Acid sites, neutralization

Addition of promoters to resist deactivation. For example, greater support stability with alumina is achieved with small amounts of added silica or zirconia, sintering and coking of platinum is reduced by adding rhenium, and acid sites neutralized with potassium. 

Bronsted acid sites and inactivated surface acid sites neutralized with 2,4,6-collidine... 

Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites... 

Alkaline earth metals in general, and sodium in particular, are detrimental to the FCC catalyst. Sodium permanently deactivates the catalyst by neutralizing its acid sites. In the regenerator it causes the zeolite to collapse, particularly in the presence of vanadium. Sodium comes from two prime sources ... 

Sodium decreases the hydrothermal stability of the zeolite. It also reacts with the zeolite acid sites to reduce catalyst activity. In the regenerator, sodium is mobile. Sodium ions tend to neutralize the strongest acid sites. In a dealuminated zeolite, where the UCS is low (24.22°A to 24.25°A), the sodium can have an adverse affect on the gasoline octane (Figure 3-7). The loss of octane is attributed to the drop in the number of strong acid sites. 

Vanadium and sodium neutralize catalyst acid sites and can cause collapse of the zeolite structure. Figure 10-5 shows the deactivation of the catalyst activity as a function of vanadium concentration. Destruction of the zeolite by vanadium takes place in the regenerator where the combination of oxygen, steam, and high temperature forms vanadic acid according to the following equations ... 

Additional experiments were carried out to examine the formation of toluene from n-heptane When the heptane was labeled with in the 1-position, half the radioactivity was in the methyl group and the other half was in the aromatic ring of the toluene. This clearly indicates that ring formation involves 1,6 or 2,7 closure and excludes the possibility of any interconversion between five membered and six membered rings. Perhaps this is not surprising since the catalyst s acid sites are neutralized by the sodium. 

All acid sites were first neutralized with ammonia and ammonia was then desorbed at 277°C (550K) for 30 min. The ratio A55o/Ao, where A550 and Ao are the intensities of NH4+band (at 1450 cm 1) after and before desorption, represents a fraction of NH4+ ions which survived the desorption and was taken as a measure of the acid strength. For the... 

As evidenced by experiments carried out on partially poisoned catalysts with 2,6-di-tert-butylpyridine, a significant decrease in the catalytic activity (of about 65-70 %) occurs because of a partial neutralization of the external acid sites. This means that the alkylation takes place predominantly on the external surface. 

Pahit source fluids, based on on-site colorimetry (crater lake, thermal acid springs, neutral spring and stream) (a). Mixing curves for the same groups (b). 

In catalyst preparation, one can use this knowledge to determine the relative contributions of various hydroxyl groups before and after application of the active phase onto the support. In this way Sibeijn etal. [31] established that rhenium oxide attached to acidic sites of the alumina support exhibits higher activity for the metathesis of olefins than rhenium oxide on neutral or basic sites. As, however, rhenium species preferentially exchange with basic hydroxyls, one needs to increase the loading above a certain value (6 wt% for an alumina of 200 m2/gram) before the catalyst exhibits appreciable activity [31]. 

Teeth decay after eating sugar, and one of the first metabolites from sugar is lactic acid Rubbing a dock leaf (which contains an organic base) on the site of a nettle sting (which contains acid) will neutralize the acid and relieve the pain... 

As described in Section 3.2.3, the use of acidic supports such as A1203 favors the dehydration of ethanol to ethylene, which leads to a severe carbon deposition.66,76,78,85 Reactions with lower H20/ethanol ratio can also favor several side reactions mentioned above and result in carbon deposition on the catalyst surface. Possible strategies to reduce the carbon deposition include (i) neutralization of acidic sites responsible for ethanol dehydration to ethylene and/or modification of the support nature, including less acidic oxides or redox oxides, (ii) use of a feed containing higher H20/ethanol molar ratio, and (iii) addition of a small concentration of air or 02 in the feed. 

Complete clustering, or segregation, of tautomers may occur during crystal growth. This can lead, for example, to crystals that consist of two tautomers in equal proportions and located at crystallographically independent sites. This is the case for isocytosine, 47 (104). Similarly, in crystalline anthranilic acid the neutral molecule and the zwitterion coexist (10S). Another example, 39, has been mentioned already (82). 

Aluminas, which were prepared from sodium aluminate and which retained about 0.1 % of sodium ions, had a large amount of weakly acid sites, and were therefore excellent dehydration catalysts. At the same time these aluminas did not isomerize cyclohexene, owing to the absence of strong acid sites, which were neutralized by the alkali metal ions. Pines and Haag (36) determined that the upper limit of the total number of acid sites, capable of dehydrating butanol, and of the number of strong acid sites, capable of isomerization of cyclohexene, was 10 X 10 and 8 X 10 sites per cm, respectively. 

The dehydration of menthols over alumina, prepared from aluminum isopropoxide and having intrinsic acidic sites, was accompanied by double bond migration of the cycloalkenes produced. The isomerization was, however, suppressed by the preferential neutralization of the strong acid sites with bases. The neutralization of acidic sites thus preventing isomerization was confirmed by von Rudloff (61) who evacuated pyridine-treated alumina for 6 hours, when about 0.8% base was retained. 

The function of piperidine was to neutralize the relatively strong acidic sites of the alumina and still leave the weak acidic sites to act catalytically. In the presence of acidic alumina a reversible isomerization of camphene to tricyclene occurs. 

When an alumina catalyst contains a small amount of alkali metal ions, it loses its olefin isomerizing properties, inasmuch as the relatively strong acidic sites of the alumina are neutral. Most of the dehydration reactions are usually performed over such aluminas. Consequently, the sequence of reaction types as discussed for weakly acidic media seem also to apply to dehydration over alumina catalysts. 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

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