(Fig. 13a)
the TRAPDOR difference spectrum (DS) showed that only the signal
at 30.5 ppm corresponds to PeOeAl units in AlPO4. The signal
at 0.6 ppm was attributed to a magnesium phosphate structure,
whereas signals at 16.7 ppm and 19 ppm were assigned to aand
b-forms of magnesium pyrophosphate, respectively, as it has
been reported in the literature [44]. The shape of the signal for
Mg3(PO4)2 was very narrow, which was attributed to the crystalline
structure. Furthermore it was observed that those peaks overlapped
a broad signal underneath, which indicated the presence of
an amorphous inorganic phosphate network. Since the 13C MAS
NMR experiment showed that very little carbonaceous char is
formed, it is concluded that most of the residue formed comes from
magnesium phosphates. Additionally, clear evidence of chemical
interaction between AlPi and MH is observed, which is correlated to
the possible synergistic effect.
The spectrum for TPE-S/AlPi/ZB/PPO is presented in Fig. 13b.
Narrow signals at 3.3 ppm and 1.6 ppm were assigned to crystalline
zinc phosphate [45]. Three signals at 16.6 ppm, 19.5 ppm
and 27.7 ppm corresponded to crystalline zinc pyrophosphate
[46]. At 31 ppm a broad signal appeared. Analysis of the differential
spectrum suggested that this signal corresponds to the AlPO4
structure. If we have a closer look at this signal it can be seen to
consist of two parts e a broad, strong resonance and a narrower
one. It was assumed that besides zinc and aluminium phosphate,
crystalline borphosphate formed during the burning. Furthermore,
additional 11B MAS NMR