碱催化剂g-C3N4在液相反应中溶胀特性的研究

doi:10.3866/PKU.WHXB201904066

摘要/Abstract

摘要：

通过三聚氰胺、双腈胺、硫脲或尿素的高温热解制备了四种比表面不同的石墨型碳化氮材料(g-C₃N₄)，利用X射线衍射(XRD)、扫描电子显微镜(SEM)、傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)、热重分析(TGA)和N₂吸附方法对所得材料进行结构与表面性质的表征，用CO₂程序升温脱附(CO₂-TPD)和酸碱滴定方法测试了其碱性。考察了这些材料对于不同溶剂中苯甲醛与丙二腈Knoevenagel缩合反应的催化活性。g-C₃N₄在弱极性溶剂甲苯中的催化活性很低，且活性大小与其比表面和表面碱性密切相关，而在极性溶剂乙醇中四种g-C₃N₄的催化活性则相差不大，且都远高于甲苯中的活性。上述结果无法用常规的碱性表征数据来解释。进一步的实验证明在极性溶剂中g-C₃N₄会发生溶胀，使得表面暴露更多的碱性位，因而催化活性大大提高。不同溶剂中的反应结果表明g-C₃N₄的溶胀效应随反应溶剂的极性增加而增强。重复利用实验表明g-C₃N₄在液相反应中具有良好的稳定性，其对于苯甲醛的转化率在重复利用三次后由74.2%下降至63.8%。

关键词: g-C₃N₄, 碱催化, Knoevenagel缩合, 溶胀效应

Abstract:

Graphitic carbon nitrides (g-C₃N₄) with different surface areas were prepared by pyrolysis using different precursors including melamine, dicyandiamide, thiourea and urea, and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and N2 adsorption. Their basicities were measured by temperature-programmed desorption of CO₂ (CO₂-TPD) and acid-base titration. The catalytic properties for the Knoevenagel condensation of benzaldehyde and malononitrile were investigated in various solvents. In non-polar toluene solution, the benzaldehyde conversions of the g-C₃N₄ catalysts were low and changed according to their respective surface areas and basicities. However, in polar ethanol solution, the benzaldehyde conversions of all catalysts were similar, and much higher than those in toluene. This could not be explained by the results obtained from either of the two conventional basicity measurements. Further experimental results proved that g-C₃N₄ catalysts swelled in polar solutions, and more basic sites were exposed on the surface of the swollen catalysts, leading to the imminent increase in catalytic activity. This was proved by the catalyst poisoning data, which showed that the g-C₃N₄ catalyst lost its activity completely in toluene by adding 40.9 mmol·g^-1 benzoic acid, while the same catalyst was still active in ethanol until the added amount exceeded 143.3 m·g^-1. Additionally, the reaction tests in various solutions showed that the swelling effect was enhanced according to the polarity of the solvent used. A similar conclusion could be reached for the Knoevenagel condensation of furfural and malononitrile in various solvents. The reusability of g-C₃N₄ catalyst in Knoevenagel condensation was also studied, which showed that g-C₃N₄ was stable in liquid-phase reactions, whose activity dropped from 74.2% to 63.8% after three regeneration processes.

Key words: g-C₃N₄, Base catalysis, Knoevenagel condensation, Swelling effect

MSC2000:

O643

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吴结群, 华伟明, 乐英红, 高滋. 碱催化剂g-C₃N₄在液相反应中溶胀特性的研究[J]. 物理化学学报, 2020, 36(1): 1904066.

Jiequn Wu, Weiming Hua, Yinghong Yue, Zi Gao. Swelling Characteristics of g-C₃N₄ as Base Catalyst in Liquid-Phase Reaction[J]. Acta Physico-Chimica Sinica, 2020, 36(1): 1904066.

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Sample	w_C/%	w_N/%	w_H/%	x	y	z	C/N ratio
MA	34.4	63.6	2.0	5.7	9.1	4.0	0.63
DA	34.4	63.4	2.2	5.7	9.1	4.4	0.63
TU	34.0	62.9	2.2	5.7	9.0	4.4	0.63
UR	34.1	62.5	2.2	5.7	8.9	4.4	0.64

Sample	Textural properties		Amount of base sites
Sample	Surface areas/ (m²·g^-1)	Pore volume/ (cm³·g^-1)	CO₂-TPD/ (mmol·g^-1)	Titration/ (mmol·g^-1)
MA	7.9	0.087	0.08	0.75
DA	5.2	0.060	0.09	0.81
TU	23.7	0.175	0.21	1.31
UR	103	0.601	0.24	1.36

Catalysts	Conversion(%)
Catalysts	in ethanol ^a	in toluene
MA	95.4(61.5)	9.7
DA	94.3(64.6)	9.0
TU	92.2(66.9)	15.2
UR	96.8(74.2)	23.7

Solvent	Polarity	Boilingpoint/℃	Conversion/%
Solvent	Polarity	Boilingpoint/℃	benzaldehyde	furfural
dimethyl formamide	6.4	153	77.5^a	68.5
ethanol	4.3	79	74.2^a	62.7
toluene	2.4	111	23.7	14.7
cyclohexane	0.1	81	6.7	5.4

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碱催化剂g-C₃N₄在液相反应中溶胀特性的研究

Swelling Characteristics of g-C₃N₄ as Base Catalyst in Liquid-Phase Reaction

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