Effect of Na exchange of a HBEA zeolite on the activity and the selectivity of a bifunctional Pt-HBEA catalyst for n-hexadecane hydroisomerization. Comparison with a Pt-HZSM-22 catalyst

Abstract

The transformation of n-hexadecane was carried out in a fixed-bed reactor at 220°C under 30 bar total pressure on bifunctional Pt-zeolite catalysts. The Pt-HBEA catalyst was very active but yielded much cracked products due to a rather strong protonic acidity. Pt-Na-HBEA catalysts obtained through partial exchange by sodium ions of the protonic sites of the HBEA zeolite were less active than Pt-HBEA but allowed to decrease significantly cracking. The effect of Na exchange on the activity of the catalysts was quantified using a kinetic model. Pt-HZSM-22 was much less active than Pt-HBEA but more selective for isomerization. However, the selectivity for isomerization of the Pt-Na-HBEA was very similar to that of Pt-HZSM-22 up to a 80% n-hexadecane conversion, while its activity was 2.5 times higher.

Catalytic Hydroisomerization of Long-Chain Hydrocarbons for the Production of Fuels

“…The formed monobranched products exhibited high cracking rate. An enhanced formation of C1, C2, and C14 compound was confirmed to occur via hydrogenolysis on Pt sites present in the Pt-H-Beta catalyst [22]. One conclusion of that study was that acidity of Pt-H-Beta is too high, leading to the undesired cracking products.…”

“…more cracking products were formed [22]. This result was explained by ability of reactant to diffuse into large zeolite pores and further cracking reactions of the formed products.…”

“…Different types of zeolites have been used in hydroisomerization including both 8, 10, and 12 membered structures [22]. The following zeolites possessing a one-dimensional structure, i.e., ZSM-22, ZSM-23, ZSM-48 [19], and SAPO-11 exhibit shape selectivity (Table 2), whereas 12 MR zeolites are devoid of shape selectivity.…”

“…Shape selectivity is very characteristic for 10 MR zeolites, when a long-chain alkane cannot diffuse into the pores of, for example, Pt-H-ZSM-22 [22]. In this case, the substrate is reacting at the pore entrance giving higher selectivity to monobranched products.…”

“…One Pt-H-Beta zeolite catalyst with optimized acidity (Pt-H-Beta with Si/Al ratio of 26) was giving a 76% yield of isomers at 94% conversion and 220 °C under 30 bar with the molar feed ratio of H2/feed of 20 [16]. However, over Pt-H-Beta with Si/Al molar ratio of 15 the yield of isomerization products in the hydroisomerization of hexadecane exhibited a maximum, 44 wt % at 60% conversion, thereafter, more cracking products were formed [22]. This result was explained by ability of reactant to diffuse into large zeolite pores and further cracking reactions of the formed products.…”