Role of drying techniques on the development of porosity in silica gels

Silica gel has been prepared by the hydrolysis of TEOS under three differen t pH conditions. These gels have been subjected to drying in an air oven, u nder humidity, and also by exposure to microwaves, followed by further calc ination at 500 degrees C under slow heating schedule(3 degrees C/min). Sili ca samples thus obtained are characterised by B.E.T. specific surface area and pore volume data. The thermal decomposition behaviour of the precursor gels are also reported. The adsorption isotherms of these samples indicate different behaviour related to the method of synthesis. Increase in pH of h ydrolysis of the TEOS from 3 to 8 results in increase in specific surface a rea in tune with the earlier reports. However more significant observation is the variation in pore size distribution as evidenced from adsorption iso therms related to method of drying. Silica gels prepared at pH 3 show Type I behaviour irrespective of the method of drying. However gel prepared at p H 6 shows Type II behaviour when dried under microwave with specific surfac e area as high as 635 m(2)/g and pore volume 0.9733 cc/g. The precursor gel s prepared at still higher pH exhibit Type IV behaviour when subjected to m icrowave drying. The pH conditions of synthesis of precursor gels along wit h drying techniques appear to affect not only the surface areas and porosit ies but also the resultant adsorption isotherms, in sol-gel silica.