NaBH<sub>4</sub> Sodalites, Synthesized by Modified Methods: (1) Autothermal Synthesis and (2) Crossover Reaction from Gel to Melt Flow

Abstract

NaBH 4 sodalites were obtained by two new modified methods of crystallization: (1) autothermal synthesis and (2) crystallization with crossover from gel to melt flow in NaOH flux. Syntheses results were presented according to XRD, SEM and FTIR. Besides important features of both synthesis procedures product properties like crystal size and morphology were investigated. Spherical agglomerates of microcrystalline sodalite of composition Na 7 [AlSiO 4 ] 6 BH 4 (H 2 O) 2 were already observed after 4 h without any external heating by the autothermal procedure. Sodalites of the same average composition but in form of agglomerated nanoparticles are crystallized after very short times (2 h 30') by the crossover reaction from gel to melt flow. Hydrogen release by heating was further studied for two selected samples with comparable composition from each synthesis procedure. Total hydrogen release by hydrolysis reaction with the internal cage water was found during heating of the autothermal product in synthetic air up to 550˚C. In contrast hydrogen release from the nanocrystalline sample of crossover synthesis was not completed when heated under the same conditions. These differences were discussed in terms of crystal size and an earlier loss of the internal water from the nanocrystals of the crossover synthesis.

Crystallization of Hydrosodalite Na<sub>6</sub>[AlSiO<sub>4</sub>]<sub>6</sub>(H<sub>2</sub>O)<sub>8</sub> and Tetrahydroborate Sodalite Na<sub>8</sub>[AlSiO<sub>4</sub>]<sub>6</sub>(BH<sub>4</sub>)<sub>2</sub> inside the Openings

“…The experimental conditions are summarized in [2]. Besides the role as Al-Si-source the function of Zeolite 13-X as "water provider" in the gel step of the crossover reaction is of main importance here.…”

“…Solvent free synthesis of zeolites [11] [12] [13] and other compounds [12] [13] [14] [15] [16] is one route for insertion of modified reaction conditions. Other interesting new methods are the autothermal synthesis, where the whole process energy is allocated by the enthalpy of a tailored reaction system itself [17] as well as the crossover synthesis from gel to melt flow [1] [2].…”

“…The present study uses the new method first demonstrated in [1] where halide-, borate and nitrite sodalites as well as BH 4 -sodalite [2] have been obtained by crystallization of a dense pressed pellet of solid educts. The procedure is a one-pot process under open conditions.…”

“…The present study presents experiments on hydrosodalite as well as BH 4 -sodalite formation as steel mesh supported thin wafers. Preparation of both sodalite wafers is performerd by the crossover synthesis (CS) from solution to melt flow, first described in [1] [2]. Whereas the space filling of the steel mesh with BH 4 -sodalite crystals was proved to be a complete close and stable package, hydrosodalite reached only a somewhat weaker quality.…”