Designed Synthesis of Mesoporous Solid-Supported Lewis Acid-Base

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Surfaces, Interfaces, and Applications

Designed Synthesis of Mesoporous Solid-Supported Lewis Acid-Base Pairs and their CO2 Adsorption Behavior Maria Zakharova, Nima Masoumifard, Yimu Hu, Jongho Han, Freddy Kleitz, and Frédéric-Georges Fontaine ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00640 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Materials & Interfaces

Designed Synthesis of Mesoporous Solid-Supported Lewis Acid-Base Pairs and their CO2 Adsorption Behavior Maria V. Zakharova,a,b Nima Masoumifard,b Yimu Hu,a,b Jongho Hanc,d Freddy Kleitz*b,e and Frédéric-Georges Fontaine*a

a

Département de Chimie, Centre en Catalyse et Chimie Verte (C3V)

Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V 0A6, Canada E-mail: [email protected] b

Département de Chimie, Centre de Recherche sur les Matériaux Avancés (CERMA) Université Laval, Québec, QC G1V 0A6, Canada c

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic of Korea d

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon, 305701, Republic of Korea e

Department of Inorganic Chemistry – Functional Materials, Faculty of Chemistry, University of Vienna, Währinger Str. 42, 1090 Vienna, Austria E-mail: [email protected]

KEYWORDS: Lewis acid-base pairs, mesoporous silica, SBA-15, carbon dioxide capture, isosteric enthalpy of adsorption, surface functionalization

Abstract:

Conventional

amines

and

phosphines,

such

as

diethylenetriamine,

diphenylpropylphosphine, triethylamine, and tetramethylpyperidine, were grafted or impregnated on the surface of metalated SBA-15 materials, such as Ti-, Al-, Zr-SBA-15, to generate air-stable solid-supported Lewis acid-base pairs. The Lewis acidity of the metalated materials before and

1

ACS Paragon Plus Environment

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

after the introduction of Lewis bases was verified by means of pyridine adsorption-FTIR technique. Detailed characterization of the materials was achieved by solid state 13C, 31P and 1H MAS NMR spectroscopy, low temperature N2 physisorption, XPS and EDX mapping analyses. Study of their potential interactions with CO2 was performed using CO2 adsorption isotherm experiments, which provided new insights into their applicability as solid CO2 adsorbents. A correlation between solid supported Lewis acid-base pair strength and the resulted affinity to CO2 is discussed based on the calculation of isosteric enthalpy of adsorption.

Introduction In order to limit the impact of climate change, novel technologies are developed to reduce the anthropic emissions of CO2. Carbon capture and storage has been proposed as a practical way to reduce CO2 emissions, but proves to be cost prohibitive and difficult of implementation.1 An interesting alternative is the use of CO2 as a C-1 feedstock to produce industrially relevant molecules. Although several homogeneous 2-6 and heterogeneous4,7-16 catalysts have been reported to transform CO2 into products of interest, such as methane, methanol and formaldehyde, there is still an important place left for catalyst development to increase the efficiency of these processes. In the past decade, the advent of metal-free catalysis, notably heralded by the discovery of Frustrated Lewis Pairs (FLPs) by Douglas Stephan, has led to a paradigm shift in catalysis.17 The combination of sterically congested Lewis pairs leads to a dramatic increase in reactivity in the activation of many substrates, such as hydrogen and carbon dioxide, but recent developments have shown that frustration is not always required and many classical Lewis pairs can catalyze FLP transformations efficiently. 18 In the past few years, several FLP systems have been shown to act as catalysts for the reduction of CO 2 into useful chemicals, in some cases surpassing the activity of transition metal catalysts. 1920

Since the immobilization of catalysts on solid supports allows for better recyclability

and product purification, the heterogeneization of FLPs is of interest for the implementation of industrial processes. Even though the concept of FLP was not formally introduced in surface science at the time, several reports on CO2 chemistry were articulated around the basic principles of FLP catalysis: the combination of a Lewis acid and a Lewis base for the activation of a 2


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substrate. Jones described the synthesis of hybrid poly(ethyleneimine)-impregnated mesoporous silica SBA-15 materials containing heteroatoms (Al, Ti, Zr, and Ce) in their silica matrices and examined the CO2 adsorption simulating flue gas (10% CO2 in Ar) and ambient air (400 ppm CO2 in Ar) to assess the effects of heteroatom incorporation on the adsorption properties.21 He demonstrated the stabilizing effect of the metals in these materials, enhancing adsorbent capacity, adsorption kinetics, recyclability, and stability of the supported aminopolymers over multiple cycles. However, the metallic centers were used as amine stabilizers rather than functional groups for CO 2 activation. Thus, only reactivity typically observed for amines, such as the formation of carbamates, was observed. Similar results were reported by Notestein who reported CO2 adsorption studies on Ti-SiO2 materials with grafted aminopropyltriethoxysilane (APTES) and (3triethoxysilylpropyl)-tert-butylcarbamate (TESPtBC).22 By combining experimental results and density functional theory (DFT) calculations, Coperet and Sautet showed that thermally pretreated γ- and δ-Al2O3 exhibit unique tricoordinate AlIII Lewis acid sites, which are present as metastable species exclusively on the major (110) termination of aluminum oxide particles, corresponding to the “defect” sites.23 In combination with specific surface oxygen atoms, they form extremely reactive Al, O Lewis acid−base pairs, which are, in fact, largely responsible for the splitting of CH4 and H2. Furthermore, Guo demonstrated that a gold surface could serve as a Lewis acid to construct effective frustrated Lewis pairs (FLPs) when coupled with Lewis bases (e.g., imine and nitrile) to activate H2 and subsequently to achieve hydrogenation of small imines and nitriles.24 He demonstrated computationally that the gold surface does not tightly adsorb Lewis bases because of the repulsion between the nitrogen lone pair and the filled d-band of gold. Recently, Ozin demonstrated computationally and experimentally while studying the photochemical reduction of CO2 to CO with In2O3 that CO2 dissociates to release CO only at specific surface sites containing adjacent Lewis basic In−OH groups and Lewis acidic In atoms. 25-27 Light-induced electronic transitions that are delocalized on the In and O atoms of the In2O3−x(OH)y surface enhance their Lewis acidic and Lewis basic character. Similarly, a robust indium–organic metal-organic framework (InOF-15) decorated with a quinoline-based dicarboxylic acids demonstrated synergistic effect between the open metal sites and the Lewis basic sites, providing high 3



selectivity for CO2 adsorption over CH4 and N2 (CO2/CH4: 98.2 and CO2/N2: 7.7) at 273 K and 1.0 bar.28

Molecular approaches in the generation of solid-state FLPs have also emerged. Taoufik et al. reported the immobilisation of a (4-hydroxyphenyl)diphenyl phosphine on a trisisobutyl aluminum modified silica support, which after treatment with B(C6F5)3 or HB(C6F5)2 produces surface bound FLPs. This material exhibits highly Z-selective reduction of 3-hexyne under forcing conditions (40 bars, 80 °C).29 O'Hare also proposed the direct synthesis of a solid-phase frustrated Lewis pair (s-FLP) by combining a silicasupported Lewis acid (RSiOB(C6F5)2, s-BCF) with a Lewis base (tBu3P) to give [RSiOB(C6F5)2][tBu3P]. Reaction of this s-FLP with H2 under mild conditions led to the heterolytic H–H bond cleavage, which was extensively characterized spectroscopically. 30 In order to use solid-state FLP systems for industrial applications, in particular related to CO2 capture and conversion, some issues need to be resolved. First, the Lewis acid part of the FLPs needs to be stable to hydrolysis, excluding the use of most borane derivatives used thus far. Also, synthesis needs to be greatly simplified. As such, it is entailing to consider the metalation of mesoporous materials as a way to generate strong water tolerant Lewis acids.31-35 In this contribution, we explored the synthesis of a stable solid supported Lewis pair systems based on Al-, Ti- and Zr-SBA-15 mesoporous solids. The Lewis bases, such as phosphines and amines, were either anchored on the surface by direct grafting through modified triethoxysilane analogues, or simply impregnated in the pores of the materials. The affinity of the obtained solid ambiphilic systems for CO2 was tested through the calculation of isosteric enthalpy of CO2 adsorption, to quantify energetically the synergistic or antagonistic effects between the Lewis acid and the Lewis base moieties. Potential reactivity with carbon dioxide will be discussed, as well as issues and challenges in the materials characterization.

Experimental Methods Materials.

4


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ACS grade organic solvents were used for the synthesis of metalated mesoporous SBA-15 materials without further purification. Titanium iso-propoxide, zirconium butoxide and aluminum iso-propoxide were purchased from Sigma-Aldrich and used as received. For the synthesis and the grafting or impregnation of Lewis bases, toluene, hexane and diethyl ether

solvents

were

sodium/benzophenone

pre-treated before

and

use.

distilled

under

a

N2

atmosphere

over

N1-(3-trimethoxysilylpropyl)diethylenetriamine

(technical grade, Sigma-Aldrich), vinyltrimethoxysilane (98%, Sigma-Aldrich), 1-bromo4-iodobenzene (98%, Sigma-Aldrich), bis(2,4,6-trimethylphenyl)phosphorus chloride (95%, Sigma-Aldrich), tert-butyllithium solution (1.7 M in pentane, Sigma-Aldrich), (3iodopropyl)trimethoxysilane (≥95.0%, Sigma Aldrich), diphenylphosphine (98%, Sigma Aldrich), 2,2,6,6-tetramethylpiperidine (≥99%, Sigma Aldrich), triethylamine (≥99%, Sigma Aldrich), tri-tert-butylphosphine (98%, Sigma Aldrich), amphiphilic block copolymer P123 (Sigma-Aldrich, MW = 5800) and tetraethylorthosilicate (TEOS, SigmaAldrich) were purchased and used without further purification. Pyridine (99.8%) was purchased from Sigma-Aldrich and distilled under an N2 atmosphere.

Synthesis of SBA-15 silica. Mesoporous silica SBA-15 was synthesized using amphiphilic block copolymer P123 (Sigma-Aldrich, MW = 5800) as a structure-directing agent and tetraethylorthosilicate (TEOS) as a silica source.36 In a typical synthesis, 10 g of P123 was dissolved in a mixed solution of distilled water (75 g) and HCl (2 M, 250 mL) at 35 °C during 2 h. TEOS (21.5 g) was then added into the clear solution and the mixture was stirred at 35 °C for 20 h, followed by heating the solution at 100 °C for 24 h under static conditions. The resulting white precipitate was stirred in water (100 mL) containing few drops of concentrated HCl for 30 minutes, filtered off and washed with water (100 mL). Calcination of the resulting silica at 550 °C for 5 h in air resulted in mesoporous silica SBA-15.

Metalation of SBA-15. Ti-, Al- and Zr-SBA-15 materials with different molar ratios were synthesized using a post-grafting method previously reported.31 In a typical synthesis, 6 g of P123 was dissolved in a mixed solution of distilled water (114 g) and HCl (37 wt%, 3.5 mL) at 35 5



°C for 2 h. TEOS (21.5 g) was then added into the clear solution and the mixture was stirred at 35 °C for 20 h, followed by heating at 100 °C for 24 h under static conditions. The resulting suspension containing precipitated SBA-15 (5 g) was added to a mixture of distilled water (145 g) and ethanol (15 g) at room temperature. After the pH of the solution was adjusted to 10.0 with diluted ammonia, a mixture of ethanol (20 g), the respective metal precursor (titanium iso-propoxide, zirconium butoxide and aluminum iso-propoxide) and acetyl acetone (AA) (AA/M = 3.15) were slowly added to the mixture at 15 °C and then stirred for 2 h. After filtration and washing with ethanol, the material was calcined at 550 °C for 5 h in order to immobilize the metallic species on the silica surface. The resulting Ti4+-, Zr4+- and Al3+-deposited mesoporous silica samples with different metal contents were denoted as M-x-SBA-15 (M=Ti, Zr, Al; x = metal molar % obtained from X-ray photoelectron spectroscopy (XPS) surface analysis). The silica yield is assumed to be 100 %.

Passivation of M-SBA-15. The passivation of surface silanol groups was performed using tetramethyldisilazane (TMDS) as a silylating agent.37 In a typical protocol, 1 g of SBA-15 material preliminary degassed at 160 °C in a vacuum oven overnight was dispersed in 50 mL of dry hexane and 2 mL of TMDS was added dropwise under vigorous stirring. The mixture was left stirring for 24 h at room temperature. The excess TMDS was removed by Soxhlet extraction for 12 h at 60 °C using dichloromethane as a solvent. The obtained passivated materials were then dried in a 60 °C oven for 12 h. Synthesis of 2-diphenylphosphinetrimethoxysilane.38 According to a Knochel protocol, to a stirring solution of sodium methoxide (93 mg, 1.7 mmol, 30 mol% in NMP) in dimethyl sulfoxide (DMSO) (2 mL) were successively added under inert atmosphere diphenylphosphine (1.1 g, 5.7 mmol) and vinyl trimethoxysilane (850 mg, 5.7 mmol). The reaction was stirred at room temperature overnight. Dichloromethane (25 mL) and water (25 mL) were added and the reaction mixture was quenched with brine (10 mL), dried over MgSO4, and concentrated under vacuum, giving the desired product (1.3 g, 71% yield). 6


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1

H NMR (400 MHz, CDCl3):  7.17-7.39 (m, 10H), 2.84 (s, 3H), 2.04-1.95 (m, 2H),

0.59-0.63 (m, 2H).

13

C{1H} NMR (75 MHz, CDCl3): 138.5, 132.6, 128.4, 50.3, 41.0,

20.4. 31P{1H} NMR (162 MHz, CDCl3):  -9.5.

Grafting of Lewis bases on the surface of SBA-15. Prior to synthesis, the metalated silica materials were dried at 150 °C in a vacuum oven overnight. The metalated silica (500 mg) was loaded in a Schlenk flask and purged under an inert atmosphere of N2. Dry toluene (50 mL) was then added and stirred vigorously for 10 minutes in order to obtain a well dispersed suspension. Lewis basic amines or phosphines modified with trimethoxy- or triethoxysilane anchoring groups (100 mg) were introduced under vigorous stirring. The mixture was then heated under reflux overnight. The solid material was purified from the ungrafted amines or phosphines under Soxhlet extraction for 12 h at 60 °C using dichloromethane as a solvent. The obtained materials were then dried in a 60 °C oven for 12 h and stored in a 80 °C vacuum oven.

Impregnation of Lewis bases. Prior to synthesis, the metalated silica materials were dried at 150 °C in a vacuum oven overnight. The metalated silica (1.3 g) was loaded in a Schlenk flask and purged under inert atmosphere. Dry hexane (50 mL) was then added and stirred vigorously for 10 minutes in order to obtain a well dispersed suspension. The Lewis base of choice (50-160 mg) was introduced in the system under vigorous stirring. The mixture was left to stir for 1 h at room temperature, followed by slow concentration under reduced pressure. The obtained materials were then stored in a vacuum oven at 30 °C.

Characterization. The nitrogen adsorption-desorption isotherms were determined at -196 °C using an Autosorb-1 sorption analyzer. Prior to analysis, the samples were outgassed at 200 °C (for non-passivated materials) and at 80 °C (for passivated materials) for 12 h. The pore size distribution was obtained by the non-local density functional theory (NLDFT) method and calculated using the Autosorb-1 1.55 software, supplied by Quantachrome Instruments, USA. The NLDFT kernel selected considers sorption of N2 on silica at -196 °C assuming a cylindrical pore geometry and 7



Page 8 of 35

the model of equilibrium isotherm based on the desorption branch and compared to the value obtained from adsorption branch (using the model of metastable adsorption). Attenuated total reflectance infrared (ATR-IR) spectra were recorded using a Nicolet Magna 850 Fourier transform spectrometer equipped with a liquid nitrogen cooled narrow band MCT detector. Each spectrum was obtained from the acquisition of 128 scans at 4 cm-1 resolution from 4000 to 700 cm-1 using a Happ–Genzel apodization. Diffuse-reflectance UV–vis spectra were recorded using a Varian Cary 500 spectrophotometer equipped with a Praying Mantis accessory. A Spectralon reflectance standard was used as a reference. 13C cross-polarization (CP/MAS) and 27Al and 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) analyses were carried out on a Bruker Advance 300 MHz spectrometer (Bruker Biospin Ltd, Milton, Canada) at 75.4 MHz for 13

C, 59.6 MHz for

29

Si, and 78.172 MHz for

27

Al nuclei, correspondingly.

29

Si MAS NMR

spectra were recorded with a spin echo sequence to avoid instrument background with a recycle delay of 30 seconds in a 4 mm rotor spin at 10 kHz.

13

C CP/MAS NMR spectra were recorded

with a 4 mm MAS probe with a spinning rate of 10 kHz and contact time of 1 ms. Chemical shifts were referenced to tetramethylsilane (TMS, δ = 0.0) for 29Si and adamantane (δ = 28.6) for 13

C NMR. Liquid NMR spectra were recorded on a Varian Inova NMR AS400 spectrometer at

400.00 MHz (1H NMR). Thermogravimetric analysis (TGA) measurements were carried out under flowing air on a Netzsch STA449C thermogravimetric analyzer with a maximum heating rate of 10 °C min-1. For energy-dispersive X-ray (EDX) mapping analysis, the samples were dispersed in acetone. The solution was dropped on a holey carbon grid. The analysis was carried out with an aberration-corrected Titan ETEM G2 equipped with EDAX #PV97-61850-ME spectrometer, operated at 300 kV. X-ray photoelectron spectroscopy (XPS) spectra were collected on a Kratos Axis-Ultra electron spectrometer (UK) using a monochromatic Al Kα Xray source (Al K = 1486.6 eV) at a power of 300 W and operated at a base pressure of 10−10 Torr. Charge compensation was performed using a low-energy electron beam perpendicular to the surface of the samples. Survey spectra used for determining the elemental composition were collected at a pass energy of 160 eV.

8


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CO2 Adsorption. The low-pressure CO2 adsorption measurements (up to 760 torr ~ P/P0 = 0.03) were carried out at 0, 10, and 25 °C (273, 283 and 298 K) using a Quantachrome Autosorb-iQ2 sorption analyzer. About 100 mg of the samples was put into a sample cell and was outgassed under high vacuum, provided by a turbomolecular pump, for at least 12 h. A temperature controlled heating system was used to adjust the heating rate and dwelling time at certain temperatures. The outgassing temperature was raised with a rate of 1°C min-1 to 80 °C in a single ramping step for organic-containing samples or to 200 °C with 30 min isothermal periods at 80, 120 and 150 °C for purely inorganic ones. To ensure adequate outgassing of the sample, the pressure rise in the cell was automatically monitored before terminating the outgassing process. For the analysis part, the bath temperature was precisely controlled using a MX VWR thermostatic circulating system containing a mixture of water and ethylene glycol as bath fluid. After each measurement, the sample was regenerated applying the same outgassing procedure described above. After collecting all the data, the isosteric enthalpy of adsorption was determined using Quantachrome ASiQwin software.

Pyridine Adsorption FT-IR experiments. The adsorption tests were performed on 50 mg of solid materials freshly degassed at 160 °C for 24 h, which was dispersed in 3 mL of a 5 % pyridine solution in dry hexane. After stirring the solid suspension for 1 h, the solvent was removed under reduced pressure and the solids were continuously outgassed first at 100 °C for 1 h and then at room temperature for 16 h. As a final step, the white powders were analyzed by FT-IR spectroscopy.

Results and Discussion Synthesis. Following a previously reported protocol, the solid Lewis acidic materials were synthesized by metalation of SBA-15 silica using Al, Ti or Zr acetylacetonate molecular precursors at 15 °C (Figure 1A).31 The synthesized Ti4+-, Zr4+- and Al3+-deposited 9



mesoporous silica samples with different metal contents were denoted as M-x-SBA-15 (M=Ti, Zr, Al; x = metal molar % obtained from XPS surface analysis). The use of these Al, Ti and Zr precursors chemically modified with organic ligands results in high metal dispersion on the silica surface. It has been previously shown that no bulk metal oxide was formed during synthesis and that an even metal distribution on the surface was achieved.30-34 The Lewis acidity of the materials was explicitly substantiated by titration techniques using Lewis acidic indicators with Hammet constant values up to -3.31 The materials which exhibited the strongest Lewis acidic properties, mainly Al-4%-SBA-15, Ti-15%-SBA-15 and Zr-7%-SBA-15, were selected for the synthesis of solid ambiphilic systems.

It was possible to add the Lewis base through a classical post-grafting procedure, allowing the formation of a solid ambiphilic system (Figure 1B). It was also possible to introduce the Lewis bases inside the pores by a wet-impregnation technique (Figure 1C). It is expected for the Lewis base to interact more readily with the Lewis acid sites in the latter system since the Lewis base has more freedom, whereas the Lewis pair in the former system might be further apart if the Lewis basic functional group is grafted far from the Lewis acid site

10


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Figure 1. Development of solid ambiphilic systems. (A) Metalation of SBA-15. (B) Grafting of Lewis bases on the surface of the metalated material. (C) Impregnation of Lewis bases in the pores of the metalated materials.

Diphenylphosphine modified with a trimethoxysilane anchoring group (I) was synthesized through the P-H bond addition to the double bond of vinyltrimethoxysilane (Scheme 1) according to an established protocol.38 The addition of phosphine I to Ti- and Zr-SBA-15 metalated materials dispersed in anhydrous toluene under inert atmosphere resulted in the formation of Ti-PPh2-SBA-15 and Zr-PPh2-SBA-15 solid ambiphilic systems.

It

was

also

possible

to

graft

commercially

available

N1-(3-

trimethoxysilylpropyl)diethylenetriamine (DIETA) using analogous reaction conditions which resulted in the formation of Ti-DIETA-SBA-15 and Zr-DIETA-SBA-15 solid ambiphilic systems.

11



Page 12 of 35

Scheme 1. Synthesis of 2-diphenylphosphinetromethoxysilane.

P H

OMe Si OMe OMe

+

NaOMe, NMP

P

DMSO MeO

MeO Si OMe

I

In order to study the alternate strategy of impregnation of Lewis bases, the silanol groups on the silica materials were passivated after the metalation step by addition of tetramethyldisilazane (TMDS),

31,37

thus ensuring the coordination of the Lewis bases

only to the metallic centers without interference from the silanol groups. 39 For our study, tetramethylpyperidine (TMP), NEt3 and tBu3P were used as Lewis bases to prepare TiSBA-15-TMDS-TMP, Zr-SBA-15-TMDS-TMP, Al-SBA-15-TMDS-TMP, Ti-SBA-15TMDS-NEt3, Zr-SBA-15-TMDS-NEt3, Al-SBA-15-TMDS-NEt3, and Ti-SBA-15TMDS-tBu3P materials. As expected, the two approaches gave very different Lewis base loadings. In the covalent grafting approach, the maximum loading was affected by the number of silanol moieties on the surface and the nature of the spacer, which might prevent the Lewis pair interaction. However, if correctly oriented, a significant entropic gain for the activation of various substrates is expected from the preorganization of the Lewis pair. In the Lewis base impregnation approach, the amount of Lewis acidic sites should determine the Lewis pair concentration, although some basic molecules could remain in the bulk phase inside the pores. A larger number of Lewis pair interactions are expected, but the dissociation and the leaching of the Lewis base is likely to occur. The representation of the materials is illustrated in Figure 2.

12


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Figure 2. Solid ambiphilic systems obtained after the grafting (A) and impregnation (B) of Lewis bases.

Materials Characterization. The porosity of solids was analyzed using low temperature N2 physisorption. Notably, an increase of the surface area, pore size and volume was observed for SBA-15 materials after the metalation step. Such phenomenon was reported by us previously and attributed to the restructuring of the microporous section of the pore walls at low metal loading.31 Decrease of the pore size was detected after the introduction of Lewis bases (Table 1). Amongst the solid materials with grafted Lewis bases, DIETA-based ambiphilic systems exhibit the most dramatic decrease of porosity. For example, Ti-DIETA-SBA-15 surface area decreases by 4 times comparably to pristine SBA-15 material, reaching the value of 270 m2/g after metalation and Lewis base grafting steps. A significant change in pore size and pore volume was also observed. Zr-DIETA-SBA-15 materials exhibit a pore size decrease of 3.1 nm compared to the starting SBA-15 material. This observation goes along with the thermogravimetric analysis (TGA) results, showing an important mass loss of 16 and 17 wt% for Ti-DIETA-SBA-15 and ZrDIETA-SBA-15 systems, respectively (Table 1). PPh2-based solid systems demonstrated a similar effect, with a pronounced decrease in pore dimensions for the Zr-PPh2-SBA-15 system,

13



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whose surface area decreased from 752 to 436 m2/g, pore size from 9.3 to 6.6 nm, and pore volume from 1.3 to 0.5 cm3/g, after grafting of the phosphine. Table 1. Physicochemical properties derived from N2 sorption measurements at -196 °C, quantitative XPS, TGA and titration results of the different samples.

Entry 1 2 3 4

Sample SBA-15 Ti-SBA-15 Zr-SBA-15 Al-SBA-15

TGA mass loss [%] 1 1 2 1

SBET [m2/g]

NLDFTads [nm]

Vt [e] [cm3/g]

814 777 752 721

8.1 9.3 9.3 9.7

1.0 1.1 1.3 1.4

Lewis base loading [mmol/g][a] ([mmol/g][f]) 0 0 0 0

Lewis acid loading [mmol/g][b] 0 0.3 0.1 0.4

Ti-SBA-15 based ambiphilic systems 5 6 7 8 9

Ti-PPh2-SBA-15 Ti-DIETA-SBA-15 Ti-SBA-15-TMDSTMP Ti-SBA-15-TMDSNEt3 Ti-SBA-15-TMDSPtBu3

17 16

758 270

8.1 7.0

1.1 0.5

0.8 (0.3) 3.3[c](3.0)

9

225

8.2

0.4

0.6

7

-[g]

-[g]

-[g]

0.7

-[d]

-[d]

-[d]

-[d]

- [d]

0.3

Zr-SBA-15 based ambiphilic systems 10 11 12 13

Zr-PPh2-SBA-15 Zr-DIETA-SBA-15 Zr-SBA-15-TMDSTMP Zr-SBA-15-TMDSNEt3

13 17

436 260

6.6 5.0 (br)[h]

0.5 0.5

0.6 (0.3) 3.6[c](2.5)

4

559

9.3

1.3

0.3

5

-[g]

-[g]

-[g]

0.5

0.1

Al-SBA-15 based ambiphilic systems 14 15

Al-SBA-15-TMDSTMP Al-SBA-15-TMDSNEt3

4

340

8.7

0.8

6

-[g]

-[g]

-[g]

0.3 0.6

0.4

[a]

Calculated from TGA results; [b] Based on quantitative titration results previously published13; [c] Calculated from TGA results, assuming that three Lewis basic centers available per one molecule; [d] Air-sensitive material, stored in Glovebox and used without analyzing; [e] Vt, total pore volume taken at relative pressure P/P0=0.95; [f] Calculated from XPS results; [g] Materials are not analyzed due to very high volatility of impregnated Lewis base, making preliminary degassing step impossible; [h] broad pore size distribution.

Not all ambiphilic systems prepared by the impregnation approach could be fully characterized. Materials Ti-SBA-15-TMDS-tBu3P (entry 9) and NEt3 (entries 8, 13 and 15) could not be analyzed by N2 physisorption due to the volatility of the ligands, which prevented the materials to be properly degassed prior the analysis. However, a decrease of porosity was observed for Ti-SBA-15-TMDS-TMP as the surface area dropped by 552 m2/g, while pore size decreased by 1.1 nm and the corresponded pore volume by 0.7 14


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cm3/g (entry 7). Zr-SBA-15-TMDS-TMP (entry 12) and Al-SBA-15-TMDS-TMP (entry 14) also showed the same behavior, although the decrease was less pronounced. Most importantly, the loading in Lewis base for these systems followed the trend expected based on the concentration of surface Lewis acidic sites: Ti-SBA-15 > Al-SBA-15 ≥ ZrSBA-15.31 TGA profiles of pristine SBA-15 and Ti-, Zr- and Al-SBA-15 materials exhibited a mass loss of 1-3%, attributed to the release of water physisorbed inside the pores or formed during the high-temperature condensation of silanols in the silica framework.40 The mass loss obtained for materials with impregnated Lewis bases is lower than for systems with grafted Lewis bases, but remains consistent with the initial Lewis base loadings and the the physisorption experiments. After converting the corresponded mass loss to a concentration of grafted/impregnated basic functions, the concentration of Lewis base incorporated is higher or equal than that of the Lewis acids, with the exception of AlSBA-15-TMDS-TMP, where the amount of base introduced is somewhat lower than the amount of Lewis acid sites (entry 14). As it was observed in our previous studies,31 the concentrations measured using XPS are significantly lower than those obtained from TGA, which is typical for the characterization of nanoporous materials since only a depth of few nm on the surface are probed by XPS. To get a better insight into the surface composition, the functionalized solids were examined by high-resolution transmission electron microscopy coupled with EDX mapping analysis, to probe the distribution of Lewis species on the surface. This method allows determining first, the nature of the metallic sites and the N/P dispersion over the sample, and second, whether the metals are homogeneously associated to their respective Lewis base (N/P). Results of these analyses are depicted in Figure 3 and Figure S1. As seen, materials containing grafted and impregnated Lewis pairs exhibit an even distribution of the Lewis basic (phosphorus and nitrogen) and acid (titanium, aluminum, and zirconium) species that relates to that of carbon, oxygen and silicon on the surface.

15



Figure 3. EDX mapping analysis of solid ambiphilic systems: A. Ti-PPh2-SBA-15; B. Zr-PPh2-SBA-15; C. Ti-DIETA-SBA-15; D. Zr-DIETA-SBA-15; E. Ti-SBA-15-TMP; F. Al-SBA-15-TMP.

16


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Good dispersion of Ti and Al is observed for all samples, while some segregation is seen for the Zr containing systems B and D. For example, Zr-PPh2-SBA-15 ambiphilic solid, as demonstrated on Figure 3B, has a distribution of Lewis species on the surface that is not homogeneous. However, the phosphines grafted after the surface metalation are distributed evenly with the zirconium centers. This can be due to the existence of Lewis acid-base interactions, presumably making surface zirconium centers capable of directing the grafting of a phosphine. Irrespective of the overall element dispersion, it appears that Lewis acids and bases are found in similar locations. The highly homogeneous distribution of elements over the SBA-15 solid supports proves the efficiency of the selected surface functionalization method and allows us to presume the existence of surface Lewis pairs. The spectroscopic characterization of these materials was also carried out using MAS NMR spectroscopy (13C, 31

31

P and 1H). The comparison of the solution

13

C{1H} and

P{1H} NMR spectra of the phosphine precursor PPh 2(CH2)2Si(OMe)3 to the

13

C

CP/MAS NMR of PPh2-grafted materials (PPh2-SBA-15, Ti-PPh2-SBA-15 and ZrPPh2-SBA-15) confirms that the ligand is grafted on the surface (see Figure 4). Indeed, all materials exhibit aromatic carbons in the range of 127 to 137 ppm. The methoxide moieties are no longer present at the solid state and the aliphatic signals around 4-20 ppm are difficult to observe because of their low intensity. However, the most significant evidence that the phosphine is present within the pores is obtained from the

31

P MAS

NMR data of the various materials where a resonance is observed around -9 ppm, which is very close to the 31P{1H} NMR of PPh2(CH2)2Si(OMe)3 (δ= -9.5)

17



Figure 4.

13

C CP/MAS (left) and

31

P MAS (right) NMR spectra of PPh2-based

ambiphilic systems; yellow asterisks are for some hydrolysis products while the red asterisks are for some remaining P123 block co-polymer.

DIETA-based solid ambiphilic systems exhibited all the characteristic signals of diethylenetriamine, as illustrated on Figure S2 (See SI). A downfield shift of the aliphatic carbons closest to the anchoring group was detected after the grafting process presumably caused by the presence of electron withdrawing effects from the nearest Lewis acidic metal centers. The MAS NMR spectra of materials impregnated with TMP are presented in Figure S4. In addition to the TMS resonance of the passivating agent at 0 ppm, resonances for TMP at 18, 32, 39, and 50 ppm were identified for all the materials, which consist in a slight upfield chemical shift compared to pristine TMP in the liquid phase. It should be noted that Zr-SBA-15-TMDS-TMP, carrying the lowest amount of TMP according to the characterization of the materials, exhibits the lowest intensity for the TMP resonances. Similar behavior was observed in the case of the NEt3 materials, with

18


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the resonances of the Lewis base being identifiable on all spectra except for the Zrcontaining materials (Figure S4). In the case of the M-SBA-15-TMDS-tBu3P materials,

31

P MAS NMR spectroscopy was

principally used (Figure S5). When tBu3P is exposed to SBA-15, a significant shift of the resonance from 61.8 ppm in solution to 56.5 ppm in the solid state spectrum is observed, which could arise from interactions with the surface, presumably by the formation of some phosphonium species such as [(tBu)3PH]+.41 However, when the surface is passivated with TMS groups, the resonance of the phosphine is closer to the original chemical shift, being close to 62 ppm for both SBA-15-TMDS-tBu3P and Ti-SBA-15TMDS-tBu3P. However, in the former case, more than one resonance was observed, with peaks at 63.2 and 61.8 ppm and a tailing at a lower frequency. For the Ti containing material, only one relatively sharp resonance was observed.

Characterization of Lewis Acidity. After the initial characterization of the materials, it was important to elucidate whether the Lewis acidity of metalated materials was preserved after the grafting of the Lewis bases. For that, we selected PPh2-type solid ambiphilic system as a model. Ti-PPh2-SBA-15 and Zr-PPh2-SBA-15 were therefore analyzed by means of pyridine adsorption-FTIR spectroscopy. This technique allows easy identification of surface Lewis or Bronsted acidic sites and to determine their relative concentration by looking at the vibrational perturbations when a basic molecule is adsorbed on a surface of metalated materials. 42 When pyridine interacts with surface Bronsted acid sites, the presence of the pyridinium species is evidenced by adsorption bands at 1540 and 1640 cm-1, while the observation of bands at 1632-1580 and 1455-1438 cm-1 are attributed to pyridine strongly adsorbed on Lewis acidic centers. Deviations of several reciprocal centimeters can be observed according to the strength of the surface acidic centers. Figure 5 illustrates the spectra of pyridine adsorbed on the surface of Ti-PPh2-SBA-15 and Zr-PPh2-SBA-15. Both of the materials show the presence of Lewis acidic centers without formation of the Bronsted sites, as evidenced by the bands at 1443-1445, and at 1594-1595 cm-1, for the Ti- and Zrcontaining materials, respectively.

19



Page 20 of 35

Figure 5. FTIR spectra of initial Ti-SBA-15 and Zr-SBA-15 materials (bottom) and pyridine adsorbed on Ti-PPh2-SBA-15 and Zr-PPh2-SBA-15 materials (top).

CO2 Adsorption Isotherms and Isosteric Enthalpy of Adsorption. The CO2 adsorption isotherms were measured at 273K for all the materials. The CO2 uptake shows a nearly linear increase with the pressure, reaching to a maximum of 1.2 mmol/g. Such results are comparable to the corresponding values reported for some porous polymers,43-45 COFs46,47 and MOFs48 materials. However, these values are significantly

lower

than

those

of

top-performing

materials,

such

as

In3O(BQDC)3(MeOH)3·(NO3) (3.48 mmol/g at 273 K),28 ALP-1 (3.25 mmol/g at 298 K),49 PPN-6-CH2DETA (3.04 mmol/g at 295 K),50 BILP-4 (2.98 mmol/g at 298 K)51 and MMPF-2 (7.59 mmol/g at 273 K)52 which could be attributed to the high amount of CO2philic functional groups of these adsorbents. In our case, however, the introduction of the 20


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high amount of Lewis pairs on the surface of SBA-15 silica was limited by the relatively low density of anchoring silanol groups as well as the steric repulsion of hindered Lewis pairs. CO2 sorption isotherms for pristine and metalated SBA-15 materials, used as references, were obtained at 273 K and the associated net isosteric enthalpy of adsorption with CO2 loading are presented in Figure S5. The isotherms obtained at 273 K for PPh2- and for DIETA-based solid ambiphilic systems are presented in Figures 6A and 6B, respectively. The CO2 adsorption isotherms for TMP- and NEt3-based materials are shown in Figure 7A and 7B, respectively.

Figure 6. A. CO2 Adsorption isotherms obtained at 273 K for PPh2-based materials; B. CO2 Adsorption isotherms obtained at 273 K for DIETA-based materials.

21



Figure 7. A. CO2 Adsorption isotherms obtained at 273 K for TMP-based materials; B. CO2 Adsorption isotherms obtained at 273 K for NEt3-based materials.

In this study, the isosteric enthalpy of carbon dioxide adsorption (Hst) was calculated from the corresponding equilibrium isotherms measured at three different temperatures (T1=283.15 K, T2=293.15 K, and T3=298.15 K) using the Clausius-Clapeyron equation for a given constant adsorbate loading (n).53 Specifically, the isosteric enthalpy of adsorption was determined by evaluating the slope of the plots of ln(P) versus 1/T at the same adsorbed amount, where P and T are the absolute pressure and temperature, respectively (Figure S6-S10). Three scenarios are possible with ambiphilic materials when the Lewis base is in close proximity of the Lewis acid site. First, no interaction is 22


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present between the Lewis pair and no ambiphilic activation is present, which should not modify the Hst of the material compared to the pristine ones. Second, a strong Lewis pair interaction is present preventing CO2 coordination, which would lead to higher Hst (less negative) and weaker adsorption. Third, ambiphilic activation of CO2 is present and a lower value of Hst (more negative) is observed, associated with stronger adsorption and higher surface affinity toward CO2. Since it is well known that the Lewis base/CO2 interaction is driving the formation of CO2 adducts with ambiphilic molecules,19-20 one does not expect a significant change in the amount of CO2 adsorbed between M-SBA-15 and M-L-SBA-15 if an ambiphilic interaction with CO2 occurs. However, CO2 adsorption should be significantly reduced in the presence of a strong Lewis pair. Calculated isosteric enthalpies vary significantly, reaching the highest value at low coverage and gradually dropping at higher coverage, thereby indicating the presence of a binding affinity to CO2.43-45 For some of the materials, an atypical drop of the enthalpy of adsorption in the range of 0-0.1 mmol/g was observed, which could be explained by the uncertainty of the method. In this case, the range of 0.2-0.8 mmol/g was extrapolated back to zero (Figure S6-S10, ESI, dashed line) and the obtained values were reported in Table 2, along with the loading of Lewis pairs and porosity of solid ambiphilic systems.

Table 2. Values of net isosteric enthalpy of CO2 adsorption, Lewis pairs loading and porosity of solid ambiphilic systems.

Entry

Sample

1

SBA-15

Lewis base loading [mmol/g] 0

2 3 4

Ti-SBA-15 Zr-SBA-15 Al-SBA-15

0 0 0

5 6 7

PPh2-SBA-15 Ti-PPh2-SBA-15 Zr-PPh2-SBA-15

0.6 0.8 0.6

8 9 10

DIETA-SBA-15 Ti-DIETA-SBA-15 Zr-DIETA-SBA-15

1.0 1.0 1.0

Lewis acid Maximum of loading CO2 adsorbed [mmol/g] [mmol/g] 0 0.9 M-SBA-15 0.3 1.2 0.1 1.0 0.4 PPh2-based systems 0 1.1 0.3 0.7 0.1 0.9 DIETA-based systems 0 1.1 0.3 0.7 0.1 1.1

23


Vt [cm3/g]

SBET [m2/g]

1.0

814

-17

1.1 1.3 1.4

777 752 721

-33 -26 -

0.9 1.1 0.5

754 758 436

-28 -26 -32

0.7 0.5 0.5

384 270 260

-35 -18 -40[b]

Hst [a] [kJ/mol]


11 12 13 14 15 16 17 18

TMP-based systems 0 0.6 0.3 0.3 0.1 0.4 0.4 0.3 NEt3-based systems NEt3-SBA-15 0.6 0 0.8 Ti-NEt3-SBA-15 0.7 0.3 0.4 Zr-NEt3-SBA-15 0.5 0.1 0.3 Al-NEt3-SBA-15 0.6 0.4 0.3 [a] Extrapolation 0.2-0.8 data back to zero coverage; [b] Extrapolation 0.2 data back to zero coverage. TMP-SBA-15 Ti-TMP-SBA-15 Zr-TMP-SBA-15 Al-TMP-SBA-15

0.4 0.6 0.3 0.3

Page 24 of 35

0.4 1.3

225 559

-25 -17 -8 -11

-

-

-25 -21 -26 -18

Ti- and Zr-SBA-15 solids, exhibiting a lower SBET than that of the pristine material, adsorb more CO2 (1.2 and 1.0 mmol/g, entries 2 and 3 respectively) than the SBA-15 silica (0.9 mmol/g, entry 1). Adsorption of CO2 on M-SBA-15 samples is stronger when compared to the pristine SBA-15 (Hst is -33 kJ/mol for Ti-SBA-15 (entry 2) and -26 kJ/mol for Zr-SBA-15 (entry 3) vs -17 kJ/mol for SBA-15 (entry 1)). We presume that strong Lewis acid sites increase the surface affinity toward CO2, resulting in more negative enthalpy and higher adsorption capacities.54 In the case of Ti-PPh2-SBA-15 (entry 6) and Ti-DIETA-SBA-15 (entry 9) ambiphilic systems, a decrease in the CO2 adsorption capacity and weaker CO2 adsorption were observed after both Lewis acidic and basic components were introduced onto the surface. Particularly, Ti-PPh2-SBA-15 adsorbs 0.5 mmol less of CO2 compared to the Ti-SBA-15 and 0.4 mmol less than PPh2-SBA-15 silica. This can be associated with decreased SBET and Vt upon the introduction of the Lewis species onto the surface. Additionally, the strength of CO2 adsorption decreases by 7 kJ/mol compared to Ti-SBA-15 silica. A similar situation is observed with Ti-DIETA-SBA-15 (entry 9), where the adsorption capacity decreased by 0.4 mmol/g and the adsorption strength decreased by 15 kJ/mol after the introduction of diethylenetriamine into the surface of Ti-SBA-15 (with a corresponding value of Hst value increased by 15 kJ/mol). Such decrease of adsorption strength indicates the decrease of surface affinity toward CO2 and can be associated with the presence of Lewis acid-base interactions, whereby the Lewis acid and base partially interact with each other. This is supported by the downfield MAS NMR shift of the aliphatic carbons of DIETA molecule, which experience the interaction with the nearest Ti atoms (see NMR spectrum in Supporting information, Figure S2). 24


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The opposite observation was made for Zr-based ambiphilic systems. Although the CO2 adsorption capacity of samples Zr-PPh2-SBA-15 (entry 7) and Zr-DIETA-SBA-15 (entry 10) remains comparable (0.9 and 1.1 mmol/g respectively) to Zr-SBA-15 (1.0 mmol/g, entry 3) and solely grafted Lewis bases (1.1 mmol/g for PPh2-SBA-15, entry 5 and 1.1 mmol/g for DIETA-SBA-15, entry 8), they show stronger CO2 adsorption (with lower value of Hst at zero coverage, such as -32 kJ/mol for Zr-PPh2-SBA-15 and -40 kJ/mol for Zr-DIETA-SBA-15, compared to -26 kJ/mol for Zr-SBA-15, -28 kJ/mol for PPh2-SBA-15 and -35 kJ/mol for DIETA-SBA-15). Additionally, a steep adsorption at low coverage is observed for Zr-DIETA-SBA-15, indicating higher affinity to CO2 compared to DIETA-SBA-15, a known polyamine-based CO2 scrubber (Figure 6 B, red).55-74 This result suggests a synergistic effect between the Zr atoms and the amine/phosphine Lewis bases, in agreement with the EDX elemental mapping analysis (for the Zr-PPh2-SBA-15 system, see Figure 3) and MAS NMR spectroscopy (for the Zr-DIETA-based system, see Figure S2). Ambiphilic materials with impregnated Lewis acid-base pairs adsorb lower amount of CO2 (≈ 0.3-0.4 mmol/g) compared to the parent M-SBA-15 (Ti, Zr-SBA-15) and the Lewis base-modified SBA-15 reference materials (TMP-SBA-15, entry 11 and NEt3SBA-15, entry 15). Their strength of CO2 adsorption is slightly inferior (-8 to -26 kJ/mol) compare to the systems with grafted Lewis pairs (-18 to -40 kJ/mol). Such observation is supported by entropic considerations, which suggest that higher entropic cost may be associated with CO2 adsorption by two-components system (solids with grafted Lewis acid is a first component and free Lewis base is a second component), compared to onecomponent system (solids with grafted Lewis acid and base). The obtained results provide a handful of important conclusions. Firstly, the materials with chemically immobilized Lewis acid-base pairs, i.e., the ones prepared by the grafting approach, seem to provide better CO2 adsorption capacity and enhanced affinity (characterized by a more negative enthalpy of adsorption) compared to the materials with simply impregnated component. However, strong grafted Lewis acid, such as Ti shows a quenching effect when it reacts with nearest Lewis bases, decreasing the affinity of the contacted Lewis pair toward CO2. Finally, Zr-based ambiphilic systems with grafted

25



amine and phosphine Lewis bases exhibit higher affinity toward CO2 as well as enhanced adsorption capacities than the corresponding Ti-analogues.

Conclusions In this contribution, we investigated the synthesis of supported Lewis pairs through the grafting and impregnation of conventional Lewis bases, such as phosphines and amines, on the surface of Al-, Ti- and Zr-SBA-15 materials. Resulted materials were analyzed by means of low temperature N2 physisorption, MAS NMR spectroscopy and EDX mapping analysis. The Lewis acidity of the systems was evaluated by means of pyridine adsorption-FTIR technique and shown to be preserved even after the addition of Lewis bases. Isosteric enthalpy of carbon dioxide adsorption was calculated on the basis of lowpressure adsorption isotherms measured at three different temperatures for all the materials. The differences in capture capacity and the adsorption enthalpy were revealed and substantiated. Solid ambiphilic systems with grafted Lewis acid and base demonstrate generally a higher CO2 sorption ability compared to the systems with impregnated Lewis base. The results suggest that titanium being a strong Lewis acid tends to interact with strong Lewis bases (amines, phosphines), resulting in inhibition of the reactivity toward CO2, making the combination of moderately strong Lewis acids, such as zirconia and Lewis bases advantageous for achieving the improved CO2 adsorption. In particular, Zr-PPh2SBA-15 and Zr-DIETA-SBA-15 solid ambiphilic systems show the highest adsorption capacity (≈1.1 mmol/g) and the strongest adsorption. We believe that our results provide an effective methodology for the synthesis of novel ambiphilic heterogeneous materials with a complex surface composition, which could be used in various catalytic transformations requiring the presence of both acid and base, including the CO 2-based transformations.

ASSOCIATED CONTENT Supporting Information Supplementary Information (SI) available: EDX elemental mapping analyses, solid-state 13C CP/MAS NMR spectra, solid-state 31P MAS NMR spectra, CO2 adsorption isotherms, and net 26


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isosteric heat of adsorption plots. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] ORCID Freddy Kleitz: 0000-0001-6769-4180 Frédéric-Georges Fontaine: 0000-0003-3385-0258 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the Fonds de recherche du Québec – Nature et technologies (FRQNT) and the Natural Sciences and Engineering Research Council of Canada (NSERC, grant N° RGPIN-2014-05821) for financial support. The authors thank Prof. R. Ryoo (IBS and KAIST, Daejeon, Korea) for providing access to high resolution transmission electron (TEM) microscopy.

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acid

with

silica-tethered

iridium

catalysts.

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Table of Content Graphic


Designed Synthesis of Mesoporous Solid-Supported Lewis Acid-Base

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