Mono-dimensional Channels Research Articles

Rational Design of 7‐Azaindole‐Based Robust Microporous Hydrogen‐Bonded Organic Framework for Gas Sorption

7‐Azaindole has been integrated as building block with complementary N‐H···N hydrogen bonding sites for the synthesis of a tetrahedral molecular tecton, namely tetra(α‐carbolin‐6‐yl)methane, TACM. The self‐assembly of this molecule results in a 3D hydrogen‐bonded organic framework (HOF). This supramolecular structure constitutes a crystalline microporous material with an extraordinary thermal and chemical robustness. Single crystal X‐ray diffraction reveals how the five‐fold catenation of diamonoid systems, stabilized by hydrogen bonds and π‐π interactions, form an interpenetrated network with monodimensional channels. The structural features of the crystalline material are also observed by transmission electron microscopy (TEM). Additionally, the microporosity of the activated TACM‐HOF is characterized by gas sorption (N2, CO2, CH4 and H2) experiments performed at different pressures. A selective adsorption is observed for CO2 uptake and TACM‐HOF also presents a good adsorption capacity for H2 among supramolecular organic frameworks.

Rational Design of 7-Azaindole-Based Robust Microporous Hydrogen-Bonded Organic Framework for Gas Sorption.

7-Azaindole has been integrated as building block with complementary N-H⋅⋅⋅N hydrogen bonding sites for the synthesis of a tetrahedral molecular tecton, namely tetra(α-carbolin-6-yl)methane, TACM. The self-assembly of this molecule results in a 3D hydrogen-bonded organic framework (HOF). This supramolecular structure constitutes a crystalline microporous material with an extraordinary thermal and chemical robustness. Single crystal X-ray diffraction reveals how the five-fold catenation of diamonoid systems, stabilized by hydrogen bonds and π-π interactions, form an interpenetrated network with monodimensional channels. The structural features of the crystalline material are also observed by transmission electron microscopy (TEM). Additionally, the microporosity of the activated TACM-HOF is characterized by gas sorption (N2, CO2, CH4 and H2) experiments performed at different pressures. A selective adsorption is observed for CO2 uptake and TACM-HOF also presents a good adsorption capacity for H2 among supramolecular organic frameworks.

Size-Selective Urea-Containing Metal-Organic Frameworks as Receptors for Anions.

Anion recognition by neutral hosts that function in aqueous solution is an emerging area of interest in supramolecular chemistry. The design of neutral architectures for anion recognition still remains a challenge. Among neutral anion receptor systems, urea and its derivatives are considered as "privileged groups" in supramolecular anion recognition, since they have two proximate polarized N-H bonds exploitable for anion recognition. Despite promising advancements in urea-based structures, the strong hydrogen bond drives detrimental self-association. Therefore, immobilizing urea fragments onto the rigid structures of a metal-organic framework (MOF) would prevent this self-association and promote hydrogen-bond-accepting substrate recognition. With this aim, we have synthesized two new urea-containing metal-organic frameworks, namely [Zn(bpdc)(L2)]n·nDMF (TMU-67) and [Zn2(bdc)2(L2)2]n·2nDMF (TMU-68) (bpdc = biphenyl-4,4'-dicarboxylate; bdc = terephthalate; L2 = 1,3-bis(pyridin-4-yl)urea), and we have assessed their recognition ability toward different anions in water. The two MOFs show good water stability and anion affinity, with a particular selectivity toward dihydrogen arsenate for TMU-67 and toward fluoride for TMU-68. Crystal structure characterizations reveal 3-fold and 2-fold interpenetrated 3D networks for TMU-67 and TMU-68, respectively, where all single interpenetrated networks are hydrogen bonded to each other in both cases. Despite the absence of self-quenching, the N-H urea bonds are tightly hydrogen bonded to the oxygen atoms of the dicarboxylate ligands and cannot be directly involved in the recognition process. The good performance in anion sensing and selectivity of the two MOFs can be ascribed to the network interpenetration that, shaping the void, creates monodimensional channels, decorated by exposed oxygen atom sites selective for arsenate sensing in TMU-67 and isolated cavities, covered by phenyl groups selective for fluoride recognition in TMU-68.

Reuse of Predesigned Dual-Functional Metal Organic Frameworks (DF-MOFs) after Heavy Metal Removal

Designing porous and functionalized adsorbents and achieving high efficiency in heavy metals removal from wastewater is in the spotlight of environmental science. On the other hand, upon removal, adsorbents are still highly hazardous requiring that great care be taken in its packaging, transporting and storing. A fundamental route in the synthesis of functional extended structures is the ability to combine different chemical entities in a controlled way in order to achieve high performance. Herein, we report the systematic design of dual-functionalized metal organic framework (TMU-81) by incorporating sulfonyl and amide groups for the removal of Cd(II), Cu(II) and Cr(II) ions from simulated aqueous solutions. TMU-81 showed significant enhancement in heavy metals uptake suggesting that the strong host − guest interactions between cations and the donor sites play a major role in adsorption process. The maximum adsorption capacity for Cd2+ was 526 mg/g which is among the highest values reported for similar MOFs and other porous materials. The good performance in uptake and selectivity of TMU-81 can be attributed to the network structure that shaping the void, create mono-dimensional channels, decorated by exposed oxygen atom sites selective for Cadmium ion. Environmental “compatibility” of a treated MOFs was studied in order to evaluate its possible recycling as a new template for different applications by using pyrolysis method. Engineering of the pore surface provides a potential for MOF with a hybrid interface to act as a versatile tool for the design of multifunctional nanoparticles to meet specific application requirements.

Toolbox of Post‐Synthetic Mordenite Modification Strategies: Impact on Textural, Acidic and Catalytic Properties

AbstractWhich post‐synthetic modification strategy is the best suited for the generation of particular textural or chemical features in mordenite for a desired catalytic application? The aim of this contribution is to shed light on the effective quality of zeolite hierarchization through the rational study of structure/catalytic property dependency. Our study focused on mordenite, as this zeolite is of particular importance due to its strong acidity and monodimensional channel system, which explains why mordenite is industrially extensively employed. A toolbox of post‐synthetic hierarchization strategies on aluminum rich and poor mordenites has been developed based on basic treatments assisted by CTAB and pyridine at various temperatures (in conventional and microwave heating) and acid HF etching in aqueous and organic medium. A systematic study of over 30 post‐synthetically modified mordenites was conducted, which allowed us to assess the impact of each hierarchization strategy on textural, morphological, chemical and catalytic properties. Major correlations between structural features and catalytic activity, selectivity and stability in both the monomolecular n‐hexane cracking and the bimolecular toluene disproportionation were deduced.

Synthesis and Structural, Electrical, and Magnetic Properties of New Iron-Aluminum Alluaudite Phases β-Na2Ni2M(PO4)3 (M = Fe and Al).

Herein we report the studies of different physical properties (structural, magnetic, thermal, morphologic, electrical, and electrochemical) of two new allotropic β-Na2Ni2M(PO4)3 (NNMP) phosphates, with M = Fe and Al. Pure orthorhombic single-phase powders were prepared under air, using an autocombustion synthesis method. They crystallize in the orthorhombic Imma space group with similar unit cell parameters [a = 10.1592(2), b = 13.0321(3), c = 6.4864(2) Å] and [a = 10.3993(1), b = 13.1966(1), c = 6.4955(1) Å] for β-Na2Ni2M(PO4)3 (NNAP) and β-Na2Ni2Fe(PO4)3 (NNFP), respectively. Crystal structures of both compounds were determined using X-ray powder diffraction and Rietveld method refinements, which indicate the occurrence of Ni2+ in the 8g site, and of M3+ in the 4a site of the structure. The structure consists of a three-dimensional anionic framework obtained by the association on MO6, NiO6, and PO4 polyhedra, sharing edges and corners. The resulting three-dimensional structure creates monodimensional channels along the [100] and [010] directions formed by face-shared oxygen polyhedra and occupied by Na+ cations. This nondisordered cationic distribution is confirmed by a significant change of magnetic properties. Thus, both NNAP and NNFP samples show paramagnetic to ferromagnetic transition at 14 and 19 K, respectively. For the two compounds, thermal stability, electrical conductivity, and electrochemical properties have been also investigated. The intercalation/desintercalation properties of NNMP compounds as positive electrode were tested in sodium-ion batteries. The first cycling curves exhibit a significant polarization for both prepared samples.

Ethylbenzene isomerization over bifunctional platinum alumina–EUO catalysts: Location of the active sites

The transformation of ethylbenzene (EB) was carried out on intimate mixtures of Pt/Al 2O 3 (PtA) and HEUO (Si/Al = 15) catalysts with different composition under the following conditions: fixed bed reactor, temperature 683 K, pressure of hydrogen and ethylbenzene equal to 8 and 2 bar respectively, weight hourly space velocity between 5 and 60. Products result from the following transformations of ethylbenzene or/and of the reaction products: the bifunctional desired isomerization of EB into xylenes (X), the acidic reactions of disproportionation, dealkylation of EB (followed by ethylene hydrogenation) and EB–X transalkylation, the hydrogenation of EB followed by ethylcyclohexane isomerization and cracking. The selectivity to isomers, which is very low on the fresh catalysts, increases significantly during an initial period of fast deactivation, essentially at the expense of disproportionation and dealkylation by products. This deactivation as well as the increase in the selectivities to isomers were shown to result from the blockage by carbonaceous deposits of the access to the inner sites of the EUO micropores. The selectivity to isomers of EB transformation over these inner sites is very low, which could be due to the trapping of ethylcyclohexene intermediates within the monodimensional channels of EUO zeolite. In contrast, over the stabilized catalysts for which only the protonic sites of large side pockets located on the outer surface can be active, the selectivity to isomers is very high, which could be related to the environment of these sites as well as to their easy access by cycloalkene intermediates.

Cracking of pentenes to C 2–C 4 light olefins over zeolites and zeotypes : Role of topology and acid site strength and concentration

The cracking of 2-methyl-2-butene was carried out as a model reaction of C 4 + olefins cracking by using of various zeolite and zeotype catalysts to obtain light olefins, such as ethylene and propylene. Only ZSM-5 and ZSM-11 zeolites with 10-MRP with intersecting channels and acid sites concentration ranging between 0.03 and 0.1 mmol/g exhibit high and stable selectivity to C 2–C 4 olefins at high conversion level. Active and selective catalysts were prepared by three different methods: direct synthesis of H-ZSM-5 with high Si/Al ratio, steaming of H-ZSM-5 and “partial” ammonium ion exchange of Na-ZSM-5. Neither H-(Al)ZSM-5 with high concentration of acid sites nor H-(Fe)ZSM-5 are suitable catalysts as they catalyze hydrogen-transfer reactions yielding paraffins and aromatics, showing the importance of low concentration and high strength of acid sites for cracking activity-selectivity. High yield of butenes in the reaction products indicates that pentenes cracking in 10-MRP proceeds via formation of voluminous carbenium ion intermediates demanding sufficient space inside of zeolites channel. Therefore, the presence of intersection of 10-MRP is very important. While in ZSM-5 and ZSM-11 zeolites the low amount of active sites mainly situated in the pores intersection is sufficient to achieve high activity-selectivity in the case of mono-dimensional 12-MRP zeolites such as mordenite and ZSM-12 and also with 10-MRP ferrierite much higher total acid concentration is necessary, as vast of the acid sites, situated out of the main channel do not take part in the reaction. AlPO-11 materials with narrow mono-dimensional channel exhibit low conversion with majority of β-scission cracking mechanism independently on the acid sites concentration.

Effects of channel structure and acidity of molecular sieves in hydroisomerization of n-octane over bi-functional catalysts

SAPO-5, −11, −31, −41, −34, ZSM-5, −22 and −23 were synthesized by using the hydrothermal method and characterized by various methods such as XRD, SEM, XRF and TPD of NH3. They are representative of large-pore, medium-pore, small-pore, weak acid, strong acid, monodimensional channel and zigzag channel type of molecular sieves. Effects of pore size, the number of acid sites over medium-pore SAPOs, acid strength and shape of medium-pore channel on hydroisomerization of n-octane were examined over Pt-loaded corresponding molecular sieves. These results indicate that the selectivity to isomerization in hydroisomerization of n-octane is highly influenced by channel structure in molecular sieves and the conversion activity of n-octane is dependent on acidity of molecular sieves. Monodimensional medium-pore molecular sieves are ideal catalytic materials for higher isomerization selectivity in hydroisomerization of n-octane regardless of acid strength, such as SAPO-11, −31, −41, ZSM-22 and −23.

A novel open-framework with non-crossing channels in the uranyl vanadates A(UO2)4(VO4)3 (A=Li, Na)

A new sodium uranyl vanadate Na(UO2)4(VO4)3 has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the tetragonal symmetry with space group I41/amd and following cell parameters: a=7.2267(4) Å and c=34.079(4) Å, V=1779.8(2) Å3, Z=4 with ρmes=5.36(3) g/cm3 and ρcal=5.40(2) g/cm3. A full-matrix least-squares refinement on the basis of F2 yielded R1=0.028 and wR2=0.056 for 52 parameters with 474 independent reflections with I⩾2σ(I) collected on a BRUKER AXS diffractometer with MoKα radiation and a CCD detector. The crystal structure is characterized by ∞2[(UO2)2(VO4)] sheets parallel to (001) formed by corner-shared UO6 distorted octahedra and V(2)O4 tetrahedra, connected by V(1)O4 tetrahedra to ∞1[UO5]4− chains of edge-shared UO7 pentagonal bipyramids alternately parallel to the a- and b-axis. The resulting three-dimensional framework creates mono-dimensional channels running down the a- and b-axis formed by face-shared oxygen octahedra half occupied by Na. The powder of Li analog compound Li(UO2)4(VO4)3 has been synthesized by solid-state reaction. The two compounds exhibit high mobility of the alkaline ions within the two-dimensional network of non-intersecting channels.

A novel open-framework with non-crossing channels in the uranyl vanadates A(UO 2) 4(VO 4) 3 ( A=Li, Na)

A new sodium uranyl vanadate Na(UO 2) 4(VO 4) 3 has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the tetragonal symmetry with space group I4 1/ amd and following cell parameters: a=7.2267(4) Å and c=34.079(4) Å, V=1779.8(2) Å 3, Z=4 with ρ mes=5.36(3) g/cm 3 and ρ cal=5.40(2) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.028 and w R 2=0.056 for 52 parameters with 474 independent reflections with I⩾2 σ( I) collected on a BRUKER AXS diffractometer with Mo Kα radiation and a CCD detector. The crystal structure is characterized by ∞ 2[(UO 2) 2(VO 4)] sheets parallel to (001) formed by corner-shared UO 6 distorted octahedra and V(2)O 4 tetrahedra, connected by V(1)O 4 tetrahedra to ∞ 1[UO 5] 4− chains of edge-shared UO 7 pentagonal bipyramids alternately parallel to the a- and b-axis. The resulting three-dimensional framework creates mono-dimensional channels running down the a- and b-axis formed by face-shared oxygen octahedra half occupied by Na. The powder of Li analog compound Li(UO 2) 4(VO 4) 3 has been synthesized by solid-state reaction. The two compounds exhibit high mobility of the alkaline ions within the two-dimensional network of non-intersecting channels.

Ordinary Diffusion and Single File Diffusion in Zeolites with Monodimensional Channels. Benzene andn-Butane in ITQ-4 and L Zeolites

A molecular dynamics study of diffusion of n-butane and benzene in ITQ-4 and L zeolites is performed with the aim of understanding the differences between single file diffusion and ordinary diffusion. The former allows the molecular passing, whereas the latter appears when narrow channels preclude molecular passing. Two zeolites with monodimensional channels with loops are selected for studying the two mechanisms of diffusion. The effects of loading, molecular shape, and channel size are investigated in relation with the corresponding diffusion mechanism.

Synthesis and structural study of K 2PbSi 3O 9·H 2O with the structure of kostylevite

A new cyclohexasilicate, K 2PbSi 3O 9·H 2O, was synthesized under mild hydrothermal conditions. The crystal structure was solved by powder X-ray diffraction data. The unit cell is monoclinic, a = 6.5623(2), b = 11.7757(3), c = 13.0483(3) Å, β = 103.57(2)°, space group P2 1/n, Z = 4. The structure consists of a framework of isolated Si 6O 18 rings with the Pb(IV) located in octahedral holes. Thus, the structural formula can be written as Pb 2K 4{uB, 6t}[Si 6O 18]·2H 2O. The structure encloses one eight membered monodimensional channel running parallel to the a axis of about 3.0 × 5.5 Å 2. This channel is filled with one of the two crystallographically independent potassium cations and the water molecules. A second smaller six-membered ring channel along a is also present and contains the second K +. This compound represents a monoclinic version of the better-known orthorhombic members of the family.

Selective synthesis of glycerol monolaurate with zeolitic molecular sieves

The purpose of this work, is to study the selective synthesis of glycerol monolaurate from lauric acid and glycerol as starting materials, employing commercial Beta, Y and Mordenite zeolites with different Si/Al ratio as catalysts. Reaction conditions such as the stirring rate, temperature, glycerol/lauric acid ratio and catalyst mass in the reaction medium were optimized to diminish homogeneous reaction and reaction limitations due to diffusive effects. In the case of zeolite Y, an increase in glycerol monolaurate yield and selectivity for the monoester were verified at high Si/Al ratio. This behavior seems to be related to the presence of stronger acid sites present in the material with low Al content and probably also because of the higher hydrophobicity exhibited by zeolites having higher Si/Al ratio. Mordenite catalyst, on the contrary, presented low activity and selectivity to glycerol monolaurate, which can be attributed to the diffusional limitations imposes by the monodimensional channel system to the organic substrate accessing the inner catalyst sites, restricting the reaction to the sites present at the external particle surface. Zeolite Beta exhibited the best results as catalyst to obtain the mono-derivative, presenting selectivities higher than 60% at 20% yield.

Synthesis and characterization of Mu-11: a porous sodium trisilicate Na2Si3O7·H2O with 10-membered ring openings

The title compound (named Mu-11) is a new type of sodium trisilicate which was synthesized by using the quasi non-aqueous route in the presence of ethylene glycol as solvent. This material was characterized by XRD, SEM, 29Si solid-state MAS NMR spectroscopy, elemental and thermal analyses. The structure was determined by single-crystal X-ray diffraction. The symmetry is monoclinic, space group P21/c, a=7.3087(8)Å, b=12.7246(13)Å, c=9.0913(11)Å, β=119.01(1)° and V=739.39(10) Å3. The structure consists of corrugated sheets built from four-, six- and eight-membered ring units. The connection of such sheets leads to a three-dimensional framework with a monodimensional channel system delimited by a 10-ring aperture. After heating to 300°C another novel phase crystallizes, which is stable up to 600°C.