Unexpectedly Promoting Effect of Carbon Nanotubes Grown During the Non-oxidative Coupling of Methane over Copper Catalysts

One of the challenges for the non-oxidative coupling of methane (NOCM) is to effectively remove the deposited coke over catalysts owing to the over-dehydrogenation of methane. Herein, we show that an insitu growth of carbon nanotubes (CNTs) instead of coke were observed during NOCM over a CuSO4/γ-Al2O3 catalyst. The as-grown CNTs depict an unexpected promoting effect for NOCM with a highest activity of 0.48 mol kg cat·h, and maintained 85% activity after 200 h running time. The equilibrium methane conversion is 9.8% with a selectivity of 78.2% for C2 (C2H4 + C2H6) products. Highly dispersed Cu nanoparticles distributed on the top of CNTs measured by transmission electron microscopy is proposed to result in high catalyst stability during NOCM for 200 h instead of deactivation in several hours. Here, we firstly prove that the as-grown CNTs can promote the catalytic activity of NOCM instead of deactivation by coking over catalysts.


INTRODUCTION
The direct, non-oxidative conversion of methane (NOCM) into light olefins or aromatics, e.g.ethylene, ethane and benzene, is a highly attractive issue for academia and industry.Recently, several advancements have been reported to develop new concepts of catalyst systems for effectively direct converting methane into aromatics or ethylene with remarkable activities, selectivity, and durability 1,2 .Moreover, the cost of the production of ethylene using NOCM was reported less than one fifth of that by using the stream cracking of crude oil 3 .Additionally, the main merits of NOCM can get rid of the complicated separation of products without oxygen as well as the generation of CO 2 .These evidences show that direct conversion of methane is promisingly more economical and environmentally friendly.However, two main challenges for NOCM are needed to be overcome: (i) the activation of methane (C-H bond strength is 434 kJ/mol) need high temperatures (>700 o C), and (ii) catalysts were deactivated quickly through kinetically preferred generation of coke.Therefore, numerous studies conducted the oxidative coupling of methane (OCM) from 1980s [4][5][6][7][8] .Usually, the presence of oxygen results in the overoxidation of methane, leading to an immense amount of the thermodynamically stable products carbon dioxide and water.Obviously, the carbon utilization efficiency of OCM is relatively low.Thus, a practical route for OCM is not available so far.Listed reactions (1)-( 4) of OCM can be represented as (3) Front.Res Recently, Guo et al. demonstrated a new type of heterogeneous iron catalyst which can directly convert methane (48.1% conversion) into higher hydrocarbons (> 99% with ethylene, benzene, and naphthalene) without the formation of coke or unwanted carbon dioxide 1 .However, preparation of the catalyst was complicated and needed high temperature (1700 o C).More recent, Morejudo et al. used a co-ionic membrane reactor to directly transform methane into benzene with a high carbon efficiency of ~80% at a relative low temperature of 700 o C 2 .Moreover, several reports have demonstrated that NOCM is a promising route to form light olefins or aromatics [9][10][11][12][13][14][15] .The main reactions of NOCM are described as Comparison of OCM, the main merit of NOCM is a suppression of over-oxidation of methane.This will significantly improve the selectivity of higher hydrocarbons instead of carbon dioxide.However, how to control the catalysis system preventing over-dehydrogenation of methane is a key step to avoid the formation of coke in NOCM 1,2,[9][10][11][12][13][14][15] .Therefore, the design concepts for active catalysts or reaction systems should avoid completely dehydrogenation of methane in NOCM.Conventionally, the as-generated carbon materials during NOCM were reported that will led to a major deactivation in catalytic activity.The active metal components of catalysts were covered with the as-grown carbon materials with highly graphitic structures, e.g.coke, graphite or carbon nanofibers, and forfeited their catalytic activity gradually 1,2,[9][10][11][12][13][14][15] .The catalytic activities of catalysts for NOCM will deactivate soon, normally in less several hours.Therefore, the design concepts of the conventional catalysts were focused on preventing the formation of carbonaceous materials during the reaction conditions in NOCM.As we know, the promotion effect of the as-grown carbonaceous materials in NOCM was not reported yet.Herein, we present a new finding that the as-grown carbon nanotubes (CNTs) during NOCM over CuSO 4 /γ-Al 2 O 3 catalysts can promote the catalytic activity with high C 2 (C 2 H 4 + C 2 H 6 ) yields for a 200 h test.The in-situ multi-walled CNTs growth following a tip-growth model through Cu nanoparticles (NPs) during the NOCM reaction was observed.The highly-dispersed Cu NPs on the top of the as-grown MWCNTs conducted an admirable activity of NOCM with a remarkable resistance of deactivation.These results present new concepts for NOCM.

Materials
Copper (II) sulfate pentahydrate (99.0%) was purchased from Signa-Aldrich.γ-Al 2 O 3 powder was supplied by Degussa Co.They were used as received without further purification.Methane, oxygen, air, and argon gas were pur-chased from Air Liquide.

Preparation of copper catalysts
Copper catalysts were prepared by wet impregnation according to our earlier work 16,17 .Typically, copper (II) sulfate pentahydrate (0.1 g) was dissolved in 10 mL pure water with a vigorous magnetic stirring under air.The copper solution was added into γ-Al 2 O 3 powder (0.4 g) step by step with a potent stirring by hand for 30 min.The final slurry solution was dried in oven at 120 o C under air overnight.The final loading of Cu was 5.0 ± 0.2 wt.%, which was determined by atomic adsorption spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS, PE-SCIEX ELAN 6100 DRC).

Oxidative conversion of methane (OCM)
Catalytic activities were carried out in a continuous flow, fixed-bed quartz tube reactor.In all OCM tests, 0.1 g of the prepared copper catalysts-5.0wt% CuSO 4 /γ-Al 2 O 3 was put in the reactor.The reaction temperatures were adjusted between 700-1100 o C. The feed gas is a mixture of argon diluted methane (CH 4 /O 2 /Ar = 60/1/19) with a flow rate of 40 mL/min.The effluent gas composition was examined by an online gas chromatography (GC, Shimadzu GC-2014), which is equipped with an FID detector with HP-DPX5 column (I.D. is 0.53 mm, 25 m in length with a 1.0 mm inner coating film).Methane conversion, hydrocarbon products selectivity and carbon deposition were calculated through the carbon balance, following previously reported methods [13][14][15] .

Non-oxidative conversion of methane (NOCM)
Catalytic activities were carried out in a continuous flow, fixed-bed quartz tube reactor.In all NOCM tests, 0.1 g of the prepared copper catalysts-5 wt% CuSO 4 /γ-Al 2 O 3 was put in the reactor.The reaction temperatures were adjusted between 700-1100 o C. The feed gas is a mixture of argon diluted methane (Ar/CH 4 = 1/3) with a flow rate of 40 mL/min.The effluent gas composition analysis and characterization of methane conversion, product distribution were carried out the same standards of OCM.

Catalyst characterization
Scanning electron microscopy (SEM) analysis was conducted using a JEOL JSM-6700F.Transmission electron microscopy (TEM, JEOL AEM-300 and JEM-2100) equipped with an energy dispersive spectrometer (EDS) were used to investigate the micro-and nano-scale structural morphologies of the as-grown samples and perform elemental analyses.High resolution TEM Images were performed on to investigate the micro-and nano-scale structural morphologies of Cu NPs.The weight percent of the as-grown carbon materials was analyzed by a thermogravimeter analyzer (TGA, TA-Q500).The oxidative characteristics of the samples were performed in TGA under air atmosphere (40 mL/min) with a ramp of 40 o C/min during 30-800 o C. X-ray Carbon balance (%) = 2 × total mole of C Raman scattering spectroscopy (JOBIN-YVON T64000) with a laser excitation wavelength of 532 nm was used to characterize the graphite-amorphous carbon features of the as-grown carbon materials.

RESULTS AND DISCUSSION
Previously, sulfate-assisted metal catalysts have been reported with high activities in OCM [18][19][20] .The main concepts of these catalysts were regarded that the sulfated-supports, e.g.SO 4 -2 -MgO or SO 4 -2 -ZrO 2 , can modify surface acidity of catalysts then conduct higher dehydrogenation rate of methane.In OCM, the present of oxygen can prevent over-dehydrogenation of methane and recover active sites of catalysts through the redox process.However, this is a trade-off in OCM and usually leads to a difficult separation of gas products.Therefore, the employment of OCM in practical utilization has been postponed for several decades.Originally, we employ CuSO 4 /γ-Al 2 O 3 as a catalyst according to several previous reports for studying OCM.However, we accidentally found that CuSO 4 /γ-Al 2 O 3 catalyst displayed remarkable activities in growth of MWCNTs with chemical vapor deposition with methane 21 , ethylene 22 and ethanol 23 .These reliable evidences lead us to conduct both OCM and NOCM over CuSO 4 /γ-Al 2 O 3 catalyst for an adequate reaction time.
To check the activity of the copper sulfated catalyst in the dehydrogenation of methane with and without oxygen, we conducted the activities of OCM firstly and then followed NOCM over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst at 800 o C. In the left-upper panel of Figure 1, which demonstrates that the reactivity of 5.0 wt% CuSO 4 /γ-Al 2 O 3 in OCM was stable and maintained approximately at the space-time yield of 0.24 mol kgcat -1 hour -1 for 2 h.After OCM, oxygen was replaced by argon, then switched to the NOCM mode.It is clearly to find that the reactivity was sharply dropped to zero in 30 min.However, surprisingly, the reactivity can recover gradually in 180 min of the NOCM process, and reached the maximum of reactivity with a yield of 0.48 mol kgcat -1 hour -1 , which is twice of the value of OCM.Moreover, in NOCM process, the first 90 minutes (reaction time during 150-240 minutes in Figure 1), the reactivity presents a lower slope (r1), and the later 60 minutes (reaction time during 240-300 minutes) demonstrates a higher slope (r2) of reactivity.Apparently, the slope of r2 is higher than r1.This indicates that the recovery behaviors of reactivity over 5.0 wt% CuSO 4 /γ-Al 2 O 3 during NOCM process depict two stages.The first is a slow-growing stage and follows a fast-growing route to reach the maximum of reactivity.We took samples from the reaction times at 30 (a), 60 (b), 120 (c), and 180 (d) min, respectively and examined the surface morphologies of them using SEM, shown in Figure 1(a-d).
It is plainly to note that the recovery of catalytic activities over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalysts is accompanying with the growth of as-grown CNTs or CNFs.The life-time testing was performed for 200 h at 800 o C, the 5.0 wt% CuSO 4 / γ-Al 2 O 3 was quite stable, and displayed only a slight decline in reaction activity, shown in upper-right panel of Figure 1.
For understanding the morphologies of as-grown fiber-like structures during NOCM over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst at 800 o C, we examined the sample using TEM and selected-area electron diffraction (SAED).) is assigned to the combustion of amorphous carbon and the higher temperature (515 o C) is assigned to that of MWCNTs 16,17 .The growth yield of the as-grown carbon soot is approximately 30.4 wt% with a composition of approximately 44% amorphous carbon and 56% MWCNTs.The graphitic quality of the asgrown MWCNTs was determined using the intensity ration of the G-band (tangential mode of graphite~1343 cm -1 ) and D-band (defect mode~1600 cm -1 ). Figure 3b  Apparently, the initial CH 4 conversion is near 29.8%, however, which descended quickly in 30 min and maintained at a stable value of 9.8% after 150 min testing time.Selectivity to ethylene plus ethane (78.2%), and butadiene (1.6%) were constant while the reaction time was above 150 minutes.Although activity and conversion of NOCM over 5 wt% CuSO 4 / γ-Al 2 O 3 is not the highest compared with previous reports 1,2 , however, the formation of carbon-related materials didn't obey the previous concept as a role of deactivation for NOCM.On the contrary, the as-grown carbon-related materials dominated the reaction process in NOCM.By comparison, the pretreatments of 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst were conducted at 800 o C under air, argon diluted hydrogen, or helium for 2 hours, the results are presented in Figure 4b I-III, respectively.Interestingly, under helium or hydrogen treatments, the maximum activities would reach in less 100 minutes, however their highest catalytic rates are still lower than that of air-treated sample.The major difference on these treatments was the formation of Cu nanoparticles (NPs).Calculated by XRD patterns of Cu (111) using Scherrer equation, the particle sizes of Cu NPs by helium and hydrogen pretreatments are 27.2 and 32.5     , which dominates the growth of CNTs and then promotes the activity in NOCM.Without SMSI, Cu NPs which derived from Cu(CH 3 COO) 2 or Cu(NO 3 ) 2 are easy to aggregate and then covered with amorphous carbon.This will cause the deactivation of catalytic activity very soon, usually in several minutes.
In order to testify our consideration, we examine surface morphologies of CuSO 4 /γ-Al 2 O 3 catalyst which conducted the NOCM for 120 min, shown in Figure 6.The SEM image demonstrates dense-growth CNTs and CNT-free area on CuSO 4 /γ-Al 2 O 3 catalyst.Semi-quantitatively elemental analysis of two areas using EDX spectra (spectrum 1 and spectrum 2) are listed.It is obviously to note that CNTfree area presents lower values of S and O elements, which strongly suggests that SO x is the determining component of CuSO 4 /γ-Al 2 O 3 catalyst.
Our observations reveal that the switch from OCM to NOCM over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst will initially generate CNTs instead of conducting the coupling of methane.Therefore, the reactivity of catalyst was quickly dropping to zero, and further instrumentally examined owing to the formation of CNTs.Possibly, this is the main reason why previously studies on NOCM did not disclose the promoting effect of the as-grown carbon materials.In this work, we reveal that the catalytic reactivity will recover gradually in NOCM using 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst with adequate reaction time.As examining the results shown from Figure 1 to Figure 6, we can suppose that the unexpected promoting effect of the as-grown MWCNTs were performed with two stages.MWCNTs were formed in the initial stage of the dehydrogenation of methane through a surface-diffusion mode over copper nanoparticles in 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst.Then Cu NPs were highly dispersed on the top of the tangle-like MWCNTs, which mainly conducted the route of coupling of methane into ethylene, ethane and higher hydrocarbons.This interesting result was not reported previous.

CONCLUSIONS
Here, we demonstrate 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst can perform both OCM and NOCM.The maximum activity of NOCM is near twice of that for OCM.The as-grown MWCNTs is firstly reported to be unexpected promoter for 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst in NOCM.The yield of the activity is 0.48 mol kgcat -1 hour -1 with a C 2 selectivity of 78.2% and an equilibrium methane conversion of 9.8% at 800 o C. A 200-h catalytic testing, the activity can maintain at 85% of the highest value.We explore a new concept for direct converting of methane with the promotion of the asgrown carbon materials instead of deactivation.

Figure 1 .
Figure 1.The left-upper and right-upper panels: Catalytic activity of 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst for oxidative coupling of methane (OCM) and non-oxidative coupling of methane (NOCM) at 800 o C. For OCM, the feeding gas is a mixture of CH 4 /O 2 /Ar = 60/1/19 with a flow rate of 40 mL/min.For NOCM, the feeding gas is a mixture of CH 4 /Ar = 3/1 with a flow rate of 40 mL/min.(a-d) are SEM images of the different reaction time position in the upper-left panel a-d for NOCM.The scale bar in (a-d) is 0.5 mm.
displays that the I G /I D (~1.0) ratio is a typical feature of MWCNTs.The yields of the as-grown MWCNTs at various reaction times over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst in NOCM in the initial 150 min was plotted in Figure 3c.A linear relationship between the MWCNT yields and reaction times was observed.Combined with the results in the left-upper panel of Figure 1, it is clearly to note that the growth of MWCNTs is the decisive step in NOCM over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst.The average particle size of Cu NPs calculated using XRD patterns of Cu (111) crystalline (2θ = 43.3o ) with various reaction times in NOCM also illustrated in Figure 3d.A slightly raising size of Cu NPs is not the main factor to dominate the catalytic activity for NOCM.For NOCM, CH 4 conversion, product selectivity and life time are three main factors to evaluate the catalytic performance of 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst.In Figure 1, we demonstrate that the life time in NOCM can maintain 200 h with a slight decay of activity.Figure 4a describes the trends of CH 4 conversions and C 2 + selectivities (C 2 H 4 , C 2 H 6 , and C 4 H 10 , mainly) with various reaction times in NOCM at 800 o C over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst.

Figure 2 .
Figure 2. (a) A typical TEM image of the as-grown CNTs, (b) TEM image for the enlarge area of (a), and (c) a selected-area electron diffraction of Cu NPs of (b), over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst in NOCM at 800 o C under a mixture of Ar diluted methane with a flow rate of 40 mL/min for 2 hours.

Figure 3 .
Figure 3. (a) Oxidative TGA profiles and (b) Raman spectra of the samples over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst in NOCM at 800 o C under a mixture of Ar diluted methane with a flow rate of 40 mL/min for 2 hours.(c) the CNT yield versus the reaction time over 5.0 wt% CuSO 4 /γ-Al 2 O 3 catalyst in NOCM at 800 o C under a mixture of Ar diluted methane with a flow rate of 40 mL/min.(d) the average particle size of Cu NPs calculated through XRD patterns of Cu (111) at 2θ = 43.3o based on Scherrer equation.