Applied Catalysis A: General 209 (2001) 107–115

Studies of the interaction between CuCl and HY zeolite forpreparing heterogeneous CuI catalyst

Zhong Lia,∗, Kechang Xiea, Robert C.T. Sladeba Institute of Chemical Engineering for Coal, Shanxi Kay Lab of Coal Science and Technology,

Taiyuan University of Technology, Taiyuan, Shanxi 030024, PR Chinab Department of Chemistry, University of Exeter, Exeter EX4 4QD, UK

Received 1 May 2000; received in revised form 25 July 2000; accepted 26 July 2000

Abstract

The interaction between CuCl and HY (Si/Al= 20) zeolite under inert nitrogen gas atmosphere has been studied byTG/TGA, XRD, 27Al MAS NMR, and29Si MAS NMR spectra. CuCl is easy to sublimate and disperse on the surface of thezeolite. But if CuCl was adsorbed on the cage of the zeolite, it was unlikely to be desorbed from the zeolite at over 800◦C.The ion-exchange of CuI in solid CuCl with H+ in HY zeolite occurred at over 300◦C and the maximum ion-exchange ratewas reached at 340◦C under a heating rate of 10◦C/min, with the consequent release of HCl gas. And it was found thatCuCl promotes the dealumination of framework aluminium of the HY zeolite and probably reacted with the extra frameworkaluminium to form (AlO)Cl. During the heating treatment, the unique structure of the Y zeolite was kept and CuCl crystallinephase was unlikely to disappear because the powder XRD pattern of CuCl crystalline phase was still observed when theCuCl/HY sample was treated at 350◦C for 60 h and 650◦C for 5 h. Over 100% ion-exchanged degree of the CuI with H+ inHY zeolite was obtained at high treatment temperature (650◦C or over) due to the reaction between defect –Si–OH and CuClto form –Si–OCu. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Solid state ion-exchange; CuCl; HY zeolite; TG/TGA; XRD

1. Introduction

The number of studies of Cu-loaded zeolite hasrecently increased, not only because of its catalyticproperties, such as conversion of NO to N2 [1,2],syngas to methanol [3], and oxidative carbonylationof methanol to dimethyl carbonate [4,5], but alsobecause of its CO adsorption properties in gasseparation [6]. CuII -zeolite is easy to prepare byion-exchange with CuII aqueous solution but this is

∗ Corresponding author. Tel.:+86-351-6018466;fax: +86-351-6041142.E-mail address:lizhong@public.ty.sx.cn (Z. Li).

not the case for CuI-zeolite because of the insolubilityof CuI salt in water and its unstable properties. CuClcould highly disperse as monolayer form on the sur-face of zeolite, such as 4A, 13X, NaY and CuY whenheating solid CuCl with zeolite for several hours atlow temperature (350◦C, less than CuCl melt pointing430◦C) [6]. A high temperature treatment of alkalineand of some transition-metal chlorides with the Hforms of A, X and Y zeolites resulted in ion-exchangebetween metal ion and H+ in zeolite, e.g. solid-stateion-exchange [7–9]. Spoto et al. [10–13] have studiedin detail the state of copper in copper-zeolite materialsmade by interactions of H-form zeolite with gaseousCuCl. They discovered by XANES, EXAFS, IR and

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-860X(00)00745-6

108 Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115

UV–VIS that the absolutely predominant CuI speciesformed when using H-ZSM-5 (Si/Al= 90–95) [12]but only isolated CuI species when using HY (Si/Al=3) [13]. It was proved that gaseous CuCl reacts withthe most acidic Brfnsted sites of H-ZSM-5 (Si/Al=90) at 300◦C (573 K) following the schemes [11]:

(1)

(2)

And the –Si–OH group remained nearly unchangedaccording to the IR spectra. It was also confirmedby Karge et al. [14] that a solid-state exchange re-action between CuI of solid CuCl and H+ of zeo-lite occurred at 397◦C (670 K) for HY and 497◦C(770 K) for H-ZSM-5, at which no substantialself-dehydroxylation of the bridging OH groups tookplace. The exchange rate is very fast in the initialstage, being proportional to the amount of copper saltin the mixture and trails off to zero after several hours.

In this study, The HY(Si/Al = 20) zeolite washeated with solid CuCl in flowing nitrogen at differenttemperatures, and TG/TGA, XRD,27Al MAS NMR,and29Si MAS NMR spectra were used to investigatethe interaction between HY zeolite and CuCl. It isfound that the interaction between CuCl and HY zeo-lite under inert nitrogen gas atmosphere includes thefollowing physical or chemical processes. (1) CuClis dispersed or adsorbed on the surface of the zeo-lite, meanwhile CuCl sublimates or vaporises. (2) H+in HY zeolite is exchanged with CuI in CuCl whenthe heating treatment temperature is over 300◦C. Andthe defect –Si–OH groups react with CuCl to form–Si–OCu when the temperature is over 650◦C. (3)CuCl promotes the dealumination of the frame workaluminium of the HY zeolite and reacts with the extraframework aluminium. (4) CuCl adsorbed on the cagesurface of the HY zeolite is very difficult to desorbwhen the heating treatment temperature drops below800◦.

2. Experimental

A mixture of CuCl (Fisher Scientific) and HYzeolite (Si/Al = 20, CBV740, Zeolyst) was groundby a pestle mortar, then heated in flowing nitrogenat a heating rate of 10◦C/min up to the desired tem-perature and then held for a certain time. Then thefurnace was cooled down to room temperature andnitrogen feed was stopped. Element analyses of Si,Al, Cu and Cl of the materials were carried out byEnergy Dispersive X-ray Fluorescence (EDXRF) atthe Earth Resources Centre (University of Exeter)using a Philips 505 instrument with EDAX 990 at30 kV. Samples were pressed into small pellets andresults were normalised to 100% to give percentagesof constituent elements. Powder XRD spectra wererecorded by a computer-driven Philips step-scanningdiffractometer with a Philips PW1050/25 goniometer(Ni-filter, Cu Ka radiation, λ = 1.54178 Å). Datawere collected for 4 s every 0.1◦ of 2θ at 30 mA and40 kV. TG and TGA analyses were carried out simul-taneously on a Stanton Redcroft STA-781 instrument.About 10 mg sample was placed in a platinum panand heated from ambient temperature to 1200◦C ata heating rate of 10◦C/min under flowing nitrogenwith a flow rate of 50 ml/min. An internal standard ofa-alumina was used. An IBM compatible computercollected data.27Al direct polarisation (DP) MASNMR spectra were recorded at 78.158 MHz at a spinrate of 9.6 kHz and pulse duration of 1.0ms with 0.5 srelaxation on a Varian Unity 300 instrument fittedwith a Doty MAS probe.29Si direct polarisation (DP)MAS NMR spectra were recorded at 59.583 MHz at aspin rate of 3.67 kHz and 90◦ pulse angle with 60.0 srelaxation on the same instrument as above.

3. Results and discussion

3.1. XRD study and EDXRF analysis

Powder XRD patterns of 20CuCl/HY (20/100 byweight) mixture before and after the heating treatmentunder flowing nitrogen for 15 h at different tempera-tures are shown on Fig. 1. All the patterns show theidentical pattern of HY zeolite. This proves that theheating treatment of HY zeolite with solid CuCl didnot affect the integrity of the Y zeolite structure, even

Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115 109

Fig. 1. XRD patterns of (a) HY zeolite; (b) physical mixture of20CuCl/HY and samples after heating treatment under flowingnitrogen for 15 h at different temperature; (c) 250◦C; (d) 350◦C;(e) 450◦C; (f) 550◦C; (g) 650◦C and (h) 750◦C (∗, pattern ofCuCl phase).

at 750◦C. The crystalline phase of CuCl (powder XRDpattern at 2θ = 28.52, 47.43, 56.29◦) in the mixturewas reduced remarkably after the heating treatment.Some of the crystalline CuCl was sublimated and somewas dispersed on the surface of the HY zeolite. At250 and 350◦C, the intensities of XRD peaks of theCuCl phase remaining on HY zeolite surface were al-most the same but at 450◦C, the peaks of CuCl phasebecome much smaller and above 550◦C, there was nothe CuCl phase XRD pattern any more. If the heatingtreatment was prolonged to 60 h, some CuCl phasestill remained on the surface of HY zeolite at 350◦Cbut no more was left at 450◦C (Fig. 2).

Fig. 3 shows that some CuCl crystalline phase stillexisted on the surface of the HY zeolite when the heat-ing treatment at 650◦C for 5 h under flowing nitrogenand no new phase was formed even though the heating

Fig. 2. XRD patterns of 20CuCl/HY sample after heating treatmentunder flowing nitrogen for 60 h at 350 and 450◦C.

Fig. 3. XRD patterns of 20CuCl/HY sample after heating treatmentunder flowing nitrogen at 650◦C.

treatment was continued for 70 h. And the structure ofY zeolite still remained the same. When the heatingtreatment was proceeding, a white material was stim-ulated on the down-flow end of the furnace tube wherethe temperature dropped. This was CuCl crystallinephase, as proved by a powder XRD pattern on Fig. 4.If water was used to absorb the outlet gases, the pHof the water declined, indicating that acidic gas wasemitted during the heating treatment, which should beHCl.

Table 1 gives the result of EDXRF analysis of20CuCl/HY samples after heating treatment underflowing nitrogen for 15 h. It shows that Si/Al (mole)of the zeolite had a little change and the amountsof copper as well as chlorine decreased with theincrements of heating treatment temperature. Whenheating treatment was done at 250◦C, the mole ratioof Cu to Cl loaded on HY zeolite was very close to 1,which means that most of the copper existed on theHY zeolite surface in CuCl form and no ion-exchangeof CuI in solid state CuCl with H+ in HY zeoliteoccurred. Less than half of the original CuCl wassublimated during the heating treatment and part ofthe remaining CuCl was dispersed on the surface of

Fig. 4. XRD patterns of remainder in the furnace tube after20CuCl/HY sample was heated under flowing nitrogen for 60 h at650◦C.

110 Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115

Table 1EDXRF analysis of 20CuCl/HY samples after heating treatment under flowing nitrogen for 15 h

Sample H.T.a (◦C) Si (wt.%) Al (wt.%) Cu (wt.%) Cl (wt.%) Si/Al (mol) Cu/Cl (mol)

HY – 44.86 2.28 – – 19 –20CuCl/HYb – 36.63 1.86 11.77 6.58 – –20CuCl/HY 250 39.38 2.42 6.92 4.12 16 0.9420CuCl/HY 450 41.42 2.52 4.66 1.83 16 1.4220CuCl/HY 750 42.57 2.15 4.16 0.52 19 4.4720CuCl/HY 650c 42.64 2.31 3.81 0.44 18 4.87

a H.T. stands for the heating treatment temperature.b The sample was a physical mixture and the data were calculated from the composition of HY and CuCl.c Heating treatment for 30 h.

the HY zeolite, so that the intensities of the XRDpattern reduced remarkably (Fig. 1).

At 450◦C and over, more copper and chlorine inthe samples were lost and the mole ratio (Cu/Cl) wasmore than 1, the losses of copper and chlorine in-creasing with the increment of the heating treatment.These results indicates that Cu+ of solid state CuClhad been exchanged with H+ in the zeolite while HClwas given off, which resulted in the mole ratio of Cu toCl loaded on the surface of Y zeolite increasing. TheSi/Al (mole) of parent material of HY zeolite obtainedby EDXRF is 19, but there are some extra frameworkAl atoms in the zeolite, as proved by27Al MAS NMRspectra. The ratio of framework Si/Al should be 25.So the ion-exchange capability of parent HY zeoliteis 2.00%CuI. If we suppose that the chlorine existedon the HY zeolite surface as CuCl, the degrees of CuI

ion exchange should be 69.3, 119.5 and 161.5%, re-spectively, at 450, 650 and 750◦C. Over 100% degreeof CuI ion-exchange was obtained at 650 and 750◦C.When the heating treatment was done at high temper-ature, such as 650◦C or over, the following reactionprobably occurred, resulting in more CuI loaded onthe surface of the HY zeolite:

(3)

3.2. TG/TGA study

Figs. 5–8 show the TG/TGA curves of HY zeolite,CuCl, 20CuCl/HY and 100CuCl/HY (100CuCl/100HY)samples at a heating rate of 10◦C/min from ambient

Fig. 5. TG/TGA of HY zeolite at 10◦C/min under flowing nitrogen.

temperature to 1200◦C under flowing N2 atmosphere.For HY zeolite sample (Fig. 5), there are two obviouspeaks on the TGA curve: a peak around 120◦C causedby the loss of zeolite-adsorbed water and a peak at1100◦C, probably caused by self-dehydroxylation of

Fig. 6. TG/TGA of CuCl sample at 10◦C/min under flowingnitrogen.

Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115 111

Fig. 7. TG/TGA of 20CuCl/HY zeolite at 10◦C/min under flowingnitrogen.

Fig. 8. TG/TGA of 100CuCl/HY zeolite at 10◦C/min under flowingnitrogen.

–OH group on the surface of the zeolite. For CuClsample (Fig. 6), the peak of TGA curve around600–700◦C resulted from the vaporisation of liquidCuCl, the other smaller peaks resulting from the lossof impurity in the CuCl sample. For CuCl/HY sam-ples (Figs. 7 and 8), there is a remarkable mass lossbetween 300–400◦C, the peak of TGA curve beingaround 340◦C. This mass loss did not happen on HYzeolite or on CuCl samples. It is shown that between

Table 2The mass loss of the sample by TG analysis

Sample <300◦C (wt.%) 300–400◦C (wt.%) 400–800◦C (wt.%) 800–1000◦C (wt.%)

CuCl 1.10 0.15 91.18 1.25HY 10.64 0.48 1.02 0.5720CuCl/HYa 8.79 (9.05) 0.91 (0.42) 10.89 (16.05) 1.06 (0.59)100CuCl/HYa 6.42 (5.87) 0.65 (0.32) 43.37 (46.10) 1.24 (0.91)

a The data in the parentheses were calculated from the corresponding mass loss data of CuCl and HY samples.

300–400◦C, the reaction between CuCl and HY, i.e.ion-exchange of CuI in solid state CuCl with H+ inHY zeolite, happened and released HCl gas simul-taneously, which went out with the flowing nitrogenand resulted in the mass loss. The maximum rate ofthe ion-exchange reaction was at 340◦C.

The TG curves of CuCl/HY sample can be dividedinto four parts according to the main substance thatparticipated in the mass loss on the TG curves: (1)temperature below 300◦C, the zeolite adsorbed waterwas lost; (2) temperature between 300 and 400◦C, thereleased HCl gas, due to the ion-exchange reactionbetween CuI in solid state CuCl and H+ in HY zeo-lite, was lost; (3) temperature between 400 and 800◦C,CuCl that was dispersed on the surface of HY zeoliteor CuCl particles were lost; (4) temperature between800 and 1000◦C, the CuCl adsorbed on the cage sur-face of the HY zeolite was lost. Table 2 shows the TGmass loss data between the different temperatures un-der flowing nitrogen and also give the mass loss datawhich were calculated from the corresponding massloss data of CuCl and HY zeolite samples.

When the temperature was between 300 and 400◦C,the real mass loss of CuCl/HY samples is slightly morethan the calculated valve. It is because the releasedHCl gas in the ion-exchange reaction between CuI insolid state CuCl and H+ in HY zeolite went out withthe flow of nitrogen. The extra mass loss should beequal to the released HCl mass, which can be usedto calculate the degree of the ion-exchange reactionbetween CuI in solid state CuCl with H+ in HY zeolite.The calculated degree of the ion-exchange is shown inTable 3. It is obvious that the degrees of ion-exchangeare only 56 and 66%, respectively, for 20CuCl/HYand 100CuCl/HY samples at the temperature between300 and 400◦C and increase with the increment ofthe amount of CuCl in the original physical mixture,which agrees with the results of Karge et al. [14].

112 Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115

Table 3The difference of mass loss between measured and calculated

Sample Difference Degree of ion-exchangea (%) CuCl-cageb (%)

300–400◦C 400–800◦C 800–1000◦C

20CuCl/HY 0.49 −5.16 0.47 56 6.92100CuCl/HY 0.33 −2.73 0.33 66 6.20

a The degree of the ion-exchange, calculated from difference of the mass loss between 300 and 400◦C as the released HCl, and2.00%CuI as the 100% degree of the ion-exchange (based on Si/Al= 25 from 29Si MAS NMR spectrum parent HY zeolite material).

b CuCl adsorbed on the cage surface of the zeolite, calculated from the difference of the mass loss between 400 and 800◦C dividedby the HY zeolite mass of the sample.

At a temperature between 400 and 800◦C, the massloss of the CuCl/HY samples is the largest loss in therange of the tested temperature. The real mass lossis less than the calculated mass loss, i.e. the differ-ence at 400–800◦C is a minus (Table 3). During thattemperature period, HY zeolite sample lost very lit-tle mass because of its self-dehydroxylation and CuClsample lost most of its mass because of the vapori-sation. Over 800◦C, CuCl sample only lost very lit-tle mass compared to the mass loss between 400 and800◦C. So CuCl sample lost almost all its CuCl at400 and 800◦C. For the CuCl/HY samples, CuCl wasdispersed on the surface of the zeolite or adsorbedon the surface of the zeolite when the temperature isless than 400–800◦C. This process is spontaneous [6]and CuCl has easy access to the cage of the zeolitethrough the channel of the zeolite when solid stateCuCl changes to liquid CuCl at temperatures beyondthe melt point of 430◦C. Supposing that, at a temper-ature between 400 and 800◦C, the CuCl which wason the surface of the zeolite in CuCl/HY samples waslost and that CuCl which was adsorbed on the cagesurface of the zeolite was not lost. The difference ofreal mass loss of CuCl/HY compared to the calculatedmass loss between 400 and 800◦C should be the CuClmass that adsorbed on the cage surface of the zeo-lite and the value should be determined by the zeolite.Table 3 confirms it. The two samples, 20CuCl/HY and100CuCl/HY give very close calculated data (6.92 and6.20%, respectively) from the minus mass loss differ-ence being divided by zeolite mass of the sample, i.e.the amount of the adsorbed CuCl on the zeolite cagesurface per zeolite mass.

Obviously, the reducing of self-dehydroxylation ofzeolite in the CuCl/HY samples was because that morethan half of the H+ had already been exchanged with

CuI at temperatures below 400◦C. The ion-exchangeof CuI of the solid or liquid CuCl with H+ of thezeolite still took place during the reaction of (1) and(2) when heating between 400 and 800◦C. And alsothe reaction (3) occurred at high temperature. Allof reactions (1), (2) and (3) drove off HCl gas andleft CuI on the surface of the zeolite, contributing tothe mass loss of the CuCl/HY samples. Over 800◦C,it was observed that the mass loss of the CuCl/HYsamples was more than the amount calculated. Thisindicates that the CuCl adsorbed on the cage of thezeolite started to desorb and this made the samplelost more mass than was calculated by the data of HYand CuCl samples (Table 3).

3.3. 27Al and 29Si MAS NMR study

27Al MAS NMR spectra are shown on Fig. 9. Someextra framework aluminium at 0 ppm was observedin the parent HY zeolite. If HY zeolite was treatedat 650◦C for 30 h under flowing N2, the signal ofextra framework aluminium at 0 ppm increased butthe framework aluminium at 58 ppm decreased. Lessthan 0.5% water was measured in the N2 gas by GCanalysis. Therefore, this dealumination of HY zeoliteduring the heating treatment was caused by water,i.e. hydrothermal dealumination [15]. But when the20CuCl/HY was treated at 650◦C for 30 and 70 hunder N2 flow, the signal of framework aluminiumdecreased and that of extra-framework aluminiumdisappeared. This dealumination is also confirmedby the deconvoluation of29Si MAS NMR shown onFig. 10. The results of the deconvoluation of29SiMAS NMR spectra are given in Table 4. Supposingthe signal at−101 ppm as the Si(1Al) signal and theother two signals below−106 ppm as the Si(0Al)

Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115 113

Fig. 9. 27Al MAS NMR spectra of the samples (a) HY zeolite after heating treatment at 650◦C for 30 h; (b) parent HY zeolite; (c)20CuCl/HY after heating treatment at 650◦C for 30 h; (d) 20CuCl/HY after heating treatment at 650◦C for 30 h.

Fig. 10. 29Si MAS NMR spectra of the samples (a) parent HY zeolite; (b)HY zeolite after heating treatment at 650◦C for 30 h; (c)20CuCl/HY after heating treatment at 650◦C for 30 h; (d) 20CuCl/HY after heating treatment at 650◦C for 30 h.

114 Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115

Table 4The results of29Si MAS NMR spectra deconvolution

Sample r (%) (29Si ppm) Si/Ala

HY 16.3 (−101.3), 75.8 (−107.4), 8.0 (−111.9) 25HYb 14.3 (−101.9), 26.9 (−106.9), 58.8 (−107.4) 2820CuCl/HYb 12.3 (−101.9), 79.6 (−107.4), 8.0 (−113.5) 3220CuCl/HYc 8.4 (−101.5), 65.0 (−107.5), 16.6 (−108.1) 48

a Calculated by Si/Al= 400/r.l. (r.l., percent of signal at−101 ppm).b 650◦C, N2, 30 h.c 650◦C, N2, 70 h.

signal [15], then we calculated the framework moleSi/Al = 25 for the parent HY zeolite. Others wereover 25, especially for 20CuCl/HY sample. From theabove brief analysis, we conclude that hydrothemaldealumination of the HY zeolite occurred during theheating treatment at high temperature 650◦C andCuCl made the extra framework aluminium signalon 27Al MAS NMR spectra disappeared and alsopromoted the dealumination of the zeolite.

At high temperature, H form zeolite lost water withthe consequent formation of Lewis sites, which in turnproduced the so-called ‘true’ Lewis sites by ejectingAl species from the framework in the continued pres-ence of water [16]. The ejected extra-framework alu-minium probably reacted with CuCl with the forma-tion of (AlO)Cl.

4. Conclusion

The interaction between CuCl and HY zeolite underinert nitrogen gas atmosphere is much more compli-cated than was expected. CuCl is likely to sublimate

and disperse on the surface of the zeolite. But if CuClis adsorbed on the cage surface of the zeolite, it isvery difficult to be desorbed from the zeolite becauseof the cage and the cage channel even over 800◦C.The ion-exchange of CuI in solid CuCl with H+ inHY zeolite occurs at heating temperature over 300◦C.The maximum rate reached at 340◦C under a heatingrate of 10◦C/min, with the consequent release of HClgas. At high heating treatment temperature, the defect–Si–OH reacts with CuCl to form –Si–OCu and re-lease HCl. And CuCl promotes the dealumination offramework aluminium of the HY zeolite and reactswith the ejected extra framework aluminium to form(AlO)Cl.

During the heating treatment of the mixture of CuClwith HY zeolite, the unique structure of the Y zeolitewas kept and CuCl crystalline phase was unlikely todisappear, since the powder XRD pattern of CuCl crys-talline phase was still observed when the 20CuCl/HY

Z. Li et al. / Applied Catalysis A: General 209 (2001) 107–115 115

sample was treated at 350◦C for 60 h and 650◦C for5 h. Over 100% ion-exchanged degree of the CuI withH+ in HY zeolite could be obtained at high treatmenttemperature 650◦C or over because of the reaction be-tween defect –Si–OH with CuCl to form –Si–OCu.

Therefore, the interaction between CuCl and HYzeolite under inert nitrogen gas atmosphere includesthe following physical or chemical processes:

1. Solid CuCl sublimates below 430◦C and liquidCuCl vaporises over 430◦C. At the same time, CuClis dispersed or adsorbed on the surface of the zeo-lite.

2. H+ in HY zeolite is exchanged with CuI in CuClfollowing reactions (1) and (2) when the heatingtreatment temperature over 300◦C. And the defect–Si–OH group reacts with CuCl to form –Si–OCufollowing reaction (3), when the temperature is over650◦C.

3. CuCl promotes the dealumination of the frameworkaluminium of the HY zeolite and reacts with theextra framework aluminium to form (AlO)Cl.

4. CuCl which is adsorbed on the cage surface the HYzeolite is very difficult to desorb when the heatingtreatment temperature is below 800◦C at the heat-ing rate of 10◦C/min.

Acknowledgements

We thank Taiyuan University of Technology, China,and Universities’ China Committee in London for fi-nancial support on Mr. Li’s academic visit, and alsothank the Material Chemistry Research Group and De-partment of Chemistry, University of Exeter, UK, forproviding facilities and friendly support.

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