Palladium mordenite catalysts

A palladium-hydrogen-mordenite catalyst with a 10.8/1 silica/alumina mole ratio was evaluated for the hydroisomerization of cyclohexane. The rate of reaction followed a first-order, reversible reaction between cyclohexane and methylcyclopentane. The energy of activation for this reaction between 400° and 500°F was 35.5 it 2.4 kcal/mole. Cyclohexane isomerization rates decreased with increasing hydrogen and cyclohexane-plus-methylcyclopentane partial pressure. These effects are compatible with a dual-site adsorption model. The change of the model constants with temperature was qualitatively in agreement with the expected physical behavior for the constants. 

This study was performed in the Petroleum Processing Laboratories, Chemical Engineering Department, Louisiana State University. The project was sponsored by Esso Research and Engineering Co. The catalyst used was prepared by the Esso Research Laboratories, Humble Oil and Refining Co., Baton Rouge, La., from mordenite crystals obtained from the Norton Co. Ammonium mordenite was impregnated with 0.5% of palladium, pilled, crushed, sized, and heated to 1000 °F in the presence of air to give Pd on H-mordenite catalyst. The properties of this catalyst are shown in Table I. 

Because of the small pores in zeolitic catalysts, reaction rates may be controlled by rates of diffusion of reactants and products. Beecher, Voorhies, and Eberly (4) studied hydrocracking of mixtures of n-decane and Decalin with mordenite catalysts impregnated with palladium. They found that acid leaching of the mordenite produces an aluminum-deficient structure of significantly higher catalytic activity. At least part of this improvement appears to be caused by the decrease in diffusional resistance. They observed that with this type of catalyst, the effective catalyst pore diameter appears to be smaller than calculated owing to the strong interaction or adsorption of hydrocarbon molecules on the pore walls. 

Catalytic stability of a Pd/H-Mordenite catalyst for C5/C6 hydroisomerization was tested in a laboratory reactor for 1000 hours. The content, chemical composition and structure of the coke formed on the catalyst discharged from a pilot reactor working in an accelerated condition was characterized using XRD, EPR, MAS-NMR, FTIR and TPO techniques. The catalyst shows stable catalytic activity and selectivity during 1000 hours. The nature of the coke and its combustion behavior depended upon time on stream and varied with the catalyst bed length. As time on stream increased, coke initially formed on palladium metals and then moved to acidic sites on the support where polyaromatic or pseudographite-like structures were formed through further acid catalyzed reactions. 

The 0.5 wt.% Pd/H-Mordenite catalyst was prepared by impregnating the commercial H-form mordenite material with a palladium amine complex solution [Pd(NH3)4]Cl2. Before the reaction, catalysts were reduced in the reactor at 400 °C under a hydrogen steam. Coke was liberated from a coked catalyst sample by dissolving the catalyst in a 40 % hydrofluoric acid solution. Finally, the coke sample was obtained by percolation, washed with deionized water and then dried. 

The three-function model introduced in the preceding section has been established on an H-mordenite (HMOR) supported cobalt—palladium catalyst [12], For the sake of demonstration, model catalysts with a unique function, i.e. FI, F2 or F3, (Figure 5.1), were prepared to separately give evidence of the major role of each active site (Figure 5.1). Let us note that three functions does not necessarily mean three different active sites, but in the case of CoPd/HMOR material, three different sites were identified. 

The noble metal component may be either palladium or platinum the effect of the concentration of both metals on methylpentane as well as on dimethylbutane selectivity in C6 hydroisomerization on lanthanum and ammonium Y-zeolite with Si/Al of 2.5 has been studied by M.A. Lanewala et al. (5). They found an optimum of metal content for that reaction between 0.1 and 0.4 wt.-%. The noble metal has several functions (i) to increase the isomerization activity of the zeolite (ii) to support the saturation of the coke precursors and hence prevent deactivation, as was shown by H.W. Kouvenhoven et al. (6) for platinum on hydrogen mordenite (iii) to support the hydrodesulfurization activity of the catalysts in sulfur containing feedstocks. 

The catalyst used for this study is of the more recent type of dualfunction catalysts, wherein the base is a crystalline zeolite, mordenite. Palladium metal was dispersed on this support. A naphthenic isomeriza-... 

To compare the hydrogenating activity of the cation forms of mordenite with that of H-form which contains the metals of column VIII, we have studied benzene hydrogenation on the catalysts 0.5% Pd/HM and 5% Ni/HM. Under the conditions indicated in Table II, the extent of benzene hydrogenation on these catalysts is 85 and 95%, respectively. Thus, the hydrogenating activity of certain cation forms of mordenite is not inferior to that of H-mordenite, which contains palladium and nickel. Benzene hydrogenation on these catalysts is accompanied by a considerable hydroisomerization to yield methylcyclopentane 30-40%. 

The cyclodimerization of cyclopropenes, a novel reaction, was foundto be catalysed by KA and NaX zeolites. Carbanion intermediates were proposed and the selectivity of the reaction was attributed to spatial constraints. Paraffin disproportionation, with isomerization, at about 500 K has been shownto occur over H-mordenite and HZSM-4 catalysts. Synthetic H-ferrierite is an active and very selective catalyst for n-paraffin cracking and hydrocracking. Palladium on zeolite L comparesfavourably with Pd/HY as a catalyst for pentane isomerization. 

Preferential oxidation catalysts usually consist of precious metals such as platinum, ruthenium, palladium, rhodium, gold and alloys of platinum with tin, ruthenium [164] or rhodium. Typical carrier materials are alumina and zeolites [164], such as zeolite A, mordenite and zeolite X. Other possible carriers are cobalt oxide, ceria, tin oxide, zirconia, titania and iron oxide [214]. A high precious metal loading usually improves catalyst performance [164]. 

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