Noble metals, zeolite catalysis

Noble metal zeolite catalysts are used in various processes, most of them occurring through bifunctional hydrogenating/acid catalysis. One exception, however, is the selective aromatization of n-alkanes (e.g. n-hexane into benzene) proceeding through monofunctional metal catalysis. Indeed the PtLTL catalyst used commercially does not present any protonic sites. 

Non-noble metals such as Ni, Co, Mo, W, Fe, Ag and Cu have been added to zeolites for use in catalysis. In addition to CO, nitric oxide (NO) has been shown to be a good adsorbate for probing the electronic environment of these metals. When NO chemisorbs on these metals, it can form mononitrosyl (M-NO) and dinitrosyl species (ON-M-NO). The monontrosyl species has a single absorption band and the dinitrosyl species has two bands due to asymmetric and symmetric vibrational modes of the (ON-M-NO) moiety. Again, there have been many studies reported in the literature on the use of NO and/or CO adsorption on non-noble metals supported on zeolites and they are too numerous to list here. Several examples have been selected and summarized to provide the reader with the type of information that can be provided by this method. 

Zeolite molecular sieves play an important role in catalysis reactions. The introduction of noble metals into zeolite pores to form bifunctional catalysts is of particular interest. To introduce noble metals into zeolites, first noble metal ions or cation complexes are... 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

The conventional amorphous silica-alumina catalysts have been substituted here by zeolites, especially of the H-ZSM-5 type [49J. Higher yields and higher pyridinc/p-picoline ratios arc obtained with zeolite catalysis. The micropores will reduce the formation of higher alkylated pyridines. The zeolites can be further improved by incorporating metal oxides (e.g. Pb, Tl, Co) or noble metals or by applying both types of promoters. As an example, a Pb-MFI catalyst, operated at 450 °C in a fixed bed reactor and fed with CH2 O/CH3CHO/NH3 in a 1.0 2.0 4.0 molar ratio gave 79 % total pyridines with a pyridine/p-picoline ratio of 7.5. Also zeolites MCM-22 and Beta [50] perform well in combined pyridine/p-picoline synthesis. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

In composite materials coatings of laterally oriented silicalite-1 crystals are assumed to be a (shape) selective component in catalysis. A catalytic membrane is obtained if a noble metal coating is applied after growth of the crystal layer, which is self-supporting or bonded to a meso-porous support. Also the catalytic site can be applied onto the support before the in situ growth of the zeolite layer. 

Figure 12 shows the S mSat process based on published information. 120,179,180 SynSat process is considered to be an iimovation across the boundary between catalysis and reactor engineering . SynSat employs several different catalyst beds within a single reactor shell with intermediate by-product gas (H2S etc.) removal, and optional counter-current gas-flow. Catalysts A and B in Figure 9 are metal sulfide catalysts such as sulfided Ni-Mo. Catalyst C is a noble metal loaded on an acidic support such as zeolite. There is an intermediate gas removal between the beds of Catalysts B and C. Nearly all the sulfur compounds must be converted and removed as H2S on beds A and B before the fuel feed reaches the noble metal catalyst bed C. 

Rosso 1, Galletti C, Saracco G, Garrone E and Specchia V (2004) Development of A zeolites-supported noble-metal catalysts for CO preferential oxidation H2 gas purification for fuel cell. Applied Catalysis B Environmental, 48, pp. 195-203. 

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