Catalyst stabilizer

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Rare earth improves the catalyst activity (Figure 3-11) and hydro-thermal stability. Catalysts can have a wide range of rare earth levels. 

Several forms are imaginable for the [Ni°(butadiene)2L] and [Ni°(butadiene)J active catalysts, depending on the monodentate (p2) or the bidentate (p4) coordination mode of butadiene from either its s-cis or its s-trans configuration. The two butadienes can be coordinated in bis(p2), p4, p2, and bis(p4) modes for the PR3/P(OR)3-stabilized catalyst complex, giving rise to formal 16e, 18e, and 20e species. On the other hand, bis(p4)- and p4,p2-butadiene species and also tris(p2)- and p4,bis(p2)-butadiene compounds are possible species for the [Ni°(butadiene)2] and [Ni°(butadiene)3] forms for the [Ni°(butadiene)J active catalyst. In general, for butadiene to coordinate in a bidentate fashion, the p4-cis mode is thermodynamically favorable relative to the p4-trans mode, while the p2-trans mode prevails for monodentate coordination. 

Monoorganotins Polyvinyl chloride stabilizers, catalysts, Sn02 precursors (CEC 1978 WHO 1980 Chau etal. 1984 Blunden etal. 1985, Blunden and Chapman 1986). 

Commercial MgO was impregnated with Ni acetate in toluene solution. Addition of 1-3 wt% K suppresses the Ni sintering and extends the catalyst stability. Catalysts were evaluated for 500 h endurance test. Process reported to be suitable for producing H2 for MCFC... 

Chemical anchoring of catalytically active metal clusters onto a support is of practical importance to stabilize catalysts against loss of activity by Ostwald ripening, i.e. metal agglomeration. Documented examples include Pt, Pd, or Rh supported on acidic oxides, in particular zeolites in their H-form. Three types of anchors have been de-... 

Relative stability Catalyst kinetics used Base Improved Improved... 

Uses. Most applications of MSC are for intermediates in the pharmaceutical, photographic, fiber, dye, and agricultural industries. There also are miscellaneous uses as a stabilizer, catalyst, curing agent, and chlorination agent. 

In an attempt to overcome the problem of accumulation of the oxidized electron donor, we have incorporated a recyclable surface-active electron donor in DODAC vesicles (12). This electron donor contains a sulfide moiety which dimerizes upon light-induced oxidation. Simultaneously, hydrogen is evolved via vesicle-stabilized, catalyst-coated, colloidal CdS particles. The dimer could be chemically reduced for additional hydrogen formation. Figure 9 is an idealized view of this cyclic process (12). 

The Sodium resistance of the active ingredients in FCC catalysts can also be tackled by high activity and stability catalysts. 

E - Severe Hydrothermal Conditions - Catalyst deactivation/ stability problems - High Activity and Stability Catalysts... 

Fig. 19. Stabilized catalyst complex. From Mejzlik and co-workers (204).

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

We have shown that oxides with a wide spectrum of donor properties are available (1). We even showed that these oxides can be ranked in a scale of donor properties. This suggests that adequate oxides can be found as donors to stabilize catalysts agains deactivation by reduction. 

Prereduced, stabilized catalyst types, introduced on the market some years ago, have gained a considerable market share. Prereduced catalysts have the full pore structure of active catalysts, although the pore surface has been oxidized to a depth of a few atomic layers to make these catalysts nonpyrophoric. 

Deactivation of light naphtha aromatization catalyst based on zeolite was studied, by kinetic analysis, micropore volume analysis and model reactions. Coke accumulates at the entrance of zeolite channel, blocks it and hinders reactant molecule to access active sites in zeolite channel. Our own stabilization technique passivates coke-forming sites at the external surface of the zeolite. This minimizes the coke formation at the entrance of zeolite channel and increases on-stream stability. The stabilized catalyst enabled us to develop a new light naphtha aromatization process using an idle heavy naphtha reformer that is replaced by CCR process. 

Kinetics of Catalyst Deactivation. In order to study the kinetics of the deactivation of stabilized catalyst, we carried out several sets of experiment varying pressure, with constant space velocity and with constant contact time, respectively. We assumed that reaction rate of light naphtha conversion conforms to first-order kinetics with respect to light naphtha concentration and that the decreasing rate of active site, which is caused by coke deposition, is expressed by first order. Then catalyst activity is described as exponential deactivation (S). 

The fact that deactivation of the stabilized catalyst is expressed as first-order kinetics with respect of concentration indicates that site coverage is responsible for the deactivation of the stabilized catalyst. 

Rate constants of the unstabilized and stabilized catalyst shown in Figure 1 were calculated. The result indicated that the unstabilized catalyst deactivated faster than expected from exponential deactivation. This suggests that the deactivation mechanism of the unstabilized catalyst is different from simple site coverage. 

Low pressure nitrogen adsorption isotherms of coked catalysts, in Figure 5 showed smaller amount of volume adsorbed in the coked unstabilized catalyst than stabilized catalyst samples. This is due to a large difference in micropore volume, shown in Table II. 

A comparison of the micropore volume of the coked catalyst samples, shown in Table II, clearly illustrates that the coked-stabilized catalyst sample possesses 70% of internal pore volume, which is almost free of coke and is accessible to nitrogen. On the other hand, the coked-unstabilized catalyst sample showed large reduction of micropore. This indicates a virtual blocking of the internal pores in the coked sample... 

Figure 3. Variation of rate constant of light naphtha conversion with increasing pressure under constant space velocity over the stabilized catalyst.

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