)onic solids are materials that are mostly produced through the combination of organic cations and anions from heteropolyacids. Cations from ILs are suitable for these catalysts because there are different types of cations available and the physi­cochemical properties can be altered by using different ILs. Keggin-type polyoxo — metalates (POMs) are favored for synthesizing these hybrid catalysts because of their good solubility in polar reaction media, which makes their recovery becomes easier (Leng et al. 2011b) . Figure 9.4 represents different use of ionic solids in chemical synthesis. Aside from its heterogeneity, the catalysts also possess high thermal stability and maintained the catalytic activity even after being recycled (Dai et al. 2010; Zhu et al. 2011).

Ionic solid catalysts can be obtained by integrating cations of ILs with anions of HPAs. Rajkumar and Ranga Rao (2008a) investigated the properties of a solid hybrid material containing [BMIM] cations and Keggin anions of phosphotungstic acid (H3PW12O40). The IL ([BMIM][Br]) was first synthesized, followed by the pro­duction of the hybrid material. It was found that it dissolves in organic solvent (DMSO), but not in water. Characterization showed that the pairing of Keggin anion and imidazolium cation leads to the formation of organic-inorganic hybrid molecu­lar solid, and most of water molecules are replaced by three imidazolium cations. Other than that, the weight loss in thermogravimetric analysis (TGA) shows the decomposition temperature was around 400°C, which proved that it is ther­mally stable even at high temperature.

An almost similar pattern was observed when imidazolium ions were paired with silicotungstic acid (Rajkumar and Ranga Rao 2008b) and phosphomolybdic acid (Ranga Rao et al. 2009) for preparation of hybrid molecular materials. Both materi­als were insoluble in water. The former showed major weight loss at 400-580°C for TGA test, while the latter decomposed at 300-500°C.

Different ionic solids can be used for catalyzing chemical synthesis. One of the applications of ionic solid catalysts was the epoxidation of alkenes with H2O2 (Leng et al. 2011b). POM-based ionic hybrid catalysts were prepared, which resulted in a semi-amorphous solid composed of nanospheres and insoluble in almost all the commonly used solvents. They hold the advantages of convenient, steady reuse, high conversion and selectivity, simple preparation, and flexible composition. The combination of amino-functionalized cations and Keggin-POM anions showed high conversion and 100% selectivity. In addition to that, the catalysts were insoluble in the reaction, highlighting the convenient step of catalysts recovery by filtration.

Leng et al. (2011a) conducted the hydroxylation of benzene with H2O2 using heteropolyanion-based ionic hybrid solid. The catalyst was the result of combining cation from divalent IL and Keggin-structured heteropolyanion. The evaluation of the catalyst proved that its activity was the highest among other catalysts for the hydroxylation process and insoluble with other reactants. Slight deactivation of the catalyst was observed during the catalytic reusability test and was caused by leach­ing of the catalyst in the early two runs.

Ionic solids also have been proven to be able to catalyze the esterification reac­tion. Leng et al. (2009) prepared ionic solid catalysts by integrating three different organic cations containing propane sulfonate (PS) from IL and anions from H3PW12O40 for esterification of carboxylic acids. [MIMPS]3PW12O40 was successful for ester production of various carboxylic acids and alcohols. An interesting point to be noted here is that the catalyst completely dissolves in the medium to form a homogeneous mixture at the beginning of the reaction, and the catalyst precipitated at the end of the process. The progress of the esterification using citric acid is

photographed in Fig. 9.5. The other two catalysts, [TEAPS]3PW12O40 and [PyPS]3PW12O40 have lower catalytic activities than [MIMPS]3PW12O40.

The absence of PS functional group from the organic cations of ILs resulted in much lower ester yield than those with PS functional group. Also, different inor­ganic anions containing the same MIMPS cation, such as SiW12O4O and PMo12O40, lead to different ester yield. The yield from the esterification of citric acid reduced from 95.4% in the first run to 84.5% in the fourth run during catalyst recycling, while the selectivity maintained at 98%.

The transesterification of Jatropha oil was conducted using catalysts based on pyridinium cation of ionic liquids (Li et al. 2010). The inclusion of [BSPy] cation with three HPAs produced [BSPy]3PW12O40, [BSPy]3PMo12O40, and [BSPy]3SiW12O40 catalysts. FAME yield for these three catalysts was around 80%, and the conversion was more than 90%. However, the reaction temperature was a little bit higher than for ILs (i. e., 120°C). Furthermore, these catalysts were less active than those [BSPy]-based ILs, probably as a result of different phase with the reactant mixtures. Biodiesel productivity was higher for ILs because they are considered as homoge­neous catalyst, thus avoiding mass transfer limitation. FAME yield was highest when using [BSPy][CF3SO3] as the catalyst (92%), obtained in 5 h reaction time at the temperature of 100°C.