Heterogeneous versus homogeneous

The use of heterogeneous supports in organic synthesis has been extensively covered, and its relevant implications on the reaction outcome were discussed in Chapter 1. A major advantage of heterogeneous versus homogeneous reactions for the generation of pool libraries is related to the handling and the purification of intermediate or final pools. 

A clear problem of heterogeneous versus homogeneous systems is the fact that there are often many different active sites on a metal particle. In regions of the particle, which have low densities of chiral modifiers, the racemic reaction over the bare surface can compete strongly with the enantioselective reaction, leading to poor ee values. In the Ni/tartrate-type system, the loss of modifier over the course... 

Liu, Y., E. Lotero J. Goodwinjr (2006) A comparison of the esterification of acetic acid with methanol using heterogeneous versus homogeneous acid catalysis. Journal of Catalysis, 242, 278-286,ISSN 00219517. 

Glasnov, T.N., Findening, S and Kappe, C.O. (2009) Heterogeneous versus homogeneous palladium catalysts for ligandless Mizoroki-Heck reactions a comparison of batch/microwave and continuous-flow processing. Chem. Eur. J., 15 (4), 1001-1010. 

Kitanosono T, Xu P, Kobayashi S. Heterogeneous versus homogeneous copper(II) catalysis in enantioselective conjugate-addition reactions ofboron in water. Chem Asian J. 2014 9 179-188. 

In the case of heterogeneous catalysis, a DCKM or microkinetic model must incorporate the added dimension of adsorbed chemical species as well as active versus non-active sites. To obtain the full predictive capability from reactant influent to product effluent, all possible reactions in the system, both heterogeneous and homogeneous, must be accounted for. To properly understand the catalytic reaction sequence, it is possible that seemingly unimportant intermediates on the surface may be what initiate gas phase reactions. To begin this elucidation, the surface chemical species and their properties must be known. 

FIGURE 10.16 Zeldovich factor for heterogeneous and homogeneous nucleation versus contact angle at different membrane porosity. (Adapted from Journal of Crystal Growth, 310, Curcio, E., Di Profio, G., and Drioli, E., Prohahilistic aspects of polymorph selection by heterogeneous nucleation on microporous hydrophobic membrane surfaces, 5364-5369, Copyright (2008), with permission from Elsevier.)... 

P. Cordoba-Torres, R. P. Nogueira, L. de Miranda, L. Brenig, J. Wallenbom, and V. Fairen, Cellular automaton simulation of a simple corrosion mechanism Mesoscopic heterogeneity versus macroscopic homogeneity, Electrochim. Acta 46 2975-2989 (2001). 

Composite materials have many distinctive characteristics reiative to isotropic materials that render application of linear elastic fracture mechanics difficult. The anisotropy and heterogeneity, both from the standpoint of the fibers versus the matrix, and from the standpoint of multiple laminae of different orientations, are the principal problems. The extension to homogeneous anisotropic materials should be straightfor-wrard because none of the basic principles used in fracture mechanics is then changed. Thus, the approximation of composite materials by homogeneous anisotropic materials is often made. Then, stress-intensity factors for anisotropic materials are calculated by use of complex variable mapping techniques. 

B. C. Childress, Properties of homogeneous and heterogeneous polyolefins Metallocene catalyzed versus Ziegler-Natta catalyzed resins, MetCon 94 Proceedings, USA, May 1994. 

The water-gas shift rates are obviously much lower when heterogenized in comparison with the Rh complexes in homogeneous solutions of the amines (also see Tables 30-33). Kinetics for nitrobenzene reduction were performed for the cis-[Rh(CO)2(2-picoline)2]PF6 catalyst, and reported in 2000. Kinetics displayed a first order dependence on Pco over the range 0-1.9 atm in the temperature range 80-120 °C. As with the kinetics previously reported by Lima Neto and coworkers,121 it was suggested that the CO addition preceded the rate limiting step. A non-linear dependence on the rate versus Rh concentration, as with the previous study, suggested participation by both mononuclear and polynuclear species. 

TABLE 5.12. Sample Data Sets Homogeneous versus Heterogeneous Data... 

The log G versus log G" plots of the various PHB/PEN/PET blends are presented in Figure 20.16, where and the slopes of the plots are measures of the homogeneity of the system [35], If the slope is zero, then the blend system is heterogeneous, and if it approaches 2, it means that the blend system is tending to a homogeneous system. The slopes of the plots in Figure 20.16 increase with the addition of excess PHB to the blend system. 

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