碱活化煤。微孔结构及对酚类化合物的吸附性能

Yu. V. Таmarkina, V. Anishchenko, A. Red'ko, V. Kucherenko
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引用次数: 0

摘要

本研究的目的是比较不同煤阶(CR)煤经碱性活化(RKOH = 1 g/g, 800℃)制备的活性炭(ACs)的微孔结构特征,并确定活性炭对水溶液中苯酚和4-氯酚的吸附能力。起始原料为碳含量(Cdaf = 80.0 - 95.6%)不断增加的煤,作为CR标准。在氩气中分三个阶段获得ac: 1)热程序升温(4 grad/min)至800°С;2)等温暴露1 h;3)冷却、洗碱、烘干。基于低温(77 K)氮气吸附-解吸等温线,计算了比表面积S (m2/g)和孔体积V (cm3/g)对平均孔径D (nm)的积分和微分依赖关系。它们被用来定义超微孔(Vumi)、超微孔(Vsmi)和微孔(Vmi)的体积。总孔隙体积Vt由相对压力p/p0 ~ 1.0下吸附的氮气量计算。超微孔(Sumi)、超微孔(Ssmi)和微孔(Smi)的S值测定方法类似。不同类型孔隙的体积和比表面积随CR的增加而减小:体积Vt -由0.59减小到0.23 cm3/g;Vmi - 0.51 ~ 0.17 cm3/g;在无烟煤AC中,超微孔体积从0.31 cm3/g到零。超微孔体积几乎与CR无关,并在Vsmi = 0.15 ~ 0.22 cm3/h的宽范围内变化。最低CR煤的AC比表面积最大(S = 1547 m2/g),随煤变质而减小,最大可达322 m2/g。微孔表面对S值的贡献占主导地位,占94.7% ~ 99.4%。在25℃条件下,研究了所有活性炭对苯酚(Ph)和4-氯苯酚(CPh)的吸附。拟二级模型和Langmuir模型最能描述吸附动力学和等温线(R2≥0.998)。随着CR的增加,最大吸附量从3.113降至1.498 mmol/g (Ph),从3.9降至2.1 mmol/g (CPh),比表面积减少约5倍,最大吸附量减少约2倍。表征1 m2表面吸附容量的Ph比容量和CPh比容量在Cdaf≤86%时变化不大,但在无烟煤AСs上显著增加(2.3 ~ 2.5倍)。Ph和CPh电容随ac比表面积的增加呈线性增加(R2≥0.966)。在Sumi和Smi参数上也发现了类似的依赖性。结果表明,在任何大小的孔隙表面,酚类物质的吸附量都是相等的。从硬煤和无烟煤中发现了一个总的趋势:CR的增加降低了活性炭的微孔隙率和表面,降低了Ph和CPh容量,但增加了比容量,即表面吸附中心的浓度。Ph和CPh的吸附包括酚醛环π电子与石墨烯层π电子的相互作用、与表面基团形成配合物以及与oh -基团形成氢键。它们的贡献取决于吸附质的性质,并随煤CR的生长而变化。
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Alkali activated coals. Microporous structure and capability to adsorb phenol compounds
The aim of the work is to compare the microporous structure characteristics of activated carbons (ACs) prepared from coals of different coals rank (CR) by alkaline activation (RKOH = 1 g/g, 800 °C) and to determine the ACs capability to adsorb phenol and 4-chlorophenol from aqueous solutions. Starting materials are coals with increasing carbon content (Cdaf = 80.0–95.6 %) selected as a CR criterion. ACs were obtained in argon in three stages: 1) thermoprogrammed heating (4 grad/min) to 800 °С; 2) isothermal exposure 1 h; 3) cooling, washing from alkali and drying. Based on low-temperature (77 K) nitrogen adsorption-desorption isotherms, integral and differential dependences of the specific surface area S (m2/g) and pore volume V (cm3/g) on the average pore diameter (D, nm) were calculated. They were used to define volumes of ultramicropores (Vumi), supermicropores (Vsmi) and micropores (Vmi). The total pore volume Vt was calculated from the nitrogen amount adsorbed at a relative pressure p/p0 ~ 1.0. The S values of ultramicropores (Sumi), supermicropores (Ssmi) and micropores (Smi) were similarly determined. The volumes and specific surfaces of different categories of pores were found to decrease with CR increase: volume Vt – from 0.59 to 0.23 cm3/g; Vmi – from 0.51 to 0.17 cm3/g; the ultramicropores volume – from 0.31 cm3/g to zero in anthracite AC. The supermicropores volume is almost independent on CR and varies in the wide range Vsmi = 0.15–0.22 cm3/h. The specific surface area is the maximum (S = 1547 m2/g) in AC from the coal of the lowest CR and decreases with coal metamorphism up to 322 m2/g. The micropores surfaces make dominant contributions to the S values: its portion is 94.7–99.4 %. For all ACs, the adsorption of phenol (Ph) and 4-chlorophenol (CPh) from aqueous solutions at 25 °C was studied. Adsorption kinetics and isotherms are best described by the pseudo-second order model and the Langmuir model (R2 ≥ 0.998). With increasing CR, the maximum adsorption capacities decrease from 3.113 to 1.498 mmol/g (Ph) and from 3.9 to 2.1 mmol/g (CPh), that is approximately ~2 times when the specific surface area decreases by ~5 times. The Ph and CPh specific capacities, characterizing the adsorption capacity of 1 m2 of surface, change little at Cdaf≤86 %, but markedly increase (2.3–2.5 times) for anthracite AСs. The Ph and CPh capacitances were determined to increase linearly (R2 ≥ 0.966) with increasing ACs specific surface area. Similar dependences were found on the Sumi and Smi parameters. The phenols were concluded to be equally adsorbed on the surface of pores of any size. A general trend was found for ACs from hard coals and anthracite: an increase in CR reduces the ACs microporosity and surface, decreases Ph and CPh capacities but increases specific capacities, i.e. concentrations of surface adsorption centers. The Ph and CPh adsorption was accepted to include the interaction of π-electrons of phenolic rings and π-electrons of graphene layers in ACs, the formation of complexes with surface groups and forming hydrogen bonds with OH-groups. Their contributions depend on adsorbate nature and change with the growth of fossil coals CR.
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