Mind the Gap: The Role of Mass Transfer in Shaped Nanoporous Adsorbents for Carbon Dioxide Capture.

Adsorptive separations by nanoporous materials are major industrial processes. The industrial importance of solid adsorbents is only expected to grow due to the increased focus on carbon dioxide capture technology and energy-efficient separations. To evaluate the performance of an adsorbent and design a separation process, the adsorption thermodynamics and kinetics must be known. However, although diffusion kinetics determine the maximum production rate in any adsorption-based separation, this aspect has received less attention due to the challenges associated with conducting diffusion measurements. These challenges are exacerbated in the study of shaped adsorbents due to the presence of porosity at different length scales. As a result, adsorbent selection typically relies mainly on adsorption properties at equilibrium, i.e., uptake capacity, selectivity and adsorption enthalpy. In this Perspective, based on an extensive literature review on mass transfer of CO2 in nanoporous adsorbents, we discuss the importance and limitations of measuring diffusion in nanoporous materials, from the powder form to the adsorption bed, considering the nature of the process, i.e., equilibrium-based or kinetic-based separations. By highlighting the lack of and discrepancies between published diffusivity data in the context of CO2 capture, we discuss future challenges and opportunities in studying mass transfer across scales in adsorption-based separations.

Development of a 3D-Printable, Porous, and Chemically Active Material Filled with Silica Particles and its Application to the Fabrication of a Microextraction Device.

We report on the first successful attempt to produce a silica/polymer composite with retained C18 silica sorptive properties that can be reliably printed using three-dimensional (3D) FDM printing. A 3D printer provides an exceptional tool for producing complex objects in an easy and inexpensive manner and satisfying the current custom demand of research. Fused deposition modeling (FDM) is the most popular 3D-printing technique based on the extrusion of a thermoplastic material. The lack of appropriate materials limits the development of advanced applications involving directly 3D-printed devices with intrinsic chemical activity. Progress in sample preparation, especially for complex sample matrices and when mass spectrometry is favorable, remains a vital research field. Silica particles, for example, which are commonly used for extraction, cannot be directly extruded and are not readily workable in a powder form. The availability of composite materials containing a thermoplastic polymer matrix and dispersed silica particles would accelerate research in this area. This paper describes how to prepare a polypropylene (PP)/acrylonitrile-butadiene-styrene (ABS)/C18-functionalized silica composite that can be processed by FDM 3D printing. We present a method for producing the filament as well as a procedure to remove ABS by acetone rinsing (to activate the material). The result is an activated 3D-printed object with a porous structure that allows access to silica particles while maintaining macroscopic size and shape. The 3D-printed device is intended for use in a solid-phase microextraction (SPME) procedure. The proposed composite's effectiveness is demonstrated for the microextraction of glimepiride, imipramine, and carbamazepine. The complex honeycomb geometry of the sorbent has shown to be superior to the simple tubular sorbent, which proves the benefits of 3D printing. The 3D-printed sorbent's shape and microextraction parameters were fine-tuned to provide satisfactory recoveries (33-47%) and high precision (2-6%), especially for carbamazepine microextraction.

Strategic Fast Induction Heating to Combat Hysteresis Barriers in a Flexible MOF for Rapid CO2 Desorption in Biogas Upgrading.

A major challenge in Cyclic Swing Separation using flexible adsorbents that have high equilibrium CO2  adsorption capacity is their very low-pressure hysteresis that hinders efficient desorption. Mg-Gallate MOF is such a flexible adsorbent that only begins to release CO2 at its pore closing pressure at 0.08 bar and 30 °C, showing very slow and inefficient desorption in pressure or temperature swing. Therefore, a novel strategy is presented that combines state of art technique Magnetic Induction Heating with a vacuum swing for fast and efficient CO2 desorption from flexible adsorbents at a moderately elevated temperature (70 °C).

Study of peak capacities generated by a porous layered radially elongated pillar array column coupled to a nano-LC system.

The performance of a porous-layered radially elongated pillar (PLREP) array column in a commercial nano-LC system was examined by performing separation of alkylphenones and peptides. The mesoporous silica layer was prepared by sol-gel processing of a mixture of tetramethoxysilane and methyltrimethoxysilane on REPs filling a 16.5 cm long, 1 mm wide channel (three lanes of 5.5 cm long channels connected by turns). The minimum plate height of 1.4 µm for octanophenone (k = 2.21) observed in isocratic mode is 5 times smaller than the smallest off-column plate height previously reported for porous pillar array columns for a retained component. This advantage is related to the earlier introduced shape of the radially elongated pillar bed that outperforms the cylindrically shaped pillar bed in terms of the plate height. In gradient mode, maximum conditional peak capacities of 220 (for a mixture of thiourea and 7 alkylphenones, tG = 180 min) and 160 (for a cytochrome c digest, tG = 150 min) were obtained. These results indicate excellent potential for implementation of this sol-gel layer in pillar array column formats.

Chromatographic Properties of Minimal Aspect Ratio Monolithic Silica Columns.

We report on a study wherein we synthesized TMOS-based silica monolithic skeletons in capillaries with an i.d. of 5 and 10 µm to produce skeleton structures with very low capillary-to-domain size aspect-ratios. These structures include the absolute minimal aspect-ratio case of a monolithic structure whose cross-section only contains a single node point. With domain-sized based reduced plate heights running as low as hmin = 1.3-1.5 for retained coumarin dyes providing a retention factor of k = 0.6-1.0, the study confirms the classic observation that ultralow aspect ratio columns generate a markedly lower dispersion than columns with a larger aspect ratio made in the past by Knox, Jorgenson, and Kennedy for the packed bed of spheres, but now for silica monoliths. The course of the reduced van Deemter curves, and more specifically the ratio of A-term versus C-term band broadening, could be interpreted in terms of the width and persistence length of the velocity bias zones in the columns. Considering the overall kinetic performance, it is found that the two best performing structures are also the structures with the lowest number of domains or node points, that is, with the lowest capillary-to-domain size aspect-ratio and, hence, resembling closest to the open-tubular format, which remains confirmed as the column format with the best kinetic performance. This is quantified by the fact that the minimal impedance values (order of Emin = 100) of the best performing ultralow aspect ratio monolithic columns are still significantly larger than the Emin values for the reference open-tubular columns (order of Emin = 15-20).

The role of crystal diversity in understanding mass transfer in nanoporous materials.

Nanoporous materials find widespread applications in our society: from drug delivery to environmentally friendly catalysis and separation technologies. The efficient design of these processes depends crucially on understanding the mass transfer mechanism. This is conventionally determined by uptake or release experiments, carried out with assemblages of nanoporous crystals, assuming all crystals to be identical. Using micro-imaging techniques, we now show that even apparently identical crystals (that is, crystals of similar size and shape) from the same batch may exhibit very different uptake rates. The relative contribution of the surface resistance to the overall transport resistance varied with both the crystal and the guest molecule. As a consequence of this crystal diversity, the conventional approach may not distinguish correctly between the different mass transfer mechanisms. Detection of this diversity adds an important new piece of evidence in the search for the origin of the surface barrier phenomenon. Our investigations were carried out with the zeolite SAPO-34, a key material in the methanol-to-olefins (MTO) process, propane-propene separation and adsorptive heat transformation.

Very High Efficiency Porous Silica Layer Open-Tubular Capillary Columns Produced via in-Column Sol-Gel Processing.

It is demonstrated that 5 µm i.d. capillaries can be coated with mesoporous silica layers up to 550 nm thickness. All the columns produced using in-column sol-gel synthesis with tetramethoxysilane provide plate height curves that closely follow the Golay-Aris theory. In 60 cm long columns, efficiencies as high as N = 150 000 and N = 120 000 were obtained, respectively, for a 300 and 550 nm thick porous layer. An excellent retention and plate height reproducibility was obtained when the recipes were subsequently applied to produce very long (1.9 and 2.5 m) capillaries. These columns produced efficiencies up to N = 600 000 plates for a retained and around N = 1 000 000 plates for an unretained component. Given the good reproducibility on the long capillaries, and considering that mesoporous silica is still the preferred support for LC, it is believed the present study could spur a renewed interest in open-tubular LC.

Shape selective properties of the Al-fumarate metal-organic framework in the adsorption and separation of n-alkanes, iso-alkanes, cyclo-alkanes and aromatic hydrocarbons.

The primary goal of this work is to study the adsorption of a wide range of hydrocarbon adsorbates in the Al-fumarate metal-organic framework in order to identify and explore trends in adsorption behaviour that can be related to the sorbate's molecular properties and as well as the properties of this MOF. The pulse chromatographic technique was used to study the adsorption properties of C5-C8 linear, branched, cyclic and aromatic hydrocarbons in vapour phase at low coverage and at high temperatures (150-250 °C). Chromatograms of alkanes having the same number of carbon atoms (C5-C8) clearly show that the linear alkane is retained the longest over its branched and cyclic isomers. Moreover, xylene isomers are also clearly separated by Al-fumarate, with retention times increasing in the order: ortho-xylene < meta-xylene < para-xylene. Differences in adsorption enthalpy of more than 10 kJ mol(-1) between linear alkanes and their di/tri-branched or cyclo-alkane isomers were observed, clearly showing that steric effects imposed by the pore structure of the adsorbent cause the difference in adsorption between linear alkanes and their isomers. In conclusion, Al-fumarate behaves as a shape selective material with respect to structural isomers of linear alkanes, with properties resembling those of medium pore size zeolites.

Comprehensive study of the macropore and mesopore size distributions in polymer monoliths using complementary physical characterization techniques and liquid chromatography.

Poly(styrene-co-divinylbenzene) monolithic stationary phases with two different domain sizes were synthesized by a thermally initiated free-radical copolymerization in capillary columns. The morphology was investigated at the meso- and macroscopic level using complementary physical characterization techniques aiming at better understanding the effect of column structure on separation performance. Varying the porogenic solvent ratio yielded materials with a mode pore size of 200 nm and 1.5 µm, respectively. Subsequently, nano-liquid chromatography experiments were performed on 200 µm id × 200 mm columns using unretained markers, linking structure inhomogeneity to eddy dispersion. Although small-domain-size monoliths feature a relatively narrow macropore-size distribution, their homogeneity is compromised by the presence of a small number of large macropores, which induces a significant eddy-dispersion contribution to band broadening. The small-domain size monolith also has a relatively steep mass-transfer term, compared to a monolith containing larger globules and macropores. Structural inhomogeneity was also studied at the mesoscopic level using gas-adsorption techniques combined with the non-local-density-function-theory. This model allows to accurately determine the mesopore properties in the dry state. The styrene-based monolith with small domain size has a distinctive trimodal mesopore distribution with pores of 5, 15, and 25 nm, whereas the monolith with larger feature sizes only contains mesopores around 5 nm in size.

Adsorption and Separation of Small Hydrocarbons on the Flexible, Vanadium-Containing MOF, COMOC-2.

COMOC-2, a flexible vanadium-containing metal-organic framework, was investigated for its adsorption and separation properties of light hydrocarbons. COMOC-2 is an extended version of the MIL-47 framework with 4,4'-biphenyldicarboxylic acid linkers instead of terephthalic acid. Adsorption isotherms of methane to propane, ethylene, and propylene were determined with a gravimetric uptake technique at temperatures between 281 and 303 K. A pronounced breathing effect was observed (in contrast to the more rigid MIL-47 framework) in which the adsorption capacity increases by more than a factor of 2 at a given breathing pressure. The breathing pressure decreases with increasing hydrocarbon molecular weight. The typical two-step isotherms are nearly identical for alkanes and alkenes, in accordance with the nonpolar nature of the material. Binary isotherms of ethane and propane were also measured with the gravimetric uptake technique at different temperatures and total pressures. The mixture isotherms and breathing transition pressures were predicted by relying on the osmotic framework adsorbed solution theory (OFAST). Finally, the separation potential of COMOC-2 for ethane/propane mixtures was looked into using breakthrough experiments for different compositions and different pressures.

Prediction of molecular separation of polar-apolar mixtures on heterogeneous metal-organic frameworks: HKUST-1.

Due to the combination of metal ions and organic linkers and the presence of different types of cages and channels, metal-organic frameworks often possess a large structural and chemical heterogeneity, complicating their adsorption behavior, especially for polar-apolar adsorbate mixtures. By allocating isotherms to individual subunits in the structure, the ideal adsorbed solution theory (IAST) can be adjusted to cope with this heterogeneity. The binary adsorption of methanol and n-hexane on HKUST-1 is analyzed using this segregated IAST (SIAST) approach and offers a significant improvement over the standard IAST model predictions. It identifies the various HKUST-1 cages to have a pronounced polar or apolar adsorptive behavior.

Adsorptive characterization of the ZIF-68 metal-organic framework: a complex structure with amphiphilic properties.

In this experimental study, the adsorption behavior of the ZIF-68 heterolinked zeolitic imidazolate framework has been explored. Vapor phase adsorption isotherms of linear C1-C6 alcohols, C6 alkane isomers, aromatics (benzene, toluene, xylene isomers, 1,3,5-trimethylbenzene, and 1,3,5-triisopropylbenzene), and polar adsorbates (water, acetonitrile, and acetone) are reported and discussed. The complex pore structure of ZIF-68, with two one-dimensional channels, each with a different polarity, displays an overall hydrophobic character. Its two-pore system results in S-shaped isotherms for small polar adsorbates (small alcohols, acetone, and acetonitrile), while longer alcohols and nonpolar molecules, such as aromatics and C6 alkane isomers, lead to type I adsorption isotherms. Bulky molecules, with a kinetic diameter significantly larger than the pore windows, are adsorbed in large amounts, which gave reason to think that this ZIF-68 material has a certain degree of framework flexibility to enlarge the free aperture of the channels. Besides, diffusion coefficients from vapor phase uptake and infrared experiments point to a different adsorption mechanism for polar and nonpolar adsorbates. Liquid phase adsorption experiments demonstrated the separation of alcohol mixtures (ethanol/1-butanol) at low concentration from water, with a clear preference for 1-butanol.

Adsorption and separation of CO2 on KFI zeolites: effect of cation type and Si/Al ratio on equilibrium and kinetic properties.

Selective separation of CO2 is becoming one of the key technologies in the (petro-) chemical industry. This study focuses on the adsorption and separation of CO2 from CH4 on a new low-silica (LS) type of the eight-membered ring KFI zeolite. A series of alkali (Li, Na, K) and alkaline-earth (Mg, Ca, Sr) exchanged samples of the new LS KFI were synthesized and characterized. LS Li-KFI showed the largest pore volume, whereas LS Na-KFI and LS K-KFI were inaccessible for Argon at 87 K. Adsorption of CO2 at 303 K demonstrated the dominant quadrupolar interaction on alkali-exchanged LS KFI samples. LS Li-KFI showed the largest capacities upon high pressure isotherm measurements of CO2 (4.8 mmol/g), CH4 (2.6 mmol/g), and N2 (2.2 mmol/g) up to 40 bar at 303 K. The performance of the new LS KFI was compared to a KFI sample (ZK-5) with a higher Si/Al ratio. Isotherm measurements and dynamic breakthrough experiments demonstrated that ZK-5 samples show larger working capacities for CO2/CH4 separations at low pressure. Li-ZK-5 and Na-ZK-5 show the highest capacities and high selectivities (similar to benchmark 13X).

Enabling hydrate-based methane storage under mild operating conditions by periodic mesoporous organosilica nanotubes.

Biomethane is a renewable natural gas substitute produced from biogas. Storage of this sustainable energy vector in confined clathrate hydrates, encapsulated in the pores of a host material, is a highly promising avenue to improve storage capacity and energy efficiency. Herein, a new type of periodic mesoporous organosilica (PMO) nanotubes, referred to as hollow ring PMO (HR-PMO), capable of promoting methane clathrate hydrate formation under mild working conditions (273 K, 3.5 MPa) and at high water loading (5.1 g water/g HR-PMO) is reported. Gravimetric uptake measurements reveal a steep single-stepped isotherm and a noticeably high methane storage capacity (0.55 g methane/g HR-PMO; 0.11 g methane/g water at 3.5 MPa). The large working capacity throughout consecutive pressure-induced clathrate hydrate formation-dissociation cycles demonstrates the material's excellent recyclability (97% preservation of capacity). Supported by ex situ cryo-electron tomography and x-ray diffraction, HR-PMO nanotubes are hypothesized to promote clathrate hydrate nucleation and growth by distribution and confinement of water in the mesopores of their outer wall, along the central channels of the nanotubes and on the external nanotube surface. These findings showcase the potential for application of organosilica materials with hierarchical and interconnected pore systems for pressure-based storage of biomethane in confined clathrate hydrates.

Fast and selective sugar conversion to alkyl lactate and lactic acid with bifunctional carbon-silica catalysts.

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.

Pulse gas chromatographic study of adsorption of substituted aromatics and heterocyclic molecules on MIL-47 at zero coverage.

The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure-property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role.

Modeling the effect of structural changes during dynamic separation processes on MOFs.

A model able to describe the effect of structural changes in the adsorbent or adsorbed phase during the dynamic (breakthrough) separation of mixtures on metal-organic frameworks (MOFs) is presented. The methodology is exemplified for a few pertinent case studies: the separation of xylene isomers and ethylbenzene on the flexible MOF MIL-53 and the rigid MOF MIL-47. At low pressures, no preferential adsorption of any component occurs on both MOFs. Contrarily, at higher pressures separation of ethylbenzene (EB) from o-xylene (oX) occurs on MIL-53 as a result of the breathing phenomenon within the MIL-53 structure. The increase in selectivity, starting from the gate-opening pressure, could be modeled by using a pressure-dependent saturation capacity for the most strongly adsorbed component oX. In the separation of m-xylene (mX) from p-xylene (pX) on the rigid MOF MIL-47, separation at higher pressures is a result of preferential stacking of pX. Here, the selectivity increases once the adsorption of pX switches from a single to a double file adsorption. By implementing a loading dependent adsorption constant for pX, the different unconventional breakthrough profiles and the observed selectivity profile on MIL-47 can be simulated. A similar methodology was used for the separation of EB from pX on MIL-47, where the separation is a result from steric constraints imposed onto the adsorption of EB.

Performance of functionalized monolithic silica capillary columns with different mesopore sizes using radical polymerization of octadecyl methacrylate.

We report on the possibility to enhance the phase ratio and retention factor in silica monoliths. According to pioneering work done by Núñez et al. [1], this enhancement is pursued by applying a stationary phase layer via radical polymerization with octadecyl methacrylate (ODM) as an alternative to the customary octadecylsilylation (C18-derivatization). The difference in band broadening, retention factor and separation selectivity between both approaches was compared. Different hydrothermal treatment temperatures for the column preparation were applied to produce monolithic silica structures with three different mesopore sizes (resp. 10, 13, and 16 nm, as determined by argon physisorption) while maintaining similar domain size (sum of through-pore and skeleton size). It has been found that the columns with the poly(octadecyl methacrylate)-phase (ODM columns) provided a 60 to 80% higher retention factor in methanol-water mixture compared to the octadecylsilylated (ODS) columns produced by starting from similar silica backbone structures. In acetonitrile-water mixture, the enhancement is smaller (15 to 30% times higher), yet significant. By adjusting the fabrication conditions (for both the preparation of the monolithic backbones and the surface functionalization), the achieved retention factors (up k = 4.89 for pentylbenzene in 80:20% (v/v) methanol/water) are obviously higher than obtained in the pioneering study on ODM monoliths of Núñez et al. [1], and column clogging could be completely avoided. In addition, also separation efficiencies were significantly higher than shown in Ref. [1], with plate heights as low as 5.8 µm. These plate heights are however inferior to those observed on the ODS-modified sister columns. The difference can be explained by the slower intra-skeleton diffusion displayed by the ODM-modified columns, in turn caused by the larger obstruction to diffusion originating from the thicker stationary phase layer.

A pulse chromatographic study of the adsorption properties of the amino-MIL-53 (Al) metal-organic framework.

Low-coverage adsorption properties of the metal-organic framework amino-MIL-53 (Al) were determined using the pulse chromatographic technique. By using n-alkanes, iso-alkanes, 1-alkenes, cyclohexane and benzene as probe molecules, the nature of the adsorptive interactions in amino-MIL-53 (Al) was studied. Henry adsorption constants and adsorption enthalpies of iso-alkanes are significantly lower than those of the linear alkanes, demonstrating the shape selective properties of amino-MIL-53. The presence of amino-groups in the pores of the material increases the electrostatic contributions with molecules containing double bonds. A simple model relates adsorption enthalpies to the number of hydrogen atoms and double bonds in the molecule. The effective pore size of the material was estimated based on the relationship between adsorption enthalpy and entropy.