The catalysts for the direct conversion of carboxylic acid to ketene and alkylketene dimer(AKD) by catalytic dehydration reaction with high yields were investigated. Direct conversion of high molecular weight carboxylic acid to ketene and AKD via abstrating a water molecule by silica catalyst could avoid the hazard of toxic chemicals and minimized unwanted wastes. Among the various metal oxides which could produce ketenes, only silicas showed the activity for AKD formation. When using typical amorphous silica powders (surface area: about 300㎡/g, with closed pores), the yields were very low regardless of pore size (60, 100, 150Å). On the other hand, mesoporous silicas (M41S, with open pores) gave higher yields of AKD in spite of smaller pore sizes (20-100Å). The higher yields were caused by the easy transfers of the reactant and products along the open pores of M41S. Higher silianol concentration also could give higher yield of AKD.