Catalyst precursors zeolites

Pt/H-MCM-22 catalysts for methane combustion have been prepared by ion-exchange of a highly crystalline H-MCM-22 zeolite using [Pt(NH3)4](N03)2. The activation procedure of the catalyst precursor has been optimized and all steps monitored by HRTEM, SEM and FTIR of CO adsorbed. The preliminary decomposition/calcination of the ion exchanged sample is very crucial in that influence the final properties of platinum active species. 

All of these literature reports mainly deal with exchanged Pt(NH3)4 on zeolite. The main difference between exchanged and impregnated [Pt (NH3)4](N03 )2 is the presence of nitric groups (N03 ) on the support in the impregnated case. As will be shown, these groups play a vital role in the pretreatment of the impregnated catalyst precursor. 

Precipitation is a method often used for producing both support precursors and catalyst precursors (including precursory forms of zeolites) and occurs when two or more solutions are mixed in a suitable way. In addition to providing general details of the method (e.g., concentration, temperature, pH, etc.) it is necessary to indicate specifically the order and rate of addition of one solution into the other, a description of the mixing procedure and the details of the ageing procedure, if... 

Reaction pathways There is general agreement as to the nature of the catalyst precursor (26. 30). It is well admitted that even under CO low pressure Rh(III)-Y was reduced to the monovalent rhodium dicarbonyl attached to the zeolite framework via one or more oxide ions irrespective of the rhodium introduction procedure onto the zeolite. 

Benzene, toluene and / -xylene are the most industrially important aromatic hydrocarbons. Their relative proportions must be adjusted according to the needs of the market, which presently requires more benzene and xylene. Dealkylation of toluene can be carried out by hydrogenolysis with H2 using Cr, Mo or Pt oxides as catalyst precursors (Equation 4). Methyl redistribution in toluene is best carried out around 420°C on ZSM-5 zeolite. Under these conditions, xylene is mainly present as the para isomer (90% selectivity. Equation 5) due to the shape-selectivity of the zeolite. 

With respect to other large pore molecular sieve supports like ALPOs very little is known about the electronic nature of the supported metals. Preliminary experiments in our laboratory indicate that the reducibility of the metal catalyst precursor and the dispersion of the metal in the final material differs significantly from that in conventional zeolites. 

Sample number Catal> st precursor Zeolite support Treatment/ catalvii /V, Tresh catalyst nm, fresh catalyH N, used catalyst X, om, used catalyst... 

Below we summarize the results obtained by using metal carbonyl clusters in zeolites as catalysts or catalyst precursors for reactions involving CO (Tkble 4-7). 

Tible 4-7. Reactions involving CO which are catalyzed by zeolites containing metal carbonyl clusters. The clusters are regarded as catalyst precursors the catalytically active species are generally not known. 

Some zeolites containing metal carbonyl dusters also are catalytically active for the shift reaction. Iwamoto et al. [SO] investigated the catalytic activity of zeolite NaY incorporating [HFe3(CO)ii] at 60-180°C and 1 bar. The catalytic activity was comparable to that reported for the solution phase reaction which uses [Fe(CO)s] as the catalyst precursor at 180 °C and 40 bar. Infrared and ultraviolet spectra indicated that [HFe3(CO)ii] was stable under the reaction conditions. A plausible mechanism was proposed to exjdain the observations (Eqs. 4.9-4.12). 

In summary, it is difficult to determine the catalytically active spedes in any supported catalyst, and there are still no well documented examples of catalysis by metal carbonyl clusters themselves in zeolites. There is, however, substantial indirect evidence that metal carbonyl clusters in zeolite cages may be either the catalysts or the catalyst precursors for a number of reactions involving CO. In some cases, these dusters are the only detectable organometallic or metallic spedes, and they are stable under the conditions of the catalytic reactions. Some of the catalysts retain the colors and the infrared spectra of the metal carbonyl dusters even after weeks of catalytic operation. In the few instances when EXAFS data were available, the presence of metal carbonyl clusters within the zeolites was indicated however, evidence for other spedes that are plaudble catalyst precursors was also obtained. 

XPS is the most commonly used and most useful surface analysis method in catalyst characterization. It can be used for both qualitative and quantitative analysis for almost all kinds of catalysts used in heterogeneously catalyzed reactions. XPS studies of oxide, sulfide, fluoride, halide, etc., catalysts,. supported metal catalysts, Raney or gauze metal catalysts and zeolite catalysts are all possible. The samples can be studied in any of the precursor, calcined, reduced, activated, deactivated, aged or poisoned states. Real industrial catalysts can be analyzed as well as fundamental model systems. Quantitative analysis is possible either with the help of empirical sensitivity factors or by standard-free methods. In the latter case appropriate theoretical models are u.sed with photoionization cross-section tables, inelastic mean free path (X.) data and individ-... 

A series of CoSx-MoSx/NaY catalysts was synthesized by intoducing Co(CO)3NO into MoSx/NaY evacuated at 673 K for 1 h, followed by second programmed sulfidation procedures. MoSx-CoSx/NaY catalysts were prepared in the reversed order of the metal sulfide accommodations into the zeolite cavities. When Co2(CO)g was used as the Co precursor, MoSx/NaY was impregnated with COj(CO)g dispersed in n-hexane, followed by evacuation at room temperature to remove the solvent. Co2(CO)g/MoSx/NaY was subsequently sulfided at 673 K to give CoSx/MoSx/NaY. The catalyst composition was determined by AAS and ICP. 

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