N-Butylamine adsorption

Although a variety of amines, particularly trimethylamine and n-butylamine have widely been used as poisons in catalytic reactions and for surface acidity determinations (20), comparably few spectroscopic data of adsorbed amines are available. As with ammonia, coordinatively adsorbed amines held by co-ordinatively unsaturated cations have preferentially been found on pure oxides (176, 193-196), whereas the protonated species were additionally observed on the surfaces of silica-aluminas and zeolites (196-199). However, protonated species have also been detected on n-butylamine adsorption on alumina (196) and trimethylamine adsorption on anatase (176) due to the high basicity of these aliphatic amines. In addition, there is some evidence for dissociative adsorption of n-butylamine (196) and trimethylamine (221) on silica-alumina. Some amines undergo chemical transformations at higher temperatures (195, 200) and aromatic amines, such as diphenylamine, have been shown to produce cation radicals on silica-alumina (201, 201a). 

A review on TLC and PLC of amino adds, peptides, and proteins is presented in the works by Bhushan [24,25]. Chromatographic behavior of 24 amino acids on silica gel layers impregnated tiraryl phosphate and tri-n-butylamine in a two-component mobile phase (propanol water) of varying ratios has been studied by Sharma and coworkers [26], The effect of impregnation, mobile phase composition, and the effect of solubility on hRf of amino acids were discussed. The mechanism of migration was explained in terms of adsorption on impregnated silica gel G and the polarity of the mobile phase used. 

A good-quality CeMCM-41 material with Si/Ce=50 was synthesized by hydrothermal method. For the purpose of comparison a pure siliceous MCM-41 was prepared using the same composition without cerium. Thermogravimetric curves for the synthesized uncalcined samples exhibit shape characteristic for the MCM-41-type materials. The specific surface area of CeMCM-41 evaluated from nitrogen adsorption was equal to 850 m2/g, whereas the pore width and mesopore volume of this material were equal to 3.8 nm and 0.8 cm3/g, respectively. In contrast to the pure silica MCM-41, the CeMCM-41 material exhibits medium and strong acid sites as revealed by thermogravimetric studies of n-butylamine thermodesorption. 

The CeMCM-41 material studied had much higher quality than the corresponding MCM-41 sample synthesized under the same conditions. While both materials exhibited analogous adsorption properties with respect to nitrogen, their interaction with n-butylamine was different. Thermogravimetric analysis of w-butylamine thermodesorption showed that CeMCM-41 possessed medium and strong acid sites in contrast to the pure silica MCM-41, the acidity of which was negligible. Thus, incorporation of cerium to MCM-41 seems to improve its hydrothermal stability and enhance the adsorption and catalytic properties. 

Lewis acid sites can coordinate with a given indicator molecule to produce an adsorption band identical in position with that produced through proton addition. Even if the indicators used are responsive only to Brpn-sted acids, most basic reagents used to titrate surface acidity (e.g., n-butylamine, pyridine) are strongly adsorbed on surface sites other than Br0nsted acid sites. In this connection, a recent study indicates that adsorption equilibrium is not fully established during titration of silica-alumina with n-butylamine because of the irreversible attachment of amine molecules by adsorption sites at which they first arrive (31). 

Yoshizumi et al. (70) determined acid strength distributions on silica-alumina catalyst calorimetrically by measuring the heat adsorption of n-butylamine from benzene solution. They found that the differential heat of adsorption of n-butylamine ranged from 3.7 kcal/mole (weak acid sites) to 11.2 kcal/mole (strongest acid sites). 

Tomida et al. (73) investigated the temperature-programmed desorption of n-butylamine from silica-alumina and alumina. The desorbed amine products were different in the two cases. n-Butylamine and n-butene were obtained from silica-alumina dibutylamine and n-butene were obtained from alumina. In a subsequent paper by Takahashi et al. (73a), the authors conclude that two types of adsorption sites on silica-alumina account for the desorption behavior of n-butylamine. One type chemisorbs the amine and the other catalyzes the decomposition of the amine to lower olefins at temperatures above 300°C. On the other hand, amine decomposition was not observed when pyridine was desorbed from silica-alumina. The effects of sodium poisoning on desorption behavior of n-butylamine and pyridine were also examined. 

Further acid site strength and concentration measurements were reported by Morita et al. (164), who related the acidity measurements to various catalytic reactions. Using Y zeolite (Linde SK-40, 90% H form) activated at 450°C, they observed no acid sites stronger than an H0 of -8.2, although the total acid site concentration was almost twice that of the former investigations (Fig. 21, curve 4). They also measured acid site concentration as a function of decomposition temperature for NH4Y, and found that n-butylamine titration values paralleled results obtained from pyridine adsorption studies (41,151). The maximum total acidity occurred... 

The initial studies demonstrating the feasibility of using probe molecules to study acid sites were conducted by Gay and coworkers from 1977 onwards [215, 216]. This approach was further developed by Dawson and coworkers who employed CP-MAS NMR to investigate the number and nature of acid sites on y-Al203 through the adsorption of n-butylamine and pyridine [217, 218]. In the case of... 

Technique. Acidity. The number of acid sites was determined by titration with n-butylamine. Hammett and arylmethanol indicators chosen according to Drushel and Sommers studies (4) were used to obtain the catalyst acidity in terms of acid strength (7). Numerical results obtained by this method are similar to those obtained by other methods, for example, adsorption (i, 14,15). 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Tables XIII I76-I79), XIV (I80-I83), and XV present a survey of micro-calorimetric studies performed for silica, alumina, and silica-alumina, respectively. Silica displays relatively low heats of adsorption for both basic probe molecules (e.g., ammonia, triethylamine, n-butylamine, pyridine, and trimethylamine) and acidic probe molecules (e.g., hexafluoroisopropanol), indicating that the surface sites on silica are both weakly acidic and basic. Most of the adsorption over silica is considered mainly to be due to hydrogen bonding and van der Waals interaction. Infrared and gravimetric adsorption measurements of pyridine adsorbed on SiO at 423 K have shown that more than 98% of the pyridine adsorbed was hydrogen bonded (62). The differential heats of ammonia 18, 74, 85, 105, 140, 147) and triethylamine (18, 71, 94. 105, 176) on silica show a considerable decrease as the adsorption temperature is increased.

In contrast, the surface of alumina has strong acid and basic sites, as demonstrated by the differential heats of adsorption of basic probe molecules such as ammonia 140,147,153, 180) and n-butylamine 48,177) or of acidic probe molecules such as carbon dioxide 147,180) and hexafluoroisopropanol 179). The temperature dependence of the heat of adsorption for alumina is characteristic of a strong acidic surface, with the initial differential heat increasing and the adsorption capacity decreasing with increasing adsorption temperature. 

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

One more method used was the titrimetric determination of n-butylamine adsorbed on the samples from toluene solution. It was found that both reduced and non-reduced samples adsorbed near 0.13 mmole of n-butylamine/g which is close to the data on ammonia adsorption. 

Optimization of the synthesis of an ultra-large pore aluminophosphate VPI-5 using di-n-butylamine (n-DBA-VPI-5) has been done. The sample synthesized was characterized by XRD, SEM, FTIR, TGA-DTA, NMR and sorption studies. The extra-large pores (larger than lOA) in VPI-5 were confirmed by adsorption measurements. N2 adsorption at 77K shows mesopores with pore diameter of 36A alongwith the micropores of n-DBA-VPI-5. 

The strongest amine discussed so far is n-butylamine, with a of 3.39. Ellis and coworkers 188] have used NMR techniques to investigate the adsorption of n-butylamine on 7-alumina. The solid-state C spectrum of n-butylamine on 7-alumina is depicted in Fig. 44a. There are xix prominent resonances, two of... 

The spectrum on n-butylamine on y-alumina is shown in the Figure 3a( ). There are six prominent resonances, two of which are readily assigned to the y-methylene and methyl carbons by direct comparison with the liquid phase spectrum. These resonances are only slightly shifted upon adsorption. The remaining four resonances arise from the two a and 3 methylene carbons, from which we conclude that at least two types of chemically different butylamine species are present on the surface. 

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