Search

Potassium/ammonium salts of 12-molybdophosphoric acid were modified by the addition of Sb3+salt, and the chemical–physical features as well as the reactivity in isobutyric acid oxidehydrogenation of the compounds obtained were studied. All the salts prepared were monophasic and the secondary structure of the compounds was characterized by a cubic crystallographic cell. The addition of antimony led to a considerable increase in the thermal structural stability of the compounds. A second effect was on the average degree of oxidation of molybdenum, which decreased in proportion to the increase in antimony content. The results of chemical–physical characterization were interpreted by hypothesizing that antimony ions occupy a cationic position in the secondary structure of the heteropolycompound in the case of the salts containing 1 atom of potassium per Keggin unit (KU), and electronically interacts with molybdenum in the Keggin anion, leading to the formation of Mo5+species. In the case of samples containing three atoms of potassium per KU the antimony is present in the form of dispersed salt or oxide. The modification of the molybdenum redox properties led to considerable worsening of the catalytic performance in isobutyric acid oxidehydrogenation, with a decrease in activity (for the samples with one atom of potassium per KU) and also in selectivity to methacrylic acid (for the samples with three atoms of potassium per KU). This effect was interpreted as due to the stabilization of a lower degree of oxidation of molybdenum in antimony-containing samples.

Changes occurring on thermal treatment of the precursor of vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation ofn-butane, were studied. The precursor was mixed with stearic acid, used as an organic binder for pelletization of the powder. The calcination of the precursor leads to a partially oxidized compound, constituted of an amorphous VIV–P mixed oxide and a crystalline hydrated VV–P–O phase. The calcined compound, when left in a 1% hydrocarbon/air stream for 100 h leads to a “nonequilibrated” catalyst, and after 1000 h to the “equilibrated” catalyst. The catalytic activity of the nonequilibrated and equilibrated catalysts inn-butane andn-pentane oxidation was studied and compared; the chemical–physical features of the two catalysts were studied by means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD. Only well crystallized (VO)2P2O7was detected in the equilibrated catalyst and a homogeneous distribution of surface centers seems to be present on its surface. In the case of nonequilibrated catalyst, a poorly crystallized (VO)2P2O7is present together with an amorphous VIV–P–O phase and γ-VOPO4; these phases define a heterogeneous distribution of at least two kind of surface centers. This surface heterogeneity gives rise to a catalyst less selective inn-butane oxidation to maleic anhydride and less specific in the conversion ofn-pentane to phthalic anhydride.

Dehydrogenation catalysts based on chromium oxide supported on commercial alumina which contained increasing amounts of chromium oxide were prepared and characterized with different techniques: XPS, UV–Vis–NIR diffuse reflectance spectroscopy, XRD, and chemical analysis. Potassium-doped catalysts containing increasing amounts of alkali metal also were prepared and characterized. The reactivity of the samples in isobutane dehydrogenation was tested, and the effect of chromium and potassium loading was examined. The results were analyzed and compared with data from the scientific literature in order to draw up a model of this catalytic system. It is proposed that in samples calcined at 600°C an initial coverage of the alumina support occurs, constituted of Cr 6+ species anchored to the support, and of dispersed Cr 6+ and Cr 3+ oxide species. This latter Cr 6+ species could be removed by a treatment with water, and thus was not grafted to the support. Crystalline α-Cr 2 O 3 was found only for the higher chomium oxide loading. In spent catalysts all the chromium was present in the Cr 3+ form; the Cr 3+ species formed by reduction in the reaction environment of the Cr 6+ species were less active than the Cr 3+ species in dispersed Cr 3+ oxide. The activity was found to be proportional to the overall amount of chromium, except for the highest CrO 3 loading (15.3 wt% CrO 3 ) which showed a lower activity. The addition of potassium led to the formation of a potassium chromate phase, the formation of which occurred at the expense of the dispersed Cr 3+ oxide and of α-Cr 2 O 3 . The catalytic activity was generally inhibited by the addition of potassium, except for the catalyst containing 15.3 wt% CrO 3 . In this case an increase in activity was achieved by addition of amounts of potassium lower than 1 wt% K 2 O.

Heteropolycompounds of the composition Kx(NH4)3−xPMo12O40 were prepared by precipitation, through addition of HNO3 to an aqueous solution of the salts. The precipitated compounds were calcined at increasing temperatures to evaluate the thermal behavior, and were characterized by means of X-ray diffraction, FTIR spectroscopy, surface area measurements, ionic chromatography, EPR spectroscopy, and reactivity in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. A monophasic system was obtained by calcination at 640 K, with patterns characteristic of the Keggin-type cubic secondary structure. The cationic composition was found to be very close to that expected for the complete salification of the Keggin anion. The structural decomposition began at temperatures close to 693 K for the ammonium and ammonium/potassium compounds, with formation of an intermediate heteropolycompound characterized by crystallographic parameters and cationic composition different from the original one and more stable than the latter. The potassium salt remained instead structurally intact up to 753 K. The data were interpreted with the assumption that the decomposition of the heteropolycompound might occur in a stepwise way, with an initial structural collapse of a part of the compound and displacement of some molybdenum ions (originally located in peripheral position in the Keggin anion) in the cationic position of the not yet decomposed compound. The highest amount of this intermediate molybdenum-salified heteropolycompound apparently formed during decomposition of the (NH4)3PMo12O40 compound. The calcination temperature remarkably affected the catalytic behavior in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. The partial structural collapse occurring by calcination at 693 K led to a decrease in the activity for the potassium/ammonium mixed salts. On the contrary, the ammonium salt increased its activity. It was assumed that this phenomenon could be due to the formation of the intermediate molybdenum-salified heteropolycompound.

A Fourier transform infrared (FT-IR) study of the adsorption of toluene, and of some possible intermediates in its oxidation and ammoxidation, has been carried out on a monolayer vanadia-titania catalyst with and without preadsorbed ammonia in the presence and in the absence of gaseous oxygen at temperatures between 300 and 600 K. The results are discussed in relation to those obtained by ammoxidation of toluene and other methylaromatics in a tubular flow reactor. An intermediate, formed on the clean surface at room temperature, is identified as an adsorbed benzyl species probably having a radical character. Its oxidation produces benzaldehyde, adsorbed benzoates, and maleic anhydride. The coadsorption of toluene and ammonia produces adsorbed benzylamine, which is also observed as a by-product in the flow reactor at low oxygen and ammonia partial pressures, and benzonitrile. Based on these data a mechanism for both oxidation and ammoxidation is proposed and discussed.