Ammonia adsorption substituted

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Addition of flourine to H-mordenite enhanced considerably the acid strength of this catalyst but decreased the ratio of Brpnsted to Lewis acidity (167). Using IR spectroscopy of adsorbed pyridine, X-Ray diffraction, catalytic activity tests for cumene cracking, and microcalorimetric measurements of ammonia adsorption, it was shown that some of the acidic hydroxyl groups were substituted with fluorine and that the inductive effect of fluorine increased the acid strength of the remaining hydroxyl groups. 

Not only the CN region gives information about the acidity but also the shift of the OH stretching modes of the zeolites upon adsorption of a base (Table 2). The high-frequency shift of the v(OH) from 3610-3640 cm after isomorphous substitution of the lattice corresponds with the decreasing heat of ammonia adsorption (Al>Fe>In). But more important the decreasing heats combine with a lower shift of the v(OH), see column 2 and... 

Kevan and co-workers investigated the framework substitution and local environment of the nickel ions in several NiAPSO-n molecular sieves in = 5, 11, 34, 41) (204) by ESR and ESEM spectroscopy. In all the systems, the Ni(II) ions are incorporated into the framework positions and various Ni(I) species are formed by reduction and adsorbate interaction. Deuterated ammonia adsorption... 

The acid properties of A1-, Ga-, and Fe-substituted MCM-41-type mesoporous silicates have been probed using ammonia adsorption at 423 K [285]. Substitution led to the formation of Bronsted and Lewis acid sites of different type and strength. Initial heats of ammonia chemisorption decreased in the same order (185 (Al) > 162 (Ga) > 144 (Fe) kJ mol ) as the degree of isomorphous substitution of framework silicon by trivalent atoms. The ratio of strong to medium-strong Brdnsted sites fell in the same order. 

A similar study performed on A1-, Ga-, and Fe-MCM-48 molecular sieves confirmed the existence of a correlation between the acid strength of BrOn-sted sites and the natme of the substituting element [286]. The average acidic strength of sites was foimd to be lower than that observed for acidic zeolites. The majority of the sites presented a medium-to-weak strength, but very weak (< 50 kj mol ) and strong (140 to 155 kj mol ) sites were also present. On pure sihceous MCM-48, the mean value of the heat of ammonia adsorption amounted to 50 kJ mol ... 

K. Teraishi et al.. Effect of Si to Al substitution at next-nearest neighbor sites on the acid strength ab initio calculation of the proton affinity and the heat of ammonia adsorption. Micro-por. Mater. 5, 233-244 (1995)... 

The catalytically active sites of isomorphous substituted MFI structures (Al-Sil and In-Sil) have been characterized by infrared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probes [261]. The hrst derivative of the heat of adsorption curves, dg/dfl, as function of the loading, a, gave maxima at about 140 and 100 kJ/mol for Al-Sil and In-Sil, respectively. Janchen and colleagues... 

The catalytically active sites of isomorphous substituted MFI structures have been characterized by inirared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH, adsorption, the NH, TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from Al>Fe>In> >silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample. 

The topographic analysis of the various oxygen atoms and the silicon substitution sites were reported based on MNDO calculations[17]. The results indicated that PI and All are the crystallographic sites where silicon substitution is energetically favourable. Incidentally, the favourable oxygen sites for the adsorption of ammonia mentioned above, are all bonded to PI and All as shown in Fig. 1. 

The acidity of dealuminated Y zeolites prepared either by conventional treatments or isomorphous substitution has been characterized in detail through a microcalorimetric study of the adsorption of ammonia. 

Figure 4. Heat of adsorption of ammonia versus coverage for the isomorphously substituted solids (IS).

We also examined the adsorption of ethylamine and n-propylamine on the SAPO-5 samples. Again, the results were very similar to those obtained on H-ZSM-5 in that some of the amine desorbed as ammonia and the corresponding olefin between 625 and 700K [4]. Of particular interest was the fact that, for a particular sample, the number of moles which reacted for each amine was the same. This implies that each sample contains a discrete number of acid sites and that each amine samples the same sites. Using this as a measure of the acid site concentration, it is interesting to compare the site concentration to the concentration of substituted metals. This is shown in Table 2 for isopropylamine on the SAPO-5 samples and on MAPO-5(1) and CoAPO-5(1). For this comparison, we used the gel concentration as a measure of the framework metal ion content. 

The mono- and bimetallic substituted MCM-41 catalysts with cobalt, vanadium or lanthanum have been prepared by direct synthesis or impregnation and characterized by various techniques, such as XRD, N2 adsorption-desorption, SEM, TEM and TGA. The catalytic activity in the selective oxidation of styrene with H2O2 has been evaluated. Adsorption of benzene and ammonia on the mono- and bimetallic mesoporous molecular sieves with cobalt and vanadium has been studied by IR spectroscopy and the results have been correlated with their catalytic properties and the characteristics of the structure. Co-incorporated catalysts prepared by direct synthesis show to be very active and selective. 

There are several probe molecules for which infrared spectroscopy can differentiate between adsorption on Bronsted and Lewis acid sites and even estimate the amounts adsorbed. Pyridine is the most widely used because it gives well-resolved bands when protonated by Bronsted acid sites (e.g., 1540 and 1640 cm ) or when coordinated to Lewis acid sites (1450 and 1620 cm ). The values of extinction coefficients are available in the literature [121] for these bands, which makes possible semiquantitative measurements, separately, of Lewis and Bronsted sites. Ammonia, with a smaller kinetic diameter that enables it to reach more easily the acid sites in smaller pores, can also be used to distinguish betwen Bronsted and Lewis acid sites however, the use of ammonia is less reliable, mainly because the resulting IR bands overlap each other [122]. Another base that can distinguish between Bronsted and Lewis acid sites is quinoline because its size is greater than that of pyridine quinoline can also be used to differentiate between acid sites at the external surface and those in pores smaller than its kinetic diameter (6 A). Bronsted sites can be selectively measured with IR methods by using substituted pyridines as probe molecules [123]. 

The availability of chemisorbed chlorine for substitution by other groups was examined by Puri et al. and Boehm. by refluxing the chlorinated carbons with 2.5 M sodium hydroxide and by treatment with ammonia, which indicated the substitution of chlorine by amino groups. The presence of amino groups imparted a basic character to the carbon surface, and there was a noticeable increase in the acid adsorption capacity (Table 1.18). Equivalence in chlorine eliminated, nitrogen... 

The active sites of isomorphously substituted MFl structures activated at 673 K have been characterized by janchen et al. [245] using microcalorimetric measurements carried out at 423 K with ammonia as a probe. Because of decreasing heats of NH3 adsorption, the Bronsted acid site strength of the modified MFl was reported to decrease in the sequence Al > Fe > In > silicalite. In... 

The overall acidic nature of Al-substituted MTS, expected on simple chemical grounds, may be proved by non-spectroscopic measurements, e.g., immersion calorimetry [31], or determination of the adsorption capacity for a base molecule (volumetric or gravimetric, e.g., [32]). Other overall methods employed to define acidity are also based on the interaction with strong bases, usually ammonia or pyridine. The interaction may be followed either by calorimetry (in the adsorption run) or through temperature-programmed desorption (TPD). Both techniques reveal the presence of sites, differing in one case by the differential heat of adsorption and in the second case by the temperature of the desorption peak. 

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