(473f) 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.