Iron-Modified Mesoporous Silica as an Efficient Solid Lewis Acid Catalyst for the Mukaiyama Aldol Reaction

Article abstract of DOI:10.1021/acscatal.7b03485

Fe-MCM-41 and Fe-SBA-15, two different iron-containing mesoporous silicas, were successfully synthesized by a straightforward and versatile method using iron acetylacetonate as a metal precursor. pH adjustment with ammonia during the synthesis was found t

Full text of DOI:10.1021/acscatal.7b03485

Subscriber access provided by READING UNIV
Article
Iron-Modified Mesoporous Silica as Efficient Solid
Lewis Acid Catalyst for the Mukaiyama Aldol Reaction
Wan Xu, Thierry Ollevier, and Freddy Kleitz
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03485 • Publication Date (Web): 08 Jan 2018
Downloaded from http://pubs.acs.org on January 8, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 17
ACS Catalysis
1
2
3
4
5
6
7
8
9
Iron-Modified Mesoporous Silica as Efficient Solid Lewis Acid
Catalyst for the Mukaiyama Aldol Reaction
Wan Xua, Thierry Ollevier*a, Freddy Kleitz*a, b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
a.
Département de Chimie, Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V 0A6, Canada
b.
Department of Inorganic Chemistry ꢀ Functional Materials, Faculty of Chemistry, University of Vienna,
Währinger Straße 42, 1090 Vienna, Austria
ABSTRACT: Fe-MCM-41 and Fe-SBA-15, two different iron-containing mesoporous silicas were successfully synthesized by
a straightforward and versatile method using iron acetylacetonate as a metal precursor. pH adjustment with ammonia dur-
ing the synthesis was found to be an efficient way to improve the iron content. Physicochemical parameters of the iron-
containing mesoporous silicas were obtained by nitrogen physisorption measurements, and the coordination environment
of iron elements was validated by UVꢀvis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The sur-
face acidity was tested by using a series of Hammett indicators. To further distinguish the Lewis acid sites on the surface,
pyridine adsorption FT-IR method was implemented. These prepared nanoporous catalysts were screened in the Mukai-
yama aldol reaction as a model reaction catalyzed by a Lewis acid. The Lewis acid catalytic activity of the materials was fine-
tuned, and the corresponding aldol products were obtained in good yield and selectivity. More importantly, the solid cata-
lysts were very stable and could be reused at least nine times while maintaining the same catalytic activity.
KEYWORDS. Mesoporous silica, grafting, iron acetylacetonate, water-tolerant Lewis acid, Mukaiyama aldol reaction
their high surface area (800-1000 m2/g), uniform and tun-
¢
INTRODUCTION
able pore diameter (usually comprised between 2 and 15
nm), ease of surface-functionalization, and well-defined
particle morphology make them potential heterogeneous
catalysts for a larger variety of organic reactions.11-19
Homogeneous Lewis acid catalysts, such as AlCl3 and
TiCl4, are applied in the production of petroleum-derived
chemicals.1,2 However, the use of these Lewis acids has
some disadvantages. For example, strictly anhydrous con-
ditions are necessary for homogeneous Lewis acids because
of their easy decomposition or complete deactivation in
water. Furthermore, higher amounts of catalysts are
usually required for reactions catalyzed by conventional
Lewis acid catalysts, which can generate large aqueous ef-
fluents during the post-synthesis workup process3 and dif-
ficulty in the recycling of the catalysts for repeated use.
Therefore, it has become a target of great interest to design
efficient heterogeneous catalysts with Lewis acid sites for
solving these issues.
Since the discovery of the Mukaiyama aldol reaction 40
years ago, different types of catalysts including both homo-
geneous and heterogeneous catalysts have been studied.20-
26 In organic synthesis, the Mukaiyama aldol reaction is an
essential way ¥Ø £®•≠Ø≥•¨•£¥©∂•¨π ≥πÆ¥®•≥©∫• E-
hydroxycarbonyl compounds.20,27,28 A few Mukaiyama al-
dol-type reactions in the presence of iron compounds have
been described since the reaction is generally catalyzed by
a Lewis acid.21,22,29,30 Also, using some metal triflates, such
as Sc(OTf)3, as catalysts for Mukaiyama aldol reaction of
aldehydes and silyl enol ethers, Kobayashi and coworkers
showed their good catalytic reactivity and selectivity
compared to metal halides.31 On the other hand,
The most prominent and early developed examples of
heterogeneous catalysts comprising Lewis acid sites are
microporous zeolites.4-6 In organic chemistry, particularly,
Kumar found that TS-1 and Ti-b zeolites could be used as
active solid catalysts in the Mukaiyama aldol reaction in
the absence of water.7,8 However, low yields led to a spec-
ulation that the too small microporous environment in ze-
olites prevents the contact of the reactants with the Lewis
acid sites, decreasing the ability for the chemical transfor-
mation. Thus, silica-based materials with larger pore sizes,
such as ordered mesoporous MCM-41 and SBA-15 silicas,9,10
have been highlighted as promising alternatives. Besides,
heterogeneous
catalysts,
such
as
Ti-containing
mesoporous materials, also showed satisfactory yields as
high as 98% in solvent-free systems, which was much
higher than that of Ti-microporous zeolites.32 Furthermore,
according to a study on mesoporous Lewis acids obtained
by immobilizing
ACS Paragon Plus Environment

ACS Catalysis
Sc(OTf)3 on mesoporous silica functionalized with so-
Page 2 of 17
¢
EXPERIMENTAL SECTION
dium-benzenesulfonate,33,34 for the Mukaiyama aldol reac-
tions, the thus-prepared materials displayed a much higher
reactivity than that of homogeneous catalysts, e.g.,
Sc(OTf)3.
1
2
3
4
5
6
7
8
MCM-41 silica support. Mesoporous silica MCM-41 was
synthesized using tetraethylorthosilicate (TEOS, 98%, Al-
drich) as the silica source and cetyltrimethylammonium
bromide (CH3(CH2)15N(Br)-(CH3)3, CTAB, Sigma-Aldrich)
as a structure-directing agent under basic aqueous condi-
tions. A typical synthesis is as follows: 9.65 g of CTAB was
first dissolved in 480 g of distilled water with vigorous stir-
ring at 35 °C to form a homogeneous solution. 36.5 mL of
NH4OH (28%, Anachemia) was subsequently added into
the solution, and the temperature of the mixture was then
reduced to 25 °C. After 15 min, 40 g of TEOS was added,
and the mixture was left stirring for 2 h. The mixture was
subsequently aged at 90 °C under static conditions and the
aging time was 3 days. The obtained white precipitate was
filtered off and washed with distilled water. After being
dried at 100 °C for 24 h to remove the surfactant, mesopo-
rous silica MCM-41 was obtained after calcination at 550 °C
for 5 h.40, 41
In comparison to other metals, iron-based catalysts are
cheap, low in toxicity, stable, and easily accessible. Fur-
thermore, the Lewis acidity of Fe(III) is another significant
benefit for the use of this metal in catalysis. As an illustra-
tion on the applicability of iron in catalysis, a series of Fe-
containing porous materials used as catalysts in different
organic reactions have been disclosed. Dhakshinamoorthy
and co-workers synthesized Fe-containing porous metal-
organic-frameworks (MOFs), which were used as solid
,•∑©≥ °£©§ £°¥°¨π≥¥≥ ¶Ø≤ ¥®• ©≥Ø≠•≤©∫°¥©ØÆ Ø¶ D-pinene oxide
into camphonelal and isopinocamphone in the absence of
solvent.35 According to other studies, Fe-containing meso-
porous silica shows promises as an efficient catalyst for sev-
eral other organic reactions. For example, studies have
demonstrated that Fe-containing mesoporous materials
are very active catalysts for Friedel-Crafts alkylations,31
which are usually catalyzed by Lewis acids. Besides being
less sensitive to water (water-tolerant Lewis acids), they
demonstrate better recycling performance in contrast to
usual Lewis acids.36-38 However, surprisingly, Fe-containing
mesoporous silica has not yet been used as a Lewis acid
catalyst for Mukaiyama aldol reactions.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SBA-15 silica support. The synthesis procedure of SBA-
15 in this study was reported by Choi et al.42 In a typical
synthesis process, the amphiphilic block copolymer Plu-
ronic P123 was adopted as the structure-directing agent
and TEOS as the silica source. First, a homogeneous solu-
tion was obtained after dissolving 13.9 g of Pluronic P123
(Aldrich, Mw = 5800 g/mol) in a mixed solution of HCl (7.7
g, 37%, Anachemia) and distilled water (252 g) at 35 °C for
2 h. After addition of TEOS into the solution, the mixture
was kept stirring at the same temperature for 24 h, fol-
lowed by aging the mixture at 100 °C for 24 h. The precipi-
tated white solid was held in a mixture containing concen-
trated HCl and ethanol (100%, Fisher Scientific) for 30 min
and then recovered by filtration. The resulting mesoporous
silica SBA-15 was obtained by heating the product at 550 °C
for 5 h.
In the present study, in order to develop an efficient
Lewis acid Fe-containing mesoporous material for the
Mukaiyama aldol reaction, a series of Fe-MCM-41 (pore
size 3-4 nm) and Fe-SBA-15 (pore size 7-8 nm) with various
iron contents were prepared by a specific post-grafting
methodology, inspired by our previous work on Ti-SBA-15
oxidation catalysts.11,39 MCM-41 and SBA-15 were chosen as
two different silica supports, to probe pore size effects, and
iron(III) acetylacetonate was used as the metal precursor
in this procedure.39 The surface iron content and bulk iron
content were analyzed using X-ray photoelectron spectros-
copy (XPS) and inductively coupled plasma mass spec-
trometry (ICP-MS). Attenuated total reflectance infrared
(ATR-IR), powder X-ray diffraction (XRD), diffuse reflec-
tance UVꢀvisible spectroscopy (DR-UV-vis), and N2 phy-
sisorption measurements were also applied for the charac-
terization of the final materials. The Lewis acidity of the
prepared materials was studied by the Hammett indicator
method to substantiate the nature of the active sites in the
materials. Pyridine sorption probed by FT-IR analysis was
performed to assess the Lewis acid sites. The above pre-
pared Lewis acid heterogeneous catalysts were then tested
in the Mukaiyama aldol reaction of benzaldehyde with 1-
Preparation of Fe-MCM-41 and Fe-SBA-15 catalysts.
Impregnation of iron species into MCM-41 and SBA-15 with
different molar ratios was performed by a simple post-
grafting method reported earlier by our group39 with some
modifications. The method used Fe(acac)3 as a metal pre-
cursor and calcined mesoporous silica MCM-41 or SBA-15
as silica supports. The procedure was as follows: the given
amount of Fe(acac)3 and 100 mL of 1-propanol (certified
ACS grade, Fisher Scientific) were mixed at 45 °C under vig-
orous stirring until a homogeneous solution was formed. 1
g of calcined mesoporous silica MCM-41 or SBA-15 was
then added into the grafting solution and finely dispersed
by stirring. The mixture was continuously stirred at the
same temperature for 2 h. In some cases, after the addition
of the silica, concentrated ammonia (28%) was used to
modify the pH of the mixture to 10. The yellow colored
product was then filtered, washed with 1-propanol, and
dried at 100 °C in the air for 24 h. The resulting material
was calcined at 550 °C for 3 h. The resulting Fe3+-deposited
mesoporous silica samples with different iron contents are
trimethylsiloxycyclohexene
or
1-phenyl-1-(trime-
thylsiloxy)-propene at ambient temperature in an aqueous
environment, obtaining high yield and selectivity, and pre-
venting the oxidation of benzaldehyde into benzoic acid.
ACS Paragon Plus Environment

Page 3 of 17
ACS Catalysis
in the range of 200 nm-800 nm using a Varian Cary 500
designated as Fe-MCM-41 (X%), Fe-SBA-15 (X%), Fe-MCM-
1
2
3
4
5
6
7
8
41 (X%, pH = 10), and Fe-SBA-15 (X%, pH = 10) (X = initial
iron molar % of the synthesis solution).
spectrophotometer equipped with a Praying Mantis acces-
sory. X-ray photoelectron spectroscopy (XPS) spectra were
collected on a Kratos AxisꢀUltra electron spectrometer
(UK), µ≥©Æß ° ≠ØÆØ£®≤Ø≠°¥©£ !¨ +D 8-ray source (Al K =
1486.6 eV) at a power of 300 W and operated at a base pres-
sure of 5×10ꢀ10 Torr. Charge compensation was performed
using a low-energy electron beam perpendicular to the sur-
face of the samples. Survey spectra used for determining
the elemental composition were collected at a pass energy
of 160 eV. The bulk iron contents of Fe-SBA-15 and Fe-
MCM-41 were achieved by ICP measurement using a Var-
ian 800 MS spectrophotometer. Scanning electron micros-
copy (SEM) images and Energy-dispersive X-ray spectros-
copy (EDX) spectra were performed using a JEOL JSM-
840A instrument. Before the analysis, a small amount of
the sample was dispersed on the aluminum sample holder,
both side of which was coated with gold and palladium.
High-resolution transmission electron microscopy (HR-
TEM) and EDX analysis were recorded on a Cs-corrected
Titan G2 ETEM, which was operated at an accelerating
voltage of 300 kV. Energy dispersive X-Ray mapping im-
ages (EDX) were obtained from a square area 15 × 15 nm of
TEM images and the acquisition time was 1 s. For the anal-
ysis, the samples were crushed in the analytical reagent
grade acetone solvent, and a few drops of the resulting sus-
pension were placed and dried on a carbon film supported
on 300 mesh grids. For the catalytic tests, the products
were analyzed using 1H and 13C nuclear resonance spectros-
copy (NMR; Varian Inova NMR AS400 spectrometer 300
MHz, 400 MHz, 500 MHz). Chloroform-d was used as an
internal standard for 1( .-2 ꢁꢁH Ë cä^b ∞∞≠ꢂꢂ °Æ§ 13C NMR
ꢁH Ë ccä^_ ∞∞≠ꢂ. The syn/anti aldol ratio was determined
by the 1H NMR analysis of the crude product. High-resolu-
tion mass spectra (HRMS) were recorded on an Agilent
6210 ESI TOF (time-of-flight) mass spectrometer.
Passivation of mesoporous silica and iron-modified
mesoporous silica. A conventional silylating agent hexa-
methyldisilazane (HMDS, 99.9%, Aldrich) was used for the
passivation of the surface silanol groups. Iron-modified
mesoporous silica was firstly degassed at 150 °C in a vac-
uum oven overnight, and 0.5 g of the material was
dispersed in 25 mL of dry hexanes, 1 mL of HMDS was
successively added into the mixture under vigorous stirring.
The mixture was stirred for 24 h at room temperature. The
excess HMDS was removed by Soxhlet extraction refluxing
for at least 6 runs at 60 °C using dichloromethane (100 mL,
certified ACS grade, Fisher Scientific) as the solvent. The
obtained passivated materials were then dried in a 60 °C
oven for 12 h. The obtained materials were named as Fe-
MCM-41-HDMS and Fe-SBA-15-HDMS.43
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Catalyst characterization and testing
Low-angle powder X-ray diffraction (XRD) patterns
were obtained using a Rigaku Multiplex instrument with
#µ +D ≤°§©°¥©ØÆ (30 kV, 40 mA). Wide-angle X-ray diffrac-
tion (XRD) patterns of all samples were collected a Sie-
≠•Æ≥ -ا•¨ $a\\\ §©¶¶≤°£¥Ø≠•¥•≤ ∑©¥® #µ +D ≤°§©°¥©ØÆ ꢁP
= 0.15496 nm). The wide-angle XRD pattern of Fe2O3 as ref-
erence was acquired from the Powder Diffraction File 2
(PDF-2) database licensed by the International Center for
Diffraction Data (ICDD). The nitrogen adsorption-desorp-
tion isotherms were measured at -196 °C with a
Quantachrome
Autosorb-1
sorption
analyser
(Quantachrome Instruments, USA). Prior to the measure-
ment, non-passivated samples were outgassed under vac-
uum at 200 °C and passivated materials at 80 °C for at least
12 h. The specific surface area was calculated through
BrunauerꢀEmmettꢀTeller (BET) equation using the ob-
tained adsorption data at P/P0 values between 0.05 and 0.2.
The amount of nitrogen adsorbed at P/P0 = 0.95 was used
to estimate the total volume of the mesopores. Non-local
density functional theory (NLDFT) method was applied to
determine the pore size distributions. The selected NLDFT
kernel considers sorption of N2 on silica at -196 °C, assum-
ing the model of equilibrium isotherm based on the de-
sorption branch and cylindrical pore geometry.44 All the
data was extracted by using the Autosorb-1 1.55 software.
Attenuated total reflectance infrared (ATR-IR) spectra
were measured with a Nicolet Magna 850 Fourier trans-
form spectrometer with a liquid nitrogen cooled narrow
band mercury cadmium telluride (MCT) detector. Each
spectrum was gathered from the acquisition of 128 scans at
4 cmꢀ1 resolution varied from 4000 to 700 cmꢀ1 using a
HappꢀGenzel apodization. All samples were dried in vac-
uum oven at 80 °C overnight before the ATR-IR measure-
ments. Diffuse-reflectance UVꢀvis spectra were recorded
Titration of the Lewis acid solids with Hammett in-
dicators. Titration with different Hammett indicators was
performed to determine the surface acidity of the prepared
catalysts, according to protocols.45 In this method, about
0.1 g of dried solid was transferred into a glass vial, after the
addition of benzene, three or five drops of a 0.1% freshly
made solution of the indicators in benzene was added. Ac-
cording to the final color of the mixture, it could be readily
determined if a catalyst was in its basic or acidic form to all
indicators or had an H0 value lying between two adjacent
indicators. Before the color tests, all the samples were dried
and stored in a glass vial in a desiccator to avoid water
adsorption, which was able to cause a shift to a lower acid
strength and decrease the color intensity of the adsorbed
indicators. In addition, the titration solutions were sus-
pended in a quartz cuvette and were analyzed using Varian
Cary 500 spectrophotometer equipped with a Praying
Mantis accessory. The benzene solvent was used as a blank
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 17
reference. All the catalysts here were tested with three dif- pended in 3 mL of 5% pyridine solution in dry hexanes. Af-
1
2
3
4
5
6
7
ferent indicators. These Hammett indicators were chosen
based on their pKa value, which is presented in Table 1 as
well as the colors of the acidic and basic forms of the
indicators.
ter stirring for 1 h, the solvent of the mixture was
evaporated under reduced pressure, and the obtained sol-
ids were dried at 100 °C for 1 h afterward and finally exposed
at room temperature for 16 h. The final gathered powders
were analyzed using ATR-IR spectroscopy.
Table 1 Indicators used for acid strength measure-
ments
¢
RESULTS AND DISCUSSION
8
9
Indicators
Basic
color
Acidic
color
pKa
Synthesis and characterization of the materials.
Both Fe-MCM-41 and Fe-SBA-15 materials were
synthesized by a post grafting method using Fe(acac)3 as
the iron source. There are several advantages of using
Fe(acac)3 for the grafting procedure. In contrast to metal
nitrides, chlorides or other metal precursors, which can
easily generate an aggregation of the metal oxide forming
large clusters, the relatively higher stability of
acetylacetonate precursors can limit their hydrolysis and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Dimethyl yellow
Yellow
Yellow
Red
+3.3
+1.5
Phenylazodiphe-
nylphosphine
Purple
Dicinnamalacetone
Yellow
Red
-3.0
Pyridine adsorption FT-IR experiments. Prior to the
tests, the iron-containing mesoporous silica was freshly de-
gassed at 150 °C for 24 h to remove the physisorbed water.
50 mg of degassed Fe-MCM-41 and Fe-SBA-15 were sus-
condensation.39,46
Furthermore,
acetylacetonate
complexes are less moisture-sensitive.47,48 There are two
documented mechanisms of the interaction of acac
compounds with the silica support surface according to
Figure 1 N2 adsorptionꢀdesorption isotherms measured at 77 K (-196 °C) for Fe-MCM-41 (A) and Fe-SBA-15 (C), with vari-
ous iron contents and the corresponding NLDFT pore size distributions for Fe-MCM-41 (B) calculated from the adsorption
branch of the isotherm, and Fe-SBA-15 (D) calculated from the desorption branch of the isotherm.
ACS Paragon Plus Environment

Page 5 of 17
ACS Catalysis
condensation of the isotherms for Fe-MCM-41 and Fe-SBA-
previous report.39 First, the Fe(acac)3 compound could be
1
2
3
4
5
6
7
8
connected to the silica surface through hydrogen bonding.
Second, the ligand exchange between the acac and silanol
groups forming a covalent FeꢀOꢀSi bond could also exist.
Therefore, a high iron dispersion on the silica surface can
be obtained using such Fe(acac)3 precursors. These previ-
ous results showed that this method was highly reproduc-
ible and could be fine-tuned by just changing the molar ra-
tio of metal-to-silicon of the grafting gel or with the addi-
tion of ammonia during the synthesis procedure. In
present study, the initial Fe/Si molar ratios of the grafting
gel were 1%, 3%, 5%, 10% and 20%. Another batch of sam-
ples was synthesized using Fe(acac)3 as the precursor with
the same iron concentrations, except for the addition of
ammonia during the grafting process.
15 with ammonia treatment shifted towards lower relative
pressures (0.2ꢀ0.3 and 0.6ꢀ0.7, respectively) demonstrat-
ing smaller average pore size of these samples compared to
native mesoporous siliceous materials. A drastic decrease
of pore volume was also observed in these samples. Such a
phenomenon could be attributed to higher loading iron
species both on the external surface and inside the chan-
nels of mesoporous silica.
9
In order to investigate the location of the iron species
grafted in the silica, the surface Fe/Si molar ratio and bulk
Fe/Si molar ratio were measured by XPS and ICP-MS, re-
spectively, and are summarized in Table 2. One can ob-
serve that all the Fe/Si molar ratio in the obtained materi-
als are lower than the initial iron concentration, suggesting
only a fraction of the iron precursors could be inserted into
the silica matrix. It was assumed that the Fe/Si molar ratio
measured by XPS should be close to the bulk Fe/Si molar
ratio measured by ICP-MS, showing a uniform grafting of
the iron precursor on the surface of silica. As shown in Ta-
ble 2, only materials with lower Fe concentration in the in-
itial gel showed a well-dispersed iron loading on the silica.
Samples obtained with pH adjustment met a strong in-
crease in Fe/Si molar ratio values. This increase in iron con-
tent on the silica might be attributed to nano-sized iron
oxide particles on the external surface of silica when the
iron loading was too high.39 It was postulated previously
that the grafting of the iron acetylacetonate could depend
on its coordination stability.39 For Fe(acac)3, the interac-
tion between the precursor complex and the silanol groups
is believed to take place through single-layer ligand ex-
change that will be limited by the number of the silanol
groups available on the silica surface. Therefore, although
20 mol% of the precursor solution was used, both surface
and bulk Fe/Si molar ratio of the resulting Fe-MCM-41 and
Fe-SBA-15 seemed to reach a plateau. This plateau may be
probably attributed
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The N2 adsorption-desorption isotherms and the respec-
tive pore size distributions of the prepared Fe-MCM-41 and
Fe-SBA-15 samples are shown in Figure 1, and structural
properties of all the prepared materials are compiled in Ta-
ble 1. All the samples exhibited similar type IV isotherms
which are the typical feature of a well-defined mesoporous
structure.10 Compared to Fe-MCM-41, isotherms of Fe-
SBA-15 showed a well-resolved H1 hysteresis loop with a
steep increase in the adsorption volume within a relative
pressure (P/P0) between 0.6 to 0.8, indicating the highly
ordered mesostructure of the catalysts with large uniform
cylindrical channels.49 Both of the pure mesoporous silica
supports, MCM-41 and SBA-15, show high BET surface area
(1168 and 1067 m2/g) and pore volume (1.0 and 1.4 cm3/g),
respectively, which slightly decrease in the cases of Fe-
MCM-41 (5%, 10% and 20%) and Fe-SBA-15 (5%, 10% and
20%), after the insertion of iron into the mesopores of the
supports. Meanwhile, when the catalysts were synthesized
through the grafting procedure performed at pH = 10, using
the same iron concentration in the initial grafting gel, a sig-
nificant reduction of the surface area, pore volume, and
pore size was observed. Besides, the onset of the capillary
Table 2 Chemical composition and the structural properties of all the prepared materials
a
SBET
(m2/g)
dpb,c
Vpd
(cm3/g)
Entry
Catalyst
Fe/Sie (mol%)
Fe/Sif (mol%)
(nm)
1
MCM-41
1168
970
991
3.8
3.9
4.0
3.8
3.5
3.8
3.5
3.8
3.5
1.0
0.8
0.8
0.9
0.5
0.9
0.6
0.9
0.5
0
1
0
2
3
Fe-MCM-41 (1%)
0.4
0.7
0.6
4
Fe-MCM-41 (3%)
1
4
5
6
7
8
9
Fe-MCM-41 (5%)
1088
683
1071
834
1088
758
1
Fe-MCM-41 (5%, pH = 10)
Fe-MCM-41 (10%)
13
2
0.7
6
Fe-MCM-41 (10%, pH = 10)
Fe-MCM-41 (20%)
13
2.4
15
0.7
10
Fe-MCM-41 (20%, pH = 10)
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 17
10
11
SBA-15
1067
879
836
960
466
868
521
8.5
8.2
8.2
8.5
8.2
8.5
8.2
8.5
8.2
1.4
1.2
1.1
0
1
0
0.4
0.8
0.5
3
1
2
3
4
5
6
7
8
Fe-SBA-15 (1%)
12
13
14
15
16
17
18
Fe-SBA-15 (3%)
1
Fe-SBA-15 (5%)
1.3
0.9
7
Fe-SBA-15 (5%, pH = 10)
Fe-SBA-15 (10%)
0.9
1.2
0.9
1.0
0.9
1.7
11
0.7
7
Fe-SBA-15 (10%, pH = 10)
Fe-SBA-15 (20%)
9
742
563
1.7
15
0.9
10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fe-SBA-15 (20%, pH = 10)
a SBET, specific surface areas calculated from the values of the relative pressure ranging from 0.05 to 0.20;
b Pore diameter calculated using NLDFT method for the materials with MCM-41 as a silica support (NLDFT adsorption condi-
tions, i.e., adsorption branch);
c Pore diameter calculated using NLDFT method for the materials with SBA-15 as a silica support (NLDFT equilibrium condi-
tions, i.e., desorption branch);
d Vp, adsorbed volume obtained at P/P0 = 0.95;
e measured by X-ray photoelectron spectroscopy (XPS);
f measured by ICP-MS.
to the decreasing amount of available silanol groups on
the silica surface as most of which have already reacted
with the iron precursor. As it was previously observed in
the case of acac-substituted titanium alkoxide precursor,46
the interactions between similar titanium precursors and
the negatively charged silica surface SiꢀOꢀ under basic
conditions could be stronger than that of SiꢀOH and
higher angle region, suggesting that the d(100) spacing and
the hexagonal unit cell parameters a0, calculated by
2d(100)ꢃØ_á §•£≤•°≥•§ ∑©¥® ¥®• ©Æ£≤•°≥•§ ©≤ØÆ ¨Ø°§©Æßä 4®•
explanation suggested by Bouazizi52 was that a slight
framework compaction could occur because of the electro-
static attraction between Fe and lattice O atoms. Besides,
the diffraction peaks of Fe-MCM-41 (10%, pH = 10) laying
in the range of 3.5-5 broadened, indicating a slight decrease
of the structural regularity of the sample. This structural
deformation can be ascribed to the fact that small iron ox-
ide clusters or nanoparticles started to form in the meso-
pores or on the external surface of those catalysts upon pH
adjustment.53 The mesoporous materials were also charac-
terized by wide-angle XRD measurements, and the results
are shown in Figure S2 (SI). No clear diffraction peaks were
observed in the wide-angle XRD patterns for the samples
synthesized neither with nor without pH adjustment. This
phenomenon indicates the absence of iron oxide species
with large crystal domain size. The presence of smaller
crystals of size below the XRD detection limit (< 4-6 nm)
however cannot be excluded.53 The absence of diffraction
peaks may also be attributed to the amorphous nature of
metal layers coated on the walls of the pores, even after
calcination at high temperature. The high dispersion of
iron species on the mesoporous surface could also be
demonstrated by other methods. SEM images of Fe-MCM-
41 and Fe-SBA-15 (SI, Figure S3) showed no large crystal
(FeO)n particles on the external surfaces of the materials,
which is in agreement with the XRD results. Fe-MCM-41 (B
and C) shown in Figure S3 exhibited spherical-like particles
°≤صƧ ] Q≠ in size as pure MCM-41 material (A) while Fe-
SBA-15 (E and F) showed typical thread-like morphology as
SBA-15 (D). The elemental composition of Fe-MCM-41 and
Fe-SBA-15 was obtained by EDX analysis. No Fe signal
+
SiOH2 groups on the silica surface at lower pH.46 pH
adjustment was confirmed to be an efficient way to
improve metal loading. This conclusion was further proved
here with Fe. Indeed, when the pH of the initial precursor
solution gel was adjusted to 10 by adding concentrated am-
monia, the iron loading substantially increased to 15%,
compared to only 2.4% for the sample with no pH adjust-
ment. Under basic conditions, decomplexation of Fe(acac)3
could occur, and the surface of silica becomes negatively
charged. There are three possible iron cationic species
produced
from
the
hydrolysis
of
Fe(acac)3,
[Fe(acac)2(H2O)2]+, [Fe(acac)(H2O)4]2+ and [Fe(H2O)6]3+,50
which were then available for the ligand exchange with the
SiOꢀ on the silica surface. Also, the products of hydrolyzed
Fe(acac)3 showed a low coordination stability, which prob-
ably leads to a multi-layer grafting, forming small iron ox-
ide particles.39 Therefore, it could be expected that the pH
adjustment method can increase the iron loading.
Low-angle XRD measurements of Fe-MCM-41 and Fe-
SBA-15 are shown in Figure S1 (SI. The diffractograms of Fe-
MCM-41 and Fe-SBA-15 are consistent with that of their
corresponding bare mesoporous silicas.51, 9 Both diffracto-
grams showed three well-resolved peaks associated with (1
0 0), (1 1 0) and (2 0 0) reflections, which originated from
the highly ordered hexagonal arranged (p6mm) mesopores.
This observation indicated that the mesostructure of
MCM-41 and SBA-15 was preserved after the insertion of
the Fe species. However, the diffraction peaks shifted to
ACS Paragon Plus Environment

Page 7 of 17
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 17
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

Page 9 of 17
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 17
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

Page 11 of 17
ACS Catalysis
Effect of the surfactant (SDS). Table S2 (SI) shows the
solvents for organic reactions have been attracting increas-
1
2
3
4
5
6
7
8
ing interests because of environmental concerns. Table 5
presents the catalytic behavior of Fe-MCM-41 (50 mg, Fe/Si
= 20%, pH = 10) for the Mukaiyama aldol reaction of ben-
zaldehyde (0.2 mmol) with 1-phenyl-1-(trimethylsiloxy)-
propene (0.4 mmol) conducted in various typical organic
solvents.
effect of the surfactant for the Mukaiyama aldol reaction.
It should be noted that both a water-SDS system and the
water-ethanol system have previously been reported as ef-
fective media for Mukaiyama aldol reactions.23,74 Both SDS
and ethanol were adopted as a surfactant or a dispersing
agent, to enhance the solubility of the hydrophobic reac-
tants in aqueous media. Fe-MCM-41 (10%, pH = 10) and Fe-
SBA-15 (10%, pH = 10) were used as the catalysts under dif-
ferent conditions to compare the water-SDS and water-
ethanol system. As one can observe there were no noticea-
ble differences in the final yields, therefore, both were suit-
able media for the Mukaiyama aldol reaction. However, a
larger amount of the surfactant SDS, which was not envi-
ronmentally friendly, was always required.74 Considering
water-ethanol as a clean, environmentally benign medium
and the surfactant-free aqueous system compared with the
water-SDS system, all further experiments in this study
were carried out at room temperature in water-ethanol.
Table 5 Effect of the solvent for Mukaiyama aldol re-
action
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Entry
Solvent
Yield (%)c
syn/anti
1a
2b
3
H2O+SDS
H2O+ethanol
THF
86
90
ꢀ
90:10
79:21
ꢀ
Effect of the iron content (Fe-MCM-41, Fe-SBA-15) on
the reactivity. MCM-41 and SBA-15 with two different
pore sizes were chosen to probe the effects of the pore size
(MCM-41 and SBA-15) and the effects of iron content (Fe/Si
= 1~20 mol%) of the catalysts for the Mukaiyama aldol
reaction of both 1-phenyl-1-(trimethylsiloxy)-propene and
1-(trimethylsilyloxy)-cyclohexene with benzaldehyde were
examined by using catalysts with various iron contents. As
shown in Table 6, the lowest yields of 3-hydroxy-2-methyl-
1,3-diphenylpropan-1-one were obtained when using cata-
lyst Fe-MCM-41 (1%, 3%) and Fe-SBA-15 (1%, 3%) with low
iron content. It suggested that only a minimal amount of
iron species was inserted into the silica supports when the
iron concentration of the grafting gel was too low. These
low yields were also in agreement with the weak acidity of
these two catalysts, as demonstrated above in the surface
acidity tests. When using catalysts with higher iron loading
Fe-MCM-41 (5%) and Fe-SBA-15 (5%) exhibiting stronger
acidity, the yield substantially increased to 85% and 90%.
This increase in yields was due to the higher iron loading
of the catalysts, indicating an increasing number of active
Lewis acid sites in the catalysts. However, it seems that the
catalytic activity reached a plateau when the Fe/Si molar
ratio was 5%. The yield fluctuated around 90% when Fe-
MCM-41 (10%, 20%) and Fe-SBA-15 (10%, 20%) were used
as catalysts. Ammonia treatment was very efficient for in-
creasing the iron loading, yet it could not improve the cat-
alytic activity of the materials. Catalysts with ammonia
treatment apparently possess higher iron loading but
barely promote the yields. Similar results have been found
in the Mukaiyama aldol reaction of 1-(trimethylsilyloxy)cy-
clohexene and benzaldehyde (Table 7). All the catalysts ex-
cept Fe-MCM-41 (1%, 3%) and Fe-SBA-15 (1%, 3%) possess
high catalytic activities, with a yield of product around 87%.
4
MeCN
ꢀ
ꢀ
5
CH2Cl2
18d
45:55
ꢀ
6
DMC
ꢀ
Reaction condition: benzaldehyde, 0.2 mmol; 1-phenyl-1-(tri-
a
methylsiloxy)-propene, 0.4 mmol; water, 0.5 mL, SDS (11.5
mg); room temperature; time, 24 h. b Water 0.25 mL, ethanol
0.25 mL; room temperature; time, 24 h. c Yield of aldol product
after purification by silica gel column chromatography; d con-
version of benzaldehyde determined by 1H NMR. SDS, sodium
dodecyl sulfate; DMC, dimethyl carbonate.
The catalyst had no or weak catalytic activity when the
reactions were carried out in polar (MeCN and THF) and
non-polar (DMC and CH2Cl2) solvents, while the
hydrophobic reactants can be easily dissolved in these
solvents.70 On the contrary, the yield of the reaction carried
out in water-SDS and water-ethanol systems reached up
to 86% and 90%, respectively, suggesting they were both
suitable media for the Mukaiyama aldol reaction, which
was also evidenced by other researchers with different
catalysts.71,72 The surfactant SDS was essential because the
Mukaiyama aldol reaction involves hydrophobic mole-
cules requiring water, and thus the yield of the product
could be enhanced by the presence of SDS.70 Shintaku et al.
reported that Ti-SBA-15 showed higher catalytic activity for
the Mukaiyama aldol reaction conducted in water-THF so-
lutions, while no catalytic activity in pure THF solvent was
observed.70 As reported by Hatanaka et al., the transition
state formed between the aldehyde activated by the Lewis
acid and the silyl enol ether could be stabilized by water
molecules, which could also contribute to the dissociation
of trimethylsilyl groups, therefore, the reaction rate can be
increased.73
Table 6 Effect of the iron content in the Mukaiyama aldol reaction of 1-phenyl-1-(trimethylsiloxy)-propene with
benzaldehyde
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 17
Entry
1
Catalyst
Fe-MCM-41 (1%)
Fe/Si (mol %)a
Yield (%)b
50
syn/anti
77:23
83:17
1
2
3
4
5
6
7
8
0.4
0.7
0.6
4
2
Fe-MCM-41 (3%)
59
3
Fe-MCM-41 (5%)
85
70:30
85:15
4
Fe-MCM-41 (5%, pH = 10)
Fe-MCM-41 (10%)
Fe-MCM-41 (10%, pH = 10)
Fe-MCM-41 (20%)
Fe-MCM-41 (20%, pH = 10)
Fe- SBA-15 (1%)
90
5
0.7
6
82
62:38
80:20
70:30
88:12
75:25
79:21
71:29
86:14
80:20
80:20
70:30
79:21
6
89
9
7
0.7
10
90
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8
81
9
0.4
0.8
0.5
3
50
10
11
Fe- SBA-15 (3%)
56
Fe-SBA-15 (5%)
90
12
13
14
15
16
Fe-SBA-15 (5, pH = 10)
Fe-SBA-15 (10)
70
0.7
7
86
Fe-SBA-15 (10, pH = 10)
Fe-SBA-15 (20)
86
0.9
10
90
Fe-SBA-15 (20, pH = 10)
93
Reaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water 0.25 mL, ethanol 0.25 mL,
room temperature; time, 24 h. a Measured by ICP-MS; b the yield of aldol product after purification by silica gel column chroma-
tography.
However, no differences in the yield of reaction using cat-
alysts exhibiting different pore sizes were observed. It in-
It can be postulated that iron-modified mesoporous sili-
dicates that the range of pore sizes from 4 nm to 9 nm has
cas have a high catalytic activity for the Mukaiyama aldol
reaction, owing to their highly dispersed active Lewis acid
The effects of pore size beyond this range need further in-
sites. Also, the adsorption and diffusion of the reactants
about the same effect on the Mukaiyama aldol reaction.
vestigation.
could be favored by the mesoporous channels of the cata-
lysts. The results showed a distinct increase in yield as the
iron content was raised from 1% to 5%. Meanwhile, no sig-
nificant difference in yield was observed for the catalysts
with iron contents in the range of 5% to 20%. Therefore,
these catalytic experiments can also lead to a conclusion
that the active site density of catalysts with iron content of
5% was getting saturated, resulting in an unchanged cata-
lytic activity of the materials with increased iron content.
In addition, to compare, FeCl3 was chosen as model ho-
mogeneous Lewis acid catalyst and also applied in the
Mukaiyama aldol reactions. As shown in Table S3 (SI), sur-
prisingly low yields of 27% and 42% were obtained when
FeCl3 was used as catalyst, showing its distinctly lower cat-
alytic activity and selectivity in aqueous media compared
to the iron-modified mesoporous materials.
Table 7 Effect of the iron content for the Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene with ben-
zaldehyde
Entry
Catalyst
Fe/Si (mol%)a
Yield (%)b
syn/anti
80:20
78:22
86:14
78:22
88:12
1
Fe-MCM-41 (1%)
0.4
0.7
0.6
4
38
49
85
78
88
84
84
2
3
4
5
6
7
Fe-MCM-41 (3)
Fe-MCM-41 (5%)
Fe-MCM-41 (5%, pH = 10)
Fe-MCM-41 (10%)
Fe-MCM-41 (10%, pH = 10)
Fe-MCM-41 (20%)
0.7
6
78:22
86:14
0.7
ACS Paragon Plus Environment

Page 13 of 17
ACS Catalysis
8
9
Fe-MCM-41 (20%, pH = 10)
Fe-MCM-41 (1%)
10
0.4
0.8
0.5
3
87
29
56
85
81
79:21
77:23
75:25
88:12
76:24
78:22
77:23
80:20
75:25
1
2
3
4
5
6
7
8
10
11
Fe-MCM-41 (3%)
Fe-SBA-15 (5%)
12
13
14
15
16
Fe-SBA-15 (5%, pH = 10)
Fe-SBA-15 (10%)
0.7
7
81
Fe-SBA-15 (10%, pH = 10)
Fe-SBA-15 (20%)
85
85
82
0.9
10
9
10
11
Fe-SBA-15 (20, pH = 10)
Reaction conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water 0.25 mL, ethanol 0.25 ml, room
temperature; time, 3 h. a measured by ICP-MS; b yield of aldol product after purification by silica gel column chromatography.
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Substrate scope. To verify the versatility of our system, a
series of aldehydes were tested with 1-phenyl-1-(trime-
thylsiloxy)-propene to explore the scope of the Fe-modi-
fied mesoporous materials (i.e., Fe-SBA-15 (5 % and 20 %))
in the catalytic Mukaiyama aldol reactions. As shown in
Table 8, the corresponding aldol products were obtained
in moderate to excellent yields in all cases. We first inves-
tigated the reaction of benzaldehyde with 1-phenyl-1-(tri-
methylsiloxy)-propene and excellent yields of 86 % and
90% were observed. Correspondingly, similar yields of cou-
pling aldols were also achieved when 4-substituted benzal-
dehydes with either an electron-donating group (-OCH3,
entries 3 and 4) or an electron-withdrawing group (-Cl, en-
tries 5 and 6) were examined as substrates. Notably, high
yields around 90% were obtained when the less reactive 4-
cyanobenzaldehyde, 4-nitrobenzaldehyde and 2-nitroben-
zaldehyde were used as substrates (entries 7-12), although
a reversed selectivity was observed, i.e., the anti-isomers
were the major product. n-Butanal as a representative ex-
ample of an aliphatic aldehyde showed lower reactivity
than the benzaldehyde or benzaldehyde derivatives, while
the aldol products were formed in 49% and 53% yields.
However, a similar yield of 64 % was previously observed
by the Ollevier group using the same aliphatic aldehyde (n-
butanal) in pure water with an homogeneous catalysts.75
3
4
A
B
A
B
A
B
A
B
A
B
A
B
4-OMe
4-OMe
4-Cl
87
90
88
86
90
93
91
79:21
83:17
76:24
88:12
34:66
33:67
40:60
39:61
34:66
31:69
67:33
70:30
5
6
4-Cl
7
4-CN
8
4-CN
9
4-NO2
4-NO2
2-NO2
2-NO2
10
11
12
13
14
92
90
91
49
53
Reaction conditions: aldehyde, 0.2 mmol; 1-phenyl-1-(trime-
thylsiloxy)-propene, 0.4 mmol; water 0.25 mL, ethanol 0.25
mL, room temperature; time, 24 h. a Yield of aldol product af-
ter purification by silica gel column chromatography
Table 8 Catalytic performances of Fe-SBA-15 (5%)(A) and
Fe-SBA-15 (20%)(B) in Mukaiyama aldol reactions (sub-
strate scope)
Reusability experiment. One of the most significant
advantages of heterogeneous Lewis acid catalysts is that
they can normally be easily removed from the reaction me-
dium and recycled. For reusability tests, 400 mg of Fe-
modified mesoporous silica (e.g., Fe-SBA-15 (5%) and Fe-
SBA-15 (20%)) were used as catalysts for the water-medium
Mukaiyama aldol reactions between benzaldehyde and 1-
(trimethylsilyloxy)-cyclohexene, respectively. In each case,
the catalyst was recollected by filtration, washed with eth-
anol and subsequently dried overnight at 150 • in a vac-
uum oven. The amount of the reactants was adjusted to the
same scale according to the amount of catalyst. As pre-
sented in Table 9 and Table S4 (SI), the recycled catalysts
could be reused for at least eight times without significant
loss in the yield and no change in selectivity. It can thus be
Entry Cat.
Aldehyde
Rï
Yield
(%)a
syn/anti
1
A
B
H
H
86
90
80:20
70:30
2
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 17
Reagents and conditions: benzaldehyde, 0.2 mmol; 1-(trime-
suggested that no elution of Fe species was taking place
during the reactions and therefore, Fe-modified mesopo-
rous silica can be viewed as highly stable, active and reus-
able heterogeneous catalysts for the Mukaiyama aldol re-
action.
1
2
3
4
thylsilyloxy)cyclohexene, 0.4 mmol; water 0.25 mL, ethanol
0.25 mL, room temperature; time, 3 h. a Yield of aldol product
after purification by silica gel column chromatography.
5
6
7
8
9
¢
CONCLUSION
Table 9 Reusability of catalysts for Mukaiyama aldol re-
action of 1-(trimethylsilyloxy)-cyclohexene with benzal-
dehyde over Fe-SBA-15 (20%) catalyst
To summarize, a versatile and easy synthesis of iron-
modified mesoporous silica using acetylacetonate as a
metal precursor was demonstrated. It was shown from
XRD that no iron oxide species with large crystal domain
size were formed during the synthesis. Furthermore, only
a fraction of the iron precursor can be grafted on the silica,
with cationic iron species preferentially dispersed on the
surface of the silica support. The Lewis acidity of the pre-
pared catalysts was explored by a quantitative titration
method using various Hammet indicators. The results of
pyridine adsorption FT-IR revealed that the iron-modified
mesoporous materials exhibited a significant majority of
Lewis acid sites compared to Brønsted sites. Finally, in the
catalytic tests, the iron species deposited on mesoporous
silica could function as highly active and selective sites for
the Mukaiyama aldol reaction of various aldehydes with 1-
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Entry
1st
Yield (%)a
syn/anti
76:24
83:17
87
88
92
93
89
91
2nd
3rd
4th
5th
6th
7th
8th
9th
75:25
77:23
78:22
79:21
93
93
94
76:24
82:18
79:21
trimethylsilyloxycyclohexene
and
1-phenyl-1-(trime-
thylsiloxy)-propene. The experimental procedure was
straightforward, the ethanol-water solution used in this
study was a clean solvent system and environmentally be-
nign, no harmful organic solvents were used. The catalysts
could be easily recovered and successively reused in the
same reaction without loss of catalytic activity. Therefore,
it is believed that iron-modified mesoporous silica is a
highly active heterogeneous Lewis acid catalyst and could
be further applied not only for the Mukaiyama aldol reac-
tion but also for various environmentally-friendly chemi-
cal transformations in organic chemistry.
Reagents and conditions: benzaldehyde, 1 equiv.; 1-(trime-
thylsilyloxy)cyclohexene, 2 equiv.; water/ethanol = 1:1, room
temperature; time, 3 h. a Yield of aldol product after purifica-
tion by silica gel column chromatography.
Catalytic behavior of passivated Fe-MCM-41 and Fe-
SBA-15. The catalytic activities of non-passivated materials
and passivated materials were compared to verify a possi-
ble effect of the silanol groups present on the surface of
silica for the activity (Table 10). High yield of the reaction
could be obtained with the presence of all catalysts, among
which the passivated materials showed a slightly better
catalytic activity with the yield rising from 84 to 89% and
from 85 to 92% for Fe-MCM-41 and Fe-SBA-15, respec-
tively. It was expected that the silylating layer on the silica
surface could provide strong surface hydrophobicity,
which may facilitate the diffusion of the organic reactants,
thus increasing possibility of contact between the hydro-
phobic reactants and the active Lewis acid sites, leading to
a higher yield of the final product.33,70
¢
AUTHOR INFORMATION
Corresponding Author
*E-mails: thierry.ollevier@chm.ulaval.ca,
freddy.kleitz@univie.ac.at
ORCID
Freddy Kleitz: 0000-0001-6769-4180
Thierry Ollevier: 0000-0002-6084-7954
Notes
The authors declare no competing financial interest.
Table 10 Catalytic test of Fe-MCM-41-HMDS (20%) and
Fe-SBA-15-HMDS (20%) catalyst
Author Contributions
The manuscript was written through contributions of all
authors. All authors have approved the final version of the
manuscript.
Catalyst
Fe-MCM-41
Yield (%)a
syn/anti
86:14
84
89
85
92
Fe-MCM-41-HDMS
Fe-SBA-15
73:27
Funding Sources
The authors thank the Fonds de recherche du QuébecꢀNature
et technologies (FRQNT), the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
80:20
80:20
Fe-SBA-15-HDMS
14
ACS Paragon Plus Environment

Page 15 of 17
ACS Catalysis
(19) Zhang, F.; Liang, C.; Wu, X.; Li, H. Angew. Chem. Int. Ed. 2014,
53, 8498-8502.
Centre in Green Chemistry and Catalysis (CGCC) for financial
support. F.K. also acknowledge the University of Vienna (Aus-
tria) for additional support.
1
2
3
4
5
6
7
8
(20) Narasaka, K.; Soai, K.; Mukaiyama, T. Chem. Lett. 1974, 3, 1223-
1224.
(21) Kitanosono, T.; Ollevier, T.; Kobayashi, S. Chem. Asian J. 2013,
8, 3051-3062.
ASSOCIATED CONTENT
ꢄ Supporting Information
(22) Ollevier, T.; Plancq, B. Chem. Commun. 2012, 48, 2289-2291.
(23) Kobayashi, S.; Nagayama, S.; Busujima, T. Tetrahedron 1999, 55,
8739-8746.
Supporting Information including characterization of the
catalysts (low-angle and wide-angle XRD patterns of the
materials, SEM images as well as the EDX spectra, UV-vis
diffuse reflectance spectra of the catalysts, and Lewis acidity
studies) and experimental procedure for the catalytic tests
(effect of the SDS surfactant, Mukaiyama aldol reaction using
FeCl3 as catalyst, reusability tests and all the NMR spectra of
the products) are available free of charge on the ACS Publica-
tions via the Internet at http://pubs.acs.org.
(24) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35, 209-217.
9
(25) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. J. Am. Chem. Soc.
2008, 130, 5854-5855.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(26) Zhang, F.; Wu, X.; Liang, C.; Li, X.; Wang, Z.; Li, H. Green Chem.
2014, 16, 3768-3777.
(27) Narasaka, K.; Soai, K.; Aikawa, Y.; Mukaiyama, T. Bull. Chem.
Soc. Jpn. 1976, 49, 779-783.
(28) Saigo, K.; Osaki, M.; Mukaiyama, T. Chem. Lett. 1976, 5, 163-164.
(29) Jankowska, J.; Mlynarski, J. J. Org. Chem. 2006, 71, 1317-1321.
¢
ACKNOWLEDGMENT
(30) Rodríguez-Gimeno, A.; Cuenca, A. B.; Gil-Tomás, J.; Medio-
Simón, M.; Olmos, A.; Asensio, G. J. Org. Chem. 2014, 79, 8263-8270.
The authors thank Prof. Ryong Ryoo and Mr. Jongho Han
(KAIST and IBS, Daejeon, Republic of Korea) for their help
with low-angle XRD and high-resolution transmission
electron microscopy analyses.
(31) Kobayashi, S. Synlett 1994, 689-701.
(32) Garro, R.; Navarro, M. T.; Primo, J.; Corma, A. J. Catal. 2005,
233, 342-350.
(33) Zhang, F.; Liang, C.; Chen, M.; Guo, H.; Jiang, H.; Li, H. Green
Chem. 2013, 15, 2865-2871.
(34) Chen, M.; Liang, C.; Zhang, F.; Li, H. ACS Sustain. Chem Eng.
2014, 2, 486-492.
¢
References
(1) Corma, A.; García, H. Chem. Rev. 2003, 103, 4307-4366.
(35) Dhakshinamoorthy, A.; Alvaro, M.; Chevreau, H.; Horcajada, P.;
Devic, T.; Serre, C.; Garcia, H. Catal. Sci. Tech. 2012, 2, 324-330.
(2) Kündig, E. P.; Saudan, C. M. Transition Metal Lewis Acids: From
Vanadium to Platinum. In Lewis Acids in Organic Synthesis; H.
Yamamoto, Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim,
2008, pp. 597-652.
(36) He, N.; Bao, S.; Xu, Q. Appl. Catal., A: General 1998, 169, 29-36.
(37) Choudhary, V. R.; Jana, S. K.; Mamman, A. S. Microporous
Mesoporous Mater. 2002, 56, 65-71.
(3) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686-
694.
(38) Jiang, Y.; Lin, K.; Zhang, Y.; Liu, J.; Li, G.; Sun, J.; Xu, X. Appl.
Catal., A: General 2012, 445ꢀ446, 172-179.
(4) Satterfield, Charles N. Heterogeneous catalysis in industrial
practice, 2nd ed; McGraw-Hill: New York, 1991.
(39) Berube, F.; Khadraoui, A.; Florek, J.; Kaliaguine, S.; Kleitz, F. J.
Colloid Interface Sci. 2015, 449, 102-114.
(5) Corma, A.; Nemeth, L. T.; Renz, M.; Valencia, S. Nature 2001, 412,
423-425.
(40) Grün, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K.
Microporous Mesoporous Mater. 1999, 27, 207-216.
(6) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199-218.
(7) Sasidharan, M.; Kumar, R. J. Catal. 2003, 220, 326-332.
(8) Sasidharan, M.; Kumar, R. Catal. Lett. 1996, 38, 251-254.
(41) Kleitz, F.; Schmidt, W.; Schüth, F. Microporous Mesoporous
Mater. 2003, 65, 1-29.
(42) Choi, M.; Heo, W.; Kleitz, F.; Ryoo, R. Chem. Commun. 2003,
1340-1341.
(9) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.;
Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552.
(43) Staub, H.; Guille-Nicolas, R.; Even, N.; Kayser, L.; Kleitz, F.;
Fontaine, F. G. Chem. Eur. J. 2011, 17, 4254-4265.
(10) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck,
J. S. Nature 1992, 359, 710-712.
(44) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105,
6817-6823.
(11) Kim, T. W.; Kim, M. J.; Kleitz, F.; Nair, M. M.; Guillet-Nicolas,
R.; Jeong, K. E.; Chae, H. J.; Kim, C. U.; Jeong, S. Y. ChemCatChem 2012,
4, 687-697.
(45) Walling, C. J. Am. Chem. Soc. 1950, 72, 1164-1168.
(46ꢂ "•≤µ¢•á &äâ .Ø®°©≤á "äâ +¨•©¥∫á &äâ +°¨©°ßµ©Æ•á 3ä Chem. Mater.
2010, 22, 1988-2000.
(12) Zakharova, M. V.; Kleitz, F.; Fontaine, F. G. Dalton Trans 2017,
46, 3864-3876.
(47ꢂ +µŠ¥≤Ø∑≥´©á 0äâ #®≠©•¨°≤∫á ,äâ $∫©•≠¢°™á 2äâ #Øبá 0äâ 6°Æ≥°Æ¥á
E. F. J. Phys. Chem. B 2005, 109, 11552-11558.
(13) Perego, C.; Millini, R. Chem. Soc. Rev. 2013, 42, 3956-3976.
(14) Duan, L.; Fu, R.; Zhang, B.; Shi, W.; Chen, S.; Wan, Y. ACS Catal.
2016, 6, 1062-1074.
(48ꢂ #®≠©•¨°≤∫á ,äâ +µŠ¥≤Ø∑≥´©á 0äâ $∫©•≠¢°™á 2äâ #Øبá 0äâ 6°Æ≥°Æ¥á
E. F. Microporous Mesoporous Mater. 2010, 127, 133-141.
(15) Das, S.; Asefa, T. ACS Catal. 2011, 1, 502-510.
(49) Kleitz, F.; Bérubé, F.; Guillet-Nicolas, R.; Yang, C.-M.;
Thommes, M. J. Phys. Chem. C 2010, 114, 9344-9355.
(16) Rodriguez-Gomez, A.; Holgado, J. P.; Caballero, A. ACS Catal.
2017, 7, 5243-5247.
(50) Capdeillayre, C.; Mehsein, K.; Petitto, C.; Delahay, G. Top. Catal.
2016, 59, 901-906.
(17) Shang, L.; Bian, T.; Zhang, B.; Zhang, D.; Wu, L. Z.; Tung, C. H.;
Yin, Y.; Zhang, T. Angew. Chem. 2014, 126, 254-258.
(51) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge,
C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.;
McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992,
114, 10834-10843.
(18) Shen, D.; Chen, L.; Yang, J.; Zhang, R.; Wei, Y.; Li, X.; Li, W.;
Sun, Z.; Zhu, H.; Abdullah, A. M. ACS Appl. Mater. Interfaces 2015, 7,
17450-17459.
15
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 17
(52) Bouazizi, N.; Ouargli, R.; Nousir, S.; Slama, R. B.; Azzouz, A. J.
Phys. Chem. Solids 2015, 77, 172-177.
(63) Yamashita, T.; Hayes, P. Appl. Surf. Sci. 2008, 254, 2441-2449.
1
2
3
4
5
6
7
8
(64) Benesi, H. J. Am. Chem. Soc. 1956, 78, 5490-5494.
(65) Corma, A. Chem. Rev. 1995, 95, 559-614.
(53) Cuello, N. I.; Elías, V. R.; Rodriguez Torres, C. E.; Crivello, M.
E.; Oliva, M. I.; Eimer, G. A. Microporous Mesoporous Mater. 2015, 203,
106-115.
(66) Yurdakoç, M.; Akçay, M.; Tonbul, Y.; Yurdakoç, K. Turk. J.
Chem. 1999, 23, 319-328.
(54) Yuan, E.; Wu, G.; Dai, W.; Guan, N.; Li, L. Catal. Sci. Tech. 2017,
7, 3036-3044.
(67) Ikemoto, M.; Tsutsumi, K.; Takahashi, H. Bull. Chem. Soc. Jpn.
1972, 45, 1330-1334.
(55) Lu, Y.; Zheng, J.; Liu, J.; Mu, J. Microporous Mesoporous Mater.
2007, 106, 28-34.
(68) Drushel, H. V.; Sommers, A. Anal. Chem. 1966, 38, 1723-1731.
(69) Parry, E. P. J. Catal. 1963, 2, 371-379.
(56) Samanta, S.; Giri, S.; Sastry, P.; Mal, N.; Manna, A.; Bhaumik,
A. Ind. Eng. Chem. Res. 2003, 42, 3012-3018.
9
(70) Shintaku, H.; Nakajima, K.; Kitano, M.; Hara, M. Chem.
Commun. 2014, 50, 13473-13476.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(57ꢂ 0œ≤•∫æ2°≠Â≤•∫á *äâ +°∞¥•©™Æá &äâ "≤²£´Æ•≤á !ä J. Catal. 2003, 218,
(71) Jankowska, J.; Paradowska, J.; Mlynarski, J. Tetrahedron Lett.
2006, 47, 5281-5284.
234-238.
(58) Hensen, E. J. M.; Zhu, Q.; Hendrix, M. M. R. M.; Overweg, A.
R.; Kooyman, P. J.; Sychev, M. V.; van Santen, R. A. J. Catal. 2004, 221,
560-574.
(72) Kitanosono, T.; Kobayashi, S. Adv. Synth. Catal. 2013, 355, 3095-
3118.
(73) Hatanaka, M.; Morokuma, K. J. Am. Chem. Soc. 2013, 135, 13972-
13979.
(59ꢂ #®≠©•¨°≤∫á ,äâ +µŠ¥≤Ø∑≥´©á 0äâ $∫©•≠¢°™á 2äâ #Øبá 0äâ 6°Æ≥°Æ¥á
E. F. Appl. Catal., B 2006, 62, 369-380.
(74) Shintaku, H.; Nakajima, K.; Kitano, M.; Ichikuni, N.; Hara, M.
ACS Catal. 2014, 4, 1198-1204.
(60) Reddy, E. P.; Davydov, L.; Smirniotis, P. G. The J. Phys. Chem.
B 2002, 106, 3394-3401.
(75) Lafantaisie, M.; Mirabaud, A.; Plancq, B.; Ollevier, T.
ChemCatChem 2014, 6, 2244-2247.
(61) De Stefanis, A.; Kaciulis, S.; Pandolfi, L. Microporous
Mesoporous Mater. 2007, 99, 140-148.
(62) Gervasini, A.; Messi, C.; Carniti, P.; Ponti, A.; Ravasio, N.;
Zaccheria, F. J. Catal. 2009, 262, 224-234.
16
ACS Paragon Plus Environment

Page 17 of 17
ACS Catalysis
TOC
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
17
ACS Paragon Plus Environment