Y2O3 Carbon Dioxide Absorbent with

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High Temperature CaO/Y2O3 Carbon Dioxide Absorbent with Enhanced Stability for Sorption-Enhanced Reforming Applications V. S. Derevschikov,†,‡ A. I. Lysikov,†,‡ and A. G. Okunev†,‡,* † ‡

Novosibirsk State University, pr.Koptyuga 1, Novosibirsk 630090, Russia Boreskov Institute of Catalysis, pr.Lavrentieva 5, Novosibirsk 630090, Russia ABSTRACT: To improve the stability of high temperature CO2 absorbent for sorption enhanced reforming applications yttria supported CaO were synthesized using two methods: calcination of mixed salt precursors and wet impregnation of yttria support. According to XRD data, CaO does not interact with the yttria matrix. However, introduction of CaO drastically changes the morphology of primary yttria particles. Increase in CaO concentration results in gradual plugging of the smaller pores and sintering of yttria support. The CO2 absorption uptake in recarbonation-decomposition cycles increases with increase in CaO content and reach 9.6 wt % at CaO content of 19.9 wt %. CaO recarbonation extent varies from 49 to 77%. CaO/Y2O3 absorbents are extremely stable under overheating and maintain their capacity in long series of decomposition-recarbonation cycles even after calcination at 1350 °C. The novel material resists moisture and retains its strength during storage in the air. According to tests, CaO/Y2O3 can be considered as a promising CO2 absorbent for fixed bed sorption enhanced hydrocarbons reforming.

’ INTRODUCTION High temperature CO2 sorbents were successfully applied for sorption-enhanced hydrocarbons reforming process (SERP). Materials of different chemical nature: alkali promoted hydrotalcites,1 13 lithium salts,14 19 or calcium oxide20 27 can reversibly absorb CO2 in the temperature range of 400 900 °C, often used in catalytic steam reforming of hydrocarbons. The SERP conditions depend on the sorption properties of the particular sorbent. Isothermal sorption regeneration cycles are used for promoted hydrotalcites,1 4 for which the CO2 desorption rate obeys a linear driving force model.7,10 On the other hand, CaO-based sorbents are regenerated usually at elevated temperature25,27 due to inherently slow kinetics of CaCO3 decomposition.28 30 Lyon and Cole presented a selfsustained thermally neutral mixed sorption-enhanced reforming unmixed combustion process.31 They used 8 atm pressure during the reforming step and an air purge at 1 atm during the regeneration step. Even a higher pressure during the reforming step would be of interest for hydrogen production in oil refining, ammonia synthesis, or coal gasification.32 However, at high steam and CO2 pressures a CaO-based sorbent undergoes accelerated sintering due to formation of the eutectic mixture of CaCO3 CaO Ca(OH)2.33 35 The problem can be solved through the use of supported or promoted sorbents with an active CaO component dispersed in the pores of a stable inert matrix. Examples of the promoters and supports used to date include individual or mixed oxides of magnesium,20,26,36 46 aluminum,46 52 lanthanum,53,54 silicon,55 59 titanium,60,61 and others.62 This paper presents the study of sorption properties of novel CaO/Y2O3 sorbent. As a support, yttria has a number of attractive properties making it a very promising material in SERP. It does not react with water or CO2 at ambient conditions, which facilitates the storage of the sorbent, does not react with either steam or carbon dioxide at elevated temperatures, and has r 2011 American Chemical Society

a melting temperature of 2415 °C. Y2O3 is among a few refractory oxides which do not react with CaO. The current price of yttria, while several times higher than the price of the alumina or titania, is, however, comparable to the price of a reforming catalyst and may not be prohibitive for small or middle scale reforming facilities.

’ EXPERIMENTAL SECTION Synthesis. Yttria supported CaO sorbents were prepared using two different techniques. The first method involved calcination of mixed precursors. The sorbent was synthesized as follows: 108.7 g of Y(NO3)3 3 6H2O (Reahim) and 33.5 g of Ca(NO3)3 3 4H2O (Laverna) were added to a 150 mL beaker containing 100 mL of distilled water and dissolved under vigorous stirring. The target Ca/Y atomic ratio in solution was 2. The solution was dried in an oven at 200 °C for 4 h, then the solids were slowly heated to 1300 °C in air and kept at this temperature for 2 h. The product was ground in a mortar and sieved to produce the fraction of 0.5 1 mm used in the experiments. This sorbent will be further referred to as CaY-calc. The second method uses the impregnation of yttria matrix with calcium nitrate solution. The matrix was synthesized as follows: 140 g of Y(NO3)3 3 6H2O were added to a 150 mL beaker containing 100 mL of distilled water and dissolved under vigorous stirring. The dried mixture was then calcined using the same calcination procedure as that described previously. The product was ground in a mortar and sieved to produce yttria grains of 1 2 mm size. The impregnation solution was prepared as follows. Ca(NO3)3 3 4H2O and distilled water were used to Received: July 15, 2011 Accepted: October 4, 2011 Revised: October 3, 2011 Published: October 04, 2011 12741

dx.doi.org/10.1021/ie2015334 | Ind. Eng. Chem. Res. 2011, 50, 12741–12749

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Figure 1. CaY-imp-5 sample weight change during the isothermal segment at 740 °C. CO2 pressure during sorption step 25 kPa, gas flow 100 cm3/min, 1 atm. total pressure. Figure 3. TEM image of CaY-calc sample.

Figure 2. XRD pattern of CaY-calc sample.

prepare saturated calcium nitrate solution in water that was stored at 20 °C. Prior to impregnation, the saturated solution was diluted with distilled water in the ratio 1:3. This solution was added dropwise to the support under intensive mixing. After the solution was added, the support became slightly wet (incipient wetness method). Afterward, the resulting precursor was dried for 4 h at 200 °C, heated to 1000 °C in air at a ramp rate of 10 °C/ min, calcined at this temperature for 2 h and, finally, calcined in air for 2 h at 1300 °C. The procedure was repeated five times. After each impregnation cycle, part of the sample was picked up and stored for characterization. This series of sorbent will be further referred to as CaY-imp-i, where i = 0 5 is the number of impregnations (i = 0 for starting Y2O3 material). Characterizations. The XRD analyses were conducted using Siemens HZG-4C powder X-ray diffractometer with a Cu Kα radiation source (wavelength, 0.15406 nm). An aluminum holder was used to support the samples in the XRD measurements. Pore volume and size distribution were calculated using mercury intrusion curves (Micromeritics Autopore IV 2500 porosimeter). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements were performed on selected sorbents to obtain information on morphology. We used electron microscopes Jeol JSM-6460 LV equipped with X-ray dispersive energy INCA X-sight and JEM-2010 equipped with X-ray microanalyzer EDAX (EDAX Co) with semiconductor detector. The samples for SEM measurements were prepared by

Figure 4. EDX data, electron diffraction patterns, and TEM images of Y2O3 (top panel), CaO (center panel), unidentified phase (bottom panel).

placing CaO sorbents on a double-sided carbon tape mounted on the sample holder. The samples used for TEM observations were 12742

dx.doi.org/10.1021/ie2015334 |Ind. Eng. Chem. Res. 2011, 50, 12741–12749


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Figure 5. SEM images of CaY-calc sorbent at different magnification.

Figure 6. Pore-size distribution for CaY-calc sample measured with low temperature nitrogen adsorption (solid line) and mercury intrusion (dashed line).

prepared by dispersing some products in absolute ethanol followed by ultrasonic vibration for 10 min, then placing a drop of dispersion onto a copper grid coated with a layer of amorphous carbon. Elemental analysis measurements were performed using a Perkin-Elmer model Optima 4300 inductively coupled plasma optical emission spectrometer. Sorbent Properties. The cyclic carbonation and calcination reactions were experimentally studied using thermogravimetric analyzer Netzsch STA 449 C. About 25 mg of the sample were put in a Pt crucible and heated at a ramp rate of 20 °C/min in Ar flow. The temperature program included two or more isothermal segments with intermediate sample heating (cooling) at 20 °C/min ramp rate. The dynamic capacity of the sorbents was measured at 740 °C and duration of 10 min for both recarbonation and regeneration steps. During the recarbonation step a mixture of CO2 and argon was fed to the sample chamber at atmospheric pressure and the total flow rate of 100 cm3/min using two mass flow controllers. During the decomposition step only argon was purged at the same flow rate. Figure 1 plots an example of the sorbent weight change during the isothermal segment of a TG run.

Figure 7. SEM EDX mapping of polished cross-section of a CaY-calc specimen. Red spots, yttrium; green spots, calcium. Size of white bar = 70 μm. The insert shows SEM EDX mapping of a crosssection of an individual particle (bar size = 600 μm).

Dynamic sorption capacity was determined as a difference between the sample weight at the end of the recarbonation step and preceding the regeneration step, normalized by the weight of the calcined sample. The weight change for empty crucible alone was also measured at the same conditions and taken into account during the capacity calculations. Before loading a new portion of the sorbent the crucible was soaked in 1 M HCl overnight, rinsed several times with distilled water and heated up to 1300 °C.

’ RESULTS AND DISCUSSION CaO Y2O3 interaction. Powder diffraction pattern of CaYcalc sample consists of CaO and Y2O3 reflexes without any unexplained reflexes which would be attributed to the mixed oxide phases (Figure 2). XRD data supports the assumption that the chemical interaction between CaO and Y2O3 at the temperatures up to 1300 °C is weak and does not result in formation of mixed oxide. FIZ/NIST Inorganic Crystal Structural Database (ICSD) also lacks the records on binary Ca Y oxides. According to the phase diagram calculated by Udalov et al. for the binary system CaO Y2O3 only the individual components exist as the phases below 2000 K.63 These calculations were 12743



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Figure 8. The SEM EDX mapping of a polished cross-section of CaY-imp-i specimens (i = 1 5). Red spots, yttrium; green spots, calcium. Numbers from 1 to 5 denote consecutive impregnations.

recently supported by Richter and Gobbels64 and Kaminaga et al.65 who did not observed binary Ca Y oxides for the samples calcinated at temperatures of 1500 and 1200 °C, respectively. On the other hand, Maeda et al.66 assumed the formation of CaY2O4 at the temperatures as low as 900 °C. Richter and Gobbels64 and Brown and Estell67 reported limited dissolution of the second component in the individual phases of CaO and Y2O3 of about 1 2% at 1400 °C. A calcined CaY-calc sample was further examined using TEM. Electron images show CaO crystals mixed with yttria support (Figure 3). Both Y2O3 and CaO domains are well-crystallized and produce characteristic electron diffraction patterns (Figure 4, top and center panels). According to EDX data, a CaY-calc sample has also domains where both Ca and Y are presented in substantial quantities. We could attribute the electron diffraction pattern of these crystallized domains neither to CaO nor to Y2O3 (Figure 4, bottom panel). Thus, we cannot completely rule out the formation of binary Ca Y oxide at our preparation conditions.

Sorbents appearance and porous structure. SEM images of CaY-calc sample shows highly irregular structure of the sorbent (Figure 5). Randomly oriented flat pieces with the thickness less than 1 μm make up a significant portion of the sample. In many places, flat pieces are sintered into dense particles. The surface of these particles is perforated with depressions of about 100 nm size. The mercury intrusion curve for a CaY-calc sample has the maximum at 7 μm (Figure 6) which is probably due to interparticle pores. The cumulative pore volume of pores less than 100 μm is 0.22 cm3 3 g 1. Low temperature nitrogen adsorption shows also number of pores of less than 100 nm with cumulative volume of 0.05 cm3 3 g 1. Some of these pores can be seen at SEM images at hollows perforating the surface of dense particles (Figure 5). The others are, probably, due to residual internal porosity of CaO particles after sintering at 1300 °C. 12744



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Figure 9. CaO fraction in the sorbent CaY-imp-i as a function of the number of consecutive impregnations.

Microstructure imaging of CaY-calc using SEM/EDX demonstrates large variations in Ca distribution throughout the specimen (Figure 7). Calcination produces agglomerates of CaO crystals of about 100 μm size (Figure 7, insert). It is of interest whether the yttria particles form an interconnected matrix which ensures sorbent resistance toward complete carbonation or hydration. SEM EDX mapping conducted at smaller magnification supports the idea that yttria provide an interconnected network, enhancing the sorbent strength. Simple experiments have been done in which the sorbent grains were put for several hours in water. After being removed from water and dried in air at ambient temperature, the sorbent grains were as strong as the original CaY-calc, confirming its resistance toward complete CaO hydratation. As compared to CaY-calc sorbent, the impregnated yttria sorbents have much smoother distribution of Ca throughout the grains (Figure 8). The amount of Ca monotonically increases with the increase in number of consecutive impregnations (Figure 9). Upon impregnations from 1 to 3, Ca occupies preferentially smaller pores of the support. After the forth impregnation, Ca starts to fill the larger pores of the yttria. After the fifth impregnation, according to the SEM EDX map, CaO plugs all the pores of the sample. SEM EDX data are in line with the changes in pore-size distribution, measured using mercury intrusion (Figure 10). The original yttria matrix has bimodal pore-size distribution with a narrow peak at 6 nm and a broad peak between 100 and 3000 nm. After the first impregnation the maximum position of the latter peak shifts to the higher pore diameters, probably due to surface sintering of yttria. Surprisingly, the shape and position of the pore-size distribution maximum at 6 nm remains unchanged during impregnations from 1 to 3. The fourth impregnation decreases the volume of these small pores twice. After the fifth impregnation smaller pores of the sorbent become inaccessible for mercury. On the contrary, the volume of large pores decreases during impregnations from 1 to 3, then remains constant. The specific surface of CaY-imp-i increases during the first and the second impregnations from 9.2 m2/g for Y2O3 matrix up to 12 m2/g for CaY-imp-2 sorbent, then linearly drops down to 1 m2/g for CaY-imp-5 (Figure 11). Taking into account the data of SEM EDX, the most plausible explanation of this drop is pore plugging with CaO. The accelerated sintering due to water

Figure 10. Pore-size distribution of CaY-imp-i calculated using mercury intrusion data.

Figure 11. Specific surface of CaY-imp-i as a function of impregnations number.

treatment during impregnation procedures may be also of importance. The fact that small pores remain unoccupied during impregnations from 1 to 3 makes us think that CaO crystals are too large to enter small pores. Instead these are located inside larger pores of more than 100 nm size. The specific surface increase during impregnations from 1 to 3 may be due to number of factors. The first is that scattered over the surface of yttria macropores, light CaO particles (3.35 g/cm3 for CaO against 5.01 g/cm3 for Y2O3) add to the specific surface of the sorbent particles without blocking the yttria surface. The intrinsic porosity of supported CaO particles may also increase the porosity of the sorbent. SEM images of yttria surface (CaY-imp-0 specimen) show oriented bunches of yttria rods (Figure 12). The rods are made of chains of small, nearly spherical particles of 200 300 nm size. The size of the particles, building up the yttria rods, coincides with the position of the maximum in pore-size distribution on the mercury intrusion curve, located between 100 and 200 nm. After five impregnations the initial texture of Y2O3 is completely lost. The sorbent CaY-imp-5 is composed of randomly packed sintered spherical particles of 500 1000 nm size (Figure 12). The ordered structure of original yttria matrix completely disappears. The increase in size of primary particles results in the simultaneous shift of the pore-size distribution maximum to higher pore diameters as compared to the pure Y2O3. 12745



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Figure 12. SEM images of CaY-imp-0 (left panel) and CaY-imp-5 (right panel) specimen.

Figure 13. Cycling performance of CaY-calc absorbent. Measurement conditions: temperature = 740 °C, 10 min duration of sorption and regeneration step, sorption in 25 kPa CO2 in Ar, regeneration in Ar. Between isothermal segments the sorbent was heated to 1350 °C and cooled down to measurement temperature at a ramp rate 20 °C/min.

CO2 Uptake and Stability. Recarbonation extent was measured at 740 °C for both recarbonation and decomposition steps. The isothermal segment was long enough to complete approximately 100 cycles. After that the sorbent was heated up to 1350 °C and cooled down to 740 °C at a ramp rate 20 °C/ min. The sorbent capacity after heating was again measured at 740 °C for approximately 100 cycles. The intermediate short excursion into the high temperature region models local overheating of the sorbent that could take place during the regeneration step of SER process. As prepared CaY—calc absorbent has the reversible CO2 uptake of 9.6 wt %, the value being stable for over 120 cycles (Figure 13). It corresponds to a CaO carbonation conversion of 61%. After a short absorbent exposure to a temperature of 1350 °C the uptake dropped down to 7.7 wt % and was remarkably stable for 190 cycles. This uptake value corresponds to 49% CaO carbonation conversion. Further calcinations had no effect either on the absorbent capacity or its stability in the cycles. It is unclear what has caused a noticeable drop after the first calcination. Indeed, during preparation the sorbent was subjected to 2 h of calcination at 1300 °C, thus short time calcination at a similar temperature would not have much effect on its capacity. On the other hand, it is known that recarbonation decomposition cycles enhance the rate of CaO sintering due to a large increase in

Figure 14. Cycling performance of CaY-imp-i absorbent. Measurement conditions were as in Figure 13. Solid lines, performance of as prepared sorbent; dashed lines, sorbent capacity after heating to 1350 °C and cooling down at a ramp rate of 20 °C/min.

the molar volume of the recarbonated phase.68 73 Probably, the combination of reaction and thermal sintering had a dramatic effect on the capacity of the CaY-calc absorbent. The effect of short time calcination was negligible for CaYimp-1 absorbent (Figure 14). Its capacity of 2.3 wt % and carbonation conversion of 61% were unchanged after calcination. The increase in CaO content in CaY-imp absorbents results in enhanced capacity drop after high temperature treatment (Table 1). For CaY-imp-5 absorbent with the highest CaO content of 14.3 wt % the CO2 capacity decreased from 7.8 to 6.9 wt % after calcination. Obviously, increased CaO content in the sorbent accelerates the rate of CaO sintering upon hightemperature treatment. The preparation method seems to have little effect on the sorption properties of CaY absorbents. On the contrary, the increase in CaO content produces a nearly linear increase in CO2 capacity. Absorbents CaY-calc and CaY-imp-5, having similar CaO content demonstrate, also, similar CO2 uptake and carbonation conversion (Table 1). Probably at concentrations up to 15 wt % CaO has little effect on the formation of Y2O3 matrix. As compared to alumina supported or alumina promoted sorbents,46,48 50,52,74,75 CaY has much lower CO2 sorption uptake (7 10 wt % against more than 40 wt %) under comparable cycling conditions. However, the novel sorbent is extremely stable upon overheating up to 1350 °C due to weak interaction of 12746



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Table 1. CaO Content and Sorption Properties of CaY Absorbents. The Measurement Conditions Were as in Figure 13 absorbent

capacity as

capacity after

carbonation conversion

carbonation conversion

CaO content, wt %

prepared, wt %

calcination, wt %

as prepared, %

after calcination, %

CaY-calc

19.9

9.6

7.7

61

49

CaY-imp-1

4.8

2.3

2.3

61

61

CaY-imp-2

6.3

3.8

3.6

77

73

CaY-imp-3

8.7

4.7

4.5

69

66

CaY-imp-4

12.2

6.5

6.1

68

64

CaY-imp-5

14.3

7.8

6.9

69

61

CaO with Y2O3, while alumina promoted sorbents lost capacity at regeneration temperature above 1000 °C due to both accelerated sintering and formation of inactive Ca3Al2O6 phase.52,74 The temperature stability of novel sorbent is similar to the stability of pure CaO, which retains its capacity after prolonged sintering at 1300 °C27 and exceeds the thermal stability of silica or alumina promoted sorbents. Probably, the high purity of the reagents used in this study for the synthesis of the sorbent provides a low content of impurities and, as a result, enhanced thermal stability. In contrast to dolomite-derived20,36,37 or La2O353,54 -based absorbents, the novel material demonstrates superior mechanical strength after the contacts with moisture or during long-term storage in air. We observed that consecutive impregnations of the yttria matrix with calcium nitrate water solutions enhanced considerably the strength of CaY-imp absorbent particles. The remarkable stability of CaY absorbent makes it a promising material for high-temperature SER applications.

’ CONCLUSIONS Several samples of CaO/Y2O3 were synthesized using two different routes. Both thermal decomposition of mixed precursor salts and CaO impregnation in Y2O3 support produce the sorbents with remarkably stable CO2 capacity in multiple sorption regeneration cycles. The absolute capacity is of about 7 wt %, that is lower than the capacity of other CaO based supported sorbents, measured under similar conditions. On the other hand, the novel material demonstrates the superior resistance to high temperature treatment of 1350 °C. Yttria matrix remains mechanically stable after 5 consecutive impregnations with aqueous solution. All these properties make CaO/Y2O3 a promising material for high temperature CO2 removal in the sorptionenhanced reforming process (SERP). ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The work was financially supported by the Young Scientist Grant program of BIC and by the Russian Federation President Grant for the Leading Scientific Schools no. NSh 3156.2010.3. The authors also thank Dr. A. N. Salanov and Dr. E. A. Suprun for SEM and SEM EDX experiments and Prof. S. V. Tsybulya for the XRD study. ’ REFERENCES (1) Hufton, J. R.; Mayorga, S.; Sircar, S. Sorption-enhanced reaction process for hydrogen production. AIChE J. 1999, 45, 248–256.

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