Nanocrystalline MgO

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

Nanocrystalline MgO and CaO also allow the destruction of chlorinated benzenes (mono-, di-, and trichlorobenzenes) at lower temperatures (700 to 900°C) than incineration.73 The presence of hydrogen as a carrier gas allows still lower temperatures to be used (e g., 500°C). MgO was found to be more reactive than CaO as the latter induces the formation of more carbon. 

Choudary, B.M. Kantam, M.L. Ranganath, K.V.S. Mahendar, K. Sreedhar, B. Bifunctional nanocrystalline MgO for chiral epoxy ketones via Claisen-Schmidt condensation-asymmetric epoxi-dation reactions. J. Am. Chem. Soc. 2004,126, 3396-3397. 

Nanocrystalline MgO and CaO with high surface area are able to absorb large amounts of chlorine, which undergo dissociative chemisorption. These can serve as rather selective, catalytic alkane chlorination reagents, which suggests that trapped Cl atoms are involved in the reaction.295 Liquid-phase low-temperature chlorination of alkanes is also possible in the presence of various alkenes as inductors and AIBN [azobis(isobutyronitrile)] 296... 

J. Klabunde, Carbon Dispersion and Morphology in Carbon-Coated Nanocrystalline MgO, Langmuir 19(24), 10426-10433 (2003). 

Until a few years back, the crystal shape of solid materials was of academic curiosity only and shape was not considered to have an effect on the chemical properties and reactivities of a material. However, recent studies clearly indicate that the shape of nanocrystals does indeed affect the chemistry. For example, it has been shown that 4 nm nanocrystalline MgO particles adsorb six molecules of S02 per nm2 at room temperature and 20 Torr pressure.30 However microcrystalline MgO adsorbs only 2 molecules of S02 per nm2 under similar conditions. Similarly, the nanocrystalline aerogel prepared, AP-MgO material adsorbs four times as much C02 as the microcrystals. There are not only differences in the amounts of gaseous molecules adsorbed on these surfaces, but also the mode of surface binding can also be different. S02 binds more predominantly as a monodentate species on the AP-MgO crystal but favors a bidentate geometry on conventionally prepared, CP-MgO microcrystals. Clearly, these results indicate that the shape and size of the crystals affect the adsorptive properties of the MgO surfaces. The high reactivities of the... 

Figure 16.7 Schematic of the preparative scheme for nanocrystalline MgO (CaO) labeled AP-MgO (CaO) and microcrystalline MgO (CaO) labeled CP-MgO (CaO).

Nanocrystalline MgO possessing superbasic sites has been prepared by the doping of potassium atoms using vapor diffusion (Sun and Klabunde, 1999a). This creates K+ and e sites on the crystal surface where the free electron associates with surface sites to produce superbasic reactive zones. Exposure of alkenes to this material result in the formation of allyl anions, formed by proton abstraction, which can alkylate ethane see reaction (17.9) ... 

Nanocrystalline MgO has demonstrated some unusual catalytic properties when chlorine gas is absorbed into the nanocrystals (Sun and Klabunde, 1999b). Contacts of this adduct with alkanes results in their catalytic chlorination see reaction (17.10). It appears that the chlorine is behaving in a manner more consistent with chlorine atoms being formed on the surface of the MgO by dissociative chemisorption. 

Nanocrystalline MgO also has a large capacity for adsorbing acid gases such as S02, C02, HC1, HBr, and S03 at near stoichiometric proportions. Work by Klabunde has demonstrated that nanocrystalline magnesium oxide has an enhanced surface reactivity over that anticipated from surface area alone. Nanocrystalline MgO chemisorbed 6 molecules of S02 per nm2, whereas larger microcrystalline MgO only chemisorbed 1.8 molecules per square nanometer (Stark and Klabunde, 1996). The proposed mechanism for the difference in adsorption capacities is due to a monodentate-type adsorption mechanism of S02 via the sulfur atom in the instance of nanocrystalline MgO, whereas microcrystalline MgO favors a bidentate adsorption mode through sulfur and an oxygen atom. 

The surface of nanocrystalline MgO has been found to interact strongly with polar organic molecules, such as aldehydes, ketones, and alcohols, by a dissociative chemisorption process, which results in the destruction of the organic molecule. This is in contrast to high-surface-area activated-carbon absorbents, which merely absorb the moiety with no resultant reaction. The chemisorption of acetaldehyde on MgO nanocrystals results in... 

Over the last two decades, Klabunde and co-workers have reported many uses of nanocrystalline MgO. These include efficient destructive chemisorbents for toxic gases, NO2, SO2, SO3, and HCl as well as chlorinated and phosphorus containing compounds, dehydrohalogenation of chlorohydrocarbons, and chlorination of alkanes. " ... 

The synthesis of nanocrystalline MgO is well established in the literature (see Section 5.3) and various types are now commercially available. Abbreviations and specific properties for various types of MgOs are listed in Table 5.1. 

Various magnesium oxide crystals [commercial MgO, CM-MgO (SSA 30 m /g), conventionally prepared MgO, NA-MgO (SSA 250 m /g), aerogel prepared MgO, NAP-MgO (SSA 590m /g)] were initially evaluated in the CSC and AE reactions separately in order to understand the relationship between structure and reactivity. All these MgO samples catalyzed both CSC of benzaldehyde with acetophenone to form chalcone quantitatively and selectively, and subsequent AE using (- -)-diethyl tartrate (DET) as a chiral auxiliary to obtain a chiral epoxy ketone in good yield and impressive ee. The nanocrystalline MgO (NAP-MgO) was found to be more active than the NA-MgO and CM-MgO in the condensation and epoxidation reactions (Figure 5.6). 

BIFUNCTIONAL CATALYSTS STABILIZED ON NANOCRYSTALLINE MgO FOR THE ONE-POT SYNTHESIS OF CHIRAL DIOLS... 

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