Selective Crystallization Of Chirat Polymorph A Enriched Eolite Beta

Zeolites and related microporous crystalline materials, which have periodicone-to-three-dimensional frameworks and well-defined pore structures, have attractedmuch interest due to their wide applications in catalysis, ion exchange, separation, andadsorption. Chiral zeolitic materials with intrinsically chiral framework are ofparticular interest because they can combine both shape selectivity andenantioselectivity, which are desirable for enantioselective catalysis andseparationand fundamental aspects in chirality. Therefore, much effort has beendevoted to the synthesis of chiral zeolitic materials in the past decades. To date,several zeolite frameworks with intrinsic chirality have been synthesized andrecognized by the Structure Commission of the International Zeolite Association (IZA)in the presence of chiral or achiral organic structure-directing agent (OSDA),including*BEA, CZP, GOO,-ITV, JRY, LTJ, OSO, SFS, and STW. To be anapplicable catalyst, the OSDA should be removed without the structure collapsing.Considering this requirement, zeolite beta (*BEA), one of the great industrial zeolitecatalysts, is almost the only candidate. Zeolite beta was first synthesized by Mobilfrom the basic aqueous aluminosilicate gel with tetraethylammonium hydroxide(TEAOH) as an OSDA in1967. Its structure was independently determined byNewsam et al.and Higgins et al.in1988, respectively. According to the structuralanalysis, zeolite beta is an intergrowth of two distinct but closely related polymorphs,i.e. A and B, in a ratio of ca.44:56. Polymorphs A and B possess the samecentrosymmetric tertiary building layer but different stacking sequences. Inpolymorph A, the stacking of the centrosymmetric tertiary building layer is in either aleft-(L) or a right-(R) handed fashion. If the layers stack in an uninterrupted sequence of RRRR…, the structure is in space group of P4122, forming oneenantiomorph of polymorph A, while if the layers stack in an uninterrupted sequenceof LLLL…, the structure is in space group of P4322, forming the other enantiomorphof polymorph A. In polymorph A, the cages are arranged in a helical fashion around afour fold screw axis forming a helical channel along the c axis of the tetragonal unitcell with a manner of either right-or left-handed. In polymorph B, the stacking is in arecurrent alternation of right and left handed fashion (RLRL…), forming an achiralstructure with the space group of C2/c. Both polymorphs occur with almost equalprobability in zeolite beta, thus giving rise to a highly faulted intergrowth of twopolymorphs. With the same building layers, Newsam and Higgins also proposed twonew hypothetical structures, i.e. polymorph C and polymorph CH. Polymorph Ccontains cage of double four-membered ring (D4R), which is not found in eitherpolymorph A or B. Later, two more related hypothetical structures, i.e. polymorph Dand E, were proposed with the same building layers. To date, pure polymorph C andalmost pure polymorph B have been synthesized. However, all other polymorphs,especially chiral polymorph A, are still hypothetical. Considering the valuablepotential applications of chiral polymorph A, much effort has been devoted to thesynthesis of pure polymorph A or polymorph A enriched zeolite beta. However, thesuccess is very limited. Some research groups reported the synthesis of polymorph Aenriched zeolite beta in the presence of chiral or achiral OSDAs. However, the profileof the experimental XRD patterns they reported is very similar to that of the normalzeolite beta. Up to now, the percentage of the polymorph A is not more than50%forthe entire synthesized zeolite beta.In this thesis, we developed a extremely high concentration route to synthesizechiral polymorph A enriched zeolite beta in the presence of differentstructure-directring agents.1. In the presence of tetraethylammonium hydroxide, we have successfullysynthesized a chiral polymorph A enriched zeolite beta in the form of pure silica(denoted Beta-TEAOH). The sample was fully characterized with XRD, NMR,HRTEM analyses and DIFFaX simulation. The ratio of A to B was determined as66:34, breaking the up limit of50%of chiral polymorph A in the normal zeolite beta.A big dominance of pure polymorph A was observed in the HRTEM image.2. In the presence of tetraethylammonium hydroxide, Ti form of chiralpolymorph A enriched zeolite beta (Ti-Beta-TEAOH) was synthesized and used as acatalyst in the asymmetric epoxidation of β-methylstyrene. The results clearly show that a significantly higher enantioselectivity for the trans-epoxide products wasobtained over the polymorph-A enriched Ti-Beta-TEAOH than over the normalTi-Beta, despite the fact that both samples showed a higher catalytic activity for theoxidation of cis-β-methylstyrene than for the oxidation of the trans-isomer,suggesting that the zeolite beta we made contains reasonable enantiomeric excess.3. In the presence of lab-made dimethyldiisopropylammonium hydroxide(DMDPOH) and N,N-dimethyl-2,6-cis-dimethylpiperdinium hydroxide (DMPOH),we have successfully synthesized chiral polymorph A enriched zeolite betas. Thepercentage of chiral polymorph A in both samples is ca.60, and70%, respectively.4. The key factors that lead to the enrichment of polymorph A duringcrystallization were investigated. Extremely low water content was found to be verynecessary for the formation of chiral polymorph A enrichment during thecrystallization. In the early stage of crystallization, the enrichment of chiralpolymorph A occurred. Using the seeds containing enriched chiral polymorph Acannot further increase the percentage of chiral polymorph A in the resulting crystals.In summary, we developed a route for the synthesis of chiral polymorph Aenriched zeolite beta with reasonable enantiomeric excess. With this route, chiralpolymorph A enriched zeolite beta can be obtained in the presence of different organicstructure-directing agents. The resulting material has a much higher enantioselectivityin the asymmetric epoxidation of-methylstyrene than normal zeolite beta. This routemay be applied in the synthesis of one enantiomorph of other chiral zeolites or onepolymorph of intergrown material.