H−ZSM-5 Modified Zeolite:  Quantum Chemical Models of Acidic Sites

Abstract

A ZSM-5 fragment, containing 52 tetrahedral moieties, each of them formed by one silicon or one aluminum atom surrounded by four oxygen atoms, was employed to model (52T systems) by quantum chemical calculations (i) the influence of the positions of the acidic sites on the energetics of 22 aluminum monosubstituted and bisubstituted 52T acidic zeolite (H−ZSM-5) systems and (ii) the local adsorption properties and acidic strength of the corresponding −OH sites. The energetics and the structural properties of simpler acid H−ZSM-5 systems containing only five tetrahedral moieties (5T systems) were also modeled for comparison. B3LYP/6-31G(d,p) partial geometry optimization routines were performed on the 5T and 52T systems. On the latter, ONIOM(B3LYP/6-31G(d,p)|AM1) calculations and an alternative approach, i.e., the ONIOM method followed by a single-point at the above B3LYP/6-31G(d,p) level, aimed to decrease the need of computational resources, were also employed to analyze the properties of the different H−ZSM-5 models. The whole results showed that the orientation and the position of the acidic hydrogen atoms within the zeolite channel strongly affect the stability of the model systems, irrespective of the starting local topology characterizing the Al ↔ Si substitution site. Brönsted gas-phase acidity strength and adsorption-ability were evaluated through the analysis of the energy involved in (i) the proton dissociation from the acidic sites and (ii) the cis-but-2-ene and trans-but-2-ene adsorption on the same acidic sites. Both were affected, although to a very different extent, by the location and number of the considered −OH acidic groups. In particular, 2 among the 12 modeled acidic sites resulted in a highly stabilized zeolite structure, pointing out that the Al ↔ Si substitutions in the synthesis of aluminated ZSM-5 zeolites, and hence the corresponding catalytic activity, could preferentially occur on special sites. The choice of the computational method along with the size and the cutoff of the mimicked structures influenced the reliability of the calculations. The suggested alternative approach (that is, the ONIOM followed by the DFT single-point calculation) provided reasonable findings at very low computational cost.

A Possible Mechanism of Hydrogen Reverse Spillover in Platinum-Zeolite Catalysts

“…Table 1 lists geometric parameters and charges for the BAS (ZOH) as well as for the anion-radical (ZOH *-) and anion (ZO -) zeolite fragments. The geometric parameters for ZOH and ZO -are in agreement with published data [17,18]. In ZOH, the Al1-O1 distance is 0.22 Å higher than that of Si1-O1.…”

Density Functional Calculations on the Distribution, Acidity, and Catalysis of TiIV and TiIII Ions in MCM‐22 Zeolite

“…There is abundant information on the distributions of triand tetravalent ions (e.g., B, Al, and Ti) in MFI-type zeolites. [16][17][18][19][20][21][22][23][24][25][26][27] Recently, the Al locations in MCM-22 zeolite were studied by Corma et al and our group. [28][29][30] It was found that these ions are not equally distributed among the various T sites.…”

“…[28][29][30] It was found that these ions are not equally distributed among the various T sites. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Herein, density functional calculations were employed to determine the Ti IV distribution in MCM-22 zeolite. The design of isolated titanium dioxide (TiO 2 ) within zeolites is currently a research focus, [31] owing to the unique photocatalytic reactivity of the Ti III sites for a variety Abstract: Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions.…”

Adsorption, reduction and storage of hydrogen within ZSM-5 zeolite exchanged with various ions: A comparative theoretical study

“…The two-layer ONIOM(B3LYP/B1:AM1) scheme [41,42] was also used to optimize the nH 2 /ZAl III O structures (n = 0, 1, 2), and the comparisons with the (B3LYP/B1:B2) method showed that this ONIOM strategy is efficient for optimizing zeolitic systems. In fact, the combination of density functional and AM1 semi-empirical methods has been widely used in zeolitic calculations [34,[43][44][45][46][47]. Hence, the ONIOM methodology was used to study the reduction and restoring barrier vs. H 2 loading over the ZAl III O species (0 6 n 6 8) as well as H 2 adsorption behaviors of on the ZAl III (OH) 2 and ZM m species (n P 0), unless otherwise noted.…”