Dehydrogenation, alkane catalyst

Johnson Matthey PLC, Alkane Dehydrogenation Catalyst, EP patent 937, 687A. 

The Development of Robust, Irldlum-Based Alkane Dehydrogenation Catalysts... 

In summary, a variety of bisphosphine (PCP) andbisphosphinite (POCOP) iridium complexes have been developed in order to have better catalyst stability and activity. It is important to note that a structure - activity relationship on the catalytic efficiencies of these modified Ir-pincer complexes has yet to be determined. What is known is that an optimal balance is required between the steric effect of the substituent and the electronic effect of the pincer ligand backbone to increase the catalytic activity. Thus, replacing the pincer ligand backbone could be an alternative strategy to increase the efficiency of this transformation. Recently, alkane dehydrogenation catalysts with oxazolinyl ligands have been reported, in which C-H bond activation occurs at the Ir(III) intermediate [132]. 

Tanaka [11], Saito [12] and Goldman [3, 13a] have reported that RhCl(CO)(PMe3)2 (5) is also an alkane dehydrogenation catalyst under photochemical conditions. Here the reactive species is probably the RhClL2 fragment formed by loss of CO (eq. 1 l)and studied in detail by Ford [14a] ... 

Jackson SD, Grenfell J, Matheson IM, et al Deactivation and regeneration of alkane dehydrogenation catalysts. In Bartholomew CH, Fuentes GAeditors Catalyst deactivation 1997, vol. Ill, Amsterdam, 1997, Elsevier, pp 167—174. 

The reversal of hydrogenation is also possible, as evidenced by the many iridium catalysts for alkane dehydrogenation to alkenes or arenes, though to date this area is of mainly academic interest rather than practical importance [19]. 

B. D. Chandler, L. 1. Rubinstein, andL. H. Pignolet, Alkane dehydrogenation with silica supported Pt and Pt-Au catalysts derived from phosphine ligated precursors, J. Mol. Catal. A Chem. 133,... 

The success of derivatives of 1 and 2 as dehydrogenation catalysts has led to the investigation of numerous different pincer ligands for iridium-catalyzed alkane dehydrogenation. The Anthraphos pincer iridium complex (3-H2) was expected to afford even greater thermal stability (Eig. 1), and indeed, the catalyst can tolerate reaction temperatures up to 250°C [42]. The catalytic activity of 3-H2, however, is much less than that of I-H2 under comparable conditions. 

Using I-H2 or 4a-H2 as the transfer-dehydrogenation catalysts, 1.25 and 2.05 M total product concentrations, respectively, were obtained from 7.6 M -hexane after 1 day at 125°C. In accord with the proposal illustrated in Scheme 2, exclusively linear -alkanes were formed (in contrast with the Basset systems). But in contrast with the idealized cycle depicted in Scheme 2, the mixtures were not restricted to C2n-2 alkane and ethane as products. 

The surface interactions hold the key to the catalyst s activity, selectivity, and stability. Unlike the situation for molecular catalysts, the bulk parameters such as particle size, shape, and mechanical strength are crucial here. Particle size is directly related to the accessible active surface area. Platinum, for example, is an excellent catalyst for alkane dehydrogenation, but it is also very expensive (ca. 32/g or 44/g... 

Van der Zande, L.M., De Graaf, E.A. and Rothenberg, G. (2002) Design and parallel synthesis of novel selective hydrogen oxidation catalysts and their application in alkane dehydrogenation. Adv. Synth. Catal.,... 

In addition to examples of stoichiometric reactions with alkanes, late transition-metal complexes also seem promising as dehydrogenation catalysts, in view of the many such complexes that catalyze olefin hydrogenation. Olefin hydrogenation is, however, a highly exothermic reaction (AH = ca. -125 kj mol ) and so there is a formidable enthalpic barrier to dehydrogenation. 

---