Adsorption performance of VOCs in ordered mesoporous silicas with different pore structures and surface chemistry

Adsorption of volatile organic compounds onto natural porous minerals

The abundance of natural porous minerals and their low cost make them the potential adsorbents for VOCs (volatile organic compounds). In this paper, three natural minerals (diatomite, stellerite and vitric tuff) and their corresponding acid-treated minerals were used as adsorbents. The adsorption performances of minerals were investigated by the adsorption breakthrough curves of VOCs. The results indicated that the properties of organic compounds such as boiling point and polarity and the surface area and pore volume of minerals had obvious effects on the adsorption of VOCs over minerals. Increasing adsorption temperature and relative humidity would have negative effects on the VOC adsorption of minerals. The adsorption capacity of 2-heptanone over acid stellerite decreased by 7.2% as the temperature rose from 25°C to 45°C. The adsorption capacity of acid stellerite for 2-heptanone reduced by 60.9% when relative humidity increased from 0% to 75%. Minerals were tested for five adsorption-regeneration cycles to study the reusability. Better fittings of Thomas model, pseudo-first order kinetics model, and Freundlich model were showed in fitting the adsorption. Overall, porous minerals with high specific surface area and pore volume have promising prospect in VOCs adsorption.

Dynamic adsorption of toluene on amino-functionalized SBA-15 type spherical mesoporous silica

Amino-functionalized spherical mesoporous silicas were successfully prepared via a convenient treatment method by using APTES, which was used for the adsorption treatment of toluene gas, showing obvious advantages.

Nanometer Confinement-Driven Promotion and Stabilization of a Hexatic Phase Intervening between Ordered Rotator Phases

Bulk phase binary mixture of two rotator phase forming alkanes, n-tricosane (C₂₃H₄₈) and n-octacosane (C₂₈H₅₈), has been previously studied. C₂₃H₄₈ exists in the R_II and R_I phases, whereas C₂₈H₅₈ exists in the R_III and R_IV phases. Over a certain range of composition, this binary mixture was found to exist in R_II, R_I and an intervening mesophase was reported to be the hexatic phase, wherein the long-range two-dimensional in-plane hexagonal lattice order of the R_II is lost and what remains is molecules present in hexagonal geometry without long-range positional correlation between individual hexagons. Upon confinement in cylindrical anodized alumina pores 200 nm wide, on the one hand, the temperature range of the hexatic phase was found to extend, and on the other hand, it underwent increased molecular ordering compared to the hexatic phase in bulk, exhibiting two counter-reacting behaviors in confinement. We provide here a temperature-dependent X-ray diffraction study and a theoretical approach combining the Landau and Flory-Huggins theories to, first, understand the underlying mechanism leading to emergence of the hexatic phase and then to explain the effect of confinement on it in the light of finite size and interfacial interaction between the alkanes and alumina pores.

Sorption of BTEX on a nanoporous composite of SBA-15 and a calcined hydrotalcite

Benzene, toluene, ethylbenzene, and p-xylene (BTEX) are hazardous volatile organic compounds mostly released from fuel combustion, paint gas emissions, and biomass burning. In this work, it is studied the BTEX sorption influence on the surface reactivity of a new kind of nanoporous composite, prepared via an in situ functionalization of SBA-15 with a Mg-Al calcined hydrotalcite (HT_C). During its preparation, Mg/Al mixed oxides are indeed formed and dispersed on the SBA-15 surface with non-blockage porosity. Furthermore, the physicochemical surface properties are exalted from its precursors and it is synergistically favorable for the BTEX sorption at low pressure and temperature.

Effect of Surface Chemistry on Confined Phase Behavior in Nanoporous Media: An Experimental and Molecular Modeling Study

It is well accepted that nanopore size is a controlling parameter in determining the phase behavior of confined adsorbate molecules. Despite this knowledge, the quantitative effect of surface chemistry on the confined phase behavior is a factor that remains obfuscated. Obtaining a complete understanding of the variables controlling confined phase behavior is a critical step in developing more complete equations of state for predictive modeling. To this end, a combined experimental and molecular modeling study was conducted to investigate the effects of surface chemistry and wetting on the confined phase behavior of propane and n-butane in modified and unmodified silica MCM-41. Isotherms were measured in four types of silica MCM-41 modified with varying sizes of alkyl groups to determine the effects of increasing surface modification. Results showed that increased pore surface coverage of carbon resulted in a notable change in the capillary condensation pressures, adsorption enthalpy, and confined critical temperature of the adsorbate. Correlations between the surface coverage of carbon and the confined critical temperature were presented and supported by thermodynamic arguments. The primary conclusions were partially supported by hybrid molecular dynamics-Monte Carlo simulations of propane adsorption in models of the four types of experimental adsorbents. Several differences were noted and explained between the experimental and modeling results. Energetic heterogeneity on the surface of the modified MCM-41 adsorbents as well as differences in adsorbate entropy induced by surface features and chemistry were suggested as primary driving factors for the observed trends. The results of this work have direct implications for improving understanding of confined phase behavior in materials of varying surface chemistries.

A review on activated carbon adsorption for volatile organic compounds (VOCs)

A number of control methods have been adopted for the removal of hazardous volatile organic compounds (VOCs) from gas streams, particularly adsorption processes which are considered more prominent in terms of feasibility, effectiveness as well as cost competence compared to other methods. In this study, most of the activated-carbon-based adsorbents are critically reviewed in terms of their advantages and limitations for VOC gas adsorption. The choice of adsorbent and process parameters depends mainly on the type of VOC used, its chemical and structural properties, in addition to the adsorbent’s characteristics. The review discusses in detail the application of fixed-bed adsorption systems. A computational simulation study using quantum-chemical conductor like screening model for real solvents is included in this review which determines the efficiency in describing and predicting the adsorption technique required for each process. This review offers a comprehensive discussion of the VOC adsorption techniques and their implementation for different applications.

Adsorption of mesitylene via mesoporous adsorbents

Mesitylene (or 1,3,5-trimethylbenzene) is a volatile organic compound emitted from various industrial processes, e.g., spray coating. Its emissions have become a critical issue because mesitylene is toxic and cannot be removed using traditional adsorbents, e.g., zeolite (H-ZSM-5; the diameter of mesitylene molecules is greater than the pore size of H-ZSM-5). Hence, an adsorbent with a large pore size, MCM-41, is used in this study to investigate its adsorption capacity for mesitylene and compare with that of H-ZSM-5. Experimental results reveal that MCM-41 without Al₂O₃ exhibits a good adsorption capacity (184 mg/g) for the gas stream containing 100 ppm of mesitylene at a relative humidity of 10%. The adsorption kinetics is well described by the Freundlich isotherm. Furthermore, experimental results reveal that MCM-41 is effective for the adsorption of low concentrations (10 ppm) of mesitylene. In addition, adsorption-desorption tests revealed that the sample MCM-41-AS is stable to sustain the adsorption capacity after 10 adsorption-desorption cycles. After 10 adsorption-desorption cycles, MCM-41-AS retains 92.4% of its initial adsorption capacity (170 vs. 184 mg/g). Finally, MCM-41 and H-ZSM-5 in series are effective for the simultaneous removal of mesitylene and toluene in the gas stream.

IMPLICATIONS

This study aims to improve the performance of adsorbent for mesitylene, which is typically applied in the spray-coating industry. The zeolite MCM-41-AS is selected as a candidate for the investigation. Experimental results reveal that MCM-41-AS exhibits a good adsorption capacity for mesitylene and that it can be integrated with H-ZSM-5-25 for the simultaneous adsorption of mesitylene and toluene.

Adsorption of VOCs onto engineered carbon materials: A review

Volatile organic compounds (VOCs) severely threaten human health and the ecological environment because most of them are toxic, mutagenic, and carcinogenic. The persistent increase of VOCs together with the stringent regulations make the reduction of VOC emissions more imperative. Up to now, numerous VOC treatment technologies have emerged, such as incineration, condensation, biological degradation, absorption, adsorption, and catalysis oxidation et al. Among them, the adsorption technology has been recognized as an efficient and economical control strategy because it has the potential to recover and reuse both adsorbent and adsorbate. Due to their large specific surface area, rich porous structure, and high adsorption capacity, carbonaceous adsorbents are widely used in gas purification, especially with respect to VOC treatment and recovery. This review discusses recent research developments of VOC adsorption onto a variety of engineered carbonaceous adsorbents, including activated carbon, biochar, activated carbon fiber, carbon nanotube, graphene and its derivatives, carbon-silica composites, ordered mesoporous carbon, etc. The key factors influence the VOC adsorption are analyzed with focuses on the physiochemical characters of adsorbents, properties of adsorbates as well as the adsorption conditions. In addition, the sources, health effect, and abatement methods of VOCs are also described.

Improving Sorbents for Glycerol Capture in Biodiesel Refinement

Biodiesel is produced by transesterification of animal fat, vegetable oil, or waste cooking oil with alcohol. After production costs, the economic viability of biodiesel is dependent on what steps are necessary to remove impurities following synthesis and the effectiveness of quality control analysis. Solid-phase extraction offers a potentially advantageous approach in biodiesel processing applications. Nanoporous scaffolds were investigated for adsorption of glycerol, a side product of biodiesel synthesis that is detrimental to engine combustion when present. Materials were synthesized with varying pore wall composition, including ethane and diethylbenzene bridging groups, and sulfonated to promote hydrogen bonding interactions with glycerol. Materials bearing sulfonate groups throughout the scaffold walls as well as those post-synthetically grafted onto the surfaces show notably superior performance for uptake of glycerol. The sorbents are effective when used in biodiesel mixtures, removing greater than 90% of glycerol from a biodiesel preparation.

Phenyl-functionalized mesoporous silica materials for the rapid and efficient removal of phthalate esters

Phthalate esters (PAEs) are a group of endocrine disrupting compounds, which have been widely used as plasticizers. To alleviate the environmental and health threats from water resources polluted by PAEs, we prepared phenyl functionalized mesoporous silica materials (ph-SBA-15) were synthesized by a simple post-modification approach for rapid and efficient removal of low concentration of di-n-butyl phthalate (DBP) from aqueous solution. Mesostructure, texture, surface chemistry and surface charges were systemically characterized. The obtained ph-SBA-15 possesses a highly ordered mesostructure, a high surface area (418m²/g), uniform mesopores (6.5nm) and high-density organic groups around 11wt.%. Batch adsorption experiments revealed that phenyl modified SBA-15 had an excellent ability to remove DBP with the maximum adsorption capacity up to ∼40mg/g at 25°C. The thermodynamics and kinetics for the adsorption were also investigated, demonstrating an exothermic, multi-layer and fast adsorption process. In addition, DBP adsorption was found to be sensitive to the pH and the uptake was observed to be greatest at around pH 7.0. Furthermore, this material can be effectively regenerated by ethanol.

Preparing highly-dispersed noble metal supported mesoporous silica catalysts by reductive amphiphilic molecules

We present an in situ reduction strategy to prepare mesoporous silica supported by highly-dispersed noble metals.

Performance of mesoporous silicas (MCM-41 and SBA-15) and carbon (CMK-3) in the removal of gas-phase naphthalene: adsorption capacity, rate and regenerability

The adsorption isotherms of gas-phase naphthalene on mesosilicas MCM-41 and SBA-15, and mesocarbon CMK-3 were determined by column tests at 125 °C, with feed concentrations ranging from 7.63 × 10⁻⁵ to 4.64 × 10⁻² mol m⁻³ (1.88 to 1140 ppm).

Use of ordered mesoporous silica-loaded phyto-phospholipid complex for BCS IV class plant drug to enhance oral bioavailability: a case report of tanshinone IIA

A CDDS was composed of PC and SD, using TS as a model drug. The marked improvements in oral bioavailability by TSPC-SD may result from comprehensive effects, including improved lg P_o/wandP_appviaPC, and increased dissolution rates from SD.

Fixed-bed adsorption of toluene on high silica zeolites: experiments and mathematical modelling using LDF approximation and a multisite model

The adsorption of toluene (TOL) as a target volatile organic compound has been studied experimentally and modelled on various hydrophobic zeolites: Faujasite (FAU), ZSM-5 (Z) and Mordenite (MOR). The influence of the nature of the compensating cation (H+ or Na+) has also been investigated for ZSM-5 zeolite, which is known to possess three kinds of adsorption sites (sinusoidal channels, straight channels and intersections). Type I isotherms observed on FAU, Na-Z and MOR fitted well with the Langmuir model. A deviation from a type I isotherm was observed for H-Z, because of the structure of this zeolite. The Successive Langmuir Model was more successful to fit the 'bump' of the experimental curve than the Double Langmuir. Classical shapes were found for MOR, FAU and Na-Z breakthrough curves that were fitted with good accuracy using the Linear Driving Force (LDF) approximation. In the case of H-Z, a change of profile was observed during the dynamic adsorption and the differences seen between the Na-Z and H-Z behaviours were explained by the strong interactions between Na+ and adsorbed TOL at the intersection sites. The Na+ cations prevented reorientation of TOL molecules at the intersection and thereby avoided the filling of the sinusoidal channel segments. Thus, a specific model was developed for fitting the breakthrough curve of H-Z. The model developed took into account these two types of adsorption sites with the overall uptake for each site being given by an LDF approximation.

An energy-efficient catalytic process for the tandem removal of formaldehyde and benzene by metal/HZSM-5 catalysts

A novel tandem temperature-pulse oxidation (TTPO) concept for energy-efficient catalytic removal of formaldehyde and benzene is proposed and realized over zeolite-hosted platinum catalysts.

Microscale scavenging of pentachlorophenol in water using amine and tripolyphosphate-grafted SBA-15 silica: batch and modeling studies

Mesoporous silica SBA-15 meets most criteria for selection of water treatment adsorbents such as high specific surface area, large pore-size, chemical inertness, repertory of surface functional groups, good thermal stability, selectivity, regenerability, and low cost of manufacture. However, its use for water treatment is still largely unexplored. SBA-15 and its functionalized derivatives of aminopropyltriethoxysilane (SA) and tripolyphosphate (ST) were synthesized, characterized, and used to investigate pentachlorophenol (PCP) removal from aqueous solutions. Functionalization improved SBA-15 capacity for PCP removal from solution in accordance with the trend SBA-15 < ST < SA. Sorption rate experiments data fit the Lagergren pseudo-second order kinetics model. Intra-particle diffusion indicated that the sorption is controlled by two mechanisms: intra-particle and equilibrium diffusion. Adsorptive pore-filling and electrostatic interactions were implicated in the removal of PCP from solution. Electrostatic interaction led to ≥75% increase in sorption upon functionalization. The equilibrium sorption data of the PCP on these mesoporous materials fits the Freundlich isotherm. Desorption hysteresis was low for the pristine SBA-15, but the functionalized SBA-15 materials showed higher hysteresis. The results imply that functionalized SBA-15 sorbents are promising materials for microscale scavenging of PCP in solution.

Americium(III) capture using phosphonic acid-functionalized silicas with different mesoporous morphologies: adsorption behavior study and mechanism investigation by EXAFS/XPS

Efficient capture of highly toxic radionuclides with long half-lives such as Americium-241 is crucial to prevent radionuclides from diffusing into the biosphere. To reach this purpose, three different types of mesoporous silicas functionalized with phosphonic acid ligands (SBA-POH, MCM-POH, and BPMO-POH) were synthesized via a facile procedure. The structure, surface chemistry, and micromorphology of the materials were fully characterized by (31)P/(13)C/(29)Si MAS NMR, XPS, and XRD analysis. Efficient adsorption of Am(III) was realized with a fast rate to reach equilibrium (within 10 min). Influences including structural parameters and functionalization degree on the adsorption behavior were investigated. Slope analysis of the equilibrium data suggested that the coordination with Am(III) involved the exchange of three protons. Moreover, extended X-ray absorption fine structure (EXAFS) analysis, in combination with XPS survey, was employed for an in-depth probe into the binding mechanism by using Eu(III) as a simulant due to its similar coordination behavior and benign property. The results showed three phosphonic acid ligands were coordinated to Eu(III) in bidentate fashion, and Eu(P(O)O)3(H2O) species were formed with the Eu-O coordination number of 7. These phosphonic acid-functionalized mesoporous silicas should be promising for the treatment of Am-containing radioactive liquid waste.

Application of ultrasound and quartz sand for the removal of disinfection byproducts from drinking water

To the best of our knowledge, little information is available on the combined use of ultrasound (US) and quartz sand (QS) in the removal of disinfection byproducts (DBPs) from drinking water. This study investigates the removal efficiency for 12 DBPs from drinking water by 20 kHz sonolytic treatment, QS adsorption, and their combination. Results indicate that DBPs with logKow≤1.12 could not be sonolysized; for logKow≥1.97, more than 20% removal efficiency was observed, but the removal efficiency was unrelated to logKow. DBPs containing a nitro group are more sensitive to US than those that comprise nitrile, hydrogen, and hydroxyl groups. Among the 12 investigated DBPs, 9 could be adsorbed by QS adsorption. The adsorption efficiency ranged from 12% for 1,1-dichloro-2-propanone to 80% for trichloroacetonitrile. A synergistic effect was found between the US and QS on DBPs removal, and all the 12 DBPs could be effectively removed by the combined use of US and QS. In the presence of US, part of the QS particles were corroded into small particles which play a role in increasing the number of cavitation bubbles and reducing cavitation bubble size and then improve the removal efficiency of DBPs. On the other hand, the presence of US enhances the DBP mass transfer rate to cavitation bubbles and quartz sand. In addition, sonolytic treatment led to a slight decrease of pH, and TOC values decreased under all the three treatment processes.

Evaluation of the potential of volatile organic compound (di-methyl benzene) removal using adsorption on natural minerals compared to commercial oxides

This study is dedicated to the investigation of the potential of volatile organic compounds (VOC) adsorption over low cost natural minerals (bentonite and diatomite). The performances of these solids, in terms of adsorption/desorption properties, were compared to commercial adsorbents, such as silica, alumina and titanium dioxide. The solids were first characterized by different physico-chemical methods and di-methyl benzene (dMB) was selected as model VOC pollutant for the investigation of adsorptive characteristics. The experiments were carried out with a fixed bed reactor under dynamic conditions using Fourier Transform InfraRed spectrometer to measure the evolution of dMB concentrations in the gaseous stream at the outlet of the reactor. The measured breakthrough curves yields to adsorbed amounts at saturation that has been used to obtain adsorption isotherms. The latters were used for determination of the heat involved in the adsorption process and estimation of its values using the isosteric method. Furthermore, the performances of the studied materials were compared considering the adsorption efficiency/cost ratio.

Novel hydrophobic PDVB/R-SiO2 for adsorption of volatile organic compounds from highly humid gas stream

A novel organic-inorganic hydrophobic polydivinylbenzene-silica adsorbent (PDVB/R-SiO2) was successfully prepared by introducing a specific amount of divinylbenzene and solvent (i.e., tetrahydrofuran) to SiO2pores and initiating polymerization under solvothermal conditions. New smaller structures and surface areas were formed in the SiO2 pores. The PDVB/R-SiO2-0.5 samples exhibited a bimodal pore size distribution with both SiO2 micropores/mesopores (0.5-2.0 nm) and mesopores (2.0-5.0 nm). The surface areas increased from 116 m(2)/g (SiO2) to 246 m(2)/g. The breakthrough curves of toluene adsorption indicated that the amount adsorbed on PDVB/R-SiO2-0.5 was 12 times higher than that on SiO2. The highly humid environment exhibited no effect on adsorption because the surface of PDVB was functionalized. The adsorbed toluene was easily desorbed in hot N2 stream at 100 °C. After 10 adsorption-desorption cycles, PDVB/R-SiO2-0.5 continued exhibiting excellent adsorption, indicating superior structural and regeneration abilities.

Rapid and efficient removal of microcystins by ordered mesoporous silica

To alleviate the environmental and health threats from water resources polluted by large-sized microcystins (MCs), we demonstrate for the first time that ordered mesoporous silica materials with large pore sizes of 2-12 nm can be used as adsorbents for rapid and efficient removal of MCs. The obvious correlations between adsorption performance of MCs and physicochemical properties of adsorbents including pore mesostructure, texture and size, and surface chemistry have been well established. Accordingly, an excellent candidate, mesoporous silica SBA-15 templated from Pluronic P123 has been sorted out, exhibiting extremely rapid rate (one minute) as well as high capacities of 5.99 and 13 mg g(-1) for removing high-concentration MC-LR and MC-RR, respectively, which are much higher than that of other silica-based adsorbents reported previously. The adsorption performance can be further improved from 50 to 95% at around pH 4 by grafting positively charged and/or hydrophobic groups onto pore surface of SBA-15. Furthermore, thermodynamic and kinetic evaluations provide additional valuable information for a better understanding of the adsorption process. Given the excellent adsorption performance, it is expected that mesoporous silica materials with unique characteristics are attractive for actual applications in removal of MCs from wastewater.