Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles: Chiral Recognition and Implications in Asymmetric Olefin Epoxidation

Article abstract of DOI:10.1002/adsc.201500441

In this work a mesoporous nanocomposite material comprising helical chiral channels and with embedded magnetic iron oxide nanoparticles in its Si-based MCM-41 type framework was synthesized. Afterwards, a bipyridine derivative was grafted to the inner surface used as ligand to coordinate a molybdenum(II) precursor complex. The Mo loading was found to be 2.42 wt% Mo, corresponding to 0.26 mmol_Mo g^-1. The successful preparation of this helical mesoporous material was evidenced by an extensive characterization process using powder XRD, SEM/TEM analysis, FT-IR and Raman spectroscopies, TGA analysis and also by SQUID measurements. Selective adsorption of enantiopure D- and L-phenylalanine shows that the material has a preference for the D- over the L- enantiomer. This provides evidence that the channels are chiral (although the particles are not) and that it is useful for chiral recognition applications. It also sets the explanation to the good results achieved in the selective epoxidation of olefins. The resulting material was tested as catalytic precursor in the epoxidation of cis-cyclooctene, styrene, R-(+)-limonene and trans-hex-2-en-1-ol, using tert-butyl hydroperoxide (tbhp) as oxygen source. The catalytic studies show that the synthesized material yields selectively the desired epoxides of the tested substrates with very good results, especially at higher temperatures and using toluene as solvent. The major achievement of the catalyst was an outstanding stereocontrol of the reaction products imposed by the confined space of the helical chiral channels. Therefore these materials offer an important contribution to asymmetric catalysis and to fields where chiral recognition is a relevant concept.

Full text of DOI:10.1002/adsc.201500441

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DOI: 10.1002/adsc.201500441
Helical Channel Mesoporous Materials with Embedded Magnetic
Iron Nanoparticles: Chiral Recognition and Implications in
Asymmetric Olefin Epoxidation
Cristina I. Fernandes,a Gavin B. G. Stenning,b James D. Taylor,b Carla D. Nunes,a,*
and Pedro D. Vaza,b,*
a
CQB, Departamento de Química e Bioquímica, Faculdade de CiÞncias da Universidade de Lisboa, Campo Grande, Ed.
C8, 1749-016 Lisboa, Portugal
Fax: (+351)-217-500-088; phone: (+351)-217-500-876; e-mail: cmnunes@ciencias.ulisboa.pt
ISIS Neutron & Muon Source, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, U.K.
b
E-mail: pedro.duarte-vaz@stfc.ac.uk
Received: May 4, 2015; Revised: August 17, 2015; Published online: October 14, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500441.
Abstract: In this work a mesoporous nanocomposite the good results achieved in the selective epoxida-
material comprising helical chiral channels and with tion of olefins. The resulting material was tested as
embedded magnetic iron oxide nanoparticles in its catalytic precursor in the epoxidation of cis-cyclooc-
Si-based MCM-41 type framework was synthesized. tene, styrene, R-(+)-limonene and trans-hex-2-en-1-
Afterwards, a bipyridine derivative was grafted to ol, using tert-butyl hydroperoxide (tbhp) as oxygen
the inner surface used as ligand to coordinate a mo- source. The catalytic studies show that the synthe-
lybdenum(II) precursor complex. The Mo loading sized material yields selectively the desired epoxides
was found to be 2.42 wt% Mo, corresponding to of the tested substrates with very good results, espe-
0.26 mmolMo gÀ1. The successful preparation of this cially at higher temperatures and using toluene as
helical mesoporous material was evidenced by an ex- solvent. The major achievement of the catalyst was
tensive characterization process using powder XRD, an outstanding stereocontrol of the reaction products
SEM/TEM analysis, FT-IR and Raman spectroscop- imposed by the confined space of the helical chiral
ies, TGA analysis and also by SQUID measure- channels. Therefore these materials offer an impor-
ments. Selective adsorption of enantiopure d- and l- tant contribution to asymmetric catalysis and to
phenylalanine shows that the material has a prefer- fields where chiral recognition is a relevant concept.
ence for the d- over the l- enantiomer. This provides
evidence that the channels are chiral (although the Keywords: asymmetric catalysis; diastereoselectivity;
particles are not) and that it is useful for chiral rec- epoxidation; heterogeneous catalysis; mesoporous
ognition applications. It also sets the explanation to materials; molybdenum
Introduction
size and shape selectivity in porous inorganic catalysts
is pivotal in a wide range of industrial and biological
Fostering the development of catalysts that yield se- applications. In this respect enzymes are the model
lective processes is the aim of much research world- systems to pursue, therefore mimicking these biologi-
wide in both academia and industry. Particularly, the cal factories in terms of exploring confined space en-
quest for selective olefin epoxidation processes is very vironments was the driving force to prepare nano-
important as epoxides are relevant building blocks structured mesoporous materials with helical mor-
across many areas. This is even more relevant when phology.[1] This opened a new field of research due to
stereoselectivity is a must and not an option, such as their potential applications such as chiral selective
in the pharmaceutical industry.
separation, recognition and enantioselective cataly-
Several options are available, metal-centred cata- sis.[2–5] Such materials share with regular mesoporous
lysts being one of the most used ways to conduct such materials (e.g., MCM-41, SBA-15, HMS) the same
transformations in both homogeneous and heteroge- physical properties – wide pore openings and narrow
neous catalysis. One advantage of the latter is that pore size distribution – along with a high surface area
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and the ability to prepare single-site active species
across the high surface area for catalytic applications.
In addition, these helical materials offer a tailored
confined space which is capable of modulating reac-
tions that are controlled by spatial constraints based
on confined space as recently reported.[6,7] At the end
of the reactions the heterogeneous catalysts can be
separated by filtration or centrifugation. More recent-
ly the advent of magnetic separation has triggered the
development of magnetically separable catalysts
which are easier to extract than using the processes
mentioned earlier. Such catalysts have been devel-
oped with the aim of combining the properties of
high surface area provided by porous materials with
those from magnetic nanoparticles for an easy separa-
tion. Some examples rely on the introduction of the
Scheme 1. Synthesis of (ClCO)2-bpy ligand for functionaliza-
tion of magMSh material.
Afterward,
a
bypiridine
(bpy)
derivative
magnetic cores after the host porous material is pre- (Scheme 1) was used to coordinate the Mo(II) centre
pared.[8,9] Although efficient, this top-down approach after being grafted to the inner silanol surface of the
may result in pore blockage and reduced catalytic ac- material.
tivity. More recently, Thielꢁs group developed
Grafting of bpy ligand was straightforward by react-
a bottom-up methodology to overcome such limita- ing (ClCO)2-bpy with a suspension of magMSh in ace-
tions.[10] This was used to prepare mesoporous materi- tonitrile. Afterward, the Mo(II) centre was introduced
als with previously prepared silica-coated magnetic by suspending the magMSh-bpy material in dichloro-
nanoparticles embedded in the framework. The strat- methane and then adding the [MoI2(CO)3(CH3CN)2]
egy proved to be adequate as the resulting catalyst precursor complex (Scheme 2). This afforded
proved to be active and stable.
magMSh-bpy-Mo material and according to elemental
Continuing our research on the development of analysis, the Mo content was found to be 2.42 wt%
stable, active and selective catalysts for olefin epoxi- Mo, corresponding to 0.26 mmolMo gÀ1. CHN elemen-
dation,[6,7] we describe in the present work the prepa- tal analysis of magMSh-bpy revealed values of 5.91%
ration of a magnetically separable mesoporous mate- C, 0.69% H, and 1.19% N. Given the N content, this
rial with helical channels. To accomplish this we fol- result also shows that the loading of bpy derivative
lowed the bottom-up strategy adopted by Thiel and inside the pores is 0.43 mmolgÀ1. This shows that the
the resulting material was derivatized with a Mo com-
plex anchored on the surface of the helical mesopo-
rous silica with embedded magnetic iron oxide nano-
particles. This material was subsequently evaluated
for its catalytic potential in olefin epoxidation reac-
tions. As will be discussed throughout this work the
strategy proved correct and the use of a material with
helical channels led to enhanced stereoselectivity of
products arising from the confined space shape of the
channels.
Results and Discussion
The helical mesoporous magnetic nanocomposite
(magMSh) was prepared using an entropy-driven pro-
cedure with achiral cationic surfactant template and
ammonia in the presence of silica-coated magnetic
nanoparticles (MNP-Si) that were co-condensed with
tetraethoxysilane (TEOS), as described in the litera-
ture.[10] In this particular case, we used cetyltrimethy-
lammonium bromide (C16TAB) and ammonia as sur-
factant and co-surfactant, respectively, according to
a literature procedure.[1]
Scheme 2. Synthesis and derivatization of magMSh material
with ligand bpy and MoII species.
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
Mo content is lower than the ligand loading reaching rods with diameter ranging between 150 to 200 nm
a ligand-to-metal ratio of 1.6 meaning that most prob- with helical morphology (Figure 1b); the images also
ably all Mo complexes are coordinated to the bpy illustrate the appearance of periodic lattice fringes
ligand.
(Figure 1b) along the rods, indicating the presence of
All materials were characterized by DRIFT, helical channels within the rods, as reported
powder XRD, SEM and TEM. Sorption/desorption before.[1,6] The periodicity of the lattice fringes indi-
N2 isotherms were also carried out for textural param- cates that there was a regular environment of the
eters estimation. All spectroscopy and textural char- channels where reactants and products may diffuse
acterization features discussed in the following lines along without much disturbance. The TEM images
were found to be in agreement with related hybrid also show darker spots which correspond to the mag-
matrix mesoporous materials.[1,6,10] The helical matrix netic nanoparticles. These nanostructures are embed-
of the as-prepared magMSh material was confirmed ded within the silica framework and evenly distribut-
by SEM and TEM measurements. SEM measure- ed across it. This makes clear that the synthetic proce-
ments (Figure 1a) show rod-shaped particles with heli- dure is adequate to prepare such mixed magnetic
cal morphology which are decorated with smaller mesoporous materials with a homogeneous distribu-
round-shaped particles. The images provide evidence tion of the nanoparticles throughout the framework
that the mesoporous silica rods have lengths ranging (Figure 1b and c). Figure 1d also provides evidence
from 100s of nm to 2 mm; the smaller round-shaped that pore openings show a regular size distribution as
structures are the magnetic iron nanoparticles at- evidenced by the pore size distribution curve dis-
tached to the silica framework. TEM images show cussed later in this work.
Figure 1. SEM image (a) of magMSh material showing the morphology of the particles along with smaller size structures due
to the magnetic iron oxide nanoparticles. TEM images are shown for the particles with helical channels (b) and magnified
view perpendicular (c) and parallel (d) of the helical channels to the electron beam. Arrows in Figure b denote lattice fringes
indicating the presence of helical channels with periodicity.
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The local structure was also confirmed by powder
X-ray diffraction measurements which agree with the
published data and also make it possible to verify that
the materials magMSh-bpy and magMSh-bpy-Mo are
mesostructured.[1,6] All resulting materials were of
good quality according to the powder X-ray diffrac-
tion (XRD) patterns (Figure 2). The XRD patterns of
the helical mesoporous magnetic material magMSh
exhibit four reflections indexed to a hexagonal cell as
(100), (110), (200) and (210) in the 1.8–108 2q range
(Figure 2a). In the 10–808 2q range a characteristic
peak for the magnetite core (Fe3O4) was detected at
around 35.58, indexed to the (311) plane, revealing
that magnetite nanoparticles are present as part of
the porous silica framework and that their crystallini-
ty was retained during the mesostructure formation
(Figure 2b). This confirms observations made by SEM
and TEM microscopy as discussed above. For materi-
al magMSh the d100 value for reflection (100) was esti-
mated to be 4.03 nm, corresponding to a lattice con-
p
stant of a=4.66 nm (a=2d100/ 3). Materials magMSh-
bpy and magMSh-bpy-Mo obtained after subsequent
stepwise functionalization with bpy and Mo(II), still
show three reflections although with a slight deviation
of the position maxima toward higher 2q values as
compared to magMSh. For magMSh-bpy material, the
d100 value is 3.95 nm, with a corresponding lattice con-
stant of a=4.56 nm; for magMSh-bpy-Mo the values
are, respectively, d100 =3.91 nm and a=45.6 nm. The
average crystallite size of the magnetic iron nanopar-
Figure 2. Powder XRD of magMSh, magMSh-bpy and
magMSh-bpy-Mo materials: (a) the small angle region (28<
ticles, estimated using the Debye–Scherrer equation, 2q<108) accounting for the mesoporous material; (b) the
wide-angle region showing the peaks from the iron oxide
magnetic nanoparticles. In both cases, most relevant peaks
are shown with the corresponding indexation.
was found to be 22 nm, based on the peak indexed to
the (311) plane at 2q=35.58.
The powder XRD patterns of the magMSh materi-
als were found to provide similar results to those al-
ready reported by us for related systems.[6,10–13]
The data from all materials are collected in Table 1,
Nitrogen sorption/desorption studies at 77 K were
summarizing the relevant textural properties of the also performed and have revealed that the magMSh
materials. The observed peak intensity reduction was sample exhibits a reversible type IV isotherm (Fig-
common to all materials, being even more significant ure 3a), typical of mesoporous solids (pore width be-
in the materials with the Mo core. This was not due tween 2 nm and 50 nm, according to IUPAC).[16] The
to a crystallinity loss, but rather to a decrease in X- calculated textural parameters (SBET and VP) of these
ray scattering contrast between the silica walls and materials (Table 1) agree with literature data.[17,18] The
the pore-filling material. This has been observed for capillary condensation/evaporation step in pristine
other types of materials and was well documented in magMSh sample appears in the 0.30–0.45 relative
the literature.[14,15]
pressure range, while the sharpness of this step re-
Table 1. Textural parameters of host and composite materials, from powder XRD data and N2 isotherms at 77 K, for all ma-
terials.
Material
2q [8]
d100 [nm]
a [nm]
SBET [m2 gÀ1][a]
DSBET [%]
VP [cm3 gÀ1]
DVP [%][b]
dBJH [nm]
magMSh
magMSh-bpy
magMSh-bpy-Mo
2.18
2.23
2.26
4.03
3.95
3.91
4.66
4.56
4.51
481
475
344
–
0.60
0.56
0.46
–
2.74
2.74
2.67
À1
À7
À28
À23
[a]
Surface area variation relative to parent magMSh.
Total pore volume variation relative to parent magMSh.
[b]
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
Figure 3. Nitrogen adsorption studies of magMSh and
magMSh-bpy-Mo materials at 77 K: (a) isotherm; (b) pore
size distribution curves. In the isotherm, both the adsorption
(closed symbols) and desorption (open symbols) are shown.
Figure 4. FT-IR spectra of magMSh, magMSh-bpy and
magMSh-bpy-Mo materials: (a) full mid-IR range; (b)
zoomed image of the nC O modes region from magMSh-
ꢀ
bpy-Mo material.
flects a uniform size distribution. The functionalized
material magMSh-bpy isotherm revealed a slightly with silica (MNP-Si) on the surface of the mesoporous
lower N2 uptake, accounting for the small decrease in material was probed by the presence of characteristic
both SBET (À1%) and VP (À7%). For the magMSh- bands of this type of nanoparticles, namely, an intense
bpy-Mo material, both SBET and VP parameters de- band at 451 cmÀ1 due to the nFe O stretching mode
À
crease more drastically by 28% and 23%, respectively. and also a broad and intense band that appears at
These results are in agreement with the p/p0 coordi- 1082 cmÀ1 due to the nSi O modes. After grafting of
À
nate decrease in the isotherm inflection points after bpy ligand, affording material magMSh-bpy, the
post-synthesis treatments.[19] Furthermore, the pore DRIFT spectrum shows an overall similar profile do-
size distribution (PSD) curve maxima (Figure 3b), de- minated by the adsorptions of the host mesoporous
termined by the BJH method – dBJH – for magMSh material. Additionally, new bands were detected evi-
based materials change from 2.74 nm to 2.67 nm on dencing the ligand presence within the pores. The
going from magMSh to magMSh-bpy-Mo (Table 1).
1630 cmÀ1 band can be related with the nC=N mode
Diffuse reflectance infrared spectroscopy (DRIFT) in addition to the OH bending mode of the matrix.
was used to characterize the hybrid materials, shown Grafting of the bpy ligand was also monitored by
in Figure 4. The DRIFT spectrum of the magMSh ma- probing its nC=O mode. Neat (ClCO)2bpy ligand
terial was typical of a silicate displaying a broad band shows this mode at 1730 cmÀ1, which agrees with the
in the 3600–2600 cmÀ1 range due to hydrogen-bonded presence of the COCl group.[21] After grafting, this
silanol groups. Another important feature is the band mode is redshifted to 1714 cmÀ1, which is compatible
at ca. 1630 cmÀ1 due to OH bending modes, while the with the expected transformation resulting in the for-
intense broad band at 1239–950 cmÀ1 is assigned to mation of silyl esters, as documented in the litera-
the asymmetric stretching vibration modes of the ture.[6,22] In this way, the absence of the 1730 cmÀ1
[20]
À À
mesoporous framework (nSi O Si). The immobili- band in the magMSh-bpy material strongly suggests
zation of magnetic iron oxide nanoparticles coated that the bpy ligand is grafted to the inorganic matrix
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in a bipodal fashion. After binding the molybdenum
complex [MoI2(CO)3(CH3CN)2], which affords mate-
rial magMSh-bpy-Mo, several changes/additional
bands in the corresponding DRIFT spectrum are de-
tected. The most striking feature is the observation of
three bands at 2049, 2007 and 1928 cmÀ1 assigned to
ꢀ
the nC O modes (Figure 4b). These bands are shifted
relatively to the [MoI2(CO)3(CH3CN)2] precursor
complex (observed at 2072, 2016 and 1921 cmÀ1),[7]
being indicative of metal binding to the bpy ligand.
Moreover, the fact that their shapes are symmetrical
is indicative that the Mo cores are isolated enough,
thus interacting through H-bonding. Additionally, the
ꢀ
bands due to the nC N vibrational modes from the
acetonitrile (CH3CN) ligands are not found, indicat-
ing that such ligands have been replaced by the im-
mobilized ligand.
Figure 5. TGA profiles of magMSh, magMSh-bpy and
magMSh-bpy-Mo materials.
Figure 5 shows the thermogravimetric analyses
(TGA) of the synthesized materials magMSh, ylation of the mesoporous structure (clearly evi-
magMSh-bpy and magMSh-bpy-Mo. All materials denced in magMSh). Both magMSh-bpy and magMSh-
show some high temperature mass loss due to hydrox- bpy-Mo materials experience mass losses (less pro-
Figure 6. SQUID magnetometry of both magMSh-bpy and magMSh.
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
nounced in the latter) corresponding to physisorbed quired on warming the system. The right hand side of
water until 398 K. The magMSh-bpy derivative dis- Figure 6 corresponds to the magnetic hysteresis loop.
plays a 3-step degradation process occurring between The hysteresis loops show similar saturation values,
393 K and 833 K, with overall weight loss of 13.5%. but a larger effect between magMSh-bpy and magMSh
Taking into account the high temperature mass loss occurs between the splitting of FC and ZFC measure-
observed in magMSh due to dehydroxylation, this ments of each.
leaves a net weight loss in magMSh-bpy material of
Figure 7 shows again magnetometry of magMSh-
12% that can be assigned to loss of the ligand. This bpy-Mo and magMSh-bpy-Mo (2) where the latter
corresponds to 0.49 mmolgÀ1 of the ligand inside the refers to an oxidized version after being exposed to
mesoporous
magnetic
nanocomposite
pores catalytic conditions. Figure 6 and Figure 7 both show
(magMSh) being in agreement with the total amount soft magnetic behaviour from the hysteresis loops
of bpy (0.43 mmolgÀ1) obtained from elemental anal- with little or no magnetic anisotropy. With differing
ysis. Similar results were obtained for the magMSh- anisotropies this could have a very profound effect on
bpy-Mo material.
this type of measurement.[23] It can be noted that the
Figure 6 (top) shows the SQUID magnetometry for saturation value and ZFC/FC measurements for
magMSh-bpy and magMSh. The left hand side of the magMSh-bpy-Mo give an increased value for when it
Figure is the temperature dependence of the samples has been oxidized, but is still smaller than those dis-
both field-cooled (FC) and zero field-cooled (ZFC). played in Figure 6. These results are consistent with
The magnetic field was set to 100 Oe for temperature those reported in the literature for a regular mesopo-
dependence and the data shown in Figure 6 were ac- rous magnetic nanocomposite.[10]
Figure 7. SQUID magnetometry of magMSh-bpy-Mo and magMSh-bpy-Mo (2).
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The derivatized mesoporous magnetic nanocompo- alysis with trans-hex-2-en-1-ol and R-(+)-limonene as
site material (magMSh-bpy-Mo) was tested as a cata- substrates using a chiral GC column.
lyst precursor for the epoxidation of two sets of sub-
In the case of cis-cyclooctene, magMSh-bpy-Mo ma-
strates. The first one comprises simple olefins, such as terial catalyzed selectively the oxidation of the sub-
cis-cyclooctene and styrene, while the second one in- strate to the corresponding epoxide, without forma-
cludes multifunctional olefins, namely, trans-hex-2-en- tion of any by-products (Table 2, entries 1–4). All cis-
1-ol and R-(+)-limonene. All reactions were carried cyclooctene epoxidation reactions gave high yields of
out using tert-butyl hydroperoxide (tbhp) as oxygen the corresponding epoxide (Figure 8a), namely, be-
donor, in different solvents, namely, acetonitrile, tolu- tween 72 and 100%; however, the maximum yield
ene and decane, at 353 K, 383 K and 393 K.
(100%) was obtained at 383 K using toluene as sol-
Special focus was dedicated to the stereoselectivity vent (Table 2, entry 3).
of products due to the specific helical features of the
Styrene conversion is very efficient for all the
matrix, as already explored in a previous work.[6] In tested temperatures and independent of the solvent.
this way, we analyzed the presence of the enantio- Despite this, selectivity for the epoxide is low, mean-
mers/diastereomers in the products resulting from cat- ing that the major product is benzaldehyde. This
Table 2. Catalytic olefin epoxidation using magMSh-bpy-Mo as catalyst.
Entry
Reaction[a]
Solvent
Temperature [K]
Conversion[b] [%]
Selectivity[b,c] [%]
de [%][d]
1
2
3
4
acetonitrile
toluene
353
353
383
393
91
80
100
72
100
100
100
100
–
–
–
–
decane
5
6
7
8
acetonitrile
toluene
353
353
383
393
97
100
81
6[e]
–
–
–
–
14[e]
31[e]
22[e]
decane
87
9
acetonitrile
toluene
353
353
383
393
32
98
93
79
100
100
100
100
70[f]
70[f]
70[f]
74[f]
10
11
12
decane
13
14
15
16
acetonitrile
toluene
353
353
383
393
98
79[g]
93[g,i]
100[j]
83[g]
>95[h]
>95[h]
>95[h]
>95[h]
97[i]
100[j]
98
decane
[a]
All reactions were carried out in the presence of 200 mol% oxidant (tbhp) and 1 mol% Mo catalyst.
Calculated after 24 h, unless otherwise stated.
Calculated as “yield of epoxide”/“conversion””100%.
Determined by chiral GC.
Benzaldehyde formed as by-product.
Value refers to diastereomeric excess (de) of the E form.
a-Hydroxy ketone formed as by-product.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
Although values of “100%” were measured, due to experimental error affecting measurements “>95%” is reported in-
stead.
[i]
After 6 h of reaction.
After 2 h of reaction.
[j]
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
conversions. For example, almost 100% of conversion
after only 2 h of reaction using toluene as solvent at
353 K and 383 K (Table 2, entries 14 and 15). The se-
lectivity was also higher for all the tested tempera-
tures. However, once again the use of toluene as sol-
vent revealed to be the best method since it was pos-
sible to obtain a very high selectivity for the epoxide
(above 95%) only after 2 h of reaction (Table 2, en-
tries 14 and 15). Kinetic profiling of trans-hex-2-en-1-
ol epoxidation shows that the catalysts present faster
and higher conversion profiles when the solvent of re-
action was toluene, for both temperatures, 353 K and
383 K (Figure 8b).
For example, in R-(+)-limonene epoxidation we ob-
served that the catalyst follows the same trend, being
sensitive to solvent and temperature. The reactions
made with toluene gave higher conversion and selec-
tivity for both temperatures (353 K and 383 K) as
shown in Figure 9.
Figure 8. Kinetics of epoxidation of (a) cis-cyclooctene and
(b) trans-hex-2-en-1-ol using magMSh-bpy-Mo as catalyst
obtained with different screening reaction.
product is formed through an oxidative cleavage
mechanism from over-oxidation of styrene epoxide
which further reacts to yield benzaldehyde. Formation
of benzaldehyde in styrene oxidation experiments has
already been reported in the literature for catalysts
featuring magnetic separation capability.[24–26]
Figure 9. Conversion for R-(+)-limonene and selectivity for
the corresponding endocyclic epoxide in the presence of
magMSh-bpy-Mo material across different reaction condi-
tions.
R-(+)-Limonene is a substrate with two unsaturat-
Recyclability of the catalyst was evaluated across
ed C=C bonds, meaning that two different epoxides, all experiments (see Table 3). In the case of cis-cyclo-
the endo- and exo-cyclic isomers, are possible. In the octene epoxidation the catalyst suffers severe deacti-
present tests, the former was the favoured epoxide by vation, with conversion decreasing from 100% to
the catalyst for all tests (Table 2, entries 9–12). It was 25% over 3 recycling experiments. The same trend
possible to observe that in all reactions limonene ep- was observed in R-(+)-limonene and trans-hex-2-en-
oxide was the sole product of R-(+)-limonene oxida- 1-ol epoxidations as well.
tion with good results (above 32%), as evidenced in
Table 2 (entries 9–12).
The exception to deactivation seems to be styrene
oxidation, whose conversion levels were not affected
These results were accompanied by very high sub- to a great extent across recycling cycles and with little
strate conversion, between 93% and 98%, when the influence on the reaction kinetics.
solvent used was toluene (Table 2, entries 10 and 11).
To complete our studies we decided to evaluate the
Even when the solvent is acetonitrile epoxide yields stereoselectivity of the magMSh-bpy-Mo catalyst in
are very high, despite conversions being modest the catalysis with R-(+)-limonene and trans-hex-2-en-
(Table 2, entry 9).
1-ol due to the induced chirality of the magnetic mes-
It was also possible to convert trans-hex-2-en-1-ol oporous material, whose results are shown in Table 3.
into its epoxide in all the tests made with very high In the R-(+)-limonene epoxidation the stereoselectiv-
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Cristina I. Fernandes et al.
Table 3. Recyclability tests for the catalytic olefin epoxidation reactions using magMSh-bpy-Mo as catalyst.
Entry
Reaction[a]
Solvent
Temperature [K]
Conversion[b,c] [%]
Selectivity[b,c,d][%]
1
2
3
4
acetonitrile
toluene
353
353
383
393
91/77/62
80/57/31
100/30/25
72/39/23
100/100/100
100/100/100
100/100/100
100/100/100
decane
5
6
7
8
acetonitrile
toluene
353
353
383
393
97/94/94
6/30/49[e]
14/21/25[e]
31/32/33[e]
22/94/51[e]
100/100/100
81/100/96
87/100/98
decane
9
acetonitrile
toluene
353
353
383
393
32/19/9
100/100/100
100/100/100
100/100/100
100/100/100
10
11
12
98/85/39
93/74/59
79/44/27
decane
13
14
15
16
acetonitrile
toluene
353
353
383
393
98/78/96
79[f]/87[f]/78[f]
93[f,g]/93[f]/100
100[h]/86[f]/89[f]
83[f]/88[f]/86[f]
97[g]/98/97
100[h]/97/97
98/98/94
decane
[a]
All reactions were carried out in the presence of 200 mol% oxidant (tbhp) and 1 mol% Mo catalyst.
Calculated after 24 h, unless otherwise stated.
[b]
[c]
[d]
[e]
[f]
Multiple values correspond to recycling runs (1 st–3rd).
Calculated as “yield of epoxide”/“conversion””100%.
In all experiments benzaldehyde formed as by-product.
Where “selectivity” <100%, a-hydroxy ketone formed as by-product.
After 6 h of reaction.
[g]
[h]
After 2 h of reaction.
ity is high (between 70% and 74% of de). The best the mesoporous helical host material. As discussed in
result was with decane as solvent at 393 K (Table 3, our previous report,[6] this arises from the described
entries 9–12). Epoxidation of trans-hex-2-en-1-ol fol- channel left-handedness, as stated in the original liter-
lows the same trend. In this case, epoxidation of the ature report[1] (Fig. 3 in ref.[1]) and the fact that the
substrate was achieved with stereoselectivity above grafted bpy ligand may lie at saddle-shaped sites in
95% (accounting for experimental errors, although in the walls of the host material. This gives rise to
all cases “100%” was measured) across all tested con- a non-planar position of both N-donor atoms chelat-
ditions (temperature and solvent), as reported in ing Mo, yielding a confined environment imposed by
Table 3 (entries 13–16). These results are in line with the rigid inorganic matrix, which leads to enhanced
our previous work.[6] It also means that the matrix stereocontrol over the catalytic reaction products.
backbone doping with magnetic iron oxide nanoparti-
In the catalytic experiments stereocontrol was ach-
cles did not influence the quality of the helical chan- ieved using the heterogeneous catalysts. Stereocontrol
nels as compared to the original non-magnetic MSh arose from the channel left-handedness similar to the
material.[1,6] Despite this, these results confirm that related materials described in the literature (cf. Fig. 3
stereoselectivity was due to the rigid confined envi- in ref.[1]). This yields a confined environment imposed
ronment leading to substrate hindrance on approach- by the rigid inorganic matrix, leading to enhanced ste-
ing the catalytic active centre in the confined space of reocontrol over the catalytic reaction products. Ac-
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
cording to the authors who reported the original syn- vious observations made by Han et al. in the synthesis
thesis of MSh materials,[1] based on TEM observations, description of related materials where TEM data
such helical materials have left-handed channels. De- showed that MSh materials are left-handed.[1] Other
spite this, further evidence at the molecular level is authors have also addressed the issue of the nature of
required to make a proof of concept on chiral recog- the chirality of the inorganic matrix in the litera-
nition for the magMSh materials reported here, and to ture.[28]
provide evidence that the chiral imprinting is not af-
This confined-space chiral concept imposed by the
fected by introduction of the magnetic nanoparticles. rigid inorganic backbone has been previously raised
Chiral recognition was accomplished by adopting and discussed to explain similar stereoselectivity en-
the literature procedure of Fernandes et al.[6] To eval- hancements,[29–35] where reactants are allowed to ap-
uate chirality of a given inorganic mesoporous materi- proach in a single orientation. Results in Table 2 sup-
al matrix we have adsorbed enantiopure d-phenylala- port this concept by the observation of high stereose-
nine and l-phenylalanine into the porous system of lectivity achieved for trans-hex-2-en-1-ol and R-(+)-li-
magMSh materials.[6] Results on the adsorption of monene. In addition, data obtained from the selective
enantiopure phenylalanine (d and l, with similar di- adsorption of enantiopure amino acids support that
mensions to the cis-cyclooctene, styrene and limonene MSh materials can be used in processes where chiral
substrates used) shows that magMSh materials selec- recognition is valuable, such as catalysis or separation
tively adsorb much more d-phenylalanine (ca. 1.4- technology.
fold) after 24 h, as evidenced in Figure 10, showing
a clear preference for d-phenylalanine over l-phenyl-
alanine. This was found to be reproducible over 5 Conclusions
batches (each one run in triplicate) and strongly sup-
ports that the matrix channels are chiral. These results In this work we report the successful preparation of
agree with previously reported results where similar a mesoporous magnetic nanocomposite with embed-
discernment of enantiomers was accomplished by sim- ded silica-coated iron oxide magnetic nanoparticles
ilar amounts.[6,27] This experiment confirms that the (22 nm average size) in a chiral mesoporous silica
observed left-handedness chirality of the channels was matrix by a one-pot direct synthesis methodology.
preserved, compared to the original materials without This approach was based on the use of an entropy-
the magnetic capability,[1,6] explaining the catalytic driven procedure with an achiral cationic surfactant
performance and the chiral recognition achievements. template Then after the functionalization of the inner
The small error bars show that the synthesis proce- surface with a bipyridine-based ligand a subsequent
dure of such materials was reproducible based on coordination to a [MoI2(CO)3] fragment was accom-
a simple entropy-driven mechanism as discussed in plished. The mesoscopic order and the helical pitch of
the literature by Han et al.[1] Therefore it strongly sug- the final materials depend on the use of ammonia.
gests that the channels in the inorganic matrix of Additionally, the solid materials were, qualitatively
magMSh materials are really chiral with a focus on and quantitatively, characterized by means of differ-
one predominant form. This study also confirms pre- ent spectroscopic techniques, which allowed a full de-
piction of the modified silica surface, corroborating
the incorporation of the ligand into the inorganic heli-
cal rod mesoporous channels. The resulting material
magMSh-bpy-Mo was found to be an adequate cata-
lyst for conducting olefin epoxidation reactions. We
also noticed strong solvent effects affecting catalytic
performance (mainly product selectivity). This issue is
currently being assessed using neutron techniques.
The application of mesoporous materials with
chiral helical channels conjugated with embedded
magnetic nanoparticles in olefin epoxidation catalysis
is reported for the first time. Compared to previous
literature reports this material, holds all the proper-
ties of regular MSh materials without embedded nano-
particles.[1,6,36]
The performance in asymmetric catalysis was found
to be outstanding across a range of olefins. In terms
of enantiocatalytic activity, the effect of the helical
channel was found to have a positive influence on the
diastereoselectivity of the epoxidation catalysis of dif-
Figure 10. Adsorption kinetic profiles of pure l-phenylala-
nine and d-phenylalanine on magMSh materials. The plot
represents the average of 5 different synthesis batches each
one tested in triplicate (error bars represent deviation from
the average measured value).
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Cristina I. Fernandes et al.
System (MPMS), capable of performing both AC and DC
magnetometry both as a function of field and temperature.
The magnetometer was able to measure in the temperature
range from 400 K down to 1.8 K with applied magnetic
fields of up to Æ7 T.
ferent substrates. Other authors have reported similar
achievements in other reactions with related materi-
als.[37] The observed stereoselectivity arises from the
confinement effect imposed by the rigid inorganic
chiral channels backbone. The introduction of the
magnetic property allows an easy separation of the
catalyst from reaction media and was found to be ad-
vantageous, as the catalysts are stable across several
catalytic cycles and recycling easiness is welcome.
Synthesis of Ligand (ClCO)2bpy
Synthesis of (ClCO)2bpy was by a two-step procedure found
in the literature;[21] the carboxylic acid was first prepared
and then the acyl chloride was obtained through reaction
with thionyl chloride.
Step 1: 4,4’-dimethyl-2,2’-bipyridine (Me2bpy) (800 mg,
4.34 mmol) was dissolved in concentrated H2SO4 (10 mL)
while cooling to 273 K. Once dissolved and cooled down,
CrO3 (2.6 g, 26.1 mmol) was very slowly added over
a period of 1 h. The resulting greenish slurry and was then
heated to 348 K for 4 h. After that time heating was turned
off and the slurry was left stirring overnight at room temper-
ature. The reaction was then quenched with ice/water. The
green precipitate was separated by centrifugation and
washed with deionized water (5”20 mL). The green powder
was resuspended in deionized water and KOH was added
under vigorous stirring until alkaline pH (~9) was reached.
A blue residue was formed which was separated by centrifu-
gation and washed with water (3”20 mL). The combined
water extracts and the solution were acidified with concen-
trated HCl to pH ~1 making the carboxylic acid to be pre-
cipitated. The resulting white powder was centrifuged,
washed with water, methanol and ether and then dried over-
night in a vacuum oven at 313 K.
Step 2: SOCl2 (5 mL, 25.7 mmol) [ATTENTION: SOCl2
is a reactive compound that can violently and/or explosive-
ly release dangerous gases upon contact with water and
other reagents. It is toxic and will produce toxic gases;
always handle in well vented fume hoods] was added to the
dry (HOOC)2bpy (1 g, 4.1 mmol) and the mixture was re-
fluxed for 3 h. The reaction was then evaporated to dryness
yielding quantitatively the desired pure product. Overall
Experimental Section
Materials and Methods
All reagents except those that follow were obtained from
Aldrich and used as received. Commercial grade solvents
were dried and deoxygenated by distillation under nitrogen
and kept over 4Š molecular sieves (3Š for acetonitrile).
The complex [MoI2(CO)3(CH3CN)2] was prepared according
to the literature method.[38] The silica coated iron oxide
magnetic nanoparticles (MNP-Si) were also prepared ac-
cording to a literature procedure.[39]
FT-IR spectra were obtained as diffuse reflectance
(DRIFT) measurements on a Nicolet 6700 in the 400–
4000 cmÀ1 range using 2 cmÀ1 resolution.
Powder XRD measurements in the 28<2q<808 range
were taken on
a Rigaku Miniflex 600 (theta/2theta)
equipped with silicon strip detector and with automatic data
acquisition, using a Cu source, using 40 kV and 15 mA. Sam-
ples were measured on glass slides.
Raman spectra were recorded on a Bruker Senterra dis-
persive microscope spectrometer equipped with cooled
charge-coupled device (CCD) detector, using an excitation
wavelength of 785 nm in the 100–4000 cmÀ1 range with
4 cmÀ1 resolution. The solid samples were put on top of
glass slides and placed on the microscope plate for focusing
the beam.
The N2 sorption measurements were obtained on a Quan-
tachrome Autosorb iQ porosimeter. BET specific surface
areas (SBET, P/P0 from 0.03 to 0.30) and specific total pore
volume Vp were estimated from N2 adsorption isotherms
measured at 77 K. The pore size distributions (PSD) were
calculated by the Barrett–Joyner–Halenda (BJH) method
from the desorption branch of the isotherm, using the modi-
fied Kelvin equation with correction for the statistical film
thickness on the pore walls.[40,41] The statistical film thickness
was calculated using the Harkins–Jura equation in the p/p0
range from 0.1 to 0.95.
Microanalyses for CHN and Mo quantitation were per-
formed at CACTI, University of Vigo. CHN analyses were
performed on a Fisons EA 1108; Mo quantification was per-
formed on a Perkin–Elmer Optima 4300DV using In as in-
ternal standard.
À
yield was 82%. IR (KBr): n=3140 (nC H, s), 1727 (nC=O,
vs), 1659 (nC=C, m), 1594 (nC=N, m), 658 cmÀ1 (nC Cl, vs);
À
1H NMR (400.13 MHz, CD3OD, r.t.): d= 9.14 (s, H-6), 9.12
(d, H-3,
J
H-3,H-4 =6.8 Hz), 8.43 (s, H-4); 13C NMR
(100.62 MHz, CD3OD, r.t.): d=164.1 (COCl), 148,3 (C-1),
147.5 (C-3), 144.6 (C-5), 126.6 (C-6), 123.3 (C-4).
Preparation of Heterogeneous Catalysts
Synthesis of material magMSh: The helical mesoporous
magnetic nanocomposite material (magMSh) was prepared
using an entropy-driven procedure with achiral cationic sur-
factant template. In this case, cetyltrimethylammonium bro-
mide (C16TAB) and ammonia as surfactant and co-surfac-
tant, respectively, according to a literature procedure.[6] In
a
typical material synthesis procedure, C16TAB (1.0 g;
2.7 mmol) (Fluka, 96.0%) was dissolved in an aqueous am-
monia solution (250 mL) (Sigma-aldrich, 25 wt%) at 323 K
until complete dissolution. Then tetraethoxysilane (TEOS)
(10 mL) (Aldrich, 98%) and MNP-Si (0.2 g) were added to
the solution and the mixture was sonicated for about 5 min
in order to disperse the nanoparticles. Following this, the so-
lution was allowed to react for 3 h at 323 K under mechani-
cal stirring in the presence of an inert N2 atmosphere. After
SEM images were obtained on a Field Emission Gun
Scanning Electron Microscope from JEOL, model JSM-
7001F. TEM images were obtained on a Hitachi microscope,
model H-8100 with a LaB6 filament using an acceleration
tension of 200 kV.
SQUID magnetometry measurements were performed on
the Quantum design XL Magnetic Property Measurement
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Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles
this period, the mixture was transferred to a Teflon-lined
stainless steel autoclave and aged at 373 K for 24 h. The
product was separated by centrifugation, washed with water
and ethanol several times and dried at 343 K in the oven.
The surfactant occluded inside the pores of the material was
then removed by calcination at 833 K for 6 h (18CminÀ1).
Prior to the grafting experiments, physisorbed water was
removed from the materials by heating at 453 K in vacuum
(10À2 Pa) for 2 h. IR (KBr): n=3413 (vs), 1630 (s), 1379 (vs),
1209 (w), 1082 (vs), 964 (w), 796 (w), 451 cmÀ1 (s); powder
XRD: 2q/8 (hkl in parenthesis)=2.18 (100), 3.87 (110), 4.36
(200), 5.99 (210), 35.50 (311).
Synthesis of material magMSh-bpy: A suspension of the
(ClCO)2bpy ligand (0.281 g; 1 mmol) in acetonitrile
(CH3CN, 10 mL) was added to a magMSh suspension (1 g)
in acetonitrile (CH3CN, 15 mL), and the mixture was stirred
under a N2 atmosphere at 358 K for 14 h. The resulting solid
was filtered off and washed twice with dichloromethane
(CH2Cl2, 2”10 mL) and then dried in vacuum at 323 K for
2 h. IR (KBr): n): 3430 (vs), 3105 (w), 1766 (m), 1719 (m),
1630 (m), 1461 (w), 1384 (s), 1088 (vs), 976 (w), 833 (w), 686
(w), 465 cmÀ1 (s); powder XRD: 2q/8 (hkl in parenthesis)=
2.26 (100), 3.91 (110), 4.51 (200), 5.92 (210), 35.70 (311); ele-
mental analysis (%): found: C 5.91, H 0.69, N 1.19.
Synthesis of material magMSh-bpy-Mo: A solution of
[MoI2(CO)3(CH3CN)2] (0.300 g; 0.56 mmol) in dry dichloro-
methane (CH2Cl2, 10 mL) was added to a suspension of 1 g
of magMSh-bpy also in dry dichloromethane (CH2Cl2,
15 mL). The reaction mixture was stirred under a N2 atmos-
phere at room temperature for 14 h. The resulting material
was then filtered off, washed with dichloromethane (CH2Cl2,
2”10 mL), and dried under vacuum for 3 h. IR (KBr): n=
3429 (vs), 3111 (w), 1712 (w), 1622 (m), 1458 (w), 1371 (w),
1088 (vs), 974 (w), 806 (w), 766 (w), 461 cmÀ1 (s); powder
XRD: 2q/8 (hkl in parenthesis)=2.22 (100), 3.81 (110), 4.47
(200), 6.23 (210), 35.62 (311); elemental analysis (%):
found: C 8.89, H 1.05, N 1.47, Mo 2.42.
which CH2Cl2 several times and dried prior to reuse in
a new catalytic cycle.
General Procedure for the Study of Channel Chirality
in MSh Materials
This study was based on a literature protocol by Fernandes
et al.[6] An aliquot of 50 mg of pure l-phenylalanine (or d-
phenylalanine) was dissolved in 20 mL of MilliQ water fol-
lowed by addition of 20 mg of template-free MSh material
previously activated. The mixture was magnetically stirred
under ambient conditions and at a constant temperature of
298 K. The concentration of l-phenylalanine (or d-phenyla-
lanine) was measured using UV spectroscopy (absorption at
260 nm) by sampling at regular time intervals. The proce-
dure was applied to 5 different synthesis batches of MSh ma-
terials and each one was tested in triplicate.
Acknowledgements
The authors thank FCT, POCI and FEDER (project UID/
MULTI/00612/2013) for financial support. CIF also thanks
FCT for a research grant (SFRH/BD/81029/2011).
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