MgO-based catalysts

In order to resolve the issue of the possible adsorption of methyl radicals on the surface of the MgO-based catalysts Campbell and Lunsford conducted an MIESR experiment with an additional layer of magnesium oxide placed downstream from the Li/MgO catalyst It was expected that, if methyl radicals were adsorbed by the MgO surface, a significant decrease in the CH3 signal would have been observed. The results of this experiment conducted at 700 °C showed that tire presence of magnesium oxide increased production of methyl radicals, suggesting that no ads(Hption of methyl radicals occurs at this tonperature. Furthermore, it was concluded that no coupling of CH3 radicals takes place on the MgO catalyst... 

Figure 16.1. A mechanism proposed for the action of CeOj-MgO based catalyst in the treatment of SO2 in FCC plants [9]. Left action performed by MgO right action performed by CeOj.

The activity of Hg(0H)2 and of HgO based catalysts is similar, as shown in Fig. 3, with 20-25% conversion and with 70% C2 selectivity. Ca(0H)2 had a slightly lower activity ( 15%), while the Si02 and 1i02 based catalysts have low activity (<5%) at 800°C. Although the y A1203 based catalyst has high activity (21%) the selectivity to C2 is only half that of the MgO based catalyst. 

A number of recent studies of CO2 reforming of methane (6,12,13) indicate that promoters may also inhibit (12,13) the dehydrogenation of the adsorbed CH -species. The results in Table 4 illustrate the difference in rates of methane activation for two catalysts (6). The Ni/MgO catalyst shows an exchange rate significantly lower than that of the Ni/MgAl204 catalyst, although the catalysts have similar nickel surface areas and show similar rates for steam reforming. The MgO based catalyst may therefore inhibit the carbon formation by this mechanism as well as by enhanced steam adsorption. 

In previous work [7] we postulated that the coke precursor species on MgO-based catalysts are highly unsaturated linear compounds, such as 2,6-dimethylhepta-2,5-dien-4-one... 

G.B. (1996) Kinetics of a gas-phase chain reaction catalyzed by a solid the oxidative coupling of methane over Li/MgO-based catalysts. Ind. Eng. Chem. Res.,... 

Ferretti CA, Apesteguia CR, Di Cosimo JI. MgO-based catalysts for monoglyceride synthesis from methyl oleate and glycerol effect of Li promotion. Appl Catal A Gen 2011 399 146-153. 

The reaction temperature can have a large effect on the activities of the catalysts. Figure 4 shows the concentration of methyl ester with the solid base and acid catalysts at different temperatures after 10 hours reaction. The results show that the concentration of methyl ester with MgO-AUOa catalyst decreased more quickly than that with Ti02-S04 and CaO catalysts. This means that the activation energy of the reverse reactions with Mg0-Al203 catalysts is higher. 

Figure 1. NHj concentration in the reactor effluent gas using a total flow of 40 Nml/min with Pnj Phs - 1 / 3 at atmospheric pressure. Traces A-E in fig.lA (from bottom to top) were obtained with Ru/AljOj, CS-RU/AI2O3, Ru/MgO, a multiply promoted iron-based catalyst, and Cs-Ru/MgO. The corresponding NH3 equilibrium concentration is displayed as dashed line. Traces A-C in fig.IB (from bottom to top) were obtained with Ru/MgO, K-Ru/MgO, and Cs-Ru/MgO.

Fig. 3 A shows the effluent NH3 concentration observed for Ru/MgO as a function of reaction temperature for three different Pn, / Phj / Paf ratios at 20 bar total pressure. It is obvious that the reaction orders for N2 and H2 have opposite signs. Fig. 3B illustrates that the reaction orders for N2 and H2 partly compensate each other in the kineticaliy controlled temperature regime. Hence an increase in total pressure with a constant Pnj / Phj 1/3 ratio does not lead to a significant increase in conversion at lower temperatures. For the plication of alkali-promoted Ru catalysts under industrial synthesis conditions, it is necessary to find a compromise between kinetics and thermodynamics by increasing the Pn, / Phj ratio. The optimum observed for Cs-Ru/MgO prepared from CS2CO3 at 50 bar is at about Pnj / Phj 40 / 60 [15]. The high NH3 concentration of about 8 % obtained with 0.138 g catalyst using a total flow of 100 Nml/min clearly shows that Ru catalysts have indeed the potential to replace Fe-based catalysts in industrial synthesis [15].

The microwave technique has been also found to be the best method for preparing strongly basic zeolites (ZSM-5, L, Beta, etc.) by direct dispersion of MgO and KF. This novel procedure enabled the preparation of shape-selective, solid, strongly base catalysts by a simple, cost-effective, and environmentally friendly process [11, 12]. New solid bases formed were efficient catalysts for dehydrogenation of 2-propanol and isomerization of cis-2-butene. 

Subrahmanyam, M., Supriya, V.T., and Reddy, P.R., Photocatalytic H2 production with CdS-based catalysts from a sulphide/sulphite An effort to develop MgO-supported catalysts, Int.. Hydrogen Energ., 21, 99,1996. 

Friedel-Crafts reactions involving electrophilic substitution of aromatic compounds have been reported on solid base catalysts such as thallium oxide and MgO. The rates of benzylation of toluene by benzyl chloride over MgO nanocrystals were found to be of the order CP-MgO > CM-MgO > AP-MgO.56 An important observation in the study was that x-ray diffraction of the spent catalyst... 

Several cross-aldol condensations have been performed with alkaline earth metal oxides, including MgO, as a base catalyst. A general limitation of the cross-aldol condensation reactions is the formation of byproducts via the self-condensation of the carbonyl compounds, resulting in low selectivities for the cross-aldol condensation product. For example, the cross-condensation of heptanal with benzalde-hyde, which leads to jasminaldehyde (a-K-amylcinnamaldehyde), with a violet scent... 

The same authors (77) also investigated the Michael addition of nitromethane to a,/l-unsaturated carbonyl compounds such as methyl crotonate, 3-buten-2-one, 2-cyclohexen-l-one, and crotonaldehyde in the presence of various solid base catalysts (alumina-supported potassium fluoride and hydroxide, alkaline earth metal oxides, and lanthanum oxide). The reactions were carried out at 273 or 323 K the results show that SrO, BaO, and La203 exhibited practically no activity for any Michael additions, whereas MgO and CaO exhibited no activity for the reaction of methyl crotonate and 3-buten-2-one, but low activities for 2-cyclohexen-l-one and crotonaldehyde. The most active catalysts were KF/alumina and KOH/alumina for all of the Michael additions tested. 

The results obtained with the Tishchenko reaction of furfural using alkaline earth metal oxides as base catalysts are presented in Table III. Because the strength of basic sites increases in the order MgOacid strengths is the reverse 73), it was concluded that CaO and SrO— which have moderate acid and base sites in comparison with MgO and BaO—are appropriate... 

Solid base catalysts were tested for this reaction, which was carried out in benzene as a solvent at 313 K. Among them, MgO, CaO, and SrO exhibited good catalytic properties, affording phthalide exclusively with yields between 86% and 100% after a short time (15min) in a batch reactor, whereas with y-alumina, phthalide was obtained with excellent yields and selectivity after 4h. The Tishchenko reaction of 2,3-naphthalenedicarbaldehyde was also carried out with CaO and with y-alumina at 333 K yields of the corresponding five-membered lactone were 94% and 100% after 2 and 20 h, respectively. 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

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