(127c) Effective Aqueous Phase Hydrogenation: GVL Production Kinetics and Stability

γ-valerolactone
(GVL) is a lignocellulosic platform chemical that offers tremendous flexibility
in down-stream applications and upgrading.  The primary route envisioned for
the production of lignocellulosic GVL is the aqueous-phase hydrogenation of levulinic
acid (LA) ¹. This reaction is generally carried out over supported
metals, most commonly Ru given its high activity for ketone hydrogenation in water.
For the process to be feasible, catalysts must deliver high intrinsic activity
towards LA hydrogenation and maintain that activity for extended times on
stream. Through an investigation of the kinetics and catalyst stability during aqueous-phase
LA hydrogenation over supported Ru nanaoparticles, we identify parameters
allowing the design of efficient catalysts for this reaction.

We first decouple
4-hydroxypentanoic acid (HPA) and angelicalactone (AL)-mediated hydrogenation
pathways to illustrate that, at low temperatures and in water, GVL formation
occurs primarily through ketone hydrogenation followed by intramolecular
esterification (Figure 1,a). This transformation appears independent of the
nature of the Ru support, and occurs with an average turnover frequency of 0.11
s^-1 at 323K, 0.5M LA, and 24 bar hydrogen on Ru/C, Ru/SiO₂,Ru/TiO₂,
and Ru/ γ -Al₂O₃. 
In the formation of GVL, the rate of ring closure of HPA was found to be the
slowest step, and its apparent barrier (70 kJ mol^-1) is larger than
that of the Ru catalyzed hydrogenation (48 kJ mol^-1). Rates of low
temperature GVL production can be improved by coupling Ru/C with Amberlyst-15,
which accelerates the rate of hydroxy-acid ring closure².

Despite
Ru/C possessing a high intrinsic activity for the hydrogenation of LA in the
aqueous phase, it deactivates rapidly once on stream, losing ~50% of initial
activity within 5 hrs. Partial regeneration of the catalyst was possible by
reducing the catalyst at 673K in a H₂ atmosphere; however, a
substantial portion of the deactivation was irreversible and attributed to
particle sintering. By comparing sintering kinetics across silica, titania, γ
-alumina and carbon, we find that the extent of sintering is inversely related
to the mean electronegativity of the support³. The source of
reversible deactivation in this system could not be conclusively identified;
however, the extent of reversible deactivation appears to correlate with
support point of zero charge (PZC) and/or the prevailing surface charge of the
catalytic material in water. The above insights establish a framework for
designing stable supported metal catalysts for use in aqueous phase
hydrogenation and hydrodeoxygenation, thus addressing a central need in biomass
processing.  

1-     Wright,
W. R. H., Palkovits, R., Development of heterogeneous catalysts for the
conversion of levulinic acid to y-valerolactone, ChemSusChem, 5,
1657-1667 (2012).

2-     O. A. Abdelrahman,
A. Heyden and J. Q. Bond, Analysis of Kinetics and Reaction Pathways in the
Aqueous-Phase Hydrogenation of Levulinic Acid To Form γ-Valerolactone over
Ru/C, ACS Catal., 2014, 4, 1171?1181.

3-     O.A.
Abdelrahman et al., Toward rational design of stable, supported metal
catalyst for aqueous phase processing: Insights from the hydrogenation of
levulinic acid, J.Catal. (2015),
http://dx.doi.org/10.1016/j.jcat.2015.04.026