Granulation and mechanic strength evaluation of nanomaterials with high potential as adsorbents and desiccants

Abstract

As an alternative to zeolites, mesoporous silica materials became spotlighted as competitive adsorbent and catalyst in different areas, due to their ordered and uniform porous structures and high surface area. In order to increase the availability of these materials, the need to granulate them becomes vital for efficient performance in industrial units. This study reports the production and characterization of clay-free granules of fumed silica and Santa Barbara Amorphous-15 (SBA-15) mesoporous silica, by applying extrusion-spheronisation technique. The traditional granulation process uses inorganic binders to ensure the mechanical strength of the final granules, but they fill the voids and pores of the material decreasing its adsorbance capacity. The main objective is to produce silica granules which can hold their features with the necessary mechanical strength and can be efficiently used as adsorbents and desiccants. The methodology suggested uses a methylcellulose polymer (methocel) as organic binder, which is removed after calcination. The effects of the granulation process, geometry, adsorbent capacity and mechanical performance were evaluated and discussed based on thermogravimetry, Fourier-transform infrared spectroscopy, nitrogen adsorption, water adsorption, axial and diametrical crushing tests, bulk crush test and microtomograph images. The granulation process was successfully achieved and produced millimetre sized pure granules of silica. Granules prepared with 20 % of methocel and calcinated at 800 °C had high mechanical strength, i.e. 8.3 MPa for SBA-15 mesoporous silica and 4.9 MPa for fumed silica (by axial crushing test). The surface area and pore volume of SBA-15 granules (calcinated at 800 °C) were around 392 m2/g and 0.53 cm³/g, respectively. The decrease of surface area, pore volume and water sorption capacity were observed for SBA-15 granules, calcinated at 800 °C, in comparison to the ones calcinated at 500°C. This can be explained by the fact that high calcination temperatures decrease the concentration of -OH groups on the surface of the material, reducing the number of micropores and enlarging some mesoporous pores