Pseudoboehmite (Al2O3, 72 wt%) was purchased from Beijing Reagents Company. Silica solution (30 wt%) and tetraethylammonium hydroxide (TEAOH, 25wt%) were purchased from Sigma-Aldrich. Orthophosphoric acid (H3PO4, 85 wt%), triethylamine (TEA, 99 wt%), di-n-propylamine (DPA, AR), aluminum isopropoxide (99 wt%), zirconium nitrate (Zr(NO)4·5H2O), zinc nitrate (Zn(NO)2·9H2O), citric acid, NH3·H2O (25 wt%), ethanol (AR) and tetraethoxysilane (TEOS, AR) were purchased from Sinopharm Chemical Reagent. H-ZSM-5 zeolite (a molar ratio of Si/Al = 200) was purchased from Nankai University Catalyst Company. The glassware was provided by Xiamen University Glassware Workshop.
The Zn-doped ZrO2 (ZnZrOx) oxide for STG reaction was synthesized with a sol-gel method with a Zn/Zr ratio of 1:8 (at/at)45. Specifically, 10 g of Zr(NO3)4∙5H2O, 0.87 g of Zn(NO3)2∙6H2O, and 9.4 g of citric acid were dissolved in 100 mL deionized water. The mixture was evaporated at 90 oC until a viscous gel was obtained. Then, the mixture was heated to 180 oC for 3 h and calcined at 500 oC in the air for 5 h. The obtained sample was denoted as ZnZrOx (1/8), where 1/8 was the molar ratio of Zn/Zr. The ZnZrOx (1/16) for STO reaction and ZnZrOx (1/100) for STA reaction were synthesized by the same method except for the Zn content. In this work, ZnZrOx was the abbreviation for three kinds of Zn-doped ZrO2 oxide for convenience although Zn content was different unless otherwise mentioned.
The ZnO oxide was prepared by a precipitation method. The NH3·H2O (25 wt%) was added into the Zn(NO3)2 solution under continual stirring until the pH of the solution was 7.0. After aging for 2 h at 70°C, the precipitate was washed three times with distilled water and separated by filtration. The filter cake was dried at 100°C for 8 h and then calcined at 500°C for 5 h.
The core-shell [email protected]2 was prepared by a modified Stöber method. Typically, 0.5 g of ZnO was dispersed in 300 mL ethanol by sonication for 1 h followed by the addition of 2.5 mL TEOS. After stirring under 450 rpm for 4 h, 5 mL of NH3·H2O and 20 mL of water were added dropwise. The mixture was stirred under 450 rpm for another 4 h. Subsequently, the product was washed three times with ethanol and dried at 100°C for 8 h. The obtained sample was denoted as [email protected]2.
SAPO-34 and SAPO-11 zeolites were hydrothermally synthesized with various gels with the molar ratio of 3.0TEA : 0.03TEAOH : 1.0Al2O3 : 0.21SiO2 : 1.0P2O5 : 50H2O and 1 Al2O3:1 P2O5:0.3 SiO2:1 DPA:50 H2O, respectively42,43. Taking the synthesis of the SAPO-34 as an example, 4.7 g of pseudo-boehmite was dissolved in 50 mL deionized water to form alumina sol after stirring under 450 rpm for 2 h. Then 0.96 g of silica sol was added to the prepared alumina sol under stirring for 2 h. Subsequently, 14.2 g of TEA, 0.81 g of TEAOH, and 7.3 g of H3PO4 were added slowly under continual stirring for another 3 h until obtaining a homogeneous gel mixture. The gel mixture was sealed in a 200 mL Teflon-lined stainless-steel autoclave, followed by heating from room temperature to 200 oC at a rate of 2 oC min− 1. The crystallization was carried out at 200 oC under autogenic pressure for 72 h. After crystallization, as-synthesized samples were obtained after centrifugal separation, washing, and drying at 100 oC for 8 h. Finally, calcination was carried out from room temperature to 550 oC with a rate of 2 oC min− 1 and kept at 550 oC for 6 h to remove the organic template.
The bifunctional catalysts composed of ZnZrOx and each zeolite were prepared by five different integration manners, classified into (i) the dual-bed with the zeolite in the upstream bed and ZnZrOx oxide in the downstream bed, separated by quartz wool with 3 mm thickness, (ii) the dual-bed with ZnZrOx oxide in the upstream bed and the zeolite in the downstream bed, separated by quartz wool with 3 mm thickness, (iii) the stacking of ZnZrOx and zeolite individual granules with 30–60 meshes (grains, 250–600 µm), (iv) the stacking of ZnZrOx and zeolite individual granules with 150–200 meshes (grains, 75–100 µm), (v) the mortar-mixing for ZnZrOx and zeolite powders. They were denoted as (i) zeoliteǁZnZrOx, (ii) ZnZrOxǁzeolite (3 mm), (iii) ZnZrOx + zeolite (400 µm), (iv) ZnZrOx + zeolite (90 µm), and (v) ZnZrOx/zeolite, respectively. For the syngas to lower olefins, gasoline fractions, and aromatics, SAPO-34, SAPO-11, and H-ZSM-5 were used as the zeolite component, respectively. SAPO-34, SAPO-11, ZSM-5 zeolites, and ZnZrOx oxides with low zinc content have been optimized for the above three different reactions in our previous work42–44. The mass ratio of ZnZrOx and the specified zeolite was fixed at 1:1. The catalysts powders were pressed, crushed, and sieved to granules of 30–60 meshes or 150–200 meshes before reaction.
Both [email protected]2/H-ZSM-5 and ZnO/H-ZSM-5 were prepared by a mortar-mixing method. ZnO-H-ZSM-5 was synthesized by an impregnation method. For these three catalysts for propylene aromatization, the weight ratio of the ZnO oxide and H-ZSM-5 zeolite was fixed at 0.015/1.
X-ray diffraction (XRD) patterns were measured on a Rigaku Ultima IV diffractometer with Cu Kα radiation (45 kV and 30 mA) as the X-ray source. N2 physisorption experiment was conducted on Micromeritics Tristar II 3020 Surface Area Analyzer. X-ray fluorescence (XRF) spectroscopy, which could provide information of Si/Al and Si/(Si + Al + P) ratio of zeolite, was analyzed with an S8 TIGER XRF instrument with rhodium target (50 kV, 50 mA). Scanning electron microscopy (SEM) measurements were performed on a Hitachi S-4800 operated at 15 kV.
To observe the spatial distribution of dual components for the sieved catalysts, the ultramicrotomy of resin-embedded catalysts was performed over Ultramicrotome Leica EM UC7 with a diamond knife. The catalyst granules were first embedded in Epofix resin, put in the oven overnight at 60°C, and cut to 70 nm sections using a diamond knife. Sections were deposited on a carbon-coated copper grid (300 mesh). Subsequently, transmission electron microscopy (TEM) measurements were carried out on a Phillips Analytical FEI Tecnai20 electron microscope operated at an acceleration voltage of 200 kV.
NH3 temperature-programmed desorption (NH3-TPD) was performed on a Micromeritics AutoChemII 2920 instrument. Typically, the sample was pretreated in a quartz reactor with purge with high-purity helium. The NH3 adsorption was performed at 100 oC in an NH3-He mixture (10 vol% NH3) for 1 h, followed by TPD in He flow by raising the temperature to 600 oC at a ramp rate of 10 oC min− 1. H2 − D2 exchange reactions were performed in a quartz reactor at 200 oC. Typically, 0.1 g sample was pretreated with He flow at 400 oC for 1 h, followed by cooling to 200 oC. The sample was treated in pure H2 gas flow for 30 min. Then, pulses of high-purity D2 were quantitatively injected into the H2 stream. H2 (m/z = 2), D2 (m/z = 4), and HD (m/z = 3) were analyzed by mass spectrometry. The rate of HD formation was normalized by the specific surface area.
The syngas conversion was performed on a fixed-bed reactor built by Xiamen HanDe Engineering Company, Ltd. Typically, 0.6 g or 1.0 g of bifunctional catalysts was loaded in a titanium reactor (inner diameter, 8 mm). Syngas with an H2/CO ratio of 2:1 and a pressure of 3.0 MPa was introduced into the reactor. Argon with a concentration of 4.0 vol% in the syngas was used as an internal standard for the calculation of CO conversion. The temperature was raised to 370–400 oC at a rate of 2 oC min− 1 to start the reaction.
The propylene aromatization was performed on the same fixed-bed reactor. Typically, 0.6 g of bifunctional catalyst was loaded in a titanium reactor. The mixture with an N2/C3H6 ratio of 10/1 and a pressure of 5 bar was introduced into the reactor. The temperature was raised to 390 oC at a rate of 2 oC min− 1 to start the reaction. Products were analyzed by an online gas chromatography equipped with a thermal conductivity detector (TCD) and two flame ionization detectors (FID). A TDX-01 packed column was connected to TCD, while RT-Q-BOND-PLOT and HP-PONA capillary columns were connected to FID. Carbon balances were all better than 95%.
All data including supplementary materials and other findings within the article are available from the corresponding author upon reasonable request.