Magnetic Nanohybrid Decorated Porous Organic Polymer: Synergistic

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02338 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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ACS Sustainable Chemistry & Engineering

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.

E-mail:

[email protected];

[email protected] †

Department of Materials Science, Indian Association for the Cultivation of Science, 2A &

2B Raja S C Mullick Road, Kolkata-700032, India. Email: [email protected] #

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore. ┴

These authors have equally contributed in this work

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 one step solvothermal route and investigated their 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,

13

C CP MAS NMR, HR-TEM, FE-SEM with the corresponding elemental

mapping and nitrogen physisorption studies. It was found that the nanohybrid PdFe3O4/PPTPA-1 catalyst exhibited a substantially enhanced activity in comparison with the monometallic catalysts (Pd/PPTPA-1 and Fe3O4/PPTPA-1). An evident of electronic interaction between Pd and Fe attributable to 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 10th successive catalytic runs, which made Pd-Fe3O4/PPTPA-1 a potential catalyst for the production of GVL in industry.

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KEYWORDS: Heterogeneous Catalyst • Magnetically Recoverable Catalyst • Biomass • γvalerolactone • Levulinic acid Hydrogenation INTRODUCTION: Depleting fossil resources motivated the researchers to focus their attention towards 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 an immense importance presenting as 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 Zrphosphonate,9 Cu-MINT,10 Ru-FLG11 etc. have showed good catalytic activity toward the selective transformation of LA into GVL. A smart strategy to fabricate Ru-nanoparticles inserted porous carbon nanofibers derived from Metal Organic Framework (MOF) has been adopted to design new catalyst for LA hydrogenation to GVL using 4.5 MPa at 150oC temperature.12 Some selective reviews present of recent progress made in the catalytic hydrogenation of LA to GVL followed by 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 a widespread attention in regulating catalytic performances (enhancement in activity, selectivity, durability) for versatile catalytic transformations which can be attributed to the synergetic effects or bi-functional effect include electronic effect, strain effect, geometric and other interfacial effects caused by the interaction of 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 towards converting LA to GVL, accompanied with desirable catalytic performances. Noble-metal-free Cu-Fe bimetallic catalyst was developed by Yan and 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 473K 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 the 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 2


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with the monometallic Ru/C catalyst but considerably lower catalytic activity was observed than the Ru/C catalyst.22 Dumesic et al. studied that alloying effect progressively enhances the catalyst stability of bimetallic Ru-Sn catalyst and GVL selectivity towards LA hydrogenation, but again displayed poor catalytic performance.23 Although hybridization effect in bimetallic catalyst play a decisive role on activity, but 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, high metal cost and these resulted in limited practical applications in GVL production from LA. In the recent years, Magnetic separation has been considered as attractive and easy technique than the filtration or centrifugation techniques in heterogeneous catalysis research area avoiding significant loss the catalyst and increasing the reusability of the catalyst.24-30 In this context, a series of magnetically-recoverable quardrapole Ni/Cu/Al/Fe catalyst has been employed in LA hydrogenation to GVL by Chen et al.31

Scheme 1. Synthesis of Pd-Fe3O4 nanohybrid decorated POP used as magnetically recoverable nanocatalyst for selective hydrogenation of Levulinic acid to γ-valerolactone.

Indeed, very recently we have demonstrated core-shell porous silica supported Pd-NPs (mSiO2@Pd@SiO2) catalyzed LA hydrogenation to GVL with 95% conversion and 96% GVL selectivity using formic acid as sustainable H2 source.7 However, the concerns related to 3



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expensive nature and rarity of noble-metal has suppressed their potential applications in industry. Therefore, to find a suitable alternative for designing highly active nanohybrid Pd catalyst alloying with easily available and inexpensive second metal for LA hydrogenation either enhancing or still maintaining catalytic performance is highly desirable. Herein, considering synergetic effect of nanohybrid and beneficial for using inexpensive metal catalyst, we have prepared well dispersed magnetically recoverable Pd-Fe3O4/PPTPA-1 nanocomposite in a one step solvothermal route (Scheme 1), which can be employed as efficient and durable nanocatalyst against selective hydrogenation of LA to GVL using HCOOH as sustainable H2 source a key product of biomass process, has emerged an increasing research interests in sustainable H2 storage due to its high energy density, excellent stability, easy handling and non-toxicity 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 a fast-growing and promising material 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 Pd-Fe3O4 nanohybrid to develop the high performing efficient catalyst which was synthesized following the method as previously reported by Mondal et al32. An enhancement in the catalytic performance for nanohybrid catalyst is observed compared with the monometallic counterparts. On the basis of detailed characterization, including XPS analysis an influence in catalytic behaviour for nanohybrid catalyst can be attributed to intrinsic synergistic effect which is caused by an obvious electronic interaction between Pd and Fe3O4. Furthermore, our finding revealed to the better stability, negligible leaching, good recyclability and no sign of deactivation of nanohybrid catalyst towards LA hydrogenation to GVL compared with the monometallic and conventional catalysts. To the best of our knowledge, it 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 bio refinery schemes employing HCOOH as sustainable H2 source. EXPERIMENTAL SECTION: Synthesis of Porous organic polymer (POP) PPTPA-1:

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Porous polytriphenylamine material PPTPA-1 was synthesized by 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 heating at 353 K. Then the blue colored 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 three days in a soxhlet apparatus. Synthesis of Fe3O4/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, 2.3mmol) was added in the previous mixture. The resulting mixture was kept stirring until everything was completely dissolved. 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 amount of MeOH by centrifugation at 7800 g for 10 minutes and dried to yield a fine brown powder of Fe3O4/PPTPA-1. Synthesis of Pd-Fe3O4/PPTPA-1 Nanohybrid (Fe:Pd = 9:1) catalyst: Co-Impregnation Method was followed to prepare Pd-Fe3O4/PPTPA-1 nanohybrid catalyst with the Fe:Pd mole ratio 9:1. PPTPA-1 (100 mg) was dispersed in 30 mL ethylene glycol and the mixture was further sonicated for additional 30 min. After that PdCl2 (35 mg, 0.189 mmol) in 5 mL 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 of 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 by stirring 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 80oC temperature to get a brown color solution. Finally the reaction mixture was autoclaved at 200oC for 8 h under static condition. After cooling at room temperature the final product was collected by through washing several times with MeOH (60 mL) by centrifugation at 7800 g for 15 minutes and dried to yield a fine brownish to black powder of PdFe3O4/PPTPA-1 material. The above same procedure was followed for the synthesis of 5



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respective other Pd-Fe3O4 nanohybrid catalysts with the mole ratio (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 ethylene glycol and the mixture was further sonicated for additional 30 min. After that PdCl2 (35 mg, 0.189 mmol) in 5 mL N,Ndimethylformamide (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 7800 g for 15 minutes and dried to yield black color powder of Pd/PPTPA-1. Synthesis of Pd/Fe3O4-PPTPA-1 catalyst by two-step reduction method: In a typical process, appropriate amounts of PPTPA-1 POP material (100 mg) was dispersed in Ethylene glycol (40 mL) in a beaker under ultrasound condition, and then PdCl2 (35 mg, 0.189 mmol) in 5 mL N,N-dimethylformamide (DMF) was added following constant stirring for 60 mins and hydrothermally treated for 8 h at 200 °C. Then a blackish brown color solution was obtained from which black color 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 as-prepared 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) was added into the solution, and the solution was kept in stirring condition for another 30 min. Later, NaOAc (0.625, 7.6 mmol) was added to the solution and heated it 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 for several times to provide a black color solid was denoted as Pd/Fe3O4-PPTPA-1. Catalytic testing: Dehydrogenation of HCOOH assisted Levulinic Acid Hydrogenation to γ-valerolactone Catalyzed by Nanohybrid Pd-Fe3O4/PPTPA-1 Catalyst: 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 catalytic amount 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 desired temperature in an oil bath for the time referred. At each interval reaction mixture was cooled to room temperature, carefully released sealed tube 6


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pressure and filtered with the 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%. Agilent 6980 Gas chromatograph equipped with a flame ionization detector and an SE54 capillary column (30 m×0.32 mm×1.0 μm) with a stationary phase based on poly(methylphenylsiloxane) has been 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 (0.5mL, 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 140oC in an oil bath for 12 h. After the completion of 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 (80oC temperature) and then directly used for the next cycle reaction successively without further purification. No any extraordinary distinct technique including by treatment with hydrogen at higher temperature, addition of acid or base, calcinations at higher temperature is adopted for the reactivation and regeneration of our catalyst. Hot filtration test: We have performed hot filtration test 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 140oC. After 6 h the catalyst was removed from the hot reaction mixture by magnetic separation technique. After 6 h of the reaction we have achieved 60% LA conversion. Then filtrate with 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: 7



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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. Ultra High 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 (FE-SEM). 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 Q-600. Solid-state

13

C 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. Quadrupole ion trap Mass Spectrometer equipped with Thermo Accela LC and Agilent 6890 GC system equipped with a flame ionization detector were 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). RESULTS AND DISCUSSION: Scheme 1 outlines the preparation of the PdFe3O4/PPTPA-1 nanocomposite, where PPTPA-1 POP was chosen as precursor for assembly of Pd and Fe3O4-NP on their surface and nanocage. Both Pd2+ and Fe3+ ions were simultaneously anchored on the porous channel as well as external surface of PPTPA-1 owing to π-bonding interaction between the N-functional groups of POP and Pd2+, Fe3+ ions, followed by solvothermal treatment to deliver metal-POP nanocomposite in presence of ethylene glycol. Here, ethylene glycol served the crucial roles as stabilizing, capping and as well as reducing agent as experimentally evidenced by recorded UV-vis spectra during the course of synthesis (Figure S1a, SI). UV-vis spectrum of 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 was completely disappeared with the appearance of a tail 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 8


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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 well as protective agent for the synthesis of nanoparticles owing to its temperature-dependent reducing power, the high boiling point, the high relative permittivity, and the ability to solvate many metal precursors.36 Uniqueness of this present strategy of preparation lies in the fact to fabricate metal nanoparticles on the surface of the highly cross-linked POP in a one-pot facile solvothermal method using ethylene glycol as the reducing agent towards the achievement of adequately robust catalyst with ease of their magnetic separation. Very less expensive and easily available Fe-precursor compared with expensive and moisture sensitive Fe(CO)5 precursor has been used here to generate homogeneous distribution of Fe3O4 nanoparticles. Ethylene glycol acts as high boiling point solvent, reductant and as well as stabilizer to control the particle growth preventing them from 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 have investigated catalytic properties of the monometallic and nanohybrid catalysts in LA hydrogenation to GVL using HCOOH as sustainable H2 source. Initially, we have tested Pd/PPTPA-1 catalyst for LA hydrogenation by conducting reaction of LA (0.1mL, 1 mmol) with HCOOH (1 mL, 25 mmol) in EtOH (2 mL) under tightly capped sealed tube condition at 120oC temperature (oil bath). We have 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 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) are not fully converted to produce GVL. We have provided the carbon balance (%) in the 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 ratio. It was found that the nanohybrid Pd-Fe3O4/PPTPA-1 with 9



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the Fe/Pd molar ratio of 9 exhibited the highest catalytic performance among all the examined nanohybrid catalysts synthesized at different Pd:Fe molar ratio (Table 1, Entry 8) providing 96% conversion with 95% GVL selectivity at 140oC. In the nanohybrid catalysts, the Pd loading was fixed at 2.1 wt % and the additive metal loading was varying 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 high catalytic activity for only Fe-catalyst is remarkable which could be attributed to the activation of O-H in H-donors due to the oxophilic nature of Fe facilitates coordination of the oxofunctionality as previously shown by Hermans et. al.37 PdFe3O4/PPTPA-1 (Fe/Pd=9) synthesized through a two-step reduction method (Experimental Section), which provided a decline in catalytic performance in comparison with the Pd1Fe9 catalyst prepared by a co-reduction method (Table 1, Entry 9), signifying the synergistic effect of Pd and Fe generated during the process of preparation. The performance 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 behaviour of Pd in hydrogenation reaction over Pd-Fe nanohybrid catalyst (Table 1, Entries 10-12). Dumesic et al. have reported the enhancement in catalytic activity for 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

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Table 1: Comparison of catalytic performance in Levulinic Acid hydrogenation to γvalerolactone over various nanocatalysts O

O

O LA (0.1 mL)

O

O

OH

O 2 mL EtOH, 1 mL HCOOH Catalyst, NEt3, 120 oC, 12 h

EL

GVL

O O

O

AL

__________________________________________________________________________________________ Selectivity Carbon Selectivity of GVL (%) Balance (%) of other(%) __________________________________________________________________________________________ 28 96 Pd/PPTPA-1 72 120 1 63 Entry

Catalyst

Temperaure (oC)

Conv (%)

2

Fe3O4/PPTPA-1

120

43

78

22

3

Pd1-Fe1/PPTPA-1

120

59

100

__

4

Pd1-Fe3/PPTPA-1

120

56

100

__

Pd1-Fe5/PPTPA-1

120

63

100

__

5

6

Pd2-Fe1/PPTPA-1

120

48

100

__

98

7

Pd1-Fe9/PPTPA-1

120

78

98

2

98

8

Pd1-Fe9/PPTPA-1

140

96

94

6

97

9

Pd/Fe3O4-PPTPA-1

140

52

95

5

97

10

Pd1-Ni9/PPTPA-1

140

26

78

22

96

11

Pd1-Co9/PPTPA-1

140

20

75

25

94

12

Pd1-Mn9/PPTPA-1

140

13

80

20

96

13

Fe3O4/PPTPA-1

140

61

62

38

95

14

Pd/PPTPA-1

140

72

65

35

95

15

Pd-Fe3O4

140

46

83

17

97

95 98

98 98

140 16 Pd/POP and Fe3O4/POP 49 92 96 8 _________________________________________________________________________________________

An enhancement in catalytic performance for the respective monometallic catalysts with the increase of temperature from 120oC to 140oC was observed (Table 1, Entries 13 & 14). 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 11



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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 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-on-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 the formation of small byproducts like 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 continuous reaction with the corresponding Pd/PPTPA-1 and Fe3O4/PPTPA-1 nanocatalysts (Figure 1A). Crucial role of temperature increasing in Levulinic acid conversion (%) has been systematically examined 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 120oC to 140oC. 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 12


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temperature. Although a drop in GVL selectivity at higher temperature was observed for catalysts which can be correlated with the carbon deposition on catalyst surface generated by some undesired side reactions.40 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 favour the adsorption of H assisted from the cleavage of C-H bonds as supported by previous report of Tsang et al.41 Similarly an enhancement in catalytic activity for 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 fact to the use of 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 helps for the facile homogeneous dispersion of nanoparticles inside the nanoporous channel and external organic framework followed by easy diffusion of organic substrates to interact with the catalytic active centres reducing the reaction time as required and enhancement of recyclability. The influence of the HCOOH on the catalytic activity for LA hydrogenation reaction has been examined by conducting reaction in different HCOOH-LA molar ratio (Figure S5, SI). 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 source favoring hydrogenation with dehydrogenation of formic acid on the nanohybrid catalyst surface during the reaction, thus resulting an obvious enhancement in catalytic reaction rate in LA conversion to GVL. Ruppert and co-workers have conducted a deep investigation towards straightforward GVL production from biomass-derived LA employing formic acid as hydrogenation source in the presence of robust Ru/C catalyst without addition of external H2 source.8 Our newly designed POP based nanohybrid catalyst was recycled in subsequent ten runs (Figure 1D) sustaining GVL selectivity and yield. In comparison monometallic Pd and Fe3O4 catalysts could also be reused for 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 re-use upto 10th cycles sustaining GVL yields (Figure 1D). Time-on stream profile (Figure 1E) suggested that catalytic activity is nicely preserved as like a fresh catalyst in each catalytic run for reusability test. Robustness and heterogeneous nature of nanohybrid catalyst was evaluated by hot filtration test (Figure 1F) with limited leaching of the metal phase, and no sign of considerable deactivation during the successive 13



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catalytic runs. After 6 h of the catalytic reaction in hot-filtartion test the reaction was stopped (Figure 1F), cooled at room temperature, followed by separation of catalyst from 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 reaction. But very trace amount Pd and Fe were detected in the reaction mixture (light brown filtrate solution) after 15 h continuous reaction as confirmed by Atomic Absorption Spectroscopy (AAS) analytical technique. Then we have poured 0.1 mL excess LA and formic acid (1 mL) solution into the filtrate with leached homogenously 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 didn’t catalyze this reaction up to full conversion. The Pd and Fe content in the respective Pd-Fe3O4/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 content in nanocatalyst were found to be 0.198 mmol/g and 1.648 mmol/g, which is still comparable with that of the fresh catalyst.

Figure 1. A) Comparison study in Catalytic performance in terms of GVL productivities as a function of time during LA Hydrogenation, Reaction conditions: LA (0.1 mL, 1 mmol), 14


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EtOH (2 mL), HCOOH (1 mL), NEt3 (catalytic amount), Temperature 140°C, Catalyst (0.201 mol% Pd); B) Influence of reaction temperature with diffrent 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 in catalytic performances with various supported Pd-Fe3O4 nanohybrid catalysts, Reaction conditions: LA (0.1 mL, 1 mmol), EtOH (2 mL), HCOOH (1 mL), NEt3 (catalytic amount), Temperature 140°C, Time 12h Catalyst (0.201 mol% Pd; D) Recycle potential diagram for catalytic hydorgenation of LA to GVL, E) Kinetic curves for different continuous hydrogenation reactions in Reusability test with PdFe3O4/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 dry sealed tube, 140oC, 12 h.; F) Hot-filtration experiments where Pd-Fe3O4/PPTPA-1 catalyst was removed after 6 h and 15 h followed by addition of 0.1 mL Levulinic Acid in the filtrate and reaction was continued for another 20 h. Reaction condition: Levulinic acid (0.2 mL, 2 mmol), Pd-Fe3O4/PPPTPA-1 catalyst (40 mg) and HCOOH (2 mL, 50 mmol) EtOH (5 mL), 140oC. We have also investigated the catalytic performance of our newly developed nanohybrid PdFe3O4/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 γ-valerolactone (GVL) has been achieved. iPrOH have been considered as moderate hydrogen source for this LA hydrogenation reaction under solvent free condition (Table 2, Entry 2) providing 65% conversion of LA to GVL as supported by previously reported results by Hermans et. al in Reductive Upgrading of Furfural in iPrOH.37 Only 12% conversion of LA to GVL was obtained with N2H4.H2O under solvent free condition (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 high pressure reactor autoclave under H2 gas at which also has been considered as poor one source. Screening of various solvents predicted that EtOH is the most potential solvent for this LA hydrogenation over nanohybrid catalyst (Table S2, SI). It is evident from the Table 2 that our designed catalyst exhibited lower catalytic performance under H2 gas than using HCOOH as source of hydrogen. In order to validate this observation and find out any promotion effect of CO2 derived from HCOOH to 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 condition in this study. Guo and co-workers have postulated the similar finding regarding the promotion effect of CO2 derived from formic acid in Ru-catalyzed 15



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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 condition (< 900 r.p.m.) leading to the reaction mixture into quasi-homogeneous in nature. Weckhuysen et. al have reported that the external mass transfer limitations was overcome in liquid phase LA hydrogenation reaction catalyzed by Ru-Pd/TiO2 catalyst running the catalytic reactions at a stirrer speed of 1,600 r.p.m.20 In our work we have examined the influence of external mass transfer limitations on catalysis with of stirrer speed considering Pd/C catalyst as standard with a stirring speed of 1500 rpm. A detailed concern of transport and kinetically controlled operating regimes, along with the calculation of dissolved H2 concentrations for gas phase LA hydrogenation reaction is presented by Bond and co-workers where they have observed that intraparticle transport limitations were negligible for smaller particles.44 On the basis of these recent reported works we can surely assume that the influence of external mass transportation for our catalytic system is negligible for the smaller particles and rapid stirring rates although Pd NPs are nicely confined inside the porous channel. Table 2: Use of different hydrogen source for LA hydrogenation to γ-valerolactone with PdFe3O4/PPTPA-1 Nanocatalysta

16


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Entry

H2-Source

Solvent

N2H4.H2O

EtOH

Temperature

Con (%)

120oC

21

PrOH

120oC

65

99

3

N2H4.H2O

120oC

12

88

4

HCOONH4

H 2O

120oC

55

75

5

H2 gas

Toluene

120oC

23

96

6

HCOONH4

EtOH

120oC

59

70

7

H2 gas

EtOH

120oC

52

71

8b

H2 gas

EtOH

120oC

55

80

1 2

i

Selectivity (%) 99

a

Reaction conditions: Levulinic acid (1 mmol, 0.1 mL), H2 source (25 mmol), PdFe3O4/PPTPA-1 catalyst (20 mg), Solvent 2 mL, time =12 h, H2 gas (5 bar pressure); bCO2 gas (5 bar pressure). The distribution of the both metals within the individual NPs with their existing electronic status should be explored in order to validate the specific reason behind the superior catalytic properties of supported nanohybrid catalysts. Slightly lower experimental values of C and N as observed from elemental analyses of the respective materials (Table S1, SI) suggested to the entrapment of guest molecules, a very common phenomenon for porous materials, which is further validated by the weight loss from TGA analysis (Figure S1b, SI). Structural compositions and different chemical environment of carbon nuclei of the POP composites have been examined by FT-IR and solid-state 13C NMR analysis. Figure 2A shows the FT-IR spectra of Fe3O4/PPTPA-1, Pd/PPTPA-1, Pd-Fe3O4/PPTPA-1 nanohybrid materials, respectively. Here all these three respective materials feature two characteristic absorption bands at 1273 cm-1 and 815 cm-1, respectively which could be assigned to the C-N stretching frequency of the tertiary amine and C-H out-of plane bending of the para-disubstituted benzene rings of the particular Porous polytriphenylamine material PPTPA-1. The additional bands centred at 2900 cm-1, 1596 cm-1, 1487 cm-1 , attributing to the phenylic C–H bond and aromatic C=C stretching vibration of the benzene ring.32 The only peak appeared at 558 cm-1 17



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which distinguishes Fe3O4/PPTPA-1 and Pd-Fe3 O4/PPTPA-1 materials from Pd/PPTPA-1 could be referred to the Fe-O stretching mode of Fe3O4 NPs.45-46 Wide angle powder XRD spectra of Pd-Fe3O4/PPTPA-1, Fe3O4/PPTPA-1 and Pd/PPTPA-1 hybrid materials, respectively are provided in the Figure 2B. PXRD pattern of Fe3O4/PPTPA-1 exhibits six sharp characteristic diffraction peaks at 2Ɵ= 30.1, 35.4, 43.0, 53.6, 56.9, and 62.5, which could be indexed to (220), (311), (400), (422), (511), and (440) reflections, corresponding to the face-centered cubic lattice of Fe3O4 nanoparticles (JCPDS no. 19-0629). In contrast, three distinct reflection patterns obtained from PXRD spectrum of Pd/PPTPA-1 material at 2Ɵ= 40.1, 46.2, 67.7, which could be assigned to (111), (200), (220) crystalline planes, suggestive to the face-centered-cubic (fcc) crystal structure of Pd nanoparticles.47-49 The weak intensities of (111) and (200) Bragg diffractions for Pd-NP in Pd/PPTPA-1 material reveal to the excellent dispersion of embedded Pd-NP with the coverage of POP shell, ascribed to the minimum exposure of Pd nanocrystal in surface. Similar finding regarding very weak XRD reflection intensities of Pd in Pd-POP based materials owing to well dispersion of Pd in Polymers was reported by Wang et al.35 The XRD pattern of Pd-Fe3O4/PPTPA-1 exhibited both the diffraction patterns of face-centered cubic (fcc) Pd and and Fe3O4, signifying that the good crystalline features with phase purity of Pd and Fe3O4 are nicely preserved during the morphological transition. It is interesting to notice that the peak intensity for Pd (111) crystalline reflection in Pd-Fe3O4/POP material is stronger than that of the corresponding Pd/POP. This observation could be ascribed to the deposition of most of the Pd-NPs on the surface of Fe3O4 nanoclusters rather than POP material, resulting a great exposure of Pd-NP in surface. Very recently, Dong and co-workers have observed the same scenario for the Ag/Fe3O4/g-C3N4 composite photocatalyst where the strongest reflection of metallic Ag (38.0°) was readily discerned owing to the deposition of Ag on Fe3O4 surface during the course of catalyst synthesis.50 A positive displacement in the diffraction patterns of Pd (111) from 39.87° to 40.46° was noticed which could be recognized for the strong interaction between Pd and Fe3O4 nanoclusters in the Pd-Fe3O4/PPTPA-1 catalyst prepared by the CoImpregnation Method. Besides all the samples present a single broad diffraction peak situated at about 2Ɵ=19.5, related to amorphous nature of porous organic polymer nanosheets.45 The XRD patterns of all the respective samples with the Pd-Fe ratio (1:1, 1:3, 1:5 and 2:1) (Figure S2) reveals to the presence of fcc Pd0 NP but no additional peaks related with the Fe3O4 NP are visibly appeared owing to the well dispersion of nanoparticles inside the polymer. The porosities of Fe3O4/PPTPA-1, Pd/PPTPA-1 and Pd-Fe3O4/PPTPA-1 hybrid materials were 18


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studied by N2 adsorption/desorption at 77 K (Figure 2C). All the materials exhibited an initial rapid N2 uptake at relatively low pressure, followed by a gradual increase in nitrogen adsorption with no hysteresis, and the steep rise in the high P/Po region signifies that the material consist of both micro-and mesopores. The specific BET surface areas of Fe3O4/PPTPA-1, Pd/PPTPA-1 and Pd-Fe3O4/PPTPA-1 are 475, 543 and 526 m2 g-1, respectively, and their corresponding total pore volumes are 0.357, 0.224 and 0.205 cm3 g-1, respectively which is comparatively lower than the reported parent as-synthesized PPTPA-1 porous polymer (BET surface area 1473 m2g-1 and pore volume 0.881 cm3g-1). Considerable decrease in the BET surface area and Pore volume with the preservation of N2 adsorption/desorption isotherm shapes for those entire respective hybrid materials indicate to partial pore filling and/or mass increment after loading of Pd and Fe3O4 NPs inside the interior cavities and nanoporous channel of the as-synthesized POP (PPTPA-1) that hinders the N2-uptake. The corresponding pore size distributions of the materials derived by using NLDFT (Non Local Density Functional Theory) method (Inset of Figure 2C) reveal the presence of hierarchical pores predominately in the microporous range. Pore size distribution of Fe3O4/PPTPA-1, Pd/PPTPA-1and Pd-Fe3O4/PPTPA-1 material exhibit a sharp peak at 2.73 nm along with at 1.75 nm, 1.40 nm and 1.58 nm, respectively, attributable to the existence of mainly micropores. Different chemical environment of carbon nuclei of Pd/PPTPA-1, Fe3O4/PPTPA-1 and Pd-Fe3O4/PPTPA-1 has been analysed by

13

C cross-

polarization magic-angle spinning (CP MAS) NMR analysis, as provided in the Figure 2D. The strong chemical shifts at δ=127.2 ppm and δ=147.1 ppm, which could be ascribed to the ortho (o-) carbon (C2) and carbon of the benzene rings (C1) attached with the N-atom of Tri phenyl amine (TPA) unit, respectively. Beside this an additional signal with low intensity at δ=135.2 was appeared, corresponding to the para (p-) carbon (C4) of each benzene ring of the TPA unit. These all the characteristic signals appeared in the three respective Pd/PPTPA-1, Fe3O4/PPTPA-1 and Pd-Fe3O4/PPTPA-1 materials, analogous with the previously reported 13

C CP MAS NMR spectrum of the as-synthesized PPTPA-1 material, confirming the

integrity of their structural backbone of polymeric framework PPTPA-1. FE-SEM images show that the nanohybrid materials are composed with the particles having granular morphology (Figure S3, SI) and the corresponding elemental mapping (Figure S4, SI) revealed the integrating even dispersion of Pd and Fe throughout the surface.

19



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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 NLDFT method have been provided in the inset of Figure 2C. To get further insight into the interactions between Pd and Fe, XPS analysis was then performed (Figure 3). All the binding energy peaks are normalized to the same intensity for clear comparisons with the reference of Pd and Iron Oxide foils. All the binding energies were calibrated via referencing to C 1s binding energy (284.6 eV). XPS survey spectra (Figure 3A) show the presence of C, N, O, Pd and Fe in the respective hybrid materials. The positive displacement in binding energies of N1S spectra (Figure 3B) by 0.5, 0.7 and 1 eV respectively, demonstrating that the interaction between the N-rich POP and nanoparticles gradually enhances with the change from monometallic (Pd or Fe3O4) to nanohybrid (PdFe3O4) NPs owing to the high electronegativity of Fe.51-52 The XPS spectrum of Pd-3d in 20


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Pd/PPTPA-1 (Figure 3C) display two peaks located at 336.0 eV and 341.2 eV are assigned to the Pd 3d5/2, Pd 3d3/2 orbits of metallic Pd0. A negative shift (~0.4 eV) in XPS binding energy in Pd-3d XPS spectra for Pd-PPTPA-1 material compared with the bare Pd-NP could be ascribed to coordination of N atoms with Pd, resulting in more electron-rich Pd(0) species (Figure 3C). Our finding regarding this trend is consistent with the experimentally observed by Wang et al. in their Pd-POP based catalyst.33 But also a significant binding energy shift to higher value (~1.4 eV) in Pd 3d XPS spectrum of Pd1Fe9 compared with the Pd/PPTPA-1 (Figure 3C) is noticed, confirming Pd-Fe nanohybrid formation.53 This XPS binding energy displacement could be attributed to the modification of electronic status of Pd caused by the intrinsic synergistic effect between Pd and Fe by charge transfer of electron from Pd to Fe due to the high electronegativity of Fe.54 We also note that this provided reason is in full agreement with the recent work of Roucoux and co-workers on Magnetically recoverable palladium (0) nanocomposite catalyst for hydrogenation reactions.55 The d orbital rehybridization of Pd arising due to the intra-or inter-atomic charge transfers between Fe and Pd can be correlated with such noticeable binding energy shift. This observation further reveals that the negative and positive binding energy displacement in Pd-3d spectra may be caused with the interaction between Pd-POP and Pd-Fe3O4, respectively. In comparison with Fe3O4/PPTPA-1, the binding energy of Fe 2p for Pd1Fe9 also presented a moderate blue shift (0.5 eV) after the nanohybrid formation (Figure 3D), indicating strong binding of electrons to the Fe, thus occurring more facile reduction of the Fe56 as experimentally evidenced by McEwen et al. in Pd-Fe2O3.54-55 In addition, the absence of satellite peak at 719.0 eV (characteristic peak of γ-Fe2O3) clearly confirms the formation Fe3O4 NPs rather than γ-Fe2O3 NPs in the corresponding nanohybrids. A similar finding regarding this characteristic satellite peak of γ-Fe2O3 was previously reported by Zhang et al. in single-crystalline γ-Fe2O3 nanowires.57 Such electronic interaction between Pd and Fe observed in XPS binding energies displacement attributable to intrinsic hybrid synergistic effect is thought to be responsible for this superior catalytic performance in LA hydrogenation reaction which is further confirmed by the catalyst evaluation results described above. Edman Tsang and coworkers have reported an enhancement in ethylene glycol hydrogenolysis to methanol with high selectivity which is triggered by the synergistic interaction between Pd and Fe in PdFe2O3 catalyst.53

21



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Figure 3. A) XPS survey spectra, B) XPS spectra N 1s region, C) Pd 3d region and D) Fe 2p region of the respective monometallic and nanohybrid catalysts (with different Pd/Fe ratio). Figure 4 provided the TEM microstructures along with the HR-TEM and SAED (Selected Area Electron Diffraction) patterns of the respective POP based nanohybrids. Both nanohybrid and monometallic NPs are strongly anchored on the external surface of PPTPA-1 nanofibers with a random distribution (Figure 4 A, B, E, F, I & J). It was clearly displayed the black color Pd and Fe3O4-NP are strongly attched on the fibrous POP surface which are represented by marking them with various colors. The morphology of the NPs was further confirmed at a closer look from the HRTEM images (Figures 4 C, G & K). Distinguished lattice fringes with the d spacing about 0.217, 0.167 and 0.189 nm are resolved from the HRTEM images corresponding to the crystalline (111), (201) and (002) planes of fcc Pd-Fe3O4 nanohybrid, respectively (Figure 4C).58-59 The (SAED) pattern presented in the Figure 4D of the nanohybrid corresponds to the respective crystalline (111), (001), (110), (201) and (200) reflections indicating Pd-Fe3O4 nanohybrid formation which is consistent with the XRD result with JCPDS data 02-1440. High-resolution TEM illustrated (Figure 4G) that the 22


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interplanar spacing for the lattice fringe was 0.228 nm, assigning to the (111) crystalline plane of Pd fcc lattice.60 Bright rings associated with distinct spots are clearly noticed (Figure 4H), which could be indexed to the (311), (400), (222), (200), (331), (220), (111) reflections planes of fcc Pd. An obvious lattice fringes with d spacing 0.248 and 0.297 nm assigning to the (311) and (220) planes (Figure 4K) and well distinguished reflections of (220), (311), (222), (511), (400), (422), (440) and (531) of fcc Fe3O4 in the marked SAED indicate successful synthesis of ultrafine Fe3O4 NPs on the fibrous polymer (PPTPA-1) surface (Figure 4L).61 In order to find out the real dispersity of nanoparticles we have carried out Zeta potential experiment of the respective bimetallic catalyst as provided in the Figures S6, SI. The measured Zeta potential value of our as-synthesized material is -33.9 mV which signifies that the nanoparticles dispersion is very stable and it will not undergo easily flocculation or agglomeration.

Figure 4. (A, B, E, F, I & J) TEM images and (C, G & K) corresponding HR-TEM images with clear lattice fringe and (D, H & L ) SAED patterns with marked facets of Pd-Fe3O4/, Pd/and Fe3O4/PPTPA-1 nanohybrid materials. 23



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Reused catalyst characterization revealed the mechanical and chemical stability, evading the associated structural degradation of the catalyst (Figure 5). It was evidently displayed a decrease in catalytic activity at the 11th catalytic run from the recycling test of the newly designed nanohybrid catalyst which may be due to the deactivation of some catalytic active sites owing to the either clogging or destruction of nanopores by carbonaceous deposition thereby hindering the diffusion of the organic substrates into the nanopores, and thus reducing the catalytic activity. This reason is experimentally evidenced with the N2 adsorption desorption study of the reused catalyst (Figure 5A). A significant decrease in the BET surface area and pore volume (273 m2g-1 and 0.187 ccg-1) of the recycled catalyst compared with the fresh one clearly indicates trapping of porous channel causing severe hindrance of N2 adsorption. Homogeneous dispersion of black color PdFe3O4 nanoparticles throughout the POP external surface avoiding agglomeration (Figure 5B & C) is observed from the HR-TEM images after the 10th catalytic run. XPS spectroscopy analysis of reused catalyst in Pd 3d core level (Figure 5D) and Fe 2p region (Figure 5E) was conducted to authenticate oxidation state of metallic Pd-Fe3O4 nanohybrid has been maintained.

13

C CP

MAS NMR studies of the reused catalyst (Figure 5F) revealed that the organic framework unit has been retained and our catalyst is stable under the catalytic reaction conditions for several cycles. A comparison study between the prepared catalyst in this work and some reported catalysts using HCOOH as H2 source has been provided in the Table S3, SI. Although widespread mechanistic pathway is not studied in deep insight at this stage but some plausible reaction pathway (Figure S8, SI) can be drawn based on our observation and previously literature reports. Two main pathways could be considered in the hydrogenation of LA to GVL including, I) esterification of the enol form of levulinic acid (LA) to angelica lactone (α-AL) followed by hydrogenation to GVL and II) hydrogenation of 4hydroxypentanoic acid followed by esterification to γ-valerolactone (GVL). With the progress of reaction initially we found small amounts of angelica lactone (α-AL), LA and GVL products, respectively that were detected by GC-MS analysis. Esterification of the enol to angelica lactone (α-AL) (First step) has been accelerated by Fe3O4 sites due to its acidic nature. The activation of O-H bond is thought to be promoted by the strong coordination of oxo functionality owing to the strong oxophilic nature of Fe3O4. Similar conclusion has been established by Hermans et.al who attributed the extraordinary activity of Pd/Fe3O4 in Furfural Hydrogenolysis reaction promoting activation of O-H bond via coordination with the Fe3O4 active centres.37 The double bond of α-AL undergoes coordination with the Pd0-NP through 24


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π-electron donation. NEt3 abstracts a labile proton from HCOOH to generate formate (HCOO-) ion which also then coordinates with the Pd0 nanoparticles. After that CO2 is liberated from coordinated formate ion and Hydride ion (H-) resides on the surface of Pd0 nanoparticles. Then this hydride ion (H-) will be transferred to reduce α-AL to GVL. A similar finding regarding this decisive role of Pd NP fabricated on POP in alkene hydrogenation has been observed by Zhao and co-workers.60 The contribution of POP could be considered as for easy diffusion of reactants for facile interaction with catalytic active centers and desorption of product, resulting and acceleration in catalytic rate. The formic acid behaves as an avialable hydrogen source not as co-catalyst due to the progress of the reaction proceeds via keto-enol to angelica lactone pathway and our result is strongly supported by Galletti and co-workers.62

Figure 5: N2-sorption A), TEM images (B-C), XPS spectra Pd-3d D) and Fe-2p regions E), and 13C CP solid state MAS NMR F) of the reused Pd-Fe3O4/PPTPA-1 catalyst after 10th catalytic run. CONCLUSION: In summary, we have successfully designed porous organic polymer (PPTPA-1) encapsulated nanohybrid magnetically retrievable Pd-Fe3O4 catalyst in one step solvothermal route with Pd:Fe (1:9) ratio which exhibited an outstanding higher activity over their monometallic counterparts for selective LA hydrogenation to GVL, a key platform 25



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molecule in many biorefinery schemes. The impressive catalytic activity was principally attributed to the intrinsic synergistic effect generated by the strong electronic interaction between Pd and Fe3O4 which was experimentally evidenced by various characterization studies including binding energy displacement in XPS spectra. Furthermore, nanohybrid catalyst showed exceptional stability maintaining GVL productivity and selectivity on multiple re-use. HR-TEM studies signified formation and uniform decoration of nanohybrid on the external surface of PPTPA-1 thereby inhibiting detachment of metal. The novelty of the newly designed catalyst system was associated with the 1) long term catalyst stability, negligible leaching, no sign of deactivation and good recyclability in LA hydrogenation to GVL compared with the conventional catalysts; 2) HCOOH has been used here as sustainable liquid H2 source to conduct the catalytic reactions which is very safe and easily hand-able avoiding highly flammable H2 gas at high pressure and temperature; 3) Easily scalable and cost-effective one-pot synthetic method has been employed for the catalyst synthesis; 4) Our catalyst is magnetically recoverable and no loss (100% catalyst recovery) took place during the course of separation for reusability test; 5) No extra procedure and reagents are required to reactivate our catalysts for next catalytic cycles. The work can encourage further research for designing of novel nanohybrid catalyst encapsulated over POP for advanced catalytic applications in biofuel and fine chemicals production using HCOOH as sustainable hydrogen source with the advantages of magnetic separation. Supporting information (SI): C, H, N analysis data, UV-vis spectrum, TGA, Wide angle PXRD, TEM, FE-SEM with the elemental mapping of the newly developed magnetic nanohybrid material, Variation in Catalytic performance, Comparison Table. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; [email protected] * E-mail: [email protected] ACKNOWLEDGEMENTS

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KD wishes to thankfully acknowledge Department of Science and Technology (DST), New Delhi for providing her DST-INSPIRE-JRF fellowship. S.M and AB wish to thank CSIR and DST, New Delhi for providing funding though DST-UKIERI project research grants and DST Unit on Nanoscience for providing instrumental facilities at IACS. RS and JM thank to Department of Science and Technology, India for DST-INSPIRE Faculty Research project grant (GAP-0522) in CSIR-IICT, Hyderabad. NOTES The authors declare no competing financial interest.

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Polymers for Alkene Hydrogenation at Room Temperature. ACS Appl. Mater. Interfaces 2016, 8, 15307-15319. 61

Patra, A. K.; Kundu, S. K.; Bhaumik, A.; Kim, D. Morphology evolution of singlecrystalline hematite nanocrystals: magnetically recoverable nanocatalysts for enhanced facet-driven photoredox activity. Nanoscale 2016, 8, 365-377.

62 Galletti, A. M. R.; Antonetti, C.; Luise, V. D.; Martinelli, M. A sustainable process for the production of γ-valerolactone by hydrogenation of biomass-derived levulinic acid. Green Chem. 2012, 14, 688-694.

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Table of Contents Graphics

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*

We have designed highly active and durable porous organic polymer encapsulated bimetallic magnetically retrievable Pd-Fe3O4 catalyst with high performance catalytic hydrogenation of Levulinic acid using formic acid as green and sustainable hydrogen source.

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Magnetic Nanohybrid Decorated Porous Organic Polymer: Synergistic

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