Formation and Characterization of Melam, Melam Hydrate, and a Melam–Melem Adduct

Abstract

Until recently, melam, [C(3)N(3)(NH(2))(2)](2)NH, has been regarded as a short-lived intermediate in the condensation process of melamine that is only detectable under special reaction conditions owing to its high reactivity. A new synthetic approach has allowed a closer look at the formation and condensation behavior of melam by using elevated ammonia pressure in autoclaves. Whereas the thermal treatment of dicyandiamide at 450 °C and 0.2 MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9 days) led to the formation of a melam-melem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 °C (24 h) yields melam hydrate, [C(3)N(3)(NH(2))(2)](2)NH⋅2 H(2)O (space group P2(1)/c; a=676.84(2), b=1220.28(4), c=1394.24(4) pm; β=98.372(2)°; V=1139.28(6)×10(6) pm(3); Z=4), which crystallizes as a layered structure that is composed of almost-planar melam molecules, thereby forming ellipsoidal rosette-like motifs. The resulting voids are filled with four water molecules, thus forming a dense network of hydrogen bonds.

Carbon nitrides: synthesis and characterization of a new class of functional materials

“…17). 95 During thermal condensation of melamine to form melem, Lotsch et al noted that the sharp peak near 800 cm À1 remained present throughout the polymerization process. 104 Sharp IR peaks also appear in the same region for PTIÁLiCl and PTIÁLiBr compounds, that are known to be built only from triazine ring units (Fig.…”

“…The nature of these vibrations has been studied using both ab initio and empirical force field calculations, as well as by reference to compounds such as s-triazine (C 3 N 3 H 3 ), melamine (C 3 N 3 (NH 2 ) 3 ) and melem (C 6 N 7 (NH 2 ) 3 ). 58,95,97,98 The simplest molecular compound containing the C 3 N 3 ring is s-triazine (C 3 N 3 H 3 ), that was first studied in detail by Larkin et al 97 Because H is linked to C rather than N atoms, it is only of limited use in understanding the vibrations of solid state and polymeric gCN structures. However, the terminology applied to its vibrational analysis is typically used to describe the vibrational mode assignments for the condensed carbon nitride phases.…”

“…However, the terminology applied to its vibrational analysis is typically used to describe the vibrational mode assignments for the condensed carbon nitride phases. 21,95,99 Larkin et al based their assignments on those for benzene, in terms of the normal modes for a planar six-membered ring. 97 The ring was split into ''quadrants'', ''sextants'', hemicircles and whole-ring vibrations (Fig.…”

Outlining the mechanism of flame retardancy in polyamide 66 blended with melamine-poly(zinc phosphate)

“…A strong broad band around 1670 cm À 1 is due to v(C¼ N) and δ(NH 2 ). The characteristic triazine ring band of melamine appeared at 780 cm À 1 [39][40][41][42]. Additional bands are identified between 1250 and 950 cm À 1 , and around 540 cm À 1 corresponding to various vibrations from ZnPO polyhedra from different sources like -meta, -pyro, -ortho phosphate and most of the bands are overlapped by melamine vibrations [43,44].…”

“…There are three main de-ammonation and condensation products of melamine-melam, melem and melon [40][41][42]45,46]. Tentatively, they are formed between 340-400, 400-450 and 450-500 1C, respectively, with certain degree of flexibility in temperature range and shown to vary with experimental condition [41].…”