Imidazolium salts, thermal decomposition

In comparison with ammonium and pyridinium salts, the thermal decomposition of alkylimidazolium salts is more difficult to predict because of the presence of two nitrogen atoms. Chan et al. [63] have sffidied the isothermal decomposition of 1,3-disubstituted imidazolium iodides in the temperature range 220-260 °C. The presence of alkylimidazoles and alkyl halides was detected in the decomposition products, indicating that the degradation proceeds mainly via a nucleophilic substitution reaction, most likely an Sn2 mechanism (Scheme 2.7). 

The surrounding gas had a distinct effect on the thermal stability of tetrafluoroborate and hexafluorophosphate imidazolium salts, whereas the presence of oxygen appeared to have no effect on the decomposition temperatures of halide salts [64], It was envisaged that the activation energy required for the thermal decomposition of the halide salts is lower than for their oxidative decomposition. 

Awad et al. [56] reported on the thermal degradation of a series of alkylimidazolium molten salts. Elemental analysis, TGA, and thermal desorption mass spectroscopy (TDMS) were used to characterize the degradation process. A correlation was observed between the chain lengths of the alkyl groups and the thermo-oxidative stability as the chain length increased from propyl, butyl, decyl, hexadecyl, and octadecyl to eicosyl, the stability decreased. Analysis of the decomposition products by FTIR provided information about the decomposition products. It suggested that the thermal decomposition of imidazolium salts followed an SN2 process (Figure 3.12). 

The decomposition temperatures can be compared with data described in the literature for imidazolium salts Cb < b < [C2BgHi2] < [PFg]- < [Co(C2B<,Hjj)2] < [BFJ- < [(CF3S02>2Nb [BioClio] < [BigClig] (Bonhote et al., 1996 Holbrey and Seddon, 1999a Huddleston et al., 2001 Ngo et al, 2000). Less nucleophilic anions are thermally more stable, because the decomposition of the imidazolium cation occurs by nucleophilic attack of the anion on the cation via a 8, 2 reaction. 

The nature of the alkyl substituents was found to rule the rate of cleavage for the N-C bonds [63, 64]. Methyl substitution in the 2 position (i.e., between the two N atoms) enhances the thermal stability. This may be due to the strong acidic character of the C-2 proton. It was observed that the thermal stability of imidazolium was also affected by the type of isomeric structure of the alkyl side group. This was evidenced by the observation that both l-butyl-2,3-dimethylimidazolium tetrafluoroborate and l-butyl-2,3-dimethylimidazolium hexafluorophosphate salts had higher onset decomposition temperatures than l,2-dimethyl-3-isobutylimidazolium tetrafluoroborate and l,2-dimethyl-3-isobutylimidazolium hexafluorophosphate salts. This reaction presumably proceeds via SnI, as shown in Scheme 2.8. 

The incorporation of imidazolium chloride salts between clay layers drastically increased their initial decomposition temperature, whereas no significant improvement was observed in the thermal stability of the intercalated tetrafluoroborate and hexafluorophosphate salts [64]. Apparently this effect was connected with the removal of the anion and depended on its nucleophilicity. Gilman and co-workers reported that the replacement of sodium in natural MMT by l-alkyl-2,3-dimethylimidazolium salts yielded organophilic MMT with a 100 °C improvement in thermal stability (in N2) as compared to the alkylammonium-treated MMT [66]. In Langat et al. s work the initial temperature of decomposition for clays modified with l-hexadecyl-3-(10-hydroxydecyl)-2-methylimidazolium and l-hexadecyl-2-methylimidazolium chloride was also very high - around 370 °C [67]. 

The onset temperature of decomposition of alkyl quaternary ammonium-modified montmorillonite, in nonoxidative thermal degradation, is about 180°C. Initial degradation of the surfactant follows either a Hoftnann elimination or an Sn2 nucleophilic substitution mechanism. Both mechanisms can affect the performance of high-processing-temperature nanocomposites and, in general, the thermal stability and combustion behavior of nanocomposites. In particular, Hofmann elimination generates acidic sites on the layered silicate that can act as a protonic acid catalyst on polymer decomposition. " Imidazolium and phosphonium salts exhibit improved thermal stability compared to ammonium salts.Alkylimidazolium salt-modified layered silicates were used successfully to prepare organoclays that exhibit an onset of decomposition temperature up to 392°C. 

On the other hand, Zhai et al. [35] described a small increase in the decomposition temperature at the earlier stages of the decomposition process for OMt PE CPN containing 3 and 5 phr of OMt modified with an ammonium salt (with a C-18 tallow), but the highest increase in the thermal stability was found for the CPN containing 1 phr of OMt, which was justified by a highest exfoliation degree and which kept monolayer in the composite containing Iphr of OMt. By other hand EM et al. [41] described an important improvement of the thermal stability of OMt PE CPN when OMt was modified with imidazolium and phosphonium salts due to the intrinsic stability of these salts as compared to conventional ammonium salts. 

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