# Ionic Liquids for Development of Heterogeneous Catalysts Based on Nanomaterials for Biocatalysis
###### Abstract  
The development of effective methods of enzyme stabilization is key for the evolution of biocatalytic processes. An interesting approach combines the stabilization process of proteins in ionic liquids and the immobilization of the active phase on the solid support. As a result, stable, active and heterogeneous biocatalysts are obtained. There are several benefits associated with heterogeneous processes, as easy separation of the biocatalyst from the reaction mixture and the possibility of recycling. Accordingly, this work focused on the supported ionic liquid phases as the efficient enzyme stabilization carriers, and their application in both continuous flow and batch biocatalytic processes. supported ionic liquid phase; supported ionic liquid-like phase; biocatalysis; enzyme; heterogeneous catalysis; immobilization; nanomaterials +
Footnote †: journal:  
* [2021] 11 2030 10 2021  
that close the protein in a support, without attachment to the matrix. For example, _Candida antarctica_ B lipase (CALB) was entrapped in electrospun poly(vinyl alcohol) (PVA) nanofibers. It was reported that entrapped CALB showed higher activity, stability, and reusability, when poly(ethylene glycol) (PEG) was added as additive for lipase immobilization [8]. CALB was encapsulated in the hybrid nanoflowers, consisting of copper (II) or manganese (II) ions, combined with magnetic and carbon nanoparticles. The enzyme performed great catalytic activity, stability, and reusability (eight cycles) in tyrosol ester production [9]. On the other hand, cross-linking is the method for the creation of an intermolecular cross-linkage between the enzyme particles and cross-linking agent, e.g., glutaraldehyde. A special example is the cross-linked enzyme aggregates (CLEAs), which are prepared by using different precipitants (e.g., ammonium sulfate, acetone, PEG) and cross-linkers. Cross-linked CALB aggregates that were obtained with PEG, exhibited the highest activity in the esterification of lauric acid with 1-propanol [10]. CALB was attached onto mesoporous silica nanowires via covalent bonding. Immobilized lipase provided a 94.3% yield (Y) of biodiesel production and a long stability (eight cycles), without a significant loss of activity [11]. The most common among enzyme immobilization methods, is the physical adsorption. This technique is inexpensive, fast, and easy to perform. The activity of the CALB immobilized on various silica supports was studied in the Baeyer-Villiger oxidation of cyclic ketones to lactones. The biocatalysts showed great stability, even in 60% hydrogen peroxide, and enabled high yields of lactones to be achieved [12]. An interesting approach, enhancing the stability and activity of enzymes, proved to be combining ionic liquids and immobilization on solid supports. In this method, the following four different types can be distinguished: supported ionic liquid catalyst (SILC), solid catalyst with ionic liquid layer (SCIL), supported ionic liquid phase (SILP), and supported ionic liquid-like phase (SILLP) (Figure 1). The supported ionic liquid catalyst technique involves the attachment of an enzyme, via covalent bonding, to ionic liquids moieties that are grafted to the solid matrix. Applications of SILC for enzyme immobilization in biocatalysis were reported. _Candida rugosa_ lipase (CRL) was covalently anchored to IL particles that were attached to the magnetic nanoparticles (Fe\({}_{2}\)O\({}_{3}\)). ILs with different chain lengths (C4, C8) and anions (Cl\({}^{-}\), [BF\({}_{4}\)]\({}^{-}\), [PF\({}_{6}\)]\({}^{-}\)) were tested both as coupling and stabilizing enzyme agents. The activity of the synthesized SILC biocatalysts was examined in the esters hydrolysis. Covalently immobilized CRL exhibited higher activity and stability than its native form [13]. _Porcine pancreas_ lipase (PPL), covalently bonded to alginate nanoparticles, modified with imidazolium ILs,  
Figure 1: Enzyme immobilized on the supports modified with ionic liquids.