Abstract
Correlation of UV/Vis Features and Reaction Profile during Alkane
Isomerization on Ordered Mesoporous and Tetragonal Sulfated ZrO2
J. Melsheimer, R. Ahmad, X. Yang, F.C. Jentoft
Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society
Faradayweg 4-6, D-14195 Berlin, Germany
Introduction
The reaction profiles during alkane isomerization on ordered mesoporous and tetrago-nal sulfated zirconia (omSZ, tSZ) are different [1,2]. In addition it was shown that the spectral answer obtained from in situ UV/Vis measurements indicates the formation of species which influence the catalytic behavior. For this reason results of the alkane isomerization on the title compounds are presented, which show a possible correlation between the reaction profile and the UV/Vis band maxima and/or band areas.
Experimental
A modified Perkin-Elmer Lambda 9 spectrometer equipped with an integrating sphere and a microreactor cell (in-house design) was used. The measurements were carried out with a scan speed of 240 nm/min, a slit width of 5.0 nm, a response time of 0.5 s, and Spectralon as the white standard. Powder samples (0.6 g of omSZ or 1.2 g of calcined XZO 682) were charged in the microreactor, activated for 90 min in He or O2 flow at 723 K, cooled down to the reaction temperature (358 to 523 K for n butane, 323 K for n pentane reaction), and fed with a 50 cm3/min flow of 5 vol% n butane or 1 vol% n pentane in He at atmospheric pressure. The apparent absorbance F(R) (Ku-belka-Munk) was calculated and difference spectra were formed from the spectra under catalytic conditions and the activated catalyst at the respective reaction tem-peratures. Overlapping bands are decomposed into Gauss curves (two or three).
Results and discussion
The activity of n-butane on tSZ becomes greater with increasing temperature, while the deactivation diminishes. In contrast, the activity of n-butane on omSZ is compa-rable with that on tSZ at 423 and 523 K in the quasi steady state. However, the deac-tivation is much lower (Tab. 1). The UV/Vis spectra at 358, 423, and 453 K are sym-metrical and those at 323 and 523 K asymmetrical with a band maximum of 290-298 nm (exception: n-pentane on tSZ at 323 K), whereby the total band areas increase with the temperature (Tab.1). In the case of n-pentane isomerization on tSZ the activ-ity is about three times as high as on omSZ; the deactivation, in contrast is approxi-mately the same. Band positions and areas in Tab. 1 were determined directly from the difference spectra (no fitting procedure). The following band maxima were deter-mined for the asymmetrical bands using the fitting procedure: 292, 310, 364 nm for n-butane on tSZ at 523 K; 279, 305, 332 nm and 309, 340 nm for n-pentane on omSZ and tSZ at 323 K. All band areas rise at the beginning while some band areas how-ever have a nearly constant course at 310, 309 nm (n-butane on tSZ at 323 and 523 K) and 332 nm (n-pentane on omSZ at 323 K) in the quasi steady state, the areas of the other bands rise at different rates.
Tabelle 1: Yield, deactivation and spectral characteristics during alkane
isomerization on omSZ and tSZ
catalyst reactant temp/ K yield/ % yield/ % deact./ % TOS/ min band-max./ nm total
band area/a.u.
i-but. i-pent.
max. min. max min. i-but. i-pent.
omSZ n-butane 453 4.3 3.16 0.31 0.15 26.5 51.6 180-745 290 0-1
tSZ 358 5.2 0.5 - - 90.4 - 200-365 294 3
tSZ 423 12.9 2 - - 84.5 - 20-385 296 4.5
tSZ 523 20.8 4.6 - - 77.9 - 2.5
747 298 18.4
95.9
omSZ n-pentane 323 0.057 0.010 0.015 <0.001 82.4 ~100 58
190 298 77.3
tSZ 323 0.198 0.024 5.7 0.45 87.9 92.1 23
300 309 4.3/15 min
73.6/310 min
Conclusions
The UV/Vis band at < 300 nm detected in all isomerizations with the exception of n-pentane on tSZ at 523 K is attributed to cyclopentenyl cations [2]; such species are responsible for the deactivation (coke deposits). No further bands are detectable for the n-butane isomerization on tSZ at T < 423 K and on omSZ at 453 K so that a simi-lar sequence of events can be assumed. At 323 K (n-pentane), 523 K (n-butane) on tSZ and 323 K (n-pentane) on omSZ different reaction pathways are favored which do not lead to the desirable main products but to a series of by-products with band positions > 300 nm.
References
[1] X. Yang, F.C. Jentoft, R.E. Jentoft, F. Girgsdies, and T. Ressler, Catal. Lett. 81 (2002), 25.
[2] X. Yang, R. Ahmad, J. Melsheimer, B. Klose, F.C. Jentoft, in: Proceedings of the DGMK Confe-
rence, G. Emig et al. (Eds), pp 217-224, DGMK, Hamburg, 2002.
