Insertion Mechanism of N₂and O₂into T_n(n= 8, 10, 12)-Silsesquioxane Framework

Abstract

The process of insertion of molecular oxygen and nitrogen into polyhedral oligomeric silsesquioxanes (POSS) has been investigated theoretically. Using ab initio methods, the N2interaction with the POSS has been described with restricted Hartree−Fock (RHF) with a triple-ζ basis set, while systems involving O2 require restricted open shell (ROHF) wave functions, to account for their open-shell ground states. This insertion process is described in terms of the energetic change that the system X2::POSS undergoes when the gas molecule passes from the exterior to the interior of the cage through the largest of its faces. The formation of the cluster occurs through a transition structure that has been characterized for each system. The barrier is a function of the dimension of the face of the POSS and, hence, of the cage dimensions. The results of the calculation are consistent with experimental observations that the O2 molecules pass through a given membrane more easily than N2. Disciplines Chemistry CommentsReprinted (adapted) with permission from

Interaction of Molecular Nitrogen and Oxygen with Extraframework Cations in Zeolites with Double Six-Membered Rings of Oxygen-Bridged Silicon and Aluminum Atoms: A DFT Study

“…The Ca 2+ (SI)-X 2 (X 2 ) N 2 , O 2 ) equilibrium internuclear distance refers to a position of the adsorbate molecule at the top of the energy barrier on its way to penetrate the T 6 O 6 ring. Though this barrier was calculated to be sufficiently low for siliceous D6R, 13 the adsorption complex could not be localized inside the D6R.…”

“…13 In general, the atom that interacts directly with the extraframework cation acquires a partial negative charge or remains neutral, while the more distant atom acquires a positive charge, see Table 5. The "bare" cations induce very high polarization in the adsorbate molecule, particularly Ca 2+ .…”

“…The penetration of N 2 and O 2 into uncharged polyhedral silsesquioxanes (HSiO 3/2 ) n has been examined by the Hartree-Fock method. 13 These theoretical studies clearly show that both N 2 and O 2 are able to penetrate the zeolite framework via six-member rings, five-member rings, and even four-member rings and find equilibrium positions inside D6R, D5R, and D4R. The calculated energy barriers to N 2 and O 2 insertion in various cages were found to decrease in the order T 8 > T 10 > T 12 , where T n is the number of tetrahedral Si atoms in the cage-shaped structures.…”

Free-standing, polysilsesquioxane-based inorganic/organic hybrid membranes for gas separations

“…The OAPS is the type of POSS with a precise pore opening in face and the aminophenyl organic functional groups at eight silicon vertices. The distance between two diametrical opposite silicon atoms was estimated to be 4.442 Å based on computational studies [60,61]. Tejerina and Gordon assessed the feasibility of inserting small gas penetrants (i.e.…”

“…O 2 and N 2 ) into the faces of three different POSSs (i.e. T 8 , T 10 , and T 12 , where the "T n "indicates "n" silsesquioxenyl (HSiO 3/2 ) units) by using Hartree-Fock calculations [60]. They demonstrated that O 2 and N 2 were not accessible to the cage of T 8 POSS due to their high insertion energy into the cage of T 8 POSS, which is comparable to the dissociation enthalpy of the Si-O bond.…”