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1. Exploring the Impact of Active Site Structure on the Conversion of Methane to Methanol in Cu‐Exchanged Zeolites

   https://doi.org/10.1002/anie.202403179  

   Göltl, Florian; Bhandari, Saurabh; Lebrón‐Rodríguez, Edgard A.; Gold, Jake I.; Hutton, Daniel J.; Zones, Stacey I.; Hermans, Ive; Dumesic, James A.; Mavrikakis, Manos (April 2024, Angewandte Chemie International Edition)

   Abstract In the past, Cu‐oxo or ‐hydroxy clusters hosted in zeolites have been suggested to enable the selective conversion of methane to methanol, but the impact of the active site's stoichiometry and structure on methanol production is still poorly understood. Herein, we apply theoretical modeling in conjunction with experiments to study the impact of these two factors on partial methane oxidation in the Cu‐exchanged zeolite SSZ‐13. Phase diagrams developed from first‐principles suggest that Cu‐hydroxy or Cu‐oxo dimers are stabilized when O₂or N₂O are used to activate the catalyst, respectively. We confirm these predictions experimentally and determine that in a stepwise conversion process, Cu‐oxo dimers can convert twice as much methane to methanol compared to Cu‐hydroxyl dimers. Our theoretical models rationalize how Cu‐di‐oxo dimers can convert up to two methane molecules to methanol, while Cu‐di‐hydroxyl dimers can convert only one methane molecule to methanol per catalytic cycle. These findings imply that in Cu clusters, at least one oxo group or two hydroxyl groups are needed to convert one methane molecule to methanol per cycle. This simple structure–activity relationship allows to intuitively understand the potential of small oxygenated or hydroxylated transition metal clusters to convert methane to methanol. 

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2. Heterometallic [Cu–O–M] ²⁺ active sites for methane C–H activation in zeolites: stability, reactivity, formation mechanism and relationship to other active sites

   https://doi.org/10.1039/d1cy00687h  

   Adeyiga, Olajumoke; Panthi, Dipak; Odoh, Samuel O. (August 2021, Catalysis Science & Technology)

   The formation and reactivities of [Cu–O–M] 2+ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O 2 elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M] 2+ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu 2 O] 2+ . Other [Cu–O–M] 2+ species, (M = Mn–Ni and Ru–Ag) have similar formation energies to [Cu 2 O] 2+ , to within ±10 kcal mol −1 . Interestingly, only [Cu–O–Ag] 2+ is more active for methane activation than [Cu 2 O] 2+ . [Cu–O–Ag] 2+ is however more susceptible to O 2 elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu 2 O] 2+ is best suited for MMC. Formation of [Cu 2 O] 2+ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assisted condensation of two hydroxo groups. To evaluate the relationship between [Cu 2 O] 2+ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH] + < paired [Cu–OH] + < [Cu 2 O] 2+ < [Cu 3 O 3 ] 2+ . Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH] + , paired [Cu–OH] + , [Cu 2 O] 2+ and [Cu 3 O 3 ] 2+ , respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH] + species is discussed. 

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3. Understanding activity origin for the oxygen reduction reaction on bi-atom catalysts by DFT studies and machine-learning

   https://doi.org/10.1039/d0ta08004g  

   Deng, Chaofang; Su, Yang; Li, Fuhua; Shen, Weifeng; Chen, Zhongfang; Tang, Qing (December 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Bi-atom catalysts (BACs) have attracted increasing attention in important electrocatalytic reactions such as the oxygen reduction reaction (ORR). Here, by means of density functional theory simulations coupled with machine-learning technology, we explored the structure–property correlation and catalytic activity origin of BACs, where metal dimers are coordinated by N-doped graphene (NC). We first sampled 26 homonuclear (M 2 /NC) BACs and constructed the activity volcano curve. Disappointingly, only one BAC, namely Co 2 /NC, exhibits promising ORR activity, leaving considerable room for enhancement in ORR performance. Then, we extended our study to 55 heteronuclear BACs (M 1 M 2 /NC) and found that 8 BACs possess competitive or superior ORR activity compared with the Pt(111) benchmark catalyst. Specifically, CoNi/NC shows the most optimal activity with a very high limiting potential of 0.88 V. The linear scaling relationships among the adsorption free energy of *OOH, *O and *OH species are significantly weakened on BACs as compared to a transition metal surface, indicating that it is difficult to precisely describe the catalytic activity with only one descriptor. Thus, we adopted machine-learning techniques to identify the activity origin for the ORR on BACs, which is mainly governed by simple geometric parameters. Our work not only identifies promising BACs yet unexplored in the experiment, but also provides useful guidelines for the development of novel and highly efficient ORR catalysts. 

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4. Direct Hydrogenolysis of Cellulose to Methane Utilizing Rare‐Earth Promoted Nickel Catalysts

   https://doi.org/10.1002/ijch.202200119  

   Koch, Christopher J.; Alagaratnam, Anushan; Goeppert, Alain; Surya Prakash, G. K. (January 2023, Israel Journal of Chemistry)

   Abstract Cellulose is one of the main components of plant matter, which makes it a viable target for biomass conversion to fuels. The direct conversion of cellulose to methane utilizing nickel‐based catalysts often has challenges associated with it. Carbon agglomeration creating nickel‐carbon nanoparticles deactivating catalytic hydrogenation of cellulose has been well reported. Utilizing rare‐earth metals as promoters increases the conversion of cellulose to methane, albeit with deactivation of the catalyst in the form of nickel‐rare‐earth‐carbon nanoparticles. Adding an additional zinc metal promoter eliminates the carbon agglomeration and allows for increased methane yields. Herein, we report an 81 % methane yield from cellulose in 48 hours utilizing a Ni/Zn/Y/Al₂O₃catalyst at 225 °C and under 50 bar H₂pressure. 

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5. Group 13 Lewis acid catalyzed synthesis of metal oxide nanocrystals via hydroxide transmetallation

   https://doi.org/10.1039/d1nr02397g  

   Gibson, Noah J.; Bredar, Alexandria R.; Chakraborty, Nilave; Farnum, Byron H. (January 2021, Nanoscale)

   null (Ed.)

   A new transmetallation approach is described for the synthesis of metal oxide nanocrystals (NCs). Typically, the synthesis of metal oxide NCs in oleyl alcohol is driven by metal-based esterification catalysis with oleic acid to produce oleyl oleate ester and M-OH monomers, which then condense to form M x O y solids. Here we show that the synthesis of Cu 2 O NCs by this method is limited by the catalytic ability of copper to drive esterification and thus produce Cu + -OH monomers. However, inclusion of 1–15 mol% of a group 13 cation (Al 3+ , Ga 3+ , or In 3+ ) results in efficient synthesis of Cu 2 O NCs and exhibits size/morphology control based on the nature of M 3+ . Using a continuous-injection procedure where the copper precursor (Cu 2+ -oleate) and catalyst (M 3+ -oleate) are injected into oleyl alcohol at a controlled rate, we are able to monitor the reactivity of the precursor and M 3+ catalyst using UV-visible and FTIR absorbance spectroscopies. These time-dependent measurements clearly show that M 3+ catalysts drive esterification to produce M 3+ -OH species, which then undergo transmetallation of hydroxide ligands to generate Cu + -OH monomers required for Cu 2 O condensation. Ga 3+ is found to be the “goldilocks” catalyst, producing NCs with the smallest size and a distinct cubic morphology not observed for any other group 13 metal. This is believed to be due to rapid transmetallation kinetics between Ga 3+ -OH and Cu + -oleate. These studies introduce a new mechanism for the synthesis of metal oxides where inherent catalysis by the parent metal ( i.e. copper) can be circumvented with the use of a secondary catalyst to generate hydroxide ligands. 

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