Catalyst behavior

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

UCS, rare earth, and sodium are just three of the parameters that are readily available to characterize the zeolite properties. They provide valuable information about catalyst behavior in the cat cracker. If required, additional tests can be conducted to examine other zeolite properties. 

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. 

Early workers viewed carriers or catalyst supports as inert substances that provided a means of spreading out an expensive material like platinum or else improved the mechanical strength of an inherently weak material. The primary factors in the early selection of catalyst supports were their physical properties and their cheapness hence pumice, ground brick, charcoal, coke, and similar substances were used. No attention was paid to the possible influence of the support on catalyst behavior differences in behavior were attributed to variations in the distribution of the catalyst itself. 

Jain, A.K., Hudgins, R.R. and Silveston, P.L., "Adsorption/ Desorption Models How Useful to Predict Catalyst Behavior under Transient Conditions", paper submitted to Seventh North American Meeting, The Catalysis Society, Boston, 1981. 

The stereoselective hydrogenation of alkynes to alkenes can be effected by a wide variety of homogeneous catalysts. The appropriate choice of catalyst and reaction conditions allows the selective formation of either the (Z)- or the (l )-a1-kene. Most of the catalysts display a very high chemoselectivity, as they are not reactive towards reducible functional groups such as carbonyl, ester, and double bonds. Many of the details related to catalyst behavior and intricate mechanistic details concerning semihydrogenation of alkynes have often not been unraveled, and will remain a topic of research for the coming years. 

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. 

To probe the mechanism the authors carried out pulsing studies, where pulses of CO and H20 were alternated under a flow of helium. Formation of C02 was never observed from CO pulses, but rather C02 and H2 were formed at the same time when H20 was added. Therefore, the authors concluded that the redox mechanism proposed by Bunluesin and coworkers386 could not account for the catalyst behavior, while an associative mechanism, likely via formates, was consistent with the experimental results. 

Diffuse reflectance is an excellent sampling tool for powdered or crystalline materials in the mid-IR and near-IR spectral ranges. Heated reaction chambers for diffuse reflectance allow the study of catalysis and oxidation reactions in situ, and can evaluate the effects of temperature and catalyst behavior. Scratching sample surfaces with abrasive paper and then measuring the spectra of the particles adhering to the paper allows for analysis of intractable solids. Perhaps one of the greatest additional benefits is that this system is amenable to automation. 

The iodide promoter effects seen in Fig. 20, and some of the catalyst behavior to be described below, can be partially understood in terms of the ruthenium chemistry involved. Iodide salts have been found (191) to react... 

Deep and exhaustive study of the catalyst behavior, especially analyzed by means of reactivity and product selectivity, allows us to finely match the catalytic performances to the synthetic needs. 

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. 

An accurate design of unsteady-state catalytic processes requires knowledge about the catalyst behavior and reaction kinetics under unsteady-state conditions, unsteady-state mass and heat transfer processes in the catalyst particle and along the catalyst bed, and dynamic phenomena in the catalytic reactor. New approaches for reactor modeling and optimization become necessary. Together, these topics form a wide area of research that has been continuously developed since the 1960s. 

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-... 

When a catalyst with practical potential is identified, further experimentation usually includes characterization of the reaction mechanism and kinetic measurements. More careful experimentation and higher accuracy are increasingly important. Subsequentiy, catalyst life tests may be required, preferably in a simulated industrial environment, to determine the long-term catalyst behavior. This may necessitate optimization of reaction conditions and further catalyst improvements. 

Because a catalyst affects the rate of reaction and not the ultimate equilibrium, it is not possible to give a general, kinetic description of catalyst behavior. Instead, a proper discussion of catalytic behavior can bo made only in terms of mechanism, which is, of course, unique for any given reaction. However, some general classification of catalysts is possible in terms of structure in relation to type of reaction mechanism involved. A useful classification of solids for this purpose is as follows ... 

Propylene conversion over three SAPO molecular sieves (SAPO-5, SAPO-11, and SAPO-34) was conducted at a variety of operating conditions. Catalyst behavior was correlated with the physical and chemical properties of the SAPO molecular sieves. The objective of this work was to determine the relative importance of kinetic and thermodynamic factors on the conversion of propylene and the distribution of products. The rate of olefin cracldng compared to the rate of olefin polymerization will be addressed to account for the observed trends in the product yields. The processes responsible for deactivation will also be addressed. 

Fig. 40. Nitrogen storage and reduction (NSR) from a lean-burn gasoline engine. Catalyst behavior is in a 30/90-s cycle. Space velocity, 15,000 inlet gas temperature, 35CfC (after225).

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. 

It is recognized that selectivity in a reaction such as that chosen could be strongly dependent on conversion level, on temperaiuie and perhaps, as noted above, on catalyst history, e.g., steaming. Basic catalyst behavior, in the absence of any hetcrocycle, is reviewed in die following table. Suffice it to say that in no case (steaming, low temperature, low conversion) did these ZSM-5 catalysts produce PET/ET (or OET/ET) ratios approaching those found on injection of helerocycle. 

The naphtha and distillate makes were significantly high. The catalyst behavior was explainable in view of the operating requirement i.e., primarily LSFO sulfur. Since the catalyst was solely HDS type, a high degree of desulfurization was obtained at lower temperature at SOR. On the other hand, the feed metals poisoned the HDS catalyst very fast, initially at the front end and then the subsequent beds. Once this happened, the WABT had to be raised steeply to attain target LSFO Sulfur. 

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