Catalyst Design for the Integration of Heterogeneous Catalysis With Biocatalysis

A promising strategy for the production of chemicals from biomass uses heterogeneous catalysts to upgrade platform chemicals that are produced biologically.  Such a scheme exploits the high efficiency of heterogeneous catalysis and couples it with the ability of biocatalysis to generate molecules not easily accessible by other means.  An important challenge for this approach is that the product solutions of biological processes may contain biologically-active compounds, such as amino acids, biomacromolecules, or vitamins, and these biologically-active species may act as poisons or inhibitors of heterogeneous catalysts used in subsequent processing steps.  In this work, we have studied the hydrogenation of triacetic acid lactone (TAL, Figure 1) as a probe reaction to examine catalyst inhibition by biogenic impurities and to explore strategies to overcome such inhibition. 

Figure 1. Hydrogenation of triacetic acid lactone (TAL, 1) to 5,6-dihydro-4-hydroxy-6-methyl-2H-pyran-2-one (2).  Sorbic acid can be produced from TAL by further hydrogenation of 2 and subsequent dehydration and ring opening.

TAL can be produced from glucose through polyketide biosynthesis, and it can be subsequently upgraded to sorbic acid.  The Pd-based hydrogenation catalyst used in this process is particularly susceptible to inhibition by amino acids.  In particular, we have observed that sulfur-containing amino acids are some of the most inhibitory compounds that are present in spent cell culture medium, with methionine causing an 80% decrease in catalyst activity at low concentration (0.01 mM).  Additionally, the same concentration of tryptophan (an aromatic amino acid) or alanine (which has only a methyl side chain) results in 37% or 30% decreases in activity, respectively.  As such, we have investigated some of the aspects of catalyst design that can make a catalyst resistant to this type of inhibition.

We have taken advantage of the different chemical properties of amino acids compared to the targeted cell-culture product (i.e., TAL) to design catalysts that are resistant to inhibition by amino acids.  We have tailored the local chemical environment surrounding the metal nanoparticles by crosslinking poly(vinyl alcohol) inside the pores of a Pd/γ-Al₂O₃ hydrogenation catalyst, thus making the adsorption of polar compounds unfavorable.  Using hydrogenation of TAL as a probe reaction, we studied this catalyst for resistance to the same three amino acids that inhibited the non-overcoated catalyst.  This novel polymer-overcoated catalyst showed good resistance to inhibition by all three, with no measurable decrease in reaction rate after aging the catalyst for 14 hours in 0.01 mM methionine, tryptophan, or alanine.  We therefore suggest that polymeric overcoating is a viable strategy for imparting amino acid tolerance to hydrogenation catalysts.