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一种具有温度响应智能表面的氧化石墨烯负载Ru催化剂及其催化性能研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/10/12 8:53:24  

摘要:氧化石墨烯(GO)是开发选择性加氢催化剂的优良载体。GO表面含有大量含氧官能团。这不仅使其呈亲水特性,而且可对其进一步功能化制备复合载体。虽然GO亲水的表面有利于金属纳米颗粒在其上的分散,但在催化加氢反应中却不利于弱极性底物分子的吸附,导致反应速率较慢。本论文将润湿性可控表面,即“智能表面”概念引入催化剂,在提高金属纳米颗粒分散性和促进底物分子吸附性能两个方面协同作用,实现高效加氢。

本论文首先利用过硫酸铵引发的自由基聚合反应,在GO表面生长温敏高分子聚(N-异丙基丙烯酰胺) (PNIPAM),得到具有温敏智能表面的复合材料GO-PNIPAM;然后以其为载体负载钌(Ru)纳米颗粒,制备催化剂Ru/GO-PNIPAM,并采用X射线衍射(XRD)、透射电子显微镜(TEM)、傅里叶变换红外光谱仪(FT-IR)、差示扫描量热仪(DSC)、热重分析(TG)、有机元素分析(OEA)、电感耦合等离子原子发射光谱仪(ICP-AES)和X射线光电子能谱分析(XPS)等手段对GO、GO-PNIPAM和Ru/GO-PNIPAM进行表征。Ru/GO-PNIPAM用于α,β-不饱和醛的选择性加氢反应,并从催化剂表面化学性质、催化剂活性中心、溶剂效应和反应温度等方面对其催化性能的影响进行研究。

结果表明,PNIPAM被成功接枝到GO表面,其接枝量占总量的60 %。Ru/GO-PNIPAM显示出明显的温敏效应,其最低临界溶液温度(LCST)约37 oC。由于在低温下相似的亲水性,Ru纳米颗粒在催化剂Ru/GO-PNIPAM和Ru/GO显示出相似的高分散度(约24%)。PNIPAM在GO上的接枝为金属纳米颗粒的沉积提供大量锚定位点,有助于金属纳米颗粒在其上的分散。

与GO负载Ru催化剂(Ru/GO)相比,Ru/GO-PNIPAM在α,β-不饱和醛(如肉桂醛和柠檬醛)选择性加氢反应中显示出较高的催化活性和对中间产物不饱和醇的较高选择性。在肉桂醛选择性加氢反应中,80 oC下,以Ru/GO-PNIPAM为催化剂的反应速率常数k达0.54 h–1,远高于Ru/GO(0.26 h–1)。接近完全反应时,Ru/GO-PNIPAM对肉桂醇的选择性为31 %,亦高于Ru/GO (9 %)。同样,Ru/GO-PNIPAM在柠檬醛选择性加氢反应中亦显示出与肉桂醛选择性加氢相似的高催化性能。80 oC下,以Ru/GO-PNIPAM为催化剂的反应速率常数k达0.24 h–1,超过Ru/GO的2倍(0.10 h–1)。当接近完全反应时,Ru/GO-PNIPAM对不饱和醇(包括香叶醇和橙花醇)的选择性为23 %,高于Ru/GO(8 %)。经计算,以Ru/GO为催化剂,柠檬醛加氢反应的活化能为33 kJ/mol。与之相比,Ru/GO-PNIPAM为催化剂的反应活化能明显降低(26 kJ/mol)。Ru/GO-PNIPAM上Ru的高分散性及对底物的优良吸附性能共同促进其催化性能的提高。

Graphene oxide (GO) is an excellent supportfor developing catalysts for selective hydrogenation. It contains abundantoxygen-containing functional groups that provide potential chemicalmodifications for the preparation of GO-based composite materials. Although thehydrophilic surface of GO helps the dispersion of metal nanoparticles on it, itis not conducive to the adsorption of the molecules with weak polarity on it incatalytic hydrogenation, which leads to a slow reaction rate. In this thesis,wettability controllable surface, i.e., smart surface, was constructed on thecatalyst to achieve enhanced catalytic performance through the strongsynergistic effects between Ru and GO-PNIPAM as well as its hydrophobic surfacefor the reaction.

Thermoresponsivepoly(N-isopropylacrylamide) (PNIPAM) was firstly grown on graphene oxide (GO)in this thesis by ammonium persulfate initiated radical polymerization. Theobtained composite GO-PNIPAM was then employed to prepare Ru catalyst(Ru/GO-PNIPAM). GO materials were characterized by some technologies such asX-ray diffraction (XRD), transmission electron microscopy (TEM), Fouriertransform infrared spectroscopy (FT-IR), differential scanning calorimetry(DSC), thermogravimetric analysis (TG), organic elementary analysis (OEA),inductively coupled plasma–atomic emission spectrometry (ICP-AES) and X-rayphotoelectron spectroscopy (XPS). Ru/GO-PNIPAM was further employed forselective hydrogenation of α,β-unsaturated aldehydes, and the effects of someimportant factors including surface chemistry of the catalyst, active center,solvent and reaction temperature on catalytic performance were investigatedhere.

Results showed that PNIPAM was successfullygrafted on GO surface, and its grafting amount accounted for 60 % of the total.Ru/GO-PNIPAM clearly exhibited thermoresponsive effect with its lower criticalsolution temperature (LCST) at about 37 oC. The hydrophilic surface ofRu/GO-PNIPAM and Ru/GO exhibited the similar high Ru dispersion (approximately24 %) due to their similar hydrophilic surface. The grafted PNIPAM on GO provideda great number of anchoring sites for the dispersion of metal nanoparticles onit.

Compared to GO supported Ru catalyst(Ru/GO), Ru/GO-PNIPAM exhibited higher catalytic performance in selectivehydrogenation of cinnamaldehyde (CAL) and citral. Specifically, in CALhydrogenation, the rate constant k over Ru/GO-PNIPAM was calculated to be 0.54h–1 at 80 °C (>LCST), much higher than that over Ru/GO (0.26 h–1). Theselectivity to cinnamyl alcohol (COL) was 31 % over Ru/GO-PNIPAM at nearlycomplete CAL conversion, higher than that over Ru/GO (about 9 %). Ru/GO-PNIPAMshowed the similar high catalytic performance in citral hydrogenation to thatin CAL hydrogenation. The rate constant k arrived at 0.24 h–1 over Ru/GO-PNIPAMat 80 °C, much higher than that over Ru/GO (0.10 h–1). The selectivity togeraniol and nerol was 23 % over Ru/GO-PNIPAM at nearly complete citralconversion, higher than that over Ru/GO (8 %). The reaction activation energywas calculated to be 33 kJ/mol over Ru/GO. In contrast, that over Ru/GO-PNIPAMdeclined obviously (26 kJ/mol). The high Ru dispersion on Ru/GO-PNIPAM as wellas its great adsorption ability to reactants together promoted the improvementon catalytic performance.

关键词:氧化石墨烯;聚(N-异丙基丙烯酰胺);智能表面;α,β-不饱和醛;选择性加氢

graphene oxide; PNIPAM; smart surface;α,β-unsaturated aldehyde; selective hydrogenation

上一篇:金属纳米簇的合成及其催化应用     下一篇:PANI-g-C3N4-TiO2复合材料光催化芳基化反应和噁二唑的合成
 
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