(373e) Synthesis of Zeolite X from Rice Husk Ash

Zeolites are materials of high industrial importance, derived from their molecular sieving and ion exchange capacity, and their catalytic properties. These materials have attracted great interest because they can be produced with different crystalline structures, with uniform pore sizes and channels, high surface area, high thermal stability and an excellent adsorption ability. In recent decades, the synthesis of zeolite has been investigated, trying to exploit more environmentally friendly raw materials under moderate processing conditions. Specifically, some biobased materials have been considered as alternative feedstock for the synthesis of zeolites, in particular the residues from biomass combustion processes.

Due to the high silica content in Rice husk ashes (nearly 90% amorphous silica) this material is suited as feedstock for zeolite production. This residue is obtained from rice drying and parboiling ovens, and it is commonly disposed in cement kilns or mixed directly into concretes as a partial cement replacement. However, production of zeolite materials would be a better value-added alternative, suitable for exploitation in Latin-America and Asia where rice is a major agricultural commodity.

Among the different possible zeolite structures that can be produced with rice husk ashes, zeolite X is of special interest given its molecular sieving capacity, and the high adsorptive selectivity in the separation of paraffin-olefin mixtures. In this regard, this work focused in the use of rice husk ashes (RHA) obtained from a Colombian milling facility (waste product of the parboiling process), as a raw material for the synthesis of zeolite X. Initially, the raw ashes with a SiO₂ content of 80% were calcined to remove the unburned carbonaceous material, reaching a 91% SiO2 content. Then, the calcined solids were processed under alkaline conditions to obtain sodium silicate. The resulting mixture was filtrated and characterized via Atomic absorption spectroscopy to determine the sodium and silicon content. In order to obtain a high content of silicon in solution, the effect of the reaction temperature and the NaOH/SiO2 ratio were assessed. Once sodium silicate was obtained, different formulations for the zeolite synthesis were evaluated to optimize yield and crystallinity degree (determined with X ray diffraction). Under the optimized conditions, a zeolite x material with a surface area of 870 ±4.6 m²/g was obtained. Finally, the adsorption capacity of the material was evaluated using methane, nitrogen and carbon dioxide at 25 °C and 1 atm.