Ultralow-Loading Subnano [Ptn–Gam]δ+ Clusters with Lewis and Brønsted Acid Functionalities for Active Diels–Alder Cycloaddition Between Biomass-Derived Furans and Normal-Pressure Alkenes

Converting renewable biomass and waste plastics into benzene, toluene, and xylene (BTX) offers a sustainable, carbon-neutral alternative to petroleum-based production. Studying catalysts that combine Lewis and Brønsted acid functionalities is critical for facilitating Diels–Alder cycloaddition (DAC) of biomass-derived furans. In this work, Pt–Ga/HZSM-5 was prepared via sequential impregnation, in which the Pt loading is ultra-low, and then adhered to cordierite to form monolithic catalysts. The metal amounts of Pt and Ga on cordierite are only 6.667 × 10^–4–1.282 × 10^–2 mol/m³ and 3.571 × 10^–1–7.143 × 10^–1 mol/m³, respectively, while the amount of Pt–Ga/HZSM-5 catalyst is 2500 g/m³. Such bimetallic Pt–Ga exhibited significant activity in the DAC between 2-methylfuran (2-MF) and normal-pressure alkenes pyrolyzed from high-density polyethylene (HDPE). The Space-time yield of BTX is up to 205.9 μmol/g_cat/min over the Pt_0.01–Ga₂/HZSM-5 catalyst, outperforming previously reported adduct systems, with 59.32% selectivity for toluene and xylene (TX), which were formed from cycloaddition between 2-MF and ethylene or propylene. Characterization results indicate electronic transfer between the Pt and Ga species, and the subnano [Pt_n–Ga_m]^δ+ clusters are highly dispersed on the HZSM-5 surface. This leads to increased Lewis acid sites and retention of Brønsted acid sites on the bimetallic catalyst, achieving a moderate Brønsted/Lewis ratio of 2.764. Kinetic studies show the promotional effect of Pt–Ga/HZSM-5 and identify the dehydration process as the rate-determining step. Density functional theory (DFT) calculations further demonstrated that the formed clusters facilitate the adsorption (Lewis acidic function) and protonation (Brønsted acidic function) of reactants. Compared to HZSM-5, Pt–Ga/HZSM-5 decreases the energy barriers for dehydration (47 and 85 kJ/mol) and product dissociation (52.06 and 11.9 kJ/mol) steps in two reaction pathways.