Pd Nanoparticles Supported on Tubular g-C3N4: Enhanced Selectivity in Acetylene Double Carbonylation.

Aiming at the problem that the selectivity of heterogeneous catalysts is difficult to be controlled and the active components are easy to be lost in the acetylene double carbonylation reaction, this paper constructs a highly efficient Pd-based catalyst through the combination of carrier morphology engineering and surface basicity regulation. Using melamine/urea as a precursor, tubular carbon nitride (TCN) was constructed by hydrothermal calcination, and then Pd/TCN-series catalysts were prepared by ultrasonic-assisted impregnation loading 5 wt% Pd and heat treatment in a N2 atmosphere. At the same time, bulk g-C3N4 (BCN) and activated carbon (AC), TiO2, and ZSM-5 supported systems were used as controls. The characterization results show that compared to BCN, TCN-500 (calcined at 500 °C) has a higher specific surface area (40.19 m2/g vs. 12.47 m2/g) and pore volume (0.1850 cm3/g vs. 0.0792 cm3/g), which provides abundant anchoring sites and efficient mass-transfer channels for Pd nanoparticles. After the introduction of Pd, the total base amount of the catalyst increased significantly from 0.4866 to 1.7172 mmol/g, and the strong Lewis-base center was significantly enhanced. TEM/XRD confirmed that Pd was uniformly dispersed on TCN-500 and mainly exposed the (111) crystal plane. XPS further revealed that there was a stronger electron-coupling effect between Pd and the support. Under the reaction conditions of 70 °C and 5 h, the selectivity of Pd/TCN-500 to dimethyl butenedioate was up to 83.7% (acetylene conversion was 59.6%), which was significantly better than that of the contrast carrier system. The cycle test showed that the selectivity and conversion of the catalyst were reduced to 54.4% and 41.1%, respectively, on the third use. The performance degradation was mainly attributed to the oxidative damage of the TCN nanotube skeleton and the passivation of the surface active defect sites. In this study, the synergistic effect of tubular morphology and surface Lewis basicity effectively stabilized the Pd active center and regulated the product selectivity, which provided a new idea for the development of efficient heterogeneous catalysts for acetylene double carbonylation in non-petroleum routes.