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

Physical and chemical properties of hetero-polyacids (HPAs), e.g., H3PMo12-O-40 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 (mpgC(3)N(4) (53 N at. %)). The ability to disperse HPAs without crystallite formation followed the trend N-C-600 < N-C-1000 activated carbon (C) < mpgC(3)N(4). 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 mpgC(3)N(4) 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.