Abstract
Ethane steam cracking is the predominant method for industrial production of ethylene in the United States. However, the economy of this process is significantly impeded by the periodic shutdown due to coke deposition. Although catalytic coking facilitated by the metallic alloy surface can be effectively suppressed by the barrier oxide layers formed upon oxidative pretreatment, radical coking still occurs at the high temperatures applied for steam cracking. To develop catalytic barrier oxide layers that further relieve radical coke accumulation through in situ steam gasification of coke under steam cracking environments, the activity and stability of various spinel oxide components from the barrier oxide layers in steam reforming of olefins and aromatics, which are model reactions for steam gasification of coke, were investigated.
Powder samples of Mn-Cr-O spinel oxides (MnxCr3-xO4) as well as single oxides of Cr and Mn that are representative of the conventional MnCr2O4/Cr2O3 protective oxide layers were synthesized and characterized. Results from X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) indicated that excess Mn in Mn1.5Cr1.5O4 exists as Mn3+ that partially substitutes the Cr3+ in the octahedral sites of the spinel lattice, whereas Cr separates into Cr2O3 when in excess. A single oxide of Mn (Mn3O4) showed the highest steam reforming rate for both ethylene and toluene at 873 K, but deactivated due to reduction to MnO. The Mn-Cr-O spinel catalysts and Cr2O3 were structurally stable under these reforming conditions. While Cr2O3 was initially more active for olefin reforming than the spinel catalysts, it deactivated rapidly due to coke deposition. The Mn-rich Mn1.5Cr1.5O4 spinel catalyst exhibited both the highest reforming rate and stability during steam reforming of aromatics, potentially attributed to the excess Mn3+ sites stabilized by the spinel lattice.
The steam reforming rate of olefins and aromatics was nearly first order in the individual hydrocarbon and slightly inhibited by H2O. The reforming kinetics are explained by a Mars-van Krevelen type mechanism, in which the inhibition by H2O is attributed to the competitive adsorption between hydrocarbon and H2O. While excess H2 did not affect the reforming rate of olefins and benzene, the rate of toluene reforming was strongly inhibited by excess H2 resulting in negative first order kinetics. Results from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of toluene adsorption on the spinel surface suggest H2 likely inhibits the oxidation of the highly reactive side methyl group of toluene. Consistent with the observed negative first order in H2, toluene steam reforming showed the highest apparent activation energy compared to olefins and benzene.
Powder catalysts of Ni-based spinel oxides (NiM2O4) that are present in the Al2O3 protective oxide layers of the Al-enhanced cracker alloy were also synthesized and characterized. Results from temperature-programmed reduction in H2 (H2-TPR) indicate the reduction of Fe3+ in NiFe2O4 accelerated the reduction of Ni into Ni-Fe alloy particles for that sample, whereas NiAl2O4 and NiCr2O4 reduced to Ni metal at much higher temperatures. Consistent with this observation, the spinel structure of NiFe2O4 degraded to Ni-Fe alloy particles that severely coked during ethylene steam reforming at 873 K. While both NiAl2O4 and NiCr2O4 remained structurally stable, the ethylene reforming rate and the amount of coke deposited increased substantially for both catalysts after re-oxidation. These findings suggest a detrimental role of Ni-based spinel oxides for the barrier oxide layers, since the oxidative pretreatment for coke removal likely also facilitates the agglomeration of small Ni metal particles into NiO aggregates, which are more prone to reduction and catalyze coke deposition when exposed to a reducing environment like steam cracking.
Finally, two Co-Cr-O spinel oxides (CoxCr3-xO4) were synthesized and characterized to explore the ethylene steam reforming activity of Co spinel oxides. The XRD and XPS results suggest the excess Co in Co1.5Cr1.5O4 exists as Co3+ that likely resides in the octahedral sites to replace Cr3+ by analogy to Mn1.5Cr1.5O4. The Co2+ sites in stoichiometric CoCr2O4 were reduced to Co metal at high temperatures. The reduction of Co1.5Cr1.5O4 with excess Co likely occurred in a stepwise manner with the reduction of Co3+ to Co2+ occurring at lower temperatures prior to reduction of Co2+. Both CoCr2O4 and Co1.5Cr1.5O4 were structurally stable during ethylene steam reforming at 873 K. The reforming rate over Co1.5Cr1.5O4 was one order-of magnitude higher than that over CoCr2O4 and was proposed to be attributed to the excess Co3+ sites. The steady-state ethylene reforming rate remained unchanged after re-oxidation for both CoCr2O4 and Co1.5Cr1.5O4, suggesting a better stability of Co-Cr-O spinel oxides against reduction to metal particles compared to the Ni-based spinel oxides under identical reforming conditions.
