Studies on ordered mesoporous materials for potential environmental and clean energy applications

Two series of ordered mesoporous materials, SBA-15 silica and CMK-3 carbon were synthesized. The ordered nanostructure of these materials was confirmed by TEM and XRD analysis. Structural parameters including the specific surface area, pore volume and pore size distribution were determined on the basis of nitrogen adsorption data at 77 K. Potential applications of these materials were explored in relation to the CO₂ sequestering, methane storage and fuel desulfurization. Initial studies of both materials showed their usefulness for environmental and clean energy applications. SBA-15 modified with triethanolamine showed a very good adsorption selectivity for CO₂ while its adsorption reversibility was retained. Also, this material after CuCl deposition was useful for removal of fuel thiophenes. However, CMK-3 was shown to be promising material for storage of natural gas. As high as 41 wt.% of methane was stored in this material in the presence of appropriate amount of water.     

Effects of textural and surface characteristics of microporous activated carbons on the methane adsorption capacity at high pressures

The objective of this study is to relate textural and surface characteristics of selected microporous activated carbons to their methane storage capacity. In this work, a magnetic suspension balance (Rubotherm, Germany) was used to measure methane adsorption isotherms of several activated carbon samples. Textural characteristics were assessed by nitrogen adsorption on a regular surface area analyzer (Autosorb-MP, by Quantachrome, USA). N₂ adsorption was analysed by conventional models (BET, DR, HK) and by Monte Carlo molecular simulations. Elemental and surface analyses were performed by X-ray photoelectronic spectroscopy (XPS) for the selected samples. A comparative analysis was then carried out with the purpose of defining some correlation among the variables under study. For the system under study, pore size distribution and micropore volume seem to be a determining factor as long as the solid surface is perfectly hydrophobic. It was concluded that the textural parameters per se do not unequivocally determine natural gas storage capacities. Surface chemistry and methane adsorption equilibria must be taken into account in the decision-making process of choosing an adsorbent for gas storage.     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

Molecular dynamics simulations of binary mixtures of methane and hydrogen in zeolite A and a novel zinc phosphate

Molecular dynamics simulations have been used to study binary mixtures of hydrogen and methane in two molecular sieve structures, zeolite NaA and a novel zinc phosphate molecular sieve, Na₃ZnO(PO₄)₃, which has a pore size tuned to light gas separations. Simulations were run at high temperature, T = 500 °C. Equimolar mixtures of methane and hydrogen in both molecular sieve structures were simulated at loadings of 2–16 molecules per unit cell. Self-diffusion coefficients were calculated using the Einstein relationship. Hydrogen was found to have higher self-diffusion Coefficients than methane in both the molecular sieve structures under study. However, in the zinc phosphate molecular sieve the methane remained immobile, allowing for purification and separation of hydrogen, whereas in Zeolite A the methane experienced appreciable cage-to-cage motion.     

SO2 adsorption capacity of K2CO3-impregnated activated carbon as a function of K2CO3 content loaded by soaking and incipient wetness

The SO₂ adsorption capacity of K₂CO₃-impregnated activated carbons, prepared by soaking carbon in large volumes of K₂CO₃ in solution of various concentrations, varies linearly with respect to the loading of K₂CO₃ on the carbon up to about 12% K₂CO₃ by weight. Above 12%, the capacity for SO₂ levels out and then decreases. This suggests that at high loadings the K₂CO₃ either aggregates and/or blocks pores of the activated carbon. In contrast, the adsorption capacity of carbons prepared by repeatedly (maximum of three times total) loading K₂CO₃ via incipient wetness is much larger than that of the soaked samples, up to 70% more, when the loading of K₂CO₃ is greater than 12%. Static and dynamic adsorption, DSC, SEM, EDX and incipient wetness studies of the samples show that the impregnant aggregates but does not block carbon pores.     

Improvement of amine-modification with piperazine for the adsorption of CO2

Both selectivity and capacity of CO₂ adsorption were considerably increased when PZ (piperazine) was added in MDEA (methyldiethylamine) that used to modify the surface of silica gels. The adsorbent saturated with CO₂ was regenerated at ambient temperature through nitrogen purge. A set of PSA (pressure swing adsorption) operation with 200 cycles was carried out and applicability of the modified adsorbent was thus illustrated. The CO₂ content in the column-top stream decreased from 13% to below 0.05% at steady state.     

Development of TREN dendrimers over mesoporous SBA-15 for CO2 adsorption

Mesoporous SBA-15 was synthesized using rice husk ash (RHA) as the silica source and their defective Si-OH groups were grafted with tris(2-aminoethyl) amine (TREN) dendrimers generation through step-wise growth technique. The X-ray diffraction (XRD) and nitrogen adsorption/desorption results of parent SBA-15 obtained from RHA, suggests its resemblance with SBA-15 synthesized using conventional silica sources. Furthermore, the nitrogen adsorption/desorption results of SBA-15/TREN dendrimer generations (G1-G3) illustrates the growth of dendrimer inside the mesopores of SBA-15 and their CO₂ adsorption capacity was determined at 25 °C. The maximum CO₂ adsorption capacity of 5-6 and 7-8 wt% over second and third dendrimer generation was observed which is discernibly higher than the reported melamine and PAMAM dendrimers. The experimental CO₂ adsorption capacity was found to be less than theoretically calculated CO₂ adsorption capacity due to inter and intra molecular amidation as result of steric hindrance during the dendrimer growth. These SBA-15/TREN dendrimer generations also exhibit thermal stability up to 350 °C and CO₂ adsorption capacity remains unaltered upon seven consecutive runs.     

Electrochemical water splitting using nano-zeolite Y supported tungsten oxide electrocatalysts

Zeolites are often used as supports for metals and metal oxides because of their well-defined microporous structure and high surface area. In this study, nano-zeolite Y (50–150 nm range) and micro-zeolite Y (500–800 nm range) were loaded with WO₃, by impregnating the zeolite support with ammonium metatungstate and thermally decomposing the salt thereafter. Two different loadings of WO₃ were studied, 3 wt.% and 5 wt.% with respect to the overall catalyst. The prepared catalysts were characterized for their morphology, structure, and surface areas through scanning electron microscope (SEM), XRD, and BET. They were further compared for their electrocatalytic activity for hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. On comparing the bare micro-zeolite particles with the nano-form, the nano-zeolite Y showed higher currents with comparable overpotentials and lower Tafel slope of 62.36 mV/dec. WO₃ loading brought about a change in the electrocatalytic properties of the catalyst. The overpotentials and Tafel slopes were observed to decrease with zeolite-3 wt.% WO₃. The smallest overpotential of 60 mV and Tafel slope of 31.9 mV/dec was registered for nano-zeolite with 3 wt.% WO₃, while the micro-zeolite gave an overpotential of 370 mV and a Tafel slope of 98.1 mV/dec. It was concluded that even with the same metal oxide loading, nano-zeolite showed superior performance, which is attributed to its size and hence easier escape of hydrogen bubbles from the catalyst.     

石墨狭缝中甲烷吸附的分子动力学模拟

用分子动力学模拟研究石墨狭缝中甲烷的吸附,考察狭缝宽度和温度对甲烷吸附的影响.模拟发现甲烷在石墨狭缝中出现分层现象,吸附层中甲烷具有类液特征,第一吸附层内甲烷中总有两个氢原子的连线与另外两个氢原子的连线分别位于平行于狭缝壁的两个平面内,游离层中甲烷呈现气体的特征;碳原子间的平均作用势说明吸附层中甲烷分子间结合能力大于游离层,吸附态是甲烷在石墨狭缝中的主要赋存形式之一;伦敦力以及由吸附层净电荷产生的电场力是甲烷吸附和分层的主要原因;甲烷的吸附量随狭缝宽度增大或温度升高而减少,当狭缝宽度小于16.46Å时,甲烷仅以吸附形态存在.甲烷在第一吸附层中的扩散能力最弱、游离层中最强,甲烷扩散系数随狭缝宽度的增大或温度的升高而增大.     

Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

Hydrogen adsorption on activated carbon nanotubes with an atomic-sized vanadium catalyst investigated by electrical resistance measurements

Activated multi-walled carbon nanotubes were prepared with appended vanadium as a hydrogen storage medium. The pore structure was significantly improved by an activation process that was studied using Raman spectroscopy, field emission transmission electron microscopy and pore analysis techniques. X-ray photoelectron spectroscopy and X-ray diffraction results reveal that the vanadium catalyst was introduced into the carbon nanotubes in controlled proportions, forming V₈C₇. The improved pore structure functioned as a path through the carbon nanotubes that encouraged hydrogen molecule adsorption, and the introduced vanadium catalyst led to high levels of hydrogen storage through the dissociation of hydrogen molecules via the spill-over phenomenon. The hydrogen storage behavior was investigated by electrical resistance measurements for the hydrogen adsorbed on a prepared sample. The proposed mechanism of hydrogen storage suggests that the vanadium catalyst increases not only the amount of hydrogen that is stored but also the speed at which it is stored. A hydrogen storage capacity of 2.26 wt.% was achieved with the activation effects and the vanadium catalyst at 30 °C and 10 MPa.     

Impact of active phase chemical composition and dispersity on catalytic behavior in PROX reaction

Iron and iron-platinum catalysts supported on activated carbon have been successfully synthesized by wet impregnation method and low-temperature treatment in inert atmosphere. The content of the supported phases corresponds to 10 wt % Fe and 0.5 wt % Pt. Four catalytic samples were synthesized: Sample A—activated carbon impregnated with Fe nitrate; Sample B—activated carbon impregnated with Pt salt; Sample C—activated carbon impregnated consequently with Fe and Pt salts; Sample D—activated carbon impregnated simultaneously with Fe and Pt salts. The as-prepared materials were characterized by Mössbauer spectroscopy, X-ray diffraction, infrared and X-ray photoelectron spectroscopy. The spectra show that the activated carbon support and the preparation procedure give rise to the synthesis of isolated metal Pt ions and ultradispersed Fe and Pt oxide species. Probably the presence of different functional groups of activated carbon gives rise to registered very high dispersion of loaded species on support. The catalytic tests were carried out in PROX reaction. A lower activity of bimetallic Pt-Fe samples was explained with the increase in surface oxygen species as a result of predomination of iron oxide on the support leading to the increase in selectivity to the H₂ oxidation. Partial agglomeration of supported iron oxide phase was registered after catalytic tests.     

Preparation of microporous activated carbon and its electrochemical performance for electric double layer capacitor

Microporous activated carbons were prepared by microwave heating petroleum coke with potassium hydroxide as activation agent. Microporous activated carbons were characterized by infrared spectroscopy, X-ray diffraction and nitrogen adsorption/desorption isotherms. Electrochemical properties of an electric double layer capacitor using microporous activated carbon as electrode materials were investigated by constant current charge-discharge and electrochemical impedance spectroscopic techniques. The results show that the specific surface area and the pore volume of microporous activated carbon increase with increasing activation time before the activation time reaches 37 min. The microporous volume totals 94.0% in the microporous activated carbons and the average pore diameter of microporous activated carbon is 2.00 nm. Microporous activated carbons prepared in the activation time of 31, 35 and 37 min are named as AC-31, AC-35 and AC-37, respectively. Compared with AC-27 electrode, the internal resistance for ions transferring in AC-31, AC-35 or AC-37 electrode is relatively small. The specific capacitance of AC-31 is the biggest among the microporous activated carbons, and it retains 279.6 F g^?1 maintaining 93.5% capacity after 200 recycling number.     

Modification of bamboo-based activated carbon using microwave radiation and its effects on the adsorption of methylene blue

Modification of bamboo-based activated carbon was carried out in a microwave oven under N₂ atmosphere. The virgin and modified activated carbons were characterized by means of low temperature N₂ adsorption, acid-base titration, point of zero charge (pH_pzc) measurement, FTIR and XPS spectra. A gradual decrease in the surface acidic groups was observed during the modification, while the surface basicity was enhanced to some extent, which gave rise to an increase in the pH_pzc value. The species of the functional groups and relative content of various elements and groups were given further analysis using FTIR and XPS spectra. An increase in the micropores was found at the start, and the micropores were then extended into larger ones, resulting in an increase in the pore volume and average pore size. Adsorption studies showed enhanced adsorption of methylene blue on the modified activated carbons, caused mainly by the enlargement of the micropores. Adsorption isotherm fittings revealed that Langmuir and Freundlich models were applicable for the virgin and modified activated carbons, respectively. Kinetic studies exhibited faster adsorption rate of methylene blue on the modified activated carbons, and the pseudo-second-order model fitted well for all of the activated carbons.     

炭分子筛吸附分离甲烷/氮混合物研究进展

高效浓缩甲烷/氮气混合气中的甲烷是合理利用煤层气、油田气和垃圾填埋气等资源时面临的重要技术难题之一。甲烷与氮气分子存在着微小的、但可操作的动力学直径差异。文中分析了以炭分子筛作为吸附剂,采用变压吸附分离技术分离甲烷/氮气的可行性,对炭分子筛的吸附分离机理、国内外的研究进展以及混合气在炭分子筛吸附床上的吸附动力学模型进行了探讨,并指出了在这一技术应用方面的一些不足。     

Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.     

Preparation of activated carbon from Enteromorpha prolifera and its use on cationic red X-GRL removal

Enteromorpha prolifera was pyrolyzed to prepare activated carbon using chemical activation by zinc chloride. The effect of activation parameters such as activation temperature, weight ratio (Enteromorpha prolifera to ZnCl₂), and activation time was investigated. The BET results showed that the surface area and pore volume of activated carbons were achieved as high as 1722 m²/g and 1.11 cm³/g, respectively, in the optimal activation conditions. Batch adsorption studies were carried out to study the adsorption properties of cationic red X-GRL onto activated carbon by varying the parameters like initial solution pH, contact time, and temperature. The kinetic studies showed that the adsorption data followed a pseudo second-order model. The isotherm analysis indicated that the adsorption data could be represented by the Langmuir isotherm model. The Langmuir monolayer adsorption capacity of cationic red X-GRL was estimated as 263.16 mg/g at pH 6.0.     

Preparation of mesoporous poly (acrylic acid)/SiO2 composite nanofiber membranes having adsorption capacity for indigo carmine dye

Mesoporous poly (acrylic acid)/SiO₂ (PAA/SiO₂) composite nanofiber membranes functionalized with mercapto groups were fabricated by a sol-gel electrospinning method, and their adsorption capacity for indigo carmine was investigated. The membranes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray powder diffraction (XRD), and nitrogen adsorption–desorption measurement. SEM and TEM observation results showed that the PAA/SiO₂ fibers had diameters between 400–800 nm and mesopores with an average pore size of 3.88 nm. The specific surface area of the mesoporous nanofiber membranes was 514.89 m²/g. The characteristic peaks for mercapto group vibration in FTIR and Raman spectra demonstrated that the mercapto groups have been incorporated into the silica skeleton. The adsorption isotherm data of indigo carmine on the membranes fit well with Redlich–Peterson model, and the maximum adsorption capacity calculated was 523.11 mg/g. It was found that the removal rate of indigo carmine by the membranes reached a maximum of 98% in 90 min and the adsorption kinetics followed a pseudo-second-order model. The high adsorption capacity of PAA/SiO₂ nanofiber membrane makes it a promising adsorbent for indigo carmine removal from the wastewater.     

Hydrophobic polymers modification of mesoporous silica with large pore size for drug release

Mesostructure cellular foam (MCF) materials were modified with hydrophobic polyisoprene (PI) through free radical polymerization in the pores network, and the resulting materials (MCF-PI) were investigated as matrices for drug storage. The successful synthesis of PI inside MCF was characterized by Fourier transform infrared (FT-IR), hydrogen nuclear magnetic resonance (¹H NMR), X-ray diffraction patterns (XRD) and nitrogen adsorption/desorption measurements. It was interesting to find the resultant system held a relatively large pore size (19.5 nm) and pore volume (1.02 cm³ g⁻¹), which would benefit for drug storage. Ibuprofen (IBU) and vancomycin were selected as model drugs and loaded onto unmodified MCF and modified MCF (MCF-PI). The adsorption capacities of these model drugs on MCF-PI were observed increase as compared to that of on pure MCF, due to the trap effects induced by polyisoprene chains inside the pores. The delivery system of MCF-PI was found to be more favorable for the adsorption of IBU (31 wt%, IBU/silica), possibly attributing to the hydrophobic interaction between IBU and PI formed on the internal surface of MCF matrix. The release of drug through the porous network was investigated by measuring uptake and release of IBU.     

Study of ethylene adsorption on zeolite NaY modified with group I metal ions

The adsorption of ethylene by zeolite NaY and zeolite NaY modified by cation exchange with potassium, rubidium, and cesium ions was studied. Cation exchanges were carried out using KNO₃, RbNO₃, and CsNO₃ in the concentration ranges of 0.2-10 mM. XRD patterns and specific surface areas illustrated that modification of NaY zeolite by very dilute solutions containing K⁺, Rb⁺ and Cs⁺ did not lead to significant changes in the crystallinity. Analysis of metals content (ICP-OES) showed that Cs⁺ can replace Na⁺ better than Rb⁺ and K⁺. Particle analysis indicated slight decreases in surface area but pore volumes and pore diameters remained unchanged. Ethylene adsorption isotherms indicated that Na-Y zeolite which was modified by 5.0 mM KNO₃, 0.5 mM RbNO₃ and 1.0 mM CsNO₃ could adsorb ethylene better than zeolite Na-Y. K-NaY zeolite adsorbed up to 102.45 cm³/g ethylene, while Rb-NaY and Cs-NaY zeolites adsorbed up to 98.50 cm³/g and 90.15 cm³/g ethylene, respectively. Ethylene adsorption capacities depended on number of adsorption sites and surface interactions.