Bronsted acid site density

The acidity distribution was obtained by pyridine adsorption-desorption (Table 3). The presence in the IR spectrum of a band at 1455 cm was assigned to the pyridine coordinatively bonded to Lewis acid sites, while a band at 1545 cm was attributed to the pyridinium ion using the known values of the extinction coefficient of the two IR signals [12] it was possible to determine the Lewis and Bronsted acid sites densities. The number of acid sites observed in the used catalyst was about half of that present in... 

The activity of some Y zeolites showing different Lewis and Bronsted acid sites density is still studied in the acylation of dimethoxyarenes (with particular attention to veratrole. Scheme 4.7) with different acyl chlorides as a function of zeolite acidity and the lipophilic nature of acyl chlorides. Because of the particularly soft reaction conditions, namely, 65°C and 1 h reaction, 3,4-dimethoxyphenyl ketones 12 are the sole isomers recognized in the reaction mixture. Results of the catalytic tests (yield of compounds 12) confirm that the best catalyst is Y(14), characterized by an optimum ratio between Lewis and Bronsted acidity (medium-strength acid sites density for Lewis and Bronsted acid = 17.0 and 11.0 mmol x g- py. 

Figure 2 shows a comparison between the Bronsted acid site density measured with the anhydride method and a classical propylamine adsorption... 

Table 4. Effect of alumina on Bronsted acid sites density in SZA.

Catalyst AI2O3 (% mol) Bronsted acid site density (mmolH+/g) ... 

Fig. 8). The Os result was explained on the basis of an interaction between the Bronsted acid sites and Fe + species. The same type of Interaction is believed to indirectly expand the s- ave function and to decrease the s-electron density at the 57pe nucleus, thus giving an Increase in 6 at the same "Si02/Al203 ratio" of 17. 

Acrolein Zirconium and niobium mixed oxides have been shown to catalyze the dehydration of glycerol to acrolein, at 300°C in the presence of water with high selectivity (72%) at nearly total glycerol conversion [50]. Silica-supported niobia catalysts can also be used with similar catalytic performance [51]. Catalytic results for small-sized H-ZSM 5 zeolites showed that the high density of Bronsted acid sites favors acrolein production [52]. Acrolein production from glycerol has also been carried out in subcritical water at 360°C and 34 MPa with catalytic quantities of ZnS04 (791 ppm [g/g]) [52],... 

After sulfidation and reduction, pyridine adsorption was performed to determine acid site density (Table 4) the number of Bronsted acid sites increased with the extent of the H2S treatment, e.g. the number of Lewis acid sites increased from 0 to 139 with increasing in S/Pt, while at higher extents of sulfiding no significant changes were observed. In particular the BR. signal attributed to the presence of Bransted acid sites reached a maximum at 1545 cm in S/Pt=0 and 2 samples, while a new band at 1553 cm"... 

Hydrogen sulfide also inhibits HDS activity by modifying the catalyst surface. For example, high concentrations of H2S are known to increase the density of Bronsted acid sites on a commercial catalyst. Several researchers have found H2S to poison mainly hydrogenolysis sites on a sulfided C0-M0/7-AI2O3 catalyst. ... 

Bronsted acidity is increased only for alumina-bound Y and does not change for alumina-bound ZSM-5, as shown in Fig. 2. The reaction of AI2O3 from the binder with Si02 in the zeolite at high temperature such as at calcination, which results in new Bronsted acid sites[5,6], is responsible for the increase in Bronsted acid density. The reason that the alumina-bound Y samples were with increased Bronsted acid sites may be due to free Si02 (non-frame silica) present in the zeolite or easier insertion of alumina into Y zeolite than into ZSM-5. 

At high temperatures, the reaction of silica from the binder with extra-framework alumina, which has the characteristics of Lewis acid, results in a new Bronsted acid sites [7]. Thus the decrease in Lewis acid and the increase in Bronsted acid would be expected to be equal in quantity and the quantity depends at least on the amount of extra-framework alumina of the zeolite. The more the extra-framework alumina, the more increase in Bronsted acid sites and decrease in Lewis acid sites would result. By XRD diffraction, the zeolite Y sample showed more crystalline than the ZSM-5 samples. Therefore, silica-bound Y samples would have less reduce in Lewis acid density and less increase in Bronsted acid density than silica-bound ZSM-5 samples. That also means the total acid density of silica-bound Y sample should have less change than silica-bound ZSM-5 samples. However, the Ludox HS-40 contained some sodium ions, which can neutralize Bronsted acid sites[ll], resulting a decrease in Bronsted acid density and total acid density. The sodium in Ludox HS-40 would reduce Bronsted acid sites by about O.lmmol/g-zeolite if all of the sodium could neutralize the acid sites. Therefore, the total acid density and Bronsted acid density of zeolite Y with few extra-framework alumina are reduced. 

Deactivation of the Mo/HZM-5 catalyst by carbonaceous deposits is a serious drawback for the possible commercialization of the MDA process, and therefore, great efforts have been devoted to improving the stability of the catalyst during the reaction by reducing the formation of coke. This has been achieved through the addition of small amounts of CO, CO2, O2, or NO into the reactant stream [10,11,12,13,14] and by reducing the density of Bronsted acid sites of the HZSM-5 zeolite by steam dealumination followed by acid washing before incorporation of Mo... 

Acidity was examined from pyridine desorption measurements in the 200-500°C temperature range, results are given in Figure 14. Acid site density (as measured from the integrated absorbance at 1542 and 1453 cm ) decreased with increasing SiOp/AlpO, ratio in the H-mordenites. However, acid sites strength increasecl with the crystal s SiO-Ml-O, ratio. Thermodesorption of pyridine from Bronsted acid Sites was more facile than from Lewis sites and at 500°C, pyridine was retained mainly on Lewis acid centers. In mordenites synthesized in the presence of TOA, the Si CL/Al O, ratio has little effect on the relative distribution of B andS acio sites and the B/L ratio monotonically decreased with desorption temperature. 

Within the series of Mg-Fe mixed oxides, no Bronsted acid sites are detected. Pyridine interacts essentially with Lewis acid sites and disappears almost completely after evacuation at temperatures higher than 21°C, indicating the weak nature of this bonding. The acid sites are coordinatively unsaturated Fe (octahedral) cations pairs of coordinatively unsaturated Fe in octahedral and tetrahedral coordination are also present in the samples at lower Mg/Fe atomic ratio. With increasing Mg/Fe ratio, a decrease in the total Lewis acid sites density is observed. The MgO single oxide, like the Mg/Al=2.0 mixed oxide, does not show the presence of any acid sites able to chemisorb pyridine. The acidity in the Fc203 samples was not detected because of the poor transparency of this material in the IR region. 

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