Ethene, hydrogenation deactivation

A little work on structure-insensitive reactions has been reported [18]. Both catalysts were very active for ethene hydrogenation, and rapid deactivation occurred even at 176 K. Ethyne and 1,3-butadiene react in a more controlled manner study of ethyne hydrogenation using both l4C-labeled ethyne and ethene showed that ethane formation took place directly from adsorbed ethyne, without the intervention of gas-phase ethene. 

Causes of deactivation are basically three-fold chemical, mechanical or thermal— hereby six different routes of deactivation of catalyst material are described (some have been introduced before, without further explanation) poisoning (i.e. CO on Pt), fouling (i.e. coke formation during ethene hydrogenation on Pt), thermal degradation, vapor compound formation accompanied by transport, vapor-solid and/or solid-solid reactions, and attrition/crushing [162, 163]. 

Coking, widely experienced in the catalysis of hydrocarbon conversion (7), can deactivate both metallic and acid catalytic sites for hydrocarbon reactions (2). Accumulation of such carbonaceous deposits affects selectivity in hydrocarbon conversion (5). Adsorbed ethene even inhibits facile o-p-Hj conversion over Ni or Pt (4 ), the surface of which it appears is very nearly covered at lower temperatures in such deposits. H spillover may enhance hydrocarbonaceous residue formation (6). Accumulated carbonaceous residues can be removed by temperature programmed oxidation, reduction and hydrogenation TPO, TPR, TPH, etc (7) as part of catalyst regeneration. 

Removal of 1,3-butadiene by selective hydrogenation from industrial olefin feedstocks before hydroformylation or polymerization is an important process in preventing catalyst deactivation. Amorphous Cu70Zr3o exhibits an excellent ability to do this (152). At 348 K a mixture of butenes containing 3% 1,3-butadiene could be converted to a diene-free product with only 1.63% butane. This catalyst also hydrogenates 1,3-butadiene in ethene with a selectivity of 95% with no hydrogenation of ethene. 

The purpose of this section is to provide an overview of the principal kinetic features of the hydrogenation of ethene and of propene, as providing a framework (or at least part of one) within which discussion of mechanisms must be conducted. Their reactions with hydrogen (and with deuterium) are quite comparable the addition of the methyl group leads to somewhat higher reactivity, due to weaker chemisorption as might be predicted from its lower heat of hydrogenation (Table 7.1). Relative rates for other alkenes will be considered later. The problem of deactivation by carbon deposition has already been mentioned, but quantitative... 

---