Solid Acid Catalysts in Ionic Liquid Solutions
Solid acid catalysts have numerous advantages over liquid catalysts regarding activity, selectivity, catalyst life, and most importantly, ease in recovery and reuse. They are widely studied as direct replacements for liquid acids to reduce pollutants and to lower operating costs. However, most solid acids do not function effectively for cellulose hydrolysis because the surfaces of these solids do not have strong acid sites or cannot allow close contact with ^-1,4-glucans. Solid acid catalysts have been used in the depolymerization of cellulose suspended in water as well as cellulose dissolved in ionic liquid media. The case of application of solid acids in ionic liquid solutions will be discussed in this section, and the solid acids in water and other mediums will be discussed in Section 7.5.
Zhang and Zhao reported [62] that solid acid catalyzed hydrolysis of cellulose in ionic liquid can be promoted by microwave heating, where H-form zeolites with a lower Si/Al molar ratio and a larger surface area showed a relatively higher catalytic activity. These solid catalysts exhibited better performance than the sulfated ion-exchanging resin NKC-9. Compared with conventional oil bath heating mode, microwave irradiation at an appropriate power significantly reduced the reaction time and increased the yields of reducing sugars. A typical hydrolysis reaction with Avicel cellulose produced glucose in around 37% yield within 8 min [62]. In another example, Amberlyst 15DRY has been shown as a solid acid catalyst for the depolymerization of cellulose and wood solubilized in 1-”butyl-3-methylimidazolium chloride (BMIMCl) at 100°C [63, 64].
The other kind of solid acids that can be used effectively in ionic liquid medium are acidic ion exchange resins. In 2011, Qi et al. reported that high glucose yields of above 80% could be obtained from the cellulose hydrolysis by a strong acidic cation exchange resin in 1-ethyl-3-methyl imidazolium chloride (EMIMCl) with gradual addition of water [65].
In 2010, Amarasekara and Owereh reported the use of a sulfonic acid group functionalized acidic ionic liquid immobilized on silica catalyst (AIL-SiO2) shown in Figure 7.9 for the hydrolysis cellulose. This silica-supported acid catalyst was shown to be effective in the hydrolysis of cellulose (DP ~ 450) dissolved in 1-”butyl-3-methyl — imidazolium chloride at 70°C, producing glucose and total reducing sugars in 26 and 67% yields, respectively [66]. Their comparison of the catalytic activity of Bronsted acidic ionic liquid immobilized on silica catalyst (AIL-SiO2) with sulfonated silica (SO3H-SiO2), and sulfuric acid in the depolymerization of cellulose dissolved in 1-”bu- tyl-3-methylimidazolium chloride is summarized in Table 7.1. [66].