Vol.42 2021年5月
No.5
1589~1597 CHEMICAL JOURNAL OF CHINESE UNIVERSITIES
高等学校化学学报
原位限域生长策略制备有序介孔碳负载的
超小MoO3纳米颗粒
reaction mass王常耀，王帅，段林林，朱晓航，张兴淼，李伟
（复旦大学化学系，上海200433）
摘要采用原位限域生长策略制备了一系列有序介孔碳负载的超小MoO3纳米颗粒复合物(OMC-US-MoO3).其
中，有序介孔碳被用作基质来原位限域MoO3纳米晶的生长.依此方法制备的MoO3纳米晶具有超小的晶粒尺
寸(<5nm)，并在介孔碳骨架内具有良好的分散度.制得的OMC-US-MoO3复合物具有可调的比表面积(428~796
m2/g)、孔容(0.27~0.62cm3/g)、MoO3质量分数(4%~27%)和孔径(4.6~5.7nm).当MoO3纳米晶的质量分数为7%
时，所得样品OMC-US-MoO3-7具有最大的孔径、最小的孔壁厚度和最规整的介观结构.该样品作为催化剂时,
表现出优异的环辛烯选择性氧化性能.
关键词有序介孔碳；氧化钼纳米晶；纳米材料；限域生长
中图分类号O611.4文献标志码A
In situ Confinement Growth Strategy for Ordered Mesoporous Carbon
Support Ultrasmall MoO
3
Nanoparticles
WANG Changyao，WANG Shuai，DUAN Linlin，ZHU Xiaohang，
ZHANG Xingmiao，LI Wei*
（Department of Chemistry，Fudan University，Shanghai200433，China）Abstract Ultrasmall particle sizes and excellent dispersity of the MoO3active species on support majorly dominate their catalytic performances.Herein，a series of ordered mesoporous carbon support ultrasmall mo⁃lybdena nanoparticles（OMC-US-MoO3）composites was synthesized through an in situ confinement growth strategy.Ordered mesoporous carbon was used as the matrix to in situ confine the growth of MoO3nanocrystals. The obtained MoO3nanocrystals show ultrasmall particle sizes（<5nm）and excellent dispersity on the meso-porous carbon frameworks.The obtained OMC-US-MoO3exhibits tunable specific surface areas（428―796 m2/g），pore volumes（0.27―0.62cm3/g），MoO3contents（4%―27%，mass fraction）and uniform pore sizes （4.6―5.7nm）.As a typical example，the obtained sample with7%MoO3（denoted as OMC-US-MoO3-7）shows the largest pore size，smallest thickness of pore wall and most regular mesostructures.When being used as a catalyst，the OMC-US-MoO3-7exhibits an excellent catalytic activity for selective oxidation of cyclooctene with a high stability.
Keywords Ordered mesoporous carbon；MoO3nanocrystal；Nanomaterials；Confinement growth
doi：10.7503/cjcu20200303
收稿日期：2020-05-28.网络出版日期：2020-09-24.
基金项目：国家自然科学基金(批准号:21975050)、国家重点研发计划纳米科技重点专项(批准号:2016YFA0204000, 2018YFE0201701)和中国博士后科学基金(批准号:2019M651342)资助.
联系人简介：李伟，男，博士，教授，主要从事介孔材料的合成及应用研究.E-mail:*******************
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Vol.42
高等学校化学学报
Epoxides，an important industrial chemicals，has been widely used in the fields of food additives，phar⁃maceutical intermediates，etc.［1，2］.Catalytic epoxidation of olefin is one of the essential route to produce epo-xides，which oxygenation of carbon-carbon double bond to form cyclic epoxide
groups.The kind of catalyst plays a key role on the epoxidation reaction.Among all catalysts，precious metal of gold based one illustrates high activity for olefin epoxidations［3，4］.However，gold is limited resource and very expensive，even though it shows high conversion efficiency.Molybdenum oxide（MoO3），as one of the low cost，non-toxic and environ⁃mentally benign transition metal oxides，is widely used as heterogeneous catalysis for Friedel-Crafts alkyla⁃tion［5］，hydrogenation reaction［6，7］，epoxidation reaction［8，9］，hydrogen evolution reaction［10］，electrochemical energy storage for lithium-ion batteries［11，12］，and gas sensors［13，14］，etc..Gratifyingly，MoO3has been reported by several groups which have high activity for epoxidation of olefins in recent years［15，16］.
It is obvious that the size and morphology of MoO3active species are critical factors that affect their prop⁃erties for application［17~20］.However，the synthesis and reaction process often easily causes serious sintering，migration and agglomeration of the MoO3nanoparticles，leading to the degradation of catalytic activity.Sup⁃ports are necessary for the immobilization of active species.Carbon has been widely used as an outstanding matrix to control the size and dispersity of supported metal oxides attributing to its advantages of intrinsical chemical inertness，high thermal stability，non-toxic and wide-sources［21~23］.Molybdena supported carbon have been reported and
show excellent performance as the catalyst for cyclooctene epoxidation［24，25］.Recently，Chen group［26］fabricatedγ-Fe2O3@C@MoO3core-shell structured nanoparticles as a magnetically recyclable catalyst for the epoxidation reaction of olefins.The coated carbon layer play an efficient role for the stabiliza⁃tion of magnetic core.Biradar group［8］also reported a carbon microspheres-supported molybdena nanoparticles catalyst which also show outstanding effect for the epoxidation of olefins.However，above-mentioned catalysts are less porosity.Porous supports，especially，mesoporous carbon have been reported on many catalytic areas because of its large surface area，pore volume and pore size，which can not only improve the load capacity but also enlarge the reaction progress，where the diffusion process may be the rate-limiting step［26~28］.Up to now，it is still urgent to fabricate mesoporous carbon supported MoO3catalyst with ultrasmall particle size and excel⁃lent dispersity.
Herein，we construct an ordered mesoporous carbon support ultrasmall MoO3nanoparticles（OMC-US-MoO3）composites via an in situ confinement growth strategy.In this strategy，the ordered mesoporous carbon works as a matrix to in situ confine the growth of MoO3nanocrystals.The obtained MoO3nanocrystals show ultrasmall particle size（<5nm）and excellent dispersity on the mesoporous carbon frameworks.The content （mass fraction）of MoO3can be tuned from4%to27%.The obtained
OMC-US-MoO3shows tunable specific surface areas（428―796m2/g），pore volumes（0.27―0.62cm3/g）and uniform pore size（4.6―5.7nm）.As a typical example，the obtained sample with7%MoO3（denoted as OMC-US-MoO3-7）shows largest pore size，smallest thickness of pore wall and most regular mesostructures.When being used as a catalyst，the OMC-US-MoO3-7exhibits an excellent catalytic activity for selective oxidation of cyclooctene with a high stability.
1Experimental
1.1Chemicals and Materials
Pluronic F127（EO106PO70EO106，M w=12600）was purchased from Aldrich.All others chemicals were obtained from Aladdin company and used directly.Deionized water was used in all experiments.
1.2Synthesis of Ordered Mesoporous Carbon Support Ultrasmall Molybdena Nanoparticles
In detail synthesis procedure，1.0g of Pluronic F127powders was added into10.0g of ethanol solution and stirred to a homogeneous clear solution at40℃.Afterwards，5.0g of20%（mass fraction）preformed
No.5王常耀等：原位限域生长策略制备有序介孔碳负载的超小MoO 3纳米颗粒phenolic resins ethanol s
olution and 1.0mL of peroxomolybdenum precursor solution were added into the ho⁃mogeneous system （5—200mg/mL ）.The preformed phenolic resins was synthesized based on the reported method ［27，28］.Peroxomolybdenum precursor solution ［29］was prepared by dissolving different contents of molyb⁃denum trioxide into 10.0mL of 30%hydrogen peroxide.The mixture solution was poured into dishes after 2h and then the dishes were heat treated at 40and 100℃for 8and 20h ，respectively ，forming the as -made com⁃posites consisting of Pluronic F127，phenolic resins ，and Mo species （denoted as as -made sample ）.Then ，the calcination of as -made sample was implemented in a tubular furnace under N 2atmosphere.The temperature program was set from 25℃to 350℃with a ramp of 1℃/min ，maintenance for 3h ，and then to 600℃with 1℃/min ，maintenance for 2h.The obtained sample after pyrolysis was named as ordered mesoporous carbon support ultrasmall molybdena nanoparticles （OMC -US -MoO 3-x ），wherein x represent the actual mass fraction of MoO 3.1.3Activity Test The selective oxidation reaction of cyclooctene was carried out in the round -bottom flask （50mL ）.In which ，40.0mmol of cyclooctene ，40.0mmol of 5.5mol/L TBHP in decane ，10mg of OMC -US -MoO 3-7cata⁃lyst （0.0048mmol/L of MoO 3），6.0g of 1，2-dichloroethane as solvent ，and 15.0mmol of chlorobenzene as internal standard.The reaction temperature is 80℃.At different time intervals ，conversion was calculated by sampling.The samples were analyzed on an Agilent 7890A gas chromatograph equipped with a HP -5column and products were confirmed by GC -MS.TOF values （mol of reacted cyclooctene per mol of catalyst and hour ）
was calculated at about half conversion of the reaction.The catalyst was reused after washing by water and drying.The test condition was kept same to the first time on the cyclic test.2Results and Discussion
2.1Synthesis and Characterizaiton
The developed in situ confinement growth strategy is employed to the preparation of ordered mesoporous carbon support ultrasmall molybdena nanoparticles （OMC -US -MoO 3）composites （Fig.1）.In the synthesis sys⁃tem ，Pluronic F127is used as the structure -directing agent （soft -template ），preformed phenolic resins is used as carbon resource ，peroxomolybdenum solution is used as precursor ，and ethanol/H 2O is used as co -solvent ，respectively.The as -made sample and product OMC -US -MoO 3composites can be obtained after heat -treatment at 100and 600℃，respectively.The mass content of MoO 3in the OMC -US -MoO 3composites can be well tuned through adjusting the amount of peroxomolybdenum precursor in the synthesis system.
TGA curves （Fig.2）show that the mass fractions of MoO 3species in the OMC -US -MoO 3composites are
Fig.1Illustration of the construction of OMC ⁃US ⁃
MoO 3composites via the in situ confinement
growth strategy Fig.2TGA curves of the OMC ⁃US ⁃MoO 3composites with different MoO 3contents obtained after
pyrolysis at 600℃,respectively
Mass fraction of MoO 3(%):a .4;b .7;c .10;d .16;e .27.1591
Vol.42高等学校化学学报4%，7%，10%，16%and 27%（Table 1），respectively ，when adjusting the amount of molybdenum precursors in the synthesis system.The mass loss below 100℃is caused by the volatilization of adsorbed water in the composites.A slight mass increasement can be detected between 100and 300℃，demonstrating the existence of trace amount of MoO 2and abundant MoO 3in the composites.The mass increasement can be attributed to the oxidation of the trace amount MoO 2.Subsequently ，the huge mass loss above 300℃can be observed attribu -ting to the remove of carbon species in the composites.The mass loss between 100and 600℃is approximate to the mass fraction of MoO 3species in the composites.
The SAXS patterns ［Fig.3（A ）］of OMC -US -MoO 3-4and OMC -US -MoO 3-7composites show two scattering
diffraction peaks at 0.391and 0.782nm −1，and 0.412and 0.824nm ‒1，respectively ，indexing to the （100）and （200）reflections of a hexagonal mesosturtures with space group P 6mm .With the increasement of MoO 3content ，the q values of the （100）diffraction peaks shift to 0.532，0.617，and 0.678nm −1，for samples OMC -US -MoO 3-10，OMC -US -MoO 3-16，and OMC -US -MoO 3-27，respectively.The corresponding cell parame⁃ters of five composites are calculated to be about 18.5，17.6，13.6，11.7，and 10.7nm with the increased MoO 3content ，respectively.WAXRD patterns ［Fig.3（B ）］of five composites all show no diffraction peaks of MoO 3phase ，suggesting the ultrasmall particle size of MoO 3nanocrystals in the frameworks.This result demonstrates that the ordered mesoporous carbon frameworks can confine the size of MoO 3nanocrystals to an ultrasmall size even at a high MoO 3content effectively.Nitrogen adsorption -desorption isotherms of five OMC -US -MoO 3composites obtained after calcined at 600℃in N 2all display representative type -Ⅳcurves with H2hysteresis loops ［Fig.4（A ）］，in agreement with the previously reported ordered mesoporous materials ［30~32］.Sharp capillary condensation steps in the relative pressure （p /p 0）of 0.41―0.70are observed for five composites ，demonstrating the narrow pore size distribu⁃tion.The Brunauer -Emmett -Teller （BET ）surface area and pore volume of five composites are calculated and listed on Table 1.The surface area and pore volume decrease with the increased MoO 3content ，which can be attributed to the partial destroy and disappear of pore structures.The av
erage pore sizes of five composites are also calculated and listed on Table 1from their pore size distribution curve ［Fig.4（B ）］derived from the adsorption branch based on BJH model.The average pore sizes are 4.7，5.7，5.5，5.4，and 4.6nm ，Table 1Structural and textural parameters for OMC -US -MoO 3with different content Sample No.12345MoO 3content （%，mass fraction ）47101627S BET /（m 2·g -1）796693652574428V /（cm 3·g -1）0.620.540.490.410.27D /nm 4.75.75.55.4  4.6
Fig.3SAXS(A)and WA ⁃XRD(B)patterns of the OMC ⁃US ⁃MoO 3composites with different
MoO 3contents obtained after pyrolysis at 600℃Mass fraction of MoO 3(%):a .4;b .7;c.10;d .16;e .27.
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No.5王常耀等：原位限域生长策略制备有序介孔碳负载的超小MoO 3纳米颗粒respectively.According to the cell parameters results ，the pore walls of five composites are calculated to be 14.1，11.9，8.1，6.3，and 6.1nm ，respectively.
SEM images （Fig.5）show that OMC -US -MoO 3-4and OMC -US -MoO 3-7composites own the most regular mesostructures.Notably ，the regular ［100］and ［110］directions can be clear observ
ed from the SEM images of OMC -US -MoO 3-7composites ［Fig.5（B ）and （F ）］.In addition ，the mesopores are opened and no obvious big metal nanoparticles can be observed from the surface.With further increasement of MoO 3content ，the reg⁃ular mesostructures is partial destroyed.TEM images of OMC -US -MoO 3-7composites ［Fig.6（A ）—（C ）］taken along the ［100］and ［110］directions manifest a well -defined 2D hexagonal mesostructures in agreement with the result of the SAXS pattern ［Fig.2（A ）］.The lattice spacing is measured to be 0.35nm from the HRTEM image ［Fig.6（D ）］，attributing to the （040）crystalline planes of α-MoO 3［33］.The average size of MoO 3nano⁃crystals is estimated to be （4.1±1.0）nm from the size statistics diagram.The survey spectrum of the OMC -
US -MoO 3-7composites shows the presence of only Mo ，O and C elements ［Fig.7（A ）］.The high -resolution Mo 3d core level XPS spectra ［Fig.7（B ）］show four peaks at 230.5，232.7，233.6，and 235.9eV ，demon⁃strating the co -existence of Mo 4+and Mo 6+species ［34~36］.The ratio of Mo 4+/Mo 6+is calculated to be about 13%.Only a few Mo 4+signals can be detected from the spectrum ，in agreement with the TGA
results.Fig.4N 2adsorption⁃desorption isotherms(A)and pore size distributions(B)of the OMC⁃US⁃MoO 3
composites with different MoO 3contents obtained after pyrolysis at 600℃Mass fraction of MoO 3(%):a .4;b .7;c.10;d .16;e .27.Fig.5SEM images of OMC⁃US⁃MoO 3composites with different MoO 3contents obtained after
pyrolysis at 600℃
Mass fraction of MoO 3(%):(A)4;(B)7;(C)10;(D)16;(E)27.1593

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