SiO2 glass density to lower-mantle pressures

The convection or settling of matter in the deep Earth’s interior is mostly constrained by density variations between the different reservoirs. Knowledge of the density contrast between solid and molten silicates is thus of prime importance to understand and model the dynamic behavior of the past and present Earth. SiO2 is the main constituent of Earth’s mantle and is the reference model system for the behavior of silicate melts at high pressure. Here, we apply our recently developed x-ray absorption technique to the density of SiO2 glass up to 110 GPa, doubling the pressure range for such measurements. Our density data validate recent molecular dynamics simulations and are in good agreement with previous experimental studies conducted at lower pressure. Silica glass rapidly densifies up to 40 GPa, but the density trend then flattens to become asymptotic to the density of SiO2 minerals above 60 GPa. The density data present two discontinuities at ∼17 and ∼60  GPa that can be related to a silicon coordination increase from 4 to a mixed 5/6 coordination and from 5/6 to sixfold, respectively. SiO2 glass becomes denser than MgSiO3 glass at ∼40  GPa, and its density becomes identical to that of MgSiO3 glass above 80 GPa. Our results on SiO2 glass may suggest that a variation of SiO2 content in a basaltic or pyrolitic melt with pressure has at most a minor effect on the final melt density, and iron partitioning between the melts and residual solids is the predominant factor that controls melt buoyancy in the lowermost mantle

Thermodynamic Assessment of the Cu-Pt System

A CALPHAD assessment of the Cu-Pt system has been carried out. Two and four sublattice models were applied to describe the Gibbs free energies of ordered phases where the contribution of SRO is taken explicitly into account through the reciprocal parameters. The disordered fcc A1 and liquid phases were treated as substitutional solutions. A consistent set of parameters for the phases in the Cu-Pt system as obtained, and those parameters can satisfactorily reproduce the experimental phase equilibria and thermodynamic properties, such as enthalpies, activity of Cu, and long-range order parameters

Methanol intrusion in MFI-zeolites at high pressure

MFI-zeolites are currently used as catalysts in some olefins-production processes, representing an appealing alternative to the high-energy demanding Steam Cracking process, which accounts for 95% of the total worldwide olefins production (Sadrameli 2016; Arvidsson et al., 2016). More recently, MFI-zeolites have been used in the promising methanol-to-olefins (MTO) synthesis process, which, being able to obtain olefins directly from methanol in place of oil, bears a potential breakthrough industrial impact. At ambient conditions, only the surfaces of the zeolite crystallites are believed to be active in the methanol-to olefins process. However, pressure may favour the intrusion and diffusion of methanol molecules through the zeolitic channels, as observed in other zeolites (Gatta et al., 2018). This phenomenon may bear a significant impact in the industrial applications of MFI zeolites as catalysts, as a \u201ccold\u201d intrusion of methanol into the zeolite cavities might pave the way to increase the efficiency of the MTO conversion process. In this regard, we have synthesized and investigated, by in situ synchrotron powder-XRD, the high-pressure behaviour of six MFI-zeolites with different chemical compositions (framework-Si partially replaced by Al or B and counterbalanced by Na or H as extra-framework cations), by using methanol and silicone oil (as a reference) as P-transmitting fluids, respectively. All the synthesized zeolites are monoclinic (space group P21/n11) at ambient pressure, although a monoclinic-to-orthorhombic phase transition (MOPT) is observed to occur at \uf07e P > 0.5 GPa. Based on the experimental X-ray diffraction patterns and on the high-P evolution of the unit-cell parameters, we ascertain that: i) all the MFI zeolites compressed in silicone oil (acting as non-penetrating fluid) have, overall, the same bulk compressibility, ii) pressure induces the intrusion of methanol through the structural voids and, among the different zeolites, the magnitude of the adsorption phenomenon is different, iii) the MOPT is influenced by both the crystal chemistry and the sorbate (methanol) loading. The experimental findings of this study represent the first step to select the optimal chemical composition of a potential MFI-catalyst for the MTO conversion process operating at high-pressure conditions. References: Arvidsson M., Haro P., Morandin M., Harvey S. 2016. Comparative Thermodynamic Analysis of Biomass Gasification-Based Light Olefin Production Using Methanol or DME as the Platform Chemical. Chem. Eng. Res. Des., 115, 182\u2013194. Gatta G. D., Lotti P., Tabacchi G. 2018. The Effect of Pressure on Open-Framework Silicates: Elastic Behaviour and Crystal\u2013fluid Interaction. Phys. Chem. Miner., 45, 115\u2013138. Sadrameli S. M. 2016. Thermal/Catalytic Cracking of Liquid Hydrocarbons for the Production of Olefins: A State-of-the-Art Review II: Catalytic Cracking Review. Fuel, 173, 285\u2013297

The elastic behavior of zeolitic frameworks : The case of MFI type zeolite under high-pressure methanol intrusion

The high-pressure behavior of six synthetic zeolites with the MFI topology, characterized by different chemical composition (framework-Si partially replaced by Al or B and counterbalanced by Na or H as extra-framework cations), has been investigated by in-situ powder synchrotron X-ray diffraction using silicone-oil and methanol as hydrostatic pressure-transmitting fluids. For each sample, the compressibility in silicone-oil has been found to be considerably higher than that in methanol. This difference in terms of bulk elasticity is due to the adsorption of methanol already at P < 0.1 GPa, with different magnitudes as a function of the sample crystal-chemistry. The high number of experimental pressure points allowed an accurate determination of the monoclinic-to-orthorhombic phase transition (MOPT), detected between 0.3 and 0.7 GPa in the samples compressed in silicone-oil, whereas the orthorhombic Pnma polymorph has been found to be stable already at 3c 0.1 GPa in four samples compressed in methanol. This suggests that the adsorption of methanol may increase the P-stability range of the orthorhombic Pnma phase. A comparative analysis of the effect of pressure on the methanol adsorption by MFI-zeolites with different chemical composition is provided, which offers potentially useful information on their application as catalysts in the methanol-to-olefins conversion processes and in industrial high-pressure processes