Supplementary Materials for

Transcription

1 advances.sciencemag.org/cgi/content/full/3/6/e /dc1 Supplementary Materials for MnTiO3-driven low-temperature oxidative coupling of methane over TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst This PDF file includes: Pengwei Wang, Guofeng Zhao, Yu Wang, Yong Lu Published 9 June 2017, Sci. Adv. 3, e (2017) DOI: /sciadv Supplementary Text fig. S1. CH4 conversion and C2-C3 selectivity for the pure supports and the supported Mn2O3-Na2WO4 catalysts. fig. S2. SEM and EDX mapping images. fig. S3. XRD patterns and testing results of the 2.7Mn2O3-5.0Na2WO4/Ti-MWW and 2.7Mn2O3-5.0Na2WO4/TS-1 catalysts under different reaction conditions. fig. S4. XRD patterns and testing results for various samples with different Si:Ti molar ratio (or Ti content). fig. S5. XPS spectra of various samples. fig. S6. Raman spectra of various samples. fig. S7. Testing results of the catalysts with different active components. fig. S8. Ea calculations. fig. S9. Testing results of the catalysts with different active components prepared by the grinding method. fig. S10. H2-TPR profiles and XRD patterns. fig. S11. Effects of Mn2O3 plus TiO2, and Na2WO4 loadings on the OCM performance for the Mn2O3-TiO2-Na2WO4/SiO2 catalyst. fig. S12. Stability testing of the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst. fig. S13. Testing results and XRD patterns for the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst under different reaction conditions. table S1. Detailed treatment history of some catalysts for XRD and Raman measurements. table S2. Specific surface areas and real contents of Mn, W, Na, and Ti of all used catalysts.

2 table S3. Surface contents of W, Mn, Na, Ti, Si, O, and C measured by XPS for the used catalysts. table S4. CH4 conversion and C2-C3 selectivity over representative catalysts. References (38 49)

3 Supplementary Text Activation energy (Ea) calculations According to the reaction network of Stansch et al. (38), methane is converted in three parallel reactions, the formation of C2-C3 products (C2H6, C2H4, C3H8, and C3H6) by oxidative coupling of methane, the nonselective total oxidation of methane to carbon dioxide, and the partial oxidation of methane to carbon monoxide. Methane has been recognized to be oxidized on the catalyst surface through an adsorption step followed by a single hydrogen abstraction, C-H bond cleavage and methyl (CH3 ) radical coupling to form C2-C3 products in the OCM process. The pathway from CH4 to C2-C3 products (equation 1) was used to calculate the Ea of the OCM process CH4 + O2 C2H6 + C2H4 + C3H8 + C3H6 + H2O (1) The reaction rate of the OCM process on the Mn2O3-Na2WO4/Ti-MWW and Mn2O3- Na2WO4/SiO2 catalysts was determined according to equation 1 in the temperature range of o C, in which kinetics limit the rate (39). The reaction rate is defined as (2) where FCH4,in (mol s -1 ) is the molar flow rate of CH4 at the inlet and W (g) the mass of catalyst filled in the reactor. The kinetic experiments can be divided into two parts to determine the Ea. During the experiments, we kept the total gas flow constant and changed the weight of the catalysts. Fig. S8A shows the variation of the conversion of CH4 to C2-C3 products (i.e., C2-C3 products yield) against W/FCH4,in. The CH4 conversion to C2-C3 products changed linearly with W/FCH4,in and the reaction rate was calculated on the basis of the slope of the straight line at each temperature. Fig. S8B also shows the linear relationship of the Ln function of the reaction rate with the reciprocal of temperature for the Mn2O3-Na2WO4/SiO2 catalyst. According to the Arrhenius equation (Lnk = LnA - Ea/RT), Ea is the slope of the straight line in fig. S8B and was estimated to be kj mol -1, consistent with the reported results for the Mn2O3-Na2WO4/SiO2 catalyst ( 200 kj mol -1 ) (19, 38). For the Mn2O3-Na2WO4/Ti-MWW catalyst, Ea was estimated to be kj mol -1 (fig. S8, C and D).

4 Evolution of Mn species from MnTiO3 (or MnWO4) to Mn2O3 in an O2 stream and Mn2O3 reversed into MnTiO3 (or MnWO4) in a CH4 stream The Mn 2+ in the Mn2O3-Na2WO4/SiO2 catalyst reacted exclusively to MnWO4, while in the Mn2O3-Na2WO4/Ti-MWW catalyst Mn 2+ reacted to MnTiO3 and to a trace of MnWO4 (Fig. 2A). Mn 3+ on the other hand, reacted exclusively to Mn2O3 for both catalysts. Therefore, we chose the change in Mn 3+ amount to show the evolution of Mn species of the two catalysts in the stream of O2 or CH4. The Mn 3+ content was calculated from the height of the Mn2O3 XRD peak at 2θ = 33 o. All Mn species of the Mn2O3-Na2WO4/Ti-MWW catalyst (mainly MnTiO3 and some MnWO4), transformed fully and fast into Mn2O3 in the O2 stream (i.e., in 1 min at 800 o C, 2 min at 760 o C, and 4 min at 720 o C), and the height of the Mn2O3 XRD peak of this sample was taken as reference height (Mn 3+ content of 100%). The Mn 3+ contents of the other samples treated in the O2 stream was calculated by dividing the height of their Mn2O3 XRD peaks by the height of the reference peak. Subsequently, the Mn2O3 was mainly reduced to MnTiO3 as well as to a trace of MnWO4 in the CH4 stream, and the Mn 3+ contents of these samples treated in CH4 stream was calculated in the same way from the heights of the XRD peaks. All MnWO4 species of the Mn2O3-Na2WO4/SiO2 catalyst could be fully transformed into Mn2O3 in the O2 stream, but relatively slowly (i.e., 3 min at 800 o C, 15 min at 760 o C, and 30 min at 720 o C). The height of the Mn2O3 XRD peak of this sample was taken as reference height (Mn 3+ content of 100%). The Mn 3+ contents of the other samples treated in the O2 stream were calculated by dividing the height of their Mn2O3 XRD peaks by the height of the reference peak. Subsequently, the Mn2O3 was reduced to MnWO4 in the CH4 stream, and the Mn 3+ contents of the resulting samples was calculated in the same way from the heights of the XRD peaks. Transition rate of Mn 2+ to Mn 3+ The half-cycle transition rate of Mn 2+ to Mn 3+ (%/min) was defined as the change of Mn 3+ (%) per minute within the linear portion of the curve (Fig. 2, D to F) in the O2 stream. For example, the transition rate of Mn 2+ to Mn 3+ for the Mn2O3-Na2WO4/Ti-MWW catalyst at 760 o C (Fig. 2E) was calculated from the linear portion of this curve (i.e., the

5 time range from 0 to 2 min). With a change of Mn 3+ (%) of 74% in this range, the transition rate was 37%/min (74% divided by 2 min). Similarly, for the Mn2O3- Na2WO4/SiO2 catalyst at 760 o C (Fig. 2E), the linear portion is from 0 to 10 min with a Mn 3+ (%) change of 70%, so that the transition rate is 7%/min (70% divided by 10 min). The transition rates of the Mn 3+ species for both catalysts at 720 and 800 o C were obtained by the same method. Notably, selecting the data within the linear portion is to make the calculation results more accurate. H2-TPR and XRD analysis The Mn2O3-TiO2-Na2WO4/SiO2 catalysts with different Mn2O3 loadings (4.3, 7.1, and 10 wt%) were characterized by H2-TPR (fig. S10A). Two peaks at o C and o C were generated for each Mn2O3-TiO2-Na2WO4/SiO2 catalyst. Na2WO4 contributes to the peak at o C because pure MnWO4 (not shown) and the Na2WO4/SiO2 sample generated only one H2-TPR-peak in this range (purple line in fig. S10A). Moreover, clear MnWO4 diffraction (2θ of 29.9 and 30.3 o ) was shown in the XRD patterns of the three catalysts (fig. S10B). Therefore, the peaks at o C of our catalysts should be generated by both MnWO4 and Na2WO4. The peaks at o C exhibited an increased area with increasing XRD-intensity of Mn2O3 (fig. S10B) and, accordingly, Mn2O3 should contribute mainly to these peaks. In addition, the XRD patterns in fig. S10B showed clear MnTiO3 (2θ of 32.2 and 35 o ) and TiO2 (2θ of 25.3 o ) diffractions. As a reference, the pure sample of MnTiO3 as well as Mn2O3 and TiO2, was also characterized by H2-TPR (fig. S10C). Their reduction temperatures are much lower than that of Na2WO4. Combining the fact that there was only one peak at o C, the Mn2O3 and MnTiO3 as well as TiO2 should be reduced at the same temperature. Therefore, the peaks at o C should be mainly generated by Mn2O3 as well as by MnTiO3 and TiO2. However, the reduction temperature of o C is much higher than those of pure Mn2O3 at 542 o C and MnTiO3 at 473 o C. Such increases in Mn2O3 and MnTiO3 reduction temperatures are likely caused by the Mn2O3- Na2WO4 and MnTiO3-Na2WO4 interactions, which increase their reduction temperatures with the result of improved catalyst selectivity (9).

6 fig. S1. CH4 conversion and C2-C3 selectivity for the pure supports and the supported Mn2O3-Na2WO4 catalysts. The pure supports include Ti-MWW and TS-1 zeolites (Si:Ti molar ratio of 40:1), pure anatase-tio2, Ti2O3, amorphous SiO2-gel, and perovskite CaTiO3. The catalysts were prepared by loading 2.7 wt% Mn2O3 and 5.0 wt% Na2WO4 on these pure supports. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air. Note: The C3 selectivity was 3-5%, 2-3%, and 0-2% for all catalysts at a C2-C3 total selectivity above 60%, between 40 and 60%, and below 40%, respectively.

7 fig. S2. SEM and EDX mapping images. SEM and EDX mapping images of W, Mn, and Na elements of the used Mn2O3-Na2WO4/Ti-MWW (denoted as Cat-Ti-MWW), Mn2O3-Na2WO4/TS- 1 (denoted as Cat-TS-1), and Mn2O3-Na2WO4/SiO2 (denoted as Cat-SiO2) catalysts (see detailed treatment history in table S1). The morphologies of the three catalysts are different, but the EDX mapping images indicate that their dispersions are similar.

8 fig. S3. XRD patterns and testing results of the 2.7Mn2O3-5.0Na2WO4/Ti-MWW and 2.7Mn2O3-5.0Na2WO4/TS-1 catalysts under different reaction conditions. XRD patterns of the 2.7Mn2O3-5.0Na2WO4/Ti-MWW catalyst (red line) and the 2.7Mn2O3-5.0Na2WO4/TS-1 catalyst (light dark line) after directly undergoing OCM reaction at 720 o C, and the 2.7Mn2O3-5.0Na2WO4/TS-1 catalyst after undergoing OCM reaction at 800 o C and subsequently at 720 o C (navy line), as well as CH4 conversion (CCH4) and C2-C3 selectivity (SC2-3) at 720 o C over these catalysts. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air. Note: the C3 selectivity was 4% for the 2.7Mn2O3-5.0Na2WO4/Ti-MWW catalyst when directly tested at 720 o C, 1-2% for the 2.7Mn2O3-5.0Na2WO4/TS-1 catalyst when directly tested at 720 o C, and 3-4% for the 2.7Mn2O3-5.0Na2WO4/TS-1 catalyst after reaction at 800 o C followed by testing at 720 o C.

9 fig. S4. XRD patterns and testing results for various samples with different Si:Ti molar ratio (or Ti content). (A) XRD patterns of the as-prepared Ti-MWW zeolites with Si:Ti molar ratio from 80:1 to 40:1 as well as the full Si zeolite also with MWW structure. (B) XRD patterns of the as-prepared TS-1 zeolites with Si:Ti molar ratio from 80:1 to 40:1 as well as the full Si zeolite also with MFI structure. (C) The Mn2O3-Na2WO4/TS-1 catalysts with different Si:Ti molar ratio in TS-1 zeolites after directly running at 720 o C. (D) The Mn2O3-Na2WO4/TS-1 catalysts with different Si:Ti molar ratio in TS-1 zeolites after running at 800 o C for 2 hours and subsequently at 720 o C for 0.5 hour. (E) CH4 conversion for the two series of catalysts in fig. S4, C and D at 720 o C. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air.

10 fig. S5. XPS spectra of various samples. (A) Mn 2p XPS of pure Mn2O3 and MnTiO3. (B) Mn 2p XPS, (C) W 3d XPS, (D) Na 1s XPS, (E) O 1s XPS, and (F) Si 1s XPS of the Cat-Ti-MWW, Cat-TS-1, and Cat-SiO2 catalysts (see detailed treatment history in table S1). Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air.

11 fig. S6. Raman spectra of various samples. Raman spectra of pure MnOx (MnO, Mn3O4, Mn2O3 and MnO2), TiOx (Ti2O3, rutile-tio2 and anatase-tio2), Na2WO4, WO3, MnWO4, and MnTiO3, and of the Mn2O3-Na2WO4/SiO2 (denoted as Cat-SiO2), Mn2O3-Na2WO4/Ti-MWW (denoted as Cat-Ti-MWW), and Mn2O3-Na2WO4/TS-1 catalysts. Note: the Mn2O3-Na2WO4/SiO2 (denoted as Cat-SiO2) and Mn2O3-Na2WO4/Ti-MWW (denoted as Cat-Ti-MWW) catalysts experienced the OCM reaction at 800 o C for 2 hours and subsequently at 760 and 720 o C for 0.5 hour; the Mn2O3- Na2WO4/TS-1 catalyst experienced the OCM reaction directly at 720 o C for 0.5 hour.

12 fig. S7. Testing results of the catalysts with different active components. CH4 conversion (blue bar) and C2-C3 selectivity (gray bar) over the MnTiO3/SiO2, Mn2O3-Na2WO4/SiO2, Mn2O3/SiO2, Na2WO4/SiO2, and Mn2O3-Na2WO4/Ti-MWW catalysts. Fresh catalyst was employed for each testing at the appointed temperature. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air. Note: The C3 selectivity was 3-5%, 2-3%, and 0-2% for all catalysts at a C2-C3 total selectivity above 60%, between 40 and 60%, and below 40%, respectively.

13 fig. S8. Ea calculations. (A) Variation of the fractional conversion of CH4 to C2-C3 products (i.e., C2-C3 yield) against W/FCH4, and (B) Ln(Rate) as a function of 1000/T for the Mn2O3- Na2WO4/SiO2 catalyst. (C) Variation of fractional conversion of CH4 to C2-C3 products (i.e., C2- C3 yield) against W/FCH4, and (D) Ln(Rate) as a function of 1000/T for the Mn2O3-Na2WO4/Ti- MWW catalyst. Reaction conditions: a feed of 50% CH4 in air.

14 fig. S9. Testing results of the catalysts with different active components prepared by the grinding method. CH4 conversion (blue bar) and C2-C3 selectivity (gray bar) over the following catalysts prepared by the grinding method: MnTiO3-Mn2O3-Na2WO4, MnWO4-Mn2O3-Na2WO4, MnTiO3-Mn2O3, and MnWO4-Mn2O3. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air. Note: The C3 selectivity was 3-5%, 2-3%, and 0-2% for all catalysts at a C2-C3 total selectivity above 60%, between 40 and 60%, and below 40%, respectively.

15 fig. S10. H2-TPR profiles and XRD patterns. (A) H2-TPR profiles and (B) XRD patterns for the Mn2O3-TiO2-Na2WO4/SiO2 catalysts prepared by the grinding method (different Mn2O3 loadings of 4.3, 7.1, and 10 wt%). The loading of Na2WO4 was 10 wt%, and amorphous SiO2-gel as well as TiO2 made up the balance (with Si:Ti molar ratio of 20:1). (C) H2-TPR profiles for the pure Mn2O3, MnTiO3, and rutile-tio2.

16 fig. S11. Effects of Mn2O3 plus TiO2, and Na2WO4 loadings on the OCM performance for the Mn2O3-TiO2-Na2WO4/SiO2 catalyst. The catalysts with varied Mn2O3 plus TiO2 loadings (i.e., with stoichiometric ratio of Mn2O3 to TiO2 to be fully transformed into MnTiO3) and Na2WO4 loadings for the Mn2O3-TiO2-Na2WO4/SiO2 catalysts were prepared by grinding method. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air, 720 o C. Note: The C3 selectivity was 3-5%, 2-3%, and 0-2% for all catalysts at a C2-C3 total selectivity above 60%, between 40 and 60%, and below 40%, respectively.

17 fig. S12. Stability testing of the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst. CH4 conversion and C2-C3 selectivity along with the time on stream at 800 o C. Reaction conditions: GHSV of 8000 ml gcat. -1 h -1 of a feed of 50% CH4 in air. The C3 selectivity was 3-5%.

18

19 fig. S13. Testing results and XRD patterns for the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst under different reaction conditions. (A) CH4 conversion (blue bar) and C2-C3 selectivity (gray bar) for the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst directly running at 650, 680, 700, and 720 o C (fresh sample was employed for each testing experiment), and then 650 o C again (i.e., after reaction at 720 o C for 1 h and then at 650 o C for another 1 h). (B) XRD patterns of the 6.2Mn2O3-6.3TiO2-10Na2WO4/SiO2 catalyst after directly running at 650 o C (for 1, 4, 8, and 12 hours) and 720 o C (for 1 and 8 hours). Note: The C3 selectivity was 3-5%, 2-3%, and 0-2% for all catalysts at a C2-C3 total selectivity above 60%, between 40 and 60%, and below 40%, respectively.

20 table S1. Detailed treatment history of some catalysts for XRD and Raman measurements. Catalyst Treatment history The catalysts in the section of In-situ MnTiO3 formation dependent low-temperature activity improvement Mn2O3-Na2WO4/Ti-MWW denoted as Cat-Ti-MWW Mn2O3-Na2WO4/TS-1 denoted as Cat-TS-1 Mn2O3-Na2WO4/SiO2 denoted as Cat-SiO2 These catalysts firstly reacted in the OCM reaction at 800 o C for 2 hours. Then, the catalyst bed temperature was gradually reduced to 720 o C, and these catalysts reacted for 0.5 hour. The catalysts in the section of MnTiO3-triggered Mn 2+ Mn 3+ chemical cycle the nature of low-temperature OCM catalysis Mn2O3-Na2WO4/Ti-MWW denoted as Ti-MWW catalyst in Fig. 2A-C,G-I Mn2O3-Na2WO4/SiO2 denoted as SiO2 catalyst in Fig. 2A-C,G-I The two catalysts (under working state) were reduced at 800 o C for 0.5 hour in CH4 stream. Then, the catalysts were treated respectively at 800, 760, 720 o C for some time in O2 stream. Finally, the catalysts were reduced respectively at 800, 760, 720 o C for some time in CH4 stream. table S2. Specific surface areas and real contents of Mn, W, Na, and Ti of all used catalysts. Catalyst a SSA (m 2 /g) Pore Size (nm) Contents (wt%) b Mn W Na Ti Mn2O3-Na2WO4/Ti-MWW (Cat-Ti-MWW) Mn2O3-Na2WO4/TS-1 (Cat-TS-1) Mn2O3-Na2WO4/SiO2 (Cat-SiO2) a The OCM reaction was run over all catalysts for 2 hours at 800 o C followed by at 720 o C for 0.5 hour using 50 vol% CH 4 in air at 720 o C with a total GHSV of 8000 ml g -1 h -1. b All contents were determined by AES-ICP measurements.

21 table S3. Surface contents of W, Mn, Na, Ti, Si, O, and C measured by XPS for the used catalysts. Catalyst a Elements contents (mol %) Na Mn W Ti Si O C Mn2O3-Na2WO4/Ti-MWW Mn2O3-Na2WO4/TS Mn2O3-Na2WO4/SiO a The OCM reaction was run over all catalysts at 800 o C for 2 hours and subsequently at 720 o C for 0.5 hour using a feed of 50 vol% CH 4 in air with a total GHSV of 8000 ml g -1 h -1.

22 table S4. CH4 conversion and C2-C3 selectivity over representative catalysts. Catalyst Temp. ( o C) v a (ml/(min g)) C:O:X b Conv. (%) Select. (%) C2-3Yield (%) Ref. Li/MgO :8: Ba/MgO :1: NaMnO4/MgO :10: La2O :2: La2O3/MgO :20: SrLa2O :9: LiSrLa2O :15: LiCa2Bi3O4Cl :10: Bi1.5Y0.3Sm0.2O3-x :10: Rb2WO4/SiO : Mn-Na2WO4/SiO :2: Mn-Na2WO4/SiO :1: Mn-Na2WO4/SBA :1: Mn2O3-6.3TiO2-10Na2WO4/SiO :1: c 20.5 p.w. d :1: c 17.5 p.w. d :1: c 13.6 p.w. d a Gas hourly space velocity. b CH 4/O 2/balance (N 2, He, or Ar in the corresponding reference). c Including C 3 selectivity of 3-5%. d Present work.