Selective oxidation catalysts catalytic behavior

Correlation Between Spectroscopic Measurements and Catalytic Behavior of Selective Oxidation Catalysts... 

The relatively low reactivity or inertness of the carbon surface is also very useful in the preparation of bimetallic catalysts, since the low interaction between the carbon surface and the two metals or metal precursors facilitates their mutual interaction. This is especially interesting when the objective is the formation of bimetallic particles. One clear example is the preparation of bimetallic Pt-Sn catalysts for selective hydrogenations. The catalytic behavior of this system is determined by at least three aspects [29,30] that determine the catalytic activity and the selectivity toward the desired product (1) the oxidation state of tin in... 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

Effects of Li content on the catalytic behaviors and structures of LiNiLaOx catalysts The dpendence of performance of LiNiLaOx catalysts on Li content at 1073K was shown in Fig.l. When D/Ni mole ratio was 0, the relatively acidic LaNiOx had the highest CH4 conversion(92.0%), but no C2 yielded. The products were CO, CO2 and H2, and CO selectivity was 98.3%. It is not an OCM catalyst but a good catalyst for partial oxidation of methane(POM). With Li content and the baric property of LiNiLaOx catalysts increasing, CH4 conversion and CO selectivity decreased, but there was still no C2 formed imtil Li/Ni mole ratio was 0.4. There was a tumpoint of catalytic behavior between 0.2 and 0.4 (Li/Ni mole... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

After a steady catalytic behavior was reached, the catalyst was treated in air at 350°C, in order to reoxidize it. Thereafter, the reaction was run again under isobutane-rich conditions (Figure 14.5), in order to understand the role of the POM reduction level on catalytic performance. The reoxidized catalyst exhibited a selectivity to methacrylic acid that was initially around 20%, and approximately 20-30 hours were necessary to recover the original performance of the equilibrated, reduced catalyst. On the contrary, the activity of the catalyst was almost the same as before the oxidizing treatment. This confirms that a partially reduced POM is intrinsically more selective to methacrylic acid than a fully oxidized one, and that one reason for the progressive increase in selectivity to methacrylic acid that occurs during the equilibration period was the increase in the POM reduction level, as a consequence of the operation under isobutane-rich conditions. 

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