Chemistry

Low-temperature selective oxidation of methane over distant binuclear cationic facilities in zeolites

DFT calculations

The doable exercise of binuclear cation M(II) buildings within the abstraction of O from N2O and the next formation of the α-oxygen was investigated by periodic DFT calculations for Co(II) and Fe(II) cations. The calculations had been carried out for a mannequin with two M(II) cations accommodated in two adjoining β websites of ferrierite3 similar to the totally ion exchanged ferrierite zeolite with M(II) cations. The computed mechanisms are the identical for each Co(II) and Fe(II). Solely the outcomes for Co(II) are described intimately (Fig. 1) whereas these computed for Fe(II) are mentioned intimately in our prior study3 (see Supplementary Desk 1) which nonetheless used one other DFT useful and completely different technical settings (particulars are in Part digital construction calculations).

Fig. 1figure1

Optimized buildings. a The two adjoining β websites of Co-ferrierite 1 after molecular dynamics simulations. b The Co…NNO Co advanced 2 fashioned within the two adjoining β websites. c The transition state TS created within the two adjoining β websites. d The Co…NN O=Co intermediate three fashioned within the two adjoining β websites. e The Co O=Co product four created within the two adjoining β websites. The distances are in Å. Silicon atoms are in grey, oxygen atoms in crimson, aluminum atoms in yellow, cobalt atoms in violet, and nitrogen atoms in blue. Schematic vitality profile (in kcal mol−1) f The formation of the α-oxygen

Subsequently, the outcomes for Fe(II) had been recalculated to permit the direct comparability of the computational outcomes for Co(II) with these for Fe(II).

Firstly, N2O adsorbs by the terminal N atom to create a [Co…NNO Co] advanced (2) by which the O atom of N2O is properly positioned to assault the naked Co(II) cation within the adjoining β web site. The calculated adsorption vitality is −15.four kcal/mol (−16.1 kcal/mol for Fe(II)-ferrierite). Consequently, the N–O bond is cleaved and the naked Co(II) cation within the adjoining β web site is oxidized to yield a [Co…NN O=Co] intermediate (three) by way of the [Co-NNO-Co] transition state (TS). The corresponding computed barrier is 25.zero kcal/mol. This worth signifies that the oxidation of Co(II) to offer the α-oxygen must be facile however considerably extra sluggish than the identical response step on Fe(II)-ferrierite (i.e., the barrier of 14.5 kcal/mol). The calculated vitality for this response step is −6.7 kcal/mol (−14.2 kcal/mol for Fe(II)-ferrierite). Subsequently, N2 desorbs, which prices 11.5 kcal/mol (12.5 kcal/mol for Fe(II)-ferrierite), and the [Co O=Co] product (four) that includes the α-oxygen is fashioned. Moreover, our calculations reveal that N2O doesn’t adsorb on Co(II) by the oxygen atom indicating that the α-oxygen can’t be created on remoted Co(II) cations.

Distant binuclear M(II) cationic facilities

The formation of distant binuclear M(II) cationic facilities in zeolites requires fulfilling three circumstances. (i) The presence of two adjoining 6-rings or Eight-rings capable of type cationic websites for naked divalent cations. The 2 adjoining rings should face one another. (ii) Every of the 2 rings has to include two Al atoms (i.e., 4 Al atoms in whole). The 2 rings, subsequently, can type two adjoining cationic websites for naked divalent cations. (iii) The occupation of the 2 adjoining cationic websites by two divalent cations. Numerous reactions on distant binuclear M(II) cationic facilities require completely different optimum distances between the 2 adjoining cations.

Two adjoining 6-rings creating the β websites within the Eight-ring channel of M(II)-ferrierite (the calculated M–M distance is ca 7.four Å) are candidates to type the distant binuclear M(II) cationic facilities. The Si/Al ratio of the ferrierite sample3,17 is Eight.6 which means that there are in common three.75 Al atoms per unit cell. The prior research confirmed that the focus of Al pairs within the β web site is excessive (50% of all of the Al atoms)17 so ca. 94% of the 6-rings of the β web site can accommodate naked divalent cations. Subsequently, no less than 88% (zero.94**2) of the β websites are capable of type binuclear M(II) structures3. The investigated Co(II)-ferrierite and Ni(II)-ferrierite are ion exchanged (M/Al ≥ zero.25) near the utmost loading of naked divalent cations (M/Al zero.33)17. Subsequently, virtually all of the β websites are occupied and binuclear M(II) cationic species are unambiguously fashioned. Assuming the worst-case situation that each one different cationic websites are occupied by M(II) earlier than the formation of the binuclear species, it’s assured that no less than 50% of M(II) cations are within the type of the binuclear species when M/Al > zero.2217. Conversely, Fe(II) cations had been launched into the ferrierite pattern utilizing acetylacetonate2,three,18. This process ensures the creation of binuclear Fe(II) buildings in all occupied β websites even on the lowest Fe loadings Fe/Al zero.04 employed to stop the formation of Fe-oxidic species2,three,18,19.

Dissociation of N2O over distant binuclear cations

The computational outcomes (Fig. 1) present that this ferrierite zeolite can serve for the preparation of the α-oxygen on Co-zeolites. Subsequently, we experimentally examined our theoretical predictions relating to Co(II) utilizing the identical ferrierite father or mother zeolite as that employed within the prior studies2,three,17. As well as, one other divalent transition metallic cation Ni(II), which was not calculated for technical causes, was investigated as properly. Furthermore, Fe(II)-ferrierite in addition to a second Fe(II)-ferrierite pattern, containing isotopically enriched 57Fe (i.e., 57Fe(II)-ferrierite) used for Mössbauer spectroscopy experiments, had been explored for comparability. Moreover, we additionally investigated the character of the α-oxygen fashioned on the Co(II)-ferrierite, Ni(II)-ferrierite, and Fe(II)-ferrierite samples. The selective oxidation of methane was used as a check of the α-oxygen.

FTIR spectroscopy of naked divalent cations

FTIR spectroscopy of the shifted antisymmetric T-O-T stretching vibrations of the lattice induced by binding naked Co(II) cations to the framework oxygens was employed to analyze the formation of the α-oxygen and its exercise within the oxidation reactions. This methodology permits evaluation of (i) naked divalent cations accommodated within the particular person cationic websites and (ii) the formations of complexes of divalent cations with visitor molecules2,19,20,21,22,23,24.

Though the construction of the α-oxygen in Fe-zeolite was not too long ago analyzed by the mix of assorted spectroscopies supported by quantum chemical calculations4, the α-oxygen on Fe is outlined because the lively oxygen species fashioned by the N2O oxidation and succesful to oxidize hydrogen, CO1, benzene25,26, and methane1,27. Furthermore, the unambiguous spectroscopic characterization of the α-oxygen on different transition metallic cations than Fe by the strategy reported by Snyder et al4. would signify a frightening activity, so the affirmation of the oxidation exercise of the α-oxygen species is crucial. Methane represents a check molecule of the primary selection because of the financial significance of the selective oxidation of CH4, and furthermore, the requirement of testing the excessive exercise of the α-oxygen for selective oxidations. As well as, evaluation of the FTIR spectra measured after the interplay of the oxidized M-zeolite with methane permits the detection of the oxygen-containing merchandise.

The FTIR spectra within the area of the shifted antisymmetric T−O−T stretching lattice modes (Fig. 2) confirmed (i) the presence of naked Co(II), Ni(II), and Fe(II) cations within the ferrierite matrix, (ii) the interplay of all of the three M(II) cations with N2O at ambient temperature and at 200 °C, and (iii) the successive interplay with methane at ambient temperature and at 200 °C.

Fig. 2figure2

FTIR T−O−T spectra of the M-ferrierites. a Co-ferrierite. b Ni-ferrierite. c Fe-ferrierite. The samples are evacuated at 450 °C for three h. The bands at 943, 940, and 935 cm−1 correspond to Co(II), Ni(II), and Fe(II) cations, respectively, accommodated within the α web site. The wavenumbers at 920, 920, and 915 cm−1 relate to Co(II), Ni(II), and Fe(II) cations, respectively, sure within the β web site. The bands at 900 cm−1 are linked with the γ websites in Co-ferrierite and Ni-ferrierite, whereas this cationic web site just isn’t occupied in Fe-ferrierite because of the low Fe/Al ratio. The tick mark labels on the y-axis are at zero.zero, zero.2, zero.four, …

Mössbauer spectroscopy experiments

Mössbauer spectroscopy was used for evaluation of the oxidation state and the coordination of the Fe species within the 57Fe-ferrierite pattern and to help the interpretations of the FTIR spectra. Mössbauer spectra of the 57Fe-ferrierite pattern recorded underneath the identical circumstances because the FTIR ones are proven in Fig. three, and the corresponding Mössbauer parameters are listed in Desk 1.

Fig. threefigure3

Mössbauer spectra with their suits of 57Fe-ferrierite. a 57Fe-ferrierite after the next therapy: three h evacuation at 450 °C for three h. b 57Fe-ferrierite after the next therapy: an evacuation at 450 °C for three h then an interplay with N2O (40 Pa) at 200 °C for 30 min and after that a N2O desorption at 200 °C for five min. c 57Fe-ferrierite after the next therapy: an evacuation at 450 °C for three h, then an interplay with N2O (40 Pa) for 30 min at 200 °C, and subsequently, a N2O desorption at 200 °C for five min, and after that a CH4 (40 Pa) adsorption for 30 min at 200 °C, and subsequently, a CH4 desorption at ambient temperature

Desk 1 The parameters of the Mössbauer spectra

FTIR spectroscopy of cations after the interplay with N2O

The FTIR spectra of the M-ferrierite samples evacuated at 450 °C exhibited two most important bands within the area of the shifted antisymmetric T−O−T stretching lattice modes at round 940 and 920 cm−1, which verify the presence of M(II) cations within the α and β cationic websites, respectively (Fig. 2). The shifted antisymmetric T-O-T stretching vibrations describe the redox conduct of the M species. FTIR spectroscopy of the evacuated samples revealed that the M(II) cations accommodated in ferrierite are predominantly situated within the β web site for all of the three M-ferrierite samples. As well as, Mössbauer spectroscopy of the 57Fe-ferrierite matrix confirmed the presence of Fe within the α and β websites as properly (Desk 1 and Fig. three).

The FTIR measurements of the interplay of N2O with the evacuated samples at ambient temperature (zero.5–14 min) (the spectra (i) in Fig. four) and after the desorption at 200 °C for five min (the spectra (ii) in Fig. four) reveal a disappearance of the bands of the M(II) cations within the β cationic positions, and moreover, a formation of two new bands at ca 950 and ca 880 cm−1.

Fig. fourfigure4

FTIR T−O−T spectra of the oxidized M-ferrierites. a Co-ferrierite. b Ni-ferrierite. c Fe-ferrierite. The M-ferrierite samples work together with N2O at ambient temperature for zero.5–14 min (i). The spectra after the next desorption of N2O at 200 °C for five min (ii). The tick mark labels on the y-axis are at zero.zero, zero.2, zero.four, …

The previous band might be attributed to the ligand advanced M(II) with N2O22 whereas the latter band, which is weaker, signifies the rise of the cation–zeolite interplay and might be attributed to the [Me=O]2+ advanced that includes the α-oxygen analogously to the Fe-ferrierite19. The newly fashioned species similar to the band at about 880 cm−1 is proof against an evacuation at 200 °C for five min (the spectra (ii) in Fig. four). The formation of the α-oxygen within the 57Fe-ferrierite pattern is noticed within the Mössbauer spectrum as properly (Desk 1). The corresponding Mössbauer parameters have been already reported4,19,28. Be aware that the formation of the α-oxygen for M-zeolite (M=Co and Ni) with a low M loading (i.e., M/Al ~ zero.1) was not noticed though the ratios ([α]/[β]) of the occupations of the α and β websites by M(II) had been much like these of the extremely exchanged M(II)-ferrierites. Conversely, the α-oxygen is created within the Fe-ferrierite pattern with the Fe/Al ratio of zero.04 because the process of the introduction of Fe(II)three,18 ensures the creation of binuclear Fe(II) buildings in all occupied β websites even at this very low Fe(II) loading.

The interactions of N2O with the evacuated M(II)-samples had been investigated by FTIR at ambient temperature and 200 °C (Fig. four) to check the reactivity of the three M(II) pairs within the formation of the α-oxygen. The depth of the band at round 880 cm−1 similar to the α-oxygen fashioned over the Fe-sample reached its regular state in four min whereas it took 6 and 14 min for the Co-ferrierite and Ni-ferrierite samples, respectively, to attain the regular state. It must be famous that the α-oxygen just isn’t the ultimate product of the response.

Examine of the character of the α-oxygen

The α-oxygen fashioned on the M(II)-ferrierite samples was investigated using the selective oxidation of methane monitored by FTIR spectroscopy (Fig. 5).

Fig. 5figure5

FTIR T−O−T spectra the oxidized and lowered M-ferrierites. a Co-ferrierite. b Ni-ferrierite. c Fe-ferrierite. The M-ferrierite samples after the desorption of N2O at 200 °C for five min (i), the interplay with CH4 at ambient temperature for 1 min (ii), the interplay with CH4 at 200 °C for 25 min (iii), the desorption of CH4 at 200 °C for five min (iv). The tick mark labels on the y-axis are at zero.zero, zero.2, zero.four, …

The M(II)-zeolites had been oxidized by N2O at ambient temperature for four–14 min, adopted by the N2O desorption at 200 °C for five min (the spectra (i) in Fig. 5) and consequently methane was launched and reacted with the α-oxygen at ambient temperature for 1 min (the spectra (ii) in Fig. 5) and at 200 °C for 25 min (the spectra (iii) in Fig. 5). The FTIR spectra (the spectra (i), (ii), and (iii) in Fig. 5) clearly reveal a lower of the depth of the bands at round 880 cm−1 similar to the α-oxygen.

Evaluation of the FTIR spectra (Fig. 6) within the area of the C–H stretching band (3000–2830 cm−1) recorded after 25 min of the interplay with methane at 200 °C reveals the presence of the band at 2960 cm−1 similar to formate species sure to the iron cation, the band at 2853 cm−1 attributed to CH3OH, and the band at 2920 cm−1 which is assigned to methoxy group sure to the iron cation (Fig. 6a–c)7,10,13.

Fig. 6figure6

FTIR spectra of the M-ferrierite samples. a Co-ferrierite. b Ni-ferrierite. c Fe-ferrierite. d Co-ferrierite. e Ni-ferrierite. f Fe-ferrierite. The spectra are proven within the areas 3000–2830 cm−1 and 1800–1290 cm−1. The spectra are recorded after the interplay of the M-ferrierite samples with N2O at 200 °C for 25 min adopted by the 5 min N2O desorption at 200 °C, and the next interplay with CH4 at 200 °C for 10 min. g The mass spectrometry outcomes for the Co-ferrierite, Ni-ferrierite, and Fe-ferrierite. Methanol was detected after the oxidation of the samples by N2O at 200 °C after which the fashioned α-oxygen was titrated by methane on the similar temperature. The *band within the area 1600–1621 cm−1 represents the bending mode of adsorbed water13

Bands within the area of δCH3, δCOH, and νCO vibrations (1800–1290 cm−1) seem at (i) 1666 and 1355 cm−1 assigned to formate anions sure to the iron cation and (ii) 1642 cm−1 attributed to formaldehyde adsorbed on the iron cation (Fig. 6a–c)13. It must be burdened that though the FTIR spectra recorded after methane oxidation over the M-ferrierites function bands of equally low depth as these already reported for Fe-ZSM-5 underneath comparable conditions7,10, the previous spectra are considerably extra advanced because of the formation of a variety of oxidation merchandise on the M-ferrierites. The presence of the bands characterizing methanol, formaldehyde, and formate point out that the protonation of methoxy teams within the M-ferrierites with binuclear M(II) species is extra facile in comparison with remoted Fe(II) cations in Fe-ZSM-5 for which solely methoxy species had been acknowledged within the FTIR spectra7,10. Subsequently, an extraction by water steam was required to acquire methanol from Fe-ZSM-57,10. Conversely, the presence of methanol and different oxidation merchandise signifies that such an extraction might not be wanted for all of the three investigated M-ferrierite samples. This truth is additional confirmed by FTIR spectra of all of the three M-ferrierites recorded after the interplay with methane at 200 °C and the successive evacuation at 200 °C. The FTIR spectra reveal (i) full disappearance of the band at ca 880 cm−1 similar to the α-oxygen and (ii) the reappearance of the band at round 920 cm−1 characterizing naked M(II) cations accommodated within the β web site (the spectra (iv) in Fig. 5). It must be famous that the α-oxygen is steady throughout the desorption at 200 °C for five min (spectra (ii) in Fig. four). Subsequently, the emergence of the band at about 920 cm−1 which characterizes naked divalent cations situated within the β web site additional signifies the protonation of methoxy teams, and subsequently, the formation of unstable merchandise of the oxidation of methane and confirms the identification of the merchandise of methane oxidation within the FTIR spectra. This conclusion is additional supported for the 57Fe-ferrierite pattern by Mössbauer spectroscopy (Desk 1 and Fig. 3c) which evidences the predominant presence of naked Fe(II) cations with the parameters an identical with these noticed for the evacuated 57Fe-ferrierite pattern earlier than the oxidation by N2O. The above reported greater protonation exercise of all of the three M-ferrierite samples (i) leads to the creation of the protonated oxidation merchandise (i.e., methanol, formaldehyde, and formic acid) and (ii) might be defined by a better exercise of the ferrierite zeolite to type protonated adducts29.

A titration of the α-oxygen by methane was carried out by through-flow experiment with the merchandise of methane oxidation monitored by a quadrupole mass spectrometer to substantiate the protonation of methoxy teams and to elucidate the composition of the merchandise of the oxidation of methane. All of the three M-ferrierite samples (i.e., M=Co, Ni, and Fe) after the activation in helium had been oxidized by N2O at 200 °C. Then, the fashioned α-oxygen was titrated by methane on the similar temperature. The formations of methanol (m/z = 31), different doable merchandise (m/z = 29) of the oxidation of methane (i.e., formaldehyde, formic acid, and dimethyl ether), and CO2 (m/z = 44) had been monitored by mass spectrometry (Fig. 6). The yields of the produced methanol per gram of zeolite are listed in Desk 2. The quantity of formaldehyde, formic acid, and dimethyl ether all collectively per one M(II) cation was estimated to be lower than ca zero.04 mol/molM and solely traces of CO2 had been noticed. The manufacturing of methanol associated to at least one M(II) cation (Desk 2) didn’t exceed zero.5 molCH3OH/molMe. This consequence additional confirms that the lively websites for the formation of the α-oxygen and methanol oxidation signify two cooperating M(II) cations.

Desk 2 Chemical composition of the studied M-ferrierite samples


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