掺杂金属离子可进一步提高MOFs的气体吸附能力[8~11]。Han等[12]研究了Li掺杂的Li·Cu3(BTC)2和Li·MIL-101(Cr)的H2吸附能力，发现在77 K和100 kPa下其储氢能力分别提高到145%、143%。Yang等[13]将NOTT-200浸渍在锂的氯盐溶液中，发现其储氢量大大提高。Cao等[14]研究了Li+、Na+和K+掺杂对HKUST-l的CO2吸附性能的影响，发现在298 K和1800 kPa下其CO2吸附量最高可达8.64 mmol/g(1K-HKUST-l)，比纯HKUST-l提高11%。Poloni等[15]基于密度泛函理论(DFT)计算了Ca、Mg和9个二价过渡金属阳离子对M-BTT和M-MOF-74的CO2吸附行为的影响，发现二价阳离子的特定电子结构和金属配位点在CO2结合时产生的对称性有利于对气体的吸附。
本文以对苯二甲酸为配体，钛酸异丙酯为金属源用溶剂热法制备金属有机骨架材料MIL125，然后用盐溶液浸渍进行后功能化改性，制备一系列Li+、Na+和K+掺杂的M@MIL125-t(M代表碱金属：Li，Na和K；t代表氯盐溶液浸渍处理时间：6，9，12 h)，研究其在293 K和101.325 kPa条件下对CO2吸附性能。
1 实验方法
1.1 实验用试剂和仪器
实验用试剂：对苯二甲酸(CAS：100-21-0，98%)；钛酸异丙酯(CAS：546-68-9，97%)；N,N-二甲基甲酰胺(DMF，CAS：68-12-2，分析纯)；甲醇(CAS：67-56-1，分析纯)；氯化锂(CAS：7447-41-8，97%)；氯化钠(CAS：7647-14-5，99.5%)；氯化钾(CAS：7447-40-7，99.5%)。
实验用仪器：傅里叶变换红外光谱仪(FT-IR，Bruker AXS TENSOR-27)；X射线衍射仪(XRD，Bruker-D8 Advance)；热重分析仪(TG，TA-SDT Q600)；扫描电子显微镜(SEM，SU8020场发射扫描电子显微镜)；气体吸附(Quantachrome Instruments Autosorb-1物理吸附仪)，在77 K下进行氮气等温吸脱附测试，在293 K下进行CO2吸附测试。
1.2 MIL125和M@MIL125-t 样品的制备
MIL125的制备：将1.3108 g的对苯二甲酸，加入到25 mL的N,N-二甲基甲酰胺/甲醇(体积比4∶1)混合溶剂中，得到均匀混合液；将1.440 mL钛酸异丙酯在0℃冰水浴条件下缓慢加入到混合液中，得到白色悬浊液；然后将悬浊液移入100 mL内含聚四氟乙烯的高压反应釜中，在110℃下晶化72 h，冷却后用DMF浸泡活化处理过夜后离心分离，最后将样品在60℃真空干燥6 h，然后甲醇用相同的步骤浸泡活化处理，得到MIL125。
M@MIL125-t的制备：用碱金属氯盐配置成摩尔浓度为0.02 mol/L的浸渍液。将0.0424 g氯化锂、0.0585 g氯化钠和0.0745 g氯化钾分别溶解在50 mL无水乙醇/DMF混合溶剂(体积比4∶1)中。将9份质量为1 g的MIL125粉末分别浸渍在上述三种浸渍液中，浸渍时间分别为6 h、9 h和12 h。浸渍完成后将产物用无水乙醇洗涤三次，抽滤后在60℃真空干燥8 h，将得到的九组掺杂样品记为M@MIL125-t。
2 结果和讨论
2.1 样品的傅里叶红外光谱
图1
红外光谱中出现在1400-1590 cm-1范围内的吸收峰对应COO-的对称伸缩振动和O-C-O的不对称伸缩振动，出现在745 cm-1附近的吸收峰对应Ti-O的伸缩振动，出现在505~760 cm-1范围内的吸收峰对应Ti-O-Ti的对称伸缩振动[17]。光谱中出现在3200~3600 cm-1范围内的吸收峰来自样品吸附的水，这种低强度的宽峰表明水的含量很低。
2.2 样品的物相组成
图2
对比不同浸渍金属Li+、Na+和K+和浸渍时间6h、9 h和12 h后MIL125的晶体结构，发现在每种碱金属的氯盐溶液中浸渍一定时间后MIL125的衍射峰强度均稍有减弱。其原因是，盐溶液浸渍处理并干燥后，不同尺寸的碱金属离子沉积在MIL125的孔道内外，特别是滞留在孔道内的碱金属离子堵塞了孔道，使晶格尺寸发生变化，引起MIL125晶体结构的细微变形。
2.3 热稳定性
图3给出了碱金属掺杂改性MIL125前后(在空气中)的热失重曲线。可以看出，掺杂Na+和K+的MIL125其失重曲线基本相同，后者的质量损失略有减小。掺杂Li+的MIL125其质量损失十分明显。其原因是，用于掺杂的Li+半径较小，洗涤干燥后掺杂的比例降低。此外，随着浸渍时间的延长MIL125的总质量损失有所降低，因为掺杂时间的延长有利于Li+进入孔道和稳定晶体骨架。
图3
2.4 MIL125，Li@MIL125-9和K@MIL125-9的形貌
图4
图4
MIL125，Li@MIL125-9和K@MIL125-9的SEM照片
Fig.4
SEM images of MIL125, Li@MIL125-9 and K@MIL125-9
图5
以Na@MIL125-t为例，研究了Na+浸渍掺杂时间对MIL125微观形貌的影响。浸渍液的摩尔浓度均为0.02 mol/L，浸渍时间分别为6 h、9 h、12 h，结果如图5所示。可以看出，随着浸渍时间的延长Na@MIL125-t均为方盘状。腐蚀的强度与电解质溶液浓度和浸渍时间有关。随着浸渍时间的延长腐蚀程度提高，粒径变小的颗粒数量逐渐增多。
为了验证碱金属盐溶液后浸渍掺杂的有效性和碱金属在MIL125骨架中的存在情况，以K@MIL125-9为例，进行SEM-EDS面扫描研究了Ti、K和Cl元素的分布。从图6可见，K+均匀地分布在K@MIL125-9中。这表明，碱金属离子浸渍及活化处理后仍能很好的结合在MIL125的骨架结构当中，表明这种后掺杂简单有效。Na@MIL125-9的EDS结果也表明，Na+的分布均匀。
图6
2.5 样品的N2 等温吸脱附
图7
图7
MIL125和M@MIL125-t的N2等温吸脱附曲线
Fig.7
N2 adsorption isotherms of MIL125 and M@MIL125-t at 77 K
图8
掺杂后MIL125的比表面积均发生了变化，主要是碱金属盐溶液浸渍腐蚀了MIL125的骨架结构。掺杂不同碱金属时，M@MIL125-t的最佳浸渍时间均为9 h。掺杂Li+时，浸渍时间较短时大量Li+因半径较小而聚集在孔道中，使孔道堵塞而减小了比表面积。随着浸渍时间的增加骨架和溶液中的Li+达到动态平衡，浸渍液置换孔道中未配位的有机配体，使MIL125-t的结构发生了变化。
2.6 样品的CO2 吸附性能
图9给出了MIL125和M@MIL125-t在293 K和101.325 kPa条件下的CO2吸附曲线，图10给出了掺杂不同种类碱金属离子时CO2吸附量与比表面积的关系。可以看出，MIL125的CO2吸附量为0.82 mmol/g，掺杂Li+、Na+、K+后CO2吸附量均有提高，其原因可能是CO2优先吸附在碱金属离子的周围。浸渍处理时间为9 h时，Li@MIL125-t、Na@MIL125-t和K@MIL125-t的比表面积和CO2吸附量达到最大。Na@MIL125-9的CO2吸附量最高为1.41 mmol/g，提高了约72.0%；Li@MIL125-9的CO2吸附量为1.21 mmol/g，提高了47.6%；K@MIL125-9的CO2吸附量为1.18 mmol/g，提高了43.9%。
图9
图9
MIL125和M@MIL125-t的CO2吸附曲线
Fig.9
CO2 adsorption isotherms of MIL125 and M@MIL125-t (293 K, 101.325 kPa)
图10
图10
CO2吸附量与MIL125、M@MIL125-t中SBET的关系
Fig.10
Relationship between CO2 uptake and SBET in M@MIL125-t(293 K and 101.325 kPa)
可以看出，碱金属掺杂影响MIL125的CO2吸附性能，CO2的吸附量与比表面积的关系很大。Na@MIL125-9的比表面积和CO2吸附量都最大，Li@MIL125-9的比表面积变小，但CO2吸附量增大。值得注意的是，Li@MIL125-12和K@MIL125-9的CO2吸附量接近，但是其比表面积分别不同(分别为861 m2/g和1521 m2/g)。其原因可能是，Li+的第一电离能和Lennard-Jones势更低，在101.325 kPa条件下对CO2的吸附性能更显著[19]。
3 结论
(1) 用不同种类的碱金属和浸渍时间对MIL125掺杂，对其成键情况、物相结构和热稳定性没有明显的影响。碱金属浸渍液的腐蚀，使MIL125的形貌逐渐由圆饼状向方饼状转变，晶粒尺寸显著减小。
(2) 用Li+、Na+和K+三种碱金属掺杂，碱金属盐溶液浸渍9 h后M@MIL125-t的比表面积最大，Na@MIL125-t系列的比表面积最高。
(3) 用碱金属掺杂后，M@MIL125-t的CO2吸附量比MIL125均有不同程度的提高，因为CO2优先吸附在新引入的Li+、Na+和K+周围。
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