ARTICLE pubs.acs.org/IECR
Sorption-Enhanced Reforming of Ethanol over Ni- and Co-Incorporated MCM-41 Type Catalysts Seval Gunduz and Timur Dogu* Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey ABSTRACT: Ni- and Co-incorporated MCM-41 type mesoporous materials with Ni/Si and Co/Si molar ratios of 0.12 were synthesized, characterized, and tested in both steam reforming of ethanol (SRE) and sorption-enhanced steam reforming of ethanol (SESRE) reactions. Characterization results showed that Co and Ni were successfully incorporated and well-dispersed in the mesoporous MCM-41 support. Ni- and Co-incorporated MCM-41 catalysts had surface area values of 449.0 and 303.6 m2/g, respectively. They also had narrow pore size distributions, with average pore diameters of 2.2 and 1.98 nm, respectively. SESRE results obtained with these catalysts showed that in situ capture of CO2 during ethanol reforming reaction significantly enhanced hydrogen yield in the temperature range of 500600 °C. The catalytic performance of Ni-incorporated MCM-41 was much better than the Co-incorporated MCM-41, in hydrogen production by ethanol reforming. The highest hydrogen yield value obtained over the Ni-incorporated MCM-41 catalyst was achieved at 600 °C as 5.6 in SESRE reaction. This was ∼94% of the maximum possible hydrogen yield value of 6.0.
1. INTRODUCTION The increase in world population, together with developments in technology, have caused a major increase in global energy demand and fuel consumption. Fast depletion of fossil resources and environmental restrictions, such as global warming due to greenhouse gas emissions, has initiated new pathways of research on alternative renewable energy sources, with high efficiency and low environmental impact.14 Unlike fossil fuels, hydrogen burns cleanly with no emission of environmental pollutants, and it possesses the highest energy content per unit of weight.5 Besides these advantages of hydrogen, its transportation and storage problems are the major drawbacks to be resolved for efficient and safe use of it as a transportation fuel in fuel-cell-derived cars. Storage difficulty of highly volatile and explosive hydrogen is a major drawback of using it as a transportation fuel. Conventionally, it is produced via the steam reforming of natural gas or oil, which is not necessarily very clean and also depends on the availability of such resources. Methanol or ethanol reforming are considered as alternative routes for the production of hydrogen.511 On-board hydrogen production via the reforming of alcohols is a possible alternate to hydrogen storage in fuel-cell-derived cars.6,7 Being almost CO2neutral and safer than methanol, ethanol reforming is considered to be an attractive route for hydrogen production. Ethanol can be produced from biowaste and cellulose through bioprocesses. The production of hydrogen from such non-fossil-fuel resources is considered to be quite attractive. Another advantage of ethanol steam reforming (SRE) over the reforming of natural gas or petroleum-derived fuels is its lower reforming temperature. In addition, it is easy to store and transport ethanol. Also, ethanol is not highly toxic to human health.5 It can be considered to be a very good source for hydrogen storage. One liter of ethanol contains 103 g of hydrogen and, as a result of its steam reforming, 6 mol of H2 could, in principle, be produced per mole of ethanol. C2 H5 OH þ 3H2 O T 2CO2 þ 6H2 r 2011 American Chemical Society
ð1Þ
However, it is not easy to achieve such a hydrogen yield value, because of the occurrence of undesired side reactions, coke formation, and equilibrium limitations. Two important reactions taking place in this system are the reforming reaction, yielding synthesis gas, and the water-gas shift reaction (WGSR). C2 H5 OH þ H2 O T 2CO þ 4H2
ð2Þ
CO þ H2 O T CO2 þ H2
ð3Þ
The presence of CO in the product stream and the formation of CH4, acetaldehyde, and ethylene by ethanol cracking, dehydrogenation, and dehydration reactions, respectively, cause a decrease in hydrogen yield, from the maximum value of 6.0 per mol of converted ethanol. Dehydration of ethanol to ethylene is expected to take place in an acidic environment, while ethanol dehydrogenation to acetaldehyde takes place over basic catalysts. The Boudouard reaction 7 may also cause coke formation in this system. C2 H5 OH T C2 H4 þ H2 O
ð4Þ
C2 H5 OH T CO þ CH4 þ H2
ð5Þ
C2 H5 OH T C2 H4 O þ H2
ð6Þ
2CO T CO2 þ C
ð7Þ
Because of these side reactions, hydrogen purity of the product gas is reduced. A new reaction process, called sorption-enhanced Special Issue: CAMURE 8 and ISMR 7 Received: August 18, 2011 Accepted: November 22, 2011 Revised: November 22, 2011 Published: November 22, 2011 8796
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steam reforming (SESRE), has been proposed to improve hydrogen production.1218 This sorption-enhanced process is based on Le Chatelier’s principle, in which the reaction equilibrium will be shifted to favor an increase of the reactant conversion by in situ removal of one of the products.13,1924 A comprehensive review of sorption-enhanced hydrogen production is given in the nice review by Harrison.21 In the present study, CO2 produced during the steam reforming of ethanol (SRE) was captured by CaO to increase hydrogen yield. Removal of CO2 from the reaction medium during reforming is expected to shift WGSR to the product side. The overall SESRE reaction and CO2 capture with CaO can be illustrated as C2 H5 OH þ 3H2 O þ 2CaO T 2CaCO3 þ 6H2
ð8Þ
CaOðsÞ þ CO2 T CaCO3
ð9Þ
In addition to high hydrogen production yield, low CO and CO2 concentrations in the product stream and less coke formation on the catalyst surface are some of the expected results of the SESRE process. The high cost of noble-metal catalysts diverted the research activities to the development Ni- and Co-based catalysts for the reforming reactions. Such metals were shown to give good catalytic performance in steam reforming reactions. Various studies on investigation of supported catalysts on different supports have been reported using nickel2528 and cobalt10,2933 as the active metals, for steam reforming of ethanol. MgO, ZrO2, Al2O3, CeO2, TiO2, and SiO2 are among the supports used in the synthesis of such catalysts. Discovery of MCM-41 and SBA-15 type silicate structured mesoporous materials with ordered pore structures opened new avenues in the development of new catalysts with much less transport limitations than microporous zeolites.34,35 Such materials have very high surface area values and narrow pore size distributions in the mesopore range. These silicate structured materials generally are not quite active for catalytic applications. Usually, metals or metal oxides were incorporated into their structure to enhance their catalytic performance. Such incorporation can be achieved following either one-pot (direct synthesis) or post-synthesis (impregnation, grafting etc.) procedures.36,37 The use of Ni-incorporated MCM-41 type catalysts modified by Ru and Rh in dry reforming reactions were illustrated in recent publications.38,39 In the present study, Coand Ni-incorporated MCM-41 type mesoporous materials were synthesized, characterized, and tested in both SRE and SESRE of ethanol reactions, using CaO as the CO2 adsorbent.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. Ni- and Co-incorporated MCM-41 type catalysts (Ni@MCM-41 and Co@MCM-41, respectively) were prepared by a wet impregnation method. First, MCM-41 type support material was synthesized following a hydrothermal procedure, according to the route described in our earlier publications.36,37 For MCM-41 synthesis, surfactant solution (13.2 g of CTMABr in 87 mL of deionized water) was prepared at 30 °C until a clear solution was obtained. Then, 11.3 mL sodium silicate solution was added to the surfactant solution and the pH was adjusted to 11.0. The final solution was stirred for 1 h and then put into a Teflon bottle, which was placed into a stainless-steel autoclave. Hydrothermal synthesis was performed at 120 °C for 96 h. The solid product produced was washed and filtered until the pH of the wash water
became almost neutral. After washing, solid product was dried at 40 °C under vacuum. Finally, calcination was performed at 600 °C for 6 h. For the impregnation of Ni or Co, calcined MCM-41 material was suspended in deionized water. A sufficient amount of Ni(NO3)2 3 6H2O or Co(NO3)2 3 6H2O was dissolved in deionized water for the synthesis of Ni@MCM-41 or Co@ MCM-41, respectively. A metal solution was added to MCM-41 suspension dropwise while stirring, and the resultant mixture was stirred for 24 h and then evaporated to dryness. The solid product was calcined at 600 °C for 6 h and reduced under a hydrogen atmosphere at 550 °C for 4 h. The synthesized materials were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), nitrogen adsorptiondesorption, and scanning electron microscopy (SEM) techniques. XRD patterns were obtained using a Rigaku Model D/MAX2200 difractometer in the Metallurgical Engineering Department of Middle East Technical University. EDS analysis was also performed in the same laboratory using a JEOL Model JSM-6400 instrument. SEM images were obtained using the Model JSM-6400 electron microscope, equipped with a NORAN System 6 microanalysis system. Nitrogen adsorptiondesorption isotherms and related pore structure analysis is made using a Quantachrome Autosorb-6 device available at the Central Laboratory of Middle East Technical University. Thermal analysis (TGA-DTA) of the used catalysts was performed using a Shimadzu Model TA-60 WS thermal analyzer, equipped with a differential thermal analysisthermogravimetry (DTATG) apparatus, to monitor coke formation on the catalyst during reaction. 2.2. Reaction Tests. Synthesized catalyst was placed into a quartz tubular reactor with an inner diameter of 10 mm. This fixed-bed reactor was placed into a tubular furnace. Reaction temperature was adjusted by the temperature controller of the tubular furnace. Liquid feed mixture was prepared with a H2O/ C2H5OH molar ratio of 3.2, and this mixture was introduced to the evaporator of the system by a liquid injection pump. Liquid feed was mixed with argon gas and evaporated in the evaporator, which was kept at 150 °C. Total flow rate and the ethanol mole fraction of the vapor phase feed stream to the fixed-bed reactor were kept as 50 mL/min and 0.095, respectively. The space time was kept at 0.18 s g cm3 (evaluated at room temperature), in the SRE experiments. In the case of SESRE experiments, space time evaluated by the division of catalyst weight to the total vapor flow rate was again 0.18 s g cm3. However, space time evaluated by the division of total mass of solid mixture in the reactor (mass of catalyst + CaO) to the total flow rate of the inlet stream was 1.98 s g cm3. A condenser was placed between the reactor outlet and the gas chromatograph, which was connected online to the reactor exit stream. Liquefied species were collected in the condenser and gaseous products were analyzed by a gas chromatograph (Agilent, Model 6850), which was equipped with a thermal conductivity detector (TCD) and a Porapak S column. Chemical analysis of the liquefied stream was also made chromatographically at certain intervals.40
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization Results. The XRD patterns of Ni@MCM-41 and Co@MCM-41 type catalytic materials containing Ni/Si and Co/Si molar ratios of 0.12 are given in Figure 1. In the same figure, the XRD pattern of the MCM-41 support that has been synthesized in this work is also shown. 8797
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Figure 1. XRD patterns of (a) MCM-41, (b) Ni@MCM-41, and (c) Co@MCM-41.
Figure 2. Nitrogen adsorptiondesorption isotherms of (a) MCM-41, (b) Ni@MCM-41, and (c) Co@MCM-41.
The XRD pattern of MCM-41 showed the characteristic diffraction pattern of this material, giving the main peak (corresponding to d100) at a 2θ value of 2.2° and the three reflection peaks at 3.8°, 4.4°, and 5.8°. This is a clear indication of the formation of the mesopore structure with long-range order. As shown in Figures 1b and 1c, the characteristic main peak of MCM41 was also observed in the XRD patterns of both Co@MCM-41 and Ni@MCM-41 catalysts. However, disappearance of the reflection peaks of MCM-41 structure after Ni or Co impregnation indicated some loss of the ordered mesopore structure, due to deposition of these metals within the mesopores. In Figure 1b, the sharp peaks observed at 2θ values of 44.48° and 51.82° correspond to metallic Ni clusters. The small peak observed at 76.38° indicated the presence of some unreduced NiO within the catalyst. In the case of the XRD pattern of Co@MCM-41 (Figure 1c), small peaks observed at 44.32° and 51.52° correspond to metallic Co clusters. Another small peak observed at 42.4° corresponds to CoO. Cluster sizes of CoO and Co were estimated as ∼5 and ∼11 nm, respectively, from the Scherrer equation. These results indicated that most of the cobalt was very well-dispersed within the mesopores of MCM-41. The broad band observed between 20 and 30° in both Figures 1b and 1c corresponds to amorphous silica, which constitutes the main body of MCM-41. Chemical compositions of the synthesized materials were checked by EDS analysis, and the results indicated that Ni/Si
and Co/Si atomic ratios in Ni@MCM-41 and Co@MCM-41 were both 0.12. Nitrogen adsorptiondesorption isotherms of MCM-41, Ni@MCM-41, and Co@MCM-41 are given in Figures 2a, 2b, and 2c, respectively. Results shown in Figure 2a also proved the formation of an ordered mesoporous structure of MCM-41, with Type IV adsorptiondesorption isotherms. Well-defined and steep hysteresis loops with parallel adsorption and desorption branches is the characteristic of formation of parallel ordered mesopores, with a very narrow pore size distribution. Such a hysteresis loop was observed for MCM-41, in the relative pressure range of 0.30.4. However, in the cases of Ni- and Co-impregnated MCM-41 type catalytic materials, this characteristic behavior with a sharp and parallel hysteresis loop is essentially lost, indicating deformations in the long-range order of mesopores, because of the deposition of Ni or Co on mesopore surfaces. Similar conclusions were reached from the pore size distributions, indicating significant decrease of intensity of the differential pore size distribution peak observed at a pore size range of 2.03.0 nm, via the incorporation of Ni or Co (Figure 3). The surface area, average pore volume, and average pore diameter values of MCM-41, Ni@MCM-41, and Co@MCM-41 are summarized in Table 1. The results reported in this table also showed that both surface area and pore volume values decreased 8798
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Figure 4. Scanning electron microscopy (SEM) images of (a) Co@MCM-41 and (b) Ni@MCM-41. Figure 3. Pore size distributions of (a) MCM-41, (b) Ni@MCM-41, and (c) Co@MCM-41.
Table 1. Some Properties of MCM-41, Co@MCM-41, and Ni@MCM-41 Surface Area (m2/g) BJH desorption catalyst
BET
BJH desorption
BJH pore volume (cm3/g) pore diameter (nm)
MCM-41 1040.0 1334.0 Co@MCM-41 303.6 271.6
1.07 0.26
2.75 1.89
Ni@MCM-41
0.29
2.17
449.0
351.3
more than 50% by the impregnation of Ni or Co into MCM-41. Some decrease in the average pore diameter was also observed after the impregnation step of Ni or Co into MCM-41. These results indicated that some of the pores of MCM-41 were blocked by the impregnated metals, which penetrated into these pores. Some fraction of Ni or Co, having cluster sizes of 0.9) at the initial reaction times (see Figure 9). This value decreased 8800
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Figure 9. Vapor-phase product stream composition in the SESRE reaction over Ni@MCM-41 at 500 °C.
Figure 10. Vapor-phase product stream composition in the SRE reaction over Ni@MCM-41 at 550 °C.
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Figure 12. Vapor-phase product stream composition in the SRE reaction over Ni@MCM-41 at 600 °C.
Figure 13. Vapor-phase product stream composition in the SESRE reaction over Ni@MCM-41 at 600 °C.
500 °C to 600 °C. Another factor that might be effective in decreasing the CH4 mole fraction at higher temperatures could be the increased contribution of dry reforming reaction. It is very well-known that Ni-incorporated catalysts were also quite effective in the dry reforming of methane. CH4 þ CO2 T 2CO þ 2H2
Figure 11. Vapor-phase product stream composition in the SESRE reaction over Ni@MCM-41 at 550 °C.
to ∼0.6 at reaction times over 100 min, clearly indicating the saturation of CaO at longer times, as a result of reaction with CO2. No CO or CO2 were observed in SESRE runs until ∼80 min of reaction time, indicating efficient capture of CO2 by CaO. However, the formation of methane was also observed in the SESRE reaction at 500 °C (see Figure 9). Similar results were obtained at 550 °C (Figures 10 and 11) and 600 °C (Figures 12 and 13). As shown in these figures, some increase of hydrogen mole fraction was observed with an increase in temperature in the SRE reaction results. This increase of hydrogen mole fraction was accompanied with a decrease in CH4 mole fraction and with some increase in CO mole fraction. These results indicated that the relative contribution of the ethanol decomposition reaction (reaction 5), with respect to SRE reaction (reaction 1), decreased as the temperature increased from
ð10Þ
The occurrence of this dry reforming reaction is expected to cause some increase in CO and H2 mole fractions, in parallel to some decrease in CH4. Another result of increase of temperature could be caused by the decrease of the conversion of CO to CO2 through the water-gas shift reaction, because of thermodynamic limitations. It is interesting to note that, in the presence of CaO (the SESRE reaction), formation of CH4 is more than the corresponding results obtained in the SRE reaction, especially at higher temperatures. For instance, at 600 °C, the methane mole fraction in the SESRE reaction was ∼10% (Figure 13), while the corresponding value in SRE reaction was 6%. Higher CH4 mole fraction values observed in SESRE runs than the SRE experiments may also be explained by the retardation of dry reforming reaction (see reaction 10), because of the removal of CO2 by CaO. XRD analysis of the used catalysts indicated that the mesoporous structure of the catalysts was not significantly distorted during the reaction. However, both XRD and TGA-DTA analysis results of the used catalysts indicated coke formation on the catalyst in SRE runs. The XRD pattern of the used Ni@MCM-41 catalyst (at 600 °C in SRE runs) is shown in Figure 1b, together with the XRD pattern of fresh catalyst. The XRD peak observed at 2θ ≈ 26° in the pattern of the used catalyst indicated the presence of coke. Thermal analysis (TGA-DTA) of this material 8801
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Figure 14. TGA-DTA analysis results of used catalysts at 600 °C (a) Ni@MCM-4 after SRE run and (b) Ni@MCM-41 + sorbent after the SESRE run.
also showed a weight loss of ∼55% with an exothermic DTA peak at ∼625 °C (Figure 14a). This weight loss corresponds to the combustion of coke during thermal analysis. However, thermal analysis of the used material mixture (catalyst + limestone), after the SESRE run at 600 °C, showed a very small exothermic DTA peak (at
