Isosintesis

Journal of Molecular Catalysis A: Chemical 175 (2001) 267–275

Effect of calcium salts on isosynthesis over ZrO2 catalysts
Yingwei Li, Dehua He∗ , Zhenxing Cheng, Caili Su, Junrong Li, Qiming Zhu
State Key Laboratory of C1 Chemistry and Technology, Department of Chemistry, Tsinghua University, Beijing 100084, China Received 27 January 2001; received in revised form 24 April 2001; accepted 22May 2001

Abstract The promoting effects of various calcium salts on the activity and selectivity of ZrO2 in isosynthesis were studied in this work. Calcium salts were added into zirconia by mechanical mixing methods. Catalytic tests were performed under relatively mild operation conditions (673 K, 650 h−1 , 5.0 MPa). CaF2 and CaSO4 were found to be effective additives, which could remarkablyenhance the i-C4 selectivity in total hydrocarbons while maintaining the activity of pure ZrO2 when being added into zirconia. However, Ca(NO3 )2 , Ca(BO2 )2 and CaCl2 wholly changed the distribution of hydrocarbons to favor the methanation. The results of temperature-programmed desorption (TPD) of NH3 and CO2 indicated that the performance of the catalysts depended on the acid–base properties ofthe catalysts. The appropriate amount of acid and base and the ratio of the basic to acidic sites on the catalysts are significant for the synthesis of i-C4 hydrocarbons from CO hydrogenation. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Isosynthesis; Zirconia; Calcium salts; Promoting effect; Acid and base properties

1. Introduction Isosynthesis is the reaction that convertssynthesis gas (CO + H2 ) to isobutene and isobutane (i-C4 ). It was first reported by Pichler and Ziesecke [1–4] using several transition metal oxides (ThO2 , CeO2 ) or mixed-oxide systems (ThO2 –Al2 O3 , ThO2 –ZnO and so on) to catalyze CO hydrogenation under very severe conditions (150–1000 atm). Zirconia has been shown to be active in the isosynthesis with high selectivity for isobutene [5]. It hasthe advantage over thoria because of no radioactivity. So recent investigations on the activity and selectivity of isosynthesis have been performed over zirconium dioxide-based catalysts. Maruya et al. [6] studied the promoting effects of various oxide compounds. They found that the
∗ Corresponding author. Fax: +86-10-6277-0304. E-mail address: dchzqm@mail.tsinghua.edu.cn (D. He).

addition ofhighly electronegative oxide such as SiO2 and Nb2 O5 resulted in the decrease of isobutene and the increase of methane. ZrO2 modified with NaOH could promote the selective formation of C4 hydrocarbons (about 85%) while decreasing overall activity. They suggested that the electronegativity of the dopant oxide was significant for the formation of C4 hydrocarbons, especially isobutene. This work wasperformed at very low conversions (0–5%). Jackson and Ekerdt [7] investigated the effect of oxygen vacancies and acidity on the isosynthesis reaction conducted at 698 K and 35 atm. The oxygen vacancy availability was altered by the addition of dopants Y2 O3 and CaO in varying concentrations. The acid–base strength of the catalysts was altered by additives including H2 SO4 , Sc2 O3 and etc. The mostactive catalysts were those in which ionic conductivity was at a maximum, suggesting that vacancies in the crystal lattice play an important role in the reaction.

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Y. Li et al. / Journal of Molecular Catalysis A: Chemical 175 (2001) 267–275

A balance ofacidic and basic properties on ZrO2 was suggested to be necessary for isobutene formation. But in a similar manner to the previous work by Pichler and Ziesecke over a 20% Al2 O3 /ThO2 catalyst [1], methane was the predominant product with a low i-C4 selectivity in total hydrocarbons. Postula et al. [8] prepared zirconia-based catalysts by precipitation or hydrothermally method and tested the...