Catalysts destructive

One of the major problems in all the ammoximation processes using aqueous H202 + TS-1 with NH3 is that, under the basic conditions (pH > 10) prevailing during the reaction, some of the lattice Si ions of the zeolite structure in TS-1 are leached into solution, leading to catalyst destruction. This leaching is a common characteristic of all silicates. Innovative catalyst formulations and process modifications are needed to overcome this problem. 

Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolation within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. 

As in the ethylene oxide system kinetics are complex and do not lend themselves to exact interpretation (19). The boron fluoride — water catalyst system appears to be most effective at a boron fluoride/water ratio of about three, a surprising and probably fortuitous similarity to the efficiency of this catalyst in the isomerization of some hydrocarbons (20). At low water concentrations the number of polymer molecules formed equals the number of water molecules added and chain transfer may be assumed, though it has not actually been demonstrated. There is some indication of a maximum molecular weight of 15,000—20,000 at — 20° C but the present data are inadequate to establish this point. The order in monomer appears to be first at low water concentrations rising to second at higher water levels, but it seems quite possible that this apparent change in order is due to some factor such as catalyst destruction. 

HDM3 Catalytic cracking feedstocks Avoid metals deposition Avoid coke buildup Avoid catalyst destruction... 

Much of the interest in polymerization initiated by lithium compounds is caused by the formation of highly specific products in nonpolar solvents. Under these conditions a highly cis-1,4-polyisoprene is formed, and methyl methacrylate is polymerized to a largely isotactic product. It is reported that isotactic polystyrene can be formed at low temperatures (2, 9), but this seems to form only in the presence of lithium hydroxide formed by catalyst destruction (28). 

A knowledge of the action of lithium alkoxides in these systems is important because they are always present to some extent, formed by fortuitous catalyst destruction by traces of oxygen. The other likely impurity is lithium hydroxide produced from traces of water in the system. It apparently decreases the initiation rate [53] but primarily [55] because it reacts with organo-lithium compounds... 

These porphyrins with chiral superstructures are expected to be insufficiently robust against oxidative catalyst destruction when H2O2 is used as the source... 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

The two types of catalysts merge with the development of what are called tethered catalysts, where a homogeneous catalyst is amended so that it is able to be attached covalently to an inert surface, such as silica. This is also called a supported catalyst. Having the active component available as part of a solid can assist processes where carrying the catalyst forward in solution to another stage of the process may lead to contamination or catalyst destruction. Further, surface attachment also can alter catalytic activity favourably in certain cases. 

Chromium oxide (Cr203). Up to 0.1% colorimetrically with diphenylcarbizide at 540 nm. Above 0.1% but as a minor constituent, colorimetrically with EDTA at 550 nm. As a major constituent by oxidation to dichromate by peroxodisulfuric acid using a silver nitrate catalyst, destruction of permanganate with HCl and titration against ferrous ammonium sulfate using diphenylamine-4-sulfonate indicator. 

Until recently, catalyst coking has been generally considered as a harmful side process that causes catalyst deactivation and, in some cases, catalyst destruction [1, 2J. However, catalyst coking (carbonization) is reported in many studies as intentionally performed in order to obtain new systems with useful properties. Thus, alumina was covered by a carbon layer to prepare more inert carriers for desulfurization catalysts [3]. R. Leboda et (see, e.g.,[4]) showed a good performance of sibca with partially carbonized surface as chromatographical adsorbent. Carrott and Sing [5] used carbonized silica to obtain standard isotherms of nitrogen adsorption. 

From this point of view the understanding of methylidene decomposition and stability is important for designing a more stable catalytic system. Usually, the catalyst destruction was studied based only on kinetic experiments without isolation and identification of products of decomposition [8,10,11]. Recently, Grubbs and coworkers isolated the major product formed after heating of (6a) in benzene solution (Eq. 2) [9]. The structure of (7) was proved by x-ray crystallography. Complex (7) was isolated with yield 46%. It has been shown that this complex is responsible for the C=C double bond isomerization [9]. 

Tlw influence of substrate. There are just a few reported experimental studies which focus on that problem [18]. All of them are concerning of the C=C double-bond isomerization proeess whieh usually was catalyzed by Ru hydride species formed in the reaction mixture after metathesis catalyst destruction. Recently, the theoretical results about substrate-induced catalyst decomposition involving P-hydride transfer from ruthenocyclobutane intermediate (Scheme 3) were published [19]. 

Decomposition and hydrogenation Catalyst Destructive hydrogenation Many stocks... 

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