Construction of Acid–Base Synergetic Sites on Mg-bearing BEA Zeolites Triggers the Unexpected Low-Temperature Alkylation of Phenol

Article abstract of DOI:10.1002/cctc.201601127

Novel Mg-bearing BEA zeolites are synthesized to simultaneously endow significantly enhanced basicity without compromising acidity over the zeolite framework. Serving as efficient solid acid–base bifunctional catalysts, they achieve the liquid-phase selective methylation of phenol with methanol to produce o- and p-cresol (o/p=2) under mild conditions. The method is readily extendable to the alkylation of phenols with various alcohols. Stereo- and regioselectivity (>95 % for p-product) was attained on the alkylation of phenol with bulky tert-butyl alcohol, rendering the first acid–base cooperative shape-selective catalysis relying on the basicity of zeolites. A preliminary mechanistic analysis reveals that the remarkable activity and shape-selectivity come from the superior special acidic–basic synergetic catalytic sites on the uniform microporous channels of the BEA zeolite.

Full text of DOI:10.1002/cctc.201601127

DOI: 10.1002/cctc.201601127
Full Papers
Construction of Acid–Base Synergetic Sites on Mg-bearing
BEA Zeolites Triggers the Unexpected Low-Temperature
Alkylation of Phenol
Jingyan Xie,[a] Wenxia Zhuang,[a] Wei Zhang,[a] Ning Yan,[b] Yu Zhou,*[a] and Jun Wang*[a]
Novel Mg-bearing BEA zeolites are synthesized to simultane-
ously endow significantly enhanced basicity without compro-
mising acidity over the zeolite framework. Serving as efficient
solid acid–base bifunctional catalysts, they achieve the liquid-
phase selective methylation of phenol with methanol to pro-
duce o- and p-cresol (o/p=2) under mild conditions. The
method is readily extendable to the alkylation of phenols with
various alcohols. Stereo- and regioselectivity (>95% for
p-product) was attained on the alkylation of phenol with bulky
tert-butyl alcohol, rendering the first acid–base cooperative
shape-selective catalysis relying on the basicity of zeolites. A
preliminary mechanistic analysis reveals that the remarkable
activity and shape-selectivity come from the superior special
acidic–basic synergetic catalytic sites on the uniform micropo-
rous channels of the BEA zeolite.
Introduction
Alkylation of aromatic hydrocarbons is a large industrial pro-
cess with the products covering numerous petrochemicals and
fine chemicals.[1–3] The processes are usually carried out over
acid catalysts.[4–6] For example, acid-catalyzed alkylation of ben-
zene with ethylene or propylene is used to produce two im-
portant bulk chemicals of ethylbenzene and isopropylben-
zene.[7,8] Compared with liquid acids, solid acids are preferred
owing to their various advantages, but the majority of them
take place at high temperature and in the gas phase.[9–11]
Nowadays, energy saving and environmental friendliness are
the two major elements involved in the development of sus-
tainable chemical processes,[12–17] preferring low-temperature
liquid-phase alkylation processes.[18,19] However, solid acids usu-
ally suffer from low activity in such cases, thus it is greatly at-
tractive and a challenge to develop new efficient solid acid cat-
alysts towards the target low-temperature liquid-phase alkyla-
tion process.
metal oxides,[20–22] zeolites,[23–25] sulfates,[26] mixed metal
oxides,[27–30] and hydrotalcites.[31] MCM-22 with different Si/Al
has only approximately 20% selectivity for the C-alkylation of
phenol at low pressure.[32] H-Beta zeolites with different crystal
sizes have been investigated for their effect on catalytic per-
formance for phenol methylation. Deactivation is triggered by
phenol and polyalkylated phenol derivatives with extending re-
action time.[23] Zeolite Zn2+/NaY generates strong Lewis acid
sites that efficiently activate the phenol methylation reaction.
However, the zeolite activity decays increasingly with the con-
comitant increase of sample acidity.[33] Zeolites as catalysts pro-
mote the O-alkylation at medium temperatures. Low-tempera-
ture liquid-phase reactions can relieve deactivation resulting
from cooking. Only one report of the liquid-phase alkylation of
phenol with methanol has been reported, in which zeolite cat-
alysts HZSM-5 and HMCM-22 were used, giving a maximum
yield of approximately only 2%.[34]
Among the various alkylation reactions, methylation of
phenol with methanol is one industrially available pathway to
produce cresols, important intermediates for manufacturing
functional polymers, antioxidants, pharmaceuticals, and agro-
chemicals. Various catalysts have been developed, such as
Zeolites are well-known microporous molecular sieves that
can be used as environmentally benign catalytic materials with
large-scale applications in industry, particularly in refinery/pet-
rochemical processes and the production of fine chemi-
cals.[35–38] They are usually used as solid acids because the
framework tetrahedral aluminum species provide internal
Lewis acid sites and the protons present as the counter cations
show Brønsted acidity.[36] However, their acid strength is far
from that of liquid acids (such as H2SO4, HCl, HF), giving rise to
inferior activity in low-temperature reactions, which is so far
one unresolved issue. Many approaches have been proposed
to improve the performance of solid acids under mild condi-
tions. One efficient way is the fabrication of acid–base bifunc-
tional sites that can reach high activity and selectivity, reduce
the operating temperature and side reactions.[39–41] Neverthe-
less, the approach is rarely successful for zeolites owing to the
[a] J. Xie, W. Zhuang, W. Zhang, Dr. Y. Zhou, Prof. Dr. J. Wang
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemical Engineering
Nanjing Tech University (former Nanjing University of Technology)
Nanjing, Jiangsu 210009 (P.R. China)
E-mail: njutzhouyu@njtech.edu.cn
junwang@njtech.edu.cn
[b] Prof. Dr. N. Yan
Department of Chemical and Biomolecular Engineering
National University of Singapore, 4 Engineering Drive 4
Singapore 117585 (Singapore)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/cctc.201601127.
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lack of robust basic sites, let alone for the construction of effi-
cient acid–base synergetic active sites in the zeolite channels.
In this work, superior framework acidity and basicity are si-
multaneously constructed by incorporating Mg ions into the
framework of BEA zeolites in a hydrothermal synthesis path-
way involving a special acid co-hydrolysis route. Modification
through ion-exchange with transitional metal ions is per-
formed to improve their basicity. Catalyzed by these Mg-bear-
ing zeolites, the low-temperature liquid-phase methylation of
phenol with methanol to produce o-/p-cresols is achieved with
high cresols yield and stable reusability. The influence of reac-
tion conditions such as temperature, time, and molar ratio of
substrates are systematically investigated. Various control cata-
lysts are tested in parallel to gain insight into the reaction. In
situ FTIR spectra of adsorbed substrates are performed to fur-
ther understand the mechanism. Alkylation of phenol with var-
ious alcohols is attained with the same method. With the
bulky substrate tert-butyl alcohol, high stereo-/regioselectivity
(>95% for the para product) is achieved thanks to the special
nano-confinement in the zeolite channels.
Results and Discussion
Mg-bearing BEA zeolites were synthesized in a one-pot hydro-
thermal process involving an acidic co-hydrolysis route
(Scheme S1 in the Supporting Information). We have revealed
that the acidic co-hydrolysis route favors the incorporation of
heteroatoms in the zeolite synthesis.[42,43] Controlling the pH
value in the initial co-hydrolysis/condensation of tetraethyl-
orthosilicate (TEOS) and magnesium nitrate gives the highly
crystallized zeolites with BEA phase, as confirmed by the XRD
patterns (Figure 1A and Figure S1 in the Supporting Informa-
tion). By contrast, no zeolite phase is detected from the direct
hydrolysis of the magnesium salt and TEOS under basic condi-
tions (Figure S1 in the Supporting Information). For the hydrol-
ysis and condensation of silica precursors, acidic conditions
offer a lower rate than basic ones; when together with metal
ions, the acidic conditions (rather than basic ones) allows more
isolated metal ions to be created, which readily form bond
linkages between the metal ions and silanol groups.[42,43] In this
work, the acid co-hydrolysis route is thus more likely to gener-
ate the desirable Si-O-Mg-O-Si-O-Al units, that is, the primary
building blocks for further growth of the Mg-containing zeolite
crystals, which may account for the indispensable role of the
acidic conditions in this synthesis. The directly synthesized Mg-
containing samples are named Mgb(n) (n=100, 55, or 33, de-
noting the Si/Mg molar ratio in the gel). Chemical composition
analyses by inductively coupled plasma (ICP; Table 1) demon-
strate the incorporation of Mg species in Mgb(n) with the
Si/Mg molar ratios of 151, 109, and 115 for n=100, 50, and 33.
All the samples have similar SiO2/Al2O3 molar ratios of approxi-
mately 31. Characterizations by SEM, TEM, thermogravimetric
analysis (TGA), nitrogen sorption measurements, and 29Si and
27Al NMR spectroscopic analyses confirm the crystal structure
has the morphology of ellipsoid-globose of 1–2 mm, has
a homo-dispersion of Mg ions, high thermal stability, and
abundant open microporosity (Table 1, Figure 1, and
Figure 1. A) XRD patterns of Mg-bearing zeolites and b. B),C) SEM images of
Mgb(50). D),E) TEM images with Si, Al, and Mg elemental mapping images
of Mgb(50). F) IR and G) UV/Vis spectra of different samples. H) 29Si and
I) 27Al MAS NMR spectra of Mgb(50).
Table 1. Textural properties of various BEA zeolites and contrast samples.
[b]
Sample
Si/Me[a]
[molmolÀ1
SiO2/Al2O3
Surface area
Pore volume
[cm3 gÀ1
]
]
[m2 gÀ1
]
Mgb(100)
Mgb(50)
Mgb(33)
Cr(ex)-Mgb
b
151
109
115
95[c]/131[d]
–
32.75
30.09
31.24
31
519
598
566
548
622
0.27
0.33
0.32
0.43
0.25
36.57
[a] Molar ratio of Si to Me for the final solid products. [b] Molar ratio of
SiO2 to Al2O3 for the final solid products. [c] Molar ratio of Si to Cr for the
final solid. [d] Molar ratio of Si to Mg for the final solid. All the molar
ratios are determined by ICP.
Figures S2–S4 and Table S1 in the Supporting Information). Fur-
ther, the unit-cell volume of Mgb(n) is apparently enlarged
(Table S2 in the Supporting Information); also, the samples ex-
hibit an IR band shift to approximately 1072 cmÀ1 for the tetra-
hedral TO4 units and new UV/Vis absorbance at approximately
245 nm.[44] In contrast, these variations are all absent for the
parent b, control samples of Mg(ex)-b (Mg ion-exchanged b)
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and
hanced basicity of Mgb(n). As for the acidic properties of
Mgb(n), NH3-TPD results (Figure 2D) reflect that they exhibit
similar NH3-TPD curves as the parent b with only a slight de-
crease of the shoulder peak at 2508C, suggesting that the ma-
jority of the acidity is preserved after Mg incorporation. All the
above phenomena reveal much enhanced basicity with well
retained super acidity for the Mgb(n) samples.
Mg(im)-b (MgO-loaded b by impregnation) (Figure 1F and G).
The observations are similar to previous Mg-containing zeo-
lites,[45] suggesting the formation of framework Mg ions that
differ from ion-exchanged and impregnated Mg species.
The X-ray photoelectron spectroscopy (XPS) core-level spec-
tra were collected on the typical sample Mgb(50) plus the con-
trols of b, Mg(ex)-b, and Mg(im)-b. In the O1s spectra, peaks
appear at 532.85, 533.5, 534.5, and 536 eV, respectively, for
Mgb(50), Mg(im)-b, Mg(ex)-b, and b (Figure 2A). A significant shift
A chromium ion-exchanged sample, Cr(ex)-Mgb, was prepared
from Mgb(50). Structural characterizations by XRD, SEM, and
nitrogen sorption experiments reveal the preservation of the
parent structure (Figure 1A, Figures S2 and S4 in the Support-
ing Information, and Table 1). The ICP analysis shows that
Cr(ex)-Mgb contains a similar Mg content as Mgb(50) and has
the Si/Cr molar ratio of 95. XPS and TPD results indicate that
Cr(ex)-Mgb exhibits additionally enhanced basicity and acidity
compared with its parent counterpart Mgb(50) (Figure 2).
The Mg-bearing BEA zeolites were next used as heterogene-
ous catalysts in the low-temperature liquid-phase methylation
of phenol with methanol. The reaction and even the side reac-
tions do not occur in the absence of catalyst, but Mgb(n) can
efficiently catalyze the methylation reaction. Mgb(50) (Table 2,
Table 2. Liquid-phase methylation of phenol with methanol.[a]
Entry Catalyst
Cphenol Ycresol Yanisole Smethyl Acid sites
[%][b] [%][c] [%][d] [%][e] [mmolgÀ1
Base sites
[mmolgÀ1
]
[f]
[g]
]
1
2
3
4
5
6
7
8
9
Mgb(50)
Cr(ex)-Mgb
Mg(ex)-b
Mg(im)-b
b
35.8 15.7[h] 19.1 97.2 1.58
49.8 30[h] 15.3 91.0 1.15
0.93
1.28
0.60
0.52
0.50
0.48
0.06
0.21
0.51
9.6
0.6
0.5
1.4
4.1
0.8
2.8[h]
0
5.7 88.5 1.42
0.2 33.3 1.22
Figure 2. A) O1s and B) Si2p XPS spectra for samples. C) CO2-TPD and
D) NH3-TPD curves for samples.
0.04
0.6
1.1
0
0
8.0 1.21
0.6 85.7 1.41
3 99.9 0.32
Hb
gAl2O3
MgO
to lower binding energies is observed for Mgb(50) in contrast
to b, which is attributable to the incorporated Mg ions with
weak electronegativity, which increases the electron density of
O atoms.[46] The Si2p peak is centered at around 107.1 eV for
b, which shifts to 105.5, 104.2, and 103.7 eV for Mg(ex)-b,
Mg(im)-b, and Mgb, respectively (Figure 2B). The observed simi-
lar shifts towards lower binding energies of Si2p can be as-
signed to the change in the polarizability of oxygen, indicating
the formation of a less stoichiometric SiO2 phase.[47,48] A more
significant shift of the O1s and Si2p signals occurs for Mgb(50)
than for Mg(im)-b and Mg(ex)-b, suggesting that in situ incorpo-
rated Mg species favors more negative O species, in accord-
ance with our previous observations.[49,50]
0.5 62.5 0.07
4.6 52.8 0.25
MgO@gAl2O3 9.0
0.15
[a] Reaction conditions: 10 mmol phenol, 35 mmol methanol, 1908C,
18 h, 0.1 g catalyst. [b] Conversion: [mmol (phenol converted)]/[mmol
(phenol initial)]ꢁ100. [c] Yield: [mmol (cresols)]/[mmol(initial phenol)]ꢁ
100. [d] Yield: [mmol (anisole)]/[mmol (initial phenol)]ꢁ100. [e] Selectivity
of the methylation product: [mmol (cresols+anisole)]/[mmol (initial
phenol)]ꢁ100. [f] Total amount of acid sites measured by NH3-TPD.
[g] Total amount of base sites measured by CO2-TPD. [h] The molar ratio
of o- to p-cresol is 2.
entry 1) gives 15.4% yield of cresols (o/p=2) at the optimized
conditions (Figure 3), which is slightly higher than the Mgb(33
and 100) samples (Table S3, entries 1–2). Besides the C-methyl-
ation products, cresols, there is also the O-methylation prod-
uct, anisole, another industrially important chemical intermedi-
ate. The yield of anisole over Mgb(50) is 19.1%. Few coking or
other byproducts were detected, with the total selectivity for
methylation products being 97.2%. The catalytic performance
of Mgb(50) under different conditions was explored by varying
the temperature, time, and phenol/methanol molar ratio
The basicity of zeolites is closely related to the negativity of
O species and more negative O species usually mean en-
hanced basicity.[45] In the CO2 temperature-programmed de-
sorption (TPD) curves, the Mgb(n) samples demonstrate three
CO2 desorption peaks in the ranges of 110–1308C, 270–3008C,
and 500–5208C, respectively identified to the weak, medium,
and strong basic sites (Figure 2C).[51] On the contrary, bare
b only presents a peak centered at 1708C, confirming the en-
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phase conditions (Table S3 in the Supporting Information, en-
tries 3–8).[52] Cr(ex)-Mgb (Table 2, entry 2) demonstrates a high
yield of 30% for cresols and 15.3% for anisole with the methyl-
ation selectivity of 91.0%, whereas the others are inert (<3%,
Table S3 in the Supporting Information). Moreover, stable activ-
ity is observed over Cr(ex)-Mgb in a five-run recycling test (Fig-
ure 4B), thanks to the well preserved structure (Figures S4–S6
and Table S1 in the Supporting Information).
The effect of catalysts was investigated over various solid
acid and base catalysts (their structural information is given in
Figures S2, S4, S7, and Table S1 in the Supporting Information).
As shown in Table 2, Mg(ex)-b gives much low activity (entry 3,
2.8%) whereas Mg(im)-b is inactive (entry 4, 0%). No or trace
amounts of cresols were detected over b (entry 5, 0%), solid
acids like Hb (entry 6, 0.6%) and gAl2O3 (entry 7, 1.1%), com-
mercial solid base MgO (entry 8, 0%), or solid acid–base cata-
lyst MgO@gAl2O3 (entry 9, 0.15%); although they were active
in the high-temperature gas-phase methylation reaction.[22,23]
These observations suggest that neither solid base nor solid
acid are effective for the reaction. Instead, methylation requires
both acid and base sites. Nonetheless, basicity introduced
through impregnation (that is, Mg(im)-b and MgO@gAl2O3)
almost damage the activity, demonstrating that such separated
acid and base sites fail to promote the catalysis. Ion-exchange
with alkaline or alkaline-earth metal ions favors the increase of
zeolite basicity, in which the basic sites are adjacent to the
acid ones, forming cooperative acid–base pairs with more neg-
ative O than its parent. Thus, catalytic activity, though still low,
is observed over Mg(ex)-b. Similarly, low activity is detected over
other analogues (Table S3 in the Supporting Information, 3.1,
2.2, 1.6, and 1.1% for Ca, Ba, Sr, and Cs(ex)-b). Their low activity
is attributable to the weak basicity and decreased acidity (Fig-
ure S8 and Table S4 in the Supporting Information). For our
newly prepared Mgb series, incorporation of Mg ions into the
framework of the BEA zeolite generates acid–base pairs with
not only well-retained original acidity but much more negative
O, denoting enhanced basicity, which thus promotes the reac-
tion. Further improved performance of Cr(ex)-Mgb can be as-
signed to additionally enhanced basicity and acidity. Based on
the above comparison, we propose that sufficient activation of
the substrates in the low-temperature liquid-phase methyla-
tion of phenol requires synergetic interaction of adjacent acid–
base pairs with both superior acidity and basicity within the
zeolite framework.
Figure 3. A) Cresol yields as a function of reaction time over Mgb(50). Reac-
tion conditions: 10 mmol phenol, 35 mmol methanol, 1908C, 0.1 g catalyst.
B) Cresol yields as a function of reaction temperature over Mgb(50). Reaction
conditions: 10 mmol phenol, 35 mmol methanol, 18 h, 0.1 g catalyst.
C) Effect of molar ratio of phenol and methanol on yield over Mgb(50). Reac-
tion conditions: 1908C, 18 h, 0.1 g catalyst.
(Figure 3). The yield as a function of each parameter presents
volcanic-type curves. With longer reaction time (>18 h) or
higher temperature (>1908C), the yield declines owing to ad-
ditional side reactions of the formed cresols.
After reaction, the catalyst can be facilely separated by filtra-
tion and reused. The three-run recycling assessment for
Mgb(50) reveals a clear decrease in activity (Figure 4A). The
Other Mg-bearing zeolites MgS-1, Mg-ZSM-5, and Mg-MOR
are totally inactive (0%) in the reaction (Table S3, entries 7–9 in
the Supporting Information), because incorporation of Mg ions
improves their basicity but decreases the acidity (Figure S8 and
Table S4 in the Supporting Information). These results, on the
one hand, support the above supposition that the reaction re-
quires both superior acidity and basicity; on the other hand, it
implies the uniqueness of incorporating Mg ions into the BEA
framework, which enhances the basicity while simultaneously
preserving the initial acidity of the zeolites.
Figure 4. Catalytic reusability of A) Mgb(50) and B) Cr(ex)-Mgb in the methyla-
tion of phenol. Reaction conditions: 10 mmol phenol, 35 mmol methanol,
1908C, 18 h, 0.1 g catalyst.
reason can be assigned to the decreased surface area, Mg con-
tent, and corresponding basicity of the reused sample
(Table S1 and Figure S5A in the Supporting Information). Ion-
exchange with metal ions is one facile and widely adopted
way to modify zeolites. Various metal ions (Mg, Cs, Cr, Ce, V,
Cu, and Nb) were selected as counter cations of Mgb(50) by
referring to their promotion effect for the reaction under gas-
In situ FTIR spectra were performed to provide insight into
the above liquid-phase methylation of phenol with methanol
(Figure 5). The band assignments are listed in Tables S5–S7 in
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The FTIR spectra of adsorbed phenol (Figure 5B) reveal that
strong adsorption of phenol takes place on Mgb(50) and
Cr(ex)-Mgb. Significant variation of the adsorption bands from
those of gas-phase phenol is observable. The band assigned to
phenolic OÀH deformation vibrations splits into two compo-
nents at approximately 1400 and 1350 cmÀ1 [53,54]
An intense
.
band at 1253 cmÀ1 is attributable to the CÀO stretching vibra-
tion of adsorbed phenol with strong interaction with the sur-
face acid–base pairs. These phenomena reflect a strong chemi-
cal adsorption of phenol on Mgb(50) and Cr(ex)-Mgb. No bands
are observed from 1800 to 2000 cmÀ1, revealing a parallel ad-
sorption of phenol on these two samples.[53] Weak adsorption
of phenol takes place on MgO and gAl2O3, and these bands are
close to those of gas-phase phenol, reflecting their weak inter-
actions with phenol. Certain chemical adsorptions occur on
Mg(im)-b and Mg(ex)-b. The weak band located at 1250 cmÀ1 re-
veals a small amount of deformed phenol molecules on the
surface. The bands at 2008 and 1857 cmÀ1 for the out-of-plane
CÀH bending vibration is an indication of the perpendicular
orientation of adsorbed phenol on Mg(im)-b.[53] In the case of
a mixed solution (phenol/methanol=1:3.5, the same as in the
alkylation of phenol with methanol), strong co-adsorption of
Figure 5. FTIR spectra of A) methanol, B) phenol, and C) mixture of phenol
and methanol with the mole ratio of 1:3.5 adsorbed on different samples at
1908C.
the Supporting Information. Figure 5A shows the FTIR spectra
of adsorbed methanol. The bands of adsorbed methanol on
MgO are similar to gas-phase methanol, suggesting weak inter-
actions between adsorbed methanol and MgO under these
conditions. gAl2O3 also shows weak adsorption but exhibits
negative bands at approximately 3750–3650 cmÀ1, which can
be assigned to the consumption of surface AlÀOH as a result
of the interaction of adsorbed methanol with these hydroxyl
groups. Correspondingly, the negative bands around 1400–
1600 cmÀ1 also come from the consumption of these hydroxyl
groups. Such phenomena indicate that methanol is mainly
weakly adsorbed on surface of gAl2O3 through the interaction
of methanol with the surface hydroxyl group. Mg-bearing zeo-
lites demonstrate strengthened adsorption to methanol with
certain variations of the bands differing from the gas phase.
Similar negative peaks as gAl2O3 are observed for these sam-
ples, implying that some methanol is adsorbed through inter-
action with surface hydroxyl groups. Apparent shifting or split-
ting is observable for the bands around 1350–1500 cmÀ1,
which are attributable to the CÀH stretching vibration and
COH deformation vibration.[22,53] The band at approximately
1250 cmÀ1 is attributable to OÀH deformation vibration per-
ceived from the zeolite catalyst. These observations reflect the
deformation of adsorbed methanol on these zeolites with the
elongation of CÀOÀH, which causes the activation of adsorbed
methanol.
methanol and phenol is still observable on Mgb(50) and Cr(ex)
-
-
Mgb, whereas weak adsorption occurs on MgO, gAl2O3, Mg(im)
b, and Mg(ex)-b (Figure 5C). These phenomena suggest that
Mgb(50) and Cr(ex)-Mgb can chemically adsorb the methanol
and phenol in the mixed solution to reach sufficient activation.
Basing on the above in situ FTIR results, a possible route for
the Mg-bearing BEA zeolites to catalyze the methylation of
phenol with methanol is proposed in Scheme 1A. Methanol
and phenol are chemically adsorbed on Mgb(50) and
Cr(ex)-Mgb. Conjunct acid–base pairs within the 12-membered-
ring (12-MR) channel provide sufficient driving force for the ad-
sorption through the mode in Scheme 1A. The positive Al
(acid site) and negative O species (basic site) respectively exert
strong interactions with the hydroxyls of methanol and
phenol, in which phenol is adsorbed in a parallel fashion.[53] Si-
multaneous activations of the two substrates is attained as
a result of such strong adsorption. The carbocation is formed
from methanol on the Lewis acid site of Al cations, whereas
the electron donor effect of the phenol OH group increases
Scheme 1. A) Possible reaction mechanism for the methylation of phenol over Mgb(50). B) Product selectivity mechanism.
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the electronic density of the o- and p-positions, strengthened
by Mg-derived negative O basic site. Then, taking the p-posi-
tion as an example, the carbocation attacks the nearby parallel
adsorbed phenol ring and one benzene ring proton is released
to the hydroxyl of methanol with the formation of water. Final-
ly, by such electrophilic substitution, p-cresol is produced. Each
attacking probability at the o- or p-position is equal, thus caus-
ing the 2:1 molar ratio of o- and p-cresol. In addition, the car-
bocation can also attack the O-position to form anisole. No
cresol forms by using anisole to replace phenol, thus excluding
the possible route from the rearrangement of anisole and fur-
ther supporting the above mechanism. By contrast, only weak
adsorption takes place on gAl2O3 and MgO, suggesting that
solid acid or base alone cannot provide enough host–guest in-
teractions to activate the reactants, especially in the coexis-
tence of phenol and methanol as a result of competing ad-
sorption. Mg(im)-b and Mg(ex)-b can adsorb methanol but weakly
capture phenol, thus also failing to efficiently activate them.
Therefore, the reaction cannot efficiently proceed over these
catalysts.
serve an acid–base synergetic shape-selective catalysis with
the Mg-bearing BEA zeolite.
Conclusions
Mgb zeolites possessing both superior acidity and basicity
were hydrothermally synthesized, which results in their use in
the highly efficient alkylation of phenol with alcohols under
the desirable low-temperature liquid-phase conditions. The un-
expectedly high activity comes from the synergetic effect of
adjacent acid–base pairs on the BEA zeolite framework. Stable
recyclability with higher activity is achieved by modifying Mgb
with Cr ion-exchange. Alkylation of phenol with different alco-
hols is also achieved on these Mg-bearing zeolites. Remarkable
stereo- and regioselectivity is obtained on a bulky substrate,
rendering an efficient shape-selective catalysis.
Experimental Section
Materials and methods
The reaction scope was extended by evaluating other alco-
hols including ethanol, isopropanol, and tert-butyl alcohol
(Table 3). All of them show good activities with the yields over
Magnesium (Mg)-bearing BEA zeolites (Mgb) were synthesized
through an acidic co-hydrolysis route. The process was performed
as follows. The mixed solution of magnesium nitrate hexahydrate
(Mg(NO3)2·6H2O, 99 wt%, Sinopharm Chem. Reagent Co., AR) and
tetraethylorthosilicate (TEOS, 28.4 wt% SiO2, Sinopharm Chem. Re-
agent Co., AR) was hydrolyzed under moderately acidic conditions
(pHꢀ0.6) by adding concentrated hydrochloric acidic (HCl,
36.5 wt%, Shanghai Chem. Reagent Co., AR) dropwise at 908C
within 4 h. Then, tetraethylammonium hydroxide (TEAOH, 35 wt%
aqueous solution, Jintan Huadong Chem. Res. Institute, AR) and
sodium aluminate (NaAlO2, 41 wt% Al2O3, Shanghai Chem. Reagent
Co., AR) were added into the above mixture one by one. The ob-
tained slurry was aged at room temperature for 24 h. The final
molar composition of the gel was 1SiO2/ 0.025Al2O3/ 0.02MgO/
0.35TEAOH/ 22.5H2O with the pH value of 12.5. Finally, the result-
ing gel was transferred into a Teflon-lined stainless steel autoclave
and heated statically at 1408C for 14 d. The products were separat-
ed by centrifugation, washed with deionized water, and dried at
1008C for 12 h. After that, they were calcined at 5508C for 5 h in
air. The obtained Mgb zeolites were denoted as Mgb(n) (n=Mg/Si
molar ratio in the synthesis gel).
Table 3. Liquid-phase alkylation of phenol with different alcohols.[a]
Entry Substrate
Catalyst
Cphenol Yalkylphenol o/p[d] Yether Salkyl
[%][b] [%][c]
[%][e] [%][f]
1
Mgb(50)[g]
62.7
41.0
42.2
65.2
62.6
25.1
26.0
63/37 16.4 91.5
60/40 16.9 84.8
ethanol
2
Cr(ex)-Mgb[h] 69.7
3
Mgb(50)[i]
Cr(ex)-Mgb[h] 81.5
Mgb(50)[j]
63.9
Cr(ex)-Mgb[h] 65.8
82.7
77/23
68/32
6.0 86.1
5.1 83.1
2-propanol
4
5
6
5/95 18.7 68.5
5/95 19.5 69.1
tert-butanol
[a] Reaction conditions: 10 mmol phenol, 35 mmol alcohol, 0.1 g catalyst.
[b] Conversion: [mmol (phenol converted)]/[mmol (phenol initial)]ꢁ100.
[c] Yield: [mmol (alkylphenols)/[mmol (phenol initial)]ꢁ100. [d] The molar
ratio of o- to p-product. [e] Yield: [mmol (aromatic alkyl ether)]/[mmol (in-
itial phenol)]ꢁ100. [f] Selectivity of the alkylation product: [mmol (alkyl-
phenols+ether)]/[mmol (initial phenol)]ꢁ100. [g] 1858C, 18.5 h.
[h] 2058C, 18 h. [i] 1958C, 18 h. [j] 2008C, 18.5 h.
Characterization
X-ray diffraction (XRD) patterns were characterized with a Smart
Lab diffract meter (Rigaku) equipped with a 9 kW rotating anode
Cu source (45 kV, 100 mA, 5–508, 0.28sÀ1). Morphologies were in-
vestigated with a field-emission scanning electron microscope (FE-
SEM) instrument (HITACHI S-4800). Transmission electron microsco-
py and corresponding elemental mapping images were taken with
a JEM-2100F electron microscope with an acceleration voltage of
200 kV. Nitrogen sorption experiments were used to detect the po-
rosity. The isotherms were measured at 77 K (the temperature of
liquid nitrogen) with a BELSORP-MINI analyzer. Before analysis, the
samples were degassed at 3008C for 3 h. Fourier transform infrared
(FTIR) spectra from 4000 to 400 cmÀ1 were recorded with an Agi-
lent Cary 660 instrument by using KBr disks. Solid-state UV/Vis
spectra were recorded with a SHIMADZU UV-2600 spectrometer
with barium sulfate (BaSO4) as the internal standard. Inorganic
chemical compositions were analyzed with a Jarrell-Ash 1100 in-
Cr(ex)-Mgb higher than or comparable to that over Mgb(50).
Further, the product distribution for ethanol and isopropanol is
same as that for methanol, suggesting that they follow a similar
catalytic route as that in Scheme 1A. Amazingly, excellent
stereo- and regioselectivity (95% for p-product) is detected for
tert-butyl alcohol. This arises from the special nano-confine-
ment of the limited zeolitic micropores: the 12-MR channels of
the BEA zeolite mostly hinder attack at o-position towards the
bulkier product o-tert-butylphenol, as reflected by Scheme 1B.
Zeolites as solid acids have exhibited shape-selectivity in many
acid reactions,[55,56] but this shape-selective behavior is scarcely
found in base or acid–base reactions. For the first time, we ob-
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ductively coupling plasma (ICP) spectrometer. The surface chemical
composition was analyzed by X-ray photoelectron spectra (XPS)
with a PHI 5000 Versa Probe X-ray photoelectron spectrometer
equipped with AlKa radiation (1486.6 eV). 29Si and 27Al magic-angle
spinning (MAS) nuclear magnetic resonance (NMR) spectra were re-
corded with a Bruker AVANCE III 600 spectrometer. Thermogravi-
metric (TG) curves were determined by using a STA409 instrument
in dry air at a heating rate of 108CminÀ1. Ammonia (NH3) and
carbon dioxide (CO2) temperature-programmed desorption (TPD)
curves were detected by using a Catalyst Analyzer BELCAT-B. Sam-
ples were pretreated at 5508C for 2 h, and then cooled to 508C
under helium (He) gas. Adsorption of NH3 or CO2 was carried out
at 508C for 30 min under NH3/He or CO2. After the samples were
purged under He gas for 30 min, the temperature was increased to
7008C (88CminÀ1). The desorbed gas was determined by using
a Gow-Mac thermal conductivity detector (TCD). In situ FTIR spec-
tra were recorded with an Agilent Cary 660 spectrometer with
a mercury cadmium telluride (MCT) detector at a resolution of
4 cmÀ1. The samples were placed in a Harrick Scientific-made
DRIFTS cell with a ZnSe window (Model WMD-U23) and in situ
heated to 823 K for 5 h at a heating rate of 10 KminÀ1 in 99.99%
pure N2 stream (8 mLminÀ1). The sample was then cooled to 463 K
and 200 mL of the target compound was introduced separately for
10 min in the N2 flow. The spectra were collected at 1, 5, 10, 20,
and 30 min after the introduction of adsorbents until the spectra
remained unchanged. Only the unchanged spectrum collected in
the last scan is presented to avoid repetition.
Keywords: alkylation of phenols · bifunctional heterogeneous
catalysis · green chemistry · shape selectivity · zeolites
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Manuscript received: September 9, 2016
Final Article published: && &&, 0000
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FULL PAPERS
Acid–base synergetic sites with en-
hanced basicity without compromising
acidity are constructed on the Mg-bear-
ing BEA zeolite by incorporating Mg
ions into the framework in a direct syn-
thesis route. As a result, the Mg-bearing
zeolite triggers the first efficient low-
temperature liquid-phase alkylation of
phenol with high yield, stable reusabili-
ty, good substrate compatibility, and
unique shape selectivity.
J. Xie, W. Zhuang, W. Zhang, N. Yan,
Y. Zhou,* J. Wang*
&& – &&
Construction of Acid–Base Synergetic
Sites on Mg-bearing BEA Zeolites
Triggers the Unexpected Low-
Temperature Alkylation of Phenol
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These are not the final page numbers! ÞÞ