Hydride transfer activation

The introduction of triethylphosphine as axial ligand increases the hydride transfer activity of [TPPRhinH] and changes the H2 evolution mechanism, since it is then able to catalyze this reaction.20... 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

Fast hydride transfer reduces the lifetime of the isooctyl cations. The molecules have less time to isomerize and, consequently, the observed product spectmm should be closer to the primary products and further from equilibrium. This has indeed been observed when adamantane, an efficient hydride donor, was mixed with zeolite H-BEA as the catalyst (78). When 2-butene/isobutane was used as the feed, the increased hydride transfer activity led to considerably higher 2,2,3-TMP and lower 2,2,4-TMP selectivities, as shown in Fig. 5. 

In another article by Corma et al. (178), ITQ-7, a three-dimensional large-pore zeolite, was tested as an alkylation catalyst and compared with a BEA sample of comparable Si/Al ratio and crystal size. The ratio of the selectivities to 2,2,4-TMP and 2,2,3-TMP, which have the largest kinetic diameter of the TMPs, and 2,3,3-TMP and 2,3,4-TMP, which have the lowest kinetic diameter, was used as a measure of the influence of the pore structure. Lower (2,2,4-TMP + 2,2,3-TMP)/ (2,3,3-TMP + 2,3,4-TMP) ratios in ITQ-7 were attributed to its smaller pore diameter. The bulky isomers have more spacious transition states, so that their formation in narrow pores is hindered moreover, their diffusion is slower. The hydride transfer activity, estimated by the dimethylhexane/dimethylhexene ratio,... 

BEA (Si/Al2 = 30), FAU (Si/Al2 = 8.6) and EMT (Si/Afi = 8.6) framework types were compared for i-butane/2-butylene alkylation. During the lifetime of the catalyst the butylene turnover number (TON) was approximately the same for each of the three zeolites and the acid sites were equivalent from the standpoint of stability in each case. With EMT the lowered selectivity to consecutive reaction products 2,2,4-TMP -I- 2,3,4-TMP relative to 2,2,3-TMP -i- 2,3,3-TMP and the lowered selectivity to heavies relative to BEA was interpreted as higher hydride transfer activity. 

Spectroscopic Probes of Hydride Transfer Activation by Enzymes... 

Dynamics of Protein Catalysis and Hydride Transfer Activation... 

Corma et al. [204] studied the conversion of -heptane over a series of dealuminated Y zeolites. They did not correlate the hydride transfer activity to the presence of adjacent sites, as often done in the literature, but to the more hydrophobic nature of a more severely dealuminated zeolite. The tendency to adsorb a more polar molecifle (i.e., an olefin) decreases compared to the less polar molecule (i.e., a paraffin) and, therefore, the conversion of adsorbed olefins to paraffins will decrease. By a change in adsorption properties hydride transfer, which is a bimolecular mechanism, will be much more influenced than cracking, which proceeds via a monomoleciflar mechanism. This explains the stronger decrease of hydride transfer compared to cracking. 

The last two contributions show the controversy about the mechanism and involvement of acid sites in the hydride transfer reactions over zeohtes. If the hydride transfer activity depends on the hydrophobicity, the rate should correlate to the acid strength of the sites, but not to the acid site density, i.e., the concentration of adjacent active sites. If the reaction is expected to proceed via two carbenium ions adsorbed on adjacent sites or one adsorbed carbenium ion on one site and a feed molecule influenced by the second adjacent site, the acid site density would be probed, but the information about the acid strength would be less obvious. However, in both cases it does not seem that these test reactions can be applied to compare large, medium, and small pore zeolite structures, due to the large (bimolecular) transition state proposed for hydride transfer reactions (unless very small molecules are used [203]). 

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