Cellulose conversion to isosorbide in molten salt hydrate media

Full text of DOI:10.1002/cssc.200900260

DOI: 10.1002/cssc.200900260
Cellulose Conversion to Isosorbide in Molten Salt hydrate Media
Rafael Menegassi de Almeida,[a, b] Jianrong Li,[a] Christian Nederlof,[a] Paul O’Connor,[c] Michiel Makkee,*[a] and
Jacob A. Moulijn[a]
Effective ways to convert lignocellulosic biomass are of univer-
sal interest.[1,2] Such biomass contains typically around 40 wt%
of cellulose,[1] a polymer of d-glucose condensed by glycosidic
bonds. Intramolecular and intermolecular hydrogen bonding
networks result in a partially crystalline, non-accessible supra-
molecular structure, making conversion a challenge.[3]
compounds are byproducts)[13] and DMA with LiCl.[14] A disad-
vantage of these processes is that HMF is quite reactive in
acidic media, leading to soluble degradation products and in-
soluble humins.[15]
The conversion process described in this paper aims to solve
the difficulties of cellulose non-accessibility and monosacchar-
ides reactivity by effecting hydrolysis, hydrogenation, and fur-
ther conversion in the same salt hydrate dissolution media, re-
sulting in a less polar, recoverable, stable compound. The pro-
posed concept is shown in Scheme 1 for lignocellulosic bio-
Cellulose can be depolymerized in high severity thermo-
chemical processes such as hydrothermal liquefaction and fast
pyrolysis. In hydrothermal liquefaction, temperatures higher
than 3008C are necessary to loosen the crystalline structure
into amorphous cellulose that promptly hydrolyzes.[4] In addi-
tion to energy requirements in such severe conditions, mono-
saccharides (such as glucose) are very reactive, leading to a
multitude of products.[1,2,5,6] From a practical standpoint, this
process is not attractive for producing chemicals or transporta-
tion fuels.[7] There is a need for effective chemical conversion
processes at mild conditions.
Acid hydrolysis of cellulose and hemicellulose to monosac-
charides at lower temperatures can be enhanced in proper
medium, in particular, some molten salt hydrates.[8–10] Molten
salt hydrates such as ZnCl2, CaCl2, and LiCl are able to solubi-
lize cellulose due to the interaction between ionic species and
hydroxyls, breaking the hydrogen-bonding network.[11] Process-
ing sugarcane bagasse and fast cellulose to glucose conversion
of up to 98% were reported.[9] However, the next step, separa-
tion of the polar monosaccharides, is troublesome, which re-
quires expensive methods such as electrodialysis or chroma-
tography.[9,10] This discourages the industrial implementation of
hydrolysis in a molten salt hydrate medium. An alternative
would be the further conversion of glucose to a more easily
separable compound. Some authors proposed the hydrolysis
of cellulose and dehydration of glucose to 5-hydroxymethylfur-
fural (HMF) and derivatives using dissolution and hydrolysis
media such as organic cation ionic liquids ([EMIM]Cl with CuCl2
and CrCl2)[12], dissolution media such as LiCl in HCl (chlorinated
Scheme 1. Process concept. Lignocellulose is treated in a molten hydrate
medium in which cellulose is dissolved and reactions proceed: hydrolysis,
hydrogenation, and dehydration to isosorbide.
mass, consisting of cellulose, hemicellulose, and lignin. In a
first step, hemicellulose is removed. Then the ZnCl2 salt hy-
drate (optionally with some HCl) dissolves and hydrolyzes the
cellulose, forming glucose; lignin is inert and removed as a
solid. Next, hydrogenation of glucose into glucitol (also known
as sorbitol) occurs. Then glucitol is dehydrated into dianhydro-
hexitol (isosorbide). Isosorbide can be conveniently separated
from the salt hydrate media; water produced (per mol of glu-
cose obtained, 1 mol consumed in hydrolysis and 2 mol pro-
duced in dehydration) has to be removed. The molten salt hy-
drate medium is recycled, allowing a continuous process. The
main reactions are presented in Scheme 2.
[a] Dr. R. M. de Almeida, J. Li, C. Nederlof, Dr. M. Makkee, Prof. J. A. Moulijn
Catalysis Engineering, Delft Chem Tech
Delft University of Technology
Julianalaan 136, 2628 BL, Delft (The Netherlands)
Fax:(+31)152785005
E-mail: m.makkee@tudelft.nl
[b] Dr. R. M. de Almeida
Petrobras, Centro de Pesquisas e Desenvolvimento
Leopoldo A. Miguez de Mello (CENPES)
Av. Horꢀcio de Macedo 950, Ilha do Fund¼o, 21941-915, Rio de Janeiro
(Brazil)
[c] P. O’Connor
Dissolution and hydrolysis of cellulose in ZnCl2 hydrate is
known in literature.[8–10] The novelty is the transformation of
glucose in this medium into a clean separable product, allow-
ing an integrated process involving chemical conversion and
BIOeCON BV
Hogebrinkerweg 15e, 3871 KM Hoevelaken (The Netherlands)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.200900260.
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325

plete (Figure 2). After 90 min, no
intermediate glucose oligomers
were observed in the HPLC spec-
trum, and no cellulose was pre-
cipitated when diluting sample
with water. No darkening or in-
soluble products were observed
in the reaction product. Some
glucose decomposition was re-
ported to occur,[9,10] however, at
the conditions employed, it was
below the detection limit. It can
be concluded that cellulose was
completely converted to glu-
cose. Addition of HCl was neces-
sary for satisfactory hydrolysis
rates at low temperatures, but
Scheme 2. Hydrolysis of cellulose to d-glucose, hydrogenation to glucitol, dehydration to sorbitan (1,4-anhydro-
glucitol pictured), and sequential dehydration to isosorbide.
separation. Each step has been separately investigated to dem-
onstrate the feasibility of the process.
Figure 1 presents the cellulose hydrolysis in a ZnCl2 hydrate
medium. The conversion is very much dependent on the ZnCl2
concentration. At lower ZnCl2 concentrations, the rate of hy-
Figure 2. Glucose yield from cellulose as a function of hydrolysis time (858C,
70 wt% ZnCl2 molten salt hydrate medium, cellulose/hydrate=1:12,
0.4 molal HCl)
recent results showed no need of an acid at temperatures
higher than 1008C.
Figure 1. Glucose yield from cellulose as a function of ZnCl2 content in the
molten salt hydrate medium (30 min, 858C, cellulose/hydrate=1:12,
0.4 molal HCl)
It might also be possible to increase the rate of hydrolysis
by using solid acid or in situ generated acid metal catalysts
under hydrogenation conditions.[16–18] This however suffers
from some drawbacks. Direct contact of biomass with a solid
catalyst in a dissolution medium is possible, but there is the
further problem of separation of non-reacted insoluble lignin
and ashes from the solid catalysts. A first delignification is pos-
sible although expensive, but ashes that could plug the pores
of the catalysts would remain. Separation of lignin and ashes
from cellulose dissolved in a molten salt hydrate, without any
conversion of cellulose, can prove difficult. A first acidic hydrol-
ysis step with further separation of insoluble lignin and ashes
drolysis to glucose is low, probably because dissolution of cel-
lulose does not take place.[11] Increasing the ZnCl2 concentra-
tion improves the dissolution rate, thus enhancing hydrolysis.
The S-shaped curve is consistent with the change of a dissolu-
tion-determined rate to a homogeneous hydrolysis at higher
ZnCl2 concentrations.
With sufficient time, at 858C, 0.4 molal of HCl, and cellulose
to molten salt hydrate ratio of 1:12 (w/w), hydrolysis was com-
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by filtration, prior to contact with solid catalysts in the next
process hydrogenation step, avoids difficulties like pore-plug-
ging, catalyst deactivation, and separation.
the molten salt hydrate media, even at high temperatures
(2308C for 4 h, less than 2% yield lost).
Polyols dehydration in aqueous media has been studied ex-
tensively.[24–26] The mechanism involves electron acceptor ionic
species, strongly interacting with hydroxyl groups, weakening
the CÀO bond, and catalyzing the intramolecular nucleophilic
substitution (internal dehydration). We assume that in the
ZnCl2 medium the same mechanism is operative. The high se-
lectivity to isosorbide is striking. Apparently, the 2,5-sorbitan
intermediate, that does not give isosorbide, is not favored, in
contrast with acidic dehydration in aqueous media. It is known
that hydrogen bonding and the resulting molecular conforma-
tions play a role in polyols dehydration selectivity.[27] The selec-
tivity probably arises from the sorbitol conformation in the
medium due to the hydroxyls interacting with the ZnCl2 salt
hydrate ionic species. This was reported for glucitol dehydra-
tion in pyridinium chloride.[28]
The hydrogenation of glucose to glucitol is a well-known
process, usually accompanied by some isomerization to fruc-
tose, yielding glucitol and mannitol.[19] Ruthenium on activated
carbon was used to withstand the presence of chlorides with-
out leaching, although we realize that further development
might be required to achieve a stable catalyst for a continuous
process. The yield of glucitol in 60 min was 70% under the fol-
lowing conditions: 5.0 MPa, 858C, 70 wt% ZnCl2 medium, glu-
cose to medium 1:12 (w/w) ratio, Ru/C to glucose ratio of 0.5:1
(w/w), and no HCl. The reaction rate dropped with ZnCl2. A
purely aqueous glucose solution, other conditions remaining
equal, full hydrogenation was obtained in 30 min. In the ZnCl2
salt hydrate medium, no mannitol was observed in the hydro-
genation products, illustrating the high selectivity and the ab-
sence of isomerization to fructose.
Dehydration of glucose can occur in ionic liquids.[29]Ionic liq-
uids are also known to dissolve cellulose, but in strictly anhy-
drous conditions,[30] which make them less suitable for the hy-
drolysis step.
The presence of HCl inhibited hydrogenation (yield dropped
from 70% to 38% and 21% with 0.2 and 0.4 molal HCl, respec-
tively), but it was not lethal to the catalyst. Fortunately HCl can
be recovered before the hydrogenation, for example, by strip-
ping, as no azeotrope is formed with the ZnCl2 hydrate.[9,20]
Using proper conditions, it was possible to fully hydrogenate
glucose to sorbitol in the molten salt hydrate medium with ac-
ceptable rates. Glucose was fully converted at a higher temper-
ature (1008C) with other conditions being the same as de-
scribed previously: 60 min reaction, 5.0 MPa, 70 wt% ZnCl2
medium, glucose to medium 1:12 (w/w) ratio, Ru/C to glucose
ratio of 0.5:1 (w/w), and no HCl.
Isosorbide can be separated by extraction in an organic sol-
vent.[31,32] Solubility in hydrocarbons such as xylene is low at
room temperature but increases significantly with temperature.
Therefore, it can be extracted in an organic phase at higher
temperatures and further precipitated or separated in an aque-
ous phase at temperatures lower than 608C. Distillation or
stripping can also be employed.[33]
Both the hydrolysis and hydrogenation steps can occur with
complete conversion, and the dehydration can be higher than
95%. In principle, it is feasible to convert 95% of the carbon
present in cellulose into isosorbide.
The hydrogen consumption (1 mol of H2 per mol of glucose
or 124.4 Nm3 ton to obtain isosorbide) is significantly smaller
than in presently developed processes, viz. in the aqueous-
phase hydrogenolysis of glucose to produce hydrocarbons
(7 mol of H2 per mol of glucose or 870.9 Nm3 tonÀ1 to obtain
hexane), or the production of bio-oils by a combination of fast
pyrolysis and subsequent hydrodeoxygenation.[7,21]
It can be desirable to further reduce the polarity of the prod-
uct molecule. The remaining hydroxyl groups can be convert-
ed into other functional groups, such as ethers and esters, or
removed by further dehydration into dihydrodifuran.[34,35] Re-
search is currently in progress.
Isosorbide is produced industrially by acidic dehydration of
glucitol, obtained via hydrogenation from starch-produced glu-
cose.[22] The dehydration advances sequentially; in the first
step, 1,4 and 3,6 and 2,5 anhydroglucitols, known as sorbitans,
are formed and in the second step, isosorbide results. The se-
lectivity to isosorbide is limited as the 2,5 anhydro intermedi-
ate does not undergo a second dehydration. In addition, inter-
molecular dehydration of sorbitol to oligomers, decomposition,
and charring side reactions occur.[23]
The water produced needs to be removed to keep a con-
stant ZnCl2 concentration in the recycle. Besides water, undesir-
able organic compounds might accumulate in the molten salt
hydrate medium, so it might be necessary to treat part or the
whole of the molten salt hydrate recycle. Organics can be re-
moved by pyrolytic methods such as gasification or oxida-
tion[36] (avoiding chlorinated compounds emission). Other suit-
able methods exist to separate/convert organics from molten
salts, such as adsorption, extraction, electrochemical methods,
chemical oxidations, and a combination of these. These meth-
ods need to be further addressed working with real biomass
and solvent recycle to quantify accumulating organics and
select proper methods of removal.
We discovered that ZnCl2 hydrate media alone or with addi-
tional catalysts, such as CuCl2 and NiCl2 allowed a clean dehy-
dration of glucitol into mainly isosorbide, at mild conditions.
At a 90 min reaction time, 1808C, 4.0 MPa (hydrogen), glucitol
to ZnCl2 hydrate ratio of 1:12 (w/w), a molar ratio of CuCl2 to
glucitol of 0.5:1, and a 70 wt% ZnCl2 salt hydrate, glucitol was
fully converted to more than 95% isosorbide. At higher tem-
peratures (210–2308C), no cocatalyst was required but the
yield was slightly lower. A two-step dehydration procedure, in
order to achieve full isosorbide selectivity without the addition
of a co-catalyst, is under investigation. Isosorbide is stable in
The process suggests the viability of a large-scale isosorbide
production and thus large-scale uses, as a base or platform
chemical in the materials and energy (fuel) sector. At present,
isosorbide and its derivatives are already demanded chemicals,
as sustainable diols for polymer industry, intermediates in phar-
maceutical products, polyvinyl chloride (PVC) plasticizers, die-
ster surfactants, and as solvents such as dimethyl isosorbide
ChemSusChem 2010, 3, 325 – 328
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(DMI).[22,23] Possible uses of isosorbide and its derivatives as
fuels (mainly diesel) are also being evaluated.
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In this Communicaion, we assumed that hemicellulose was
separated in a pre-step. In principle, hemicellulose can also be
hydrolyzed in a ZnCl2 hydrate medium. Giving mainly pentoses
that can be hydrogenated to pentitols and further dehydrated
to monoanhydropentitols, but being more polar than dianhy-
drohexitols, they are more difficult to separate.[37] Lignin is
inert under the process conditions. Therefore, in principle real
biomass can be processed without a pre-separation step. How-
ever, a stage-wise process might be better, involving a first
step in which depolymerization of hemicellulose and separa-
tion of its products takes place, benefiting from the relatively
easy hydrolysis step.[1] This process is under investigation.
The technology described can be applied broadly. Saccha-
rose can be hydrolyzed to glucose and fructose, which can in
turn be converted to dianhydrohexitols (isosorbide and iso-
mannide). Thus, biomass such as sugarcane can be processed.
It can be coupled to ethanol from C5 sugars fermentation
(hemicellulose) and lignin conversion processes. Therefore, the
technology can be the basis of a high-yield biorefinery, analo-
gous to distillation combined with dedicated catalytic process-
es in an oil refinery.
In summary, in ZnCl2 hydrate media, the dissolution of cellu-
lose and chemical conversions can be carried out without in-
termediate separation steps, leading to a product that can be
easily recovered. This method enables an efficient continuous
process and is a good example of process intensification. De-
pending on the chemical conversion steps, attractive products
with unprecedented high yields can become viable on a large
scale, isosorbide being a good first example.
ˇ
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Experimental Section
Materials: Cellulose fibrous, long chains, d-glucose, d-sorbitol,
ZnCl2, HCl, and Ru/C catalyst are all detailed in the Supporting In-
formation.
Products were analyzed by using a HPLC equipped with an RI ana-
lyzer and a Rezex Ca column. Compounds were confirmed by elu-
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Authors acknowledge Petrobras for taking a sabbatical at TU-
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Keywords: biomass
·
carbohydrates
·
hydrogenation
·
Received: November 9, 2009
hydrolysis · sustainable chemistry
Published online on February 23, 2010
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