Direct conversion of methane-to-methanol: transition-metal dimer sites in small-pore zeolites: First-principles calculations and microkinetic modeling

Direct conversion of methane to methanol is a highly desired reaction. Partially oxidizing methane into a liquid fuel at ambient temperature and pressure would enable utilization of natural gas and biogas to a much larger extent than what is possible today. This is desirable since natural gas is the cleanest fossil energy source, and when in the form of biogas (or biomethane) has a net-zero carbon emission. The direct conversion of methane requires a catalyst; however, no material with high enough activity and selectivity towards methanol has been identified. Mimicking the enzyme methane monooxygenase (MMO), copper-exchanged zeolites are considered promising candidates. A plethora of different active sites have been suggested, but neither the detailed structure and composition of the active site, nor the mechanism for the reaction, are known.In this thesis, the catalytic properties of transition metal dimers in small-pore zeolites are studied using first-principles calculations, ab initio thermodynamics, and microkinetic modeling. As a first step, the stability of the Cu dimer structure in SSZ-13 is investigated under direct conversion conditions. The zeolite is found to be very humid, and the structure of the proposed active site is highly dependent on the temperature and partial pressure of relevant gases. The Cu2O and Cu2OH structures are found to be the energetically most preferred. The reaction over the sites is limited by a high free energy barrier of the C-H bond in methane and a slow methanol desorption rate. Adding water to the reaction facilitates desorption of the products, increasing the activity of the Cu2O site. The reaction mechanism for an entire reaction cycle over the Cu-dimer, including the formation of the active site, is investigated in dry and wet conditions. The oxidation of the Cu monomers, using molecular oxygen, is limited by the diffusion of the Cu species along the zeolite framework and the activity is increased when water is added to the reaction. To further investigate the composition of the active dimer site, transition-metal and transition-metal alloy configurations are investigated. The adsorption energy of atomic oxygen is identified as a descriptor for the activity of the dimer systems. Identified motifs showing activity towards direct methane to methanol conversion are the 2Cu, along with the AuPd and PdCu alloy dimer systems. The activity of these systems is comparable and, when excluding competing reactions, meets the high turn-over needed for a commercially viable catalyst

Direct conversion of methane-to-methanol in Cu-exchanged small-pore zeolites

Fossil fuel consumption continuous to increase worldwide and of all the fossil fuels, natural gas is growing the most. The combustion of natural gas, which mainly contains methane, is more environmentally friendly than oil or coal thanks to its high specific energy. More energy gained and less CO2 released per kg fuel drives an increase in production. This in turn increases the demand on manageability, resulting in the common practice to liquefy natural gas. The gas is kept in liquid form, via high pressure or low temperature, for transport and distribution. In an effort to reduce the energy cost of managing gaseous energy resources, the conversion of methane into its liquid counterpart methanol is highly desired. The established procedure for the conversion is a large scale, multi-step, power-plant process, and there is aneed for a small scale, direct conversion alternative. Copper-exchanged zeolites are considered promising candidates for the methane-to-methanol reaction, where mono-, dimer, and trimer Cu-clusters have been suggested to be the active site. In this thesis, the catalytic properties of Cu-dimers in zeolites are studied using first-principles calculations, ab initio thermodynamics, and micro kinetic modeling. As a first step, the stability of the Cu-dimer structure in SSZ-13 is investigated under direct conversion conditions. The zeolite is found to contain water and the structure of the proposed active site highly dependent on temperature and partial pressure of relevant gases. Under reaction conditions, the Cu2O and Cu2OH structures are found to be energetically preferred. Evaluating the reaction path for direct conversion over the identified active sites, reveals a low activity for the reaction, stemming from a high activation barrier of the C-H bond in methane and an inability for methanol to desorb. The activity of the Cu2O site is, however, increased when water is added into the reaction mechanism. Presence of water enables desorption of the reaction products and results in an endergonic reaction path. The Cu2OH site responds in an opposite manner with respect to water, becoming less active. The new insights on the nature of the active site and the reaction mechanism provide a deeper understanding, which will aid the future search for new catalytic materials with high activity and selectivity

Reaction mechanism for methane-to-methanol in CU-SSZ-13: First-principles study of the Z2[Cu2O] and Z2[Cu2oh] motifs

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

Complete Reaction Cycle for Methane-to-Methanol Conversion over Cu-SSZ-13: First-Principles Calculations and Microkinetic Modeling

The steadily increasing consumption of natural gas imposes a need to facilitate the handling and distribution of the fuel, which presently is compressed or condensed. Alternatively, reduced volatility and increased tractability are achieved by converting the chemical energy of the main component, methane, into liquid methanol. Previous studies have explored direct methane-to-methanol conversion, but suitable catalysts have not yet been identified. Here, the complete reaction cycle for methane-to-methanol conversion over the Cu-SSZ-13 system is studied using density functional theory. The first step in the reaction cycle is the migration of Cu species along the zeolite framework forming the Cu pair, which is necessary for the adsorption of O2. Methane conversion occurs over the CuOOCu and CuOCu sites, consecutively, after which the system is returned to its initial structure with two separate Cu ions. A density functional theory-based kinetic model shows high activity when water is included in the reaction mechanism, for example, even at very low partial pressures of water, the kinetic model results in a turnover frequency of ∼1 at 450 K. The apparent activation energy from the kinetic model (∼1.1 eV) is close to recent measurements. However, experimental studies always observe very small amounts of methanol compared to formation of more energetically preferred products, for example, CO2. This low selectivity to methanol is not described by the current reaction mechanism as it does not consider formation of other species; however, the results suggest that selectivity, rather than inherent kinetic limitations, is an important target for improving methanol yields from humid systems. Moreover, a closed reaction cycle for the partial oxidation of methane has long been sought, and in achieving this over the Cu-SSZ-13, this study contributes one more step toward identifying a suitable catalyst for direct methane-to-methanol conversion

Reaction Mechanism for Methane-to-Methanol in Cu-SSZ-13: First-Principles Study of the Z2[Cu2O] and Z2[Cu2OH] Motifs

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

First-principles study of oxidation state and coordination of Cu-Dimers in Cu-SSZ-13 during methane-to-methanol reaction conditions

Direct methane-to-methanol conversion is a dream reaction which presently can be realized via a three-step cycle over copper-exchanged zeolites; an activation phase, a reaction phase, and finally an extraction phase. Here we use ab initio molecular dynamics and first-principles thermodynamics to examine oxidation state and coordination of Cu-dimers in Cu-SSZ-13 under relevant experimental conditions. A multitude of Cu2(HxOy) clusters are exergonic at room temperature. However, at the relevant reaction conditions only Cu2O and Cu2(OH) remain as thermodynamically stable structures for the activation and extraction phase, respectively