Selective direct oxidation of methane to methanol is
an important
reaction for efficient utilization of natural gas. Photocatalytic
routes for methane oxidation processes have advantages over traditional
thermal catalysis as they can be more easily deployed at small-scale
and remote off-grid sites, potentially reducing the rate of methane
release and flaring. Hu and co-workers identified single-site vanadium-doped
mesoporous amorphous silica photocatalysis which demonstrated methane
conversion with high methanol selectivity (J. Photochem. Photobiol., A20132644855). Photocatalysts using amorphous SiO2 frameworks
are significantly less studied than other heterogeneous photocatalysts.
As a result, computational studies are important for elucidating mechanisms
in order to gain fundamental understanding as to how amorphous SiO2-based photocatalysts can be used for partial oxidation of
methane. This work uses density functional theory to establish the
reaction barriers associated with the photocatalytic methane to methanol
reaction combined with microkinetic modeling. The work is able to
elucidate the role of terminal versus bridging oxygens at the photocatalytic
center in enhancing photocatalytic efficiency. In addition, avoiding
non-Mars–van Krevelen reaction pathways, in which the oxygen
vacancy is filled before methanol is formed, is found to be important,
owing to overstabilization of reaction intermediates leading to subsequent
high energy reaction barriers. Finally, the amorphous SiO2 surface is found to modify reaction barriers by stabilizing intermediates
through dispersion and preventing the photocatalytic center from forming
the optimum coordination geometry.
The click‐chemistry capture of volatile aldehydes and ketones by ammonium aminooxy compounds has proven to be an efficient means of analyzing the carbonyl subset in complex mixtures, such as exhaled breath or environmental air. In this work, we examine the carbonyl condensation reaction kinetics of three aminooxy compounds with varying β‐ammonium ion substitution using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). We determined the activation energies for the reactions of the aminooxy compounds ATM, ADMH and AMAH with a panel of ketones and aldehydes that included acrolein and crotonaldehyde. The measurements indicate that the activation energies for the oximation reactions are quite low, less than 75 kJ mol−1. ADMH is observed to react the fastest with the carbonyls studied. We postulate this result may be attributed to the ADMH ammonium proton effecting a Brønsted‐Lowry acid‐catalyzed elimination of water during the rate‐determining step of oxime ether formation. A theoretical study of oxime ether formation is presented to explain the enhanced reactivity of ADMH relative to the tetraalkylammonium analog ATM.
Single-site transition-metal-doped photocatalysts can potentially be used for partial oxidation of methane (POM) at remote sites where natural gas is extracted and methane is often flared or released to the atmosphere. While there have been several investigations into the performance of vanadium, there has been no general survey of the performance of other metals. This work aims and examines Cr, Nb, and W metal oxide materials embedded in amorphous SiO 2 to determine the viability of each metal in catalyzing the POM. Photoexcited states are examined to determine the nature of the photoactivated species, and then the subsequent POM reaction mechanisms are elucidated. Using the calculated energies of reaction intermediates and transition states, the rate of methanol formation is evaluated through the use of a microkinetic model. The findings indicate that all three metals are potentially more suitable for catalyzing POM than vanadium but that niobium shows the most favorable energy profile.
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