Supercritical n Alkane Adsorption Data On 5A Zeolite Using the Gaussian Adsorption Isotherm Model

Supercritical adsorption data available in the literature for C₁ to nC₁₂  n alkanes on 5A zeolite is correlated using the Gaussian Adsorption model.  This isotherm model has three parameters: q_max, the saturation loading; the 50% adsorbate loading pressure P₅₀; and the standard deviation σ. The latter two parameters are deduced from a log plot of the isotherm pressure P versus the inverse Gaussian cumulative frequency distribution plot Φ^-1(q). The slope of this plot is σ and the y-intercept is log P₅₀.  The temperature dependency of P₅₀ is deduced from a plot of log P₅₀ versus 1/T: this is actually the isosteric heat of adsorption plot at 50 % loading with slope proportional to the heat of adsorption at 50 % loading (4.5 g/100g Z).

 The model is tested using 63 supercritical n alkane isotherms from the literature on 5A zeolite. The 63 isotherms from 18 different studies are fitted with the model using a q_max of 9 g/100g Z applicable to n alkane / 5A isotherms in the supercritical temperature range. The mean standard deviation σ is observed to be 0.960 with a standard deviation of 0.127. All 63 isotherms reduce to one characteristic curve when θ is plotted against (P/P₅₀)^1/ϭ.

The isosteres of P₅₀ indicate heats of adsorption (-ΔH₅₀) that are equal to or greater than literature values at zero loading (-ΔH₀); the difference between the two is negligible  for low carbon numbers rising to over 10 kcal/mole at higher carbon numbers (nC₁₀). This is expected as the heat of adsorption is calculated at 50% loading.