Catalysis of Organic..

Bacchini et al . 113these samples had the largest fraction of sites with the higher strength. Sample eqspdid not possess Brønsted acidity at all, while sample ox3sp had Brønsted sites whichstill retained pyridine after evacuation at high T, and that therefore can be classifiedas strong ones. In the case of sample ox1sp, no pyridine remained adsorbed onBrønsted sites after evacuation at 300°C.Table 2. Main chemical-physical features of catalysts after reactivity tests.Sample S. area, m 2 /g n in V n+eqsp 21 3.99ox1sp 21 3.98-3.99ox2sp nd 4.020-4.025ox3sp 16 4.2880eqspox1sp1residual pyridine, μmol/g604020ox1speqspox3spox3sp0,80,60,40,2Lewis sites / total sites000 100 200 300 400 500desorption temperature, °CFigure 3. Total amount of residual adsorbed pyridine (full symbols) and fraction ofLewis sites with respect to the total number of acid sites (open symbols) as afunction of the desorption temperature (after adsorption of pyridine at roomtemperature).The stronger Lewis-type acidity in the equilibrated catalyst agrees with themany literature evidences about the existence of these sites in well-crystallized VPP(14). Also, a discrete amount of V 3+ ions may contribute to an enhancement ofLewis-type acidity, due to the presence of defects associated to the anionic vacancies(5). The Brønsted acidity in the oxidized catalyst (sample ox3sp) can be associated to