Dehydrogenations reaction velocity

The effect of orientation is illustrated in an interesting manner by some experiments of Palmer and Constable on the rate of dehydrogenation of alcohols in presence of metallic copper. Primary alcohols appear to be adsorbed with the -CH2OH group attached to the catalyst. The hydrogen is lost from this group in the chemical change, so that it appears reasonable to suppose that this is the portion of the molecule which must be activated. The hydrocarbon chain, therefore, would not be expected to have much influence on the process, and it was indeed found by experiment that the rates of reaction of five primary alcohols are equal. Moreover, the temperature coefficients of the reaction velocity are also equal. 

At the temperatures ordinarily used these two reactions occur with about equal velocities in the case of propane. Both of the olefins which are formed tend to polymerize and undergo further decomposition at this temperature (700° to 800° C.), the propylene at a much higher rate than the ethylene, with the result that either low yields are obtained or low conversions per pass through the cracking reactor must be accepted. A process which would enable a paraffin hydrocarbon to be converted to an olefin of the same number of carbon atoms by a dehydrogenation reaction would be highly desirable in some cases. 

The ethylbenzene dehydrogenation reaction was carried out using the pulse method. The catalyst weight (60 80 mesh) was 150 mg. The carrier gas was highly purity Nz with a velocity of 35 ml.min at 293 K. 

In Table 2 it can be seen that, as expected, an increase in space velocity results in a decrease in the overall conversion. However the yield of butenes remains approximately constant at the equilibrium conversion. Therefore the main reaction affected by the increase in space velocity is not the dehydrogenation reaction but the carbon deposition reactions. As carbon laydown is one of the major other reactions occurring we analysed... 

Figure 4 shows the evolution of the initial conversion versus temperature at a space velocity of 0.03 h l. The equilibrium conversion of isobutane to isobutene is 100% in our conditions. An increase of the conversion with temperature up to 773-823 K is observed. When metals were added, we also noted a large increase in isobutane dehydrogenation. Table 2 gives initial isobutane conversions, isobutene selectivities and yields of the reaction at 823 K for the three tested samples. 

The selectivity and activity of these preparations in the dehydrogenation and dehydration of n-butyl alcohol were determined. The reactions were conducted at atmospheric pressure with temperatures between 400° and 460°C., and an hourly space velocity of 1.3 which precluded thermodynamic equilibrium. The activities were expressed in milliliters (STP) of gaseous product formed by each of the reactions per milliliter of alcohol... 

The deactivation was obtained by n-heptane dehydrogenation. Prior to the catalytic tests, the samples were dried with N2 in situ at 423 K and then reduced with H2 at a heating rate of 10 K/min up to 773 K. The reaction was performed in a flow microreactor at atmospheric pressure and at 773 K. The molar ratio was H2/n-C7H]6 = 16 and the space velocity WHSV=2-7 h l. After 4 h under reaction, the catalysts were purged with N2 flow for 30 min and cooled to room temperature. 

The reaction of non-oxidizing dehydrogenation of n-hexane was carried out in a flow quartz reactor with a stationary layer of catalyst at atmospheric pressure in a stream of high purity helium. The optimum conditions of the reaction have been found by variation of volume velocity from 2.4 to 12 h and temperature in the range 500-700 C. Reaction products were analyzed by chromatography (Chrom-5), chromatomasspectroscopy (MX 1331) and IR-spectroscopy (Specord) methods. 

Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbVOs, SbV04 and Nb2Mo30n are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500 C, propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 

The conversion and selectivity in the catalytic reactions were obtained from the product distribution, as follows conversion (%) = 100 - wt% of reactant in products product selectivity (%) = [(wt% of particular product in products)/(100 - wt% of reactant in products)] X 100. The aromatization/cracking (A/C) and dehydrogenation/cracking (D/C) activity ratios were obtained from the products selectivities as follows. A/C activity ratio = [(selectivity for aromatics)/(selectivity for Ci, C2= and C2)] and D/C activity ratio = [(100 - selectivity for C,, C2= and C2)/(selectivity for Cj, C2= and C2)]. The space velocity reported here is based on the zeolite content of the catalyst. 

Figure 1. Effect of Sr/La ratio on the ethane and O2 conversion, product selectivity and CO/CO2 ratio in the oxidative dehydrogenation of ethane to ethylene over Sr-La2O3/SA-5205 catalyst [ Reaction condition Temperature = 700°C, C2H5/O2 = 6.0, steam/C2H6 = 1.0, space velocity = 100,104 cm. g. h ].

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