Elsevier

Journal of Catalysis

Volume 324, April 2015, Pages 25-31
Journal of Catalysis

Aminopropyl-silica-supported Cu nanoparticles: An efficient catalyst for continuous-flow Huisgen azide-alkyne cycloaddition (CuAAC)

https://doi.org/10.1016/j.jcat.2015.01.014Get rights and content

Highlights

  • Room temperature preparation of 3-aminopropylsilica (APSiO2)-supported Cu nanoparticles.

  • Use of metal vapor synthesis technique to prepare very small Cu nanoparticles (1.0–4.5 nm).

  • The effective use of Cu/APSiO2 in continuous-flow click reaction.

  • Negligible Cu leaching (<9 ppm) in click reaction under continuous-flow conditions.

Abstract

Cu nanoparticles prepared by metal vapor synthesis (MVS) were immobilized on 3-aminopropyl-functionalized silica at room temperature. HRTEM analysis of the catalyst showed that the copper nanoparticles are present with mean diameters limited in the range 1.0–4.5 nm. TPR analysis was performed in order to study the oxidation state of the supported copper nanoparticles. The supported catalyst was used both in batch and in a packed-bed reactor for continuous-flow CuAAC reaction. The activation of the copper catalyst by reduction using phenyl hydrazine in continuous-flow conditions was demonstrated. Along with the high catalytic activity (productivity up to 1689 mol/mol), the catalyst can be used several times with negligible Cu leaching in the product (<9 ppm), less than allowed Cu contaminant in pharmaceuticals. The applicability of packed-bed flow reactor was showed by sequentially converting different substrates in their corresponding products using same column.

Introduction

The regioselective Cu(I) catalyzed 1,3-dipolar azide-alkyne cycloaddition leading to 1,4-disubstituted-1,2,3-triazoles (CuAAC) was independently introduced by the groups of Meldal and Sharpless [1], [2], [3]. Since then, the broad applicability, ease reliability, and high efficiency of this reaction stimulated an increasing interest for a wide range of synthetic applications, from drug discovery to material science and chemical biology [4], [5], [6], [7], [8], [9], [10], [11], [12]. Different copper sources have been described for the CuAAC, including simple halide salts and coordination complexes in homogeneous or heterogeneous form [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. From the last decade, nanostructured Cu systems are being explored in both homogeneous and heterogeneous form in CuAAC reactions [13], [14], [23], [24], [25], [26], [27], [28], [29], [30]. The combination of supported Cu-based catalysts and flow reactor technologies represents a step forward in terms of reliability, time, safety, and costs over traditional batch reaction conditions. Recently, considerable efforts in this direction have been devoted to the preparation of innovative Cu-based catalysts by using different strategies to perform CuAAC in flow [31], [32], [33], [34], [35], [36], [37], [38]. However, the development of highly recyclable heterogeneous systems with a copper contamination content into the final product at the acceptable levels still remains a challenge [35].

Here we report the preparation of Cu nanoparticles supported on 3-aminopropyl-functionalized silica (APSiO2) by metal vapor synthesis (MVS) technique and their use as catalyst both in batch and in continuous-flow Huisgen azide-alkyne cycloaddition (CuAAC). Supported Cu catalyst showed remarkable activity and stability in continuous-flow CuAAC with a very low Cu leaching in the product. In addition, the activation and regeneration of the catalyst by flowing phenyl hydrazine as a reducing agent for copper were demonstrated. In order to better understand the morphology and the active phase of the supported copper nanoparticles, high-resolution electron microscopy and temperature-programmed reduction (TPR) analysis, both in the pristine state and after use in catalysis, were performed.

Section snippets

General

All the reactions involving sensitive compounds were carried out under dry argon by using conventional Schlenk technique. All anhydrous solvents were either purchased in Sure/Seal™ bottles or dried by standard procedure. Cu shots, 2–8 mm, 99.999% trace metals basis, were purchased from Strem Chemicals. Acetone (Aldrich product) was dried over molecular sieves and stored under dry argon. 3-Aminopropyl-functionalized silica gel (40–63 μm, extent of labeling: ∼1 mmol/g NH2) was purchased from

Preparation and characterization of Cu nanoparticles supported on 3-aminopropyl-functionalized silica (Cu/APSiO2)

Cu nanoparticles were prepared by MVS technique [41], [42] and supported on APSiO2 (see Section 2.2). The MVS approach allowed to deposit very small copper nanoparticles (<5 nm) at room temperature directly in its reduced form, so that, calcination and activation processes, which can eventually modify the structure of hybrid organic/inorganic support, were avoided. The choice of amino-functionalized silica was dictated by the ability of primary alkyl amines to stabilize Cu nanoparticles [43],

Conclusions

New 3-aminopropylsilica supported copper catalysts have been developed, which showed remarkable activity and recyclability in continuous-flow CuAAC. With respect to other immobilized Cu catalysts reported to date, including nanostructured ones [35], [36], [73], [74], [75], [76], [77], [78], these novel systems displayed improved performances especially for what it concerns the low metal leaching and extended reuse under optimized conditions. Arguably, these unique properties can be traced back

Acknowledgment

This work was supported by the Italian Ministry of University and Scientific Research (MIUR) under the FIRB 2010 Program (RBFR10BF5V).

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