Direct Hydrogen Cyanide Synthesis and Water-gas Shift Reaction

Direct Hydrogen Cyanide Synthesis and Water-gas Shift Reaction 

The selectivity of modified hydrotalcite material for carbon dioxide with respect to water vapor (an important prerequisite for the water-gas shift reaction) was good, but it adsorbed large quantities of HCN, even though carbon dioxide is the more acidic component, making it wholly unsuitable for the direct HCN synthesis, especially as it subsequently proved impossible to recover the adsorbed HCN as such [21], 

Moving on to gas-solid reactions for CO2 capture, magnesium oxide can be rejected due to the lability of the corresponding carbonate and the greater stability of its hydroxide, while the old chestnut of calcium oxide requires temperatures in excess of 750 °C to regenerate the carbonate (Fig. 7.10). There are, nevertheless, other solid materials which can form carbonates in the temperature window of interest and be regenerated at lower temperatures of around 600 °C, for example lithium zirconate [38] which can theoretically adsorb 22.3 wt% CO2  

Our own studies with lithium zirconate have demonstrated the critical importance of the ratio between the lithium salts and zirconia used in its preparation [21]. Further, it appears that the reaction is strongly inhibited by the superficial formation of solid products curtailing capacity and impairing kinetics to the point where characteristic adsorption times are measured in hours rather than the seconds necessary. Very slow rates of CO2 adsorption can also be observed in the early published data on lithium zirconate and, while some progress has been made [39], developing a 

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

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