Acid centers

A significant concern in all nitration plants using mixed acids centers on the disposal method or use for the waste acids. They are sometimes employed for production of superphosphate ferti1i2ers. Processes have also been developed to reconcentrate and recycle the acid. The waste acid is frequently first stripped with steam to remove unreacted HNO and NO. Water is then removed by low pressure evapori2ation or vacuum distillation. 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

The completion of the synthesis of 1 required installation of the (R)-nipecotic moiety. The original method used (R)-ethyl nipecotate L-tartrate 21, which was commercially available, but the availability of this intermediate on multi-kilogram scale required long lead times and cost was a major factor. In addition, it was also discovered that saponification of the ethyl ester in the final stages of the synthesis, as shown in Scheme 7.3, was accompanied by small amounts of epimerization at the carboxylic acid center of 1, resulting in diastereomeric contamination of the final product. 

The NMR spectra of the resulting poly(MMA/MAA) and prepolymers show that the carbonyl spectra of the prepolymer and the tin-stripped polymer are nearly identical. The syndiotactic com-positonal triads are assigned in Figure 5 with A representing the free acid centered triad and B representing the methyl ester centered triad. The ratio of the intensities of the A-centered triad are approximately correct for a random composition. 

Fig. 16. Chiral 4th-generation dendrimers from para- or meto-(chloromethyl)-phenyl substituted acetonides and a 1,3,5-benzenetricarboxylic acid center piece [72]... 

It was an interesting idea to create the giant tube-like structures 46 capped with a Lewis acid center by the homologation reaction of 42 with dimethylsulfoxonium methylide and their deboronation to the three-armed star polymethylene polymers 47 incorporating a m, r-l,3,5-trisubstituted cyclohexane core (Scheme 12) <2003JA12179, 20010L3063>. 

Recrystallization procedure applied to the amorphous aluminosilicates of different chemical composition resulted in the formation of the dispersed zeolitic domains of the FAU and BEA structure in porous matrices. The structural transformation into the composite material was proved with TEM, XRD and 27Al and 29Si MAS NMR spectroscopies. The IR data revealed that strong Bronsted acid centers were main active sites generated in the composite materials, irrespectively of the Al content. 

We can state reasonably activity in alcohol dehydration and high selectivity to ethers of large pore H-zeolites. The concentration and strength of acid sites is important. The mesoporous aluminosilicate showed very little activity despite very large pores because of low number and low strength of acid centers. 

The LLB catalyst system needs a rather long reaction time and the presence of excess ketone to get a reasonable yield. Yamada and Shibasaki63 found that another complex, BaBM (91), was a far superior catalyst. Complex 91 also contains a Lewis acidic center to activate and control the orientation of the aldehyde, but it has stronger Bronsted basic properties than LLB. The preparation of BaBM is shown in Scheme 3-35. 

In conclusion, dehydrated TS-1 (and presumably other titanosilicates) most likely does not have Brpnsted acid centers. The observed activity for acid-catalyzed reactions that yield undesired side products is, therefore, inferred to be created under reaction conditions in the presence of aqueous H2O2 (vide infra). 

With the Ti4+ ions acting as Lewis acid centers, a strong interaction with ammonia and water with these centers is expected. There is in fact abundant spectroscopic evidence for the coordination of NH3 and H20 molecules to tetrahedral Ti4+ centers and for the corresponding expansion of their coordination spheres. 

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

Acid centers, structure, sulfate-supported metal oxides, 37 192-196 Acidic catalysis, 6 241 montmorillonite, 38 266-268 Acidic dissociation constant, probe molecules, 38 210... 

It is worth noting that anodic oxidations often give electrophilic and/or acidic centers, whereas cathodic reductions lead to nucleophihc and/or basic centers. Moreover, we must keep in mind that the mechanisms of many anodic and cathodic cyclizations are not accurately known. 

The study of the product distribution from the isomerization of 3,3-dimethylbutene proved useful for evaluating the strength of the acid centers in aluminas (36). Pure alumina from aluminum isopropoxide which was calcined at 700° showed optimum activity. Heating at higher temperatures decreased the number of acid sites as well as their acid strength. Aluminas obtained from potassium or sodium aluminate contained alkali in amounts of 0.08 to 0.65%, depending on the way of precipitation and on the number of washings. 

The presence of acidic centers in the catalyst promotes acid-catalyzed processes in addition to the above reactions. Of course, their rates may be much higher than those of metal catalyzed ones. 

With gem-dimethyl hydrocarbons, isomerization and demethylation compete with each other. Isomerization predominates in the presence of acidic centers. Over platinum supported by acidic alumina 1,1-dimethylcyclo-hexane gave 18% toluene and 60% o-xylene (percentages of the total aro-... 

Silicoaluminophosphates (SAPOs), along with their crystalline aluminum phosphate counterparts (ALPOs), first discovered by Union Carbide workers in the early 1970s [41, 42], derive their acidity through the substitution of framework phosphorous by silicon thereby creating the charge imbalance which, when compensated for by protons, creates acidic centers. SAPOs in general have seen limited use in bond-breaking applications primarily due to weaker acidity, framework stability, or technoeconomic reasons. Of the rich variety of structures available,... 

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