Response to Comment on “Selective anaerobic oxidation of methane enables direct synthesis of methanol”

Abstract

Periana argues that the stepwise reaction of methane with water is thermodynamically unfavorable and therefore impractical. We reply by presenting an in-depth thermodynamic analysis of each step in the process and show that the surface concentrations of the reactants and products as well as the stabilizing effect of additional water molecules, as discussed in the original paper, fully support the feasibility of the proposed reaction. In the Technical Comment, Periana (1) presents a thermodynamic analysis of two transformations in the oxidation of methane to methanol with water. Whereas the reaction of copper (II) oxide with methane (the first step in chemical looping) was accepted as being thermodynamically possible (Eq. 1, DG 473K = -12 kJ/mol), the reaction of copper(I) oxide with water (the second step in chemical looping) was concluded to be thermodynamically impossible (Eq. 2, DG 473K = +121 kJ/mol). The conclusions reached by Periana about the thermodynamics of bulk oxides were then projected to the Cu I -□-Cu I sites that interact with water in the mordenite zeolite.The thermodynamics represented in Eqs. 1 and 2 are correct under standard conditions; however, the models of the two reaction steps, the conditions, and the reactive species described by Periana are not representative of the material, nor do they fully represent the reaction steps in the chemical looping system described in our paper. The following assumptions were made by Periana, which led to erroneous conclusions: 1) Bulk crystalline copper oxides were used to model the copper sites in mordenite zeolite.2) Gibbs free energy for the system in equilibrium, with total pressure of 1 bar for each component, was applied.3) Specific adsorption of reacting and formed molecules over the surface of the copper-exchanged mordenite zeolite (CuMOR) material was not considered.4) Enthalpic stabilization of intermediate species by water molecules was not taken into account.5) An activation step, including the dehydration of the material, was omitted. We present the thermodynamic analysis of each of the subreactions, taking into account the adsorption phenomena typical of heterogeneous systems. We estimate the Gibbs free energy under the conditions given in our paper, in contrast to the standard DG.Taking into account the experimental details of chemical looping over the CuMOR material, our simplified reaction scheme consists of two main steps (Eqs. 3 and 4), each of which is required for the reaction to occur. The Cu-O-Cu mono-m-oxo sites are assumed to be active sites responsible for such transformations. Equation 3 corresponds to the interaction of activated CuMOR with methane at 7 bar and 473 K . Equation 4 represents the continuous flowing of water vapor at 473 K followed by desorption of methanol and the formation of hydrogen with the reoxidation of the copper sites. All these observations are based on experimental evidence. The standard Gibbs free energy was calculated by density functional theory, and Fig. 3 in (2) presents all the intermediates wit...

The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

“…Furthermore,t he larger distance between the two copper atoms in the case of the monomer system will necessarily lead to larger activation barriers.Thus, asingle water molecule cannot initiate the reaction of copper reoxidation and simultaneous hydrogen release.W emodeled the formation of hydrogen with the subsequent stabilization of the formed species with three additional water molecules, according to the stabilization model described in our previous work. [14] This is consistent with the experimentally observed amount of water added to the system. Thus the formation of hydrogen involves an increase in DG in the case of monomers while the corresponding energy profile for dimers rapidly goes down owing to the possible formation of very stable bis(dihydroxy)dicopper sites.Although the energy of the final water-stabilized species depends on the number of water molecules,i tis clearly much easier to stabilize the dimer structure following the release of hydrogen.…”

“…[13] Whereas the presence of isolated monomeric species leads to high energy barriers of up to 175 kJ mol À1 and therefore renders the process energetically unfeasible, [17] the presence of two copper atoms in the form of ad imer facilitates the water-mediated release of methanol with ab arrier as low as 85 kJ mol À1 . [14] This is consistent with the experimentally observed amount of water added to the system. Figure 3shows that the methane activation step is similar for both the pair of monomers and the dimer,w ith the monomer system exhibiting higher stabilization of the methoxy species (214 kJ mol À1 vs.1 34 kJ mol À1 for the dimer).…”

“…[13] Shortly after water contact with the methane-reacted materials,t he generation of hydrogen was observed in all three cycles (Figure 1, insets). [13][14][15] Similarly,for CuMOR(10), methanol and hydrogen were present during the cycling of the material in methane and water vapors.T reatment in an oxygen-free environment also had asignificant impact on the autoreduction process,r esulting in the formation of additional Cu I species as evidenced by FTIR spectroscopy of adsorbed NO ( Figure S12). …”

“…[5][6][7][8] Most crucially,e xisting methods often suffer from low selectivity or the costliness of the oxidants used. [14,15] Instead of using oxygen, only the presence of water is required, while the reactivation of the zeolite material is done in an inert atmosphere.S uch aw ater-facilitated redox process requires at least two copper atoms to stoichiometrically oxidize methane into methanol, thus suggesting the presence of active sites containing several copper atoms,s uch as dimers [5][6][7] and trimers, [8] stabilized by the framework aluminum atoms. [14,15] Instead of using oxygen, only the presence of water is required, while the reactivation of the zeolite material is done in an inert atmosphere.S uch aw ater-facilitated redox process requires at least two copper atoms to stoichiometrically oxidize methane into methanol, thus suggesting the presence of active sites containing several copper atoms,s uch as dimers [5][6][7] and trimers, [8] stabilized by the framework aluminum atoms.…”

“…[9][10][11][12] We recently proposed an alternative solution to the above problem by showing that selective anaerobic oxidation of methane is possible; [13] water can be used both to provide oxygen to regenerate the zeolite active sites and to stabilize reaction intermediates,w hich uniquely occurs in the copper zeolite,t od rive the otherwise endothermic Cu I oxidation reaction. [14,15] Instead of using oxygen, only the presence of water is required, while the reactivation of the zeolite material is done in an inert atmosphere.S uch aw ater-facilitated redox process requires at least two copper atoms to stoichiometrically oxidize methane into methanol, thus suggesting the presence of active sites containing several copper atoms,s uch as dimers [5][6][7] and trimers, [8] stabilized by the framework aluminum atoms.…”