2. EXPERIMENTAL SECTION
2.1. Chemical. All chemicals were purchased from chemical companies and used without further purification: Iron (III) nitrate nonahydrate (Fe (NO3)3·9H2O, AR 98.5%, Sinopharm Chemical Reagent Co., Ltd. China), Zinc nitrate hexahydrate (Zn (NO3)2·6H2O), AR 99%, Sinopharm Chemical Reagent Co., Ltd. China), Sodium hydroxide (NaOH, AR, 96%, DAMAO CHEMICAL REAGENT FACTORY. China), Potassium hydroxide (KOH, AR, 85%, Sinopharm Chemical Reagent Co., Ltd. China), ZSM-5 ((SiO2)x(Al2O3)y, SiO2/Al2O3 (mole ratio) ≈ 25-30, Shanghai MackLin Biochemical Technology Co., Ltd, China), LUDOX®SM Colloidal Silica (30 wt%, Grace Trading (Shanghai) Co., Ltd. China), Potassium nitrate (KNO3, > 99.0%, Sinopharm Chemical Reagent Co., Ltd. China), Cerium nitrate hexahydrate (Ce(NO3)3·6H2O, AR, 99.5%, Shanghai MackLin Biochemical Technology Co., Ltd, China), were selected to fabricate the samples.
2.2 . Synthesis of ZnFe2O4. The synthesis of spinel-like ZnFe2O4 catalyst was referred to our previous report by a solvent-thermal method. Typically, 2.02g iron (III) nitrate hexahydrate and 0.74g zinc nitrate hexahydrate (Zn: Fe=1: 2 molar radio) were dissolved in distilled water (40 mL), and 0.1 mol NaOH was added into aqueous solutions to ensure alkaline solution. Transferring the solution to a 100 mL Teflon tube being set in a stainless autoclave and placing it in an oven at 180 °C for 8 h for the synthesis reaction. After the product was cooled to ambient temperature, the product was washed with 0.5 L deionized water to control the amount of Na and dried at 60 °C, denoted as Na-ZnFe2O4. Instead of NaOH with KOH, K-ZnFe2O4 can be obtained by the same procedure.
2.3. Synthesis of ZnFe2O4@H-ZSM-5. The ZnFe2O4@H-ZSM-5 composite catalyst with a core-shell structure coupled modified spinel-like ZnFe2O4 catalyst and H-ZSM-5 catalyst by spin coated method. Transferring the 0.2 g ZnFe2O4 catalyst (Na or K) wetted with colloidal silica (30 wt %) into a round-bottom flask. Subsequently, 0.15 g ZSM-5 was added into the flask, and spinning round-bottom flask made H-ZSM-5 and ZnFe2O4 in intimate contact, forming a composite catalyst with a capsule structure.
2.4. Synthesis of M-ZSM-5 (K, Ce). Surface acid properties of parent H-ZSM-5 zeolite (Si/Al = 25-30) was treatment through different ions-exchange strategies. Prior to treatment, H-ZSM-5 was calcinated at 550 °C to remove adsorbed water molecules. Subsequently, H-ZSM-5 (1.0g) was directly treated by one of nitrate solution of K or Ce ion (100 mL, 0.2 mol/L) at 80 °C for 12 h. The filtered H-ZSM-5 was rinsed several times with plenty of deionized water, and then it was calcined at 550 °C for 5 h. Finally, the obtained products were labelled as M-ZSM-5, in which M stands for the exchanged ion. 1 g K-ZSM-5 was treated in 100ml of NH3·H2O (0.2 mol/L) solution at 80 °C for 12 h, to obtain H-ZSM-5*.
2.5. Catalysis evaluation. The catalytic reactions were carried out in a stainless steel fixed-bed reactor. Typically, 0.35 g of composite catalyst (20-40 mesh), M-ZnFe2O4@M-ZSM-5, was used. Prior to reaction, the catalyst was in situ reduced in pure H2 with 50 mL/min at atmospheric pressure, 400 °C for 6 h. After reduction, the temperature was cooled to 320 °C and the reactant gas with H2/CO2 ratio = 3/1 (24.15% CO2, 71.71% H2, and 4.14% Ar was employed as an internal standard) was switched into the reactor. The reactions were conducted at 320 °C, 2.0 MPa with a reaction gas flow rate of 20 mL/min.
CO2 conversion and CO selectivity were analyzed by an online gas chromatograph (GC) using a thermal conductivity detector (TCD, Anhui Chromatographic instrument GC5190). The light hydrocarbons were analyzed by an online GC with a flame ionization detector (FID, Anhui Chromatographic instrument GC5190). N-octane (C8) as solvent was equipped to capture the heavy hydrocarbons in the effluents, and the products were analyzed by an off-line GC using an FID. The detailed product distribution was calculated as previously reported.
CO2 conversion, CO selectivity, and hydrocarbon selectivity were calculated on a molar carbon basis:
\begin{equation} \mathrm{\text{CO}}_{\mathrm{2}}\mathrm{\text{\ conversion}}\mathrm{\ }\mathrm{(\%)=}\frac{\mathrm{\text{CO}}_{\mathrm{2\ in}}\mathrm{-}\mathrm{\text{CO}}_{\mathrm{2\ out}}}{\mathrm{\text{CO}}_{\mathrm{2\ in}}}\mathrm{\times 100\%}\nonumber \\ \end{equation}
Where the CO2in and CO2out represent the moles of CO2 at the inlet and outlet, respectively.
\begin{equation} \mathrm{\text{CO\ selectivity}}\mathrm{\ }\mathrm{(\%)=}\frac{\mathrm{\text{CO\ }}_{\mathrm{\text{out}}}}{\mathrm{\text{CO}}_{\mathrm{2\ in}}\mathrm{-}\mathrm{\text{CO}}_{\mathrm{2\ out}}}\mathrm{\times 100\%}\nonumber \\ \end{equation}
Where the COout represent the moles of CO at the outlet.
The selectivity of hydrocarbons (Ci) in total hydrocarbons was obtained by the following formula:
\begin{equation} \mathrm{C}_{\mathrm{\text{i\ }}}\mathrm{\text{hydrocarbon\ selectivity}}\mathrm{\ }\mathrm{(C-mol\%)=}\frac{\mathrm{\text{mol\ of}}\ \mathrm{C}_{\mathrm{i}}\mathrm{\ \times i}}{\sum_{\mathrm{i=1}}^{\mathrm{n}}{\mathrm{\text{mol\ of}}\ \mathrm{C}_{\mathrm{i}}\mathrm{\times i}}}\nonumber \\ \end{equation}
The space-time yield (STY) of C5+ hydrocarbons on catalyst was obtioned by the following formula:
\begin{equation} \mathrm{STY=}\frac{\mathrm{\text{CO}}_{\mathrm{2}}\mathrm{\ Conversion\times\ }\left(\mathrm{1-CO\ selectivity\ }\right)\mathrm{\times}\mathrm{C}_{5+}\mathrm{\text{hydrocarbon\ selecitivity}}}{\mathrm{Catalyst\ mass\ (g)}}\nonumber \\ \end{equation}
2.6. Catalyst Characterization. X-ray diffractometer (SmartLab 9kW) with Cu Kα radiation was used to obtain diffraction patterns. The surface morphologies of the catalysts were investigated using scanning electron microscopy (SEM, Regulus 8230). Transmission electron microscopy (TEM, JEOL JEM-2100F) was used to observe particle size sand distribution at an acceleration voltage of 100 kV. The adsorption behavior of the catalyst was measured by using a Micromeritics AutoChem II Chemisorption Analyzer 2920. AutoChem II analyzer with a thermal conductivity detector (TCD) was used to acquire H2temperature programmed reduction (H2-TPR) profiles. A 50 mg sample was first pretreated with He for 1 h at 200 °C. When the temperature was cooled to 50 °C, a 5vol % H2/Ar gas mixture was supplied to the reactor (30 mL/min). Finally, H2-TPR profiles were obtained at temperatures ranging from 50 to 700 °C, with a heating rate of 10 °C/min. The same equipment was also used to investigate the CO2 or NH3 temperature-programmed desorption (TPD). A 50-mg sample was reduced for 2 h at 400 °C under a H2 gas flow (30mL/min). The temperature of the catalysts was reduced to 50 °C under a He gas flow (30 mL/min) after reduction. There actor was subsequently filled with a 5% CO2/He or 5% NH3/He gas mixture for 1h. He was then inserted into the reactor to remove the physically adsorbed CO2 or NH3. The CO2-TPD and NH3-TPD profiles were recorded from 50 to 800 °C with a heating rate of 10 °C/min. A Thermo Fischer Scientific ESCALAB 250 Xi instrument with a catalyst pretreatment chamber for altering the gas composition was used to conduct the X-ray photoelectron spectroscopy (XPS) analysis. The XAFS was recorded in table XAFS by Chuang Pu specreation. TG was tested by STA 449C Jupiter®. N2 physisorption was performed on a ASAP 2460 surface area & pore size analyzer. Prior to texts, the samples were degassed at 200 °C under vacuum conditions. Pyridine adsorption (Py-IR) spectra were recorded with a Thermo IS50 IR spectrometer from Thermo Scientific to the measurement, the sample cell was vacuumed to 10-2 Pa at 400 °C for 1 h, and the IR spectra were recorded at 350 °C. The 27Al magic-angle spinning (MAS) NMR was recorded by Bruker Avance 600 AV. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded on a Thermo iS50 FT-IR spectrometer. Before test, the sample was in situ reduced at 400 °C under 10 vol% H2 in Ar (50 mL/min) for 2 h and then switched to He for 15 minutes (50 mL/min). After that, the reaction gas was introduced into the reaction cell for reaction. Finally, at atmospheric and 320 °C, 20 mL/min of mixture gas (24 vol% CO2, 70 vol% H2 and 6 vol% Ar) passed through and DRIFT spectra were recorded. To the M-ZSM-5, the sample was in situ heating at 320 °C under He for 15 minutes (50 mL/min) and then switched to 20 mL/min of mixture gas as 10 vol% C3H6, 90 vol% Ar passed through and DRIFT spectra were recorded at atmospheric, 320 °C. After 10minutes, the mixture gas is stopped and the data is recorded for 20 minutes. Before performing DFT calculations, the composition and structure of the smallest monomer can be determined from literature and experimental results. Based on the determined minimum monomer structure, the smallest monomer is extracted from the polyatomic structure using the super cell builder in CASTEP, a sampling tool provided in the calculation software. This usually requires the setting of periodic boundary conditions and the adjustment of unit cell parameters to ensure the periodicity and compatibility of the smallest monomer structure. Structural optimization: after extracting the smallest monomer structure, structural optimization can be performed in the calculation software to determine the most stable zeolite structure. Then the adsorption energy is calculated.