Long-term degradation of adsorbed natural gas storage in the MOF Basolite C300 due to C_n≥2 alkanes accumulation

Abstract

Adsorbed natural gas (ANG) storage using metal-organic frameworks (MOFs) is a promising alter- native for efficient natural gas storage at moderate pressures. However, the presence of higher alkanes in natural gas mixtures can significantly affect storage performance by reducing methane adsorption capacity. Basolite C300, a well-studied MOF, offers high volumetric methane storage, but its long-term efficiency in real-world conditions remains a challenge due to potential pore blockage from hydrocarbon accumulation. This study investigates the long-term impact of C_n≥2 alkanes on the adsorption capacity of Basolite C300. Volumetric storage capacities of methane, individual alkanes, and a natural gas mixture were measured at 20 °C. The material underwent 100 adsorption-desorption cycles to assess the progressive impact of C_n≥2 alkanes on methane storage. The experimental results revealed a 63% reduction in methane storage capacity after 100 cycles, highlighting the detrimental effect of alkane accumulation. Higher alkanes were preferentially adsorbed within Basolite C300 micropores, leading to progressive pore blockage and decreased methane uptake. These findings underscore the critical role of gas composition in ANG systems and emphasize the need for mitigation strategies, such as selective pre-adsorption or regeneration techniques, to maintain long-term storage efficiency in MOF-based gas storage applications.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-10131-w.

Introduction

Natural gas (NG) is a viable alternative to gasoline and diesel for automotive applications, offering lower emissions and widespread availability. However, its adoption is limited by its low energy density at ambient conditions. Adsorbed natural gas (ANG) technology provides an efficient storage and transportation solution by enabling NG storage at moderate pressures and room temperature. Compared to Liquefied Natural Gas (LNG) and Compressed Natural Gas (CNG), which require extreme cooling or high-pressure compression, ANG offers a safer and more energy-efficient alternative^1–3. ANG tanks are designed to operate within a pressure range of 35–65 bar, enabling single-stage compression and allowing for flexible tank de-signs. Research in ANG technology primarily aims to enhance sorbent storage capacity to meet the performance benchmarks established by the Methane Opportunities for Vehicular Energy (MOVE) program under the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E). These targets include a gravimetric storage capacity of 0.5 g/g and a volumetric energy density of 12.5 MJ/L^4–14. Due to their exceptional microporosity and high surface area, activated carbon (AC) and metal-organic frameworks (MOFs) are widely recognized as leading materials for efficient natural gas adsorption and storage^15–18. Basolite C300 has demonstrated the highest recorded volumetric storage capacity, reaching 150 g/L at 35 bar². Despite its advantages, the widespread adoption of ANG technology faces several challenges. One major limitation is the discrepancy between laboratory-measured storage capacities and actual tank.

performance. Storage capacities for powdered sorbents are typically estimated based on the single-crystal density of MOFs or the intragranular porosity of activated carbon (AC). However, these values do not account for factors such as packing density and structural integrity, leading to an overestimation of practical storage performance in real-world applications^19–21. To enhance volumetric storage capacity, researchers have explored advanced packing and pelletization methods for sorbent materials. However, the densification process often compromises the structural integrity of MOFs, leading to reduced performance. Effective thermal management remains a critical challenge for the broader implementation of ANG technology. During charging, heat generated by the adsorption process raises the system temperature, while during discharging, desorption leads to cooling. These temperature fluctuations negatively impact storage efficiency by reducing the amount of deliverable gas, making thermal regulation a key factor in optimizing ANG system performance.

Natural gas consists of a complex mixture of hydrocarbons and other gases, which can significantly influence the performance of ANG storage systems. In particular, higher alkanes tend to strongly adsorb onto sorbent materials due to their greater binding energy, gradually accumulating within micropores. This progressive buildup reduces the number of available adsorption sites for methane, leading to a decline in overall storage capacity. Despite its critical impact, the effect of C_n≥2 alkanes on ANG systems remains poorly understood, with limited experimental and computational studies addressing this issue. Previous modeling efforts by Mu and Mota et al. have explored the role of gas composition in storage performance, but further research is needed to fully quantify and mitigate these effects^22,23. A guard-bed system has been proposed as a filtration mechanism to remove non-methane components from natural gas (NG) before entering the adsorbed natural gas (ANG) tank. Positioned at the tank’s inlet, the guard bed selectively prevents C_n≥2 alkanes from accumulating within the sorbent material, thereby minimizing contamination. During the desorption phase, these hydrocarbons are redirected back into the NG stream rather than being retained in the tank. In this study, we investigate the influence of C_n≥2 alkanes on NG storage performance and analyze the long-term cycling behavior of NG mixtures to quantify their impact on storage efficiency^24–26.

Methods

Materials

In this study, Basolite C300 (Merck 688614), also known as HKUST-1and MOF-199, was utilized as the adsorbent material. This commercial MOF has been studied extensively for volumetric methane storage reaching 147–150 g/L at 35 bar and room temperature^2,27. According to Zerner’s calculations, the minimum pore width allowing molecular adsorption varies, with values ranging from 3.0 Å for nitrogen (N₂) to 4.0 Å for hexane (C)²⁸. HKUST-1 is constructed from Cu₂(H₂O)₂ dimer units and tridentate trimesic acid (benzene-1,3,5-tricarboxylate) linkers, forming an intricate three-dimensional framework. This structure features an interconnected network of channels, comprising alternating cavities with diameters of 11 Å and 14 Å, linked by narrower pores measuring 10 Å in diameter. Additionally, smaller cavities of approximately 5 Å are interspersed throughout the framework, situated between the larger voids²⁹. As a result, all natural gas components can access the porous structure, ensuring that any observed deterioration in storage capacity is not constrained by pore accessibility. The NG mixture utilized in this study primarily consists of methane, accounting for 74.063 wt% of the total composition. Other hydrocarbon constituents include ethane (7.877 wt%), propane (2.363 wt%), iso-butane (0.942 wt%), n-butane (1.240 wt%), iso-pentane (0.339 wt%), n-pentane (0.308 wt%), and C_n≥6 hydrocarbons, which collectively constitute 0.725 wt%. Non-hydrocarbon components, specifically nitrogen and carbon dioxide, comprise 10.795 wt% and 1.348 wt%, respectively. Additionally, water content (H₂O) was determined to be 1.704 wt% prior to removal for micro gas chromatography measurement. The higher heating value (HHV) of the dry gas was measured as 37.66 MJ/m³ (1010 BTU/SCF) under standard conditions of 101.0 kPa and 288.7 K, while the HHV of the water-saturated mixture was slightly lower at 36.97 MJ/m³ (992 BTU/SCF). The calculated molecular weight of the gas mixture is 18.51 g/mol. This composition represents a typical wet natural gas, suitable for assessing the impact of hydrocarbons on natural gas storage performance in Basolite C300.

Gas adsorption

NG molecules undergo physisorption within the microporous structure, forming a dense adsorbed phase near the sorbent surface. To assess storage capacity, an excess adsorption isotherm is first measured and later converted into both gravimetric and volumetric storage values. The gravimetric excess adsorption , expressed in grams per kilogram (g/kg), is measured at 20 °C using an automated Sieverts apparatus, a well-established technique in adsorption studies^30,31. The gravimetric storage capacity (g/kg) is derived from excess adsorption, taking into account the gas density at the corresponding pressure and temperature as well as the pore volume , as defined in Eq. (1). Since this study primarily investigates the effects of C_n≥2 hydrocarbons rather than optimizing sorbent packing within the storage tank, the volumetric storage capacity is calculated by multiplying the gravimetric storage capacity by the crystal density of ,, rather than using the powder packing density²⁷as described in Eq. (2).

1

2

Prior to the cycling process, the storage vessel was subjected to a vacuum treatment at 400 °C for four hours to eliminate any residual moisture. During each cycle, the vessel was pressurized with gas from 1 bar to 35 bar, followed by a controlled depressurization back to 1 bar. A stabilization period of 10 min for adsorption and 25 min for desorption was allowed to ensure temperature and pressure equilibrium before measurements were taken. The volumetric storage capacity was recorded at 20 °C and 35 bar across multiple cycles, specifically at cycle numbers 1, 5, 20, 40, 80, and 100.

Micro gas chromatography and XRD

Following the 50th and the 100th adsorption (35 bar)-desorption (1 bar) cycles, Basolite C300 was subjected to a degassing procedure under a vacuum of 1.3 × 10⁻⁷ bar at 400 °C for 2 h to remove hydrocarbons trapped within the material’s pores. The desorbed gas was subsequently collected in gas sampling bags. The composition of the gas retained in Basolite C300 pores, representing the fraction that resisted desorption during cycling and consequently reduced storage capacity, was analyzed using micro gas chromatography GPA-2261 M. Additionally, X-ray diffraction (XRD) measurements were performed after the 100th adsorption-desorption cycle using a Bruker D8 Advance diffractometer, equipped with a Cu X-ray source operated at 40 kV and 40 mA. Locked-coupled scans were conducted in the Bragg–Brentano geometry to detect any structural alterations in Basolite C300 resulting from repeated cycling.

Results and discussion

Fig. 1 presents the adsorption and desorption isotherms for the first cycle of individual NG components, methane, ethane, and propane, along with the NG mixture at 20 °C. The volumetric storage capacities of the NG mixture are slightly higher than those of pure methane. However, the adsorption and desorption mechanisms differ significantly between the two. While pure methane is stored under supercritical conditions at 20 °C, the adsorption of a multi-component NG mixture involves a combination of supercritical and subcritical adsorption, depending on the specific sorbate within the micropores. The NG mixture consists of methane, ethane, propane, butane, pentane, hexane, carbon dioxide, and nitrogen. Fig. 2 provides the thermo-physical properties of these NG components, as reported by the National Institute of Standards and Technology (NIST)^32,33. At 20 °C, methane adsorption occurs significantly above its critical temperature (− 83 °C), placing it in a supercritical state. Under these conditions, the adsorbed methane exists as a dense fluid-like phase that, even at elevated pressures, remains confined to a monolayer on the sorbent surface. This behavior arises from the fact that above the critical temperature, there is no distinct liquid-gas phase transition, and intermolecular forces are insufficient to overcome thermal energy for the formation of subsequent layers in the confined space of the micropores. In contrast, C_n≥2 alkanes, with critical temperatures exceeding the experimental temperature of 20 °C (ethane: 32 °C, propane: 97 °C, butane: 152 °C, pentane: 197 °C, and carbon dioxide, often present in NG, with a critical temperature of 31 °C), can undergo phase transitions within the pores and exhibit the potential for multilayer formation via mechanisms such as capillary condensation at sufficiently high pressures. The stability of the supercritical methane monolayer throughout adsorption and desorption cycles ensures a highly reversible storage process, crucial for practical applications requiring repeated cycling. In a single-component methane system, rapid equilibration is observed following temperature and pressure stabilization. This is attributed to the high diffusivity of the small methane molecules within the micropores and the homogeneous nature of the interactions within the single-component system, allowing the adsorbed phase and the gas phase to readily achieve equilibrium. However, the presence of multiple components in natural gas mixtures introduces significant complexities due to the phenomenon of preferential adsorption. C_n≥2 hydrocarbons, characterized by stronger van der Waals interactions with the sorbent surface, exhibit higher binding energies and are thus preferentially adsorbed over methane. This selective adsorption results in the preferential accumulation of C_n≥2 components near the pore surfaces due to their stronger binding energies. While classical concentration gradients cannot form across the narrow pore diameters, the resulting differential site occupation extends the time required for the system to reach equilibrium, especially in multi-component mixtures with competing adsorption kinetics. The increasing binding energies with chain length (19 kJ/mol for methane, 26 kJ/mol for ethane, 30 kJ/mol for propane, 37 kJ/mol for butane, 44 kJ/mol for pentane, and 46 kJ/mol for hexane) quantitatively illustrate the enhanced affinity of C_n≥2 alkanes for the sorbent^34,35. This competitive adsorption among NG constituents not only affects the overall storage capacity for methane but also contributes to a gradual decline in performance over time, as the less readily desorbed C_n≥2 hydrocarbons can occupy high-energy adsorption sites, effectively inducing irreversible adsorption effects on the timescale of typical operating cycles. Fig. 3 demonstrates the progressive reduction in volumetric storage capacity with repeated adsorption-desorption cycling, a critical factor in evaluating the long-term viability of this storage technology. Following the initial treatment of the sorbent material via vacuum treatment at 400 °C, which effectively removes any pre-existing adsorbates and ensures that the micropores are completely free, the initial storage capacity for methane is maximal. However, with each subsequent adsorption and desorption cycle, a gradual but persistent decrease in capacity is observed, eventually reaching a significant 63% reduction after 100 cycles. This capacity degradation is a complex process influenced by several interconnected factors, including the concentration of C_n≥2 alkanes present as impurities in the natural gas stream. Strongly adsorbed C_n≥2 alkanes can effectively displace lighter components, primarily methane, from energetically favorable adsorption sites, forcing them back into the gas phase and reducing the overall methane uptake.

Fig. 1.

Adsorption and desorption isotherms for the first cycle of methane, ethane, propane, and natural gas mixture on HKUST-1 at 20 °C.

Fig. 2.

Thermophysical properties of individual natural gas components at 20 °C.

Fig. 3.

Deterioration of natural gas storage capacities with successive cycling.

Fig. 4 illustrates the gas composition retained within the micropores of Basolite C300 following 50 and 100 adsorption-desorption cycles using a multi-component natural gas (NG) mixture. It shows a marked preferential retention of C_n≥4 hydrocarbons, notably n-butane, iso-butane, n-pentane, iso-pentane, and hexane, compared to lighter components such as methane, ethane, and nitrogen. Although methane initially represented more than 74 wt% of the NG mixture, its concentration in the retained gas is significantly lower, underscoring effective desorption of methane during the cycling process. Conversely, C_n≥2 hydrocarbons, though initially present in minor proportions, dominate the retained gas composition due to their persistent adsorption and entrapment within the microporous framework of the MOF.

Fig. 4.

Composition of the gas retained in the pores after 50 and 100 adsorption-desorption cycles.

This selective retention can be attributed to the stronger van der Waals interactions and significantly higher binding energies of C_n≥2 alkanes. For instance, hexane exhibits a binding energy of approximately 46 kJ/mol compared to 19 kJ/mol for methane, enhancing its adsorption affinity and resistance to desorption under typical regeneration conditions (1 bar, 20 °C). Over successive cycles, even minimal amounts of C_n≥2 hydrocarbons progressively accumulate, blocking micropores and reducing available adsorption sites, which results in a pronounced decline in methane storage capacity. Moreover, the kinetics of adsorption and desorption vary significantly between methane and C_n≥2 hydrocarbons. Methane’s rapid diffusion and weaker interaction allow for efficient cycling, whereas C_n≥2 hydrocarbons diffuse slowly due to their larger molecular size and stronger interactions, becoming progressively more difficult to remove during cycling. While the main reason that larger alkanes are not desorbed is the large binding energy, this differential behavior due to diffusion also contribute to cumulative entrapment of C_n≥2 alkanes and severely impacting the effective natural gas storage capacity over time. Consequently, a substantial reduction in methane storage capacity, reaching 63% after 100 cycles (Fig. 3), was observed. Importantly, XRD presented in the supplementary material, showed no detectable crystallographic changes and confirmed that this reduction is not due to structural degradation of the MOF but rather due to the cumulative and partially irreversible adsorption of C_n≥2 alkanes.

The practical implications of these findings are significant for the deployment of ANG storage systems, particularly in transportation and remote energy applications. The disproportionate retention of C_n≥2 hydrocarbons, even at trace concentrations, necessitates careful gas composition management. Effective mitigation strategies are therefore essential, including employing guard-bed filtration systems upstream to selectively remove C_n≥2 alkanes before they contact the primary sorbent material. Alternatively, more aggressive regeneration approaches, such as elevated-temperature vacuum swing desorption, could be implemented to restore the full adsorption capacity between cycles. Without these interventions, the viability of Basolite C300 and similar MOFs for long-term ANG applications is significantly compromised.

Conclusions

This study demonstrates the significant and progressive degradation of methane storage capacity in Basolite C300 when exposed to realistic, multi-component natural gas (NG) mixtures over extended cycling. Despite its high initial storage performance, Basolite C300 exhibits a marked 63% reduction in methane uptake after 100 adsorption-desorption cycles. This decline is attributed primarily to the selective and cumulative retention of C_n≥2 hydrocarbons within the microporous framework compounds that exhibit higher binding energies and slower desorption kinetics. XRD analyses confirmed that this degradation is not a result of structural damage but rather of pore blockage due to persistent alkane accumulation. These findings highlight the crucial influence of gas composition on the long-term viability of adsorbed natural gas (ANG) storage systems. They also underscore the necessity of integrating effective mitigation strategies, such as selective pre-filtration of C_n≥2 alkanes or advanced regeneration techniques, to preserve sorbent performance. Future ANG technology development must account for these composition-dependent effects to ensure sustained storage capacity and system reliability in practical applications.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

J.R. and M.S. contributed to the conceptualization of the study. S.A., R.R., and H.H. conducted the formal analysis. J.R. secured funding for the research. The methodology was developed by J.R., S.A., and R.R. Project administration was managed by J.R. J.R. led the writing of the manuscript, with M.S. providing review and editing.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper.

Declarations

Competing interests

The authors declare no competing interests.