The Formation of PdCx over Pd-Based Catalysts in Vapor-Phase Vinyl Acetate Synthesis: Does a Pd–Au Alloy Catalyst Resist Carbide Formation?

The formation of Pd carbide (PdC _x ) during the synthesis of vinyl acetate (VA) was investigated over Pd/SiO₂ catalysts with two different Pd particle sizes, as well as over a Pd–Au/SiO₂ mixed-metal catalyst. XRD data show that PdC _x was produced in the pure Pd catalysts after reaction based on the downshift of the Pd(111) and (200) XRD features. The smaller Pd particles showed greater resistance to the formation of PdC _x . The XRD and XPS data are consistent with formation of a PdC _x species at the surface of the Pd–Au catalyst, however, the primary contributor to the downshift of the Pd(111) feature subsequent to reaction in the mixed-metal catalyst is believed to arise from reaction-induced alloying of Au with Pd. The alloying of Au with Pd is apparently very effective in preventing PdC _x formation in Pd-based catalysts for VA synthesis.     

Observations on the synthesis and structure of wide pore aluminum phosphate molecular sieves

The synthesis of wide pore 18-ring aluminum phosphates has been explored using several amines as templates. The systems studied in more detail involve the use of dipentylamine (DPTA), a combination of tributylamine (TBA) and dipentylamine and dipropylamine (DPA). In the latter two systems a thermally unstable phase with an XRD pattern similar to VPI-5 was obtained which has been identified as the H1 phase. In the DPTA system both thermally stable and unstable phases can be formed, depending on the reaction conditions used. By controlling the gel formation in the DPA system ALPO₄-8 was synthesized by hydrothermal methods. The products in the first two cases contain significantly higher amounts of the amine than has been reported in the synthesis of VPI-5. The structure of the thermally unstable H1 phase has been solved from X-ray powder data collected using a synchrotron X-ray source and shown to contain 6-coordinate aluminum in the 4-rings.     

The phases formed by heating Mn(II) and Zn(II) exchanged α-zirconium phosphate

The phases formed upon heating α-zirconium phosphate, Zr(HPO₄)₂·H₂O, loaded to different levels of Mn(II) and Zn(II) were determined by a combination of thermal and X-ray powder methods. The fully exchanged phases, ZrM(PO₄)₂·4H₂O lose water stepwise at progressively higher temperatures until anhydrous. Above 500°C the X-ray pattern indicates a triphosphate-like phase of the type M_0.5Zr₂(PO₄)₃ is formed. At lower loadings, solid solutions with relatively broad composition regions are observed. The data are summarized in phase diagrams which, however, are not equilibrium diagrams.     

Ionic conductivities of phosphorus-substituted β-eucryptite ceramics

Lithium ion conduction in β-eucryptite (LiAlSiO₄) and β-eucryptite-AlPO₄ solid solutions has been investigated by an ac impedance method between 423°K and 673°K over a frequency domain of 10 Hz - 500 kHz. Adding AlPO₄ increases conductivity and lowers the activation energy. This finding suggests that phosphate addition creates some sites in which the lithium ions are less tightly bound to the lattice oxide ions than in the parent β-eucryptite structure; it is these ions that account for the enhanced conductivity. The solid solutions have a composition of Li_1?x AlSi_1?xP_xO₄, where 0 ≤ x ≤ 0.4.     

Ionic conductivity of Ag+ exchanged α-zirconium phosphate

The conductivity of silver ion in silver exchanged α-zirconium phosphate, Zr(AgPO₄)₂, was measured in the temperature range 80 to 330°C. The data were found to conform to an Arrehnius type behavior in this temperature range with an activation energy of 0.53eV and ⁰300°C = 1.94 × 10^?4 ohm^?1 cm^?1. The conductivity values for Ag⁺ are higher than the corresponding values obtained for Na⁺ and K⁺ conduction in their respective exchanged forms. This enhancement is attributed to the greater polarizibility of the silver ion.     

Supermicroporous alumina–silica zinc oxides

A new family of porous mixed oxides with pores largely in the 8–20 Å range have been prepared. TEOS acts as a solvent and as a source of silica to which aluminum butoxide and transition metal acetates are added. Neutral amines are added as templates and to effect hydrolysis. This paper describes the ZnO–Al₂O₅–SiO₂ system but similar results have been obtained with other transition metal oxides. An interesting feature of the technique is that the larger the amine template the greater is the surface area of the mixed oxide with only a slight increase in the average pore diameter. Both NMR and atomic pair distribution functional methods have been used to prove the homogeneity of the mixed oxide products. This preparative method complements our earlier report in Chemical Communications on mixed oxides prepared with ZrO₂ and TiO₂ incorporating transition metal oxides.     

Preparation of (NH4)Zr2(PO4)3 and HZr2(PO4)3

(NH₄)Zr₂(PO₄)₃ has been prepared, hydrothermally, from α-zirconium phosphate in three different ways; (1) from amine intercalates at 300°C, (2) from mixtures of ZrOCl₂·8H₂O in excess (NH₄)H₂PO₄ and (3) reaction of NH₄Cl with Zr(NaPO₄)₂. Ammonium dizirconium triphosphate is rhombohedral with a = 8.676(1) and $\text{c}\text{= 24.288(5)}\text{A}\text{?}$. It decomposed on heating to HZr₂(PO₄)₃. Below 600°C a complex, as yet unindexed, X-ray pattern was obtained. A very similar X-ray pattern was obtained by washing LiTi_0.1Zr_1.9(PO₄)₃ with 0.3N HCl. Heating this phase or NH₄Zr₂(PO₄)₃, above 600°C resulted in the appearance of a rhombohedral phase of HZr₂(PO₄)₃ with cell dimensions a = 8.803(5) and $\text{c}\text{= 23.23(1)}\text{A}\text{?}$. The protons were not completely removed until about 1150°C. Decomposition of (NH₄)Zr₂(PO₄)₃ at 450°C yielded an acidic gas whereas at 700°C NH₃ was evolved. A possible explanation for this behavior is presented.     

Separation of americium from curium by oxidation and ion exchange

Nuclear energy has the potential to be a clean alternative to fossil fuels, but in order for it to play a major role in the US, many questions about the back end of the fuel cycle must be addressed. One of these questions is the difficult separation of americium from curium. Here, we report the oxidation of Am in two systems, perchloric acid and nitric acid and the affect of changing the acid has on the oxidation. K(d) values were observed and a direct separation factor was calculated and was seen to be as high as 20 for four metal(IV) pillared phosphate phosphonate inorganic organic hybrid ion exchange materials. These ion exchangers are characterized by very low selectivity for cations with low charge but extremely high uptake of ions of high charge.