Supercages Of NaY Zeolite Research Articles

Investigations of adsorbed organic molecules in Na-Y zeolite by xenon-129 NMR

The adsorption of n-hexane, benzene, and trimethylbenzene in Na-Y zeolite supercages has been investigated by use of xenon-129 NMR. As shown by Fraissard and co-workers, xenon-129 is an excellent probe of its environment in zeolites and is sensitive to the number of adsorbed guest molecules. The xenon-129 chemical shift, when extrapolated to zero effective xenon pressure, is 89.7 {plus minus} 0.1 ppm for benzene, 89.0 {plus minus} 1.3 ppm for trimethylbenzene, and 107.7 {plus minus} 1.2 ppm for n-hexane at a concentration of two guest molecules per zeolite supercage compared to 58.9 {plus minus} 0.5 ppm for empty Na-Y zeolite supercages. This approach is useful for determining the number of adsorbed molecules inside zeolite supercages and given information about their arrangement within the supercages.

Selective hydroformylation of ethene and propene catalysed on nay zeolite-entrapped Rh 6 and bimetallic RhFe clusters and their structural characterization by extended x-ray absorption fine structure and fourier transform infrared spectroscopy

Tailored metal—bimetal catalysts were prepared from Rh 6(CO) 16, a physical mixture of Rh 6(CO) 16 and HFe 3 (CO) 11 − and bimetallic RhFe carbonyl clusters which were presynthesized inside NaY zeolite supercages. The structural characterization and location of zeolite-entrapped metal clusters were studied by extended X-ray absorption fine structure and Fourier transform infrared spectroscopy to reveal uniformity of the size distribution of Rh clusters less than 10A˚in diameter. The catalysts derived from [Rh 6(CO) 16]/NaY and #x0005B;Rh 6(CO) 16]+#x0005B;HFe 3(CO) 11 −/NaY gave a mixture of aldehydes as the main oxygenated products in the hydroformylation of ethene and propene. In contrast, the bimetallic RhFe/NaY catalyst exhibited higher yields and selectivities towards straight-chain alcohols. The results suggest that the promotion of Fe in the RhFe/NaY catalyst is associated with adjacent heteronuclear RhFe ensembles formed within the NaY zeolite framework to enhance CO insertion with metal-alkyl and M&z.sbnd;H groups which favour linear alcohol formation in olefin hydroformylation.

Ensemble and ligand effects in selective alkane hydrogenolysis catalysed on well characterised RhIr and RhFe bimetallic clusters inside NaY zeolite

A series of metal/bimetal catalysts has been prepared from molecular carbonyl clusters such as Rh6–x Irx(CO)16(x= 0–6) and Rh4Fe2(CO)162– which were synthesized inside NaY zeolite supercages. Na Y-entrapped Rh6, Ir6, Rh4Ir2, Rh3Ir3 and Rh2Ir4 crystallites derived from the precursor carbonyl clusters have been well characterized by f.t.i.r., EXAFS, 129Xe n.m.r. and CO/H chemisorption on their metallic structures and electronic states. The results suggested that the reduced crystallites inside NaY zeolite consist of cluster ensembles < 10 Å in size with controlled metal compositions. These ensembles are fairly stable through several cycles of oxidation, reduction and alkane hydrogenolysis. As the probing reactions, hydrogenolysis of n-butane and ethane, and benzene hydrogenation were conducted. There is a dramatic decrease of hydrogenolysis activity by four orders of magnitude across the series of clusters on increasing the number of Ir atoms in the active Rh ensembles, while a slight enhancement of activity is observed for benzene hydrogenation. The remarkable activity suppression of hydrogenolysis has been interpreted not simply in terms of the geometric Rh ensemble-size effect with a small amount of Ir atoms but of the different electronic states of the RhIr bimetallic clusters in decreasing the electron deficiency through the series clusters. This was demonstrated by the unusually large chemical shifts of n.m.r. signals of 129Xe adsorbed on NaY-Rh-rich clusters in reflecting the higher electron deficiency. For butane hydrogenolysis, the smaller Rh ensembles in bimetallic RhIr clusters gave the maximum selectivities towards the central C—C bond scission to give C2H6, higher than those on the Rh6 and Ir6 inside zeolites. In contrast, RhFe/NaY derived from [Rh4Fe2(CO)16]2–/NaY exhibited conversely a high selectivity towards terminal C—C bond scission to give CH4+ C3H8. The Fe promotion for the terminal C—C bond scission is suggested to be associated with the electronic ligand effect on the heteronuclear sites consisting of Rh–Fe3+ located on the internal zeolite cages.

TEMPERATURE DEPENDENCE OF THE Pt L3-EDGE EXAFS OF PLATINUM CLUSTERS SUPPORTED ON NaY-ZEOLITE

The temperature dependence o f the Pt L3-edge EXAFS spectra of the platinum clusters encaged in the cavities of the Nay-zeolite werz observed by use of synchrotron radiation. A contraction of Pt-Pt bond by 0.07 A from the distance in the bulk Pt metal is observed, which is likely to be associated w ith a strong electron deficiency of the platinum clusters. The temperature dependence of the mean square relative displacement (MSRD) shows that the static disorder in this system is large, while the thermal d isorder is significantly smaller than that of the bulk metal. The estimatedDebyete mperatureis 310 K, which is much higher than that of the bulk metal (240 K). This unusual phenomenom seems to be associated with the increase of the binding energy between platinum atoms and with the strong interaction between a cluster and zeolite-framework (the cage effect). EXAFS spectroscopy is a suitable technique to investigate the s tructural difference between a fine metal particle and a bulk metal. So far, much attention has been paid for the metal-metal bond distance and it has been often concluded that the bond distance becomes the shorter as the particle size is smaller. Another interesting structural aspect of a fine metal particle is the particle-size effect of thermal and/or static disorder. Recently Marques et a1 [l] measured the temperature dependence of EXAFS of supported clusters, and suggested that such an experiment is useful to investigate thermal vibrations and structural disorders as well as to determine the interatomic distance more correctly. In the present study we investigated the temperature dependence of EXAFS on platinum clusters encaged in the supercages of Nay-zeolite. The bond distance and disorder in this system are determined and discussed by comparing with those of the bulk metal and of the metal particles supported on A1203.

Carbon monoxide chemisorption on a Pt–NaY catalyst. Part 1.—Determination of the distribution of the chemisorbed carbon monoxide phase by a 129Xe-nuclear magnetic resonance study of adsorbed xenon

By using 129Xe as an n.m.r.-detectable probe it has been possible to determine quantitatively the distribution of CO molecules chemisorbed on small platinum particles (containing on average six atoms) located in the supercages of NaY zeolite. At 25 °C CO molecules are chemisorbed on the first particles encountered on entering the zeolite crystallite, with a coverage of one CO molecule per two Pt atoms. In contrast, at 300 °C one can obtain, at low coverage, a homogeneous distribution corresponding to only one CO molecule per particle.