Structure-activity-selectivity relationship and isomer distribution in dipropylene glycol synthesis over HZSM-5 zeolites

Dipropylene glycol (DPG) is a versatile fine chemical whose quality is governed by the isomeric ratio, particularly the DPG2 content. However, the structure-activity-selectivity relationship among catalyst acidity, DPG yield, and isomer selectivity remains ambiguous. Herein, a combination of catalytic performance tests and characterization (SEM/TEM, 27 Al MAS NMR, NH 3 -TPD, Py-IR and N 2 adsorption-desorption) on HZSM-5 with varying Si/Al ratios is employed to systematically probe the correlation. It is found that the DPG yield reaches a maximum at an optimal B/L ratio. The DPG2 selectivity plateaus beyond a threshold B acid density, but this plateau can be surpassed by an appropriately enlarged pore structure. Crucially, based on 1 H-NMR and 13 C-NMR evidence, a regioselective mechanism governing the formation of the three DPG isomers is unraveled: propylene oxide (PO) protonated at Brønsted acidic sites forms a positively charged carbon resonance structure, which then undergoes nucleophilic attack by 1,2-propylene glycol (PG) to yield DPG isomers in the order of DPG2 > DPG1 > DPG3. The regioselectivity arises from the synergistic interaction between the electrophilicity of PO ring carbons and the terminal hydroxyl nucleophilicity of PG. Kinetic analysis shows that the reaction is second-order ( E a,exp = 47.81 kJ·mol -1 ). DFT calculations on a 104T cluster model at the ONIOM (B3LYP/6–31G(d,p):PM6) level further identify nucleophilic attack as the rate-determining step ( E a,cal = 53.13 kJ·mol -1 ), and the isomer E a order (DPG2 < DPG1 < DPG3) agrees well with the experimental results. This quantitative agreement validates the proposed regioselective mechanism, establishing a molecular-level framework for selectivity control in acid-catalyzed etherification to provide a theoretical basis for the targeted regulation of DPG isomer ratio.