Catalysis of Organic..

Bacchini et al . 1111 summarizes the main chemical features, surface area and distribution of V speciesas determined by chemical analysis, for catalysts employed in the present work.Sample eq was a fully equilibrated catalyst, while samples oxn were obtained bycontrolled oxidizing treatments of sample eq. The procedure adopted for thistreatment is also given in the Table; more severe conditions led to the generation of ahigher amount of V 5+ , while sample eq did not contain V 5+ at all.Table 1. Main chemical-physical features of catalysts before catalytic tests.Sample T of treatment in air (°C), time (h) S. area, m 2 /g n in V n+eq none 21 3.99ox1 450, 4 17 4.035ox2 500, 4 nd 4.052ox3 550, 4 14 4.47Figure 1 reports the corresponding UV-Vis Diffuse Reflectance spectra. Theband at 400 nm is relative to the CT between O 2- and V 5+ ; its intensity wasproportional to the amount of V 5+ . On the other hand, the intensity of the band at 900nm decreased, relative to a d-d transition for V 4+ in the vanadyl moiety. The XRDpattern of all samples, but ox3, corresponded to that of well-crystallized VPP; in thecase of sample ox3, additional reflections were due to the presence of δ-VOPO 4(JCPDS 37-0809). Therefore, the treatment of the VPP led to the generation ofsurface V 5+ in samples ox1 and ox2, and to the growth of crystalline VOPO 4 insample ox3.Table 2 compares the chemical analysis of the spent samples, downloaded afterreaction (samples eqsp and oxnsp); Figure 2 reports the corresponding UV-Vis DRspectra. It is shown that catalysts after reaction were more reduced than thecorresponding samples before reaction. Sample ox1sp was fully reduced, and alsocontained a low amount of V 3+ ; samples ox2sp and ox3sp, instead, yet retained somefraction of V 5+ . In electronic spectra, the intensity of the band at 400 nm was lowerthan in corresponding fresh samples (Figure 1); in the case of catalysts eqsp andox1sp, this band was absent. Therefore, the V 5+ species generated by the oxidizingtreatment of the VPP was not stable in the reaction environment, and wasprogressively reduced to V 4+ . In addition, a weak band at ≈ 500 nm, attributed to theV 3+ species (13), was present in samples eqsp and ox1sp.Figure 3 reports the amount of residual pyridine adsorbed on acid sites as afunction of temperature, for samples eqsp, ox1sp and ox3sp. Tests were carried outby adsorbing pyridine at room temperature, and by recording the FT-IR spectrumafter desorption at increasing temperatures. The amount of pyridine retained wasproportional to the total number of acid sites (Lewis + Brønsted). The Figure alsoshows the corresponding fraction of Lewis-type sites; the latter were the predominantacid sites in all samples. However, the residual amount of adsorbed pyridine afterevacuation at 400°C was very low; this indicates that the acid sites were of mediumstrength.