Oxides alkaline earth metals

Chemical Properties. In addition to the reactions Hsted in Table 3, boron trifluoride reacts with alkali or alkaline-earth metal oxides, as well as other inorganic alkaline materials, at 450°C to yield the trimer trifluoroboroxine [13703-95-2] (BOF), MBF, and MF (29) where M is a univalent metal ion. The trimer is stable below — 135°C but disproportionates to B2O2 and BF at higher temperatures (30). 

Now consider strong and weak bases. The common strong bases are oxide ions and hydroxide ions, which are provided by the alkali metal and alkaline earth metal oxides and hydroxides, such as calcium oxide (see Table J.l). As we have seen,... 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

Stabilization of MoO is achieved by incorporation of alkaline-earth metal oxides (e.g. BaO, MgO) or rare-earth metal oxides (e.g., La20 ) or by reaction with base metal oxides (e.g., NiO, CoO, CuO etc.). Since MoO is an acidic oxide, its reaction with a basic oxide should lead to a mixed oxide... 

Fig. 14. CH4 conversion (a) and CO yield (b) in the C02 reforming of CH4 catalyzed by reduced 16.7-wt% NiO/alkaline earth metal oxides. Before reaction, each catalyst was reduced in flowing H2 at 773 K for 14 h. Reaction conditions pressure, 1 atm temperature, 1063 K feed gas molar ratio, CH4/C02 = 1/1 GHSV, 60,000 mL (g catalyst)-1 h-1 (239).

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

The electron-rich oxygen anions exhibit basic electron donor capacity. Basic metal oxides are commonly used for neutralizing or scrubbing acidic gases. Alkaline earth metal oxides have been used for the removal of NOx. The surfaces of cubic alkaline metal oxide such as MgO, CaO, and BaO are dominated by the Lewis basicity of surface oxide anions. The basicity increases down the alkaline earth family as the metal ion radii become larger and the chaige on the metal ion becomes more positive. 

Bauman, J. E. "Alkali- and Alkaline-Earth Metal Oxides and Hydroxides in Water" in "Solubility Data Series ... 

This review is a summary of the work done and potential opportunities for inexpensive and easily accessible base catalysts, such as alkaline earth metal oxides and hydroxides, as well as alkali metals and oxides supported on alkaline earth metal oxides. Preparation methods of these materials, as well as characterization of basic sites are reported. An extensive review of their catalytic applications for a variety of organic transformations including isomerization, carbon-carbon and carbon-oxygen bond formation, and hydrogen transfer reactions is presented. 

Concerning the nature of Lewis basic sites, little work has been done to establish general rules and models, except for alkaline earth metal oxides and zeolites. With respect to the former, i.e., the nature of oxygen Lewis basic sites on alkaline earth metal oxide catalysts, a charge-density model predicts that the strength of the sites decreases in the order > OH > H2O > H30. ... 

If a correlation between the nature of the various sites and their catalytic activities and/or selectivities has to be established, methods for characterizing the different basicities will be required. Therefore, in the following sections, we discuss the methods for preparation of alkaline earth metal oxides as well as the principal characterization techniques used to evaluate their basicities. 

III. Synthesis of High-Surface-Area Alkaline Earth Metal Oxides... 

Alkaline earth metal oxides are generally prepared by thermal decomposition of alkaline earth compounds, such as hydroxides, chlorides, sulfates, and carbonates. 

The most general methodology followed to prepare alkaline earth metal oxides as basic catalysts consists of the thermal decomposition of the corresponding hydroxides or carbonates in air or under vacuum. Decomposition of hydroxides is frequently used to prepare MgO and CaO, whereas BaO and SrO are prepared from the corresponding carbonates as precursor salts. Preparation of alkaline earth metal... 

Among the spectroscopic techniques, one of the most widely used to characterize the basic properties of alkaline earth metal oxides is infrared (IR) spectroscopy of adsorbed probe molecules (41,47-49) this is described below. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Alkaline earth metal oxides have been used as solid base catalysts for a variety of organic transformations. Excellent reviews by Tanabe 4) and Hattori 2,3,7) provide detailed information about the catalytic behavior of alkaline earth metal oxides for several organic reactions of importance for industrial organic synthesis. In this section, we describe in detail reactions that have been reported recently to be catalyzed by alkaline earth metal oxides. 

Alkaline earth metal oxides are active catalysts for double bond isomerization. For example, SrO exhibits high activity and selectivity for the isomerization of a-pinene to /1-pinene 110). MgO and CaO have excellent activities for isomerization of 1-butene and 1,4-pentadiene and, particularly, for isomerization of compounds containing heteroatoms, such as allylamine or 2-propenyl ethers 111-115). Recently... 

Baba and Endou 117) reported that CaO is an active catalyst for isomerization of VBH to EBH when it is evacuated at temperatures above 800 K, whereas MgO did not show any activity for this process. However, some discrepancies have been reported by Kabashima et al. 10), who found that MgO, CaO, SrO, and BaO catalyze the isomerization of VBH to EBH, with the order of activity being CaO > MgO > SrO > BaO. This order in activity is attributed to the trends in base strength of oxides (BaO > SrO > CaO > MgO) and the surface area, the latter decreasing in the order MgO > CaO > SrO > BaO (Table II). The activity of the CaO was the highest among these alkaline earth metal oxides, and the activity of the MgO varied with the pre-treatment temperature, reaching a maximum it was 873 K ... 

Results Characterizing Isomerization of VBH to EBH on Alkaline Earth Metal Oxide Catalysts in a... 

Condensation of butanol has been carried out on alkaline earth metal oxides at 273 K (13,121). This condensation reaction yields 2-ethyl-3-hydroxy-hexanal as a main product other products, such as 2-ethyl-2-hexenal (arising from the dehydration of 2-ethyl-3-hydroxy-hexanal), n-butyl-K-butyrate (arising from the Tishchenko reaction of butyraldehyde), and 2-ethyl-3-hydroxy- -hexyl butyrate (arising from the Tishchenko reaction of 2-ethyl-3-hydroxy-hexanal), are also formed (Scheme 12). 

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... 

Various nitro compounds have been condensed with carbonyl compounds in reactions catalyzed by alkaline earth metal oxides and hydroxides 145). It was found that the reactivities of the nitro compounds were in the order nitro-ethane > nitromethane > 2-nitropropane, and those of carbonyl compounds were propionaldehyde > isobutyraldehyde > pivalaldehyde > acetone > benzaldehyde > methyl propionate. Among the catalysts examined, MgO, CaO, Ba(OH)2, and Sr(OH)2, exhibited high activity for nitroaldol reaction of nitromethane with propionaldehyde. In reactions with these catalysts, the yields were between 60% (for MgO) and 26% (for Sr(OH)2) at 313 K after 1 h in a batch reactor. On Mg(OH)2, Ca(OH)2, and BaO, the yields were in the range of 3.8% (for BaO) and 17.5% (for Mg(OH)2). Investigation of the influence of the pre-treatment... 

Dimerization of methyl crotonate has been carried out with alkaline earth metal oxides as basic catalysts 15). The reaction proceeds by Michael addition, which is initiated by abstraction of an allylic hydrogen of methyl crotonate by the basic site to form the allylic carbanion, which attacks a second methyl crotonate molecule at the jS-position to form a methyl diester of 3-methyl-2-vinylglutaric acid. The diester undergoes a double bond migration to form the final E- and Z- isomers of 3-ethylidene-3-methylglutaric acid dimethyl ester (MEG) (Scheme 22). 

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 Guerbet reaction is an important industrial process for increasing the carbon numbers of alcohols. Thus, a primary or secondary alcohol reacts with itself or another alcohol to produce a higher alcohol (Scheme 23). Alkaline earth metal oxides have been used as catalysts for the condensation of alcohols. Ueda et al. (158,159) reported the condensation of methanol with other primary or secondary alcohols having a methyl or methylene group at the )S-position they used MgO, CaO, and ZnO as catalysts. The reactions were performed with gas-phase reactants at 635 K only MgO was found to be both active and selective (>80%). 

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