Pyridine, adsorption constants

Fig. 2. The temperature dependence of the inhibitor adsorption constant. O quinoline, differential reactor X, quinoline, integral reactor A, pyridine, differential reactor 0, pyridine, integral reactor.

Using also H-ZSM-5, Parrillo et al. [80] have demonstrated that the differential heats of adsorption for ammonia and pyridine are constant up to a coverage of one molecule per Al site, and that the differential heats for coverages below one/Al are 145 5 kJ mol for ammonia and 200 5 kJ mol for pyridine. 

Three samples of H-ZSM-5 prepared with varying aluminum content and by different synthesis methods were examined by Parrillo et al. [50] using microcalorimetry of ammonia and pyridine adsorption. Two samples with bulk Si/Al ratios equal to 34 and 58 were prepared employing a TPA-Br template in hydrothermal solution, the third sample (Si/Al 26) was synthesized without a template. The results showed that the heats of adsorption for both ammonia and pyridine were constant with values of 145 kj mol for ammonia and 200 5 kJ mor for pyridine up to a coverage of one molecule per framework Al, and were, in fact, independent of the Si/Al ratio. From these results, the authors suggested that the Brbnsted acid sites in unsteamed H-ZSM-5 are independent of the sample and equal in concentration to the framework aluminum content. 

Parrillo et al. [242] have used micro calorimetric measurements of ammonia and pyridine adsorption to compare the acid sites in H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5. On each of the molecular sieves, the differential heats of adsorption for both ammonia and pyridine were constant up to a coverage of one molecule per Brbnsted site. The differential heats at coverages below 1 1 were identical on each of the materials, with values for ammonia of 145 5, 150 5, and 145 5 kjmol- on H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5, respectively, and for pyridine of 195 5, 200 5, and 200 5kjmol i on H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5, respectively. The authors [242] concluded that the microcalorimetric heats of adsorption for ammonia and pyridine at Brbnsted acid sites formed by framework Fe(III) and Ga(III) were very similar to heats of adsorption at Al(OH)Si sites, and that the three samples were effectively equivalent proton donors. In contrast, they foimd very different reactivity measurements for n-hexane cracking and propene oligomerization on the same materials. The authors claimed that heats of adsorption for strong bases do not reflect differences in inherent acid strength and may not be related to catalytic activity in any simple manner. 

Finally, two sets of physical properties have been correlated by the Hammett equation. Sharpe and Walker have shown that changes in dipole moment are approximately linearly correlated with ct-values, and Snyder has recently correlated the free energies of adsorption of a series of substituted pyridines with u-values. All the reaction constants for the series discussed are summarized in Table V. 

Combinations of Bi203 and Mo03, promoted by P2Os at a constant P/Mo ratio (0.2) were studied over a full composition range by Ai and Ikawa [6], Acidity (and basicity) were measured directly by adsorption of compounds like ammonia, pyridine and acetic acid. The effect of the Bi/Mo ratio on the acidity (Fig. 14) parallels the effect on the overall butene oxidation activity [presented in Fig. 5, Sect. 2.3.2(a)(i)]. 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

As a typical example of Method 1, Nishide el al. cross-linked poly(4-vinyl-pyridine) (PVP) with 1,4-dibromobutane (DB) in the presence of metal ions as templates [2-4], The adsorption behaviour of Cu(II), Co(II), Zn(II), Ni(II), Hg(II) and Cd(II) on the obtained resins was studied. The resins preferentially adsorbed the metal ion which had been used as template. PVP was cross-linked by alkylation of pyridine groups in PVP with DB to yield insoluble PVP resins, of which free pyridines were utilised to coordinate metal ions (Scheme 9.2). The stability constant of the Cu(II) complex of the resin was largest for the resin prepared with Cu(II) as template, being due to its large entropy change for the complexation. 

This chapter describes the results of the acidity characterization of a selected silica surface with VT-DRIFT spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength. 

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