Design of a green chemoenzymatic cascade for scalable synthesis of bio-based styrene alternative

In line with the call for sustainable production, researchers are focusing their efforts on the utilization of renewable resources and the development of environmentally friendly manufacturing methods. Bio-based polymers are emblematic and has potential in terms of polymerization and material characteristics.

Many products are using hydroxystyrene monomer these days. Hydroxystyrenes are particularly appealing as a replacement or addition to styrene-based polymer chemistry since they are renewable lignin building blocks. These monomers are made by decarboxylating phenolic acids, and their phenolic hydroxy groups are frequently subjected to chemical changes to enhance polymerization behavior. A straightforward, scalable, and entirely (chemo)catalytic synthesis of acetylated hydroxystyrene is still difficult to come by.

With functionalized polystyrene's range and potential, the question arises of how scalable and sustainable the respective monomers are that can be synthesized?

Contributing to answering the above question we will discuss a recent research project INTERfaces that developed a green, one-pot, two-step approach to producing acetylated hydroxystyrenes from raw lignin. In this case, the acylated hydroxystyrenes were designed as environmentally friendly monomers for making certain polystyrenes. Also, authors suggestions on a novel chemoenzymatic pathway that uses phenolic acid decarboxylase (PAD) would be discussed.

A novel chemoenzymatic pathway

Authors suggest a novel chemoenzymatic pathway that makes use of a phenolic acid decarboxylase (PAD). Authors have hypothesized that limitations could be circumvented by a tailored combination of a more active decarboxylation catalyst, milder reaction conditions and a compatible reaction medium. As a renewable, non-toxic, and highly active catalyst, phenolic acid decarboxylase from Bacillus subtilis (BsPAD) was the biocatalyst of our choice to embark on this process development. Herein, we describe our systematic efforts to design an alternative, scalable, chemo-enzymatic route to access bio-based styrene alternatives in an environmentally friendly and efficient manner.

An approach to process development entails a computational solvent assessment that provides information on solubilities and feasible reactor operation modes, experimental solvent screening, cascade engineering, heterogenization of biocatalyst, customization of acetylation conditions, and reaction upscale in a rotating bed reactor.

Procedure

Decarboxylation of phenolic acids, which can be generated from lignin, to the equivalent hydroxystyrene is the initial step. As a green decarboxylation catalyst, phenolic acid decarboxylase from Bacillus subtilis (bsPAD) was used. A significant amount of computational and experimental work was put into finding a solvent and water saturation that allowed for high enzyme catalytic activity in the non-conventional media while also offering good solubility of the phenolic acid reactant and hydroxystyrene product. MTBE and CPME that were water-saturated had all required characteristics. The more eco-friendly option was CPME because of higher boiling point, which was beneficial in the second step.

The hydroxystyrene intermediate was directly acylated without switching solvents in the second stage of the one-pot, two-step synthesis. The effectiveness of the acylation of the phenol in a wet solvent was confirmed. The ideal parameters for an experiment were found to be low catalyst concentrations (0.03 eq NaOAc), moderate anhydride concentrations (2.0 eq Ac2O), and relatively high temperatures (90°C).

The enzyme bsPAD was immobilized to allow for reuse and to stop it from obstructing the second step and work-up. Immobilization on Purolite ECR8415F as a carrier material was the outcome of extensive evaluation of enzyme carrier materials and chemistries for high immobilization yield, enzymatic activity, and longevity.

Using the RBR

The reaction was scaled up to 1 L to prove its feasibility. SpinChem RBR S3 was used to charge the immobilized bsPAD-8415F enzyme for the reaction. The application of RBR, resulted in a developed clean, one-pot, two-step procedure that makes use of bio-based phenolic acid educts, reusable immobilized PAD, and the renewable solvent CPME. On a 1 L scale, the entire chemoenzymatic reaction casscadded to produce 18.3 g of 4-acetoxy-3-methoxystyrene with a 96% isolated yield.

As a result of the rotating bed reactor, the carrier beads were protected and the enzyme could be simply removed between steps one and two, thus preventing the enzyme from interfering with the second step and to be reused in the decarboxylation step.

The RBR made the process fast and simple, provided efficient sampling and monitoring of the process and kept the immobilized catalyst safely confined.

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