274920 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.