Catalysts site accessibility

Metal-assisted enantioselective catalytic reactions are one of the most important areas in organic chemistry [1-3]. They require the appropriate design and the preparation of chiral transition metal complexes, a field also of major importance in modern synthetic chemistry. These complexes are selected on both their ability to catalyze a given reaction and their potential as asymmetric inducers. To fulfill the first function, it is absolutely required that the catalysts display accessible metal coordination sites where reactants can bind since activation would result from a direct interaction between the metal ion... 

Cleaning and sweeping of the Ti02 surface due to acoustic microstreaming allows for an access to more active catalyst sites at any given time. 

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

From the H/M values for the catalysts NiSn-BM (Sn/Ni = 0.29) and PtSn-BM (Sn/Pt = 0.71), and the H/M values for the corresponding monometallic ones, it can be inferred that Sn blocks about 70% of the originally accessible M atoms. For these systems, based on the dispersion measured for Pt and Ni, the atomic ratios Sn/M correspond to values higher than 1. Notably, even in these cases, an important portion of the metallic surface has sites accessible to hydrogen dissociative adsorption, which is essential for the phase to be active in hydrogenation reactions. 

To date, many soluble transition metal-based homogeneous catalysts including POMs have been developed for H202- and 02-based green oxidations. They are usually dissolved in reaction solutions, making all catalytic sites accessible to... 

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

This leads to a poor conversion and higher coke and fuel gas yields. In order to improve the conversion of large molecules, new modifications in the FCC Catalyst Architecture and Active Site Accessibility, are required in terms of Pore Size and Pore Acidity distribution. 

Carlier32,33 used various functional alkylsilane groups on silica as co-monomer, transfer agent or initiator for grafting of a functional polymer. These functional polymers may be used to anchor a catalyst. The polymer polyphenylsilsesquioxane was grafted onto porous silica and sulfonated, to obtain catalysts of high stability with enhanced site accessibility and increased number of sites, as well as high acidity level.34 This catalyst is used for esterification and phenol alkylation. Other catalysts have been reviewed by Pinnavaia,35 and are summarized in table 8.5. 

The most general methodology followed to prepare alkaline earth metal oxides as basic catalysts consists of the thermal decomposition of the corresponding hydroxides or carbonates in air or under vacuum. BaO and SrO are prepared from the corresponding carbonates as precursor salts, whereas decomposition of hydroxides is frequently used to prepare MgO and CaO. Preparation of alkaline earth metal oxides with high surface areas is especially important when the oxide will be used as a basic catalyst, because the catalytic activity will depend on the number and strength of the basic sites accessible to the reactant molecules, which is dependent on the accessible surface area. 

The catalytic coking reaction may require dual catalyst sites, whereas small size reactants which are less subjective to coking may be able to access the sites covered by the coke. 

According to a recent study, the highest values of isomerization selectivity can be obtained with bifunctional Hmordenite catalysts with high values of the ratio between the concentrations of Pt and protonic sites accessible by reactant molecules, i.e. with catalysts on which the acid step is the limiting step of ethylbenzene isomerization. Furthermore, because of the reduction of the disproportionation activity, Na exchange of HMOR has a positive effect on the selectivity. 

In both competitive and noncompetitive inhibition, the reaction is of order between zero and minus one with respect to the inhibitor. However, there is a kinetic difference between competitive and noncompetitive inhibition. In the former, the action of the inhibitor can be effectively countered by an increase in reactant concentration direct competition by the reactant for a catalyst site can "crowd out" the inhibitor. In noncompetitive inhibition, this is not the case even a large excess of reactant does not impair the inhibitor s access to the cycle member Xj. [Mathematically, in competitive inhibition the new and retarding denominator terms have as factor, the sum of the first matrix row and only row that lacks the coefficient Xq, the only coefficient with CA as co-factor. In contrast, in noncompetitive inhibition the terms have DSI as factor and contain Xqj and thus CA as co-factor the result is that an increase in CA, apart from a direct beneficial effect on the rate, also strengthens the adverse effect of the noncompetitive inhibitor.]... 

The data in Fig. 28 clearly show that intermediate values of x, which limit olefin removal and enhance secondary readsorption reactions but still permit unrestricted and rapid access of CO and H2 to reaction sites, lead to maximum C5+ selectivity. They also show that eggshell catalysts allow access to these intermediate values of x for any pellet size. The design of eggshell pellets with values of x between 0.2 and 2.0 x 10 m leads to high C5+ selectivity (Fig. 28a) and maintains catalytic rates and activation energies near their intrinsic kinetic values (Table VII). 

The term aging generally describes a loss in the activity—or selectivity—observed in a catalytic process after a certain period of reaction time. Aging may result from some change in the nature or number of catalyst sites, or in the accessibility of the sites to reactant molecules. Thus, such factors as formation of hydrogen-deficient organic residues ( coke ), selective adsorption of impurities from the charge ad-... 

Geometric constraints and related factors including active site accessibility, steiic effects of transition states and diffusion limitation of reactants and products play a crucial role in liquid phase catalyzed reactions [240]. Several examples are presented hereafter for illustration (i) TS-1 is more active in the oxidation of linear vs. branched alcohols [241], and in the epoxidation of linear vs. cyclic olefins [153] (ii) in the presence of TBHP, TS-1 has no activity [242], while Ti-6 is less active than Ti-MCM-41 [243] (iii) large TS-1 particles are less effective catalysts than smaller ones [244]. 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

We can support the foregoing by evaluating two catalysts differing in active site accessibility. The delta in bottoms conversion increases with higher feed CCR, higher metal levels, and lower CTO ratios. The catalyst with the lowest number of accessible sites is most sensitive to coke and metal poisoning (Figure 10). From equation (1) there are two possible solutions to the problem ... 

In summary, the quantity of soft coke seems to increase with the surface area in the small-pore range (zeolite and matrix), while the stripping rate is determined inversely by the accessibility of the catalyst sites and increases with larger and nonconstrained pore systems. We can conclude that for delta coke limited RFCC catalyst selection it will be essential to assess the diferences in all the factors contributing to commercial delta coke. 

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