發(fā)布時(shí)間：2017-12-27 15:27

  本文關(guān)鍵詞：溶液燃燒合成Ni基催化劑結(jié)構(gòu)調(diào)控及催化甲烷化性能研究　出處：《太原理工大學(xué)》2017年博士論文　論文類型：學(xué)位論文

  更多相關(guān)文章： 煤制天然氣 CO甲烷化 Ni基催化劑 溶液燃燒法 燃料

【摘要】：近年來(lái),煤炭的直接燃燒利用排放了大量的SO_x、NO_x及VOC等污染物,由此引發(fā)了一系列嚴(yán)重的環(huán)境污染問(wèn)題�？沙掷m(xù)發(fā)展,環(huán)境友好型發(fā)展成為我國(guó)當(dāng)下迫在眉睫、亟待解決的難題之一。相比之下,煤炭經(jīng)過(guò)氣化、凈化、變換和甲烷化等過(guò)程轉(zhuǎn)化為清潔燃料甲烷(煤制天然氣),能夠?qū)⑽廴疚飪艋摮?是煤炭清潔高效利用的重要途徑之一。甲烷化反應(yīng)是煤制天然氣轉(zhuǎn)化技術(shù)的核心工藝,其中甲烷化催化劑制備及其催化活性、選擇性和壽命是該工藝過(guò)程的關(guān)鍵因素,決定了催化反應(yīng)工藝,以及上游和下游相關(guān)工藝的配套和設(shè)計(jì)。目前,工業(yè)甲烷化技術(shù)通常使用固定床反應(yīng)器,鑒于甲烷化反應(yīng)強(qiáng)放熱的特點(diǎn),采用低溫漿態(tài)床反應(yīng)器更有利于反應(yīng)熱的傳遞,提高反應(yīng)效率。然而,常規(guī)的浸漬法或沉淀法制備得到的Ni基催化劑具有顆粒尺度較大、分散不均等結(jié)構(gòu)缺陷,在漿態(tài)床反應(yīng)工藝中活性低,穩(wěn)定性差,易于失活。溶液燃燒法是一種快速、高效的合成方法,可用于合成各種類型的納米材料,其所制備的材料具有顆粒尺度小、形貌規(guī)則及分散均勻等優(yōu)點(diǎn),是制備催化材料較為新穎的技術(shù),應(yīng)用前景廣泛�；诖�,本文重點(diǎn)考察了微波輔助溶液燃燒合成Ni基催化劑過(guò)程中燃料種類、燃料加入量、燃料添加劑及可溶惰性鹽對(duì)前驅(qū)體溶液、燃燒熱、氣體釋放量等反應(yīng)過(guò)程因素的影響,并和催化劑微觀結(jié)構(gòu)進(jìn)行了有機(jī)關(guān)聯(lián)。通過(guò)研究催化劑織構(gòu)性質(zhì)以及活性物種Ni表面形態(tài)、尺寸、化學(xué)結(jié)構(gòu)等因素,建立了其對(duì)催化甲烷化反應(yīng)的構(gòu)效關(guān)系。具體研究結(jié)論如下:(1)在溶液燃燒合成Ni-Al_2O_3催化劑過(guò)程中考察了尿素、甘氨酸、乙二醇和檸檬酸四種燃料,研究發(fā)現(xiàn)燃料類型的不同對(duì)燃燒過(guò)程中產(chǎn)生的熱量、氣體量、燃燒強(qiáng)度以及持續(xù)時(shí)間具有顯著的影響。其中,尿素和Ni(NO3)2以及Al(NO3)3在加熱過(guò)程中幾乎同時(shí)熱分解,從而引發(fā)了溫和、持續(xù)的燃燒反應(yīng),有利于熱量均勻散逸進(jìn)而避免Ni顆粒高溫?zé)Y(jié)。另外,以尿素為燃料的體系產(chǎn)生的熱量最少,氣體釋放量最多。表征發(fā)現(xiàn),以尿素為燃料制備的催化劑中,Ni顆粒高度分散且平均晶粒尺寸最小(9.2 nm),其BET比表面積和Ni金屬比表面積分別達(dá)到了256.2 m2/g和34.6 m2/g,此外還形成了更多易于還原的Ni物種。其在漿態(tài)床CO甲烷化反應(yīng)中,溫度300°C,壓力1.0 MPa,空速3000 m L/gcat·h條件下的CO轉(zhuǎn)化率和CH4選擇性分別達(dá)到了95.7%和96.2%,并且在較高空速8000 m L/gcat·h條件下反應(yīng)300 h未失活,比其它燃料制備的催化劑及商業(yè)催化劑具有更好的活性和穩(wěn)定性。(2)尿素有利于促進(jìn)和穩(wěn)定Ni~(2+)的分散,提高催化劑中Ni的分散。在尿素與硝酸鹽形成的溶液中,尿素分子和Ni~(2+)離子絡(luò)合形成鎳胺絡(luò)合物,形成原子分散的前驅(qū)體,促進(jìn)了燃燒過(guò)程中高分散Ni物種的形成,且隨著尿素加入量的增加,提高了絡(luò)合程度,更有利于分散。在溶液燃燒過(guò)程中,燃燒產(chǎn)生的熱量和釋放的氣體對(duì)催化劑性質(zhì)的控制呈現(xiàn)為協(xié)同、競(jìng)爭(zhēng)關(guān)系。具體而言,當(dāng)RV/OV(還原價(jià)態(tài)/氧化價(jià)態(tài))≤0.75時(shí),釋放氣體產(chǎn)生的驅(qū)散作用對(duì)形成高比表面積來(lái)分散Ni納米顆粒起了決定性的作用;而熱效應(yīng)隨著RV/OV值的增加而增長(zhǎng),當(dāng)RV/OV≥0.75時(shí),較高的燃燒溫度促進(jìn)了Ni O遷移進(jìn)入Al_2O_3體相中以至于生成了低活性前驅(qū)體Ni Al2O4尖晶石;當(dāng)RV/OV=0.75時(shí),釋放氣體和反應(yīng)熱促使燃燒過(guò)程形成了的催化劑具有最大的Ni金屬比表面積(62.6 m2/g)和最小的Ni顆粒尺寸(10.8 nm),其CO轉(zhuǎn)化率和CH4選擇性在溫度280°C,壓力1.0 MPa及空速3000 m L/gcat·h條件下分別達(dá)到94.5%和91.3%。(3)燃料添加劑能夠調(diào)節(jié)燃燒過(guò)程,影響催化劑的結(jié)構(gòu)和催化活性。在尿素和硝酸鹽以最優(yōu)比例混合形成的燃燒溶液中,分別加入醋酸銨、淀粉和聚乙二醇(PEG4000)三種燃料添加劑。結(jié)果表明,醋酸銨較強(qiáng)的絡(luò)合能力提高了Ni~(2+)離子的絡(luò)合程度,進(jìn)而促進(jìn)了Ni物種在燃燒過(guò)程中分散。在前驅(qū)體溶液的燃燒過(guò)程中,含有 NH_2基團(tuán)的醋酸銨比含有 OH基團(tuán)的淀粉和PEG更具活性,加入醋酸銨的燃燒體系反應(yīng)也更為迅速。另外,醋酸銨、尿素和硝酸鋁在寬泛的溫度區(qū)間內(nèi)同時(shí)熱分解產(chǎn)生氣相產(chǎn)物,使得燃燒在理論上有無(wú)數(shù)多的起火點(diǎn),使得燃燒的引發(fā)更為均勻。相比之下,在淀粉和PEG加入的體系中幾乎不存在反應(yīng)物同時(shí)分解的情況,所形成的非均相燃燒反應(yīng)主要發(fā)生在氣固相,少量的起火點(diǎn)使得火焰蔓延不均勻。另一方面,醋酸銨的分解是吸熱反應(yīng),使得其可以像“滅火器”一樣降低燃燒熱。而淀粉和PEG在加熱過(guò)程中放熱,進(jìn)一步加劇了燃燒的劇烈程度。表征發(fā)現(xiàn),在加入醋酸銨的燃燒體系中形成的Ni O顆粒尺寸較小且高度分散,催化劑在漿態(tài)相CO甲烷化反應(yīng)中表現(xiàn)出較好的活性和穩(wěn)定性。而在淀粉和PEG加入的燃燒體系中,Ni O顆粒較大并高度團(tuán)聚,且體系中較高的燃燒溫度使得Ni O和Al_2O_3載體的相互作用力較強(qiáng)以致形成了Ni Al2O4尖晶石,導(dǎo)致其活性和穩(wěn)定性均較差。(4)在尿素和硝酸鹽混合而成的前驅(qū)體溶液中引入五種不參與反應(yīng)的可溶惰性鹽(Li Cl、Na Cl、KCl、Mg Cl2及Ca Cl2)。熱力學(xué)研究表明,加熱過(guò)程中Na Cl的融化效應(yīng)吸收了大量的燃燒熱,使得相應(yīng)體系的燃燒溫度最低。且由于Na Cl的熔點(diǎn)和體系的燃燒溫度接近,因而熔融的Na Cl在同步生成的Ni O顆粒表面形成了包覆層,極大程度上抑制了Ni O顆粒的生長(zhǎng)和團(tuán)聚。另一方面,微波加熱和傳統(tǒng)電加熱兩種模式下加熱環(huán)境的差別很大程度上影響了燃燒過(guò)程中Ni O顆粒的生長(zhǎng)和遷移。采用微波加熱且加入Na Cl惰性鹽制備的催化劑具有最小的Ni晶粒尺寸(7.8 nm)、最大的BET比表面積和Ni金屬比表面積(分別為416.5 m2/g和52.7 m2/g),其CO轉(zhuǎn)化率在310°C、1.0 MPa和9200 m L/gcat·h的苛刻條件下達(dá)到87%,100h內(nèi)未見(jiàn)失活。
[Abstract]:In recent years, a large number of pollutants such as SO_x, NO_x and VOC have been discharged from the direct combustion of coal, which has caused a series of serious environmental pollution problems. Sustainable development and environmental friendly development have become one of the most urgent and urgent problems to be solved in our country. In contrast, coal is transformed into clean fuels, such as methane, coal and natural gas through gasification, purification, transformation and methanation. It can remove pollutants and is an important way for clean and efficient utilization of coal. Methanation is the core technology of coal to natural gas conversion technology. Methanation catalyst preparation and its catalytic activity, selectivity and lifetime are the key factors in the process, which determines the catalytic reaction process and the matching and design of upstream and downstream related processes. At present, industrial methanation technology usually uses fixed bed reactor. In view of the strong exothermic characteristics of methanation reaction, the low temperature slurry bed reactor is more conducive to the transfer of reaction heat and the efficiency of reaction. However, the Ni based catalyst prepared by conventional impregnation or precipitation method has large particle size and uneven dispersion structure. It has low activity and poor stability in slurry reactor technology, and it is easy to deactivate. Solution combustion method is a fast and efficient synthetic method. It can be used for the synthesis of various kinds of nanomaterials. The prepared materials have the advantages of small particle size, regular morphology and uniform dispersion, which is a relatively new technology for preparing catalytic materials, and has wide application prospects. Based on this, this paper investigated the effect of microwave assisted solution combustion synthesis of Ni catalyst in the process of fuel type, fuel quantity, fuel additives and soluble inert salt of precursor solution, heat of combustion, emissions and other factors of the reaction process, and the microstructure of catalyst and organic association. The structure-activity relationship of catalytic methanation was studied by studying the texture characteristics of Ni and the surface morphology, size and chemical structure of active species. The specific conclusions are as follows: (1) in the solution combustion synthesis of Ni-Al_2O_3 catalyst was investigated in urea, glycine, ethylene glycol and citric acid of four kinds of fuel, found that the different types of fuel combustion heat, gas volume, combustion intensity and duration has significant effect. Among them, urea and Ni (NO3) 2 and Al (NO3) 3 in the process of heating at almost the same time thermal decomposition, causing a mild and sustained combustion reaction, is conducive to uniform heat dissipation and avoid high temperature sintering of Ni particles. In addition, the system that uses urea as the fuel produces the least heat and the largest amount of gas released. It was found that the Ni particles were highly dispersed and the average grain size was the smallest (9.2 nm), and the BET specific surface area and Ni Ni specific surface area reached 256.2 m2/g and 34.6 m2/g, respectively. The slurry bed CO methanation reaction, temperature 300 C, pressure 1 MPa, 3000 m L/gcat h velocity under the condition of CO conversion and CH4 selectivity reached 95.7% and 96.2%, and 300 h was not inactivated in response to higher space velocity of 8000 m under the condition of H L/gcat, the catalyst than other fuels preparation and commercial catalyst has better activity and stability. (2) urea is beneficial to promote and stabilize the dispersion of Ni~ (2+) and improve the dispersion of Ni in the catalyst. The solution formed in the urea and nitrate, urea molecules and Ni~ (2+) ion complex to form nickel amine complex, forming precursor atomic dispersion, promoted the formation of highly dispersed Ni species in the combustion process, and with the increase of the amount of urea, improves the degree of complexation, more conducive to the spread of. In the process of the combustion of the solution, the heat generated by the combustion and the release of the gas on the nature of the catalyst show a synergistic and competitive relationship. Specifically, when RV/OV (reduced valence / oxidation state is less than or equal to 0.75), the release of gas generated disperse effect on the formation of dispersed Ni nanoparticles plays a decisive role in high surface area and thermal effect; with the increase of RV/OV value and growth, when RV/OV is larger than 0.75, higher combustion temperature promoted Ni O migration into the Al_2O_3 body phase that generates low active precursor Ni Al2O4 spinel; when RV/OV=0.75, catalyst and reaction heat to release the gas combustion process has formed the largest surface area of the metal Ni (62.6 m2/g) and the minimum Ni grain size (10.8 nm), the conversion rate of CO and CH4 in the selective temperature 280 C, pressure 1 MPa and 3000 m L/gcat - H space velocity conditions were respectively 94.5% and 91.3%. (3) the fuel additive can regulate the combustion process and influence the structure and catalytic activity of the catalyst. Three kinds of fuel additives, ammonium acetate, starch and polyethylene glycol (PEG4000), were added in the mixture of urea and nitrate in the optimal proportion of combustion solutions. The results show that the stronger complexing ability of ammonium acetate improves the complexation degree of Ni~ (2+) ions, and thus promotes the dispersion of Ni species during the combustion process. In the combustion process of precursor solution, ammonium acetate containing NH_2 group is more active than starch and PEG containing OH group, and the combustion system adding ammonium acetate also has more rapid reaction. In addition, ammonium acetate, urea and aluminum nitrate are thermally decomposed at the same time in the wide temperature range to produce gaseous products, which makes combustion theoretically have numerous ignition points, making combustion more uniform. In contrast, in the system of starch and PEG addition, there is almost no simultaneous decomposition of reactants. The heterogeneous combustion reaction mainly occurs in gas-solid phase, and a small number of ignition points make the flame spread unevenly. On the other hand, the decomposition of ammonium acetate is a endothermic reaction, making it like a "fire extinguisher" to reduce the burning heat. When the starch and PEG are heated during the heating process, the intensity of the burning is further aggravated. surface
【學(xué)位授予單位】：太原理工大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：O643.36;TQ546

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