Dispersion and Fixation of 12-Tungstophosphate Anion on a Silica Surface Modified with Silane Agents Having an Amine Group and Their Catalytic Properties

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Abstract
Silica gel (SiO2) was modified by a dip-cure method with silane coupling agents having an amine-group: (3-anilinopropyl), (3-aminopropyl), and N-[3-(2-aminoethyl)aminopropyl] trimethoxysilanes. 12-tungstophosphate (PW12) was deposited on the carriers as a test catalyst material. The amounts and the dispersed states of the silane agents and PW12 on the modified SiO2 were successfully analysed by gravimetry as well as UV and FT-IR spectrometries. The amount of PW12 deposited increased in proportion to the amount of the silane agents on SiO2 in the case of deposition under a constant concentration of PW12-solution, although only a little PW12 was deposited directly on SiO2. The acidic site of PW12 was confirmed to interact directly with the amine-group on the modified SiO2, and the strength of the interaction was correlated with the basicity of the amine-group. The activity per gram of catalyst for the dehydration of 2-propanol as a test reaction is generally higher than that on a directly deposited one, although the activity per PW12 unit certainly decreases with the SiO2-modification. The activity was confirmed to increase by a pretreatment at high temperatures, such that the amine-group was completely decomposed. It was clearly shown that the present modification method could be effectively applied for preparing a highly dispersed heteropoly acid catalyst on carriers.