On calcium carbonate

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

A solution of 0.3 of 3j9-acetoxy-21-ethoxy-17a-pregn-5-en-20-yn-17/3-ol in 7 ml of pure pyridine is hydrogenated over 90 mg of 2.5% palladium-on-calcium carbonate at room temperature and atmospheric pressure. The reaction mixture is filtered and evaporated and the residue recrystallized to give 80 % of the 17a-vinyl product. 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

Catalytic reduction of quinazolines unsubstituted in position 4 using palladium-charcoal, palladium on calcium carbonate, Raney nickel, or Adam s platinum has been used for preparing 3,4-dihydro-... 

Support has been shown to influence selectivity and some workers have obtained higher yields of cis isomer over palladium-on-calcium carbonate or palladium-on-barium sulfate 21), whereas others find carbon satisfactory. In general, carbon support makes the more active catalyst and it is, therefore, more prone to become hydrogen poor. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

Dalas E (2001) The effect of ultrasonic field on calcium carbonate scale formation. J Cryst Growth 222(l) 287-292... 

A recent study in these laboratories (75) on calcium carbonate precipitation from Wyodak coal has confirmed the relationship between ion-exchangeable calcium and the appearance of calcium carbonates during liquefaction. These experiments were performed on samples of the subbituminous coal which had been demineralized, to ensure that all carboxylic acid groups were in the acidic form, and subsequently exchanged with varying amounts of calcium ions. 

Lindlar s catalyst metallic palladium deposited on calcium carbonate and is poisoned with lead acetate and quinoline. 

Z)-Methyl styrene was easily obtained by hydrogenation of 1-phenyl-1-propyne using Lindlar s catalyst (5% palladium on calcium carbonate, poisoned with lead) in //-hexane under an atmospheric pressure of hydrogen. The mixture, containing 90% of (Z)-methyl styrene and 10% of the overreduced alkane, was used without further purification. 

Finally, it is intriguing that in terms of biomineralization, invertebrates have based their reliance on calcium carbonates, while vertebrates appear to have used almost exclusively calcium phosphate. We say almost, because, while the use of calcium phosphates for biomineralization is an invention of some vertebrates, they still use calcium carbonate for the formation of otoliths4 of the inner ear. It remains to be established if the equivalent of the gene starmaker required for otolith formation in zebrafish has homologues among invertebrates. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

RP-HPLC with gradient elution was employed for the study of the influence of theaflavins and thearubigins on the adsorption of black tea on calcium carbonate. Separation of tea constituents was performed in an ODS column (250 X 4.9mm i.d. particle size 5 im). Aqueous solvent was 1 per cent citric acid, pH adjusted to 2.8 with sodium hydroxide and the organic solvent was ACN. The gradient initiated at 8 per cent ACN, was increased to 31 per cent in 50min. Theaflavins and thearubigins were detected at 460 nm, while total polyphenolics were detected at 280 nm. The flow rate was 1.5 ml/min. The results demonstrated the involvement of theaflavins and thearubigins in the adsorption process [185],... 

S. Suzuki, Black tea adsorption on calcium carbonate a new application to chalk powder for brown powder materials. Coll. Surf. A Physicochem. Eng. Asp. 202 (2002) 81-91. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

On the other hand, some compounds slow down the uptake of hydrogen and may even stop it at a certain stage of hydrogenation. Addition of lead acetate to palladium on calcium carbonate makes the catalyst suitable for selective hydrogenation of triple to double bonds (Lindlar catalyst) [36] (Procedure 9, p. 206). 

Ketones containing acetylenic bonds were reduced selectively at the triple bond by catalytic hydrogenation. Over 5% palladium on calcium carbonate in pyridine (which decreases the activity of the catalyst) 17-ethynyltestosterone was reduced in 95% yield to 17-vinyltestosterone, while over palladium on charcoal in dioxane 80% of 17-ethyltestosterone was obtained the carbonyl in position 3 and the conjugated 4,5-double bond remained intact [386]. 

The reduction of quinazolinone 242 with hydrogen at 3 atm in the presence of a palladium-on-calcium-carbonate catalyst furnished perhydroquin-azolinone 243. The stereochemistry of 243, with four chiral centers, was not investigated (70M1767). 

Understanding the adsorption mechanism of the polymer on calcium carbonate particles is also something important which should help the design of an improved dispersant. Increasing the level of adsorption of the polymer on particles will greatly enhance the dispersing efficiency. 

Figure 8 Radius of gyration of Na polyacrylate in solution and adsorbed on calcium carbonate surface as a function of ionic strength...

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