【作者】 邱江华；

【导师】 王光辉；

【作者基本信息】 武汉科技大学 ， 化学工艺， 2010， 博士

【摘要】 随着人们环境保护意识的日益提高,燃油中硫含量的控制指标不断提高。实际上,燃油中硫的零排放将成为未来世界各国寻求的最终目标。当前,传统的加氢脱硫工艺(HDS)对噻吩硫的脱除率低已成为燃油深度脱硫的瓶颈,而氧化脱硫方法(ODS)以其反应条件温和、脱硫效率高和容易脱去HDS难脱除的噻吩类含硫化合物等特点受到广泛关注。本论文通过复分解方法制备了八种磷钼杂多化合物,并通过傅立叶红外光谱(FT-IR)、X射线衍射图谱(XRD)、紫外可见光谱(UV-vis)、热重分析(TG-DSC)和扫描电镜分析(SEM)等手段对这些化合物的结构和热稳定性进行了分析;以这些磷钼杂多化合物作催化剂,研究了模型油(苯并噻吩和二苯并噻吩)和柴油的氧化脱硫,并对柴油氧化后的分离方法和磷钼杂多化合物催化柴油氧化脱硫的反应机理进行了探讨。由磷钼酸分别与氯化镧、氯化铈、氯化氧化钒、氯化铬经复分解反应制得四种磷钼酸金属盐。FT-IR图谱表明,磷钼酸掺杂金属盐后其Keggin结构未受破坏;XRD图谱显示,镧、铈、钒、铬等金属离子的添加对磷钼酸盐的结构形成和完善有利;UV-vis图谱显示,磷钼酸掺杂金属盐后金属元素进入抗衡位,其特征吸收峰位置发生少许红移,使得Ob,Oc→Mo的跃迁能降低;TG-DSC图谱表明,镧、铈、钒、铬等金属盐掺入后,磷钼杂多化合物的表面和杂多阴离子的结构发生了较大改变。考察了分别以磷钼酸、磷钼酸镧盐、磷钼酸铈盐、磷钼酸钒盐与磷钼酸铬盐作催化剂时模型油和柴油的氧化脱硫。结果表明,催化剂用量、H2O2初始浓度、反应温度和反应时间等因素对氧化脱硫均有影响,且二苯并噻吩(DBT)和苯并噻吩(BT)的氧化反应都符合表观一级反应动力学规律;在相同的反应条件下,微波辐射加热时DBT、BT的脱除率相比较于普通加热都得到了显著提高;使用磷钼酸镧盐作催化剂时,脱硫效果较好,当用V(DMF)/V(diesel)为1/4的DMF萃取一次,直馏柴油的脱硫率达到72.2 %,回收率为97.6 %,柴油硫含量从994 g/g降至276 g/g,达到欧洲柴油质量Ⅲ号标准。由磷钼酸分别与四甲基氯化铵、十二烷基三甲基氯化铵、十六烷基三甲基氯化铵、十八烷基三甲基氯化铵经复分解反应制得四种磷钼酸季铵盐。FT-IR图谱表明,磷钼酸掺杂季铵盐后其Keggin结构未受破坏;XRD结果显示,磷钼酸掺杂季铵盐后活性中心得到了分散,并且分散程度随着季铵盐中烷基链长度的增加而提高;UV-vis图谱显示,磷钼酸掺入季铵盐后,Mo=O键、组内Mo—Ob—Mo桥氧键和组间Mo—Oc—Mo桥氧键的强度都得到了明显降低,供氧能力增强;TG-DSC图谱表明,磷钼酸掺入季铵盐后,磷钼杂多化合物的表面和杂多阴离子的结构发生了较大改变,磷钼杂多化合物的热稳定性降低;SEM图谱显示,磷钼酸掺入季铵盐后,形成了类似于分子筛式的蓬松结构,催化活性中心得到高度分散,有利于提高催化效率。考察了分别以磷钼酸四甲基铵、磷钼酸十二烷基三甲基铵、磷钼酸十六烷基三甲基铵和磷钼酸十八烷基三甲基铵作催化剂时模型油和柴油的氧化脱硫。结果表明,催化剂用量、H2O2初始浓度、反应温度和反应时间等因素对模型油和柴油的氧化脱硫均有影响,并且在相同的反应条件下DBT比BT更容易被氧化;动力学研究表明,DBT和BT的氧化都符合表观一级反应,磷钼酸十二烷基三甲基铵、磷钼酸十六烷基三甲基铵和磷钼酸十八烷基三甲基铵催化DBT氧化的表观活化能分别为31.4 kJ/mol、26.8 kJ/mol和22.5 kJ/mol,而BT催化氧化的表观活化能分别为45.7 kJ/mol、54.5 kJ/mol和62.4 kJ/mol;磷钼酸四甲基铵中的碳链太短,不能形成稳定的乳化体系,因而催化活性较低;十八烷基的长链能够将DBT环抱到催化反应中心并形成稳定的乳化体系,因而磷钼酸十八烷基三甲基铵对DBT表现出了较好的催化活性;十二烷基的长链对BT分子具有较好的环抱能力和具有较低的位阻效应,因而磷钼酸十二烷基三甲基铵对BT表现出了较好的催化活性;以磷钼酸十八烷基三甲基铵作催化剂,在m(催化剂) / m(柴油) = 1. 8 %、V ( H2O2 ) / V (柴油) = 2.5 %、反应温度为70℃的条件下反应3 h,柴油的脱硫率达88. 7 %,回收率不低于99 %。探讨了柴油氧化后萃取剂种类、萃取剂用量和萃取次数等分离条件对柴油的脱硫率和回收率的影响。结果显示,选择DMF作萃取剂时,柴油同时具有较高的脱硫率和回收率;减少单次萃取剂的用量并增加萃取的次数,既可以减少萃取剂的总用量,又可以同时提高柴油的脱硫率和回收率;增加萃取次数,脱硫率提高了,但柴油的回收率却明显下降。因此,柴油的高脱硫率和高回收率是一对矛盾。运用气相色谱和气相色谱/质谱联用手段追踪了DBT和BT的氧化产物,并推测了磷钼杂多化合物催化柴油氧化脱硫的反应机理。首先,过氧化氢亲核进攻催化剂的MoVI活性中心,形成活性的氢过氧化MoVI,然后含硫化合物中的硫原子亲核进攻氢过氧化MoVI同时释放出一个水分子后形成MoVI过氧自由基,紧接着MoVI过氧自由基进攻硫原子,形成亚砜和释放出催化剂的活性中心MoVI。接下来,亚砜经过与上述相似的催化氧化过程进一步氧化成砜,释放出的催化剂活性中心MoVI进入新的催化氧化循环。由于杂多化合物催化柴油氧化脱硫为多相反应,而磷钼酸季铵盐和柴油能够形成稳定的催化乳化体系,因此磷钼酸季铵盐的催化活性大大高于磷钼酸金属盐。更多还原

【Abstract】 With people’s environmental awareness increasing, the control target of sulfur content in fuel oil continues to increase. In fact, zero-emission of sulfur in fuel oil will be the ultimate goal for the future of the world. The conventional hydrodesulfurization process (HDS) is inefficient for removing of thiophene sulfurs and has become the bottleneck for deep desulfurization of fuel oil. However, the oxidative desulfurization method (ODS) has been widely concerned for its mild reaction conditions, efficiency and easy to take off thiophene sulfur which is difficult to be removed by HDS. In the paper, eight types of phosphomolybdates have been prepared by double decomposition, and the samples’constructions and thermal properties have been characterized by Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), thermogravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM). Moreover, the oxidative desulfurization of model oil (benzothiophene and dibenzothiophene) and diesel fuel catalyzed by the samples, the separation methods for the oxidized diesel fuel and the catalytic reaction mechanism of oxidative desulfurization catalyzed by phosphomolybdates have been discussed.Metallic phosphomolybdates have been prepared by mixing of phosphomolybdic acid and lanthanum chloride, cerium chloride, vanadium oxide chloride and chromium chloride via double decomposition reaction, respectively. FT-IR analysis shows that the phosphomolybdates keep the Keggin structures. XRD analysis indicates that the additions of lanthanum, cerium, vanadium and chromium are favorable for the structure formation and improve of the phosphomolybdates. UV-vis. analysis shows that the characteristic absorption peaks become a little red shift with mixing of metallic elements into the counter positions, which cut down the transition energies of the Mo-Ob-Mo and Mo-Oc-Mo bonds. TG-DSC analysis shows that the surface and heteropoly anion structures of the phosphomolybdates have been changed with mixing of lanthanum, cerium, vanadium and chromium, respectively.The catalytic oxidative desulfurizations of model oil and diesel fuel have been investigated by phosphomolybdic acid, lanthanum phosphomolybdate, cerium phosphomolybdate, vanadium oxide phosphomolybdate and chromium phosphomolybdate, respectively. The results show that the amount of catalyst, initial concentration of H2O2, reaction temperature and reaction time are impacted on the oxidative desulfurization, and the oxidative reactions of dibenzothiophene (DBT) and benzothiophene (BT) are in conformity with apparent-first order kinetics. The conversion rates of DBT and BT under microwave radiation heating are significantly higher than those of traditional heating with the same reaction conditions. When catalyzed by lanthanum phosphomolybdate and extracted once with V(DMF)/V(diesel) of 1/4, the better desulfurization rate of diesel fuel reaches 72.2 % with the recovery rate of 97.6 %, and the sulfur content in the diesel fuel reduces from 994 g/g to 276 g/g, which meets the European III standard.Quaternary ammonium phosphomolybdates have been prepared by mixing of phosphomolybdic acid and tetramethyl ammonium chloride (TMAC), dodecyl trimethyl ammonium chloride (DTAC), hexadecyl trimethyl ammonium chloride (HTAC) and octadecyl trimethyl ammonium chloride (OTAC) via double decomposition reaction, respectively. FT-IR analysis shows that the quaternary ammonium phosphomolybdates keep the Keggin structures. XRD anslysis indicates that HPMo clusters are finely dispersed with mixing of quaternary ammonium salts, and the dispersion levels increase with longer alkyl chains in the catalysts. UV-vis analysis shows that the bond strengths of Mo=O, Mo-Ob-Mo and Mo-Oc-Mo lower with mixing of quaternary ammonium salts, which indicates the favorable ability for oxygen supply. TG-DSC analysis indicates that the surface and heteropoly anion structures of the phosphomolybdates have been changed with mixing of quaternary ammonium salts and the thermal stabilization of the quaternary ammonium phosphomolybdates has been cut down. SEM anslysis shows that the catalysts form the loose structure like the molecular sieves, which leads to the higher dispersion of the catalytic active centers and higher catalytic properties.The catalytic oxidative desulfurizations of model oil and diesel fuel have been investigated by tetramethyl ammonium phosphomolybdate, dodecyl trimethyl ammonium phosphomolybdate, hexadecyl trimethyl ammonium phosphomolybdate and octadecyl trimethyl ammonium phosphomolybdate, respectively. The results show that the amount of catalyst, initial concentration of H2O2, reaction temperature and reaction time are impacted on the oxidative desulfurization, and the catalytic oxidative reactivity towards DBT is much higher than that of BT under the same reaction conditions. The oxidative reactions of dibenzothiophene (DBT) and benzothiophene (BT) are in conformity with apparent-first order kinetics. The apparent activation energies of DBT catalyzed by dodecyl trimethyl ammonium phosphomolybdate, hexadecyl trimethyl ammonium phosphomolybdate and octadecyl trimethyl ammonium phosphomolybdate are 31.4 kJ/mol, 26.8 kJ/mol and 22.5 kJ/mol, respectively, and the apparent activation energies of BT are 45.7 kJ/mol, 54.5 kJ/mol and 62.4 kJ/mol, respectively. As the alkyl chains in the tetramethyl ammonium phosphomolybdate are too short to form stable emulsion system, so the catalytic reactivy of the catalyst is low. Octadecyl chains are more favorable to wrap up DBT to the catalytic center and form stable emulsion system with the better conversion rates of DBT. The shorter dodecyl chains can wrap up BT more suitably and bring smaller steric hindrance, which displays the better conversion rates of BT. Under the condition of m(catalyst)/m(diesel) 1.8 %, v(H2O2)/v(diesel) 2.5 %, 70°C and 3 h, the desulfurization rate of diesel fuel reaches 88.7 % with the recovery rate of no less than 99 % catalyzed by octadecyl trimethyl ammonium phosphomolybdate.The effects of the separation methods, such as the types of extractant, extractant dosage and extraction times, for the desulfurization rates and the recovery rates of the diesel fuel have been investigated. The results show that the diesel fuel can get the high desulfurization rate and the high recovery rate with using DMF as the extractant. To cut down the solvent/diesel ratio and increase the extraction times, the total amount of the extraction agent can be reduced and the higher desulfurization rate and recovery rate can be got. With the extraction times increasing, the desulfurization rates increase, but the recovery rates of diesel decrease significantly. Therefore, the high desulfurization rate and the high recovery rate are contradictory for the oxidative desulfurization of diesel fuel.The oxidized products of DBT and BT have been traced by gas chromatography (GC) and gas chromatography/mass spectrum (GC/MS), and the reaction mechanism of oxidative desulfurization of diesel fuel has been speculated. Firstly, hydrogen peroxide attacks nucleophilicly the active center MoVI of the catalyst to form the active hydrogen peroxide MoVI. Secondly, the sulfur atom in the sulfur compounds attacks nucleophilicly the hydrogen peroxide MoVI and gives out one water molecule to form the MoVI peroxy radicals. Thirdly, the MoVI peroxy radicals attack the sulfur atoms to form sulfoxides and give out the active center MoVI. Then, the sulfoxides can be oxidized to sulfones with a similar oxidation process of the previous, and the leased active site MoVI enters a new cycle of catalytic oxidation. As the catalytic oxidation of diesel fuel is a multi-phase reaction and the efficient catalytic emulsion system can be formed with mixing of quaternary ammonium salts, the catalytic reactivity of quaternary ammonium phosphomolybdates are much higher than those of metallic phosphomolybdates.更多还原