Selective Oxidation of Cinnamyl Alcohol to Cinnamaldehyde Over Functionalized Multi-Walled Carbon Nanotubes Supported Silver-Cobalt Nanoparticles

catalysts

Article Selective Oxidation of Cinnamyl Alcohol to Cinnamaldehyde over Functionalized Multi-Walled Carbon Nanotubes Supported Silver-Cobalt Nanoparticles

Zahoor Iqbal 1,* , Muhammad Sufaid Khan 1,*, Rozina Khattak 2, Tausif Iqbal 3, Ivar Zekker 4, Muhammad Zahoor 5 , Helal F. Hetta 6 , Gaber El-Saber Batiha 7 and Eida M. Alshammari 8

1 Department of Chemistry, University of Malakand, Chakdara 18800, Pakistan 2 Department of Chemistry, Shaheed Benazir Bhutto Women University, Peshawar 25000, Pakistan; [email protected] 3 Center for Computational Materials Science, University of Malakand, Chakdara 18800, Pakistan; [email protected] 4 Institute of Chemistry, University of Tartu, 14a Ravila St., 50411 Tartu, Estonia; [email protected] 5 Department of Biochemistry, University of Malakand, Chakdara 18800, Pakistan; [email protected] 6 Department of Internal Medicine, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0595, USA; [email protected] 7 Department of Pharmacology and Therapeutics, Faculty of Veterinary Medicine, Damanhour University, Damanhour 22511, El-Beheira, Egypt; [email protected] 8 Department of Chemistry, College of Sciences, University of Ha’il, Ha’il 81451, Saudi Arabia; Citation: Iqbal, Z.; Khan, M.S.; [email protected] Khattak, R.; Iqbal, T.; Zekker, I.; * Correspondence: [email protected] (Z.I.); [email protected] (M.S.K.) Zahoor, M.; Hetta, H.F.; El-Saber Batiha, G.; Alshammari, E.M. Abstract: The selective oxidation of alcohols to aldehydes has attracted a lot of attention because of Selective Oxidation of Cinnamyl its potential use in agrochemicals, fragrances, and ﬁne chemicals. However, due to homogenous Alcohol to Cinnamaldehyde over catalysis, low yield, low selectivity, and hazardous oxidants, traditional approaches have lost their Functionalized Multi-Walled Carbon efﬁciency. The co-precipitation method was used to synthesize the silver-cobalt bimetallic catalyst Nanotubes Supported Silver-Cobalt Nanoparticles. Catalysts 2021, 11, 863. supported on functionalized multi-walled carbon nanotubes (Ag-Co/S). Brunauer Emmet Teller https://doi.org/10.3390/ (BET), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray catal11070863 diffraction (XRD) were used to characterize the catalyst. For the oxidation of cinnamyl alcohol (CA) with O2 as an oxidant, the catalyst’s selectivity and activity were investigated. The impacts of several Academic Editors: Tannistha parameters on catalyst’s selectivity and activity, such as time, temperature, solvents, catalyst dosage, Roy Barman and Manas Sutradhar and stirring speed, were comprehensively studied. The results revealed that in the presence of

Ag-Co/S as a catalyst, O2 could be employed as an effective oxidant for the catalytic oxidation of Received: 21 June 2021 cinnamyl alcohol to cinnamaldehyde (CD) with 99% selectivity and 90% conversion. In terms of cost Accepted: 9 July 2021 effectiveness, catalytic activity, selectivity, and recyclability, Ag-Co/S outperforms the competition. Published: 19 July 2021 As a result, under the green chemistry methodology, it can be utilized as an effective catalyst for the conversion of CA to CD. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: silver-cobalt bimetallic catalyst; CNTs; selective oxidation; cinnamyl alcohol; cinnamaldehyde published maps and institutional afﬁl- iations.

1. Introduction The functional transformation of molecules to carbonyl compounds contributes sig- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. nificantly to the organic synthesis. The selective oxidation of alcohols in the presence of This article is an open access article different stoichiometric reagents, such as chromium and manganese has been well investi- distributed under the terms and gated [1]. The oxidation of cinnamyl alcohol, a simple aromatic allylic alcohol, is one of conditions of the Creative Commons the most studied heterogeneously catalyzed oxidation reactions. The carbonyl compound Attribution (CC BY) license (https:// cinnamaldehyde is a significant chemical intermediate in organic transformations and has creativecommons.org/licenses/by/ a wide range of applications in the food and fragrance industries. It is also used in animal 4.0/). repellant, plant protection against nematodes, flea repellent, and antibacterial agent. The

Catalysts 2021, 11, 863. https://doi.org/10.3390/catal11070863 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 863 2 of 11

synthesis of metal catalysts for the catalytic the oxidation of target molecules in the presence of molecular oxygen as an oxidant has received more attention due to the atom economy and related environmental issues under the protocol of green chemistry. Few catalysts with suitable reactivity and stability are currently available for large-scale applications of aerobic oxidation of alcohols. So far, monometallic catalysts with platinum-group metals have dominated for the oxidation of cinnamyl alcohol. Furthermore, bimetallic nanoparticles have also received significant attention in comparison to monometallic particles because of their multifunctionalities, selectivity, and activity, and their use in a variety of research fields such as optics, water purification, electronics, and catalysis [2,3]. As a result, scien- tists are interested in synthesizing bimetallic nanoparticles in various shapes in order to improve their efficiency through different approaches, such as chemical approaches, sol-gel and precipitation methods [4]. Silver-cobalt (Ag-Co) bimetallic nanoparticles has become notable in several industries and various fields of science including catalysis, biotechnology and energy storage [5]. Hydrothermal reduction, sol-gel, and co-precipitation procedures have been used to prepare the bimetallic Ag-Co nanoparticles [6]. Erdogan and his co-workers used a co-precipitation approach to synthesize Ag-Co bimetallic nanoparticles and examined their role in the oxidation of carbon monoxide [7]. Alonso et al. investigated the antiseptic properties of Co and Ag mono metallic nanoparticles as well as Ag/Co composites [8]. Lima et al. prepared bimetallic nanoparticles of silver and cobalt in various molar ratios and studied their ability to reduce oxygen in alkaline environments [9]. Herein, the Ag and Co nanoparticles were synthesized through co-precipitation method and supported on functionalized multi walled carbon nanotubes [10]. The functionalized multi walled carbon nanotubes (FCNTs) was used as a support for nanoparticles due to its mechanical properties, stability, surface area, compara- tively excessive oxidation stability, conductivity and selectivity [11]. Ag and Co bimetallic nanoparticles on the surface of FCNTs (Ag-Co/S) were used to catalyze the oxidation of cinnamyl alcohol to cinnamaldehyde. According to the literature review, the utilization of Ag-Co/S for the oxidation of cinnamyl alcohol in this study is a novel strategy because the supported nanoparticles showed increased selectivity, % conversion, recyclability, and stability. This FCNTs-supported Ag-Co catalyst, on the other hand, might be used well in industries for alcohol oxidation.

2. Results and Discussion 2.1. Characterization of of Ag-Co/S Nanoparticles The supported nanoparticles were morphologically studied using SEM. Figure1 shows that silver and cobalt bimetallic nanoparticles are present on the surface of FCNTs in a well-dispersed form. Kazici et al. [12] also conducted a similar investigation and reported the same findings. The existence of silver, cobalt, carbon, and oxygen was confirmed in the elemental composition of the supported bimetallic nanoparticles, as shown in ,. Metal oxide production and MWCNTs functionalization with organic acids (COOH) caused oxygen peaks in the results. The Ag-Co/S surface area measured by BET was 217.5 m2/g, which is consistent with the literature. The MWCNTs are identified by the diffraction peak at 2θ = 26.1 in Figure3. The diffraction peaks at 2 θ = 38.1◦, 66◦ and 78.4◦ correspond to Ag crystal lattices, while the peaks at 2θ = 41◦, 44◦, 47◦ and 76◦ correspond to cobalt crystal lattices. Kanwal et al. [13] also studied Co-Ag nanoparticles and found similar results. Catalysts 2021, 11, 863 3 of 11

Figure 1. Scanning electron microscopy (SEM) image of Ag-Co/S nanoparticles.

Figure 2. Energy dispersive X-ray spectrum of Ag-Co/S nanoparticles.

Figure 3. X-ray diffractometry image of Ag-Co/S nanoparticles.

2.2. Oxidation of Cinnamyl Alcohol as a Model Reaction To conﬁrm the absence of autocatalysis of cinnamyl alcohol, a substrate solution (CA in ethanol) was introduced in the reactor without Ag-Co/S, and the percent conversion and selectivity were calculated at 75 ◦C, 750 rpm, 760 torr (O2), and 80 min. The reaction was next studied in the presence of Ag-Co/S in order to determine its catalytic effect on Catalysts 2021, 11, 863 4 of 11

CA oxidation under the same experimental circumstances. Furthermore, multiple tests were carried out to study the effect of various parameters on percent conversion and selectivity, such as time duration, temperature, catalyst quantity, solvent, stirring speed, and recyclability. The percent conversion and selectivity were calculated by applying Equations (1) and (2).

moles of reactant disappeared % Conversion = × 100 (1) moles of initial reactant

Ccompound % Selectivity = × 100 (2) ∑ Cproduct In the current investigation, Ag-Co/S was determined to be an outstanding catalyst in terms of selectivity and perecnt conversion when compared to the other reported catalysts (Table1).

Table 1. Comparative study of the catalytic performance of our synthesized nanoparticles to that reported in the literature for the oxidation of CA to CD.

Catalysts Conv/Sel (%) Reaction Conditions References Temp; 60 ◦C, Solvent; Toluene, Bi-Pt/AC 34/84 [14] Time; 2 h Temp; 80 ◦C, Solvent; Water, Fe O /AC 44/89 [15] 2 3 Time; 2 h Bi-Pt/Alumina >90/>90 Temp; 100 ◦C, Detergents; [16] Temp; 100 ◦C, Solvent; Toluene, Au–Pd/TiO 82/64 [17] 2 Time; 7 h Temp; 75 ◦C, Solvent; Ethanol, Ag-Co/S 90/99 The current research Time; 80 min

2.3. Impact of Different Reaction Parameters 2.3.1. Effect of Time To study the effect of time on both percent conversion and selectivity of the oxidation of CA to CD, the reaction was periodically monitored. The reaction was carried out at 75 ◦C with 25 mg of the catalyst added to the substrate solution (1 mmole CA/10 mL ethanol) at 1 atm and 750 rpm stirring. Figure4 showed that the percent conversion and selectivity grew in a linear fashion up to 80 min, after which both percent conversion and selectivity gradually dropped due to the formation of by-products, as demonstrated by GC. As a result, the remaining processes were assigned an optimal response time of 80 min. A comparable study was conducted by Zhao et al. [18]. CatalystsCatalysts2021 2021, 11, 11, 863, x FOR PEER REVIEW 55 ofof 11 11 Catalysts 2021, 11, x FOR PEER REVIEW 5 of 11

%Conv %Sel %Conv %Sel 100 100 80 80 60 60 40 40 %Conv/Sel %Conv/Sel 20 20 0 0 0 20406080100 0 20406080100 Time (min) Time (min)

Figure 4. Time effect on percent conversion and selectivity of CA to CD with Ag-Co/S as a catalyst. Figure 4. Time effect on percent conversion and selectivity of CA to CD with Ag-Co/S as a catalyst. FigureConditions 4. Time of effectReaction: on percent CA; 1 mmol, conversion Cat; 25 and mg, selectivity Stirring speed; of CA to750 CD rpm, with oxidant; Ag-Co/S O2 as(760 a catalyst.torr), Conditions of Reaction: CA; 1 mmol, Cat; 25 mg, Stirring speed; 750 rpm, oxidant; O2 (760 torr), Conditionsand temperature; of Reaction: 75 °C. CA; 1 mmol, Cat; 25 mg, Stirring speed; 750 rpm, oxidant; O (760 torr), and and temperature; 75 °C. 2 temperature; 75 ◦C. 2.3.2. Effect of Temperature 2.3.2.2.3.2. Effect Effect of of Temperature Temperature The temperature has a significant effect on percent conversion and selectivity. By The temperature has a significant effect on percent conversion and selectivity. By adoptingThe temperature experimental has conditions a signiﬁcant constants, effect onboth percent percent conversion conversion and and selectivity. selectivity By for adopting experimental conditions constants, both percent conversion and selectivity for adoptingcinnamyl experimental alcohol to cinnamaldehyde conditions constants, oxidation both were percent investigated conversion at andtemperatures selectivity rang- for cinnamyl alcohol to cinnamaldehyde oxidation were investigated at temperatures rang- cinnamyling from alcohol25 to 100 to cinnamaldehyde°C in the presence oxidation of ethanol were as investigateda solvent. As at demonstrated temperatures in ranging Figure ing from 25 to◦ 100 °C in the presence of ethanol as a solvent. As demonstrated in Figure from5, both 25 topercent 100 C conversion in the presence and selectivity of ethanol asrose a solvent.until 75 As°C, demonstrated after which they in Figure both declined5, both 5, both percent conversion and selectivity rose ◦until 75 °C, after which they both declined percentdue to conversionsolvent evaporation and selectivity and the rose generation until 75 ofC, by-products. after which they As a both result, declined for CA due to CD to solventdue to evaporationsolvent evaporation and the generation and the generation of by-products. of by-products. As a result, As for a CAresult, to CD for oxidation,CA to CD oxidation, all subsequent reactions were carried out at 75 °C. Wu et al. conducted a similar alloxidation, subsequent all subsequent reactions were reactions carried were out carried at 75 ◦C. out Wu at et75 al.°C. conducted Wu et al. conducted a similar analysis a similar analysis and found that cinnamaldehyde had a maximum selectivity of 64 percent at 100 andanalysis found and that found cinnamaldehyde that cinnamaldehyde had a maximum had a maximum selectivity selectivity of 64 percent of 64 at percent 100 ◦C[ 17at ].100 °C [17]. °C [17].

% Conv % Sel % Conv % Sel

85 85

65 65

% Conv/ Sel% Conv/ 45 % Conv/ Sel% Conv/ 45

25 25 25 45 65 85 25 45 65 85 Temperature (ºC) Temperature (ºC)

FigureFigure 5. 5.Effect Effect of of temperature temperatureon on percentpercent conversionconversion andand selectivityselectivity of CA to CD in the presence presence ofFigure Ag-Co/S. 5. Effect Conditions of temperature of reaction: on percent Cat; 25 conversion mg, CA; 1 mmoland selectivity in 10 mL ofsolvent, CA to Solvent;CD in the ethanol, presence ofof Ag-Co/S. Ag-Co/S. Conditions of reaction: reaction: Cat; Cat; 25 25 mg, mg, CA; CA; 1 1mmol mmol in in10 10mL mL solvent, solvent, Solvent; Solvent; ethanol, ethanol, Oxidant; O2 (1 atm), Time; 80 min, Stirring speed; 750 rpm. Oxidant;Oxidant; O O2 2(1 (1 atm),atm), Time;Time; 80 80 min, min, Stirring Stirring speed; speed; 750 750 rpm. rpm.

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2.3.3.2.3.3. Effect Effect of of Stirring Stirring Speed Speed TheThe stirring stirring speed speed has has a a considerable considerable effect effect on on the the percent percent conversion conversion and and selectivity selectivity ofof CA CA to to CD. CD. The The experiment experiment was was carried carried out without with a range a range of stirring of stirring speeds speeds (200–1000 (200–1000 rpm) whilerpm) keepingwhile keeping all other all experimentalother experimental variables variab constantles constant in order in order to optimize to optimize the stirring the stir- speed.ring speed. Figure Figure6 indicated 6 indicated that percent that percent conversion conversion and selectivity and selectivity both increasedboth increased in the in earlythe early phases, phases, with with the maximum the maximum percent percent conversion conversion andselectivity and selectivity occurring occurring between between 750 and750 800and rpm, 800 rpm, and aand little a little drop drop occurring occurring above above 800 rpm 800 duerpm todue by-product to by-product production. production. All subsequentAll subsequent trials trials were were carried carried out at out a stirring at a stirring speed speed of 750 of rpm 750 to rpm eliminate to eliminate the probability the prob- ofability mass of transfer. mass transfer. Sadiq et Sadiq al. synthesized et al. synthesized cinnamaldehyde, cinnamaldehyde, which which is consistent is consistent with with the ﬁndingsthe findings of this of studythis study [19]. [19].

% Conv % Sel 100

40 % Con/Sel 20

0 100 300 500 700 900 Stirring (rpm)

FigureFigure 6. 6.Effect Effect of of stirring stirring speed speed on on percent percent conversion conversion and and selectivity selectivity of of CA CA to to CD CD in in the the presence presence of Ag-Co/S. Conditions of reaction: Cat; 25 mg, CA; 1 mmol, Solvent; ethanol, Oxidant; O2 (1 atm), of Ag-Co/S. Conditions of reaction: Cat; 25 mg, CA; 1 mmol, Solvent; ethanol, Oxidant; O (1 atm), Time; 80 min, Temperature; 75 °C. 2 Time; 80 min, Temperature; 75 ◦C.

2.3.4.2.3.4. Catalyst Catalyst Dose Dose Study Study ByBy maintaining maintaining the the entire entire parameters parameters constant, constant, a a catalyst catalyst dose dose study study was was done done to to evaluateevaluate the the percent percent conversion conversion and and selectivity selectivity of of CA CA to to CD CD oxidation. oxidation. Up Up to to 25 25 mg mg of of catalyst,catalyst, the the percent percent conversion conversion rose rose linearly linearly with with the the amount amount of of catalyst catalyst used, used, with with no no decreasedecrease in in selectivity. selectivity. A A catalyst catalyst dose dose of of 25 25 mg mg resulted resulted in in a a maximum maximum conversion conversion of of 90% 90% withwith 99 99 percent percent selectivity, selectivity, as as illustrated illustrated in in Figure Figure7. 7. Due Due to to the the adsorption adsorption of of oxidized oxidized productsproducts on on the the surface surface of of the the Ag-Co/S Ag-Co/S catalyst, catalyst, increasing increasing the the catalyst catalyst loading loading causes causes a a noticeablenoticeable decrease decrease in in both both conversion conversion and and selectivity. selectivity. As a As result, a result, subsequent subsequent experiments experi- werements carried were outcarried using out 25 using mg of 25 the mg catalyst. of the cataly Breenst. et Breen al. also et used al. also the used Ir/C the catalyst Ir/C forcatalyst the selectivefor the selective hydrogenation hydrogenation of CD to of CA CD [20 to]. CA [20].

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Figure 7. The effect of catalyst quantity on percent conversion and selectivity of CA to CD in the Figure 7. The effect of catalyst quantity on percent conversion and selectivity of CA to CD in presence of Ag-Co/S. Conditions of reaction: CA; 1 mmol, Solvent; ethanol, Time; 80 min, Temper- the presence of Ag-Co/S. Conditions of reaction: CA; 1 mmol, Solvent; ethanol, Time; 80 min, Figureature; 757. The °C; effectOxidant; of catalyst O2 (1 atm),quanti Stirringty on percent Speed; conversion 750 rpm. and selectivity of CA to CD in the Temperature; 75 ◦C; Oxidant; O (1 atm), Stirring Speed; 750 rpm. presence of Ag-Co/S. Conditions of2 reaction: CA; 1 mmol, Solvent; ethanol, Time; 80 min, Temper- 2.3.5.ature;2.3.5. Effect75 Effect °C; Oxidant; of of Solvents Solvents O2 (1 atm), Stirring Speed; 750 rpm. 2.3.5.As AsEffect shown shown of Solvents in in Figure Figure 8, the8, the nature nature of solventsof solven hasts has a signiﬁcant a significant impact impact on the on conversion the conversion of CA to CD. This graph compares the effects of several solvents such as water, n- of CAAs to shown CD. This in Figure graph 8, comparesthe nature theof solven effectsts ofhas several a significant solvents impact such onas the water, conver- n-hexane, acetonitrile,sionhexane, of CA acetonitrile, to toluene, CD. This and toluene, graph ethanol compares and on ethanol the the conversion effects on the of conversion ofseveral CA to solvents CD of while CA such to maintaining CDas water, while n- maintain- all other variableshexane,ing all acetonitrile,other constant. variables Amongtoluene, constant. and the solventsethanol Among on used, the the conversion ethanolsolvents yielded ofused, CA to theethanol CD highest while yielded maintain- conversion the highest rate. Acetone,ingconversion all other water, rate.variables acetonitrile, Acetone, constant. water, toluene, Among acetonitrile, and the ethanolsolvents toluene, wereused, and seen ethanol ethanol in ascending yielded were the seen order highest in of ascending percent conversion.conversionorder of percent rate. The Acetone, solubility conversion. water, of oxygen Theacetonitrile, solubility in different toluene, of oxygen solventsand ethanol in hasdifferent were an impactseen solvents in ascending on thehas CA an toimpact CD conversion.orderon the of CA percent to Guo CD conversion. et conversion. al. studied The Guo thesolubility effect et al. of ofstudi oxygen solventsed thein ondifferent effect the catalyticof solvents solvents hydrogenation has on an the impact catalytic of CD hy- toondrogenation CA the andCA cameto CD of toCDconversion. the to sameCA and Guo conclusion came et al. tostudi [the21ed]. sa Nyamundatheme effect conclusion of solvents et al. [21]. investigated on Nyamunda the catalytic the et inﬂuencehy- al. investi- of differentdrogenationgated the solvents influence of CD onto CAof the different and selectivity came solvents to the of cinnamaldehydessa meon conclusionthe selectivity [21]. and Nyamundaof foundcinnamaldehydes that et al. cinnamaldehyde investi- and found hadgatedthat an cinnamaldehyde the 84 influence percent selectivityof different had an [solvents 1484]. percent on the selectivity selectivity [14]. of cinnamaldehydes and found that cinnamaldehyde had an 84 percent selectivity [14].

Figure 8. The effect of various solvents on percent conversion and selectivity of CA to CD in pres- Figure 8. The effect of various solvents on percent conversion and selectivity of CA to CD in presence ence of Ag-Co/S. Conditions of reaction: Cat; 25 mg, CA; 1 mmol, Solvent; ethanol, Oxidant; O2 (1 ofatm),Figure Ag-Co/S. Time; 8. The 80 Conditions min, effect Stirring of various of sp reaction:eed; solvents 750 rpm, Cat; on 25Temperature; percent mg, CA; conversion 1 75 mmol, °C. Solvent; and selectivity ethanol, of Oxidant; CA to CD O2 in(1 pres- atm), Time;ence 80of Ag-Co/S. min, Stirring Conditions speed; 750 of reaction: rpm, Temperature; Cat; 25 mg, 75 CA;◦C. 1 mmol, Solvent; ethanol, Oxidant; O2 (1 atm), Time; 80 min, Stirring speed; 750 rpm, Temperature; 75 °C.

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2.4. Leaching and Recyclability of Ag-Co/S 2.4. LeachingTo ensure and that Recyclability the solvents of Ag-Co/S were inert, Ag-Co/S was mixed and stirred in them at optimalTo conditions.ensure that The the appearance solvents were of a inert, single Ag-Co/S signal in was the chromatographmixed and stirred confirmed in them the at solvents’optimal conditions. inert nature. The CA appearance was added toof thea single filtrate signal and stirredin the chromatograph under the same confirmed experimental the conditionssolvents’ inert to check nature. the CA heterogeneous was added natureto the filtrate of the catalysts.and stirred There under was the no same CA toexperi- CD conversion,mental conditions indicating to check that the Ag-Co/S heterogeneous was stable nature in theof the chosen catalysts. solvents There and was had no CA true to heterogeneousCD conversion, behavior. indicating The that reaction Ag-Co/S took was place stable when in 25the mg chosen of Ag-Co/S solvents was and added had true to theheterogeneous solution (CA/ethanol) behavior. underThe reaction the given took conditions, place when and 25 cinnamaldehyde mg of Ag-Co/S was was produced added to inthe a solution significant (CA/ethanol) amount with under 90% the conversion. given conditions, In ethanol, and cinnamaldehyde the recyclability was of Ag-Co/S produced wasin a tested.significant The amount catalyst with was 90% separated conversion. from In the ethanol, reaction the mixture, recyclability washed, of Ag-Co/S and dried was attested. about The 100 catalyst◦C before was beingseparated exposed fromto the the reaction cinnamyl mixture, alcohol washed, solution and for dried up at to about five cycles100 °C without before being experiencing exposed any to the noticeable cinnamyl decrease alcohol in solution catalytic for activity up to infive terms cycles of percentwithout selectivity,experiencing but any a 10%noticeable decrease decrease in conversion in catalyti wasc activity observed in terms (Figure of9 percent). These selectivity, results suggestedbut a 10% that decrease Ag-Co/S in conversion can be used was effectively observed several (Figure times 9). forThese the oxidationresults suggested of cinnamyl that alcohol.Ag-Co/SSadiq can be et used al., on effectively the other several hand, investigatedtimes for the theoxidation recyclability of cinnamyl and stability alcohol. of Sadiq Zn- Mnet al., bimetallic on the other nanoparticles hand, investigated as a catalyst the forrecyclability CA to CD and oxidation stability under of Zn-Mn mild bimetallic reaction conditionsnanoparticles [20]. as a catalyst for CA to CD oxidation under mild reaction conditions [20].

Figure 9. Reusability of Ag-Co/S for oxidation of CA to CD. Figure 9. Reusability of Ag-Co/S for oxidation of CA to CD.

3.3. MaterialsMaterials andand MethodsMethods 3.1.3.1. MaterialsMaterials andand ChemicalsChemicals TheThe chemicalschemicals andand reagentsreagents usedused inin thisthis studystudy werewere ofof high high purity purity (Sigma(Sigma Aldrich,Aldrich, SaintSaint Louis,Louis, MO,MO, USA,USA, AlfaAlfa Aesar,Aesar,Thermo Thermo Fisher,Fisher,Kandel, Kandel, Germany,Germany, and and Merck, Merck, New New Jersey,Jersey, United United States). States). The The multi-walled multi-walled carbon carbon nanotubes nanotubes (MWCNTs; (MWCNTs; O.D. O.D.× L ×6–13 L 6–13 nm nm× µ 2.5–20× 2.5–20m) µm) were were furnished furnished by by Sigma Sigma Aldrich. Aldrich. Gases Gases such such as as oxygen oxygen and and nitrogen nitrogen were were suppliedsupplied by by the the British British Oxygen Oxygen Company Company (BOC), (BOC), West West Wharf Wharf Karachi, Karachi, Pakistan. Pakistan. To remove To re- tracesmove from traces the appliedfrom the gases, applied certain gases, filters certain such as (C.R.S.Inc.202223)filters such as (C.R.S.Inc.202223) and (C.R.S.Inc.202268) and were(C.R.S.Inc.202268) used. were used. 3.2. Synthesis of Ag-Co/S Catalyst 3.2. Synthesis of Ag-Co/S Catalyst Following the co-precipitation approach, equimolar solutions of metal salts such as Following the co-precipitation approach, equimolar solutions of metal salts such as AgNO3·2H2O and CoCl2·2H2O (0.01 M) were prepared and titrated against ammonium AgNO3∙2H2O and CoCl2∙2H2O (0.01 M) were prepared and titrated against ammonium hydroxide (NH4OH). As a result, dense metal hydroxides developed in the form of precipitates.hydroxide The precipitates (NH4OH). wereAs a filtered,result, dense washed metal with hydroxides deionized waterdeveloped several in times,the form and of then pre- treatedcipitates. with The 0.1 precipitates N HCl in a were modified filtered, Soxhlet washed apparatus with deionized using a glass water thimble several until times, the pHand wasthen neutral. treated Thenwith 0.1 it was N HCl let to in dry a modified overnight. Soxhlet Chemically apparatus altering using the a glass surface thimble of carbon until nanotubesthe pH was (CNTs) neutral. can Then give themit was a let desired to dry feature. overnight. In general, Chemically functionalization altering the surface has been of carbon nanotubes (CNTs) can give them a desired feature. In general, functionalization

Catalysts 2021, 11, 863 9 of 11

accomplished through the use of oxidants such as HNO3, KMnO4, and others. Carboxyl groups, on the other hand, serve to functionalize CNTs defects and ends. The present goal of the study was to design a simple and quick approach for functionalizing carbon nanotubes. For the functionalization of multi-walled carbon nanotubes (MWCNTs) with an amine-containing group, we used a covalent chemical method. As a result, MWCNTs were treated with p-aminobenzoic acid to make them functional. Functionalized multi- walled carbon nanotubes (FCNTs) were separated by centrifugation, washed multiple times with deionized water until pH was neutral, and then dried for 12 h at 100 ◦C. The synthesized silver-cobalt nanoparticles were dispersed in ethanol/water (50% v/v) and a speciﬁc amount of FCNTs were added to the suspension to incorporate the prepared bimetallic nanoparticles on the FCNTS. At room temperature, the resultant mixture was sonicated for an hour. Bimetallic nanoparticles of silver and cobalt supported on FCNTs were separated by centrifugation, washed, dried and stored in a desiccator [22].

3.3. Characterization of Ag-Co/S Catalyst The morphology of the catalyst Ag-Co/S was investigated using scanning electron microscopy (SEM, JSM 5910, JEOL, Tokyo, Japan). Elemental analysis of the supported nanoparticles was carried out by Energy dispersive X-ray spectroscopy (EDX, JSM 5910, JEOL, Tokyo, Japan). X-Ray diffractometer (XRD, JDX-3532, JEOL, Tokyo, Japan) with operating voltage of 20–40 kV and 2θ range of 0–160◦ was used to analyze the phase of nanoparticles. The BET surface area of the Ag-Co/S nanoparticles was measured using a surface area and pore size analyzer (NOVA2200e, Quantachrome, Boynton Beach, FL, USA).

3.4. Catalytic Test In a 100 mL three necked double walled round bottom batch reactor with a quick fit thermometer and a condenser, a substrate solution (CA/10 mL ethanol) and 25 mg Ag-Co/S as a heterogeneous catalyst were added. As shown in Scheme1, the reaction mixture was vigorously stirred (200–1000 rpm) for 80 min at 75 ◦C while delivering a steady supply of oxygen (40 mL/min). For the analysis of the reaction mixture, two types of analytical methods were applied. Gravimetric and gas chromatographic methods. In gravimetric method, phenyl hydrazine was used for the detection of the product. The supported nanoparticles were separated from reaction mixture and then 1 mL of phenyl hydrazine was added to 10 mL reaction mixture. This resulted in the formation of a precipitate in an hour, which was filtered out and dried at 70–80 ◦C. Gas chromatography (GC, Clarus 580, Perkin Elmer, Waltham, MA, USA) with a flame ionized detector and capillary column (cross-linked methyl siloxane capillary column; length: 30 m, ID: 0.32 mm, and film thickness: 0.25 µm) was used to measure percent conversion and selectivity. A nitrogen generator (G6010E, Parker Domnick hunter, Team Valley Trade Est, NE11 0PZ Gateshead, UK) and a hydrogen generator (PGXH2 100, Perkin Elmer, Waltham, MA, USA) were used to provide nitrogen and hydrogen gases. In gas chromatographic technique, the sample was spiked with authentic standard, the sharpening of the given peak at that particular retention time, confirmed the formation of the desired product.

Scheme 1. Oxidation of CA to CD with 25 mg catalyst, 10 mL substrate solution (1 mmole of CA/10 mL ethanol) at 75 ◦C

for 80 min under stirring 750 rpm in the presence of O2. Catalysts 2021, 11, 863 10 of 11

4. Conclusions In the present study, Ag-Co/S was synthesized effectively by co-precipitation method and characterized through SEM, EDX, XRD, and the BET surface area analyzer. The prepared nanoparticles were tested as a heterogeneous catalyst for the selective oxidation of CA to CD using different solvents such as water, acetone, acetonitrile, toluene and ethanol under mild reaction conditions in the presence of cost effective and ecofriendly oxidant. The Ag-Co/S has shown increased potential in terms of percent conversion (90%) with selectivity (99%) at optimal conditions such as catalyst; 25 mg, cinnamyl alcohol; 1 mmol, solvent; ethanol, temperature; 75 ◦C, time; 80 min, stirring speed; 750 rpm and oxidant; O2 at 1 atm. The Ag-Co/S is a potent heterogeneous catalyst for the oxidation of cinnamyl alcohol to cinnamaldehyde because of their ease of preparation, high catalytic activity, cost effectiveness, heterogeneous behavior, and recyclability. Future research into the kinetics of adding an oxidant as well as changing the catalyst support could result in major changes in catalyst selectivity/activity and may diverse their applications in organic transformation of other alcohols.

Author Contributions: Conceptualization, Z.I. and M.S.K.; methodology, Z.I. and M.S.K.; software, Z.I. and T.I.; validation, Z.I, T.I. and I.Z.; formal analysis, Z.I., M.S.K., R.K. and I.Z.; investigation, Z.I. and M.S.K.; resources, M.Z., H.F.H., E.M.A., and G.E.-S.B.; data curation, Z.I., T.I. and M.S.K.; writing— original draft preparation, Z.I.; writing—review and editing, R.K., H.F.H., E.M.A., M.Z. and G.E.-S.B.; visualization, Z.I.; supervision, Z.I.; project administration, H.F.H., E.M.A.; funding acquisition, M.Z.; revisions. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: No data associated with this publication to be link. All the data associated in presented in this paper. Conﬂicts of Interest: The authors declare no competing interest.

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