Brønsted/Lewis Acid Synergy in Methanol-to-Aromatics Conversion. on Ga-Modified ZSM-5 Zeolites as Studied by Solid-State NMR

Transcription

1 Brønsted/Lewis Acid Synergy in Methanol-to-Aromatics Conversion on Ga-Modified ZSM-5 Zeolites as Studied by Solid-State NMR Spectroscopy Pan Gao,,, Qiang Wang,, Jun Xu,*, Guodong Qi, Chao Wang, Xue Zhou,, Xingling Zhao,, Ningdong Feng, Xiaolong Liu, Feng Deng*, State Key Laboratory Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan , P. R. China. University of Chinese Academy of Sciences, Beijing , P. R. China. *

2 1. Materials and Sample Preparation H-ZSM-5 zeolite (Si/Al=12.5, purchased from the Catalyst Plant of Nankai University, China) and Ga(NO 3 ) 3 xh 2 O (99.99% metal basis, purchased from Aladdin Reagent Co., Ltd.) were used as starting materials. In a typical impregnation experiment, 3g of H-ZSM-5 was added into 5 ml of Ga(NO 3 ) 3 solutions containing g Ga(NO 3 ) 3 xh 2 O. The resulting mixture was dried at 323 K under stirring and subsequently calcined under flowing air at 823 K for 8 h. The obtained zeolite was denoted as Ga/ZSM-5(IM), containing a Ga loading of ~6 wt.%. Thereafter, 1g of the Ga/ZSM-5(IM) was pressed and crushed into pellets between 40~60 mesh and placed into a stainless tubular fixed-bed reactor (6 mm ID, 26 cm length) for the reduction-oxidation treatment. The reduction-oxidation cycle was performed at 723 K under 50 sccm of pure hydrogen and dry air for 1.5 h. The obtained sample was denoted as Ga/ZSM-5(redox). For the Ga-modified ZSM-5 zeolites with a Si/Al ratio of 50 (H-ZSM-5, purchased from the Catalyst Plant of Nankai University, China), both impregnation and reduction-oxidation treatments were performed by the same procedure as discribed above. For the preparation of Ga 2 O 3 /ZSM-5, 2 g of H-ZSM-5 and 0.24 g of Ga 2 O 3 (99%, purchased from Aladdin Reagent Co., Ltd.) were dispersed in 5 ml of deionized water. The mixture was stirred and dried at 323K and subsequently calcined at 823 K under flowing air for 8 h.

3 2. Catalyst characterization. 2.1 X-ray power diffraction (XRD) The structure and crystalline nature of H-ZSM-5 and other Ga-modified zeolites were examined by X-ray diffractometer (X Pert 3 Powder) using a CuKα radiation with a step of 0.02 at a respective voltage of 40 kv and a current of 40 ma. 2.2 ICP analysis The Ga chemical composition of the as-prepared samples were determined by an inductively coupled plasma-atomic emission spectrometer (ICP-AES) method using a Thermo icap 6300 equipment. 2.3 Solid-state NMR analysis 1 H single-pulse MAS, 1 H- 1 H SQ-DQ MAS and 1 H- 71 Ga S-RESPDOR and 71 Ga QCPMG solid-state NMR experiments were performed at 11.7 T on a Bruker Avance III 500 spectrometer with resonance frequencies of and MHz for 1 H and 71 Ga, respectively. Prior to all 1 H MAS NMR experiments, 0.15 g of the samples were kept at 723 K under high vaccum (<10-3 Pa) for 12 h to achieve complete dehydration. After cooling down to room temperature, the dehydrated samples were transferred to 4 mm zirconia rotors under dry nitrogen atmosphere in a glovebox for subsequent 1 H MAS NMR measurements. Single-pulse 1 H MAS NMR experiments were performed using a 1 H 90-degree pulse length of 3.9 μs and a repetition time of 5 s. The concentration of the BAS on the samples were quantified by careful integration of the peak at 4.3 ppm with

4 DMFIT software, the obtained peak areas were then referenced to a standard curve established by using admantane as external standard. In 2D 1 H- 1 H SQ-DQ MAS NMR experiments, double-quantum coherences were excited and reconverted with the R pulse sequence with 1H = 3 R = 37.5 khz, and the excitation and reconversion time were set to 0.48 ms. The increment interval in the indirect dimension was set to 80 μs, and 72 t1 increments and 160 scan accumulations for each t1 increment were used. In 1 H- 71 Ga S-RESPDOR experiments, the duration of 71 Ga saturation pulse was set to 1.5 rotor periods with 1 ( 71 Ga) = 110 khz, The 1 H rf-field nutation frequency of the π/2 and pulses was 1H = 62.5 khz, which yields /2 and pulse lengths of 4 and 8 μs, respectively. The 1 H- 71 Ga dipolar couplings were reintroduced by applying SR4 2 1 heteronuclear recoupling pulse sequence on the 1 H channel with 1H = 2 R = 25 khz scans were collected for each spectrum with a recycle delay of 2 s. The scan-to-scan alternation of the collection of S and S 0 spectra was used to reduce the effect of spinning speed fluctuation. 1 For measuring the Brønsted acid strengths of zeolites, [D5]pyridine ( 2 H-enrichment of 99.5%, Cambridge Isotope Laboratories, Inc.) was introduced onto the dehydrated samples on vacuum system, the sample was then transferred into a 4 mm zirconia NMR rotor sealed with a Kel-F end-cap in a glove box under dry N 2 atmosphere for subsequent 1 H single-pulse MAS NMR experiments. For all the 1 H MAS NMR spectra, the magic angle spinning rate were set to 12.5 khz, and the 1 H chemical shifts were referenced to adamantane (1.78 ppm).

5 The 27 Al MAS NMR spectra were also acquired on the same 4 mm probe by small-flip angle technique with a pulse length of 0.3 μs (<π/12) and a recycle delay of 1s. The magic angle spinning rate was set to 12.5 khz. The 27 Al chemical shifts were referenced to 1M Al(NO 3 ) 3 aqueous solution (0 ppm). 71 Ga WURST-QCPMG experiments 2 were performed by a 1.9 mm probe with samples spinning at 40 khz. In the experiments, the CT signal was enhanced by irradiating a single ST sideband using WURST pulse before the CT-selective pulse. Rotor-synchronized inter-pulses and inter-acquisition delays were employed in the QCPMG scheme. The acquisition time for each full echo was 0.56 ms and the number of Meiboom-Gill loops was Ga WURST-QCPMG spectra of Ga-modified ZSM-5 were acquired by averaging about 92,000 transients with a recycle delay of 1 s. The 71 Ga chemical shifts were referenced to 1 M Ga(NO 3 ) 3 aqueous solution (0 ppm). 29 Si MAS NMR experiments were carried out at 7.1 T on a Varian Infinity plus-300 spectrometer with resonance frequencies of and MHz for 1 H and 29 Si, respectively. Single-pulse 29 Si MAS NMR spectra with high power proton decoupling were recorded on a 7.5 mm probe, using a π/2 pulse of 5 μs, a recycle delay of 80 s and a spinning rate of 4 khz. The 29 Si chemical shifts were referenced to kaolinite (-91.5 ppm). 2.4 Temperature-programmed desorption of ammonia (NH 3 -TPD) The NH 3 -TPD measurements were performed using a FINESORB-3010 chemisorption instrument. Typically, 100 mg of sample was pre-treated at 773 K for 2 h under 25 sccm of He. After the sample cooled down to 373 K, NH 3 was introduced

6 and held for 1 h. The temperature was then elevated from 373 to 873 K at a ramping rate of 10 K/min and the desorbed NH 3 was detected by a thermal conductivity detector (TCD). 2.5 FT-IR of pyridine adsorption The FT-IR of pyridine adsorption measurements were performed on a Bruker Tensor 27 spectrometer. The catalysts were first activated at 773 K under high vacuum (<10-5 Pa) for 12 h. After cooling down to 313 K, a background spectrum was collected. Excessive amount of pyridine was then introduced to the infrared cell and held for 2 h to allow equilibrium. The residual pyridine was removed by vaccum. The FT-IR spectra of pyridine-adsorbed samples were measured at 313 K after evacuation at 423, 523, 623, and 723 K, respectively. 3. Catalytic testing. The Methanol-To-Aromatics (MTA) reactions were carried out in a stainless tubular fixed-bed reactor with a inner diameter of 6 mm and a length of 24 cm. The catalysts were pressed into wafers and then crushed and sieved to obtain the 40~60 mesh pellets. In a typical run, 120 mg of catalyst sample was loaded into the reactor and activated at 773 K for 2 h under 120 sccm helium flow. After the sample was cooled down to the predetermined reaction temperature, methanol vapor was introduced into the reactor by passing 30 sccm of helium through a methanol evaporator ml of methanol was pumped into the evaporator per minute, which equals to a feed weight-hourly space velocity (WHSV) of 2 h -1. The total carrier gas flow rate was set to 90 sccm and the reaction pressure was 1.25 bar. The effluent

7 product flow was split and routed to two parallel gas chromatographs for online analysis. A GC-FID chromatograph (Shimadzu GC-2010) equipped with a Supelco Supel-Q TM PLOT capillary column (30 m 0.32 mm 15 um) was adopted for organics analysis, meanwhile, a GC-TCD chromatograph (Shimadzu GC-2014) equipped with a stainless TDX-01 packed column (2 m 3 mm) was utilized to investigate the inorganic part of the products. The flow routed to the GC-FID was kept at 473 K to avoid condensation of the products while the other arriving at the GC-TCD chromatograph went through a cold trap to eliminate the effect of organics. 4. Analytical simulations of the 1 H- 71 Ga dephasing curves. For an ideal case in which the the heteronuclear dipolar dephasing is only induced by an isolated nuclear spin pair, analytical formula of S-RESPDOR is expressed as: 3 where I is the spin quantum number of the quadrupolar nucleus and J ±1/4 denotes ±1/4 order Bessel function of the first kind. The τ is the dephasing time. This formula depends on the dimensionless parameter λ=τ* D IS and the pre-factor parameter f. Note that the parameter f represents the fraction of sample in which the spin states of quadrupolar nucleus are saturated by the saturation pulse, and is equal to 1 only for 100%-abundant I nuclei experiencing a complete saturation. Since the adiabaticity parameter α = (ν 1Ga )2/2ν Q ν R is around 1 for the intermediate condition, and neither sudden (<<1) nor slow (>>1) adiabatic passage is fulfilled, the parameter f mainly

8 represents the proportion of 71 Ga isotopes being in close spatial proximity to protons. Meanwhile, the D IS is the heteronuclear dipolar interaction between two spins, reflecting the distance between two interactive nuclei. In the present case, the 1 H- 71 Ga dipolar dephasing curves were fitted by the above analytical formula. The parameter f and D IS were determined by best fitting the experimental dephasing curves, and the respective 1 H- 71 Ga distances of the BAS, AlOH and GaOH were accordingly obtained (Figure S6~S8). The maximum dephasing fraction (ΔS/S 0, max ) obtained from the 1 H- 71 Ga S-RESPDOR experiment, 71 Ga isotope abundance (0.398) and an additional scaling factor (~0.75 for I =3/2 nuclei) 3 were used for calculating the concentration of 1 H- 71 Ga spin pairs. The fraction of 1 H- 71 Ga pair was calculated by the formula 1 : H *(ΔS/S 0, max )/0.398/0.75. H represents the concentration of the remained BAS on the Ga- modified ZMS-5, which is determined by 1 H MAS NMR spectrum.

9 Figure S1. XRD patterns of H-ZSM-5 and different Ga-modified ZSM-5 zeolites. The dots denote the reflections of bulk Ga 2 O 3 particles.

10 Figure S2. 27 Al (a) and 29 Si (b) MAS NMR spectra of H-ZSM-5 and different Ga-modified ZSM-5 zeolites

11 Table S1. NMR parameters of the 71 Ga QCPMG NMR spectra and the relative concentration of different Ga species for Ga 2 O 3 /ZSM-5, Ga/ZSM-5(IM) and Ga/ZSM-5(redox). sample δ iso (ppm) C Q (MHz) relative peak intensity Ga 2 O 3 /ZSM Ga/ZSM-5(IM) Ga/ZSM-5(redox) All the values are obtained by the simulation of the 71 Ga QCPMG spectra with a Czjzek model using the DMFIT software. 4

12 Figure S3. NH 3 -TPD of (a) H-ZSM-5, (b) Ga 2 O 3 /ZSM-5, (c) Ga/ZSM-5(IM) and (d) Ga/ZSM-5(redox).

13 Figure S4. FT-IR spectra of pyridine adsorbed on (a) H-ZSM-5, (b) Ga 2 O 3 /ZSM-5, (c) Ga/ZSM-5(IM) and (d) Ga/ZSM-5(redox) after evacuation at different temperatures. The B, L, and Py-H peaks represent Brønsted acid sites, Lewis acid sites and hydrogen-bonded pyridine, respectively.

14 Figure S5. 1 H- 71 Ga S-RESPDOR spectra of Ga/ZSM-5(IM). S 0 and S represent two independent 1 H NMR spectra acquired without and with 71 Ga irradiation; ΔS denotes the difference spectrum of (S 0 S); ΔS ref is obtained by a control experiment in which the spectra S 0 and S were both acquired without 71 Ga irradiation and accumulated with the same number of scans as that used to obtain ΔS. Thus, the ΔS ref reflects the signal difference produced by the instability of NMR instrument. Since no signal is observable in the ΔS ref spectrum, the weak difference signals in the ΔS spectrum should be due to the dipolar interaction between hydroxyl groups and neighboring Ga species.

15 Figure S6. 1 H- 71 Ga S-RESPDOR dephasing fraction of BAS in Ga/ZSM-5(redox) fitted by analytical formula with (a) fixed D IS with adjustable f, (b) fixed f with adjustable D IS. The best fit of the experimental results was determined to be f =0.081 and D IS =280 Hz, corresponding to a 1 H- 71 Ga internuclear distance of 5.05 Å.

16 Figure S7. 1 H- 71 Ga S-RESPDOR dephasing fraction of AlOH in Ga/ZSM-5(redox) fitted by analytical formula with (a) fixed D IS with adjustable f, (b) fixed f with adjustable D IS. The best fit of the experimental results was determined to be f =0.185 and D IS =120 Hz, corresponding to a 1 H- 71 Ga internuclear distance of 6.72 Å.

17 Figure S8. 1 H- 71 Ga S-RESPDOR dephasing fraction of GaOH in Ga/ZSM-5(redox) fitted by analytical formula with (a) fixed D IS with adjustable f, (b) fixed f with adjustable D IS. The best fit of the experimental results was determined to be f =0.280 and D IS =1300 Hz, corresponding to a 1 H- 71 Ga internuclear distance of 3.04 Å.

18 Figure S9. 1 H- 1 H SQ-DQ MAS NMR spectra of (a) H-ZSM-5, (b) Ga/ZSM-5(IM) and (c) Ga/ZSM-5(redox) with a Si/Al ratio of 50. The DQ recoupling time was set to 0.96 ms.

19 Figure S10. 1 H- 71 Ga S-RESPDOR NMR spectra of Ga/ZSM-5(redox) with a Si/Al ratio of 50. The recoupling time was set to 8 ms.

20 Figure S11. Aromatic selectivity (red) and H 2 production (blue) of the MTA reaction over H-ZSM-5 and Ga-modified HZSM-5 samples with a Si/Al ratio of 50. The product selectivity are given on carbon basis, and the amount of H 2 production are normalized to that of the H-ZSM-5 sample.

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