Microporous and Mesoporous Materials

Published by Elsevier
Print ISSN: 1387-1811
The representative entropy, H(q, ) superimposed on the phase diagram of the diffusion process as represented by the MLF as the characteristic function.
In this high-resolution magnetic resonance imaging (MRI) study at 17.6 Tesla of a fixed rat brain, we used the continuous time random walk theory (CTRW) for Brownian motion to characterize anomalous diffusion. The complex mesoporus structure of biological tissues (membranes, organelles, and cells) perturbs the motion of the random walker (water molecules in proton MRI) introducing halts between steps (waiting times) and restrictions on step sizes (jump lengths). When such waiting times and jump lengths are scaled with probability distributions that follow simple inverse power laws (t (-(1+α)), |x|(-(1+β))) non-Gaussian motion gives rise to sub- and super- diffusion. In the CTRW approach, the Fourier transform yields a solution to the generalized diffusion equation that can be expressed by the Mittag-Leffler function (MLF), E α (- D α, β|q|(β)Δ(α)). We interrogated both white and gray matter regions in a 1 mm slice of a fixed rat brain (190 μm in plane resolution) with diffusion weighted MRI experiments using b-values up to 25,000 s/mm (2), by independently varying q and Δ. When fitting these data to our model, the fractional order parameters, α and β, and the entropy measure, [Formula: see text], were found to provide excellent contrast between white and gray matter and to give results that were sensitive to the type of diffusion experiment performed.
The design of carbon sorbents traditionally focuses on the control of pore structure and the number and type of surface functional groups. The present paper explores the potential of also controlling the carbon crystal structure, or graphene layer orientation, in the immediate vicinity of the internal surfaces. We hypothesize that this crystal structure influences the properties of the carbon surfaces and affects the number and type of active sites for functionalization. Here a series of mesoporous carbons are fabricated by capillary infiltration of mesophase pitch (naphthalene homopolymer) into a series of controlled pore glass templates of different characteristic pore size followed by carbonization and template etching. The liquid crystalline mesogens are known to adopt perpendicular alignment (anchoring) at liquid/silica interfaces, which after carbonization lead to a high concentration of graphene edge sites at the inner surfaces. These surfaces are shown to have elevated chemical reactivity, and the pore structures are shown to be consistent with predictions of a quantitative model based on the negative replica concept. Overall, the use of mesophase pitch for templated mesoporous carbons allows systematic and simultaneous control of both pore structure and interfacial crystal structure through the well-defined rules of liquid crystal surface anchoring.
The low-temperature synthesis (90°C) of nanoparticle forms of a pure phase smectic clay (saponite) and zeolite (cancrinite) is reported, along with phase mixtures thereof. A synthesis gel corresponding to the Si:Al:Mg unit cell composition of saponite (3.6:0.40:3.0) and a NaOH/Si ratio of 1.39 affords the pure phase clay with disordered nanolayer stacking. Progressive increases in the NaOH/Si ratio up to a value of 8.33 results in the co-crystallization of first garronite and then cancrinite zeolites with nanolath morphology. The resulting phase mixtures exhibit a compound particulate structure of intertwined saponite nanolayers and cancrinite nanolaths that cannot be formed through physical mixing of the pure phase end members. Under magnesium-free conditions, pure phase cancrinite nanocrystals are formed. The Si/Al ratio of the reaction mixture affects the particle morphology as well as the chemical composition of the cancrinite zeolite. Ordinarily, cancrinite crystallizes with a Si/Al ratio of 1.0, but a silicon-rich form of the zeolite (Si/Al=1.25) is crystallized at low temperature from a silica rich synthesis gel, as evidenced by (29)Si NMR spectroscopy and XEDS-TEM. Owing to the exceptionally high external surface areas of the pure phase clay (875 m(2)/g) and zeolite end members (8.9 - 40 m(2)/g), as well as their unique mixed phase composites (124 - 329 m(2)/g), these synthetic derivatives are promising model nanoparticles for studies of the bioavailability of poly-aromatic hydrocarbons immobilized in silicate bearing sediments and soils.
Solid-state CPL measurements were performed for the first time on hybrid, laminar materials based on γ-ZrP pillared with organic diphosphonates. Ad hoc optically pure diphosphonates were synthesized and the luminescence properties of their complexation with Tb(III) were verified in solution. CD and CPL measurements showed that the bistriazolylpyridine chromophores bonded to the metal provided an effective chiral environment that produced significant signals. In the case of the γ-ZrP-derived materials, experimental evidence and simple molecular modeling hinted to the occurrence of supramolecular chirality in the particles, induced by the intrinsic dissymmetry of the organic diphosphonates or by the intercalation of chiral species such as 1-phenethylamine. Chirality at the supramolecular level was revealed in the solid state by the CPL signals measured from reporter Tb(III) ions intercalated in the hybrid matrix.
Mesoporous silicas have been extensively used for entrapping small chemical molecules and biomacromolecules for drug delivery. We hypothesize that the loading density of biomacromlecules such as proteins in mesoporous silicas could be limited due to disordering in the pore structure and long diffusion time in the pore channels. We shattered mesoporous silicas non-destructively resulting in improved intramesoporous structures and reduced particle sizes in aqueous solutions by a powerful sonication, where the mesoporous structures were still well maintained. The sonication-shattered mesoporous silica can increase the protein loading density to nearly 2.7 times as high as that of the non-shattered one, demonstrating that significantly more mesopore space of the silica could be accessible by the protein molecules, which may result in more sustained protein drug delivery.
Large-pore mesoporous silica with 3D wormhole framework structures (denoted MSU-J) are prepared through a supramolecular hydrogen-bonding assembly pathway from low-cost sodium silicate as the silica source and commercially available mono- and triamine Jeffamine and Surfonamine surfactants as structure-directing porogens. The calcined mesostructures exhibit large pore sizes (up to 8.2 nm), surface areas (632-1030 m(2)/g) and pore volumes (0.5-2.0 cm(3)/g), depending on the surfactant chain length and synthesis temperature (25-65 °C). The textural properties of these new wormhole mesostructures are comparable to those of hexagonal SBA-15 derivatives and large pore MCM-48. However, unlike the SBA-15 structure type, wherein the 3D pore network is formed by connecting 1D cylindrical mesopores through micropores, MSU-J mesophases have wormhole framework structures containing fully interconnected 3D mesopores that can minimize the diffusion limitations often encountered in adsorption and chemical catalysis. Also, unlike large pore MCM-48, which requires cost-intensive tetraethylorthosilicate as a silica source and the use of a co-surfactant as a pore expander under strong acid conditions, MSU-J mesostructures are assembled from low cost sodium silicate in the presence of a single Jeffamine or Surfonamine porogen at near-neutral pH.
We present a new methodological basis for selectively illuminating a dilute population of fluid within a porous medium. Specifically, transport in porous materials can be analyzed by now-standard nuclear magnetic resonance (NMR) relaxometry and NMR pulsed field gradient (PFG) diffusometry methods in combination with with the prominent NMR signal amplification tool, dynamic nuclear polarization (DNP). The key components of the approach introduced here are (1) to selectively place intrinsic or extrinsic paramagnetic probes at the site or local volume of interest within the sample, (2) to amplify the signal from the local solvent around the paramagnetic probes with Overhauser DNP, which is performed in situ and under ambient conditions, and (3) to observe the ODNP-enhanced solvent signal with 1D or 2D NMR relaxometry methods, thus selectively amplifying only the relaxation dynamics of the fluid that resides in or percolates through the local porous volume that contains the paramagnetic probe. Here, we demonstrate the proof of principle of this approach by selectively amplifying the NMR signal of only one solvent population, which is in contact with a paramagnetic probe and occluded from a second solvent population. An apparent one-component T 2 relaxation decay is shown to actually contain two distinct solvent populations. The approach outlined here should be universally applicable to a wide range of other 1D and 2D relaxometry and PFG diffusometry measurements, including T 1-T 2 or T 1-D correlation maps, where the occluded population containing the paramagnetic probes can be selectively amplified for its enhanced characterization.
The open framework zinco-cobalto-borophosphate, NH4[(Zn1−xCox)BP2O8] (0⩽x⩽0.14), has been synthesized as pale blue powder under hydrothermal conditions. The crystal structure was determined at 293 K from single-crystal X-ray diffraction data (triclinic (no. 2), NH4[(Zn0.88Co0.12)BP2O8]: a=7.4319(11) Å, b=7.5997(5) Å, c=7.8402(2) Å, α=118.9940(9)°, β=101.6597(9)°, γ=103.4308(14)°, V=350.01 Å3, Z=2). The structure is characterized by channels of eight-membered rings of tetrahedra running along [1 0 1] and [1 1 0] directions, respectively. The title compound is an isotype of the zinco-borophosphates A[ZnBP2O8] (A=NH4+, Rb+, Cs+) which exhibit a gismondine-type framework topology.
The effect of UV-irradiation on (0 0 1) rutile wafers, used as a model support for in-situ zeolite A layer growth on TiO2-coated membrane supports, is reported. From previous studies it is known that UV-radiation increases the number of defect sites and hydroxyl groups which promote the hydrophilicity of the rutile surface. Our work reports on the increase of the nucleation rate of zeolite A on this support due to these improved wetting properties. Because of the larger number of zeolite A precursor nuclei, the resulting zeolite A phase on an irradiated TiO2 support appears to be constant in thickness and monolithic, as well as perfectly attached to the support.
The tris(1,2-diaminoethane)nickel(II) complex was found to be a novel structure-directing agent for the preparation of the chabazite-like AlPO4-34 molecular sieve. During crystallization the complex undergoes a ligand exchange, thus helping stabilization of the AlPO4's chabazite structure by hydroxyls that have exchanged one 1,2-diaminoethane (en) molecule from the Ni(II) coordination sphere. The exchanged complex, [Ni(en)2(OH)2], remains in the chabazite host causing a triclinic deformation of the rhombohedral chabazite framework. Upon calcination the triclinic phase completely transforms into the rhombohedral chabazite-like AlPO4-34 as evidenced from the in situ XRD pattern.
By adding benzene-1,2-diol as a complex agent for silicon into the system: SiO2–TPABr–NaOH–R–H2O (R: benzene-1,2-diol), single crystals of zeolites silicalite-1 with different crystallite sizes ranging from 9×3×2 μm to 165×30×30 μm were obtained. Experimental results showed that crystals synthesized in the presence of benzene-1,2-diol have a much larger size and better morphology than those synthesized in the absence of benzene-1,2-diol, and their sizes were largely influenced by the content of benzene-1,2-diol in the reaction system. The role of benzene-1,2-diol in the hydrothermal reaction mixture has been characterized by 29Si-NMR, and it was shown that silicon–benzene-1,2-diol complex was formed in the reaction system, which was very important for the synthesis of large single crystals of zeolites.
The framework structure and extraframework atoms of calcined and dehydrated cancrinite synthesized in 1,3-butanediol are characterized by powder neutron diffraction and 23Na nuclear magnetic resonance (NMR) spectroscopy. The cancrinite structure is refined in the hexagonal space group P63 (No. 173) with lattice parameters a=12.659 Å and c=5.153 Å. Carbonate anions are found occluded in the pores of the cancrinite structure. Although there are two different crystallographic cation sites found by the Rietveld refinement, there are three peaks in the 23Na magic-angle spinning (MAS) NMR spectrum. These peaks correspond to sodium cations found in site I inside the cancrinite cages, cations in site II inside the cancrinite pore without neighboring carbonates, and cations in site II with neighboring carbonates. Quadrupole coupling constants (QCC) obtained by a simple point-charge model agree well with the simulation of the 23Na MAS NMR spectra.
Flexible tripodal ligand 1,3,5-tris(imidazol-1-ylmethyl)benzene (L) was used to react with various zinc(II) salts ZnX [X = (BF4)2, SO4, Cl2, Br2, I2] to afford a series of coordination polymers with different structures. {[Zn(L)2](BF4)2}n (1) has an infinite 2D cationic double layered structure, while {[Zn4(L)3(SO4)4] · 9H2O}n (2) possesses a 3D framework structure with two different kinds of channels. The structure of [Zn3(L)2Cl6]n (3) is 2D network and the one of {[Zn3(L)2Br6] · CH3OH}n (4) is an infinite 1D zigzag chain in a plywood-like stacking fashion. {[Zn(L)I]I}n (5) has 2D network structure which is further linked by hydrogen bonds to give rise to a fascinating 3D interlocked framework with twofold interpenetration. The results demonstrated that the counteranions have drastic effects on the structure of the coordination polymers. On the other hand, the flexible ligand L acts as a three-connecting node to connect three zinc(II) centers with different conformations. In 1 and 5, L has cis, cis, cis-conformation, while in 2, it adopts cis, trans, trans-conformation. It is interesting that two different conformations (cis, cis, cis and cis, trans, trans) of L coexist in 3 and in the case of 4, the ligand L adopts a special trans-conformation. The results attest that the flexible ligand L can adopt different conformations to form complexes with varied structures. In addition, the uncoordinated tetrafluoroborate anions in 1 can be exchanged by nitrate or nitrite anions, which mean that 1 has anion exchange property.
Novel mesoporous MCM-41 type hybrid materials were synthesised by co-condensation of a “flexible” ligand derived from 1,4-diazobutadiene (1) and tetraethyl orthosilicate in three different ratios (0.027, 0.05, and 0.20) and in the presence of cetyltrimethylammonium bromide as templating agent. Surfactant extraction under mild conditions produced hybrid mesoporous materials with uniform mesoporous scale channels, large pore volumes, and high specific surface areas. These mesoporous organosilicas were characterised by powder X-ray diffraction, nitrogen gas adsorption, 13C and 29Si solid state NMR and thermogravimetric analysis. Results from DFT calculations were used for discussing NMR and IR experimental data. The organosilica materials with lower content of organic molecules present uniform 2D-hexagonal mesoporous arrays with pore diameter ranging from 2.0 to 4.0 nm and a surface area ranging from 693 to 803 m2/g. The introduction of a larger amount, such as 20%, of organic building block seems to decrease the order of the material and reduce the space available within the pore.
Highly ordered periodic mesoporous organosilicas (PMOs) with the 1,4-diureylenebenzene moieties incorporated into the silica framework walls were synthesized through co-condensation of bissilylated 1,4-diureylenebenzene and tetraethyl orthosilicate using triblock copolymer P123 as a structure-directing agent in acidic solutions. These PMOs were characterized by powder X-ray diffraction, N2 adsorption/desorption measurements, 13C and 29Si solid state NMR spectra, FT-IR, SEM, TEM and elemental analysis. It is observed that the PMOs containing 1,4-diureylenebenzene groups inside the pore walls possess large surface area ranging from 423 to 223 m2/g and uniform mesopores between 4.7 and 6.2 nm. With the increasing component of the bissilylated precursor to 10 mol%, PMOs with a rod-like morphology were obtained. When the fraction of the bridged organosilane exceeds 10 mol% in the start mixture, the orderness of the PMOs decreases and the mesoporous wall structure collapses under the adopted synthetic conditions.
Composite membranes of sodium alginate (NaAlg) prepared by solution casting method after incorporating with SBA-15 and Fe-SBA-15 molecular sieves have been crosslinked with glutaraldehyde. These membranes have been tested for the pervaporation (PV) dehydration of isopropanol and 1,4-dioxane from their aqueous solutions at ambient temperature (30 °C) to judge their performance capabilities over that of the pristine NaAlg membrane. Infinite selectivity values with moderate fluxes have been observed for the composite membranes in the investigated feed compositions of 10–20 wt.% of water. A complete removal (100 wt.%) of water has been possible on the permeate side with a slight compromise in flux values. For the range of feed water compositions investigated, fluxes of the composite membranes have been slightly lower than the pristine NaAlg membrane. Pristine NaAlg could remove up to a maximum of 97 wt.% of water at higher feed water concentrations with considerably much lower values of selectivity. Dual pore system of SBA-15, having both hydrophilic micropores and hydrophobic mesopores, in addition to its molecular sieving effect as well as its interaction with the NaAlg matrix might have been responsible for an increased performance of the composite membranes. The results of this study would serve as a useful guideline to recommend the possible applications of these membranes for large-scale commercial exploitation.
The preparation of palladium and rhodium complexes of the Schiff base salen (N,N′-bis-(salicylidene)-ethylenediamine) in the intracrystalline cavities of zeolites with the FAU and the EMT framework topology is reported. The host/guest compounds obtained are active catalysts for the selective hydrogenation of 1,5-cyclooctadiene in the liquid phase at 60°C and under a hydrogen pressure of 1.5 MPa. In addition, the immobilization of palladium complexes of the chiral salen-type ligand R,R-N,N′-bis-(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine in zeolites FAU and EMT is reported for the first time. These materials too are active catalysts for the selective hydrogenation of 1,5-cyclooctadiene. However, their ability to catalyse enantioselective hydrogenations remains to be demonstrated.
By reacting strongly acidic aqueous mixtures of ZnCl2 (or ZnO), H3BO3, H3PO4 and triethylenetetraamine under mild hydrothermal conditions (160 °C, pH < 2) the new zincoborophosphate (H4TETA)1.5[Zn6B6P12O48] · 1.5H2O (1) has been obtained. 1 is the first metalloborophosphate with a zeolite tetrahedral framework (CZP framework type) synthesized in the presence of an organic amine and structurally closely related to a known sodium zincoborophosphate hydrate. The non-framework H4TETA4+ cations and water molecules are trapped in helical channels. Upon heating 1 loses all water molecules in the temperature range from 90 to 290 °C with retention of the framework structure. The framework collapses, however, in the course of decomposition reactions of the organic cations at temperatures above 390 °C. 1 was characterized by single-crystal X-ray crystallography, 11B and 13C MAS NMR and FT-IR spectroscopy, thermogravimetry and variable-temperature powder X-ray diffraction. Crystal data for 1: Hexagonal, space group P6522, Z = 1, a = 9.6685(4) Å, c = 14.8879(6) Å (291 K).
The adsorption of n-alkanes (n-pentane to n-nonane) on the mesoporous MIL-100(Cr) and the MIL-101(Cr) rigid solids has been performed with a view of evaluating the influence of the alkyl chain length in terms of sorption capacity and sorbate/sorbent affinity. MIL-100(Cr) exhibits at first sight a classical type I behaviour towards the sorption of n-alkanes. These sorption isotherms have proven to be fully reversible. A direct relationship between amounts of alkane adsorbed at saturation and number of carbon atoms present in the adsorbed species has been found. At very low pressure (p/p0 < 0.025), sub-steps are present in the isotherm curve, due to the presence of two sets of cages (≈24 Å and 29 Å) and windows (≈5 Å and 9 Å). In the case of MIL-101(Cr), the sub-steps are shifted to higher partial pressures (p/p0 < 0.10) due to the larger size of the cages (≈27 Å and 34 Å) and windows (≈12 Å and 16 Å). For the same apolar sorbates, this outlines the effect of pore dimensions of the two MOFs, which are larger in the case of the MIL-101(Cr) structure, and gives rise to important differences in terms of affinity or sorption mechanisms. This is confirmed by the determination of the enthalpy of adsorption of n-hexane or n-heptane much lower in the case of MIL-100 as compared to MIL-101, at low coverage, −65 kJ mol−1 and −30 kJ mol−1, respectively. The kinetics of the sorption process has also been discussed by comparing the equilibration times of the different sorbates during their adsorption. n-Nonane exhibited much longer equilibration times as to compared to shorter n-alkanes.
Chronotherapy, a new approach for treating pathological conditions, is based on circadian rhythm. Present work conceptualizes a specific technology, based on combining floating and pulsatile principles to develop drug delivery system, intended for chronotherapy in arthritis. This approach was achieved by using low density microporous polypropylene, Accurel MP 1000®, as a multiparticulate carrier along with drug of choice ibuprofen. Carrier amount and solvent volume was kept invariant in designing this simple system by adsorbing drug via melting or solvent evaporation using different carrier: drug ratios. In solvent evaporation, methanol (M) and dichloromethane (DCM) were used. Drug loaded multiparticulate system was subjected to various characterization and evaluation parameters showing influence of adsorption process. Drug release study was performed in acidic environment using pH 1.2 HCl IP medium for 6 h to mimic gastric condition for evaluating gastroretention followed by basic environment using appropriate medium as phosphate buffer pH 7.2 IP for 3 h resembling transit. The release pattern showed influence of drug adsorption methods characterized by ever changing pore geometry with total release ranges in acidic medium as 10.7–27.6% and final release as 55.6–88.6%. Present drug delivery system devoid of any additives/excipients influencing drug release show distinct behaviour from other approaches/technologies in chronotherapy by (a) observing desired low drug release (11%) in acidic medium (b) overcoming the limitations of process variables caused by multiple formulation steps (c) reducing time consumption due to single step process (d) can be extended to controlled release also.
The influence of pore size of MCM-41 materials on drug delivery rate has been studied. In order to achieve this objective, small pore size MCM-41 materials have been synthesised from mixtures of two different alkyltrimethylammonium surfactants with chain length of 8 and 10 carbon atoms. The analgesic ibuprofen was introduced into the pore channels and its delivery to the media has been measured and compared with the delivery from bigger pore-sized MCM-41 obtained from surfactants of 12 and 16 atoms of carbon. This study has revealed that the delivery rate of ibuprofen in a simulating body fluid solution decreases as the pore size decreases in the range of 3.6–2.5 nm.
The pore structures of silicoaluminophosphates (SAPO), SAPO-5, SAPO-11 and their transition metal substituted modifications were studied by physisorption of nitrogen at 77 K. The pore volume distributions in these materials were computed by applying the expanded form of the BJH equation to the N2 desorption data. The N2 adsorption–desorption hysteresis loops, the pore volume distributions and other physical properties show that these molecular sieves contain nonuniform pores spreading over a wide range of pore radii upto 300 Å with appreciable contributions from micropores (pore radii <2 nm) to pore volumes of a majority of them. The generation of mesopore structures in these materials with different surface areas should be attributed to the changes in structures and textures brought about by evolution of water vapour and oxides of carbon and to the mobility of cations during calcination at 500°C in air to remove organic template after hydrothermal synthesis and drying. An attempt is made to correlate the physical properties of the catalysts to the ionic radii of the cations substituted in them. The probable influence of these cations on the reactivity of the surfaces of the metal substituted SAPO-5 molecular sieves is assessed by normalizing the activities of the catalysts in the reaction between 1-naphthol and methanol to produce 2-methyl-1-naphthol and 1-methoxy naphthalene with respect to surface areas.
Structural and dynamic properties of confined hydrogen isotopes (H2, HD, D2) in model porous materials are investigated by neutron scattering. Among our results concerning the D2/AlPO4-N family system, we found the structure of confined D2 in AlPO4-5 to be composed of a “quadrimers” chain, which is partially commensurate with the inner surface sorption sites. Concerning the zeolite mixture sample composed of AlPO4-54 (40%), AlPO4-8 (40%) and AlPO4-11 (20%), we determined the filling sequences by D2 molecular sorbate which surprisingly is: AlPO4-11 filling first, AlPO4-54 filling second and filling last, AlPO4-8.
Sorption of xylenes in two aluminophosphate molecular sieve structures, AlPO4-5 and AlPO4-11, has been studied using Monte Carlo simulations. Pure-component adsorption isotherms were predicted and compared with previously reported experimental data. Our simulated isotherms for xylenes in AlPO4-5 showed a qualitative agreement with the experimental data available especially the ortho-selectivity. However, for AlPO4-11, despite the qualitative agreement between the simulated and experimental isotherms, ortho-selectivity could not be precisely reproduced. A detailed structural analysis is also presented for each component in both molecular sieves. The analysis was performed using the detailed structures for the crystals and molecules, the statistics of the molecules centers of mass during the simulation, and the orientation of the normal vector to the plane of the molecules aromatic ring. Based on this analysis, the adsorption sites and molecules positioning in the crystals channels were proposed. The molecules positioning confirmed the configurations proposed from experimental data by other authors. The simulations also showed that the ortho-selectivity previously observed in experimental results could be related to the variations in the channels diameters and the corresponding interaction energy of the molecule–crystal lattice.
Zeolite synthesis was performed from low water containing initial gels. The batch composition was: aM2O–bNa2O–bAl2O3–150SiO2–2bTAABr–490H2O with M=Li, Na, K; TAA is tetramethyl, tetraethyl and tetrabutyl-ammonium; 2.25⩽a⩽8.82 and 1.87⩽b⩽30. Self-bonded pellets of high mechanical resistance of mordenite and ZSM-11 were obtained from two systems: the first one did not contain any organic cations, while the second one was made in presence of tetrabutylammonium bromide. The other systems led either to nonpelleted mordenite, ZSM-5 or sodalite.
AlPO4-11 nanocrystals, in size, were synthesized by optimizing the following chemical parameters: the crystallization temperature and time, the H2O content, the molar ratio of phosphorus to organic template (P2O5/R), and the HF content. The products were characterized by powder X-ray diffraction and scanning electron microscope. AlPO4-11 nanocrystals could not be obtained by optimizing only the crystallization temperature, time and the water content, but obtained by optimizing the molar ratio of P2O5/R, the organic template, and the HF content.
Series of CoAPO-5, CoAPO-11 and CoAPO-34 molecular sieves have been synthesized starting from a synthesis gel varying in its amount of Co2+ and type of solvent molecule. Four protonic solvents have been investigated: water, ethanol, ethylene glycol and glycerol. The obtained crystalline materials were characterized with X-ray diffraction; diffuse reflectance UV–Vis–NIR spectroscopy; infrared spectroscopy; elemental analysis; electron microscopy microprobe analysis and thermo-gravimetrical analysis. It was found that the type of solvent has a strong influence on the crystallization behavior and the substitution degree of Co2+ for Al3+ in the framework of microporous aluminophosphates. Ethanol, ethylene glycol and glycerol seem to be the best solvents for the synthesis of single-phase and highly crystalline Co-rich CoAPO-34, CoAPO-11 and CoAPO-5 molecular sieves, respectively. By varying the type of solvent molecule, Co content and template amount in the synthesis gel it was possible to increase the substitution degree of framework Co2+ in microporous aluminophosphates. In this manner, around 10%, 25% and 36% of Al3+ could be replaced by Co2+ in the framework of CoAPO-11, CoAPO-5 and CoAPO-34, respectively. These substitution degrees are substantially higher than those for CoAPO materials synthesized in the presence of water.
Xenon NMR is a useful method for probing structure and dynamics of micro-porous materials due to the sensitivity of xenon’s chemical shifts to its local interactions, and the diffusion property of xenon atoms. Here, we report a study of solvation, interaction and diffusion of xenon atoms inside the HPLC column materials, Zorbax SB-C18 and XDB-C18 which were made of siloxane surface coatings of porous silica, by variable-temperature dependent (VT) 129Xe, 1H–129Xe cross-polarization (CP), and two-dimensional exchange (2D EXSY) NMR experiments. The VT NMR experiment showed the solvation and dynamics of xenon atoms in the column materials. The CP experiment at low temperature provided evidence for probing the direct interaction of xenon atoms with the hydrocarbon chains of the stationary phase, and helped for assigning the 129Xe peaks in the VT NMR spectra. The 2D EXSY NMR experimental result showed the diffusion of xenon atoms within the accessible spaces in the column materials. Combined with our previous study, a full picture of xenon’s behavior inside the column materials has been described. This study provides a basic understanding of xenon NMR of the column materials, which enables us to conduct further investigation of retention mechanisms of column materials in terms of molecular interaction and diffusion by xenon NMR method.
The dynamic behavior of methane and xenon adsorbed in H-mordenite (H-MOR) has been investigated by 1H and 129Xe NMR in a wide range of temperatures (4.2–290 K). Previous results for Na-MOR (Si/Al=7.5) showed that methane adsorbs in both the main channels and the side-pockets of mordenite and that adsorption of methane in the side-pockets is favored relative to adsorption in the main channels. When Na-MOR is converted to its protonated form (H-MOR; Si/Al=8.6), the rotational barrier of methane in the side-pocket increases from 2.0 to 5.0 kJ mol−1, whereas for methane in the main channel, the rotational barrier decreases slightly from 1.0 to 0.53 kJ mol−1. This indicates that methane molecules in the side-pockets interact with acidic Brønsted sites via some sort of “unique” hydrogen bonding. For dealuminated H-MOR (Si/Al=40), the temperature dependence of the 1H spin-lattice relaxation time, T1, for adsorbed methane indicates that dealumination leads to an enlargement of the pore sizes of both the main channel and side-pocket. This effect is more pronounced for the side-pocket than for the main channel. This observation is consistent with literature results indicating that dealumination opens side-pockets from adjacent channels, thereby forming a secondary pore structure connecting the main channels. 129Xe NMR spectra were also measured for both Na- and H-MOR. Chemical exchange between the two possible locations, i.e. main channel and side-pocket, takes place so that only one broad resonance is observed at room temperature in H-MOR (Ea=18±2 kJ mol−1). The exchange process is slower in Na-MOR (Ea=21±2 kJ mol−1). From this observation and the 1H T1 results, it is concluded that Na+ cations restrict the side-pockets more efficiently than H+, and it is suggested that Brønsted acid sites are located near the opening of the side-pockets.
NMR spectroscopy of xenon gas adsorbed in clays and pillared clays has been used to glean information on the interlayer gallery height of clays before and after pillaring. Two clay minerals were studied, a Ca2+-montmorillonite and Bentonite L. The NMR results indicate that the effective interlamellar spacing of the montmorillonite increased from 5.4 to 8.0 Å after pillaring with aluminum polyoxohydroxy Keggin cations. These data are consistent with X-ray powder diffraction results, which show a corresponding increase in gallery height from 5.6 to 8.4 Å.
A first example of a purely organic material showing an open pore size of 12 Å for empty channels was obtained for micrometer sized crystals of 2,4,6-tris-(4-bromophenoxy)-1,3,5-triazine (BrPOT). The zeolitic-like form of the known inclusion compound was formed by a solid to solid transformation from micrometer large particles of the rhombohedral modification of BrPOT taking up gaseous CS2. Long-term thermal stability of the empty channel structure was found up to about 60 °C. Functionalisation of the pore volume by including aromatic amines allowed sorption of molecular iodine. It is proposed that micrometer sized channel inclusion crystals are a promising entry to produce open pore modifications, which in the case of large crystallites are known to collapse into the solvent-free structure by the process of desolvation.
Accommodation of various organic molecules into a one-dimensional nanochannel of tris(ethylenediamine) cobalt(III) chloride, [Co(en)3]Cl3, anhydrated racemic crystal was examined using weight increment measurements, thermogravimetric–differential thermal analysis (TG–DTA), powder X-ray diffraction (PXRD) and 13C cross-polarization/magic angle sample spinning (CP/MAS) NMR techniques. Results showed that the (±)-[Co(en)3]Cl3 ionic crystal accommodates n-alkanes, n-alkylalcohols, n-alkylamines, and acetonitrile in the 1D nanochannel without decomposition of the crystal lattice. The guest molecules are in all-trans conformation in the nanochannel. Furthermore, the activation energies of uni-axial molecular reorientation of n-alkanes and n-alkylalcohols in the nanochannel were determined using temperature dependence of the 1H spin–lattice relaxation time in the rotating frame (T1ρ). Results demonstrated that the activation energy per CH2 unit (6.5 kJ mol−1) in n-alkylalcohols was twice that in n-alkanes (3.3 kJ mol−1), implying the possibility of dimer formation of n-alkylalcohols in the nanochannel by hydrogen bonding through OH group. These findings strongly suggest that the (±)-[Co(en)3]Cl3 ionic crystal has adsorption ability to various organic molecules. It will open new avenues to explore new types of zeolites that include ionic crystals.
Templated by 4,4′-bipyridine (bipy), a new indium phosphate with an In/P ratio of 9/14 has been synthesized under hydrothermal conditions. The compound, In9(HPO4)14(H2O)6F3 · (C10N2H9)3 · (H3O) · (H2O)2, crystallizes in the trigonal system with the space group P-31c (No. 163) having cell parameters a = 13.9306(5) Å, c = 24.0435(12) Å, V = 4040.8(3) Å, and Z = 2 with R1 = 0.0508. The overall structure consists of two types of secondary building units (SBUs): SBU-1, 6∗1, with a formula of [In3(HPO4)4(H2O)3], constructs into a 4.6.12-net layer; and SBU-2, [In3(HPO4)6F3], features two [3.3.3] propellane-like chiral motifs with Δ and Λ configurations, forming an unprecedented racemic “pillar” unit. The layers are stacked in an ABAB sequence, penetrated by the pillar units perpendicularly, forming a 3-D pillared layer structure with a rectangular 14-ring channel system in two dimensions (along the [1 0 0] and [0 1 0] directions). Organic amine and fluorine ions play key roles in directing the formation of the structure, and the compound presents the same solid fluorescence properties as the bipy molecules attributing to the intraligand π–π∗ transition.
Mesoporous materials play an important role in many technical aspects such as enzyme immobilization. Hence, Chloroperoxidase (CPO) from Caldariomyces fumago was immobilized in SBA-16 mesoporous materials either by physical adsorption or covalently. The strategy of covalent immobilization of chloroperoxidase was based on substrate accessibility to the active site, protein size and surface properties of the enzyme. The stability of free, physically and chemically adsorbed enzymes against temperature and urea exposure was measured in aqueous solution. An improvement in catalytic activity was obtained after orienting the enzyme active site to the substrate through two ways: (1) surface impregnation with Cs+ ions and (2) CPO covalent immobilization with an organosilane derivative. The stability of immobilized enzymes against urea improved as the pore size became large enough to accommodate enzyme molecules. Chemical adsorption also favored stability against denaturants. Although Cs+-doped material increased the amount of adsorbed enzyme, this preparation resulted as sensible to urea as the free enzyme. None CPO-material showed a clear tendency concerning stability against temperature.
We have synthesized mesoporous silica SBA-16 at room temperature and low pH condition by using the surfactant system C8TMAB/SDS/F127 as template. Faceted single crystals of rhombdodecahedron form can be obtained under well-controlled conditions. Synthetic factors in controlling the morphology, including stirring, water content, the ionic surfactants and acidity are explored. A schematic theory for the formation of the equilibrium crystal structure based on surface roughening transition is proposed to explain the formation of faceted single crystal.
Copper-containing SBA-16 (with Si/Cu molar ratios 22, 43 and 61) has been synthesized by one step novel “internal pH-modification method” using hexamethylenetetramine (HMTA) as complexing agent and internal pH-modifier. HMTA plays a critical role in material synthesis; it releases NH3 during hydrothermal treatment to increase the internal pH of the gel that results in introduction of more amount of copper into silica framework. The synthesized material has been characterized by XRF, XRD, HRTEM, N2 adsorption/desorption, SEM, FT-IR, diffuse reflectance UV–visible spectroscopy and 29Si MAS NMR spectroscopy. The characterization results reveal that the synthesized materials possess ordered cubic mesoporous structure up to Si/Cu molar ratio 43; further increase in copper contents (Si/Cu = 22) lead to disordering of mesostructure. 29Si MAS NMR results indicate that copper has been successfully incorporated into silica framework. Owing to its high surface area (983–315 m2/g) and copper contents, the material was applied for the adsorption desulfurization of dibenzothiophene (DBT), wherein it exhibited good performance and reduced the sulfur contents of 40 mL of model oil with 550 ppm of sulfur (DBT) to a level of less than 1 ppm.
Functionalized SBA-16 mesoporous silica with –SH groups was synthesized using one-pot method. The resulting material was characterized by powder X-ray diffraction, nitrogen gas sorption, FT-Raman spectroscopy. The solid was employed as a Cu(II) adsorbent from aqueous solutions at room temperature. The effects of several variables (stirring time, pH, metal concentration and presence of other ions in the medium) were studied using the batch technique. The results showed that by controlling an optimum molar ratio of 3–4 between tetraethyl orthosilicate (TEOS) and 3-mercaptopropyltriethoxysilane (TMMPS), the synthesized material possessed high order and adsorption capacity for Cu(II) ions though the pore size decreased probably due to the attachment of the organic functional groups in the mesopore channels. The maximum Cu(II) adsorption on this adsorbent occurred in the range of pH 5–6 with an adsorption maximum of 36.38 mg/g. The adsorption of Cu(II) on functionalized SBA-16 mesoporous fitted well to the Redlich–Peterson isotherm equation (r2 = 0.9999) followed by the Langmuir equation (r2 = 0.9847). The involved mechanism might be the adsorption through ligand exchange with the –SH group. The adsorptive competing cations in the aqueous solution had a little effect on the adsorption of Cu(II) on this adsorbent. The presence of different anions (Cl−, , , OAc− and Cit−) influenced the Cu(II) adsorption in the order of . Even after seven regeneration cycles, functionalized SBA-16 mesoporous had a Cu(II) adsorption amount of over 23 mg/g and could be easily regenerated through acid washing, showing a promising application for the treatment of wastewater containing Cu(II) ions.
Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into “liquid particles” (spheres or polyhedra), as observed by optical microscopy. The “liquid particles”, which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.
SEM micrographs of mesoporous SBA-16 type silica spheroidal microparticles prepared with the triblock copolymer surfactant PEO140PPO39PEO140 between 80 and 125 °C and under different acid concentrations (HCl 0.2, 0.4 and 0.8 M), indicate that they are formed of coalesced nanoparticles. The coalescence degree is shown to increase with the temperature and with the presence of the co-surfactant cetyltrimethylammonium bromide (CTMABr), whereas the reverse trend is observed at high acidity. The nanoparticles formation from the initial triblock micelles and the silica source (TEOS) was followed in solution by dynamic light scattering (DLS) before the flocculation step (phase separation corresponding to the formation of micron-sized liquid particles by aggregation and fusion of the silica coated micelles). The variation of the scattered intensity and of the hydrodynamic diameter could be related to the TEOS hydrolysis degree leading to the formation of polycondensed siliceous species in the hydrophilic corona of the nanoparticles (composite colloids). Depending on the synthesis conditions, four types of nanoparticles could be defined according to the “hardness” of the siliceous species, which is related to their polycondensation degree and their distribution inside the corona of the nanoparticles. The morphology and texture of the resulting materials will depend on the type of nanoparticles and on the temperature at the phase separation (flocculation). In the absence of CTMABr, three kinds of materials are obtained: gel-like materials (low siliceous species hardness and low temperature), microparticles of mesoporous silica (medium hardness and medium temperature), blocks of aggregated nanoparticles (high hardness and high temperature). In the presence of CTMABr which acts like a buffer for the siliceous species hardness, well coalesced microparticles of mesoporous silica are obtained independently on the temperature and acidity conditions.
The surface structure, pore size distribution and pore wall thickness of the mesoporous material FSM-16 have been studied by X-ray powder diffraction (XRD), 1H and 29Si MAS NMR and 1H liquid-state NMR, and by applying surface silylation as a probe. The concentrations of surface hydroxyl groups for FSM-16 are estimated from 29Si MAS NMR, and amount to 3×1021 g−1, corresponding to approximately 3 nm−2. O2 molecules contribute to 29Si spin–lattice relaxation of Q2 and Q3 as well as Q4, suggesting thin walls. 1H-MAS-NMR spectra indicate the presence of isolated and hydrogen-bonded hydroxyl groups. Both hydroxyl groups are silylated, and the silylated fraction is about 50%. The spatial distribution of surface hydroxyl groups is estimated from the line width in 1H static spectra. A rather homogeneous distribution is demonstrated in one of the samples. The sample with a less homogeneous distribution has a larger affinity for moisture. The pore size and pore wall thickness were determined by 1H NMR measurements on water-saturated FSM-16 samples, and the results are in good agreement with literature values obtained by N2 adsorption isotherms and transmission electron micrographs on a similar sample. In benzene-saturated samples, a non-freezing surface layer of benzene is much thicker than that of water which indicates a stronger interaction between benzene and the FSM-16 surface.
The synthesis of Fe-SBA-16 with isolated framework Fe species was achieved by simple adjustment of molar ratio of co-surfactant n-butanol, TEOS, nH2O/nHCl ratio, Si/Fe ratio, and aging condition, respectively. The extent of mesopore structural ordering and the coordination of iron are confirmed by X-ray diffraction, N2 physisorption, 29Si-MAS NMR, SEM, TEM, electron spin resonance (ESR), and NH3-TPD analysis. The material obtained constitutes micro- (∼316 m2/g) and mesopore surface area (∼1000 m2/g), with high-mesopore sizes (∼9.8 nm). The micropore volume present up to 0.28 cm3/g, while the total pore volume range ∼0.90 cm3/g at the maximum. Among various Si/Fe ratios synthesized, Fe-SBA-16 with Si/Fe ratio 19 exhibited good catalytic activity for the oxidation of cyclohexene at the optimized reaction condition.
A new open-framework zinc phosphate, Zn3PO4(HPO4)3 · C6N3H18 (denoted FJ-11), was synthesized under solvothermal conditions in the presence of 1-(2-aminoethyl)piperazine as structure-directing agent. Its structure was determined by single-crystal X-ray diffraction and further characterized by FTIR, elemental analysis, ICP analysis, powder X-ray diffraction and thermogravimetric analysis. The compound crystallized in the triclinic space group, P-1, with a=8.2807(2) Å, b=9.13750(10) Å, c=13.3463(2) Å, α=83.7700(10)°, β=84.4460(10)°, γ=78.0250(10)°, V=979.17(3) Å3, Z=2. The structure consists of ZnO4, PO4 and HPO4 tetrahedra with shared corners, forming intersecting three-dimensional channels. The triply protonated 1-(2-aminoethyl)piperazine molecules reside in the large 16-ring channels and interact with the inorganic framework by extensive hydrogen bonds.
Small pore (∼2.6 nm) thick-walled (7.7 nm) cage-like cubic mesoporous silica materials SBA-16 (ST-SBA-16) have been synthesized by using oligomeric surfactant Brij700 (polyoxyethylene (100) stearyl ether, C18H37EO100) with ultra-long hydrophilic chains. Their ordered mesostructures were characterized by X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), and nitrogen sorption analysis. Nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) results show that ST-SBA-16 products have large numbers of silanol groups. The mesoscopic ordering of ST-SBA-16 can be greatly improved by the addition of suitable amount of co-surfactants, such as 1,3,5-trimethylbenzene (TMB). The effects of the synthesis parameters on the structure properties of ST-SBA-16 have also been systematically investigated. It is shown that the total pore volume can be enlarged from 0.31 to 0.51 cm3 g−1 by the addition of TMB, pore diameter can be augmented from 2.6 to 4.0 nm by increasing the synthesis temperature from 5 to 40 °C and the surface area can be effectively tailored by changing the hydrothermal treatment time and the calcination temperature.
Cubic mesoporous silica SBA-16 materials with a rhombdodecahedral or decaoctahedral shape were synthesized by microwave reactions within 2 h using sodium silicate as the silica source and a triblock copolymer F127 as structure directing agents. The synthesis parameters, stirring time of the precursor solution before microwave reaction, microwave reaction time and temperature, were systematically varied and their influences on the structure and morphology were evaluated. The stirring time and reaction temperature govern the structure of the product and the reaction time influences the particle size and morphology. Through this study, we have established the optimal conditions for highly crystalline SBA-16 with rhombdodecahedral shape as stirring time of 30 min and microwave irradiation for 120 min at 373 K.
This paper is devoted to the geometry of mesoporous mesophase systems (MMSs) of the MCM-41 or FSM-16 texture types. The interrelations among surface areas inside and outside the mesoporous blocks of the MMS, of the size and the volume of mesopores are discussed. The equations for calculating these textural characteristics are derived. These equations are based on adsorption data and X-ray diffraction studies, and are applied for considerations of the silica and zirconia hexagonal MMS texture. The approach was used for a critical review of the results published by other authors for similar systems.
A new rare-earth dicarboxylate [Pr(H2O)]2[O2C(CH2)2CO2]3·H2O (MIL-17) was hydrothermally prepared (180°C, 3 days) by reaction of succinic acid with the metal chloride in the ratio 1:1. The crystal structure of the compound has been determined by single-crystal X-ray diffraction. [Pr(H2O)]2[O2C(CH2)2CO2]3·H2O crystallizes in the monoclinic space group C2/c (No. 15) with a=20.2586(8) Å, b=7.9489(3) Å, c=13.9716(5) Å, β=121.641(1)° (final agreement factors R1=0.0282, wR2=0.0764). The organic–inorganic network is three-dimensional and consists of edge-sharing rare-earth polyhedra LnO8(H2O) chains linked together by the carbon chains along two directions as already encountered in [Pr(H2O)]2[O2C(CH2)3CO2]3·4H2O. The connection in [Pr(H2O)]2[O2C(CH2)2CO2]3·H2O involves the formation of tunnels large enough to incorporate one weakly bonded water molecule.
Zeolite A provides a unique opportunity to explore the relationship between structural parameters and 17O nuclear magnetic resonance (NMR) isotropic chemical shifts (δISO) because differences in site multiplicity for framework oxygens often permit unambiguous site assignments. Strontium exchanged zeolite A and hydrated and dehydrated K- and Na-A were investigated using 17O triple quantum and five quantum magic angle spinning (3QMAS and 5QMAS) NMR. The results concur with previous observations that δISO is influenced by T–O–T bond angle, but also suggest that interactions between framework oxygens and extraframework species (water molecules and cations) affect δISO as well. Apparent chemical shifts for zeolitic water sites also exhibit relative chemical shifts as a function of extraframework cation contents consistent with shifts observed for water sites and non-bridging oxygens in glasses. Abundances of Si–O–Al sites in dehydrated forms indicate that oxygen isotope exchange rates can vary between crystallographic sites, as well as between Si–O–Si and Si–O–Al sites.
Polyoxyethylene tridecylethers (C13EOm, m=6, 12 and 18) have been used as templating agents for synthesis of large pore mesoporous materials. The effect of surfactant/silicium molar ratio, heating time, and temperature, and the number of oxyethylene units on the mesoporous material syntheses has been studied in detail. Final compounds were characterized by different techniques such as SEM, TEM and nitrogen adsorption–desorption analysis. The present work shows that the surfactant/tetramethoxysilane molar ratio has a strong effect on the pore diameter for a given surfactant. It is evidenced that both tetramethoxysilane (TMOS) and surfactant can play a role of swelling agent depending on the surfactant/TMOS molar ratio. It is also shown that the pore diameter depends strongly on the heating time and temperature, oxyethylene unit number and surfactant/TMOS molar ratio. All these factors can affect jointly or separately the pore diameter of obtained materials. It is revealed that the surfactant conformation can be changed with the heating temperature. At higher temperature, a more extended molecular conformation can be obtained, which leads to materials with larger pore size.
A new method for obtaining 18 Å Al-pillared vermiculites is reported. In a first step, the vermiculite is submitted to an acid treatment followed by calcination. These treatments lead to a permanent reduction of the global negative charge of the mineral layers. The extra-framework species are removed by a subsequent leaching with a complexing acid, and the solid is converted to the sodium form by ion exchange. The charge-reduced Na (or Ca)-vermiculite is then brought in contact with the aluminum pillaring solution. The calcined Al-pillared vermiculites exhibit the characteristic features of this class of materials, namely, 18 Å spacing, high surface area, microporosity, and both Brønsted and Lewis acidities. These textural properties are preserved at higher temperatures (up to 800°C) compared with Al-pillared smectites, owing to the higher thermal stability of the vermiculite structure (higher dehydroxylation temperatures). The Brønsted acid content of the Al-pillared vermiculites established by IR spectroscopy of adsorbed pyridine is about twice as high as that of an Al-pillared saponite.
Zeolite RUB-24 was prepared by heating a precursor phase, the intercalated layer silicate R-RUB-18 (R = alkylammonium cations) in air at 500 °C. The microporous silica framework of RUB-24 forms by topotactic condensation of the silicate layers of the crystalline precurser R-RUB-18. A Rietveld structure refinement of X-ray powder diffraction data of RUB-24 which was performed in space group I41/amd confirmed the topology of the structure model. RUB-24, Si32O64 per unit cell (a0 = 7.6677 (2) Å, c0 = 27.063 (1) Å) is a new small pore zeolite with a one-dimensional pore system consisting of straight and non-intersecting 8-ring channels. The structure represents a new zeolite framework type (the code assigned by the International Zeolite Association is RWR) having a high framework density of 19.2 T/1000 Å3 (with respect to idealized cell parameters).Although the structure analysis showed that the pores of the (idealized) silica framework are empty, nitrogen sorption experiments showed that there is no “free” access to the pore volume. According to 29Si MAS NMR spectroscopy a few defects are present caused by incomplete or random condensation of the silanol groups which might block access to the pores.
Top-cited authors
Stefan Kaskel
  • Technische Universität Dresden
J. Caro
  • Claude Bernard University Lyon 1
Michael Stöcker
Johan Groen
  • Delft Solids Solutions
Freek Kapteijn
  • Delft University of Technology