The non-renewability of fossil energy and the increasingly prominent global environmental problems caused by it have made the development of biomass energy highly valued by governments of all countries(Danish and Wang 2019; Cao et al. 2020). Lignocellulose is the most widely distributed and abundant biomass resource on earth(Chandel et al. 2018; Karumuri et al. 2015). Through certain processing, cellulose can be transformed into a series of products with great application value, such as fuel(Ci et al. 2017), chemical raw materials(Ji et al. 2019) and pharmaceutical products(Zhang et al. 2015a). The utilization and transformation of cellulose is of great significance to solve the world energy crisis and environmental pollution problems.
Cellulose is a linear polymer compound bound by glucose molecules with β-1,4-glycosidic bonds, and generally requires pretreatment to be degraded into small molecular sugars before it can be used. Lignocellulose can be degraded by physical method(Chen et al. 2017b), chemical method(Zhang et al. 2020), biological method(Wang et al. 2020). Biological method generally uses cellulase to hydrolyze lignocellulose into glucose. It has the advantages of mild conditions, environmental friendliness, strong specificity, high catalytic efficiency, and low unit energy consumption, meeting the sustainable development of today’s society demand. However, the use cost of cellulase accounts for 50% of the total cost of cellulose enzymatic hydrolysis and saccharification(Gokhale and Lee 2012). Therefore, obtaining immobilized enzymes with more stable performance and recyclable utilization is an effective way to reduce the application cost of cellulase.
The catalytic performance of immobilized enzymes is highly dependent on the property of carrier materials(Drout et al. 2019; Hu et al. 2018). The desired carrier materials should have the characteristics of good stability, easy regulation of physical and chemical properties. A series of carrier materials, such as natural products(Wang et al. 2016; Aigner and Scheibel 2019), inorganic carriers(Grewal et al. 2017; Zhang et al. 2019), organic polymers(Lozano et al. 2014), magnetic carriers(Hosseini et al. 2018; Zhou et al. 2020), and metal organic framework(Qi et al. 2018; Phipps et al. 2020), have been developed for enzyme immobilization. As a new type of porous material, metal-organic framework is a kind of porous coordination polymer with two-dimensional or three-dimensional pore structure connected by metal nodes and organic ligands through coordination bonds(Cui et al. 2018). MOF materials have the advantages of high porosity and high specific surface, so its application is in full swing, especially in the field of enzyme immobilization(Liang et al. 2015).
There are relatively few studies on the use of mesoporous MOF materials for the immobilization of cellulase. The diffusion of macromolecular cellulose into micro/mesoporous MOF is often limited. The currently reported immobilization of cellulase on MOF is mainly surface adsorption(Khoshnevisan et al. 2011) and covalent binding(Huang et al. 2020). Multi-step reactions are often involved in covalent bonding process, and the preparation of carrier materials is complicated. The physical adsorption method has remarkable characteristics of simple and mild immobilization process, reducing the loss of activity during immobilization process. However, the immobilized enzyme is easily detached from the carrier material due to weak binding force, which is not conducive to the recycling of enzyme.
UIO-66-Zr is a MOF material with highly stable secondary building units Zr6O4(OH)4, which makes UIO-66-Zr highly chemically and thermally stable(Schaate et al. 2011; Decoste et al. 2013). Lipase was immobilized on UIO-66-Zr by surface adsorption with 202.4 mg g− 1 loading capacity(Liu et al. 2015). Soybean epoxy-hydrolase (SEH) can be effectively fixed on the amino surface of UIO-66-NH2 MOF by cross-linking(Cao et al. 2016). To the best of our knowledge, the immobilization of cellulase onto bare UIO-66-Zr has seldom been reported. Meanwhile, the synthesis of nanocrystals of mesoporous UiO-66-Zr for enzyme immobilization is not straightforward. Although the NH2 functionalized UIO-66-Zr has been developed as carrier for anchoring cellulase(Ahmed et al. 2018; Zhou et al. 2019), the adsorption mechanism of cellulase onto UIO-66-Zr and how to enhance adsorption needs to be further explored.
In this paper, a mesoporous UIO-66-Zr was first synthesized by biomineralization method using dextran as template, and the prepared MOF was used as support material for cellulase immobilization by physical adsorption. Furthermore, a small amount of PVP was used to improve the stability of immobilized cellulase. The resultant mesoporous MOF before and after enzyme immobilization were characterized by BET, FT-IR, XRD, TGA, DLS, SEM and TEM. The stability, reusability, and catalytic efficiency of immobilized cellulase were investigated.