Simultaneous Adsorption of H2S and CO2 on Triamine-Grafted Pore

Feb 4, 2011 - However, the CO2/H2S selectivity was not mentioned. ... After vessel 2 was filled with the bulk compound (CO2), the whole installation, ...
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Simultaneous Adsorption of H2S and CO2 on Triamine-Grafted Pore-Expanded Mesoporous MCM-41 Silica Youssef Belmabkhout,† Nicolas Heymans,‡ Guy De Weireld,‡ and Abdelhamid Sayari*,† † ‡

Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Thermodynamics Department, Faculty of Engineering, University of Mons, 20 Place du Parc, Mons 7000, Belgium ABSTRACT: The feasibility of using triamine-grafted pore-expanded mesoporous MCM-41 silica (TRI-PE-MCM-41) as a potential adsorbent for simultaneous removal of CO2 and H2S was investigated. Adsorption isotherms of CO2 and H2S were measured at different temperatures, whereas adsorption at low H2S concentrations from 700 to 10000 ppm with balance CO2 was studied at 298 K. It was found that both CO2 and H2S adsorb and the selectively toward H2S versus CO2 decreases at an increasing H2S concentration. Because TRI-PE-MCM-41 hardly adsorbs any of the common gases, such as CH4, H2, N2, O2, and CO, it was inferred that this material may be suitable for the purification of a variety of gas streams containing CO2 and H2S. These findings lay the ground for the development of a single-stage process for the removal of CO2 and H2S from biogas, using amine-supported PEMCM-41.

’ INTRODUCTION The deleterious effect of fossil fuels on the environment and human health triggered a worldwide race for the development of renewable energy sources. Although natural gas (NG) is a fossil fuel, it is recognized to be an excellent alternative before the transition to cleaner energy vectors. However, NG often contains significant amounts of CO2 and H2S1 that have to be removed to less than 1% for CO2 and 4 ppm for H2S2 to meet the fuel gas specifications for pipeline transportation. The transformation of biomass into biogas, a mixture of typically 50% CO2 and 50% CH4 with some water vapor, H2S, and other minor impurities, through chemical or enzymatic conversion made great strides in recent years.3 Biogas is now being developed on a large scale for the production of fuels for stationary power generation as well as for the transportation sector. It is also being upgraded to biomethane and fed into the natural gas grid or in dedicated pipelines. The production of high-quality biomethane from biogas requires the separation of CO2, H2S, and water vapor to meet strict specifications. To decrease the amount of H2S in the biogas, air is often injected in the anaerobic digester, which leads to additional complexity of nitrogen and oxygen removal. CO2 and H2S are also the major impurities in some refinery gases as well as in synthesis gas for hydrogen production. In addition, H2S is a potent poison to many transition-metal-based catalysts used in downstream processes, such as Ni-based reforming catalysts, Cu-based water-gas-shift catalysts, as well as the Pt-based catalysts in fuel cells.4-6 Current technologies for biogas upgrading and NG treatment are often multi-stage processes, requiring separate steps for the removal of H2S, CO2, and H2O, and thus, costly. The most common method for acid gas removal is the liquid-phase chemical scrubbing with amines.7-9 However, this technology suffers a number of shortcomings, including high regeneration cost, excessive corrosion, loss of amine via oxidative degradation, and evaporation.10,11 Although significant progress was made for upgrading biogas and natural gas using polymer membrane r 2011 American Chemical Society

technology,1,12 this process is limited by the low CO2/CH4 and H2S/CH4 selectivities, which generally lead to the loss of methane. Moreover, the selective removal of CO2 and H2S requires two types of polymer membranes with pressurized streams.1 High-pressure water scrubbing, driven by the much larger solubility of CO2 and H2S in water compared to CH4, is known to be the simplest technique for biogas upgrading, allowing for 99% of CH4 purity. However, it has H2S-containing water and air streams that should be in turn treated. In addition, because of changing pH during processing, the H2S removal may be reduced. Adsorption is recognized to be an energy-efficient technology for CO2 and H2S removal, provided that a material with high and stable adsorption capacity and high selectivity toward acid gases under mild conditions is available. In recent years, considerable research effort was made to develop novel CO2 and H2S adsorbents with such attributes including metal oxides,13-15 modified mesoporous silicas,16-26 activated carbons,27,28 and metal organic frameworks.29-31 However, only few investigations focused on the development of adsorbents with the ability to remove both CO2 and H2S, selectively and simultaneously. Early work of Huang et al.32 described CO2, H2S, and CH4 adsorption on 3-aminopropyl-functionalized MCM-48. However, the CO2/H2S selectivity was not mentioned. Recently, Ma et al.23 reported that polyethylenimine (PEI)-impregnated MCM-41 or SBA-15 is capable of selective adsorption of both CO2 and H2S. However, it was reported that not only is the adsorption of CO2 diffusion-limited but also the optimum temperature for CO2 adsorption was 75 versus 25 °C for H2S. Moreover, CO2 seems to strongly inhibit the H2S adsorption at room temperature. In earlier contributions, we showed that triamine-grafted pore-expanded MCM-41 silica (TRI-PE-MCMReceived: November 21, 2010 Revised: January 12, 2011 Published: February 04, 2011 1310

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Energy & Fuels 41) exhibits very high affinity toward H2S and CO233 and excellent CO234,35 and H2S33 adsorption cyclability. The current work was devoted to adsorption measurements for CO2 and H2S at different temperatures (both adsorption and desorption) and concentrations, aiming to improve our understanding regarding the most appropriate adsorption-desorption conditions using temperature or pressure swing adsorption (TSA or PSA). Adsorption equilibrium measurements for CO2/H2S mixtures revealed a preferential adsorption toward H2S, which is a highly desirable behavior toward the development of a single-stage procedure for the removal of CO2 and H2S from methanecontaining gases.

’ EXPERIMENTAL SECTION

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the equilibrium state is repeated. When the mass is stable, valve V8 is opened until a new thermodynamic equilibrium is reached. The mass, temperature, and pressure are then recorded. This method is used to avoid the dead volume issue, i.e., partial or non-homogenization of the gas mixture in this volume. Valves V5 and V6 are then switched in vertical position, and valves V17 and V18 are opened. The gas phase is analyzed by gas chromatography coupled with mass spectrometery. Using an appropriate (p-v-T) EOS for the H2S/CO2 mixture,40 in addition to the pressure, temperature, and gas mixture composition, the gas density of the mixture can be determined. Further details on the adsorbed quantity determination and the correction of the buoyancy effect may be found elsewhere.36 The selectivity of H2S over CO2 in binary mixtures is determined using the following equation: SH2 S=CO2 ¼

Materials. The detailed preparation procedure and structural characterization of the adsorbent TRI-PE-MCM-41 were reported elsewhere.21 The Brunauer-Emmett-Teller (BET) surface area, pore volume, pore size, and particle density of TRI-PE-MCM-41 were as follows: 367 m2/g, 0.87 cm3/g, 9.4 nm, and 0.93 g/cm3, respectively. The typical amine loading of TRI-PEMCM-41 determined by TGA was 7.9 mmol g-1. For single-gas adsorption, carbon dioxide (99.99%) and helium (99.999%) were supplied by BOC Canada and hydrogen sulfide was supplied by Praxair Belgium (99.8%). For multicomponent adsorption experiments, gas mixtures were achieved using carbon dioxide (99.996%), helium (99.999%), and hydrogen sulfide (99.8%) supplied by Praxair Belgium. Adsorption Measurements. Single-Component Gas Adsorption. Adsorption equilibrium measurements for pure H2S and CO2 were performed using a Rubotherm (Bochum, Germany) gravimetric-densimetric apparatus. Further details about the experimental setup and procedure may be found elsewhere.21 Pretreatment of the adsorbent was carried out in vacuum (510-4 mbar) in the temperature range of 348-423 K. Adsorption of Binary Mixtures. Adsorption measurements of CO2H2S mixtures were carried out using a homemade setup comprised of a Rubotherm magnetic suspension balance (Figure 1).36 This setup allows for accurate measurements of pressure in the range of 0-50 bar using two pressure transducers, namely, MKS Baratron 627B, 0-1000 Torr (accuracy of 0.12% of the reading value) and Endress Hauser Cerabar S PMP 635, 0-100 bar (accuracy of 0.1% of the full scale). The adsorbent sample (up to 1.5 g) is placed in a 12 cm3 crucible and outgassed at a maximum temperature of 393 K before the actual adsorbent mass is measured. At the beginning of an experiment, the whole installation is under vacuum. Then, the first gas, i.e., the trace gas (H2S), is injected in vessel 1, and the temperature and pressure are measured. Knowing the vessel volume and using an appropriate (p-v-T) equation of state (EOS),37-39 the amount of gas introduced is determined. Vessel 1 is then isolated by closing valves V3 and V4; the rest of the installation is outgassed, and the same operation is repeated for the second gas in vessel 2. After vessel 2 was filled with the bulk compound (CO2), the whole installation, except the two vessels, is evacuated. The two gases are then mixed, and the circulation pump is switched on to homogenize the gas mixture. Once the mixture is completely homogeneous, it is directed in the adsorption cell by opening valves V1, V2, V3, and V4. The threeway valves V5, V6, V9, and V10 are in the horizontal position; i.e., flow goes from V4 to the adsorption cell and then to the circulation pump. With the tube between V9 and V10 being under vacuum, the gas mixture circulates through the adsorption cell but not in the volume loop. Because of dead volume issues, the pressure transmitters remain under vacuum (valves V7 and V8 closed). Once the adsorption equilibrium is reached, the circulation pump is switched off and the mass is monitored with the magnetic balance, with the mass being recorded every 3 min for 15 min. If the standard deviation is under 80 μg, the value is recorded; otherwise, the circulation pump is switched on for 2 h, and the control of

xH2 S =xCO2 yH2 S =yCO2

where x and y refer to the molar composition of the adsorbed phase and the gas phase, respectively.

’ RESULTS AND DISCUSSION Single-Component Adsorption Data. In this work, we used two different regeneration modes and pretreatment methods. Pressure-temperature regeneration operation (PTRO) means that the regeneration (or pretreatment) temperature is higher than the adsorption temperature, while pressure regeneration operation (PRO) means that the regeneration (or pretreatment) are carried out at the same adsorption temperature; i.e., the temperature is constant over the whole process of pretreatment, adsorption and desorption. PTRO. Figures 2 and 3 show the adsorption isotherms of pure CO2 and H2S on TRI-PE-MCM-41 at 298, 308, and 328 K and 298, 308, 323, and 353 K, respectively. The material was pretreated at 393 K in vacuum (ca. 5  10-4 mbar). CO2 adsorption isotherms showed steep slopes at low partial pressure, while less pronounced slopes were observed for the H2S adsorption isotherms. This may be indicative of relatively stronger interactions of CO2 with amine adsorption sites in comparison to H2S. At the same partial pressure, the adsorption capacity was higher for CO2 than H2S over the whole temperature range, particularly at low pressure, where chemical adsorption is predominant, but the trend is reversed at relatively high pressure, where physical adsorption is most likely more dominant in the case of H2S.33 Moreover, it is seen that, within the range 298-323 K, the CO2 adsorption capacity is comparatively less sensitive to the temperature than H2S. This may be explained by the (i) difference in the chemical adsorption mechanism for CO2 (zwitterion mechanism) and H2S (proton-transfer mechanism) and (ii) the higher contribution of physical adsorption for H2S compared to CO2, as pointed out in our previous contribution.33 Indeed, CO2 adsorption on amine-modified materials is dominated by chemisorption as a result of the direct interaction between the adsorbate and amine groups. Whether under dry or humid conditions, there was a striking similarity with CO2-amine chemistry in solution.20,41 It is thus inferred that the interaction takes place via the zwetterion mechanism for carbamate formation. The occurrence of carbamate is consistent with the appearance of a 13C nuclear magnetic resonance (NMR) peak at 165 ppm (not shown).35 Being an acid gas, the behavior of H2S as an adsorbate is qualitatively similar to CO2. At low pressure, H2S chemisorbs on the amine groups via the protontransfer mechanism. 1311

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Figure 1. Gravimetric-chromatographic setup.

Figure 2. Adsorption isotherms for CO2 on TRI-PE-MCM-41 at different temperatures (regeneration at 393 K).

Analysis of single-gas adsorption data indicated that, under room temperature, the H2S/CO2 molar ratio of adsorbed gases is below 1 at low pressure and higher than 1 at pressures above ca. 0.5 bar, indicating that the relative affinity of the adsorbent toward the two adsorbates switches from CO2 to H2S. Moreover, at higher temperatures, the intersection point of the H2S and CO2 adsorption isotherms (Figures 2 and 3) occurs at higher pressures. This is in good agreement with the trend of H2S/CO2 selectivity at an increasing temperature in liquid amine absorption.2,7 Because the adsorbed molar ratio of CO2/H2S is a qualitative indicator, not taking into account the competitive adsorption of CO2 and H2S, this parameter is often quite different from the actual H2S versus CO2 adsorption selectivity. The experimental CO2 versus H2S selectivity will be discussed later in this work. Figure 4 shows the evolution of the CO2 and H2S isosteric heat (Qisos) of adsorption, calculated using the Clauysius Clapeyron equation and the adsorption data at different temperatures shown in Figure 2 and 3, respectively. At the lowest adsorption 1312

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Figure 3. Adsorption isotherms for H2S on TRI-PE-MCM-41 at different temperatures (regeneration at 393 K).

Figure 4. Isosteric heat of CO2 and H2S adsorption on TRI-PE-MCM41.

capacity recorded, i.e., 0.94 mmol/g for CO2 and 0.2 mmol/g for H2S, the values of Qisos were 92 and 42 kJ/mol for CO2 and H2S, respectively. Consistent with the heat of adsorption in liquid amine absorption,42,43 these results show that CO2 exhibits stronger interactions with amine groups than H2S. This is also consistent with the fact that H2S is a weaker acid than CO2, resulting in weaker interaction with TRI-PE-MCM-41. Nonetheless, H2S acidity is strong enough to allow for highly selective adsorption of H2S on TRI-PE-MCM-41 in the presence of CH4.33 Interestingly, Figure 4 shows a faster decrease of Qisos versus loading in the case of CO2, indicating that CO2 probes TRI-PE-MCM-41 as a more energetically heterogeneous material than H2S. PRO. The purpose of this section is to carry out a preliminary study on the feasibility of using an isothermal regime for H2S adsorption and desorption operation, as was performed earlier for CO2 .44 Figures 5 and 6 show the adsorption-desorption isotherms of H2S at 323 and 343 K on TRI-PE-MCM-41 pretreated in vacuum at the same temperature (PRO). The adsorption and desorption isotherms at 323 and 343 K after pretreatment in vacuum and 393 K (PTRO) are also included for comparison. Interestingly, despite the low pretreatment temperature at 323 and 343 K, H2S exhibited only slightly higher capacity than the case when higher pretreatment temperature (393 K) was applied. In addition, there are only slight differences between the

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Figure 5. Adsorption-desorption isotherms for H2S at 323 K (pretreatment at 323 and 393 K).

Figure 6. Adsorption-desorption isotherms of H2S at 343 K (pretreatment at 343 and 393 K).

adsorption and desorption data in the case of PRO compared to PTRO possibly because of the slightly higher number of adsorption sites when PTRO is applied. Thus, as reported previously for CO2,44 H2S adsorption-desorption on TRI-PEMCM-41 using PRO operation seems to be reversible and H2S uptake at 323 and 343 K is as high as in the case of PTRO operations. However, it is important to notice that PTRO and PRO isotherms exhibit a more pronounced hysteresis at 323 K than at 343 K. This is most likely because the H2S adsorptiondesorption rate is lower at 323 K than at 343 K, so that the equilibrium criteria used, which corresponds to a mass change for less than 0.02 mg in 5 min, was met before a real equilibrium has been established, particularly for 323 K data. CO2-H2S Mixture Adsorption Data. To investigate the competition of CO2 and H2S, adsorption tests were carried out at 298 K using mixtures of different low concentrations of H2S. The material was pretreated at 348 K in vacuum. Low concentrations of H2S were obtained by applying a high CO2 partial pressure in the preparation step of the CO2-H2S mixtures. The overall pressure of the mixtures was 11 bar. Figures 7 and 8 show the equilibrium adsorption capacity of H2S and CO2 as well as H2S/CO2 selectivity as a function of the H2S concentration in the mixture. As seen, the H2S adsorption capacity increased by 66%, while the CO2 adsorption decreased by 2.7% when the H2S concentration increased from ca. 700 to 10 000 ppm. Although 1313

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The financial support of the Natural Science and Engineering Council of Canada (NSERC) is acknowledged. Y.B. thanks NSERC for a postdoctoral fellowship, and A.S. thanks the Federal Government for the Canada Research Chair in Nanostructured Materials for Catalysis and Separation (2001-2015). ’ REFERENCES Figure 7. Equilibrium adsorption capacity for CO2 and H2S in CO2H2S mixtures over TRI-PE-MCM-41 at 11 bar and 298 K.

Figure 8. H2S versus CO2 adsorption selectivity as a function of the H2S concentration at 11 bar and 298 K.

the CO2 concentration was dominant, the H2S/CO2 selectivity was higher than 1 in the whole range of concentrations studied. It decreased from 68 to less than 10 at an increasing H2S concentration from ca. 700 to 10 000 ppm. This result is in good agreement with the H2S/CO2 adsorbed molar ratio discussed earlier. It is thus inferred that, under appropriate conditions, H2S/CO2 selectivity may be in favor of H2S, indicating that TRIPE-MCM-41 could be a promising material for the simultaneous removal of H2S and CO2. Because, in most practical cases, the H2S concentration is much lower than CO2, the higher the H2S/ CO2 selectivity, the better. Further effort is underway to tune the H2S/CO2 selectivity for low (biogas) and high (natural gas and syngas) pressure applications.

’ CONCLUSION In this work, we thoroughly investigated the adsorption of CO2 and H2S and mixtures thereof on TRI-PE-MCM-41. It was found that TRI-PE-MCM-41 exhibits CO2 and H2S capacities as well as H2S versus CO2 selectivity suitable for simultaneous removal of CO2 and H2S. In addition, isothermal adsorptiondesorption tests showed that the optimum temperature for vacuum swing operation is in the range of 323-343 K for both CO2 and H2S.

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