Cooperative and Competitive Occlusion of Organic and Inorganic Structure-Directing Agents within Chabazite Zeolites Influences Their Aluminum Arrangement

Abstract

We combine experiment and theory to investigate the cooperation or competition between organic and inorganic structure-directing agents (SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites and to rationalize the effects of SDA siting on biasing the framework Al arrangement (Al–O­(−Si–O) x –Al, x = 1–3) among CHA zeolites of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized using mixtures of TMAda+ and Na+ contain one TMAda+ occluded per cage and Na+ co-occluded in an amount linearly proportional to the number of 6-MR paired Al sites, quantified by Co2+ titration. In contrast, CHA zeolites crystallized using mixtures of TMAda+ and K+ provide evidence that three K+ cations, on average, displace one TMAda+ from occupying a cage and contain predominantly 6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis media containing more than 10-fold higher inorganic-to-organic ratios with K+ than with Na+ before competing crystalline phases form, providing a route to decrease the amount of organic SDA needed to crystallize high-silica CHA. Density functional theory calculations show that differences in the ionic radii of Na+ and K+ determine their preferences for siting in different CHA rings, which influences their energy to co-occlude with TMAda+ and stabilize different Al configurations. Monte Carlo models confirm that energy differences resulting from Na+ or K+ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements, respectively. These results highlight opportunities to exploit using mixtures of organic and inorganic SDAs during zeolite crystallization in order to more efficiently use organic SDAs and influence framework Al arrangements.

Multi-objectivede novomolecular design of organic structure-directing agents for zeolites using nature-inspired ant colony optimization

“…The predicted candidates contain experimentally proven OSDAs, molecules similar to experimentally proven ones, and novel molecules owing to the unique advantage of ACO molecular design of simultaneously sampling structural analogues of known molecules and completely new ones. 43 Several recent studies have revealed that OSDAs can control morphology, 27 chirality, 26 and atomic congurations 30,66 of zeolites, which can be evaluated by computational methodologies. It is therefore anticipated that multi-objective design of OSDAs to simultaneously optimize these zeolite properties can be achieved by ACO in the near future.…”

Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Aluminum Distributions Preferentially Stabilize Alkanol Dehydration Transition States

“…that separate CHA cages (7.4×9.8 Å) . CHA can be synthesized with a specific acid site density (Si:Al ratio) but with varying distributions of proximal Al‐Al site ensembles, for example by varying the ratios of N , N , N ‐trimethyl‐1‐adamantylammonium (TMAda + ) to Na + structure‐directing agents (SDAs) present during crystallization at fixed total SDA cation concentration . This synthesis procedure enables systematically varying the number of Al‐Al pairs within a 6‐MR (as measured by Co 2+ titration), thus avoiding effects caused by simultaneously varying bulk acid site density to allow better elucidation of acid site proximity effects on catalysis .…”

Benchmarking binding energy calculations for organic structure-directing agents in pure-silica zeolites

“…However, most known OSDAs are positively charged and direct the formation of zeolites with heteroatoms in their backbone. Theoretical studies on how OSDAs affect the position of the heteroatoms have been developed, [80][81][82][83][84][85] although at a high computational cost. Nevertheless, we suggest that the current analysis should be transferable to cationic OSDAs as well.…”