NLDFT adsorption models for zeolite porosity analysis with particular focus on ultra-microporous zeolites using O2 and H2.

The most often used gases for the PSD characterization of zeolites are N2 and Ar. According to the IUPAC Technical Report (2015), Ar is recommended as more reliable than N2, which molecules possess a significant quadrupole moment that may influence the adsorption isotherms on polar surfaces. In practice, however, using Ar as a cryogen for Ar adsorption measurements is not preferred due to its higher cost than liquid N2. We propose using O2 which has a much lower quadrupole moment than N2, and its adsorption can be measured using standard liquid N2 as a coolant. To support using O2, we demonstrate good agreement between the PSDs calculated from O2 and Ar adsorption. In the present work, we develop several semiempirical models based on nonlocald density functional theory (NLDFT) for the PSD characterization of several types of zeolites. We discuss the underlying difficulties in modeling gas adsorption on zeolites which adsorption potential field depends on both the pore width and the chemical structure of the zeolite. For the analysis of ultra microporous zeolites such as Chabazite and molecular sieve 5A, we apply H2 in combination with O2 at 77 K. H2 molecule has a smaller diameter than O2 and diffuses faster into ultra micropores, reducing the time of isotherm measurement. Moreover, we show that the dual gas analysis method can be used with O2 isotherms measurements omitting low-pressure points, making the analysis faster.

Differential hysteresis scanning of non-templated monomodal amorphous aerogels.

We perform Differential Hysteresis Scanning (DHS) Porosimetry of amorphous silicon oxycarbide aerogels to quantify hierarchical connectivity in these porous materials. We contrast high-resolution argon sorption scanning isotherms of samples obtained through a non-templated synthesis using different solvents, and characterize respective changes after calcination at 1000 °C. The multi-scan DHS data sets are analyzed through non-negative least-squares deconvolution using a kernel of theoretically derived isotherms for a selection of hierarchical geometries using non-local density functional theory (NL-DFT). We obtain two-dimensional contour plots that characterize mesopores according to the ratio between pore diameter and its connecting window. Combined information from DHS and complementary BET and BJH approaches reveals one system with monomodal distribution both in pore diameters and in window diameters. Hence, this amorphous material exhibits a uniformity usually only observed for crystalline systems. We demonstrate that DHS analysis provides quantitative data analyzing the hierarchical structure of mesoporous materials and unlocks pathways towards tailored materials with control of surface heterogeneity, localization, and sequential accessibility - even for amorphous systems.

Consistency of carbon nanopore characteristics derived from adsorption of simple gases and 2D-NLDFT models. Advantages of using adsorption isotherms of oxygen (O2) at 77 K.

The pore size distribution (PSD) of porous carbons is most often derived from the analysis of standard N2 and Ar adsorption isotherms measured at 77 and 87 K. From the two gases, Ar is recommended (IUPAC Technical Report 2015) as more reliable for the PSD analysis due to its minimal specific interactions with the surface polar groups. Such interactions may influence the adsorption of N2 molecules due to its significant quadrupole moment. In practice, however, using liquid Ar as a cryogen for Ar adsorption measurements may be challenging because of its high cost and limited availability in various parts of the world. In this study, we propose using O2 adsorption isotherms for the PSD characterization of porous carbons. The quadrupole moment of O2 is less than one-third of the value reported for N2, and thus its susceptibility to specific interactions with polar groups is much smaller than that of N2. We demonstrate a quantitative agreement between the PSD results derived from the adsorption isotherms of O2 and N2 measured at 77 K, and Ar at 87 K on four representative carbon samples. The PSD calculations are performed using adsorption models based on the two-dimensional non-local density functional theory (2D-NLDFT).

Structural analysis of hierarchically organized zeolites.

Advances in materials synthesis bring about many opportunities for technological applications, but are often accompanied by unprecedented complexity. This is clearly illustrated by the case of hierarchically organized zeolite catalysts, a class of crystalline microporous solids that has been revolutionized by the engineering of multilevel pore architectures, which combine unique chemical functionality with efficient molecular transport. Three key attributes, the crystal, the pore and the active site structure, can be expected to dominate the design process. This review examines the adequacy of the palette of techniques applied to characterize these distinguishing features and their catalytic impact.

Unified method for the total pore volume and pore size distribution of hierarchical zeolites from argon adsorption and mercury intrusion.

A generalized approach to determine the complete distribution of macropores, mesopores, and micropores from argon adsorption and mercury porosimetry is developed and validated for advanced zeolite catalysts with hierarchically structured pore systems in powder and shaped forms. Rather than using a fragmented approach of simple overlays from individual techniques, a unified approach that utilizes a kernel constructed from model isotherms and model intrusion curves is used to calculate the complete pore size distribution and the total pore volume of the material. An added benefit of a single full-range pore size distribution is that the cumulative pore area and the area distribution are also obtained without the need for additional modeling. The resulting complete pore size distribution and the kernel accurately model both the adsorption isotherm and the mercury porosimetry. By bridging the data analysis of two primary characterization tools, this methodology fills an existing gap in the library of familiar methods for porosity assessment in the design of materials with multilevel porosity for novel technological applications.

Toward understanding reactive adsorption of ammonia on Cu-MOF/graphite oxide nanocomposites.

The adsorption of ammonia on HKUST-1 (a metal-organic framework, MOF) and HKUST-1/graphite oxide (GO) composites was investigated in two different experimental conditions. From the isotherms, the isosteric heats of adsorption were calculated from the Clausius-Clapeyron equation following the virial approach. The results on HKUST-1 were compared with those obtained using molecular simulation studies. All materials exhibit higher ammonia adsorption capacities than those reported in the literature. The ammonia adsorption on the composites is higher than that measured separately on the MOF component and on GO. The strong adsorption of ammonia caused by chemical interactions on different adsorption sites is evidenced by the trends in the isosteric heats of adsorption. The molecular simulations conducted on HKUST-1 support the trends observed experimentally. In particular, the strong chemisorption of ammonia on the metallic centers of HKUST-1 is confirmed. Nevertheless, higher adsorption capacities are predicted compared with the experimental results. This discrepancy is mainly assigned to the partial collapse of the MOF structure upon exposure to ammonia, which is not accounted for in the simulation study.