Dehydrogenation selective poisoning effects

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

Under dehydrogenation conditions (385 °C ratio H2/HC = 4), an increase in the selectivity for aromatics with PtSn,(/Si02 catalyst has been observed. The increase in aromatic selectivity with tin content seems to be due to a geometric effect, favoring aromatic desorption. When the catalyst contains only small amounts of tin, an important poisoning by coke has been observed. As a consequence, it is possible that coke comes from adsorbed aromatic degradation. If aromatic formation starting from olefins had already and previously been proposed in the literature, their formation mechanism was still unknown. The coexistence of two possible dehydrocycHzation mechanisms has been proposed (Scheme 3.24). 

Irreversible deactivation can have a similar effect on the hydrothermal deactivation by deteriorating coke selectivity (for instance for nickel poisoning). The hydrothermal deactivation on its turn will now also have an effect on the catalyst poisons, as for instance on the mobility of vanadium and on the deactivation of vanadium and nickel as dehydrogenation catalysts [2]. 

These problems have been studied on Fe304 by Mars (7). It was found that at 300°C the reaction proceeds as to 70-85% in the direction of dehydrogenation, giving a H2/CO ratio which is lower than would have to be expected if the watergas shift reaction had reached equilibrium. Even a long residence time of the gas in the reactor has no effect upon the selectivity, which shows that a secondary reaction does not take place. Moreover it was shown that formic acid completely poisons the conversion of additional CO + H20. Evidently, formic acid is adsorbed very strongly on the surface as a result, the decomposition of formic acid is zero-order. 

Several adatoms effects were observed, with various reactions. This effect can usually be depicted as poisoning of undesirable sites, resulting simultaneously in a decrease of the global catalytic activity and in a significant increase of the selectivities for the desired products. We describe here three examples, the hydrogenation of a, -unsaturated aldehydes (the same reaction as above but now in the presence of a very small amount of tin), the isomerization of 3-carene into 2-carene, and the dehydrogenation of butan-2-ol into methyl ethyl ketone. 

A similar effect was observed during the dehydrogenation reaction of butan-2-ol into methyl ethyl ketone on Raney nickel (Scheme 11). Raney nickel is a very efficient catalyst for this reaction and leads to methyl ethyl ketone with a selectivity of ca 90%. This result is good but for industrial applications higher selectivities are required. This can be achieved by poisoning some sites by reaction with tetrabutyl tin (the best results are obtained with an Sn/Ni ratio of 0.02). Indeed, the reaction... 

Xhe presence of CO2 causes various kinetic effects it accelerates the reaction rate, enhances the selectivity, alleviates the chemical equilibrium, suppresses the unwanted total oxidation products, prevents the hot spots on the catalyst surface, poisons the non-selective sites of the catalysts, and the equilibrium yield of styrene dehydrogenation is much higher in the presence of CO2 than in that of steam. 

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