The influence of multiwalled carbon nanotubes and graphene oxide additives on the catalytic activity of 3d metal catalysts towards 1-phenylethanol oxidation

Accepted Manuscript Title: The influence of multiwalled carbon nanotubes and graphene oxide additives on the catalytic activity of 3d metal catalysts towards 1-phenylethanol oxidation Author: Ana Paula C. Ribeiro Emmanuele Fontolan Elisabete C.B.A. Alegria Maximilian N. Kopylovich Roberta Bertani Armando J.L. Pombeiro PII: DOI: Reference:

S1381-1169(16)30265-5 http://dx.doi.org/doi:10.1016/j.molcata.2016.07.015 MOLCAA 9949

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

25-4-2016 28-6-2016 6-7-2016

Please cite this article as: Ana Paula C.Ribeiro, Emmanuele Fontolan, Elisabete C.B.A.Alegria, Maximilian N.Kopylovich, Roberta Bertani, Armando J.L.Pombeiro, The influence of multiwalled carbon nanotubes and graphene oxide additives on the catalytic activity of 3d metal catalysts towards 1-phenylethanol oxidation, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.07.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The influence of multiwalled carbon nanotubes and graphene oxide additives on the catalytic activity of 3d metal catalysts towards 1-phenylethanol oxidation

Ana Paula C. Ribeiro,1 Emmanuele Fontolan,1,2 Elisabete C.B.A. Alegria,1,3,* Maximilian N. Kopylovich,1 Roberta Bertani,2 Armando J.L. Pombeiro1,*

1

Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Portugal. 2 3

Department of Industrial Engineering, University of Padova, Padova, Italy.

Chemical Engineering Departament, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Portugal.

Dedicated to Prof. Georgiy B. Shul'pin on the occasion of his 70th birthday as a recognition for his relevant scientific achievements.

*

Corresponding authors. E-mail: [email protected], [email protected]

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Graphical Abstract

Ball milling Up to 49% increase

Significant improvement in catalytic activity of CoCl2-based catalysts was achieved by addition of 0.1-5% of carbon nano additives

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Highlights

  

Simple preparation of catalytic composites by ball milling Carbon nanomaterials (CNTs and GO) provide significant yields improvement Mechanochemical preparation of catalysts and catalytic reactions

Abstract 3d metal (Cu, Fe, Co, V) containing composite catalysts for the solvent-free microwave-assisted transformation of 1-phenylethanol to acetophenone with tert-butyl hydroperoxide (TBHP) as oxidant were prepared by ball milling. The influence of multiwalled carbon nanotubes (CNTs) and graphene oxide (GO) additives on the catalytic activity of the catalysts was studied. CNTs or GO were mixed by ball milling with the metal salts (CoCl2), oxides (CuO, Fe2O3, V2O5) or binary systems (Fe2O3-CoCl2, CoCl2-V2O5, CuO-Fe2O3). For CoCl2-based catalytic systems, addition of small amounts (0.1–5%) of CNTs or GO leads to significant improvement in catalytic activity, e.g. 1% of the CNTs additive allows to rise yields from 28 to 77%, under the same catalytic conditions. The CoCl2-5%CNTs composite is the most active among the studied ones with 85% yield and TON of 43 after 1 h. Keywords: Carbon nanotubes; mechanochemistry; catalysis; oxidation; alcohols.

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1. Introduction The design and synthesis of micro- and nanoscale materials with specific shapes, sizes and morphologies is one of the most promising directions in chemical technology. Due to their small sizes, micro- and nanocatalysts have high surface-to-mass ratio and exposed active sites, and thus they can exhibit unique properties, in particular a high catalytic activity [1-8]. However, in many instances, high cost, fast deactivation and difficult recovery of the dispersed catalysts make their practical application prohibitive [Error! Bookmark not defined.,Error! Bookmark not defined.]. In this respect, the following points should be addressed towards the development of new (micro, nano) dispersed catalytic systems: (i) how to decrease the price of the catalysts and improve their stability; (ii) how to recover the catalysts by a cheap and effective way. The usage of cheap and available starting materials and simple synthetic procedures can be proposed as a response for point (i), while magnetic recovery can be an answer for the (ii) challenge [9,10]. One of the well-known approaches to construct smart materials is the preparation of composites from compounds of various classes [11,12]. Carbon nanomaterials, such as carbon nanotubes or graphene, can -interact with aromatic substituents of a substrate and facilitate electron transfer in catalysis [13], while the inorganic component of composite, e.g. a transition metal, can be responsible for performing a redox or another transformation of substrate [14]. If the overall action is synergistic, a significant improvement in activity can occur [Error! Bookmark not defined.]. As result, the preparation of catalytic systems composed of transition metals and carbon nanomaterials is of great potential. For example, a sandwich-like N-doped graphene/Co3O4 hybrid catalytic system was prepared by solvothermal method and applied for selective oxidation of olefins and alcohols [15], while graphene oxide-SnO2 composite demonstrated superior catalytic efficiency for the synthesis of beta-enaminones and beta-enaminoesters [Error! Bookmark not defined.]. However, in these and other related cases [Error! Bookmark not defined.,Error! Bookmark not defined.], the rather costly nanocarbon materials were generally used as a support rather than an active part of the catalytic systems. In this respect, we decided to study if the cost of the dispersed catalytic system can be decreased by minimizing the amount of CNTs and GO and by using cheap and available metal salts or oxides as starting materials. On the other hand, further economical and environmental advantages could be achieved by application of green, simple, time- and energysaving mechanochemical ball-milling synthetic procedures. Additionally, we aimed to prepare the catalyst and perform the catalytic reaction in one pot, namely by the exclusive application of ball milling under ambient conditions. 4 

 

As a model catalytic reaction, we chose the oxidation of 1-phenylethanol to acetophenone in view of our scientific interests [16], generality of alcohol oxidations [17,18] and their importance for many domino and cascade transformations, e.g. for preparation of aryl nitriles from benzylic alcohols [19], dehydrogenative couplings of primary alcohols [20], or direct synthesis of amides from alcohols and ammonia [21-23].

2. Experimental

2.1. Preparation of the dispersed catalysts During the dry ball milling (BM) treatment, the commercial salts or oxides (Sigma Aldrich) were mixed mechanically, during 1 h, in a PM100/200, Retsch GmbH, planetary ball mill, equipped with a 250 mL grinding bowl and 10 stainless steel balls of 10 mm size (for homogenization). The rotational speed was 450 rpm, with rotational inversions every 5 minutes. All metallic composites were prepared in the absence of any added solvent (dry milling). The amounts of salts used in each mixture were previously weighted and directly added to the reactor. For the MW - one pot method (performing the preparation of catalyst and the oxidation reaction in one-pot only under microwave-irradiation without any mechanochemical involvement), the calculated amounts of the metal salt and CNTs (Bayer Materials) were directly added to a cylindric Pyrex tube to be used in the MW reactor, whereafter the substrate (1-phenyethanol) and the oxidant (TBHP, 70% aq. solution) were added and the reaction performed as indicated below in section 4.4 (catalytic studies).

2.2. Preparation of GO A modified Hummers’ method [24] was used to synthesise GO, in which graphite powder (1.0 g) was poured into a solution of NaNO3 (0.5 g) in concentrated H2SO4 (30 mL) and cooled to 0 ºC. KMnO4 (3.0 g) was then added, during which the temperature of the mixture was maintained below 20 ºC. Successively, the mixture was stirred at 35 ºC for 1 h, and then diluted with deionised water (46 mL) by keeping the temperature at 85 ºC and then increasing it to 100 ºC for 30 min. Warm deionised water (140 mL) was then added to the mixture, followed by the dropwise addition of a 50% aqueous solution of H2O2 (15 mL), and the solution was stirred for 30 min. The mixture was then centrifuged and washed with a 5% aqueous solution of HCl (1 L) to remove metal ions,

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followed by deionised water (1 L) to remove the acid. The resulting solid was dried in air for 12 h at 60 ºC. 2.3. Characterization studies The synthesized heterometallic mixtures, with or without additives, were characterized using SEM, EDX, FEGSEM and TEM techniques. TEM measurements were performed on a Transmission Electron Microscope Hitachi 8100 with ThermoNoran light elements EDX detector and digital image acquisition. Morphology and distribution of metal composites were characterized using a scanning electron microscope (SEM, FEGSEM and EDX) (JEOL 7001F with Oxford light elements EDX detector and EBSD detector).

2.4. Catalytic studies The microwave-assisted (MW) solvent-free peroxidative oxidations of 1-phenyethanol catalytic tests were performed in focused Anton Paar Monowave 300 reactor using a 10 mL capacity reaction tube with a 13 mm internal diameter, fitted with a rotational system and an IR temperature detector. The alcohol (5 mmol) as substrate, 100 μmol metallic composite as catalyst (2 mol% vs. substrate) and a 70% aqueous solution of t-BuOOH as oxidant (10 mmol) were introduced in a cylindric Pyrex tube that was sealed. This tube was then placed in the microwave reactor and the system was left under stirring and under irradiation (1-20 W), at 30-150 °C for 0.25-6 h. Finally, 300 µL of benzaldehyde (internal standard) and 5 mL of MeCN (to extract the substrate and the organic products from the reaction mixture) were added. The obtained mixture was stirred for 10 min and then a sample (1 µL) was taken from the organic phase and analysed by GC using the internal standard method. Blank experiments, in the absence of any catalyst, were performed under the studied reaction conditions and no significant conversion was observed. Gas chromatographic measurements were carried out using a FISONS Instruments GC 8000 series gas chromatograph with a FID detector and a capillary column (DB-WAX, column length: 30 m; internal diameter: 0.32 mm) (He as the carrier gas) and the Jasco-Borwin v.1.50 software. The temperature of injection was 240 °C. The initial temperature of the column was maintained at 120 °C for 1 min, then raised 10 °C/min up to 200 °C, and held at this temperature for 1 min. Attribution of peaks was made by comparison with chromatograms of genuine samples and, in some cases, by GC-MS analyses using a Perkin Elmer Clarus 600 C instrument (He as the carrier gas), equipped with a 30 m × 0.22 mm × 25 μm BPX5 (SGE) capillary column.

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3. Results and discussion

3.1. Preparation of the catalysts Catalysts were prepared using commercially available metal salts or oxides (CoCl2, CuO, Fe2O3 and V2O5) and nanocarbon additives (CNT or GO) [loadings from 0.1 to 5 wt% or ca. 1000-100 metalper-carbon molar ratio]. The remarkable mechanical properties of carbon nanotubes, such as high elastic modulus and tensile strength, make them promising reinforcements of resulting composites [25]. Additionally, the nanocarbon additives have a high specific surface area, providing interfacial interaction with organic substrate (e.g. due to multiple - interactions) [26]. In general, the use of CNTs or GO as additives in microcatalysts is still quite understudied, and in this paper we aim to shed more light on this topic. The catalysts preparation was performed in a conventional planetary ball milling reactor [27], by mechanical neat treatment for 1 h at 450 rpm, with rotational inversions every 5 minutes. CNTs were mixed by ball milling with simple salts or oxides (CoCl2, CuO and Fe2O3) or with binary systems (Fe2O3-CoCl2, CoCl2-V2O5). GO was also tried as an additive with CoCl2. All the sample handling was undertaken under ambient conditions, without additional purification of the commercial reagents.

3.2. Characterization of the dispersed material Ball mill solvent-free and liquid assisted grinding are environmentally benign methods which have been applied, in recent years, in a variety of fields e.g. organic [28,29] and organometallic reactions [30-32], preparation of pharmaceuticals [33] or functional materials [Error! Bookmark not defined.], among others [34]. Mechanochemical procedures can involve reagents in any aggregation state (solid, liquid or even gas) but are mostly applied to solid state processes [35]. During the milling process, a high pressure is generated locally due to collisions of rigid balls. Ballmilling can be successfully applied to disperse or shorten CNTs [Error! Bookmark not defined.d] and, hence, the grinding of CNTs with inorganic material could effectively break the entangled CNTs and thus result in their uniform dispersion [Error! Bookmark not defined.b]. GO, with its 2D structure with a high specific surface area should also serve as an effective dispersion agent for the mixture of metallic salts or oxides. To follow the morphological and chemical changes during the ball-milling process, the microstructure and elemental distribution of the milled samples were characterized by scanning 7 

 

electron microscopy (SEM), field emission gun scanning electron microscopy (FEGSEM), energydispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). From SEM images (Figure 1), one can conclude that the synthesized mixtures are well dispersed. It is noteworthy to mention that the type and amount of additive (CNTs or GO) has a pronounced effect on the sizes and morphology of the final composites. The CoCl2-V2O5 (3:1) and Fe2O3-CoCl2 (3:1) mixtures without the nanocarbon additives were analysed (Figure 1) and non-uniform crystallites of different shapes were detected. The particles are less scattered without an additive than with a small amount of additive. Generally, the produced mixtures appear to be nanocrystalline, and this feature can be advantageous in terms of catalytic performance and chemical resistance [36]. Additionally, the TEM analysis of the synthetized GO additive (Figure 1h) reveals both single and double lamellar layer GO sheets. The EDX analysis was performed to determine the elemental composition and uniformity of some composites prepared by ball milling (e.g. the CoCl2-V2O5 one, Tables S1 and S2, ESI). The mechanochemical treatment of the solid compounds by ball milling results in uniform mixtures since Co and V peaks are evenly distributed. To evaluate the degree of dispersion after the milling treatment at different loadings of the nanocarbon additives and confirm the absence of broad areas (pockets) in the CoCl2–containing materials, FEGSEM studies were performed for CoCl2-5%CNTs, CoCl2-1%CNTs and CoCl20.1%CNTs composites (Figures S1, S2 and S3, respectively). The absence of concentrated areas of CNTs indicates that they are rather uniformly scattered at the surface.

3.3 Catalytic studies The catalytic activity of the above described composites and, for comparative purposes, of the similarly dispersed metal oxides or salts and their mixtures was studied for the peroxidative oxidation of 1-phenylethanol (model substrate) following a previously described procedure [37-40] and using tert-butyl hydroperoxide (t-BuOOH, aq. 70%, 2 eq.) as an oxidant (Scheme 1). Typical conditions were as follows: 80 ºC, 5 W MW irradiation, 1 h reaction time, absence of any added solvent or other additive. Acetophenone was the major product, and the high selectivity (typically  98%) was confirmed by mass balance. The influence of different percentages of CNTs on the catalytic activity (Table 1) was studied. The nanotube additive affects differently the catalytic activity of the studied inorganic materials (Table 1, Figures 3 and 4). Thus, for CoCl2 a significant yield growth from 28 to 50, 77 and 85% is 8 

 

observed as the percentage (wt%) of CNTs increases from 0 to 0.1, 1 and 5%, respectively (entries 16, 3-5, Table 1, Figures 3 and 4). However, for the CoCl2-V2O5 (3:1) combination, the presence of CNTs appears not much affect the activity since the yields observed for (CoCl2-V2O5)-0.1%CNTs, (CoCl2-V2O5)-1%CNTs and (CoCl2-V2O5)-5%CNTs are very similar (44, 46 and 54%, entries 1113, Table 1, respectively) and close to the value obtained in the absence of CNTs (52%, entry 15, Table 1). In case of the Fe2O3-CoCl2 (3:1) system, even a significant depletion in catalytic activity is observed in the presence of CNTs. Actually, yields of 66, 71 and 73% were obtained for (Fe2O3CoCl2)-0.1%CNTs, (Fe2O3-CoCl2)-1%CNTs and (Fe2O3-CoCl2)-5%CNTs (entries 8-10, Table 1), while a conversion of 75% is observed for Fe2O3-CoCl2 (3:1) in the absence of any CNTs (entry 14, Table 1).

Hence, the promoting effect of the CNTs additive is observed only for some catalytic systems (Table 1, Figures 3 and 4). We also observed that another nanocarbon material, viz. graphene oxide, could be an activating additive for at least some mechanochemically synthesized materials. Thus, CoCl2-1%GO and CoCl2-5%GO composites allow to convert 1-phenylethanol into acetophenone with 67 and 72% yields, respectively (entries 6 and 7, Table 1, Figure S3), while simple CoCl2 gives 28% yield under the same catalytic conditions (entry 16, Table 1). To further study the influence of reaction conditions on the catalytic activity, we have chosen the most active composite, CoCl2-5%CNTs (Table 2).

3.3.1. Effect of Reaction Time The dependence of the product yield on the reaction time (within 0.25-6 h) is shown in Figure 4 (entries 1-5, Table 2). The yield reaches a maximum of 89% after 6 h, while after 1 h it is already quite close (85%) (Figure 4). After 0.5 h of reaction, already 73% of yield was reached.

3.3.2. Effect of Temperature Temperature has a pronounced effect on the yield which has the maximum at 80 ºC (entries 3, 6 and 8-10, Table 2, Figure 5) for the CoCl2-5%CNTs system. Heating the catalytic system to 100 or 150 ºC results in a yield decrease (entries 9 and 10, Table 2), from 85% at 80 ºC to 69% at 100 ºC and 54% at 150ºC, conceivably due to decomposition of the t-BuOOH oxidant and peroxo intermediates. On the other hand, performing the oxidation of 1-phenylethanol in the presence of 9 

 

CoCl2-5%CNTs at 50ºC and at room temperature resulted in a marked acetophenone yield drop relatively to that performed at 80 ºC (from 85% at 80 ºC to 19% at 50 ºC and 2% at room r.t., entries 3, 6 and 8, respectively, Table 2, Figure 5).

3.3.3. Effect of Energy Type Input The influence of mechanochemical ball milling (BM) and microwave irradiation (MW) methods on the synthesis of the catalysts and on the catalytic reaction was studied. For that, we performed the following combinations of energy inputs to compare the catalytic outcomes: i) preparation of the catalyst by ball milling and its subsequent use in catalytic reaction under microwave irradiation (BM+MW); ii) performing the preparation of catalyst and the oxidation reaction in one-pot only under microwave-irradiation without any mechanochemical involvement (MW - one pot); and iii) performing the preparation of catalyst and the oxidation reaction in mechanochemical one-step, without MW-irradiation (BM-one pot). For the studied combinations, the BM+MW method seems to be the most appropriate one, since 85% conversion of 1-phenylethanol to the corresponding ketone was achieved after 1 h reaction under microwave-irradiation, at 80 °C (entry 3, Table 2). When both the catalyst synthesis and the catalytic reaction were performed under microwave irradiation (MW - one pot method) only 21, 48 and 55% yields were achieved after 1, 3 and 6 h, respectively (entries 17-19, Table 2, Figure 6).

One-pot catalyst synthesis and catalytic reaction performed in ball mill at room temperature allowed to reach yields of 10, 18 and 24% of acetophenone after 1, 3 and 6 h reaction times, respectively (entries 21-23, Table 2, Figure 6). In fact, the value obtained by the BM - one pot method at room temperature (10%, entry 21, Table 2) is higher than that obtained by the BM+MW performed at room temperature (2%, entry 6, Table 3) or by the MW - one pot (5%, entry 15, Table 2, Figure 7).

3.3.4. Influence of TEMPO Additive The influence of 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO), a nitroxyl radical that is a known [41,42] radical promoter for the aerobic oxidation of alcohols to the corresponding carbonyl compounds due to their high efficacy and selectivity, was evaluated. For the three studied systems (BM+MW at 80°C, MW - one pot at 80ºC and BM - one pot at 25°C), different TEMPO effects were observed. For the BM+MW method at 80ºC, an inhibiting effect with a yield drop from 85% (entry 3, Table 2) to 76% (entry 12, Table 2) was noticed when the oxidation reaction was carried out in the presence of TEMPO. However, for the MW - one pot method, a promoting effect is 10 

 

observed and the yield increases from 21% in the absence of any additive (entry 17, Table 2) to 76% in the presence of TEMPO (entry 20, Table 2). The TEMPO promoted reaction conceivably involves its coordination, as well as of the alcohol substrate, followed by Co-centered oxidative dehydrogenation of the alcohol upon H-abstraction [Error! Bookmark not defined.e, Error! Bookmark not defined., Error! Bookmark not defined.,Error! Bookmark not defined.].

3.3.5. Recycling studies The recycling of CoCl2-5%CNTs was tested and for that upon completion of each cycle, the products were analyzed and the catalyst was recovered by centrifugation with following filtration. The subsequent cycles (under the same reaction conditions) were initiated upon addition of new portions of the starting materials (5 mmol of 1-phenylethanol and 10 mmol of 70% aqueous solution of t-BuOOH). In the 1st, 2nd and 3rd cycles, CoCl2-5%CNTs led to 85, 68 and 66% yield, respectively, while in the 4th cycle only 29% yield was achieved (Table 3, Figure 9). A significant loss of activity was observed during the first three cycles however being more pronounced in the 4th cycle. Moreover, the amount of Co was quantified after the first catalytic run and an important decrease of the initial content was observed, which indicates the occurrence of leaching phenomena during the catalytic reaction or catalyst recovery.

4. Conclusions In this work we have successfully prepared 3d metal (Cu, Fe, Co, V) containing composites with nanocarbon materials (CNTs and GO) by a simple and solvent-free mechanochemical method, i.e., ball-milling. The prepared heterometallic systems where shown to act as catalysts for the solventfree, microwave assisted oxidation of an alcohol (1-phenylethanol) with t-BuOOH. The influence of CNTs and GO on the catalytic activity of the 3d metal catalysts was studied and it was demonstrated that the presence of such additives result, in some cases, in a significant improvement in the catalytic performance. For instance, in the case of CoCl2 catalyst, a significant yield growth from 28 to 50, 77 and 85% is observed as the percentage of CNTs increases from 0 to 0.1, 1 and 5 wt%, respectively. Additionally, it was shown that the catalyst preparation and the catalytic reaction can be achieved by using the mechanochemical energy input. Further study of mechanochemically induced catalysis and the understanding of the effect of the nanocarbon materials are of clear need and are planned within our ongoing project.

Electronic Support Information 11 

 

FEGSEM mapping and EDX studies for some mixtures prepared by ball milling.

Acknowledgments Financial support from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, is gratefully acknowledged for fellowship SFRH/BPD/90883/2012 to A.P.C.R., research contract to M.N.K. within the "Investigador 2013" program, and the IF/01270/2013/CP1163/CT0007 and the UID/QUI/00100/2013 projects.

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[17] M.N. Kopylovich, A.P.C. Ribeiro, E.C.B.A. Alegria, N.M.R. Martins, L.M.D.R.S. Martins, A.J.L. Pombeiro, Catalytic oxidation of alcohols: recent advances, Adv. Organomet. Chem., 63 (2015) Ch.3 91-174. [18] Y.Y. Karabach, M.N. Kopylovich, K. T. Mahmudov, A. J. L. Pombeiro, Microwave-assisted catalytic oxidation of alcohols to carbonyl compounds, ed. Pombeiro A. J. L., Advances in Organometallic Chemistry and Catalysis, Wiley-VCH, Weinheim (2014) Ch. 22 285-294.  [19] R.A. Molla, K. Ghosh, K. Tuhina, Sk M. Islam, New J. Chem, 39 (2015) 921-930. [20] T. Zweifel, J.V. Naubron, H. Grützmacher, Angew. Chem. Int. Ed. Engl. 48 (2009) 559-563. [21] S. Gaspa, A. Porcheddu, L. De Luca, Org. Biomol. Chem., 11 (2013) 3803-3807. [22] K. Yamaguchi, H. Kobayashi, T. Oishi, N. Mizuno, Angew. Chem. Int. Ed. 51 (2012) 544-547. [23] M. Arefi, D. Saberi, M. Karimi, A. Heydari, ACS Comb. Sci. 17 (2015) 341-347. [24] J. Chen, B. Yao, C. Li, G. Shi, Carbon 64 (2013) 225-229. [25] B. Arash, Q. Wang, V.K. Varadan, Scientific Reports 4, Article number: 6479 (2014).   [26] Non-covalent interactions in the synthesis and design of new compounds, A.M. Maharramov, K.T. Mahmudov, M.N. Kopylovich, A.J.L. Pombeiro (eds.), Wiley, 2016. [27] a) J. Wang, S. Ganguly, S. Sabyasachi, N.D. Browning, S.M. Kauzlarich, Polyhedron 58 (2013) 156-161; b) Z. Wronski, R.A. Varin, C. Chiu, T. Czujko, A. Calka, J. Alloys Compd. 434435 (2007) 743-746; c) T.D. Shen, C.C. Koch, T.L. McCormick, R.J. Nemanich, J.Y. Huang, J.G. Huang, J. Mater. Res. 10 (1995) 139-148; d) Z.Y. Liu, S.J. Xu, B.L. Xiao , P. Xue, W.G. Wang, Z.Y. Ma, Composites: Part A 43 (2012) 2161-2168. [28] G.-W. Wang, Chem. Soc. Rev. 42 (2013) 7668-7700. [29] W. Su, J. Yu, Z. Li, Z. Jiang, J. Org. Chem. 76 (2011) 9144-910. [30] T.L. Cook, J.A. Walker, J. Mack, Green Chem. 15 (2013) 617-619 [31] J. Stojakovic, B. Farris, L. MacGillivray, Chem. Commun. 48 (2012) 7958-7960. [32] T. Friscic, Chem. Soc. Rev. 41 (2012) 3493-3510. [33] D. Hasa, B. Perissutti, C. Cepek, S. Bhardwaj, E. Carlino, M. Grassi, S. Invernizzi, D. Voinovich, Mol. Pharm. 10 (2013) 211-224. [34] A. Stolle, B. Ranu, Ball Milling Towards Green Synthesis : Applications, Projects, Challenges, Royal Society of Chemistry, 2015. [35] A.M. Belenguer, G.I. Lampronti, D.J. Wales, J.K. M. Sanders, J. Am. Chem. Soc. 136 (2014) 16156-16166. [36] N.K. Singh, M. Hardia, V. P. Balema, Chem. Commun., 49 (2013) 972-974. [37] I. Timokhin, C. Pettinari, F. Marchetti, R. Pettinari, F. Condello, S. Galli, E.C.B.A. Alegria, L.M.D.R.S. Martins, A.J.L. Pombeiro, Cryst. Growth Des., 15 (2015) 2303-2317. 14 

 

[38] P.J. Figiel, M.N. Kopylovich, J. Lasri, M.F.C. Guedes da Silva, J.J.R. Fraústo da Silva, A.J.L. Pombeiro, Chem. Commun. 46 (2010) 2766-2768.  [39] M. Sutradhar, M.M.D.R.S. Martins, M.F.C. Guedes da Silva, E.C.B.A. Alegria, C.-M. Liu, A.J.L. Pombeiro, Dalton Trans., 43 (10) (2014) 4009-4020.  [40] M. Alexandru, M. Cazacu, A. Arvinte, S.C. Turta, B.C. Simionescu, A. Dobrov, E.C.B.A Alegria, L.M.D.R.S. Martins, A.J.L Pombeiro, V.B. Arion, Eur. J. Inorg. Chem. (2014) 120-131. [41] (a) R.A. Sheldon, Chem. Commun. 2008, 3352-3365; (b) R.A. Sheldon, I.W.C.E. Arends, Adv. Synth. Cat. 346 (2004) 1051-1071. (c) W. Adam, C.R. Saha-Möller, P.A. Ganeshpure, Chem. Rev. 101 (2001) 3499-3548. (d) M. Shibuya, Y. Osada, Y. Sasano, M. Tomizawa, Y.J. Iwabuchi, Am. Chem. Soc. 133 (2011) 6497-6500. [42] (a) Z. Ma, L. Wei, E.C.B.A. Alegria, L.M.D.R.S. Martins, M.F.C. Guedes da Silva, A.J.L. Pombeiro, Dalton Trans. 43 (2014) 4048-4058; (b) M. Sutradhar, L.M.D.R. S. Martins, M.F.C. Guedes da Silva, E.C.B.A. Alegria, C.-M. Liuc, A.J.L. Pombeiro, Dalton Trans. 43 (2014) 39663977; (c) P.J. Figiel, M. Leskelä, T. Repo, Adv. Synth. Cat. 349 (2007) 1173-1179.  

15 

 

  (a) 

(b)

(c)

(d)

(e)

(f)

(g)

(h)

 

 

Figure 1. SEM (a-g) and TEM (h) images: (a) CoCl2-V2O5 (3:1), 100 µm scale; (b) Fe2O3-CoCl2 (3:1), 1 µm scale; (c) CuO-1wt%CNTs, 100 nm scale; (d) synthesized GO, 10 µm scale; (e) ball milled CNTs, 1 µm scale; (f) CoCl2-1%GO, 10 µm scale; (g) CoCl2-5%CNTs, 10 µm scale; (h) TEM image of synthesized GO, 500 nm scale.

16 

 

Yield (%)

90 80

1 - CNTs

70

2 - GO

60

3 - CoCl2

50

4 - CoCl2-0.1%CNTs

40

5 - CoCl2-1%CNTs

30

6 - CoCl2-5%CNTs

20

7 - CoCl2-1%GO

10

8 - CoCl2-5%GO

0 1

2

3

4

5

6

7

8

Figure 2. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone with CoCl2 with various CNTs or GO percentages.

90 80

Yield (%)

70 60 50

CoCl2 CoCl

40

Fe2O3‐ CoCl2(3:1) Fe2O3‐CoCl2 (3:1)

2

CoCl2‐V2O5 (3:1) CoCl2‐V2O5 (3:1)

30 20 10 0 0% CNTs 0.1% CNTs 1% CNTs

5% CNTs

Figure 3. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone with some 3d metal containing materials with various different CNTs percentages.

17 

 

100

Yield (%)

80 60 40 20 0 0

1

2

3 Reaction time (h)

4

5

6

Figure 4 - Dependence of acetophenone yield on the reaction time. Reaction conditions: 1phenylethanol (5 mmol), CoCl2-5%CNTs catalyst (0.1 mmol) and t-BuOOH (10 mmol), at 80 ºC, under MW-irradiation.

18 

 

100

Yield (%)

80 60 40 20 0 1 2 3 4 5 r.t.                  50                80                  100              150   

Temperature (ºC) Figure 5 - Effect of the temperature on acetophenone yield in the solvent-free MW-assisted oxidation of 1-phenylethanol, catalyzed by the CoCl2-5%CNTs material. Reaction conditions: substrate (5 mmol), 0.1 mmol of catalyst and 10 mmol of t-BuOOH, 1 h.

19 

 

100

Yield (%)

80 60

1 h

40

3 h 6 h

20

6 h

3 h

0 BM+MW (80ºC)

1 h MW ‐ one pot BM ‐ one pot (80ºC) (25ºC)

Figure 6. Comparison of different energy type inputs and their combination on the acetophenone yield in the oxidation of 1-phenylethanol. Reaction conditions: substrate (5 mmol), 0.1 mmol of CoCl2-5%CNTs and 10 mmol of t-BuOOH, 1, 3 and 6 h reaction time.

10

Yield (%)

8 6 4 2 0

BM+MW (r.t.)

MW ‐ one pot (r.t.)

BM ‐ one pot (r.t.)

Figure 7. Acetophenone yield in the oxidation of 1-phenylethanol as function of the energy type input at room temperature: BM+MW, MW - one pot and BM - one pot. Reaction conditions: substrate (5 mmol), 0.1 mmol of CoCl2-5%CNTs and 10 mmol of t-BuOOH, 1 h at 25ºC.

20 

 

100 80 60 40 20 0 1st cyle

2nd cycle

3rd cycle

4th cycle

Figure 9. Recycling of CoCl2-5%CNTs in the MW-assisted solvent-free oxidation of 1phenylethanol to acetophenone. Reaction conditions: 5 mmol of substrate, 10 mmol of t-BuOOH (aq. 70%), 80 ºC, 1 h, microwave irradiation (5 W).

21 

 

 

Scheme 1. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone.

22 

 

Table 1. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone by simple, binary and composite catalytic systemsa Entry

Catalytic system

Ratio

Yield (%)b

TONc

1

CuO-1% CNTs

-

59

30

2

Fe2O3-1% CNTs

-

32

16

3

CoCl2-0.1% CNTs

-

50

25

4

CoCl2-1% CNTs

-

77

39

5

CoCl2-5% CNTs

-

85

43

6

CoCl2-1% GO

-

67

34

7

CoCl2-5% GO

-

72

36

8

(Fe2O3-CoCl2)-0.1% CNTs

(3:1)

66

33

9

(Fe2O3-CoCl2)-1% CNTs

(3:1)

71

36

10

(Fe2O3-CoCl2)-5% CNTs

(3:1)

73

37

11

(CoCl2-V2O5)-0.1% CNTs

(3:1)

44

22

12

(CoCl2-V2O5)-1% CNTs

(3:1)

46

23

13

(CoCl2-V2O5) -5% CNTs

(3:1)

54

27

14

Fe2O3-CoCl2

(3:1)

75

38

15

CoCl2-V2O5

(3:1)

52

26

16

CoCl2

-

28

14

17

CuO

-

16

8

18

Fe2O3

-

10

5

19

V2 O5

-

45

23

20

CNTs

-

8

4

21

GO

-

14

7

Reaction conditions: 5 mmol of substrate, 100 mol metal composite (2 mol% vs. substrate), 10 mmol of tBuOOH (aq. 70%), 80 ºC, 1 h, microwave irradiation (5 W). CNTs = multiwalled carbon nanotubes; GO = graphene oxide. b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate, determined by GC. c Turnover number = number of moles of product per mol of metal catalyst. a

23 

 

Table 2. MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone by CoCl25%CNTs. a Entry

a

Method

Reaction time

Temperature

(h)

(ºC) in MW

Additive

Yield (%)b

TONc

1

BM+MW

0.25

80

-

44

22

2

BM+MW

0.5

80

-

73

37

3

BM+MW

1

80

-

85

43

4

BM+MW

3

80

-

87

44

5

BM+MW

6

80

-

89

45

6

BM+MW

1

25

-

2

1

7

BM+MW

2

25

-

4

2

8

BM+MW

1

50

-

19

10

9

BM+MW

1

100

-

69

35

10

BM+MW

1

150

-

54

27

11

BM+MW

1

25

TEMPO

9

5

12

BM+MW

1

80

TEMPO

76

38

13d

BM+MW

1

80

TEMPO

2

1

14e

BM+MW

1

80

TEMPO

1

1

15

MW - one pot

1

25

-

5

3

16

MW - one pot

2

25

-

7

4

17

MW - one pot

1

80

-

21

11

18

MW - one pot

3

80

-

48

24

19

MW - one pot

6

80

-

55

28

20

MW - one pot

1

80

TEMPO

76

38

21

BM - one pot

1

25

-

10

5

22

BM - one pot

3

25

-

18

9

23

BM - one pot

6

25

-

24

12

23

BM - one pot

1

25

TEMPO

15

8

24f

BM - one pot

2

25

-

8

4

25g

BM - one pot

2

25

-

9

5

Reaction conditions: 5 mmol of substrate, 100 mol metal composite (2 mol% vs. substrate), 10 mmol of t-

BuOOH (aq. 70%), 25-150 ºC, 0.25-6 h, microwave irradiation (5-20 W). CNTs = carbon nanotubes; BM = Ball Milling. b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate determined 24 

 

by GC. c Turnover number = number of moles of product per mol of metal catalyst. dwithout t-BuOOH. e

without t-BuOOH and catalyst. e2 ball mill spheres. f20 ball mill spheres.

Table 3. Recycling of CoCl2-5%CNTs in MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone.a Cycle

Yield (%)b

TONc

1st

85

43

2nd

68

34

3rd

66

33

4th

29

15

Reaction conditions: 5 mmol of substrate, 10 mmol of t-BuOOH (aq. 70%), 80 ºC, 1 h, microwave irradiation (5 W). b Molar yield (%) based on substrate, i.e. moles of product per 100 mol of substrate determined by GC. a

C

Turnover number = number of moles of product per mol of metal catalyst.

25