Palladium exchanged zeolite

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. 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

Metallic palladium on zeolite gives PhNCO fi om nitrobenzene with 35 % selectivity in dichlorobenzene at 240 °C and 200 atm [29], It has also been reported that zeolite Y exchanged with palladium (II), alone or in the presence of pyridine, gives PhNCO from nitrobenzene at 220-240 °C and 250 atm [30]. At... 

The material being impregnated by the palladium precursor, once all the internal exchanged positions have been already occupied by Co2+, PdO is formed on the external surface of the zeolite grain, as observed by TEM (not shown, [12]). 

Palladium metal clusters confined in zeolites have been prepared from Pd (CO)y species accommodated in zeolite cages. These species can be obtained by carbon-ylation of impregnated or exchanged palladium compounds such as PdCh or [Pd(NH3)4] [47, 48]. 

A Linde NaY zeolite without binder was ion exchanged in an ammonia-cal solution of PdCl2 which provides exchangeable (Pd(NH3)4)2+ cations. The solution was stirred at room temperature for 24 hr and then filtered. The zeolite was washed with ammonia solution to eliminate Cl ions. The desired exchange level was readily obtained by allowing the zeolite to equilibrate in a solution where a suitable amount of palladium has been introduced. Chemical analysis for palladium and sodium showed the composition of the calcined sample to be Pdtf.sNaiQ.sHn.sAlfieSiiaeCW (10 wt % of Pd proton concentration determined by difference). 

Materials. A series of zeolites was prepared from NaY by exchanging part of the Na+ ions by NH4+, Ca2+, Ce3+, Mg2+, and La3+. Palladium was exchanged into these zeolites from a Pd(NH3)2Cl2 aqueous basic solution to yield a palladium content of approximately 2 wt % (approximately 2 Pd2+ ions per unit cell). The chemical compositions of the catalysts so obtained are given in Table I. During the exchange by the palladium salt part of the sodium is replaced by ammonium ions. 

From the general inaccessibility of both the sodium and TMA ions, we postulate that most of the acidic sites generated by thermal treatment of the derived NH4+/TMA+ zeolite will also be inaccessible to reactant molecules. Likewise, catalytically active metals such as Pt and Pd introduced by ion exchange are expected to be located in or near these same inaccessible sites. This may explain the poor approach to equilibrium observed with the isomerization catalysts, and the poor hydrogenation activity of the hydrocracking catalyst indicated by excessive coking and catalyst decline, even in the presence of a massive 3.1 wt % palladium. 

Figueras et al. (105) found some direct evidence for electron-deficient palladium clusters on various cation-exchanged forms of zeolite Y from CO adsorption experiments. In particular, a correlation was observed between the turnover number for benzene hydrogenation and the CO stretching frequency. The shift toward higher frequency with increasing support acidity was considered as evidence for increased electron acceptor properties of the support. Further studies will, however, be required to provide a more detailed understanding of this phenomenon. 

H. Robson Palladium was deposited on the zeolite powder by exchange with Pd(NH3)4Cl2 solution. We did not observe significant differences in product distribution between cationic forms of erionite, but of course we were looking primarily for the disappearance of n-pentane and n-hexane. 

Bimetallic species have also been placed into zeolites. A Pd-Ni/Y catalyst was prepared by exchange with Pd(NH3)4" 2 and Ni++. The presence of palladium enhanced the reducibility of the nickel.2 0 2l 1 Impregnation at high pH placed the nickel ions in the supercages where they were in close proximity to the reduced... 

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