Utilization of rice husk silica for synthesis of mesoporous molecular sieve MCM-41 applied for catalytic hydrodechlorination of chlorinated volatile organic compounds

Abstract

High silica containing in rice husk was utilized as silica source for MCM-41 synthesis. Rice husk was refluxed in 5 M hydrochloric acid at 80C for 1 h and then calcined at 650 C for 4 h. This method provided more than 99% of silica content. RH-MCM-41 was synthesized using sodium silicate prepared from rice husk as silica source and hexadecyltrimethylammonium bromide (CTAB) as template. The molar composition was 1.0SiO2:1.1NaOH: 0.13CTAB: 0.12H2O. The mesoporous structure was completely crystallized within 48 h aging at pH value of 10. The RH-MCM-41 possessed uniformly hexagonal structure. The BET surface area was around (800+8) m2 g-1 with average pore diameter of 29.5 A and narrow pore size distribution. The estimated cost of 1 kg RH-MCM-41 was approximately 26,000 Baht. This material was applied to the adsorption studiesof some chlorinated valatile organic compounds (CVOCs) such as trichloroethylene (TCE), tetrachloethylene (PCE), and carbon tetrachloride (CT). The adsorption of TCE and PCE was proved to be physical, while the adsorption of CT was stronger. The adsorption capacity of RH-MCM-41 for CVOCs was higher than commercial mordenite and activated carbon. The adsorption isotherm of carbon tetrachloride (CT) at 25C on the RH-MCM-41 was determined by using a magnetically coupled microbalance. The CT isotherms were classified as reversible Type V and the nitrogen adsorption isotherm was Type IVc. Pore size distributions (PSD) of nitrogen isoththerm for the RH-MCM-41 calculated by using the BJH and Naono methods showed quite narrow pore diameter distributions, centered around 27 and 29 A, respectively. Similarly, the peak pore diameters calculated from CT isotherms using the BJH and Naono methods were 24 and 28 A. The RH-MCM-41 was tested as a catalyst support of palladium for the hydrodechlorination of chloroform. The RH-MCM-41 supported palladium showed the best performance with the conversion enhanced up to 80-90% at 150-200 C compared to silica and silica-alumina.