Energy & Fuels 1990,4,467-474
467
Nature and Structure of Calcium Dispersed on Carbon A. Linares-Solano,* C. Salinas-Martinez de Lecea, and D. Cazorla-Amorbs Departamento de Quimica Znorgiinica e Ingenierza Qulmica, Uniuersidad de Alicante, Alicante, Spain
J. P . Joly Laboratoire de Catalyse Organique, ESCIL, Universitg Claude Bernard de Lyon, Lyon, France
H. Charcosset Institut de Recherches sur la Catalyse du CNRS, Villeurbane, France Received March 26, 1990. Revised Manuscript Received June 18, 1990
A pure phenol-formaldehyde polymer carbon has been oxidized by HN03, air,and H202,to introduce different amounts and types of oxygen surface groups. The HN03-oxidized carbon has been loaded with different calcium contents by both the ion-exchange and impregnation processes from calcium acetate solution. A maximum value of 3.7% calcium content is found to be ion exchanged. A thermal programmed desorption, mass spectrometry technique (TPD-MS) has been used to investigate the initial state of the calcium species after ion exchange or impregnation. Information about the nature of the calcium species and its distribution in the carbon matrix, as a function of the calcium loading, has been obtained by comparing the TPD profiles of the raw and oxidized carbons with those obtained for the calcium-carbon samples. Two different hypothetical structures for the ion-exchanged calcium have been proposed. The structure depends on the calcium to carbon carboxylic group ratio. For low calcium loading, the ion-exchanged calcium completes its coordination sphere with H20 and C 0 2 molecules. For high calcium loadings its coordination becomes similar to that of calcium acetate because each Ca2+ion acts as a nucleation site where a crystal of acetate grows. The TPD interpretation is consistent with reactivity measurements obtained as a function of calcium content.
Introduction The catalytic activity of calcium in carbon-gas reactions depends, among other factors, on its concentration and dispersion into the carbon matrix.' The porous texture and surface chemistry of the carbon may control dispersion and hence catalytic activity. R e ~ e n t l y , isothermal ~-~ C 0 2 and steam reactivities on pure polymer carbons have been studied as a function of their calcium contents. Calcium was loaded by both ionexchange and impregnation methods, from calcium acetate solution. Reactivities in both atmospheres increase linearly with calcium content up to a loading saturation level (LSL) of about 4 w t % , above which the reactivities are constant. For comparable calcium loadings, the catalyst addition method (ion exchange or impregnation) did not produce any significant difference. The different catalytic behavior of calcium above and below the LSL was interpreted as being due to the catalyst dispersion conditioned by the carboxylic groups of the carbon. In any case, only the ion-exchanged calcium has catalytic activity, whereas the excess calcium has low dispersion and therefore has no additional catalytic activity. From the above result^,^-^ it is clear that the surface chemistry of the carbon plays an important role in de(1) Linares-Solano,A,; Hippo, E. D.; Walker, P. L., Jr. Fuel 1986,65,
.." ..".
77G-779
(2) Salinas-Martinez de Lecea, C.; Linares-Solano, A.; hela-Alarcbn, M.; Charcosset, H.; Joly, J. P.; Marcilio, N. R. Proceedings: Carbon 88, Newcastle upon Tyne, UK, 1988; McEnaney, B.,Mays, T. J., Eds.; IOP Publishing: Bristol, UK, 1988; pp 392-395. (3)Linares-Solano, A.;Salinas-Martinez de Lecea, C.;Almela-Alarcbn, M. 1987lnternational Conference on Coal Science;Moulijn, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier: Amsterdam, 1987; pp 559-562. (4)Salinas-Martinez de Lecea, C.; h e l a - A l a r d n , M.; Linares-Solano, A. Fuel 1990,69, 21-27.
0887-0624/90/2504-0467$02.50/0
termining the calcium-carbon contact during sample preparation as well as the initial state of the catalyst. In spite of the importance of the initial state of the catalyst, a survey of the literature indicates that very little work has been conducted to assess it. Therefore, considerable uncertainties still exist related to the nature and the structural forms of the catalyst after the sample preparation stage. An exception to this lack of research is the work done by Huggins et al."' who analyzed the state of alkali-metal and alkaline-earth species in coals and chars as well as after pyrolysis treatment. This paper analyzes the thermal behavior of carboncontaining calcium in order to investigate the nature and structure of the calcium species dispersed on the carbon matrix after ion exchange or impregnation. Calcium distribution and calcium-carbon interaction have also been analyzed to interpret the above LSL. This study is carried out with TPD experiments (MS) in the same calcium-carbon samples used in previous
work^.^-^ Experimental Section Sample Preparation. A pure polymer carbon (A) with minimal ash content, high surface mea, and well-developed porous network has been used for the investigation. The carbon was prepared by carbonization of phenol-formaldehyde polymer resin in N2at 5 K/min to 1273 K with 1 h soak time. In order to introduce different amounta of superficial oxygen groups, carbon (5) Huffman, G. P.; Huggins, F. E.; Shoenberger, R. W.; Walker, J. S.; Lytle, F. W.; Greegor, R. B. Fuel 1986, 65, 621432. (6) Huggins, F.E.; Huffman, G. P.; Shah, N.; Jenkins, R. G.; Lytle, F. W.; Greegor, R.B. Fuel 1988,67, 938-941. (7) Huggins, F. E.; Shah, N.; Huffman, G. P.; Lytle, F. W.; Greegor, R. B.;Jenkins, R. G . Fuel 1988, 67, 1662-1667.
0 1990 American Chemical Society
468 Energy & Fuels, Vol. 4 , No. 5, 1990
Linares-Solano et al.
. t 6
lbVOV0
a
6
I
Figure 1. TPD-MS spectrum of raw polymer carbon (A) (-,
co; - - -, COJ.
A was treated with three oxidizing agents: (i) air flow at 573 K (2 h), carbon Al; (ii) 15 N HN03 solution at 353 K to dryness, carbon A2; and (iii) 2 N H202 at 298 K (48 h), carbon A3.
Calcium was ion exchanged from a calcium acetate solution
(1.5 M, 4 h). The Ca-carbon sample was washed until water was free of Ca2+ions. Carbon A2 has also been loaded by impregnation with a wide range of calcium content, from 1.5 to 9.4 wt %, using an appropriatecalcium acetate solution. The impregnatedsamples were not washed. All Ca samples were dried at 383 K under
vacuum. In the nomenclature I1 stands for ion exchanged and I for impregnation, and the calcium loading (wt %) is also included. The details of carbon preparation and oxidation,calcium loading, and COP and steam reactivity have been described el~ewhere.~ Experimental Procedure. About 200 mg of sample was heated to 1225 K under He flow (60 mL/min) with a heating rate of 20 K/min. A large portion of the evolved gas was evacuated with a rotary pump, and a small amount passed the leak valve. The gases (COz,CO, HzO) and the mass 43 were analyzed with a quadrupole mass spectrometer. TPD results are expressed as percent volume per unit weight of carbon sample. The details on the mass spectrometer, data acquisition, and treatment are described elsewhere."1° Under the experimental conditions used (heating rate of 20 K/min), it can be assumed that secondary reactions between evolved COz and carbon-active sites (C(f)) and between evolved CO and carbon-occupied sites (C(0)) are small." In this regard, negligible differences were found in the amount of COzand CO evolved,when a heating rate of 5 K/min instead of 20 K/min was used.
Results and Discussion TPD Spectra of Carbon Samples. TPD spectra for raw and oxidized carbon samples were analyzed and quantified to assess how the calcium is bonded to the carbon surface after ion exchange or impregnation. (8)Bianchi, D.; Joly, J. P. Bull. SOC.Chim. Fr. 1985, 4, 664-667. (9) Bianchi, D.; Joly, J. P. Bull. SOC.Chim. Fr. 1985, 4, 668-671. (10)Joly, J. P.; Cazorla-Amorbs,D.; Charcosset, H.;Linares-Solano, A.; Marcilio, N. R.; Martinez-Alonso, A.; Salinas-Martinez de Lecea, C. Fuel 1990,69,87&844. ( 1 1 ) Hall, P. J.; Calo, J. M. ; Lilly, W. D. Proceedings: Carbon 88, Newcastle upon Tyne, UK, 1988; McEnaney, B., Mays, T . J., Eds.; IOP Publishing: Bristol, UK, 1988; pp 77-79.
---..-. --- ---..__
l W , ; 0
340 M10 700 so0 troo 1Figure 2. TPD-MS spectra of polymer carbon treated with air (Al): (a) fresh sample; (b) aged sample (-, CO; - - -,COP).
Figures 1-4 show the TPD spectra obtained for the raw and oxidized polymer carbons that are typical of those found for carbonaceous materials treated with similar oxidizing agents.12 Different oxidizing treatments give rise to important differences in the TPD profiles in both the amount of CO and C 0 2 evolved, as well as in their shapes (mainly for the COOpeak shape). In this way, HN03 (Figure 3) and H202(Figure 4) produce a large amount of carboxylic groups responsible for the COPevolved at low temperature (500 K). In contrast, C 0 2 evolution of the air-treated sample (Figure 2a) occurs a t a much higher temperature (850 K) than it does with samples oxidized with HN03 or H 2 0 2 . Because of the temperature used in the air oxidation, anhydride instead of carboxylic groups are formed.13J4 The small contribution of the lower tem(12) Otake, Y.; Jenkins, R. G.Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1987, 32, 310.
Energy & Fuels, Vol. 4, No. 5, 1990 469
Nature and Structure of Calcium Dispersed on Carbon ‘VOl
500
Table I. Desorbed C 0 2 from Superficial Complexes and Ion-Exchanged Ca by Raw and Oxidized Carbon Samples
600
700
#K)
no01640
TW
Figure 3. TPD-MS spectrum of polymer carbon treated with HNOB (A2) H2O; -, CO; ---,C02). (*.e,
%W/O
sample
CO,, Mmol/g
Ca, wt %
CO,/Ca
A A1 A2 A3
180 330 1810 310
0.40 1.60 3.65 0.60
1.8 0.8 2.0 2.0
implies that in the aged sample an important part of the anhydride groups has indeed been hydrolyzed to carboxylic acid. The quantification of desorbed C 0 2 evolved from superficial complexes is compiled in Table I together with the amounts of ion-exchanged calcium on the carbons. From the results of Table I, it is evident that the ion-exchanged calcium is a function of the chemistry of the carbon surface; the oxidizing effect increases in the order H202C air C HN03, as is also found for the ion-exchange capacity of the resulting carbons. Except for Al, a C02/Ca ratio close to 2 is observed, which indicates that two H+ from carboxylic groups are ion exchanged by one calcium ion. For the air-treated sample, the C02/Ca ratio shown in Table I can be recalculated considering that each anhydride group of the fresh sample, upon heat treatment during the TPD experiment, will decompose to one C02 and one CO molecule, whereas the ion exchange process, in aqueous solution, will lead to two carboxylic groups. This reasoning is in agreement with TPD differences observed for the fresh sample with respect to the aged one. 1 anhydride
I
-
1 anhydride
4
100
700
800
1100
1900
T(K)
Figure 4. TPD-MS spectrum of polymer carbon treated with H202 (A3) (-, CO; - - -,C02). perature C02 peak (related to carboxylic groups) may be due to a partial hydration of anhydride during sample preparation by water vapor of the atmosphere. To test this possibility, the same oxidized sample was analyzed a long time after being oxidized. The TPD of this “aged sample” is given in Figure 2b. Comparing its TPD profile with that of the fresh sample (Figure 2a), it was observed that besides the C02 peak at high temperature there was also a significant C02 peak at low temperature. This low-temperature peak, which is not present in the fresh sample, (13) Zawadzki, J. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1988; Vol. 21, pp 147-380. (14) Calemma, V.; Rausa, R.; Margarit, R.; Girardi, E. Fuel 1988,67, 764-770.
TPD
water
1C0,
+ 1CO
2(COOH)
This result means that for each COPevolved in TPD from Al, there will be two carboxylic groups which, in aqueous calcium acetate solution, will be ion exchanged by calcium. According to this a C02/Ca ratio close to 2 is also obtained for sample A1 (1.65). These results show that in our carbons regardless of the oxidizing agent used to increase the amount of superficial oxygen, two H+ ions from carboxylic groups are ion exchanged by one calcium. These findings are in agreement with the existence of carboxylic-bound calcium observed in low-rank coal using XAFS s p e c t r ~ s c o p y and ~ ~ Jwith ~ a larger number of investigators who have obtained a similar 2H+/Ca2+ratio, e.g., Schafer.17 However, this ratio should not be generalized to all the carbons and therefore need to be tested before using. TPD Spectra for Carbon A2 with Different Calcium Loadings: Description. TPD spectra have been obtained for carbon A2 with different weight percent calcium. Figures 5-10 show TPD profiles for samples with the weight percent calcium varying from 1.5 to 9.4. Two important points should be noted: First, the TPD spectrum of carbon A2 without calcium (Figure 3) differs significantly from those obtained for this carbon when calcium is present (Figures 5-10), regardless of the calcium loadings. Second, interesting differences are observed in comparing TPD spectra of samples having calcium contents less and greater than the LSL. The presence of calcium makes the TPD profiles much more complex but provides the means to investigate how (15) Huffman, G. P.; Huggins, F. E. Chemistry of Low-Rank Coals; Schobert, H. H., Ed.; ACS Symposium Series 264; American Chemical Society: Washington, DC, 1984; pp 159-174. (16) Huggins, F. E.;Huffman, G. P.; Lytle, F. W.; Greegor, R. B. Proceedings-lnternational Conference on Coal Science; Center for Conference Management: Pittsburgh, PA, 1983; pp 679-682. (17) Schafer, H. N. Fuel 1970, 49, 197-213.
Linares-Solano et al.
470 Energy & Fuels, Vol. 4, No. 5, 1990
30m 26
-
20-
16
-
600
700
800
t(oouoo
boo
700
m) Figure 5. TPD-MS spectrum of sample A2-1-1.5
co; - - -, COZ). 30
900
1 m
1800
m 4. There is a noticeable increase in the CO peak with the calcium content, and its maximum appears at the same temperature as the 1000 K COPpeak. This additional CO peak will be
Figure 11. Calcium acetate decomposition followed by (a) T G DTA gnd (b) TPD-MS.
referred to as “extra CO”. These three peaks (mass 43, high-temperature COP,and extra CO) occur together; they are not present in samples with weight percent calcium < LSL and they increase in a similar way with the calcium content. TPD Spectra Interpretation for Ca > 4 wt %. Thermal decomposition of calcium acetate permits interpretation of the TPD spectra of calcium-carbon samples with calcium contents higher than 4 wt %, where it has been established that their CO and COz profiles differ significantly from those found for lower calcium loadings. The decomposition of Ca(CH3C00)2has been studied by TG-DTA under Nz atmosphere and by TPD-MS (under the conditions described in the Experimental Section). The results are shown in Figure l l a and Figure l l b , respectively. As the temperature increases, thermal
Linares-Solano et al.
472 Energy & Fuels, Val. 4 , No. 5, 1990
Table 11. "Calcium in Excess" and Ion-Exchanged Calcium Quantified from Peak Mass 43 and CaCOSDecomposition Ca in excess, wt%
samde A2-1-4.3 A2-1-7.3 A2-1-9.4
M43 0.56 3.45 6.04
CaCO, 0.59 3.41 6.57
ion-exchanged Ca, wt % M43 CaCOl 3.70 3.70 3.80 3.84 3.33 2.80
decomposition of this compound shows the following sequence:
- +-
1
Ca(CH3C00)2~xHz0 Ca(CH3C00)2
2
3
CaC03 CaO C 0 2
The TPD-MS data complement the TG-DTA results by showing that at =700 K (step 2) a mass 43 peak is found in the Ca(CH3C00)2decomposition; rupture of evolved CH3COCH3obviously gives rise to this peak (also mass 15 should appear, which is not shown in Figure 11). The COP peak a t high temperature comes from CaC03 decomposition. From these results, the TPD spectra of carbon with calcium loadings > 4 wt % can be interpreted by means of the presence of calcium acetate species; the amount of calcium acetate present in the carbon increases with calcium loading above the LSL. These findings are in agreement with earlier XRD and TG studies conducted on the same ~ a m p l e s . The ~ * ~presence of calcium acetate was observed (not quantified) in samples with calcium contents higher than 4 wt %; samples with weight percent calcium < 4 wt '70 had no diffraction peaks of calcium acetate. Therefore, the presence in these high calcium content samples of a peak a t mass 43, C02, and CO is interpreted as follows: (i) the peak at mass 43 corresponds to a species in the decomposition of Ca(CH3C00)2,(ii) the C02peak at high temperature comes from CaC03decomposition, and (iii) the CO peak comes from C 0 2 carbon gasification proceeding from C032-decomposition. Quantification of the mass 43 peak and of C02 and "extra" CO from Figure 11 permits the calculation of the amount of calcium acetate present in these samples; hereafter, this calcium will be referred as "calcium in excess". Knowing the calcium acetate present and the weight percent calcium content of each sample, the amount of ion-exchanged calcium by the carboxylic groups of the oxidized carbon can be determined. Table I1 compiles the "excess calcium" as well the ionexchanged calcium (by difference from the total calcium content) calculated from the mass 43 peak and from COz evolution. Two important features merit further comment. First, quantification data obtained from both mass 43 peak and COPgive very similar results. Second, the amount of ion-exchanged calcium for samples with calcium loading larger than the LSL is almost constant with a mean value of 3.5 wt %. This value is very close to the experimental ion-exchange capacity of carbon A24 and to its theoretical maximum ion-exchange capacity (deduced from desorbed COPby assuming that its 1800 Fmol/g of C02come from carboxylic groups that exchange two H+ by one Ca2+). Note that TPD of sample with a 3.7 wt '70 calcium does sample A2 A2-11-2.9 A2-1-3.6 A2-1-7.3 A2-1-9.4
not give peak mass 43 (see Figure 7), in agreement with the fact that the maximum value of ion-exchanged calcium is 3.5 wt %. The constancy found in the ion-exchanged calcium by both methods of preparation (ion exchange or impregnation) is indicative that the ion-exchange process is fast. These results confirm a previous s t u d 9 where the calcium ion exchange capacity of carbon A2 was investigated with 1.5 M aqueous calcium acetate solution for different contact times. It was shown that 80% of its Ca2+exchange capacity is reached in only 10 min of contact time. By increasing the pH of the calcium acetate solution to 10, the ion-exchange capacity of this carbon reaches a value of 4 wt %, which closely corresponds to the LSL. TPD Spectra Interpretation for Ca < 4 wt %. When carbon A2 is put in contact with an aqueous solution of calcium acetate, either by an impregnation or an ion-exchange process, a rapid exchange of Ca2+ions takes place with available carboxylic groups on the carbon. If all the carboxylic groups are ion exchanged as would happen with a 3.7 w t % calcium content, an atomic distribution of calcium is expected after sample preparation. Of course, the coordination sphere of each exchanged calcium will be completed with water molecules. As will be described and quantified shortly, during the drying process, which was performed at 383 K and at atmospheric pressure, minor changes in the calcium coordination sphere occur and C02 molecules from the atmosphere can interact with the ion-exchanged calcium and partially replace water molecules from the coordination sphere. Figures 6 and 7 show two well-defined COP peaks of similar intensities at ~ 5 7 and 3 ~ 6 7 K. 3 The higher C02 peak should be related to the decomposition of the more stable calcium carboxylate. The lower COz peak may therefore be ascribed to COz uptake by the atomically distributed calcium during the drying process. In agreement with this model is the fact that increasing the weight percent calcium above the LSL gives rise to a continuous decrease in the first COPpeak. This is because the presence of calcium acetate will change the coordination sphere of the exchanged Ca2+ion and fewer C 0 2 molecules will reach it; for high calcium content this coordinated C 0 2 disappears. If the ion-exchange capacity of the carbon is not exceeded, e.g., with the 1.5 wt % calcium sample, in addition to calcium carboxylate and the coordinated C02there will also be free carboxylic groups. Hence, thermal decomposition will lead to three different COz peaks as is shown in Figure 5. TPD Spectra Quantification. The quantification of all TPD spectra is compiled in Table III. The table shows the C02/Ca, HPO/Ca, and (C02+ H20)/Ca ratios calculated as follows: (i) the amounts of H 2 0and C02from A2 have been subtracted, so only the amount of COPcorresponding to the first peak and the H20associated with the presence of calcium have been considered (it must be pointed out that the amount of COz of carbon A2 has been associated with its carboxylic groups and so they are now as calcium carboxylate); (ii) the calcium used in these ratios is the ion-exchanged calcium calculated from data for the
Table 111. Quantification of TPD Spectra from Samples with Different Calcium Contents" coz co M43 H,O H,O/Ca CO,/Ca (COP+ H,O)/Ca 1810 2830 2710 2020 2530
2330 2640 2070 2940 4830
930 1660
830 2930 3500 1320 2290
All these amounts are expressed in pmol/g of carbon sample, catalyst free.
2.8 2.8 0.8 1.6
1.4 1.0 =O
0.1
4.2 3.8 0.8 1.7
Nature and Structure of Calcium Dispersed on Carbon
(3)
Figure 12. Hypothetical structures for ion-exchanged calcium: (1) in solution; (2) and (3) after drying process.
mass 43 peak, as shown in Table 11. The ratios of Table I11 therefore refer only to the ion-exchanged calcium, to determine its coordination in the carbon surface as a function of the weight percent calcium content of the sample. Two important trends are observed in the data shown in Table 111. (1)For calcium contents lower than 4 wt %, the ratio (H20+ C02)/Cais nearly 4 and the C02/Ca ratio is between 1 and 2. (2) For calcium contents larger than the LSL, the (H20+ CO,)/Ca ratio is decreased and the C02/Ca ratio goes to 0. Hypothetical Structure of Ion-Exchanged Calcium. The above results indicate that the calcium distribution, as well as its structure and coordination, should be very different above and below the LSL. Below it, the amount of ion-exchanged calcium is controlled by the available carboxylic groups and therefore an atomic distribution of calcium will result. Above the LSL, in addition to the ion-exchanged calcium (which is constant and near 3.5 wt % ) there will be calcium acetate. 1. Proposed Structure for Calcium Loading < 4 wt 70. Figure 12 shows the hypothetical structures of calcium resulting from an ion-exchangeprocess deduced from TPD measurements. Each calcium ion should have a total coordination number of 6 in agreement with previous findings of Huggins et a1.16 As discussed above, each calcium has exchanged two H+ from the two nearest carboxylic groups; the other four coordination sites should be completed either by HzO or by C 0 2 molecules in agreement with the results of Table I1 where a (HzO + COZ)/Caratio of almost 4 is found. During catalyst preparation in aqueous calcium acetate solution, structure 1 is the most probable. During drying treatment of the calcium loaded carbon samples, H 2 0 molecules might be replaced by C02 molecules leading to structures 2 and 3. It is well-known that the C02molecule is a poor ligand; however, it can exist as a ligand if appropriate stabilization interactions are possible. The insertion of a CO, molecule into a o metal-carbon bond is known to occur.18 For example, C02 has been inserted by a 1,2-addition mechanism into a titanium organometallic complex; the resulting product has a distorted C02bonded to a C (of a phenyl group) and to the Ti atom. In our case, the C atom from the C 0 2 molecule may interact with the 0 of the carbonyl group (18) Lukerhart, C. M. Fundamental Transition Metal Organometallic Chemistry; Ceoffroy, C. L., Ed.; Brooks/Cole Series in Inorganic Chemistry: Brooks/Cole: Monterrey, CA, 1985: pp 234-245.
Energy & Fuels, Vol. 4, No. 5, 1990 473
of the carboxylate complex; the other oxygen (anion) of the carboxylic group is bonded to Ca2+. Figure 12 (2 and 3) shows possible stabilization interactions of C 0 2 molecules through the carboxylic group of the calcium carboxylate species. Under these conditions, the maximum number of C 0 2 molecules that can be coordinated to calcium cannot be higher than two, in agreement with the data in Table I1 (see C02/Ca ratio values). Furthermore, the fact that the (H20+ COz)/Ca ratio for samples with calcium content < 4 wt % is almost 4 confirms the above hypothetical structures and the idea that each calcium atom ion exchanged throughout the carbon matrix is, after sample preparation, hexacoordinated. This expected coordination has been used before in the coal science field.16 Among others, Spiro and K o ~ k y ,in ' ~ their proposed low rank coal structure model, incorporated two ion-ex. changeable carboxylate groups coordinated to an aquated metal ion (transition metal as well as alkaline earth because all show a strong affinity for carboxylic groups). The authors do not add further comments in relation to this point. 2. Proposed Structure for Calcium Content > 4 wt 5%. The proposed coordination model for samples with calcium loading > 4 w t 9o has to be different. On the one hand, the data in Table I1 indicate that the amount of ion-exchanged calcium is almost constant and close to the limiting value of 3.5 wt 70,so the excess of loaded calcium has to be distributed around this ion-exchanged calcium. On the other hand, the data in Table 111 show, for these high calcium content samples, that the coordination of their Ca2+ions with C 0 2 and H 2 0 decreases noticeably, making the (H20+ C02)/Ca ratio much less than 4. The fact that the CO,/Ca ratio is near 0 and that the H20/Ca ratio varies between 1 and 2 confirms that Ca2+ions have different structures above and below the LSL. It is thought that once all the H+ from the carboxylic groups has been ion exchanged by Ca2+ions, the calcium in excess will interact with the ion-exchanged calcium sites which act as centers of nucleation for the growth of calcium acetate crystals. Therefore, the coordination sphere of Ca2+is not like that depicted in Figure 12 and neither H 2 0 nor COz molecules will be coordinated. The ion-exchanged calcium will have now a coordination closer to that corresponding to calcium acetate. This could explain why the CO, and H 2 0 coordinations decrease. The fact that HzO/Ca ratio is not 0 and has a value between 1 and 2 indicates that the growth of the calcium acetate crystal occurs around the ion-exchanged calcium with xH20 molecules of Recall that TPD of samples with high calcium contents (A2-1-7.3 and A2-1-9.3, Figures 9 and 10, respectively) show a new H20 peak at 473 K. TG decomposition of hydrated calcium acetate (Figure 1la) also shows that at -473 K dehydration of calcium acetate takes place. In summary, a high level of calcium loading by an impregnation process involves two processes: (i) an ion-exchange process in which two H+ from carboxylic groups are ion exchanged by one Ca2+and (ii) a process in which the calcium in excess interacts with the calcium already exchanged and growth of calcium acetate takes place. At first glance, these results could be related to and would complement the observation of Huffman et al.,I5 who, using XAFS spectroscopy, have observed a systematic change from carboxylic-bound calcium to calcite in coal with increasing rank. In our case, due to the acetate solution used, (19) Spiro, C. L.; Kosky, P. G. Fuel 1982,61, 108C-1084. (20) Handbook of Chemistry and Physics, 59th 4.;CRC Press: Boca Raton, FL, 1978; p B104.
474 Energy & Fuels, Vol. 4, No. 5, 1990
calcium acetate is found when no more carboxylic groups are available. LSL Interpretation. If we assume that our interpretation about the calcium distribution through the carbon matrix (asa function of the calcium loading) is correct, two different reactivities should be found above and below the LSL. As mentioned in the Introduction, two different catalytic activities of calcium have recently been found below and above the LSL.'p4 Below the LSL where calcium is present only as ion-exchanged calcium, the higher the calcium loading (wt ?% < 4) the higher is the reactivity and hence its catalytic activity. Conversely, above the LSL there is a constant maximum of ion-exchanged calcium (3.5 wt %) with extra calcium as calcium acetate. As each ion-exchanged calcium acts as a nucleation center for calcium acetate, its contact with the carbon matrix remains constant and independent of the calcium loading, and hence, the reactivity will be also constant. This is exactly what was ~bserved.~,~ Isothermal COz (0.1 MPa) and steam (19.7 kPa) reactivities as a function of calcium loadings were investigated. Reactivities in both atmospheres increased linearly with calcium content up to a LSL of about 4 wt %. In those earlier papers, the different catalytic behavior of calcium above and below the LSL was interpreted as due to the catalyst dispersion being limited by the availability of carboxylic groups and it was suggested that only the ion-exchanged calcium is active, whereas the excess calcium has no additional catalytic activity. TPD measurements presented in the present paper confirm these results and allows the statement that the excess
Linares-Solano et al. calcium has no additional catalytic activity because it does not increase the contact between calcium and the carbon. Only the ion-exchanged calcium is effective as a catalyst. Conclusions Quantification of COPevolved from TPD of the raw and oxidized carbons, and of the amounts of ion-exchanged calcium (from a 1.5 M calcium acetate solution), leads to the conclusion that all carboxylic groups are ion exchanged by Ca2+ions with a 2/ 1 stoichiometry. For calcium contents lower than the LSL, the ion-exchanged calcium has its coordination sphere completed with HzO and COz molecules with a coordination number of six, indicative of having an atomic distribution through the carbon matrix. High levels of calcium loading prepared by the impregnation method involve two processes: a fast ion-exchange process to a constant value of 3.5 wt % and nucleation of calcium acetate about the exchanged Ca2+ions. Therefore, for high levels of calcium loading, the Ca2+ion has a coordination sphere more similar to that of calcium acetate. Interpretations of TPD data given in this paper are consistent with reactivity measurements obtained previously as a function of calcium loading. Acknowledgment. We thank the Spanish-French Joint Grant (No. 89.1421, DGICYT (Project No. PB 860286 and No. PB 88-02951, and MEC for the thesis grant of D.C.A. Registry No. Ca, 7440-70-2; carbon, 7440-44-0.
