Preparation of [Fe]-SSZ-24 through the isomorphous substitution of [B]-SSZ-24 with iron, and its catalytic properties in the isopropylation of biphenyl

Abstract

[Fe]-SSZ-24, a ferrosilicate with AFI topology, was prepared through an isomorphous substitution of [B]-SSZ-24 with iron, and applied for the isopropylation of biphenyl (BP) to understand the mechanism of shape-selective catalysis. The substitution of [B]-SSZ-24 with an aqueous solution of a limited amount of Fe(NO 3) 3·6H 2O effectively gave [Fe]-SSZ-24, and its XRD gave clear patterns of AFI topology without the peaks assigned to Fe 2O 3. [Fe]-SSZ-24 exhibited enhanced catalytic activity for the isopropylation of BP. Shape-selective formation of 4,4′-diisopropylbiphenyl (4,4′-DIPB) occurred at moderate temperatures (250–300 °C); however, the decreases of the selectivity for 4,4′-DIPB occurred at high temperatures (325–350 °C). On the other hand, the selectivities for 4,4′-DIPB in encapsulated products remained almost constant ( ca 75%), irrespective of the reaction temperature and the SiO 2/Fe 2O 3 ratios. The differences in the selectivities for 4,4′-DIPB between bulk and encapsulated products indicate that shape-selective formation of 4,4′-DIPB occurs in the [Fe]-SSZ-24 channels, and these channels prevent the isomerization of 4,4′-DIPB, even at 350 °C. These results suggest that the channels of SSZ-24 can discriminate 4,4′-DIPB from other possible DIPB isomers at their transition states although high reaction temperatures cause isomerization at external acid sites. Large pore molecular sieves of AFI topology, [Fe]-SSZ-24, [Al]-SSZ-24, MgAPO-5, ZnAPO-5, and SAPO-5, gave similar levels of selectivities for 4,4′-DIPB in the isopropylation of BP. These results indicate that the framework of AFI topology primarily controls shape-selective formation of 4,4′-DIPB, although catalytic activities of the materials were dependent on acidic properties.