Alkaline earth metal oxides catalytic activity

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. 

Conjugate addition of methanol to a,/l-unsaturated carbonyl compounds forms a new carbon-oxygen bond to yield valuable ethers (Scheme 26). Kabashima et al. (12) reported the conjugate addition of methanol to 3-buten-2-one on alkaline oxides, hydroxides, and carbonates at a temperature of 273 K. The activities of the catalyst follow the order alkaline earth metal oxides > alkaline earth metal hydroxides > alkaline earth metal carbonates. All alkaline earth metal oxides exhibited high catalytic activities and, as in alcohol condensations and nitroaldol reactions, their catalytic activities were not much affected by exposure to CO2 and air. 

Peterson and Scarrah 165) reported the transesterification of rapeseed oil by methanol in the presence of alkaline earth metal oxides and alkali metal carbonates at 333-336 K. They found that although MgO was not active for the transesterification reaction, CaO showed activity, which was enhanced by the addition of MgO. In contrast, Leclercq et al. 166) showed that the methanolysis of rapeseed oil could be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pre-treated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37% with 97% selectivity to methyl esters was achieved after 1 h in a batch reactor. The authors 166) showed that the order of activity was Ba(OH)2 > MgO > NaCsX zeolite >MgAl mixed oxide. With the most active catalyst (Ba(OH)2), 81% oil conversion, with 97% selectivity to methyl esters after 1 h in a batch reactor was achieved. Gryglewicz 167) also showed that the transesterification of rapeseed oil with methanol could be catalyzed effectively by basic alkaline earth metal compounds such as calcium oxide, calcium methoxide, and barium hydroxide. Barium hydroxide was the most active catalyst, giving conversions of 75% after 30 min in a batch reactor. Calcium methoxide showed an intermediate activity, and CaO was the least active catalyst nevertheless, 95% conversion could be achieved after 2.5 h in a batch reactor. MgO and Ca(OH)2 showed no catalytic activity for rapeseed oil methanolysis. However, the transesterification reaction rate could be enhanced by the use of ultrasound as well as by introduction of an appropriate co-solvent such as THF to increase methanol solubility in the phase containing the rapeseed oil. 

The preparation of lactones via intramolecular catalytic esterification can be carried out by the Tishchenko reaction of dialdehydes. Alkaline earth metal oxides have been shown to be active not only for the intermolecular Tishchenko reaction, but also for the intramolecular Tishchenko lactonization. Thus, these solid catalysts have been applied for the Tishchenko reaction of o-phthalaldehyde to phthalide 182) (Scheme 34). 

Alkaline earth metal oxides and hydroxides have also been tested in transesterification reactions. Ca(OH)2 did not show significant catalytic activity in the transesterification of rapeseed oil with methanol at conditions normally used to prepare biodiesel.Peterson et al. reported relative alcoholysis activities of a series of supported CaO catalysts under near reflux conditions of methanol-rapeseed oil mixtures at 6 1 molar ratios.Among the catalysts tested, the most active was CaO (9.2 wt% CaO) on MgO. For instance, in a 12 h reaction the total oil conversion using this catalyst was over 95%, similar to... 

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. BaO and SrO are prepared from the corresponding carbonates as precursor salts, whereas decomposition of hydroxides is frequently used to prepare MgO and CaO. Preparation of alkaline earth metal oxides with high surface areas is especially important when the oxide will be used as a basic catalyst, because the catalytic activity will depend on the number and strength of the basic sites accessible to the reactant molecules, which is dependent on the accessible surface area. 

Alkaline or alkaline earth metal oxides are well known basic catalysts and much of work has been devoted to their characterization and to the study of their catalytic activity [64,65]. Taking into account the strong basic character of these oxides it is possible to enhance the basicity of zeolites by over-exchanging them with alkali [66-69] or alkaline earth metals [70-72] and producing after thermal de-... 

Purity The purity of activated carbon is essential for the performance of the final catalyst. Impurities of activated carbon originate from the raw material and the process conditions. Ash contents of up to 20% can be possible. Wood-based activated carbons have ash contents as low as 1 wt% [7]. The ash content can be lowered further by acid treatment of the activated carbon [8]. Typically, the ash consists of alkaline and alkahne earth metal oxides, silicates, and smaller amounts of other compounds (e.g., iron). The presence of the alkaline and alkaline earth metal oxides makes those carbons more basic in nature, so that some additional adjustments are necessary during catalyst manufacturing to meet the constant quality requirements. Since the supports are used in catalysts, the presence of catalytically active compounds that could have a potential influence on the performance of the final catalyst has to be considered as well. For the manufacture of catalysts, activated carbon based on wood, peat, nut shells, and coconut are commonly used. Due to a relatively high sulfur content in activated carbons derived from coal, those carbons are typically not used as catalyst support. 

It is seen from Fig. 6.24 that the alkali metal hydroxide can be easily absorbed by carbon support due to its low melting point and its large fluidity. Therefore, only with enough quantity, the alkali metal can be accreted on the interface between ruthenium and carbon support and then plays the promotional roles effectively. As alkaline earth metal oxides have high melting point and poor fluidity, small amounts of them can be accreted on the interface between ruthenium and carbon support, which can produce effective active sites. The excessive promoters might cover the active sites of catalyst smface, which can influence the effective contact between active sites of ruthenimn smface and reactant gases and therefore decrease the catalytic activity. 

According to the Lewis theory, alkaline earth metal hydroxides are weaker bases than their oxides, the order of the strength of the basic sites being Ba(OH)2> SrO(OH)2 > Ca(OH)2 > Mg(OH)2. The hydroxides have been used recently as solid catalysts for organic transformations, such as the conjugate addition of methanol to a, S-unsaturated carbonyl compounds (12), cyanoethylation of alcohols (163,164), and transesterification reactions (166,167,171,172) which are described above. The extensive work of Sinisterra et al. (282) on the number and nature of sites and on the catalytic activity of the most basic alkali metal hydroxide, Ba(OH)2, is emphasized. It was found that commercial barium hydroxide octahydrate can be converted into... 

Tsuruya and co-workers (83,84) recently reported that addition of alkaline earth metals (e.g., Ca, Sr, and Ba) to an Ag/SiOi catalyst by a coimpregnation method enhanced the catalytic activity of the partial oxidation of benzyl alcohols into benzaldehydes, with production of only small amounts of byproducts (carbon dioxide, toluene, and benzene). The formation of carbonaceous material was thought to be inhibited by the alkaline earth metals, which also helps to disperse the metallic silver and facilitate oxygen adsorption. This effect causes the formation of an oxygenated silver surface that is generally believed to be responsible for the partial oxidation of benzyl alcohol. 

In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of dkali and alkaline earth metal doped ceria catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [ 13]. Zhang and Baems explained the observed dependence of selectivity on the Ca content in terms of oxygen-ion conductivity... 

The main cause of deactivation are elements or compounds which chemically attack the catalytically active material or its support. Also, structural changes and pore blocking are important issues of deactivation. A variety of poison compounds containing elements such as halogens, alkah metals, alkaline earth metals, arsenic, lead, phosphorus, and sulfur are mentioned in the hterature. AS2O3 is the most severe poison in coal-fired power plant operation in Germany. In power plants equipped with wet-bottom boilers alkah metal oxides mostly remain in the molten ash, whereas AS2O3 tends to escape into the flue gas and deposits on the catalyst. 

The conversions of HC, CO, and NOx on the Pd and Pd/Sr catalysts plotted as a function of X in simulated exhaust gases at 300°C are shown in Figs.l and 2, respectively. The catalytic activity on the Pd/Sr catalyst was superior to that on the Pd catalyst, in particular, under reducing conditions defined as A.<1. The conversion of HC, as representation of hydrocarbon oxidation activity, on the catalysts with alkaline earth metals and that with alkali metals plotted as a function of X in simulated exhaust gases are shown in Figs.3 and 4,... 

The alkaline earth metal addition to the Pd catalyst improved the hydrocarbon oxidation activity. Similar phenomena have been observed on Pd/Ba and Pd/La catalysts, and it is concluded that the suppression of hydrocarbon chemisorption on Pd by the addition of Ba or La allows the catalytic reaction to proceed smoothly under reducing conditions( 16,20). On the other hand, the alkali metal addition, especially K or Cs, to the Pd catalyst deteriorated the hydrocarbon oxidation activity. 

In the present work the behaviour of zirconia samples doped with oxides of alkali metals and alkaline-earth metals was investigated, in order to better understand the role of both the nature and the amount of the doping cation. Li-, K-, Ca-, and Ba-doped zirconia samples were prepared. Their surface acid-base properties were assessed by means of adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Their catalytic activity for the 4-methylpentan-2-ol dehydration was tested in a flow microreactor. 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

Other supports with different acidic characteristics and the addition of alkali or alkaline-earth metals and other promoters are used [2, 3]. For example, the addition of tin (Sn), which affects the amount of the oxidized chromium, results in a significant decrease of the amount of deposited coke. The Sn addition also improves selectivity, but, unfortunately, reduces catalytic activity [3]. Alternative catalysts are extensively investigated and some of them are already in commercial use (e.g., Pt-based). 

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