本文關(guān)鍵詞： 鐵 ZSM- 脫水 乙醇 乙烯　出處：《催化學(xué)報》2016年11期 　論文類型：期刊論文

【摘要】：乙烯是一種重要的大宗化工原料.目前國內(nèi)外乙烯的生產(chǎn)方法主要是石腦油裂解法.但是,隨著全球性石油資源供求關(guān)系日趨緊張,以及該生產(chǎn)過程存在較大環(huán)境污染,該工藝面臨嚴(yán)峻挑戰(zhàn).生物乙醇是一種可以通過生物質(zhì)發(fā)酵獲得的可再生資源.因此,生物質(zhì)乙醇催化脫水制乙烯工藝受到越來越多研究者關(guān)注.該技術(shù)的關(guān)鍵在于高性能乙醇脫水制乙烯催化劑的開發(fā).研究發(fā)現(xiàn),Si/Al比大于40的Fe改性ZSM-5分子篩在乙醇轉(zhuǎn)換制碳?xì)浠衔锏拇呋磻?yīng)中具有較高活性,當(dāng)反應(yīng)溫度大于400 ℃時,可生成C_1-C_9的烷烴、烯烴和芳香烴,其中以C_3產(chǎn)物和芳香烴產(chǎn)物為主.本文研究了Si/Al比為25-300的Fe離子交換ZSM-5分子篩在乙醇脫水制乙烯反應(yīng)中的催化活性,并利用XRD,NH_3-TPD,吡啶吸附FT-IR和DRS UV-VIS等表征手段,研究了催化劑的晶相結(jié)構(gòu)、表面組成及酸性位點等,進而探究了該催化反應(yīng)的反應(yīng)機理.我們首先考察了Si/Al比為25-300的HZSM-5分子篩.隨著分子篩Si/Al比增大,乙醇轉(zhuǎn)化率先增加后降低,在Si/Al比為100時獲得最高值;但是乙烯收率隨著Si/Al比的增加而持續(xù)下降,Si/Al比為25時有其最高值47%.經(jīng)產(chǎn)物分析,HZSM-5(25)和HZSM-5(300)雖具有相似的乙醇轉(zhuǎn)化率,但前者產(chǎn)生大量C_(3+)產(chǎn)物,而后者產(chǎn)物只有乙烯和乙醚.據(jù)文獻報道,乙醚是乙醇脫水制乙烯的中間產(chǎn)物,它的進一步脫水產(chǎn)生乙烯,而乙烯可進一步轉(zhuǎn)化生成C_(3+)產(chǎn)物.因此,由于HZSM-5(300)表面酸性較弱,主要生成反應(yīng)中間體,而HZSM-5(25)較強的表面酸性又導(dǎo)致乙烯進一步轉(zhuǎn)化,生成C_(3+)產(chǎn)物.然后我們考察了經(jīng)過3次離子交換處理的Fe-ZSM-5催化劑.隨著Si/Al比上升(25-300),乙醇轉(zhuǎn)化率和乙烯收率下降,Si/Al比為25時為其最高值;隨著反應(yīng)溫度上升,乙醇轉(zhuǎn)化率在260 ℃時達到近100%,之后維持不變,乙烯收率也在260℃時為其峰值,溫度繼續(xù)上升造成乙烯收率再次下降;催化劑空速增大降低乙醇轉(zhuǎn)化率和乙烯收率.經(jīng)產(chǎn)物分析,溫度較低和空速較大時產(chǎn)生大量的反應(yīng)中間體乙醚,而溫度較高時導(dǎo)致乙烯進一步轉(zhuǎn)化生成C_(3+)產(chǎn)物.在反應(yīng)溫度為260 ℃、空速為0.81 h~(-1)時,Fe-HZSM-5(25)催化劑上乙醇轉(zhuǎn)化率為98%-99%、乙烯收率為97%-99%,并可實現(xiàn)長達1440 h的單程使用壽命,該值是HZSM-5(25)催化劑的20余倍,具有很好的工業(yè)應(yīng)用前景.為探究Fe-ZSM-5(25)催化劑高催化活性和長催化壽命的原因,我們表征了催化劑.從XRD結(jié)果可以看出,離子交換沒有損壞HZSM-5的晶體結(jié)構(gòu),也沒有新的可檢測到的物相產(chǎn)生.從NH3-TPD結(jié)果看,HZSM-5(25)的CH/CL(強酸/弱酸)比為0.7,Fe-ZSM-5(25)的CH/CL比為0.29,可知Fe離子交換降低了分子篩的表面酸性,特別是強酸性位.從吡啶吸附FT-IR結(jié)果看,HZSM-5(25)的B/L(Br?nsted酸性位/Lewis酸性位)比為1.42,Fe-ZSM-5(25)的B/L比為0.25,可知Fe離子交換主要減少的是分子篩表面的Br?nsted酸性位.文獻報道,乙醇脫水制乙烯主要發(fā)生在弱酸性位上,而乙烯進一步轉(zhuǎn)化為C_(3+)產(chǎn)物發(fā)生在強酸性位上.所以,催化劑上強酸性位的減少有利于乙烯的生成反應(yīng).另據(jù)文獻報道,Br?nsted酸性位是乙烯聚合、迅速覆蓋催化活性位點產(chǎn)生積炭的催化活性中心.因此,Br?nsted酸性的降低可認(rèn)為是Fe-HZSM-5(25)催化劑單程使用壽命長較HZSM-5(25)分子篩顯著延長的原因.從UV-VIS結(jié)果得知,Fe-ZSM-5上的Fe物種主要以骨架內(nèi)和骨架外Fe~(3+)為主,此外含有少量低聚合的Fe_xO_y,但幾乎沒有Fe_2O_3顆粒存在.文獻記載,Fe~(3+)物種是乙烯形成的活性物種,而FeO_x催化產(chǎn)生乙烯和乙醛.因此,催化劑中大量骨架內(nèi)和骨架外Fe~(3+)物種的存在也可認(rèn)為是該催化劑具有較強乙醇脫水制乙烯催化活性的原因之一.
[Abstract]:Ethylene is a kind of important chemical raw materials. At present the main method of domestic production of ethylene is naphtha pyrolysis method. However, with the increasingly tense relationship between supply and demand of global oil resources, environmental pollution and the large production process, the process is facing severe challenges. Bio ethanol is a kind of can be obtained by fermentation of biomass renewable resources. Therefore, the biomass catalytic dehydration of ethanol to ethylene process has attracted more and more attention of researchers. The key of this technology lies in the development of high performance catalyst for preparing ethylene by ethanol dehydration. The study found that the Si/Al ratio is greater than 40 Fe modified ZSM-5 molecular sieve with high activity in the catalytic reaction of ethanol conversion for hydrocarbons, when the reaction temperature is greater than 400 C, can generate C_1-C_9 alkanes, olefins and aromatics, which C_3 products and aromatic hydrocarbon product. This paper studies the Si/Al ratio of 25-300 F E ion exchange ZSM-5 molecular sieve catalytic activity in the reaction of ethanol dehydration to ethylene, and the use of XRD, NH_3-TPD, FT-IR and DRS UV-VIS pyridine adsorption characterization methods, investigated the crystal structure, surface composition and acidic sites, and then explore the reaction mechanism of the catalytic reaction. We first examined the Si/Al ratio for the 25-300 HZSM-5 molecular sieve with molecular sieve Si/Al ratio increases, ethanol conversion increased first and then decreased, the Si/Al ratio of 100 was the highest value; but the ethylene yield increases with the Si/Al ratio continued to decline, the Si/Al ratio of 25 has the highest value of 47%. by product analysis, HZSM-5 (25) and HZSM-5 (300) is similar to the conversion of ethanol, but the large amount of C_ (3+) product, the latter only product of ethylene and ether. It is reported that ether is the intermediate product of ethanol dehydration to ethylene, it further dehydration to produce ethylene, and Ethylene can be further transformed into C_ (3+) product. Therefore, the HZSM-5 (300) surface acidity is weak, the main reaction intermediates, and HZSM-5 (25) surface strong acid and ethylene resulted in further transformed to generate C_ (3+) product. Then we investigated the effect of Fe-ZSM-5 catalyst after 3 ion exchange treatment with. Si/Al Rose (25-300), ethanol conversion and ethylene yield decreased, Si/Al ratio was 25 for the highest value; as the temperature increases, the conversion rate of ethanol reached nearly 100% at 260 DEG C, then remain unchanged, the ethylene yield also at 260 DEG C for its peak value, the temperature continues to rise caused by ethylene yield decreased again the increase of space velocity of catalyst; ethanol conversion and ethylene yield decreased. By product analysis, resulting in a large number of ether reaction intermediates of low temperature and high space velocity, and higher temperature leads to ethylene was further transformed into C_ (3+) in the product. 鍙嶅簲娓╁害涓，

本文編號：1548756