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Abstract

Adsorbents consisting of Mn_xO_y, Zn_xO_y, and Mn_xO_y–Zn_xO_y supported on activated carbon (AC) have been successfully prepared and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD), Fourier-transform infrared spectroscopy (FTIR), specific surface area (BET) and SEM. The results show that Mn is present in the forms of Mn³⁺ and Mn⁴⁺, and Zn is present in the form of Zn²⁺. Mn and Zn mainly exist in the forms of MnO₂ and ZnO respectively, as the active components of the adsorbents. MnO₂–ZnO/AC was found to exhibit the best H₂S desulfurization performance. At 313 K, the breakthrough time was 108 min, the saturated sulfur capacity was 334.7 mg/g, and the degree of desulfurization was 74.2%. The H₂S removal capacities of the different adsorbents decreased in the order: MnO₂–ZnO/AC > ZnO/AC > MnO₂/AC > AC. The MnO₂–ZnO/AC adsorbent retained some activity for removal of H₂S after regeneration at 643 K for 2 h in air.

Keywords

ZnO activated carbon adsorptive desulfurization

Introduction

With ever-increasing energy demands, there is clearly a pressing need to maximize usage efficiency. Natural gas represents a high-quality, efficient, and clean energy source, and has been widely used as such in most developed countries (Zou et al., 2013). However, natural gas derived from mining processes is often contaminated by acidic gases such as hydrogen sulfide (H₂S). Toxic H₂S is not only harmful to human health, but can also corrode equipment in industrial production, poison catalysts, and so on, which has caused great damage and loss. Therefore, the H₂S content of the gas must be reduced to acceptable levels before it can be used (Chaichanawong et al., 2010; Huang et al., 2006; Koriakin et al., 2012; Jaiboon et al., 2014).

At present, the main methods applied for gas desulfurization can be divided into dry and wet processes. Wet desulfurization is used to high concentrations of H₂S, it has the advantage of being applicable on a large scale, but its disadvantages high power consumption, bulky equipment, high running costs, and the need for strictly controlled conditions. In dry adsorption, desulfurization contact is maintained between the gas phase and a solid phase, H₂S is either physisorbed or chemisorbed on a solid adsorbent. Dry process desulfurization technology has a long history. Its advantages are the simple technological process, low energy consumption, and high desulfurization efficiency and precision. The most widely used materials in dry process desulfurization are activated carbon, silica, and molecular sieves (Lim et al., 2008; Sun et al., 2007; Wang et al., 2014).

By virtue of the high specific surface area and porosity of activated carbon, it can be loaded with more metal cations to form an efficient adsorbent, and is therefore the material of choice for dry desulfurization. However, adsorption on unmodified activated carbon is mainly physisorption, and hence the adsorption performance is poor. As reported by Fang, an adsorbent made of activated carbon loaded with metal oxide shows superior performance in H₂S removal (Fang et al., 2013). Indeed, more and more researchers are focusing on activated carbon loaded with metal oxides for desulfurization.

In the present work, Zn_xO_y/AC, Mn_xO_y/AC, and Mn_xO_y–Zn_xO_y/AC have been prepared, and their adsorptive desulfurization performances toward a simulated low H₂S concentrations gas have been studied. The Mn_xO_y–Zn_xO_y/AC adsorbent showed a higher sulfur capacity and a faster desulfurization rate, and performed at a low adsorption temperature, making it the most economic and efficient adsorbent for the removal of H₂S from natural gas. It is hoped that this work will not only provide theoretical insight into the operation of metal-oxide-modified activated carbon in the deep desulfurization of natural gas, but also guidance for its industrial application.

Experimental

Preparation of adsorbents

Preparation of activated carbon-supported manganese oxide (Mn_xO_y/AC) AC is cocoanut activated carbon from Shanxi Xinhua Activated Carbon Factory. Its BET surface area is 685.7 m²/g and particle sizes are within 0.5–1 mm. A certain amount of potassium permanganate was dissolved in 100 mL of deionized water and stirred at room temperature for some time. Then, a calculated amount of AC was added into the resulting solution and stirred for 30 min (The mass ratio of manganese dioxide to AC were 1:1.). About 150 mL of hydrogen peroxide was dripped into the previous suspension with magnetic stirring. The titration speed was carefully controlled at 0.5–1 drop per second to make the potassium permanganate reacted with hydrogen peroxide completely during titration (equation (1)).

{2 KMnO}_{4} + H_{2} O_{2} = {2 MnO}_{2} + {2 O}_{2} ↑ + 2 KOH

(1)

Activated carbon supported zinc oxide (Zn_xO_y/AC) was synthesized by impregnating AC with zinc nitrate solution (5 wt%). After filtration and drying in air, the solid was calcined at 573 K for 2 h to obtain Zn_xO_y/AC adsorbent with a Zn:AC mass ratio of 0.5:1.

Activated carbon-supported manganese oxides and zinc oxides (Mn_xO_y–Zn_xO_y/AC) were synthesized by impregnating PMA with zinc nitrate solution (5 wt%). After filtration and drying in air, the solid was calcined at 573 K for 2 h to obtain Mn_xO_y–Zn_xO_y/AC adsorbent with an Mn:AC:Zn mass ratio of 1:1:0.5.

Characterization of adsorptions

XPS experiments

X-ray photoelectron spectra were obtained by using an K-Al pha (Thermofisher Scienticfic) with Al Ka X-radiation (the resolution of the instrument is 1.15 eV).

X-ray diffraction experiments

X-ray diffraction was obtained by using an D/max-2200PC (Rigaku Corporation) at 40 kV and 30 mA using monochromatized Cu-Ka radiation. The patterns were measured over the 2θ range from 10° to 80° at a scan step of 10°/min.

Fourier transform infrared spectroscopy experiments. Fourier transform infrared spectroscopy (FTIR) were obtained using the KBr disk technique on a TENSOR 27 (BRUKER) FTIR spectro photometer (the resolution of the instrument is 4 cm⁻¹). Background and sample were scanned in a minute by using 24 DigiTect TM detector system and ROCKSOLIDTM interferometer.

BET experiments

The specific surface area and total pore volumes of catalyst were calculated on a BET apparatus (NOVA2000e) with N₂ adsorption. All the samples were outgassed at 180℃ until the vacuum pressure was 6 mm Hg. The adsorption isotherms for nitrogen were measured at −196℃ (liquid N₂). The surface area (S_BET) of each adsorbent was calculated from the adsorption isotherm data using the Brunauer–Emmett–Teller (BET) equation at a relative pressure (P/P₀) between 0.05 and 0.35. The total pore volume (V_total) was assessed from the volume of nitrogen at P/P₀ = 0.95.

SEM experiments

SEM was obtained in the vacuum conditions with accelerating voltage of 100 V to 30 kV by using a SIGMA scanning electron microscopy (Carl Zeiss AG, Germany) instrument to enlarge the sample to 2000 ∼ 10,000 times.

Adsorption experiments

Measurements of adsorbent activity were conducted in a fixed-bed reactor. The fixed-bed reactor, which consisted of a quartz glass tube of internal diameter 10 mm and height 300 mm (the effective bed height of adsorbent was 90 mm), was positioned vertically in a clam shell furnace. 0.2 g of the prepared adsorbent was loaded into the center region of the reactor. The up and down sides of the adsorbent were filled with quartz sand. The model fuel gas from a feed tank passed through a reducing valve and entered into the reactor at a flow rate of 20 mL·min⁻¹. Any unabsorbed H₂S gas in the effluent gas would be absorbed by the liquid in the absorption bottle, and the H₂S content of the absorption liquid was determined by iodometry at intervals of 5 min. The breakthrough concentration was defined as that at which the H₂S concentration in the effluent gas reached 10% of the initial gas concentration, that is 2 mg/L. The H₂S capacity, saturated H₂S capacity, and H₂S removal rate were calculated according to the concentration of sulfur in the absorption liquid after the experiment.

The H₂S saturation capacity is defined as the amount of adsorbed H₂S per gram of adsorbent when the H₂S concentrations in the effluent and in the initial gas are the same. It can be calculated as follows

q = \frac{c_{o} Qt - Q \int_{0}^{t} c d t}{m}

(2) where q is the H₂S breakthrough capacity, mg·g⁻¹; c_o is the initial concentration of H₂S, mg/L; Q is the gas flow rate of the model fuel gas, mL·min⁻¹; c is the H₂S concentration in the effluent gas, mg/mL; t is the adsorption time, min; and m is the mass of adsorbent, g.

H₂S saturation capacity is defined as the amount of adsorbed H₂S per gram of adsorbent when the H₂S concentrations in the effluent and in the initial gas are the same. It can be calculated as follows

q_{m} = \frac{V \times ω_{H_{2} S} - (c_{I_{2}} v_{I_{2}} - 0.5 c_{{Na}_{2} S_{2} O_{3}} v_{{Na}_{2} S_{2} O_{3}}) \times 34}{m}

(3) where q_m is the H₂S saturation capacity, mg·g⁻¹;

ω_{H_{2} S}

is the H₂S concentration in the sample gas, mg/L; V is the total gas volume, L; and 34 is the molar mass of H₂S, g·mol⁻¹.

The H₂S removal rate ŋ can be expressed as

η = \frac{V \times ω_{H_{2} S} - (c_{I_{2}} v_{I_{2}} - 0.5 c_{{Na}_{2} S_{2} O_{3}} v_{{Na}_{2} S_{2} O_{3}}) \times 34}{V \times ω_{H_{2} S}} \times 100 %

(4)

Results and discussion

Adsorbent characterization

XPS analysis

The Mn_xO_y/AC and Mn_xO_y–Zn_xO_y/AC adsorbents were studied by XPS, and the Mn 2p spectra are shown in Figure 1(a). It is well known (Jang et al., 2012; Li et al., 2009) that Mn³⁺ can be identified by an Mn 2p3/2 binding energy in the range 641.1–641.5 eV, and that Mn⁴⁺ can be identified by a binding energy in the range 642.3–642.8 eV. The Mn 2p3/2 lines for Mn_xO_y–Zn_xO_y/AC were shifted by 0.4 and 0.5 eV to lower binding energy compared to those for Mn_xO_y/AC. The Mn 2p content was decreased from 26.4% in Mn_xO_y/AC to 2.22% in Mn_xO_y–Zn_xO_y/AC (Table 1). The Zn 2p3/2 spectra of Zn_xO_y/AC and Mn_xO_y–Zn_xO_y/AC are shown in Figure 1(b). For Mn_xO_y–Zn_xO_y/AC, the Zn 2p3/2 line was shifted by 0.2 eV to lower binding energy compared to that for Zn_xO_y/AC (Morozov et al., 2015). The XPS patterns of Zn_xO_y/AC and Mn_xO_y–Zn_xO_y/AC show two peaks and the shake-up satellite. For Zn_xO_y/AC, the binding energy at 1021.3 eV can be attributed to Zn²⁺ species. The Zn 2p content was increased from 2.1% in Zn_xO_y/AC to 4.63% in Mn_xO_y–Zn_xO_y/AC (Table 1) (Al-Gaashani et al., 2013).

Figure 1.

XPS spectrum of Mn_xO_y/AC, Zn_xO_y/AC, and Mn_xO_y–Zn_xO_y/AC adsorbents.

Table 1.

Spectral parameters of AC, Mn_xO_y/AC, Zn_xO_y/AC, and Mn_xO_y–Zn_xO_y/AC by XPS analysis.

Adsorbent	Zn_2p	Mn_2p		Percentage/%
	Binding energy (eV)	Binding energy (eV)		Percentage/%
	Zn²⁺	Mn³⁺	Mn⁴⁺	C_1s	N_1s	O_1s	Zn_2p	Mn_2p
AC	—	—	—	84.7	3.7	11.6	—	—
Zn_xO_y/AC	1021.1	—	—	77.6	1.6	18.7	2.1	—
Mn_xO_y/AC	—	641.5	642.8	32.5	2.7	38.4	—	26.4
Mn_xO_y–Zn_xO_y/AC	1021.3	641.1	642.3	60.69	0.01	32.45	4.63	2.22

The electronic absorption abilities of the different species decrease in the order: Zn²⁺> Mn⁴⁺> Mn³⁺. For Mn_xO_y–Zn_xO_y/AC, the shift in the peak positions of the Mn and Zn species can be attributed to an increase in the electron cloud density, which is caused by sharing electron pairs through covalent bonds because of interaction between the respective species.

XRD results

The XRD patterns of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC are shown in Figure 2. The diffuse peaks at 2θ = 20−30° (002) and 2θ = 40−50° (101) can be assigned to activated carbon, and indicate an irregular microcrystalline graphite structure therein. For the MnO₂/AC adsorbent, peaks at 2θ = 12.8° (110), 18° (200), 28.8° (310), 37.6° (211), 41.9° (301), 49.8° (411), and 60.2° (521) show that the adsorbent in the active group was divided into MnO₂. For ZnO/AC, the peaks at 2θ = 36.3° (002), 38.7° (100), 43.1° (101), and 54.8° (102) can be attributed to birnessite ZnO (Garces et al., 2010). For MnO₂–ZnO/AC, the peaks at 2θ = 27.9° and 44.7° can be attributed to graphitic carbon, indicating that the regularity of the activated carbon structure was greatly enhanced. A sharp peak at 2θ = 27.9° is seen for MnO₂–ZnO/AC, which shows that the structure of activated carbon rules is greatly enhanced, ZnO in the adsorbent characteristic peak does not reduce, and the intensity of characteristic peak of MnO₂ are greatly reduced, may be due to load first ZnO, ZnO occupies the first position in a lot of space, greatly reduce the load of MnO₂ (Heiba et al., 2012).

Figure 2.

XRD patterns of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents.

FTIR results

Figure 3 shows the FTIR spectra of the AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents. For each of the samples, a peak at ν = 3420 cm⁻¹ was observed, which could be attributed to the O–H stretching vibrations of hydroxyl, carboxyl, and surface-adsorbed water. The peaks at ν = 2926 and 1629 cm⁻¹ could be attributed to the symmetric and asymmetric stretching vibrations of C–H and C=O of the lactone base, respectively. The peaks at ν = 1383 and 1039 cm⁻¹ could be attributed to asymmetric and symmetric COO⁻ stretching vibrations and skeleton C–O stretching vibrations of the activated carbon, respectively. The spectra of the MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC absorbents retained all of the characteristic peaks seen for the unmodified activated carbon, indicating that the types of functional groups thereon were unchanged. Compared with AC, a peak at ν = 601 cm⁻¹ was detected for MnO₂/AC, which could be attributed to the characteristic absorption of Mn-O (Wang et al., 2014). For ZnO/AC, a peak at ν = 570 cm⁻¹ could be attributed to the characteristic absorption of Zn-O (Bhunia et al., 2015). For MnO₂–ZnO/AC, narrow and intense characteristic peaks at ν = 550 and 619 cm⁻¹ could be attributed to adsorption peak of Zn-O and Mn-O, respectively, providing evidence for the incorporation of Mn and Zn into the framework (Fang et al., 2010). These findings were in accordance with the results obtained by XRD and XPS.

Figure 3.

FTIR spectra of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents.

BET results. The pore parameters of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC derived from BET analysis are summarized in Table 2. The BET surface area and pore volume of the AC used were evaluated as 685.7 m²·g^–1and 0.36 mL·g^–1, respectively. The BET surface areas and pore volumes of the MnO₂/AC and ZnO/AC adsorbents decreased significantly, showing channel plugging. However, the pore volume of MnO₂–ZnO/AC was slightly increased, showing that although MnO₂ can clog pores inactivated carbon, bimetal loading with both ZnO and MnO₂ can actually expand the pores, thereby enhancing the pore size and radius and decreasing the surface area. The surface areas of the various adsorbents decreased in the order: AC (685.7 m²·g^–1) > ZnO/AC (622.6 m²·g^–1) > MnO₂–ZnO/AC (450.3 m²·g^–1) > MnO₂/AC (190.1 m²·g^–1).

Table 2.

Pore properties of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents.

Adsorbent	BET surface area	Pore volume	Average pore diameter
Adsorbent	A/ (m²·g^–1)	V/ (mL·g^–1)	D/nm
AC	685.7	0.36	1.0
MnO₂/AC	190.1	0.15	2.0
ZnO/AC	622.6	0.33	1.1
MnO₂–ZnO /AC	450.3	0.38	2.9

SEM analysis

Figure 4 shows SEM images of the AC and MnO₂–ZnO/AC adsorbents. It can be seen that loading with MnO₂ and ZnO has a great influence on the microstructure of AC. The SEM image of the AC reveals a relatively smooth surface, particles of many different sizes, and relatively few channels. However, the SEM image of MnO₂–ZnO/AC shows that there are great changes in the modified activated carbon surface. The surface of channel distribution of activated carbon is dense, the holes is filled with solid particles, and in the process of loading, pore-forming completes, although the channels increase, making pore volume larger. These observations are consistent with the results of BET analysis (Zhang et al., 2012).

Figure 4.

SEM analysis of the AC and MnO₂–ZnO/AC adsorbents.

Desulfurization performances of the adsorbents

All of the adsorbents were evaluated at an adsorption temperature of 313 K with a gas velocity of 20 mL/min using a fixed-bed plug flow reactor. The results are shown in Figure 5 and Table 3. For AC, the desulfurization performance, with a breakthrough time of 20 min and a saturated adsorption time of 70 min, was poor. Compared to those for AC, the breakthrough time (108 min) and saturated sulfur capacity (334.7 mg/g) for MnO₂–ZnO/AC were increased tremendously. The desulfurization performances of the various adsorbents decreased in the order: MnO₂–ZnO/AC > ZnO/AC > MnO₂/AC > AC.

Figure 5.

Breakthrough curves of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents.

Table 3.

Desulfurization performance of AC, MnO₂/AC, ZnO/AC, and MnO₂–ZnO/AC adsorbents.

Adsorbent	Breakthrough time (min)	The saturated adsorption time (min)	Saturated sulfur capacity (mg/g)	The desulfurization rate (%)
AC	20	70	37.7	25.1
MnO₂/AC	50	101	229	59.6
ZnO/AC	74	149	289.3	70.2
MnO₂–ZnO/AC	108	176	334.7	74.2

Regeneration of MnO₂–ZnO/AC adsorbent

Regeneration experiments were also performed in the packed-bed microreactor. The adsorbent was first purged in a flowing nitrogen gas stream for about 30 min at 643 K. Regeneration was then achieved by maintaining it at 643 K for 2 h in air. The breakthrough curve of the MnO₂–ZnO/AC adsorbent after regeneration is shown in Figure 6. The saturated sulfur capacity for MnO₂–ZnO/AC after regeneration decreased from 334.7 to 205.4 mg/g. The recovered saturated sulfur capacity of 205.4 mg/g for MnO₂–ZnO/AC after regeneration was still much higher than that of AC. It was therefore shown that the adsorbent retained certain activity for H₂S removal after regeneration at 643 K for 2 h in air.

Figure 6.

Breakthrough curves of MnO₂–ZnO/AC adsorbent after regeneration.

Conclusions

Adsorbents consisting of MnO₂, ZnO, and MnO₂–ZnO supported on activated carbon have been successfully prepared. XPS results showed that Mn was present in the forms of Mn³⁺ and Mn⁴⁺, and that Zn was present in the form of Zn²⁺. The specific surface area of MnO₂–ZnO/AC was evaluated as 450.3 m²/g. Compared with the original activated carbon, the specific surface areas of all of the modified samples were lower, decreasing in the order: AC > ZnO/AC > MnO₂–ZnO/AC > MnO₂/AC. The results of fixed-bed adsorption desulfurization experiments indicated that MnO₂–ZnO/AC showed the best H₂S desulfurization performance. The H₂S removal capacities of the various adsorbents decreased in the order: MnO₂–ZnO/AC > ZnO/AC > MnO₂/AC > AC. The saturated sulfur capacity for MnO₂–ZnO/AC after regeneration decreased from 334.7 to 205.4 mg/g, but was still much higher than that for AC, showing that the adsorbent was still active after regeneration.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Synthesis and characterization of Mn x O y –Zn x O y /AC adsorbents for adsorptive removal of H 2 S from natural gas

Abstract

Keywords

Introduction

Experimental

Preparation of adsorbents

Characterization of adsorptions

XPS experiments

X-ray diffraction experiments

BET experiments

SEM experiments

Adsorption experiments

Results and discussion

Adsorbent characterization

XPS analysis

XRD results

FTIR results

SEM analysis

Desulfurization performances of the adsorbents

Regeneration of MnO2–ZnO/AC adsorbent

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

Regeneration of MnO₂–ZnO/AC adsorbent