Synthesis and Characterization of Zeolites via a Multiple Inorganic Cation Approach

Abstract

Zeolites and molecular sieves, a class of crystalline microporous materials with unique shape-and/or surface-selective properties, due to the extreme uniformity in size and shape of their cavities and channels, are of major importance for industrial catalysts and adsorbents. If so, the development of new synthesis methods for already known zeolites is of technological relevance, as well as of scientific interest: there are many examples where the original synthesis procedure includes the use of expensive organic structure-directing agents (OSDAs) and/or heteroatoms. In this thesis we have synthesized zeolites with novel framework topologies and compositions via the multiple inorganic cation approach in the presence of already known OSDAs. We have described the small gases (e.g., CO2, CH4, and N2) adsorption properties of alkali cation-exchanged forms of our novel materials. Finally, we have investigated the catalytic properties of our novel zeolites in Brønsted acid- and metal-catalyzed reactions. i.e., as catalyst supports as well as catalysts. 1. We report the synthesis and structure of PST-29, the second generation of the RHO family of embedded isoreticular zeolites, the structure of which was proposed about 50 years ago but has remained undiscovered until now. We were able to synthesize this missing zeolite using N,N´-dimethyl-diazabi-cyclo[2.2.2]octane dications as an organic structure-directing agent (SDA) in the presence of both Na+ and K+ ions. When adding a small amount of seed crystals to the synthesis mixture, we were also able to synthesize PST-29 with a higher Si/Al ratio (4.5 vs 3.5), and thus with a higher stability, in the same mixed-SDA system. The Na+ form of this PST-29 has a comparable CO2 uptake to the corresponding cation form of zeolite rho, but is characterized by much faster adsorption kinetics, suggesting its high potential as a CO2 adsorbent. A comparative study was performed not only on the proton form of rho and PST-29, the first and second generations of the RHO family of embedded isoreticular zeolites, respectively, but also on their steam-dealuminated analogues, as catalysts for methylamines synthesis at 400 oC from ammonia and methanol. A similar yield in the sum of monomethylamine plus dimethylamine was observed for H-rho and H-PST-29. Steam treatment at 600 oC gave both zeolites an increase in catalyst lifetime, but which is much more apparent in H-PST-29, probably due to a larger decrease in strength of medium and strong acid sites, together with the mesopore formation. Dealumianted PST-29 showed a somewhat lower catalyst stability than H-mordenite, the former commercial catalyst for this reaction. However, it was found to exhibit a much higher yield in smaller methylamines and a comparable regenerability. 2. The multiple inorganic cations approach with already known OSDAs in the aluminosilicate system have enabled to crystallize a series of unprecedented zeolite structure. We report that the use of Me2-DABCO in presence of both Na+ and Cs+ cations has been successfully introduced for the synthesis of aluminosilicate version of SBT-type zeolite, which have large cages connected by 8- and 12-ring intersecting channels in three dimensions. This topology and these pore dimensions present interesting opportunities for catalyst in reactions of industrial relevance. 3. We report that RHO, KFI and OFF zeolites have been synthesized using tetrarethylammnoium ions as an organic structure-directing agent via a multiple inorganic cation approach. Under the synthesis conditions studied here, the crystallization field each of these zeolites was found to be narrow. 4. The structure model of an ABC-6 family zeolite with the 4-layer stacking sequence AABC was first proposed in 1981, but has remained undiscovered so far. Here we report the synthesis of PST-33, a small-pore aluminosilicate (Si/Al = 4.1) zeolite with this framework topology, using 5-azonia-spiro[4.4]nonane ions as an organic structure-directing agent via the so-called multiple inorganic cation approach, and its structure solved by single-crystal X-ray crystallography. PST-33 contains a two-dimensional pore system consisting of 8-hedral ([4662]) double 6-ring (d6r), 14-hedral ([4668]) sod, and 17-hedral ([496583]) lev cages. The Na+ form of PST-33 showed comparable CO2 adsorption properties to the corresponding cation form of zeolite A and chabazite.