One-Pot Direct Synthesis of b-Axis-Oriented and Al-Rich ZSM-5 Catalyst via NH4NO3-Mediated Crystallization for CO2 Hydrogenation

Al-rich NH4-ZSM-5 with highly oriented crystals was directly synthesized through a one-pot hydrothermal technique, using ammonium nitrate as a metal-free mineralizer. The samples were characterized by XRD, N2 adsorption–desorption, SEM, FTIR, Py-FTIR, 27Al MAS NMR, 29Si MAS NMR, 1H MAS NMR, and TGA techniques. The impact of aluminum source, ammonium source, and H2O/SiO2 molar ratio was studied. XRD results showed that the ZSM-5 catalyst with a low Si/Al ratio (13) was successfully synthesized without any amorphous phase, including a microporous/mesoporous structure. A low H2O/SiO2 molar ratio (75) resulted in coffin-shape surface morphology, large b-axis-oriented particles (ca. 19 µm), and high specific surface area (>300 m2 g−1), providing a large portion of straight channels (90.5%). The catalytic activity of the catalysts was evaluated in the CO2 hydrogenation reaction in tandem configuration with a Na/Fe2O3 catalyst. The results confirmed that highly b-oriented crystals improved the product shape selectivity to p-xylene by affecting the diffusion resistance. Therefore, the developed catalyst provided high CO2 conversion (45%) and high aromatic selectivity (77%), with p-xylene accounting for 82% of the produced xylene compounds, over a long-term time on stream (17 h). These results demonstrate the effectiveness of the direct synthesis strategy in producing Al-rich ZSM-5 catalysts with tailored textural and acidic properties for tandem and shape-selective catalysis.