Pd-Y zeolite

Figure 1. ESR spectra of Pd-Y zeolites at 77°K heated at 500°C (.1) in vacuo, (2) in oxygen...

Effects of Hydrogen Adsorption on Pd-Y Zeolite Samples. The oxidized samples in which Pd(III) ions have been detected, when exposed to hydrogen at 25 °C, turned black instantaneously, and ESR measurements showed that the Pd(III) ions had disappeared. Simultaneously, a strong ESR signal developed. 

Figure 2. ESR spectrum at 77°K of Pd-Y zeolite heated at 500°C after hydrogen adsorption at 25° C...

On the other hand, the participation of Pd2+ and/or Pd+ species as active sites in the reaction of dimerization of ethylene was suggested some years ago (222). Recently, two independent groups established that Pd + species in Pd/X and Pd/Y zeolites are sites of highest activity (124-126, 223). Starting with a Pd2+/Y or Pd2+/X catalyst, they monitored its activity in the course of reaction. Parallel study with XPS, ESR, and solid-state NMR confirmed that Pd+ is an active species in the reaction. Its concentration grows with time on-stream. 

ESCA measurements and assignments depended on comparison of Pd 3 and 3d lines for specimens and standard compounds. Measurements from Pd line intensities were in good agreement for the Pd-Y zeolite with chemically determined Pd content. 

The investigated catalysts [9] were Pt-Pd/USY zeolite (SiO2/Al203 ratio 33.5, total- and mesopore surface areas 650 m2/g and 51 m2/g, total metal content 0,9%, Pd/Pt mass ratio 6 1 to 1 3, dispersion 0.41-0.55, acidity 0.20 mmol/g). Metal contents and Pd/Pt ratios are summarized in Table 1. As reference catalysts bimetallic Pt-Pd/Si02-Al203 (platinum-content 0.3%, palladium-content 0.6%, A1203 content 15%, surface area 292 m2/g, dispersion 0.41, acidity 0.18 mmol/g) and Pt-Pd/y-Al203 (platinum-content 0.3%, palladium-content 0.6%, surface area 182 m2/g, dispersion 0.35, acidity 0.12 mmol/g) were used. 

Figure 16.10 Second-stage activity versus metal content of Pd/stabilized Y zeolite catalysts (a) 0.8% Pd, (b) 2.4% Pd.

Pd/Cu zeolite Y associations were found to be selective catalysts for oxidation of olefins in the presence of steam at temperatures ranging from 373 to 433K [22-30]. Acetone and acetaldehyde were obtained by propylene and ethylene oxidation, with selectivities of at least 90%. Neither Pd/Y nor Cu/Y showed good activity in these reactions. The conversion of different olefins under the same experimental conditions decreases in the following order [23] ethylene > propylene > 1-butene > cis-2-butene - trans-2-butene. 

Pd/Cu-zeolites are also catalysts for the oxidative acetoxylation of propylene to allylacetate [32-39]. The best results are obtained on a catalyst which is pretreated with an alkali solution to neutralize the acidic centres and containing Pd and Cu in an atomic ratio of 1.1 [37]. The alkali treatment suppresses the acid catalyzed addition of acetic acid to propylene, resulting in the formation of isopropyl acetate, which is observed over non-neutralized Na- and H-Y, as well as over unreduced and reduced Pd/Cu-NaY. Experiments with... 

Recently, a Pd(0)-Y zeolite system has been reported by Artok and Bulut. In general, aryl bromides coupled with arylboronic acids at room temperature in a DMF/H2O solvent mixture.The catalyst could be recovered by filtration, but in order to obtain high yields of coupling product the temperature had to be raised to 50 °G. Regeneration of the catalyst by consecutive treatments with O2 and H2 was required to obtain high yields after the second use. 

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. 

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. 

Table I. Hydroisomerization of n-Pentane Over Pd-H-Zeolite Y Influence of Sodium on Catalytic Activity...

It has been claimed that noble metal dual function catalysts based on H-mordenite are more active for paraffin isomerization than their counterparts based on H-zeolite Y (25). For both zeolites the isomerization activity depends strongly on the degree of sodium removal and comparison of low sodium Pd-H-mordenite and low sodium Pd-H-zeolite Y for isomerization of n-hexane at 250° C shows that both materials have about the same activity (Table IV), the Y sieve based material being slightly more active. 

Table IV. Isomerization of n-Hexane Over Pd-H-Zeolite Catalysts Comparison of Mordenite and Zeolite Y...

Previous results(2) had shown that a Pd-Ni-SMM catalyst was effective for hydrocracking hexane as well as a raffinate feed. Conclusions showed that this catalyst system when containing two nickel atoms per unit cell (15 wt % nickel) was approximately 15 times more active than a Pd-rare earth-Y zeolite catalyst and 1.2 times more active than Pd-H-mordenite. This same catalyst system (0.7 wt % Pd-15 wt % Ni-SMM) was chosen for our raffinate processing studies. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

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