Thermally stable organically modified layered silicates based on alkyl imidazolium salts

A series of imidazolium salts having various substituents and functional groups were synthesized and characterized by FTIR and NMR spectroscopy. Organic modification of natural and synthetic layered silicates involving montmorillonite (MMT), laponite (lap), and synthetic mica (mica) was carried out by ion-exchange reaction. The obtained organo-clays were characterized by FTIR and powder X-ray diffraction techniques. Results indicate that these organically modified clays have much higher thermal stabilities compared to their corresponding imidazolium halides. It was also observed from TGA analysis that thermal stability does not depend on the functional group present at the 3-position of the imidazolium salts. These studies strongly supports premise that the removal of halide is necessary to improve the thermal stability of the organo-clay produced.     

Synthesis, characterisation and properties of ferrocenylalkylimidazolium salts

New ferrocenylalkylimidazolium salts [Fc(CH₂)_n(C₃H₃N₂)R]X⁻ were synthesised through the incorporation of green chemistry principles of atom economy and when feasible under solvent-free conditions. The products comprise a series of salts all characterised by the ferrocenyl moiety with variations in the length of the linker alkyl chain (n), the size of the imidazolium alkyl substituent (R) or the electronic nature of the counter-ion (X⁻). The dependence of the physical and electronic properties of the salts on the three main structural variants was studied. It was found that variation in the steric size of the R group has the most profound influence on the melting points of the ionic liquids. The compounds were fully characterised by IR, ¹H and ¹³C NMR, MS and melting point determinations.     

Structural design of imidazolium and its application in PP/montmorillonite nanocomposites

Four kinds of imidazolium surfactants with high thermal stability were designed and synthesized accordingly. The structures of these surfactants were characterized by ¹H NMR spectra. The TGA results indicated that the thermal stabilities of these surfactants with saturated alkyl groups were relatively high and the initial decomposition temperatures at 5% weight loss (T_0.05) were higher than 250 °C. Imidazolium(O) modified montmorillonite (MMT) was prepared by cation exchange. TGA results showed that the OMMT showed obviously higher thermal stability than the surfactants themselves and the T_0.05 values of OMMT were higher than 330 °C. The dihexadecane imidazolium (DHI) with two long tails has the ability to enlarge the interlayer spacing to a bigger degree compared with other imidazolium surfactants with only one long tail. Polypropylene(PP)/OMMT nanocomposites were prepared by solution blending and the effects of these surfactants with different structures on the silicate layer dispersion in PP matrix were measured.     

Ambient temperature imidazolium-based ionic liquids with tetrachloronickelate(II) anions

Deep blue tetrachloronickelate(II) salts of 1-alkyl-3-methylimidazolium cations [C_nmim]⁺ with intermediate chain lengths (n = 5, 7, 8 and an n = 4/6 mixture) have been prepared from imidazolium chlorides and NiCl₂. They are liquid at ambient temperatures (ca. 20 °C) and above, and display viscosities and thermal stability comparable to related [BF₄]⁻ compounds. The similarities may reflect the compensating effects of the dinegative charge of the [NiCl₄]²⁻ ion and its larger size.     

Organocatalysis of nucleophilic substitution reactions by the combined effects of two promoters fused in a molecule: oligoethylene glycol substituted imidazolium salts

Oligoethylene glycol substituted imidazolium salts were synthesized as promoters for a range of S_N2 reactions, and their efficiency was examined. These tailor-made organic promoters enhanced the nucleophilicity of alkali metal salts significantly through the combined effects of two promoters (oligoethylene glycols and imidazolium salts) in a single molecule. The effects of the oligoethylene glycol side chain length, ionic liquid anions, nucleophiles, and substrates were investigated systematically. [hexaEGmim][OMs] and [dihexaEGim][OMs] showed the highest efficiency for S_N2 reactions using alkali metal salts. The role of the terminal hydroxyl groups of the oligoethylene glycol moiety was assessed by examining the relative S_N2 yields of chlorination and bromination. The results showed that the hydrogen bonding strength of the hydroxyl groups with the nucleophile is very important. The mechanism for the excellent promotion of S_N2 reactions by oligoEGILs was examined by quantum chemical calculations. The results showed that the oxygen atoms in the oligoethylene glycol portion and the ionic liquid anion act on the counter cation K⁺ or Na⁺ as a Lewis base, to enhance the reactivity of the metal salts significantly.     

Isomerization of endo-tetrahydrodicyclopentadiene over clay-supported chloroaluminate ionic liquid catalysts

Various halide salts with different alkyl lengths were allowed to intercalate into the layer structure of sodium montmorillonite clay through an ion exchange reaction. Intercalation of 1-hexadecyl-3-methylimidazolium chloride, hexadecyltrimethylammonium bromide, dihexadecyldimethylammonium bromide, and tributylhexadecylphosphonium bromide could expand the spacing of the silicate layers from 12 to 37–41 Å (measured by X-ray diffraction). The modified clays were pretreated with the pyridine hydrochloride/AlCl₃ mixture and used for suitably supporting a chloroaluminate ionic liquid catalyst for the isomerization of endo-tetrahydrodicyclopentadiene into the corresponding exo-isomer. Nearly quantitative conversion to the desired product and nearly quantitative selectivity were observed for the newly developed clay-supported ionic liquid catalysts, which were proven to be recyclable.     

Grignard allylic substitution catalyzed by imidazol-2-ylidene- and imidazol-4-ylidene-magnesium complexes

In the presence of a catalytic amount of 1,2-disubstituted or 1,2,3-trisubstituted imidazolium salts, γ-substituted allyl chlorides reacted with alkyl Grignard reagents to undergo substitution reactions in an S_N2′-selective fashion, where the magnesium ate complexes [(N-heterocyclic carbene-MgR₃)⁻(MgX)⁺] of imidazol-2-ylidenes or imidazol-4-ylidenes, generated in situ, were postulated as the active species. It was observed that the reactions with imidazol-4-ylidene catalysts were faster than those with imidazol-2-ylidenes. Enantioselective catalysis using a chiral imidazolium salt was preliminarily investigated.     

Synthesis and Structure of Ionic Liquids Containing the ­[Al(OC6H4CN)4]– Anion

The synthesis, structure, and physical properties of ionic liquids (IL) bearing the novel [Al(O–C₆H₄–CN)₄]^– ion as counterion to the commonly used [NR₄]⁺, [PR₄]⁺ and imidazolium ions are reported. Both the influence of the alkyl chain length as well as the functionalization with cyano groups is studied. These ILs are easily obtained by reaction of Ag[Al(O–C₆H₄–CN)₄] with the corresponding ammonium, phosphonium, and imidazolium halides. The stability towards electrophilic cations was investigated. All prepared salts have a window for the liquid phase of ca. 200 °C and are thermally stable up to 450 °C. The solid‐state structures reveal only weak cation ··· anion and anion ··· anion interactions in accord with the observed low melting points (glass transition points).     

Application of ionic liquids as low-volatility plasticizers for PMMA

Room temperature ionic liquids (ILs) based on imidazolium salts, were found to be excellent plasticizers for poly(methyl methacrylate), with improved thermal stability, and the ability to reduce glass transition temperatures to near 0 °C. Because ILs have environmentally benign properties, they can be used in place of traditional chemicals in numerous products and processes. In this work, PMMA was formulated using dioctyl phthalate, DOP, as a traditional plasticizer, and properties were compared to PMMA plasticized with two ILs: butyl methylimidazolium/hexafluorophosphate, [bmim⁺][PF₆⁻], and hexyl methylimidazolium/hexafluorophosphate, [hmim⁺][PF₆⁻]. Formulations incorporated up to 30 vol.% DOP and 50 vol.% ILs. Bulk and plasticized polymers were characterized for glass transition temperature, elastic modulus, and the thermal stability of the plasticizers.     

Crystallographic view of fluidic structures for room‐temperature ionic liquids: 1‐butyl‐3‐methyl­imidazolium hexa­fluoro­phosphate

Shock‐induced crystallization of the supercooled ionic liquid 1‐butyl‐3‐methyl­imidazolium hexa­fluoro­phosphate, C₈H₁₅N₂⁺·PF₆⁻, allows for the first time precise X‐ray diffraction analysis directly pertinent to the fluid state. This inter­mediate‐chain‐length structure shows features of both short‐ and long‐chain analogs. Two types of inter­planar distances between imidazolium rings are observed. The anions are located in channels formed by the imidazolium rings and alkyl chains.     

Modification of kaolinite with alkylimidazolium salts

We have demonstrated that the d-spacing and thermal stability of the alkylimidazolium intercalated kaolinite compounds can be controlled by adjusting chain length of the alkyl groups. The composites were synthesized by displacement method using selected imidazolium ionic liquids bearing different short alkyl chains as guest molecules. The effects of the length of alkyl side chain on d-spacing and thermal stability of the ionic liquids–kaolinite intercalations were investigated by XRD and TG-DSC. Results revealed that in these composites, increasing the length of alkyl substituent led to larger d-spacing and decreased thermal stability. Temperature was found to have influence on the intercalation ratio but not on the d-spacing of final product. The present study shows an easy way of tailoring the performance of these intercalations via small variation of guest ionic liquids, providing the possibility of finding “species-specific” modifier for fully exfoliated nanocomposites.     

Kinetic study of the thermal degradation of PS/MMT nanocomposites preparedwith imidazolium surfactants

Styrene and montmorillonite organically modified with imidazolium surfactants (MMT) at various alkyl chain lengths (C₁₂, C₁₆ and C₁₈) were used to prepare the corresponding PS/MMT/C₁₂, PS/MMT/C₁₆ and PS/MMT/C₁₈ nanocomposites by in situ polymerization. XRD and TEM analyses evidenced the formation of both intercalated and exfoliated structures. The glass transition temperatures (T _g) of nanocomposites, as well as that of neat PS, were obtained by DSC measurements. The thermal degradations were carried out in the scanning mode, in both inert and oxidative environments, and the initial temperatures of decomposition (T _i) and the apparent activation energies of degradation (E _a) were determined. Due to an oxidative degradation mechanism, the T _i and E _a values in air atmosphere were lower than those under nitrogen. The results indicated that nanocomposites are more thermally stable than polystyrene, and suggested an increasing degree of exfoliation as a function of alkyl chain length of surfactant, associated with enhancing thermal stability.     

Tetraalkylphosphonium-based ionic liquids

Ionic liquids are salts that are liquid at or near room temperature. Their wide liquid range, good thermal stability, and very low vapor pressure make them attractive for numerous applications. The general approach to creating ionic liquids is to employ a large, unreactive, low symmetry cation with and an anion that largely controls the physical and chemical properties. The most common cations used in ionic liquids are N-alkylpyridinium and N,N′-dialkylimidazolium. Another very effective cation for the creation of ionic liquids is tetraalkylphosphonium, [PR₁R₂R₃R₄]⁺. The alkyl groups, R_n, generally are large and not all the same. The halide salts of several phosphonium cations are available as starting materials for metathesis reactions used to prepare ionic liquids. The large phosphonium cations can combine with relatively large anions to make viscous but free flowing liquids with formula mass greater than 1000 g mol⁻¹. Some other more massive salts are waxes and glasses. The synthesis and the physical, chemical, and optical properties of phosphonium-ionic liquids having anions with a wide range of masses were measured and are reported here.     

Head-to-head and head-to-tail multilayer n-alkylsilsesquioxane films

Two lamellar organosilica films made up of stacks of head-to-head and head-to-tail alkyl bilayers were synthesized by a UV-driven polymerization of n-C₁₈H₃₇SiCl₃ and n-C₁₈H₃₇Si(OCH₃)₃, respectively. In addition to thermogravimetry and small-angle X-ray scattering, the characterization of these multilayer systems was focused on a variety of multinuclear solid-state NMR methods (¹H, ²⁹Si, ¹³C, 2D WISE). The chemical structure of the siloxane interlayer, the alkyl chain conformation and thermal stability were systematically investigated and compared. Through its marked sensitivity to molecular motion, solid-state NMR was used to investigate the effect of interdigitation on alkyl chain dynamics.     

Polystyrene‐Clay Nanocomposites Prepared with Polymerizable Imidazolium Surfactants

Polystyrene/montmorillonite nanocomposites were prepared via in situ bulk polymerization. New imidazolium salts were used to modify montmorillonite obtaining an improvement in thermal stability in comparison with a montmorillonite modified with a standard alkylammonium cation. The synthesized cations facilitate the formation of partially exfoliated structures because of the presence in their structure of a polymerizable group. The polystyrene/montmorillonite nanocomposites were characterized by X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

The new thermally stable, reactive surfactant imidazolium salts (C₁₂, C₁₆ and C₁₈) used to organically modify montmorillonite in this study.     

Effect of counterions and central cores of tripodal imidazolium salts on palladium-catalyzed Suzuki-Miyaura cross-coupling reactions

Seven tripodal imidazolium salts were synthesized and used as catalyst precursors in cross-coupling reactions of aryl halides with arylboronic acids. Effect of counterions and central cores of seven tripodal imidazolium salts was investigated. The tripodal imidazolium salt anchored to benzene with anion PF₆⁻ was found most effective with Pd(OAc)₂ for the cross-coupling of aryl bromides with arylboronic acids.     

Effects of alkyl chain length on properties of 1-alkyl-3-methylimidazolium fluorohydrogenate ionic liquid crystals

A series of 1‐alkyl‐3‐methylimidazolium fluorohydrogenate salts (C_xMIm(FH)₂F, x=8, 10, 12, 14, 16, and 18) have been characterized by thermal analysis, polarized optical microscopy, IR spectroscopy, X‐ray diffraction, and anisotropic ionic conductivity measurements. Liquid crystalline mesophases with a smectic A interdigitated bilayer structure are observed from C₁₀ to C₁₈, showing a fan‐like or focal conic texture. The temperature range of the mesophase increases with the increase in the alkyl chain length (from 10.1 °C for C₁₀MIm(FH)₂F to 123.1 °C for C₁₈MIm(FH)₂F). The distance between the two layers in the smectic structure gradually increases with increasing alkyl chain length and decreases with increasing temperature. Conductivity parallel to the smectic layers is around 10 mS cm^?1 regardless of the alkyl chain length, whereas that perpendicular to the smectic layers decreases with increasing alkyl chain length because of the thicker insulating sheet with the longer alkyl chain.     

Synthesis, characterization and thermal properties of small R2R2NX-type quaternary ammonium halides

Twenty-one R₂R^′₂N⁺X⁻ -type (R=methyl or ethyl, R^′=alkyl, X=Br or I) quaternary ammonium (QA) halides have been prepared by using a novel one-pot synthetic route in which a formamide (dimethyl-, diethylformamide, etc.) is treated with alkyl halide in the presence of sodium or potassium carbonate. The formation of QA halides was verified with ¹H-NMR, ¹³C-NMR, MS and elemental analysis. The crystal structures of four QA halides (two bromide and two iodide) were determined using X-ray single crystal diffraction, and the powder diffraction method was used to study the structural similarities between the single crystal and microcrystalline bulk material. The thermal properties of all compounds were studied using TG/DTA and DSC methods. The smallest compounds decomposed during or before melting. The decreasing trend of melting points was observed when the alkyl chain length was increased. The liquid ranges of 120-180 °C were observed for compounds with 5-6 carbon atoms in the alkyl chain. The low melting points and wide liquid ranges suggest potential applicability of these compounds for example as ionic liquids precursors.     

Supercritical CO2-ionic liquid mixtures for modification of organoclays

The use of supercritical CO(2) as solvent in the modification of montmorillonite by imidazolium and phosphonium ionic liquids bearing long alkyl chains (C(18)) known for their excellent thermal stability is described. The objective is to combine the environmentally friendly character of ionic liquids and supercritical carbon dioxide for the organophilic treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites, cationically exchanged under supercritical carbon dioxide with or without co-solvent, have been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and compared to montmorillonites treated by conventional cationic exchange.     

Study of the Alkyl Chain Length on Laccase Stability and Enzymatic Kinetic with Imidazolium Ionic Liquids

The activity and stability of laccase and their kinetic mechanisms in water soluble ionic liquids (ILs): 1-butyl-3-methyl imidazolium chloride [C₄mim][Cl], 1-octyl-3-methyl imidazolium chloride [C₈mim][Cl], and 1-decyl-3-methyl imidazolium chloride [C₁₀mim][Cl] were investigated. The results show that an IL concentration up to 10% is satisfactory for initial laccase activity at pH 9.0. The laccase stability was well maintained in [C₄mim][Cl] IL when compared to the control. The inactivation of laccase increases with the length of the alkyl chain in the IL: [C₁₀mim][Cl] > [C₈mim][Cl] > [C₄mim][Cl]. The kinetic studies in the presence of ABTS as substrate allowed calculating the Michaelis–Menten parameters. Among the ILs, [C₄mim][Cl] was the suitable choice attending to laccase activity and stability. Alkyl chains in the ions of ILs have a deactivating effect on laccase, which increases strongly with the length of the alkyl chain.