Delopment of Composite Carbon–Silicate Materials, Their Research and Testing for the Preparation of Heterogeneous Biocatalysts for Low-Temperature Synthesis of Esters

Composite carbon–silica materials (CCSM), differing in the content of carbon and silica components, were obtained using two silicon dioxide precursors (silica sol and silane) and multi-walled carbon nanotubes (MWNTs). At the initial stage of obtaining CCSM by method 1, impregnation of finely dispersed MWCNT powder with silica sol was used, method 2 was carried out using treatment of MWCNTs with tetraethoxysilane followed by hydrolysis and polycondensation. The content of silica (SiO₂) in the composites varied from 3 to 60 wt %. After drying and appropriate heat treatment at 250–350°C, the composite materials were studied by various physicochemical methods: nitrogen porosimetry, electron microscopy, X-ray fluorescence analysis, and synchronous thermal analysis. Significant differences in parameters were found depending on the chemical composition of CСSM, including textural characteristics. Thus, with an increase in the SiO₂ content, the specific surface area of composite materials increased (by a factor of 2), and maxima were observed on the distribution curves over pore diameters (at 20–40 nm).The composite carbon–silica materials were tested as adsorbent for the preparation of heterogeneous biocatalysts (BC) for the low-temperature synthesis of esters; the active component of these BC was lipase immobilized exclusively on the carbon surface of nanotubes. With a decrease in the content of MWCNTs in the composite materials, the enzymatic activity and operational stability of biocatalysts, measured in the reaction of esterification of heptanoic acid (C₇) with butanol (C₄), decreased monotonically, reaching a 2–8-fold drop in activity at the maximum content of SiO₂ (58 wt %).