Hydrogenation

Catalysts and Processes

S. David Jackson

• Publisher: De Gruyter
• Year: 2018
• eBook ISBN: 9783110543759
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Environmental Science

This book gives a comprehensive overview of modern hydrogenation methods used in organic synthesis.
In clearly structured chapters, the authors cover the catalysts, scope and limitations of their application, and the techniques for hydrogenation of carbon-carbon, carbon-heteroatom and heteroatom-heteroatom multiple bonds.

C. Martin Lok

1 Structure and performance of selective hydrogenation catalysts

1.1 Introduction

Hydrogenation is one of the major reactions for the synthesis of pharmaceuticals, agrochemicals, fine chemicals, flavors, fragrances, and bulk chemicals like methanol and ammonia. The reaction generally is highly selective and easy to work up. It is commonly employed to reduce or saturate organic compounds in the presence of a catalyst such as Ni, Pd, or Pt. Rylander called it “one of the most powerful weapons in the arsenal of the synthetic organic chemist” [1]. An important feature is the atom economy, which for several reactions is 100% if the reaction is carried out by hydrogen, the cleanest of all reducing agents. The process usually does not generate any waste provided that the catalyst is selective and the excess hydrogen (and solvent) can be recycled. Moreover, heterogeneous catalysts can be recovered for reuse or for metal reclamation.

The earliest hydrogenation is that of the Pt-catalyzed reaction of hydrogen with oxygen that was commercialized in the Döbereiner’s Lamp as early as 1823. Later in 1897, the French scientist Sabatier discovered that in a gas-phase reaction, Ni catalyzed the addition of hydrogen to hydrocarbons [2]. Shortly afterward, in 1902, Normann was awarded a patent for hydrogenation in the liquid phase [3]. This was the beginning of edible oil and fatty acid hydrogenation which now is a worldwide industry. Normann first used Ni catalysts in the “hardening” of liquid oleic acid to the more valuable solid stearic acid and subsequently applied these catalysts in the hydrogenation of oils and fats. He explored commercialization of the process, first at Joseph Crosfield & Sons in the United Kingdom, and later in Ölwerke Germania in Germany [4]. The catalysts consisted of finely dispersed Ni, supported on, at first, pumice and later on a kieselguhr carrier. In 1926, Murray Raney, when involved in the hydrogenation of cottonseed oil, made his classical discovery of a catalyst based on a Ni/Al alloy [5]. Despite having been used for over 80 years, Raney® or “sponge” catalysts are still essential for a wide variety of industrial applications, such as the manufacture of sorbitol, sulfolane, fatty and alkylamines, hexamethylenediamine, 1,4-butanediol, and various fine chemicals and pharmaceuticals.

The early twentieth century was a landmark period for industrial catalysis and within decades, major processes for the production of methanol, ammonia, and liquid hydrocarbons, all based on hydrogenation processes, were discovered. The commercially important Haber–Bosch process for production of ammonia, first described in 1905, involves hydrogenation of nitrogen allowing the large-scale production of fertilizer. In 1913, Mittasch and Schneider patented the conversion of mixtures of carbon monoxide and hydrogen in the presence of heterogeneous catalysts such as supported Co resulting into the formation of liquid hydrocarbons in a reaction now commonly described as the Fischer–Tropsch (FT) process [6]. This discovery was not immediately followed up because priority was given to the commercialization of the methanol and ammonia processes [7, 8]. Since then, hydrogenation is widely applied for a variety of compounds.

Gaseous hydrogen is by far the most common source of hydrogen and is produced industrially from hydrocarbons by steam reforming or, on a smaller scale, by electrolysis. In organic synthesis, transfer hydrogenation is also used for hydrogenation of polar unsaturated substrates, such as ketones, aldehydes, and imines from donor molecules such as hydrazine, formic acid, isopropanol, and dihydronaphthalene [9]. These hydrogen donors undergo dehydrogenation to, respectively, nitrogen, carbon dioxide, acetone, and naphthalene.

Hydrogenation is a strongly exothermic reaction. The hydrogenation of an alkene involves a Gibbs free energy change of −101 kJ.mol−1. Even for the partial hydrogenation of a large molecule as a triglyceride (MW 890), the heat generated can be sufficient to raise the temperature of the oil by 50–100 °C. Bulk chemicals hydrogenation can be done in the gas or in the liquid phase. Mostly, no solvents are used.

In 1934, Horiuti and Polanyi proposed a reaction scheme assuming that the hydrogenation of alkenes occurs in three steps [10]:

• (a) Binding of both the unsaturated bond and the hydrogen molecule onto the catalyst surface. The hydrogen dissociates into atomic hydrogen.
• (b) Addition of one atom of hydrogen to the adsorbed olefin. In this step, the intermediate formed can rotate and, after releasing a hydrogen atom, may detach from the catalyst surface.
• (c) Addition of the second atom; this step is irreversible under hydrogenating conditions.

Free rotation of alkyl groups in the half-hydrogenated intermediate state results in cis–trans isomerization. At low hydrogen pressure, the rate of isomerization can even be higher than the hydrogenation rate. In addition, a double bond shift is possible by H abstraction from an adjacent CH2 group. These are general phenomena in partial hydrogenation.

The main classes of hydrogenation catalysts are homogeneous catalysts, stabilized metal nanoparticles, and heterogeneous catalysts. Homogeneous catalysts are often based on platinum group metals (PGMs), e.g., Wilkinson’s catalyst, RhCl(PPh3)3. Homogeneous catalysts are superior in asymmetric synthesis by the hydrogenation of prochiral substrates. An early demonstration of this approach was the Rh-catalyzed hydrogenation of enamides to produce the drug L-DOPA. In principle, asymmetric hydrogenation can be catalyzed by chiral heterogeneous catalysts too, but selectivity often is inferior to that of homogeneous catalysts. Homogeneous and enantioselective hydrogenations have recently been reviewed by De Vries et al. [11] and Blaser et al. [12,13,14]. The current chapter focuses exclusively on heterogeneous catalysts which are the catalysts most commonly used in commercial hydrogenation. For recent reviews on general hydrogenation, see Refs. [15,16,17,18,19].

1.2 Selective hydrogenations

Typical substrates for hydrogenation are alkenes, alkynes, aldehydes, ketones, esters, carbon monoxide, nitriles, and nitro-compounds which are converted into the corresponding saturated or partial-hydrogenated compounds, i.e., alkanes, alkenes, alcohols, and amines. Often complete saturation has to be avoided and the reaction should be stopped in, e.g., the mono-ene stage for polyolefins. Almost always a single hydrogenation product is required.

Examples of selective hydrogenation are as follows:

• – Partial hydrogenation of vegetable oils consisting of esters of glycerol and long-chain mono-enoic, di-enoic, and tri-enoic fatty acids. Depending on the target product, the reaction has to be terminated in the mono-ene or di-ene stage.
• – Production of pyrolysis gasoline. Selective hydrogenation of di-enes to mono-enes without saturating the aromatics.
• – Hydrogenation of triple bonds while avoiding full saturation. Examples are the removal of acetylene in ethylene and the partial hydrogenation of phenylacetylene to styrene.
• – Conversion of alkyl nitriles into primary amines without formation of secondary and tertiary amines.
• – Unsaturated carbonyl hydrogenation to unsaturated alcohols while minimizing saturated aldehydes and saturated alcohols.
• – Carbon monoxide and carbon dioxide hydrogenation to exclusively methane, methanol, or alkanes.
• – Hydrogenation of benzene to cyclohexane while avoiding formation of by-products like methylcyclopentane, a.o.
• – Asymmetric hydrogenation to chiral compounds.

In general, the requirements for a heterogeneous hydrogenation catalyst are high activity, excellent selectivity, long lifetime and reusability, and/or recyclability of the catalyst. In addition, fast filtration for powder catalysts is required. Activity and selectivity are strongly dependent on the choice of the main active metal. The main metal influences the strength of adsorption of reactants, the rate of desorption of reaction products, and the rate of chemical transformations. Metals most frequently used in heterogeneous catalytic hydrogenation are Pd, Pt, Rh, Ru, Ni, Co, and Cu. More rarely, Ir, Os, and Re are used. Subsequently, promoters (or selective poisons), additives, and supports have to be selected. For some reactions, selection of the carrier with the right porous structure and particle size distribution can be extremely critical. In addition, selectivity often is strongly dependent on the selection of reactor type and process conditions like temperature and pressure. Thus in the rhodium-catalyzed hydrogenation of chloronitrobenzenes, higher pressures favor the selective reduction of the nitro group, while at low pressur...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• List of Contributors
• 1 Structure and performance of selective hydrogenation catalysts
• 2 The Raney® catalyst legacy in hydrogenation
• 3 Model studies on hydrogenation reactions
• 4 Aromatic hydrogenation
• 5 Nitrile hydrogenation
• 6 Fischer–Tropsch synthesis – carbon monoxide hydrogenation
• 7 Heterogeneously catalyzed ammonia synthesis
• Index