Adsorption of HCl on Calcined Ca and Zn Hydrotalcite-like

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Adsorption of HCl on Calcined Ca and Zn Hydrotalcite-like Compounds (HTLs) at Medium-High Temperature in Flue Gas Jun Cao, Tianyu Chen, Baosheng Jin,* Yaji Huang, and Chunhong Hu

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Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: The high level of HCl generated from municipal solid-waste incinerators at 350−650 °C is considered a primary chlorine source for emitted toxic polychlorinated dibenzodioxins and polychlorinated dibenzofurans. Herein, calcium (Ca) and zinc (Zn) hydrotalcite-like compounds (HTLs) were prepared to remove HCl at different reaction temperatures and initial HCl concentrations. The mechanism of the adsorption of HCl by the HTLs and calcined HTLs (HTOs) was investigated using thermogravimetric analysis, X-ray powder diffraction, scanning electron microscopy, and surface-area measurements. The Ca- and ZnHTLs had similar structures and physicochemical properties. Both sorbents had good adsorption capacities for HCl that exceeded 90% over 456 min at 550 °C in an HCl concentration of 750 mg/g. The adsorption capacity of ZnHTOs was better than that of Ca-HTOs at 350 °C. The removal efficiency of Ca-HTLs was higher than that of ZnHTLs at temperatures above 550 °C. The Freundlich isotherm provided a good fit to the experimental data for the HTLs.

1. INTRODUCTION The high level of HCl generated from various municipal solidwaste incinerators is a major cause of environmental pollution and corrosion of incineration equipment. The process forms low-melting metal chlorides in the presence of heavy metals, which undesirably increases the heavy-metal content of the fly ash.1,2 Capturing HCl from industrial flue gas is one of the major issues in China. This can be achieved at low operating temperatures, but damage caused by HCl at high incineration temperatures is challenging to mitigate. Extensive research has demonstrated that at temperatures in the range of 250−450 °C the HCl concentration in flue gas plays an important role in the formation of toxic polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).3−5 In China, because of the lack of garbage classification and recovery, the HCl concentration at the outlet of waste incinerators is much higher (815−1630 mg/m3) than that in Western countries (163−815 mg/m3).6 Therefore, it is important to establish a means to remove HCl in flue gas at moderate temperatures. The major treatment processes to remove HCl include controlling the generation of HCl at the source and removing HCl from the flue gas.7−9 Semidry and wet methods have been widely used in industry to remove HCl from flue gas, but both approaches are expensive and cause difficulties in sludge handling. Because off these limitations, the dry method has become more attractive. Various low-cost adsorbents have been studied, including CaO, NaHCO3, and ZnO.10−12 However, many of these sorbents are unsatisfactory in terms of adsorption © XXXX American Chemical Society

capacity, especially at high temperatures. Additionally, their HCl adsorption capacities decrease drastically with increasing temperature. Consequently, recent research has focused on developing efficient sorbents displaying high adsorption capacity at medium-high temperatures. Layered double hydroxides or hydrotalcite-like compounds (HTLs) are interesting classes of anionic clays that have attracted the attention of academia and industry.13 HTLs can be represented by the general formula [M2+1−xM3+x(OH)2]x+[An−x/n]x−·zH2O where M2+ and M3+ are divalent and trivalent cations, respectively,14 x is equal to the molar ratio of M3+/(M2+ + M3+), and n is the valence of the interlayer anion A. Significant progress has been made in the application of HTLs to remove pollutants. Extensive research has demonstrated the use of HTLs, especially calcined HTLs (HTOs), as catalysts, catalyst precursors, or catalyst supports for the adsorption and removal of CO2,15−17 NOx,18 and harmful oxyanions13 in ion-exchange processes.19−22 Hydrotalcite-type materials have high adsorption capacities for anionic species.23−25 Our previous study investigated the HCl adsorption capacity of Ca−Mg−Al-HTOs.26 This sorbent had a high adsorption capacity for HCl, and the removal efficiency exceeded 95%, even reaching 99%, under the operating Received: Revised: Accepted: Published: A

July 9, 2018 November 23, 2018 December 11, 2018 December 11, 2018 DOI: 10.1021/acs.iecr.8b03092 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research conditions considered. The Ca−Mg−Al-HTLs have not been well characterized and the kinetics of their pyrolysis not extensively studied. Specifically, the activation energy for the pyrolysis of Ca−Mg−Al-HTLs has not been determined. Calcined Zn-HTLs (Zn-HTOs) are commonly used to adsorb pollutants, such as CO2,27 and H2S,28−30 at different temperatures. These sorbents have good efficiencies, thermal stabilities, and chemical reactivities. We report herein our investigation of Zn-HTOs to remove HCl at moderate temperatures. Our Ca- and Zn-HTLs were prepared by the coprecipitation method. Their adsorption capacity was compared with the common sorbent (CaO). The performance for the removal of HCl was explored at moderate reaction temperatures (350−650 °C) and initial HCl concentrations (450−1500 mg/g). X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and surface area measurements were used to characterize the original and calcined sorbents. Thermogravimetric analysis (TGA) was used to investigate the kinetics of HTLs pyrolysis.

Figure 1. Adsorbent performance system. (1) Electrical heating furnace; (2) filter; (3) condenser; (4) HCl gas infrared analyzer; (5) adsorption solutions; (6) computer; (7) N2; (8) mass flowmeter controller; (9) N2; (10) HCl.

2. EXPERIMENTAL SECTION 2.1. Materials. Raw materials to prepare the sorbent were all analytical-grade reagents. Mg (NO3)2·6H2O, Ca (NO3)2·4H2O, Al(NO3)3·9H2O, and Zn(NO3)2·6H2O were purchased from Longxi Chemical Reagent Factory, China. NaHCO3 and Na2CO3 were purchased from Shanghai Lingfeng Chemical Reagent Factory, China. HCl (1500 mg/m3) and N2 (99.999%) were obtained from Nanjing Shangyuan Gas Product Co. Ltd., China. The double-distilled Millipore deionized water was used during the synthesis. 2.2. Synthesis of HTLs. The HTLs were synthesized with reference to the procedure of a previous investigation.26 Before the experiment, all the vessels were washed by deionized water and dried in constant temperature drier. An aqueous solution containing Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, and Al(NO3)3· 9H2O was then kept under 60 °C and continuously added to the solution of NaHCO3 and Na2CO3, with molar ratios of Ca/Mg = 2, M2+/M3+ = 3, [OH−] = 2([M2+]+[M3+]) and [CO32−] = 1/ 2[M3+]. The two solutions were simultaneously titrated into the same vessel in 3 min and stirred vigorously for 30 min at 60 °C. The pH was then maintained at a constant value of 10. The pH of the suspension was measured with a pH meter (Sinomeasure, SIN-Ph-100). The suspension was aged at 80 °C for 1080 min. After that, the obtained solid products were collected and washed with deionized water until a pH value of 7 was obtained. The washed products were dried for 1440 min using a drier at 85 °C. The obtained sample had a nominal structural formula of Ca2MgAl1.2(OH)4.2(CO3)0.6·nH2O, was called CaMgAl-HTLs (Ca-HTLs) as Ca/Mg/Al as atomic ratio determined by energydispersive X-ray (EDX) facility. Repeating the above steps to prepare ZnMgAl-HTLs (Zn-HTLs) with the formula Zn1.8Mg0.9Al (OH)3.7(CO3)0.5·nH2O. The original and calcined HTLs (hereafter referred to as HTOs) samples were ground and sieved to 0.15−0.355 mm. 2.3. Adsorption Experiments. The HCl adsorption capacity of each sample was tested in an adsorbent performance system, shown in Figure 1. Adsorbent performance tests consisted of (1) introduction of gas into the system, (2) performance testing, and (3) HCl analysis. The introduced gas consisted of N2 and HCl, each controlled with a separate mass flowmeter. One part of N2 was mixed with HCl to obtain the desired initial HCl concentration. The remaining N2 was used as

balance gas. Performance tests were carried out on a fixed bed reactor with an electrical heating system accurate to ±1 °C. The concentration of HCl was determined online, after filtering and drying, with a model 7900FM HCL GFC analyzer provided by Signal Company. The gas analyzer was calibrated to an accuracy of 0.02 mg/g. The adsorption capacity of calcined HTLs samples was determined as follows. An amount of HTLs (0.5 g) was placed in the center of the quartz glass tube (length 90 mm, internal diameter 16 mm) of a fixed bed reactor. After leak checking, N2 was pumped into the reactor at 110 mL/min for 30 min. The reactor was then heated to a specified temperature. The concentration of HCl was adjusted between 450 and 1500 mg/g, which is a typical range encountered in the exhaust gas of incinerators in China. Specific concentrations of HCl were obtained by mixing the pure gases (HCl and N2). The flow rate of the mixed gas was 0.5 L/min. Residual HCl was continuously analyzed after being condensed and filtered. Gas from the reactor was released into the environment after passing through an NaOH adsorption solution. HCl removal efficiencies and the average removal efficiency of each type of sample were calculated using eqs 1 and 2, respectively, ji C − Ct zyz η = jjj 0 z × 100% j C0 zz k { η̅ =

1 N

(1)

N

∑ ηi

(2)

i=1

where η is the removal efficiency of the sorbent, η̅ is the average removal efficiency of the sorbent, C0 is the initial HCl concentration (mg/m3), Ct is the outlet concentration of HCl at time t (mg/m3), N is the number of measurements, and ηi is a single measurement of removal efficiency. The adsorption capacity of samples qt were calculated by eq 3, qt = mg of HCL/g of HTLs = 1 × 10−3 B

∫0

t

(C0 − Ct )V dt mHTLs

(3)

DOI: 10.1021/acs.iecr.8b03092 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research where V is the flow rate of the flue gas (L/min), mHTLs is the mass of HTLs (g), t is the reaction time (min), and qt is the total amount of HCl adsorbed on the HTLs from time zero to t. Each test was performed in identical conditions to guarantee reproducibility, and the margin of error between replicated experiments was maintained below 3%. 2.4. Characterization of HTL Samples. X-ray Diffraction. The crystalline phases within Ca-HTLs and Zn-HTLs samples were characterized across 2θ from 5 to 80° using a D8 Advance Diffractometer (Bruker AXS Ltd., Germany) with a Cu−Kα Xray source (γ = 0.15406 nm) operating at 40 kV and 100 mA. The scanning speed was 2°/min in increments of 0.02°. Diffraction patterns from all samples were collected at room temperature, and after calcination at 350 and 550 °C in a N2 atmosphere. Scanning Electron Microscopy (SEM) Analysis. Scanning electron microscopy (SEM) of HTLs and calcined HTLs was performed on an S-4800 microscope (Hitachi, Japan) equipped with an energy-dispersive X-ray (EDX) silicon detector. Analyses were performed with an acceleration voltage of 15 kV. Surface Area Measurements. The surface areas of Ca-HTLs and Zn-HTLs, both before and after calcination at 350 and 550 °C, were determined from N2 adsorption data measured at −196 °C using an ASAP 2020 sorptometer (Micromeritics, USA). Specific surface areas were calculated according to the Brunauer−Emmett−Teller (BET) method using adsorption isotherms. Pore size distributions were calculated from desorption curves using the Barrett, Joyner, and Halenda method.31,32 Thermogravimetric Analysis (TGA). Thermogravimetric analyses were performed using a turbocharged thermogravimetric system at Southeast University. The system consisted of three parts: a thermogravimetric analyzer (TherMax 500; Thermo Cahn, USA), a temperature-controlled heating system equipped with a weighing mechanism, and a data acquisition and processing system. The weighing mechanism is sensitive to 8 mV /μm based on eddy current dampening. Using Al2O3 as a reference material, the thermal balance accuracy is 1 μg. The TGA analyses were performed under a flow of N2 at a heating rate of 20 °C/min from room temperature to 700 °C, which was held for 20 min.

3. RESULTS AND DISCUSSION 3.1. Dechlorination. The HCl adsorption breakthrough values of the sorbents were measured. The effects of reaction temperature and initial HCl concentration on the removal of HCl gas were also investigated (Figure 2). 3.1.1. Adsorption Capacity. Hydrogen chloride generated from the treatment of municipal solid waste and coal thermal treatment is most commonly treated using CaO.33 In this study, the HCl adsorption capacities of laboratory-made sorbents (Caand Zn-HTOs) and CaO were measured at 550 °C in flowing (0.5 L/min) HCl gas at a concentration of 750 mg/g. The sorbent sample size was fixed at 0.5 g. The breakthrough curves and removal efficiencies are shown in Figure 2a. At the beginning of the experiments, the removal efficiency of CaO was low (85.52%), whereas the removal efficiencies of the Caand Zn-HTOs were substantially higher (96.9 and 97.7%, respectively). The removal efficiency of CaO slightly increased over the first 200 min. Its removal efficiency rapidly declined. The removal efficiencies of both the Ca- and Zn-HTOs were high, exceeding 90%, and remained at such high levels for 456 min. The removal efficiency of the Zn-HTOs increased during

Figure 2. Effects of operating conditions on dechlorination test. (a) Removal efficiencies and breakthrough curves for three sorbents. (b) Effect of temperature on the average removal efficiency of sorbents. (c) Effect of initial HCl concentration on the average removal efficiency of sorbents. C


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Figure 3. TGA/DTGA analysis results for Ca-HTLs and Zn-HTLs under N2.

the first 94 min, then declined, whereas that of the Ca-HTOs remained relatively constant. The laboratory-prepared sorbents had better removal properties than the traditional CaO sorbent. 3.1.2. Effect of Reaction Temperature. Figure 2b shows the average removal efficiencies of the HTOs (0.5 g) after reacting with flowing (0.5 L/min) HCl gas (750 mg/g) at temperatures ranging from 350 to 650 °C. The removal efficiencies of the Caand Zn-HTOs both first increased as the temperature increased from 350 to 450 °C, then decreased as the temperature increased further. At 350 °C, the removal efficiency of the ZnHTOs was slightly higher (96.35%) than that of the Ca-HTOs, which was attributed to the low decomposition temperature of ZnCO3. The removal efficiencies of both kinds of HTOs increased when the temperature was raised to 550 °C. This behavior was likely due to greater release of H2O and CO2 through open pores and adsorption sites, which enabled HCl to diffuse to the inner regions of the sorbents. Additionally, the decomposition and reaction rates increased with increasing temperature. Higher concentrations of produced CaO, CaCO3, and ZnO promoted dechlorination, thereby increasing the removal efficiency of the sorbent. 3.1.3. Effect of Initial HCl Concentration. Figure 2c shows the performance of the Ca- and Zn-HTOs (0.5 g) after reaction at 550 °C with HCl gas flowing at 0.5 L/min and inlet HCl concentrations of 450, 750, 1125, and 1500 mg/g. Figure 2c reveals that the initial HCl concentration had only a slight effect on the removal efficiency by the Ca-HTOs. When the initial concentration was increased from 450 to 1,500 mg/g, the average removal efficiencies by the Ca-HTOs decreased slightly, but nevertheless exceeded 90%. However, the removal efficiency of the Zn-HTOs declined steadily from 94.77 to 89.75% under the same conditions. A probable explanation is the onset of CaCO3 decomposition at 550 °C to form CaO and CO2. This generated more open pores through which HCl could diffuse between the layers of the sorbent, thereby enabling more CaO to react with the flowing HCl. Accordingly, the removal efficiency slowly decreased with increasing HCl concentration. However, the thermal decomposition of ZnCO3 was nearly complete by 350 °C, and so the removal efficiency of the Zn-HTOs declined. Both Ca- and Zn-HTOs thus displayed good adsorption capacities toward HCl. These HTOs were further characterized to clarify the mechanism of HCl adsorption.

3.2. Characterization. 3.2.1. Pyrolysis Analysis of HTLs. Figure 3 shows the TGA and differential TGA (DTGA) curves for the pyrolysis of the Ca- and Zn-HTLs in flowing nitrogen gas. The DTGA curves displayed three weight loss stages below 700 °C. Stage I (room temperature to 100 °C) was attributed to the evaporation of moisture that did not alter the layered structure. Stage II of the Ca-HTLs differed from that of the Zn-HTLs. For the latter, it occurred at a lower temperature (116−423 °C) with a broad peak at 186 °C, while it appeared at 166−341 °C with a peak at 269 °C for the Ca-HTLs. Stage II was attributed to the removal of water (mostly structural water intercalated in the interlayer galleries) and loss of CO2 produced by the decomposition of the carbonate anions present in the metal hydroxide layers.34,35 XRD and SEM analyses supported this concept. During this stage, the Zn-HTLs displayed the highest weight loss of ca. 11.5%. Stage III of both Ca- and Zn-HTLs occurred over a temperature range of 470−670 °C with peak weight loss at 540 °C. This process was attributed to carbonate decomposition and dihydroxylation of brucite-like sheets, consistent with the results of Chen et al.36 and Chang et al.27 The greatest weight loss of Ca-HTLs was 21.24%. Moreover, after Stage I, the weight loss of the Zn-HTLs was greater than that of the Ca-HTLs. The TGA results led us to choose samples calcined at 350 and 550 °C for further study. 3.2.2. XRD and SEM Analysis. The XRD patterns of the Caand Zn-HTLs and their calcined forms are shown in Figure 4. The sharp and symmetric basal reflections of the (003) and (006) crystal planes at low 2θ values (10−25°) and the peaks for the (009), (015), and (018) planes at higher 2θ values (30−45°) indicated well-formed crystalline layered structures. These were in excellent agreement with the JCPDS file 48−1023 for HTLs. The in-plane diffraction peaks corresponding to the (110) and (113) planes appeared at 2θ values ranging from 60 to 66.2°, which indicated good dispersion of metal ions in the hydroxide layers.37 The calculated lattice parameters (a = 2d110 and c = 3d003) are listed in Table 1. The d003 spacing is equivalent to the thickness of the unit layers. The d003 values of the Ca- and ZnHTLs were 0.758 and 0.763 nm, respectively. The d003 value of the Zn-HTLs is similar to the 0.766 nm reported by Yang.38 The thickness of the brucite-like layers was 0.477 nm39 and the calculated gallery heights were 0.281 and 0.286 nm, respectively. These heights are close to the size of the CO32− anion, which indicated monolayer arrangement of CO32− ions in the D


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The d110 values were 0.152 and 0.153 nm (Table 1), which indicates that the charge density on the crystal surface of the Caand Zn-HTLs was essentially identical. The d003 values and gallery heights of the prepared Ca- and Zn-HTLs were all slightly less than those of Mg−Al hydrotalcite (0.776 and 0.293 nm, respectively). This is likely due to the difference in ionic radii of the metals. The ionic radius of Mg2+, Zn2+ and Ca2+ are 0.065, 0.074, and 0.099 nm, respectively.40 Added Ca and Zn ions twist the hydrotalcite layers and increase the force between the laminate and interlayer anions. Figure 4 reveals that each of the Ca- and Zn-HTLs was a mixture of a crystalline HTL phase and an amorphous phase; the latter were identified as CaCO3 and Zn4(CO3)(OH)6·nH2O, respectively. Figure 5a, b shows SEM micrographs of the prepared Ca- and Zn-HTLs, respectively. The micrographs show well-developed layered and platelet structures. Images of the Ca-HTLs show hexagonally shaped particles, as well as particles of other shapes that are believed to be CaCO3. The micrographs of the ZnHTLs show a homogeneous distribution of hexagonally shaped particles, but also a few irregular particles that are believed to be complex Zn oxides. The Zn-HTLs particles appeared more regular and uniform than those of the Ca-HTLs. Additionally, particle size of Zn-HTLs (6−12 μm) was much larger than that of Ca-HTLs (ca. 2−5 μm). Heating the Ca- and Zn-HTLs samples to 350 and 550 °C caused destruction of their respective structures (Figure 4). The crystalline phase of the Ca-HTLs partially transformed into amorphous phases, as indicated by the broad and indistinct peaks in the XRD pattern. Possible components include Ca-HTL, Ca−Mg mixed oxides, CaCO3, and CaO. The Zn-HTLs likely partially transformed into Zn− Mg mixed oxides and ZnO. The respective oxides of the sorbents reacted with HCl to generate the corresponding chlorides. At 550 °C, the main decomposition products were CaO and ZnO. Unlike the Ca-HTOs, the Zn-HTOs formed at 350 °C were nearly carbonate-free because the decomposition temperature of ZnCO3 is much lower than that of CaCO3 (ca. 300 °C). This explained why Zn-HTOs had better removal efficiency than CaHTOs at 350 °C. Figure 5c, d provide images of the HTLs calcined at 350 °C; the hexagonal sheets are notable. Other morphological changes occurred in the Ca- and Zn-HTOs. Calcination partially converted plate-like particles into small irregular particles. A significant number of new channels formed, especially in the CaHTLs particles. There are fewer hexagonally shaped particles in the Ca-HTLs images (Figure 5a) than in images of the CaHTOs (Figure 5c), and the particles of Ca-HTLs appear more uniform and looser than the Ca-HTOs particles. The SEM images of Ca- and Zn-HTOs showed agglomerates of irregular morphology. As revealed by the XRD results discussed above, metal ions assisted the decomposition of OH− and CO32− ions during calcination to form metal mixed oxides, CaCO3, CaO, and ZnO, which changed the structures of the HTLs. 3.2.3. Nitrogen Adsorption−Desorption Analysis. Nitrogen adsorption−desorption isotherms of the Ca- and Zn-HTLs were measured at −196 °C (Figure 6). The isotherms were Type IV according to the IUPAC classification, which indicated mesoporous adsorbent structures with strong adsorbent− adsorbate interactions.32,41 Figure 6a shows that the hysteresis loop of the Ca-HTLs was Type H4, and Figure 6b shows that the hysteresis loop of the Zn-HTLs was Type H1. Hysteresis loops are associated with capillary condensation in the mesostructures. The different hysteresis loops observed for the Ca- and ZnHTLs were attributed to differences in pore size shape and

Figure 4. XRD patterns of origin and calcined HTLs. (a) Ca-HTLs; (b) Zn-HTLs.

Table 1. Structure Parameters of Ca-HTLs and Zn-HTLs sample CaHTLs ZnHTLs

d003 (nm)

d110 (nm)

lattice parameter a (nm)

lattice parameter c (nm)

gallery height (nm)

0.758

0.152

0.304

2.274

0.281

0.763

0.153

0.306

2.289

0.286

interlayer. Table 1 indicates that the thickness of the unit layers (d003) and the gallery height of the Zn-HTLs were all higher than those of the Ca-HTLs. This suggests that the Ca-HTLs require more energy, i.e., higher temperature, than the Zn-HTLs for complete decomposition. This also explained the greatest weight loss transition for the Ca-HTLs, which occurred at higher temperature and was smaller than that of the Zn-HTL. However, Figure 3 shows that at high temperature, the decomposition rate of the Ca-HTLs greatly increased and was higher than that of the Zn-HTLs. The d110 spacing corresponds to the size of adjacent metal ions. It reflects the charge density of laminates, which is related to the atomic composition of the crystal plane. The value of d110 decreases with increasing density of the laminate atoms. E


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Figure 5. SEM micrographs of samples (a) original Ca-HTLs; (b) original Zn-HTLs; (c) calcined Ca-HTLs; (d) calcined Zn-HTLs.

larger surface areas. Table 2 indicates that the Ca- and Zn-HTLs had relatively low surface areas, but calcination caused these to increase because of decomposition caused by water molecules and CO32− anions in the interlayer and the formation of openings. The experimental and characterization results were used to identify the chemical transformations of the main functional groups in the Ca- and Zn-HTLs. At ca. 200 °C, their layered structures began to collapse because of dihydroxylation of the sheets42 and transformation of CO32− (reactions 1a and 2a). Calcination of the Zn-HTLs produced ZnCO3, which also began to decompose at this temperature into ZnO (reaction 3a). Pyrolysis of the Ca-HTLs at 430 °C generated CaCO3,43 which

homogeneity. The Ca-HTLs loop closed at ca. 0.45 relative pressure (P/P0), whereas the Zn-HTLs loop closed at a higher relative pressure of ca. 0.6. This indicated that the Zn-HTLs pore size was larger than that of the Ca-HTLs (Figure 6). The average pore-size distributions and surface areas for the Ca- and Zn-HTLs and HTOs are listed in Table 2. The pore diameter of the Zn-HTLs (28.57 nm) was larger than that of the Ca-HTLs (26.47 nm), which is in good agreement with the nitrogen adsorption−desorption isotherms. The different pore diameter was attributed to differences in aggregation of the primary particles. The larger measured surface area of the CaHTLs was attributed to a smaller crystallite size. The results suggest that hydrotalcites having smaller crystallite sizes have F


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Figure 6. N2 adsorption−desorption isotherms at −196 °C for Ca- and Zn-HTLs.

Table 2. BET Surface Area and Pore-Size Distrbution for Caand Zn-HTLs sample Ca-HTLs Zn-HTLs calcined Ca-HTLs 350 calcined Zn-HTLs 350 calcined Ca-HTLs 550 calcined Zn-HTLs 550

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

44.53 38.84 120.38

0.27 0.30 0.32

26.46 28.57 34.86

114.67

0.34

38.73

140.12

0.30

36.93

136.95

0.29

39.48

Ca‐Mg mixed oxides + HCl → Ca‐Mg mixed chloride + H 2O

2CaCO3 + 3HCl → CaCl 2 + H 2O + CaClOH + 2CO2 (9)

CaO + 2HCl → CaCl 2 + H 2O Zn‐HTLs + HCl → Zn‐Cl HTLs + CO2 + H 2O

→ Zn‐Mg mixed chloride + H 2O

198 − 369° C

(1a)

Zn1.8Mg 0.9Al(OH)3.7 (CO3)0.5 ·nH 2O 245 − 450° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn‐HTLs + Zn‐Mg mixed oxides + ZnCO3 (2a)

≥ 350° C

(3a)

436 − 680° C

Ca‐HTLs ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaCO3 + CaO + CO2 + H 2O + Ca‐Mg mixed oxides (4) (5)

Zn‐HTLs ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→

ZnO + 2HCl → ZnCl 2 + H 2O

(14)

qmKLC0 1 + KLC0

(Langmuir isotherm model)

where qe is the amount absorbed at equilibrium (mmol/g), C0 is the initial HCl concentration (mmol/g), KL is the adsorption equilibrium constant (L/mmol), and KF and n are the Freundlich constants that are related to the adsorption capacity and the intensity of adsorption, respectively. Nonlinear curve fitting provided the parameters of the adsorption isotherm models (Table 3). The high correlation coefficients (R2)

(6)

The HCl gas mainly reacts with HTO through the following reactive formulas, For Ca-HTO: Ca‐HTLs + HCl → Ca‐Cl HTLs + CO2 + H 2O

(13)

qe = KFqe1/ n(Freundlich isotherm model)

441 − 621° C

Zn‐Mg mixed oxides + ZnO + CO2 + H 2O

ZnCO3 + 2HCl → ZnCl 2 + H 2O + CO2

qe =

≥ 550° C

CaCO3 ⎯⎯⎯⎯⎯⎯⎯→ CaO + CO2

(12)

At 350 °C, the Ca-HTOs removed HCl according to reactions 7, 8, and 9, whereas the Zn-HTOs removed HCl according to reactions 11, 12, 13, and 14. The Zn-HTOs contained more adsorbing sites than the Ca-HTOs for HCl molecules because of the higher polarity and chemical reactivity of the surface. Consequently, the Zn-HTOs had the higher adsorption capacity. At 550 °C, the Ca-HTOs removed HCl primarily via reactions 7, 8, and 10; reaction 9 was a minor pathway. The ZnHTOs removed HCl via reactions 11, 12, and 14. Continued decomposition of the Ca-HTLs, Ca-HTOs, and CaCO3 increased the rate of the chemical reactions via increased internal and external diffusion mechanisms, as supported by the XRD, SEM, and TGA analyses. 3.2.4. Adsorption Isotherm. Figure 7 shows the adsorption isotherms of the HTOs toward HCl. The data were fitted using the Langmuir and Freundlich isotherm models as follows:

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca‐HTLs + Ca‐Mg mixed oxides + CaCO3

ZnCO3 ⎯⎯⎯⎯⎯⎯⎯→ ZnO + CO2

(11)

Zn‐Mg mixed oxides + HCl

Ca 2MgAl1.2(OH)4.2 (CO3)0.6 ·nH 2O

+ ZnO + CO2 + nH 2O

(10)

For Zn-HTO:

similarly decomposed (reactions 4 and 5). The carbonates nearly completely disappeared to form ZnO during pyrolysis of the Zn-HTLs37 (reactions 3a and 6).

+ CO2 + nH 2O

(8)

(7) G



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-25-83794744. Fax: +8625-83795508. ORCID

Baosheng Jin: 0000-0003-2115-8758 Yaji Huang: 0000-0002-0176-4358 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the natural science foundation of China (51476032 and 51706042). We thank the contribution of Songshan Cao, from School of Energy and Environment, Anhui University of Technology, for the revision and improvements of the writing of this work.

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Table 3. Adsorption Isotherm Parameters for HCl Adsorption by Calcined HTLs Langmuir isotherm model

Freundlich isotherm model

samples

qm

KL

R2

KF

1/n

R2

Ca-HTLs Zn-HTLs

49.5184 45.4908

0.02487 0.02492

0.992 0.979

2.418 2.145

0.633 0.644

0.994 0.990

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Figure 7. Adsorption isotherms of HCl by calcined HTLs.

indicated that both types of isotherm models fitted the experimental points relatively well, especially the Freundlich model, R2 was 0.99. Table 3 indicates that the Ca-HTLs displayed a slightly better adsorption density than the Zn-HTLs. The results further indicate that both the Ca- and Zn-HTLs had good adsorption capacity toward HCl. The values of 1/n were both less than 1, which further demonstrated that the Ca- and Zn-HTOs had good adsorption properties toward HCl.

4. CONCLUSIONS The calcined products of the Ca-HTLs and Zn-HTLs were applied in HCl removal and both have shown good removal efficiencies. The prepared Ca- and Zn-HTLs had the similar structure and physicochemical properties, and the chemical formula is Ca2MgAl1.2(OH)4.2(CO3)0.6·nH2O and Zn1.8Mg0.9Al(OH)3.7(CO3)0.5·nH2O, respectively. The results reveal that when the temperature falls below 350 °C, the adsorption capacity of Zn-HTOs was better than Ca-HTOs, then the opposite pattern was observed. This is because the activation energy of Zn-HTLs is lower than Ca-HTLs. At lower temperatures, more new pores were formed and Zn carbonate were decomposed to remove HCl. With the increasing in temperatures, the removal efficiencies of Ca-HTOs became higher, whereas in the case of the consumption of the Zn-HTOs sorbents, its removal efficiencies dropped. Freundlich isotherm fitted well to the experimental data of both sorbents, since the correlation coefficients show satisfactory values. The values of 1/ n were both less than 1, further prove both Ca- and Zn-HTOs have good adsorption properties for removing HCl. H


Article

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Adsorption of HCl on Calcined Ca and Zn Hydrotalcite-like

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