An Examination of Confinement Effects in High-Silica Zeolites

A Universal Descriptor for the Entropy of Adsorbed Molecules in Confined Spaces

Confinement of hydrocarbons in nanoscale pockets and pores provides tunable capability for controlling molecules in catalysts, sorbents, and membranes for reaction and separation applications. While computation of the enthalpic interactions of hydrocarbons in confined spaces has improved, understanding and predicting the entropy of confined molecules remains a challenge. Here we show, using a set of nine aluminosilicate zeolite frameworks with broad variation in pore and cavity structure, that the entropy of adsorption can be predicted as a linear combination of rotational and translational entropy. The extent of entropy lost upon adsorption is predicted using only a single material descriptor, the occupiable volume (V _occ). Predictive capability of confined molecular entropy permits an understanding of the relation with adsorption enthalpy, the ability to computationally screen microporous materials, and an understanding of the role of confinement on the kinetics of molecules in confined spaces.

Location, distribution and acidity of Al substitution in ZSM-5 with different Si/Al ratios – a periodic DFT computation

Systematic periodic density functional theory computations including dispersion correction (GGA-PBE-D3) have been carried out to characterize the location, distribution and acidity of Al substitution in ZSM-5 with different Si/Al ratios (up to 8 Al atoms).

Measuring the Brønsted acid strength of zeolites--does it correlate with the O-H frequency shift probed by a weak base?

Brønsted-acid zeolites are currently being used as catalysts in a wide range of technological processes, spanning from the petrochemical industry to biomass upgrade, methanol to olefin conversion and the production of fine chemicals. For most of the involved chemical processes, acid strength is a key factor determining catalytic performance, and hence there is a need to evaluate it correctly. Based on simplicity, the magnitude of the red shift of the O-H stretching frequency, Δν(OH), when the Brønsted-acid hydroxyl group of protonic zeolites interacts with an adsorbed weak base (such as carbon monoxide or dinitrogen) is frequently used for ranking acid strength. Nevertheless, the enthalpy change, ΔH(0), involved in that hydrogen-bonding interaction should be a better indicator; and in fact Δν(OH) and ΔH(0) are often found to correlate among themselves, but, as shown herein, that is not always the case. We report on experimental determination of the interaction (at a low temperature) of carbon monoxide and dinitrogen with the protonic zeolites H-MCM-22 and H-MCM-56 (which have the MWW structure type) showing that the standard enthalpy of formation of OH···CO and OH···NN hydrogen-bonded complexes is distinctively smaller than the corresponding values reported in the literature for H-ZSM-5 and H-FER, and yet the corresponding Δν(OH) values are significantly larger for the zeolites having the MWW structure type (DFT calculations are also shown for H-MCM-22). These rather unexpected results should alert the reader to the risk of using the O-H frequency shift probed by an adsorbed weak base as a general indicator for ranking zeolite Brønsted acidity.

Incorporation and electron transfer of anthracene in pores of ZSM-5 zeolites. Effect of Brønsted acid site density

The sorption course of anthracene (ACENE-3) into Brønsted-acidic medium pore MFI zeolites was monitored by in situ EPR and diffuse reflectance UV-visible absorption over one year. Weighed amounts of solid ACENE-3 were merely exposed to H(n)ZSM-5 (H(n)(AlO(2))(n)(SiO(2))(96-n)), with the following Brønsted acid site (BAS) densities, n = 0.0, 0.17, 0.57, 0.95, 2.0, 3.4, 6.6, dehydrated at 623 K under argon. The weighed amounts correspond to 1 ACENE-3 per zeolite unit cell. ACENE-3 is found to be incorporated as intact molecules in purely siliceous MFI (silicalite-1). Monte Carlo simulations indicate that ACENE-3 lies in the intersection of straight and zigzag channels. In contrast, the presence of BASs on the inner surface of channels induces spontaneous ionization of ACENE-3 (ionization potential = 7.44 eV). The charge separation as ACENE-3*(+)@H(n)ZSM-5*(-) is caused by the strong Coulombic field gradient of Si-O(-)(H(+))-Al BAS in the absence of any Lewis acid site. The rate and yield of ionization are found to increase dramatically with BAS density increase. The stabilization of ACENE-3*(+)@H(n)ZSM-5*(-) is explained by the tight fit between the rod-shape ACENE-3 and the channel dimensions and especially by the compartmentalization of ejected electrons as AlO(4)H*(-) centers away from the initial site of ionization. The final charge recombination occurs after more than one year and leads to ACENE-3 occluded in the straight channel in close proximity to BAS without any protonation of ACENE-3 (pK(a) = -13.5).

Thermodynamic Study of Water Adsorption in High-Silica Zeolites

The joint use of microcalorimetric and computational approaches has been adopted to describe H2O interaction with cus Al(III) Lewis and Si(OH)+ Al- Brønsted acidic sites within H-BEA and H-MFI zeolites (both with approximately 6 Al/unit cell). Adsorption data obtained at 303 K were compared to experimental model systems, such as all-silica zeolites, amorphous silica, and silico-alumina, transition alumina. In parallel, ab initio molecular modeling was carried out to mimic, in a cluster approach, Lewis and Brønsted acidic sites, as well as a variety of Si-OH species either with H-bonding interacting (nests and pairs) or isolated. H-BEA and H-MFI water affinity values were found to be almost equivalent, in both quantitative and energetic terms, in that dominated by Al-containing sites population, more than by nanocavity topology or by acidic site nature. Both H-zeolites, saturated with approximately 5 Torr of H2O vapor, bind approximately 4 H2O per Al site, almost one of which is tightly bound and not eliminated by RT pumping-off. A 160 < q(diff) < 80 kJ/mol interval was measured for the adsorption up to 1H2O/Al. The zero-coverage heat of adsorption (q0 approximately 160 kJ/mol, for both H-zeolites) was assigned to H2O/Lewis complex formation, which dominates the early stage of the process, in agreement with the ab initio computed H2O/Lewis sites binding energy. The rather broad q(diff) interval was interpreted as due to the simultaneous adsorption of H2O on both structural Brønsted sites and strongly polarized H2O already adsorbed on Lewis sites. For this latter species, BE = 74 kJ/mol was computed, slightly higher than BE = 65 kJ/mol for H2O/Brønsted sites interaction, showing that H2O coordinated on cus Al(III) Lewis sites behaves as a structural Brønsted site. The investigated all-silica zeolites have been categorized as hydrophilic in that the measured heat of adsorption (100 < q(diff) < 44 kJ/mol) was larger than the heat of liquefaction of water (44 kJ/mol) in the whole coverage examined. Indeed, polar defects present in the hydrophobic Si-O-Si framework do form relatively stable H2O adducts. Crystalline versus amorphous aluminosilicate q(diff) versus n(ads) plots showed that the measured adsorption heat is lower than expected, due to the extraction work of Al atoms from the amorphous matrix to bring them in interaction with H2O. On the contrary, such an energy cost is not required for the crystalline material, in which acidic sites are already in place, as imposed by the rigidity of the framework. Modeling results supported the experimental data interpretation.

Effects of Confinement on the Adsorption Behavior of Methane in High-Silica Zeolites

Adsorption isotherms of methane on high-silica zeolites at ambient temperature and up to high pressure were experimentally measured. The isotherms were analyzed on the basis of thermodynamics and statistical mechanics, and the relationship between microscopic properties and the macroscopic adsorption behavior was investigated. A comparison between the confined and unconfined phases revealed that molecular motion is restricted in the pores. As a result, the adsorbed phase is entropically destabilized, which cannot be neglected in comparison with the energetical stabilization that occurs as a result of the solid-molecule interaction. Our findings also indicate that the smaller slope (drho/d ln p) of the adsorption isotherms compared to that of the isotherm of the bulk at the same density is due to the smaller intermolecular interaction in the pores. The pore-size dependence is indicated not only in solid-molecule interactions but also in intermolecular interactions and molecular motion. Of these, the solid-molecule interactions strongly influence the adsorption behaviors in pores of different sizes. The origin of the restriction of molecular motion in the pores is well-explained by the one-dimensional transition and two-dimensional vibration (1D-trans, 2D-vib) model.