Synthetic Methods to Control Aluminum Proximity in Chabazite Zeolites and Consequences for Acid and Redox Catalysis

John R Di Iorio, Purdue University

Abstract

Zeolites contain distinct Bronsted acid site (H+) ensembles that arise from differ- ences in the arrangement of framework Al atoms (AI-O(-Si-O),-Al) between isolated (x>3) and paired (x=1,2) configurations, the latter defined by their ability to ex- change certain divalent cations (e.g., Cu2+, Co2+). Manipulation of the synthesis conditions used to prepare MFI zeolites has been proposed to influence the proximity of framework Al atoms, but in a manner that is neither determined randomly nor by any simple predictive rules. Moreover, the effects of proton proximity have been studied for hydrocarbon catalysis in MFI zeolites, but interpretations of cataly tic phenomena are convoluted by effects of the distribution of framework Al atoms among different crystallographic tetrahedral sites (T-sites) and diverse pore environ- ments (i.e., confining environments) present in MFI. This work instead focuses on the chabazite (CHA) framework, which contains a single cryst allogr aphically-distinct lattice tetrahedral site (T-site) that allows clarifying how synthesis conditions influence Al proximity, and in turn, how H+ site proximity influences catalysis independent of T-site location. Selective quantification of the number and type of H+ site ensembles present in a given zeolite allows for more rigorous normalization of reaction rates by the number of active sites, but also for probing the number and identity of active sites on bi- functional catalysts that contain mixtures of Bronsted and Lewis acid sites. Gaseous NH3 titrations can be used to count the total number of protons on small-pore CHA zeolites, which are inaccessible to larger amine titrants (e.g., pyridine, alkylamines), and can be used to quantify the exchange stoichiometry of extraframework metal cations (e.g., Cu2+, [CuOH2+ that are stabilized at different framework Al arrange- ments. Additionally, paired Al sites in CHA zeolites can be titrated selectively by divalent Co2+ cations, whose sole preseuce is validated by measuring UV-Visible spec- tra, counting residual protons after Co2+ exchange, and titration of paired Al with other divalent cations (e.g., Cu2+). These different titration procedures enabled reli- able and reproducible quantification of different Al arrangements, and recognition of the effects of different synthetic methods on the resulting arrangement of framework Al atoms in CHA zeolites. Upon the advent of this suite of characterization and titration tools, different synthetic methods were developed tocrystallize CHA zeolites at const ant composition (eg., SiJAl = 15) but with systematic variation in their paired Al content. The substitution of N,N,N-trimethyl-l-adam antylammonium (TMaAdat+) cations for Na+ in the synthesis media (Nat /TMAda+ <2), while holding all other synthetic variables constant, resulted in CHA zeolites of similar composition (Si/Al = 15) and organic content (ca. 1 TMAdat per cage), but with percentages of paired Al (044%) that increased with the total amount of sodium retained on the zeolite product. This result suggests that sodium atoms are occluded near the ammonium group of TMAda~ leading to the formation of a paired Al site. Replacement of Na+ by K+ in the syuthesis media allowed for the crystallization of CHA (Si,'Al = 15) at much higher ratios of alkali to TMAda+ (Kt /TMAdat+ <20), likely due to the suppression of alternate crystalline phases by Kt. Incorporation of K+ during crystallization of CHA did not correlate with the formation of paired Al sites, but instead resulted in the displacement of one TMAda* molecule by two K+ cations, which likely assist in the stabilization of the CHA framework at low TMAda+ concentrations.

Degree

Ph.D.

Advisors

Gounder, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Medical imaging|Optics

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