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A special setup was assembled to study solvent adsorption by metal organic frameworks (MOFs). A blank test was run to examine the noise level and the kinetic desorption curves for solvent molecules under different carrier gas (N 2 ) flow rates were measured with the instrument error lying within a limited range. The desorption process could be adequately characterized by the transport of adsorbates in pores within MOFs. For the MOF samples NH 2 -MIL-101(Fe), HKUST-1, MIL-101(Cr), ZiF-67, and ZIF-8, the intercrystalline diffusion coefficient of ethanol, toluene, and propan-2-ol at 25, 35, and 45 °C were calculated and the activation energy was evaluated. The diffusion data obtained are in agreement with those reported earlier.

In this study we present a new methodology for correcting experimental Zero Length Column data, to account for contributions to the measured signal arising from extra-column volumes and the detector. The methodology considers the experimental setup as a series of mixing volumes with diffusive pockets whose contributions to the overall measured signal can be accurately described by simple model functions. The composite effect of the individual contributions is subsequently described through the method of convolution. It is shown that the model parameters are closely related to the physical characteristics of the setup components and as such they remain valid over a range of process conditions. The methodology is firstly validated through fitting to experimental experiments without adsorbent present. The inverse procedure of deconvolution can in turn be applied to experimental data with adsorbent, to yield corrected data which can readily be modelled using standard tools for equilibrium and kinetic analysis. A number of case studies is finally presented exemplifying the effect of applying accurate blank corrections, demonstrating also the application to a nonlinear adsorption system.

A simple kinetic model based on the zeolite acid strength, the number of Brønsted acid sites, and the catalyst efficiency was developed for the cracking of n-hexane. A series of HY zeolites with a mesopore volume from 0.04 to 0.32 cm3/g was synthesized and characterized by various physical-chemical methods and tested for n-hexane cracking. The generation of mesoporosity influenced several other important parameters, such as acidity and extra-framework aluminum. Zero-length column diffusion measurements for mesitylene showed a large decrease in the characteristic diffusion time upon the introduction of mesoporosity, which changed only slightly with a further increase in mesoporosity. Similar n-hexane physisorption enthalpies were measured for all samples. The highest initial activity for n-hexane cracking per catalyst volume was observed for the sample with an intermediate mesopore volume of 0.15 cm3/g. The three mesoporous H-USY zeolites showed the same value of the intrinsic rate constant and the same activation energy. The difference in initial activity of the mesoporous zeolites was caused by the difference in the number of Brønsted acid sites. The increase in initial activity for the mesoporous zeolites compared to a microporous zeolite was caused by an increase in the acid strength.

Adsorption isotherms and zero length column (ZLC) desorption curves of alkylbenzene (toluene and isopropylbenzene) in a series of microspherical ZSM-5 zeolites with inter- and intracrystalline mesopores were measured for revealing structural and surface characteristics of the hierarchical zeolites. The isotherms showed that adsorption capacities of toluene and isopropylbenzene increase with increase of mesoporosities in the range of higher relative pressure. Henry’s constants (KH) and initial heats of adsorption (Qst) turned out that introduction of mesoporous structure into the zeolites weakens interaction between alkylbenzene molecules and zeolite surface. Moreover, isopropylbenzene and toluene have different adsorption and diffusion behavior in the zeolites. The adsorption capacity, KH and Qst for isopropylbenzene to microporous and mesoporous ZSM-5 zeolites are lower than these for toluene. The effective diffusion time constants (Deff/R2) of alkylbenzene from the ZLC desorption curves increase with increase of mesoporosities in the ZSM-5 zeolites, but diffusion activation energy (Ea) expresses a contrary tendency. Due to low interaction between isopropylbenzene and zeolite surface, Deff/R2 values of isopropylbenzene with lower activation energy in all ZSM-5 samples are higher than these of toluene. The molecular sizes of two alkylbenzenes are not the key factor for their adsorption and diffusion in the mesozeolites.

Diffusion is generally faster at higher temperatures. Here, a counterintuitive behavior is observed in that the movement of long-chain molecules slows as the temperature increases under confinement. This report confirms that this anomalous diffusion is caused by the “thermal resistance effect,” in which the diffusion resistance of linear-chainmolecules is equivalent to that with branched-chain configurations at high temperature. It then restrains the molecular transportation in the nanoscale channels, as further confirmed by zero length column experiments. This work enriches our understanding of the anomalous diffusion family and provides fundamental insights into the mechanism inside confined systems.

The zero length column technique has been developed over the past 30 years as a versatile experimental method to measure adsorption equilibrium and kinetics. In this review we discuss in detail the theory that forms the basis for the technique in order to understand how to design and operate efficiently a system. Experimental checks that should be performed to ensure the correct interpretation of the dynamic response are presented and examples are used to identify how to avoid major errors in determining diffusion time constants. The review concludes with an overview of all experimental studies available in the literature to date and a set of recommendations that should help improve the standard in the reported equilibrium and kinetic properties.

The problem of measuring sorption kinetics in microporous adsorbents and distinguishing experimentally between surface resistance and internal diffusion is discussed and reviewed with reference to several commonly used experimental techniques; direct uptake rate measurements, zero length column (ZLC), frequency response, PFG NMR and interference microscopy.

Mass transport in nanoporous materials is a key property that allows to improve the performance of many gas separation processes and design more efficient heterogeneous catalytic reactors. In many instances a combination of surface resistance and internal diffusion are present. The combined model for surface barrier and diffusion in a ZLC system is discussed in detail and the analytical solutions valid for the traditional and the partial loading experiments have been derived for the spherical and slab geometries. The model reduces to the limiting forms of pure diffusion when , and pure surface barrier when . This study has shown that most literature studies have analysed ZLC responses incorrectly based on an effective combined dimensionless parameter. Two methods are described to obtain the parameters from the long-time asymptotic behaviour of the response curves. Both approaches have been demonstrated on curves generated from the full model solution and experimental data on an etched sample of Y zeolite. Both the analysis of the model and of the experimental results confirm that to characterize combined surface barriers and diffusion one should perform at least experiments at two different flowrates where the system is kinetically controlled, and crucially a partial loading experiment with a time to the switch which should be at least an order of magnitude smaller than the smallest of the diffusion and surface barrier times.

The adsorption kinetics of carbon dioxide (CO 2 ) in three cationic forms of binderless pellets of Y-types zeolites (H-Y, Na-Y, and TMA exchanged Na-Y) are studied using the zero-length column (ZLC) technique. The measurements were carried out at and using different flowrates and an initial CO 2 partial pressure of – conditions representative of post-combustion CO 2 capture applications. The mass transport within the adsorbent pellets was described using a 1-D Fickian diffusion model accounting for intra- and inter-crystalline mass transport. For the latter, the parallel pore model formulation was used to explicitly account for the adsorbent’s macropore size distribution in estimating the volume-averaged diffusivity of the gas. Experiments carried out using different carrier gases, namely helium and nitrogen, were used (i) to determine that these systems are macropore diffusion limited and (ii) to simplify the parameter estimation to a single parameter - the macropore tortuosity. The latter ( ) was in good agreement with independent measurements using MIP ( ). The associated diffusion coefficient, , was found to vary due to differences in the materials’ macropore size distributions and overall porosity. Upon combining the parallel pore model formulation with the temperature dependencies for the pore diffusivities derived from molecular theories of gases, we predict with depending on the macropore size distribution. Notably, for the range of temperature tested in this study, varies approximately linearly with temperature ( )– in contrast to the commonly reported correlation of , which may be more appropriate for systems where molecular diffusion dominates and Knudsen diffusion is negligible. The binderless pellets of Y-type zeolites studied exhibit generally higher values for the effective macropore diffusivity of CO 2 compared to previously reported results on commercial FAU zeolites.

Understanding the dynamics of mixed-gas adsorption is essential for industrial-scale adsorptive separations. Also, obtaining binary adsorption isotherms is essential to gain a better understanding of adsorbate-adsorbent interactions in multicomponent gas mixtures. Porous organic cages (POCs) are highly porous, crystalline materials, with a small average pore window that make them promising candidates for separation applications. In this work, we utilized zero-length column (ZLC) technique to not only determine the kinetics of CO 2 /H 2 separation over CC3 but also obtain the binary adsorption isotherms. The ultracrystalline diffusivities of 1.49 × 10 –4 and 8.31 × 10 –5 cm 2 .s -1  were estimated from the ZLC desorption profiles for unary CO 2 and H 2 gases at 293 K, respectively, whereas in binary CO 2 /H 2 mixture, these diffusion values were reduced by 18 and 19 times, respectively. Moreover, our results indicated that the adsorption capacities estimated from binary runs diminished by approximately 36.0% and 33.6% for CO 2 and H 2 , respectively, relative to the unary runs. The findings of this investigation highlight the importance of the ZLC technique in providing valuable insights on mixture adsorption equilibrium and dynamics without the need for tedious and complex experiments.