Pyridine, nonlinear effects

The results given in Table 2.1 show that the addition of even a small amount of pyridine greatly increased the conductivity of the solution. This increase was nonlinear as seen in Fig. 2.11. The viscosity, on the other hand, after the first jump at 0.5% concentration, increased linearly with pyridine addition (Fig. 2.12). The initial jump is thought to be due to the formation of large ionic complexes between the pyridine and acetic acid molecules and the increase thereafter due to the increase in the number of complexes formed in proportion to the pyridine amount. The effect of pyridine on the value of surface tension was small (Fig. 2.13). 

Table i shows the solution measurements for the polymers which were later cast as thin films. A comparison of samples 4 and which were measured as very dilute solutions, shows that nonlinearity increases by more than 2.5 times when pyridine is substituted for both benzene rings. This effect was seen in our earlier work however, the increase was not as dramatic when substitutions in analo-gous single-arene repeat unit polymers were compared. 

Substituting Mo + addenda atoms for V + (i.e., effectively reducing the catalyst) also leads to improvement of the catalytic performance [42], but in a nonlinear way. There is a maximum at one V atom. Further, neutralization of the Brpnsted acidity by exchanging the protons with different metal cations (Cs/Fe, Sb, Nb, etc.) brings the acrylic acid yield up to 13% (entries 3 to 6 in Table 13.3). This is a better way of reducing the acidity than with pyridine, because the latter tends to elude from the catalyst into the reaction effluent with time and the metal cations (Cs, in particular) significantly improves the thermal stability ( 450°C) and the surface area. 

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