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Journal of Molecular Catalysis A-Chemical, Vol.350, No.1-2, 1-8, 2011
Preparation of [Fe]-SSZ-24 through the isomorphous substitution of [B]-SSZ-24 with iron, and its catalytic properties in the isopropylation of biphenyl
[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)center dot 6H(2)O effectively gave [Fe]-SSZ-24, and its XRD gave clear patterns of AFI topology without the peaks assigned to Fe(2)O(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 degrees C); however, the decreases of the selectivity for 4,4'-DIPB occurred at high temperatures (325-350 degrees 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(2)O(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 degrees C. These results suggest that the channels of SSZ-24 can discriminate 4,4'-DIPB from other possible DIPS 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. (C) 2011 Elsevier B.V. All rights reserved.