SURFACE ORGANOMETALLIC CHEMISTRY ON METALS - SELECTIVE DEHYDROGENATION OF ISOBUTANE INTO ISOBUTENE ON BIMETALLIC CATALYSTS PREPARED BY REACTION OF TETRA N-BUTYLTIN ON SILICA-SUPPORTED PLATINUM CATALYST
F. Humblot et al., SURFACE ORGANOMETALLIC CHEMISTRY ON METALS - SELECTIVE DEHYDROGENATION OF ISOBUTANE INTO ISOBUTENE ON BIMETALLIC CATALYSTS PREPARED BY REACTION OF TETRA N-BUTYLTIN ON SILICA-SUPPORTED PLATINUM CATALYST, Journal of catalysis (Print), 179(2), 1998, pp. 459-468
The selective dehydrogenation of isobutane into isobutene was studied
on silica-supported bimetallic Pt-Sn. Several bimetallic catalysts wer
e carefully prepared by selective hydrogenolysis of Sn(n-C4H9)(4) on P
t. Previous EXAFS studies have shown that this hydrogenolysis is a ste
pwise transformation of a Pt-Sn(n-C4H9)(3) fragment into a surface all
oy. It was shown that after hydrogen treatment at 550 degrees C, tin a
nd platinum are in reduced form (zero-valent oxidation state) and that
the tin atoms are located on the surfrace of the metallic particles.
The presence of tin on platinum caused a decrease in hydrogen or carbo
n monoxide chemisorption, but an increase of the oxygen consumption. T
he decrease of Wt and CO chemisorption is explained by the decrease of
the number of accessible platinum atoms due to the increased number o
f surface tin atoms. The increase in the Oz chemisorption was explaine
d by the following reaction which represents a phase segregation: PtsS
nx/SiO2 + 1/2(y + xy')O-2 --> (PtOy)(s)(SnOy')(x)/SiO2. The values of
y and y' was about 1 and 2 at respectively 25 degrees C and 300 degree
s C. Thermodesorption of adsorbed CO on several reduced PtSn catalysts
showed no shift of the v (CO) frequency, suggesting negligible electr
onic effect of tin atoms on the platinum atoms when both are reduced.
At 550 degrees C under atmospheric pressure of hydrogen and isobutane,
the presence of tin increases drastically, both the selectivity and t
he activity of the isobutane conversion into isobutene (for Sn/Pt-s =
0.85, the selectivity is higher than 99% and the TOF, based on total p
latinum atoms, is greater than 6 s(-1). The increase in selectivity co
uld be explained by the ''site isolation effect'' and the increase in
activity could be due to the inhibition of the coke formation (which p
oisons the active surface). A mechanism of dehydrogenation and hydroge
nolysis of isobutane based on elementary steps of organometallic chemi
stry has been proposed which accounts both for the high selectivity an
d activity of the bimetallic catalysts as compared to pure Pt/SiO2. (C
) 1998 Academic Press.