Rare earth basic oxides

Similarly, Bond et al. [4] confirmed that the microwave stimulation of methane transformation reactions in the presence of a number of rare earth basic oxides to form C2 hydrocarbons (ethene, ethane) was achieved at a lower temperature and with the increased selectivity. Microwave irradiation resulted in an increase of the ethene/ethane ratio, which was desirable. The results obtained were explained by the formation of hot spots (Sect. 10.3.3) of higher temperature than the bulk catalyst. This means that methane is activated at these hot spots. 

Stabilization of MoO is achieved by incorporation of alkaline-earth metal oxides (e.g. BaO, MgO) or rare-earth metal oxides (e.g., La20 ) or by reaction with base metal oxides (e.g., NiO, CoO, CuO etc.). Since MoO is an acidic oxide, its reaction with a basic oxide should lead to a mixed oxide... 

The first ceramic oxygen membranes were discovered by Nernst [2] in 1899 in the form of mixrnres of zirconia and rare-earth metal oxides. Basically, oxygen transport in oxide ceramics can be realized in three variants (Fig. 1). Materials... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Lanthanide, as a pure metal, is difficult to separate from its ores, and it is often mixed with other elements of the series. It is mosdy obtained through an ion-exchange process from the sands of the mineral monazite, which can contain as much as 25% lanthanum as well as the oxides of several other elements of the series. The metal is malleable and ductile and can be formed into many shapes. Lanthanum is considered the most basic (alkaline) of the rare-earth elements. 

According to Fraser (1975a), rare earth oxides may dissociate into the melt with both basic and acidic behavior—i.e.. 

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

Samarium ore usually is digested with concentrated sulfuric or hydrochloric acid. The extraction process is similar to other lanthanide elements. Recovery of the metal generally consists of three basic steps. These are (1) opening the ore, (2) separation of rare earths first to various fractions and finally to their individual compounds, usually oxides or halides, and (3) reduc-... 

The rare earth oxides have a number of distinguishing properties important in catalytic applications. The oxides are basic O) compared to alumina, lanthanum oxide (La203) being the most basic. The oxides also have good thermal stability, a valuable characteristic in most industrial applications. Some rare earths including cerium, praseodymium, and terbium form non-stoichiomet-ric oxides ( ), an important property shared by many good oxidation catalysts. These mixed valence state compounds are typically polymorphic. 

The other pathway leading to the formation ofoxogroups in the coordination sphere of the metal atom is provided by uncontrolled oxidation of the basic alkoxides such as alkali, alkaline earth metal, and quite probably the rare earth metal ones by oxygen dissolved in solvents and present in the atmosphere. The primary oxidation products are peroxides and hydroperoxides — M(OOR)n and M(OOH)n, whose decomposition gives water among the other... 

The method20 consists essentially of heating a mixture of rare earth oxides and excess ammonium chloride to a temperature of 200°C. or higher. Hydrolysis of the rare earth chlorides with formation of basic compounds is effectively prevented by the presence of excess ammonium chloride. The remaining ammonium chloride is then removed completely by heating in a vacuum at 300 to 320°C. 

Eecent work by L. M. Dermis2 and his co-workers has shown that electrolysis may be of considerable value in effecting a complete or partial separation of the oxides of the rare earth metals. Prom a neutral solution of the nitrates of neodymium, praseodymium, lanthanum, and samarium, nearly all the lanthanum is deposited as hydroxide in the last fractions discharged on the cathode. The hydroxides are deposited fractionally in order of their basicity, and the deposition is not dependent upon the... 

On heating, all of the anhydrous sulfates of the trivalent rare-earth elements and yttrium, type formula R2(S04)3 decompose, without first melting, to basic salts (oxysulfates) of the type R203.S03, then to an oxide. The oxide final product is R203 for all the elements except cerium, praesodymium,... 

Most of the knowledge about aluminate and alkylaluminum coordination stems from X-ray crystallographic studies. The basic idea of this section is to compile a rare-earth metal aluminate library categorizing this meanwhile comprehensive class of heterobimetallic compounds. Main classification criteria are the type of homo- and heterobridging aluminate ligand (tetra-, tri-, di-, and mono alkylaluminum complexes), the type of co-ligand (cyclopen-tadienyl, carboxylate, alkoxide, siloxide, amide), and the Ln center oxidation state. In addition, related Ln/Al heterobimetallic alkoxide complexes ( non-alkylaluminum complexes) are surveyed. Emphasis is not put on wordy structure discussions but on the different coordination modes (charts) and important structural parameters in tabular form. An arbitrary collection of molecular structure drawings complements this structural report. 

This is related to the temperature range (see Scheme 4.2 [116]) and the type of the catalyst. Type (1) reactions prevail (but are not exclusive to) with early transition metal oxides, strongly basic oxides like MgO and rare earth oxides type (2) reactions prevail on the oxides of the late transition metals (group 6 and higher) and on Cu oxides. 

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