Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes as Catalysts for Hydrogenati
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Comparative Study of Homogeneous and Silica Immobilized N^N and N^O Palladium(II) Complexes as Catalysts for Hydrogenation of Alkenes, Alkynes and Functionalized Benzenes Saphan O. Akiri1 · Nondumiso L. Ngcobo1 · Stephen O. Ojwach1 Received: 6 December 2019 / Accepted: 18 March 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract This work reports the use of homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl2-((3(triethoxysilyl)propyl)imino)ethanol) (L1), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol) (L2), [L1MCM-41] (L1im), and [L2-MCM-41] (L2im) as catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The homogeneous complexes [Pd(L1)2] (Pd1), [Pd(L2)2] (Pd2), [Pd(L1)(Cl2)] (Pd3), and [Pd(L2)(Cl2)] (Pd4), and their respective silica immobilized complexes [Pd(L1)2]-MCM-41] (Pd1im), [Pd(L2)2)-MCM-4] (Pd2im), [Pd (L1) (Cl2)-MCM-41] (Pd3im) and [Pd(L2)(Cl2)]-MCM-41] (Pd4im) formed active catalysts in the molecular hydrogenation of these substrates. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed a pseudo-first order dependence on styrene substrate for both the homogeneous and immobilized catalysts. Significantly, the selectivity of both homogeneous and immobilized catalysts were comparable in the hydrogenation of both alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum loss of catalytic activity and showed absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established that complexes Pd1–Pd4 were largely homogeneous in nature, while the immobilized complexes Pd1im–Pd4im formed Pd(0) nanoparticles as the main active species. Graphic Abstract
Keywords Palladium · Immobilization · Hydrogenation · Alkenes · Benzenes · Recycling Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-020-03192-1) contains supplementary material, which is available to authorized users. Extended author information available on the last page of the article
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1 Introduction Molecular hydrogenation reaction of saturated substrates is among the vital catalytic processes in synthetic organic chemistry, both in industrial and academic laboratories, largely due its cleaner process [1]. The industrial applications of hydrogenation reactions range from pharmaceuticals, petrochemical and fine chemicals syntheses. Traditionally, platinum metal derivatives have been applied as catalysts in the hydrogenation of alkenes, alkynes, and other unsaturated hydrocarbons [2–4]. However, palladium catalysts have been widely applied since they possess a higher surface to volume ratio, than the platinum catalysts. A classic example is the Lindlar’s catalyst, which has been widely applied in food industries in the hydrogenation of alkynes [5]. Phosphine-donor palladium catalysts are the
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