耐水热与毒素的费托蜡合成用纳米复合催化材料

Cobalt-based catalytic materials have importantant applications in Fischer–Tropsch synthesis (FTS) of wax products. The FTS process is usually operated with significantly high steam pressure. Under the consideration of the sources of syngas, the research frontier of Fischer–Tropsch catalysis has been pushed towards processing of coal/biomass-derived syngas, which urges the studies on poison-resistance. Traditional cobalt-based FTS catalytic materials load cobalt over simple oxide supports (e.g., Al2O3、SiO2), which may not be durable to the hydrothermal environments and poisons of new FTS conditions. This proposal will solve two scientific problems, of which one is the improvement of steam-resistance of the FTS catalytic materials and the other one is the enhancement of the materials’ poison-resistance. A fomulation of “Co/spinel-type complex oxide support/ poison-resistant promoter” is proposed to construct the FTS catalytic materials. Complex oxides (e.g., spinels) have better resistance against steam than simple oxides and thus a spinel-type complex oxide (e.g., ZnAl2O4) will be used to support cobalt and gain better steam-resistance. The poison-resistant promoter, ZnO, will be involved in the catalytic materials to enhance their poison-resistance. The combination of coprecipitation and stepwise reduction may further the well dispersion of nanosized components of catalytic composites, relieve the sintering of Co crystallites, and avoid the formation of cobalt-containing spinels. This research may significantly improve the cobalt-based FTS catalytic materials’ resistance to steam as well as poisons. Therefore, this research is of scientific significance and has application values.

钴基催化材料在费托蜡合成领域有重要应用。费托反应体系存在较高水蒸汽分压；对合成气来源的考虑又把费托催化研究前沿推进到加工源自煤和生物质的合成气，使其抗毒研究愈显迫切。传统钴基费托催化材料把钴担载在简单氧化物（如Al2O3、SiO2）上，不利于长久耐受新费托反应条件的水热环境和毒素。本项目重点解决钴基费托催化材料的耐水热性能提高和抗毒性能提高这两个科学问题。提出构筑“Co/尖晶石型复合氧化物载体/抗毒助剂”费托合成催化材料，即利用复合氧化物更耐水热环境的特点，把钴担载在尖晶石型载体上（如ZnAl2O4），提高耐水热性能；引入抗毒助剂（如ZnO），提高抗毒性能；使用共沉淀制备法，结合还原气氛精密热处理，实现复合催化材料各纳米尺度组分的高度分散，减轻Co晶粒的烧结，并避免生成含钴尖晶石相。通过本项目的研究，可望显著提高钴基费托催化材料对水热环境和毒素的耐受性。因此，具有重要的科学意义和研究价值。

Zn掺杂可提高Co/(Al)–O–(Zn)复合材料的表面碱度，从而能够对活性和选择性产生重要影响。Zn含量过高时催化剂在费托合成反应条件下结构不够稳定，而Zn含量适中的Co/ZnAl2O4•Al2O3催化剂则结构非常稳定。与Co/γ-Al2O3相比，Co/ZnAl2O4•Al2O3催化剂具有很低的甲烷烃产物分布、相差不多的C5+烃产物分布、较高的C2-C4烃产物分布以及显著增高的C2-C4烯烷比，因此适合FTS过程。原位水添加可抑制加氢及促进C–C耦合，导致CH4和C2-C4烃分布降低，而C5+烃分布和C2-C4烯烷比则升高。水对费托合成生成长链烃的反应具有自催化作用。Co/ZnAl2O4•Al2O3催化剂在原位添加水的费托合成中表现出优异的耐水热烧结能力。原位导入100ppmv当量硫的CS2能够对Co/ZnAl2O4•Al2O3和Co/γ-Al2O3产生毒化作用。引入0.1 ppmv当量S对Co/ZnAl2O4•Al2O3催化剂的活性有轻微促进作用；引入1 ppmv当量S时没有显著的负面影响；引入10 ppmv当量S时有轻微负面影响；引入100 ppmv当量S时有显著负面影响；引入300 ppmv当量S时具有破坏性影响。引入0.1～100 ppmv当量S对Co/ZnAl2O4•Al2O3催化剂的结构没有明显的影响，而引入300 ppmv当量S则可以分解载体中的ZnAl2O4相。引入100ppmv当量S可以降低Co/ZnAl2O4•Al2O3的CO转化率、降低CO2选择性、抑制重质烃(C5+)的生成，同时会提高C2-C4烯烷比、促进轻碳烃(C1-C4)的生成。在100ppmv当量硫添加量下，Co/ZnAl2O4•Al2O3催化剂的硫吸收速率为Co/γ-Al2O3催化剂的硫吸收速率的3.1倍。低浓度当量硫(0.1~1ppmv)时Co/ZnAl2O4•Al2O3催化剂能够从合成气中吸收显著更高比例的硫。Co/ZnAl2O4•Al2O3催化剂的硫中毒失活具有一定程度的可逆性。Co/ZnAl2O4•Al2O3很可能成为装载在固定床顶端清除合成气中硫杂质的催化剂(或吸附剂)，具有水汽变换活性、甲烷选择性低、C2-C4烯烷比高、C5+选择性适宜以及耐水热和硫毒的特性，无需额外的水汽变换单元来提高H2与CO的比例，是很有前景的可直接从煤/生物质基富CO合成气制取有机产品的催化剂。