Hydroxyalkylation of phenol to bisphenol F over Al-pillared clay

Article abstract of DOI:10.1039/c6ra01959e

Hydroxyalkylation of phenol to bisphenol F over the intercalation of aluminum hydroxy oligomeric into layered montmorillonite K10 was investigated. A remarkably high product yield (89.2%) and selectivity to bisphenol F (92.7%) has been achieved at a 110 °C reaction temperature and reaction time of 80 min with a Al-MMT(6) catalyst. A series of catalysts were prepared and characterized by FT-IR, XRD, BET, NH₃-TPD and Py-IR. Characterization results showed that the catalytic performance of these catalysts depended on weak and moderate acidity and the textural properties (specific surface areas). The effect of the catalyst calcination temperature to this reaction was also studied. Moreover, the influences of various reaction parameters like mole ratio, catalyst concentration, reaction temperature and reaction time on the product yield and selectivity to bisphenol F were investigated. Finally, the reusability of the catalyst was studied and a plausible mechanistic pathway was proposed.

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Hydroxyalkylation of phenol to bisphenol F over Al-pillared clay
＊
Xianzhang Wu, Xinnian Xia , Ran Liu and You Chen
Received 00th January 20xx,
Accepted 00th January 20xx
Hydroxyalkylation of phenol to bisphenol F over the intercalation of aluminum hydroxy oligomeric into layered
montmorillonite K10 was investigated. A remarkably high product yield (89.2%) and selectivity to bisphenol F (92.7%) has
been achieved at 110℃ reaction temperature and reaction time of 80min with Al-MMT(6) catalyst. A series of catalysts
DOI: 10.1039/x0xx00000x
3
were prepared and characterized by FT-IR, XRD, BET, NH -TPD and Py-IR. Characterization results showed that the catalytic
performance of these catalysts depended on weak and moderate acidity and the textural properties (specific surface
areas). The effect of catalyst calcination temperature to this reaction was also studied. Moreover, the influences of various
reaction parameters like mole ratio, catalyst concentration, reaction temperature and reaction time on product yield and
selectivity to bisphenol F were investigated. Finally, the reusability of catalyst was studied and a plausible mechanistic
pathway was proposed.
www.rsc.org/
1
Introduction
affection in the weak and moderate acidity for bisphenol F synthesis
were also evaluated.
The synthesis of bisphenol F has been given an extensive attention
due to the increasing demand and applicability in plastics, resins
and rubber industries.
1
-3
2
Experimental
Conventionally, hydroxyalkylation of
phenol with formaldehyde to bisphenol F could be catalyzed by
mineral acids, such as phosphoric acid, hydrochloric acid, sulfuric
acid or other inorganic acids. Even though the promising results
2
.1 Materials
4
-6
have been attained, these homogeneous catalysts suffer from Commercially available montmorillonite K10 as the raw Material
serious drawbacks such as hazards of handling, and corrosion of supplied by Sinopharm Chemical Reagent Co, Ltd, China. Analysis of
equipment. In addition, the recovery of homogeneous catalysts is
its mineralogy composition showed it to be 95% montmorillonite.
7
an extremely formidable obstacle. Our group are highly desirable
Its anhydrous structural (layer) formula that formed from one
for solid catalysts to exhibit activities and selectivity at least
comparable to homogeneous catalysts for bisphenol F synthesis. octahedral sheet between two tetrahedral sheets 2:1 layer type.
Solid acid catalysts that are insoluble, easily recovery, reuse, and
highly active for eco-friendly chemical processes. A series of studies
have been reported about the design of solid acid catalysts for
bisphenol F synthesis, which spans over a broad range of catalytic
The montmorillonite K10 had a basal (001) spacing of 0.96 nm and
according to
the formula of [Si7.86Al0.14][Al2.84Fe0.30Mg0.86]O20(OH)
4
1
4
the literature. Montmorillonite K10, which is calcium-rich in
commodities form and was converted into the Na-MMT (denoted
as MMT) by treatment with NaCl (1 mol/L NaCl solution, 100 mL
solution/g of clay, 80℃ for 2 h); phenol, formaldehyde (37–40%),
aluminum chloride hexahydratere were purchased from Sinopharm
8
9
materials including modified mesoporous silicas,
clay,
organometallic framework, and zeolites. Additionally, the
PWA/MCM-41 was reported to be active for bisphenol
10
11
F
12
synthesis. The studies indicate that the concentration of acidic
sites was a more predominant factor than its surface area for
catalytic activity. The use of the PTA@MIL-101 (Fe or Cr) was also C h e m i c a l R e a g e n t C o , L t d , C h i n a .
reported as efficient solid catalysts to produce bisphenol F by Chen
10
et al. and leading to different bisphenol F isomer distributions.
However, the synthesis of these materials is complex and need
complicated equipment. In other work, one can underline the
special affection in acid site for bisphenol F isomer distributions.
Specifically, DTP/montmorillonite K10 were tested by Garade et
al. , leading to different bisphenol F isomer distributions. However,
the low catalytic activity severely restricted its application in the
reaction that hydroxyalkylation of phenol with formaldehyde to
bisphenol F.
2
.2 Catalyst preparation
A series of Al-MMT(x) were synthesized by aluminum hydroxy
oligomeric intercalated in montmorillonite K10 according to
previous report (x represents the molar of Al in per gram of MMT).
Typically, Al-MMT(6) was prepared as follows: (i) Preparation of the
Al-pillaring agent. 0.1mol/L solution of NaOH was slowly added to
0
15
1
3
.1 mol/L AlC1 solutions prepared from AlCl ·6H O to obtain a final
3 3 2
− 3+
hydrolysis ratio OH /Al = 2.4. Aging at 50℃for 24 h. (ii) Syntheses
In this work, we describe the catalyst prepared that Al-pillared of Al-MMT(6). The Al-pillaring agent was slowly added to MMT
clays via aluminum hydroxy oligomeric intercalated in slurry of 2g/100 ml (6 mmol of Al per gram of MMT). The final
montmorillonite K10, and evaluated for the hydroxyalkylation of suspension was stirred at room temperature for 24 h, then was
phenol to bisphenol F. Furthermore, the effect of solid catalyst transferred to dialysis tubing and dialyzed in distilled water for 3
−
calcination temperature (673−1033 K) was also studied. Finally, the days to remove C1 . The dialysis water was changed each 24 h until
−
the water-tested C1 free by the AgNO test. The resultant mixture
3
was subsequently separated from suspension by filtration to obtain
powders, dried in an air and calcined at 400℃for 4 h. Other
samples were prepared as the above procedures.
College of Chemistry and Chemical Engineering, Hunan University, Changsha
4
8
10082, P. R. China. E-mail address: xnxia@hnu.edu.cn; Tel./fax: + 86 731
8821549
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.3 Characterization
Journal Name
2
X-ray diffraction (XRD) was performed on a D8-Advance with a
Bruker diffractometer using Cu Ka radiation ( 40 kV , 40 mA ) in the
range between 2° and 30° and Cu-Ka radiation (40 kV, 40 mA) in the
range between 10° and 80°. Diffraction data were recorded with a
scanning speed of 0.5° of 2θ/s and 2θ = 0.02° at a temperature of
25℃. The chemical composition of the sample was determined by
energy-dispersive X-ray (EDX) analysis using an FEI QUANTA-200.
Nitrogen dsorption isotherms were performed at 77 K on an ASAP
2010 sorption system (Micromeritics). The samples were degassed
at 200℃ for 10 h before measurement. The specific surface area
was estimated by the Brunauer−Emmeꢀ−Teller (BET) equaꢁon. The
strength and amount of acid sites in the solid catalysts were
3
estimated by using NH -TPD measurements with a Micromeritics
AutoChem II 2920 V3.05 instrument. Prior to analysis, the catalysts
sample were heated from room temperature to 200℃ with a ramp
of 10℃/min and purged with helium gas for 30 min. The sample
was cooled down to 60℃ under a flow of helium gas and then
followed by adsorption of NH
helium gas flow from 60 to 700℃ with a ramp of 10℃/min and the
signals of NH -TPD were recorded with an online thermal
conducted detector (TCD). Infrared spectra were recorded in the
3 3
for 60 min. NH is desorbed in
3
-1
wavelength range of 800–4000 cm using a Bruker vector 22 FT-IR
spectrophotometer using KBr disk technique. Pyridine adsorption
monitored by in situ infrared FTIR spectra (Py-IR) of the catalyst
samples were recorded with a Brucker Vector 22 spectrometer in
−
1
the absorption mode with a resolution of 4cm . Self-supporting
wafers were made and loaded in an IR cell. The wafers were
pretreated at 400℃ under flowing oxygen for 2 h,. Background
spectra were recorded after the sample was cooled to room
temperature. Adsorption of pyridine was then conducted until
saturation. Py-IR spectra were recorded after degassing for 0.5 h
2.4 Catalytic activity tests
The hydroxyalkylation of phenol with formaldehyde to bisphenol F
was performed in a magnetically stirred glass reactor fitted with a
reflux condenser and an arrangement for temperature control. In a
typical procedure, 82.86 mmol of phenol, 5.73 mmol of
formaldehyde and catalyst (0.12 g) were added into the reactor.
The reaction mixture was heated to 383 K. After 80 min, the
reaction was stopped and cooled down to room temperature, then
the solid catalysts were separated by filtration. The products were
analyzed by a HPLC system with a Shimadzu LC-20AT system
coupled with a SPD-20A UV/Vis detector and a Phenomenex Luna
C18 column (250 × 4.6 mm, 5 μm) and Column oven temperature
was 25℃, and mobile phase was methanol: water with 65:35v/v, a
flow rate of 0.6 mL/min. All the reaction conditions followed the
above procedures unless otherwise stated.
Fig. 1. XRD patterns of MMT and Al-MMT(x) samples.
ratio, indicating an expansion of the basal spacing of the catalysts.
All of the catalysts have a 001 reflections corresponding to basal
spacings of approximately 1.7-2.1 nm, which is larger than that of
MMT (shows the thickness of the clay layer of 0.96 nm). Note that
3
+
the A1 /clay ratio is an important factor in the enlargement of the
basal spacing of the catalysts. Fig. 1(B) shows wide-angle XRD
patterns of MMT and Al-MMT(x). The XRD pattern of all solid
catalysts, typical peaks of trioctahedral subgroup of 2:1
phyllosilicates are observed, which are ascribed to (110), (020),
Performances of MMT, Al-MMT(x), and Al-MMT(6)-Y(Y=773 K,
923 K, 1033 K) were evaluated in terms of percentage of product
yields and percentage of product selectivity, which the equations
are defined as follows:
1
6-18
(
004), (130), (200), (330) and (060) diffractions.
When more Al-
pillaring agent were intercalated in the clay layers, all samples did
not exhibit any peaks assignable to Al species, suggesting that the
Al-pillaring agent was well intercalated into interlayered clay and no
long-range change in the crystal structure during the reaction.
yield %=100×N/M
Selectivity %=100×N/F
where N is the actual moles of product formed in the reactor, M
denotes expected moles of product formed based on formaldehyde
consumed and F denotes the amount (mol) of all the products.
FT-IR spectra of the catalysts MMT and Al-MMT(x) are presented in
Fig. 2. It can be clearly seen from Fig. 2 that the bands appearance
3
Results and discussion
–1
–1
3
.1 Catalyst characterization
The basal spacings of the samples were calculated according to the
001 value measured by XRD (small angle). Fig. 1(A) shows the XRD
patterns of MMT and Al-MMT(x) prepared with the different
around 3448 cm and 1640 cm are assigned to the stretching
19
and bending vibration of the OH group in water molecules. In all
–
1
curve, the band located at 1033 cm was indexed to the
asymmetric stretching bonds of Si-O-Si, associated with the
d
1
5,20
3
+
motion of oxygen in Si-O-Si antisymmetric stretching.
The band
A1 /clay ratio treated at 673 K. The Al-MMT(x) exhibited the 001
reflection at a lower diffraction angle with an increase in A1 /clay
-1
3
+
at around 3640 cm is due to the vibration of Al-OH, and the
absorption at 849 cm is due to the Si-OH in the tetrahedral
layer. Besides, significant decreases of the peaks intensity
-1
2
1
2
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Fig. 2. FT-IR spectra of MMT and Al-MMT(x) samples.
3
Fig. 3. NH -TPD profiles of MMT and Al-MMT(x) samples.
−1
-1
near 3448cm and 1640 cm with increase Al-pillaring agent, which
2
2
means a decrease of water content in the interlayer space.
Compared to the FT-IR spectra of pure MMT, significant increase of moderate acid sites of Al-MMT(x) enhanced considerably from 0.29
ratio from 2 to 6. At the same time, the amount of weak and
-1
the peaks intensity at 3642 cm with increasing content of Al- to 0.38 mmol/g and from 0.58 to 1.41 mmol/g, respectively.
pillaring agent, which is attributable to the appearance of Al-OH However, the amount of strong acid sites decreased from 2.14 to
bending vibration and the intensity is also enhanced. The above 1.88 mmol/g. When further increase the A1 /clay ratio up to 8,
results revealed that Al-pillaring agent were successfully increased slightly in the amount of surface acid sites was observed.
3
+
intercalated into the interlayer of MMT.
This is because the decrease in the specific surface areas, which is
The profiles of NH -TPD to characterize acidic properties of closely related to the amount of surface acid sites. The acidic
3
MMT and Al-MMT(x) are shown in Fig. 3. The amount of surface property of the catalyst is a key factor for hydroxyalkylation of
acid sites based on NH desorption are presented in Table 2. As phenol with formaldehyde to bisphenol F.
3
shown in Fig. 3, the solid catalyst showed three types of desorption
The acidic properties were also studied by Py-IR spectroscopy to
peaks from 90℃ to 700℃ region which could be assigned to weak, evaluate the Lewis and Brönsted character. The spectra are
2
3-25
moderate and strong acid sites.
sites on MMT or Al-MMT(x) is related to the amount of sites are shown in Table 2 .Two types of adsorbed pyridine species
The weak and moderate acid presented in Fig. 4 and the amount of the Lewis and Brönsted acid
−1
mntmorillonite interlayer aluminum ions, which engender sites of were observed. The band at 1540 cm can be assigned to Brönsted
weak and moderate acidity in the form of the OH groups bonded to acid sites and the band at 14480 cm can be attributed to Lewis
the pillars Al ions (Al-OH). Meanwhile, the strong acid sites is acid sites according to the literatures
associated with the OH groups bonded to tetrahedrally coordinated cm originated from the coadsorption of pyridine on both Brönsted
−1
2
9-30
. While the band at 1490
−1
2
6-28
Al ions (Al -OH).
Interestingly, the total amount of acid sites of and Lewis acid sites. The Al-pillaring agent significantly affected the
Td
Al-MMT(x) increase obviously with increasing Al/clay
acidity of the MMT materials. Fig. 4 clearly shows that both the
concentration of Brönsted and Lewis acid sites increase,
Table 1 Textural properties and elemental composition of various solid acid catalysts
Composition
2
Catalyst
BET surface area (m /g)
Si (wt%)
36.5
O (wt%)
57.9
Mg (wt%)
0.7
Al (wt%)
15.8
MMT
61
Al-MMT(2)
Al-MMT(4)
Al-MMT(6)
Al-MMT(8)
115
169
207
194
34.9
56.7
0.5
18.7
34.6
53.3
0.6
21.4
33.7
51.1
0.5
24.9
34.4
50.8
0.6
27.3
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Table 2 Acidic properties of various solid acid catalysts
Acidity by type
Brönsted
Catalyst
Lewis
(mmol g )
Lewis/Brönsted
(L/B)
-1
-1
(
mmol g )
MMT
0
0
0
0
0
.16
.24
.26
.27
.39
0.14
0.87
Al-MMT(2)
Al-MMT(4)
Al-MMT(6)
Al-MMT(8)
0.22
0.25
0.32
0.48
0.92
0.10
1.18
1.23
Fig. 4. Py-IR spectra of MMT and Al-MMT(x) samples.
with the presence of Aluminum species, confirming that catalytic
activity is a result of Aluminum species presence in the MMT.
Among various solid catalysts, the MMT showed the least activity
and the L/B ratio increases almost linearly with increasing Al-
pillaring agent, which indicates that the addition of Al-pillaring
agent resulted in the redistribution of the two types of acid sites.
(
10% yield), there could be due to its the lowest specific surface
area and presence of the lowest acidity ( Table 2). As compared to
MMT, Al-MMT(x) showed higher product yield due to its higher
This result is well consistent with the above NH -TPD result.
3
3
1
acidity. The bisphenol F yield increasing from 10.6 to 89.2% with
3
+
The specific surface areas of MMT and Al-MMT(x) samples are increase in A1 /clay ratio from 2 to 6. This is attribute to enhanced
presented in Table 1. Among various solid catalysts, MMT showed amount of weak and moderate acid sites and higher L/B ratio
2
the lowest (60 m /g) specific surface areas. The specific surface resulted in higher catalytic activity, which are confirmed by the
areas of Al-MMT(x) shows a significantly increase compared with results of NH -TPD and Py-IR (Table 2 and Table 3). When further
3
3
+
parent MMT, which is because the expansion of the basal spacing of increase A1 /clay ratio up to 8, there is a decrease of bisphenol F
clay layers after being intercalated by Al-pillaring agent. The solid yield and selectivity (yield.%=87.3, sel.%=90.4). Generally, a higher
catalysts were found to be in the sequence of Al-MMT(6) > Al- Al-pillaring agent content means a higher amount of both weak and
MMT(8) > Al-MMT(4) > Al-MMT(2) > MMT. Compared with other moderate acid sites as well as higher specific surface areas.
3
+
samples, Al-MMT(8) showed decrease in the specific surface areas. However, a further increase of A1 /clay ratio gives rise to a decline
This is due to the excess Al-pillaring agent occupied the spaces of of specific surface areas, and slight increase in weak and moderate
gallery channels and blocked the pore of the clay layers.
acid sites. In other words, excess Al-pillaring agent occupied the
spaces of gallery channels and blocked the pore of clay layers, and
3.2 Activity measurement
3
results in decline of specific surface areas. As can be seen from NH -
TPD characterization of various catalysts and the results of catalytic
reaction showed that both weak and moderate acid sites of Al-
MMT(x) are critical for achieving the highest yield of bisphenol F in
hydroxyalkylation of phenol.
Activity of MMT and Al-MMT(x) screening experiments for the
hydroxyalkylation of phenol were be implemented and the results
are presented in Table 3. The yield of bisphenol F significantly rose
Table 3 Catalytic activities of Al-pillared clay for bispenol F
Isomer distribution/%
NH -TPD amount of acidic sites (mmol/g)
3
Yield/
%
Selectivity
/%
Catalyst
Total
2.67
3.02
3.27
3.70
3.74
3.27
3.21
3.01
Weak
0.22
0.29
0.35
0.41
0.46
0.30
0.25
0.21
Moderate Strong
4,4'-Isomer
17.2
2,4'-Isomer
52.5
2,2'-Isomer
30.3
MMT
0.37
0.58
0.97
1.41
1.52
1.08
0.86
0.53
2.08
2.14
1.94
1.88
1.76
1.89
2.10
2.27
10.8
69.2
74.5
89.2
87.3
80.2
70.4
66.8
90.3
92.2
91.3
92.7
90.4
91.6
92.2
91.5
Al-MMT(2)
20.5
51.1
28.4
Al-MMT(4)
31.2
49.2
19.6
Al-MMT(6)
30.5
50.4
19.1
Al-MMT(8)
32.2
48.5
19.3
Al-MMT(6)-773
Al-MMT(6)-923
Al-MMT(6)-1033
30.1
49.3
20.6
26.4
44.2
29.4
25.6
46.4
28.0
Reaction conditions: phenol/formaldehyde molar ratio, 15:1; catalyst concentration, 0.030 g/g; reaction temperature, 383 K; reaction
time, 80min;
4
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.3 Effect of Catalyst Calcination Temperature
3
Calcination temperature is closely related to the interlayer space
height and the amount of surface acid sites. Therefore, the effect of
catalyst calcination temperature was investigated. The Al-MMT(6)
catalysts calcined at 673, 773, 923, and 1033 K were used in the
hydroxyalkylation of phenol to bisphenol F, and the results are
shown in Table 3. The yield of bisphenol F decreased from 89.2 to
6
6.8% with calcination temperature is increased from 673 to 1033
K. The reason for this result may be that the decrease of the surface
acid sites and collapse of clay layers. To confirm the above
speculation, the catalysts calcined at 673, 773, 923, and 1033 K
were subsequently explored using NH -TPD, XRD, and FT-IR. NH -
3 3
TPD patterns of catalyst calcined at 673, 773, 923, and 1033 K are
shown in Fig. 5(A). After calcination temperature increased from
6
73 to 1033 K, the weak and moderate acid sites peak intensity
...
were diminished. This is due to Al-OH O-Si-hydrogen bonds were
replaced during thermal treatments by covalent bonds that form Al-
O-Si linkages between the pillars and the clay layers. Hence, it lead
to the decrease of surface acid sites.
3
2
Fig. 6. XRD patterns of Al-MMT(6) at different calcination
temperatures (773 K, 923 K, 1033 K).
Furthermore, FT-IR spectra of these catalysts are presented in
Fig. 5(B). It could be clearly found that significant decrease of the
peak intensity at 3642 cm (corresponding to the Al-OH) with
calcination temperature increased from 673 to 1033 K,
-1
which is attributed to decomposition of the Al-polyoxocations at
3
3
higher temperatures. Fig. 6 shows the XRD patterns of the Al-
MMT(6) calcined at 673, 773, 923, and 1033 K. For catalysts
calcined at 773 K and 923 K, exhibits widening and decreasing
intensity, 001 reflection peak. Moreover, the 001 reflection peak is
shifted toward higher diffraction angles after calcination at 1033 K.
These results indicate that the crystalline structure of the clay layer
3
4
has been destroyed and clay layers appeared collapse.
3
.4 Optimization of Reaction Parameters.
The effects of phenol/formaldehyde ratio on product yield and
bisphenol F selectivity for Al-MMT(6) catalyst are illustrated in Fig.
7
; The product yield is increased significantly from 33 to 85% with
increase in the phenol/formaldehyde ratio from 5 to 15; Further
increase in mole ratio up to 20 does not result in any significant
changes. The lower product yield was mainly because of the further
reaction of bisphenol F with formaldehyde to give tarry products
such as trimers and higher homologues under the condition of low
Fig. 7. Effect of phenol/formaldehyde molar ratio on yield,
selectivity and isomers’ distribution of bisphenol F. Reaction
conditions: catalyst, Al-MMT(6); reaction time, 80 min; reaction
temperature, 383 K; catalyst concentration, 0.030 g/g.
Fig. 5. (A) NH -TPD profile and (B) FT-IR spectra of Al-MMT(6) at
different calcination temperatures (773 K, 923 K, 1033 K).
3
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Fig. 8. Effect of reaction temperature on the yield, selectivity and
isomers’ distribution of bisphenol F. Reaction conditions: catalyst,
Al-MMT(6); mole ratio of phenol/formaldehyde, 15; reaction time,
Fig. 10. Effect of reaction time on the yield, selectivity and isomers’
distribution of bisphenol F. Reaction conditions: catalyst, Al-
MMT(6); mole ratio of phenol/formaldehyde, 15; reaction time, 20-
8
0 min; reaction temperature, 343-383 K; catalyst concentration,
.030 g/g;
9
0 min; catalyst concentration, 0.030 g/g;
0
3
5
phenol/formaldehyde ratio. The 4,4′-isomer increased from 27 to
0% and 2,2′-isomers decreased from 25 to 21%, whereas the
amounts of 2,4′- isomers remained almost unchanged, with
increase in phenol/formaldehyde ratio from 5 to 20. Thus, the
optimal molar ratio was 15.
formaldehyde to promote the formation of bisphenol F. However, if
further increase in temperature, formaldehyde to evaporate from
the mixture, which is not conducive reaction proceeds. Thus, 383 K
was chosen as the optimal reaction temperature.
3
Results showing the effects of the catalyst concentration on
product yield, selectivity to bisphenol F and isomer distribution are
presented in Fig. 9. The product yield increased markedly from 77
to 88% as the catalyst concentration increased from 0.01 to 0.030
g/g. when further increase in catalyst concentration, the product
yield and selectivity to bisphenol F only shows a minor change. This
is expected due to the bisphenol F conversion into a trimer as
higher catalyst concentration. The amount of 4,4′-isomer increased
from 25 to 28% , while no significant change was observed in the
The effects of reaction temperature on product yield, bisphenol F
selectivity and isomer distribution over Al-MMT(6) was investigated
and shown in Fig. 8. It was clearly observed that the product yield
significantly increased from 36 to 89.2%, while remained almost
unchanged in bisphenol F selectivity with increased temperature
from 343 to 383 K. The amount of 4,4′-isomer decreased from 49 to
3
4
2%, however, the amount of 2,4′- and 2,2′-isomers increased from
2 to 47% and 9 to 21% , respectively. It was demonstrated that
high temperatures is needed for hydroxyalkylation of phenol with
Fig. 11. Reusability of catalyst study. Reaction conditions: catalyst,
Al-MMT(6); mole ratio of phenol/formaldehyde, 15; catalyst
concentration, 0.030 g/g; reaction temperature, 383 K; reaction
time, 80 min;
Fig. 9. Effect of catalyst concentration on the yield, selectivity and
isomers’ distribution of bisphenol F. Reaction conditions: catalyst,
Al-MMT(6); mole ratio of phenol/formaldehyde, 15; catalyst
concentration, 0.010-0.040 g/g; reaction time, 80 min; reaction
temperature, 383 K;
6
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PAPER
OH
OH
O
Lewis acid
Cat
C
H
H
OH
2,2' isomer
Cat provide proton
Cat
Cat
HO
CH2
CH2
OH
HO
OH
OH
OH
OH
OH
C
OH
OH
OH
H
C
OH
H
H
HO
2,4' isomer
H
4,4' isomer
Scheme 1. Proposed plausible mechanism for hydroxyalkylation of phenol and formaldehyde to bisphenol F.
amount of 2,2′- and 2,4′-isomers, as increase the catalyst electrophilic attack of hydroxy benzyl carbocation on the second
concentration from 0.01 to 0.04 g/g. Thus, 0.030 g/g was selected molecule of phenol followed by proton transfer. The
as the suitable catalyst dose. hydroxyalkylation of phenol to bisphenol F is catalyzed by the
a
Fig. 10 depicts the effects of reaction time on product yield, synergy of Brönsted acid sties and Lewis acid sties. This result was
bisphenol F selectivity and its isomer distribution. The results shows confirmed by Py-IR.
that the product yield increased from 46 to 88% with increase in
reaction time from 20 to 90 min. No significant change was
discovered in the selectivity of bisphenol F over the entire range of
4
Conclusions
the reaction course; There is a slight change for the distributions of
isomer during the whole reaction progress. Based on the results, we
consider that 80 min is the reaction time for optimal performance.
The catalyst stability of Al-MMT(6) was tested by performing
recycle experiments, which were carried out at 383 K with phenol
to formaldehyde mole ratio of 15. The testing results are presented
in Fig. 11, which the activity of the catalyst shows no significant
change even after four successive runs showing 84.3% product yield
and 91.5% bisphenol F selectivity. The slight decrease of the
product yield could be because of the leaching out of the Al-
pillaring agent from the clay interlayers and losses of the catalyst
during reuse. Although, the synthesized Al-MMT(6) shown excellent
stability, which was greatly important to its practical applications.
3+
We have prepared a series of Al-MMT(x) composites with A1 /clay
ratio from 2 to 8 and tested them for the hydroxyalkylation of
phenol. Among the various solid acid catalysts, Al-MMT(6) exhibited
the highest catalytic activity for the hydroxyalkylation of phenol,
because its weak and moderate acid sites and larger specific surface
areas. Moreover, the effect of catalyst calcination on the amount of
weak and moderate acid sites was studied, the results showed that
both weak and moderate acid sites quantity appears the reduction
under the high temperature. The catalytic performance of these
catalysts depended on weak and moderate acidity and the textural
properties (specific surface areas). In addition, the effects of various
reaction parameters like mole ratio, catalyst concentration,
reaction temperature and time on product yield and the selectivity
to bisphenol F were investigated, and an 89.2% yield and 92.7%
selectivity to bisphenol F was obtained under optimal conditions.
Finally, the catalyst was found to retain its activity even after a
fourth recycle experiment.
3.5 Plausible mechanistic pathway
A proposed mechanistic pathway to illustrate the processes of
phenol with formaldehyde to give bisphenol F by Al-MMT(6) is
shown in Scheme 1. Indeed, it has been shown that such contact
3
6
with an acid catalyst. The monomeric form of formaldehyde that
presents in aqueous solution is mostly in the form of methylene
glycol and oligomer. In the first step, the protonation of
Acknowledgments
This research was financially supported by the National Natural
Science Foundation of China (no. 51378187, J1210040), Innovative
Research Team Development Plan-Ministry of Education of China
formaldehyde is activated by Brönsted acid sites of the catalyst to
+
form the CH OH , then converted to carbocations. At the same time,
2
−
the carbocations were produced by the abstraction of H over Lewis
(
no. IRT1238), the Key Project of Hunan Provincial Education
acid sites of the catalyst. Then, the molecules of hydroxy benzyl
alcohol were synthesized with the carbocations may attack phenol.
Next, the hydroxymethyl group of the o/phydroxy benzyl alcohol
abstracts a proton from the catalysts to form hydroxy benzyl
carbocation. Ultimately, the bisphenol F may be formed as the
Department (no.13CY001), and Hunan Provincial International
Cooperation Project of China (no. 2014WK3030).
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
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Graphic Abstract
Hydroxyalkylation of phenol to bisphenol F catalyzed by Al-pillared clay
3+
The effect of Al /clay ratio on yield of bisphenol F
Catalyst
Al-MMT(2)
Al-MMT
(
4)
Al-MMT(6) Al-MMT(8)
Yield of bisphenol F (%)
74.5
87.3
69.2
89.2
weak acid
moderate acid
strong acid
2.14
1.94
2
1.88
1.76
1.52
1.41
0.97
0.58
0.41
0.46
0.35
0
.29
0
2
4
6
8
3+
Al /clay ratio of the Al-MMT cataiyst
The distribution acid sites of Al-MMT