Catalytic role of metals supported on SBA-16 in hydrodeoxygenation of chemical compounds derived from biomass processing

Article abstract of DOI:10.1039/d0ra06696f

Hydrodeoxygenation (HDO) carried out at high temperatures and high hydrogen pressures is one of the alternative methods of upgrading pyrolytic oils from biomass, leading to high quality biofuels. To save energy, it is important to carry out catalytic proc

Full text of DOI:10.1039/d0ra06696f

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Catalytic role of metals supported on SBA-16 in
hydrodeoxygenation of chemical compounds
derived from biomass processing†
Cite this: RSC Adv., 2021, 11, 9505
a
a
b
Paulina Szczyglewska,
Agnieszka Feliczak-Guzik,
Mietek Jaroniec
a
*
and Izabela Nowak
Hydrodeoxygenation (HDO) carried out at high temperatures and high hydrogen pressures is one of the
alternative methods of upgrading pyrolytic oils from biomass, leading to high quality biofuels. To save
energy, it is important to carry out catalytic processes under the mildest possible experimental
conditions. The aim of our research was the synthesis of ordered mesoporous SBA-16 type silica
materials modified with transition metal atoms (Ir, Ru, Pd, Pt), their physicochemical characterization and
use as catalysts in hydrodeoxygenation of model chemicals (guaiacol, syringol, creosol). The HDO
process was carried out under mild experimental conditions at temperatures in the range from 90 to
130 ꢀC and hydrogen pressures in the range from 25 to 60 bar. The catalytic tests revealed differences in
the catalytic properties of the samples studied. The catalytic systems used assured highly efficient
transformations of the examined molecules as well as high selectivity towards chemical compounds with
lower O/C ratio and higher H/C ratio as compared to those in the initial substrates. High activity of the
catalysts containing precious metals in the experimental conditions applied suggests their potential to
improve bio-oil production for biofuels.
Received 3rd August 2020
Accepted 19th February 2021
DOI: 10.1039/d0ra06696f
rsc.li/rsc-advances
majority of cheap and abundant non-food materials available
from plants.3,5 Taking into account high bioavailability, low
Introduction
costs, low emission of harmful chemical compounds and
inedible character, the lignocellulosic biomass has a high
potential to be used for production of liquid fuels of the 2nd
generation.6,7 According to literature data, scientists worldwide
deal with this subject, which has resulted in a considerable
number of reviews and research works. In order to obtain bio-
energy from biomass of quality similar to that obtained from
fossil fuel combustion, it is necessary to transform it into
chemical compounds characterized by low molecular mass,
hydrophobic nature and anaerobic structure.8 Bioenergy can be
produced from lignocellulose biomass in various ways: by
physical, thermochemical or biochemical methods.9 Special
attention has been paid to pyrolysis, which is one of the tech-
nologies used for direct biomass liquefaction. In this process, as
a result of high temperature action without access to air,
macromolecular chemical compounds decompose to the
chemical compounds of lower molecular weight from which
solid, liquid or gas fuels can be obtained.10 However, the liquid
pyrolytic oil produced is acidic (signicant content of carboxylic
acids), viscous and chemically unstable due to high content of
oxygen functional groups (total oxygen content 35–50% by
weight) and water (15–30% by weight).11,12 Moreover, pyrolytic
oils derived from biomass do not mix with conventional
hydrocarbon fuels and are too reactive to be fractionated by
distillation. Thus, to make the pyrolysis process a viable
Development of the biofuel sector is a realistic alternative to the
gradually depleting sources of fossil fuels. Increasing number of
developed and developing countries consider biofuels to be of
the key importance for reduction of the dependence on oil,
greenhouse gas emissions and achievement of rural develop-
ment goals.1,2 Currently, the production of biofuels is limited to
the 1st generation biofuels, based on the well-established
technologies, whose actual exploitation has been improved.
The advantages of the 1st generation biofuels include, among
others, the possibility of using their blends with oil-derived
fuels in the existing internal combustion engines. In spite of
numerous advantages of the fuels in question, they arouse high
scepticism among scientists because they are obtained from
edible plants and thus compete with the food market.3,4
Therefore, it is expected that the 2nd generation biofuels will
become the dominant fuel products. Such fuels are produced
from plant biomass (i.e., agricultural residues, organic waste or
intentionally cultivated energy plantations) and use the
aAdam Mickiewicz University, Poznan, Faculty of Chemistry, Uniwersytetu
´
´
´
Poznanskiego 8, 61-614 Poznan, Poland. E-mail: nowakiza@amu.edu.pl
bKent State University, Department of Chemistry and Biochemistry, Kent, Ohio, 44242,
USA. E-mail: jaroniec@kent.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06696f
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technological platform for the production of hydrocarbon fuels chemical compounds subjected to the hydrodeoxygenation
from biomass the effective methods for improvement of pyro- process. Iridium also has catalytic activity in HDO reactions of
lytic oil through catalytic oxygen removal should rst be devel- bio-oil. Nevertheless, its activity is not as good as that of Pd, Pt
oped.13 One of the most effective methods used to improve or Ru and therefore, as already mentioned, the studies of Ir
oxygen compounds is catalytic hydrodeoxygenation (HDO), catalysts have been noticeably rarer.28 As mentioned above,
which increases the energy value of bio-oil by transforming palladium and platinum are widely used components of
potential energy carriers into simple hydrocarbons.14,15 This is heterogeneous catalysts; the application of ruthenium as
due to simultaneous deoxidation and hydrogenation of organic another element from the group of precious metals is denitely
compounds in the presence of hydrogen gas supplied to the less frequent. The application of ruthenium in hydrogenation
system under increased pressure and temperature.16,17 The key reactions or Fischer–Tropsch synthesis is best known.29 Never-
issue in the whole process is selection of an appropriate cata- theless, Ru-based catalysts are increasingly used in the
lytic system, capable of reducing the O/C ratio, while increasing production of biodiesel, which can also be deduced from the
the H/C ratio.18 The catalysts used so far are of heterogeneous data summarized in Table 1. Due to the higher affinity of Ru to
nature, consisting of a solid support and active substance organic compounds containing oxygen atoms than other
dispersed on it.19,20 Precious metals are known for their ability to precious metals, the selectivity of Ru-based catalysts to such
increase the efficiency of many advanced catalytic systems.21,22 compounds is relatively high.28 The popularity of precious
The solid supports with incorporated species of these metals metals used especially in catalytic processes results from the
catalyse the hydrogenation reaction during hydro- fact that the d orbitals of these metals are not completely lled,
deoxygenation, which also results in the removal of oxygen from while the support surface easily adsorbs reactive compounds
the bio-oil. It is commonly known that precious metals are and therefore, these metals exhibits high catalytic activity
expensive, which results in costly production of the improved towards hydrogenation at low temperatures.28 Many precious
fuels. However, it should be noted that they are recyclable, and metals are deposited on reducible oxides, such as CeO2, TiO2,
in addition, they are selective towards such chemicals as ZrO2 and on non-reducible media, such as zeolites, silica.26 The
cyclohexane, methoxycyclohexane or aliphatic hydrocarbons. solid supports with well-developed specic surface area and
Three metals, i.e., ruthenium, palladium, platinum, have porosity are particularly widely used because such porous
already been thoroughly tested in many catalytic reactions. networks allow for high dispersion of metallic catalysts and
Therefore, they are considered the most suitable metal catalysts additionally stabilize the whole catalyst.30
to optimally remove oxygen and thus improve the efficiency of
In this work we examined the catalytic activity of the systems
hydrocarbons production.23 The use of expensive noble metal containing precious metals (Ir, Ru, Pd, Pt) dispersed in meso-
catalysts in very small quantities is common in crude oil porous SBA-16 support towards hydrodeoxygenation of guaia-
rening in such processes as hydrocracking, catalytic reforming col, syringol, creosol – key chemical compounds present in
or isomerization as well as in the production of specialized pyrolytic oil. The aim of this work was to synthesize the above-
chemicals. Excellent catalytic efficiency of precious metals and mentioned metallic catalysts and to study their catalytic
available technologies of recycling of the used catalysts justify performance in the hydrodeoxygenation reaction under mild
their application in various industrial catalytic processes despite experimental conditions (temperature 90–130 ꢀC, hydrogen
higher cost in comparison to non-precious metals.24 Table 1 pressure 25–60 bar). The catalysts were comprehensively char-
presents the literature data for hydrodeoxygenation reactions of acterized by X-ray diffraction (XRD), low temperature nitrogen
selected model chemical compounds catalysed by various systems adsorption/desorption, transmission and scanning electron
containing precious metals, such as: Ir, Ru, Pd, Pt.
microscopy (TEM, SEM), scanning electron microscopy coupled
Palladium is a valuable metal used repeatedly in the hydro- with a mass detector (SEM/EDX). Inductively coupled plasma
deoxygenation of pyrolytic oil.23 Very oen it is a component of optical emission spectrometry (ICP-OES) and X-ray photoelec-
bifunctional and bimetallic catalysts. As can be seen in Table 1, tron spectroscopy (XPS) were also used. Special emphasis was
the palladium catalysts are particularly highly active in hydro- placed on the comparison of the effectiveness of the catalysts
genation of multiple bonds between carbon atoms. It has also with different active phases.
been demonstrated that these catalysts are much less active in
hydrogenation reactions of aromatic hydrocarbons and C]O
bonds, which is particularly true for aliphatic aldehydes and
ketones.20 In turn, platinum is well known as a highly active
Materials and methods
Chemicals
metal in hydrogenation reactions at much lower temperatures All the listed chemicals were of analytical purity and were used
than, for example, sulphide catalysts.25 Among many precious as purchased without further purication. The chemicals used
metals, Pt-based catalytic systems have been most frequently in the carrier synthesis were as follows: tetraethyl orthosilicate
studied, whereas the catalytic activity of systems based on e.g., Ir denoted as TEOS (Aldrich), copolymer of ethylene oxide and
have been rarely described.26 One of the reasons of high popu- propylene oxide known as Pluronic F127 (Sigma) and hydro-
larity of Pt catalysts is their good stability even under very chloric acid (POCH). The chemicals used to modify the carriers
demanding reaction conditions.27 Analysis of the data pre- were: hexachloroiridic acid (Alfa Aesar), ruthenium(III) chloride
sented in Table 1 shows that the presence of platinum in (Aldrich), palladium(II) chloride (Alfa Aesar), hexachloroplatinic
a catalytic system oen permits removal of oxygen atoms from acid (Alfa Aesar), ethanol (POCH). The chemicals used for
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Table 1 Summary of HDO reactions of model chemical compounds carried out using catalysts containing Pd, Ru, Pt or Ir
Operating
conditions
Catalyst
T [ꢀC]
p [bar]
Model compound
Phenol
Main reaction product
Selectivity [%]
Conversion [%]
Ref.
31
Pd/Nb2O5
200
400
100
220
200
200
90
1
Cyclohexanone
Benzene
p-Cresol
Methoxycyclohexanol
Cyclohexanol
Cyclohexanol
2,4-Dimethylhexane
Cyclohexanone
81
100
95
78
98
62
51
57
61
38
67
18
90
94
51
70
44
38
51
77
26
96
100
6
10
100
80
100
87
100
4
Pd/C
Pd/TiO2
Pd/C
30
30
50
50
25
45
Vanillin
Guaiacol
Phenol
Phenol
Phenol
Phenol
24
32
33
33
34
35
Pd/SiO2
Pd/SBA-16
Ru/SBA-15
Ru/TiO2
Ru/MCM-41
Ru/H-Beta
Ru/H-Beta
Ru/C
Ru/SBA-16
Ru/MCM-41
Pt/C
300
4
16
55
48
100
100
70
100
70
64
100
100
5
140
100
100
130
130
100
285
250
400
90
40
Diphenyl ether
Cyclohexane
Cyclohexanol
36
30
60
60
30
40
40
1
25
6
30
60
Vanillin
Phenol
Anisole
Vanillin
Guaiacol
Guaiacol
Anisole
Phenol
Stearic acid
Phenol
Vanillin alcohol
2,4-Dimethylhexane
1-Methoxycyclohexene
Vanillin alcohol
Cyclohexane
Cyclohexane
Benzene
2,4-Dimethylhexane
1-Heptadecene
Cyclohexane
24
34
37
24
38
39
40
34
41
26
42
Pt/TiO2
Pt/Al-MCM-48
Pt/H-Beta
Pt/SBA-16
Ir/SiO2
Ir/ZSM-5
Ir/SiO2
300
200
50
32
60
Hydroxymethyl-furfural
2,5-Bis(hydroxymethyl)furan
reactivity tests were: syringol (Aldrich), guaiacol (SAFC), creosol was poured into the weighed portion of the SBA-16 support. The
(Sigma-Aldrich), decaline (Honeywell). The gases used were: Ar mixtures prepared in this way were tightly wrapped with
(Purity N5.0, Linde Gas Poland), H2 (Purity N5.0, Linde Gas polyolen-paraffin foil Paralm® and le for 24 h. Aer that
Poland).
time, the beaker with impregnated support was placed under
the extractor (room temperature) until complete evaporation of
ꢀ
ethanol. Then the obtained catalyst was dried for 1 h at 30 C,
Synthesis of SBA-16 material
ꢀ
ꢀ
1 h at 40 C, 18 h at 60 C. The catalysts obtained in this way
were reduced in the tube furnace in hydrogen atmosphere. The
rst stage of this process was to pass argon (50 cm3 minꢁ1, 0.5
h) at room temperature through the catalyst to remove air from
the system. Then the stream of inert gas was switched to
hydrogen (50 cm3 minꢁ1, 0.5 h) and the furnace heating was
switched on. The catalysts were reduced for 3 h at temperature:
400 ꢀC (iridium catalyst), 250 ꢀC (ruthenium catalyst), 350 ꢀC
(palladium catalyst), 250 ꢀC (platinum catalyst). The optimal
temperature of metal precursor reduction was selected based on
H2-TPR tests.
SBA-16 was synthesized using Pluronic F127 (EO106PO70EO106
)
block copolymer as a structure directing agent and TEOS as
a silicon source. In typical synthesis 4.8 g of Pluronic F127 were
dissolved in 46.6 g of deionized water and 192.0 g of HCl (2 M).
The prepared mixture was stirred at room temperature using
a magnetic stirrer until complete dissolution of polymer (about
1 h). Then 20.2 g of TEOS were added in small portions and the
whole mixture was stirred for 0.5 h without temperature change.
Aer drying in a laboratory dryer (48 h, 90 C), the solids were
drained and then washed several times with deionised water.
Aer drying at room temperature overnight, the products were
calcined for 6 h at 550 C. The resulting sample is denoted as
ꢀ
ꢀ
SBA-16/K.
Characterization of catalysts
X-ray diffraction data were collected at low angles to obtain
information about structural properties of the synthesized
Modication of SBA-16 material
The synthesized silica materials were subjected to modication materials, i.e., the degree of structural ordering. The XRD
by wetting impregnation to obtain samples with a metal content method was also used to determine the distance between
of 3% wt. All metal precursors, i.e., iridium (H2IrCl6$H2O), parallel crystal lattice planes (d100) and the distances between
ruthenium (RuCl3$H2O), palladium (PdCl2) and platinum (H2- mesopore centres (a0). The elementary cell parameters were
PtCl6$6H2O) were dissolved in appropriate amount of ethyl determined from the formula: a0 ¼ dO2. Low-angle X-ray scat-
alcohol and the solutions were le in ultrasonic cleaner for 1 h. tering measurements were performed using a Bruker AXS D8
Then the ethanol solution of the appropriate metal precursor Advance having Johannson monochromator with CuKa
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radiation, which generated the wavelength of l ¼ 0.154 nm. All liquid reaction products from the solid catalyst. Aer the
measurements were made with a step 0.02ꢀ in the range 2q ¼ centrifugation process, the reaction mixtures were transferred
0.6–8ꢀ. Nitrogen adsorption/desorption isotherms and pore size to 1.5 ml vials and subjected to chromatographic analysis.
ꢀ
distributions (PSD) were determined at ꢁ196 C with a Quan-
tachrome Nova 1200e analyser. All samples were degassed for 24
hours at 90 ꢀC before the actual measurements. The specic
surface area of the samples studied was determined using the
BET method and the PSD functions were obtained using the
BJH algorithm. Microscopic images of the materials were taken
using a transmission electron microscope JEOL JEM-1200 EX II
at 80 kV. This type of microscopy was used to determine the
degree of ordering of mesopores and their distribution. The
TEM image, aer setting the scale and threshold for the whole
image, were analyzed using “Analyze Particles” option to
generate a table with data of the particle areas and diameters.
These data were exported into Origin program to construct the
histogram plots. The samples were also analysed using a Zeiss
scanning electron microscope type EVO at 17 kV. The imaging
data were used to evaluate the surface texture and morphology
of all synthesized catalysts. Scanning electron microscopy with
X-ray energy dispersion option was used to assess the structure
of ordered silica modied with transition metal atoms. For this
purpose, the LEO 1430 VP microscope (SEM measurement) and
Quantax 200 with XFlash 4010 detector by Bruker AXS (EDX)
were used. The content of transition metal was determined
using inductively coupled plasma optical emission spectrom-
etry with ICP-OES VISTA-MPX spectrometer by Varian. Prior to
measurement the samples were mineralized in aqua regia in
a microwave oven. Spectroscopic measurements of X-ray excited
photoelectrons were performed using the SPECS UHV analysis
system with AlKa radiation (E ¼ 1.487 keV). All samples were
neutralized using a low energy electron gun. The measurements
permitted determination of the total content of metals in the
analysed samples and their oxidation degree.
Chromatographic analysis
The reaction mixtures were analysed using two chromato-
graphs: a VARIAN 3900 GC gas chromatograph equipped with
automatic sample dispenser, ame ionization detector, 25
meter capillary column CPWAX57CB with internal diameter of
0.32 mm and 1.2 mm lm and a gas chromatograph coupled to
a VARIAN 4000 GC mass spectrometer equipped with an auto-
matic sample dispenser, mass sensor and a similar capillary
column (CPWAX57CB). The substrates and reaction products
were identied in two ways by using: the retention times of
commercially available standards and the NIST chemicals
library data.
Study of catalyst reuse
The procedure of regeneration of the SBA-16/3%Pd catalyst,
previously used in the HDO process of guaiacol, was carried out
for 4 hours at 130 ꢀC under 60 bar hydrogen pressure. The
catalyst was initially ltered through a high graded lter (to
separate the catalyst from the liquid reaction products). Then
the catalyst, which remained on the lter, was washed with
ethanol, and nally dried in a laboratory dryer. The regenerated
catalyst was reused in the guaiacol hydrodeoxygenation process
under identical conditions. The same catalyst was used four
times because the amount of the regenerated catalyst was
sufficient to perform four tests only. The amount of guaiacol
used in the reaction was adjusted in proportion to the amount
of the remaining catalyst.
Results and discussion
X-ray diffraction studies
Hydrodeoxygenation reaction
Diffractograms of SBA-16 materials modied with transition
A high-pressure reactor (CAT 24 HEL) placed on a magnetic metal species and SBA-16/K material are shown in Fig. S1 (see
stirrer was used to carry out the hydrodeoxygenation reaction of ESI†). The XRD proles are very similar and characteristic for
the model substrates. The reactor was equipped with a ther- mesoporous SBA-16 structure (Im3m symmetry) with regularly
mocouple to control the process temperature and a manometer arranged pores. Most oen diffractograms of SBA-16 show the
to determine the initial reaction pressure. A catalyst (0.05 g) and presence of three reections with indices (110), (200) and
guaiacol (1 g) or creosol (1 g) were placed in a dedicated glass (210).43–45 The diffractograms of the materials synthesized by us
reaction vessel with a magnetic stirrer inside. As syringol was in show a strong peak at a low angle 2q ¼ 0.82–0.88ꢀ. Its position
solid state it was necessary to use a solvent, so a catalyst (0.005 corresponds to the peak observed for multi-walled particles and
g), syringol (0.05 g) and decaline (1 g) were placed in the vessel. indexed by the plane (110). Only in the material modied with
The reaction mixtures prepared in this way were placed in iridium, the intensity of the main reection was lower than for
a high-pressure reactor, which was then closed and rinsed three the other materials, which indicates that this sample is poorly
times with argon to remove air from inside the reactor. In the ordered. A weak intensity of higher-order reections (which for
next step, the reactor was ushed three times with hydrogen SBA-16/Ir silica are completely undetectable) may be a conse-
and nally lled with hydrogen to 25 bar, 40 bar or 60 bar quence of the high ratio of the wall thickness to pore size or the
pressure. The reactions were carried out at different tempera- roughness of the walls due to the large number of micropores in
tures, i.e., 90 ꢀC, 110 ꢀC or 130 ꢀC for 4 h and the reaction time the structure of the samples studied.46
was measured from the moment of achieving the desired
In addition to the main reection position, Table 2
temperature (stirring at 700 rpm). Aer 4 h the reactor was summarises the parameters of the elementary cell. The values of
cooled, the accumulated gas was released and the reaction the 2q angle of all silicas corresponding to the rst order
mixtures were centrifuged (Micro Star 17, VWR) to separate the reection are similar, which implies similar distances between
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Table 2 Positions of the main reflections, distances between the Table 3 Parameters of the porous structure of mesoporous SBA-16
parallel lattice planes (d100) and distances between the centres of silica samples impregnated with precious metal species and unmod-
mesopores (a0) in SBA-16 impregnated with transition metal atoms ified SBA-16 material
and SBA-16/K material
Surface area
Pore size
[nm]
Total pore volume
Catalyst
2q [ꢀ]
d100 [nm]
a0 [nm] Catalyst
[m2 gꢁ1
]
[cm3 gꢁ1
]
SBA-16/K
0.88
0.88
0.82
0.86
0.86
10.01
10.01
10.77
10.25
10.25
14.16
14.16
15.23
14.50
14.50
SBA-16/K
796
849
769
743
705
6.33
5.46
6.34
6.36
6.32
0.59
0.58
0.58
0.55
0.54
SBA-16/3%Ir
SBA-16/3%Ru
SBA-16/3%Pd
SBA-16/3%Pt
SBA-16/3%Ir
SBA-16/3%Ru
SBA-16/3%Pd
SBA-16/3%Pt
the parallel lattice planes (10.01–10.77 nm) and similar 0.59 cm3 gꢁ1. In the materials tested the mesopore diameters
distances between the centres of mesopores (14.16–15.23 nm). corresponding to the maximum pore distribution functions are
similar and are about 6.3 nm for all the samples except SBA-16/
3%Ir, for which this dimension is about 5.5 nm. Also, all PSD
curves in Fig. 2 show that the most of mesopores feature
diameters in the range of 5–8 nm.
Low-temperature nitrogen sorption studies
The nitrogen adsorption/desorption isotherms for ordered
mesoporous SBA-16 type silica materials modied with transi-
tion metal atoms and unmodied SBA-16 material are pre-
sented in Fig. 1. The shape of all isotherms is type IVa according
to IUPAC classication,47 typical for SBA-16 samples. The
observed sharp step at the relative pressure range p/p0 ¼ 0.4–0.7
corresponds to capillary condensation occurring in uniform
mesopores.43 The shape of the hysteresis loop indicates that the
silicates tested contain pores with narrow openings and wide
interiors (type H2 according to IUPAC classication).47,48 It can
be also concluded that the modication of silica with metal
atoms has no effect on the deterioration of the structure, which
is indicated by the similarity of the isotherms of the unmodied
and all modied materials.
The specic surface area, pore size and pore volume values
estimated from the adsorption isotherm data for all synthesized
samples are collected in Table 3. All samples have a large
specic surface area in the range 705–849 m2 gꢁ1. The total pore
volume of the synthesized silicas is in the range from 0.54 to
Transmission electron microscopy studies
Images from transmission electron microscope show that the
synthesized materials are characterized by high structural
ordering (Fig. S2, see ESI†). The TEM images are in agreement
with the XRD analysis showing diffractograms of the silicas with
characteristic reections for ordered mesostructures. Addi-
tionally, dark spots visible on the images of the samples with
metal species conrm the presence of precious metals intro-
duced by impregnation method. The metal nanoparticles have
approximately spherical shapes and exhibit relatively narrow
and monomodal size distribution histograms as shown in ESI
(Fig. S2†). These histograms were obtained by analyzing images
with a few hundreds of individual nanoparticles. The presented
distribution histograms of metallic nanoparticles were used to
compute the mean particle size for each catalyst: Ir (3.52 ꢂ 1.64
Fig. 2 Pore size distribution (obtained by BJH method using the
Fig. 1 N2 adsorption/desorption isotherms measured for the SBA-16 adsorption branch data) for the SBA-16 materials modified with tran-
materials modified with precious metal atoms and unmodified SBA-16 sition metal atoms and unmodified SBA-16 material. Each PSD curve
material. Each isotherm curve has been shifted successively by 100 has been shifted successively by a fixed value of 0.1 cm3 gꢁ1 nmꢁ1 from
cm3
g
ꢁ1 from the previous one.
the previous one.
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nm) y Ru (3.82 ꢂ 1.45 nm) z Pd (4.63 ꢂ 1.55 nm) # Pt (8.02 ꢂ platinum modied silica samples show even distribution of
2.14 nm). Only histogram for platinum nanoparticles exhibits crystallites. The average palladium content in the examined
a broad distribution of the nanoparticle sizes, however low sample is 2.80%, while the platinum content is 4.30%. These
standard deviation can suggest that these nanoparticles are well results are not far from the assumed values. Moreover, similarly
dispersed in the SBA-16 silica.
as for the samples containing iridium, no larger clusters of both
metals are visible.
Scanning electron microscopy imaging
The observations made with the scanning electron microscope Study of chemical composition by inductively coupled plasma
show, as expected, spherical nanoparticles (Fig. S3, see ESI†). All optical emission spectrometry
SEM images show a very similar morphology of mesostructured
Table 4 presents a comparison of the assumed percentage of the
materials. Moreover, the materials studied are composed of
introduced metal with its actual content in the sample. The
particles of very similar sizes.
results show that the wetting impregnation method used for
modication of silicates was successful and allowed for intro-
duction of metal species in the quantities similar to those
assumed.
Study of metal distribution by scanning electron microscopy
coupled with X-ray energy dispersion
Table S1 (see ESI†) and Fig. 3 present the results of scanning
electron microscopy with X-ray energy dispersion. The image
obtained for the sample of mesoporous silica modied with
X-ray photoelectron spectroscopy
XPS analysis was used to determine the presence and stoi-
iridium (Fig. 3a) clearly conrms the presence of metal on the
chiometric ratios of elements in subsurface layers of catalysts.
surface. The results of this analysis also indicate an even
By comparing these results with the results of volumetric
distribution of the introduced metal species. The average
analysis by ICP-OES (Table 4), one can comment on the metal
iridium content in the sample is 5.34% – this value is higher
incorporation into the SBA-16 mesostructure. If the percentage
than the assumed value (3%). No larger metal clusters are
of metal in the surface layer is higher than the average value for
observed on the mesoporous matrix surface. Fig. 3b shows an
the entire sample volume, then we can conclude that the pore
image of the mesoporous silica impregnated with ruthenium.
penetration of metal clusters is negligible. The high specic
This image conrms that the process of impregnation of silica
surface area of the catalysts, resulting from its porosity, does
has been successful. The formation of ruthenium aggregates is
not play a signicant role then, and such a catalyst will work in
a similar way as a bulk material (metal nanopowder). However,
observed on the surface of SBA-16 (point 2 marked on the
image), which is reected by the average percentage of ruthe-
if the content of a given metal in the surface layer is the same or
nium (about 12.2%) present on the surface of the examined
less than the average value for the entire sample volume, then
fragment of the sample. This value is four times higher than the
we can conclude that the metal penetrated the pores effectively.
assumed one. However, it should be noted that the values ob-
Assuming a good dispersion of metal in the volume of samples,
tained by the EDX method based on the SEM images represent
the porosity of such material will have a signicant impact on
local measurements and do not always reect the situation for
the catalytic function. The latter case was observed if one
the entire sample. However, larger metal concentrations on the
compares data in Tables 4 and 5.
Table 5 presents the results of XPS analysis of silica samples
modied with transition metal ions. The percentage content of
surface may have a catalytic effect. The subsequent photographs
marked on Fig. 3c and d reect structures of the silica modied
with palladium and platinum, respectively. Both palladium and
metals in the surface layer and their oxidation state was deter-
mined. For the catalysts modied with Ru, Pd, Pt, the minimum
metal content in the surface layer was shown, which proves the
effective penetration of metals into pores of the SBA-16 silica.
Only in the case of the sample containing Ir, the measured
metal content was higher than in the other samples and
amounted to nearly 0.5%. Analysis of the data collected in this
table, clearly shows that all metal species have been reduced to
Table 4 The nominal and actual loading of SBA-16 supports with
transition metal atoms
Measured metal
Catalyst
Assumed metal content [%]
content [%]
SBA-16/3%Ir
SBA-16/3%Ru
SBA-16/3%Pd
SBA-16/3%Pt
3
3
3
3
2.81
2.33
2.72
2.72
Fig. 3 Elemental maps showing the distribution of metal, silicon and
oxygen on the SBA-16 support obtained with the use of EDX detector:
(a) SBA-16/3%Ir, (b) SBA-16/3%Ru, (c) SBA-16/3%Pd, (d) SBA-16/3%Pt.
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Table 5 XPS results for SBA-16 silicas modified with transition metal atoms
Metal content [wt%]
Binding energy
Catalyst
Assumed
Measured
(reference for metal) [eV]
Metal oxidation state
SBA-16/3%Ir
SBA-16/3%Ru
SBA-16/3%Pd
SBA-16/3%Pt
3
3
3
3
0.48
0.13
0.18
<0.10
59.8 (60.9 Ir4f7/2
278.3 (280.2 Ru3d5/2
334.9 (335.0 Pd3d5/2
70.4 (71.0 Pt4f7/2)
)
0
0
0
0
)
)
zero oxidation state, which conrms that the applied reduction the presence of methoxycyclohex-1-ene (formed in the highest
temperature for the respective metal precursors has been well amounts), 1,2-cyclohexanediol and 2-methylcyclohexanol
ꢀ
ꢀ
chosen. The analysis of the binding energy (BE) shows that all (Table S2†). For processes carried out at 90 C and 110 C, the
core levels of metals are not shied to higher BE in comparison conversion of guaiacol did not exceed 30%. In turn, when the
ꢀ
to the reference data. Thus, no metal oxides are present in the reaction temperature was 130 C, 100% conversion of guaiacol
catalysts.
took place. At 90 ꢀC, the formation of methoxycyclohex-1-ene
prevails, whereas the selectivity towards 2-methylcyclohexanol
and 1,2-cyclohexanediol increase with increasing temperature.
The results presented in Fig. 4c show that the palladium
catalyst deposited on SBA-16 exhibits a high activity in the HDO
process of guaiacol. With increasing conversion of guaiacol, the
selectivity to methoxyclohex-1-ene decreases and that to 1,2-
cyclohexanediol increases (Table S2†). At 100% conversion of
the model chemical, the selectivity to methoxyclohex-1-ene is
49–72% and to 1,2-cyclohexanediol 22–40%. Analysis of the data
indicates that the values of hydrogen temperature and pressure
had a signicant inuence on the process. No 2-methyl-
cyclohexanol was found in the post-reaction mixture when
palladium modied catalysts were used.
Analysis of the data presented in Fig. 4d shows that the
platinum catalyst deposited on SBA-16 exhibits a moderate
activity in the HDO process of guaiacol. In each case, the highest
selectivity is observed towards methoxycyclohex-1-ene (Table
S2†). Higher conversion of guaiacol is observed for the reactions
carried out at 90 ꢀC, at which the conversion of guaiacol
increases with increasing hydrogen pressure. The maximum
observed conversion was 46%.
The reported data for the HDO process of guaiacol show the
highest selectivity to cyclohexane or cyclohexanol, however the
temperature and pressure values used were much higher than
those applied by us. Deutsch et al.49 in their HDO reaction of
guaiacol on copper chromite (225–275 ꢀC, hydrogen pressure 50
bar) reported methoxycyclohexane as one of the products. Its
presence in the post-reaction mixture was explained by the
probable occurrence of the alkylation process, which involved
the addition of methyl group to the hydroxyl group with
generation of 1,2-dimethoxybenzene. Then, anisole was formed
due to demethoxylation, which was subsequently hydrogenated
to methoxycyclohexane. In our study the highest selectivity is
observed for methoxyclohex-1-ene – a chemical compound with
a double bond. Such a reasoning may explain the presence of
this compound in the post-reaction mixtures.
Results for the HDO reactions studied
Guaiacol
The products formed during hydrodeoxygenation of guaiacol on
the SBA-16 catalysts containing precious metals are anisole,
methoxyclohex-1-ene, methoxycyclohexane, 1,2-cyclohexanediol
and 2-methylcyclohexanol (Scheme 1).
The graph presented in Fig. 4a shows that the SBA-16 silica
modied with iridium has no catalytic activity in the HDO
guaiacol process. The catalyst did not show any improvement in
the activity even under changed reaction conditions. In all
cases, the conversion of guaiacol was less than 1% and the
highest selectivity was obtained to methoxyclohex-1-ene and
1,2-cyclohexanediol (Table S2, see ESI†). Because of such low
conversion, the results are not discussed as they are subject to
a very high measurement error.
Fig. 4b presents the results obtained for a series of HDO
reactions for guaiacol on the ruthenium modied SBA-16. The
catalytic tests performed show that the reaction leads to the
formation of three products. The GC and GC-MS analysis shows
Syringol
GC and GC-MS chromatography of the products of hydro-
deoxygenation reactions for syringol reveal the presence of four
Scheme 1 Products of the hydrodeoxygenation reaction of guaiacol:
(1) anisole, (2) methoxycyclohex-1-ene, (3) methoxycyclohexane, (4)
1,2-cyclohexanediol, and (5) 2-methylcyclohexanol.
products:
anisole,
1,2,3-trimethoxycyclohexane,
1,2-
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cyclohexanediol and cyclohexanol, whose structures are pre-
sented in Scheme 2.
The iridium catalysts turned out to be completely ineffective
in the process, which was concluded from the results presented
in Fig. 5a. Only for the reaction at 130 ꢀC at 60 bar, an increase
in the conversion to 5% was observed, which still is an unsat-
isfactory result. Among the reaction products formed, only two
chemicals were found: cyclohexanol and 1,2,3-trimethox-
ycyclohexane (Table S3, see ESI†). However, since the selectivity
to the reaction products was much awed due to low conver-
sions, their comparison would be doubtful.
The mesoporous silica material with ruthenium as the active
phase shows the highest activity among all investigated cata-
lysts in the process of syringol hydrodeoxygenation, which is
illustrated in Fig. 5b. These catalysts were characterized by
noticeably higher activity already at 110 ꢀC, while at 130 ꢀC
almost 100% conversion of syringol was achieved. The product
formed in the highest amounts in all the reactions is 1,2,3-tri-
methoxycyclohexane (Table S3†). Note that with increasing
conversion of syringol, the amounts of anisole formed are
higher at the expense of decreasing amounts of 1,2,3-trime-
thoxycyclohexane. Although anisole features an unsaturated
bond system, it is a more desirable reaction product than 1,2,3-
trimethoxycyclohexane because of a lower content of oxygen
atoms in the structure. Analysis of the results shows that the
amount of chemical compounds belonging to the group of
alcohols increases with increasing pressure and temperature.
Thus, the increase in temperature and pressure, apart from
higher syringol conversion, also causes a total decrease in the O/
C ratio of the post-reaction mixture, which is the desired effect.
The catalysts with palladium as the active phase exhibit
a slightly worse activity than ruthenium catalysts in the HDO
reaction of syringol as shown by the results in Fig. 5c. The
highest syringol conversion of 48% is achieved for the process
carried out at 130 ꢀC and 60 bar. It was found that the selectivity
Fig. 4 Guaiacol conversion in the HDO process at temperatures in the
range of 90–130 ꢀC and hydrogen pressure of 25–60 bar in the
presence of catalysts modified with precious metals: (a) SBA-16/3%Ir,
(b) SBA-16/3%Ru, (c) SBA-16/3%Pd, (d) SBA-16/3%Pt.
Scheme 2 The products of the hydrodeoxygenation reaction for
syringol: (1) anisole, (2) 1,2,3-trimethoxycyclohexane, (3) 1,2-cyclo-
hexanediol, and (4) cyclohexanol.
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reactions catalysed by the ruthenium-containing systems (Table
S3†).
When a platinum catalyst is used, higher selectivity to ani-
sole is achieved than that in the reactions catalysed by the
systems containing Ir, Ru or Pd (Table S3†). Nevertheless, this
catalyst is not very active in the HDO process of syringol, which
is indicated by low degree of transformation reaching the
maximum conversion of syringol of 5% only (Fig. 5d).
Under the applied reaction conditions, it was impossible to
obtain a fully hydrodeoxygenated chemical compound – all the
resulting molecules had oxygen atoms in their structure. The
molecule of 1,2,3-trimethoxycyclohexane has the same number
of oxygen atoms in its structure as that of the initial compound,
but it has saturated bonds. In turn, 1,2-cyclohexanediol has one
oxygen atom less than syringol and has a higher ratio of H/C
atoms. The remaining compounds, i.e., anisole and cyclo-
hexanol, have as much as 2 oxygen atoms less in the structure,
which indicates a signicant hydrodeoxygenation. To sum up,
the total oxygen content because of the reactions carried out
decreases while the ratio of H/C atoms increases. The chemical
compound formed in the greatest amounts in the processes
carried out in this study is 1,2,3-trimethoxycyclohexane is
probably formed as a result of the methyl group attachment to
the oxygen atom of the hydroxyl group and then the process of
hydrogenation of the aromatic ring. In the available literature
on the HDO reaction of syringol, we could not nd a report on
the reaction, in which 1,2,3-trimethoxycyclohexane is formed.
Only Shu et al.50 reported for hydrodeoxygenation of syringol
catalysed by Ni/Al2O3 (200 ꢀC, 20 bar) formation of 1,2,3-tri-
methoxybenzene – an aromatic compound, as one of the
products. The presence of this molecule in the post-reaction
mixture was explained by the occurrence of side reactions,
which were inevitable because of low activation energy of the
main process.
Creosol
For the HDO reaction of creosol, ve chemicals were identied:
1,2-cyclohexanediol,
1,2-dimethoxycyclohexane,
4-methyl-
cyclohexanol, methoxycyclohexane and anisole (Scheme 3).
The graphs presented in Fig. 6a show the results of the HDO
reaction of creosol in the presence of mesoporous SBA-16 silica
modied with Ir. The iridium catalyst did not show any
improvement in the activity under changing reaction condi-
tions. The conversion of creosol ranged from 1 to 5%. In the
presence of all catalytic systems studied three reaction products
were identied: 4-methylcyclohexanol, 1,2-cyclohexanediol and
anisole (Table S4, see ESI†). No formation of 1,2-dimethox-
ycyclohexane was observed. Due to the low degree of trans-
formation of the model chemical compound, the selectivity
values are charged with high error and are not discussed.
The results of the HDO reaction of creosol catalysed by
ruthenium-containing SBA-16 are shown in Fig. 6b. The catalyst
used was very inactive during the processes carried out at 90 ꢀC
– the conversion of creosol did not exceed 3%. The activity of
ruthenium-modied catalyst was recorded only at higher
temperatures, where the maximum 56% conversion of creosol
Fig. 5 Syringol conversion in the HDO process at temperatures in the
range of 90–130 ꢀC and hydrogen pressure of 25–60 bar in the
presence of catalysts modified with precious metals: (a) SBA-16/3%Ir,
(b) SBA-16/3%Ru, (c) SBA-16/3%Pd, (d) SBA-16/3%Pt.
of reaction products did not depend strictly on the applied
reaction conditions; no signicant differences in the selectivity
are observed. The distribution of the products is like that for the
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Scheme 3 Products of hydrodeoxygenation reaction of creosol: (1)
1,2-cyclohexanediol, (2) 1,2-dimethoxycyclohexane, (3) 4-methyl-
cyclohexanol, (4) methoxycyclohexane and (5) anisole.
was achieved. In all the experiments the highest selectivity was
obtained for 4-methylcyclohexanol and then for 1,2-cyclo-
hexanediol (Table S4†). Note that with increasing conversion of
creosol, the selectivity of catalysts towards 1,2-dimethox-
ꢀ
ycyclohexane decreases. For the process carried out at 130 C,
this compound was not formed in the amount greater than 2%.
Based on the results for the HDO reaction of creosol, cata-
lysed by the systems containing palladium atoms (Fig. 6c),
several conclusions can be drawn. Firstly, the conversion of
creosol increases with increasing temperature and hydrogen
pressure. Thanks to the catalyst used, the creosol molecule can
be completely transformed in the process carried out at 110 ꢀC
and 60 bar hydrogen pressure and for the process carried out at
130 ꢀC. Secondly, with decreasing conversion of creosol, the
amounts of 1,2-dimethoxycyclohexane and 4-methylcyclohex-
anol increase (Table S4†). On the other hand, as the conversion
of creosol increases, the selectivity to 4-methylcyclohexanol
increases and that of 1,2-dimethoxycyclohexane decreases in
favour of higher amounts of 1,2-cyclohexanediol. Higher values
of pressure and temperature favour demethylation processes.
Results for the hydrodeoxygenation reaction of creosol on
platinum catalysts are presented in Fig. 6d. As can be seen on
this gure, platinum catalyst shows medium activity in the
process. The conversion of creosol increases with increasing
temperature and increasing hydrogen pressure. It reaches the
value of 31% and is the highest among all conducted processes.
The reaction product for which the highest selectivity is ob-
tained is 4-methylcyclohexanol (Table S4†). Its amount
increases with higher conversion degree. Note that the amount
of 1,2-dimethoxycyclohexane decreases with increasing
conversion of creosol. The main product of the HDO reaction of
creosol in all conducted experiments is 4-methylcyclohexanol.
The latter compound is probably formed as a result of the cre-
osol demethoxylation and then hydrogenation of the aromatic
ring. There are very few literature reports on HDO of creosol.
Fig. 6 Creosol conversion in the HDO process at temperatures in the
range of 90–130 ꢀC and hydrogen pressure of 25–60 bar in the
presence of catalysts modified with precious metals: (a) SBA-16/3%Ir,
(b) SBA-16/3%Ru, (c) SBA-16/3%Pd, (d) SBA-16/3%Pt.
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introduced precious metals on the surface allow for the
following conclusions.
(i) The synthesized materials are characterized by good
structural and textural parameters, which is conrmed by XRD,
TEM, SEM, SEM/EDX, low-temperature sorption N2, ICP-OES
and XPS studies.
(ii) Among the HDO reactions of guaiacol studied in the
presence of SBA-16 silica impregnated with precious metals, the
highest selectivity was obtained towards methoxycyclohex-1-ene
(molecule with higher degree of hydrogenation and deoxygen-
ation than guaiacol).
The formation of 1,2-cyclohexanediol
– a chemical
compound with higher H/C ratio than that in guaiacol – was
also observed in signicant amounts. The inuence of the type
of active phase deposited on SBA-16 support on the reaction
efficiency is reected by the following order: Pd > Ru > Pt > Ir.
(iii) The model molecule, which is syringol, in the HDO
process catalysed by SBA-16 type silica modied with precious
Fig. 7 Reaction cycle using regenerated SBA-16/3%Pd catalyst.
Nevertheless, Deutsch et al.49 carried out the HDO process of
creosol using copper chromite as a catalyst at 225–275 ꢀC and 50
bar hydrogen pressure and also they obtained 4-methyl-
cyclohexanol as the main product of the reaction. In turn,
Jongerius et al.51 reported for the HDO reaction of creosol on
metals is transformed to 1,2,3-trimethoxycyclohexane
–
a chemical compound with higher H/C ratio than that in the
substrate. The formation of anisole in larger amounts is also
observed, especially in the presence of the catalysts containing
Ru and Pt. The inuence of the active phase type deposited on
SBA-16 on the reaction efficiency is reected by the following
order: Ru > Pd > Pt > Ir.
ꢀ
CoMo/Al2O3 (300 C and 50 bar) the formation of p-cresol, i.e.,
the equivalent of 4-methylcyclohexanol with aromatic character
as the main product of the reaction.
Study of catalyst reuse
(iv) In the HDO reaction of creosol catalysed by SBA-16 type
silica modied with precious metals the highest selectivity was
observed for 4-methylcyclohexanol. Moreover, in the presence
of silica modied with Pd ions a signicant amount of 1,2-
dimethoxycyclohexane was obtained, whereas in the presence
of silica containing Ru species, the dominant product was 1,2-
cyclohexanediol. The inuence of the type of active phase
deposited on SBA-16 on the reaction efficiency is reected by the
following order: Pd > Ru > Pt > Ir.
(v) The catalytic system, which showed the worst perfor-
mance in all reactions is the silica modied with iridium. The
reason for this is probably its less developed porous structure
than those of the remaining catalysts, which is reected by the
XRD results. Diffractograms of the remaining catalytic systems
show smaller crystallites and better metal dispersion, which
translates directly into better catalytic performance. The XPS
analysis also proved that iridium species are not impregnated
effectively as in the case of the remaining metals (a large
number of iridium atoms are identied in the surface layer,
which indicates their weaker penetration into the pores). In the
HDO reaction of guaiacol and creosol, the activity of the cata-
lysts studied is as follows: Pd > Ru > Pt > Ir, whereas in the HDO
reaction of syringol the ruthenium catalysts are the best (Ru >
Pd > Pt > Ir). It is likely that the increase in the ruthenium
activity is due to the fact that the catalyst consists of silica
modied with this metal agglomerates (as shown in SEM/EDX
images). Most likely, the ruthenium catalyst used for the HDO
reaction of syringol showed much higher content of ruthenium
than the metal content in other catalytic systems tested for the
In conclusion, aer the analysis of the data presented in Fig. 7,
the SBA-16/3%Pd catalyst used four times in the HDO reaction
of guaiacol shows similar activity. The conversion of guaiacol
was 100% each time. It should be emphasized that the regen-
eration process causes a signicant loss of the catalysts related
to the loading and unloading of lters. Therefore, we were
unable to carry out any more processes due to the large catalyst
loss. Nevertheless, the catalyst destruction is compensated by
its high catalytic performance, which makes the entire process
protable. Moreover, if the catalyst is used in the HDO process
in larger amounts, its regeneration would be easier to carry out.
Reaction mechanism
In view of the complexity of the HDO process and under-
standing of the complete reaction network, an analysis of the
calculated empirical data would be benecial. However, the
atomistic processes occurring at the catalyst surface are unclear
because computational studies dealing with aromatic oxygen-
ates are not as numerous as those dealing with aliphatic
oxygenates.52 The adsorption energy (calculated by the standard
Gaussian 16 program with Hartree–Fock method) on the
palladium surface was in the following order: syringol < cre-
osole < guiacol, the same as the catalytic activity. More detailed
study of a “real” catalyst, which will contain defect sites is
however necessary.
Conclusions
The results of the experiments carried out in this study, HDO reactions of guaiacol and syringol.
including synthesis, physicochemical characterization and
(vi) Higher temperature and higher hydrogen pressure
catalytic tests of ordered mesoporous silica materials with improve the conversion and facilitate the formation of products
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with lower total oxygen content and higher hydrogen content. At 13 A. J. Foster, P. T. M. Do and R. F. Lobo, Top. Catal., 2012, 55,
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and they have greater chance to overcome the energy barrier, 14 E. F. Iliopoulou, E. V. Antonakou, S. A. Karakoulia,
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19 L. Faba, E. Dia and S. Ordonez, Renewable Sustainable Energy
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ˇ
´
´
´
´
20 D. Prochazkowa, P. Zamostn´y, M. Bejblova, L. Cerven´y and
Conflicts of interest
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J. Cejka, Appl. Catal., A, 2007, 332, 56–64.
There are no conicts to declare.
21 F. Cheng and C. E. Brewer, Renewable Sustainable Energy
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22 Z. He and X. Wang, Catal. Sustainable Energy, 2012, 1, 28–52.
23 A. S. Ouedraogo and P. R. Bhoi, J. Cleaner Prod., 2020, 253, 1–
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Acknowledgements
This work was supported by the National Science Centre
(project no: DEC 2013/10/M/ST5/00652).
24 J. L. Santos, M. Alda-Onggar, V. Fedorov, M. Peurla,
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¨
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