Shape selectivity vapor-phase conversion of lignin-derived 4-ethylphenol to phenol and ethylene over acidic aluminosilicates: Impact of acid properties and pore constraint

Abstract

Selective dealkylation of alkylphenols, the opposite reaction of the more commonly studied phenol alkylation, may represent an important reaction in the production of base chemicals like phenol and olefins from fossilized and raw lignocellulosic matter. This study reports the first thermodynamics and kinetics studies of the vaporphase conversion of ethylphenol (EP) over acidic γ-Al2O3, amorphous (ASA) and crystalline aluminosilicates (like ferrierite, ZSM-22, ZSM-5, beta, and USY) in the absence of hydrogen and noble metals, as a way to produce phenol and ethylene. The reaction was studied deliberately in presence of steam to get stable time on stream catalysis. The thermodynamic analysis shows an endothermal EP conversion to phenol and ethylene, favoured at high reaction temperature, while isomerisation, disproportionation and transalkylation are thermodynamically preferred at low temperature. The kinetic study examines the role of the catalytically active sites; it reveals the importance of site constraint on the activity, selectivity and stability, and shows the complex temperature dependency of the dealkylation. Both Brønsted and Lewis acid sites are active, but multifactor dependency (such as acid strength and site accessibility) complicates the establishment of simple quantitative relationships with the acid type. EP does not enter the micropores of ferrierite and ZSM-22, as suggested by adsorption experiments. Kinetics without significant diffusion limitations were obtained with ZSM-5, beta and USY. Thus, in absence of intracrystalline diffusion limitation (as verified by calculations using reported effective diffusivities, and substantiated by a comparably high apparent activation energy for all zeolites), the increased reaction turnover rate with increasing pore size from medium to large pore zeolites is largely explained by a change in reaction pathway (from monolecular to bimolecular) to convert EP to phenol and ethylene. A pathway proceeding through fast thermodynamically favourable bimolecular reactions occurs in the spatially non-constrained pores and crystal surface, whereas monomolecular reactions take place in the micropores of ZSM-5. Despite the lower rate, the selectivity over ZSM-5 strongly benefits from active site confinement, being responsible to achieve quantitative formation of phenol and ethylene from ethylphenol. The excellent performance of ZSM-5 thus accords with its shape selective property that avoids undesired side reactions such as the sterically demanding bimolecular reactions like disproportionation, transalkylation and CC cracking, and severe cokes formation.

Product‐oriented Direct Cleavage of Chemical Linkages in Lignin

“…Among various solid acid materials, HZSM‐5 zeolite showed the best performance for the selective elimination of alkyl group, which resulted from a transition state shape selectivity effect, avoiding disproportionation and transalkylation . In addition, the presence of water is essential to ensure the stable catalysis of ZSM‐5, avoiding the condensed product formation in the pores through reducing the lifetime of intermediate at the active site owing to the competitive adsorption between water and intermediate …”

A Comparison of Phenolic Monomers Produced from Different Types of Lignin by Phosphotungstic Acid Catalysts

“…This indicates that this selective process of lignin is highly dependent on the PTA catalysts, and phenolic monomers basic units are formed by different types of lignin β‐O‐4 aryl ether bonds cleavage. The 4‐ethylphenol is a critical starting material for fine chemical industry and it can be readily converted to phenol and ethylene over acid aluminosilicates . And the 4‐ethyl‐2‐methoxyphenol also have a promising application for the pharmaceutical field and food industries .…”

“…The 4ethylphenol is a critical starting material for fine chemical industry and it can be readily converted to phenol and ethylene over acid aluminosilicates. [19] And the 4-ethyl-2-methoxyphenol also have a promising application for the pharmaceutical field and food industries. [20] As abovementioned, the PTA catalyst is also the key to selective catalysis of lignin.…”

Catalytic Processes For Lignin Valorization into Fuels and Chemicals (Aromatics)

“…Al-MCM-41). The results show how the pores structure of the catalysts, as well as the nature, density and strength of their acidic sites influence the catalytic activity, selectivity and stability of the materials [134][135][136]. Bi-phenolic compounds are other interesting aromatic compounds for chemical industry.…”