Chiral Templating of Polycarbonate Membranes by Pinene Using the Modified Atomic Layer Deposition Approach.

In this article, chiral templating of a polycarbonate (PC) membrane by (-)-α-pinene using the atomic layer deposition (ALD) approach is investigated. The templating with the enantiomer of (-)-α-pinene, used as a case compound, was performed either on the original commercial PC membrane or on the PC membrane with a beforehand deposited Al2O3 layer. The efficiency of the templating was assessed by a difference in the membrane ability to adsorb/absorb (-)-α-pinene, (+)-α-pinene, and their racemic mixture, using a very sensitive gas sorption analyzer. The results clearly show that the solution-diffusion mechanism rather than the sieving mechanism applied for adsorption/absorption of (-/+)-α-pinene enantiomers, which have the same size of the molecule. The PC membrane with the predeposited Al2O3 before the (-)-α-pinene templating shows significantly higher sorption of (-)-α-pinene compared to (+)-α-pinene and racemate, which clearly demonstrates the presence of a chiral recognition effect.

Ligand Excess "Inverse-Defected" Zr6 Tetrahedral Tetracarboxylate Framework and Its Thermal Transformation.

A new porous coordination polymer (PCP/MOF), ZRTE-10, based on a tetrahedral 1,3,5,7-tetra(carboxyphenyl)benzene ligand (H4L4) was synthesized using formic or acetic acids as modulators. The low symmetry (C2/c) framework, [Zr6(µ3-O)4(µ3-OH)4(L4)(HL4)2(OH)2(H2O)2], is built upon a rare 10-connected Zr6 cluster. Two-thirds of the ligands bear one nondeprotonated carboxy group, and the framework has a complex trinodal 3,4,10-c, {414.624.87}{43}2{45.6}, underlying net. Supercritical CO2 drying and mild degassing at 120 °C yielded a porous material with SBET = 1190 m2 g-1. When heated up to ∼200 °C, ZRTE-10 converts to another crystalline framework, ZRTE-11. The latter was identified to be identical to the expected fluorite (flu) observed previously for other tetrahedral ligands. The high symmetry (I4/m) framework is built upon 8-connected Zr6 clusters and has a formula of [Zr6(µ3-O)4(µ3-OH)4(OH)4(L4)2]. The complicated trinodal network of ZRTE-10 and the simple flu net in ZRTE-11 are topologically interrelated via the operation of the merging of two neighbor three-connected nodes to one four-connected one. The thermally induced conversion of ZRTE-10 proceeds with expulsion of one ligand per Zr6 node in the pores of the framework, resulting in a relatively low SBET = 585 m2 g-1 for the activated H4L4@ZRTE-11. A mixed ligand approach for ZRTE-10,11 was attempted using 1,3,5-tetra(carboxyphenyl)benzene (H3L3), which is a truncated analog of H4L4 with one missing branch. The monocrystalline sample of ZRTE-10 obtained in small yields demonstrated only minor inclusion of H3L3. However, the high-yielding (∼80%) procedure with HCl as a modulator allows near proportional incorporation of the ligands. The formed materials are semiamorphous with powder XRDs intermediary between pure ZRTE-10 and -11. Thermal treatment of the semiamorphous materials increases their crystallinity and allows SBET = 400-550 m2 g-1 surface areas to be reached for pure H4L4 and H3L3 or their mixture alike. The approach proposes a viewpoint on the H3L3 trifunctional ligand as a model of a ligand platform, suitable for bearing a large functionality on the fourth "truncated" branch. The significance of ZRTE-10 as a material for postsynthetic introduction of metal-based cluster functionality and as a model of functionality encapsulation, an alternative to the ship-in-the-bottle method, is discussed.

The effect of the zeolite pore size on the Lewis acid strength of extra-framework cations.

The catalytic activity and the adsorption properties of zeolites depend on their topology and composition. For a better understanding of the structure-activity relationship it is advantageous to focus just on one of these parameters. Zeolites synthesized recently by the ADOR protocol offer a new possibility to investigate the effect of the channel diameter on the adsorption and catalytic properties of zeolites: UTL, OKO, and PCR zeolites consist of the same dense 2D layers (IPC-1P) that are connected with different linkers (D4R, S4R, O-atom, respectively) resulting in the channel systems of different sizes (14R × 12R, 12R × 10R, 10R × 8R, respectively). Consequently, extra-framework cation sites compensating charge of framework Al located in these dense 2D layers (channel-wall sites) are the same in all three zeolites. Therefore, the effect of the zeolite channel size on the Lewis properties of the cationic sites can be investigated independent of other factors determining the quality of Lewis sites. UTL, OKO, and PCR and pillared 2D IPC-1PI materials were prepared in Li-form and their properties were studied by a combination of experimental and theoretical methods. Qualitatively different conclusions are drawn for Li(+) located at the channel-wall sites and at the intersection sites (Li(+) located at the intersection of two zeolite channels): the Lewis acid strength of Li(+) at intersection sites is larger than that at channel-wall sites. The Lewis acid strength of Li(+) at channel-wall sites increases with decreasing channel size. When intersecting channels are small (10R × 8R in PCR) the intersection Li(+) sites are no longer stable and Li(+) is preferentially located at the channel-wall sites. Last but not least, the increase in adsorption heats with the decreasing channel size (due to enlarged dispersion contribution) is clearly demonstrated.

Measuring the Brønsted acid strength of zeolites--does it correlate with the O-H frequency shift probed by a weak base?

Brønsted-acid zeolites are currently being used as catalysts in a wide range of technological processes, spanning from the petrochemical industry to biomass upgrade, methanol to olefin conversion and the production of fine chemicals. For most of the involved chemical processes, acid strength is a key factor determining catalytic performance, and hence there is a need to evaluate it correctly. Based on simplicity, the magnitude of the red shift of the O-H stretching frequency, Δν(OH), when the Brønsted-acid hydroxyl group of protonic zeolites interacts with an adsorbed weak base (such as carbon monoxide or dinitrogen) is frequently used for ranking acid strength. Nevertheless, the enthalpy change, ΔH(0), involved in that hydrogen-bonding interaction should be a better indicator; and in fact Δν(OH) and ΔH(0) are often found to correlate among themselves, but, as shown herein, that is not always the case. We report on experimental determination of the interaction (at a low temperature) of carbon monoxide and dinitrogen with the protonic zeolites H-MCM-22 and H-MCM-56 (which have the MWW structure type) showing that the standard enthalpy of formation of OH···CO and OH···NN hydrogen-bonded complexes is distinctively smaller than the corresponding values reported in the literature for H-ZSM-5 and H-FER, and yet the corresponding Δν(OH) values are significantly larger for the zeolites having the MWW structure type (DFT calculations are also shown for H-MCM-22). These rather unexpected results should alert the reader to the risk of using the O-H frequency shift probed by an adsorbed weak base as a general indicator for ranking zeolite Brønsted acidity.

Investigation of Brønsted acidity in zeolites through adsorbates with diverse proton affinities.

Understanding the adsorption behavior of base probes in aluminosilicates and its relationship to the intrinsic acidity of Brønsted acid sites (BAS) is essential for the catalytic applications of these materials. In this study, we investigated the adsorption properties of base probe molecules with varying proton affinities (acetonitrile, acetone, formamide, and ammonia) within six different aluminosilicate frameworks (FAU, CHA, IFR, MOR, FER, and TON). An important objective was to propose a robust criterion for evaluating the intrinsic BAS acidity (i.e., state of BAS deprotonation). Based on the bond order conservation principle, the changes in the covalent bond between the aluminum and oxygen carrying the proton provide a good description of the BAS deprotonation state. The ammonia and formamide adsorption cause BAS deprotonation and cannot be used to assess intrinsic BAS acidity. The transition from ion-pair formation, specifically conjugated acid/base interaction, in formamide to strong hydrogen bonding in acetone occurs within a narrow range of base proton affinities (812-822 kJ mol-1). The adsorption of acetonitrile results in the formation of hydrogen-bonded complexes, which exhibit a deprotonation state that follows a similar trend to the deprotonation induced by acetone. This allows for a semi-quantitative comparison of the acidity strengths of BAS within and between the different aluminosilicate frameworks.

Brønsted acidity in zeolites measured by deprotonation energy.

Acid forms of zeolites have been used in industry for several decades but scaling the strength of their acid centers is still an unresolved and intensely debated issue. In this paper, the Brønsted acidity strength in aluminosilicates measured by their deprotonation energy (DPE) was investigated for FAU, CHA, IFR, MOR, FER, MFI, and TON zeolites by means of periodic and cluster calculations at the density functional theory (DFT) level. The main drawback of the periodic DFT is that it does not provide reliable absolute values due to spurious errors associated with the background charge introduced in anion energy calculations. To alleviate this problem, we employed a novel approach to cluster generation to obtain accurate values of DPE. The cluster models up to 150 T atoms for the most stable Brønsted acid sites were constructed on spheres of increasing diameter as an extension of Harrison's approach to calculating Madelung constants. The averaging of DPE for clusters generated this way provides a robust estimate of DPE for investigated zeolites despite slow convergence with the cluster size. The accuracy of the cluster approach was further improved by a scaled electrostatic embedding scheme proposed in this work. The electrostatic embedding model yields the most reliable values with the average deprotonation energy of about 1245 ± 9 kJ·mol-1 for investigated acidic zeolites. The cluster calculations strongly indicate a correlation between the deprotonation energy and the zeolite framework density. The DPE results obtained with our electrostatic embedding model are highly consistent with the previously reported QM/MM and periodic calculations.

Bismuth Oxychloride Nanoplatelets by Breakdown Anodization.

Herein, the synthesis of BiOCl nanoplatelets of various dimensions is demonstrated. These materials were prepared by anodic oxidation of Bi ingots in diluted HCl under dielectric breakdown conditions, triggered by a sufficiently high anodic field. Additionally, it is shown that the use of several other common diluted acids (HNO3, H2SO4, lactic acid) resulted in the formation of various different nanostructures. The addition of NH4F to the acidic electrolytes accelerated the growth rate resulting in bismuth-based nanostructures with comparably smaller dimensions and an enormous volume expansion observed during the growth. On the other hand, the addition of lactic acid to the acidic electrolytes decelerated the oxide growth rate. The resulting nanostructures were characterized using SEM, XRD and TEM. BiOCl nanoplatelets received by anodization in 1 M HCl were successfully employed for the photocatalytic decomposition of methylene blue dye and showed a superior performance compared to commercially available BiOCl powder with a similar crystalline structure, confirming its potential as a visible light photocatalyst.

SiO2 Fibers by Centrifugal Spinning with Excellent Textural Properties and Water Adsorption Performance.

Facile and innovative route for large-scale synthesis of SiO2 fibers with excellent textural properties and H2O adsorption performance is presented. At first, a three-dimensional network of SiO2 precursor fibers was produced from tailored spun solutions (without any toxic elements and surfactants) by centrifugal spinning, which is a very modern fiber-synthesis technique, with numerous advantages over electrospinning. Upon thermal annealing of the precursor fibers, mesoporous amorphous SiO2 fibers with an ultrahigh surface area of up to 824 m2/g and pore size distribution in the range of 2-10 nm were produced. Owing to the high number of OH groups available on the surface, the produced SiO2 fibers showed significantly better performance in H2O adsorption compared to that of the reference silicagel.

One-pot synthesis of iron doped mesoporous silica catalyst for propane ammoxidation.

Iron doped hexagonal mesoporous silica (Fe-HMS) was successfully prepared by one-pot direct synthesis at ambient temperature by using dodecylamine as a templating agent. The catalyst has been characterized by various techniques including XRD, DR-UV-Vis, FTIR, SEM and N2 adsorption. The obtained material exhibits very large surface area (above 1000 m(2) g(-1)), mesoporosity with narrow distribution of pores (around 4 nm) and nano-crystallites with the 20-30 nm in size. The concentration of iron in the catalyst was 0.95 wt%. The prepared material was tested in the direct ammoxidation of propane. It was shown that direct synthesis leads to better dispersion of the metal compared to the material prepared by impregnation of pure HMS silica. It is also demonstrated that this material is able to catalyze the direct ammoxidation of propane and shows quite high selectivity to acrylonitrile (52% at 11% conversion of propane).

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study.

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).

The vibrational dynamics of carbon monoxide in a confined space-CO in zeolites.

Based on theoretical calculations, and a survey of infrared spectra of CO adsorbed on different cation exchanged zeolites, a model is proposed to explain the influence of the zeolite framework on the vibrational behaviour of CO confined into small void spaces (zeolite channels and cavities). The concepts developed should help to understand a number of details relevant to both, precise interpretation of IR spectra and a better understanding of the vibrational dynamics of small molecules in a confined space.