Paraffin/olefin ratio, hydrogen

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. 

The decreasing selectivity for C3 products, but also for C4 and C5 products, indicates that the cracking reaction is more deactivated than the other reactions of the network. The decrease of the paraffin/olefin ratio with the coke content illustrates that the hydrogen transfer reactions, which play an important role in the production of paraffins, are less deactivated. The same can be concluded for the isomerisation reactions leading to the hexane isomers. 

From comparative studies of H/ZSM-5 and Zn/ZSM-5 it is known that the acid sites are poor at removing hydrogen after C-H bond activation. However, metal sites not only increase the rate of dehydrogenation, but also the rate of hydrogenation. Since the feed is already olefinic, it is expected that the metal sites should promote both hydrogenation and aromatisation. The decrease in paraffin to olefin ratio therefore suggests a preferential deactivation of the metal sites. 

Hydrogenations of 1-octene and cyclohexene were studied at 70°C and 172 bar in the 100 mL reactor using the same experimental conditions. The catalyst employed was RhCl(TANi5DPPA)3. The mole fractions of the olefins and hydrogen were 0.001 and 0.033, respectively. The substrate to catalyst molar ratio was 400. The total conversion of cyclohexene was slower compared to 1-octene, but because no isomers were produced the yield profile toward the saturated hydrocarbon (paraffin) is practically the same as is observed in Figure 10. Therefore, the catalytic activity was not affected by the degree of substitution in the double bond in these two olefrns at 70 C and 172 bar. 

Also, concerning the effect of the temperature on the reaction rates, different assumptions were made here with respect to our previous work.10 In that case, only the hydrogen and CO adsorption were regarded as activated steps, in order to describe the strong temperature effect on CO conversion. In contrast, due to the insensitivity of the ASF product distribution to temperature variations (see Section 16.3.1), other steps involved in the mechanism were considered as non-activated. In the present work, however, this simplification was removed in order to take into account the temperature effect on the olefin/paraffin ratio. For this reason, Equations 16.7 and 16.8 were considered as activated. 

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). 

Respect to olefins/parafFins ratio, it can be mentioned that the it electron bonds present in the unsaturated hydrocarbons can interact with the surface electrons of the carbon microdomains these surface electrons act as hydrogenation catalyst in the same way that Platinum surface electrons act in conventional catalytic hydrogenation processes The observed values of olefins/paraffins ratio decrease from CON to C-155, and practically olefins are not present as a product when C-155 is used, suggesting for the new developed materials the presence of high electronic densities surrounding the carbon microdomains. 

The goal is to improve the ratio of straight chain to branched chain products (n iso ratio) and to keep the hydrogenation of olefin to paraffin and aldehyde to alcohol as low as possible. 

Reaction products obtained by low-temperature hydroformylation (100°-120°C) of linear a-olefins with an equimolar amount of CO and Hd at 280 atm were (% ) aldehyde, 94 1 (n iso ratio 2, 56 to 3, 16) formate, 4 0.5 alcohol, 2 0.5. Furthermore, 1% of the olefin feed is hydrogenated to paraffin and 2-3% is converted to high boiling products (aldols, ketones, and acetals). 

Results for C4 products (Table 6) indicate that, whilst the totel C4 yield is slightly higher with ZSM-5 (due to the greater conversion compared to KEHY), the ratio of paraffins to olefins is significantly lower, and is particularly so with the iso compounds. This reflects the greater facility for hydrogen transfer in KEHY, as opposed to ZSM-5, vdiich can be explained in terms of the influence of site proximity [12], or the effects of sorption preference [13] both of which favour hydrogen transfer in more alumina-rich zeolites. 

When light hydrocarbons terminate predominantly as paraffins (kh>>ko), or when a-olefins are rapidly hydrogenated in secondary reactions (ks>kr), we should obtain a light product distribution with a low and constant value of a. We describe below two such systems. A Fe-based catalyst (a-Fe2C>3) at very high H2/CO ratios (-9) gives only C to C5 paraffins with a constant chain growth... 

The initial increase in C5+ selectivity as x increases arises from diffusion-enhanced readsorption of a-olefins. At higher values of CO transport restrictions lead to a decrease in C5+ selectivity. Because CO diffuses much faster than C3+ a-olefins through liquid hydrocarbons, the onset of reactant transport limitations occurs at larger and more reactive pellets (higher Ro, 0m) than for a-olefin readsorption reactions. CO transport limitations lead to low local CO concentrations and to high H2/CO ratios at catalytic sites. These conditions favor an increase in the chain termination probability (jSr, /Sh) and in the rate of secondary hydrogenation of a-olefins (j8s) and lead to lighter and more paraffinic products. 

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