Synthesis of Chiral Nematic Mesoporous Metal and Metal Oxide Nanocomposites and their Use as Heterogeneous Catalysts

Article abstract of DOI:10.1002/ejic.202000673

We report the synthesis of chiral nematic mesoporous metal and metal oxide nanocomposites containing zinc or copper prepared through a vacuum-assisted loading of the metal precursor into a chiral nematic mesoporous silica template followed by high-tempera

Full text of DOI:10.1002/ejic.202000673

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Chiral Nematic Composite Materials | Very Important Paper |
Synthesis of Chiral Nematic Mesoporous Metal and Metal Oxide
Nanocomposites and their Use as Heterogeneous Catalysts
Christopher M. Walters,[a] Keegan R. Adair,[a] Wadood Y. Hamad,[b] and
Mark J. MacLachlan*[a,c,d]
Abstract: We report the synthesis of chiral nematic mesopo- metal oxide films with surface areas up to 200 m2/g. To demon-
rous metal and metal oxide nanocomposites containing zinc or strate the utility of these high surface area materials, we used
copper prepared through a vacuum-assisted loading of the the prepared CuO material as a heterogeneous catalyst for click
metal precursor into a chiral nematic mesoporous silica tem- reactions. Our prepared CuO showed excellent activity (95 %
plate followed by high-temperature treatment. Significant metal conversion after seven hours) when compared to commercially
loading into the template was achieved after a single loading available CuO nanoparticles (20 % conversion after 24 hours)
cycle (up to 55 % of the pore volume) as confirmed by nitrogen owing to its considerably higher surface area. Moreover, the
sorption measurements. Growth of the metal nanoparticles was prepared CuO material maintained high activity (>70 % conver-
contained within the porous silica template, and subsequent sion) after four cycles.
removal of the template afforded free-standing chiral nematic
Introduction
Hard templating (sometimes called “casting”) has emerged as
a powerful general technique for constructing new solid-state
Although naturally occurring metals are achiral substances, im-
materials with periodic order.[8] In this approach, a well-defined,
parting chirality into metals and metal surfaces, and studying
periodic mesoporous material - often a silicate, such as MCM-
these systems is necessary for developing new catalysts, sen-
41[9] or SBA-15[10] – is used as the template. After the target
sors, and photonic materials. In addition to adding chiral modi-
material is constructed within the pores of the host, removal of
fiers onto metal surfaces[1] and chiral seeding,[2] there are a
the template leaves a nanostructured material with the same
variety of other routes, such as molecular adsorption,[3] imprint-
morphology as the original template. This strategy has been
ing,[4] and electrodeposition in chiral environments,[5] that have
integral to unlocking a variety of different carbon based materi-
been used to create chiral metallic structures. While these ap-
als,[11] metal oxides,[12] polymers,[13] and other materials.[14]
A
proaches have resulted in materials with fascinating properties,
in many cases, the chirality is expressed over a restricted do-
main, and lacks long-range order with appropriate surface
area.[6] Further, when these methods are used to create hetero-
geneous catalysts, they introduce new challenges and costs not
encountered in their homogeneous equivalents.[7]
possible approach to construct chiral metals is by using hard
templating with a chiral host to form a metal oxide, which can
be reduced to a metal. In 2004, Che et al. used a surfactant-
templated synthesis to create ordered chiral mesoporous silica
that was then used to fabricate chiral Co and Pt nanowires,
demonstrating the ability of this silica material in enantioselec-
tive applications.[15] In another example, An et al. used a chiral
mesoporous silica to template the formation of In2O3 and
Co3O4 through a solvent-assisted infiltration of the pores. Fol-
lowing removal of the template, they showed that these free-
standing materials had unique chiroptical properties that could
be beneficial in optical and magnetic micro-devices.[16] Despite
these advances, there still only remain a few examples in the
literature where these methods have been used to construct
chiral materials, even though such materials may be intriguing
substrates for catalysis, photonics, and separations.[17]
[a] C. M. Walters, K. R. Adair, Prof. M. J. MacLachlan
Department of Chemistry, University of British Columbia,
2036 Main Mall, Vancouver, BC V6T 1Z1, Canada
E-mail: mmaclach@chem.ubc.ca
https://groups.chem.ubc.ca/maclachlan
[b] Dr. W. Y. Hamad
Transformation and Interfaces Group, Bioproducts Innovation Centre of
Excellence, FPInnovations,
2665 East Mall, Vancouver, BC V6T 1Z4, Canada
[c] Prof. M. J. MacLachlan
Stewart Blusson Quantum Matter Institute, University of British Columbia,
2355 East Mall, Vancouver, BC V6T 1Z4, Canada
[d] Prof. M. J. MacLachlan
Cellulose nanocrystals (CNCs) are a widely available nanoma-
terial with highly attractive properties for the construction of
functional composite materials.[18] CNCs derived from the hy-
drolysis of biomass using sulfuric acid are typically ca. 100–
300 nm long and ca. 5–20 nm wide.[18a] Importantly, CNCs self-
WPI Nano Life Science Institute, Kanazawa University,
Kanazawa, 9201192, Japan
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000673.
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assemble into a left-handed chiral nematic (cholesteric) liquid terials, the CNMS films were first fully submerged in aqueous
crystalline phase in water.[19] Following evaporation of the wa- solutions of the respective metal salts and placed under vac-
ter, CNCs retain the chiral nematic organization in the solid uum. By reducing the pressure in the system, the porous net-
state, leading to iridescent films when their helical pitch work was evacuated of gas, as indicated by the formation of
matches the wavelength of visible light.[20] If CNCs are com- bubbles at the film surface. Once bubbling ceased, the vessel
bined with a sol-gel precursor (e.g., tetramethyl orthosilicate was rapidly returned to atmospheric pressure, causing the pre-
(TMOS)), the TMOS undergoes hydrolysis and condensation cursor solution to fill the porous network. By repeating this
while the CNCs assemble, leading to a glass/CNC composite in process three times, adequate loading of the pores could be
which the CNCs are organized into a chiral nematic structure.[21] achieved. This method is advantageous compared to other
Removal of the cellulose via calcination generates chiral nem- loading techniques (e.g., incipient wetness loading), which of-
atic mesoporous silica (CNMS) – a silica film with pores arranged ten only achieve poor infiltration of the pore network and can
in a chiral nematic structure.[22] Not only is CNMS an inexpen- require extensive surface modification of the silica tem-
sive and easily synthesized material, but it has potential for sep- plate.[24a,27] Furthermore, unlike traditional loading methods,
aration of enantiomers in applications such as gas and liquid the films do not need to be crushed into fine powders to pro-
chromatography.[23] Moreover, CNMS has served as a platform mote precursor absorption. This strategy to load pores of chiral
to template mesoporous organic and inorganic materials with mesoporous materials is attractive for its efficiency and ability
long-range chirality.[12a,24] Surprisingly, there have only been a to create large films.
handful of reports demonstrating the synthesis of high surface
area chiral metals despite their potential for chiral sensing, ca-
talysis, and separation.[12a,25] Therefore, it is of great interest to
use CNMS as a hard-template to create chiral materials with
high surface area.
Growth of nanoparticles was promoted through calcination
of the precursors in air (CNMS-CuO and CNMS-ZnO) or through
H2 reduction (CNMS-Cu); attempts to synthesize CNMS-Zn
through H2 reduction were unsuccessful and only yielded the
corresponding oxide. The resulting films were iridescent (Fig-
Transition-metal oxide materials are already used in numer- ure 1 A-D) due to the reflection of left-handed circularly-polar-
ous applications, ranging from cosmetics to antimicrobials. In ized light by the chiral nematic structure.[22] The appearance of
particular, zinc oxide (ZnO) and copper oxide (CuO) are attract- this iridescence suggests that the chiral nature of the films was
ive materials for catalysis, photocatalysis, and use as semicon- preserved during crystallite growth. Evidence for the chiral
ductors.[26] While there has been work using CNMS as a hard nematic structure was also obtained from SEM images. Figure 1
template, traditional loading methods such as drop-casting and E-H shows the SEM images of cross sections of the composites.
incipient wetness impregnation led to poor infiltration of the The structure observed is similar to CNMS, indicating that the
precursors into the template and required surface modification chiral ordering of the template is maintained.
to immobilize metal nanoparticles in the porous support.[27] Be-
cause of these limitations, removing the chiral template re-
sulted in total loss of the chiral structure of the guest material.
In this paper, we report a simple method that has allowed
us to create chiral nematic mesoporous silica – metal oxide
composite materials using zinc and copper for the first time.
Using a vacuum-assisted loading technique, we achieved a high
loading of precursor into the mesoporous template without the
need for surface modification. The simplicity and speed of this
method, and the ability to achieve deep pore penetration pro-
vides significant advantages over traditional loading meth-
ods,[24a] and allows for the formation of a more complete chiral
structure. Simple thermal treatment yields the metal oxide com-
posites, which can then be extracted from the silica template
while maintaining appreciable surface area and chirality. To
demonstrate the potential of these materials, we used the
mesoporous copper(II) oxide as a heterogeneous catalyst in
click reactions. The catalyst was highly active, showed excellent
stability in aqueous and organic media, and was recyclable.
Figure 1. Photographs of (A) CNMS, (B) CNMS-CuO, (C) CNMS-Cu, and (D)
CNMS-ZnO films. Scale bars are 1 cm. Scanning electron micrographs of cross
sections of (E) CNMS, (F) CNMS-CuO, (G) CNMS-Cu, and (H) CNMS-ZnO. Scale
bars are 2.00 μm.
One important finding is that there are very few defects in
the films, indicating the growth of the metal oxide nanoparti-
cles was restricted within the pores of the CNMS template. Sub-
stantially larger crystallites could be observed when viewing
the surface and edges of the films (as opposed to the cross-
section), suggesting these areas underwent unrestricted crystal
growth (Figure S1). In addition to SEM, circular dichroism meas-
urements also confirmed left-handed chirality of the nanocom-
posites (Figure S2; further discussion in the SI).
Results and Discussion
Chiral nematic mesoporous silica (CNMS) films were prepared
according to previous literature procedures using CNCs as a
template.[22] CNMS was used as a template to form composite
materials with zinc oxide (CNMS-ZnO), copper oxide (CNMS-
Powder X-ray diffraction (PXRD) patterns (Figure 2) of the
CuO), and copper (CNMS-Cu). To prepare these composite ma- composite films show the presence of the crystalline nanocom-
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Figure 2. PXRD patterns of chiral nematic nanocomposite films. The PXRD patterns of (A) CNMS-CuO, (B) CNMS-Cu, and (C) CNMS-ZnO match those of tenorite
(PID: 00–048–1548), copper (PID: 00–004–0836) and, zincite (PID: 00–036–1451), respectively. Similarly, the patterns after removal of the CNMS template of
(D) CuO, (E) Cu, and (F) ZnO match those of tenorite (PID: 00–048–1548), [cuprite (PID: 00–005–0067) + copper (PID: 00–004–0836)] and, zincite (PID: 00–036–
1451), respectively.
posites and the patterns for CNMS-ZnO, CNMS-CuO, and CNMS-
Cu matched the known PXRD patterns of zincite, tenorite, and
copper metal, respectively, confirming that the desired nano-
materials formed. An amorphous halo observed near 20° 2θ
originates from the sample holder. Crystallite sizes were esti-
mated using the Scherrer equation (Table S1) and ranged be-
tween ca. 15–27 nm. We did not observe any change in the
crystalline domains when the temperature of thermal decom-
position was changed from 300 to 500 °C; however, an increase
in peak intensity was observed as the concentration of the
metal salt precursor was increased from 1 to 4
M (Figure S3).
The extent of pore loading was quantified through nitrogen
sorption analysis. Gas sorption isotherms for all films indicated
a type IV isotherm with a hysteresis loop consistent with meso-
porous materials with cylindrical and spherical pores (H2) (Fig-
ure 3).[28]
The surface areas of CNMS, CNMS-CuO, CNMS-Cu, and
CNMS-ZnO are 497 m2/g, 223 m2/g, 218 m2/g, and 225 m2/g,
respectively, and all pore sizes are ca. 4 nm. In general, there
was a significant decrease in the pore volume of up to 55 % of
the CNMS template after loading the pores with the precursors
(Table S2) suggesting that the metal-containing nanoparticles
infiltrated deep into the pore network of the template.
Figure 3. Nitrogen adsorption (square)/desorption (circle) isotherms for CNMS
(black), CNMS-CuO (red), CNMS-Cu (green), and CNMS-ZnO (blue). All materi-
als show a type IV isotherm.
than the composites. SEM images (Figure 4C, D) revealed that
the layered helical structure was retained after template re-
Further, the films maintained a high surface area and the moval, however, some defects could be observed in the CuO
pores remained accessible, which may allow for future modifi- films (Figure S4).
cation. Overall, nitrogen sorption, and EDX (Table S3) data sug-
gest a higher loading of metal salts into the CNMS films when
compared to other silica-metal composite materials.[29]
Additionally, when the ZnO films were dried, they appeared
iridescent, whereas the dried CuO films did not. In the case of
the CNMS-Cu films, removal of the silica yielded a fine powder.
Removal of the silica template by base-etching was con- When observed under SEM, this powder contained nanoparticle
firmed through EDX, which showed less than 3 % silicon re- and rod-like structures and agglomerates with layered organiza-
maining (Table S3). Both ZnO and CuO were recovered as free- tion; however, a chiral nematic structure was not observed (Fig-
standing films (Figure 4A, B) that were noticeably more brittle ure S5). These features were not observed in the CuO and ZnO
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shapes are consistent with cylindrical pores (H1) after etch-
ing.[28] The recovered Cu powder was not analyzed by nitrogen
sorption.
PXRD of the samples obtained after removal of the template
(Figure 2D–F) verified that the phase of the crystalline metal/
metal oxide did not change for CuO and ZnO. Interestingly,
despite etching with KOH in EtOH, it appears the etching proc-
ess resulted in partial oxidation of the Cu0. It is possible that this
change in oxidation state resulted in a change in nanoparticle
morphology that contributed to the collapse of the intercon-
nected nanoparticle network thus resulting in the recovered
powder.
Figure 4. Photographs of A) CuO and (B) ZnO films after etching with 2
M
NaOH. CuO films are no longer iridescent. Scale bar is 0.5 cm. Scanning elec-
tron micrographs of free-standing chiral nematic (C) CuO and (D) ZnO films.
Scale bar is 2.00 μm. E) Nitrogen adsorption (square)/desorption (circle) iso-
therms for ZnO (black), CuO (blue). Both materials show a type IV isotherm.
Taking advantage of the high surface area and porosity of
these new materials, we demonstrated their promise as hetero-
geneous catalysts. Click chemistry is one of the most important
reactions for cycloadditions and has been extensively stud-
ied.[30] However, difficulties recovering or removing copper
complexes from the reaction, and increasing environmental
concerns about waste disposal present challenges for homoge-
neous click reaction catalysis.[31] Consequently, heterogeneous
systems have garnered more attention as a way to address
these issues.[32] The mesoporous CuO films presented herein
were effective in catalyzing the reaction between benzyl azide
and phenylacetylene, and had 95 % conversion to the product
films and it is likely that the observed morphological change
of the metal nanoparticles during reduction from Cu2+ to Cu0
disrupted the nanoparticle connectivity, and removal of the sil-
ica scaffold caused the structure to collapse.
The CuO and ZnO films were then analyzed by nitrogen sorp-
tion (Figure 4E), which showed a moderate decrease in the sur-
face area of the CuO films, as well as an increase in the pore
size (110 m2/g, 16.5 nm) due to what we believe was partial
destruction of the chiral network and removal of the template
(Table S2). In contrast, ZnO films maintained their high surface
1
after 7 h as indicated by H NMR spectroscopy (Figure 5).
We note an induction period in the first hour where no prod-
area and underwent an increase in pore size (198 m2/g, uct was formed. We believe during this time the phenylacetyl-
11.4 nm) due to the removal of the silica template. Notably, the ene is coordinating to the copper surface. A control reaction
films are still mesoporous (Type IV isotherm), but the pore without catalyst showed no product formation after 7 h of reac-
Figure 5. 1H NMR (CDCl3, 25 °C, 300 MHz) spectra of the copper oxide-catalyzed click reaction of benzyl azide and phenylacetylene to form 1-benzyl-4-phenyl-
1H-[1,2,3]triazole.
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plied by FPInnovations. Tetramethyl orthosilicate (TMOS), zinc(II)
nitrate hexahydrate, copper(II) nitrate hemi(pentahydrate), phenyl-
acetylene, sodium azide, and benzyl chloride were purchased from
Sigma Aldrich.
tion. Moreover, we found our catalyst to have similar perform-
ance to previous literature reports of mesoporous copper(II) ox-
ide.[26b] We also observed less than 20 % product formation
when using commercially available copper(II) oxide nanoparti-
cles after 24 h of reaction (Figure S6). We attribute the higher
Preparation of CNC/Silica Composite Films (CNC-TMOS): An
activity of our catalyst to the high surface area not seen in aqueous suspension of CNCs (20 mL) was sonicated for 10 min.
commercially available CuO NPs (10 m2/g – Figure S7). To test
TMOS (1 mL) was added dropwise to the CNC solution while it was
being stirred. The mixture was stirred for 90 min, transferred into a
the recyclability of our mesoporous CuO catalyst, we recovered
100 mm diameter polystyrene petri dish, and left to dry under am-
bient conditions for 4 d.
the catalyst after the first reaction and reused it for three subse-
quent reactions. The largest decrease in catalytic activity oc-
curred between the first and second cycles. Likely, non-inter-
Preparation of Chiral Nematic Mesoporous Silica (CNMS): The
locked nanoparticles dislodged from the catalyst and were re- cellulose nanocrystal template was removed from the CNC/silica
composites by heating the films to 100 °C at 2 °C min–1, holding at
moved during subsequent washing steps. This may be ex-
100 °C for 2 h, heating to 540 °C at 2 °C min–1, and holding at
plained by the partial fragmentation we observed in SEM (Fig-
540 °C for 6 h before slowly cooling the sample to room tempera-
ure 4E). Small decreases in catalytic activity in subsequent cy-
ture.
cles are most likely the result of some catalyst being removed
during product extraction (Table S4); however, after 4 cycles we
Loading of Precursors into CNMS Films: CNMS-ZnO and CNMS-
still observed ca. 70 % product conversion. To demonstrate the CuO films were prepared by submerging the films in aqueous solu-
tions of 3
M
Zn(NO3)2 and 4
M
Cu(NO3)2·2.5H2O, respectively. In both
versatility of this mesoporous CuO catalyst, we also tried cata-
lyzing the reaction of benzyl azide with either 4-ethynylbenzoic
acid or methyl 4-ethynylbenzoate. THF was used since the start-
ing reagents are not soluble in chloroform. After 24 hours of
reaction time, we observed 100 % conversion of the 4-ethynyl-
benzoic acid (Figure S8) and 93 % conversion for methyl
4-ethynylbenzoate as calculated by H NMR spectroscopy (Fig-
ure S9). Overall, the performance of this material demonstrates
the promise of these easily fabricated mesoporous metals,
cases, the submerged films were placed in a vacuum flask con-
nected to a Schlenk line and evacuated until no air bubbles could
be seen. At this point, the flask was rapidly returned to atmospheric
pressure. The vacuum cycle was repeated three times to ensure
thorough wetting of the films. The loaded films were then isolated
from the excess solution by filtration and placed in an oven. CNMS-
CuO was formed by heating the films to 350 °C at 2 °C min–1 and
then holding the temperature at 350 °C for 4 h. A similar procedure
was used to obtain CNMS-ZnO at 450 °C. CNMS-Cu was prepared
1
which may find use in electronics, catalysis, storage, and sens- by heating CNMS-CuO to 250 °C at 2 °C min–1 under 4 % H2 gas
flow and holding it at 250 °C for 4 h.
ing. Further work exploring the photocatalytic properties of our
ZnO materials is currently being investigated.
Removal of Silica Template: To create free-standing chiral nematic
films, all composite films (except CNMS-Cu) were placed in an aque-
Conclusion
ous solution of 2
M
NaOH to remove the silica. The etching solution
NaOH. This
was decanted every 1.5 h and replaced with fresh 2
M
We have demonstrated a hard-templating technique to gener-
ate chiral zinc and copper oxides, potential precursors to chiral
metals. These materials were constructed using a vacuum-as-
sisted infiltration of aqueous metal solutions into chiral nematic
mesoporous silica. This technique is fast, simple, and does not
require surface modification. Improved pore loading by the
metal precursor resulted in composite CNMS-CuO and CNMS-
ZnO films that show long-range order and have high surface
area and porosity. The CNMS-CuO film was reduced under
hydrogen gas to yield a composite CNMS-Cu metal film. Re-
moval of the silica template afforded free-standing chiral nem-
atic films, except for the copper metal films, which degraded
after removal of the silica scaffold. We demonstrated that the
CuO films could be used as a heterogeneous catalyst for click
chemistry. The catalysts showed over 95 % conversion in 7 h in
model reactions and could be recycled. Although this work took
advantage of the high surface area in the porous CuO, future
efforts will explore the potential for the chirality to influence
the outcome of catalytic reactions. These novel transition metal
oxides and composites with chiral nematic order may find prac-
tical use in applications such as chiral sensing or catalysis.
was repeated three times, after which the films were gently washed
with deionized water to remove any residue. CNMS-Cu was etched
using 2
M KOH in ethanol to prevent oxidation of the copper.
Synthesis of Benzyl Azide: Benzyl azide was prepared according
to
literature procedure.[33] Briefly, sodium azide (715 mg,
a
11.0 mmol) was dissolved in 200 mL of DMSO, then benzyl bromide
(1.71 g, 1.19 mL, 10 mmol) was added to the solution. The reaction
was vigorously stirred for 8 h at room temperature before the addi-
tion of 200 mL of water. The solution was extracted with diethyl
ether (3 × 100 mL) and the organic layers were combined and
washed with water (2 × 100 mL), followed by brine (100 mL). Finally,
the solution was dried with MgSO4 and the ether was evaporated
under a flow of nitrogen at room temperature. The product was
isolated as a colorless oil and was not further purified. 1H-NMR
(300 MHz, CDCl3): δ (ppm) = 7.56–7.28 (m, 5H, Ph), 4.36 (s, 2H, CH2).
The NMR data matched the literature data.[33]
Synthesis of 1-Benzyl-4-phenyl-1H-[1,2,3]triazole: 1.0 mg of
mesoporous CuO films (0.013 mmol, 1 mol-%) was added to a
10 mL round-bottomed flask containing benzyl azide (0.15 mL,
1.2 mol), phenylacetylene (0.195 mL,1.77 mmol), and CDCl3 (3.6 mL).
The reaction mixture was refluxed at 50 °C and stirred at 400 RPM.
Aliquots were removed from the reaction vessel at various time
points and analyzed by 1H NMR spectroscopy. 1H NMR (300 MHz;
CDCl3) δ (ppm): 5.54 (2 H, s, CH2), 7.2–7.4 (8 H, m, Ar), 7.65 (1 H,
s, CH), and 7.78–7.85 (2 H, m, Ar). The product matches literature
Experimental Section
Materials: All chemicals were used as received without further puri-
fication. Aqueous CNC suspensions (4.0 wt.-%, pH ca. 2.5) were sup- reports.[34]
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Synthesis of 4-(1-Benzyl-1H-1,2,3-triazol-4-yl)benzoic Acid:
1.0 mg of mesoporous CuO films (0.013 mmol, 4.4 mol-%) was
added to a 10 mL round-bottomed flask containing benzyl azide
(36.6 μL, 292 μmol), 4-ethynylbenzoic acid (64 μg, 443 μmol), and
THF (5 mL). The reaction mixture was refluxed at 50 °C and stirred
at 400 RPM for 24 h. An aliquot was taken and dried under a flow
of nitrogen before being analyzed by 1H NMR spectroscopy. 1H NMR
(300 MHz; [D6]DMSO) δ (ppm): 5.68 (2 H, s, CH2), 7.38–7.39 (5 H, m,
Ar), 8.00 (4 H, m, Ar), and 8.79 (1 H, s, CH).
Keywords: Chirality · Copper · Zinc · Heterogeneous
catalysis · Mesoporosity
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Catalytic Activity: Prior to use, CuO films were first passed through
a 35.5 μm sieve (ASTM E-11 specification) to ensure uniform particle
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1
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We thank NSERC for funding (Discovery grant; CREATE NanoMat
grant).
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Received: July 14, 2020
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