用TPD、TPSR研究苯、丙烯和异丙苯在Hβ沸石上的吸附和脱附行为

STUDY ON THE ADSORPTION AND DESORPTION BEHAVIOR OF BENZENE, PROPENE AND CUMENE ON Hβ ZEOLITE WITH TPD AND TPSR TECHNIQUES

用程序升温脱附(TPD)和程序升温表面反应(TPSR)技术研究了苯,丙烯和异丙苯在Hβ沸石上的吸附和脱附行为。结果表明,苯的脱附峰为一单峰,T_M值为260℃。在TPSR过程中,丙烯和异丙苯在Hβ沸石上分别发生聚合和裂解反应。用氨中毒法研究表明,强酸中心对聚合和裂解反应影响明显。文中讨论苯在Hβ沸石上的吸附模型和改进催化剂稳定性的可能途径。

The adsorption and desorption behavior of benzene, propene and cumene on Hβ zeolite was studied with the apparatus as shown in Fig. 1 . After flowing through the thermal conductivity cell, the desorbed products from Hβ zeolite at different temperature were collected by gas-collecting tubes. Then, gas chromatography was used to analyze the desorbed products qualitatively and quantitatively. TPSR curves were obtained by plotting the response values of the G.C. peaks against the desorption temperatures. The thermal desorption curve of NH3 for H@ zeolite at 393 K has two peaks. One is a sharp peak, representing weak acidic centres on HP zeolite. Another is a small broad one, representing strong acidic centers. The desorption amount of NH3 ( adsorbed at 393 K ) was considered as representing the amount of total acidic centers on Hβ zeolite. The TPSR spectrum of benzene is a symmetric single peak and Tm is 533 K. As NH3 is pre-adsorbed on Hβ zeolite, the Tm values increases. A model of benzene adsorption on HP zeolite is put forward to explain this. The interaction energy between benzene and Hβ zeolite is considered to be the mutual attraction between the π electrons of benzene and the acidic centers on Hβ. Benzene adsorption on the clean HP zeolite is a one-site adsorption, that is, one molecule of benzene is adsorbed at one acidic center. Benzene adsorption on the NH3 pre-adsorbed HP zeolite is a two-site adsorption, that is, one molecule of benzene is adsorbed between an acidic center and a NH4+ complex center. The interaction energy of a two-site adsorption is certainly greater than that of a one-site adsorption. So the values of Tm increase. The propene adsorbed on HP zeolite polymerizes into a dimer during the TPSR process.Desorption curve of propene has a single peak and the value of Tm is 623 K, while desorption curve of the dimer has double peaks and the values of Tm are 563 K and 673 K respectively. Pre-adsorption of some NH3 on HP zeolite decreased the amount of dimer formed as shown by the spectra of TPSR. When Hβ zeolite is poisoned by NH3 entirely ( pre-adsorption of NH3 on Hβ zeolite at 393 K), there is neither propene nor its dimer flowing out in the TPSR process. This seems to mean that propene is only adsorbed on the acidic centers and that eliminating some acidic centers (especially strong acidic centers) can reduce the dimerization of propene. There is no cumene flowing out in the TPSR process. Cumene adsorbed on HP zeolite is cracked into benzene and propene. This means that adsorption of cumene on the HP zeolite is very strong and cracking of it is easier than its desorption from HP zeolite. Pre-adsorption of HN3 on HP zeolite raises the flowing out temperature of the cracking products. Obviously, cracking reaction occurs easily at the strong acidic centers. Dimerization of propene and cracking of cumene are both disadvantageous to the alkylation of benzene and propene. Polymerization of propene on the HP zeolite affetcs the life of zeolite catalyst greatly. So eliminating some strong acidic centres on the HP zeolite can raise the yield of cumene and improve the stabilization of HP zeolite catalyst.