The pore size of hexagonal mesoporous carbon nitride (HMCN) materials were engineered with different pore sizes of 3.5, 4.7, 5.9, 7.1, 7.9 and 9.2 nm using SBA-15 hard template synthesised at 80, 100, 120, 150, 160 and 180 degrees C, respectively. HMCN (x) and SBA-15 (x) materials were characterized by XRD, N-2 sorption, FESEM, CHN, FTIR, Raman and DR UV-Vis spectroscopy. N-2 sorption isotherms of HMCN (x) and SBA-15 (x) samples showed a type IV isotherm with H1 hysteresis loop having a region of steep increase due to capillary condensation. The catalytic performance of HMCN (x) catalysts were studied on the synthesis of alpha, beta-unsaturated carboxylic acid and its derivatives via Knoevenagel-Doebner reaction in metal/pyridine-free, azeotropic conditions (Dean-Stark apparatus) by an environmental friendly approach. Among the HMCN (x) catalysts studied, HMCN (5.9) showed an excellent performance of 94.5% p-hydroxybenzaldehyde conversion with 100% selectivity of p-hydroxycinnamic acid (91% isolated yield) within 60 min. The reaction parameters of temperature, time and catalyst amount were studied on p-hydroxycinnamic acid synthesis. The derivatives of alpha, beta-unsaturated carboxylic acid were synthesised under optimized reaction parameters. The experimental results of our study proved that the pore size engineering of HMCN is important for the catalytic reaction to get maximum product yield for the reactants with different molecular dimensions.