Crystal structure of an acetylene sorption complex of zeolite 4A

Abstract

The crystal structure of an acetylene sorption complex of zeolite A has been determined by single-crystal X-ray techniques. Fully vacuum-dehydrated zeolite A of approximate composition Na12A112Si1~048 per unit cell was exposed to dry acetylene for 24 hr at a pressure of 650 Torr. The cubic space group Pm3m was used with a = 12.260(5) A. Approximately six CzH2 molecules are absorbed per unit cell and occupy two or perha.ps three kinds of sites in the large cavity. Three C2H2 molecules participate in a symmetric approach to threefold axis sodium ions; each carbon atom in each of these three molecules is 2.8(1) A from a sodium ion. Two other carbon positions have been determined and refined, but the assignment of molecular positions to these is uncertain. One C2H2 appears to be symmetrically associated with the twofold axis sodium ion at distances of 2.6(1) A and simultaneously asymmetrically associated with a sodium ion in the eight-oxygen window with distances of 2.6(1) and 3.0(1) A. Crystallographically refined occupancy parameters indicate that the remaining two CzHz molecules are (nearly) equivalent to this one except that each associates with only one ion, one of the remaining two sodium ions in the eight -oxygen windows. However, a chemically more reasonable distribution of occupancy parameters for these two carbon positions would allow each of these latter two C2H2 molecules to be symmetrically associated (2.6(1) A) with an eight-window sodium ion. In each case, the sorption mechanism involves an ion to induced dipole interaction between Naf and the laterally polarizable A system of acetylene. IntroductionZeolites find extensive commercial application as catalysts for hydrocarbon cracking or for specific high yield rearrangement reartions involving hydrocarbons and other organic mo1ecules.l In order to determine the sorption sites (these may be catalytic sites as well in some cases) selected by a hydrocarbon in a zeolite, this structure was studied. Because unsaturated hydrocarbons are sorbed more tenaciously than saturated ones, and because the carbon atoms i~n acelylene were likely to be equivalent in the complex, C2W:z was selected as the sorbate material. Previous sorption nieasurements2 ,3 indicated that approximately six CzH:! molecules are sorbed per unit cell in zeolitcl 4A; the effect of less absorbent binder materials contained in the commercial pellets was taken into accountThe mechanism for sorption could involve a direct ion to induced dipole interaction4 between the cations and the polarizable T sysitems in these hydrocarbons as spectroscopic5-7 and nuclear magnetic resonance8 evidence indicates, and/or weak hydrogen bonding between the very weakly acidic protons, whose acidity might be enhanced by the former interattion, and the negatively charged zeolite framework oxygen atoms. Specific results9 for acetylene on variounly exchanged zeolite A samples support the former mechanism for zeolite 4A. Ethylene sorbed onto variously exchanged samples1° was found to be freely rotating except in the cas...

The uniaxial rotation of ethylene adsorbed by sodium 13X zeolite

“…Figure 2 shows the principal axes of this conventional model for coordinated ethylene. This geometry has been demonstrated by X-ray crystallography in the system formed by adsorbing acetylene into sodium zeolite 4 A [9]. It is also that which has been calculated for cis-2-butene by a semi-empirical C.N.D.O.…”

“…We may assume that the molecules adsorbed at low coverages will adsorb preferentially upon the sites of four-fold coordination because of their accessibility and their higher calculated field strengths [8]. However, because we know that in practice our scattering models are not particularly sensitive to changes in the angle between equivalent orientations of the molecule when this is less than 90 ~ , an error in this assumption will not affect our conclusions 9 We also assume that the ethylene molecules lie with their zr-electrons oriented towards the sodium ions in the cage walls. Figure 2 shows the principal axes of this conventional model for coordinated ethylene.…”

Monte Carlo Simulations of Ne, Ar, Kr, and Xe Encapsulated in Cs3Na9−A Zeolite

“…For the purpose of the storage of small molecules and for the illumination of the behavior of encapsulated molecules, encapsulation of small nonpolar molecules (H 2 , O 2 , N 2 , CH 4 , and inert gases) and small organic molecules (small hydrocarbons and some aromatic compounds) by zeolites, especially A-type, was extensively studied experimentally [1][2][3][4][5][6][7][8][9] and by computer simulations. [10][11][12][13] Adsorption of small molecules and organic compounds by faujasite has been investigated by several workers both experimentally [14][15][16][17][18][19] and computationally.…”

Distribution of metal and adsorbed guest species in zeolites

“…Hydrocarbons adsorbed on zeolites have been extensively studied by X-ray scattering [97], infrared spectroscopy [98], and neutron diffraction (99] To address these issues, we have employed a combination of 129 Xe and proton multiple-quantum NMR to obtain detailed information on the distributions of adsorbed organic molecules in NaY zeolite. By virtue of a 129 Xe chemical shift range that exceeds 5000 ppm, slight variations in local environment can have a profound influence on the 129 Xe resonance frequency.…”

“…Preparation-dependent properties of adsorbate mobility and local structure have been studied for small, strongly sorbed hydrocarbon molecules in A-type zeolites [97,123,124,165,166). Bakaev has used statistical mechanics arguments to predict the time-averaged distribution of mobile sorbate molecules in zeolites from which equilibrium cage occupancies can be obtained.…”