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Use of Nitrogen-Containing Carbon Supports To Control the Acidity of Supported Heteropolyacid Model Catalysts

Zhao, Xiaowen, Li, Xin-Hao, Chen, Jie-Sheng, Barteau, Mark A.
Industrial & engineering chemistry process design and development 2018 v.57 no.42 pp. 13999-14010
X-ray diffraction, acidity, ammonium, carbon nitride, catalysts, crystallites, graphene, methanol, models, nitrogen, nitrogen content, oxidation, porous media, process design
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.