2.1. Materials
The fly ash used in this study came from Erdos, Inner Mongolia, China; the iron tailing slag came from the Anshan Mining Group, China; and the diatomite came from Fengsheng Mining, China. The reagents used included sodium hydroxide, concentrated nitric acid, sodium aluminate, tetrapropyl ammonium bromide, triethylamine, 3-aminopropyltriethoxysilane (APTES) and tetraethylenepentamine (TEPA), all of which were of analytically pure grade, purchased from the Beijing Chemical Company (Beijing, China) and the Tianjin Yongda Chemical Company (Tianjin, China).
2.2. Instruments
X-ray fluorescence spectrometry (XRF-1800, Shimadzu Corporation, Kyoto, Japan) was used to analyze the chemical compositions of the raw materials. An X-ray diffractometer (XRD, Ultima IV, Rigaku Corporation, Tokyo, Japan) was used to analyze the diffraction patterns of the synthetic materials. A field emission electron microscope (SEM, SU8010, Hitachi Ltd., Tokyo, Japan) and energy-dispersive spectrometer (SEM-EDS, Oxford Instruments Company, Oxford, UK) were used to observe the surface morphologies and microstructural characteristics of zeolites and analyze the elements. A surface area and pore analyzer (ASAP 2460, Micromeritics Instrument Corporation, Norcross, GA, USA) was used to measure the N2 adsorption–desorption isotherms before and after adsorbent modification. The N content of the modified zeolite was measured by EA (UNICUBE, Elemental Analysis Systems Co., Ltd., Hanau, Germany). Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher Company, Waltham, MA, USA) was used to measure the changes in the functional groups of adsorbents before and after modification. The adsorption of CO2 by the modified zeolites was measured by a thermogravimetric analyzer (TG-209, Neichi Scientific Instrument Trading (Shanghai) Co., Ltd., Shanghai, China). Before being measured by the above instruments, all samples were ground and passed through a 200-mesh sieve, and then dried in a drying oven at 80 °C for 8 h.
2.4. CO2 Adsorption Experiments
The adsorption isotherms of CO2 adsorbed by zeolites before and after modification were determined using ASAP 2460. The CO2 adsorption test of the modified zeolites was carried out on TG-209. Before CO2 adsorption, 50 mg samples were placed in an alumina crucible and activated at 150 °C in pure He gas for 30 min (50 mL/min) to remove adsorbed substances (H2O, CO2). Then, CO2 was absorbed at different temperatures (30, 50, 70, 90 °C) and gas flow rates (50, 70, 125 mL/min) for 30 min. The amount of CO2 adsorption was determined by the change in the mass of the adsorbent.
The adsorption selectivity of the amine-modified zeolites was tested on a BSD-MAB device (Bester Instrument Technology Co., Ltd., Beijing, China). Prior to the test, 1 g sample was filled into the penetrating column with a length of 66 mm and purged and activated for 1 h at 200 °C with a 10 mL/min He airstream. After activation, the penetrating column was removed from the activation furnace and placed in a water bath at 25 °C for cooling, until the temperature and mass spectrum signal were stable. Ensure that the gas entering the penetrating column is independent of each other, and start distributing gas after the signal is stable. MFC was used to control the flow rate of CO2 at 1.5 mL/min and N2 at 8.5 mL/min. From the beginning of the test, the gas signals of each component were detected and recorded by the mass spectrometer, and the penetration curve was drawn.
The reuse of the amine-modified zeolites was realized by the adsorption–desorption process. During the CO2 adsorption process, the airflow was switched from CO2 to He (50 mL/min), and the temperature was set at 150 °C for 30 min of desorption. The cycle experiment was completed through five adsorption–desorption processes.
2.6. Model and Calculation
To study the adsorption characteristics and mechanism of CO2 adsorption of the amine-modified zeolites, an isothermal adsorption model, adsorption kinetics model and thermodynamic model were used to fit the experimental data.
Adsorption isotherm models mainly include the Langmuir model [
21], Freundlich model [
22] and Toth model [
23]. The Langmuir model holds that in the process of adsorption of the gas phase by the solid phase, the gas molecule is simultaneously adsorbed and desorbed on the solid surface and finally reaches a dynamic adsorption equilibrium [
24]. The Langmir model is given by Equation (2).
where
is the equilibrium constant of the Langmuir model;
is the saturation adsorption amount, mmol/g;
p is the equilibrium adsorption pressure, kPa;
is the equilibrium adsorption capacity, mmol/g.
The Freundlich isotherm model is represented by Equation (3) [
25].
where
is the equilibrium constant of the Freundlich model;
is a constant that depends on the adsorption temperature;
is the equilibrium adsorption capacity, mmol/g;
p is the equilibrium adsorption pressure, kPa.
The Toth model is an extension of the Langmuir model. Different from the Langmuir model, the Toth model is more suitable for the adsorption of non-uniform surfaces. The Toth model is a three-parameter isothermal adsorption model obtained after introducing the non-uniform parameter
n on the basis of the Langmuir model [
26]. The Toth isotherm model is given by Equation (4).
where
is the equilibrium adsorption capacity, mmol/g;
is the saturation adsorption amount, mmol/g;
b is the equilibrium constant of the Langmuir model;
p is the equilibrium adsorption pressure, kPa;
n is a constant related to the degree of uneven adsorption on the surface of the adsorbent.
The pseudo-first-order kinetic model, pseudo-second-order kinetic model and Avrami kinetic model were used to fit the CO
2 adsorption kinetics of the amine-modified zeolites. The pseudo-first-order model considers that the adsorbate adsorption rate of an adsorbent is linearly related to the difference between the equilibrium adsorption capacity of the adsorbent and the adsorption capacity at a certain time [
27], which can be expressed as in Equation (5):
where
is the adsorption amount at time
t, mmol/g;
is the equilibrium adsorption capacity, mmol/g;
is a quasi-first-order rate constant, min
−1.
The pseudo-second order kinetic model assumes that the reaction rate is proportional to the volume fraction of the two reactants, and chemisorption is described as the main control step and rate-limiting step of the adsorption process [
28,
29], which is expressed as in Equation (6):
where
is the adsorption amount at time
t, mmol/g;
is the equilibrium adsorption capacity, mmol/g;
is a quasi-second-order rate constant, min
−1.
The Avrami dynamic model is a semi-empirical model based on particle nucleation theory, and it has been successfully used to describe the adsorption process of CO
2 by PE-MCM-41 and carbon nanotubes [
30]. The expression of the Avrami dynamic model is given by Equation (7):
where
is the adsorption amount at time
t, mmol/g;
is the equilibrium adsorption capacity, mmol/g;
is the Avrami rate constant, min
−1;
is the series of Avrami equations.
To accurately calculate the relevant data of the adsorption thermodynamic characteristics of amine-modified zeolites, the study obtained the expression of the adsorption isotherm through the Freundlich–Langmuir equation [
31], which can be expressed as
where
V is the adsorption amount, cm
3/g;
p is the pressure, bar;
a is the saturation adsorption capacity, cm
3/g;
b is the Langmuir constant;
c is a constant.
Then, the Clausius–Clapeyron Equation (9) was used to fit the single adsorption isotherm of CO
2 at different adsorption temperatures, and the equivalent adsorption heat was obtained [
32].
where
Q is the equal heat of adsorption, J/mol; R is the gas constant, 8.314 J/mol;
T1 and
T2 are the measured adsorption temperatures, K;
P1/
P2 is the adsorption pressure corresponding to the same adsorption amount on the measured isotherm of the sample at the above two adsorption temperatures, bar.