Catalytic upgrading of carboxylic acids as bio-oil models over hierarchical ZSM-5 obtained: Via an organosilane approach

Article abstract of DOI:10.1039/c7ra03890a

Biomass is an interesting renewable energy resource as it is widespread in nature and low cost. The development of bio-oil derived from biomass as a fuel is still a scientific and industrial challenge. In this context, we demonstrate the synthetic method of bio-oil upgrading catalysts based on hierarchical zeolites and open up interesting perspectives for bio-oil upgrading processes. The hierarchical ZSM-5 zeolite has been successfully prepared via a direct hydrothermal synthesis with the aid of a commercial organosilane surfactant (TPOAC). The influences of TPOAC content and Si/Al ratio on hierarchical structures were also systematically studied. To illustrate their catalytic performances, an esterification reaction of various organic acids such as (acetic acid and levulinic acid) and alcohols was performed as the model reaction representing the bio-oil upgrading application. The synergic effect of acidity and the hierarchical structure of catalysts can greatly enhance the catalytic performance in terms of activity, product yield, coke formation, and reusability of the catalysts. For example, they can convert almost 100% of reactant in 8 h in the esterification of acetic acid and alcohols, whereas the conventional zeolite reveals significantly lower activity (<20%). Interestingly, the hierarchical zeolite can also greatly improve the catalytic activity of the esterification of levulinic acid and ethanol to produce ethyl levulinate that can be used as a diesel miscible biofuel (DMB). In addition, the efficiency of hierarchical catalysts obtained by different synthesis methods is also discussed. This first example demonstrates that the hierarchical zeolite obtained via a direct synthesis approach can benefit bio-oil upgrading applications via the esterification of various carboxylic acids.

Full text of DOI:10.1039/c7ra03890a

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Catalytic upgrading of carboxylic acids as bio-oil
models over hierarchical ZSM-5 obtained via an
organosilane approach†
Cite this: RSC Adv., 2017, 7, 35581
b
Kamonlatth Rodponthukwaji,a Chularat Wattanakit,
Thittaya Yutthalekha,b
*
Sunpet Assavapanumat,b Chompunuch Warakulwit,a Wannaruedee Wannapakdeeb
and Jumras Limtrakulc
Biomass is an interesting renewable energy resource as it is widespread in nature and low cost. The
development of bio-oil derived from biomass as a fuel is still a scientific and industrial challenge. In this
context, we demonstrate the synthetic method of bio-oil upgrading catalysts based on hierarchical
zeolites and open up interesting perspectives for bio-oil upgrading processes. The hierarchical ZSM-5
zeolite has been successfully prepared via a direct hydrothermal synthesis with the aid of a commercial
organosilane surfactant (TPOAC). The influences of TPOAC content and Si/Al ratio on hierarchical
structures were also systematically studied. To illustrate their catalytic performances, an esterification
reaction of various organic acids such as (acetic acid and levulinic acid) and alcohols was performed as
the model reaction representing the bio-oil upgrading application. The synergic effect of acidity and the
hierarchical structure of catalysts can greatly enhance the catalytic performance in terms of activity,
product yield, coke formation, and reusability of the catalysts. For example, they can convert almost
100% of reactant in 8 h in the esterification of acetic acid and alcohols, whereas the conventional zeolite
reveals significantly lower activity (<20%). Interestingly, the hierarchical zeolite can also greatly improve
the catalytic activity of the esterification of levulinic acid and ethanol to produce ethyl levulinate that can
be used as a diesel miscible biofuel (DMB). In addition, the efficiency of hierarchical catalysts obtained by
different synthesis methods is also discussed. This first example demonstrates that the hierarchical
zeolite obtained via a direct synthesis approach can benefit bio-oil upgrading applications via the
esterification of various carboxylic acids.
Received 5th April 2017
Accepted 11th July 2017
DOI: 10.1039/c7ra03890a
rsc.li/rsc-advances
conversion of biomass is one of the most fascinating methods
because it is widespread in nature and low cost. Typically,
Introduction
biomass resources can be divided into one of the following
three groups: (i) wastes; (ii) forest products; (iii) energy crops.2
The lignocellulosic biomass is one of the most abundant sour-
ces to be used for the production of liquid fuels or bio-oils.
Although the bio-oil derived from the lignocellulosic biomass
is a good candidate to replace a petroleum fuel, it typically
consists of high oxygen content (20–50 wt%) and acidity (pH ¼
2.5–3), resulting in undesirable properties, such as low heating
value, high viscosity, thermal instability, and corrosiveness.3 To
overcome these problems, the upgrading of bio-oils is crucial to
improve their properties. In general, the bio-oil upgrading
processes can be obtained by hydrodeoxygenation (HDO),4
ketonisation,5 aldol condensation,6 and esterication.7 The
most popular one is based on the catalytic upgrading via an
esterication of organic acids in the presence of alcohol mole-
cules, resulting in the reduction of oxygen content and acidity,
thus improving the oil stability.8
The development of renewable energy sources has attracted
a great deal of attention from both academic and industrial
points of view due to the shortage of fossil fuels. Although
renewable energy can be obtained from many sources, such as
solar, wind, geothermal, hydroelectric, and biomass,1 the
aDepartment of Chemistry, NANOTEC Center for Nanoscale Materials Design for Green
Nanotechnology, Kasetsart University, Bangkok 10900, Thailand
bDepartment of Chemical and Biomolecular Engineering, School of Energy Science and
Engineering, Vidyasirimedhi Institution of Science and Technology, Rayong 21210,
Thailand. E-mail: Chularat.w@vistec.ac.th; Fax: +66 3301 4445 extn 2176
cDepartment of Materials Science and Engineering, School of Molecular Science and
Engineering, Vidyasirimedhi Institution of Science and Technology, Rayong 21210,
Thailand
† Electronic supplementary information (ESI) available: Calibration curves (S1),
particle size distribution (S2), XRD patterns (S3), 27Al MAS NMR spectra (S4), IR
spectra of pyridine adsorption experiments (S5), weight loss proles (S6), O2
TPO proles (S7), Raman spectra (S8), benzyl alcohol conversion (%) on
different samples obtained by different methods (S9), Tables S1–S4. See DOI:
10.1039/c7ra03890a
Although a common acid catalyst, such as sulfuric acid
(H2SO4), exhibits an excellent catalytic activity in an
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esterication, such an acid is not recommended as it is corro- range of 25–50 is the most favourable by a post-treatment, while
sive and non-environmentally-friendly. Alternatively, various a low mesoporosity and a loss of crystallinity are obtained in the
solid acid catalysts, such as sulfated zirconia,9 functionalized case of higher and lower Si/Al ratios, respectively.45,46 To the best
mesoporous silica,10 heteropoly acid,11 and zeolite12,13 have been of our knowledge, the development of hierarchical zeolites by
applied. In particular, zeolites have been widely used for an a direct synthesis based on an organosilane approach for the
esterication due to their outstanding properties, such as high bio-oil upgrading application has not yet been demonstrated,
surface area, high thermal/chemical stability, suitable acidity, even though it is one of the most interesting alternative ways to
and shape selective property.
extend the scope of hierarchical zeolite researches.
Over the past decade, zeolites have played an important role
Herein, we demonstrate not only the synthesis of hierar-
in various potential applications, especially in petrochemical chical ZSM-5 by a direct synthesis approach with the aid of
reactions, such as aromatization,14,15 isomerization,16,17 alkyl- a commercial organosilane surfactant, dimethyloctadecyl[3-
ation,18,19 dehydration,20 and esterication.21,22 However, the (trimethoxysilyl)propyl] ammonium chloride (TPOAC), as
presence of sole micropores in their structures oen suffers a hierarchical pore-directing agent but also its catalytic
from the transportation limitation of guest molecules, perspectives for the bio-oil upgrading application using a model
including reactants, intermediates, and products. As a conse- reaction. The catalytic performances in terms of activity,
quence, the catalyst deactivation can be easily observed due to product selectivity, and coke formation were explored via the
the coke formation inside micropores. To improve the efficiency esterication of various carboxylic acids such as acetic acid and
of a conventional zeolite, many researchers have paid a lot of levulinic acid and alcohols. Complementary, the reusability of
effort to develop the zeolite structure by either reducing the hierarchical catalysts was also examined and compared with
crystal size23 or introducing the hierarchical pores into its that of the conventional one. In addition, the catalytic activity of
structures.24–26 In the latter case, the fabrication of hierarchical the hierarchical catalyst obtained by different synthesis
zeolites has been successfully obtained by one of the following methods including direct- and post synthesis approaches was
two major routes: (i) post-treatment;27–29 (ii) direct synthesis.30–37 discussed in details.
In the direct synthesis, a secondary template was applied by
either hard or so templates to control the hierarchical struc-
ture. According to the so templating approach, an organo-
silane surfactant has been widely used as the mesopore-
directing agent because it provides a strong interaction with
Experimental
Preparation of catalysts
the zeolitic gels, thus preventing the phase separation between The hierarchical ZSM-5 was hydrothermally synthesized by
zeolite precursors and mesoporous template molecules. using the dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammo-
However, it sometimes also suffers from the disadvantage nium chloride (TPOAC) (42 wt% in methanol, Sigma-Aldrich) as
because it requires many complicated steps of the synthesis of a mesopore structure-directing agent. For a typical procedure,
a designed organosilane template,38 while there are only a few we developed the synthesis method from our previous work by
hierarchical zeolites which have been successfully produced by using the TPOAC instead of carbon material as the hierarchical
using a commercial organosilane surfactant as a mesoporous mesopore-directing agent.47 In the modied mixture, a 1 g of
structure-directing agent.35,39–41
silica gel (SiO2, Merck), 0.02 g of sodium aluminate (53 wt%
¨
Recently, hierarchical zeolites have been applied as an Al2O3, 43 wt% Na2O; Riedel-deHaen), 0.26 g of tetrapropy-
interesting catalyst for an esterication. For example, the mes- lammonium bromide, TPABr, (98% C12H28BrN, Sigma-Aldrich),
oporous ZSM-5 and H-ZSM-5 monolith were prepared by an 0.11 g of sodium hydroxide (98% NaOH, Carlo Erba), and 7 ml
alkali treatment and using corn or sorghum stem piths as of deionized (DI) water were mixed and stirred at room
sacricial solid templates, respectively. Interestingly, the ob- temperature for 1 h. Then, the desired amount of TPOAC was
tained ZSM-5 can signicantly improve catalytic performances dropped slowly into the solution. The nal molar composition
of the esterication of benzyl alcohol and hexanoic acid.42 In was 178 SiO2/1 Al2O3/16 NaO2/9 TPABr/4251H2O/x TPOAC,
addition, the esterication of o-cresol with acetic acid over which x refers to the amount of TPOAC. The mixture was stirred
mesoporous ZSM-5 zeolites obtained by an alkaline treatment further for 2 h and transferred to a Teon-coated stainless steel
or post treatment has been reported.43 The remarkably autoclave. Then, it was heated at 453 K for 3 days. The product
improved activity in the liquid-phase esterication of alcohol was ltered and washed with deionized water until the pH of
and acid was observed over the mesoporous ZSM-5 via deme- ltrate less than 9. It was then dried in an oven at 373 K over-
tallation in alkaline media.
night and subsequently calcined in air at 823 K for 9 h. The
An alternative way to produce the ordered mesoporous obtained samples were designated as ZSM-5_(x)TPOAC in which
zeolite has been achieved by a direct synthesis or a templating x refers to an amount of TPOAC in molar composition. To
approach. This strategy can obviously solve several limitations compare the benets of modied and unmodied structures,
of hierarchical zeolites obtained from a post-treatment and in a conventional sample, the ZSM-5 was also synthesized in the
particular it can be used to synthesize hierarchical zeolites with absence of TPOAC and the commercial ZSM-5 (SH-55 from ALSI-
a wide range of Si/Al ratios.44 In contrast to this, a post- PENTA Zeolithe GmbH, Si/Al ratio of 24) was also used to
treatment has been applied to the limited initial Si/Al ratio of compare with the synthesized samples. They are designated as
parent zeolites. Typically, the zeolite having the Si/Al ratio in the C_ZSM-5 and Commercial ZSM-5, respectively.
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To prepare the acid catalyst, all synthesized samples were minꢁ1) and it was subsequently cooled down to 323 K. The
ion-exchanged into the proton-formed ZSM-5 by repeating the sample was equilibrated with 5 vol% of NH3 in He for 60 min
treatment three times with 1 M of NH4NO3 at 353 K for 2 h. and then purged under the ow of He (30 ml minꢁ1) for 1 h. To
Then, the sample was dried at 373 K and calcined at 823 K for record NH3 TPD proles, the temperature was increased from
6 h.
323 K to 1073 K with the heating rate of 10 K minꢁ1. To deter-
To evaluate the inuence of the number of acid sites, we mine the density of acid sites in zeolites, FTIR spectra of pyri-
therefore prepared the hierarchical ZSM-5 samples with various dine adsorption were recorded at a 2 cmꢁ1 resolution and
Si/Al ratios. For a typical synthesis, the above-mentioned averaged over 64 scans performed on a Bruker Vertex V70v
procedure with varying the amount of sodium aluminate was instrument. Firstly, the zeolite sample was heated to 823 K with
used. The amount of Al2O3 was varied in the range of 1 to 3.6 the heating rate of 2 K minꢁ1 under 20 vol% O2 in He. Pyridine
molar ratio. The samples are designated as Hierarchical ZSM- was then introduced for 10 min into the chamber at its vapor
5(A) in which A refers to the Si/Al ratio.
pressure at room temperature. Subsequently, pyridine was
To compare the effect of hierarchical structures obtained by removed by evacuation for 1 h at 573 K. To calculate the number
different synthesis approaches on their catalytic performances, of Brønsted and Lewis acid sites, molar extinction coefficient
an alkaline post treatment was also chosen for the preparation of values of 0.73 and 1.11 cm mmolꢁ1 were applied, respectively.
hierarchical ZSM-5 as a representative of a top-down synthesis
The amount of coke formation in the zeolite network aer
approach. A 1 g of the commercial zeolite (SH-55 from ALSI- catalytic tests was evaluated by the thermogravimetric analysis
PENTA Zeolithe GmbH, Si/Al ratio of 24) was mixed with 30 ml (TGA) and the O2 temperature-programmed desorption (O2-
of 0.5 M NaOH and it was stirred at 353 K for 1 h. Then, the TPO). As for TGA experiments, it was performed on a Perkin
product was washed with DI water until the pH of ltrate less Elmer Pyris 1 TGA instrument in the temperature range of 373–
than 8. The ltered product was dried and subsequently ion- 1073 K with a heating rate of 10 K minꢁ1 under an oxygen (O2,
exchanged into the proton-formed catalyst. The alkaline 99.99% purity, Praxair, Thailand) with the ow rate of 40 ml
treated sample is designated as the Hierarchical ZSM-5_AT.
minꢁ1. Prior to the O2-TPO measurement, used catalysts were
pretreated at 523 K for 2 h under the ow of He. The catalysts
were then heated under the ow of 5% (v/v) O2 in He. The TPO
proles were recorded in the temperature range of 373–1073 K
Characterization of catalysts
Powder X-ray diffraction (XRD) patterns were recorded by a Bruker with the ramp rate of 5 K minꢁ1. The Raman spectra of spent
D8 ADVANCE instrument using CuKa radiation (30 kV, 40 mA) catalysts were collected at room temperature, performing at
with step size of 0.02ꢀ and scan rate of 10ꢀ minꢁ1 in the 2q range of least 3 different positions. A laser beam was used at 532 nm
5–50ꢀ. The relative crystallinity of zeolite samples was calculated by focused on the sample by a microscope.
the following equation: (% XRD relative crystallinity of the desired
sample ¼ (Hx/Hr) ꢂ 100), where Hx and Hr are the height of the
Esterication of benzyl alcohol with acetic acid
most intense peak for the desired samples and the reference
Prior to the catalytic test, the proton-formed ZSM-5 samples
sample, respectively. The morphologies of materials were exam-
were activated at 823 K for 6 h. For the experimental procedure,
ined by a scanning electron microscopy (SEM) performed with
the reaction was performed in a three-necked round bottom
a Hitachi-3400 instrument. The samples for SEM studies were
ask with a reux condenser at 373 K under an atmospheric
pressure. A 15 ml of toluene was added into the reactor
dispersed on carbon tape and coated with a thin layer of gold.
Nitrogen adsorption–desorption isotherms were measured
following by 0.10 ml of decane as a solvent and an internal
at 77 K performed on a Micromeritics ASAP 2020 physisorption
analyzer. Before the measurement, the samples were evacuated
at 575 K for 20 h. The specic surface area was calculated by
using the Brunauer–Emmett–Teller (BET) theory (SBET).48 The
standard, respectively. A 3.10 ml of benzyl alcohol and 2.06 ml
of acetic acid were added. A 0.45 g of catalyst was then placed
into the reactor. The reaction mixtures were collected at
a desired time and analyzed by gas chromatography (GC) per-
total pore volume (Vtot) was calculated at P/P0 ¼ 0.98. The t-plot
formed on an Agilent 7820A GC instrument using a FID detector
method was used to calculate the micropore volume (Vmicro),
and a DB-1 capillary column (100 m ꢂ 0.5 mm ꢂ 0.25 mm). The
percentage of conversion was calculated based on the total
percentage of benzyl alcohol converted into the product. The
quantitative analysis was investigated by using the individual
calibration curves of benzyl alcohol and its products as shown
in Fig. S1 in ESI.† The mass balance was checked for all
experiments (97.29 ꢃ 1.50%).
and external surface area (Sext).49 The mesopore size distribu-
tions were examined by using the Barrett–Joyner–Halenda (BJH)
model derived from the adsorption branch of isotherms.50
The aluminum structures were determined by using 27Al
MAS NMR spectroscopy performed on an AVANCE 300 MHz
Digital NMR spectrometer (Bruker Biospin; DPX-300) at 78
MHz. The Al2O3 was used as a reference (set to 0 ppm). The Si/Al
ratio of products was investigated by ICP-OES technique per-
formed on the Agilent Technologies 715 model.
Results and discussion
Characterization of catalysts
The NH3 temperature-programmed desorption (NH3-TPD)
curves were recorded by using a xed-bed reactor system
equipped with a TCD detector. Typically, a 1 g of zeolite sample
Effect of TPOAC content. According to the XRD results in
was pretreated at 823 K for 2 h under the ow of N2 (30 ml Fig. 1, the patterns reveal that all samples contain characteristic
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Fig. 1 XRD patterns of all ZSM-5 samples obtained at various TPOAC
contents: (a) Commercial ZSM-5, (b) C_ZSM-5, (c) ZSM-5_(4.8)
TPOAC, (d) ZSM-5_(9.6)TPOAC, and (e) ZSM-5_(38.4)TPOAC.
Fig. 2 SEM images of ZSM-5 samples obtained at various amounts of
TPOAC: (a) C_ZSM-5, (b) ZSM-5_(4.8)TPOAC, (c) ZSM-5_(9.6)TPOAC,
and (d) ZSM-5_(38.4)TPOAC.
peaks of the MFI zeolite structure with high crystallinity, while
no crystalline impurity phase is observed. It suggests that
zeolite samples were successfully synthesized under a simple
hydrothermal process even in the presence of a mesopore
structure-directing agent. For example, the samples obtained by
adding a small amount of TPOAC (ZSM-5_(4.8)TPOAC and ZSM-
5_(9.6)TPOAC) exhibit high intense peaks comparable to the
reference sample (C_ZSM-5), having the relative crystallinity of
61.5% and 69.5%, respectively (see Table S1 in the ESI†).
However, when the TPOAC was further added with higher
content, the relative crystallinity of samples, ZSM-5_(38.4)
data, showing a dramatic decrease in crystallinity compared
with the reference sample (C_ZSM-5). In particular, the simul-
taneous presence of the coffin-like crystals and the amorphous
phase is observed in the case of the ZSM-5_(38.4)TPOAC sample
due to the use of overdosed TPOAC.
To conrm the presence of mesoporosity, the textural
properties of all samples were measured by N2 physisorption
technique. N2 adsorption/desorption isotherms (Fig. 3)
demonstrate that synthesized ZSM-5 products show the
isotherms with hysteresis loop at P/P0 in the range of 0.4–0.8
due to the mesopore lling, indicating the presence of addi-
tional mesopores. In contrast to the synthesized samples, the
hysteresis loop does not exist in the isotherm of the commercial
ZSM-5 and the C_ZSM-5, while the adsorption at the relatively
low pressure (P/P0 ꢄ 0.2) with a long horizontal plateau is clearly
observed due to the micropore lling, suggesting the presence
of sole micropores in the zeolite structure. Moreover, the BJH
mesopore size distribution in Fig. 4 reveals the presence of
a mesoporous structure in the hierarchical ZSM-5 samples. The
BJH curve of ZSM-5_(9.6)TPOAC sample shows a wide distri-
bution in the range of 10–20 nm. However, when the TPOAC is
TPOAC, is signicantly decreased (about 40%).
A large
amount of surfactant directly causes the signicant reduction of
crystallinity, suggesting the incompatibility of surfactant and
zeolite gels to hinder the crystallization process (from 9.6 to
38.4 mol of TPOAC). This makes it clear that the amount of
a mesoporous structure-directing agent strongly affects the
formation of zeolite structures.
To verify the morphologies of synthesized zeolites, the
scanning electron microscopy (SEM) images reveal that the
TPOAC apparently affects the morphology of products (Fig. 2).
In the absence of TPOAC, the C_ZSM-5 sample exhibits smooth
surface morphology, having particle sizes of 3.6 ꢃ 0.5 mm (see
the particle size distribution in Fig. S2 in the ESI†). In strong
contrast to this, the sample obtained in the presence of low
TPOAC content, ZSM-5_(4.8)TPOAC, shows non-uniform parti-
cles with various shapes, including coffin-like and spherical
morphologies. Due to the variety of shapes and non-uniform
particle size distribution (8.8 ꢃ 2.0 mm see in Fig. S2 in the
ESI†), this would allow us to increase the TPOAC content to
produce more uniform particles. Consequently, the ZSM-5_(9.6)
TPOAC product was fabricated by increasing the amount of
TPOAC. The sample exhibits stacking plate-like shapes,
providing a high surface areas with average crystal sizes of 10.6
ꢃ 2.0 mm (Fig. S2 in the ESI†). To examine the limitation of
TPOAC content, the ZSM-5_(38.4)TPOAC was also synthesized
by further increasing the amount of TPOAC by the fourfold
compared with the ZSM-5_(9.6)TPOAC. The mixture of crystal-
line and amorphous phases is observed, when an excess
Fig. 3 N2 adsorption/desorption isotherms of synthesized ZSM-5
samples obtained at various TPOAC contents: (a) Commercial ZSM-5,
(b) C_ZSM-5, (c) ZSM-5_(4.8)TPOAC, (d) ZSM-5_(9.6)TPOAC, and (e)
amount of TPOAC is added. This is consistent with the XRD ZSM-5_(38.4)TPOAC.
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Fig. 5 SEM images of: (a) Commercial ZSM-5, and (b) Hierarchical
ZSM-5(34), the representative sample having low Si/Al ratio: (c) Hier-
archical ZSM-5(90), the representative sample having high Si/Al ratio.
structure as shown in Fig. S3 in the ESI.† However, its crystal-
linity is lower than that of the commercial ZSM-5 due to the
effect of TPOAC template on the morphology of crystal growth.
SEM images in Fig. 5 also conrm the differences in morphol-
ogies of three catalysts, the commercial ZSM-5, the hierarchical
ZSM-5(34), and the hierarchical ZSM-5(90). Interestingly, the
hierarchical ZSM-5(34) sample exhibits the self-assemblies of
stick-like nanocrystals, while the hierarchical ZSM-5(90) sample
consists of stacking plate-like crystals. Both samples are ex-
pected to show high catalytic performance due to their high
surface areas and mesoporosity as shown in Table 1.
In addition to the number of acid sites, the nature of Al sites
was also monitored by 27Al MAS NMR spectroscopy (as shown in
Fig. S4 in the ESI†). As the presence of the single peak at about
54 ppm of all samples, it clearly shows that the samples contain
only tetrahedral coordinated Al in frameworks, whereas the
presence of the octahedrally extra-framework Al species can be
excluded.
The acidity of catalysts was also examined by NH3-TPD
technique. From NH3-TPD proles (Fig. 6), all curves were tted
and obtained with two visible peaks, appearing at low temper-
ature, LT (ca. 493 K) and high temperature, HT (ca. 723 K). The
LT peak is assigned to weak acid sites due to the interaction
between NH3 molecules desorbed from weakly acidic hydroxyl
sites, which also refer to the defect and external silanol groups.
However, the HT peak corresponds to the desorption of NH3
from strong acid sites, which are Brønsted acid sites (BAS) and
Lewis acid sites (LAS).51 Among synthesized samples, the hier-
archical ZSM-5(34) sample exhibits the lower acidity, which is of
35.5% compared with the commercial ZSM-5. As expected, the
hierarchical ZSM-5(90) shows the lowest acidity due to a small
number of acid sites and it decreases approximately by 35%
Fig. 4 Mesopore size distribution derived from BJH adsorption branch
of synthesized ZSM-5 samples obtained at various TPOAC contents:
(a) Commercial ZSM-5, (b) C_ZSM-5, (c) ZSM-5_(4.8)TPOAC, (d) ZSM-
5_(9.6)TPOAC, and (e) ZSM-5_(38.4)TPOAC.
increased to 38.4 mol, the pore size distribution becomes
narrow in the range of 5–10 nm. In contrast, the porous struc-
ture in the range of mesopore size was not observed in the cases
of the commercial ZSM-5 and the C_ZSM-5. This result again
conrms the presence of hierarchical mesopores in the hierar-
chical ZSM-5 samples and the presence of sole micropores in
the conventional ZSM-5, which is consistent with the N2 sorp-
tion isotherm results.
From these observations, the results indicate that a suitable
amount of TPOAC is equal to 9.6 mol, which produces excellent
textural properties (Table 1), providing the highest meso-
porosity. It is therefore reasonable to assume that in the case of
the catalytic study, it should be possible to be used as a candi-
date catalyst.
Effect of Si/Al ratios
To verify the role of acidity on the catalytic performances, we
also carried out several additional experiments to prepare the
hierarchical ZSM-5 samples with various Si/Al ratios via a direct
synthesis. The catalysts, having low Si/Al ratio, Si/Al ¼ 34
(hierarchical ZSM-5(34)), and high Si/Al ratio, Si/Al ¼ 90, (ZSM-
5_(9.6)TPOAC or hierarchical ZSM-5(90)), were also synthesized
by using the suitable amount of TPOAC obtained from the
previous section to generate secondary mesopores. The hierar-
chical ZSM-5(34) also shows characteristic peaks of the MFI
Table 1 Textural properties and Si/Al ratios of all synthesized ZSM-5 samples
Si/Ala
SBET
Smicro
Sext
Vtotal
Vmicro
Vext/meso
Vmeso/Vtotal
b
c
d
e
f
g
h
Samples
Commercial ZSM-5
C_ZSM-5
ZSM-5_(4.8)TPOAC
ZSM-5_(9.6)TPOAC
ZSM-5_(38.4)TPOAC
Hierarchical ZSM-5(34)
19
115
88
90
97
34
363
387
374
396
393
434
276
249
162
176
163
287
87
0.171
0.180
0.240
0.346
0.321
0.338
0.110
0.099
0.066
0.072
0.068
0.115
0.060
0.081
0.174
0.273
0.253
0.223
0.351
0.450
0.725
0.789
0.788
0.660
137
212
220
229
147
a
Si/Al ratio obtained by ICP-OES technique. b SBET: BET specic surface area. c Smicro: micropore surface area. d Sext: external surface area. e Vtotal
total pore volume. f Vmicro: micropore volume. g Vext/meso ¼ Vtotal ꢁ Vmicro; all surface areas and pore volumes are in the units of m2 gꢁ1 and cm3 gꢁ1
:
,
h
respectively. Fraction of mesopore volume.
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Fig. 6 NH3-TPD curves obtained on different samples: (a) Commer-
cial ZSM-5, (b) Hierarchical ZSM-5(34), and (c) Hierarchical ZSM-5(90).
Fig. 7 Benzyl alcohol conversion (%) (red) and benzylacetate selec-
tivity (%) (black) as a function of reaction time on different samples:
Commercial ZSM-5, Hierarchical ZSM-(90), and Hierarchical ZSM-
5(34).
compared with the hierarchical ZSM-5(34) sample. The amount
of acid sites is known to relate to Al atoms in a framework in
which the charge is compensated by acidic protons.52
To further investigate the amount of BAS and LAS, FTIR
spectra of pyridine adsorption are shown in Fig. S5 and Table
S2.† The peak at around 1545 cmꢁ1 is assigned to pyridinium
ions formed by protonation at BAS, while a band at 1455 cmꢁ1
corresponds to pyridine adsorbed on LAS. It was found that the
contribution of BAS of both catalysts is very high compared with
LAS. This parameter strongly affects many potential catalytic
applications, especially in an esterication.53 Consequently, this
catalyst is expected to lead to enhancing the catalytic activity of
the esterication.
hierarchical porous network can reduce the diffusion
constraints inside micropores. Interestingly, the hierarchical
sample containing the lower amount of acidic density (high Si/
Al ratio) also exhibits higher catalytic activity than that of the
commercial ZSM-5, implying that the higher catalytic activity
can be contributed by the hierarchical porous network. This
result again conrms that the promising property of hierar-
chical structures can promote the catalytic activity of an ester-
ication of carboxylic acid.
To demonstrate the efficiency of catalysts, the reusability
studies of the hierarchical ZSM-5 and the corresponding
conventional zeolite were also carried out at various reaction
cycles (Fig. 8). The conversion of alcohol is about 90% at 8 h
when the hierarchical ZSM-5(34) sample was used as a catalyst
for the rst cycle. In contrast, the commercial sample only
converts the reactant into the product by 22%. For the second
cycle, the hierarchical ZSM-5(34) sample still exhibits an excel-
lent performance by showing about 85% of alcohol conversion.
However, the commercial catalyst performs similarly low cata-
lytic activity compared to itself in the rst cycle. In the third
cycle, the hierarchical catalyst still shows, as expected, a good
catalytic performance (80% conversion of alcohol), while the
Catalytic performance of catalysts
To illustrate the benets of hierarchical catalysts for the cata-
lytic upgrading of carboxylic acids, the prepared samples were
tested with an esterication of benzyl alcohol and acetic acid as
a model study (as shown in Scheme 1). As shown in Fig. 7, the
catalytic activities of an esterication of different catalysts are
demonstrated as a function of reaction time. The major product
of this reaction is benzylacetate, which is almost 100% selec-
tivity for all catalysts (Fig. 7). Interestingly, the hierarchical
ZSM-5(34) sample exhibits an excellent activity compared with
the other catalysts. It can convert the reactant into product
about 90% in 8 h. In contrast, the commercial ZSM-5 shows very
low activity, which gives only 22% of the conversion of alcohol,
although it contains higher acidic density compared with the
hierarchical ZSM-5(34). The different activities between two
catalysts can be attributed to the fact that the presence of the
Fig.
8 Reusability experiments of Hierarchical ZSM-5(34) and
Scheme 1 Esterification reaction of benzyl alcohol and acetic acid on Commercial ZSM-5 catalysts for three cycles (catalytic test condition:
zeolite catalysts.
373 K and 1 atm).
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commercial one only converts about 15% of the alcohol toward
This makes it clear that the catalytic activity for the esteri-
the product. It is noteworthy that the inferior catalytic activity of cation of carboxylic acids can be greatly improved in the case of
the commercial sample is due to the presence of sole micro- hierarchical ZSM-5 samples even though they exhibit lower
pores in the structure and this could eventually lead to accel- Brønsted acid density. Together with the presence of meso-
erate the coke formation.
porous networks connected to the microporous framework, it
To further verify in the term of catalyst deactivation, the gives the benets to improve the catalytic performances in
amount of deposited coke was also measured by the tempera- terms of catalytic activity, desired product yield, and catalyst
ture thermogravimetric analysis (TGA) and the O2 temperature- stability/reusability, due to the improved accessibility to active
programmed oxidation (O2-TPO) as shown in Fig. S6 and S7 in sites. On the other hand, the sole microporous structure of
the ESI.† From TGA proles, it was found that a carbon weight commercial ZSM-5 does not show the advantage for the reaction
loss over the hierarchical ZSM-5(34) sample is much less than because of not only inhibiting the diffusion of molecules to
that of the commercial one (11.5% and 32.4% for the hierar- active sites but also facilitating the coke formation inside
chical ZSM-5(34) and the commercial ZSM-5, respectively) aer microporous network, especially for catalysts with very high
the reaction time of 24 h. As expected, the presence of hierar- Brønsted acid density, which is required for the acid catalysed
chical pores obviously improves the catalytic activity by reaction, such as an esterication of organic acids.
rendering molecular transportation, resulting in a reduction in
coke formation.
To compare the catalytic performances of hierarchical
zeolites obtained by various synthesis approaches (Fig. S9 in the
In contrast, the commercial sample contains a large amount ESI†), the hierarchical ZSM-5 sample obtained by a direct
of deposited coke due to the presence of sole micropores in its synthesis, hierarchical ZSM-5(34), shows signicantly
a
structure, causing the pore blocking. Besides, O2-TPO proles improved catalytic activity compared with those obtained by
demonstrate the location and the type of carbonaceous species. using a conventional alkaline treatment, even though both
At the low temperature about 673 K, it is attributed to the lowest samples contain the similarity in textural properties, such as
aromatic condensation, which are easily oxidized and mostly micro- and mesoporosity (as shown in Table S3 in the ESI†). The
located on the outermost surface of zeolite (coke I). Meanwhile, hierarchical ZSM-5(34) sample exhibits an excellent conversion
at higher temperature about 773–873 K, it is ascribed to the high of alcohol (about 90% and high selectivity of benzylacetate
aromatic condensation, the polynuclear aromatic coke species almost 100%).
or graphitic like coke (cokes II and III), which are originated
In order to further illustrate the benets of hierarchical
from the microporous zeolite structure.54–56 The deconvolution zeolites for the catalytic upgrading of carboxylic acids, an
of TPO curves shows an amount of coke content of three esterication of levulinic acid, which can be formed by the acid-
different species. The signicant decrease of cokes I and II over catalyzed dehydration of monosaccharide, and ethanol has also
the hierarchical ZSM-5 sample was observed, indicating a lower been investigated (Table S4 in the ESI†). Interestingly, the
degree of condensation of coke species. The commercial sample hierarchical zeolite can greatly improve the catalytic activity to
exhibits a large amount of developed polynuclear aromatic coke/ produce the ethyl levulinate that can be used as diesel miscible
graphitic species deposited on external surfaces and inside biofuel (DMB). The hierarchical ZSM-5 have the two-three fold
zeolite structures with the total coke content of 10 mmol gꢁ1. In advantage of the catalytic conversion of levulinic acid compared
contrast, the hierarchical ZSM-5 sample dramatically reduces with the conventional zeolite as well as preserved shape selec-
the coke formation of all types with the total coke content of tivity of zeolite framework. This work opens up interesting
3 mmol gꢁ1 due to shortening the diffusion path length. On the perspectives for the development of hierarchical mesoporous
other hand, a large content of coke II and III species in the zeolites via a direct synthesis approach using an organosilane
commercial sample indicates that the fast deactivation of cata- template for the catalytic upgrading of various carboxylic acids.
lyst is the results of pore blockage by high aromatic condensa-
tion. This is also consistent with the TGA results, revealing that
the hierarchical samples can signicantly reduce the content,
Conclusions
resulting in an increase of catalytic activity. Fig. S8† shows The hierarchical ZSM-5 samples have been successfully
Raman spectra of coke species deposited on catalysts aer the synthesized under a simple direct hydrothermal method. With
reaction. It was found that two visible peaks appearing at 1610 the aid of TPOAC, the synthesized hierarchical ZSM-5 products
and 1389 cmꢁ1 are assigned to the G band corresponding to the exhibit promising catalytic performances in terms of increasing
presence of graphitic materials57 and the D band due to disor- activity, desired product yield, and catalyst reusability for an
dered carbon structures, respectively. Obviously, both catalysts esterication of carboxylic acids. In particular, the hierarchical
exhibit the presence of both graphite-like carbon species and ZSM-5(34) sample performs a remarkable activity by converting
disordered amorphous polyaromatic species. Interestingly, the almost 90% of benzylalcohol to benzylacetate for an esterica-
ratio of area of D band and G band (ID/IG) of commercial ZSM-5 tion of acetic acid and benzylalcohol within 8 h corresponding
and hierarchical ZSM-5(34) is 0.52 and 0.39, respectively. These to the presence of hierarchical porous networks in the zeolite
correspond well with O2-TPO results that the higher ratio of coke structure.
II to coke III content was observed in the deposited coke ob-
tained by a commercial catalyst.
To illustrate the ability of hierarchical structures, the sample
with a low content of acidic density also shows a good perfor-
mance by catalyzing 70% of reactant in 24 h. This might be
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vefold catalytic activity even aer three cycles.
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