Alkali metal | EPFL Graph Search

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element. The alkali metals are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. They can all be cut easily with a knife due to their softness, exposing a shiny surface that tarnishes rapidly in air due to oxidation by atmospheric moisture and ox

Effect of alkali, aluminium and equilibration time on calcium-silicate-hydrates

Yu Yan

Calcium silicate hydrate (C-S-H) is the main hydration product in Portland and blended cements, and greatly affects durability and mechanical properties of the hydrated cement. In the presence of Al-rich supplementary cementitious materials (SCMs), C-(A-)S-H can contain more Al than C-S-H in plain Portland cements. The use of SCMs in cementitious system not only lowers CO2 emission, but may also enhance the mechanical properties in the long term, its durability and can help to suppress alkali silica reaction. However, important questions regarding the effect of Al, alkali and equilibration time on C-A-S-H structure and solubility are still not fully understood. This thesis focuses on the interplay between aluminium, alkali hydroxides in solution, the structure of C-A-S-H and aluminium, alkali uptake by C-A-S-H and kinetics of C-A-S-H formation. For aluminium free C-S-H, KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.For C-A-S-H with Ca/Si=1, the presence of Al led to higher amounts of secondary phases, larger interlayer distance, longer silicate chain, higher dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases is reduced at higher pH; strätlingite and Al(OH)3 dominate at lower pH, and katoite at higher pH values. High pH is also associated with shortened silicate chain length, increased Al and Si concentrations, maximum Al/SiC-S-H and lower Ca concentrations. The Al uptake in C-A-S-H increased with aluminum concentration in solution, indicating the predominance of a common Al sorption mechanism independent of dissolved Al concentration or pH values. XANES data showed the presence of both Al(IV) and Al(VI) with different chemical environments in C-S-H.The effect of equilibration time on C-A-S-H was investigated from 6 hours to 3 years. Rapid changes within the first day were observed in both the solid phase and the aqueous phase. Portlandite was formed in all samples with different Ca/Si and its amount decreased rapidly within the first day and remained stable after more than 7 days. Gismondine-P1 precipitated between 1 and 3 years and destabilized the C-A-S-H phases at target Ca/Si 0.6. In the solid phase, the Al migration from secondary phases to C-A-S-H and an increasing fraction of Al(V) and Al(VI) in C-S-H were observed with longer equilibration time. The Ca/Si ratio of C-S-H played an important role on the extent of changes in aqueous concentration: an increasing of Al concentrations with time was observed in C-A-S-H with Ca/Sitarget=0.6, while a decrease was observed for C-A-S-H with Ca/Sitarget=1.0. The uptake of Al and Na in C-A-S-H was observed to decrease after 4 days and 1 day separately, which indicates a rearrangement of C-A-S-H with time.

EPFL2022

Advances in heterogeneous catalysis for the synthesis of polyoxymethylene dimethyl ethers

Christophe Jean Baranowski

Our society is engaged in a major shift towards more sustainable industries. The required transition from fossils to renewable resources with low greenhouse gas emission is specifically challenging in the transportation sector where high thermal efficiency and energy density are key requirements. Due to their appealing properties, polyoxymethylene dimethyl ethers (OME) have recently received increasing attention as a new type of Diesel additive or substitute. However, the OME technology being only at an early stage of research, there is a limited understanding of the catalyst structure-activity relationships for OME synthesis. The aim of this thesis is to progress in the development of knowledge in heterogeneous catalysis for the sustainable production of OME. We focused on rational catalysts synthesis and in-depth characterization to understand how their properties influenced OME synthesis. Zeolites have demonstrated a high catalytic potential for the synthesis of OME. By modifying accessibility to the active sites in an H-ZSM-5 zeolite, we demonstrated that the reaction suffered from severe internal diffusion limitations of the reactants and products in the zeolite micropores. Controlled introduction of an intercrystalline network of mesopores significantly enhanced the zeolite's activity. Subsequently, we introduced tin in montmorillonite clay. This resulted in a hierarchical material composed of tin oxide inserted between the clay layers, with Brønsted and Lewis acidity. Its advantageous textural and acidic properties resulted in an active catalyst for OME synthesis from trioxane (TRI) and dimethoxymethane (OME1). To investigate the role of Lewis and Brønsted acidity for this reaction, a series of Beta zeolites with varying amounts of Brønsted and Lewis acid sites were synthesized. A synergy between these two types of acid sites resulted in a large turnover frequency increase accompanied with a reduction in by-product formation. Then, we studied the causes of inhibition of the reaction kinetics by water for the synthesis of OME from TRI and OME1 over an H-Beta zeolite. The presence of water as an impurity severely affected the reaction kinetics. The main OME growth mechanism shifted from direct TRI insertion to formaldehyde incorporation in OME, as the level of water in OME1 increased. The cause of deactivation was the hampered adsorption of TRI on the zeolite active sites by the presence of water. Lastly, the water free, non-oxidative catalytic dehydrogenation of methanol to formaldehyde (FA) was investigated as a source of oxymethylene groups, and thus an appealing alternative to TRI utilization. Grafting on amorphous silica in methanol was selected as the method of choice to study the activity of alkali metals. The resulting catalysts displayed an increased activity for the catalytic dehydrogenation of methanol. A large gap in selectivity towards FA between ions with a high and low charge density was observed. Na grafted on silica yielded the best combination of moderate conversion and high selectivity. As a result of the work presented in this thesis, important catalyst features can now be considered when developing catalysts or designing production processes for OME synthesis. More energy-efficient processes will require alternatives to the usage of TRI as the oxymethylene source. Alkali metals grafting on silica has the potential to catalyze this reaction and this method could pave the way for future research.

EPFL2020