【作者】 赵燕熹；

【导师】 李金林；

【作者基本信息】 苏州大学 ， 物理化学， 2014， 博士

【摘要】 费-托合成(Fischer-Tropsch synthesis，FTS)是将煤、天然气或生物质间接转化为液体燃料的重要工艺过程。尽管对费-托合成反应的研究已经有九十多年历史，但费-托反应是一个较复杂的反应体系，同时随着世界对能源需求的增加，费-托合成反应的研究也越来越受到研究者的关注。目前研制开发具有高活性、高稳定性和合理选择性的费-托合成催化剂仍是费-托合成研究的焦点。本文工作以P123和F127为模板剂，采用水热法制备了有序介孔分子筛SBA-16，同时采用两步‘调pH法’，制备了铝掺杂的SBA-16，并以此为载体负载钴基费-托合成催化剂。采用粉末X-射线衍射(XRD)、氮气物理吸附-脱附、透射电子显微镜(TEM)、氢气程序升温还原(H2-TPR)、氢气程序升温脱附(H2-TPD)、氧滴定、氨气程序升温脱附(NH3-TPD)、固体核磁(ssNMR)等技术对催化剂进行了表征。在固定床反应器和浆态床反应器中系统地考察了催化剂的费-托合成反应的活性、稳定性以及产物的选择性，研究所得的结论如下：(1)以SBA-16为载体采用满孔浸渍法制备的钴基催化剂，Co3O4纳米颗粒被负载到了SBA-16的三维笼状孔道中，平均粒径大小约为12nm，未随着钴负载量的增加而发生明显变化。在固定床反应器中考察了其费-托合成反应活性，随着钴负载量的增加，CO的平均转化率先增加后变化不大。与SiO2载体相比，SBA-16负载的催化剂表现出了较高的活性。(2)在固定床反应器中分别考察了20%Co/SBA-16和20%Co/SiO2催化剂在高转化率情况下的费-托合成反应的稳定性，与SiO2相比SBA-16负载的钴基催化剂表现出了较高的催化活性和反应的稳定性。这是由于SBA-16负载的催化剂，钴物种的分散度更高，钴颗粒由于受到SBA-16载体笼状结构的限制，颗粒较难通过笼之间的小孔发生迁移、团聚以及进一步的烧结，因而显示了高的催化活性和稳定性。而SiO2负载的催化剂，Co3O4纳米颗粒大部分都负载在二氧化硅载体的表面，在反应的过程中活性金属发生团聚和烧结，催化反应以后钴颗粒变大。在浆态床反应器中分别考察了在加水的情况下SBA-16、SiO2和SBA-15负载的钴基催化剂的活性和稳定性。SBA-16和SBA-15负载的催化剂，水的加入降低了反应物和产物在孔道内的扩散限制，增加了CO的转化率。SiO2负载的钴基催化剂，由于钴物种负载到载体的表面，水的加入未增加CO的转化率。停止加水后SBA-16负载的钴基催化剂仍保持了较高的活性和稳定性。在整个反应过程中，SBA-15和SiO2负载的钴基催化剂，均呈现了失活的趋势。SBA-16的三维笼状孔道结构不仅有利于金属的分散，阻止催化剂金属的团聚，而且有利于反应物和产物在孔道内的传输。(3)采用两步调pH的方法制备了具有不同铝含量，相同的孔径大小的Al-SBA-16载体；在载体中铝主要以四配位的形式存在；载体的酸性随着铝含量的增加而增加；负载钴基催化剂后，钴物种具有相同的颗粒大小，并未随着铝含量的增加而发生较大变化；其还原度随着铝含量的增加而减小，而分散度变化不大。费-托合成反应结果表明，随着铝含量的增加催化剂的活性和C5+选择性逐渐减小，甲烷的选择性和烯烃/烷烃比逐渐增加。汽油(C5–C12)段产物的含量也随着铝含量的增加而增加，产物选择性随铝含量变化的主要原因是由于Al-SBA-16载体的酸性，阻止了α-烯烃在活性位上进一步的吸附以及链增长。更多还原

【Abstract】 The Fischer–Tropsch synthesis (FTS) is an important technology for the production ofclean transportation fuels and chemicals from syngas (CO+H2). Although it was firstdeveloped90years ago, some challenges still remain. The development of FTS catalystswith high activity, high stability, and, in particular, high selectivity, remains one of the keygoals of current research in this area.In this paper, ordered mesoporous molecular sieves SBA-16have been synthesizedusing P123and F127as template agent. Aluminum-substituted mesoporous SBA-16(Al-SBA-16) materials were also synthesized using “pH-adjusting” method. The catalystswere prepared by using incipient wetness impregnation method and characterized by usingX-ray diffraction(XRD), nitrogen adsorption-desorption, transmission electronmicroscopy(TEM), hydrogen temperature programmed reduction(H2-TPR), hydrogentemperature programmed desorption(H2-TPD), oxygen titration, ammonia temperatureprogrammed desorption(NH3-TPD) and solid-state27Al NMR(ssNMR). The catalyticproperties scuh as activity, stability and selectivity for FTS were investigated in afixed-bed reactor and a continuously stirred tank reactor (CSTR). The main conclusionsare as follows:(1) For SBA-16supported cobalt catalysts, most of the Co3O4nanoparticles areuniformly distributed inside the interconnected cages of SBA-16. The average diameter ofthe nanoparticles is12nm, which does not increase largely with cobalt loading. Thecatalytic activity of the Co/SBA-16and Co/SiO2catalysts was studied in a fixed-bedreactor. The activity increases with increasing cobalt loading from10to20wt%. A furtherincrease in cobalt loading from30to40wt%increased the CO conversion only slightly.The CO conversion of20%Co/SBA-16is about three times higher than that of20%Co/SiO2catalyst. This higher activity is related to the higher dispersion of cobalt onthe SBA-16surface because cobalt nanoparticles on SBA-16have a smaller particle sizeand a higher surface area. (2) A comparison of the stability of20%Co/SBA-16catalyst with that of20%Co/SiO2catalysts at a high initial CO conversion was studied in a fixed-bed reactor. The20%Co/SBA-16catalyst is more active and stable than the20%Co/SiO2catalyst, which isattributed to the high dispersion of cobalt species and low mobility of cobalt particles inthe SBA-16cages, respectively. Owing to the spatial restriction of the isolated nanocagesand smaller pore entrances of SBA-16, the aggregation and the sintering of cobaltnanoparticles were efficiently prevented. However, for20%Co/SiO2catalyst, the spatialrestriction was unable to prevent the growth of cobalt nanoparticles.The stability of the20%Co/SBA-16,20%Co/SiO2and20%Co/SBA-15catalysts withthe addition of water was investigated in a CSTR. For the20%Co/SBA-16and20%Co/SBA-15catalyst, the CO conversion was found to be higher with the water thanthat without water. For the20%Co/SiO2catalyst, the water addition did not affect the COconversion. The enhanced CO conversion by water addition was due to a diffusion effect inthe pores. After switching back to the standard operating conditions, the CO conversion ofthe20%Co/SBA-16catalyst is still higher and stability than the other two’s. The highstability of the Co/SBA-16catalyst is attributed to the effective stabilization of cobaltnanoparticles in the three-dimensional mesoporous silica cages of SBA-16—known as thepore confinement effect.(3) Al-SBA-16supports with the unique pore size, different aluminum content wereprepared using “pH-adjusting” method. The acidity of the support increases withincreasing aluminum content. The majority of the aluminum is tetrahedrally coordinatedAlO4groups and only a small amount of aluminum is present outside the framework. TheCo3O4nanoparticles have similar particle size (12nm) and are highly dispersed within theAl-SBA-16cages. The reducibility of the catalyst decreased from65.4to59.6%withdecreasing Si/Al ratios from30to10, which is attributed to the increased interactionbetween cobalt and the support. The acidity of the Al-SBA-16supports played a criticalrole in controlling the selectivity of the FTS. The selectivity shifted towards lighterproducts with lower Si/Al ratios, which was mainly attributed to the increased acidity ofthe Al-SBA-16supports, thereby hindering the olefins from undergoing furthertransformations at lower temperatures on the cobalt catalysts.更多还原