pH-Regulated Aqueous Catalytic Hydrogenation of Biomass Carbohydrate Derivatives by Using Semisandwich Iridium Complexes

Article abstract of DOI:10.1002/cctc.201601009

Semisandwich Ir complexes [Cp*Ir-(di-OMe-bpy)(OH₂)][SO₄] (di-OMe-bpy=4,4′-dimethoxyl-2,2′-bipyridine) and [Cp*Ir-(di-OH-bpy)(OH₂)][SO₄] (di-OH-bpy=4,4′-dihydroxyl-2,2′-bipyridine) are excellent efficient catalysts for the aqueous hydrogenation reactions of biomass carbohydrate derivatives. We show that pH value plays an important role in the hydrogenation reaction. By adjusting the pH value of the solvent, we can improve the catalytic efficiency and control the product distribution. The turnover frequencies (TOF) of hydrogenation of furfural up to 13 877 h^?1 and TOF of hydrogenation of levulinic acid up to 12 200 h^?1 at 120 °C were achieved. Converting furfural into γ-valerolactone in one step is observed at strong acidic aqueous system, which makes it easy to achieve a distribution-controlled product by pH adjustment.

Full text of DOI:10.1002/cctc.201601009

Accepted Article
Title: pH-Regulated Aqueous Catalytic Hydrogenation of Biomass
Carbohydrate Derivatives by Using Semi-sandwich Iridium
Complexes
Authors: Wei-Peng Wu, Yong-Jian Xu, Shang-Wei Chang, Jin Deng, and
Yao Fu
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ChemCatChem
10.1002/cctc.201601009
FULL PAPER
pH-Regulated Aqueous Catalytic Hydrogenation of Biomass
Carbohydrate Derivatives by Using Semi-sandwich Iridium
Complexes
[
a]
[a]
[a][b]
Jin Deng,* [a] and Yao Fu*[a]
Wei-Peng Wu, Yong-Jian Xu, Shang-Wei Chang,
Abstract:
bpy)(OH )][SO
Cp*Ir-(di-OH-bpy)(OH
Semi-sandwich
] (di-OMe-bpy = 4,4’-dimethoxyl-2,2’-bipyridine) and
)][SO (di-OH-bpy 4,4’-dihydroxyl-2,2’-
Ir
complexes
[Cp*Ir-(di-OMe-
derivatives are water-soluble, highly-reactive and low volatile [10]
.
2
4
In addition, water plays an important role as the solvent or
[
2,
11]. Therefore,
[
2
4
]
=
reactant in biomass treatment process
carbohydrates derivatives are suitable for aqueous reaction [
.
10]
bipyridine) are excellent efficient catalysts for the aqueous
hydrogenation reactions of biomass carbohydrate derivatives. We
show that pH value plays an important role in the hydrogenation
reaction. By adjusting pH value of solvent, we can improve the
catalytic efficiency and control the product distribution. The turnover
Considering the advantages of hydrogenation and aqueous
reaction, the aqueous hydrogenation reaction is an essential
method to convert biomass into fuels and fine chemicals [12]
.
Among the hydrogenation products of carbohydrates
derivatives, furfuryl alcohol and γ-valerolactone (GVL) are
drawing ever more attention. Furfuryl alcohol, the selective
reduction product of furfural [13], is mainly used for synthesizing
widely-adopted furfuryl alcohol resins and the production of
polyurethane foams and polyesters [14]. GVL, known as platform
-
1
frequencies (TOF) of hydrogenation of furfural up to 13877 h and
-
1
TOF of hydrogenation of levulinic acid up to 12200h at 120 °C.
Converting furfural into GVL in one step is observed at strong acidic
aqueous system, which makes it easy to achieve
distribution-controlled by pH adjustment.
a product
[
15]
molecules
, can be used the synthesis of 2-methyl-
[
16]
[17]
, liquid alkanes [18], ionic
tetrahydrofuran
, 1,4-pentanediol
liquids [
19-21]
and polymers [22]. Also, GVL was suggested as a fuel
Introduction
additive [23] and a green solvent [24-28] due to distinctive physical
chemistry properties.
Nowadays, the rapid development of human society is at the
expense of great consumption of the non-renewable resources.
The gradual depletion of fossil resources have attracted global
attention to the research of exploring alternatives to organic
As a cheap and abundant solvent for organic reactions,
water attracts particular attention due to its superior
physicochemical properties over traditional organic solvents like
non-toxic, non-carcinogenic, non-combustible and non-explosive
[
1]
carbon sources
.
Biomass is the sole readily-available
[29].
Moreover, water has the Brønsted amphoteric behaviour,
renewable carbon source to produce liquid fuels and fine
chemicals [ . The major component of lignocellulosic biomass is
carbohydrate polymers [4], which contain pentose monomer and
hence the reaction rates and selectivity can be regulated by the
adjusting of pH value [30]. Recently, various homogeneous
2-3]
catalytic systems used in aqueous hydrogenation reaction of
biomass carbohydrate derivatives have been reported. Typically,
[
5]
hexose monomer . Both carbohydrates and their derivatives
have rich oxygenated functional groups [6-7]. Considering efficient
utilization of biomass, it is necessary to remove oxygenated
functional groups efficiently and selectively [8-9]. Hydrogenation is
the preferred method for biomass processing owing to its atom
economy. Normally, the raw biomass is rich in water and its
[18,31,32]
[33,34]
Ru complexes
and Ir complexes
performed well in
homogeneous catalysis. In particular, iridium complexes have
drawn considerable attentions in homogeneous catalytic
hydrogenation because of their robustness and highly activity in
respect of turnover frequencies (TOFs) and turnover numbers
(
TONs).[35,36] However, researchers found the inorganic acid such
as sulphuric acid, which is necessary in biomass hydrolysis
system, would weaken the catalytic efficiency [37]. Thus, the
tolerance of catalysts to acidity is very important in biomass
treatment process.
[
a]
W.-P. Wu, Y.-J. Xu, Dr. J. Deng, Prof. Dr. Y. Fu
iChEM, CAS Key Laboratory of Urban Pollutant Conversion Anhui
Province Key Laboratory of Biomass Clean Energy
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (P. R. China)
Fax: (+86) 551-6360-6689
E-mail: dengjin@ustc.edu.cn
fuyao@ustc.edu.cn
[
30,38-43]
Himeda and coworkers
and Fukuzumi and
coworkers [ have developed a series of semi-sandwich iridium
complexes catalysts, which show good water-solubility and acid
resistance. Also, our group have reported that water-soluble
semi-sandwich iridium complexes can efficiently convert levulinic
44]
[
b]
S.-W. Chang
acid (LA) into GVL under mild conditions (120 °C and 1.01 MPa)
Nano Science and Technology Institute
University of Science and Technology of China
Suzhou 215123 (P. R. China)
[34]
in pure aqueous system
. In addition, pH was recently
demonstrated to effectively regulate the catalytic efficiency and
distribution of products of hydrogenation/ring-opening reaction of
Supporting information for this article is given via a link at the end of
the document.
5
-HMF catalyzed by semi-sandwich iridium catalyst [45]
.
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ChemCatChem
10.1002/cctc.201601009
FULL PAPER
Figure 1. TOF of reactions under different pH values.
Reaction conditions: a) Furfural (0.60 mmol), aqueous solution of catalyst (0.5 mmol/L, 100 μL), phosphate buffer solution (2 mL), 1.0 MPa H
1
2
, 120 °C,
hour. The catalyst/substrate molar ratio of highest TOF (7340 h ) is 0.0017% (A fifth of the general reaction condition). b) LA (0.49 mmol), aqueous
solution of catalyst (0.5 mmol/L, 50μL), phosphate buffer solution (2 mL), 1.0 MPa H , 120 °C, 1 hour. c) Furfural (0.60 mmol), aqueous solution of
catalyst (0.5 mmol/L, 100 μL), formate buffer solution (1mol/L, 2 mL), 1.0 MPa H , 120 °C, 1 hour. The catalyst/substrate molar ratio of highest TOF
13800h ) is 0.0017% (A fifth of the general reaction condition). d) LA (0.49 mmol), aqueous solution of catalyst (0.5 mmol/L, 50μL), formate buffer
solution (1mol/L, 2 mL), 1.0 MPa H , 120 °C, 1 hour.
-1
2
2
-1
(
2
Herein, we developed the pH-regulated aqueous
hydrogenation of biomass-derived molecules over semi-sandwich
iridium complexes. By adjusting pH value of solvent and electronic
density of substituent on ligand, we can improve the catalytic
efficiency and control the product distribution. Full conversion of
dihydroxyl-2,2’-bipyridine) (catalyst 2), were used to perform our
experiments. Furfural, which is the typical product of dehydration
of pentose, is hydrogenated under 1MPa H
hour over 0.0083mol% catalyst 1. And the pH value from 0.5 to
7.0 was regulated with H SO /H PO or phosphate buffer solution
2
at 120 °C for one
2
4
3
4
-
1
furfural to furfuryl alcohol (TOF up to 13877 h ) and full
(PBS). Under above reaction conditions, the TOFs versus
reaction pH value are shown in Chart a) of Fig. 1, Table s1-s2.
The results showed that, under pH of 3.5, we obtained the highest
-
1
conversion of LA to GVL (TOF up to 12200 h ) were achieved
respectively. In addition, under strong acidic aqueous conditions,
the hydrogenation product of furfural, furfuryl alcohol, could be
hydrolyzed into LA, and LA could be further converted into GVL.
Hence to achieve the one-pot conversion of GVL from furfural.
-
1
yield of furfuryl alcohol and the maximum TOF of 1600h . When
we use hydroxyl as the substitute (catalyst 2), the highest TOF is
7340 h-1 under pH of 5.0. Our previous research found that the
electron-donating group on catalysts would promote catalytic
activity significantly [34,45]. A small amount of GVL was observed
when pH<1.5. And soluble furfuryl alcohol resins and insoluble
humins were formed at pH 2-4. As pH value raises above 3, the
hydroxyl group of 4,4’-dihydroxyl-2,2’-bipyridine ionizes into aryl
oxygen anion (Figure 2, species 2), which results in a remarkable
improvement in electron-donating ability. Thus, the activity of
catalyst 2 is more highly than catalyst 1. Goldberg and coworkers
Results and Discussion
Initially, we investigated the influence of electronic effect of
substituents on TOF under different pH values. Two of the most
active catalysts in our previous work 34,45], [Cp*Ir-(di-OMe-
[
bpy)(OH
2
)][SO
4
] (di-OMe-bpy = 4,4’-dimethoxyl-2,2’-bipyridine)
)][SO ] (di-OH-bpy = 4,4’-
+
suggested that there are [Cp*Ir-(di-OMe-bpy)(H)] intermediate in
(catalyst 1) and [Cp*Ir-(di-OH-bpy)(OH
2
4
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ChemCatChem
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FULL PAPER
Figure 2. Catalyst 2 and its reaction intermediates under different pH.
the reaction process, which would react with protonized
rates. Using furfural as substrate, the ratios of the highest TOFs
between Cat.2 and Cat.1 are 4.4 (PBS) and 3.8 (FBS)
respectively. While the ratios of the highest TOFs are 1.5 (PBS)
and 1.8 (FBS) respectively with LA as substrate. The values of
furfural are higher than the values of LA under different buffer
solutions. Therefore, hydrogenation of furfural is more influenced
by electronic effect of the substituents than hydrogenation of LA,
and the aldehydic carbonyl group of furfural is more active than
the ketone carbonyl of LA.
[
46]
carboxylic compounds . At higher pH values, the concentration
of protonated carbonyl compound in the system is decreasing,
while proton characteristics of intermediate is increasing. This
causes the decrease of the catalytic activity [47]
LA is the typical dehydration product of hexose. Under
identical conditions (0.0083 mol% catalyst, 1.0 MPa H , 120 °C , 1
.
2
hour), TOF of hydrogenation of LA is 2864 h-1 (pH=1.5) by using
catalyst 1 and 4326 h-1 (pH=2.5) by using catalyst 2 (see Chart b)
of Fig. 1, Table s3-s4). The TOF change trend of hydrogenation
of LA is same as the hydrogenation of furfural. This result confirms
the above speculation.
Table 1. Product distribution in strong acidic system.
yield, %
It has been reported that semi-sandwich iridium complexes
can catalyse the decomposition of formic acid (FA) [48], which is
the by-product of acidic hydrolysis of biomass raw material. Thus,
it is economical to use formic acid as a hydrogen source in the
reduction of carbohydrate derivatives. Furfural is hydrogenated at
(TOF)
hydrogen
source
entry
Cat.
c(Ir)
furfuryl
alcohol
LA
GVL
1[a]
2[a]
3[a]
4[a]
0.0083 mol%
4.8 ppm
1 mol/L
HCOOH
0
(0)
14
(422)
0
(0)
1
2
1
2
1
2
1
2
1
20 °C for 1 hour in 1mol/L FA buffer solution (pH value ranges
0.0083 mol%
4.8 ppm
1 mol/L
HCOOH
0
(0)
1
(30)
0
(0)
from 2.0 to 7.0). The TOFs versus reaction pH value are shown
in Chart c) of Fig. 1, Table s5-s6. TOF of catalyst 2 is higher than
0.083 mol%
48 ppm
1 mol/L
HCOOH
0
(0)
7
(21)
4
(12)
-
1
that of catalyst 1.The value is 13877 h , which is higher than the
reported TOF we have known [13,49-54] (see Table s9). Under the
0.083 mol%
48 ppm
1 mol/L
HCOOH
2
(6)
6
(18)
3
(9)
-
1
same conditions, the efficiency of catalyst 2 (TOF=12200 h ) in
the hydrogenation of levulinic acid is higher than catalyst 1 (TOF=
-
1
5[b]
0.0083 mol%
4.8 ppm
1 MPa H
1 MPa H
1 MPa H
1 MPa H
2
0
(0)
24
(724)
35
(1056)
6
800 h ), which is better than the highest value that we previously
[
17,18,31,33,34,55-60]
reported
(see Table s10). The TOFs versus
6[b]
0.0083 mol%
4.8 ppm
0
(0)
41
(1237)
9
2
2
2
reaction pH value are shown in Chart c) of Fig. 1, Table s7-s8. In
our reaction system, catalyst is mainly in the form of the partial
deprotonated species (Figure 2, species 2). As formic acid ionizes
into formate, the electrostatic attraction makes it easier to
combine with the catalyst in homogeneous phase than hydrogen
in biphasic system. This results in a higher reaction efficiency
using formic acid as hydrogen source.
(272)
7[b]
0.083 mol%
48 ppm
0
(0)
<1
(3)
55
(166)
8[b]
0.083 mol%
0
(0)
1
(3)
47
(142)
4
8 ppm
Reaction Conditions: Furfural (0.60 mmol), aqueous solution of catalyst (0.5
mmol/L or 5 mmol/L, 100 μL), 1.0 MPa H , 120 °C, 4 hour. [a] formate buffer
solution (pH=1.0, 1mol/L, 2 mL), [b] phosphate buffer solution (pH=1.0,
.1mol/L, 2 mL)
2
The ratios of the highest TOFs between Cat.2 and Cat.1
show the correlation between substrates reactivity and reaction
0
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In a strong acidic phosphate buffer system with pH of 1.0
0.0083 mol% catalyst, 1.0 MPa H , 120 °C , 1 hour), we found
strong acidity (pH<1.5) using phosphate buffer solution. Since the
species 5 (Figure 4) is instable at such acidity, it tends to combine
with hydron. That causes the competition of FA decomposition
with the hydrogenation reaction. The system cannot provide
enough hydride when FA is used as hydrogen source. In order to
(
2
that furfural can be converted into γ-valerolactone directly in one-
pot without any intermediate product separation step. While GVL
was not detected in formate buffer system. As mentioned above,
a loss of catalytic activity was detected at strong acidic solution,
hence we carried out the cascade reaction from furfural to GVL
with an increase in catalyst concentration to 0.083 mol% and an
increase in reaction time to 4 hour. When hydrogenation of furfural
was performed in formate buffer solution (pH=1.0) with
2
improve the yield of GVL, we enhance H pressure up to 1.5 MPa.
But no improvement had been detected (Figure s3). Previous
studies [61-62] indicated that hydrolysis of furfuryl alcohol competed
with its polymerization, and humins was observed in our reaction,
both of which caused the decreasing of GVL yield. And humins
0
.0083mol% catalyst for 4 hour, only LA was obtained (Table 1,
increase with the substrate concentration (Figure s4).
Goldberg and coworkers [
46,63]
have pointed out that 2-
entry 1 and 2). A GVL yield less than 10% was achieved by
increasing the catalyst concentration to 0.083 mol% in formate
buffer solution (Table 1, entry 3 and 4). When furfural was
methyltetrafuran could be obtained by hydrogenation of LA and
deep hydrogenation of GVL over semi-sandwich iridium
complexes catalysts. We carried out the hydrogenation reaction
of GVL under our optimum experimental conditions (0.083 mol%
reduced under 1 MPa H
at 120 °C , a LA yield of 24% and a GVL yield of 35% was obtained
Table 1, entry 5). While LA in 41% yield and GVL in 9% yield was
2
for 4 h using 0.0083 mol% of catalyst 1
(
2
Cat.1, 1 MPa H , 120 °C , and 4 hour). Less than 1% yield of 2-
obtained using 0.0083 mol% of catalyst 2 (Table 1, entry 6). In the
case of 0.083 mol% catalyst use in phosphate buffer solution at
methyltetrafuran was obtained. Since our reaction conditions (1
MPa and 4 hour) are milder than Goldberg’s work (30bar and 18
hour), hydrogenation reaction of GVL was difficult to process. Xu
pH 1.0,
a 55% yield of GVL and a 3% yield of 2-
and coworkers [ and Zhu and coworkers
64]
[65]
showed the direct
methyltetrahydrofuran with catalyst 1, and a 47% yield of GVL
with catalyst 2 were obtained respectively (Table 1, entry 7 and
hydrogenolysis of furfuryl alcohol with subsequent hydrogenation
into 2-methyltetrafuran under hydrogen atmosphere. Thus, the 2-
methyltetrafuran in our reaction system came from
hydrogenolysis of furfuryl alcohol.
8
). These results are consistent with our previous work [34]
,
catalyst 1 performed slightly better than catalyst 2 in strong acidic
system.
From the above results, H
2
is better hydrogen source than
Li and coworkers [66] found that the reduction efficiency of
furfural and levulinic acid in methanol system is better than
reactions in water. Dumesic and coworkers [67] pointed out that
GVL solubilized humins during cellulose deconstruction process.
We performed the hydrogenation loading organic solvents as
additive in phosphate buffer solution. The formation of humins
significantly decreased by adding organic solvents while GVL
yield was not improved (Figure s2) due to the selectivity
decreased.
formic acid for the one-pot conversion of furfural to GVL in strong
acid aqueous system (a) of Scheme 1). However, in our previous
work [ , formate buffer solution performed better than phosphate
45]
buffer
solution
in
catalytic
(5-HMF)
conversion
of
5-
hydroxymethylfuraldehyde
to 1-hydroxy-2,5-
hexanedione (HHD, b) of Scheme 1). This is a result of pH rise
with HCOOH decomposition during the hydrogenation/hydrolysis
reaction of 5-HMF in formate buffer solution, which avoid the
degradation of 2,5-bis-(hydroxymethyl)furan (BHMF) and HHD.
Moreover, acid-catalyzed hydrolysis of BHMF could still occur
with a mild acidity in the pH change process. In contrast, Conclusions
hydrolysis of furfuryl alcohol to LA requires a sustained strong
acidity, and then LA could be reduced to GVL. Besides, GVL is
stable in strong acidic solution. Furthermore, furfuryl alcohol can
easily polymerize to soluble furfuryl alcohol resins and insoluble
humins in the range of pH 2-4. Hence, it is necessary to keep
In conclusion, we report a pH-regulated aqueous catalytic
hydrogenation of biomass carbohydrate derivatives by using
semi-sandwich iridium complexes. We observed the best catalytic
efficiency and TOF so far under mild conditions. Moreover, by
simply regulating the pH values, we can control the product
distribution and achieve the one-pot aqueous reaction for the
preparation of γ-valerolactone from furfural. Our system is
compatible with pure water system and low concentration of
substrate, which shows a robustness of semi-sandwich Ir
complexes for acidic aqueous system and a remarkable viability
for practical hydrolysis system.
Scheme 1. The cascade reactions of furfural to GVL and 5-HMF to HHD.
Experimental Section
Materials. All catalysts and chemicals are commercially available. LA
(99%), FA (98%), and GVL (98%) were purchased from Aladdin Reagent
Co. Ltd. Furfural was generous gifts from Hefei Leaf Energy
Biotechnology Co., Ltd. and used with purification by distillation.
Figure 3. The competition of FA decomposition with the hydrogenation
(
2 2
Pentamethylcyclopentadienyl) iridium (III) chloride dimer [(Cp*IrCl ) ,
reaction.
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ChemCatChem
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FULL PAPER
9
4
2
8%] was purchased from Suzhou Sinocompound Technology Co., Ltd.
,4’-dimethoxy-2,2’-bipyridine were purchased from TCI. 4,4’-dihydroxy-
,2’-bipyridine, were synthesized according to the previously reported
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.
Preparation of 0.1 mol/L phosphate buffer solution. pH range
from 0.5 to 1.5. Dripping concentrated sulphuric acid into the solution of
phosphoric acid (0.1 mol/L) while measuring the pH with a pH meter. pH
range from 2.0 to 7.0. Making a solution of monosodium orthophosphate
[
[
[
[
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,
6
(0.1 mol/L) with the same concentration as the orthophosphoric acid or
disodium hydro-gen phosphate and mixing while measuring the pH with a
pH meter.
,
Preparation of 1 mol/L formate buffer solution. pH range from
.5 to 1.5. Dripping concentrated sulphuric acid into the formic acid
0
[23] I. T. Horváth, H. Mehdi, V. Fábos, L. Boda, L. T. Mika, Green Chem.,
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concentration as formate while measuring the pH with a pH meter.
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General catalytic hydrogenation of LA or furfural with H
0.49 mmol) or furfural (0.60 mmol), aqueous solution of catalyst (0.5
mmol/L, 50μL for LA, 100 μL for furfural), and phosphate buffer solution
2 mL) were added to a 10 mL zirconium alloy high-pressure reaction
tube, and stirred at a rate of 900 rpm under 1.0 MPa H , The mixture was
2
. LA
(
[
(
2
464.
27] V. Fa
Csefalvay, V. Kova
2
[
́
bos, M. Y. Lui, Y. F. Mui, Y. Y. Wong, L. T. Mika, L. Qi, E.
cs, T. Szucs, I. T. Horváth, ACS Sustainable
heated to 120 °C for 1 hour. The mixture of substrates and catalyst were
heated to the desired temperature in 10 min and cooled down in water to
room temperature after the reaction. The liquid products were diluted with
acetonitrile and analysed by using GC on a Shimadzu GC-2014 gas
chromatograph equipped with a DB-FFAP capillary column (30 m×0.320
mm×0.25μm) and a flame ionization detector. An internal standard (1-
methyl-2-pyrrolidone) was used to determine the amount of product. The
typical GC chart of the internal standard was showed in Figure s1.
General catalytic hydrogenation of LA or furfural with FA. LA
́
́
̋
Chem. Eng., 2015, 3, 1899-1904.
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[
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(
0.49 mmol) or furfural (0.60 mmol), formate buffer solution (2mL),
aqueous solution of catalyst (0.5 mmol/L, 50μL for LA, 100 μL for furfural)
were loaded in sealed glass tube. The catalytic conversion proceeded as
de-scribed above, but in the absence of hydrogen.
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FULL PAPER
Wei-Peng Wu, Yong-Jian Xu,
Shang-Wei Chang, Jin Deng,* and Yao Fu*
Page No. – Page No.
pH-Regulated Aqueous Catalytic
Hydrogenation of Biomass Carbohydrate
Derivatives by Using Semi-sandwich
Iridium Complexes
-
1
Full conversion of furfural to furfuryl alcohol (TOF up to 13877 h ) and full
-
1
conversion of levulinic acid to γ-valerolactone (TOF up to 12200 h ) were
reached respectively. In addition, the one-pot aqueous reaction of furfural for
the preparation of γ-valerolactone was achieved.
This article is protected by copyright. All rights reserved.