Removal of sulfur compounds from LPG by heteropoly acid-modified Al–MCM-41 mesoporous molecular sieves

Abstract

Al–MCM-41(30) was synthesized using Al₂(SO₄)₃ as the aluminum source, Na₂SiO₃ as the silicon precursor and cetyltrimethylammonium bromide (CTAB) surfactant as the template. Heteropolyacid supported on the mesoporous sieves was prepared using the incipient wetness method. The heteropolyacid-modified Al–MCM-41 adsorbents significantly enhanced the desulfurization ability of LPG through a dynamic adsorption method in a fixed bed. Several factors that affect desulfurization, including PW₁₂ loading, calcination temperature and the type of heteropolyacid, were investigated. The adsorbents were characterized by power X-ray diffraction, nitrogen adsorption, FTIR spectroscopy, thermogravimetric analysis and Py-IR spectroscopy. Al–MCM-41 mesoporous molecular sieves, with a hexagonal phase and a large specific surface area (1265.033 m² g⁻¹), were obtained. The experimental results showed that the PW₁₂-modified Al–MCM-41 adsorbents with a 25 wt% PW₁₂ loading had an optimum desulfurization ability. Also, a higher calcination temperature better promoted the removal of sulfur compounds by improving the acid amount of adsorbents. In addition, PW₁₂-modified Al–MCM-41 is better than PMo₁₂ modified Al–MCM-41 adsorbents in removing sulfur compounds from LPG. A total acid site number within a certain range on the adsorbent surface facilitated the desulfurization, and these values were obtained via Py-IR analyses.

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1. Introduction

Liquid petroleum gas (LPG) consists mainly of C3 and C4 compounds, which can be used as household fuels and petrochemical feedstock. Isobutylene from LPG is mainly used for the production of methyl tert-butyl ether (MTBE), which is commonly added to gasoline to boost its octane number.¹ However, LPG obtained from a fluid catalytic cracker unit a delayed coking process usually contains large amounts of sulfur compounds.^2,3 Therefore, it is necessary to remove these sulfur compounds with a pretreatment process before synthesizing MTBE since they result in the high sulfur content of subsequent products and poisoning of catalysts.

Adsorptive desulfurization (ADS) is regarded as one of the most competitive methods for removing sulfur, especially for an ultra-deep desulfurization of fuels. In the past decade, various porous adsorbents, such as activated carbons (ACs),^2,4–6 modified composite oxide,⁷ zeolite,^1,8,9 mesoporous silica,^10,11 and metal–organic frameworks,^12,13 were studied; however, mesoporous materials of the M41S (Mobil Composition of Matter) family, which are regarded as a breakthrough in the development of porous materials, have attracted widespread attention.¹⁴ These materials have unique properties, such as a large specific surface area and pore volume, ordered structure, tunable pore size and narrow pore size distribution. All these features are useful in many applications such as adsorption^15–17 and catalysis.^18–20 Among the members of this class of materials, MCM-41 is an ideal support for active functionalities, mainly because of its pseudo-crystalline and textural properties, such as the hexagonal arrangement of one dimensional channels, pore diameters in the range of 2–10 nm and large surface areas.²¹ However, the reactivity, ion-exchange capacity, and acidity of pure siliceous mesoporous materials are poor because of the absence of active sites and Brønsted acidity in their matrix and surface.²² These drawbacks limit the practical application of mesoporous materials. Therefore, to have a variety of applications, studies on the mesostructured materials have been focused on the surface and framework modifications of these materials.

The incorporation of heteroatoms, such as transition metals or Al, within the MCM-41 structure has recently gained considerable interest because it promotes the appearance of active catalytic sites. It was reported that the introduction of aluminum ions into the silicate framework is an efficient way to modify the surface acidity of MCM-41.^23–25 Luan et al.²⁶ reported that the Brønsted acidity of Si–MCM-41 could be significantly enhanced after modification with aluminum ions. The Brønsted acidity of aluminosilicates generally arises due to the presence of accessible hydroxyl groups or “bridging” Si–(OH)–Al hydroxyl groups (structural hydroxyls) associated with 4-coordination framework aluminum.²⁷

The surface acidity of Si–MCM-41 can be also increased by introducing some strong acidic groups and compounds like sulphate ions,²⁸ sulphated zirconia²⁹ or heteropolyacids with a Keggin structure.^30,31 Recently, heteropolyacids have been widely investigated due to their strong acidity.^32–34 Carriazo et al.³⁵ have prepared heteropolyacids supported on MCM-41 silica to study the adsorption of 2-butanol. Wang et al.^36–38 have reported Al–MCM-41 modified with 12-tungstophosphoric acid, which produced heteropoly compound/Al–MCM-41 hybrid catalysts containing a large number of Bronsted acid sites, thus exhibiting high acid strength.

The main objective of this study was to investigate the efficiency of a modified Al–MCM-41, obtained by the surface incorporation of a heteropolyacid, for removing various organic sulfur compounds from LPG through a dynamic adsorption method in a fixed bed. The adsorbents were characterized by nitrogen adsorption, power X-ray diffraction (PXRD), Py-IR spectroscopy, thermogravimetric analysis (TGA/DSC) and FTIR spectroscopy.

2. Experimental section

2.1. Materials and feedstocks

Liquid petroleum gas was obtained from the delayed coking process carried out at the Quanzhou oil refinery (Fujian Province, China). The composition of the LPG is presented in Table 1.

Table 1 Compositions of LPG

Name	Content (wt%)	S concentration (mg m^−3)
Paraffin	77.5
N-Butane	24.1
Iso-butane	14.2
Propane	39.2
Olefin	14.2
Proplyene	3.9
Butylene	10.3
Others	8.3
Total	100
Sulfur compound		29.94
Carbonyl sulfide		7.32
Carbon disulfide		0.67
Dimethyl disulfide		7.45
Methanethiol		10.66
Ethanethiol		3.42
Hydrogen sulfide		0.42

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2.2. Adsorbent preparation

2.2.1. Synthesis of Al–MCM-41. Al–MCM-41 samples were prepared using Al₂(SO₄)₃ as the aluminum source, Na₂SiO₃ as the silicon precursor and cetyltrimethylammonium bromide (CTAB) as the synthesis template. A typical preparation process of an Al–MCM-41 sample with a Si/Al molar ratio of 30 is discussed. First, three solutions were prepared: the first solution was prepared by adding 35.88 g of Na₂SiO₃ (34 wt%) into an appropriate amount of distilled water with stirring; the second solution was prepared by adding 20.79 g of CTAB into a moderate amount of deionized water (at around 50 °C) with stirring; and the third solution was prepared by dissolving 0.57 g of Al₂(SO₄)₃ into an appropriate amount of distilled water. Then, the first solution was added into the second solution and vigorously stirred for 2 h to form a gel. Subsequently, the third solution was added dropwise into the abovementioned gel and further stirred for 1 h. The pH value of this mixture was maintained at 10–11 by adding diluted 1 M sulfuric acid during the agitation. Afterwards, the mixture was transferred into a Teflon bottle and heated at 100 °C for 72 h. The resultant white solid was filtered and extensively washed with a large amount of deionized water and alcohol for four times and then dried in air at 100 °C for 24 h. Finally, the resultant solid was calcined at 600 °C for 6 h in air at a heating rate of 1 °C min⁻¹.

2.2.2. Adsorbent preparation. A typical preparation method is described as follows: first, different amounts of H₃PW₁₂O₄₀ (PW₁₂) were dissolved in different amounts of deionized water. Typically, 1 g of Al–MCM-41 was suspended in 10 mL of PW₁₂ aqueous solution and stirred for 12 h at room temperature. Then, this suspension was transferred into a water bath preheated at 80 °C. After the water was completely evaporated, the material was dried overnight in an oven at 120 °C. Finally, the sample was calcined in a muffle furnace at 550 °C in air for 4 h, and then crushed and screened through a 20–40 mesh to give the PW₁₂/Al–MCM-41 adsorbent. The other adsorbents were prepared using a similar procedure.

2.3. Adsorptive desulfurization experiments

The adsorptive desulfurization performance of the adsorbents was evaluated in a fixed-bed flow sorption system with a stainless steel column (8 mm I.D. (internal diameter) × 400 mm length). The adsorbent particles were packed into the middle of the column and the remaining space was filled with quartz sand. The experiments were conducted under the following conditions: 0.6 MPa, room temperature, and a weight hourly space velocity (WHSV) of 10.0 h⁻¹. The sulfur compound concentrations at the inlet and outlet were analyzed hourly after each test started. The total sulfur concentration at the outflow was analyzed by ultraviolet fluorescence using a sulfur/nitrogen analyzer.

2.4. Characterization of adsorbents

2.4.1. Power X-ray diffraction (PXRD). The crystalline structure of the adsorbents was confirmed by power X-ray diffraction using a Siemens D-500 X-ray diffractometer equipped with Ni-filtrated CuK_α radiation (40 kV tube voltage, 100 mA tube current and λ = 0.154 nm). Small angle XRD (SA-XRD) patterns were obtained over the 2θ range of 1.5° to 10°, whereas the wide angle XRD (WA-XRD) patterns were obtained over the 2θ range of 3–80° with a step of 0.02° per s.

2.4.2. Nitrogen isotherms. The Brunauer–Emmett–Teller (BET) specific surface area of bentonite was measured by nitrogen adsorption at −196 °C using a micromeritics JW-BK112 instrument. Prior to analysis, the samples were degassed for 1.5 h at 300 °C under a vacuum of P < 10⁻² Pa at a constant pressure. The BET specific surface areas were determined by N₂ adsorption at the relative pressures of 0.05 < P/P₀ < 0.35. The pore size distribution (PSD) was obtained from the adsorption branch of the N₂ sorption isotherms according to the Barrett–Joyner–Halenda (BJH) formula.³⁹ The total pore volumes (V_t) were estimated from the volume of N₂ (as liquid) held at a relative pressure (P/P₀) of 0.99.

2.4.3. FTIR spectroscopy. Fourier transform infrared (FTIR) spectra of the adsorbents were obtained using a Nicolet (Magna-IR550) instrument with a KBr pellet. About 4 mg of the sample was ground with 200 mg of spectral grade KBr to form a mixture, which was then made into a pellet using a hydraulic press. This pellet was used for obtaining the infrared spectra in the range of 2000–400 cm⁻¹.

2.4.4. TGA/DSC. TGA/DSC analysis of samples (>20 mg and 200 mesh) was performed by a STA 449 F3 instrument, which allowed both TGA and DSC curves to be obtained simultaneously. Samples were analyzed under air atmosphere and at a heating rate of 10 °C min⁻¹ in the temperature range 25–800 °C.

2.4.5. Py-IR spectroscopy. Using pyridine as the probe molecule, the type and number of acid adsorbents on the surface were analyzed by Fourier transform infrared (FTIR) spectroscopy. First, the loaded sample in the in-site cell was pretreated at 380 °C under vacuum to remove moisture. Second, excess of pyridine was adsorbed after cooling down to 80 °C. Finally, the adsorbed pyridine was desorbed at 200–450 °C for 2 h.

3. Results and discussion

3.1. Power XRD

The SA-XRD pattern for Al–MCM-41 is shown in Fig. 1. It can be seen that this solid exhibits an intense peak at a 2θ of about 2.23°, assigned to the (100) plane, and three weak peaks at a 2θ of 4.04°, 4.64° and 6.03°, assigned to higher order (110), (200) and (210) reflections, respectively, which are characteristic of a mesoporous hexagonal phase.²¹ As shown in Fig. 2, comparison of the XRD patterns of pristine Al–MCM-41 and PW₁₂/Al–MCM-41 reveals that the mesoporous structure is preserved and the incorporation of Keggin-type structures does not have adverse effects on the ordered structure of Al–MCM-41. However, the intensity of the peak corresponding to the (100) plane decreases with an increase in PW₁₂ loading, which might be due to the decrease in the scattering domain size instead of structural degradation.⁴⁰ Also, lines appearing above 22° (2θ) correspond to the crystalline PW₁₂ phase as the PW₁₂ loading increased.

Fig. 1 SA-XRD pattern of Al–MCM-41.

Fig. 2 WA-XRD patterns of (a) Al–MCM-41; (b) 15 wt% PW₁₂/Al–MCM-41; (c) 20 wt% PW₁₂/Al–MCM-41; (d) 25 wt% PW₁₂/Al–MCM-41 and (e) 30 wt% PW₁₂/Al–MCM-41.

3.2. Nitrogen isotherms

Fig. 3 and 4 show the N₂ adsorption–desorption isotherms and pore size distribution curves for Al–MCM-41, 25 wt% PMO₁₂/Al–MCM-41, and PW₁₂/Al–MCM-41 with different PW₁₂ loadings. All samples exhibited type IV isotherms with capillary condensation step and a strong peak on the pore size distribution curves, which are characteristic of typical mesoporous materials, according to the IUPAC classification.⁴¹

Fig. 3 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution curves for Al–MCM-41 and PW₁₂/Al–MCM-41 with different PW₁₂ loadings.

Fig. 4 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution curves for Al–MCM-41, 25 wt% PMO₁₂/Al–MCM-41 and 25 wt% PW₁₂/Al–MCM-41.

Table 2 shows the structural parameters of the adsorbents from the nitrogen adsorption isotherms. As shown in Table 2, the BET surface area, pore size and total pore volume decreased with the increase in PW₁₂ loading in PW₁₂/Al–MCM-41 adsorbents, and the Al–MCM-41 adsorbent had the largest BET surface area, pore size and total pore volume. The decrease in the BET surface area, pore size and total pore volume might be due to the deposition of heteropoly anion species onto the surface, which would block the channels of Al–MCM-41 mesoporous zeolite, indicating that the heteropolyacid interacts with the Al–MCM-41 surface.^42,43 In addition, the 25 wt% PW₁₂/Al–MCM-41 adsorbent had a larger BET surface area, pore size and total pore volume than the 25 wt% PMO₁₂/Al–MCM-41 adsorbent.

Table 2 Structural parameters of the adsorbents obtained from the nitrogen adsorption isotherms

Sample	BET surface area (m^2 g^−1)	Vtotal (cm^3 g^−1)	BJH pore size (nm)
Al–MCM-41	1265.033	1.153	3.987
15 wt% PW12/Al–MCM-41	974.363	0.894	3.714
20 wt% PW12/Al–MCM-41	915.500	0.768	3.695
25 wt% PW12/Al–MCM-41	886.740	0.750	3.678
30 wt% PW12/Al–MCM-41	818.890	0.657	3.427
25 wt% PMO12/Al–MCM-41	696.147	0.567	3.580

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3.3. FTIR analysis

FTIR spectra of PW₁₂, Al–MCM-41 and PW₁₂/Al–MCM-41 samples in the region of 2000–400 cm⁻¹ are shown in Fig. 5. The PW₁₂ bands that appear at approximately 595, 793, 890, 983 and 1080 cm⁻¹ can be assigned to the characteristic Keggin anion vibrations of δ(O–P–O), v_as(W–O_c–W), v_as(W–O_b–W), v_as(W=O_t) and v_as(P–O), respectively.²³ The Al–MCM-41 and PW₁₂/Al–MCM-41 adsorbents show characteristic strong IR bands in the range of 474–1100 cm⁻¹. The vibration bands at 804, 930 and 1100 cm⁻¹ are assigned to v_as(Si–O–Si), whereas the vibration band at 474 cm⁻¹ corresponds to δ(Si–O–Si).^30,31 There is an intense band located at 1650 cm⁻¹, assigned to the flexion vibration of the OH group. However, peaks broadening at 474, 804 and 1100 cm⁻¹ for the PW₁₂/Al–MCM-41 adsorbent were observed. This might be due to the overlapping of heteropolyacid bands with the bands of Al–MCM-41 sieves.⁴⁴ In all cases, an additional band can be observed at 1620 cm⁻¹, which is assigned to the bending vibrations of v_as(H₂O) in the secondary structure of the Keggin species.⁴⁵

Fig. 5 FTIR spectra for PW₁₂, Al–MCM-41 and PW₁₂/Al–MCM-41.

3.4. Effects of the amount of PW₁₂ loading on desulfurization performance

Dynamic tests were carried out to examine the effect of the PW₁₂ loading amount on the desulfurization performance. For this, four PW₁₂/Al–MCM-41 adsorbents with different PW₁₂ loading amounts of 15, 20, 25 and 30 wt% were prepared via the impregnation method. Fig. 6 shows the experimental results for the PW₁₂-modified Al–MCM-41 adsorbents. As shown in Fig. 6, compared to raw Al–MCM-41 adsorbent, the modified adsorbents could remove larger amounts of sulfur. In addition, at low PW₁₂ loadings, the sulfur removal level of the adsorbents increased with the increase in the PW₁₂ loading. When the PW₁₂ loading was 25 wt% the desulfurization performance was optimum. However, in the case of 30 wt% PW₁₂/Al–MCM-41, the sulfur removal level clearly decreased. As shown in Table 2, compared to the raw Al–MCM-41 adsorbent, the modified adsorbents' total pore volume and BET surface area decreased. However, the performances of modified adsorbents were still better than that of raw Al–MCM-41. This result may be explained by the presence of large Keggin structures with a high density of acid sites inside the mesopores of Al–MCM-41.⁴⁶ However, the sulfur removal level clearly decreased when the PW₁₂ concentration was above a certain threshold, due to the deposition of heteropolyanion species onto the channel surface of Al–MCM-41 mesoporous zeolite and subsequent blockage of the active sites. This indicates that there are some trade-off effects between PW₁₂ loading and the amount of sulfur that can be removed. Therefore, the amount of sulfur to be removed can be controlled by choosing an appropriate concentration of the PW₁₂.

Fig. 6 Breakthrough curves for ADS with PW₁₂/Al–MCM-41 sorbents containing different PW₁₂ loadings.

To explore the relationship between acidity and adsorption, FTIR spectra of THE adsorbed pyridine were obtained. The FTIR spectra in Fig. 7 and 8 show the presence of both Lewis and Brønsted acidity in the samples. The band at 1540 cm⁻¹ was assigned to the Brønsted acid sites, whereas the band at 1455 cm⁻¹ was assigned to the Lewis acid sites. For comparison, the acidity data obtained are presented in Table 3. For PW₁₂/Al–MCM-41 samples, the acidity increases gradually with increase in the PW₁₂ loading. This steady increase in the acidity at higher PW₁₂ loadings suggest that there is no collapsing of the structure wall even at higher loadings. This was further confirmed by XRD studies (Fig. 2). G. Karthikeyan et al.⁴⁷ reported that Bronsted acid sites are due to the presence of small clusters of heteropolyacids, whereas Lewis acid sites might be due to the interaction of heteropolyacids with framework SiO₂.⁴⁷ When combining with the results shown in Fig. 6, we found that the increased acidity had contributed to the desulfurization of LPG, but at same time, an excessive amount of acid was not good for desulfurization.

Fig. 7 Adsorbed pyridine FT-IR spectra for the adsorbents with different PW₁₂ loadings at 200 °C: (a) Al–MCM-41; (b) 15 wt%; (c) 20 wt%; (d) 25 wt% and (e) 30 wt%.

Fig. 8 Adsorbed pyridine FT-IR spectra for the adsorbents with different PW₁₂ loadings at 450 °C: (a) Al–MCM-41; (b) 15 wt%; (c) 20 wt%; (d) 25 wt% and (e) 30 wt%.

Table 3 Acid properties of adsorbents (×10⁻³ mol g⁻¹)^a

Adsorbents	T	TL	TB	SL	SB	WL	WB
a T-total acid; TL-total Lewis acid; TB-total Brønsted acid; SL-strong Lewis acid; SB-strong Brønsted acid; WL-weak Lewis acid; WB-weak Brønsted acid.
Al–MCM-41	2.02	0.36	1.66	0.25	1.25	0.11	0.41
15 wt% PW12/Al–MCM-41	5.83	0.51	5.32	0.38	4.35	0.13	0.97
20 wt% PW12/Al–MCM-41	6.70	0.68	6.02	0.54	4.93	0.14	1.09
25 wt% PW12/Al–MCM-41	7.78	0.75	7.03	0.60	5.58	0.15	1.45
30 wt% PW12/Al–MCM-41	8.73	0.90	7.83	0.73	6.24	0.17	1.59

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3.5. Effects of calcination temperature on the adsorptive desulfurization of LPG

Another important characteristic of these mesoporous solids is that when comparing adsorbents of the same nature, it is observed that the calcination temperature is an important factor for the ADS activity. Fig. 9 shows that the results of breakthrough curves for ADS at 250 and 550 °C calcination for 25 wt% PW₁₂/Al–MCM-41. Fig. 10 shows the WA-XRD patterns for PW₁₂; when PW₁₂ is heated at different calcination temperatures, the main diffraction signal was retained. This shows the relative thermal stability of these compounds at the treatment temperatures. These results were associated with results from the thermal analysis, conducted by TGA/DSC (Fig. 11).

Fig. 9 Breakthrough curves for ADS at different calcination temperatures for 25 wt% PW₁₂/Al–MCM-41.

Fig. 10 WA-XRD patterns for PW₁₂ (a) uncalcined; (b) at 250 °C and (c) 550 °C.

Fig. 11 DSC–TGA curves in air for (a) PW₁₂ and (b) Al–MCM-41.

Thermogravimetric analysis was carried out to determine the thermal stability of the adsorbents. Fig. 11 shows the differential scanning calorimetry (DSC)–TGA curves for PW₁₂ and Al–MCM-41. DSC curves for PW₁₂ showed two endothermal signals between 80 and 190 °C; these are accompanied by a considerable weight loss in the TGA curve corresponding to the elimination of crystallization and constitution water, respectively. Previous studies suggested that this loss of water involves a deprotonation of the adsorbent with the concurrent loss of lattice oxygen, however, the primary Keggin unit is retained.⁴⁸ Additionally, weight changes and thermal effects were not observed at the intermediate temperatures, suggesting the formation of anhydrous heteropolyanions. However, an exothermal peak at 600 °C indicates the decomposition of HPW into their constituent oxides, as a result of the total decomposition of the Keggin units. All these results are in agreement with those reported.⁴⁹ However, a different behavior appears after supporting PW₁₂ on the Al–MCM-41 surface. The thermal decomposition of the PW₁₂/Al–MCM-41 occurs in one step by the elimination of adsorbed water at around 60 °C. The surface acidity of the adsorbents was measured by in situ FTIR spectroscopy of the adsorbed pyridine.

In Table 4, it can be seen that higher calcination temperature promotes higher acidity. The same result was reported by F. J. Méndez et al.²³ When comparing with the results shown in Fig. 9, we concluded that higher calcination temperature promoted the removal of sulfur compounds by increasing the acid site concentration in the adsorbents.

Table 4 Acid properties of 25 wt% PW₁₂/Al–MCM-41 baking at 250 and 550 °C (×10⁻³ mol g⁻¹)

Temperature (°C)	T	TL	TB	SL	SB	WL	WB
550	7.775	0.746	7.029	0.601	5.584	0.145	1.445
250	6.325	0.701	5.624	0.547	4.287	0.154	1.337

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3.6. Effects of different heteropolyacids on adsorption desulfurization of LPG

To examine the effect of different heteropolyacids on the adsorptive desulfurization of LPG, two modified Al–MCM-41 adsorbents were prepared via the impregnation method. As shown in Fig. 12, PW₁₂/Al–MCM-41 had a greater ability to remove sulfur than PMo₁₂/Al–MCM-41. As can be seen in Table 2, the 25 wt% PW₁₂/Al–MCM-41 adsorbent has a larger BET surface area, pore size, and total pore volume than the 25 wt% PMO₁₂/Al–MCM-41 adsorbent. This may be one reason for the greater desulfurization performance of PW₁₂/Al–MCM-41.

The surface acidity of the adsorbents was measured by in situ FTIR spectroscopy of THE adsorbed pyridine at 200 and 450 °C. The FTIR spectra in Fig. 13 and 14 show the presence of both Lewis and Brønsted acid sites in the samples. In Table 5, it can be seen that PW₁₂/Al–MCM-41 had the highest acidity. In addition, it was observed that the supported adsorbents have a larger number of acid sites, especially Brønsted acid sites, in comparison with Al–MCM-41, due to the incorporation of heteropolyacids. It is worth noting that the total number of Lewis and Brønsted acid sites on the surface of PW₁₂/Al–MCM-41 was greater than that for PMo₁₂/Al–MCM-41. Thus, we concluded that the number of total acid sites on the surface of the adsorbents facilitated the adsorption of sulfur compounds from LPG.

Fig. 12 Breakthrough curves for ADS with PMo₁₂- and PW₁₂-modified Al–MCM-41.

Fig. 13 FTIR spectra of adsorbed pyridine for three different adsorbents at 200 °C: (a) Al–MCM-41; (b) PMo₁₂/Al–MCM-41; and (c) PW₁₂/Al–MCM-41.

Fig. 14 FTIR spectra of adsorbed pyridine for three different adsorbents at 450 °C: (a) Al–MCM-41; (b) PMo₁₂/Al–MCM-41; and (c) PW₁₂/Al–MCM-41.

Table 5 Acid properties of adsorbents (×10⁻³ mol g⁻¹)

Adsorbents	T	TL	TB	SL	SB	WL	WB
PW12/Al–MCM-41	7.775	0.746	7.029	0.601	5.584	0.145	1.445
PMo12/Al–MCM-41	5.233	0.451	4.782	0.407	3.287	0.045	1.495
Al–MCM-41	2.017	0.364	1.653	0.252	1.246	0.112	0.407

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4. Conclusions

Al–MCM-41 mesoporous molecular sieves, with a hexagonal phase and large specific surface areas (1265.033 m² g⁻¹), were obtained via the sol–gel method. The experimental results showed that the PW₁₂-modified Al–MCM-41 adsorbents had an optimum desulfurization ability with a PW₁₂ loading of 25 wt%. Additionally, higher calcination temperatures promoted the removal of sulfur compounds by increasing the amount of acid sites on the adsorbents. In addition, PW₁₂-modified Al–MCM-41 was better than PMo₁₂-modified Al–MCM-41 adsorbents at removing sulfur compounds from LPG. A total acid site number within a certain range on the surface of the adsorbents facilitated the desulfurization and these values were obtained via Py-IR analyses.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21276086) and Sinopec Zhenhai Refining & Chemical Company.

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