Copper Supported Mesoporous Carbon Cu/CMK-3 for Catalytic Oxy-carbonylation of Methanol in Vapor Phase

WANG Rui-Yu LIU Ya-Li FAN Xing＊, WEI Xian-Yong

(1Low Carbon Energy Institute,China University of Mining&Technology,Xuzhou,Jiangsu 221006,China)

(2Key Laboratory of Coal Processing and Efficient Utilization,Ministry of Education,China University of Mining&Technology,Xuzhou,Jiangsu 221116,China)

Abstract:Ordered mesoporous carbon CMK-3 was synthesized via the nanocasting route,and used to prepare Cu/CMK-3 catalyst for dimethyl carbonate(DMC)synthesis by oxidative carbonylation of methanol in a gas-phase reaction.The effect of activation temperature on the catalyst structure and catalytic performance were investigated.N2adsorption-desorption,X-ray diffraction (XRD)and transmission electron microscopy(TEM)results revealed that the Cu/CMK-3 catalysts were mesoporous,the active copper species dispersed well in the surface and pore channels of CMK-3,their diameter were between 10～20 nm,far less than that of Cu/activated carbon (Cu/AC).The corresponding catalytic activity in a fixed-bed reactor increased with the activation temperature and the Cu/CMK-3 catalyst prepared at 450℃exhibited the best catalytic activity.The space time yield (STY)was 286 mg·g-1·h-1and the selectivity for DMC was 76%in 10 h running.A long periodic test also confirmed a better catalytic stability of Cu/CMK-3 compared to Cu/AC,the STY of DMC declined by 20%after 50 h reaction and 28%after 75 h reaction.

Keywords:heterogeneous catalysis;supported catalysts;nanocasting;mesoporous Cu/CMK-3;dimethyl carbonate;methanol;oxidative carbonylation

0 Introduction

Dimethy carbonate is an environmentally benign chemical product[1],it has attracted great attention and widely used as the methylation or carbonylation agent,solvent,and fuel additive[2-4].Among several routes for DMC synthesis,the most promising one is the gasphase oxidative carbonylation of methanol due to its several distinct advantages such as inexpensive and easy available raw materials,mild operation conditions and a simple environmentally friendly process[5].

The activated carbon (AC)supported CuCl2or Wacker (CuCl2-PdCl2)catalysts exhibited excellent catalytic activities,but their stability is poor,the loss of chlorine with time on stream causes the deactivation of catalyst during the reaction[6-8].In recent years,chlorine-free catalysts have been investigated as a means of avoiding catalyst inactivation by Cl-loss[9-12].Wang et al.[10]prepared the AC support CuO,Cu2O and Cu catalyst by heating the Cu2(NO3)(OH)3/AC precursor in an inert atmosphere,and found that the catalytic activities increased in the order of CuO＞Cu2O＞Cu.Ma et al.[11]found that the active sites of the catalysts were the Cu2O nanoparticles that coordinated to the oxygen containing groups (OCGs)on the AC surface,the optimal Cu loading as well ascatalyticactivity increased linearly with the amount of OCGs.Ren et al[12]prepared Cu/C catalyst from Cu(NO3)2and starch,by the sol-gelmethod,subsequenthigh temperature carbonization and KOH activation. Under the optimized preparation conditions,a resulting surface area of 1 690 m2·g-1and microporosity of 72.4%were achieved,from which the catalyst exhibited the highest activity.Although CuOx/AC catalyst exhibits beginning catalytic performance,the Cu particles located outside the AC support aggregate over the reaction period,which lead to the decrease of catalytic activities[13].Subsequently,many works focus on the improvement of CuOxdispersion of on AC support[14-16].Li et al.[14]found thatthe treatmentofAC with ammonia increased the amount of the surface OCGS on AC,leading to easy access of metal salt solutions with the surface of AC during the impregnation,and further improved the dispersion of active species.Shi et al.[15]prepared CuOx/AC composite by vapor-phase methanol reduction under mild conditions.The obtained CuOx/AC catalyst with good copper dispersion exhibited an enhanced catalytic performance in oxidative carbonylation reaction of methanol.Zhang et al.[16]synthesized Cu/AC catalyst via impregnation method using nitric acid treated AC as support,the crystalline size of the resulting Cu particles decreased from 32 to 12 nm.However,the microporous dominating structure of AC still restrict the dispersion of active copper species which is related to the catalytic activity.

Support plays an important role in the dispersion state of active components on catalyst,and it has been widely accepted that high surface area and large pore volume are beneficial for the dispersion of active component[17-18].As one of the most promising material,ordered mesoporous carbons have attract numerous attentions for their applications in electrochemistry,energy storage and catalysis[19-21].They possess several unique properties,includinghigh-specificsurface,large volumes,uniform and tunable pore size,high thermal and mechanical stability and chemical inertness[22-25].

In this study,ordered mesoporous carbon CMK-3 was prepared via the nanocasting route,and used as support to prepare the Cu2O or Cu supported catalyst for DMC synthesis by the gas-phase oxidative carbonylation of methanol.The dispersion state of Cu components was assessed using the N2adsorptiondesorption,XRD and TEM techniques,the relationship between surface properties of the catalyst and catalytic performance was also studied.

1 Experimental

1.1 Materials

Tetraethyl orthosilicate (TEOS)and Pluronic P123 triblock copolymer(EO20PO70EO20,Mav=5 800 g·mol-1)purchased from Sigma Aldrich were used for the synthesis of SBA-15.Glucose,sulphuric acid(H2SO4,98%(w/w)),hydrochloric acid(HCl,37%(w/w)),anhydrous methanol,copper nitrate trihydrate(Cu(NO3)2·3H2O)and hydrofluoric acid (HF,40%(w/w))were purchased from Sinopharm Chemical Reagent(China)Co.,Ltd.All reagents were of analytical reagent grade and used without further purification.Carbon monoxide(99.99%),nitrogen(99.99%)and oxygen(99.99%)were supplied by Xuzhou Tezhong Gas(China),Ltd.

1.2 Catalyst preparation

SBA-15 was synthesized under acidic conditions using pluronic P123 triblock copolymer as a template and TEOS as a silica source according to the procedure described previously[26-27].Briefly,8 g pluronic P123 was dissolved in a solution containing 60 g distilled water and 120 g 2 mol·L-1HCl at 35 ℃.The mixture was vigorously stirred until complete dissolution of pluronic P123.Then,17 g TEOS was added dropwise to the clear solution under vigorous stirring.Finally,the slurry was kept under stirring at 35℃for 20 h,and then hydrothermally treated under static conditions at 90℃for another 24 h.The resulting white precipitate was isolated by filtration without washing,dried at 60℃overnight and calcined in air at 550℃for 12 h.

The obtained SBA-15 material was used as a hard template in the synthesis of CMK-3 replica[28-29].1 g SBA-15 was impregnated with a solution containing 5.00 g water,1.25 g sucrose and 0.14 g sulfuric acid.The sample was dried at 100℃for 6 h and then at 160℃for another 6 h.The SBA-15 containing partially polymerised and carbonised sucrose was impregnated for a second time by a solution with 0.80 g sucrose and 0.09 g sulfuric acid dissolved in 3.00 g water.The sample drying procedure was repeated.Subsequently,the dried material was calcined in flowing N2by rising temperature (1℃·min-1)up to 750℃and kept at the temperature for 12 h.The carbon-silica composite obtained after pyrolysis was washed by HF(10%(w/w))solution at room temperature to remove the silica template.

Copper was introduced onto the CMK-3 support(10%(w/w)CMK-3)by the incipient wetness method.0.19 g Cu(NO3)2·3H2O was dissolved in 50 mL deionized water.0.5 g CMK-3 was added and the resultant mixture was stirred vigorously at room temperature for 4 h.After drying at 120℃,the residual mixture was calcined in a tubular furnace at different temperatures in the atmosphere of nitrogen for 4 h.Catalysts made by this process were denoted as Cu/CMK-3(n).Herein“n” represents the calcination temperature of the Cu/CMK-3 catalyst.

1.3 Catalyst characterization

N2adsorption-desorption isotherms were measured with a JW-BK122W adsorption analyzer at-196℃.Specific surface area were calculated according to the isotherms obtained by N2adsorption-desorption test,and the cumulative volumes of pores were obtained by the BJH method from the desorption branches of the adsorption isotherms.

X-Ray diffraction (XRD)patterns were recorded on a Rigaku D-Max 2500 diffractometer,using Cu Kα radiate on(λ=0.154 nm)at 40 kV and 200 mA,and with a scanning rate of 2°·min-1in 2θ range of 1°～10°and 8°·min-1in 2θ range of 10°～70°.

Transmission electron microscopy (TEM)was carried out using a Tecnai G2F20S-Twin microscope operating at 200 kV.TEM samples were prepared by immersing C-coated Cu grids in ethanol solutions of samples,and drying at room temperature.

1.4 Catalyst evaluation

DMC synthesis was carried out in a fixed-bed reactor.A stainless steel tube reactor with inner diameter of 6 mm was used.The catalyst loading was 0.45 g.Liquid methanol was introduced by a micropump and then heated sufficiently to vaporize.The resulting gaseous methanol was thoroughly mixed with CO and O2,each of which was controlled individually by mass gas flow controllers.Liquid products were analyzed by a gas chromatography (9790,Zhejiang Fuli Co.,China)coupled with a flame ionization detector(FID)and a HP-INNO-WAX(30 m×530 μm×1 μm)column.Conditions in the fixed-bed reactor were 140℃and atmospheric pressure over a duration of 10 h.Reactant flow rates of CH3OH,CO and O2were 0.02,28 and 2.8 mL·min-1.

2 Results and discussion

2.1 XRD analysis

Fig.1 shows the low angle XRD patterns of CMK-3 and Cu/CMK-3.Both the patterns of CMK-3 and Cu/CMK-3 showed strong reflections in the low angle,indicating the uniform pore structure.The sharp and intense peak at 2θ=1.08°((100)plane)and two less intense peaks at 2θ=1.8°((110)plane)and 2.04°((200)plane)were observed in XRD pattern of CMK-3,which are indexed in the hexagonal lattice with the crystallographic space group of P6nm[22].Cu/CMK-3 also show sharp and intense peak at 2θ=1.1°((100)plane)and two less intense peaks at 2θ=1.8°((110)plane)and 2.04°((200)plane).However,the(100)plane peaks of Cu/CMK-3 exhibited a shift towards high-angle compared to that of CMK-3,indicating that the pore size of the catalysts reduced after loading of active Cu components.

Fig.1 Small angle XRD patterns of(a)Cu/CMK-3(350),(b)Cu/CMK-3(400),(c)Cu/CMK-3(450),(d)Cu/CMK-3(500)and(e)CMK-3

Fig.2 presents the wide angle XRD patterns of Cu/CMK-3 at different activation temperatures.There was no diffraction peak in the XRD pattern of Cu/CMK-3 (350),indicating a good dispersion of Cu components on the surface of CMK-3.For Cu/CMK-3(400),Cu/CMK-3(450)and Cu/CMK-3(500),weak characteristic peaks for the crystallized Cu at 2θ=43°and 50°,and Cu2O at 2θ=36°were observed on the XRD patterns,indicating the formation of Cu2O and Cu nano clusters outside the mesochanels of CMK-3.The Cu2O may be generated during the XRD test from the nano clusters of Cu.The intensity of Cu diffraction peaks slightly increased with calcinations temperature,because Cu particles were sintered into larger clusters at higher calcinations temperatures.Crystal Cu2O was only detected on Cu/AC (400),and crystal Cu was detected on Cu/AC (450)[10].Good dispersion of Cu componentsonCMK-3maybeascribedtothe reduction of calcinations temperature for Cu/CMK-3.For Cu/CMK-3(400),Cu/CMK-3(450)and Cu/CMK-3(500),the particle size of Cu estimated from the Scherrer equation was between 10 to 20 nm,which were much smaller than that of Cu/AC catalyst prepared at the same temperatures(42.6 nm at 450℃and 46.9 nm at 500℃)[10].

Fig.2 Wide angle XRD patterns of(a)Cu/CMK-3(350),(b)Cu/CMK-3(400),(c)Cu/CMK-3(450)and(d)Cu/CMK-3(500)

2.2 Porosity

Fig.3 presents the N2adsorption-desorption isotherms and pore size distributions of Cu/CMK-3 prepared under different activation temperatures.In Fig.3a,all catalysts show similar typeⅣisotherms with H1-type hysteresis loop which is typical for mesoporous materials[26].A knee at relative pressure(p/p0)＜0.1 indicated a certain amount of micropores,and an obvious capillary condensation step(hysteresis loop)at p/p0＞0.4 indicated the existence of a large number of mesoporous[30].In Fig.3b,it indicates that theCu/CMK-3 catalystsweretypicalmesoporous materials with pore size around 3.8 nm.A series of micropores were also indentified with size distributed in the range of 1～1.8 nm.

Fig.3 (a)N2adsorption-desorption isotherms and(b)pore size distribution of Cu/CMK-3 at different activation temperatures

Table 1 Textural parameters of Cu/CMK-3 catalysts

Specific surface areas and pore structure parameters of CMK-3 and Cu/CMK-3 are summarized in Table 1.CMK-3 had a high specific area and large pore volume,and the mesopore volume was close to ninety percentofthe totalvolume.When Cu components were loaded on the support,both specific area (SBET)and pore volume of the catalysts reduced remarkably.As the activation temperature increased from 350 to 500℃,the SBETof catalyst decreased slightly from 920 to 816 m2·g-1,and the total pore volume(Vtotal)decreased from 1.2 to 0.97 cm3·g-1.Pore diameters of CMK-3 and Cu/CMK-3 catalysts were almost same,suggesting that the Cu components entered into the pore channels of CMK-3 without blocking the pore.In general,the loading of Cu components and activation temperature are two importantfactorswhich are responsible forthe changes of texture properties of Cu/CMK-3 catalysts.

2.3 TEM analysis

High resolution TEM (HRTEM)images of the CMK-3 support and the Cu/CMK-3 catalysts are shown in Fig.4.From the Fig.4a,the CMK-3 support showed an ordered mesoporous structure and contained a transparent central area that runs longitudinally along the cylinder.From the TEM images of Cu/CMK-3(Fig.4(b～e)),the copper species were evenly dispersed on the surface of CMK-3.The crystal structure gets perfect with the increase of preparation temperature,and the average diameter of copper particles were in a range of 10～20 nm,which was in accordance with the XRD results.For Cu/CMK-3 prepared under 500℃,larger copper particles could be observed because of the sintering process.

Fig.4 HRTEM micrographs of(a)CMK-3,(b)Cu/CMK-3(350),(c)Cu/CMK-3(400),(d)Cu/CMK-3(450)and(e)Cu/CMK-3(500)

2.4 Catalytic activity

Fig.5 Catalytic characteristics of Cu/AC and Cu/CMK-3 prepared at different temperatures:(a)STY and(b)selectivity of DMC

Fig.5 shows the catalytic performances of Cu/AC and Cu/CMK-3 catalysts during a 10 h running.The catalytic activities of Cu/AC and Cu/CMK-3 catalysts show the similar tendency.For example,the STY of DMC increased with the preparation temperature and reached the maximum value at 450℃,and then decreased.The DMC selectivity of the Cu/AC and Cu/CMK-3 catalysts showed little dependence on preparation temperatures.For the Cu-based catalysts in vapor phase oxy-carbonlation of methanol,the preparation temperature has great influences on its catalytic performance[9-10].With the preparation temperature increased to 450℃,the Cu components existed in a low valence state and exhibited a good structure of crystal phase.After then,the Cu particles sintered with the further increase of preparation temperature,and lead to a decrease of catalytic activities,which could be observed from the TEM results.From Fig.5,it also can be seen that the catalytic activities of Cu/CMK-3 catalysts were higher than that of Cu/AC catalysts;Cu/CMK-3(450)exhibited the highest STY of DMC (282 mg·g-1·h-1)with a DMC selectivity of 76%.It is believed that the good dispersion of Cu components in CMK-3 makes the Cu/CMK-3 exhibit a good catalytic performance in DMC synthesis.

Fig.6 shows the catalytic performance of Cu/AC(450)and Cu/CMK-3(450)for the synthesis of DMC via oxidative carbonylation of methanol for a long periodic test.It can be found that the Cu/AC(450)had a high initial catalytic activity at a STY value of 270 mg·g-1·h-1.After 50 h of time on stream,the STY of DMC decreased by about 50%and reached a relative stable value of 140 mg·g-1·h-1.Comparatively,Cu/CMK-3(450)had a better catalytic stability.It showed a decline of STY of DMC by only 20%after 50 h reaction,and 28%after 75 h reaction.The selectivity of DMC based on methanol increased slowly in the testing period,from 76%to 77%over Cu/AC(450),and 76%to 78%over Cu/CMK-3(450).Thus,Cu/CMK-3 catalyst exhibited favorable catalytic activity and durability for the synthesis of DMC from oxidative carbonylation of methanol.

Fig.6 Catalytic properties for Cu/AC(450)and Cu/CMK-3(450):(a)STY of DMC;(b)Selectivity of DMC

3 Conclusions

CMK-3 was successfully synthesized from SBA-15,a hard template,and sucrose,a carbon source,via the nanocasting route.Cu/CMK-3 was prepared by wet impregnation and activated in inert atmosphere.The impregnation with copper nitrate followed by thermal decomposition in inert atmosphere had a slight effect on the characteristic of support CMK-3.Structural properties of CMK-3 including high-surface area,largepore volume and uniform pore size facilitate the dispersion of Cu components,which in turn makes Cu/CMK-3 exhibit a good catalytic performance in DMC synthesis.Compared to Cu/AC catalysts,catalytic activities and stability forCu/CMK-3 havebeen improved remarkably.

Acknowledgement:This work was supported by the Fundamental Research Funds for the Central Universities(Grant No.2017XKZD10).