Бурение грунтовых зондов, установка энергетических колодцев

It is well established that the reduction of CO by hydrogen to yield ethanol can be greatly enhanced by the addition of promoters [25,26]. A variety of metal ions from alkali metals, first group tran­sition metals to rare earths have been used as promoters, and one or more of these metal oxides can be used in combination with Rh on a silica, alumina, titania or zirconia support. Selected examples of promoter-modified Rh catalysts used in the synthesis of ethanol from syngas are shown in Table 13.2.

The promoter action of rare earth oxides (La2O3, CeO2, Pr2O3, Nd2O3, and Sm2O3) in the Rh-catalyzed conversion of syngas to ethanol has been studied by Yu-Hua and coworkers using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). They found that ethanol production selectiv­ity can be increased by the addition of rare earth oxides, espe­cially CeO2, and Pr6O11. High temperature reduction of the catalyst

favored selectivity for ethanol in all rare earth oxides promoted rhodium catalysts. XPS measurements revealed that CeO2, in Rh-CeO2/SiO2 mostly exists as Ce2O3 after reduction, in contrast to that in rhodium-free CeO2/SiO2. From these experiments Yu-Hua and coworkers concluded that rhodium assists in the reduction of CeO2, and furthermore, they reported a correlation between the selectivity for ethanol and the reducibility of rare earth oxides in these promoter-assisted CO reductions [26].

In a more recent study [38] electronic properties of oxide promot­ers in Rh-catalyzed selective synthesis of oxygenates from synthesis gas has been investigated. In this study a series of promoter oxides (M = Fe, V, Nb, Ta, Ti, Y, Pr, Nd, Sm) were used and a broad range of products were observed in the reduction reactions. Interestingly they have been able to correlate the selectivity parameter with the electronic properties of the MOx promoters (i. e., electron-donating/ electron-withdrawing capacity) for an extensive series of catalysts. Low-temperature and at-work CO-FTIR experiments suggested that the high activity and hydrocarbon selectivity displayed by cat­alysts was promoted by more electron-withdrawing (acidic) oxide promoters (e. g., TaOx). These activity enhancements were related to a higher proportion of bridged Rh2(CO)B adsorption sites and to a higher electron density (i. e., a higher electron back-donation ability) of the Rh0 surface sites, both factors promoting CO disso­ciation events. In contrast, linear CO adsorption on Rh0 sites dis­played decreased electron back-donation in catalysts promoted by electron-donating (basic) oxides (e. g., PrOx, SmOx). This was likely related to nondissociative CO activation and thus to the selective formation of oxygenates. TEM, XPS, and CO-FTIR results pointed to differences in morphology, rather than size or partial electronic charge, of the nanosized Rh0 crystallites as the likely cause for the different proportions of CO adsorption sites.

Alkali metals like Li, Na, K, and Cs are also known to act as pro­moters in the CO hydrogenation. These alkali promoters enhance oxygenate formation by suppressing the hydrogenation activ­ity of Rh catalyst [25]. Nevertheless, according to the mechanism shown in Figure 13.2, C2+ oxygenate formation also involves the hydrogenation of the intermediates. Therefore, the alkali promo­tion is effective only if the hydrogenation suppression decreases the formation of methane more than that of C2+ oxygenates as seen in the proposed mechanism. In a comparison study, Wender has tested a series of alkali promoters on Rh catalyst supported on TiO2 and reported that their ability to enhance selectivity to oxygenates increased in the order of unpromoted < Li < K = Cs, while over­all CO conversion decreases in the order of unpromoted > Li > K > Cs. Later, Spivey’s group reported [27] that the addition of 0.10 wt% Li to Rh supported on TiO2 more than doubled the CO con­version for CO hydrogenation, while increasing ethanol selectiv­ity. They found that addition of Li also increases formation of C2 oxygenates at the expense of C1 species methanol and methane. This is attributed to enhanced dispersion of Rh by Li that appears to reduce dissociation of CO, which previous studies have shown to require large ensembles of Rh atoms on the surface. Further, they suggested that Li promotion appears to increase the associatively adsorbed CO, allowing for increased H2 chemisorption on the sur­face compared to the dissociative adsorption of the same number of CO atoms [27].

The use of more than one metal ion or multiple promoters is a common feature in a number of recent studies. Another publication from Spivey’s group reported [35] the synergistic effect of using two promoters, namely La and V. The data from the experiment demonstrating the selectivity enhancement in using a mixture of two promoters is shown in Table 13.3. The use of single promoter gave 25.6 and 31.8% ethanol selectivities for V and La, respectively, whereas using a mixture of V and La increased the selectivity to 39.0% as shown in Table 13.3.

Furthermore, in this study [35] they compared the effect of tem­perature, H2/CO ratio, space velocity, and pressure on ethanol selectivity in a Rh/SiO2 catalyst system. During these experiments the highest ethanol selectivity achieved was 51.8% at a CO conver­sion of 7.9%, with a corresponding methane selectivity of 15.4% at 270°C, 14 bar and H2/CO = 2 over the Rh-La/V/SiO2 catalyst. Additionally, they found that combined La/V promotion reduces methane selectivity and increases C2+ oxygenates selectivity com­pared to the singly promoted catalysts by increasing the rate of CO insertion. Contrary to earlier studies, higher pressures led to a dramatic increase in methane selectivity at the expense of etha­nol, indicating increased CO dissociation activity at higher pres­sures, leaving fewer active CO molecules for insertion. The chain growth factor (a) for higher oxygenates differed significantly from that for hydrocarbons, suggesting that formation of these two types of products either proceed by different mechanisms or on different active sites [35].

The highest reported ethanol selectivity for a Rh catalyst on a silica support was found in a 2007 publication from Hu’s group at Pacific Northwest National Laboratory in the United States [31]. In this work Rh-Mn/SiO2 (powdered, 70-100 mesh) coated on a FeCrAlY metallic felt substrate was used as the catalyst, which was subsequently integrated into a microchannel reactor. Ethanol selectivity of 56.1% was reported at T = 280°C, P = 5.4 MPa, and H2/CO = 2:1, with a 24.6% CO conversion (mol%) [31]. This high (56.1%) ethanol selectivity could be attributed to unique design of the microchannel reactor and enables highly exothermic CO hydro­genation reaction to be operated in an isothermal mode to achieve high productivity. A comparison list of high ethanol selectivities in the reduction of CO using Rh-based catalyst supported on silica are shown in Table 13.4.

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