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

Abstract

Adsorption of ethylene by sodium forms of mordenite and Y zeolite is reversible and results in no chemical activation of olefin. Therefore, diffuse reflectance infrared Fourier transform spectra of ethylene adsorbed by these zeolites do not differ much from those of liquefied or frozen C 2 H 4 . In contrast, adsorption of ethylene by HY and HMOR results in the strong hydrogen bonding with acidic hydroxyl groups and in subsequent oligomerization of ethylene already at room temperature. The hydrogen bonding strongly perturbs OH frequencies but exerts only a weak influence on the frequencies and intensities of the fundamental C-H stretching vibrations of ethylene. In contrast, intensities of the infrared (IR) bands from combinations of the double bond stretching vibrations with the bending vibrations of CH 2 groups increase significantly. According to previously published quantum chemical calculations, these combinations contribute most significantly to the reaction coordinates of protons added to adsorbed ethylene. Therefore, the results obtained allow formulation of a new spectral criterion of the reactivity index for concerted proton transfer to adsorbed ethylene; the IR bands corresponding to combination frequencies of most strongly polarized chemical bonds involved in this elementary step have unusually high intensities. It is also suggested that this criterion is of more general significance and also can be applied to other elementary steps of acid or acid-base catalytic reactions when chemical activation of adsorbed molecules results from simultaneous polarization of several chemical bonds.

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.…”

The Acid Catalyzed Reaction of α-Pinene Over Y-Zeolite

“…The presence of Brønsted and Lewis sites was studied using the adsorption of pyridine with monitoring by FT-IR [11]. The band at 1543 cm -1 was assigned to the pyridinium ion formed on a Brønsted acid site, while the band at 1442 cm -1 corresponded to the pyridine coordinated to Lewis side centers (Fig.…”

Chemical diversity of zeolite catalytic sites

“…10,11 and recently the ethoxide has been identified also using NMR spectroscopy . 12 The structure II illustrates the geometry of the transition state, and shows-what is now generally accepted, but was not until the 90s-that transition states on zeolites are carbenium-ion-like.…”