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Abstract:
Catalytic oxidation of ethane is studied intensively in the scientific literature, but it is still far from an industrial approach. Among the possible catalysts supported vanadium oxides are very attractive because they are active already at temperatures less than 600 °C. The generalized
reaction network consist of the parallel formation of ethylene and carbon oxides and the consecutive over-oxidation of ethylene to carbon oxides [1,2,3]. However, at a more detailed level, distinguishing between CO and CO2, the pathways are still under intensive discussion [4,5,6]. In a previous paper [7] we proposed a network consisting of five partial reactions, the oxidative dehydrogenation of ethane (1), the direct oxidation of ethane to CO2 (2), the
consecutive oxidation of ethylene to CO (3) and CO2 (4) and finally, the consecutive oxidation of CO to CO2.
C2H6 + 0.5 O2 → C2H4 + H2O (1)
C2H6 + 3.5 O2 → 2 CO2 + 3 H2O (2)
C2H4 + 2 O2 → 2 CO + 2 H2O (3)
C2H4 + 3 O2 → 2 CO2 + 2 H2O (4)
CO + O2 → CO2 (5)
The scope of this contribution is to analyze how the different partial reactions are influenced by the catalyst properties. For this reason several series of catalysts were prepared by the impregnation of γ-alumina (BET surface area 200 m²/g) with VO(acac)2 in acetone (series A) or with V2O5 in a mixture of ammonia, water and acetone (series B), followed by subsequent calcination under air at 650 °C for 8 h. Catalysts were characterized by AAS, XRD, BET/N2 porosimetry, XPS, thermal analysis and TPR/TPO. Catalytic tests were performed in a fully automated laboratory fixed bed reactor plant varying temperature between 400 and 650 °C at two W/F levels and two different initial oxygen concentrations. The same program was repeated for ethylene and CO oxidation to characterize the conversion of these intermediates in the reaction network. Checking for reactant product interactions, additional experiments were performed for the oxidation of ethane and ethylene by CO2.
XRD patterns show that first V2O5 reflexes appear 9.1 % V for catalysts from A series (Fig. 1) and 5.5 % V for catalysts from B series (Fig. 2). This means, that by the catalyst preparation from VO(acac)2 a higher V dispersion can be reached than by the inorganic route. This conclusion is supported by the results from the BET measurements, where the surface drop caused by pore clogging with V2O5 is shifted to higher V loadings (Fig. 3). For the fresh catalysts (presently only A series was analyzed) XPS results show an increase of the average V oxidation state from 4.3 to 4.8 with increased V loading. V dispersion also influences significantly reducibility catalysts, in the TPR data plots (Fig. 4, presently available only for A series) there can be distinguished four different reduction peaks, attributable to isolated (200 °C) and the different polymeric vanadate species (250-350 °C) and to bulk-like V2O5 (>350 °C). The H/V ratio measured after repeating several reduction-reoxidation cycles shifts from 1.3 for 1.7-6.1% V to 1.8 for the higher loaded samples. This means that in contrast to V2O5, vanadate species do not undergo the full redox cycle between V(V) and V(III), a significant vanadate part remains at +4 after re-oxidation (TPO). Also in TPO dispersed vanadite and bulk-like V2O3 can be clearly distinguished.
Activity measurements are not finished yet, but at the present state, with oxygen as the oxidant initial ethylene selectivity between 60 and 80 % was observed and the maximum ethylene yield is higher than 20 % (W/F = 0.273 g s ml-1). The results showed that for oxygen shortage conditions further reactions should be added to the network: the oxidation of ethane to ethylene by CO2 (6), the oxidation of ethylene by CO2 (7), ethylene pyrolysis yielding soot and finally, the Boudouard equilibrium reactions. For CO2 as the oxidant ethylene yields can reach similar values, ethylene selectivity is around 80 %.
C2H6 + CO2 → C2H4 + CO + H2O (6)
C2H4 + 4 CO2 → 6 CO + 2 H2O (7)
The financial support of the DFG (Forschergruppe 447, “Membranunterstützte Reaktionsführung” is acknowledged.
REFERENCES
[1] M. A. Banares, Catal. Today, 51 (1999), 319
[2] M. Baerns, O. V. Buyevskaya, Petrolchemie, 116/1 (2000), 25
[3] F. Cavani, F. Trifirò, Catal. Today, 51 (1999), 561
[4] N. F. Chen, K. Oshihara, W. Ueda, Catal. Today, 64 (2001), 121
[5] S. T. Oyama, A. M. Middlebrook, G. A. Somorjai, J. Phys. Chem. 94 (1990), 5029
[6] M. P. Casaletto, L. Lisi, G. Mattogno, P. Patrono, G. Ruoppolo, G. Russo, Appl. Catal. A, 226 (2002), 41
[7] F. Klose, M. Joshi, C. Hamel, A. Seidel-Morgenstern, Appl. Catal. A: General 260 (2004) 101
