Selectivity, ammonia

Note that no stereochemistry has been introduced so far. Reduction (sodium and liquid ammonia) selectively gives trans chrysanthemic alcohol which can be oxidised to the acid with CrOs. Draw out the whole synthesis as a chart. 

GP 10] [R 18] The ammonia selectivity of 62% at a p-gauze platinum catalyst is in the range of the technical process (Figure 3.48) the methane selectivity of 30% is only half the industrial performance (70 ml h methane 70 ml h ammonia ... 

Vreactor=70 ml VCh=Vrcn=10 ml (0.045 mole) 111 1 =0.3 g PH2=80 bar (at RT) was not maintained during reaction NH3/RCN=0.25 without ammonia, selectivity of SB is higher at lower temperatures. The selectivity to RNH2 decreased with reaction time for the experiment performed without NH3. The apparent activation energy of the hydrogenation of RCN on RNi-L catalyst was 30.5 kJ/mol, which is close to the value 46 kJ/mol measured in the liquid phase hydrogenation of acetonitrile on CoB amorphous alloy catalyt [7], RNi-C is more active than RNi-L catalyst (compare Table 1 No 4 and 6 and Table 2 No 7 and 8). 

At present the most effective available after-treatment techniques for NO, removal under lean conditions are ammonia selective catalytic reduction (SCR) [1-3] and NO, storage reduction (NSR) [4—6]. Indeed, three-way catalysts (TWCs) are not able to reduce NO, in the presence of excess oxygen, because they must be operated at air/ fuel ratios close to the stoichiometric value. Also, non-thermal plasma (NTP) and hydrocarbon-selective catalytic reduction (HC-SCR) are considered, although they are still far from practical applications. 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

Selective Catalytic Reduction (SCR) process is very similar to SNCR with the exception that a catalyst is used to accelerate the reactions at lower temperatures allowing it to be applied to both full and partial-burn regenerators. An SCR system consists of a catalyst bed installed in the flue gas line of a combustion system. Ammonia is injected into the flue gas with air in the presence of a catalyst. The catalyst is typically oxide forms of vanadium and tungsten. The ammonia selectively reacts with NOx to form molecular nitrogen and water via an exothermic reaction that has achieved > 90% reduction in NO when applied to an FCCU. 

Ammonium chloride is analyzed by treatment with formaldehyde (neutralized with NaOH) and the product HCl formed is analyzed by titration using an acid-base color indicator such as phenolphthalein. Alternatively, it may be mixed with caustic soda solution and distdled. The distillate may be analyzed for NH3 by titration with H2SO4 or by colorimetric Nesslerization or with an ammonia-selective electrode (APHA, AWWA, WEF. 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC, American Pubhc Health Association). The presence of ammonia or any other ammonium compound would interfere in the test. The moisture content in NH4CI may be determined by Karl—Fischer method. 

Elemental composition Be 49.11%, N 50.89%. Analysis may be performed by treatment with HCl. The soluble BeCL solution is then measured for Be by AA or ICP techniques. The ammonia liberated is determined by titrimetry, colorimetry or by ammonia-selective electrode (see Ammonia). 

Elemental composition Na 58.93%, N 35.90%, H 5.17%. The compound may he decomposed cautiously with water (reaction is violent) under cooling to yield sodium hydroxide and ammonia. (Or it may he decomposed with anhyrous alcohol to form ammonia and sodium alcoholate. The alcoholate then may he treated with water to form sodium hydroxide). Ammonia liberated is dissolved in water and the solution is measured using an ammonia-selective electrode. Alternatively, ammonia is collected over horic acid solution containing a small quantity of methyl red indicator. The solution is titrated with a standard solution of sulfuric acid. Sodium hydroxide is measured hy titration with a standard solution of hydrochloric or sulfuric acid. 

In a similar vein, reaction of the readily accessible xanthine (39-1) with the ubiquitous phosphorus oxychloride affords the enol chloride (39-2). Alkylation of the anion obtained from that product with a base with the chloro deoxy sugar (39-3) leads to the glycosylated product (39-4). Treatment with ammonia selectively replaces the halogen at the 6 position. The protecting groups on the sugar are cleaved in the course of the reaction to afford cladribine (39-4) [41]. 

The cation exchange capacity (CEC) of the saponite was determined from the ammonium content in solution after exchange with NaOH using an ammonia selective electrode. 

HMSO (UK) [17] have published a method for the determination of ammonia, nitrate and nitrite in potassium chloride extracts of soil extracts. An aliquot of the extract is made alkaline and the ammonia released, originating from ammonium ions, is determined either with an ammonia-selective probe or, after removal by distillation, by titration (Crompton TR, private communication). 

A method [17] has been described for the determination of nitrate and nitrite nitrogen and ammonium ions in the 2 M potassium chloride extracts of moist soils. Firstly, an aliquot of the extract is made alkaline and the released ammonia determined via an ammonia-selective probe or titrimetrically. The nitrate in the ammonia-free extract is then reduced to ammonia with Devarda s alloy and the ammonia removed by distillation and determined titrimetrically. The concentration of nitrite in the extract is then determined spectrophoto-metrically as the red dye formed by coupling diazotised sulfanilic acid with N naphthylethylenediamine hydro chloride. 

The gas-sensing electrodes also are used for the potentiometric measurement of biologically important species. An enzyme is immobilized at or near the gas probe. The gas sensor measures the amount of characteristic gas produced by the reaction of the analyzed substance with the enzyme. For example, an enzyme electrode for urea [NH2C(0)NH2] determination is constructed by the immobilization of urease onto the surface of an ammonia-selective electrode. When the electrode is inserted into a solution that contains urea, the enzyme catalyzes its conversion to ammonia ... 

Fig. 4.23. Manifolds used for speciation analysis of solid samples. (A) Mercury speciation (as Hg- and PhHgH). (B) Nitrogen speciation (as ammonia and urea). GLS gas-liquid separator, MWS microwave system, ISE ammonia-selective electrode, RE reference electrode, WB water bath thermostat. Other abbreviations as in previous figures. (Reproduced with permission of Elsevier.)...

Saliva Freeze at -20 °C for up to 2 weeks, or for 1 hour at 4 °C, or analyze immediately Membrane based ammonia- selective electrode Not reported Not reported Huizenga et al. 1994... 

Ammonia selectivity of platinum and platinum-nickel catalysts for NOx reduction varies with the nature of the supporting oxide. Silica, alumina, and silica-alumina supports on monolithic substrates were studied using synthetic automotive exhaust mixtures at 427°-593°C. The findings are explained by a mechanism whereby the reaction of nitric oxide with adsorbed ammonia is in competition with ammonia desorption. The ease of this desorption is affected by the chemistry of the support. Ammonia decomposition is not an important reaction on these catalysts when water vapor is present. 

We explored the mechanistic implications of the apparent importance of acidity to ammonia selectivity, beginning with the assumption that ammonia is an intermediate in the reduction of nitric oxide over precious metal catalysts. (Attempts to explain our observations on the alternative assumption that chemisorbed isocyanate is the active intermediate (7) were fruitless since nickel should not change isocyanate s behavior. However, an acidic surface would probably lower isocyanate stability.) Since the total conversion of NO increased with nickel addition, it is likely that some conversion takes place on the nickel, probably by the reaction of NO with hydrogen spilled over from the platinum. Disappearance of this ammonia could occur by ammonia decomposition (Reaction 1) as hy-... 

The enthalpies of ammoniation of gaseous cations relative to that of Na+, and of gaseous anions and the electron relative to that of I , have been determined from an analysis of enthalpies of solution of salts and metals in liquid ammonia. Selected experimental enthalpies of formation, A/f /kcal mol , of ion pairs in liquid ammonia at —33 °C are shown in Table 3. The AJTj of a... 

Also for oxidation reactions, the choice of the alumina support mainly depends on two criteria the stability of the phase at the reaction temperature and the reactivity (or better the lack thereof) toward feed components and products. For example, ethylene oxychlorination to ethylenedichloride is performed at approximately 220-250° C and 5-6 atm in the presence of a y-Al203-supported catalyst, which has a surface area of from 100 to 200 m and contains 10wt% CUCI2 and 3 wt% KCl, (423,424). Another example is a process called ammonia selective oxidation (ASO, or also selective catalytic oxidation, SCO), which converts small amounts of NH3 from waste gases to N2 at reaction temperatures of 150—300 °C. The process is used to abate the ammonia sHp after a selective catalytic reduction process with ammonia or urea in diesel-engine-exhaust after-treatment (425). The patented catalyst consists of Y-AI2O3 (60—300 m g ) loaded with 0.5-4 wt% platinum and 0.5—4 wt% vanadia and is coated onto the surface of a ceramic or metallic monoftthic structure (426). 

---