Adsorbate acidic strength

At RT, NH3 adsorbed on Lewis acid sites, Zr and V. Accordingly, the intensity of bands from NH3 decreased little with the V-content, by 15% at most, as expected on account of the similar Lewis acid strengths of Zr V and VV. The symmetric bending... 

The density of Bronstcd and Lewis acid sites was determined by IR spectroscopy (Nicolet 710) of adsorbed pyridine, after desorption at 250°C, using the molar extinction coefficients previously obtained by Emeis [11]. The acid strength distribution of selected zeolites was studied by NH3-TPD in an Autochem 2910 Equipment (Micromeritics) coupled to a quadrupole mass spectrometer. First, NH3 was adsorbed at 175°C until saturation and then desorbed by increasing the temperature up to 800°C at a heating rate of 10°C/min. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

The variety of the different framework structures result in different adsorbent characteristics acid strength, size of molecule adsorbed, adsorption/desorption rate of different molecules, capacity and stability. As a result, these differences characterize the adsorbent s selectivity to a specific molecule and adsorbent-adsorbate interactions. Take for example, the difference in selectivity of BaY and Ba-Mordenite [24] to p-xylene (PX), m-xylene (MX) and o-xylene (OX) ... 

Ba-Modenite s selectivity to MX is higher than OX, but the opposite is true for BaY. This reversal in selectivity is a result of differences in adsorbent framework characteristics mordenite has higher acid strength compared to Y zeolite. Adsorption and desorption rates of xylenes are expected to be faster in BaY compared to Ba-Mordenite because Mordenite is a one-dimensional channel system while Y zeoUte is a three-dimensional channel. With the reason stated, a three-dimensional channel ZeoUte is the preferred mass separating agent of choice compared to one-or two-dimensional channels for the liquid adsorption separation. 

Utilization of zeolite acidic strength in Cg-aromatics (xylens) systems is illustrated next. In the presence of strong acids, xylene isomers have varying basicity (Table 6.5), with m-xylene being the most basic and p-xylene the least basic among the Cg-aromatics [27]. Based on the basicity of the xylenes, the acidity of each zeolite can be properly adjusted to selectively adsorb m-xylene or p-xylene. As demonstrated in Figure 6.4, a more acidic zeolite such as NaY will selectively adsorb m-xylene from other Cg-aromatics [28, 29], In contrast. Figure 6.5 shows that a weaker acidic zeolite such as KY will selectively adsorb p-xylene from other Cg-aromatics [30, 31]. In both systems, toluene was used as the desorbent. 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

Acidic properties of zeolite L were observed to correlate well with its structural disorders. The Si-MAS-NMR spectrum of zeolite L having a Si/AI ratio different from 3 revealed that Al distribution deviated from the ideal and suggested the presence of six different boat-shaped 8-ring patterns. Differential molar heats of adsorption of ammonia changed step-wise with the adsorbed amount, which reflects the difference in the acid strength of protons located in structurally different 8-rings. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

Nonaqueous methods for the determination of surface acidity represent a considerable improvement over aqueous methods because the solvents used (e.g., benzene, iso-octane) do not react with catalyst surfaces as previously described in the case of water. Of the available types of nonaqueous methods (1-3), the simplest is that employing adsorbed indicators. It can be used to determine acid strengths and amounts of surface acids as described in the following section. 

After the acid strength of a catalyst surface has been bracketed by means of colors of adsorbed indicators, the next logical step in the determination of surface acidity is the measurement of the number of acidic groups. This is generally done by titrating a suspension of the catalyst with a solution of a suitable amine in an inert solvent the previously described indicators are used to determine endpoints. 

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