Study of the Adsorption Properties of MCM-41 Molecular Sieves Prepared at Different Synthesis Times

The variation of surface properties of SiMCM-41 and AlMCM-41 nanoporous materials as function of synthesis time was examined. The main properties studied were: surface area, pore diameter, pore volume, mesoporous parameter, and wall thickness. Siliceous MCM-41 molecular sieves were synthesized starting from hydrogels with the following molar compositions: 4.58SiO₂:0.435Na₂O:1 CTMABr:200 H₂O for SiMCM-41, and 4.58SiO₂:0.485 Na₂O:1 CTMABr:0.038 Al₂O₃:200 H₂O, for AlMCM-41. Cetyltrimethylammonium bromide (CTMABr) was used as the structural template. The crystallographic parameters were obtained from XRD data and by nitrogen adsorption using the BET and BJH methods. The results obtained showed a significant variation of the surface properties of the MCM-41 materials as a function of the synthesis time reaching silica wall thickness of ca. 2 nm on the fourth day.     

Model-free kinetics applied for the removal of CTMA<Superscript>+</Superscript> and TPA<Superscript>+</Superscript> of the nanostructured hybrid AlMCM-41/ZSM-5 material

The nanostructured hybrid AlMCM-41/ZSM-5 composite was synthesized starting from a hydrogel with molar composition SiO₂:0.32Na₂O:0.03Al₂O₃:0.20TPABr:0.16CTMABr:55H₂O. The cetyltrimethylammonium bromide (CTMABr) and tetrapropylammonium bromide (TPABr) were used as templates. The above mentioned material presents morphological properties with specific characteristics, such as the surface area of the composite which is approximately half of the surface area of the conventional MCM-41. Another interesting feature is the formation of walls with the double of the density of the MCM-41 structure, which characterizes the hybrid material, resulting in a high stability material for catalytic application. The aim of this study is obtain optimized structures of the hybrid material and for this purpose variations in the synthesis time were carried out. A comparative analysis was performed including X-ray diffraction, Fourier transform infrared spectroscopy, and Thermogravimetry measurements. The model-free kinetic algorithms were applied in order to determinate conversion and apparent activation energy of the decomposition of the CTMA⁺ and TPA⁺ species from the hybrid AlMCM-41/ZSM-5.     

嫁接Al改性MCM-41介孔分子筛催化合成双酚F

以十六烷基三甲基溴化铵和正硅酸乙酯为原料,合成纯硅MCM-41介孔分子筛。 再利用九水硝酸铝为改性剂来嫁接改性纯硅MCM-41介孔分子筛,NH3-TPD结果表明,嫁接后的AlMCM-41产生了中强酸。 用改性后不同n(Si)/n(Al)的AlMCM-41催化合成双酚F,在n（苯酚）/n（甲醛）=30、反应时间5 h、反应温度90 ℃、m（甲醛）/m（AlMCM-41）=7的反应条件下,在一定范围内,双酚F的产率随Al含量增加而增加,但在n（Si）/n（Al）＜50时,双酚F的产率反而下降,当n（Si）/n（Al）=50时,双酚F的产率最高,为42.28%。     

Influence of cross profile of PE fibers on thermal properties of materials

Molecular sieves MCM-41 were synthesized from rice husk ash (RHA) as alternative sources of silica, called RHA MCM-41. The material was synthesized by a hydrothermal method from a gel with the molar composition 1.00 CTMABr:4.00 SiO₂:1.00 Na₂O:200.00 H₂O at 100 °C for 120 h with pH correction. The cetyltrimethylammonium bromide (CTMABr) was used as a structure template. The material was characterized by X-ray powder diffraction, FTIR, TG/DTG, and surface area determination by the BET method. The kinetics models proposed by Ozawa, Flynn–Wall, and Vyazovkin were used to determine the apparent activation energy for CTMA⁺ species decomposition from the pores of MCM-41 material. The results were compared with those obtained from the MCM-41 synthesized with silica gel. The synthesized material had specific surface area, size, and pore volume as specified by mesoporous materials developed from conventional sources of silica.     

Thermogravimetric Investigations During the Synthesis of Silica-based MCM-41

MCM-41 material was synthesized starting from hydrogel containing colloidal fumed silica, sodium silicate, cetyltetramethylammonium bromide(CTMABr) as surfactant, and distilled water as solvent. These reactants were mixed to obtain a gel with the following composition: 4SiO₂:1Na₂O:1CTMABr:200H₂O. The hydrogel with pH=14 was hydrothermally treated at100°C, for 4 days. Each day, the pH was measured, and then adjusted to 9.5–10 by using 30%acetic acid solution. Thermogravimetry was the main technique, which was used to monitor the participation of the surfactant on the MCM-41 nanophase, being possible to determine the temperature ranges relative to water desorption as well as the surfactant decomposition and silanol condensation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic study of template removal of MCM-41 nanostructured material

The siliceous MCM-41 molecular sieve was synthesized starting from a hydrogel with the following molar composition: 4.58SiO₂:0.437Na₂O:1CTMABr:200H₂O. The cetyltetramethylammonium bromide (CTMABr) was used as structure template. A kinetic study of template removal after the syntheses was performed by Vyazovkin model-free kinetic method obtaining apparent activation energy of 166±8.2 kJ mol^-1. This revised version was published online in August 2006 with corrections to the Cover Date.     

Calorimetric determination of enthalpy changes for the proton ionization of glycine, N,N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) in water-methanol mixtures

Enthalpies for the two proton ionizations of glycine, N,N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) were obtained in water-methanol mixtures with methanol mole fraction (X_m) from 0 to 0.360. With increasing methanol the ionization enthalpy for the first proton (ΔH₁) of glycine increased from 4.4 to 9.4 kJ mol⁻¹ with a minimum of 4.1 kJ mol⁻¹ at X_m = 0.059. The ionization enthalpy of the second proton (ΔH₂) for glycine decreased from 46.3 to 38.1 kJ mol⁻¹. ΔH₁ of bicine increased from 3.5 to 7.6 kJ mol⁻¹ at X_m = 0.273 before dropping to 4.1 kJ mol⁻¹ at X_m = 0.360. ΔH₂ of bicine increased from 24.9 to 29.4 kJ mol⁻¹. For tricine, ΔH₁ increased from 6.7 to 9.8 kJ mol⁻¹ at X_m = 0.194 then dropped to 7.4 kJ mol⁻¹ at X_m = 0.360. ΔH₂ for tricine first dropped from 30.8 to 28.5 kJ mol⁻¹ at X_m = 0.059 before increasing to 33.3 kJ mol⁻¹ at X_m = 0.273. The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent-solvent and solvent-solute interactions.     

Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX₃)₂ and Pd(X₂PCH₂CH₂PX₂) where X = Me, H, Cl. It was shown that the C_ipso-Br activation energy by Pd(PX₃)₂ correlates well with the rigidity of the X₃P-Pd-PX₃ angle and increases via the trend X = Cl < H < Me. The more rigid the X₃P-Pd-PX₃ angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX₃ bond strength increasing faster than the Pd(0)-PX₃ bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX₃)₂, the C_ipso-Br activation energy by Pd(X₂PCH₂CH₂PX₂) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d_π orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X₂PCH₂CH₂PX₂ than for the case of PX₃. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl).     

Structure and reactivity of derivatives of dihalogenomethyl indium(III) halides, X2InCHX2 (X = Cl, Br, I)

The reactions of indium monohalides, InX with haloforms, CHX₃, in 1,4-dioxane (diox), produce the dioxane adducts of dihalogeno-dihalogenomethyl-indium(III), X₂In(diox)_nCHX₂ (X = Cl, Br, n = 1; X = I, n = 2) compounds. The ionic derivative [(C₂H₅)₄N] [Cl₃InCHCl₂] was prepared and its crystal structure determined by X-ray means. The reactions of the X₂In(diox)_nCHX₂ compounds are significantly different from those of the related X₂InCH₂X compounds. The dihalogenomethyl derivatives react with strong electrophiles suggesting dihalogenomethyl substituents of mild nucleophilic character, while the carbon atoms in the halogenomethyl derivatives are electrophilic.     

A rare multi-coordinate tellurite, NH4ATe4O9·2H2O (ARb or Cs): The occurrence of TeO3, TeO4, and TeO5 Polyhedra in the same material

Two new tellurites, NH₄RbTe₄O₉·2H₂O and NH₄CsTe₄O₉·2H₂O have been synthesized and characterized. The compounds were synthesized hydrothermally, in near quantitative yields, using the alkali metal halide, TeO₂, and NH₄OH as reagents. The iso-structural materials exhibit layered, two-dimensional structural topologies consisting of TeO_x (x=3, 4, or 5) polyhedra separated by NH₄⁺, H₂O, Rb⁺ or Cs⁺ cations. Unique to these materials is the presence of TeO₃, TeO₄, and TeO₅ polyhedra. Thermogravimetric and infrared spectroscopic data are also presented. Crystal data: NH₄RbTe₄O₉·2H₂O: Monoclinic I2/a (no. 15), a=18.917(3) Å, b=6.7002(11) Å, c=21.106(5) Å, β=101.813(2)°, V=2618.5(9) Å³, Z=8; NH₄CsTe₄O₉·2H₂O: Monoclinic I2/a (no. 15), a=18.9880(12) Å, b=6.7633(4) Å, c=21.476(2) Å, β=102.3460(10)°, V=2694.2(3) Å³, Z=8.     

Determination of thermodynamic properties of aqueous mixtures of MgCl2 and Mg(NO3)2 by the potentiometric method at T = 298.15

In this research, thermodynamic properties of the ternary electrolyte system (MgCl₂ + Mg(NO₃)₂ + H₂O) were investigated using a potentiometric method. The galvanic cell used had no liquid junction of type: Mg-ISE|MgCl₂ (m_A), Mg(NO₃)₂ (m_B), H₂O|Ag/AgCl. The measurements were performed at T = 298.15 K and at total ionic strengths from 0.001 to 8.000 mol/kg for different series of salt ratios r=mMgCl₂/mMg₂(NO₃) =1.00, 2.50, 5.00, 7.50, 10.00 and 15.00. The PVC based magnesium ion-selective electrode (Mg-ISE) and the Ag/AgCl electrode used in this work were prepared in our laboratory and showed a reasonably good Nernst response. The Pitzer ion interaction model and Harned rule were used to illustrate the ternary electrolyte system investigated. The experimental results showed that both Pitzer model and Harned rule were suitable to be used satisfactorily to describe this ternary system.     

Calorimetric determination of enthalpy changes for proton ionization of N-[2-hydroxyethyl]piperazine-N′-[2-ethane sulfonic acid] (HEPES) and N-[2-hydroxyethyl]piperazine-N′-[2-hydroxypropane sulfonic acid] (HEPPSO) in water-methanol mixtures

Enthalpies for the two proton ionizations of the biochemical buffers N-[2-hydroxyethyl]piperazine-N′-[2-ethane sulfonic acid] (HEPES) and N-[2-hydroxyethyl]piperazine-N′-[2-hydroxypropane sulfonic acid] (HEPPSO) were obtained in water-methanol mixtures with methanol mole fraction (X_m) from 0 to 0.360. With increasing methanol, the ionization enthalpy for the first proton (ΔH₁) of HEPES increased steadily from 8.4 to 15.3 kJ mol⁻¹ whereas that for HEPPSO rose to a maximum of 21.0 kJ mol⁻¹ at X_m = 0.123 before dropping to 18.4 kJ mol⁻¹ at X_m = 0.360. The ionization enthalpy for the second proton (ΔH₂) of HEPES varied from 20.8 kJ mol⁻¹ in water to 13.6 kJ mol⁻¹ at X_m = 0.360 with a maximum of 24.8 kJ mol⁻¹ at X_m = 0.194. For HEPPSO, ΔH₂ increased steadily from 23.4 to 29.2 kJ mol⁻¹. The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent-solvent and solvent-solute interactions.     

DFT and ab initio theoretical study for the CF3S + CO reaction

The potential energy surface for the reaction of CF₃S with CO is calculated at the G4//B3LYP/6-311++G(d,p) level of theory. The results show that F-abstraction and addition-elimination mechanisms are involved, and the latter one is dominant thermodynamically and kinetically. The dominant channel is the reactant addition to form CF₃SCO, and then decomposes to CF₃ + OCS. While the direct F-abstraction channel and CF₃SCO isomerization channel are not significant for the title reaction due to higher barriers involved. The comparisons among four reactions of CX₃Y + CO (X = H, F; and Y = O, S) are made to imply the similar and different properties and reactivities of the same family elements and the F- and S-substituted derivatives.     

From discrete entity to three-dimensional architecture: Syntheses,crystal structures and optical properties of a series of transition metal complexes constructed from 4-carboxymethylbenzoic acid and 4,4′-bipyidine

Five new interesting transition metal coordination polymers [MnL₂(bpy)₂(H₂O)₂]_n (1) (H₂L = 4-carboxymethylbenzoic acid) (bpy = 4,4′-bipyidine), [CoL(bpy)(H₂O)₃ · H₂O]_n (2), [CdL(bpy)(H₂O)₃ · H₂O]_n (3), [Cu₂L(bpy)₂ · 3H₂O]_n (4) and [Zn₂L₂(bpy) · H₂O]_n (5) have been synthesized under solvothermal conditions and structurally characterized. This series of complexes has shown an intriguing variety of architectures, which firstly form zero- to two-dimensional frameworks by metal–ligand interactions, and secondly form three-dimensional supramolecular frameworks by intermolecular interactions such as hydrogen bonds. Compound 3 shows strong blue fluorescent emissions in the solid state upon photo-excitation at 359 nm at room temperature and may be an excellent candidate for blue-fluorescent materials. Compound 4 appears to be a good candidate for new hybrid inorganic–organic NLO materials.     

Chelating ionic versus bridged molecular structures of group 13 metal complexes with bidentate ligands

Complexes of aluminum and gallium trihalides with ethylenediamine (en) and N,N,N′,N′-tetramethylethylenediamine (tmen) of 2:1 composition have been synthesized and structurally characterized by single crystal X-ray diffraction analysis. In contrast to known molecular complexes of hydrido and methyl-substituted analogs, these solid complexes adopt ionic structures of the general type [M¹X₂LL]⁺[M²X₄]⁻ (X = Br, I; M¹, M² = Al or Ga; LL = en, tmen).     

Influence of some reaction conditions on the obtaining of tetra- and dinuclear zinc complexes of some Schiff bases derived from 2,6-diformyl-4-alkyl-phenols

A template 2:2:4 condensation of 2,6-diformyl-4-methyl-phenol, triethylenetetramine and zinc acetate gave rise to the crystallisation of [{Zn₄(H₄L¹)(OAc)₄}{Zn(OAc)₃(H₂O)}(OAc)] · 7H₂O (1 · 7H₂O), being H₆L¹ a macrocyclic diphenolate Schiff base ligand. Changing some operation conditions, other template reactions yielded dinuclear complexes of the type Zn₂(Lⁿ)(OAc) · xH₂O, where H₃Lⁿ (n = 2, 3) are podant triphenolate Schiff base ligands derived from a 3:1 condensation of the corresponding 2,6-diformyl-4-alkyl-phenol (alkyl = Me or Bu^t, respectively) and triethylenetetramine. After recrystallisation, these two latter complexes could be X-ray characterised as Zn₂(L²)(OAc) · 1.25H₂O · 0.5MeCN (2 · 1.25H₂O · 0.5MeCN), and Zn₂(L³)(OAc) (3). Furthermore, after addition of a 3:1 molar ratio of 2-amino-4-methyl-phenol to 3, this underwent imidazolidine hydrolysis and a double imine condensation, yielding Zn₂(L⁴)(OAc)(HOAc) · 2H₂O (4 · 2H₂O), where H₃L⁴ is an acyclic pentadentate Schiff base derived from the 1:2 condensation of 2,6-diformyl-4-tert-butyl-phenol and 2-amino-4-methyl-phenol.     

Lanthanide N,N′-piperazine-bis(methylenephosphonates) (Ln=La, Ce, Nd) that display flexible frameworks, reversible hydration and cation exchange

Hydrothermal syntheses of lanthanide bisphosphonate metal organic frameworks comprising the light lanthanides lanthanum, cerium and neodymium and N,N′-piperazine bis(methylenephosphonic acid) (H₂L(1) and its 2-methyl and 2,5-dimethyl derivatives (H₂L(2) and H₂L(3)) gives three new structure types. At elevated starting pH (ca. 5 and above) syntheses give ‘type I’ materials with all metals and acids of the study (MLnLxH₂O, M=Na, K, Cs; Ln=La, Ce, Nd; x≈4: KCeL(1)·4H₂O, C2/c, a=23.5864(2) Å, b=12.1186(2) Å, c=5.6613(2) Å, β=93.040(2)°). The framework of structure type I shows considerable flexibility as the ligand is changed, due mainly to rotation around the -N-CH₂- bond of the linker in response to steric considerations. Type I materials demonstrate cation exchange and dehydration and rehydration behaviour. Upon dehydration of KCeL·4H₂O, the space group changes to P2₁/n, a=21.8361(12) Å, b=9.3519(4) Å, c=5.5629(3) Å, β=96.560(4)°, as a result of a change of the piperazine ring from chair to boat configuration. When syntheses are performed at lower pH, two other structure types crystallise. With the ‘non-methyl’ ligand 1, type II materials result (LnL(1)H₂L(1)·4.5H₂O: Ln=La, P−1, a=5.7630(13) Å, b=10.213(2) Å, c=11.649(2) Å, α=84.242(2)°, β=89.051(2)°, γ=82.876(2)°) in which one half of the ligands coordinate via the piperazine nitrogen atoms. With the 2-methyl ligand, structure type III crystallises (LnHL(2)·4H₂O: Ln=Nd, Ce, P2₁/c, a=5.7540(9) Å, b=14.1259(18) Å, c=21.156(5) Å, β=90.14(2)°) due to unfavourable steric interactions of the methyl group in structure type II.     

Thermodynamic investigation of the moderately dilute liquid Bi-Fe-Sb alloys

The molar enthalpies of formation of liquid ternary Bi-Fe-Sb alloys have been determined at 1065 K by direct reaction calorimetry using the drop method. Measurements were performed along two sections: X_Bi/X_Sb = 1/9 and 1/4. The experimental calorimetric data and estimated values with a Redlich-Kister formulation are compared.     

The effect of polyether terminal chains in the liquid crystalline behavior of ortho-palladated complexes

A series of benzylideneanilines bearing terminal polyether chains, HL (HL = R-C₆H₄-CHN-C₆H₄-R′: R = OC₈H₁₇, R′ = O(CH₂CH₂O)₂C₂H₅; R = O(CH₂CH₂O)₂C₂H₅, R′ = OC₈H₁₇; R = R′ = O(CH₂CH₂O)₂C₂H₅; R = OC₁₂H₂₅, R′ = O(CH₂CH₂O)₃C₂H₅; R = O(CH₂CH₂O)₃C₂H₅, R′ = OC₁₂H₂₅; R = R′ = O(CH₂CH₂O)₃C₂H₅) have been prepared. Their dinuclear, [Pd(μ-X)L]₂ (X = OAc, Cl, Br, SC₈), [Pd₂(μ-SCn)(μ-X)L₂] (X = OAc, Cl; n = 8, 2) and mononuclear orthopalladated derivatives, Pd(acac)L, Pd(Ala)L, are reported and their mesogenic properties are compared with those of the analogous compounds with alkoxy chains. In general a great lowering in the melting points is produced for all the products. The free ligands and the alanine complexes are not liquid crystals. The chloro-bridged complexes bearing alkoxy and short polyether chains (O(CH₂CH₂O)₂C₂H₅) show the larger improvement of mesogenic properties. Longer polyether chains (O(CH₂CH₂O)₃C₂H₅) result usually in a destabilization of the mesophases. If only polyether chains are present, the destabilization is important regardless of the chain length. The ability of these molecules as ionic extractants and transporters was qualitatively evaluated for the more propitious cis-dinuclear complexes, which in fact showed some extracting ability, modest but improved compared to the free ligands.