From propene and isobutene, the organic molecules irreversibly retained in the micropores of 5A zeolite are constituted of carbonaceous compounds (“coke”). Practically, all the components of coke formed at low temperature (T≤250°C) are soluble in CH2Cl2 after dissolution of the zeolite in a solution of HF: no polyaromatic compounds, insoluble in CH2Cl2, were found. However, part of coke components is very volatile and can be eliminated during the dissolution of zeolite in HF [7]. For T≥350°C, the formation of polyaromatic compounds insoluble in CH2Cl2 was observed. At low temperature (T≤150°C), coke is mainly constituted of oligomers formed on the acid sites of the adsorbent and located in the a cages of the zeolite (Table 1). These oligomers present a carbon number multiple of 3 (propene) and of 4 (isobutene), indicating that only oligomerization and hydrogen transfer reactions occur. Indeed at these low temperatures and in presence of weak protonic acid sites, resulting of partial dissociation of water [9, 10], the cracking of oligomers are not favored. As it is impossible to extract these compounds by a soxhlet treatment with CH2Cl2, C12 to C20 oligomers are necessary retained in the zeolite pores. This retention is mainly due to their high degree of branching, especially favored with isobutene. When the temperature of adsorption increases, the aromaticity of the soluble coke molecule is enhanced (Table 1). From 350°C methyl-pyrene molecules were found trapped in the α cages of the 5A zeolite and cannot escape out of these cavities. Indeed their size (8.5 Å) is lower than the size of the α cages (11.4 Å) and much greater than the pore apertures (4.2 Å).
Table 1. Main Families of Components in the Coke Soluble in Methylene Chloride, obtained at various temperatures from propene and isobutene
T(°C)Coke formed fromPropeneIsobutene80°CThe weak difference of coke composition observed from propene and isobutene (Table 1) confirms that at low temperature (T=100°C), the more pronounced effect of “coke” from isobutene on the adsorption capacity (Fig. 3a) is due to the difference in coke location inside the zeolite pores. The distribution of oligomer molecules inside the zeolite crystallites would be more heterogeneous from isobutene. Indeed, owing to the olefinic character of isobutene and this slower diffusion rate in the zeolite pores, oligomers will be preferentially formed in the a cages close to the outer surface of the crystallites.
From isopentane, only isopentane molecules are found in the organic material trapped at low temperatures (50-150°C) in the 5A zeolite pores (Table 2). At higher temperatures, isopentane is still the main compound retained in the 5A zeolite, accompanied by a small amount of cracked products (olefins and paraffins), oligomers at 250°C, and aromatics at 420°C.
Table 2. Composition of the adsorbed phase from isopentane
T(°C)50–150250420Constituents (%)iC5 (100%)iC5(90%)The presence of isopentane in the zeolite pores shows that this molecule can enters the pores of zeolite and that its desorption is very low. Furthermore this result also confirms that isobutene can enters the 5A zeolite pores. At 50°C, isopentane molecules occupy a large part of the volume of the zeolite. It was calculated that at saturation (weight adsorbed = 12 wt%), each α cage of the 5A zeolite is able to accommodate 3 molecules of isopentane [11]. Therefore it was concluded that the isopentane molecules occupy all the α cages. This result can be confirmed by extrapolation of the curve obtained in Figure 3b. Indeed, the complete pore blockage of the zeolite can be estimated to about 12 wt% for isopentane against of 9 wt% for isobutene. These results confirm that the presence of methyl group is not the main parameter responsible to a rapid deactivation of the zeolite but that the association of methyl group and double bond (e.g isobutene) leads more rapidly to a pore blockage.
At higher temperature (T≥250°C), a small part of trapped isopentane molecules are transformed into “coke” (oligomers and aromatic compounds) by successive reaction such as cracking, oligomerisation, cyclisation and hydrogen transfer on the weak acid sites of 5A zeolite.