Influence of Polymer Content and Reactant Concentration
In Fig. 8.11 the global observed reaction rate against methanol concentration is depicted for four catalyst samples with different polymer content. In general, higher polymer content leads to lower activity for MTBE synthesis per active site. Only at a very low polymer content is the typical MTBE kinetic pattern observed. This difference from regular MTBE kinetics, as measured by Rehfinger and others [11, 17], is because the transport of isobutene may become mass-transport limited, as well as the mass-transport limitations of methanol within the macropores of the catalyst. The reason for mass-transfer limitations for isobutene may be gel phase diffusion inside the polymer particles. The polymer spheres formed during precipitation polymerization are non-porous particles much bigger than the microspheres in commercial ion-exchange resins. By increasing the polymer content, larger particles are formed. However, the polymer spheres of the new catalysts are much smaller than commercial ion-exchange resin beads.
The dimerization of isobutene (DIB formation) is the only observed side reaction. Fig. 8.12 shows the influence of methanol concentration on DIB formation, which increases with decreasing methanol concentration. In general this is expected, because at low methanol concentration active sites are available for this side reaction. In commercial macroporous ion-exchange catalysts (such as Amber-lyst 15) the by-product formation starts when MTBE synthesis reaches its maximum rate. This is only the case for the GFP-15 catalyst with the lowest polymer content. At higher polymer content the behavior changes. The reaction shows no ignition behavior. In general the dimerization rate falls with increasing polymer content. This indicates a mass-transfer limitation for isobutene in the polymer spheres.
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