Boehm method

Boehm and Johnson Matthey titrations. Both titrations give information on the acidic or basic character. The procedures are described in [13] and [14]. Boehm method classifies the surface acidic sites in three types (called phenolic, lactonic, carboxylic), by growing strength of acidity. However, this arbitrary classification fails to attribute one surface acidic group to one type of acidity. Moreover, this method does not give very accurate results with solids containing few acidic sites. Nevertheless, it appears for our active carbon that "phenolic" sites are a majority. The amount of each type of acidic sites does not change very much with the nitric treatment. 

In Table 7, the effect of nitric acid pretreatment at room temperature is illustrated for an OL carbon from Calgon. A 30% converted methyl acetate feed solution was selected for the hydrolysis study. When not treated with nitric acid, the carbon required a contact time between 26 to 52 minutes to reach 40% conversion. When the same carbon is treated with nitric acid, a 42% conversion was reached at a contact time of 12 minutes or less. Dimethyl ether was not observed in either experiment (10,11). The surface acidity was determined by using the Boehm method. The consumed NaHCOa determined the amount of carboxylic groups on the surface of the carbon. The nitric acid treated carbon possessed 6.3 times more carboxylic groups. 

The Boehme method was also used to determine the increase in carboxylic groups on a Bamaby Sutcliffe activated carbon. Table 8 summarizes this determination. 

Table 8 Surface Acidity of Bamaby Sutcliffe Carbon by Boehme Method...

A quantitative comparison between direct potentiometric titration and the classical Boehm method for characterization of the acidity of activated carbons has been drawn by various researchers [85,215,218], Although a rather acceptable agreement is generally accomplished for both methods, when applied to a carbon snrface where oxygen is the major constituent, some discrepancies exist. Figure 2.7 shows a correlation between the total acidic groups detected on an activated carbon using both Boehm and potentiometric titrations. 

However, perusal of the literature shows some ambiguities and discrepancies in the reported results from Boehm method when it is applied to carbon materials [38, 200-202]. It is due to the fact that the surface heterogeneity of carbon materials makes extremely difficult any attempt to accurately determine the surface functionalities on carbons, according only to their acidic and base properties. This issue has been acutely discussed by Leon y Leon [40]. 

The well-known Boehm method [47,48], based on the titration of acidic centers using several basic solutions of increasing strengths (EtONa, NaOH, Na2C03, and NaHCOj) has become very popular (see Table 3.1) owing to its simplicity its protocol is exhaustively repeated in the literature (see, for example. Reference... 

The authors pointed out the importance of the accessibility to the functional groups for the understanding of their behavior in adsorption and catalysis. The mere evaluation of the chemical functionalities (e.g., by using the Boehm method) and surface areas is not always sufficient to arrive at reliable conclusions. 

Boehm, H.J., Boehringer, M., Bur, D., Gmuender, H., Huber, W., Klaus, W., Kostrewa, D., Kuehne, H., Luebbers, T., Meunier-Keller, N., and Mueller, F. Novel inhibitors of DNAgyrase 3D structure based biased needle screening, hit validation by biophysical methods,... 

Boehm, H. P. et al Z. Anorg. Chem., 1965, 335, 74-79 The oxide (an intercalated laminar material) is thermally unstable and on rapid heating it will deflagrate at a temperature dependent on the method of preparation. This temperature is lowered by the presence of impurities, and dried samples of iron(III) chloride-impregnated oxide explode on heating. 

As combinatorial chemistry has been fully integrated into the modern drug discovery process, more computational search methodologies against large virtual combinatorial compound spaces have been steadily developed in recent years (11-16). A detailed summary and comparison of those published methods are reported in the Section 5 and in Table 13.6. A good review on this subject could also be found in the publication by Boehm and coworkers (16). 

Another technique widely used for size separation of humic materials is field-flow fractionation (FFF) (e.g., Baalousha et al., 2006 Boehme and Wells, 2006 Geckeis et al., 2003 Hassil ov et al., 2007 Siripinyanond et al., 2005 Suteerapataranon et al., 2006 Zanardi-Lamardo et al., 2002). This technique was developed and introduced in 1966 by Giddings (1966) as a method for the separation and characterization of materials ranging in size from macromolecules to particulates. Similar to SEC, FFF... 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Among the experimental methods, that allow us to establish the concentration of surface hydroxyl groups are spectroscopy, methods based on chemical reactions, adsorption, and isotope exchange. The detailed survey of the applied experimental methods is presented by Boehm and Knozinger [36]. The densities of surface hydroxyl groups are presented in Table 2. Some of them, of the iron oxide structure, are beyond accepted hydroxyl group densities [39]. 

Most carbon blacks have a low affinity for water, i.e. they are hydrophobic. However, the level of hydrophobicity is reduced by the presence of chemisorbed oxygen and certain functional groups (Walker and Janov) 1968 Bradley et al., 1995). The relative extents of the polar and hydrophobic areas of carton blacks have been studied by various methods (Boehm, 1994), including energy of immersion measurements (Barton and Harrison, 1975) and by liquid flow calorimetry (Groszek,... 

Boehm G etal., 2001, Method and Apparatus for Operating a Fuel Cell. WO 01/43216. 

Oxidation and incorporation of nitrogen usually affect the surface chemistry of carbons [12, 20, 24]. The types and numbers of basic and acidic groups evaluated using Boehm titration [3, 4]. are collected in Table 2. The data reported indicate that the initial carbons differ in their acidity as a result of the activation method and the type of an organic precursor [1]. BAX is more acidic than BPL. This is expected for a phosphoric acid activated carbon [25]. After modification with urea and heat treatment at 723 and 1223 K, the overall surface chemistry... 

The pH of MCM-41 was measured with ASTM D 3838. About l.Og of dry MCM-41 was added to 20 ml of distilled water, and the suspension was shaken overnight to reach equilibrium. Then the sample was filtered, and pH of the solution was measured. The surface functional groups of the samples were determined by Boehm s titration method [13]. In the case of acid value, about l.Og of the sample was added to 100 ml of O.IN NaOH solution and the mixture was shaken for 24 h. Then the solution was filtered through a membrane filter and titrated with 0. IN HCI. Likewise, the base value was determined by converse titration. 

Comparisons of a variety of methods were made on cycloheptadecane by Saunders et al. (91) and it was concluded that the stochastic method was most efficient. In one of the few independent comparisons of the effectiveness of these procedures, Boehm et al. (122) studied the sampling properties on the model system caprylolactam, a nine-membered ring, and concluded that systematic search was both inefficient and ineffective at finding the minima found by the other methods when the number of conformers examined was limited. 

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