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Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on amine-functionalized UiO-66
Accepted Manuscript Title: Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on amine-functionalized UiO-66 Authors: Chinna Krishna Prasad Neeli, Pillaiyar Puthiaraj, Yu-Ri Lee, Young-Min Chung, Sung-Hyeon Baeck, Wha-Seung Ahn PII: DOI: Reference:
S0920-5861(17)30599-0 http://dx.doi.org/10.1016/j.cattod.2017.09.002 CATTOD 11007
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
23-5-2017 10-8-2017 2-9-2017
Please cite this article as: Chinna Krishna Prasad Neeli, Pillaiyar Puthiaraj, Yu-Ri Lee, Young-Min Chung, Sung-Hyeon Baeck, Wha-Seung Ahn, Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on aminefunctionalized UiO-66, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on amine-functionalized UiO-66 Chinna Krishna Prasad Neeli,1 Pillaiyar Puthiaraj,1 Yu-Ri Lee,1 Young-Min Chung,2 Sung-Hyeon Baeck,1 Wha-Seung Ahn1, * 1
Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea
2
Department of Nano and Chemical Engineering, Kunsan National University, Kunsan 573-701,
South Korea
*E-mail:[email protected] Graphical Abstract
Pd nanoparticles supported on NH2-UiO-66 were effective in transfer hydrogenation of nitrobenzene using formic acid under mild conditions in water.
1
Highlights of the present work
Pd NPs with size 2.2 nm supported on NH2-UiO-66 were prepared. The catalyst was efficient in transfer hydrogenation of nitrobenzene to aniline. In-situ hydrogen utilization through the formic acid decomposition. Reaction operated at mild pressure and temperature in water.A plausible reaction mechanism was proposed
Abstract Pd nanoparticles with a mean diameter of 2.2 nm on amine-functionalized UiO-66 were synthesized by facile anionic-exchange followed by chemical reduction. The prepared Pd/NH2-UiO-66 catalyst was effective in the catalytic transfer hydrogenation of nitrobenzene to aniline using formic acid as a hydrogen source under mild reaction conditions in water with a maximum nitrobenzene conversion of 98% and an aniline selectivity of 99%. The superior catalytic activity of Pd/NH2-UiO-66 was attributed to the cooperative effects of nano-sized Pd and its stabilization offered by the NH2-UiO-66 support. The heterogeneous nature of the catalytic system was confirmed by a hot-filtering test and the catalyst maintained stable performance in 4 repeated cycles. A plausible reaction mechanism was proposed.
Keywords: Palladium, nitrobenzene, transfer hydrogenation, aniline, water.
1. Introduction Aniline (AN) is important intermediate in the production of agrochemicals, pharmaceuticals, dyes, polyurethanes, pigments, and pesticides [1-4]. Conventionally, the 2
preparation of amines relies on the homogeneous catalytic hydrogenation of nitro compounds, mostly in the presence of an organic solvent and/or activation ligands [5-7]. Separation, excess metallic/reagent waste, and recycling issues associated with homogeneous catalysis hinder their use in practical applications. To overcome these problems, a number of heterogeneous catalysts such as Pt [8], Pd [9], Au [10], Ru [11], and Rh [12] have been developed for the direct hydrogenation of nitrobenzene to aniline. However, drawbacks such as the use of environmentally harmful organic solvents, long reaction times, and high temperature/high pressure reaction conditions are still problems, and a facile and ecofriendly procedure that avoids the use of toxic solvents and hazardous molecular hydrogen in high pressure is desirable [13]. Catalytic transfer hydrogenation (CTH), an alternative to direct hydrogenation through the use of hydrogen donors, e.g., formic acid, is operationally simple, and can avoid the use autoclaves and high pressure hydrogen gas [14]. In particular, formic acid is an economical, safe and easy to handle renewable liquid hydrogen carrier that can be produced from biomass. As the same time, the development of metal catalyzed reactions in aqueous media [15] has been motivated because water represents the most benign, abundant, and inexpensive solvents [16]. The use of water as a reaction medium has reportedly led to unique reactivity and selectivity that cannot be obtained in conventional organic solvents [17]. From the view of green chemistry principles, there is a definite need to develop environmentally benign and clean protocols, such as CTH, which allow effective hydrogen transfer under mild conditions in aqueous medium. Metal organic frameworks (MOFs), which are a new class of high crystalline organicinorganic hybrid materials, have attracted tremendous attention owing to their novel 3
coordination structures, diverse topologies and potential applications in diverse areas including catalysis and adsorption [18-24]. One of the current challenges in MOFs catalysis is the development of stable MOF structures incorporated with functional sites that can be used for encapsulation of active species either directly or via post-synthetic modification [25]. In this respect, the zirconium-based UiO-66 [26] has emerged as a highly desirable host matrix for the encapsulation of metal nanoparticles (NPs) owing to its large specific surface area as well as good chemical resistance to water and organic solvents. The conjunction of a solid porous structure of MOFs and metal NPs can offer high catalytic activity compared to conventional catalysts. Recently, palladium supported on different amine-functionalized MOF catalysts has been used for Suzuki-Miyaura coupling [27], Heck coupling [28], acetalization [29], dehalogenation of arylhalides [30] and hydrogenation reactions [31]. Few reports are available for the CTH of nitrobenzene using formic acid over a palladium-based catalyst [32] and there is scope to explore the potential improvement in catalytic performance over different support systems. Herein, a catalysts made of Pd NPs encapsulated in amine-functionalized UiO-66 was proposed for the CTH of nitrobenzene to aniline using formic acid as a hydrogen source in water medium under mild reaction conditions. Highly active and selective catalysis was observed, and it was demonstrated that the replacement of organic solvents with neat water led to significant improvements in both the activity and selectivity in the reaction. A plausible catalytic mechanism of nitrobenzene CTH was also proposed based on the reaction results and independent formic acid dehydrogenation studied.
2. EXPERIMENTAL SECTION 4
2.1. Chemicals Palladium chloride (PdCl2, 99.9%), zirconium tetrachloride (ZrCl4, 99.5%), terephthalic acid (99%), 2-aminoterephthalic acid (99%), formic acid (FA, 95%), methanol (MeOH, 99.9%), NaBH4 (99%), Pd/C (5 wt.%), and anhydrous N,N-dimethylformamide (DMF, 98%) were purchased from Sigma Aldrich. Hydrochloric acid (HCl, 36%) and ethanol (EtOH, 99.95%) were supplied from Samschun (Korea) and Merck. Nitrobenzene (NB, 98%) and ethylacetate (EtOAc, 99.5%) were purchased from TCI and Duksan (Korea). All chemicals were used as received. Deionized water was produced using a Millipore Milli-Q ultrapure water purification system. 2.2. Catalyst Preparation 2.2.1. Preparation of NH2-UiO-66 NH2-UiO-66 was prepared under the solvothermal conditions reported by Cavka et al. [26] and Hupp et al. [33] with some modification. In a typical synthesis, ZrCl4 (1.16 g, 5 mmol) and HCl (0.30 mL, 10 mmol) were dissolved in anhydrous DMF (60 mL) at room temperature. 2-Aminoterephthalic acid (0.90 g, 5 mmol) was then added to the clear solution with vigorous stirring and further sonicated at room temperature for 10 min. The resulting mixture was transferred to a 100 mL Teflon lined stainless steel autoclave, which was sealed and heated to 120 °C for 24 h. After 24 h, the substrate mixture was cooled to room temperature and the precipitate was collected by centrifugation. The obtained yellow solid was washed with DMF (3 x 30 mL) and absolute ethanol (2 x 30 mL) followed by drying under vacuum at 80 °C for 12 h; the sample was designated as NH2-UiO-66. Pure UiO-66 was synthesized using the same method mentioned for the preparation of NH2-UiO-66 except using terephthalic acid as the organic ligand. 5
2.2.2. Preparation of Pd NPs encapsulated on NH2-UiO-66 Pd/NH2-UiO-66 was prepared according to the anionic exchange method [30] followed by chemical reduction [34, 35]. Prior to encapsulation, NH2-UiO-66 was activated at 120 °C for 4 h under vacuum to remove the residual solvent in the pores. Subsequently, 0.5 g of activated NH2-UiO-66 was dispersed in 50 mL of methanol followed by the addition of a PdCl2 suspension (30 mg in 1 mL of 2 M HCl aqueous solution) and stirred continuously at RT for 12 h. The resulting orange solid was isolated by centrifugation, washed with EtOH and dried at 80 °C. The solid was then dispersed in 50 mL water and reduced with a 0.1 M NaBH4 aqueous solution at RT for 4 h. Finally, a greyish brown suspension was washed with water and ethanol followed by vacuum drying at 80 °C for 12 h to obtain the Pd/NH2-UiO-66 catalyst. Pd/UiO-66 was also synthesized by replacing the UiO-66-NH2 with an equivalent amount of UiO-66 to compare the catalytic activity. The Pd particle was found to be 0.39 wt.% in Pd/UiO-66 by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin-Elmer optima 7300 DV). To increase the Pd loading level on UiO-66, following impregnation method was used. A 0.5 g of UiO-66 was dispersed in 25 mL de-ionized (DI) water followed by adding 1 mL of Na2PdCl4 solution (0.1 mol/L) and stirred at 80 oC for 1 h. Then the NaOH solution (1mol/L) was added into the reaction mixture to reach the pH 12, and the solution was stirred at 80 oC for another 2 h. Subsequently, NaBH4 (0.3 g in 10 mL water) was added dropwise into the reaction mixture under continuous stirring, which produced ultrafine Pd/UiO-66. The Pd/UiO-66 was collected by filtration, washed with DI water, and dried at 80 oC for 12 h. The Pd loading on UiO-66 was 1.68 wt.% by ICP-OES. 2.3. Catalyst characterization The X-ray diffraction (XRD, Rigaku) patterns were measured using Cu Kα radiation (λ= 6
1.54 Å) within the range, 5–50° 2θ, at a scanning rate of 0.5° min−1. The N2 adsorptiondesorption isotherms were obtained at −196 °C using a BELsorpII-Max (BEL, Japan). Prior to the adsorption measurements, the samples were out-gassed at 120 °C for 12 h under vacuum. The specific surface area and pore size distribution of the samples were calculated using the BET (Brunauer-Emmett-Teller) method and Horvath–Kawazoe (HK) model, respectively. Fourier transform infrared spectroscopy (FT-IR, Bruker VERTEX 80v) was performed on KBr disks at room temperature. Field emission transmission electron microscopy (FE-TEM, FEI TECNAI 30 G2 S-TWIN) was carried out at an accelerating voltage at 100 kV. The TEM samples were prepared by dispersing the catalyst powder in ethanol under ultrasonic radiation for 10 min, and placing a drop of the resulting solution onto a copper grid followed by slow evaporation of the solvent under vacuum at room temperature. Elemental analysis for nitrogen was determined on an Elementar Vario ELIII analyzer. The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo VG ESCALAB250) using Al Kα radiation and a hemispherical analyzer. The XPS data was calibrated internally, fixing the binding energy of C 1s to 284.6 eV. 2.4. Transfer hydrogenation of nitrobenzene The catalytic activity of Pd/NH2-UiO-66 in the transfer hydrogenation of nitrobenzene was evaluated by performing the reactions in a 15 mL ACE pressure tube. Typically, 1 mmol of nitrobenzene, 3.5 mmol of formic acid, and Pd/NH2-UiO-66 catalyst (50 mg, 0.7 mol% of Pd) in 5 mL of H2O were charged into the pressure tube and sealed. The reaction mixture was stirred at 60 °C for 7 h. After the reaction was complete, the catalyst was separated by centrifugation and the products were extracted with ethyl acetate. The reaction products were then analyzed using a GC (HP6890 model, M/s. Agilent Technologies, USA) equipped with a 7
flame ionization detector and a HP-5 capillary column. The products were confirmed by GCMS (Varian 1200L single quadrupole with 3800GC model, M/s. Agilent Technologies, USA) using a DB-5 column. The effects of the reaction conditions with respect to the solvent, temperature, formic acid to nitrobenzene molar ratio, time, and heterogeneity of the reaction and catalyst recyclability were examined. 2.5. Formic acid dehydrogenation Formic acid dehydrogenation was performed in a 100 ml three-necked round flask with water jacket. Before use, catalyst was dried at 110 ℃ for 4 h under vacuum. In a typical run, a reactor containing catalyst (100 mg) and water (15 ml) was connected to a gas burette, pressure-equalizing funnel containing formic acid (6 mmol), and Ar gas line. After heated to 60 ℃, the reaction medium was magnetically stirred for 30 min to achieve thermal equilibrium under inert condition. A reaction was initiated by injecting the formic acid in the connected pressure-equalizing funnel to the reaction medium under vigorous stirring. The volume of the evolved gas was monitored by recording the displacement of water in the gas burette. The reaction temperature was maintained by using water circulator connected to the reactor. The released gas during a reaction was collected and analyzed by Agilent 6890 Gas Chromatography equipped with Carboxen-1010 Plot capillary column and TCD/methanizerFID detector.
3. Results and Discussion 3.1. Catalyst characterization Pd NPs were immobilized on UiO-66 by a direct anionic exchange method followed by 8
reduction [30]. To this end, the chloride anions present in the complex, [NH3+-UiO-66][Cl]-, formed by the dissolution of the HCl into NH2-UiO-66, were exchanged with the PdCl2 precursor to form a [NH3+-UiO-66]2[PdCl4]2- ion pair via electrostatic attraction and subsequently reduced with a 0.1 M aqueous NaBH4 solution at room temperature to obtain the Pd nanocatalyst supported on NH2-UiO-66. The actual Pd loadings in the fresh Pd/NH2UiO-66 and used Pd/NH2-UiO-66 catalyst after 4 cycles estimated by ICP-OES were 1.54 wt. % and 1.49 wt. %, respectively. On the other hand, only 0.39 wt. % Pd was detected in Pd/UiO-66 despite the same Pd precursor loading of 2.0 wt. % applied in the feed solution. Unlike UiO-66, the rapid absorption of PdCl2 in Pd/NH2-UiO-66 was observed with the appearance of an orange color, suggesting the crucial role of amine groups in NH2-UiO-66 for the effective Pd loading and stronger Pd–N binding interaction. From elemental analysis, the N content in NH2-UiO-66 was ca. 4.75 wt. %. Fig. 1 presents the N2 adsorption-desorption isotherms of the various samples measured at −196 °C, exhibiting type I isotherms with no hysteresis. Table 1 lists the textural properties determined from these isotherms. The BET surface area of Pd/NH2-UiO-66 was ca. 790 m2 g−1 with a micropore volume of 0.32 cm3 g−1, which was lower than that of the NH2-UiO-66 support (1052 m2 g−1 with a micropore volume of 0.43 cm3 g−1) due to the encapsulation of Pd NPs into NH2-UiO-66. Fig. 2 shows the XRD patterns of NH2-UiO-66, fresh Pd/NH2-UiO-66, and reused Pd/NH2-UiO-66 samples. These samples showed similar major XRD peaks at 7.4° and 8.58° 2θ. These were assignable to the (111) and (200) planes of highly crystalline NH2-UiO-66, respectively, which are in good agreement with the simulated XRD patterns of UiO-66 [36]. No loss of crystallinity in Pd/NH2-UiO-66 was detected, and the characteristic XRD peaks for 9
Pd were not observed due to the low loading and high dispersion of palladium NPs inside the MOF matrix [37]. Fig. 3 presents the FT-IR spectra of NH2-UiO-66, fresh Pd/NH2-UiO-66, and reused Pd/NH2-UiO-66 samples. A broad band at 2969 cm−1, which was characteristic of the –OH stretching vibration of the carboxylic acid group in the NH2-H2BDC ligand, was shown by all the samples. In addition, two bands at approximately 3475 and 3363 cm−1, which were assignable to the asymmetric and symmetric stretching vibrations of the primary amine group, could be distinguished clearly over the prepared NH2-UiO-66 and Pd/NH2-UiO-66 (curves a and b in Fig. 3). The decrease in intensity of these bands after Pd incorporation was attributed to the presence of an electrostatic interaction between Pd and the amine group of NH2-UiO66 [38]. The presence of a NH2 group within the framework of NH2-UiO-66 was analyzed using the N 1s spectra from the XPS data (curve c in Fig. 5); the N 1s peak of the amine group was observed at 399.68 eV. As shown in the TEM image of Pd/NH2-UiO-66 (Fig. 4a), the Pd NPs were dispersed homogeneously in the pore channels of NH2-UiO-66. The particle size distribution (Fig. 4a as an inset) of the fresh Pd/NH2-UiO-66 revealed Pd NPs with a mean diameter of 2.2 nm. On the other hand, the TEM image of the reused Pd/NH2-UiO-66 catalyst (Fig. 4b) after 4 cycles showed an increase in the mean particle size to 8 nm (Fig. 4b as inset). XPS was performed to analyze the surface chemical state of the synthesized Pd/NH2UiO-66 and the results are presented in Fig. 5. Deconvolution of the Pd 3d spectrum resulted in two peaks at 335.39 and 340.78 eV, which were assignable to Pd 3d5/2 and Pd 3d3/2, confirming the existence of metallic Pd (curve b in Fig.5). These peaks appeared at slightly higher energy level due to the interaction between the Zr and Pd [39]. The signal intensity of 10
Pd was low due to the relatively low Pd content in the MOF structure. No obvious peak for Pd+2 was observed, indicating the complete formation of Pd NPs through the reduction of PdCl4−2. Two additional peaks in curve b in Fig. 5 located at 332.58 and 346.17 eV were ascribed to Zr 3p3/2 and Zr 3p1/2 respectively [40]. The curves of the Zr 3d region could be deconvoluted into two peaks for Zr 3d5/2 and Zr 3d3/2 centered at approximately 182.04 and 184.35 eV, respectively (curve d in Fig. 5). Moreover, the peaks corresponding to oxygen, carbon, nitrogen, zirconium, and palladium were also clearly observed in the XPS elemental survey of the catalyst (curve a in Fig. 5). 3.2. Transfer hydrogenation of nitrobenzene The activity of the Pd/NH2-UiO-66 catalyst in the transfer hydrogenation of nitrobenzene to aniline using formic acid as the hydrogen source in aqueous media was investigated (Scheme 1). Controlled and blank experiments were carried out in water at 60 °C to optimize the conditions for the CTH of nitrobenzene over UiO-66, NH2-UiO-66 and without a catalyst using formic acid; the results are summarized in Table 2. No meaningful
conversion of
nitrobenzene was observed using the parent UiO-66, NH2-UiO-66 and without a catalyst (Table 2, entries 1–3), confirming the need for a metal to dehydrogenate formic acid and to carry out the hydrogenation of nitrobenzene. The maximum nitrobenzene conversion of 98% to aniline with 99% selectivity (Table 2, entry 4) was observed when Pd/NH2-UiO-66 was used as a catalyst, showing the presence of palladium was indispensable for high catalytic activity. In contrast, the Pd/UiO-66 catalyst with 0.39 wt.% and 1.68 wt.% Pd loading exhibited low catalytic activity with 15% and 69% of nitrobenzene conversion, respectively (Table 2, entries 5 and 6). These could be explained by the low loading of Pd NPs in the cavities of UiO-66 as determined by ICP-OES and the contribution of amine moiety in the 11
support material. A similar result was observed elsewhere [38]. There have been previous reports that the primary (NH2-) and secondary ((CH3)2N-) amine groups in the vicinity of Pd on supporting materials interact with the adsorbed formic acid molecules, and accelerate the rate of formic acid deprotonation [41, 42]. We also compared the catalytic activity with 15 mg of commercial Pd/C (5 wt.%) catalyst in the CTH of nitrobenzene. It was detected that the Pd/C gave 82% of nitrobenzene conversion (Table 2, entry 7), which was slightly lower than Pd/NH2-UiO-66. When the reaction was carried out with Pd/NH2-UiO-66 in an open flask reaction tube, only 33% conversion (Table 2, entry 8) was achieved indicating the evolved H2 gas also involved in the reaction to convert the nitrobenzene. Initially, the influence of solvent in the CTH of nitrobenzene was investigated using 5 mL of different solvents: water, toluene, ethanol and methanol over Pd/NH2-UiO-66 catalyst (0.7 mol% of Pd), 1 mmol of nitrobenzene and 3 mmol of formic acid at 60 °C for 6 h. The results listed in Table 2 (entries 9–12) showed that there was a significant influence of solvents in nitrobenzene conversion and aniline selectivity. The activity of the catalyst increased with increasing polarity of the solvent, which resulted in the high solubility of formic acid with the promotion of hydrogen transfer over the Pd surface. Water proved to be the best solvent with 86% nitrobenzene conversion and 99% aniline selectivity. Using water as a solvent, the effects of the reaction temperature (40, 60 and 80 °C) were then examined, and the results are shown in Fig. 6. At 40 °C, the conversion of nitrobenzene was approximately 52%, which increased significantly to 86% when the reaction temperature was increased to 60 °C with a marginal change in the aniline selectivity from 97 to 99%. This increase in conversion is likely to be a consequence of the increased hydrogenation reaction rate between the adsorbed nitrobenzene and H species generated by palladium formate 12
intermediate decomposition on Pd surface (see mechanism later). With increasing reaction temperature from 60 to 80 °C, however, nitrobenzene conversion decreased to 74% accompanied by aniline selectivity to 95%. In addition, decrease in aniline selectivity with the by-product formation of azoxybenzene at 80 °C was confirmed by GC-MS. A similar decrease in activity with increasing temperature was found in previous report [43], and the optimum reaction temperature was 60 °C. The effect of the formic acid to nitrobenzene molar ratio was then investigated with 1 mmol of nitrobenzene and 5 mL of H2O at 60 °C for 7 h, and the results are shown in Fig. 7. With increasing formic acid/nitrobenzene ratio from 2 to 3, nitrobenzene conversion increased from 68% to 92% with a constant aniline selectivity of > 99%, because of the high availability of H species generated through the decomposition of palladium formate molecules. At the formic acid:nitrobenzene mole ratio of 3.5, a maximum of nitrobenzene conversion 98% with aniline selectivity 99% was achieved with very low N-formylaniline (FAN) of < 1% as a byproduct. With further increase in the amount of formic acid from 3.5 to 4, nitrobenzene conversion remained constant to 98%, but the aniline selectivity decreased to 96% with FAN selectivity to 4%. The increase in the FAN selectivity was due to the formic acid:nitrobenzene mole ratio 4 (higher than the stoichiometric molar ratio of 3), which led to the formation of FAN by the reaction of aniline with excess formic acid under the reaction conditions. Therefore, the optimal amount of formic acid was approximately 3.5 moles per mole of nitrobenzene. To examine the effect of the reaction time on the CTH of nitrobenzene, the distribution of reactants and products were analyzed and plotted as a function of time in Fig. 8. When the reaction time was increased from 1 to 7 h, the concentration of aniline increased from 0.06 to 13
0.19 mmol/mL i.e. aniline selectivity of > 99% with a very low concentration (0.002 mmol/mL) of N-formyl aniline (FAN) as the only byproduct; it must be formed by the reaction of aniline with excess formic acid. Upon further reaction to 8 h, the concentration of FAN increased to 0.01 mmol/mL. To verify the pathway of the CTH of nitrobenzene using formic acid as a hydrogen source, simple dehydrogenation of formic acid over the Pd/NH2-UiO-66 was also carried out in the absence of nitrobenzene, as shown in Fig. 9. In the formic acid dehydrogenation experiment, approximately equal amount of H2 and CO2 gas (1.35 mmol) was generated after 1 h of formic acid dehydrogenation and no further reaction took place afterwards, whereas the CTH reaction proceeded over 7 h. Therefore, formic acid dehydrogenation was not likely to be the rate determining step in the CTH reaction. Accordingly, a reaction mechanism was proposed as illustrated in Fig. 10. In the initial step, formic acid dehydrogenation takes place over the nano-sized palladium surface of Pd/NH2-UiO-66. The amine groups present in the catalyst weakly dissociate the acid –OH of formic acid, which results in the formation of palladium formate and ammonium ion by the addition of H+ species with amine group of the NH2-UiO-66. The dehydrogenation of palladium formate leads to the formation of CO2 and H− (3 in Fig. 10). The ammonium ion containing H+ and H− species adsorbed on Pd(0) surface are transferred to electron-rich O and electron deficient N of the nitro group, respectively to form 4 in Fig. 10. This results in the formation of nitrosobenzene intermediate (5 in Fig. 10) by the elimination of water. Subsequently, dehydrogenation of the second formic acid molecule generating H− and H+ attacks the N and O atoms of nitrosobenzene to form phenylhydroxyl amine (6 in Fig. 10). Upon dehydrogenation of the third formic acid molecule, transfer of H− and H+ species to nitrogen atom and hydroxyl group (see 7 in Fig. 10) 14
results in the formation of aniline with simultaneous elimination of another water molecule, which is desorbed from the Pd surface. In the presence of excess formic acid, N-formyl aniline is formed via the reaction of aniline with formic acid by removal of water. The high activity of the Pd/NH2-UiO-66 catalyst may be due to the strong electrostatic interactions between the polar nitro group with H+ and H− on Pd/NH2-UiO-66. This strong Pd and nitrogen interaction leads to the effective reduction of nitrobenzene to aniline without the formation of other side products. These experimental results showed 98% nitrobenzene conversion with a high selectivity to aniline ( > 99%) under mild reaction conditions in aqueous media under optimal reaction conditions of Pd/NH2-UiO-66 (0.7 mol% Pd), 5 mL of H2O and formic acid/nitrobenzene molar ratio of 3.5 at 60 °C for 7 h. To determine if the catalytic process is heterogeneous, the transfer hydrogenation of nitrobenzene was conducted by following the hot filtering protocols and the results are shown in Fig. 11. Under the optimized reaction conditions, the Pd/NH2-UiO-66 catalyst was removed after 3 h and the reaction was continued for another 7 h. No further conversion in nitrobenzene was detected, which confirms the heterogeneity of the catalyst. Furthermore, ICP-OES also detected practically no trace of Pd species in the mother liquor. The recyclability of the Pd/NH2-UiO-66 catalyst was evaluated in the transfer hydrogenation of nitrobenzene under the optimized reaction conditions. After the catalytic reaction, the catalyst was isolated from the liquid phase by centrifugation, washed thoroughly with ethanol, vacuum dried at 80 °C, and reused as a catalyst in subsequent runs under identical reaction conditions. The results shown in Fig. 12 indicate no significant loss of the hydrogenation activity of nitrobenzene for at least four repeated cycles. To check the stability of the reused catalyst, Pd/NH2-UiO-66 recovered after the CTH reaction was characterized by XRD, FT-IR, 15
N2 adsorption, and FE-TEM. No significant crystallographic changes were observed for the reused catalyst (Fig. 2), preserving the high crystallinity of the pristine NH2-UiO-66. The IR bands assigned to –NH2 groups were retained even after the catalytic reactions (Fig. 3), which is consistent with the high chemical stability for NH2-UiO-66, as reported previously. On the other hand, a decrease in the amount of adsorbed N2 (Fig. 1) at 77K (BET surface area decreased from 790 to 534 m2g−1) was observed after use, which can be attributed to pore blockage by the residual reactants or products. The reduced surface area, inhibits the access of the reactant molecules to the active sites along with the increased Pd particle size due to partial agglomeration, as confirmed by FE-TEM analysis (Fig. 4b), may be responsible for the decrease in catalytic activity in further cycles. Finally, we compared the catalytic activity of Pd/NH2-UiO-66 in hydrogenation of nitrobenzene to aniline with some of other heterogeneous catalysts reported previously (Table 3). From Table 3, our Pd/NH2-UiO-66 catalyst exhibited higher activity in aqueous medium with formic acid as a hydrogen source compared to other solid catalysts. The previously reported catalysts were required high pressure H2 gas, and/or higher temperature. These comparison results confirmed that Pd/NH2-UiO-66 catalyst is effective for CTH of nitrobenzene.
Conclusions Pd/NH2-UiO-66 was synthesized by a facile anionic exchange method followed by chemical reduction. The highly dispersed Pd NPs were efficient in the catalytic transfer hydrogenation of nitrobenzene to aniline using forming acid as a hydrogen source under mild reaction conditions in a water medium without base additives or ligands. The catalyst was 16
recycled successfully for up to 4 cycles without significant loss of catalytic activity. A reaction mechanism explaining the experimental result was proposed.
Acknowledgement This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2015R1A4A1042434).
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17. N. Shapiro, A. Vigalok, Angew. Chem. Int. Ed. 47 (2008) 2849-2852. 18. J.L.C. Rowsell, E.C. Spencer, J. Eckert, J.A.K. Howard, O.M. Yaghi, Science 309 (2005) 1350-1354. 19. G. Ferey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A. Percheron-Guegan, Chem. Commun. (2003) 2976-2977. 20. E. Haque, J.E. Lee, I.T. Jang, Y.K. Hwang, J.S. Chang, J. Jegal, S.H. Jhung, J. Hazard. Mater. 181 (2010) 535-542. 21. J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450-1459. 22. D.A. Yang, H.Y. Cho, J. Kim, S.T. Yang, W.S. Ahn, Energy Environ. Sci. 5 (2012) 64656473. 23. A.C. McKinlay, R.E. Morris, P. Horcajada, G. Ferey, R. Gref, P. Couvreur, C. Serre, Angew. Chem. Int. Ed. 49 (2010) 6260-6266. 24. S.M. Humphrey, P.T. Wood, J. Am. Chem. Soc.126 (2004) 13236-13237. 25. J. Juan Alcaniz, J. Gascon, F. Kapteijn, J. Mater. Chem. 22 (2012) 10102-10118. 26. J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, J. Am. Chem. Soc. 130 (2008) 13850-13851. 27. Y. Huang, Z. Zheng, T. Liu, J. Lu, Z. Lin, H. Li, R. Cao, Catal. Commun. 14 (2011) 2731. 28. S.N. Kim, S.T. Yang, J. Kim, J.E. Park, W.S. Ahn, CrystEngComm. 14 (2012) 4142-4147. 29. X. Li, T.W. Goh, L. Li, C. Xiao, Z. Guo, X.C. Zeng, W. Huang, ACS Catal. 6 (2016) 3461-3468. 30. Y. Huang, S. Liu, Z. Lin, W. Li, X. Li, R. Cao, J. Catal. 292 (2012) 111-117. 31. L. Chen, H. Chen, R. Luque, Y. Li, Chem. Sci. 5 (2014) 3708-3714. 32. J. Tuteja, S. Nishimura, K. Ebitani, RSC Adv. 4 (2014) 38241-38249. 33. M.J. Katz, Z.J. Brown, Y.J. Colon, P.W. Siu, K.A. Scheidt, R.Q. Snurr, J.T. Hupp, O.K. Farha, Chem. Commun. 49 (2013) 9449-9451. 34. C.K.P. Neeli, S. Ganji, V.S.P. Ganjala, S.R.R. Kamaraju, D.R. Burri, RSC Adv. 4 (2014) 14128-14135. 35. C.K.P. Neeli, M. Ravi Kumar, G. Saidulu, K.S. Rama Rao, D.R. Burri, J. Chem. Technol. & Biotechnol. 90 (2015) 1657-1664. 36. M. Kandiah, M.H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E.A. Quadrelli, F. Bonino, K.P. Lillerud, Chem. Mater. 22 (2010) 6632-6640. 37. M.S. El Shall, V. Abdelsayed, A.E.R.S. Khder, H.M.A. Hassan, H.M. El-Kaderi, T.E. Reich, J. Mater. Chem. 19 (2009) 7625-7631. 38. F. Zhang, S. Zheng, Q. Xiao, Y. Zhong, W. Zhu, A. Lin, M. Samy El Shall, Green Chemistry 18 (2016) 2900-2908. 39. L. Shen, W. Wu, R. Liang, R. Lin, L. Wu, Nanoscale 5 (2013) 9374-9382. 40. B. Yuan, Y. Pan, Y. Li, B. Yin, H. Jiang, Angew. Chem. Int. Ed. 49 (2010) 4054-4058. 41. K. Mori, M. Dojo, H. Yamashita, ACS Catal. 3 (2013) 1114-1119. 18
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Figures
F Fig. 1. N2 adsorption isotherms of NH2-UiO-66, fresh Pd/NH2-UiO-66 and reused Pd/NH219
UiO-66 after fourth run reaction.
Fig. 2. XRD diffraction patterns of NH2-UiO-66, fresh Pd/NH2-UiO-66 and reused Pd/NH2UiO-66 catalysts.
20
Fig. 3. FT-IR spectra of (a) NH2-UiO-66, (b) fresh Pd/NH2-UiO-66 and (c) reused Pd/NH2UiO-66 catalysts.
21
Fig. 4. FE-TEM images of (a) fresh Pd/NH2-UiO-66 and (b) reused Pd/NH2-UiO-66 catalysts with particle size distribution as inset.
22
Fig. 5. XPS patterns of the Pd/NH2-UiO-66 catalyst.
23
Fig. 6. Effect of temperature in CTH of nitrobenzene reaction.
24
Fig. 7. Effect of formic acid to nitrobenzene mole ratio in CTH reaction.
25
Fig. 8. Kinetic plot distribution of reactant and products as a function of time in CTH reaction.
26
Fig. 9. Formic acid dehydrogenation results in absence of nitrobenzene.
27
Fig. 10. Plausible reaction mechanism for the CTH of nitrobenzene through formic acid dehydrogenation.
28
Fig. 11. Heterogeneity test for Pd/NH2-UiO-66 catalyst in CTH reaction.
29
Fig. 12. Recyclability of Pd/NH2-UiO-66 catalyst in CTH reaction.
Scheme 1. Schematic representation of CTH of nitrobenzene to aniline.
Table 1. Physicochemical characteristics of NH2-UiO-66, fresh and reused palladium loaded NH2-UiO-66 catalysts from N2 adsorption isotherms. 30
Sample
SBETa (m2g─1)
Vporeb (cm3g─1)
Pore widthc (nm)
NH2-UiO-66
1052
0.43
0.41
Fresh Pd/NH2-UiO-66
790
0.32
0.39
Reused Pd/NH2-UiO-66
534
0.24
0.36
a
BET surface area Total pore volume measured at P/P0 =0.9998 c Pore width calculated by HK plot b
Table 2. Optimized reaction conditions in CTH of nitrobenzene.a
7
Nitrobenzene Conv. (%) nr
Aniline Sel. (%) –
H2O
7
nr
–
NH2-UiO-66
H2O
7
nr
–
4
Pd/NH2-UiO-66
H2O
7
98
99
5b
Pd/UiO-66
H2O
7
15
83
6c
Pd/UiO-66
H2O
7
69
96
7d
Pd/C
H2O
7
82
97
8e
Pd/NH2-UiO-66
H2O
7
33
88
9f
Pd/NH2-UiO-66
H2O
6
86
99
10f
Pd/NH2-UiO-66
Toluene
6
17
91
11f
Pd/NH2-UiO-66
EtOH
6
35
82
12f
Pd/NH2-UiO-66
MeOH
6
54
87
Entry
Catalyst
Solvent
Time (h)
1
No catalyst
H2O
2
UiO-66
3
a
Reaction conditions: Nitrobenzene (1mmol), catalyst (50 mg), formic acid (3.5 mmol), H2O (5 mL), temperature (60 °C), time (7 h); bPd/UiO-66 (0.39 wt.%); cPd/UiO-66 (1.68 wt.%), d 15 mg of Pd/C (5 wt.%); eopen flask reaction tube; fformic acid (3 mmol).
Table 3. Comparison catalytic activity of Pd/NH2-UiO-66 with other heterogeneous catalysts for liquid phase hydrogenation of nitrobenzene to aniline. Catalyst
H2 source
Solvent 31
Temp. (°C)
Nitrobenzene Aniline Conv. (%) Sel. (%)
Ref.
Pd/B-MCM-41 Ru/RGO Rh-Fe3O4 Cubane Mo3S4 Pt/MWNT Au/ZrO2 Au/TiO2 Ni/SiO2 Pd/NH2-UiO-66
H2 (20 bar) H2 (20 bar) N2H4·H2O HCOOH/Et3N H2 (40 bar) H2 (40 bar) H2 (9 bar) N2H4·H2O HCOOH
scCO2 EtOH/H2O EtOH THF Aniline EtOH Toluene EtOH H2O
32
50 110 80 70 60 150 110 100 60
100 100 100 99 100 95 99 97 98
99 99 99 99 99 97 98.5 98.2 99
9 11 12 14 44 45 46 47 This work
S0920-5861(17)30599-0 http://dx.doi.org/10.1016/j.cattod.2017.09.002 CATTOD 11007
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
23-5-2017 10-8-2017 2-9-2017
Please cite this article as: Chinna Krishna Prasad Neeli, Pillaiyar Puthiaraj, Yu-Ri Lee, Young-Min Chung, Sung-Hyeon Baeck, Wha-Seung Ahn, Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on aminefunctionalized UiO-66, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Transfer hydrogenation of nitrobenzene to aniline in water using Pd nanoparticles immobilized on amine-functionalized UiO-66 Chinna Krishna Prasad Neeli,1 Pillaiyar Puthiaraj,1 Yu-Ri Lee,1 Young-Min Chung,2 Sung-Hyeon Baeck,1 Wha-Seung Ahn1, * 1
Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea
2
Department of Nano and Chemical Engineering, Kunsan National University, Kunsan 573-701,
South Korea
*E-mail:[email protected] Graphical Abstract
Pd nanoparticles supported on NH2-UiO-66 were effective in transfer hydrogenation of nitrobenzene using formic acid under mild conditions in water.
1
Highlights of the present work
Pd NPs with size 2.2 nm supported on NH2-UiO-66 were prepared. The catalyst was efficient in transfer hydrogenation of nitrobenzene to aniline. In-situ hydrogen utilization through the formic acid decomposition. Reaction operated at mild pressure and temperature in water.A plausible reaction mechanism was proposed
Abstract Pd nanoparticles with a mean diameter of 2.2 nm on amine-functionalized UiO-66 were synthesized by facile anionic-exchange followed by chemical reduction. The prepared Pd/NH2-UiO-66 catalyst was effective in the catalytic transfer hydrogenation of nitrobenzene to aniline using formic acid as a hydrogen source under mild reaction conditions in water with a maximum nitrobenzene conversion of 98% and an aniline selectivity of 99%. The superior catalytic activity of Pd/NH2-UiO-66 was attributed to the cooperative effects of nano-sized Pd and its stabilization offered by the NH2-UiO-66 support. The heterogeneous nature of the catalytic system was confirmed by a hot-filtering test and the catalyst maintained stable performance in 4 repeated cycles. A plausible reaction mechanism was proposed.
Keywords: Palladium, nitrobenzene, transfer hydrogenation, aniline, water.
1. Introduction Aniline (AN) is important intermediate in the production of agrochemicals, pharmaceuticals, dyes, polyurethanes, pigments, and pesticides [1-4]. Conventionally, the 2
preparation of amines relies on the homogeneous catalytic hydrogenation of nitro compounds, mostly in the presence of an organic solvent and/or activation ligands [5-7]. Separation, excess metallic/reagent waste, and recycling issues associated with homogeneous catalysis hinder their use in practical applications. To overcome these problems, a number of heterogeneous catalysts such as Pt [8], Pd [9], Au [10], Ru [11], and Rh [12] have been developed for the direct hydrogenation of nitrobenzene to aniline. However, drawbacks such as the use of environmentally harmful organic solvents, long reaction times, and high temperature/high pressure reaction conditions are still problems, and a facile and ecofriendly procedure that avoids the use of toxic solvents and hazardous molecular hydrogen in high pressure is desirable [13]. Catalytic transfer hydrogenation (CTH), an alternative to direct hydrogenation through the use of hydrogen donors, e.g., formic acid, is operationally simple, and can avoid the use autoclaves and high pressure hydrogen gas [14]. In particular, formic acid is an economical, safe and easy to handle renewable liquid hydrogen carrier that can be produced from biomass. As the same time, the development of metal catalyzed reactions in aqueous media [15] has been motivated because water represents the most benign, abundant, and inexpensive solvents [16]. The use of water as a reaction medium has reportedly led to unique reactivity and selectivity that cannot be obtained in conventional organic solvents [17]. From the view of green chemistry principles, there is a definite need to develop environmentally benign and clean protocols, such as CTH, which allow effective hydrogen transfer under mild conditions in aqueous medium. Metal organic frameworks (MOFs), which are a new class of high crystalline organicinorganic hybrid materials, have attracted tremendous attention owing to their novel 3
coordination structures, diverse topologies and potential applications in diverse areas including catalysis and adsorption [18-24]. One of the current challenges in MOFs catalysis is the development of stable MOF structures incorporated with functional sites that can be used for encapsulation of active species either directly or via post-synthetic modification [25]. In this respect, the zirconium-based UiO-66 [26] has emerged as a highly desirable host matrix for the encapsulation of metal nanoparticles (NPs) owing to its large specific surface area as well as good chemical resistance to water and organic solvents. The conjunction of a solid porous structure of MOFs and metal NPs can offer high catalytic activity compared to conventional catalysts. Recently, palladium supported on different amine-functionalized MOF catalysts has been used for Suzuki-Miyaura coupling [27], Heck coupling [28], acetalization [29], dehalogenation of arylhalides [30] and hydrogenation reactions [31]. Few reports are available for the CTH of nitrobenzene using formic acid over a palladium-based catalyst [32] and there is scope to explore the potential improvement in catalytic performance over different support systems. Herein, a catalysts made of Pd NPs encapsulated in amine-functionalized UiO-66 was proposed for the CTH of nitrobenzene to aniline using formic acid as a hydrogen source in water medium under mild reaction conditions. Highly active and selective catalysis was observed, and it was demonstrated that the replacement of organic solvents with neat water led to significant improvements in both the activity and selectivity in the reaction. A plausible catalytic mechanism of nitrobenzene CTH was also proposed based on the reaction results and independent formic acid dehydrogenation studied.
2. EXPERIMENTAL SECTION 4
2.1. Chemicals Palladium chloride (PdCl2, 99.9%), zirconium tetrachloride (ZrCl4, 99.5%), terephthalic acid (99%), 2-aminoterephthalic acid (99%), formic acid (FA, 95%), methanol (MeOH, 99.9%), NaBH4 (99%), Pd/C (5 wt.%), and anhydrous N,N-dimethylformamide (DMF, 98%) were purchased from Sigma Aldrich. Hydrochloric acid (HCl, 36%) and ethanol (EtOH, 99.95%) were supplied from Samschun (Korea) and Merck. Nitrobenzene (NB, 98%) and ethylacetate (EtOAc, 99.5%) were purchased from TCI and Duksan (Korea). All chemicals were used as received. Deionized water was produced using a Millipore Milli-Q ultrapure water purification system. 2.2. Catalyst Preparation 2.2.1. Preparation of NH2-UiO-66 NH2-UiO-66 was prepared under the solvothermal conditions reported by Cavka et al. [26] and Hupp et al. [33] with some modification. In a typical synthesis, ZrCl4 (1.16 g, 5 mmol) and HCl (0.30 mL, 10 mmol) were dissolved in anhydrous DMF (60 mL) at room temperature. 2-Aminoterephthalic acid (0.90 g, 5 mmol) was then added to the clear solution with vigorous stirring and further sonicated at room temperature for 10 min. The resulting mixture was transferred to a 100 mL Teflon lined stainless steel autoclave, which was sealed and heated to 120 °C for 24 h. After 24 h, the substrate mixture was cooled to room temperature and the precipitate was collected by centrifugation. The obtained yellow solid was washed with DMF (3 x 30 mL) and absolute ethanol (2 x 30 mL) followed by drying under vacuum at 80 °C for 12 h; the sample was designated as NH2-UiO-66. Pure UiO-66 was synthesized using the same method mentioned for the preparation of NH2-UiO-66 except using terephthalic acid as the organic ligand. 5
2.2.2. Preparation of Pd NPs encapsulated on NH2-UiO-66 Pd/NH2-UiO-66 was prepared according to the anionic exchange method [30] followed by chemical reduction [34, 35]. Prior to encapsulation, NH2-UiO-66 was activated at 120 °C for 4 h under vacuum to remove the residual solvent in the pores. Subsequently, 0.5 g of activated NH2-UiO-66 was dispersed in 50 mL of methanol followed by the addition of a PdCl2 suspension (30 mg in 1 mL of 2 M HCl aqueous solution) and stirred continuously at RT for 12 h. The resulting orange solid was isolated by centrifugation, washed with EtOH and dried at 80 °C. The solid was then dispersed in 50 mL water and reduced with a 0.1 M NaBH4 aqueous solution at RT for 4 h. Finally, a greyish brown suspension was washed with water and ethanol followed by vacuum drying at 80 °C for 12 h to obtain the Pd/NH2-UiO-66 catalyst. Pd/UiO-66 was also synthesized by replacing the UiO-66-NH2 with an equivalent amount of UiO-66 to compare the catalytic activity. The Pd particle was found to be 0.39 wt.% in Pd/UiO-66 by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin-Elmer optima 7300 DV). To increase the Pd loading level on UiO-66, following impregnation method was used. A 0.5 g of UiO-66 was dispersed in 25 mL de-ionized (DI) water followed by adding 1 mL of Na2PdCl4 solution (0.1 mol/L) and stirred at 80 oC for 1 h. Then the NaOH solution (1mol/L) was added into the reaction mixture to reach the pH 12, and the solution was stirred at 80 oC for another 2 h. Subsequently, NaBH4 (0.3 g in 10 mL water) was added dropwise into the reaction mixture under continuous stirring, which produced ultrafine Pd/UiO-66. The Pd/UiO-66 was collected by filtration, washed with DI water, and dried at 80 oC for 12 h. The Pd loading on UiO-66 was 1.68 wt.% by ICP-OES. 2.3. Catalyst characterization The X-ray diffraction (XRD, Rigaku) patterns were measured using Cu Kα radiation (λ= 6
1.54 Å) within the range, 5–50° 2θ, at a scanning rate of 0.5° min−1. The N2 adsorptiondesorption isotherms were obtained at −196 °C using a BELsorpII-Max (BEL, Japan). Prior to the adsorption measurements, the samples were out-gassed at 120 °C for 12 h under vacuum. The specific surface area and pore size distribution of the samples were calculated using the BET (Brunauer-Emmett-Teller) method and Horvath–Kawazoe (HK) model, respectively. Fourier transform infrared spectroscopy (FT-IR, Bruker VERTEX 80v) was performed on KBr disks at room temperature. Field emission transmission electron microscopy (FE-TEM, FEI TECNAI 30 G2 S-TWIN) was carried out at an accelerating voltage at 100 kV. The TEM samples were prepared by dispersing the catalyst powder in ethanol under ultrasonic radiation for 10 min, and placing a drop of the resulting solution onto a copper grid followed by slow evaporation of the solvent under vacuum at room temperature. Elemental analysis for nitrogen was determined on an Elementar Vario ELIII analyzer. The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo VG ESCALAB250) using Al Kα radiation and a hemispherical analyzer. The XPS data was calibrated internally, fixing the binding energy of C 1s to 284.6 eV. 2.4. Transfer hydrogenation of nitrobenzene The catalytic activity of Pd/NH2-UiO-66 in the transfer hydrogenation of nitrobenzene was evaluated by performing the reactions in a 15 mL ACE pressure tube. Typically, 1 mmol of nitrobenzene, 3.5 mmol of formic acid, and Pd/NH2-UiO-66 catalyst (50 mg, 0.7 mol% of Pd) in 5 mL of H2O were charged into the pressure tube and sealed. The reaction mixture was stirred at 60 °C for 7 h. After the reaction was complete, the catalyst was separated by centrifugation and the products were extracted with ethyl acetate. The reaction products were then analyzed using a GC (HP6890 model, M/s. Agilent Technologies, USA) equipped with a 7
flame ionization detector and a HP-5 capillary column. The products were confirmed by GCMS (Varian 1200L single quadrupole with 3800GC model, M/s. Agilent Technologies, USA) using a DB-5 column. The effects of the reaction conditions with respect to the solvent, temperature, formic acid to nitrobenzene molar ratio, time, and heterogeneity of the reaction and catalyst recyclability were examined. 2.5. Formic acid dehydrogenation Formic acid dehydrogenation was performed in a 100 ml three-necked round flask with water jacket. Before use, catalyst was dried at 110 ℃ for 4 h under vacuum. In a typical run, a reactor containing catalyst (100 mg) and water (15 ml) was connected to a gas burette, pressure-equalizing funnel containing formic acid (6 mmol), and Ar gas line. After heated to 60 ℃, the reaction medium was magnetically stirred for 30 min to achieve thermal equilibrium under inert condition. A reaction was initiated by injecting the formic acid in the connected pressure-equalizing funnel to the reaction medium under vigorous stirring. The volume of the evolved gas was monitored by recording the displacement of water in the gas burette. The reaction temperature was maintained by using water circulator connected to the reactor. The released gas during a reaction was collected and analyzed by Agilent 6890 Gas Chromatography equipped with Carboxen-1010 Plot capillary column and TCD/methanizerFID detector.
3. Results and Discussion 3.1. Catalyst characterization Pd NPs were immobilized on UiO-66 by a direct anionic exchange method followed by 8
reduction [30]. To this end, the chloride anions present in the complex, [NH3+-UiO-66][Cl]-, formed by the dissolution of the HCl into NH2-UiO-66, were exchanged with the PdCl2 precursor to form a [NH3+-UiO-66]2[PdCl4]2- ion pair via electrostatic attraction and subsequently reduced with a 0.1 M aqueous NaBH4 solution at room temperature to obtain the Pd nanocatalyst supported on NH2-UiO-66. The actual Pd loadings in the fresh Pd/NH2UiO-66 and used Pd/NH2-UiO-66 catalyst after 4 cycles estimated by ICP-OES were 1.54 wt. % and 1.49 wt. %, respectively. On the other hand, only 0.39 wt. % Pd was detected in Pd/UiO-66 despite the same Pd precursor loading of 2.0 wt. % applied in the feed solution. Unlike UiO-66, the rapid absorption of PdCl2 in Pd/NH2-UiO-66 was observed with the appearance of an orange color, suggesting the crucial role of amine groups in NH2-UiO-66 for the effective Pd loading and stronger Pd–N binding interaction. From elemental analysis, the N content in NH2-UiO-66 was ca. 4.75 wt. %. Fig. 1 presents the N2 adsorption-desorption isotherms of the various samples measured at −196 °C, exhibiting type I isotherms with no hysteresis. Table 1 lists the textural properties determined from these isotherms. The BET surface area of Pd/NH2-UiO-66 was ca. 790 m2 g−1 with a micropore volume of 0.32 cm3 g−1, which was lower than that of the NH2-UiO-66 support (1052 m2 g−1 with a micropore volume of 0.43 cm3 g−1) due to the encapsulation of Pd NPs into NH2-UiO-66. Fig. 2 shows the XRD patterns of NH2-UiO-66, fresh Pd/NH2-UiO-66, and reused Pd/NH2-UiO-66 samples. These samples showed similar major XRD peaks at 7.4° and 8.58° 2θ. These were assignable to the (111) and (200) planes of highly crystalline NH2-UiO-66, respectively, which are in good agreement with the simulated XRD patterns of UiO-66 [36]. No loss of crystallinity in Pd/NH2-UiO-66 was detected, and the characteristic XRD peaks for 9
Pd were not observed due to the low loading and high dispersion of palladium NPs inside the MOF matrix [37]. Fig. 3 presents the FT-IR spectra of NH2-UiO-66, fresh Pd/NH2-UiO-66, and reused Pd/NH2-UiO-66 samples. A broad band at 2969 cm−1, which was characteristic of the –OH stretching vibration of the carboxylic acid group in the NH2-H2BDC ligand, was shown by all the samples. In addition, two bands at approximately 3475 and 3363 cm−1, which were assignable to the asymmetric and symmetric stretching vibrations of the primary amine group, could be distinguished clearly over the prepared NH2-UiO-66 and Pd/NH2-UiO-66 (curves a and b in Fig. 3). The decrease in intensity of these bands after Pd incorporation was attributed to the presence of an electrostatic interaction between Pd and the amine group of NH2-UiO66 [38]. The presence of a NH2 group within the framework of NH2-UiO-66 was analyzed using the N 1s spectra from the XPS data (curve c in Fig. 5); the N 1s peak of the amine group was observed at 399.68 eV. As shown in the TEM image of Pd/NH2-UiO-66 (Fig. 4a), the Pd NPs were dispersed homogeneously in the pore channels of NH2-UiO-66. The particle size distribution (Fig. 4a as an inset) of the fresh Pd/NH2-UiO-66 revealed Pd NPs with a mean diameter of 2.2 nm. On the other hand, the TEM image of the reused Pd/NH2-UiO-66 catalyst (Fig. 4b) after 4 cycles showed an increase in the mean particle size to 8 nm (Fig. 4b as inset). XPS was performed to analyze the surface chemical state of the synthesized Pd/NH2UiO-66 and the results are presented in Fig. 5. Deconvolution of the Pd 3d spectrum resulted in two peaks at 335.39 and 340.78 eV, which were assignable to Pd 3d5/2 and Pd 3d3/2, confirming the existence of metallic Pd (curve b in Fig.5). These peaks appeared at slightly higher energy level due to the interaction between the Zr and Pd [39]. The signal intensity of 10
Pd was low due to the relatively low Pd content in the MOF structure. No obvious peak for Pd+2 was observed, indicating the complete formation of Pd NPs through the reduction of PdCl4−2. Two additional peaks in curve b in Fig. 5 located at 332.58 and 346.17 eV were ascribed to Zr 3p3/2 and Zr 3p1/2 respectively [40]. The curves of the Zr 3d region could be deconvoluted into two peaks for Zr 3d5/2 and Zr 3d3/2 centered at approximately 182.04 and 184.35 eV, respectively (curve d in Fig. 5). Moreover, the peaks corresponding to oxygen, carbon, nitrogen, zirconium, and palladium were also clearly observed in the XPS elemental survey of the catalyst (curve a in Fig. 5). 3.2. Transfer hydrogenation of nitrobenzene The activity of the Pd/NH2-UiO-66 catalyst in the transfer hydrogenation of nitrobenzene to aniline using formic acid as the hydrogen source in aqueous media was investigated (Scheme 1). Controlled and blank experiments were carried out in water at 60 °C to optimize the conditions for the CTH of nitrobenzene over UiO-66, NH2-UiO-66 and without a catalyst using formic acid; the results are summarized in Table 2. No meaningful
conversion of
nitrobenzene was observed using the parent UiO-66, NH2-UiO-66 and without a catalyst (Table 2, entries 1–3), confirming the need for a metal to dehydrogenate formic acid and to carry out the hydrogenation of nitrobenzene. The maximum nitrobenzene conversion of 98% to aniline with 99% selectivity (Table 2, entry 4) was observed when Pd/NH2-UiO-66 was used as a catalyst, showing the presence of palladium was indispensable for high catalytic activity. In contrast, the Pd/UiO-66 catalyst with 0.39 wt.% and 1.68 wt.% Pd loading exhibited low catalytic activity with 15% and 69% of nitrobenzene conversion, respectively (Table 2, entries 5 and 6). These could be explained by the low loading of Pd NPs in the cavities of UiO-66 as determined by ICP-OES and the contribution of amine moiety in the 11
support material. A similar result was observed elsewhere [38]. There have been previous reports that the primary (NH2-) and secondary ((CH3)2N-) amine groups in the vicinity of Pd on supporting materials interact with the adsorbed formic acid molecules, and accelerate the rate of formic acid deprotonation [41, 42]. We also compared the catalytic activity with 15 mg of commercial Pd/C (5 wt.%) catalyst in the CTH of nitrobenzene. It was detected that the Pd/C gave 82% of nitrobenzene conversion (Table 2, entry 7), which was slightly lower than Pd/NH2-UiO-66. When the reaction was carried out with Pd/NH2-UiO-66 in an open flask reaction tube, only 33% conversion (Table 2, entry 8) was achieved indicating the evolved H2 gas also involved in the reaction to convert the nitrobenzene. Initially, the influence of solvent in the CTH of nitrobenzene was investigated using 5 mL of different solvents: water, toluene, ethanol and methanol over Pd/NH2-UiO-66 catalyst (0.7 mol% of Pd), 1 mmol of nitrobenzene and 3 mmol of formic acid at 60 °C for 6 h. The results listed in Table 2 (entries 9–12) showed that there was a significant influence of solvents in nitrobenzene conversion and aniline selectivity. The activity of the catalyst increased with increasing polarity of the solvent, which resulted in the high solubility of formic acid with the promotion of hydrogen transfer over the Pd surface. Water proved to be the best solvent with 86% nitrobenzene conversion and 99% aniline selectivity. Using water as a solvent, the effects of the reaction temperature (40, 60 and 80 °C) were then examined, and the results are shown in Fig. 6. At 40 °C, the conversion of nitrobenzene was approximately 52%, which increased significantly to 86% when the reaction temperature was increased to 60 °C with a marginal change in the aniline selectivity from 97 to 99%. This increase in conversion is likely to be a consequence of the increased hydrogenation reaction rate between the adsorbed nitrobenzene and H species generated by palladium formate 12
intermediate decomposition on Pd surface (see mechanism later). With increasing reaction temperature from 60 to 80 °C, however, nitrobenzene conversion decreased to 74% accompanied by aniline selectivity to 95%. In addition, decrease in aniline selectivity with the by-product formation of azoxybenzene at 80 °C was confirmed by GC-MS. A similar decrease in activity with increasing temperature was found in previous report [43], and the optimum reaction temperature was 60 °C. The effect of the formic acid to nitrobenzene molar ratio was then investigated with 1 mmol of nitrobenzene and 5 mL of H2O at 60 °C for 7 h, and the results are shown in Fig. 7. With increasing formic acid/nitrobenzene ratio from 2 to 3, nitrobenzene conversion increased from 68% to 92% with a constant aniline selectivity of > 99%, because of the high availability of H species generated through the decomposition of palladium formate molecules. At the formic acid:nitrobenzene mole ratio of 3.5, a maximum of nitrobenzene conversion 98% with aniline selectivity 99% was achieved with very low N-formylaniline (FAN) of < 1% as a byproduct. With further increase in the amount of formic acid from 3.5 to 4, nitrobenzene conversion remained constant to 98%, but the aniline selectivity decreased to 96% with FAN selectivity to 4%. The increase in the FAN selectivity was due to the formic acid:nitrobenzene mole ratio 4 (higher than the stoichiometric molar ratio of 3), which led to the formation of FAN by the reaction of aniline with excess formic acid under the reaction conditions. Therefore, the optimal amount of formic acid was approximately 3.5 moles per mole of nitrobenzene. To examine the effect of the reaction time on the CTH of nitrobenzene, the distribution of reactants and products were analyzed and plotted as a function of time in Fig. 8. When the reaction time was increased from 1 to 7 h, the concentration of aniline increased from 0.06 to 13
0.19 mmol/mL i.e. aniline selectivity of > 99% with a very low concentration (0.002 mmol/mL) of N-formyl aniline (FAN) as the only byproduct; it must be formed by the reaction of aniline with excess formic acid. Upon further reaction to 8 h, the concentration of FAN increased to 0.01 mmol/mL. To verify the pathway of the CTH of nitrobenzene using formic acid as a hydrogen source, simple dehydrogenation of formic acid over the Pd/NH2-UiO-66 was also carried out in the absence of nitrobenzene, as shown in Fig. 9. In the formic acid dehydrogenation experiment, approximately equal amount of H2 and CO2 gas (1.35 mmol) was generated after 1 h of formic acid dehydrogenation and no further reaction took place afterwards, whereas the CTH reaction proceeded over 7 h. Therefore, formic acid dehydrogenation was not likely to be the rate determining step in the CTH reaction. Accordingly, a reaction mechanism was proposed as illustrated in Fig. 10. In the initial step, formic acid dehydrogenation takes place over the nano-sized palladium surface of Pd/NH2-UiO-66. The amine groups present in the catalyst weakly dissociate the acid –OH of formic acid, which results in the formation of palladium formate and ammonium ion by the addition of H+ species with amine group of the NH2-UiO-66. The dehydrogenation of palladium formate leads to the formation of CO2 and H− (3 in Fig. 10). The ammonium ion containing H+ and H− species adsorbed on Pd(0) surface are transferred to electron-rich O and electron deficient N of the nitro group, respectively to form 4 in Fig. 10. This results in the formation of nitrosobenzene intermediate (5 in Fig. 10) by the elimination of water. Subsequently, dehydrogenation of the second formic acid molecule generating H− and H+ attacks the N and O atoms of nitrosobenzene to form phenylhydroxyl amine (6 in Fig. 10). Upon dehydrogenation of the third formic acid molecule, transfer of H− and H+ species to nitrogen atom and hydroxyl group (see 7 in Fig. 10) 14
results in the formation of aniline with simultaneous elimination of another water molecule, which is desorbed from the Pd surface. In the presence of excess formic acid, N-formyl aniline is formed via the reaction of aniline with formic acid by removal of water. The high activity of the Pd/NH2-UiO-66 catalyst may be due to the strong electrostatic interactions between the polar nitro group with H+ and H− on Pd/NH2-UiO-66. This strong Pd and nitrogen interaction leads to the effective reduction of nitrobenzene to aniline without the formation of other side products. These experimental results showed 98% nitrobenzene conversion with a high selectivity to aniline ( > 99%) under mild reaction conditions in aqueous media under optimal reaction conditions of Pd/NH2-UiO-66 (0.7 mol% Pd), 5 mL of H2O and formic acid/nitrobenzene molar ratio of 3.5 at 60 °C for 7 h. To determine if the catalytic process is heterogeneous, the transfer hydrogenation of nitrobenzene was conducted by following the hot filtering protocols and the results are shown in Fig. 11. Under the optimized reaction conditions, the Pd/NH2-UiO-66 catalyst was removed after 3 h and the reaction was continued for another 7 h. No further conversion in nitrobenzene was detected, which confirms the heterogeneity of the catalyst. Furthermore, ICP-OES also detected practically no trace of Pd species in the mother liquor. The recyclability of the Pd/NH2-UiO-66 catalyst was evaluated in the transfer hydrogenation of nitrobenzene under the optimized reaction conditions. After the catalytic reaction, the catalyst was isolated from the liquid phase by centrifugation, washed thoroughly with ethanol, vacuum dried at 80 °C, and reused as a catalyst in subsequent runs under identical reaction conditions. The results shown in Fig. 12 indicate no significant loss of the hydrogenation activity of nitrobenzene for at least four repeated cycles. To check the stability of the reused catalyst, Pd/NH2-UiO-66 recovered after the CTH reaction was characterized by XRD, FT-IR, 15
N2 adsorption, and FE-TEM. No significant crystallographic changes were observed for the reused catalyst (Fig. 2), preserving the high crystallinity of the pristine NH2-UiO-66. The IR bands assigned to –NH2 groups were retained even after the catalytic reactions (Fig. 3), which is consistent with the high chemical stability for NH2-UiO-66, as reported previously. On the other hand, a decrease in the amount of adsorbed N2 (Fig. 1) at 77K (BET surface area decreased from 790 to 534 m2g−1) was observed after use, which can be attributed to pore blockage by the residual reactants or products. The reduced surface area, inhibits the access of the reactant molecules to the active sites along with the increased Pd particle size due to partial agglomeration, as confirmed by FE-TEM analysis (Fig. 4b), may be responsible for the decrease in catalytic activity in further cycles. Finally, we compared the catalytic activity of Pd/NH2-UiO-66 in hydrogenation of nitrobenzene to aniline with some of other heterogeneous catalysts reported previously (Table 3). From Table 3, our Pd/NH2-UiO-66 catalyst exhibited higher activity in aqueous medium with formic acid as a hydrogen source compared to other solid catalysts. The previously reported catalysts were required high pressure H2 gas, and/or higher temperature. These comparison results confirmed that Pd/NH2-UiO-66 catalyst is effective for CTH of nitrobenzene.
Conclusions Pd/NH2-UiO-66 was synthesized by a facile anionic exchange method followed by chemical reduction. The highly dispersed Pd NPs were efficient in the catalytic transfer hydrogenation of nitrobenzene to aniline using forming acid as a hydrogen source under mild reaction conditions in a water medium without base additives or ligands. The catalyst was 16
recycled successfully for up to 4 cycles without significant loss of catalytic activity. A reaction mechanism explaining the experimental result was proposed.
Acknowledgement This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2015R1A4A1042434).
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Figures
F Fig. 1. N2 adsorption isotherms of NH2-UiO-66, fresh Pd/NH2-UiO-66 and reused Pd/NH219
UiO-66 after fourth run reaction.
Fig. 2. XRD diffraction patterns of NH2-UiO-66, fresh Pd/NH2-UiO-66 and reused Pd/NH2UiO-66 catalysts.
20
Fig. 3. FT-IR spectra of (a) NH2-UiO-66, (b) fresh Pd/NH2-UiO-66 and (c) reused Pd/NH2UiO-66 catalysts.
21
Fig. 4. FE-TEM images of (a) fresh Pd/NH2-UiO-66 and (b) reused Pd/NH2-UiO-66 catalysts with particle size distribution as inset.
22
Fig. 5. XPS patterns of the Pd/NH2-UiO-66 catalyst.
23
Fig. 6. Effect of temperature in CTH of nitrobenzene reaction.
24
Fig. 7. Effect of formic acid to nitrobenzene mole ratio in CTH reaction.
25
Fig. 8. Kinetic plot distribution of reactant and products as a function of time in CTH reaction.
26
Fig. 9. Formic acid dehydrogenation results in absence of nitrobenzene.
27
Fig. 10. Plausible reaction mechanism for the CTH of nitrobenzene through formic acid dehydrogenation.
28
Fig. 11. Heterogeneity test for Pd/NH2-UiO-66 catalyst in CTH reaction.
29
Fig. 12. Recyclability of Pd/NH2-UiO-66 catalyst in CTH reaction.
Scheme 1. Schematic representation of CTH of nitrobenzene to aniline.
Table 1. Physicochemical characteristics of NH2-UiO-66, fresh and reused palladium loaded NH2-UiO-66 catalysts from N2 adsorption isotherms. 30
Sample
SBETa (m2g─1)
Vporeb (cm3g─1)
Pore widthc (nm)
NH2-UiO-66
1052
0.43
0.41
Fresh Pd/NH2-UiO-66
790
0.32
0.39
Reused Pd/NH2-UiO-66
534
0.24
0.36
a
BET surface area Total pore volume measured at P/P0 =0.9998 c Pore width calculated by HK plot b
Table 2. Optimized reaction conditions in CTH of nitrobenzene.a
7
Nitrobenzene Conv. (%) nr
Aniline Sel. (%) –
H2O
7
nr
–
NH2-UiO-66
H2O
7
nr
–
4
Pd/NH2-UiO-66
H2O
7
98
99
5b
Pd/UiO-66
H2O
7
15
83
6c
Pd/UiO-66
H2O
7
69
96
7d
Pd/C
H2O
7
82
97
8e
Pd/NH2-UiO-66
H2O
7
33
88
9f
Pd/NH2-UiO-66
H2O
6
86
99
10f
Pd/NH2-UiO-66
Toluene
6
17
91
11f
Pd/NH2-UiO-66
EtOH
6
35
82
12f
Pd/NH2-UiO-66
MeOH
6
54
87
Entry
Catalyst
Solvent
Time (h)
1
No catalyst
H2O
2
UiO-66
3
a
Reaction conditions: Nitrobenzene (1mmol), catalyst (50 mg), formic acid (3.5 mmol), H2O (5 mL), temperature (60 °C), time (7 h); bPd/UiO-66 (0.39 wt.%); cPd/UiO-66 (1.68 wt.%), d 15 mg of Pd/C (5 wt.%); eopen flask reaction tube; fformic acid (3 mmol).
Table 3. Comparison catalytic activity of Pd/NH2-UiO-66 with other heterogeneous catalysts for liquid phase hydrogenation of nitrobenzene to aniline. Catalyst
H2 source
Solvent 31
Temp. (°C)
Nitrobenzene Aniline Conv. (%) Sel. (%)
Ref.
Pd/B-MCM-41 Ru/RGO Rh-Fe3O4 Cubane Mo3S4 Pt/MWNT Au/ZrO2 Au/TiO2 Ni/SiO2 Pd/NH2-UiO-66
H2 (20 bar) H2 (20 bar) N2H4·H2O HCOOH/Et3N H2 (40 bar) H2 (40 bar) H2 (9 bar) N2H4·H2O HCOOH
scCO2 EtOH/H2O EtOH THF Aniline EtOH Toluene EtOH H2O
32
50 110 80 70 60 150 110 100 60
100 100 100 99 100 95 99 97 98
99 99 99 99 99 97 98.5 98.2 99
9 11 12 14 44 45 46 47 This work