Crystal phase dependent metal-support interaction in Pt/SO4-ZrO2 catalysts for hydroconversion of n-alkanes

J.M. GRAU; J.C. YORI; C.R. VERA; F.C. LOVEY; A.M. CONDÓ; J.M. PARERA

APPLIED CATALYSIS A-GENERAL

Elsevier Sci. B.V.

Lugar: Amsterdam; Año: 2004 vol. 265 p. 141 - 152

0926-860X

            In order to elucidate the influence of the crystal structure of zirconia on the properties of the metallic and acid function of Pt/SO42--ZrO2, catalysts with different zirconia crystal phases were synthesized, fully tetragonal, fully monoclinic, and with a mixture of the tetragonal and monoclinic phases.  Their catalytic properties were studied in the metal-catalized reaction of cyclohexane dehydrogenation (300°C, 0.1 MPa, WHSV= 10 h-1, H2/C6H12= 30), the acid-catalyzed isomerization of n-butane (350°C, 0.1 MPa, WHSV= 1 h-1, H2/C4H10 = 6), and the bifunctional hydroconversion of n-octane (300°C, 1.5 MPa, WHSV= 4 h-1, H2/C8H18= 6).  TPR, XRD and FTIR of chemisorbed CO were also used in order to characterize the catalysts.  The results showed a strong influence of the crystal phase on the activity of the acid function.  A less marked effect was found for the metal-catalyzed reaction.  An opposite relation between the two functions was seen with respect to this crystal structure influence.  Among the sulfated catalysts, monoclinic Pt/SO42--ZrO2 had the lowest activity in n-C4 isomerization and the highest activity in cyclohexane dehydrogenation.  Tetragonal Pt/SO42--ZrO2 catalysts were the most active in isomerization of n-butane.  They had the lowest activity in cyclohexane dehydrogenation and their metal properties were negligible.  They were also the most active in n-C8 conversion, producing mainly i-C4.  Monoclinic catalysts had low cracking activity and produced mainly isooctane.  Mixed phase catalysts had an intermediate behavior.              While S poisoning of Pt was present as a uniform effect on all sulfated catalysts, the metal-acid behavior and the different Pt properties could be explained by a metal-support interaction between tetragonal SO42--ZrO2 and Pt.  The Pt-support interaction was analyzed both with a model of Pt particles encapsulation and a model of electron depletion.  The electronic deficiency of Pt particles supported on SO42--ZrO2 was evident in the shift of the IR bands of adsorbed CO. Further experimental work is however needed to get conclusive evidence about the nature of the Pt–support interaction. Keywords:    SO42--ZrO2, platinum, metal-support interaction, n-butane isomerization, n-octane hydroisomerization-cracking, metal function, acid function