Catalysts activity in hydrogenation

Fig. 16. Nanophase PtyTiOa catalysts (a) finely dispersed Pt/TiOa at room temperature, (b) In situ dynamic catalyst activation in hydrogen imaged at 300°C. The (111) lattice atomic spacings (0.23 nm) are clearly resolved in the platinum metal particle (P) under the controlled reaction conditions, (c) The same particle of platinum (P) imaged at 450°C, also in H2. Catalyst deactivation with growth of the support oxide monolayer indicated by a larger arrow, and the development of nm-scale single-crystal clusters of platinum metal (which show no coating as they emerge) with 0.2-nm lattice spacings indicated by smaller arrow (87).

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

Raman spectroscopy is a powerful technique for characterization of solids and surfaces, and is well suited for examining oxides and supported oxide catalysts. Over the past few years, our group successfully examined a number of catalytic systems using UV Raman spectroscopy. The use of UV excitation prevents fluorescence from the Raman spectra by exciting the sample at a frequency where fluorescence does not occur. In this study, Raman spectra were obtained from the 1% chromium on alumina catalyst during exposure to propane or propene under reaction conditions. Calcined catalysts were compared to catalysts activated in hydrogen. 

Fig. i.n Transmission electron micrograph (left) of a molecular sieve with supported PdNPs (right). The catalyst, active in hydrogenation of olefins, contains 20 wt.% IL whose average layer thickness is 0.4 nm. (Reprinted with permission from Ref [41b], Han group, Angew.Chem. Int. Ed. 2004, 43, 1397). 

Aldehydes and ketones are similar in their response to hydrogenation catalysis, and an ordering of catalyst activities usually applies to both functions. But the difference between aliphatic and aromatic carbonyls is marked, and preferred catalysts differ. In hydrogenation of aliphatic carbonyls, hydrogenolysis seldom occurs, unless special structural features are present, but with aryl carbonyls either reduction to the alcohol or loss of the hydroxy group can be achieved at will. 

Increase in the catalyst activity and hydrogen transfer properties... 

Fe/Ir catalysts In situ Fe and Ir Mossbauer spectroscopy of silica-supported Fe/Ir catalysts with different iron to iridium ratios following pretreatment in hydrogen show that the reduction of the Fe component is enhanced by the presence of Ir metal. The presence of Ir was found to increase the catalytic activity in hydrogenation of carbon monoxide and also to influence selectivity... 

Recent work done by Xiong et al.84 on Co/AC (activated carbon) catalysts showed that a Co2C species formed during the catalyst reduction in hydrogen at 500°C. Evidence for the carbide in the Co/AC catalysts was obtained by x-ray diffraction and XPS measurements, and the formation of this Co2C species reduced the FTS activity over the Co-based catalysts. The presence of bulk carbide also seems to enhance alcohol selectivity.85... 

From the H/M values for the catalysts NiSn-BM (Sn/Ni = 0.29) and PtSn-BM (Sn/Pt = 0.71), and the H/M values for the corresponding monometallic ones, it can be inferred that Sn blocks about 70% of the originally accessible M atoms. For these systems, based on the dispersion measured for Pt and Ni, the atomic ratios Sn/M correspond to values higher than 1. Notably, even in these cases, an important portion of the metallic surface has sites accessible to hydrogen dissociative adsorption, which is essential for the phase to be active in hydrogenation reactions. 

Figure 5.13. In situ atomic-resolution ETEM image of Pt/titania catalyst (a) finely dispersed Pt particles (b) in situ real-time dynamic activation in hydrogen imaged at 300 C. The 0.23 nm (111) atomic lattice spacings are clearly resolved in the Pt metal particle, P and (c) the same particle imaged at 450 C, also in H2. SMSI deactivation with a growth of a Ti-oxide overlayer (C), and the development of nanoscale single-crystal clusters of Pt, with atomic resolution (arrowed). (After Gai 1998.)...

Attempts have also been made to develop biphasic methodologies for the hydrogenation of aromatics. The hydrogenation of benzene derivatives was studied using various Ru complexes.468,469 A trinuclear cluster cationic species was isolated as the tetrafluoroborate salt and showed increased activity in hydrogenation.470 The air/ moisture-stable [bmim][BF4] ionic liquid and water with [Rh r -CgHg).,] [BF4] as the catalyst precursor is an effective system under usual conditions (90°C, 60 atm)471 A system composed of stabilized Rh(0) nanoparticles proved to be an efficient catalyst in the hydrogenation of alkylbenzenes 472... 

In developing the mechanism, it was assumed that the adsorbed state of buta-1 2-diene active in hydrogenation was the di-7r-adsorbed species C in Fig. 32. The close similarity in the ZV-profiles for each butene formed in a given reaction suggests that each was formed as a primary product and the general mechanism shown in Fig. 33 was proposed. The difference in behaviour of the type A and B catalysts was explained by proposing that, at the type A surface, a-bonded and 7r-olefinic species are of importance as... 

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