Rh/Sn-catalysts

A bimetallic catalyst can be obtained by the reaction of tetrabutyltin with Rh/Si02 catalyst. The partial hydrogenolysis leads to the Rhs[Sn(n-C4H9)2]/ Si02 surface organometallic complexes, which proved to be fully selective in the hydrogenation of unsaturated aldehydes into the corresponding unsaturated alcohols.318... 

Copper-catalyzed Suzuki cross-coupling reactions using mixed nanocluster catalysts have been studied recently. Copper-based catalysts were shown to be effective as reagents that can present an inexpensive and environmentally friendly alternative to noble metal catalysts. In the hydrogenation of cinnamic acid to corresponding alcohol, the selectivity can be varied by doping Sn with Rh colloid catalysts. A selectivity of 86% was achieved using a colloidal Rh/Sn (Rh/Sn = 1.5 1) catalyst on... 

The Rh(0.5)-Sn(0.23)/SiO2-OM catalyst showed a selectivity to the unsaturated alcohol in the range 10-16%, whereas with the Rh(0.5)-Sn(0.23)/SiO2-BM catalysts selectivities were close to 30%. In the Rh-Sn-OM samples, the presence of butyl groups seemed to contribute to a significant increase in the selectivity to crotyl alcohol due to both electronic and steric effects. 

As a consequence of the experimental results for catalyst Sets A and B, appropriate rhodium-containing catalysts were tested as Set C. Figure 3.38 shows the reactor outlet concentration for propylene oxide, acrolein and acetone. A large number of the catalysts tested produce high concentrations of propylene oxide of up to 2000 ppm at 1% conversion of propylene. The combinations Rh-Sn and Rh-In are very effective for propylene oxide formation. In most cases the binary catalysts have higher activity at lower propylene loading. In Figure 3.38, it can also be seen... 

In vapor-phase hydrogenation of crotonaldehyde over Rh-Sn-Si02 catalysts, the selectivity to trans- and cA-crotyl alcohol increased strongly with the tin content, reaching 62-69% for the trans compound with the Sn/(Sn+Rh) atomic ratio higher than 40%.83... 

Some time ago, selective Pt/Y zeolite catalysts were developed for the liquid-phase hydrogenation of cinnamaldehyde into cinnamyl alcohol (selectivity >96% [449]) and an organometallic derived Rh[Sn-(n-C4H9)2]Si02 catalyst for the conversion of citral to the corresponding a, (f-unsaturated alcohols,... 

When Rh/Si02 catalyst is modified by Sn(n-C4H9)4, there is a drastic improvement of the selectivity for phenyl-1 ethanol, as seen in figures lb, c and d. Depending on the amount of tetrabutyltin introduced and on the time of the reaction in the catalyst modification, the catalytic activity of the bimetallic sample were strongly reduced (figures Ic and Id) or clearly increased (figure lb). 

As expected, the total normalized area for 20 mg samples decreased as a function of dispersion and Ge grafted amount. The evolution of brigded CO species frequencies indicated a modification in the CO coordination to Rh as the wide band corresponds at least to two species Rh 3(CO)2 and Rh aCCO) respectively at about 1920 and 1870 cm [9]. Thus, an increase of band frequencies can be correlated to a higher contribution of Rh 3(CO)a species and consequently to a growth of particle size. A decrease of band frequencies corresponding to identical particles can be attributed to the presence of Ge inducing site isolation and favored low-coordinated CO species. Finally, the frequencies of linear and gem-dicarbonyl species exhibited no modification that can be explained by the absence of an electronic effect between Rh and Ge in our catalysts whereas such an effect was proposed for Pt-Ge (Ge as electron acceptor) [15] and Rh-Sn (Sn as electron donor) [16]. 

The bimetallic catalysts are obtained by reaction of M (n-C4H9)4 (M =Ge, Sn or Pb) with the reduced Rh/Si02 catalyst in the liquid phase (n-heptane) under hydrogen pressure (5 MPa) at 373 K. The amount of M fixed on the Rh/Si02 catalyst... 

Table 1 Amount of M (Ge, Sn or Pb) fixed on the Rh/Si02 catalyst (b), as a function of the reaction time (in braket) and of the amount of M (n-C4H9)4 introduced MVRhs (a)...

The results presented above show that a partially reduced catalyst performed better in CO oxidation and NO reduction. Partially reduced centres are apparently necessary to enhance low temperature O2 and NO dissociation. In the case of Rh single crystal sur ces. Wolf et al. [35] found direct evidence that vacancies are required for the dissociation of NO molecules. A high coverage of molecularly adsorbed NO inhibits the reaction rate at low temperatures because of the limited availability of vacancies on the surface of the noble metal. Partially reduced metal oxides may provide for the dissociation centres for NO. This was also shown by Tomishige et al., who studied the promoting effect of Sn on NO dissociation and NO reduction with H2 over Rh-Sn/Si02 catalysts [41]. They found that NO even dissociated at room temperature on a reduced Rh-Sn/Si02 catalyst and that the rapid dissociation stopped when one-third of sur ce Sn atoms was oxidised by NO. 

REDUCTION BEHAVIOR OF Rh-Sn/SiOi BIMETALLIC CATALYSTS AND ITS CO OXIDATION ACTIVITY... 

Rh oxide supported on Si02 was reduced at c.a. 360 K. The spectra b to e show DTA-TG lines over Rh-Sn/Si02. The DTA spectra indicate that two major peaks, one was sharp near 370 K, the other was a broad beyond 420 K. The first sharp peak was similar to reduction of Rh oxide over Rh-Sn/Si02, which accompanied no weight loss (TG). The formed H2O was not desorbed fi om the catalyst below 400 K. The position of peaks in the DTA lines obtained over Rh-Sn/Si02 catalyst are summarized in Table 1. 

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