Conversion of cellulose to hexitols catalyzed by ionic liquid-stabilized ruthenium nanoparticles and a reversible binding agent

Article abstract of DOI:10.1002/cssc.200900235

A reversible cellulose binding agent, 1-(4'-(4''-(2'''-boronobenzyl)piperazinyl)-2'-butenyl)-3-n-butylimidazolium chloride 1, is synthesized by reacting 2-((4'-allylpiperazinyl)methyl) phenylboronic acid with 3-allyl-1-n-butylimidazolium chloride. 1 is mi

Full text of DOI:10.1002/cssc.200900235

DOI: 10.1002/cssc.200900235
Conversion of Cellulose to Hexitols Catalyzed by Ionic Liquid-Stabilized
Ruthenium Nanoparticles and a Reversible Binding Agent
Yinghuai Zhu,*[a] Zhen Ning Kong,[a] Ludger Paul Stubbs,[a] Huang Lin,[a] Shoucang Shen,[a] Eric V. Anslyn,[b] and
John A. Maguire[c]
Biomass has recently attracted wide interest because it is rec-
ognized as a candidate for both new energy resources and as
feedstocks to replace fossil fuels.[1] Because cellulose is the
principal component of biomass, the conversion of cellulose to
valuable chemicals is a major focus in biomass conversion.[1]
Yet, the multiple hydroxyl groups of cellulose that form hydro-
gen bonds between adjacent polymers, and that create crystal-
line structures giving plants structural strength, also make cel-
lulose particularly difficult to digest.[2] Three methods have
been employed to overcome this problem:[2,3] a) acid-catalyzed
cellulose hydrolysis, b) enzyme-catalyzed cellulose hydrolysis,
and c) subcritical or supercritical water-catalyzed cellulose hy-
drolysis. Mineral acids are used to catalyze cellulose hydrolysis,
creating glucose even under mild conditions. However, remov-
al or neutralization of the acid is required after treatment, and
the use of corrosive mineral acids causes environmental prob-
lems.[3] Furthermore, enzyme-catalyzed cellulose hydrolysis is
not practical at present because it is expensive, and no satis-
factory methods have been developed to recover the enzymes
from the hydrolysate mixture.[3] Lastly, although hydrolysis in
supercritical water at 3748C is fast, it is difficult to inhibit sec-
ondary degradation of the generated monosaccharides. This
degradation causes both low yields of glucose and subsequent
inhibition of fermentation to ethanol because of the enzymati-
cally toxic 5-hydroxymethylfurfural (5-HMF) produced.[3] Hence,
there is a need to develop viable catalysts for rapid cellulose
degradation that gives high conversion to hexitols with mini-
mal environmental impact.
conversion to polyols.[6] The resulting sorbitol products were
easily separated by extraction.[7]. However, the poor solubility
of cellulose still hinders these catalytic processes.
To assist in solubility, ionic liquids have been used as a sol-
vent in cellulose pretreatment and transformation.[8–11] In addi-
tion, the ionic liquid species can stabilize transition metal
nanoparticles to sustain their small size, high surface area, and
inhibit nanoparticle leaching.[12]
Further, it is well known that o-(N,N-dialkylaminomethyl) ar-
ylboronic acids can interact with sugars and diols through re-
versible covalent bonds.[13] We anticipated that boronic acids
could break up the crystal packing of cellulose by reversible
binding with the multiple hydroxyl groups, thereby improving
solubility and catalytic activity. This should be especially effec-
tive when combined with the cellulose dissolving ability of
ionic liquids, and hence we designed conjugate 1.
Compound 1 was created by metathesis between boronic
acid 2 and an n-butyl-3-imidazolium cation 3 (Scheme 1; see
the Supporting Information for characterization). This cation
Transition metal nanoparticles as catalysts for organic trans-
formations is attracting widespread attention.[4] This is because
nanoparticle-based catalytic systems exhibit superior catalytic
activities relative to their corresponding bulk materials,[5] and
often higher selectivity when compared with conventional het-
erogeneous catalysts.[5] The particle size and surface structure
are the most important factors dominating the catalytic selec-
tivity of metal nanoparticle-based catalysts.[ 5g] For example,
nanoparticles of Ru, Pd, and Pt are active catalysts for cellulose
Scheme 1. Synthesis of boronic acid binding agent.
mimics that found in the ionic liquid 1-n-butyl-3-methylimida-
zolium chloride ([BMIM]Cl). Therefore, while compound 1 is in-
soluble in water, it is fully miscible with the ionic liquids 1-n-
butyl-3-methylimidazolium chloride ([BMIM]Cl) and trihexylte-
tradecylphosphonium dodecylbenzene sulfonate ([THTdP]-
[DBS]).
[a] Dr. Y. Zhu, Z. N. Kong, Dr. L. P. Stubbs, Dr. H. Lin, Dr. S. Shen
Institute of Chemical and Engineering Sciences
1 Pesek Road, Jurong Island, 627833 (Singapore)
Fax: (+65)6796-3801
E-mail: zhu_yinghuai@ices.a-star.edu.sg
After achieving 1, we checked if it would stabilize metal
nanoparticles as do free ionic liquids. Ruthenium nanoparticles
were prepared and stabilized with [BMIM]Cl and [THTdP][DBS]
as previously reported (mole ratio of [BMIM]Cl/1 or [THTdP]-
[DBS]/1=45:1).[12] The resulting Ru nanoparticles were ana-
lyzed with transmission electron microscopy (TEM) and X-ray
photoelectron spectroscopy (XPS) to determine their particle
[b] Prof. Dr. E. V. Anslyn
Department of Chemistry and Biochemistry
University of Texas at AustinAustin, Texas 78712 (USA)
[c] Prof. Dr. J. A. Maguire
Department of Chemistry, Southern Methodist University
Dallas, Texas 75275-0314 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.200900235.
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67

size and oxidation states. The TEM image shows that the nano-
particles are small with an average size of approximately
4.0 nm, with a narrow size distribution (Figure 1). These results
the nanoparticles were easily recovered from the reaction mix-
ture by diluting with water, decanting the aqueous phase, and
washing the obtained residue with water to remove any traces
of hexitols. Activity was maintained after 5 runs with sodium
formate as the hydrogen donor, whereas the Ru catalysts loss
was less than 3 ppm, based on the inductively coupled plasma
(ICP) analysis.
Based on previous reports and our results, we propose the
following steps are involved in the mechanism. Firstly, upon
boronic acid-based receptor 1 incubation with cellulose, bind-
ing occurs with the 1,2-diols present along the polymeric chain
of cellulose. Compared with a mixture of [THTdP][DBS] and 1
in CD2Cl2, after suspending with cellulose at 308C for 2 h, a
broad peak at d=À1.1 ppm emerged in the 11B NMR spectra
(see the Supporting Information, Figure S5), which supports
complex formations between 1 and multiple hydroxyl groups
in cellulose. The affinity of boronic acids to diols is enhanced
at neutral pH,[13] such as in most of our conditions. However,
when formic acid/Na+ formate is used, the reaction mixture is
slightly acidic (pH ca. 5–6), which lowers the affinity between 1
and the cellulose. Under any pH we use, the binding of 1 with
cellulose is proposed to solubilize the polysacchloride. Cellu-
lose hydrolysis to glucose is then followed by glucose hydro-
genation to hexitols by the Ru nanoparticle catalysts. Further-
more, when formic acid/Na+ formate is used as the hydrogen
donors, the ionic liquid stabilizes ruthenium nanoparticle cata-
lysts, which likely also enhances the hydrogen release; it has
been reported that the ruthenium complexes are effective cat-
alysts for this release.[17] Lastly, dissociation of binding agent 1
from the free glucose and hexitol allows 1 to play its role
again. Our proposed catalytic process for cellulose hydrolysis
and hydrogenation is shown in Figure 2.
Figure 1. TEM images and particle histograms of [BMIM]Cl/1 (mole
ratio=45:1) stabilized Ru0 nanoparticles (150 particles counted).
are comparable with other reported results.[14,15] The XPS spec-
trum (see the Supporting Information, Figure S1) shows typical
Ru0 absorptions at 280.20 and 284.40 eV for 3d5/2 and 3d3/2, re-
spectively, with a D=4.20 eV, which is consistent with the liter-
ature results.[16] All preparative work was carried out in an
argon atmosphere to guard against oxide formation, and ac-
cordingly no evidence was found in the XPS results for the ex-
istence of RuO contamination.[14]
In the absence of 1, a conversion of 15% was achieved for
cellulose hydrogenation to hexitols using a Ru nanocluster cat-
alyst in [BMIM]Cl.[8] However, when using varying hydrogen
donors in the presence of 1, cellulose was successfully convert-
ed to hexitols in high yield (Table 1). It was found that both
ionic liquids [BMIM]Cl and [THTdP][DBS] were equally effective
in runs in which the hydrogen source was sodium formate
(Table 1, entries 4 and 6). Furthermore, in the absence of ruthe-
nium nanoparticles and hydrogen, cellulose was smoothly hy-
drolyzed to a-glucose using compound 1 with a 95% conver-
sion to glucose after 5 h at 808C, which shows that 1 is crucial
in activating the cellulose polymer chains. Compound 1 and
Table 1. Catalytic hydrogenation of cellulose[a]
Entry
Catalyst
Conv. [%]
Yield [%][b]
G S M
1
2
3
4
5
6
[BMIM]Cl
[BMIM]Cl+1
[BMIM]Cl+1+Ru
[BMIM]Cl+1+Ru
[BMIM]Cl+1+Ru
[THTdP][DBS] +1+Ru
<5
95
90 (FA)
100 (SF)
98 (H2)
100 (SF)
87
76
7[c]
3[c]
Figure 2. Catalytic conversion of cellulose to hexitols by reversible interac-
tion.
94
89
93
[a] FA=HCO2H, SF=HCO2Na, G=glucose, S=sorbitol, M=mannitol. Re-
action conditions: 1 (0.1 mmol), Ru0 nanoparticles (10.0 mmol), [BMIM]Cl
(0.8 g, 4.5 mmol) or [THTdP][DBS] (1 mL), cellulose (1 g), sodium formate
(0.1 g, 1.5 mmol) or formic acid (70 mg, 1.5 mmol) or H2 (10 atm); reaction
temperature 808C; reaction time 5 h. The hexitol products were analyzed
by NMR spectroscopy, GC and EI-MS. [b] Isolated yield. [c] Based on GC
analysis.
In conclusion, we have successfully synthesized a conjugate
between an ionic liquid moiety and a boronic acid binding
agent 1. Compound 1 is an effective catalyst when combined
with an ionic liquid-stabilized ruthenium nanoparticle catalyst
for cellulose conversion to hexitols. The catalyst can be recy-
cled with sustained activity.
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ChemSusChem 2010, 3, 67 – 70

vigorous stirring for 2 h at 808C. The resulting catalyst composite
was then added to a mixture of pretreated cellulose (1 g) and
sodium formate (0.1 g, 1.5 mmol) or formic acid (70.0 mg,
1.5 mmol) in deionized water (15 mL). When hydrogen was used,
the reaction vessel was then saturated with H2 (10 atm). The reac-
tion was conducted at 808C for 5 h before quenching with cold
methanol followed by diluting with deionized water (40 mL). For a
control experiment, neither 1 nor Ru0 nanoparticles were added;
only [BMIM]Cl was used. Any unreacted cellulose was then collect-
ed by filter or centrifugation and dried in vacuum. Cellulose con-
version was determined by the change in weight of cellulose used
before and after the reactions. Catalyst composite with ionic liquid
was recovered by decanting the solution. The recovered catalyst
was washed with deoxygenated deionized water and dried in
vacuum for the subsequent run. The results are listed in Table 1.
The hexitols products were purified by flash chromatography
(Al2O3-SiO2, eluted with a mixed solvent of methanol/dimethyl sulf-
oxide/water 3:5:1) and analyzed by NMR spectrscopy, GC, and MS.
For GC analysis, samples were prepared according to literature.[20]
The above hexitols (20.0 mg) were added to a solution of acetic
anhydride/pyridine 10:1 (10 mL) for 4 h at 808C. The solutions
were then concentrated by evaporation under a stream of argon
before subjecting to GC analysis. Results are shown in the Support-
ing Information (Figure S6).
Experimental Section
General Procedure
All operations were operated under argon atmosphere with a
glove box or standard Schlenk line. a-Cellulose (microcrystalline,
powder), 1-n-butyl-3-methylimidazolium chloride ([BMIM]Cl) and
other reagents were purchased from Aldrich. The ionic liquid tri-
hexyltetradecylphosphonium dodecylbenzenesulfonate [THTdP]-
[DBS] was provided by IL-TECH Inc. Ruthenium nanoparticles were
produced according to literature.[12] 1H and 13C NMR spectra were
recorded on a Bruker Fourier-Transform multinuclear spectrometer
at 400 and 100.6 MHz, relative to an external Me4Si (TMS) stan-
dard. 11B NMR spectra were recorded on a Bruker 400 analyzer at
128.38 MHz. Infrared (IR) spectra were measured by using a BIO-
RAD spectrophotometer with a KBr pellets technique. The MS was
measured on a Thermo Finnigan MAT XP95 analyzer using the EI
model. ICP analysis was carried out on a VISTA-MPX, CCD simulta-
neous ICP-OES analyzer. XPS was carried out on an ESCALAB 250
analyzer, and TEM measurements were carried out on a JEOL
Tecnai-G2, FEI analyzer at 200 kV. GC analysis was performed on a
PerkinElmer, Clarus 500 GC analyzer on a DB-1 column (30 mꢁ
0.32 mmꢁ1.00 mm) with an isothermal temperature of 2508C.
Detail experimental procedures are available in the Supporting In-
formation.
Synthesis of Compound 2: A literature procedure was used to syn-
thesize compound 2.[13a] In brief, 2-formylbenzeneboronic acid
(0.309 g, 2.0 mmol) was dissolved in dry methanol (10 mL), and a
solution 1-allylpiperazine (0.258 g, 2.0 mmol) in methanol (5 mL)
was added dropwise over 30 min at room temperature in a glove-
box. After stirring for 2 days at room temperature, sodium borohy-
dride (0.139 g, 3.5 mmol) was added to the reaction mixture in two
parts within 2 h, and the mixture was further reacted for 4 h. The
solvent was removed in vacuum, and dichloromethane (20 mL)
was added to the residue. The solid precipitate was removed by fil-
tration and the filtrate was concentrated to 5 mL followed by pu-
rification with chromatography (SiO2, eluted with a mixted solvent
of methanol/dichloromethane 1:10) to produce 2 (0.395 g, 76%).
Synthesis of Compound 3: 1-Butylimidazole (5.0 mL, 37.3 mmol)
and allyl chloride (40 mL, 485.9 mmol, great excess) were added to
a 100 mL round bottom flask equipped with a magnetic stir bar.
The resulting mixture was left reacting for one week with continu-
ous stirring. After removing all the solvent under reduced pressure,
the obtained residue was washed with hexane and diethyl ether
followed by drying in vacuum to produce pure product 3 (6.5 g,
86.8%).
Synthesis of Compound 1: A literature procedure was used to con-
duct the reaction.[18] Compounds 2 (0.260 g, 1.0 mmol) and 3
(0.200 g, 1.0 mmol) were dissolved in dichloromethane (10 mL).
The second generation Hoveyda–Grubbs catalyst (62.7 mg,
0.1 mmol) was added to the solution. The resulting mixture was
left stirring for one week in a glove box at room temperature.
After removing all the solvents under reduced pressure, the ob-
tained residue was purified by precipitation from dichloromethane
solution with pentane followed by drying in vacuum to give 1
(0.27 g, 62%).
Acknowledgements
This work was supported by theInstitute of Chemical and Engi-
neering Sciences (ICES), Singapore. We are also grateful for the
contributions made by our colleagues at ICES.
Keywords: biotransformations
nanoparticles · receptors · ruthenium
·
carbohydrates
·
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Received: October 9, 2009
Published online on December 18, 2009
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