Synthesis of Pd/Fe₃O₄ nanoparticle-based catalyst for the cross-coupling of acrylic acid with iodobenzene

Abstract

In this study, catalyst Pd/Fe₃O₄ based on magnetic body with the size of 8 nm was prepared and characterized by TEM, HRTEM, XPS and VSM methods. The results show that Pd⁰ atom produced after the first reduction is bound to APTS-coated Fe₃O₄ nanoparticles by the coordination of –NH₂ ligand with Pd⁰ and then grows into bigger clusters with the increase in the reduction times. However, when the reduction is performed for more than three times, Pd⁰ atom produced in subsequent reduction is dispersed in aqueous solution instead of depositing around preexisting Pd⁰ atom or clusters. In order to further investigate its catalytic behavior, Heck reaction of the cross-coupling of acrylic acid with iodobenzene was employed. The activity measurements show that the product yield decreased greatly from 81% for the first times to 53% for the fifth times and then kept constant in the subsequent re-use. One of the important reasons for this is that the catalyst aggregates into big particle and becomes more difficult to be dispersed with the increase in reaction times. In a word, this preliminary work provides the primary understanding of the catalyst based on Fe₃O₄ nanoparticles and its application in Heck reaction.

Introduction

In recent years, there are growing interests on the catalytic properties of transition metal nanoparticles because of their large surface area and a great ratio of atoms remaining at the surface [1], [2], [10]. Although much work has been done [3], [4], [5], [6], [7], [8], there is still the paramount challenge for the wide application of transition metal nanoparticles as catalysts in the industry, i.e., how to separate them completely from products. For example, due to the industrial importance of the Heck chemistry, which is traditionally associated with the palladium–phosphine-catalysed reaction of aryl bromides or iodides with olefins and presents one of the simplest ways to obtain various substituted olefins [5], many researches have been focused on designing recyclable catalyst based on Pd nanoparticles [9]. And an advance among those researches is the successful synthesis of dendritic catalyst that can be separated from the reaction medium by the nanofiltration membrane [2], [3], [4]. However, such separation does not meet the large-scale application in industry and another problem related to this approach is the leaching of heterogeneous dendritic catalyst [2].

At present, the main idea presented for recyclable systems may be built in liquid–liquid and solid–liquid modes [1]. For a liquid–liquid system, due to the high interfacial tension between water and low-polar organic liquids, the area of the interface is small even with vigorous stirring. Therefore, separation and catalytic efficiency come to a contradiction. Another approach to separate and recycle metal nanoparticles is to immobilize them on solid supports, such as organic polymeric (resin) [10] and inorganic microsphere (Pd/C, Pd/SiO₂, Pd/Al₂O₃, etc.) [1], [11], making it really easy and simple to separate the catalyst from the mixtures of the products. However, with solid catalysts suspended within liquids, the transport of reactants within the liquid to the catalyst bodies as well as the transport of them within porous catalyst bodies would be rate limiting, because the transport rate to the surface of the catalyst bodies is proportional to 1/D (D is the diameter of catalyst body) [12]. To raise the rate of the reaction, it is therefore attractive to utilize small catalyst bodies.

Because the conventional centrifugation or filtration, which calls for the particle with the size of at least about 3 μm, cannot be applied to separate the catalyst bodies of the nanometer size [12], it is necessary to develop new procedure, such as magnetic separation. However, there are two difficulties with the utilization of magnetic bodies as the catalyst supports, viz., the magnetostatic attraction between ferromagnetic particles and the effect of their surface properties on the catalyst.

To overcome the above deficiencies, we chose Fe₃O₄ nanoparticles with the size of 8 nm as the catalyst body. Before the preparation of the catalyst Pd/Fe₃O₄, its surface was firstly modified with the 3-aminopropyl triethoxysilane (APTS). And then the catalytic site Pd⁰ was bound to the surface of Fe₃O₄ nanoparticles by means of the coordination of –NH₂ ligand with Pd⁰. Finally, the catalytic behavior of Pd/Fe₃O₄ nanoparticles was measured by the Heck reaction of the cross-coupling of acrylic acid with iodobenzene (please see Scheme 1).