波谱学杂志, 2023, 40(4): 376-384 doi: 10.11938/cjmr20233061

研究论文

93Nb核磁共振研究铌酸纳米棒的水热晶化机理

申长志, 张琳琳, 李新,*, 申万岭,#

河南工业大学 化学化工学院,河南 郑州 450001

Hydrothermal Crystallization of Niobium Oxide Nanorods Studied by 93Nb Nuclear Magnetic Resonance

SHEN Changzhi, ZHANG Linlin, LI Xin,*, SHEN Wanling,#

College of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, China

通讯作者: * Tel: 0371-67756718, E-mail:lixin_chemi@haut.edu.cn;# Tel: 0371-67756718, E-mail:wshen@haut.edu.cn.

收稿日期: 2023-03-27  

基金资助: 国家自然科学基金资助项目(21773056); 国家自然科学基金资助项目(21703056); 河南省科技攻关项目(212102210608); 河南省科技攻关项目(202102110289); 河南工业大学青年骨干教师项目(0503/21420046); 河南工业大学青年骨干教师项目(0503/21420110); 河南工业大学创新基金支持计划专项资助(2021ZKCJ01)

Corresponding authors: * Tel: 0371-67756718, E-mail:lixin_chemi@haut.edu.cn;# Tel: 0371-67756718, E-mail:wshen@haut.edu.cn.

Received: 2023-03-27  

摘要

氧化铌(Nb2O5)及其水合物铌酸(Nb2O5·nH2O)代表了一类重要的多相催化剂,然而其水热合成过程和机理尚不明确.本文采用93Nb NMR,结合X射线粉末衍射、透射电镜等技术和量化计算,研究了以草酸铌铵为前驱体水热生成铌酸纳米棒的晶化过程.结果表明草酸铌铵水热晶化过程中发生了水解-聚合反应,先是草酸铌铵水解,接着铌氧单体之间发生缩水聚合反应,最终生成了层内无序、层间有序结构的固体铌酸纳米棒.晶化过程符合“液相成核”机理,层状结构铌酸纳米棒在180 ℃下1 h内即可生成,后续水热处理并未使其明显长大,新的固体产物不断由溶液中独立生成.

关键词: 93Nb核磁共振; 纳米氧化铌; 草酸铌铵; 水热; 晶化机理

Abstract

The crystallization process of hydrothermally synthesizing niobium oxide nanorods with ammonium niobium oxalate precursor was investigated using 93Nb NMR, combined with X-ray powder diffraction, transmission electron microscopy and quantum chemistry calculations. The results showed that the hydrothermal crystallization of ammonium niobium oxalate underwent a hydrolysis-polymerization reaction, starting with the hydrolysis of ammonium niobium oxalate, followed by the polymerization of the niobium-oxide monomers by condensation, resulting in dimers, trimers and multimers, and finally generating solid niobium oxide nanorods with disordered intra-layer and ordered inter-layer structures. The mechanism of crystallization process is “liquid phase nucleation”, with the layer-structured niobium oxide nanorods being produced within 1 h at 180 ℃. The subsequent hydrothermal treatment does not lead to significant growth of the nanorods, and new solid products are continuously generated independently from the solution.

Keywords: 93Nb NMR; nano niobium oxide; ammonium niobium oxalate; hydrothermal; crystallization mechanism

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本文引用格式

申长志, 张琳琳, 李新, 申万岭. 93Nb核磁共振研究铌酸纳米棒的水热晶化机理[J]. 波谱学杂志, 2023, 40(4): 376-384 doi:10.11938/cjmr20233061

SHEN Changzhi, ZHANG Linlin, LI Xin, SHEN Wanling. Hydrothermal Crystallization of Niobium Oxide Nanorods Studied by 93Nb Nuclear Magnetic Resonance[J]. Chinese Journal of Magnetic Resonance, 2023, 40(4): 376-384 doi:10.11938/cjmr20233061

引言

氧化铌(Nb2O5)及其水合物,即铌酸(Nb2O5·nH2O),是一类重要的多相催化剂,在一系列酸催化的含氧官能团反应中(如水合反应、脱水缩合或烷基化反应、羧酸及其衍生物的亲核取代反应、选择性氧化/脱氢反应)表现出高活性和高选择性[1].特别重要的是,氧化铌和铌酸不溶于一般的无机酸和碱,性质稳定,无毒,环境友好,且具有一般固体材料所不具备的在水溶液中依然可以部分保留的Lewis酸性的特点,是一种“耐水固体酸催化剂”[2],其表面的配位不饱和Nb5+可以起到Lewis酸性位的作用,在水系溶液中催化一系列生物质来源多元醇的脱水反应,如甘油脱水为丙烯醛[3],二羟基丙酮转化为乳酸[4],葡萄糖、蔗糖或木糖合成5-羟甲基-2-糠醛(HMF)等[5,6].基于Nb2O5的催化剂也被用于酯化、酯交换和酯水解反应等[7].Nb2O5还有活化氢的能力,可以被用于催化丙烷无氧脱氢制丙烯反应[8],甚至作为加氢催化剂催化丁酸和生物质的高值化加氢反应等[9-12].另外,Nb2O5是一种n型半导体(禁带宽度约为3.4 eV),能够吸收紫外波段的光,产生电子-空穴对,可以作为光催化剂用于光催化反应[13],如染料污染物降解和光催化氧化等[14-17].Nb2O5还广泛应用于气体传感器[18]、光学[19]以及电致变色[20]等.

上述应用的前提和核心是Nb2O5材料的合成制备.Tanabe等[21]早期在氧化铌材料的合成中做出过许多开创性工作.与块状三维材料相比,具有纳米尺寸的催化剂可以增加其比表面积,以及位于表面、台阶、棱和边角等配位不饱和原子的暴露位点数量,从而有利于提高其催化活性.因此,研究者通过液相合成方法合成了各种形状的低维铌氧化物,如零维球状、一维棒/线/纤维状和二维片/膜状纳米材料[22-25].然而,合成中用到的一些铌源是氯化物/氟化物,它们容易潮解,对空气中操作、运输和储存不利[26-28];一些是昂贵的金属铌或氧化铌,合成中需要加入双氧水或有毒的氢氟酸,这对工业生产不利[27,29-33].此外,在许多合成中需要使用有机试剂,如油酸、胺、表面活性剂或聚酰胺,作为结构导向剂、溶剂和封端配体,这些价格较贵,且与绿色化学的概念相悖.使用草酸铌铵作为除水溶剂以外的单一反应物,不需要添加任何额外的化学试剂,水热处理后可以生成铌酸纳米棒,并可作为有良好催化活性的固体酸[34].然而,由于水热过程的复杂性和原位表征技术的挑战性,其水热晶化机理尚未有研究报道,至今仍处于“黑箱”阶段.本工作采用93Nb NMR,结合X射线粉末衍射(XRD)和透射电子显微镜(TEM)等技术,非原位研究了以草酸铌铵为前驱体水热生成铌酸纳米棒的过程和机理.

1 实验部分

1.1 水热晶化实验

将0.75 g草酸铌铵(NH4[NbO(C2O4)2nH2O,以金属计纯度99.9%,长沙华京材料)溶于18 mL蒸馏水,搅拌30 min后,将溶液转移到25 mL的特氟隆内衬高压釜中,180 ℃下水热处理特定时间后(1、2、4、6和8 h),取出冷却.然后从聚四氟乙烯内衬中取出反应液,进行抽滤.滤液在15 000 r/min离心7 min,取上层清液部分室温蒸发完水分,所得固体即为液相部分产物,使用2.5 mL重水将其溶解,以备后续NMR测试.将过滤和离心所得的固体部分混合后于120 ℃烘干,即为固相部分产物铌酸固体.

1.2 表征测试

固相部分产物的XRD测试采用MiniFlex600 X射线衍射仪,射线源为Cu Kα,扫描速度为10°/min,步长为0.02°.

固相部分产物的TEM表征在JEOL JEM-2100F仪器上进行,工作电压为200 kV.

重量分析过程为将液相和固相部分产物分别放在马弗炉中以5 ℃/min升温至800 ℃焙烧1 h,所得产物经分析天平称重后按照纯Nb2O5组分计算其中的Nb元素含量m,再以m液相/(m液相 + m固相)和m固相/(m液相 + m固相)分别得出固相和液相产物中Nb元素相对含量.

93Nb NMR在Bruker AVANCEIII HD 500超导NMR波谱仪上进行,93Nb在此场强下的共振频率为122.43 MHz,采用单脉冲序列,90°脉宽为11 μs,循环延迟为0.5 s,谱宽为200 kHz,采样次数为1 024次.93Nb化学位移定标参照草酸铌铵重水溶液.

1.3 计算模型与方法

采用B3LYP泛函,C、H、O原子采用TZVP基组,Nb原子采用包含赝势的def2tzvp基组进行结构优化.

优化后的结构利用GIAO方法进行Nb的化学位移计算[35],采用B3LYP泛函,C、H、O原子采用def2tzvp基组,Nb原子采用全电子相对论Sapporo-DKH3-TZP基组.所有的计算均在深圳超算中心工作站上利用Gaussian09程序包完成[36].

2 结果与讨论

2.1 XRD和重量分析

Nb2O5具有六方、斜方和单斜等丰富的晶体结构.以草酸铌铵为铌源,水热晶化后产物经过550 ℃以上温度焙烧后,可以转变为赝六方相(TT),这是一种层状化合物.铌原子之间通过氧原子互相连接.铌氧八面体和十面体在层内通过共用顶点和共边结构排列,层与层之间通过共顶点的方式有序堆积[34].

我们在草酸铌铵溶液水热晶化的不同时间,分离出其中的固相部分对其进行了XRD测试.如图1(a)所示,在衍射角2θ为22.9°和46.5°处有两个比较尖锐的衍射峰,按照布拉格公式计算,对应的晶面间距d分别为3.88 Å和1.99 Å.它们可以归结为(001)和(002)衍射,这显示固体产物拥有沿晶轴c方向的有序层状结构.除了这两个明显的尖峰,在衍射角2θ为27°、35°和56°附近出现了一些宽包信号,表明此时的固体在a-b平面的排列是无定形的.水热反应以后的产物是TT相的层状前驱结构,只是在层间排列上具有晶体学有序性,层内长程无序,整体上还没有形成严格的三维有序晶体[34].水热反应1 h到8 h固相部分产物的XRD衍射图基本没有变化,说明在本文采用的水热温度下延长反应时间不会改变固体的晶相结构.这种层状结构在180 ℃下最初的1 h内即可形成.但固相部分产物铌酸的产率变化较大,我们以Nb元素在固相和液相产物中的比例变化来反映晶化过程的完成度.如图1(b)所示,将反应开始时溶液中的Nb含量设为100%,通过重量法分析得到液相产物中的Nb元素在2 h内迅速下降至27.7%,6 h后下降至6.0%,8 h后下降至0.8%.与此对应,固相产物中的Nb元素在2 h内迅速升至72.3%,6 h后升至94.0%,8 h后高达99.2%,基本完成了整个晶化过程.

图1

图1   (a)不同水热反应时间的固相部分产物的粉末XRD图谱;(b)固相和液相部分产物中Nb含量随水热时间的变化.虚线为伪一级动力学拟合曲线

Fig. 1   (a) Powder XRD patterns of solid product of different hydrothermal reaction time; (b) Nb content in solid product and solution as a function of hydrothermal time. The dashed line is the fitting curve of the pseudo-first-order kinetic


2.2 TEM

水热晶化1 h和8 h后固相部分产物的TEM表征结果如图2所示,可以看到1 h和8 h时所制备的固体产物均为纳米棒状形貌,直径约为4~10 nm,长度在50~150 nm范围.形貌和尺寸随着水热时间的延长,并没有太大变化.经大视场大范围搜索,未发现其他形貌或无定形颗粒,说明固体产物为纯的铌酸纳米棒.结合固体产率的变化,说明后续大量生成的层状结构固体产物并不是在初始“晶核”上继续外延生长,使固体长大,而是在溶液中独立生成的,即类似于晶体合成中的“液相成核”机理.

图2

图2   水热反应(a) 1 h和(b) 8 h固相部分产物的透射电镜图

Fig. 2   TEM images of the solid product after hydrothermal reaction of (a) 1 h and (b) 8 h


2.3 NMR

XRD对长程有序敏感,但不能探测晶化过程液相环境中发生的变化,而NMR对原子周围配位环境的短程有序敏感,可以测量液体样品,适合于研究水热体系的晶化机理[37-39].两者互相补充,可以得到体系更全面的信息.93Nb天然丰度为100%,核自旋为9/2.较大的四极相互作用会使谱线严重展宽,不利于其固体NMR谱的采集.但在液相中分子的自由运动平均掉了四极耦合作用,可以得到各向同性的谱线.且其化学位移范围较大,可以较为灵敏的反映Nb原子周围的化学环境变化,较为适合用来研究铌酸的水热晶化机理.

我们将草酸铌铵溶液水热晶化不同时间后液相部分产物分离出来,进行了93Nb NMR测试.如图3所示,晶化开始前的草酸铌铵溶液的谱中出现了一个峰宽较窄、峰形对称的单峰,对应于游离的草酸合铌酰根离子[NbO(C2O4)2]-,将其化学位移定为0.该峰高场侧δNb -400 ppm处也出现了一些强度不为零的宽包信号,可能来源于聚合的[NbO(C2O4)2]-杂质[40].水热处理1 h后,由于[NbO(C2O4)2]-的水解,草酸根被羟基取代,主峰的化学位移向低场移动至δNb 15.同时,主峰高场侧的宽包信号也更为显著,对应水解的[NbO(C2O4)2]-继续缩水聚合生成了一些多聚物.水热晶化2 h后,[NbO(C2O4)2]-继续水解,主峰的化学位移继续向低场移动到δNb 32,同时强度降低,对应晶化后液相中Nb的减少和固体的析出.水热晶化4 h后,主峰的化学位移转而向高场移动到了δNb 21,强度继续降低,对应液相中Nb的继续减少和固体产物的持续析出.水热处理6 h和8 h后,晶化过程接近完成,原来的主峰几乎消失,谱中只剩下中心位于δNb 40和 δNb -300的两个宽包信号.

图3

图3   不同水热反应时间液相部分产物的93Nb NMR谱

Fig. 3   93Nb NMR spectra of the product in solution with different hydrothermal reaction time


93Nb NMR谱中所有的Nb信号面积积分,定量得到液相产物中的Nb元素相对于反应开始前体系中Nb元素总量的相对含量,结果如图1(b)所示,可以看到液相产物中Nb元素相对含量随水热时间的变化趋势与重量分析法一致,即随着水热时间的延长,溶液中的Nb元素含量经历了一个先快速后慢速降低的过程,呈现伪一级动力学的指数下降特征,即液相中Nb元素的减少速率正比于液相中Nb元素的含量.

$-\frac{\text{d}{{c}_{\text{Nb}}}}{\text{d}t}=k{{c}_{\text{Nb}}}$

(1)式中${{c}_{\text{Nb}}}$为溶液中Nb元素的浓度,t为水热时间,k为伪一级动力学常数.其积分形式为一个指数衰减函数,即

${{c}_{\text{Nb}}}={{c}_{\text{Nb},0}}{{\text{e}}^{-kt}}$

(2)式中${{c}_{\text{Nb},0}}$为溶液中Nb元素的初始浓度.对93Nb NMR法测得的溶液中Nb元素含量随水热时间的变化用最小二乘拟合[图1(b)],结果表明溶液中Nb元素含量降低过程较好的符合伪一级反应动力学(决定系数大于99.6%).意味着层状结构固体产物是在溶液中独立生成的,其生成速率与液相中的构晶离子含量成正比;而不是在初始“晶核”上连续外延长大,否则固体产物的生成速率将正比于“晶核”数量和液相中构晶离子含量的乘积,而不是呈现出伪一级动力学曲线,与前文的推测一致.

2.4 量化计算

量化计算模拟已经广泛应用于化学位移等NMR参数的计算,与实验结合可以辅助谱峰归属,并更深刻地揭示反应机理[41-44].为了进一步探讨水热晶化过程中93Nb NMR信号的化学位移变化规律和谱峰归属,我们对其做了量化计算模拟.水热晶化后的固体产物并不是完全的晶体,只是在c方向层间有序,而在a-b面内有序度较低,对应于XRD中的宽包衍射信号,因此,一般根据其XRD的相似性用类似于Mo3VOx的结构模型来表示此种铌酸的面内结构[34].我们选取了其中一些典型的铌氧单体、二聚体、三聚体、四聚体和五聚体进行了考察,对其结构优化后计算了其93Nb化学位移.Nb原子采用全电子相对论Sapporo-DKH3-TZP基组,该基组已经被广泛用于计算重原子(如Ag、Au、I、Ru、Pt、Nd和Hg等)的化学位移、XRD和离解能等性质,并且与实验值相比取得了较为一致的效果[45-47].本文的计算结果如图4所示.对于单体1来说,相对于[NbO(C2O4)2]-δNb 0,其化学位移向低场方向移动到了δNb 62.56,说明水解会使其化学位移增大.二聚体2位于δNb -57.12,说明水解后继续缩水聚合会使化学位移减小.三聚体345δNb既有16 ~ -27的情况,也出现了-183和-240这种向高场偏移幅度较大的Nb位.对于结构更为复杂的四聚体和五聚体,计算得到的δNb涵盖了从17到-150范围以及-203 ~ -254较大高场偏移的情况.理论计算的δNb和实验值总体趋势一致,即草酸铌铵在水热条件下水解成单体,δNb向低场移动,单体继续缩聚成二聚体和多聚体,反而向高场移动,二者共同作用,造成了主峰化学位移先向低场后向高场移动的趋势;各种Nb位的δNb分布很宽,对应实验谱中较宽的谱峰和较低的分辨率;多聚体中出现的-200以上较大幅度偏向高场的δNb,与实验谱中右侧宽包信号的化学位移接近.量化计算为谱峰的归属和水热产物铌酸模型的合理性提供了理论支持.93Nb NMR实验和理论计算结果表明,草酸铌铵水热晶化过程中发生了水解-聚合反应,先是草酸铌铵水解,接着铌氧单体之间通过缩水聚合,最终生成了层内无序、层间有序结构的固体铌酸纳米棒.

图4

图4   量化计算得到的一些典型铌氧单体、二聚体、三聚体、四聚体和五聚体的93Nb化学位移

Fig. 4   93Nb chemical shift of some typical niobium-oxygen monomer, dimer, trimers, tetramers and pentamers calculated by quantum chemistry


3 结论

以草酸铌铵作为单一来源前体,利用水热法合成了铌酸纳米棒.采用93Nb NMR,结合XRD、TEM等技术和量化计算,研究了以草酸铌铵为前驱体水热生成铌酸纳米棒的晶化过程.结果表明草酸铌铵水热晶化过程中发生了水解-聚合反应,先是草酸铌铵水解,接着铌氧单体之间通过缩水聚合,最终生成了层内无序、层间有序的固体铌酸纳米棒.晶化过程符合“液相成核”机理,层状结构铌酸纳米棒在180 ℃下1 h内即可生成,后续水热处理并未使其明显长大,新的固体产物不断由溶液中独立生成.

致谢

感谢国家自然科学基金(21773056,21703056),河南省科技攻关项目(212102210608,202102110289),河南工业大学青年骨干教师项目(0503/21420046,0503/21420110).

利益冲突

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