Y. Hu, P. Shah
Keywords: silica gel, silica support, porosity, surface area, pore volume, catalysis, polymerization
Summary:Most commercial silica gel products are not nanomaterials per se but their pore structure is distinctively nanoscale. Silica gel has found many industrial applications including catalysis, adsorption, surface coating, ink jet printing, personal care, polymer additive, thermal insulation, desiccant, etc. One of the main factors that contribute to the versatility of silica gels in these applications is its tunable porosity – surface area (SA) from ~10 to 1000 m2/g, pore volume (PV) from ~0.3 to >3 cc/g, and average pore size (APD) from a few to several hundred nanometers. In catalysis, the porosity of the silica gel support can affect the activity, selectivity and stability of the catalyst. In the particular case of silica gel supported Cr catalyst for ethylene polymerization, the porosity of the silica gel support plays a key role in determining the molecular structure of the polyethylene produced. Thus ability in tailoring the porosity of silica gel is critical for its industrial applications. For catalysis applications we divide silica gel into five types based on its SA and PV: (1) Ultra-wide-pore silica gel (UWPS) with ~10-50 m2/g SA and ~0.5-1.5 cc/g PV; (2) Wide-pore silica gel (WPS) with ~50-250 m2/g SA and ~0.7-1.2 cc/g PV; (3) medium PV silica gel (MPV) with ~250-700 m2/g SA and ~0.7-2.0 cc/g PV; (4) High PV silica gel (HPV) with ~250-700 m2/g SA and ~2.0 to >3.0 cc/g PV; and (5) High SA silica gel (HiSA) with ~700-1000 m2/g SA and ~0.7 to >3 cc/g PV (see Figure 1). While UWPS is especially suitable for catalysis applications involving large molecules such as oligomers and polymers, HiSA gel is ideal for catalysis applications that require high concentration of active sites and are not subjected to diffusion-limitation. The wide range of porosity required for various catalysis applications of silica gel is only achievable by a combination of process technologies. Traditional silica gel process technology – aging silica hydrogel in an aqueous medium at alkaline pH and at elevated temperature – is well suited for the production of MPV and HPV types of silica gel but is incapable or non-ideal for the production of HiSA, WPS and UWPS types of silica gel. In this presentation we will give examples of process technologies suitable for the commercial production of each type of silica gel mentioned above, with particular focus on the new process technology we developed recently to produce HiSA silica gel and its application as support for Cr/Silica catalyst for ethylene polymerization.