In Situ Characterization of Gas Adsorption Mechanisms in Hierarchical Zeolites

Abstract:
Hierarchical zeolites, which integrate microporosity with meso- or macroporosity, offer enhanced mass transport and improved accessibility to active sites compared to conventional microporous zeolites. Understanding gas adsorption mechanisms in these complex structures is crucial for optimizing their performance in gas separation, storage, and catalysis. This article reviews recent advances in the in situ characterization of gas adsorption processes within hierarchical zeolites. By employing techniques such as in situ infrared (IR) spectroscopy, in situ Raman spectroscopy, and in situ small-angle X-ray scattering (SAXS), researchers can now probe the dynamic behavior of adsorbed molecules at the molecular level. Key mechanisms discussed include: (i) sequential pore filling in micro- and mesopores, (ii) surface-mediated phase transitions, (iii) confinement effects on adsorbate structure, and (iv) cooperative adsorption between different pore regimes. These insights reveal how hierarchical architectures alter adsorption thermodynamics and kinetics, leading to improved selectivity and capacity. The article concludes with perspectives on emerging multimodal in situ approaches and their potential to guide the rational design of hierarchical zeolites for targeted gas adsorption applications.

1. Introduction

Zeolites are crystalline aluminosilicates with well-defined micropores (typically <2 nm), which grant them molecular sieving properties and high surface areas. However, their exclusive microporosity often leads to diffusion limitations, particularly for bulky molecules, reducing the utilization of internal active sites. Hierarchical zeolites address this limitation by introducing secondary porosity (mesopores of 2–50 nm and/or macropores >50 nm), creating a multi-level pore network that facilitates faster mass transport while retaining the intrinsic microporous framework.

Gas adsorption in hierarchical zeolites is not simply a superposition of micro- and mesopore behavior. Instead, the proximity and connectivity between pore regimes give rise to synergistic effects. For instance, mesopores can act as reservoirs or transport channels that feed molecules into micropores, while micropores can impose selectivity on the overall adsorption process. To unravel these complex mechanisms, in situ characterization—measuring adsorption under realistic conditions (pressure, temperature, gas composition)—is essential. Unlike ex situ methods, in situ techniques capture transient states, intermediate species, and structural rearrangements during gas uptake.

2. Key In Situ Characterization Techniques

2.1 In Situ Infrared (IR) Spectroscopy
In situ IR spectroscopy monitors vibrational modes of adsorbed molecules and zeolite framework functional groups (e.g., silanols, Brønsted acid sites) as a function of gas pressure or time. For hierarchical zeolites, IR can distinguish between molecules adsorbed in micropores (confined, often perturbed bands) versus mesopores (more bulk-like bands). The shift in C–O or O–H stretching frequencies, for example, reveals hydrogen bonding interactions with pore walls. Time-resolved IR further enables tracking of site competition: gas molecules first occupy high-energy micropores, followed by weaker mesopore adsorption sites.

2.2 In Situ Raman Spectroscopy
Complementary to IR, Raman spectroscopy is particularly sensitive to non-polar adsorbates and symmetric vibrations. In hierarchical zeolites, in situ Raman can probe the formation of molecular clusters or condensed phases within mesopores. For instance, during CO₂ or CH₄ adsorption, the appearance of lattice modes in the Raman spectrum signals pore confinement effects. Spatially resolved Raman (confocal Raman microscopy) has been used to map adsorbate distribution across different pore regimes in individual zeolite crystals.

2.3 In Situ Small-Angle X-ray Scattering (SAXS)
SAXS provides information on electron density variations, making it ideal for studying gas-induced changes in pore filling. In situ SAXS, combined with adsorption isotherms, can quantify the fraction of mesopores filled at a given relative pressure. Anomalous SAXS (ASAXS) using contrast variation (e.g., tuning X‑ray energy near absorption edges of probe molecules like Xe or Kr) allows selective visualization of gas density within pores. This technique has revealed that in hierarchical zeolites, mesopores often fill before micropores—a reverse order compared to purely microporous materials—due to capillary condensation in mesopores.

2.4 In Situ Neutron Scattering
Neutrons are highly sensitive to light elements (H, C, O) and can penetrate high-pressure cells. In situ neutron diffraction and quasielastic neutron scattering (QENS) elucidate the positions and mobilities of adsorbed molecules. For hierarchical zeolites, QENS has demonstrated that translational diffusion in mesopores is orders of magnitude faster than in micropores, confirming the “highway” role of mesopores.

3. Adsorption Mechanisms Revealed by In Situ Studies

3.1 Sequential and Cooperative Pore Filling
Classical models assume independent adsorption in micro- and mesopores. However, in situ IR and SAXS show a more nuanced picture: at low relative pressures, micropores fill first due to strong adsorbent–adsorbate interactions. Once micropores saturate, mesopores begin to fill via multilayer adsorption and capillary condensation. In hierarchical zeolites, the transition between these regimes is often smoother than in physical mixtures of micro- and mesoporous materials, indicating a cooperative effect. For example, the mesopore wall provides a “pre-layer” of adsorbed molecules that can migrate into adjacent micropores through pore mouths, maintaining a near-equilibrium chemical potential across the hierarchy.

3.2 Confinement-Induced Structural Changes
In micropores, molecules are forced into specific orientations or even distorted conformations. In situ Raman has identified that benzene adsorbed in the micropores of hierarchical ZSM‑5 exhibits a red shift of its ring-breathing mode compared to liquid benzene, indicating strong confinement. In mesopores (e.g., 10–20 nm diameter), confinement is weaker but still sufficient to induce layering near the pore wall. This layering has been directly imaged by in situ SAXS through oscillatory scattering patterns.

3.3 Surface-Mediated Phase Transitions
Capillary condensation in mesopores is a first-order phase transition that depends on pore diameter, surface chemistry, and temperature. In situ SAXS hysteresis loops reveal that hierarchical zeolites often exhibit a reduced hysteresis width compared to ordered mesoporous silicas, due to the presence of micropores that act as “nucleation sites” for condensation. Furthermore, in situ IR has shown that in the presence of polar surface groups (e.g., silanols), water or alcohol adsorption proceeds via cluster formation rather than continuous film growth, altering the condensation pressure.

3.4 Competitive and Selective Adsorption in Mixtures
For gas mixtures (e.g., CO₂/N₂, CH₄/N₂), in situ IR combined with mass spectrometry allows real-time monitoring of composition changes in the adsorbed phase. In hierarchical zeolites, the selectivity is not simply a function of micropore size. Mesopores can pre-concentrate the more polarizable component (e.g., CO₂) through weaker van der Waals forces, delivering a higher local concentration to micropore entrances. This “antenna effect” enhances overall selectivity and capacity, as demonstrated for CO₂ capture using hierarchical zeolite 13X.

4. Case Study: CO₂ Adsorption in Hierarchical ZSM‑5

To illustrate the power of in situ characterization, consider the adsorption of CO₂ in hierarchical ZSM‑5 prepared by desilication. In situ IR at 298 K reveals: (i) at P < 0.01 bar, asymmetric stretching of CO₂ appears at 2342 cm⁻¹, characteristic of adsorption on extra-framework Al species in micropores; (ii) as P increases to 0.1 bar, a second band at 2335 cm⁻¹ emerges, assigned to CO₂ in mesopores; (iii) simultaneously, the bending mode of zeolite framework (550 cm⁻¹) shifts, indicating lattice relaxation due to adsorbate stress. In situ SAXS shows that mesopores start filling at P/P₀ ≈ 0.2, but the micropores continue to uptake CO₂ until P/P₀ ≈ 0.6, revealing that the two regimes operate in parallel rather than sequentially. Neutron diffraction further locates CO₂ molecules preferentially near Al-rich regions in the micropores and at silanol nests in mesopore walls.

5. Challenges and Future Directions

Despite progress, several challenges remain. First, time resolution: many in situ techniques trade off temporal vs. spatial resolution. Fast processes (seconds) require synchrotron or neutron sources with high flux. Second, sample heterogeneity: hierarchical zeolites often contain a distribution of pore sizes; in situ methods need to disentangle contributions from different pores. Third, operando conditions: combining adsorption measurements with catalytic reaction (operando) is still rare for hierarchical zeolites.

Future developments include:

• Multimodal in situ cells: Simultaneous IR + SAXS or Raman + neutron diffraction on the same sample under identical conditions.

• Machine learning-assisted data analysis: Extracting component-specific signals from complex, overlapping spectra.

• In situ electron microscopy: Environmental TEM (ETEM) with differential pumping to image gas adsorption at the single-particle level, though careful electron beam effects must be mitigated.

6. Conclusion

In situ characterization has transformed our understanding of gas adsorption in hierarchical zeolites. Far from being a simple combination of independent pore systems, these materials exhibit cooperative mechanisms—sequential filling, confinement-induced structuring, surface-mediated condensation, and antenna effects—that arise from the intimate coupling between micro- and mesopores. Techniques such as in situ IR, Raman, SAXS, and neutron scattering provide complementary windows into the molecular behavior of adsorbates under realistic conditions. The insights gained are already guiding the rational design of hierarchical zeolites with tailored pore architectures for energy-efficient gas separation, carbon capture, and hydrocarbon storage. As in situ methods become faster, more sensitive, and easier to combine, we can expect a new era of precision engineering of hierarchical nanoporous solids.