Increase of the catalyst activity

A solid-state promotion of CuH-ZSM-5 zeolite by cobalt ions results in the over-additive increase of the catalyst activity in C2Hg total oxidation, and leads to the rise of the thermostability of the bi-cationic catalyst. The insertion of cobalt cations into zeolitic channels with stabilization of Cu " active sites is assumed. 

The effects of catalyst amount and reaction time were investigated as shown in Fig 2. While other conditions were kept constant, 2.5 wt% HPA (1 g in 40 g reaction mixture) showed fairly good activity. Further increase of the catalyst amount does not have serious effect on the activity. One hour was enough for the reaction to complete as illustrated in Fig. 2 (b). 

As shown in Table 3, after a pretreatment performed at 333 K, the activity of the K3P sample increased with time on stream (TOS), giving rise to a high production of dimethylhexanes (DMH) and of olefins (Cg" ). After a dehydratation performed at 423 K, the conversion of C4= and the selectivities towards TMP were initially high. As generally observed in the aliphatic alkylation reaction with solid acids, the decrease of the catalyst activity was accompanied by a concomitant decrease of the selectivity in TMP and an increase of the selectivities in DMH and olefins (C4 dimerization) indicating... 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

The narrow molecular weight distributions accomplished by the supported catalysts were attributed to the absence of any organoaluminium co-catalyst dissocia-tion/reassociation processes at the heterogenized active neodymium centers. Furthermore, the order of the grafting sequence seemed to have minor implications for the catalyst performance. Control experiments have been conducted to explain the lower activity [0.9 (47) and 1.1 kg-PBD molNd h (48)] of the supported neodymium catalyst. Accordingly, an increase of the catalyst concentration (48) and use of a nonporous silica support (49) suggested that monomer diffusion and accessibility of the Nd centers are limited by the relatively small mesopores [dp = 2.4 (47) and 2.5 nm (48), after grafting]. 

A complex nanostructured catalyst for ammonia synthesis consists of ruthenium nanoclusters dispersed on a boron nitride support (Ru/BN) with barium added as a promoter (33). It was observed that the introduction of barium promoters results in an increase of the catalytic activity by 2—3 orders of magnitude. The multi-phase catalyst was first investigated by means of conventional HRTEM, but this technique did not succeed in identifying a barium-rich phase (34). It was even difficult to determine how the catalyst could be active, because the ruthenium clusters were encapsulated by layers of the boron nitride support. By HRTEM imaging of the catalyst during exposure to ammonia synthesis conditions, it was found that the... 

The mechanism of the catalyst activation by sulfur is not understood. The amount of sulfur compounds necessary to maintain or increase the catalyst activity depends in some cases on the stability of the heavy-metal sulfide component of the catalyst. Thus molybdenum sulfide seems to require a higher hydrogen sulfide concentration than tungsten sulfide. However, some catalysts that do not contain elements that can form sulfides under reaction conditions also showed an increased activity when sulfur compounds were added to the feed. Hydrogen sulfide in many cases decreases the catalyst sensitivity to nitrogen compounds and thus causes an activity increase. Sufficient data for pure compounds are not available to permit segregation of these effects. 

The use of solid catalysts and especially zeolites in Fine Chemical synthesis introduces another complication with respect to homogeneous reactions. There is always a progressive decrease of the catalyst activity with increasing reaction time.1191 In some reactions, this deactivation can be due to irreversible chemical transformation of the zeolite catalyst, e.g. reactions with acid reactants causing dealumination and sometimes collapse of the framework. However, in most cases, deactivation results from poisoning of the active sites by the desired reaction... 

The active portion of the Nd precursor was determined in various studies. There is common agreement that the fraction of active Nd is very low. Some recent studies are available, however, which provide evidence that Nd-efficiency can be considerably increased. The use of supported catalysts and the modification of the readily available NdV catalyst allow for significant increases of overall catalyst activities. These increases can be explained by increases of the active portion of the Nd precursor used. 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

The addition of water to the reaction feed was investigated [136]. This led to two significant effects being noted. The selectivity to maleic anhydride increased (with increased yields of acetic and acrylic acids). There was also an increase in the surface area of the catalysts activated with the n-butane/water/air feed compared to the dry activated catalyst Arnold and Sundaresan excluded the possibility of the water vapor acting as a diluent by performing experiments with an n-butane/N2/air feed. Instead they proposed that water is adsorbed onto the surface, blocking sites that are responsible for over-oxidation of the products. 

Surface Area (SA, mf/g). The snrface area is the measure of the catalyst activity (as long as the same catalyst types are compared) and has a strong effect on the performance of an Flnidized Catalytic Cracking Unit (FCCU). High surface area also results in increased adsorption of hydrocarbons, and a higher steam rate in the stripper may be reqnired. The zeolite and matrix surface areas of a catalyst can be analysed separately. Matrix pores provide access of the hydrocarbons to the active zeolite sites and matrix surface area often correlates with the bottoms conversion activity of the catalyst or the Light Cycle Oil (LCO) yield at constant conversion. 

Quite stable catalytic reaction solutions were obtained in THF with the starting pressure for ethylene of 6-6.5 MPa at a reaction temperature of 120 °C. Under these conditions and with the ratios piperidine/rhodium of 100 1 and 1000 1 in 36 and 72 h, yields of 70 and 50 % ethylpiperidine were reached, which correspond to TONs of 2 and 7 mol amine/(mol Rh) per h, respectively. Total conversion is also possible if the reaction time is prolonged further. As a side reaction, ethylene dimerization to butene was observed. This indicates the formation of a hydrido rhodium(III) complex in the hydroamination reaction, as formulated in Scheme 3, route (b). Hydrido rhodium(III) complexes are known as catalysts for ethylene dimerization [19], and if the reductive elimination of ethylpiperidine from the hydrido-y9-aminoethyl rhodium(III) complex is the rate-limiting step in the catalytic cycle of hydroamination, a competitive catalysis of the ethylene dimerization seems possible. In the context of these mechanistic considerations, an increase of the catalytic activity for hydroamination requires as much facilitation of the reductive elimination step as possible. 

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