Catalyst behavior, activation

No differences in operability and catalyst behavior (activity and deactivation) in the two plants were discernible. The expected catalyst lifetime in a commercial plant, calculated from the movement of the temperature profile down the catalyst bed with time, in both cases will be more than 16,000 hrs under the design conditions. 

The opposite behavior was observed after the treatment of the two catalysts in the steam-containing stream, at 380°C. The catalyst P/V 1.06 did not show any change of catalytic performance, whereas in the case of P/V 1.00 the treatment rendered the catalyst less active but more selective than the sample equilibrated in the reactive atmosphere at 380°C. This means that with P/V 1.00, the active layer is not fully hydrolyzed under reaction conditions, and that a hydrolyzed surface is more selective than the active surface of the equilibrated P/V 1.00 catalyst. On the contrary, the active surface of catalyst P/V 1.06 either was already hydrolyzed under... 

The catalytic behavior of Fe-MTW zeolites in the direct ammoxidation of propane was investigated. The obtained catalytic results are compared with behavior of Fe-silicalite catalysts whose activity in propane ammoxidation was recently published. It was found that Fe-MTW catalysts exhibit the similar activity as Fe-silicalites but the selectivity to acrylonitrile was substantially lower. On the other hand, Fe-MTW catalysts produce higher amount of propene and have better acrylonitrile-to-acetonitrile ratio. 

The characteristics of the hydrogenation of norbornadiene, substituted butadienes and conjugated and cyclic dienes are all very similar. In the case of conjugated dienes, there appears to be hardly any isomerization activity, while in the case of 1,4-dienes an isomerization step to form the corresponding 1,3-diene is assumed prior to hydrogenation. The catalyst behavior changes after the diene has been completely converted to the monoene, whereupon the rhodium catalyst resumes its normaF monoene hydrogenation behavior. 

Catalytic test. The catalytic behavior was evaluated for the gas phase isobutene trimerization reaction using a fixed bed reactor, with dimensions of 2 cm of diameter and 55 cm of length respectively. The operation conditions and evaluation procedure were as follows the catalyst was activated at 400°C in flowing air (1 ml/s) during 8 hours. After the activation treatment, temperature was lowered to 40°C and a mixture of isobutane/isobutene 72 28 w/w was feed. The GHSV value was varied to 8, 16, 32 and 64 h respectively. The average time of reaction was 11 h. The time of reactor stabilization after the beginning of the catalytic evaluation was 2 h. 

For SK-500 the rate at 573°K and 400 sec after the initiation of reactant flow is independent of reactant mole ratio for Ce C2 = 0.7 to 10. Under these conditions the 400-sec point is just beyond the maximum in the rate curve. Similar behavior was observed at one other condition. Initial rate of reaction estimated by extrapolating the decay portion of the rate curves for this data to zero time (see below) indicates a maximum in the rate at C6 C2 == 3.5 (Figure 2). Error bars represent estimated 95% confidence limits. The observed activity for HY is about twice that of SK-500, that for LaY is about two-thirds that of SK-500 (Figure 2). This is consistent with the trend expected (7) since all catalysts were activated to the same temperature. The temperature dependence of the observed rate is large for all systems studied indicating the absence of external mass transfer limitations. 

This review will only focus on the modeling efforts in pore diffusion and reaction in single-catalyst pellets which have incorporated pore plugging as a deactivation mechanism. A broad literature exists on the deactivation of catalysts by active site poisoning, and it has been reviewed by Froment and Bischoff (1979). The behavior of catalytic beds undergoing deactivation is... 

For the same catalyst, the activation energy is drastically decreased and the reaction rate enormously increased with increasing electron concentration and decreasing work function of the support. This behavior... 

Many studies, mainly by spectroscopic methods and calculation, have been devoted to the conformational behavior of the Inoue catalyst 1 (and 2) and its interactions with HCN and the substrate aldehydes [26, 34—36]. As noted originally by Inoue et al., however, the diketopiperazine 1 does not have catalytic activity and selectivity in homogeneous solution, i.e. in molecular dispersion. Instead, the diketopiperazine 1 is a heterogeneous catalyst - the active/selective state is a gel which forms, for example, in benzene or toluene, or just a suspension (e.g. in ether). As a consequence, catalyst performance is strongly influenced by the amorphous or crystalline character of the diketopiperazine from which the gel is formed. The best performance was achieved when amorphous materials were employed. The latter can... 

In addition to a proper membrane, CMRs also need a good catalyst. Due to the specific conditions under which catalysts are placed in CMRs, conventional active phases could behave differently from when under classical conditions. For example, in dehydrogenation reactions, due to the removal of H2, the hydrogen hydrocarbon ratio is smaller in CMRs when compared to other reactors, which will probably affect the stability of the catalyst. The low oxygen partial pressure used in CMRs for selective oxidation (Section A9.3.3.2) could also lead to some changes in catalyst behavior. These aspects could necessitate the specific design of catalysts for CMRs. 

The turnover numbers and activation energies for methanation over all four metals freshly reduced and under sulfur-free conditions (Table XVI) were in good agreement with values reported for supported metal catalysts (220). At 673 K, Ni and Ru exhibited only very slow losses in activity apparently due to slow carbon deposition, whereas Co and Fe underwent rapid, severe carbon deactivation after maintaining their fresh catalyst activity for a few hours. After rapid deactivation the final steady-state activity, which was about 100-fold lower than the activity of the fresh catalyst, was approached slowly this activity region was referred to as the lower pseudosteady state. Likewise, the fresh catalyst behavior was referred to as the upper pseudo-... 

As a further aspect, it must be considered that in the context of a type of structure, differences in shape and positions of LMCT bands can monitor the occurrence of peculiar local geometries or distortions. Such spectral features can usually be analyzed in more detail, with a consequent higher information outpuL in the case of catalysts with active centers that are quite homogeneous in structure and with simpler spectra, as more commonly occurs in the case of highly isolated species. These are the conditions for the observation of spectral behavior that can be rationalized in terms of differences in the bond angles connecting the metal... 

Autocatalysis is a distinctive phenomenon while in ordinary catalysis the catalyst re-appears from the reaction apparently untouched, additional amounts of catalyst are actively produced in an autocatalytic cycle. As atoms are not interconverted during chemical reactions, this requires (all) the (elementary or otherwise essential) components of autocatalysts to be extracted from some external reservoir. After all this matter was extracted, some share of it is not introduced in and released as a product but rather retained, thereafter supporting and speeding up the reaction(s) steadily as amounts and possibly also concentrations of autocatalysts increase. At first glance, such a system may appear doomed to undergo runaway dynamics ( explosion ), but, apart from the limited speeds and rates of autocatalyst resupply from the environment there are also other mechanisms which usually limit kinetics even though non-linear behavior (bistability, oscillations) may not be precluded ... 

A large portion of the information is of wide applicability to past and present catalytic investigation, and a thorough understanding of it should serve to guide the experimenter in the determination of catalytic activity constants, of activation energies, orders of reaction, and of other modes of description of catalyst behavior. 

Numerous parameters can affect the catalytic behavior of zeolite catalysts. The activity of a zeolite is related to its Br0nsted acidity due to presence of Al in the framework, while selectivity is guided mainly by the topology of the intracrystalline network [6-11]. These dependencies are strongly influenced by all reaction conditions. They affect the catalytic mechanism in a concerted way. Hence, the substantial parameters that are necessary to characterize the catalytic behavior of a zeolite cannot be considered separately. 

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