宗益峰，王如意，楊毓?jié)?，劉?b，趙輝,b

纖維素基輕質(zhì)多孔材料的研究進(jìn)展

宗益峰a，王如意a，楊毓?jié)峚，劉楊a,b，趙輝a,b

（廣西大學(xué) a.輕工與食品工程學(xué)院 b.廣西清潔化制漿造紙與污染控制重點(diǎn)實(shí)驗(yàn)室，南寧 530004）

纖維素基輕質(zhì)多孔材料具有質(zhì)輕、孔隙率高、成本低等優(yōu)點(diǎn)，被廣泛應(yīng)用于吸附、催化、隔熱等領(lǐng)域，但易燃、耐水性差等缺點(diǎn)限制了它的應(yīng)用范圍。通過(guò)復(fù)合改性可以改善上述缺點(diǎn)，并賦予其新的特性，因此需要充分了解功能化改性方法和復(fù)合輕質(zhì)多孔材料的廣泛應(yīng)用。通過(guò)追蹤國(guó)內(nèi)外纖維素基輕質(zhì)多孔材料的功能化改性研究和應(yīng)用進(jìn)展，概述纖維素基輕質(zhì)多孔材料的基本性質(zhì)和性能，重點(diǎn)分析纖維素基復(fù)合輕質(zhì)多孔材料的功能化改性方法和應(yīng)用，詳細(xì)介紹纖維素基復(fù)合輕質(zhì)多孔材料在眾多領(lǐng)域的應(yīng)用。將有機(jī)或無(wú)機(jī)材料與纖維素進(jìn)行復(fù)合制成輕質(zhì)多孔材料，可以實(shí)現(xiàn)阻燃、吸附、電磁屏蔽、導(dǎo)電、疏水、抗菌等功能，拓寬了纖維素基輕質(zhì)多孔材料在包裝、醫(yī)用、電池等領(lǐng)域的應(yīng)用范圍。

纖維素；復(fù)合材料；功能化

纖維素基輕質(zhì)多孔材料于1971年由Weatherwax等[1]首次制備完成，此后便很少有人關(guān)注。直到30年后的2001年，Tan等[2]以醋酸纖維素和乙酸丁酸纖維素為原料，以二異氰酸酯為交聯(lián)劑，得到了具有高沖擊強(qiáng)度的纖維素基輕質(zhì)多孔材料。2004年，Jin等[3]首次使用非衍生性溶劑，將纖維素在硫氰酸鈣水溶液中溶解和再生來(lái)制備纖維素多孔材料，隨后，各種非衍生性溶劑體系被開(kāi)發(fā)出來(lái)，包括NaOH水溶液體系[4]、N,N?二甲基乙酰胺/氯化鋰(DMAc/LICI)體系[5]、堿/尿素或硫脲/水體系[6]、胺氧化物體系[7]、離子液體體系[8]等。2006年，從植物源或細(xì)菌纖維素（BC）中分離的納米纖維素也被用來(lái)制備纖維素多孔材料[9]。纖維素基輕質(zhì)多孔材料是一類密度低（＜100 mg/cm3）、孔隙率高（＞50%）、比表面積大、孔結(jié)構(gòu)從納米到微米的固體材料[10]，在傳統(tǒng)的多孔材料特性[11]（低密度、高比表面積、高孔隙率及孔徑分布小等）基礎(chǔ)上增加了纖維素的生物性優(yōu)勢(shì)[12]（可生物降解性和生物相容性），以及高的力學(xué)強(qiáng)度等優(yōu)異性能，并帶有豐富的羥基基團(tuán)。

纖維素基輕質(zhì)多孔材料的制備主要分為2步，首先將纖維素通過(guò)溶劑分散并凝膠化得到水凝膠，然后通過(guò)干燥去除溶劑成型。在凝膠過(guò)程中，纖維素的溶膠?凝膠轉(zhuǎn)變通過(guò)物理或化學(xué)交聯(lián)形成三維纖維網(wǎng)絡(luò)[13]。這里最主要的問(wèn)題是如何保持制備過(guò)程中形成的微觀結(jié)構(gòu)[14]，可通過(guò)添加表面活性劑、穩(wěn)泡劑等來(lái)解決[15]。纖維素基輕質(zhì)多孔材料的孔結(jié)構(gòu)還取決于干燥工藝的選擇。超臨界干燥可有效避免3D多孔結(jié)構(gòu)的坍塌[16]，但存在干燥周期長(zhǎng)、產(chǎn)量低、成本高等缺點(diǎn)。相對(duì)而言，冷凍干燥使冰晶直接升華，更加安全且相對(duì)經(jīng)濟(jì)。目前也有研究采用冰模板定向冷凍干燥的方法來(lái)制備具有各向異性多孔結(jié)構(gòu)和力學(xué)性能的纖維素泡沫/氣凝膠[17]。在冷凍干燥過(guò)程的冷凍和升華階段，大冰晶會(huì)導(dǎo)致納米纖維的聚集，使纖維素多孔材料的比表面積顯著降低，可以通過(guò)將溶劑改為叔丁醇來(lái)改善[18]。除了上述2種主要的干燥方法外，烘箱干燥方法因其成本低、工藝簡(jiǎn)單而被認(rèn)為是最有前景的方法。在干燥過(guò)程中存在溶劑表面張力引起的黏附性，固體基質(zhì)的毛細(xì)作用會(huì)導(dǎo)致內(nèi)部網(wǎng)絡(luò)結(jié)構(gòu)崩塌[19]等問(wèn)題的出現(xiàn)。目前也有研究致力于改善這些問(wèn)題，從而降低制備材料的成本[20]。

纖維素基輕質(zhì)多孔材料在應(yīng)用過(guò)程中存在易燃、耐水性差、不易回收、可重復(fù)使用性能差等問(wèn)題，可以通過(guò)化學(xué)或物理改性等方法來(lái)改善這些問(wèn)題，并賦予其新的功能，從而應(yīng)用于吸附[21]、抗菌[22]、疏水[23]、電磁屏蔽[24]、阻燃[25]等領(lǐng)域，這也是目前的研究熱點(diǎn)。開(kāi)發(fā)環(huán)境友好、成本低、可回收，以及具有特殊性能的纖維素基復(fù)合多孔材料將成為未來(lái)的主流研究方向。文中將圍繞纖維素基輕質(zhì)多孔材料的改性方法、結(jié)構(gòu)特點(diǎn)、性能及應(yīng)用等方面，著重闡述纖維素基復(fù)合輕質(zhì)多孔材料在吸附、醫(yī)用、電磁屏蔽、阻燃等領(lǐng)域的研究現(xiàn)狀，為纖維素基復(fù)合輕質(zhì)多孔材料研究提供一定的理論參考。

1 纖維素基輕質(zhì)多孔材料的復(fù)合改性

纖維素基復(fù)合輕質(zhì)多孔材料是以纖維素為基體，以有機(jī)或無(wú)機(jī)材料為功能體或增強(qiáng)體，通過(guò)復(fù)合制成的性能優(yōu)良的新型功能材料[26]。功能體或增強(qiáng)體與纖維素的復(fù)合改性方法有很多，按與纖維素復(fù)合的材料種類分類，可分為有機(jī)材料復(fù)合、納米碳材料復(fù)合、金屬納米粒子復(fù)合、氧化物復(fù)合、礦物復(fù)合等。

1.1 有機(jī)材料復(fù)合

與纖維素基輕質(zhì)多孔材料復(fù)合的有機(jī)材料大多為高分子聚合物，如殼聚糖[27]、甲殼素[28]、共價(jià)有機(jī)骨架COF（Covalent Organic Framework）粉末材料[29]、單寧、海藻酸鈉等。也有如甲醇[30]等非高分子聚合物的有機(jī)材料，用甲醇將纖維素進(jìn)行再生，制成纖維素?甲醇凝膠，通過(guò)超臨界干燥方法，可以制備出比表面積大、密度低（0.058 g/cm3）和孔隙率高（96%）的納米孔徑（<2 nm）氣凝膠。

殼聚糖的化學(xué)結(jié)構(gòu)與纖維素十分相似，其分子鏈上存在大量氨基和羥基，能與纖維素上豐富的羥基形成氫鍵[31]。Zhang等[32]制備了一種殼聚糖?纖維素包裹磁性碳泡沫吸附劑，將纖維素與殼聚糖混合后，殼聚糖水凝膠球的表面孔隙率增加，材料的微觀結(jié)構(gòu)得到改善。這些變化將促進(jìn)重金屬溶液滲透到聚合物網(wǎng)絡(luò)結(jié)構(gòu)中，從而提高金屬離子的吸附性能和吸附速率。這里總結(jié)了部分有機(jī)材料與纖維素的復(fù)合原理，復(fù)合后的結(jié)構(gòu)特點(diǎn)、性能和應(yīng)用見(jiàn)表1。

表1 幾種有機(jī)材料與纖維素的復(fù)合原理及應(yīng)用

Tab.1 Principle and application of the compounding of several organic materials and cellulose

MOF/纖維素復(fù)合光催化材料在酸性環(huán)境中還原六價(jià)鉻通過(guò)式（1）—（2）實(shí)現(xiàn)，其中過(guò)量的H+有利于將Cr(Ⅵ)還原為Cr(Ⅲ)[37]。在堿性環(huán)境中，Cr(Ⅵ)的存在形式是CrO42–，相應(yīng)的Cr(Ⅵ)還原通過(guò)式（3）完成。

NH2-MIL-101(Fe)+→(h++e?) (1)

Cr2O72?+14H++6e?→(2Cr3++7H2O) (2)

CrO42?+4H2O+3e?→(Cr(OH)3+5OH?) (3)

共價(jià)有機(jī)骨架（COF）材料是一種結(jié)晶的、高度多孔的二維或三維聚合物，具有可調(diào)的拓?fù)浣Y(jié)構(gòu)和功能[38]。已經(jīng)開(kāi)發(fā)了COF材料的許多用途，包括催化劑載體[39]、超靈敏傳感器[40]等。COF材料通常以粉末狀存在，很難直接形成結(jié)晶多孔材料[41]。粉末本身不易均勻摻雜和進(jìn)一步加工，且難以回收，所以將COF與纖維素復(fù)合，以對(duì)COF材料賦形，拓寬其應(yīng)用領(lǐng)域。郭從寶[41]將NFC（Nano Fibrillated Cellulose）、CNC（Cellulose Nanocrystals）和COF材料相結(jié)合，制備了COF?CMC（Carboxymethyl Cellulose）/CNC復(fù)合氣凝膠。將CNC和CMC進(jìn)行功能化改性和化學(xué)交聯(lián)，制備的氣凝膠具有分層結(jié)構(gòu)，并顯示出良好的力學(xué)性能、熱穩(wěn)定性、機(jī)械穩(wěn)定性和柔韌性。

1.2 納米碳材料復(fù)合

納米碳具有優(yōu)異的導(dǎo)電性和導(dǎo)熱性[42]，由于聚集性的問(wèn)題[43]，它們很難單獨(dú)應(yīng)用。纖維素可以對(duì)各種納米碳材料起到穩(wěn)定的作用，還可以作為納米碳材料優(yōu)異的載體。碳納米管與納米纖維素之間具有良好的親和力，可以制成性能優(yōu)異的復(fù)合材料[44]。目前研究較多的就是碳納米管和氧化石墨烯。

碳納米管（CNT）的質(zhì)量較輕，具有良好的導(dǎo)電性、導(dǎo)熱性，出色的力學(xué)性能和高柔韌性，但CNT在水中不能均勻分散，這是急需解決的問(wèn)題。Mougel等[45]將纖維素納米晶體（CNCs）和碳納米管（CNT）分散在水中制成三維大孔導(dǎo)電泡沫。由于CNCs的化學(xué)結(jié)構(gòu)和其表面帶有負(fù)電荷的影響，CNCs在水性介質(zhì)中具有出色的膠體穩(wěn)定性，從而具有靜電穩(wěn)定性，可以起到穩(wěn)定CNT的作用。氧化石墨烯（GO）是一種單層石墨氧化物，力學(xué)性能較好，剛度強(qiáng)，比表面積大，具有優(yōu)異的導(dǎo)電性和較高的電催化活性。由于纖維素基多孔材料具有較高的比表面積和孔隙率，所以具備一定的吸附能力。如果將纖維素基多孔材料與氧化石墨烯復(fù)合，就可以制備出吸附能力更強(qiáng)且具有導(dǎo)電性能的纖維素基復(fù)合材料[46]。

1.3 金屬納米粒子復(fù)合

金屬納米粒子具有獨(dú)特的光學(xué)性質(zhì)[47]、電學(xué)性質(zhì)和良好的催化效果，在許多領(lǐng)域引起了人們的廣泛關(guān)注。纖維素?納米粒子復(fù)合多孔材料主要由Au、Ag、Cu[48]、Pb[49]等與纖維素復(fù)合而成，其中研究得最多的是金納米離子和銀納米粒子。

最簡(jiǎn)單的金屬納米粒子與纖維素復(fù)合的方法就是物理沉積，一般通過(guò)靜電作用來(lái)實(shí)現(xiàn)[50]。Zhou等[51]以MFC氣凝膠為模板，通過(guò)簡(jiǎn)單的浸涂方法合成了MFC/PPy/Ag混合氣凝膠。作為一種成本低、高效且環(huán)保的工藝，可實(shí)現(xiàn)納米顆粒在纖維素氣凝膠中的精細(xì)分散。所制備的混合氣凝膠表現(xiàn)出增強(qiáng)的抗菌、導(dǎo)電和壓力響應(yīng)特性。

與物理負(fù)載相比，原位化學(xué)沉積更有利于實(shí)現(xiàn)納米粒子在納米纖維素上的均勻分布。納米纖維素的表面電荷密度是影響貴金屬離子靜電吸附的一個(gè)關(guān)鍵因素，對(duì)納米纖維素的形狀、形貌和懸浮穩(wěn)定性有著重要影響[52-53]。例如，部分脫硫處理的NCC比未脫硫的NCC能更好地穩(wěn)定AgNPs（Ag Nanoparticles）。AgNPs的尺寸分布隨著脫硫程度的增加而顯著改善[54]。此外，通常需要還原劑來(lái)誘導(dǎo)納米纖維素上金屬前體的成核和控制納米纖維素上金屬納米顆粒的形態(tài)，如硼氫化物[55]、抗壞血酸[56]、多巴胺（PDA）[57]等。

1.4 氧化物復(fù)合

與纖維素基復(fù)合的氧化物一般為ZnO、TiO2、SiO2、Fe3O4等。氧化物和納米纖維素的復(fù)合材料通常通過(guò)溶膠?凝膠[58]、自組裝[59]、原子層沉積[60]和原位合成[61]制備。氧化物的晶體結(jié)構(gòu)、大小、形狀和分布在很大程度上取決于納米纖維素的形貌和表面官能團(tuán)[62]。這里總結(jié)了部分氧化物與纖維素復(fù)合的制備方法，以及制備后的結(jié)構(gòu)特點(diǎn)、性能及應(yīng)用，見(jiàn)表2。

表2 幾種氧化物與纖維素的復(fù)合方法及應(yīng)用

Tab.2 Several composite methods and applications of oxides and cellulose

Fe3O4納米粒子是傳統(tǒng)的磁性材料，是一種雙復(fù)介質(zhì)，存在密度大、頻帶窄等缺點(diǎn)[67]。纖維素具有質(zhì)輕、強(qiáng)度高、比面積大等優(yōu)點(diǎn)，因此將這2種材料結(jié)合起來(lái)能夠取長(zhǎng)補(bǔ)短，使制成的復(fù)合材料具備2種材料的優(yōu)點(diǎn)。He等[68]利用Pickering乳液技術(shù)結(jié)合冷凍干燥技術(shù)制備了納米原纖維（CNF）/CNT/聚乳酸（PLA）/Fe3O4泡沫。復(fù)合材料具有質(zhì)輕、隔熱性能好、電磁波吸收性能好等優(yōu)點(diǎn)。得益于納米Fe3O4顆粒、碳納米管的特性以及有機(jī)?無(wú)機(jī)組分組裝產(chǎn)生的新型界面極化之間的協(xié)同效應(yīng)，復(fù)合材料的最佳回波損耗為?65.14 dB。

TiO2是另一種被廣泛研究的負(fù)載在納米纖維素上的金屬氧化物材料。Lu等[69]利用超臨界干燥制備了TiO2/纖維素復(fù)合氣凝膠。纖維素基質(zhì)中的多孔結(jié)構(gòu)和羥基通過(guò)靜電和氫鍵相互作用，增強(qiáng)了二氧化鈦的穩(wěn)定性。二氧化鈦/纖維素復(fù)合氣凝膠在紫外光照射下表現(xiàn)出良好的光催化活性。這項(xiàng)研究為便攜式和柔性光催化劑的開(kāi)發(fā)提供了一種途徑。

1.5 天然礦物復(fù)合

由于纖維素?氧化物基復(fù)合材料引入了金屬氧化物，可能會(huì)產(chǎn)生二次污染，而礦物質(zhì)材料無(wú)污染[70]且來(lái)源豐富、成本低，因此不會(huì)出現(xiàn)上述的問(wèn)題。礦物質(zhì)纖維素基復(fù)合吸附材料主要由沸石[71]、蒙脫土[72]、硅藻土[73]、坡縷石[74]等與多種纖維素復(fù)合而成。

蒙脫土（MMT）是一種天然粘土礦物質(zhì)，具有層狀結(jié)構(gòu)和離子交換性能[75]，是一種被廣泛研究的吸附材料。如果只是單純以MMT為原料進(jìn)行吸附，則吸附后存在難以回收等問(wèn)題。將纖維素與蒙脫土進(jìn)行復(fù)合制成輕質(zhì)多孔材料，不僅會(huì)提高材料的吸附性能，而且吸附后可以回收。PAN等[76]以超細(xì)碳酸鈣為造孔劑，通過(guò)一種新的方法制備了胺化纖維素/蒙脫土介孔復(fù)合微珠（AceMt），作為綠色吸附劑，可用于染料的去除。在乙酸溶液中，經(jīng)過(guò)5次吸附?解吸循環(huán)，復(fù)合材料的吸附容量也保持相對(duì)穩(wěn)定。

羥基磷灰石（HAP）是脊椎動(dòng)物體內(nèi)骨骼和牙齒的主要無(wú)機(jī)成分，具有生物相容性高、無(wú)毒和易燃等優(yōu)點(diǎn)。利用HAP和纖維素可以制備具有絕緣、阻燃、生物相容性良好的材料。Guo等[77]使用丙基三甲氧基硅烷和聚乙烯亞胺將HAP與CNF進(jìn)行交聯(lián)，采用冷凍干燥法制備CNF/HAP復(fù)合泡沫。復(fù)合泡沫具有熱導(dǎo)率較低和阻燃性能良好等優(yōu)點(diǎn)，在垂直燃燒測(cè)試中擁有自熄能力和非常低的峰值熱釋放率，具有良好的熱穩(wěn)定性。復(fù)合泡沫表現(xiàn)出優(yōu)異的防火性能，可以用作絕緣和阻燃建筑材料。

2 功能化應(yīng)用

將纖維素與有機(jī)或無(wú)機(jī)材料復(fù)合后可以賦予其特定的功能性，極大地拓展了材料的應(yīng)用領(lǐng)域。相較于未功能化纖維素基輕質(zhì)多孔材料，功能化纖維素基輕質(zhì)多孔材料在電磁屏蔽、吸附、抗菌、阻燃隔熱等領(lǐng)域有較大的應(yīng)用前景，見(jiàn)圖1。

圖1 纖維素復(fù)合輕質(zhì)多孔材料的應(yīng)用領(lǐng)域

2.1 電磁屏蔽材料

電子產(chǎn)品的大量使用會(huì)造成嚴(yán)重的電磁效應(yīng)，會(huì)對(duì)人體的健康造成嚴(yán)重的影響，并且還會(huì)引起信息泄露及干擾其他電子設(shè)備的正常運(yùn)行[78]。電磁屏蔽材料可以減輕或者消除上述不良影響，但傳統(tǒng)的電磁屏蔽材料存在質(zhì)量大、屏蔽機(jī)理單一、二次污染嚴(yán)重、生產(chǎn)過(guò)程煩瑣等缺點(diǎn)[79]。纖維素基電磁屏蔽材料具有輕質(zhì)靈活、保溫隔音、綠色無(wú)污染、循環(huán)可再生等優(yōu)點(diǎn)，正成為目前研究的熱點(diǎn)。

Zeng等[80]將CNF用來(lái)構(gòu)建具有定向仿生細(xì)胞壁的過(guò)渡金屬碳化物和氮化物（Mxene）氣凝膠，采取了一種仿生的微結(jié)構(gòu)設(shè)計(jì)方法（見(jiàn)圖2），由珍珠狀細(xì)胞壁的CNF“砂漿”黏結(jié)的MXene“磚”具備機(jī)械強(qiáng)度高、導(dǎo)電性好和界面極化的特性，從而使MXene/ CNF氣凝膠具有超高的電磁屏蔽性能。在密度僅為8.0、1.5 mg/cm3的情況下，氣凝膠的屏蔽效能（SE）分別達(dá)到74.6 dB和35.5 dB，躋身于目前研發(fā)的其他電磁屏蔽材料的第一梯隊(duì)。

圖2 MXene/CNF仿生細(xì)胞壁結(jié)構(gòu)和定向冷凍干燥示意圖[80]

Zeng等[81]制備了超輕、高柔性的生物聚合物氣凝膠，它由纖維素納米纖維和銀納米線形成的仿生細(xì)胞微結(jié)構(gòu)組成，通過(guò)冷凍干燥方法成型。結(jié)合原位壓縮產(chǎn)生的屏蔽變換和建筑單元的控制，優(yōu)化的層狀多孔生物高聚物氣凝膠具有非常好的電磁屏蔽效果，電磁屏蔽的原理見(jiàn)圖3。在密度為6.2 mg/cm3時(shí)，X波段屏蔽效能為70.5 dB，電磁屏蔽性能優(yōu)異。

圖3 CNF/Ag氣凝膠層狀多孔結(jié)構(gòu)實(shí)現(xiàn)電磁屏蔽效果的機(jī)制[81]

介紹的2種電磁屏蔽材料采取了不同的結(jié)構(gòu)設(shè)計(jì)思路，并且材料都具有輕巧靈活、電磁屏蔽性能良好等優(yōu)點(diǎn)，對(duì)以后設(shè)計(jì)電磁屏蔽材料有一定借鑒意義。

2.2 吸附材料

隨著石油工業(yè)和化學(xué)工業(yè)的發(fā)展，漏油和化學(xué)品泄漏正在成為人類面臨的環(huán)境問(wèn)題[82-83]。吸附是一種成本低且操作簡(jiǎn)單的方法[84]。低成本、輕質(zhì)、可循環(huán)使用是作為優(yōu)異吸附材料必備的性能條件[85]。纖維素質(zhì)輕且強(qiáng)度高，利用纖維素與其他材料復(fù)合可以制備出質(zhì)輕且吸附性能優(yōu)異的復(fù)合材料。目前，纖維素基復(fù)合吸附材料的研究主要集中在設(shè)計(jì)特定的細(xì)胞微結(jié)構(gòu)[86]和使用可彎曲但堅(jiān)固的固體成分等方面[87]。

Mi等[88]以纖維素和石墨烯為原料，采用雙向冷凍干燥方法，制備了高彈性、可壓縮和各向異性的纖維素/石墨烯氣凝膠（CGAs）。制備的復(fù)合材料超輕（密度為5.9 mg/cm3）且具有高比表面積（47.3 m2/g）。經(jīng)過(guò)超疏水改性的纖維素/石墨烯氣凝膠（MCGA）具有相當(dāng)于其質(zhì)量80～197倍的顯著吸附能力，并且可以通過(guò)簡(jiǎn)單的機(jī)械擠壓快速高效地回收吸收的油。

Wei等[89]將納米纖維素與氧化鐵（Fe3O4）納米顆粒集成在一起，制備了一種磁性雜化氣凝膠。制備的雜化氣凝膠對(duì)Cu2+、Pb2+、Cr6+的吸附容量分別為0.4、1.25、2.2 mg/g，見(jiàn)圖4，可以有效地吸附水中的重金屬離子，實(shí)現(xiàn)磁性條件下的可控回收。

通過(guò)與不同的材料復(fù)合，賦予新材料對(duì)不同污染物的吸附性能，如吸附重金屬離子、印染廢水和石油等，這對(duì)處理環(huán)境污染問(wèn)題具有重要的意義。

2.3 電極材料

Fe3O4是一種優(yōu)秀的電池材料，可以作為鋰離子電池的負(fù)極材料。由于Fe3O4的導(dǎo)電性較差，且在應(yīng)用中體積會(huì)膨脹，因而限制了它在實(shí)際中的應(yīng)用。Sun等[90]通過(guò)分解纖維素/酒石酸鐵絡(luò)合物體系，制備了Fe3O4@C復(fù)合材料。酒石酸鐵絡(luò)合物經(jīng)冷凍干燥和高溫炭化后，分解成Fe3O4，均勻沉積在纖維素衍生的多孔炭中。多孔炭具有良好的導(dǎo)電性，在電化學(xué)研究中發(fā)現(xiàn)它可以適應(yīng)復(fù)合材料的體積變化。復(fù)合材料作為鋰電池的負(fù)極在100 mA/g下的充電容量可以達(dá)到864.9 mA·h/g，初始充電容量較高。在1 000 mA/g下循環(huán)300次后，容量保持率為86.4%。該方法為制備性能優(yōu)良的Fe3O4@C復(fù)合材料提供了一種新的途徑，并有望在未來(lái)的鋰離子電池中得到實(shí)際應(yīng)用。

Zheng等[91]制備了一種以纖維素納米纖維（CNF）/氧化石墨烯（RGO）/碳納米管（CNT）雜化氣凝膠為電極（見(jiàn)圖5），以H2SO4/聚乙烯醇（PVA）凝膠為電解質(zhì)的新型高柔性全固態(tài)超級(jí)電容器。首先將氧化石墨納米片（GONS）與碳納米管（CNT）均勻混合。由于GONS是水溶性的，含有大量的氧原子，因而極易與CNT形成均勻的溶液。再加入纖維素納米纖維CNF形成均勻的溶液，通過(guò)預(yù)凍和冷凍干燥方法制成CNF/RGO/CNT氣凝膠，并通過(guò)熱還原形成最終的超級(jí)電容器電極材料。得益于氣凝膠電極的多孔結(jié)構(gòu)和CNF優(yōu)異的電解質(zhì)吸收性能，所制備的柔性超級(jí)電容器具有較高的比電容（216 mF/cm2）和優(yōu)異的循環(huán)穩(wěn)定性（在1000次充放電循環(huán)后仍保持99.5%以上的比電容）。CNF/RGO/CNT氣凝膠電極具有優(yōu)良的電化學(xué)性能、成本低、易于大規(guī)模生產(chǎn)、環(huán)境友好等特點(diǎn)，在柔性儲(chǔ)能裝置的開(kāi)發(fā)中具有廣闊的應(yīng)用前景。

目前，電池性能制約了手機(jī)、汽車行業(yè)的發(fā)展，提高電池的容量和抗衰減性能顯得至關(guān)重要?？顾p性能的提高可以有效地延長(zhǎng)電池的使用時(shí)間、節(jié)約成本。電池容量的提高可以減輕人們對(duì)石油等不可再生資源的依賴，使電動(dòng)汽車的使用體驗(yàn)逐漸媲美燃油車，推動(dòng)了人類在可持續(xù)能源領(lǐng)域的高質(zhì)量發(fā)展。

2.4 疏水材料

由于存在大量的羥基，纖維素基輕質(zhì)多孔材料具有較強(qiáng)的親水性[92]，這會(huì)導(dǎo)致材料的耐水性差、強(qiáng)度降低，并且影響油水分離性能[93]。為了提高纖維素基輕質(zhì)多孔材料的耐水性能，可通過(guò)提高粗糙度（微米或納米級(jí)的粗糙度）和引入低表面能物質(zhì)來(lái)實(shí)現(xiàn)[94]。目前，常用的纖維素基輕質(zhì)多孔材料的疏水化工藝包括化學(xué)氣相沉積、原子層沉積、冷等離子體處理[95]、溶膠?凝膠[96]、酯化[97]和氟化等，主要以TiO2、SiO2、烷氧基硅烷、氯硅烷、烷基烯酮二聚體[98]、三氯甲基硅烷、全氟十二烷基三氯硅烷[99]、硬脂酰氯和棕櫚酰氯[100]等為疏水改性原料。

圖4 納米纖維素?Fe3O4混合氣凝膠的吸附效率[89]

圖5 CNF/RGO/CNT氣凝膠電極的制造過(guò)程[91]

在最新的研究中，研究人員發(fā)現(xiàn)使用木質(zhì)素也可制備疏水纖維素基輕質(zhì)多孔材料。Ferreira等[101]通過(guò)堿處理從木質(zhì)纖維素中提取木質(zhì)素，然后再將木質(zhì)素沉積到木質(zhì)纖維素中，通過(guò)烘箱干燥制備疏水纖維素泡沫。泡沫的接觸角為117°±8°，最高測(cè)量值為127°（見(jiàn)圖6），呈現(xiàn)出較好的疏水性。泡沫的疏水性主要?dú)w因于木質(zhì)纖維素中殘留木質(zhì)素、再沉積木質(zhì)素、纖維和泡沫表面粗糙度等的共同作用。從圖6可以看出，再沉積的木質(zhì)素增加了纖維表面的粗糙度，提高了泡沫的疏水性能。制備所用烘箱也具有成本低、操作簡(jiǎn)單等優(yōu)點(diǎn)，為大規(guī)模的工業(yè)化生產(chǎn)提供了良好的借鑒意義。

圖6 檸檬酸交聯(lián)木質(zhì)纖維素泡沫的接觸角和表面特性[101]

2.5 抗菌材料

羥基磷灰石（HAP）與天然骨組織有著相似的結(jié)構(gòu)和化學(xué)成分[102-103]，將它植入骨缺損的部分無(wú)不良反應(yīng)，但其自身強(qiáng)度低，難以起到支撐作用[104]。纖維素具有強(qiáng)度高的特點(diǎn)，可為HAP提供支撐作用，再與其他材料一起復(fù)合可以得到結(jié)構(gòu)穩(wěn)定、抗菌性能好、生物相容性好的復(fù)合支架[105]。Khan等[106]以細(xì)菌纖維素（BC）和?葡聚糖為原料，采用自由基聚合將?HAP和GO接枝到聚合物網(wǎng)絡(luò)中，并通過(guò)冷凍干燥技術(shù)制備了應(yīng)用于骨組織工程的納米復(fù)合支架。復(fù)合支架對(duì)于革蘭氏陰性菌和革蘭氏陽(yáng)性菌具有良好的抗菌性能，并且顯示出較強(qiáng)的生物化學(xué)親和力，以及細(xì)胞黏附、增殖和生物相容性，這對(duì)于應(yīng)用抗菌性能的骨組織支架具有深遠(yuǎn)的意義。

作為天然抗菌劑，精油（Eos）既有抗菌作用，又有抗氧化作用，因此被廣泛用于抗菌材料。Zhang等[107]采用了一種簡(jiǎn)單而環(huán)保的方法，用CNF和百里香精油（EO）制備出納米纖維?百里香精油泡沫（CNF?EO），通過(guò)酶解預(yù)處理和TEMPO氧化預(yù)處理可以制備碳納米纖維。將酶解纖維素納米纖維（EHCN）和氧化纖維素納米纖維（TOCN）浸泡在EO納米乳液中，然后進(jìn)行冷凍干燥，就可以制備出CNF?EO復(fù)合材料。在食品保鮮方面，采用CNF?EO泡沫保鮮的牛肉在風(fēng)味、色澤等感官上均發(fā)揮出較好的效果，與同期不加精油的對(duì)照組相比，采用TOCN?EO泡沫保鮮牛肉的貨架期延長(zhǎng)了5 d，見(jiàn)圖7。這種CNF?EO泡沫體系的制備方法對(duì)新鮮食品的保鮮具有重要意義。

研發(fā)具有抗菌性能的環(huán)境友好型材料，可以應(yīng)用到食品保鮮和醫(yī)用材料等領(lǐng)域，優(yōu)異的抗菌性能可延長(zhǎng)食品的保質(zhì)期，用作醫(yī)用材料可以降低患者被感染的風(fēng)險(xiǎn)。

圖7 EHCN?EO和TOCN?EO的抗菌性能對(duì)比[107]

2.6 隔熱、阻燃材料

纖維素氣凝膠具有密度低、隔熱性能好、導(dǎo)熱性低等特點(diǎn)，被廣泛應(yīng)用于建筑的隔熱保溫材料。由于該材料易燃，因而限制了它在部分場(chǎng)景下的應(yīng)用。由此，賦予纖維素氣凝膠優(yōu)異的阻燃性能，可以極大程度地拓展此材料的應(yīng)用領(lǐng)域。

Wang等[108]將四甲氧基硅烷（TMOS）和磷腈縮合，合成了含有環(huán)三磷腈橋連基團(tuán)的介孔有機(jī)二氧化硅（PMOPZ），然后將它摻入CNF懸浮液中，通過(guò)冷凍澆鑄法制備了PMOPZ/CNF復(fù)合泡沫材料。相較于傳統(tǒng)的CNF泡沫，復(fù)合泡沫在水平（圖8a）和垂直（圖8b）方向的燃燒結(jié)果都表明它具有優(yōu)異的阻燃和自熄滅性能，極氧指數(shù)高達(dá)31%。SiO2的加入使復(fù)合泡沫塑料的放熱率（PHRR）和總放熱率（THR）分別降低了52.0%和61.3%，其優(yōu)異的阻燃性能滿足安全和節(jié)能的需要。

He等[109]受到硼酸鹽在植物中的交聯(lián)作用的啟發(fā)，通過(guò)硼酸鹽交聯(lián)，制備出力學(xué)性能和阻燃性能均優(yōu)異的輕質(zhì)、高孔隙率紙漿泡沫。與易燃原漿泡沫相比，低導(dǎo)熱系數(shù)（約0.045 W·m?1·K?1）的含硼紙漿泡沫具有更好的阻燃性能和良好的自熄滅性能。當(dāng)硼的質(zhì)量分?jǐn)?shù)為3.45%時(shí)，制得的紙漿泡沫完全不燃，為制備具有優(yōu)良阻燃和隔熱性能的高強(qiáng)度、多孔紙漿泡沫提供了一條經(jīng)濟(jì)有效、簡(jiǎn)單易行的路線。

圖8 PMOPZ/CNF泡沫在水平和垂直方向燃燒測(cè)試前后的照片[108]

賦予輕質(zhì)多孔材料阻燃的性能可以將它作為建筑物或家用電器中的熱絕緣體，提高材料的耐用性，降低材料的使用成本，從而降低社會(huì)的能耗需求。

3 結(jié)語(yǔ)

纖維素是一種縱橫比高、比表面積大、力學(xué)性能優(yōu)異，具有生物相容性和生物降解性的生物友好型材料。纖維素與有機(jī)或無(wú)機(jī)材料復(fù)合制備的輕質(zhì)多孔材料可以實(shí)現(xiàn)阻燃、吸附、電磁屏蔽、導(dǎo)電、抗菌等功能，極大地拓展了其應(yīng)用領(lǐng)域。隨著研究的深入，未來(lái)會(huì)出現(xiàn)越來(lái)越多性能更加優(yōu)異的纖維素基復(fù)合多孔材料，但是纖維素基復(fù)合多孔材料在成本控制、大規(guī)模生產(chǎn)、回收利用等方面還面臨挑戰(zhàn)。此外，還應(yīng)認(rèn)真考慮更環(huán)保、更綠色的合成方法，以滿足可持續(xù)發(fā)展的要求。研發(fā)綠色、環(huán)保、可再生的材料是大勢(shì)所趨，在碳中和的背景下顯得尤為重要，所以纖維素基復(fù)合輕質(zhì)多孔材料的開(kāi)發(fā)、制備及其推廣應(yīng)用仍將是未來(lái)研究的重點(diǎn)。

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Research Progress of Cellulose-based Lightweight Porous Materials

ZONG Yi-fenga, WANG Ru-yia, YANG Yu-jiea, LIU Yanga,b, ZHAO Huia,b

(a. College of Light Industry and Food Engineering b. Guangxi Key Laboratory of Clean Pulp and Paper and Pollution Control, Guangxi University, Nanning 530004, China)

Cellulose-based lightweight porous material has the advantages of light weight, high porosity, and low cost. So they are widely used in adsorption, catalysis, heat insulation and other fields. However, problems such as flammability and poor water resistance limit its scope of application. Compound modification can improve the above shortcomings and give it new characteristics. Therefore, it is necessary to fully understand the functional modification method and the wide application of lightweight porous composite materials. In this paper, by tracking the research and application progress of functional modification of cellulose-based lightweight porous materials at home and abroad, the basic properties and performance of cellulose-based lightweight porous materials are summarized, and the functions of cellulose-based composite lightweight porous materials are analyzed. Chemical modification methods and applications, the opportunities and challenges of cellulose-based composite lightweight porous materials in many fields are introduced in detail. The organic or inorganic materials are combined with cellulose to make lightweight porous materials. These materials can achieve flame retardant, absorption, electromagnetic shielding, electrical conductivity, hydrophobic, antibacterial and other functions, which can broaden the application range of cellulose-based lightweight porous materials in packaging, medical, battery and other fields.

cellulose; compound material; functionalization

TS71+1；TB484

A

1001-3563(2022)13-0066-13

10.19554/j.cnki.1001-3563.2022.13.009

2021?10?26

國(guó)家自然科學(xué)基金（22068004，21534007）；廣西自然科學(xué)基金（2020GXNSFAA159027，2020GXNSFBA159023）

宗益峰（1997—），男，本科，主攻纖維素輕質(zhì)多孔材料。

劉楊（1979—），女，博士，廣西大學(xué)副教授，主要研究方向?yàn)槟举|(zhì)纖維素基功能型先進(jìn)材料。

責(zé)任編輯：彭颋