Cations lanthanum

According to the hypothetical catalytic cycle (Figure 36), the lanthanum atom is believed to function as a Lewis acid and a lithium binaphthoxide moiety as a Brpnsted base. The nature of the coordination of the aldehyde appears to be of first importance. This coordination provides activation of the aldehyde for reaction with the hypothetical LLB-enolate (II) (which on the basis of pKa values can be present at most in low concentration), and also controls of the orientation of the aldehyde for enantioselective reaction. A H NMR study also supports the existence of the coordination between aldehydes and the lanthanum cation.89... 

Aluminum cation % Phosphorus cation Oxygen atom Lanthanum cation Calcium cation... 

Adsorption of pyridine subsequent to spectrum c gave rise to a band at 1452 cm typical of pyridine coordinatively bonded to lanthanum cations only a tiny pyridinium ion band at 1542 cm was observed. When, however, generation of spectrum d (i.e. after HgO contact of the sample) was followed by pyridine adsorption, the band of acidic OH groups at 3630 cm was completely removed and a strong band at 1542 cm (pyridium ions) appeared. 

Finally, La-Y zeolites were obtained by solid-state ion exchange between LaCl3 and the sodium form of Y-type zeolites as well [25]. This was proven by chemical analysis, IR, X-ray diffraction (XRD) and a test reaction. Details of the preparation and characterisation are outlined in Ref. [25]. Chemical analysis gave evidence for a partial replacement of sodium by lanthanum cations. IR showed the formation of acidic OH groups. Finally, XRD demonstrated, via the appearance of reflections of crystalline NaCl, that obviously Na cations were expelled from the interior of the zeolite crystals by in going La. Outside the zeolite particles they had formed small NaCl crystallites. 

On the other hand, if zeolites are treated at high temperatures, a complete ion exchange can be achieved. The radius of the hydrated La3+ ion is 3.96 A whereas the free diameter of the entrances to the network of the small cavities is 2.4 A (Herreros et al. 1992). However, when the zeolite is heated to about 320°C, the hydration sphere is lost (the Pauling radius of La3+ is 1.13 A) and some of the lanthanum cations migrate into the sodalite cages. 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

Faujasites exchanged with rare earth cations, and particularly with La " ", have shown good performance as solid alkylation catalysts, as nicely reported in the pioneering works carried out in the late 1960s by researchers at Mobil Oil (89) and Sun Oil (96), and later on by Weitkamp s group (132,133). This fact is most likely related to the ability of La-FAU zeolites to catalyze hydride transfer reactions. The Bronsted acidity in La-exchanged FAU zeolites can be associated with the protons generated upon hydrolysis of water in the hydrated lanthanum cations when snbjected to thermal activation treatment at temperatures between 60 and 300°C (134-136) ... 

Lithium can be inserted into the material up to at least 0.08 Li" " per formula unit. This level of intercalation is insufficient for the number of lithium and lanthanum cations to exceed unity and so the A sites of the perovskite structure still contain some vacancies at this stoichiometry. Whilst this intercalation process is reversible, experiments using this electrolyte in conjunction with a graphite electrode show that an irreversible oxidation process occurs. The reduction of Ti" " narrows the band gap and leads to electronic conductivity of 0.01 S cm at room temperature. This reactivity and electronic conduction would lead to a rapid discharge via short circuit of a stored battery and so makes these materials unsuitable for use as an lithium electrolyte in these applications. 

When the lithium content is reduced further the structure becomes still more complex. A slow-cooled sample of Lio.i6Lao.62Ti03 has been extensively studied and shows a distribution of lanthanum cations across two sites to give a layered structure shown in Figure 3.22. This contains sheets where the lanthanum site is almost filled which alternate with layers containing 70% vacancies on the lanthanum position. These lanthanum-poor layers accommodate the lithium in square-planar coordination environments similar to the disordered lithium-rich compositions. 

According to the previous conclusions it was expected that the maximum selectivity should coincide with the maximum basicity. This was, however, not true as can be derived from Figure 11, showing not only selectivity but also the ratio of acetone-to-propene formed from isopropanol and the rate of propene formation. This reaction is often used as a probe reaction for determining acidity and basicity propene formation is assumed to be indicative for acidic sites while both acidic and basic sites are needed for acetone formation. The maximum selectivity is close to the maximum of the propene formation rate. From this it was concluded that besides surface basicity a certain amount of surface acidity is required for maximizing selectivity. This is achieved by incorporating the lanthanum cation which may act as a weak acid in the host matrix. 

However, in this fluorine sensor, the base electrolyte is composed of the lanthanum cation with the fluorine anion. In addition, one unusual property is that the sensor can operate even when directly immersed into aqueous solutions at room temperature. The realization of the application is greatly dependent on two facts Lap3 is stable in water and it shows an excellent fluorine conduction even at room temperature. These remarkable characteristics permit the use of Lap3 as a simple and portable sensor and have led to its commercial use on a worldwide scale. 

In the case of the formation of ATLS from lanthanum oxide, the structure of lanthanum hydroxide as an intermediate allows the formation of apatite through the topotactic mechanism. Thus, the coordination number (CN) of lanthanum in lanthanum oxide is 7, while there are 7- and 9-coordinated lanthanum cations in the apatite (Table 6). The lanthanum oxide hydration leads to increase of lanthanum CN from 7 to 9. This is also accompanied by the lattice expansion in the a and b dimensions and its contraction in the c dimension (Table 6). Subsequent acid-base interaction between the lanthanum hydroxide and silica or intermediate amorphous silicate results in the silicon incorporation into the structure to form apatite structure. This leads to the additional lattice expansion in three dimensions and the decrease of CN from 9 to 7 for 64% of lanthanum cations. 

Nery JG, Mascarenhas YP, Bonagamba TJ, Mello NC, Souza-Aguiar EF. Location of cerium and lanthanum cations in CeNaY and LaNaY after calcination. Zeolites 1997 18 44-9. 

This compound can take two crystallographic structures, bixbyite and perovskite. In the bixbyite structure the lathanum and the yttrium anions are both placed in a mean el t-fold oxygen environment and in perovskite structure the lanthanum cation is strictly placed in a 12-fold oxygen environment and thus yttrium in a 6-fold environment. For a low temperature process we reach firstly the bixbyite structure and the perovskite... 

---