Support and promoter effects in the selective oxidation of ethane to acetic acid catalyzed by Mo-V-Nb oxides

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Abstract

Catalysts based on Mo-V-Nb oxides were examined in bulk and supported forms for the oxidation of ethane to ethene and acetic acid. Bulk Mo0.61V0.31Nb0.08Ox powders showed rates and selectivities similar to those in previous reports. Precipitation in the presence of colloidal TiO2 led to a 10-fold increase in ethene and acetic acid rates (per active oxide) without significant changes in selectivity relative to unsupported samples. Precipitation in the presence of colloidal ZrO2 and Al2O3 suspensions, however, introduced unselective combustion sites without improving ethane oxidation rates. Mo5O14 structures, containing low-valent metal cation centers were detected in bulk Mo0.61V0.31Nb0.08Ox and TiO2-supported samples by Raman and UV–visible spectra and consistent with X-ray diffraction patterns, but not in Al2O3- or ZrO2-containing catalysts. The introduction of trace amounts of Pd (0.0025–0.01 wt.%), as a physical mixture of separate 0.3 wt.% Pd/SiO2, led to the near complete depletion of ethene intermediates and to a significant increase in acetic acid synthesis rate. Small PdOx catalyze ethene oxidation to acetaldehyde, but require the rapid scavenging of these molecules by Mo-V-Nb oxides to prevent acetaldehyde combustion and loss of selectivity. Dispersed VOx domains on TiO2 were able to catalyze all steps required for ethane oxidation to acetic acid. COx selectivities, however, were much higher than on bulk and TiO2-supported Mo0.61V0.31Nb0.08Ox catalysts. Dispersed MoOx domains were essentially inactive at these reaction conditions but their concurrent presence with VOx increased acetic acid selectivity by titrating unselective sites and stabilizing more reducible VOx species.

Graphical abstract

The rate and selectivity of acetic acid synthesis from ethane-O2 reactants were markedly increased by depositing active Mo0.61V0.31Nb0.08Ox onto colloidal TiO2 and combining them with a Pd/SiO2 co-catalyst (≤0.01 wt.% Pd). TiO2 leads to active structures with higher area but surface reactivity similar to that on bulk powders. PdOx species catalyze selective oxidation of ethene intermediates to acetic acid.

Introduction

Methanol carbonylation catalyzed by Rh and Ir organometallic complexes (BP-Monsanto Cativa process) and iodide as co-catalysts is the most widely practiced approach for the synthesis of acetic acid [1]. The presence of toxic and corrosive iodide species, the use of expensive and difficult to recover noble metals, and the high CO partial pressures required have led to a search for alternate routes. The synthesis of acetic acid via selective oxidation of ethane [2], ethene [3], [4] or ethanol [5], [6], [7], [8] on metal oxide domains provides such alternate routes.

Selective oxidation of ethane to ethene and acetic acid was reported by Thorsteinson et al. [2] on mixed metal oxide catalysts containing Mo, V, and another element (Nb, Sb, Ti, Ta, Sn, As, W, Fe) at relatively high pressures (0.5–2.0 MPa) and temperatures (550–600 K). Vanadium phosphate catalysts (VPO), typically used for butane oxidation to maleic anhydride, also form acetic acid from ethane [9], [10], [11], [12], but with significant selectivity to COx byproducts. Polyoxometallate clusters act as mere precursors for ethane oxidation catalysts, because they decompose to ill-defined mixed oxides at the temperatures and pressures required for practical acetic acid yields from ethane oxidation, and give low acetic acid productivities (<100 g/kg-cat-h) [13], [14]. Partially decomposed molybdovanadophosphoric acid clusters exchanged with pyridine also catalyze ethane oxidation to acetic acid albeit with relatively low rates and yields [15], [16]. A summary of these previous studies is included in Table 1.

The simple synthetic protocols, high acetic acid yields, and excellent stability of mixed Mo-V-Nb oxides have led to several previous publications and patents. The co-precipitation of noble metals (e.g., Re [17], Pd [18], [19]) with Mo-V-Nb oxides increased acetic acid synthesis rates and shifted ethene selectivities towards acetic acid. Previous attempts at supporting Mo-V-Nb oxides on α-Al2O3 [2], [20] and on SiO2-TiO2 [21] were unsuccessful at increasing the surface area of the active phase or increasing acetic acid productivities over those observed on bulk mixed oxide catalysts (Table 1).

Here, we report Mo-V-Nb oxide catalysts structurally promoted by introducing titania during precipitation. These materials give unprecedented acetic acid synthesis rates when promoted with trace amounts of Pd, present as a physical mixture in the form of a separately prepared Pd/SiO2 catalyst. Even after many studies, the compositional and structural complexity of bulk Mo-V-Nb oxides has prevented the unequivocal elucidation of the specific function of each component in these oxidation catalysts. In view of this, we also examine here the specific functions of Mo and V components present as dispersed MoOx and VOx and their mixtures on various supports. We find that dispersed VOx domains can activate ethane and convert it to acetic acid, while MoOx by itself is essentially unreactive.

Section snippets

Synthesis of mixed Mo-V-Nb oxides (Mo0.61V0.31Nb0.08Ox)

Mo0.61V0.31Nb0.08Ox powders were prepared using a slurry method [2]. A solution containing C4O8NbOH·NH3 (ammonium niobate(V) oxalate hydrate; 2.42 g; Aldrich; 99.99%) was added drop-wise to another solution containing C2O4H2 (oxalic acid; 7.2 g; Fluka, 99%), NH4VO3 (ammonium (meta)vanadate; 3.63 g; Sigma–Aldrich, 99%) and (NH4)6Mo7O24·4H2O (ammonium heptamolybdate tetrahydrate; 10.77 g; Aldrich, 99.98%) at ambient temperature while stirring. The water was then evaporated at 363 K while stirring

Catalyst characterization

Measured surface areas and VOx surface densities (calculated from V contents and BET areas) are shown in Table 2. Mo0.61V0.31Nb0.08Ox gave modest surface areas (7.8 m2 g−1), suggesting that only a small fraction of the active components are accessible to reactants.

γ-Al2O3 and ZrO(OH)2 supports have surface areas of 148 and 378 m2 g−1, respectively and TiO2 has a surface area of 49 m2 g−1. Surface areas for all supported samples were slightly smaller than for the fresh supports, but only because of

Conclusions

The catalytic activity of ethane oxidation to ethene and acetic acid on multi-component oxide, Mo0.61V0.31Nb0.08Ox, was enhanced by the structural dispersion of active oxides on TiO2; more than 10 times higher reaction rates were achieved with similar selectivities to all the products, ethene, acetic acid and COx. However, negative effects were observed when supporting on Al2O3 and ZrO2 because of the introduction of unselective linkages which encouraged the combustion reactions of ethene

Acknowledgements

We acknowledge with thanks the financial support by ExxonMobil Research and Engineering Co. and helpful technical discussions with Dr. Nan Yao.

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