CO2 reforming of methane to syngas: I: evaluation of hydrotalcite clay-derived catalysts
Introduction
Many important chemical processes require syngas (H2/CO) in various H2 to CO ratios. Examples include the production of methanol, acetic acid, and Fischer–Tropsch synthesis (Kung, 1981; Eby and Singleton, 1983; Anderson, 1984; Courty and Chaumette, 1987). Syngas is usually made from natural gas (Rostrup-Nielsen, 1984) via steam reforming (Eq. (1)):
This process produces higher H2 to CO ratio (3:1) synthesis gas. Energy is supplied to drive this endothermic reaction by heating the reactor externally or by other means.
Another way of making syngas from methane is the direct oxidation of methane (Eq. (2)). This reaction is exothermic and H2 to CO ratio of the product is more desirable (2:1) for downstream process.
Steam or oxygen in both , may be replaced by CO2. The corresponding CO2 reforming reaction (Eq. (3)) is of industrial interest because of the low H2 to CO ratio in the product gas. CO2 reforming is practised in industry (Tenner, 1987; Stal et al., 1992). This process has also attracted interest as a CO2 consuming reaction (Ashcroft et al., 1991a, Ashcroft et al., 1991b). CO2 reforming may be combined with steam reforming or partial oxidation to achieve a desirable H2 to CO ratio for a downstream chemical process.
All the three syngas forming reactions discussed above are catalyzed by nickel catalysts supported on alumina, MgO or metal aluminates. A number of excellent studies on this subject are available (Dissanayake et al., 1991; Hickman and Schmidt, 1992; Rostrup-Nielsen and Bak Hansen, 1993; Blom et al., 1994; Zhang and Verykios, 1995; Hu and Ruckenstein, 1996). It is believed that the nickel particles grow in size as a function of time on stream. Larger crystallites of nickel promote coke formation causing catalyst deactivation. Sulfur-passivated CO2 reforming, as practised in the Haldor Topsoe SPARG process, solves the problem of carbon formation by nickel ensemble control which means that the sites for carbon formation are blocked while sufficient sites for the reforming reactions are maintained. This effect is obtained by adding a small amount of H2S to the process feed. The goal of this study was to slow down the nickel particle growth and hence the excessive coke formation without using hydrogen sulfide. For this, we have prepared and activated a series of NiMgAl hydrotalcite-type materials (Bhattacharyya et al., 1995a, Bhattacharyya et al., 1995b, Bhattacharyya et al., 1997a, Bhattacharyya et al., 1997b). We chose hydrotalcite-type materials because of their unique structure where the 2+ and 3+ metal ions are randomly distributed in a layered structure. The structure of hydrotalcite is very similar to that of brucite, Mg(OH)2. In brucite, each magnesium cation is octahedrally surrounded by hydroxyls. The resulting octahedron shares edges with neighboring Mg(OH)6 octahedra to form extended sheets having no net charge. In hydrotalcite, Mg6Al2(OH)16CO3⋅4H2O, some of the Mg2+ are replaced by Al3+ in the brucite sheet, resulting in a net positive charge on the clay sheets. The positively charged Mg–Al double hydroxide sheets (or layers) are charge-balanced by the carbonate anions residing in the interlayer sections of the clay structure. This hydrotalcite structure is schematically represented in Fig. 1.
The structure of hydrotalcite can accommodate wide variations in the Mg2+/Al3+ molar ratio, the type of interlayer anions, and different 2+ and 3+ cations (Feitknecht and Fischer, 1935; Frondel, 1941; Feitknecht, 1942; Miyata and Kumura, 1973; Miyata, 1975, Miyata, 1983; Miyata and Okada, 1977; Miyata, 1978; Van Olphen, 1977; Richle, 1986; Drezdzon, 1988; Thevenot et al., 1989; Bhattacharyya and Hall, 1992, Bhattacharyya and Hall, 1997; Clause et al., 1992; Bhattacharyya, 1993). A hydrotalcite-type clay may be represented by the following general formula:where M2+ and M3+ are metal cations, A is an anion, x=charge of the anion, n>m, and y=number of interlayer water molecules. The size and orientation of the interlayer anion determines the layer separation or interlayer spacing (Fig. 1). For this study we used Ni and Mg as M2+, Al as M3+ and CO32− as the interlayer anion. Since Ni ions are randomly distributed in the layered structure and somewhat insulated by Mg and Al ions, it was believed that nickel aggregation will be minimized in the clay-derived catalysts. We have examined the reforming activity of these hydrotalcite-derived catalysts containing various amounts of Ni (Bhattacharyya and Kaminsky, 1994; Bhattacharyya et al., 1995a, Bhattacharyya et al., 1995b, Bhattacharyya et al., 1997a, Bhattacharyya et al., 1997b). Here we report the preparation, thermal activation and evaluation of Ni and NiMg containing hydrotalcite-type materials for producing syngas from methane by CO2/steam reforming.
Section snippets
Catalyst preparation
Preparation of hydrotalcites such as Ni4Al2(OH)12CO3⋅4H2O, Ni2Mg2Al2(OH)12CO3⋅4H2O and NiMg5Al2(OH)16CO3⋅4H2O and their calcined compositions Ni4Al2O7, Ni2Mg2Al2O7 and NiMg5Al2O9, respectively are described elsewhere (Bhattacharyya et al., 1995a, Bhattacharyya et al., 1995b, Bhattacharyya et al., 1997a, Bhattacharyya et al., 1997b). The catalysts were reduced in situ with about 8 volume percent H2 and balance N2 at 850°C for 2 h. For severe pretreatment the catalyst was reduced by hydrogen
Results and discussion
A hydrotalcite-type material undergoes dehydration when heated. At about 200°C, the interlayer water leaves and at about 450°C, the layer hydroxides dehydrate. If the interlayer anion in the clay is carbonate, it also decomposes at about 400°C. This dehydrated material, however, retains the memory of the layered structure as hydration in the presence of a carbonate anion reconstitutes the layered structure. The surface area of the thermally activated material is usually higher than the original
Acknowledgements
We thank Tamila J. Barnes and Frank C. Witbrod for catalyst preparations, Carl A. Udovich for valuable suggestions, and Amoco for the permission to publish.
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