(264d) Roles of Metal Cations in Static and Continuous Synthesis of Phillipsite and Tobermorite

The identity of metal cations plays crucial roles as Al counterbalancing cations and as inorganic structure-directing agents (SDA) in growth solutions that undergo high temperature treatments (≥ 373 K) to crystallize inorganic, porous materials in the absence of organic SDAs. We vary the metal cation identity (e.g., K⁺, Na⁺, and Ca²⁺) and composition to interrogate crystallization mechanisms associated with phillipsite and tobermorite precipitation. XRD patterns of solid precipitates recovered after hydrothermal treatments (373 or 393 K) of synthesis growth solutions containing sodium and potassium (PHI-TOB-373/393-0) result in the selective crystallization of phillipsite zeolites. Substitution of the cationic charge imparted by sodium and potassium with calcium (PHI-TOB-373/393-1), however, results in the crystallization of tobermorite silicate hydrates. Elemental analysis (probed via ICP-OES) of PHI-TOB-373/393-0 and PHI-TOB-373/393-1 indicates that phillipsite crystals retain metal cations at molar ratios that exclusively account for Al counterbalance roles ((Na+K)/Al ≤ 1); yet, tobermorite silicate hydrates retain inorganic cations compensating for Al counterbalance and SDA roles (Ca/Al ≥ 1). Systematic replacement of sodium and potassium with calcium in synthesis gels results in the crystallization of mixed phases, and a systematic increase in the total metal cation content ((K+Na+Ca)/Al) and the micropore volume (~0.003-0.07 g cm^-3, N₂ adsorption at 77 K). These results suggest that growth solutions containing calcium selectively nucleate aluminum and silicon complexes that crystallize tobermorite during high temperature treatments (373-393 K), and complexes that lead to phillipsite precipitation are predominantly nucleated in the presence of sodium and potassium. Guidelines developed in batch reactors were extrapolated to fundamental learnings, critical for the synthesis of phillipsite zeolites in a continuous plug flow reactor (473 K, 2 MPa). These findings highlight the roles of metal cations during hydrothermal treatments in the absence of organic SDAs, and their relevance in crystallization mechanisms that dictate zeolite polymorphism.