English
 
User Manual Privacy Policy Disclaimer Contact us
  Advanced SearchBrowse

Item

ITEM ACTIONSEXPORT
 
 
DownloadE-Mail
  In situ diffuse reflectance UV/Vis spectroscopy investigations of alkane activation catalyzed by sulfated zirconia

Ahmad, R., Melsheimer, J., & Jentoft, F. C. (2002). In situ diffuse reflectance UV/Vis spectroscopy investigations of alkane activation catalyzed by sulfated zirconia. Poster presented at XXXV. Jahrestreffen Deutscher Katalytiker, Weimar.

Item is

Basic

show hide
Item Permalink: http://hdl.handle.net/11858/00-001M-0000-0011-1575-5 Version Permalink: http://hdl.handle.net/11858/00-001M-0000-0011-1575-5
Genre: Poster

Files

show Files
hide Files
:
ahmad_weimar_2002.pdf (Any fulltext), 31KB
Name:
ahmad_weimar_2002.pdf
Description:
-
Visibility:
Public
MIME-Type / Checksum:
application/pdf / [MD5]
Technical Metadata:
Copyright Date:
-
Copyright Info:
-
License:
-
:
rafat_poster_weimar20023.ppt (Any fulltext), 434KB
 
File Permalink:
-
Name:
rafat_poster_weimar20023.ppt
Description:
-
Visibility:
Restricted ( Max Planck Society (every institute); )
MIME-Type / Checksum:
application/vnd.ms-powerpoint
Technical Metadata:
Copyright Date:
-
Copyright Info:
-
License:
-

Locators

show

Creators

show
hide
 Creators:
Ahmad, Rafat1, Author              
Melsheimer, Jörg1, Author              
Jentoft, Friederike C.1, Author              
Affiliations:
1Inorganic Chemistry, Fritz Haber Institute, Max Planck Society, ou_24023              

Content

show
hide
Free keywords: Project 1: Acid-base & redox properties of promoted sulfated zirconia
 Abstract: In Situ Diffuse Reflectance UV/Vis Spectroscopy Investigations of Alkane Activation Catalyzed by Sulfated Zirconia Rafat Ahmad, Jörg Melsheimer, Friederike Jentoft, Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany Introduction Sulfated zirconia (SZ) is highly active for n-butane isomerization [1,2] but deactivates rapidly. Because isomerization and disproportionation of n-butane seem to occur concomitantly it is not possible to recognize if deactivation is the result of one or the other reaction. For n-pentane, isomerization and disproportionation activity reach their maximum at different times on stream, allowing the separate observation of mono- and bimolecular reaction pathways. In situ UV/Vis spectroscopy has been used to study the interaction of n-butane and n-pentane with SZ. Experiments and Summary of Results In situ diffuse reflectance measurements during alkane reaction were conducted using a home-made microreactor cell with a quartz window facing the integration sphere. The setup was fitted into a modified Perkin–Elmer Lambda 9 UV/Vis spectrometer, operated at a scan speed of 240 nm/min, a slit width of 5.0 nm, a response time of 0.5 s, and with Spectralon® as a reference. SZ was obtained by calcining sulfated zirconium hydroxide (MEL Chemicals) in flowing air for 3 h at 823 K. The calcined catalyst (≈1.3 g) was loaded into the cell and activated in 30 ml/min O2 for 1.5 h at 723 K. The feed mixture was 5 vol-% n-butane in He or 0.25 or 0.50 vol-% n-pentane in He with a total flow of 50 ml/min at reaction temperatures of 358 – 523 K for n-butane and 298 – 308 K for n-pentane. Product analysis was performed by on-line GC with FID. UV/Vis spectra showed a single broad band centered at 328 nm, which started to form after about 90 min of n-pentane reaction; the final intensity of this band (after 13 h) was independent of n-pentane concentration and reaction temperature. The formation of a band at 310 nm was detected during n-butane reaction (all temperatures, inset in Fig. 1). This band was previously observed [3] on deactivated sulfated zirconia and assigned to monoenic allylic cations. At 523 K, additional bands appeared at 370 nm and 430 nm after 20 min and 6 h on stream, respectively. Analysis of the product stream during n-pentane reaction revealed an initial carbon loss of 16 to 44 %, suggesting considerable adsorption without the formation of spetroscopically detectable species. At 298 K and 0.25 vol-% n-pentane, the maximum rate of formation of n-pentane preceded the maximum in production of i-butane (Fig. 2). Hexanes, n-butane, and propane were byproducts. Increasing the n-pentane concentration to 0.50 vol-% did not change the conversion but led to an increase in i-pentane selectivity and a decrease in i-butane selectivity. Interestingly, as the rate of disproportionation became significant (Fig. 2), a band at 330 nm appeared in the UV/Vis spectrum (Fig. 1). In the reaction of n-butane, the maximum conversion and the rate of deactivation increased with increasing temperature while the selectivity towards i-butane decreased (range 70 to 99 %). Conclusions The broad UV/Vis absorption band formed during n-pentane reaction is presumably composed of two bands, maybe representing different isomers of monoenic allylic cations. These bands reach significant intensity only after the rate of i-pentane formation passes through a maximum, excluding these allylic species from being identified as isomerization intermediates (Figs. 1 & 2). During the induction period, i-pentane is predominantly formed through a monomolecular mechanism. The formation of i-butane at longer times on stream indicates a bimolecular mechanism, i.e. the disproportionation of C10-intermediates, which are formed in an alkylation step requiring alkenes. However, this bimolecular pathway is disadvantageous because the alkenes generated by the catalyst are precursors for site-blocking species. 1. M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc. 101 (1979) 6439. 2. F.C. Lange, T.-K. Cheung, B.C. Gates, Catal. Lett. 41 (1996) 95. 3. D. Spielbauer, G.A.H. Mekhemer, E. Bosch, H. Knözinger, Catal. Lett. 36 (1996) 59. 4. S. Rezgui and B. C. Gates, Catal. Lett. 37 (1996) 5.

Details

show
hide
Language(s): eng - English
 Dates:
 Publication Status: Not specified
 Pages: -
 Publishing info: -
 Table of Contents: -
 Rev. Type: -
 Identifiers: eDoc: 1766
 Degree: -

Event

show
hide
Title: XXXV. Jahrestreffen Deutscher Katalytiker
Place of Event: Weimar
Start-/End Date: 2002-03-20 - 2002-03-22

Legal Case

show

Project information

show

Source

show