Acid site titrations

Although materials prepared by post-aliunination procedures did not feature acid sites stronger than those obtained by direct synthesis, the catalytic activity of the former (evaluated for ciunene cracking [115,116], isopropanol dehydration [120,121] or aldehyde condensation [117]) was found systematically higher than that of the latter. Such a result must be attributed to a better accessibility of the acid sites in the case of post-aluminated materials. This fact was actually shown by CoUart et al. [118,119] in the case of [Si]-MCM-48s treated with Al(acac)3. The munber of acid sites titrated by ammonia amounted to 100% and 68% of the total number of aliuninum atoms deposited in samples containing 1.2wt% (Si/Al = 37) and 5.1 wt% (Si/Al = 17) Al, respectively. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

This potential reflects itself in the titration curves of weak polyacids such as poly(acrylic acid) and poly(methacrylic acid) [32]. Apparent dissociation constants of such polyacids change with the dissociation degree of the polyacid because the work to remove a proton from the acid site into the bulk water phase depends on the surface potential of the polyelectrolyte. 

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. 

The titration of silica-alumina acid sites with neodymium oxide is clearly seen in Figure 4. The relative acid strength, as estimated by the 2,3DMB2/4MP2 ratio during 2MP2... 

From the data reported in Fig. 8, it clearly emerges that the acidity of the silicalite-l/H20 and of the TS-I/H2O systems are remarkably different (compare open and full circles in Fig. 8). This difference can be explained as follows TS-1 has two main acidic sites, Ti(IV) Lewis sites and silanols, mainly located in the internal defective nests (see Sect. 3.8), while only the latter are present in silicalite-1. Addition of H2O2 to siUcaUte-l does not modify the titration curve (compare open circles with open squares in Fig. 8). This means that no additional acidic sites appear in the siUcaUte-l system upon adding H2O2, i.e., that hydrogen peroxide molecules coordinated to internal silanol do not modify their acidity. Conversely, addition of H2O2 to TS-1 moves the whole titration curve toward lower pH values, (compare full circles with full... 

Figure 10-4. The double- and single-site titration models for His and Asp groups [42]. (A) In the double site model, only one X is used for describing the equilibrium between the protonated and deprotonated forms, while the tautomer interversion process is represented by the variable x. (B) In the single-site model, protonation at different sites is represented by different X variables. HSP refers to the doubly protonated form of histidine. HSD and HSE refer to the singly protonated histidine with a proton on the h and e nitrogens, respectively. ASP1 and ASP2 refer to the protonated carboxylic acid with a proton on either of the carboxlate oxygens...

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

Using calorimetric and n-butytlamine titrations, Ghosh and Curthoys (105) have reported that aluminum-deficient mordenite exhibits strong acid sites. However, the location of these sites is uncertain. 

Fatty acid, MRNi hydrogenation, 32 243-245 Faujasites, 34 160-183 acidic sites, 27 151-154 alkaline and rare earth forms, 27 160-165 amine titration, 27 163 infrared smdies, 27 160-163 surface acidity and catalytic activity, 27 163-165... 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

Generation of acidity in silica gel by ionizing radiation was reported by Barter and Wagner (274). p-Dimethylaminoazobenzene was adsorbed out of anhydrous solution in CCI4 in the red, acid form. About 3 /aeq/gm of acid sites were generated by a dose of 3 10 rad, as determined by titration with butylamine. 

Concerning the nature of the acid sites on dehydrated alumina, the chemical evidence is more in favor of Lewis-type than of Bronsted-type acids. Trambouze and Perrin (343) estimated the content of Lewis acid sites by thermometric titration of a slurry in benzene with dioxane. The authors stated that Lew is acidity was not observed in boehmite and its dehydration products, only in the products obtained from hydrargillite. As mentioned earlier, Webb (339) found no indication of Bronsted acidity ammonia did not form ammonium salts. The quantity of ammonia chemisoi-bed per unit area in the range from 175° to 500° was not a function of the hydrogen content. After exten.sive dehydration, more ammonia was chemisorbed at 100 mm pressure, even at 500°, than corresponded to the hydrogen present (341b). 

Selected entries from Methods in Enzymology [vol, page(s)] Active site titration [by 2-hydroxy-5-nitro-a-toluenesulfonic acid sultone, 19, 6-14 by p-nitrophenyl ester substrates, 19, 14-20 by rapidly reversible, covalently bound substrates, 19, 14-20 by slowly reversible, covalently bound inhibitors, 19, 6-14] assay,... 

Analysis of the reaction products was canied out with a Hewlett-Packai d 5780 gas chromatograph. Details on the experimental set up and on the analysis of the products are given in our previous papers (5,8). The number of basic sites was determined by adsoiption of CO2 by dosing at 300 Ton and room temperature (10,11), while the number of acidic sites was measured by n-butylamine titration (12). Conversions are defined as moles of ethane reacted per min/mol ethane fed per min. Selectivity S is defined as mol of products formed per min/mol ethane reacted per min. Yield is given as mol products formed per min/ mol ethane fed per min. 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

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