K-edge XANES Spectra Research Articles

Enhanced arsenic(III) sequestration via sulfidated zero-valent iron in aerobic conditions: Adsorption and oxidation coupling processes

Sulfidated zero-valent iron (S-ZVI) has shown significant potential for the removal of arsenic(III). However, little attention has been paid to the mechanism of As(III) sequestration enhancement and how the phase transformation for S-ZVI strengthens this process in aerobic conditions. In this work, sulfidated ZVI was created by ball-milling (S-ZVIbm) and liquid-mixing (S-ZVIlm) of ZVI with elemental sulfur(S0) to investigate the performance and mechanisms of As(III) sequestration in air-saturated water. Sulfidation was found to significantly enhance the As(III) removal rate constant, which was 2.8 ∼ 6.7 times (S-ZVIbm) and 3.1 ∼ 17.1 times (S-ZVIlm) higher than that without sulfidation. FeS was identified as the predominant sulfur species in the S-ZVI samples using S K-edge XANES spectra. The enhanced electron transfer and ZVI corrosion after sulfidation were verified via electrochemical tests. XANES and Mössbauer spectra suggested that lepidocrocite(γ-FeOOH) was the predominant corrosion product generated on the ZVI surface with the presence of oxygen, and DFT calculations further confirmed the improved performance of γ-FeOOH for As(III) sequestration. Besides, As(III) oxidation occurred dominantly on the heterogeneous surface rather than in solution, and the As(III) sequestration pathway of adsorption followed by oxidation was proposed. This study provides new insight into the enhanced As(III) sequestration by S-ZVI in aerobic conditions.

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model.

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Changes in the sulfur forms of subbituminous coals during oxidation with hydrogen peroxide and peracetic acid

We have investigated chemical desulfurization of subbituminous coals to prepare ultra-low sulfur coal for the converter process in the steel industry. Herein, we report the oxidative desulfurization of thiophene contained in two subbituminous coals using two peroxides, hydrogen peroxide and peracetic acid. Sulfur K-edge X-ray absorption near edge structure analysis was carried out to follow the change in sulfur form during oxidation. Sulfur form distribution was determined quantitatively by deconvolution of the pyrite, thiophene, sulfoxide, sulfone, and sulfate peaks in the Sulfur K-edge XANES spectra of the raw and oxidized samples using the least-squares method. In the presence of hydrogen peroxide, oxidation was promoted only at 60 °C. Furthermore, an oxidation treatment was carried out at 20 °C using peracetic acid generated by mixing hydrogen peroxide and acetic anhydride, which selectively reacted with the sulfur atoms. Treatment with peracetic acid promoted the oxidation of thiophene and improved the desulfurization extent with low carbon loss. Sulfone-type sulfur formed by the oxidation of thiophene could be completely volatilized by heat treatment at 400 °C under a N2 gas stream. By combining pre-oxidation with peracetic acid treatment and pyrolysis, 74 % of total sulfur and 72 % of thiophene could be removed while recovering approximately 90 % of total carbon as char.

DFT combined with XANES to investigate the sulfur fixation mechanisms of H2S on different CaO surfaces

In this study, Erdos (ED) coal with high inorganic sulfur content was selected to investigate the absorbed effects of H2S on the perfect or defective surfaces of CaO (001). Pyrolysis connected with gas chromatography (Py-GC) was used to analyze H2S release behavior during coal pyrolysis. S K-edge XANES spectrum can prove that CaO fixing sulfur mainly generates CaS. The O vacancy of CaO possesses the best adsorption effect of H2S. Through the electronic structure and orbital analysis, O atom can be confirmed to the active site on the surfaces of CaO (001), and the O vacancy belongs to a strong electron-donating system. Different paths of H2S fixation can undergo on both perfect surface and O vacancy of CaO. After 700 °C, the most possible paths would happen and generate CaS and clean energy H2 on the surface of the O vacancies. However, it is impossible for Ca vacancies to be generated and to further fix H2S during pyrolysis under inert atmosphere. These calculated results are very consistent with these experimental results. Thus, this study would provide theoretical support for the sulfur fixation mechanisms on the surface of CaO (001) and for the subsequent improvement of the CaO sulfur fixation agent.

Structural and Optical Properties of Mn-Incorporated BiVO4 Nanoparticles Synthesized by Sonochemical Process

In this work, Mn-incorporated BiVO4 nanoparticles with different Mn contents (0–5%) were synthesized by one-step sonochemical process. Structural phase of Mn-incorporated BiVO4 nanoparticles was extensively carried out by X-ray diffraction technique. The XRD results show that the structural phase of BiVO4 belongs to the monoclinic principal structure while the secondary phases of MnO, Mn2O3, and Mn3O4 structures are noticed in Mn-loaded samples. Chemical bondings of Mn-incorporated BiVO4 nanoparticles were characterized with Raman spectroscopy. Relevant chemical bondings confirm the existence of VO4 3− tetrahedron and the V–O band by Raman spectra. Diffuse reflection spectroscopy and FE-SEM were employed to examine their optical and morphological properties, respectively. The results indicate that the incorporation of Mn can induce the red-shift of optical absorption edge in visible range, which can decrease the optical band gap of BiVO4. Morphology of all samples demonstrates the difference in shape of the compounds with the incorporation of Mn contents. Oxidation number and local structure of Mn-incorporated BiVO4 nanoparticles were investigated by X-ray absorption spectroscopy. XAS results demonstrate that oxidation number of all elements correspond to Mn2+/Mn3+, Bi3+, and V5+ ions. The normalized Mn K-edge XANES spectra suggest that Mn atoms would not locate at the local site of Bi or V in BiVO4 crystal because the specific feature of experimental spectra are inconsistent with simulated spectra, which is corresponded to XRD results indicating the formation of Mn-based oxide structures with various oxidation states.

Investigation of Local Structure of ZnO with High Doping Copper Concentration (≥13 %mole) Prepared by Co-Precipitation Method

Cu-doped ZnO (CZO) with Cu %mole of 0, 13, 14, and 15% have been synthesized by the co-precipitation method with controlled pH at 9. The diffraction pattern of CZO shows that a single phase of ZnO with wurtzite structure (w-ZnO) was achieved in the parent compound. In contrast, small fractions of the secondary phase of CuO monoclinic (m-CuO) were identified in the doped compounds. Rietveld-refinement on the X-Ray Diffraction (XRD) patterns reveals that the crystallite size of CZO is estimated in the range of 84 - 148 nm with instrumental correction factor and that CZO14 exhibits the most considerable fraction of m-CuO among the samples. Interestingly, lattice constants decrease by Cu doping. The effects of Cu doping on the valence state and the local structure were investigated by X-Ray Absorption Spectroscopy (XAS). Based on our analysis on both Cu K-edge and Zn K-edge XANES spectra, oxidation states of Cu and Zn ions are about +2 with no evidence of other valence states. Cu atom is likely to incorporate into wurtzite crystal lattice by occupying Zn sites in a small portion, and the doped compounds create a few oxygen vacancies.

XANES spectroscopy of sulfides stable under reducing conditions

Abstract X-ray absorption near-edge structure (XANES) spectroscopy is a powerful technique to quantitatively investigate sulfur speciation in geologically complex materials such as minerals, glasses, soils, organic compounds, industrial slags, and extraterrestrial materials. This technique allows non-destructive investigation of the coordination chemistry and oxidation state of sulfur species ranging from sulfide (2–oxidation state) to sulfate (6+ oxidation state). Each sulfur species has a unique spectral shape with a characteristic K-edge representing the s → p and d hybridization photoelectron transitions. As such, sulfur speciation is used to measure the oxidation state of samples by comparing the overall XANES spectra to that of reference compounds. Although many S XANES spectral standards exist for terrestrial applications under oxidized conditions, new sulfide standards are needed to investigate reduced (oxygen fugacity, fO2, below IW) silicate systems relevant for studies of extraterrestrial materials and systems. Sulfides found in certain meteorites (e.g., enstatite chondrites and aubrites) and predicted to exist on Mercury, such as CaS (oldhamite), MgS (niningerite), and FeCr2S4 (daubréelite), are stable at fO2 below IW-3 but rapidly oxidize to sulfate and/or produce sulfurous gases under terrestrial surface conditions. XANES spectra of these compounds collected to date have been of variable quality, possibly due to the unstable nature of certain sulfides under typical (e.g., oxidizing) laboratory conditions. A new set of compounds was prepared for this study and their XANES spectra are analyzed for comparison with potential extraterrestrial analogs. S K-edge XANES spectra were collected at Argonne National Lab for FeS (troilite), MnS (alabandite), CaS (oldhamite), MgS (niningerite), Ni1–xS, NiS2, CaSO4 (anhydrite), MgSO4, FeSO4, Fe2(SO4)3, FeCr2S4 (daubréelite), Na2S, Al2S3, Ni7S6, and Ni3S2; the latter five were analyzed for the first time using XANES. These standards expand upon the existing S XANES end-member libraries at a higher spectral resolution (0.25 eV steps) near the S K-edge. Processed spectra, those that have been normalized and “flattened,” are compared to quantify uncertainties due to data processing methods. Future investigations that require well-characterized sulfide standards, such as the ones presented here, may have important implications for understanding sulfur speciation in reduced silicate glasses and minerals with applications for the early Earth, Moon, Mercury, and enstatite chondrites.

X-ray absorption study of the local structure of Mg in hydroxyapatites thin films deposited by RF magnetron Co-Sputtering

Mg-doped hydroxyapatite (HAp) were deposited on Ti–6Al–4V substrates by radiofrequency (RF) magnetron co-sputtering of HAp and Mg targets. The as-deposited films were amorphous. Therefore, they were annealed at 1000 °C in ambient air. After annealing, the crystal structure, local structure, coatings morphology, and composition were investigated by X-ray diffraction technique (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDX), respectively. The XRD results show the amorphous phase for as-deposited and the major phase of TiO2 with a few secondary phase. The Mg K-edge XANES spectra was examined in order to identify the local site of Mg in hydroxyapatite thin films, which is formed as MgO in as-deposited sample and as MgAl2O4 in annealed sample. The information on the locally determined structures were obtained and discussed. The surface morphologies show the spherical shape particles with uniform distribution on the surface and new agglomeration of particles of MgAl2O4 on the surface of as-deposited and annealed samples, respectively.

Solution XAS Analysis for Exploring Active Species in Syndiospecific Styrene Polymerization and 1-Hexene Polymerization Using Half-Titanocene–MAO Catalysts: Significant Changes in the Oxidation State in the Presence of Styrene

Mechanistic studies through Ti K-edge XANES and EXAFS spectra of the catalyst solution for 1-hexene polymerization using Cp*TiX2(O-2,6-iPr2C6H3) [X = Cl (1), Me (2)]–MAO catalysts and for syndiospe...

The influence of no-till coupled with straw return on soil phosphorus speciation in a two-year rice-fallow practice

Conservation tillage is well known to reduce the loss of soil phosphorus (P). However, the effects of different conservation practices on soil P speciation are still unclear. To delineate these effects, two field experiments were conducted during 2-year rice-fallow seasons with four treatments: conventional tillage/straw harvest (CTH), conventional tillage/straw return (CTR), no-till/straw harvest (NTH), and no-till/straw return (NTR). A combination of Liquid-state 31P-NMR and P K-edge XANES was used to investigate the variations in P species present in two paddy soils. Liquid-state 31P-NMR results showed that P monoesters significantly increased under NTR, and after the 2-year rice-fallow practice, the proportion of P monoesters relative to total P in the different treatments ranked as: NTR (23.25%) > NTH (22.18%) > CTR (20.29%) > CTH (19.79%) and NTR (8.61%) > NTH (8.49%) > CTR (8.31%) > CTH (8.03%) in a loam and a clay loam soil, respectively. Orthophosphate (ortho-P) also increased over time, but with no differences among treatments for the two soil types at the end of the experiment. P K-edge XANES spectra further revealed that the relative proportions of iron oxide-bound P (Fe-Pi) and calcium dihydrogen phosphate (Ca-Pi) were highest in the NTR treatment, higher by 27.6% and 11.1% in a loam soil and by 10.4% and 12.7% in a clay loam soil than that in the first rice season. However, the relative proportion of aluminum oxide-bound P (Al-Pi) was lower by 38.6% and 40.7% in the loam and the clay loam soil, respectively. Our results suggested that no-till coupled with straw return can significantly increase the soil pool of organic P, and that this treatment was beneficial to the conversion of inorganic P species to labile forms in rice paddy soils. This study highlights the transformational relationship between different P species in conservation tillage systems.

High-temperature X-ray absorption spectroscopy study of thermochromic copper molybdate

X-ray absorption spectroscopy at the Cu and Mo K-edges was used to study the effect of heating on the local atomic structure and dynamics in copper molybdate (α-CuMoO4) in the temperature range from 296 to 973 K. The reverse Monte-Carlo (RMC) method was successfully employed to perform accurate simulations of EXAFS spectra at both absorption edges simultaneously. The method allowed us to determine structural models of α-CuMoO4 being consistent with the experimental EXAFS data. These models were further used to follow temperature dependencies of the local environment of copper and molybdenum atoms and to obtain the mean-square relative displacements for Cu–O and Mo–O atom pairs. Moreover, the same models were able to interpret strong temperature-dependence of the Cu K-edge XANES spectra. We found that the local environment of copper atoms is more affected by thermal disorder than that of molybdenum atoms. While the MoO4 tetrahedra behave mostly as the rigid units, a reduction of correlation in atomic motion between copper and axial oxygen atoms occurs upon heating. This dynamic effect seems to be the main responsible for the temperature-induced changes in the O2−→Cu2+ charge transfer processes and, thus, is the origin of the thermochromic properties of α-CuMoO4 upon heating above room temperature.

Phosphorus speciation in a long-term manure-amended soil profile – Evidence from wet chemical extraction, 31P-NMR and P K-edge XANES spectroscopy

Long-term application of manure can lead to an enrichment of phosphorus (P) in agricultural soils. To which extent this P leaches into drainage systems and thereby potentially contributing to eutrophication of surface waters, depends on the distribution and speciation of P present in the soil. In this study the P speciation and related soil characteristics were investigated for a sandy loam soil that had been receiving manure for >40 years. A combination of solution-state 31P-NMR, P K-edge XANES and wet chemical extractions was applied.Topsoil P contents were with 42 mmol kg−1 more than twice as high as in the sub soil (14–18 mmol kg−1). Linear combination fitting (LCF) of P K-edge XANES spectra indicated that P accumulation in the topsoil occurred predominantly via adsorption of inorganic P to Fe/Al hydroxide surfaces, resulting in substantial P saturation of these mineral phases. Up to 76% of the topsoil P was associated with Al or Fe. The organic P content was low (5–10%), but an additional important topsoil P pool identified with XANES-LCF was amorphous calcium phosphate (30%), which may have been added with manure or formed in situ. Towards the deeper subsoil the relative portion of primary apatite increased progressively and reached 80% of total P at a depth of 70 to 80 cm. These results highlight the importance of Fe and Al mineral phases for P retention in long-term manure-amended soils also under non-acidic conditions.

Phosphorus speciation and release kinetics of swine manure biochar under various pyrolysis temperatures.

Converting swine manure to biochar is an effective way to recycle valuable nutrients, but there are few reports on its feasibility as a phosphorus (P) source. The objective of this study was to clarify the unique nature, including P speciation, of manure biochar products under various pyrolysis temperatures. We used solution 31P nuclear magnetic resonance and P K-edge X-ray adsorption near-edge spectroscopy (P XANES) to characterize P species in swine manure biochar. For every 100°C increment starting from 300°C, the P content in manure biochar increased by 2.16 to 3.37gkg-1. However, above 400°C, organic P species did not appear anymore, and only inorganic P, including orthophosphate and pyrophosphate, existed. P K-edge XANES spectra further showed all biochar samples had higher percentages of Ca3 (PO4)2 and NaP2O7, and lower percentages of FePO4, AlPO4, and inositol hexaphosphate compared to manure. Interestingly, percentages of Ca3(PO4)2, FePO4, and AlPO4 in MB400 (indicating manure pyrolysed at 400°C) were comparable with those in MB700 while the percentage of NaP2O7 was higher in MB400. Phosphorus release from MB400 maintained a relatively high level at 0.33gkg-1 during the whole 300-h observation period. These results suggest that with a suitable pyrolysis temperature, it was feasible for manure biochar to be a P source alternative. Graphical abstract ᅟ.

Effect of gold nanoparticles on lead zinc niobate–barium titanate structure by X-ray absorption spectroscopy

ABSTRACTThe 0.93PZN-0.07BT ceramics containing 0.5 wt% AuNPs sintering was carried out for 3 – 5 h at various temperatures ranging from 1100 to 1150°C. The effect of AuNPs on 0.93PZN-0.07BT local structure and dielectric properties were investigated. The Au K-edge XANES spectrum indicated that AuNPs are not substituted into 0.93PZN-0.07BT structure. The dielectric constants and dielectric loss of 0.93PZN-0.07BT ceramics containing 0.5 wt% AuNPs after sintering at 1050°C for 3–5 h are higher and lower than that of 0.93PZN-0.07BT ceramics without AuNPs after sintering at 1100°C for 3–5 h, respectively, which can be described by assuming the connectivity series between the ceramics and the metal phases. In addition, Curie temperature shifting of 0.93PZN-0.07BT ceramics containing 0.5 wt% AuNPs due to the higher degree of chemical heterogeneity caused by the AuNPs addition. Therefore, the including of AuNPs into 0.93PZN-0.07BT ceramics can be improving dielectric properties with lower temperature and shorter time in sintering process.

Electronic structure determination using an assembly of conventional and synchrotron techniques: The case of a xanthate complex

The electronic properties of the coordination complex nickel (II) bis-n-propylxanthate, Ni(CH3(CH2)2OC(S)S)2, were studied by a combination of complementary experimental (both laboratory and synchrotron based techniques) and theoretical methods. Energy differences between HOMOs and LUMOs were determined from UV–visible spectroscopy. The assignment of the transitions were performed with the aid of TD-DFT calculations and based in symmetry considerations. The analysis of the Raman excitation profiles of selected vibrational modes of the complex, taken in resonance with a particular electronic transition, was found to reinforce the electronic assignment. Experimental binding energies of inner and core electrons were determined by PES measurements. Ni K-edge, S K-edge, Ni L-edge, O K-edge and C K-edge XANES spectra were interpreted in terms of the promotion of core electrons to unoccupied electronic levels. An experimental quantitative molecular orbital diagram was constructed using the information extracted from the different techniques.

Correlated XANES, TEM, and NanoSIMS of presolar graphite grains

We report correlated XANES, TEM, and NanoSIMS measurements of twelve presolar graphite grains extracted from primitive meteorites and for which isotopic data indicate predominantly Type-II supernovae origins. We find continued evidence for isotopic heterogeneities in presolar graphite grains, including the first observation of a radial gradient in the inferred initial 26Al/27Al within a presolar graphite grain. The XANES spectra of these samples show a variety of minor absorbances near the C K-edge, attributable to vinyl-keto, aliphatic, carboxyl, and carbonate molecules, as well as possible damage during sample preparation. Each sample exhibits homogeneous C K-edge XANES spectra within the graphite, however, showing no correlation with isotopic heterogeneities. Gradients in the isotope ratios of C, N, O, and Al could be due to both processes during condensation, e.g., mixing in stellar ejecta and granular transport, and post-condensation effects, such as isotope dilution and exchange with isotopically normal material in the early Solar System or laboratory, the latter of which is a significant issue for high-density presolar graphite grains. It remains unknown whether the mechanisms behind isotope exchange would also affect the local chemistry and therefore the XANES spectra. Ti L-edge XANES from most Ti-rich subgrains match standard spectra for TiC and potentially TiCN. A rare rutile (TiO2) subgrain has been identified, though it lacks the lowest energy L3 peak typically seen in standard spectra. Ca has also been identified by EDXS in TiC subgrains, likely due to the decay of live 44Ti at the time of formation. Future NanoSIMS measurements will determine the variability of initial 44Ti in TiC subgrains, an important constraint on mixing in the ejecta of the grains’ parent supernovae.

Reactivities study of titanium sites in titanosilicate frameworks by in situ XANES

This study focused on the study of titanium sites, their local structure and reactivities in nanoporous titanosilicates frameworks by using in situ X-ray absorption spectroscopy. XANES provides the information on the oxidation state of the titanium species as well as their change in coordination geometry during de- and re-hydration. Results from in situ XANES studies shows that TS-1 is much more hydrophobic than TiAl or TiFe systems. Pre-edge features of Ti K-edge XANES spectra of these materials also suggested that TiIV is the dominant Ti coordination in dehydrated system whilst TiVI is the dominant in the re-hydrated one.

Temperature dependence of Zr and Ti K-edge XANES spectra for para- and ferro-electric perovskite-type PbZrO3, PbTiO3 and BaTiO3

Zr and Ti K-edge XANES spectra of PbZrO3, PbTiO3 and BaTiO3 perovskite-type compounds were measured in the temperature range from 10K to 850K. Quantitative comparisons for the near-edge spectra were performed in a wide temperature range using the absorption intensity invariant point (AIIP) standardization. Clear temperature dependence for pre-edge shoulder is identified by the calculating the temperature difference of the XANES spectrum intensity. Decrease of pre-edge shoulder and peak intensity is observed only in the para- and ferro-electric phases and draw curves, not straight lines. The gradients for shoulder and pre-edge peak intensity are rich in a variety. The decrease in absorption of pre-edge peak and shoulder is speculated due to the shift from the off-centre position of the Zr atom with respect to the oxygen octahedron to center position. The Zr ion in the PbZrO3 para-electric phase has same temperature behaviors of Ti ions in the ferroelectric perovskite.

EPR and X-Ray Spectroscopy Characterization of Reported Mono-Ruthenium Water Splitting Catalysts

Technology to convert Solar energy into fuel is being developed to search for alternate energy sources. This includes electrochemical cells based on artificial water splitting catalysts. By studying their water splitting mechanism we can hope to optimize the energy conversion process and also learn more natural water splitting complexes like Photosystem II. In this study, we analyzed two reported single site Ruthenium based catalysts, [RuII(bpy)(tpy)H2O]2+ and [RuII(bpy)(tpy)Cl]+. [RuII(bpy)(tpy)H2O]2+ is a widely studied water splitting catalyst for which [RuV(bpy)(tpy)= O]3+ has been detected as a rate limiting intermediate. However, using EPR, we see that this intermediate is not present when the sample is measured with CeIV in D2O or HNO3. Using, Ru-K edge EXAFs, we also see that the Ru=O distance measured after adding CeIV is more consistent with its assignment as a [RuIV(bpy)(tpy)=O]2+ intermediate and not with RuV=O.We also found that [RuII(bpy)(tpy)Cl]+ is not a catalyst and that its oxidation past RuIII is impeded by a lack of proton coupled electron transfer. Using L-edge and K-edge XANES spectra, we show that the edge position of the [RuII(bpy)(tpy)Cl]+ spectra is more consistent with the presence of [RuIII(bpy)(tpy)Cl]+. EXAFS analysis of [RuII(bpy)(tpy)Cl]+ also show shortening of the Ru-Cl bond (2.41 A to 2.34 A), implying a transition from RuII to RuIII. This is further tested in an Oxygen evolution assay where the sample is incubated in H2O for different time periods. We show that samples that are not incubated for long times show no activity, hence implying that a Cl- to H2O ligand exchange is necessary for [RuII(bpy)(tpy)Cl]+ to show catalytic activity. These studies give us crucial insight into the water splitting mechanism of these catalysts.

Preparation and catalytic properties of mesoporous titanosilicate of cubic Pm3n structure

Mesoporous titanosilicate of cubic Pm3n structure was prepared for the first time using cetyltriethylammonium (CTEA+) bromide as pore-directing agent in alkaline condition (pH around 9) with the aid of NaCl salt. The alkaline synthesis condition facilitated the incorporation of Ti(IV) into the silica framework as well as the assembly of mesoporous silica with zeolite seeds. Two kinds of silica sources were used. The material prepared with sodium silicate was named Ti-SBA-1-alk, and the other named Ti-ZSBA-1 was prepared with the seeds of titanosilicalite-1 (TS-1). The studies by UV–Vis and Ti K-edge XANES spectra indicated that the Ti(IV) in Ti-SBA-1-alk was favorably in tetrahedral (Td) coordination and should be incorporated into the framework of mesoporous silica, whereas a larger amount of Ti(IV) in Ti-ZSBA-1 was in octahedral coordination. In oxidation of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethylbenzoquinone (TMBQ) using hydrogen peroxide as the oxidant, the mesoporous titanosilicate materials were found to be efficient catalysts, while only trace TMP conversion was observed over TS-1 due to its small zeolitic pores. In comparison of mesoporous titanosilicate materials in similar pore diameters, Ti-SBA-1-alk and Ti-ZSBA-1 both of cubic Pm3n structure gave higher TMBQ selectivities than Ti-MCM-41 of 2D channeling pores, attributing to the better diffusion of the phenoxyl radicals in the three-dimensional pores. Moreover, Ti-SBA-1-alk which contained more Td-coordinated Ti(IV) showed higher catalytic activities than Ti-ZSBA-1.