Microwave‐Assisted Cross‐Coupling and Hydrogenation Chemistry by Using Heterogeneous Transition‐Metal Catalysts: An Evaluation of the Role of Selective Catalyst Heating

Abstract

The concept of specific microwave effects in solid/liquid catalytic processes resulting from the selective heating of a microwave-absorbing heterogeneous transition-metal catalyst by using 2.45 GHz microwave irradiation was evaluated. As model transformations Ni/C-, Cu/C-, Pd/C-, and Pd/Al2O3-catalyzed carbon-carbon/carbon-heteroatom cross-couplings and hydrogenation reactions were investigated. To probe the existence of specific microwave effects by means of selective catalyst heating in these transformations, control experiments comparing microwave dielectric heating and conventional thermal heating at the same reaction temperature were performed. Although the supported metal catalysts were experimentally found to be strongly microwave absorbing, for all chemistry examples investigated herein no differences in reaction rate or selectivity between microwave and conventional heating experiments under carefully controlled conditions were observed. This was true also for reactions that use low-absorbing or microwave transparent solvents, and was independent of the microwave absorbtivity of the catalyst support material. In the case of hydrogenation reactions, the stirring speed was found to be a critical factor on the mass transfer between gas and liquid phase, influencing the rate of the hydrogenation in both microwave and conventionally heated experiments.

Pd/C Catalysis under Microwave Dielectric Heating

“…However, in 2009, Kappe and coworkers reproduced the Suzuki-Miyaura coupling reactions shown in Scheme 1 obtaining mostly identical results both in terms of conversion and selectivity with MW irradiation under simultaneous cooling and conventional heating at the same temperature and in the same reaction times [3]. This observations were later confirmed by Leadbeater's group demonstrating how simultaneous cooling has no effect on large scale Suzuki couplings [18].…”

“…Furthermore, charcoal is one of the most active MW absorbing materials known so far. It is able to efficiently absorb MWs becoming an internal heat source by the very high surface temperatures generated [3]; as most of the heterogeneous catalyzed reactions occur on the active surface of the catalyst, Pd/C MW-assisted transformations are highly accelerated [4]. The electric discharge phenomena in metal-solvent mixtures have recently been reviewed [5], supporting the hypothesis that arcing is simply limited to large metal particles [6].…”

“…The previously observed differences between conventionally and microwave heated hydrogenations could be ascribed more to the efficient heating and the resulting higher H2 dissolution into the solvent than to the selective MW heating of the catalyst [3,47].…”

Microwave‐Assisted Selective Hydrogenation of Furfural to Furfuryl Alcohol Employing a Green and Noble Metal‐Free Copper Catalyst

“…Microwave irradiation enables a fast, uniform, and efficient dielectric heating of the reaction media, generating an increase in reaction rates as well as reducing the energy consumption 29, 30, 31, 32, 33, 34. The use of this technique represents a breakthrough in terms of sustainability, efficiency, development of new materials, and cost reduction 31, 35, 36, 37.…”

Iron Nanoparticles Supported on Chemically‐Derived Graphene: Catalytic Hydrogenation with Magnetic Catalyst Separation

“…The hydrogenation of olefins and alkynes is typically promoted by heterogeneous catalysts based on precious metals such as palladium or platinum. [1][2][3][4][5] In order to render catalytic hydrogenation reactions more efficient, many efforts have been made to decrease the costs for these reactions either by obtaining highly active catalytic species or by substituting inexpensive metals such as iron for the commonly used noble metals Pd and Pt. [6][7][8][9][10] The broad availability of iron makes it a desirable metal for catalysis.…”