# Ionic Liquids for Development of Heterogeneous Catalysts Based on Nanomaterials for Biocatalysis
## 3 Supported Ionic Liquid Phases in Biocatalysis
The next approach employs D-glucose-based ionic liquids that are adsorbed on MWCNTs, for CALB immobilization. The CALB-SILLP system was used in the esterification reaction of acrylic acid and n-butanol. The biocatalyst provided a high ester yield (99%), selectivity, and long stability. Lipase retained its activity for five cycles and three cycles, with over a 90% yield of n-butyl acrylate [78]. The next studies of this group included two approaches of the chemical modification of MWCNTs, as presented in Figure 5.  
Ionic liquids were anchored via amide or imine bonds. Prepared SILLPs were tested as carriers for CALB, CRL, and _Aspergillus oryzae_ lipase immobilization. Biocatalysts were used in the Baeyer-Villiger oxidation of 2-adamantanone (Figure 6). All the immobilized lipases demonstrated great activity and stability in the aggressive environment of hydrogen peroxide. Four reaction cycles, without a loss of the enzyme's activity, were performed [79]. Silica materials are the most common group of supports used in SILP and SILLP techniques in biocatalysis. Silica mesoporous materials are characterized by ordered porosity, well-defined pore geometry, large surface area, and possibility of surface modification,  
Figure 5: Scheme of MWCNTs chemical modifications with ionic liquids. Figure 6: Scheme of Baeyer–Villiger oxidation of 2-adamantanone catalyzed by supported ionic liquid phase biocatalysts. which enable high enzyme loading [80]. In the beginning of the section, the first reports about SILP based on silica materials (Celite) were described. The other approach includes supported SILLP for enzyme immobilization. PPL was adsorbed on SILLP, based on mesoporous silica SBA-15. Further, 1-methyl-3-(3-trimethoxysil-propyl)imidazolium tetrafluoroborate was attached onto hydroxyl groups on the silica surface via covalent bonding. The enzyme activity in triacetin hydrolysis was improved, from 594 to 975 U/g, after support modification with IL [81]. The next studies of this group focused on the influence of alkyl chain length in the SILLP' s imidazolium ring on PPL stabilization. Once more, the enzyme showed higher stability and reusability if a longer alkyl chain (C8) was presented in the SILLP structure [82]. Further studies included the effect of various functional groups, such as alkyl, amino, and carboxyl, in the IL structure, attached via covalent bonding onto the mesoporous silica, for PPL activity. Lipase was adsorbed on SILLP and examined in triacetin hydrolysis. Each synthesized SILLP improved protein activity. PPL-SILLP, consisting of IL functionalized with an amino group, expressed the highest activity, which was almost 13 times higher compared with PPL-SBA-15 [83]. Another investigation concentrated on the following different techniques for PPL immobilization: via physical adsorption and covalent bonding. For this purpose, SBA-15 was modified with IL functionalized with a carboxyl group. Both of the biocatalysts presented better stability compared with PPL-SBA-15, but lipase that was grafted into SILLP retained 81.25% of its original activity, when the lipase adsorbed on SILLP retained 52.5% after 20 days of incubation and four reaction cycles [84]. The following experiments checked PLL activity, when the lipase was attached onto the IL anion via ionic binding and cross-linking. The activity of the lipase that was immobilized on the support consisting of imidazolium IL with L-lysine anion, did not decrease after five cycles, so leaching of the enzyme was effectively inhibited [85]. The next studies of this group examined the influence of the particle type grafted into mesoporous silica, for enzyme immobilization and activity in triacetin hydrolysis. The following various supports were prepared: C1-SiO\({}_{2}\), C8-SiO\({}_{2}\), C16-SiO\({}_{2}\), SH-SiO\({}_{2}\), Ph-SiO\({}_{2}\), NH\({}_{2}\)-SiO\({}_{2}\), CH\({}_{3}\)IL-SiO\({}_{2}\) (methyl group in imidazolium ring), and COOHIL-SiO\({}_{2}\) (alkyl chain in imidazolium ring functionalized with carboxyl group).