,only several of them show the fascinating chiral structural features,including*BEA,CZP,GOO,-ITV,JRY,LTJ,OSO and STW.Zeolite Beta with the*BEA topology has been achieved great industrialization,which is one of the most promising chiral zeolitic materials.Zeolite beta was first synthesized in 1967 by Mobil using tetraethylammonium hydroxide?TEAOH?as the organic structure directing agent?OSDA?.However,due to the limit of characterization methods and the structural complexity of zeolite beta,it took approximately 21 years to resolve the structure of zeolite beta.Its structure was determined as an intergrowth by several distinct but closely related polymorphs?A,B and C?along the[001]direction.The lattice energies of polymorph A and B was basically the same,therefore the growth of polymorph A and B is almost disorder,in a ratio of ca.44:56.In contrast,polymorph C,which contains double four-membered rings?D4R?cages with highly strained Si-O-Si bonds,has a higher energy than polymorph A and B,thus is commonly negligible in the intergrowth of zeolite beta.The three polymorphs have different channel features along[001]direction.Polymoroh A?P4₁22 or P4₃22?has a chiral helical channel while polymorph B?C2/c?has an achiral sinusoidal channel and polymorph C?P4₂/mmc?has a straight channel.To date,researchers have reported the synthesis of pure polymorph C?BEC type?and 85%polymorph B-enriched zeolite beta.Considering the potential value of chiral polymorph A,researchers have devoted much effort to the selective synthesis of chiral polymorph A,including the addition of chiral OSDAs and chiral inducers,however,the synthesis has met with limited success.Recently,the proportion of chiral polymorph A has been significantly improved to approximately 65%by Tong et al.,Guo et al.,and Zhang et al.using achiral OSDAs independently,which indacate that the chiral OSDA was not essential to the enrichment of chiral polymorph A.Therefore,the identification of the key factor promoting the enrichment of chiral polymorph A is central to the synthesis of polymorph A highly enriched zeolite beta.In this thesis,we identify the essential factor promoting the enrichment of chiral polymorph A in zeolite beta and report a“inhibition of nucleation”strategy to synthesize polyorph A highly enriched zeolite beta.The main research results of this thesis are as follows:1.By controling the experimental variables,we have made a thorough investigation on the essential factor promoting the enrichment of chiral polymorph A in zeolite beta.The acidic medium was confirmed crutial to the enrichment of polymorph A.From this point of view,we have developped a generalized acid-assisted route and polymorph A highly enriched?greater than 70%and near 80%?zeolite beta can be achieved with this route.The acidic medium inhibits the nucleation of zeolite beta during the crystallization,which might be the essential reason for the enrichment of polymorph A.2.Based on the“inhibition of nucleation”strategy,we have screened several alcohols as additives and developped a novel green route to synthesize polymorph A-enriched zeolite beta.With this route,highly corrosive and toxic concentrated HF was avoided.Combine the DIFFaX simlation and HRTEM statistics analyses,the proportion of polymotph A was determined as 65%⁷0%,which is among the highest values reported so far.A continuous area of polymorph A was observed in the HRTEM images.3.Under particular dry gel conditions,well crystallized polymorph A-enriched all silica zeolite beta was synthesized.When Al species are introduced into the starting mixture,the proportion of polymorph A decreases significantly.The crystal size of Al-incorporated zeolite beta decreases with the decrease in Si/Al ratio.Moreover,the crystallization of zeolite beta is accelerated significantly because of the incorporation of Al species.The promoting nucleation by Al during the crystallization might be the reason for the missing polymorph A.In addition,the formation of five-coordinated Al intermediate was observed during crystallization of Al-incorporated zeolite beta,which might also alter the enrichment of polymorph A during crystallization of Al-incorporated beta zeolite.In summary,we explored the factors that influence the crystallization of chiral polymorph A in zeolite beta and designed two new route for the synthesis of polymorph A-enriched zeolite beta.This“inhibition of nucleation”strategy introduces a new perspective to the enrichment of chiral polymorph A in zeolite beta.With this strategy,the proportion of chiral polymorph A in zeolite beta might be further improved.