Shape-selective N-methylation of melamine with methanol by use of Ru/mordenite catalyst in the liquid phase Original Research Article

A Ru/mordenite (Ru/M) catalyst with catalytically-active ruthenium species retained in the mordenite pores has been prepared by the ion-exchange of Na-mordenite with [Ru(NH3)6]3+, followed by a very mild thermal treatment. While extended X-ray absorption fine structure (EXAFS) analysis showed the presence of both Ru–N and Ru–Ru bonds, HRTEM photographs evidenced the absence of metal particles larger than the diameter of a mordenite pore. The remaining NH3 ligand was also observed from the characteristic IR bands appearing at 1640 and 1356 cm−1; the IR spectrum of adsorbed CO indicated the presence of multicarbonyl-type surface species (such as Run+(CO)3), consistent with the well-dispersed or coordinatively-unsaturated nature of ruthenium species. After the complete preparation procedure, a high degree of crystallinity was maintained for mordenite (XRD), and the decrease of SBET from 362 to 313 m2 g−1 ensured the existence of ruthenium species in the mordenite pores. It was demonstrated that, by using this catalyst, N-methylation of melamine with methanol was able to proceed in a moderately shape-selective manner (suppression of the formation of N,N′,N″-tri-methylated melamine (3) as compared to that of N-methylated or N,N′-di-methylated melamine (1 or 2)), which is in harmony with the sizes of relevant molecules relative to the size of a mordenite pore.