Brønsted acid sites in zeolites. FTIR study of molecular hydrogen as a probe for acidity testing

Abstract

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.

Intensities of combination IR bands as an indication of the concerted mechanism of proton transfer from acidic hydroxyl groups in zeolites to the ethylene hydrogen-bonded by protons

“…As has been earlier reported [11], ethylene adsorption by the hydrogen forms of zeolites results in the strong broadening and low-frequency shifts of O-H stretching bands of acidic hydroxyl groups. In our case, for ethylene adsorption on HY at the equilibrium pressure of 10 torr the maximum of this band shifts from 3660 to 3285 cm -1 .…”

A Bifunctional Mechanism for Ethene Dimerization: Catalysis by Rhodium Complexes on Zeolite HY in the Absence of Halides

“…Mass spectra of the gasphase products show that the butenes were characterized by C 3 H 5 and C 4 H 8 fragments (m/z 41 and 56, respectively, corresponding to the typical fragmentation of n-butenes) with no deuterated dimers. [18][19][20][21][22][23][24] (Scheme SI-1 illustrates the reported reaction mechanisms for ethene dimerization on these catalysts.) [18][19][20][21][22][23][24] (Scheme SI-1 illustrates the reported reaction mechanisms for ethene dimerization on these catalysts.)…”

“…H 2 promotes the dimerization reaction, which is accompanied by a slower hydrogenation of the C=C bond, and we infer that the ethyl species expected to be intermediates in the hydrogenation reaction [11] are not involved in the formation of the C À C bonds. [18][19][20][21][22][23][24] The observation that site-isolated rhodium complexes supported on zeolite HY, initially present as [Rh(C 2 H 4 ) 2 ] (Table 1), catalyze the dimerization of ethene in a oncethrough plug-flow reactor at 1 bar and 303 K (with or without H 2 in the feed-the reaction is faster with H 2 ) is contrasted with our observation that the isostructural rhodium complexes supported on highly dehydroxylated MgO (Table SI-1 in the Supporting Information) are 100 % selective for hydrogenation of the C=C bond in the presence of H 2 and inactive under our conditions when C 2 H 4 is used in the absence of H 2 ( Table 2). [18][19][20][21][22][23][24] The observation that site-isolated rhodium complexes supported on zeolite HY, initially present as [Rh(C 2 H 4 ) 2 ] (Table 1), catalyze the dimerization of ethene in a oncethrough plug-flow reactor at 1 bar and 303 K (with or without H 2 in the feed-the reaction is faster with H 2 ) is contrasted with our observation that the isostructural rhodium complexes supported on highly dehydroxylated MgO (Table SI-1 in the Supporting Information) are 100 % selective for hydrogenation of the C=C bond in the presence of H 2 and inactive under our conditions when C 2 H 4 is used in the absence of H 2 ( Table 2).…”

“…IR spectra show that the alkene interacted, on the one hand, with the zeolite through the acidic Al-OH groups, as indicated by the reduction in intensity of the OÀH stretching bands at 3630 and 3565 cm À1 (Figure 1, top), which became strongly broadened and shifted to lower frequencies (Figure SI-1) [18,19] and, on the other hand, with the rhodium sites as well, as indicated by the disappearance of the bands at 3084, 3060, and 3016 cm À1 characterizing the CÀH stretching vibrations in the ethene initially p-bonded to the Rh atoms ( Figure 1, bottom). [27] The C 2 H 4 -zeolite interaction is inferred to be weak, in agreement with the results of previous investigations, [25,28] as the C = C bond was retained upon ethene adsorption, as indicated by the appearance of an IR band at 1615 cm À1 (Figure SI-2), assigned to the C=C stretching vibration of ethene p-bonded to the acidic sites of the zeolite.…”

Ab Initioinvestigation of the interaction of H2with lithium exchanged low-silica chabazites

“…[1][2][3] However, a safe and easy to handle media for hydrogen storage is a primary requirement to link efficiently production and end use. [4] Among the proposed methods for hydrogen storage, molecular adsorption in microporous materials has been proposed as a viable solution [5][6][7][8] because the simplicity and the moderate temperature and pressure conditions of the process. For the role of host material in this process, Al-substituted chabazites [9] have been suggested as promising candidates.…”