Ethylene formation selectivity

Comparing the effects of alkali cations of various sizes applied in reduction of C02 in HCOJ solution with a Cu cathode, Na+, K+, and Cs+ were shown to favor the formation of hydrocarbons.138 The selectivity of ethylene formation surpasses that of methane with increasing cation size. Deactivation of the Cu cathode... 

Some terminal alkenes are oxidized to aldehydes depending on their structure. As described before, acrylonitrile and acrylate are oxidized to acetals of aldehydes in alcohols or ethylene glycol.Selective oxidation of terminal carbons in 4-hydroxy-1-alkenes (18) gave the five-membered hemiacetals (19), which can be converted to y-butyrolactones by PCC oxidation (Scheme 4). Formation of a tricyclic six-membered hemiacetal (62%) from a 5-hydroxy-1-alkene system was used for the synthesis of rosa-ramicin. Formation of aldehydes as a major product from terminal alkenes using (MeCN)2Pd(Cl)(N02) and CuCU in r-butyl alcohol under selected conditions was reported. The vinyl group in the -lactam was oxidized mainly to the aldehyde as shown below (equation 12). ... 

Group analysis was carried out to see which combination of acid-base groups gave rate and selectivity in best agreement with experimental results. For ethylene formation, the best group association appeared to be an acid and a weak base site, with dissociative ethanol adsorption. For ether formation, two acid-base site-pairs were required, allowing a dissociatively adsorbed... 

Table 9.2 compares the selectivity over all studied catalysts. The ethylene oxide selectivity of either the Ag/y-AhOa or Au-Ag/y-AhOa catalysts drastically decreased with increasing reaction temperature. This is because the total oxidation of ethylene is favorable at high temperatures. The addition of an appropriate amount (<0.63 wt.%) of Au to the 13.2 wt.% Ag catalyst (which exists in separate Au particles on Ag particles) was found to promote the ethylene epoxidation reaction by weakening the Ag-O bond in the reaction temperature range of 493-528 K [18,19]. When the Au loading was higher than 0.63 wt.%, the activity of the ethylene epoxidation decreased because of the formation of an Au-Ag alloy which resulted in a decrease in the adsorption capacity of molecular oxygen [18]. 

Solution Equation (11-93) is applicable for the diffusion-limited situation. The pellet selectivity for ethylene formation with respect to ethane disappearance is... 

Figure 3. Conversion, selectivity, and yield of ethylene formation by ethane oxidation on Pt coated alumina ceramic foam monoliths. Up to 70% C2H4 selectivity is obtained at -70% C2H6 conversion for a single pass yield of -55%. Addition of Sn to Pt increases the selectivity and alkane conversion significantly.

The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3 1 6 9) reacts on B-MFI at 330 °C and WHSV 0.17 h 1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. 

Discussion at the First European Workshop on Catalytic Methane Conversion also addressed the effect of chloride addition to catalysts. It was stated that the addition of halides significantly enhanced Cj selectivities and that the enhancement could be observed by cofeeding chlorine or methyl chloride. The mechanism of ethylene formation over chloride-containing catalysts has been described in terms of a purely gas-phase dehydrogenation of ethane. ... 

It appeared that overoxidation decreased with ketal formation selectivity. Thus, whereas the reaction product obtained in the case of ethylene glycol as... 

Different classes of catalysts have been claimed for the oxidation of ethane to acetic acid [3], but the catalyst that gives the best performance is made of a mixed oxide of Mo/V/Nb (plus other components in minor amounts). This compound was first described in a paper by Thorsteinson et cd. 3a] - a paper that is considered nowadays a milestone in the field of the selective oxidation of alkanes, in view of the number of active phases that have been developed starting from catalysts described therein. Several patents were also issued by Union Carbide [3a-f], now Dow Chemical, regarding this system and the ETHOXENE process. The activity in ethane oxidation was attributed to the development of a crystalline phase characterized by a broad X-ray diffraction reflection at d = 4.0 A. The best composition was claimed to be Moo.73Vo.i8Nbo.o90 c, which reached 10% conversion of ethane at 286 °C with almost total selectivity to ethylene the selectivity decreased with increasing temperature, due to the formation of carbon oxides. The main peculiarity of this catalyst is its capability to activate the paraffin at low temperatures (<250 °C). 

The ethane conversion, on a molybdenum-based catalyst (Mo/Si-Ti), varies from 2.4% at 0.46 mg min/cm to 4.1% at 1.0 mg min/cm. Selectivity decreases with increasing contact time. Ethylene selectivities are 75% and above, CO selectivities are 15% and below. CO2 selectivity varies from 2.5% to 3.8% at 2.4% and 4.2% ethane conversion, respectively. The rate of ethylene formation increases with increasing N2O concentration. The activation energy when using N2O is 98 kJ/mol, whereas that using O2 is 41 kJ/mol. ODHE performances are attributed to the less oxidized state of the catalyst when using N2O. The catalyst is fully oxidized to... 

The activity for ethanol formation of SiCh—AI2O3, which has a comparatively large acid amount at i/o 3, was much lower than that expected from the linear relation shown in Fig. 4.9. Since ethylene polymer and acetaldehyde formed as byproducts, the ethylene formation was decreased. The decrease in the selectivity for ethylene formation is considered due to the existence of too strong acid sites of /foil 8.2 on the surface of SiCh — AI2O3. In fact, ethylene polymer and acetaldehyde was also formed over the alumina and aluminum phosphate catalysts which have strong acid sites of //o < — 8.2, but not over solid phosphoric acid and boron phosphate which have no such strong acids. These results combined with those mentioned in Section 4.5.1 A indicate that the effective acid strength for ethanol formation is -8.2view point of both activity and selectivity. 

Figures 60 and 61 show the effect of adding trace amounts of C2H4CI2 to the feed under NEMCA conditions. Dichloroethane suppresses the formation of acetaldehyde at negative potentials and leads, in conjunction with NEMCA, to ethylene oxide selectivity values of up to 75% for positive potentials (Fig. 62). As shown in the next section, even higher ethylene oxide selectivity values can be obtained using sodium, instead of O ", as the promoting ion.

In a very recent study " at temperatures between 250 and 3(X)°C and higher pressures (5 bar), it was found that technologically important ethylene oxide selectivity values can be obtained in the presence of C2H4CI2 moderators. Figure 84 shows a typical galvanostatic experiment. Negative currents, that is, Na supply to the catalyst, enhances the rate of epoxidation without affecting the rate of CO2 formation. Consequently, the selectivity to ethylene oxide increases substantially (Fig. 85). 

At low temperature, propylene is mostly formed by DME propagation as well as ethylene methylation. In conditions of reduced DME conversion, propylene formation occurs nearly exclusively and via methylation of the added ethylene with selectivity close to 100% on SAPO-34 and H-ZSM-5. It was also demonstrated that DME propagation does not proceed via ethylene, on both topologies. The addition of ethylene to the DME feed switches the MTO mechanism toward a mechanism where propylene is a true primary product from DME. At higher temperature, the propylene is a product of the ethylene methylation, aromatic/ coke and olefins cycles. 

The metals are impregnated together or separately from soluble species, eg, Na2PdCl4 and HAuCl or acetates (159), and are fixed by drying or precipitation prior to reduction. In some instances sodium or potassium acetate is added as a promoter (160). The reaction of acetic acid, ethylene, and oxygen over these catalysts at ca 180°C and 618—791 kPa (75—100 psig) results in the formation of vinyl acetate with 92—94% selectivity the only other... 

MPa (15—20 atm), 300—400 kg benzene per kg catalyst per h, and a benzene ethylene feed ratio of about 30. ZSM-5 inhibits formation of polyalkjlated benzenes produced with nonshape-selective catalysts. With both ethylene sources, raw material efficiency exceeds 99%, and heat recovery efficiency is high (see Xylenes and ethylbenzene). 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

---