Catalyst performance reactivity

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

The systems described in this chapter possess properties that define supramolecular reactivity and catalysis substrate recognition, reaction within the supermolecule, rate acceleration, inhibition by competitively bound species, structural and chiral selectivity, and catalytic turnover. Many other types of processes may be imagined. In particular, the transacylation reactions mentioned above operate on activated esters as substrates, but the hydrolysis of unactivated esters and especially of amides under biological conditions, presents a challenge [5.77] that chemistry has met in enzymes but not yet in abiotic supramolecular catalysts. However, metal complexes have been found to activate markedly amide hydrolysis [5.48, 5.58a]. Of great interest is the development of supramolecular catalysts performing synthetic... 

High enantiomeric excess in organocatalytic desymmetrization of meso-diols using chiral phosphines as nucleophilic catalysts was achieved for the first time by Vedejs et al. (Scheme 13.21) [36a], In this approach selectivity factors up to 5.5 were achieved when the C2-symmetric phospholane 42a was employed (application of chiral phosphines in the kinetic resolution of racemic secondary alcohols is discussed in Section 12.1). A later study compared the performance of the phos-pholanes 42b-d with that of the phosphabicyclooctanes 43a-c in the desymmetrization of meso-hydrobenzoin (Scheme 13.21) [36b], Improved enantioselectivity was observed for phospholanes 42b-d (86% for 42c) but reactions were usually slow. Currently the bicyclic compound 43a seems to be the best compromise between catalyst accessibility, reactivity, and enantioselectivity - the monobenzoate of hydrobenzoin has been obtained with a yield of 97% and up to 94% ee [36b]. 

A wide variety of techniques are usually employed in investigations of heterogeneous catalysis. These investigations typically involve one or more of the following experimental approaches (i) synthesis and testing of catalytic materials, (ii) characterization of bulk and surface properties, (iii) evaluation of surface adsorptive properties and chemical reactivity, and (iv) assessment of catalyst performance. The recent advances in... 

We begin with a summary of the importance of XAFS spectroscopy in catalyst characterization science, using examples from the literature to illustrate each point. This introduction to the field includes all types of XAFS spectroscopy of catalysts in reactive atmospheres and is not restricted to investigations in which activity data were measured simultaneously with catalyst performance. This section is meant to familiarize the reader with the types of relevant information that can be provided by XAFS data. 

Almost all group 4 metal complexes require a cocatalyst to generate an active metal-alkyl cationic species. Ordinary alkylaluminums - used in conventional Ziegler Natta catalysts - are insufiicient to activate these compounds on their own. The principal activator nsed is methylalumoxane (MAO), a structurally enigmatic material with a mixture of nuclearities. Its purpose is to alkylate the metal dichloride and to abstract one of the reactive hgands to form the ion pair active catalyst. The interaction is dynamic and a large excess of MAO is needed for effective catalyst performance, thus inhibiting a comprehensive characterization of these catalysts. 

Detailed data on the manufacturing steps most important to the catalyst performance, reduction, prereduction, passivating, and reactivation, appear in [33], including a discussion of the most important literature in this field. 

A study has been undertaken to compare the effectiveness of molybdenum and uranium oxide and iron sodalite catalysts with the homogeneous gas-phase oxida-hon of methane [54]. Catalyst performance was evaluated in a high-pressure annular reactor and data were compared to the reactivity of the empty reactor. It was concluded that none of the catalysts gave any advantage over the homogeneous reaction. Indeed, using a catalyst only reduced the selectivity to the desired partial oxidation products. Similar conclusions have been reached for many catalysts used for the partial oxidation of methane, and therefore it is perhaps not surprising that uranium oxide catalysts are no different... 

During recent years an increasing evidence has accumulated that discernible types of centers exist in Z—N catalysts, particularly in their heterogeneous versions. The centers may differ in their kp values, monomer coordination abilities, stereospecificities and reactivities in copolymerization. This concept can explain — at least qualitatively — wide MWD of polyolefins, composition heterogeneity of copolymers and specific responses of the catalyst performance to electron-donor additives. The origin of the differently behaving centers should be seen in a diversity of chemical processes... 

The pore diameters of MFI-type zeolites are comparable to the size of many commercially important molecules, such as aromatics or linear or branched hydrocarbons [1]. Thus, the study of the difiusion of reactive molecules in the channel system of zeolite catalysts is of considerable interest for the understanding of the catalyst performance. A variety of methods has been developed and applied to the measurement of diffii-sion coefficients, amongst others gravimetric techniques [2], neutron scattering [3], NMR [4] and Frequency Response [5]. The FTIR technique offers the possibility to study sorption and sorption kinetics under conditions close to those of catalytic experiments. By the use of an IR microscope, single crystals have become accessible to the FTIR technique. 

Graf et al.108 performed a comparative study of steam reforming of methane, ethane and ethylene on Pt, Rh, and Pd supported on YSZ. They observed that the reactivity and product distribution depends on the type of noble metal loaded. Over Rh/YSZ catalyst, the reactivity decreased in the order C2H6 > CdE > CH4. On the other hand, over Pt/YSZ, methane reacted much faster than the C2 hydrocarbons and the order of reactivity is CH4 > C2H4 > C2H6 (Fig. 2.8). The higher reactivity of Rh... 

Catalyst beds were completely saturated with sulfur during poisoning tests. In each experiment the catalyst bed was exposed to the desired HjS concentration for 4 - 6 hours to st ilize the catalyst performance. After the experiments, the catalyst beds were cooled to room temperature under dry nitrogen flow before opening the reactor vessel. In pressurized tests the pressure let down was not started until 20-30 min after nitrogen flushing to be sure that all remaining reactive gas components had been removed. 

The catalytic combustion of Diesel soot was studied performing reactivity tests of soot-catalyst mixtures in a tubular flow reactor. The dependence of the reaction rate on the temperature was found. With respect to the uncatalysed combustion the reactivity of the soot in the presence of catalyst increased of some orders of magnitude while the apparent activation energy was found to be less than half. 

In this paper results of soot reactivity tests and of catalysed and uncatalysed ceramic filter regeneration experiments are presented. The aim is to investigate the influence of the catalyst features and of the carbon-catalyst contact on the catalyst performances. This is accomplished by comparing the results of combustion tests of mixtures of carbon and catalyst particles, specifically prepared by thorough pounding of the two components in a mortar, with those attainable in a more realistic filter regeneration system. 

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