Efficient Transformation of Anisole into Methylated Phenols over High-Silica HY Zeolites under Mild Conditions

Article abstract of DOI:10.1002/cctc.201500479

Transformation of anisole (methoxybenzene), a typical component of bio-oil, into phenol and methylated phenols was studied over HY zeolites with framework Si/Al ratios of 5, 15, 25, and 35. It was demonstrated that the amounts of Bronsted acid and Lewis acid sites, the ratio of Lewis acid and Bronsted acid sites, and the textural properties of the zeolites were all crucial parameters that influenced the catalytic performance. The Bronsted acid and Lewis acid sites of the zeolites catalyzed the reaction cooperatively and efficiently promoted the transmethylation. The hierarchical channel system with fully open micropore-mesopore connectivity was favorable to reduce the amount of coke generated.

Full text of DOI:10.1002/cctc.201500479

DOI: 10.1002/cctc.201500479
Communications
Efficient Transformation of Anisole into Methylated
Phenols over High-Silica HY Zeolites under Mild
Conditions
Qinglei Meng, Honglei Fan, Huizhen Liu, Huacong Zhou, Zhenhong He, Zhiwei Jiang,
Tianbin Wu, and Buxing Han*[a]
Transformation of anisole (methoxybenzene), a typical compo-
nent of bio-oil, into phenol and methylated phenols was stud-
ied over HY zeolites with framework Si/Al ratios of 5, 15, 25,
and 35. It was demonstrated that the amounts of Brønsted
acid and Lewis acid sites, the ratio of Lewis acid and Brønsted
acid sites, and the textural properties of the zeolites were all
crucial parameters that influenced the catalytic performance.
The Brønsted acid and Lewis acid sites of the zeolites catalyzed
the reaction cooperatively and efficiently promoted the trans-
methylation. The hierarchical channel system with fully open
micropore–mesopore connectivity was favorable to reduce the
amount of coke generated.
Figure 1. Methoxy group in the common lignin linkages adapted from
Pollution problems associated with the use of fossil fuels and
the dwindling reserves of these fuels have resulted in in-
creased attention on alternative sources. Biomass is an impor-
tant renewable carbon resource for fuels and chemicals.[1]
Among the various components of biomass, the transforma-
tion of lignin has attracted much attention in recent years.
Lignin is considered to be the major aromatic resource of bio-
based economy.[2] Some lignin structural linkage units are
shown in Figure 1.[3] The direct use of bio-oil as a liquid fuel is
currently not feasible owing to its high oxygen content and in-
stability.[4] The transformation of methoxyphenols into liquid al-
kanes for bio-oil upgrading by cracking or hydrolysis, catalytic
reduction, and catalytic oxidation has been extensively studied.
Unfortunately, current upgrading process are usually accompa-
nied by the generation of CH4,[5] CH3OH,[5,6] or other highly vol-
atile products by demethylation or demethoxylation of the
methoxy groups, which approximately account for 10–25% of
lignin.[7] Transmethylation offers an atom-economic way to pre-
vent carbon from being lost as a high-volatile product, and the
methylated phenolic products can be further transformed into
Huber et al.[3]
high-octane fuels by hydrodeoxygenation, and they are also
fundamental raw materials of organic chemicals.[8]
Zeolites are widely used to catalyze various kinds of reac-
tions. They have also been used in the transmethylation of ani-
sole (methoxybenzene), which is a typical component of bio-
oil.[9a] For example, Zhu et al. studied the transmethylation of
anisole over HZSM-5 zeolite at approximately 200–5508C.[9a] It
was shown that the reaction temperature had a significant
impact on the yields of the methylated products. At 2508C,
only 23% conversion of anisole was achieved, and the yields
of the methylated products cresol and 2,4-xylenol were ap-
proximately 6 and 1%, respectively. By contrast, 95% conver-
sion was achieved to provide cresol in 36% yield and 2,4-xyle-
nol in 12% yield at 4508C. They also reported the bifunctional
transmethylation and hydrodeoxygenation of anisole over
a Pt/HBeta catalyst at 4008C.[9b] Wang et al. investigated the se-
lective synthesis of para-cresol by conversion of anisole on
HZSM-5 zeolites at 4508C.[10a] The selectivity for para-cresol in-
creased significantly by silylation and steaming treatments.
Prasomsri et al. investigated the conversion of pure anisole
and its mixtures with tetralin over HY and HZSM-5 zeolites at
4008C.[10b] The dominant reaction of pure anisole was transal-
kylation, which produced phenol, methylated phenols, and
methylanisoles as the main products, with significant catalyst
deactivation. Co-feeding tetralin had a beneficial effect on ani-
sole conversion and reduced coke formation. In general, reac-
tions over zeolites have been studied under harsh reaction
conditions, and the effects of the properties of zeolites on the
transmethylation need to be studied further.
[a] Q. Meng, H. Fan, H. Liu, H. Zhou, Z. He, Z. Jiang, T. Wu, Prof. B. Han
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Colloid and Interface and
Chemical Thermodynamics
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: hanbx@iccas.ac.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500479.
This publication is part of a Special Issue on Carbon in Catalysis. Once
the full issue has been assembled, a link to its Table of Contents will
appear here.
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Table 1. Textural and acid properties of the HY and OHY zeolites.
[a]
Entry
Catalyst
Si/AlF
Surface area
Pore volume
Total amount [mmolgÀ1, 2008C] and distribution of acid sites[f]
[m2 gÀ1
]
[cm3 gÀ1
]
[b]
[c]
[d]
[e]
Smicro
Smeso
Vmicro
Vmeso
Brønsted
Lewis
B+L[g]
L/B[h]
1
2
3
4
5
HY5
4.97
14.1
23.4
34.3
23.4
665
467
382
319
380
154
355
423
483
445
0.275
0.185
0.156
0.121
0.155
0.090
0.162
0.192
0.226
0.204
1125
355
229
157
206
239
96
125
60
1364
451
354
217
227
0.21
0.27
0.55
0.38
0.10
HY15
HY25
HY35
OHY25
21
[a] Si/AlF denotes the framework Si/AlF ratio of the HY zeolite obtained by XRD. [b] The t-plot micropore surface area. [c] The t-plot mesopore surface area.
[d] The t-plot micropore volume. [e] BJH mesopore volume. [f] The total amount of acid sites in the samples was calculated according to equations in
Ref. [15] [g] “B+L” represents the sum of the amounts of Brønsted acid and Lewis acid sites. [h] ”L/B” denotes the ratio of the amounts of Lewis acid and
Brønsted acid sites.
In this work, we studied the transmethylation of anisole over
HY zeolites with different properties under mild conditions. It
was found that the acidic and textural properties of the zeo-
lites influenced the reaction significantly. HY zeolites with
larger Si/AlF ratios (framework Si/Al ratio, mol) showed excel-
lent transmethylation performance because of suitable textural
properties and the cooperative effects of the Brønsted acid
and Lewis acid sites in the catalysis.
scripts denote the Si/Al ratios) were all very high under the ex-
perimental conditions. However, the product distribution de-
pended considerably on the properties of the zeolites used. A
comparative experiment was conducted by using HZSM-5 zeo-
lite as the catalyst (Table 2, entry 6). The conversion of anisole
and yields of methylated phenols over the HZSM-5 zeolite
were much lower than those on the high-silica HY zeolites at
2408C. Taking into account the topology of the zeolite, the
framework of HZSM-5 zeolite contains two perpendicularly in-
tersecting channel systems composed of straight channels
with dimensions of 0.51”0.54 nm and zigzag channels with
free dimensions of 0.54”0.56 nm and only one type of frame-
work hydroxy groups, which are probably located near the
channel intersections.[11a–c] A high temperature is required to
overcome the diffusional resistance of anisole from the pore
openings to the intersections, where the transmethylation can
be catalyzed on the Brønsted acid sites.[11d] However, the fauja-
site structure of the HY zeolites has a 3D channel system with
supercages (1.3 nm) connected tetrahedrally through 12-mem-
bered silicate ring openings that are 0.74 nm in diameter.[11e]
The hierarchical channel system with fully open micropore–
mesopore connectivity of the HY zeolites is better for the diffu-
sion of reactant and product.
The HY zeolites used were characterized by pyridine-ad-
sorbed Fourier transformed infrared spectroscopy (Py-FTIR), X-
ray diffraction (XRD), magic-angle spinning (MAS) nuclear mag-
netic resonance (NMR) spectroscopy, N2 adsorption/desorption,
and transmission electron microscopy (TEM). Details are dis-
cussed in the Supporting Information (Figures S1–S6 and Ta-
bles S1 and S2). Table 1 presents their Si/AlF ratios and their
textural and acid properties.
The transformation of anisole over different zeolites was
studied, as shown in Scheme 1 and Table 2. The conversions of
anisole on the HY catalysts (HY5, HY15, HY25, and HY35; sub-
Besides the textural structure, the properties of the acid
sites are also important parameters for the transmethylation
reaction. At a given Si/AlF ratio, differences in the zeolite acidi-
ty between various sites and frameworks is generally due to
differences in both the local structures and crystal potenti-
als.[12a] As reported previously, the framework SiÀOÀAl bond
angle in the local structure of the bridging OH group, which
depends on the zeolite structure type, always affects the acid
strength of the Brønsted acidic hydroxy protons.[12b] In HZSM-5
zeolite, the SiÀOÀAl bond angles vary from 137 to 1778,
whereas hydroxy protons are preferably bound to the more ac-
cessible bridging oxygen atoms of the double six-membered
ring (O1 positions) in zeolite HY, which limits the range of SiÀ
OÀAl bond angles from 138 to 1478, and this makes the O(1)H
sites of high-silica HY zeolite more acidic than the “regular”
acidic sites in HZSM-5 at lower temperatures.[12] So, the high-
silica HY zeolite exhibits much better performance than HZSM-
5 under mild conditions. The HY25 zeolite with a Lewis acid/
Brønsted acid ratio of 0.55 (Table 1, entry 3) exhibited the high-
Scheme 1. Transformation of anisole on the HY zeolites.
Table 2. Transformation of anisole on different zeolite catalysts.[a]
Entry
Catalyst
Conversion
[wt%]
Yield [wt%]
1
2
3
4
coke[b]
1
2
3
4
5
6
HY5
HY15
HY25
HY35
OHY25
HZSM-5
94.3
96.3
96.6
94.6
68.0
40.3
22.8
31.0
31.7
36.2
25.9
17.2
40.4
45.2
51.5
48.0
19.0
5.3
2.5
2.0
1.9
2.7
8.4
5.4
3.3
3.6
4.1
2.8
2.7
0.8
18.3
10.5
5.3
3.9
5.5
7.5
[a] Reactions conditions: Anisole (0.010 mol), catalyst (0.1 g), N2
(0.5 MPa), 2408C, 3 h. [b] Equation (3) was used to calculate the coke
yield.
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est yields of the methylated phenols (Table 2, entry 3), al-
though HY5 had the maximum total amount of Lewis acids
and Brønsted acids. This suggests that Brønsted acids and
Lewis acids cooperatively promote the transmethylation. Re-
cently, the Gates group compared the activity of Pt/Al2O3 and
Pt/HY catalysts in the hydrodeoxygenation of anisole and
guaiacol.[13,14] Their results showed that the transalkylation ac-
tivity of the catalysts was significantly affected by the type of
acidic sites. Zhu et al. further confirmed the effect of acidic
sites on transalkylation activity in the hydrodeoxygenation of
anisole.[9b]
The properties of the Brønsted acids or Lewis acids in the
HY zeolites is generally related to the location of the aluminum
atoms within the zeolites.[16] With an increase in the Si/AlF
ratio, the number of Al atoms per unit cell (NAl/UC, Table S1) and
the relative content of the Si(OAl)n(OSi)4Àn units (Table S2, n=1,
2 and 3) in the HY zeolites decreases; this correlates with
a gradual decrease in the Brønsted acid sites of the HY zeolites
and proves the function of framework aluminum (FAl) species
as the predominant source of Brønsted acid sites.[17] Besides, it
is generally accepted that framework Al species in the micro-
porous skeleton are also the suppliers for the Lewis acid
sites.[18] The content of extra-framework oxoaluminum (EFAl) is
only 0.6%, and the amount of Lewis acid can still reach
239 mmolgÀ1 for HY5 zeolite (Table 1). In contrast to the HY5
and HY15 zeolites, HY25 possesses a lower content of FAl spe-
cies but a higher EFAl/FAl ratio (0.28, Table S2), which indicates
Figure 2. Proposed catalytic mechanism for the transmethylation of anisole
over high-silica HY zeolite.
As previously reported, solid acid catalyzed alkylation is
always accompanied with coke formation.[24] A comparison of
coke yield at nearly the same conversion of anisole is given in
Table 2. The amount of coke produced over the zeolites fol-
lows the order HY5 >HY15 >HY25 >HY35. As shown in Fig-
ure S5a, the uptake of nitrogen at a relative pressure (P/P0) ex-
ceeding 0.8 is justifiably attributed to the occurrence of a wide
pore-size distribution,[25] which is clearly evidenced by the mes-
opores with another broad distribution centered at approxi-
mately 20 nm in the HYn (n=15, 25, and 35) zeolites. The TEM
images (Figure S6) also reveal that as the Si/AlF ratio of the HY
zeolites increases, the integrity and angular shape of the crys-
tals get progressively worse and ill defined, respectively, which
coincides with the XRD analysis (Figure S2, Table S1). Especially
noteworthy is that the structures of HY15, HY25, and HY35 are
more mesoporous than that of HY5 zeolite (Figure S6). Con-
cretely, as the Si/AlF ratio of the HY zeolite increases from 5 to
35, the microporous surface area and volume decrease from
665 to 319 m2 gÀ1 and from 0.275 to 0.121 cm3 gÀ1, respectively
(Table 1). Furthermore, the mesoporous surface area and
volume of HY35 are almost 3.1 and 2.5 times higher than those
of the HY5 zeolite, respectively (Table 1). Therefore, it can be
deduced that the hierarchical porous HY25 and HY35 zeolites
with more mesopores and fully open micropore–mesopore
connectivity could provide more diffusional paths for the fast
diffusion of phenol and methylated phenols formed on the
acid sites located in the micropores and mesopores, and this,
therefore, retards coke formation.[26] In contrast, the amount of
coke was very large if the HY5 zeolite was used as the catalyst.
The main reason may be that it has very large amounts of FAl-
derived Brønsted acid sites in the long and narrow micropores
but a small ratio of Lewis acid and Brønsted acid sites. Conse-
quently, the primary products generated on the FAl-derived
acid sites within the HY5 zeolite are subjected to deep reac-
tions and the formation of coke precursors through a transalky-
lation route owing to diffusion limitations of the micropores.[27]
Subsequently, oligomerization of the coke precursors trapped
inside the micropores results in a higher coke yield and lowers
the yields of phenol and methylated phenols.[28]
that six-coordinated EFAl, such as AlO+, Al(OH)2+, and AlOH2+
,
or some neutral species, AlOOH and Al(OH)3 resonating in the
27Al MAS NMR spectrum of HY25 zeolite at dꢀ0 ppm (Fig-
ure S4),[19] should be responsible for the larger Lewis acid
quantity.[20] According to a previous study on electrophilic aro-
matic substitution reactions, Lewis acids can activate the ben-
zene ring, which tends to make one of the carbon atoms of
the benzene ring highly nucleophilic, and this facilitates elec-
trophilic aromatic substitution.[21] Lewis acidic EFAl species in
the HY25 zeolite can be considered as more effective activators
of the aromatic ring, owing to their readily accessible deposi-
tion in the voids of the zeolite.[21c–d]
In the catalysis of the high-silica HY zeolites, the cooperative
effect between the Brønsted acid and Lewis acid sites contrib-
utes significantly to the high yield of the desired product. The
cooperative mechanism of the Brønsted acids and Lewis acids
is shown in Figure 2. First, the methoxy group is transferred to
the Brønsted acid site of the HY zeolite to form a methyl car-
bocation, and then the benzene ring is attacked to form the
product through an electrophilic substitution reaction.[22] It is
the Lewis acid sites that promote the aromatic electrophilic
substitution reaction. To further demonstrate the contribution
of the Lewis acid sites, HY25 zeolite was treated with oxalic acid
(defined as OHY25, entry 5 of Table 1) to decrease the Lewis
acid quantity.[23] Compared with HY25, the conversion of anisole
and yield of methylated phenols were clearly lower over OHY25
zeolite (Table 2, entry 5). The results convincingly demonstrate
that Lewis acid sites, especially the Lewis acidic EFAl positions,
contribute to activating the aromatic ring and are indispensa-
ble to highly effective methylation.
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The above results indicate that HY25 is the best catalyst for
the transmethylation reaction among the zeolites tested owing
to its hierarchical channel system and suitable ratio of Brønst-
ed acid sites and Lewis acid sites. Therefore, we studied the ef-
fects of reaction temperature and time on the reaction per-
formance of HY25 zeolite. The conversion of anisole and the
yields of the products as a function of temperature are plotted
in Figure 3a. It can be seen that the conversion of anisole in-
creased from 45.7 to 99.6% as the temperature was changed
from 180 to 2808C. In this temperature range, the yield of
methylanisole began to decrease at 2008C. By contrast, the
yields of phenol, cresol, and xylenols kept increasing with tem-
perature from 180 to 2408C. The amount of coke increased
markedly at temperatures above 2208C, because oligomeriza-
tion of the coke precursors became more severe. The results in
Figure 3a show that the optimum temperature for the trans-
formation of anisole is 2408C. The effect of reaction time on
the reaction at 2408C is illustrated in Figure 3b. The yield of
methylanisole increased in the time range of 0 to 20 min and
then decreased, and the yield of the phenolic products such as
phenol, cresol, and xylenols increased with time until 180 min.
The yields of phenol and methylated phenolics decreased
slightly after 180 min, because the side reactions that produce
byproducts (e.g., xanthene and coke) became more pro-
nounced.
In conclusion, we studied the transformation of anisole into
phenol and methylated phenols over HY zeolites. It was found
that high-silica HY zeolites, such as HY25 and HY35, are highly
efficient for the transmethylation reaction under mild condi-
tions. HY25 zeolite showed the best transmethylation per-
formance, because it has both the proper ratio of Lewis acid
sites and Brønsted acid sites and a suitable porous structure.
The Brønsted acid and Lewis acid sites of the zeolites have an
excellent cooperative effect in promoting the transmethylation
reaction. The hierarchical channel system with fully open mi-
cropore–mesopore connectivity is favorable to reduce the
amount of coke generated. We believe that high-silica HY zeo-
lites have potential applications in atom-economic transmethy-
lation and lignin transformations.
Experimental Section
Typical experimental procedure
Anisole (1.0813 g) and the catalyst (0.10 g) were charged into
a 25 mL stainless-steel reactor. After flushing the reactor with N2
(3”), it was placed in an air bath of the desired temperature and
the stirrer was started. After a known time, the reactor was
quenched in an ice-water bath, and the organic products were ex-
tracted with acetonitrile. Quantitative analysis of the mixture was
performed by using gas chromatography (GC, HP 4890) with
a flame ionization detector and a DB-5 capillary column (30 m”
250 mm). The products were also identified by GC–MS (Shimadzu
QP 2010S) and by comparing the retention times of the standards
in the GC traces. The conversion and selectivity were calculated
from the GC data by using biphenyl as an internal standard.
The conversion was calculated by using Equation (1), the yield was
calculated by using Equation (2), and the coke yield was calculated
on the mass basis of the fresh and used catalysts [Eq. (3)]:[29]
ðmass of converted raw materialÞ
Conversion ½wt %Š ¼
 100 %
ðtotal mass of raw materialÞ
ð1Þ
ð2Þ
ðmass of each productÞ
Yield ½wt %Š ¼
 100 %
ðtotal mass of raw materialÞ
ðmass of used catalystÀmass of fresh catalystÞ
Coke yield ½wt %Š ¼
ðmass of loaded anisoleÞ
Â100 %
ð3Þ
Acknowledgements
The authors thank the National Natural Science Foundation of
China (21173234, 21373230, 21321063) and the Chinese Academy
of Sciences (KJCX2.YW.H30).
Figure 3. The effect of a) reaction temperature and b) time on the conver-
~
!
&
*
sion of anisole ( ) and on the yields of phenol ( ), cresol ( ), xylenols ( ),
^
tri/tetra/pentamethylphenol (3), methylanisole ("), xanthene ( ) and coke
&
( ). Reaction conditions for a) Anisole (0.010 mol), HY25 (0.1 g), N2 (0.5 MPa),
Keywords: Brønsted acids
transmethylation · zeolites
·
Lewis acids
·
lignin
·
180 min. Reaction conditions for b) Anisole (0.010 mol), HY25 (0.1 g), N2
(0.5 MPa), 2408C.
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Published online on July 16, 2015
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