Hydrophobic aluminosilicate zeolites as highly efficient catalysts for the dehydration of alcohols

Article abstract of DOI:10.1016/j.catcom.2016.02.006

Efficient dehydration of alcohols to olefins, acting as a control step in the upgrading of phenolic biofuel into alkane fuels, is an important topic in biomass conversion. Here, we report the design and synthesis of hydrophobic aluminosilicate ZSM-5 zeolites by an organosilane-modification approach (ZSM-5-OS). Water-droplet contact angle tests confirm the formation of hydrophobic surface after the modification. Interestingly, the obtained ZSM-5-OS catalysts exhibit excellent catalytic properties in dehydration of various alcohols into the corresponding olefins in water solvent. The approach reported in this work would be potentially important for developing more efficient catalysts for biomass conversion in the future.

Full text of DOI:10.1016/j.catcom.2016.02.006

Hydrophobic aluminosilicate zeolites as highly efficient catalysts for the
dehydration of alcohols
Shaodan Xu, Huadong Sheng, Tao Ye, Dan Hu, Shangfu Liao
PII:
DOI:
Reference:
S1566-7367(16)30053-X
doi: 10.1016/j.catcom.2016.02.006
CATCOM 4587
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
24 November 2015
3 February 2016
5 February 2016
Please cite this article as: Shaodan Xu, Huadong Sheng, Tao Ye, Dan Hu, Shangfu Liao,
Hydrophobic aluminosilicate zeolites as highly efficient catalysts for the dehydration of
alcohols, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.02.006
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ACCEPTED MANUSCRIPT
Hydrophobic Aluminosilicate Zeolites as Highly Efficient Catalysts
for the Dehydration of Alcohols
Shaodan Xu a,b, Huadong Sheng a, Tao Ye a,c, Dan Hu a, Shangfu Liao a,*
a Zhejiang Institute of Quality Inspection Science, Hangzhou 310018, China
Tel: +86-0571-85129819;
E-mail: liaoshangfu057@163.com
b Key Laboratory of Applied Chemistry of Zhejiang Province, Department of
Chemistry, Zhejiang University, Hangzhou 310027, China
cMOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
310027, China.
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Abstract: Efficient dehydration of alcohols to olefins, acting as a control step in the
upgrading of phenolic biofuel into alkane fuels, is an important topic in biomass
conversion. Here, we report the design and synthesis of hydrophobic aluminosilicate
ZSM-5 zeolites by an organosilane-modification approach (ZSM-5-OS).
Water-droplet contact angle tests confirm the formation of hydrophobic surface after
the modification. Interestingly, the obtained ZSM-5-OS catalysts exhibit excellent
catalytic properties in dehydration of various alcohols into the corresponding olefins
in water solvent. The approach reported in this work would be potentially important
for developing more efficient catalysts for biomass conversion in the future.
Keywords: ZSM-5 zeolite, dehydration of alcohols, hydrophobic, organosilane
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1. Introduction
The fast pyrolysis of lignocellulosic biomass, which is regarded as a renewable
feedstock, has attracted much attention for the production of phenolic bio-oil as a
promising alternative to fossil fuels [1-8]. However, the direct use of the as-prepared
phenolic bio-oil is impossible, because of the unfavorable features of high oxygen
content, low energy density, low stability, and high viscosity [9-14]. In order to solve
these problems, the upgrading of phenolic bio-oil by hydrodeoxygenation strategy has
been developed to obtain high-quality alkane-oil [10,15]. Typically, the hydrogenation
process over combined metal (e.g. Ru, Pt) and acid catalysts (e.g. HCl, acid resins,
zeolites) contains consecutive steps including hydrogenation of phenolic molecules to
alcohols, dehydration of alcohols to olefins, and hydrogenation of olefins to alkanes.
In this process, the hydrogenation and dehydration reactions occurred on the metal
and acid sites, respectively. Particularly, the step of acid-catalyzed alcohol
dehydration is greatly important for the cleavage of C-O bonds, which is reported to
be a rate-control step in the bio-oil upgrading process [9]. Therefore, developing
highly efficient catalyst for dehydration of alcohols in water solvent is important for
obtaining high-quality alkane-oils from biomass [16-20].
Recently, many homogeneous acid catalysts have been found to be active for the
dehydration of alcohols, including phosphoric acid and acidic ionic liquids [9, 21].
However, their homogeneous feature produces difficulty in catalyst separation and
regeneration from the reaction system. More recently, aluminosilicate zeolites (e.g.
ZSM-5, Beta) as typical examples of solid acids have been employed to catalyze the
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dehydration of alcohols in bio-oil upgrading [22-25]. Because of the obvious
advantages of high stability, abundant acidic sites, and heterogeneous feature, zeolites
are regarded to be one of the most promising catalysts for the dehydration of alcohols.
However, it should be noted that the catalytic activity of aluminosilicate zeolites still
could not satisfy all the requirements of an efficient catalyst, even some methods have
already been used to enhance the zeolite activity, high reaction temperature (≥ 150
C) was still necessary to obtain the olefin products [22, 25].
In this work, we report an efficient approach method to enhance the catalytic
activities of aluminosilicate zeolite in dehydration of alcohols into olefins by
improving the zeolite hydrophobicity. The hydrophobic zeolites were synthesized by
an organosilane-modification approach (ZSM-5-OS). The ZSM-5-OS catalysts with
hydrophobic surface could promote the fast diffusion of water species from the acid
sites, and motivate the reaction balance to the formation of olefin products, giving
10-fold higher activities than the conventional ZSM-5 catalyst. More importantly, the
ZSM-5-OS catalyst is stable and shows high activities even after several recycling
steps. Furthermore, by combining with Ru nanoparticles, the obtained Ru/ZSM-5-OS
catalysts are highly efficient for the direct hydrodeoxygenation of phenolic molecules
into alkanes, because of the efficiency of hydrophobic ZSM-5-OS catalysts for
dehydration of alcohols, which is a key step in the hydrodeoxygenation processes.
2. Experimental
2.1 Sample preparation
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All reagents were of analytical grade and used as purchased without further
purification. The zeolites are all in H-form.
Synthesis of ZSM-5 zeolite. As a typical run, 8 ml of tetrapropyl ammonium
hydroxide (TPAOH, 20 wt%) and 0.09 g of sodium aluminate (NaAlO2) were added
into 20 ml of water. After stirring at room temperature for 2 h, 7 ml of tetraethyl
orthosilicate (TEOS) was added and the resulted mixture was stirred overnight. Then
the gel was transferred into an autoclave to crystallize at 180 C for 3 days. After
filtrating, drying, and calcining at 550 C for 4 h, the ZSM-5 zeolite was obtained.
Synthesis of ZSM-5-OS samples. As a typical run, 1g of ZSM-5 was dried at 120
ºC under vacuum for 3 h, followed by the addition of 50 ml of anhydrous toluene
containing 1.2 g of organosilane. The mixtures were refluxed overnight and collected
by rotary evaporation, followed by washing with a large amount of anhydrous toluene
and ethanol. The sample obtained was designated as ZSM-5-OS. The experiments
above were carried out in anhydrous conditions to avoid the reaction between
organosilane and H2O.
Synthesis of Ru/ZSM-5 and Ru/ZSM-5-OS-C16. As a typical run, 1 g of the
ZSM-5 was stirred in 50 ml of RuCl3 solution with desired Ru concentration for 12 h,
followed by evaporating the excess water at 80 °C, heating at 100 °C for 12 h, and
washing with a large amount of water. Then the solid powder was calcined in air at
400 °C for 2 h, and reduced by H2 at 250 °C for 2 h to obtain Ru/ZSM-5.
Ru/ZSM-5-OS-C16 was obtained by modifying Ru/ZSM-5 with
n-Hexadecyltrimethoxysilane. By ICP analysis, the Ru loadings were established at
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0.78 and 0.81wt% for Ru/ZSM-5 and Ru/ZSM-5-OS-C16, respectively. Additionally,
the dispersion of Ru atom (number of Ru atoms on the nanoparticle surface / total Ru
atoms) was determined by the pulse CO adsorption tests, obtaining 23.3 and 20.1%
for Ru/ZSM-5 and Ru/ZSM-5-OS-C16, respectively.
2.2 Sample characterization
X-ray diffraction (XRD) data were collected on a Rigaku D/MAX 2550
diffractometer with Cu KR radiation (=1.5418 Å). Si/Al ratios were determined by
inductively coupled plasma (ICP) analysis (Perkin-Elmer 3300DV). Nitrogen sorption
isotherms were measured using a Micromeritics ASAP 2020 system. The contact
angles of water and 1-hexene droplets on the solid surface were measured on an
Optical Contact Angle Meter (SL200KB). Scanning electron microscopy (SEM) was
performed using a Hitachi SU 1510. FT-IR spectra were recorder on a Bruker 66V
FTIR spectrometer. The pulse CO adsorption test was performed on FINETEC
Finesorb-3010. The adsorption capacity of water and cyclohexanol was tested in a
static reactor at room temperature; the solid samples were degased at 180 C under
vacuum for 14 h before the test. 50 mg of catalyst was added into a mixture of alcohol
(2 mmol) and water (10 ml). Under stirring at room temperature, the concentration of
alcohol in water was analyzed by GC analysis using phenol as internal standard.
2.3 Catalytic tests
The dehydration reactions were performed in a 100-mL high-pressure autoclave
with a magnetic stirrer (900 rpm). The substrate and powder catalyst were mixed in
the reactor, then N2 was introduced and kept at desired pressure. The reaction system
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was heated to a given temperature. After the reaction, the product was taken out from
the system and analysed by gas chromatography (GC-17A, Shimadzu, using a flame
ionization detector) with a flexible quartz capillary column coated with OV-17 and
FFAP.
The unconverted alcohol substrate and olefin product could be quantified by
using dodecane as internal standard. The alcohol conversion and olefin selectivity
were calculated according to the following equation:
Conversion = [1 – (mol of unconverted alcohol / mol of alcohol in the starting
liquor)] * 100%
Selectivity = [mol of olefin product / (mol of alcohol in the starting liquor - mol
of unconverted alcohol)] * 100%
The hydrodeoxygenation reactions were performed under similar conditions by
using H2 in the autoclave. The recyclability of the catalyst was tested by separating it
from the reaction system by centrifugation, washing with large quantity of ethanol
and drying at 100 C for 4 h.
3. Results and Discussion
The ZSM-5-OS samples were easily synthesized by modifying the zeolite
surface with various organosilane in anhydrous toluene solvent. The samples modified
with different organosilanes of dimethyl dimethoxy silane, n-propyltrimethoxysilane,
dimethoxydiphenylsilane, and n-hexadecyltrimethoxysilane are denoted as
ZSM-5-OS-C1, ZSM-5-OS-C3, ZSM-5-OS-C6, and ZSM-5-OS-C16, respectively.
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Figure 1A shows the XRD patterns of various synthesized ZSM-5 samples, exhibiting
good crystallinities associated with typical MFI structure [26]. Figure 1B shows the
SEM images of these synthesized ZSM-5 samples, no significant difference in
morphology was observed before and after the modification treatment. Further
analysis show that all ZSM-5 samples have similar surface area (380~415 m2/g, Table
1), Si/Al ratios (33-38, Table 1), and micropore sizes (0.54-0.56 nm, Table S1). These
results demonstrated that the structure of ZSM-5 zeolite was well maintained during
the synthesis of ZSM-5-OS samples [26].
Figure 2 shows the FT-IR spectra of the ZSM-5 based samples. Compared the
spectrum of as-synthesized ZSM-5, the spectra of various ZSM-5-OS samples gave
significant bands at 2845-2981 cm-1, associating with the C-H bonds of the methyl,
methene and aromatic groups [27]. Additionally, the ZSM-5-OS-C16 gave additional
bands at 779 and 1000 cm-1, associated with the presence of benzene ring [27]. These
results indicate the successful modification of the organic groups on ZSM-5 zeolite.
The loading of organosilane on the samples were studied by total organic carbon
(TOC) analysis, obtaining 0.40, 0.33, 0.30, 0.26 mmol/g on ZSM-5-OS-C1,
ZSM-5-OS-C3, ZSM-5-OS-C6 and ZSM-5-OS-C16 samples, respectively. Furthermore,
the probe reactions of condensation of benzaldehyde with 2-hydroxyacetophenone
(CBH) and condensation of benzaldehyde with 1-pentanol (CBP) were performed on
the catalysts, which are typical reactions occurring on the external acid sites of ZSM-5
crystals because of the bulky product molecules. Interestingly, the ZSM-5-OS-C1 and
ZSM-5-OS-C16 catalysts exhibited much lower substrate conversion than
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as-synthesized ZSM-5 (See Figure S1 in Supporting Information for details),
suggesting the external acid sites of the zeolite crystal are inhibited by the grafting of
organosilanes. It is noted that the organosilanes should be grafted on the external
surface of zeolite crystal instead of on the internal framework, because the Sn-OH
groups, which reacted with organosilane precursor, are mainly presented on the extern
surface. Moreover, we performed water-droplet contact angle test to study the
hydrophobicity/hydrophilicity of the zeolite samples. The conventional ZSM-5 zeolite
showed CA of 15, assigning to the hydrophilic surface. Interestingly, after
organosilane-modification treatment by dimethyl dimethoxy silane and
n-propyltrimethoxysilane, the ZSM-5-OS-C1 and ZSM-5-OS-C3 samples showed CA
of 85 and 88 (Table 1), indicating improvement in the zeolite surface hydrophobicity.
When dimethoxydiphenylsilan and n-hexadecyltrimethoxysilane were employed in
the modification treatment, the obtained ZSM-5-OS-C6 and ZSM-5-OS-C16 showed
higher CA at 121 and 134 (Table 1), because long-chain alkane groups (C16) and
benzene group (C6) are more hydrophobic than the short-chain group (C1 and C3).
These results indicate the successfully changed hydrophobicity of the ZSM-5-OS
samples by organic group modification method (Figure S2).
The study of the catalytic performances of the ZSM-5-OS catalysts started from
employing dehydration of cyclohexanol to cyclohexene as a model reaction (Table 2).
Interestingly, the ZSM-5-OS catalysts exhibited significantly improved cyclohexanol
conversion compared with the conventional ZSM-5 catalyst. For example, as listed in
Table 2, the ZSM-5-OS catalysts exhibit cyclohexanol conversion at 56.3-76.0%,
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much higher than 19.2% over ZSM-5 catalyst under the same reaction conditions.
Notably, the ZSM-5-OS-C16 catalyst exhibited the highest cyclohexanol conversion
(76.0%) than the other ZSM-5-OS catalysts. For comparison, the homogeneous
H3PO4 catalyst, which is reported to be highly active dehydration catalyst at high
temperature (200 C), exhibited a low conversion of 1.2% [21]. Additionally, the
conventional acid catalysts, including homogeneous H2SO4 and heterogeneous
Al-SBA-15 and Amberlyst-15 catalysts all exhibit lower cyclohexanol conversions
(2.2-19.2%) than ZSM-5-OS-C16. These results indicate the excellent catalytic
activities of the organosilane-modified catalysts, especially ZSM-5-OS-C16 catalyst.
Notably, the turnover frequency (TOF) of ZSM-5-OS-C16 could reach 27.0 h-1 (Figure
3A), which is 10-fold higher than that (2.5 h-1) of conventional ZSM-5 catalyst.
Considering that ZSM-5-OS-C16 and conventional ZSM-5 have the same zeolite
framework, the superior catalytic activity of ZSM-5-OS-C16 should be attributed to
the hydrophobic surface, which could efficiently enrich the alcohol substrate and keep
water away from the acid sties. These features are favorable for the dehydration
reactions, thus promoting the shift of reaction balance to the formation of cyclohexene
[21] (Figure S2). We also studied the carbon balance before and after the reaction.
After the dehydration of cyclohexanol over ZSM-5-OS-C16 catalyst, by a simple
washing with methanol, the carbon balance is analyzed and calculated to be >99.5%.
Importantly, the ZSM-5-OS-C16 could be recycled very easily by filtrating from the
reaction system, followed by washing and drying. Even in the fifth run, the
ZSM-5-OS-C16 catalyst still showed cyclohexanol conversion at 73.4%, which is
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comparable with the as-synthesized ZSM-5-OS-C16 catalyst (76.0%, Figure 3B). After
reaction, the used ZSM-5-OS-C16 catalyst showed a water-droplet CA of 126 (Figure
3D), which is comparable with that of the as-synthesized catalyst (134, Figure 3C),
suggesting that the hydrophobic surface is stable during the reaction process.
Furthermore, by treating the ZSM-5-OS-C16 catalyst in boiling water for 12 h and
filtrating to separate the solid, no organic species were detected by GC analysis in the
obtained liquor, suggesting no leaching of C16 group. These results confirm the high
stability and recyclability of ZSM-5-OS-C16 catalyst.
We also studied the catalytic performances of the hydrophobic catalysts in
dehydration of other alcohols, including cyclopentanol, 2-hexanol, and phenylethyl
alcohol (Table 2). In all these cases, the ZSM-5-OS-C16 always exhibited much higher
alcohol conversion (34.4-69.0%) than the conventional ZSM-5 zeolite (4.3-12.2%),
indicating the wide substrate scope of ZSM-5-OS-C16 in the dehydration reactions.
The features of ZSM-5-OS-C16, including wide substrate scope, easy preparation,
high stability and recyclability, make it potentially important for the wide application
in the future.
The success in alcohol dehydration motivated us to extend the use of
ZSM-5-OS-C16 catalyst in the direct hydrodeoxygenation phenol into alkane. The
phenol was employed as a bio-oil model molecule (Scheme S1) [9, 21]. By combining
with Ru sites, the obtained Ru/ZSM-5-OS-C16 exhibit a cyclohexane yield of 97.9% at
130C in water solvent, which is higher than that (66.4%) over the conventional
ZSM-5 supported Ru (Ru/ZSM-5, Figure 3E), and even comparable with the reported
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highly effective catalysts used at relatively high temperatures (150-220C) [21]. The
excellent catalytic performance of ZSM-5-OS-C16 catalyst in phenol
hydrodeoxygenation is reasonably attributed to the hydrophobic surface, which could
enhance the activity in cyclohexanol dehydration, a control step in phenol
hydrodeoxygenation.
4. Conclusion
In summary, the ZSM-5 zeolites with hydrophobic surface were designed and
synthesized by an organosilane-modification approach (ZSM-5-OS). The hydrophobic
ZSM-5-OS catalysts exhibit 10-fold higher catalytic activities than the conventional
ZSM-5 catalyst, because the hydrophobic surface could efficiently enrich the alcohol
substrate and keep water away from the acid sties, which are favorable for the
dehydration reactions. More importantly, the ZSM-5-OS catalyst is stable and easily
recyclable. Furthermore, by combining with Ru nanoparticles, the obtained
Ru/ZSM-5-OS catalysts are highly efficient for the direct hydrodeoxygenation of
phenolic molecules into alkanes, because of the efficiency of hydrophobic ZSM-5-OS
catalysts for dehydration of alcohols, a key step in the hydrodeoxygenation processes.
It is believed that the approach developed in this work would be potentially important
for synthesizing more efficient catalysts for dehydration of biomass feedstocks in the
future.
Acknowledgements
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This work is supported by the Science and Technology Project of General
Administration of Quality Supervision, Inspection and Quarantine of the People's
Republic of China (AQSIQ, No. 2014QK203).
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Captions
Table 1. Dehydration of alcohols to olefins over various catalysts in water solvent.
Table 2. Catalytic dehydration of alcohols to olefins over various catalysts in water
solvent.a
Figure 1. (A) XRD patterns and (B) SEM images of various ZSM-5 samples. (a)
Conventional ZSM-5, (b) ZSM-5-OS-C1, (c) ZSM-5-OS-C3, (d) ZSM-5-OS-C6, and
(e) ZSM-5-OS-C16.
Figure 2. FT-IR spectra of various samples.
Figure 3. (A) Turnover frequencies (TOFs) of various synthesized catalysts in
dehydration of cyclohexanol. The reaction conditions are the same to those in Table 2
except reaction time of 20 min. The TOFs are calculated based on the number of Al
atoms in the reaction system. (B) Recycle tests of ZSM-5-OS-C16 in the dehydration
of cyclohexanol. Water-droplet CA of (C) as-synthesized and (D) recycled
ZSM-5-OS-C16 catalyst. (E) Cyclohexane yield of Ru/ZSM-5 and Ru/ZSM-5-OS-C16
in hydrodeoxygenation of phenol to cyclohexane.
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Table 1. Dehydration of alcohols to olefins over various catalysts in water solvent.
SBET
(m2/g)b
399
Crystal sizes
(nm)c
300
CA
()d
15
Sample
Organosilane
Si/Ala
ZSM-5
--
38
35
37
33
35
ZSM-5-OS-C1
ZSM-5-OS-C3
ZSM-5-OS-C6
Dimethyl dimethoxy silane (C1)
n-Propyltrimethoxysilane (C3)
Dimethoxydiphenylsilane (C6)
388
290
88
380
309
85
415
315
121
134
ZSM-5-OS-C16 n-Hexadecyltrimethoxysilane (C16)
400
320
a Molar ratio of Si/Al by (Inductively Coupled Plasma) ICP analysis;
b BET surface area;
c Mean sizes of the zeolite crystals, by counting more than 50 crystals in SEM images;
d Water-droplet contact angle (CA) on the solid surface.
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Table 2. Catalytic dehydration of alcohols to olefins over various catalysts in water
solvent.a
Entry
Substrate
Product
Catalyst
Conversion Selectivity
T(C)
(%)
(%)
1
2
ZSM-5
150
150
150
150
150
120
150
150
150
150
19.2
>99.0
ZSM-5-OS-C1
ZSM-5-OS-C3
ZSM-5-OS-C6
ZSM-5-OS-C16
ZSM-5-OS-C16
56.3
56.7
67.0
76.0
70.7
1.2
>99.0
>99.0
>99.0
>99.0
>99.0
69
3
4
5
6
b
7
H3PO4
8
H2SO4
19.2
2.2
97.5
9
Al-SBA-15
Amberlyst-15
>99.0
>99.0
10
9.4
Benzenesulfonic
acid
11
150
16.9
>99.0
12
13
ZSM-5
150
150
12.2
69.0
98.9
98.1
ZSM-5-OS-C16
14
15
ZSM-5
150
150
4.3
>99.0
>99.0
ZSM-5-OS-C16
34.4
16
17
ZSM-5
165
165
11.4
65.0
>99.0
>99.0
ZSM-5-OS-C16
a Reaction conditions: 1 mmol of alcohol, 50 mg of catalyst, 5 ml of water, 3 MPa of N2, reaction
time of 20 h.
b The data are from Ref. [21].
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(B)
(A)
a
c
b
d
ZSM-5
ZSM-5-OS-C1
ZSM-5-OS-C3
ZSM-5-OS-C6
e
ZSM-5-OS-C16
5
10 15 20 25 30 35 40
2Theta (Degree)
Figure 1. (A) XRD patterns and (B) SEM images of various ZSM-5 samples. (a)
Conventional ZSM-5, (b) ZSM-5-OS-C1, (c) ZSM-5-OS-C3, (d) ZSM-5-OS-C6, and
(e) ZSM-5-OS-C16.
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2981
2929
2845
ZSM-5-OS-C16
779
1000
2953
ZSM-5-OS-C6
ZSM-5-OS-C3
ZSM-5-OS-C1
ZSM-5
4000
3500
3000 1500
1000
500
Wave number (cm-1)
Figure 2. FT-IR spectra of various samples.
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(E)
(B)
(C)
(A)
80
60
40
20
0
30
25
20
15
10
5
100
80
60
40
20
0
CA=134 
(D)
CA=126 
0
1
2
3
4
5
Catalyst
Catalyst
Runs
Figure 3. (A) Turnover frequencies (TOFs) of various synthesized catalysts in
dehydration of cyclohexanol. The reaction conditions are the same to those in Table 2
except reaction time of 20 min. The TOFs are calculated based on the number of Al
atoms in the reaction system. (B) Recycle tests of ZSM-5-OS-C16 in the dehydration
of cyclohexanol. Water-droplet CA of (C) as-synthesized and (D) recycled
ZSM-5-OS-C16 catalyst. (E) Cyclohexane yield of Ru/ZSM-5 and Ru/ZSM-5-OS-C16
in hydrodeoxygenation of phenol to cyclohexane.
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Graphical abstract：
Hydrophobic surface
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Research highlights：
ZSM-5-OS samples were synthesized by organic-silane modification
method.
ZSM-5-OS samples have hydrophobic surfaces.
ZSM-5-OS catalysts are efficient for dehydration of alcohols.
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