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

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.