Where is the Catalyst

There is increasing awareness that catalysts introduced into a system as a solid may partially dissolve or react to give the real catalyst in the liquid phase. Conversely, species introduced in solution may react to produce some solid that is actually the catalyst. This Section will discuss the methods that may be used to identify the phase of the active catalyst. 

A standard chemical test for catalytic activity of the M(0) species, which seems to have been applied first in this general area by Whitesides and co-workers, is to add Hg(0) to the system. The principle is that Hg(0) will amalgamate or coat the surface of the M(0) species and greatly reduce or eliminate the catalytic activity. It is necessary to add a substantial excess of Hg(0), with vigorous stirring to ensure mixing with the M(0) species, and to test that Hg(0) does not react with any of the reactants or products. 

A second but less widely used test is to poison the M(0) surface with additives such as CS2, thiophene or PPhj. The principle is that the poison will adsorb at the active sites on the metal and inhibit or stop other catalytic reactions. The assumption is that the poison is essentially irreversibly adsorbed, but this becomes doubtful if modest to high temperatures ( 50°C) are required. Finke and co-workers have pointed out that less than 1 equivalent of poison per metal should be effective because a substantial amount of the metal will be in the bulk rather than at active sites. Of course, it must again be shown that the poison does not react with the reactants or products. 

A method that is complementary to the above two was proposed and tested by Anton and Crabtree. They found that several homogeneous Rh, 

Another characteristic of systems that form heterogeneous catalysts adventitiously in situ is that they tend to give irreproducible rates. This might not be the case if all the conditions, such as concentrations, temperature and mixing, were carefully controlled during the formation of the catalytic solid, but this is not usually the case if the initial assumption is that no significant solid is forming. The importance of the synthetic conditions is shown in the series of studies by Finke and co-workers on the 

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An effective experimental design is to measure the pseudo-first-order rate constant k at constant pH and ionic strength as a function of total buffer concentration 6,. Very often the buffer substance is the catalyst. Let B represent the conjugate base form of the buffer. Because pH is constant, the ratio (B]/[BH ] is constant, and the concentrations of both species increase directly with 6 where B, = [B] -t-[BH"]. 

Anonymous Several speakers found iron deposited on the nickel catalyst. Where is the iron believed to come from, and what steps will be taken to prevent the deposition ... 

In this equation, Y is the catalyst performance, the variables X and ni are normalized variables representing the reaction conditions and catalyst s metal weight loadings, respectively. The model coefficients C, a , and (3 , are functions of the catalyst composition, as shown in Eqns (6) and (7), where m.j refers to the nominal weight loading of Pt, Ba, or Fe. The equation for (3 takes the same form as Eqns (6) and (7). 

Another achievement in recent asymmetric reaction study is the so-called chiral autocatalysis—where the product itself catalyzes its own asymmetric synthesis. In this process, the chiral catalyst and the products are the same in an asymmetric autocatalytic reaction. The separation of chiral catalyst from the product is not required, because the product itself is the catalyst. Starting from an optically active product with very low ee, this process allows the formation of a product with high ee values.106,114... 

Cyclic voltammetry is a useful alternative to RDEV, particularly in the present case, where binding the catalyst to the electrode surface and rotation of the electrode may not be compatible in a number of practical cases. Moreover, scan rates in cyclic voltammetry can be varied over a much wider range than rotation rates in RDEV. 

It is possible to reverse the formation of an ester by a catalyzed hydrolysis reaction. In this reaction, the water reenters the molecule where it was removed. If an acid is the catalyst, the original acid and alcohol will reform. If a base is the catalyst, the alcohol will reform however, the acid will react further to produce its conjugate base. The base-catalyzed hydrolysis of an ester is saponification. 

Micropore mass transfer resistance of zeoUte crystals is quantified in units of time by r /Dc, where is the crystal radius and Dc is the intracrystalline diffusivity. In addition to micropore resistance, zeolitic catalysts may offer another type of resistance to mass transfer, that is resistance related to transport through the surface barrier at the outer layer of the zeoHte crystal. Finally, there is at least one additional resistance due to mass transfer, this time in mesopores and macropores Rp/Dp. Here Rp is the radius of the catalyst pellet and Dp is the effective mesopore and macropore diffusivity in the catalyst pellet [18]. 

For the single straight cylindrical pore the surface-to-volume ratio was 4/r/p, but for a porous catalyst slab, this ratio is SgPb) where SgP, is the catalyst surface area per unit volume of catalyst, as defined previously. 

Catalytic evaluation of the different pillared clays was performed using a microactivity test (MAT) and conditions described in detail elsewhere (5). The weight hourly space velocity (WHSV) was 14-15 the reactor temperature was 510 C. A catalyst-to-oil ratio of 3.5-3.8 was used. The chargestock s slurry oil (S.O., b.p. >354 C), light cycle oil (LCGO, 232 C < b.p. <354 C) and gasoline content were 62.7 vol%, 33.1 vol% and 4.2 vol% respectively. Conversions were on a vol% fresh feed (FF) basis and were defined as [VfVp/V ] x 100, where is the volume of feed... 

Catalytic Oxidation of Butane. The dependences of the conversion of butane on W/F for (VO)2P207 and Si02/(V0)2P207 are given in Figure 5, where W is the catalyst weight and F is the flow rate of butane. The conversion increased with the increase in W/F. The catalytic activity and the selectivity of oxidation of butane are summarized in Table 2. 

In Figure 4 we have represented citral conversions (Xcit) as a function of tW/n°j( where W is the catalyst weight, t the reaction time, and n , the initial moles... 

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