Kinetics of producing vanillin and 4-hydroxy benzaldehyde from the hydrolysis residue of rice straw by photocatalysis

Abstract

Alkali soluble lignin present in rice straw hydrolysis residue (RSHR) is subjected to photocatalytic treatment to study its degradation rates and production of different value-added oxygenated organic compounds. TiO₂ and ZnO are used as photocatalysts. ZnO could degrade lignin faster than TiO₂ in pseudo-first order photocatalytic degradation. The maximum observed photocatalytic lignin degradation is 83.4% using 2 g/L ZnO dose with pseudo-first order rate constant 0.1386 h⁻¹. Vanillin and 4-hydroxy benzaldehyde are the two important value-added products formed during photocatalysis. Both these compounds, themselves, are susceptible to photocatalytic degradation. Vanillin degradation followed pseudo-first order kinetics and the highest rate constant of 0.1415 h⁻¹ is achieved using 2 g/L ZnO as photocatalyst. While the degradation kinetics of 4-hydroxy benzaldehyde is pseudo-first order with TiO₂ and ZnO as a photocatalyst. Higher catalyst doses increased the reaction rate constant. With multiple reactions in operation, the concentration of both vanillin and 4-hydroxy benzaldehyde in the reaction mixture first increased, attained their maxima, and thereafter decreased. With 1 g/L TiO₂ as a photocatalyst, the maximum attained vanillin concentration is 22.4 mg/L after 7 h photocatalysis. Maximum attained vanillin concentration, after 8 h photocatalysis, is significantly higher at 51.2 mg/L when 2 g/L ZnO is used as a catalyst. 4-Hydroxy benzaldehyde is produced in lesser amounts. Its maximum observed concentration in the reaction mixture is 20.4 mg/L, obtained with 1.5 g/L TiO₂ after 10 h photocatalysis.