Article pubs.acs.org/IECR
Highly Dispersed HKUST‑1 on Milimeter-Sized Mesoporous γ‑Al2O3 Beads for Highly Effective Adsorptive Desulfurization Libo Qin, Yunshan Zhou,* Dianqing Li, Lijuan Zhang,* Zipeng Zhao, Zareen Zuhra, and Cuncun Mu State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China
Ind. Eng. Chem. Res. 2016.55:7249-7258. Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/07/19. For personal use only.
S Supporting Information *
ABSTRACT: HKUST-1 was impregnated effectively on millimeter-sized mesoporous γ-Al2O3 beads under hydrothermal conditions, resulting in formation of a composite material HKUST-1@γ-Al2O3 that features high specific surface area, remarkable enhanced mechanical strength, chemical and thermal stability, and low cost. The composite material exhibited excellent performance with the adsorptive desulfurization capacity of 59.7 mg S/g MOF (versus 49.1 mg S/g MOF for bare HKUST-1) for a model oil composed of dibenzothiophene (with the initial S-content being 1000 ppmwS) and n-octane. Experimental results also revealed that HKUST-1@γ-Al2O3 could reduce 35 ppmwS sulfur content of the model oil lower than 9.6 ppmwS at a ratio of HKUST-1@γ-Al2O3 to oil over 30 wt %, indicating effectiveness for deep adsorptive desulfurization. The Gibbs free energy for DBT adsorption by HKUST-1@γ-Al2O3 was found smaller than that by HKUST-1 due to efficient utilization of active centers, shorter diffusion channels and larger specific surface area of nanosized HKUST-1 particles formed under confined environment of γ-Al2O3 channels/pores. Remarkably, the used HKUST-1@γ-Al2O3 beads can easily be regenerated by acetone washing and the adsorptive desulfurization capacity just slightly decreased after experiencing five recycles. The results indicate that the as-synthesized HKUST-1@γ-Al2O3 beads have great potential as an adsorbent for adsorptive desulfurization in practical applications. oxygen.16−18 Currently adsorbents such as active carbon,19,20 silica gel, zeolites,21,22 and metal oxide23 have already been reported in model and actual oils to adsorb stubborn sulfurcontaining compounds. It has been found that the activated carbon,24 NaX, and NaY25−27 have less selectivity for thiophenes as they can also adsorb benzene and toluene. On the contrary, zeolites like ZSM-5 and the HY28−31 are highly selective for thiophenes; however strong interactions with thiophenes hinder the total regeneration of the adsorbents. Metal−organic frameworks (MOFs) are a genre of porous crystalline materials with periodic multidimensional network self-assembled via coordination bonds among central metal and organic ligands.32,33 The extraordinary specific surface area and unsurpassed porous structure empowers MOFs to retain superior adsorption capacity in contrast to active carbon, silicate, and zeolite.32,34 Cychosz et al.17 deliberated the adsorption capacities of five different MOFs (UMCM-150, HKUST-1, MOF-5, MOF-177, and MOF-505) for BT, DBT, and 4,6-DMDBT in model oil, and the results showed that UMCM-150, HKUST-1, and MOF-505 have significantly higher adsorption capacities to sulfur-containing compounds
1. INTRODUCTION Sulfur-containing compounds, the major noxious wastes of fuel oils, not only prompt acid rain but also poison human health, so presently it is a thought-provoking issue to reduce the content of sulfur-containing compounds in gasoline and diesel oils in the motorized and power industries. Hydrodesulfurization is currently the key approach to confiscate the sulfur-containing compounds from gasoline and diesel in industry,1 but it is comparatively futile to remove the stubborn sulfur-containing compounds including benzothiophene (BT), dibenzothiophene (DBT), 4,6-dimethyldibenzothiopene (4,6-DMDBT), and their derivatives.2,3 Besides, the hydrodesulfurization process requires high temperature (300−400 °C) and pressure (20−100 atm of H2) and can cause loss of octane number because of hydrogenation via consuming a huge quantity of H2.4−6 Due to these complications, people are considering about some substitutional technologies for alternative desulfurization processes like adsorptive desulfurization,7,8 oxidative desulfurization,9−12 extractive desulfurization,13,14 and biodesulfurization.15 Among them, adsorptive desulfurization is obviously fascinating due to its favorable route for accomplishing wide/ deep desulfurization with high selectivity for sulfur-containing molecules as compared to major components of fuel oils through the probability of adjusting the pore structure and altering the surface functional groups on the adsorbents as well as mild operation conditions without custom hydrogen and © 2016 American Chemical Society
Received: Revised: Accepted: Published: 7249
March 19, 2016 June 6, 2016 June 14, 2016 June 27, 2016 DOI: 10.1021/acs.iecr.6b01001 Ind. Eng. Chem. Res. 2016, 55, 7249−7258
Article
Industrial & Engineering Chemistry Research than zeolite. Shi et al.35 dipped MOF-5 with molybdenum carbide as adsorption sites to remove sulfur-containing compounds and the modified MOF-5 reinforced the adsorption influence to DBT in model oil (isooctane). Achmann et al.36 further studied the adsorption capacities of MOF-5, CuDABCO-MOF, a-MOP-1, and HKUST-1 to thiophene compounds in actual oil, and these four different MOFs have a quite noteworthy adsorption capacity even in actual oil. In conclusion, MOFs have showed a superior performance of adsorptive desulfurization both in model and actual oil. However, the following tiresome problems exist in the MOFs material as the adsorbent: (1) MOFs are formed by coordination bonds between organic ligands and metal ions. The coordination bond energy is usually less than other chemical bond energy, so MOFs’ thermal stability is often not high enough. In high temperature water, heat, and other harsh conditions, the partial coordination bond will crack leading to heterogeneous restructuring. (2) MOF materials are usually powder with poor mechanical strength, a character which is difficult for recycling and reusing in actual industrial operation. (3) When MOF powders act as sorbents for desulfurization, steric and dynamic hindrance allow sulfur molecules to adsorb mainly on the surface rather than to access to whole MOF particle entity, resulting in low utilization of MOFs pore volumes and/or surface areas (For example, the adsorption of DBT only occupies 4% of surface area of MOF-535). Therefore, improving the thermal stability, pore volume and/or surface area utilization, and adsorption capacity of MOFs and solving the problem of easy recovery and reuse are important scientific aspects which remain to be studied. Obviously, the solution to these problems is very important for MOF materials to be applied in the oil desulfurization industry. In view of the above questions, we thought of an idea, namely, growth/deposition of MOFs on the surface of internal confined spaces of millimeter-sized mesoporous γ-Al2O3 beads as carriers which feature high specific surface area, fine pore structure, high mechanical strength, excellent chemical stability, good fluidity, and low price.37 So highly dispersed nanosized MOFs can be the result under a confined environment of internal channels of the γ-Al2O3 carrier (Scheme 1), leading to
(BT), 3-methylthiophene (3-MT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiopene (4,6-DMDBT) in the model oil was systematically studied in this work. The results showed that the prepared composite MOFs@γ-Al2O3 shows superior desulfurization efficiency, remarkable convenience in separation, recovery and recycling, and therefore shows great potentials as an adsorbent for desulfurization in industrial applications.
2. EXPERIMENT 2.1. Materials. γ-Al2O3 beads (SBET, 216.4 m2·g−1; Vtotal, 0.684 cm3·g−1; average pore size, 13.38 nm; diameter, ca. 2 mm) were homemade by the State Key Laboratory of Chemical Resource Engineering (Beijing University of Chemical Technology, China), washed with distilled water, and pretreated at 200 °C for 2 h in muffle furnace every time before use to remove water and other contaminants. All the other chemicals were purchased from commercial sources: cupric nitrate trihydrate (AR grade) and n-octane (AR grade) from Tianjin Guangfu Technology Development Co. Ltd.; 1,3,5benzenetricarboxylic acid (H3BTC, 98%), methanol (HPLC grade), ethanol (AR grade), BT (99%), 3-MT (99%), DBT (99%), and 4,6-DMDBT (99%) from Beijing Chemical Reagent Co. Ltd. All reagents were used as received without further purification. HKUST-1 was prepared according to ref 38. 2.2. Characterization. The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet FTIR-170SX spectrometer with KBr pellets in the range of 400−4000 cm−1. The powder X-ray diffraction (XRD) data was collected on a Rigaku D/max 2500 X-ray diffractometer at a scanning rate of 10°/min in the 2θ range from 5° to 70° with graphitemonochromatic Cu Kα radiation (λ = 0.15405 nm). Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) mapping data were obtained using scanning electron microscope Zeiss Supra55 at an accelerating voltage of 20 kV. HRTEM images were obtained using a JEOL JEM2100 transmission electron microscope at an accelerating voltage of 200 kV. Elemental analyses for C, H, and N were performed on a PerkinElmer 240C analytical instrument. Analyses for Al and Cu were performed by the SPECTRO ARCOS type inductively coupled plasma spectrometer. TG-DTA curves were recorded on a Netzsch STA449, and thermogravimetric analysis was carried out with a temperature rate of 10 °C min−1 in the temperature range from 25 to 800 °C under air flow. The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP-2020 (Micrometrics USA). The specific surface area (SBET) was determined from the linear part of the BET equation (P/P0 = 0.05−0.3). The mechanical strength of the beads before and after loading of HKUST-1 was measured on a YHKC-2A particle strength tester (Yan Zheng Shanghai Experimental Instrument Co. Ltd.). The pore size distribution was derived from the desorption branch of the N2 isotherm using the Barrett−Joyner−Halenda (BJH) method. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of ca. 0.99. The sulfur contents were analyzed by Agilent HPLC 1100 Series with C18 column, diameter 4.6 mm, length 250 mm, diameter of filler 5 μm, 10% water and 90% methanol as the initial mobile phase, and gradient elution to 100% methanol in 10 min with flow rate of 1.0 mL·min−1. 2.3. Preparation of the Composite Material HKUST-1@ γ-Al2O3. In a typical synthesis, 1.8 mM of Cu(NO3)2·3H2O
Scheme 1. Schematic Representation for the Structure of HKUST-1@γ-Al2O3 Composite
formation of composite material MOFs@γ-Al2O3, where more efficient utilization of active centers, shorter diffusion channels, and larger specific surface area of nanosized HKUST-1 can be realized. So owed to the merits mentioned above of γ-Al2O3 carrier and the highly dispersed nanosized MOFs on the carrier, the aforementioned tiresome problems existing in the MOFs material as the adsorbent are expected to be solved in principle. On the basis of the analysis above, HKUST-1, a very promising desulfurization absorbent,17 was chosen as a prototype to be loaded on the millimeter-sized mesoporous γ-Al2O3 beads forming a composite material HKUST-1@γAl2O3, and its adsorptive performance for benzothiophene 7250
DOI: 10.1021/acs.iecr.6b01001 Ind. Eng. Chem. Res. 2016, 55, 7249−7258
Article
Industrial & Engineering Chemistry Research
which are assigned to the out-of-plane vibrations of BTC3−.41 The resulted HKUST-1@γ-Al2O3 composite not only showed the vibration bands of γ-Al2O3, but also the vibration bands coming from HKUST-1 apart from the characteristic band at 1300−600 cm−1 covered by the strong band of Al2O3, indicating the successful loading of the HKUST-1 on the γAl2O3 beads, a conclusion which is further confirmed based on the comparison of the powder XRD patterns of the γ-Al2O3 beads, standard HKUST-1, synthesized HKUST-1 and HKUST-1@γ-Al2O3 composite (Figure 2), where the XRD
was mixed with 1.0 mM of H3BTC in 50 mL of 50:50 (V:V) H2O:EtOH and 1.0 g of γ-alumina beads in a 100 mL Teflonlined stainless-steel autoclave and the mixture was heated at 110 °C for 20 h. After cooling the autoclaves to room temperature, the products were removed by decanting from mother liquor and washed in deionized water (3 × 10 mL). The material was then evacuated for 12 h to remove water molecules and stored in desiccator. The loading of HKUST-1 is 18.7 wt % according to elemental analysis (Al 43.04, Cu 5.93, C 6.43, H 0.63%). 2.4. Adsorption Test. The model oil was prepared by dissolving DBT in n-octane, with the initial S-content being 1000 ppmwS and 35 ppmwS, respectively. All these stock solutions were used directly in the following adsorption experiments. Prior to adsorption experiments, γ-Al2O3, HKUST-1 and HKUST-1@γ-Al2O3 were degassed under vacuum at 150 °C overnight to remove water and other contaminants. Their desulfurization performances were tested with the reaction temperature controlled by a water bath at atmospheric pressure under stirring. The model oil was mixed with the adsorbent in the conical flask under stirring for 1h. The liquid phase was then separated from the adsorbent by filtration and the S-content of the treated oil was determined by HPLC. The adsorption capacity under different experimental conditions was calculated by the following formula: W (C0 − Ci) × 10−3 Qi = M where Qi is the adsorption capacity of sulfur adsorbed on the adsorbent (mg S·g−1 MOF), W is the mass of model oil (g), M is the mass of the MOF used (g), and C0 and Ci are the initial and final S-concentrations in the model oil (μg/g), respectively.
Figure 2. X-ray diffraction patterns for the samples of the γ-Al2O3 beads (a = simulated, b = prepared), HKUST-1 (e = simulated, d = synthesized), and HKUST-1@γ-Al2O3 composite (c).
3. RESULTS AND DISCUSSION 3.1. Characterization of the Composite HKUST-1@γAl2O3. As shown in Figure 1, the IR spectrum of γ-Al2O3
pattern of HKUST-1@γ-Al2O3 showed the relatively strong peaks at 2θ values of about 46.6 and 66.7° which are attributed to the (400) and (440) reflections of γ-Al2O3 (JCPDS 10− 0425)42 and a few weak peaks at 2θ values in 7.3°, 9.6°, 12.3°, and 13.7° which are attributed to the HKUST-143 formed on the γ-Al2O3 carrier. Besides, the obvious color change found from white for bare γ-Al2O3 beads (Figure 3a) to blue for the composite HKUST-1@γ-Al2O3 (Figures 3b and c) (like preserved duck eggs) is another intuitional and strong evidence of the successful loading of the HKUST-1 on the γ-Al2O3 beads. Apart from the above, any visible size change (Figures 3a and b: inset) was not observed between the γ-Al2O3 and HKUST-1@γ-Al2O3 in terms of their diameter being ca. 2 mm. In order to characterize distribution of the impregnated HKUST-1 on γ-Al2O3 carrier, cross-section EDX mappings were carried out by slicing the beads into two roughly equal parts. From Figures 3d and e, it can be seen that HKUST-1 was distributed on the γ-Al2O3 beads uniformly. Cu penetration depth of HKUST-1@γ-Al2O3 beads was more than 556 μm (Figure 3f), i.e., HKUST-1 penetration depth was more than half of γ-Al2O3 beads in view of the diameter being approximately 2 mm. In addition, the lattice fringe distance is found to be 0.2301 nm corresponding to {880} crystal surface of HKUST-1 (Figure 3g), which further confirmed the successful loading of the HKUST-1 on the γ-Al2O3 beads. Meanwhile, it is found that the lattice diffraction fringes of HKUST-1 appear discontinuously (Figure 3h), so it can be speculated that continuous films of HKUST-1 is not obtained probably due to the fast nucleation of HKUST-1 on the inner surface of γ-Al2O3 beads, instead, discontinuous nanoparticles of HKUST-1 are formed/deposited.
Figure 1. IR spectra of the γ-Al2O3 beads, HKUST-1, and HKUST-1@ γ-Al2O3 composite.
showed bands at around 3440 and 1620 cm−1 which are ascribed to the vibration bands of hydroxyl groups and H−O− H from surface-adsorbed water on Al2O339 as well as the two broad bands from 900 to 500 cm−1 which are ascribed to the stretching and bending modes of octahedral aluminum (AlO6).40 The IR spectrum of HKUST-1 showed the asymmetric stretching of the carboxylate groups in BTC3− appearing at 1508−1623 cm−1, the symmetric stretching of the carboxylate groups in BTC3− appearing at 1384 and 1405 cm−1 together with several bands in the region of 1300−600 cm−1 7251
DOI: 10.1021/acs.iecr.6b01001 Ind. Eng. Chem. Res. 2016, 55, 7249−7258
Article
Industrial & Engineering Chemistry Research
Figure 3. Overview SEM images and photographs [(inset) Highlighting the spherical appearance and color change from white to blue] of γ-Al2O3 (a) and HKUST-1@γ-Al2O3 (b), cross-section SEM image and photograph [(inset) Highlighting the appearance of preserved duck eggs] of HKUST-1@γ-Al2O3 (c), cross-section EDX mapping of Al (blue dots) (d) and Cu (green dots) (e) of HKUST-1@γ-Al2O3, cross-section SEM-EDX line scans of HKUST-1@γ-Al2O3 (f), and HRTEM images of HKUST-1@γ-Al2O3 (g, h) highlighting the lattice fringe.
3.2. Adsorptive Desulfurization Capacity of HKUST1@γ-Al2O3. First, the experiments were conducted to decide an appropriate adsorption time by using model oil (20 g), HKUST-1 (0.05 g), or HKUST-1@γ-Al2O3 (0.5 g, wherein 0.05 g of HKUST-1 was loaded) adsorbents at 30 °C. It should be pointed that adsorptive desulfurization capacity of γ-Al2O3 as shown in Figure 4 could be ignored due to its very low
Second, the possible effect of temperature on adsorptive desulfurization for DBT was investigated in the range from 20 to 50 °C as shown in Figure 5. The adsorptive desulfurization
Figure 5. Effect of temperature on adsorptive desulfurization capacity for DBT over HKUST-1 and HKUST-1@γ-Al2O3. Reaction condition: model oil, 20 g; adsorbent, HKUST-1 0.05 g, HKUST-1@γ-Al2O3 0.5 g; [DBT], 1000 ppmwS; temperature, 30 °C; time, 60 min. Figure 4. Effect of time on adsorptive desulfurization capacity for DBT over γ-Al2O3, HKUST-1, and HKUST-1@γ-Al2O3. Reaction conditions: model oil, 20 g; adsorbent, γ-Al2O3 0.5 g, HKUST-1 0.05 g, HKUST-1@γ-Al2O3 0.5 g; [DBT], 1000 ppmwS; temperature, 30 °C.
capacity increased slightly in temperature from 20 to 30 °C but after 30 °C it started to decrease slowly. Thus, the optimum temperature was found to be 30 °C. The phenomenon can be explained as a result of the combined effects of the physisorption and chemisorption.44 At low temperature physisorption played a significant role by weak van der Waals interactions, however, at high temperature chemisorption became dominant. Third, it was found that the adsorptive desulfurization capacity increased when the mass ratio of oil to HKUST-1 for both pristine HKUST-1 and HKUST-1@γ-Al2O3 was increased (Figure 6).44 The increasement of adsorbent concentration signified that the DBT concentration around adsorbent molecule decreased and hence led to decrease in adsorption desulphurization capacity. It was also found that the adsorptive desulfurization capacities for DBT of HKUST-1@γ-Al2O3 were
adsorption value of 2.4 mg S/g at adsorption equilibrium. The adsorptive desulfurization capacity for DBT increased rapidly in the initial 10 min, 41.6 mg S/g for HKUST-1 and 48.4 mg S/g for HKUST-1@γ-Al2O3, respectively. Then the adsorption capacity increased steadily and almost reaches aplateau in 60 min, being 49.1 and 59.7 mg S/g for HKUST-1 and HKUST1@γ-Al2O3, respectively. When the time was extended to 120 min, the adsorptive desulfurization capacities were increased only by 0.1 and 0.2 mg S/g for HKUST-1 and HKUST-1@γAl2O3, correspondingly. Hence the optimum reaction time was concluded to be about 60 min. 7252
DOI: 10.1021/acs.iecr.6b01001 Ind. Eng. Chem. Res. 2016, 55, 7249−7258
Article
Industrial & Engineering Chemistry Research
DBT molecules occupied 15.1% of total surface area of pristine HKUST-1, while the occupancy value increased significantly to 18.2% when HKUST-1@γ-Al2O3 was used corresponding to a promoted surface utilization rate by 20%. Very impressively, by comparison with some conventional zeolites, activated carbon, MOFs, and some other adsorbents (Table 1), the adsorptive desulfurization capacity of HKUST-1@γ-Al2O3 was found to have excellent adsorptive desulfurization capacity superior to most of other conventional adsorbent and all of the MOFs, i.e. MIL-101, MOF-5, UMCM-150. What is noteworthy is that the HKUST-1@γ-Al2O3 composite also has a significant advantage in view of economics over other MOFs like UMCM-150 which need expensive organic ligands for preparation and, therefore, has great potential for practical applications. In addition, deep desulfurization experiments for DBT performed at the optimum reaction conditions (Table 2)
Figure 6. Effect of different adsorbent amounts on adsorptive desulfurization capacity for DBT over HKUST-1 and HKUST-1@γAl2O3. Reaction condition: model oil, 20 g; [DBT], 1000 ppmwS; reaction time, 60 min; temperature, 30 °C.
Table 2. Deep Desulfurization Experimentsfor DBT over HKUST-1@γ-Al2O3 as Adsorbenta
higher than HKUST-1 and the adsorption reaction of HKUST1@γ-Al2O3 reached equilibrium quickly than HKUST-1 indicating more efficient utilization of the surfaces/pores of HKUST-1 on the γ-Al2O3. On the basis of the above results, it is known that under the optimum experimental conditions, the adsorptive desulfurization capacities of pristine HKUST-1 and HKUST-1@γ-Al2O3 for DBT were 49.5 and 59.7 mg S/g, respectively, i.e., the adsorptive desulfurization capacity of HKUST-1@γ-Al2O3 increased by 17.8% compared to pristine HKUST-1. According to the dynamic diameter being 5.97 Å, the dynamic molecular size being 0.26 nm2 of DBT molecule, and the BET surface area being 1601 m2/g for the HKUST-1, it can be calculated that the
Madsorbent (g)
C0 (ppmwS)
Ci (ppmwS)
Madsorbance/Moil (wt %)
MMOF/Moil (wt %)
5 6 10 15
35 35 35 35
10.8 9.6 4.6 3.8
25 30 50 75
4.68 5.61 9.35 14.03
a
Reaction conditions: model oil 20 g, HKUST-1@γ-Al2O3 0.5 g, temperature 30 °C, time 60 min.
showed that, when the ratio of adsorbent HKUST-1@γ-Al2O3 to oil was more than 30 wt %, which corresponded to MMOF/ Moil being >5.6 wt %, the DBT content in model oil decreased
Table 1. Adsorptive Desulfurization Capacity of Different Adsorbents for DBT adsorbent
sovlent
C0 (ppmwS)/ system
Qmax (mg S/g MOF)
refs
MOF-5 MOF-5 MOF-505 MOF-505 UMCM-150 UMCM-150 MIL-101 ZIF-8-derived ZIF-8-derived Cr-BDC Cr-BTC activated Al2O3 CMK-3 CMK-5 Carbon aerogel Activated carbon activated carbon spheres Cu(I)−Y zeolite Co−Y zeolite Ce/Ni−Y zeolite AC loaded Na,Co,Ag or Cu MC-Z (Cu, Co, or Fe)a PT-Ag-MASNb HKUST-1 HKUST-1@γ-Al2O3
iso-octane iso-oct:toluene (85:15) iso-octane iso-oct:toluene (85:15) iso-octane iso-oct:toluene (85:15) n-octane n-hexane n-hexane:para-xylene (9:1) n-octane n-octane n-hexane n-hexane n-hexane n-hexadecane n-octane n-octane n-octane n-octane n-octane n-hexane n-hexane n-decane n-octane n-octane
300/fixed bed 300/fixed bed 300/fixed bed 300/fixed bed 300/fixed bed 300/fixed bed ∼714/batch
