Dehydrogenation and rehydrogenation reactions

Overall dehydrogenation reactions of alkali borohydrides are described as follows  

Reaction Hydrogen (mass%) Condition temp. T iK) AHd (kJ/mol H2) References 

After the dehydrogenation reaction of LiBIij, the end-products, lithium hydride (LiH) and boron, were rehydrogenated at 873 K under 35 MPa for 12 hours or at 1000 K under 15 MPa for over 10 hours to form LiBH4. The rehydrogenation reaction was confirmed by Raman spectroscopy and PXD measurement. The modified lithium borohydrides, LiBH475% -i- Ti0225% 

The dehydrogenation properties of alkaline-earth borohydrides M(BH4 2 are also shown in Table 15.7. Mg(BH4)2 was dehydrogenated according to the following reaction  

The theoretical gravimetric density of the effective hydrogen is 9.6 mass%. The thermogravimetric analysis for prepared Ca(BH4)2 shows that the total mass loss up to 800 K is 9.2 mass%, which supports the overall reaction of Eq. 15.13. On the other hand, the in situ X-ray diffraction analysis indicated that the dehydrogenation reaction of Ca(BH4)2 is as follows  

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Another important research for dehydrogenation and rehydrogenation is for multicomponent (or mixed) system. For example, the dehydrogenation and rehydrogenation reactions of complex hydride-metal hydride e.g. M(BH4) -M Hm have been recently investigated ... 

Returning again to the acid-base adduct thought model, the intermediate of the dehydrogenation and rehydrogenation LisAlHe can be loosely considered a coordination compound between three basic LiH molecules and an acidic AIH3. It is found that for the reaction ... 

Pillared clays with pillars based on A1 and also different elements (e.g., Cr, Fe) or with mixed pillars have been extensively investigated in numerous catalytic reactions of hydrocarbons, principally in proton-catalyzed reactions. Catalytic aspects have been reviewed in several articles (3,91,92). Comparison of the catalytic performances of acid solids with potential catalytic applications preferably with a reference catalyst is a straightforward way to obtain a preliminary information on their efficiency. With this approach, the activity of Al-pillared materials prepared with different smectites and pillaring methods have been compared, using the hydroisomerization of linear paraffins over Pt-impregnated pillared samples. In these bifiinctional catalysts, the metal function is necessary to dehydrogenate the paraffin and rehydrogenate the branched olefins, and the protons aid the formation of carbenium ions, which isomerize via protonated cyclopropane... 

In the ISHE it was possible to connect PCX diagrams which characterize the bulk properties of a system, with diffraction studies that help to determine which phases are involved in reactions when more than two components are present in the mixture. The measurement of PCX diagrams tends to go first, as they demonstrate whether the destabilization is achieved. Diffraction study can clarify the mechanism of such destabilization , i.e. the reaction mechanism. In particular, the dehydrogenation process has to be analyzed for new intermediate phases, and for new hydrogen-poor (or hydrogen-free) decomposition products. The latter may be tested as starting compounds in rehydrogenation processes. New compounds may show different properties and maybe even reversibility ... 

At 435°C the olefin concentration is found to be only about 0.02% at 30 atm. partial pressure of hydrogen. Thus, if we were to carry out paraffin isomerization by successive and separate steps of dehydrogenation of ri-paraffin to n-olefin, followed separately by skeletal isomerization of the n-olefin produced to iso-olefin (and subsequent rehydrogenation), the over-all conversion of such a scheme could be, at best, 0.02%. Thus, the paraffin isomerization, if accomplished in a bifunctional reaction system with a high conversion as might be described by formula (2), is an example of a nontrivial case as defined by (3) above. 

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