Influence of Molecular Structure of Quaternary Phosphonium Salts on Thai Bentonite Intercalation

Abstract

AbstractA comprehensive study of the intercalation of organo-phosphonium salts into Thai bentonite (Mt) was conducted to investigate the influence of the molecular structures of organic moieties, including chain types (alkyl vs. aryl), chain length, and structural symmetry, on their intercalation. A series of quaternary phosphonium salts with systematically varied molecular structures (tetraphenyl phosphonium, TPP-Br; tetrabutyl phosphonium, TBP-Br; tetraoctyl phosphonium, TOP-Br; methyl triphenyl phosphonium, MTPP-Br; and butyl triphenyl phosphonium, BTPP-Br) was intercalated into Mt via an ion-exchange reaction. From thermogravimetric analysis results, tetrabutyl phosphonium-modified Mt (TBP) with shorter alkyl chain length began to decompose at a slightly lower temperature (263 vs. 351°C), yet showed comparable thermal stability (i.e. maximum decomposition temperature) at 470°C, compared to tetraoctyl phosphonium-modified Mt (TOP). Aryl phosphonium-modified Mt (TPP) showed a higher thermal decomposition temperature (576 vs. 470°C) than those of alkyl phosphonium-modified Mts (TBP and TOP). Introducing short alkyl chains into the aryl phosphonium moiety (MTPP, BTPP) caused a slight decrease in thermal decomposition temperature, but an increase in cation loadings of their modified Mts (71 and 73%, respectively). X-ray diffraction analysis showed that the flexibility of alkyl chains in TBP yielded smaller increases in basal spacing, i.e. lower degree of intercalation, compared to the rigid aryl structure in TPP. Increasing chain length resulted in greater basal spacing in alkyl phosphonium-modified Mts (1.67 nm. in TBP vs. 2.46 nm. in TOP). Such an effect, however, was less significant in aryl phosphonium-modified Mt.