FTIR spectroscopy is used for the investigation of hydrogen bonding of the Bronsted hydroxyls in H-ZSM-5 zeolite with a number of weak bases (Ar, H2,02, N2, CH4, C2H6, CsHs, CO, C2H4). The following correlation for the increase in intensity of the hydroxyl band, due to its shift resulting from perturbation, is observed: M I A 0 = 0.018Auo~. Applying this correlation to the HF-LF pair in the infrared spectra of H-Y, H-EMT, and H-EM0 zeolites, it is shown that the distribution of Bronsted hydroxyls between super and @cages is approximately 1: 1 whereas in SAPO-37 samples this ratio is close to 3: 1. This is in agreement with results for the deconvolution of the hydroxyl region of the lH MAS N M R spectra. The estimated values of the extinction coefficients for the HF and LF band in H-Y zeolite (Si/Al = 2.7) are 3.2 and 8.5 cmpmol-l, respectively.

No abstract

No abstract

No abstract

Proton affinities of Brsnsted hydroxy groups in H-forms of zeolites and SAPOs are determined by an FTIR study of the low-temperature adsorption of carbon monoxide. This method reveals the range of heterogeneity of Brsnsted hydroxy groups associated with unmodified zeolites where only one band interacting with carbon monoxide is evident in the infrared spectrum. Three types of acid hydroxy groups associated with different IR bands are detected in a modified zeolite (US-Y), (i) strongly acidic [3599 cm-', proton affinity (€,J = 1112 kJ mol-'1, (ii) medium acidic (3627 cm-', E,, = 1142 kJ mol-') and (iii) weakly acidic (ca. 3740 and 3675 cm-', ED, = 1192 f 29 kJ mol-I).

Hydrogen is used as a probe molecule for characterisation of Brsnsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTlR (OH--H2 complex formation), simultaneously. The physisorption of hydrogen is a function of the pore size of a zeolite whereas the OH. * .H2 interaction reflects the strength of the acid sites. Hydrogen complexes observed by FTlR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H , bands. For the zeolites studied the behaviour of these parameters is consistent and shows the following order of acidity: SAPO-37 < H-Y < H-EMT < H-ZSM-5 < H-MOR. Hydrogen is also compared with carbon monoxide, another probe widely used for acidity determination.

Using in situ FTlR and GC kinetic studies, we have examined the mechanism of dehydration of isobutyl alcohol to butene on well characterized ZSM-5 zeolite (number of active sites determined by various methods). Dehydration takes place on Brsnsted-acid sites. The kinetics of water evolution from butanol is followed by in situ FTlR and both the rate constant and the activation energy of water evolution are estimated [k, = k , , exp (-€,/R77, where k , , = 2 x lo9 s-' and E, = 19 3 kcal mol-'1. At low temperatures (45-7OoC), elimination of water is accompanied by simultaneous formation of isobutyl ether (which at the given temperatures is adsorbed and desorbed with difficulty, but is able to form inside the channels). At higher temperatures (125"C), there is a shift in the equilibria of various reaction steps, resulting in the formation of butene. This butene may desorb into the gas phase with traces of ether (in conditions of excess alcohol, flow GC experiments) or form oligomers which remain adsorbed in the zeolite (no excess of alcohol, static IR experiments). The measured rate constant and activation energy [k, = k , , exp(-€,/RT), where k , , = 3 x lo1, s-' and E, = 32 2 kcal mol-'1 for butene formation are effective values, containing contributions from several reaction steps, which explains the rather high value of E,.We have already reported the start of a series of studies devoted to shape-selectivity and pore-confinement effects in the dehydration of C, alcohols on zeolites, in particular on H-ZSM-5.'"vb The molecular dimensions of C, alcohols are rather similar to the dimensions of the channels in ZSM-5,2 and so this choice of reaction and reactants is particularly suitable. Furthermore, using the four butanol isomers, it is possible to modify systematically the molecular structure and rtxtivity of the reactant. The aim of the current work is to elucidate the mechanism of isobutyl alcohol dehydration on a molecular level, and in particular, to clarify how confinement of reactants, transition/intermediate complexes and products in a channel system of molecular dimensions, influence the rate and the reaction pathway. In order to do this, it is essential to: (1) thoroughly characterize the active centres of the zeolite, (2) separate the diffusion and kinetic regimes of reaction and (3) determine the mechanism of molecular conversion at the active centres. High-silica type ZSM-5 zeolites are highly active catalysts for the conversion of alcohol^.^ In recent years, most studies have concentrated on the conversion of methanol to hydroc a r b o n~.~.~ Here, interest has mainly focused on the formation of the first C-C bond, formation of higher olefins and aromatics, and the influence of the zeolite shape selectivity on these final products. Even so, the mechanism of reaction is still under debate. The initial dehydration reaction has received less attention, other than the observation of rapid formation of dimethyl ether under reaction conditions. Less work has been carried out on higher alcohols. There are stud...

No abstract