14475-63-9Relevant articles and documents
Influence of the preparation method on the morphological and composition properties of Pd-Au/ZrO2 catalysts and their effect on the direct synthesis of hydrogen peroxide from hydrogen and oxygen
Menegazzo, Federica,Signoretto, Michela,Manzoli, Maela,Boccuzzi, Flora,Cruciani, Giuseppe,Pinna, Francesco,Strukul, Giorgio
, p. 122 - 130 (2009)
Bimetallic Pd-Au samples supported on zirconia were prepared by different methods and tested for the direct synthesis of hydrogen peroxide under very mild conditions (room temperature and atmospheric pressure), outside the explosion range and without halides addition. Further catalytic tests were performed at higher pressure using solvents expanded with CO2. Samples were characterized by N2 physisorption, metal content analysis, XRD, HRTEM combined with X-ray EDS, TPR, and FTIR. The effect of the addition of gold to Pd in enhancing the yield of H2O2 is sensitive to the preparation method: the best catalytic results were obtained by depositing gold by deposition-precipitation (DP) and by introducing in a second step Pd by incipient wetness impregnation. The origin of the differences between samples is discussed. The role of Au in the catalytic reaction seems to be a complex one, changing the chemical composition of the metallic particles, their morphology, and charge of the exposed Pd sites.
Synthesis of lead zirconate titanate from an amorphous precursor by mechanical activation
Junmin, Xue,Wang, John,Weiseng, Toh
, p. 139 - 146 (2000)
Many of the chemistry-based processing routes for functional ceramics inevitably involve calcining the chemical-derived precursors at an intermediate/high temperature, in order to form the designed ceramic phase. This is very undesirable, although widely used, as the calcination can result in an extensive degree of crystal growth and particle coarsening at the calcination temperature and therefore ruins almost all the advantages offered by the chemistry-based processing routes, such as an ultrafine particle size and high sintering-reactivity. Using a specifically designed PZT precursor prepared by co-precipitation, it is demonstrated that the precursor-to-ceramic conversion can alternatively be realized by mechanical activation. In this connection, a single phase, nanocrystalline perovskite PZT powder has been successfully derived from an amorphous hydroxide precursor by mechanical activation. The resulting PZT powder was well dispersed, and the particle size was in the range of 30-50 nm, as observed using the scanning electron microscopy and transmission electron microscopy. This is in contrast to the poor particle characteristics, represented by very coarse and irregular particle and agglomerate sizes, for the powder derived from calcination at 750 °C. The activation-triggered PZT powder was sintered to a density of 97.6% theoretical density at 1150 °C for 1 h. Sintered PZT ceramic exhibits a dielectric constant of 927 at room temperature and a peak dielectric constant of approximately 9100 at the Curie point of 380 °C when measured at the frequency of 1 kHz.
FT-IR and laser Raman spectroscopic investigation of the formation and stability of low temperature t-ZrO2
Stefanic,Music,Popovic,Sekulic
, p. 391 - 394 (1997)
The influence of the preparation chemistry on the formation and the stability of metastable t-ZrO2 was studied. γ-Irradiation has very small (if any) influence on the t-ZrO2 → m-ZrO2 transition, while increased temperature and pressure caused gradual transition of pure t-ZrO2 to m-ZrO2. Metastable t-ZrO2 containing SO4/2- impurities proved to be more stable, and its transition was not uniform. The mentioned effects were monitored by FT-IR and laser Raman spectroscopy. X-ray diffraction was used as a complementary technique.
Influence of precipitation chemistry and ball-milling on the thermal behavior of zirconium hydroxide
Stefanic,Music,Sekulic
, p. 119 - 133 (1996)
Zirconium hydroxide precipitates, obtained by rapid precipitation at pH 2.5, 7.5, and 10.5, were ball-milled for up to 60 h and then heated inside a differential scanning calorimeter (DSC) at temperatures of up to 600°C. Crystal phases produced after heating were analyzed by FTIR and laser Raman spectroscopy. It was found that without regard to the precipitation pH the first stage of ball-milling caused an increase of the crystallization temperature that resulted in the formation of pure t-ZrO2. The second stage of ball-milling caused a decrease of the crystallization temperature resulting in the formation of m-ZrO2. The ball-milling process also influenced the dependence of the crystallization enthalpy of zirconium hydroxide on the precipitation pH. In the case of zirconium hydroxide precipitated at pH 2.5, the ball-milling caused dehydration and an increase in its hygroscopy. The nature of these effects was discussed. The extension of FTIR spectra to the far infrared region made it possible to distinguish between t-ZrO2 and m-ZrO2 polymorphs by this technique. Also, the influence of laser power on the identification of ZrO2 polymorphs by Raman spectroscopy was elaborated.
Kraus, K. A.,Phillips, H. O.
, p. 249 - 249 (1956)
Evolution of the Catalytic Activity in Pt/Sulfated Zirconia Catalysts: Structure, Composition, and Catalytic Properties of the Catalyst Precursor and the Calcined Catalyst
Manoli, Jean-Marie,Potvin, Claude,Muhler, Martin,Wild, Ute,Resofszki, Gabor,Buchholz, Thomas,Paal, Zoltan
, p. 338 - 351 (1998)
A 3% Pt/sulfated zirconia catalyst was prepared and characterized before and after calcination at 900 K by XRD, XPS, EM, and in the catalytic hydroisomerization of n-hexane. The fresh sample exhibited small but definite catalytic properties. Calcination brought about a dramatic increase of the activity with practically constant high (90-100%) selectivity for hydroisomerization versus cracking. This increased activity was accompanied by the transformation of the predominantly amorphous support to predominantly tetragonal crystals and the wrapping up of most parts of surface Pt atoms into the bulk, as shown by the physical characterization methods. Hence metallic Pt particles exhibited mainly Pt-O rather than Pt-S interactions. S was present as sulfate. Pt-sulfated zirconia was different from traditional bifunctional metal catalysts on acidic supports. We attributed its higher catalytic activity and favorable isomerization selectivity to a few but very active centers, formed by interaction of Pt sites with sulfate groups on the high Miller-index surfaces of ZrO2. Calcination must be essential to create these active sites. H2 dissociating on Pt sites would provide the hydride species that are necessary for isomerization occurring on the acidic (sulfate-zirconia) part of that ensemble. We proposed the name compressed bifunctional sites for these centers of acid-metal cooperation. The assumption of such active sites, the maximum activity as a function of the hydrogen pressure, can also be explained in a consistent way.
Heterogeneous Liquid-Phase Oxidation of Alcohols with Solid Oxidizing Reagents of Vanadium(V) Oxide and Chromium(VI) Oxide Supported on Zirconium(IV) Oxide
Nakamura, Hideo,Matsuhashi, Hiromi
, p. 997 - 1000 (1995)
The oxidizing reagents in a heterogeneous system were obtained by impregnating Zr(OH)4 with NH4VO3 and (NH4)2CrO4 followed by calcination in air at 773 K.These materials (V2O5/ZrO2 and CrO3/ZrO2) converted alcohols into their corresponding aldehydes or ketones at moderate temperatures in a solvent with very high selectivity.Changes in the oxidation states of vanadium and chromium were investigated by a temperature-programmed reduction method (TPR).A linear relation was found between the yield of cyclohexanone formed from cyclohexanol and the amount of reduced vanadium or chromium, determined by TPR.It was confirmed that the active species of V(V) and Cr(VI) were reduced to V(II) and Cr(IV), respectively, in the oxidation process.
Synthesis of solid superacid catalyst with acid strength of H0 ≤ -16.04 1
Hino, Makoto,Arata, Kazushi
, p. 851 - 852 (1980)
A solid superacid catalyst with an acid strength of H0 ≤ -16.04, which was active for reactions of propane and butane, was obtained by exposing Zr(OH)4, prepared by the hydrolyses of ZrOCl2 and ZrO(NO3)2, to 1 N H2SO4 and then calcining in air at 575-650°C.
Influence of the preparation and of the activation treatments on the catalytic activity of mechanical mixtures of sulfated zirconia and Pt/Al2O3
Tichit,El Alami,Figueras
, p. 18 - 27 (1996)
Zirconium hydroxides with specific surface areas in the range of 60 to 300 m2/g were obtained by varying between 6 and 12 the final pH of precipitation of the gels and the nature of the precursor salt. These compounds were sulfated by impregnation with 15 and 35 ml of aqueous 0.5 MH2SO4. Their sulfur content was 3 to 5 wt%. Sulfation increases the thermal stabilities of the zirconias by nearly 200 K. The solids remain amorphous up to at least 673 K and then crystallize in the tetragonal and monoclinic phases. The properties of mechanical mixtures of the sulfated zirconias and a Pt/alumina catalyst were evaluated for the isomerization of n-hexane. The study of the conditions of activation show that the highest performances of these catalysts result from a pretreatment under hydrogen at 573 and 623 K when the amounts of H2SO4 added are, respectively, of 35 and 15 ml/g of zirconium hydroxide, whatever their specific surface areas. A close correlation is found between the activities of the catalysts and their specific surface areas. This suggests that the active sites are located at the low coordination sites of zirconia and that their efficiency increases with their dispersion. The acidity determined by IR spectroscopy of NH3 adsorption is essentially of Lewis type after evacuation at 573 K. However, Bronsted acidity results from a reversible redox equilibrium between hydrogen and zirconia. The isomerization of n-hexane is considered to follow a classical bifunctional mechanism.
Metal-reinforced sulfonic-acid-modified zirconia for the removal of trace olefins from aromatics
Kong, Decun,Peng, Qian,Shi, Li,Wang, Xin,Meng, Xuan,Hu, Xiude,Liu, Naiwang
, p. 1644 - 1653 (2020)
Metal-reinforced sulfonic-acid-modified zirconia catalysts were successfully prepared and used to remove trace olefins from aromatics in a fixed-bed reactor. Catalysts were characterized by ICP-OES, N2 adsorption–desorption, X-ray diffraction, thermogravimetric analysis (TGA), and pyridine-FTIR spectroscopy. Different metals and calcination temperatures had great influence on the catalytic activity. Alumina-reinforced sulfated zirconia exhibited outstanding catalytic performance, stable regeneration activity, and giant surface area, and are promising in industrial catalysis. TGA showed that the decomposition of methyl could be attributed to Br?nsted acid sites, and pyridine-FTIR spectroscopy proved the weak Br?nsted sites on these synthesized metal-reinforced sulfated zirconia. Also, a relation between the reaction rate and weak Br?nsted acid density is proposed.
Hydrogen effect on n-butane isomerization over sulfated zirconia-based catalysts
Sayari, Abdelhamid,Yang, Yong,Song, Xuemin
, p. 346 - 353 (1997)
Iron- and manganese-promoted sulfated zirconia (SFMZ) has been tested as an n-butane isomerization catalyst in the temperature range of 35 to 180°C. The catalytic activity exhibits an induction period whose length is dependent on the reaction conditions. The presence of H2 in the feed stream strongly suppresses n-butane conversion over unpromoted sulfated zirconia (SZ) and over Pt-containing SFMZ (PtSFMZ). However, hydrogen had no effect on n-butane isomerization over SFMZ. These findings were interpreted on the basis of a bimolecular mechanism where unsaturated intermediates (carbenium ions and/or butene) are formed during the break-in period. The role of promoters (Fe and Mn) is not only facilitating the formation of hydrogen-deficient intermediates and their accumulation on the catalyst surface, but also enhancing their stability. The negative effect of hydrogen over PtSFMZ is attributed to the occurrence of atomic hydrogen via the dissociative adsorption of H2 on Pt.
Zirconia supported phosphotungstic acid as an efficient catalyst for resorcinol tert-butylation and n-heptane hydroisomerization
Devassy, Biju M.,Halligudi,Elangovan,Ernst,Hartmann,Lefebvre
, p. 113 - 119 (2004)
The alkylation of resorcinol with tert-butanol was carried out using zirconia supported phosphotungstic acid (PTA) as catalyst in liquid phase conditions. Among the different PTA loaded catalysts, the 15% PTA/ZrO2 calcined at 750 °C was found t
Effect of ZnO additives and acid treatment on catalytic performance of Pt/WO3/ZrO2 for n-C7 hydroisomerization
Liu, Yan,Guan, Yejun,Li, Can,Lian, Juan,Gan, Geok Joo,Lim, Eng Chew,Kooli, Fethi
, p. 17 - 23 (2006)
The effect of WO3 (5-50 wt%) and ZnO (0.7-22 wt%) on the catalytic properties of Pt/WO3/(ZnO)-ZrO2 for n-heptane (n-C7) hydroisomerization was investigated. The optimized WO3 and ZnO contents are 20 wt% and 3.4 wt%, respectively. The catalytic performance is achieved at 81% n-C7 conversion and 89% C7 isomer selectivity at 250 °C, which is reproducible and can be kept constant over 82 h under reaction conditions. Both WO3 and ZnO can stabilize the tetragonal phase of ZrO2. The Bronsted acid-to-Lewis acid ratio should be optimized to achieve high catalytic performance. The activity for Pt/WO3/ZrO2 using Zr(OH)4 as the catalyst support (n-C7 conversion, 88% at 250 °C) is much higher than that for Pt/WO3/ZrO2 with ZrO2 as the support (n-C7 conversion, 9% at 250 °C) with the same Pt and WO3 loadings. BET, SEM-EDX, and pyridine-FTIR analyses show that acid treatment can successfully enhance the surface area (from 73 to 91 m2/g), increase the number of Bronsted acid sites, and lower the surface Zn:Zr ratio (from 0.43 to 0.15) for ZnO-ZrO2 with 22 wt% ZnO. The yield of C7 isomers is increased from nil to 47% at 300 °C on Pt/WO3/ZnO-ZrO2 catalyst after acid treatment. It is suggested that n-heptane hydroisomerization activity is related to acidity, surface area, and crystalline phase of ZrO2.
Catalytic and surface properties of ZrO2 modified with sulfur compounds
Sohn, Jong Rack,Kim, Hae Won
, p. 361 - 374 (1989)
A series of ZrO2 catalysts modified with sulfur compounds were prepared by treating Zr(OH)4 with sulfate ion and by treating ZrO2 with H2S, CS2, and SO2 followed by oxidation. The redoxidized sulfur species on ZrO2 and the oxidation state of sulfur were investigated by infrared and X-ray photoelectron spectroscopies. It was found that the SO42- species in the highest oxidation state was responsible for enhancing the acidity and catalytic activity of the catalyst. There was a close relationship between the oxidation state of sulfur and the catalytic activity, when 1-butene isomerization was carried out as a test reaction. The surface properties of ZrO2/SO42-, such as specific surface area, phase transition temperture, crystalline structure, acidity and acid strength, were very different from those of ZrO2 not modified with sulfur compounds.
Preparation of a novel catalyst UDCaT-5: Enhancement in activity of acid-treated zirconia - Effect of treatment with chlorosulfonic acid vis-a-vis sulfuric acid
Yadav, Ganapati D.,Murkute, Ambareesh D.
, p. 218 - 223 (2004)
UDCaT-5, a zirconia-based catalyst, with high sulfur content (9% w/w) but preservation of the tetragonal phase of zirconia was synthesized for the first time, by using chlorosulfonic acid as a new source for sulfate ions. UDCaT-5 was characterized by using elemental analysis, FTIR, ammonia-TPD, XRD, and BET surface area. Its catalytic activity and stability were evaluated and compared with S-ZrO2 in three different reactions. The characterization and reaction studies show that UDCaT-5 exhibits more superacidity vis-a-vis conventional sulfated zirconia prepared by using sulfuric acid.
Probing the low temperature initiation sites in Fe-, Mn-promoted sulfated zirconia via CO and H2 adsorption
Sayari, Abdelhamid,Yang, Yong
, p. 186 - 190 (1999)
Exposing freshly activated Fe-, Mn-promoted sulfated zirconia to CO at ≤ 50°C induced permanent loss of activity, while adding CO after butane isomerization had a reversible effect, regardless of whether the butane flow was interrupted or not. Similar experiments using dissociated hydrogen instead of CO led to irreversible poisoning in all cases. These findings were analyzed, based on the occurrence of initiation sites that are consumed stoichiometrically and very rapidly on exposure to butane, such initiation sites which are also consumed by CO or dissociated hydrogen, CO which competes effectively for adsorption sites without affecting accumulated reaction intermediates, and such intermediates that are removed in the presence of dissociated hydrogen.
Thiolation of dimethyl sulfide to methanethiol over WO3/ZrO 2 catalysts
Chen, Shiping,Wang, Weiming,Zhang, Yuanhua,Wei, Yucai,Fang, Weiping,Yang, Yiquan
, p. 60 - 65 (2012)
The thiolation of dimethyl sulfide with H2S over a variety of tungsten-zirconia (WO3/ZrO2) catalysts with different contents of WO3 was studied. The maximum yield of methanethiol was obtained at the reaction temperature of 633 K in the presence of 10 wt.%WO 3/ZrO2 catalyst. XRD, BET and TPD characterization results reveal that supporting WO3 species on ZrO2 gives rise to the improvement of both in structure stability and surface acidity. The MT yield increased first and then decreased with the increase of reaction temperature for all of the catalysts due to the decomposition of methanethiol and dimethyl sulfide. The optimum loading of WO3 was found to be 5-10 wt.% (with a surface density of 3.5-4.5 w-atom nm-2). Furthermore, the WO 3/ZrO2 catalyst exhibits high resistance to water.
Transesterification of diethyl malonate with benzyl alcohol catalyzed by modified zirconia: Kinetic study
Thimmaraju,Mohamed Shamshuddin,Pratap,Venkatesh
, p. 55 - 65 (2014)
Zirconia and its modified forms such as 10%Mo(VI)/ZrO2, 10%V(V)/ZrO2, 10%W(VI)/ZrO2 and SO4 2-/ZrO2 were prepared and characterized for their physico-chemical properties such as BET for surface area, NH3-TPD and n-butylamine back titration method for total surface acidity, PXRD technique for crystallinity and ICP-OES technique for elemental analysis. These materials were used as catalysts in liquid phase transesterification reaction of diethyl malonate (DEM) with benzyl alcohol (BA). Optimization of reaction conditions such as reaction time, reaction temperature, weight of the catalyst and molar ratio of reactants were carried out to obtain highest possible transester yield. Dibenzyl malonate (DBM) and benzyl ethyl malonate (BEM) were obtained as major products. Highest total transester yield of (88%) was obtained in the presence of 0.75 g of SZ catalyst at a molar ratio of DEM: BA = 1:3, reaction temperature of 393 K and reaction time 5 h. Kinetic studies were carried out to find out the rate of the reaction and energy of activation values for zirconia catalysts, in order to identify a facile catalyst system for this reaction. A possible reaction mechanism was proposed based on the kinetic data and it was observed that Eley-Rideal mechanism fits well for this reaction. Reactivation and reusability studies of the catalysts were also taken up.
Stoecker, Michael
, p. 371 - 378 (1985)
Silicotungstic acid supported zirconia: An effective catalyst for esterification reaction
Parida,Mallick, Sujata
, p. 77 - 83 (2007)
A series of solid acid catalysts were synthesized by incipient wetness impregnation method by varying the wt% of silicotungstic acid on hydrous zirconia (ZSTA). The prepared catalysts were characterized by PXRD, FTIR, UV-vis DRS, EPMA, BET surface area, acid sites, etc. FTIR and UV-vis DRS studies indicate that the material retain the Keggin-type structure of silicotungstic acid up to 500 °C. The suitability of the materials was studied for acid catalysed esterification reactions using formic, acetic, propionic, n-butyric acid and n-butyl alcohol (NBA), isobutyl alcohol (IBA) and sec-butyl alcohol (SBA). Material with 15 wt% STA on hydrous zirconia having high surface area and acid sites acts as better catalyst for esterification reactions. The esterification of acids with NBA was found to be higher than IBA and SBA. In all the cases, the selectivity for the corresponding esters is nearly100%. The straight-line plot of -ln(1 - conversion) versus reaction time for the reactions carried out at 98 °C supports that the esterification reaction obeys first order kinetics with respect to acid concentration. The reusability study justifies that the catalyst is stable and active.
Hydrated surface structure and its impacts on the stabilization of t-ZrO2
Wang, Hui,Li, Guangshe,Xue, Yanfeng,Li, Liping
, p. 2790 - 2797 (2007)
We first optimized the preparation conditions to 3.6-6.0 nm ZrO2 in a pure tetragonal structure (t-phase). All samples were characterized by X-ray diffraction, high-resolution transmission electron microscope, thermal analysis, Raman spectra, and infrared spectra. It is found that the surfaces of t-ZrO2 nanostructures were terminated by an amorphous hydration layer co-existing with small amounts of carbonate molecules. With the removal of hydrated surface layers under hydrothermal conditions at T>150 °C, t-ZrO2 nanostructures became thermodynamically unstable, which partially transformed into monoclinic ZrO2 (m-phase). Such a transformation occurs initially at surface regions and then develops into the bulk. High-temperature annealing in air could also remove the hydrated surface layers, which is however followed by a gradual transformation of t-ZrO2 into m-ZrO2 in both bulk and surface regions. These observations are explained in terms of the difference in surface free energies of m-ZrO2 and t-ZrO2 upon H2O adsorption.
Synthesis of Solid Superacid of Tungsten Oxide supported on Zirconia and its Catalytic Action for Reactions of Butane and Pentane
Hino, Makoto,Arata, Kazushi
, p. 1259 - 1260 (1988)
A solid superacid catalyst with an acid strength of H0 =/ -14.52 was obtained by impregnating Zr(OH)4 or amorphous ZrO2 with aqueous ammonium metatungstate followed by calcining in air at 800-850 deg C (13 wt.percent W); this catalyst was active for the isomerisations butane to isobutane at 50 deg C, and pentane to isopentane at 30 deg C.
Estimation of oxyhydroxide specific surface area using the amounts of OH groups adsorbed
Pechenyuk,Matveenko,Semushin
, p. 1582 - 1588 (2001)
A procedure for pH-metric determination of the limiting adsorption of OH groups (AOH) by FeIII, ZrIV, CrIII, and InIII oxyhydroxide hydrogels from 0.1 and 1.0 M solutions of NaCl is described. Data on the molecular area occupied by a single OH group on the hydrogel surface (SOH) and the Sspec values, which were calculated from AOH and SOH. are presented. The Sspec value does not depend on the pH of hydrogel precipitation: the true Sspec value can be determined only from sorption of the OH groups at the actual point of zero charge of the hydrogel. The AOH values for hydrogels were found to change only slightly during aging of hydrogels in electrolyte solutions.
Methanol Synthesis from CO2 and H2 over CuO-ZnO Catalysts Combined with Metal Oxides under 13 atm Pressure
Xu, Zheng,Qian, Zaihu,Mao, Liqun,Tanabe, Kozo,Hattori, Hideshi
, p. 1658 - 1663 (1991)
The synthesis of methanol from CO2 and H2 in a flow reactor was studied over CuO-ZnO catalysts combined with ZrO2, MgO, Al2O3, or Cr2O3 under a pressure of 13 atm.CuO-ZnO-Al2O3, CuO-ZnO-ZrO2, and CuO-ZnO-MgO showed higher activity and higher selectivity for methanol than did CuO-ZnO.The selectivity for methanol formation became higher as the reaction temperature was lower, or any of the H2/CO2 ratio, the pressure and the space velocity were higher.On the basis of analyses by XRD, XPS and ultraviolet diffused reflectance spectroscopy, the active centers of CuO-ZnO-ZrO2 seem to be a Cu+-ZnO species stabilized by ZrO2.
Catalytic dehydration of fructose to 5-hydroxymethylfurfural over a mesoscopically assembled sulfated zirconia nanoparticle catalyst in organic solvent
Wang, Ningning,Yao, Yuan,Li, Wei,Yang, Yan,Song, Zhanxin,Liu, Wentao,Wang, Haijun,Xia, Xiao-Feng,Gao, Haiyan
, p. 57164 - 57172 (2014)
The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) in DMSO was performed over a sequence of mesoscopically assembled sulfated zirconium nanostructures (MASZN) derived from zirconyl chloride with a template as a fastening agent. The materials were characterized by X-ray diffraction, FTIR spectroscopy, NH3 temperature-programmed desorption, pyridine FTIR spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, and N2 sorption. The heterogeneous catalyst MASZN with Lewis-Bronsted acid sites had a superior performance in the dehydration of fructose to HMF. With MASZN-3 as catalyst, a HMF yield of 91.9% with a 98.5% fructose conversion was obtained at 110°C for 120 min in DMSO. Finally, the catalyst MASZN-3 was recycled in four consecutive cycles with scarcely any loss of activity. The excellent catalytic properties together with its easy synthesis, low cost, and nontoxic nature make this MASZN a promising catalyst for the development of new and efficient processes for biomass-based chemicals.
Fluorite-like hydrolyzed hexanuclear coordination clusters of Zr(IV) and Hf(IV) with syn-syn bridging N,N,N-trimethylglycine in soft crystal structures exhibiting cold-crystallization
Matsuoka, Moe,Takao, Koichiro,Tsushima, Satoru
, (2021/09/27)
Tetravalent metal ions are hydrolyzed under presence of N,N,N-trimethylglycine hydrochloride (betaine hydrochloride, [Hbet]Cl) in aqueous solutions to afford [M6(μ3-O)4(μ3-OH)4(μ-bet)8(κ-bet)4(H2O)4]12+ (M4+ = Zr4+ (1), Hf4+ (2)) as hydrated perchlorate salts. These compounds were characterized by single crystal X-ray diffraction, elemental analysis and IR spectroscopy. As a result, we have found that fluorite-like [M6O8] coordination clusters are formed through octahedral arrangement of six M4+ linked by μ3-O atoms. Additionally, each pair of neighboring M4+ are connected by the μ-bet ligand through a syn-syn bridging coordination of its carboxylate moiety. This interaction seems to prevent further growth of the fluorite structure leading to formation of MO2. It was difficult to directly distinguish each μ3-O atom to be μ3-OH? or μ3-O2? due to its strongly anisotropic thermal displacement in the obtained structures. Bond valence sum analysis suggested that four μ3-OH? and four μ3-O2? are alternately arranged in the [M6O8] core motifs. Indeed, such a symmetric structure of [M6(μ3-O)4(μ3-OH)4] was confirmed in another phase of 1 at 296 K, where 1 transforms to a monoclinic structure (1′). The number of ClO4? counteranions found in the structure determination is not enough to compensate + 12 charge of [M6(μ3-O)4(μ3-OH)4(μ-bet)8(κ-bet)4(H2O)4]12+ in any unit cells of 1, 1′ and 2. Instead, large solvent/ion accessible voids have been actually observed in their crystal structures, indicating that the missing ClO4? are located therein and are significantly disordered to make them invisible in the crystallography. DSC analysis revealed that ClO4? and H2O/H3O+ in the crystal lattice of 1 undergo unique structure relaxation and rearrangements pronounced by cold-crystallization to induce the phase transition from 1 to 1′ with elevating temperature.
Tuning the Lewis acidity of ZrO2for efficient conversion of CH4and CO2into acetic acid
Li, Yufeng,Liu, Bing,Liu, Jie,Wang, Ting,Shen, Yu,Zheng, Ke,Jiang, Feng,Xu, Yuebing,Liu, Xiaohao
, p. 8978 - 8985 (2021/06/02)
The conversion of CH4 and CO2 into acetic acid is a dream reaction, but it remains a great challenge owing to the inertness of both CH4 and CO2. The formation of acetic acid requires efficient activation of CH4 and CO2. In this work, we demonstrated that enhanced acetic acid production from CH4 and CO2 is achieved via improving the Lewis acidity of ZrO2-containing catalysts. Definitely, the best catalyst (SZ-3) exhibits about 14 times higher activity for acetic acid formation than that of pure ZrO2, owing to its strongest Lewis acidity that facilitates the activation of both CH4 and CO2. The mechanism of acetic acid formation is revealed via DFT calculations. CH4 is activated at Lewis acid sites to form Zr-CH3 and O-H species, and subsequently, the O-H species could readily hydrogenate CO3 species formed from CO2 activation at Lewis acid sites to give HCO3, followed by facile coupling with Zr-CH3 yielding acetic acid with a lower energy barrier.
