Carboxylic groups in mesoporous silica and ethane-bridged organosilica: effect of the surface on the reactivity

Proton-donor ability of carboxylic groups incorporated by co-condensation into SBA-15 and ethane-bridged periodic mesoporous organosilica (PMO) has been studied through IR spectroscopy by dosing ammonia, which forms reversibly COO(-) groups and NH(4)(+) ions. The related equilibrium constants, determined by elaboration of IR data, reveal a lower reactivity of -COOH groups at the surface of PMO than on SBA-15, when the two samples have been outgassed at the same temperature. This finding is interpreted in terms of different dielectric constants and intermolecular interactions engaged with the surface species. Carboxylic groups on ethane-bridged organosilica react with silanols upon thermal treatment at 473 K to form a mixed anhydride species Si-O-C(O)-, at variance with the same groups on SBA-15.     

Synthesis of carboxyl-modified rod-like SBA-15 by rapid co-condensation

Carboxyl-modified SBA-15 rod-like mesoporous materials have been synthesized by a facile rapid co-condensation of tetraethylorthosilicate (TEOS) and 2-cyanoethyltriethoxysilane (CTES), followed by hydrolysis of cyanide groups in sulfuric acid. The concentration of carboxylic groups was varied by changing the silica source ratio of CTES/TEOS from 0.05 to 0.3. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the uniform ordered mesoporous structure and rod-like morphology of SBA-15 have been preserved even at the high concentration of carboxylic groups employed. Characterization by Fourier transformed infrared spectroscopy (FTIR), solid-state NMR investigation indicated that carboxylic groups have been successfully grafted onto the surface of SBA-15 through siloxane bonds [(O(3))SiCH(2)CH(2)COOH. The negative charges of the modified SBA-15 materials were enhanced by the presence of the carboxylic groups on the surface. The capacity of lysozyme adsorption of the modified SBA-15 materials were found to be significantly improved as compared with pure silica SBA-15. The maximum amount of lysozyme adsorption on carboxyl-modified was increased with the pH of solution increased from 5.5 to 9.0.     

Controlled drug release from bifunctionalized mesoporous silica

Serial of trimethylsilyl-carboxyl bifunctionalized SBA-15 (TMS/COOH/SBA-15) have been studied as carriers for controlled release of drug famotidine (Famo). To load Famo with large capacity, SBA-15 with high content of carboxyl groups was successfully synthesized by one-pot synthesis under the assistance of KCl. The mesostructure of carboxyl functionalized SBA-15 (COOH/SBA-15) could still be kept even though the content of carboxyl groups was up to 57.2%. Increasing carboxyl content could effectively enhance the loading capacity of Famo. Compared with pure SBA-15, into which Famo could be hardly adsorbed, the largest drug loading capacity of COOH/SBA-15 could achieve 396.9 mg/g. The release of Famo from mesoporous silica was studied in simulated intestine fluid (SIF, pH=7.4). For COOH/SBA-15, the release rate of Famo decreased with narrowing pore size. After grafting TMS groups on the surface of COOH/SBA-15 with hexamethyldisilazane, the release of Famo was greatly delayed with the increasing content of TMS groups.     

Direct evidence for interactions between acidic functional groups and silanols in cubic mesoporous organosilicas

A facile one-pot synthesis route for preparation of a well-ordered cubic mesoporous silica SBA-1 functionalized with -COOH functional groups is reported for the first time. The results of 29Si{1H} HETCOR NMR provide direct evidence for the interactions between the carboxylic acid and silanol groups in carboxylic acid functionalized SBA-1. Density functional theory calculations indicate that the T3-Q4-Q3 motif is the favorable framework composition in the material and the carboxylic protons in the T3 species can form hydrogen bonds with the spatially proximate oxygen atom in the Q3 Si-OH species.     

Retention of heavy metal ions on SBA-15 mesoporous silica functionalised with carboxylic groups

Three samples of SBA-15 functionalised with -(CH(2))(3)COOH groups have been prepared by co-condensation, starting from solutions of TEOS and 4-(triethoxysilyl)butyronitrile, acting as -(CH(2))(3)COOH precursor, of different molar compositions. Materials were characterised by X-ray diffraction, nitrogen adsorption, and FT-IR spectroscopy. The pK(a) and the acidic capacity were measured for all samples by potentiometric titration. The acidic capacity increases with increasing amount of -COOH precursor in the synthesis mixture only up to 10% molar of total alkoxysilane. The value for the pK(a)(4.75) is independent of the acidic capacity of the material. The sample prepared starting from an amount of -COOH precursor equal to 10% molar of total alkoxysilane was chosen to test selective interactions with heavy metals of environmental importance (Ag(+), Cd(2+), Fe(3+), Cu(2+), Zn(2+)) at different pH values and ionic strengths. The significant and selective adsorption exhibited by the material has been exploited in a preliminary cation-exchange chromatographic application showing the possibility of eluting the metal ions at different retention times.     

Synthesis,Bifunctionalization, and Remarkable Adsorption Performance of Benzene‐Bridged Periodic Mesoporous Organosilicas Functionalized with High Loadings of Carboxylic Acids

Highly ordered benzene‐bridged periodic mesoporous organosilicas (PMOs) that were functionalized with exceptionally high loadings of carboxylic acid groups (COOH), up to 80 mol % based on silica, have been synthesized and their use as adsorbents for the adsorption of methylene blue (MB), a basic dye pollutant, and for the loading and release of doxorubicin (DOX), an anticancer drug, is demonstrated. These COOH‐functionalized benzene? silicas were synthesized by the co‐condensation of 1,4‐bis(triethoxysilyl) benzene (BTEB) and carboxyethylsilanetriol sodium salt (CES), an organosilane that contained a carboxylic acid group, in the presence of non‐ionic oligomeric surfactant Brij 76 in acidic medium. The materials thus obtained were characterized by a variety of techniques, including powder X‐ray diffraction (XRD), nitrogen‐adsorption/desorption isotherms, TEM, and ¹³C and ²⁹Si solid‐state NMR spectroscopy. Owing to the exceptionally high loadings of COOH groups, their high surface areas, and possible π? π‐stacking interactions, these adsorbents have very high adsorption capacities and extremely rapid adsorption rates for MB removal and for the controlled loading/release of DOX, thus manifesting their great potential for environmental and biomedical applications.     

SBA-15分子筛羟基结构性质及其催化活性的理论研究

采用密度泛函方法对SBA-15分子筛簇模型化合物进行了计算模拟,重点研究了簇模型化合物羟基的几何构型和电子结构性质。从键级、前线分子轨道、静电势和质子化能等方面探讨了表面羟基的酸碱及氧化还原性质。分子筛模型化合物的静电势图显示SBA-15分子筛表面孤立羟基的H原子为分子筛表面的L酸性位,而氢键羟基的O原子则为分子筛表面的B碱性位。分子筛的前线分子轨道研究发现,表面羟基是分子筛表面的氧化还原活性位。计算得到的羟基SiO-H键级及形成的氢键键级分别在0.677 5～0.710 5和0.055 7～0.092 6范围,计算羟基OH的质子化能在1 471～1 589 kJ.mol-1范围。考察分子筛表面的质子化能显示未参与形成氢键的羟基H质子具有较强的B酸性。     

Adsorption and structural properties of channel-like and cage-like organosilicas

Channel-like and cage-like mesoporous silicas, SBA-15 (P6mm symmetry group) and SBA-16 (Im3m symmetry group), were modified by introducing single ureidopropyl surface groups, mixed ureidopropyl and mercaptopropyl surface groups, and single bis(propyl)disulfide bridging groups. These hexagonal and cubic organosilicas were prepared under acidic conditions via co-condensation of tetraethyl orthosilicate (TEOS) and proper organosilanes using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) amphiphilic block copolymer templates, P123 (EO₂₀PO₇₀EO₂₀) and F127 (EO₁₀₆PO₇₀EO₁₀₆). The modified SBA-15 and SBA-16 materials were synthesized by varying the molar ratio of organosilane to TEOS in the initial synthesis gel. The removal of polymeric templates, P123 and F127, was performed with ethanol/hydrochloric acid solution. In the case of SBA-15 the P123 template was fully extracted, whereas this extraction process was less efficient for the removal of F127 template from the SBA-16-type organosilicas; in the latter case a small residue of F127 was retained. The adsorption and structural properties of the resulting materials were studied by nitrogen adsorption-desorption isotherms at −196^∘C (surface area, pore size distribution, pore volumes), powder X-Ray diffraction, CHNS elemental analysis and high-resolution thermogravimetry. The structural ordering, the BET specific surface area, pore volume and pore size decreased for both channel-like and cage-like mesoporous organosilicas with increasing concentration of incorporated organic groups.     

Significant improvement in the pore properties of SBA-15 brought about by carboxylic acids and hydrothermal treatment

A comparative study of the pore properties of SBA-15 samples prepared under nonhydrothermal and hydrothermal conditions, in the absence and presence of carboxylic acids such as succinic, tartaric and citric acids has been carried out. In the absence of carboxylic acid, flake-like and spheroid particles were generally obtained irrespective of the preparative procedures. On the other hand, stirring of the pre-mix induces a rod-like morphology in presence of carboxylic acids. The samples prepared under non-hydrothermal conditions exhibit a higher degree of silicate condensation compared to those synthesized under hydrothermal conditions. SBA-15 samples prepared under hydrothermal conditions show higher values of the d (100) spacing independent of the presence of carboxylic acids. Presence of carboxylic acids as well as hydrothermal treatment improves the pore properties of SBA-15.     

高分散TiO_2/SBA-15的制备及光催化性能

提出了一种制备高分散TiO2/SBA-15光催化剂的新方法.以钛酸四正丁酯(TB)和羧基改性的SBA-15(COOH/SBA-15)为原料,利用COOH/SBA-15表面上高分散的羧基与TB的配合作用将TB锚定,经过溶剂热及焙烧处理制得TiO2/SBA-15光催化剂.采用粉末X射线衍射(XRD),N2吸脱附,傅里叶变换红外光谱(FT-IR),透射电镜(TEM)等对所得催化剂进行了表征.结果表明:所得TiO2/SBA-15光催化剂为高结晶度的锐钛矿晶型,TiO2均匀地分散在SBA-15表面,TiO2/SBA-15光催化剂保持较好的介孔特征结构,具有较大比表面积.以降解罗丹明B(RhB)为探针反应,考察了所得TiO2/SBA-15光催化剂的光催化性能.与后处理浸渍法制备的光催化剂相比,本文制备的TiO2/SBA-15光催化剂表现出了更加优越的光催化性能.     

Zr-SBA-15介孔微球的制备、表征及其甲醇脱水性能研究(英文)

在SBA-15的合成体系中直接引入锆原子硝酸盐,用氨水调节体系的pH值到4.0~5.0,通过水热晶化处理后得到一系列不同含量的Zr-SBA-15介孔微球。利用XRD、氮吸附、SEM、TEM-EDS、ICP、FT-IR、和漫反射UV-vis等技术对该材料的组成、结构及形貌进行了表征。结果表明,这种杂化材料具有纳米介孔结构、高比表面积和空心微球等特征,空心微球形貌的形成可能与金属杂原子的引入、溶液pH值的调节有关,锆原子被高效率地引入到纳米介孔结构中,表面酸性与未改性前相比有了显著的提高,随着锆含量的增加,样品结构和表面酸性的变化直接影响着催化剂在甲醇脱水反应中的表现。     

Theoretical and experimental studies on the carbon-nanotube surface oxidation by nitric acid: interplay between functionalization and vacancy enlargement

The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.     

Correlation between pore-expanding and dye adsorption of platelet C/SBA-15 prepared by carbonization and oxidation of P123-TMB/SBA-15 composites

Platelet SBA-15 with significantly shortened and larger mesopores were prepared with different dosage of trimethylbenzene (TMB) in the assembly process, and then functionalized with acidic oxygen groups by oxidation of carbon layer that obtained by carbonization of P123-TMB organic species occluded in the silica pores. The preparation procedure involved three steps, namely, (a) synthesis platelet SBA-15 with larger mesopores, (b) carbonization, using P123 in the pore directly as the carbon source, and (c) oxidation with K₂S₂O₈. The resulting oxidation and carbonization of platelet SBA-15 (CST-ox, where C = carbon, S = SBA-15, T = trimethylbenzene, and ox refers to oxidation) composites contained of carbonaceous matter with acid oxygen groups (e.g. –COOH, –C=O and –OH) attached onto the deposited carbon layer. The structural characteristics of the parent silica were retained in the oxidized composite materials, which exhibit high surface area, large pore volume and well-ordered porosity. The oxygen-functionalized CST-ox composites with larger mesopores were found to be excellent adsorbents towards methylene blue. Especially, the adsorption equilibrium time was significantly reduced from 60 to 20 min, and the maximum adsorption capacity was increased from 379 to 420 mg/L, for which may be closely associated with the larger pore size, highly shortened meso-channels and the functionalized carbon layers. The adsorption kinetic data were analysed using pseudo-first-order, pseudo-second-order and Weber’s intraparticle diffusion models. Also equilibrium data were fitted to the Langmuir, Freundlich isotherm models and isotherm constants were determined. Thermodynamic parameters such as changes in the free energy of adsorption (ΔG ⁰), enthalpy (ΔH ⁰) and entropy (ΔS ⁰) were calculated.     

Nb-P/SBA-15催化剂的制备及其对果糖水解制5-羟甲基糠醛的催化性能

果糖催化脱水制5-羟甲基糠醛（5-HMF）是生物质转化制高附加值化合物过程中的一个重要反应。采用自制的介孔SBA-15,浸渍法制备了Nb/SBA-15催化剂,再经磷酸化处理制得Nb-P/SBA-15催化剂,研究了该催化剂在果糖脱水制5-HMF反应中的性能。SEM、TEM、BET和XRD表征结果表明,Nb/SBA-15和Nb-P/SBA-15完好地保留了SBA-15的微观结构,其内孔道直径为10 nm,铌酸在孔内表面分布均匀;负载铌和磷酸化后,孔壁变薄。NH₃-TPD结果显示,Nb/SBA-15经磷酸预处理后,不仅弱酸性位有所增加,而且产生了中强酸和强酸性位,使得在含水两相体系果糖脱水反应中,Nb-P/SBA-15比Nb/SBA-15具有更高的催化活性和5-HMF选择性。同时考察了反应温度、溶剂比例、反应时间对果糖脱水的影响,结果表明,以水/MIBK（V/V=1/2）为溶剂时,160 ℃下反应1.5 h,果糖转化率和5-HMF收率分别高达96.1%和92.6%。Nb-P/SBA-15经循环使用四次后仍具有较好的催化活性,表明该催化剂具有较高的耐水稳定性。     

Synthesis,spectroscopic characterization,X-ray structure and magnetic properties of [Cu(hmquin-7-COOH)2(MeOH)] complex

The reaction of copper(II) chloride dihydrate and 8-hydroxy-2-methylquinoline-7-carboxylic acid (Hhmquin-7-COOH) leads to [Cu(hmquin-7-COOH)₂(MeOH)]. The compound has been studied by IR, UV–Vis, EPR spectroscopy and X-ray crystallography. X-ray studies confirm bidentate coordination mode of the hmquin-7-COOH anions via the pyridine nitrogen atom and deprotonated hydroxyl group. The COOH groups of the hmquin-7-COOH ligands are potentially available for further conjugation. The title complex has been additionally studied by magnetic measurement. The TDDFT/PCM calculations have been employed to discuss the electronic spectrum of [Cu(hmquin-7-COOH)₂(MeOH)] in more detail.     

Chemical reactions in dense monolayers: in situ thermal cleavage of grafted esters for preparation of solid surfaces functionalized with carboxylic acids

The thermodynamics of a chemical reaction confined at a solid surface was investigated through kinetic measurements of a model unimolecular reaction. The thermal cleavage of ester groups grafted at the surface of solid silica was investigated together with complementary physicochemical characterization of the grafted species. The ester molecules were chemically grafted to the silica surface and subsequently cleaved into the carboxylic acids. A grafting process of a reproducible monolayer was designed using the reaction of monofunctional organosilane from its gas phase. The thermal deprotection step of the ester end-group was investigated. The thermal deprotection reaction behaves in quite a specific manner when it is conducted at a surface in a grafted layer. Different organosilane molecules terminated by methyl, isopropyl and tert-butyl ester groups were grafted to silica surface; such functionalized materials were characterized by elemental analysis, IR and NMR spectroscopy, and thermogravimetric analysis, and the thermodynamic parameters of the thermal elimination reaction at the surface were measured. The limiting factor of such thermal ester cleavage reaction is the thermal stability of grafted ester group according to the temperature order: tert-butyl < i-propyl < methyl. Methyl ester groups were not selectively cleaved by temperature. The thermal deprotection of i-propyl ester groups took place at a temperature close to the thermal degradation of the organofunctional tail of the silane. The low thermolysis temperature of the grafted tert-butyl esters allowed their selective cleavage. There is a definite influence of the surface on the reaction. The enthalpy of activation is lower than in the gas phase because of the polarity of the reaction site. The major contribution is entropic; the negative entropy of activation comes from lateral interactions with the neighbor grafted molecules because of the high grafting density. Such reaction is an original strategy to functionalize the silica surface by carboxylic acid groups by means of a simple, reproducible, and efficient process involving in situ thermolysis of ester groups.     

Preparation, characterization and catalytic activity towards green reactions of sulfonic functionalized SBA-15

Acidic heterogeneous catalysts based on the anchorage of sulfonic groups on SBA-15 mesoporous silica were synthesized. In a first synthesis step, samples containing mercapto groups were prepared by co-condensation of tetraethylorthosilicate with 3-mercaptopropyltrimethoxysilane, in presence of ethylene-propylene block copolymer as mesoporous silica structure director. In other samples, mercapto groups were introduced by post-functionalization of the traditional calcined SBA-15. In a second step, these mercapto groups were oxidized in order to get sulfonic acid groups on the surface. Characterization of the samples was carried out by N₂ adsorption-desorption, FTIR, XPS and acid-base titration. Spectroscopic techniques showed that the effective incorporation of sulfonic groups depends on the synthesis methodology used. In turn, the SBA-15 post-synthesis functionalization produces changes in structural characteristics like a decrease in BET surface and changes in the pore size distribution. The as-prepared materials were tested as acid catalysts in the alkylation of isobutane with 1-butene, and in the esterification of free fatty acids with methanol. The results obtained show a lack of activity in the alkylation reaction which can be associated with the formation and stabilization of the intermediate carbocation species.     

Synthesis of Cu nanoparticles in mesoporous silica SBA-15 functionalized with carboxylic acid groups

In this study, the mesoporous silica SBA-15 materials containing carboxylic acid groups were used as an effective support to synthesize Cu nanoparticles. Various Cu loading levels from 4% to 13% on SBA-15 catalysts produced an average particle size of 2.8 to 3.1 nm, regardless of the Cu content.     

Surface modification of functional self-assembled monolayers on 316L stainless steel via lipase catalysis

Lipase catalyzed esterification of therapeutic drugs to functional self-assembled monolayers (SAMs) on 316L stainless steel (SS) after assembly has been demonstrated. SAMs of 16-mercaptohexadecanoic acid (-COOH SAM) and 11-mercapto-1-undecanol (-OH SAM) were formed on 316L SS, and lipase catalysis was used to attach therapeutic drugs, perphenazine and ibuprofen, respectively, on these SAMs. The reaction was carried out in toluene at 60 degrees C for 5 h using Novozyme-435 as the biocatalyst. The FTIR spectra after surface modification of -OH SAMs showed the presence of the C=O stretching bands at 1745 cm(-1), which was absent in the FTIR spectra of -OH SAMs. Similarly, the FTIR spectra after the reaction of the -COOH SAM with perphenazine showed two peaks in the carbonyl region, a peak at 1764 cm(-1), which is the representative peak for the C=O stretching for esters. The second peak at 1681 cm(-1) is assigned to the C=O stretching of the remaining unreacted terminal COOH. XPS spectra after lipase catalysis with ibuprofen showed a photoelectron peak evolving at 288.5 eV which arises from the carbon (C=O) of the carboxylic acid of the drug (ibuprofen). Similarly for -COOH SAMs, after esterifiation we see a small, photoelectron peak evolving at 286.5 eV which corresponds to the C in the methylene groups adjacent to the oxygen (C-O), which should evolve only after the esterification of perphenazine with the -COOH SAM. Thus, lipase catalysis provides an alternate synthetic methodology for surface modification of functional SAMs after assembly.     

Acid-Containing Tyrosine-Derived Polycarbonates: Wettability and Surface Reactivity

Tyrosine-derived polycarbonates having carboxylic acid pendant groups were characterized by water contact angle and X-ray photoelectron spectroscopy (XPS). A pronounce decrease of receding angle as well as contact angle hysteresis as a function of acid composition strongly indicated that the acid groups are more accessible at the water/polymer interface after hydration. pH dependent contact angle confirmed an existence of carboxylic acid groups in the surface region. The receding angle transition appearing in the pH range of 4-6 was a consequence of hydrophilicity change due to interconverting from carboxylic acid (-COOH) to carboxylate ion (-COO⁻). The surface compositions of imidazole-labeled polymers as analyzed by XPS were consistent with the bulk stoichiometry of the polymers. Reactivity of acid groups towards chemical reaction at the surface was also investigated. The acid groups at the surface of polymers were capable of adsorbing a significant amount of calcium ion from simulated body fluid and being activated by a reaction with N-hydroxysuccinimide.