Benzene formation, pressure

The effect of microwave irradiation on the catalytic hydrogenation, dehydrogenation, and hydrogenolysis of cydohexene was studied by Wolf et al. [81]. Optimum conditions for benzene formation were a hydrogen flow, N-CaNi5 catalyst, atmospheric pressure, and 70 s irradiation time. Cydohexane was the main product when the irradiation time was 20 s, or in a batch/static system. 

Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5. which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cyclohexene (ring contraction)... 

With respect to catalyst contact time, the effects of temperature and pressure on the yields are shown in Figs. 18, 19, and 20. Activity (as measured by the C5- gas make) is a strong function of temperature, as shown in Figs. 18 and 19. Again, the higher-temperature operation favors benzene formation. KINPTR s prediction of activity as a function of pressure is shown in Fig. 20. Lower-pressure operation favors the yield of benzene. 

CJ. Pope and J.A. Miller. Exploring Old and New Benzene Formation Pathways in Low-Pressure Premixed Flames of Aliphatic Fuels. Proc. Combust. Inst., 28 1519-1527,2000. 

Sexton et al. (66) also examined the activity for the dehydrogenation of cyclohexane and conversion of methylcyclopentane of a series of PtSn alumina catalysts where the Sn/Pt ratio was varied. They found that the activity decreased as the Sn/Pt ratio increased. Selectivity for benzene formation from methylcyclopentane increased to a maximum at ca. 1.5 to 2.5 wt.% Sn (Sn/Pt = 4.9 to 8.2) and then declined. These conversions were conducted at normal pressures. 

The samples in Table I were tested in the reaction of n-hexane at 733 K and atmospheric pressure. Figure 3 shows the selectivity to benzene formation (calculated as the yield of benzene divided by the conversion of n-hexane) as a function of n-hexane conversion for Pt-LI and Pt-VI as circles and squares, respectively. Results recently reported for a 0.88 wt% Pt (H/Pt = 0.49) catalyst prepared by impregnation of aqueous Pt(NH3)4Cl2 are included for comparison (8). However, the results in (8) were obtained at 750 K, H2/n-hexane = 6 (6 kPa n-hexane) and diluent He at atmospheric total pressure which are slightly different from the experimental conditions used in the current work. 

Figure 3. Selectivity to benzene formation as a function of n-hexane conversion at 733 K, H2/C6H14 molar ratio of 6, and atmospheric total pressure. Circles and squares correspond to results for Pt-LI and Pt-VI, respectively. Catalytic results at 750 K for Pt/Mg(Al)0 prepared from Pt(NH3)4Cl2 are included as triangles for comparison (Adapted from ref. 8).

Zelikoff and Aschenbrand found that the gaseous products included benzene, diacetylene, hydrogen, ethylene and vinylacetylene. These authors photolysed C2H2 at pressures from 2 to 75 torr at 1849 A. The quantum yield of benzene formation increases markedly with pressure. Although the quantum yield for the disappearance of acetylene also increases with pressure it remains very much larger than the quantum yield of the sum of the products. The quantum yields of di-... 

The mechanism of benzene and higher polymer formation remains uncertain with further work on isotopic mixtures needed to help determine the processes which occur. It is interesting to note that, in the P- and X-ray radiolysis of C2H2-C2D2 mixtures, Mains et observe all benzenes do to in the products and conclude that a C-H rupture must occur in the radiolysis. Dorfman and Wahl have shown that in the helium-sensitized radiolysis of acetylene, where only ionized states of acetylene are formed, there is no formation of benzene. The strong pressure-dependence of the benzene formation in the direct photolysis still provides the strongest evidence for a molecular mechanism such as given in the reaction scheme. 

Hindin et al. (18) published data showing benzene formation to proceed readily from methylcyclopentane over mechanical mixtures of platinum bearing particles and silica-alumina, at atmospheric pressure and near 500°C. temperature. Under these conditions the equilibrium constant for conversion of a cyclopentane to a cyclopentene is of the order of unity. Consequently, the first step, if it is catalyzed by X, can itself proceed with... 

The benzene formation rate on Pd(l 11) under catalytic conditions (i.e., P(acetylene) -20 Torr) is rather insipid with a turnover frequency of -10" reactions/site/s [39]. This reaction is first order in acetylene pressure and has a relatively low reaction activation energy of -2 kcal/mol. 

Fig. 1.4 Plot of the benzene formation rate as a function of hydrogen pressure catalyzed by Pd(lll). (With kind permission from Springer Science and Business Media.)...

Wibaut and his colleagues260 studied the pyrochemical formation of biphenyl from benzene under pressure and in presence of catalysts (nickel or iodine), which permit the use of lower temperatures. However, the proportion of side reactions then increases, e.g., formation of methane when nickel is used as catalyst. 

A Ni metal catalyst was recently used to produce benzene from methane under ultrahigh vacuum conditions(equation 1) by activation of CH4 physisorbed on the Ni catalyst at 47 K by molecular beam techniques at pressures less than lO torr. The selectivity of benzene formation under these conditions is 100% and the reaction proceeds via collision-induced dissociative chemisorptionThe collision of Kr atoms with physisorbed CH4 leads to dissociation into adsorbed H-atoms and methyl radicals the latter dissociate to CH fragments which successively recombine to adsorbed C2H2. Trimerization of C2H2 on the metal surface yields quantitatively C Hg. 

Hydrogenation of benzene under pressure using a metal catalyst such as nickel results in the addition of three molar equivalents of hydrogen and the formation of cyclohexane (Section 14.3). The intermediate cyclohexadienes and cyclohexene cannot be isolated because these undergo catalytic hydrogenation faster than benzene does. 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Examples of the hydroquinone inclusion compounds (91,93) are those formed with HCl, H2S, SO2, CH OH, HCOOH, CH CN (but not with C2H 0H, CH COOH or any other nitrile), benzene, thiophene, CH, noble gases, and other substances that can fit and remain inside the 0.4 nm cavities of the host crystals. That is, clathration of hydroquinone is essentially physical in nature, not chemical. A less than stoichiometric ratio of the guest may result, indicating that not all void spaces are occupied during formation of the framework. Hydroquinone clathrates are very stable at atmospheric pressure and room temperature. Thermodynamic studies suggest them to be entropic in nature (88). 

Environmental aspects, as well as the requirement of efficient mixing in the mixed acid process, have led to the development of single-phase nitrations. These can be divided into Hquid- and vapor-phase nitrations. One Hquid-phase technique involves the use of > 98% by weight nitric acid, with temperatures of 20—60°C and atmospheric pressure (21). The molar ratios of nitric acid benzene are 2 1 to 4 1. After the reaction is complete, excess nitric acid is vacuum distilled and recycled. An analogous process is used to simultaneously produce a nitrobenzene and dinitrotoluene mixture (22). A conversion of 100% is obtained without the formation of nitrophenols or nitrocresols. The nitrobenzene and dinitrotoluene are separated by distillation. 

More up-to-date data of this process are employed in a study by Rase (Fixed Bed Reactor Design and Diagnostics, Butterworths, 1990, pp. 275-286). In order to keep the pressure drop low, radial flow reactors are used, two units in series with reheating between them. Simultaneous formation of benzene, toluene, and minor products is taken into account. An economic comparison is made of two different catalysts under a variety of operating conditions. Some of the computer printouts are shown there. 

Other purification procedures include the formation of the picrate, prepared in benzene soln and crystd to constant melting point, then decomposed with warm 10% NaOH and extracted into ether the extract was washed with water, and distd under reduced pressure. The oxalate has also been used. The base has been fractionally crystd by partial freezing and also from aq 80% EtOH then from absolute EtOH. It has been distd from zinc dust, under nitrogen. 

In 1927 Putochin studied the effect of temperature on the nature of the products formed when the formylation reaction was carried out in benzene and observed that 1-formyl derivatives were the major products obtained at low temperatures, whereas the 3-formyl derivatives predominated at higher temperatures. Britton et al. in 1947 claimed that the formation of the 3 -formylindole derivative is probably favored, relative to the alternate 1-formylation process, by elevated temperatures and pressures.However, it was apparently not possible to suppress completely the formation of the 1-formyl derivatives and yields of the order of 40% of both products were usually obtained. 

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