Sorption of pyridine

Infrared spectra of Rb-offretite before and after room temperature sorption of pyridine. Outgassing temperatures (a) 100, (b) 200, (c) 400 °C. Curve (d) refers to the Rb-offretite crystals used... 

IR spectra of H-boralites show four hydroxyl bands at 3450, 3680, 3720 and 3740 cm. These bands were assigned [7,11] to Si—OH—O, B—OH, Si—OH -B, and terminal Si—OH respectively. In the case of H-ZSM-5, two bands were observed 3610 (Si—OH— Al) and 3740 cm (terminal Si—OH). The sorption of pyridine resulted in a reaction of Si—OH - B and Si—OH—A1 groups and the formation of PyH ions. The maximal intensity of PyH band was used to calculate the content of Brdnsted acid sites in the boralites and zeolites studied. These concentrations are presented in Table 2. After the neutralization of all Bronsted acid sites pyridine was desorbed. This was accompanied by the diminishing of PyH and reappearance of acidic hydroxyl bands (3720 cm in the case of boralites or 3610 cm in the case of H-ZSM-5). The intensities of both PylT and OH bands are presented in Figure 1. 

Three different ZSM-5 crystal types have been studied by synchrotron infrared micro-spectroscopy, namely parent zeolite (ZSM-5-P), mildly treated zeolite (ZSM-5-MT) and severely treated zeolite (ZSM-5-ST). The goal is to compare the influence of different dealumination conditions on the molecular diffusion and reactivity of individual ZSM-5 crystals. Synchrotron-based IR micro-spectroscopy [34] was used in combination with pyridine adsorption. Pyridine, having a molecular dimension of 0.57 nm, is able to diffuse throughout the micropore system of ZSM-5 and allows the detection of all acidic sites present within the zeolite material [35]. The sorption of pyridine was introduced at room temperature and the samples were subsequently heated up to 573 K and 673 K. The Bronsted acid sites, giving rise to IR spectra of adsorbed pyridine at around 1545 cm , allow the visualization of differences in the nature and strength of the acid sites in the zeolite crystals under investigation. Fig. 4 shows the IR spectra of the adsorbed pyridine collected at room temperature, 573 K and 673 K for the ZSM-5-P, ZSM-5-MT and ZSM-5-ST crystals. 

This clearly indicates that the support is able to convert a fraction of methane but contributes primarily to coking To investigate the role of acid sites in the conversion of methane to coke and hydrogen, the acid sites of the catalysts were characterized by sorption and temperature programmed desorption (t.p.d) of pyridine T.p.d. of pyridine (see Fig. 4) suggest a higher... 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

SSZ-35 the reactions would be influenced by the presence of very strong Lewis sites. Quantitative sorption of ammonia, pyridine and d3-acetonitrile in both zeolites showed that the real number of acidic groups was close to values, derived form the number of aluminum atoms (taken from AAS analysis) in the idealized unit cell. Obtained values are 1.1 H+/u.c. for SSZ-33 with idealized unit cell composition H2.9[Al2.9Si53.iOii2] (plus 1.3 Lewis sites per u.c.) and 0.3 H+/u.c. for SSZ-35 with ideal formula Ho.4[Alo.4Sii5 6032] (plus 0.05 Lewis sites per u.c.). 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

Pyridine Sorption. After pyridine sorption and degassing in vacuo at (to remove physisorbed pyridine), the intensity of the... 

Surprisingly, in the case of CTO the admixture of a small amount of pyridine to the reaction mixture had no significant influence. Moreover, the increase in the pyridine amoimt leads to a decrease of the CTO conversion, maybe by blocking of sorption sites. A similar effect can also be seen when pyridine is added to a TO feed. The selectivity is only slightly increased. A significant improvement of the catalytic performance is obtained when pyridine is added... 

A radiotracer study of the adsorption and electrocatalytic reduction of nicotinic acid at a platinized platinum electrode in acid medium furnished evidences proving the significant differences between the sorption behavior of pyridine ring and that of the saturated (piperidine) ring. 

The effect of soil pH on the sorption of organic bases is well illustrated by observations with pyridine, quinoline, and acridine, three heterocyclic nitrogen compounds, using subsurface materials whose properties are listed in Table 3.1. The Freundlich K values and exponents are summarized for two subsoils in Table 3.10. 

Figure 3.12 Sorption of quinoline at 1 5 solid-to-solution ratio with and without pyridine. [Reproduced with permission from J. M. Zachara, C. C. Ainsworth, C. C. Cowan, and...

The assumption that the strong interaction between the strongly acidic adsorbent and the strongly basic adsorbate under our experimental conditions was the reason for the failure to achieve reversibility was confirmed when adsorbents with the same structure but lower (Li-ZSM-5 or Na-ZSM-5) or zero acidity (silicalite-1) were employed [26]. Figure 12 demonstrates the perfect reversibility of pyridine sorption into the slightly acidic Li-ZSM-5 zeolite. 

Fig. 12 Curves obtained upon reversible uptake sorption (left side) and desorption (right side) of pyridine into and out of Li-ZSM-5...

In the AD expanded saponite, acidity increases, Hgure 5-9C. As expected, the amount of pyridine retained on the clay acid centers monotonically decreases with temperature. If AD is replaced by SCD, Bronsted acidity is enhanced. Figure 5-9D. The steam-aged clays retain their sorption capacity for pyridine. However, their acid site strength is greatly reduced, and after degassing in vacuo for 2h at 200 C, pyridine is essentially removed from the steam-aged expanded clay samples. 

Figure 5-9. IR spectra of pyridine sorbed on expanded montmorillonites that have been (A) air dried or (B) supercritical dried with CO2. Spectra for expanded saponites that have been air dried or supercritical dried with CO2 are given in C and D, respectively. All samples have been degassed in vacuo at (a) 200 C, (b) 300 C, (c) 400 C, and (d)500 C for two hours at each temperature. In (e) is the clay spectra before pyridine sorption.

Another noticeable example of an MOF constructed from a polytopic carboxylic acid and the binuclear metal-carboxylate paddlewheel-like cluster MBB, equivalent to a square planar SBU, is the one commonly referred to as HKUST-1 MOF, with the general formula [Cu3(TMA)2 (H20)3] where TMA denotes trimesic acid (1,3,5-benzene-tricarboxylic acid) (Figure 13, ). The MBB is defined by four carboxylate ions coordinating two Cu(II) ions in a paddlewheel-like structure with four points of extension defining a square-planar SBU. The authors reported the capability of exchanging the axial water molecules coordinated to the Cu(II) paddlewheel, postsynthetically, with pyridine molecules, opening the door for further investigations toward the viability of preferential sorption of small molecules and/or catalysis on coordinatively unsaturated metal centers. 

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