Oxidation of aldehydes to carboxylic acids over geopolymer supported CuO
Graphical abstract
Introduction
Oxidation is one of the prominent reactions for the synthesis of a variety of organic compounds. Oxidation of aldehydes to their corresponding carboxylic acids is an essential transformation during organic synthesis because the derivatives of carboxylic acids play a crucial role in pharmaceuticals and agrochemicals [1]. Majority of these transformations involve numerous well established oxidizing reagents such as CrO3, K2Cr2O7 and quinolinium dichromate [2,3], KMnO4 [4,5], H2O2 [6], KHSO5 [7] and NaClO [8] to name a few. A major limitation of the utilization of these reagents is the formation of toxic by-products after oxidation. Therefore, attention needs to be paid towards the development of efficient, eco-friendly, economic and greener oxidation methodologies for the oxidation reaction. In this regard, aerobic oxidation with heterogeneous catalysts is an alternative and widely used protocol which offers a unique advantage over the traditional ones of the availability of oxygen from air. This also been reported to show better atom economy [9].
Copper-based catalysts [10,11], both in homogeneous [12] and heterogeneous [13] forms, are well established for the aerobic oxidation of aldehydes to the corresponding carboxylic acids. CuO based materials are attractive owing to their low toxicity and high stability as compared to the other well-established reagents including CrO3, KMnO4, KHSO5 and NaClO. Liu and Li [12] reported aerobic aldehyde oxidation over copper-catalysts which proceeded under mild aqueous conditions with oxygen as the sole oxidant. Even though homogeneous catalysts exhibit good activities, heterogeneous catalysts are desirable because of the ease of their separation from the reaction mixture and their reusability. Solid catalysts such as Ag2O/Cu2O, Ag2O/CuO [13] and Ru/CeO2 [14] have been reported to be environment friendly because of the ease of separation and recyclability. However, such catalysts are not always cost effective. CuO can be combined with different supports like ZrO2 [15], CeO2 [16], Ag2O and hyper-cross-linked polystyrene [17] to serve as effective catalysts for similar reactions. Still, there are concerns regarding the activity and reusability of such compositions.
While they are not as active as homogeneous or single-atom catalysts reported for oxidation reactions [18], [19], supported catalysts provide a step towards the search for eco-friendly and economic catalyst development. In this context, geopolymers could provide a good platform for CuO. In this work, efforts have been made to synthesize an economic, stable and scalable supported catalyst for the oxidation of aldehydes to carboxylic acids. Geopolymers are a promising choice in this regard as they can be synthesized with ease from natural minerals like kaolinite and industrial waste materials like slag and fly ash [20], [21], [22], [23], [24], [25]. Their synthesis is scalable and they have been used as the supports for several heterogeneously catalyzed organic reactions [25], [26], [27], [28]. In this communication, we report a novel catalyst based on CuO supported over geopolymers. Geopolymers have a unique advantage that they are highly economical alongside their high surface area and mesoporosity which are beneficial for catalysis. This class of materials is less explored in heterogeneous catalysis as a support. Using the support, we can minimize the metal/metal oxide content without sacrificing the activity thereby making the process economical. Use of a support also helps preventing leaching of the active metal. The catalyst (CuO/Geo) was employed for the oxidation of aldehydes. The catalyst showed good activity and recyclability. The reaction was observed to take place in oxygen atmosphere under mild conditions with water as the only and green solvent. Density functional theory (DFT) based theoretical calculations were employed to quantify the energetics of the reaction. Our theoretical analysis provided detailed structural and mechanistic insights, thereby providing an understanding of the catalytic processes associated with the reaction.
Section snippets
Experimental details
All chemical reagents were obtained from Sigma-Aldrich, Alfa Aesar and S. D. Fine. All the chemicals were used for the experiments directly without further purification. Kaolinite, fumed silica and potassium hydroxide were used for the synthesis of the geopolymer support. Deionized water was used for the reaction and washing the catalyst. Sodium hydroxide as a base and 37% hydrochloride acid were used for the synthesis. Thin layer chromatography (TLC) plates (Merck silica gel 60 F254) were used
Structure and morphology of the synthesized catalyst
XRD of the geopolymer, CuO/Geo and pure copper oxide are shown in Fig. 1. The geopolymer showed a broad peak in the range of 20°−35° conforming the formation of the geopolymer. The presence of crystalline quartz, mallite and illite in the metakaolinte was also noticed which matched well with the reports in the literature [22,30]. The diffraction pattern of CuO/Geo was similar to the pure geopolymer with no significant diffraction peaks of CuO. This could be due to low concentration and small
Conclusion
In summary, a novel heterogeneous catalyst incorporating copper oxide in mesoporous geopolymer as a support was synthesized. Geopolymer was used as a stable and efficient support for the synthesis of CuO/Geo catalyst. The resulting catalyst was found to be active for the oxidation of aldehydes to their corresponding carboxylic acids with molecular oxygen as an oxidant and water as solvent. The catalyst was reused for six times for the same reaction and showed comparable activities with each
Credit authorship contribution statement
Jyoti Upadhyay: Synthesis, characterization, writing – original draft. Swayam Prabha Misra: Formal analysis, computational investigation, writing – original draft. Sudhanshu Sharma and Parag A. Deshpande: Conceptualization, methodology, resources, investigation, supervision, writing – review & final draft.
Declaration of Competing Interest
Authors declare that they have no conflict of interest.
Acknowledgements
PAD and SS gratefully acknowledge DST SERB project ((EMR/2016/005957) for the research funding and IITGN CIF for the instrumentation facilities and SAIF IIT Mumbai for ICP-AES analysis. SM and PAD gratefully acknowledge the Param Shakti-National Supercomputing Mission (NSM), Government of India, for providing the computational resources at Indian Institute of Technology Kharagpur, India.
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These authors have contributed equally to the study.