Abstract
Investigating Zirconia Catalysts = Handling Sensitive Materials
B.S. Klose, R.E. Jentoft, A. Hahn, T. Ressler, 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
Sulfated zirconia (SZ) based catalysts are active for n-butane isomerization at 373 K [1]. Despite intense research, no convincing structure-activity relationship for these materials has evolved. Still in question is the role of the bulk phase of ZrO2 which in its room temperature stable form is monoclinic [2], m-ZrO2. The presence of sulfate [3] and the incorporation of cationic promoters such as Mn [4] increase the fraction of the tetragonal phase, t-ZrO2. Transformations between the two phases are achieved for pure zirconia through ball milling [5] and pressing [6]. Our research focused on the influence of mechanical stress treatments on the catalytic activity. For SZ and manganese promoted SZ (MnSZ) the effect of milling, grinding, and pressing, on the bulk phase composition and the catalytic activity was investigated [7].
Experimental
The precursor was (NH4)2SO4 doped hydrous zirconia (XZO 682/01, MEL Chemi-cals). The promoter was introduced by the incipient wetness method to give a Mn content of 0.5 or 2.0 wt%. The raw materials were calcined in 17 ml batches [8] in flowing air for 3 hours at 823 (SZ) or 923 K (MnSZ). For pressing, flat-surfaced stainless steel tools and a manually operated hydraulic press were used; milling was performed in a vibrating mill using 1.5 ml stainless steel capsules and 300 mg of sample (all Perkin-Elmer). Grinding was performed manually in an agate mortar. X-ray diffractrograms were recorded using a STOE STADI-P-diffractometer, Debye-Scherrer geometry, and Cu K radiation. Catalytic tests were run in a fixed bed flow reactor. Samples (500 mg) were activated at 723 K in N2. The feed was 1.0 vol.% n-butane in N2 (80 ml min-1) at atmospheric pressure; reaction temperatures were 378 K (SZ), 323 K (2 wt% MnSZ), or 338 K (0.5 wt% MnSZ). The effluent stream was analyzed by on-line GC with flame ionization detection. Further characterization in-cluded surface area and thermal analysis.
Results and discussion
During 10 min pressing at 540 MPa the phase composition of SZ changed from t-ZrO2 to 33 wt% of m-ZrO2. Also at more moderate pressing conditions (130 MPa, 1 min) an increase in the monoclinic fraction was detectable. For treatment of SZ and MnSZ (2.0 wt% Mn) in a vibrating mill the fraction of m-ZrO2 increased with increas-ing milling time from <5 wt% in the original sample to ca. 30 wt% after 10 min of mill-ing. Grinding 0.5 wt % MnSZ in a mortar for 10 min resulted in a partial transition of t-ZrO2 to m-ZrO2; the fraction of m-ZrO2 was increased from about 8 wt% to either 19 or 57 wt%, depending on the operator.
Catalytic tests focused on untreated, ground, or milled samples because these mate-rials need no further sample manipulation. In all cases the catalytic performance was severely reduced by treatment. For milled SZ the maximum isobutane formation rate was only about 25% of its value for untreated SZ (Fig. 1). The initial maximum isobu-tane conversion for 2 wt% MnSZ was similarly reduced by milling; the long term ac-tivity (>2 h) remained unaffected. Ground 0.5 wt% MnSZ showed a prolonged induc-tion period. The isomerization rate was never more than 30% of values measured for the untreated catalyst.
For many experiments (e.g. IR spectroscopic), powders need to be pressed or ground. All of these "standard laboratory practice" treatments alter not only the structural, but also the catalytic properties of sulfated zirconia catalysts. This can lead to problems in data correlation because data obtained in experiments that involve different sample handling procedures can reflect the properties of different materials.
References
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[7] B.S. Klose, R.E. Jentoft, A. Hahn, T. Ressler, J. Kröhnert, S. Wrabetz, X. Yang, F.C. Jentoft, J. Catal., accepted.
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