Palladium nanoparticles supported in the nanospaces of imidazolium-based bifunctional PMOs: The role of plugs in selectivity changeover in aerobic oxidation of alcohols

Article abstract of DOI:10.1021/acscatal.5b00237

Novel heterogeneous catalyst systems comprised of palladium nanoparticles immobilized into the nanospaces of imidazolium-based bifunctional plugged and unplugged periodic mesoporous organosilicas (BFPMO) have been described for the selective aerobic oxidation of alcohols in water. BFPMOs were characterized by N₂ adsorption-desorption analysis, transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), thermal gravimetric analysis (TGA), ₂₉Si and ₁₃C cross-polarization magic angle spinning (CP MAS) NMR spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), elemental analysis, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity of all plugged and unplugged catalysts was investigated in the aerobic oxidation of benzylic alcohols by emphasizing the effect of different physiochemical properties as well as the plugs on the reaction selectivity. While unplugged catalysts exhibited much better activity for the selective oxidation of benzyl alcohol to benzaldehyde, the selectivity pattern shifts to the benzoic acid in high yield and selectivity in the presence of plugged catalyst under the exact same reaction conditions. The studies showed for the first time that varying the hydrophobic-hydrophilic balance with concomitant control of plugs in the interior of the mesochannels of the described catalyst enabled tuning of both the catalyst performance and the reaction selectivity, possibly through a cooperative mechanism. A possible model has been proposed to explain this unprecedented observation.

Full text of DOI:10.1021/acscatal.5b00237

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Article
Palladium Nanoparticles Supported in the Nanospaces of
Imidazolium-Based Bifunctional PMOs: The Role of Plugs
in Selectivity Changeover in Aerobic Oxidation of Alcohols
Babak Karimi, Mojtaba Khorasani, Hojatollah Vali, Carolina Vargas, and Rafael Luque
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00237 • Publication Date (Web): 22 May 2015
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Palladium Nanoparticles Supported in the
Nanospaces of Imidazolium-Based Bifunctional
PMOs: The Role of Plugs in Selectivity Changeover
in Aerobic Oxidation of Alcohols
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Babak Karimi,*† Mojtaba Khorasani,† Hojatollah Vali,‡ Carolina Vargas,§ and Rafael LuqueЪ
†
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), PO-Box
45195-1159, Gavazang, Zanjan 45137-6731, Iran
‡
Department of Anatomy and Cell Biology and Facility for Electron Microscopy Research,
McGill University, Montreal, Quebec, H3A 2A7 Canada
§
Departamento de Tecnologia Ambiental (ESCET III), Universidad Rey Juan Carlos, Madrid,
Spain
Ъ
Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A,
Km 396,Córdoba, Spain
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KEYWORDS Supported Palladium, Alcohols Oxidation, Selectivity Control, Plugged
Hexagonal Templated Silica (PHTS), Bifunctional Periodic Mesoporous Organosilica (BFPMO),
Plugged PMO
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ABSTRACT
Novel heterogeneous catalyst systems comprised of palladium nanoparticles immobilized into
the nanospaces of imidazolium-based bifunctional plugged and unplugged periodic mesoporous
organosilicas (BFPMO) have been described for the selective aerobic oxidation of alcohols in
water. BFPMOs were characterized by N2 adsorption-desorption analysis, transmission electron
microscopy (TEM), powder X-ray diffraction (PXRD), thermal gravimetric analysis (TGA), 29Si
and 13C cross-polarization magic angle spinning (CP MAS) NMR spectroscopy, diffuse
reflectance infrared Fourier transform spectroscopy (DRIFT), elemental analysis, scanning
electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Catalytic activity of
all plugged and unplugged catalysts was investigated in the aerobic oxidation of benzylic
alcohols by emphasizing the effect of different physiochemical properties as well as the plugs on
the reaction selectivity. While unplugged catalysts exhibited much better activity for the
selective oxidation of benzyl alcohol to benzaldehyde, selectivity pattern shifts to the benzoic
acid in high yield and selectivity in the presence of plugged catalyst under the exactly same
reaction conditions. The studies showed for the first that varying the hydrophobic-hydrophilic
balance with concomitant control of plugs in the interior of the mesochannels of the described
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catalyst enabled tuning of both the catalyst performance and the reaction selectivity possibly
through a cooperative mechanism. A possible model has been proposed to explain this
unprecedented observation.
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Introduction
Selective oxidation of alcohols into their corresponding carbonyl compounds is a very
attractive and challenging transformation from both synthetic and industrial points of view,
because the related products are versatile intermediates of valuable compounds, such as
pharmaceuticals, agricultural, and fine chemicals.1 In the view of the stringent environmental
issues, green chemistry, and especially atom efficiency, a plethora of research in this area has
been recently directed toward the development of new and efficient heterogeneous and
recyclable catalysts to accomplish the efficient oxidation of alcohols with molecular oxygen or
air as the final oxidant.2 Along this line, a significant number of metal-catalyzed aerobic
oxidation processes based on palladium3, platinum4, gold5, ruthenium6 and copper7 as well as
organocatalysts8 have achieved a promising results. Although, the goal of such processes is often
to prevent over-oxidation in order to stop the reaction at aldehyde stage, sometimes it is more
desirable to produce pure carboxylic acids from alcohols under conditions analogous to those
employed for the selective formation of aldehyde. Aldehyde is the first product of this process
whereas carboxylic acids can generally be obtained in two steps in an aqueous medium, via the
in situ generation of geminal diol through hydration of aldehyde followed by oxidation into the
corresponding carboxylic acid (Scheme 1).9 Although, numerous studies have focused on
developing new and eco-friendly catalytic systems on the basis of either transition-metals or
organocatalysts for selective aqueous oxidation of primary alcohols,3-8 designing a more
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sophisticated catalyst system for the selective generation of aldehyde or carboxylic acid, remains
a challenge. In this context, it was discovered in the aerobic oxidation of primary alcohols
catalyzed by Pt/C, reaction selectivity could be adjusted to selectively produce aldehydes or
carboxylic acids in dioxane or water-dioxane, respectively.10 However, the possibility of
controlling this reaction to selectively produce aldehyde or carboxylic acid in water under
identical reaction conditions has not been explored. Considering the remarkable difference in
lipophilic character of aldehydes and their gem-diols, it is conceivable that the reaction
selectivity might be controlled to a great extent by adjusting the hydrophobic-hydrophilic
balance on the catalyst surface.
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Scheme 1. Possible pathway for direct oxidation of alcohols to the related carboxylic acids.
The discovery of periodic mesoporous organosilica (PMO) materials was clearly one of the
biggest advances in materials science.11 These materials are prepared from the hydrolysis and
condensation of bridged organosilica precursors [(R’O)3Si-R-Si(OR’)3] in the presence of varied
surfactant assemblies as the structure directing agent (SDA). In the last decade, hybrid organic-
inorganic mesoporous materials including PMOs have been employed by several approaches and
strategies in catalysis and catalyst design owing to their well-ordered mesoscopic structure,
exceptionally high surface area, high thermal-hydrothermal stability, tunable pore size and
physiochemical properties.12 Depending on the applications, functionalized silica materials or
PMOs with high surface area and available open pore volume are preferred12 to materials with
confined, constricted and/or sometimes closed pore systems. Nevertheless, functionalized
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mesoporous materials having constricted channels appeared to be good candidates for specific
applications including selective absorption of organic and important biological compounds,
designing drug delivery systems and ability to encapsulate guest molecules with controlled
release rates.13 Van Der Voort et al. reported the first synthesis of periodic mesoporous
silica/organosilica with confinement channels namely plugged hexagonal templated silica
(PHTS) materials.14 One of the most important features of these materials arose from their non-
uniform walls and often constricted pores caused by the microporous silica plugs inside the
PHTS channels. The presence and extent of plugs could often be evaluated by N2 adsorption-
desorption analysis.15 Furthermore, the possibility of plug formation inside the pores of these
materials not only provides additional improved stability and appreciable degree of structure
tailorability,16 but potentially makes the plugged mesoporous materials attractive as
encapsulation media for increasing catalytic activity as well as stereoselectivity in some
enantioselective reactions through a cooperative confinement mechanism.17 However, the
synthesis of plugged PMOs by employing two bulky groups is quite rare, and to our knowledge
the use of plugged PMOs in which the chemical functionality, surface hydrophobic/hydrophilic
balance, and the plug properties could be tuned for catalytic applications has yet to be
unexplored.
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On the basis of our interest in heterogeneous catalysis, we found the selectivity of a number of
important chemicals transformations could be effectively controlled by adjusting the
hydrophobic-hydrophilic balance inside the pores of mesoporous silica materials, where the
catalytically active site are incorporated.18 We also discovered that mono-functional PMO with
bridging imidazolium groups (PMO-IL) can be successfully employed as a powerful functional
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support for the immobilization and stabilization of metal nanoparticles in various synthetically
important transformations.19 Inspired by the attractive confinement properties around the plugs
and within the mesoporous channels, we contemplated whether combining plug technology and
built-in bridging functional sites in PMOs might be used to cooperatively control (or adjust) the
product selectivity depending on the reaction profile in a certain catalytic transformation.
Although, the use of metal nanoparticles and/or metal catalysts in particular palladium
supported on either functionalized ordered mesoporous materials or PMOs is well-documented
in the aerobic oxidation of alcohols,20 to the best of our knowledge there has not been any study
concerning the impact of either surface physiochemical properties or channel plugging of these
materials in the selectivity control of this transformation. In this study, we report a novel
methodology for the preparation of bifunctional PMO (BFPMO) incorporating ionic liquids (ILs)
with either bridged functional groups [1,4-bis(triethoxysilyl)benzene (BTEB) or 1,4-
bis(trimethoxysilyl)ethane (BTME)] featuring highly ordered structures plugging the final
materials. In order to prove whether the surface physiochemical properties and plugs could
synergistically influence selectivity in the aerobic oxidation of alcohols, we first prepared a set of
supported Pd catalysts on the basis of plugged BFPMO-IL as outlined in Scheme 2. In addition,
to discriminate between the fractional influence of the plugs and organic groups (either phenyl or
ethyl) on the overall selectivity of the process, two types of supported Pd were prepared; a
supported Pd catalyst on a monofunctional PMO-IL comprising only imidazolium bridge in the
PMO framework (Pd@PMO-IL) and a Pd supported catalyst on an unplugged BFPMO-IL.21 The
catalytic performance of the two latter supported Pd catalysts were tested and compared to the
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plugged catalysts in the aerobic oxidation of benzyl alcohol under essentially the same reaction
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conditions. Preliminary investigations showed both the hydrophobic/hydrophilic balance and
plug formation inside the pores of these plugged catalysts have a remarkable effect on the
efficiency as well as the selectivity of the titled reaction under the described reaction conditions.
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Scheme 2. Schematic of plugged (A) and unplugged (B) Pd catalysts.
Results and discussion
Four phenyl-BFPMOs (denoted as BX-BFPMO-A, where B, X, and A indicates phenyl,
phenyl content, and method A, respectively) were first synthesized with organic contents ranging
from 5 to 20 % BTEB and 10% 1,3-bis(3-trimethoxysilylpropyl)-imidazolium chloride
(BTMSPCl) precursors22 i.e. all silica precursors such as bridged ionic liquid, BTEB, and
tetramethoxyorthosilicate (TMOS) were simultaneously added into solution.21 N2 adsorption-
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desorption isotherms and BJH (Barrett-Joyner-Halenda) pore size distributions for all BFPMO
are summarized in Figure 1. Nitrogen adsorption isotherms for all synthesized materials
exhibited type IV isotherm with a sharp capillary condensation step at relative pressure (P/P0) of
0.6-0.7, which confirmed the uniformity of cylindrical mesoporous channels in the materials
(Figure 1A). Interestingly, N2 desorption isotherms showed two steps similar to that reported for
PHTS materials (sub step at the P/P0 ≈ 0.45). Such adsorption-desorption isotherms are very
similar to those materials having both open and plugged cylindrical mesopores. Open pores are
gradually desorbed at reduced relative pressures but blocked pores will remain filled until the
vapor pressure is lowered to P/P0 ≈ 0.45 (lower limit for hysteresis P/P0) when condensed N2
eventually desorbs. While the first segment of the N2 desorption branch provides information on
open pores, the last segment can used to evaluate of pore size distribution in confined and
plugged sections.23 Results are in good agreement with the published literature.14-15 Calculations
from the adsorption branch using the BJH method (Figure 1B) show plugged B-BFPMOs have a
very narrow pore size distribution, with a small fraction of pore constriction. Increasing the
Si/P123 molar ratio leads to a decreases in both pore size distribution and pore volume (from 9.2
to 8.0 nm and 0.85 to 0.73 cm3 g-1, respectively). Micropore volume systematically increased
from 0.07 to 0.11 cm3 g-1 by the very gradual growth of microplugs in the B-BFPMO pores,
remarkably is similar to that reported for PHTS.14-15 Other textural properties determined by N2
physisorption and X-ray diffraction (XRD) data including BET area (SBET), unit cell (a0) and
pore wall thickness (tw) for all BX-BFPMO-A samples are summarized in Table 1. Estimated
BET area and wall thickness values vary from 518 to 569 m2 g-1 and 3.8 to 5.5 nm, respectively.
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Overall, the textural properties and shapes of the resulting isotherms suggested that the nature of
plugging is likely very similar for all synthesized B-BFPMOs (Figure 1A).
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Figure 1: (A) Nitrogen adsorption-desorption isotherms and (B) pore size distributions (B) for
BX-BFPMO-A (X: 5, 10, 15, and 20) series. Plot are offset on the vertical axis by 0, 210, 410
and 610 cm3 g-1 STP, and 0, 0.28, 0.5 and 0.9 cm3 g-1 nm-1, respectively.
Table 1. Textural properties of BX(5-20)-BFPMO-A and EX(5-20)-BFPMO-A materials
determined from nitrogen physisorption and XRD data.a
c
d
f
g
Organic
groups (%)
a0
SBET
Vte
Vmic
DBJH
Wth
Materials
Si/P123b
(nm) (m2g-1) (cm3g-1) (cm3g-1) (nm) (nm)
B5-BFPMO-A
BTEB (5)
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83
87
90
80
83
87
90
13.9
13.0
13.1
13.1
13.8
13.7
13.3
13.7
611
518
529
569
694
530
534
569
0.76
0.73
0.73
0.73
1.20
0.94
0.87
0.91
0.07
0.09
0.08
0.11
0.06
0.07
0.07
0.08
9.2
9.2
4.7
3.8
3.9
5.0
3.2
3.1
4.1
4.5
9
B10-BFPMO-A BTEB (10)
B15-BFPMO-A BTEB (15)
B20-BFPMO-A BTEB (20)
9.2
8.1
E5-BFPMO-A
BTME (5)
10.6
10.6
9.2
E10-BFPMO-A BTME (10)
E15-BFPMO-A BTME (15)
E20-BFPMO-A BTME (20)
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aAll materials are contained 10% IL precursors and also synthesized according to the method A
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b
which described in the Experimental Section. total mmol of Si in TMOS, BTEB (or BTME)
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and IL precursors.ca0 = unite cell derived from d100 values using a0=2d100/√3. SBET = specific
e
surface area was determined from the linear part of the BET plot (P/P0 ≈ 0.05-0.15). Vt = total
pore volume on the basis of N2 adsorbed at P/P0 ≈ 0.995. fVmic = micropore volume calculated by
the t-plots method. gDBJH = pore size distribution calculated from the adsorption branch using the
BJH methods (may underestimated values of mesopores). hwt = pore wall thickness calculated
using wt = a0- DBJH (may underestimate values of pore wall thickness)
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In the next stage, four ethane-containing BFPMO-A were also synthesized with various
organic contents ranging from 5 to 20% (denoted as EX (5-20)-BFPMO-A). N2 adsorption-
desorption isotherms and pore size distribution for these materials have also been performed and
summarized in Figure 2. A remarkable increase in plugging was observed in these materials with
increasing ethane contents. Plugging was strongly enhanced by increasing the Si/P123 molar
ratio in E-BFPMO as suggested by the broad hysteresis loop with a moderately delayed capillary
evaporation step that may be attributed to micro-plugs with distributed particle size. These
observations are consistent with other reports on the synthesis of PHTS materials prepared by
increasing Si/P123 molar ratios.14-16 However, the reason for the different behavior of BTME and
BTEB in creating plugs is not clear for us at this stage and we are currently working to find a
logical explanation. All E-BFPMOs exhibited very narrow pore size distribution and highly
ordered mesostructures regardless of the observed plugging and the presence of a significant
proportion of confined pores at high Si/P123 molar ratios (Figure 2B). Related textural properties
for all E-BFPMO materials are summarized in Table 1. Predictably, pore size distribution,
specific surface area and pore volume for these materials decreased from 10.6 to 9.2 nm, 694 to
534 m2g-1 and 1.2 to 0.87 cm3g-1, at increasing ethane content, respectively. Micro-porosity and
wall thickness were found to systematically increase from 0.06 to 0.08 cm3 g-1 and 3.1 to 4.5 nm,
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respectively, which are in good agreement with the published literature for PHTS materials.
These findings may be attributable to the presence of micro-plug encapsulation or non-
uniformity in the walls at increasing Si/P123 molar ratios.
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Figure 2: (A) Nitrogen adsorption-desorption isotherms and (B) pore size distributions (B) for
EX-BFPMO-A (X: 5, 10, 15, and 20) series. Plots are offset on the vertical axis by 0, 420, 750
and 975 cm3 g-1 STP, and 0, 0.2, 0.7 and 1.0 cm3g-1 nm-1, respectively.
It should be noted that we have conducted the synthesis of all materials under strongly acidic
conditions (pH<0.1) in order to ensure silica precursors are immediately converted to their
corresponding hydrolyzed species as soon as their methanolic mixture (solution) was added into
the initial HCl/Pluronic solution. Methanol was highly beneficial for adjusting the rate (slowing
down) of co-condensation of the precursors.14c,d It also suppressed excessive non-templated
polymerization in highly acidic solutions ensuring the formation of a well-ordered structure and
homogeneous distribution of all precursors inside the PMO framework.
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Transmission electron microscopy (TEM) images of B and E-BFPMO-A materials are shown
in Figures 3 and 4, respectively. The materials exhibited a highly ordered structure with well-
defined cylindrical, 2D-hexagonal pores, in good agreement with results obtained by N2
physisorption analysis. Ordered nanostructures preserved the long range ordering even at large
loadings of functional groups (up to 30%, B20-BFPMO-A, Figure 3f) or E20-BFPMO-A (Figure
4d). Energy dispersive X-ray spectroscopy (EDS) of the solvent extracted B- or E-BFPMO-A
materials did not show traces of KCl residue (Figure S3-S10), suggesting that KCl (used in the
synthesis) was not contributed as plugging agent.17a
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Figure 3. TEM images of (a) B5-BFPMO-A (scale bar: 5 nm), (b) (c) B10-BFPMO-A (scale
bar: 5 nm), (d) (e) B15-BFPMO-A (scale bar: 20 and 5 nm, respectively) and (f) B20-BFPMO-A
(scale bar: 100 nm).
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Figure 4. TEM images of (a) E5-BFPMO-A (scale bar: 10 nm), (b) E10-BFPMO-A (scale bar:
10 nm), (c) E15-BFPMO-A (scale bar: 25 nm) and (d) B20-BFPMO-A (scale bar: 5 nm).
Powder X-ray diffraction (PXRD) analysis was also carried out on both phenyl and ethyl-
containing BFPMO-A (Figures S11-12). Interestingly, the XRD patterns of all samples exhibited
three peaks with the d-value ratio of 1:1/√3:1/2 that can be indexed as d100, d110 and d200
reflections of the highly ordered 2D hexagonal symmetry (P6mm), respectively.24 These findings
are in good agreement with the N2 adsorption-desorption data and TEM images. Moreover no
KCl peaks were observed in the diffractogram over the range 20 to 70° 2θ (Figure S13), which
clearly indicated successful removal during P123 extraction with acidic ethanol.17a
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The integrity of the organic functional groups in the framework of the materials was studied by
13C cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy. Figure S14 shows
the solid phase NMR spectrum of E15-BFPMO-A. Examination of this spectrum clearly leads to
the conclusion that both ethyl and propyl imidazolium fragments remained intact during the
preparation and template removal stages. Notably, all 6 peaks that one might expect for E15-
BFPMO-A are observed. The 13C CP-MAS NMR spectrum for E15-BFPMO-A shows a
resonance peak at 4.8 ppm which is attributed to the methylene carbons in the ethane bridge
(Figure S14B). The peaks at 9.4, 24.2, and 52.4 ppm are attributed to C3, C2, and C1 in the
propyl groups bridged to the imidazolium ring and Si atoms in the E15-BFMPO-A
framework.22,25 The signals at 123.4 and 135.9 arise from carbons in CHCH and NHCN of the
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imidazolium ring, respectively. Moreover, no additional carbon peaks were detected in C CP-
MAS NMR of E15-BFPMO-A. This suggests that all of the Si-C bonds were retained during the
synthesis and surfactant extraction stages. However, the presence of two weak peaks at ≈70 and
13
≈ 73 ppm in the C CP-MAS NMR spectra of our E10-BFPMO-A sample are attributed to
carbons of the surfactant residue (marked by asterisks in Figure S14A), which likely remained in
the samples during the surfactant extraction by ethanol.26 The peak at 4.8 ppm tends to be
enhanced by increasing the content of BTME with respect to BTMSPCl in the bifunctionalized
samples E10-BFPMO-A, E15-BFPMO-A, and E20-BFPMO-A. Figure S14D shows a typical 13C
CP-MAS NMR spectrum of the bifunctionalized sample B20-BFPMO-B. A sharp peak at 133.7
with side bands of phenyl carbons, overlaps with the resonance from carbons in the NCHN of
imidazolium moiety. The spectrum also exhibits four resonances at 123.1, 52.5, 24.0, and 9.6
ppm that are characteristics of CHCH in imidazolium rings and C1, C2, and C3 carbons of the
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propyl bridges. The weak peaks at ≈ 75 ppm, is likely due to the carbons of the Pluronic P123
residue.26 However, this spectrum also suggests that organic precursors such as BTEB and
BTMSPCl have been embedded in the BFPMO framework.
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29Si NMR CP-MAS NMR spectroscopy experiments were carried out to evaluate the
incorporation of organic groups i.e. imidazolium, phenyl and ethyl groups inside the pore walls
of the BFPMO-As (Figures S15-16). 29Si NMR spectra of all phenyl- PMO-A materials
displayed three distinctive peaks at δ= -59.5, -68, and -78 ppm, which are attributed to covalently
bonded Si species to carbon atoms T1 [C-Si-(OSi)(OH)2], T2 [C-Si-(OSi)2(OH)] and T3 [C-Si-
(OSi)3], respectively.27 E-BFPMO-A spectra jointly display two signals at -58 and -65 ppm that
correspond to Si atoms covalently bonded to carbon atoms T2 [C-SiO2(OH)] and T3[C-SiO3],
respectively.27 The absence of a T1 signal in E-BFPMO confirmed that the degree of network
29
condensation for these materials is larger compared to BX-BFPMO materials. Furthermore, Si
NMR spectra for all BFPMO show three peaks at δ= -91, -100, and -109 ppm, related to Q2[Si-
(OSi)2(OH)2], Q3[Si-(OSi)3(OH)] and Q4[Si-(OSi)4] silica network species, respectively.28 The
degree of framework condensation was found to be high as evident from the higher percentage of
fully cross-linked T3 and Q4 silicon sites. Comparison of the solid state 29Si NMR spectra
(Figures S15-S16 and Table S1) shows that with increasing content of BTEB or BTME in the
initial gel, the intensity of Tn bands with respect to Qn bands gradually increased which illustrates
successful condensation and incorporation of IL moieties into the mesoporous walls.
The presence of organic groups in the materials was also confirmed by diffuse reflectance
infrared Fourier transform spectroscopy (DRIFT) measurements (Figures S17-19). All phenyl or
ethyl-BFPMO-A exhibited similar patterns with differences only in peak intensity. Sharp and
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wide peaks appeared at ca. 925 and 1096 cm-1, corresponding to asymmetric and symmetric
stretching vibrations of Si-O-Si bonds.29 The peak at 3125 cm-1 may be assigned to the
asymmetric stretching vibration of C-H in the phenyl ring and/or imidazole groups.30 Ethyl and
propyl functional groups in BFPMO-A contribute with several peaks in the 2930-3059 cm-1
range (aliphatic C-H stretching).29 Moreover, the presence of imidazolium functionality can be
generally confirmed by peaks at 1620 cm-1 that are assigned to the stretching vibration of C=N
bonds.30 Additionally, the bands at 1560 cm-1 which correspond to the stretching vibration of
C=C bonds confirmed the presence phenyl groups in the materials.30 Peaks at 700-750 and
3300 cm-1 can be ascribed to bending vibrations of Si-C bonds and stretching vibration of O-H in
surface silanol groups,30,31 respectively, which in addition to aforementioned peaks confirmed
the stability of PMO precursors during the Sol-Gel process, aging and surfactant extraction
stages.
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Thermal stability of BFPMO-A was evaluated by thermal gravimetric analysis (TGA). TG
patterns of synthesized materials were recorded in the range of 25-800 °C (Figures S20-27). The
first weight loss (ca. 5-8 %, below 100 °C) can be correlated to physisorbed water and alcohol
desorption occluded in the pores or on the surface of the materials. Phenyl-containing materials
have been proved to be slightly more stable as compared to their ethyl-containing counterparts
owing to the insignificant weight loss between 100°C and 300°C.32 In any case, all BFPMO-A
exhibited high thermal stability, with total measured weight loss of organic functional groups
totally removed at temperatures >400 ºC as measured by TGA, which is in close agreement with
theoretical organic loadings and elemental analyses data (Table S2). These data clearly revealed
that the all bridged precursors were preserved after the Sol-Gel process and even after extraction
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of SDA under acidic ethanol conditions. These findings were also in good agreement with results
from 29Si CP-MAS NMR (Figures S15-16).
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Elemental analysis (CHN) was carried out to evaluate the integrity of the organic bridging
functional groups inside the framework of our materials. As can be seen in Table S2, the weight
percentage of carbon increase and according to the nitrogen percentages decreased with
increasing either the BTME or BTEB content in the EX(5-20)-BFPMO-A and BX(5-20)-
BFPMO-A series, respectively, which again confirms the incorporation of all bridging groups
into the framework of the materials.
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Finally, a set of supported Pd catalysts based on our plugged BFPMO-IL were successfully
2-
prepared by replacing the chloride ions in parent BFPMO with PdCl4 using a modified ion
exchange technique (Scheme 2).33 In addition, to discriminate between the fractional influence of
the plugs and organic groups (either phenyl or ethyl) on overall selectivity of the aerobic
oxidation of alcohols (vide infra), a set of Pd supported catalyst based on unplugged BFPMO-IL
(Pd@U-B20, Pd@u-E10 and Pd@U-E15) with very close metal loading were also prepared by
adjustment the synthesis condition (stepwise addition of silica precursors).21 Using inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) of acid washed samples, the amount
of Pd in either synthesized plugged or unplugged catalysts was determined to be in range from
0.08 to 0.096 mmol g-1 (Table 2).
Table 2: Chemical and physical properties of catalysts used in screening studies.a
d
Entry Cat. name
F.G (%)b SBETc (m2 g-1) Vt (cm3 g-1) DBJHe (nm) Pdf (mmol.g-1)
1
2
3
Pd@U-B20g
Pd@U-E10h
Pd@U-E15
B (20)
E (10)
E (15)
400
371
395
0.5
0.82
0.83
10.6
10.6
10.6
0.085
0.096
0.086
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Pd@P-E10i
Pd@P-E15
Pd@P-E20
E (10)
E (15)
E (20)
450
378
452
0.88
0.67
0.78
10.6
9.2
9.2
0.080
0.084
0.085
aAll materials are contained 10% bridged IL and synthesized according to the either method A
b
(plugged) or B (unplugged) which described in the Experimental Section. Bridge functional
group. cSBET = specific surface area determined from the linear part of the BET plot (P/P0 ≈ 0.05-
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d
e
0.15). Vt = total pore volume based on the N2 adsorbed at P/P0 ≈0.995. DBJH= pore size
distribution calculated from the adsorption branch using BJH methods. Loading of Pd in the
either plugged or unplugged catalyst as determined by ICP-AES of acid washed sample. U=
unplugged catalyst and B: phenyl functional group. E= ethyl functional group. P= plugged
catalyst.
f
g
h
i
The catalytic activity of all catalysts (0.5 mol%) was first examined in the aerobic oxidation of
benzyl alcohol by using water as a green solvent at 90 °C. In our earlier study, we showed that
Pd nanoparticles supported on mono functional PMO-IL (Pd@PMO-IL) was an effective catalyst
for the selective oxidation of alcohols to the corresponding carbonyl compound with the
triflourotoluene (TFT) as the solvent.34 However, this catalyst exhibited ineffective catalytic
activity in an aqueous medium (Table 3, entry 1 and 2). Therefore, the unplugged catalyst
Pd@U-B20 which contained up to 20% phenyl group was evaluated under the same reaction
condition. This reaction produced 43% benzaldehyde after 20 h (Table 3, entry 3). These results
suggests the presence of the phenyl groups inside the nanospaces of the unplugged-BFPMO
framework plays a decisive role in improving catalytic activity likely by enhancing the surface
hydrophobicity, which in turn favors faster diffusion of benzyl alcohol into the mesoporous
channels where the Pd species are located. To obtain further insight into this issue, we
investigated whether the use of BFPMO having appropriate concentration of ethyl functional
groups would result in to a similar catalyst improvement. Although, the aerobic oxidation of
benzyl alcohol in the presence of Pd@U-E10 (0.5 mol%) comprised of 10% imidazolium and
10% ethyl functional groups inside the framework (E10-BFPMO-A) produced 44%
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benzaldehyde yield within 15h at 90 °C (Table 3, entry 4), the same reaction using Pd@U-E15
(0.5 mol% as well) with 10% and 15% imidazolium and ethyl functionalities, respectively,
furnished 90% benzaldehyde yield and an excellent selectivity of >99% (Table 3, entry 5).
Considering the hydrophobic nature of PMOs through the sequence ethylene >phenylene>
methylene > silica,35 the outstanding catalytic performance of Pd@U-E15 might be amenable to
the hydrophobic-hydrophilic balance arising from the ethyl units inside the mesochannels of the
unplugged BFPMO-IL support. This in turn favors easier diffusion of the starting alcohol into
the nanospace of the catalyst, thereby enhancing the catalytic activity Pd@U-E15 under the
described aqueous reaction conditions. Encouraged by these promising results, we then directed
our attention toward the oxidation of some substituted benzylic alcohols in water. p-methoxy and
p-methylbenzyl alcohols were oxidized to the corresponding aldehydes in excellent yields and
selectivities without any over-oxidation to the carboxylic acids (Table 3, entries 6 and 7).
Moreover, p-chlorobenzyl alcohol was also oxidized to the related aldehyde with good yield and
high selectivity (Table 3, entry 8). Particularly, unsupported catalyst (Na2PdCl4) with the same
catalytic loading displayed no activity under the identical reaction conditions (Table 3, entry 9).
This data implies that the presence of hydrophobic groups (ethyl moiety) and the active catalyst
in immobilized form is indispensable to attaining high activity in water by employing the present
catalyst system.
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What is appealing in these investigations is that despite using water as the reaction solvent the
amount of the by-product benzoic acid remained below 5% even after prolonged reaction times
at 90 °C and under pure O2 atmosphere in the presence of unplugged Pd@U-E15 (Table 3, entry
8). Therefore, we next investigated whether the confinement effect caused by the plugged
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mesopores as well as the physiochemical properties in the proximity the plugs could also
synergistically contribute to the selectivity of aerobic oxidation of alcohols under the presented
reaction conditions.
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Table 3. Aerobic oxidation of benzylic alcohols catalyzed by unplugged Pd@BFPMO.a
Entry
1c
R
H
Catalyst
T (h)
24
Conversion (%)
-
A(%)b
-
B(%)b
-
Pd@PMO-IL
2d
H
H
Pd@PMO-IL
Pd@U-B20
Pd@U-E10
Pd@U-E15
Pd@U-E15
Pd@U-E15
Pd@U-E15
Na2PdCl4
24
15
15
15
15
15
15
24
14
43
44
90
95
>99
79
-
14
43
44
90
95
>99
74
-
-
-
3
4
H
-
5
H
-
6
CH3
OCH3
Cl
-
7
-
8
<5
-
9e
H
aReaction conditions: 0.5 mmol alcohol, 1.5 mmol K2CO3, 1 atmosphere O2, 0.5 mol%
b
c
catalyst, and 1.5 ml H2O at 90 °C. GC yield using internal standard method. Pd catalyst based
on mono-functional PMO -IL at 80 °C, Ref 36.dAt 90°C, Ref. 36.e0.5 mol% Na2PdCl4 was used.
Three supported Pd catalysts with approximately the same Pd loading were prepared via
simple impregnation of plugged E20-BFPMO-A, E15-BFPMO-A, and E10-BFPMO-A with an
aqueous solution of Na2PdCl4 to produce the corresponding plugged catalyst Pd@P-E20, Pd@P-
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E15, and Pd@P-E10 (Table 2), respectively, in a way similar to that previously described for the
preparation of unplugged catalysts (Scheme 2). It is worth noting that the extent of the plugging
in these catalysts was estimated to be in the order Pd@P-E20≈ Pd@P-E15> Pd@P-E10, as
evidenced by N2 sorption analysis. In addition, no apparent loss of N2 adsorption-desorption
isotherms was found upon Pd incorporation within the indicated range of Pd content in the
materials, suggesting the mesoporous structure and degree of plugging of the materials was
mainly preserved under the described doping conditions (Figures S32-S42). The choice of
pristine plugged bifunctional PMOs was motivated by the initial promising results obtained with
the Pd supported catalyst on the ethyl functionalized BFPMO-IL (catalyst Pd@U-E15) in the
aerobic oxidation of alcohols (Table 3). Therefore, in order to understand how plugs may impact
the selectivity of the process, plugged ethyl-bridged BFPMO-IL, was employed as the support
for the remainder of the study.
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After initial characterization of the plugged catalysts Pd@P-E10, Pd@P-E15, and Pd@P-E20
(Figure 32-42), their catalytic performance (0.5 mol% Pd) was tested and compared in the
aerobic oxidation of benzyl alcohols under essentially the same optimized reaction conditions
presented in Table 3 for the unplugged catalysts Pd@U-B20, Pd@U-E10, and Pd@U-E15.
Unless otherwise noted, to compare the performance (both selectivity and reactivity) of all
catalysts, the reaction time was fixed at 15 h. Interestingly, while the oxidation of benzyl alcohol
in the presence of Pd@P-E10 (0.5 mol%) having 10% ethyl bridges afforded 76% conversion,
the yield of benzaldehyde dropped to 30% with concomitant formation of benzoic acid at a 46%
yield (Table 4, entry 1). Catalyst Pd@P-E15 with a slightly higher ethyl content (15%) in the
mesoporous network exhibited improved activity leading to a 95% benzyl alcohol conversion
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(Table 4, entry 2). There was, however, a noticeable change in product selectivity in compared to
the unplugged catalyst Pd@U-E15 (Table 3, entry 5) having almost the same ethyl loading under
essentially identical reaction condition. Hence, we suspected here that the key factor explaining
the remarkable selectivity difference between Pd@U-E15 and Pd@P-E15 might be ascribed to
the presence of plugs inside the mesochannels of Pd@P-E15. This surprising difference in
behavior between two catalyst systems encouraged us to further investigate the possible
synergistic role of plugs and organic (ethyl) bridge in our system. This was further stimulated by
the fact that the effective methods for direct oxidation of benzylic alcohols into their benzoic
acid in the presence of Pd catalysts are extremely rare,36 since, it has been well-postulated that
even low concentration of benzyl alcohol can effectively inhibit the over oxidation of
benzaldehyde into benzoic acid.37 Therefore, our next step was to determine whether the
observed product selectivity could be produced by applying plugged catalyst Pd@P-E15 under
the same reaction conditions for the oxidation of a range of benzylic alcohols similar to that
demonstrate in Table 3 (Table 4, entries 3-5). It was found that the enhanced benzoic acid
formation could also be achieved in water suggesting the presence of plugs in catalyst Pd@P-
E15 might indeed have some marked impact on the reaction selectivity (Table 4, entries 3-5). To
collect more information on the synergistic role of plugs and ethyl bridge in our system, the
oxidation of the same range of alcohols was carried out using catalyst Pd@P-E20 having 20%
ethyl moiety in the mesoporous network (~5% higher ethyl concentration than of Pd@P-E15)
under the same reaction conditions (Table 4, entries 6-9).
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Table 4. Aerobic oxidation of benzylic alcohols catalyzed by plugged Pd@BFPMOa.
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Entry
R
Catalyst
T (h) Conversion (%)
A(%)b
30
B (%)b
46
1
2
3
4
5
6
7
8
9
H
H
Pd@P-E10 15
Pd@P-E15 15
Pd@P-E15 24
Pd@P-E15 24
Pd@P-E15 20
Pd@P-E20 15
Pd@P-E20 24
Pd@P-E20 24
Pd@P-E20 24
76
95
2
93
75
60
86
78
53
50
69
CH3
OCH3
Cl
>99
>99
>99
>99
88
25
40
14
21
35
35
5
H
CH3
OCH3
Cl
85
74
aReaction condition: 0.5 mmol alcohol,1.5 mmol K2CO3, 1 atmosphere O2 , 0.5 mol% catalyst,
and 1.5 ml H2O at 90 °C. GC yield using the internal standard method of caliration.
b
By increasing the ethyl bridge content in the material, a meaningful improvement in the overall
consumption of starting alcohols (up to >99% conversion) was observed. This is due to the
gradual increase in alcohol diffusion into the system pore space that likely arises from a
favorable interaction of the hydrophobic corona of alcohol molecules with the hydrophobic
mesopores of the catalyst. Although consideration of this interaction help to explain the
relationship between the degree of hydrophobicity of the mesochannels in our catalysts based on
the ethyl bridge-plugged BFPMO-ILs and their catalytic performance (i.e benzyl alcohol
conversion) in water, it does not explain the increased selectivity to benzoic acid derivatives
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when ethyl content was increased from 10 to 15% and the noticeable drop when increased to
20%. By comparing the data in Table 3 and 4 of the oxidation of primary benzylic alcohols using
unplugged (Pd@U-E15) and plugged (Pd@P-E15), respectively, we speculated that while an
appropriate concentration of ethyl bridge in our system was indispensable to ensuring acceptable
conversion, the presence of plugs resulted in the dramatic shift in reaction selectivity to the
benefit of benzoic acid generation. There was the critical consideration of how the plug
characteristics and the physicochemical nature of the nanospaces in the mesoporous solid Pd@P-
E15 as well as reaction profile could be rationalized. It is well-known that in the preparation of
plugged SBA-15, plugs can form at high TEOS (tetraethoxyorthosilicate)/template ratios,14 as
was the case in the synthesis of our materials. This would suggest that the plugs in our materials
mainly consisted of amorphous silica nanocapsules likely originated from the hydrolysis and
condensation of TEOS. Hence, it is reasonable to assume that the silica nanocapsules have a high
surface concentration of silanol groups which provide markedly hydrophilic nano-regions inside
the hydrophobic mesochannels of the BFPMO-IL (Scheme 3). In considering the reaction profile
of alcohol oxidation (Scheme 3), we hypothesized that a hydrophilic nano-region can strongly
interact with hydrated aldehyde (gem-diol), presumably through hydrogen bond interaction. As a
consequence, it appears that the presence of plugs in the interior of the nanospaces of catalyst
Pd@P-E15 would enable a longer retention of hydrated aldehyde inside the system pore space in
close proximity to available Pd active site, which thus over oxidized to produce the
corresponding acid. This model may also explain the consistent decrease in selectivity of benzoic
acid formation when more hydrophobic Pd@P-E20 (with 20% ethyl bridge) is used rather than
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catalyst Pd@U-E15. At an ethyl bridge content >15% (i.e~ 20%), the hydrophobic nature of the
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catalyst become dominant rendering the mesoporous environment less favorable to interact with
the hydrated aldehydes and prohibiting to some extent penetration of the hydrated aldehyde into
the pore system. This would certainly decrease the concentration of hydrated aldehyde in the
proximity of active sites, thereby limiting the formation of acidic products and leading to a drop
in the reaction selectivity toward the formation of carboxylic acid. Based on this model, the
excellent selectivity toward aldehyde formation in the case of unplugged catalyst Pd@U-E15
might be attributed to the pronounced hydrophobic character of the open pore system in Pd@U-
E15, which makes diffusion of hydrated aldehyde from the aqueous phase into the mesopores
more difficult, thus lowering its over oxidation to the corresponding carboxylic acids. The role of
the physiochemical and structural characteristic of the described material in reaction selectivity
was further confirmed by the lack of detectable oxidation products in the control experiments in
the absence of either plugged or unplugged BFPMO-IL (Table 3, entry 9). Therefore, from these
observation it should be noted the optimum amounts of ethyl bridge (15% ethyl), bridge
imidazolium moiety (10%), and concomitant control of plugs in the interior of the mesochannels
of the catalyst are reached at compromise between high catalysts activity and reaction selectivity
(toward the formation of either aldehyde or carboxylic acid), most likely through a synergistic
(cooperative) mechanism.
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An alternative explanation may be also that for the plugged systems the effective diffusion of
generated aldehyde from the system pores becomes lower in comparison with unplugged
catalysts, giving rise to enhanced concentration of aldehyde (or gem-diol) at available Pd active
site in the mesopores and thus causing its more pronounced reaction to corresponding carboxylic
acid.
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Scheme 3. Schematic representation of catalyst Pd@U-E15 (A) and Pd@P-E15 (B)
In the final part of this study, recyclability of either plugged or unplugged catalysts was
examined in the aerobic oxidation of benzyl alcohol under the described reaction conditions. In
this regard, after each reaction run, the catalysts Pd@U-E15 and Pd@P-E15 were recovered
using simple filtration and reuse for four subsequent reactions without any remarkable decrease
in the catalyst activity and selectivity under the same reaction conditions (Figures S60 and S61).
To determine the leaching of Pd species into solution, a hot filtration test was performed. The
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Pd@P-E15 was filtered out after the aerobic oxidation of benzyl alcohol had run for 8h
(conversion 37%). The solid-free filtrate was then welded to another reaction vessel and
continually stirred under standard reaction conditions (oxygen atmosphere, 90 °C). After 24h, the
analysis of the catalyst-free reaction showed that <3% benzoic acid was produced. This data was
confirmed by atomic adsorption analysis of the filtrate solution and showed no Pd within the
limit of detection (< 1 ppm).
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To obtain information about the nature of the active Pd species in the above mentioned
catalysts, samples of fresh and recovered catalyst Pd@P-E15 were studied using X-ray
photoelectron spectroscopy (XPS). Figure 5 shows the XPS spectra of the region corresponding
to the binding energy range of 320-360 eV, which includes Pd 3d3/2 and Pd 3d5/2 peaks.
Predictably, the fresh catalyst Pd@P-E15 shows peaks located at 336 and 342 eV that were
assigned to the Pd(II) 3d5/2 and Pd(II) 3d3/2 levels, respectively.38 This binding energy clearly
supported the Pd species in the fresh catalyst only as the Pd(II) form. On the other hand, the
recovered catalyst Pd@P-E15 exhibited a similar pattern with slightly lower bonding energy
which is evidence of presence the Pd catalyst in the metallic form.38 Therefore, it is reasonable to
speculate that the Pd species is most likely in the metallic form and the active component in our
catalyst system.
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Figure 5. XPS spectra of the fresh and reused Pd@P-E15
Figure 6 is a typical TEM image of the recovered Pd@P-E-15 catalyst after the 4th reaction
cycle. The image shows that the 2-D-hexagonal structure of the pristine BFPMO-IL is mostly
intact and Pd nanoparticles as well distributed throughout the regular channels. These clearly
demonstrate the mesoporous support provides the means for distributing and stabilizing the Pd
nanoparticles inside the mesochannels of the catalyst system during the reaction.
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Figure 6. TEM image of recovered Pd@P-E15 (Scale bar: 20 nm)
Conclusion
We presented the preparation and characterization of a set of novel Pd catalysts based on
plugged and unplugged bifunctional imidazolium-phenyl and -ethyl PMOs by varying the
content of phenyl or ethyl bridges from 5 to 20%. Analysis showed these materials have 2D
P6mm symmetry with narrow pore size distributions and a varied degree of plugging in the
mesopores channels. The co-existence of all described organic functionalities in the samples was
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successfully quantified by employing C CP-MAS NMR, Si CP-MAS NMR, DRIFT-IR, and
elemental analysis, which also revealed that the relative organic contents can be varied within the
indicated range without deterioration of the ordered structure. By modifying the condition of
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