The nature of active sites on zeolites VI. The influence of calcination temperature on the structural hydroxyl groups and acidity of stabilized hydrogen Y zeolite

The sorption of phosphate by montmorillonite at 10, 30, and 50 oC were investigated aiming to mainly determine thermodynamic parameters for the formation of surface complexes in the adsorption of phosphate ions by montmorillonite. Data were collected by adsorption edge experiments investigating the effect of pH, adsorption isotherms enabling the effect of sorbate concentration, and acid-base titration calculating protons released or taken up by adsorption process. Data analysis was carried out using surface complexation model to fit the data collected in this study using the parameters obtained from previous study, as well as to calculate the values of ΔH and ΔS. Previous study reported that phosphate ions formed two outer-sphere surface complexes with active sites of montmorillonite through hydrogen bonding. In the first complex, [(XH)0– H2L─]─, the phosphate was held to permanent-charge X─ sites on the tetrahedral siloxane faces, and the second complex, [[(SO─)(SOH)]– – [H2L]─] 2─ was formed through the interaction between the phosphate and variable charge surface hydroxyl groups at the edges of montmorillonite crystals and on the octahedral alumina faces. The values of ΔH for the first and second reactions are 39.756 and 3.765x10-7 kJ mol‒1 respectively. Since both reactions have positive enthalpy values, it can be concluded that the reactions are endothermic. Large energy for the first reaction is needed by X─ sites (permanent negatively charge sites of montmorillonite) to be partially desolvated, on which K+ or other surface cations are replaced by H+ ions in the surface protonated process, and are then ready to interact phosphate ions in the solution. Small values of ΔH for the second reactions indicates that hydrogen bonds formed by phosphate and SOH sites in the second reaction are easily broken out, and the phosphate can easily desorbed from the surface. The values of ΔS for the first and second reactions are 122.523 and 2.393 x10-2 J K‒1 mol‒1, which are greater than -10 kJ mol‒1 and indicates that the surface reactions occurs through dissociative mechanisms.Keywords: montmorillonite, adsorption edge, extended constant capacitance, surface complexation model, enthalpy, reaction mechanisms

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Nanocomposites SnO2/SiO2:SiO2 Impact on the Active Centers and Conductivity Mechanism

Materials ◽

10.3390/ma12213618 ◽

2019 ◽

Vol 12 (21) ◽

pp. 3618 ◽

Cited By ~ 2

Author(s):

Dayana Gulevich ◽

Marina Rumyantseva ◽

Artem Marikutsa ◽

Tatyana Shatalova ◽

Elizaveta Konstantinova ◽

...

Keyword(s):

Active Sites ◽

Nitrogen Adsorption ◽

Hydroxyl Groups ◽

Pure Sno2 ◽

Active Centers ◽

Paramagnetic Resonance ◽

Surface Hydroxyl ◽

Chemisorbed Oxygen ◽

Temperature Dependences ◽

Sio2 Nanocomposites

This paper is focused on the effect of the stabilizing component SiO2 on the type and concentration of active sites in SnO2/SiO2 nanocomposites compared with nanocrystalline SnO2. Previously, we found that SnO2/SiO2 nanocomposites show better sensor characteristics in CO detection (lower detection limit, higher sensor response, and shorter response time) compared to pure SnO2 in humid air conditions. Nanocomposites SnO2/SiO2 synthesized using the hydrothermal method were characterized by low temperature nitrogen adsorption, XRD, energy dispersive X-ray spectroscopy (EDX), thermo-programmed reduction with hydrogen (TPR-H2), IR-, and electron-paramagnetic resonance (EPR)-spectroscopy methods. The electrophysical properties of SnO2 and SnO2/SiO2 nanocomposites were studied depending on the oxygen partial pressure in the temperature range of 200–400 °C. The introduction of SiO2 results in an increase in the concentration of paramagnetic centers Sn3+ and the amount of surface hydroxyl groups and chemisorbed oxygen and leads to a decrease in the negative charge on chemisorbed oxygen species. The temperature dependences of the conductivity of SnO2 and SnO2/SiO2 nanocomposites are linearized in Mott coordinates, which may indicate the contribution of the hopping mechanism with a variable hopping distance over local states.

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Modified xanthan gum for crystal violet uptake: kinetic, isotherm, and thermodynamic behaviors

Water Science & Technology ◽

10.2166/wst.2019.038 ◽

2019 ◽

Vol 79 (1) ◽

pp. 165-174 ◽

Cited By ~ 3

Author(s):

Meixia Zheng ◽

Fengli Lian ◽

Yujing Zhu ◽

Bo Liu ◽

Zheng Chen ◽

...

Keyword(s):

Crystal Violet ◽

Active Sites ◽

Xanthan Gum ◽

Maximum Adsorption Capacity ◽

Hydroxyl Groups ◽

X Ray Diffraction ◽

Uptake Kinetic ◽

Freundlich Isotherms ◽

Pseudo Second Order ◽

Kinetics Of

Abstract Modified xanthan gum (XG-AM-TTE) was employed as an adsorbent to study the adsorption behavior, thermodynamics and kinetics of crystal violet (CV) from an aqueous solution. Fourier transform infrared spectroscopy analysis indicates that the functional groups present in the adsorbent, such as carboxyl, ester and hydroxyl groups, are included on the external surface of the material, and these groups are potential active sites for interaction with CV. According to X-ray diffraction results, the structure of XG-AM-TTE after CV adsorption became more disordered, and the microstructure change is an indication of effective adsorption of CV to the surface, with CV becoming remarkably dispersed in the adsorbent according to the scanning electron microscopy observations. The adsorption kinetics and adsorption equilibrium were best described by the pseudo-second-order model and Freundlich isotherms, respectively. The thermodynamic parameters, as the Gibbs-free energy (ΔG), enthalpy (ΔH) and entropy (ΔS), indicated that the adsorption is a spontaneous, endothermic and entropy increase process. The maximum adsorption capacity of XG-AM-TTE was 183 ± 12 mg/g, suggesting that XG-AM-TTE is an efficient adsorbent.

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