Hexane-H2 reactions

Fig. 12. Effect of self-poisoning (i hr at 450 C, by the reaction mixture) on hexane/H2 reactions. AT is the temperature increase necessary to achieve the same overall conversion after poisoning over that before poisoning. AT is plotted as a function of the average particle size of various Pt/SiO2 catalysts. From V. Ponec et al, in Catalyst Deactivation, p. 93, Elsevier, Amsterdam (1980).

Under the present oxygen treatment conditions, treated WC did not show any change in bulk structure as seen by XRD when the treatment temperature was below 773 K. At 773 K, bulk oxide was formed and the sample was inactive for -hexane-H2 reactions. Hence, four WC samples were compared fresh WC (WC/fresh) and WC treated in 02 at RT, 473 K, and 673 K (denoted by WC/treatment temperature). Bulk oxidation occurred at much lower temperatures for Mo2C and thus the maximum temperature of the oxygen treatment was 473 K (Mo2C/473). 

Another difference between WC and Mo2C catalysts in n-hexane-H2 reactions is their selectivity-conversion relationship as shown in Figure 21.3. Isomerization selectivity over WC did not show any dependence on n-hexane conversion. It depended only on the oxygen treatment temperature. In contrast, isomerization selectivity over Mo2C showed a linear decrease with increasing -hexane conversion. 

Table 21.2 Rates and product distribution after 10 h on stream for n-hexane-H2 reactions over Mo2C at 623 Ka...

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

Oxygen treatment did not cause much change in BET areas, but a marked decrease in CO chemisorption was observed for oxygen treated WC or Mo2C. Figure 21.4 shows XRD patterns of WC catalysts. As mentioned, oxygen treatment did not alter the bulk structure of WC below 773 K except the formation of metallic W. Above 773 K, the formation of W03 and W was extensive and the catalyst was inactive for n-hexane-H2 reactions. Similar observations were made for Mo2C. 

Figure 21.3 The relationship between n-hexane conversion and isomerization selectivity in n-hexane-H2 reactions at 623 K. (a) WC and (b) Mo2C catalysts. Conversions were varied by changing flow rates of 6% n-hexane-H2 mixture. Top panel , WC/fresh , WC/RT . WC/200 O, WC/400 Botton panel , Mo2C/RT , Mo2C/fresh.

Figure 21.7 TPR of Mo2C/RT under He flow (a) before and (b) after n-hexane-H2 reactions at 623 K for 10 h. The samples were reduced in H2 at 673 K for 1 h before the TPR.

TPR of the samples in flowing He or H2 were performed in a Pyrex flow system which was also used for catalytic reactions. Acid properties of the samples were probed by TPD of NH3 preadsorbed at RT. The analysis of gaseous products was made by an on-line mass spectrometer or a thermal conductivity detector. Reactions of n-hexane in the presence of excess H2 were carried out at 623 K and atmospheric pressure. A saturator immersed in a constant temperature bath at 273 K was used to produce a reacting mixture of 6% n-hexane in H2. Reaction products were analyzed by an online gas chromatograph (HP-5890A) equipped with a flame ionization detector and an AT-1 (Alltech) capillary column. 

Figure 2. Product distribution for cyclohexane conversion to either benzene or hydrogenolysis (>90% n-hexane) products over (a) pure nickel on alumina and (b) the same catalyst after treatment with hexamethyldisilane in H2. Reaction conditions are discussed in references (6) and (8).

Catalytic activity experiments were carried out in a stainless steel stirred tank reactor operated at 353 K and with a stirring velocity of 600 rpm. The hydroformylation of 1-octene was carried out at a total pressure of 4-5 MPa (with H2 CO ratio of 1 1) in a solution of 5 vol% of the olefin in hexane. The reaction was carried out for seven hours. Reactants and products were analysed by gas chromatography and H NMR spectroscopy. Heterogeneous samples were characterized by P solid state NMR and XPS. 

D. 4a(S),8a(R)-2-Benzoyloctahydro-6(2H)-isoquinolinone (4). Palladium (Pd), 10% on carbon, 4.0 g, (Note 21) is placed in a 500-mL Parr bottle under N2 and carefully wetted with 50 mL of cold denatured ethanol (EtOH). A slurry of 34.7 g of enone 3 (0.14 mol) in denatured EtOH (250 mL) is added and the Parr shaker apparatus assembled. After the system is purged with nitrogen-hydrogen (N2/H2), the reaction is shaken at 50 psi H2 and 50°C until H2 uptake is complete (1 hr, Note 22). The catalyst is filtered over a Celite pad (Note 23) and rinsed with warm chloroform (CHCI3) (4 x 75 mL). The filtrate is concentrated under reduced pressure, dissolved in 90 mL of CH2CI2 and crystallized with 200 mL of hexanes. The crystalline solid is filtered, rinsed with hexanes and dried to afford 34.3 g (98%, Note 24) of the ketone 4, representing a 51% yield over four steps. 

The shift of curves, as shown in Fig. 3.9, is unsurprising since the larger fuel molecules and their intermediates tend to break down more readily to form radicals that initiate fast reactions. The shape of the propane curve suggests that branched chain mechanisms are possible for hydrocarbons. One can conclude that the character of the propane mechanism is different from that of the H2—02 reaction when one compares this explosion curve with the H2—02 pressure peninsula. The island in the propane-air curve drops and goes slightly to the left for higher-order paraffins for example, for hexane it occurs at 1 atm. For the reaction of propane with pure oxygen, the curve drops to about 0.5 atm. 

In a typical experiment, the appropriate IL (2.0 ml), the iridium complex 11 (3 X 10 mmol) and the substrate 8 (11 8 = 500 1) were loaded under argon in a window-equipped stainless steel autoclave (V = 12 ml). The reactor was then pressurised with H2 and the desired amount of CO2, followed by heating under stirring to 40 °C for a standard reaction time of 22 h. The products were collected for GC and HPLC analysis by extraction of the IL phase with hexane after cooling and venting, or alternatively isolated by CO2 extraction. Representative results are summarised in Table 3. 

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