(129f) Bi-Functional Catalysts: A Path to Improved Catalytic Performance for the Conversion of Biomass Feedstocks

The conversion of sustainably derived bio-feedstocks to fuels and chemicals generally involves myriad
individual reactions.  To effectively
realize the goal of an integrated bio-refinery for the conversion of biomass, a
strategy for intensifying the number of catalysts, conditions, and separations
involved in moving from plant to product without sacrificing overall yield is required.  One strategy involves the development of bifunctional catalysts capable of performing two or more
distinct conversions, ideally more selectively than if
the two catalytic functionalities were kept discreet. 

One type of bifunctional catalyst
is the combination of a traditional noble metal catalyst with a more oxophilic metal.  As
opposed to traditional bimetallic catalysts, where the addition of another
metal merely augments the rate or selectivity of the reaction, for example by
reducing the binding of catalyst poisons, a bifunctional
catalyst introduces an additional reactive site to the catalyst.  A RhRe/C catalyst employs
Rh as a traditional hydrogenation catalyst, and utilizes Re in a partially
oxidized state (as Re(OH)_x) to introduce an
acid active site.  This catalyst has been
used successfully for selective ring opening of compounds like 2-methylpyran
and has shown acidity by dehydrating fructose to hydroxymethylfurfural.   The
inherent proximity of the active sites in a bifunctional
catalyst allows for the potential to increase selectivity by decreasing side
reactions of intermediates or allowing the quick conversion of otherwise
unstable intermediates.  For example, it
will be shown that the use of a RhRe/C
catalyst can improve the selectivity of the reaction of furfuryl
alcohol to 2-methylfuran compared to a Rh/C catatlyst
paired with either a heterogeneous or homogeneous acid catalyst.  Additionally, the Re(OH)_x
functionality provides a way to augment the acidity of the catalyst by
providing an anchor site for atomic layer deposition (ALD) of different
functionalities, such as Nb₂O₅-xH2O or ?SO₃H.  The augmentation of the acid site in the bifunctional catalyst provides another handle with which
rate and selectivity and be manipulated.

Utilizing ALD provides another route to bifunctional
catalysts.  The atomic layer control of
ALD allows for the careful addition of the extra functionality.  For example, ALD overcoating
of Cu catalysts with alumina prevents the leaching of Cu in the liquid phase
hydrogenation of furfural to furfuryl alcohol.  However, it will be shown that by utilizing
ALD Bronsted acidity can be added to the overcoating by sandwiching a layer of Nb₂O₅
among the alumina overcoat layers. 
This Bronsted acidity introduces a bifunctional site that allows for the hydrogenation of
furfural to furfuryl alcohol on the Cu and the alcoholysis of furfuryl alcohol
to levulinate esters. 
The bifunctionality of this catalysts allows
for this conversion over a single catalyst and prevents the accumulation of the
very reactive furfuryl alcohol intermediate, thus
improving overall yield compared to pairing Cu with a homogeneous acid or a
discrete heterogeneous solid acid in either the same or separate reactors.

Figure 1: A potential graphical representation of how Rh and
Re(OH)_x work in tandem to produce 2-methylfuran from furfuryl alcohol.