One-step room-temperature synthesis of [Al]MCM-41 materials for the catalytic conversion of phenylglyoxal to ethylmandelate

Article abstract of DOI:10.1002/cctc.201300375

Mesoporous [Al]MCM-41 materials with n_Si/n_Al ratios of 15 to 50 suitable for the direct catalytic conversion of phenylglyoxal to ethylmandelate have been successfully synthesized at room temperature within 1 h. The surface areas and pore sizes of the obtained [Al]MCM-41 materials are in the ranges of 1005-1246 m² g^-1 and 3.44-3.99 nm, respectively, for the different n_Si/n_Al ratios. For all [Al]MCM-41 catalysts, most of the Al species were tetrahedrally coordinated with Si in the next coordination sphere of atoms. ¹H and ¹³C magic-angle spinning NMR spectroscopic investigations indicated that the acid strength of the SiOH groups on these [Al]MCM-41 catalysts and the density of these surface sites are enhanced with increasing Al content in the synthesis gels. These surface sites with enhanced acid strength were found to be catalytically active sites for phenylglyoxal conversion. The [Al]MCM-41 material with n_Si/n_Al=15 showed the highest phenylglyoxal conversion (93.4 %) and selectivity to ethylmandelate (96.9 %), whereas the [Al]MCM-41 material with n_Si/n_Al=50 reached the highest turnover frequency (TOF=99.3 h^-1). This is a much better catalytic performance than that of a dealuminated zeolite Y (TOF=1.7 h^-1) used as a reference catalyst, which is explained by lower reactant transport limitations in mesoporous materials than that in the microporous zeolite. Mesopores flex their catalytic muscles: Mesoporous [Al]MCM-41 materials with n_Si/n_Al ratios of 15 to 50 suitable for the direct catalytic conversion of phenylglyoxal to ethylmandelate were successfully synthesized at room temperature within 1 h. Copyright

Full text of DOI:10.1002/cctc.201300375

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DOI: 10.1002/cctc.201300375
One-Step Room-Temperature Synthesis of [Al]MCM-41
Materials for the Catalytic Conversion of Phenylglyoxal to
Ethylmandelate
Zichun Wang,[a] Yijiao Jiang,[b] Rafal Rachwalik,[c] Zhongwen Liu,[a] Jeffrey Shi,[a]
Michael Hunger,*[d] and Jun Huang*[a]
Mesoporous [Al]MCM-41 materials with nSi/nAl ratios of 15 to
50 suitable for the direct catalytic conversion of phenylglyoxal
to ethylmandelate have been successfully synthesized at room
temperature within 1 h. The surface areas and pore sizes of the
obtained [Al]MCM-41 materials are in the ranges of 1005–
1246 m2 gꢀ1 and 3.44–3.99 nm, respectively, for the different
nSi/nAl ratios. For all [Al]MCM-41 catalysts, most of the Al spe-
cies were tetrahedrally coordinated with Si in the next coordi-
tent in the synthesis gels. These surface sites with enhanced
acid strength were found to be catalytically active sites for
phenylglyoxal conversion. The [Al]MCM-41 material with nSi/
nAl =15 showed the highest phenylglyoxal conversion (93.4%)
and selectivity to ethylmandelate (96.9%), whereas the
[Al]MCM-41 material with nSi/nAl =50 reached the highest turn-
over frequency (TOF=99.3 hꢀ1). This is a much better catalytic
performance than that of a dealuminated zeolite Y (TOF=
1.7 hꢀ1) used as a reference catalyst, which is explained by
lower reactant transport limitations in mesoporous materials
than that in the microporous zeolite.
1
nation sphere of atoms. H and 13C magic-angle spinning NMR
spectroscopic investigations indicated that the acid strength of
the SiOH groups on these [Al]MCM-41 catalysts and the densi-
ty of these surface sites are enhanced with increasing Al con-
Introduction
Mesoporous MCM-41 materials are silica-based systems with
hexagonally arranged mesopores with long-range order, which
were initially developed in 1992.[1] Generally, the ordered meso-
pores are formed through the condensation of silicates around
the self-assembled micelles by surfactant molecules or through
liquid-crystal phases influenced by the silicate. During the past
decades, numerous routes for the synthesis of MCM-41 materi-
als have been developed.[1b,2] With modification of the synthe-
sis conditions, such as the type of solvents, silica sources, addi-
tion of other metal species (e.g., Al), surfactant type and rela-
tive concentration, pH value, synthesis temperature, aging and
drying conditions, etc., the morphology and catalytic proper-
ties of the obtained MCM-41 materials can be controlled.[2a,3]
Among them, the direct room-temperature synthesis of sili-
ceous MCM-41[3c,f,h,4] has drawn great interest as it is a rapid
method under mild conditions with less energy and time con-
sumed compared with classic hydrothermal synthesis (heating
normally required at 60–1508C for 1–6 d).[1,5]
Huo et al.[6] synthesized mesoporous silica spheres by using
different silica sources under basic conditions with stirring for
15–30 h at room temperature. They found that lower alkoxysi-
lanes, such as tetramethyl orthosilicate (TMOS) and tetraethyl
orthosilicate (TEOS), yielded only small MCM-41 particles,
whereas higher alkoxysilanes, such as tetrabutyl orthosilicate
(TBOS), led to larger MCM-41 spheres. These spheres were ob-
tained at a low stirring speed (<200 rpm), whereas the forma-
tion of small particles occurred at a high stirring speed
(>450 rpm). Cai et al.[3c] and Grun et al.[3h] synthesized siliceous
MCM-41 particles in a diluted solution of surfactants with tet-
raethoxysilane under basic conditions at room temperature
with stirring for 2 h. The resulting particle size was in the
range of micrometers (1 mm) to nanometers (100 nm) con-
trolled by using different basic additives (NaOH/NH4OH)[3c] and
surfactants (n-hexadecyltrimethylammonium bromide and n-
hexadecylpyridiniumchloride monohydrate).[3h] Recently, Pes-
carmona and co-workers[3a,7] synthesized [Ti]MCM-41 materials
with particle sizes of 80–160 nm by using TEOS, cetyltrimethy-
lammonium bromide (CTAB), NaOH, and titanium(IV) isoprop-
oxide with stirring at room temperature for 2 h and found that
[a] Z. Wang, Dr. Z. Liu, Dr. J. Shi, Dr. J. Huang
Laboratory for Catalysis Engineering
School of Chemical and Biomolecular Engineering
The University of Sydney
NSW 2006 (Australia)
Fax: (+61)2-9351-2854
E-mail: jun.huang@sydney.edu.au
[b] Dr. Y. Jiang
School of Chemical Engineering
The University of New South Wales
NSW 2052 (Australia)
[c] Dr. R. Rachwalik
Institute of Organic Chemistry and Technology
Cracow University of Technology
31-155 Krakꢀw (Poland)
[d] Prof. Dr. M. Hunger
Institute of Chemical Technology
University of Stuttgart
70550 Stuttgart (Germany)
Fax: (+49)711-685-64081
E-mail: michael.hunger@itc.uni-stuttgart.de
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these are suitable catalysts for the epoxidation of cyclohexene
with H2O2. In addition, [Al]MCM-41 materials were synthesized
under similar conditions by using a mixture of TEOS and alumi-
num isopropoxide (AIP) as precursors under basic (aqueous
ammonia) or acidic (HCl) conditions.[8]
MCM-41 materials with and without doping with metal spe-
cies (B, Ga, Ti, Al, and Fe) have been widely utilized in hetero-
geneous catalysis, drug delivery, and adsorption technolo-
gies.[1,2b,9] Most of these applications require materials with
well-defined morphologies, adsorption properties, and acidi-
ty.[1b,2b,3a,b,9,10] Therefore, the synthesis of MCM-41 materials
with the desired properties has attracted much attention. One
of the most important modifications of MCM-41 materials is
the incorporation of Al into the silica framework to lead to the
formation of surface SiOH groups with enhanced acid strength.
These acidic [Al]MCM-41 materials are promising as solid-acid
catalysts as well as supports of nanometal catalysts for bifunc-
tional catalysis because of their better reactant/product diffu-
sion properties for big molecules inside the mesopores[8a,11]
compared with widely used microporous zeolites. Therefore,
the incorporation of Al into the framework of mesoporous
[Al]MCM-41 materials is a convenient method to tune the
Brønsted acidity for desired catalytic reactions, and in the pres-
ent work, these catalysts have been prepared by a rapid, one-
step, room-temperature synthetic route.
Figure 1. Small-angle XRD patterns of a) [Si]MCM-41 and [Al]MCM-41 with
nSi/nAl of b) 50, c) 30, d) 20, and e) 15.
For the development of green processes in the chemical in-
dustry, an increasing number of solid acids have replaced
liquid catalysts. In fine chemistry for example, solid acids were
recently applied in the conversion of a-keto aldehyde to a-hy-
droxyl carboxylates in alcohols.[12] In this case, phenylglyoxal
(PG) is converted in ethanol to yield ethylmandelate (EM),
which is an essential chemical in the pharmaceutical and fine-
chemical industries.[13] Recently, an EM yield of 95% with a se-
lectivity of 97% was obtained on a USY zeolite.[12c]
(110), (200), and (210) reflections become small and broad
(Figure 1b–e), which hints at a disturbance of this order. This
finding as well as the broadening and shift of the (100) reflec-
tion to smaller angles that occurs for decreasing nSi/nAl ratios
of the [Al]MCM-41 materials is a consequence of the incorpora-
tion of Al into the silica framework.[16]
The N2 adsorption–desorption isotherms and the corre-
sponding Barrett–Joyner–Halenda (BJH) pore size distribution
curves of the MCM-41 samples under study are depicted in
Figure 2. All samples show type IV isotherms, which indicates
a mesoporous structure according to the IUPAC classifica-
tion.[17] The pore size distributions, evaluated by the BJH
model, correspond to narrow and uniform mesopores for all
MCM-41 materials under study (inset in Figure 2).
In this work, we present a one-step method for the synthesis
of [Si]MCM-41 and [Al]MCM-41 materials with different nSi/nAl
ratios at room temperature. These materials were characterized
by XRD, N2 adsorption–desorption, and electron microscopy
for their long-range order, texture, and morphology. As a pow-
erful technique for the characterization of aluminosilicates,[14]
solid-state NMR spectroscopy was used for the investigation of
the local structure of the [Al]MCM-41 framework and the sur-
face acid sites. Finally, the conversion of PG to EM was em-
ployed on [Al]MCM-41 catalysts as a test reaction.
The structural data obtained by XRD and the results of N2
adsorption are summarized in Table 1. Interestingly, the wall
thicknesses (the difference between the average pore size and
the unit cell parameter) of the [Al]MCM-41 materials (0.72–
1.17 nm) are very similar to those of [Si]MCM-41 in the present
study (0.91 nm) and those described in the literature
(ꢁ1 nm).[3c] This may indicate that the mechanical strength of
the Al-containing MCM-41 materials prepared in this work is
comparable to that of conventional MCM-41 materials.
Results and Discussion
Characterization of textural and morphological properties
XRD patterns recorded in the small-angle region (2q=1.5–108)
of the MCM-41 materials with different nSi/nAl ratios are shown
in Figure 1. A typical pattern that indicates a hexagonal frame-
work was observed for [Si]MCM-41 in Figure 1a as evidenced
by the strong (100) reflections at low angles and weak (110),
(200), and (210) reflections at 4.1, 4.7, and 6.38, respectively.
These reflections are a result of the long-range order of the
MCM-41 materials.[15] With an increasing Al content, the weak
The SEM images of the [Al]MCM-41 samples presented in
Figure 3 show a uniform particle shape with particle size in the
range of 50 to 200 nm, independent of the Al content. In the
transmission electron microscopy (TEM) image of the [Si]MCM-
41 material in Figure 4a, a typical regular mesoporous struc-
ture with hexagonal pores can be observed clearly.[3c,19] The
TEM images of the Al-containing MCM-41 materials shown in
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Figure 2. N2 adsorption–desorption isotherms and pore size distributions of a) [Si]MCM-41 and [Al]MCM-41 with nSi/nAl of b) 50, c) 30, d) 20, and e) 15.
content was also observed from XRD patterns and
Table 1. Structural and textural properties of the MCM-41 materials under study: d(100)
spacing, unit cell parameter a0, BET surface areas, total pore volume, average pore
TEM images.
size, and pore wall thickness.
Solid-state NMR investigation
Sample
d(100)
a0
A
V
d
Pore wall
[c]
[c]
[nm][a] [nm][b] [m2 gꢀ1
]
[cm3 gꢀ1
]
[nm][c] thickness [nm]
29Si magic-angle spinning (MAS) NMR spectroscopy
was used to investigate the Si coordination inside
the MCM-41 framework. The 29Si MAS NMR spectrum
of the [Si]MCM-41 material shown in Figure 5a con-
sists of strong signals at d29Si =ꢀ110 and ꢀ101 ppm
attributed to Q4 (Si(OSi)4) and Q3 (Si(OSi)3OH) species,
respectively, and the small hump at d29Si =ꢀ92 ppm is
attributed to Q2 (Si(OSi)2(OH)2) species. Q4 species are
caused by tetrahedrally coordinated Si atoms, where-
as one and two hydroxyl groups are bound to Si
atoms that give rise to Q3 and Q2 signals, respectively.
Generally, the Q4/(Q3+Q2) ratio is used as a measure
of the degree of condensation of the silica frame-
[Si]MCM-41
3.77
4.35
4.77
4.43
4.56
4.71
–
1042
1246
1036
1005
1130
671
0.897
1.324
0.959
0.881
1.127
0.388
3.44
3.6
3.6
3.5
3.99
1.1
0.91
1.17
0.83
1.06
0.72
–
[Al]MCM-41/50 4.13
[Al]MCM-41/30 3.84
[Al]MCM-41/20 3.95
[Al]MCM-41/15 4.08
deAl-HY[18]
–
[a] The d(100) spacing was calculated by using the d(100) reflection with Bragg’s equa-
tion (2dsinq=l, l=0.1542 nm). [b] Hexagonal unit cell parameter a0 =(2/ 3)d(100)
which represents the distance between two neighboring pores. [c] Specific surface
area (A), total pore volume (V), and average pore size (d) were determined by N2 ad-
sorption–desorption isotherms.
p
,
Figure 4b–f exhibit more irregular pores, however, only with
slight effects on their long-range order as demonstrated by
the XRD investigations. The pore size of the material shown in
Figure 4c (ꢁ2.8 nm) is smaller than the mean pore size of
[Al]MCM-41/50 (3.6 nm), which could be caused by the detec-
tion of some small pores that are in the range of the pore size
distribution shown in Figure 2b (inset). In comparison with
[Al]MCM-41 materials reported in the literature,[8,20] the
[Al]MCM-41 materials obtained in this work show typical hex-
agonal structural properties. The N2 adsorption–desorption iso-
therms and BET analysis evidenced the mesoporous structure
of these [Al]MCM-41 materials, similar to those reported previ-
ously.[8,20] Similar to other [Al]MCM-41 materials,[16] the disturb-
ance of the long-range order of [Al]MCM-41 with increasing Al
work.[21] Upon determining the contents of the different Qn
(n=0–4) species by deconvolution and quantitative evaluation
of the 29Si MAS NMR spectrum of the [Si]MCM-41 material,
a Q4/(Q3+Q2) ratio of 1.4 was obtained, which is similar to that
in previous work.[5b,7]
As an example of the Al-containing MCM-41 materials, the
29Si MAS NMR spectrum of [Al]MCM-41/30 is shown in Fig-
ure 5b. The strong broadening of the 29Si MAS NMR signals is
because of the disturbance of the local Si structure by incorpo-
ration of Al into the MCM-41 framework, which agrees with
the observations made by XRD and TEM (vide supra). There-
fore, in addition to the Q4, Q3, and Q2 signals of Si atoms exclu-
sively coordinated to OꢀSiꢀ and OH species, a signal from Si
atoms coordinated to framework Al atoms (Si(1Al) signals) also
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Figure 3. SEM images of [Al]MCM-41 with nSi/nAl of a) 50, b) 30, c) 20, and
d) 15.
Figure 5. 29Si MAS NMR spectra of a) [Si]MCM-41 and b) [Al]MCM-41 with nSi/
nAl =30.
Figure 6. 27Al MAS NMR spectra of [Al]MCM-41 with nSi/nAl of a) 50, b) 30,
c) 20, and d) 15.
The 27Al MAS NMR spectra of the [Al]MCM-41 materials
shown in Figure 6 are dominated by signals of tetrahedrally co-
ordinated AlIV species (Al(4Si)) at d27Al =54 ppm. With the in-
creasing Al content of the synthesis gel that corresponds to
a decreasing nSi/nAl ratio from 50 to 15, the AlIV signals become
significantly stronger. Hence, a larger number of Al atoms have
been incorporated into the silica framework. The weak
27Al MAS NMR signals at d27Al =0 ppm are explained by octahe-
drally coordinated AlVI species that are not incorporated into
the silica framework or are located at defect sites.[22]
Figure 4. TEM images of a) [Si]MCM-41 and [Al]MCM-41 with nSi/nAl of b) and
c) 50, d) 30, e) 20, and f) 15.
contributes to the spectrum shown in Figure 5b. This signal
may be the reason for the increase of the signal at approxi-
mately ꢀ101 ppm (Q3+Si(1Al)) and the slight shift of the maxi-
mum from ꢀ110 to ꢀ107 ppm in comparison with the spec-
trum of [Si]MCM-41 shown in Figure 5a.
As discussed for dealuminated zeolites, specific types of oc-
tahedrally coordinated extraframework Al species may act as
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Lewis acid sites.[18,23] For [Al]MCM-41 materials, the Lewis acidic
properties of AlVI species were supported by comparison of the
results of FTIR spectroscopy upon adsorption of pyridine as
a probe molecule and 27Al MAS NMR spectroscopy.[24] There-
fore, the content of the AlVI species in [Al]MCM-41 is interest-
ing for the catalytic properties of these materials. In the pres-
ent work, an increase the of AlVI/AlIV ratio from 4:96 for
[Al]MCM-41/50 to 12:88 for [Al]MCM-41/15 was observed.
Clearly, higher Al contents lead to an enhanced probability of
AlVI species and possibly to the occurrences of Lewis acid sites.
However, the amount of Lewis sites is very low on [Al]MCM-41
materials because of the low content of AlVI species.
Solid-state NMR investigations of various noncrystalline alu-
minosilicates have demonstrated that tetrahedrally coordinat-
ed framework AlIV species in the local structure of SiOH groups
can enhance the acid strength of these hydroxyl groups (see
Ref. [14b] and references therein). For the application of
[Al]MCM-41 materials as catalysts or supports for metal cata-
lysts, therefore, the determination of the density and strength
of these acid sites is essential. Suitable methods to characterize
1
these surface sites are H and 13C MAS NMR spectroscopy upon
adsorption of probe molecules.
1
The H MAS NMR spectra of the dehydrated MCM-41 sam-
ples are dominated by strong signals of silanol groups at d1H
=
1.8 ppm (Figure 7, bottom traces).[14b] No low-field signals of
bridging SiOHAl groups at 3.6–4.3 ppm,[14b] as typically ob-
served in the spectra of acidic zeolites, were observed in the
spectra of [Al]MCM-41. Therefore, the determination of the
density of surface OH groups with enhanced acid strength was
1
performed by quantitative H MAS NMR spectroscopy of am-
monium ions formed upon adsorption of ammonia on dehy-
drated samples of the MCM-41 materials under study (Figure 7,
top traces).[14b,25] In this case, the presence of SiOH groups with
an enhanced Brønsted acid strength is indicated by the ap-
pearance of an ammonium signal at d1H =6.6 ppm. As demon-
strated by the spectrum of the ammonia-loaded [Si]MCM-41
material (Figure 7a, top trace), no ammonium ions are formed
if no Al is incorporated into the silica framework. For the
[Al]MCM-41 materials, however, 1H MAS NMR signals of the am-
monium ions increase with increasing Al content (Figure 7b–e,
top traces). The quantitative evaluation of these signals gave
the densities of SiOH groups with enhanced Brønsted acid
strength listed in column 2 of Table 2, which increase from
0.013 mmolgꢀ1 for [Al]MCM-41/50 to 0.121 mmolgꢀ1 for
[Al]MCM-41/15, that is, with increasing Al content. As demon-
strated by earlier transfer of population in double resonance
(TRAPDOR) experiments,[26] some of the Al species incorporat-
ed into the silica framework of MCM-41 materials are located
in the vicinity of SiOH groups, which is probably the reason for
the enhanced acid strength mentioned above.
Figure 7. 1H MAS NMR spectra of a) [Si]MCM-41 and [Al]MCM-41 with nSi/nAl
of b) 50, c) 30, d) 20, and e) 15.
with acetone-2-13C with a dominant signal at 210 ppm is
shown in Figure 8a. The small signals that appear at d13C =70–
75 ppm in the 13C MAS NMR spectra are probably caused by
the conversion of acetone to diacetone alcohol.[27] The signals
at approximately 30 and 160 ppm are a result of the nonen-
riched methyl groups and the enriched C atoms of other ace-
As a scale of the acid strength of surface OH groups on solid
catalysts, often the adsorbate-induced resonance shift of the
NMR signals of probe molecules, such as acetone-2-13C, is tone condensation products, respectively. The signal at d
used.[14b] In this case, a high adsorbate-induced low-field shift
of the 13C MAS NMR signal of acetone-2-13C corresponds a high
acid strength of interacting surface OH groups and vice versa.
The 13C MAS NMR spectrum of the [Si]MCM-41 sample loaded
=
13C
210 ppm agrees very well with that of acetone-2-13C adsorbed
on SiOH groups of pure silica.[25] For the Al-containing MCM-41
materials under study, this 13C MAS NMR signal shifted to
212 ppm for [Al]MCM-41/50 and to 213 ppm for [Al]MCM-41/
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41, which is exclusively characterized by nonacidic
surface SiOH groups, is not able to convert PG to EM
(Table 2, columns 4–6). The total conversion of PG is
zero after 6 h. For [Al]MCM-41 materials characterized
by SiOH groups with enhanced acid strength, howev-
er, the conversion of PG starts immediately even for
the [Al]MCM-41/50 material with the lowest Al con-
tent. After 6 h, a PG conversion of 77.4% with a selec-
tivity to EM of 87.5% were obtained for this catalyst.
Upon further increasing the Al content, which is ac-
companied by an increase of the density and
strength of acid sites, a maximum PG conversion of
93.4% and a selectivity to EM of 96.9% were reached
for the [Al]MCM-41/15 material. At a PG conversion
of 50%, all [Al]MCM-41 materials had a similar EM se-
lectivity of 96.5–98.2%, which is higher than 90.2%
on zeolite deAl-HY.
Table 2. Densities of SiOH groups with enhanced acid strength, contents of octahe-
drally coordinated Al species, and catalytic performance of the MCM-41 materials with
different nSi/nAl ratios in the conversion of PG to EM.
Sample
Amount of Brønsted Ratio[a] CPG
SEM
TOF
S’EM
[d]
acid sites [mmolgꢀ1
]
AlVI/AlIV [%][b] [mol%][c] [hꢀ1
]
[mol%][e]
[Si]MCM-41
0
0
0
0
0
0
[Al]MCM-41/50 0.013
[Al]MCM-41/30 0.07
[Al]MCM-41/20 0.101
[Al]MCM-41/15 0.121
4/96
6/94
9/91
12/88
–
77.4 87.5
87.8 94.2
92.5 96.1
93.4 96.9
99.3
21
15.5
13
98.2
96.5
97.5
97.5
90.2
deAl-HY[18]
0.865
90
90
1.7
[a] Determined by 27Al MAS NMR spectroscopy. [b] Conversion of PG (0.4m) in ethanol
(1.25 mL) using 50 mg of catalyst at 363 K for 6 h in a batch reactor to yield EM.
[c] SEM =selectivity to EM. [d] TOF calculated based on the number of Brønsted acid
sites and PG conversion as an averaged value after 6 h reaction. [e] S’EM =selectivity to
EM at 50% PG conversion.
The turnover frequency (TOF) is often used to evaluate the
performance of active sites on the surface of a catalyst during
the reaction. The calculated TOFs are summarized in Table 2. If
we compare the TOFs of the reactants at the Brønsted acidic
surface sites (Table 2), the [Al]MCM-41 materials have an 8–58
times higher activity than zeolite deAl-HY. Clearly, the [Al]MCM-
41 materials show a higher activity for PG conversion com-
pared with zeolite deAl-HY, although the zeolite catalyst has
a much higher density of Brønsted acid sites with a much
higher acid strength.[14b] The possible reason for the different
catalytic performances of [Al]MCM-41 materials and the acidic
zeolite deAl-HY is that the mesopores (pore size of up to
ꢁ4 nm and pore volume of ꢁ1 cm3gꢀ1) of MCM-41 materials
offer much better reactant and product diffusion properties
than the micropores (pore size of 1.1 nm and pore volume of
ꢁ0.4 cm3 gꢀ1) of zeolites Y.
Figure 8. 13C CP MAS NMR spectra of a) [Si]MCM-41 and [Al]MCM-41 materi-
als with nSi/nAl of b) 50 and c) 15.
In addition, it is important to note that the weak acid sites
of [Al]MCM-41 are already able to convert PG to the desired
product EM. The much stronger acid strength of the acid sites
on zeolite deAl-HY, therefore, can have negative effects, such
as hindering the product desorption by strong adsorption on
acid sites. If we consider this argument, the differences in the
catalytic performances of the [Al]MCM-41 materials under
study can also be explained. The slightly higher acid strength
of the SiOH groups of [Al]MCM-41/15 in comparison with
those of [Al]MCM-41/50 (Figure 8) is not significant for the cat-
alytic activity (TOF). However, the relatively higher content of
octahedrally coordinated Al species in [Al]MCM-41/15 (Table 2,
column 3) partially act as Lewis acid sites. This may hinder the
reactant and product desorption, which leads to an improved
PG conversion to EM (Table 2, columns 4 and 5) and, on the
other hand, to a lower TOF value (Table 2, column 6).
15. In comparison, the 13C MAS NMR signals of acetone-2-13C
adsorbed at acidic bridging OH groups (SiOHAl), for example,
of SAPO-type zeolites, zeolite H-ZSM-5, and heteropoly acids
were observed at 217, 225, and 235 ppm, respectively.[14b]
Hence, the incorporation of Al into the silica network of MCM-
41 leads to an enhancement of the acid strength of neighbor-
ing SiOH groups, which is, however, weak in comparison with
zeolitic bridging OH groups.
The solid-state NMR investigations of the MCM-41 materials
under study demonstrate that, as already shown in previous
studies,[25] tuning the nSi/nAl ratio leads to changes in the densi-
ty and acid strength of surface OH groups and the occurrence
of a small amount of octahedrally coordinated Al species,
which partially may act as Lewis acid sites. These findings are
important for the catalytic properties of the [Al]MCM-41 mate-
rials under study.
Conclusions
[Al]MCM-41 acidic catalysts were successfully synthesized at
room temperature within 1 h. The obtained [Al]MCM-41 mate-
rials are characterized by a highly ordered hexagonal structure
with high specific areas of 1005–1246 m2 gꢀ1 and narrow pore
size distributions of 3.44–3.99 nm. 29Si and 27Al magic-angle
Catalytic conversion of PG
The catalytic performance of the MCM-41 materials was tested
for the conversion of PG to EM in ethanol at 363 K. [Si]MCM-
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were recorded by using a FESEM, Zeiss Ultra+. TEM images were
obtained by using a Philips CM120 BioFilter with samples that
were mounted on a carbon-coated copper grid by drying a droplet
of a suspension of the ground sample in ethanol. The surface area,
average pore size, and total pore volume of the MCM-41 materials
were determined by N2 adsorption–desorption isotherms by using
an Autosorb IQ-C system. Each sample (50 mg) was degassed at
423 K for 12 h under vacuum before the measurements and then
recorded at 77 K.
spinning (MAS) NMR spectroscopic studies confirmed the well-
condensed silica structure and the incorporation of Al atoms
1
into the silica framework. H MAS NMR investigations of am-
monia-loaded samples showed that the surface of siliceous
MCM-41 materials ([Si]MCM-41) is covered by SiOH groups,
which are not able to protonate ammonia, that is, of nonacidic
properties. The incorporation of Al species into the silica
framework of [Al]MCM-41 materials was found to be accompa-
nied by an enhanced acid strength of nearby silanol groups
able to protonate ammonia to ammonium. With the increasing
Al content of the [Al]MCM-41 materials (nSi/nAl ratios of 50 to
15), the density of the SiOH groups with enhanced Brønsted
acid strength increased from 0.013 to 0.121 mmolgꢀ1. Using
acetone-2-13C as a probe molecule for 13C MAS NMR investiga-
tions of these surface sites, the enhanced acid strength of
SiOH groups in [Al]MCM-41 materials could be supported but
was found to be significantly lower than that of bridging OH
groups (SiOHAl) in acidic zeolites.
Solid-state NMR investigations
For the 27Al and 29Si MAS NMR investigations, all samples were fully
hydrated by exposure to the saturated vapor of a Ca(NO3)2 solution
at ambient temperature overnight in a desiccator. Before the 1H
and 13C MAS NMR experiments, the samples in glass tubes were
dehydrated at 723 K for 12 h at a pressure lower than 10ꢀ2 bar.
These dehydrated samples were sealed in the glass tubes or direct-
ly loaded with ammonia or acetone-2-13C (99.5% 13C-enriched,
Sigma–Aldrich) by using a vacuum line. The loaded samples were
evacuated at 393 K for 1 h (for ammonia) or at RT for 2 h (for ace-
tone) to remove weakly physisorbed molecules. Subsequently, the
samples were transferred into the MAS NMR rotors under dry N2
inside a glove box.
In the catalytic studies, [Si]MCM-41 was not able to initiate
the PG conversion within a reaction time of 6 h, but the
[Al]MCM-41 materials could immediately start the PG conver-
sion to EM. The turnover frequencies (TOFs) of the reactants at
the catalytically active surface sites of the [Al]MCM-41 materials
hint at their 8–58 times higher activity than that of a dealumi-
nated zeolite Y used as a reference catalyst. The different cata-
lytic performances of the [Al]MCM-41 materials and the zeolite
Y is explained by the large size of the mesopores of the MCM-
41 materials, which offers much better reactant and product
diffusion properties than the micropores of zeolite Y.
1H, 27Al, and 13C MAS NMR spectroscopy was performed by using
a Bruker Avance III 400 WB spectrometer at resonance frequencies
of 400.1, 104.3, and 100.6 MHz, respectively, with a sample spin-
ning rate of 8 kHz using 4 mm MAS rotors. The spectra were re-
corded after single-pulse p/2 and p/6 excitation with repetition
1
times of 20 and 0.5 s for the H and 27Al nuclei, respectively. Quan-
titative 1H MAS NMR spectroscopy was performed by using a zeolite
H,Na-Y (35% ion exchanged) as an external intensity standard. 13C
cross-polarization (CP) MAS NMR spectra were recorded with a con-
tact time of 4 ms and a repetition time of 4 s. 29Si MAS NMR spec-
tra were recorded on the same spectrometer at a resonance fre-
quency of 79.5 MHz with a sample spinning rate of 4 kHz using
a 7 mm MAS rotor. For the 29Si MAS NMR spectroscopic studies,
single-pulse p/2 excitation, high-power proton decoupling, and
a recycle delay of 20 s were applied.
Experimental Section
Catalyst preparation
All chemicals used for the MCM-41 synthesis, such as an ammoni-
um hydroxide solution (28% NH3 in H2O), tetraethylorthosilicate
(TEOS, >98%), hexadecyltrimethylammoniumchloride solution
(CTACl, purum, ꢁ25% in H2O), and aluminum sulfate octadecahy-
drate (>98%) were obtained from Sigma–Aldrich. For the typical
preparation of [Si]MCM-41 material, CTACl was mixed with an am-
monium hydroxide solution and TEOS in a volume ratio of 1:1:1 in
demineralized water (500 mL) and stirred at RT to form a white gel.
For the synthesis of [Al]MCM-41 materials with different nSi/nAl
ratios, calculated amounts of aluminum sulfate were added to the
above gels. The obtained gels were completely mixed with vigo-
rous stirring for 1 h. The resulting solids were collected by filtra-
tion, washed with distilled water, and then in an oven at 353 K. Fi-
nally, the obtained MCM-41 materials were calcined at 823 K with
a heating rate of 1 Kminꢀ1 in the presence of static air for 6 h. The
nomenclature of [Al]MCM-41 is defined as [Al]MCM-41/x, where
x represents the nSi/nAl ratios of 15, 20, 30, and 50. Dealuminated
zeolite deAl-HY (nSi/nAl =5.4), used as a reference catalyst, was ob-
tained by steaming a zeolite H-Y (nSi/nAl =2.7) at 748 K for 2.5 h.[18]
Catalytic reaction
The conversion of PG (Sigma–Aldrich, >97%) to EM was used to
study the catalytic performance of the MCM-41 materials. Dealumi-
nated zeolite deAl-HY was used as a reference catalyst. This has
the same catalytic properties as dealuminated zeolite HY reported
in the literature as the best catalyst for this reaction.[12c] Therefore,
using zeolite deAl-HY and the mesoporous [Al]MCM-41 material
under the same reaction conditions is a better way to compare
their catalytic properties than to compare with the reported dealu-
minated zeolite HY. The MCM-41 catalyst (0.05 g) or zeolite deAl-
HY was employed and activated in a U-tube with N2 (50 mLminꢀ1
)
at 723 K overnight. After cooling the catalyst under flowing N2 gas,
it was transferred into a glass reactor with a volume of 15 mL. Sub-
sequently, an ethanol solution (1.25 mL) that contained PG (0.4m)
and octane (0.05m; as the GC internal standard) was added to the
glass reactor filled with activated catalyst. The reaction was per-
formed in the tightly closed glass reactor, which was heated in
a stirred oil bath at 363 K for 6 h. The reaction products were ana-
lyzed by using GC (Shimadzu GCMS-QP2010 Ultra equipped with
a Rtx-5 MS, 30 mꢁ0.25 mmꢁ0.25 mm) with MS detection and GC
(25QC3/BP1, 25 mꢁ0.32 mmꢁ5 mm) with flame ionization detec-
Characterization of textural and morphological properties
Small-angle XRD, SEM, and TEM were employed to characterize the
structure and morphology of the MCM-41 materials. The XRD stud-
ies were performed by using a Siemens D5000 with CuKa radiation
in the range of 1.5–108 with scanning steps of 0.028. SEM images
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Acknowledgements
This work was supported by the Early Career Research Scheme
and the Major Equipment Scheme from the University of Sydney.
M.H. thanks the Deutsche Forschungsgemeinschaft and the
Baden-Wꢁrttemberg Stiftung for financial support.
Keywords: acidity · aluminum · heterogeneous catalysis ·
mesoporous materials · nmr spectroscopy
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Revised: June 25, 2013
Published online on September 9, 2013
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ChemCatChem 2013, 5, 3889 – 3896 3896