Activation of Sibunit Carbon Materials with Nitrogen(II) Oxide: Studying the Physicochemical and Adsorption Properties of Nitrogen-Containing Supports and Testing the Resulting Adsorbents for Preparing Biocatalysts for the Low-Temperature Synthesis of Esters

The activation of the Sibunit carbon material by etching with nitric oxide (NO) enabled the synthesis of nitrogen-containing supports with a content of introduced nitrogen (N) varied in a range of 2.3–10.7 at % (according to XPS). Etching was implemented in a high-pressure reactor (15–29 atm) at elevated temperatures (180 and 230°C). The physicochemical and adsorption properties of the synthesized nitrogen-containing carbon supports were studied. Thus, studies of the texture of the supports showed that with an increase in the nitrogen content, the specific surface area of the materials decreases from 470 to 65 m2/g, while the average pore diameter increases from 7.4 to 18.1 nm. The analysis of XP spectra revealed that the N atom in the carbon materials is present in five forms. For the material with the maximum nitrogen content (11.7 at %), the proportions of states (α) of different forms are the following: α = 0.47 (pyridinic), α = 0.38 (pyrrolic), α = 0.13 (graphitic N), and α = 0.011 and 0.0077 (as a part of O=NC and C–NO2 groups, respectively.. The synthesized nitrogen-containing carbon materials were studied as adsorbents for the immobilization of the recombinant rPichia/lip lipase. Using SAXS and SEM methods, it was shown that the adsorbed lipase is capable of forming octamers and larger complex-structure aggregates (composed of more than 100 molecules). The content of individual (nonaggregated) enzyme molecules increased (by 3.3 times) with an increase in the nitrogen content (up to 11 at %) in the carbon material. Biocatalysts (BCs) prepared by the adsorptive immobilization of the recombinant rPichia/lip lipase were tested in the low-temperature esterification of heptanoic acid (C7) with butyl alcohol (C4) in hexane. It was found that the introduction of nitrogen during a single-stage post-activation of the Sibunit material with NO has a significant positive effect on the enzyme activity and stability of the prepared BCs. The optimum content of introduced nitrogen is 7–10 at %; in this case, the biocatalytic activity increases by 2–3 times (as compered to the original Sibunit), and the heptanoic acid conversion is 84% within 6 h at 20 ± 2°C.