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.

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).

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.

Evaluation of particle and bed integrity of aqueous size-exclusion columns packed with sub-2 µm particles operated at high pressure.

The performance of columns packed with 1.7 µm particles for aqueous size-exclusion chromatography was assessed at high-pressure conditions and linked to particle- and column-bed integrity. Decreasing the particle size from 3.5 µm to 1.7 µm increases the resolution due to the improved mass-transfer characteristics, allowing to significantly speed-up analysis without compromising the selectivity. A sub-minute separation of intact proteins was realized on a 4.6 mm i.d × 75 mm long column packed with 1.7 µm SEC particles applying a flow rate of 1.8 mL/min, corresponding to a column pressure of 530 bar. Ultra-high pressure operation (exceeding manufacturer's recommendations) resulted in peak deformation, a shift towards earlier retention times, and an alteration in selectivity. To gain insights in the mechanisms of column deterioration, short 30 mm long columns were operated at UHPLC conditions, maximizing the pressure drop over individual particles. This resulted in the presence of fractured particles situated at the column outlet, as verified by scanning electron micrographs. Mercury-intrusion porosimetry and argon-adsorption measurements did not reveal significant differences in intraparticle volume between particle batches sampled before and after pressure stress testing. As particles at the column outlet fracture (but not collapse) at high pressure operation, a void was formed at the column inlet. The degradation of the separation performance appeared to be the result of a decrease in interparticle pore volume.

Chromatographic study of the structural properties of mesoporous silica layers deposited on radially elongated pillars.

We report on the ability to change the layer properties of porous layered radially elongated pillar (PLREP) array columns and its relevance to the separation efficiency. The adjustment of the preparation condition resulted in the formation of a 1.2-fold thicker layer than the layer produced in the preceding study. The mesoporosity of the layer was controlled by changing the hydrothermal treatment temperature from 105 °C to 80 °C. Chromatographic characterization was performed on a commercial nano-LC system using the octadecylsilylated PLREP columns having the aforementioned characteristics, i.e. (1) different layer thickness (df = 180-220 nm) and (2) different mesoporosity (dp = 7.6-11.2 nm, Pore volume (Vp) = 0.733‒0.838 cc/g and Surface area (SA) = 364‒611 m2/g). For isocratic separations of an alkylphenone mixture, the change in both the layer thickness and the mesoporosity caused no significant difference in the column efficiency, while the thicker layer and the reduction of mesopore size resulted in a 1.3-fold increase and a 1.4-fold increase in the retention capacity, respectively. Based on the result of the examination using scanning electron microscopy and argon physisorption technique, the formar enhancement was in agreement with the increase in the layer thickness, and the latter one was attributed to the larger surface area. When applying a column with 16.5 cm long to gradient separations, the combination of the thicker layer and the smaller mesopores provided the peak capacity of 365 for the alkylphenone mixture at a 180 min gradient, while the combination of the thinner layer and the larger mesopores provided the peak capacity of 315. For peptide separations, it appeared that the thicker layer was still favorable, however, the lager mesopores were more advantageous for MWs of larger than 1000, providing a conditional peak capacity of 245 for a commercially available peptide mixture because of less content of small pores which hinder the diffusion of large molecules in pores in the layer.

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.

Silica-based hybrid porous layers to enhance the retention and efficiency of open tubular capillary columns with a 5 µm inner diame9ter.

We report on possibility to enhance the hydrophobicity of octadecylsilylated silica-based porous layered open tubular (PLOT) columns with an inner diameter (i.d.) of 5 µm by applying hybrid tetramethoxysilane (TMOS)/methyltrimethoxysilane (MTMS) layers with inserted methyl groups. Due to this higher hydrophobicity, thinner porous layers suffice to achieve similar retention factor (k) as in octadecylsilylated silica-based PLOT columns synthesized using TMOS only. Since thinner layers have a lower intra-layer mass transfer resistance, this in turn allows to obtain superior column efficiencies in comparison with separations carried out with TMOS-based PLOT columns at the same retention. Since layer thickness contributes to the C-term type of band broadening, this is most pronounced at high velocities. Typical gains in column efficiency at a reduced velocity of νi = 30 are on the order of 15%. Preparing the hybrid PLOT columns in 5 µm i.d.-capillaries with a length of 0.4 m using different TMOS/MTMS preparation mixtures leads to different layer thickness in the capillaries. It is shown that column efficiencies for the most retained compound (k = 0.9-1.5) went from N = 101,000 for PLOT columns with a layer thickness (df) of 250 nm, over N = 95,000 for df = 320 nm to N = 89,000 for df = 400 nm, corresponding to plate heights (H) in the order of 3.5-3.9 µm (reduced plate heights (h = 0.8-1.0)). By applying the same preparation mixtures for much longer capillaries of 1.3 m, a high repeatability of the volumetric phase ratio (m) (difference <1%) and the k-values (difference <5%) was observed between the 0.4 m and 1.3 m PLOT columns. In addition, also a very similar band broadening was obtained, as the h-values in the longer columns coincided well (order of a few % difference) with the reduced plate height curves measured in the shorter columns. The effect of the retention factor and layer thickness on these reduced plate height curves furthermore fits well with the Golay-Aris theory. Depending on the layer thickness, plate numbers in the longer capillary columns were varying from N = 282,000 to N = 379,000 for the most retained compound.

Exploring the effect of mesopore size reduction on the column performance of silica-based open tubular capillary columns.

We report on a modification in the hydrothermal treatment process of monolithic silica layers used in porous-layered open tubular (PLOT) columns. Lowering the temperature from the customary 95 °C to 80 °C, the size of the mesopores reduced by approximately about 35% from 12-13.5 nm to 7.5-9 nm, while the specific pore volume essentially remains unaltered. This led to an increase of the specific surface area (SA) of about 40%, quasi-independent of the porous layer thickness. The increased surface area provided a corresponding increase in retention, somewhat more (48%) than expected based on the increase in SA for the thin layer columns, and somewhat less than expected (34%) for the thick layer columns. The recipes were applied in 5 µm i.d.-capillaries with a length of 60 cm. Efficiencies under retained conditions amounted up to N = 137,000 for the PLOT column with a layer thickness (df) of 300 nm and to N = 109,000 for the PLOT column with df = 550 nm. Working under conditions of similar retention, the narrow pore/high SA columns produced with the new 80 °C recipe generated the same number of theoretical plates as the wide pore size/low SA columns produced with the 95 °C recipe. This shows the 80 °C-hydrothermal treatment process allows for an increase in the phase ratio of the PLOT columns without affecting their intrinsic mass transfer properties and separation kinetics. This is further corroborated by the fact that the plate height curves generated with the new and former recipe can both be well-fitted with the Golay-Aris equation without having to change the intra-layer diffusion coefficient.

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).

Preparation and evaluation of mesoporous silica layers on radially elongated pillars.

The present paper describes the application of a sol-gel procedure on radially elongated pillars (REPs) using tetramethoxysilane and methyltrimethoxysilane. After octadecylsilylation, the quality of the porous layered REP (PLREP) columns was evaluated by in-situ determination of migration velocities and band broadening of coumarin dyes with fluorescence microscopy in reversed-phase liquid chromatography. Based on the increase in retention due to the sol-gel process, an increase in accessible specific surface by a factor of 112 was observed. Argon physisorption measurements on bulk monoliths prepared with the same method revealed a predominant pore size of 91Å. Plate heights as low as 0.4-0.8µm (k=0-1.97) could be obtained thanks to the very low dispersion of the REP format and to the fact that the applied silica layer was conformally and uniformly deposited on the flow-through channels. A kinetic plot analysis demonstrated that the studied PLREP column will deliver more theoretical plates per unit of time than a 5µm core shell packed bed when more than 1.0×104 theoretical plates are required.

Intensified Biobutanol Recovery by using Zeolites with Complementary Selectivity.

A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.

Gel-based morphological design of zirconium metal-organic frameworks.

The ability of metal-organic frameworks (MOFs) to gelate under specific synthetic conditions opens up new opportunities in the preparation and shaping of hierarchically porous MOF monoliths, which could be directly implemented for catalytic and adsorptive applications. In this work, we present the first examples of xero- or aerogel monoliths consisting solely of nanoparticles of several prototypical Zr4+-based MOFs: UiO-66-X (X = H, NH2, NO2, (OH)2), UiO-67, MOF-801, MOF-808 and NU-1000. High reactant and water concentrations during synthesis were observed to induce the formation of gels, which were converted to monolithic materials by drying in air or supercritical CO2. Electron microscopy, combined with N2 physisorption experiments, was used to show that irregular nanoparticle packing leads to pure MOF monoliths with hierarchical pore systems, featuring both intraparticle micropores and interparticle mesopores. Finally, UiO-66 gels were shaped into monolithic spheres of 600 µm diameter using an oil-drop method, creating promising candidates for packed-bed catalytic or adsorptive applications, where hierarchical pore systems can greatly mitigate mass transfer limitations.

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.

Separation properties of the MIL-125(Ti) Metal-Organic Framework in high-performance liquid chromatography revealing cis/trans selectivity.

Monodisperse MIL-125(Ti) Metal-Organic Framework crystals were synthesized and studied as stationary phase in high performance liquid chromatography (HPLC). Different pure compounds and model mixtures (including stereoisomer mixtures) were injected from which chromatographic parameters, including selectivities and resolution factors, were determined to evaluate the adsorption properties and separation performance of MIL-125(Ti) in liquid phase. The MIL-125(Ti) framework displayed a trans selectivity for cis/trans difunctionalized cyclohexane molecules with high selectivity and resolution for 1,3-dimethylcyclohexane and 4-ethylcyclohexanol. The slurry-packed column was further characterized via van Deemter analysis. Fitting of the van Deemter equation through the experimental points allowed to define the contributions of the different processes to plate height with a significant proportion of the A-term, reflecting the importance of a good crystal packing. Although high in comparison to commercial HPLC stationary phases, a very good plate height of around 50µm was found.

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.

Effect of polyethylene glycol on pore structure and separation efficiency of silica-based monolithic capillary columns.

Monolithic silica materials (first unclad monolith rods, then monolithic capillary columns) were prepared using various amounts of polyethylene glycols (PEGs) with different molecular weight (MW). The monolith rods were used to examine the mesoporosity by argon physisorption technique, and the macroporosity by mercury intrusion porosimetry. Subsequently, silica-based monolithic capillary columns with an inner diameter of 100 µm were produced using the same preparation conditions as used for the rods. The results obtained with the monolith rods showed the following important findings: (1) it is feasible to fabricate monolithic silica rods possessing macropore size of 0.5-1.4 µm by tuning the amount of PEGs (independently of the MW), whereas the macropore volume and the mesoporosity remain similar. (2) the smallest macropore size (0.5 µm) rod prepared with PEG having a MW=20,000g/mol provided a narrower macropore size distribution than with PEG with MW=10,000g/mol. The monolithic capillary columns produced with the different PEG type showed similar retention factors for hexylbenzene (k=2.3-2.4) and similar t0-based column permeability (Kv0=2.3-2.4×10(-14)m(2)) in 20:80% (v/v) water:methanol, as expected from the results obtained with the monolith rods. The column prepared with PEG of MW=20,000g/mol gave a plate height of H=4.0 µm for hexylbenzene at an optimal linear velocity of u0=2.6mm/s in 20:80% (v/v) water containing 0.1% formic acid:acetonitrile. To the best of our knowledge, this is the lowest plate height ever recorded for a monolithic column. Comparing the kinetic performance at 30MPa shows that the best monolithic silica column obtained in the present study performs better than the second-generation monolithic silica columns developed up till now in the practically most relevant range of plate numbers (N≤40,000). In this range, the performance is now similar to that of 2.7 µm core-shell particle columns.

Molecular separations with breathing metal-organic frameworks: modelling packed bed adsorbers.

Various metal-organic framework (MOFs) adsorbents show peculiar adsorption behaviour as they can adopt different crystal phases, each phase with its own adsorption characteristics. Besides external stimuli such as temperature or light, different species of guest adsorbate can trigger a transition (breathing) of the host structure at a different pressure. Such phase transitions also occur during dynamic separations on a packed bed of adsorbent, where the concentrations of the adsorbates vary throughout axial column distance and time. This work presents a general strategy to model the adsorption behavior of such phase changing adsorbents during column separations and focuses on remarkable model predictions for pure components and binary mixtures in diluted and non-diluted conditions. During binary breakthrough experiments, the behaviour of flexible adsorbents is quite complex. A succession of complete or even partial phase transformations (resulting in phase coexistence) can occur during the adsorption process. A variety of unusual breakthrough profiles is observed for diluted binary mixtures. Simulations reveal at least five types of breakthrough profiles to emerge. The occurrence of these cases can be rationalized by the hodograph technique, combined with the phase diagram of the adsorbent. The remarkable experimental breakthrough profiles observed for ortho-xylene/ethylbenzene (diluted) and CO2/CH4 (non-diluted) separation on the flexible MIL-53 framework can be rationalized by application of the proposed model strategy.

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.

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.