Some important catalytic challenges in the bioethanol integrated biorefinery

Article abstract of DOI:10.1016/j.cattod.2014.02.016

The concept of integrated biorefinery is presented and discussed. Integrated biorefineries demand the use of innovation, or rather, new chemical routes must be introduced in order to add value to intermediates and residues. The concept of integrated biorefinery is then applied to an ethanol producing facility and a flow sheet of the main catalytic routes to promote modifications of residues is proposed. CO₂ and bagasse are considered the most promising residues to undergo catalytic transformations. Hydrogenation of CO₂ to produce syngas/methanol is an interesting alternative to add value to this molecule. Nickel supported on a mixed oxide NiCeZr is presented as an excellent catalyst to produce syngas out of CO₂. Furthermore, the potential use of two main components of bagasse, lignin and hemicellulose, is discussed, lignin being deployed as a feedstock to produce activated carbons and acidic sulfonated carbons, Acid sulfonated carbons are shown to be excellent catalysts for hydrolysis/dehydration of biomass derivatives such as polysaccharides and polyols. Moreover, activated carbons may also play an important role as outstanding supports for metal-supported catalysts, which may be used in the hydrogenation of sugars.

Full text of DOI:10.1016/j.cattod.2014.02.016

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Catalysis Today
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Some important catalytic challenges in the bioethanol
integrated biorefinery
Eduardo Falabella Sousa-Aguiara,b,∗, Lucia Gorenstin Appelc, Priscila Costa Zonettic,
Adriano do Couto Fragaa, Alex Azevedo Bicudoa, Isabel Fonsecad
a PETROBRAS S.A., CENPES R&D Centre, Cidade Universitária, Av. Horácio Macedo, 950, Cidade Universitária, Rio de Janeiro, RJ CEP 21.941-915, Brazil
b Department of Organic Chemistry, School of Chemistry, Federal University of Rio de Janeiro, Brazil
c Catalysis and Chemical Processes Division, National Institute of Technology (INT), Av. Venezuela 82/518, 21081-312 RJ, Brazil
d Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia, Secc¸ ão de Engenharia Química e Bioquímica, Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The concept of integrated biorefinery is presented and discussed. Integrated biorefineries demand the use
of innovation, or rather, new chemical routes must be introduced in order to add value to intermediates
and residues. The concept of integrated biorefinery is then applied to an ethanol producing facility and a
flow sheet of the main catalytic routes to promote modifications of residues is proposed. CO2 and bagasse
are considered the most promising residues to undergo catalytic transformations. Hydrogenation of CO2
to produce syngas/methanol is an interesting alternative to add value to this molecule. Nickel supported
on a mixed oxide NiCeZr is presented as an excellent catalyst to produce syngas out of CO2. Furthermore,
the potential use of two main components of bagasse, lignin and hemicellulose, is discussed, lignin being
deployed as a feedstock to produce activated carbons and acidic sulfonated carbons, Acid sulfonated
carbons are shown to be excellent catalysts for hydrolysis/dehydration of biomass derivatives such as
polysaccharides and polyols. Moreover, activated carbons may also play an important role as outstanding
supports for metal-supported catalysts, which may be used in the hydrogenation of sugars.
© 2014 Elsevier B.V. All rights reserved.
Received 2 December 2013
Received in revised form 27 January 2014
Accepted 8 February 2014
Available online xxx
Keywords:
Ethanol
Biorefinery
Hydrogenation
Carbon dioxide
Lignin
Active carbon
Hemicellulose
Xylitol
Anhydroxylitol
1. Introduction
will depend on many factors. Indeed, the refiner of the future will
have to face multiple challenges, the following deserving special
The increasing necessity for clean-burning fuels – with very low
or even no sulphur, with the minimum content of aromatics and
with minimum formation of nitrogen oxides, soot and unburned
hydrocarbons – is changing in a rather drastic way the traditional
goals of the refining industry. Although the generation of cleaner
fuels may be achieved by introducing further processing capacity
in refineries, such as desulphurization, these new units are very
energy consuming and reduce the overall thermal efficiency of the
refinery. Furthermore, desulphurization processes usually require
hydrogen, whose production via the Shift reaction also produces
CO2. Hence, an improvement in air quality due to the use of cleaner
diesel may come at the expense of higher greenhouse-gas emis-
sions during such diesel production. The survival of the oil industry
attention [1,2]:
•
Increasing stringent environmental regulation;
Growing demand for cleaner fuels;
Globalization;
Increase in the production of derivatives from declining quality
oil;
•
•
•
•
Uncertainty about the consumer’s choice;
Growing pressure of several segments of the society aiming at
the reduction of GHG (greenhouse gases);
•
•
Maintenance of its profitability.
Therefore, the search for alternative feedstock such as biomass
has become a must in order to cope with more stringent regulations.
In this new peculiar scenario, biorefineries play an outstanding role
[3].
Biorefineries represent a concept encompassing the facilities
and processes though which, via renewable biomass conversion,
biofuels and other classical products of the traditional oil refining,
∗
Corresponding author at: PETROBRAS S.A., CENPES R&D Centre, Cidade Uni-
versitária, Av. Horácio Macedo, 950, Cidade Universitária, Rio de Janeiro, RJ CEP
21.941-915, Brazil.
E-mail address: efalabella@petrobras.com.br (E.F. Sousa-Aguiar).
http://dx.doi.org/10.1016/j.cattod.2014.02.016
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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Fig. 1. The integrated bioethanol biorefinery.
such as petrochemicals, may be produced [4,5]. It must be borne
in mind that biorefineries may also produce energy. Biorefinery
may also be understood as the evolution of technologies based
on biomass, which comprise biological, thermal and chemical
processes. The concept of biorefinery takes advantage of the
current synergies between the different technologies, resulting in
a complete range of products. Recently, the concept of integrated
biorefinery has been coined [6]. An integrated biorefinery is capa-
ble of efficiently converting a broad range of biomass feedstocks
into affordable biofuels, biopower, and other bioproducts. In that
sense, integrated biorefineries are similar to conventional refiner-
ies, generating a variety of products to optimize both the use of
the feedstock and production economics. Furthermore, integrated
biorefineries imply the use of innovation, or rather, new chemical
routes must be developed in order to reduce costs, improve
competitiveness and, above all, explore the potential of residues.
Several raw materials may be used for integrated biorefiner-
ies. Ethanol producing facilities seem to be an excellent example
of how different by-products and residues may undergo chemical
transformations aiming at increasing the overall profitability of the
biorefinery, as clearly demonstrated in Fig. 1. It must be borne in
mind that several routes of chemical transformations are innova-
tions, thereby requiring the development of innovative catalysts.
The aim of this work is to study the some important catalytic
challenges when the concept of integrated biorefinery is applied to
an ethanol producing facility.
(a.2) saccharification to produce sugars, mainly hexoses and
pentoses;
(a.3) lignin recovery and transformation.
(b) CO2 recovery and transformation.
All these routes are catalytic, some deserving special attention
since innovative catalysts, not commercial yet, are to be developed
Regarding thermochemical routes, technologies can be cat-
egorized as gasification, pyrolysis, or hydrothermal processing,
whose intermediate products include clean syngas (CO + H2), bio-
oil (pyrolysis or hydrothermal product), and gases rich in methane
or hydrogen. These intermediates can be further converted into
gasoline, diesel, alcohols, ethers, synthetic natural gas etc. and also
high-purity hydrogen, which can be used as fuels and for electric
power generation.
Concerning gasification, the main catalytic challenges are
related to gas cleaning, mainly the secondary methods, involving
hot gas cleaning via catalytic conversion. Catalysts must present
high tar conversion, deactivation resistance, easy regeneration and
low cost, being also capable of promoting methane. Several cat-
alysts such as non-metallic oxides (olivine, dolomite, magnesite),
Ni-containing catalysts and noble metal-containing supported cat-
alysts (M/CeO2/SiO2, where M = Rh, Pd, Pt, Ru, Ni) have been tested
with good results.
As far as saccharification is concerned, this reaction of hydrolysis
may employ either enzymes or heterogeneous catalysts. Recently,
molten salts have been proposed as alternative catalysts [7], but
ionic liquids [8–10] as well as ball milling [11,12] have also been
tested with good results. Increasing the acidity of the molten salt
used as catalysts in the hydrolysis step is a challenge, since higher
acidity may improve hydrolysis. Still, other important challenges
are in the transformation step, or rather, hydrogenation and dehy-
dration reactions to produce alcohols and cyclic compounds, which
present a much higher added value. An innovative catalyst capable
of promoting both reactions in one step is still a matter of research.
Lignin is a recurrent problem. Recent studies revealed that
lignin may be an excellent raw material for the production of
active carbons, as well as functionalized carbons. Such materials
may be used either as a support for several types of catalysts or
2. The ethanol integrated biorefinery
When the concept of integrated biorefinery is applied to an
ethanol producing plant, as depicted in Fig. 1, it becomes clear that
the major concerns are the following:
(a) The problem of lignocellulosic residues.
Indeed, bagasse and straw are generated in huge quantities
and chemical transformations must be carried out in order to
add value to these potential raw materials. Several options may
be found in the literature, deserving special attention:
(a.1) thermochemical treatment to produce synthesis gas;
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3
as catalysts themselves. Indeed, high surface carbons (in which
acidic sites have been generated) have been tested in the reaction
of dehydration of alcohols with outstanding results. Furthermore,
they may also be used as supports for noble-metal supported
catalysts, which can catalyse hydrogenation reactions, mainly the
transformation of sugars into polyols.
Finally, the problem of CO2 must be addressed. Indeed, alcohol
producing plants do produce enormous quantities of CO2, which,
however, is rather pure and concentrated, thus being a convenient
feedstock for several chemical routes. The hydrogenation thereof
to produce syngas (CO + H2) seems to be a very promising one. The
catalytic conversion of CO2 via the reverse water gas shift reaction
(RWGS) is an interesting alternative for the production of synthesis
gas. Indeed, several catalysts have been reported to be active in the
RWGS reaction.
3. The potential use of CO2
Fig. 2. Rates of CO2 consumption, rates of CO and methane generation at 550 ◦C and
1 atm.
As well known, the fermentation of the sugar cane juice for
the production of ethanol generates CO2 as a byproduct. Indeed,
1000 kg of ethanol produces 950 kg of high purity CO2. Brazil’ sugar-
cane harvest of 2012/2013 season manufactured 21.4 Mt of ethanol,
and consequently 20.3 Mt of CO2 [15]. Moreover, in the near future,
the fermentation of hexoses obtained from bagasse and straw (2G
ethanol), will generate much more CO2.
Carbon dioxide can be employed as raw material for the gen-
eration of many different chemicals. The literature suggests the
synthesis of formic acid, polycarbonate, dimethyl carbonate, urea,
sodium bicarbonate, acetic acid, methilamines, alcohols and oth-
ers. Hydrocarbons for diesel or gasoline production, methanol and
dimethyl ether (DME) can also be generated from CO2 [16–18].
Taking into account the biofuel future demand of the developed
and developing countries and the huge amount of CO2 available
in the sugar cane bio-refineries (huge supply), the generation of
fuels seems to be the most suitable option for the CO2 use as a raw
material.
Hydrocarbons for the diesel or gasoline production as well as
methanol and dimethyl ether (DME) are or can be produced from
synthesis gas (CO + H2). This gaseous mixture can be synthesized
by the reverse water gas shift reaction (RWGS), followed by well
known processes of production of these fuels [19]. The genera-
tion of synthesis gas can also be considered as a mechanism step
of the synthesis (new processes) of hydrocarbons (F-T reaction),
methanol and dimethyl ether from CO2 [20].
The availability of non-expensive and renewable H2 is a key
issue for the fuel generation from CO2. The water electrolyses
can be considered an option when this process is associated with
hydroelectricity [21] or nuclear power generation [19] units at low
demand periods. It is worth mentioning that the most challenging
and promising process of hydrogen generation from water is pho-
tolysis [22]. Despite the efforts to generate hydrogen from water
employing solar energy this process is not industrially feasible yet.
used in the WGS reaction, they have also been considered for the
reverse (RWGS) [27]. When the RWGS catalysts are employed for
the CO + H2 generation, they should be stable at high temperatures,
selective to CO, and the rate of the methanation reaction should
be low. The thermal stability is not very relevant when the “RWGS
catalyst” is a component of the catalytic systems employed in the
F-T reaction. The same can be suggested for the methanol or DME
synthesis. This occurs because these reactions are carried out at low
temperatures (T < 400 ◦C). In this case, the Cu-based catalysts can
be employed despite their low thermal stability.
Fig. 2 shows the rate of CO2 consumption and the rates of
methane and CO generation of NiCeZr, Ni/NiCeZr and Ni/SiO2 at
550 ◦C and 1 bar. The first one is a mixed oxide (NixCe0.75-xZr0.25O2)
prepared by coprecipitation. The supported catalysts were synthe-
sized by dry impregnation of Ni(NO3)2 on SiO2 or NiCeZr (0.5 wt.%
Ni). The NiCeZr mixed oxide shows high reducibility due to the
presence of Ni in the Ce0.75Zr0.25O2 lattice [23]. The results show
that these catalysts are very selective to CO, being the Ni/NiCeZr
catalyst the most active one followed by NiCeZr, and Ni/SiO2. Thus,
Ni at very low concentrations and supported on mixed oxides that
show high reducibilities, are very promising catalysts for the RWGS
reaction.
Since the 90’ the mechanism WGS has been thoroughly studied.
However, until now there is no consensus about the main mecha-
nism of this reaction or about its reverse reaction. Thus, currently
the main challenge related to the RWGS reaction is the determina-
tion of its detailed mechanism. This valuable piece of information
will surely provide the researchers with sufficient knowledge to
carry out the synthesis of a new generation of very selective, active
and stable catalysts for transforming CO2 into CO.
3.2. The RWGS as a step of the methanol synthesis from CO2
3.1. The RWGS and the generation of synthesis gas
Table 1 shows the thermodynamic data calculated employing
UniSin for the RWGS reaction. Also displayed is the data of the
methanol synthesis from CO2 being the RWGS reaction the first
step followed by the methanol generation from CO + H2. This table
also shows the direct synthesis of methanol from CO2. Table 2 dis-
plays the catalytic results employing the Cu/ZnO/Al2O3 catalyst,
three different space velocities and two H2/CO2 ratios (A = 6 and
B = 3) at 50 bar and 250 ◦C.
Being the RWGS an endothermic reaction, when increasing
the temperature the equilibrium conversion increases as well. At
700 ◦C, the CO2 conversion is around 68% and it does not vary with
pressure, once there are no changes in the mol numbers of reagents
and products [23].
The literature shows that many different systems can be
employed as catalysts for the RWGS reaction. The following mate-
rials are good examples of RWGS catalysts: Pt, Pd, Ni and Co
supported on CeO2 [24], Au/CeO2 [25], Au, Cu or Ni on TiC [26]
and Ni on NiCeZr solid solution [23]. As copper-based catalysts are
As can be observed in Table 2, when employing ratio A the
increase of the contact time almost does not change the conversion
values nor the selectivities to CO and methanol. Comparing these
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Table 1
generates a solid residue, consisting mainly of lignin, which is a
fraction representing 20–30 wt.% of bagasse [32]. Several projects of
ethanol 2G plants are currently planned to be constructed in Brazil,
representing a generation of a large amount of residual lignin in
the near future. The valorization of this waste could be a way to
improve the economic balance of ethanol plants. Several papers
deal with the potential use of lignin, such as the conversion thereof
into low-boiling-point arenes [13] or the aqueous phase reforming
to produce hydrogen and aromatics [14]. An important alternative
for the generation of high added value materials could be the obten-
tion of activated carbons and other carbonaceous derivatives as
solid acids from these residues, which have a high carbon content.
In this context, lignin has been studied as a precursor for activated
carbons.
Thermodynamic data generated at 50 bar and 250 ◦C, H2/CO2 = 6:1 (A) and
H2/CO2 = 3:1 (B).
H2/CO2
XCO (%)
SMeOH (%)
2
CO2 + H2 ↔ CO + H2O
A
B
22.2
16.2
CO2 + H2 ↔ CO + H2O
CO + 2H2 ↔ CH3OH
A
B
37.9
25.5
75.9
69.4
CO2 + 3H2 ↔ CH3OH + H2O
A
B
34.0
21.8
Table 2
The activation of lignin may be interesting because activated
carbons prepared from materials with high lignin content showed
a higher amount of larger pores when compared with activated
carbon obtained from materials with high content of cellulose
that are primarily microporous [33]. Materials with large surface
areas and larger pore sizes can be used in noble applications as
catalysts or catalyst supports. Furthermore, the high aromatic car-
bon content in lignin is also a desirable feature for obtaining high
yields in activated carbon production [34]. Specific surface values
around 2000 m2/g with pore volume of about 1 cm3/g have been
reported [35]. Thus, the chemical activation of lignin derived from
the hydrolysis process of bagasse has been studied. The influence
of activating agents such as phosphoric acid and zinc chloride at
two temperature levels (300 and 450 ◦C) and four levels of activa-
tion time (4, 8, 24, 48) has been determined. Figs. 3 and 4 depict
the results of lignin activation. The influence of activating agents
such as phosphoric acid and zinc chloride at two temperature lev-
els (300 and 450 ◦C), four levels of activation time (4, 8, 24, 48) and a
fixed activating agent to lignin mass ratio of 3, has been determined.
Figs. 3 and 4 depict the results of lignin activation.
CO2 conversion (XCO ), selectivity to methanol (SMeOH) and CO (SCO), H2/CO2 = 6:1
(A) and H2/CO2 = 3:12(B) and space velocity (V) at 50 bar and 250 ◦C.
H2/CO2, V
XCO (%)
SMeOH (%)
SCO (%)
2
A, 90 min−1
A, 225 min−1
A, 900 min−1
B, 90 min−1
B, 225 min−1
B, 900 min−1
26.0
26.0
23.0
11.3
10.7
6.9
15.1
16.4
18.0
40.7
20.7
24.5
84.9
83.6
82,0
59.3
79.3
75.5
three results with the ones displayed in Table 1, it can be suggested
that the reaction reached the equilibrium condition of the RWGS
reaction with only a slight shift due to the methanol formation. This
may occur due to the fact that the RWGS is 5–10 times faster than
the methanol synthesis for Cu-based catalysts [27].
Table 2 depicts the results employing ratio B (H2/CO2 = 3). The
CO2 conversion values almost do not modify with the residence
time. When employing this same catalyst at 1 bar, 250 ◦C and
90 min−1, only CO and H2O are generated and the conversion value
is almost the same of the one observed at 50 bar. Thus, it can be
suggested that the results for H2/CO2 = 3 at 50 bar are once more
related to the RWGS reaction. Indeed, the thermodynamic data sug-
gest that the RWGS is the main reaction. It can be observed that in
this case the conversions values are lower than the thermodynamic
data displayed in Table 1. This might occur due to the fact that at
low conversions (compared with H2/CO2 = 6), the amount of water
generated is low, and consequently, the rates of the RWGS are low
too. This happens because this reaction is “catalyzed” by water [28].
It is interesting to observe that at high contact time (90 min−1) the
selectivity to methanol increases without changing the conversion,
showing that the rate of methanol generation is not able to shift the
RWGS reaction.
It is observed that activated carbons (AC) made with zinc chlo-
ride showed higher specific surface than carbons prepared with
phosphoric acid at all activation times, as similarly reported pre-
viously [36,37]. Another difference between the activating agents
was the average pore size obtained, 35A for zinc chloride ACs and
55A for phosphoric acid ACs. This difference represents a greater
amount of mesopores activation with phosphoric acid.
For both activating agents the severity of pyrolysis at 300 ◦C
was very low, not providing sufficient attack to the lignin struc-
ture to increase its porosity. Thus, this temperature does not seem
to be promising for future investigations. However, at 450 ◦C high
specific surfaces were obtained.
The almost constant values of average pore size obtained for the
carbons was correlated to the template effect that the molecules of
phosphoric acid and zinc chloride promote on the carbonaceous
structure. Indeed, this pore size control is one of the main charac-
teristics of the chemical activation process.
All in all, it can be inferred that the thermodynamic properties
and the kinetic behavior of the RWGS reaction strongly affect the
methanol synthesis from CO2.
4. The potential use of lignin
The highest specific surface area obtained for carbons prepared
with phosphoric acid was approximately 900 m2/g. This value is
higher than that obtained by Hayashi et al. [37], 750 m2/g with
500 ◦C and is lower than that obtained by Romero-Anaya et al. [39]
1385 m2/g with 450 ◦C. Sun et al. [38] activating at 500 ◦C obtained
an activated carbon with specific surface of 820 m2/g, pore volume
of 0.8 cm3/g, and type IV isotherm, very similar to those obtained in
the present work. Therefore, carbons prepared with zinc chloride
showed specific surfaces close to those reported by Hayashi group.
Both phosphoric acid as zinc chloride showed good performance
as activating agents, zinc chloride generating more micropores
whereas phosphoric acid increasing the mesopore contribution. In
the same conditions of temperature and time of pyrolysis (450 ◦C,
24 h) both agents produce high specific surface carbons. At these
conditions with phosphoric acid, an AC with specific surface of
4.1. Lignin as raw material for activated carbons
As previously mentioned, since 1975 Brazil has been investing in
the production of ethanol by fermentation of sugars existing in the
sugar cane juice. This production route generates millions of tons
of agricultural waste, mainly bagasse, which is partially burned to
generate power for the mills [29,30]. More recently, investments
in a new route of ethanol production, which uses as raw material
bagasse generated in the sugar and ethanol mills in operation, have
been announced. Through this route the cellulose and hemicellu-
lose fractions which are present in the residue are converted into
sugars, which are then fermented, resulting in ethanol [31]. This
route for ethanol production, known as second generation ethanol,
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Fig. 3. Effect of pyrolysis time and temperature on the textural properties of zinc chloride activated carbons.
900 m2/g and average pore diameter of 59A was obtained. For zinc
chloride, ACs showing a specific surface of 1147 m2/g and average
pore diameter of 34A were obtained.
of large amounts of wastewater, which require treatments. The
change of processes based on homogeneous catalysis for heteroge-
neous catalysis involves the replacement of liquid by solid catalysts,
which may be more easily recovered. Such materials, however,
must possess high acidity and stability in the reaction conditions,
in addition to be affordable [40].
4.2. Lignocellulosic waste as raw material for solid acid catalysts
The solids acids are employed in various reactions such as
esterification, hydrolysis, dehydrogenation, cracking, isomeriza-
tion, polymerization, among others. However, some features are
desirable, such as thermal stability, tolerance aqueous medium,
acidity suitable according to the type of reaction (predominance
Another possible application for lignocellulosic wastes is the
generation of solid acids, by preparing sulfonated carbons. As a
matter of fact, each year millions of tons of liquid inorganic acids
are used as catalysts in various chemical processes. The recov-
ery of these acids is often impossible, leading to the generation
Fig. 4. Effect of pyrolysis time and temperature on the textural properties of phosphoric acid activated carbons.
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Fig. 5. Effect of carbonization temperature on the sulfonic groups content.
of Lewis or Bronsted acid) [41]. Within the group of solid acids, sul-
fonated carbons are gaining prominence due to promising results
for esterification and hydrolysis of polysaccharides, among others
[40,42–46]. In this work, the influence of carbonization temper-
ature of lignocellulosic residues in the final content of sulfonic
acid groups is discussed Moreover, the catalysts were tested in the
hydrolysis of cellobiose, a dimer of glucose often used as a model
compound representing cellulose for the hydrolysis of beta-1,4-
glycosidic bond. The carbonization temperature of a lignocellulosic
waste was varied between 300 and 550 ◦C, then the carbonized
material was sulfonated at 120 ◦C with 98% sulfuric acid, with a
mass ratio acid:residue = 10 for 6 h.
Fig. 6 presents the results of hydrolysis of cellobiose over sul-
fonated carbons. The reactions were carried out at 110 ◦C for 24 h,
using a catalyst to cellobiose mass ratio of 0.5. A clear linear depend-
ence between the content of sulfonic acid groups in the sulfonated
carbons and conversion level is achieved. The sulfonic acid groups
content appears to be the most important variable for the hydrol-
ysis of cellobiose. However, when compared with other acidic
materials, it is obvious that the amount of sulfonic acid groups
alone does not explain the success of the application of sulfonated
carbons as catalysts in the hydrolysis of cellobiose.
A commercial sulfonic resin (Amberlyst 36) having 5.4 mmol/g
of sulfonic acid groups was used as a reference. This resin showed
an average conversion of 79%. Despite having almost 5 times
the amount of sulfonic acid groups of the best sulfonated carbon
obtained, its conversion was a few percentage points lower. Indeed,
similar results are widely observed in the literature [42,55,56] and
the most accepted explanation for this phenomenon is the exist-
ence of other functional groups, such asphenolic OH or CO2H groups
Fig. 5 shows the variation of the concentration of sulfonic acid
groups on the carbon with the carbonization temperature of the
material.
The highest levels of sulfonic acid groups were obtained for the
samples that were generated from carbonized material at 300 and
350 ◦C respectively. One observes the same behavior mentioned in
the literature, which reports that, in low carbonization tempera-
ture, degradation products remain in the carbonaceous structure,
not allowing a complete sulfonation. On the other hand, at higher
temperatures, both the size and organization of aromatic sheets
increase, so the sites for insertion of sulfonic acid groups are less
accessible, bringing about a lesser extent sulfonation. A higher tem-
perature carbonization also produces a more rigid carbonaceous
structure, which hinders the access of reagents to the acid sites
[44,46–48]. Thus, a point of maximum efficiency for the sulfona-
tion of lignocellulosic carbonized residue is attained and this value
appears to be in the range of 300–350 ◦C. The values obtained for
the concentration of sulfonic acid groups are in agreement with the
range reported in the literature for various materials and conditions
of sulfonation [38,44,49–53]. Higher concentrations of sulfonic acid
groups seems to be possible by using more drastic conditions of
sulfonation, such as higher temperatures and times, and the use
of fuming sulfuric acid; in fact, sulfonated carbons as presented
herein were prepared at 120 ◦C for 6 h with concentrated sulfuric
acid.
Fig. 6. Correlation between sulfonic groups content and cellobiose conversion.
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Table 3
7
Main differences between cellulose and hemicellulose [54].
Cellulose
Hemicellulose
Consists of glucose units
Consists of several units of pentoses
and hexoses
High degree of polymerization
Form fibrous arrangement
Presents crystalline and
amorphous regions
Low degree of polymerization
Does not form fibrous arrangement
Presents only the amorphous regions
It is slowly attacked by mineral
acid at elevated temperatures
Alkali insoluble
Is rapidly attacked by dilute mineral
acid at elevated temperatures
Alkali soluble
ramifications, as well as the presence of different monomer units,
contribute to the structural complexity of hemicellulose and its
different conformations [59,60].
Table 3 summarizes the main differences of the polysaccha-
ride components in lignocellulosic materials. Understanding these
characteristics is of fundamental importance in order to define
strategies for the use of these biomass as raw materials for the
production of second generation ethanol and other chemicals.
Obtaining chemicals from saccharides (sucrochemistry), in
order to use natural polymers rich in oxygen (starch, cellu-
lose, hemicellulose), is an alternative to generate oxygenated
products, traditionally produced via processes of hydration and
hydroxylation of unsaturated petroleum hydrocarbons. Having
high concentrations in the biomass, the natural carbohydrates can
be transformed by various methods, supplying organic compounds
(polyols, diols, mono-alcohols, etc.) intermediates of industrial
interest.
Fig. 7. Turnover number as a function of sulfonic groups content.
in addition to SO3H groups in sulfonated carbons, that may interact
with cellobiose molecule through hydrogen bonds. This interaction
leads to the loss of hydrogen bonds within and between molecules
observed in the cellobiose, which protect the ␤-1,4-glycosidic bond,
making it more easily hydrolyzed.
Shuai and Pan [57] correlated this with the mechanism of the
reaction observed for the cellulase enzymes, where there are two
active sites. The first one is called the binding domain that would be
the equivalent to the functional groups of the sulfonated carbons;
the second one would be the catalytic domain, which effectively
promotes hydrolysis and would be the equivalent of sulfonic acid
groups. In fact, adsorption tests were carried with cellobiose at
25 ◦C and 3–4% of cellobiose were adsorbed by the sulfonated car-
bons. No adsorption was observed in the resin. These results are
in agreement with the findings of Kitano et al. [55] and Ormsby,
Kastner and Miller [58] who found 2–4% cellobiose adsorbed onto
sulfonated carbons and correlate this fact to the higher hydrolysis
reaction rate compared to other solid acids.
Using the conversion data and the amount of available sulfonic
sites, the turnover number (TON) was calculated. Interestingly, as
clearly demonstrated in Fig. 7, the TON decreases with the increas-
ing number of active sites. One possible explanation is that, when
there are fewer sites, such sites are located in privileged positions,
being more accessible for the hydrolysis. A higher density of active
sites can also hinder the adsorption and desorption of cellobiose
and glucose, reducing the speed of these phenomena, which ulti-
mately causes a reduction in the average number of cellobiose
molecules converted per site.
Although one of the main products to be obtained from hemicel-
lulose is surely ethanol, through the fermentation of xylose carried
out by some specific microorganisms, the goal of this work is to
present other chemicals produced via non-biological routes.
5.1. Xylitol and anhydroxylitol
The conversion of lignocellulosic feedstock, rich in cellulose
and hemicellulose, in derivatives for industrial use or as fuel, have
aroused interest in the development of new processes. Thus, some
studies have shown interesting results, suggesting hydrolysis of
these materials in molten salts, obtaining the separately hexoses,
the pentoses (from hemicellulose), whose major component is
xylose and finally lignin.
Xylose and other pentoses, are usually little or even not used for
conversion into products with higher added value. This is due to the
difficulty in determining a specific pre-treatment that makes it pos-
sible to selectively hydrolyze cellulose and hemicellulose, without
excessive production of unwanted byproducts (e.g. furans). Fur-
thermore, developments in biochemical regarding microorganisms
capable of consuming pentoses, excreting most interesting prod-
ucts are still relatively limited, and not yet dominated from the
point of view of industrial application.
In that sense, the hydrolysis of sugarcane bagasse in molten
ZnCl2 has been proposed [62]. The cellulosic fraction is hydrolyzed
to glucose and hydrogenated using Ru/C (5% ruthenium on carbon)
catalyst, converting most of glucose into sorbitol. Sorbitol is then
dehydrated by the action of Amberlyst acidic resin in order to obtain
isosorbide. The same concept could also be applied to xylose, pro-
ducing pentose derivatives with higher added value, such as xylitol
and anhydroxylitol.
Xylitol is a sweetening agent with interesting properties and
diverse applications. It is very soluble in water (approximately
60 wt.% at 25 ◦C) and its crystals are similar to the sugars plane
crystals. Xylitol is stable when stored, doesn’t caramelizes in high
temperatures, has a melting point between 92 and 96 ◦C and has a
5. The potential use of hemicellulose
Hemicellulose is intimately associated with cellulose in
plant tissues, being both of them the more abundant compo-
nents in plants. These macromolecules, unlike cellulose, exhibit
heteropolysaccharide nature and a considerable degree of rami-
fication, therefore not presenting crystalline regions.
They are composed mostly of a mixture of polysaccharides
with a low molecular mass: xylan, arabinans, arabinoxylans, man-
nans and galactomannans. The fundamental units (monomers)
are essentially molecules of d-xylose, d-mannose, d-galactose,
d-glucose, l-arabinose, d-glucuronic acid, d-galacturonic acid, ␣-
d-4-O-methylglucuronic acid, and also some oxidation products,
such as, for example, acetates.
Unlike cellulose, hemicellulose structure does not present high
crystallinity and, therefore, are more susceptible to chemical
hydrolysis under milder conditions. The varieties of bondings and
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Fig. 8. The structures of starting materials, byproducts and products of xylose hydrogenation.
sweetening capacity 20–25% higher than the sucrose [63]. Thus, it
has become a popular sweetener, particularly due to its anticario-
genic properties. Its uses are many in the pharmaceutical, cosmetic,
food and synthetic resins [64].
Xylitol can be prepared from the hydrogenation of xylose in
batch, in a triphasic process. High pressures of hydrogen and rela-
tively high temperatures are used. The high pressures are required
to promote the solubility of hydrogen in the liquid and high tem-
peratures are due to the fact that the reaction is temperature
dependent, as the rate of hydrogenation increases considerably
with the increasing temperature.
The main byproducts are d-arabinitol, d-xylulose, acid d-xylonic
and furfural. The formation of these byproducts is influenced pri-
marily by the temperature, pH and the mass transfer of hydrogen.
The higher the temperature, the more intense the byproducts for-
mation is. Alkaline conditions increase the formation of d-xylonic
acid via Cannizzarro reaction. It is observed that xylulose is formed
at the beginning of the reaction and is subsequently hydrogenated
into xylitol and d-arabinitol [65].
Fig. 9. Xylitol dehydration scheme.
In the experiments below 90 ◦C neither xylulose nor d-arabinitol
is formed. Typically, the formation of furfural was moderate. At
higher temperatures, some furfural formation occurred [66]. Fig. 8
shows the structures of starting materials, byproducts and prod-
ucts.
reactors were closed and submitted to a constant H2 pressure of
50 bar. Dehydration experiments have been carried out in a Met-
tler Easy Max reactor (150 ml) containing 1.5 g of catalyst and 60 g
of xylitol. As expected, xylose may be hydrogenated to xylitol on
Ru-containing catalysts, as shown in Fig. 10. Several hydrogena-
tion catalysts have been tested, such as Ru/C, Ru/Al2O3 and Ru/Y.
Ru supported on activated carbon is the best catalyst, as far as xyli-
tol yield is concerned (Table 4). Also, when testing Ru/C catalyst, no
degradation products (lyxose, xylulose, furans) were detected.
Furthermore, aiming at investigating the dehydration of xylitol,
several catalysts, such as Amberlysts 36WET and 70, and some zeo-
lites have been tested. It has been observed that xylitol may easily
undergo dehydration to anhydroxylitol on acidic solid catalysts, at
low temperatures, as shown in Fig. 11.
Regarding the anhydroxylitol, it is expected that it has analogous
properties to isosorbide, being then used also as a fuel booster or
an intermediate in organic synthesis. Fig. 9 shows the xylitol dehy-
dration scheme and its respective molecules. The dehydration of
polyols in aqueous solution has been studied extensively [67,68].
The mechanism involves protonation of the hydroxyl by a Brönsted
or Lewis acid, with subsequent water loss and formation of a carbo-
cation. In fact, the selectivity of the desired products is influenced
by the type of acid site [69]. Acidic resins, due to their high acid sites
density and considerable thermal stability, are interesting catalysts
for the dehydration step. Also, given the acidity required for the
occurrence of this reaction, the possibility of using zeolites must
not be disregarded, due to its characteristics of shape selectivity,
surface acidity and ion exchange capacity [70].
In this work, the production and kinetics of anhydroxylitol
via hydrogenation of xylose, followed by dehydration is studied.
Hydrogenation reactions have been carried out in a Parr Multi
Reactor Heater System (MRHS) 5000, comprising 6 reactors, 75 ml
each, with magnetic agitation. Each reactor was loaded with 0.5 g
of catalyst and a 50% aqueous solution of xylose. After loading,
The kinetics of xylitol dehydration was also determined. Dehy-
dration clearly displays a first order kinetics (Ea = 57 kJ/mol on
Amberlyst 36), as proven in Fig. 12.
Table 4
Xylitol yield and activation energy of hydrogenation catalysts.
Catalyst
% Xylitol
Ea (kJ/mol)
Ru/C 5%
Ru/Al2O3 5%
Ru/Y 5%
37.9
8.0
12.3
27
78
–
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Fig. 10. Hydrogenation of xylose utilizing Ru/C 5% – conversion × time.
Fig. 11. Dehydration of xylitol utilizing Amberlyst 36WET – conversion × time.
Fig. 12. Amberlyst 36WET kinetic order determination.
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materials. Regarding CO2, the hydrogenation of this molecule to
produce synthesis gas (CO + H2)seems to be a rather interesting way
of adding value to this molecule. In fact, a new catalytic system
(Ni/NiCeZr) was very active, selective and stable for the reaction
of reverse water gas shift (RWGS), which is apparently the reac-
tion path to the hydrogenation of CO2 to syngas. Furthermore, it is
demonstrated that the kinetics of the RWGS reaction is a limiting
step for the production of methanol from CO2.
Concerning the bagasse and straw components, lignin has been
transformed into active carbons with excellent properties, which
makes evident the fact that such carbonaceous raw material is a
potential one for the production of adsorbents. Moreover, these
active carbons underwent treatment with sulphuric acid, gener-
ating acidic sulphonic sites on the surface. Such solid acid was
employed as an alternative catalyst in the hydrolysis of cellobiose
with good results. Finally, the potential use of hemicellulose was
studied.
The hydrogenation of xylose to xylitol, followed by the dehy-
dration thereof to nhydroxylitol was carried out. The kinetics of
these reactions has been determined and several catalysts have
been tested, showing the feasibility of using pentoses such as xylose
to the production of new added value derivatives.
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