图1 MOF-1的Cd离子配位环境图(a)、1D共价链(b)和3D氢键超分子网络(c)
纸质出版日期:2024-04-25,
收稿日期:2023-09-19
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工业废水处理对于维护生态平衡、实现可持续发展具有重要意义,研究开发新的污水处理剂,实现污染物催化降解是从根本上解决废水问题的有效方案。基于此,从晶体工程原理出发,以选材广泛、结构可调的MOFs材料为研究对象,通过选用亚甲基双羟萘酸(H2pam)和2,5-双(4-吡啶基)-1,3,4-噻二唑(bpt)作为构件,成功制备了一例MOFs材料,即[Cd(bpt)(H2O)4]n·n(pam)·2nH2O (MOF-1)。结构分析和性能表征说明,MOF-1具有一维共价链状和复杂的超分子构型,其合成方法简单、品质好,且形貌较为单一,尺寸为0.5~2 μm;主体框架在310 ℃以下稳定存在并具有较好的化学稳定性,在强酸和中性水溶液中可保持完整形貌不少于60 d和163 d;呈现明亮的绿色荧光,可在较宽温度范围内作为分子发光温度计。催化降解工业染料废水的模拟研究显示,MOF-1材料的禁带宽度Eg=2.85 eV,带隙值小,对罗丹明B的脱色率达到了85.94%,可显著降解。因此,MOF-1材料是一种具有较大比较优势的染料污水处理剂。
Industrial wastewater treatment is of great significance for maintaining ecological balance and achieving sustainable development. Developing new removal agents with the feature of catalytic degradation of pollutants is an effective solution to the wastewater treatment. To address this, metal-organic frameworks (MOFs) with inherent characteristics of wide material sources and adjustable structures were proposed and demonstrated according to the principle of crystal engineering. Therefore, a MOF of [Cd(bpt)(H2O)4] n·n(pam)·2nH2O (MOF-1) was prepared by using pamoic acid(H2pm) and 2,5-bis (4-pyridyl)-1,3,4-thiadiazole (bpt) as components.The structural analysis and physico-chemical characterization showed that MOF-1possessed complex supramolecular network with one-dimensional covalent chain, the simple synthesis method, good product quality, and the uniform morphology with the size of about 0.5~ 2 μm. The main framework of MOF-1 was thermostable till 310 ℃. The crystal products owned excellent chemical stability of not less than 60 days and 163 days in strong acid and neutral aqueous solution, respectively. Moreover, MOF-1 displayed bright green fluorescence and could be used as a molecular luminescence thermometer over a wide temperature range.The catalytic degradation of organic dyes performed by MOF-1 in simulated industrial wastewater revealed that the materials had a narrow bandgap width Eg=2.85 eV and a large decolorization rate of Rhodamine B with 85.94%. Hence, MOF-1 is a brilliant wastewater treatment agent with greater comparative advantages with lots of known materials.
近年来,突发性水体污染事件频发,严重影响了饮水安全和生态圈的自清洁循环,有效处理污染水体成为时代之需。目前,水体治理基本通过物理吸附处理或水体自洁,然而污染水体成分复杂,传统方法存在选材窄、价格昂贵、处理不彻底等局限性[
在MOFs催化剂的合成中,与单一构件相比,两类配体协同作用,能满足不同金属中心的配位和电荷需求,增加MOFs材料的稳定性和框架结构的可调性,提高了微结构的可预测性[
实验所用的化学试剂,如亚甲基双羟萘酸、Cd(OAc)2·2H2O、2,5-双(4-吡啶基)-1,3,4-噻二唑、罗丹明B、NaOH、甲醇、浓盐酸、双氧水(H2O2)均为市售所得,并直接用于实验,所用去离子水为实验室自制。
仪器型号:APEX II SMART CCD X射线单晶衍射仪(德国布鲁克公司);240C元素分析仪(美国珀金埃尔默公司);Miniflex 600型粉体X射线衍射仪(日本理学株式会社);TGA/STDA 851e型综合热分析仪(瑞士梅特勒-托利多公司);Gemini 300型场发射扫描电子显微镜(德国蔡司公司);X-MAX能谱仪(英国牛津仪器公司);H7700场发射透射电子显微镜(日本日立公司);F-4600荧光分光光度计(日本日立公司);TCB1402C荧光高温附件(上海天美科学仪器有限公司);UV-3600Plus UV-VIS-NIR分光光度计(日本岛津公司);UV-2600型UV-VIS分光光度计(日本岛津公司)。
将Cd(OAc)2·2H2O (0.2 mmol,0.042 5 g)、H2pam (0.2 mmol, 0.077 7 g)、bpt (0.2 mmol, 0.048 0 g)和水/甲醇(7/2 mL)充分混合,再滴加0.025 M的NaOH水溶液直至体系pH为5~6,室温下继续搅拌10 min,移入10 mL西林瓶中,然后置于有聚四氟乙烯内衬的25 mL不锈钢反应釜中,在120 °C下保温72 h,控制冷却速度1 ℃/h匀速降温至室温,最终得到浅黄色块状晶体MOF-1,产量为0.142 2 g,产率为84%(以Cd为基准)。元素分析(%, C35H33.5CdN4O12S)计算值:C 49.65,H 3.99,N 6.62,S 3.79;实验测得:C 49.81,H 3.84,N 6.49,S 3.67。
室温下挑选结晶度高的晶体样品进行单晶结构表征。晶体结构使用SHELXS-2013程序通过直接法解得[
罗丹明B(rhodamine B,RhB)是一种在工业生产中广泛使用的人工合成染料,利用罗丹明B作为催化降解对象具有一定的代表性。催化降解过程为:配置适量3×10-5 mol/L的罗丹明B水溶液,量取75 mL罗丹明B溶液和0.075 0 g的MOF-1粉末样品加入到光催化反应器中,同时滴加1滴浓盐酸(模拟工业染料废水多呈酸性的实际情况)。将混合体系避光搅拌30 min并在暗室中静置24 h,使MOF-1粉体与染料溶液达到吸附脱附平衡。然后滴加俘获剂(质量分数30% H2O2)若干,并置于500 W Xe灯下进行催化反应,间隔10 min取样、离心分离,测定溶液的吸光度。催化效果采用染料溶液的脱色率D表示:D=[(A0-A)/A0]×100%,式中A0、A分别为染料溶液光照前、后的吸光度。
化合物MOF-1的晶体学数据:实验式C35H33.5CdN4O12S,Mr=846.62,三斜晶系Pī空间群,a=7.772(2)Å,b=13.647(4)Å,c=18.820(6)Å,α=79.145(4)°,β=82.266(5)°,γ=86.803(5)°,V=1941.5(10) Å3,Z=2,Dc=1.448 g·cm-3,总衍射数14 825,独立衍射数7 163,R(int)=0.100 7, 2.45°<θ< 25.50°,GOOF=1.059,R1[I>2σ(I)]=0.119 0,wR2 [I>2σ(I)]=0.300 4。CCDC号:2245687。MOF-1的部分键长和键角数据列于
键长/Å或键角/° | |
---|---|
Cd1-O3 | 2.254(11) |
Cd1-O4 | 2.281(10) |
Cd1-N1 | 2.298(10) |
Cd2-O1 | 2.277(13) |
Cd2-O2 | 2.280(15) |
Cd2-N4 | 2.319(12) |
O3-Cd1-O4 | 95.3(4) |
O3-Cd1-N1 | 89.8(4) |
O4-Cd1-N1 | 86.2(4) |
O1-Cd2-O2 | 93.0(6) |
O1-Cd2-N4 | 87.5(4) |
O2-Cd2-N4 | 90.3(5) |
单晶结构分析表明,MOF-1的最小不重复单元中,有1个Cd离子(Cd1和Cd2各取半个位置)、1个配位的中性bpt分子、4个配位水分子、1个游离的pam2-阴离子以及2个游离水分子〔见
图1 MOF-1的Cd离子配位环境图(a)、1D共价链(b)和3D氢键超分子网络(c)
Fig. 1 Coordination modes of Cd(II) ions (a), 1D covalent chain (b) and 3D H-bonding supramolecular network (c) in MOF-1
注: 对称代码:A-x+1,-y,-z+1;B-x+1,-y+2,-z
2.3.1 物相纯度
室温下,对MOF-1的批量粉体样品进行了粉末X射线衍射测试(PXRD),与Mercury软件导出的单晶体衍射图谱相比,除基线平整度外,批量产品实际测试的衍射图谱与单晶体样品模拟的衍射图谱在峰位、峰形、峰数等方面高度吻合(见
图2 MOF-1单晶体样品模拟和合成产物的PXRD谱图
Fig. 2 PXRD patterns of single-crystal structure determination and as-synthesized product of MOF-1
2.3.2 形貌分析
通过水热法制备的MOF-1样品结晶度较好,可使用场发射扫描电子显微镜(FESEM)和场发射透射电子显微镜(FETEM)观察样品的微形貌。
图3 MOF-1的FESEM图(a)和FETEM图(b)
Fig. 3 FESEM(a) and FETEM(b) images of MOF-1
图4 化合物MOF-1的元素分布图
Fig. 4 Element mapping of MOF-1
2.3.3 稳定性分析
样品MOF-1在空气中稳定存在,并能保持晶体的完整性。为了检验MOF-1的热稳定性,对粉体样品进行热重分析(TG),如
图5 MOF-1的热重曲线图(插图为热重分析最终产品)
Fig. 5 TG curve inlet with the final product of MOF-1
为了检验MOF-1的化学稳定性,室温下挑选4份适量的块状晶体样品置于玻璃瓶中,并分别注入pH值为1、6.5、10、14的水溶液,其中盐酸和NaOH作为酸碱度调节剂。通过体视显微镜逐天观察晶体样品的形貌,可粗略判断样品的化学稳定性(见
图6 MOF-1晶体在pH=1、6.5、10、14的水溶液中的形貌
Fig. 6 Single-crystal shapes of MOF-1 in aqueous solutions with pH 1、6.5、10 and 14 respectively
具有良好的热稳定性和化学稳定性是材料实现产业化的前提和基础,样品MOF-1优异的稳定性,能满足基于实际生产和应用的需要。
2.3.4 荧光特性
室温下对MOF-1粉体进行了固体荧光光谱测试(见
图7 MOF-1的发射光谱(a)和CIE色度坐标(b)
Fig. 7 Fluorescent emission spectrum (a) and CIE chromaticity coordinates (b) inlet with a luminous image under ultraviolet irradiationof MOF-1
固态样品良好的荧光性能,促使我们继续探索在温度变化情况下粉体样品的发光行为。如
图8 MOF-1在不同温度下激发、发射光谱(a)和最强谱峰强度比-温度的线性关系(b)
Fig. 8 Excitation and emission spectra at different temperatures (a) and linear relationship between maximum spectral peak intensity ratio and temperature (b) of MOF-1
2.4.1 紫外-可见漫反射光谱分析
紫外-可见漫反射吸收光谱(UV-Vis DRS)能够直接检测半导体材料的光响应范围。室温下以高纯BaSO4作参比扣出背景,扫描范围为200~800 nm,扫描速度为1 nm/s。
图9 MOF-1的紫外-可见漫反射谱图和Tauc曲线图(插图)
Fig. 9 UV-Vis diffuse reflection spectra and Tauc diagram(inset) of MOF-1
2.4.2 染料的催化降解
MOF-1材料中2类配体都具有较大的共轭π平面,并且材料的禁带宽Eg较小,有很强的吸收光子的能力,可作为光催化材料。选用经过酸化后的RhB水溶液作为污染模型评价MOF-1的光催化活性,RhB的最大吸收波长为554 nm,可作为特征吸收峰用来实时监测染料吸附和光催化降解全过程。新制RhB溶液在554 nm处的吸光度为0.974,加入MOF-1后,经过搅拌陈化,使RhB染料分子在MOFs粉体表面实现吸附脱附平衡,测得在554 nm处的吸光度为0.758,MOF-1的粉体吸附染料高达22.18%。
为详细研究MOF-1的光催化能力,分别针对RhB染料、RhB染料/H2O2、MOF-1粉体/RhB染料、MOF-1粉体/RhB染料/H2O24组实验进行了比较研究。MOF-1粉体/RhB染料的比例为1 g/L,75 mL染料溶液中H2O2的添加量约为5 μL。4组样品的紫外-可见吸收光谱如
图10 RhB(a)、RhB/H2O2(b)、MOF-1/RhB(c)、MOF-1/RhB/H2O2(d)的紫外-可见吸收光谱图
Fig. 10 UV-Vis absorption spectra of RhB (a), RhB/H2O2(b), MOF-1/RhB (c), MOF-1/RhB/H2O2(d)
根据朗伯-比尔定律,当溶质浓度较低时,吸光度A=abc,A与溶质浓度c存在线性关系。基于此,可将4组实验进行数据处理,以吸附平衡的染料溶液(浓度为c)在无光照下测试的吸光度作为A0,则实时测试的染料浓度c=(A0/A) C0。如
图11 氙灯照射下不同RhB溶液体系的光降解曲线(a)和MOF-1粉体/RhB染料/H2O2的准一级动力学曲线(b)
Fig. 11 Photodecomposition of RhB dyes in different solutions with Xe light irradiation (a) and the pseudo-first order kinetic curve of MOF-1/RhB/H2O2(b)
本研究以2例具有大共轭π平面的芳香基有机物为构件,以过渡金属Cd为中心离子,通过晶体工程原理构筑了一例低维MOFs材料,确定了晶体结构。该材料合成简单、产率大、纯度高、形貌规则,表现出优异的热稳定性、化学稳定性和荧光特性,可在较宽温度范围内用作分子发光温度计。同时,材料的带隙值小,对罗丹明B具有良好的光催化活性,可作为高效降解染料废水的处理剂。后续,我们将着手开展延伸研究,通过合理调控MOFs材料的结构类型,深入探索材料的微结构与催化降解性能之间的构效关系,揭示催化降解机理,继续筛选出更多质优价廉的污水处理剂,从而为废水处理领域提供更多选择。
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