Computational study of the effect of confinement within microporous structures on the activity and selectivity of...

The effect of confinement within some zeolitic structures on the activity and selectivity of metallocene catalysts for the ethylene oligomerization has been investigated using grand canonical Monte Carlo simulations (GCMC). The following zeolite (host) frameworks displaying different pore sizes, have been studied as solid hosts: mazzite (MAZ), AIPO-8 (AET), UTD-1F (DON), faujasite (FAU), and VPI-5 (VFI). Intermediates and transition states involved in the ethylene trimerization reaction catalyzed by a Ti-based catalyst [(η(5)-C(5)H(4)CMe(2)C(6)H(5))TiCl(3)/MAO] have been used as sorbates (guests). We have demonstrated linear correlations with slope a(H,j) between the adsorption enthalpy and the molecular volume V(m) of the sorbates, each holding for a given microporous host below a host-specific threshold V(mmax,j). Beyond this maximal molecular volume, the adsorption vanishes due to steric exclusion. a(H,j) increases, and V(mmax,j) decreases with decreasing host pore size, in line with the confinement concept. We moreover showed that, in the limit of vanishing loading (Henry regime), the enthalpies and entropies of adsorption in a given host are linearly correlated. We have defined a host-specific confinement compensation temperature a(j), which refers to a temperature where the stabilizing adsorption enthalpic interactions are canceled out against the loss in entropy. However, calculated a(j) are much larger than the operating temperatures. With a setup microkinetic model, we predict that the activity and selectivity of the confined Ti-catalyst in ethylene oligomerization can be significantly altered with respect to homogeneous phase conditions, since the adsorption free energies of transition states and intermediates also become functions of a(H,j) and V(m). We have applied this theory to predict the optimum host pore size to get maximum α-octene production, instead of α-hexene, which is primarily produced in the homogeneous phase. We also predict a significantly increased activity for confined catalysts.