Structure of Hydrothermally Stable Acid Sites and their Catalytic Role in P-Modified ZSM-5 Zeolite Revealed by Solid-State NMR Spectroscopy.

The ZSM-5 zeolite is the key active component in high-severity fluid catalytic cracking (FCC) catalysts and is routinely activated by phosphorus compounds in industrial production. To date, however, the detailed structure and function of the introduced phosphorus still remain ambiguous, which hampers the rational design of highly efficient catalysts. In this work, using advanced solid-state NMR techniques, we have quantitatively identified a total of seven types of P-containing complexes in P-modified ZSM-5 zeolite and clearly revealed their structure, location, and catalytic role. The experimental findings indicate that the introduced phosphorus can stabilize a portion of the aluminum atoms in the lattice by forming an energetically favorable framework-bound Si-OH-Al-O-P structure inside the zeolite channels, preserving more acidic bridging hydroxyl groups under the working conditions of the catalyst. Besides, two other types of hydrothermally stable phosphoric acid species (H3PO4 and H4P2O7) captured by neighboring silanol groups (H3PO4···HO-Si(SiO4)3 and H4P2O7···HO-Si(SiO4)3) are identified. For the FCC process, the framework-bound Si-OH-Al-O-P structure is proven to be the active site in the conversion of the FCC reactant, while the two phosphoric acid species can promote the yield of C2-C4 olefins.