Tuning Ca-Al-based catalysts' composition to isomerize or epimerize glucose and other sugars

Article abstract of DOI:10.1039/c9gc02823d

One of the key reactions to achieve good productivity in the transformations of cellulose derived from biomass feedstocks is the isomerization of glucose to fructose, the latest being the platform molecule for obtaining other important derivatives. In this work, Ca-Al containing catalysts based on hydrotalcite-type derived materials were used to perform the selective isomerization of glucose to fructose, and the selective epimerization of glucose to mannose, using water as the solvent under mild reaction conditions. The catalysts showed high activity (conversion = 51-87%), and excellent selectivity (63-88%) towards fructose, compared with the current industrial process based on the glucose transformation via biocatalysis. It was also possible to modulate the selectivity towards fructose or mannose by tuning the amount of basic sites of the catalysts and their composition. The combination of basic and acid sites present in the Ca-Al-based catalysts plays a key role in the reaction, a fact that is discussed in the text together with other important operational parameters. The stability and recyclability of the catalysts were tested, detecting only a small activity loss after 5 consecutive runs. The synthesis of the catalysts and their characterization are also discussed since they are one of the few cases found in the literature of this kind of hydrotalcite-type material with such a high level of Ca incorporation. Some green metrics, such as E-factor, have been calculated to evaluate our system as an environmentally friendly process.

Full text of DOI:10.1039/c9gc02823d

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DOI: 10.1039/C9GC02823D
ARTICLE
Tuning Ca-Al-based catalysts composition to isomerize or
epimerize glucose and other sugars
a
b
b
a
Maria Ventura,* Juan A. Cecilia, Enrique Rodríguez-Castellón, and Marcelo E. Domine*
Received 00th January 20xx,
Accepted 00th January 20xx
One of the key reactions to achieve good productivity on the transformations of the cellulose-derived from biomass
feedstocks is the isomerization of glucose to fructose, being this latest the platform molecule for obtaining other important
derivatives. In this work, Ca-Al containing catalysts based on hydrotalcite-type derived materials were used to perform the
selective isomerization of glucose to fructose, and the selective epimerization of glucose to mannose, using water as solvent
under mild reaction conditions. The catalysts showed high activity (conversion = 51-87%), and excellent selectivity (63-88%)
towards fructose, compared with the current industrial process based on the glucose transformation via biocatalysis. It was
also possible to modulate the selectivity towards fructose or mannose by tuning the amount of basic sites of the catalysts
and its composition. The combination of basic and acid sites present in the Ca-Al-based catalysts plays a key role in the
reaction, fact that is discussed in the text together with other important operational parameters. The stability and
recyclability of the catalysts were tested detecting only a small activity loss after 5 consecutive runs. The synthesis of the
catalysts and their characterization is also discussed since they are one of the few cases found in the literature of this kind
of hydrotalcite-type materials with such a high level of Ca incorporation. Some green metrics, such as E-factor have been
calculated to evaluate our system as an environmental friendly process.
DOI: 10.1039/x0xx00000x
chemical catalysts show the advantages of significantly shorter
reaction time and greater endurance to impurities. Glucose
isomerization can be catalysed by Lewis acids (i.e., electron pair
Introduction
4
+
3+
acceptor such as Sn and Al ) via intramolecular hydride shift
6
,7
–
Fructose is a well-known sweetener used as an alternative to from C-2 to C-1, by Brønsted (i.e., OH ) or Lewis bases via
-2
-1 8,9
conventional table sugar (sucrose) as it is 1.8−2.0 fold sweeter hydrogen transfer from O to O , or by a combination of both
than sucrose. It has also emerged as a key starting compound functionalities. Homogeneous catalysts provide good catalytic
in biorefineries for the synthesis of value-added chemicals such performance, but often raise concerns about equipment
as 5-hydroxymethylfurfural, 2,5-furandicarboxylic acid, levulinic corrosion as well as downstream product separation and
acid or lactic acid, chemicals that can replace those synthetized solvent treatment, which has promoted the development of
1
10
1
1
2
,3
12
from fossil feedstocks. Currently, fructose is industrially heterogeneous solid catalysts for biomass conversion. The use
produced by the use of a biocatalytic process, employing xylose of fructose as intermediate for the production of valuable furan
isomerase as enzyme. At the end of this process and after derivatives and platform chemicals (Scheme 1) has spurred the
several HPLC enrichments the so-called high fructose syrup is quest for sustainable and cheap catalysts to replace the
4
13,14
attained. Although the enzymatic process is recognized as an enzymatic systems.
As it is shown in Scheme 1, the
effective method, it incurs high cost owing to the demand for isomerization of glucose to fructose is accompanied by a variety
purified feeding material, being biomass feedstock not suitable of by-products. Consequently, highly selective, efficient and
for this process; narrow operating conditions (pH and inexpensive catalysts are essential to the control of undesirable
5
temperature); and irreversible deactivation of spent enzymes.
reactions improving the reaction kinetics and to develop a
Glucose synthetized from green resources, such as system economically more favourable.
lignocellulosic biomass, represents an alternative to the
classical feedstock; therefore, more emphasis should be placed
Mg-Al mixed oxides have been used as effective catalysts for
this process reporting good fructose selectivity at glucose
on developing new systems, able to work with biomass derived
substrates, based on more efficient and selective processes as
well as more resistant to the reaction medium. In comparison,
1
2
conversion below 40%. These metallic mixed oxides derived
from hydrotalcites, which are layered double hydroxides
II
III
consisting of a mixture of M and M , have also exhibited good
catalytic performance (HT-Mg/Al) with a high selectivity (83%)
at high glucose conversion (61%) by using ethanol:water
a. Instituto de Tecnología Química, ITQ (UPV–CSIC). Universitat Politècnica de
València. Consejo Superior de Investigaciones Científicas. Avda. Los Naranjos S/N,
1
5
mixture as solvent. Many other heterogeneous catalysts have
been developed for this important process but none of them
are comparable to enzymatic isomerization of glucose that
4
6022, Valencia, Spain.
b.
Departamento de Química Inorgánica, Cristalografía y Mineralogía (Unidad
Asociada al ICP-CSIC), Facultad de Ciencias, Universidad de Málaga, Campus de
Teatinos, 29071 Málaga, Spain.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
selectively synthetizes fructose with an almost quantitative Main textural and physicochemical properties ofViCewa-AArtilclebOansleinde
yield (ca. 50%). materials with different Ca/Al molar ratio
D
O
s
I:
s
1
y
0
n
.1
t
0
h
3
e
9
s
/Ciz
9
e
G
d by82th3De
C0
2
co-precipitation method (see Experimental procedure section)
and used in this work are summarized in Table 1. It is worth to
noting that Ca-Al (0.8) sample possesses a surface area of 116
Glyceraldehyde
Dihydroxyacetone
2
m /g, which is almost 20 folds of that of the other Ca-Al mixed
Glycolaldehyde
Erythrose
oxides presented in this study. As Ca/Al molar ratios in solids
surpass the value of 1, surface areas dramatically decrease
because of the presence of high amount of Ca. The same effect
was observed for the pore volume that drastically decreases
with the increased amount of Ca.
Retro-Aldol
Condensation
OH
O
HOH2C
OH
Isomerization
O
HO
HO
HO
CH2OH
OH
OH
HO Fructose
Glucose
Table 1. Main textural and physicochemical properties of Ca-Al
mixed oxides.
Dehydration
Dehydration
O
O
O
O
O
O
Ca/Al
molar
ratio
0.8/1
2.4/1
3.0/1
3.4/1
Mixed
oxide
Surface
area
(m /g)
116
6
5
Pore
OH
HO
H
H
Sample
volume
OH
Furfural
OH
5-hydroxymethylfurfural
a
2
b
3
b
structure
Ca0.8AlO2.3
Ca2.4AlO3.9
Ca3.0AlO4.5
Ca3.4AlO4.9
(cm /g)
1
,6-anhydroglucose
Ca-Al (0.8)
Ca-Al (2.4)
Ca-Al (3.0)
Ca-Al (3.4)
0.420
0.029
0.032
0.041
Scheme 1. Isomerization of Glucose to Fructose and side
reactions taking place during the process.
7
a
b
Values obtained from ICP measurements. Values calculated from N
adsorption isotherms (BET method).
2
Glucose epimerization to mannose arises in nearly every
investigation of glucose conversion and competes with the
isomerization reaction. Just a few examples have been found in
the literature about the selective production of mannose
catalyzed by heterogeneous catalysts, both acid and basic
systems. Sn-Beta zeolites were used to react glucose in
methanol solvent where mannose was produced selectively via
The crystalline structures of the as synthetized HT-precursors
and the Ca-Al mixed oxides (with different Ca:Al molar ratios)
obtained after calcination were characterized by XRD
measurements, and their XRD patterns are shown in Figure 1.
1
6
a mechanism known as the Bilik reaction. Although other
homogeneous and heterogeneous catalysts able to catalyze the
1
7,18
epimerization of glucose to mannose have been reported,
basic catalysts have not been explored so far because of the
poor selectivity towards mannose, being the isomerization to
fructose always favoured. To the best of our knowledge,
metallic mixed oxides derived from hydrotalcite-type
precursors have not been explored to mediate this reaction.
In the present work, metallic mixed oxides containing Calcium
and Aluminium were synthetized via hydrotalcite-type
precursor structure, with a very high yield (≥75%), this being
one of the first examples of the synthesis of these materials
1
9,20
minimizing or even avoiding the presence of CaCO
3
.
Then,
the Ca-Al-based materials were used to isomerize or epimerize
glucose under mild reaction conditions. Catalysts were first fully
characterized, followed by a catalysts screening study to
modulate the selectivity of the reaction towards fructose or
mannose. The effects of temperature, pressure as well as
catalysts composition were investigated. Some insights of the
mechanisms were also explored by following the products
formation on the time reaction course. The application scope of
the Ca-Al-based materials was demonstrated, catalysts showing
good performance in the isomerization of other sugars such as
xylose or lactose.
Results and discussion
HT-precursors and Ca-Al-based catalysts characterization:
Figure 1. XRD patterns of A. HT-precursor materials; and B. Ca-
Al mixed oxides obtained from the calcination of HT-precursors.
3
Legends: stars: CaCO ; squares: CaO.
2
| J. Name., 2012, 00, 1-3
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ARTICLE
The patterns for HT-precursors series (0.8; 2.4; 3.0; and 3.4)
clearly confirm the layered structure of the materials, which is
View Article Online
DOI: 10.1039/C9GC02823D
consistent with others hydrotalcite-type structures
characterization.2
0,21
Three relatively narrow peaks appear at
low 2θ values corresponding to the 003, 006 and 012
reflections. The interlayer distance can be calculated from the
diffraction peak at 10.1-10.3°, which originates from the (003)
plane of HT-precursors (Table 2).
Table 2. Interlayer distance calculated based on Bragg’s law.
Sample
HT (0.8)
HT (2.4)
HT (3.0)
HT (3.4)
2θ
d (Å)
8.71
8.56
8.62
8.56
10.15
10.32
10.25
10.34
Figure 2. FT-IR spectrum of the HT (3.0) sample.
FT-IR measurements of the HT-precursors, before calcined,
were investigated in order to know the composition of these
new structures. Figure 2 shows the FT-IR spectrum of HT (3.0)
sample. Only one spectrum is shown because of the similarity
As it is shown in Table 2, the interlayer distance is higher to that
observed for classical hydrotalcite comprising of a mixture of
2
2
Mg:Al (≈7.8 Å), because of the higher atomic radius of Ca (2.31
Å) compared with the value of Mg (1.73 Å). This fact increases
the interlayer space making it bigger and increasing at the same
time the crystallinity of the solid. No other phases were identify
in the X-rays diffraction patterns; although HT (0.8) sample
shows a pattern identified as a mixture of the HT structure and
other non-identified phase. It is worth to highlight the absence
-1
between all of them (Figure S1). The peak at 3635 cm is
assigned to the hydroxyl group vibration in the Ca-Al HT layer,
-1
and the shoulder signal at 3200 cm corresponds to hydrogen
-
3 3
bonding between interlayer water and NO in the Ca-Al-NO
-1
HT-type structure. The peak at 3474 cm could be only
attributed to the hydrogen bonding between N and H, which
of CaCO
3
, usually present in this kind of hydrotalcite-type
2-
confirms the absence of CO
part, the synthesis of these hydrotalcite-type materials is done
under inert atmosphere, to avoid the formation of CaCO
3
. As is written in the experimental
materials.1
control in the atmosphere is often observed the segregation of
a CaCO phase instead of the formation of the desired HT-like
phase. This fact is presumably attributed to the greater
insolubility of CaCO formed by the reaction of calcium and
Na CO , or ambient CO
the interlayer space of the hydrotalcite,
incompatibility of the atomic radius of Al and Ca (1.43 versus
.31 Å). The hydrotalcite-precursors synthesized here did not
present practically any rest of CaCO (or its amount is very low),
9,21
When the synthesis is performed without any
3
.
3
Therefore, the presence of CO
2-
is highly minimized, but does
present in the
3
not rule out the possibility that part of the CO
2
3
water of the nitrate solutions could participate in the reaction
helping to create the layers of the hydrotalcite. In fact, the peak
2
-
2
3
2
, commonly used to introduce CO
3
in
2
3
and the
-1
at 1340 cm can be also attributed to the vibration of interlayer
2-
-1
CO
3 2
. The feature at 1616 cm represents the H O bending of
2
26
-
the interlayer water present in HT layers. Peaks below 800 cm
could be attributed to the vibration of the metal-O bonds.
3
1
as it was demonstrated in the XRD pattern. After calcination the
XRD patterns obtained for Ca-Al mixed oxides with intermediate
Ca/Al molar ratios (2.4 and 3.0) shows only pure CaO phase To elucidate and quantify the basic and acid sites present in
Figure 1B). Calcination treatment of the samples was made at each synthesized Ca-Al mixed oxide (derived from the
temperatures (723 K) lower than the needed for the calcination of HT-precursor), which may be a key factor in the
decomposition of CaCO (1023 K) that further shows the isomerization/epimerization of glucose, TPD analysis were
absence of CaCO in the as-prepared HT-precursors. In the cases performed by using CO and NH as gaseous probe molecules
of the highest [HT (3.4)] or lowest [HT (0.8)] molar ratios of for basic and acid sites, respectively. In the case of evaluation of
Ca/Al a bit proportion of CaCO was detected. For the Ca-Al (0.8) the total number of CO molecules desorbing from the solid
mixed oxide, it was impossible to calculate the proportion of surface, samples were carefully analyzed by considering the
CaCO because of the very poor crystallinity of the sample. formation of carbonate species before and after analysis;
(
3
3
2
3
3
2
3
However, the high crystallinity shown for the solid having the therefore, data shown herein correspond to those adsorb on
highest Ca/Al molar ratio [Ca-Al (3.4)], allowed to make an the surface and not coming from the carbonates formed. Table
estimation of the value giving a 7.3% of CaCO
3
. Some authors 3 shows the concentration of the basic sites (and also acid sites)
postulated that for the HT-precursors made up by Ca and Al, the of the Ca-Al mixed oxides. Because of the desorbed
2
6,27
metastable HT phase formed in these solids, can evolve to a one temperature, and according to the previous reports,
these
2
4
more stable identified as katoite. In our case, we could dismiss peaks could be assigned to oxygen anions, which desorb at
its formation by comparison of our XRD patterns with a temperatures higher than 773 K. No other peaks were found in
2
5
reference pattern from the literature.
anywhere else temperature region (see Figure S2 in SI). Due to
the absence of peaks NH -TPD has not been included.
3
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J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Table 3. Concentration of basic and acid sites measured by CO
Journal Name
2
-
Table 4. Catalytic activity of Ca-Al mixed oxides in the
View Article Online
isomerization of glucose.a
3
and NH -TPD analysis of the Ca-Al mixed oxides.
DOI: 10.1039/C9GC02823D
Fructose
Yield /
Selectivity (%)
Mannose
Yield /
Selectivity (%)
C
Balance
(%)
57
97
2
2
Sample
mol CO /m
mol NH /m
Conversion
Catalysts
2
3
0.6
0.0
4.0
0.0
(
%)
Ca-Al (0.8)
Ca-Al (2.4)
Ca-Al (3.0)
Ca-Al (3.4)
4.3
142.0
181.0
185.0
CaOb
59
<1
26 / 49
<1 / 98
7 / 12
0 / 0
b
Al O
2
3
Ca-Al (0.8)
Ca-Al (2.4)
Ca-Al (3.0)
Ca-Al (3.4)
35
41
50
89
32 / 94
30 / 74
43 / 87
23 / 26
<1 / 8
8 / 21
6 / 13
65 / 73
95
92
99
98
As it can be seen in Fig. S2 (in SI), strong basic sites dominate on
the surface of all of the mixed oxides, which show a very high
amount of basic sites, except (as might be expected) the Ca-Al
(
0.8) sample having the lowest amount of Ca. A correlation was
a
Reaction conditions: [Glucose] = 0.55M, 0.05 g catalyst, 5 mL H
2
O, PN
2
= 10
observed between Ca/Al molar ratio and the amount of basic
sites, as amount of Ca increases total basicity increases too.
Nevertheless, it is important to bear in mind that no so high
b
bar, temperature = 363 K, reaction time = 90 min. Commercial samples.
differences were found between Ca-Al mixed oxides with Ca/Al First, single oxides (i.e. CaO, Al O ) were employed to perform
2
3
molar ratios ≥3.0. It was also possible to correlate catalytic the isomerization of glucose, as shown in Table 4. The reaction
activity with these basicity values, which is going to be discussed with commercial CaO achieves good conversion but poor
in the next section of the manuscript.
selectivity towards fructose, with a low carbon balance.
Aluminum oxide did not give activity getting less than 1% of
glucose conversion with good selectivity to fructose and carbon
balance. Performing the isomerization of glucose with the Ca-Al
catalysts containing different amounts of Ca leads to different
results based on the Ca:Al molar ratio of the sample. As Ca:Al
molar ratio increases conversion of glucose increases reaching
almost a quantitative conversion [Ca-Al (3.4)]. However,
selectivity towards fructose does not follow the same trend.
Good selectivity to fructose was achieved by using the catalysts
with Ca:Al molar ratio between 0.8 to 3.0. Nevertheless,
increasing the Ca:Al molar ratio >3 promotes changes in the
activity of the catalysts and the production of mannose was
enhanced, thus reaching a mannose selectivity of 73%. By
contrast, the lowest Ca:Al molar ratio gives the highest
selectivity to fructose, although the conversion of glucose (35%)
was the lowest [Ca-Al (0.8)].
The SEM micrographs of the as-produced HT (3.0) precursor and
the Ca-Al (3.0) mixed oxide are shown in Figures 3a and 3b,
respectively. The rest of the HTs and mixed oxides micrographs
are not showing because of their similarity (see Figure S3 in SI).
The micrograph in Fig. 3a shows the layers typical of a HT-type
phase, while after calcined these layers are collapsed because
the dehydration at high temperatures (Fig. 3b). In addition,
hexagonal sheets are also detected (red cycle in Fig. 3a). The
successful homogeneous distribution of Ca and Al in the solid
was confirmed by SEM-EDX mapping (see Fig. 3 c).
2
7
As can be seen from the data of Table 4 and under the same
reaction conditions (at 363K and 10 bars of N ), the catalysts
2
with Ca:Al molar ratio between 0.8 to 3.0 afforded quite similar
yields of fructose 32-43%, because all of them reach the
equilibrium of glucose isomerization. Therefore, the initial
isomerization rates, fare from the equilibrium, were
investigated in order to make a proper comparison and choose
the best catalyst for next studies. Figure 4 shows a correlation
Figure 3. SEM images of a. HT (3.0) before calcination; b. Ca-Al
(
3.0) mixed oxide (after calcination); and c. SEM-EDX mapping
of mixed oxide Ca-Al (3.0), Al (red) Ca (green).
-1
-1
between isomerization rate, expressed as mmol fructose·g ·h
Glucose isomerization and epimerization over Ca-Al mixed oxides
A. Glucose-to-Fructose isomerization
in function of the Ca:Al molar ratio of catalysts. It is easy to note
the increasing in reaction rate as Ca content in the solids
increase. Due to the fact that the fastest catalyst [Ca-Al (3.4)]
gives mannose as main product, we decided to choose the Ca-
Al (3.0) sample for next studies. In order to accurate the
reliability of the results, the standard deviation for each group
of experiments was calculated getting values from 0.007 to
In order to minimize residues and energy, water was used as
solvent by working under mild reaction conditions
(
2
temperature = 373 K and pressure = 10 bars of N ). Usually this
1
2
reaction is performed at low glucose concentration, but going
one step forward and in order to maximize product yield with a
view to a future industrial application, a concentration higher or
equal than 0.5 mol/L was used. Table 4 shows the catalytic
activity of Ca-Al mixed oxides here prepared at similar reaction
conditions.
0
.0005 (see Figure S4 in SI).
4
| J. Name., 2012, 00, 1-3
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ARTICLE
To clarify this last conclusion, a reaction in presenViceweAortficlCeaOCnlOine
was performed, together with the catalyst ivity
3
D
O
t
I:
h
1
a
0
t
.1
h
0
a
39
v
/
e
C
n
9G
o
C
a
0
c
2
t
8
23
D
towards mannose, this means Ca-Al (0.8) sample. The
production of mannose was enhanced achieving from <1% to
1
3
0% of yield with CaCO addition. The same experiment was
performed using the Ca-Al (3.0) catalyst, also detecting ≈20%
increase of mannose yield with Ca salt addition. From these
results it can be concluded that the presence of CaCO together
3
with the high basicity and the lack of acidity observed for these
solids play a key role in the epimerization of glucose to
mannose.
In addition, it is important to note the kind of basic sites present
in this solids, all of them have strong basicity characterized by
the presence of peaks at high temperature ≥550 °C detected by
Figure
. Correlation between Ca:Al molar ratio of catalysts and
glucose isomerization rate.
4
2
means of CO -TPD analysis (see Figure S2 in SI), which can be
2
8
assigned to oxygen anions according to the literature. Strong
basic sites dominate the surface of the solids, however the
solids having the intermediate (3.0) and the highest (3.4) Ca:Al
molar ratios, exhibit two different peaks (Fig. S2 in SI) assigned
to the presence of strong and intermediate-strength basic sites
in both solids. Peak for intermediate-strength basicity is more
predominant in the case of Ca-Al (3.4) sample, also having the
highest total basicity that leads to a very high yield of mannose,
fact that could be explained by the need of a balance between
strong/intermediate basicity to perform the epimerization, and
a mixture of strong/intermediate sites, being the strong
predominant, probably together with some few amounts of
-
1
Turnover frequencies, TOF (h ), for the different catalysts
essayed in the glucose isomerization were calculated from the
initial isomerization rate divided by the density of basic sites
(
see Table 3). An interesting correlation exists between fructose
selectivity and TOF, as shown in Figure 5.
2
9
acid sites to perform the isomerization to fructose.
Despite the large surface area of the Ca-Al (0.8), it did not show
a superior activity in the isomerization of glucose, mainly in
terms of conversion. The activity of the Ca-Al-based catalysts is
mostly determined by the chemical composition and type of the
material, which might offer the adequate and accessible basic
sites to carry out the desired reaction.
Taking into consideration that Ca-Al (3.0) exhibited one of the
best
catalytic
performance
for
glucose-to-fructose
isomerization and a high isomerization rate (2313 mmol/gcath),
the influence of varying temperature and pressure of the
process was then evaluated. Figure 6 shows the catalytic
performance (in terms of glucose conversion and selectivity to
fructose) on the different reaction conditions tested by using
Ca-Al (3.0) catalyst.
Figure 5. Correlation between TOF, fructose yield and basicity
2
(in terms of mol CO /g) vs Ca content of the solids.
Some interesting conclusions could be extracted from the data
given in Figure 5, such as:
a. Highest yield of fructose was observed using the Ca-Al (3.0)
catalyst having a high density of basic sites (c.a. 906 mol/g),
and a few amount of acids (c.a. 20 mol/g). Therefore, the
adequate combination of basic and acid sites appears to be
crucial to give a good performance on the isomerization of
glucose to fructose.
b. A very high density of basic sites leads to the highest TOF but
also to a low selectivity towards fructose, as can be seen in the
case of Ca-Al (3.4) catalyst, owing to the formation of mannose
as the main product of reaction. This highest mannose Figure 6. Reaction condition screening for the isomerization of
production was achieved with the catalyst possessing the fructose at A. 363 K and B. 393 K.
highest density of basic sites and no acid functionalities. In
3
addition, the presence of CaCO in the Ca-Al (3.4), as it can be
seen in its XRD pattern, can help in the formation of mannose.
On one hand, at 363 K glucose conversion kept constant when
pressure was increased from 1 to 20 bar, while selectivity to
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fructose increased to reach a maximum of 87% at 10 bar of isomerized to glucose and then the epimerizationVietwaAkreticslepOlnalcinee
pressure; after that selectivity decreased when the pressure of forward mannose (see Scheme 3).
reaction was augmented until 20 bar. On the other hand, an
increase on the temperature (from 363 to 393 K) let to a
decrease in the selectivity to fructose, being some
decomposition products detected in the reaction medium.
Among all of the suitable decomposition products,
glycolaldehyde (GLY) was detected in a higher proportion, by
DOI: 10.1039/C9GC02823D
1
3
both HPLC (Figure S5 in SI) and C-NMR (Figure S6 in SI)
measurements. These side reactions taking place during the
isomerization of glucose are those typically observed in the
3
0,33
presence of basic catalysts (see Scheme 2).
O
HO
OH
dihydroxyacetone
OH
retro-aldol
condensation
HOH2C
O
OH
O
isomerization
HO
HO
HO
OH
CH2OH
HO Fructose
OH
O
Glucose
OH
O
retro-aldol
concensation
HO
O
Figure 7. Kinetics of the isomerization of glucose-to-fructose
glyceraldehyde
pyruvaldehyde
catalyzed by Ca-Al (3.0).
O
OH
glycolaldehyde
Pathway A
Scheme 2. Side reactions occurring during the isomerization of
glucose to fructose.
When is reached fructose
OH
OH
HOH2C
O
OH
Epimerization
yield= 44%
O
HO
HO
HO
O
HO
HO
HO
CH OH Time = 90min
OH
2
OH
Glucose
Mannose OH
HO Fructose
Further investigations were developed in order to find out the
role that glycolaldehyde GLY plays in the reaction. In this sense,
a series of experiments in presence of GLY (2 mmol) were done
Pathway B
OH
When is reached fructose HOH2C
O
OH
OH
in the established optimal conditions (at 363K and 10 bar of N
2
)
Isomerization
O
HO
O
HO
HO
yield= 44%
HO
HO
HO
using the Ca-Al (3.0) mixed oxide as catalyst. In these
experiments, a decrease in both activity and selectivity towards
fructose were observed (see Figure S7 in SI); therefore, we can
suggest that the presence of GLY in the reaction medium
inhibits the formation of fructose, because its higher
transformation ratio, as other authors have already
postulated.11
OH
OH
CH2OH
Fructose
Time = 90min
Glucose
HO
Mannose OH
Scheme 2. Possible pathways in the production of mannose
from glucose or fructose.
Once the optimal reaction conditions were set (363 K and 10 bar
of N ) and the most active catalyst, Ca-Al (3.0) was selected,
2
In order to elucidate this complicate issue some reactions
having fructose as starting reactant were performed, and the
results are shown in Table 5.
reaction kinetics were further investigated to get some insights
on the reaction mechanism (see Figure 7). Catalytic experiments
were repeated several times to accurate the given values were
correct (see Figure S18 in SI). As can be seen, glucose conversion
achieved around 50% at 90 min of reaction, and then it was kept
constant over the time even if a small decrease in its value was
observed mainly due to the deactivation of the catalyst, fact
that will be discussed in more detail in the next section.
Nevertheless, an unexpected profile, almost a sinusoidal graph,
was obtained for the selectivity towards the two main products,
fructose and mannose, respectively (HPLC of the points at 90,
Table 5. Catalytic activity of Ca-Al (3.0) with fructose as
substrate.
Glucose
Yield
(%)
GLY
Yield
(%)
Time
Temperature
(K)
Conversion
(%)
Mannose
Yield (%)
Entry
(
min)
1
2
3
4
5
6
7
45
41
53
61
63
61
87
87
34
34
38
36
37
38
35
7
7
7
8
180
210
300
45
180
210
11
15
17
12
39
40
3
63
7
1
80 and 270 min are shown in Figure S8, in SI). From these
11
10
11
results, it seems like glucose is firstly isomerized to fructose
achieving a maximum selectivity of 88%, point where the
equilibrium is reached; therefore, the reaction does not evolve
in this direction (towards the production of fructose), although
393
the catalyst continues to be active. Thus, the question to ask Two different temperatures were checked, 363 and 393 K,
now is if mannose is consecutively formed via the isomerization respectively. At 363 K mainly glucose was detected as product
of fructose, the most logical pathway or rather, fructose is (entries 1-4 in table 5), but it is interesting to highlight that
6
| J. Name., 2012, 00, 1-3
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conversion increased with the time of reaction, as well as the to decrease slightly, the equilibrium between VgielwucArotisclee Oanlninde
mannose yield, while a decrease in the glucose concentration fructose is achieved (time = 45 min); ther
D
O
e
I:fo10
r
.
e
10
,
3
t
9
h
/
e
C9
p
G
r
C
o
0
c
2
e
8
ss is
2
3
D
was observed at the same time. When increasing the reversed to glucose production and then glucose-to-mannose
temperature (393 K), conversion and mannose production were epimerization occurs until reaching a maximum in both glucose
increased, thus observing the same tendency as the reaction conversion (≈90%) and mannose selectivity (≈70%). Afterwards,
performed at low temperatures: an increase in the mannose the equilibrium between glucose and mannose is achieved, so
yield is observed with the reaction time. To corroborate that it is once again reversed to glucose and then the isomerization
mannose was not formed by the isomerization of fructose, to fructose takes place. Taking into account all these results
liquid samples were taken off from the reaction mixture and together with those shown in Table 5 and Figure 7, it could be
analyzed at very short reaction times. Glucose and mannose stated that fructose is isomerized to glucose and then under the
yields obtained at very short reaction times are drawn in Figure correct basic environment epimerization takes places forming
8
. At the beginning of the reaction, fructose is mainly isomerized mannose. In basic medium, glucose isomerization into fructose
to glucose, and then, when the glucose concentration in the requires little reorganization of the intermediate, whereas
reaction medium is enough, glucose epimerization to form formation of mannose takes place through rotation around the
mannose takes place. In light of all of these results it is C2-C3 bond (Scheme 4). The rotation is connected with
reasonable to conclude that mannose is mostly formed by the substantial reorganization of the water shell, and thus,
3
4
3
5,36
epimerization of glucose more than by the isomerization of mannose is formed more slowly than fructose.
If the catalyst
fructose, although this latest pathway cannot be discarded has only strong basic sites the evolution to mannose would be
entirely.
more slowly because of the stronger coordination to the metal
center. On the contrary, if this solid has also medium strength
basic, as in the case of Ca-Al (3.4) sample, this coordination
becomes weak making easier the rotation around C2-C3.
Figure 8. Glucose and mannose yields at short reaction times
using fructose as substrate.
B. Glucose-to-Mannose epimerization.
Figure 9. Kinetics of the epimerization of glucose using Ca-Al
Taking into account that the catalyst having the highest calcium (3.4) as catalyst.
content, Ca-Al (3.4) showed the highest selectivity towards
mannose, the performance of this catalyst in the selective
epimerization of glucose was investigated by carrying out the
kinetics of the reaction (see Figure 9). Catalytic experiments
H2C OH
Isomeric ketose =
Fructose
C
O
R
H
O
were repeated several times to accurate the given values were
correct (see Figure S15 in SI).
C
C
R
HC
C
O
H
H
OH
O
H
R
H
O
C
Glucose
Enediol
anion
Epimeric aldose =
Mannose
Similar trends as before observed with glucose-to-fructose
isomerization can be stated, with the difference that when
mannose reached the maximum production (at 90 min),
conversion is also reaching the maximum as well. After that,
both conversion and selectivity to mannose fell, and
simultaneously an increase in fructose selectivity is observed.
Therefore, we have two parallel equilibriums taking place: one
between glucose and mannose, and the other between glucose
HO
C
H
R
Scheme 4. Fructose and Mannose formation via enediol anion
intermediate.
and fructose. At the beginning of the reaction (time <50 min), Taking into account the kinetics observed in Figures 7 and 9,
glucose is isomerized to fructose and also epimerized to rate constants for the glucose isomerization to fructose (k1) and
mannose, the former reaction rate being higher than that of the for the glucose epimerization to mannose were calculated,
-1
glucose epimerization. When conversion of glucose reached obtaining the following values: k1 = 0.020 min and k2 = 0.021
-1
around 45%, and although initially mannose selectivity started min (see Figures S16 and S17 in SI). As both constants have
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practically the same value, this could be consistent with the in a new run or catalytic re-use without calcinationVtierweaAtrtmicleeOnntlionef
kinetic model of Competitive First Order Reactions, where the the solids.
equations (developed by a well-known mathematical model)
allow to calculating the fructose and mannose concentrations
during the process (see SI).
DOI: 10.1039/C9GC02823D
In order to get more insights in the mechanism of the process,
the reaction using glucose-2-d1 as substrate was performed in
the same conditions as those described for the isomerization of
glucose to fructose with unlabeled glucose. In such a way, it was
possible to elucidate the rate-determining step in the glucose
isomerization due to the decrease in the isomerization rate
when using glucose-2-d1. This indicates the C2–H bond cleavage
as the rate-determining step of the process, as can be seen in
Table S7 (in SI). Further NMR spectroscopic studies were
performed to clarify the mechanism, but the results were not
conclusive (see SI). Apparently, both mechanisms, basic and
acid-catalyzed glucose isomerization could be involved, the first
mechanism being the more plausible for our catalytic system.
Catalysts recyclability
One of the great advantages of the glucose isomerization
process when carried out heterogeneously consists in its simple
catalyst separation and reutilization, an essential requisite to
make any process economically viable. The recyclability of Ca-Al
(3.0) catalyst in the glucose isomerization to fructose and of Ca-
Al (3.4) catalyst in the glucose epimerization to mannose was
evaluated by reusing the solids in several consecutive runs (see
Figure 10). In a typical experiment and at the end of reaction,
the solid catalyst was separated from the solution after
previous centrifugation of the reaction mixture and ulterior
filtration, then washed three times with water, and finally
reused in a new catalytic experiment. A decrease in the activity
of the catalysts was observed during the first re-use in both
cases. After checking the recovered solids by XRD
measurements, it was detected that hydrotalcite-type structure
was regenerated (see Figure S9, in SI), this structure showing
less activity towards isomerization than the Ca-Al mixed oxides.
With the aim to raise the catalytic activity again, a thermal
treatment (calcination) of the recovered catalysts before the
new run was made. In this way, the activity was almost
recovered in the second re-use and maintained constant up to
five consecutive re-uses, although a small decay is observed
compared to the initial fresh catalyst. It is worth to note that
when the catalyst loses its activity (mainly when it was re-used
without previous calcination treatment) glycolaldehyde (GLY)
was generated as main side-product.
Figure 10. Repeated uses of Ca-Al-based catalysts for both A.
Glucose isomerization to fructose and B. Glucose epimerization
to mannose before and after regeneration (2n.c means non-
calcined material). Reaction conditions: [Glucose]
g catalyst, 5 mL H O, PN = 10 bar, at 363 K.
i
=0.55M, 0.05
2
2
During the catalytic re-cycling tests, a small leaching of Ca was
detected by checking the solids after the first and the last (fifth)
run by means of ICP analysis (see Table S1 in SI). The measured
loss of Ca (with respect to the total initial amount of Ca in the
fresh solids) for both Ca-Al (3.0) and Ca-Al (3.4) catalysts was of
2
.5% and 2.8%, respectively, values given after the first and fifth
runs being practically identical, this meaning that the principal
leaching of Ca species occurs in the first run with a practically
negligible increase in the value after several re-uses. In order to
elucidate a possible catalytic activity of the leached Ca ions, an
additional test with hot filtration step was carried out. Thus, the
catalytic reaction was conducted during 45 min in the presence
of Ca-Al (3.0) sample until the conversion of glucose reached ca.
2
+
By considering the results of Figure 10 and in order to accurate
that hydrotalcite-type structure is less active towards both
isomerization and epimerization of glucose, some additional
experiments using hydrotalcite-type materials as catalysts were
performed. Indeed, a decrease in the glucose conversion and
the selectivity to the desired products was observed by using HT
3
5%. After that, the solid catalyst was removed by filtration and
the filtrate was held under typical reaction conditions during
another 45 min. No changes in the concentrations of glucose or
fructose could be detected by HPLC analysis, these evidences
supporting that the isomerization of glucose is catalyzed
heterogeneously over the Ca-Al-based catalysts. However, after
two hours of incubation of the reaction mixture in the absence
of the solid catalyst, the pH of the filtrate decreased to a more
(3.0) and HT (3.4) for glucose isomerization and epimerization,
respectively. Thus, glucose conversion of 15% and selectivity to
fructose of 62% were achieved for isomerization to fructose,
while glucose conversion of 45% and selectivity to mannose of
2
+
acid value confirming that Ca species are not active for the
isomerization reaction, but can catalyze the decomposition of
6
1% were reached for epimerization to mannose, being both
carbohydrates into acidic by-products getting
a high
results lower than the values achieved with the catalysts used
concentration of saccharic acid, as it was checked by HPLC
8
| J. Name., 2012, 00, 1-3
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a
analysis of the liquids at the end of the reaction. Similar results
were obtained by other authors when a solution of NaOH was
used as homogeneous catalyst.
Reaction conditions: [Substrate]
=10bar, at 363 K.
i 2
=0.5M, 10wt% of catalyst, 5 mL H O,
View Article Online
PN
2
DOI: 10.1039/C9GC02823D
3
7
The Ca-Al-based catalysts prepared in this study were tested in
the isomerization of xylose-to-xylulose and lactose-to-lactulose,
and the attained results are shown in Table 6. On one hand, the
Ca-Al (3.0) catalyst was able to isomerize both substrates
(xylose and lactose), but with lower catalytic performance
compared to that reached in the glucose-to-fructose
isomerization. Particularly, for the isomerization of xylose,
results were as expected since the isomerization of xylose to
It is clear that our reaction conditions are so far from the
physiological ones employed with enzymatic systems where
proportions of glucose/fructose/mannose below the
equilibrium values are reported. In addition, the conversion of
glucose during epimerization attained with our catalytic system
is so high because glucose is transformed into two products in
parallel reactions, being the yield of fructose far from the
1
2
4
4
xylulose had been classically performed by acid catalysts. In
fact, when the isomerization reaction was performed in
presence of a more acid Ca-Al mixed oxide, such as Ca-Al (0.8)
equilibrium. Other authors observed something similar in their
systems,38 glucose conversion increased in the reaction course
because of the consumption of fructose; although this fructose
transformation was not clear in the paper, since any other by-
product was detected in the reaction mixture.
(Entry 1), better values of conversion and selectivity were
achieved. More interestingly, longer reaction times let to a
decrease in the selectivity of the isomer and non-identified
degradation products were obtained. On the other hand,
as different and better results were obtained in the case of the
2
More interestingly, it was also reported that using Ca(OH)
catalyst for this reaction, mannose yields superior to the isomerization of lactose. Good selectivity to lactulose and
3
9
thermodynamic equilibrium values can be reached. Authors lactose conversion were reached at 90 min of reaction time by
2
+
explain that the formation of a complex between Ca species using the Ca-Al (3.0) catalyst (Entry 7). By increasing the Ca:Al
and mannose allows getting
kinetic (instead of molar ratio from 3.0 to 3.4, the catalytic activity increase too,
a
thermodynamic) control of the system, thus surpassing the 50% achieving good conversion (53-56%) and excellent selectivity to
mannose yield theoretically expected. Taking into account that the isomer (95-96%) by working at different reaction times
2
+
the Ca-Al (3.4) catalyst possesses a high amount of Ca species (Entries 9, 10), although no more improvements were observed
exposed on the solid surface, the possibility of this complex at longer reaction times. Remarkably, these results are better
formation at the solid-liquid interface could not be ruled out. than those found in the bibliography, from our knowledge.
2
+
This does not mean that the Ca species are leached from solid These results represent a step forward in the isomerization of
to the solution during reaction. In fact, as it was clearly lactose since this reaction is usually performed with the
demonstrated above, most of the Ca remains in the solid addition of external bases or by using homogeneous catalysts,
4
0
catalyst even after fifth consecutive catalytic cycles.
thus making difficult their recovering and recyclying. In
addition, any cleavage or decomposition products were
detected in the reaction mixture, while the corresponding
epimer was also not detected in any reaction. Due to these very
promising results, further investigations on the selective
isomerization of lactose over Ca-Al-based materials are
currently on-going in our research group.
Catalytic performance of Ca-Al-based materials in the
isomerization of other sugars
With the aim to explore the performance of these Ca-Al-based
catalysts more in deep, the isomerization of other biomass-
derived sugars was performed. Xylose and lactose were chosen
for this studies since: i) its isomerization still represents a
Green metrics: comparison with other systems
challenging task;4
0-42
and ii) because their respective isomers
Green metrics is a useful tool to evaluate the sustainability of a
process and to make a fair comparison with the classical or
already developed systems. Among all of the tools used for this
purpose, we have chosen three:
are useful platform molecules for the synthesis of high added-
value products, thus increasing at the same time the catalysts
applicability.43
1
. Atom economy (AE):45,46 It is the classical tool and is defined
Table 6. Isomerization of xylose and lactose over Ca-Al-based
as the ratio of the molecular weight of the target molecule to
the sum total of the molecular weights of all the substances
produced in the stoichiometric equation for the reaction
involved. It takes into account the amount of the reagents
incorporated into the end product (see SI).
catalysts.a
Isomer
select.
C
Catalyst
Time
Conversion
(%)
Entry Substrate
balance
(%)
(
min)
(
%)
Ca-Al
1
90
65
70
85
4
7
(0.8)
2. Environmental factor (E-factor): It is the ratio of the weight
of generated waste to the total weight of the end product. It is
a useful tool for rapid evaluation of processes based on
generated waste (see SI).
3. Ecoscale:48 It is a simplified algorithm for evaluation of
chemical reactions on laboratory scale developed by Koen Van
Aken et al. It is a semi-quantitative tool that unifies economic
and ecological aspects of a given reaction (see SI).
2
Ca-Al
(3.0)
Ca-Al
(3.4)
Ca-Al
90
180
90
45
63
42
55
21
18
12
9
81
74
84
72
xylose
3
4
5
180
6
90
45
21
48
(2.4)
7
Cal-Al
(3.0)
Ca-Al
(3.4)
90
180
90
65
75
53
56
72
55
95
94
98
82
98
97
lactose
8
9
1
0
180
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Analysis of green metrics have been done for our catalytic (≥95%), were purchased from Sigma-Aldrich and used as
View Article Online
system using Ca-Al-based materials by making a comparison
with the enzymatic system used at industrial level (with xylose
DOI: 10.1039/C9GC02823D
received. Water (Milli-Q quality, Millipore) was used as solvent.
Hydrotalcite-precursors were prepared by using the proper
4
9
isomerase as catalyst),
heterogeneous system that uses Sn-Beta zeolite as catalyst.
and one of the most cited
metallic precursor bought from Sigma-Aldrich: Ca(NO
≥99%), Al(NO ·9H O (≥98%), and an aqueous solution of
NaOH (98%) in milli-Q water. Single oxides CaO and Al were
3 2 2
) ·4H O
5
0
(
)
3 3
2
Figure 11 shows a comparative estimation of the three green
parameters for the three selected isomerization systems (see
SI).
2 3
O
purchased also from Sigma-Aldrich. Nitrogen (99.999%) was
acquired from Abelló Linde S.A.
Catalysts preparation:
Due to the few examples of this Ca-Al hydrotalcite-type
materials found in bibliography, a complete optimization of
their synthesis was performed. Optimization as well as XRD
analysis of each sample is depicted in the Supporting
Information (Table S6, and Figures S10-13). The hydrotalcite-
precursors were prepared by adaptation of the co-precipitation
1
9
method described in ref. Once main parameters of synthesis
i.e. aging temperature, base concentration, etc.) were
(
optimized, the standard procedure for hydrotalcite-precursors
preparation was as follows: Starting from two different
solutions, solution A, containing the metallic species (Ca, Al) in
the desired concentrations and solution B containing the basic
-
2+
solution in the wanted concentration that is: [OH ] = 2[Ca ].
These two solutions were added at room temperature, at the
Figure 11. Comparison of AE, E-factor and Ecoscale, green
parameters for our system and other two systems used in the
glucose-to-fructose isomerization.
same rate (20 mL/h) to an empty flask under N
with a stirring speed of 500 rpm. The precipitates were aged for
8h at 333 K in an oven, keeping the N atmosphere. After that,
2
atmosphere
1
2
the solids were washed with deionized water until pH ≤8 and
calcined, prior to their use in the catalytic experiments. The
calcination treatment was done from room temperature until
Comparison made in Figure 11 clearly shows that it is possible
to compare our catalytic system with the enzymatic system in
terms of AE parameter, even taking into account the limitations
of the process itself where 50% conversion would be the
maximum value. With respect to E-factor parameter our system
has not a bad number (considering the ideal E-factor of zero),
value being higher than the enzymatic system, but much better
than Sn-Beta system. In terms of a more general scale, the
7
23 K with a temperature rate of 2 °C/min under air
atmosphere, and then kept at 723 K during 4 h under N
atmosphere (see Fig. S18 and comments in SI). Finally, the solids
were cooling down during 8h under N atmosphere.
2
2
Ecoscale shows that the enzymatic system is better than our Catalysts characterization:
system (with a difference of 18.3 points), but it should be taken
HT-precursors and Ca-Al mixed oxides were characterized by
inductively coupled plasma (ICP) with a Varian 715-ES ICP-
into account that for this calculation the need of using inert
atmosphere in the synthesis of the catalysts was considered in
our system. However, our catalytic process could be considered
as a green process and at least greener than the process
catalyzed by Sn-Beta. In conclusion, our process is within the
green concept, better than other systems based in
heterogeneous catalysts, and close to the industrially used,
xylose isomerase enzymatic route.
Optical emission spectrometer, after solid dissolved in HNO
3
aqueous solution. Phase purity was determined by X-ray
diffraction (XRD) in a Philips X`pert MPD diffractometer
equipped with a PW3050 goniometer (CuKa radiation, graphite
monochromator). Surface areas of the solid samples (150 mg)
were calculated by applying the BET method to the N
2
adsorption isotherms obtained by carrying out liquid nitrogen
adsorption experiments at 77 K, in a Micromeritics flowsorb
apparatus.
Experimental
Chemicals:
The basicity of the catalysts was studied by temperature-
programmed desorption of CO (CO -TPD). Approximately
2 2
Glucose and fructose (Sigma-Aldrich, 99.5%), lactose and xylose 100ꢀmg of sample was pretreated with a stream of helium at
Sigma-Aldrich, 99.9%) were used as reagents in the 1073 K during 30ꢀmin (10 K/min and 60ꢀmL/min). The reaction
(
isomerization and epimerization tests. For the analysis, temperature was then decreased to 373 K, and a flow of pure
mannose (99.99%), galactose (99.99%), glycolaldehyde dimer CO (60ꢀmL/min) was subsequently introduced into the reactor
2
(
99.99%), glyceraldehyde (≥90%), xylulose (≥98%), and lactulose during 30ꢀmin. The CO -TPD reaction was carried out between
2
1
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3
73 and 1073 K under a flow of helium (10 K/min, 30ꢀmL/min), water at a flow rate of 0.5 mL/min. Analysis were dVoienweArbticyleuOsnilninge
and the amount of CO
2
evolved was analyzed using a external calibration curves generated fDoOrI:a10ll.1o03f9t/Ch9eGCsu0i2t8a2b3lDe
quadrupole mass spectrometer (Balzer GSB 300 02) equipped products separately using standard solutions with known
with a Faraday detector (0–200ꢀU), which monitors the mass of concentrations. The results are reported in terms of substrate
CO
2
(44ꢀU) during the experiment.
conversion, and products yield and selectivity, which are
calculated as follows:
The temperature-programmed desorption of ammonia (NH
3
-
TPD) was carried out to evaluate the total surface acidity of the Substrate conversion (%)
=
(Moles of substrate
catalysts. After cleaning of the catalysts (80 mg) with helium and converted)/(Initial moles of substrate) x 100
subsequent adsorption of ammonia at 373 K, the NH
measurement was performed by raising the temperature from
73 to 823 K, under a helium flow of 40mL/min, with a heating
rate of 10 K/min and maintained at 823 K during 15 min. The
evolved ammonia was analyzed using a TCD detector of a gas
chromatograph (ShimadzuGC-14A).
3
-TPD
Products yield (%) = (Moles of product produced)/(Initial moles
of substrate) x 100
3
Products selectivity (%) = (Moles of product produced)/(Moles
of substrate converted ) x 100
A scanning electron microscope (SEM, Zeiss instrument, AUR-
IGA Compact) has been employed to determine the morphology
of the solid samples before and after calcination. FT-IR
Conclusions
spectroscopic analyses of the fresh catalyst samples were In this work, some hydrotalcite-precursors and their derived
performed on a Perkin Elmer FT-IR spectrometer, and the mixed oxides containing Ca and Al were successfully prepared
-1
by a sustainable method and evaluated in the isomerization of
various sugars. Hydrotalcite-type phase was obtained as the
main crystalline phase in all the HT-precursors prepared with
spectra were recorded in the region of 4000-500 cm .
Catalytic experiments:
Ca:Al molar ratios ranging from 0.8 to 3.4, and no rest of CaCO
3
Catalytic tests were performed in a 12 mL stainless steel was observed in almost all of the solids, with the exception of
autoclave type reactor with PEEK (polyether-ethyl-ketone) the solid having the highest Ca:Al molar ratio [Ca-Al (3.4)], in
interior, equipped with a magnetic bar, pressure control and a which a 7.3% of CaCO
valve for either liquid or gas sample extraction, and an electrical CaCO played a key role to tune the catalyst activity and drive
heating jacket. Typically, 2.7 mmol of the desired substrate the reaction to the production of mannose instead of fructose.
glucose: 0.5 g; fructose: 0.5 g) was dissolved in 5 mL of distilled A superior activity was observed in the glucose isomerization to
3
was detected. The presence of this
3
(
water and placed in the reactor together with 0.05 g of the fructose for the catalysts with an intermediate Ca:Al molar ratio
catalyst. The autoclave was then closed and purged three times and intermediate basicity [i.e. Ca-Al (3.0)]. More interestingly,
with N
pressure of nitrogen and heated to the desired temperature sugars, such as lactose and xylose, thus extending their
363 or 393 K). Stirring was stopped at fixed intervals of time application scope. Recyclability of the catalysts was also
and liquid samples (0.1 mL) were withdrawn and then analyzed evaluated for both isomerization and epimerization of glucose
. Then, the reactor was charged with the appropriate these Ca-Al-based materials are capable to isomerize other
2
(
by HPLC by following reactants and products evolution.
showing a good resistance under reaction conditions over
several catalytic runs. Some green metrics were also evaluated,
the here proposed catalytic system fits very well with green
concepts compared with the enzymatic system, industrially
employed, while showing superior sustainability than other
systems based on solid acid catalysts.
Recycling tests were performed using the desired catalysts at
the fixed optimal conditions. After a catalytic run, the reaction
mixture was centrifuged and the solid catalyst was recovered by
filtration, then washed with ultrapure water and reused in a
new run under the same conditions. Because a drop in the
activity was observed, a previous calcination treatment was
made by applying the same conditions as in the synthesis of the
mixed oxides.
Conflicts of interest
There are no conflicts to declare.
Reaction mixtures analyses:
The liquid samples collected from the experiments were
analyzed using HPLC (Varian) equipped with a refractive index
detector (RI) Varian Star with internal temperature of 323 K to
determine the amount of products and substrates. Bio-Rad
Aminex HPX-87H column was used in the case of glucose-to-
fructose (or mannose), while HPX-87P (9m, 7.8 x 300 mm)
column was employed for xylose-to-xylulose and lactose-to-
lactulose reactions. In both cases, the columns were maintained
at 333 K and the mobile phase was 100% milli-Q HPLC grade
Acknowledgements
Financial support by the Spanish Government (PGC2018-
0
97277-B-I00 and SEV-2016-0683), Generalitat Valenciana
GVA, PROMETEO/2018/006), and especially the
Unesco/IUPAC/PhosAgro partnership within the program
Green Chemistry for Life” (Grant #139) is gratefully
(
“
acknowledged. Authors also thank the ITQ for the technical
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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