Aldehyde Self-Condensation Catalysis by Aluminum

Apr 1, 2013 - Aminoterephthalate Metal−Organic Frameworks Modified with. Aluminum ..... after the modification with Al-i-Pro, albeit the contents of...
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Aldehyde Self-Condensation Catalysis by Aluminum Aminoterephthalate Metal−Organic Frameworks Modified with Aluminum Isopropoxide Lev Bromberg, Xiao Su, and T. Alan Hatton* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Porous materials based on aluminum(III) 2aminoterephthalate metal organic frameworks (MOFs NH2MIL101(Al) and NH2MIL53(Al)) and their composites with aluminum isopropoxide (Al-i-Pro) are studied as sorbents of vapors of volatile aldehydes and catalysts of acetaldehyde dimerization to ethyl acetate via the Tischenko reaction. MOF/Al-i-Pro composites obtained by simple impregnation of the MOFs with hydrocarbon solutions of Al-i-Pro are stable due to the formation of bonds between the MOF carbonyls and Al-i-Pro. The specific BET surface areas of the MOFs NH2MIL101(Al) and NH2MIL53(Al) ranged from 1650 to 1980 and 670−780 m2/ g, respectively, and were lowered 6−12-fold by impregnation of Al-i-Pro into the MOF pores. However, the acetaldehyde and acrolein uptake by the MOF/Al-i-Pro composites from saturated vapor atmosphere is comparable to that of their respective parent MOFs and exceeds the aldehyde uptake of activated carbon or molecular sieves. Due of the propensity of the Al-i-Pro to catalyze dimerization of acetaldehyde to ethyl acetate, the latter is the main product of the reaction between acetaldehyde and MOF/Al-i-Pro materials, whereas crotonaldehyde is found in the products of the acetaldehyde self-condensation on the parent MOF NH2MIL101(Al). The kinetics of acetaldehyde dimerization into ethyl acetate catalyzed by NH2MIL101(Al)/Al-i-Pro in deuterated benzene at room temperature are measured over three consecutive cycles. The apparent second-order reaction rate is 5.2 × 10−5 M−1s−1, which is of the same order as in the analogous reaction catalyzed by a homogeneous solution of Al-i-Pro. The MOF/Al-i-Pro materials are proven to be recyclable heterogeneous catalysts. KEYWORDS: aluminum 2-aminoterephthalate, aluminum isopropoxide, metal−organic framework, composite, aldehyde dimerization, Tischenko reaction, catalysis

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capturing targeted molecules, self-detoxification, etc.9 The presence of the amino group in each linker molecule enhances the ability of the MOFs to capture CO2.11,12 Herein, we explored, for the first time, the performance of the aminocontaining MOFs in sorbing volatile aldehydes, with potential applications as sorbents for the preconcentration of trace analytes13 or in bio-oil purification and upgrade.14 Further, we functionalized the amino-containing MOFs by aluminum isopropoxide, a well-known catalyst in the dimerization of aldehydes to the analogous carboxylic esters, or the Tischenko reaction.15−18 Aluminum isopropoxide (Al-i-Pro) is a versatile and inexpensive oligomeric Lewis acid catalyst typically employed in Meerwein−Ponndorf−Verley−Oppenauer (MPVO) reactions, which are a standard, mild, and relatively inexpensive method for reduction of aldehydes and ketones in the organic chemistry community.18 Furthermore, Al-i-Pro is a known catalyst for the catalytic copolymerization of epoxides and CO219 and ring-opening polymerization of lactides and lactones.20−22 In this work, we concentrated on the Tischenko reaction catalysis for acetaldehyde dimerization, as we

INTRODUCTION Exceptionally high porosity, crystallinity, compositional and structural variability, large surface area and acceptable thermal and chemical stability of metal−organic frameworks (MOFs) make them ideal materials to satisfy the needs of various applications such as catalysis, gas and vapor sorption, chemical separation, and so forth.1−7 Postsynthesis modification of the MOFs with catalytic species presents a versatile route toward rational design of catalysts. Homogeneous catalysts can be incorporated into the MOF structures, enabling integration of the functionality of well-defined, single-site catalysts into the MOF micropores to perform shape-, size-, chemo-, or enantioselective reactions.8 Moreover, the ability to separate and reuse a heterogeneous catalyst is highly desirable in largescale reactions, where separation and waste disposal can be costly. In the present work, we based our catalyst and sorbent design on recently introduced aluminum aminoterephthalate MOFs that consist of nontoxic metal and linker.9−11 Such MOFs with MIL-53 or MIL-101 crystal topology possess a large content of primary amino groups available for utilization as a handle to covalently modify the MOF with functional compounds or attach the MOF particles to other materials in order to engineer functional surfaces capable of sensing or © XXXX American Chemical Society

Received: January 3, 2013 Revised: March 25, 2013

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°C for 4 h resulting in a gelled paste, which was briefly sonicated and then vacuum-evaporated and dried. The resulting dry powder was suspended in excess hexane and extracted in a conventional 50-mL jacketed Soxhlet extractor (Sigma-Aldrich Corp.) for 8 h with three changes of hexane. The solvent was removed from the solids under vacuum. The resulting solid was subjected to NMR analysis in DMFd7. Elemental Analysis, Calcd. (for C20H33Al2NO8): C, 51.2; H, 7.09; Al, 11.5; N, 2.98; Found: C, 50.4; H, 7.35; Al, 11.2; N, 3.06. Methods. General. MOF surface area and pore parameters were measured using a Micromeritics ASAP 2020 Accelerated Surface Area and Porosimetry Analyzer (Micromeritics Corp., Norcross, GA) and an Autosorb-iQ Automated Gas Sorption Analyzer (Quantachrome Instruments, Boynton Beach, FL) at 77 K. The samples were outgassed and activated prior to the measurements at 60 °C for 5 h and at 80 °C for 10 h. In the measurements on Autosorb-iQ Gas Sorption Analyzer, the surface areas were calculated using a multipoint BET method for relative pressures of 0.05 and 0.30. The pore size distributions were calculated using Quantachrome ASiQwin software, where the nonlocalized DFT method (NLDFT) was applied using the N2 sorption on silica at 77 K with cylindrical pores as the model kernel. Surface areas resulting from the DFT calculations correlated well with the values obtained using BET method. 1 H NMR spectra were collected at 25 ± 0.5 °C using a Bruker Avance-400 spectrometer operating at 400.01 MHz. FTIR spectroscopy was performed with a Nicolet 8700 FTIR spectrometer (Thermo Scientific Inc.). For attenuated total reflection (ATR) FTIR measurements, a Golden Gate ATR accessory (Specac Ltd., Cranston, RI) was applied, with 64 scans performed at a resolution of 1 cm−1. Thermogravimetric analysis (TGA) and simultaneous differential scanning calorimetry (DSC) were conducted using a Q600 TGA/ DSC instrument (TA Instruments, Inc.). Concentration of aluminum in organic solutions was measured in the 5−50 mg/L concentration range with a PinAAcle 900 Atomic Absorption Spectrometer (PerkinElmer, Inc., Shelton, CT), after solvent evaporation and sample redissolution in HCl/HNO3 mixture using a wavelength of 309.3 nm, according to the EPA Method 202.1. Capture and Condensation of Aldehyde Vapors on MOF Materials. For aldehyde capture, borosilicate glass vials, each containing a weighed amount of dry powder of a solid sample were placed next to an open 5-mL wide mouth jar containing 2 g of liquid aldehyde, initially poured into the dish at −20 °C. Both the vials and the jar were situated in a small glass desiccator, which was sealed immediately after pouring liquid aldehyde into the jar. Aldehyde evaporated from the jar at room temperature, with the vapors contained within the sealed desiccator. The open vials were kept in the desiccator for 24 h to 14 days at room temperature, and the uptake of the aldehyde was determined periodically by withdrawing the vials from the desiccator, immediately sealing them and measuring their weight. Liquid aldehyde was added into the jar within the desiccator each time the desiccator was opened for the sample withdrawals, to maintain the saturated vapor atmosphere inside the desiccator. Equilibrium weight uptake was reached after 24 h, at which point no further weight increase of the vials was observed. The weight uptake, measured in triplicate, was calculated as follows:

conceptualized not only to sorb aldehydes by our functionalized MOFs, but simultaneously to convert aldehydes to esters. This reaction is ideal for the bio-oil upgrading process as it can efficiently create carbon−carbon bonds between the low molecular weight aldehydes, so that they are not eliminated as potential liquid fuels during hydrodeoxygenation. Aluminum alkoxides including Al-i-Pro are typically employed as homogeneous catalysts of the Tischenko reaction, and an excess of the Al-i-Pro compound is commonly required.16 In order to convert Al-i-Pro into a heterogeneous catalyst that affords reuse, the aluminum alkoxide moieties have been grafted onto mesoporous silicate (MCM-41) via siloxide linkages, producing materials with enhanced catalytic activity in the MPVO reduction of cyclic ketones.23 In the present work, we adopted a similar concept, wherein amino-MOFs served as a solid support for the Al-i-Pro. We serendipitously discovered that a simple impregnation of the MOFs with a hydrocarbon solution of Al-i-Pro followed by heating to 60 °C results in the Al-i-Pro attachment to the MOF, leading to heterogeneous catalysts of acceptable chemical stability. These catalysts are capable of quantitative dimerization of acetaldehyde at room temperature, as described below.

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EXPERIMENTAL SECTION

Materials. 2-Aminoterephthalic acid (99%, 2-ATA), aluminum chloride hexahydrate (99%), acetaldehyde (≥99.5%), acrolein (99%), butyraldehyde (99%), aluminum isopropoxide (Al-i-Pro, ≥98%), N,Ndimethylformamide (DMF, 99.9%), N-nitroso-di-n-butylamine (99%) were all obtained from Sigma-Aldrich Chemical Co. and used as received. All other chemicals, solvents and gases were of the highest purity available and were received from commercial sources. MOF Synthesis. NH2MIL101(Al). A solution of aluminum chloride hexahydrate (0.51 g, 2 mmol) and 2-ATA (0.56 g, 3 mmol) in DMF (40 mL) was kept at 130 °C for 72 h in a Teflon-lined autoclave bomb. Then the solids were separated from the solution by centrifugation (5000 g, 10 min) and washed with DMF under sonication for 20 min. This was followed by washing with methanol at room temperature, washing with excess hot (70) methanol for 5 h, and drying under vacuum at 80 °C until constant weight was achieved. Elemental analysis, Calcd. (for unit cell, Al816C6528H4896N816O4352): Al, 11.8; N, 6.13%; Found: Al, 12.1; N, 6.34%. The resulting MOF was designated NH2MIL101(Al)auto. NH2MIL53(Al). This material was synthesized by thermal treatment of the MOF components through autoclaving. A solution of aluminum chloride hexahydrate (2.55 g, 10 mmol) and 2-ATA (2.8 g, 15.5 mmol) in DMF (99.9%, 40 mL) was kept at 130 °C for 72 h in a Teflon-lined autoclave bomb. The solids were separated from the solution by centrifugation (5000 g, 10 min) and washed with DMF under sonication for 20 min. This was followed by washing with methanol at room temperature, washing with excess hot (70 °C) methanol for 5 h, and drying under vacuum at 80 °C until constant weight was achieved. Elemental analysis, Calcd. (for unit cell, Al4C32H24N4O20): Al, 12.1; N, 6.27%; Found: Al, 12.7; N, 6.75%. The resulting MOF was designated NH2MIL53(Al)auto. Amino-Containing MOF Functionalized by Aluminum Isopropoxide (Al-i-Pro). NH2MIL101(Al)auto or NH2MIL53(Al)MW MOF (0.88 g) was mixed with a solution of 0.88 g (4.3 mmol) of aluminum isopropoxide in 10 mL toluene. The suspension was kept at 60 °C for 4 h, briefly sonicated and analyzed for the solvent composition by HS-GC (see below), which detected formation of acetone. The suspension was then dried at 70 °C in a vacuum oven until constant weight. The resulting dry powder was suspended in excess hexane with brief sonication, separated by centrifugation (5000 g) and dried. Elemental analysis is reported in the text. Reaction between 2-Aminoterephthalic Acid and Al-i-Pro. 2ATA powder (0.44 g) was suspended in a solution of 0.44 g Al-i-Pro in 5 mL toluene with a 5-min sonication. The suspension was kept at 60

WU, % = 100 ×(sample weight after equilibration − initial sample weight)/initial sample weight Analysis of Acetaldehyde and Products. Acetaldehyde and products of its chemical conversion on MOF and MOF/Al-i-Pro composites were quantified using headspace sampling coupled with gas chromatography (HS−GC). The MOF samples were immersed in acetonitrile at 10 mg/mL solid concentration. The suspensions were sonicated briefly, solids were separated by centrifugation (15000 g, 1 min), and the supernatant was diluted with deionized water (acetonitrile/water ratio, 1:2 v/v), placed into Perkin-Elmer headspace vials and sealed with PTFE/butyl rubber septa. The vials were placed into a headspace sampler (TurboMatrix HS-40 Trap) and kept at 60 °C for 1 h. The products were detected using a Clarus 600 GC−MS (PerkinElmer, Inc.). The gas chromatograph was equipped with flame B

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ionization detector and an Elite BAC-1 column (30 m × 0.32 mm × 1.8 μm) (PerkinElmer). The transfer line and needle temperature was 100 °C, carrier pressure, 40 psi. The injector and FID temperatures were 100 and 150 °C, respectively. The GC was run from 45 to 150 °C at a rate of 40 °C/min. The retention times for acetone, acetaldehyde, crotonaldehyde, ethyl acetate and isopropanol were 0.6, 0.7, 1.0, 1.2, and 1.5 min, respectively. N-nitroso-di-n-butylamine was used as internal standard. Calibration curves for acetaldehyde were developed and the products were quantified using the instrument’s software. Kinetics of Acetaldehyde Conversion to Ethyl Acetate. Acetaldehyde conversion catalyzed by NH2/MIL101(Al)/Al-i-Pro was studied in C6D6 as a solvent at 25 °C. Benzene, which is a good solvent for Ali-Pro, was chosen as a liquid medium due to the reported kinetic data on homogeneous catalysis in this solvent.24 A weighed amount of the catalyst was suspended in the solvent in a sealable glass vial with a brief (5 s) sonication. A measured volume of acetaldehyde was injected via syringe, and the reaction commenced. The glass vials were stirred by small magnetic stirrers. The initial C6D6 volume was 10 mL; 0.7-mL samples were withdrawn periodically using a membrane filter-fitted syringe and immediately subjected to 1H NMR measurements. Three consecutive cycles of acetaldehyde conversion were conducted. The catalyst was separated from the reaction medium 7 h after the start of the reaction by centrifugation (15 000 g, 45 s). The solids were suspended in excess hexane, separated from hexane by centrifugation and dried under vacuum. The recovered catalyst was subjected to the next reaction cycle, identical to the first one. In a separate series of experiments, the catalyst and supernatant were separated by centrifugation at the end of each cycle and the supernatant was subjected to aluminum content determination by elemental analysis. Less than 1% of aluminum initially present in the MOF was leached in each cycle of these experiments. More precise measurement was not available due to the limitations of the assay. During the course of the reaction, the signal for the aldehyde group proton (−HCO) at 9.7 ppm diminished and eventually disappeared, while a strong signal due to the methylene group of ethyl acetate appeared at 4.0 ppm and grew. These signals were a good reference for calculation of the aldehyde conversion (F), which was obtained from the expression

F = 0.5Iα /(0.5Iα + Iε)

the MOF pores was achieved by a simple mixing of the MOF and Al-i-Pro solution in toluene, followed by drying and washing off the excess aluminum isopropoxide by hexane, at room temperature. The specific BET surface areas of various batches of the MOFs NH2MIL101(Al) and NH2MIL53(Al) ranged from 1650 to 1980 and 670−780 m2/g, respectively, and were lowered 6−12-fold by impregnation of Al-i-Pro into the MOF pores. The surface area values for the MOF NH2MIL101(Al) obtained using BET model were close to the ones computed using density functional theory (DFT). Typical nitrogen sorption isotherms and pore size distributions for the parent NH2MIL101(Al) material and the MOF impregnated by aluminum isopropoxide are shown in the Supporting Information, SI (S-1,S-2). Pore size distribution results indicate that most of the pores in the parent MOF range between 10 and 20 Å, which correlates well with the reported pore diameters for MOF NH2MIL101(Al).9 A dramatic decrease in the BET surface area and the fraction of the micropores upon impregnation indicated that most of the micropores in the MOF were occupied by Al-i-Pro. Weight-average particle sizes of either the parent MOFs or their MOF/Al-i-Pro composites were measured by dynamic light scattering in hexane and ranged from 80 to 250 nm. XRD patterns of the MOFs impregnated by Al-i-Pro are shown in Figure 1. The patterns demonstrate that the initial

(1) Figure 1. X-ray diffraction patterns of NH2MIL101(Al) and NH2MIL53(Al) MOFs and corresponding composite materials. MOFs functionalized by aluminum isopropoxide are designated NH2MIL101(Al)/Al-i-Pro and NH2MIL53(Al)/Al-i-Pro.

where Iα and Iε are relative integrations of the corresponding methylene and aldehyde protons of the product (ethyl acetate) and reactant (acetaldehyde), respectively, measured at time t. Since the initial rate of acetaldehyde dimerization in benzene catalyzed by Al-iPro has been reported to be first-order in acetaldehyde,24 the aldehyde conversion was fitted to eq 2 to obtain the observed rate constant (kobs):

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ln(1 − F ) = − kobst

MIL-101 and MIL-53 topologies did not change appreciably after the modification with Al-i-Pro, albeit the contents of the amorphous phase increased. No peaks characteristic of Al-i-Pro in the 2Θ = 20−30° range25 were present, indicating that this species was molecularly dispersed in the MOF network. TGA and DSC thermograms of as-received aluminum isopropoxide (Al-i-Pro) exhibited sharp endothermic melting and decomposition at T > 120 °C, with ∼75% weight loss at T ≥ 300 °C (S-3 of the SI). The NH2MIL101(Al) MOF samples released up to 8 wt % of solvent at temperatures ≤100 °C and decomposed only at temperatures above 450 °C, following endothermic melting of the 2-ATA-aluminum complexes.9 Thermograms of the NH2MIL101(Al)/Al-i-Pro materials were similar to those of the parent MOF, but with 10−12% larger weight loss in the 250−471 °C range, which can be explained by the decomposition of the alkanol groups of aluminum isopropoxide. Due to the broad decomposition (occurring in the same temperature ranges) of the parent MOF and MOF/Al-i-Pro, we were unable to estimate the Al-i-Pro content in MOF/Al-i-Pro using thermograms. Instead, we employed elemental analysis to

(2)

RESULTS AND DISCUSSION Following the concept of NH2-containing MOF functionalization with catalytic compounds, we incorporated aluminum isopropoxide (Al-i-Pro) into the MOFs by their impregnation with Al-i-Pro hydrocarbon solutions. Aluminum isopropoxide (Al-i-Pro) used herein for the MOF functionalization was received as a solid crystalline material. Several oligomeric forms of Al-i-Pro have been reported, and the species with Tm of 127 °C has been shown by XRD to be a tetramer of formula Al[(μOiPr)2Al(OiPr)2]3.18,19 The melting point of the as-received material was measured to be 127−128 °C by a capillary melting apparatus, in agreement with the data previously reported.25 The Al-i-Pro crystals were freely soluble in toluene at 50 °C, soluble at room temperature in hexane at concentrations of at least 1 mg/mL and freely soluble in benzene and isopropanol at concentrations of at least 10 mg/mL. The material loading into C

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by reaction of the carbonyls with Al-i-Pro, which possessed only weak to medium-intensity bands in this area. In addition, the model reaction between 2-ATA and Al-i-Pro in toluene (conducted under conditions identical to the MOF impregnation procedure, see Experimental Section) followed by exhaustive extraction of the product by hexane yielded a product, the elemental analysis and 1H NMR spectrum of which (S-5 of the SI) corresponded well to those anticipated for the MPVO reaction. Note that since the unreacted Al-i-Pro is soluble in hexane while 2-ATA is totally insoluble, aluminum isopropoxide should have been fully removed from the 2-ATA/ Al-i-Pro product by the Soxhlet extraction. Al-i-Pro is well-known to form bridged structures, both with itself as well as with other metal carboxylates. Analogous reactions of Al-i-Pro with carboxylic and amino acids leading to the formation of aluminum carboxylates are well-documented and are a source of a variety of important pharmaceuticals and other products.28,29 Further, a similar quantitative reaction between carboxylic acid and aluminum and titanium alkoxides has been reported for carboxylated polymers, which results in the formation of multifunctional networks.30 The above experiments provide an explanation for the stability of the MOF/Al-i-Pro materials as catalysts. MOF and MOF/Al-i-Pro Materials in Reactive Aldehyde Sorption. The composite NH2MIL101(Al)/Al-i-Pro and NH2MIL101(Al)/Al-i-Pro and their parent MOFs were tested for their ability to sorb aldehydes from the saturated vapor phase. The uptake of saturated vapors of acetaldehyde was much higher overall than that of acrolein and butyraldehyde (Figure 3), and the overall uptake values were in the sequence acetaldehyde > acrolein > butyraldehyde.

determine the Al-i-Pro contents in the MOF/Al-i-Pro composites, in order to access the composite stability. MOF impregnated with Al-i-Pro, washed by hexane and dried under vacuum as described in the Experimental Section was characterized by the following elemental analysis: C, 48.5; H, 6.91; Al, 12.8; N, 2.93. Calculated elemental analysis of the Al-iPro monomer is C, 52.9; H, 10.4; Al, 13.2; N, 0.0, that of NH2MIL101(Al) MOF, C, 42.1; H, 2.65; Al, 11.8; N, 6.13, and NH2MIL53(Al) MOF, C, 43.06; H, 2.71; Al, 12.09; N, 6.28. Using the above measurements and calculations, including unit cell analyses,9 we estimated that as-prepared MOF/Al-i-Pro contained 5 monomers of Al-i-Pro per unit cell of the MOF crystal. The accuracy of the estimation was not sufficient to distinguish between likewise impregnated NH2MIL101(Al) and NH2MIL53(Al) materials, as the elemental compositions of these MOFs are not sufficiently different. After extensive washing with toluene, comprising four cycles of 200-rpm stirring of MOF/Al-i-Pro suspensions (10 mg/mL) for 3 days followed by separation of solids by centrifugation, drying under vacuum until constant weight and resuspension in toluene, the elemental analysis of the resulting washed material was C, 46.8; H, 5.95; Al, 12.6; N, 3.81. This measurement afforded an estimate of three Al-i-Pro monomers per one unit cell of MOF. This is a considerable stability against leaching, given that the loading/impregnation of the MOF with Al-i-Pro was conducted using the same hydrocarbon solvent. The nature of such stability appears to be complexation between Al-i-Pro and the aluminum atoms of the MOF. Such complexation can be in the form of bridging interaction between ionic carboxylates of MOF and Al atoms of Al-i-Pro as well as due to the Meerwein−Ponndorf−Verley−Oppenauer (MPVO) reaction occurring between carboxylates of MOF and Al-i-Pro (S-4 of the SI). The MPVO reaction, which is known to occur under conditions such as those we employed in the process of the MOF/Al-i-Pro material synthesis,26 leads to the complex formation between MOF and triisopropoxyaluminum and release of acetone, which was detected. ATR FTIR spectra (Figure 2) present evidence of the reduction in the content of the carbonyl groups on the MOF surface after the impregnation procedure. The ratio of the intensity of the band at 1578 cm−1 (stretching vibrations of the carbonyls of the aluminum carboxylate27) to the intensity of the band at 1668 cm−1 attributed to the NH2-vibrations,9 was significantly diminished

Figure 3. Equilibrium saturated aldehyde vapor uptake by composites NH2MIL101(Al)/Al-i-Pro and NH2MIL53(Al)/Al-i-Pro and their parent MOFs NH2MIL101(Al) and NH2MIL53(Al). Temperature, 25 °C, time of exposure to vapors, 48 h.

The aldehyde uptake by the MOF/Al-i-Pro composites was comparable to that of their respective parent MOFs, despite the fact that the BET surface areas of the tested batches of MOFs NH2MIL101(Al) and NH2MIL53(Al) of 1950 and 780 m2/g, respectively, were reduced to 280 and 130 m2/g, respectively, in the corresponding MOF/Al-i-Pro materials. The uptake of acetaldehyde vapors on our MOFs and MOF/Al-i-Pro materials was approximately 5-fold higher than physisorption on wood origin-carbon with a BET surface area of 2266 m2/g, chemically activated with phosphoric acid, under saturated vapor conditions.31 The uptake of acrolein by the MOF or MOF/Al-i-Pro was ∼2-fold higher than the saturation adsorption capacity of heat-activated 3A molecular sieves (synthetic zeolite), reported to be ∼25% under saturated vapor conditions.32 The composition of the acetaldehyde vapors sorbed on the MOF and MOF/Al-i-Pro materials was analyzed in acetonitrile

Figure 2. Attenuated total reflectance (ATR) FTIR spectra of Al-i-Pro, NH2MIL101(Al), NH2MIL101(Al)/Al-i-Pro and the composite NH2MIL101(Al)/Al-i-Pro that sorbed acetaldehyde (AA) vapors at 25 °C for 48 h. Dashed lines show bands at 3488, 3377, 1668, and 1578 cm−1, corresponding to the asymmetric and symmetric NH2vibrations (N−H stretch), vibrations of the amino groups of 2-ATA in MOF, and stretching vibrations of the aluminum carboxylate groups, respectively. D

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Kinetics of Acetaldehyde Dimerization in Liquid Medium. When the acetaldehyde dimerization was conducted in the liquid medium, we were able to measure its kinetics. Typical 1H NMR spectra representing the conversion of acetaldehyde to ethyl and isopropyl acetates (Tischenko reaction) catalyzed by NH2MIL101(Al)/Al-i-Pro suspended in C6D6 at 25 °C are shown in Figure 5.

extracts by headspace GC-MS as described in the Experimental Section. It was found that after 1 h of exposure of the MOF/Ali-Pro materials to acetaldehyde vapors, the ratio of acetaldehyde to ethyl acetate was approximately 10:1. After 8 h of exposure, the ratio of acetaldehyde to ethyl acetate in the extracts was approximately 1:5. After 24 h, ethyl acetate comprised the majority of the product; crotonaldehyde was not found. This result was corroborated by the 1H NMR measurements of the extracts in deuterated acetonitrile (Figure.4).

Figure 5. Conversion of acetaldehyde to ethyl and isopropyl acetates catalyzed by NH2MIL101(Al)/Al-i-Pro (Tischenko reaction) in C6D6 at 25 °C. 1H NMR spectra of the reaction mixture were taken at t = 0 (1) and after 48 h (2). NH2MIL101(Al)/Al-i-Pro loading, 10 mg/mL.

Figure 4. 1H NMR spectra of extracts from NH2MIL101(Al) (1) and NH2MIL101(Al)/Al-i-Pro (2) materials exposed to an atmosphere of saturated acetaldehyde vapors for 24 h at 25 °C. After the exposure to the vapors, the materials were suspended in CD3CN at 10 mg/mL concentration and briefly sonicated. The solids were removed by centrifugation and the supernatant was subjected to the NMR measurements.

In the control experiments, conducted under identical conditions but in the presence of NH2MIL101(Al) devoid of aluminum isopropoxide, no conversion of acetaldehyde to ethyl acetate was observed within 48 h. 1H NMR spectra measured within the first 8 h in the reaction conducted in the presence of NH2MIL101(Al)/Al-i-Pro revealed only the appearance of ethyl acetate as a product. GC−MS measurements demonstrated that isopropyl acetate comprised less than 2% of the ethyl acetate product after 8 h. However, after 24 and 48 h, the formation of minor products such as isopropyl acetate (δ = 5.02 ppm, methine group α to −O−CO) and ethoxydiisopropoxyaluminum (3.55 ppm, methine and methylene groups α to −C and α to −O−Al) was detected. After 48 h, these products constituted up to 10 mol % of the reaction’s main product, ethyl acetate. These observations are in agreement with the previous reports on homogeneous catalysis of the Tischenko reaction by aluminum isopropoxide involving a rapid acetaldehyde dimerization into ethyl acetate and a slower transfer of alkoxide ion to aldehyde, eventually producing isopropyl acetate (Figure 6).24,36,37 The kinetics of acetaldehyde dimerization into ethyl acetate catalyzed by NH2MIL101(Al)/Al-i-Pro in deuterated benzene in three cycles are evident in Figure 7. Within the first 8 h of the reaction, no discernible formation of isopropyl acetate was detected. As is seen, the reaction kinetics showed linear fits (R > 0.98 in all cases) in the ln(1 − F) vs time coordinates (eqs 1 and 2). The observed rate constant (kobs) was measured to be 5.2 × 10−5 s−1 in the first cycle and 4.7 × 10−5 s−1 in the 3-rd cycle, diminishing by 9.5%. With an initial acetaldehyde concentration of 1 M, the apparent second-order reaction rate in the first cycle was estimated to be 5.2 × 10−5 M−1s−1, which is approximately 4-fold lower than in analogous reactions catalyzed by a homogeneous solution of Al-i-Pro24 and 3- to 6fold slower than the reaction in the presence of isopropoxyaluminum 1,1′-biphenyl-2-oxy-2′-perfluorooctanesulfonamide,39 also a homogeneous catalyst. However, in our experiments, a ∼2-fold lower effective concentration of Al-iPro (∼20 mM) was utilized, so that the catalytic efficiency of our NH2MIL101(Al)/Al-i-Pro material was similar to that of

In the NMR measurements, after 24 h of exposure of the MOF/Al-i-Pro to acetaldehyde vapors, ethyl acetate comprised over 95% of the products, with approximately 5% of isopropyl acetate as a minor product. In contrast, after 24-h contact between NH2MIL101(Al) and or NH2MIL53(Al), the ratio of acetaldehyde to crotonaldehyde was approximately 8:1, while ethyl acetate was not detected. The appearance of crotonaldehyde in that case can be explained by the Mannich-type catalysis of aldol condensation of acetaldehyde by the amino groups through the formation of imines. Such a reaction was apparently avoided in the presence of MOF/Al-i-Pro composite materials. Note that the N−H stretching bands of the NH2groups in the FTIR spectra in the 3350−3500 cm−1 area (Figure 2) that are very sensitive to intra- and intermolecular H-bond formation as well as adsorption on and reactions with the amino groups,33 were not changed after the exposure of the MOF/Al-i-Pro materials to acetaldehyde vapors. These bands were significantly diminished after the reaction of the NH2MIL101(Al) material with acetaldehyde. To summarize these observations, we demonstrated that because of the propensity of the Al-i-Pro to catalyze dimerization of acetaldehyde to ethyl acetate, the latter was the main product of the reaction between acetaldehyde and MOF/Al-i-Pro materials. This result differs dramatically from the one we recently reported with MOF NH2MIL101(Al) impregnated with superacidic phosphotungstic acid (PTA), where the majority of acetaldehyde vapor molecules sorbed on the MOF/PTA composite were rapidly converted into crotonaldehyde and higher molecular weight products through the repeated aldol condensation mechanism.34 Likewise, dimerization of acetaldehyde catalyzed by silica−alumina catalysts results in crotonaldehyde.35 We thus found a route toward engineering the catalytic properties of the MOF materials and the outcome of the catalysis by changing the functionality of the material incorporated into the MOF network. E

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NH2MIL101(Al)/Al-i-Pro continued and reached approximately 80% after 7 h. This experiment demonstrates that the reaction was catalyzed by NH2MIL101(Al)/Al-i-Pro material rather than any Al-i-Pro that might leach into the solvent during the reaction. The slight decrease in the reaction rate after the first cycle can be explained by the alkoxide transfer (see Figure 6) that starts to contribute at high ethyl acetate yields and can potentially diminish the effective Al-i-Pro concentration in the reaction zone due the appearance of ethoxydiisopropoxyaluminum and other reaction products. After the second cycle, no further decrease in reaction rate was observed.

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CONCLUSIONS A priori knowledge about the catalytic activity of a certain species is a convenient route toward designing novel heterogeneous catalysts by fixation of such species in the framework.42 Utilizing this approach, we have previously introduced several MOF-based catalysts that were functionalized with polyoxometalate molecules.43,44 In the present work, aluminum aminoterephthalate MOFs are modified with aluminum alkoxide, a known catalyst for the Tischenko reaction. Aluminum isopropoxide is inexpensive and nontoxic. These modified MOFs appear to be stable against leaching and possess sorption capacities for volatile aldehydes that exceed those of activated carbon or molecular sieves. Acetaldehyde, which is quite susceptible to undesired side reactions such as self-aldol condensation, was efficiently and quantitatively converted to ethyl acetate by the aluminum isopropoxidemodified MOF at room temperature. The modified MOF was recyclable, implying the possibility of practical applications.

Figure 6. Mechanism of the MOF/Al-i-Pro-catalyzed Tischenko reaction of acetaldehyde dimerization. Ogata−Kawasaki dimerization of acetaldehyde into ethyl acetate is the main reaction,24,37,38 while isopropoxide acetate formation via alkoxide transfer from ethyl acetate to Al-i-Pro3 takes place at high degrees of dimerization conversion.

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Figure 7. Kinetics of acetaldehyde conversion into ethyl acetate in the presence of NH2MIL101(Al)/Al-i-Pro and NH2MIL101(Al) (control) at 25 °C in C6D6. Initial acetaldehyde and catalyst concentrations in each cycle were 1 M and 10 mg/mL, respectively. Catalyst removal is shown by an arrow.

ASSOCIATED CONTENT

* Supporting Information S

Nitrogen adsorption isotherms, pore size distributions, TGA and DSC thermograms, complexations schematics, NMR spectra of 2-aminoterephthalic acid (2-ATA) and the product of reaction between 2-ATA and aluminum isopropoxide, and XRD patterns (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

the Al-i-Pro in the homogeneous reaction. The turnover number and turnover frequency of the dimerization reaction catalyzed by NH2MIL101(Al)/Al-i-Pro were estimated to be approximately 35 and 5 h−1, respectively, in line with the results on the homogeneous catalysis of the Tischenko reaction by homoleptic lanthanide amides, M[N(SiMe3)2]3 (M = Sc, Y, La), which have been reported as highly active catalysts for the dimerization of aromatic and aliphatic aldehydes to the carboxylic esters.16,40,41 The structure of the NH2MIL101(Al)/Al-i-Pro material did not change appreciably after the reaction with acetaldehyde in C6D6 (conditions as in Figure 5), judging by the XRD patterns (S-6 of the SI). In order to ascertain whether the NH2MIL101(Al)/Al-i-Pro material is a heterogeneous catalyst, we removed the solid material after 1.5 h of the reaction by centrifugation (15 000 g, 45 s), then placed the supernatant back into the reaction vial and continued monitoring the reaction progress for another 5.5 h. The presence of aluminum in the wash-outs after centrifugation and catalyst removal was measured by atomic absorption spectrometry (see Experimental Section) and was found to be less than 1% of the initial aluminum present in the MOF/Al-i-Pro material initially placed in the reaction zone. After the solids removal, the conversion (F) essentially stopped at 30% total, while the reaction in the presence of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded, in part, by Philip Morris International; however, the opinions and conclusions do not necessarily reflect the position of Philip Morris International. The authors are grateful to Dr. Katie Cychosz (Quantachrome Instruments) for helpful discussions about interpretation of the adsorption results.

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REFERENCES

(1) Stock, N.; Biswas, S. Chem. Rev. 2012, 112, 933−969. (2) Li, J.-R.; Sculley, J.; Zhou, H.-C. Chem. Rev. 2012, 112, 869−932. (3) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724− 781. (4) Cohen, S. M. Chem. Rev. 2012, 112, 970−1000. (5) Bétard, A.; Fischer, R. A. Chem. Rev. 2012, 112, 1055−1083.

F

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(6) Jiang, H.-L.; Xu, Q. Chem. Commun. 2011, 47, 3351−3370. (7) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196− 1231. (8) Shultz, A. M.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. J. Am. Chem. Soc. 2009, 131, 4204−4205. (9) Bromberg, L.; Klichko, Y.; Chang, E. P.; Speakman, S.; Straut, C. M.; Wilusz, E.; Hatton, T. A. ACS Appl. Mater. Interfaces 2012, 4, 4595−4602. (10) Ramos-Fernandez, E. V.; Pieters, C.; van der Linden, B.; JuanAlcañ i z, J.; Serra-Crespo, P.; Verhoeven, M. W. G. M.; Niemantsverdriet, H.; Gascon, J.; Kapteijn, F. J. Catal. 2012, 289, 42−52. (11) Serra-Crespo, P.; Ramos-Fernandez, E. V.; Gascon, J.; Kapteijn, F. Chem. Mater. 2011, 23, 2565−2572. (12) Couck, S.; Denayer, J. F. M.; Baron, G. V.; Rémy, T.; Gascon, J.; Kapteijn, F. J. Am. Chem. Soc. 2009, 131, 6326−6327. (13) Gu, Z.-Y.; Wang, G.; Yan, X.-P. Anal. Chem. 2010, 82, 1365− 1370. (14) Deng, L.; Fu, Y.; Guo, Q.-X. Energy Fuels 2008, 23, 564−568. (15) Törmägankas, O. P.; Koskinen, A. M. P. Rec. Res. Dev. Org. Chem. 2001, 5, 225−255. (16) Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Procopiou, P. A. Org. Lett. 2007, 9, 331−333. (17) Tsunetake, S.; Hideshi, H. Catal. Surv. Asia 2003, 7, 145−156. (18) Jerome, J. E.; Sergent, R. H. In Catalysis of Organic Reactions; Morrell, D.G., Ed.; Marcel Dekker: New York, 2003; Chapter 9. (19) Zevaco, T. A.; Janssen, A.; Sypien, J.; Dinjus, E. Green Chem. 2005, 7, 659−666. (20) Dubois, P.; Jacobs, C.; Jerome, R.; Teyssie, P. Macromolecules 1991, 24, 2266−2270. (21) Long, W.; Gill, C. S.; Choi, S.; Jones, C. W. Dalton Trans. 2010, 39, 1470−1472. (22) Stridsberg, K. M.; Ryner, M.; Albertsson, A.-C. Adv. Polym. Sci. 2002, 157, 41−65. (23) Anwander, R.; Palm, C.; Gerstberger, G.; Groeger, O.; Engelhardt, G. Chem. Commun. 1998, 17, 1811−1812. (24) Ogata, Y.; Kawasaki, A. Tetrahedron 1969, 25, 929−935. (25) Folting, K.; Streib, W. E.; Kenneth, G.; Caulton, K. G.; Poncelet, O.; Hubert-Pfalzgraf, L. C. Polyhedron 1991, 10, 1639−1646. (26) Corma, A.; Garcia, H. Chem. Rev. 2002, 102, 3837−3892. (27) Volkringer, C.; Loiseau, T.; Guillou, N.; Férey, G.; Elkaïm, E.; Vimont, A. Dalton Trans. 2009, 12, 2241−2249. (28) McKenna, A. L. Aluminum Carboxylates. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 2000. (29) Trout, T. J. U.S. Patent 4,707,429, Nov. 17, 1987. (30) Kazama, K.; Tezuka, Y.; Imai, K. Macromolecules 1991, 24, 122− 125. (31) El-Sayed, Y.; Bandosz, T. J. J. Colloid Interface Sci. 2001, 242, 44−51. (32) Gold, A.; Dubé, C. E.; Perni, I. B. Anal. Chem. 1978, 50, 1839− 1841. (33) Stavitski, E.; Pidko, E. A.; Couck, S.; Remy, T.; Hensen, E. J. M.; Weckhuysen, B. M.; Denayer, J.; Gascon, J.; Kapteijn, F. Langmuir 2011, 27, 3970−3976. (34) Bromberg, L.; Su, X.; Hatton, T. A. ACS Appl. Mater. Interfaces 2013. (35) Arandes, J. M.; Romero, A.; Bilbao, J. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 828−831. (36) Lin, I.; Day, A. R. J. Am. Chem. Soc. 1952, 74, 5133−5135. (37) Ogata, Y.; Kawasaki, A.; Kishi, I. Tetrahedron 1967, 23, 825− 830. (38) Seki, T.; Nakajo, T.; Onaka, M. Chem. Lett. 2006, 35, 824−829. (39) Ooi, T.; Ohmatsu, K.; Sasaki, K.; Miura, T.; Maruoka, K. Tetrahedron Lett. 2003, 44, 3191−3193. (40) Berberich, H.; Roesky, P. W. Angew. Chem., Int. Ed. 1998, 37, 1569−1571. (41) Burgstein, M. R.; Berberich, H.; Roesky, P. W. Chem.−Eur. J. 2001, 7, 3078−3085.

(42) Czaja, A. U.; Trukhan, N.; Müller, U. Chem. Soc. Rev. 2009, 38, 1284−1293. (43) Bromberg, L.; Hatton, T. A. ACS Appl. Mater. Interfaces 2011, 3, 4756−4764. (44) Bromberg, L.; Diao, Y.; Wu, H.; Speakman, S. A.; Hatton, T. A. Chem. Mater. 2012, 24, 1664−1675.

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