Zero Length Column Method Research Articles

Determination of intracrystalline diffusion coefficient of organic solvents in metal organic framework adsorbents by zero-length column method

Determination of intracrystalline diffusion coefficient of organic solvents in metal organic framework adsorbents by zero-length column method

External surface Al-rich ZSM-48 zeolite with high efficient n-dodecane hydroisomerization activity

ZSM-48 zeolites with exterior Al-rich (Z-48-Sx) are synthesized by the recrystallization method. The crystal structure of Z-48-Sx with the Al-rich outer layer is verified by analyzing the crystallization process of Z-48-Sx and the EDS of the ultrathin section. The diffusion properties of Z-48-S3 and Z-48-C are investigated by the zero-length column (ZLC) method, and the synthesized ZSM-48 zeolite with an Al-rich external layer effectively improves the diffusion ability. The effective diffusion constants of Z-48-S3 are 53 %–102 % higher than those of the conventional ZSM-48 (Z-48-C). Meanwhile, the values of turnover frequency (TOF) of Z-48-S3 are increased by 14 %–63 % at similar n-dodecane conversions compared with Z-48-C, which is closely related to the diffusion characteristics. In addition, the depth of hydroisomerization (Iiso) of Z-48-S3 is decreased by 65.3 %, and the isomerization selectivity is increased by 52.5 % at 340 °C compared with that of Z-48-C. Hence, the ZSM-48 molecular sieves synthesized by the recrystallization hydrothermal method are beneficial for controlling the depth of hydroisomerization of the catalyst and reducing the occurrence of the cracking reaction. Meanwhile, the isomer distribution does not change significantly compared with that of the conventional ZSM-48, and the 5-methylundecane is still the main isomerization product.

Effect of particle size of single-crystalline hierarchical ZSM-5 on its surface mass transfer in n-heptane catalytic cracking

Single-crystalline hierarchical ZSM-5 zeolites with different particle sizes (namely 100, 140, and 200 nm) were successfully prepared by adjusting the amount of tetrapropylammonium hydroxide (TPAOH), and investigated in n-heptane catalytic cracking reaction. Diffusional measurements by zero-length column (ZLC) method showed that the apparent diffusivities of n-heptane decreased with the reduction of particle size, indicating the existence of surface barriers. Moreover, with the decrease of particle size, the additional diffusion path length increased, which meant the influence of surface barriers became more apparent. Despite the change of surface barriers, the intracrystalline diffusion still dominated the overall diffusion. Catalytic performance showed that the zeolite with smaller particle size had better stability.

Quantifying molecular surface barriers and intracrystalline diffusion in nanoporous materials by zero‐length column

AbstractThe zero‐length column (ZLC) method has been frequently used to measure the effective diffusivity of guest molecules in nanoporous crystalline materials. Recent studies unveiled that the mass transport of guest molecules could originate from either intracrystalline diffusion and/or surface barriers. Directly quantifying the intracrystalline diffusivity and surface permeability of guest molecules. therefore, is essential for understanding the mass transfer and thus optimizing the design of nanoporous crystalline materials. In this work, we extended the ZLC method, based on a derived theoretical expression of the desorption rate, to decouple the surface barriers and intracrystalline diffusion from effective diffusion of guest molecules in nanoporous materials. The diffusivities of ethane, propane and n‐pentane in SAPO‐34 and Beta zeolites have been experimentally measured to verify the effectiveness of this method.

Reducing the diffusion barriers of Pt/Beta catalyzed n-hexane isomerization by SBA-15 addition and high-energy milling

Aiming at improving reaction selectivity of zeolite Beta-catalyzed isomerization of normal alkane, we reported a facile but efficient route to prepare the Beta@SBA-15 composite supports with low surface diffusion barriers in isomers formation from normal alkane. Microporous Beta zeolite and mesoporous SBA-15 zeolite were physically mixed and milled by high-energy planetary ball-mill. The addition of SBA-15 and milling, which have little effect on the acid properties of Beta zeolites, disaggregate the Beta particles and anchor the Beta nanocrystals on the surface of SBA-15 mesostructure. The apparent diffusivities of 2-methyl pentane measured by zero-length column method increased by 50%. n-Hexane isomerization tests show that the Pt/Beta@SBA-15 catalysts exhibit much higher selectivity to isomers, especially the dibranched isomers and lower selectivity to cracking reaction, owning to the reduced transport limitation of composite supports.

Regulating mesopore structures of support toward enhanced selective hydrogenation of dimethyl oxalate to methyl glycolate on Ag catalysts

Precisely regulating mesopore structures toward enhanced diffusion of reactants and targeted product is of great significance for achieving simultaneously improved activity and selectivity, which is exemplified by this work employing mesoporous silica with distinct structured mesopores as support for Ag catalysts to selectively hydrogenate dimethyl oxalate (DMO) into methyl glycolate (MG). Mesoporous silica nanosphere (MSNS) with center-radial pores shows shorter pore length than SBA-15 with parallel mesopores, which is beneficial for the diffusion process and thus gives rise to enhanced hydrogenation activity. Inspired by these insights, MSNSs sized in 50, 90, 120 and 160 nm are further synthesized to regulate pore length and systematically explore their effects on the reaction. The diffusion process is enhanced with decreasing the size of MSNS from 160 to 50 nm, as elucidated by the effective diffusion time constant (De/R2) determined by zero-length column (ZLC) method. Accordingly, DMO can facilely diffuse into the mesopores with shorter length to access the active sites while the formed MG product can easily diffuse out to avoid the over-hydrogenation process, which is corroborated by the comparison for performances of these Ag/MSNSs catalysts. Notably, a moderately short pore length, balancing well the diffusion and hydrogenation process, is more preferable for achieving both higher activity and better selectivity to MG, and MSNS sized in 90 nm supported Ag catalyst thus exhibits the greatest performances, with MG selectivity up to 96.6 % at 99.7 % of DMO conversion.

Considerations when determining Counter-diffusion constants in liquid phase catalytic reactions using the Zero Length column (ZLC) method

The confusion with diffusion: In heterogeneous catalysis often, the catalytic material is housed within a porous support structure. It is important therefore to determine whether this support structure exerts limiting behaviors. In this paper, desorption and diffusion effects are separated under reactions conditions, to provide accurate effective diffusion constants in swelling and gate-opening catalysts. • A Zero Length Column (ZLC) method was developed using liquid chromatography. • Diffusion in porous catalyst pellets as small as 5 mg can be directly characterised. • Reactant counter-diffusion coefficients were determined for the Heck reaction. • Clear differences in diffusion rates for reactants versus non-reactant species. A Zero Length Column (ZLC) method was developed using a liquid chromatographic system to calculate reactant counter-diffusion coefficients in porous, solvent swollen catalytic pellets and gate-opening support materials. Reactant diffusivities within these porous materials were determined at the reaction conditions of 80 °C and 0.3–0.75 mL min −1 . For all materials, molecules of a similar size to the pore apertures, or molecules with strong interactions with the material surfaces, catalysts, were observable by the ZLC method, with both micro and macropore diffusion observed. Differences between Pd(II) and Pd(0) forms of the EnCat30 catalyst were examined to determine the effects of catalytic deactivation. The deactivated catalyst showed iodobenzene macroporous diffusion constants similar to the relatively inert toluene molecules. Finally, pulse field gradient NMR was utilised to compare and validate ZLC diffusion measurements for solvent swollen reaction systems. This study presents much needed novel findings on diffusivity data for liquid phase catalytic systems.

Investigation of the secondary porosity in ill-crystallized or desilicated ZSM-5 zeolites and its performance on MTH reaction

The use of zeolites in several fields, especially in base or acid-catalyzed reactions and adsorption, depends on their physical and chemical properties tuned by modification of the synthesis (bottom-up methods) or post-synthesis procedures (top-down). Among the various possibilities, three stand out: (1) the embryonic zeolites, (2) partially formed or ill-crystallized structures, and (3) the desilication of consolidated zeolites by consuming part of the zeolitic structure in an alkaline solution. The modifications of zeolites aim to obtain materials with secondary porosity, mainly for improved applications as catalysts in reactions that form deactivating coke, as usually seen in the conversion of methanol to hydrocarbons (MTH). Herein, amorphous, partially crystallized, and fully-grown ZSM-5 zeolites were prepared and later characterized to establish the synthesis-property function in MTH. Finally, the completely crystallized ZSM-5 zeolite was desilicated, and acid leached to create different structural and morphological properties. These modifications aimed to evaluate the various porosities and correlate them with the accessibility to catalytic sites and their effect on catalyst deactivation. The Zero-Length Column (ZLC) method was used to evaluate the diffusional behavior of methanol in all fresh synthesized samples. After the MTH, external and internal coke within the spent catalyst samples were extracted and analyzed by GC-MS. The size distribution of occluded carbonaceous molecules was associated with the porosity of the catalysts. Besides the typical polyaromatic compounds, oxygen-containing cycloalkenes were also formed inside the zeolitic pores. On the other hand, bulky linear oxygenates and paraffin were predominant on the outer surface. The hierarchical zeolites were more resilient to deactivation by coke, even after 28 h of reaction, keeping selectivity to light and heavy hydrocarbons longer than the purely microporous catalysts. As a result, a more outstanding production of aromatic compounds is directly related to increased material pore volume.

Diffusivities of Hydrocarbons in the Fresh and Aged Hydrotreating Catalysts Studied by Zero-Length Column Method

Diffusivities of Hydrocarbons in the Fresh and Aged Hydrotreating Catalysts Studied by Zero-Length Column Method

Application of ZLC technique for a comprehensive study of hydrocarbons’ kinetics in carbon molecular sieves and zeolites

The mass transfer of hydrocarbon gases was studied in a commercial CMS and zeolite 5A (as a reference adsorbent). The zero length column (ZLC) technique was applied to determine the mass transfer resistances to gas sorbates. To confirm the ZLC results, a comprehensive study involving uptake and kinetic behavior of the constituent gas components was also carried out using the gravimetric technique. The comparison between ZLC and gravimetric results established the strong dominance of surface barrier for both adsorption and desorption of sorbates in CMS. As a result of surface barrier resistance in the CMS, the adsorption kinetics is extremely slow. The ZLC method required a considerably shorter measurement time compared to the gravimetric method, e.g., less than 14% of the time required for the gravimetric method, which proved the advantage of the ZLC as a fast screening technique to rank a large number of adsorbents particularly those with large surface barrier effects. To apply the ZLC technique for measuring the kinetics in CMS adsorbent, surface resistance model and modified form of ZLC model (combined surface and diffusion resistances model) were applied. Since the surface resistance is the main barrier to the sorbate molecules diffusing out from the CMS pores, the application of combined model resulted in a considerable inaccuracy in the kinetic parameter’s estimations. This was primarily due to the existence of low experimental value of the L' criterion, representing the ratio of diffusion to surface resistance, e.g., <0.33, thus contradicting the model criteria required by the ZLC method, i.e., L′ > 0.33. Consequently, only surface barrier was considered using the ZLC model for the CMS kinetic parameter estimates. According to the experimental results, the combined model is suitable for ZLC analysis of adsorbents with minor to moderate surface resistance. In these adsorbents, the increase in L', e.g., higher than 0.33, indicates greater contribution of diffusion resistance compared to surface barrier. As a result, the zeolite 5A, which has a minor surface resistance effect (L' ≫ 0.33), was applied to verify the consistency of the combined model in predicting the kinetics of hydrocarbon gases.

Adsorption and diffusion of n-heptane and toluene over mesostructured ZSM-5 zeolitic materials with acidic sites

In order to understand the effects of the surface acid sites in mesoporous zeolites on the adsorption and diffusion of hydrocarbons, the adsorption and diffusion of n-heptane and toluene (two C7 hydrocarbons) over the mesoporous HZSM-5 zeolites (SiO2/Al2O3 = 36) were investigated. The adsorption isotherms of two C7 hydrocarbons over the mesoporous HZSM-5 zeolites showed the experimental data good fitting to dual-site Langmuir-Freundlich model (DSLF) similar to mesoporous NaZSM-5. Henry's constants and the initial heats of adsorption displayed stronger interactions in HZSM-5 than in NaZSM-5, especially for n-heptane, due to the presence of surface acid sites. The influence of acidity to the adsorbate-adsorbent interactions decreases with increasing of the mesoporosity. The investigation on the diffusion of n-heptane and toluene in mesoporous HZSM-5 samples by the zero-length column (ZLC) method showed the strong interaction of adsorbates and HZSM-5 samples by effective diffusivity and activation energy. Moreover, the diffusions of adsorbates in zeolites were advanced with the introduction of mesopores in the zeolitic samples, though the dramatic influence of mesopore to diffusion is not arisen in mesoporous NaZSM-5, reflecting larger influence of acid sites on diffusion in mesoporous HZSM-5 samples.

Adsorption and Diffusion of Xylene Isomers on Mesoporous Beta Zeolite

A systematic and detailed analysis of adsorption and diffusion properties of xylene isomers over Beta zeolites with different mesoporosity was conducted. Adsorption isotherms of xylene isomers over microporous and mesoporous Beta zeolites through gravimetric methods were applied to investigate the impact of mesopores inside Beta zeolites on the adsorption properties of xylene isomers in the pressure range of lower 20 mbar. It is seen that the adsorption isotherms of three xylene isomers over microporous and mesoporous Beta zeolites could be successfully described by the single-site Toth model and the dual-site Toth model, respectively. The enhanced adsorption capacities and decreased Henry’s constants (KH) and the initial heats of adsorption (Qst) for the all xylene isomers are observed after the introduction of mesopores in the zeolites. For three xylene isomers, the order of Henry’s constant is o-xylene &gt; m-xylene &gt; p-xylene, whereas the adsorption capacities of Beta zeolite samples for xylene isomers execute the following order of o-xylene &gt; p-xylene &gt; m-xylene, due to the comprehensive effects from the molecular configuration and electrostatic interaction. At the same time, the diffusion properties of xylene isomers in the mesoporous Beta zeolites were also studied through the desorption curves measured by the zero length column (ZLC) method at 333–373 K. It turned out that the effective diffusion time constant (Deff/R2) is a growing trend with the increasing mesoporosity, whereas the tendency of the activation energy is just the reverse, indicating the contribution of mesopores to facilitate molecule diffusion by shortening diffusion paths and reducing diffusion resistances. Moreover, the diffusivities of three xylene isomers in all Beta zeolites follow an order of p-xylene &gt; m-xylene &gt; o-xylene as opposed to KH, conforming the significant effects of adsorbate-adsorbent interaction on the diffusion.

Automatic estimation of kinetic and isotherm parameters from ZLC experiments

In order to automate and improve the analysis of the experimental Zero Length Column (ZLC) data a general adsorption simulator is applied to the simulation of the ZLC system. The output of this ZLC simulation is linked to a hybrid optimisation strategy: combining a global Genetic Algorithm (GA) with a local Nelder–Mead algorithm ensures an efficient and accurate optimisation procedure. By minimising the deviation between the experimental data and the simulation the kinetic and equilibrium parameters of the adsorbent can be estimated. In the first step the parameters of the blank experiment are fitted; thus taking the dynamics of the detector and the system piping into account. In the second step several experiments at different flow rates and temperatures are fitted simultaneously. This increases the accuracy and reliability of the results. The automated parameter estimation was applied to ZLC runs with carbon dioxide on zeolite 13X pellets. The results agree with the traditional analysis methods and with independent measurements. In addition it is demonstrated that it is possible to approximate the correct adsorption isotherm from ZLC runs under kinetic control. The automatic analysis tool allows the inclusion of nonlinear effects in the fitting and lowers the access barrier to the application of the ZLC method.

Efficient and Rapid Screening of Novel Adsorbents for Carbon Capture in the UK IGSCC Project

Here we present the results of the rapid screening of novel adsorbents for CO2 capture. The materials were synthesized as part of the Innovative Gas Separations for Carbon Capture (IGSCC) project, a UK wide consortium which involves the University of Edinburgh in collaboration with the University of St. Andrews, Cardiff University, Imperial College London, the University of Manchester and University College London. For the first time a wide range of materials relevant for carbon capture are investigated with regard to the CO2 capacity at the same conditions and using the same technique. More than 120 samples, belonging to different classes of adsorbents, were investigated: MOFs, zeolites, PIMs, carbons and silicas. The conditions chosen are the ones of interest for post- combustion applications: 0.1bar and 35°C. All the samples were tested using the Zero Length Column (ZLC) method. Relative to other technique, the ZLC presents the advantage of requiring a very small amount of sample (< 15mg) allowing to obtain rapidly kinetic and equilibrium properties of the novel materials.For each class of materials one typical sample will be shown and compared to typical commercial materials as benchmarks.

Diffusion of n-alkanes in mesoporous 5A zeolites by ZLC method

Diffusion properties of mesostructured zeolite 5A were investigated by employing n-alkanes as probe molecules using the zero length column (ZLC) method. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (Deff/R2) increased with mesoporosity in the zeolites between 308 K and 393 K, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance.

Gas-phase diffusion of linear and multi-branched alkanes on a carbon molecular sieve by the ZLC method

This study compares the individual desorption kinetics in terms of effective diffusional time constants (Deff/r2) of 20 linear and multi-branched alkanes through a carbon molecular sieve (CMS) material obtained by the zero length column (ZLC) technique.As is typical for microporous materials with pore sizes under 0.6nm, the effective diffusivity decreased as the carbon number increased in each hydrocarbon family. However, comparing hydrocarbons with the same carbon number, the effective diffusivity tended to increase as the number of branches increased. In this case, the physisorption of the different alkanes on the carbon molecular sieve may be the result of multiple pore mouth adsorption modes. Each mode depends on the number of carbon atoms inside and outside the pore mouth cavity and the interactions between these branched hydrocarbons and the carbon walls.

Flux response technology applied in zero length column diffusivity measurements

Flux Response Technology (FRT) has been successfully adapted as an in situ perturbation technique in dynamic gas sorption measurements to extract hydrocarbon diffusion coefficients in alumina/CeZrOx washcoats of cordierite monoliths. FRT works by measuring minuscule changes in flowrate between two gas streams for any gaseous process involving a change in volume (δV/δt). Ad- and desorption transients, which can be collected during the same experimental run have been analysed using the zero length column (ZLC) method to study propane diffusivity within an alumina/CeZrOx washcoat as a function of temperature. Extracted diffusivities and activation energies compare favourably with literature data.

Flowrate correction for the determination of isotherms and Darken thermodynamic factors from Zero Length Column (ZLC) experiments

The ZLC method is a simple and rapid approach to measure adsorption equilibrium isotherms. When extending this technique to the measurement of large concentration steps, the primary problem is how to estimate the outlet flowrate from the measured concentration signal. In this paper, two different corrections to calculate the outlet flowrate are introduced. Based on simulations of the ZLC system that are in good agreement with reported experimental results, a series of desorption cases were simulated. Isotherms were calculated using the two approximations of the outlet flowrate. The comparisons are used to identify the range in which the approximations are applicable. Darken’s thermodynamic correction factors were also calculated to verify the validity of the flow corrections.

Diffusion of linear paraffins in silicalite studied by the ZLC method in the presence of CO2

The diffusion behavior of C4–C10n-alkanes in silicalite-1 has been investigated by using the Zero Length Column method. The diffusivities derived from measurements at different purge rates with different purge gases confirming intracrystalline diffusion control. Data are compared with results reported in the literature for MFI zeolites. The diffusivities were found to be consistent and agree well with data previous obtained by ZLC. However, these data showed a remarkable disagreement with other reported techniques (PFG-NMR, QENS and Permeation). The eventual influence of carbon dioxide (CO2) adsorption on diffusion properties of n-alkanes in silicalite was also investigated. For this purpose, a series of experiments was performed involving hydrocarbons mixed with CO2. Data were obtained at 303 K and flow rates between 20 and 80 mL/min. The presence of CO2 does not seem to influence the intracrystalline transport rate of the investigated light hydrocarbons (n-C4 and n-C6). On the other hand, the situation for n-C8 and n-C10 is more complex. The diffusivity values are higher compared to the previously reported values.

Kinetic analysis of anionic surfactant adsorption from aqueous solution onto activated carbon and layered double hydroxide with the zero length column method

Low cost adsorption technology offers high potential to clean-up laundry rinsing water. From an earlier selection of adsorbents, layered double hydroxide (LDH) and granular activated carbon (GAC) proved to be interesting materials for the removal of anionic surfactant, linear alkyl benzene sulfonate (LAS), which is the main contaminant in rinsing water. The main research question is to identify adsorption kinetics of LAS onto GAC-1240 and LDH. The influence of pre-treatment of the adsorbent, flow rate, particle size and initial LAS concentration on the adsorption rate is investigated in a zero length column (ZLC) set-up. The rate determining step is obtained by fitting an adsorption model and an ion exchange model describing intraparticle diffusion to the experimental data. GAC-1240 is well described with the adsorption model following Fick's second law. The effective diffusion coefficient of GAC-1240 is 1.3 × 10 −10 ± 0.2 × 10 −10 m 2/s and is not influenced by particle sizes or initial LAS concentrations. The ion exchange of LAS onto LDH is not well described by the ion exchange model. The rate determining step is obtained by comparing several models to different experimental data. A double layer model resulted in a good description of the experimental data. At the outer surface of LDH a stagnant film resistance originating from an electric double layer is assumed. The double layer mass transfer coefficient is 7 × 10 −5 ± 2 × 10 −5 m/s.