Fluctuating Storage of the Active Phase in a Mn‐Na2WO4/SiO2 Catalyst for the Oxidative Coupling of Methane

Abstract Structural dynamics of a Mn‐Na2WO4/SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2WO4 with Mn7SiO12. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.


Introduction
Theoxidative coupling of methane (OCM) to ethane and ethene represents an attractive alternative to current crudeoil-based processes in order to access value-added chemicals. [1] Since the pioneering works of Keller and Bhasin, [2] Hinsen and Baerns, [3] and Ito and Lunsford [4] in the early 1980s,t he multi-phase Mn-Na 2 WO 4 /SiO 2 catalyst has established itself as ah igh-performance system that exhibits extended on-stream stability at high reaction temperatures. [5] Despite extensive investigations,b oth the active site and working mechanism of the catalyst remain much debated. [6] In general, most research has been directed towards the structural characterization of the catalyst before and after the reaction or in aq uenched state. [7] Only limited efforts have,h owever,b een made to elucidate the nature of the catalyst under working conditions.
In this report, aM n-Na 2 WO 4 /SiO 2 catalyst (Supporting Information, Table S1, Figures S1-S3), synthesized in large scale by Simon et al., [5e] was investigated with the aim of identifying structural motifs and phase transitions directly under relevant reaction conditions.Amulti-method approach, featuring electron microscopy (SEM-EDX), thermal analysis (TG-DTA-MS), in situ and operando Raman spectroscopy and in situ X-ray diffraction, was adopted. Reference compounds,i nf orm of Na 2 WO 4 ·2 H 2 O, MnWO 4 ,a nd natural braunite (Mn 7 SiO 12 ) [8] were also examined (Supporting Information, Figures S4-S13). With pure Na 2 WO 4 known to undergo ap hase transition from solid to liquid at 695 8 8C (Supporting Information, Figure S11), [9] the formation of ac atalytically active liquid component, containing alkali and transition metal oxides,was investigated.

Results and Discussion
Thea s-synthesized catalyst is am acro-porous material with low specific surface area (2.9 m 2 g À1 )( Supporting Information, Figure S3). Its crystalline fraction is constituted by a-cristobalite (92 wt %), quartz (0.5 wt %), Na 2 WO 4 (4.1 wt %), and mixed-valent Mn 7 SiO 12 (3.4 wt %) (Supporting Information, Figure S1). Them anganese silicate mostly contains the transition metal in the oxidation state + 3 (Mn 2+ Mn 3+ 6 SiO 12 ). Another phase,M nWO 4 ,w hich only features Mn in the oxidation state + 2, was sporadically detected at certain spots by Raman spectroscopy ( Figure 1A, bottom, Supporting Information, Figure S2). By investigating the aforementioned reference compounds using Raman spectroscopy (Supporting Information, Figures S4-S10), all observed bands were successfully allocated for the multiphase Mn-Na 2 WO 4 /SiO 2 catalyst system. Thel ow concentration of MnWO 4 is expected to arise from the higher stability of Mn 7 SiO 12 ,relative to MnWO 4 ,under oxidizing conditions. SEM-EDX revealed an inhomogeneous distribution of manganese,s odium, and tungsten on the catalyst surface and inside the silica support, with Na 2 WO 4 preferentially forming separate domains (appearing in yellow-orange in Figure 1B). Furthermore,l arge areas of the catalyst are only constituted by manganese,s ilicon and oxygen. Higher concentrations of Mn, in close vicinity to the Na 2 WO 4 phase,m ay suggest as tructural interaction of the Mn-containing phases with Na 2 WO 4 .F reely existing Wc an also be observed, suggestive of silica-supported WO x species.H owever,t ypical bands for WO x species dispersed on silica do not appear in the Raman spectrum of the catalyst ( Figure 1A). [10] Surface inhomogeneities are also reflected in the Raman spectra recorded at different spots,w hich evidently feature varying concentrations of Na 2 WO 4 and Mn 7 SiO 12 ( Figure 1A,top and middle).
After testing the catalyst under relatively mild conditions (Supporting Information, Figure S14), as lightly increased concentration of quartz was observed, yet the general phase composition of the catalyst remained unaltered (Supporting Information, Figure S1). Both the support (in its high-temper-ature b-cristobalite modification #>225 8 8C) as well as the Mn 7 SiO 12 phase did not undergo structural changes in the temperature range of 400-750 8 8Cinsynthetic air (Figure 2A; Supporting Information Figure S15). TheN a 2 WO 4 phase,o n the other hand, displayed significant structural dynamics. When heating to 600 8 8C, the reflections of cubic Na 2 WO 4 were replaced by an unidentified phase that is apparently formed during the transition of cubic to orthorhombic Na 2 WO 4 ( Figure 2A;S upporting Information Figure S15). Thel atter was first observed at 650 8 8Cand subsequently remained stable until 680 8 8C. Further heating to 690 8 8Cr esulted in the complete disappearance of the Na 2 WO 4 reflections,w hich is suggestive of melting. This stands in agreement with the onset melting temperature of pure sodium tungstate (# m = 695 8 8C; Supporting Information, Figure S11). An endothermic event, indicated by aDTA signal at 689 8 8Cinthe thermal analysis of the catalyst in synthetic air (Supporting Information, Figure S16), further confirms the melting of Na 2 WO 4 on the surface of the catalyst support. Forthe catalyst, the relatively small endothermic signal arises from the low concentration of Na 2 WO 4 but is,n evertheless,c learly verified by its reversibility during cooling. Solidification of supported, molten Na 2 WO 4 evidently results in the formation of an amorphous phase (Supporting Information, Figure S15). Furthermore, the crystallization of two unidentified phases is observed in the temperature range of 660-600 8 8Cw hile cubic Na 2 WO 4 only reappeared at 450 8 8C. Similar results were reported by Hou et al., [6j] who observed aweakening of the reflections for Na 2 WO 4 in air from 500 8 8Co nwards and ac omplete disappearance at 700 8 8C. In addition to this,asignificant broadening of the n sym (WÀO) stretching mode of Na 2 WO 4 at 923 cm À1 was observed in the Raman spectrum of the catalyst above the melting point of Na 2 WO 4 ( Figure 1A,t op). Both, Takanabe et al., [6o] and Yu.e tal., [11] reported similar observations,d escribing ar eversible disappearance and intensity loss of the characteristic Raman bands for Na 2 WO 4 supported on TiO 2 and CeO 2 ,r espectively.R aman spectroscopy at different sampling positions clearly proves that the melt does not completely wet the catalyst surface.W hile in some areas only signals of b-cristobalite and Mn 7 SiO 12 were evident ( Figure 1A,t op,g ray spectrum), other areas also featured ab roadened spectrum of Na 2 WO 4 ( Figure 1A,t op,b lack spectrum). In summary,m elting of the crystalline Na 2 WO 4 phase in the Mn-Na 2 WO 4 /SiO 2 catalyst was clearly observed in synthetic air by in situ XRD and TG-DTAa nalysis. However,according to Raman spectroscopy,afull wetting of the catalyst surface with molten Na 2 WO 4 does not occur,that is,s urface inhomogeneities clearly persist at temperatures higher than 700 8 8C.
In contrast to this,t he structural evolution changes drastically in inert atmosphere.C ubic Na 2 WO 4 disappears at 600 8 8Ci nf avor of an unknown transient phase ( Figure 2B; Supporting Information, Figure S17), which only partially resembles the transient phase formed in air at 600 8 8C ( Figure 2A). Instead of detecting the formation of orthorhombic Na 2 WO 4 at 650 8 8C, as observed in synthetic air (Figure 2A), the patterns of the transient peaks are subject to further change.The intensity of the reflection near 338 8 2q,assigned to Mn 7 SiO 12 ,starts to decrease,while peaks due to MnWO 4 arise ( Figure 2B). Thed evelopment of the MnWO 4 peaks occurs simultaneously with the complete disappearance of the Mn 7 SiO 12 reflection. As ignificant formation of MnWO 4 ,b y reaction of Mn 7 SiO 12 with Na 2 WO 4 and/or WO x ,w as also evident via Raman spectroscopy for nitrogen feed ( Figure 3A). Theb and of Mn 7 SiO 12 at 958 cm À1 is no longer distinguishable at higher temperatures due to shift or broadening of the band of Na 2 WO 4 at 927 cm À1 .T he latter is most likely caused by phase transition and melting of Na 2 WO 4 or by formation of tetrahedrally coordinated, silica-supported WO x species.T he disappearance of Mn 7 SiO 12 and cubic Na 2 WO 4 in the XRD,s tarting at 600 8 8C, is,t herefore,m ost likely connected to ap artial or complete conversion of the two phases to MnWO 4 .T hermal analysis of the Na 2 WO 4 , MnWO 4 ,a nd Mn 7 SiO 12 reference compounds (Supporting Information, Figures S11-S13) only revealed as ignificant oxygen release (m/z = 32) at elevated temperatures for the Mn 7 SiO 12 phase,with an onset at 807 8 8C( Figure 3B;Supporting Information, Figure S13). Several different thermal events were observed for the Mn-Na 2 WO 4 /SiO 2 catalyst system in argon ( Figure 3B;S upporting Information, Figure S18). The DTAcurve features an endothermic event (1) at 226 8 8C, which is assigned to the phase transition of the a-cristobalite support to b-cristobalite. [12] Oxygen evolution from the catalyst is shifted to significantly lower temperatures when compared to the Mn 7 SiO 12 reference,with the onset recorded at 653 8 8C (2). This could be associated with the phase transition of cubic Na 2 WO 4 into unknown transient phases at 600 8 8C, the commencing formation of MnWO 4 at 650 8 8Ca sw ell as the disappearance of Mn 7 SiO 12 ,a lso observed from 650 8 8C onwards ( Figure 2B). Instead of aw ell-defined endothermic peak, the DTAcurve only displays minor irregularities in the temperature regime between 600 8 8Ca nd 700 8 8C( Figure 3B; Supporting Information, Figure S18). It is possible that the superposition of the melting and redox processes,asobserved via in situ XRD in the absence of air in this temperature range,limits the ability of DTAt oclearly detect melting.
Progressive surface mobility,i nitiated by restructuring of supported Na 2 WO 4 above 600 8 8Cand facilitated by melting of residual Na 2 WO 4 at higher temperatures,c an enable the reaction between Mn 7 SiO 12 and Na 2 WO 4 as shown in

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Equation (1). This,inturn, leads to the reduction of Mn 3+ (in Mn 7 SiO 12 )t oM n 2+ (in MnWO 4 ), the release of molecular oxygen with its maximum at 927 8 8C( (3) and (4) in Figure 3B), and the formation of amorphous or dispersed sodium oxide. Theb road temperature range for the oxygen release covers the onset temperature of oxidative coupling of methane (650 8 8C) as well as the reaction temperatures that are typically applied (700-800 8 8C). Aw eight loss of 0.2 % (0.22 mg) was caused by the event, with the DTAc urve indicating endothermicity (4). It is postulated that the process described in Equation (1) provides mobile lattice oxygen under steady-state operation of the catalyst. This is in agreement with O 2 -TPD experiments performed by Gordienko et al., [13] who identified two forms of lattice oxygen that may potentially contribute to catalysis upon desorption at relevant temperatures.T he source of oxygen has been attributed to manganese oxide, [6j, 7] or any unspecified lattice oxygen. [14] Potentially more decisive is the release of an active form of sodium oxide species that has been proposed to catalyse the oxidative coupling of methane by generation of OH radicals at high temperatures, [6l,m,o] presumably under involvement of homogeneous gas-phase reactions [15] due to the high volatility of sodium compounds under operation conditions. [16] Evidence,t hat the redox process described in Equation (1) occurs under reaction conditions and is indeed reversible,w as provided by operando Raman spectroscopy at different reaction temperatures and feed compositions (Figure 4; Supporting Information Figure S19). In all operando spectra ( Figures 4A and B), the region of 100-400 cm À1 is dominated by ab road band of the b-cristobalite support, which is formed via phase transition from a-cristobalite at 225 8 8C. [12a,c, 17] Thes pectra recorded under steady-state conditions in aC H 4 /O 2 /N 2 (4:1:4) feed at various temperatures ( Figure 4A)p redominantly feature bands of b-cristobalite and the remaining steady-state concentrations of Mn 7 SiO 12 (450-700 cm À1 )a nd Na 2 WO 4 (920-930 cm À1 ). At the highest reaction temperature,t he formation of MnWO 4 becomes apparent, as indicated by av ery weak band at 874 cm À1 . Carbon oxides,ethane,and water were identified as the main reaction products via online mass spectrometry and gas chromatography in the effluent gas of the Raman cell ( Supporting Information, Figures S20,S21). Significant formation of coke was not observed ( Figure 4A). The selectivity to C 2+ products was much lower compared to analogous experiments conducted in aq uartz fixed bed reactor without dilution of the catalyst (Supporting Information, Figure S14), which is attributed to consecutive reactions of the desired C 2+ products on the hot stainless-steel walls of the operando cell and the inadequate reactor geometry. However,comparison of the catalytic tests conducted with the empty operando cell and the same cell filled with the catalyst under identical reaction conditions clearly showed al ower activity and selectivity to C 2+ products for the empty reactor ( Figure 4C).
Changes to the spectral composition were observed upon switching to ar educing CH 4 /N 2 (4:5) feed at 700 8 8C ( Figure 4B). This was accompanied by as ignificant decline in catalyst performance and the formation of hydrogen and carbon monoxide as main products,s uggesting that lattice oxygen is consumed and gas-phase reactions as well as methane pyrolysis prevail under these conditions (Supporting Information, Figures S20 and Figure S21). Thec omplete removal of oxygen leads to the formation of MnWO 4 ,a s indicated by the band at 878 cm À1 (Figures 4B and D). Thus, ar eaction of Na 2 WO 4 and/or WO x with Mn 7 SiO 12 and/or MnO x to yield MnWO 4 occurs under reducing conditions.The formation of coke is excluded here based on Raman spectroscopy ( Figure 4A). During catalyst regeneration using the initial reaction feed (CH 4 /O 2 /N 2 = 4:1:4), braunite is suddenly reformed at the expense of MnWO 4 after approximately 8h ( Figure 4D). Moreover,d ue to this reversible phase transformation, the formation of C 2 products is reinitiated (Supporting Information, Figure S20 and Figure S21 after 8h), thereby highlighting the significance of the Mn 7 SiO 12 phase in maintaining catalytic activity.The presence of Mn 7 SiO 12 is also clearly responsible for the formation of CO 2 in place of CO (Supporting Information, Figure S21). As can be seen in Figure 4A,t he spectrum initially observed under steady-state conditions,i sr estored entirely by switching back to aCH 4 /O 2 /N 2 feed ( Figure 4A,red spectrum).

Conclusion
In conclusion, the present in situ and operando experiments disclosed reversible redox activity of the Mn 7 SiO 12 , MnWO 4, and Na 2 WO 4 phases under operation conditions in the oxidative coupling of methane over Mn-Na 2 WO 4 /SiO 2 .A new concept is proposed that involves the fluctuating storage and release of an active phase in heterogeneous catalysis. According to Equation (1), active sodium oxide species, which are responsible for high activity and selectivity in the oxidative coupling of methane, [6o] are generated in the catalytically relevant temperature regime in small amounts. Theextent of this reaction is controlled by the oxygen partial pressure in the gas phase and the redox chemistry on the surface (Scheme 1). While the structural synergy of all phases is responsible for the high stability and activity of the catalyst, the supported Na 2 WO 4 phase acts as storage phase responsible for transient generation of active sodium oxide species that would, in absence of the stabilizing Mn 7 SiO 12 -MnWO 4 redox couple,s uffer from steady sublimation, [16] thus leading to catalyst deactivation. [6b,g] As long as the oxygen partial pressure in the reactor is not too low,asmall steady-state concentration of the active phase is formed according to Scheme 1.
Phase transitions and melting of Na 2 WO 4 enable the generation of the active phase by providing mobile sodium species.F urthermore,t he supported Mn 7 SiO 12 phase was observed to function as oxygen-donor at working temperatures,w hich further enhances sodium oxide formation and, thus,h as implications for the reactivity.H owever,t he availability of adsorbed or lattice oxygen due to the presence of redox-active elements alone [6j, 7, 13, 14] cannot be responsible for the outstanding performance of the Mn-Na 2 WO 4 /SiO 2 catalyst and does not explain the mechanistic role and importance of Na in this system. On the other hand, ap ure silicasupported sodium oxide would rapidly deactivate under the severe reaction conditions applied in the oxidative coupling of methane. [6g] Only the chemical complexity of the Mn-Na 2 WO 4 /SiO 2 catalyst guarantees long-term stability.T he synergistic element combination discovered by chance is so successful because melting and redox reactions occur in the same temperature window (Figure 2a nd Figure 3).
In the presence of gas-phase oxygen, the phase transition of Mn 7 SiO 12 and Na 2 WO 4 into Na 2 Oand MnWO 4 [Eq. (1)] is largely impeded and generates only transient amounts of active [6o] sodium oxide species.A pparently,o nly as mall concentration of oxygen is necessary to keep the system in this highly active state (> 88 %o xygen conversion in the steady state,see Supporting Information, Figure S21, t > 8h). Such al ow concentration of oxygen in the gas phase is beneficial in terms of the selectivity.Only in total absence of oxygen is the system disturbed and MnWO 4 formed in Figure 4. Operando Raman experimentconducted on the Mn-Na 2 WO 4 /SiO 2 catalyst using 457 nm excitation. The catalyst was studied in atemperature range of 670-700 8 8Cu nder CH 4 /O 2 /N 2 = 4:1:4(A, black spectra), followed by exposure to CH 4 /N 2 = 4:5at700 8 8C(B) and subsequent regeneration under CH 4 /O 2 /N 2 = 4:1:4at700 8 8C(A, red spectrum) (total flow = 10 mL min À1 ;W /F = 0.0030 gmin mL À1 ). The Raman spectra under steady-state conditions(A) were collected using an exposure time of 30 min. X(CH 4 )a nd S(C 2 )indicate that the catalyst was active in the given temperature regime (C). By comparingt he collectedm ass spectrometry data that is representative for C 2 formation and the band intensities for Mn 7 SiO 12 (670 cm À1 )a nd MnWO 4 (878 cm À1 ), the re-formation of Mn 7 SiO 12 from MnWO 4 was found to be associated with an increase in catalytica ctivity during the catalyst regeneration phase. Scheme 1. Impeded phase transition:T ransient release of the active phase controlledb ythe partial pressure of oxygen in the gas phase and the surface redox chemistry. noticeable amounts.T he oxygen donor Mn 7 SiO 12 (Figure 3B), however, evidently hinders ac omplete and rapid transformation into MnWO 4 and Na 2 O( Figure 4B), which would lead to al oss of Na 2 Od ue to sublimation and progressive catalyst deactivation. Thed escribed scenario may also be considered as displacement of the redox chemistry from the organic to the inorganic part of the hybrid reaction system. Therefore,i nf uture concepts of catalyst design it might be reasonable to take into account that the activation of methane could also proceed via an acid-base reaction by using the strong base O 2À as catalyst avoiding radical chemistry in the selective pathway.
Our experiments clearly show that MnWO 4 is aproduct of catalyst deactivation, which is formed under strongly reducing reaction conditions.However,regeneration by increasing the partial pressure of oxygen in the feed again is possible ( Figures 4D and Supporting Information, Figure S21).
Thestudy in this report is an example for how in situ and operando Raman spectroscopy techniques can be applied as effective,n on-invasive tools to obtain valuable information on high-temperature catalysts under relevant operation conditions.B ased on experimental evidence,i ti sc learly explained how the chemical complexity of the Mn-Na 2 WO 4 / SiO 2 catalyst warrants ah igh yield of C 2 products and longterm stability in the oxidative coupling of methane.