12-钼磷酸的Brnsted酸位和Lewis酸位

BRONSTED AND LEWIS ACID SITES OF 12-MOLYBDOPHOSPHORIC ACID

研究了加热、抽空等预处理对固态12-钼磷酸(PMA)酸位的影响。发现了PMA上除存在B-酸位外,还可生成L-酸位。研究了PMA上L-酸位的成因,水在两类酸位形成过程中的作用,L-酸位与B-酸位的相对强度,以及PMA上酸强度的分布。据此,对Miscno等认为PMA中只存在质子酸和酸强度分布均匀的观点提出了不同的看法。

A sample of dodecamolybdophosphoric acid (PMA) with the Keggin structure and a chemical formula of H3 CPMo12O40)·34.9H2O was prepared for studying the acid sites of solid PMA by varying pretreatment conditions and using IR, TPD and in-situ IR techniques with Pyridine and ammonia as probe molecules. It was revealed that both Lewis (L-) and Brbnsted(B-) acid sites can exist on / in PMA. Infrared studies of pyridine adsorbed onto PMA indicate that only B-acid sites exhibit with strong acidic property for PMA containing a definite quantity of H2O (Fig.1 and Fig. 2-a). Heating PMA in a dry N2 flow (Fig.4) and vacuum outgassing of PMA at high temperatures (Fig. 2-b-e) both can make a part of water and lattice oxygen of PMA being desorbed, and it results in the formation of baring ions of Mo6+Mo5+). which are L-acid sites. It is proved that the L-acid sites can be converted to B-acid sites while an adequate quantity of H2O exists (Fig.2-e, f). In-situ IR spectra of pyridine adsorbed onto PMA, recorded in a TPD run, show that the acid strength of B-acid sites is much stronger than that of L-acid sites (Fig.3). TPD spectra of NH3 adsorbed onto PMA (Fig.4) display three peaks of NH3, which are assigned to the weak acid sites (peak 1), medium strong (peak 2') and strong(peak 30 acid sites, respectively, leading to the conclusion that the distribution of acid strength in PMA is heterogeneous. Based on the findings of this paper, some results of references [2,3] art discussed and resonable explanations are proposed.