Dehydrogenation bimetallic catalysts

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

In bimetallic catalysts, Cu-Ru is an important system. Combinations of the Group Ib metal (Cu) and Group VIII metal (Ru)-based catalysts are, for example, used for the dehydrogenation of cyclohexane to aromatic compounds and in ethane hydrogenolysis involving the rupture of C-C bonds and the formation of C-H bonds (Sinfelt 1985). Here we elucidate the structural characteristics of supported model Cu-Ru systems by EM methods, including in situ ETEM. 

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [1] reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. 

Results from these studies are important for the ongoing debates on the existence and utility of Sn/Pt alloy phases in bimetallic Pt-Sn supported catalysts. For example, our observation of dramatically decreased carbon buildup on the alloy surfaces from acetylene (a coke-precursor), and the enhanced yield of aromatics and alkenes from alkane dehydrogenation mimics important aspects of the chemistry of commercial Pt-Sn supported catalysts used for reforming. On the contrary, it seems unlikely that Sn/Pt alloy phases are solely responsible for the high selectivity observed in crotonaldehyde hydrogenation using Pt-Sn bimetallic catalysts. 

It was found originally by Swift and Bozik in an early study of supported bimetallic catalysts that the addition of tin to a nickel-silica catalyst greatly promoted the activity and gave a longer catalyst life for the dehydrogenation of cyclohexanol or cyclohexanone to phenol, especially with a... 

The effect of irreversible sulfur on catalyst activity was investigated in the course of cyclohexane dehydrogenation (Table 3). The comparison of relative activities (activity of sulfided catalyst/activity of fresh catalyst) shows that dehydrogenation is more strongly inhibited by sulfur adsorption on bimetallic catalysts. However, the higher the Pt-Re interaction, the higher the remaining activity of sulfided bimetallic catalysts. 

Catalyst presulfidation induces a strong decrease of the amount of coke deposited on bimetallic Pt-Re catalysts, the effect being mote pronounced on catalysts prepared by catalytic reduction (Table 4). However, coke deposits are less toxic for cyclohexane dehydrogenation on catalysts activated by direct reduction, i.e. on catalysts where Pt-Re interaction is high. 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

During our research on bimetallic catalysts, it was evident very early that the activities of a metal catalyst for different reactions could be altered to markedly different degrees by the incorporation of a second metallic element into the catalyst. The discussion begins with a brief review of our early exploratory studies of this selectivity phenomenon, with emphasis on hydrocarbon reac tions such as isomerization of alkanes, aromatization of alkylcydopentanes, dehydrogenation of cyclohexanes to aromatic hydrocarbons, and hydro-genolysis of alkanes and cycloalkanes. 

This review is followed by a consideration of some of the features characteristic of hydrocarbon reactions on catalysts comprising individual metals from Groups VIII and IB of the periodic table. Finally, the activities of a series of unsupported nickel-copper alloys for hydrogenolysis and dehydrogenation reactions are discussed. These latter studies were made to obtain information on the selectivity phenomenon with bimetallic catalysts of known structure. The nickel-copper alloys were characterized by a variety of chemical and physical probes. 

The consideration of rates and mechanistic aspects of reactions such as hydrogenolysis, dehydrogenation, and isomerization provides a basis for interpreting selectivity data on bimetallic catalysts. 

Monometallic Pt (0.4% w/w) and bimetallic Pt-Sn (0.4% 0.49% w/w respectively) catalysts supported on alumina have been modified with alkali metals ( Li, Na, K,Rb Cs, Pt alkali molar ratio of 1 40) have been investigated by TPR, TPD (ammonia and hydrogen), Pt dispersion and TPCO measurements and evaluation of activity for dehydrogenation of n-decane. Activity of alkali promoted mono and bimetallic catalysts are shown in Fig. 6 7 In the case of monometallic catalysts, Pt-Li system exhibits comparable initial actvity while the stability improves significantly. Other alkali elements do not show... 

As expected from the above discussion, the amounts of cracked products and methane increase by a factor of 2 to 2.5. However, the DHC products (naphthenes and toluene) increase by a factor of 12. Under the experimental conditions, DHC can be both mono- or bi-([metal + alumina]-) functional. The rate of dehydrogenation of MCH to toluene (not shown) actually decreases for the bimetallic catalyst, from 45 to 12 mol/h/kg. 

J. Haro, R. Gomez, and J.M. Ferreira. The Role of Palladium in Dehydrogenation of Cyclohexane over Pt-Pd/Al203 Bimetallic Catalysts. J. Catal. 45 326 (1976). 

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