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Nadji ab , A. Delgado a , R. Issaadi b , E. Rodriguez-Aguado c , E. E-mail: jmlopez itq. Under our operating conditions, all the catalysts were active and selective in the transformation of glycerol to acrolein, which was always the main reaction product. In addition, the role of oxygen in the feed on catalytic performance of these catalysts is also discussed. Several type of catalysts have been proposed in the last years, 4—7 including zeolites, 9,10 heteropoly acids and acidic heteropoly salt, 11 phosphate-based materials 12 and oxides. Nevertheless, the deactivation of the catalyst during the reaction is one of the major drawbacks. In addition, it has been also observed that oxygen not only decreases the coking rate, but also improves the selectivity to acrolein. Among all the catalysts proposed for glycerol dehydration to acrolein, supported tungsten oxide catalysts are one of the most efficient. Maksasithorn et al. Non-hydrolytic sol—gel NHSG route based on the reaction of chloride or alkoxide precursors with diisopropyl ether has shown to provide an excellent control over the stoichiometry and homogeneity of mixed oxide gels. Thus, these non-hydrolytic routes are attracting increasing attention for the preparation of W-containing mixed oxide catalysts. Spectra were collected at an accelerating voltage of 12 kV at a counting time of s. The total acidity of fresh catalysts was evaluated by means of temperature-programmed desorption of ammonia TPD-NH 3. In order to quantify the amount of ammonia desorbed, the equipment was previously calibrated. Typically, 20 mg of dried sample were mixed with mg of potassium bromide KBr and pressed to obtain a pellet. Then the spectra were recorded. Samples were excited at nm corresponding to visible green light in the electromagnetic spectrum with a power of 15 mW on the samples. Binding energy values were referenced to C 1s peak Data treatment was performed with CasaXPS software. N 2 -adsorption isotherms of calcined catalysts are shown in Fig. Adsorption—desorption profiles show hysteresis loops typical of mesoporous materials, 29 giving average pore sizes in the 3—11 nm range Table 1. In this way, both the surface area and the pore volume initially increase when low tungsten amount is added 5W—Si and 5W—10ZrSi , but they decrease dramatically with the W-loading for samples with high W-contents. In the case of x W—Si series Fig. The absence of tungsten oxide reflections can be explained by the low W content and the high dispersion of tungsten species in this catalyst. A similar trend is observed for x W—10ZrSi series Fig. This suggests that the Zr—Si—O can disperse tungsten oxide better than silica in spite of its lower surface area. This interaction could prevent the formation of well-ordered tungsten or zirconium oxide phases, as it was previously reported for other systems based on W and Zr oxides. This means that the proportion of terminal W O species in the catalyst is higher at lower W content, which is consistent with a higher dispersion of the tungsten oxide formed but also a lower crystallite size. In this way, higher surface areas and higher dispersion of WO 3 are achieved at lower W-loading Table 1. Interestingly, these bands show a broader profile, which is in agreement with the amorphous nature of the catalysts observed by XRD. In the case of Zr-containing samples, signals related to zirconium oxide are only found in the 15W—Zr sample, being absent in the other catalysts. Measuring the amount of adsorbed ammonia allows us to determine the total concentration of surface acid sites. Also, by analysing the desorption profiles measured at increasing temperatures it is possible to estimate the specific acid strength of those sites. Thus, NH 3 will be desorbed at higher temperatures as the strength of the acid sites increases. Then, the amount of acid sites progressively decreases for catalysts with higher W contents. However, if we consider the surface density of acid sites, i. These features are in agreement with the decrease in surface area observed at increasing W-loadings and the low acid characteristics of the undoped SiO 2 sample Table 1. In the rest of the series, i. In this way, Zr-containing catalysts, i. For comparison, Fig. All the materials display bands related to Lewis L, at ca. The chemical nature of surface species of selected catalysts was elucidated by XPS Fig. Indeed, this fact has already been reported for W-promoted silica catalysts, in which the lower the W-loading, the higher the differential charging effect. On the other hand, pure tungsten oxide, WO 3 , shows high catalytic activity and relatively high selectivity to acrolein, with a yield to acrolein of ca. The selectivity to acrolein initially increases with W-loading, achieving a maximum selectivity of ca. The effect of reaction temperature on the catalytic behaviour of the catalysts has been also studied. The most representative catalytic results are presented in Fig. In general, a decrease in the selectivity to acrolein at increasing reaction temperatures is observed, together with a concomitant increase in the selectivity to carbon oxides. Nevertheless, the decrease in acrolein selectivity is lower for 15W—Si and 35W—10ZrSi catalysts, which are in fact one of the most selective materials. In addition, the selectivity to other by-products is also lower for these two catalysts Fig. Interestingly, there exists a correlation between the nature of surface acid sites in the materials and the selectivity to acrolein Fig. Indeed, a higher number of Lewis acid sites could promote the re-adsorption of the product, leading to consecutive reactions to form carbon oxides, heavy by-products and other oxygenated molecules. Important differences have been found, regarding both the glycerol conversion and the selectivity to acrolein. When oxygen is present in the feed, total conversion of glycerol and a constant selectivity to acrolein ca. These results are in good agreement to previous ones in which it has been observed that co-feeding oxygen could drastically reduce catalyst deactivation and prevent the formation of acetol as by-product. The maximum selectivity to acrolein observed using the optimal catalysts was ca. W—Si—O and W—Zr—Si—O catalysts show acid sites, with low and medium—high acid strength in which the composition of the catalyst strongly influences the acid characteristics. More interestingly, and considering the FTIR spectra of adsorbed pyridine, we can conclude that the most selective catalysts, i. In this way, and according to the results of Fig. Therefore, a trade-off in the tungsten loading is necessary to achieve the highest acrolein formation. Thus, the maximum is obtained at intermediate W-loadings. On the other hand, the acrolein formation also decreases when the Zr-content of the catalysts increases. Then, at high Zr-loadings the formation of carbon oxides and other oxygenated by-products are favoured to the detriment of the acrolein formation. It must be noted that just the amount of acid sites does not determine neither the catalytic activity nor the acrolein formation. In fact, no correlation has been observed between the amount of ammonia adsorbed in the TPD experiments and the glycerol conversion or the selectivity to acrolein. In this way, the one-pot non-hydrolytic sol—gel method is an effective way for preparing active, selective and stable catalysts for the aerobic transformation of glycerol to acrolein. On the other hand, strong differences in catalytic performance have been observed when the reaction is carried out in the presence or in the absence of oxygen in feed. Thus, in the absence of oxygen, an important deactivation of the catalyst occurs, probably due to coke deposition on the surface. DOI: Received 21st February , Accepted 4th April Experimental conditions: 0. Characteristics of catalyst in Table 1. For comparison, it has been also included the Raman spectrum of pure WO 3. Experimental conditions as in Fig. See DOI:

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