Effect of freezing treatment on structure and adsorption characteristic of soy protein porous materials crosslinked by aldehydes

Abstract: The main goal of this work was to prepare porous materials using renewable soy protein as raw material. Soy protein was first blended with sodium hydroxide solution and crosslinked by aldehydes (formaldehyde, glyoxal and glutaraldehyde) to fabricate soy protein gel. The gel was then treated by freezing in refrigerator and liquid nitrogen to obtain porous materials after dry vacuum. A series of dynamic rheological tests were conducted for the gel, which included the strain sweep ranging from 0.5% to 120% at 1 Hz and the frequency sweep ranging from 0.13 to 3.6 Hz at 2% strain. Pore properties of these porous materials including the BET specific surface area, average pore diameter, pore size distribution and pore volume were characterized by the DFT method based on the nitrogen adsorption/desorption isothermal. The microstructure of porous materials was investigated by the field emission scanning electron microscope (FESEM). The thermostability of porous materials from ambient temperature to 600 ℃ in nitrogen was investigated at the heating rate of 10℃/min by thermo gravimetric analysis technology. The results showed that the aldehydes crosslinked soy protein was efficient in forming gel. The storage modulus and loss modulus of soy protein gels were all increased as the scanning frequency increased, and the storage modulus was always higher than the loss modulus; the glutaraldehyde crosslinked soy protein gel had the highest storage modulus, as evidenced by rheological behavior analysis. These indicated that glutaraldehyde crosslinked protein was better than formaldehyde or glyoxal. Pore properties analysis results showed that aldehydes treatment porous materials had a higher BET specific surface area, a larger pore volume, and a smaller porous size than the control sample. Compared with the sample treated in refrigerator, the similar tendency was also observed on the porous materials treated by liquid nitrogen, which was that the diameter of nanoscale pore size was lower than 80 nm, the pore volume of mesoporous accounted for at least 50% of total pore volume, and the pore volume percentage decreased with the order of mesoporpus > macropore > micropore. All the refrigerator-treated porous materials showed the diameter of nanoscale pore size was lower than 70 nm. The BET specific surface area and pore volume of porous materials treated by liquid nitrogen had the decreased tendency with the order of glutaraldehyde > glyoxal > formaldehyde, indicating glutaraldehyde could be the crosslinking agent for soy protein porous material, but the freezing treatment had a greater influence on the pore structure than aldehydes did. This also could be evidenced by the nitrogen adsorption/desorption isothermal. The FESEM observations suggested that the pore patterns of all soy protein porous materials were micron grade circular pore and nanoscale slit pore, but the liquid nitrogen treatment resulted in more circular pore than the refrigerator treatment. Two times the mass loss (before 100℃ and after 200℃) occurred during the heating process of all samples, but soy protein porous material prepared from glutaraldehyde and liquid nitrogen treated soy protein showed better thermal stability after 440℃, indicating a better thermostability as evidenced by the thermo gravimetric analysis. There were 14.5% p-nitrophenol and 5.6% hexavalent chromium absorbed when glutaraldehyde and liquid nitrogen treated soy protein porous materials were soaked into the solution of p-nitrophenol and potassium dichromate for 3 h, respectively. Therefore, glutaraldehyde and liquid nitrogen treated soy protein porous materials could be potentially applied as adsorption or thermal insulation materials, or prepare other porous materials such as carbon aerogel.