Ammonia adsorption-desorption over the strong solid acid catalyst H3PW12O40 and its Cs+ and NH4+ salts - Comparison with sulfated zirconia

Ammonia thermodesorption using a thermal conductivity detector does not allow an unambiguous study of the acidic properties of strong solid acids such as heteropolycompounds and sulfated zirconia. As a matter of fact, these strong acids release compounds other than NH3 during TPD experiments which obviously involves side-reactions. In the present work, in order to circumvent the preceding problems, NH3 adsorption and TPD experiments were performed using a dual mass spectrometry (MS)-TCD detection system. The equipment was applied to the characterization of the acidic properties of porous heteropolycompounds. CsxH3-xPW12O40 and (NH4)(2.8)H0.2PW12O40, by comparison with the parent acid H3PW12O40 and another strong solid acid, sulfated zirconia. It was observed that gaseous NH3 reacted and formed ionic species, leading to (NH4)(3)PW12O40 in the case of heteropolyacids (HPA) and probably (NH4)HSO4 in the case of sulfated zirconia. Consequently, based on the amount of NH3 irreversibly adsorbed at 373 K the number of acid sites has been deduced. For all HPA samples, TPD profiles of NH3 appeared quite similar suggesting that the TPD curves were more connected to the stability of these ammonium salts rather than to the acid strength of the solid.