Zeolites hydrogen transfer

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

With hydrofluoric acid (23,50), and to a lesser degree also with zeolites (14,81, 87-89), a significant fraction of the product stems from self-alkylation, which is sometimes also termed hydrogen transfer. The importance of this mechanism depends on the acid, the alkene, and the reaction temperature. Self-alkylation... 

Fig. 15 A proposed reaction network of direct ring opening of decalin reaction over acidic zeolites. PC Protolytic cracking HeT Hydride transfer HT Hydrogen transfer I Isomerization P /i-scission DS Desorption TA Transalkylation. Adapted from ref. 47.

It is generally accepted that aluminum deficient structures derived from type Y zeolite alter the extent of hydrogen transfer reactions which ordinarily favor the formation of paraffins and aromatics at the expense of olefins and naphthenes. This octane reducing reaction is controlled principally by the silica/alumina ratio of the zeolite and its rare earth content(1). 

An approximation of the extent of hydrogen transfer reactions occurring compared to cracking reactions and the net effect on product distribution can be initially seen by a consideration of the zeolite properties of the catalysts tested in the present study ... 

The main reactions of the MTG/MTO process can be summarized as follows the first is the dehydration of methanol to DME on acidic zeolite catalysts. The equilibrium mixture of methanol, DME, and water is then converted to light alkenes, which react further to form higher alkenes, n- and Ao-alkanes, aromatics, and naphthenes by hydrogen transfer, alkylation, polycondensation, isomerization, and other secondary reactions. 

The effect of rare-earth HY zeolites is somewhat similar, except that it produces less olefin as a result of enhanced hydrogen transfer reactions. Decreased hydrogen transfer is the main feature of USY zeolites, yielding a product significantly richer in olefins. It is slightly richer in aromatics because of the retardation of their secondary transformations (condensation, coke formation). 

Reduction of ketones and aldehydes by hydrogen transfer from alcohols is catalyzed by a variety of oxides [7,10]. For example, alumina catalysts turned out to be active in this reaction both in the gas-solid [11,12] and liquid-solid conditions with the alcohol preadsorbed on the catalyst [13], Also zeolites [14] and hydroxyapatite [15] have been employed in this reaction. However, a,/9 conjugated ketones resulted to be quite resistant to the reduction and poor yields of allylic alcohols or only saturated ketones could be obtained [10-13]. 

Over-cracking of PCC gasoline with either ZSM-5 or REHY results, in both cases, in a preferential loss of heavier olefin components. The major differences between the two zeolites is the increased C3/C4 ratio with ZSM-5 which has been assigned to pore size effects, and enhanced bimolecular hydrogen transfer reactions with REHY, resulting in a higher paraffin/olefin ratio. 

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