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In-Situ Formed Fe Cation-Modified Ir/MgO Catalyst for Selective Hydrogenation of Unsaturated Carbonyl Compounds Masazumi Tamura, Dai Yonezawa, Teruhisa Oshino, Yoshinao Nakagawa, and Keiichi Tomishige ACS Catal., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017
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ACS Catalysis
In-Situ Formed Fe Cation-Modified Ir/MgO Catalyst for Selective Hydrogenation of Unsaturated Carbonyl Compounds Masazumi Tamura* Dai Yonezawa, Teruhisa Oshino, Yoshinao Nakagawa, Keiichi Tomishige* Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai, 980-8579 (Japan). ABSTRACT: Selective hydrogenation of α,β-unsaturated ketones to α,β-unsaturated secondary alcohols is a challenging reaction for heterogeneous catalysts because of their non-uniformity of the surface active sites, and high-yield synthesis of the target α,β-unsaturated secondary alcohols has not been achieved by heterogeneous catalysts. Herein, we found that Fe cation-modified Ir/MgO acted as an effective heterogeneous catalyst for the reaction, providing high yield (up to 90%) of α,β-unsaturated secondary alcohols. The activity is high (initial TOF = 12 min-1 and TOFs (TOF per surface Ir sites) = 84 min-1), and the TOF is more than 10 times higher than those of the reported heterogeneous catalysts. To the best of our knowledge, the TOFs is the highest among the reported homogeneous and heterogeneous catalysts under additional basefree conditions. The catalyst was easily fabricated by only mixing Ir/MgO and Fe-cation precursor in the reaction media, and the active sites were proposed to be the interface among Ir metal, Ir4+ and Fe2+ species over MgO support based on kinetic studies and catalyst characterizations such as XPS, TPR, XRD, TEM and ICP. The cation-anion pair sites of Fe2+ and O2- formed at the active sites will bring about formation of active hydrogen species (H- and H+), leading to high selectivity and activity. The catalyst was also applicable to selective hydrogenation of α,β-unsaturated aldehydes, providing the corresponding α,β-unsaturated primary alcohols quantitatively (yield >99%). Keywords: Hydrogenation, Iridium, Iron, Magnesium oxide, Unsaturated ketone, Unsaturated alcohol
■ INTRODUCTION
OH
Highly efficient (highly selective and highly active) catalysts are essential for sustainable society, and heterogeneous catalysts will be preferable among various catalysts including homogeneous catalysts and enzymes owing to the high durability, reusability and ease of separation from the reaction mixture and so on1. However, it is generally difficult for heterogeneous catalysts to achieve high selectivity due to the non-uniformity of the surface active sites. Various techniques for tuning active sites have been intensively developed such as control of size, shape, component, and assemble structure2, but, cumbersome catalyst preparation methods are mostly required for these approaches. In-situ fabrication of well-designed heterogeneous catalysts by mixing easily-obtainable solid catalysts and additives is a promising methodology because it is simple and easy-to-use. Organic modifiers are often used for modification of catalytic property of heterogeneous catalysts3,4. Quinoline-modified Lindlar or Rosenmund catalysts3 and cinchona alkaloid-modified noble metal catalysts4 were well-known as amine modifier-improved catalysts. Metal salts such as Ge, Fe, Sn and Co salts were also used as modifiers, and catalytic performance of Pt or Co metal was enhanced by modification with the metal salts5. However, these approaches require a large amount of additives and/or additional treatment such as longtime mixing of the metals and metal oxides to tune the surface active sites precisely. Therefore, development of effective and facile methods for tuning the active sites on
R1 R2 α,β-unsaturated alcohol
O R1 R2 α,β-saturated carbonyl compound
O
R1 R2 α,β-saturated carbonyl compound
OH R1 R2 α,β-saturated alcohol
Scheme 1 Hydrogenation of α,β-unsaturated carbonyl compounds
the heterogeneous catalysts is highly desirable. Selective hydrogenation of α,β-unsaturated carbonyl compounds to α,β-unsaturated alcohols is an important reaction since α,β-unsaturated alcohols are useful chemicals and intermediates for perfumes, pharmaceuticals and agrochemicals6, and this reaction is a challenging task for chemists because hydrogenation of the C=C bond is thermodynamically and kinetically more favorable than that of the C=O bond (Scheme 1)6,7. Following the outstanding discovery by Noyori and Ohkuma that Ru complexes with both amines and strong inorganic bases drastically accelerated hydrogenation of the C=O bond in unsaturated carbonyl compounds but decelerated the hydrogenation of the C=C bond8, many excellent homogeneous catalyst systems including Ru9, Cu10 and Fe11 complexes were developed, and high selectivity (up to >99%) and activity (turnover frequency (TOF), up to 40 min-1) were achieved in hydrogenation of benzylideneacetone as a typical model reaction. After the discovery, inorganicbase-free Ru complex catalysts were also reported by Noyori and co-workers and Morris and co-workers12,
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however, the activity was much lower than that with strong inorganic bases such as t-BuOK, namely the bases are essential for obtaining high activity and selectivity, leading to production of inorganic salts. Recently, basefree effective catalysts13, in particular heterogeneous catalysts14 for the selective hydrogenation of “unsaturated aldehydes”, have been developed, although the activity is low (Typically, TOF99 2 26 35 49 >99 50 97 >99 12 11 10 5 9 7 12 12 58 38 70 20
3
Selectivity (%) 1 13 10 9 9 2
