Why are zeolites harder to make than MOFs? A study of the sodalite system

Abstract

The mechanochemical synthesis of zeolites is far less developed than that of MOFs, despite the need for more sustainable production of these commodity materials. Grinding (potentially involving mechanochemistry) followed by heating has been investigated but in many cases the individual roles of these activation methods have not been elucidated. Also, reactants that might corrode steel vessels (e.g. NaOH), have often been used. Here, we systematically investigate the mechanochemical synthesis of sodalite (SOD) to elucidate the key factors. To aid diffusion and minimise reaction times we have investigated liquid Si sources, (tetraethoxysilane,TEOS; vinyltriethoxysilane VTEOS; and phenyltriethoxysilane, PTEOS) with NaOH and Al(OAc)2OH. With TEOS, ball milling without heating gave a quantitative reaction (by 27Al MAS NMR) in as little as 50 min, but only amorphous phases were formed (as shown by PXRD). Organosilanes enabled solvent-free synthesis of organic-functionalised SOD phases. Acetate-impregnated phases, e.g. [Al3Si3O12]2[Na4OAc]2, were obtained arising from the use of Al(OAc)2OH as Al source. 3D Electron Diffraction (3DED) was used to characterise TSOD and VSOD giving a = 9.0311(7) for TSOD and 9.0718(9) for VSOD. The larger cell of the latter is consistent with the accommodation of vinyl groups. Use of Na2SiO3·5H2O as Si source to avoid NaOH, in closed steel vessels with short heating periods at moderate temperatures (e.g., 0.5-4 h at 100-300oC) gave the acetate-impregnated SOD phase [Al3Si3O12]2[Na4OAc]2 in high crystallinity (up to 94%) and with N2-BET surface areas similar to the highest literature values from hydrothermal synthesis (e.g. 44.7 m2/g). Overall, we note that milling the reactants prior to heating often gave no clear advantage compared to simple hand-mixing prior to heating. Notably, heating was always required to crystallise the product into the zeolite structure. We ascribe this to the high Si-O and Al-O bond strengths which necessitate heating to cause recrystallisation into ordered phases in contrast to MOFs based on more labile bonds. This study has clarified some of the technical challenges and factors to consider in mechanochemical zeolite synthesis and presents an efficient method for solventless SOD synthesis in closed vessels. The potential for developing continuous zeolite synthesis by twin-screw extrusion (TSE) is also discussed.