Some rigid carbon materials like carbon nanotubes, graphene, ordered mesoporous carbon, and activated carbon are relatively hard to be sulfonated. Concentrated H2SO4 and fuming H2SO4 are used as the sulfonating agents. However the CBSC prepared with fuming H2SO4 possess much higher catalytic activity (Dehkhoda et al. 2010; Kastner et al. 2012). Researchers have extensively studied the influence of different parameters like sulfonating agent, sulfonation time, and carbon precur­sor on the activity of such catalysts by direct sulfonation. Directly pyrolysis of the carbon precursors and sulfonation is concentrated at high H2SO4 temperatures, pro­duces sulfonated carbons with low in acid density, lower specific surface area, and poor reusability (Konwar et al. 2013). Figure 15.5 shows the way for the preparation of SO3H-carbon (Konwar et al. 2014).

Studies also indicated that the catalytic activity is primarily determined by total acid density, — SO2H density, surface functional groups, and pore structure. It is established that a high — SO3H density and pore volume favored high activity. These properties in turn are directly influenced by the carbonization temperature (Konwar et al. 2014). Apart from these there are numerous other reports on the preparation of similar sulfonated carbons catalysts, by one-step hydrothermal carbonization/

sulfonation (Xiao et al. 2010), polymerization followed by sulfonation and carbon­ization (Zhao et al. 2010) , by the thermal treatment of p-toluene sulfonic acid (Zhang et al. 2010) etc.