Functionalized carbon dot adorned coconut shell char derived green catalysts for the rapid synthesis of amidoalkyl naphthols

Accepted Manuscript Functionalized Carbon Dot Adorned Coconut Shell Char Derived Green Catalysts for the Rapid Synthesis of Amidoalkyl Naphthols Divya P. Narayanan, Sudha Kochiyil Cherikallinmel, Sugunan Sankaran, Binitha N. Narayanan PII: DOI: Reference:

S0021-9797(18)30233-9 https://doi.org/10.1016/j.jcis.2018.02.077 YJCIS 23349

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

31 October 2017 24 February 2018 27 February 2018

Please cite this article as: D.P. Narayanan, S. Kochiyil Cherikallinmel, S. Sankaran, B.N. Narayanan, Functionalized Carbon Dot Adorned Coconut Shell Char Derived Green Catalysts for the Rapid Synthesis of Amidoalkyl Naphthols, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.02.077

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Functionalized Carbon Dot Adorned Coconut Shell Char Derived Green Catalysts for the Rapid Synthesis of Amidoalkyl Naphthols Divya P Narayanana, Sudha Kochiyil Cherikallinmela, Sugunan Sankaranb, Binitha N Narayanana* a

Department of Chemistry, Sree Neelakanta Government Sanskrit College Pattambi, Palakkad-679306, Kerala, India Ph: +91 466-2212223. Fax: +91 466-2212223, *[email protected]

b

Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 22, Kerala, India

Abstract A one pot synthesis of carbon dot incorporated porous coconut shell char derived sulphonated catalyst is reported here for the first time and is effectively used in the multicomponent synthesis of amidoalkyl naphthol. Macroporous nature of the char is revealed from scannig electron microscopic (SEM) analysis, whereas the dispersion of the carbon dots (CDs) on the porous coconut shell char is confirmed from the high resolution transmission electron microscopic (HRTEM) analysis. Fluorescence emission spectrum further confirmed the presence of CDs in the catalyst. Fourier-transform infrared (FTIR) spectral analysis of the materials indicated that sulphonation occurred both to the carbon dot and to the porous char. Xray photo electron spectroscopic (XPS) analysis of the active catalyst confirmed the presence of both sulphonic acid and carboxylic acid groups in the catalyst. The coconut shell char derived materials prepared by varying the amount of H2SO4 are successfully utilized as efficient alternative green catalysts for the multicomponent reaction, where excellent activity in amidoalkyl naphthol synthesis is obtained within short periods under solvent free reaction conditions. A maximum yield of 98% is obtained in the synthesis of N-[Phenyl-(2-hydroxynaphthalen-1-yl)-methyl]-benzamide, the representative amidoalkyl naphthols, with one of the present catalyst within 3 min of reaction. The catalyst is highly active for the reactions carried out with varieties of aldehydes and amides with a product yield in the range of 88-98%. The best catalyst system retained more than 90% of its initial activity even after 6 repeated runs.

1

Keywords: Carbondots; porous materials; heterogeneous catalysis; multicomponent reactions; solvent free synthesis. Introduction To protect the environment, modern synthetic chemists should strictly follow the principles of green chemistry while conducting organic transformations and should stick on to developing new chemical methods with atom economy, without the use of hazardous and toxic solvents or chemicals [1]. In chemical industry, acid catalyzed reactions play a major role and inorganic mineral liquid acids such as H2SO4, HNO3, HF, etc are extensively being used as homogeneous catalysts in many organic syntheses [2]. However these acid catalysts are highly toxic and corrosive in nature and also homogeneous catalyzed reaction faces the problem of difficulty in the separation of the product after the reaction. The use of heterogeneous catalysts had evoked extensive research interest owing to their non-toxicity, ease of handling, reusability and environmentally benign nature [3, 4]. The main limitation of the conventional heterogeneous catalysts is the low accessibility of reactant molecules to the active sites of the catalyst surface. This hurdle can be conquered by supporting the active materials on suitable porous materials such as clays, silica, zeolites, activated carbons etc. [5-10]. Immobilization of the active functionalities present in homogeneous catalysts on the solid supports garners great applause because it makes the catalyst heterogeneous. From the economic and environmental point of view, they gain much attention due to their ease of separation and reusable nature. Development and the use of highly efficient green catalysts from renewable sources have been intensified in recent years. Waste materials from agricultural operations and abundant available natural materials can act as potential sources of many low cost solid supports [11]. Among the various support materials, carbon based materials are well suited since they have advantages such as high chemical stability, thermal stability, ease of recovery from the reaction mixture, easy availability and low cost [12]. Carbon based materials from agricultural wastes are widely studied as adsorbents and it can also be used as an efficient catalyst support [13-15]. The tunable porosity and surface chemistry of carbonaceous substances are key factors that help them to function as better support materials in catalytic applications. Coconut shell (Cocos nucifera L) is an agricultural solid waste and is found vastly all around the tropical regions [16]. Different utilization avenues of coconut shell are well studied [17-19]. The most 2

effective use of coconut shell is the utilization of its char as an adsorbent, especially in pollutant removal [20-22]. Kaur and Kaur have reported a review on effective application of coconut shell as coarse aggregates in mass concrete [23]. In 2015, Hidayat et al. reported the catalytic application of sulphated coconut shell char for the synthesis of biodiesel by the esterification of palm fatty acid distillate [24]. Recently, Azizah et al reported the production of biodiesel over coconut shell derived solid acid catalyst [25]. Multicomponent reactions (MCRs) have acquired a spectacular position in the synthetic organic chemistry because of their inherent peculiarities like synthesis of complex molecules by the formation of several bonds in a one pot process without isolating the intermediates or without the addition of further reagents. [26-29]. The use of better synthetic methods in the existing MCRs has gained great interest in recent years. Among the various MCRs, amidoalkyl naphthol (AAN) synthesis by the condensation of an aromatic aldehyde, beta naphthol and an amide is one of the most studied reactions due to the immense biological, medicinal and pharmacological activities of amido alkyl naphthol derivatives [30-34]. Thus the development of new methods with high efficiency for the synthesis of AANs is of supreme importance. Many new synthetic strategies involving catalysts such as Ce(SO4)2 [35], I2 [36], RuCl2(PPh3)3 [37], POCl3-Na2B4O7 [38], H4SiW12O40 [39], ZnO [40], HClO4-SiO2 [41], MoO3-ZrO2 [42], chloroaceticacid [43], nickel-doped SnO2 [44], hexanesulphonic acid [45] etc have been reported in amidoalkyl naphthols synthesis. Even while these methods have a lot of potential, they have some of the hurdles to attend in the synthesis such as prolonged reaction time, low product yield, harsh reaction conditions, requirement of expensive and/or non reusable catalyst, and incongruity with green chemistry protocol. Therefore, huge efforts have been and still are being made to find new highly efficient environmentally benign catalysts for amidoalkyl naphthol synthesis. As a part of our investigations in the development of environmentally benign catalysts for MCRs [46], herein we report the catalytic application of sulphonated coconut shell char in the multicomponent amidoalkyl naphthol synthesis for the first time. Sulphonation of the coconut shell char additionally lead to the formation of sulphonic acid and carboxylic acid functionalized carbon dots (CDs) in the developed functionalized porous coconut shell char. The obtained CDs have the size limit of 2-10 nm [47]. The reaction was performed under solvent free conditions and the catalyst was found to be highly efficient and reusable. 3

2. Experimental 2.1 Materials Coconut shell was procured from local coconut farms. All other chemicals used in the present study, their suppliers and purity were as follows. H2SO4 (NICE chemicals Pvt Ltd, 98%), benzaldehyde (NICE chemicals Pvt Ltd, 98%), β-naphthol (NICE chemicals Pvt Ltd, 98% ), benzamide (NICE chemicals Pvt Ltd, 98% ), acetamide (NICE chemicals Pvt Ltd, 97% ), 4chloro benzaldehyde (NICE chemicals Pvt Ltd, 99%), 4-hydroxy benzaldehyde (Loba Chemie Pvt Ltd, 99%), 4- methoxy benzaldehyde( Loba Chemie Pvt Ltd, 98%), 4-methyl benzaldehyde (NICE chemicals Pvt Ltd, 98%), 2- nitro benzaldehyde (Spectrochem, 99%), 3- nitro benzaldehyde (NICE chemicals Pvt Ltd, 98%), 4- nitro benzaldehyde (Spectrochem, 98%) and urea (NICE chemicals Pvt Ltd, 99%). All chemicals were used without further purifications. 2.2 Preparation of sulphonated coconut shell char The coconut shell was cleaned, washed with deionized water, dried and burned in open air to obtain coconut shell char (CSC). For sulphonating the coconut shell char, 30 weight % of conc. H2SO4 was added in to CSC and the mixture was stirred at 150 °C for 5 h. After cooling, the contents were poured into 100 ml distilled water. The modified char was then filtered and washed thoroughly with distilled water until the filtrate was is free from sulphate ion so as to remove the unbound acid. The precipitate was dried and further subjected to a heat treatment at 150 °C for 3 h. The prepared sulphonated coconut shell char was then designated as SCSC30. The other systems such as SCSC10, SCSC20 and SCSC40 were also prepared in the same way as described above but by varying the amount of sulphuric acid. Here the numbers indicate the weight (g) of sulphuric acid taken to sulphonate 1 g of CSC. For the separation of CDs from the prepared catalyst, 10 ml isopropanol was added into 100 mg of SCSC30 and it was sonicated for 2 h, which resulted in the dispersion of CDs. The settled char was removed via filtration.

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2.3 Characterization techniques The prepared catalysts were characterized using various techniques. Functional groups on the catalysts were determined using Fourier Transform Infrared (FTIR) Spectroscopy on Jasco FT/IR (4100) spectrophotometer with KBr pellet method. X-ray diffraction (XRD) measurements of the catalysts were performed on Rigaku Miniflex 600 X-ray powder diffractometer equipped with Cu Kα radiation. XPS analysis of the active catalyst SCSC30 was done over AXIS ULTRA X-ray Photoelectron Spectrometer (KRATOS ANALYTICAl) using C1s at 284.6 eV as internal standard. For the investigation of surface morphology of the prepared systems, SEM images were taken using JEOL model JSM-6390LV. HRTEM images of the representative catalyst sample was taken using JEOL/JEM 2100. The fluorescence analysis of SCSC30 was done using HORIBA FLUOROLOG Fluorescence Spectrometer. The percentage of elements C, H, N and S in the synthesized catalytic systems, CSC, SCSC10, SCSC30 and SCSC40 were done on the CHNS analyzer of model Elementar Vario EL III. 2.4 Synthesis of amidoalkyl naphthols A typical procedure for the synthesis of 1-amidoalkyl-2-naphthols over sulphonated coconut shell char was performed as follows unless otherwise mentioned. To 0.1 g of SCSC, 1 mmol of benzaldehyde, 1mmol of β-naphthol and 1.3 mmol of benzamide were added and heated at 125 °C in an oil bath under solvent free condition and stirred until the mixture became solidified. The time required for solidification is taken as the duration of the reaction. After the contents got completely solidified and further mixing became impossible, hot ethanol was added to dissolve the organic compounds and the mixture was filtered to remove the catalyst. The alcohol was then separated and the crude product thus obtained was purified by recrystallization using ethanol. The products were identified by H1 NMR and comparison of its melting points with standard data. 2.5 Reusability of sulphonated coconut shell char The reusability studies of the synthesized sulphonated coconut shell char were done by conducting the reaction between benzaldehyde, β-naphthol and benzamide over SCSC30. After each run, hot ethanol was added to the reaction mixture to dissolve the reaction mixture and the catalyst was filtered and washed several times with hot ethanol. The catalyst was then dried and 5

treated at 150 °C for 3 h before each subsequent reaction. The catalyst was effectively reused up to 6 repeated cycles. 3

Results and Discussions Present procedure targets the development of highly efficient environmentally benign

sulphonated coconut shell char derived catalysts and their potential application for multicomponent amidoalkyl naphthol synthesis. Here coconut shell char was effectively used as an active support for hanging sulphonic acid groups. Acid treatment of coconut shell lead to the formation of functionalized carbon dot adorned porous coconut shell char. The schematic representation of formation of functionalized carbon dot adorned coconut shell char catalyst is shown in scheme 1. The plausible mechanism of material formation is as follows. Coconut shell contains cellulosic and lignin polymeric units and the pyrolysis of coconut shell resulted in the removal of water moieties with the formation of complex amorphous carbon network [48]. The oxidative acid treatment leads to the breakdown of carbon structures into nanostructured small domains of carbon atoms ie. the carbon dots [49]. It also leads to the oxidation and leaching of some carbonaceous network as well as acid sensitive components, resulting in the formation of macropores. In addition, treatment with concentrated sulphuric acid leads to the anchoring of -SO3H and -COOH groups to the surface of the carbon species [49].

6

Scheme 1: Schematic representation of formation of SCSC catalyst The synthesized systems were characterized by various techniques such as FTIR spectroscopy, XRD, XPS analysis, SEM, TEM and CHNS analyses so as to investigate the active centers responsible for catalysis. FTIR spectral and fluorescence measurements of the separated CDs were also taken. 3.1 Structural characterization of the catalyst In order to assess the assimilation of acidic groups on the synthesized catalysts, the FTIR spectra (Fig. 1) of coconut shell char and sulphonated systems have been analyzed. The FTIR spectra revealed the successful anchoring of -SO3H group on the carbon surface. The most prominent band responsible for sulphonic acid group is observed at 1170 cm-1, which arises due to the symmetric stretching vibrations of -SO3H group. The band at 1030 cm-1 is resultant of the presence of -S=O moiety [50-52]. In addition, the band around 1710 cm-1 indicates the presence of carbonyl groups suggesting the presence of carboxylic acid functionality.

7

Fig. 1. FTIR spectra of CSC and Sulphonated CSC catalysts Fig. S1 represents the XRD patterns of raw coconut shell char and sulphonated coconut shell char catalysts. The broad peak points out the absence of well-ordered crystalline phase and confirms the amorphous nature of the catalysts. As the percentage of sulphonation increases, the intensity of the diffraction bands increases, thereby suggesting a path of conversion towards crystalline nature from the disordered amorphous phase. The broad peaks around 2θ values of 16° to 30° and 40° to 50° can be due to the diffraction from (002) and (100) planes of amorphous carbon with a low amount of graphitic layers [53, 54]. These broad peaks may be aroused as a result of polymerization of amorphous carbonaceous matter such as lignin, hemicelluloses and amorphous cellulose present in coconut shell char after their decomposition during ignition. XPS analysis of SCSC30 catalyst was performed to find out the functional groups present in the material. The XPS analysis of SCSC30 confirmed the presence of C, O and S in the sample. The wide spectra and deconvoluted C, O and S spectrum are given in Fig. 2. The C (1s) spectrum is deconvoluted in to four peaks centered at 284.4 eV, 285.1 eV, 286.2 eV and 288.7 8

eV attributed to the sp2 hybridized carbon, sp3 hybridized carbon, C-O/C-S bond and -COOH carbon respectively [55-57]. The results indicate that the treatment of char with H2SO4 resulted in the formation of more sp2 hybridized carbon atoms [54]. There are some reports that highlighted the formation of -COOH groups in sulphuric acid treated chars [56, 57]. The deconvoluted oxygen spectrum composed of three peaks located at 531.0 eV, 531.8 eV and 533.2 eV that can be assigned to -COOH, S-OH and -S=O functionalities respectively [55-57]. The deconvoluted S spectra showed two prominent binding energy curves at 168.4 eV and 169.6 eV attributed to S2p(3/2) and S2p(1/2) binding energies of sulphonic acid group (- SO3H). The peak at 167.1 eV can be assigned to the binding energy of C-S bond [56]. Therefore XPS measurements confirmed the presence of both -SO3H and -COOH acidic functionalities in SCSC30.

Fig. 2. (a) XPS survey spectrum of SCSC30, deconvoluted spectrum of (b) C (1s), (c) S (2p) and (d) O (1s). 9

Fig. 3. SEM images of CSC and SCSC catalysts SEM photographs (Fig. 3) show the lumps of coconut shell char. The images show a change in morphology in the sulphuric acid treated coconut shell char systems. Sulphonated coconut shell chars have macroporous morphology. The harsh acid treatment may lead to the 10

formation of several macropores in the catalytic systems due to the leaching of acid sensitive components. The porous nature (cavernous pores) increases with increase in the percentage of H2SO4 taken for the preparation, which is evident from the images (Fig. S2). The presence of cavernous pores can help in the easy adsorption of reactant molecules in the catalyst surfaces so as to promote the catalytic reaction [56].

Fig. 4: HRTEM images of SCSC30 [(a) and (b)] and Carbon dots [(c) and (d) The TEM images of SCSC30 revealed (Fig. 4 (a) and (b)) the porous agglomerated nature of the catalyst with some horn like structure. The TEM images also revealed the most fascinating result obtained by the treatment of coconut shell char with sulphuric acid. 11

Sulphonation led to the formation of carbon dots in the synthesized catalytic system as seen in the images. Carbon dots are dispersed in the catalyst. Upon continuous sonication in isopropyl alcohol (IPA), the carbon dots preferentially got dispersed whereas the char got deposited. Figure (5 (c) and (d)) shows the TEM image of the separated carbon dots. It is found that the carbon dots are of various sizes with diameter within the range of 5-10 nm (Fig. S3). There are reports on the formation of carbon dots by the acid treatment of activated carbon [58]. Since it is expected that sulphonation can be the major reason for the catalytic performance of coconut shell char, we investigated the presence of sulphonic acid groups both in the deposited char (settled SCSC30) as well as in the dispersed carbon dot so as to find out the active component of the catalyst material. The FTIR spectra (Fig. 5) indicated the presence of sulphonic acid and carboxylic acid groups in the dispersed carbon dot as well as on the deposited porous char. Sonication is the only way to separate the dots; and thus the catalyst can be stable in the reaction. Thus the active catalyst of the present study is found to be functionalized carbon dot adorned porous functionalized coconut shell char as evident from the characterization results discussed above.

Fig. 5. FTIR spectra of SCSC30, settled SCSC30 and separated carbon dot

12

Fig.6. Fluorescence emission spectra of CDs For the confirmation of presence of carbon quantum dots (CQDs) in the prepared catalyst SCSC30, fluorescence emission spectra of the dispersion obtained from SCSC30 were taken (Fig. 6). From the spectra it can be seen that CDs gave strong emission between 350 nm and 600 nm. The maximum emission intensity of CDs is observed at 437 nm with an excitation wavelength of 330 nm. The fluorescence spectra obtained in the present study is comparable with the reported data of CDs [58]. Thus fluorescence emission spectrum further confirmed the presence of CDs in the SCSC30 catalyst. The CHNS analysis results of CSC and sulphonated CSC catalysts reveal that the coconut shell char contains highest amount of carbon, which after treatment with sulphuric acid got decreased as a result of oxidation (Table S1) [56]. As the amount of sulphuric acid used for the treatment with coconut shell char increases, the percentage of carbon content decreases indicating leaching as well as functionalization. Sulphonation of the coconut shell char results in the increase in the amount of sulphur in it as evident from the data shown in Table S1. The amount of sulphuric acid used in sulphonation does not have any major role in the sulphur content of various sulphonated systems since even the lowest amount of H2SO4 in preparation is very high, i.e., 10 times of the weight of the char. 13

3.2 Multicomponent amidoalkyl naphthol synthesis The synthesized catalytic systems were very effectively used in the rapid multicomponent synthesis of 1-amidoalkyl-2-naphthols under solvent free condition (neat) (Scheme 2). Since it is an acid catalyzed reaction, sulphonation can result in catalytic properties to the coconut shell char in the amido alkyl naphthols synthesis.

Scheme 2: Synthesis of Amidoalkyl naphthols over sulphonated coconutshell char The synthesis of N-[Phenyl-(2-hydroxy-naphthalen-1-yl)-methyl]-benzamide by the condensation of benzaldehyde, β-naphthol and benzamide under solvent free condition at 125 °C is taken as the model reaction to find out the best catalyst among the different sulphonated coconut shell char systems and bare coconut shell char. The results are given in Table 1. Table 1 Results on the synthesis of N-[Phenyl-(2-hydroxy-naphthalen-1-yl)-methyl]-benzamide over different catalytic systems a Catalyst

Yield (%)b

CSC

-

Solidification

Error in

Time (min)

Yield (%)

-

-

SCSC10

95

10

±1

SCSC20

97

7

±1

14

SCSC30

98

3

±1

SCSC40

98

3

±1

a Reaction conditions: benzaldehyde:-naphthol: benzamide – 1 mmol : 1 mmol : 1.3 mmol, over 0.1 g catalyst at 125 C, b Isolated Yield

From the table, it is clear that sulphonation of coconut shell char results in catalytic reaction, whereas coconut shell char was found to be catalytically inactive. The reaction time decreases and product yield increases with the amount of H2SO4 used in the preparation up to 30g, which remains constant thereafter. Therefore, the catalyst, SCSC30 is chosen for further investigations to study the effect of reaction parameters on the product yield. The effect of catalyst weight in the synthesis of amidoalkyl naphthol is shown in Fig. 7. It is clear that as the weight of catalyst increases, product yield increases with decrease in the reaction time up to a catalyst weight of 0.10 g. Thereafter the increase in the weight of catalyst has no pronounced effect on the reaction yield or time of reaction. So a catalyst weight of 0.10 g was is chosen for further studies.

Fig. 7: Effect of catalyst weight

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The potency of a reaction generally varies with temperature. So the effect of temperature on the reaction was also investigated. The rate of the reaction increases with increase in temperature up to 125 °C. Further increase in temperature resulted in slight decrease in the yield of product, which may due to the undesired side reactions at higher temperature (Fig.S4). Effect of molar ratio of the reactant molecules was also studied for the amidoalkyl naphthol formation; for this, the molar ratio of benzamide was varied. The product yield increased with increase in the amount of benzamide. Highest yield of 98% is obtained with low reaction time at a molar ratio of 1:1:1.3 of benzaldehyde, beta naphthol and benzanmide respectively (Fig. S5) After studying the influence of different reaction parameters like, catalyst weight, reaction temperature and reactants’ molar ratio, the reaction over SCSC30 is extended to different varieties of benzaldehyde and amides. It can be seen that the catalyst is highly effective in the synthesis of a range of amidoalkyl naphthols using different aldehydes and amides. The structure of the synthesized amidoalkyl naphthols, their yield and time taken for the reaction are given in table 2. From the data shown in table 2, it can be seen that the catalyst is found to be very active with different varieties of reactants, whereas aldehydes with electron withdrawing groups gave high yield of product within short interval of time when compared to aldehyde with electron donating groups [59,60] supporting acid catalyzed AAN synthesis.

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Table 2: Results on the synthesis of amidoalkyl naphthols over SCSC30a

a Reaction conditions: aldehyde (1 mmol), -naphthol (1mmol), amide/urea (1.3 mmol), SCSC30 (0.10 g) at 125 C

It is well known that recyclability and reusability are the major essential factors to assess the performance of a heterogeneous catalyst. The reusability studies of the synthesized catalyst 17

SCSC30 were done in the synthesis of N-[Phenyl-(2-hydroxy-naphthalen-1-yl)-methyl]benzamide as a model reaction. The catalyst is found to be reusable even up to six consecutive cycles with only about 7% loss in the initial yield of the reaction; however the reaction time slightly increased after each run (Fig. 8), which may be due to the blocking of a few of the active sites during each reaction.

Fig. 8: Reusability data FTIR spectral measurements of the reused catalysts after each run are taken to study the stability of the catalytic system, SCSC30, and to investigate the leaching of active groups to the reaction mixture or during the work up. Fig. S6 represents the FTIR spectral patterns of the reused catalyst after each cycle. The spectra revealed the retention of both sulphonic acid and carboxylic acid groups from their characteristics bands in all reused systems with a confirmation that the catalyst is highly stable without leaching of the active groups. The plausible mechanism for the synthesis of amidoalkyl naphthols over sulphonated carbon dot adorned coconut shell char is represented in scheme 3. The reaction proceeds through different steps most of which are catalyzed by the acidic groups (both –SO3H and –COOH functionalities; and for simplicity only –SO3H is shown in the scheme) anchored on the carbon dot as well as on the char. In the first step, aldehyde (1) react with the acidic functionality of the catalyst to form activated aldehyde (2), followed by the nucleophilic attack of the β-naphthol (3). 18

The formed intermediate (4) then got dehydrated to form α,β unsaturated carbonyl compound named as ortho-quinone methides (o-QMs) (5). The o-QMs, in the presence of catalyst will act as a Michael acceptor and then react with the amide (6) via conjugate nucleophilc addition to form intermediate (7), which is then converted to the corresponding amidoalkyl naphthol (8). The sulphonated catalyst acts as a strong Bronsted acid and can render the active sites that facilitate different steps in the reaction path way. Results shown in Table 3 are in agreement with the proposed mechanism, because aldehyde with electron withdrawing groups gave high yield of product within short interval of time, since electron withdrawing group bearing aldehydes can fascilitate nucleophilic addition of β- naphthol there by promoting further reactions [59,60].

Scheme 3: Proposed mechanism for the synthesis of amidoalkyl naphthols over SCSC catalyst The efficiency of our catalyst over some of the reported catalysts was also evaluated (Table 3). The table reflects that functionalized carbon dot adorned porous coconut shell char of present investigation is superior over most of the reported catalysts in many aspects such as reaction time, product yield, catalyst reusability etc.

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Table 3 Comparison of SCSC30 with other reported catalysts Catalyst

Molar ratio of reactants, Amount of

Time

catalyst, Reaction conditions

Yield

Ref

(%)

Ce(SO4)2

1:1:1, 1 mmol, CH3CN, Reflux

36 h

72

35

Iodine

1:1:1.3, 5 mol% ,125 °C

5.5 h

85

36

RuCl2(PPh3)3

1:1.2:1.3, 0.05 mmol, Toluene

12 h

85

37

POCl3-Na2B4O7

1:1:1.2, 0.1g, 120 °C

30 min

90

38

H4SiW12O40

1:1;1.2, 5 mol%, 110 °C

20 min

88

39

ZnO

1:1:1.2, 20 mol%, 120-130 °C

35 min

88

40

HClO4-SiO2

1:1:1.1, 0.02 g, dichloroethane, reflux

6h

90

41

MoO3-ZrO2

1:1:1.1, 0.1g, 80 °C

120 min

80

42

Chloroacetic acid

1:1:1.2. 0.2 mmol, 125 °C

9 min

83

43

Nickel-doped SnO2

1:1:1.3, 0.05g, 100 °C

110 min

87

44

Hexanesulphonic acid

1:1:1.5, 10 mol%,Microwave

8 min

92

45

SCSC30

1:1:1.3, 0.1g, 125 °C

3 min

98

This work

20

4

Conclusions The present study describes the one-pot synthesis of functionalized carbon quantum dot

incorporated porous coconut shell char for the first time. The paper also spells out the first time report of a multicomponent reaction over coconut shell char based catalyst. The principal achievement of the present study is the development of highly efficient cost effective and environmentally benign catalyst from naturally available waste material, coconut shell, for the solvent free synthesis of amidoalkyl naphthols. Characterization studies revealed the presence of sulphonic acid and carboxylic acid groups as the active sites of the catalyst. The TEM images provide interesting information regarding the presence of carbon dots in the prepared catalytic systems. The catalyst composed of carbon dot dispersed porous amorphous carbon char whereas acidic –SO3H and –COOH groups were anchored on both the char as well as on the carbon dot. The fluorescence measurements showed a strong fluorescence band with a λmax of 437 nm that confirm the presence of carbon quantum dots (CQDs) in the catalyst. The representative catalyst SCSC30 was very active with varieties of aldehydes and amides for the synthesis of libraries of amidoalkyl naphthols with a product yield in the range of 88-98%. The catalytically active sites were retained even after 5 cycles of continuous run as evident from the FTIR spectral investigation of the used catalysts after each run. Compared to most of the other reported catalysts for amidoalkyl naphthols synthesis, the present catalyst is highly active in terms of reaction time, product yield, reusability of catalyst etc. [35-42].The present work also has a bright future outlook since the CDs prepared by the present method can also be explored for various applications like bio-imaging since they can act as carbon nanolights. The prepared catalyst can also be used for various other multicomponent reactions as well as acid catalyzed organic transformations. Acknowledgement Authors acknowledge Sree Neelakanta Govt. Sanskrit College Pattambi and University of Calicut for providing the facilities for carrying out the research work. Binitha N. Narayanan is grateful to the University Grants Commission, New Delhi, India, for UGC Research Award (2012 - 2014). ACNSMM, AIMS Kochi and SAIF, STIC, CUSAT, Kochi, India are acknowledged for XPS and CHNS, SEM and TEM analysis respectively.

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Graphical abstract

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