【摘要】：低碳烯烴(C2=-C4=)是基礎(chǔ)且重要的有機化工原料,工業(yè)上主要通過石腦油裂解或烷烴脫氫獲得。對于我國"富煤、貧油、少氣"的能源結(jié)構(gòu)而言,開發(fā)基于煤、生物質(zhì)等來源的合成氣(低H2/CO比)經(jīng)費托合成直接制備低碳烯烴(FTO)工藝,對均衡合理利用我國資源、確保國家能源安全具有十分重要的戰(zhàn)略意義。近年來,負載型Fe基催化劑因能直接轉(zhuǎn)化煤、生物質(zhì)等來源的合成氣高選擇性制備低碳烯烴,且兼具成本低廉、強度高、抗中毒性強等優(yōu)點,而備受關(guān)注。本文針對目前Fe基FTO催化劑中Fe物種與助劑之間的不均勻分布、表界面結(jié)構(gòu)特性與催化機理不明晰等問題,采用KMnO4或K2FeO4氧化修飾CNTs制備Fe基復(fù)合物納米催化劑,通過調(diào)變熱處理方式來調(diào)變金屬與助劑之間的作用方式,實現(xiàn)了微觀尺度上鐵物種與助劑之間的均勻混合,建立了催化劑表界面結(jié)構(gòu)與FTO性能之間的構(gòu)-效關(guān)系。此外,結(jié)合DFT計算與穩(wěn)態(tài)同位素瞬變動力學(xué)分析(SSITKA),提出了 Fe基催化劑上新的費托合成機理。(1)基于KMnO4氧化修飾CNTs獲取的MnK-CNTs復(fù)合物納米材料隨溫度、氣氛變化表現(xiàn)出獨特的結(jié)構(gòu)轉(zhuǎn)變性質(zhì),即隨熱處理溫度的升高,CNTs表面的層狀MnO2可自發(fā)轉(zhuǎn)變?yōu)榍蛐渭{米顆粒,且在一定還原條件下,可保證錳的晶相由MnO2完全轉(zhuǎn)變?yōu)镸nO,制備了一種新型Fe基復(fù)合物納米催化劑(Fe/MnK-CNTs)。相較于傳統(tǒng)共浸漬法制備的FeMnK/CNTs催化劑,該類新型催化劑不僅具有較小粒徑和分散均勻的鐵納米顆粒、微觀尺度上均勻混合的鐵物種與助劑,而且金屬-載體相互作用較弱、載體表面缺陷較多。(2)新型Fe/MnK-CNTs催化劑相較于FeMnK/CNTs催化劑具有較短的誘導(dǎo)期以及較高的反應(yīng)活性、C2=-C4=選擇性和C2-C4烴類中烯烴/烷烴之比,且反應(yīng)后的催化劑中鐵顆粒仍均勻分散,未發(fā)生明顯的團聚現(xiàn)象,并生成更多的活性相x-Fe5C2。這表明利用MnK-CNTs復(fù)合物納米材料隨溫度、氣氛變化獨特的結(jié)構(gòu)轉(zhuǎn)變性質(zhì)是一種制備高效Fe基FTO催化劑的可行思路。(3)調(diào)變Fe/MnK-CNTs催化劑焙燒溫度可以顯著改變催化劑的微觀形貌、金屬-載體相互作用、載體表面缺陷、催化劑的碳化能力和穩(wěn)定性等,發(fā)現(xiàn)當(dāng)焙燒溫度為220℃時,相應(yīng)的C-220催化劑表現(xiàn)出較高的活性、C2=-C4=選擇性和烯烴/烷烴之比等,且在較優(yōu)反應(yīng)條件(270℃、2.0MPa以及GHSV=30000mL·h-1·gcat-1)下可獲得較高的FTY(337.2 μmolco·gFe-1·s-1)和51.3%C的C2=-C4=選擇性。進一步研究了 Mn負載量對催化劑FTO性能的影響,發(fā)現(xiàn)提高Mn含量盡管有利于提高低碳烯烴選擇性,但同時增加了催化劑還原和碳化的難度,延長了反應(yīng)誘導(dǎo)期。此外,研究發(fā)現(xiàn)較低的碳化溫度或較高的%,有利于催化劑活性相的生成。(4)基于K2FeO4和CNTs之間的氧化還原反應(yīng),一步法制備了新型Fe-K基催化劑FeK-OX。系統(tǒng)的催化劑結(jié)構(gòu)與FTO性能表征結(jié)果表明:新型FeK-OX催化劑相較于傳統(tǒng)共浸漬法制備的FeK-IM催化劑具有粒徑較小且分布均勻的鐵納米顆粒、獨特的金屬與助劑接觸方式、較多的載體表面缺陷等,因而表現(xiàn)出較高生成低碳烯烴的、穩(wěn)定的比質(zhì)量活性以及較短的誘導(dǎo)期。在此基礎(chǔ)上,通過浸漬法進一步引入K助劑,可以顯著提高催化劑的FTY,降低甲烷選擇性。這預(yù)示著通過浸漬法向FeK-OX催化劑引入K助劑是一種有效的提高反應(yīng)活性和抑制甲烷生成的方法。(5)考慮到費托合成反應(yīng)路徑的復(fù)雜性,發(fā)展了針對Fe基催化劑的多組分SSITKA方法,將分析方法拓展到Fe基催化劑上的C2-C6產(chǎn)物,從表面活性物種的測定、TOF、同位素分布曲線等方面對傳統(tǒng)的SSITKA分析方法進行修正。結(jié)合基于SSITKA實驗的動力學(xué)分析和DFT計算,辨認出SSITKA測試條件下鐵基催化劑上CO較優(yōu)的活化路徑以及速率決定步驟等。(6)結(jié)合上述研究結(jié)果提出了 Fe基FTO催化劑主要活性相x-Fe5C2上可能的反應(yīng)機理:碳化鐵表面上CO活化產(chǎn)生的CHx與表面C原子或其加氫物種CHy結(jié)合發(fā)生C-C偶聯(lián)反應(yīng),其產(chǎn)生的碳空穴有利于CO活化,恢復(fù)完美的碳化鐵表面,然后遵循上述相似的C-C偶聯(lián)機理實現(xiàn)鏈增長,且最終形成的烴類產(chǎn)物端位C原子來自于原始碳化鐵表面上的C原子。
[Abstract]:Low carbon olefin (C2=-C4=) is a basic and important organic chemical raw material. It is obtained by naphtha cracking or dehydrogenation of alkanes in industry. For the energy structure of "rich coal, poor oil and less gas" in our country, the process of preparing low carbon olefin (FTO) based on coal, biomass and other sources of synthetic gas (low H2/CO ratio) is developed. Using our resources to ensure national energy security is of great strategic significance. In recent years, the supported Fe based catalysts have attracted much attention because they can directly convert coal, biomass and other sources of synthetic gas to produce low carbon olefins with high selectivity, which has the advantages of low cost, high strength and strong toxic resistance. This paper is aimed at the current Fe based FTO. The uneven distribution of Fe species and auxiliaries in the chemical agent, the structure characteristics of the surface interface and the catalytic mechanism are not clear. The nano catalyst of Fe based complex is prepared by KMnO4 or K2FeO4 oxidation modification, and the action formula between metals and additives is adjusted by the adjustment heat treatment, and the iron species and the auxiliary agent in the micro scale are realized. The structure effect relationship between the surface structure of the catalyst and the properties of FTO was established. In addition, the new Fischer Tropsch synthesis mechanism on the Fe based catalyst was proposed by DFT calculation and steady-state isotope transient dynamics analysis (SSITKA). (1) the MnK-CNTs composite nanomaterials obtained by KMnO4 oxidation modified CNTs with the temperature and atmosphere change table With the increase of heat treatment temperature, the layered MnO2 on the surface of CNTs can be transformed into spherical nanoparticles spontaneously, and under certain reduction conditions, the crystalline phase of manganese can be completely transformed from MnO2 to MnO, and a new Fe based nanocompound (Fe/MnK-CNTs) is prepared. Compared with the traditional co impregnation method, it is prepared. FeMnK/CNTs catalyst, the new type of catalyst not only has small particle size and dispersed uniform iron nanoparticles, but also has a uniform mixing of iron species and auxiliaries on the micro scale, but the interaction of metal carrier is weak, and the surface defects of the carrier are more. (2) the new type of Fe/MnK-CNTs catalyst has a shorter induction period than the FeMnK/CNTs catalyst. And higher reactive activity, C2=-C4= selectivity and the ratio of olefin / alkanes in C2-C4 hydrocarbons, and the iron particles are still dispersed evenly in the catalyst after reaction, and there is no obvious agglomeration, and more active phase x-Fe5C2. is generated. This indicates that the unique structure transformation properties of the MnK-CNTs complex nano material with the temperature change are a kind of unique structure transformation properties. The feasible idea of preparing high efficiency Fe based FTO catalyst. (3) the temperature of the modified Fe/MnK-CNTs catalyst can significantly change the micromorphology of the catalyst, the metal carrier interaction, the surface defect of the carrier, the carbonization ability and stability of the catalyst, and found that the corresponding C-220 catalyst showed higher activity when the calcination temperature was 220, C2= -C4= selectivity and the ratio of alkene / alkane, and higher FTY (337.2 molco. GFe-1. S-1) and 51.3%C C2=-C4= selectivity under the better reaction conditions (270, 2.0MPa and GHSV=30000mL. H-1. Gcat-1). Further study the effect of the amount of Mn load on the performance of the catalyst. Olefin selectivity, but also increased the difficulty of the catalyst reduction and carbonization, extended the reaction induction period. In addition, the study found that lower carbonation temperature or higher%, is beneficial to the generation of active phase of the catalyst. (4) based on the redox reaction between K2FeO4 and CNTs, a new Fe-K based catalyst FeK-OX. system catalyst was prepared by one step method. The characterization results of the structure and FTO show that the FeK-IM catalyst prepared by the new FeK-OX catalyst has smaller size and uniform distribution of iron nanoparticles than the traditional co impregnation method, the unique contact mode of metal and auxiliaries, more surface defects of the carrier and so on, thus showing a stable ratio of quality to Gao Shengcheng's low carbon olefin. On this basis, further introduction of K additives by impregnation can significantly increase the FTY of the catalyst and reduce the selectivity of methane. This indicates that the introduction of K additive to the FeK-OX catalyst by impregnation is an effective way to improve the reaction activity and inhibit the formation of methane. (5) the complex reaction path of the Fischer Tropsch synthesis is taken into account. The multi component SSITKA method for Fe based catalysts was developed. The analytical method was extended to the C2-C6 products on the Fe based catalyst. The traditional SSITKA analysis methods were corrected from the determination of the surface active species, the TOF, the isotopic distribution curve and so on. In combination with the kinetic analysis based on the SSITKA experiment and the DFT calculation, the SSITKA measurement was identified. The better activation path of CO and the rate determination step on the iron base catalyst. (6) the possible reaction mechanism on the main active phase x-Fe5C2 of the Fe based FTO catalyst was proposed in combination with the above results: the CHx produced by CO activation on the surface of the iron carbide and the C-C coupling reaction of the surface C atom or the CHy binding of the hydrogenated species were produced by the C-C coupling reaction. The carbon vacancies are beneficial to CO activation, restore the perfect surface of the iron carbide, and follow the similar C-C coupling mechanism to achieve chain growth, and the end C atoms of the final hydrocarbon products are derived from the C atoms on the surface of the original iron carbide.
【學(xué)位授予單位】：華東理工大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TQ426;TQ529.2
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本文編號：2167945