Glycerol: An Optimal Hydrogen Source for Microwave-Promoted Cu-Catalyzed Transfer Hydrogenation of Nitrobenzene to Aniline

Abstract

The search for sustainable alternatives for use in chemical synthesis and catalysis has found an ally in non-conventional energy sources and widely available green solvents. The use of glycerol, an abundant natural solvent, as an excellent "sacrificial" hydrogen source for the copper-catalyzed microwave (MW)-promoted transfer hydrogenation of nitrobenzene to aniline has been investigated in this work. Copper nanoparticles (CuNPs) were prepared in glycerol and the efficacy of the glycerol layer in mediating the interaction between the metal active sites has been examined using HRTEM analyses. Its high polarity, low vapor pressure, long relaxation time, and high acoustic impedance mean that excellent results were also obtained when the reaction media was subjected to ultrasound (US) and MW irradiation. US has been shown to play an important role in the process via its ability to enhance CuNPs dispersion, favor mechanical depassivation and increase catalytic active surface area, while MW irradiation shortened the reaction time from some hours to a few minutes. These synergistic combinations promoted the exhaustive reduction of nitrobenzene to aniline and facilitated the scale-up of the protocol for its optimized use in industrial MW reactors.

Keywords: copper nanoparticles; glycerol; microwaves; transfer hydrogenation; ultrasound.

Figure 1

(a) HRTEM image of CuNPs, (b) zoom-in of the region shown in (a), and (c) schematic representation of nanoparticle morphology and graph reporting the measurements of the spacings between the observed diffraction fringes. Instrumental magnification 3,00,000 ×.

Scheme 1

CuNPs-catalyzed reduction of nitrobenzene (1a) using glycerol as reducing agent to obtain aniline (2a).

Figure 2

Particle-size distributions, PSD. Blue curve freshly prepared CuNPs. Green curve freshly prepared CuNPs after heating by MW irradiation for 10 min (Anton Paar Monowave 300, T = 130°C, PMax = 400 W). Red curve CuNPs after sonication for 10 min [Hielscher UP50H, F(kHz):30, P(W):50].

Figure 3

Nitrobenzene reduction profile with and without previous CuNP US-sonication. Pretreatment: CuNPs, US irradiation 10 min [Hielscher UP50H, F(kHz):30, P(W):50]. Reaction conditions: nitrobenzene (1 mmol), KOH (2 mmol), glycerol (40 mmol), CuNPs (5 mol%), 130°C. The % of aniline was determined by GC-MS.

Figure 4

Temperature and power profile curves registered by MW instruments: (A) Anton Paar Monowave 300, Program: 2 min P_max = 100% heated as quickly as possible to reach 130°C, then T = 130°C; (B) Milestone MicroSynth, Program: 2 min P_max = 400 W heated as quickly as possible to reach 130°C, then T = 130°C; (C) Anton Paar Monowave 300, Program: 2 min P_max = 100% heated as quickly as possible to reach 130°C, then P: 4W; (D) Milestone MicroSynth, Program: 2 min P_max = 400 W heated as quickly as possible to reach 130°C, then P: 80 W.

Figure 5

CuNPs behavior when applying MW-irradiation (Anton Paar Monowave 300, Program: 2 min Pmax = 100% heated as quickly as possible to reach 130°C, then T = 130°C). Recorded using USB Digital Microscope Supereyes B003+.

Figure 6

First generation multimode MW reactor. Internal cavity: 72L Power: 1.2 kW. MEAM Explorer VP.

Figure 7

Histogram density of temperature recorded by IR camera Optris IP Connect. Reaction time: 10 min.

Figure 8

(A) Power profile while working at constant power (25–30 W). (B) Power profile while working at varying power (40–0 W) (the power meter displays 0–4 W when the magnetron is completely off).