Active catalysts formation from precursor

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Hexaruthenium carbonyl complexes have been used to prepare Ti02-supported mthenium catalysts for the sulfur dioxide reduction with hydrogen [112, 113], A catalyst derived from [Ru6C(CO)i6] showed higher activity in the production of elemental sulfur at low temperatures than that prepared from RUCI3 as precursor. This catalytic behavior is related with the formation of an amorphous ruthenium sulfide phase that takes place during the reaction over the ex-carbonyl catalyst [112]. 

Finally, a third means of ligand formation from an imidazolium cation, described by Dupont and co-workers, should be mentioned here [34]. They investigated the hydrodimerization/telomerization of 1,3-butadiene with palladium(II) compounds in [BMIM][BF4] and described the activation of the catalyst precursor complex [BMIM]2[PdCl4] by a palladium(lV) compound formed by oxidative addition of the imidazolium nitrogen atom and the alkyl group with cleavage of the C-N bond of the [BMIM] ion, resulting in bis(methyHmidazole) dichloropalladate (Scheme 5.2-5). However, this reaction was only observed in the presence of water. 

The reduction of the molar ratio of Al/MNd constitutes another key to the reduction of overall catalyst costs. Especially in NdV-based catalyst systems a considerable portion of the expensive Al- alkyl is consumed in the activation of Nd-carboxylates and other reactions prior to the formation of the active Nd-species (Sect. 4.2) [178]. The formation of the active Nd-species from Nd-phosphate precursors requires a comparably low amount of Al-alkyls [272] as Nd-phosphate-based catalyst systems are active at very low molar ratios of Al/ Nd < 5 (Sects. 2.1.1.4 and 2.1.4) [268,269,272],... 

A. Following the Formation of Active Catalysts from Precursor Structures... 

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