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Alternatively, the nitrogen functional groups were introduced from the polymeric resins such as melamine-formaldehyde and urea-formaldehyde. The resins were synthesized by the addition-condensation reactions. Firstly, melamine/urea and formaldehyde were mixed in aqueous media in the molar ratio 1:3/1:2. NaOH was added to this mixture to bring the pH to 11-12 at 60° C., which initiates the addition reaction to form methylolurea/methylolmelamine intermediates. Then the pH was adjusted to around 4-5 using H2SO4 to initiate the condensation/polymerization reaction resulting in the respective resins. The sample was dried using a rotary evaporator at 80° C. under vacuum, and heat-treated in an Ar atmosphere at 800° C. for 1 h.

1. customThe resins were synthesized by the addition-condensation reactions
2. customat 60° C.
3. additionthe addition reaction