Catalytic Reforming of Methane in the Presence of CO2 and H2O at High Pressure

Exploiting the greenhouse gas CO2 as raw material could be more advantageous than its long-term storage. For instance, CO2 can be used on large scale for the production of COrich syngas with a H2/CO ratio of unity. Such a syngas can be used for the synthesis of commodity chemicals like aldehydes and ketones obtained through carbonylation of alkenes in hydroformulation and can be produced by CO2 reforming of methane. The actual CO2 reforming processes are conducted at low pressure (below ca. 7 bar), which is not suitable if the upstream as well as the downstream processes are operated at high pressures (typically the case in petrochemical platforms). The investigations conducted in this study consist of the development of active catalysts as well as a process concept for the valorization of CO2 within the CO2 reforming of methane under high pressure (21 bar) in order to achieve a CO-rich syngas with a H2/CO ratio of one. Simulations of reforming equilibria highlights the fact that water is always produced in the CO2 reforming of methane; therefore, CO2 reforming is conducted with addition of the amount of water produced at the thermodynamic reforming equilibrium to enable coke gasification on the catalyst with water. The use of high pressures above 10 bar, high temperatures above 1173 K (900°C), and high partial pressure of methane leads to significant impact of the gas-phase reactions and the formation of coke precursors, like ethylene. The addition of more than 30 vol.% H2 in the feed stream can significantly limit the formation rate of coke precursors whereas the addition of water shows no particular influence on coke-precursor formation rates. Various Pt-based catalysts are studied with regard to the dependence of activity and stability in steam reforming with various zirconia supports. The Pt/ZrO2 catalyst exhibits strong deactivation under steam reforming due to a fast decrease of its specific surface area. The modification of the support of this Pt/ZrO2 with silica is likely to form Pt particles embedded in silica, resulting in a low activity in steam reforming. On the other hand, the performance of a Pt-based catalyst supported on ZrO2, which is modified with 10 wt.% La2O3 and 6 wt.% CeO2, exhibits an enhanced stability and activity when used close to the thermodynamic equilibrium of steam reforming of methane under 21 bar and at 1123 K (850°C). In a kinetic study, this Pt-based catalyst, tested in steam reforming in the absence as well as in the presence of CO2, shows that at 1023 K (750°C), atmospheric pressure, and a GHSV (Gas Hourly Space Velocity) above 33000 h−1 the activation of methane is performed with steam while CO2 reacts with H2 and/or methane. In addition, the TOFs (Turn Over Frequency) of methane conversions are found to be higher as the steam content in the feed increases, while the TOFs of CO2 conversions increase as a higher CO2 content in the feed is used. Furthermore, the TOFs of methane conversions are always superior to the TOFs of CO2 conversions. The Ni-based spinel catalyst prepared by molten-salt impregnation of hydrotalcite-like carrier with the stoichiometry of Mg spinel (Mg/Al=1/2) exhibits, after final calcination at 1223 K (950°C) for 4 h, the formation of mixed oxides, (Ni/Mg)O and (Ni/Mg)Al2O4, with a relatively high specific surface area (42 m2.g−1). After preformation in reducing atmosphere above 1073 K (800°C), Ni nanoparticles highly dispersed on MgAl2O4 are produced. This preformation step is found to be very sensitive to the presence of 10 vol.% H2O or 5 vol.% CO2, which inhibits the preformation mechanism. Compared to the samples prepared by precipitation and spray drying, the molten-salt impregnation performed by using a hydrotalcite-like carrier displays a much higher specific surface area and a much smaller crystallite size of both mixed oxides (< 20 nm). In CO2 reforming of methane with addition of 10 vol.% H2O at 1223 K (950°C) and 21 bar, the molten-salt-impregnated catalyst with a corundum prebed can achieve stable performances with CH4 and CO2 conversions 3-4 % below the thermodynamic equilibrium of reforming after a long preformation using the H2 test protocol. In comparison, the use of the H2O test protocol leads to the inhibition of the catalyst preformation, and performances are far from thermodynamic equilibrium, with methane conversions below 40%. The performance of Ni hexaaluminate as catalyst in CO2 reforming strongly depends on the choice of the mirror plane cation as well as the degree of substitution of Al3+ with Ni2+ in the hexaaluminate lattice. In addition, moderate calcination of hexaaluminate samples at 1473 K (1200°C) permits retention of the high specific surface area of 15 m2.g−1 necessary for the stabilization of the metallic Ni nanoparticles. Only Sr,Ni and Ba,Ni hexaaluminates are highly active, while the Ni loading should be kept below the sensitivity threshold of y=0.25 in SrNiyAl12−yO19−δ and in BaNiyAl12−yO19−δ to maintain the high dispersion of metallic Ni nanoparticles that exhibits highly stable performances under CO2 reforming of methane at 1123 K (850°C) and moderate pressure (11 bar) for 10 h.