Highly active and spherical natured mesoporous aluminosilicate nanoparticles materialized for: T -butylation of phenol

Article abstract of DOI:10.1039/c6ra09745f

Aluminium-containing spherical mesoporous silica nanoparticle (AlSMSN) materials were synthesized for the first time using cetyltrimethylammonium bromide (CTMABr) as a structure-directing agent under nonhydrothermal conditions at room temperature. The effect of the nature of the Al source on the synthesis of the AlSMSN materials was investigated using a variety of Al sources. The effect of higher Al ion incorporation on the structural and textural properties of the AlSMSN materials was evaluated by varying the n_Si/n_Al ratios, and an efficient chemical treatment method was developed for the removal of superficial octahedral Al ions from the surface of the AlSMSN materials. The synthesized AlSMSN materials were characterized using ICP-AES, XRD, N₂ adsorption, ²⁷Al & ²⁹Si MAS NMR, FTIR, TEM, and FE-SEM techniques. ICP-AES results revealed incorporation of a high Al ion content (n_Si/n_Al = 24) into the spherical mesoporous silica nanoparticle materials. For all of the calcined AlSMSN materials, the XRD reflection peaks shifted to lower angle with increasing Al ion content, resulting in an increase of the unit cell parameter. The textural parameters of the AlSMSN materials, except for the surface area, found from nitrogen adsorption measurements, increase with an increase in the amount of aluminium isopropoxide in the synthesis gel. The location and coordination of Al ions on the walls of the AlSMSN materials and the amount of Al ions incorporation were determined from the ²⁷Al MAS NMR measurements. TEM and FE-SEM images demonstrated that the AlSMSN materials comprise spherical particles with hexagonally packed uniform mesoporous channels. Furthermore, the AlSMSN(20) with a higher Al content exhibits better hydrothermal stability than the AlSMSN(100) congener with a lower Al content. Catalyst recyclability studies showed that the washed catalyst (W-AlSMSN(20)) exhibited exceptional catalytic activity during t-butylation of phenol with 95.4% 4-t-butylphenol (4-TBP) selectivity due to removal of superficial octahedrally coordinated Al ions on the surface of active sites.

Full text of DOI:10.1039/c6ra09745f

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PAPER
Highly active and spherical natured mesoporous
aluminosilicate nanoparticles materialized for t-
butylation of phenol†
Cite this: RSC Adv., 2016, 6, 60983
M. Selvaraj*
Aluminium-containing spherical mesoporous silica nanoparticle (AlSMSN) materials were synthesized for
the first time using cetyltrimethylammonium bromide (CTMABr) as a structure-directing agent under
nonhydrothermal conditions at room temperature. The effect of the nature of the Al source on the
synthesis of the AlSMSN materials was investigated using a variety of Al sources. The effect of higher Al
ion incorporation on the structural and textural properties of the AlSMSN materials was evaluated by
varying the nSi/nAl ratios, and an efficient chemical treatment method was developed for the removal of
superficial octahedral Al ions from the surface of the AlSMSN materials. The synthesized AlSMSN
27
29
2
materials were characterized using ICP-AES, XRD, N adsorption, Al & Si MAS NMR, FTIR, TEM, and
FE-SEM techniques. ICP-AES results revealed incorporation of a high Al ion content (nSi/nAl ¼ 24) into
the spherical mesoporous silica nanoparticle materials. For all of the calcined AlSMSN materials, the XRD
reflection peaks shifted to lower angle with increasing Al ion content, resulting in an increase of the unit
cell parameter. The textural parameters of the AlSMSN materials, except for the surface area, found from
nitrogen adsorption measurements, increase with an increase in the amount of aluminium isopropoxide
in the synthesis gel. The location and coordination of Al ions on the walls of the AlSMSN materials and
27
the amount of Al ions incorporation were determined from the Al MAS NMR measurements. TEM and
FE-SEM images demonstrated that the AlSMSN materials comprise spherical particles with hexagonally
packed uniform mesoporous channels. Furthermore, the AlSMSN(20) with a higher Al content exhibits
better hydrothermal stability than the AlSMSN(100) congener with a lower Al content. Catalyst
recyclability studies showed that the washed catalyst (W-AlSMSN(20)) exhibited exceptional catalytic
activity during t-butylation of phenol with 95.4% 4-t-butylphenol (4-TBP) selectivity due to removal of
superficial octahedrally coordinated Al ions on the surface of active sites.
Received 15th April 2016
Accepted 7th June 2016
DOI: 10.1039/c6ra09745f
www.rsc.org/advances
stability. Furthermore, the mesoporous MCM-41 materials are
easily destroyed by exposure to boiling water and steam as
Introduction
In 1992, Beck et al. discovered highly ordered mesoporous a result of rapid hydrolysis of the relatively thin amorphous
3,4
materials, denoted MCM-41, which are comprised of an amor- silica pore walls. To circumvent these limitations, transition
phous silica wall with a regular mesopore system consisting of metal-ions such as Al, Fe, Ga, Mn, Cr, Zr, and V have been
an array of hexagonally shaped mesopores. Compared to incorporated into the amorphous silica walls of MCM-41,
microporous silica materials, these MCM-41 materials have a process that is mandatory for the formation of catalytically
5–7
garnered signicant interest due to their potential catalytic active sites in mesoporous molecular sieve materials.
applications based on their high surface areas, large pore sizes,
The highly tailored mesoporous MCM-41 materials have
1,2
and large pore volumes. Although MCM-41 materials offer been widely used in catalysis research. Depending on the type of
many advantages for the conversion of bulky molecules during transition metal species, the coordination, and the number of
catalysis, the pure mesoporous MCM-41 materials have not yet metal species incorporated into the framework of the MCM-41
found practical application as catalysts or catalyst supports in silica walls, both the Brønsted and Lewis acid sites and their
industry due to their lack of acidic sites and poor hydrothermal acid strengths might be varied. Generally, mesoporous mate-
rials have a lower content of acid sites than zeolites. Because of
its uniform pore size, mesoporous M-MCM-41 (where M
denotes transition metal-ions) is a promising catalyst for
synthesis of bulky ne chemicals in various industries. For
instance, Selvaraj and coworkers developed a hydrothermal
Nanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, Kuala
Lumpur 50603, Malaysia. E-mail: selvaraj@um.edu.my; Fax: +60 379676956; Tel: +60
1
†
1
76882929
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra09745f
synthesis procedure for the preparation of an efficient Al-MCM-
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1 catalyst that had a large number of Brønsted acid sites with Synthesis of AlSMSN materials
Paper
4
enhanced hydrothermal stability, for a variety of catalytic reac-
tions. The conventional hydrothermal method is applicable for
The AlSMSN materials were synthesized using CTMABr as
a structure-directing agent under non-hydrothermal conditions
at room temperature. In a typical synthesis, 5.8 g of CTMABr
was added to 50 mL of water to produce a clear white solution.
Thereaer, the required amount of EtOH was added, and the
mixture solution was again stirred for 20 minutes at 250 rpm to
produce a clear solution. Prior to addition of an appropriate
8
synthesis of Al-MCM-41, but requires an extensive reaction time
and careful control of the temperature and pH for exchange of
sodium cations to ammonium ions and subsequent calcination
9
at 873 K in air to generate the protonated form. Grun et al.
proposed a room temperature synthesis procedure for the
preparation of MCM-41 using tetraethylorthosilicate as a silica
source and ethanol as a solvent in the presence of aqueous
amount of NH OH, 10.4 g of TEOS and a required amount of the
4
desired Al source solution were simultaneously added drop-
wise within a few minutes, and the resulting mixture was stir-
red for 2 h. Finally, the gel solution was aged for 20 h at room
temperature. A rst set of catalytic samples was prepared using
different Al sources with a xed nSi/nAl ratio of 40. These
samples are denoted as AlSMSN(AlC), AlSMSN(AlP), AlSMS-
N(AlN), and AlSMSN(AlS), where the abbreviation of AlC, AlP,
AlN, and AlS are the abbreviated forms of aluminium chloride
hexahydrate, aluminium isopropoxide, aluminium nitrate
nonahydrate, and aluminium sulfate octadecahydrate, respec-
tively. A second set of catalytic samples was prepared by varying
the molar ratio of nSi/nAl (nSi/nAl ¼ 20, 40, and 100). These
samples are denoted as AlSMSN(20), AlSMSN(AlN)/AlSMSN(40),
and AlSMSN(100).
10
ammonia catalyst. Aqueous ammonia has been conrmed to
be a good catalyst in the preparation of Al-MCM-41 via the
11
+
graing method, at room temperature. The incorporated NH
4
is a potentially acid generating cation, and therefore no further
ion exchange would be required to maintain Brønsted acid sites
on the surface of Al-MCM-41. Although using hydrothermal
method the hexagonally well-ordered mesoporous Al-MCM-41
prepared with tailored morphology and regular arrays of
uniform channels is known, aluminium-containing spherical
mesoporous silica nanoparticle (AlSMSN) materials with rich Al-
content, enhanced hydrothermal stability and catalytic activity
have not been clearly documented in open literature.
In the present study, for the rst time, an effective and
convenient nonhydrothermal procedure has been used for
synthesis of AlSMSN materials with a very high loading of Al
All of the solid products prepared above, having the molar
gel composition of 1TEOS/0.01–0.05Al O /0.32CTMABr/13NH /
2
3
3
ions by simple adjustment of the molar ratio of n /n at room
Si Al
1
50H O/60EtOH, were recovered by ltration, and washed
2
temperature. The effect of the nature of the Al source on the
synthesis of the AlSMSN materials has been also investigated by
using different Al sources, i.e., aluminium chloride hexahydrate
several times with distilled water. Finally, the samples were
calcined in air at 813 K for 6 h for complete removal of the
surfactant template.
(AlC), aluminium isopropoxide (AlP), aluminium nitrate non-
ahydrate (AlN), and aluminium sulfate octadecahydrate (AlS).
The hydrothermal stability of the AlSMSN materials has been
Evaluation of hydrothermal stability
also evaluated by treatment in boiling water. An efficient The hydrothermal stability of the AlSMSN catalysts was inves-
chemical treatment method for removal of the octahedral Al tigated by treatment with pure boiling water at 373 K for 168 h at
ions on the surface of the AlSMSN materials has been effectively autogenous pressure.
used. The efficacy of the synthesized AlSMSN materials has
been demonstrated based on extensive use as catalysts in vapor-
Removal of octahedral aluminium
phase t-butylation of phenol under optimized conditions.
To ensure complete removal of non-framework octahedral Al
ions on the surface of AlSMSN(20), the calcined sample was
Experimental section
Chemicals
washed with 1.0 M NH
AlSMSN(20)/10 mL of 1.0 M NH
ture for 1 h. This treatment was repeated twice with fresh NH
solution. Aer completion of the treatments, the sample was
ltered, washed with copious amounts of deionized water, and
4
Cl aqueous solution (1 mg of
Cl solution) at room tempera-
Cl
4
4
For the synthesis of AlSMSN materials, all chemicals, including
cetyltrimethylammonium bromide (CTMABr, >99%), tetraethy-
lorthosilicate (TEOS, 98%), ammonium hydroxide solution

air-dried at 373 K for 1 h. The washed AlSMSN(20) is denoted as
W-AlSMSN(20).
(
(
(
NH OH, >99.99%), 2-propanol (99.5%), absolute ethanol
EtOH, >99.5%), AlC (99%), AlP (>98%), AlN (99.997%), and AlS
>98%), were purchased from Aldrich Chemical Inc. All chem-
4
Characterization
icals were used as received without further purication. Milli-
pore water was used in all experiments.
The elemental composition of the resultant solid products was
For the vapor-phase t-butylation of phenol, all chemicals, analyzed by ICP-AES (Perkin-Elmer, Optima 3000) aer disso-
including phenol (puried by redistillation, >99%), tert-butanol lution of the catalytic samples in HF solution. Small-angle XRD
(
BuOH, >99.7%), and authentic chemicals for comparative patterns were recorded under ambient conditions on a Shi-
analyses of the reaction products, 2-t-butylphenol (2-TBP, 99%), madzu XRD-6000 instrument with Cu Ka radiation (l ¼ 1.5406
˚
3
2
-t-butylphenol (3-TBP, 99%), 4-t-butylphenol (4-TBP, 99%), and A). The X-ray tube was operated at 40 kV and 30 mA and the
ꢀ
,4-di-t-butylphenol (2,4-DTBP, 99%), were purchased from diffractograms were recorded in the 2q range of 0.6–9 with a 2q
Aldrich Chemical Inc.
step size of 0.01 and a step time of 10 s. Nitrogen adsorption/
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desorption measurements were conducted on a Quantachrome analyzed along with authentic samples by gas chromatography
Autosorb-1 system by N physisorption at 77 K. All samples were using SE-30 and AT1000 columns. The identity of the products
2
ꢀ
ꢁ5
outgassed for 3 h at 250 C under vacuum (p < 10 hPa) in the was further conrmed using a combined gas chromatography-
degas port of the sorption analyzer. The BET specic surface mass spectrometry system (GC-MS; Hewlett G1800A).
areas of the catalytic samples were calculated in the range of
relative pressures between 0.05 and 0.3. The pore size distri-
butions were calculated from the adsorption branch of the
isotherm using the thermodynamics-based Barrett–Joyner–
Results and discussion
Effect of aluminium sources
Halenda (BJH) method. The total pore volume was determined The AlSMSN materials were synthesized using CTMABr as
from the adsorption branch of the N isotherm at P/P ¼ 1. FTIR a template and ethanol as a solvent using a modication of
2
0
spectra of the solid mesoporous samples were acquired in the St ¨o ber's method, at room temperature, to generate mono-
ꢁ
1
12
range of 4000–400 cm via the KBr disc technique using an disperse silica spheres. In this study, TEOS served as a silica
FTIR spectrometer (Shimadzu-FTIR800) by utilizing 40 scans source, and NH OH solution was used as the catalyst to balance
4
ꢁ
1 27
and a resolution of 4 cm
.
Al MAS NMR spectra were recor- the hydrolysis rate of TEOS and Al precursors. The combined
ded at 104.3 MHz on a Bruker DRX-400 spectrometer equipped solution of NH OH and ethanol promotes the formation of
4
with a magic angle spin probe at room temperature. AlCl
3
$6H
2
O
spherical mesoporous silica. The effect of the nature of the Al
2
9
was used as a reference. Si nuclear magnetic resonance (NMR) ion source was evaluated by using AlC, AlP, AlN, and AlS as the
spectra were recorded at 11.75 T on a Varian INOVA 500 NMR Al sources for the synthesis of AlSMSN materials. There is
2
9
spectrometer with a CPIMS probe. Si MAS spectra were a small variation in the n /n ratio of the calcined AlSMSN
Si Al
measured at 99.3 MHz using a SiN rotor at 3 kHz. The chemical materials as a function of the Al source used in the synthesis gel
shis were estimated in ppm using tetramethylsilane (TMS) as with a xed n /n molar ratio of 40. Based on the ICP-AES
Si Al
2
9
a reference for Si. Field-emission scanning electron micros- measurements (Table 2), the n /n ratios of the calcined
Si Al
copy (FESEM) images were obtained with a JEOL JSM-6700F AlSMSN materials, starting from an n /n ratio of 40 in the
Si Al
microscope at an accelerating voltage of 5.0 kV. Transmission synthesis gel, were 41 (AlSMSN(AlN)), 42 (AlSMSN(AlP)), 43
electron microscopy (TEM) images were collected on a JEOL (AlSMSN(AlS)), and 46 (AlSMSN(AlC)). The AlSMSN materials,
2
2
010 electron microscope operated at an acceleration voltage of i.e., AlSMSN(AlS), AlSMSN(AlP), and AlSMSN(AlN) were charac-
00 kV.
terized by a very high amount of Al ion incorporation. This is in
The acid sites of various AlSMSN catalysts were determined good agreement with the synthesis of AlMCM-41, showing that
by temperature programmed desorption (TPD) of pyridine. In the use of AlN, AlS, and AlP facilitates the incorporation of Al
8,13
a typical experimental of AlSMSN(AlP), the catalyst (100 mg) was ions into the silica pore walls. Because the use of AlC reduces
evacuated for 3 h at 523 K under vacuum (p < 10 hPa). Further, the pH of the synthesis gel slightly from 10.8 to 9.5, there was
ꢁ
5
the sample was cooled to room temperature under dry nitrogen a slight decrease in the amount of Al ions incorporated on the
8
followed by an exposure of pyridine stream in nitrogen for 30 silica pore walls. As Selvaraj's group reported, a less basic
min. Subsequently, the physisorbed pyridine from the sample synthesis gel (pH < 10) is not favorable for heteroatom incor-
was removed by heating at 393 K for 2 h in a nitrogen ow. The poration. The amount of Al ions incorporated into AlSMSN(AlC)
TPD of pyridine for the sample was performed by heating in was much lower than that incorporated into AlSMSN(AlS),
ꢁ1
a nitrogen ow (50 mL min ) from 393 to 973 K with a rate of AlSMSN(AlP), and AlSMSN(AlN) because of the slight change in
ꢁ
1
1
0 K min using a high-resolution thermogravimetry analyzer pH of the synthesis gel from high basicity to low basicity (pH <
SETARAM setsys 16MS). The weight loss observed from the 10) and the slight dissociation of the Al–O–Si bonds due to the
sample was used to measure the strength of acid sites, increase in the rate of hydrolysis of AlC with TEOS.
assuming that each mole of pyridine corresponds to one mole Fig. 1 shows the powder XRD patterns of the AlSMSN mate-
of protons. The same experiment procedure was used to other rials prepared by employing different Al sources using the
(
AlSMSN catalysts for identication of their acid sites.
ethanol–NH OH synthesis procedure at room temperature. The
4
XRD patterns of the AlSMSN materials show several Bragg peaks
at low angles, and the peaks are much broader than those ob-
t-Butylation of phenol
tained for regular AlMCM-41 materials, as reported by Selvaraj
8
The catalytic t-butylation of phenol with BuOH was carried out et al. The second and third diffraction peaks of the AlSMSN
using 500 mg of AlSMSN material using a xed-bed vertical ow materials overlap, indexed as (110) and (200) peaks, respec-
reactor. Prior to the reaction, the AlSMSN material was activated tively, whereas in the case of the regular MCM-41 material,
1,2,14
at 773 K in owing air for 10 h, followed by cooling to the these two peaks are distinctly separate.
However, the XRD
reaction temperature (450 K) under nitrogen. Aer an hour, the pattern of the AlSMSN materials is characterized by a single
reactant mixture of phenol and BuOH, with a desired molar broad Bragg peak and the other two peaks are diminished. The
ratio and weight hour space velocity (WHSV, WHSV ¼ mass ow unit cell parameters of the AlSMSN materials (Table 1) varied
ꢁ
3ꢁ
7
2ꢁ
of reactant divided by mass of catalyst) was fed into the reactor based on the counterions (such as NO , C H O , SO4 , and
3
3
ꢁ
using a dual syringe pump (Harvard Apparatus) with nitrogen as Cl ) of the Al sources used in the preparation of the AlSMSN
a carrier gas. The condensed liquid products obtained from the materials given that the counterions affect the interaction
gaseous products were collected at different times, and between the surfactant and the silicate species. This
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than that observed for AlSMSN(AlC). It is also notable that the
value of the unit cell (a ) size was equal to the internal pore
o
diameter (V ) plus one pore wall thickness (T ). The unit cell
p
w
size of the AlSMSN materials is thus larger than that of pure
3
+
SMSN because the size of Al (radius 53 pm) is larger than that
4
+
of Si (radius 40 pm).
The N sorption isotherms of the AlSMSN materials prepared
via the ethanol–NH OH synthesis procedure at room tempera-
2
4
ture using different Al sources can be classied as type IV
isotherms according to the IUPAC classication (Fig. 2). All of
the AlSMSN materials exhibited a high degree of structural
ordering with a very narrow hysteresis loop, as indicated by the
capillary condensation step of the respective adsorption
isotherms. The values of the textural parameter of AlSMSN(AlN),
AlSMSN(AlP), and AlSMSN(AlS) were much lower than that of
AlSMSN(AlC) given that the unit cell parameters increase with
increasing Al ion content, as noted in Table 1. However, the
pore size of AlSMSN(AlP) was signicantly larger than that of the
other AlSMSN materials because isopropanol or propanol
formed from AlP at the micelle/water interface is accumulated,
and acts as a swelling agent to form large pores. Moreover, the
pore wall thickness of AlSMSN(AlN) is slightly higher than those
Fig. 1 XRD powder patterns of AlSMSN materials prepared with
various synthetic parameters.
observation is explained with an example as follows. Among the
counterions, the nitrate ion provides the strongest counterion
association, which neutralizes the micellar charge. In the
4
ethanol–NH OH synthesis medium, the stronger charge
neutralization would in turn promote the formation of more
rigid micelles with monodisperse spherical structure thus
reducing the synthesis time. The size of the unit cells of
AlSMSN(AlN), AlSMSN(AlP), and AlSMSN(AlS) is slightly higher Fig. 2 Nitrogen adsorption isotherms of AlSMSN materials prepared
with different Al sources, such as AlC, AlN, AlP and AlS.
a
Table 1 Structural and textural parameters of AlSMSN materials prepared with different Al sources, and with different nSi/nAl ratios
nSi/nAl
b
2
ꢁ1
3
ꢁ1
˚
(A)
˚
(A)
˚
p
Catalysts
Gel
Product
a
o
A
BET (m g
)
V
p
(cm g
)
d
p
T
w
¼ a
o
ꢁ d
(A)
AlSMSN(AlC)
AlSMSN(AlS)
AlSMSN(AlN)
AlSMSN(AlP)
40
40
40
40
20
100
—
46
43
41
42
24
101
—
41.0
41.5
41.7
41.6
42.7
40.6
39.8
1155
1120
1171
1152
1011
1274
1300
0.79
0.80
0.80
0.80
0.85
0.77
0.77
30.7
30.8
30.9
31.0
31.5
30.6
30.2
10.3
10.7
10.8
10.6
11.2
10.0
09.6
c
AlSMSN(20)
c
AlSMSN(100)
SMSN
a
b
a
o
, unit cell parameter; ABET, Specic surface area; V
p
, pore volume; d
p
, pore diameter, T
w
, wall thickness.
nSi/nAl ratios of products are
c
determined by ICP-AES. The catalysts were synthesized using AlP as an aluminum source.
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2
7
Fig. 3
Al MAS NMR spectra of calcined AlSMSN materials prepared
29
Fig. 4
Si MAS NMR spectra of calcined AlSMSN materials prepared
with different Al sources, such as AlC, AlP, AlN and AlS.
with different Al sources, such as AlP, AlN, AlS and AlC.
of the other AlSMSN materials. It has been reported that the
the ethanol–NH OH synthesis procedure at room temperature.
4
ꢁ
1
˚
charge/radius (Z/r, A ) ratio of the central cation of the coun-
The broad peak at ꢁ110 ppm is assigned to the Si(OSi) (Q )
4
4
15
terions controls the rate of silica condensation. The Z/r ratio of
the nitrate ion may be twice that of the other counterions, and
hence, the nitrate ion enhances silica condensation and
promotes the formation of thick silica walls (Table 1). Taken
together, these observations indicate that the structural order,
textural parameters, and the incorporation of Al ions into the
SMSN material can be modied by using different sources of Al.
sites, and the peaks at ꢁ103 and ꢁ93 ppm are attributed to the
3 3 2 2 2
Si–(OSi) OH (Q ) and Si–(OSi) (OH) (Q ) sites, respectively. The
spectra of the calcined AlSMSN catalysts are less well-resolved
than that of the SMSN material. There is a progressive reduc-
tion in the intensity of the Q peaks (due to the large number of
3
silanol groups in the SMSN material, the intensity of the Q3
peaks is higher) as the amount of Al ions in the AlSMSN
materials increases. The same effect has been observed by Sel-
varaj and Lee for incorporation of both V and Mo into MCM-41
2
7
The Al MAS NMR spectra of the calcined AlSMSN materials
prepared from different aluminium sources using the ethanol–
NH
4
OH synthesis procedure at room temperature are shown in
7c
using a direct hydrothermal method. Furthermore, the
Fig. 3. The spectra of the calcined AlSMSN materials synthe-
sized using the various Al sources exhibit a strong and sharp
signal around 55 ppm as well as a weak and broad peak around
29
deconvoluted Si MAS NMR spectra of the AlSMSN materials,
including SMSN (Fig. 4), demonstrate that there is a progressive
reduction in the intensities of the Q and Q species with
3 2
0
ppm, irrespective of the Al ion content. The signal at 55 ppm is
increasing incorporation of Al ion into the material. Based on
these results, it is deduced that the Al ions are strongly bonded
to the pore walls of the SMSN materials.
SEM was used to determine the particle size and particle
morphology of the AlSMSN materials prepared from different
aluminium sources. The SEM images show that the calcined
AlSMSN materials comprise almost uniformly sized spherical
particles with sizes ranging from 600 to 1000 nm. Fig. 5 shows
the uniform-sized spherical particles of calcined AlSMSN(AlN)
as an example.
TEM analysis demonstrated the uniform pore size of the
AlSMSN material and the uniformly sized spherical particles,
illustrating that the spherical particles were comprised of
hexagonal arrays of pores. As an example, the uniform pore size
and hexagonal arrays are shown for calcined AlSMSN(AlN) in
assigned to tetrahedrally coordinated Al ions substituted into
the framework of the SMSN material, whereas the signal at
0
ppm is ascribed to non-framework Al ions that are octahe-
drally coordinated on the silica walls of the AlSMSN materials.
The presence of tetrahedral and octahedral Al ions can be been
clearly conrmed in the AlMCM-41 catalysts synthesized from
different Al sources via the direct hydrothermal method based
13
27
on the published literature. On the basis of Al MAS NMR
studies, it is noteworthy that the calcined AlSMSN(AlN) has
a slightly higher content of tetrahedral Al ions relative to the
4
other AlSMSN materials, and the ethanol–NH OH synthesis
procedure is found to be an efficient method for synthesis of
AlSMSN materials from various Al sources.
2
9
Fig. 4 shows the Si MAS NMR spectra of the calcined
AlSMSN materials synthesized from different Al sources using
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Fig. 5 SEM image of calcined mesoporous AlSMSN(AlN).
Fig. 6. Moreover, the average thickness of the wall and the pore
˚
˚
diameter for AlSMSN(AlN) were ca. 11.2 A and 30.7 A, respec-
tively, which is consistent with the N adsorption measurement
2
data. In addition, the AlSMSN materials prepared in the co-
solvent system of water–ethanol, at room temperature, were
characterized by small pore sizes given that ethanol is a polar
protic solvent and has the ability to reduce the repulsive inter-
actions between the template headgroups, and reduces the
Fig. 6 TEM images of calcined mesoporous AlSMSN(AlN).
10
diameter of the micelle templates.
acid sites, whereas, the surface hydroxyl groups form the weak
acid sites. The total number of acid sites (medium and strong
acid sites) depends on the nSi/nAl ratios and increases with
increasing the tetrahedral Al-ions framework on the silica walls.
The number of weak, medium and strong acid sites of AlSMSN
catalysts increases in the following order: AlSMSN(AlN) >
AlSMSN(AlP) > AlSMSN(AlS) > AlSMSN(AlC). The number of
strong and medium acid sites is almost similar in AlSMSN(AlN),
AlSMSN(AlP) and AlSMSN(AlS), except AlSMSN(AlC), which has
lower aluminium content as compared to other AlSMSN
materials.
Based on the TEM images, the formation of the AlSMSN
materials can be explained as follows: in the synthesis of the
AlSMSN materials, ethanol was added to the aqueous reaction
4
mixture in the presence of NH OH solution, which was used as
a catalyst. Initially, the ethanol solution enhances the interac-
tions among the headgroups to rapidly generate rod-like
micelles with a small diameter, and simultaneously decreases
the rate of hydrolysis of TEOS. The NH OH catalyst then
4
enhances the formation of rod-like micelles with hexagonal
arrangements in different segments. The micelles react with
silicic acid or its oligomeric derivatives to form SMSN materials.
Al–OH formed from the Al source then reacts rapidly with the
hydroxyl groups of SMSN to produce AlSMSN materials. The Effect of n /n ratios
Si Al
hexagonal pore arrangement may vary if the distribution of the
ICP-AES was used to analyze the elemental compositions of the
length of the rod-like micelles is varied. The pore channels may
be arranged as follows: the longer pore channels with uniform
hexagonal pore size distribution are centered in the spheres,
whereas the small pore channels formed between the longer
pore channels coordinate to form the spheres. Careful investi-
gation of the TEM images remarkably reveals that the AlSMSN
materials have uniform pore sizes with hexagonal arrays of
pores.
calcined AlSMSN materials synthesized using AlP as the Al
source while varying the nSi/nAl ratios (Table 1). The nSi/nAl ratios
of the calcined materials were generally slightly higher than the
nSi/nAl ratios of the synthesis gel. In this synthesis method, the
n
Si/nAl ratio changes from 24 to 101 due to the higher Al
incorporation on the silica pore walls by the formation of more
Al hydroxyl groups.
In Fig. 1, which shows the powder XRD patterns of the
AlSMSN materials with different nSi/nAl ratios, three Bragg peaks
were observed at low angles for all of the samples, which were
indexed to the (100), (110), and (200) reections. The XRD peaks
shi to lower angle with increasing Al ion content, with
a concomitant increase in the d-spacing values and unit cell
parameters. The expansion of the unit cell dimensions with
The acid sites of a variety of AlSMSN catalysts were deter-
mined using TPD of pyridine, and their data are noted in Table
S1,† as weak (weight loss between 393 and 623 K), moderate
(623–723 K) and strong (723–873 K) are found in all catalytic
materials. The Al-ions incorporated with tetrahedral coordina-
tion on the silica pore walls create the medium and/or strong
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decreasing Si/Al ratio may be attributed to the larger radius of exposed on the pore wall surface, behaving as impregnated
3
+
4+
Al relative to Si , and/or the longer Al–O bond distance metal-ion complexes on the pore walls of the AlSMSN material.
compared to the Si–O bond distance. These results demonstrate These metal ions may then interact with surface hydroxyl
that the mesostructural order could be maintained with incor- groups and cause the pore wall to contract by coordination with
poration of trivalent Al ions into the framework structure of the two or three hydroxyl groups, causing the pore size to
16
SMSN materials.
decrease. However, if these metal ions are incorporated deeply
Table 1 summarizes the N
2
sorption isotherm data for the within the silica framework, this pore shrinkage might be
calcined AlSMSN materials prepared with different nSi/nAl precluded given that the metal ions would be unable to interact
ratios. A sharp condensation step was observed at relative with the surface hydroxyl groups, and the pore size might
pressures in the range of 0.3–0.98 in the isotherms of all instead increase due to the increased metal–oxygen bond
samples. It can also be seen that the capillary condensation step length, similar to the situation observed for zeolites. Based on
is shied to higher relative pressures with increasing Al ion the above hypothesis, this unusual result strongly suggests that
content in the calcined samples (not shown the gure), result- the Al ions were incorporated into the silica framework. More-
ing in an increase of the pore diameter with increasing Al ion over, the pore size increases without any ascertainable change
content in the samples. Moreover, as the nSi/nAl ratio of the in the structural order of the AlSMSN materials with increasing
calcined samples decreases, the pore diameter, pore volume, AlP content in the synthesis gel. In addition, the pore volumes
and pore wall thickness increase, whereas the specic surface of all of the AlSMSN materials prepared with various nSi/nAl
area decreases (Table 1). The unit cell size also increases ratios from 24 to 101 were higher than that of the purely sili-
monotonically as the n /n ratio of the calcined catalytic ceous SMSN material. These results suggest that AlSMSN
Si Al
samples decreases. The same effect was observed when AlP was materials can be prepared with very high Al content (n /n
¼
Si Al
8
a
used as the Al source in the synthesis of MCM-41. It is evident 24) without affecting the structural order of the resulting
that the enlargement of the unit cell size is not a consequence of AlSMSN materials. Furthermore, the structural and textural
Al ion incorporation into the pore walls given that the increase properties of W-AlSMSN(20) are maintained aer washing, as
the unit cell size is only observed when AlP is used as the Al shown in Fig. 1 and Table 2.
source. The observed increase in the pore size of AlSMSN(AlP)
FTIR spectra of the as-synthesized and calcined AlSMSN
can be explained as follows: in crystalline zeolites, the pore size materials are shown in Fig. 7. The incorporation of metal ions
5
b
increases slightly with incorporation of metal ions because of has been studied in depth in one of Selvaraj group reports. The
the longer bond length of the metal ion–oxygen bond relative to as-synthesized AlSMSN materials exhibit absorption bands
ꢁ
1
the Si–O bond length. Isopropanol or propanol is formed by around 2921 and 2851 cm , corresponding to n-C–H and d-C–
hydrolysis of AlP at the micelle/water interface; accumulation of H vibrations of the surfactant molecules. The broad bands near
ꢁ
1
this alcohol, which acts as a swelling agent, results in an 3500 cm are ascribed to water molecules adsorbed on the
increase of the pore size. However, this does not apply to surface silanol groups (Si–OH) but not on the Si–O–Si groups,
AlSMSN, which has an amorphous structure where both the because the non-polar Si–O–Si groups exhibit hydrophobic
5
b
bond length and bond angle may change. It is generally character derived from dehydroxylation of the silica surface.
observed that the pore size of AlSMSN materials decreases Moreover, the pores of the mesoporous materials are con-
subsequent to incorporation of metal ions, but thus far, there structed from tetrahedral SiO , and ideal pores formed from Si–
4
has been no clear explanation for this observation. Although O–Si bonds may give rise to the hydrophobic nature. With
AlSMSN has thicker pore walls than zeolites, not all metal ions increased substitution/incorporation of Al ions into the SMSN
can be completely incorporated or substituted into the silica materials, the hydrophobicity increases with a concomitant
framework. This is because some of these metal ions may be increase in the number of tetrahedral Al ions, which creates
Table 2 Structural and textural parameters of AlSMSN materials prepared after hydrothermal treatment using boiling water, and recyclable
a
AlSMSN catalysts used for the t-butylation of phenol
nSi/nAl
b
2
ꢁ1
3
ꢁ1
˚
(A)
˚
(A)
˚
p
Catalysts
Gel
Product
a
o
A
BET (m g
)
V
p
(cm g
)
d
p
T
w
¼ a
o
ꢁ d
(A)
c
AlSMSN(20)
—
—
—
—
—
25
101
25
25
101
42.6
39.5
42.6
42.6
40.4
1050
1001
998
980
1200
0.85
0.70
0.86
0.86
0.77
31.5
30.1
31.5
31.5
30.6
11.1
9.40
11.1
11.1
09.8
c
AlSMSN(100)
d
W-AlSMSN(20)
e
AlSMSN(20)
e
AlSMSN(100)
a
b
a
o
, unit cell parameter; ABET, Specic surface area; V
p
, pore volume; d
p
, pore diameter, T
w
, wall thickness.
nSi/nAl ratios of products are
d
c
determined by ICP-AES. Hydrothermal stability of the samples were investigated with boiling water at 373 K for 168 h. The catalyst was
washed using ammonium chloride solution for the removal of non-framework octahedral aluminum species. The results were obtained for
the reusable catalysts used for the 4 run in the t-butylation of phenol.
e
th
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27
Fig. 7 FTIR spectra of as-synthesized and calcined AlSMSN materials Fig. 8
Al MAS NMR spectra of calcined AlSMSN(20) prepared by an
synthesized with different nSi/nAl ratios.
efficient chemical treatment method for the removal octahedral
aluminum, and with nSi/nAl ¼ 20.
negative charges on the SMSN materials according to the
following order: AlSMSN(20) > AlSMSN(AlP) > AlSMSN(100). The indicates that there is a greater Al ion incorporation in
3
+
anti-symmetric Si–O–Si vibration bands and anti-symmetric Si– AlSMSN(20) than in AlSMSN(100). The additional Al ions in
ꢁ
1
O–Al vibration bands shi from 1065 to 1095 cm and 960 and the AlSMSN materials with lower nSi/nAl ratios are tetrahedrally
ꢁ1
4+
9
65 cm , respectively (Fig. 1) in the order: AlSMSN(20) > coordinated to Si on the silica walls. The weak shoulder
AlSMSN(AlP) > AlSMSN(100). The most plausible reason for observed around 0 ppm for AlSMSN(20) is not observed subse-
these shis is that the mean bond distance of Si–O in the walls quent to the washing process (for W-AlSMSN(20)), as shown in
increases due the increased substitution of silicon (radius 40 Fig. 8. This evidence strongly supports the suggestion that the
3
+
17
pm) by the larger Al ions (radius 53 pm). Increased Al small amounts of non-framework octahedral Al ions are
incorporation leads to a shi of most of the bands to higher dispersed on the surface of the pore walls in the Al-rich SMSN
8a
wavenumbers, as reported by Selvaraj et al. Additionally, an materials, whereas the non-framework octahedral Al ions
ꢁ
1
absorption band in the range of 960–965 cm , assigned to interact readily with water molecules to become six-coordinate,
a stretching vibration of the Si–O–Al linkage, is clearly observed which in turn affects the catalytic activity of the Brønsted acid
(
Fig. 7). This is generally considered to be evidence that the sites created by tetrahedral Al ions (see, section on catalytic
heteroatom is incorporated into the framework of the SMSN studies). The non-framework octahedral Al ions are also
18
materials. Cambler et al. reported similar stretching vibrations completely removed by the washing process as demonstrated in
of Si–OH groups with defect sites. The disappearance of the Fig. 8. Similar behavior has been reported for AlMCM-41
ꢁ1
11
peaks at 2851 and 2921 cm clearly indicate that the organic materials. Remarkably, TEM analysis demonstrates that the
template molecules used for preparation of the AlSMSN mate- sizes of the spherical nanoparticles increase with increasing Al
rials were completely removed by the calcination process, as ion content, which suggests that the incorporation of a larger
shown in Fig. 7.
quantity of tetrahedral Al ions on the surface of the SMSN
The solid state Al MAS NMR spectra of the AlSMSN mate- structure can be regulated by controlling the ratio of TEOS to
2
7
rials prepared with different nSi/nAl ratios are characterized by AlP.
a strong and sharp signal at 53 ppm as well as a weak and broad
The AlSMSN(20) has more acid sites as compared to
11
peak at 0 ppm, as shown in Fig. 8. As reported in literature, the AlSMSN(100) due to increasing the lots of tetrahedral Al-ions
intensity of the peaks at 53 and 0 ppm (that are respectively incorporated into silica pore walls, whereas, the W-
assigned to tetrahedral and octahedral Al ions in the framework AlSMSN(20) is still maintained their acid sites similar to
of the SMSN materials) increases as the n /n molar ratio of the AlSMSN(20), aer removing of the octahedral aluminium ions
Si Al
product decreases because the Q3 silanol groups interact by washing treatments, as noted in Table S1.† On the basis of
strongly with the Al ions. The higher intensity of the peaks in acidity determination results, it is obviously conrmed that the
the AlSMSN(20) spectrum relative to that of AlSMSN(100) acid sites (medium and strong acid sites) and the stability of
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solid acid catalysts increase with tetrahedral Al-ions coordina- water at 373 K for 168 h. FE-SEM and TEM observations
tion, which are highly promoted to Brønsted acid sites on the demonstrate the preservation of the spherical morphology of
silica pore walls.
the AlSMSN(20) particles even aer hydrothermal treatment at
73 K for 168 h, further conrming that AlSMSN(20) exhibits
3
excellent hydrothermal stability with maintenance of the well-
ordered spherical morphology.
Hydrothermal stability
The hydrothermal stability of the AlSMSN materials, i.e.,
AlSMSN(20) and AlSMSN(100), was evaluated by treating the
samples in boiling water at 373 K for 168 h. The hydrothermal
stability is an important factor for determining the distinct
structural and textural parameters of AlSMSN materials, and
may be useful in evaluation of the catalytic stability, which is
pertinent to the use of AlSMSN materials in the synthesis of
bulk aromatics in chemical industries.
Vapor-phase tert-butylation of phenol
The AlSMSN materials synthesized from different Al sources
were used as catalysts in the tert-butylation of phenol under
suitable reaction conditions as noted in Table 3. The tert-
butylation of phenol with BuOH is an electrophilic substitution
reaction on the aromatic ring. The tert-butylation of phenol
catalyzed by acids or solid acid zeolites is commonly considered
Fig. 1 shows the XRD patterns of the AlSMSN materials aer
treatment in boiling water at 373 K for 168 h. Aer hydro-
thermal treatment, the (100) diffraction peak of AlSMSN(100)
shis slightly to the higher 2q region and the relative intensity
of the diffraction peaks decreases (not shown in Fig. 1).
Nevertheless, the XRD spectrum of AlSMSN(100) still displays
one strong and well-resolved (100) diffraction peak with over-
lapping of the other diffraction peaks, suggesting a highly
ordered spherical mesostructure. The unit cell parameter of
AlSMSN(100) decreases aer hydrothermal treatment (Table 1)
given that this sample contains a high number of free Si–O–Si
bonds that are susceptible to hydrolysis, whereas in the case of
AlSMSN(100) having a low amount of incorporated Al ions, the
unit cell parameter remains constant. These results suggest that
hydrothermal treatment of AlSMSN(100) may cause a certain
degree of shrinkage of the mesopores and structural disorder.
However, aer hydrothermal treatment of AlSMSN(20) under
similar conditions, the unit cell parameter remains constant
due to the high number of Si–O–Al bonds, which are relatively
stable to further attack by boiling water.
20
to proceed via carbonium ion mechanisms. The reaction of
phenol with BuOH is represented in Scheme 1. Depending upon
the textural and structural properties with the rich tetrahedral
aluminium ions incorporated on the silica pore walls, the
results of the catalytic studies presented in Table 3 clearly show
that the three AlSMSN catalysts, i.e. AlSMSN(AlP), AlSMSN(AlN),
and AlSMSN(AlS), produce good phenol conversion with rela-
tively high 4-TBP selectivity, whereas the AlSMSN(AlC) catalyst
gives rise to much lower phenol conversion than the other
AlSMSN catalysts because of its relatively lower Al ion content,
whereas mesoporous ZSM-5 produces a better 4-TBP selec-
21
tivity. The 4-TBP selectivity of the former three AlSMSN cata-
lysts increases in the order: AlSMSN(AlP) > AlSMSN(AlN) >
AlSMSN(AlS). By-products (3-TBP, 2-TBP, and 2,4-DTBP) are
generated with all of the AlSMSN catalysts. Compared to the
other by-products, the formation 3-TBP is more probable given
that the reaction of t-butanol to give more sterically saturated
phenol molecules is favored, because the strong acid sites on
the surface of the AlSMSN catalysts increases with increasing
21
aluminium content. This may initiate the secondary reaction,
The remarkable improvement in the hydrothermal stability
of the AlSMSN materials derived from higher Al ion incorpo-
viz., formation of the m-isomer through isomerization of the o-
ration is further evidenced by N
2
adsorption measurements
(
Table 1) subsequent to treatment with boiling water for 168 h.
a
Table 3 Vapor-phase t-butylation of phenol over AlSMSN catalysts
Aer hydrothermal treatment of AlSMSN(100), the textural
properties, i.e., surface area, pore diameter, pore volume, and
pore wall thickness, decrease, but those of AlSMSN(20) remain
Selectivity (%)
Conversion of
e
almost constant. Because AlSMSN(20) contains a much larger Catalysts
phenol (%)
4-TBP 2-TBP 2,4-DTBP Others
19
amount of Al ions in the framework of the silica pore walls, it
has more Si–O–Al bonds that are relatively stable to further
attack by boiling water. Furthermore, AlSMSN(20) contains
AlSMSN(AlN)
AlSMSN(AlP)
AlSMSN(AlS)
75.5
76.7
74.4
69.4
85.5
50.5
90.4
90.3
50.9
86.5
87.5
85.4
80.5
89.2
57.8
95.4
95.3
58.3
55.4
95.5
7.4
7.5
7.4
7.0
8.0
1.0
3.0
2.9
1.5
2.3
1.0
6.1
5.0
7.2
5.5
2.8
1.0
1.6
1.8
1.7
42.3
3.5
—
—
—
7.0
—
39.2
5.8
5.6
38.5
—
a larger amount of tetrahedral Al ions, which can create more AlSMSN(AlC)
ꢁ
negative charges on the surface of the pore walls to repel OH
ions, and hence prevent hydrolysis of the siloxane bonds, and
can also result in an increase in the number of acid sites, as
AlSMSN(20)
AlSMSN(100)
W-AlSMSN(20)
AlSMSN(20)
AlSMSN(100)
f
f
b
19
reported elsewhere. Because the tetrahedral Al ions are irre-
b
c
versibly incorporated into the structure of AlSMSN(20), it is W-AlSMSN(20) 95.4
reasonable that AlSMSN(20) exhibits higher hydrothermal W-AlSMSN(20) 33.5
d
—
stability than AlSMSN(100). These results also further conrm
that the pore wall of AlSMSN(20) becomes thicker due to the
incorporation of more tetrahedral Al ions into the framework of
the silica walls, and the thick pore wall of AlSMSN(20) displays
a
ꢁ1
Reaction conditions: 500 mg of catalyst, WHSV ¼ 4.86 h , nt-butanol-to-
phenol ratio ¼ 2, time-on stream ¼ 1 h, reaction temperature, 450 K.
n
b
th
The results were obtained using recyclable catalysts for 4 run.
c
e
d
ꢁ1
n
t-butanol-to-nphenol ratio ¼ 4 at 3 h TOS. WHSV ¼ 7.58 h
.
f
Others: 3-TBP, butane and butane dimer products. Unreacted
remarkable hydrothermal stability upon treatment in boiling reactants.
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Scheme 1 The formation of t-butylated phenol products by t-butylation of phenol over AlSMSN.
and p-isomers. Moreover, the AlSMSN(AlP) catalyst with spher- high surface area and tailored pore diameters, which leads to an
ical morphology, synthesized at room temperature, produces increased number of accessible active sites, and the large
higher phenol conversion and 4-TBP selectivity relative to number of tetrahedral Al ions incorporated into the framework
2
7
AlMCM-41(AlP) that is produced by the hydrothermal method of the SMSN catalysts, as conrmed by Al MAS NMR studies,
13
and possesses a rod-like morphology. The conversion as well gives rise to high numbers of Brønsted sites to enhance the
as 4-TBP selectivity has been still maintained with different pore catalyst activity in the vapor-phase t-butylation of phenol. This
sizes of AlSMSN catalysts prepared using different aluminium reaction was also carried out using an SMSN catalyst synthe-
sources. On the basis of comparative reports, it is concluded sized by an easy method at room temperature. In that case, 5%
that the well-ordered spherical mesoporous catalyst with high phenol conversion was achieved (not shown in Table 3), indi-
Al ion loading acts as an important catalytic element in the cating that the high activity of the Al-incorporated SMSN cata-
production of 4-TBP with higher selectivity.
lysts is derived from the tetrahedral Al ions substituted on the
The AlSMSN catalysts synthesized by varying the molar ratios silica pore walls. Overall, AlSMSN(20) exhibits superior catalytic
of nSi/nAl using AlP as an Al source were utilized in the vapor activity in the synthesis of 4-TBP relative to other mesoporous
phase t-butylation of phenol (Scheme 1) under the reaction catalysts, and is found to be a promising heterogeneous catalyst
conditions noted in Table 3; their catalytic activities and 4-TBP for t-butylation of phenol.
selectivity rank in the following order: AlSMSN(20) > AlSMS-
Among the spherical mesostructured AlSMSN catalysts
N(AlP) AlSMSN(100). Among the AlSMSN catalysts, evaluated, AlSMSN(20) and AlSMSN(100), respectively, exhibit
>
AlSMSN(20) exhibits the best catalytic activity, and also the highest and lowest 4-TBP selectivities. The reusability of the
produces a higher 4-TBP selectivity (with a higher phenol AlSMSN(20) and AlSMSN(100) catalysts were thus evaluated
conversion) (Table 3) due to the higher loading of tetrahedral Al under the reaction conditions listed in Table 3, to determine
ions on the surface of the silica pore walls. The various types of their catalytic stabilities. Generally, used AlSMSN catalysts
mesoporous catalysts used in this catalytic reaction also exhibit exhibit lower product selectivity because most of the active sites
13,21,22
superior catalytic activity to those reported in literature,
are blocked by unreacted organics, which is one of the factors
but AlSMSN(20) exhibits the best performance with a phenol contributing to the sintering effect on the catalyst surface.
conversion of 85.5% and a 4-TBP selectivity of 89.2%. These Consequently, prior to reuse, the catalysts must be regenerated
values are signicantly higher than that of other mono- and/or by washing and calcination. The used AlSMSN catalysts were
bi-metal incorporated mesoporous catalysts under the opti- washed four times with acetone and dried overnight at 393 K.
13,21,22
mized reaction conditions.
The higher catalytic activity of Finally, the catalysts were calcined at 773 K for 6 h in air for
AlSMSN(20) is also tentatively ascribed to its spherical shaped- removal of formed organics (as coke) and unreacted molecules.
mesostructure with excellent textural features such as very The recovered recyclable and active AlSMSN(20) and
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AlSMSN(100) catalysts were reused in the reaction (Table 2). The
catalyst preparation processes were carried out for each run
before initiation of the catalytic reaction. Aer two runs, the
phenol conversion and 4-TBP selectivity of AlSMSN(20)
increased to 90.3% and 95.3%, respectively, given that the non-
framework octahedral Al ions deposited on the surface of the
catalyst can be completely removed with repeated use, and the t-
butylation of phenol can be more efficiently promoted by the
AlSMSN(20) catalyst. From the fourth run, the phenol conver-
sion and the 4-TBP selectivity remained constant, as shown in
Table 2, indicating that the active tetrahedral Al ions cannot be
further leached from the mesoporous matrix; this is in good
agreement with the ICP-AES results for the ltrate solutions, in
which no active aluminium was detected. The non-framework
octahedral aluminium ions are almost inactive in the t-butyla-
13
tion of phenol, as reported in the literature. W-AlSMSN(20)
was therefore used as a ecofriendly spherical mesostructured
catalyst in this reaction to determine its catalytic stability. The
Fig. 9 Effect of nt-butanol/nphenol ratios on the catalytic activity of W-
ꢁ1
catalytic effect of W-AlSMSN(20) was nearly the same as that AlSMSN(20) (reactions conditions: WHSV ¼ 4.8 h and temperature ¼
achieved in the fourth run using the regenerated AlSMSN(20) 450 K and time-on-stream ¼ 1 h).
catalyst, as shown in Table 3. Based on this result, it is
concluded that the active tetrahedral Al ions could not be
further leached from the surface of the SMSN matrix. Addi- because the mixture of reactants reaches an equilibrium state at
tionally, ICP-AES analysis conrms the absence of Al ions in the an nBuOH/nphenol ratio of 2, and the reactants react mutually with

ltrate. W-AlSMSN(20) therefore has higher catalytic activity each other at this ratio on the catalyst surface without deacti-
than the unwashed congener in the eco-friendly heterogeneous vation. Although higher phenol conversion was achieved at an
2
7
process. Importantly, the Al MAS NMR spectra (Fig. 8) clearly
conrm that the few non-framework octahedral Al ions, to at this ratio, with an increase in the 2-TBP selectivity. When the
which the signal at 0 ppm is assigned, are completely removed
BuOH/nphenol ratio was increased from 3 to 5, the 2,4-DTBP
by the washing treatments. Furthermore, ICP-AES, XRD, N
selectivity increased, accompanied by a decrease in the 4-TBP
sorption isotherm, and Al MAS NMR spectroscopy results selectivity, whereas the higher mesoporous BEA enhances to
nBuOH/nphenol ratio of 1, the 4-TBP selectivity was slightly lower
n
2
-
2
7
21
prove that the structural and textural features, including the produce a better 2,4-DTBP selectivity. This observation was
3
+
Al
ions tetrahedrally coordinated to the surface of W- further conrmed when the reaction was carried out for 3 h
AlSMSN(20), are maintained (Table 2 and Fig. 8). The recy- using TOS, as shown in Table 3. Moreover, the low phenol
cling results demonstrate that W-AlSMSN(20) is unprecedented conversion obtained at the high nBuOH/nphenol ratio may be due
as a green eco-friendly catalyst among mesoporous metal- to the blocking of active sites by the adsorption of either excess
13,21,22
losilicate catalysts.
tert-butanol or the water molecules produced from the dehy-
Given that W-AlSMSN(20) is the most active of the AlSMSN dration of t-butanol. The low phenol conversion could also be
catalysts, it was further used in the reaction to determine the attributed to the reduced diffusion rate of phenol due to the
optimal reaction parameters, namely
n
BuOH/nphenol ratio, counter diffusion effect of tert-butanol and its products, and
temperature, and weight hourly space velocity (WHSV). Fig. 9 deactivation of the catalyst by oligomerization of excess olens
shows the effect of various nBuOH/nphenol ratios on the t-butyla- produced by dehydration of tert-butanol.
tion of phenol in terms of phenol conversion and product
The t-butylation of phenol was carried out over W-
selectivity at the reaction temperature of 450 K and WHSV of AlSMSN(20) at various reaction temperatures using a WHSV of
ꢁ
1
ꢁ1
4
.86 h in the presence of the W-AlSMSN(20) catalyst. The 4- 4.86 h and an nBuOH/nphenol ratio of 2, as shown in Fig. 10, to
TBP selectivity is markedly increased from 62 to 95.4% with an determine the effect of the reaction temperature on the phenol
increase in the nBuOH/nphenol ratio from 1 to 2. Recently, Vinu conversion and product selectivity. The phenol conversion and
et al. reported achieving higher 4-TBP selectivity using a nBuOH
/
4-TBP selectivity gradually increased as the reaction tempera-
n
phenol ratio of 3 in the presence of various mesoporous catalysts ture was increased up to 450 K. However, the phenol conversion
22
under similar reaction conditions. The catalytic reaction was decreased drastically from 95.4 to 28% when the reaction
thus carried out using the same nBuOH/nphenol ratio of 3 in the temperature was further increased from 450 to 540 K. The low
presence of W-AlSMSN(20), but in this catalytic system the phenol conversion at high temperature may be due to the
production of 2,4-DTBP increased with a decrease in the 4-TBP predominance of dealkylation over alkylation at high tempera-
selectivity, as shown in Fig. 9. In accord with the results of ture, and also to the diminishing availability of t-butanol with
1
3,21
catalysis studies published by Selvam et al.,
we found that progress of side reactions such as oligomerization or aromati-
23
higher 4-TBP selectivity is achieved using the nBuOH/nphenol ratio zation. The selectivity for 4-TBP increases to reach a maximum
of 2 relative to other ratios, as shown in Table S2.† This may be of 95.4% at the higher reaction temperature of 450 K. Similar
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size of the AlSMSN materials can be increased by increasing the
2
7
29
Al ion content. Al and Si MAS NMR studies conrm that
3
+
increased incorporation of Al ions with tetrahedral coordi-
nation (active sites) can be achieved by the use of AlN and AlP as
Al sources, making them the preferred source of Al ions among
those studied. FE-SEM and TEM studies conrm that the
AlSMSN materials exhibit a spherical morphology and hexago-
nally packed uniform mesopore diameter. Furthermore, evalu-
ation of the hydrothermal stabilities of the AlSMSN materials by
treatment in boiling water at 373 K for 168 h shows that
AlSMSN(20) with a higher Al content exhibits superior hydro-
thermal stability relative to AlSMSN(100). Non-framework
octahedral Al ions were completely removed from the surface
of the AlSMSN materials by utilizing the efficient washing
2
7
method presented herein, as conrmed by Al MAS NMR
studies. The catalytic activity of green/washed AlSMSN(20) (W-
AlSMSN(20)) was higher than that of calcined AlSMSN(20)
owing to an increase in the number of active tetrahedral Al sites,
Fig. 10 Effect of reaction temperature on the phenol conversion and
selectivity of products over W-AlSMSN(20) (reactions conditions: resulting from the removal of the non-framework octahedral Al
ꢁ
1
WHSV ¼ 4.8 h and nt-butanol/nphenol ratio ¼ 2 and time-on-stream ¼
ions that were deposited on the active surface of AlSMSN(20). By
comparing the catalytic studies with many previously reported
evaluations, it is remarkably found that W-AlSMSN(20) is
unprecedented as a green eco-friendly catalyst among meso-
porous metallosilicate catalysts.
1
h).
observations have been reported elsewhere for different meso-
porous catalysts.
13,21,22
The p-isomers are preferentially gener-
ated due to their different diffusional kinetics determined via
24
computational modeling, as reported elsewhere. Moreover, it
must be noted that the selectivity for 2,4-DTBP and 2-TBP is
much higher at lower reaction temperature and decreases with
an increase in the reaction temperature. This may be due to the
higher stability and the availability of the tert-butyl cation at low
reaction temperature. Moreover, the formation of 4-TBP is
thermodynamically favored at higher reaction temperature, as
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