Continuous selective oxidation of methane to methanol over Cu- and Fe-modified ZSM-5 catalysts in a flow reactor
Graphical abstract
Introduction
Natural gas has been hailed as a bridging feedstock for society's transition away from a petroleum-dependent economy [1]. However, the direct catalytic upgrading of its principal components; methane and ethane, to oxygenated products has yet to be realised under mild, environmentally benign conditions. Whilst processes for conversion of methane to higher value products have been commercialised, high substrate stability (ΔHC–H = 439.57 kJ mol−1) means that harsh conditions are often employed for activation [2], [3]. Direct upgrading of methane via oxidation to a more energy dense product, such as methanol, is therefore an attractive prospect. Such a process must operate under mild reaction conditions to prevent further oxidation of methanol to products like formic acid and CO2. Several approaches to methane oxidation have been reported. The enzymatic direct oxidation of methane to methanol has been studied, with metalloenzymes shown to perform this reaction selectively under mild conditions with molecular oxygen as the terminal oxidant [4], [5]. This has stimulated the search for synthetic analogues of these enzymes’ active sites [6], [7]. Indeed, the gas phase oxidation of methane over Cu-modified zeolites activated in O2 [8], [9], [10], [11], [12], [13], [14] or NO/N2O [15] has also been studied, with proposed active sites for the O2 treated catalysts not unlike the binuclear Cu site found in methane monooxygenase enzymes. Meanwhile non-catalytic activation using N2O/O2 treated Fe/ZSM-5 has also been reported (with α-oxygen as the in situ generated oxidant) [16], [17], [18]. Periana and co-workers have extensively studied the oxidation of methane in acidic media-yielding methyl esters at temperatures of <200 °C [19], [20], [21]. These indirect, methyl ester-yielding processes typically require homogeneous metal complexes which activate methane through electrophilic attack of the CH bond, affording high reaction selectivity yet incurring additional hydrolysis steps to yield methanol. Such homogeneous processes have been extensively reviewed by Periana et al. and Shilov et al. [22], [23] and heterogenised by Schüth and co-workers [24], [25]. A number of homogeneously [26], [27], [28] and heterogeneously [29], [30], [31], [32] catalysed aqueous processes for the direct oxidation of methane with H2O2 have recently been reported. These benefit from the clean decomposition of the oxidant to H2O as an environmentally benign byproduct. The importance of efficient, selective utilisation of methane as a feedstock for the synthesis of bulk chemicals is explored in recent reviews of the catalytic upgrading of methane [33], [34], [35], [36], [37]. Unfortunately despite extensive research in the field, no approach has yet been deemed commercially viable, with methanol still produced through an energy intensive two-step process which proceeds via synthesis gas.
It has previously been reported that ZSM-5 materials containing trace amounts of iron (as dimeric μ-oxo-hydroxo iron species) can catalyse the direct conversion of methane and ethane to oxygenated products, utilising hydrogen peroxide as the oxidant [31], [32], [38], [39], [40]. The oxidation of methane to methanol was shown to proceed via formation of methylhydroperoxide (CH3OOH), and deep oxidation to formic acid and CO2 was observed [31]. Appreciable methane conversion (10%) and high oxygenate selectivity (>90%) have been reported at temperatures as low as of 50 °C. Furthermore, incorporation of Cu2+ into the reaction as either a homogeneous additive or heterogeneous component of the zeolite catalyst allows tuning of reaction selectivity to favour methanol (>85%) as the major product. Catalytic reaction pathways determined for the oxidation of methane with H2O2 are shown in Scheme 1 [31]. Previous studies have suggested that the intrinsic activity of ZSM-5 is derived from the presence of octahedral (extra framework) Fe species, formed during high temperature activation of the zeolite. The role of Cu2+ in effecting high primary product selectivity has been studied, and is attributed to catalytic termination of hydroxyl radicals [41].
In this paper we aim to translate the catalyst system from operation in a batch autoclave to a continuous flow reactor in order to further study catalyst deactivation and determine whether high selectivity to methanol might be achieved under mild reaction conditions.
Section snippets
Catalyst preparation
Fe and Cu were impregnated onto ZSM-5 (Zeolyst, SiO2/Al2O3 = 23, 30 or 80) via chemical vapour impregnation (CVI) according to the procedure previously reported [39], [42]. NH4-ZSM-5 was calcined in a flow of air (550 °C, 20 °C min−1, 3 h) to yield H-ZSM-5. This was then either (i) activated in static air (3 h, 550 °C) and tested without further modification or (ii) modified through chemical vapour impregnation. The procedure for simultaneous impregnation of ZSM-5 with 1.5 wt% Fe and 1.5 wt% Cu follows;
Characterisation of catalysts
It has previously been reported that trace iron in ZSM-5, present as cationic μ-oxo-hydroxo species located at exchange sites, can catalyse the oxidation of methane to methanol by H2O2 [31], [41]. Increased catalyst productivity can also be achieved through post-synthesis addition of Fe to ZSM-5 (30) through various techniques [31], [42]. One post-deposition method which has been successfully applied is CVI, a solvent and halide-free vapour deposition technique, which can be applied in the
Conclusions
It has been demonstrated that bimetallic FeCu/ZSM-5 effectively catalyses the direct oxidation of methane to methanol under continuous flow conditions. Mild reaction conditions and a green oxidant are used without additional promoters or pH control. Reaction selectivity is high towards the primary oxidation products with limited over oxidation (>73% methanol selectivity), showing low sensitivity to experimental variables. It has been shown that maximising the ion exchange capacity of ZSM-5
Acknowledgements
We thank Cardiff University and The Chinese Academy of Science for their financial support.
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