Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam

This contribution studies the steam-assisted dealkylation of 4-<i>n</i>-propylphenol (4-<i>n</i>-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (<i>e.g</i>., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (<i>i</i>) analyzing products distribution under varying temperature and contact time conditions, (<i>ii</i>) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (<i>iii</i>) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed to a pore confinement, avoiding disproportionation/transalkylation as a result of a transition state shape selectivity. The presence of water maintains a surprisingly stable catalysis, especially for ZSM-5 with low acid density. The working hypothesis of this stabilization is that water precludes diphenyl ether(s) formation in the pores by reducing the lifetime of the phenolics at the active site due to the high heat of adsorption of water on H-ZSM-5, besides counteracting the equilibrium of the phenolics condensation reaction. The water effect is unique for the combination of (alkyl)­phenols and ZSM-5

Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum Activation

Incorporating aluminum is the most widely applied and industrially relevant method to functionalize amorphous silica. However, established protocols yield predominately poorly distributed and inaccessible Al species, and as a result only ∼10–15% of the present aluminum gives rise to the acid sites, hampering the overall catalytic potential. Herein, the influence of alkaline activations with aqueous NaOH and NH₄OH on the porosity, acidity, and catalytic properties of Al-MCM-41 is studied. By performing room temperature activations in 0.01–0.1 M NaOH or 0.5 M NH₄OH, the Ostwald ripening of silica in alkaline media is exploited, which results in high mass retention yields (100–74%) and a controlled transformation of the 3.6 nm mesopores of the parent material to a broad pore range from 3 to ∼12 nm. Electron microscopy indicates the presence of additional interconnected intraparticle porosity, whereas no significant change in the shape and size of the original particles is observed. Elemental analysis reveals that the optimal alkaline activation with 0.05 M NaOH leads to a decrease in the Si/Al ratio at the surface, despite an increase in the bulk Si/Al ratio. ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy demonstrates a large conversion of octahedral Al into tetrahedral Al, doubling the purely tetrahedral fraction from 30 to 60%. Pyridine-probed Fourier transformed infrared spectroscopy shows a doubling of the Brønsted and Lewis acidity after activation. The compositional and spectroscopic results are ratified by monitoring the relative accessibility of the acid sites, i.e., effective acidity (mol acid sites per mol Al). The alkaline activation enhances the effective acidity by increasing access to the Al sites trapped inside the pore wall and by reincorporation of the octahedral Al as accessible tetrahedral sites. As a result, an unprecedented effective acidity is obtained after the Al incorporation, which is substantiated using a novel accessibility concept. The catalytic potential of the activation protocol is demonstrated by quadrupling the catalytic activity for the acid-catalyzed alkylation of toluene with benzyl alcohol, an over-50% activity gain, a slightly enhanced selectivity, and a strongly reduced coking in the acid-catalyzed coupling of furfural with sylvan