(687d) In-Situ X-Ray Absorption Spectroscopy of Supported Transition Metal Catalysts for Hydrogenolysis of Sorbitol and Oxidation of Glycerol

            Sorbitol
and glycerol are listed as two of the Department of Energy's top 12 chemicals
that will serve as key feedstocks in future biorefineries.  This work applies heterogeneous catalysis to
the aqueous phase hydrogenolysis of sorbitol and the aqueous phase oxidation of
glycerol for the production of value-added chemicals.  Heterogeneous catalysis operation in the aqueous phase can be
complicated by metal particle instability, resulting in particle growth or
leaching.  Moreover, catalysis may occur
on the solid catalyst surface or in the surrounding solution. 

To evaluate the
stability of Ru particles operating under aqueous conditions at 473 K a variety
of catalysts (Ru/SiO₂, Ru/TiO₂, Ru/Al₂O₃,
and Ru/C) were characterized in-situ with X-ray absorption spectroscopy
(XAS).  Treatment of the samples at 473
K with 40 bar H₂-saturated water resulted in significant particle
growth for the Ru/SiO₂ and Ru/Al₂O₃ samples,
while both the Ru/TiO₂ and Ru/C samples remained highly
dispersed.  Addition of a 0.4 M NaOH solution
to the previous conditions resulted in continued Ru particle growth for the
alumina sample while the carbon and titania supported samples remained
stable.  We then prepared a number of
Ru-Pt/C bimetallic catalysts and characterized them with XAS.  Results for the as-prepared samples revealed
that for a Pt rich sample (11.1 wt% Pt, 3.2 wt% Ru) the Ru and Pt are well
alloyed, while a 5.3 wt% Pt and 5.3 wt% Ru bimetallic catalyst had Ru
segregated to the particle surface. 

The hydrogenolysis
of sorbitol (10 wt% sorbitol, 473 K, 40 bar H₂, CaO added to
maintain pH = 11.9) has been performed over monometallic Ru (5 wt%) and Pt (3
wt%) catalysts.  Following 5 h of
reaction, the conversion of sorbitol over both catalysts was similar: 71% over
Ru/C and 82% over Pt/C.  The product
selectivities, defined as the moles_{carbon in product} / moles_{carbon
reacted in feed}, varied slightly between the two catalysts.  The major products observed during the
reaction were lactic acid, propylene glycol, ethylene glycol, methanol,
glycerol, and methane.  Interestingly,
the hydrogenolysis of sorbitol over the Pt/C was observed at temperatures as
low as 423 K whereas a minimum temperature of 473 K was necessary over the Ru/C
catalyst.  In the absence of base, the
Ru/C catalyzed approximately 60% conversion of sorbitol to lower molecular
weight C4 and C5 polyols along with carbon dioxide and methane, while over the
Pt/C a low conversion of sorbitol was observed (<15%) with no gas phase
products detected.

The aqueous phase
oxidation of glycerol over carbon-supported Au catalysts (333 K, 1-10 atm O₂,
0.3 M glycerol, 0.6 M NaOH) has also been investigated.  Initial rates are in good agreement with the
published literature, however the observed selectivities to glyceric acid are
poor for a wide range of Au particle sizes. 
To investigate the effect of a second metal on activity and selectivity,
a Au on Pd bimetallic catalyst was prepared via a surface redox process.  The XAS results showed that Au was
coordinated to both Pd and Au.  Reactivity
studies suggest the bimetallic system is more stable than monometallic Pd.  Additional kinetic studies are currently
underway.