Dynamic confinement of SAPO-17 cages on the selectivity control of syngas conversion

Abstract The OXZEO (oxide−zeolite) bifunctional catalyst concept has enabled selective syngas conversion to a series of value-added chemicals and fuels such as light olefins, aromatics and gasoline. Herein we report for the first time a dynamic confinement of SAPO-17 cages on the selectivity control of syngas conversion observed during an induction period. Structured illumination microscopy, intelligent gravimetric analysis, UV-Raman, X-ray diffraction, thermogravimetry and gas chromatography-mass spectrometer analysis indicate that this is attributed to the evolution of carbonaceous species as the reaction proceeds, which gradually reduces the effective space inside the cage. Consequently, the diffusion of molecules is hindered and the hindering is much more prominent for larger molecules such as C4+. As a result, the selectivity of ethylene is enhanced whereas that of C4+ is suppressed. Beyond the induction period, the product selectivity levels off. For instance, ethylene selectivity levels off at 44% and propylene selectivity at 31%, as well as CO conversion at 27%. The findings here bring a new fundamental understanding that will guide further development of selective catalysts for olefin synthesis based on the OXZEO concept.

UV-Raman spectra were collected using a home-built spectrometer [3]. The system was composed of a 266 nm constant-wave laser, a 25 mm diameter off-axis parabolic mirror as the lightcollecting element, an edge filter to filter the Rayleigh scattered light, a spectrograph, and a UV-CCD camera produced by Andor. All spectra were calibrated by placing the main Raman peak of monocrystalline Si at 520 cm −1 . To ensure optical throughput, the slit width was set at 150 μm, resulting in a spectral resolution of ∼7 cm −1 . For most experiments, the laser power at the sample was kept below 2 mW to prevent burning effects. The typical accumulation time per spectrum was ∼30 s.
Structured illumination microscopy (SIM) was carried out on a Nikon N-SIM super-resolution microscopy system with a motorized inverted microscopy ECLIPSE Ti2-E, a×100/numerical aperture 1.49 oil-immersion total internal reflection fluorescence objective lens (CFI HP) and an ORCA-Flash 4.0 sCMOS camera (Hamamatsu Photonics K.K.) [4,5]. The wavelengths of illumination and emission detection of SIM were 405 (detection at 435-485 nm), 488 (detection at 500-545 nm), 561 (detection at 570-640 nm), and 640 nm (detection at 663-738 nm), respectively. The software NIS-Elements Ar and N-SIM Analysis were used to analyze the collected images and to computationally reconstruct the super-resolution image.
The adsorption kinetics were analyzed according to Fick's second law of diffusion [6]. Eq. S1 For short time:

Eq. S2
Where m t /m ∞ is the normalized loading, D is diffusion coefficient, t is diffusion time, and r represents characteristic diffusion length. The SAPO-17 zeolite crystals were treated as spherical adsorbent particle, and the equivalent radius r = 2.5 μm.

Catalytic reaction tests
Syngas conversion was performed in a continuous-flow fixed-bed stainless steel reactor furnished with a quartz lining. Typically, 420 mg catalyst (40-60 mesh) with ZnCrOx/SAPO-17 = 1/1 (mass ratio) was used. Syngas contained 5% Ar as the internal standard for online gas chromatography (GC) analysis. Reaction was carried out under conditions of H2/CO = 2.5, 4.0 MPa, 400℃ and 5000 mL·gcat -1 ·h -1 unless otherwise stated. Products were analyzed by an online GC (Agilent 7890B) equipped with a TCD and an FID. Hayesep Q and 5 Å molecular-sieve-packed columns were connected to the TCD while HP-FFAP and HP-AL/S capillary columns were connected to the FID. Oxygen-containing compounds and hydrocarbons up to C17 were analyzed by the FID, while CO, CO2, CH4, C2H4, and C2H6 were analyzed by the TCD. CH4 and C2H4 were taken as a reference bridge between the FID and TCD.
CO conversion was calculated on a carbon atom basis using the following equation Eq. S3 Where COinlet and COoutlet represent moles of CO at the inlet and outlet, respectively. CO2 selectivity based on C atom (SelCO2) was calculated according to: Where CO 2 outlet denotes moles of CO2 at the outlet.
The selectivity of individual hydrocarbon CnHm (SelCnHm) among hydrocarbons was obtained according to          Additional discussion: The dual-cycle mechanism is widely accepted for MTH reaction. Ethylene and propylene are produced in a similar quantity in the classical aromatic cycle while propylene and higher hydrocarbons dominate following olefinic cycle. Following the aromatic cycle, Haw and co-workers further showed that ethylene would dominate the products if the aromatic compounds in H-SAPO-34 cage contain less substituent groups e.g., trimethylbenzene and tetramethylbenzene whereas propylene would be favored from pentamethylbenzene and hexamethylbenzene [7]. A similar phenomenon was reported by Bjørgen and coworkers for the aromatic cycle over H-ZSM-5. In addition, pentamethylbenzene and hexamethylbenzene were reported in the cages of H-beta, which also favored formation of propylene [8].
In this study, the aromatics species are dominated in SAPO-17 cage during the entire induction period of syngas conversion. Trimethylbenzene, tetramethylbenzene and methylnaphthalene predominate during the first 10 h of the induction period ( Supplementary Fig. S10). Thereafter, tetramethyl-diphenyl and dimethyl-anthracene increase while trimethylbenzene and tetramethylbenzene decrease gradually. According to the aromatic cycle of MTH, ethylene should be more favored during the first 10 h and then propylene selectivity should increase while ethylene selectivity should decrease. However, Figure 1B shows an opposite trend. Therefore, the selectivity change during the induction period of syngas conversion is not controlled by the dual-cycle mechanism.
The difference in mechanism between OXZEO and MTH has also been previously observed. For instance, in our previous study on ZnCrOx-SAPO-18 bifunctional OXZEO catalyst, a stronger acid strength and higher acid density of SAPO-18 leads to a higher C3/C2 ratio in the products [9]. By contrast, a stronger acid strength and higher acid density benefit the aromatic cycle, giving a lower C3/C2 ratio in MTH catalyzed by various zeolites, e.g., SAPO-34, SSZ-13, and ZSM-5 [10][11][12].