Catalytic activity product selectivity

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

Catalyst testing and evaluation have been revolutionized by computers, automated test reactors, and analytical methods. With modem equipment, researchers can systematically prepare and screen many catalysts in a short time and efftciendy deterrnine, not only the initial catalytic activity and selectivity, but also the stabiUty and the appearance of trace products that may indicate some new catalytic properties worthy of further development. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The present work deals with the study of the liquid phase phenol alkylation by (-butanol over the three types of catalysts derived from MWW-precursor MCM-22, MCM-36 and ITQ-2. It was assumed that by pillaring and/or delamination the contribution of acid sites located on the hemicages will increase and it could be evidenced during the alkylation of phenol by (-butanol, process involving large reaction intermediates and products which are difficult to be accommodated within sinusoidal channels. The reaction pathway involves many parallel and/or successive steps, the main reactions being O-alkylation and C-alkylation. The catalytic activity and selectivity of these materials are discussed. A general scheme of the process is proposed on the basis of the structural and acidic features of the catalysts. 

The unconsumed reactants and products were both easily removed from the reaction mixture by extraction with K-hexane, which is not miscible with [BMIMJPFg. The brown-red ionic liquid phase containing the catalyst was reused five times with PhI(OAc)2 as the oxidant. The recovered catalyst gave catalytic activity comparable to that of the original. In epoxidation of styrene and of cyclohexene, both catalytic activity and selectivity fell slightly after five reuses. In the conversion of hept-l-ene, the reused catalyst showed the same activity and selectivity as the fresh catalyst, and the catalyst was shown to be unchanged after the reaction. 

With this preparation method, a high ligand/Rh ratio (10 or 20) was shown to be essential for catalyst activity and for high nji ratios in the product aldehyde. In the presence of ionic liquid, the nji ratio was as much as 23.7, compared to a value of 16.9 observed in the absence of ionic liquids. Surprisingly, an L/Rh ratio of 2.5 resulted in an inactive catalyst. The catalytic activity and selectivity decreased steadily during the 5-h test, independent of the type of ionic liquid and the L/Rh ratio. 

Our initial studies in this area were based on the reasoning that, since the reduction of carbon monoxide to C2 products is a complex, multi-step process, the use of appropriate combinations of metals could generate synergistic effects which might prove more effective (in terms of both catalytic activity and selectivity) than simply the sum of the individual metal components. In... 

Replacement of tppts by a ligand containing less -S03Na groups such as tppms gave rise to a dramatic drop in the catalytic activity and selectivity to ibuprofen. A palladium catalyst generated from the sulfonated diphosphine 29 (Table 2 x=3 m=n=0 86% and m=0,n=l 14%) exhibited low catalytic activity and the major product was 3-IPPA (78%).451... 

In describing catalytic activities and selectivities and the inhibition phenomenon, we will use a common format, where possible, which is based on a common reaction pathway scheme as outlined in Scheme 1. In contrast to the simple one- and two-ring sulfur species from which direct sulfur extrusion is rather facile, in the HDS of multiring aromatic sulfur compounds such as dibenzothiophene derivatives, the observed products are often produced via more than one reaction pathway. We will not discuss the pathways that are specific for thiophene and benzothiophene as this is well represented in the literature (7, 5, 8, 9) and, in any event, they are not pertinent to the reaction pathways involved in deep HDS processes whereby all of the highly reactive sulfur compounds have already been completely converted. 

The catalyst system originated from the Knapsak catalyst (76) for the am-moxidation and catalysts found in Nippon Kayaku (78-80) for simple oxidation. A number of catalyst systems have been indicated in the patents in the past 25 years, and some of them are used practically in the industrial production. Strictly speaking, almost all catalyst systems may be designed and prepared on the same principle irrespective of their different compositions. The catalyst system is generally expressed as shown in Fig. 5. The first four elements are essential and consist of a fundamental structure of the catalyst system, and the other elements are added for the enhancement of the catalyst life and mechanical strength and minor improvement of the catalytic activity and selectivity. 

The study of pyridine-piperidine reactions under high pressure conditions has given much information concerning the kinetics of HDN, but these results are however complicated by alkyl transfer (disproportionation) reactions, and thus the possibility of using such reactions as an easy test for determination of mechanism and as a catalyst probe is partly excluded. The study of polycyclic amines (quinoline, etc.) for the same purpose is limited by the complexity and the number of different possible routes, but is a very interesting test reaction for an overall study of catalytic activity or selectivity toward HDN in industrial conditions. Because no disproportionation occurs and the numbers of products and routes are reasonable, the studies of pyridine-piperidine and alkylpyridine-alkylpiperidine HDN under normal H2 pressure and low amine pressure (< lOTorr) are very powerful test reactions both for mechanism determination and catalyst study, although these conditions are far removed from those of industrial practice. 

The deactivation of catalysts, especially zeolites, during cracking, hydrocracking, methanol conversion, etc, is one of the major technological and economic problems of the chemical industry (1). The interest of these materials lies not only in their high catalytic activity and selectivity but also in the possibility of regenerating them several times so that their Lifetime" is compatible with the cost of their production. Consequently, it is necessary to understand the manner and the rate of catalyst deactivation as well as the nature of the carbonaceous residues formed, commonly called coke". 

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