Palladium-loaded

Two most firequently applied palladium precursor, H2PdCl4 and Pd(NH3)4Cl2, were usrf to prepare Pd/CNF by west impregnation. Results of foe CTA hydrogenation showed that under the same palladium loading ratio (0.5Pd%, see Table 2), foe Pd/CNF fi-om H2PdCl4... 

In order to probe the influence of Au and KOAc on the vinyl acetate synthesis chemistry, four different catalysts were synthesized. All of these catalysts were prepared in a manner exemplified in prior patent technology [Bissot, 1977], and each contained the same palladium loading in an egg-shell layer on the surface of a spherical silica support. The palladium content in the catalyst was easily controlled by adjusting the solution strength of palladium chloride (PdClj) added to the porous silica beads prior to its precipitation onto the support by reaction with sodium metasilicate (Na SiOj). The other two catalyst components (Au and KOAc) were either present or absent in order to complete the independent evaluation of their effect on the process chemistry, e.g., (1) Pd-i-Au-hKOAc, (2) Pd-i-KOAc, (3) Pd-hAu, and (4) Pd only. 

The palladium-loaded chelating groups are located exclusively at the surface of the particle they are accessible and there is no problem of diffusion. TONs of up to 210,000 were obtained for the vinylation of aryl iodides and aryl bromides. However, no recycling is reported and the authors did not exclude the participation of a dissolved (homogeneous) Pd species in the catalytic activity. 

The Pd-ZSM-5 catalysts are prepared by impregnation and by solid exchange methods on the carrier of HZSM-5 and NaZSM-5 (Si/Al = 26) with variable palladium loading and different pre-treatment gas (He and O2). N2-physisorption, DRX and CH4-TPR are the main techniques used to characterise these catalysts. Furthermore, total methane oxidation is used to test their catalytic activity. Among the preparative variables, the solid exchange method, the NaZSM-5 support and the increase of the palladium loading improve considerably the activity of the Pd-ZSM-5 catalysts in methane oxidation. 

Keywords Total methane oxidation, Pd-ZSM-5 catalysts, impregnation, solid exchange method, pre-treatment and palladium loading. 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

T10%) are respectively 355 and 285°C. Finally, the increase of the designed palladium loading from 0.5 to 2 % is observed to increase the methane conversion. The ignition temperatures are 350, 320 and 285°C respectively for the Pdo.sNaZS02, the PdiNaZS02 and the Pd2NaZS02 catalysts. 

The N2-physisorption characterisation results show that, no significant variations (less than 5%) are observed on the BET surface area, the total pore volume and the micropore volume of the different Pd-ZSM-5 catalysts, when the preparation method, the pretreatment gas, the charge-balancing cations and the palladium loading are modified. This result suggests that the ZSM-5 texture is stable with respect to the preparative parameter variations and that the observed activity differences are not related to any... 

Fig. 2 Plot of H2 STY at 200 °C versus palladium loading for some catalysts reported in the literature.

Various supports have been used, but the results reported here have been exclusively obtained with 5-AI2O3 with a specific surface area of 130 m /g and a porous volume of 1.04 cm /g. The palladium loading is about 0.1% by weight. After impregnation, the samples are dried overnight at 393 K. 

Better results were obtained by using in situ prepared palladium complexes of a G4 dendrimer (calculated molecular weight 20 564 Da for 100% palladium loading of the 32 diphosphines). After 100 residence times, the conversion had decreased from 100% to approximately 75% (Fig. 3). A small amount of palladium was leached from the catalyst during this experiment (0.14% per residence time), which only partly explains the decrease in conversion. The formation of inactive PdCl2 was proposed to account for the additional drop in activity. A sound conclusion about the effect of this dendritic catalyst requires more experiments. 

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. 

A. Anson et al., Hydrogen capacity of palladium-loaded carbon materials. J. Phys. Chem. 

Several catalysts with various degrees of palladium loading and metal dispersions were submitted to thiophene adsorption. The dissociatively... 

A system for the Heck reaction between methyl acrylate and iodobenzene has been developed comprising an imidazolium-functionalized polystyrene monolith, initially for use in batch. This system could be reused six times before any reduction in yield was observed. Accordingly, a continuous-flow reactor system was developed using DMF at 200 °C with a residence time of 3-4 min to achieve full conversion. This system was characterized by very low palladium loadings (0.02 mol%), and ICP-MS of the solution aliquots showed leaching of less than 1 ppm. Attempts at using EtOH as solvent to provide a more environmentally and procedurally benign protocol resulted in maximum yields of 85% [144]. 

Tsyganok, A.I. and Otsuka, K. (1999) Selective dechlorination of chlorinated phenoxy herbicides in aqueous medium by electrocatalytic reduction over palladium-loaded carbon felt. Appl. Catal. B. Environ. 22, 15-26. 

While potassium hydroxide is beneficial in catalysts containing palladium loads near 5% with low palladium loads it appears that potassium hydroxide is not always necessary. The semihydrogenation of dehydrolinalool (19) gives 100% of linalool (20) over 0.5% PCI/AI2O3 in alcoholic solvents as shown in Eqn. 16.25. Perhaps with lower palladium-loaded catalysts reactant diffusion is not a factor in the reaction, so the presumed enhanced adsorption of the potassium hydroxide complex is not as important. 

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