Catalyst performance case studies

This example illustrates one of the ways in which the catalyst chemist must study and use the work of others it illustrates the importance of constraints not involving the catalyst directly. Other cases can be found where the catalyst performance has a more direct bearing on the easing of constraints. The synthesis of methanol provides such an example ... 

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

The hydrogenation of enamides and enol acetates without acid function is often more demanding, and at present is not applied widely. Besides a bench-scale application by Roche with a Ru-biphep catalyst [55], two examples are of interest a pilot process for a cyclic enol acetate by Roche [55], and a feasibility study by Bristol-Myers Squibb [56], both using Rh-DuPhos catalysts (Fig. 37.11). In the latter case, despite very good ee-values, a chiral pool route was finally chosen. Chiral Quests Rh-f-KetalPhos (see Fig. 37.9) has been shown to hydrogenate a variety of substituted aryl enamide model substrates at r.t., 1 bar, with very promising catalyst performance (ee 98-99%, TON 10000) [47]. 

Further hydrogenations of a variety of C=C substrates depicted in Figure 37.22 range from a pilot process to several feasibility studies. Of special interest are PhanePhos, originally reported by Merck, an unsymmetrically substituted phos-pholane developed by Pfizer, and the rare case of an Ir-diphosphine complex active for the hydrogenation of a C=C bond. Nevertheless, the catalyst performances of most processes summarized in Figure 37.22 are probably not (yet) sufficient for manufacturing purposes. Indeed, several of the reports explicitly mention that further development was stopped, either because another route proved to be superior or because the compound was abandoned. 

The effects of non-uniform distribution of the catalytic material within the support in the performance of catalyst pellets started receiving attention in the late 60 s (cf 1-4). These, as well as later studies, both theoretical and experimental, demonstrated that non-uniformly distributed catalysts can offer superior conversion, selectivity, durability, and thermal sensitivity characteristics over those wherein the activity is uniform. Work in this area has been reviewed by Gavriilidis et al. (5). Recently, Wu et al. (6) showed that for any catalyst performance index (i.e. conversion, selectivity or yield) and for the most general case of an arbitrary number of reactions, following arbitrary kinetics, occurring in a non-isothermal pellet, with finite external mass and heat transfer resistances, the optimal catalyst distribution remains a Dirac-delta function. 

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

Among all catalysts studied, Ru, Rh, and Ni supported on metal oxides exhibit the highest activity, and Ru has been reported to be the most selective.59-61 The supports exert a marked influence on both the adsorption and specific activity of the metals. In most cases, Ti02 is the best support material to achieve good catalyst performance,62 63 which is attributed to a strong electronic interaction between the metal and the support. 

We shall prepare the various building blocks of the catalyst surface and study them separately. Then we put the parts together and the resultant structure should have all of the properties of the working catalyst particle. Just as in the case of synthetic insulin or the B12 molecule, the proof that the synthesis was successful is in the identical performance of the synthesized and natural products. Our building blocks are crystal surfaces with well-characterized atomic surface structure and composition. Cutting these crystals in various directions permits us to vary their surface structure systematically and to study the chemical reactivity associated with each surface structure. If we do it properly, all of the surface sites and microstructures with unique chemical activity can be identified this way. Then, by preparing a surface where all of these sites are simultaneously present in the correct configurations and concentrations the chemical behavior of the catalyst particle can be reproduced. The real value of this synthetic approach is that ultimately one should be able to synthesize a catalyst that is much more selective since we build into it only the desirable active sites in a controlled manner. 

For the present case study, a first attempt to predict quantitatively performances of WGS catalysts by ANN regression technique led to a rather poor correlation between predicted and experimental CO conversion values (Fig. 10.13). This suggests that, in addition to noisy data, the used descriptors, which were restricted to the single elemental composition of the catalysts, do not contain per se sufficient... 

Dumesic and co-workers have presented various case studies in which they have applied the above concept of microkinetic analysis in order to propose new catalysts or to explain the different catalytic performance of different catalyst formulations for given reactions ... 

In summary, the compilation of relevant case studies shows that XRD of working catalysts is a widely applicable technique. It gives rich and useful information about synthesis and activation of catalysts as well as deactivation by structural transformations. The pertinent question about the structure of the active sites is not accessible directly by this method despite such claims in the literature. It must be pointed out that this shortcoming of a technique involving characterization of samples in reactive atmospheres is common to all methods when one is concerned with high-performance catalysts in which the active sites are a small fraction of the active surface. Model systems do a better job in this respect, provided that they are active for the reaction of interest and not only in proxy reactions. 

This complexity determines that investigations on heterogeneous photo-catalytic processes sometimes report information only on dark adsorption and use this information for discussing the results obtained under irradiation. This extrapolation is not adequate as the characteristics of photocatalyst surface change under irradiation and, moreover, active photoadsorption centers are generated. Nowadays very effective methods allow a soimd characterization of bulk properties of catalysts, and powerful spectroscopies give valuable information on surface properties. Unfortunately information on the photoadsorption extent under real reaction conditions, that is, at the same operative conditions at which the photoreactivity tests are performed, are not available. For the cases in which photoreaction events only occur on the catalyst surface, a critical step to affect the effectiveness of the transformation of a given compound is to understand the adsorption process of that compound on the catalyst surface. The study of the adsorbability of the substrate allows one to predict the mechanism and kinetics that promote its photoreaction and also to correctly compare the performance of different photocatalytic systems. 

Hydrogenation studies were undertaken on the parent iron-tin treated coal (Drum 289) as well as the THF insolubles, preasphaltene, asphaltene and oil derived from a continuous reactor run as previously discussed. Studies with no additional catalyst added (case A) and with the addition of a sulphided nickel molybdate catalyst supported on alumina (case B) were performed. The results are presented in Table 1. The Ni/Mo catalyst in case B did not increase the conversion of the coal or the THF insolubles beyond that for case A because sufficient amounts of iron and tin materials were already... 

Large pilot plants remain indispensible only when in the development of an unconventional process sufficient amounts of novel product have to be made available for application studies, or when complex interactions between elements of the process have to be studied in an integrated way. In the latter case, a pilot plant will be a scaled-down version of an actual complex industrial plant, rather than just a reactor unit as required in catalyst performance testing or kinetic process studies. 

In this paper we have investigated the performance of zeolites Y, EMT, beta and ZSM-5 for the reaction of isoprene (1) with methylvinylketone (2) to 4-acetyl-1-methyl-1-cyclohexene (3) and 4-acetyl-2-methyl-1-cyclohexene (4) shown in fig. 1. Furthermore, we used a silica binder in order to avoid experimental problems which arise when zeolite powder is taken as catalyst. In some cases binders have a significant influence on the catalyst activity and selectivity sometimes the activity can be enhanced, e.g., by additional active sites on the binder surface. Again, a loss of selectivity can be observed, if the spatial restrictions inside the zeolite framework, which are often required for selectivity, are not present on the binder surface. The nature of the binder and its effect on catalyst performance were also a subject of this study. 

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