Synthesis of ordered and disordered silicas with uniform pores on the border between micropore and mesopore regions using short double-chain surfactants

Mechanistic Principles for Engineering Hierarchical Porous Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have generated tremendous research interest in the past two decades, due to their high surface areas, tailorable active sites, and tunable structures. Hierarchical porous MOFs (HP-MOFs) with two or more pore systems are particularly attractive, benefiting from improved active site accessibility and enhanced mass diffusivity in applications involving bulk molecules. This review outlines the mechanistic principles used for the rational design of HP-MOFs, current techniques used to measure their hierarchical porosities, as well as their emerging applications. We then critically summarize the current challenges in this field and provide a contemporary perspective on the technological innovations that would address current synthetic challenges in the field of HP-MOFs. The aim of this review is to provide an in-depth understanding of the formation mechanisms, materials chemistry, and structural and chemical properties of HP-MOFs while exploring ways to enhance the performance of current MOF materials in a range of fields.

Rapid access to ordered mesoporous carbons for chemical hydrogen storage

Ordered mesoporous carbon materials offer robust network of organized pores for energy storage and catalysis applications, but suffer from time-consuming and intricate preparations hindering their widespread use. Here we report a new and rapid synthetic route for a N-doped ordered mesoporous carbon structure through a preferential heating of iron oxide nanoparticles by microwaves. A nanoporous covalent organic polymer is first formed in situ covering the hard templates of assembled nanoparticles, paving the way for a long-range order in a carbonaceous nanocomposite precursor. Upon removal of the template, a well-defined cubic mesoporous carbon structure was revealed. The ordered mesoporous carbon was used in solid state hydrogen storage as a host scaffold for NaAlH 4 , where remarkable improvement in hydrogen desorption kinetics was observed. The state-of-the-art lowest activation energy of dehydrogenation as single step was attributed to their ordered pore structure and N-doping effect.

Synthesis and Shape-Selective Catalytic Application of Ordered Cubic Ia3̅d Supermicroporous Materials Templated by Rosin-Derived Quaternary Ammonium Salt with a Hydroxyl Radical in the Headgroup

We described the comprehensive synthesis, characterization, and catalytic performance of a novel type of the ordered cubic Ia3̅d supermicroporous silicas by using tetraethyl orthosilicate as a silicon source and a hydroxyl-functionalized quaternary ammonium salt as a template under alkali conditions. The effects of various reaction conditions on the pore structure and morphology of the silica materials were thoroughly investigated. Our results showed that under a wide range of reaction conditions, supermicroporous silicas with a highly ordered cubic Ia3̅d structure can be produced with a large BET specific surface area of 1741 m²/g, high pore volume of 0.91 cm³/g, concentrated pore size at 19.1 Å, and crystalline morphology. After Al doping, the obtained aluminosilicates preserved a highly ordered cubic supermicroporous structure. By using the H-form aluminosilicates as catalysts, we selectively dimerized β-pinene. The catalysts exhibited an excellent catalytic activity for β-pinene dimerization with a conversion yield up to 100%. Compared with conventional mesoporous H-form Al-MCM-48 catalysts, the prepared supermicroporous catalysts exhibited superior catalytic performance due to their excellent shape-selective properties, producing the β-pinene dimer in a yield up to 72.4% with dimer/oligomer ratios in the range of 7.5-10.1. This study featured a detailed preparation and characterization of supermicroporous silica with novel microstructures and showed its utility in catalytic dimerization.

Synthesis of ordered lamellar supermicroporous silica with rigid neutral and long-chain cationic composite templating route

Using a mixture of neutral primary amine dehydroabietylamine (DHAA) and long-chain cetyltrimethyl ammonium bromide (CTAB) as the template, ordered lamellar supermicroporous silicas were synthesized with NaOH as the base source and tetraethylorthosilicate (TEOS) as the silica source. The concentrations of DHAA, CTAB, and NaOH in the synthesis system had great effects on the structural properties of the samples. When the molar ratio of components was nTEOS:nCTAB:nDHAA:nNaOH:nH₂O = 1:0.114:0.00457:0.5:60, the material showed a lamellar phase with the highest ordering degree. By adding only a trace amount of DHAA into the synthesis system, the structure of the samples could be transformed from cubic phase to lamellar phase, since the added DHAA solubilized in CTAB micelles to change the effective surfactant ion pair packing parameter. The dosage of CTAB should be moderate; too high or too low will decay the ordering degree of the lamellar structure.A much higher concentration of NaOH resulted in an ethanol-rich solvent in which the DHAA did not solubilize in the micelles of CTAB, but adsorbed at the hydrophilic headgroup–solvent interface. Accordingly, a structural transformation from lamellar phase to hexagonal phase occurred.

Tuning the Mesomorphism and Redox Response of Anionic-Ligand-Based Mixed-Valent Nickel(II) Complexes by Alkyl-Substituted Quaternary Ammonium Cations

The combination of the redox-active mesogenic anion [Ni^II (Bdt)(BdtSQ)]^- (Bdt=1,2-benzenedithiolato; BdtSQ=1,2-dithia-semi-benzoquinonato) with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of the type [NR₄ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NC16₄ Ni) and C8,10 (NC8,10₄ Ni); C8,10=6-octylhexadecyl] or [NMe₂ R₂ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NMe₂ C16₂ Ni) and C8,10 (NMe₂ C8,10₂ Ni)]. X-ray crystallographic analyses of NMe₂ C16₂ Ni and NC16₄ Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe₂ C16₂ Ni and NC16₄ Ni commonly form crystalline phases at room temperature, whereas complexes NMe₂ C8,10₂ Ni and NC8,10₄ Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe₂ C16₂ Ni, NMe₂ C8,10₂ Ni, and NC16₄ Ni commonly form a smectic A phase (SmA) at 375, 317, and 342 K, respectively. For the four complexes, well-defined cyclic voltammetry responses, derived from ligand-based oxidation and reduction, were observed in solution and the condensed phases, that is, upon casting these complexes on an indium-doped tin oxide working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl-substituted quaternary ammonium cations, while maintaining the well-defined redox responses of the anions even in the condensed phases.

Ordered lamellar supermicroporous titania templating by rosin-derived quaternary ammonium salt

By using dehydroabietyltrimethyl ammonium bromine (DTAB), a novel rosin-derived quaternary ammonium salt, as template and peroxotitanium acid as precursor, ordered lamellar supermicroporous titania has been synthesized via a hydrothermal process. The template agent:titanium source molar ratio in the synthesis system and the hydrothermal temperature have great impact on the microstructure characteristics of the samples. The increase of DTAB:TiO₂ molar ratio from 0.04:1 to 0.10:1 is favorable to the increase of regularity of pore structures, but has no significant effects on the crystalline structures. The increase of the hydrothermal temperature from 343 to 393 K can induce an increase in crystallinity of the samples. However, the exorbitant hydrothermal temperature will reduce the regularity of pore structures. When the mole ratio of DTAB:TiO₂ is 0.10:1 and the hydrothermal temperature is 373 K, the as-synthesized sample possesses pore structure with the highest level of long-range order, as well as pore wall with semicrystallized anatase phase. The pore size and the pore wall thickness are about 2.0 nm and 1.0 nm, respectively.

Segregation of confined ionic liquids inducing the formation of super-micropores in the silica matrix

Super-microporous silica with highly-branched framework and disordered worm-like pore formed by the segregation of confined ionic liquid.

The importance of raw graphite size to the capacitive properties of graphene oxide

The importance of raw graphite size to the capacitive properties of graphene oxide was discussed.

A simple, phosphine free, reusable Pd(ii)–2,2′-dihydroxybenzophenone–SBA-15 catalyst for arylation and hydrogenation reactions of alkenes

An efficient, phosphine and co-catalyst free Pd(ii)–DHBP@SBA-15 catalyst for arylation and hydrogenation reactions of alkenes was synthesized and characterized.

Shape selectivity extending to ordered supermicroporous aluminosilicates

Shape selectivity was observed in the pores of ordered supermicroporous aluminosilicates for the first time and the superior selectivity for target products was attributed to the particular pore diameter.

Synthesis of ordered porous SiO2 with pores on the border between the micropore and mesopore regions using rosin-based quaternary ammonium salt

Ordered hexagonal supermicroporous silica with nanosheet morphology has been synthesized with a type of rosin-based quaternary ammonium salt.

Size control and biological properties of monodispersed mesoporous bioactive glass sub-micron spheres

Monodispersed mesoporous bioactive glass sub-micron spheres with a controllable size and good biocompatibility were fabricated by an improved sol–gel method.

An economic method for synthesis of highly ordered porous silica

We report an economic method for synthesis of highly ordered silica with a mixing surfactant system containing short-chain cationic surfactant (decyltrimethyl ammonium bromide, denoted C10TMAB) and short-chain anionic surfactant (sodium octyl sulfate, denoted SOS) as the templating agents. Highly ordered supermicroporous silica was synthesized by judiciously chosen mixing ratio of surfactants. The samples were characterized by small-angle X-ray diffraction, transmission electron microscopy, and N2 adsorption-desorption. The results showed that the pore structure of the resulting silica belongs to the two-dimensional hexagonal structure (space group 2D-p6mm) with a pore size of ca. 2.2nm. Moreover, the method proposed herein is expected to facilitate the synthesis of not only porous silicas but also materials with other framework compositions.

Fluorinated Surfactant Templating of Ordered Nanoporous Silica

We will describe the first synthesis of ordered mesoporous silica using a fluorinated surfactant template. Nitrogen adsorption, x-ray diffraction and TEM results will be presented that show that at room temperature, we form particles with well-ordered 2D hexagonal pores using a cationic surfactant with a partially fluorinated octyl (C8) tail. The pore diameter determined by the inflection in the nitrogen adsorption isotherm is on the order of 2.6 nm and the pore size distribution is very narrow. Consistent diameters are found with a modified Kelvin equation approach and with nonlocal density functional theory. The same pore size and long-range order are found for a range of surfactant to silica ratios.

Ultralow temperature synthesis of ordered hexagonal smaller supermicroporous silica using semifluorinated surfactants as template

Ordered hexagonal smaller supermicroporous silica (JLU-14L and JLU-11L) has been successfully synthesized at ultralow temperature (0 approximately -20 degrees C) by semifluorinated surfactants, which is extensively investigated by X-ray diffraction, N2 adsorption, and transmission electron microscopy. X-ray diffraction shows that the peak intensity of the samples increases with decreasing synthesis temperatures in the range of 0 approximately -20 degrees C, indicating that the ordering of porous silica improves with decreasing synthesis temperature. N2 adsorption/ desorption isotherms show that JLU-14L has both ordered smaller hexagonal supermicropores (1.27 and 1.28 nm) and disordered micropores (0.67 nm). Such hierarchically porous materials with micro/supermicroporosity should be potentially important for fast diffusion of reactants and products in catalytic reactions. The ultralow temperature synthesis is a crucial factor for the formation of ordered smaller supermicropores and the control of microporosity in JLU-14L.

Designed synthesis of nanostructured siloxane–organic hybrids from amphiphilic silicon-based precursors

This paper reports on recent progress in the synthesis of nanostructured siloxane-organic hybrids based on the self-assembly of amphiphilic silicon-based precursors. A variety of ordered hybrid materials have been obtained by molecular design of the precursors. Alkoxysilanes and chlorosilanes with covalently attached hydrophobic organic tails are hydrolyzed to form amphiphilic molecules containing silanol groups, leading to the formation of layered (lamellar) structures. Transparent and oriented thin films of lamellar hybrids were prepared by the reaction in the presence of tetraalkoxysilane. In addition, the design of molecules having alkyl chains and large oligosiloxane heads led to the formation of mesophases consisting of cylindrical assemblies, providing a direct pathway to ordered porous silica. The synthesis, structural features, and formation processes of these hybrid mesostructures are discussed.

Self-Assembly of Designed Oligomeric Siloxanes with Alkyl Chains into Silica-Based Hybrid Mesostructures

A novel self-assembly route to ordered silica-organic hybrids using well-defined siloxane oligomers with alkoxy functionality and covalently attached alkyl chains has been investigated. Various hybrid mesostructures were obtained by hydrolysis and polycondensation without the use of any structure-directing agents. The oligomers 1(Cn), having an alkylsilane core and three branched trimethoxysilyl groups, formed highly ordered lamellar phases when n = 14-18, while those with shorter alkyl chains formed cylindrical assemblies, slightly distorted two-dimensional (2D) hexagonal structures (n = 6-10), and a novel 2D monoclinic structure (n = 12). Furthermore, the mixtures of 1(Cn) with different chain lengths yielded well-ordered 2D hexagonal phases, possibly due to the better packing of the precursors. The hybrids consisting of cylindrical assemblies were converted to ordered porous silica with tunable pore sizes upon calcination to remove organic groups. The liquid-state 29Si NMR analysis of the hydrolysis and polycondensation processes of 1(Cn) revealed a unique intramolecular reaction yielding primarily the oligomer with a tetrasiloxane ring which is a new class of amphiphilic molecule having both self-assembling ability and high cross-linking ability. We also found that the mesostructure (lamellar or 2D hexagonal) was strictly controlled by varying the number of siloxane units per alkyl chain. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of oligosiloxane precursors.

Large- and small-nanopore silica prepared with a short-chain cationic fluorinated surfactant

A cationic partially fluorinated surfactant with four carbons in the chain 1-(3,3,4,4,4-pentafluorobutyl)pyridinium chloride is employed as a structure-directing agent to synthesize nanoporous silica. Samples are prepared in dilute ammonia solutions at room temperature with a range of surfactant:Si ratios. The sample with the largest surfactant:Si ratio forms particles with wormhole-like micropores with an average diameter of 1.6 nm, which corresponds to the anticipated small size of the surfactant aggregates. On the other hand, the sample with the smallest surfactant:Si ratio forms a gel that, upon drying, has uniform 11.1 nm pores. The formation and stabilization of the latter large-mesopore structure is unusual for a sample prepared and dried under ambient conditions, and may reflect favourable roles of the surfactant both in inducing gelation and in stabilizing the pore structure during drying.

Fluorinated Carbon with Ordered Mesoporous Structure

Fluorination and structural change of highly ordered mesoporous carbons were studied. Mesoporous and fluorinated carbons with ordered cubic structure were synthesized and characterized with TEM, FTIR, adsorption, and EDX.

Evaluation of pore size distribution in boundary region of micropore and mesopore using gas adsorption method

This paper discusses an accurate method of pore size distribution evaluation in boundary regions of micropores and mesopores using the gas adsorption process on the basis of the capillary condensation theory, which is liable to be underestimated with the existing BJH and DH methods. A typical nitrogen adsorption isotherm for highly ordered mesoporous silica, which has cylindrical pores with diameter smaller than 4 nm, is considered to be type IV and it is well known for the steep increase of the amount adsorbed through capillary condensation in the region of the relative pressure P/P0 smaller than 0.4. In calculating the distribution of the pore size from the change of the amount adsorbed due to capillary condensation, it is important to accurately predict both the multilayer thickness t of the adsorbed nitrogen molecules and the critical radius rc where capillary condensation occurs. It is necessary to consider the curvature of the adsorption layer-gas phase interface when predicting the multilayer thickness t of nitrogen adsorbed within the pore of highly ordered mesoporous silica. Revision of the Kelvin equation is also required when rc is to be predicted. While the predicted value of t based on the Broekhoff and de Boer theory is matched well with the value of t which is actually measured using highly ordered mesoporous silica, and the predicted value of rc based on the GTKB-Kelvin-cylindrical equation that has been revised considering the effect of the interfacial curvature on the interfacial tension of the adsorption layer-gas phase interface is matched with the value of rc which is actually measured using highly ordered mesoporous silica. A combination method of the Broekhoff and de Boer equation and the GTKB-Kelvin-cylindrical equation is proposed as a means of accurately evaluating, from the nitrogen adsorption isotherm, the pore size distribution in the highly ordered mesoporous silica in boundary region of micropore and mesopore. The proposed new method of pore size evaluation features high accuracy and offers the convenience of obtaining the pore size distribution without repeated calculations by employing the same algorithm as DH method. The pore size predicted by the Halsey equation and the Kelvin equation of the conventional DH method is about 20% smaller than the pore size predicted by the newly proposed evaluation method.

Suppression of binary nucleation in amorphous La-Fe-Sb mixtures

The nucleation energy of a series of La(x)Fe(y)Sb(z) modulated elemental reactants was measured as a function of the Fe/Sb ratio over a large composition range while holding the La content constant. The nucleation energy of the ternary compound La(0.5)Fe(4)Sb(12) with the skutterudite crystal structure was found to depend very strongly on the Fe/Sb ratio in the modulated elemental reactant, with a higher nucleation energy as the Fe/Sb ratio is moved away from the 1:3 stoichiometric value. When the results of this study are compared with those from Fe(y)Sb(z) modulated reactants, the addition of lanthanum was found to suppress the nucleation of FeSb(2), thereby broadening the Fe/Sb composition range in which the ternary skutterudite compound La(x)Fe(4)Sb(12) nucleates. This suppression of nucleation of a binary phase on addition of a ternary component to an amorphous intermediate is in agreement with theoretical arguments. The observed suppression of nucleation also provides rational for the observed nucleation of metastable ternary and higher-order compounds from homogeneous amorphous reactants.

Nickel(II) macrocyclic complexes with long alkyl pendant chain: synthesis, x-ray structure, and anion exchange property in the solid state

A nickel(II) pentaaza macrocyclic complex containing a 1-hexadecyl pendant chain, [Ni(C(25)H(55)N(5))](ClO(4))(2).H(2)O (1), was synthesized by a one-pot metal-template condensation reaction. Crystal data for 1: triclinic, Ponemacr;, a = 8.333(4) A, b = 8.356(3) A, c = 28.374(9) A, alpha = 81.865(19) degrees, beta = 86.242(18) degrees, gamma = 63.871(17) degrees, Z = 2. Solid 1 forms hydrophobic layers that are constructed by the long alkyl chains of the macrocycles. Solid 1 exchanges ClO(4)(-) with NCS(-), PF(6)(-), C(2)O(4)(2-), NO(3)(-), and CF(3)SO(3)(-) that are dissolved in water. From the reaction of [Ni(C(25)H(55)N(5))Cl(2)] with Et(3)NH(TCNQ)(2) in EtOH/DMF/acetone solution, [Ni(C(25)H(55)N(5))(TCNQ)(2)](TCNQ).(CH(3)COCH(3)) (2) (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) was prepared. Crystal data for 2: triclinic, Ponemacr;, a = 8.459(0) A, b = 13.945 (1) A, c = 26.833(2) A, alpha = 88.744(2) degrees, beta = 84.536(2) degrees, gamma = 80.089(4) degrees, Z = 2. In 2, TCNQ anions coordinate nickel(II) at the axial sites, which form pi-stacked TCNQ(-) dimers to give rise to 1-D chains. The neutral TCNQ molecules are included between the dimerized TCNQ(-) species, which construct a pi-stacked group of six TCNQ units as blocked by the long alkyl chains. Compound 2 is an electric insulator. It shows a weak signal in the EPR spectrum. The magnetic susceptibility data of 2 measured at 5-300 K exhibit a simple paramagnetism at low temperatures (<100 K) but an increase in the magnetic moment at higher temperatures due to the contribution of a thermally accessible triplet state for the antiferromagnetically coupled [TCNQ](2)(2-).

Metamorphosis of ordered mesopores to micropores: periodic silica with unprecedented loading of pendant reactive organic groups transforms to periodic microporous silica with tailorable pore size

Ordered porous silicas with unprecedented loadings of pendant vinyl groups have been synthesized via co-condensation of tetraethyl orthosilicate (TEOS) and triethoxyvinylsilane (TEVS) under basic conditions in the presence of cetyltrimethylammonium surfactant. The resulting organosilicate-surfactant composites exhibited at least one low-angle X-ray diffraction (XRD) peak up to the TEVS:TEOS molar ratio of 7:3 (70% TEVS loading) in the synthesis gel. The surfactant was removed from these composites without any structural collapse. Nitrogen adsorption provided strong evidence of the presence of uniformly sized pores and the lack of phase separation up to TEVS:TEOS ratios as high as 13:7 (65% TEVS loading), whereas (29)Si MAS NMR and high-resolution thermogravimetry showed essentially quantitative incorporation of the organosilane. Thus, a hitherto unachieved loading level for pendant groups, considered by many to be impossible to achieve for stable organosilicas because of the expected framework connectivity constraints, has been obtained. The resulting vinyl-functionalized silicas exhibited gradually decreasing pore diameter (from 2.8 to 1.7 nm for TEVS loadings of 25-65%) and pore volume as the loading of pendant groups increased, but the specific surface area was relatively constant. Because of the reactivity of vinyl groups, ordered silicas with very high loadings of these groups are expected to be robust starting materials for the synthesis of other organic-functionalized ordered microporous materials. Herein, we demonstrate that these starting materials can also be transformed via calcination into ordered microporous silicas with pore diameters tailorable from 2.5 to as little as 1.4 nm simply by using an appropriate loading of the vinyl-functionalized precursor. This ease of the micropore size adjustment and the attained degree of structural ordering (as judged from XRD) have not been reported before. The novel ordered microporous materials reported herein are promising as adsorbents and catalyst supports.