(678g) Investigating the Role of Oxophilic Modifiers in Bimetallic Catalysts for Enhanced Hydrodeoxygenation

Introduction

 In the search for
renewable energy sources, lignocellulosic biomass has
emerged as a promising alternative to typical petroleum-based fuels. 
However, due to the complex and recalcitrant nature of the lignin fraction,
current catalyst technology is inefficient for upgrading bio-oil to liquid
fuels and value-added products.  Recent advances have demonstrated that
bimetallic catalysts consisting of a noble metal and an oxophilic modifier may
improve catalyst performance while resisting deactivation under the harsh
conditions of bio-oil upgrading^1,2.  Several theories have been
proposed for the role of the modifier species in the hydrodeoxygenation
(HDO) of lignin-derived oxygenates.  Some groups have suggested that the
increased performance derives primarily from an electronic interaction between
the oxophilic metal and the noble metal³.  Others have
hypothesized that each metal species performs a unique function in the HDO
reaction.   In this case, the modifier species may act as an
anchoring site for the oxygen-containing functionality, with the strength of
the metal-oxygen interaction being a key predictor for catalyst performance⁴. 
Alternatively, the modifier may act primarily as a Bronsted
or Lewis acid site, in which case the strength and density of acid sites on the
surface would be expected to correlate with improved HDO activity⁵. 
Thorough characterization of the reactivity and surface structure of multiple
bimetallic catalysts is needed to pinpoint which factors can be used most
reliably to predict their performance in HDO upgrading. 

Materials and Methods

normal">In order to study the effect of a modifier on the HDO activity of
bimetallic catalysts, samples of Pt-W catalysts were synthesized at varying
modifier loadings through incipient wetness impregnation of a γ-Al₂O₃
support.  Reaction experiments were carried out in a tubular plug-flow
reactor using benzyl alcohol as the probe molecule.  Surface
characterization was examined through inductively coupled plasma – mass
spectrometry (ICP-MS), CO chemisorption, CO diffuse reflectance infrared
Fourier-transform spectroscopy (DRIFTS), temperature programmed reduction
(TPR), and pyridine DRIFTS.

Results and Discussion

                   
Recent results demonstrate that bimetallic Pt-W/Al₂O₃
samples show improved HDO activity with increasing W loading up to about 6 wt.%
W.  However, samples with higher W loadings experience a sharp reduction
in overall catalytic activity, as seen in Figure 1.  Additionally, the
selectivity towards the undesirable decarbonylation
(DC) product, benzene, is suppressed at these high W loadings. This behavior
suggests that the presence of W in these bimetallic systems introduces at least
two competing factors that affect the HDO activity of the catalyst. 

Figure 1.
Benzyl alcohol HDO and DC activity for bimetallic Pt-W/Al₂O₃
samples

Further characterization of the physicochemical
properties of the surface have offered hints as to which factors are correlated
with increases and decreases in catalyst performance.  For example,
pyridine DRIFTS has revealed that Brønsted acid site
density is strongly correlated with HDO activity for our Pt-W/Al₂O₃
catalysts while Lewis acid site density has no correlation.  Additionally,
CO DRIFTS of these samples indicate that W species are in close coordination
with Pt at all weight loadings.  In conjunction, these results imply that
the W in our samples tends to coordinate with Pt nanoparticles on the surface
of the catalyst, which may result in the formation of new metal-acid bifunctional sites that are active for the HDO of benzyl
alcohol.  However, despite these beneficial interactions at moderate W
loadings, the results of CO chemisorption suggest that high W loadings tend to
reduce the catalytically active surface area by covering Pt atoms.

Significance

                   
HDO of aromatic oxygenates is a necessary step in the upgrading of biomass to
alkane fuels and products, and bimetallic catalysts have shown promise for
activating this process while minimizing side products.  However, the
fundamental role of each metal species in this reaction is currently poorly
understood.  The identification of key factors for improving catalyst
performance would allow for the rational design of bimetallic catalysts with
optimized activity and selectivity for desired products.

References

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