Photocatalytic Properties of Tetraphenylporphyrins Immobilized on Calcium Alginate Aerogels

We have prepared photocatalytic systems based on tetraphenylporphyrins (TPP) immobilized on calcium alginate solid gels in the conditions of thermal drying on air (xerogel), freeze drying in vacuum (cryogel) and supercritical drying in the supercritical carbon dioxide (scCO₂) medium (aerogel). As a test reaction to measure the prepared systems' efficiency, we studied tryptophan photooxidation in the aqueous medium. We have demonstrated that the systems with aerogel as a carrier exhibited the highest photocatalytic efficiency. In that case, the rate constant for the test substrate (tryptophan) oxidation exceeds the corresponding rate constants of similar systems based on xerogel and cryogel by more than 5 times. Moreover, the aerogel-based photocatalytic systems demonstrated enhanced functional stability and a possibility of multiple use of such a catalyst in tryptophan oxidation. Based on the data of small-angle X-ray scattering and thermooxidative destruction, we have made a conclusion about the relationship between the high photocatalytic activity of aerogel-immobilized TPP and formation of a developed porous aerogel structure in the conditions of drying in the scCO₂ medium, which is stabilized due to formation of additional complex bonds of calcium ions with fragments of glycoside rings.