Recent Advances in Polyoxometalate-Catalyzed Reactions

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H2) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis–structure–function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer–Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer–Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO2 and recycle side-products in the process are described in the third section.

Status and prospects in higher alcohols synthesis from syngas

This paper summarizes recent advances in α-alkylation reactions based on hydrogen borrowing methodologies using alcohol as an alkylating agent. This review provides a summary of recent progress toward the α-alkylation of carbonyl substrates, as well as relatively unactivated substrates bearing fewer acidic α-hydrogens, such as acetonitriles, acetamides, esters, methylpyrimidines, and methylquinolines. A summary of recent improvements in α-methylation strategies based on hydrogen borrowing methodologies has also been provided. Particular emphasis has been placed on highly practical and green chemistry approaches involving modified catalytic systems, including metal-supported heterogeneous catalysts and nanoparticle-based catalysts, as well as reactions conducted in the absence of a transition-metal catalyst. A review of recent achievements in methylation strategies using methanol as a methyl source, and their application to the α-methylation of ketones using transition-metal catalyzed hydrogen borrowing me...

Recent Advances in α-Alkylation Reactions using Alcohols with Hydrogen Borrowing Methodologies

This review presents recent developments in some important industrial processes. For homolytic systems, the similarities and specificities of the transformation of cyclohexane into adipic acid (via the formation and isolation of the cyclohexanol–cyclohexanone mixture) and of the process for the preparation of terephthalic acid (by oxidation of p-xylene) are analysed from a prospective viewpoint. Innovations in heterolytic chemistry are also presented. For example, the epoxidation of propene currently performed by the Arco–Lyondell method, which uses a molybdenum catalyst and tert-butyl hydroperoxide, is in competition with other systems involving recycling of the co-product, or the use of hydrogen peroxide as oxygen donor. Access to diphenols, from phenol and H2O2, is becoming more selective. New oxoperoxo complexes of titanium, rhenium, tungsten and molybdenum are presented with their potential for catalytic oxidation. Achievements of our group in the development of catalysts for C–C bond cleavage by dioxygen, and for epoxidations by H2O2 are also given. Biomimetic oxidations and systems without metal catalysts feature among the innovations. These novel systems could compete with transition-metal-based chemistry for fine chemicals, but industry increasingly focuses research and development on fully inorganic systems, and there is little impact on the large scale of catalytic complexes with organic ligands.

Transition-metal complexes for liquid-phase catalytic oxidation: some aspects of industrial reactions and of emerging technologies

Carbon dioxide is ubiquitous and a vital molecule for maintaining life on our planet. However, the ever-increasing emission of anthropogenic CO2 into our atmosphere has provoked dramatic climate changes. In principle, CO2 could represent an important one-carbon building block for the chemical industry, yet its high thermodynamic and kinetic stability has limited its applicability to only a handful of industrial applications. On the other hand, carbon monoxide represents a more versatile reagent applied in many industrial transformations. Here we review the different methods for converting CO2 to CO with specific focus on the reverse water gas shift reaction, main element reductants, and electrochemical protocols applying homogeneous and heterogeneous catalysts. Particular emphasis is given to synthetic methods that couple the deoxygenation step with a follow-up carbonylation step for the synthesis of carbonyl-containing molecules, thus avoiding the need to handle or store this toxic but highly synthetically useful diatomic gas. CO is a vital building block in organic synthesis but, due to its toxicity, storage and transport can be problematic. This review focuses on the methods — both chemical and electrochemical — for the in situ generation of CO from CO2, and its subsequent incorporation into chemicals through catalytic means.

Chemically and electrochemically catalysed conversion of CO 2 to CO with follow-up utilization to value-added chemicals

Monsanto initiated development of its rhodium- and iodide-catalysed process for the carbonylation of methanol to acetic acid in 1966. Ownership of the technology was acquired in 1986 by BP Chemicals who have further extended it. The work of the Sheffield group, in developing a deeper understanding of the mechanism of the process, is reviewed. The rate-determining step in the rhodium–iodide catalysed reaction is the oxidative addition of methyl iodide to [Rh(CO)2I2]–1a: the product from this reaction, the reactive intermediate [MeRh(CO)2I3]–2a has been detected and fully characterised spectroscopically. The rates of the reversible reactions linking 1a, 2a and the acetyl complex [(MeCO)Rh(CO)I3]–3a, as well as activation parameters for several of the processes involved, have been measured. The efficiency of methanol carbonylation arises primarily from rapid conversion of 2a into 3a, leading to a low standing concentration of 2a, and minimising side reactions such as methane formation. By contrast, in the iridium-catalysed carbonylation, for which similar cycles can be written, the reaction of [MeIr(CO)2I3]–2b with CO to give [(MeCO)Ir(CO)2I3]–4b is rate determining. Model studies show that while kRh/kIr is ca. 1 : 150 for the oxidative addition, it is ca. 105–106 : 1 for migratory CO insertion. The migratory insertion for iridium can be substantially accelerated by adding either methanol or a Lewis acid (SnI2); both appear to facilitate substitution of an iodide ligand by CO, resulting in easier methyl migration. The carbonylation of higher alcohols (ROH) has also been successfully modelled; the corresponding alkyl iodides (RI) react much more slowly than MeI with [M(CO)2I2]–, but again give the acyls [(RCO)M(CO)I3]– for M = Rh and the alkyls [RM(CO)2I3]– for M = Ir. The greater stability of [MeIr(CO)2I3]– compared with [MeRh(CO)2I3]– accounts for the very different characters of the reactions catalysed by the two metals. It is suggested that the broad features of the Rh/Ir reactivities can be rationalised since the M–C bond to a 5d metal is generally stronger than that to the corresponding 4d metal; thus if metal–ligand bond making plays a key role in a step, then the 5d metal is more likely to react faster (e.g. in the oxidative addition), but if a metal–ligand bond-weakening or -breaking step plays a key role in a process (e.g. in the migration), it is likely that the 4d metal will be faster.

Methanol carbonylation revisited: thirty years on.

The major features of three catalyst systems for ‘acetyls’ processes developed by BP Chemicals are described: 1. the homogeneous, promoted iridium methanol carbonylation system for acetic acid manufacture recently commercialised in the USA and Korea (CATIVA), 2. a non-commercialised ruthenium promoted rhodium system, also for methanol carbonylation, 3. a vapour phase reaction of ethylene with acetic acid over silicotungstic acid supported on silica giving commercially viable activity and catalyst lifetime for the manufacture of ethyl acetate. All three examples illustrate the importance of exploring process conditions to reveal the advantages of new catalyst systems, or transform known catalysts into commercial viability.

Mark J. Howard

Papers

Methanol carbonylation revisited: thirty years on.

New acetyls techonologies from BP chemicals