Palladium Y zeolite

Okitsu K, Yue A, Tanabe S, Matsumoto H (2002) Formation of palladium nanoclusters on Y-zeolite via a sonochemical process and conventional methods. Bull Chem Soc Jpn 75 449 155... 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

Pd2+ tend to be the most dispersed among the eight sodalite cages, and bridged palladium ions are unlikely to occur. In conclusion, palladium ions exhibit behavior similar to that of Cu2+ (3) whereas Ni2+ ions mainly occupy SI sites in Y zeolites (7-8). 

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 800°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface. 

The frequency of the linear CO is higher than that found for Pd films (j co = 2085 cm-1) (27) or for supported palladium (Pd/Si02), vco = 2060 cm-1) (28). The increase in frequency reported in this study is the result of the decrease of the backdonation from the d metal orbitals to the 7T orbital of CO. Y zeolites have very strong Lewis acid sites these sites should be able to decrease the electronic density of the palladium atoms bonded to CO. The decrease of the intensity of the band at 2100 cm-1 by increasing the hydrogen reduction temperature could be explained by the formation of agglomerates of palladium still in interaction with a Lewis acid site. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

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. 

Nishizaka, Y. and Misono, M. (1994) Essential role of aridity in the catalytic reduction of nitrogen monoxide by methane in the presence of oxygen over palladium-loaded zeolites. Chem. Lett., 2237-2238. 

H. Jobic A. Renouprez (1987). J. Less-Common Met., 129, 311-316. Formation of hydrides in small particles of palladium supported in Y-zeolite. 

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