A novel SiO 2 supported Pd metal catalyst for the Heck reaction
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Catalysis Letters Vol. 112, Nos. 3–4, December 2006 (Ó 2006) DOI: 10.1007/s10562-006-0206-8
A novel SiO2 supported Pd metal catalyst for the Heck reaction L. Huang,* Z. Wang, T.P. Ang, J. Tan, and P.K. Wong Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore, 627833, Singapore
Received 20 August 2006; accepted 19 October 2006
A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO2 (O)) derived by calcination of Pd(acac)2/SiO2 in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd2+ is reduced to Pd0 on SiO2 by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd0/SiO2 is highly active for the Heck reaction, the remaining Pd2+/SiO2 is not responsible for the high activity. Pd/SiO2 (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO2 (H)) prepared by reduction of Pd(acac)2/SiO2 in H2 as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO2 (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO2 (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature. KEY WORDS: SiO2-supported Pd; Heck reaction; ligand-free; recyclable; highly active.
1. Introduction Among the various ways to synthesize arylated olefins, the construction of C(sp2)–C(sp2) single bonds through the Heck coupling is probably the most attractive [1–4]. Traditional Pd complex catalysts employ costly organic ligands such as phosphines. Since ligand-free Pd turned out to be feasible for the Heck reaction on aryl iodides [5], extensive studies of ligandfree Pd catalyzed Heck reactions on aryl halides have increasingly been appearing [2,6–10]. Due to the drawbacks with homogeneous catalysts, it has been desirable to develop heterogeneous Heck reaction catalysts for industrial applications and notable progress has been seen in this area [6,9,10]. Instead of organic ligands, ligand-free catalyst systems use inexpensive bases such as NaOAc, Na2CO3, NaHCO3, Ca(OH)2, K3PO3, and amines, etc. In homogeneous catalysis, the high activity and selectivity of catalysts can readily be achieved, but the problem of catalyst separation and the metal contamination of products cannot be avoided using such coupling reactions in the fine chemical industry. In heterogeneous catalysis, the circumstances are contrary. While the difficulties of catalyst separation and product purification can be overcome, the improvement of activity and selectivity would be realized by designing heterogeneous analogs of homogeneous catalysts. SiO2 is one of the most commonly used supports for preparing heterogeneous metal catalysts. Nevertheless, less attention has been drawn to the investigation of amorphous SiO2-supported metal catalysts toward the *
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