Acid-Mediated Synthesis of Ordered Mesoporous Aluminosilicates: The Challenge and the Promise

Article abstract of DOI:10.1002/chem.201604368

A new intrinsic hydrolysis method was employed, for the first-time, to synthesize well-ordered H-AlSBA-15 with trivalent aluminium exclusively in the tetrahedral framework structure of SBA-15. Unlike other methods, which involve incorporation of aluminium ions in both the framework (Br?nsted) and non-framework (Lewis) sites of the silicate matrix, the intrinsic hydrolysis method isomorphously substitutes aluminium ions in the tetrahedral network even at high aluminium content. This unique approach relies mainly on the hydrolysis rates of the inorganic (silicon and aluminium) precursors used for the preparation in such a way that the condensation occurs simultaneously so as to overcome the usually encountered difficulties in stabilizing aluminium ions in the silicate matrix. In this way, we could successfully synthesize high quality Br?nsted acidic H-AlSBA-15, hitherto not reported. The synthesized materials were systematically characterized by various analytical, spectroscopic, and imaging techniques, including XRD, Brunauer–Emmett–Teller (BET) surface area measurements, TEM, SEM,²⁹Si and²⁷Al magic angle spinning NMR spectroscopy, X-ray fluorescence (XRF), and NH₃temperature-programmed desorption (TPD). The characterization results reveal the presence of a highly porous structure (with narrow pores) and tetrahedrally coordinated trivalent aluminium in the silicate matrix with more medium to strong Br?nsted acid sites. The resulting high quality catalysts exhibit excellent activity for tert-butylation of phenol with high selectivity towards para-tert-butyl phenol and 2,4-di-tert-butyl phenol.

Full text of DOI:10.1002/chem.201604368

A Journal of
Accepted Article
Title: ACID-MEDIATED SYNTHESIS OF ORDERED MESOPOROUS
ALUMINOSILICATES: THE CHALLENGE AND THE PROMISE
Authors: Selvam Parasuraman and Nunna Vamsi Krishna
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Nunna V. Krishna† and Parasuraman Selvam†,‡*
† National Centre for Catalysis Research and Department of Chemistry, Indian Institute of Technology-Madras,
Chennai 600 036, India
‡New Industry Creation Hatchery Center, Tohoku University,6-6-10, Aoba, Aramaki, Sendai 980 8579, Japan
KEYWORDS: Acid-mediated synthesis, intrinsic hydrolysis method, mesoporous aluminosilicates, H-AlSBA-15, tertiary-butylation
of phenol.
ABSTRACT: A novel intrinsic hydrolysis method was employed, for the first-time, to synthesize well-ordered H-AlSBA-15 with
trivalent aluminium exclusively in tetrahedral framework structure of SBA-15. Unlike other methods, which involve incorporation
of aluminum ions in, both, framework (Brønsted) and non-framework (Lewis) sites of the silicate matrix, the intrinsic hydrolysis
method isomorphously substitutes aluminium ions in the tetrahedral network even at high aluminium content. This unique approach
relies mainly on hydrolysis rates of inorganic (silicon and aluminium) precursors used for the preparation in such a way that the
condensation occurs simultaneously so as to overcome the usually encountered difficulties in stabilizing aluminium ions in the silicate
matrix. In this way, we could successfully synthesize high quality Brønsted acidicH-AlSBA-15, hitherto, not reported. The synthe-
sized materials were systematically characterized by various analytical, spectroscopic and imaging techniques, viz., XRD, BET, TEM,
SEM, 29Si &27Al MAS-NMR, XRF and NH3-TPD. The characterization results reveal the presences of highly porous structure (with
narrow pores) and tetrahedrally coordinated trivalent aluminium in the silicate matrix with more medium-to-strong Brønsted acid
sites. The resulting high quality catalysts exhibit excellent activity for tertiary-butylation of phenol with high selectivity towards
para-tertiary-butyl phenol and 2,4-di-tertiary-butyl phenol.
In the case of pH adjusting and hydrolysis methods, the pH adjust-
ment leads to disturbance of double-walled hydrogen interaction
Ordered mesoporous silicates (OMS), e.g., MCM-n, SBA-n, KIT-
(S0H+[X]H+I0; S-surfactant, X-anion, I-silicon), thereby causing
n, IITM-n, etc., have potential applications in diverse fields such as
broad pore size distribution and thinner walls, which affects the
adsorption, separation, catalysis, drug delivery, photonics and elec-
acidic strength significantly resulting poor stability of the materi-
tronics[1-5]. Among the various 2D-OMS, the acid-medium synthe-
als. Hence, in order to overcome such problems, in this investiga-
sized SBA-15 is considered one of the promising candidates in
tion, we have adopted an innovative approach in which the hydrol-
view of its thicker walls, that impart hydrothermal stability, high
ysis rates of both silicon and aluminium sources are controlled in
crystallinity that makes it thermally stable, stronger activity upon
such way to achieve complete condensation of the inorganic spe-
incorporation hetero atoms, which are suitable for catalyzing bulky
cies. The pH of the gel is maintained below the isoelectric point of
molecules[6,7]. In particular, the aluminium analogue (ordered mes-
silica so that more aluminium gets in to the framework without af-
oporous aluminosilicate, OMAS) with Brønsted acidity, i.e., H-
fecting the interaction between the surfactant and silicon species.
AlSBA-15, has a greater potential as heterogeneous (solid) acid
A set of aluminium sources with hydrolysis rates and the corre-
catalyst[8,9]. However, the synthesis of high quality H-AlSBA-15,
sponding stability constants are presented in Table S1.[22-25]
.
with tetrahedral aluminium, via direct acid-mediated synthesis[10-
12] is highly difficult due to the presence of trivalent aluminium ions
as well as the differences in the hydrolysis rates of both silicon and
aluminium sources. In addition, the ease dissociation of AlOSi
bond under reaction conditions makes aluminium incorporation in
the tetrahedral silicate framework highly unfavorable. In the past,
several attempts, e.g., direct method[10-12], post-synthesis method[13-
15], pH adjusting method[16,17], hydrolysis method[18], have been
made for the synthesis of H-AlSBA-15 using various inorganic (sil-
icon and aluminium) precursors[18,20] but without much success. In-
deed, one can graft more aluminium by adopting direct method[10-
12] and post-synthesis method[13-15], nonetheless they always lead to
the formation of undesired extra-framework aluminium (weak
Lewis acidity) and/or non-framework aluminium (strong Lewis
Scheme 1 depicts a general synthesis procedure of H-AlSBA-15 by
intrinsic hydrolysis method. A preliminary account of this work
was presented elsewhere[26,27]. Among industrially important al-
kylation reactions, tertiary-butylation of phenol[28-31] is of great im-
portance, as one of the products, viz., 4-t-BP,is used as precursor in
the production of phenolic resins, synthetic lubricants, antioxi-
dants, polymerization inhibitors, durable surface coatings, wire
enamels, printing inks, lube additives as well as in the production
of substituted triaryl phosphates[28,30-34]. Heterogeneous catalysts,
e.g., superacids, polymer resins, heteropoly acids, ionic liquids,
clays, microporous (zeolites and zeo-types) as well as ordered mes-
oporous aluminosilicate materials have been employed for tertiary-
butylation of phenol[35-37]. Microporous and mesoporous alumino-
silicates are promising heterogeneous catalysts but suffer from dif-
fusion limitation and low/ moderate acidity.
acidity) which also cause pore blockage[21]
.
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10.1002/chem.201604368
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Scheme 1. Pictorial representation of the formation of H-AlSBA-15 by intrinsic hydrolysis method.
Nonetheless, it is possible to improve the activity by proper choice
of the catalyst with tunable acidic sites and by optimizing the reac-
tion conditions/parameters[35-37].In addition, the t-butylation of
phenol is also used as model reaction to understand the nature of
acid sites as well acidic strength of various catalysts. Therefore, in
the present work, an attempt has been made to synthesize H-
AlSBA-15 with different silicon-to-aluminium ratios (nSi/nAl) and
study the performance of the catalysts prepared by intrinsic hydrol-
ysis method using appropriate aluminium source.
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 pre-
cipitate 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.
The various aluminium complexes, viz., aluminium citrate, alumi-
num acetate and aluminium acetylacetonate were prepared by
adopting he reported procedure [23,24,27]
.
Aluminium Citrate: 2.8 g of aluminium nitrate nonahydrate and
3.1 g of citric acid monohydrate were dissolved in 140 mL of water.
This mixture was stirred overnight at 40-50°C. Water was removed
from the mixture by using rotary evaporator. The viscous material
obtained was dissolved in minimum amount of water and pH was
adjusted to 8 using dil. NH3. In order to obtain crystalline material,
absolute ethanol was added at 4°C. Anal. Calcd for C12H32O16N5Al:
C, 27.20; H, 6.04; N, 13.22.Found: C, 26.78; H, 5.63; N, 12.81.
Aluminium Acetate: 20 mL of glacial acetic acid and 20 mL of
acetic anhydride were refluxed at 110°C for 30 min to remove wa-
ter. To this hot solution, 3.0 g of anhydrous aluminium chloride
was added and the mixture was cooled to room temperature. The
obtained fine white precipitate was filtered, washed with dry ace-
tone and dried at 110°C. The obtained product was kept in vacuum
desiccator over phosphorous pentoxide for further use. Anal. Calcd
for C6H9O6Al: C, 35.30; H, 4.45. Found: C, 35.04; H, 4.38.
All the chemicals used were of analytical grade. The surfac-
tant(Pluronic P123) and silica source, tetraethoxysilane (TEOS)
were procured from Sigma-Aldrich. Aluminium precursors (alu-
minium isopropoxide, aluminium nitrate nonahydrate, anhydrous
aluminium chloride, aluminium sulphate hexadecahydrate, ligands
(citric acid monohydrate, glacial acetic acid, acetic anhydride, ace-
tyl acetone), auxiliary chemicals (HCl, 32%; NH3, 25%), solvents
(ethanol, acetone, cyclohexane) and reactants (phenol and tertiary
butanol) were obtained from Merck. The authentic samples of or-
tho-tertiary butyl phenol (2-t-BP), para-tertiary butyl phenol (4-t-
BP) and2,4-di-tertiary butyl phenol (2,4-di-t-BP) were acquired
from Sigma-Aldrich.
SBA-15 was synthesized using a previously reported method[36]
.
Aluminium Acetylacetonate: 3.3 g of acetylacetone was taken in a
conical flask to which 40 mL of water followed by the addition of
8 mL of 5 M ammonia. 3.0 g of aluminium sulphate hexadecahy-
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 and stirred for 2 h and pH was adjusted to 1.5. Then,
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10.1002/chem.201604368
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drate in 30 mL of water was taken in another conical flask. Am-
monical solution of acetylacetone was added to aluminium sul-
phate solution in small portions with swirling. The pH of the result-
ing solution was adjusted to 7-8 using 5 M. The product was fil-
tered, washed with 10 mL of water, dried using suction and was
recrystallized from cyclohexane. Anal. Calcd for C15H21O6Al: C,
55.85; H, 6.48.Found: C, 55.36; H, 6.03.
reactant mixture, viz., phenol and tertiary butyl alcohol (t-BA),
with a desired ratio and weight hour space velocity (WHSV) was
fed into the reactor using a liquid injection pump (Miclins) and ni-
trogen as the carrier gas. The gaseous products were cooled and the
condensed liquid products were collected after 2 h. The products,
viz., 2-t-BP, 4-t-BP and 2,4-di-t-BP, were identified using gas chro-
matography (Perkin Elmer) with ZB-1 capillary column. The prod-
ucts were further confirmed by GC-MS (Agilent 7890A) with DB-
5 capillary column.
Intrinsic Hydrolysis Method: 4.0 g of Pluronic P123 was added to
30 mL of water and stirred till a clear solution was obtained. 70 mL
of 0.086 MHCl was added to the solution, 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 complexes were added at 2 h interval so that
the mixture will have nSi/nAl(molar ratio) 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 treat-
ment at 100°C for 24 h. The obtained precipitate was filtered,
washed, dried at 60°C for 6 h and calcined at 550°C for 1 h in ni-
trogen and 5 h in air with a heating rate of 1°C min-1 in order to
obtain surfactant-free samples. The samples were designated as H-
AlSBA-15(CT/IP/AC/AA), where CT, IP, AC, AA denote alumin-
ium complexes of citrate, isopropoxide, acetate and acetyl ace-
tonate, respectively. Subsequently, H-AlSBA-15 with nSi/nAl of
30, 60 and 90 (in the synthesis gel) were prepared using TEOS and
aluminium citrate in a similar manner as mentioned above. The
calcined samples were designated as H-AlSBA-15(x), where x de-
notes nSi/nAl in the gel. For a comparison, the sample H-AlMCM-
XRD patterns of as-synthesized (Figure S1) and calcined (Figure
1A) samples of both SBA-15 and H-AlSBA-15 exhibit well-re-
solved reflections typical of mesoporous 2D-hexagonal struc-
ture[38]. As expected, the latter showed an increase in unit cell pa-
rameter as that of the former (see Table S2), which could be at-
tributed to isomorphous substitution of trivalent aluminum having
larger ionic radius[39]. In addition, it could also be ascribed to fac-
tors such as increased wall-thickness and large pores as well as to
ion-exchanged or grafted aluminium ions. At this juncture, it is
also interesting to note the aluminium content in H-AlSBA-15 is
consistently less than the initial gel composition which could be
ascribed to high solubility of aluminium in acidic medium. How-
ever, by choosing aluminium sources with a stability constant close
to that of the silicon precursor (cf. Table S1), e.g., aluminium cit-
rate, a better condensation (as only a part of the precursors is solu-
ble) can be achieved and thereby stabilizing aluminium in the tet-
rahedral framework structure of the silicate matrix.N2 sorption iso-
therms of various H-AlSBA-15 samples understudy are shown in
Figure 1B along with pore size distributions. The corresponding
textural properties are summarized in Table S2. As expected, all
the samples under study exhibited high surface area and pore vol-
ume analogous to the siliceous analogues, viz., SBA-15. It can also
be seen from this figure that all materials show type IV isotherm
with a sharp H1 hysteresis suggesting the presence of large meso-
porous channels with narrow pore size distribution[40]. However,
except H-AlSBA-15(CT), all other samples show capillary conden-
sation at high relative pressure (cf. Figure 1B; inset) which may be
due to poor condensation of the constituent elements as a result of
different hydrolysis rates.
41 was also synthesized using a previously reported procedure[36]
.
All the synthesized materials with various aluminium sources were
thoroughly characterized to understand their structural, textural and
acidic properties. Powder X-ray diffraction (XRD) patterns of all
the samples were recorded using a Rigaku Miniflex II diffractom-
eter using Cu-K (λ= 0.15418 nm) in the range of 0.5 to 7.5 with a
2 step size of 0.01°and step time of 2 s. The nSi/nAl ratios were
quantified on a Rigaku Primini X-ray fluorescence (XRF) spec-
trometer.CHN analyses of aluminium complexes were performed
with Perkin Elmer 2400 series elemental analyser. 29Si MAS NMR
and 27Al MAS NMR spectra were recorded on Bruker DSX 300
spectrometer and the chemical shifts were referenced to tetrame-
thylsilane and aqueous aluminum nitrate solution (1M) respec-
tively.Transition electron microscope (TEM) images were obtained
using Philips CM12 microscope operated at 120 keV with a point-
to-point resolution of 3.4 Å. The surface morphology of the cata-
lysts was studied using a QUANTA 450-FEI scanning electron mi-
croscopy (SEM). Nitrogen sorption isotherms were recorded at 77
K using Micromeritics ASAP 2020 surface area analyzer, in which
the samples were outgassed at 200°C for 4 h prior to the measure-
ments. Temperature programmed desorption of ammonia (NH3-
TPD) was carried out on a Micromeritics Autochem II chemisorp-
tion analyzer. Acid strengths of the samples were determined by
temperature programmed desorption using ammonia gas as probe.
Samples were activated at 550°C for 1 h in helium flow, later they
were cooled and maintained at 120°C prior to their exposure to am-
monia vapor followed by purging with helium for 30 min. The de-
sorption of ammonia was carried out by heating the reactor up to
450°C at a uniform rate of 10°Cmin1.
The tertiary-butylation of phenol reaction was carried out using
500 mg of catalyst in a fixed-bed flow reactor. The catalyst was
activated at 500°C in a flow of air for 6 h followed by cooling to
reaction temperature (160°C) in N2atmosphere. After an hour, the
Figure 1. XRD patterns (A) and N2 sorption isotherms with pore
size distribution (B) of: (a) SBA-15; (b) H-AlSBA-15(CT); (c) H-
AlSBA-15(AC); (d) H-AlSBA-15(AA); (e) H-AlSBA-15(IP).
3
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Figure 2. TEM images of: (a,b) SBA-15; (c,d) H-AlSBA-15(CT); (e,f) H-AlSBA-15(AC); (g,h) H-AlSBA-15(AA); (i,j) H-AlSBA-15(IP).
Figure 3. FE-SEM images of: (a) SBA-15; (b) H-AlSBA-15(CT); (c) H-AlSBA-15(AC); (d) H-AlSBA-15(AA); (e) H-AlSBA-15(IP).
Figure 2 depicts TEM images which can be viewed along both par-
allel and perpendicular directions of pore axis of samples. The
well-ordered arrays (parallel direction) of one-dimensional meso-
channels with a typical honeycomb (perpendicular direction) struc-
ture is typical characteristics of SBA-15 matrix[41]. These images
further validate, in particular, the excellent 2D-hexagonal
mesostructural ordering of H-AlSBA-15 prepared by intrinsic hy-
drolysis method. As indicated earlier (see Figure 1B), except H-
AlSBA-15(CT) sample, all others possess larger pore channels
which is in excellent agreement with TEM images as well as their
wall thickness. Figure 3 illustrates the FE-SEM images of SBA-15
and H-AlSBA-15 samples. It can be seen from this figure that the
siliceous sample (Figure 3a) exhibits a typical morphology, i.e.,
bundles of long rope-like features, characteristic of high quality
SBA-15, which is in good agreement with literature[42]. However,
such a feature is not observed in the corresponding aluminium an-
alogues except the one prepared using aluminium citrate (see Fig-
ure 3b). The similarity in the microstructures of SBA-15 and H-
AlSBA-15(CT) may be attributed to comparable hydrolysis rates
of both TEOS and CT (cf. Table S1) which could facilitate the for-
mation such bundles without causing strain in polymeric inorganic
species[12,43]. In contrast, in the case of samples prepared using
other aluminium sources, viz., IP, AC and AA, the rope-like bun-
dles were partially formed and/or disintegrated (see also Figure 3c-
3e) due to an increase in surface curvature of micelles caused by
differences in the hydrolysis rates.
corresponding to (3Si,1Al) environment[44]. A similar trend is also
observed for all the aluminosilicate samples under investigation.
Likewise, a decline in the Q3 sites, which is normally prominent in
the siliceous samples (see Figure 4A; curve a), could be noticed.
In other words, the lower Q4/Q3 ratio of H-AlSBA-15(CT) or high
amount of Q3 sites indicate more aluminium content in the frame-
work structure[44,45]
.
Figure 4 shows the 29Si and 27Al MAS-NMR spectra of various
samples. It is noticed from 29Si MAS-NMR spectra of aluminosil-
icates (Figure 4A; curves b-e) that some of the Q4 sites (110 ppm)
and Q3 sites (100 ppm) are partially replaced as Q3 (106 ppm) sites
Figure 4. 29Si MAS-NMR (A) and 27Al MAS-NMR (B) spectra of:
(a) SBA-15; (b) H-AlSBA-15(CT); (c) H-AlSBA-15(AC); (d) H-
AlSBA-15(AA); (e) H-AlSBA-15(IP).
4
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Figure 4B presents 27Al MAS-NMR spectra of various samples. It
is interesting to note that, in case of H-AlSBA-15(CT), trivalent
aluminium is exclusively located in tetrahedral coordination (Fig-
ure 4B; curve b; AlT; 53 ppm). On the other hand, for all other
samples (Figure 4B; curves c-e), in addition to the signal due to
tetrahedral coordination (AlT; 53 ppm), an extra feature (AlP; 35
ppm)[46]appears which could be attributed to extra-framework
(penta-coordinated) aluminium species. Such species arise owing
to the varied hydrolysis rates of aluminium sources and TEOS. On
the other hand, it is missing in the case of H-AlSBA-15(CT) as the
monomeric tetrahedral species generated condense immediately
leading to the formation of framework aluminium uniquely. At this
juncture, it is also interesting to note the absence of non-framework
(hexa-coordinated; Al2O3 aggregates or second phase) aluminium
(AlO; 0 ppm) in these materials.
Figure 5 presents NH3-TPD profiles of various H-AlSBA-15. The
distribution of acid sites is calculated fromdeconvolution of de-
sorption data using Gaussian function and the results are given in
Table 1. The weak acid sites, designated as type (i), are attributed
to surface hydroxyl groups (silanols), and weak Brønsted acid sites,
referred as type (ii), are generated due to presence of aluminium
close to the surface. On the other hand, moderate Brønsted acid
sites, designated as type (iii), and strong Brønsted acid sites, desig-
nated as type (iv), arise due to aluminium in two different frame-
work positions whereas weak Lewis acid sites, referred as type (v),
are due to penta-coordinated aluminium[47](cf. Scheme 1). How-
ever, it is interesting to note from Table 1 and Figure 4 that the TPD
trace for H-AlSBA-15(CT) show more medium-to-strong Brønsted
acid sites compared to other samples. This could be attributed to
Figure 5. NH3-TPD traces of: (a) SBA-15; (b) H-AlSBA-15(CT);
(c) H-AlSBA-15(AC); (d) H-AlSBA-15(AA); (e) H-AlSBA-
15(IP).
Figure 6 depicts TEM images of H-AlSBA-15(x) prepared with
various nSi/nAl ratios. All the materials show well-ordered arrays
(parallel direction) of one-dimensional mesochannels with a typical
honeycomb (perpendicular direction) structure characteristics of
SBA-15[41] validating the excellent 2D-hexagonal mesostructural
ordering of H-AlSBA-15 prepared by intrinsic hydrolysis method.
As indicated earlier (see Figure S3B), H-AlSBA-15 samples with
various nSi/nAl ratios, have pore channels of nearly same size as that
of SBA-15 which is in excellent agreement with TEM as well as
with wall thickness. Figure 7illustrates the FE-SEM images of
SBA-15 and H-AlSBA-15(CT). It can be seen from these images
that the latter with various (nSi/nAl) ratios (seeFigure 7b-d) exhibits
a typical morphology, i.e., bundles of long rope-like features simi-
lar to the corresponding siliceous analogue, i.e., SBA-15 (cf. Figure
7a)[42]. The similarity in the microstructures of both SBA-15 and
H-AlSBA-15(CT) further confirms the use of aluminium source
having similar hydrolysis rate that of the silicon source (cf. Table
S1) facilitate the formation such bundles without causing strain in
the thicker walls and presence of more framework aluminium[48]
.
In other words, owing to the very different hydrolysis rates of alu-
minium and silicon sources, most of the aluminium is present near
to surface in the case of H-AlSBA-15(AC/AA/IP) instead of get-
ting incorporated in the interior and therefore the weak acid sites
are prominent.
XRD patterns of as-synthesized (Figure S2) and calcined(Figure
S3A) samples of both SBA-15 and H-AlSBA-15(CT) with various
silicon-to-aluminium (nSi/nAl) ratios exhibit well-resolved reflec-
tions in the low-angle region typical of mesoporous2D-hexagonal
structure with p6mmsymmetry[38].These long-range ordered dif-
fraction patterns indicate that mesostructural order of SBA-15 is
sustained even at higher aluminium loadings. Further, as expected,
H-AlSBA-15 showed an increase in unit cell parameter as that of
SBA-15 (Table S3), which could be attributed to isomorphous sub-
stitution of larger trivalent aluminum ion in the smaller tertravalent
polymeric (inorganic) species even at high aluminium content[12,43]
.
Figure 8 shows 29Si and 27Al MAS-NMR spectra of H-AlSBA-
15(CT) with various nSi/nAl ratios. As expected, in all these sam-
ples, the Q3site appears and increases with decrease in silicon-to-
aluminium ratio at the cost of Q3 sites. This indicates that more
aluminium gets incorporated in the framework structure (Figure
8A; curve b-d)[44,45]. At this juncture, it is also interesting to note
that, even at high aluminium content, the trivalent aluminium is
exclusively located in tetrahedral coordination (Figure 8B; curve
b-d; AlT; 53 ppm). Unlike the samples prepared using other alu-
minium sources, i.e., AC, AA and IP, the H-AlSBA-15(CT) sam-
ples do not show extra-framework aluminium (penta-coordi-
nated; AlP; 35 ppm) indicating the absence of such species (Figure
silicon[39]
. Figure S3B depicts the N2 sorption isotherms of H-
AlSBA-15 with various nSi/nAl ratios. All materials exhibit type IV
isotherm with a steep H1 hysteresis suggesting large mesoporous
channels with narrow pore size distribution[40]. It can also be seen
from this figure that the materials with various silicon-to-alumin-
ium ratios show capillary condensation step at the same relative
pressure that of SBA-15 indicating that the pore size remains unaf-
fected even at high aluminium loading. This could be due to iden-
tical ionic strength of the synthesis gel owing to similar hydrolysis
rates of both silicon (TEOS) and aluminium(CT) precursors used
for the preparation. However, it is interesting to note that the wall
thickness of H-AlSBA-15 is larger than SBA-15 and it is found to
increase with increase in aluminium content. In contrast, the wall
thickness of H-AlSBA-15, synthesized by other methods10-19, de-
creases with increase in aluminium content as the condensation of
silica gets affected due to the change in ionic strength22. The tex-
tural properties of the various samples are listed in Table S3.
7B; curve b-d)[46]
. In other words, in the case of H-AlSBA-
15(CT), the aluminium is exclusively incorporated in the frame-
work structure owing to similar hydrolysis rates of the constituent
inorganic sources generate monomeric species of both aluminium
and silicon which condense instantaneously to form the regular
aluminosilicate network[49]
.
5
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10.1002/chem.201604368
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in acidic strength can also be noticed with increase in aluminium
content, i.e., desorption traces gets shifted to higher temperatures
which is in good agreement with literature[48]
.
Figure 8. 29Si MAS-NMR (A) and 27Al MAS-NMR (B) spectra of:
(a) SBA-15; (b) H-AlSBA-15(90); (c) H-AlSBA-15(60); (d) H-
AlSBA-15(30).
Table 1 presents the tertiary-butylation reaction results of H-
AlSBA-15 under optimized experimental conditions. As expected,
among the various catalysts, H-AlSBA-15(CT) exhibits high phe-
nol conversion with good selectivity towards 4-t-BP and 2,4-di-t-
BP. The higher activity of H-AlSBA-15(CT) could be attributed to
strong acidity and presence of more medium-to-strong Brønsted
acid sites(cf. Table 1) as a consequence of framework alumin-
Figure 6. TEM images of: (a,b) SBA-15; (c,d) H-AlSBA-15(90);
(e,f) H-AlSBA-15(60); (g,h) H-AlSBA-15(30).
ium[37]
.
Indeed, phenol adsorbs in horizontal orientation on
Brønsted acid sites forming 4-t-BP largely due to steric hindrance
though it can get attacked in both ortho/para positions[50].On the
other hand, the catalyst, H-AlSBA-15(AA) shows the formation of
considerable amount of 2-t-BP owing to the presence of relatively
high amount of weak Lewis acid (penta-coordinated aluminium).
Phenol adsorbs in vertical orientation on these sites, and therefore
results in the ortho-product at the expense of phenol conversion.
Figure 7. FE-SEM images of: (a) SBA-15; (b) H-AlSBA-15(90);
(c) H-AlSBA-15(60); (d) H-AlSBA-15(30).
Figure 9depicts the NH3-TPD traces of various H-AlSBA-15(CT)
samples. As depicted in this figure, the distribution of acid sites is
calculated based on deconvolution of desorption data using Gauss-
ian function and the results are listed in Table 1. It can be seen
from the table and Figure 9 that the observed increase in medium-
to-strong Brønsted acid sites with a decrease in nSi/nAl ratio could
be attributed to the increase in wall thickness as result of high alu-
minum content in the framework structure. In addition, an increase
Figure 9. NH3-TPD profiles of: (a) SBA-15; (b) H-AlSBA-15(90);
(c) H-AlSBA-15(60); (d) H-AlSBA-15(30).
6
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10.1002/chem.201604368
Chemistry - A European Journal
Table 1. Acidity and tertiary butylation of phenol data of H-AlSBA-15a.
Acidic sites
nSi/nAl
Selectivity (%)
Phenol
Conv.
(%)
e
hw
(mmol g1) / TDes (°C)f
d
nSi/nAlт
Material
(nm)
Aluminium
source
2-t-
BP
4-t-
2,4-di-
gelb Productc
Total
(iii)
--
(iv)
--
BP
t-BP
SBA-15
4.06
4.77
4.95
5.30
4.15
4.11
0.06
0.16
0.18
0.24
0.17
0.10
0.0
0.0
11.8
10.4
8.4
0.0
0.0



CT
H-AlSBA-15
90
60
30
60
60
248
140
85
248
140
85
0.09/313 0.02/382
0.10/327 0.04/387
0.10/327 0.04/394
0.08/317 0.02/384
0.05/307 0.01/378
38.7
43.7
55.5
39.0
24.0
73.6
72.8
76.5
76.8
70.5
14.6
16.8
15.1
13.9
13.0
AC
AA
IP
H-AlSBA-15
H-AlMCM-41
120
168
153
234
10.4
16.5
60
90
125
93
157
93
4.06
1.60
0.16
0.17
0.06/314 0.02/382
0.06/276 0.02/356
34.2
26.4
10.2
12.9
76.4
72.5
13.4
14.5
ASg
aReaction conditions: Catalyst = 500 mg; nt-BA:nPheno1= 2:1; 160°C; WHSV = 7.0 h1; TOS = 2 h. bInitial (nominal) composition; c Determined by XRF;
dDetermined by 27Al MAS-NMR. ePore wall-thickness. fDesorption temperature. gAluminiumsulphate.
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Chemistry - A European Journal
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8
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Chemistry - A European Journal
Table-of-Contents
A novel intrinsic hydrolysis method was
employed for the synthesis of ordered
mesoporous aluminosilicates, H-AlSBA-
15 with trivalent aluminium in tetrahedral
framework structure. Unlike many other
approaches, the present method exhibit
isomorphous substitution of the alumin-
ium ions in the mesoporous silicate ma-
trix with high aluminium content. The re-
sulting materials exhibit excellent cata-
lytic activity for acid-catalyzed reactions.
Nunna V. Krishna and
Parasuraman Selvam*
ACID-MEDIATED SYNTHESIS OF
ORDERED MESOPOROUS
ALUMINOSILICATES: THE
CHALLENGE AND THE PROMISE
9
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