Hafnium Species Research Articles

High efficiency catalytic transfer hydrogenation of ethyl levulinate to γ‑valerolactone over Hf-based catalyst

The transformation of biomass platform compounds to value-added products is of great significance in biomass development. Herein, a series of wHfO2/SBA-15 (w = 0.7–12.5 wt%) catalysts were synthesized and applied to the catalytic transfer hydrogenation (CTH) of ethyl levulinate (EL) to γ-valerolactone (GVL) reaction. The physicochemical properties of the prepared samples have been characterized by XRD, N2 adsorption-desorption, ICP, FT-IR, SEM, TEM, XPS, UV–vis, 29Si NMR, NH3-TPD, and Pyridine-FTIR. The introduction of hafnium species could produce abundant Lewis acid sites (LAS) without altering the ordered mesoporous structure of SBA-15. In addition, part of Hf species entered the SBA-15 framework and formed a small amount of Brønsted acid sites (BAS). Among these catalysts, 10.4HfO2/SBA-15 showed an excellent catalytic performance, providing a very high EL conversion (96 %) and GVL selectivity (91 %) at 130 °C in 3 h using isopropanol as the H-donor. The high catalytic performance maybe related to the high surface area and large amount of LAS. Furthermore, this catalyst also showed a good stability and reusability.

On the effect of hafnium loading and location in mesoporous silica on the activity of catalysts in oxidation of dibenzothiophene

Oxidation of thiophenes to sulfones (easily extracted from fuel) is of crucial importance in petrochemical and automotive industry. The rational design and synthesis of stable catalysts of this process still remains one of the biggest challenges. In this work, a series of HfSBA-15 mesoporous materials with different hafnium content (3, 5 or 10 wt.%) has been prepared and studied in the liquid phase oxidation of dibenzothiophene (DBT) with H2O2. The activity of HfSBA-15 samples prepared by one-pot method was compared to that of Hf/SBA-15 synthesized by impregnation and that of HfMCF type of silica. It has been shown that hafnium loading and its location in the silica structure (framework and/or extra-framework positions) affect the efficiency of DBT oxidation. Moreover, the reaction conditions (i.e. oxidation temperature, reaction time, oxidant/sulfur ratio and catalyst amount) were optimized to achieve maximum conversion of dibenzothiophene (99 %). The location of hafnium species in SBA-15 and MCF materials was influenced by the loading of the metal and the type of silica structure (determined by the preparation method). Hafnium located in the framework of SBA-15 promoted generation of a higher number of active Lewis acid sites (LAS) and higher activity in oxidation of dibenzothiophene compared to extra-framework HfO2 species anchored in HfMCF material. It indicates that the location (framework vs extra-framework) and formation of isolated Hf4+ species are decisive for high activity of hafnium-doped mesoporous molecular sieves. This finding opens up a pathway for the development of highly efficient, stable and relatively cheap solid acid catalyst for oxidation of thiophenes to sulfones.

Effective Synergistic Hafnium-Aluminum Bimetallic Oxides catalysts for the Synthesis of 5-Hydroxymethylfurfural from Glucose and Fructose

5-Hydroxymethylfurfural (5-HMF) is considered as a bridge connecting biomass and petrochemical products with excellent application potential and market prospects. In this work, a series of hafnium-aluminum bimetallic oxide (Al-HfOx) catalysts were synthesized with the coprecipitation method, possessing outstanding catalytic effect in the conversion of glucose or fructose to 5-HMF. 5-HMF yield of 49.31% was acquired from glucose in a methyl isobutyl ketone (MIBK)/saturated salt water (SSW) biphasic solvent at 170 ℃ after 2 h. When fructose was the substrate, 5-HMF yield was 74.85% in DMSO at 150 ℃ after 75 min. In addition, the reaction mechanism was speculated that the isomerization process could be promoted by the synergistic effects of aluminum and hafnium species, in which Hf-OH and Al-OH groups, and Hf active sites served as Brønsted and Lewis acidic sites, respectively. After at least four cycles, there was no remarkable wastage in the catalytic activity of Al-HfOx. This paper offers a prospective catalytic strategy for heterogeneous bimetallic oxide catalysts used in the effective conversion of carbohydrates to 5-HMF.

Stability of Hydroxo/Oxo/Fluoro Zirconates vs. Hafniates—A DFT Study

We performed density functional theory (DFT) calculations on binary and ternary oxo/fluoro crystals of the geochemical twin pair zirconium and hafnium to evaluate and compare their stabilities. This is the first DFT study on bulk ZrF4 or HfF4, as well as on a hypothetical ZrOF2 or HfOF2 bulk crystal. For α-MO2, β-MF4 and MOF2, we have found significantly higher cohesive energies for the respective hafnium species. This suggests a considerable gap in affinity toward fluorine and oxygen between the twin pair in the solid state. In agreement with experimental findings, this gap is slightly more pronounced for fluorine. This study is also the first to evaluate the theoretical, endothermic mono-hydroxylation of the respective fluorides or oxyfluorides to model the difference in affinity toward fluoride versus hydroxide. For these, we could also find a slight energetic preference for the hafnium compound.

Anionic hafnium species: an active catalytic intermediate for the coupling of epoxides with CO2?

A series of hafnium complexes were structurally identified showing high activity (up to 500 h-1) in the selective alternated copolymerization of epoxides with CO2 under low pressure.

Palladium-Catalyzed Reductive Coupling Reaction of Terminal Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a Hafnium Hydride Precursor Leading to trans-Alkenes.

Herein, we describe a reductive cross-coupling of alkynes and aryl iodides by using a novel catalytic system composed of a catalytic amount of palladium dichloride and a promoter precursor, hafnocene difluoride (Cp2 HfF2 , Cp=cyclopentadienyl anion), in the presence of a mild reducing reagent, a hydrosilane, leading to a one-pot preparation of trans-alkenes. In this process, a series of coupling reactions efficiently proceeds through the following three steps: (i) an initial formation of hafnocene hydride from hafnocene difluoride and the hydrosilane, (ii) a subsequent hydrohafnation toward alkynes, and (iii) a final transmetalation of the alkenyl hafnium species to a palladium complex. This reductive coupling could be chemoselectively applied to the preparation of trans-alkenes with various functional groups, such as an alkyl group, a halogen, an ester, a nitro group, a heterocycle, a boronic ester, and an internal alkyne.

Zeolites with isolated-framework and oligomeric-extraframework hafnium species characterized with pair distribution function analysis.

Zeolites containing framework heteroatoms (e.g., Ti, Sn, and Hf) with open coordination sites behave as solid-state Lewis acids and exhibit remarkable catalytic properties unachievable with bulk oxides. However, direct evidence confirming the incorporation of such heteroatom species into the zeolite framework is difficult to obtain because of the limited number of analytical methods capable of discerning framework incorporation from extraframework species. In this work, the structural environments of hafnium (Hf) framework and extraframework species added post-synthetically into *BEA zeolites were analyzed using coupled pair distribution function (PDF) and diffuse reflectance (DR) UV-vis measurements. PDF analysis enabled the visualization and identification of framework and extraframework HfOx species, both of which were undetectable by traditional X-ray and neutron diffraction methods. Reactivity data from the aldol condensation of benzaldehyde and acetone confirmed that framework Hf species are responsible for catalytic activity.

Zr/Hf ratio in supercritical chloride fluids: Experimental study of zirconium and hafnium complexation at 450°C and 0.6–1 kbar

The solubility method was used to study the Zr and Hf behavior in supercritical aqueous chloride fluids (m(HCl) = 0.5–1.5, m(NaCl) 1 and 2 mol kg−1 H2O) at 450°C and 0.6–1 kbar. The crystals of natural baddeleyite (Kovdor Deposit) served as a solid phase in all experiments. An increase in HCl and NaCl concentrations leads to the increase of Zr and Hf contents in solutions. Zirconium mainly occurs as ZrOHCl3(aq)0 and NaZrOHCl4(aq)0 complexes, while dominant hafnium species are HfOHCl3(aq)0, HfCl4(aq)0, and NaHfOHCl4(aq)0 or NaHfCl5(aq)0. An increase of acid concentration is accompanied by the decrease of Zr/Hf ratio (Hf enriches fluid), which results from difference in acid-base properties of these elements and may lead to their separation in natural processes.

Tracer studies relating to alloying element behaviour in porous anodic alumina formed in phosphoric acid

Abstract The behaviours of hafnium, molybdenum and hafnium tracer species in porous anodic alumina films are compared in order to investigate the relationship between the migration rate of alloying element species and their amounts and distributions in the films. The study employs substrates consisting of a thin layer of Al–Hf, Al–Mo or Al–Nd alloy deposited by magnetron sputtering on to electropolished aluminium, with a top layer of aluminium deposited above the alloy layer. The substrates are then anodized at constant current density in phosphoric acid to incorporate the tracer species into the porous anodic film. The tracer distributions in the film are investigated by scanning electron microscopy, and the amounts of tracer quantified by Rutherford backscattering spectroscopy. The distributions of the species within the films differ for the three tracer species. Molybdenum species, which migrate slower than Al 3+ ions, are retained within the films, since they are unable to migrate to the pore base regions of the films. In contrast, hafnium and neodymium species, which respectively migrate at rates similar to and faster than Al 3+ ions, are able to reach the pore base regions where they are lost to the electrolyte; the loss of the faster migrating-neodymium species is greater than that of hafnium species. The behaviours are related to the influence of flow of the alumina within the barrier layer on the transport of the tracer species.

Growth of porous anodic films on sputtering-deposited aluminium incorporating Al–Hf alloy nanolayers

Formation of porous anodic films on sputtering-deposited aluminium incorporating Al–Hf tracer layers has been examined at constant current in sulphuric and phosphoric acids. Hafnium was selected as the tracer species since the migration rates of Hf 4+ and Al 3+ ions are similar in barrier-type anodic alumina. The distribution of hafnium in the films was determined using ion beam analysis, scanning electron microscopy and transmission electron microscopy. Increases in the anodizing voltage and barrier layer thickness accompany the oxidation of hafnium and the migration of Hf 4+ ions through the barrier layer region of the porous film. Hf 4+ and Al 3+ ions that migrate to the pore bases are lost to the electrolyte. Other Hf 4+ ions are incorporated into the cell walls. For films formed in phosphoric acid, with relatively thick barrier layers, channelling of the ion current leads to accelerated outward transport of Hf 4+ ions toward the pore base, while a U-shaped inner edge of the hafnium distribution beneath the pores is associated with more slowly transported hafnium species. The tracer behaviours for films formed in both acids are consistent with the transport of Hf 4+ ions in the barrier layer regions by a combination of flow of film material and ion migration, the flow being a key factor in the development of the pores. The percentage losses of Hf 4+ and Al 3+ ions from the films to the electrolyte are relatively similar, correlating with their similar migration rates, and contrast with the retention in the film of slow migrating W 6+ ions, found previously, due to a more dominant role of flow.

Contamination of silicon dioxide films by aqueous zirconium and hafnium species

Zirconium and hafnium oxides and silicates have emerged as potential replacements for SiO2 as gate dielectric material. Patterning of these materials by wet etching in fabrication areas originally designed for SiO2 gates may give rise to contamination of SiO2 by aqueous zirconium and hafnium species. This paper summarizes the work carried out to characterize the adsorption behavior of aqueous zirconium and hafnium species onto thermally grown silicon dioxide. Electrokinetic and adsorption measurements were carried out to understand the extent and nature of interaction. The adsorption of both Zr and Hf species showed a maximum at pH 5.5. Significant reduction in the adsorption of both Zr and Hf occurred upon addition of fluoride ions to the solution. Using appropriate speciation diagrams, an adsorption model has been developed to explain the experimental data.

HfO2–ZrO2 Doped Silica Thin Films by XPS

Silica thin films embedding ZrO2 and HfO2 were prepared by spin-coating on silica glass via a modified sol-gel processing. The novel synthetic route is based on the co-polymerization of two organically modified oxozirconium and oxohafnium clusters (M4O2(OMc)12 with M = Zr, Hf and OMc = OC(O)–C(CH3)=CH2)) with (methacryloxypropyl)trimethoxysilane (MAPTMS). The crystalline clusters, which are the precursors for the corresponding metal oxides (MO2) were prepared via the sol-gel route by reacting zirconium or hafnium butoxide with methacrylic acid. The copolymerization of the cluster with previously prehydrolyzed methacrylate-functionalized siloxane, allows the anchoring of the oxoclusters to the forming silica network. Thin films were prepared starting from a THF (tetrahydrofurane) solution with molar ratios Hf4O2(OMc)12: Zr4O2(OMc)12:MAPTMS of 1:1:88. After deposition, the films were annealed 3 h at 800 °C in air to promote the decomposition of the hafnium and zirconium oxoclusters to give the corresponding HfO2 and ZrO2 oxides. The obtained HfO2–ZrO2–SiO2 films resulted transparent, homogeneous and displayed a very good adhesion to the substrate. The composition of the films was investigated by secondary ionization mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS). The depth profiles evidenced a very homogenous distribution of both zirconium or hafnium species within the whole silica films and sharp film-substrate interfaces. As far as XPS analyses are concerned, the main XPS core-levels were analyzed for the annealed sample and the formation of hafnium and zirconium oxides was evidenced.

Synthesis and reactivity of alkenyl and alkynyl derivatives of permethylzirconocene and permethylhafnocene. Rearrangement of alkenyl derivatives involving β-H elimination from an sp2-hybridized carbon

The insertion of unactivated acetylenes into the metal-hydride bonds of (η 5-C 5Me 5) 2MH 2 (M = Zr, Hf) proceeds rapidly at low temperature to form mono- and/or bisinsertion products, dependent on the steric bulk of the acetylene substituents. 2-Butyne and t-butylacetylene react to form (η 5-C 5Me 5) 2M(H)(C(Me)CHMe) and (η 5-C 5Me 5) 2M(H)(CHCHCMe 3) (M = Zr, Hf). Propyne and 1-butyne react to form (η 5-C 5Me 5) 2M(CHCHCH 3) 2 (M = Zr, Hf) and (η 5-C 5Me 5) 2Zr(CHCHCH 2CH 3) 2. Phenylacetylene yields both (η 5-C 5Me 5) 2M(H)(CHCHPh) and (η 5-C 5Me 5) 2M(CHCHPh) 2 (M = Zr, Hf). To extend the study of the unsaturated carbon ligands, the bis(propynyl) complexes (η 5-C 5Me 5) 2M(CCCH 3) 2 (M = Zr, Hf) are prepared by treating (η 5-C 5Me 5) 2MCl 2 with LiCCCH 3. The reactivity of many of these complexes with carbon monoxide and dihydrogen is surveyed. The mono(2-butenyl)zirconium complex (η 5-C 5Me 5) 2Zr(H)(C(Me)CHMe) rearranges, forming the crotyl-hydride species (η 5-C 5Me 5) 2Zr(H)(η 3-C 4H 7) or, in the presence of 2-butyne, the zirconacyclopentene (η 5-C 5Me 5) 2- ZrCH 2CH(CH 2CH 3)C(CH 3)C (CH 3). The hafnium analog (η 5-C 5Me 5) 2Hf-(H)(C(Me)CHMe) also rearranges, although more slowly and less cleanly, to the analogous crotyl-hydride hafnium species. The bis(propenyl) and bis(butenyl)zirconium complexes also rearrange, forming zirconacyclopentenes (η 5-C 5Me 5) 2 ZrCH 2CH(R)CHC R and (η 5-C 5Me 5) 2 ZrCH 2CH(R)C(R)C H (R = CH 3, CH 2CH 3). The bis(propenyl)hafnium complex is stable to rearrangement. Mechanistic and kinetic studies of these transformations demonstrate the occurrence of an unusual β-hydrogen elimination from an sp 2-hybridized carbon.

A method for the determination of surface areas of hydrophobic colloids

Abstract The adsorption of hydrolyzed hafnium species on a polyvinyl choride (PVC) latex in aqueous solutions was determined as a function of pH using the radioactive tracer Hf181. At equilibrium, monolayer coverage was obtained at pH > 4.5. Under these conditions hafnium is in the solution in the form of neutral, soluble Hf(OH)4 species. Using the particle size distribution and the number concentration of PVC, as obtained from light scattering, a cross-sectional area of Hf(OH)4 in the close packed adsorption layer is calculated to be 23 A2. This is in agreement with the value obtained using powdered glass (24A2).

Interactions of silver halide sols with hydrolyzed hafnium species

Coagulation and reversal of charge effects of freshly prepared and heated solutions of hafnium tetrachloride have been studied as a function of the pH using aged silver halide sols and solsin statu nascendi. It was shown that the critical coagulation concentration and the critical stabilization concentration (due to charge reversal) increased with increasing pH. These observations have been related to the hydrolysis of the hafnium ion. At pH values above ∼4 essentially the entire amount of hafnium is present in the form of the neutral soluble, species Hf(OH)4. This accounts for the inability of the hafnium solutions to reverse the charge of the sols at higher pH values. The adsorption measurements carried out with the aid of the radioactive isotope181Hf showed that the neutral hydrolyzed species are strongly adsorbed on negatively charged silver iodide particles. The adsorbed amounts of hafnium on a AgI sol are considerably larger than in the case of charged hydrolyzed ions (such as hydrolyzed thorium ions) on a similar sol. This is explained by the ability of the neutral hafnium species, Hf(OH)4. to form a close-packed adsorbed layer. The results confirm previous findings that the enhanced adsorption of hydrolyzed ions is caused by the presence of the hydroxyl group, whereas the ionic charge plays a negligible role in this case.

The aqueous chemistry of hafnium (IV) solvent extraction and ion exchange studies

Some aspects of the solution chemistry of hafnium (IV) have been investigated by TTA solvent extraction and shallow bed ion exchange resin techniques. The stability constants of hafnium (IV) with chloride ( β 1 0·7, β 2 0·48), nitrate ( β 1 0·8), sulphate ( β 1 110, β 2 5 × 10 3) and fluoride ( β 1 4·2 × 10 4) ions have been determined and they are all lower as compared to the corresponding zirconium (IV) complexes. The rates of exchange of hafnium species on a cation exchange resin at various molarities in HCl, HNO 3 and HClO 4 media have also been determined. A model for the variation of diffusion coefficients with acidity has been suggested.