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        小麥秸稈纖維素均相醚化制備羧甲基纖維素工藝優(yōu)化

        2017-11-13 03:27:15楊全剛諸葛玉平劉春增
        關(guān)鍵詞:硫脲羧甲基氧化鋅

        楊全剛,諸葛玉平,曲 揚(yáng),劉春增

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        小麥秸稈纖維素均相醚化制備羧甲基纖維素工藝優(yōu)化

        楊全剛,諸葛玉平※,曲揚(yáng),劉春增

        (山東農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,泰安 271018)

        為了利用小麥秸稈通過(guò)均相反應(yīng)制備羧甲基纖維素(CMC,carboxymethyl cellulose),采用氫氧化鈉和過(guò)氧化氫回流加熱法制備小麥秸稈纖維素。以加入氧化鋅的氫氧化鈉/尿素/硫脲體系為溶劑,采用凍融循環(huán)法溶解小麥秸稈纖維素,利用正交試驗(yàn)獲得了該溶解體系的最佳組成。在溶解了小麥秸稈纖維素的氫氧化鈉/尿素/硫脲/氧化鋅的體系中,以氯乙酸鈉為醚化劑制備CMC,并對(duì)其進(jìn)行紅外光譜分析和取代度(degree of substitution,DS)測(cè)定。結(jié)果表明:在固液比為1∶20 g/mL,質(zhì)量分?jǐn)?shù)為10%NaOH,反應(yīng)溫度為85 ℃,回流3.5 h和固液比為1∶30 g/mL,質(zhì)量分?jǐn)?shù)為3%H2O2,反應(yīng)溫度為85 ℃,回流3 h處理小麥秸稈,纖維素提取率最高為84.61%,同時(shí)能較好的脫除半纖維素和木質(zhì)素;最佳溶解體系為:質(zhì)量分?jǐn)?shù)為7%NaOH,11%硫脲,5%尿素,0.05%氧化鋅,0 ℃時(shí),最大溶解度為2.880 1 g。紅外光譜試驗(yàn)表明小麥秸稈纖維素與微晶纖維素特征吸收峰基本一致,醚化反應(yīng)生成的CMC與商品CMC的特征吸收峰基本一致。CMC的取代度受纖維素用量、溫度和氯乙酸鈉與纖維素葡萄糖單元(AGU,anhydroglucose unit)的摩爾比影響,隨纖維素用量和氯乙酸鈉與纖維素AGU的摩爾比增大而提高,隨醚化溫度的增加先增大(<55 ℃)后降低。研究結(jié)果為以小麥秸稈為原料,經(jīng)NaOH和H2O2處理獲得纖維素,溶解在加入氧化鋅的氫氧化鈉/尿素/硫脲體系中,與氯乙酸鈉經(jīng)過(guò)均相醚化反應(yīng)合成取代度較高的CMC提供參考。

        秸稈;纖維素;優(yōu)化;溶解度;羧甲基纖維素(CMC)

        0 引 言

        中國(guó)秸桿資源豐富,產(chǎn)量大、分布廣、種類(lèi)多,但主要以小麥、水稻和玉米秸稈為主。2015年全國(guó)主要農(nóng)作物秸稈可收集資源量為9.000億t,利用量為7.209億t,秸稈綜合利用率為80.1 %。其中,用作工業(yè)原料的僅占2.7%左右,90%以上作為還田肥料、燃料、飼料和廢棄焚燒[1]。目前秸稈肥料化利用率為43.2%(主要為秸稈機(jī)械化直接還田),僅為發(fā)達(dá)國(guó)家的一半[2]。如東北地區(qū),由于相關(guān)部分政策模糊、氣候、地域環(huán)境、技術(shù)資金和農(nóng)戶認(rèn)識(shí)水平等原因,導(dǎo)致該地區(qū)無(wú)法在短期內(nèi)完全遏制秸稈焚燒現(xiàn)象[3]。以哈爾濱市為例,秸稈資源中,用于生活燃料為20%左右;用作培養(yǎng)基質(zhì)和工業(yè)原料為2%左右;用作飼料為5%左右。能源化綜合利用不足30%;其余70%以上的秸稈資源被廢棄,很大一部分秸稈被隨意丟棄或焚燒[4]。綜上所述,農(nóng)作物秸稈利用方式簡(jiǎn)單,資源浪費(fèi)嚴(yán)重,同時(shí)污染環(huán)境,因此進(jìn)一步拓寬和推進(jìn)秸稈的工業(yè)化利用具有重要意義。羧甲基纖維素(carboxymethyl cellulose,CMC)是最具代表性的離子型纖維素醚,具有增黏、乳化、懸浮、降失水、抗鹽以及熱穩(wěn)定性好等優(yōu)良特性。廣泛用于石油、采礦、造紙、紡織、印染、洗滌劑、食品、化妝品等多個(gè)行業(yè),享有“工業(yè)味精”的美譽(yù)[5]。然而,由于CMC制備的主要原料精制棉成本高,因此需要尋求新的原材料來(lái)制備CMC。目前常用的主要有農(nóng)作物秸稈和工業(yè)副產(chǎn)物,如玉米秸稈[6-7]、小麥秸稈[8-9]、棉花稈[10]、稻草[11]、桑枝皮[12]、木屑[13]、竹屑[14]、甜菜[15]、香蕉莖稈[16]、橘皮[17]、廢棄棉織物[18]、甘蔗渣[19]、廢糖粕[20]、海帶廢渣[21]、馬鈴薯淀粉渣[22]、化纖廠廢堿液等[23]。但其制備方法是把這些原料通過(guò)堿液或酸液處理后提取出纖維素,然后制備CMC,或者通過(guò)氣爆、機(jī)械活化后制備CMC。制備中需要加入有機(jī)稀釋劑和催化劑,反應(yīng)條件復(fù)雜,均為異相反應(yīng)體系,沒(méi)有進(jìn)行纖維素的溶解,直接進(jìn)行醚化反應(yīng)。這就需要進(jìn)行多次加堿和較高的反應(yīng)溫度,使醚化反應(yīng)的副反應(yīng)增強(qiáng),獲得的CMC取代度在0.4以上才能溶于水[5,24]。這種用異相方法合成的CMC,由于反應(yīng)只發(fā)生在纖維素?zé)o定形區(qū)和結(jié)晶表面,是一個(gè)由表及里的過(guò)程,反應(yīng)過(guò)程難以精確控制,產(chǎn)物性能無(wú)法預(yù)測(cè)[25]。因此尋求進(jìn)行均相反應(yīng)制備CMC是目前的研究熱點(diǎn),常用的溶解纖維素的均相溶劑體系有LiCl/DMAc、DMSO/TBAF、離子液體和堿/尿素/硫脲溶劑體系[26]。

        堿/尿素/硫脲溶劑體系是一種纖維素水溶體系,該溶劑體系預(yù)冷至?5~?12 ℃后可迅速溶解纖維素,得到透明的溶液[27]。以微晶纖維素或棉短絨為原料在該體系下均相合成了羥乙基纖維素[28]、兩性纖維素醚[29]、季銨化纖維素陽(yáng)離子聚電解質(zhì)醚[30-31]、甲基纖維素和羥丙基纖維素[32]等纖維素改性物質(zhì),而且反應(yīng)條件溫和,不需要有機(jī)溶劑,同時(shí)在該體系中合成的CMC取代度在0.20時(shí)即可溶于水[33]。

        本文利用小麥秸稈制備纖維素,選擇加入氧化鋅的堿/尿素/硫脲溶劑體系(NaOH /urea/thiourea/ZnO aqueous solution,SUTZ)對(duì)制備的小麥秸稈纖維素進(jìn)行溶解。并通過(guò)正交試驗(yàn)確定其最大溶解度時(shí)的各組分的最佳含量。探討在該溶解體系中直接對(duì)纖維素醚化制備CMC的方法,減少了加堿次數(shù),降低了堿的用量,不再加入有機(jī)溶劑。同時(shí)對(duì)在該體系下影響CMC取代度的因素進(jìn)行了初步探討。獲得了一種利用溶液法由小麥秸稈制備CMC的生產(chǎn)工藝。研究結(jié)果為以小麥秸稈為原料,經(jīng)過(guò)處理獲得纖維素,再將小麥秸稈纖維素溶于SUTZ中,均相合成CMC提供參考。

        1 材料與方法

        1.1 試驗(yàn)材料及主要試劑

        原料:小麥秸稈取自山東農(nóng)業(yè)大學(xué)馬莊試驗(yàn)基地。

        試劑:尿素、硫脲、氧化鋅、氫氧化鈉、丙酮、無(wú)水乙醇、乙酸、氯乙酸鈉、甲醇、鹽酸、酚酞,均為國(guó)產(chǎn)分析純?cè)噭?,微晶纖維素和商品羧甲基纖維素(CMC)為生物試劑。

        1.2 主要試驗(yàn)儀器

        DF-101S集熱式恒溫加熱磁力攪拌器(北京同德創(chuàng)業(yè)科技有限公司);真空干燥器(上海君翼儀器設(shè)備有限公司);ND7-1L球磨機(jī)(南京萊步科技實(shí)業(yè)有限公司);IR-810紅外分光光度計(jì)(JASCO日本分光株式會(huì)社)。

        1.3 試驗(yàn)方法

        1.3.1 工藝流程

        小麥秸稈→預(yù)處理→40目小麥秸稈粉末→NaOH處理→H2O2處理→小麥秸稈纖維素→凍融循環(huán)法和正交試驗(yàn)→小麥秸稈纖維素均相溶液→55 ℃條件下氯乙酸鈉醚化反應(yīng)→羧甲基纖維素。

        1.3.2 小麥秸稈的預(yù)處理

        將小麥秸稈去葉,切成1~2 cm長(zhǎng)小段,用清水漂洗3~5次除去塵土等雜質(zhì)。再用去離子水清洗3~5次,將蒸餾水煮沸后,加入上述已初步處理的小麥秸稈煮1 h,取出晾干。再在烘箱中烘干,用高速粉碎機(jī)粉碎,過(guò)40目篩,得到小麥秸稈粉末,放入干燥器中備用[34]。按照參考文獻(xiàn)[35]的方法測(cè)定小麥秸稈中纖維素、半纖維素和木質(zhì)素質(zhì)量分?jǐn)?shù)分別為:48.32%、23.26%和21.14%。

        1.3.3 小麥秸稈纖維素的制備

        分別配制質(zhì)量分?jǐn)?shù)為6%、8%、10%、12%的氫氧化鈉溶液[36],按1∶20 g/mL的固液比將過(guò)40目篩的小麥秸稈粉末加入到氫氧化鈉溶液中,攪拌,混合均勻后,在85 ℃下回流加熱3.5 h。反應(yīng)結(jié)束后過(guò)濾,用水洗滌至中性,移入烘箱干燥[37]。配制質(zhì)量分?jǐn)?shù)為3%過(guò)氧化氫溶液,將上述用氫氧化鈉溶液處理過(guò)的小麥秸稈粉末按1∶30 g/mL的固液比混合。加入少量硅酸鈉作為穩(wěn)定劑,在85 ℃回流加熱3 h,過(guò)濾,用水洗滌,移入烘箱干燥。將上述過(guò)氧化氫處理過(guò)干燥的小麥秸稈粉末用球磨機(jī)研磨1.5 h[38]。按照參考文獻(xiàn)[35]的方法測(cè)定纖維素、半纖維素和木質(zhì)素含量,計(jì)算半纖維素和木質(zhì)素脫除率(脫除率=(秸稈總半纖維素或木質(zhì)素含量-處理后粉末中半纖維素或木質(zhì)素含量)/秸稈總半纖維素或木質(zhì)素含量×100%)。

        1.3.4 小麥秸稈纖維素的最佳溶解條件的確定

        以氫氧化鈉、硫脲、尿素、氧化鋅在溶劑體系中的質(zhì)量分?jǐn)?shù)為影響因素,每個(gè)因素取5個(gè)水平(表1)。以SUTZ對(duì)制備纖維素的溶解度為評(píng)價(jià)指標(biāo),設(shè)計(jì)正交試驗(yàn)[39],以確定SUTZ的最佳組成。配制100 g一定組成的氫氧化鈉/尿素/硫脲/氧化鋅混合溶劑,加入制備的小麥秸稈纖維素4 g(0)強(qiáng)烈攪拌(7 000 r/min)1 min,得到纖維素懸浮液。將其在?20 ℃的冰箱中放置4 h,取出在冰水浴中解凍后強(qiáng)烈攪拌(7 000 r/min)。將得到的纖維素溶液在高速離心機(jī)(8 000 r/min)下離心20 min,將上清液與不溶的部分分離。不溶部分用水反復(fù)洗滌至中性,再用丙酮洗滌后真空烘干,稱(chēng)其質(zhì)量(1,g),并計(jì)算小麥秸稈溶解度(,g),=o?1。

        表1 正交試驗(yàn)因素水平表

        1.3.5 羧甲基纖維素的制備

        分別將1.4、2.1、2.8 g制備的小麥秸稈纖維素溶解在100 g最佳SUTZ中。取上清液,加入5 mL10%的NaOH堿化,設(shè)定溫度35 ℃,攪拌回流1 h進(jìn)行堿化反應(yīng),繼續(xù)升溫達(dá)到設(shè)定的醚化溫度55 ℃時(shí),加入定量的氯乙酸鈉,使氯乙酸鈉與纖維素葡萄糖單元(AGU,anhydroglucose unit)的摩爾比分別為:3.5∶1、7∶1、10.5∶1,攪拌回流5 h[33]。將所得醚化產(chǎn)物用少量乙酸中和,過(guò)濾,用無(wú)水乙醇反復(fù)洗滌固體3~5次。后移入烘箱105 ℃干燥2 h,得到羧甲基纖維素產(chǎn)品[40]。

        1.3.6 紅外光譜測(cè)定

        用溴化鉀壓片法測(cè)定醚化前秸稈纖維素、微晶纖維素、醚化后CMC和商品CMC的紅外光譜特征,波長(zhǎng)范圍為500~4 000 nm。

        1.3.7 取代度的測(cè)定

        用質(zhì)量分?jǐn)?shù)為70%的甲醇溶液配制1 mol/L的HCl/CH3OH溶液,取0.5 g醚化纖維素浸于20 mL上述溶液中,攪拌3 h,使CMC完全酸化,抽濾,用蒸餾水洗至溶液無(wú)氯離子,用150 mL標(biāo)準(zhǔn)NaOH溶液溶解,得到透明溶液,以酚酞作指示劑,用標(biāo)準(zhǔn)鹽酸溶液滴定至終點(diǎn),重復(fù)3次,求平均值[41]。

        用下式計(jì)算取代度[41]:

        取代度=0.162/(1?0.058)

        式中為每克羧甲基纖維素消耗的NaOH毫摩爾數(shù)。=(NaOH用量(mmol)-鹽酸用量(mmol))/醚化纖維素質(zhì)量(g)。

        1.4 數(shù)據(jù)處理與分析方法

        用Excel進(jìn)行整理分析試驗(yàn)數(shù)據(jù)并作圖,用Origin 8繪制紅外光譜特征圖,用SPSS 21.0軟件進(jìn)行正交試驗(yàn)方差分析。

        2 結(jié)果與分析

        2.1 制備小麥秸稈纖維素NaOH質(zhì)量分?jǐn)?shù)的確定

        NaOH質(zhì)量分?jǐn)?shù)在10%以下時(shí),纖維素提取率、半纖維素和木質(zhì)素的脫除率隨NaOH含量的增加而提高,在NaOH質(zhì)量分?jǐn)?shù)為10%時(shí),小麥秸稈粉末纖維素提取率最高為84.61%,半纖維素和木質(zhì)素的脫除率分別為84.44%、91.14%。NaOH質(zhì)量分?jǐn)?shù)為12%時(shí),半纖維素的脫除率和纖維素提取率均降低,木質(zhì)素脫除率提高到92.06%,但只比NaOH質(zhì)量分?jǐn)?shù)為10%時(shí)的脫除率高0.92個(gè)百分點(diǎn),因此選擇NaOH質(zhì)量分?jǐn)?shù)為10%和3%過(guò)氧化氫溶液來(lái)脫除半纖維素和木質(zhì)素,制備纖維素(圖1)。出現(xiàn)這種情況的原因可能是NaOH過(guò)量,引起纖維素和半纖維素降解,導(dǎo)致在處理后的小麥秸稈粉末中含量下降。但木質(zhì)素卻沒(méi)有降解,從而其脫除率繼續(xù)升高[34]。

        圖1 纖維素提取率與半纖維素、木質(zhì)素脫除率

        2.2 小麥秸稈纖維素最佳溶解條件的確定

        對(duì)正交試驗(yàn)結(jié)果進(jìn)行分析(表2),值表示各因素水平對(duì)應(yīng)試驗(yàn)指標(biāo)和的平均值,通過(guò)值大小可以判斷最佳水平和最佳組合。在SUTZ中,通過(guò)對(duì)比NaOH5個(gè)水平的平均溶解度發(fā)現(xiàn),隨NaOH含量的增加,對(duì)小麥秸稈纖維素的溶解能力越大。在質(zhì)量分?jǐn)?shù)為8%時(shí),小麥秸稈纖維素的溶解度達(dá)到最大為2.578 3 g,但在質(zhì)量分?jǐn)?shù)為7%,其溶解度為2.577 4 g,與質(zhì)量分?jǐn)?shù)為8%時(shí)相差0.000 9 g,差別幾乎可以忽略。為減少加堿量,降低成本,減少?gòu)U堿液的排放,因此選擇NaOH質(zhì)量分?jǐn)?shù)7%為最佳。

        在SUTZ體系中,對(duì)比硫脲(CO(SH2)2)和尿素(CO(NH2)2)的5個(gè)水平的平均溶解度發(fā)現(xiàn),隨兩者質(zhì)量分?jǐn)?shù)的增加,硫脲對(duì)小麥秸稈纖維素的溶解度先降低后增大,再降低。硫脲在質(zhì)量分?jǐn)?shù)為11%對(duì)小麥秸稈纖維素的溶解能力最大。尿素對(duì)小麥秸稈纖維素溶解度先降低后一直增大。尿素在質(zhì)量分?jǐn)?shù)為13%時(shí)溶解度最大為2.232 5 g,但與5%時(shí)的溶解度2.224 8 g,相差0.007 7 g,差別較小。為節(jié)約原料和降低成本,選擇尿素最佳質(zhì)量分?jǐn)?shù)為5%。

        表2 小麥秸稈纖維素在SUTZ體系中的溶解度正交試驗(yàn)結(jié)果

        氧化鋅(ZnO)在SUTZ體系中對(duì)小麥秸稈纖維素溶解能力的表現(xiàn)與NaoH、硫脲和尿素都不同。比較其5個(gè)水平的平均溶解度發(fā)現(xiàn),溶解度隨氧化鋅質(zhì)量分?jǐn)?shù)的增大先升高后降低,再升高。最大溶解度出現(xiàn)在質(zhì)量分?jǐn)?shù)為0.05%時(shí),與0.1%的溶解度相差不大。同時(shí)氧化鋅質(zhì)量分?jǐn)?shù)為0時(shí)與0.2%對(duì)小麥秸稈纖維素的溶解度相差不大,都低于氧化鋅質(zhì)量分?jǐn)?shù)為0.05%時(shí)的溶解度,因此選擇氧化鋅的最佳溶解質(zhì)量分?jǐn)?shù)為0.05%。

        正交試驗(yàn)中,各因素值表示因素對(duì)試驗(yàn)指標(biāo)的影響大小,值越大,影響越大,越重要。SUTZ體系中NaOH、硫脲、尿素、氧化鋅值的分別為1.190 7、0.217 4、0.170 8、0.219 9,因此對(duì)溶解度影響最大的因素為NaOH。其次為氧化鋅、硫脲。影響最小的為尿素。方差分析也表明(表3),NaOH為影響最顯著因素,其次為氧化鋅。

        根據(jù)以上分析,綜合NaOH、硫脲、尿素、氧化鋅的最佳溶解含量,SUTZ體系的最佳組成是:NaOH質(zhì)量分?jǐn)?shù)為7%,硫脲質(zhì)量分?jǐn)?shù)為11%,尿素質(zhì)量分?jǐn)?shù)為5%,氧化鋅質(zhì)量分?jǐn)?shù)為0.05%。用最佳組成進(jìn)行小麥秸稈纖維素溶解試驗(yàn),測(cè)定其溶解度為2.880 1 g。

        表3 小麥秸稈纖維素在SUTZ體系中的溶解度方差分析

        注:**,極顯著。

        Note:**,Very significant

        2.3 氯乙酸鈉與纖維素AGU的摩爾比對(duì)CMC取代度的影響

        CMC的取代度與醚化反應(yīng)中氯乙酸鈉與纖維素AGU的摩爾比有關(guān)(表4)。當(dāng)小麥秸稈纖維素的量不變時(shí),隨摩爾比的增大,CMC的取代度逐步提高。在小麥秸稈纖維素用量為1.4 g時(shí),摩爾比分別從3.5∶1增加到10.5∶1,氯乙酸鈉用量增加3倍,CMC的取代度分別從0.12增加到0.31。小麥秸稈纖維素用量為2.1和2.8 g時(shí)有相同規(guī)律。當(dāng)摩爾比不變情況下,纖維素的用量增加,羧甲基纖維素的取代度明顯提高,提高幅度高于增加氯乙酸鈉的用量。在相同摩爾比3.5∶1情況下,當(dāng)纖維素用量從1.4 g增加2倍到2.8 g時(shí),取代度從0.12增加到0.30。在纖維素醚化過(guò)程中,存在兩個(gè)競(jìng)爭(zhēng)反應(yīng),一是堿纖維素與氯乙酸鈉的醚化反應(yīng),二是氯乙酸鈉與堿的反應(yīng)生成乙醇酸鈉,當(dāng)氯乙酸鈉過(guò)量時(shí),纖維素用量增加,生成的堿纖維素增加,促進(jìn)了醚化反應(yīng)的進(jìn)行,因此取代度提高[5,33]。

        2.4 溫度對(duì)CMC取代度的影響

        溫度對(duì)CMC的取代度有顯著影響(圖2)。DS隨溫度的升高先升高后下降,在溫度較低時(shí)取代度很低,只有0.12。隨溫度升高,取代度逐漸升高,在溫度為55 ℃時(shí),取代度最高,達(dá)到0.28。而后取代度隨溫度升高而降低。主要原因可能在于溫度過(guò)高,首先引起纖維素溶解度下降,其次導(dǎo)致纖維素降解,同時(shí)副反應(yīng)加快,不利于主反應(yīng)的進(jìn)行[19,22,33]。因此醚化溫度在55 ℃較佳,纖維素為2.8 g,氯乙酸鈉與纖維素AGU摩爾比為10.5:1時(shí),取代度為0.45(表4)。

        表4 SUTZ最佳溶解體系下CMC取代度

        注:醚化反應(yīng)溫度為55 ℃,摩爾比=氯乙酸鈉:纖維素AGU(纖維素質(zhì)量/纖維素AGU相對(duì)分子質(zhì)量)。

        Note: The etherification reaction temperature of 55 ℃. molar ratio =sodium chloroacetate: cellulose AGU (Cellulose quality /the relative molecular weight of cellulose).

        注:纖維素為1.4g,氯乙酸鈉與纖維素AGU摩爾比為7:1。

        2.5 紅外光譜結(jié)果分析

        分析圖3可以看出,小麥秸稈纖維素(a)與微晶纖維素紅外光譜(b)比較,兩者基本重合,2 913 cm-1處的吸收峰歸屬為C-H的伸縮振動(dòng)峰,1 055 cm-1附近的吸收峰是纖維中醚鍵的特征峰,894 cm-1為環(huán)狀C-O-C不對(duì)稱(chēng)面外伸縮振動(dòng)或CH2(CH2OH)非平面搖擺振動(dòng)產(chǎn)生的特征峰[42]。通過(guò)對(duì)比醚化前小麥秸稈纖維素(a)和醚化后羧甲基纖維素(c)紅外光譜,3 442 cm-1附近的吸收峰是OH基的伸縮振動(dòng)吸收,醚化后在1 600 cm-1處有明顯的羰基吸收峰,強(qiáng)吸收帶歸屬于羧酸鹽中的–COO–伸縮振動(dòng)[43];醚化后羧甲基纖維素(c)1 420處吸收峰變強(qiáng),由此可知–CH2基團(tuán)的存在,表明羧甲基化反應(yīng)完成[44]。初步證明產(chǎn)物為羧甲基纖維素。比較小麥秸稈醚化得到的CMC(c)和商品純CMC(d)的紅外譜圖,可以看到二者的特征峰位基本吻合,從而進(jìn)一步證明從小麥秸稈在SUTZ中均相合成了CMC。在1 000~1 100 cm-1處是醚鍵的對(duì)稱(chēng)與不對(duì)稱(chēng)振動(dòng)吸收峰,醚化前有較強(qiáng)的峰,醚化后峰基本消失,說(shuō)明在制備羧甲基纖維素的過(guò)程中纖維素的聚合度有較大變化,聚合度降低,CMC的溶解度增加,這也是通過(guò)均相SUTZ合成的CMC在取代度小于0.40情況下能溶于水的原因之一[45]。

        a. 小麥秸稈纖維素;b. 微晶纖維素;c. 小麥秸稈醚化后得到的羧甲基纖維素;d. 商品羧甲基纖維素

        3 討 論

        在SUTZ溶解度正交試驗(yàn)中,硫脲與尿素的各水平質(zhì)量分?jǐn)?shù)相同,水平間溶解度變化規(guī)律不同。通過(guò)水平間溶解度對(duì)比能夠確定在K時(shí)硫脲溶解度最大。但尿素是否在質(zhì)量分?jǐn)?shù)比K小或比K大時(shí)溶解度繼續(xù)增大,還需進(jìn)一步進(jìn)行試驗(yàn)驗(yàn)證。兩者可能通過(guò)與NaOH作用或與纖維素形成氫鍵來(lái)促進(jìn)纖維素的溶解。查純喜等研究認(rèn)為硫脲、尿素與NaOH有著相互作用,并且硫脲與NaOH的作用更強(qiáng)烈[46]。而有研究認(rèn)為在SUTZ中,硫脲和尿素促進(jìn)纖維素溶解的原因在于分子能夠擴(kuò)散到纖維素的結(jié)晶區(qū), 分子中的C=O、C=S、–NH2基團(tuán)與纖維素形成氫鍵而拆散纖維素結(jié)晶,從而使纖維素溶解[39]。根據(jù)硫脲和尿素各水平溶解度在本研究中變化情況,質(zhì)量分?jǐn)?shù)低時(shí)硫脲和尿素可能先與NaOH生成某種絡(luò)合物,包裹住纖維素大分子,阻止其分子間或分子內(nèi)形成氫鍵,保持纖維素溶液的穩(wěn)定。隨質(zhì)量分?jǐn)?shù)升高時(shí),消耗NaOH過(guò)多,降低了NaOH作用,溶解度下降。質(zhì)量分?jǐn)?shù)繼續(xù)增大,過(guò)量的硫脲和尿素進(jìn)入纖維素分子內(nèi)部,與纖維素形成氫鍵,促進(jìn)纖維素溶解,溶解度增大。質(zhì)量分?jǐn)?shù)繼續(xù)增大,由于硫脲與NaOH作用強(qiáng)烈,消耗過(guò)量NaOH,使NaOH作用下降,導(dǎo)致溶解度下降。而尿素與NaOH作用較硫脲低,因此在試驗(yàn)質(zhì)量分?jǐn)?shù)范圍內(nèi)溶解度先降低后升高,未出現(xiàn)再降低現(xiàn)象。

        本研究發(fā)現(xiàn)隨氧化鋅的質(zhì)量分?jǐn)?shù)的增加,氧化鋅在SUTZ中的溶解度下降。當(dāng)其質(zhì)量分?jǐn)?shù)過(guò)高時(shí)(大于0.2%),氧化鋅無(wú)法完全溶解在該體系中。可能原因是氧化鋅的溶解與NaOH的質(zhì)量分?jǐn)?shù)大小有關(guān)。氧化鋅在SUTZ中促進(jìn)纖維素溶解的原因在于能夠與NaOH反應(yīng)生成[Zn(OH)4]2-,0.05%ZnO在水和NaOH溶液中完全溶解,并以[Zn(OH)4]2-的形式存在,反應(yīng)式為:ZnO+ 2NaOH+H2O→Na2[Zn(OH)4],[Zn(OH)4]2-離子可能起到了與Na+水合離子同樣的作用,即穩(wěn)定纖維素的羥基[47],從而促進(jìn)纖維素的溶解。

        4 結(jié) 論

        1)在質(zhì)量分?jǐn)?shù)為10%NaOH,固液比為1∶20 g/mL,反應(yīng)溫度為85 ℃,回流3.5 h,以及質(zhì)量分?jǐn)?shù)3%H2O2,固液比為1∶30 g/mL,反應(yīng)溫度為85 ℃,回流3 h條件下提取小麥秸稈纖維素效果較好,達(dá)到84.61%,半纖維素和木質(zhì)素脫除率分別為84.44%和91.14%。

        2)在堿/尿素/硫脲溶劑體系中加入氧化鋅能促進(jìn)小麥秸稈纖維素的溶解,對(duì)該體系溶解能力的影響由大到小依次為氫氧化鈉、氧化鋅、硫脲、尿素。其最佳組成是:質(zhì)量分?jǐn)?shù)分別為氫氧化鈉7%,硫脲11%,尿素5%,氧化鋅0.05%,最佳組成條件下,小麥秸稈纖維素的溶解度為2.880 1 g。

        3)在SUTZ均相溶劑體系中,合成了CMC,其取代度隨氯乙酸鈉與纖維素AGU的摩爾比增加而提高,隨溫度的升高先增加后降低。

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        Process optimization for preparation of carboxymethyl cellulose by homogeneous etherification of wheat straw

        Yang Quangang, Zhuge Yuping※, Qu Yang, Liu Chunzeng

        (,,271018,)

        In this study, carboxymethyl cellulose (CMC) was synthesized from wheat straw cellulose by a homogeneous reaction. The main process was as follows: wheat straw was cut into 1–2 cm-long pieces, rinsed with water 3–5 times to remove dust and other impurities, and washed first with deionized water for a further 3–5 times and then with boiling distilled water. The wheat straw was then preliminarily treated for 1 h, and dried in an oven. Crushing was then performed using a high-speed grinder with a 40-mesh screen to obtain the wheat straw powder. Sodium hydroxide and hydrogen peroxide were used to treat the wheat straw powder to obtain cellulose. An alkali/urea/thiourea system was used to dissolve ZnO. The extracted wheat straw cellulose was dissolved by freezing and melt circulation, following which the cellulose was stored for 4 h at ?20°C, then thawed, and a clear cellulose solution was obtained by rapid stirring in an ice water bath. The optimal composition of the alkali/urea/thiourea aqueous solution was obtained through orthogonal experiments. The orthogonal design were four factors, five levels. The four factors with five levels were sodium hydroxide (5%, 6%, 7%, 8%, and 9%), thiourea (5%, 7%, 9%, 11%, and 13%), urea (5%, 7%, 9%, 11%, and 13%), and zinc oxide (0, 0.05%, 0.1%, 0.15%, and 0.2%). After dissolving wheat straw cellulose with the optimal dissolving system, CMC samples were prepared with sodium chloroacetate. The CMC samples were characterized by Fourier transform-infrared (FTIR) spectroscopy and the degree of substitution (DS). The results can be summarized as follows: first, the wheat straw powder was treated at a solid-liquid (10% NaOH solution) ratio of 1:20 g/mL, at a reaction temperature of 85 ℃, reflux time of 3.5 h, and with a 10% sodium hydroxide solution, followed by treatment with a 10% hydrogen peroxide solution. For the reaction conditions of a solid-liquid (the wheat straw powder treated with 10% NaOH and 3% H2O2) ratio of 1:30 g/mL, at a reaction temperature of 85 ℃, and a reflux time of 3 h, the highest proportion of cellulose that can be extracted from the wheat straw was 84.61 %. The removal rate of hemicellulose was 84.44%, while that of lignin was 91.14%. In the orthogonal experiment, we assessed the influence of thevalue of each factor on the experimental indicators, with a greatervalue indicating a greater impact. Thevalues for the dissolution system (NaOH, thiourea, urea, ZnO) were 1.190 7, 0.217 4, 0.170 8, and 0.219 9, respectively; thus, the most influential factor was NaOH, followed by ZnO, thiourea, and urea with minimal impact. Variance analysis also showed that NaOH was the most influential factor, followed by ZnO. After comparing NaOH, thiourea, urea, and ZnO at the level of solubility, a comprehensive consideration of the cost and environmental factors can be developed. The optimum values of the mass fraction for the solution components were 7%, 11%, 5%, and 0.05% for NaOH, thiourea, urea and ZnO, respectively, for a total solubility of 2.8801 g. The FTIR spectra of the cellulose straw and cellulose showed characteristic absorption peaks of pure cellulose. The etherification reaction of wheat straw cellulose results in the formation of CMC. The characteristic absorption peaks of wheat straw CMC and pure CMC were very similar. The DS of CMC was dependent on the cellulose dosage, temperature, and the molar ratio of sodium chloroacetate to cellulose AGU. Increased cellulose dosage, molar ratios of sodium chloroacetate to cellulose AGU, and temperature initially increased the DS and then caused it to decrease. For a temperature of 55 ℃, the amount of cellulose is 2.8 g, the molar ratio of sodium chloroacetate and cellulose AGU is 10.5:1, and the DS is the highest at 0.45.

        straw; cellulose; optimization; solubility; carboxymethyl cellulose (CMC)

        10.11975/j.issn.1002-6819.2017.20.038

        S19

        A

        1002-6819(2017)-20-0307-08

        2017-05-27

        2017-10-15

        國(guó)家科技支撐計(jì)劃子課題:水肥鹽相互作用關(guān)系與調(diào)控技術(shù)研究(2013BAD05B03);山東省自主創(chuàng)新及成果轉(zhuǎn)化專(zhuān)項(xiàng):黃河三角洲鹽堿地快速改良技術(shù)研發(fā)集成與示范(2014ZZCX07402)

        楊全剛,山東省泰安市人,博士生,主要從事秸稈利用與液體地膜方向研究。泰安 山東農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,271018。 Email:sttzzy@sdau.edu.cn

        ※通信作者:諸葛玉平,山東臨沂人,博士,教授,博士生導(dǎo)師,主要從事土壤生態(tài)環(huán)境和植物營(yíng)養(yǎng)肥料方向研究。泰安 山東農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,271018。Email:zhugeyp@sdau.edu.cn

        楊全剛,諸葛玉平,曲 揚(yáng),劉春增. 小麥秸稈纖維素均相醚化制備羧甲基纖維素工藝優(yōu)化[J]. 農(nóng)業(yè)工程學(xué)報(bào),2017,33(20):307-314. doi:10.11975/j.issn.1002-6819.2017.20.038 http://www.tcsae.org

        Yang Quangang, Zhuge Yuping, Qu Yang, Liu Chunzeng. Process optimization for preparation of carboxymethyl cellulose by homogeneous etherification of wheat straw[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(20): 307-314. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2017.20.038 http://www.tcsae.org

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