nanomaterials Review Ionic Liquids for Development of Heterogeneous Catalysts Based on Nanomaterials for Biocatalysis Anna Wolny and Anna Chrobok * Department of Chemical Organic Technology and Petrochemistry, Faculty of Chemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-237-2917 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. Keywords: supported ionic liquid phase; supported ionic liquid-like phase; biocatalysis; enzyme; heterogeneous catalysis; immobilization; nanomaterials Citation: Wolny, A.; Chrobok, A. 1. Introduction Ionic Liquids for Development of Increasing the ecological awareness of the society and subsequent restrictive regula- Heterogeneous Catalysts Based on tions concerning environmental protection, push the chemical industry to develop clean Nanomaterials for Biocatalysis. technologies. In order to achieve the United Nations sustainable development goals 2030, Nanomaterials 2021, 11, 2030. https:// each material or product should be safe and sustainable. The use of volatile organic sol- doi.org/10.3390/nano11082030 vents, hazardous substances, production of large amounts of hazardous wastes, and the Academic Editor: Haralambos need to provide large amounts of energy, are the main problems for the environment, Stamatis associated with chemical processes [1]. Additionally, nanotechnology is often mentioned as a technology that could enable a green growth [2]. Therefore, solutions reducing both Received: 10 July 2021 the harmful effects of chemical processes on the environment and enhancing synthesis Accepted: 5 August 2021 effectiveness are desirable. Green chemistry rules deliver the clues for sustainable and eco- Published: 10 August 2021 logical development. One of the most important paths of chemical industry development is the search for a new effective catalysts or/and biocatalyst [3,4]. Publisher’s Note: MDPI stays neutral Enzymes represent a great alternative to conventional catalysts generating hazardous with regard to jurisdictional claims in wastes [4]. Enzymes are protein particles that enable various chemical processes to be published maps and institutional affil- carried out under mild conditions, and provide high effectiveness of synthesis, due to high iations. enantio-, regio- and chemo-selectivity. Unfortunately, there are some limitations associated with protein applications. Enzymes are sensitive to temperature, pressure, pH changes, and organic solvents. A non-aqueous environment can lead to their folded three-dimensional structure being destroyed, and, in consequence, to biocatalyst deactivation [5]. For that Copyright: © 2021 by the authors. reason, lots of enzymes stabilization methods were developed, e.g., via ionic liquids (ILs), Licensee MDPI, Basel, Switzerland. which will be described in this work. Among them, the most common method for enzyme This article is an open access article stabilization is immobilization in, or on, a solid matrix. Immobilization not only provides distributed under the terms and enzyme stabilization, but also enables the easy separation of heterogeneous biocatalysts conditions of the Creative Commons from the reaction system. The following methods of enzyme immobilization have been Attribution (CC BY) license (https:// developed: entrapment, encapsulation, cross-linking, and adsorption or covalent attach- creativecommons.org/licenses/by/ ment onto the insoluble carriers [6,7]. Enzyme entrapment and encapsulation are methods 4.0/). Nanomaterials 2021, 11, 2030. https://doi.org/10.3390/nano11082030 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 20 Nanomaterials 2021, 11, 2030 or covalent attachment onto the insoluble carriers [6,7]. Enzyme entrapment and encap-2 of 19 sulation are methods 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)that close (PVA) the nanofibers. protein in a It support, was reported without that attachment entrapped to CALB the matrix. showed For higher example, activity,Can- stabilitydida antarctica, and reusability,B lipase (CALB)when poly(ethylene was entrapped glycol) in electrospun (PEG) was poly(vinyladded as additive alcohol) for (PVA) li- pasenanofibers. immobilization It was reported[8]. CALB that was entrapped encapsulated CALB in the showed hybrid higher nanoflowers activity,, consisting stability, andof copperreusability, (II) or whenmanganese poly(ethylene (II) ions, com glycol)bined (PEG) with was magnetic added and as additivecarbon nanoparticles. for lipase immobi- The enzymelization performed [8]. CALB great was encapsulatedcatalytic activity, in the stability hybrid, and nanoflowers, reusability consisting (eight cycles) of copper in tyro- (II) solor ester manganese production (II) ions,[9]. On combined the other with hand, magnetic cross-linking and carbon is the method nanoparticles. for the Thecreation enzyme of anperformed intermolecular great catalyticcross-linkage activity, between stability, the andenzyme reusability particles (eight and cycles) cross-linking in tyrosol agent, ester e.gproduction., glutaraldehyde. [9]. On A the special other example hand, cross-linking is the cross-linked is the methodenzyme foraggregates the creation (CLEAs) of an, whichintermolecular are prepared cross-linkage by using betweendifferent theprecipitants enzyme particles (e.g., ammonium and cross-linking sulfate, agent,acetone, e.g., PEG)glutaraldehyde. and cross-link Aer specials. Cross example-linked is CALB the cross-linked aggregates enzymethat were aggregates obtained (CLEAs),with PEG which, ex- hibitedare prepared the highest by using activity different in the precipitantsesterification (e.g., of lauric ammonium acid with sulfate, 1-propanol acetone, [10] PEG). CALB and wascross-linkers. attached on Cross-linkedto mesoporous CALB silica aggregates nanowires that via covalent were obtained bonding. with Immobilized PEG, exhibited lipase the providedhighest activitya 94.3% inyield the (Y) esterification of biodiesel of production lauric acid withand a 1-propanol long stability [10]. (eight CALB cycles) was attached, with- outonto a significant mesoporous loss silica of activity nanowires [11]. via The covalent most common bonding. among Immobilized enzymelipase immobilization provided a methods94.3% yield, is the (Y) physical of biodiesel adsorption. production This and technique a long stability is inexpensive, (eight cycles), fast, and without easy ato signifi- per- form.cant The loss a ofctivity activity of [the11]. CALB The most immobilized common amongon various enzyme silica immobilization supports was studied methods, in isthe the Baphysicaleyer–Villiger adsorption. oxidation This of technique cyclic ketones is inexpensive, to lactones. fast, The and biocatalysts easy to perform. showed The great activity sta- bilityof the, even CALB in 60% immobilized hydrogen onperoxide various, and silica enabled supports high was yield studieds of lactones in the to Baeyer–Villiger be achieved [12]oxidation. of cyclic ketones to lactones. The biocatalysts showed great stability, even in 60% hydrogenAn interesting peroxide, approach and enabled, enhancing high yields the stability of lactones and to activity be achieved of enzymes [12]. , proved to be combiningAn interesting ionic liquids approach, and immobilization enhancing the stabilityon solid andsupports. activity In ofthis enzymes, method proved, the fol- to lowingbe combining four different ionic liquidstypes can and beimmobilization distinguished: supported on solid supports. ionic liquid In thiscatalyst method, (SILC), the solidfollowing catalyst four with different ionic liquid types layer can be (SCIL), distinguished: supported supported ionic liquid ionic phase liquid (SILP) catalyst, and (SILC), sup- portedsolid ionic catalyst liquid with-like ionic phase liquid (SILLP) layer (Figure (SCIL), 1). supported ionic liquid phase (SILP), and supported ionic liquid-like phase (SILLP) (Figure1). Figure 1. Enzyme immobilized on the supports modified with ionic liquids. Figure 1. Enzyme immobilized on the supports modified with ionic liquids. The supported ionic liquid catalyst technique involves the attachment of an enzyme, viaThe covalent supported bonding, ionic to liquid ionic liquidscatalyst moieties technique that involves are grafted the attachment to the solid of matrix. an enzyme Appli-, viacations covalent of SILC bonding for enzyme, to ionic immobilization liquids moieties in that biocatalysis are grafted were to the reported. solid matrix.Candida Appli- rugosa cationslipase of (CRL) SILC was for covalentlyenzyme immobilization anchored to ILin particlesbiocatalysis that were were reported. attached Candida to the magnetic rugosa lipase (CRL) was covalently