CH4 oxidation to oxygenates with N2O over iron-containing Y zeolites:Effect of preparation☆

Jing Zhu,Lisong Fan,Lina Song,Fengqiu Chen,Dangguo Cheng*

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

Keywords:Ferrocene Iron-containing zeolites Preparation Distributions Partial oxidation

ABSTRACT Developing effective iron-incorporated zeolites and determining their active centers for the direct oxidation of CH4 to oxygenates have remained challenging topics so far.In this paper,we successfully prepare the highly-dispersed iron supported Y zeolites by a facile solid-state ion-exchange method with ferrocene,which was conducted under water-free conditions followed by a series of calcination.Moreover,extra-framework dinuclear Fe2+complexes are identified as so-called active α-iron sites on zeolites.ICP-OES,N2 adsorption-desorption test,X-ray diffraction,solid-state 27AlNMR,N2Otitration,TEM,EPRand 57Fe M?ssbauerspectra were carried outto characterize properties of sample structure,acid sites,as well as the supported iron species.Characterization results indicate that high-temperature treatments have no effect on the typical structure feature of zeolites.Compared with catalysts synthesized by conventionalimpregnation,the samples prepared by the facile approach possess abundantdinuclear Fe2+complexes but no Fe2O3 bulks and show weak acidity.These lead to a higher oxygenate selectivity in CH4 oxidation to oxygenates.Remarkably,the oxygenate(HCHO and CH3OH)selectivity of 6.5%at 375°C can be eventually obtained.

1.Introduction

CH4is the main component of natural gas while CH3OH is one of the most important feedstocks in the petrochemical industry.Hence,to develop a technology that transforms the raw material CH4to a strategic chemical CH3OH in an economical way has been spotlighted since its discovery[1].Currently,the industrialproduction ofCH3OH,which firstconverts CH4into syngas via steam reforming and then converts syngas to CH3OH in high-pressure catalytic process,is indirect,thermal-inefficient and expensive[2].In contrast,direct conversion of CH4to oxygenates seems to be a more economically viable strategy[3].

Nevertheless,the direct route is feasible in thermodynamics but dif ficult to realize in kinetics,since the energy of C--H bond is 440 kJ·mol-1in CH4while 393 kJ·mol-1in CH3OH[2].Thus,the desired chemicals are usually further oxidized.N2O,as one ofthe oxidants[4,5],has been widely investigated due to its mild oxidation capacity.Besides,itplays a vitalrole in benzene hydroxylation and seems to be promising in methane hydroxylation[6].However the yield of oxygenates has been rarely high in partial oxidation of CH4with N2O.Lobe et al.[7]prepared a series of Cu-SSZ-13 and tested them at 250-300°C.As temperature raised,CH4conversion was increased from 0.016%to 0.75%while total selectivity(CH3OH and HCHO)was decreased from 24%to 2.4%,thus the oxygenate yield was always below 0.02%.Recently,Park et al.[8]reported Fe/FER as an effective catalystwith CH4conversion reaching 0.81%and oxygenates(CH3OHand DME)yield up to 0.38%at 280°C.

For CH4oxidation with N2O,iron-containing zeolites are studied extensively due to the unique performance at low temperature.It is observed that special α-iron sites stabilized in zeolite matrix are able to decompose N2O and produce active α-oxygen sites,which can hydroxylate CH4at room temperature[9,10].Further research manifests that the α-iron sites are extra-framework Fe species connected with framework Al and formed after high temperature treatments[11].However,the structure and nuclearity of α-iron sites are still a matter of debate.Some authors point out that binuclear oxygen-bridged iron sites derived from M?ssbauer spectra are α-iron sites,while others propose that mononuclear iron species are responsible[12,13].

It is commonly known that,there are a variety of iron species over zeolites prepared by conventional impregnation,including isolated Fe2+,Fe3+,mononuclear Fe,binuclear Fe,Fe2O3bulks,FeOxnanoparticles,etc.[14].Thus,the investigation of specific Fe compound becomes difficult.Furthermore,not all of these complexes are conducive to the selective partial oxidation,especially the Fe2O3bulks,which can over-oxidize CH4and result in low yield of oxygenates[15].During the preparation of impregnation,the dissolved iron precursors can travel with the capillary flow of solvent.So when it comes to the drying step,a number of iron complexes are removed to the external edge of catalyst body and deposited,with the movement of liquid water[16,17].Due to the poor distribution of precursors,there are generally large clusters of Fe oxides over the hosts after impregnation.

Thus,in this paper,we prepared the highly dispersed iron-containing Y zeolites under water-free conditions by employing ferrocene as the precursor,and attempted to gain deeper insight into the structure of active α-iron sites for selective oxidation of CH4.

2.Experimental

2.1.Catalyst preparation

The highly dispersed FeY was prepared by a facile solid-state ionexchange method using ferrocene(99%purity,Aladdin)as iron precursor.HY zeolite(Si/Al=5,Nankai University catalyst plant)was first dried in oven at 110°C for 2 h,then sealed and transferred into a glove box for further treatments.To prepare a sample of 2-wt%iron content,3-g HY was thoroughly mixed with 0.2-g ferrocene in an agate mortar within 5 min,and the mixture was sealed in a weighing bottle at room temperature for 5 h to insure the complete adsorption.Finally,the resulting powder was thermally treated in a tube furnace in three stages[21,22]:(i)treatment under flowing N2at 150°C for 2 h to eliminate one cyclopentadienylgroup,and 500°C for4 h to eliminate the other(ii)treatmentunder air atmosphere at400°C for 4 h to remove the surface carbon(iii)treatment under flowing N2at 900°C for 1 h to activate the catalyst[12].The product was labeled as FeY-O.

For comparison,conventional FeY was prepared by incipient wetness impregnation.To ensure a 2%iron content,0.45-g Fe(NO3)3·9H2O(98.5%purity,Sinopharm)was dissolved in deionized water and 3-g dried HY zeolite was added.The mixture was stirred for 8 h and kept static at room temperature for 2 h.Then the obtained sample was dried at 120°C for 2 h and subsequently calcined in three stages mentioned above.The product was marked as FeY-I.And the HY zeolites,which were treated in the same three stages,were marked as HY-900.

2.2.Catalyst characterization

The iron content of FeY zeolites was analyzed by ICP-OES(ICPA 7400)after dissolving samples in hot aqua regia and HF compound.

The specific surface area and pore volume were determined by N2adsorption-desorption test(Micromeritics ASAP 2020 Analyzer)at 77 K and were calculated via Brunauer Emmettand Teller(BET)method and t-plot method,respectively.

The structure and crystallinity were confirmed by X-ray diffraction(Shimadzu XED-6000 diffractometer)using Cu Kα radiation(λ =0.154 nm)in the range of 3°-50°,with a counting time of 0.3 s and step size of 0.02°.

The states of Al in FeY zeolites were performed by solid-state27Al NMR spectroscopy(Varian In finityPlus 300)at room temperature with a magnetic field of 7.1 T.The spectra were obtained at a resonance frequency of 78.13 MHz and spinning rate of 8.0 kHz.The NMR spectra were calibrated by 1-mol·L-1Al(NO3)3at δ =0.

The activeα-iron sites were carried outby N2Otitration(Micrometrics AutoChem II2920 instrument).Typically,0.07-g sample waspretreated at 500°C for 1 h under flowing He followed by cooling down to 250°C,then the pulses of N2O(10 vol%N2O-He)were continuously introduced in to react with the sample while the ef fluent was analyzed by an on-line mass spectrum.The amount of α-iron was measured from the accumulated amountof N2O consumed using a stoichiometry ofN2O/α-iron=1.

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Transmission electron microscopy(TEM)images and EDX mapping were obtained on a Philips Tecnai G2 F20 S-TWIN electron microscope equipped with Oxford X-MaxN,using a 200 kV acceleration voltage.

The Fe3+states of FeY zeolites were observed by EPR(Bruker EMX 10/2.7 spectrometer)at-153°C with a microwave power of 5.0 mW and modulation frequency of 100 kHz.

The further information of chemical state of iron and its coordination in FeY zeolites was operated by57Fe M?ssbauer spectra(Wissel)with a velocity range of-11.2 to+11.2 mm·s-1at room temperature using57Co in a Pd matrix.

2.3.Activity measurements

The partial oxidation of CH4was performed on a fixed-bed reactor(TJPX,BTRS-2,8 mm i.d.)over 0.2-g catalyst(0.85-0.425 mm)at atmosphere pressure.Before activity test,the catalyst was pretreated in flowing He at550°C for 1 h and then cooled to room temperature.The reactant gas mixture was introduced into the reactor at the flow rate of 60 ml·min-1with a fixed proportion(n(CH4):n(N2O):n(He)=1:1:1).And products were detected by two on-line gas chromatographs.Porapak Q column was used for the separation of CH4,N2O and CO2,while Molecular Sieve 5A in the same gas chromatograph was employed for the analysis of CH4,N2and O2.The separation of HCHO,CH3OCH3and CH3OHwas performed on the othergas chromatograph by Porapak T column.All the on-line recording was carried out by three thermal conductivity detectors(TCD).

3.Results and Discussion

3.1.Structure property

The actual iron content of synthesized FeY zeolites was determined by ICP and listed in Table 1.It is obvious that the two samples show almost equal Fe loadings with values of 1.89 wt%and 1.87 wt%for FeY-O and FeY-I,respectively.

Table 1 also shows the textural properties obtained from N2adsorption-desorption test.Compared with HY and HY-900,the BET surface area and microporous volume were both decreased significantly with respect to iron-containing Y zeolites.This is because a number of iron compounds enter pore structure of Y zeolites during the preparation and block some micropores[23].Notably,the surface area of FeY-O is 10.6%lower than that of FeY-I while the microporous volume of FeY-O is 12.7%lower.This suggests that much more iron species of FeY-O enter pore channels and have a better dispersity over the zeolite matrix.

XRD patterns of powders(2θ =3°-50°)are shown in Fig.1.All the samples exhibit the classical FAU features of Y zeolite structure[24],and the diffraction peaks of FeY zeolites are only a little weaker in contrast to HY and HY-900.Therefore,it can be concluded that the crystal phase of Y zeolite is almost unaffected by the import of iron.Besides,no obvious characteristic diffraction peaks of iron oxides are detected in FeY zeolites,probably resulting from the low iron loading or the small crystalline size of supported species.

Table 1 Physicochemical properties of different samples

Fig.1.XRD patterns of different samples(a)FeY-I(b)FeY-O(c)HY-900(d)HY.

3.2.Acidity property

The acidic types of powders were investigated by solid-state27Al NMR,depicted in Fig.2.It can be seen that two strong signals at around 0 ppm and 58 ppm were observed for all samples.The former corresponds to six-fold coordinated extra-framework aluminum species in Lewis acidic centers,and the latter is attributed to aluminum species of tetrahedral coordination in Br?nsted acidic centers[25].Obviously,in27Al NMR spectra the intensity of two peaks are both decreased dramatically after the introduction of iron,especially the peak correlated with Br?nsted acid sites in parent zeolites,suggesting that the iron precursors mainly interact with Br?nsted acid sites.From the27Al NMR results,the amount of Br?nsted acid sites on FeY-O is less than that of FeY-I.Since the Fe complexes are better dispersed over FeY-O,which is supported by N2adsorption-desorption test,there are more opportunities for iron precursors to interact with Br?nsted acid sites inside channels[8]and thus the unoccupied Br?nsted acid sites are decreased.

Fig.2.Solid-state 27Al NMR spectra of samples(a)HY(b)FeY-I(c)FeY-O*spinning side bands.

3.3.Characterization of iron species

The number of α-iron sites in iron-containing zeolites was quantitatively measured by N2O titration.The schematic displayed in Fig.3 shows the typical response to the pulses of N2O over FeY-O at 250°C.The peaks of N2could be continuously detected because of trace amounts of N2impurities in N2O pulses.It is accepted that α-iron can bond to the atomic oxygen generated from N2O decomposing at temperature below 250°C[13].From Fig.3,one can see that no oxygen was detected in the reactor ef fluent,indicating the species estimated are all active α-iron sites.As is summarized in Table 1,the amount of accessible α-iron sites is 3.03×1019site·g-1and 1.86×1019site·g-1for FeY-O and FeY-I,respectively.The sample acquired by facile solid-state ion-exchange method contains ca.63%more α-iron sites.During the impregnation[17],a number of Fe precursors were gathered on the surface of Y and thus less Fe precursors were remained inside zeolites.As a result,more α-iron sites were produced over FeY-O since the α-iron sites are bonded to framework Al inside zeolites.

Fig.3.N2O titration curves over FeY-O at 250°C(a)O2(b)N2O(c)N2.(1 Torr=133.322Pa).

According to TEM images of iron-containing zeolites shown in Fig.4,large amorphous blocks can be clearly observed.For FeY-O,the crystalline surface is uniform with almost no distinguished aggregates,and further analysis of EDX in Fig.4 also confirms the well dispersion of Fe species.For FeY-I,as can be seen from Fig.4(a),large clusters are present over the host and EDX analysis detects the presence of iron aggregates.It is obviously indicated that,when iron precursors were introduced by conventionalimpregnation,they were able to depositon the outersurface of catalyst and aggregated into large bulks.On the contrary,the facile approach in solid state avoids the effect of capillary forces and lead to a relatively uniform Fe distribution throughout the zeolites.

The EPRspectra ofFeYsamples are shown in Fig.5 with the observed signals at g=2.0 and g=4.3.According to previous study[26,27],the number and position of discernible EPR signals often vary by different magnetic interaction of metal species and their local environment,hence the attribution of detected signals is not straightforward.However,mostopinions have been metwith thatthe g=2.0 EPR signal can be assigned to highly symmetric isolated Fe3+or small oligomers with weak dipolar coupling,and the g=4.3 EPR signal can be related to Fe3+in tetrahedral coordination[26,28].

Compared with FeY-I,the signal for FeY-O at g=2.0 is stronger and the one at g=4.3 is observably weaker,which implies that the state of supported ferric species is considerably affected by different preparation methods.Another interesting finding is that,for FeY-O,the signal at g=4.3 is almost hard to be distinguished which indicates there are few tetrahedral coordinated Fe3+.A similar phenomenon is found by Long[21].They stated that binuclear iron clusters,which are dominant iron species on their synthetic sample,are silent for EPR analysis.This hence suggests that,in FeY-O,there are considerable ESR-silent iron species analogous to the “binuclear iron clusters”mentioned above,since the amount of EPR-visible Fe3+is far less than that of FeY-I while the total iron content between two samples are almost equal.

Fig.5.EPR spectra of samples at-153°C(a)FeY-I(b)FeY-O.

To better understand the state of iron species,the57Fe M?ssbauer spectra were recorded at room temperature and are showed in Fig.6.From the spectra,there are two intense doublets and one well-defined sextet in FeY-I,representing three individual iron species(Doublet-A,Doublet-B,Sextet-I),whereas FeY-O shows only two components(Doublet-A,Doublet-B).The results ofM?ssbauer parameters are summarized in Table 2.The isomershift(IS)and quadruple splitting(QS)are used to distinguish the oxidation state as well as the coordination of Fe[29].In oxygen-coordinated compounds,states with 0.1＜IS＜0.6 mm·s-1usually relate to Fe3+,while those with 0.7＜IS＜1.4 mm·s-1are assigned to Fe2+[29,30].On the basis of data from literatures[29-32],Doublet-A(IS=0.96-1.3 mm·s-1,QS=1.97-2.44 mm·s-1)corresponds to Fe2+of octahedral coordination in dinuclear Fe2+complexes(Fe2+-O-Fe2+).Doublet-B(IS=0.38-0.53 mm·s-1,QS=0.7-0.75 mm·s-1)is attributed to Fe3+of octahedral coordination,in small FeOxnanoparticles with superparamagnetism inside channels.And Sextet-I(IS=0.36-0.50 mm·s-1,Bhf=39.2-51.4 T)relates to Fe2O3bulks with a size larger than 10 nm outside zeolites.

Fig.4.TEM and EDX images of samples(a)FeY-I(b)FeY-O.

Fig.6.57Fe M?ssbauer spectra at room temperature of samples(a)FeY-I(b)FeY-O.

The present results indicate that,conventional impregnation method can produce multiple iron complexes,including a considerable amount of large Fe2O3particles on the surface.In contrast,less species of Fe complexes and no Fe2O3bulks are found in FeY-O.This is consistent with the resultfrom TEMimages where large bulks ofiron particles are detected in FeY-I sample alone.For Fe3+,as shown in Table 2,26%of it in FeY-I are large Fe2O3particles and 74%are small FeOxnanoparticles inside channels.On the contrary,in FeY-O,all of Fe3+are superparamagnetic FeOxnanoparticles with weak dipolarcoupling or in highly symmetric isolated states,supported by EPR analysis.For Fe2+,there are only one ferrous component in two samples and its amount in FeY-O is 56%higher than that in FeY-I.Note that the amount of α-iron sites calculated by N2O titration is 63%higher in FeY-O,which is extraordinary approaching to the result acquired by57Fe M?ssbauer,and α-iron sites are known to be ferrous[12].We hence consider that the special α-iron sites are actually extra-framework dinuclear Fe2+complexes.

Overall,combined with conclusions from TEM,EDX,ESR,N2O titration and57Fe M?ssbauer,it is evident the difference of iron species between two FeY zeolites.FeY-O possesses more extra-framework dinuclear Fe2+complexes,small FeOxnanoparticles inside channels and no Fe2O3bulks outside.Its comparatively narrow range of iron species can be attributed to high distribution of Fe complexes throughout zeolites during preparation.In contrast,for FeY-I,iron precursors are poor distributed affected by capillary forces in the drying step.The precursors are able to aggregate near the external edge of zeolite bodies and do not sufficiently occupy Br?nsted acid sites inside.Therefore,there are fewer dinuclear Fe2+complexes and FeOxnanoparticles,but a lot of large Fe2O3particles over the parent zeolites.

3.4.Catalytic performance

To evaluate the catalytic behavior of FeY zeolites,the partial oxidation of CH4with N2O was carried out at 325-425°C.The catalytic results are displayed in Fig.7.It can be clearly seen that HCHO and CH3OH are the only partial oxidation products over FeY.Expectedly,as temperature raised(higher than 375°C),the selectivity to oxygenates,especially CH3OH,is decreased rapidly due to facile over-oxidation to CO and CO2at high temperature.For CH4conversion,it is higher in FeY-I,while for product selectivity,FeY-O performs better.Notably,FeY-O shows comparative superiority not only in CH3OH selectivity but also in HCHO selectivity,and its total oxygenate selectivity is almost three times as high within the temperature range tested.

Fig.7.CH4 conversions and product selectivity during partial oxidation of CH4 by N2O at different temperatures(a)FeY-I(b)FeY-O.

Table 2 57Fe M?ssbauer parameters of catalysts

From previous study[8],Br?nsted acid sites have a great effect on the conversion of reactants by activating C--H bonds of CH4.Based on the results of27Al NMR,there are more Br?nsted acid sites on FeY-I,resulting in relatively higher catalytic activity.As for the selectivity to oxygenates,it is considered that there is a strong correlation between the supported Fe species and the product distribution.The extraframework dinuclear Fe2+complexes,which possess the feature of α-iron,are generally considered the active iron centers for a selective oxidation of CH4by N2O[12].Note that FeY-O owned 43.2%of highly dispersed dinuclear Fe2+complexes inside channels,which is 1.6 times the amountofFeY-I.Consequently,more HCHOand CH3OHare generated over FeY-O at the same temperature.Furthermore,it has been proved that the Fe2O3bulks outside the FeY-I zeolite lead to the deep oxidation[15],so do Br?nsted acid sites[8].As a result,the low oxygenate selectivity of FeY-I attributes not only to low content of dinuclear Fe2+complexes,but also to the presence of Fe2O3and a large amount of Br?nsted acid sites.Additionally,it is important to highlight that various Fe species can all participate in the partial oxidation of CH4and in fluence the catalytic performance to a different extent[32-34].Therefore,FeY-O reaches its best product selectivity to 6.5%at 375°C,while the best oxygenate selectivity for FeY-I is 1.8%at 350°C.

4.Conclusions

The iron-supported Y zeolites with highly dispersed Fe complexes have been successfully synthesized by a facile solid-state ion-exchange method with ferrocene.Compared with FeY-I prepared by impregnation,solid-state ion exchange in water-free conditions enable ferrocene to distribute more uniformly and thus more Fe complexes inside are able to interactwith bare acid sites rather than aggregate to large clusters outside the zeolite.As a result,there are fewerspecies ofiron as wellas fewer Br?nsted acid sites on FeY-O after preparation.For a partial oxidation of CH4by N2O over FeY-I and FeY-O,the amount of extra-framework dinuclear Fe2+complexes,Fe2O3bulks and Br?nsted acid sites are crucial factors to affect product distribution along with CH4conversion.With weak acidity,no Fe2O3bulks,but abundant extra-framework Fe2+-O-Fe2+complexes as mainly active centers,FeY-O zeolites inevitably show high oxygenate selectivity.Actually,in the preparation of FeY-O,when ferrocene with two cyclopentadienyl groups enters the pore channel of Y zeolites,Fe elements are able to keep a distance between each other due to the large groups.The Fe elements,anchored with framework Al,are able to maintain the distance and keep well-dispersed throughout the whole preparation since the series of calcination remove rings step by step.Overall,our findings suggest that preparing the metalsupported zeolite with an organometallic precursor in stepwise calcination might acquire a uniform distribution of metal and potentially enhance the catalytic activity.

Acknowledgments

We acknowledge the supportfromthe Zhejiang ProvincialEngineering Research Center of Industrial Boiler&Furnace Flue Gas Pollution Control,Hangzhou,China.