催化裂化条件下噻吩与改性Y分子筛的作用机制

以HY、NiY和稀土离子改性的Y分子筛(REY)为研究对象,采用固定床装置评价噻吩模拟油催化裂化性能;运用气相色谱-氢火焰离子发光检测器(GC-FID)、气相色谱-硫化学发光检测器(GC-SCD)和原位红外光谱技术分析产物,关联分子筛的酸性,研究催化裂化条件下噻吩与改性Y分子筛的作用机制。实验结果表明,催化裂化条件下,噻吩与分子筛的作用机制差异主要取决于与B酸或L酸相关的非骨架铝物种或金属离子物种的存在形式。其中,NiY分子筛中,噻吩主要是吸附在与NiOH⁺物种相关的L酸中心,而Ni₄AlO₄³⁺等物种减弱B酸性中心从而降低其裂化性能。对HY来说,噻吩易在与AlO⁺等物种相邻的B酸中心上聚合形成三联噻吩,并发生一定的氢转移和裂化反应;而对REY而言,分子筛中与RE物种相关的L酸位会促进噻吩在与非骨架铝羟基等物种(如Al(OH)₂⁺、Al(OH)²⁺等)相邻的B酸中心形成的二联噻吩发生氢转移和裂化反应。

Mechanisms of thiophene conversion over the modified Y zeolites under catalytic cracking conditions

The mechanisms of thiophene conversion over various Y zeolites, viz., HY, Ni-modified NiY and rare earths metal-modified REY, were investigated under catalytic cracking conditions in a fix-bed reactor. The final products of thiophene conversion were analyzed by using a gas chromatography with flame ionization detector (GC-FID), a gas chromatography with sulfur chemiluminescence detector (GC-SCD) and an in situ infrared spectrometer. The results were then correlated with the acidic properties of the zeolites, which indicated that the mechanisms of thiophene interacting with these Y zeolites are dependent on the state and occurrence mode of the modified metallic cation and/or the extraframework aluminum species, which can modulate the Brnsted and Lewis acidities in the zeolites. For the NiY zeolites, the thiophene molecules are mainly adsorbed on the Lewis acidic sites associated with the NiOH⁺ species; the species of Ni₄AlO₄³⁺ and those generated from the Ni²⁺ and the extraframework aluminum can weaken the Brnsted acidity and then reduce the cracking activity of the zeolite. Polymerization and cracking reactions are observed for the conversion of thiophene over the HY and REY zeolites. For the HY zeolites, 2,2',5',2'-terthiophene can be found, which is ascribed to the Brnsted acid sites adjacent to extraframework aluminum species (AlO⁺ or so on); some subsequent reactions of hydrogen transfer and cracking can also be detected. Over the REY zeolite, hydrogen transfer and cracking reactions of thiophene oligomer species derived on the Brnsted acid sites adjacent to extraframework aluminum species (Al(OH)₂⁺, Al(OH)²⁺ and so on) can be promoted by the Lewis acid center related to the RE species.