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H. Zhang et al. / Journal of Catalysis 375 (2019) 56–67
catalyst system can be recycled for additional batches. When both
Zr-HY-15-20 and Al-HY-15 were added at the start of the reaction
(Fig. 8a), furfural was rapidly converted to furfuryl ether (FE),
levulinate ester (LE) and b-angelica lactone (b-AL). The concentra-
tion of FE reached a maximum after 10 min while those of LE and
b-AL peaked at 20 and 30 min, respectively. Unlike FE, the concen-
trations of LE and b-AL decreased slowly with longer reaction
times. This suggests that their transformation to GVL is rate-
limiting, especially for b-AL. After 5 h, the yield of GVL was only
65 mol % and ꢀ4.2 mol% of LE, 9.6 mol% of b-AL and 6.2 mol% 4-
hydroxypentanoic ester (PE) were still present. When the Brønsted
acid catalyst, Al-HY-15, was present from the beginning, substan-
tial amounts of the side product (SP), up to 15 mol%, were formed.
This side reaction between furfural and the solvent competes for
the acid sites required to transform LE and b-AL to GVL as well
as the lactonization of PE, thus lowering the GVL yield.
To reduce the formation of the side product, Al-HY-6 with
weaker Brønsted acidity (as deduced from NH3 TPD measure-
ments) was tested. When Zr-HY-15-20 and Al-HY-6 were added
simultaneously at the start of the reaction, no FA could be
detected; instead FE, LE and b-AL were observed, showing the fast
reaction rates for steps 1 and 2 in the cascade reaction (Fig. 8b).
Their concentrations decreased with time to give 85.1 mol% GVL
after 5 h. The side product (SP) was significantly lower, constitut-
ing 4.5 mol% of the products. From the low concentrations of b-
AL, LE and PE at the end of the reaction, it can be inferred that
the Brønsted acidity of Al-HY-6 is adequate for the transformation
of these molecules but not sufficiently strong to catalyze the side
reaction. Hence, the weaker Brønsted acid catalyst, Al-HY-6, in
combination with Zr-HY-15-20 formed a very effective combina-
tion to achieve high GVL yields within a few hours. The influence
of the concentration of Lewis to Brønsted acid sites was studied
by varying the ratio of the two catalysts (Fig. S14). At a weight ratio
of Zr-HY-15-20/Al-HY-6 = 1, the conversion of levulinate ester and
b-angelica lactone became limiting, resulting in a GVL yield of 59%.
However, with Zr-HY-15-20/Al-HY-6 = 5, there was a buildup of
furfuryl ether due to the limited number of Brønsted acid sites
for step 2 reactions (elimination, addition and ring-opening), and
only 38.5% GVL yield was obtained. Hence, at Zr-HY-15-20/Al-
HY-6 = 2 we found the optimum in the density of Lewis and
Brønsted acid sites for the one-pot cascade reaction.
ity, pore size and hydrophobicity proved to be a highly efficient
catalyst for this step (1).
In step (2), the FE is protonated by the Brønsted acid and
releases 2-pentanol to form A which can undergo two pathways
depending on whether it reacts with water or 2-pentanol. In path-
way (1), attack at C1 of A by water forms the b-angelica lactone
intermediate, followed by ring-opening transesterification with
2-pentanol to yield the levulinate ester. In pathway (2), an initial
attack by 2-pentanol at C1 of A, followed subsequently by water
forms the levulinate ester. With a more hydrophobic catalyst, pref-
erential adsorption of 2-pentanol over water at the surface leads to
the formation of LE rather than b-angelica lactone. Both pathways
occur for the Al-Beta and Al-HY zeolites although with different b-
angelica lactone/LE ratios, reflecting the different degree of
hydrophilicity of the samples.
In step (3), LE undergoes MPV reduction to 4-hydroxy pentanoic
ester (PE) at the Lewis acidic Zr site. This is followed by intramolec-
ular lactonization with the extrusion of 2-pentanol to form GVL
which desorbs from the Lewis acid site. Alternatively, Brønsted
acid-catalyzed intramolecular lactonization of PE with subsequent
extrusion of 2-pentanol forms the protonated intermediate B
which deprotonates to give GVL.
4. Conclusion
The one-pot transformation of furfural to GVL involving a cas-
cade of reactions was achieved by using a physical combination
of solid Zr-HY Lewis acid and Al-HY Brønsted acid catalysts. The
Zr-containing zeolites were synthesized via
a post-synthesis
method involving the dealumination of the parent Al-containing
zeolites followed by Zr incorporation. NH3 TPD and pyridine
adsorption showed that the Zr-HY samples have weaker acid
strength and a higher ratio of Lewis to Brønsted acid sites than
Zr-Beta samples. In the MPV reduction of furfural and levulinate
ester with 2-pentanol as hydrogen donor, the Zr-HY (Si/Zr 5–20)
showed much higher activity than Zr-Beta samples (Si 12.5–150).
This was attributed to a larger pore size and a more hydrophobic
nature of Zr-HY (Si/Zr 20) as compared to Zr-Beta (Si/Zr 12.5).
The catalytic system comprising Zr-HY-15-20 and Al-HY-6 exhib-
ited excellent activity for the one pot cascade reaction, with 85%
GVL yield after only 5 h at 120 °C. A 2:1 wt ratio of Zr-HY-15-20
and Al-HY-6 was optimum. The catalyst mixture could be easily
recovered and reused for at least 3 successive runs without any
obvious drop in activity or GVL yield. A mechanism for the one-
pot cascade transformation from furfural to GVL was proposed
based on the roles of Lewis and Brønsted acid catalysts.
3.3.6. Recyclability of catalysts
After reaction, the catalyst combination of Zr-HY-15-20 and Al-
HY-6 was recovered, washed with acetone, dried and recalcined at
500 °C for 6 h to remove any adsorbed organics. The activity for
two subsequent runs was steady, with 83% GVL yields after 5 h
(Fig. S15). Therefore, the mixture of Zr-HY-15-20 and Al-HY-6
formed a highly robust and stable catalyst system for the one-
pot conversion of furfural to GVL.
Declaration of Competing Interest
The authors declare no conflicts of interest.
Acknowledgements
3.4. Proposed mechanism
An overall mechanism for the cascade reaction catalyzed by a
combination of Lewis and Brønsted acid catalysts is proposed
(Fig. 9). In step (1), FF reacts with 2-pentanol to form FA by MPV
reduction involving a six-membered transition state at the catalyst
surface in which both 2-pentanol and FF are coordinated to the
H. Zhang thanks -National University of Singapore (NUS), Singa-
pore for a research scholarship. This work is supported by Faculty
of Science, NUS ARC Tier 1 grant R-143-000-667-114.
Appendix A. Supplementary material
same Lewis-acidic Zr center. The
a-hydrogen is transferred from
2-pentanol to FF forming FA. Protonation of FA by a Brønsted acidic
proton associated with Si-(OH)-Al followed by an attack on the
Supplementary data to this article can be found online at
electrophilic
a-carbon with extrusion of water and subsequent
deprotonation forms FE. FA can also coordinate to the Lewis-acid
Zr site, which enhances its electrophilicity and allows for nucle-
ophilic attack by OH of 2-pentanol forming FE with the release of
water. Zr-HY-15-20 with an optimized combination of Lewis acid-
References