Review on recent advances in nanocellulose aerogels for oil-water separation
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摘要:
石油泄漏事件频繁发生，对环境和人类健康造成了极大的危害，因此亟需对含油废水进行有效的处理。目前的吸油材料存在局限性，如吸附量小、成本高和对环境有害等，而纳米纤维素气凝胶由于具有高孔隙度、高比表面积和低密度等特点，经疏水改性后能够吸附大量的油，在油水分离中有着显著优势。本文综述了纳米纤维素气凝胶的制备和疏水改性方法；介绍了纳米纤维素气凝胶结构特点及对其吸附性能的影响，综述了近年来纳米纤维素气凝胶在油和有机溶剂的吸附及油水混合物分离中的应用；最后提出了纳米纤维素气凝胶的发展现状及对未来的展望。
Abstract:The frequent occurrence of oil spills has caused great threat to environment and human health, so it is urgent to treat oily wastewater effectively. Current oil absorbing materials have limitations, such as low absorption capacity, high cost and environmental harm. Nanocellulose aerogel can absorb a large amount of oil because of its high porosity, high specific surface area and low density. After hydrophobic modification, nanocellulose aerogel has a significant advantage in oil-water separation. The preparation and hydrophobic modification of nanocellulose aerogel are reviewed in this paper. The structural characteristics of nanocellulose aerogel and their influence on absorption performance were introduced. The application of nanocellulose aerogel in adsorption of oil and organic solvent and separation of oil-water mixture in recent years were reviewed. Finally, the development status and future prospects of nanocellulose aerogel are presented.
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Keywords:
- aerogels /
- nanocellulose /
- absorption /
- hydrophobicity /
- oil absorption /
- oil-water separation
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膜分离技术具有高能效、易操作、环境友好和占地面积小等优点，近年来在气体分离领域受到广泛关注[1-4]。传统气体分离膜多以聚合物膜为主，然而由于聚合物分离膜固有的选择性和渗透性的制约关系(Trade-off效应)，使其性能很难再提升[5-6]。研究者发现将多孔材料与聚合物基体共混制成混合基质膜，通过结合两种材料的优点，能够同时提升膜的气体渗透性和选择性，从而突破聚合物膜的Trade-off效应[7-8]。因此，制备混合基质膜是改善膜气体分离性能的一种有效方法。
对于混合基质膜，填料和聚合物基体材料的选择尤为重要。聚酰亚胺由于其优异的热稳定性、良好的力学性能以及可加工性，已经在气体分离膜领域发展多年，是混合基质膜聚合物基体候选材料之一[9-11]。对于填料材料的选择，共价有机框架材料(COFs)是一种由有机单元通过共价键构成的多孔材料，由于其具有优异的稳定性，易功能化、永久空隙率以及高比表面积等优点，在气体分离领域展现出不俗的潜力[12]。由于COFs全有机的性质，使其能够均匀地分散在聚合物基质中，减少了混合基质膜中由于界面缺陷产生的非选择性孔[13-14]。然而，大部分COFs的孔径很难做到2 nm以下，相对于气体分子动力学直径(N2：0.36 nm；O2：0.35 nm；CO2：0.33 nm)还是较大，难以实现对气体的高效分离，从而降低了气体选择性[15-16]。因此，需要对COFs的孔径大小调控或引入一些功能性吸附位点进行改善。研究表明，引入氟原子能够有效改善COFs的孔径大小且能够提供与气体相互作用的吸附位点。Alahakoon等[17]通过使用含氟单体制备出两种氟化COFs，将氟化COFs与未氟化的相比，发现氟化COFs具有更大的比表面积、更小且更明确的孔径。Gao等[18]报道了3种具有—H、—Me和—F取代基的同构三维共价有机骨架，对比不含氟的COFs，氟化COFs具有更高的CO2亲和力，对CO2/N2有着更高的IAST选择性。Yang等[19]制备了一种氟化CTF，通过氟原子的强静电作用以及C—F键与CO2的偶极-四极作用，使其具有优异的CO2吸附能力。
基于上述讨论，本文合成出一种具有较小孔径、高比表面积的氟化共价有机框架材料(TpPa-CF3)。随后，以TpPa-CF3为填料，聚酰亚胺(6FDA-ODA)为基体，制备出不同负载量的TpPa-CF3/6FDA-ODA混合基质膜。表征了其结构和表面、截面的微观形貌，探究了其热性能、力学性能以及疏水性能，最后讨论了混合基质膜的气体渗透性以及在烟道气分离(CO2/N2)和空气分离(O2/N2)上的应用前景。
1. 实验材料及方法
1.1 原材料
4,4-二氨基二苯醚(ODA，98%)、4,4-(六氟异丙烯)二酞酸酐(6FDA，98%+)、2,4,6-三甲酰间苯三酚(Tp，98%)、2-三氟甲基-1,4-苯二胺(Pa-CF3，97%)、1,3,5-三甲基苯(99%+)、1,4-二氧六环(99%)均购自上海阿达玛斯试剂有限公司；乙酸(AR)、N,N-二甲基甲酰胺(DMF，AR)均购自西陇科学股份有限公司；间甲酚(m-Cresol，99%)、异喹啉(97%)均购自上海阿拉丁试剂有限公司；丙酮(Acetone，AR)，成都市科隆化学品有限公司；工业酒精(95%)，弘昊实验设备有限公司。
1.2 TpPa-CF3的合成
将Tp (63.0 mg，0.30 mmol)、Pa-CF3 (79.0 mg，0.45 mmol)、1,3,5-三甲基苯(1.5 mL)、1,4-二氧六环(1.5 mL)依次加入到Pyrex管(25 mL)中。为使混合物均匀分散，超声处理0.5 h，再加入3 mol/L乙酸溶液(0.5 mL)。随后，用液氮将Pyrex管骤冻抽出空气，再在室温下解冻，此操作循环3次。密闭封管，将Pyrex管在120℃下油浴3天。反应完毕后冷却至室温，过滤收集产物，先用DMF溶液搅拌洗涤3次，再通过索氏提取法进行提纯(提纯溶剂采用丙酮)。随后，收集产物，在真空烘箱中120℃下干燥12 h后，得到橘红色粉末样品TpPa-CF3。
1.3 聚酰亚胺(6FDA-ODA)的合成
在N2氛围下，向装有机械搅拌、冷凝回流的150 mL三口烧瓶内依次加入ODA (2.00 g，9.99 mmol)、间甲酚(28 mL)，待ODA完全溶解后再依次加入6FDA (4.44 g，9.99 mmol)、间甲酚(28 mL)，随后将温度升到50℃待反应物完全溶解后，滴加5~6滴异喹啉后升温至80℃反应3 h，120℃反应3 h，180℃反应3 h，最后200℃反应12 h。反应结束冷却至室温后，将聚酰亚胺溶液缓慢倒入大量工业酒精中拉丝沉淀，过滤收集产物，在真空烘箱中150℃干燥8 h。随后，使用适量DMF重新溶解并进行二次沉淀以除去聚酰亚胺中残留杂质。
1.4 TpPa-CF3/6FDA-ODA混合基质膜的制备
取一定量的TpPa-CF3粉末分散在DMF (3 mL)中，使用细胞粉碎机，在300 W功率下超声0.5 h后再搅拌6 h，保证TpPa-CF3粉末在DMF溶液中分散均匀。同时，称取0.2 g 6FDA-ODA溶解在DMF (2 mL)中，用针式滤头(0.45 μm，尼龙)过滤除去杂质。随后，将TpPa-CF3的分散液滴加到6FDA-ODA溶液中搅拌12 h，确保TpPa-CF3和6FDA-ODA充分混合。最后，将混合溶液缓慢流延到光滑平整的玻璃板(5 cm×5 cm)上，在80℃下蒸发12 h除去溶剂，待冷却至室温后在温水中脱膜，最后在150℃的真空烘箱中干燥12 h以除去残留的溶剂分子。按以上步骤分别制备含量为0wt%、1wt%、3wt%、5wt%、7wt%的TpPa-CF3/6FDA-ODA混合基质膜。
1.5 表征测试
X射线衍射(XRD)：采用荷兰帕纳科公司的X'Pert Pro型X射线衍射仪对制备的TpPa-CF3粉末与薄膜进行晶型及结构表征，扫描范围在3°~40°，扫描速度为2°/min。
傅里叶变换红外光谱(FTIR)：采用美国尼高力公司的Nicolette 6700-NXR型傅里叶变换红外光谱仪分析TpPa-CF3和薄膜的化学键组成及官能团。对于粉末样品通过溴化钾压片的方式测试，对于薄膜样品通过制成厚度约为20 μm的薄膜直接测试。扫描范围为400~
4000 cm−1，扫描次数64次以上。固态核磁共振(ssNMR)：通过德国布鲁克公司的Avance Neo 400 WB型固体核磁共振波谱仪测试TpPa-CF3的13C NMR，分析其化学键连接方式。所需样品压实后的体积应多于0.5 cm3。
X射线光电子能谱(XPS)：采用美国热电公司的Escalab 250 Xi型X射线光电子能谱仪分析TpPa-CF3的化学元素及化学态。采用粉末压片的方式制样。
扫描电子显微镜(SEM)：通过日本日立公司的SU 4800型扫描电子显微镜表征TpPa-CF3和薄膜表面、截面的微观形貌。对于粉末样品，用牙签将少量样品涂在导电胶上制样；对于薄膜样品，膜表面直接粘在导电胶上，膜断面通过液氮脆断选取平整截面制样。全部样品在测试前通过喷金处理提高样品导电性。
N2吸附-脱附测试：通过美国康塔公司的Autosorb IQ型比表面及孔隙度分析仪器表征TpPa-CF3的比表面积及孔径分布。采用BET (Brunauer-Emmett-Teller)法计算比表面积，密度泛函理论(DFT)计算孔径分布。
热重分析(TGA)：通过德国耐驰公司的STA 449C型综合热分析仪测试TpPa-CF3和薄膜的热稳定性。在N2氛围下测试，升温速率为10℃/min，测试范围在50~800℃。
差示扫描量热分析(DSC)：通过德国耐驰公司的DSC 214型差示扫描量热仪测试薄膜的玻璃化转变温度，在N2氛围下以10℃/min的速率升温，测试范围在30~350℃，所有结果均采用消除热历史后的二次升温曲线。
力学性能：通过美国美特斯公司的CMT2103型万能试验机来表征薄膜的力学性能。薄膜样品尺寸为50 mm×10 mm，拉伸载荷为5 kN，拉伸速率为2 mm/min，标距为20 mm。
水接触角测试：通过中国承德优特仪器有限公司JY-PHb型接触角分析仪测定薄膜的亲疏水性能。薄膜样品尺寸为20 mm×20 mm，测试次数至少3次。
气体渗透性测试：通过中国济南兰光公司的VAC-V1型气体渗透仪测试薄膜的气体渗透性能。测试方法为恒体积变压法，测试气体为高纯气体(CO2、O2、N2)，测试条件为4 bar，35℃。测试过程如下，将厚度均匀的待测薄膜装入膜腔中，测试前将上下腔气压抽至20 Pa以下，随后下腔关闭，上腔通入待测纯气体形成压差。压差推动气体自上腔(高压侧)向下腔(低压侧)渗透，通过系统计算得到膜的气体渗透系数P。
2. 结果与讨论
2.1 TpPa-CF3的表征
通过粉末X射线衍射(PXRD)对合成的TpPa-CF3的晶体结构进行分析，图1(a)中2θ=4.79°处出现的强峰对应于COF的(100)晶面，其他峰也出现在2θ=7.81°、26.14°处，分别对应于(200)、(001)晶面，其中(001)晶面也是其π-π堆叠峰，通过布拉格方程计算得出其堆叠层间距为0.33 nm。将测试结果与模拟的晶体模型的衍射峰进行对比，结果表明二者衍射峰的位置与强度均匹配良好。TpPa-CF3在Pawley精修后得到的晶胞参数为a=
2.290351 nm，b=2.236760 nm，c=0.423813 nm，α=89.56415 ，β=89.73479 ，γ=120.51471 。实验结果与精修后的PXRD之间的残差值较小，Rwp=1.24%，Rp=0.91%。以上结果初步说明成功合成出了目标晶体结构，且具有良好的结晶性。为进一步说明TpPa-CF3成功合成，通过FTIR对TpPa-CF3以及其构筑单元测试，从图1(b)中可以看到，构筑单元2,4,6-三甲酰间苯三酚(Tp)在
2894 cm−1处醛基的CH=O特征峰和构筑单元2-三氟甲基-1,4-苯二胺(Pa-CF3)在3318 cm−1和3210 cm−1处的—NH2特征峰在产物TpPa-CF3中消失，表明醛胺缩合反应完全。在1282 cm−1处的C—N的特征吸收峰表明烯醇-酮异构的发生。因为框架是以酮的形式存在，结构中有强的分子内氢键及共轭作用，所以在1592 cm−1处的C=C的特征峰和1610 cm−1处的C=O特征峰合并呈肩状[20]。在1128 cm−1处出现了C—F的特征峰。13C固体NMR分析如图1(c)所示，图中显示化学位移在184.2×10−6和108.1×10−6处有两个较明显的信号峰，分别对应于烯醇-酮异构反应所形成的C=O键和C—N键上的C原子，123.6×10−6处归属于C—F上的C原子。其余在119.0×10−6、134.1×10−6和146.5×10−6处的信号峰则归属于芳香单元上的C原子。
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图 1 TpPa-CF3的PXRD精修谱图(a)、FTIR谱图(b)、13C固体核磁谱图(c)Figure 1. PXRD refined spectra (a), FTIR spectra (b), 13C solid-state NMR spectrum (c) of TpPa-CF3“*”标记的峰为测试仪器产生的旋转边带峰FTIR和固体核磁分析结果证实TpPa-CF3的成功合成且以稳定的β-酮胺形式存在。
通过XPS测量TpPa-CF3的全谱和各个元素的光谱，由图2(a)的全谱可知TpPa-CF3是由C、N、O、F 4种元素组成的。图2(b)是C1s的高分辨率XPS光谱，其能够被卷积为4个峰，分别对应于TpPa-CF3中的C=C/C—C(284.8 eV)、C—N(286.2 eV)、C=O(288.9 eV)和C—F(292.9 eV)键，N1s的高分辨率XPS如图2(c)所示，其被卷积为2个峰，分别归属于N—C(400.2 eV)和N—H(403.9 eV)键，F1s的高分辨率XPS如图2(d)所示，其只有一个卷积峰归属于C—F(688.3 eV)。所有以上结果说明TpPa-CF3形成目标结构，由C1s和N1s证明该结构发生了烯醇-酮异构。
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图 2 TpPa-CF3的XPS图谱：(a)全谱；(b) C1s；(c) N1s；(d) F1sFigure 2. XPS spectra of TpPa-CF3: (a) Survey spectrum; (b) C1s; (c) N1s; (d) F1s通过扫描电镜(SEM)观察TpPa-CF3的微观形貌。如图3(a)所示，TpPa-CF3具有均匀的微观形貌，呈现为“米粒”形颗粒堆积形成的团簇，每一颗“米粒”的尺寸在(100±30) nm。
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图 3 TpPa-CF3的微观形貌(a)、N2吸附-脱附曲线(b)、孔径分布图(c)、TGA和DTG曲线(d)Figure 3. Microscopic morphology (a), N2 adsorption-desorption curves (b), pore size distribution (c), TGA and DTG curves (d) of TpPa-CF3dV/(dlogD)—; STP—为了解TpPa-CF3的多孔性，对其进行N2吸附-脱附测试。图3(b)中N2吸脱附曲线呈现出I型曲线特征，TpPa-CF3在相对压力较低的区域(p/p0＜0.1)，N2的吸附量快速增加，说明材料中存在丰富的微孔结构。通过计算分析得出TpPa-CF3具有较大的比表面积(791.83 m2·g−1)，图3(c)显示TpPa-CF3具有较小的孔径(1.18 nm)。这归因于TpPa-CF3中氟原子的高电负性增强了框架中芳香环之间的相互作用力，这种相互作用力有助于COF形成较大的比表面积以及较小的孔径[17]。
通过热重分析TpPa-CF3的热性能。热重曲线如图3(d)所示，从图中看到热损失分为两个阶段，大约在400℃之前的损失可能为残留在孔道里的高沸点溶剂(DMF)的挥发。400℃后出现明显的质量损失，从DTG曲线上可以看到在416℃质量损失的速度最快，这主要归因于TpPa-CF3框架的分解。以上结果可以看出TpPa-CF3具有较好的热稳定性。
2.2 TpPa-CF3/6FDA-ODA混合基质膜的表征
通过XRD对膜结构表征，评价了填料对聚合物链排列的影响。从图4(a)中可以看到所有曲线在2θ=15°左右均出现典型的聚合物宽峰。通过布拉格方程计算，得到膜的分子链间距。纯6FDA-ODA膜的分子链间距为0.574 nm，随着填料TpPa-CF3负载量的增加，链间距呈现先增大后下降的趋势，7%TpPa-CF3/6FDA-ODA膜的链间距最小(0.566 nm)。分子链间距先增大主要是由于小负载量的掺入破坏了分子链的堆积，随着负载量的增大，填料与聚合物基质的相互作用逐渐增强，限制了分子链的迁移率，链间距减小有利于提高气体的选择性。同时，在TpPa-CF3/6FDA-ODA混合基质膜中没有观察到TpPa-CF3粉末的特征峰，这主要是由于在超声搅拌过程中COF填料的部分剥落[21]。
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图 4 6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜的XRD (a)和FTIR图谱(b)Figure 4. XRD patterns (a) and FTIR spectra (b) of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranes混合基质膜的FTIR图谱如图4(b)所示，所有膜都表现出6FDA-ODA的特征峰，包括C=O的对称(
1783 cm−1)和不对称拉伸(1733 cm−1)、C—N的拉伸振动(1378 cm−1)、C—O—C的拉伸振动(1157 cm−1)、C—F键的吸收峰(1110 cm−1)，以及酰亚胺环的弯曲振动(721 cm−1)，值得注意的是在1597 cm−1处的特征峰，随着填料的增加而增强，这主要归因于TpPa-CF3和6FDA-ODA中芳香环上的C=C的吸收峰重叠[22]。以上结果说明聚酰亚胺基体和填料之间具有良好的相容性，填料的加入并没有破坏聚酰亚胺的结构。为分析TpPa-CF3的加入对混合基质膜热稳定性的影响，对6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜进行了热重测试。如图5(a)所示，混合基质膜的分解分为两个阶段，第一阶段是400℃左右TpPa-CF3框架的分解，第二阶段是500℃左右6FDA-ODA基体膜的分解，填料的加入对膜的热稳定性影响不大。所有混合基质膜都表现出高达500℃的良好热稳定性，远高于工业中膜的操作温度，表明这些膜具有良好的适用性。DSC曲线用于分析膜的玻璃化转变温度(Tg)。如图5(b)所示，6FDA-ODA膜的Tg出现在297.5℃。随着TpPa-CF3负载量的增加，TpPa-CF3/6FDA-ODA膜的Tg从297.5℃逐渐增加到302.1℃，说明TpPa-CF3与6FDA-ODA之间具有良好的界面相互作用，这有利于提升混合基质膜的气体选择性[23]。
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图 5 6FDA-ODA和TpPa-CF3/6FDA-ODA混合基质膜的TGA曲线(a)和DSC曲线(b)Figure 5. TGA curves (a) and DSC curves (b) of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranes膜表面、截面的扫描电镜表征能够反映出填料在膜内的分散情况。如图6(a1)~6(e1)所示，与表面光滑平整的纯膜相比，混合基质膜的表面随着填料负载量的增加逐渐变得粗糙，在负载量达到7wt%时可以看到膜表面出现不平整及大颗粒团聚的现象。图6(a2)~6(e2)为纯膜及其混合基质膜的截面扫描电镜图，纯膜的截面表现出均匀、致密的微观结构，在1wt%~5wt%混合基质膜的截面图中能够观察到随着负载量的增加其截面形貌逐渐变粗糙，同时在膜内能够观察到TpPa-CF3颗粒很好地被聚合物包裹且分散均匀。当填料负载量达到7wt%时膜内出现填料与聚合物基质相分离的现象，说明此时负载量已经达到聚合物基质所能承受的上限，5wt%为其最优负载量。
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图 6 膜表面((a1)~(e1))和膜截面((a2)~(e2))的SEM图像((a)~(e)分别代表不同TpPa-CF3的负载量：0wt%、1wt%、3wt%、5wt%、7wt%)Figure 6. SEM images of membrane surface ((a1)-(e1)) and cross-section ((a2)-(e2)) ((a)-(e) represent different TpPa-CF3 loadings: 0wt%, 1wt%, 3wt%, 5wt%, 7wt%, respectively)通过接触角测试仪分析纯膜及其混合基质膜的水接触角(θw)。如图7(a)和表1所示，6FDA-ODA膜的水接触角为79.9°，TpPa-CF3/6FDA-ODA混合基质膜的水接触角为81.4°~89.1°，呈现逐渐增大的趋势。这主要归因于TpPa-CF3框架中含有—CF3疏水基团，因此随着TpPa-CF3含量的增加相应负载量的混合基质膜水接触角也逐渐增加。提升膜的疏水性能有助于阻止水汽进入，提升其气体传输性能。
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图 7 不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的接触角(a)和应力-应变曲线(b)Figure 7. Contact angle (a) and stress-strain curves (b) of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadings表 1 不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的力学性能Table 1. Mechanical properties of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadingsTpPa-CF3 loadings/wt% Tensile strength/MPa Elongation at break/% Young's modulus/GPa θw/(°) 0 74.1 10.1 1.59 79.9 1 79.6 9.7 1.63 81.4 3 82.9 8.6 1.70 83.1 5 93.0 7.8 1.82 84.1 7 84.5 7.3 1.76 89.1 Note: θw—Water contact angle. 对混合基质膜进行拉伸实验以此来检验其力学性能。测试结果如图7(b)和表1所示。从表中可以看出，TpPa-CF3/6FDA-ODA混合基质膜的抗拉强度和杨氏模量随着TpPa-CF3负载量的增加呈现出先增加后下降的趋势，而断裂伸长率呈现逐渐下降的趋势。这主要归因于，在混合基质膜中TpPa-CF3与6FDA-ODA之间较好的相互作用力使得填料与聚合物之间具有良好的界面相容性，增强了膜的刚性。然而，当负载量达到7wt%时，抗拉强度和杨氏模量略微下降，这主要是过量的TpPa-CF3颗粒之间发生团聚，使得界面出现缺陷导致应力集中，降低了膜的力学性能[24]。
利用3种纯气体(CO2、O2、N2)渗透测试来评估不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的渗透性及CO2/N2和O2/N2的理想选择性。结果如表2所示，每一种膜气体渗透系数的大小均与气体分子动力学直径呈反比，即膜的3种气体渗透系数大小排列为P(CO2)>P(O2)>P(N2)，3种气体分子动力学直径大小排列为N2(0.36 nm)>O2(0.35 nm)>CO2(0.33 nm)。同6FDA-ODA膜相比所有混合基质膜的气体渗透性都有所提升。由图8(a)中可得，随着TpPa-CF3含量的增加，膜的气体渗透性呈现出先增大后下降的趋势，其中5%TpPa-CF3/6FDA-ODA膜气体渗透性能最佳，P(CO2)提升了149%，P(O2)提升了138%，P(N2)提升了98%。这主要归因于TpPa-CF3的高孔隙率提高了TpPa-CF3/6FDA-ODA膜的比表面积及固有孔隙率，为气体传输提供了快速通道。TpPa-CF3负载量到7wt%时，气体的渗透性明显下降，但仍然比6FDA-ODA膜高。这主要是由于负载量过大，造成TpPa-CF3在膜内团聚堵塞了气体传输的孔道。
表 2 6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜的气体渗透系数P和理想选择性Table 2. Gas permeability coefficient P and ideal selectivity of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranesMembrane Permeability/Barrer Ideal selectivity α CO2 O2 N2 α(CO2/N2) α(O2/N2) 6FDA-ODA 12.47 2.55 0.64 19.5 4.0 1%TpPa-CF3/6FDA-ODA 16.91 3.76 0.98 17.2 3.8 3%TpPa-CF3/6FDA-ODA 22.77 4.43 1.03 22.0 4.3 5%TpPa-CF3/6FDA-ODA 31.08 6.08 1.27 24.5 4.8 7%TpPa-CF3/6FDA-ODA 18.62 4.16 1.11 16.8 3.8 Notes: 1 Barrer=10−10 cm3(STP)·cm·cm−2·s−1·cmHg−1; Ideal selectivity α=P(A)/P(B), A and B are two different pure gases. ![]()
图 8 不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的气体渗透性(a)、气体选择性(b)、72 h连续渗透性测试(c)Figure 8. Gas permeability (a), gas selectivity (b), 72 h continuous permeability performance test (c) of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadingsTpPa-CF3/6FDA-ODA膜的气体选择性变化趋势和气体渗透性变化趋势不同，如图8(b)所示，CO2/N2和O2/N2均呈现出先下降后上升再下降的趋势，CO2/N2及O2/N2的理想选择性范围分别在16.8~24.4和3.8~4.8。其中，当TpPa-CF3负载量为5wt%时，混合基质膜的CO2/N2和O2/N2选择性最好，分别是6FDA-ODA膜的125%和119%。理想选择性的提高主要归因于两个方面：一个方面是CO2和O2的分子动力学直径要小于N2的分子动力学直径，从而CO2和O2分子倾向于优先通过。另一个方面，TpPa-CF3中富含大量对CO2具有亲和力的N、O和F等电负性原子，同时框架内还存在能与CO2发生偶极-四极相互作用的强极性C—F键，因此CO2/N2选择性相较于O2/N2的提升更明显[25]。然而，当负载量到7wt%时，混合基质膜的CO2/N2和O2/N2选择性大幅下降，略低于6FDA-ODA膜，这主要归因于当TpPa-CF3的负载量增加一定程度时，其在膜内发生团聚，并和聚合物基质产生部分相分离，产生一些非选择的孔，从而造成CO2/N2和O2/N2理想选择性的大幅下降。
对TpPa-CF3/6FDA-ODA混合基质膜进行72 h的连续气体渗透性测试，以验证膜的稳定性。如图8(c)所示，该膜在72 h的运行试验中P(CO2)下降了18%，CO2/N2的选择性下降了16%，总体表现出了良好的分离稳定性。
为了评估混合基质膜的气体分离性能，图9显示了不同负载量的TpPa-CF3/6FDA-ODA混合基质膜的气体分离性能与Robeson上限的对比。当负载量为5wt%时，其气体分离性能更靠近Robeson上限，气体的渗透性与选择性同步提升。说明适量的引入TpPa-CF3能够改善聚合物膜的气体分离性能。此外，表3显示了文献[26-30]中报道的混合基质膜气体分离性能与本工作的对比，TpPa-CF3/6FDA-ODA混合基质膜显示出适中的气体渗透性以及适中的气体选择性，说明TpPa-CF3/6FDA-ODA混合基质膜还有进一步提升的潜力。
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图 9 不同负载量的TpPa-CF3/6FDA-ODA混合基质膜CO2/N2 (a)和O2/N2 (b)的Robeson上限图Figure 9. Robeson upper bound plots of TpPa-CF3/6FDA-ODA mixed matrix membranes with different loadings CO2/N2 (a) and O2/N2 (b)表 3 文献中报道的混合基质膜气体分离性能与本工作的对比Table 3. Comparison of gas separation performance of mixed matrix membranes reported in the literature with the present workMembrane type P(CO2)/Barrer P(O2)/Barrer α(CO2/N2) α(O2/N2) Ref. TpPa-1-nc/Pebax 21 — 72 — [26] COFp-PVAm 270 — 86 — [27] TpBD@PBI-BuI 14.8 — 23 — [28] ZIF-7-I/(BPDA/6FDA-ODA) — 2.9 — 0.19 [29] PBI-PI-based carbon 293.5 93.1 8.3 2.6 [30] 5%TpPa-CF3/6FDA-ODA 31.08 6.08 24.4 4.8 This work Notes: TpPa-1-nc—; COFp—; TpBD—; BPDA—; Pebax—Poly(ether-block-amide); PVAm—Polyvinylamine; PBI-BuI—Tert-butylpolybenzimidazole; ZIF-7-I—Wide-pore ZIF-7; PBI—Polybenzimidazoles; PI—Polyimide. 3. 结 论
(1)采用溶剂热法合成了一种氟化共价有机框架材料(TpPa-CF3)，其具有高比表面积(791.83 m2·g−1)，较小且均一的孔径(1.18 nm)以及良好的热稳定性。
(2)采用共混法成功制备TpPa-CF3/聚酰亚胺(6FDA-ODA)混合基质膜。通过表征得出，所得膜具有良好的界面相容性以及较高的热稳定性(热分解温度在500℃左右)。水接触角的范围在81.4°~89.1°，且膜具有良好的力学性能，有利于膜在分离过程中的稳定性。
(3) TpPa-CF3的掺入提高了混合基质膜的气体渗透性，随着膜中TpPa-CF3负载量的增加，混合基质膜的气体渗透性呈现先减小后增大再减小的趋势。其中，5%TpPa-CF3/6FDA-ODA膜的气体分离性能最好，其CO2和O2的渗透性能分别提高了149%和138%，CO2/N2和O2/N2的分离性能分别是6FDA-ODA基体膜的125%和119%。
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图 2 干燥过程孔壁应力分析[36]
F1, F2, F3, F4—Capillary pressure, resulting from the evaporation of solvent at the gas-liquid interface will transmit to the pore walls; F1′, F2′, F3′, F4′—Resultant forces on the pore walls in the horizontal direction caused by capillary pressure
Figure 2. Stress analysis of pore wall during drying process[36]
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图 3 (a) 化学气相沉积法(CVD)改性细菌纳米纤维素(BNC)气凝胶示意图[39]；(b)改性前后细菌纳米纤维(BNC)气凝胶的疏水性及相应水接触角[39]；(c)不同甲基三甲氧基硅烷(MTMS)添加量改性纤维素纳米纤维(CNF)气凝胶(TCNF-Si)的水接触角[40]
BC—Bacterial cellulose
Figure 3. (a) Illustration for the chemical vapor deposition (CVD) process of bacterial nanocellulose (BNC) aerogel[39]; (b) Hydrophobicity and corresponding water contact angle of bacterial nanocellulose (BNC) aerogel before and after modification[39]; (c) Water contact angle of cellulose nanofibers (CNF) aerogels (TCNF-Si) at different methyltrimethoxy-silane (MTMS) supplemental levels[40]
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图 4 (a)不同交联方式和不同前驱体浓度下纳米纤维素气凝胶的比表面积[46]；(b)不同纳米纤维素(NC)/Al2O3质量比气凝胶对噻吩的吸附能力[49]
S—Specific surface area
Figure 4. (a) Specific surface area of nanocellulose aerogel under different crosslinking methods and different precursor concentrations[46]; (b) Adsorption capacity of aerogel for thiophene at different mass ratios of nanocellulose (NC) to Al2O3[49]
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图 5 (a)通过冷冻干燥制备气凝胶的SEM图像[50]；(b)通过超临界CO2干燥制备气凝胶的SEM图像[50]；(c)不同甲基三乙氧基硅烷(MTES)含量下气凝胶的孔隙结构及对应吸附能力[50]
DMF—Dimethylformamide; DMSO—Dimethyl sulfoxide
Figure 5. (a) SEM images of the aerogels obtained by freeze drying[50]; (b) SEM images of typical aerogels obtained by supercritical CO2 drying[50]; (c) Pore structure and adsorption capacity of aerogel with different methyl triethoxysilane (MTES) contents[50]
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图 6 (a)不同干燥方法下气凝胶的应力-应变曲线[52]；(b)不同聚多巴胺(PDA)含量下CNF气凝胶的应力-应变曲线[53]；(c) CNF/聚乙烯醇(PVA)/四乙氧基硅烷(ETOS)定向冷冻干燥过程[54]；(d) 100次压缩-卸载循环下气凝胶的应力-应变曲线[54]；(e) 40次吸附-解吸循环下气凝胶的吸附能力[54]
F—Freezing in a refrigerator; L1—Non-directional freezing in liquid nitrogen; L2—Directional freezing in liquid nitrogen; E1—Non-directional freezing in ethanol; E2—Directional freezing in ethanol; ε—Compressive strain; Ee—Linear elastic region; CNF—Cellulose nanofibers; PCNF—polydopamine (PDA)/CNF composite aerogel, the number represents the PDA concentration
Figure 6. (a) Stress-strain curves of aerogel under different drying methods[52]; (b) Stress-strain curves of CNF aerogel with different polydopamine (PDA) contents[53]; (c) Directional freeze drying process of CNF/polyvinyl alcohol (PVA)/tetraethoxysilane aerogel (ETOS)[54]; (d) Stress-strain curves of aerogel under 100 compression-unload cycles[54]; (e) Adsorption capacity of aerogel after 40 adsorption and desorption cycles[54]
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图 7 (a) 气凝胶油水分离机制[56]；(b) CNC/壳聚糖(CS)复合气凝胶对油和有机溶剂的吸附能力[58]；(c) CNC/CS复合气凝胶吸附能力与溶剂密度关系[58]
Δp—Intrusion pressure; γ, r, θ—Surface tension of the liquid, liquid meniscus radius and liquid contact angle, respectively; O—Center of the circle
Figure 7. (a) Oil-water separation mechanism of aerogel[56]; (b) The ability of CNC/Chitosan (CS) composite aerogel to adsorb oils and organic solvents[58];(c) Relationship between adsorption capacity of CNC/CS composite aerogel and solvent density[58]
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图 8 (a) CNF/富含果胶的小叶(PML)气凝胶对机油、柴油和原油的吸附容量-时间曲线及伪一阶模型和伪二阶模型[60]; (b) CNF/PML气凝胶对柴油的循环吸附测试[60]
Qe—Equilibrium adsorption amount; Qt—Amount of adsorption at time t
Figure 8. (a) Adsorption capacity-time curves of CNF/pectin rich lobule (PML) aerogel on machine oil, diesel and crude oil: Pseudo-first-order model and pseudo-second-order model[60]; (b) Cyclic adsorption test of CNF/PML aerogel on diesel oil[60]
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图 9 (a) CNC/石墨烯(GO)复合气凝胶快速吸附乙酸乙酯和四氯甲烷[64]；(b) CNC/GO复合气凝胶对各种油和有机溶剂的吸附能力[64]；(c) CNC/GO复合气凝胶的重复使用性能[64]
CNT—Carbon nanotube
Figure 9. (a) CNC/graphene (GO) composite aerogel quickly adsorbed ethyl acetate and tetrachloromethane[64]; (b) Ability of CNC/GO composite aerogel to adsorb oils and organic solvents[64]; (c) Reusable performance of CNC/GO composite aerogel[64]
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图 10 (a) 气凝胶厚度与通量的关系[67]；(b) 泵驱动下的连续油水分离过程[68]；(c) CNF/聚二甲基硅氧烷(PDMS)气凝胶对正己烷、甲苯和甲基环己烷的分离通量[68]；(d)纯气凝胶(A0)和复合气凝胶(AM4)分离乳液过程及分离前后液滴尺寸[70]；(e) 气凝胶分离乳液机制 [70]；(f)气凝胶初始分离通量、总分离量和分离效率[70]；(g) 气凝胶对4种油包水乳液的分离效率[71]
Figure 10. (a) Relationship between aerogel thickness and flux[67]; (b) A continuous oil-water separation process driven by a pump[68]; (c) Separation fluxes of CNF/polydimethylsiloxane (PDMS) aerogel for n-hexane, toluene and methylcyclohexane[68]; (d) Pure aerogel (A0) and composite aerogel (AM4) separation emulsion process and droplet size before and after separation[70]; (e) Mechanism of aerogels separating emulsion[70]; (f) Initial separation flux, total separation amount and separation efficiency of aerogel[70]; (g) Separation efficiency of aerogel for four water-in-oil emulsions[71]
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图 11 (a)纤维素纳米纤维(CNF)/石墨烯(GO)/聚乙烯醇(PVA)气凝胶在10次吸附-燃烧循环下对乙醇的吸附能力[75]；(b) 纤维素纳米晶体(CNC)/赤泥(RM)气凝胶可燃性测试照片[76]
Figure 11. (a) Adsorption capacity of cellulose nanofiber (CNF)/graphene (GO)/polyvinyl alcohol (PVA) aerogel for ethanol under 10 sorption-combustion cycles[75]; (b) Digital photographs of cellulose nanocrystals (CNC)/Red Mud (RM) aerogel for flammability tests[76]
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表 1 不同原料和干燥方法下纳米纤维素气凝胶的基本性能
Table 1 Basic properties of nanocellulose aerogels under different raw materials and drying methods
Material Content/wt% Density/(mg·cm−3) Porosity/% Specific surface area/(m2·g−1) Drying Ref. BNC 1 90 93.6 660 Supercritical drying [31] CNF 0.5 9.42 99.26 362.7 Freezing drying [33] CNF 1.5 58.82 – 22.4 Atmospheric pressure drying [36] BNC 0.4 46 97.7 – Freezing drying [41] CNC 0.5 5.6 99.6 – Freezing drying [42] CNC 2 21.7 98.6 250 Freezing drying [42] CNF 0.5 4 99.8 42 Freezing drying [43] CNF 0.6 8 99.5 30 Freezing drying [44] CNF 2 23 99 90 Freezing drying [45] Notes: BNC—Bacterial nanocellulose; CNF—Cellulose nanofiber; CNC—Cellulose nanocrystal. 
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表 2 不同纳米纤维素复合气凝胶的性能及油水分离性能比较
Table 2 Comparison of the properties and oil-water separation performances of different nanocellulose composite aerogel
Materials Density/
(mg·cm−3)Oil types Maximum
absorption
capacity/(g·g−1)WCA/(°) Porosity/% Specific surface
area/(m2·g−1)Reusability/
timesMaximum oil
flux/(L·m−2·h−1)Ref. CNF/CS 8.4 Trichloromethane 253 148 96 – 50 – [58] CNF/PML 5.1 Tetrachloromethane 160 129 – 9.8 15 – [60] CNC/RGO 4.98 Tetrachloromethane 276 – 99.6 272.2 10 – [64] CNF/PEI/EGDE 53.8 Tetrachloromethane 28.03 130 95.73 – 10 5400 [67] CNF/PDMS 22.7 Toluene 48 163.5 98.4 – 20 145 [68] CNF/SiO2 6.43 – 168.4 129 99.6 108.6 20 1910 [70] CNF/TA/ICO 24 Dichloromethane 113.8 134.8 98.32 – – 4783.8 [71] BNC/PMSQ 5.74 Trichloromethane 203 168 99.59 – 10 473.8 [72] CNF/SA 24.2 Silicone oil 88.91 144.5 97.85 149.64 20 – [73] CNF/CS/ZIF-8 15.87 Trichloromethane 74.55 132.6 99.01 5.51 20 13167.5 [74] Notes: CS—Chitosan; PML—Premna microphylla leaves; RGO—Reduced graphene oxide; PEI—Polyethyleneimine; EGDE—Ethylene glycol diglycidyl ether; PDMS—Polydimethylsiloxane; TA—Tannic acid; ICO—Castor oil; PMSQ—Polymethylsilsesquioxane; SA—Sodium alginate; WCA—Water contact angle; ZIF-8—Zeolitic imidazolate framework-8.
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目的
随着海上石油运输频率和海上油田数量的增加，石油泄漏事件频繁发生。大量的石油泄露对环境以及人类健康造成了极大的危害。与常规吸附材料相比纳米纤维素气凝胶具有高孔隙度、高比表面积和低密度等特点。同时，纳米纤维素具有高比表面积和长径比，其表面丰富的羟基功能基团易于表面修饰或与特定的功能材料复合制备出适用于油水分离领域的气凝胶。本文系统地综述了目前国内外应用于油水分离的纳米纤维素气凝胶研究进展情况。
方法通过归纳整理近年来国内相关文献，对目前用于油水分离的纳米纤维素气凝胶的研究现状进行了简述。总结了纳米纤维素气凝胶的制备和改性方法，并分析了不同制备和改性方法的优缺点；总结了纳米纤维素气凝胶的结构特点以及结构对其性能的影响；综述了纳米纤维素气凝胶在油和有机溶剂的吸附以及油水混合物分离中的应用。最后提出了纳米纤维素气凝胶存在的科学问题以及未来发展建议。
结果从现有研究可以看出，目前制备纳米纤维素气凝胶的步骤主要为将纳米纤维素前驱体转化为高度交联的凝胶后通过超临界干燥、冷冻干燥或常压干燥的方法制备成气凝胶。纳米纤维素具有易于修饰的特点，可通过浸涂法、化学气相沉积法和酸水解硅烷改性法对其进行疏水改性。纳米纤维素气凝胶可作为吸附剂吸附水面或水底的油，也可作为过滤材料分离油水混合物或者乳液，在油水分离领域中具有广泛的应用。除此之外，纳米纤维素气凝胶还可兼具其他性能，如阻燃性、磁性、智能响应性能、光热和抗菌性能等，拓宽了纳米纤维素气凝胶的应用范围。
结论纳米纤维素气凝胶独特的结构和优异的性能，使得其在油水分离领域已经取得一些进展。但仍有一些科学问题有待解决，如(1)纳米纤维素气凝胶的吸附机理有待进一步研究。(2)缺乏对纳米纤维素气凝胶结构和性能之间关系的系统性研究。此外，对优化纳米纤维素气凝胶的实际应用提出了三点建议(1)可将廉价的生物废弃物作为原材料提取纳米纤维素，改进纳米纤维素的制备和与处理工艺。(2)提升纳米纤维素气凝胶长期耐久性和抗污染能力。(3)开发功能性纳米纤维素气凝胶，拓宽其应用范围。
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