Elsevier

Journal of Catalysis

Volume 388, August 2020, Pages 104-121
Journal of Catalysis

Boosting multiple photo-assisted and temperature controlled reactions with a single redox-switchable catalyst: Solvents as internal substrates and reducing agent

https://doi.org/10.1016/j.jcat.2020.04.026Get rights and content

Highlights

  • CoCr2O4-HNT as a redox switchable catalyst with multifunctional catalytic activity.

  • Replacement of acetaldehyde by ethanol in Claisen-Schmidt condensation.

  • In situ oxidation of isopropanol to acetone at room temperature and under the influence of UV light.

  • EPR-evidence on the creation of positive holes at cobalt site of the catalyst.

  • Density functional theory calculation on the mechanism of ethanol oxidation to ethanal.

Abstract

An alternative and economically viable process for the synthesis of β-aryl enals, enones and the aryl amines has been developed by partial oxidation of ethanol, isopropanol and N, N-dimethyl formamide (DMF). The formation of β-aryl enals, enones and the aryl amines was catalyzed by a mixed metal oxides layer of cobalt and chromium supported on halloysite nanotubes, designated as CoCr2O4-HNT. The Csingle bondC and Csingle bondN bond formation reactions were found to be influenced by temperature and the nature of base. The condensation of aldehyde with in situ generated acetaldehyde by ethanol oxidation forming β-aryl enals occurred selectively at 120 °C. The partial oxidation of isopropanol to acetone and its condensation with aldehydes forming β-aryl enones occurred at room temperature. Increase in temperature caused the liberation of hydrogen gas from isopropanol and allowed the reversible reduction of aldehydes to alcohols. Increase in temperature in isopropanol and increase in base concentration in ethanol causes the selective reduction of aldehydes to alcohols. Besides being active for the Claisen-Schmidt type of reactions and the aryl halides amination process, the synthesized catalyst was also found to be highly active for the photocatalytic oxidation of benzyl alcohols in absence of any external oxidizing agent. The positive holes (h+) generated at the Co(II) site as evident from EPR analysis was considered to be responsible for high photocatalytic activity of the material reducing the recombination rate of holes and electrons (e). Density Functional Theory calculations were performed to understand the mechanism of ethanol oxidation to acetaldehyde.

Graphical abstract

Boosting of multiple reactions by CoCr2O4-HNT catalyst.

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Introduction

Catalysts, either in homogeneous or heterogeneous form are mostly designed to promote a reaction with high selectivity and good productivity [1], [2]. In general, a particular catalyst is prepared to catalyze one particular reaction either of industrial or academic interest [3]. However, with the advancement in the field of catalysis, the traditional way of doing catalysis has been encouraged in different directions [4]. Presently, it is motivated towards the development of a newer type of catalysts with multifunctional activities that can effectively catalyze multiple reactions [5], [6], [7]. Such types of catalysts are nowadays highly demanding in industries as they minimize the cost of the catalyst and also reduces the time required for the synthesis of different catalysts for various reactions [8], [9]. The industries are currently looking for solid and reusable catalysts, mostly of metals or metal oxides in nanometer-scale supported on solid matrix of high surface area that is capable to accelerate multicomponent, cascade, tandem or domino type reactions [10], [11], [12], [13], [14]. Among the various form of multiple component reactions or single pot reactions, the sequential catalytic reactions are gaining much interest in chemical industries [15], [16]. Tuning the catalytic performances of such type of solid catalysts for concomitant activation of different substrates in a single pot is however difficult and challenging as it is limited by certain factors. Mostly, the distance between the homo or hetero metals, the ability to undergo spillover mechanism, control diffusion of the substrates, transport of the intermediate species are the main factors that manifest the catalytic ability of a catalyst to participate in multicomponent reactions [17], [18].

Apart from the catalyst, the solvents are also known to play a critical role in various aspects of chemical reactions and catalytic processes [19]. In a chemical reaction or in a catalytic process, the choice of solvent can also have direct influence on the feasibility of the reaction [20]. There are several catalytic reactions where the role of solvent on product selectivity is well mentioned. Just like in the homogeneous catalysis, the role of solvent is also studied over heterogeneous catalysts [21]. In this regard, aluminosilicates like ZSM-5 are considered to be one of the suitable matrices where some classical studies have been made by different researchers in understanding the influence of solvents on the rate of conversion. For example, Haw et al. first observed the influence of nitromethane on the conversion of acetone over ZSM-5 [22], [23]. The acidic and basic sites available on these aluminosilicates plays an important role on the adsorption behavior of solvent molecules and also their conversion to various other components. Very recently, we have also demonstrated the role of zeolite-Y in the electrochemical oxidation of methanol [24]. Like the zeolites, halloysites also possesses external acidic and internal basic sites with high external (20–30 nm) as well as internal (30–70 nm) diameter that can be advantageous for absorption of different solvent molecules and their selective conversion to useful substrate molecules. But so far, the halloysites has not been explored in the activation of solvent molecules like ethanol, isopropanol and DMF.

Among the various solvents, ethanol, isopropanol and N, N-dimethyl formamide (DMF) are three of the solvents which have gained high attention in various catalytic reactions [25]. Ethanol as well as isopropanol have gained much interest in fuel cell applications and also have played an important role in various catalytic reactions or in reactions like Meerwein-Ponndrof-Verley (MPV) reduction [26]. Although recently several studies have been made on the understanding of the selective oxidation of ethanol to acetaldehyde, their direct applications in the formation of β-aryl enals and also as green reducing agent for selective reduction of aldehydes to alcohols has not been much explored [27]. Our group probably reported for the first time for the synthesis of different cinnamaldehydes (β-aryl enals) with high percentage yield through condensation of benzaldehydes with partially oxidized ethanol over Pd-Au nanoalloy supported on zeolite-Y (Pd-Au-Y) [28]. The Pd-Au-Y catalyst, although provided a high yield of cinnamaldehyde (upto 92%), but the use of two costly metals Pd and Au might limit the practical application of such catalysts. Therefore, it will be much more effective if it could be replaced by some low-cost catalysts. Similar to ethanol and isopropanol, DMF plays an important role in various organic reactions under various catalytic conditions [29]. For example, they have been employed as a reactant for amination reactions, as source for CN and also for Csingle bondN bond formation reactions [30]. But to the best of our knowledge, the direct synthesis of 4-dimethylamino benzaldehyde and its acid form using DMF as the source of amine through C-X (X = Cl, Br, I, F) bond activation is very sparse [31], [32], [33].

Considering the importance of the above reactions herein we have tried to design a cobalt-chromium oxide catalyst supported on HNT designated as CoCr2O4-HNT. Such complex system has been considered because Co and Cr are low-cost transition metals in comparison to many other metals [34]. Apart from this, HNT low cost material, they are good templating agents for the stabilization of metal oxide nanoparticles. Besides, halloysites exist in nanotubular form and have good surface area. Due to the interior lumen of HNT, different metal nanoparticles in specific dimension can be entrapped inside the HNT.35 They introduce high selectivity towards the products and the transition state. They provide a surface that favors the spill over mechanism and because of their presence of specific pores and channels they have a high control over the diffusion of substrate molecules to their active sites [35]. Furthermore, finding a suitable catalyst that can promote multiple reactions with high product selectivity will be highly beneficial as it will reduce the cost of catalyst preparation, man power and mostly the time. Therefore, we tried to design such a complex system that can be used for different catalytic reactions

Section snippets

Materials

Halloysite nanoclay (HNT) was procured from Sigma-Aldrich. Cobalt (II) chloride hexahydrate (CoCl2·6H2O) was brought from E-Merck. Chromium (III) chloride hexahydrate (CrCl3·6H2O) was received from SRL. HPLC grade acetonitrile, ethanol, isopropanol (ipr-OH), DMF and all other solvents were procured from E-Merck. Benzyl alcohols, benzaldehydes, and their family derivatives were also brought from Sigma-Aldrich. All the bases for the synthesis of β-aryl enals (cinnamaldehydes) and β-aryl enones

Results and discussion

The CoCr2O4-HNT material was characterized by various physicochemical and spectroscopic techniques. The FTIR (Fourier transformed Infrared) spectra and the XRD (X-Ray Diffraction) pattern of CoCr2O4-HNT was found to be similar to that of pristine HNT with a slight shifting of the XRD signals at 2θ value of 11.6, 20.0 and 24.7 [36], [37]. In the XRD pattern, the signals of CoCr2O4-HNT were found to be more intense in comparison to the pristine HNT (Fig. 1a). The increase in peak intensity was

Catalytic study

In this manuscript we have considered two different forms of reactions. One form is temperature-controlled reaction and the other is photo-assisted reaction. Under the thermal condition we have considered the synthesis of β-aryl enals, enones from in situ oxidation of ethanol and isopropanol respectively, selective reduction of benzaldehydes and amination of aryl halides with DMF. Furthermore, under the photo-assisted condition, we have studied the photochemical oxidation of benzyl alcohols,

Plausible mechanism for photochemical oxidation of BA

Based on the above analysis a plausible mechanism for the oxidation of BA is depicted in Scheme 6. As the light falls on the catalyst there probably occurs the creation of positive holes (h+) and the electrons (e) due to photo-excitation from the valence band to the conduction band of CoCr2O4-HNT. At the same time the holes created at the Co-site [Co(III) ions as evident from EPR studies] helped in reducing the recombination rate between h+ and e-. The generated e- reacted with the solvated

Recyclability of the catalyst

In heterogeneous catalysis recyclability of the catalyst is an important factor to judge the suitability as well as the heterogeneity of the catalyst. In order to test the recyclability of the catalyst, we considered the photochemical BA oxidation reaction under similar conditions. After running the 1st run, the catalyst was separated by filtration technique and dried to reuse for the next cycle. In a similar manner, we were successful to recycle the catalyst without any effect in % conversion

Conclusion

In summary, it can be said that a highly efficient low-cost catalyst has been found to induce the number of important reactions that allows the formation of different β-aryl and enones, dimethyl amine substituted benzaldehyde and benzoic acid. The catalyst was found to be highly active for the selective reduction of benzaldehydes without the use of any toxic reducing agents. At the same time, the same catalyst appeared to be an efficient photocatalyst for highly selective photochemical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

KKB and MS acknowledge the Department of Science and Technology (DST), Science and Engineering Research Board, (SERB), India for the financial grant (NO SB/EMEQ-463/2014, CRG/2019/00096). MJB thanks Department of Science and Technology (DST), Govt. of India for the DST-INSPIRE Fellowship (No. DST/INSPIRE Fellowship/2018/IF180217). MS also acknowledges CSIR-HRDG, New Delhi for the SRF fellowship (No 09/796(0097)/19-EMR-I). BD also thanks UGC-MHRD, Govt. of India for National Fellowship

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