Bronsted acids modification

As shown in Table 1, the amount of irreversibly adsorbed pyridine dropped to one third its original value after modification, which is caused by the methylation of surface Bronsted acidic sites through equation (1). The results were confirmed by the presence of surface methoxyl groups and the absence of Bpy peaks of adsorbed pyridine in i.r. spectra. The TPD of ammonia in Figure 3 indicates that the modification influenced mainly the number of surface acidic sites. The results in Table 1 aslo show that the drop in acidity paralleled that... 

Based on the above results, we conclude that the diazomethane modification of zeolites is an effective method to change selectively the amount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature 300 0 catalytic reactions. 

The effects of post-synthesis alumination on purely siliceous MCM-41 material with A1(NC>3)3 on acidity have been studied by FTIR, NH3-TPD, and IPA decomposition reaction. The FTIR results of pyridine absorption show that both Lewis and Bronsted acid sites are increased by the post-modification. The amount of NH3 adsorbed on the alumina-modified MCM-41 samples increases with the loading of Al onto the surface of MCM-41. Due to the improved acidity, the alumina-modified MCM-41 materials show considerably higher catalytic activity for dehydration of isopropanol than purely siliceous MCM-41. In addition, XRD and N2 adsorption results show that all MCM-41 samples maintained their uniform hexagonal mesoporous structure well after they have been subjected to post-synthesis alumination with the loading of Al species on Si-MCM-41 varied from 0.1 wt. % up to 10 wt. % (calculated based on AI2O3). 

The modification of mesoporous silicate FSM-16 by metal ion-exchange and sulfiding with hydrogen sulfide was studied through the isomerization of 1-butene, cis-2-butene and cyclopropane. It was revealed that the catalytic activities of MeFSM-16 were remarkably enhanced by sulfiding with hydrogen sulfide due to the formation of new Bronsted acid sites... 

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

Chemical modification of the ALPOs is required to create a new class of catalysts. In the metalloaluminophosphates (MeALPOs), the framework contains metal (Me), aluminium and phosphorus. Thus, it becomes possible to produce a wide range of active catalysts with Lewis and Bronsted acid sites and redox properties by the partial replacement of Al3+ by Me2+ ions (e.g. Co, Cu, Mg, Zn, etc.) in an ALPO framework (Thomas, 1995 Martens etal, 1997). 

The modification of Rh/USY with alkali metal salts such as NaOH, NaNOs, Na2C03, NaCl, etc. was, therefore, performed in order to control the strength and number of strong Bronsted acid sites of Rh/USY catalyst. It was revealed that the catalyst deactivation of Rh/USY was remarkably improved by the addition of small amount of alkali metal salts Modification with NaOH was the most effective and 0.5wt% addition of Na using NaOH was optimal amount for the improvement of the catalyst deactivation with reaction time as shown in Figure 4... 

Structural motif of chiral Bronsted acid catalysts. Further modification of the chiral diamine backbone or the substituents on the nitrogen atoms of the catalyst could lead to its becoming an efficient enantioselective catalyst. 

The chemical modification of alumina both by alkali and acid leads to the textural as well as surface chemistry changes on the surface of alumina. The total Lewis acidity and Bronsted acidity and basicity are altered to the desired levels. The textural changes like surface area and pore size distribution can be deduced from the adsorption and desorption of N2 at 77K on the four alumina samples. The surface area calculated by BET method for the four adsorbents is given in Table 3. 

The surface properties of amorphous silicas are largely influenced by the nature of the surface silanol (SiOH) groups (1-3). Lewis acid-base sites are absent unless the silica has been activated at very high temperatures, Bronsted acidity at the gas-solid interface is low or nonexistent, and the siloxane bridges are relatively unreactive toward most molecules. This chapter discusses some methods that employ chemical modification and H-D exchange to probe the nature of the surface hydroxyl groups on... 

The metathesis of propene on WOs/Si02 is speeded up by pretreatment of the catalyst with HCl (Pennella 1974 Aliev 1977) but the product but-2-ene undergoes considerable isomerization to but-l-ene (Aliev 1978). The inclusion of 1% cycloocta-1,5-diene (COD) in the propene stream also increases the rate of metathesis and reduces the break-in time from 20 min to less than 5 min at 500°C. The latter effect disappears when the additive is removed, so it is not due to reduction of the catalyst. The effects of both HCl and COD have been attributed to a favourable modification in the metal d-orbital levels as the result of the presence of new ligands (Pennella 1973, 1974). Pretreatment with hexamethyldisilazane (HMDS) at 250°C also has a remarkable effect on the activity, increasing it as much as 140 times for the metathesis of propene at 427°C. The same treatment of silica alone completely eliminates its capacity to isomerize rra/j5-but-2-ene at 427°C, so it is concluded that the Bronsted acidic hydroxyl groups, poisoned by HMDS, are not likely to be the precursors for the active sites in propene metathesis over W03/Si02 (van Roosmalen 1980a, 1982). 

The hydridic character of the rhenium-bonded hydrogen of 34a, b facilitated the hydride abstraction upon treatment with either Bronsted acids or Lewis acids [32]. Depending on the nature of the phosphine substituent, the 16e Re(—I) dinitrosyl cations [Re(NO)2(PR3)2][BAr 4] (35, R = iPr a, Cy b) could be accessed from the reaction of either 34a with [H(OEt2)2][BAr 4] or 34b with [PhaC] [BAi 4]. Further anion modification to access the corresponding [B(C6F5)4] salts [Re(NO)2(PR3)2l[B(C6F5)4] (36, R = Pr a, Cy b) showed enhanced solubilities in nonpolar solvents [105]. 

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