Rh-supported catalysts

Chemisorption on nonmetallic catedysts should provide the number of catal3rtic sites and for comparative purposes a single Mg can be taken as a catalytic site on metals. This permits the calculation of turnover frequencies which was a new concept in post ICC 1 and which permitted intercomparison of catalyst activities. For the first time then, one has been able, for example, quantitatively to discuss support effects in Rh/support catalysts. 

Catalytic behaviour of Rh-supported catalysts on lamellar and zeolitic structures by anchoring of organometallic compound... 

A Rh supported catalyst was chosen because Rh has been shown to be one of the most active and selective catalysts for methane partial oxidation [6-8]. A 3% Rh/Ti02 was the most active catalyst, which ignited at 320°C in a fixed bed microreactor when using methane and oxygen feed rates of 500 and 250 cc/min respectively. It yielded a methane conversion of 70% and a CO selectivity of 85%[10]. It was also found that 100% oxygen conversion is achieved in all cases and that the ignition temperature could be even lower for lower methane/oxygen feed ratios. Experiments were performed initially in the fixed bed reactor so that results obtained in the membrane reactor could be compared to those obtained in the fixed bed reactor. 

Xi-Si02 samples have been synthesized by grafting Si02 with either TiCU or Ti[OCH(CH3)2]4 as Ti source. These samples have been used as supports for the preparation of Rh-supported catalysts, which have been studied in the POM reaction. A higher Rh dispersion as well as a more reduced Rh was found for the catalyst prepared om TiCU, which exhibits also a higher selectivity towards H2 and CO. 

Preparation of Rh-supported catalysts. 3.8 g of the support (Ti-modified Si02) were dipped into 400 cc of deionized water and the adequate amount of (NH4)3RhCl6 3H20 (Acros) was added in order to obtain theoretically Iwt. % of Rh catalysts. After stirring at room temperature for 3h, the solvent was evaporated in a rotavapor at 35°C. The recovered solid was dried at 110"C 16h before its calcination in air at 700°C for 4 h. Samples are denoted as Rh/Ti-Si02/I and Rh/Ti-Si02/0. 

XRD analyses were performed on the fresh (after calcination), reduced (after in situ pre-treatment in pure Ha) and used (after POM reaction) samples. XRD patterns of the fresh samples show peaks corresponding to anatase in addition two broad reflections appear at about 20=33 and 35°, which could be due to the presence of Rh203 crystallites. In the reduced samples, the peaks of anatase appear more narrow and slightly shifted compared to the liesh samples. The main peak of crystalline Rh at 20=41° is also detected in both samples. The X-ray diffraction profiles of used samples revealed the presence of anatase and crystalline Rh, but not of Rh203. Rh dispersion is clearly higher on the inorganic support than on the organic one. The XPS parameters of Rh-supported catalysts are reported in Table 3. 

Figure 11.3. Schematic of the experimental setup used (a) to induce electrochemical promotion (via YSZ) on Ir02 and Ir02-Ti02 porous catalyst films (b) to compare the electrochemical promotion induced on Pt via YSZ and via Ti02 and (c) to compare the electrochemical promotion behaviour induced by varying UWR on a Rh porous catalyst film (left) and on a fully dispersed Rh catalyst supported on porous (80 m2/g) YSZ support.22...

Figure 11.8. Effect of po2 on the rate (TOF) of C2H4 oxidation on Rh supported on five supports of increasing d>. Catalyst loading 0.5wt%.22,27 Inset Electrochemical promotion of a Rh catalyst film deposited on YSZ Effect of potentiostatically imposed catalyst potential Uwr on the rate and TOF dependence on po2 at fixed Pc2H4-22,33 Reprinted with permission from Elsevier Science (ref. 27) and Academic Press (ref. 33).

As was the case for the silica-supported Ru-Rh bimetallic catalysts, there was no significant surface enrichment in either metal over the entire range of bimetallic catalyst compositions. 

Conditions Catalyst 0.15g, Rh/support (2.5wt.%), RhPt/support (Rh 2.0wt.%, Pt 0.75wt.%, Pt/Rh = 0.2), C4H10/H2 = 1/9, GHSV = 3800h 101 kPa. Results are obtained after 4h reaction time under steady-state conditions. 

A series of anchored Wilkinson s catalysts were prepared by reacting the homogeneous Wilkinson catalyst with several alumina/heteropoly acid support materials. These catalysts were used to promote the hydrogenation of 1-hexene. The results were compared with those obtained using the homogeneous Wilkinson and a l%Rh/Al203 catalyst with respect to catalyst activity and stabihty as well as the reaction selectivity as measured by the amount of double bond isomerization observed. The effect which the nature of the heteropoly acid exerted on the reaction was also examined. 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

Several supported metalhc catalysts were evalrrated for the selective hydrogenolysis of glycerol. Initially, the reactions were performed tmder acidic conditions in order to promote the formation of 1,3-PDO. Rutheniirm-based catalysts were found to be the most active catalysts but significant amount of tmdesired products resulted from C-C cleavages were detected. On the contrary, Rh/C catalysts were found selective to C-O cleavages. As far as the selectivity to 1,3-PDO was concerned, we previously reported that the addition of iron salts in the medium improved the l,3-PDO/l,2-PDO selectivity (11). A systematic study on the influence of additives was therefore carried out in the present investigation. Mineral and organic acids were evaluated for this purpose (Table 35.1). 

The hydrogenation processes were performed at a relatively low temperature and pressure in the presence of promoted Raney Ni 2400 and Raney Co 2724 catalysts (13) in this study but any common nitrile hydrogenation catalysts (e.g. Fe, Ru, Rh, bulk or supported catalysts) could be used. The advantage of using a low temperature and pressure process is that it lowers the investment cost of an industrial process. Raney Ni 2400 is promoted with Cr and Raney Co 2724 is promoted with Ni and Cr. The particle sizes for both catalysts were in the range 25 - 55 pm. The BET surface area of Raney Ni 2400 and Raney Co 2724 are 140 m2/g and 76 m2/g, respectively, and the active surface area of the Ni and Co catalysts are 52 and 18 m2/g, respectively, based on CO chemisorption (Grace Davison Raney Technical Manual, 4th Edition, 1996). 

The catalysts used throughout the research were 2.5 % Rh/Si02 catalysts prepared by incipient wetness. Grace Catalysts supplied the catalyst supports and the catalysts were prepared by Johnson Matthey. 

The commercial 5 wt % Rh supported on y-A1203 catalysts were supplied in the reduced form by Engelhard (Rh/Al203-1) and Johnson Matthey (Rh/Al203-2), and were used without further treatment. Quinoline (Q), 1,2,3,4-... 

Table 4 Characterization data for the two commercial, 5 wt% Rh supported on y-Al2Q3 catalysts. ...

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