Synthesis of mesoporous silica with different pore sizes for cellulase immobilization: pure physical adsorption

Abstract

To discuss the physical adsorption mechanism of the adsorption process of cellulase, a commercial enzyme cocktail sourced from Acremonium was immobilized in mesoporous silica with various pore sizes by pure physical adsorption in this study. Mesoporous silica materials with 17.6 nm and 3.8 nm pore sizes (hereafter referred to as MS-17.6 nm and MS-3.8 nm, respectively) were synthesized in the manner of a seeded-growth method. Other available mesoporous silica materials denoted as H-32 and diatomite were also used as sorbents. Then, the sorbents were characterized via small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), the Barrett–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) method to confirm their mesostructure. Furthermore, the adsorption abilities of different sorbents and enzymatic activities of immobilized cellulase were studied. The adsorption amounts exhibited a clear correlation with the pore size of the sorbents; i.e., the adsorption amount of MS-17.6 nm (410 mg g⁻¹) with the pore size similar to the long axes of cellulase molecules was higher than that of MS-3.8 nm (315 mg g⁻¹) with the pore size approximated to the short axes of cellulase (which was realized at 50 °C). Besides, the adsorption behavior of diatomite (with a pore size of about 200 nm) revealed a periodicity because the pore size was significantly larger than cellulase molecules. Moreover, the pore size was suggested to be a critical factor for the enzymatic activity of cellulase. When the average pore size of MS-3.8 nm just matched the short axes of cellulase molecules, immobilized cellulase preserved the active sites of cellulase intactly and showed the best activity (i.e. 63.3% of free cellulase activity at 50 °C). Consequently, the pore size of the sorbents had a significant influence on cellulase immobilization.