Surface basic sites

This is the case as shown in Table 1, runs 4,5, provided that basic sites are free from adsorbed H20 and C02 (compare runs 3 and 4, Table 1). Notably, once again the ratio hexan-3-one/hexen-3-ols appears unaffected by the catalyst precursor and/or pretreatment (runs 3,4, Table 1). These observations suggest that in the transfer hydrogenation of 4-hexen-3-one, the substrate is coordinated on a weak acid site while propan-2-ol must be coordinated on an adjacent surface basic site [7,24]. This is confirmed by the lack of reduction products observed over Mg(OH)2 and MgCl2 (runs 8,9, Table 1). 

One gram of carbon sample was placed in 50 mL of the following 0.05N solutions sodium hydroxide, sodium carbonate, sodium bicarbonate and hydrochloric acid. The vials were sealed and shaken for 24 h and then 10 mL of each filtrate was pipetted and the excess of base or acid was titrated with HCl or NaOH. The numbers of acidic sites were calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups Na2C03 - carboxyl and lactonic and NaHCOj only carboxyl groups [3, 4]. The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the crubon. 

El-Sayed, Y. and Bandosz, T.J. (2004). Adsorption of valeric acid from aqueous solutions on activated carbons role of surface basic sites. J. Colloid Interface Sci. 

As the olefins and to a lesser extent the alkanes are basic one may expect the desorption to be favored by surface basic sites. In other words oxidative dehydrogenation of alkanes is expected to be easier on surface exhibiting basic properties. As a matter of fact the results given in table 5 from ref 41 show that Mg2V207 which is more basic as shown in table 4 is more selective for olefins in propane conversion and to a lesser extent for n-butane and isobutane oxidation reactions than the other two phases. Such a feature is even more pronounced for the samples with excess MgO at least for propane oxidation, samples which were also shown to present higher basicity (table 4). 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing the BET method with Sorptomatic 1900 (Carlo-Erba). X-ray diffraction (XRD) patterns of powdered catalysts were carried out on a Siemens D5(K) (0/26) diffractometer with Cu Ko monochromatic radiation. The total amount of surface basic sites was determined via surface titration by benzoic acid from dry hexane solution as reported elsewhere [14]. Before the titration catalyst samples were dried at 300°C for 2 hours. 

Many authors correlated the SO2 adsorption capacity with the basic centers on the carbon surface [107,108,116,118-120]. However, it has been pointed out that more SO2 was adsorbed than corresponded to the basic centers [102]. This is not surprising since the surface basic sites are quite weak, and SO2 is not a strong acid. Also, no good correlation of SO2 conversion per square meter of surface or nitrogen content was found. The correlation was much better with the N-6 content [108]. It was also reported that the SO2 oxidation capacity increased with increasing N-6 and N-Q contents [118-121]. 

It has been demonstrated that surface basic sites of PCPs successfully catalyze polymerization in the nanochannels. Thus, the pillared-layer complex [Cu2(pzdc)2bpy] acted as a host for spontaneous polymerization of substituted acetylenes in a specific manner (Fig. 5) [67]. In the case of acidic monosubstituted acetylenes, the basic oxygen atoms from carboxylate ligands in [Cu2(pzdc)2bpy] produced reactive acetylide species that subsequently initiated anionic polymerization in the nanochannel. Compared with a control experiment using a discrete model catalyst... 

Similar to homogeneous one-phase systems, the acidity of solid surfaces manifests itself only in relation to a base that must be present the strength of the acid-base interaction therefore depends on the particular base involved. This is true for both types of acid sites, Bronsted or Lewis. The reverse can be said, of course, for surface basic sites. 

Alternatively, deprotonation enthalpies can be evaluated from probe adsorption calorimetric data or from temperature programmed desorption (TPD) measurements. The strengths of surface Lewis acid sites and of surface basic sites can also be evaluated, in principle, by the heat of probes adsorption or desorption. In all cases, however, probes adsorption on solids can result in multiple interactions for example, van der Waals interactions can be superimposed to true acid-base interactions, which can also be multiple and finally give rise to some kind of solvation effects, in particular, in the zeolite cavities [23-25]. Thus, the pure acidity/basicity... 

Even in reactions which have been recognized to be catalyzed only by acid sites on a catalyst surface, basic sites also act more or less as active sites in cooperation with acid sites. The catalysts having suitable acid-base pair sites sometimes show... 

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