Electron Diffraction Enables the Mapping of Coke in ZSM‐5 Micropores Formed during Methanol‐to‐Hydrocarbons Conversion

Abstract Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM‐5 catalysts used in methanol‐to‐hydrocarbons conversion by single‐crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high‐quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke‐associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain.


S2. Catalyst and coke characterization
The scanning electron microscopy (SEM) imaging and the analysis of silicon and aluminium continent were performed by using a SEM JSM-7100F scanning electron microscope equipped with Ametek-EDAX EDX detector (Si(Li) 30 mm 2 ) that is operated at 15.0 keV with a working distance of 10.0 mm. A thin layer of the sample was spread on top of aluminum stump covered by a carbon tape.
The elemental analysis was obtain by averaging six EDS spectra collected from the sample surface.
Micropore volumes and surface areas of the catalysts were calculated from the nitrogen isotherms that were recorded at 77 K using a Micromeritics 3D Flex automatic surface area and pore-size analyzer after degassing the samples (ca. 60 mg) at 573 K and 10 mbar for 12 h. The powder X-ray diffraction (PXRD) patterns were recorded in a Bruker D8 ADVANCE diffractometer using a Bragg-Brentano geometry and Cu Kradiation ( = 1.54 Å) generated by 2.2 kW Cu anode, which was operated at V = 40 kV and I = 20 mA. The data was recorded in the 2 range of 5-70 degrees, using a step size of 0.014 degrees and acquisition time of 1.6 s per step. All datasets were outlined for a baseline and carefully analyzed using Lines and TOPAS 6 software, respectively. [1,2] The position of Bragg reflections S2 were refined in the space group Pnma by Pawley method to obtain the lattice parameters. The peak shapes and specimen displacements were fitted to individual sets with Pseudo-Voight function and condition  = z × cos(), respectively. The Rwp, Rp, Rexp, and goodness-of-fit for the refinement of models were in the ranges of 4.137-5.160, 3.065-3.881, 3.287-4.586, and 1.125-1.510, respectively.
Coke content was determined by thermogravimetric analysis (TGA), which was performed in a NETZSCH STA 449 Jupiter analyzer. The samples (ca. 10 mg) were dried under flowing nitrogen (80 cm 3 STP min -1 ) at 573 K for 30 min and then heated in an oxygen-nitrogen mixture (120 cm 3 STP min -1 , molar ratio O2:N2 = 1:2) up to 1173 K (heating rate 10 K min -1 ). TGA determined weight loss was used to calculate the bulk carbon content of the catalysts by assuming the molar C:H ratios of coke of 1. [3] 27 Al and 1  After cooling down to a room temperature, it was contacted with 4 mbar of pyridine vapors, which is followed by the cell evacuation at 423 K (ca. 0.6 h) to desorb the weakly adsorbed molecules.
Thereafter, the cell was cooled down to room temperature to collect the spectrum. The concentration of Brønsted (BAS) and Lewis (LAS) acid sites was determined by using the previously reported molar integral excitation coefficients. [4] Raman spectra of the self-supported sample discs were obtained in a Horiba LabRAM HR Evolution instrument by using 325 nm laser (source power 25 mW), 1800 mm -1 grating, 40x objective lens (Thorlabs LMU-40X UVB), edge and holographic notch filters, and Horiba Synapse CCD detector, and an acquisition time of 30 s. The absence of laser-induced sample damage was ensured by visual inspection. The confocal fluorescence microscopy of the zeolite crystals was performed by using Zeiss LSM880 microscope with Airyscan and the oil immersion objective (Plan-Apochromat 63×/1.4 oil DIC M27). The samples were illuminated with 405, 488, 561 and 633 nm lasers and the emitted light was detected by GaAsP-PMT area detector (Airyscan), which collects a pinhole-plane image at every scan position. [5] The image acquisition and spatial profile analyses were performed by using Zeiss ZEN black edition and Fiji software, respectively. [6] X-ray photoelectron spectroscopy (XPS) analysis of the carbon and silicon concentrations in the surface region of the catalysts was carried on a Physical Electronics Quantera X-ray photoelectron spectrometer using monochromatic Al Kα radiation that is generated from an electron beam operated at 15 kV, and equipped with a hemispherical capacitor electron-energy analyzer. The solids were analyzed at an electron takeoff angle of 45 degrees and a pass energy of 55 eV. The atomic concentrations were calculated from the Si 2p and C 1s photoelectron peak integrals obtained after Shirley background subtraction using PHI-MultiPak software and the built-in relative sensitivity factors that are corrected for the system S3 transmission function. Electron diffraction data were collected at 200 keV on a CM200 transmission electron microscope (TEM, Philips, Amsterdam, the Netherlands) with a LaB6 electron source. A 512 × 1024 pixel JUNGFRAU detector with a 320 μm thick silicon sensor and a pixel size of (75 μm) was used. Due to an aperture underneath the fluorescence screen, the detector area was restricted to a circle with about 320 pixels diameter. A low-and a high-resolution data were therefore collected at effective detector distances of 780 and 280 mm for Z1h-Z19.5h, and at 395 nm for Zc respectively, in order to cover the full resolution range of the samples. The detector distance for every data set was refined in the CORRECT step of the program XDS. This leads to more precise unit cell parameters. The rotation angle of the goniometer stage was logged every 0.3 s with a GATAN Digital Micrograph script, and the resulting value was fitted to a straight line with GNUPLOT. [7] This results in the standard uncertainties listed below. Exposure time is equal to 0.05 s frame -1 for all of the data sets. Other details regarding the collection of the electron diffraction data used in this work are summarized in Tables S1-S5. Data were processed with XDS. [8] The high degree of data quality of the applied system allows for the refinement of the detector distance, which results in a significantly better precision of the unit cell parameters.
Structures were solved by using SHELXT, while the refinement and model building were done with SHELXL and ShelXle, respectively. [9,10] Further details on data acquisition, conversion, and processing as well as solution and refinement of the structures can be found in our previous publication. [11] Visualization and analysis of the coke-associated signal from direct and difference maps was performed by standardizing the signal according to the Eq. S2,   average s , -( ) in which Is represents the standardized signal intensity at point with coordinates x, y, and z, Iaverage is the average signal intensity for the whole sample, and SD(I) is the standard deviation of the signal. The term signal refers to the electrostatic potential, which describes the charge distribution around a molecule generated by its nuclei and electrons, and is broadly used in literature on ED. [12,13] The channel volume, Vc, was calculated according to the geometric condition defined by Eq. S3, in which the xn, yn, and zn denote the coordinates and rn the radius of the nearest framework atom (T site, rn = 1.18 Å or oxygen site, rn = 0.74 Å) and dt denotes the threshold distance (dt = 1.75 Å).
The relative fraction of the signal in the channel sections, fIs,Vs, was calculated from the direct maps according to Eq. S4,  19.5h , respectively. Data were cut where CC 1/2 drops below 70%. All data sets diffracted to better than 1 Å resolution, i.e., better than atomic resolution. The latter is a threshold resolution of the data set of a crystal structure enabling to resolve the individual atoms. [14] Some differences in the values of the unit cell parameters between the powder X-ray diffraction ( Table 1 of the main manuscript) and electron diffraction can be ascribed to the much smaller sample population size in the case of electron diffrication and thus more pronounced heterogeneities in the coke distribution between the individual ZSM-5 crystals. Al MAS NMR spectra display an intensive resonance at ca. 56 ppm and a weak resonance at ca. 1 ppm, which can be ascribed to the tetrahedral (Td) and octahedral (Oh) aluminum sites, respectively. Consistent with the latter results, the FTIR spectra acquired after pyridine adsorption exhibit the bands associated with internal modes of pyridine interacting with Lewis acid sites (LAS, 1456 cm -1 ) and BAS (1490 cm -1 , 1546 cm -1 ). [15] The integrals of 1546 cm -1 and 1456 cm -1 bands were used to quantify the BAS and LAS, the concentrations of which were found to be 345 mol gcat -1 and 130 mol gcat -1 , respectively. deconvoluting the C 1s signal. [20] The MTH reaction was performed at reduced weight-hourly space velocity, increased methanol concentration, and increased temperature, which led to a complete (100%) methanol conversion. Under these conditions, the rate of coking is enhanced, which resulted in a high content of coke of a Zc, as shown in Table 1 of the main manuscript. Raman spectra display all the characteristic bands of coke, with G band (ca. 1605 cm -1 ) being shifted to ca. 1605 cm -1 , suggesting the formation of more condensed external and/or bent internal coke. [16][17][18] Consistent with this, C 1s X-ray S11 photoemission spectra evidence the accumulation of coke species in the surface region of Zc catalyst.
In comparison to Z0h material, Zc displays an increased fraction of the peak component associated with C-C and C=C bonds and reduced contribution of the components arising from C-O bonds, implying the formation of more condensed PAH. [20] Likewise, the PXRD exhibit overlapping of the reflection doublets at 2 of ca. 23.5 and 45.5 degrees, indicating significant accumulation of internal coke. [19] S12 Figure S3. High-resolution transmission electron microscopy (HR-TEM) images of some of the Zc crystals analyzed by electron diffraction. Consistent with the SEM analysis of the fresh catalysts presented in Figure S1, the analyzed crystals of the coked catalysts were in the size-range of ca. 0.2-1 m.
S13 Figure S4.  of the PAH (ca. 10-45 ppm) from Z1h to Z19.5h. [21,22] With raising coke content, the resonances stemming from the unsubstituted aromatics (ca. 115-132 ppm) increase more prominently than those originating from the substituted aromatic rings (ca. 132-150 ppm), which is also accompanied with the lower S17 increment of the signal of the aliphatic carbons with respect to aromatic carbons. [22] Similarly, the Raman spectra indicate an increase of the bands associated with methylated benzenes (ca. 1280 cm -1 ), PAH (ca. 1170, 1370, 1420 and 1620 cm -1 ), and graphitic carbons (T band at ca. 1100, D band at ca. 1400, and G band at ca. 1600 cm -1 ) when going from Z1h to Z19.5 h. [16,23,24] Therein, the position of the most intense Raman peak shifts from ca. 1620 cm -1 in Z1h to ca. 1605 cm -1 in Z19.5h, suggesting the formation of more condensed external and/or bent internal PAH. The fluorescence signals measured after illuminating the samples with the 405, 488, 561 and 633 nm lasers are associated with mono-, bi-, tri-, and tetracyclic arenes, respectively. [25,26] The signal profiles point towards the uniform distribution of these compounds across the particle. S18 Figure S8. Si 2p (left) and C 1s (right) X-ray photoemission spectra of deactivated Z1h, Z5h, Z12h, and Z19.5h catalysts. The X-ray photoemission spectra of the fresh Z0h material are shown for the reference.
Bar plots shown as insets display the relative fractions of peak components that are associated with C-C and C=C (284.2 eV) C-O, C=O (285.4 eV), and O-C=O (288.2 eV) bonds, which are calculated by deconvoluting the C 1s signal. [20] In comparison to Z0h material, deactivated Z1h-Z19.5h catalysts display progressively increasing intensity of the C 1s with respect to Si 2p signal, which indicates the increasing coke content in the surface region of a catalyst (Table 1 of the main manuscript). Herein, the fraction of the peak component associated with C-C and C=C bonds increases, while the contributions of the components arising from C-O bonds decline when going from fresh to more deactivated samples, indicating that the evolution of the more condensed PAH species in the surface region of the catalyst. S19 Figure S9. PXRD  . The change of lattice parameters can be ascribed to the strain that is introduced by the deposition of the internal coke. [19] S20 Figure S10. a) Distribution of the standardized signals in the direct maps of Z1h, Z5h, Z12h, and Z19.5h catalysts. The signals for which Is > 1.0 were considered as coke-associated and plotted for b) Z1h, c) Z5h, d) Z12h, and e) Z19.5h. As shown with the insets in a), the signal range Is > 1.0 is splitted into four sub-ranges, compraising the values between 1 < Is ≤ 1.2, 1.2 < Is ≤ 1.5, 1.5 < Is ≤ 2.5, and 2.5 < Is, which are ploted from left to right and indicated in light gray, dark gray, black, and light blue, respectively. The straight and sinusoidal channels, and their intersections are indicated with blue, red, and pink isosurfaces, respectively. Crystal orientiation is indicated with respective coordinates. The signals in the direct maps that are recorded in the zeolite channels are of similar intensity as those observed at the S21 positions corresponding to the framework atoms, indicating that they are associated with coke molecules entrapped in these crystalline voids. The maps also illustrate that the coke-associated signals become more intense with increasing extent of coking, with most intensive features appearing in the channel intersections.
S22 Figure S11. a) Distribution of standardized signals in the difference maps of Z1h, Z5h, Z12h, and Z19.5h catalysts. The signals for which Is > 2.5 were considered as coke-associated and plotted for b) Z1h, c) Z5h, d) Z12h, and e) Z19.5h. As shown in inset in a) and legend in b-d), the signal range Is > 2.5 is splitted into four sub-ranges, compraising the values between 2.5 < Is ≤ 3, 3 < Is ≤ 3.5, 3.5 < Is , which are ploted from left to right and indicated in light gray, dark gray, and black, respectively. The tetrahedral (aluminum or silicon) and oxygen framework sites are colored in green and white, respectively. The straight and sinusoidal channels, and their intersections are indicated with blue, red, and pink isosurfaces, respectively. Crystal orientation is indicated with respective coordinates.