Hydrogenation to hydrocarbons

T. Inui, and T. Takeguchi, Effective conversion of carbon dioxide and hydrogen to hydrocarbons, Catalysis Today 10, 95-106(1991). 

Hydrodearomatization (HDA) was investigated on the mentioned catalysts in temperature range 260-360°C, at total pressures 35-80 bar, hydrogen to hydrocarbon ratios (H2/HC thereafter) 200-1000 Nm3/m3 and liquid hourly space velocity (thereafter LHSV) 0.75-4.0 h"1. The experiments were carried out on catalysts of steady-state activity, in continuous operation. 

Fig. 20 Ratio of substituted C-C cleavage to unsubstituted C-C cleavage of 1,3-DMCH over Ir on different supports. Reaction was conducted at 593 K and 3540 kPa. Hydrogen to hydrocarbon ratio was kept at 30. Without considering secondary hydrogenolysis, the ratio (2-MC7 + 4-MC7)/ 2,4-DMC6 would represent the ratio of (a + b)/c cleavage. The statistical value of (a + b)/c is 2. A = Ir/Al203 O = Ir/Si02 = Ir/Ti02. Adapted from ref. 49.

While alcohols and acids are considered to be primary products (see Figure 4) ketones are probably formed in secondary reactions which only occur at higher temperatures 2). In Table II it can be seen that as the temperature increases, ketone production increases at the expense of alcohols up to a point after which both decrease (due to hydrogenation to hydrocarbons which under FT conditions are thermodynamically more stable (2), It has been suggested (2) that ketones result from the direct reaction between alcohols and surface carbon atoms and/or from the dehydrogenation of secondary alcohols (eg under high temperature FT conditions acetone and isopropyl alcohol are in thermodynamic equilibrium (2). 

As a result of the studies discussed above, a reasonably consistent picture of the kinetics and mechanism of the dehydrocyclization reaction over oxide catalysts has evolved. However, as pointed out earlier in this section, relatively few kinetic data have been reported for dehydrocyclization over platinum-alumina reforming-type catalysts. The data which have been reported include those of Hettinger and co-workers (H7), who studied the dehydrocyclization of re-heptane over platinum catalysts. These investigators found that the rate of dehydrocyclization decreased with increasing total pressure at a constant hydrogen-to-hydrocarbon ratio (Fig. 9). They also reported that the extent of dehydrocyclization was substantially greater for re-nonane than for re-heptane, which is consistent with the results obtained on oxide catalysts. In a later study of the kinetics... 

In a series of studies, the variation of the turnover number for the dehydrogenation reaction (the number of product molecules/platinum surface atoms/second) with the hydrogen to hydrocarbon ratio at a constant hydrocarbon pressure of 4 x 10"8 Torr was determined. The results are shown in Fig. 20 for the several stepped surfaces studied. The reaction rates increase with increasing hydrogen to hydrocarbon ratio. If no hydrogen is introduced into the reaction chamber, the catalyst behaves very differently. No benzene... 

Figure 1. Effect of sulfur in Pd-Nir-SMM raffinate hydrocracking. 7000 kPa (1000 psig), 2 LHSV, 5 hydrogen-to-hydrocarbon mole ratio. Feed—Q, raffinate , raffinate + 1500 ppm sulfur. Catalyst—Q, reduced 0.7 wt % — 15 wt % Ni—SMM , sulfided 0.7 wt% — 15 wt % Nir-SMM.

Average Product Distributions at 371°C (700°F), 7,000 kPa (1,000 psig), 2 LHSV, 5 Hydrogen-to-Hydrocarbon Mole Ratio... 

Also depending on drcumstances, the conversion takes place between 300 and 500, and usually between 400 and 48(PC at a pressure between 1 and 3.10 Pa absolute. The hydrogen to hydrocarbon molar ratio is about 8 to 10, and the LHSV is 1 to 2 h Since the isomerization reaction is slightly exothermic in the shift towards p-xylene production, it is carried out in adiabatic reactors. 

The catalyst is regcnerable, but can be employed for more than one year without regeneratioo. The temperature at the beginning of the run reaches 370 C, and is gradually increased, vp to 450 C at the end of the run, to maintain the activity of the catalyst system. The pressure ranges from 1.5 to 3. lO Pa absolute, the LHSV between 0.5 and 3 h and tise hydrogen to hydrocarbon molar ratio at the reactor inlet between 3 and 8. 

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