高烯烷比、低甲烷、耐热且组成稳定的FTO用纳米复合催化材料及其作用机理

The Fischer-Tropsch synthesis coal-to-oil process prefers the production of long-chain hydrocarbons, while the Fischer-Tropsch to lower olefins (FTO) process prefers the production of short chain hydrocarbons with high olefin/paraffin ratios. Both the FTS process and the FTO process wish to suppress the formation of methane. Although internationally advanced Na and S modified catalytic materials for FTO exhibit good product selectivities, problems emerge from the gradual leaching of these promoters. This proposal will solve two scientific problems, of which one is to gain high product selectivities of the catalytic materials for FTO and the other one is to achieve high stability of compositions of these materials. A formulation of “Fe/Al–O–M (M = Zn, Mg, Ca, Sr, Ba, Mn)” is proposed for the catalytic materials. Doping the Al–O support with suitable amount of M–O Lewis base may neutralize the Lewis acidity of the support phase and weaken the promotional effects on both hydrogenation and C–C coupling caused by original metal/Lewis acidic support interfaces. As a result, high product selectivities such as high olefin/paraffin ratio, less methane and more short-chain hydrocarbons may be achieved. By reacting the M–O and Al–O species, nanosized MAl2O4 spinel phases that are water-insoluble and chemically/thermally stable may be generated to completely avoid the loss of components and simultaneously enhance the sintering-resistance. The feasibility of this idea is supported by advance research results. This research may achieve both high product selectivities and high stability of compositions of catalytic materials for FTO as well as analysis of mechanisms. Therefore, this research is of scientific significance and has application values.

费托合成煤制油过程需要富产长链烃，而费托合成制低碳烯烃(FTO)过程则需富产高烯烷比的短链烃。两者共同点在于希望少产甲烷。目前国际领先的Na、S改性FTO催化材料虽选择性高，但是存在助剂因具有溶水性而逐渐流失的问题。本项目重点解决FTO催化材料获取高选择性和组成高稳定性这两个科学问题。提出构筑“Fe/Al–O–M (M=Zn,Mg,Ca,Sr,Ba,Mn)”催化材料，即通过掺杂适量的M–O固相Lewis碱中和Al–O载体上的Lewis酸，削弱原来的金属/Lewis酸性载体界面对于加氢及C–C键偶联的促进作用，实现提高烯烷比、抑制甲烷生成、增多短链烃的高选择性；通过使M–O与Al–O生成不溶于水且化学及热稳定的纳米MAl2O4尖晶石相，杜绝组分流失，并提高耐热性。预研结果支持可行性。通过本项目研究，可望实现FTO催化材料的高选择性、组成高稳定性及机理分析。因此，具有重要的科学意义和研究价值。

构筑了复合氧化物载体担载的“Fe/(K)/Al–O–M (M=Zn, Mg, Ca, Sr, Ba, Mn)”催化材料，探索了载体的1200℃高温钝化方法，探明了表面碱性与催化性能之间的构效关系，从而开发出了高选择性、耐热且组成稳定的费托合成制低碳烯烃（FTO）用钝化型含锌尖晶石载铁催化剂。首先研究了普通煅烧条件下（即350℃）(Al)–O–(Zn)载体担载铁、担载钴的性能，发现纯ZnAl2O4尖晶石相能够转化合成气，但也发现其缺乏C–C偶联能力，而铁比钴更适于FTO反应。适当提升Zn/Fe原子比（例如，达到1.11并且名义组成为15%Fe/85%ZnAl4O7）可以有效地抑制C5+烃的产生；在较低温度下还原加上温和的碳化可以使铁基催化剂更有效地抑制C–C偶联。开展了ZnAlCrO4尖晶石相担载SAPO系磷铝分子筛的尝试，发现混粒工艺制备FTO复合催化剂可以有效保留SAPO-34分子筛组分的晶体结构。通过共沉淀后350℃煅烧，再浸渍K制备了(Zn)–O–(Al)尖晶石负载的钾促进型铁基催化剂，发现钾的添加可去除铁基催化剂表面的强碱性位点，增加弱碱性位点的数量和强度；ZnAl2O4相可以有效抑制C–C耦合；2%K2O含量催化剂的C2=-C4=烃类选择性可达53.4%。采用1200℃高温来部分钝化ZnAl2O4•xAl2O3（x = 1, 2, 3）载体，制备了简化型“铁/钝化载体”催化剂，发现高温钝化的功能不是抑制加氢能力，而是抑制碳链增长；钝化后的ZnAl2O4相亦可抑制碳链增长；钝化后的氧化铝相可起到固体碱和抑制加氢的作用；催化剂的C2-C4烯烷比与CO2-TPD高温峰、低温峰强度比呈正相关；发现1200℃钝化的ZnAl2O4•3Al2O3是铁基FTO催化剂中最具低碳烯烃选择性的载体材料，载铁后形成的无助剂催化剂具有达到58%的C2=-C4=烃类选择性。引入钾助剂后可进一步有效抑制二次加氢，显著提高烯烷比；所制15%Fe/2%K2O/83%(ZnAl2O4•3Al2O3)催化剂的C2=-C4=烃类选择性高达64.4%，C2、C3、C4中烯烃含量分别达到88%、92%、89%。对其他掺杂元素所制催化剂也开展了FTO测试，发现Zn是最适于FTO的元素， C2=-C4=烃类选择性最大值次序为Zn > Mg > Mn > Ca > Ba > Sr（以掺杂元素代表催化剂）。