Research Collection Journal Article Hemicellulose arabinogalactan hydrolytic hydrogenation over Ru-modified H-USY zeolites Author(s): Murzin, Dmitry Y.; Kusema, Bright; Murzina, Elena V.; Aho, Atte; Tokarev, Anton; Boymirzaev, Azamat S.; Wärnå, Johan; Dapsens, Pierre Y.; Mondelli, Cecilia; Pérez-Ramírez, Javier; Salmi, Tapio Publication Date: 2015-10 Permanent Link: https://doi.org/10.3929/ethz-a-010792434 Originally published in: Journal of Catalysis 330, http://doi.org/10.1016/j.jcat.2015.06.022 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection . For more information please consult the Terms of use . ETH Library
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Research Collection50067/eth-50067-01.pdf · High-resolution magic angle spinning 27Al nuclear magnetic resonance (MAS NMR) spectroscopy was carried out using a Bruker AVANCE 700
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conducted using a PANalytical X’Pert PRO-MPD diffractometer. Data were recorded in
the 5-70° 2θ range with an angular step size of 0.05° and a counting time of 7 s per step.
High-resolution magic angle spinning 27Al nuclear magnetic resonance (MAS NMR)
spectroscopy was carried out using a Bruker AVANCE 700 NMR spectrometer
equipped with a 2.5-mm probe head and a 2.5-mm ZrO2 rotor at 182.4 MHz. Spectra
were acquired using a spinning speed of 20 kHz, 4096 accumulations, and a recycle
delay of 1 s. The acid properties of the catalysts were evaluated by Fourier transform
infrared spectroscopy (FTIR) using pyridine (Sigma-Aldrich, ≥99.5 %, a.r.) as the probe
molecule. The measurements were performed using an ATI Mattson spectrometer
equipped with an in situ cell containing ZnSe windows. The samples were pressed into
thin self-supporting wafers (20 mg, radius = 0.65 cm). Pyridine was first adsorbed for
30 min at 373 K and then desorbed by evacuation for 20 min at different temperatures
(523, 623, and 723 K). Spectra were recorded by co-addition of 32 scans with a
resolution of 2 cm−1. The Brønsted-acid sites (BAS) and Lewis-acid sites (LAS) were
quantified based on the intensities of the bands at 1545 and 1450 cm−1, respectively, and
using the molar extinction coefficients reported in the literature [28]. Transmission
electron microscopy (TEM) imaging was undertaken with a FEI Tecnai F30 microscope
7
operated at 300 kV (field emission gun). The samples were prepared by depositing a
few droplets of zeolites suspension in methanol onto a carbon-coated copper grid,
followed by evaporation at room temperature.
Catalytic evaluation
Hydrolysis in the presence of hydrogen over protonic zeolites and hydrolytic
hydrogenation experiments over metal-modified zeolites were carried out in a 300-cm3
Parr autoclave reactor connected to a 200-cm3 pre-reactor. The autoclave was equipped
with sampling outlet featuring a 1-m filter to prevent even very fine catalyst particles
from escaping it. The temperature was measured with a thermocouple and controlled
automatically (Brooks Instrument). At the beginning of each experiment, 400 mg of AG
were dissolved in 90 cm3 of deionized water and loaded into the pre-reactor. Thereafter,
200 mg of catalyst with a particle size below 63 m (to avoid internal diffusion
limitations) were loaded into the reactor containing 10 cm3 of deionized water. The
reactor was pressurized with hydrogen and heated to 458 K, reaching a total pressure of
31 bar. Based on the vapor pressure of the solvent at this temperature, the partial
pressure of hydrogen was 20 bar. The stirring (1000 rpm, to minimize external mass
transport limitations) was then started and the reactant solution was fed from the pre-
reactor into the reactor. This was considered as the initial reaction time. Liquid samples
from the reaction mixture were periodically withdrawn for analysis. The decrease in
volume of the reaction mixture was taken into account in the calculations of reactant
and products concentrations.
Product analysis
8
The liquid samples from the reaction mixture were quantitatively analyzed without any
pretreatment in a high-performance liquid chromatography (HPLC) system using two
different columns and a refractive index (RI) detector. A Bio-Rad Aminex HPX-87C
column heated at 353 K was used to analyze AG, sugars, sugar alcohols, and furan
compounds. A diluted (1.2 mM) CaSO4 solution flowing at 0.4 cm3 min−1 was
employed as the mobile phase. An Aminex cation H+ column heated at 338 K was used
to analyze acidic compounds and other degradation products. In this case, the mobile
phase comprised a 0.005 M H2SO4 solution flowing at 0.5 cm3 min−1.The individual
components were identified by gas chromatography-mass spectrometry (GC-MS) as
discussed in detail in [19]. The carbon mass balance was calculated considering the
concentration of AG, sugars, polyols, furfurals, and low molecular-weight compounds
analyzed by HPLC.
The weight average molar mass (Mw) and number average molar mass (Mn) were
determined via gel permeation (size exclusion) chromatography (SEC) using a system
equipped with two columns in series (300 7.8 mm Ultrahydrogel liner, Waters,
Milford, USA), a multi-angle laser light scattering MALLS unit (miniDAWN, Wyatt
Technology, USA), and RI and UV detectors. A 0.1 M NaNO3 aqueous solution
flowing at 0.5 cm3 min−1 served as the eluent. The samples were filtered with a 0.45-m
syringe Acrodisc filter. The injection volume was 100 μl. The Astra software (Wyatt
Technology) was used for data analysis.
Results and Discussion
USY and Ru/USY zeolites
The FAU-type zeolites used in this study feature a bulk Si/Al ratio of 15 (USY-15) and
30 (USY-30). According to the NMR spectroscopic analysis (not shown), both zeolites
9
contain a significant amount of distorted tetrahedral Al species as well as
extraframework penta- and, especially, hexacoordinated Al centers, which are generated
upon stabilization of the pristine Y zeolite via steaming and acid washing. Both samples
feature a mesoporous surface area (Smeso) of 125-128 m2g−1 and a micropore volume
(Vmicro) of 0.29-0.31 cm3 g−1, as typically observed for these materials (Table 1). Their
acidity was evaluated via FTIR studies using pyridine as a probe molecule. Upon
adsorption and desorption of pyridine at different temperatures, the Brønsted-acid sites
(BAS) and Lewis-acid sites (LAS) could be quantified and classified based on their
relative strength. Thus, USY-15 and USY-30 possess a similar concentration of BAS
and LAS (335 and 65 μmol g−1 and 310 and 51 μmol g−1, respectively), but the former
catalyst contains a slightly higher number of medium and strong sites of either type
(Table 1).
Table 1. Characterization data of the protonic and Ru-modified zeolites.
Catalysts BASa
(μmol g−1)
LASa
(μmol g−1)
Smesob
(m2 g−1)
Vmicrob
(m3 g−1)
Ru
contentc
(wt.%)
Cryst.d
(%)
523 K 623 K 723 K 523 K 623 K 723 K
USY-15 163 126 46 41 17 7 128 0.29 - 100
Ru(1)/USY-15 233 11 2 32 3 1 117 0.29 1.4 -
Ru(2.5)/USY-15 231 7 4 42 3 1 115 0.28 2.3 82
Ru(5)/USY-15 197 0 0 35 0 0 104 0.27 4.8 -
USY-30 152 133 25 45 5 1 125 0.31 - 100
Ru(2.5)/USY-30 207 7 0 39 5 0 118 0.31 2.0 78 aDetermined by FTIR of adsorbed pyridine. bDetermined by the t-plot method. cDetermined by XRF. dDerived from XRD.
Ruthenium was deposited onto both zeolites to attain bifunctional catalysts in an amount
of 1.4, 2.3, and 4.8 wt.% for USY-15 and 2.0 wt.% for USY-30. The mesoporous
surface area and microporous volume of the aluminosilicates were substantially retained
upon metal incorporation (Table 1), likely due to the rather low loading, while XRD
analysis (not shown) indicated a slight decrease in crystallinity (Table 1). The absence
10
of reflections specific to Ru in the patterns suggested the presence of a well-dispersed,
nanostructured metal phase. The structural features of the catalysts were further
investigated by TEM (Figure 1). As expected, both protonic zeolites comprised crystals
featuring intraparticle mesoporosity. With respect to the Ru-modified samples, the
crystallinity of the zeolites appeared hardly modified and the supported metal phase was
detected in form of nanostructures with a broad size distribution. Small nanoparticles
(3-4 nm) were visualized along with much larger structures (50-60 nm). Only few of the
latter were found in Ru(1)/USY-15, whereas they were more abundant in the catalysts
with higher metal loadings. Based on these observations, the moderate decrease in
crystallinity upon metal deposition determined by XRD seems to be mainly related to
the presence of the secondary metal phase in addition to the aluminosilicate.
Figure 1a. TEM micrographs of protonic (P) and Ru-modified USY-15 zeolites.
11
Figure 1b. TEM micrographs of protonic (P) and Ru-modified USY-30 zeolites.
The total acidity of the Ru-containing samples was substantially modified compared to
the metal-free counterparts. In particular, the BAS of medium and high strength almost
vanished, while the amount of weak BAS moderately increased (Table 1). This evidence
is in line with previous studies for various zeolite supported metal catalysts, which
pointed that introduction of metals onto the zeolite support results in redistribution of
acid sites strength [33-35]. The origin of such changes was discussed in detail in the
previous work and can be attributed to interactions between the metal crystallites and
the support material as well as changes in the support properties during catalyst
preparation due to exposure of the zeolite to the metal precursor solution.
Interestingly, the change in acid properties was rather comparable regardless of the
ruthenium loading. The absence of strong acid sites is expected to suppress side
reactions such as sugar dehydration to HMF and furfural, thus favoring the selectivity to
polyols.
XPS analysis of the reduced catalysts indicated that the maximum of the Ru3d5/2 peak
is shifted to binding energy less than 280 eV when the charging (ca. 2.7 eV) is taken
into account meaning that oxidation state of ruthenium is close to 0. The peak
assignment was based on the values reported by Pedersen and Lunsford [36].
12
Non-catalytic hydrolysis and hydrolytic hydrogenation
The key role of acidity in hydrolysis is in donating a proton to the glycosidic bond
between the sugar units in the polysaccharide chain, enabling the liberation of the
monosaccharides [19]. Disadvantageously, acid species additionally catalyze sugar
dehydration to furfural and HMF under the hydrolysis conditions. This comprises a
competitive reaction to the further desired conversion of the sugars to polyols.
Acid solids are expected to influence the hydrolysis of AG in similar manner to how
homogeneous mineral acids drive the hydrolysis of (hemi)cellulose provided that the
molecules involved in the process can have access to the acid sites. It can be speculated
that the external surface acid sites will exclusively provide the Brønsted acidity required
for the hydrolysis of the macromolecule, since the latter cannot penetrate inside the
pores, but that the cleavage of short-chain depolymerization products could also occur
on acid sites situated in the channels.
However, as the pKw value of water decreases with increasing temperature, H3O+ ions
generated in situ can also homogenously contribute to the overall hydrolysis process and
to sugar dehydration.
Thus, prior to the utilization of parent and metal-modified zeolites, a non-catalytic
experiment was conducted for the title reaction at 458 K and a total pressure of 31 bar
(Figure 2) to serve as a reference.
13
0 50 100 150 200 250
0
10
20
30
40
50
60
70
80
90
100 AG nd humins
Oligomers
Galactose
Arabinose
Ethylene glycol
HMF
Furfural
Unknown
pro
du
ct d
istr
ibu
tio
n, %
time, min
Figure 2. Non-catalytic hydrolysis of AG at 458 K and 31 bar.
Evidently, even in the absence of any acid catalyst, a significant decrease in the
concentration of hemicellulose, visible formation of oligomers, arabinose, and galactose
monosaccharides as well as furans were detected.
The peculiar shape of the AG-humins curve derives from the fact that upon the course
of the reaction polymeric humins were formed while AG was consumed and the two
classes of species cannot be separated by HPLC analysis. Polymeric humins derive from
acid-catalyzed transformations of HMF [20-23] and aggregation of galactose when
arabinose is removed from the side chain of the hemicellulose [19].
It was thus interesting to analyze the molecular mass of the polymers by SEC. Since
SEC of water-soluble polymers is less straightforward than analysis of polymers in
organic media, due to interactions between the stationary phase and polar carbohydrates
[29], electrolyte solutions of sufficiently high ionic strength were used as the mobile
phase to prevent such secondary effects.
In polymer analysis, besides for the peak apex molecular weight Mp characterizing the
sample only in a single point, relevant parameters comprise the number
14
average molecular mass of polymers Mn (more sensitive to molecules of low molecular
mass) and the molecular weight average Mw (more sensitive to molecules of high
molecular mass), which are determined in the following way:
)(
)(
Mh
MMhM n
,
MMh
MMhMW
)(
)( 2
(1)
where h(M) is the slice height at a molecular weight M when the eluted peak is divided
into several equidistant volume slices. Another significant parameter is dispersity
(Mw/Mn), which gives an indication about the distribution in the polymer, approaching
unity when the polymer chain approaches a uniform chain.
The values of the molecular weight average Mw are displayed for the non-catalytic
hydrolysis experiment in Figure 2.
0 50 100 150 200 250
0
5
10
15
20
25
30
35
40
Mw
, kD
a
time, min
a)
Figure 3. (a) Molecular weight of polymers/oligomers in the reaction mixture as a
function of time. Experimental conditions for this and subsequent figures are the same
as for Figure 2.
15
0 50 100 150 200 250
0
2
4
6D
isp
ers
ity,
[-]
time, min
Figure 3. (b) Dispersity (Mw/Mn) of polymers/oligomers in the reaction mixture as a
function of time. Experimental conditions for this and subsequent figures are the same
as for Figure 2.
These data evidence a very clear disaggregation of AG with a relatively high molecular
mass (ca. 40 kDa) and a dispersity index of ca. 5 in the first 50-75 min of reaction
leading to a polymer of ca. 2.5 kDa molecular weight and ca. 1.5 dispersity. Thereafter,
there is a clear increase of the molecular weight and of the dispersity of the resulting
polymer in excellent agreement with HPLC data which display a sudden increase in the
concentration of the macromolecule and a decrease in that of the oligomers. The UV-vis
detector confirmed the presence of aromatic moieties in the newly formed compound,
which can thus be ascribed to a humin-type polymer. As already discussed in [30],
hydrolysis of AG is, moreover, associated with the removal of arabinose from the side
chain, relaxing the steric hindrance in the hemicellulose and allowing galactose
molecules to oligomerize resulting in aggregates. This process would inevitably lead to
an increase of dispersity. It should be noted that differentiation and quantification of
humins and other potential aggregates was not in the main focus of the work, therefore
it was not pursued further.
16
Hydrolysis over acidic zeolites
The results for hydrolysis of AG carried out over USY-15 and USY-30 are presented in
Figure 4 and demonstrate that, similarly to a non-catalytic experiment (Figure 4), AG
was hydrolyzed into oligomers and monomers.
a)
0 50 100 150 200 250
0
20
40
60
80
100
AG and humins
Oligomers
Galactose
Arabinose
Ethylene glycol
HMF
Furfural
Unknown
Pro
du
ct d
istr
ibu
tio
n, %
time, min
Figure 4a. Hydrolysis of AG over USY-15.
b)
0 50 100 150 200 250
0
20
40
60
80
100
AG and humins
Oligomers
Galactose
Arabinose
Ethylene glycol
HMF
Furfural
Unknown
Pro
du
ct d
istr
ibu
tio
n, %
time, min
Figure 4b. Hydrolysis of AG over USY-30.
17
Still, a higher conversion level was attained in the presence of both zeolites compared to
the blank run. Thus, application of solid acid catalysts is necessary to enhance the
efficiency of the hydrolysis.
An interesting observation besides an increase in the rates was the substantial
suppression of humins formation, since the peak of AG constantly decreased with time.
Such evidence with USY catalysts differs not only from the blank experiment but also
from the previous data on beta zeolites [19], over which humins were generated to a
large extent. At the moment it can be only speculated that the differences in the
behavior of beta compared to USY could be related to lower Lewis acidity and
moreover additional mesoporosity of the latter allowing easier release of sugars and