Methane, the simplest alkane, is the principal component of natural gas, which is one of the cheapest and most abundant sources of hydrocarbons. Its direct conversion to an oxygenate, such as methanol, a large-scale product in the chemical industry, is an exciting goal. However, the low chemical reactivity of methane and the tendency of methanol towards oxidation to carbon oxides mean that the direct oxidation of methane with oxygen in one step is challenging, if not impossible.[1] One of the approaches to the selective oxidation of methane to methanol is so-called “chemical looping”, implying cyclic exposure of the material possessing redox properties to an oxidant and methane at different temperatures with the subsequent extraction of the oxidation products.[2-5] Despite multiple studies in this field and the partial identification of the active species and reaction mechanism, it has not been possible to design the best material and determine the ideal process conditions for industrial implementation. Our present study shows how to design a significantly better performing material and the reaction parameters to enhance the methanol yield. This led to the highest methanol productivity to date, using a material that until recently was considered to be inactive, opening a novel direction for the design of active materials.

We performed a systematic study of the copper-exchanged zeolites with MOR, MFI, BEA and FAU structures with similar Si/Al ratio and copper loading in the conversion of methane to methanol. FTIR spectroscopy of adsorbed nitrogen monoxide and molecular hydrogen determined the copper speciation, while the redox properties of the materials were evaluated by means of the temperature-programmed reaction (TPR) with methane, monitored by mass spectrometry and in situ X-ray absorption spectroscopy. The redox properties of the Cu(II)-oxo sites in copper-exchanged zeolites, governed by the zeolite framework and copper speciation, correlate with activity and selectivity in the direct synthesis of methanol from methane. Such redox properties can be extracted with ease from TPR experiments and then serve as a guide for selecting the conditions when testing new materials and predicting the optimal temperature for activation of methane. The data on the copper-oxo reducibility can be used to optimize the reaction conditions to achieve the highest methanol yield. In this way, it was established that the reaction of CuFAU(Si/Al = 3) with methane in an isothermal regime at 633K and 15 bar leads to a methanol yield of 360 µmol/g, which is the highest methanol yield achieved over copper-exchanged zeolite in one cycle.

The discovered process answers needed advancement for industrial feasibility of the direct conversion of methane to methanol and provides a route for further optimization of the reactive material and reaction conditions.

[1]   M. Ravi, M. Ranocchiari, J. van Bokhoven, Angew. Chem. Int. Ed., 2017, 56, 16464-16483. [2]  M. H. Groothaert, P. J. Smeets, B. F. Sels, P. A. Jacobs, R. A. Schoonheydt, J. Am. Chem. Soc., 2005, 127, 1394. [3]    J. S. Woertink, P. J. Smeets, M. H. Groothaert, M. A. Vance, B. F. Sels, R. A. Schoonheydt, E. I. Solomon, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 18908.  [4]    S. Grundner, M. A. C. Markovits, G. Li, M. Tromp, E. A. Pidko, E. J. M. Hensen, A. Jentys, M. Sanchez-Sanchez, J. A. Lercher, Nat. Commun., 2015, 6, ncomms8546.   [5]   V. L. Sushkevich, D. Palagin, M. Ranocchiari, J. A. van Bokhoven, Science, 2017, 356, 523.   [6]  V. L. Sushkevich, D. Palagin, J. A. van Bokhoven, Angew. Chem. Int. Ed., 2018, 57, 8906-8910.