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Selective Catalysis for Room-temperature Hydrogenation of Biomassderived Compounds over Supported NiPd Catalysts in Water SK Nazmul Hasan MD Dostagir, Mahendra Kumar Awasthi, Ankit Kumar, Kavita Gupta, Silke Behrens, Abhijit Shrotri, and Sanjay Kumar Singh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00486 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Sustainable Chemistry & Engineering

Selective Catalysis for Room-temperature Hydrogenation of Biomass-derived Compounds over Supported NiPd Catalysts in Water SK Nazmul Hasan MD Dostagir,†,§ Mahendra Kumar Awasthi,† Ankit Kumar,† Kavita Gupta,† Silke Behrens,‡ Abhijit Shrotri§ and Sanjay Kumar Singh*,† †Catalysis

Group, Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore453552, M.P., India. ‡Institute

of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. §Institute

for Catalysis, Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021,

Japan Email: [email protected] (SKS)

Abstract Utilizing biomass-derived furan-based platform chemicals for the production of a wide range of value-added components for application as fuel/fuel blenders and other fine chemicals are gaining much attention. Here, we demonstrated an efficient room-temperature selective hydrogenation of furan-based long chain aldol compounds over supported NiPd/SiO2, NiPd/RHA (RHA is rice-husk ash) and NiPd/Z4A (Z4A is zeolite 4A) catalysts in water. A wide range of furan-based compounds, such as 1,5-bis(furan-2-yl)penta-1,4-dien-3-one (1), 4(furan-2-yl)but-3-en-2-one (3), 1,3-bis(furan-2-yl)prop-2-en-1-one (4), 1-(furan-2-yl)-3-(5methylfuran-2-yl)prop-2-en-1-one (5) and 3-(furan-2-ylmethylene)pentane-2,4-dione (6), were conveniently hydrogenated to the corresponding saturated ketone-products using the present protocol. Our findings inferred that the studied supported NiPd catalysts selectively promoted the hydrogenation of C=C bond over C=O bond to yield the corresponding saturated ketone products. The observed tuned catalytic properties can be attributed to the crucial role of the support in controlling the substrate-to-surface interactions, presumably, by disfavoring the interaction of polar carbonyl functional groups with the catalyst surface, and hence facilitating the hydrogenation of C=C over C=O bonds. Moreover, the support RHA facilitated the high dispersion of NiPd nanoparticles (~4 nm) for NiPd/RHA catalyst. Hence, NiPd/RHA catalyst displayed high stability the under the catalytic reaction condition and was reused for six consecutive catalytic runs without any significant loss in the catalytic activity. KEYWORDS: Nickel-Palladium, Bimetallic, Silica-based supports, Selective hydrogenation, Room-temperature, Aqueous condition 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction The depleting fossil fuel resources triggered the exploration of lignocellulosic biomass, as an alternative, inexpensive and widely available carbon source, to obtain a variety of platform chemicals for wide applications in fuel and pharmaceuticals.1-3 However, the high structural complexity of lignocellulosic biomass limits its direct usage as transportation fuel, and therefore its transformation into furan-based platform chemicals is considered as one of the prominent routes for efficient utilization of lignocellulosic biomass.4-9 In particular, furanbased platform chemicals such as furfural and 5-hydroxymethylfurfural (5-HMF) are being extensively explored for the synthesis of a wide range of value-added chemicals such as 2methylfuran (2-MF), γ-valerolactone (GVL), levulinic acid, diketones and so on.4-11 Though, these short chain furan-based platform chemicals (C5-C6) are not suitable to produce efficient fuel components with sufficiently high energy density, in this direction utilization of these (C5C6) chemicals as raw materials to produce a variety of long carbon chain (C8-C18) compounds is indeed an efficient approach (Figure 1).5,8,10,12-15

Figure 1. Transformation of lignocellulosic biomass to long chain fuel components Noteworthy work of Dumesic et al. on utilizing aldol condensation reaction for extending the carbon chain and branching, starting from furfural or 5-HMF using ketones, has promoted this approach as a facile route to produce long chain (C7 to C15) hydrocarbons.13 They have utilized bifunctional catalysts based on Pd supported on Mg-Al-oxide to induce onepot aldol condensation and hydrogenation to produce alkane precursors at 120 °C and 55 bar

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H2.13,14 The hydrogenated products of 1,5-bis(furan-2-yl)pent-1,4-dien-3-one such as 1,5bis(tetrahydrofuran-2-yl)pentan-3-one, 4-(tetrahydrofuran-2-yl)butan-2-ol and others have enormous application not only in the production of fuel components having long carbon chain (C8-C18), but also in the synthesis of 1-octanol, ethyloctylether (important chemicals in perfume industry).16-18 Among several catalytic methodologies explored to upgrade these platform chemicals, those based on Pd have shown high performance for the hydrogenation of furan derivatives. For instance, Pd/Co-Al HT-like catalyst facilitated both aldol condensation of furfural with acetone and subsequent total hydrogenation to 4-(tetrahydrofuran-2-yl)butan-2-ol at 120 °C and 24 MPa H2.19 On the other hand, hydrogenation and hydrodeoxygenation of furan-acetone aldol adducts was also investigated over Pt-based catalysts (Pt/Al2O3, 220 °C, 55 bar H2), wherein saturated carbonyl intermediate was also observed during the formation of the product n-tridecane.20 Nakagawa et al. employed Ni-Pd/SiO2 catalyst (Ni/Pd = 7) for the total hydrogenation of furan derivatives at 40 °C and 80 bar H2 in the presence of acetic acid, but the Ni-Pd/SiO2 catalyst readily deactivated due to the significant leaching of Ni in acetic acid.21 Recently, we achieved an efficient room-temperature total hydrogenation of a wide range of furan-based compounds to tetrahydrofuran analogous over NiPd catalysts (Ni/Pd = 9) in water under H2 atmosphere.22 The high catalytic activity exhibited by NiPd catalyst was attributed to the efficient adsorption of C=O bond over Ni sites, while electron rich Pd species promoted the C=C bond adsorption and the activation of H2 molecules. Moreover, the studied NiPd catalysts displayed high stability towards leaching in water and the reaction condition. On the other hand, selective hydrogenation of C=C bond in the furan-aldol mono-/diadducts resulted in the formation of saturated aldehydes, which can subsequently selfcondensed to complex precursors to produce liquid C8 to C10 hydrocarbon.13 Cu/Mg-Al2O4 was reported for the partial hydrogenation of the exocyclic C=C bond along with the carbonyl group 3 ACS Paragon Plus Environment


of furfural-acetone adduct to 3-hydroxybutyl-5-methylfuran at 200 °C at 5 MPa H2. However, the catalyst suffered from poor stability and reusability due to the leaching of Cu.23 Pt/MWCNT-Al2O3 (100 °C, 300 psi H2) and Pt/SWNT (100 - 230 °C, 80 - 900 psi H2) also catalyzed the hydrogenation of furfural-acetone aldol adduct through the intermediates 4(furan-2-yl)butan-2-one and 4-(furan-2-yl)butan-2-ol.24 Earlier, Gorden et al. also reported Pd catalyzed selective hydrogenation of exocyclic C=C bond in 4-(4-methylfuran-2-yl)butan-2one and 4-(4-hydroxymethylfuran-2-yl)butan-2-one in acetic acid at 65 °C under 1 atm H2.25 Removal of the exocyclic C=C bond was considered to be a key step to prevent fragmentation of the substrate during hydrodeoxygenation of ketone, and hence facilitated the formation of long chain hydrocarbons.25 Analogously, Li et al. employed Pd/CN@MgO catalyst for the selective hydrogenation of the furan ring and the exocyclic olefinic bond of the furan-acetone aldol adduct at 60 °C and 0.1 MPa H2.26 The observed higher selectivity for saturated ketone was attributed to the fine dispersion of Pd nanoparticles over the CN@MgO support. Interestingly, in sharp contrast to the Ru@SILP catalyst, which exhibited high selectivity for the exocyclic olefin and the furan ring hydrogenation of furfural-acetone aldol adduct to yield the saturated ketone, the bimetallic Fe-Ru@SILP catalyst showed high selectivity for the hydrogenation of C=O and exocyclic C=C bonds over furan ring at 100 °C and 20 bar H2.27 Such saturated ketones, 1,5-bis(furan-2-yl)pentan-2-one, can also be converted into several important hydrocarbon based lubricant oils over Ru/Al2O3 catalyst at 110 °C.28 Though a wide range of catalysts have been explored for hydrogenation reactions, catalytic systems for selective hydrogenation of these furfural-aldol adducts under mild reaction conditions yet not explored.


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O R

O

(0.5 equiv.)

O

O O

R = H, Me

O

R

O R = H (1) Me (2)

O O

O

O

3

H2 / Cat.

O O

O O

O O O O

O O

H2 / Cat. 6 R

O 1b

R = H (4) Me (5)

H2 / Cat.

H2 / Cat.

O

O R

R

3b

O O

O

O

O O

R = H (4b) Me (5b)

6a

Scheme 1. Transformation of furan-based platform chemicals into long chain (C8C18) compounds and their subsequent catalytic hydrogenation We, herein, demonstrated the selective hydrogenation of C=C bonds over C=O bonds for a wide range of furan-based compounds, such as 1,5-bis(furan-2-yl)penta-1,4-dien-3-one (1), 4-(furan-2-yl)but-3-en-2-one (3), 1,3-bis(furan-2-yl)prop-2-en-1-one (4), 1-(furan-2-yl)-3(5-methylfuran-2-yl)prop-2-en-1-one (5) and 3-(furan-2-ylmethylene)pentane-2,4-dione (6), to the corresponding saturated ketones over supported NiPd catalysts at room-temperature in water under H2 atmosphere. Results evidenced the significant role of support materials in tuning the selectivity towards ketone products, presumably by tuning the adsorption behavior of substrates over the catalyst surface. Experimental Section Preparation of NiPd/support catalysts. Synthesis of the supported NiPd catalysts comprised of two steps: first the synthesis of NiPd nanoparticles and second stabilization of the NiPd nanoparticles over different support materials. Bimetallic (Ni/Pd = 9:1) nanoparticles were 5 ACS Paragon Plus Environment


synthesized following our earlier reported method.29,30 In a typical procedure, an aqueous solution of NaBH4 (0.0125g, in 5 mL of water) was added dropwise to a 5 mL aqueous solution of K2PdCl4 (0.0016 g, 0.005 mmol), NiCl2.6H2O (0.0107 g, 0.045 mmol) and PVP (0.0250 g). The content of the flask was sonicated for 10 min to obtain a black suspension of NiPd (Ni/Pd = 9:1) nanoparticles, which were collected by centrifugation twice at 5000 rpm for 5 min. Further to stabilize the freshly synthesized NiPd nanoparticles, 0.050 g of the support was added in a suspension of NiPd nanoparticles in 10 mL mixture of distilled water and ethanol (3:1 v/v). The above suspension was stirred for 4 h under argon atmosphere. The supported NiPd catalyst was collected by centrifugation twice at 5000 rpm for 5 min, and was washed with distilled water. Rice husk ash (RHA), SiO2 and Zeolite 4A (Z4A) were used as support to obtain NiPd/RHA, NiPd/SiO2 and NiPd/Z4A catalysts. General procedure for the catalytic room-temperature selective hydrogenation of furanbased aldol adducts. Typically for the hydrogenation reaction, 1.0 mmol of furan-based aldol adduct was added to 10 mL aqueous suspension of NiPd/support nanoparticles catalyst (5 mol%), and reaction mixture, equipped with a H2 balloon, was stirred at 30 °C for the desired reaction time. The catalyst was recollected using centrifugation at 5000 rpm for 10 min and the product in the solution was extracted by ethyl acetate (5 x 10 mL). Organic layer was dried over anhydrous Na2SO4 and all of the volatiles were removed under reduced pressure. The isolated products were analyzed by NMR and HRMS. Conversion and selectivity of the obtained products were confirmed by 1H NMR. Results and Discussion At an outset, we synthesized bimetallic NiPd alloy nanoparticles (Ni:Pd 9:1) and supported these nanoparticles over silica, rice-husk ash (RHA) and zeolite 4Å supports, in aqueous ethanol solution at room temperature. Consistent with our previous reports, the presence of a peak at 2θ value of 40° in the powder X-ray diffractogram (PXRD) of NiPd 6 ACS Paragon Plus Environment

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nanoparticles was assigned to the face centered cubic (fcc) (111) plane of the NiPd nanoparticles.22,29,30 Upon stabilizing the synthesized NiPd nanoparticles over the support materials, the characteristic peaks for NiPd was hardly observed in the PXRD of the supported NiPd nanoparticles (NiPd/SiO2, NiPd/RHA and NiPd/Z4A) under the background of intense peaks for the respective support (Figures S1-S3). The observed pattern can also be attributed to the high dispersion of NiPd nanoparticles over the support.

(a)

(c)

(b)

(d) 500 400

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

Ni Pd Si

200 100 0

0

50

100

150

200

Distance (nm)

Figure 2. (a-c) TEM images of (a) NiPd/SiO2, (b) NiPd/RHA and (c) NiPd/Z4A. (d) STEM-HAADF image and the corresponding EDS line-scan analyses of NiPd/RHA catalyst. Though the presence of NiPd nanoparticles stabilized over the supports was not observed by scanning electron microscopic (SEM) images, the SEM-energy dispersive X-ray 7 ACS Paragon Plus Environment


spectroscopic (SEM-EDS) analyses infered the presence of both Ni and Pd (Ni/Pd atomic ratio of 9:1) for all the studied supported NiPd catalysts (Figures S1-S3). Moreover, SEM-EDS analyses also evidenced the presence of Si for NiPd/SiO2 and NiPd/RHA, and both, Si and Al for NiPd/Z4A (Figures S1-S3). Figure 2 depicted the transmission electron microscopic (TEM) images of the studied NiPd/SiO2, NiPd/RHA and NiPd/Z4A catalysts (Figures S4-S6). TEM images revealed the stabilization of NiPd nanoparticles of size ca. 4 nm over the respective supports for all the supported NiPd catalysts (Figure 2a-c, and Figures S4-S6). Moreover, TEM images evidenced high dispersion of NiPd nanoparticles for NiPd/RHA in comparison to NiPd/SiO2 and NiPd/Z4A catalysts (Figure 2a-c). Nevertheless, STEM-HAADF images and the corresponding EDS analysis also supported the presence of NiPd in the synthesized NiPd/SiO2, NiPd/RHA and NiPd/Z4A catalysts (Figures S7-S9). EDS line scan analysis for NiPd/RHA further evidenced the uniform distribution of NiPd (Ni:Pd 9:1) nanoparticles over the RHA support (Figure 2d).

Figure 3. (a) PXRD pattern and (b-c) XPS profiles for Ni and Pd of NiPd/RHA catalyst. X-ray photoelectron spectroscopic (XPS) analysis of NiPd/RHA catalyst inferred the presence of the characteristic signals for both metallic Pd (3d5/2) and Ni (2p3/2) at 335.39 eV and 856.70 eV, respectively, suggesting the alloy composition of Ni and Pd in NiPd/RHA catalyst (Figure 3). The observed binding energy values for Ni and Pd are well in accordance with the reported values of Ni0.1Pd0.9 alloy nanoparticles, suggesting that the NiPd alloy 8 ACS Paragon Plus Environment

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nanoparticles retain its alloy composition in the NiPd/RHA catalyst. These results are consistent with the TEM and TEM-EDS results showing the presence of highly dispersed NiPd nanoparticles over the RHA support. Catalytic efficacy of the synthesized supported NiPd catalysts was investigated using a aldol di-adduct of furfural and acetone, 1,5-di(furan-2-yl)penta-1,4-dien-3-one (1), as a model substrate at room temperature in water under H2 atmosphere. Complete conversion of compound 1 was achieved in 8 h for all the catalysts (NiPd/SiO2, NiPd/RHA and NiPd/Z4A), but with varying trend in the selectivity towards the hydrogenated products, 1,5bis(tetrahydrofuran-2-yl)pentan-3-ol (1a), 1,5-bis(tetrahydrofuran-2-yl)pentan-3-one (1b) and 1,5-di(furan-2-yl)pentan-3-one (1c).

Figure 4. (a) Product selectivity (1a/1b/1c) during catalytic hydrogenation of 1 over NiPd/RHA catalyst at room temperature. (b) Product selectivity (1a/1b) over supported (NiPd/RHA, NiPd/SiO2, NiPd/Z4A) and un-supported NiPd (Ni/Pd 9:1) catalyst for the room-temperature hydrogenation of compound 1. Interestingly, all the studied catalysts exhibited higher selectivity for 1,5bis(tetrahydrofuran-2-yl)pentan-3-one (1b) product, wherein the higher preference for C=C bonds (exocyclic olefin bond and furan ring) over C=O bond was observed. During the initial 8 h of the catalytic reaction, NiPd/RHA catalyst exhibited 20% selectivity for the total



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hydrogenated product (1a), while the selectivity for the ketone product (1b) was as high as 63% along with the 17% selectivity for the partially hydrogenated ketone product (1c) (Table 1, entry 1) (Figure 4a). Further extending the reaction duration to 16 h, furan ring of the compound 1c also hydrogenated to increase the selectivity for 1b to 75% (Table 1, entry 2). Analogous trend in the product selectivity for the hydrogenation of compound 1 was also observed with NiPd/SiO2 (1a/1b/1c = 20:80:0) and NiPd/Z4A (1a/1b/1c = 25:75:0) catalysts (Table 1, entries 4-9) (Figure 4b). Therefore, the above results evidenced that NiPd/RHA, NiPd/SiO2 and NiPd/Z4A catalysts exhibited high selectivity for 1,5bis(tetrahydrofuran-2-yl)pentan-3-one (1b) in 16 h, and no significant change in the selectivity for 1b was observed with the further increase in reaction duration to 24 h. Notably, hydrogenation of compound 1 could not proceed in the presence of Ni/RHA, whereas Pd/RHA, containing Pd content equivalent to that present in NiPd/RHA, displayed only 30% conversion of compound 1 under analogous reaction condition. These results clearly evidenced the crucial role of the bimetallic NiPd alloy nanoparticles to achieve high catalytic activity for the facile hydrogenation of furfural-acetone aldol di-adduct (1). Table 1. Catalytic hydrogenation of compound 1 over supported NiPd catalysts at room temperaturea entry

catalysts

t (h)

conversion (%)

selectivity (%)

yield (%)

(1a:1b:1c)

(1b)

1

NiPd/RHA

8

>99

20:63:17

61

2

NiPd/RHA

16

>99

25:75:0

73

3

NiPd/RHA

24

>99

30:70:0

65

4

NiPd/SiO2

8

>99

18:68:14

63

5

NiPd/SiO2

16

>99

20:80:0

75

6

NiPd/SiO2

24

>99

20:80:0

73

7

NiPd/Z4A

8

>99

23:71:6

64

8

NiPd/Z4A

16

>99

25:75:0

71


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9

NiPd/Z4A

24

>99

25:75:0

52

10b

Ni/RHA

16

n.r.

-

-

11

Pd/RHA

16

30c (>99)d

25:55:20

n.d.

12e

NiPd

24

>99

>99:0:--

-

aReaction

condition: compound 1 (1 mmol), catalyst (5 mol% NiPd), H2 balloon, water (10

mL), 30 °C. b5 mol% Ni. c0.5 mol% Pd, equivalent to the Pd content present in NiPd/RHA (N/Pd 9:1). d5 mol% Pd. eUnsupported NiPd (Ni/Pd = 9:1) catalyst, product 1c was not observed (Ref. 22). n.r. = no reaction. n.d. = not determined.

It is worth mentioning here that, previously we demonstrated that the unsupported NiPd (Ni/Pd 9:1) nanoparticle catalyst displayed high selectivity for the hydrogenated product (1a), with only 30% selectivity for the saturated ketone product, 1,5-bis(tetrahydrofuran-2yl)pentan-3-one (1b), under analogous reaction condition.22 Hence, stabilizing these active NiPd nanoparticles on silica-containing supports (NiPd/RHA, NiPd/SiO2 and NiPd/Z4A) significantly tuned the selectivity of the hydrogenated product towards the saturated ketone products. The observed enhanced selectivity (75% – 80%) for the ketone product (1b) achieved over the supported NiPd/RHA, NiPd/SiO2 and NiPd/Z4A catalysts illustrated the significant role of the supports, RHA/SiO2/Z4A, in tuning the selectivity of the products towards the selective hydrogenation of C=C bond over C=O bond (Figure 4b). Considering that rice-husk ash (RHA) is a waste material, and that the NiPd/RHA catalyst exhibited catalytic activity as par with other NiPd/SiO2 and NiPd/Z4A catalysts, we further explored NiPd/RHA catalyst to investigate the tuned selectivity for other furan-based compounds (2-8) under the optimized reaction condition (Scheme 3). In sharp contrast to the efficient hydrogenation achieved for compound 1, the analogous compound 2 (1,5-bis(5methylfuran-2-yl)penta-1,4-dien-3-one) having methyl substituted furan rings, remained unreactive for the hydrogenation reaction under analogous reaction condition. This behavior can be attributed to the steric hinderance due to the methyl groups which presumably hindered 11 ACS Paragon Plus Environment


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the facile interaction of the compound 2 with the catalyst surface. The furfural-acetone aldol mono-adduct (compound 3) was also efficiently hydrogenated, where the saturated ketone product 4-(tetrahydrofuran-2-yl)butan-2-one (3b) was obtained as the major product (55% yield), along with 4-(tetrahydrofuran-2-yl)butan-2-ol (3a) in 40% selectivity in 16h. Further, for lesser reaction duration (6h), >99% selectivity for the ketone products was acheived, where 3b along with 4-(4,5-dihydrofuran-2-yl)butan-2-one (3c) were obtained in 1:1 ratio. Catalytic hydrogenation of the compound 4, aldol condensed adduct of furfural and 2-acetylfuran, over NiPd/RHA catalyst also displayed high tolerance towards C=O bond hydrogenation and resulted in the formation of ketone products (4b-4d) with a combined selectivity of 74%. Results inferred that the exocyclic C=C bond instantly hydrogenated to produce following ketone

products,

1,3-bis(tetrahydrofuran-2-yl)propan-1-one

(4b),

1-(furan-2-yl)-3-

(tetrahydrofuran-2-yl)propan-1-one (4c) and 1-(tetrahydrofuran-2-yl)-3-(furan-2-yl)propan-1one (4d). The total hydrogenated product 1,3-bis(tetrahydrofuran-2-yl)propan-1-o1 (4a) was obtained with 26% selectivity. Placing a methyl substituent on the furan ring as in compound 5, a close analogue of compound 4, significantly retarded the hydrogenation of the furan ring having the methyl substituent, as we observed for compound 2 (Scheme 3). Hence, 1(tetrahydrofuran-2-yl)-3-(5-methylfuran-2-yl)propan-1-one (5d) was obtained with 54% selectivity,

while

the

other

ketone

products,

1-(tetrahydrofuran-2-yl)-3-(5-

methyltetrahydrofuran-2-yl)propan-1-one (5b) and 1-(furan-2-yl)-3-(5-methyltetrahydrofuran2-yl)propan-1-one (5c) were obtained respectively with 16% and 23% selectivity. Therefore for compound 5, a combined selectivity of 93% for the ketone products (5b-5d) was observed, while

the

complete

hydrogenated

product

1-(tetrahydrofuran-2-yl)-3-(5-

methyltetrahydrofuran)propan-1-o1 (5a) was obtained with only 7% selectivity.


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OH O

O O

O

O 1a

NiPd/RHA (5 mol%) H2O, H2 balloon, 30 °C

1

O

O

O

O

O

O 1c

1b O

O O

O

NiPd/RHA (5 mol%)

O 2b No reaction

H2O, H2 balloon, 30 °C

2

O

OH

O NiPd/RHA (5 mol%)

O

O

O


3

3a 38% sel.

3b 62% sel. (55% yield)

OH

R R

O

O

R

O

O

O

O O

NiPd/RHA (5 mol%)

O

4b (27% sel.) 5b (16% sel.)

4a (26% sel.) 5a (7% sel.)


R = H (4) Me (5)

O

R

O

O

R

O

O

O

O 4d (18% sel.) 5d (54% sel.)

4c (29% sel.) 5c (23% sel.)

O O O

O NiPd/RHA (5 mol%) H2O, H2 balloon, 30 °C

6

O O 6a >99% sel. (93% yield) R

R O

NiPd/RHA (5 mol%) O

O O


O

O

R = H (7) Me (8)

R = H (7a) Me (8a) No reaction

Scheme 3. Various furan-based compounds (1 ‒ 8) explored for room-temperature hydrogenation over NiPd/RHA catalyst. Reaction condition: substrate (1 mmol), catalyst (5 mol%), 30° C, H2 balloon, water (5 mL), 16 h. Further, the hydrogenation of compound 6, aldol product of furfural and acetylacetone, was also attempted, where the only produce 3-((tetrahydrofuran-2-yl)methyl)pentane-2,4dione (6a) was obtained with >99% selectivity (93% yield). Notably, in the product 6a, both the exocyclic bond and the furan ring was hydrogenated while C=O bond remained unaffected. Further, for the non-planar sterically hindered compounds 7 and 8, reaction could not proceed 13 ACS Paragon Plus Environment


under the analogous reaction condition (Scheme 3). This could be due to the sp3 central carbon atom connecting the three furan rings restricted the facile adsorption of the furan ring over the catalyst surface or the support. Encouraged by the high catalytic performance shown by the NiPd/RHA catalyst, we further conducted the recyclability experiments for the selective hydrogenation of compound 6 to 6a over NiPd/RHA catalyst under the optimized reaction condition (Figure 5a). Results inferred no significant loss in the catalytic activity of NiPd/RHA catalysts, and complete conversion of the compound 6 was achieved over the NiPd/RHA during recyclability test up to six catalytic runs. Moreover, the TEM and PXRD analysis of the spent NiPd/RHA catalyst inferred no significant changes in the catalyst, suggesting the high stability of NiPd/RHA catalyst under the catalytic reaction condition (Figure 5b and Figure S10).

Figure 5. (a) Recyclability experiment for the catalytic hydrogenation of compound 6 over NiPd/RHA catalyst. (b) TEM image of the recovered NiPd/RHA catalyst. Our findings inferred that the studied supported NiPd catalytic system efficiently catalysed the room-temperature hydrogenation of several furfural-based unsaturated ketones to saturated ketone products with high selectivity in water using H2 balloon, on the other hand other catalytic systems required either high temperature (>100 °C) or high pressure of H2 gas for analogous catalytic transformations (Table S1).20-27 For instance, Pd/CN@MgO catalyst selectively hydrogenated the furan ring and the exocyclic olefinic bond of the furan-acetone 14 ACS Paragon Plus Environment

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aldol adduct at 60 °C and 0.1 MPa H2.26 At 200 °C and 5 MPa H2, Cu/Mg-Al2O4 also catalyzed hydrogenation of the exocyclic C=C bond along with the carbonyl group of furfural-acetone adduct to yield 3-hydroxybutyl-5-methylfuran, but the catalyst suffered from poor stability due to the leaching of Cu.23 Saturated carbonyl products of dialdol adduct of furfural-acetone were also observed over several supported Pt catalysts at 220 °C and 2.5 MPa.20 Selective hydrogenation of exocyclic C=C bond in 4-(4-methylfuran-2-yl)butan-2-one and 4-(4hydroxymethylfuran-2-yl)butan-2-one was also reported over Pd catalyzed in acetic acid at 65 °C under 1 atm H2.25 In this regard, Ru@SILP catalyst exhibited high selectivity for the saturated ketone product by facilitating the preferential hydrogenation of the exocyclic olefin and the furan ring at 100 °C and 20 bar H2.27 However, the bimetallic Fe-Ru@SILP catalyst selectively catalyzed the hydrogenation of C=O and exocyclic C=C bonds over furan ring under analogous condition.27 Recent studies suggest electronic charge transfer from Ni to Pd center resulting an electron rich Pd center in the NiPd alloy nanoparticles.22,29 Such electron-rich Pd centers are considered as favorable sites for the facile activation of H2 to achieve efficient hydrogenation of furan-based compounds over unsupported NiPd nanoparticle catalysts even at roomtemperature.22 The observed higher activity of unsupported NiPd nanoparticle catalysts was attributed to the high tendency of Pd metal to interact preferably with C=C bond(s) and oxyphilic Ni with C=O double bond, facilitated the favorable interaction of the planar furan aldol adducts over the Ni-Pd nanoparticles.22 However, stabilizing these active NiPd nanoparticles over silica-containing supports, presumably disfavored the interaction of polar carbonyl functional groups with the catalyst surface, which may significantly tune the adsorption behavior of the studied furan-based compounds and hence facilitated the preferred hydrogenation of C=C over C=O bonds. In this context, literature revealed that the selective C=C bond hydrogenation may depends on the catalyst particle size, exposure of (111) facets,



specific non-covalent interaction of the substrate and the catalyst/support surface, nature of the support.31-36 It is anticipated that these factors may tune the reactivity of the catalyst towards a specific bond by controlling the orientation of the substrate over the catalyst surface. For instance, large Pt particles favors C=O hydrogenation over C=C bond in cinnamaldehyde, attribute to the hindered unfavorable interaction of C=C with Pt(111) facets.32,33 Recent studies also revealed that the support polarity may also influence the preferential C=C versus C=O hydrogenation.31,34,35,36 Lee et al. also demonstrated that Pt/SiO2 favors C=C hydrogenation in cinnamaldehyde to yield saturated aldehyde.31 These reports hypothesized that the non-polar supports such as carbon or fumed silica may disfavor the adsorption of the polar C=O function at the adjacent site of the active metal center, and hence favor selective C=C hydrogenation. Hence, for the studied NiPd/RHA catalyst, the highly dispersed NiPd nanoparticles (~ 4 nm) over RHA support governs the high catalytic activity, while the observed selective hydrogenation of C=C bond over the C=O bond evidenced a strong support effect. These observations can be attributed to the non-polar nature of RHA support, favoring C=C adsorption over the active NiPd nanoparticles highly dispersed over the silica-rich RHA support. Moreover, steric hindrance at the furan ring of the furan-based aldol adducts may also disfavored the facile adsorption of the substrate over the catalyst surface. Hence, the trend for the hydrogenation of the studied furan-based substrates follows the order: exocyclic C=C double bond > furan ring > C=O double bond. Further, the successive addition of hydrogen to the substrate decreases the adsorption ability of the intermediate hydrogenated compounds with the catalyst surface, and hence resulted in an easy removal of the desired products.22

Conclusions Herein, we explored the catalytic efficacy of supported NiPd catalyst for roomtemperature hydrogenation of several furan-based compounds with tuned selectivity for the saturated ketone products. Upon stabilizing NiPd nanoparticles over silica-containing supports, 16 ACS Paragon Plus Environment

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rice husk ash (RHA), SiO2 and zeolite 4A, advantageously we were able to retain the high activity of NiPd catalyst for hydrogenation reaction and could also tune the product selectivity with the aid of the support to preferentially hydrogenate C=C bond (furan ring and olefinic arm) over C=O bond. We anticipated that the silica-containing support played a crucial role in tuning the substrate adsorption over catalyst surface by retarding the C=O bond adsorption, and hence notable control over the selectivity for saturated ketone products was achieved. Moreover, we utilized a waste material, Rice Husk Ash (RHA), as a potential support material to achieve high dispersion of NiPd nanoparticles in NiPd/RHA, and hence observed high catalytic activity and tuned product selectivity. Moreover, NiPd/RHA catalyst exhibited appreciably high activity and stability under the catalytic reaction condition and hence was reused for six consecutive catalytic runs. Therefore, the explored mild protocol to obtain saturated ketones, at room temperature in water using H2 balloon, may find application to produce several useful compounds, as precursors for high-density fuel additives and lubricants, from biomass-derived furan-based compounds under mild reaction conditions. Acknowledgements Authors thank IIT Indore, SERB-DST (EMR/2016/005783) and CSIR, New Delhi for the financial support. Instrumentation facilities from SIC, IIT Indore, India, IKFT, KIT, Germany and Institute for Catalysis, HU, Japan are gratefully acknowledged. M.K.A., A.K. and K.G. thanks IIT Indore, SERB-DST and CSIR, New Delhi, respectively for their fellowships. Supporting Information Recyclability experiment. Synthetic procedure for compounds 1-8. SEM, TEM, STEMHAADF, EDX and PXRD characterization of NiPd/SiO2, NiPd/RHA and NiPd/Z4A catalysts. Comparative chart for the catalytic hydrogenation of furan derived compounds over several metal catalysts. 1H NMR, 13C NMR and HRMS of the substrate and products.



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Synopsis

Selective hydrogenation of C=C bonds over C=O bonds for various furan-based compounds was successfully achieved over supported NiPd catalyst in water at room temperature.


Selective Catalysis for Room-Temperature ... - ACS Publications

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