Hydrophobicity Of Zeolites Research Articles

Tailoring hydrophobicity vs. water capacity of adsorbents for adsorption applications: role of composites

Water adsorption capacities of various adsorbents reported in the literature were investigated to define a hydrophobicity index that was plotted vs. water capacity. In this plot, logarithmic curves were proposed to be used as indicators of performance limits of adsorbents, especially for adsorption heat pumps. In spite of their useful adsorption properties, zeolites generally exhibited quite low hydrophobicity, remaining well below the logarithmic curve. In this study, the use of composites of zeolite NaY was examined both theoretically and experimentally for improvements in the water capacity and hydrophobicity. Salt impregnation and hydrothermal synthesis experiments were performed to prepare composites of zeolite NaY with LiCl/MgCl2 salts and activated carbon, respectively. Water capacity and hydrophobicity of zeolite NaY composites were generally superior to those of pure zeolite. Zeolite composites may be advantageous for enhancing adsorption capacity and hydrophobicity of zeolites while eliminating low stability and slow adsorption kinetics of other adsorbents. Interface between two different phases might indicate another opportunity to provide improved adsorption properties for zeolite composites.

A two-step organic modification strategy for improving surface hydrophobicity of zeolites

The organic modification using a silane coupling agent can significantly improve the surface hydrophobicity of zeolite powders. However, insufficient hydroxyl group content on surface of the zeolites greatly limits the immobilization dosage of the agent, making the samples less hydrophobic. In this study, a two-step organic modification strategy using 1,2-bis(triethoxysilyl)ethane (BTESE) and dodecyltrimethoxysilane (DTMS) was adopted and developed to synthesize highly hydrophobic organic modified zeolite powders. The results showed that the BTESE modification could significantly increase the hydroxyl group content on the surface of zeolite powders, and relatively higher amount of DTMS could be then modified on the BTESE-modified samples. The mean critical surface tension, water vapor adsorption capacity, and water drop contact angle/penetration time of the 0.5% DTMS modified 10% BTESE-zeolite powders were 46.20 mN /m, 27 mg/g, and 142°/˃25 s, while those of the 0.5% DTMS modified zeolite powders were > 71.60 mN/m, 45 mg/g, and 124°/19.48 s, respectively, indicating that the sample synthesized by this two-step method was much more hydrophobic than the sample synthesized by traditional method. This two-step organic modification method appears to be a simple and effective way to prepare highly hydrophobic zeolite powders.

Enhancing the accessible TiO6 concentration and the hydrophobicity of hierarchical TS-1 zeolites for alkene epoxidation and oxidative desulfurization

The accessible TiO6 concentration and the hydrophobicity of hierarchical TS-1 zeolites are both considered as critical factors for promoting the catalytic performance of selective oxidation. However, simultaneously enhancing these two factors still remains a challenge. Herein, hierarchical TS-1 zeolites with high hydrophobicity and concentration of accessible TiO6 were fabricated via sequential amphiphilic organosilane-assisted pre-treatment, dry-gel conversion and hydroxyl radical post-treatment, using amphiphilic organosilane and surfactant as meso-porogens. The synthesis mechanism was investigated, and it was found that the interaction between the amphiphilic organosilane and Ti species formed during the pre-treatment benefited the formation of accessible TiO6 during the subsequent dry-gel conversion. During the post-treatment of zeolite with H2O2 solution under ultraviolet light irradiation, appropriate H2O2 concentration facilitated the generation of hydroxyl radical and thus enhanced the hydrophobicity of zeolite without significantly influencing the TiO6 concentration. The hierarchical TS-1 displayed excellent catalytic performances in 1-hexene epoxidation and oxidative desulfurization, with conversion improvements of 91.2%±2.8% and 348%±17%, respectively, compared with the conventional counterpart.

Synthesis of high hydrophobicity USY zeolite with excellent VOCs adsorption performance under humid condition: Combined strategy of high temperature steam and acid treatment

Y Zeolite and its ultra-stabilized hierarchical derivative (USY) as absorbents have low adsorption capacity for VOCs under humid conditions thus limits their industrial applications. It is well known that high SiO2/Al2O3 ratio (SAR) can improve the hydrophobicity of zeolite, and the increase of pore volume can enhance the adsorption performance. In the present work, SAR and the pore size of CY were enlarged by oxalic acid-high temperature steam-citric acid treatment, as the framework aluminum in the zeolite was removed and maintaining its structural integrity. The surface morphology and structure of samples were characterized by XRD, BET, TEM, SEM and TG. 27Al MAS NMR, XPS and Pyridine infrared analysis the mechanism of aluminum removal from the CY framework. The dynamic adsorption properties of the samples were systematically investigated with toluene selected as representative volatile organic compounds. The hydrophobicity of the samples was investigated by static water adsorption, and the adsorption kinetics model of the samples were plotted to study their adsorption characteristics. N2 adsorption-desorption results showed that the modified CY material has larger surface area, wider pore size and larger microporous volume than the raw material. On the other hand, with the improved SiO2/Al2O3 ratio and hydrophobicity, the modified USY also exhibited excellent adsorption performance under humid conditions, with 4.2 times more capacity to adsorb toluene. Therefore, this method can provide effective adsorption of VOCs in humid environments.

Insights into the efficiency of hydrogen peroxide utilization over titanosilicate/H2O2 systems

The key to high efficiency of utilization of hydrogen peroxide (H2O2) over titanosilicate/H2O2 systems was discovered thanks to a comprehensive catalytic investigation. The H2O2 utilization efficiency depended chiefly on the competition between the epoxidation and decomposition pathways of the H2O2 reaction. In terms of the epoxidation pathway, the formation of TiOOH species could be promoted by enhancing the intrinsic catalytic activity of titanosilicates by improvement of Lewis acid strength. Meanwhile, the ability of active “O” to transfer to reactants could be enhanced by improving the nucleophilicity of alkene (reactant), the surface hydrophobicity of zeolites, and the Oα–Oβ polarizations of TiOOH species. These were beneficial for the increase of H2O2 utilization efficiency. In the ineffective decomposition pathway, the self-decomposition of H2O2 hardly occurred within a short period of time and at a low temperature, while the oxidative decomposition of H2O2 (OD) with TiOOH species was mainly responsible for low H2O2 utilization efficiencies. The stability of TiOOH species could be improved by enhancing the hydrogen-bonding interactions with the addition of hydrogen-bond acceptors and the proper solvent, which was helpful for suppressing OD. Thus, the synergistic effect of regulating the intrinsic catalytic activity of titanosilicates for the formation of TiOOH species, adjusting the hydrogen-bonding interactions for the stability of TiOOH species, and enhancing the ability of active “O” to transfer to reactants was the key to achieving high utilization efficiency of H2O2 over titanosilicates/H2O2 systems. These discoveries could provide beneficial ideas for designing a highly efficient titanosilicates-catalyzed epoxidation system.

Modified Jordanian zeolitic tuff in hydrocarbon removal from surface water

This work aims to investigate the potential of Jordanian raw zeolitic tuff (RZT) as oil adsorbent for oil-contaminated water. As hydrophobic properties are the primary determinants of effective oil adsorbents, the hydrophobicity of RZT was enhanced by dealumination process; since the degree of hydrophobicity of zeolites is directly dependent on their aluminum content. The microemulsion modification of the dealuminated zeolitic tuff (TZT) was also applied to increase its hydrophobicity. The raw and modified tuffs were characterized in terms of the surface area and porosity (BET), mineral composition (XRD), microstructure and morphology using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). In this work, a mixture of water and kerosene was used to examine the hydrophobic/organophilic character of raw and modified zeolitic tuff. Water/dodecane and water/octane mixtures were used to study the kinetics of the adsorption over zeolitic tuff. The results revealed that the sorption capacity using kerosene as a mixed model (water-oil) was enhanced by three- and four-fold for TZT and micro-emulsified zeolitic (MeTZT) tuff respectively. The adsorption capacity of modified zeolitic was compared with that of activated carbon adsorbents.

Influence of Hydronium Ions in Zeolites on Sorption

AbstractIn the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm3. In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level.

An undergraduate research experience: Synthesis, modification, and comparison of hydrophobicity of zeolites A and X

Two procedures using a template-free synthesis of zeolites A and X were compared in an effort to assess how modification of the porous structure of zeolites with various ligands impacts the property of hydrophobicity. Modification with ligands such as octyltrichlorosilane (OTCS), dichlorodiphenylsilane (DCDPS), chlorotrimethylsilane (CTMS), hexamethyldisiloxane (HMDS) as well as with a combination of CTMS and HMDS, was performed. Both procedures led to the formation of water repellant materials. The hydrophobicity of zeolite A increased when zeolite A was modified with DCDPS and with OTCS. Zeolite X demonstrated superior hydrophobic properties when modified with all ligands, except with HMDS.Infrared spectroscopy (IR), macroscopic analysis, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) data are presented.

Photocatalytic oxidation of NOx over TiO2/HZSM-5 catalysts in the presence of water vapor: Effect of hydrophobicity of zeolites

TiO2 hybridized with HZSM-5 zeolites photocatalysts were prepared by a simple solid state dispersion method. The physicochemical properties of the catalysts were characterized by X-ray diffraction, UV–vis diffuse reflectance and FT-IR spectroscopy. The photocatalytic oxidation of NOx over TiO2/HZSM-5 having different Si/Al ratios was carried out under various levels of humidity and different pre-adsorption times in dark. The TiO2/HZSM-5 composite catalysts exhibited higher NO conversion and lower NO2 formation than pure TiO2. Pre-adsorption with water vapor and the high humidity during the photoreaction were harmful to the reactivity of TiO2 hybridized with hydrophilic HZSM-5 zeolite. However, the photocatalytic reactivity of TiO2 hybridized with hydrophobic zeolite varied little with increase in humidity. The results indicated that the high photocatalytic reactivity of TiO2/HZSM-5 catalysts is largely depended on the hydrophobicity of the zeolites.

TiO2 photocatalyst for degradation of organic compounds in water and air supported on highly hydrophobic FAU zeolite: Structural, sorptive, and photocatalytic studies

Preparation of highly hydrophobic FAU zeolite has been achieved by a two-step preparation method that comprises predealumination treatment using concentrated mineral acids and subsequent calcination treatment. The hydrophilic/hydrophobic character of the zeolites was estimated by detailed adsorption measurements using water and toluene molecules as adsorbates, which showed the strict hydrophobic nature of the prepared FAU zeolites. By means of TG, FT-IR, and 29Si MAS NMR analyses, it was concluded that the enhancement in hydrophobicity originates from the healing of silanol defect sites and the formation of a refined silica surface with fewer adsorption sites. Thus-prepared FAU zeolite, with high crystallinity, structural and thermal stability, and hydrophobicity, significantly improved the photocatalytic degradation rates of 2-propanol in water and acetaldehyde in air on TiO2 when it was used as a catalyst support. A distinct correlation was found between the hydrophobicity of zeolites and photocatalytic activity of the supported TiO2.

The effect of dydrophobic modification of zelites on CO2 absorption in different solvents

The hydrophobicity of zeolite was achieved by a method of hydrothermal synthesis combined with acid treatment for the enhancement of CO2 absorption. The influence of zeolite before and after modification on the absorption of CO2 in three kinds of solvents (water, cyclohexane and soy bean oil) was investigated and many different effects were observed. In aqueous solution, modified zeolite showed a very remarkable enhancement with a maximum enhancement factor of 2.2 for a ratio of SiO2 and Al2O3 equal to 86. However, no measurable enhancement effect was found in cyclohexane solution by adding modified zeolite, although unmodified zeolite showed a weak increase in the absorption rate. In soybean oil, almost no change in CO2 absorption was obtained for both unmodified and modified zeolites. These results indicate that the particles with an enhancement effect for CO2 absorption rate need to have two properties, hydrophobicity and a high adsorption capacity for the solute. For predicting the enhancement factor of CO2 absorption by solid particles, a three-dimensional heterogeneous microcosmic mass transfer model was developed, in which the influence of several parameters and the interaction between particles were mainly considered. The results predicted by the present model show good conformity to experimental data, indicating that the model is rational and applicable for the enhancement process of CO2 absorption by modified zeolite particles.

Influence of post-synthesis treatment on BEA zeolites hydrophobicity assessed under static and dynamic conditions

A series of BEA samples were dealuminated from a parent sample (Cal) by steaming and acid leaching. The hydrophobicity of these samples was compared to the hydrophobicity of a purely siliceous BEA. The hydrophobicity was assessed by Weitkamp original or modified relation for water and toluene competitive and non-competitive adsorption under static or dynamic conditions. According to sample dealumination method, water adsorption sites were identified to be Brønsted acidic sites, silanols nests, extra-framework aluminum species (EFAl), microporous and mesoporous available volume. Toluene molecules were mainly physisorbed by capillary condensation in microporous and mesoporous volume. Hydrophobicity indexes achieved by non-competitive adsorption under static or dynamic conditions were not a good assessment of zeolites hydrophobicity in operating industrial conditions. On the other hand water and toluene competitive adsorption under dynamic conditions as described by Weitkamp remains an appropriate method to estimate the hydrophobicity of zeolites. According to the nature of treatment, hydrophobicity index (HI) increases when aluminum content decreases. This increase is moderate for acid leached samples and pronounced for steamed samples.

The Effect of Hydrophobic Modification of Zeolites on CO 2 Absorption Enhancement

Two methods of the modification of zeolite were employed: framework element modification and surface coating, and the influence of the zeolites before and after modification on the CO 2 absorption was investigated. It was found that although hydrophobicity of zeolite could be obtained by means of the surficial organic coating inthe method of surface coating modification, partial channel of zeolite would be plugged, as a result, leading to the surface area reducing greatly. Distinctively, the framework element modification method could maintain not only complete lattice structure and adsorption capability of zeolite, but would also obtain a good hydrophobic property. Consequently, significant enhancement on gas absorption by this modified zeolite was achieved and up to a maximum enhancement factor of 2.62. This shows that the solid particles with good enhancement role to gas absorptionneed not only good adsorptive capability but also certain hydrophobicity. An unsteady heterogeneous model wasemployed to predict enhancement factor and the calculated results agree well with the experimental data.

Photocatalytic oxidation of acetaldehyde with oxygen on TiO 2/ZSM-5 photocatalysts: Effect of hydrophobicity of zeolites

TiO 2 photocatalysts hybridized with ZSM-5 were prepared by the impregnation of (NH 4) 2[TiO(C 2O 4) 2] aqueous solution. This is one of the most ideal green processes in establishing TiO 2 photocatalysts because of an organic solvent-free preparation method. The highly hydrophobic H-ZSM-5 with low Al 2O 3 content was effective as an adsorbent for acetaldehyde molecules. These results suggest the close relationship between the hydrophobicity of the zeolite and the high photocatalytic reactivity of TiO 2 prepared on zeolites compared with unloaded TiO 2 catalysts. However, acetaldehyde molecules were strongly trapped on the Na + sites of ZSM-5 and their efficient diffusion onto the TiO 2 sites was inhibited, resulting in low photocatalytic reactivity. The photocatalytic oxidation of acetaldehyde over TiO 2/ZSM-5 was improved in the coexistence of H 2O vapor. The role of the H 2O molecules was estimated to be the efficient formation of OH radicals and the prevention of strong adsorption of acetaldehyde on the TiO 2 sites.