The present research introduces the photocatalytic process for the conversion of 4-carboxybenzaldehyde to terephthalic acid through the synthesis of phosphomolybdic acid (PMo) supported on TiO2 and MCM-41 as the new class of photocatalysts for this application. As synthesized photocatalysts were characterized through XRD, FT-IR, N2 adsorption–desorption, and FESEM-EDS. It was found that the maximum conversion of 4-carboxybenzaldehyde to para-toluic acid was achieved by 25% PMo/TiO2 photocatalyst with a value of 97.85%. To investigate the effect of parameters such as UV irradiation power intensity, temperature, catalyst dosage, and persulfate concentration, Response Surface Methodology (RSM) was used. Finally, the experimental results showed the UV irradiation power intensity of 2.3 W/mL, the temperature of 24 °C, and the concentration ratio of terephthalic acid to persulfate and catalyst of 3.1 mg/mg and 7.2 mg/mg, respectively as the optimum values. At this operating condition, the concentration of 4-carboxybenzaldehyde decreased from 1870 ppm to 40.2 ppm after 90 min.