t-ブチルベンゼン分解反応におけるシリカアルミナ触媒の活性劣化
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シリカアルミナ触媒を用いてトプチルベンゼソの分解反応を行い，活性劣化速度および反応終了後採取した使用済触媒の比表面積および細孔分布を測定した。その結果，比表面積，細孔分布は反応開始後1時間以内に大きく変化し，その後ほぼ一定となることがわかった。さらに，使用済触媒の気相ピリジン吸着量の吸着温度依存性を調べ，以下の点を明らかにした。すなわち，吸着量の温度係数は反応時間と共に大きくなり，コーク沈着量に対してほぼ直線的に増大する。これらの結果から本反応に対する触媒の活性劣化は，触媒の物理的形状の変化によらず，酸性点，特に大きな酸強度を有する酸点がコークにより被覆されることに起因することがわかった。The vapor phase de-i-butylation of t-butylbenzene has been carried out over a commercially obtained silicaalumina catalyst in a fixed-bed reactor under the following conditions: reaction temperature, 573-623 K;contact time (W/v), 18-180 kg-s-mol-1; diluent, H2 or N2; and feedstock/diluent ratio, 12/88. Variation in surface area, micropore distribution, acidity, and acid strength distribution of the catalyst with process time were also measured to obtain information about catalyst de activation. The relationship between rate constants in de-t-butylation and process time (Fig. 4) is represented by Eq. (3), and the values ofk0 and a are summarized in Table 1, where k0 is a rate constant at zero coke deposit and a is the deactivation coefficient. The value of k0 obtained under H2 dilution is twice that obtained under N2 dilution, but a under H2 dilution is almost the same as that under N2 dilution. The coke content deposited on the used catalyst at process time 12 ks was measured with a thermanogravimetric balance. Effects of W]v, reaction temperature, and kinds of diluents on coke content are shown in Fig. 5. The content does not depend upon Wjv and reaction temperature, but it is dependent upon diluents; under H2 dilution, it decreases to about two thirds of that under N2 dilution. A typical linear plot of coke content against process time on a log-log scale is shown in Fig. 6. The content can be represented by process time Eq. (4). The slope of the line in Fig. 6 becomes about 0.5 as suggested bv Voorhies.15) The nitrogen adsorption isotherm of the catalyst was measured, and the surface area and micropore distribution, were determined by the BET and Cranston-Inkley methods, respectively. The relationship between surface area and process time is shown in Fig. 7. It decreased about 10% within the process time of 1ks, but after that no significant decrease was observed. The micropore distribution curves shown in Fig. 8 changed only in the early periods of the reaction due to blockage of the fine micropores less than 3nm by coke deposition on the catalyst. Fig. 9 shows a typical linear relationship between adsorption temperature and the amount of pyridine chemisorbed by the catalyst. The amount of chemi-sorbed pyridine decreased with increasing process time. Fig. 10 gives the relationship that can be represented by Eq. (5), between chemisorbed pyridine and adsorption temperature. In Eq. (5), Q. calculated from the slopes of the lines in Fig. 10 means the temperature dependency factor of the amount of chemisorbed pyridine. Since Q, represents the acid strength distribution of the catalyst, the fact that the same Q_can be used for used catalyst as well as for fresh one implies thatcatalyst deactivation occurs uniformly throughout the whole range of acid strength. However, it is found that values of Q_ of used catalysts tend to increase with the process time as shown in Table 3. It means that sites of stronger acid strengths rather than those of weaker acid strengths are attacked by the coke. This consideration is supported by the linear relationship found between Q_ and coke content in the used catalyst as shown in Fig. 11.
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