Solid hydride transfer reaction

Most strong Bronsted acids effectively facilitate the hydride transfer reaction required to make alkylate (4), but avoiding the formation of high molecular weight coke precursors has proven more difficult to achieve. This second hurdle is particularly important for the deactivation of solid acid catalysts and has proven to be a stumbling block for the technology. 

Faujasites exchanged with rare earth cations, and particularly with La " ", have shown good performance as solid alkylation catalysts, as nicely reported in the pioneering works carried out in the late 1960s by researchers at Mobil Oil (89) and Sun Oil (96), and later on by Weitkamp s group (132,133). This fact is most likely related to the ability of La-FAU zeolites to catalyze hydride transfer reactions. The Bronsted acidity in La-exchanged FAU zeolites can be associated with the protons generated upon hydrolysis of water in the hydrated lanthanum cations when snbjected to thermal activation treatment at temperatures between 60 and 300°C (134-136) ... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

The isopentene produced will either be protonated or be added to another carbenium ion. With a butyl cation, this would lead to a nonyl cation. The resultant carbenium ion fragment can accept a hydride and form a product heptane, or it can possibly add a butene to form a Cn cation. With hydride transfer, another alkane with an odd number of carbon atoms is produced. Just this example is sufficient to show the huge variety of possible reactions. By means of gas chromatographic analysis, Albright and Wood (82) found about 100-200 peaks in the C9-C16 region, regardless of the alkene and acid employed. A similar number of products can be observed for solid acid-catalyzed alkylation. 

Aluminum methoxide Al(OMe)3 is a solid which sublimes at 240 °C in vacuum. Aluminum isopropoxide melts in the range 120-140 °C to a viscous liquid which readily supercools. When first prepared, spectroscopic and X-ray evidence indicates a trimeric structure which slowly transforms to a tetramer in which the central Al is octahedrally coordinated and the three peripheral units are tetrahedral.162,153 Intramolecular exchange of terminal and bridging groups, which is rapid in the trimeric form, becomes very slow in the tetramer. There is MS and other evidence that the tetramer maintains its identity in the vapour phase.164 Al[OCH(CF3)2]3 is more volatile than Al[OCH(Me)2]3 and the vapour consists of monomers.165 Aluminum alkoxides, particularly Al(OPr )3, have useful catalytic applications in the synthetic chemistry of aldehydes, ketones and acetals, e.g. in the Tishchenko reaction of aldehydes, in Meerwein-Pondorf-Verley reduction and in Oppenauer oxidation. The mechanism is believed to involve hydride transfer between RjHCO ligands and coordinated R2C=0— A1 groups on the same Al atom.1... 

Eliminate all sources of ignition. Wear butyl rubber gloves, large and heavy face shield, goggles, and laboratory coat. Cover spill with dry sand. Scoop into a container and transfer to the fume hood. Decompose by cautious addition of dry butanol (40 mol butanol to 1 mol hydride or 26 mL/g hydride) until reaction ceases. Slowly add mixture to a pail of cold water. Let stand for 24 hours. Neutralize with 6 M hydrochloric acid (cautiously add concentrated acid to an equal volume of cold water). Decant the solution to the drain. Treat the solid residue as normal refuse.4 5... 

The NaBH4 reduction of a pentacyclic cage diketone in EtOH afforded a 38 62 mixture of endo-endo and exo-endo diol. " In contrast, solid-state NaBH4 reduction of diketone provided exclusively the corresponding endo-endo diol. " This indicates that the hydride transfer occurs exclusively at the exo-face of the carbonyl group in solid state reaction (Scheme 5.52). 

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

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