Utilization of Waste Biomass for the Synthesis of Functionalizable

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Utilization of Waste Biomass for the Synthesis of Functionalizable Support for Covalent Anchoring of Active Organo Catalyst Dhananjay S. Doke, Jacky Advani, Dhanaji Naikwadi, Manoj B. Gawande, Pravin S Walke, Shubhangi Bhalchandra Umbarkar, and Ankush V. Biradar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04430 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Sustainable Chemistry & Engineering

Utilization of Waste Biomass for the Synthesis of Functionalizable

Support

for

Covalent

Anchoring of Active Organo Catalyst Dhananjay S. Doke, b,c, † Jacky H. Advani, a, c, † Dhanaji R. Naikwadi, a Manoj B. Gawande, d Pravin Walke, e Shubhangi B. Umbarkar, b and Ankush V. Biradar, a,

c, *

aInorganic Material and Catalysis Division, CSIR-Central Salt and Marine Chemical Research

Institute, Bhavnagar 364002, Gujarat, India. bCatalysis

Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune,

411008, Maharashtra, India. cAcademy

d

of Scientific and Innovative Research (ACSIR), Ghaziabad- 201002, India.

Regional Centre of Advanced Technologies and Materials, Palacký University, Šlechtitelů

27, 783 71 Olomouc, Czech Republic. e

National Centre for Nanoscience and Nanotechnology, University of Mumbai, Vidyanagari,

Kalina, Santacruz (E), Mumbai-400098, India. † contributed equally *Corresponding author: Tel.: +91 278 2567760 Ext: 7170; Fax: +91 278 2566970. Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT A single step synthetic procedure for carbon microspheres from agricultural waste residue (sugarcane bagasse) by low-temperature hydrothermal carbonization using oxalic acid as a hydrating/dehydrating agent is developed. The FTIR and XPS spectroscopy analysis indicates the presence of -OH, -COOH, C=O functional groups on the surfaces of carbon spheres. These functional groups of the carbon spheres were utilized as a novel route to anchor 3aminopropyl-triethoxysilane

and

3-(2-aminoethylamino)propyl)-trimethoxysilane

via

condensation of triethoxy/trimethoxy silanes for the synthesis of organo base supported on carbon catalysts. The catalytic activity of the obtained supported organo-base catalyst was demonstrated for C-C bond forming (Henry) reaction. Among all prepared catalysts, 3-(2aminoethylaminopropyl)-trimethoxysilane grafted in toluene showed high conversion (up to 100%) of aldehydes with excellent selectivity towards β-nitrostyrene. The catalyst was reused five times without losing significant activity for the same reaction. KEYWORDS: Carbon spheres, Base catalyst, Supported catalyst, Heterogeneous catalysis, Henry reaction.


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INTRODUCTION Functionalized silica materials/nanomaterials are attractive candidates and have been extensively utilized in catalysis.1-2 However, special attention is required for the synthesis of different meso- and micropores silica structures. Also, these days an enormous amount of research is being carried out to find alternative precursors that are cheaper, abundant and as stable as silica materials. Abundantly available carbon can find similar potential as that of silica if their surfaces are functionalized. Conventionally, the activated carbon is synthesized by hightemperature carbonization of coconut shell, petroleum coke, and wood under insufficient air, which leads to the formation of sp² carbon with the high surface area and pore volume.3 Thus synthesized carbon materials find applications in electrocatalysis, energy storage, gas separations, composite materials, pollution control treatment, water treatment and as support for heterogeneous catalysts.4-6 Notably, in heterogeneous catalysis, carbon has been used for dispersion of active metal components and has very less influence on catalysis. Also, the leaching of active metals is a major problem leading to catalyst deactivation.7-8 Such leaching can be avoided either by protecting active metal by functionalizing the inert sp2 carbon with hydroxyl groups or grafting various functional groups onto a surface of carbon9-12 Traditionally, the -C=O, -OH, or -COOH functional groups can be directly introduced by the oxidation of C-C bond of carbon surfaces with different oxidants. However, there are some reports available on the direct functionalization of inert carbon surface by aryl radicals, amines, alkynes and azide coupling at high temperatures.13 Also, Downard et al.14 reported the successful coupling of carbon with aryl diazonium salts. However, some of these processes required harsh reaction conditions and an equivalent amount of corrosive reagent employed in the protocol may result in extensive pollution. Furthermore, direct functionalization of organoamines on activated carbon was reported by Zhu et al. However, it is a notable fact that the directed functionalization of the sp2 hybridized carbon is rather difficult. This fact was



illustrated by the TEM images from the same report, which shows the irregularly arranged selfassembled silica particles on the carbonaceous material.15 Thus it is evident that there is no report available on direct covalent functionalization of carbon. Synthesis of new nanostructured carbon materials from naturally abundant carbon source has become the subject of interest due to its cost-effectiveness and the promising applications in catalytic transformations.16-18 A large number of researchers have devoted their efforts for the synthesis of carbon materials with different structures such as carbon sphere19, and hollow carbon sphere20-24 employing low-temperature hydrothermal (LTH) synthesis by readily available sugars (e.g., glucose). Notably, LTH methods lead to uniform carbon spheres with hydrophilic functionalities like -OH, -C=O and -COOH groups on the carbon spheres as they offer easy functionalization of many catalytically active organocatalysts.25-26 Recently, functionalization of polypyrrole modified carbon sphere with dithiocarbamate was reported using readily available sugars. Though their catalytic applications are widely untapped.27 Considering all these facts, there is an enormous scope to explore and find alternate methods for the synthesis of stable support materials from the abundantly available bio-resources and their use for the functionalization of active catalytic species. In this regards, the abundantly available biomass waste (rice hull, corn husk, and hazel nutshell, etc.) were utilized for the synthesis of carbon materials.28 Particularly, countries like India grows a substantial amount of many bio-wastes from which the sugarcane is the second largest seasonal crop produced. After sugar extraction from these sugarcanes, large quantities of sugarcane bagasse is annually generated by sugar mills, which consists of high carbon content and primarily used as fuel for the steam generation which is partially used for electricity production emitting large quantities of CO2 into the atmosphere. Also, bagasse can be valorized to produce carbon like materials. To this end, some efforts have been made to synthesize activated carbon by physical activation and carbonizing the raw fiber pellets at a different


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temperature.29 However, there is still a need in state of the art to provide a process for the synthesis of uniform carbon spheres and their utilization as a catalyst support. In continuation to our efforts for the sustainable synthesis of nanomaterials and its applications,30 herein we report the synthesis and characterization of carbon spheres from readily available sugarcane bagasse as well as d-glucose for comparison and covalent functionalization by organoamine onto the carbon surfaces via condensation of the hydroxyl group with trimethoxysilanes. The successful formation of solid base catalyst was demonstrated by using the prepared catalyst for Henry reaction

EXPERIMENTAL Synthesis of bagasse carbon microspheres (BCM). Sugarcane bagasse fibers were first milled to a fine powder. The obtained bagasse powder (4 g), oxalic acid (1 g) and 50 mL of distilled water were charged into a 100 mL Teflon coated stainless steel high-pressure autoclave. Then the reactor was closed with screw cap nuts. The packed autoclave was heated at 210 °C for 12 h in an electric furnace for hydrothermal carbonization. After completion of the reaction, the reactor was cooled to room temperature. Then obtained black solid was filtered by simple filtration and washed with plenty of water till reddish color disappeared from the filtrate and finally washed with ethanol. The obtained solid (Carbon microspheres) was dried at 80 °C. The final product was named as BCM. The yield of carbon spheres was 1.39 gm (35%). Synthesis of glucose carbon microspheres (GCM). A similar method was used as described above using glucose as a carbon source instead of bagasse. The yield of carbon spheres was 1.32 gm (33%). Functionalization of BCM with 3-aminopropyl)triethoxysilane (APTES). In the typical experimental procedure, 250 mL round bottom flask was charged with 1 g of BCM and well



dispersed

in

150

mL

of

isopropyl

alcohol

by

sonication.

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Then

1

mL

(3-

aminopropyl)triethoxysilane (APTES) was added to the reaction mixture, and the whole assembly was purged with argon. Then the reaction mixture was refluxed at 80 °C for 12 h. Resultant mixture was filtered, and the residue was washed with IPA followed by 1,2dichloroethane and ethanol. The obtained material was dried at 80 °C for 12 h. Final compound was named as APTES-I-BCMs. The yield of APTES-I-BCMs was 1.3 gm. Similarly (3aminopropyl)triethoxysilane (APTES) was grafted in toluene instead of IPA at 110 °C and obtained catalyst was named as APTES-T-BCM. The yield of APTES-T-BCM was 1.31 gm. Functionalization

of

BCM

with

(3-(2-aminoethylamino)propyl)-trimethoxysilane

(AEAPTS). For the functionalization of AEAPTS onto BCM, a similar method was used. Instead of APTES, (3-(2-aminoethylamino)propyl)-trimethoxysilane (AEAPTS) was used in isopropyl alcohol, and the final catalyst was named as AEAPTS-I-BCM. The yield of AEAPTS-I-BCM was 1.38 gm. Similarly, 3-(2-aminoethylamino) propyl)-trimethoxysilane was grafted in toluene instead of IPA at 110 °C and obtained catalyst was named as AEAPTST-BCM. The yield of AEAPTS-T-BCM was 1.42 gm. Functionalization of GCM with APTES and AEAPTS groups. For the functionalization of GCM with APTES and AEAPTS onto GCM, a similar method was used and the final catalyst was named as APTES-GCM and AEAPTS-GCM respectively. Catalytic activity. Typically, the Henry reaction was carried out in 50 mL two-necked round bottom flask equipped with a magnetic stirrer, oil bath, and water condenser. The substituted benzaldehyde, nitromethane and catalyst were added. The reaction mixture was stirred at 90 °C until the complete conversion of one of the reactant. The samples were withdrawn periodically and analyzed on Agilent 7890B Gas Chromatograph equipped with HP-5 column coated with 5% phenyl 95% dimethylpolysiloxane (60 m length, 0.25 mm diameter and 0.25


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µm film thicknesses) with flame ionization detector. Some of the products were analyzed by GCMS (SI, Fig.-S10) and 1H NMR (SI, Fig.-S11). RESULTS AND DISCUSSION 3-aminopropyl-triethoxysilane

(APTES)

and

3-(2-aminoethylamino)propyl)-

trimethoxysilane (AEAPTS) were used for covalent functionalization of bagasse carbon microspheres (BCM) and their catalytic activity studied for C-C bond forming reaction. The aforementioned BCM was prepared by low-temperature hydrothermal carbonization (LTHC) of sugarcane bagasse which contains cellulose (45-55%), hemicellulose (20-25%) and lignin (18-24%) and generated in large quantities by the sugar industries. The reaction occurs between the finely powdered bagasse fibers and oxalic acid in the presence of deionized water at hydrothermal conditions (Figure. 1). Hydrothermal treatment of finely milled fibers of bagasse led to the formation of carbon spheres. The cellulose in bagasse undergoes hydrolysis catalyzed by a strong dicarboxylic acid, i.e., oxalic acid31 to yield glucose and by further dehydration to yield 5-(hydroxymethyl)furfural (5-HMF) which further condensed and polymerized to form solid carbon spheres with -OH, -C=O and -COOH groups onto the carbon surface. Owing to the high autogenous pressure, the polymerized product reduces its surface energy by forming a spherical morphology.32 Then obtained hydrophilic BCMs were used for the functionalization of catalytically active organoamines (APTES and AEAPTS). The functionalization was carried out by condensation of the terminal -OH or -COOH groups present on the synthesized BCM with trimethoxy or triethoxysilanes in different solvents, i.e., toluene and IPA to explore the role of solvent in the degree of functionalization of APTES and AEAPTS.



Figure 1. Schematic presentation of the formation of carbon microspheres from sugarcane bagasse and solid base catalyst by post grafting of amines. The morphology and structural composition of the prepared pristine BCM was characterized by SEM, EDAX, and TEM (Figure. 2). The EDAX and TEM analysis (SI, Fig.S8) of GCM was carried out for the comparative purpose. The SEM and TEM images display that the BCM is monodispersed with 4-5 µm in size. The spherical morphology is attained by the mono dispersed particles to reduce their surface energies which arise by the autogenous pressure in the vessel during the LTH process. However, to minimize the effect of this pressure, a polymerized product of spherical shape were formed.31


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Figure 2. a, b) SEM images at 50 and 4 µm scale and c, d) shows the TEM images of BCM prepared from bagasse at 0.5 µm and 5 nm scale. The SEM-EDAX analysis of grafted amines sample showed the 4.77 and 5.19 N wt % and 2.86 and 3.05 wt % of Si loading of APTES-I-BCM and APTES-T-BCM respectively and 5.59 and 7.76 N wt% 2.09 and 3.05 wt% of Si loading of AEAPTS-I-BCM, AEAPTS-T-BCM respectively (SI, Fig.-S1). However, APTEST-GCM, AEAPTS-GCM samples showed the absence of nitrogen in the sample (SI, Fig.-S2). The X-ray diffraction pattern of the BCM (SI Figure. S2) showed the Bragg diffractions at 2θ = 16, 22 and 34º corresponding to (101), (002) and (040) facets of carbon.33 TGA results showed some early weight loss up to ~130 °C in all the cases due to the physisorbed water (Figure-3). Furthermore, about 5% weight loss in the range of 130-325 °C could be due to the adsorbed organic moieties on the surfaces of carbon microspheres. Additionally, when samples were heated in the range of 325-380 °C rapid weight loss was found in all the samples. However, the loss was different in all the grafted BCM, where APTES-I-BCM and APTES-T-BCM samples gave 9.88% and 14.43% weight loss respectively. Also, AEAPTS-I-BCM and AEAPTS-T-BCM sample were observed with 9.86% and 13.48% weight losses respectively. This indicates that the functionalization was more in toluene.



100 90 % weight loss

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

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80 70 60 a

50

b d c e

40 30

100

200

300

400

500

600

o

Temperature in C

Figure 3. Thermogravimetric analysis (TGA) profile of a) BCM; b) APTES-I-BCM; c) APTES-T-BCM; d) AEAPTS-I-BCM and e) AEAPTS-T-BCM. The FTIR analysis (Figure. 4 and full range spectra SI, Fig.-S3) of all the samples showed distinct vibrational bands of Si-O, C-C, C-H, N-H and Si-C in the fingerprint region and overlapped with carbon bands hence parent carbon spectrums was subtracted from grafted carbons. The FTIR band at 897, 713 cm−1 can be assigned to C−H out of plane bending vibrations and the bands at 1459 cm-1 can be assigned to C-H deformation, while the band at 1334 cm-1 was assigned to C-N stretching vibration, whereas bands at 987 and 713 cm-1 were assigned to C-H out of plane bending vibrations. The bands at 1705 and 1609 cm-1 can be attributed to C=O (carbonyl, quinone, ester, or carboxyl) and C=C respectively. The additional peaks include the -Si-O-C symmetric stretching at 1030 cm-1 and bending mode of –CH2 groups at 1300 cm-1. The Si-C vibrational band was observed at 1166 cm-1. As the carbon chain length increased, the Si-C stretching frequency shifted to shorter wave number. Si-O-Si and Si-O bonding was typically seen in the range 1130-1000 cm-1 which shifted to 1110 and 1060 cm-1 when grafted onto carbon spheres.34


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1705

1609

1369 1110 1030 1459 1334 1166 1060 985 897

670 713 e

Absorbance

a b

c d

1800

1600

1400

1200

1000

800

600

Wavenumber (cm-1)

Figure 4. FTIR spectra of a) APTES-I-BCM; b) APTES-T-BCM; c) AEAPTS-I-BCM; d) AEAPTS-T-I-BCM after subtraction and e) BCM. The backbone structure of carbon spheres prepared from bagasse and d-glucose precursors was investigated using 13C CP-MAS NMR spectroscopy. Figure-5a showed peaks around 50-60 ppm, 70-75 ppm, and 80 ppm due to cellulose carbon. A weak peak at 150 and 110 ppm corresponds to furanic type backbone of carbon structures. Weak NMR peak at 205 ppm corresponds to the carbonyl carbon. The NMR analysis of carbon prepared from glucose precursor showed two major peaks: one at 150 and 110–118 ppm due to the furanic rings, and another peak at 205 ppm due to carbonyl carbon (SI, Fig.-S4).32 From this analysis, we can conclude that there are more sp2 carbons present in GCM.

8.42

19.83

Ankush_CCS_13C-7.jdf

40.57

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


a b c d e 160

140

120

100 80 60 Chemical Shift (ppm)

40

20

0

Figure 5. 13C NMR of the spectrum of a) APTES-I-BCM; b) APTES-T-BCM; c) AEAPTSI-BCM; d) AEAPTS-T-BCM and e) BCM.



The successful grafting of amines onto BCM was confirmed by 13C CP-MAS and 29Si MAS NMR analysis (Figure. 5-6). The APTES grafted carbon spheres showed the additional peaks at 9 ppm corresponding to the propyl carbons of (C-O-Si-CH2-CH2-CH2-NH2), 19 ppm (C-O-Si-CH2-CH2-CH2-NH2), and 40 ppm (C-O-Si-CH2-CH2-CH2-NH2).35 Also, AEAPTS amine grafted carbon spheres showed the peaks at 8 ppm corresponds to the propyl carbons of (C-O-Si-CH2-CH2-NH-CH-CH2-NH2), 17 ppm (C-O-Si-CH2-CH2-NH-CH-CH2-NH2), and 38 ppm (C-O-Si-CH2-CH2-NH-CH-CH2-NH2). No additional peak for (C-O-Si-CH2-CH2-NHCH-CH2-NH2) was observed due to overlapping of backbone carbon.

0

-20

-40

-103.28 -111.08

-69.89

29Si-3.001.1r.esp

-62.33

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

-60 -80 -100 Chemical Shift (ppm)

-120

-140

Figure 6. 29Si MAS NMR AEAPTS-T-BCM sample. The representative solid-state

29Si

NMR spectrum of AEAPTS grafted in toluene is

displayed in Figure. 6 and other spectra are given in Figure. S5. All spectra showed a very weak NMR peak at -112 and -103 ppm which corresponds to the Q4 and Q3 silicate species. This NMR study indicates no self-condensation of both the silica precursors occurred. The strong peak at -69 ppm corresponding to T3 silicates of (C-O)3-Si-(CH2)3-NH2 and a shoulder at -63 ppm corresponding to (C-O)2-Si(OH)-(CH2)3-NH2 of T2 silicate species was observed. This result suggests that more silane may covalently attach to the carbon surfaces. Furthermore, 13C

CP-MAS (SI, Fig.-S4) and

29Si

MAS NMR analysis (SI, Fig.-S6) of APTES-GCM and

AEAPTS-GCM samples showed the absence of propyl and Q4 and Q3 peaks in the respective analysis, which directly confirmed that grafting of amines did not occur onto glucose-derived


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carbon. The low signal to noise ratio can be attributed to the absence or low grafting of the amines onto the glucose-derived carbon.

Intensity (a.u.)

C1s

1000

800

600

(c)

(b)

O1s

400

200

0

Intensity (a.u.)

(a) Intensity (a.u.)

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


292

290

288

286

284

282

Binding Energy (eV)

Binding Energy (eV)

540

538

536

534

532

530

528

526

Binding Energy (eV)

Figure 7. XPS spectra of carbon microsphere (BCM) a) Survey mode; b) deconvoluted C1s spectra of carbon and c) O1s spectrum. Figure 7 and8 displays the X-ray photoelectron spectra of BCM and APTES-BCM. XPS spectrum of AEAPTS-BCM is given in supporting information (SI, Fig. S7). All the spectra showed the presence of carbon at BE 284.6 ± 0.3 eV. The broad survey scan of ungrafted carbon (Figure7a) demonstrated the presence of C1s and O1s peak. The deconvoluted C1s spectrum of BCMs is shown in Figure. 7b. BCM showed three different components in C1s peaks. A major part of the peak centered at 284.6 eV, which corresponds to sp2 C-C in graphitic carbon. The second peak at 286.2 eV was associated with C-O carbon and a third minor peak at 287.8 eV can be assigned to C=O carbon. The O1s spectra (Figure. 7c) revealed the presence of two peaks at 532.7 and 534 eV corresponding to -C-O and –C=O. These results indicate that the BCM has a backbone with sp2 carbon and some functional groups such as -C-O, -COOH, or C-O-C present on the surface or in the framework of carbon.



O1s

Intensity (a.u)

Intensity (a.u.)

C1s

Intensity (a.u.)

N1s Si 2p 1000

800

600

400

200

0

290

288

Binding Energy (eV)

286

284

404

282

402

400

398

396

Binding Energy (eV)

Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

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

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538

536

534

532

530

528

Binding Energy (eV)

526

106

104

102

100

98

96

Binding Energy (eV)

Figure 8. XPS spectra of APTES-T-BCM a) Survey mode; b) deconvoluted C1s spectra of carbon; c) N1s spectrum; d) O1s spectrum and e) Si2p spectrum. The survey mode analysis of APTES-BCM (Figure. 8a) and AEAPTS-BCM (SI, Fig.S5) showed the presence of C, Si, N and O. The deconvoluted C1s spectra’s for both, APTESBCM and AEAPTS-BCM, revealed four different components in a C1s peak at 284.3, 285.7, 286.6 and 287.7 eV which corresponds to sp2 carbon, -C-N, -C-O, -COOH, or C-O-C carbon respectively. The O1s XPS spectra’s (Figure 8d and SI, Fig.-S7d) gave similar results as that of BCM. However, the emergence of a new peak at 531.7 eV can be attributed to –Si-O bond. The binding energy at 102 eV corresponds to the presence of Si2p. The N1s spectra of APTESBCM (Figure 7c) revealed a single peak at 399.8 eV corresponding to the -C-NH. The N1s XPS peak for AEAPTS-BCM (SI, Fig. S7) at 400.38 and 399.2 eV corresponding to C-NHand C-NH2 bond respectively.34 In both samples, XPS spectra showed lower binding energy corresponds to carbon spheres. This could be the grafting of primary and secondary amine over carbon microsphere. The lower binding energy of N1s XPS peak of secondary amine could be due to the presence of electron donating group attached to nitrogen (-NH-CH2). Thus the


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combined results of XPS showed the successful grafting of the APTES and AEAPTS on carbon microsphere. All the above characterization results of FTIR, NMR, XPS, and TGA confirmed the successful formation of spherical carbon microspheres with 4-5 µM diameter from sugarcane bagasse with hydrophilic surfaces consist of -OH, -COOH, -C=O functional groups and covalently anchoring of propylamine onto the surfaces of carbon microspheres to obtained the solid base catalysts. The synthesized base catalysts were used for the Henry reaction. This reaction is one of the key steps in the formation of C-C bonds. For instance, hydroxynitroalkanes, 1, 2-amino alcohols and hydroxycarboxylic acids, have been used for the synthesis of pharmaceutical products,

insecticides,

fungicides,

bactericides,

rodent-repellent,

antitumor

agents,

prostaglandins, pyrroles, and porphyrins.36-39 This reaction is known to be catalyzed by various homogeneous (bases, organometallic) and heterogeneous catalysts.40-46 This signposts the growing interest in the newer heterogeneous base catalyst development. In this report, we have synthesized and characterized the carbon spheres from readily available sugarcane bagasse as well as D-glucose for comparison and covalent functionalization by organo-amine onto the carbon surfaces via condensation of the hydroxyl group with trimethoxysilanes. The asprepared solid base organocatalyst was further employed for Henry reaction. The catalytic activity of prepared catalysts was investigated for base catalyzed Henry reactions (Figure.9). NO2

OH CHO CH3NO2 X

Catalyst, Temp.

X

X X= H, OH, NO2

NO2

NO2

A

Figure 9. Schematic representation of the Henry reaction.


NO2

X B

C


As such there are no reports on direct functionalization of amines over as-synthesized carbon and reaction results can’t be compared directly. But, in the related family of carbons including graphene oxide (GO) grafted with different amines were used as base catalysts in C-C bond forming a reaction. For instance, Xue et al. reported functionalization of ethylene diamine over GO and utilized for Knoevenagel condensation reaction with excellent catalytic activity.47 Furthermore, acid-base bifunctional organoamines grafted graphene oxide (GO) was used for one-pot Henry-Michael reactions with good activity to give synthetically valuable multifunctionalized nitroalkanes.48 Zhang et al.49 have reported amine-functionalized graphene oxide prepared by a facile one-step silylation approach and used as an acid−base bifunctional catalyst in one-pot cascade reactions containing successive acetal hydrolysis and Knoevenagel condensation. Where 95% of acetal conversion with 95% selectivity for (E)-ethyl-2-cyano-3phenyl acrylate has been reported.50 Fan et al.51 have reported the synthesis of primary and tertiary amines grafted on graphene oxide (GO) for typical trans-β-nitrostyrene forming reaction. They have reported 99.5% benzaldehyde conversion and 100% trans-β-nitrostyrene selectivity. In another report, acid-base bifunctional catalyst with acid, the ureidopropyl (UDP) group, and 3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP) group was grafted onto GO surfaces and utilized for Henry reaction. The yield of the α,β-dinitrostyrene was 88.1%.52 Still, a significant challenge remains for large-scale synthesis of GO, use of hazardous chemicals and associated pollution. Hence, it is expected that our methodology can be a good example for the synthesis of carbon-based catalysts. The reaction of substituted benzaldehydes with nitroalkanes in the presence of solid base undergoes condensation via ion pair or imine formation and yields hydroxyl nitrostyrene (A), β-nitrostyrene (B) or Michael product (C).41 First, the reaction was carried out using pnitrobenzaldehyde, nitromethane as substrates with unfunctionalized BCM to understand the role of parent carbon on the Henry reaction. Even after stirring the reaction mixture for 24 h. it


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didn’t show any conversion (Table 1 entry 1). This result indicates that there is no role of parent carbon on the reaction. Furthermore, APTES-T- GCM was used as a catalyst then to our surprise no conversion was obtained. Careful analysis of catalyst by 13C CP-MAS, 29Si MASNMR, and SEM-EDAX (SI, Fig. S5) showed the absence of propyl carbon and nitrogen in the sample. The same fact was also revealed by FTIR spectra (SI, Fig. S9). This result suggests that amine was not grafted onto GCM. When APTES-T-BCM was used as a catalyst, it gave 78% conversion in 35 h with exclusively β-nitrostyrene as a product (100%), whereas APTESI-BCM gave 100% conversion in 35 h with 29% selectivity for β-nitro alcohol and 71% selectivity for β-nitrostyrene (Table 1, entries 3 and 4). The enhancement in the conversion using IPA grafted sample was due to the effect of the amine as well as surface hydroxyl groups. When amines were grafted in IPA, it resulted in less amine loading, and some hydroxyl groups remain unchanged, which assisted cooperatively in enhancing the rate of reaction. Whereas, a change in selectivity pattern of the product using the more amine grafted samples were observed and which could lead to further dehydration of β-nitro alcohol product.35 Table 1. Results of Henry reaction using various amine functionalized CM catalysts. [a] Entry Benzaldehyde Catalyst Time (h) % Conv. Sel.% TOF h-1 A B 1.

p-NO2

BCM

24

0

-

-

-

2.

p-NO2

APTES-TGCM

35

0

-

-

-

3.

p-NO2

APTES-TBCM

35

78

-

100

0.4

4.

p-NO2

APTES-IBCM

35

100

29

71

0.6

5.

m-NO2

APTES-IBCM

35

92

32

68

0.5

6.

o-NO2

APTES-IBCM

35

28

40

60

0.2



Page 18 of 29

7.

p-NO2

AEAPTS-TBCM

9

99

100

-

1.3

8.

p-NO2

AEAPTS-IBCM

9

68

100

-

1.2

9.

-H

APTES-TBCM

6

40

59

41

1.7

10.

-H

AEAPTS-TBCM

4

98

43

57

4.2

11.

p-OH

APTES-TBCM

0.5

100

100

0

44.1

12.

p-OH

AEAPTS-TBCM

0.5

100

6

94

29.6

13.

m-OH

AEAPTS-TBCM

0.5

100

-

100

44.1

14.

m-OH

AEAPTS-TBCM

0.5

100

-

100

44.1

15b.

-OH

NH2-SBA15

14

57

8

92

20.5

[a]

Reaction condition: Substrate: 0.01 mol; nitromethane: 8 mL; catalyst: 10 wt% of the

substrate; temperature: 90 °C; T- toluene, IPA-isopropyl alcohol.

[b]catalyst

used was

primary amine grafted SBA-15 (See SI, S12 for synthesis procedure). The reactivity of nitro at a different position on benzaldehyde was investigated. The electron withdrawing nature of the nitro group in the nitrobenzaldehyde enhances the electrophilicity of the carbonyl group, thus increasing rate of reaction to give β-nitrostyrene as a sole product and the order of reactivity was found to be p > m >o. The higher activity of parasubstituted nitro benzaldehyde gives better yield due to nitro having strong electron withdrawing character it attracts all electron density from aldehydic carbonyl which was enhanced electrophilicity. (Table 1, entries 4-6). When AEAPTS-T-BCM was utilized for the same reaction, it gave 99% conversion in only 9 h with 100% selectivity for β-nitro alcohol (Table 1, entry 7). This result suggests that even in the presence of primary and secondary amines on the same sample, only AEAPTS


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amine dominates the reaction to give the hydrated product. When used AEAPTS-I-BCM as a catalyst, it gave as expected less conversion (68%) without affecting the selectivity. Furthermore, when benzaldehyde was used as a substrate, it gave 40% conversion of benzaldehyde in 6 h with 59 and 41% β-nitrostyrene and nitroalcohol selectivity using APTEST-BCM catalyst (Table 1, entry 9). However, for the same reaction when using AEAPTS-TBCM gave about 98% benzaldehyde conversion 43 and 57% selectivity for hydroxyl nitrostyrene and nitrostyrene product respectively in 4 h (Table 1, entry 10). Furthermore, when electron donating substituent was present on the substrate (p-hydroxybenzaldehyde), very high conversion (100%) of the substrate was obtained in 30 min, but the product received was different in both the APTES-T-BCM, AEAPTS-T-BCM catalyst (Table 1. entries 11 and 12). In the case of hydroxyl-substituted benzaldehydes, the electron donating ability of hydroxyl group decreases the reactivity of the carbonyl group which is in accordance with the observed results which gave β-nitrostyrene as a sole product when used AEAPTS-T-BCM catalyst and order of reactivity was found to be m > p > o. In the case of ortho- hydroxyl benzaldehyde, it may have intramolecular hydrogen bonding like 2-hydroxy benzaldehyde which decreases the reactivity compare to para and meta-substituted benzaldehyde. (Table 1, entries 13 and 14). These results are also consistent with the literature reports that the APTES and AEAPTS amines as bases operate with different reaction mechanism and given different products.36 The efficacy of the synthesized materials was also compared with the amine grafted SBA-15 (NH2SBA-15) for the Henry reaction. The use of amine grafted SBA-15 as a catalyst gave only 57% aldehyde conversion in 14 h with 92% and 8% selectivity towards β-nitrostyrene and nitroalcohol respectively (Table 1, entry 15). The low activity attributed to amine grafted silica was due to the possibility of amines grafted inside the pores of the SBA-15 channel and difficult to diffuse the reactant may lead to a slower reaction rate.



The reusability of the catalyst was also tested using the best catalyst, i.e., AEAPTSBCM and p-nitrobenzaldehyde as a substrate, notably 70% conversion of reactant were observed. Our results showed that even after five cycles catalyst gives consistent conversion and selectivity (SI, Table S1). The amine contain remained almost the same after the reaction. (SI-13, Fig. SI12 &13).

CONCLUSIONS Carbon microspheres (BCM) were successfully synthesized from agricultural waste residue (sugarcane bagasse) by a low-temperature hydrothermal process (LTH) using oxalic acid as a dehydrating agent in one step. The obtained BCM were uniform in size and bares many -OH, -COOH, C=O functional groups, that provide ease in functionalization to create a hydrophilic surface using alkylamine which generated basic catalyst. This solid base catalyst was highly efficient for the Henry reaction. The conversion and selectivity was dependant on substrates and the type of catalysts used in the reaction. The catalyst was successfully recycled up to five cycles without losing activity and selectivity. This result suggests that naturally, abundant available carbon resources could replace silica precursors for the synthesis of the catalyst support. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ Synthesis of SBA-15 and functionalization protocol, EDAX profiles of various carbons, XRD of BCM, 13C and 29Si Solid-state NMR of BCM and GCM samples, XPS of AEAPSTBCM, HR-TEM of GCM, FTIR of BCM’s and GCM’s, Reusability table for AEAPST-BCM, GC-MS profile of the product mixture and 1H and 13C NMR of isolated product.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through the contributions of all authors. All authors have approved to the final version of the manuscript. Funding Sources Science & Engineering Research Board India under the project SERB/F/2139/2017-2018. Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS AVB acknowledges finical assistant SERB (DST) Government of India for EMR project no SERB/F/2139/2017-2018. Analytical and Environmental Science Division and Centralized Instrument Facility for providing all the necessary analytical facility. CSIR-CSMCRI Communication No. 145/2017. REFERENCES 1. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature, 1992, 359(6397), 710-712, DOI 10.1038/359710a0. 2. Wight, A. P.; Davis, M .E. Design and preparation of organic-inorganic hybrid catalysts. Chem. Rev., 2002, 102(10), 3589-3614, DOI 10.1021/cr010334m. 3. Azevedo, D. C.; Araujo, J. C. S.; Bastos-Neto, M.; Torres, A. E. B.; Jaguaribe, E. F.; Cavalcante, C. L. Microporous activated carbon prepared from coconut shells using chemical



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Bio-waste (Bagasse) was used for the synthesis of a solid base catalyst which showed comparable activity in Henry reaction 271x125mm (150 x 150 DPI)


Utilization of Waste Biomass for the Synthesis of Functionalizable

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