(730d) Mechanistic Insights into the Hydrogenolysis of Levoglucosanol over Bifunctional Platinum Silica-Alumina Catalysts in Tetrahydrofuran Solvent

The cellulose-derived platform molecule levoglucosenone (LGO) can be
converted to a variety of high-value oxygenated chemicals. [1] LGO
can be quantitatively hydrogenated to levoglucosanol (Lgol) over metal
catalysts. [2] Herein, we report on the hydrogenolysis of Lgol to
tetrahydrofurandimethanol (THFDM) and tetrahydropyran-2-methanol-5-hydroxyl
(THP2M5H) over bifunctional platinum or palladium catalysts supported on silica-alumina
in tetrahydrofuran solvent. [3,4] THFDM is an α,ω-diol and potential
polymer precursor, while both THFDM and THP2M5H can be further upgraded to the
commodity chemical 1,6-hexanediol.

A combination of
experimental reaction kinetics studies and first principles calculations were
used to investigate the mechanism and the roles of the metal and acid sites in
this reaction. THFDM and THP2M5H are produced in parallel pathways, and the
ketone intermediate THP2M5one is a precursor to THP2M5H. [3] The THFDM
cis/trans ratio is independent of the Lgol threo/erythro ratio, suggesting that
the mechanism passes through an acyclic intermediate which erases the
stereochemistry of the Lgol reactant. ¹³C radio-labeling was used to
confirm the ring rearrangement forming THFDM. The catalyst undergoes moderate, reversible deactivation with
time-on-stream in a continuous flow reactor. The reaction rates and product
selectivities are comparable at 1.1% and 5.3% Pt loadings, indicating that the reaction
rate is acid-limited at these metal loadings; the metal-catalyzed hydrogenation
step is not rate-limiting (Figure 1). [4] The measured zero-order dependence in
hydrogen indicates that the rate-limiting acid-catalyzed step precedes the fast
hydrogenation step. As the Pt loading in Pt/SiAl catalysts is decreased,
or when the bare SiAl support is separated from a Pt/SiO₂ catalyst
in a dual-layer configuration, the selectivity towards identified products
decreases (Figure 1). These results suggest that degradation reactions are
avoided when the reactive intermediates formed over acid sites are rapidly
hydrogenated over metal sites. First principles simulations were performed to
investigate the energetics of the proposed reaction pathway consistent with the
experimental results.

Figure 1. Lgol consumption rate (left y-axis)
and product selectivity (right y-axis) versus Pt loading for Pt/SiO₂-Al₂O₃
catalysts.

These insights into the
mechanism of Lgol hydrogenolysis provide a foundation for rationally designed
processes to produce high-value chemicals from lignocellulosic biomass using
bifunctional metal-acid catalysts.

References.

1. Cao, F.; Schwartz, T. J.; McClelland,
D. J.; Krishna, S. H.; Dumesic, J. A.; Huber, G. W., Dehydration of cellulose
to levoglucosenone using polar aprotic solvents. Energy & Environmental
Science 2015, 8 (6), 1808-1815.

2. He, J.; Huang, K.; Barnett,
K. J.; Krishna, S. H.; Alonso, D. M.; Brentzel, Z. J.; Burt, S. P.; Walker, T.;
Banholzer, W. F.; Maravelias, C. T.; Hermans, I.; Dumesic, J. A.; Huber, G. W.,
New catalytic strategies for [small alpha],[small omega]-diols production from
lignocellulosic biomass. Faraday Discussions 2017, 202
(0), 247-267.

3. Krishna, S. H.; McClelland,
D. J.; Rashke, Q. A.; Dumesic, J. A.; Huber, G. W., Hydrogenation of
levoglucosenone to renewable chemicals. Green Chemistry 2017, 19
(5), 1278-1285.

4. Krishna, S. H.; Assary, R.
S.; Rashke, Q. A.; Schmidt, Z. R.; Curtiss, L. A.; Dumesic, J. A.; Huber, G.
W., Mechanistic Insights into the Hydrogenolysis of Levoglucosanol over
Bifunctional Platinum Silica–Alumina Catalysts. ACS Catalysis 2018,
3743-3753.