ISOMERIZATION OF N-BUTANE OVER DEUTERATED SULFATED ZIRCONIA

Isotopic labeling with deuterium of the surface of a sulfated zirconia catalyst that was precalcined at 650 degrees C and evacuated at 450 d egrees C, shows that this surface contains a small number of Bronsted sites; the number ratio of sulfate ions to Bronsted sites is 12.5/1. I somerization of n-butane to iso-butane over this catalyst with a C-4 f eed from which olefins and dienes were rigorously removed (estimated o lefin/paraffin ratio, 10(-15)) is slow and so is the rate of catalyst deactivation. After 25 min reaction the turnover frequency, based on t he number of surface protons detected by exchange against deuterium, i s similar to 10(-4) s(-1). At a conversion of 6.6% n-butane to iso-but ane, each surface site has turned over one molecule; all D atoms have been exchanged against H atoms from the butane. The iso-C-4 product in itially contains roughly 20% iso-C-4-d(1). At the same stage, the n-C- 4 contains 4% n-C-4-d(1). In view of the large n-C-4/iso-C-4 ratio at this stage, this means that more D atoms have moved into n-C-4 molecul es than into iso-C-4 molecules. This indicates the participation of su rface Bronsted sites in exchange reactions and events such as the prev iously detected (V. Adeeva et al. Catal. Lett. 33, 135 (1995)) intramo lecular rearrangement of C atoms in n-butane via a protonated cyclopro pane intermediate. The absence of any molecules with more than one D a tom indicates that all olefin molecules at the surface of this catalys t are tied up as ''carbenium ions'' or as surface alkoxy groups. (C) 1 997 Academic Press