Catalytic hydrolysis of lignin-derived aryl methyl ethers over zeolites

The transition to a sustainable society needs renewable alternatives to replace fossil fuels. Lignocellulose, being a renewable resource and available on a global scale, has the potential to synthesize carbon-based materials, fuels, and chemicals. Lignin, one of the major components of lignocellulose, consists of phenolic units with high content of methoxyl groups. Thus, converting lignin fraction into phenolic materials or chemicals is of great interest. Unfortunately, the methoxyl groups at the ortho position of phenolic units hamper the reactivity of lignin-derived compounds, requiring further upgrading to unlock their phenolic functionalities. 

For this purpose, acid-catalyzed O-demethylation (hydrolysis) was carried out to increase the phenolic functionality. We first developed a strategy to cleave the ether bonds of 4-alkylguaiacols by using Beta zeolite as a catalyst in hot liquid water, yielding 4-alkylcatechols and methanol. Combining with our previously reported technologies, lignin-first (reductive catalytic fractionation: RCF) and C-dealkylation, we successfully synthesize bio-catechol in high yield from raw pine wood. Notably, zeolites showed much higher reactivity in the hydrolysis step than other catalysts without microporous structures, hinting at the importance of the molecular-sized confines in catalysis.

To better understand the underlying catalysis that occurred in molecular-sized confinements. The integration of operando molecular dynamics modeling and the experimental kinetic study was applied to analyze the molecular environment and solvent dynamics (hydronium ion clusters), suggesting that the localized hydronium ions in a confined space of zeolites are more reactive than the ones homogeneously distributed in bulk water. A one-step, concerted SN2 mechanism was proposed, in contrast to the previously reported mechanism with stable protonated intermediates. Besides, the independence to the ionic strength of hydrolysis turnover rates is in high contrast to the literature-reported rate dependency on ionic strength in the dehydration of alcohols in confined and unconfined hydronium ions. But the acid density determines the distance between neighboring hydronium ion clusters. As the distance is close to the kinetic size of TS complex (reactants), a better fit within such free reaction space (cavity) offers a stronger Van der Waals stabilization to TS complex, lowering the activation free energy barrier of relevant reactions. 

Accordingly, guaiacols with various kinetic diameters were used as guest molecules to explore such cavity (host) fit effects on turnover rates of hydrolysis. In contrast to the slight variation (or even reduction) of rates with unconfined hydronium ions in bulk water, the growth in kinetic diameter of guaiacols (guaiacol, 4-methylguaiacol, 4-ethylguaiacol, and 4-propylguaiacol) promotes the turnover rates in confined space, which is beyond conventional size discrimination. This enhancement can also be attributed to the increased Van der Waals interaction between transition states and the negatively charged framework of zeolite. As was revealed by the relationship between turnover rate and fit percentage, the better the fit, the lower the Gibbs free energy barrier, leading to higher turnover rates. A ‘prefect-fit’ concept that leads to the highest hydrolysis rates is demonstrated here.

Although the high activity of zeolites was observed in guaiacol hydrolysis, the poor hydrothermal stability of zeolite limits its future as being catalyst respecting biomass conversion in hot liquid water. We, therefore, chose cellulose-derived carbohydrates as precursors to create carbon deposits for healing the zeolite silanol defects that determine the stability of zeolites in hot liquid water. Glucose showed a promising ability to selectively cover the silanol defects of zeolites, majorly internal ones. Notwithstanding that a small fraction of acid sites were lost during the formation of carbon deposits, the catalytic activity of glucose-modified zeolites was limitedly impacted. Overall, the use of renewable glucose to enhance the hydrothermal stability of zeolites is feasible.

This study provides an attention-grabbing example of achieving a high yield of drop-ins from renewable woody biomass. It adds a new layer to catalysis taken inside a confined space. Besides, the insights of this research help understand the fact of confinement in catalysis and inspire future catalyst design.