1. Field of the Inventive Concepts
The inventive concepts disclosed and/or claimed herein relate generally to catalysts and, more particularly, but not by way of limitation, to a heterogeneous, metal-free hydrogenation catalyst containing frustrated Lewis pairs. In one non-limiting embodiment, the heterogeneous, metal-free catalyst comprises hexagonal boron nitride (h-BN) having frustrated Lewis pairs therein.
2. Brief Description of Related Art
The addition of molecular hydrogen (H2) across an unsaturated organic compound such as an olefin is an important step in the processing of petroleum and bio-based chemicals. Although the conversion of olefins to alkanes is thermodynamically favored under appropriate conditions, such reactions are sluggish and a catalyst is required to facilitate hydrogenation. It was first observed that traces of nickel could mediate the addition of H2 to olefins. Later, it was found that palladium, platinum, rhodium, and other precious metals exhibited similar catalytic activity owing to the overlap between these metal's d-orbitals and the molecular orbitals of hydrogen.
Two disadvantages in the use of precious metals such as palladium, platinum, and rhodium are their cost and the potential leaching of such metals into the hydrogenation products. In order to avoid the high costs associated with palladium, platinum, and rhodium containing catalysts, industrial or commercial scale hydrogenation catalysts have generally been nickel-based. Such nickel-based hydrogenation catalysts have been used in a variety of processes, and especially for the hydrogenation of oils. Nickel-based catalyst systems are, however, readily poisoned in operation. While many potential poisons are removed during preprocessing, small quantities still remain, leading to catalyst degradation over time and operation. Additionally, measurable quantities of metal found in the catalyst are incorporated into the final hydrogenation product. The presence of these impurities in the hydrogenation products (or downstream items produced from such hydrogenation products) can have undesirable effects and may have increased toxicity in humans and animals. These and other factors make a nonmetallic or “metal-free” hydrogenation catalyst an attractive alternative for commercial scale hydrogenation processes.
Frustrated Lewis Pair (FLP) catalysts are potentially useful as one type of metal-free catalyst. Catalytic hydrogenation utilizing FLPs as a catalyst material is, therefore, a subject of growing interest because of the opportunity for the development and use of nickel and/or transition-metal-free hydrogenations reactions. In 2007, Stephan and co-workers, for example, developed the first non-transition metal system capable of releasing and absorbing molecular hydrogen using FLPs. Stephan determined that when a sterically encumbered Lewis acid approaches a bulky Lewis base, adduct formation is hindered and gives rise to electronic “frustration”. Such frustration effectively mimics the donor-acceptor properties of transition metals. Stephan and his team demonstrated that upon exposure to 1 atm H2 at 25° C., a solution of red phosphino-borane [(C6H2Me3-2,4,6)2P(C6F4)BF(C6F5)2]transformed to the colorless zwitterionic salt [(C6H2Me3-2,4,6)2PH(C6F4)BH(C6F5)2]. Upon thermolysis at 150° C., the salt lost H2 and converted back to the original phosphine-borane substrate. Such phosphonium borates (as well as similar compounds) have been shown to successfully catalyze the hydrogenation of select imines, enamines, aldehydes, and olefins.
To date, only homogeneous FLP catalytic systems have been studied, i.e., FLP catalysts and the resulting reactions have previously involved the FLP catalyst being in the same phase as the reactants. For example, the homogeneous FLP catalyst is typically co-dissolved in a solvent with the reactants. Heterogeneous catalysis, on the other hand, is performed with the catalyst in a different phase from that of the reactants. One example of heterogeneous catalysis is the petrochemical alkylation process where the liquid reactants are immiscible with a solution containing the catalyst. Heterogeneous catalysis offers the advantage that products may be readily separated from the catalyst. Typically, heterogeneous catalysts are more stable and degrade much slower than homogeneous catalysts.
In view of the foregoing, there is a need for a heterogeneous, metal-free hydrogenation catalyst having improved stability and separation characteristics. In particular, a heterogeneous, metal-free hydrogenation catalyst having an FLP-type electronic structure would be a valuable addition to catalysts currently available for industrial and commercial use. It is to such a heterogeneous, metal-free hydrogenation FLP catalyst that the presently disclosed and/or claimed inventive concept(s) is directed.