Use of Nitrogen-Containing Carbon Supports To Control the Acidity of Supported Heteropolyacid Model Catalysts
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© 2018 American Chemical Society. Physical and chemical properties of heteropolyacids (HPAs), e.g., H3PMo12O40 supported on nitrogen-containing carbon materials, were investigated. Supports included nitrogen-doped graphitic carbons (N-C-1000 (2 N at.%) and N-C-600 (19 N at.%)) and mesoporous graphitic carbon nitride (mpgC3N4 (53 N at.%)). The ability to disperse HPAs without crystallite formation followed the trend N-C-600 < N-C-1000 activated carbon (C) < mpgC3N4. HPAs preferentially interact with pyridinic nitrogen and surface NH2 groups; the latter lead to ammonium HPA salt crystallites observed via X-ray diffraction (XRD). At low coverage, HPAs are molecularly dispersed on all four supports. At comparable polyoxometalate coverages, C and N-C-1000 showed similar dehydration/oxidation activities in methanol oxidation. However, N-C-600 and mpgC3N4 exhibited lower activities for both reactions. The much-greater decrease in dehydration vs oxidation activity on nitrogen-rich supports led to higher catalyst oxidation selectivities. This work demonstrates that acid site populations of HPA catalysts can be controlled via support nitrogen content.
author list (cited authors)
Zhao, X., Li, X., Chen, J., & Barteau, M. A.