(473e) Ruthenium-Catalyzed Hydrogenation of Levulinic Acid: Influence of the Support On Selectivity and Stability

Ruthenium-Catalyzed Hydrogenation of Levulinic acid: Influence of the Support on Selectivity and
Stability

Wenhao
Luo,¹ Upakul Deka,¹ Andrew M.
Beale,¹ Ernst R. H. van Eck,² Pieter C.A. Bruijnincx,^1*
Bert M. Weckhuysen^1*

¹Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, The
Netherlands

²Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen, The Netherlands

* p.c.a.bruijnincx@uu.nl; b.m.weckhuysen@uu.nl

Introduction

Levulinic acid (LA) has been identified as a promising,
renewable platform molecule that can be converted by catalytic hydrogenation/deoxygenation to other valuable chemicals, such as γ-valerolactone (GVL), methyltetrahydrofuran
(MTHF), pentanoic acid (PA) or its esters (PE) [1,2].
Here, we present our results on the influence of supports of varying acidity on
the selectivity and stability of Ru catalysts in the
hydrogenation of LA under increasingly harsh conditions in three solvents (dioxane, the LA mimic 2-ethylhexanoic acid (EHA), and neat  LA) [3].

Experimental details

1wt% ruthenium catalysts on TiO₂, Nb₂O₅,
HZSM-5 and H-Beta were prepared by wetness impregnation. The fresh and spent
catalysts were extensively characterized by NH₃-TPD, TEM, N₂physisorption, XRD, AAS, TGA, IR after pyridine
adsorption, and solid state NMR. Catalytic tests were run at 40 bar H₂
and 200  °C in
a Parr batch autoclave system.

Results and Discussion

Figure 1.  Catalytic hydrogenation of 10 wt% LA in dioxane.

Large differences
in activity and selectivity are observed with the different catalysts (Figure
1). Notably, the zeolite-supported Ru-catalysts are the
first examples of catalysts for the direct, one-pot conversion of LA to pentanoic acid (PA) and its derivatives (max. yield 45.8 %
in dioxane) at this fairly low temperature. The acid
sites on the support accelerate both the dehydration step from LA to GVL as
well as the GVL ring opening, which is the most difficult step on the pathway
to PA. Indeed, reactions with the intermediates GVL and pentenoic
acid (PEA) in EHA showed that the acid-supported catalysts are capable of
catalyzing the further conversion of GVL.

Spent catalysts were extensively studied to assess
stability and detect any changes in either the metal phase or the support. The Ru/TiO₂ catalyst was highly selective to GVL and
proved remarkably stable, even under the most severe condition (i.e. in neat
LA). Gradual deactivation of the zeolite-supported catalysts was observed in
EHA and neat LA. Coke build-up resulted in a decrease in surface area and pore
volume for the niobia- and zeolite-supported
catalyst, but not for the titania-supported one. TGA
measurements showed that carbon build-up to decrease in the order H-ZSM5 >
H-Beta > Nb₂O₅ > TiO₂ and that angelicalactone is involved in coke formation. Remarkably,
coke build-up on the Ru/HZSM-5 catalyst
preferentially occurred only in the zigzag channels, as evidenced by systematic
shifts in the XRD patterns. While limited leaching and sintering of ruthenium
does occur after long runs under severe conditions, dealumination
is considered to be the main reason for deactivation, as demonstrated by pyridine
IR studies and 2D ²⁷Al MQ MAS NMR (Figure 2).

Figure 2. 1D and 2D ²⁷Al MQ MAS NMR spectra of fresh and spent Ru/H-Beta.

Conclusions

The obtained results provide further insight into the
required properties for selective levulinic acid
hydrogenation and show that Ru catalysts on strongly
acidic supports are capable of the direct conversion of levulinic
acid to pentanoic acid. The insights obtained in the
catalyst deactivation mechanisms can be used for further improvement of these
first promising examples.

References

1.      J.Q. Bond, D.M. Alonso, D. Wang, R.M. West, and J.A. Dumesic, Science 2010, 327, 1110

2.      J.-P. Lange, R. Price, P.M. Ayoub,
J. Louis, L. Petrus, L. Clarke, and H. Gosselink,  Angew. Chem. Int. Ed. 2010, 49, 4479.

3.      W. Luo, U. Deka, A.M. Beale, E.R.H. van Eck, P.C.A. Bruijnincx, and
B.M. Weckhuysen, J.
Catal. 2013,
301, 175.