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Green Chemistry
Page 2 of 8
ARTICLE
Journal Name
Development of the metal-free system
DOI: 10.1039/D0GC02676J
First, LA was reacted with excess hydroiodic acid. LA was
converted completely and yielded 62.3% PA at 180 °C for 1 h
(Table 2, entry 1). The reaction solution became brownish-red
after the reaction, and the presence of I2 was confirmed by the
potassium iodide–starch method. The formation of I2 as a side
product and the high concentration of HI make this reaction
impractical. Therefore, the system was further improved
according to the metal-free catalytic systems17-18 with low-
concentration of HI mentioned above. First, a system with 0.36
mmol H2SO4, 4 mmol NaI as the catalyst, and MTHF as the
solvent was applied. However, only a 12.3% yield of PA was
obtained with 48.2% LA conversion at 180 °C for 1 h (Table 2,
entry 2), which was probably due to the instability of the solvent,
as a small amount of the iodinated product of MTHF was
detected after the reaction. When the solvent was changed to
acetonitrile (Table 2, entry 3), 80.6% of LA was converted, but
only an 11.6% yield of PA was obtained. This low selectivity
could result from the low acidity of the system. Then acetic acid
was used as the solvent (Table 2, entry 4), in which LA was
completely converted, and a 70.5% yield of acetoxypropionic
acid (AOPA) and 17.0% yield of PA were obtained. AOPA might
be the intermediate of the reaction. Increasing the temperature
to 200 °C and 220 °C (Table 2, entry 5&6), LA converted
completely in both conditions and yielded 64.4% AOPA and
26.2% PA at 200 °C while 3.3% AOPA and 84.3% PA were
obtained at 220 °C. Although the yield of PA is slightly higher
than that obtained without using H2SO4, the reaction system is
more corrosive due to the addition of a strong acid. After
prolonged the reaction at 180 °C from 1 h to 10 h (Table 2, entry
7), all of LA was consumed to yield 54.1% PA and 28.3% AOPA.
It was not a satisfactory result. Surprisingly, when only 2 mmol
NaI was used without additional strong acid in the acetic acid
solvent system (Table 2, entry 8), the reaction also yielded 9.8%
PA and 30.7% AOPA with 43.8% LA conversion. According to
previous studies16-18, a proton is necessary to catalyze iodide
substitution of the hydroxyl group. In this reaction, the proton
must come from the acetic acid. The good carbon balance under
these conditions encouraged us to optimize the reaction
without using extra strong acid.
First, the reaction temperature was increased to 240 °C
(Table 2, entries 9-11). The yield of PA slightly increased to
23.8% at 200 °C (Table 2, entry 9) and climbed to 72.5% at 220 °C
(Table 2, entry 10). Encouragingly, 96.2% PA could be obtained
at 240 °C in 1 h (Table 2, entry 11). Thus, high selectivity to PA
from LA was realized at a relatively low reaction temperature in
our system compared with the previous literature.
To better understand the reaction system, LA conversion
with 2 mmol NaI at 220 °C for 1 h was selected as the reaction
conditions for the following study. First, under the standard
reaction condition (Table 2, entry 10), no I2 was detected after
the reaction, which confirmed that I- is the catalyst and H2 is the
reducing reagent. When H2 was replaced by He (Table 2, entry
12), a 49.6% yield of PA was still obtained, but a large amount
of iodine was found after the reaction, which rendered the
solution brown-black. Thus, without H2, HI (generated by NaI in
Scheme 1. Two hydrogenation pathways of LA
The low selectivity for removing the α-hydroxy group of LA
by direct catalytic hydrogenation is probably due to the fact that
theoretically13, the reaction enthalpy of α-hydroxyl
hydrogenation (∆Hr = -118 kJ/mol) is higher than that of
carboxyl hydrogenation (∆Hr = -85 kJ/mol) (Scheme 1). Thus, a
different methodology must be used to further improve the
selectivity.
Hydroiodic acid is a strong reducing agent and iodide is a
good nucleophilic and leaving group, which makes it good at
reducing hydroxyl groups. As early as 186015, HI was used to
remove the hydroxyl group of LA to prepare PA. Recently, our
work16 also reported that PA could be prepared from LA in
hydroiodic acid over noble metals, which could catalyze the in-
situ regeneration of HI from I2 under H2. In the latest research,
an HI/I- mediated metal-free catalytic system was developed for
the selective reduction of hydroxyl groups by H2. In 2017,
Vlachos and Xu17 reported that tetrahydrofuran-2,5-diformic
acid could yield 89% adipic acid using approximately 4
equivalents HI acid under the synergistic effect of PA
solvent/HI/H2 without a metal catalyst. They proposed that H2
was activated by HI. Subsequently, our group found that using
4 equivalents of NaI in 2-methyltetrahydrofuran (MTHF) as a
solvent, 5-hydroxymethylfurfural could be directly reduced to
5-methylfurfural with an 80% yield in the NaI/H2SO4/H2O/H2
system without metal catalysts18. The mechanism study
speculated that the main reduction process was the free radical
reaction. Therefore, it is hopeful to develop an efficient metal-
free method to produce PA from LA with HI acid.
Herein, we further improved the system and developed a
metal-free system to convert LA to PA catalyzed by NaI. The
product PA was used as the solvent to both provide acidity and
simplify the product separation step. Under the optimum
reaction conditions, a >99% yield of PA was obtained from LA at
220 °C for 6 h without adding extra strong acid. Compared with
the previous system, the amount of I- added is greatly reduced
to 8 mol% equivalent. A NaI separation and recycling
methodology based on the extraction of PA with cyclohexane
was proposed, which makes the production process green and
sustainable. Furthermore, based on reported work regarding LA
production from cellulose7b, a two-step process was developed
to produce PA from cellulose, which provides a simple and
economically feasible method to obtain PA from sustainable
biomass resources. To the best of our knowledge, this is the first
time that this metal-free system has been reported to produce
PA from biomass with excellent selectivity and yield. The whole
process is well in line with the design of a green chemistry
future19.
Results and Discussion
2 | J. Name., 2012, 00, 1-3
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