Ultrastable HY zeolites

The starting NaY zeolite was an SK-40 from Union Carbide with a framework Si/Al ratio of 2.4. Ultrastable HY zeolites (HYUS) were prepared by steam-calcination of partially ammonium exchanged zeolites at atmospheric pressure and 550-750 °C during 3-20 hours. After dealumination they were exchanged twice with an NH solution at 80 C for one hour and then calcined at 550 °C for 3 hours. In this way dealuminated samples containing less than 2% of the original Na were obtained. One of these (HYUS-8) was subjected to different treatments (1) washed with a solution of citric acid or HCl (pH=3) at 25 °C for one hour (samples HYUSAC and HYUSl, respectively) (2) washed with a solution 0.1 M of NaOH at 40 °C for one hour (HYUSN), and (3) washed with a 38% v/v solution of acetylacetone in ethanol at 20 °C for 2 hours (HYUSA). 

Table I gives, as an example of the efficiency of Y zeolites, the results obtained over an ultrastable HY zeolite (Si/Al=2.5), under the same conditions (200°C) for both reactions.

The 2,6-dicyclohexylnaphthalene, a crystalline compound (m.p.= 152°C) with a crystallographic center [20], is thus obtained in a 27% yield from the mixture of the cyclohexylation reaction of naphthalene with cyclohexyl bromide over the ultrastable HY zeolite. 

SAPO-37 molecular sieve which has the crystalline structure of faujasite differs from this zeolite by the presence of phosphorus in the structure (1). It was shown that this element increases the thermal and hydrothermal stability of the structure (2). With regards to acidity, the SAPO-37 materials have acidic properties (1,3,4) with two OH groups very similar to those of faujasites (1,4). It was also observed that the SAPO-37 materials have besides acid centers of medium strength a small number of protonic sites stronger than in HY or even than those of an ultrastable LZY-82 (4). 

The highest proton-donor strengths are exhibited by zeolites with the lowest concentrations of AlOY tetrahedra such as H-ZSM-5 and the ultrastable zeolite HY. These are superacids, which at high temperatures (ca. 500 °C) can even protonate alkanes. It was foimd that the acid strength depends on the number of A1 atoms that are adjacent to a silanol group. Since the A1 distribution is nonuniform, a wide range of acid strengths results. 

Vazquez M.I., Escardino A. and Ancejo A. (1988) Hydrocracking of n-Heptane with a NiO-MoOj/HY Ultrastable Zeolite as Catalyst. The Network of the Reaction , Ind. Eng. Chem. Res. 27, 2039-2043. 

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