Types Of Silica Gel Research Articles

Theoretical and experimental investigations of isosteric heats for water adsorption on silica gel surfaces

The knowledge of the isosteric heats (Qsto) is essential to design porous adsorbents for calculating the performances of adsorption-assisted cooling, separation and gas storage systems. This paper presents a thermodynamic framework to calculate the interaction potentials and isosteric heats for water adsorption on SiO2 structures. Here both Lennard Jones (LJ) and electrostatic potentials are considered. It is found that (i) Qsto varies from 1.37 eV (∼131 kJ/mol) to 0.54 eV (∼52 kJ/mol) for the adsorption of one water molecule on various pore sizes of SiO2 structure, and (ii) Qsto depends on the pore size (H). Qsto is found to be very high in the super-micro-pore regions. The density functional theory (DFT) is applied to calculate Qsto for the adsorption of water on silica. Here water-water interactions are not considered. Later Qsto for five types of silica gels are obtained from experimentally-measured isotherms data at low pressures (up to 0.15 kPa) and wide range of temperatures. The simulation result agrees well with the experimental data.

High performances of mesoporous g-C3N4 for adsorptive desulfurization in model gasoline (iso-octane) solutions

The desulfurization performance and mechanisms of mesoporous g-C3N4 as adsorbent in iso-octane solutions are presented. Mesoporous g-C3N4 was synthesized at 600 °C with a hard-template method using three types of silica-gels as template agent. XRD diffraction patterns show the interlayer distances of g-C3N4 are enlarged due to the intercalation of H+. The as-prepared adsorbents possess a large sulfur capacities and excellent selective adsorption for thiophene in a benzene/iso-ocatane mixed solution. TPD and regeneration experimental tests demonstrate that the larger sulphur-capacities could attribute to H+-intercalation. The adsorptive mechanisms of the thiophene over mesoporous g-C3N4 are interpreted by a novel donor-acceptor (D-A) interaction, where the donor is the S-atom of thiophene and the acceptor is the C-atoms in g-C3N4. Gaussian computation indicates the ortho-C atoms of N-atoms bonded with H+ are the main active sites for D-A interactions.

Physical properties and adsorption kinetics of silica-gel/water for adsorption chillers

The choice of a suitable adsorbate/adsorbent pair is critical for an adsorption cooling cycle. The surface characteristics and thermo-physical properties of the adsorbent, and the adsorption rate of adsorbate are key parameters in making the choice. Through literature review, it is found that there are disagreements among the experimental measurements and various equations/models used to describe adsorption isotherms and surface diffusivity of water in silica-gel. In this work, an experimental set-up is built to measure the isotherms and kinetics of vapor adsorption for any working pair. Using the newly measured data, those from the manufacturers and from the literature, these inconsistencies are eliminated by utilizing the Dubinin-Astakhov (D-A) model to fit the entire adsorption isotherm curve. The Brunauer-Emmett-Teller(BET) method is used to calculate the surface area, pore volume and pore diameter of two different types of silica-gel. Based on the adsorption rate and the adsorbent temperature measured simultaneously, a new approach is proposed to measure the surface diffusivity in the temperature and pressure ranges typical of those during the operating conditions of adsorption cooling systems. Analysis of the results indicates that the surface diffusivity follows the Arrhenius-form equation. The calculated activation energy at different adsorption conditions varies from 40.0 to 41.2 kJ/mol and the pre-exponential factor varies from 2.5 × 10−4 to 2.8 × 10−4 m2/s. These values are close to those previously reported in the literature. Thus, the proposed approach can be used to measure the surface diffusivity in nanoporous materials.

Dynamic catalytic adsorptive desulfurization of real diesel over ultra‐stable and low‐cost silica gel‐supported TiO2

To develop dynamic ultra‐deep catalytic adsorptive desulfurization (CADS) of real diesel using ultra‐stable and low‐cost silica gel‐supported TiO2 is the aim of the work. A two‐stage dynamic breakthrough model was built to describe the CADS process, varied with H/R ratio and O/S ratio. The desulfurization capacity reached 1.3 mg‐S/g‐A at the breakthrough concentration of 5 ppm‐S. Various types of silica gel were screened as the substrate for TiO2, and the textural/acidic properties and CADS capacity were correlated in high relevancy. The effectiveness of diverse oxidants on CADS and the oxidation path were elucidated via combined experiment/simulation. Adsorption enthalpy derived from fitted isotherm data was calculated as 33.4 kJ/mol. The TiO2/silica gel‐based sorbent demonstrated remarkable recyclability/stability in 10 cycles. An effective and economic route to eliminate the trace amount of stubborn sulfur compounds in low‐sulfur diesel is provided, which can be potentially implemented as the final polishing step for ultra‐clean diesel production. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2146–2159, 2018

Analysis of Distribution and Intraparticle Diffusion of a Fluorescent Dye in Mesoporous Silica Gel by Confocal Fluorescence Microspectroscopy.

A micrometer-sized spherical silica gel microparticle (pore diameter, ∼7 nm) was injected into an aqueous rhodamine 6G solution using microcapillary manipulation-injection technique, and the dye distribution in the single microparticle was measured as the fluorescence depth profile by confocal fluorescence microspectroscopy. The fluorescence depth profile was simulated by the convolution and deconvolution methods to correct the contribution of the spatial resolution of the experimental system. The dye homogeneously or heterogeneously distributed in the microparticle at the adsorption equilibrium, dependent on the type of silica gel. The intraparticle diffusion coefficient of the dye distributed homogeneously in the silica gel was analyzed by the simulations of the time dependence of the fluorescence depth profile based on the external and intraparticle diffusion model. The results indicated that the intraparticle diffusion of the dye in the silica gel is governed by the pore diffusion.

Reprint of “Investigations of electrochemical luminescence characteristics of ZnO/TiO2 nanotubes electrode and silica-based gel type solvents”

Abstract In this work, we suggest a simple preparation process for advanced electrochemical luminescence (ECL) cell-based on silica gel type solvents using ZnO/TiO2 nanotubes (ZnO/TNT) electrode. The ionic liquid gel solvent was prepared by adding silica nanopowder in liquid Ru(II) complex. The ZnO/TNT nanotubes were fabricated by anodic oxidation method. The ECL cell-based silica gel type on ZnO/TNT electrode is composed as F-doped SnO2 (FTO) glass/Ru(II) complex [Ru(bpy)32 +] mixed silica gel type/ZnO/TNT electrode film/FTO glass. The morphological and structural properties of ZnO/TNT electrode film were confirmed to be successful by a scanning electron microscope (SEM), X-ray diffraction and a spectral brightness analyzer. The threshold voltage at light emission start was 2.0 V for ZnO/TNT that was lower than 2.24 V for zinc oxide (ZnO), 2.25 V for TiO2 nanotube (TNT) and 2.75 V for bare FTO. The luminance and electrical properties of the ECL cell-based silica gel type with ZnO/TNT electrode were 253 cd/m2 for light intensity, 64 mA for output current, and 0.55 lm/W for ECL efficiency at 4 Vac 60 Hz. The peak intensity of the ECL cell-based silica gel type with ZnO/TNT electrode at wavelength was 622 nm which responded to dark orange color and had no effect on electrode layers. The ZnO/TNT electrode for ECL cell can increase an ECL efficiency and long lifetime stability.

Adsorbent-assisted supercritical CO2 extraction of carotenoids from Neochloris oleoabundans paste

Neochloris oleoabundans emerges as alternative source of bioactives that complies with the algae-based biorefinery concept, which consists of a platform that offers a multitude of algae bioproducts. The development of an integrated extractive processes in line with the green chemistry principles have motivated the use of Supercritical CO2 (scCO2), as an alternative to toxic organic solvents, for the extraction of bioactives. However, process integration and optimization is challenging because microalgae are grown in liquid cultures, therefore is often necessary a drying step prior to scCO2 extraction. Moreover, this step is usually energy intensive and risks damaging the compound's bioactivity. An alternative is the simultaneous extraction process of the microalgae paste (containing around 70–80% water), nevertheless little information is available that explores this type of extraction. This work aims to explore the direct extraction of microalgae paste and to evaluate the effect of water on carotenoid extractions of N. oleoabundans. To study the extraction under a batch-wise system, an indirect extraction system was developed by mixing the microalgae paste with low cost adsorbents as support medium. Two types of silica gels, two different chitosans and active carbon were tested as adsorbents; sea sand was used as inert control. All of the materials showed different adsorbent capacity, being chitosan adsorbents those with higher capacity. However, oleoresin yield and recovery was negligible in a system with only scCO2 as a solvent and ethanol as co-solvent was required to improve the extraction yield. Although the overall oleoresin recoveries were low for all adsorbents (ranging from 2 to 10%), chitosan-assisted extraction showed the highest carotenoid recoveries (60–140% g/g) surpassing acetone benchmark extraction in case of chitosan microspheres. These results are interesting for the development of low energy consumption processes, since there is no need to dry the microalgal paste.

Biosurfactant as a Promoter of Methane Hydrate Formation: Thermodynamic and Kinetic Studies.

Natural gas hydrates (NGHs) are solid non-stoichiometric compounds often regarded as a next generation energy source. Successful commercialization of NGH is curtailed by lack of efficient and safe technology for generation, dissociation, storage and transportation. The present work studied the influence of environment compatible biosurfactant on gas hydrate formation. Biosurfactant was produced by Pseudomonas aeruginosa strain A11 and was characterized as rhamnolipids. Purified rhamnolipids reduced the surface tension of water from 72 mN/m to 36 mN/m with Critical Micelle Concentration (CMC) of 70 mg/l. Use of 1000 ppm rhamnolipids solution in C type silica gel bed system increased methane hydrate formation rate by 42.97% and reduced the induction time of hydrate formation by 22.63% as compared to water saturated C type silica gel. Presence of rhamnolipids also shifted methane hydrate formation temperature to higher values relative to the system without biosurfactant. Results from thermodynamic and kinetic studies suggest that rhamnolipids can be applied as environment friendly methane hydrate promoter.

Binder Influence on the Water Adsorption Uptake of Silica Gel

The heat transfer rate of an adsorbent embedded heat exchanger can be significantly improved by as much as three folds using powder adsorbent coated by binder on the fin surfaces of exchangers. The overall performance of such exchangers in an adsorption chiller, however, is determined both by the heat transfer and adsorption mass transport phenomena, which are also highly interlaced in the chiller operations. If a severe suppression on the latter is observed as a result of the binder, the heat exchanger may eventually underperform. This work evaluates the overall effect of the binder on the mass transport. It focuses on a common adsorbent-adsorbate pair utilized in the chiller, i.e. silica gel type 3A-water. A hydroxyethyl cellulose (HEC) is used as the binder. Experiment is conducted to measure the water equilibrium uptake of the pure and binder added silica gel at various temperatures. Based on the results, the net influence of the binder on the water uptake rate in the adsorption heat transfer process is determined.

Calorimetric Assessment of Rates of Desorption

This paper concerns the further testing of a new double calorimeter. We earlier reported initial development of the calorimeter and thereafter inferred rates coefficients for adsorption of moisture under a “large pressure jump.” (The test materials were silica gel type A and water.) The paper presents a further check on the calorimeter by demonstrating that measured desorption and adsorption rates were compatible. A disappointing aspect was an unintended near step change in condenser pressure at the start of the experiment. The condenser design was deficient, and better drainage is needed in the future to maintain vapor in immediate contact with cold surfaces. Notwithstanding the lower than intended pressure driving force, the heat addition to the silica gel was measurable, following an exponential pattern against time with regression coefficient better than 99%. The rate coefficients that fitted each data set were broadly in line with coefficients reported for adsorption (within the limits of experimental error).

Performance of adsorbent-embedded heat exchangers using binder-coating method

The performance of adsorption (AD) chillers or desalination cycles is dictated by the rates of heat and mass transfer of adsorbate in adsorbent-packed beds. Conventional granular-adsorbent, packed in fin-tube heat exchangers, suffered from poor heat transfer in heating (desorption) or cooling (adsorption) processes of the batch-operated cycles, with undesirable performance parameters such as higher footprint of plants, low coefficient of performance (COP) of AD cycles and higher capital cost of the machines. The motivation of present work is to mitigate the heat and mass “bottlenecks” of fin-tube heat exchangers by using a powdered-adsorbent cum binder coated onto the fin surfaces of exchangers. Suitable adsorbent–binder pairs have been identified for the silica gel adsorbent with pore surface areas up to 680m2/g and pore diameters less than 6nm. The parent silica gel remains largely unaffected despite being pulverized into fine particles of 100μm, and yet maintaining its water uptake characteristics. The paper presents an experimental study on the selection and testing processes to achieve high efficacy of adsorbent–binder coated exchangers. The test results indicate 3.4–4.6 folds improvement in heat transfer rates over the conventional granular-packed method, resulting a faster rate of water uptake by 1.5–2 times on the suitable silica gel type.

Investigation of novel waste glass and limestone binders using statistical methods

This paper presents a study on new alternative binders based on waste glass and limestone activated by NaOH and Na2CO3. The Taguchi method was applied, to optimize the experimental work while keeping statistical significance, considering the factors: CaO/SiO2 ratio, % Na2O and curing temperature; their effect was analyzed on the compressive strength using signal to noise ratios. The optimal conditions included CaO/SiO2=0.5, activated at 40°C with 9% Na2O using Na2CO3/NaOH; the strength of the confirmation experiments agreed with the predicted values. The reaction products were: calcium silicate hydrate, a silica gel type phase, and crystalline phases like pirssonite and gaylussite.

A chiral stationary phase coated by surface molecularly imprinted polymer for separating 1,1′-binaphthalene-2,2′-diamine enantiomer by high performance liquid chromatography

This report describes novel pathway for the development of a new type of CSP. A chiral stationary phase (CSP) prepared by coating a molecularly imprinted polymer (MIP) on the surface of silica gel (SMIP-CSP) showed a high separation factor (3.39) in separating racemic 1,1′-binaphthalene-2,2′-diamine (DABN) by high performance liquid chromatography (HPLC). Being the crosslinking agent, ethylene glycol dimethacrylate (EGDMA) was copolymerized with the monomer, methacrylic acid (MAA), in the presence of (R)-DABN as the template molecules on the surface of the silica gel particles to produce the SMIP-CSPs. The effects of the pretreatment temperature, the (R)-DABN content and the type of silica gel and monomer in the SMIP-CSPs on the separation of the racemic DABN were systematically investigated.

Identification of N,N-bis(1-pentylindol-3-yl-carboxy)naphthylamine (BiPICANA) found in an herbal blend product in the Tokyo metropolitan area and its cannabimimetic effects evaluated by in vitro [35S]GTPγS binding assays

During our careful survey of unregulated psychotropic drugs in June 2013 in the Tokyo metropolitan area, we found a new compound in a herbal product. It was identified as an analog of NNEI (MN-24) and differed in that its molecule possessed another N-pentyl indole-carbonyl group: N, N-bis(1-pentylindol-3-yl-carboxy)naphtylamine (BiPICANA, compound 2). Compound 2 was purified by silica and octadecyl group bonded type silica gel (C18, ODS) columns and confirmed by liquid chromatography–mass spectrometry, accurate mass spectrometry, nuclear magnetic resonance spectroscopy, and X-ray crystallography. No pharmacological information on compound 2 has been reported previously; our experiments on the binding ability of compound 2 to cannabinoid receptors revealed that it has affinities for CB1 and CB2 receptors (EC50 = 3.80 and 8.77 × 10−7 M, respectively). This is the first report identifying compound 2 in a dubious herbal product and demonstrating its binding affinities to cannabinoid receptors. Binding affinities of azepane isomers (compounds 3 and 4) of AM-1220 and AM-2233, also found in commercial products in Japan, are also presented in this report.

Hydrothermal stability of SAPO-34 for refrigeration and air conditioning applications

Hydrothermal stability is one of the crucial factors in applying SAPO-34 molecular sieve to adsorption refrigration. The SAPO-34 was synthesized by a hydrothermal method using diethylamine as a template. Both a vacuum gravimetric method and an intelligent gravimetric analyzer were applied to analyze the water adsorption performance of SAPO-34. Cyclic hydrothermal performance was determined on the modified simulation adsorption refrigeration test rig. Crystal phase, morphology, and porosity of SAPO-34 were characterized by X-ray diffraction, scanning electron microscopy, and N2 sorption, respectively. The results show that, water adsorption strength on SAPO-34 is between that on 13X and A type silica gel. During 100–400Pa, the water uptake on SAPO-34 increases sensitively to pressure, and equilibrium water uptake reaches 0.35kg/kg, 25% higher than 13X. SAPO-34 shows no significant reduced cyclic water uptake over 60 cycles. Most of the initial SAPO-34 phase is restored, while the regular cubic-like morphology is well maintained, and the specific surface area only decreases by 8.6%.

Performance Comparison of Three-Bed Adsorption Cooling System With Optimal Cycle Time Setting

This article presents the optimal cycle time and performance of two different types of silica gel–water-based three-bed adsorption chillers employing mass recovery with heating/cooling scheme. A new simulation program has been developed to analyze the effect of cycle time precisely on the performance of the systems. The particle swarm optimization (PSO) method has been used to optimize the cycle time and then the optimum performances of two chillers are compared. Sensitive analysis of cycle time has been conducted using the contour plot of specific cooling power (SCP) with driving heat source temperature at 80°C. It is found that the center point of the contour indicates the maximum SCP value and optimal cycle time, which are comparable with the quantitative values obtained for the PSO method. Both three-bed mass recovery adsorption cycles can produce effective cooling at heat source temperature as low as 50°C along with a coolant at 30°C. The optimal SCP is similar for both cycles and is greater than that of the conventional two-bed adsorption system employing the same adsorbent–refrigerant pair. Consequently, the proposed comparison method is effective and useful to identify the best performance of adsorption cycles.

Adsorption-desorption characteristics of water on silico-alumino-phosphate-34 molecular sieve for cooling and air conditioning

SAPO (silico-alumino-phosphate) molecular sieves might be promising for adsorption refrigeration driven by low-grade heat source due to their weak polar frameworks. The SAPO-34 molecular sieve was synthesized using a hydrothermal method. Its crystal phase, morphology, and microstructure were characterized by X-ray diffraction, scanning electron microscopy, and N2 sorption isotherms, respectively. Vacuum gravimetric method and thermo-balance were applied to analyze the water adsorption and desorption performance, respectively. Results show that the synthesized molecular sieve has characteristic diffraction peaks of SAPO-34, regular crystal grains, a microporous structure with size of 0.4–0.6 nm, and a specific surface area of 545.8 m2/g. Water adsorption strength on SAPO-34 is between that on 13X molecular sieve and A type silica gel. During 100–400 Pa, the water uptake on SAPO-34 increases sensitively to pressure, and equilibrium water uptake reaches 0.35 kg/kg, 25% higher than that on 13X. Dubinin-Astakhov equation is able to well fit SAPO-34 water uptake isotherm to some extent, with a correlation coefficient R of 96.3%. Water is completely desorbed from SAPO-34 at 155 °C, which is lower than that of 13X by 145 °C. Correlation of SAPO-34 structure to its adsorption/desorption behavior would provide fundamentals in developing high efficiency adsorbent for adsorption refrigeration system.

FUEL CELL WASTE HEAT POWERED ADSORPTION COOLING SYSTEMS

In the present paper, the effect of desorption temperature on the performance of adsorption cooling systems driven by waste heat from fuel cells was analyzed. The studied adsorption cooling systems employ activated carbon fiber (ACF) of type A-20–ethanol and RD type silica gel–water as adsorbent–refrigerant pairs. Two different temperature levels of waste heat from polymer electrolyte fuel cell (PEFC) and solid oxide fuel cell (SOFC) are used as the heat source of the adsorption cooling systems. The adsorption cycles consist of one pair of adsorption–desorption heat exchanger, a condenser and an evaporator. System performance in terms of specific cooling capacity (SCC) and coefficient of performance (COP) are determined and compared between the studied two systems. Results show that silica gel–water based adsorption cooling system is preferable for effective utilization of relatively lower temperature heat source. At relatively high temperature heat source, COP of ACF–ethanol based adsorption system shows better performance than that of silica gel–water based adsorption system.

Distribution of grafted β-cyclodextrin in porous particles for bone tissue engineering

The spatial distribution of β-cyclodextrin grafted to a range of porous materials was investigated. (3-glycidyloxypropyl)trimethoxysilane was used to facilitate the covalent grafting of β-cyclodextrin. The spatial distribution was analysed by combining results from confocal laser scanning microscopy and electron microscopy fitted with a focused ion beam and an energy-dispersive X-ray detector. Three materials were analysed: Two types of silica gel and one calcium phosphate. The results show that by combining the two methods it is possible to deduce the spatial distribution of the grafted cyclodextrins. Silica gel with small pores could be grafted to a depth of approximately 10μm, whereas calcium phosphate with larger pores indicated a penetration depth larger than 30μm.

Preparation of silica gel supported amidoxime adsorbents for selective adsorption of Hg(II) from aqueous solution

Two types of silica gel supported amidoxime (SG-HE-AO and SG-HO-AO) adsorbents were prepared via the reactions of silica gel supported acrylonitrile functional group (SG-AN) with hydroxylamine under alkaline conditions. The SG-AN was obtained by so-called “heterogeneous” and “homogeneous” methods and named SG-HE-AN and SG-HO-AN, respectively. All the structures of adsorbents and intermediate were characterized by FT-IR, SEM, porous structure analysis and elemental analysis. The amidoxime group content of SG-HE-AO and SG-HO-AO were 1.28 and 0.77mmolg−1, respectively, indicating that the heterogeneous method is more effective than the homogeneous method to anchored functional groups onto the surface of silica gel in this study. In terms of Hg(II) adsorption, SG-HE-AO exhibited better capability than SG-HO-AO. The adsorption kinetics of the two adsorbents followed the pseudo second-order process in the temperature range of 5–35°C. Increased temperature was beneficial to adsorption. Langmuir and Freundlich isotherm models were both applied to experimental data analysis, and the former proved to be a better fit. Finally, in Hg(II)–Pb(II), Hg(II)–Ag(I), Hg(II)–Cu(II) and Hg(II)–Ni(II) binary systems, both SG-HE-AO and SG-HO-AO exhibited excellent adsorption selectivity for Hg(II).