Comparative Study of the Effect of Defects on Selective Adsorption of Butanol from Butanol/Water Binary Vapor Mixtures in Silicalite-1 Films.

A promising route for sustainable 1-butanol (butanol) production is ABE (acetone, butanol, ethanol) fermentation. However, recovery of the products is challenging because of the low concentrations obtained in the aqueous solution, thus hampering large-scale production of biobutanol. Membrane and adsorbent-based technologies using hydrophobic zeolites are interesting alternatives to traditional separation techniques (e.g., distillation) for energy-efficient separation of butanol from aqueous mixtures. To maximize the butanol over water selectivity of the material, it is important to reduce the number of hydrophilic adsorption sites. This can, for instance, be achieved by reducing the density of lattice defect sites where polar silanol groups are found. The density of silanol defects can be reduced by preparing the zeolite at neutral pH instead of using traditional synthesis solutions with high pH. In this work, binary adsorption of butanol and water in two silicalite-1 films was studied using in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy under equal experimental conditions. One of the films was prepared in fluoride medium, whereas the other one was prepared at high pH using traditional synthesis conditions. The amounts of water and butanol adsorbed from binary vapor mixtures of varying composition were determined at 35 and 50 °C, and the corresponding adsorption selectivities were also obtained. Both samples showed very high selectivities (100-23 000) toward butanol under the conditions studied. The sample having low density of defects, in general, showed ca. a factor 10 times higher butanol selectivity than the sample having a higher density of defects at the same experimental conditions. This difference was due to a much lower adsorption of water in the sample with low density of internal defects. Analysis of molecular simulation trajectories provides insights on the local selectivities in the zeolite channel network and at the film surface.

Adsorption of Butanol and Water Vapors in Silicalite-1 Films with a Low Defect Density.

Pure silica zeolites are potentially hydrophobic and have therefore been considered to be interesting candidates for separating alcohols, e.g., 1-butanol, from water. Zeolites are traditionally synthesized at high pH, leading to the formation of intracrystalline defects in the form of silanol defects in the framework. These silanol groups introduce polar adsorption sites into the framework, potentially reducing the adsorption selectivity toward alcohols in alcohol/water mixtures. In contrast, zeolites prepared at neutral pH using the fluoride route contain significantly fewer defects. Such crystals should show a much higher butanol/water selectivity than crystals prepared in traditional hydroxide (OH-) media. Moreover, silanol groups are present at the external surface of the zeolite crystals; therefore, minimizing the external surface of the studied adsorbent is important. In this work, we determine adsorption isotherms of 1-butanol and water in silicalite-1 films prepared in a fluoride (F-) medium using in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. This film was composed of well intergrown, plate-shaped b-oriented crystals, resulting in a low external area. Single-component adsorption isotherms of 1-butanol and water were determined in the temperature range of 35-80 °C. The 1-butanol isotherms were typical for an adsorbate showing a high affinity for a microporous material and a large increase in the amount adsorbed at low partial pressures of 1-butanol. The Langmuir-Freundlich model was successfully fitted to the 1-butanol isotherms, and the heat of adsorption was determined. Water showed a very low affinity for the adsorbent, and the amounts adsorbed were very similar to previous reports for large silicalite-1 crystals prepared in a fluoride medium. The sample also adsorbed much less water than did a reference silicalite-1(OH-) film containing a high density of internal defects.The results show that silicalite-1 films prepared in a F- medium with a low density of defects and external area are very promising for the selective recovery of 1-butanol from aqueous solutions.

Effect of phosphate on heterogeneous Fenton oxidation of catechol by nano-Fe3O4 Inhibitor or stabilizer?

The effect of phosphate on adsorption and oxidation of catechol, 1,2-dihydroxybenzene, in a heterogeneous Fenton system was investigated. In situ attenuated total reflectance infrared spectroscopy (ATR-FTIR) was used to monitor the surface speciation at the nano-Fe3O4 catalyst surface. The presence of phosphate decreased the removal rate of catechol and the abatement of dissolved organic compounds, as well as the decomposition of H2O2. This effect of phosphate was mainly due to its strong reaction with surface sites on the iron oxide catalyst. At neutral and acid pH, phosphate could displace the adsorbed catechol from the surface of catalyst and also could compete for surface sites with H2O2. In situ IR spectra indicated the formation of iron phosphate precipitation at the catalyst surface. The iron phosphate surface species may affect the amount of iron atoms taking part in the catalytic decomposition of H2O2 and formation of hydroxyl radicals, and inhibit the catalytic ability of Fe3O4 catalyst. Therefore, phosphate ions worked as stabilizer and inhibitor in a heterogeneous Fenton reaction at the same time, in effect leading to an increase in oxidation efficiency in this study. However, before use of phosphate as pH buffer or H2O2 stabilizer in a heterogeneous Fenton system, the possible inhibitory effect of phosphate on the actual removal of organic pollutants should be fully considered.

Adsorption Behavior of Cellulose and Its Derivatives toward Ag(I) in Aqueous Medium: An AFM, Spectroscopic, and DFT Study.

The aim of this study was to develop a fundamental understanding of the adsorption behavior of metal ions on cellulose surfaces using experimental techniques supported by computational modeling, taking Ag(I) as an example. Force interactions among three types of cellulose microspheres (native cellulose and its derivatives with sulfate and phosphate groups) and the silica surface in AgNO3 solution were studied with atomic force microscopy (AFM) using the colloidal probe technique. The adhesion force between phosphate cellulose microspheres (PCM) and the silica surface in the aqueous AgNO3 medium increased significantly with increasing pH while the adhesion force slightly decreased for sulfate cellulose microspheres (SCM), and no clear adhesion force was observed for native cellulose microspheres (CM). The stronger adhesion enhancement for the PCM system is mainly attributed to the electrostatic attraction between Ag(I) and the negative silica surface. The observed force trends were in good agreement with the measured zeta potentials. The scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analyses confirmed the presence of silver on the surface of cellulose microspheres after adsorption. This study showed that PCM with a high content of phosphate groups exhibited a larger amount of adsorbed Ag(I) than CM and SCM and possible clustering of Ag(I) to nanoparticles. The presence of the phosphate group and a wavenumber shift of the P-OH vibration caused by the adsorption of silver ions on the phosphate groups were further confirmed with computational studies using density functional theory (DFT), which gives support to the above findings regarding the adsorption and clustering of Ag(I) on the cellulose surface decorated with phosphate groups as well as IR spectra.

Adsorption of water and butanol in silicalite-1 film studied with in situ attenuated total reflectance-Fourier transform infrared spectroscopy.

Biobutanol produced by, e.g., acetone-butanol-ethanol (ABE) fermentation is a promising alternative to petroleum-based chemicals as, e.g., solvent and fuel. Recovery of butanol from dilute fermentation broths by hydrophobic membranes and adsorbents has been identified as a promising route. In this work, the adsorption of water and butanol vapor in a silicalite-1 film was studied using in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy to better understand the adsorption properties of silicalite-1 membranes and adsorbents. Single-component adsorption isotherms were determined in the temperature range of 35-120 °C, and the Langmuir model was successfully fitted to the experimental data. The adsorption of butanol is very favorable compared to that of water. When the silicalite-1 film was exposed to a butanol/water vapor mixture with 15 mol % butanol (which is the vapor composition of an aqueous solution containing 2 wt % butanol, a typical concentration in an ABE fermentation broth, i.e., the composition of the gas obtained from gas stripping of an ABE broth) at 35 °C, the adsorption selectivity toward butanol was as high as 107. These results confirm that silicalite-1 quite selectively adsorbs hydrocarbons from vapor mixtures. To the best of our knowledge, this is the first comprehensive study on the adsorption of water and butanol in silicalite-1 from vapor phase.

The facile assembly of nanocrystals by optimizing humidity.

The ambient humidity and the nature of substrates are considered coordinately in the assembly of nano-sized crystals. The nanocrystal monolayers show large-area uniformity without any aggregates. Zeolite and hematite monolayers with thicknesses of 20-100 nm and excellent orientations are produced.

Influence of Zn(II) on the adsorption of arsenate onto ferrihydrite.

Addition of iron oxide to arsenic-contaminated soil has been proposed as a means of reducing the mobility of arsenic in the soil. Arsenic and zinc are common coexisting contaminants in soils. The presence of zinc therefore may affect the adsorption properties of arsenic on iron oxide, and may thus affect its mobility in the soil. The influence of Zn(II) on the adsorption of arsenate ions on iron oxide was studied. Batch adsorption experiments indicated that Zn(II) increased the arsenate removal from a solution by ferrihydrite at pH 8. However, ATR-FTIR spectroscopy showed that no adsorption of arsenate on a ferrihydrite film occurred at pD 8 in the presence of Zn(II). Precipitation of zinc hydroxide carbonate followed by arsenate adorption onto the precipitate was found to be a plausible mechanism explaining the arsenate removal from a solution in the presence of Zn(II) at pH/pD 8. The previously suggested mechanisms attributing the enhanced removal of arsenate from solution in the presence of Zn(II) to additional adsorption on iron oxides could not be verified under the experimental conditions studied. It was also shown that at pH/pD 4, the presence of Zn(II) in the system did not significantly affect the adsorption of arsenate on ferrihydrite.

Surface complexation modeling of Fe3O4-H+ and Mg(II) sorption onto maghemite and magnetite.

The surface acid/base properties of magnetite (Fe(3)O(4)) particles and the sorption of Mg(2+) onto magnetite and maghemite (γ-Fe(2)O(3)) have been studied using high precision potentiometric titrations, batch experiments, and zeta potential measurements. The acid/base properties of magnetite were found to be very similar to maghemite except for the difference in surface site density, N(s) (sites nm(-2)), 1.50±0.08 for magnetite, and 0.99±0.05 for maghemite. The experimental proton exchange of the magnetite surface increased from pH 10 and above, indicating dissolution/transformation reactions of magnetite at alkaline conditions. Thus, magnetite with its Fe(II) content proved to be less stable toward dissolution in comparison with pure Fe(III) oxides also at high pH values. Three different ratios between surface sites and added Mg(2+) were used in the sorption experiments viz. 0.5, 1, and 2Mg(2+)site(-1). Surface complexation modeling of the Mg(2+) sorption onto maghemite and magnetite was restricted to pH conditions where the interference from Mg(OH)(2)(s) precipitation could be ruled out. The model calculations showed that Mg(2+) sorb onto the magnetite and maghemite surfaces as a mixture of mono- or bidentate surface complexes at 0.5Mg(2+)site(-1) and as monodentate complexes at 1 and 2Mg(2+)site(-1) conditions. Mg(2+) was also found to adsorb more readily at the maghemite surfaces in comparison with magnetite surfaces. For experiments with excess Mg(2+) relative to the number of surface sites, the calculations suggested the formation of polynuclear surface complexes on maghemite.

Sorption of phosphate onto mesoporous γ-alumina studied with in-situ ATR-FTIR spectroscopy.

BACKGROUND: Due to the extensive use of phosphates in industry, agriculture and households, the phosphate - γ-alumina interactions are important for understanding its detrimental contribution to eutrophication in lakes and rivers. In situ Fourier transform infrared (FTIR) spectroscopy can provide more detailed information on the adsorbate-adsorbent interaction and the formation of hydrogen bonds. RESULTS: In situ ATR-FTIR spectroscopy was used to identify phosphate complexes adsorbed within the three-dimensional network of mesoporous γ-alumina at pH 4.1 and 9.0. The integrated intensity between 850 cm-1 and 1250 cm-1 was used as a relative measure of the amount of adsorbed phosphate. The integrated intensity proved to be about 3 times higher at pH 4.1 as compared with the corresponding intensity at pH 9.0. The adsorption of phosphate at the two pH conditions could be well described by the Langmuir adsorption isotherm at low concentrations and the empirical Freundlich adsorption isotherm for the whole concentration range, viz. 5 - 2000 µM. CONCLUSIONS: From the band shape of infrared spectra at pH 4.1 and pH 9.0, it was proposed that the symmetry of the inner-sphere surface complex formed between phosphate and γ-alumina was C1 at the lower pH value, whilst the higher value (9.0) implied a surface complex with C2v or C1 symmetry. The difference in adsorbed amount of phosphate at the two pH values was ascribed to the reduced fraction of ≡ AlOH2+ surface sites and the increased fraction of ≡ AlO- sites upon increasing pH from 4 to 9.

In situ ATR-FTIR studies on the competitive adsorption of arsenate and phosphate on ferrihydrite.

In the present study, in situ ATR-FTIR spectroscopy was used for the first time to study the competitive adsorption of phosphate and arsenate on ferrihydrite. Deuterium oxide was used as solvent to facilitate the interpretations of recorded infrared spectra. It was found that arsenate and phosphate adsorbed more strongly at lower pD-values, showing similarities in the adsorption behavior as a function of pD. However, arsenate complexes were found to be more strongly adsorbed than phosphate complexes in the pD range studied. About five times higher concentration of phosphate in solution was needed to reduce the absorbance due to pre-adsorbed arsenate to the same relative level as for pre-adsorbed phosphate, which was desorbed using a solution containing equal (molar) concentrations in arsenate and phosphate. At pD 4, two phosphate complexes were adsorbed on the iron oxide, one deuterated and one de-deuterated. When phosphate was pre-adsorbed and arsenate subsequently added to the system, the deuterated phosphate complex desorbed rapidly while the de-deuterated phosphate complex was quite stable. At pD 8.5, only the de-deuterated phosphate complex was adsorbed on the iron oxide. Moreover, the arsenate adsorbed was also predominantly de-deuterated as opposite to the arsenate adsorbed at pD 4. During the substitution experiments the configuration of these complexes on the iron oxide surface did not change. To the best of our knowledge, this is the first time this difference in stability of the different phosphate complexes is reported and shows the power of employing in situ spectroscopy for this kind of studies.

Competition between sodium oleate and sodium silicate for a silicate/oleate modified magnetite surface studied by in situ ATR-FTIR spectroscopy.

Attenuated Total Reflection (ATR) IR spectroscopy was utilized to monitor adsorption of sodium oleate and sodium silicate onto synthetic magnetite at pH=8.5, both individually and in a competitive manner. Oleate was adsorbed within a concentration range of 0.01-0.5 mM. It was observed that adsorption of oleate increased linearly with increasing concentration of oleate in solution up to a concentration of 0.1 mM. The infrared spectrum of oleate showed a broad single band at 1535 cm(-1) assigned to the asymmetric stretching vibration of carboxylate, implying chemisorption of oleate to the magnetite surface. The kinetics of oleate adsorption followed a pseudo first-order reaction with an apparent rate constant of k(1)=0.030+/-0.002 min(-1). Competitive adsorption of silicate and oleate was performed either by adding silicate solution to a magnetite film initially equilibrated with 0.1 mM oleate or adding oleate solution to magnetite treated with silicate solutions in the concentration range 0.1-5 mM. It was shown that silicate, within reasonable time, had only minor effect on the amount of oleate already adsorbed on magnetite. On the other hand, oleate did not efficiently compete with silicate if the latter substance was already adsorbed on the iron oxide.

An ATR-FTIR study of sulphate sorption on magnetite; rate of adsorption, surface speciation, and effect of calcium ions.

The adsorption of sulphate on magnetite was studied in-situ using ATR-FTIR spectroscopy. Synthetic magnetite particles were deposited on a ZnSe internal reflection element and the spectra of sulphate adsorbed at pH 4-8.5 were recorded. Two different ionic strengths were used viz. 0.01 M and 0.1 M NaCl. The spectra of adsorbed sulphate on magnetite coated ZnSe were compared with the spectra of sulphate solutions at the same pH values and in contact with uncoated ZnSe. The spectrum of adsorbed sulphate at pH 4 showed three maxima at 979, 1044, and 1115 cm(-1) indicating a monodentate adsorption in which the T(d) symmetry of SO(4)(2-) is lowered to C(3v). At pH 6.5, sulphate adsorbed as an outer-sphere complex with two weak bands appearing at 1102 and 980 cm(-1). Moreover, spectra of the adsorbed sulphate at pH 4 were recorded as a function of time and sulphate concentration. The equilibrium absorbance at different concentrations fitted a Langmuir type adsorption isotherm. The Langmuir affinity constant K at pH 4 was determined from the slope and intercept of the Langmuir plot to be K=1.2344x10(4) M(-1) and the Gibbs free energy of adsorption DeltaG(ads)(0) was estimated from this value to be -33.3 kJ/mol. Kinetic analysis indicated that adsorption at pH 4 is fast, whilst the desorption kinetic at the same pH is very slow. In addition, the effect of Ca ions on sulphate adsorption was also studied. It was shown that Ca ions increased the sulphate adsorption on magnetite at pH 8.5.

A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy.

The sorption of sodium silicate by synthetic magnetite (Fe3O4) at different pH conditions (pH 7-11) and initial silicate concentrations (1 x 10(-3) and 10 x 10(-3) molL(-1)) was studied using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The analysis of infrared spectra of sodium silicate in solution as well as adsorbed on magnetite nano-particles clearly showed the evolution of different silicate species depending on pH and silica concentration. The silicate concentration studied (10 x 10(-3) molL(-1)) contained polymeric or condensed silicate species at lower pH as well as monomers at high pH, as evident from infrared spectra. Condensation of monomers resulted in an increased intensity of absorptions in the high frequency part (>1050 cm(-1)) of the spectral region, which contains information about both silicate in solution and sorbed silicate viz. 1300 cm(-1)-850 cm(-1). In the pH range studied, infrared spectra of sorbed silicate and sorbed silicate during desorption both indicated the presence of different types of surface complexes at the magnetite surface. The sorption mechanism proposed is in accordance with a ligand exchange reaction where both monodentate and bidentate complexes could exist at low surface loading level, the relative proportion of the complexes being due to both pH and concentration in solution. Oligomerization occurred on the magnetite surface at higher surface loading.

Study of potassium O,O'-Dibutyldithiophosphate combining DFT, 31P CP/MAS NMR and infrared spectroscopy.

Dithiophosphates are used in many different industrial applications. To explain their functions and properties in these applications, a fundamental understanding on a molecular level is needed. Potassium O, O'-Dibutyldithiophosphate and its anion have been investigated by means of a combination of DFT and (31)P CP/MAS NMR and infrared spectroscopy. Several low-energy conformations were studied by DFT. Three different conformations with significantly different torsion angles of the O-C bond relative to the O-P-O plane were selected for further studies of infrared frequencies and (31)P NMR chemical-shift tensors. A good agreement between theoretical and experimental results was obtained, especially when the IR spectra or (31)P chemical shift tensor parameters of all three conformations were added, indicating that, because of the low energy difference between the conformations, the molecules are rapidly fluctuating between them.

A theoretical and experimental study of vibrational properties of alkyl xanthates.

Geometrical structure and vibrational modes of potassium and sodium ethyl/heptyl xanthates were studied, using both theoretical and experimental methods. Both Hartree-Fock and density functional theory were used. The experimental method used was infrared absorption spectroscopy (FTIR). Our work showed that vibrational frequencies calculated with density functional theory, using the local density approximation, are in very good agreement with experiments. The results were not improved by using the more sophisticated and computationally demanding B3LYP functional.

n-Heptyl xanthate adsorption on a ZnS layer synthesized on germanium: an in situ attenuated total reflection IR study.

Adsorption of n-heptyl xanthate on synthesized zinc sulfide was followed in situ by monitoring the methylene absorption band at 2925 cm(-1). The zinc sulfide surface used in the adsorption experiments was synthesized on a germanium internal reflection element using the chemical bath deposition method. Characterization of the adsorbent surface was performed by X-ray photoelectron spectroscopy. The time needed to reach adsorption equilibrium varied with the initial concentration of the aqueous potassium heptyl xanthate solution. The amount of adsorbed xanthate ions increased with the concentration of the solution within the range studied (10 microM-50 mM). The experimental data are reasonably well described by the Langmuir adsorption isotherm. Polarized infrared attenuated total reflection (ATR) was used to determine the average orientation of the heptyl chains by measuring the absorbance of the infrared beam polarized perpendicularly and parallely to the plane of incidence. Measured absorbances were corrected for contribution from heptyl xanthate in bulk solution.

Zeolite coated ATR crystals for new applications in FTIR-ATR spectroscopy.

Thin silicalite-1 films were grown on ATR crystals and used for detection of low amounts of organic molecules in a gas flow by FTIR spectroscopy.