Mesoporous Xerogel Research Articles

Xerogel mesoporous materials based on ultrastable Blatter radicals for efficient sorption of nitric oxide

The reduction of hazardous nitric oxide emissions remains a significant ecological challenge. Despite the variety of possibilities, sorbents able to capture low concentrations of NO from flue gas with high selectivity are still in demand. In this work a new type of mesoporous xerogel material highly loaded with ultrastable Blatter radicals (BTR, >60 % by mass) that act as selective NO sorption sites is developed. Electron Paramagnetic Resonance (EPR) spectroscopy evidences reversible NO sorption in nanometer-scale pores of BTR-based xerogels and indicates the high NO capacity of such radical-rich sorbent. Efficient NO capture from model flue gas mixture is also evidenced in experiments with a fixed bed reactor. Such advanced properties of new materials as selectivity, strong binding with NO and an ability for mild regeneration via thermodesorption promote them for future ecological applications.

THE ACTIVATION OF ANIONIC DYE ADSORPTION AFTER UV IRRADIATION OF INDIVIDUAL AND BINARY XEROGELS OF ALUMINUM AND ZIRCONIUM OXIDES

Mesoporous xerogels of hydrated individual and binary oxides of aluminum (A) and zirconium (Z) were obtained by hydrolysis sol-gel method after drying at 180 °C of a hydrogel synthesized from aluminum nitrate and zirconium oxychloride in the presence of polyvinylpyrrolidone. The samples were examined by IR spectroscopy, SEM, XRD, BET/BJH. It was found that the xerogels are X-ray amorphous. The particles have a hierarchical structure, the specific surface in a series (number and mol.%) of samples A (1) < 65AZ (2) < 35AZ (3) < Z (4) increases exponentially from 21 to 298 m2/g. The pore diameter is 4-6 nm. Photoactive aluminum oxohydroxide (δAlOOH = 1070 cm-1) is registered only in samples 1 and 2. Adsorption from an aqueous solution using the example of anionic dye methyl orange was determined by spectrophotometer for long-time (24 h) and dynamic (10 min) regime. In both cases, Gibbs adsorption of Гm (μmol/g) increases in a number of samples 1 < 2 < 3 < 4. The value of ГS (μmol/m2) in the same series decreases sharply. Therefore, the adsorption centers of Al+3 and Zr+4 have different activity. It is shown that only in the case of AZ samples a photo activated adsorption rise after UV irradiation of sorbents with a mercury lamp for 10 min of duration. Kinetic adsorption curves for samples without and with UV treatment correspond to the pseudo-second-order equation with Langmuir-type two-fold adsorption model. The parameters of this equation Гmax (monolayer capacity) and W0 (initial adsorption rate) increase in a series of non-irradiated samples 1 < 2 < 3 < 4. The effect of UV activation, as the ratio of Гmax and W0 after and before irradiation, augments in the sequence Z < A < 35AZ < 65AZ (for 65AZ sample the growth of Гmax and W0 is 2.4 and 1.8 times, while the adsorption rate constant k2 decreases tenfold). Comparison of adsorption after 24 h with Гmax showed a linear decrease in the ratio Г24/Гmax for unirradiated sorbents A (2.9) > 65AZ (2.2) > 35AZ (1.6) > I(1.0). This result is explained by adsorption-induced deformation of the xerogel framework, which is greatest in Al2O3 and absent in ZrO2. For citation: Vakhrushev N.E., MIkhalenko I.I., Podzorova L.I. The activation of anionic dye adsorption after UV irradiation of individual and binary xerogels of aluminum and zirconium oxides. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 6. P. 61-68. DOI: 10.6060/ivkkt.20236606.6744.

Selective Impact of MTMS-Based Xerogel Morphology on Boosted Proliferation and Enhanced Naphthoquinone Production in Cultures of Rindera graeca Transgenic Roots.

In situ extraction is a method for separating plant secondary metabolites from in vitro systems of plant biomass cultures. The study aimed to investigate the MTMS-based xerogels morphology effect on the growth kinetics and deoxyshikonin productivity in xerogel-supported in vitro culture systems of Rindera graeca hairy root. Cultures were supplemented with three types of xerogel, i.e., mesoporous gel, microporous gel, and agglomerated precipitate, in the disintegrated or monolithic form. Structure, oil sorption capacity, and SEM analyses for xerogel-based additives were performed. Application of monolithic macroporous xerogel resulted in the highest biomass proliferation, i.e., 5.11-fold fresh biomass increase after four weeks of the screening culture. The highest deoxyshikonin production (i.e., 105.03 µg) was noted when hairy roots were maintained with particles of disintegrated mesoporous xerogel. The detailed kinetics investigations (6-week culture) revealed the highest growth of hairy root biomass and secondary metabolite production, equaling 9.46-fold fresh weight biomass and 204.08 µg deoxyshikonin, respectively. MTMS-based xerogels have been recognized as selective biocompatible scaffolds for boosting the proliferation of transgenic roots or for productivity enhancement of naphthoquinones without detrimental effects on biomass growth, and their successful applicability in in situ removal of secondary plant metabolites has been experimentally confirmed.

Organic Template‐Free Synthesis of Mesoporous ZnO Microparticles by Sol‐Gel Method and Low‐Temperature Hydrothermal Treatment

Mesoporous zinc oxide microparticles were synthesized by the sol‐gel method, followed by hydrothermal treatment at three different temperatures (100°C, 120°C, and 140°C), confirming that the method is fast, simple, reproducible, and ecological. To carry out the synthesis of the samples, zinc acetate was used as a precursor and sodium hydroxide as a precipitating agent. From the results obtained, the sample that shows the highest mesoporosity was the one synthesized at a hydrothermal treatment temperature of 140°C, obtaining an average particle size of 5 μm and a pore diameter of 1.7 nm. The importance of the results obtained was that mesoporous xerogel microparticles were obtained without using any type of organic template.

Mechanically Strong, Scalable, Mesoporous Xerogels of Nanocellulose Featuring Light Permeability, Thermal Insulation, and Flame Self-Extinction.

Scalability is a common challenge in the structuring of nanoscale particle dispersions, particularly in the drying of these dispersions for producing functional, porous structures such as aerogels. Aerogel production relies on supercritical drying, which exhibits poor scalability. A solution to this scalability limitation is the use of evaporative drying under ambient pressure. However, the evaporative drying of wet gels comprising nanoscale particles is accompanied by a strong capillary force. Therefore, it is challenging to produce evaporative-dried gels or "xerogels" that possess the specific structural profiles of aerogels such as mesoscale pores, high porosity, and high specific surface area (SSA). Herein, we demonstrate a structure of mesoporous xerogels with high porosity (∼80%) and high SSA (>400 m2 g-1) achieved by exploiting cellulose nanofibers (CNFs) as the building blocks with tunable interparticle interactions. CNFs are sustainable, wood-derived materials with high strength. In this study, the few-nanometer-wide CNFs bearing carboxy groups were structured into a stable network via ionic inter-CNF interaction. The outline of the resulting xerogels was then tailored into a regular, millimeter-thick, board-like structure. Several characterization techniques highlighted the multifunctionality of the CNF xerogels combining outstanding strength (compression E = 170 MPa, σ = 10 MPa; tension E = 290 MPa, σ = 8 MPa), moderate light permeability, thermal insulation (0.06-0.07 W m-1 K-1), and flame self-extinction. As a potential application of the xerogels, daylighting yet insulating, load-bearing wall members can be thus proposed.

Synthesis of high surface area aluminophosphate and -phosphonate xerogels by non-hydrolytic sol-gel reactions

We disclose the preparation of high-surface-area mesoporous aluminophosphates and aluminophosphonates by the non-hydrolytic sol-gel reactions (NHSG) of Al(NMe 2 ) 3 with trimethylsilylated phosphate OP(OSiMe 3 ) 3 , phosphonates RP(O)(OSiMe 3 ) 2 (R = Me, t Bu, Ph), and bis-phosphonates (Me 3 SiO) 2 (O)P–X–P(O)(OSiMe 3 ) 2 (X = C 6 H 4 , (C 6 H 4 ) 2 ) in dry toluene. The reactions proceed by silylamine elimination of Me 3 SiNMe 2 to provide organic-inorganic hybrid xerogels with properties influenced by organic substituents and the Al:P ratio of the precursors. Dried xerogels exhibit large surface areas (up to 1000 m 2 g −1 ) and matrices based on condensed Al–O–P networks. They stay stable under relatively harsh thermal conditions. 27 Al, 13 C, and 29 Si solid-state NMR spectroscopy was employed to characterize the aluminum coordination and the residual amido and trimethylsilyl groups. The catalytic performance of NHSG prepared material was examined in gas-phase dehydration of ethanol to ethylene exhibiting conversion and selectivity comparable to weak solid acid benchmark catalysts. The number of unreacted surface groups was determined by gravimetric measurements and by thermal analysis (TG-DSC). These residual groups have the potential to be used in post-synthetic grafting of catalytically active metal centers. • Aluminophosphates and aluminophosphonates are prepared by non-hydrolytic sol-gel synthesis. • Mesoporous xerogels with large surface areas (1030 m 2 g -1 ). • Gas-phase dehydration of ethanol to ethylene with good conversion and selectivity.

Enhanced Catalytic Properties of Carbon supported Zirconia and Sulfated Zirconia for the Green Synthesis of Benzodiazepines

AbstractThis work reports for the first time a new series of promising porous catalytic carbon materials, functionalized with Lewis and Brønsted acid sites useful in the green synthesis of 2,3‐dihydro‐1H‐1,5‐benzodiazepine – nitrogen heterocyclic compounds. Benzodiazepines and derivatives are fine chemicals exhibiting interesting therapeutic properties. Carbon materials have been barely investigated in the synthesis of this type of compounds. Two commercial carbon materials were selected exhibiting different textural properties: i) Norit RX3 (N) as microporous sample and ii) mesoporous xerogel (X), both used as supports of ZrO2 (Zr) and ZrO2/SO42− (SZr). The supported SZr led to higher conversion values and selectivities to the target benzodiazepine. Both chemical and textural properties influenced significantly the catalytic performance. Particularly relevant are the results concerning the non‐sulfated samples, NZr and XZr, that were able to catalyze the reaction leading to the target benzodiazepine with high selectivity (up to 80 %; 2 h). These results indicated an important role of the carbon own surface functional groups, avoiding the use of H2SO4. Even very low amounts of SZr supported on carbon reveal high activity and selectivity. Therefore, the carbon materials herein reported can be considered an efficient and sustainable alternative bifunctional catalysts for the benzodiazepine synthesis.

Comparative Study of Powders Based on the ZrO2–Y2O3–СeO2 System Obtained by Various Liquid Phase Methods of Synthesis

Using the liquid-phase methods of synthesis—the coprecipitation of hydroxides and the hydrothermal method—mesoporous xerogels are obtained based on the ZrО2–Y2О3–CeО2 system with 5–8-nm particles and powders (after the heat treatment of xerogels at 600°C) with a coherent scattering region (CSR) size of 9 to 10 nm and Ssp = 96–156 m2/g. After calcination at 1400°C, the powders are transformed into tetragonal solid solutions with a CSR size of 65 nm in the synthesis by the coprecipitation method, and they are transformed into solid solutions with a CSR size of 84 nm with a high degree of tetragonality of c/a = 1.438–1.431 in the synthesis by the hydrothermal method.

Synthesis and comparative studies of xerogels, aerogels, and powders based on the ZrO2–Y2O3–СeO2 system

The technology of liquid-phase synthesis of mesoporous xerogels and aerogels based on ZrO2–Y2O3–CeO2 is developed. Xerogels are obtained by the coprecipitation of hydroxides, while aerogels are obtained in accordance with the sol–gel technology: the average pore size is 1.5–17.2 nm and the specific surface area is 120–878 m2/g. Aerogels are characterized by a high degree of porosity: the pore volume attains 1–4 cm3/g. Based on precursor xerogels, nanopowders of a tetragonal solid solution of the (ZrО2)0.92(Y2О3)0.03(CeО2)0.05 composition with a particle size of 5–9 nm and S spec = 74 m2/g were fabricated. Due to the high values of their specific surface area, the synthesized xerogels and aerogels are promising as sorbents, catalysts, or catalyst supports.

Synthesis and study of mesoporous xerogels and nanopowders of a metastable solid solution 97ZrO2–3Y2O3 for the fabrication of catalyst substrates

The—technology of the liquid-phase synthesis of metastable phases in the ZrO2–Y2O3 system has been developed. Mesoporous xerogels with the specific surface area of ~350 m2/g and monophase nanopowders (5–10 nm) of the tetragonal solid solution (ZrO2)0.97(Y2O3)0.03 have been obtained and their structural peculiarities have been revealed. The efficiency of the suggested technology and good prospects of the synthesized ZrO2-based precursors in creating catalysts’ substrates have been demonstrated.

Waterborne Colloidal Polymer/Silica Hybrid Dispersions and Their Assembly into Mesoporous Poly(melamine-formaldehyde) Xerogels.

The acid-catalyzed polycondensation of oligo(melamine-formaldehyde) in aqueous phase and in the presence of silica nanoparticles leads to a stable dispersion of coexisting silica and polymer nanoparticles. The dispersion can be processed into mesoporous xerogels (SBET ≈ 200 m(2) g(-1)), whose porosity can be enhanced by etching of silica up to specific surface areas of >400 m(2) g(-1). The formation mechanism and the characteristics of the hybrid dispersion are crucial to the materials derived from it and analyzed in detail using a variety of experimental techniques (electron and force microscopy, light and X-ray scattering, ultracentrifugation, and spectroscopy). The transformation of the dispersion into xerogels by electrostatic destabilization is described. Furthermore, the obtained materials are characterized with regard to their porosity and morphology using microscopy and porosimetry. The impact of selected synthesis parameters on the obtained properties is discussed, and it was found (most interestingly) that stable porosity was only observed if silica nanoparticles were present within the dispersion.

Templated non-hydrolytic synthesis of mesoporous zirconium silicates and their catalytic properties

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous zirconium silicate xerogels is presented. The condensation between silicon acetate, Si(OAc)4, and Zr(NEt2)4 resulting in acetamide elimination leads to homogeneous zirconium silicate xerogels containing Si–O–Zr linkages. The addition of Pluronic P123 template provides stiff gels that are after template removal by calcination at 500 °C in air converted to stable mesoporous xerogels with wormhole-type pores, high surface area over 500 m2 g−1, and tetrahedrally coordinated Zr atoms in the framework. The composition and morphology of the xerogels, volatile reaction byproducts, and thermal transformations were followed by elemental analysis, IR spectroscopy, thermal analysis TG-DSC, nitrogen adsorption, 13C and 29Si solid-state NMR spectroscopy, DRUV–Vis spectroscopy, SAXS, and HT powder XRD. These potential catalysts were tested for the Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone and for aminolysis of styrene oxide with aniline. Resulting reaction systems display good activity and selectivity.

Sol-gel synthesis and adsorption properties of mesoporous manganese oxide

Sol-gel synthesis of mesoporous xerogels of manganese oxide with different phase compositions has been performed. The manganese oxide sols were obtained by redox reactions of potassium permanganate with hydrogen peroxide or manganese(II) chloride in aqueous solutions. The isotherms of the low-temperature adsorption-desorption of nitrogen with manganese oxide xerogels treated at 80, 200, 400, and 600°C were measured. The samples were studied by electron microscopy and thermal and XRD analysis. The phase transformation and the changes in the adsorption and capillary-condensation properties of manganese oxide were shown to depend on the sol synthesis conditions and the temperature of the thermal treatment of the gel. The X-ray amorphous samples heated at 80°C were shown to have low values of the specific surface; at higher temperatures, the xerogel crystallized into mixed phases with various compositions and its surface area increased at 200–400°C and decreased at 600°C.

A biomimetic approach to strengthen and protect construction materials with a novel calcium-oxalate–silica nanocomposite

A novel biomimetic nanocomposite composed of amorphous silica and calcium oxalate has been efficiently synthesised by incorporating materials of low toxicity and cost, imitating a stable binary system occurring in well-preserved areas of monuments, plant biomineralisation and the industrial processing of sugar. A simple two-step and cost-effective reaction route involving calcium hydroxide and oxalic acid dihydrate in isopropanol as precursors yielded a colloidal solution of nano-calcium oxalate; this solution was mixed with tetraethoxysilane to produce a crack-free mesoporous xerogel with pore radius of approximately 15nm. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential thermal-thermogravimetric analysis (DTA-TG) and scanning electron microscopy (SEM) studies demonstrated that the nano-calcium oxalate embodiment into the silica matrix resulted in a crack-free nanocomposite. This nanocomposite penetrated deeply and was distributed homogeneously within the treated porous medium. The hygric properties and tensile strength of treated samples were improved without affecting the microstructural characteristics. The colour and water vapour permeability changes were ranged within acceptable limits. The physico-chemical stability and compatibility of the nanocomposite with stone created a potential strengthening agent with a partial protective effect for inorganic porous building materials.

Polysulfone mixed matrix gas separation hollow fibre membranes filled with polymer and carbon xerogels

This work involves the preparation, tensile testing and gas separation characterization of polysulfone mixed matrix hollow fibres filled with polymeric sol based, and subsequently carbonised, xerogels. The pore characteristics of the xerogels were determined using a surface area and porosity analyser. The xerogel materials were reduced to submicron particles by grinding and wet milling, and the resultant particle size was determined using dynamic light scattering. Using dry/wet forced convection spinning, mixed matrix hollow fibre membranes (MMMs) were spun from solutions of polysulfone loaded with the submicron xerogel particles. At 5% loading, all MMMs exhibited higher strain at break and higher strength than unfilled membranes. Compared to unfilled fibres, MMMs were stiffer when filled with hard xerogel inclusions but became more pliable when filled with soft xerogel particles. Knudsen diffusion becomes an important gas transport mechanism in the membranes filled with mesoporous xerogels. When compared to the unfilled hollow fibres, these membranes showed a strong increase in the permeation of low molecular weight, high kinetic diameter gases, leading to a decline in fast/slow gas selectivities. All types of MMM gave higher CO2/O2 (fast/fast) and CH4/N2 (slow/slow) selectivities than unfilled hollow fibres. The MMMs filled with a microporous xerogel gave a higher CO2 pressure normalized flux when compared to the unfilled fibres without sacrificing the CO2/CH4 selectivity. Future work should focus on the tailoring of the pore size of the xerogels and on the wet milling procedure to obtain smaller filler particles.

One-pot synthesis of hybrid mesoporous xerogels starting with linear polymethylhydrosiloxane and bridged bis-(trimethoxysilyl)ethane

A convenient one-pot synthesis pathway for hybrid mesoporous xerogels is presented. Hybrid xerogels with organic groups embedded in and anchored onto a framework were facilely prepared using 1,2-bis(trimethoxysilyl)-ethane (BTME) and polymethylhydrosiloxane (PMHS) as organosilica precursors in the absence of traditional structure-directing agents; and the dependence of textural characteristics on PMHS usage in the syntheses was investigated in detail. As-prepared hybrid materials were characterized by solid state 13C cross polarization (CP)/29Si magic angle spinning nuclear magnetic resonance spectroscopy (13C CP/29Si MAS NMR), transmission electron microscope (TEM), scanning electron microscope (SEM), N2 adsorption–desorption isotherms, thermogravimetric analysis (TGA-DSC), Fourier transform infrared spectroscopy (FT-IR) and hydrophobicity tests. It was shown that the representative hybrid materials possessed a significant hydrophobic interface and stable framework structure, and high porosity with tunable high specific surface areas and pore volumes. This kind of hybrid material may find potential applications in the separation/adsorption of organic substances, controllable delivery of hydrophobic drugs, and specific surface coatings due to its fascinating properties and designable synthesis pathway.

Non-hydrolytic sol–gel routes to heterogeneous catalysts

Oxides and mixed oxides have a tremendous importance in the field of heterogeneous catalysis, serving either as catalysts or as supports for active species. The performance of a catalyst depends directly on its composition, texture, structure and surface properties, which have to be precisely controlled and adapted to each application. In this context, the sol-gel process is a unique tool for the preparation and understanding of catalytic materials, owing to its exceptional versatility. In the last 10 years, the non-hydrolytic sol-gel (NHSG) or non-aqueous sol-gel process based on nonhydrolytic condensations in nonaqueous media has established itself as a simple and powerful method for the design of a wide range of oxide, mixed oxide and hybrid materials with controlled composition, morphology, texture and structure. NHSG proved particularly interesting for the preparation of catalytic materials, notably mesoporous xerogels, single site catalysts and highly crystalline nanoparticles. This critical review addresses the application of NHSG to the preparation of heterogeneous catalysts, emphasizing the specificities of this process, and giving a comprehensive overview of the literature (251 references).

Development of a Novel CaP-Containing Mesoporous Silica-Based Xerogels Used as Hemostatic Material with Good Biodegradability and Low Heat Generation

A novel Ca/P-containing mesoporous silica-based xerogels (CaMSX) with good degradability and low heat generation were synthesized for hemorrhage control by a modified sol-gel process and physicochemically characterized. The mesoporous silica-based xerogels without Ca/P (MSX) was also prepared as comparison. The in vitro whole blood coagulation time, activated partial thromboplastin time (APTT) and prothrombin time (PT) of the CaMSX were measured with MSX, Chinese traditional medicin-Yunnan medical powder (YM) and traditional zeolite. In vivo bleeding model was tested with rabbit’ ear side veins. The results illustrated that the CaMSX could remarkably promote the intrinsic and extrinsic blood coagulation processes which greatly depend on the dosage and the content of Ca in the CaMSX. The CaMSX had better efficiency in promoting coagulation process than that of MSX, YM and zeolite.

Graphene and carbon nanotube structures supported on mesoporous xerogel carbon as catalysts for oxygen reduction reaction in proton-exchange-membrane fuel cells

Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity.

Non-hydrolytic SiO 2–TiO 2 mesoporous xerogels—Efficient catalysts for the mild oxidation of sulfur organic compounds with hydrogen peroxide

SiO 2–TiO 2 mixed oxides with molar ratios Ti/(Si + Ti) ranging from 0.03 to 0.09 were prepared by a non-hydrolytic sol–gel process using SiCl 4 and TiCl 4 precursors. The texture and the distribution of Ti atoms in the resulting xerogels strongly depended on the Ti concentration as precursor and the reaction temperature. Mesoporous xerogels with very high surface area (up to 1200 m 2 g −1) and pore diameters (up to 7.9 nm) can be obtained at 150 °C with a Ti loading of 6 mol%. These mesoporous materials showed excellent catalytic performances in the sulfoxidation reaction of different bulky sulfides (methyl phenyl sulfide, diphenyl sulfide), and thiophenes (benzothiophene, dibenzothiophene, tetrahydrothiophene, 4,6-dimethyl dibenzothiophene) with H 2O 2. In all the cases a very high selectivity toward H 2O 2 consumption as oxidizing agent was observed (>98%).