catalysts Article Reaction Mechanism for Methane-to-Methanol in Cu-SSZ-13: First-Principles Study of the Z2[Cu2O] and Z2[Cu2OH] Motifs Unni Engedahl 1,2,∗ , Adam A. Arvidsson 1,2 , Henrik Grönbeck 1,2 and Anders Hellman 1,2,∗ 1 Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden; [email protected] (A.A.A.); [email protected] (H.G.) 2 Competence Center for Catalysis, Chalmers University of Technology, 412 96 Göteborg, Sweden * Correspondence: [email protected] (U.E.); [email protected] (A.H.) Abstract: As transportation continues to increase world-wide, there is a need for more efficient utilization of fossil fuel. One possibility is direct conversion of the solution gas bi-product CH4 into an energy-rich, easily usable liquid fuel such as CH3OH. However, new catalytic materials to facilitate the methane-to-methanol reaction are needed. Using density functional calculations, the partial oxidation of methane is investigated over the small-pore copper-exchanged zeolite SSZ-13. The reaction pathway is identified and the energy landscape elucidated over the proposed motifs Z2[Cu2O] and Z2[Cu2OH]. It is shown that the Z2[Cu2O] motif has an exergonic reaction path, provided water is added as a solvent for the desorption step. However, a micro-kinetic model shows that neither Z2[Cu2O] nor Z2[Cu2OH] has any notable activity under the reaction conditions. These findings highlight the importance of the detailed structure of the active site and that the most stable motif is not necessarily the most active. Keywords: DFT; reaction mechanism; micro-kinetic model; small-pore zeolite; chabazite; SSZ-13; copper; methane-to-methanol; direct conversion Citation: Engedahl, U.; Arvidsson, A.A.; 1. Introduction Grönbeck, H.; Hellman, A. Reaction The growing need for transportation makes a more efficient usage of combustion fuel Mechanism for Methane-to-Methanol in Cu-SSZ-13: First-Principles Study of necessary [1]. Methane, which is extracted together with crude oil, is a not fully utilized bi-product. As methane is a gas at standard conditions with high sunlight absorption the Z2[Cu2O] and Z2[Cu2OH] Motifs. Catalysts 2021, 11, 17. https:// properties, methane is often flared into CO2 instead of being utilized [2]. Such a scenario dx.doi.org/10.3390/catal11010017 can be avoided if it were possible to directly convert methane into methanol, which is a liquid at standard conditions. The management and distribution system is already Received: 15 November 2020 in place to handle liquid fuels. The current method for methane-to-methanol (MTM) Accepted: 15 December 2020 conversion is based on a large-scale two step process operating at high temperature and Published: 25 December 2020 high pressure [3]. A major development would be a catalytic material that can transform methane into methanol in one step (direct methane-to-methanol (DMTM)) on a small scale, Publisher’s Note: MDPI stays neu- at low temperature and standard pressure. tral with regard to jurisdictional claims The search for a catalyst for the partial oxidation of MTM has been the topic of many in published maps and institutional studies [4–7]. One line of research has been to mimic the porous structure and ionic affiliations. metal sites of the naturally occurring enzymatic methane monooxygenases (MMOs) [8], which has been accomplished by metal ion-exchanged zeolites. The basic building-blocks of zeolites are corner-sharing SiO4 and AlO4 tetrahedrons, Copyright: © 2020 by the authors. Li- with Si and Al tetrahedrally coordinated (T-site). A porous framework with pore sizes censee MDPI, Basel, Switzerland. This ranging from two to 12 membered rings (MRs) is formed by connecting tetrahedrons. article is an open access article distributed One important characteristic of the zeolite system is the Si:Al ratio, which is commonly in under the terms and conditions of the the range of 5–20:1. Creative Commons Attribution (CC BY) Experimentally, direct conversion of MTM over zeolites has been observed using a license (https://creativecommons.org/ quasi-catalytic three step process: activation of the oxidant (O2,N2O or NO) at high tem- ◦ ◦ licenses/by/4.0/). perature (>350 C); reaction phase at 50–210 C and with the addition of CH4; extraction Catalysts 2021, 11, 17. https://dx.doi.org/10.3390/catal11010017 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 17 2 of 12 at 25–210 ◦C, where the reactants are flushed out using water/ethanol or some other sol- vent [9,10]. For instance, studies of the MFI zeolite ZSM-5 have shown catalytic activity for DMTM conversion, with the cation system of Cu-ZSM-5 being the most active for conver- sion [11]. However, the detailed configuration of the active site is still under debate [12–14]. An additional use of Cu-ZSM-5 is as a catalyst for the selective catalytic reduction reaction (SCR) of NOx using NH3 as the reducing agent [15]. However, the NH3-SCR reaction is preferably performed over the small-pore chabazite system of Cu-SSZ-13 [16]. It is, thus, interesting to investigate whether Cu-SSZ-13 also would show activity for partial oxidation of methane [17–20]. Over the Cu2O motif in Cu-ZSM-5, the reaction mechanism for direct conversion has been shown to occur via a three step reaction path: activation of the C–H bond in CH4 over the active site and a rotation of the reactant, which leads to the formation of the CH3OH-complex [11]. The active site in Cu-SSZ-13 has been suggested to be CuOH [21,22]. The proposed reaction mechanism [21] for CuOH is more complex than for Cu2O in MFI. Direct insertion of a methyl radical can be done either with a possible two step reaction path or a five (alternatively six) step reaction path. The longer path shows an energy profile with low barriers. Furthermore, including water in the reaction path makes methanol formation energetically favourable [21]. However, the presence of the CuOH motif has also been explained as it is a possible precursor for the formation of either Cu2O2 or Cu2O[23]. Larger Cu clusters (Cu > 2), namely Cu3O3 [24,25], have also been suggested as the active motif. The trimer structure has been shown to decrease the energy barrier for the activation of the C–H bond when compared with the smaller Cu monomer and dimer [21]. However, only large pore zeolites such as MFI and mordenite (MOR), are believed to contain these large clusters; in small pore zeolites, they have been shown to be unstable [22]. It has been shown that the active site motif is dynamic and depends sensitively on the operating conditions, such as the temperature and partial pressure of relevant gases [26,27]. Previous work [27] reports that for a quasi-catalytic reaction cycle in the zeolite Cu-SSZ-13 (with Si:Al = 10:2 and Al:Cu = 2:2, in the 12 T-site unit cell), the Cu2OH motif, as seen in Figure1a, is the energetically preferred structure for low temperature and low partial pressure of oxygen, while for high temperature and high partial pressure of oxygen, the Cu2O motif, as can be seen in Figure1b, is preferred. However, the reaction mechanism and character of the intermediates are not yet known. In this study, we use first-principles calculations to determine the reaction mechanism for the partial methane oxidation over the active site motifs of Cu2O and Cu2OH in the small-pore zeolite SSZ-13. As water is often included as a solvent in the experimental procedure, its influence on the reaction mechanism is investigated in detail, and a micro- kinetic modelling is used to explore the activity of the considered sites. Catalysts 2021, 11, 17 3 of 12 Figure 1. The two investigated active site structures and their positions in the zeolite framework are shown. In (a), the Cu2OH site in SSZ-13 and in (b) the Cu2O site in SSZ-13. 2. Results and Discussion The reaction mechanism is elucidated for the reaction and the extraction phase of the DMTM reaction. This is experimentally done at 448 K and flow reactor conditions with 2% −9 CH4 and 10 % CH3OH with respect to atmospheric pressure [10]. Under these conditions, previous work has identified Z2[Cu2O] and Z2[Cu2OH], with the active sites situated in the 8MR in the chabazite (CHA) (see Figure1a,b), as the most stable active site motifs [ 27]. In the description of the intermediate steps in the reaction mechanism, the two copper atoms of the active site are denoted as *. Hence, *X,Y implies that reaction intermediate X is coordinated to both Cu atoms, forming the current state of the active site, while reaction intermediate Y is either adsorbed on the active site or free in the zeolite. The structural properties of gas phase H2O, CH4 and CH3OH are in good agreement with the experimental values [28]; for details, see Table S1. 2.1. Reaction Mechanism over Z2[Cu2O] The identified reaction mechanisms for DMTM at 448 K over Z2[Cu2O] in SSZ-13 are presented in Figure2, where the difference in the Gibbs free energy of two different mechanisms, with and without water, is compared (total energies for the mechanism can be found in Figure S2). Starting with an empty zeolite, the free energy cost of moving one methane molecule into the zeolite framework is 0.28 eV. The first transition state (TS) is CH4 dissociation, suggested to occur via a CH3 radical [29]. Here, the methyl structure was found by constrained geometry optimizations and proved to be a saddle point through vibrational analysis and is shown as TS1 in Figure2. Catalysts 2021, 11, 17 4 of 12 Figure 2.