Ammonia sorption

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

The kinetics and thermodynamics of gaseous ammonia sorption and diffusion have been studied in detail (36,39,40). 

The removal of sodium cations from the parent [Si,Al]MAG by treatment with hydrochloric acid results in the extraction of part of the framework aluminum as indicated by the low amount of ammonia (0.23 mmol/g) evolved in the high-temperature desorption step (Figure 3) from H-[Si,Al]MAG converted to the ammonium form by ion exchange or ammonia sorption prior to TPD. The formation of strong Lewis-acid sites (1462 cm )... 

The principle of the FR technique has been described previously [15]. The frequency window used in this study was O.Ol-lOHz. 50mg zeoUte sample were placed into a sorption chamber and outgassed at 723K or lower tenq>erature for 14 homs before carrying out ammonia sorption experiments. The NH3 sorbate was admitted to pretreated samples and allowed to come to pressure equilibrium at 0.4 0.7 1.0 1.5 and 2.0 Torr in a temperature range of 373-723K Measurements were carried out in the presence and absence of sorbent zeoHte samples to obtain the difference of the respective FR parameters. 

Fig. 1 Conq)arisoii of (a) the FR spectra ( ) (Pb/Pz)cosz.B for ammonia sorption at 1 Torr with (b) OH and (c) NH bands in the FTIR spectra of eight different zeohte structures.

Figure la,b) and for the strong base ammonia sorption (Figure Ic) was found to be the rate-controlling step of transport. 

The Bronsted proton had been transferred to the acetic acid with two broad bands centred at -2870 and 2480 cm-1 observed. Similar bands have been observed for HgO sorbed at 80°C [9,10], but not for sorbed NH3. In the case of ammonia sorption the proton is completely transfered giving NH4+ and no broad bands are observed. The broad bands probably result from protons with a broad range of energies shared between the zeolite and the acetic acid. 

The catalytic activity of metal oxide composites depends both on the acidity and basicity of the surface that can be characterized by the level of ammonia sorption and The catalyst should adsorb 100-200 pmol/g NH3 and its pAT, should be in the range 15-17.5 [31]. Catalysts having low basicity and acidity of the snrface have too low activities, but a lot of undesired by-products (dioxane and PEGs) are formed when their acidities are too high. The activity of Al-Mg composite oxide catalyst increases with an increase in the calcination temperature and reaches a maximum of approximately 700°C [22]. Further increase in the calcination temperature results in reduced activities, attributed to the sintering of the catalyst surface. The catalyst activity increases with inaeasing aluminum content, but less narrow distributed products are formed. Filtration of the catalyst is not always possible. Therefore, addition of water (180 g of water/1063 g of oxyethylation product) and the use of activated clay or diatomaceous earth as a filter aid are proposed [33]. 

COFs with boronate-ester linkages show effective ammonia sorption due to the Lewis acid-base interaction. Among them, COF-10 exhibits an uptake capacity of 15 molkg at 298 K and 1 bar. COFs can be recycled several times without much loss of activity. 

CrAPO-5 Framework tetrahedral Cr +/after hydration or ammonia sorption Cr change coordination to 5 or 6 EXAFS/XANES (219)... 

In a study (2) of sorption by various zeolites, platy, fibrous, and rigid three-dimensional networks, several points of interest were established. Ammonia sorption was most rapid in the lattices of the three dimensional networks (chabasite, analcite). It was least rapid in suitably outgassed fibrous and platy zeolites (natrolite, scolecite, and heulandite). Heulandite, a... 

Another point of interest was the difference in the velocity of ammonia sorption by analcite observed by Barrer(2) and Tiselius(6). The former found sorption rapid at temperatures as low as 200 C. the latter found that sorption equilibrium could not be established at 270 C., so slow was the ammonia uptake. These experiments all serve to show the complexity of behaviour of this very interesting series of diffusion systems. 

The surface acidity of the ceria-doped SBA-15 samples was studied by a temperature-programmed desorption of ammonia (NH3-TPD). The measiuements were performed with a Micromeritics Autochem 2910 apparatus equipped with a thermal conductivity detector (TCD) and a mass quadmpole spectrometer (Thermostar, Balzers). Prior to the ammonia sorption, the samples ( 100 mg) were outgassed in a flow of O2 (5% in He) at 500°C for Ih, then, cooled to room temperature under He and saturated in a flow of NH3 (5% in He, 30 mL/min) for Ih. Subsequently, the catalysts were purged in a He flow at lOO C for Ih until a constant baseline level was reached. The ammonia desorption was carried out with a linear heating rate (10°C/min) up to 1050°C under a flow of He (30 mL/min). Cahbration of the TCD were carried out in order to evaluate the ammonia desorption peaks. 

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