Roles of texture and acidity of acid-activated sepiolite catalysts in gas-phase catalytic dehydration of glycerol to acrolein
Roles of texture and acidity of acid-activated sepiolite catalysts in gas-phase catalytic dehydration of glycerol to acrolein
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DOI：
10.1016/j.mcat.2016.12.022
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发表时间：
2017-06
影响因子：
4.6
通讯作者：
C. Zhou;Guizhou Li;Zhuang Yu Xiao;Pengju Wang;D. Tong;Huimin Yang;C. Lin;Li Li-Li;Hao Zhang;Sheng-fu Ji;W. Yu
中科院分区：
文献类型：
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作者：
C. Zhou;Guizhou Li;Zhuang Yu Xiao;Pengju Wang;D. Tong;Huimin Yang;C. Lin;Li Li-Li;Hao Zhang;Sheng-fu Ji;W. Yu
在寻找新型固体酸催化剂用于甘油气相催化脱水制丙烯醛的过程中，用盐酸活化海泡石粘土矿物制备酸活化海泡石催化剂。考察了活化工艺对酸活化海泡石的结构和酸性的影响，以及它们在甘油气相催化脱水制丙烯醛反应中的作用。用不同浓度的盐酸（0-8 mol/L）在80 ℃下处理提纯的天然海泡石，然后在100 ℃下干燥，制得一系列酸活化海泡石催化剂。采用X射线衍射、X射线荧光分析、扫描电镜、热重分析、N2吸附-脱附等温线、NH3程序升温脱附等方法对酸活化海泡石催化剂样品进行了表征。傅里叶变换红外光谱和吡啶吸附，然后是原位红外光谱。采用立式固定床反应器研究了甘油气相催化脱水制丙烯醛的催化性能。典型地，当在HCl（2 mol/L）酸活化的海泡石催化剂上在320 °C下用甘油水溶液（20 wt.%）进行甘油的气相催化脱水时，实现了92.9%的甘油转化率、59.4%的丙烯醛选择性和55.2%的丙烯醛收率。以0.10 ml/min的速率作为原料，空气作为载气，流速为20 ml/min。用盐酸活化海泡石，然后干燥，可以除去隧道中的一部分沸石水，除去海泡石八面体片中的一部分镁和铝阳离子，解开聚集的海泡石纤维，部分破坏海泡石四面体片中的Si-O-Si。这些因素增加了海泡石的孔隙率、比表面积和酸性。八面体片层中镁和铝离子的部分浸出产生八面体空位，并在八面体片层中产生Mg单键O单键Al+位，从而增加了刘易斯酸位的量。沿着H+离子通过离子交换反应和物理化学表面吸附进入海泡石层间空间，四面体片中Sisingle键O单键Si键的部分断裂产生更多的Sisingle键O单键H+，从而增加了Brønsted酸中心的数量。此外，酸活化后的盐酸活化的海泡石的酸性强度显着变化。中等强度的Brønsted酸中心（Sisingle键、H+和H+吸附在表面）有利于甘油的高转化率和丙烯醛的选择性。刘易斯酸中心可促进丙酮醇的形成。在上述反应条件下，盐酸活化的海泡石催化剂容易结焦，导致丙烯醛产率迅速下降。
In the search for novel solid acid catalysts for the gas-phase catalytic dehydration of glycerol to acrolein, sepiolite clay minerals were activated with hydrochloric acid to produce acid-activated sepiolite catalysts. The effects of the activation process on the texture and acidity of acid-activated sepiolite, and their roles in gas-phase catalytic dehydration of glycerol to acrolein, were then investigated. A series of acid-activated sepiolite catalysts were made from purified natural sepiolite treated with hydrochloric acid of different concentrations (0–8 mol/L) at 80 °C, followed by drying at 100 °C. The acid-activated sepiolite catalyst samples were characterized using X-ray diffraction, X-ray fluorescence analysis, scanning electronic microscopy, thermogravimetric analysis, N2adsorption-desorption isotherms, NH3-temperature-programmed desorption. Fourier transform-infrared spectroscopy and pyridine adsorption followed by in situ infrared spectroscopy. The catalytic performances in gas-phase catalytic dehydration of glycerol to acrolein were studied using a vertical fixed-bed reactor. Typically, a glycerol conversion of 92.9%, an acrolein selectivity of 59.4%, and a yield of acrolein of 55.2% were achieved when the gas-phase catalytic dehydration of glycerol was conducted over HCl (2 mol/L) acid-activated sepiolite catalyst at 320 °C with an aqueous glycerol solution (20 wt.%) at a rate of 0.10 ml/min as the feedstock and air as the carrier gas at a flow rate of 20 ml/min. The activation of the sepiolite with hydrochloric acid followed by drying can eliminate a portion of the zeolitic water in the tunnels, remove part of the magnesium and aluminum cations in the octahedral sheets of sepiolite, unbundle aggregated sepiolite fibers, and partly break the Si-O-Si in the tetrahedral sheets of sepiolite. These factors increased the porosity, the specific surface area and the acidity of the sepiolite. The partial leaching of magnesium and aluminum ions in the octahedral sheet generated octahedral vacancies and created Mgsingle bondOsingle bondAl+sites in the octahedral sheets, thereby increasing the amount of Lewis acid sites. Along with penetration of H+cations into the interlayered space of the sepiolite by an ion-exchange reaction and physicochemical surface adsorption, some breakage of the Sisingle bondOsingle bondSi bonds in the tetrahedral sheets generated more Sisingle bondOsingle bondH+, thus increasing the quantity of Brønsted acid sites. Furthermore, the strength of acidity of the hydrochloric acid-activated sepiolite was significantly changed after the acid activation. The medium-strong Brønsted acid sites (Sisingle bondOsingle bondH+and H+adsorbed on the surface) appeared to be beneficial for the high conversion of glycerol and the selectivity to acrolein. Lewis acid sites could facilitate the formation of acetol. Under the reaction conditions mentioned above, the coking on the hydrochloric acid-activated sepiolite catalysts readily occurred, rapidly leading to the decrease of the yield of acrolein.