Role of Strontium Cations in ZSM-5 Zeolite in the Methanol-to-Hydrocarbons Reaction

The selectivity of the methanol-to-hydrocarbons (MTH) reaction can be tuned by modifying zeolite catalysts with alkaline earth metals, which typically increase propylene selectivity and catalyst stability. Here we employed Sr2+ as its higher atomic number in comparison to the zeolite T atoms facilitates characterization by scanning transmission electron microscopy and operando X-ray absorption spectroscopy. Sr2+ dispersed in the ZSM-5 micropores coordinates water, methanol, and dimethyl ether during the MTH reaction. Complementary characterization with nuclear magnetic resonance spectroscopy, thermogravimetric analysis combined with mass spectrometry, operando infrared spectroscopy, and X-ray diffraction points to the retention of substantially more adsorbates during the MTH reaction in comparison to Sr-free zeolites. Our findings support the notion that alkaline earth metals modify the porous reaction environment such that the olefin cycle is favored over the aromatic cycle in the MTH, explaining the increased propylene yield and lower deactivation rate.


Catalyst preparation
The following zeolite catalysts were prepared for this study: HZSM-5, 0.1Sr/ZSM-5, 0.2Sr/ZSM-5, 0.4Sr/ZSM-5 and Na/ZSM-5. Notations x in Sr/ZSM-5 is the Sr content, i.e., 0.11, 0.22 and 0.44 mmol•g (1). HZSM-5 was obtained by calcining commercial NH4ZSM-5 zeolite (Si/Al = 25, Alfa Aesar) at 550 °C for 5 h. HZSM-5 modified with Sr were obtained by incipient wetness impregnation of the calcined zeolite with aqueous solutions of Sr(NO3)2 (Alfa Aesar, 99.0%) following the procedure described elsewhere 1 . The modified catalysts were prepared aiming at Sr loadings of 1, 2, 4 wt.%. The impregnated samples were dried overnight at 110 °C and calcined at 550 °C for 5 h under static air conditions. XRD data demonstrate that the MFI topology of the parent HZSM-5 zeolite is preserved in all samples (Fig. S1). 1 H MAS NMR spectroscopy of dehydrated samples was used to compare the amount of Brønsted acid sites (BAS, signal at 4 ppm, Fig. S2). The OH-stretching region of IR spectra of dehydrated samples and IR spectra of adsorbed pyridine provide further information about the acidity situation (Fig. S3). The amount of extraframework Al was estimated using 27 Al MAS NMR (Fig.   S4). The preparation procedure of ion-exchanged Na/ZSM-5 is provided elsewhere. (2)

Catalyst characterization
The elemental composition (Si/Al ratio and metal content) of the zeolite catalysts was determined by ICP-OES (Spectro CIROS CCD ICP optical emission spectrometer). The samples were dissolved in a 1:1:1 mixture of HF (40%), HNO3 (65%) and H2O prior to the ICP measurements.
The crystallinity of the zeolite samples was determined by powder X-ray diffraction (XRD). XRD measurements were performed on a Bruker D2 powder diffraction system (Cu Kα radiation, scan speed 0.01°/s, 2θ range 5−60°).
Solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded using an 11.7 T AvanceNeo Bruker NMR spectrometer operating at 500 MHz, 125 MHz and 130 MHz for 1 H, 13 C and 27 Al, respectively. 1 H and 13 C MAS NMR experiments were performed using a Bruker triple channel 4 mm MAS probe head spinning at rates between 8 and 10 kHz. Prior to 1 H measurements, the samples were dehydrated and sealed in an air-and moisture-free glovebox. Twodimensional 1 H-13 C{1H} HETCOR (HETeronuclear CORrelation) MAS NMR spectra were recorded with a ramped contact pulse time of 5 ms and an interscan delay of 3 s. 13 C direct excitation (DE) spectra were measured using a high power proton decoupling Hahn echo pulse sequence p1-τ1-p2-τ2-aq with a 90° pulse p1 = 5 μs, a 180° pulse p2 = 10 μs, a very short τ1= τ2 echo time of 0.013 μs and an interscan delay of 10 s. 13 C NMR spectra were recorded at spinning rate of 8 -10 kHz. 27 Al MAS NMR spectra were recorded using a Bruker 2.5-mm MAS probe head spinning at 25 kHz. NMR shift calibration for 1 H, 27 Al and 13 C was done using tetramethylsilane (TMS), saturated Al(NO3)3 solution, and solid adamantane, respectively.
The acidic properties of zeolites were determined by IR spectroscopy of adsorbed pyridine as a probe molecule. Spectra were taken in the 4000 -1000 cm −1 range using a Bruker Vertex 70v spectrometer. Samples were pressed into self-supporting wafers (10 -20 mg, diameter 1.3 cm) and placed in an environmental cell. The wafers were pre-treated in O2:N2 (1:4 vol. ratio) flow at 550 °C (rate 10 °C•min -1 ) to remove contaminants followed by cooling to 150 °C under dynamic vacuum (p < 10 -5 mbar). Afterwards, the samples were exposed to excess pyridine vapor until saturation. IR spectra were then recorded after desorbing pyridine at 150 °C under dynamic vacuum. For the quantification of Brønsted and Lewis acid sites, integral molar extinction coefficients (IMEC) of 0.73 cm•mol -1 and 1.11 cm•mol -1 were used. (3) Textural properties of zeolites were determined by Ar porosimetry at -186 °C using a Micromeritics ASAP2020 machine. Prior to measurements, the samples were pre-treated at 400 °C under dynamic vacuum. The micropore volume was calculated by the t-plot method using a thickness range from 3.5 The microscopy experiments were performed using a TITAN X-FEG 60-300 located at the Advanced Microscopy Laboratory, University of Zaragoza, Spain. The microscope is equipped with a field emission gun operating at an acceleration voltage of 300 kV, a CEOS spherical aberration (Cs) corrector for the electron probe, a Gatan Tridiem Energy Filter and an Oxford Silicon drift detector for chemical analysis. Bright and dark field images were simultaneously acquired by annular dark field (ADF) and annular bright field (ABF) detectors. In order to work under very low dose conditions, data acquisition was assisted by the Real Time Up-sampling Filter.(4) The electron dose used for recording the atomic-resolution data was 800-1000 e -/Å 2 . For energy-dispersive X-ray spectroscopy (EDX), JEOL EDX spectrometer hardware and JEOL software were employed to acquire and to process the data.
The spectra were acquired within 1 min using an energy of 20 keV. Prior to measurements, the samples were crushed, dispersed in ethanol and few drops of the suspension were placed on a holey carbon copper microgrid as described here (5).

Catalytic activity measurements
The catalytic performance in the MTH reaction was performed in a fixed-bed reactor. In a typical experiment, a quartz reactor was charged with 25 mg of the sieved catalyst (250-500 µm pellet size) held between two quartz wool plugs. The catalyst was subsequently pre-treated in an oxygen atmosphere (20 vol.% O2 in He) at 550 °C (ramp rate 10 °C•min -1 ) for 1 h to remove organic contaminants. After pre-treatment, the temperature was set to 450 °C in pure He. The reaction was started by changing the feed to a 30 mL•min -1 flow of methanol in He (the temperature of the thermostat was kept at 19 °C, resulting in a methanol partial pressure of 12 kPa) at a WHSV 12 h -1 .
The reactor outlet was connected to a GC (Compact Selectivity to products was calculated on the carbon atom basis. Methanol throughput is defined as the mass of methanol converted per mass of catalyst before the conversion of methanol and DME decreased below 75%). Overall carbon selectivity during the MTH reaction was provided after 1 h time on stream. Conversion of methanol was calculated using the following formula: Further calculation procedure was reproduced from here. (1,6) Operando measurements: methods

TGA-MS
The procedure for TGA-MS analysis is analogous to the one we have previously reported 2 . In short, The temperature of the thermostat was kept at -14.6 °C, resulting in a methanol partial pressure of 1.5 kPa. After dilution with the additional He flow, the methanol partial pressure was 0.75 kPa. A dry He flow of 80 mL•min -1 was supplied to the catalyst bed for dry conditions. Calibrated thermal massflow controllers (Brooks) were used to supply the gasses to the TGA chamber.

Infrared spectroscopy
To study the response of the catalysts to the switches between methanol and dry He by IR spectroscopy, we used a Bruker Vertex 70v IR spectrometer to which a setup was connected for methanol dosing.(2) Spectra were taken in the 4000 -1000 cm -1 range. Samples were pressed into self-supporting wafers (10 -20 mg, diameter 1.3 cm) and placed in an environmental cell. The wafers were pretreated in O2:He (1:2 vol. ratio) flow at 550 °C (rate 10 °C·min -1 ) to remove organic contaminants, followed by cooling to 350 °C in He. After that, IR measurements were started. First, background spectra were recorded. An automated 4-way valve was used to switch the feed every 20 min from dry He to a MeOH-containing He flow. For this purpose, a thermostated saturator was used to supply methanol vapor to the catalyst bed: a He flow of 10 mL·min -1 was led through the saturator followed by dilution with another He flow of 120 mL·min -1 . The temperature of the thermostat was kept at −14.6 °C, which yielded after dilution with the additional He flow in a methanol partial pressure of 0.12 kPa. A dry He flow was fed at a rate of 130 mL·min -1 to obtain dry conditions. Calibrated thermal mass-flow controllers (Brooks) were used to supply the gases to the IR cell. The outlet of the IR cell was connected to an MS (Pfeiffer Omnistar MS).

X-ray diffraction
Operando XRD experiments were performed at the ID31 beamline of ESRF synchrotron (Grenoble,

Operando XRD
We performed operando XRD analysis over HZSM-5 and 0.2Sr/ZSM-5. The changes in the zeolite unit cell volume were determined by Rietveld refinement of the experimental patterns. We found that the unit cell of Sr-modified catalyst expands more upon filling with adsorbates during 5 h on methanol stream as compared to HZSM-5 (Fig. S13). After the methanol was switched off, the unit cell of Sr-free HZSM-5 became smaller again, while the unit cell of 0.2Sr/ZSM-5 remained nearly unchanged, indicating that the adsorbates were retained in an irreversible manner over Sr-modified zeolite. By

MCR-ALS analysis of MeOH switching experiment at 450 °C
The detailed procedure of the analysis is ascribed here. (12,16) In short, using MCR-ALS we were able to distinguish the presence of two states of Sr upon MeOH/He → He switch at 450 °C (Figs. S18 -S19).