Fabrication of nanoscale covalent porous organic polymer: An efficacious catalyst for Knoevenagel condensation

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Highlights

  • Nanoscale spherical POP is synthesized at room temperature.

  • Effective and efficient heterogeneous catalyst for Knoevenagel condensation.

  • Almost complete product formation occurs at room temperature within 2 h.

  • Exhibits good reusability with stability after several times reuse.

Abstract

Porous organic polymer (POP) plays an important role as a catalyst for their unique properties like high porosity, excellent thermal stability and persistence towards various solvents (polar/nonpolar). In this work, a nanoscale spherical POP is fabricated by simple polymerization between triformylphloroglucinol and triaminobenzene at room temperature following Schiff-base condensation. The micro-porosity, morphology, surface charge and functionality of the synthesized POP are thoroughly characterized using the relevant techniques. The POP demonstrates efficient catalysis for Knoevenagel condensation at room temperature with higher % of conversion (~98%). The presence of imine functionality with high negative potential attracts H+ ions to facilitate the catalysis. Available active sites of the material favour to coordinate the loosed water molecule for the formation of the product. Here almost complete product formation (almost 98%) is observed within 2 h at room temperature for Knoevenagel condensation between benzaldehyde and malononitrile. The synthesized catalyst also exhibits efficient reusability with retaining good catalytic stability.

Graphical abstract

A POP is synthesized at RT following the Schiff base condensation reaction and used as sustainable heterogeneous catalyst towards Knoevenagel Condensation.

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Introduction

Recently, porous materials formed by covalent networks, such as Porous organic polymer (POP) and Covalent organic framework (COF) have gained significant attention in wide-spread research area like catalysis, adsorption, drug delivery, enzyme immobilization, semiconductor and so on [[1], [2], [3], [4], [5], [6], [7], [8]]. These porous materials usually exhibit higher surface area, various functionalities, and low density with a particular morphology. The crystalline materials are termed as COF and the amorphous materials are called POP. In recent days, POPs/COFs have been focused as a heterogeneous catalyst. The micro or meso-porous nature provides an enormous surface area, while the available binding sites are capable to hold a guest molecule. In some cases, metal ions encapsulated framework [[9], [10], [11], [12], [13], [14]], and other cases framework itself acts as a potential catalyst [[15], [16], [17], [18], [19], [20], [21], [22]] in the various organic reactions due to the availability of several functional groups with π-π interaction in their structural moiety.

These porous organic materials are generally synthesized by several methods like self-assembly polymerization, trimerization of nitriles, Schiff base reaction, boronic ester trimerization, and boronated ester formation. In general, COF or POP is synthesized by solvothermal procedure at high temperature in the presence of inert atmosphere for a prolonged time (almost 2–7 days). A few years before, R. Banerjee group presented the environment-friendly mechanochemical route by mixing an aldehyde and an amine moiety following Schiff base condensation reaction for synthesizing of COF [[23], [24], [25]]. Till now a very few works are reported where these porous materials are synthesized at room temperature by the procedure following Schiff base condensation. For example, Yang et al. fabricated COF at room temperature via solution-phase synthesis in ethanol medium [26]. C. Montoro et al. have developed a COF by mixing aldehyde and amine moiety in m-cresol or acetic acid at room temperature for potential application in fuel cell [27]. F. Zhang et al. have synthesized COF-LZU1 nanobars in CO2/Water solvent at room temperature [28]. B. Zhang et al. have reported the nitrogen-rich microporous polymers via the amine-formamido condensation reaction, a facile one-step polymerization reaction in the absence of any catalyst [29]. Although aforementioned articles demonstrate various porous organic material for the catalytic application, herein, we have presented an imine functionalized nanoscale porous organic framework via simple solution phase polymerization process by mixing 1,3,5-triformylphloroglucinol (Tp) and 1, 2, 4-triamino benzene (Tab) in DMF solvent at room temperature (RT). The framework structure of the TpTab synthesized by Schiff base condensation gets stabled by irreversible tautomerization [30]. The synthesized framework is not so crystalline; hence it can be called as porous organic polymer (POP). This TpTab POP shows effective and efficient catalytic properties for Knoevenagel condensation reaction under mild condition due to the presence of imine linkage, shown in Scheme 1.

Knoevenagel condensation, one of the most crucial organic reactions for Cdouble bondC bond formation by the reaction between aldehyde or ketone and an active methylene compound is usually catalyzed by the basic compound [[31], [32], [33]]. The reaction is mostly involved in the synthesis of key intermediate or building units of synthetic products like fragrance, dyes, carbohydrates, herbicides, etc. Practically, imine functionalized covalent frameworks act as an efficient heterogeneous catalyst for Knoevenagel condensation reaction due to their high stability as well as the availability of basic reaction sites. Till now, several covalent frameworks are reported as a catalyst in Knoevenagel condensation, where the base functionality or imine linkages performed the catalytic reaction [[34], [35], [36], [37], [38]]. But in most of these cases, the catalyst has some downsides for Knoevenagel condensation like (i) the requirement of high temperature, (ii) size selectivity of the organic moieties, and (iii) large time (generally 4–12 h) obligation. In this work, the synthesized TpTab POP showed the effective catalysis for Knoevenagel condensation at room temperature and almost completed the product formation within 2 h. The synthesized porous TpTab maintain stability after several times successive reuse.

Section snippets

Materials

1,2,4-Triaminobenzene dihydrochloride 96% (Tab) and the reagents such as malononitrile, benzaldehyde, 4-chlorobenzaldehyde, 4-methyl benzaldehyde, and 4-methoxy benzaldehyde were purchased from AlfaAesar. Dichloromethane (DCM) was purchased from TCI Chemicals Pvt. Ltd. Dimethylformamide (DMF), Ethanol and Sodium Sulphate Anhydrous extra pure (Na2SO4) were purchased from Merck India. Millipore water was used in overall work done.

Synthesize of TpTab

The TpTab was synthesized by following Schiff base condensation in

Surface morphology with particle size and charge distribution analysis

FESEM morphology reveals the porous nature of the POP and nanosized spherical morphology with an average particle diameter of 200 ± 10 nm, shown in Fig. S1. Different ratios of organic units (Tp:Tab ratio 1:0.5, 1:1, 1:1.5, 1:2) were used to study the growth of the porous polymer. The FESEM imaging of these materials results in the spherical morphology for all these four cases and the agglomeration is observed with enhancing the amine concentration. This result has also lied with the DLS

Conclusion

In conclusion, we have successfully synthesized a TpTab POP at room temperature by coprecipitation method following Schiff base condensation reaction. The porous nanosized spherical particle-like morphology of developed POP has been confirmed from the FESEM analysis. The functionality of the TpTab is studied in detail using FTIR, NMR and XPS analyses. The synthesized TpTab shows effective and efficient catalysis towards Knoevenagel condensation in room temperature due to the availability of –NH

CRediT authorship contribution statement

Arpita Samui: Methodology, Investigation, Writing - review & editing, Conceptualization. Neha Kesharwani: Formal analysis. Chanchal Haldar: Formal analysis. Sumanta Kumar Sahu: Validation, Supervision.

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.

Acknowledgment

Authors are thankful to Indian Institute of Technology (ISM) Dhanbad for giving us funding for research.

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