An elaborate structure investigation of the chiral polymorph A-enriched zeolite beta

Abstract

Polymorph A-enriched zeolite beta was hydrothermally synthesized from an extremely concentrated system using tetraethylammonium hydroxide (TEAOH) as an organic structure-directing agent (OSDA) in the presence of fluoride. The sample was investigated by powder X-ray diffraction (XRD), 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR), 19 F MAS NMR, and high-resolution transmission electron microscopy (HRTEM). The polymorph A enrichment was confirmed by the characterization, and the percentage of polymorph A was determined to be approximately 66% by DIFFaX simulation and HRTEM analyses. A pure polymorph A stacking area was detected by HRTEM, and the chiral structure of polymorph A in the zeolite beta was investigated on the basis of the HRTEM images.

Synthesis of large-pore zeolites from chiral structure-directing agents with two l-prolinol units

“…Indeed, much effort has been applied in recent years in order to enrich zeolite beta in polymorph A, which has been partially accomplished by using special synthesis conditions. [46][47][48][49][50][51][52] Besides, theoretical studies based on molecular simulations have showed that polymorph A could be obtained by using particular SDAs designed with specific geometric features. 53 In our case, no hints of a potential enrichment in polymorph A was observed by looking at the profile of the first diffraction peak at around 7.5°2θ (enrichment in polymorph A should enhance peaks at 7.0, 7.7 and 9.7°).…”

Design of Organic Structure Directing Agents for Chiral Zeolite Beta A

“…Access to enantiomerically pure zeolite beta can be a breakthrough in this field . A general strategy to synthesize enantiomerically enriched BEA through a chiral OSDA was first proposed by Davis et al., and attempts towards this goal have been reported ,,…”

Chiral Organic Structure-Directing Agents

“…3+ cations in AlPO-CJ22 displayed a chiral order in which they follow a 2 1 screw axis along the [100] direction, interacting with the AlPO chains through extensive H-bonds in a helical fashion between the N atoms and the terminal O atoms of the inorganic chain (see Figure 8, inset). Indeed, measurement of the optical rotation clearly revealed that the Co complex cations have predominantly the Λ-configuration.…”

“…On the one hand, Taborda and coworkers used an agingdrying method of the starting synthesis gels (for long times) prior to the conven-tional hydrothermal treatment which led to enrichments in polymorph A up to 68-70 % [97,98]. On the other hand, Tong and coworkers developed a similar synthesis strategy but this time using an extremely high concentration of the synthesis gels through a dramatic reduction of the water content by heating the gels at 80 ºC for several days until reaching a H 2 O/SiO 2 ratio of 0.3; this also caused a partial degradation of the tetraethylammonium cations (through Hofmann elimination) to give triethylamine [99,100]. In fact, it was later demonstrated that the presence of the tertiary amine played a crucial role for the enrichment in polymorph A [101].…”