Designing ordered mesoporous aluminosilicates under acidic conditions via an intrinsic hydrolysis method

Article abstract of DOI:10.1039/c6dt02964g

Acid-mediated synthesis of ordered mesoporous aluminosilicates (OMAS) with medium-to-strong Br?nsted acid sites and trivalent aluminium exclusively in a tetrahedral framework structure is realized by a newly devised intrinsic hydrolysis method. In this way, we have synthesized a series of well-ordered OMAS, e.g., H-AlSBA-15 and H-AlIITM-56, which are otherwise difficult to distinguish under acidic conditions owing to very different hydrolysis rates of both silicon and aluminium sources used for preparation as well as easy dissociation of thus formed Al-O-Si bonds. This novel intrinsic hydrolysis approach, however, relies mainly on similar hydrolysis rates of constituent inorganic species leading to efficient condensation. Thus, the innovative methodology using tetraethyl orthosilicate and aluminium citrate, respectively, as tetravalent silicon and trivalent aluminium as precursors facilitates the formation of high quality OMAS with a narrow pore size distribution, thicker walls, and trivalent aluminium in a tetrahedral framework structure with a high aluminium content, as evidenced by a battery of characterization techniques, viz., XRD, XRF, SEM, TEM and BET. The resulting materials, in turn, generate Br?nsted acid sites in the aluminosilicate matrix, with the absence of the usually encountered Lewis acid sites, viz., extra-framework and/or non-framework species, as confirmed by both ²⁷Al MAS-NMR and NH₃-TPD studies. All the prepared catalysts exhibit excellent activity towards the tertiary butylation of phenol, and the high activity of the catalysts is attributed to the unique and exclusive presence of medium-to-strong acid sites in the OMAS matrix.

Full text of DOI:10.1039/c6dt02964g

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Architecting ordered mesoporous aluminosilicates under acidic condition via
intrinsic hydrolysis method†
Nunna V. Krishnaa and Parasuraman Selvam*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Acid-mediated synthesis of ordered mesoporous aluminosilicates (OMAS) with medium-to-strong Brønsted acid sites and trivalent
aluminium exclusively in tetrahedral framework structure is realized by a newly devised intrinsic hydrolysis method, hitherto not
reported. In this way, we have successfully synthesized a series of well-ordered OMAS, e.g., H-AlSBA-15 and H-AlIITM-56, which are
otherwise difficult to distinguish under acidic conditions owing to very different hydrolysis rates of both silicon and aluminium sources
used for the preparation as well as the easy dissociation of thus formed Al−O−Si bonds. This novel intrinsic hydrolysis approach,
however, relies mainly on similar hydrolysis rates of constituent inorganic species leading to efficient condensation. Thus, the
innovative methodology use tetraethyl orthosilicate and aluminium citrate, respectively, as tetravalent silicon and trivalent aluminum
as precursors facilitates the formation of high quality OMAS with narrow pore size distribution, thicker wall, and trivalent aluminium in
tetrahedral framework structure with high aluminium content, as evidenced by a battery of characterization techniques, viz., XRD, XRF,
SEM,TEM and BET. The resulting materials, in turn, generates Brønsted acid sites in the aluminosilicate matrix, with the absence of
usually encountered Lewis acid sites, viz., extra-framework and/or non-framework species, as confirmed by both 27Al MAS-NMR and
NH3-TPD studies. All the prepared catalysts exhibit excellent activity towards tertiary butylation of phenol, and that the high activity of
the catalysts are attributed to the unique and exclusive presence of medium-to-strong acid sites in the OMAS matrix.
(Dedicated to Professor M.M. Sharma, FRS on the occasion of his 80th birthday)
Introduction
extrinsic hydrolysis method22 and using various sources24,25, have
Ordered mesoporous silicates (OMS) synthesized using self- been devoted for incorporation of aluminium into the acid-
assembled surfactants as templates are advantageous owing to mediated OMS, e.g., SBA-15. Various strategies are undertaken in
their large pore size, huge pore volume, high surface area and these synthesis protocols such as; usage of aluminium sources with
narrow pore size distribution. These characteristics have several slow hydrolysis rate (direct method), synthesis is done at higher pH
advantages over the corresponding microporous analogues as the (pH adjusting method and extrinsic hydrolysis method) and a usage
former shows enhanced accessibility of active sites and eliminates of preformed SBA-15 (post synthesis grafting). However, all the
diffusion limitations in catalyzing bulky substrates.1-5 In this regard, attempts have resulted in poor quality samples with much inferior
acid-mediated synthesized OMS (SBA-15, IITM-56 or KIT-6)6-8 agree (disordered/distorted) textural properties. More importantly, they
preferred over the corresponding OMS prepared under basic have also resulted in the formation of undesired (weak) structural
(MCM-41 or MCM-48)9 or neutral (HMS or MSU)10,11 conditions Lewis acid sites (penta-coordinated) as well as (strong) non-
owing to high thermal and hydrothermal stabilities as a result of framework Lewis acid sites (hexa-coordinated). In addition, the
thicker pore wall structure as well as large pores with in-built latter causes pore blockage26-29. Thus, the successive failures in the
microporosity6-8,12
. Indeed, Brønsted acidity can be generated in formation of high quality materials having exclusively tetrahedral
acid-mediated OMS upon isomorphous substitution of aluminium in framework aluminium could be attributed to the existence of
the silicate matrix, i.e., the formation of ordered mesoporous trivalent aluminium ions under acidic conditions, the differences in
aluminosilicates (OMAS), which possess great potential as the hydrolysis rates of silicon and aluminium sources used as the
heterogeneous acid catalysts13,14
. Many efforts, viz., direct precursors and the easy dissociation of thus formed Al−O−Si bond
synthesis15-17, post-synthesis method18-20, pH adjusting method21,22, make the synthesis of highly ordered AlSBA-15 with desired textural
properties is a challenge to the scientific community. Therefore, in
this investigation, an attempt has been made to unravel some of
aNational Centre for Catalysis Research and Department of Chemistry, Indian
Institute of Technology- Madras, Chennai, India. E-mail: selvam@iitm.ac.in.
these issues so as to produce high quality materials, hitherto, not
bNew Industry creation Hatchery Center, Tohoku University, Sendai 980 8579,
Japan.
reported. We report, herein, an innovative approach, viz., intrinsic
hydrolysis method (I), see Scheme 1, in which the hydrolysis rates of
(or hydrolysis constant which is equilibrium constant and that a
†Electronic Supplementary Information (ESI) available: Physico-chemical
characteristics and Acidity data. See DOI: 10.1039/x0xx00000x
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lower value (log K) implies faster hydrolysis; see Table 1)30-33 both Al−O bond in these matrix is 1.74 Å, and the average value for a
silicon and aluminium sources are matched in such a way that the tetrahedral Si−O bond is 1.60 Å44. In addition, this increase could
condensation of silicate and aluminate species occur simultaneously also be ascribed to other factors such as increased wall-thickness,
resulting in the formation of mesoporous aluminosilicate structure. large pores, and broad pore size distribution as well as to ion-
In this fashion, a series of well-ordered OMAS, viz., H-AlSBA-15 and exchanged or grafted aluminium ions in the structure. Further, it
H-AlIITM-56, were successfully synthesized which are otherwise can also be noticed from data that the aluminium content in all the
difficult to prepare under acidic conditions owing to very different samples is consistently lower than the initial gel composition owing
hydrolysis rates of diverse silicon and aluminium sources used as to high solubility of aluminium under acidic condition. N2 sorption
well as the easy dissociation of thus formed Al−O−Si bonds.
preliminary account of this work was presented elsewhere34,35
A
isotherms are shown in Fig. 1B along with pore size distributions
and the corresponding textural properties are listed in Table 2. It
can be seen from this figure that all materials show type IV
.
isotherm with H1 hysteresis suggesting large mesopore channels45
.
However, the samples prepared by pH adjusting method, i.e., H-
AlSBA-15(P), and hydrolysis method, i.e., H-AlSBA-15(H), show
broad pore size distribution and low surface area due to the
disturbance of double-walled hydrogen interaction. In addition,
they also show capillary condensation at higher relative pressure
which could be attributed to poor condensation of the constituent
elements owing to different hydrolysis rate of precursors28,46
. On
the other hand, the pore size of H-AlSBA-15(G) is lower than the
pristine sample, viz., SBA-15(D), as the aluminium ions preferably
gets grafted on already formed pore channels. Thicker walls of H-
AlSBA-15(I) in comparison to others indicate that similar hydrolysis
rates of silicon and aluminium sources are essential for better
condensation which dictates the pore walls thickness.
Scheme 1 Synthesis methodology of H-AlSBA-15/H-AlIITM-56 by intrinsic
hydrolysis method.
Table 1. Stability constants of various sources30-33
.
Inorganic Precursor
Code
log K
Tetraethyl orthosilicate
Aluminium citrate
Aluminium acetate
Aluminium isopropoxide
Aluminium acetylacetonate
TEOS
ACT
AAC
AIP
11.8
11.7
4.6
< 4.6
AAA
22.3
For comparison, a series of H-AlSBA-15 were also prepared with
various known preparation methods reported in literature, viz.,
direct method (D), pH adjusting method (P), post-synthesis grafting
method (G) and Hydrolysis method (H). Further, the adaptability of
this new methodology/procedure, i.e., intrinsic hydrolysis method,
was checked by the synthesis of yet another series of mesoporous
aluminosilicates, viz., H-AlIITM-56, with various silicon-to-
aluminium ratios (nSi/nAl). All these samples were systematically
characterized by various analytical, spectroscopic and imaging
techniques. The resulting high quality materials, viz., H-AlSBA-15
and H-AlIITM-56, were tested for industrially important tertiary-
butylation of phenol reaction, and its alkylated product, i.e., para-
Fig.
1 XRD patterns (A), and N2 sorption isotherms with pore size
distributions (B) of: (a) SBA-15(D); (b) H-AlSBA-15(I); (c) H-AlSBA-15(D); (d)
H-AlSBA-15(P); (e) H-AlSBA-15(G); (f) H-AlSBA-15(H).
tertiary butyl phenol (4-t-BP)36-43
Results and Discussion
H-AlSBA-15
.
Table 2 Properties of various samples prepared by different methods.
d
Materiala
nSi/nAl
a0
SBET
DBJH
(nm)
6.46
6.69
7.64
8.06
6.20
8.31
Vp
(cm3g−
1.04
0.82
0.96
1.44
0.98
1.48
hw
c
(nm)b
10.26
11.64
(m2g−
702
645
655
436
621
526
)
)
(nm)
3.80
4.95
4.06
3.44
4.62
4.15
1
1
XRD patterns of as-synthesized (Fig. S1) and calcined (Fig. 1A)
samples of both SBA-15 and H-AlSBA-15 show well-resolved
reflections in the low-angle region typical of mesoporous 2D-
hexagonal structure6. Further, the diffraction data indicate that
mesostructural order of SBA-15 is sustained even after aluminium
incorporation by various preparation methods, and that showed an
increase in unit cell parameter upon aluminium incorporation
(Table 2). This could be attributed to isomorphous substitution of
trivalent aluminium (0.53 Å) for tetravalent silicon (0.40 Å) in a
zeolite framework structure as the average value of a tetrahedral
SBA-15(D)
∝
H-AlSBA-15(I)
140
125
103
100
107
H-AlSBA-15(D) 11.70
H-AlSBA-15(P) 11.50
H-AlSBA-15(G) 10.82
H-AlSBA-15(H) 12.46
a Nominal (initial) gel composition is nSi/nAl ratio of 60.
b Average unit cell parameter calculated using 1/d 2 =4/3 (h2 + hk + k2/ a2).
c nSi/nAl determined by XRF; d Wall thickness, hw = a0 - DBJH
.
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Fig. 2 HR-TEM images of: (a,b) SBA-15(D); (c,d) H-AlSBA-15(I); (e,f) H-AlSBA-15(D); (g,h) H-AlSBA-15(P); (i,j) H-AlSBA-15(G); (k,l) H-AlSBA-15(H).
Fig. 2 depicts TEM images of various H-AlSBA-15, taken along both P/ H) consists of curved rope like features or bundles of small
parallel and perpendicular directions of pore axis of samples, aggregates indicate the different hydrolysis rates of Si and Al
prepared using different methods. It can be seen from this figure sources used. Such observations are in good agreement with the
that all the materials show well-ordered arrays (parallel direction) pore sizes as depicted by both TEM and BET data.
of one-dimensional mesochannels with typical honeycomb
Fig. 4A & 4B shows the 29Si and 27Al MAS-NMR spectra of various
(perpendicular direction) structure characteristics of SBA-15 samples, respectively. In general, it can be noticed from 29Si MAS-
matrix47. These images further validate, in particular, the excellent NMR spectra of aluminosilicates (Fig. 4A; curves b-f) that some of
2D-hexagonal mesostructural ordering of H-AlSBA-15 prepared by the Q4 sites (110 ppm) and Q3 sites (100 ppm) are partially replaced
various preparation methods. As indicated earlier (see Fig. 1B), as Q3 (106 ppm) sites corresponding to (3Si,1Al) environment50. A
′
except H-AlSBA-15(I) and H-AlSBA-15(G) samples, all others possess similar trend is also observed for all the aluminosilicate samples
larger pore channels which is in good agreement with BET data. under investigation. As a result, a depletion in the Q3 sites could be
However, as expected, H-AlSBA-15(G) show smaller pore size owing noticed which is normally prominent in the case of siliceous
to grafting of aluminium on the pore wall structure. At this juncture, samples (see Fig. 4A; curve a). In other words, the lower Q4/Q3
′
it is also worth noting that the samples, H-AlSBA-15(P) and H-AlSBA- ratio or high amount of Q3 sites indicate more aluminium in the
′
15(H) contain slightly irregular pore channels/pore sizes.
tetrahedral sites of the framework structure50,51
.
Such an
Fig. 3 illustrates the FE-SEM images of various samples under observation is clearly noticed for the sample, H-AlSBA-15(I). On the
study. It can be seen from this figure that the siliceous sample other hand, the presence of lesser Q3′ sites in case of H-AlSBA-15(P/
(SBA-15(D); Fig. 3a) exhibits a typical morphology, i.e., bundles of G/ H) indicates that fewer aluminium ions get incorporated in the
long rope-like features or slilicatein-type morphology, characteristic tetrahedral environment.
of high quality SBA-15, which is in good agreement with literature48
.
Fig. 4B presents 27Al MAS-NMR spectra of various samples. It is
However, such a feature is not observed in the corresponding interesting to note for this figure that, in case of H-AlSBA-15(I), the
aluminium analogues except the one prepared using intrinsic trivalent aluminium is exclusively located in tetrahedral
hydrolysis method (H-AlSBA-15(I); see Fig. 3b). Indeed, the coordination (Fig. 4B; curve b; AlT; 53 ppm). On the other hand, for
similarity in the microstructures of both SBA-15(D) and H-AlSBA- all other samples (Fig. 4B; curves c-f), in addition to the signal due
15(I) could be attributed to comparable hydrolysis rates of both to tetrahedral coordination (AlT; 53 ppm), the spectra consists of a
tetraethyl orthosilicate (TEOS; log K = 11.8) and aluminium citrate second feature (pentacoordinated; AlP; 35 ppm)52 which could be
(ACT; log K = 11.7), which could facilitate the formation such attributed to extra-framework aluminium ions. However, in the
bundles without causing strain in polymeric inorganic species16,49
.
cases of H-AlSBA-15(G) and H-AlSBA-15(H), non-framework
At the same time, it is also interesting to note that the sample, H- aluminium (hexacoordinated; Al2O3 aggregates; AlO; 0 ppm) also
AlSBA-15(G) also present a similar morphology that consists of appears. These species, in general, arise owing to varied hydrolysis
bundle of long rope like features similar to SBA-15(D) as it is rates of the aluminium sources and TEOS. On the other hand, in the
prepared by using pre-formed SBA-15. In contrast, H-AlSBA-15(D/ case of H-AlSBA-15(I), aluminium source (ACT) forms monomeric
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species which condense immediately with monomeric silica type (iii), and strong Brønsted acid sites, designated as type (iv),
generated by TEOS, resulting in the formation of well condensed arise due to aluminium in two different framework positions
aluminosilicate structure53
.
whereas weak Lewis acid sites, referred as type (v), occur due to
penta-coordinated aluminium54. However, it is interesting to note
from Table S1 and Fig. 4C that the TPD trace for H-AlSBA-15(I) show
prominent medium-to-strong Brønsted acid sites. This could be
attributed to the thicker walls and presence of more framework
aluminium in agreement with literature55. The traces corresponding
to weak Lewis acid sites are observed in H-AlSBA-15(D) and AlSBA-
15(P). Since most of the aluminium is present near to surface in
other preparation methods instead of getting incorporated in the
interior and therefore the weak acid sites are prominent. In
general, strong Lewis acid sites (vi) due to Al2O3 aggregates are
observed at higher desorption temperature (> 480°C); however,
such a feature is not observed in the cases of both H-AlSBA-15(G)
and H-AlSBA-15(H) due to its overlap with dehydroxylation peak54
.
H-AlIITM-56
XRD patterns of as-synthesized (Fig. S1) and calcined (Fig. 5A)
samples of both IITM-56 and H-AlIITM-56 with various silicon-to-
aluminium (nSi/nAl) ratios prepared using intrinsic hydrolysis method
show well-resolved reflections in the low-angle region typical of
mesoporous 2D-hexagonal structure with p6mm symmetry7,43
.
These long-range ordered diffraction patterns indicate that
mesostructural order of IITM-56 is sustained even at higher
aluminium loadings using intrinsic hydrolysis method. Further, as
expected, H-AlIITM-56 showed an increase in unit cell parameter as
that of IITM-56 and is in proportional to aluminium present in H-
AlIITM-56 materials (Table 3), which could be attributed to
isomorphous substitution of trivalent aluminium in tetrahedral
Fig. 3 FE-SEM images of: (a) SBA-15(D); (b) H-AlSBA-15(I); (c) H-AlSBA-
15(D); (d) H-AlSBA-15(P); (e) H-AlSBA-15(G); (f) H-AlSBA-15(H).
coordination owing to its larger ionic radius44
.
Further, the
aluminium content in H-AlIITM-56 samples is consistently less than
the initial gel composition (see Table 3) due to high solubility of
aluminium in acidic medium and therefore a drastic decrease in
aluminium content is noted. N2 sorption isotherms of H-AlIITM-56
with various nSi/nAl ratios are shown in Fig. 5B along with pore size
distributions and the corresponding textural properties are also
summarized in Table 3.
Fig. 4 29Si MAS-NMR spectra (A), 27Al MAS-NMR spectra (B) and NH3-TPD
profiles (C) of: (a) SBA-15(D); (b) H-AlSBA-15(I); (c) H-AlSBA-15(D); (d) H-
AlSBA-15(P); (e) H-AlSBA-15(G); (f) H-AlSBA-15(H).
Fig. 4C presents NH3-TPD profiles of various H-AlSBA-15 prepared
by different methods, tells about the types of acid sites present and
their distribution. As depicted in this figure, the distribution of acid
sites is calculated based on deconvolution of desorption data using
Gaussian function and the results are listed in Table S1. The weak
acid sites, designated as type (i), are attributed to surface hydroxyl
groups (silanols), and weak Brønsted acid sites, referred as type (ii), Fig. 5 XRD patterns (A) and N2 sorption isotherms with pore size
distributions (B) of: (a) IITM-56; (b) H-AlIITM-56(90); (c) H-AlIITM-56(60); (d)
H-AlIITM-56(30).
are generated due to presence of aluminium close to the surface.
On the other hand, moderate Brønsted acid sites, designated as
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(e,f) H-AlIITM-56(60); (g,h) H-AlIITM-56(30).
Table 3 Properties of H-AlIITM-56 with varied nSi/nAl ratios.
Fig. 6 depicts TEM images of H-AlIITM-56 prepared with various
nSi/nAl ratios by intrinsic hydrolysis method. All the materials show
well-ordered arrays (parallel direction) of 1D- mesochannels with
c
ao
SBET
(m2g−
D
Vp
hw
b
Materiala
nSi/nAl
1
1
(nm)
)
(nm) (cm3 g−
)
(nm)
IITM-56(∝)
5.19
5.64
5.89
6.03
∝
827
913
896
818
2.71
3.14
3.32
3.37
0.61
0.89
0.87
0.79
2.48
2.50
2.57
2.66
typical
honeycomb
(perpendicular
direction)
structure
characteristics of IITM-567. As in the case of H-AlSBA-15 (cf. Fig. 2),
H-AlIITM-56(90)
H-AlIITM-56(60)
H-AlIITM-56(30)
240
144
80
these images further validate the excellent of 2D-hexagonal
mesostructural ordering of H-AlIITM-56.
Further, as indicated
earlier (see Fig. 1B), H-AlIITM-56 with various nSi/nAl ratios, have
pore channels of nearly same size as that of IITM-56. Hence, it can
be concluded that, in general, for the preparation of mesoporous
materials under acidic condition, one can achieve excellent quality
by choosing appropriate silicon and aluminium sources and/or by
adapting intrinsic hydrolysis method.
a Values in parenthesis indicated the nominal (initial) gel composition of nSi/nAl ratio.
a Average unit cell parameter calculated using 1/d 2 =4/3 (h2 + hk + k2/ a2).
b nSi/nAl determined by XRF; c Wall thickness, hw = a0 - DBJH
.
All materials show type IV isotherm with steep H1 hysteresis
suggesting large mesopore channels with narrow pore size
distribution45. H-AlIITM-56 with various nSi/nAl ratios show capillary
condensation step nearly the same shape that of IITM-56 (cf. Fig.
5B; inset) owing to a consistent ionic strength of gel or a similar
hydrolysis rates of silicon and aluminium precursors, and that the
wall-thickness of the former is larger than the latter, which further
increases with increase in aluminium. In contrast, wall thickness of
H-AlSBA-15 synthesized by other methods decreases with increase
in aluminium content as silica condensation gets affected due to
change in ionic strength28,46. Because of its thicker walls, surface
area and pore volume of H-AlIITM-56 decreases with higher
aluminium content (see Table 3).
Fig. 7 illustrates the FE-SEM images of IITM-56 and H-AlIITM-56
samples. It is clear from this figure that H-AlIITM-56 with various
(nSi/nAl) ratios (Fig. 7b-d) exhibits a typical morphology, i.e., long
block-like features, similar to IITM-56 (Fig. 7a). The similarity in the
microstructures of both IITM-56 and H-AlIITM-56 depicts that the
use of ACT (log K = 11.7) as aluminium source with similar hydrolysis
rate of TEOS (log K = 11.8) facilitates the formation of such blocks
without causing strain in polymeric inorganic species17,49
.
As
described earlier, a similar observation was also made for SBA-15 as
well as H-AlSBA-15 (cf. Fig. 3), where the siliceous samples and the
corresponding aluminosilicate analogue prepared using ACT exhibit
a characteristic morphology, i.e., bundles of long rope-like features.
Thus, these results clearly support the formation of high quality
materials in terms of framework aluminium in the silicate matrix,
using TEOS and ACT as silicon and aluminium sources, respectively.
Fig. 7 FE-SEM images of: (a) IITM-56 (b) H-AlIITM-56(90); (c) H-AlIITM-
56(60); (d) H-AlIITM-56(30).
Fig. 8 shows the 29Si and 27Al MAS-NMR spectra of H-AlIITM-56
with various nSi/nAl ratios prepared by intrinsic hydrolysis method.
It can be seen from Fig. 8A that the appearance of Q3 sites (or
′
decline in the Q3 sites as IITM-56) indicate the isomorphous
substitution of aluminium in the framework structure. Further, the
corresponding signal intensities are proportional to nSi/nAl ratios
which is in agreement with literature50,51
. This observation is well
supported by 27Al MAS-NMR spectra of H-AlIITM-56 (Fig. 8B). That
Fig. 6 TEM images of: (a,b) IITM-56; (c,d) H-AlIITM-56(90);
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is, as expected, the trivalent aluminium show a single signal attacked in both ortho/para positions57
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.
H-AlSBA-15(G/ H) shows
corresponding the tetrahedral aluminium. At this juncture, it is low conversion compared to H-AlSBA-15(I) due to presence of non-
worth mentioning here that, even at high aluminium content, the framework aluminium species, i.e. Al2O3 aggregates (cf. Table S1)58
.
aluminium ion is exclusively located in tetrahedral sites (Fig. 8B; On the other hand, H-AlSBA-15(P) shows the formation of
curve b-d; AlT; 53 ppm) of the silicate matrix in agreement with H- considerable amount of 2-t-BP owing to the presence of relatively
AlSBA-15 (see also Fig. 8B, curve b)52
. Further, the use of intrinsic high amount of weak Lewis acid (penta-coordinated aluminium). In
hydrolysis method, not only give good results for the structural other words, phenol adsorbs in vertical orientation on these sites,
characteristics but also incorporate high amount of aluminium and therefore results in the ortho-product at the expense of phenol
without affecting the textural properties. At this point, it is also conversion. It shows how the presence of weak Lewis acid sites that
interesting to note that, unlike many other methods, the intrinsic are generated by aluminium in penta co-ordination have adverse
hydrolysis method does not produce the extra-framework and non- effect on 4-t-BP selectivity. This emphasizes the complete presence
framework aluminium species.
of tetra coordinated aluminium which could be achieved in both H-
AlSBA-15 and H-AlIITM-56 using intrinsic hydrolysis method. In
order to check the adaptability of intrinsic hydrolysis method
towards the synthesis of acid-mediated OMAS; H-AlIITM-56 has
been tested with various silicon-to-aluminium ratios (Fig. 9).
Table 4 Tertiary-butylation of phenol over various H-AlSBA-15.
Selectivity (%)
hw
Conv.
(%)
b
Material
nSi/nAl
(nm)
2-t-BP 4-t BP 2,4-di-t-BP
SBA-15 (D)
∝
3.80
4.95
---
---
---
---
43.2
±0.5
10.5
±0.2
72.4
±0.4
17.1
±0.2
H-AlSBA-15(I)
140
33.7
±0.5
10.0
±0.5
75.9
±0.3
14.1
±0.8
H-AlSBA-15(D)
H-AlSBA-15(P)
157
151
4.06
3.44
Fig. 8 29Si MAS-NMR spectra (A); 27Al MAS-NMR (B) spectra; NH3-TPD
profiles (C) of: (a) IITM-56; (b) H-AlIITM-56(90); (c) H-AlIITM-56(60); (d) H-
AlIITM-56(30).
32.1
±0.3
18.0
±0.5
66.5
±0.7
15.5
±0.3
Fig. 8C depicts the NH3-TPD profiles of various H-AlIITM-56 samples
and the acidity data are listed in Table S2. It can be seen from Fig.
6C and Table S2 that an increase in medium-to-strong Brønsted acid
sites with a decrease in nSi/nAl ratio could be noticed. In a similar
way, the increase in acidic strengths are also in line with decrease in
39.3
±0.7
10.1
±0.4
75.2
±0.2
14.7
±0.5
H-AlSBA-15(G)
H-AlSBA-15(H)
147
153
4.62
4.15
38.7
±0.4
10.4
±0.1
75.0
±0.5
14.6
±0.1
nSi/nAl ratio. These observations could be attributed to an increase a Reaction conditions: Catalyst = 500 mg; nt-BA:npheno1= 2:1; 160°C; WHSV = 7.0 h-1; TOS
= 2 h; b Determined by 27Al MAS-NMR. c Post-synthesis grafting with aluminium source.
in walls thickness as result of high aluminum content in the
framework structure55. Further, H-AlIITM-56 materials with various
nSi/nAl were tested for phenol tertiary butylation reaction under
optimized conditions and the results are summarized in Table 3. It
can be seen from this table that the catalyst H-AlIITM-56 show high
phenol conversion with good selectivity towards 4-t-BP and 2,4-di-t-
BP. The very high activity of this material could be attributed, as
before, to the strong acidity and the presence of more medium-to-
strong Brønsted acid sites. Further, the enhanced activity of both
H-AlIITM-56 and H-AlSBA-15 (see Table 2) is also due to the
presence of framework aluminium exclusively in tetrahedral sites as
a result of the thicker pore wall structure.
Tertiary-butylation of Phenol
Table 4 presents the tertiary-butylation of phenol reaction results
over various H-AlSBA-15 under optimized experimental conditions.
As expected, among the various catalysts, H-AlSBA-15(I) shows high
Fig. 9 Tertiary-butylation of phenol over H-AlIITM-56 with various nSi/nAl
phenol conversion with good selectivity towards 4-t-BP and 2,4-di-t-
BP. The higher activity of the catalyst is attributed to strong acidity
and presence of more medium-to-strong Brønsted acid sites (cf.
prepared by intrinsic hydrolysis method.
Conclusions
Table 2) as a consequence of framework aluminium56
. Indeed,
In summary, in this investigation, we report well-ordered
mesoporous aluminosilicates, viz., H-AlSBA-15 and H-AlIITM-56,
with tetrahedrally coordinated trivalent aluminium exclusively in
phenol adsorbs in horizontal orientation on Brønsted acid sites
forming 4-t-BP largely due to steric hindrance though it can get
6 | J. Name., 2012, 00, xxx
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the silicate framework structure synthesized by intrinsic hydrolysis added at 2 h interval to this mixture in order to have nSi/nAl of 60 in
method. That is, a similar hydrolysis rates of both silicon and the gel and the resultant mixture was stirred at 40°C for 22 h. The
aluminium sources facilitates mesostructure formation without mixture was transferred to an autoclave and then kept for
affecting surfactant-inorganic species interface which in turn allows hydrothermal treatment at 100°C for 24 h. Obtained precipitate
the formation of tetra-coordinated aluminium species to interact was filtered, washed, dried at 60°C for 6 h and calcined at 550°C for
with tetrahedral silica units immediately once they are generated. 1 h in nitrogen and 5 h in air atmosphere at a heating rate of 1°C
These novel materials posses high acidic strength along with min−1 in order to obtain surfactant free sample.
increased amount of medium-to-strong Brønsted acid sites owing
pH adjusting method: H-AlSBA-15(P) by pH adjusting method was
to larger wall thickness. Thus, they exhibit excellent catalytic
synthesized using previously reported method21. 4.0 g of Pluronic
activity for the tertiary-butylation of phenol reaction with selective
P123 was dissolved in 125 mL of 2 M HCl and stirred till a clear
formation of 4-t-BP and 2,4-di-t-BP.
solution was obtained. 8.46 g (0.04 mol) of TEOS and 0.213 g (0.34
Experimental
mmol) of aluminium sulphate hexadecahydrate was added at 2 h
interval to this mixture in order to have nSi/nAl of 60 in the gel and
the resultant mixture was stirred at 40°C for 22 h. The mixture was
transferred to an autoclave and then kept for hydrothermal
treatment at 100°C for 48 h. After which the pH of the synthesis
system was adjusted to 7.5 by adding ammonia drop wise at room
temperature and the obtained precipitate was again
hydrothermally treated at 100°C for 48 h. Obtained precipitate was
filtered, washed, dried at 60°C for 6 h and was calcined at 550°C for
1 h in nitrogen and 5 h in air atmosphere at a heating rate of 1°C
min-1 in order to obtain surfactant free sample.
Starting Materials
All the chemicals used were of analytical grade. The surfactants,
(Pluronic P123 and Brij 56) and the silica source, tetraethoxysilane
(TEOS) were procured from sigma-aldrich. Aluminium precursors,
(aluminium nitrate nonahydrate, aluminium isopropoxide,
aluminium sulphate hexadecahydrate, anhydrous aluminium
chloride), ligands (citric acid monohydrate), auxiliary chemicals
(hydrochloric acid (32%) (HCl), ammonia (25%) (NH3), ethanol
(99.9%), ammonium fluoride (98%)), and reactants (phenol and
tertiary butyl alcohol) were obtained from Merck. The authentic
samples such as phenol, 2-t-BP (ortho tertiary butyl phenol), 4-t-BP
(para tertiary butyl phenol) and 2,4-di-t-BP (2,4-di tertiary butyl
phenol), were acquired from sigma-aldrich.
Post-synthesis grafting method: H-AlSBA-15(G) by post-synthesis
grafting method was synthesized using previously reported
method19. 1.5 g (0.02 mol) of calcined SBA-15 was added to 150 mL
of ethanol and stirred till a homogeneous solution was obtained.
Then, 0.056 g (0.2 mmol) of anhydrous aluminium chloride was
added to this mixture in order to have nSi/nAl of 100 and the
resultant mixture was stirred at room temperature for 12 h.
Obtained solid was filtered, washed, dried at 60°C for 6 h and was
calcined at 550°C for 1 h in nitrogen and 5 h in air atmosphere at a
heating rate of 1°C min−1 in order to obtain sample.
Synthesis of SBA-15(D)
SBA-15 was synthesized using a previously reported method6. 4.0 g
of Pluronic P123 was added to 30 mL of water and stirred till a clear
solution was obtained. Thereafter, 70 mL of 2 M HCl was added
and stirred for 2 h. Then, 8.46 g of tetraethyl orthosilicate (TEOS)
was added and stirred at 40°C for 22 h. The mixture was
transferred to an autoclave and then kept for hydrothermal
treatment at 100°C for 24 h. Obtained precipitate was filtered,
washed, dried at 60°C for 6 h and calcined at 550°C for 1 h in
nitrogen and 5 h in air atmosphere at a heating rate of 1°C min−1 in
order to obtain surfactant free sample.
Hydrolysis method: H-AlSBA-15(H) by hydrolysis method was
synthesized using previously reported method23. 2.0 g of Pluronic
P123 and 0.025 g ammonium flouride was dissolved in 75 mL of
0.086 M HCl (pH =1.5) and was stirred at room temperature to
obtain a homogeneous solution A. Solution B was prepared by
dissolving 0.07 g (0.34 mmol) of aluminium isopropoxide (nSi/nAl
=
Synthesis of H-AlSBA-15
60) and 4.32 g (0.02 mol) of TEOS in 5 mL of 0.086 M HCl (pH= 1.5)
and were stirred for 1.5 h. Both the solutions were added and
stirred at 40°C. After 5 h of stirring (once white precipitate is
formed), the pH of the synthesis system was adjusted to 7 by
adding ammonia drop wise at room temperature and the obtained
mixture was again stirred at 40°C for 20 h. The mixture was
transferred to an autoclave and then kept for hydrothermal
treatment at 100°C for 24 h. Obtained precipitate was filtered,
washed, dried at 60°C for 6 h and calcined at 550°C for 1 h in
nitrogen followed by 5 h in air atmosphere at a heating rate of 1°C
min−1 in order to obtain surfactant free sample.
Intrinsic Hydrolysis Method: A 4.0 g of Pluronic P123 was added to
30 mL of water and stirred till a clear solution was obtained.
Thereafter, 70 mL of 0.086 M HCl was added, stirred for 2 h and pH
was adjusted to 1.5. Then, 8.46 g (0.04 mol) of TEOS and aluminium
citrate59 was added at 2 h interval to this mixture in order to have
nSi/nAl of 60 in the gel and the resultant mixture was stirred at 40°C
for 22 h. The mixture was transferred to an autoclave and then kept
for hydrothermal treatment at 100°C for 24 h. Obtained precipitate
was filtered, washed, dried at 60°C for 6 h and was calcined at
550°C for 1 h in nitrogen and 5 h in air atmosphere at a heating rate
1
of 1°C min− in order to obtain surfactant free sample which is
Synthesis of IITM-56
designated as H-AlSBA-15(I).
IITM-56 was synthesized using a previously reported method7. In a
typical synthesis, 4.0 g of Brij-56 was dissolved in 20 mL of water
and 80 mL of 2 M HCl and stirred till a clear solution was obtained.
To this solution, 8.3 g of TEOS was added and stirred at 50°C for 24
h. The mixture was transferred to an autoclave and then kept for
hydrothermal treatment at 100°C for 24 h. The molar gel
composition was 1 TEOS: 0.15 Brij-56: 11.30 HCl: 119 H2O. Obtained
Direct Method: H-AlSBA-15(D) by direct method was synthesized
using a modified version of previously reported method60. 4.0 g of
Pluronic P123 was added to 30 mL of water and stirred till a clear
solution was obtained. Thereafter, 70 mL of 0.086 M HCl was
added, stirred for 2 h and pH was adjusted to 1.5. Then, 8.46 g
(0.04 mol) of TEOS and 0.67 mmol of aluminium isopropoxide was
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precipitate was filtered, washed several times with water, dried at purging with helium for 30 min. The desorption of ammonia was
60°C for 6 h and was calcined at 550°C for 6 h for 1 h in nitrogen and carried out by heating the reactor up to 450°C at a uniform rate of
1
1
5 h in air atmosphere at a heating rate of 1°C min− in order to 10°C min−
.
obtain surfactant free sample.
Catalytic Activity Studies
Synthesis of H-AlIITM-56
The tertiary-butylation of phenol was carried out using 500 mg of
A 4.0 g of Brij-56 was added to 30 mL of water and stirred till a clear OMAs catalyst in a fixed-bed flow reactor. The catalyst was
solution was obtained. Thereafter, 70 mL of 0.086 M HCl was activated at 500°C in a flow of air for 6 h followed by cooling to
added, stirred for 2 h and pH was adjusted to 1.5. Then, 8.30 g (0.04 reaction temperature (160°C) in nitrogen atmosphere. After an
mol) of TEOS and aluminium citrate54 was added at 2 h interval to hour, the reactant mixture, viz., phenol and tertiary butyl alcohol (t-
this mixture in order to have nSi/nAl of 30/60/90 in the gel and the BA), with a desired ratio and weight hour space velocity (WHSV)
resultant mixture was stirred at 50°C for 22 h. The mixture was was fed into the reactor using a liquid injection pump (Miclins) and
transferred to an autoclave and then kept for hydrothermal nitrogen as the carrier gas. The gaseous products were cooled and
treatment at 100°C for 24 h. Obtained precipitate was filtered, the condensed liquid products were collected after 2 h. The
washed, dried at 60°C for 6 h and was calcined at 550°C for 1 h in products, viz., ortho-tertiary-butyl phenol (2-t-BP), para-tertiary-
nitrogen and 5 h in air atmosphere at a heating rate of 1°C min−1 in butyl phenol (4-t-BP) and 2,4-di-tertiary butyl phenol (2,4-di-t-BP),
order to obtain surfactant free sample which are designated as H- were identified using Perkin-Elmer gas chromatography with ZB-1
AlIITM-56(x), where x denotes nSi/nAl in the gel.
capillary column. They were further confirmed by GC-MS (Agilent
7890A) with DB-5 capillary column.
Characterization
Acknowledgements
Powder X-ray diffraction (XRD) patterns of the solid catalysts were
recorded using a Rigaku Miniflex II diffractometer using Cu-Kα (λ= The authors thank Department of Science and Technology,
0.15418 nm) in the 2θ range of 0.6 to 7.5° with a 2θ step size of Government of India for funding National Centre for Catalysis
0.01° and step time of 2 s. The elemental composition of materials, Research (NCCR), IIT-Madras, Chennai.
viz., silicon and aluminium content, the various ordered
Notes and references
mesoporous aluminosilicates, and sulphur content in the case of
sulfonic acid functionalized materials were measured on Rigaku
Primini X-ray fluorescence (XRF) spectrometer using palladium
source operating at 40 kV. Samples were packed tightly in Teflon
holder without using any binder was analysed. The elemental
analysis of starting materials, viz., aluminium complexes, were
performed using Perkin Elmer 2400 series CHN elemental analyzer,
which analyzes the organic content of the samples by classical
Pregl-Dumas technique. Magic angle spinning-nuclear magnetic
resonance (29Si MAS-NMR and 27Al MAS-NMR) spectra of samples
were recorded on Bruker DSX 400 spectrometer at a spinning
frequency of 8 KHz at room temperature with a resonance
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measured at 77 K using Micromeritics ASAP 2020 surface area
analyzer, in which the catalysts were outgassed at 200°C for 4 h
before the measurements. Temperature programmed desorption of
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A novel intrinsic hydrolysis method employed for the synthesis of well-ordered mesoporous
aluminosilicates, viz., H-AlSBA-15 and H-AlIITM-56, with trivalent aluminium, exclusively, in
tetrahedral framework structure.