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近年来,臭氧(O3)逐渐成为各省市的首要污染物[1-2]。近地面的臭氧主要是由大气环境中的挥发性有机化合物(VOCs)与其它化合物(如NOx)在阳光照射下发生光化学反应生成的[3]。因此,VOCs的捕集与治理是控制臭氧工作的难点和关键[4]。VOCs种类繁多,来源复杂[5-7]。其中,溶剂产品使用源在我国VOCs排放中占有较大比重(>20%),而苯系物又占总其排放量比重的30.1%[8]。苯系物不仅会对生态环境造成严重污染[9],同时也会对人体免疫系统造成伤害等。
很多吸附剂在处理低浓度VOCs上取得了有效突破,展现出较大的应用前景[10-14]。廖苑如[9]发现改性SBA-15对甲苯的饱和吸附容量较大,且脱附再生性能优异。黄海凤等[14]发现MCM-41分子筛对低浓度VOCs吸附效果良好。MFI型ZSM-5分子筛具有特殊的高硅三维交叉直通道,丰富可调的表面性质,是理想的催化剂载体[10];同时其孔径尺寸与苯的分子动力学直径相当,可作为苯吸附材料。
虽然许多学者在MFI型分子筛吸附苯方面进行过研究[15-19],但其苯吸附容量有限;且近些年对ZSM-5的改性也多集中于金属负载或是针对工业应用的催化改质[20-26]。而非金属元素负载在保证低成本的前提下能有效提高分子筛的吸附容量[10]。本研究以尿素为氮源制备改性ZSM-5分子筛,通过一系列表征手段考察其微观结构,以稳定的苯分子为探针物,阐明氮改性提高苯吸附性能的机理,为开发高性能、低成本的苯吸附材料提供一定的思路。
氮改性ZSM-5分子筛的苯吸附性能
Study on benzene adsorption properties on nitrogen modified ZSM-5 zeolites
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摘要: 为解决固体吸附剂ZSM-5型分子筛对VOCs(挥发性有机物)吸附容量较低的难题,本文以ZSM-5型分子筛作为载体材料,尿素作为氮源,采用一步煅烧法制备得到氮改性ZSM-5分子筛。利用X射线衍射分析仪(XRD)、傅里叶变换红外光谱仪(FTIR)、比表面与孔隙分析仪(BET-BJH)等对氮改性ZSM-5分子筛进行了表征,并采用固定床探究了氮改性ZSM-5分子筛对苯的吸附机制。结果表明,改性后分子筛的骨架结构未被破坏,氮通过尿素与分子筛上羟基间的相互作用成功引入到ZSM-5型分子筛上,所制备得到的氮改性ZSM-5分子筛具有典型的微孔结构。ZSM-5分子筛中引入的含氮官能团,不仅能作为新的吸附活性位点,还能提高苯在吸附剂孔道中的扩散速率。研究结果能为开发高性能低成本的VOCs吸附材料提供一定思路。Abstract: In this paper, modified ZSM-5 based adsorbents were prepared to enhance its VOCs (volatile organic compounds) adsorption capacity. Different N contents from urea were introduced into ZSM-5 zeolites through one-step solid-state method. The microstructures and surface physic-chemical properties of adsorbents were characterized by various techniques like X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmett-Teller-Barret-Joyner-Halender Measurement (BET-BJH) and so on. The adsorption performance of benzene was also investigated using fixed bed. The experimental results showed that the typical microporousstructure of ZSM-5 was maintained. Nitrogen contained groups were successfully introducedintoZSM-5 zeolites based on the interaction between urea and the surface hydroxyl groups of ZSM-5. The nitrogen contained groups not only showed strong inter-molecular interaction with benzene, but also modified its diffusion rate in the pores of zeolites. This work provided a novel idea for the development VOCs adsorbents with high performance and low cost.
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Key words:
- ZSM-5 zeolites /
- urea /
- modification /
- benzene adsorption
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表 1 改性ZSM-5分子筛吸附剂的织构性能和元素含量
Table 1. BET characterizations and element content analysis formodified ZSM-5 adsorbents
样品名
SamplesBET比表面积/(m2 ·g−1)
BET surface
area单点吸附孔容/(cm3 ·g−1)
Single point adsorption
pore volume平均吸附孔/nm
Average particle
sizeN/% C/% ZSM-5 380.54 0.17 1.84 − − 0.17N-ZSM-5 392.11 0.17 1.78 0.15 0.15 0.25N-ZSM-5 398.41 0.18 1.79 0.12 0.14 1N-ZSM-5 399.32 0.19 1.88 0.23 0.21 2N-ZSM-5 418.43 0.19 1.83 0.09 0.18 4N-ZSM-5 404.20 0.19 1.87 0.04 0.01 6N-ZSM-5 392.53 0.18 1.85 0.04 0.03 urea 50.27 0.13 2.04 4.08 2.87 表 2 改性ZSM-5分子筛的吸附容量及拟合曲线参数值
Table 2. Adsorption properties and logistic fitting curve parameters for modified zeolite adsorbents
样品名
SamplesA1/(mg·m−3) A2/(mg·m−3) t0/min p/min−1 R2 吸附容量/(mg·g−1)
Adsorption capacityZSM-5 −3.05 110.45 24.01 1.54 0.99 9.70 0.17N-ZSM-5 −1.23 193.69 198.80 2.91 0.99 40.51 0.25N-ZSM-5 −2.09 129.78 105.56 2.26 0.99 31.99 0.5N-ZSM-5 −0.86 135.59 132.93 2.74 0.99 38.69 1N-ZSM-5 −0.26 106.10 122.38 8.15 0.99 42.26 2N-ZSM-5 0.06 106.21 137.67 6.05 0.99 47.56 4N-ZSM-5 −2.08 112.93 43.97 2.20 0.99 14.73 6N-ZSM-5 −1.96 105.55 35.23 3.43 0.99 11.92 表 3 不同改性ZSM-5对VOCs吸附性能对比
Table 3. Comparison of VOCs adsorption capacity among different zeolites
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