Study of the acidity of carbon supported and unsupported heteropolyacid catalysts by ammonia sorption microcalorimetry

Acid property of a series of bulk and carbon-supported heteropolyacids of tungsten were studied by means of microcalorimetry applied to the ammonia sorption. The four bulk samples H3PW12O50, H4SiW12O40, H5BW12O40 and H21B3W39O132 all display a strong acidity, which can be classified in the order of H3PW12O40>H4SiW12O40>H(5)BW(12)O(40)much greater than H21B3W39O132. A dependence on acidity is shown for the catalytic activity of these bulk heteropolyacids in the methanol dehydration. For the Keggin-type samples, nearly all the protons detected display the same acidity for a given heteropolyacid. Deviation from stoichiometric ammonia sorption is observed with H4SiW12O40 and becomes more important with H5BW12O40 The acidity of the three supported Keggin-type heteropolyacids, H3PW12O40/CPX, H4SiW12O40/CPX and H5BW12O40/CPX, is slightly reduced as compared with that of corresponding bulk samples. On the contrary, acidity of H21B3W39O132 is enhanced with the CPX carbon supported sample; especially the quantity of the strong acidic sites is considerably increased. Heterogeneity in acidity is observed when the heteropolyacids are grafted on the carbon support and could be related to the heterogeneity of adsorption sites on the carbon surface. (C) 1999 Elsevier Science B.V. All rights reserved.