Article pubs.acs.org/cm
Synthesis of Structured Porous Polymers with Acid and Basic Sites and Their Catalytic Application in Cascade-Type Reactions Estíbaliz Merino,† Ester Verde-Sesto,‡ Eva M. Maya,‡ Marta Iglesias,§ Félix Sánchez,*,† and Avelino Corma*,|| †
Instituto de Química Orgánica General, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain Instituto de Ciencia y Tecnología de Polímeros, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain § Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, Cantoblanco 28049 Madrid, Spain || Instituto de Tecnología Química, UPV-CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain ‡
S Supporting Information *
ABSTRACT: We described the use of one porous polymeric aromatic framework (PPAF) with a 9,9′-spiro-bisfluorene unit as a support in heterogeneous catalysis. The material was functionalized with acid and base active sites and used as a bifunctional catalyst in a model cascade reaction. This catalyst was recycled up to eight times with only a small loss of activity.
KEYWORDS: porous aromatic frameworks, bifunctionalized solids, heterogeneous catalysis, organocatalysis, one-pot tandem reaction
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PAFs with polar organic groups (−OH, −COOH, −NH2) guided by quantum chemical calculations for CO2, H2, and N2 adsorption and have simulated the behavior of these materials using the Grand Canonical Monte Carlo emthod.21 To the best of our knowledge, there are no examples of multifunctionalization (Figure 1) of this type of material and their use in catalysis. Heterogenization of acid and base homogeneous catalysts is an attractive concept to carry out multistep cascade reactions for the synthesis of complex organic molecules.22−24 Although homogeneous acids and bases will rapidly neutralize when used together in one reactor, this can be prevented by fixing them on solid supports and tailoring spatial separation between the two
INTRODUCTION Nature’s strategy of employing multistep cascade reaction for the synthesis of complex and bioactive organic molecules in living systems has been a source of inspiration for designing artificial catalysts. Most of the time, a multistep chemical process involves multisite catalysts. However, by performing the consecutive reaction steps in one pot, costly intermediate separations and purification processes will be avoided. Therefore, also from an energy savings point of view, the design of solid catalysts with well-defined multisites are of interest to achieve more-intensive chemical processes. Efforts are being made today to prepare such multifunctional inorganic and hybrid organic solid catalysts for performing one-pot multistep reactions.1−5 Normally, those multisite catalysts are formed by well-structured microporous and mesoporous organic−inorganic hybrid materials such as metal-organic frameworks (MOFs) and periodic mesoporous organosilicas (PMOs). Sometimes, it could also be of interest to have purely multisite-organic microporous and mesoporous structured materials. This could, in principle, be achieved by suitable functionalization of covalent organic frameworks (COFs),6−13 including the porous aromatic frameworks (PAFs) with a diamond-like structure.14,15 Recently, organic porous materials have attracted considerable attention, because of potential applications in storage,16 separation,17 and catalysis.18−20 Thus, Jiang et al. have designed © 2013 American Chemical Society
Figure 1. Bifunctionalized structure of the unit of PPAF. Received: January 11, 2013 Revised: February 27, 2013 Published: February 27, 2013 981
dx.doi.org/10.1021/cm400123d | Chem. Mater. 2013, 25, 981−988
Chemistry of Materials
Article
antagonist functional sites.25 Heterogeneous multifunctional catalysts can provide a continuous range of functional groups and offer advantages, such as an enhancement of reactivity and stability of antagonist functional groups.26−28 Bifunctional materials of different nature have been described.29−39 One example can be represented by the zeolitic hybrid organic− inorganic materials with acid sites located in zeolitic counterpart and base centers in the organic component. They are used as bifunctional catalysts in a cascade reaction.2 In this paper, we report for the first time, the simultaneous functionalization of porous polymeric aromatic frameworks (PPAFs) with acid and base groups and their application to an acid−base catalyzed tandem reaction. We demonstrate that the incorporation of acid and basic groups in the same porous aromatic framework results in an excellent catalyst for one-pot cascade reaction (hydrolysis of an acetal and a Knoevenagel condensation), avoiding the inconvenience observed with homogeneous catalysts where the acid and base sites are cancelled. It will be shown that the bifunctional acid−base mesoporous material is stable and robust, allowing several recycles with a small loss of activity.
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subjected to refluxing until a yellow color was obtained. Finally, the sample was filtrated and washed with tetrahydrofuran (THF) and diethyl ether (Et2O). After drying in vacuo, a pale yellow powder was obtained. Catalytic Testing. The reactions were carried out in a micro reaction vessel (5 mL). Benzaldehyde dimethyl acetal (4) (54 μL), malononitrile (29 mg), and dodecane (41 μL) were dissolved in 2 mL of toluene and 25 μL of water, and then the mixture was stirred at 90 °C under argon with 20 mg of PPAF−SO3H−NH2. The samples of the reaction mixture were analyzed by gas chromatography (GC) (Konik, Model HCGC 5000B) using a 15-m KAP-5 capillary column and a flame ionization detector (FID). After the completion of the reaction, the bifunctionalized material was separated by filtration and washed with toluene, an AcOH/AcONa buffer (pH 4), water, THF, and Et2O, and then was reused for the above one-pot tandem reaction. Characterization Methods. Chemical Composition. The carbon, nitrogen, sulfur, and hydrogen contents were determined in a LECO Model CHNS-932 analyzer (see Table 1). Thermogravimetric and
Table 1. Elemental Analysis of PPAF and Functionalized PPAFs
EXPERIMENTAL DETAILS
PPAF was prepared using an adaptation to a literature method.40 (See Scheme 1.) Here, 259 mg (0.316 mmol) of 2,2′,7,7′-tetraiodo-9,9′-
Scheme 1. Synthesis of PPAF Support with Idealized Structures for Solid Materials
support
%C
%H
%N
PPAF-I PPAF-SO3H PPAF-SO3H-NO2 PPAF-SO3H-NH2 PPAF-NO2 PPAF-NH2
87.04 72.64 58.22 62.4 64.31 60.85
4.38 4.70 2.92 4.7 3.11 4.90
6.93 7.18 7.38 6.92
%S 4.93 2.99 2.56
differential thermal analyses (TGA-DTA) were conducted in an air stream with a TA Instruments Model TA-Q500 analyzer. The samples were heated under an air stream from 40 °C to 850 °C with a heating rate of 10 °C/min. Fourier Transform Infrared (FTIR) spectra were recorded on a Perkin−Elmer Spectrum One spectrometer and are reported in terms of the frequency of absorption (cm−1). Ultraviolet− visible light (UV-vis) spectra were recorded on a Shimadzu Model UV80 2401PC apparatus and are reported in terms of wavelength (nm). Nuclear magnetic resonance (NMR) spectra were recorded with a Bruker Model AV300 spectrometer (Larmor frequencies of 75 and 300 MHz for 13C and 1H, respectively) for liquids and a Bruker AV400 WB spectrometer (Larmor frequencies of 400 and 100 MHz, using 4mm MAS probes spinning at a rate of 10 kHz for 13C solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) measurements. The 13C CP-MAS spectra were obtained using a contact time of 3.5 ms and a relaxation time of 4 s. The number of scans used for the 13C CP-MAS spectra was 1024. Textural Characterization. Nitrogen adsorption isotherms were measured at 77 K, using a Micromeritics ASAP 2020 instrument. Prior to measurement, the samples were degassed for 12 h at 200 °C. The pore size average was determined using the Barrett−Joyner−Halenda (BJH) method. Wide-angle X-ray scattering (WAXS) was carried out using a Bruker D8 Advance diffractometer. Data were collected stepwise over the angular region of 1° ≤ 2θ ≤ 65°, using steps of 0.5 s/step accumulation time, a Vantec detector, and Cu Kα radiation (λ = 1.542 Å). Scanning electron microscopy (SEM) micrographs were obtained with a Hitachi Model SU-8000 microscope operating at 0.5 kV. The samples were prepared directly by dispersing the powder onto a double-sided adhesive surface.
spirobisfluorene (2),41 105 mg (0.632 mmol) of 1,4-phenylenediboronic acid (3), 41 mg (0.158 mmol) of triphenylphosphine, 117 mg (1.39 mmol) of sodium bicarbonate, and 3.5 mg (0.016 mmol) of palladium acetate are suspended in a mixture of dimethylformamide (DMF) (2 mL) and water (0.5 mL). The mixture was degassed by argon bubbling. Microwave heating was performed in a computercontrolled CEM Discover microwave with temperature and pressure control. Initial heating was performed at a power input of 75 W. After the pressure has reached ∼10 bar, the heating was stopped until the temperature cooled to 60 °C and the pressure disappears. After that period, heating was continued to reach the reaction temperature (∼145 °C). The reaction was conducted at that temperature and a pressure of ∼7 bar for 5 min. After cooling to room temperature, the mixture was filtrated and the crude product was washed with DMF and water. The solid was suspended in a mixture of water (H2O, 100 mL), hydrochloric acid (HCl, 1 mL), and nitric acid (HNO3, 2 mL) and was
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RESULTS AND DISCUSSION The preparation of PPAF is carried out by combining 1,4benzenediboronic acid (3) with the tetrabromo monomer (1) or tetraiodo monomer (2), easily available from 9,9′-spirobisfluorene. Thus, 2,2′,7,7′-tetrabromo-9,9′-spiro-bisfluorene (1) or 2,2′,7,7′-tetraiodo-9,9′-spiro-bisfluorene (2) reacts with 982
dx.doi.org/10.1021/cm400123d | Chem. Mater. 2013, 25, 981−988
Chemistry of Materials
Article
additives. More-accurate values are obtained from EDX analysis, which also confirms that the iodo, palladium, and phosphor contents are
