Shenglai Li ,Lisong Fan ,Lina Song,2 ,Dangguo Cheng,2, *,Fengqiu Chen,2

1 College of Chemical and Biological Engineering,Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology,Zhejiang University,Hangzhou 310027,China

2 Institute of Zhejiang University-Quzhou,78 Jiuhua Boulevard North,Quzhou 324000,China

Keywords:Extra-framework Al Methane conversion α-site Nitrous oxide Oxygenates

ABSTRACT Iron-containing zeolites play an important role in the selective oxidation of methane to oxygenates by nitrous oxide.A solid-state ion exchange method is adopted to prepare Fe-MOR zeolite catalysts with different amounts of extra-framework Al.EPR spectra and UV–vis spectra show that the percentage of iron ions in tetrahedral or octahedral coordination increases while those of clustered Fe species decrease by the addition of extra-framework Al species.Nitrous oxide titration reveals that more active Fe centers are formed,which promote the nitrous oxide consumption.The number of active centers in the catalyst with the introduction of extra-framework Al is about four times that of the catalyst without the addition of extra-framework Al.Due to this,there is an increase in the methane conversion,total selectivity and yield of oxygenates.

1.Introduction

The partial oxidation of primary carbon-hydrogen bonds is of great importance in converting crude materials to useful products in the chemical and pharmaceutical business sectors.Compared to various energy-intensive synthesis gas production processes[1],the selective oxidation of methane to valuable oxygenates,such as methanol,dimethyl ether(DME)and formaldehyde(FA),is one of the most challenging reactions.Methane is a specially ubiquitous raw material due to the strong C--H bonds(435 kJ·mol−1),which are stronger than the C--H bonds of the partial oxidation products[2].This means that the intermediate products are more likely to be oxidized to COx[3].

Researchers have found that it involves several significant steps in the direct methane oxidation reaction.According to Panov and coworkers[4,5],methane could be activated on so-called α-oxygen species,which are formed on active iron sites(α-sites) on Fe-ZSM-5 by N2O decomposition at high temperature.Due to the radical anionic nature of α-oxygen species,it allows the cleavage of methane C--H bonds by hydrogen extraction mechanisms to occur at room temperature,forming methoxy and hydroxy groups.When the temperature is lower than 200°C,the reaction is quasi-catalytic[6]as methanol must be extracted by aqueous solvents or steam.At above 200°C,the reaction returns to conventional catalytic pattern[7]and the products diffuse into the gas phase.Similar steps have been discovered by the other researchers[8],which also consist of N2O decomposition to generate αoxygen species on Fe-ZSM-5 and methoxy group generation.

It is widely accepted that α-sites of iron-containing zeolites perform the oxidation via the reversible redox transition FeII⇌FeIII[9–11].However,the structure of α-sites is still debatable in spite of many experimental and theoretical studies[12–14].It can be inferred that either the mono-nuclear or the di-nuclear Fe species is the active site [15].The formation of α-sites was easy to proceed in the zeolites of Al–Si composition(Fe-ZSM-5 and Fe-β)and difficult in the zeolites of B–Si and Ti–Si composition[16].Redox behavior and catalytic activity of Fe species in ZSM-5 were dramatically affected by the occurrence of framework Al atoms in Al–Si–Si–Al sequences(Al pairs)in the 6MRs of cationic sites [11].Bell et al.[8] reported that the active sites in this reaction are extra-framework Fe atoms associated with framework Al.Snyder and co-workers [17] reported mono-nuclear α-Fe2+in an extra-lattice site within Fe-beta zeolite(BEA)by using magnetic circular dichroism spectroscopy.It was described that the reactive intermediate was a high spin Fe4+=O species,and the confinement of the zeolite lattice promotes the reactivity observed.Wang and co-workers[18]investigated the effect of extra-framework Al species via AlCl3deposition in Fe-ZSM-35 and found that the formation of binuclear Fe sites bound to extra-framework Al species,with a feature Raman band at 875 cm−1,promotes the activity of N2O decomposition.A possible explanation given by them is that the introduction of extra-framework Al species in Fe-ZSM-35 enhanced the formation of binuclear active Fe sites bound to extra-framework Al.

These findings emphasize the importance of Al species in the metal-zeolite catalysts used in the selective methane oxidation or N2O decomposition.However,much less attention is paid to the role of the extra-framework Al species for the direct conversion of methane to oxygenates by nitrous oxide.Knowing the influence of extra-framework Al species for the partial methane oxidation can provide some guidance to future catalyst design approaches.So we did some work in this topic,and part of the aim of this study is to assess the effect of that species in the reaction.

Conventional impregnation method results in a range of iron species over zeolites,such as isolated Fe2+,Fe3+,mononuclear Fe,and binuclear Fe,Fe2O3bulks[19].Thus many efforts have been made in producing such catalysts to increase the number of α-sites.Fan et al.[20,21]reported that freeze drying limits the movement of metal precursor in the impregnation and highly dispersed iron species are observed in freeze-drying catalyst.Moreover,an iron-incorporated Y catalyst with single-dispersed iron-oxo species was successfully prepared by surface organometallic chemistry (SOMC) with ferrocene as precursor and showed good catalytic reactivity towards CO catalytic oxidation[22].Fe-ZSM-5 with well-defined iron species were prepared by the same method and exhibited the higher activity for N2O-mediated propane oxidative dehydrogenation[23].Our recent studies have shown that a facile solid-state ion-exchange method with ferrocene can be carried out to prepare the uniformly dispersed iron supported Y zeolites with no Fe2O3bulks[24].

In this work,the Fe-MOR catalysts were prepared by a solid-state ion exchange method[25].The influence of extra-framework Al on the performance of direct conversion of methane to oxygenates was investigated.It is shown that the addition of extra-framework Al species can modify the distribution of iron species and enhance the performance in the partial methane oxidation reaction without changing the amount of Bronsted acid sites in the catalysts.

2.Experimental

2.1.Catalyst preparation

Fe-MOR catalysts with 1 wt%iron content and different amounts of extra-framework Al were prepared by solid-state ion exchange method using ferrocene(99%purity,Aladdin)as iron precursor.Al-MOR catalysts without iron were prepared by the same method.Mordenite(Si/Al=25,The Catalyst Plant of Nankai University)was first calcined in air at 500°C for 4 h to remove the templating agent.To prepare the samples,Fe-MOR-x catalysts were prepared by intensively grinding a mixture of ferrocene,MOR and/or Al(NO3)3·9H2O (99.0% purity,Sinopharm) in a mortar,where x means the atomic ratios of extraframework Al/Fe.The well-mixed powder was first heated in static air by raising the temperature from 30°C to 150°C and kept at 150°C for 16 h.Then,the temperature was raising from 150 °C to 500 °C,and the sample was calcined at 500°C for 4 h.Finally the sample was calcined in a flow of N2at 900°C for 1 h[18].

2.2.Catalyst characterization

X-ray diffraction(XRD)patterns of Fe-MOR catalysts were recorded on a PANalytical X'Pert3 powder X-ray diffractometer using Cu Kαradiation(λ=0.154 nm).The surface area was determined by N2adsorption using Micromeritics ASAP 2020 Surface Area at 77 K.The Brunauer–Emmett–Teller(BET)method was adopted to calculate the specific surface area.The elemental composition of the catalyst was analyzed by inductively coupled plasma optical emission spectroscopy(ICP-OES)using ICPA 7400.

N2O titration was carried out to determine the number of active iron sites using Micrometrics AutoChem II 2920 instrument.Typically,the catalyst was placed into a quartz tube and pretreated in flowing helium at 500°C for 1 h followed by cooling down to 250°C.Continuous pulses of N2O(10 vol%N2O-He)were introduced into the quartz tube,and the effluent was analyzed by an on-line mass spectrum.N2O decomposition gives rise to the formation of gaseous N2and an equal amount of αoxygen species.The number of active iron sites was calculated by integration of the N2O peak according to the assumption that one oxygen atom was deposited on each active Fe site.

UV–visible diffuse reflectance spectra(UV–vis)of Fe-MOR catalysts were recorded on Cary Series UV–Vis–NIR instrument in the range of 200–800 nm.Transmission electron microscope(TEM)images and energy dispersive X-ray(EDX)mapping of the samples were obtained on a HT-7700 electron microscope equipped with Oxford X-MAXn65 T using a 120 kV acceleration voltage.Electron paramagnetic resonance(EPR)spectra were observed by a Bruker A300 EPR Spectrometer at room temperature with a microwave power of 20.0 mW and modulation frequency of 100 kHz.27Al magic angle spinning nuclear magnetic resonance(27Al MAS NMR)measurements were carried out on a Bruker Avance III 400 spectrometer at 78.13 MHz.The MAS spin rate was 8.0 kHz and magnetic field was 7.1 T.27Al chemical shifts were referenced to a 1 mol·L−1Al(NO3)3solution at chemical shift at 0.

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2.3.Catalyst activity measurement

The amount of the catalyst placed in a fixed-bed quartz reactor(TJGE,WF-2,I.D.=8 mm)was 0.2 g(380-830μm).The reaction was performed at 300°C under atmosphere pressure,and the gas flow controlled by mass flow controllers was 20 cm3·min−1for methane,helium and nitrous oxide respectively.The effluent gases from the reactor were detected continuously by two online gas chromatographs with three columns through thermal conductivity detectors(TCD).CH4,N2O and CO2were separated in a packed column of Porapak Q.CH4,N2and CO were separated in a packed column of Molecular Sieve 5A and CH3OH,HCHO and CH3OCH3were separated in a packed column of Porapak T.To inhibit the condensation of liquid products,some part of the valves and lines between reactor exit and gas chromatograph were heated at 110°C.

3.Results and Discussion

3.1.Structure property

The textual properties and elemental composition of the Fe-MOR catalysts are listed in Table 1.The total pore volume is similar while the BET surface area and micropore volume demonstrate a decrease tendency,which is due to the extra-framework Al species added blocking some micropores of MOR zeolites during the preparation.Analysis of the catalyst element composition shows that all samples have similar silicon-aluminum ratios,and the difference in iron content is negligible,while the aluminum content is gradually increased.

Table 1Physicochemical properties of Fe-MOR catalysts

Fig.1.XRD patterns of Fe-MOR catalysts.(1)Fe-MOR-0,(2)Fe-MOR-1,(3)Fe-MOR-2.

XRD patterns of catalysts are shown in Fig.1.The diffraction peaks of all the catalysts are sharp and in good coincidence with the pattern reported in the literature[26],indicating their crystalline nature.The diffraction peaks of Fe-MOR-0 are intense and the intensity of Fe-MOR-1 and Fe-MOR-2 is lower.It is due to the lower zeolite content in the samples[27]by the addition of extra-framework Al.No impurity peaks are observed,confirming the purity of the zeolites.The low iron loading and solid-state ion exchange method may contribute to the formation of the isolated iron species.

Fig.2 shows representative TEM images and elemental distribution of the Fe-MOR catalysts.The surface morphology of the three samples has little difference and the basic structure is a square shape of 0.5–1.0 μm.According to the EDX elemental mapping images,it is known that the Fe element is uniformly distributed in the catalyst bulk phase.No large particles of iron-containing oxide are found,which is in agreement with the results of the XRD patterns.

3.2.Property of iron species and aluminum

EPR is a non-destructive analytical technique and transition metal ions can be identified by this method.Because of the relatively strong zerofield splitting effect and the short spin–spin relaxation time[28,29],it is easy for EPR to detect Fe3+cations[30,31]and difficult to detect Fe2+cations.It indicates that in the iron-containing molecular sieves,the corresponding iron species at g ≈4.2 belongs to the tetrahedral coordination of isolated Fe3+ions[30,32]and the iron species at g ≈2.0 to the coupled Fe3+cations,which is generally present in FexOynanoclusters[31,33].

In Fig.3,all the samples have the two kinds of peaks.With an increase in the quantity of the extra-framework Al species,the signal at g ≈4.2 gets stronger(the integrated peak area is 9.1×107,1.2×108and 1.3×108respectively)and the signal at g ≈2.0 gets weaker(the integrated peak area is 3.8×108,1.8×108and 1.3×108respectively),indicating that the amount of isolated Fe3+in the tetrahedral coordination gradually increases and that of FexOynanoclusters decreases.This suggests that the iron sites in the pores may be converted to the isolated Fe3+ions by the reaction to the extra-framework Al species.

The UV–vis spectra of the Fe-MOR samples are shown in Fig.4.The three curves in the picture seem to have a great similarity.According to reports in the literature,bands below 300 nm[18,34,35]are related to isolated iron ions in tetrahedral or octahedral coordination,bands between 300 and 400 nm[34,36]to clustered Fe species whereas bands above 400 nm[37–39]to large Fe2O3particles located at the outside of zeolite pores.Alternatively,isolated Fe3+species in octahedral coordination in Al2O3contribute to the absorption features at 286 nm[40,41].For all the samples,an absorption band centered at 275 nm appears and other bands are not obvious.

Deconvolution of the UV–vis spectra into Gaussian subbands was carried out to get the percentage of different iron species,which is shown in Table 2.It has to be taken into consideration that this method[42]neglects the relation between the wavelength and the absorption coefficient,which is also hindered by overlapping bands.

It is found that the percentage of iron ions in tetrahedral or octahedral coordination increases while those of clustered Fe species decrease when having an increase in the quantity of the extraframework Al species.A possible explanation is that the introduction of extra-framework Al species is of great significance to the formation of isolated iron species in above catalysts.There is a positive correlation between the amounts of isolated iron species and α-sites[18],and it can be inferred that a part of isolated iron species is αsites.Since it is difficult to differentiate α-sites and the rest part of isolated iron species,the role of latter is still unclear in the selective methane oxidation and N2O decomposition.Actually,how clustered Fe species contribute to those reactions remains poorly understood due to the chemical inertness of that species.

The concentrations of α-sites in Fe-MOR catalysts were determined by the N2O titration method[14,43].Fig.5 is a schematic diagram of N2O pulse titration of Fe-MOR-2.It can be seen that N2O is rapidly decomposed into N2without formation of gaseous O2,which indicates that only the reaction N2O+α-sites →N2+(O)αoccurs.The N2O peak area gradually increases and the N2peak area decreases with the introduction of N2O pulse.The peak areas of the two substances keep stable after several minutes,which indicate that the decomposition rate of N2O is close to zero at this time.A constant N2peak still appears in the subsequent stage owing to the N2O used is a pure gas containing 0.1%of N2.

The obtained α-sites content is shown in Table 2 based on the consumption of N2O[20].The number of the α-sites increases from 0.28×1019g−1for Fe-MOR-0 to 1.05×1019g−1for Fe-MOR-1,and further to 1.14 × 1019g−1for Fe-MOR-2.The latter two have almost the same number of active centers,which are about four times that of Fe-MOR-0.The introduction of extra-framework Al species is beneficial for the formation of α-sites by forming an extra-framework Fe--O--Al mixed oxide phase[18].Moreover,only a part of Fe species can participate in the formation of α-sites and excessive extra-framework Al species exhibit no promotion in the number of α-sites.It can be concluded that the addition of extra-framework Al species enhances the generation of α-sites when each catalyst has the same iron content.The promotion is no longer obvious when the amount of aluminum nitrate added is twice that of iron.

27Al MAS NMR spectra of the catalysts are shown in Fig.6.The peak at 0 is assigned to octahedrally coordinated aluminums(extra-framework)[44]and the peak at 55 to the tetrahedral coordination aluminums (Bronsted acid) [45].The increase of Bronsted acid content is beneficial for the formation of α-sites though it could lead to the deep oxidation of oxygenates to COxin the selective methane oxidation[46].Bronsted acidity is required to support the Fe-based α-O for the important initial hydrogen abstraction step while excessive Bronsted acidity could result in an increased level of catalyst fouling [47].Table 3 lists the integrated area of bands.

The percentage of integrated peak area at 0 increases from 11.9%for Fe-MOR-0 to 15.0%for Fe-MOR-1,and further to 16.0%for Fe-MOR-2.It is in agreement with the results of the ICP-OES(Al content).The area of peak at 55 of Fe-MOR-2 and Fe-MOR-1 both increased by 1.0% compared with Fe-MOR-0,which means that the content of Bronsted acid is similar.

3.3.Catalytic performance

Table 4 compares the catalytic performances of Fe-MOR samples.It is evident that the extra-framework Al modified catalysts exhibit enhanced catalytic activities.The same rule applies to the values of CH4conversion,product selectivity and total oxygenates yield.Al-MOR catalysts are not shown here because no catalytic activity is found among them.

Fig.2.TEM and EDX images of Fe-MOR catalysts.(a)Fe-MOR-0,(b)Fe-MOR-1,(c)Fe-MOR-2.

Fig.3.EPR spectra of Fe-MOR catalysts at room temperature.(1)Fe-MOR-0,(2)Fe-MOR-1,(3)Fe-MOR-2.

The CH4conversion of Fe-MOR-2 and Fe-MOR-1 increases by 55.6%and 44.4%compared with Fe-MOR-0 respectively.While,there is not much difference between Fe-MOR-2 and Fe-MOR-1.Fe-MOR-2 only increases by 7.7%compared to Fe-MOR-1 in the term of CH4conversion.The overall selectivity of oxygenates also follows the above rules.The selectivity of Fe-MOR-2 and Fe-MOR-1 increases by 32.4% and 25.5%compared with Fe-MOR-0 respectively.

The CH4conversion and the total selectivity of oxygenates are mainly affected by the Bronsted acids[47]and α-sites content[48].The C--H bond can be activated by Bronsted acids and N2O decomposition occurs at α-sites followed by the formation of α-oxygen,which can oxidize CH4to CH3O-substance[6,49].The above substance can form methanol,dimethyl ether and formaldehyde through several complex steps.Sincethere is no obvious difference in the Bronsted acids contained in the three samples(Table 2),the improved catalytic performance is mainly due to the increase in the number of active Fe centers[50].

Table 2Number of α-sites and percentages of different iron species calculated from the deconvoluted UV–Vis spectra

Table 327Al MAS NMR spectra integral results of Fe-MOR catalysts

Table 4Catalytic data of Fe-MOR catalysts for CH4 partial oxidation by N2O

Fig.4.UV–Vis spectra of Fe-MOR catalysts.(1)Fe-MOR-0,(2)Fe-MOR-1,(3)Fe-MOR-2.

Fig.5.N2O titration curves over Fe-MOR-2 at 250°C(other samples are not shown here).(1)O2,(2)N2,(3)N2O.

Fig.6.27Al MAS NMR spectra of Fe-MOR catalysts.(1)Fe-MOR-0,(2)Fe-MOR-1,(3)Fe-MOR-2.

This indicates that the addition of an appropriate amount of aluminum nitrate increases the number of α-sites in the catalyst,and the αsites can promote the N2O conversion.When the amount of aluminum nitrate added is twice that of iron,the effect is no longer obvious.

4.Conclusions

Fe-MOR catalysts with different amounts of extra-framework Al were prepared by solid-state ion exchange method.It is found that a large fraction of iron ions in tetrahedral or octahedral coordination are in the catalysts,and no large α-Fe2O3particles are detected.The N2O conversion,CH4conversion,total selectivity and yield of oxygenates get improved with increasing extra-framework Al content.Addition of the aluminum nitrate in the MOR can promote the formation of more α-sites.The increase in the number of α-sites enhances the catalytic performance of samples.It is possible that the addition of appropriate amount of extra-framework Al species is beneficial for the formation of active Fe centers bound to extra-framework Al species.This method offers a way to increase the number of α-sites in iron-containing zeolites.

Acknowledgements

This research was supported by the Zhejiang Provincial Natural Science Foundation of China (LR18B060001),Basic Research Project of Sinopec Group(415025)and the Fundamental Research Funds for the Central Universities.