High pressure, increased turnover number

The use of high pressure photocatalysis resulted in greater turnover number for photocatalytic reduction of CO2 to CO with Re(bpy)(CO)3Cl and [Re(bpy)(CO)3(POiPr)3]+, in addition to greater catalyst stability [97,98]. For example, for sacrificial quenching in DMF solution the former exhibited 5.1 times the turnover number at normal pressure (0.1 Pa) using a pressure of 2.45 Pa of CO2. The latter showed a 3.8 times increase at 3.8 Pa. 

In fact, with the expected trend toward upgrading coal and the depletion of United States natural gas reserves, methanation should further increase in importance. The methanation reaction is thermodynamically favorable even at high temperatures and pressures and there are a variety of metals which catalyze this reaction. Vannice (231) compared the turnover numbers for a variety of Al203-supported group VIII metals and showed that the rate of... 

The influence of high pressure on the Heck reactions of selected alkenyl and aryl halides, respectively, i.e., 1-iodocyclohex-l-ene, iodobenzene, bromobenzene, with methyl acrylate has been investigated and the activation parameters of these reactions determined [142], Two different catalyst cocktails were used in this study, the classical system (Pd(OAc)2, NEtg, PPhg) and the one reported by Herrmann, Beller and others [16] (la). The temperature-dependent and the pressure-dependent rate coefficients both follow the order PhI/Pd(OAc)2 > 1-iodocyclohexene/Pd(OAc)2 > Phl/la > PhBr/la and the activation enthalpies as well as the activation entropies exhibit the trend 1-iodocyclohexene/Pd(OA)2 < Phl/Pd(OAc)2 < Phl/la < PhBr/la. The absolute values of the activation volumes, which were ascertained from the pressure-dependent rate coefficients, increase as follows l-iodocyclohexene/Pd(OAc)2 < PhI/Pd(OAc)2 Phl/la < PhBr/la. Under high pressure, the lifetime of the active palladium catalyst and thereby the turnover numbers are greatly enhanced [88]. 

The increase of the catalyst turnover numbers is indeed one other major area where further improvements could be expected. Such improvements have recently been achieved for the standard Heck reaction by the use of high pressure conditions [86], the use of preformed palladacycles as catalysts [87], or by using a macrocyclic tetraphole as hgand [88].Dendritic diphosphine-palladium complexes as catalysts for Heck reactions have also been reported to possess superior stabihty compared to the monomeric parent compounds [89]. Transferring such iimovations to the AHR remains an important goal. 

Much of the work done in recent years on polymer mechanochemistry has made use of the high elongational strain rates observed around collapsing cavitation bubbles in sonicated solutions, as outlined in the section on mechanoluminescence [27]. In addition to the distinctive features of sonochemically-induced mechanical reactivity described above, further attention needs to be paid to the sonication conditions in the case of mechanochemical catalysis, because catalyst lifetime and turnover number are reduced by sonochemical byproducts. Implosion of cavitation bubbles is essentially an adiabatic process which leads to formation of local hotspots within the bubble in which temperature and pressure increases drastically. The content of cavitation bubbles pyrolyses under these extreme conditions and results in formation of reactive species, such as radicals and persistent secondary byproducts acidic byproducts may also form from the degradation of the substrates [75]. Chemical impurities deactivate the reactive catalyst partially if not completely. Recent studies in our group have shown that heat capacity of gas... 

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