Research Article pubs.acs.org/journal/ascecg
Magnetic Nanohybrid Decorated Porous Organic Polymer: Synergistic Catalyst for High Performance Levulinic Acid Hydrogenation Karnekanti Dhanalaxmi,†,∥ Ramana Singuru,†,∥ Sujan Mondal,‡,∥ Linyi Bai,§ Benjaram Mahipal Reddy,† Asim Bhaumik,*,‡ and John Mondal*,† †
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 007, India Department of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S C Mullick Road, Kolkata-700032, India § Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore
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‡
S Supporting Information *
ABSTRACT: Herein we have developed a highly active, robust, and selective porous organic polymer (PPTPA-1, POP) encapsulated magnetically retrievable Pd-Fe3O4 nanohybrid catalyst in a one-step solvothermal route and investigated its catalytic performance in levulinic acid (LA) hydrogenation, a key platform molecule in many biorefinery schemes, to γ-valerolactone (GVL), employing formic acid as sustainable H2 source. The specific textural and chemical characteristics of as-synthesized nanohybrid materials were identified by XRD, XPS, FT-IR, 13C CP MAS NMR, HR-TEM, and FE-SEM with the corresponding elemental mapping and nitrogen physisorption studies. It was found that the nanohybrid Pd-Fe3O4/PPTPA-1 catalyst exhibited a substantially enhanced activity in comparison with the monometallic catalysts (Pd/PPTPA-1 and Fe3O4/PPTPA-1). Evidence of the electronic interaction between Pd and Fe attributable to the intrinsic hybrid synergistic effect is thought to be responsible for this superior catalytic performance and improvement in catalyst stability. The recycling experiments revealed that the magnetic nanohybrid catalyst sustained remarkable recycling efficiency and magnetism after being used in 10 successive catalytic runs, which made Pd-Fe3O4/PPTPA-1 a potential catalyst for the production of GVL in industry. KEYWORDS: Heterogeneous catalyst, Magnetically recoverable catalyst, Biomass, γ-Valerolactone, Levulinic acid hydrogenation
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INTRODUCTION
further upgrading of GVL to some value-added chemicals, with a focus on the mechanistic interpretation.13,14 Currently, supported nanohybrid heterogeneous catalysts have attracted widespread attention in regulating catalytic performances (enhancement in activity, selectivity, durability) for versatile catalytic transformations, which can be attributed to the synergetic effects or the bifunctional effect, including the electronic effect, the strain effect, and geometric and other interfacial effects caused by the interaction of the present two metal components.15−18 In the past decades the collection of bimetallic catalysts has not been explored, especially for the conversion of LA to GVL. Indeed, a limited set of literature reports have been directed toward converting LA to GVL, accompanied by desirable catalytic performances. A noblemetal-free Cu−Fe bimetallic catalyst was developed by Yan and
Depleting fossil resources motivated the researchers to focus their attention toward the production of fuels and value-added chemicals from renewable biomass feedstocks. Levulinic acid (LA) is a lignocellulosic biomass-derived promising platform molecule in many biorefinery schemes, and its selective hydrogenation to γ-valerolactone (GVL) has emerged as immensely important, presenting a versatile intermediate to produce renewable fuels and chemicals.1−3 Various types of metal based heterogeneous nanostructured catalysts, including Cu/ZrO2,4 Au/ZrO2,5 (Au/ZrO2-VS),6 m-SiO2@Pd@SiO2,7 Ru/C,8 Zr-phosphonate,9 Cu-MINT,10 Ru-FLG,11 etc., have showed good catalytic activity toward the selective transformation of LA into GVL. A smart strategy to fabricate Runanoparticles inserted porous carbon nanofibers derived from a metal organic framework (MOF) has been adopted to design a new catalyst for LA hydrogenation to GVL using 4.5 MPa at 150 °C.12 Some selective reviews present the recent progress made in the catalytic hydrogenation of LA to GVL followed by © 2016 American Chemical Society
Received: September 27, 2016 Revised: November 16, 2016 Published: November 24, 2016 1033
DOI: 10.1021/acssuschemeng.6b02338 ACS Sustainable Chem. Eng. 2017, 5, 1033−1045
Research Article
ACS Sustainable Chemistry & Engineering
Scheme 1. Synthesis of Pd-Fe3O4 Nanohybrid Decorated POP Used as a Magnetically Recoverable Nanocatalyst for Selective Hydrogenation of Levulinic Acid to γ-Valerolactone
prepared a well dispersed magnetically recoverable Pd-Fe3O4/ PPTPA-1 nanocomposite in a one-step solvothermal route (Scheme 1), which can be employed as an efficient and durable nanocatalyst against selective hydrogenation of LA to GVL using HCOOH as a sustainable H2 source of a key product of the biomass process, which has emerged to have increasing research interest in sustainable H2 storage due to its high energy density, excellent stability, easy handling, and nontoxicity at room temperature. Our strategy is to employ a new type of porous material, porous organic polymer (POP), as the support material instead of other conventional supports. POPs have been recognized as fast-growing and promising materials in heterogeneous catalysis, owing to their mechanically stable rigid covalent framework with designable pore structure, low skeleton density, large surface area, controllable compositions, and powerful confinement effects.33−35 Here we have employed PPTPA-1 (nanoporous polytriphenylamine) as the parent POP material for the fabrication of a Pd-Fe3O4 nanohybrid to develop the high performing efficient catalyst which was synthesized following the method as previously reported by Mondal et al.32 An enhancement in the catalytic performance of the nanohybrid catalyst is observed compared with the monometallic counterparts. On the basis of a detailed characterization, including XPS analysis, an influence on the catalytic behavior of the nanohybrid catalyst can be attributed to the intrinsic synergistic effect, which is caused by an obvious electronic interaction between Pd and Fe3O4. Furthermore, our finding revealed the better stability, negligible leaching, good recyclability, and no sign of deactivation of the nanohybrid catalyst toward LA hydrogenation to GVL compared with the monometallic and conventional catalysts. To the best of our knowledge, this is the first example of employing POP encapsulated nanohybrid magnetically retrievable Pd-Fe3O4 catalyst for performing LA hydrogenation to produce GVL, a key transformation in the biorefinery schemes employing HCOOH as a sustainable H2 source.
co-workers which exhibited efficient catalytic performance in the hydrogenation of biomass-derived levulinic acid.19 Very recently, Weckhuysen et al. reported levulinic acid (LA) hydrogenation to γ-valerolactone (GVL) using Ru-Au/TiO2 catalyst under 473 K and 40 bar H2 pressure.20 Yang et al. have designed highly efficient and durable Ru−Ni bimetallic nanoparticles inside ordered mesoporous carbons and pointed out that similar catalytic activity has been achieved with the corresponding monometallic Ru catalyst in LA hydrogenation to GVL.21 Catalyst stability is improved after alloying Ru with Re against traces of sulfuric acid in the LA feed compared with the monometallic Ru/C catalyst but considerably lower catalytic activity was observed than with the Ru/C catalyst.22 Dumesic et al. determined that the alloying effect progressively enhances the catalyst stability of bimetallic Ru−Sn catalyst and GVL selectivity toward LA hydrogenation, but again displayed poor catalytic performance.23 Although the hybridization effect in bimetallic catalyst plays a decisive role on activity, some drawbacks still associated with these reported catalysts include slow deactivation due to carbon deposition, acid-assisted metal leaching, structural changes of support, particle agglomeration, poor recycling efficiency, and high metal cost, and these resulted in limited practical applications in GVL production from LA. In recent years, magnetic separation has been considered as an attractive and easy technique compared to the filtration and centrifugation techniques in the heterogeneous catalysis research area, avoiding significant loss of the catalyst and increasing the reusability of the catalyst.24−30 In this context, a series of magnetically recoverable quardrapole Ni/ Cu/Al/Fe catalysts has been employed in LA hydrogenation to GVL by Chen et al.31 Indeed, very recently we have demonstrated core−shell porous silica supported Pd-NPs (m-SiO2@Pd@SiO2) catalyzed LA hydrogenation to GVL with 95% conversion and 96% GVL selectivity using formic acid as a sustainable H2 source.7 However, the concerns related to the expensive nature and rarity of noble-metals have suppressed their potential applications in industry. Therefore, to find a suitable alternative for designing highly active nanohybrid Pd catalysts alloying with easily available and inexpensive second metals for LA hydrogenation either enhancing or still maintaining the catalytic performance is highly desirable. Herein, considering the synergetic effect of the nanohybrid and the fact that it is beneficial for use as an inexpensive metal catalyst, we have
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EXPERIMENTAL SECTION
Synthesis of Porous Organic Polymer (POP) PPTPA-1. Porous polytriphenylamine material PPTPA-1 was synthesized by a one-step oxidative polymerization process. In a typical synthesis, a mixture of triphenyl amine (TPA; 2 mmol, 491 mg) and anhydrous FeCl3 (6 mmol, 973 mg) was first taken in a 250 mL round-bottom flask, and then dry dichloroethane (30 mL) was poured into that flask and the mixture was stirred for 20 h under N2 atmosphere with constant 1034
DOI: 10.1021/acssuschemeng.6b02338 ACS Sustainable Chem. Eng. 2017, 5, 1033−1045
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Nanohybrid Pd-Fe3O4/PPTPA-1 Catalyst. A typical catalytic transfer hydrogenation reaction was performed in a 25 mL dry sealed tube using levulinic acid (0.1 mL, 1 mmol), Pd-Fe3O4/PPTPA-1 catalyst (20 mg), EtOH (2 mL), and formic acid (1 mL, 25 mmol). Then a catalytic amount of NEt3 (3−5 drops) was added in the previous reaction mixture at room temperature. The tube was capped tightly, and the reaction mixture was allowed to stir at the desired temperature in an oil bath for the time referred. At each interval, the reaction mixture was cooled to room temperature, pressure was carefully released from the sealed tube, and the reaction mixture was filtered with centrifugation prior to being analyzed by GC. Quantitative analysis for the isolated liquid reaction mixture was performed using toluene as an internal standard followed by comparison with known standards with a standard deviation less than 2%. An agilent 6980 gas chromatograph equipped with a flame ionization detector and an SE-54 capillary column (30 m × 0.32 mm × 1.0 μm) with a stationary phase based on poly(methylphenylsiloxane) was utilized for the analysis and quantification of the reaction products. Recycling Test. In a typical recycling experiment of levulinic acid hydrogenation with Pd-Fe3O4/PPTPA-1 catalyst, a mixture of levulinic acid (0.5 mL, 5 mmol), Pd-Fe3O4/PPTPA-1 nanocatalyst (100 mg), EtOH (10 mL), and formic acid (5 mL) was heated to stir in a 100 mL dry sealed tube at 140 °C in an oil bath for 12 h. After the completion of the reaction, the catalyst was separated from the reaction mixture by magnetic separation using a magnetic bar. Then the recovered catalyst was washed with methanol followed by acetone, oven-dried (80 °C temperature), and then directly used for the next cycle reaction successively without further purification. No extraordinarily distinct technique, including treatment with hydrogen at higher temperature, addition of acid or base, or calcinations at higher temperature, is adopted for the reactivation and regeneration of our catalyst. Hot Filtration Test. We have performed hot filtration tests in order to confirm that our catalyst is indeed heterogeneous in nature and no significant leaching of the catalytic active sites took place into the solution. In this typical experiment, a mixture of levulinic acid (0.2 mL, 2 mmol), magnetic nanohybrid catalyst (40 mg), and formic acid (2 mL, 50 mmol) was heated in EtOH (5 mL) at 140 °C. After 6 h, the catalyst was removed from the hot reaction mixture by a magnetic separation technique. After 6 h of the reaction, we have achieved 60% LA conversion. Then, filtrate with the same reaction mixture was continued to run under sealed tube conditions with optimized reaction conditions for another additional 8 h. After completion of 15 h, no increase in the LA conversion beyond 60% was observed, as determined by GC analysis. This observation undoubtedly clarifies that the nanohybrid was strongly anchored, with the highly rigid organic framework making the system heterogeneous in nature and no obvious sign of metal detachment from the framework, as validated by AAS analysis measurement of the hot filtrate solution. Characterization Techniques. Powder X-ray diffraction (PXRD) patterns of different samples were recorded with a Bruker D8 Advance X-ray diffractometer operated at a voltage of 40 kV and a current of 40 mA using Ni-filtered Cu Kα (λ = 0.15406 nm) radiation. Ultrahigh resolution transmission electron microscopy (UHR-TEM) images were recorded in a JEOL JEM 2010 transmission electron microscope with operating voltage 200 kV equipped with a FEG. Field emission scanning electron microscopic images of samples were obtained using a JEOL JEM 6700 field emission scanning electron microscope (FESEM). Nitrogen sorption isotherms were obtained using a Quantachrome Autosorb 1C surface area analyzer at 77 K. Prior to the measurement, the samples were degassed at 393 K for approximately 6 h in high vacuum. FT IR spectra of the samples were recorded using a Nicolet MAGNA-FT IR 750 spectrometer Series II. Thermogravimetry (TGA) analyses of the samples were carried out using a TGA Instruments thermal analyzer TA-SDT Q600. Solid-state 13C CP-MAS NMR studies were performed using a Bruker Avance III HD 400 MHz NMR spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on an Omicron nanotech operated at 15 kV and 20 mA with a monochromatic Al Kα X-ray source. A quadrupole ion trap mass spectrometer equipped with a Thermo Accela LC and Agilent 6890 GC system equipped with a
heating at 353 K. Then the blue precipitate was poured into acetone and the deposited polymer product was filtered and washed successively with acetone, THF, and methanol. To remove iron completely from the polymer, the product was further washed with methanol for 3 days in a Soxhlet apparatus. Synthesis of Fe 3O4 /PPTPA-1 (Monometallic) Catalyst. PPTPA-1 (100 mg), anhydrous FeCl3 (90 mg), and ethylene glycol (40 mL) were mixed together in a round-bottom flask by sonication for about 2 h. Then trisodium citrate (0.09 g, 0.348 mmol) was added into the mixture and stirred at 80 °C until a clear brown solution was obtained. Subsequently, NaOAc (0.189 g, 2.3 mmol) was added to the previous mixture. The resulting mixture was kept stirring until everything was completely dissolved. A hydrothermal process was conducted at 200 °C for 8 h in a stainless steel autoclave under static conditions. The final product was washed several times with copious amounts of MeOH by centrifugation at 7800g for 10 min and dried to yield a fine brown powder of Fe3O4/PPTPA-1. Synthesis of Pd-Fe3O4/PPTPA-1 Nanohybrid (Fe:Pd = 9:1) Catalyst. A coimpregnation method was followed to prepare PdFe3O4/PPTPA-1 nanohybrid catalyst with the Fe:Pd mole ratio 9:1. PPTPA-1 (100 mg) was dispersed in 30 mL of ethylene glycol, and the mixture was further sonicated for an additional 30 min. After that, PdCl2 (35 mg, 0.189 mmol) in 5 mL of N,N-dimethylformamide (DMF) was added dropwise to the above solution with constant stirring for 30 min. On the other side, a homogeneous mixture was prepared by vigorously stirring anhydrous FeCl3 (300 mg, 1.8 mmol) and trisodium citrate (0.3, 1.16 mmol) in 5 mL of water and 10 mL of ethylene glycol at room temperature for 30 min. This solution mixture was added dropwise to the above resulting mixture with the subsequent addition of solid NaOAc (0.625, 7.6 mmol), followed by heating the mixture at 80 °C to get a brown solution. Finally, the reaction mixture was autoclaved at 200 °C for 8 h under static conditions. After cooling at room temperature, the final product was collected by thorough washing several times with MeOH (60 mL) by centrifugation at 7800g for 15 min and dried to yield a fine brownish to black powder of Pd-Fe3O4/PPTPA-1 material. The above same procedure was followed for the synthesis of the respective other PdFe3O4 nanohybrid catalysts with the mole ratios (1:1), (1:3), (1:5), and (2:1), respectively. Synthesis of Pd/PPTPA-1 (Monometallic) Catalyst. PPTPA-1 (100 mg) was dispersed in 30 mL of ethylene glycol, and the mixture was further sonicated for an additional 30 min. After that, PdCl2 (35 mg, 0.189 mmol) in 5 mL of N,N-dimethylformamide (DMF) was slowly added to the previous mixture with vigorous stirring for 60 min. The resulting mixture was hydrothermally treated at 200 °C for 8 h in a stainless steel autoclave. The final product was recovered from the resulting mixture after cooling at room temperature, washing with MeOH (60 mL) by centrifugation at 7800g for 15 min, and drying to yield a black powder of Pd/PPTPA-1. Synthesis of Pd/Fe3O4-PPTPA-1 Catalyst by a Two-Step Reduction Method. In a typical process, appropriate amounts of PPTPA-1 POP material (100 mg) were dispersed in ethylene glycol (40 mL) in a beaker under ultrasound conditions, and then PdCl2 (35 mg, 0.189 mmol) in 5 mL of N,N-dimethylformamide (DMF) was added following constant stirring for 60 min and hydrothermal treatment for 8 h at 200 °C. Then a blackish brown solution was obtained from which black material was isolated by constant washing with MeOH for several times. After the black solid powder was dried in an oven overnight, it was denoted as Pd/PPTPA-1. Then, the asprepared Pd/PPTPA-1 was dispersed in ethylene glycol (40 mL) again in a beaker under ultrasound, FeCl3 (300 mg, 1.8 mmol) and trisodium citrate (0.3, 1.16 mmol) were added into the solution, and the solution was kept under stirring conditions for another 30 min. Later, NaOAc (0.625, 7.6 mmol) was added to the solution and heated at 80 °C. The resulting mixture was hydrothermally treated at 200 °C for about 8 h, and then the residue was collected after washing with MeOH several times to provide a black solid denoted as Pd/Fe3O4PPTPA-1. Catalytic Testing: Dehydrogenation of HCOOH Assisted Levulinic Acid Hydrogenation to γ-Valerolactone Catalyzed by 1035
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Table 1. Comparison of Catalytic Performance in Levulinic Acid Hydrogenation to γ-Valerolactone over Various Nanocatalysts
Entry
Catalyst
Temp (°C)
Conv (%)
Selectivity of GVL (%)
Selectivity of other (%)
Carbon Balance (%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Pd/PPTPA-1 Fe3O4/PPTPA-1 Pd1-Fe1/PPTPA-1 Pd1-Fe3/PPTPA-1 Pd1-Fe5/PPTPA-1 Pd2-Fe1/PPTPA-1 Pd1-Fe9/PPTPA-1 Pd1-Fe9/PPTPA-1 Pd/Fe3O4-PPTPA-1 Pd1-Ni9/PPTPA-1 Pd1-Co9/PPTPA-1 Pd1-Mn9/PPTPA-1 Fe3O4/PPTPA-1 Pd/PPTPA-1 Pd-Fe3O4 Pd/POP and Fe3O4/POP
120 120 120 120 120 120 120 140 140 140 140 140 140 140 140 140
63 43 59 56 63 48 78 96 52 26 20 13 61 72 46 49
72 78 100 100 100 100 98 94 95 78 75 80 62 65 83 92
28 22
96 95 98 98 98 98 98 97 97 96 94 96 95 95 97 96
flame ionization detector was used for analysis of catalytic reactions. The loading amounts of Pd and Fe were determined using an inductively coupled plasma mass spectrometer (ICP-MS, X Series II, Thermo Scientific).
2 6 5 22 25 20 38 35 17 8
expensive and easily available Fe-precursor compared with the expensive and moisture sensitive Fe(CO)5 precursor has been used here to generate a homogeneous distribution of Fe3O4 nanoparticles. Ethylene glycol acts as a high boiling point solvent, reductant, and stabilizer to control the particle growth, preventing aggregation. Pd and Fe3O4 nanoparticles with homogeneous size distribution and also homogeneous nanohybrid are obtained employing this strategy. All the monometallic Pd/PPTPA-1 and Fe3O4/PPTPA-1 materials have been synthesized following the same synthetic strategy. The Pd-Fe3O4 with different Fe/Pd mole ratios of 1, 3, 5, 0.5, and 9 are also prepared following the similar method, which are represented as Pd1Fe1, Pd1Fe3, Pd1Fe5, Pd2Fe1, and Pd1Fe9, respectively. Then we investigated the catalytic properties of the monometallic and nanohybrid catalysts in LA hydrogenation to GVL using HCOOH as a sustainable H2 source. Initially, we tested Pd/PPTPA-1 catalyst for LA hydrogenation by conducting reaction of LA (0.1 mL, 1 mmol) with HCOOH (1 mL, 25 mmol) in EtOH (2 mL) under tightly capped sealed tube conditions at 120 °C (oil bath). We achieved 63% LA conversion to GVL after 12 h (Table 1, Entry 1) with 72% GVL selectivity. The catalytic conversion for this hydrogenation reaction could be observed at 43% after 12 h with 78% selectivity of GVL using monometallic Fe3O4/PPTPA-1 catalyst (Table 1, Entry 2). The observed poor selectivity of the desired product below 80% for the catalysts of Pd/POP and Fe3O4/ POP could be illustrated as the appearance of ethyl levulinate (EL) as undesired side product, and some extent of intermediate angelica lactone (α-AL) is not fully converted to produce GVL. We have provided the carbon balance (%) in Table 1. As observed in Table 1 (Entries 3−7), a profound rise in GVL selectivity (%) had been evidently displayed when the monometallic Pd and Fe3O4/POP catalysts were replaced by the synthesized nanohybrid Pd-Fe3O4/POP catalyst with various Pd:Fe ratios. It was found that the nanohybrid PdFe3O4/PPTPA-1 with the Fe/Pd molar ratio of 9 exhibited the highest catalytic performance among all the examined nanohybrid catalysts synthesized at different Pd:Fe molar ratios
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RESULTS AND DISCUSSION Scheme 1 outlines the preparation of the Pd-Fe3O4/PPTPA-1 nanocomposite, where PPTPA-1 POP was chosen as precursor for assembly of Pd and Fe3O4-NP on their surface and the nanocage. Both Pd2+ and Fe3+ ions were simultaneously anchored on the porous channel as well as the external surface of PPTPA-1, owing to π-bonding interaction between the Nfunctional groups of POP and Pd2+, Fe3+ ions, followed by solvothermal treatment to deliver a metal−POP nanocomposite in the presence of ethylene glycol. Here, ethylene glycol served crucial roles as stabilizing, capping, as well as reducing agent, as experimentally evidenced by recorded UV−vis spectra during the course of synthesis (Figure S1a, Supporting Information). The UV−vis spectrum of the reaction mixture shows the absorption band with the maximum at 400 nm (Figure S1a), which is referred to the ligand to metal charge transfer transition of Pd ion. After the completion of reaction, the peak at 400 nm completely disappeared with the appearance of a tail stretching across 200−800 nm in the UV−vis spectrum, indicating the reduction of Pd(II) to Pd(0) in the nanohybrid in the presence of ethylene glycol. Also, the color change observed during the course of synthesis from brown to intense black also clearly signifies the role of ethylene glycol as reducing agent. It is already well-established in the literature that ethylene glycol is employed both as reducing agent and as protective agent for the synthesis of nanoparticles, owing to its temperature-dependent reducing power, high boiling point, high relative permittivity, and the ability to solvate many metal precursors.36 The uniqueness of this present strategy of preparation lies in the fabrication of metal nanoparticles on the surface of the highly cross-linked POP via a one-pot facile solvothermal method using ethylene glycol as the reducing agent toward the achievement of an adequately robust catalyst with ease of magnetic separation. The very less 1036
DOI: 10.1021/acssuschemeng.6b02338 ACS Sustainable Chem. Eng. 2017, 5, 1033−1045
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ACS Sustainable Chemistry & Engineering
Figure 1. (A) Comparison study of the catalytic performance in terms of GVL productivities as a function of time during LA hydrogenation. Reaction conditions: LA (0.1 mL, 1 mmol), EtOH (2 mL), HCOOH (1 mL), NEt3 (catalytic amount), temperature 140 °C, catalyst (0.201 mol % Pd). (B) Influence of reaction temperature with different catalysts. Reaction conditions: LA (0.1 mL, 1 mmol), EtOH (2 mL), HCOOH (1 mL), NEt3 (catalytic amount), time (12 h), catalyst (0.201 mol % Pd). (C) Comparison study of the catalytic performances with various supported PdFe3O4 nanohybrid catalysts. Reaction conditions: LA (0.1 mL, 1 mmol), EtOH (2 mL), HCOOH (1 mL), NEt3 (catalytic amount), temperature 140 °C, time 12 h, catalyst (0.201 mol % Pd. (D) Recycle potential diagram for catalytic hydrogenation of LA to GVL. (E) Kinetic curves for different continuous hydrogenation reactions in reusability test with Pd-Fe3O4/PPTPA-1 catalyst. Reaction conditions for reusability test: levulinic acid (0.5 mL, 5 mmol), catalyst (100 mg), EtOH (10 mL), and HCOOH (5 mL), NEt3 (catalytic amount), 100 mL of dry sealed tube, 140 °C, 12 h. (F) Hotfiltration experiments where Pd-Fe3O4/PPTPA-1 catalyst was removed after 6 and 15 h followed by addition of 0.1 mL of levulinic acid in the filtrate; reaction was continued for another 20 h. Reaction conditions: levulinic acid (0.2 mL, 2 mmol), Pd-Fe3O4/PPPTPA-1 catalyst (40 mg), HCOOH (2 mL, 50 mmol), EtOH (5 mL), 140 °C.
(Table 1, Entry 8), providing 96% conversion with 94% GVL selectivity at 140 °C. In the nanohybrid catalysts, the Pd loading was fixed at 2.1 wt %, and the additive metal loading was varied by the mole ratio of the additive to Pd. Notably, a remarkable enhancement in catalytic performance for nanohybrid Pd1Fe9 catalyst was observed compared with the monometallic Pd and Fe3O4 catalysts. Although the high catalytic activity for only the Fe catalyst is remarkable, which could be attributed to the activation of O−H in H-donors due to the oxophilic nature of Fe facilitating coordination of the oxofunctionality as previously shown by Hermans et al.,37 PdFe3O4/PPTPA-1 (Fe/Pd = 9) was synthesized through a twostep reduction method (Experimental Section), which provided a decline in catalytic performance in comparison with the Pd1Fe9 catalyst prepared by a coreduction method (Table 1, Entry 9), signifying the synergistic effect of Pd and Fe generated during the process of preparation. The performances of other nanohybrid Pd-M/PPTPA-1 catalysts (M = Co, Ni, and Mn) were also investigated; all of them presented very poor activity, demonstrating Fe has a greater promotional effect on Pd than Ni, Co, and Mn, thereby influencing the catalytic behavior of Pd in the hydrogenation reaction over Pd-Fe nanohybrid catalyst (Table 1, Entries 10−12). Dumesic et al. have reported the enhancement in catalytic activity for the nanohybrid Pd-Fe catalyst with Fe/Pd ratio 18 in APR (aqueous-phase reforming) of ethylene glycol compared with the monometallic catalyst is caused by the synergistic combinations of Pd and Fe.38
An enhancement in catalytic performance for the respective monometallic catalysts with the increase of temperature from 120 to 140 °C was observed (Table 1, Entries 13 and 14). The influence of reaction temperature on catalyst activity was thoroughly surveyed. In order to validate the exact role of the porous organic polymer, we have conducted the levulinic acid hydrogenation reaction with the bare Pd-Fe3O4 nanocatalyst. In this case we have achieved only 46% LA conversion to GVL (Table 1, entry 15). From this result we can definitely conclude that the POP plays a crucial role in this catalytic reaction. Our as-synthesized porous organic polymer (PPTPA-1) afforded highly porous carbon nanofibers with hierarchical nanostructure, as confirmed by the N2 adsorption−desorption studies (Figure 2C). Such a hierarchical structure of POP would facilitate the adsorption of reagents, owing to hydrophobic and π−π stacking interactions between hydrophobic organic substrate and supporting matrix followed by transportation of products during liquid-phase reactions. Very recently, Bhaumik and co-workers have shown that the Ru-NP fabricated on the POP provided an enhancement in catalytic performance compared with the bare Ru-NP in the hydrogenation reaction.51 We have conducted the catalytic performance with the physical mixture of Pd/POP and Fe3O4/POP materials. In that case we have achieved 49% LA conversion to GVL (Table 1, entry 16) compared with the nanohybrid Pd-Fe3O4/POP catalyst. This result clearly signifies enhancement in catalytic performance is caused due to the strong synergistic effect between Pd and Fe. Figure 1A demonstrated the comparison in catalytic activity for the LA hydrogenation to GVL (time-on1037
DOI: 10.1021/acssuschemeng.6b02338 ACS Sustainable Chem. Eng. 2017, 5, 1033−1045
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ACS Sustainable Chemistry & Engineering stream profile at 140 °C), which suggested that nanohybrid catalyst displayed excellent catalytic activity with 99% conversion of LA and 95.6% GVL selectivity compared with the monometallic Pd/PPTPA-1 and Fe3O4/PPTPA-1 catalysts, respectively. Notably, a slight drop in GVL selectivity is observed for hybrid Pd-Fe3O4 nanocatalyst, which could be assumed to be due to the formation of small byproducts, such as PD (1,4-pentane diol) and MTHF (methyl tetrahydrofuran) following consecutive hydrodeoxygenation reaction.39 On the other hand, a gradual drop in product selectivity over Fe3O4/ POP and Pd/POP materials in the 60−70% range even after 15 h reaction is observed. This may be caused due to the carbon deposition of the side products on the catalyst surface, thus blocking the accessibility of active sites, leading to the loss in activity and selectivity. We have achieved 73.2% and 79.6% LA conversion to GVL after 15 h of continuous reaction with the corresponding Pd/PPTPA-1 and Fe3O4/PPTPA-1 nanocatalysts (Figure 1A). The crucial role of temperature increasing in levulinic acid conversion (%) has been systematically examined by varying reaction temperature for all nanocatalysts (Figure 1B). LA conversion of 96.5% from 78.3% was reached over the Pd-Fe3O4/PPTPA-1 nanohybrid, with the enhancement of temperature from 120 to 140 °C. Similar rising trends in LA conversion (from 43.2% to 61.5%) on Fe3O4/PPTPA-1 and Pd/PPTPA-1 catalyst (from 63.5% to 73.6%) were also observed, since hydrogenation rate is accelerated with the increase of reaction temperature. Although a drop in GVL selectivity at higher temperature was observed for catalysts which can be correlated with the carbon deposition on the catalyst surface generated by some undesired side reactions,40 the highly active nature (Figure 1C) of Pd1Fe9-POP based catalyst compared with other supported catalysts suggested that the N atoms in POP enhance electron density over the metals, which may favor the adsorption of H assisted from the cleavage of C−H bonds, as supported by the previous report of Tsang et al.41 Similarly, an enhancement in catalytic activity for the nanohybrid RuPd/CN (N-doped carbon) catalyzed hydrogenation reaction compared with other supported catalysts has been identified by Wang et al.42 A major improvemnet compared with the nanohybrid catalysts reported in the previous literature reveals the use of the three-dimensional highly cross-linked rigid POP framework with extraordinarily mechanical and thermal stability and huge surface area. The huge surface of the used POP PPTPA-1 aids the facile homogeneous dispersion of nanoparticles inside the nanoporous channel and the external organic framework, followed by easy diffusion of organic substrates to interact with the catalytic active centers, reducing the reaction time as required and enhancing the recyclability. The influence of the HCOOH on the catalytic activity for the LA hydrogenation reaction has been examined by conducting the reaction at different HCOOH−LA molar ratios (Figure S5, Supporting Information). A substantial rise in GVL yield, from 23.6% to 95.6%, was achieved with increasing HCOOH:LA mole ratio. This fact could be correlated with the accessibility of more hydrogen sources favoring hydrogenation with dehydrogenation of formic acid on the nanohybrid catalyst surface during the reaction, thus resulting in an obvious enhancement in catalytic reaction rate in LA conversion to GVL. Ruppert and co-workers have conducted a deep investigation toward straightforward GVL production from biomass-derived LA employing formic acid as hydrogenation source in the presence of robust Ru/C catalyst without addition of an external H2 source.8 Our newly designed
POP based nanohybrid catalyst was recycled in the subsequent ten runs (Figure 1D), sustaining GVL selectivity and yield. In comparison, monometallic Pd and Fe3O4 catalysts could also be reused for the seventh and sixth consecutive catalytic cycles (Figure 1D). The stability of our nanohybrid catalyst was examined by performing consecutive catalytic runs, which predicts that our catalyst is stable on multiple reuse up to 10 cycles, sustaining GVL yields (Figure 1D). The time-on stream profile (Figure 1E) suggested that catalytic activity is nicely preserved, as for a fresh catalyst in each catalytic run for the reusability test. The robustness and heterogeneous nature of the nanohybrid catalyst was evaluated by a hot filtration test (Figure 1F) with limited leaching of the metal phase, and no sign of considerable deactivation during the successive catalytic runs. After 6 h of the catalytic reaction in a hot-filtration test, the reaction was stopped (Figure 1F), and the reaction mixture was cooled at room temperature, followed by separation of catalyst from the reaction mixture by magnetic separtion. The reaction was running continuously, but no further enhancement of LA conversion to GVL was observed beyond 60%. This experimental result undoubtedly predicts that no obvious leaching of Pd and Fe took place in the reaction mixture during the course of the reaction. But very trace amounts of Pd and Fe were detected in the reaction mixture (light brown filtrate solution) after 15 h of continuous reaction, as confirmed by an atomic absorption spectroscopy (AAS) analytical technique. Then, we poured 0.1 mL of excess LA and formic acid (1 mL) solution into the filtrate with leached homogeneously Pd and Fe metals, and reaction was further carried out for another 20 h. But only 3−4% product conversion has been achieved after 20 h, which proved that leached Pd and Fe species contributed to the LA hydrogenation but did not catalyze this reaction up to full conversion. The Pd and Fe contents in the respective PdFe3O4/PPTPA-1 nanohybrid material are 0.201 mmol/g and 1.708 mmol/g, respectively, as determined by inductive coupled plasma-mass spectroscopy (ICP-MS) analysis. After the 10th catalytic run, the Pd and Fe contents in the nanocatalyst were found to be 0.198 mmol/g and 1.648 mmol/g, which are still comparable with that of the fresh catalyst. We have also investigated the catalytic performance of our newly developed nanohybrid Pd-Fe3O4/PPTPA-1 nanocatalyst in LA hydrogenation employing different hydrogen sources (Table 2). When the reaction was carried out in the presence of N2H4·H2O (Table 2, Entry 1), 21% LA conversion to γTable 2. Use of Different Hydrogen Sources for LA Hydrogenation to γ-Valerolactone with Pd-Fe3O4/PPTPA-1 Nanocatalysta Entry
H2-Source
1 2 3 4 5 6 7 8b
N2H4·H2O i PrOH N2H4·H2O HCOONH4 H2 gas HCOONH4 H2 gas H2 gas
Solvent
Temp
EtOH
120 120 120 120 120 120 120 120
H2O Toluene EtOH EtOH EtOH
°C °C °C °C °C °C °C °C
Con (%)
Selectivity (%)
21 65 12 55 23 59 52 55
99 99 88 75 96 70 71 80
a Reaction conditions: Levulinic acid (1 mmol, 0.1 mL), H2 source (25 mmol), Pd-Fe3O4/PPTPA-1 catalyst (20 mg), solvent 2 mL, time = 12 h, H2 gas (5 bar pressure). bCO2 gas (5 bar pressure).
1038
DOI: 10.1021/acssuschemeng.6b02338 ACS Sustainable Chem. Eng. 2017, 5, 1033−1045
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. (A) FT-IR spectra, (B) Wide angle powder X-ray diffraction spectra, (C) N2 adsorption/desorption isotherms and D) 13C CP solid state MAS NMR spectra of Fe3O4/PPTPA-1, Pd/PPTPA-1, and Pd-Fe3O4/PPTPA-1 hybrid materials. Pore size distributions calculated by the NLDFT method are provided in the inset of part C.
valerolactone (GVL) has been achieved. iPrOH has been considered as a moderate hydrogen source for this LA hydrogenation reaction under solvent-free conditions (Table 2, Entry 2), providing 65% conversion of LA to GVL, as supported by previously reported results by Hermans et al. on reductive upgrading of furfural in iPrOH.37 Only 12% conversion of LA to GVL was obtained with N2H4·H2O under solvent-free conditions (Table 2, Entry 3). We have carried out the same reaction employing HCOONH4 as hydrogen source in H2O medium (Table 2, Entry 4), and the reaction furnished 55% product conversion with 75% GVL selectivity. We have also checked the catalytic efficiency using a high pressure reactor autoclave under H2 gas, which has also been considered as a poor source. Screening of various solvents predicted that EtOH is the most potential solvent for this LA hydrogenation over nanohybrid catalyst (Table S2, Supporting Information). It is evident from Table 2 that our designed catalyst exhibited lower catalytic performance under H2 gas than using HCOOH as the source of hydrogen. In order to validate this observation and find out any promotion effect of CO2 derived from HCOOH on the reaction, we have conducted the catalytic reaction under the same hydrogen pressure, but additionally CO2 was introduced together with H2, and surprisingly, no obvious positive effect was observed on catalyst under the reaction conditions in this study. Guo and co-workers have postulated similar findings regarding the promotion effect of CO2 derived from formic acid in Rucatalyzed hydrogenation of LA.43 Based on the literature survey reports, we can predict that the rate of LA hydrogenation is obviously influenced with interparticle and intraparticle transport limitations, owing to the confinement of Pd NPs inside the porous channel of the POP. Such restrictions could be easily conquered under vigorous magnetic stirring conditions (
