IRON-ALUMINUM CLUSTER CATALYSTS OBTAINED BY ALKOXY SYNTHESIS .2. IRON-ALUMINUM CATALYSTS FOR CO HYDROGENATION - SELECTIVITY - STRUCTURE RELATION

A series of iron-containing catalysts with 2.5 mass% Fe (A), 5.5-8.0ma ss% Fe(B), 5.5-8.Omass% Fe + 0.2 mass% Zr(B-Zn), and 15.0-20.0 mass% F e (C) was obtained from gels produced by alkoxy synthesis and treated either under H-2 (400-degrees-C) or successively in air (500-degrees-C ) and under H-2 (400-degrees-C). The catalysts were tested in CO hydro genation (T = 350-degrees-C, P = 18 MPa, CO/H-2 = 1) and studied by in situ Mossbauer spectroscopic and magnetic susceptibility measurements . Depending on the Fe loading and activation procedure, the catalysts showed more or less selectivity, S, in C1-C3-Ph and C4-C8-Ph productio n. Catalysts A and B, treated by H-2, yielded C1-C3-Ph with S almost-e qual-to 48% and traces Of C4-C8-Ph. When successively treated with 02 and H-2, catalysts A and B markedly lost C1-C3-Ph selectivity (S almos t-equal-to 30%) with increasing S(C4-C8-Ph) to almost-equal-to 8-9%. C atalyst B-Zr showed the highest selectivity (S(C1-C3-Ph), almost-equal -to 52%, even after two-step activation. Independent of the activation procedure, catalyst C showed no selectivity at all. The formation of small (d almost-equal-to 3 nm) carbide clusters Fe(x)C and chi-Fe5C2 w as shown to be a crucial point for process selectivity. No metallic ir on phase was observed in selective catalysts. The most selective B-Zr catalyst revealed small amounts of FexC carbide along with high-spin F e3+ and Fe2+ in a gamma-Al2-x-yFexZryO3 matrix. The presence of Zr cau ses a stabilization effect, inhibiting the loss of lattice oxygen and favoring the reaction of active carbon (C(i)) with iron intermediates. The participation of anion vacancies and ''oxidic'' carbon in selecti ve catalysis is discussed. (C) 1994 Academic Press, Inc.