Influence of Organic and Inorganic Cations on Directing Aluminum Distributions in Zeolite Frameworks and Effects on Brønsted Acid Catalysis

Claire T Nimlos, Purdue University

Abstract

Zeolites are microporous crystalline solids with tetrahedrally bonded Si4+ atoms linked together with bridging oxygens, interconnected in various geometries and arrangements to generate a diversity of microporous topologies. The substitution of Al3+ into framework tetrahedral sites (T-sites) generates anionic lattice charges that can be counterbalanced by protons (Brønsted acid sites) or extraframework metal cations and complexes that can act as catalytic active sites. The local arrangement of Al ensembles can be categorized by the size of the (alumino)silicate rings and the number and order of the Al atoms they contain, which are critical structural features that influence their ability to serve as binding sites for extraframework cations of different size and oxidation state. The ability to exercise control over the isomorphic substitution of Al3+into the zeolite framework during hydrothermal crystallization has long been envisioned, but recognized to depend on complex and kinetically-controlled nucleation and crystal growth events that challenge the development of reproducible synthesis routes and predictive synthesis-structure relations. Here, we present the results of extensive experimental and theoretical investigation of the chabazite (CHA) zeolite topology, which contains a single crystallographically distinct T-site that enables studying effects of Al arrangement independent of T-site location. We then extend these findings and methodologies to investigate more complex zeolite topologies with larger numbers of distinct T-sites, including other small-pore (AEI, LEV) and medium-pore zeolites (MFI, MEL), with a specific focus on MFI zeolites because of their versatility in commercial applications. Cationic species are often present during hydrothermal zeolite crystallization, in the form of inorganic and organic structure directing agents (SDAs), to help guide formation of the intended zeolite topology and to compensate charge when Al is incorporated into the lattice. Variations in the type and amount of cationic SDAs have been shown to influence both the Al siting within different void locations of a given zeolite and the local Al arrangement. In order to make quantitative assessments of the number of Al-Al site pairs formed in a given zeolite, experimental protocols to titrate the specific Al-Al site ensembles are required. We specifically explore the use of Co2+ titrants at saturation uptakes, verifying the sole presence of Co2+ cations via spectroscopic identification and a cation site balance that is closed by quantifying residual Brønsted acid sites by NH3 titration. We then investigate the role of the cationic SDA content in the synthesis mixture on the Al arrangement in MFI zeolites. Depending on the specific mixture of theorganic cation tetrapropylammonium (TPA+) or various neutral organic molecules when used together with smaller Na+ cations, MFI zeolites can be crystallized over a range of Al content. Moreover, the fraction of Co2+-titratable Al-Al pairs correlates with the amount of occluded Na+ cations when the total Al content is held approximately constant (Si/Al ~ 50). These results are consistent with our prior reports of CHA zeolites, wherein the occlusion of smaller Na+ cations correlates positively with the formation of Al-Al pairs in six-membered ring (6-MR) locations. Unlike the N,N,N trimethyl-1-adamantylammonium (TMAda+) cation used to crystallize CHA, which alone does not form Co2+ titratable Al-Al site pairs, the organic TPA+ alone can form Al-Al site pairs in MFI. DFT calculations of Al siting energies, using a 96 T-site MFI unit cell containing either one or two Al charge-balanced by one or two occluded TPA+ respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA+ and the anionic lattice charge. DFT calculations of probable Co2+ exchange sites are used to identify a subset of Al-Al site pairs with favorable energies when compensated either by Co2+ or by two TPA+ molecules in adjacent MFI channel intersections. MFI crystallized with one cationic species (TPA+ or Na+) with a neutral organic species (ethylenediamine, pentaerythritol, or a mixture of methylamine and 1,4-diazabicyclo[2.2.2]octane) contain significantly lower fractions of Co2+-titratable Al-Al pairs at similar bulk Al content (Si/Al = 43–58), demonstrating the role of neutral organic species to occupy void spaces without providing the capacity to compensate charge, thus serving to increase the average spatial separation of framework Al sites.

Degree

Ph.D.

Advisors

Gounder, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Optics

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