Chapter 2 LDHs based Adsorbent for Carbondioxide

Transcription

1 Chapter - 2

2 The removal and recovery of CO 2 from hot gas stream is becoming increasingly significant in the field of eco-friendly energy production (2-29). The combustion of fossil fuels such as coal or natural gas releases large volumes of CO 2 to the environment resulting into pollution problem ( ). The studies carried out for CO 2 adsorption on LDHs suggest that these materials have the attributes of a good adsorbent such as high adsorption selectivity and capacity, adequate adsorption/desorption kinetics at operating conditions and mechanical strength of adsorbent particles after cyclic exposure to high-pressure streams (22-225, 9). The adsorption capacity of LDH materials is significantly influenced by their structural, textural and thermal behavior, which is further determined by the synthetic parameters. In general, higher x (x=m(iii)/m(ii)+m(iii)) values result into higher adsorption capacity (226). However, an optimum amount of Al content is found necessary for maximum adsorption. For example, by decreasing the Al content in Mg-Al-LDH from 7% to 5%, CO 2 adsorption capacity increased from.36 mmol/g to.2 mmol/g at 3 C temperature and 1 bar pressure, however, further decrease in Al content to 3% resulted into decrease of the CO 2 adsorption capacity to.26 mmol/g (29). The amount of CO 2 adsorption is also affected by the interlayer spacing, the size and charge density of the intercalated anion. The interlayer spacing can be changed depending on the size of the intercalated anions. The incorporation of large anion with high charge density depends upon the Mg-Al ratio. The interlayer anions can be easily exchanged and a wide variety of inorganic and organic anions can be incorporated in the interlayer (32-33). The surface area of the materials or the adsorption capacity for hydrocarbons or carbon dioxide can be greatly modified by the intercalation of large anions, such as Fe(CN) 3-6 or Fe(CN) - 6 (231). The layer charge and the void space will have to be taken into account for interpretation of maximum adsorption (226). 5

3 The sorption process is affected by the composition of the LDHs, which determines the layer charge, the nature of the anion present in the interlayer and the nature of the metal ions in the brucite layer. These characteristics determine the accessibility of the anionic interlayer positions and hence should influence the adsorption of the anionic contaminants in these sorbents (232). Studies in literature showed that CO 2 adsorption capacity of all LDHs is higher at elevated temperature (235). Furthermore, for wet feed conditions the CO 2 adsorption capacity is reported to be approximately 1 % higher than that under dry feed conditions at same adsorption temperature. Measurement also suggests rapid and irreversible chemisorptions on freshly packed adsorbent followed by reversible and relatively weak adsorption perhaps on the chemisorbed material. Depending upon the conditions of pretreatment of the adsorbent some variation in adsorption parameters is expected (233,23). The adsorption saturation capacity of LDH is insensitive or even is promoted in presence of water while water vapor is known to compete with CO 2 in adsorption behavior onto zeolites. So LDH has advantages in capturing CO 2 in large range of temperatures (225). LDH has been also used as coating material on the commercial zeolites to improve its adsorption quality (236). Several studies have been reported on the synthesis of Mg-Al-LDH and other LDHs using different methods such as conventional precipitation, sol-gel and urea hydrolysis (15, ) and thermal decomposition (9-99,,235,33). CO 2 adsorption studies on LDHs have been reported (17-1,29,227-22,233,239) in the literature. Above references shows that intercalated anions affect the adsorption capacity significantly and there are several reports on LDH with varied intercalated anions but not yet with acetate anion. Breakthrough CO 2 adsorption on Mg-Al LDH with carbonate anion are sparse (2,2-22). LDH with different anions have not been studied yet for breakthrough of CO 2 adsorption with the larger 51

4 range of adsorption as well as pretreatment temperatures. Scope of present work is to fill these gapes. The present chapter is focused on the synthesis of LDHs samples by co-precipitation method and their applications for CO 2 adsorption. A series of Mg-Al LDH samples were synthesized under varied synthesis conditions with carbonate as intercalated anion. The Mg-Al LDH sample with varied intercalated anion such as Fe(CN) - 6, CH 3 COO -, EDTA and without anion were also synthesized. The CO 2 adsorption and desorption measurements were carried out on all synthesized LDH samples using volumetric adsorption set-up. The effect of activation temperatures (25 C and 35 C) on CO 2 adsorption isotherms was also studied. The breakthrough CO 2 adsorption study on LDH samples has been also carried out. This Chapter is presented in three in parts Part I Synthesis and characterization of Mg-Al-CO 3 LDHs for CO 2 adsorption Part II Synthesis and characterization of Mg-Al LDHs with varied intercalated anions for CO 2 adsorption Part III Breakthrough CO 2 adsorption study on Mg-Al LDHs with varied intercalated anions 52

5 Part - I

6 2.1.1 Materials Magnesium nitrate were procured from s.d. Fine Chem. Ltd India, Aluminium nitrate was from LOBA Chemie Pvt. Ltd. India, Sodium carbonate was from National chemicals India. Sodium hydroxide was procured from Ranbaxy Lab. Ltd India and Commercial hydrotalcite (PMG63) was obtained from Sasol Germany GmbH, Hamburg. All chemicals were used as such. CO 2 gas (99.92 % purity) was from Inox Air Product Ltd. Vadodara, India Synthesis of Mg-Al-CO 3 LDHs A series of Mg-Al-CO 3 LDH samples were prepared by precipitation and co-precipitation under different synthetic conditions such as varied Mg:Al cation molar ratios i.e. 1.7:1, 2:1 and 3:1, mode of addition, addition temperature, agitation, and drying temperature (Figure 1). Mg-Al LDH sample preparation Figure 1. Schematic representation of 5

7 LDH samples were designated as Mg-Al-CO 3-1 to Mg-Al-CO 3-9 (Table 1). The yield of Mg-Al LDH samples was calculated as below: Yield (wt %) = (Obtained weight of product /Theoretical weight of product) x 1 Table 1. Synthesis of Mg-Al-CO 3 LDH samples with varied synthetic parameters Sample Cation Molar Ratio Addition Temperature ( C ) Agitation Temperature ( C ) Drying Temperature ( C ) Yield (%) Mg-Al-CO 3-1* 1.7: Mg-Al-CO 3-2* 2:1,,,,,, 79 Mg-Al- CO 3-3* 3:1,,,,,, 7 Mg-Al-CO 3-1.7:1,,,,,, 69 Mg-Al-CO 3-5,,,,,, 11 6 Mg-Al-CO 3-6,,,, ---,, 52 Mg-Al-CO 3-7,, 3 ---,, 62 Mg-Al-CO 3 -,,,, Mg-Al-CO 3-9,,,, 65,, 52 *samples prepared through precipitation method and other all samples prepared through co-precipitation Preparation of Solutions Solution A was prepared by dissolving Mg and Al nitrate salts with different molar concentration such as Mg(NO 3 ) 2 =.63M, Al(NO 3 ) 3 =.37M for Mg-Al=1.7:1, Mg(NO 3 ) 2 =.66M, Al(NO 3 ) 3 =.3M for Mg-Al=2:1 and Mg(NO 3 ) 2 =.75M and Al(NO 3 ) 3 =.25M for Mg-Al=3:1. For different molar concentration ratio of Mg and Al, x (x=m(iii)/m(ii)+m(iii)) value is.37,.3 and.25 for 1.7:1, 2:1 and 3:1 respectively. Solution B having sodium carbonate (Na 2 CO 3 :Mg(NO 3 ) 2 =1:1 molar ratio) dissolved in 1 ml NaOH (2.2 M) was prepared separately. 55

8 Mode of addition of solution A and solution B i) Precipitation Method (precipitation at variable ph): Solution A was slowly added to solution B at the flow rate of.5-1. ml min -1. The samples were designated as Mg-Al-CO 3-1 to Mg-Al-CO 3-3 in Table 1 obtained with different Mg-Al ratios. This method is defined as precipitation at variable ph because when solution A (acidic) is added to solution B (basic) the ph of resultant solution will change and ph will be variable during the precipitation process. ii) Co-precipitation Method (precipitation at constant ph): Both solution A and solution B were added into a container simultaneously at a flow rate of -1 ml min -1. This method is defined as precipitation at constant ph because when a basic and an acidic solution having molarities in similar range are added simultaneously to a common container with same flow rate, the ph of resultant solution will be constant during the precipitation process. Samples prepared by this method are designated as Mg-Al-CO 3 - to Mg-Al-CO Addition temperature of solution A and B To study the effect of temperature on precipitation of solutions, solution A and B were added at two different temperatures, i.e., at 65 C and at room temperature (~3 C). Samples prepared by addition at 65 C are designated as Mg-Al-CO 3-1 to Mg-Al-CO 3-6 and prepared by addition at room temperature are Mg-Al-CO 3-7 to Mg-Al-CO Agitation effect The samples were also prepared with agitation at 65 C for 1h without incorporating agitation step. Samples prepared with agitation were named as Mg-Al-CO 3-1 to Mg-Al-CO 3-5 and Mg-Al-CO 3-9. The samples prepared without agitation were designated as Mg-Al-CO 3-6 to Mg-Al-CO

9 Drying temperature The obtained precipitates were dried under two different conditions to determine the effect of drying temperature on structural and textural properties of LDH samples. The drying conditions are at room temperature (~3 C) under vacuum till it dried (~2-3h) over CaCl 2 and in oven at 11 C for h. The samples dried at room temperature were termed as Mg-Al- CO 3-1 and Mg-Al-CO 3 - and Mg-Al-CO 3 - to Mg-Al-CO 3-9. The samples dried at 11 C were termed as Mg-Al-CO 3-5 to Mg-Al-CO Characterization of LDHs Powder X-Ray Diffraction Studies (PXRD): The d-spacing and crystallinity of LDH samples were measured by X-ray powder diffractometer (Philips X pert) using Cu Kα radiation (λ=1.55ǻ). The samples were scanned in 2θ range to 7 at a scanning rate of. deg sec -1. It may be noted that for layered materials such as LDHs, crystallite size is defined along the a-b plane as disc diameter (). Crystallite size, i.e., disc diameter was calculated from 3 reflection (2θ = 11.6) using the Scherrer formula (17) with a shape factor (K) of.9 as below Crystallite size = Kλ/ W cos θ Where W = Wb - Ws Wb = broadened profile width of experimental sample and Ws = standard profile width of reference silicon sample. Lattice parameters of Mg-Al LDH samples, prepared with varied synthesis parameters, were compared with the reference hydrotalcite from JCPDS data. In-situ high temperature powder X-ray diffraction (HT-PXPD) measurements were done in the temperature range of 1 to 5 C with a heating rate of 2 C min -1 using XRK reactor chamber to determine the structural modifications of LDH samples during heating. 57

10 FT-IR-Spectroscopic studies: The Fourier transform infrared (FT-IR) spectra of LDH samples in the range of - cm - 1 with a resolution of cm -1 as KBr pellets were measured using Perkin-Elmer GX- Spectrometer Thermal Gravimetric Analysis: TG/DTA analysis of LDH samples was performed using TGA (Mettler Toledo Star e System) in the temperature range of 5 to 75 C with a heating rate of 1 C min -1 under N 2 flow ( cm 3 min -1 ) Surface Area Analysis: N 2 adsorption and desorption isotherms on various LDH samples were measured at 77. K using ASAP 21 Micromeritics, USA. Prior to measurement, the samples were activated in situ by heating at 15 C under vacuum (1x 1-3 mmhg) for 2 h. Surface area was determined from N 2 adsorption data using BET equation (113) Chemical Analysis: The elemental analyses of Mg and Al in the synthesized LDH samples was carried out using inductivity coupled plasma (ICP) emission spectroscopy with Perkin-Elmer optima 2 DV Optical Emission Spectrometer. The solution was prepared by dissolving ppm (for Mg/Co) and 1 ppm (for Al) of LDH samples in 5% HNO3. The total water content of LDH catalysts was analyzed by Karl Fischer titrator (Titra Master 5) and also calculated on the basis of weight loss observed during TG analysis. The water content was found in the range of.-.5 moles per mole of LDH by Karl Fischer titrator, however, theoretically calculated values are in the range of moles per mole of LDH. The values obtained by Karl Fischer titrator included in the molecular formula of LDH sample shown in Table 2. 5

11 2.1. CO 2 - Study CO 2 adsorption and desorption measurements were carried out in LDH samples using ASAP 21 Micromeritics, USA. CO 2 adsorption isotherms were measured at 3 C and 6 C upto 1 bar equilibrium pressure. Prior to adsorption measurements, the samples were activated in situ by heating at 15 C under vacuum (1x 1-3 mmhg) for 2 h. The effect of activation temperatures (25 C and 35 C) on CO 2 adsorption isotherms was also studied. The error in adsorption measurements was calculated to be in the range of ±.2%. The measured adsorption isotherms were fitted into, the Langmuir (335), Freundlich (336) and Langmuir-Freundlich (335) isotherm models given below: q = q m bp 1+bP... (1) q = KP n (2) q q = m bp n 1+bP n.. (3) Where q is the amount of gas adsorbed by the adsorbent at equilibrium, q m is the monolayer or saturated amount adsorbed, b is the Langmuir constant, and P is the equilibrium pressure of the adsorbate. The quantity q m b equals K. K and n are fitting parameters Results and Discussion Characterization of LDHs Powder X-Ray Diffraction Studies 59

12 Powder X-Ray diffractogram of Mg-Al-CO 3 LDH samples given in Figure 2a shows structure typical of hydrotalcite (JCPDS-1-191) displaying the characteristic reflections: (i) sharp and intense basal reflections (l) of 3, 6 planes in the low angle region (2θ 25 ), (ii) broad kl reflections of, 15, 1 planes in the mid angle region (2θ =3-5 ) and (iii) sharp hk and hkl reflections of 11, 113 and 1 planes in the high angle region (2θ =55-65 ). The LDH samples synthesized under different synthetic parameters have similar structure, however, variation in d 3, crystallinity and other structural features were observed to be affected with synthetic parameters as shown in Table 2 and are briefly discussed below Effect of Mg/Al ratio The d 3, crystallinity and lattice strain were observed to decrease with increasing the Al content or x value. The theoretical value of d 3 of Mg-Al-CO 3 is expected to be.76 nm, 2- considering the thickness of brucite like layer. nm and the gallery height in CO 3 containing LDH of.2 nm (1). LDH samples synthesized by precipitation and co-precipitation method were found to have d 3 values of nm depending upon the synthetic parameters. For example, d 3 decreases from.77 to.762 nm with increasing the Al content from 25 to 37 mol % (Table 2). The lattice strain was also observed to decrease from to 1.366% with increasing the Al content from 25 to 37 mol %, which results in the increase of the crystallite size, i.e., disc diameter from 11 to 35 nm (Table 2). In the samples having higher Al content, the smaller size of Al 3+ (.5Å) as compared to Mg 2+ (.65Å) leads to significant decrease in the lattice strain and also in d 3 value. LDH sample having Al content of 37 mol% was observed to show maximum CO 2 adsorption at 15 C and 1 bar (discussed later), therefore, for the present study LDH with 37 mol% Al 6

13 content, i.e., 1.7:1 Mg-Al molar ratio was chosen as optimum cation molar ratio for preparing different LDH samples Effect of mode of addition LDH samples prepared by co-precipitation mode (i.e., precipitation at constant ph) showed higher surface area (67 m 2 /g) and disc diameter (3 nm) as compared to LDH samples prepared by precipitation mode (i.e., precipitation at variable ph) showing lower surface area (65 m 2 /g) and disc diameter (35 nm) with all the other parameters being similar. Therefore, co-precipitation method was chosen to prepare other LDH samples with varied synthetic parameters Effect of agitation Among the various synthetic parameters, agitation of the precipitated sample was observed to have significant effect on the crystallinity and disc diameter of LDH sample, without affecting much the d 3 value. For example, LDH samples prepared without agitation were found to be less crystalline having lower disc diameter (1-13 nm, Table 2) as compared to samples prepared with agitation having higher crystallinity (Figure 2b) and higher disc diameter (3-51 nm). However, d 3 value was observed in the range of nm Effect of precursor addition and drying temperature Among the two temperatures (room temperature, i.e., 3 C and 65 C, Table 1) studied during the addition of two solutions A and B, room temperature addition slightly favors the crystallinity (Table 2). Similarly, drying of LDH sample at room temperature for 2 h enhanced the crystallinity of the sample as compared to oven drying at 11 C for h, whereas, d 3 value remains in the similar range. Similar observations that the lower 61

14 temperature is more effective for the formation of LDHs with high Al 3+ substitution have been reported earlier (337). Based on the above results on various synthetic parameters and CO 2 adsorption results (discussed later) the following synthetic parameters to prepared Mg-Al-CO 3 LDH with maximum CO 2 adsorption capacity; co-precipitation of solutions A (having 37 mol % Al) and B at room temperature followed by agitation of precipitated LDH at 65 C for 1 h and drying at room temperature for 2 h, were chosen as optimum synthetic parameters to prepare Mg- Al-CO 3 LDH. The sample (Mg-Al-CO 3-9) prepared using these optimized synthetic conditions was observed to be highly crystalline with d 3 =.76 nm, having highest disc diameter of 51nm (Table 2). Table 2. Characterization of LDH samples Sample d 3 (nm) Crysta - llinity (counts/s) 3 Disc Dia -meter (nm) Lattice Strain (%) Surface Area (m 2 /g) Formula Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.51H 2 O Mg-Al-CO Mg.67 Al.33 (OH) 2 (CO 3 ).5. H 2 O Mg-Al-CO Mg.75 Al.25 (OH) 2 (CO 3 ).5.52H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.57H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.55H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.5H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15.51H 2 O Mg-Al-CO Mg.63 Al.37 (OH) 2 (CO 3 ).15..5H 2 O PMG 63* Mg.63 Al.37 (OH) 2 (CO 3 ).15.5H 2 O *Commercial sample 62

15 a b Figure 2a. XRD spectra of Mg-Al-CO 3-9 LDH c d Figure 2b. Mg-Al-CO 3 LDH prepared with and without agitation 63

16 Lattice parameters Table 3 shows the lattice parameters of Mg-Al LDH samples prepared with varied synthesis conditions, XRD pattern of which were compared with the reference hydrotalcite reported in JCPDS data. It is interesting to note that LDH samples prepared with different Mg-Al molar ratio showed variation in a and c unit cell parameters, wherein, a corresponds to the average metal-metal distance within the layer and c corresponds to layer-to-layer distance (9). The sample Mg-Al-CO 3-3 having the minimum Al content (25 mol %) was found to have a =.37 nm and c = 2.32 nm with rhombohedral structure and resembles with natural hydrotalcite (JCPDS-1-191). As d 3 of the Mg-Al-CO 3-3 =.77 nm, the sample has three times layer-to-layer distance showing 3R (2) stacking of the layers. The sample Mg- Al-1 having the maximum Al content (37 mol %) was found to have a =.6 nm and c = 1.53 nm with hexagonal structure and resembles with Manasseite (JCPDS-1-525). As d 3 of the Mg-Al-1 =.762 nm, the sample has two times layer-to-layer distance showing 2H (1) stacking of the layers. However, the sample Mg-Al-2 having the Al content (33 mol %) was found to have a =.62 nm and c =.6 nm with rhombohedral structure and resembles with synthetic hydrotalcite (JCPDS-22-7). As d 3 of the Mg-Al-2 =.76 nm, the sample has six times layer-to-layer distance showing 6R (2) stacking of the layers. The other LDH samples prepared with co-precipitation method having the maximum Al content (37 mol%) were also observed to have the similar structure showing 6R stacking of the layers. Structural disorders are known to occur in LDHs, among which stacking disorders are common (-2). As the layered sheet of metal hydroxide can be stacked in various ways, a number of polytypes can be formed. Naturally occurring LDHs are known to have either a triple-layer cell with rhombohedral symmetry designated as 3R polytype or a double-layer cell in manasseite with hexagonal symmetry designated as 2H polytype (1-2). One-layer (1H) (1) stacking and six-layer (6R) polytypes (2) have also been reported. Furthermore, 6

17 polytypes having equal number of layers per unit cell, however, differ substantially in layer arrangements are theoretically possible for LDH materials (2). Table 3. Lattice parameters of LDH samples Sample Lattice parameters (nm) a c Crystal type Polytype Reference Mg-Al-CO Hexagonal Two layer (2H) Manasseite (JCPDS-1-525) Mg-Al-CO Rhombohedral Mg-Al-CO ,, Mg-A-CO ,, Six layer (6R) Three layer(3r) Six layer(6r) Hydrotalcite, syn (JCPDS-22-7) Hydrotalcite (JCPDS-1-191) Hydrotalcite, syn (JCPDS-22-7) Mg-Al-CO 3-5,,,,,,,,,, Mg-A-CO 3-6,,,,,,,,,, Mg-Al-CO 3-7,,,,,,,,,, Mg-Al-CO 3 -,,,,,,,,,, Mg-Al-CO 3-9,,,,,,,,,, PMG 63*,,,,,,,,,, Mg-O ** Cubic --- Periclase, syn (JCPDS-5-96) *Commercial LDH sample, ** Mg-Al-CO 3-9 after calcination at 5 C High Temperature-Powder X-Ray Diffraction (HT-PXRD) data Studies To study the structural modification of Mg-Al-CO 3 LDH phase on heating, in-situ heating of sample was carried out in the temperature range of 1- C. The sample heated at 1-25 C resulted into a gradual decrease in the crystallinity and d 3 (Table and Figure 3) due to dehydration of interlayer water molecules and dehydroxylation. Further heating at 3 C led to dehydroxylation along with decarbonation, which resulted in the destruction of layered 65

18 structure of hydrotalcite and sharp decrease in crystallinity with formation of mixed oxide observed. At C presence of cubic MgO observed (JCPDS-5-96) having a =.21 nm and characteristic d-spacing 2.9 (2 plane) and 1. (22 plane) at 3.1 and θ respectively. Figure 3. HT-XRD Mg-Al-CO 3-9 Table. High Temperature X-Ray Diffraction (HT-XRD) of Mg-Al-CO 3-9 Temperature ( C) d-spacing (nm) Peak Intensity (Counts/sec) Phase HT-CO HT-CO HT-CO HT-CO HT-CO HT-CO 3 + Oxide Oxide 66

19 FT-IR-Spectroscopic studies FTIR spectra of Mg-Al LDH samples prepared with varied synthesis conditions with carbonate anion are similar in nature (Table 5 and Figure ). The spectra showed all bands related to hydrotalcite structure for example an intense and broad band at ~39 cm -1 that is attributed to stretching frequency of OH groups of interlayer water as well as to OH groups bound to metal cation (Mg-Al) with a shoulder at ~37 cm -1 indicating the hydrogen bonding between interlayer water and CO 2-3 species. A broad peak at ~36 cm -1 is attributed to the bending frequency of interlayer water molecules. The incorporated CO 2-3 anions in the interlayer of LDH samples show a sharp and intense peak for anti-symmetric stretching frequency at ~1369 cm -1 (٧ 3 ), shoulder for out of plane bending at 71 cm -1 (٧ 2 ) and a intense and broad peak for in- plane bending at 656 cm -1 (٧ ). A broad peak at 7 cm -1 and a sharp peak at 555 cm -1 is associated with Al-OH translation, whereas, a shoulder at 99 cm -1 for Al-OH deformation. An intense peak at cm -1 could be assigned to M 2+ -OH (M 2+ = Mg) translation. Table 5. FT-IR peak assignments for Mg-Al-CO 3 LDHs Peak Position cm -1 Assignment 337 ν- OH, interlayer water 37 H-bonding of CO 2-3 with interlayer water 36 δ OH interlayer water 1369 ν 3 CO -2 3 antisymmetric stretching 71 ν 2 CO -2 3 out of plane bending 656 ν CO -2 3 in-plane bending 99 δ Al-OH deformation 7 Al-OH translation 55 Al-OH translation Mg-OH translation 1 Al-OH translation 67

20 6 36 %T cm Figure. FT-IR Spectrum of Co-Al and Mg-Al LDH Thermal Analysis All the Mg-Al-CO 3 LDH samples prepared under varied synthesis conditions showed the total weight loss in the range of 36-%. The DTA curve showed two endothermic peaks, first at ~2 C (weight loss=%) due to dehydration of interlayer water and the second at ~ C (weight loss=2-27%) due to decarbonation. Though, it is reported in the literature that cation ratio affects the thermal behavior of LDHs, we could not observe any significant difference on the thermal stability of the samples having different Mg-Al molar ratio. The synthetic parameters, however, seem to show slight influence on the thermal behavior. For example, the shape of decarbonation peak was found to be slightly influenced with the agitation step. The DTA curve of LDH samples prepared with agitation at 65 C for 1 h shows broad peak at C due to decarbonation having a shoulder at ~3 C indicating the dehydroxylation before complete decarbonation at C (Figure 5a), whereas, samples prepared without agitation shows a sharp and intense peak at C due to decarbonation (Figure 5b). Furthermore, LDH samples dried at 1 C were observed to show a small 6

21 additional peak at 6-65 C for decarbonation, which indicate that some CO 3 2- ions are firmly trapped in the material and evolve at higher temperature (Figure c) % W t. L o s s /M in % W t L o s s /M in % W t L o s s /M in Temperature ( C) Temperature( C) Temperature( C) a b c Figure 5. TG-DT Analysis Mg-Al-CO 3 agitation, c: dried at 11º C LDH; a: prepared with agitation, b: prepared without Surface Area Analysis Mg-Al-CO 3 LDH samples prepared by precipitation and co-precipitation addition were observed to have the surface area in the range of 29-7 m 2 /g (Table 2). Among the synthetic parameters studied, agitation of precipitated LDH was observed to have significant influence on the surface area. The LDH samples prepared without agitation were observed to have lower surface area 29- m 2 /g as compared to samples prepared with agitation having higher surface area 65-7 m 2 /g CO 2 Study Model fitting The adsorption isotherms of CO 2 on Mg-Al LDH samples prepared under varied synthesis conditions followed type III isotherm (1/n <1). In type III isotherm with 1/n <1, the amount 69

22 of gas adsorbed exponentially increases with increasing pressure. This is attributed to the formation of additional layers of physically adsorbed gas molecules ( ). The Langmuir, Freundlich and Langmuir-Freundlich parameters for the CO 2 adsorption are given in Tables 6, 7 and respectively. The parameters for CO 2 adsorption at different activation and adsorption temperatures are given in Tables 9, 1 and 11 for Mg-Al-CO 3-9 LDH and Figure 6 shows the isotherms of the experimental data and the isotherm model data for the adsorption of CO 2. For Mg-Al-CO 3 LDH the Langmuir isotherm fails to fit (variance =.32 to 3.3) with the experimental data in all the cases. Langmuir-Freundlich isotherm gives a better fit (variance =.67 to.726), but the Freundlich isotherm gives an excellent fit (variance.6 to.6) with the experimental data. The Langmuir isotherm fails to fit with experimental data because it represents monolayer chemisorption but the experimental data shows multilayer adsorption having physisorption with chemisorption, i.e., the adsorption upto langmuir saturation point shows chemisorption and after that physisorption. 25 Volume adsorbed(cc/g) Experimental Langmuir Langmuir-Freundlich Freundlich Pressure(mmHg) Figure 6. Langmuir, Freundlich, and Langmuir-Freundlich isotherm fittings for CO 2 adsorption on Mg-Al-CO 3-9 LDH 7

23 Table 6. Langmuir parameters for CO 2 adsorption at 3ºC on Mg-Al LDH samples Sample * K(ccg -1 mmhg -1 n Regression (R 2 ) Variance Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al- Mg-Al- CO Mg-Al-CO Mg-Al-CO 3-9Cal PMG *Activated at 15ºC under vacuum (1x1-3 mmhg), 2h 71

24 Table 7. Freundlich parameters for CO 2 adsorption at 3ºC on Mg-Al LDH samples Sample * q (ccg -1 mmhg -1 ) b(mmhg -1 ) Regression (R 2 ) Variance Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO 3-9Cal PMG *Activated at 15ºC under vacuum (1x1-3 mmhg), 2h 72

25 Table. Langmuir-Freundlich parameters for CO 2 adsorption at 3ºC on Mg-Al LDH samples *Activated at 15ºC under vacuum (1x1-3 mmhg), 2h Sample * q (ccg -1 mmhg -1 ) b(mmhg -1 ) n Regression (R 2 ) Variance Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO 3-9Cal PMG

26 Table 9. Langmuir parameters for CO 2 adsorption on Mg-Al-CO 3 9 LDH sample at different activation and adsorption temperatures Activation Temperature (ºC) q (ccg -1 mm Hg -1 ) 3ºC 6 C b (mm Hg -1 ) Regression (R 2 ) q (ccg -1 mm Hg -1 ) b (mm Hg -1 ) Regression (R 2 ) Table 1. Freundlich parameters for CO 2 adsorption on Mg-Al-CO 3 9 LDH sample at different activation and adsorption temperatures Activation Temperature (ºC) K (ccg -1 mmhg -1 ) 3 C 6 C n Regression (R 2 ) K (ccg -1 mmhg -1 ) n Regression (R 2 ) Table 11. Langmuir-Freundlich parameters for CO 2 adsorption on Mg-Al- CO 3 9 LDH sample at different activation and adsorption temperatures Activation Temp -erature (ºC) q (ccg -1 Mm Hg -n ) b (mm Hg -n ) 3 C 6 C n Regression (R 2 ) q (ccg -1 mm Hg -1 ) b (mm Hg -1 ) n Regression (R 2 )

27 Effect of different synthesis parameters The Mg-Al-CO 3 LDH samples prepared under varied synthesis conditions were found to adsorb CO 2 in the range of -22 cc/g at 1 bar pressure and 3 C temperature (Table ). The study reveals that the CO 2 adsorption capacity of LDH can be enhanced by variation in synthetic conditions as discussed as below. The LDH samples having lower Al content (25 mol % and x=.25) showed minimum CO 2 adsorption of cc/g, which increased to 1. cc/g for the LDH samples having higher Al content (37 mol % and x=.37) (Figure 7a & b). This shows that increased in charge density due to presence of higher Al content leads to increase in the basicity of the material. The enhanced basicity of hydrotalcite will further lead to increase in chemisorption of acidic carbon dioxide resulting into higher adsorption capacity for CO 2. The correlation between Al content and CO 2 adsorption capacity by Langmuir saturation point indicates that with an increase in Al content, chemisorption increases. For minimum Al content studied (25 mol%), the volume of CO 2 adsorbed by Langmuir saturation point is 5.32 cc/g but for the 33 and 37 mol % Al content the volume of CO 2 adsorbed are 9.7 and 1.97 cc/g respectively. The amount of CO 2 adsorbed in terms of CO 2-3 content has also been calculated. In general, 1 mol % increase in Al content results to increase CO 2-3 content by.5 mol. Among the two modes of addition of two solutions A and B, the LDH samples prepared with co-precipitation showed slightly higher CO 2 adsorption (.2 cc/g) having other synthetic parameters similar to LDH samples prepared with precipitation method showing lower CO 2 adsorption capacity (1. cc/g) (Figure 7c). It may be due to higher crystallinity of the material formed at constant ph. 75

28 Agitation of the precipitated solution plays an important role in the CO 2 adsorption capacity of LDH. For example, LDH sample prepared with agitation showed higher CO 2 adsorption (13.5 cc/g) than the sample prepared without agitation, which showed lower CO 2 adsorption (1 cc/g) having other synthetic parameters similar (Figure 7d). Furthermore, the sample prepared by room temperature addition of the two solutions and without agitation showed lower CO 2 adsorption (1.2 cc/g) than the sample prepared by addition at 65 C and with agitation showed higher CO 2 adsorption (.2 cc/g). Samples prepared through agitation are more crystalline as compared to samples prepared without agitation the higher crystallinity of agitated samples increases the basicity of the material and thus results into the increase in the adsorption capacity for CO 2. The temperature of addition of two solutions also affects the CO 2 adsorption. For example LDH sample prepared at room temperature addition showed higher CO 2 adsorption ( cc/g) as compared to sample prepared by addition at 65 C, which shows lower CO 2 adsorption (1 cc/g) with other synthetic parameters similar (Figure 7e). As room temperature addition favors the crystallinity (as discussed above) which increases the basic sites and thus resulting into enhanced adsorption capacity for CO 2 chemisorption. Drying temperature is another parameter which influences the CO 2 adsorption capacity. The samples dried at room temperature for 2 h showed higher adsorption capacity (1.2-17cc/g) as compared to samples dried at higher temperature of 11 C in oven for h showing lower adsorption (-13.5 cc/g) (Figure 7f). Drying at higher temperature causes loss of interlayer water molecules from LDH resulting a decrease in CO 2 adsorption capacity while samples dried at ambient temperature have interlayer water molecules react with CO 2 to form bicarbonates in presence of water molecules as per the following reaction which is favorable for adsorption of CO 2 and enhance the adsorption capacity (2). 76

29 M 2 CO 3 + CO 2 + H 2 O 2MHCO 3, Table. CO 2 adsorption capacities of Mg-Al LDH samples at 3ºC Sample CO 2 (cc/g) Mg-Al-CO Mg-Al-CO 3-2. Mg-Al-CO 3-3. Mg-Al-CO Mg-Al-CO Mg-Al-CO Mg-Al-CO 3-7. Mg-Al-CO Mg-Al-CO Mg-Al-CO 3 -Cal. PMG63(commercial sample)

30 Volume adsorbed(cc/g) Chemisorption +Physisorption Chemisorption Volume Adsorbed(cc/g) x=.3 x=.37 x= x-value a Pressure(mmHg) b Volume Adsorbed(cc/g) Co-precipitation Precipitation Pressure(mmHg) c Volume Adsorbed(cc/g) With agitation Without agitation Pressure(mmHg) d 1 c 1 d Volume Adsorbed(cc/g) C 65 C Volume Adsorbed(cc/g) C 11 C Pressure(mmHg) e Pressure(mmHg) f Figure 7. a) Relation between Al content and CO 2 adsorption capacity. b to f CO 2 isotherm of Mg-Al-CO 3 LDH b) having different Al content, c) prepared with precipitation and co-precipitation method d) prepared with and without agitation, e) prepared at different addition temperature, f) dried at 3ºC and 11ºC 7

31 The LDH sample (Mg-Al-CO 3-9) prepared with optimized synthetic conditions (as discussed above) showed maximum CO 2 adsorption (22 cc/g) which was found to be similar with commercial Mg-Al-CO 3 LDH sample (Pural MG63) under similar adsorption conditions of 1 bar and 3 C, (Table ). After calcination at 5 C for h, the sample (Mg-Al-CO 3-9-Cal) showed decrease in CO 2 adsorption ( cc/g). The DTA curve for Mg-Al-CO 3-9 (Figure a-b) after CO 2 adsorption showed increased weight loss of % during the decarboxylation and decarbonation, which was found similar to the weight increase after CO 2 adsorption ( 22 cc/g) /Min /Min Total Wt. Loss 1% Temperature( C) -.2 Total Wt. Loss 5% 2 6 Temperature ( O C) a b Figure. TG Analysis Mg-Al-CO 3 LDH a: Mg-Al-CO 3-9 before CO 2 adsorption b: Mg-Al- CO 3-9 after CO 2 adsorption LDH samples having higher surface area are expected to show higher adsorption capacity for CO 2, if only physisorption of gas occurs. The present study showed the multilayer adsorption having both chemisorption and physisorption therefore no linear co-relation of adsorption capacity of CO 2 with the surface area have been observed. On the other hand, higher crystallinity results into well developed layered structure having CO 3 2- anions as basic sites in the interlayer space resulting into the enhanced chemisorption of CO 2. The sample having higher crystallinity and surface area showed higher chemisorption as well as physisorption, 79

32 however, the sample having one property lower and other higher showed lower adsorption capacity for CO 2 (Figure 9a-b). Disc diameter was observed to have a correlation with CO 2 adsorption capacity. For example, LDH sample having highest disc diameter of 51 nm showed maximum CO 2 adsorption of 22cc/g. As disc diameter represents the width of the brucite layer of LDH sample therefore, with an increase in disc diameter, width of the layer increases, which leads to increase in number of basic sites as well as interstitial space which in turn increases the adsorption capacity (Figure 9c) Volume Adsorbed(cc/g) Volume Adsorbed(cc/g) Volume Adsorbed(cc/g) C rysta llinity(cou nts/sec) Surface Area(m2/g) C r y st allize siz e a b c Figure 9. Relation between CO 2 adsorption capacities of different Mg-Al-CO 3 LDH samples with a: Crystallinity ( Intensity/height of 3 plane), b: surface area, c: crystallite size (disc diameter of 3 plane The desorption pattern of all LDH samples showed the chemisorption of CO 2 over the LDH surface as complete desorption of CO 2 does not occurs by only changing pressure at the same temperature (Figure 1Aa-c). However, the calcined LDH sample (Figure 1Ad) showed complete desorption of CO 2 due to the physisorption of CO 2 over calcined LDH surface.

33 Volume adsorbed(cc/g) Pressure(mmHg) Volume adsorbed(cc/g) Pressure(mmHg) a b Volume adsorbed(cc/g) Pressure(mmHg) Volume adsorbed(cc/g) Pressure(mmHg) c d Figure 1A. CO 2 isotherm of Mg-Al-CO 3 LDH at 3ºC after activation at different temperature a:15º C, b: 25ºC, c: 35º C, d:15ºc of Mg-Al-CO 3-9Cal Volume adsorbed(cc/g) Pressure(mmHg) Volume adsorbed(cc/g) Pressure(mmHg) a b c Volume adsorbed(cc/g) Desorptio Pressure(mmHg) Figure 1B. CO 2 isotherm of Mg-Al-CO 3 LDH at 6ºC after activation at different temperature a:15º C, b: 25ºC, c: 35º C 1

34 Effect of activation temperature Furthermore, CO 2 adsorption study after different activation temperature (15, 25 and 35 C) showed the decrease in adsorption capacity from 22 to.2 cc/g with increasing the activation temperature from 15 to 35 C (Table 13 & Figure 1A & B). Table 13. CO 2 capacity of Mg-Al-CO 3-9 LDH at different activation and adsorption temperature Degassing temperature ( C) CO 2 (cc/g) 3 C 6 C It shows that CO 2 adsorption on Mg-Al-CO 3 LDH is significantly affected by the structural changes occurring during the heating of the material. Heating of the samples at 15 C results into loss of some of the interlayer water, as TGA data showed the complete removal of interlayer water at ~2 C. As small amount of water present helps in the adsorption of CO 2, the LDH samples was observed to show maximum adsorption (22cc/g). However, incomplete desorption occurs due to the chemisorption of CO 2 (Figure 1Aa). Upon heating at 2-3 C, interlayer space significantly collapses, as evident by HT-PXRD data (Figure 3 section High Temperature-Powder X-Ray Diffraction (HT-PXRD) data Studies), due to dehydration and dehydroxylation of OH groups bound with Mg 2+ ions. This increases the availability of Mg 2+ ions to react with CO 2 to form MgCO 3 resulting in chemisorption of CO 2 over LDH surface (23). Tichit et al. (33) attributed the progressive collapse of the layered structure and progressive increase in the lattice a parameter with increasing temperature due to the migration of Al 3+ ions from the framework octahedral brucite type 2

35 layers to tetrahedral sites in the interlayer. This effectively makes the framework Mg 2+ ions more accessible to react with CO 2. Therefore, the decrease in d 3 at higher temperature results decrease in adsorption capacity after activation at 25 C (1 cc/g). The increased chemisorption of CO 2 after activation at 25 C results into less desorption as compared to desorption after activation at 15 C (Figure 1Ab). Further heating at 35 C results in complete destruction of layered structure, as evident by HT-PXRD data (Figure 3 section High Temperature-Powder X-Ray Diffraction (HT-PXRD) data Studies) and the material becomes amorphous having lower adsorption of CO 2 (.2 cc/g), however, Mg 2+ ions are less available as compared to the sample heated at 25 C, thus desorption is better than the sample heated at 25 C (Figure 1Ac). Further heating at temperature higher than C results into the decomposition of CO 2-3, and at this point a three-dimensional structure is formed consisting of a close-packed oxygen network, which traps in its interstices a disordered cation configuration with octahedral (M 2+ ) and tetrahedral (M 3+ ) cations. The decreased availability of Mg favors physisorption over chemisorption at this temperature (23), which can be clearly seen in the calcined sample (Mg-Al-CO 3 -Cal-9) showing complete desorption after activation at 15 C (Figure 1Ad). CO 2 adsorption study at higher adsorption temperature (6 C) shows adsorption capacity in the similar range (Table 13 & Figure 1Ba-c), however, desorption was observed to be incomplete due to chemisorption Conclusions The CO 2 adsorption capacity of Mg-Al-CO 3 LDH is significantly influenced by the synthetic parameters used during the preparation of the material. Based on the study, 37% Al content (i.e.,1.7:1 Mg:Al ratio), room temperature addition of magnesium and aluminum precursors followed by agitation at 65 C for 1h and room temperature drying of the material have been optimized for the synthesis of Mg-Al-CO 3 LDH sample. The 3

36 sample prepared (Mg-Al-CO 3-9) using these optimized synthetic conditions were observed to be highly crystalline resulting maximum CO 2 adsorption of 22 cc/g, similar to commercial hydrotalcite sample. The adsorption data was found to be best described by the Freundlich isotherm because experimental data shows multilayer adsorption, i.e., the adsorption upto Langmuir saturation point shows chemisorption and after that physisorption. The incomplete desorption isotherm confirms the chemisorption of CO 2 over the LDH surface, whereas, the calcined material shows complete desorption due to the destruction of hydrotalcite structure and the reduced availability of Mg 2+ to form MgCO 3 with CO 2, which favor physisorption over chemisorption after calcination at C -5 C.

37 Part - II

38 2.2.1 Materials Magnesium nitrate were procured from s.d. Fine Chem. Ltd India, Aluminium nitrate was from LOBA Chemie Pvt. Ltd. India, Sodium carbonate from National chemicals India. Potassium ferrocyanide, Sodium hydroxide, EDTA was procured from Ranbaxy Lab. Ltd India and Acetic acid was from Qualigens Fine Chemicals India. All chemicals were used as such. CO 2 gas (99.92 % purity) was from Inox Air Product Ltd. Vadodara Synthesis of Mg-Al LDHs with varied intercalated anions A series of Mg-Al-LDH samples with different anions such as carbonate, ferrocyanide, acetate and EDTA were prepared. Sample was also prepared without anion (Table 1). Solution A having 1.7:1 molar concentration of Mg and Al (x =.37 where x = Al/Al+Mg), was prepared by adding magnesium nitrate (.629 M) and aluminium nitrate (.371 M). Solution B having anion precursor potassium ferrocyanide, acetic acid and EDTA having 1:1 molar ratio with Mg (NO 3 ) 2 was prepared by dissolving in 1 ml NaOH (2.2 M). Solution B was also prepared by only NaOH (2.2 M) for LDH sample without anion. The solution A and solution B were added simultaneously to a container at a flow rate of -1 ml min -1 in alkaline ph. Agitation of resultant solution was carried out at 65 C for 1h. The precipitate was filtered, washed with deionized water till neutrality. The residues were dried at room temperature under vacuum over calcium chloride. Mg-Al-Fe(CN) 6 LDH samples were also synthesized at different addition temperature and also with and without agitation. The LDH sample with EDTA was also prepared with and without agitation. 6

39 It was observed that no precipitate was formed during the synthesis of Mg-Al-CH 3 COO by addition of precursor at room temperature, therefore, addition of precursor was done at 65 C. LDH samples with CH 3 COO - anion were also prepared by precipitation of solution in acidic and neutral medium keeping other parameters similar to study the effect of ph on precipitation formation. The yield of Mg-Al LDH samples was calculated as below and given in Table 1 Yield (wt %) = (Obtained weight of product / Theoretical weight of product) x 1 Table 1. Synthesis of LDH samples with varied synthetic parameters Sample Cation Molar Ratio Cation Combination Intercalated Anion Addition Temperapture ( C ) Agitation Tempe rature ( C ) Drying Temperature ( C ) Yield (%) Mg-Al-Fe1 Mg-Al 1.7:1 Fe(CN) 6 -,,,,,, 5 Mg-Al-Fe2,,,,,,,, ---,, 52 Mg-Al-Fe 3,,,,,, 65 65,, 5 Mg-Al-E1,,,, EDTA,, ---,, 62 Mg-Al-E2,,,,,,,, 65,, 56 Mg-Al-Ace1,,,, CH 3 COO -,,,,,, 5 Mg-Al-Ace2*,,,,,,,,,,,, 7 Mg-Al-Ace3**,,,,,,,,,,,, 2 Mg-Al-WA,,,, without anion 3 ---,, 66 *Reaction ph is 7 **Reaction ph is Characterization of LDHs All prepared LDHs with varied intercalated anions were characterized by XED, FT-IR Thermal analysis, surface area and chemical analysis as described in part I. 7

40 2.2. Results and Discussion Characterization of LDHs Powder X-Ray Diffraction Studies The different LDH samples prepared with different anions showed a variation in the crystallinity and d spacing (Table 15). We have focused on d spacing of 3 plane and crystallinity in terms of peak intensity, counts/sec of 3 planes because this plane representing the total thickness of layer including d spacing as well as thickness of cationic layer. Powder X-Ray diffractogram of all Mg-Al-LDH samples prepared with varied intercalated anion show structure typical of reference hydrotalcite reported in JCPDS data (JCPDS-22-7). The gallery height is varying with variation in anions which, in turn, causes change in d 3. Powder X-Ray diffractogram of Mg-Al-Fe(CN) 6 LDH sample (Mg-Al-Fe1 given in Figure 11Aa) show shifting in 2θ because of increased d-spacing. The theoretical value of d 3 of Mg-Al-Fe(CN) 6 LDH is expected to be of nm, considering the thickness of brucite like layer.77 nm and the dimension of Fe(CN) nm (226). The d 3 for Mg- Al-Fe(CN) 6 LDH samples prepared by varied synthetic parameters (Mg-Al-Fe1 to Mg-Al-Fe 3) was found between 1.79 nm to 1.55 nm having disc diameter of in the range of nm (Table 15). The small difference in d-spacing in LDH samples possibly related to variation in hydration state. Such a d-spacing of 1.55 nm to 1.79 nm is consistent with the intercalated anion present in the expected orientation of having three fold axis perpendicular to the hydroxide layers (32). Powder X-Ray diffractogram of Mg-Al-CH 3 COO LDH sample (Mg-Al-Ace1) prepared in alkaline medium given in Figure 11Ab show d 3 =.31 nm assuring the intercalation of acetate anion. It was observed that in alkaline medium the LDH phase was formed with very

41 low crystallinity but no spinel or oxide phase was observed. However, the Mg-Al-CH 3 COO LDH sample (Mg-Al-Ace2) prepared in neutral medium (Figure 11Ab) showed HT phase (JCPDS-1-191) having crystallinity 226 counts/sec and d 3 of.6 nm with maximum of spinel form (JCPDS ) with 1% peak intensity at 2θ = 29.3 with little amount of Al 2 O 3.H 2 O (JCPDS-1-9). While the same sample prepared in acidic medium (Mg-Al- Ace3 Figure 11Ab) showed predominantly formation of HT phase (JCPDS-5-525) with crystallinity of 16 counts/sec and d 3.6 nm and less spinel ( ) and little amount of Al 2 O 3.H 2 O (2-1373). Powder X-Ray diffractogram of Mg-Al-EDTA sample prepared without agitation (Mg-Al- E1) and with agitation (Mg-Al-E2) given in Figure 11Ba showed d 3 =.762 nm, disc diameter nm, d 3 =.765 nm, disc diameter 26 nm respectively similar to LDH sample with carbonate anion. However, LDH sample with EDTA prepared with agitation (Mg-Al-E2) showed increase in crystallinity from 215 counts/sec to 1137 counts/sec as compared to sample prepared without agitation. Mg-Al-EDTA and LDH samples without anion were found d 3 similar as that of LDH sample with carbonate anion. It may be because of intercalation of atmospheric carbon dioxide as the sample was not prepared in inert atmosphere. Mg-Al-LDH sample (Mg-Al-WA) prepared without anion given in Figure 11Bb showed d 3 =.763nm, disc diameter 11 nm. 9

42 Table 15. Characterization of LDH samples Sample d 3 (nm) Crysta - llinity (counts/ s) 3 Disc Dia - meter (nm) Lattice Strain (%) Surface Area (m 2 /g) Formula Mg-Al-Fe Mg.63 Al.37 (OH) 2 [Fe(CN) 6 ] H 2 O Mg-Al-Fe Mg.63 Al.37 (OH) 2 [Fe(CN) 6 ] H 2 O Mg-Al-Fe Mg.63 Al.37 (OH) 2 [Fe(CN) 6 ].9275.H 2 O Mg-Al-Ace Mg.63 Al.37 (OH) 2 (CH 3 COO) H 2 O Mg-Al-Ace Mg.63 Al.37 (OH) 2 (CH 3 COO) H 2 O Mg-Al-Ace Mg.63 Al.37 (OH) 2 (CH 3 COO) H 2 O Mg-Al-E Mg.63 Al.37 (OH) 2 (CO 3 ).15.7H 2 O Mg-Al-E Mg.63 Al.37 (OH) 2 (CO 3 ).15.5H 2 O Mg-Al-WA Mg.63 Al.37 (OH) 2 (CO 3 ).15.2H 2 O 9

43 Figure 11Aa. XRD spectra of Mg-Al-Fe1 LDH Figure 11Ab. XRD spectra of Mg-Al-CH 3 COO LDH prepared under varied ph conditions 91

44 Figure 11Ba. XRD spectra of Mg-Al-E2 LDH Figure 11Bb. XRD spectra of Mg-Al-WA LDH 92

45 Lattice parameters Table shows the lattice parameters of Mg-Al LDH samples with varied intercalated anions, XRD pattern of which were compared with the reference hydrotalcite reported in JCPDS data. Table. Lattice parameters of LDH samples Sample Lattice parameters (nm) a c Crystal type Polytype Reference Mg-Al-Fe Rhombohedral Four layer(r) Hydrotalcite, syn (JCPDS- 22-7) Mg-Al-Fe2,,,,,,,,,, Mg-Al-Fe3,,,,,,,,,, Mg-Al-E1,,,,,, Six layer (6R),, Mg-Al-E2,,,,,,,,,, Mg-Al-WA,,,,,,,,,, Mg-Al-Ace1,,,,,, Five layer(5r),, Mg-Al-Ace ,, Three layer (3R) Hydrotalcite (JCPDS-1-191) Spinel (JCPDS ) Al 2 O 3 (JCPDS-1-9) Mg-Al-Ace Hexagonal Two layer (2H) Manasseite (JCPDS-1-525) Spinel (JCPDS ) Al 2 O 3 (JCPDS ) 93

46 It was observed that LDH samples prepared with various intercalated anions resemble with reference hydrotalcite (JCPDS-22-7) reported in JCPDS data. It showed similarity in a and c unit cell parameters wherein, a corresponds to the average metal-metal distance within the layer and c corresponds to layer-to-layer distance (9). The various polytypes have been observed for LDH samples with varied intercalated anions are namely 2H, 3R, R, 5R and 6R by the comparison with reference hydrotalcite given in JCPDS data. Structural disorders are known to occur in LDHs, among which stacking disorders are common (-2) High Temperature-Powder X-Ray Diffraction (HT-PXRD) Studies To study the structural modification of hydrotalcite phase on heating, in-situ heating of samples with different anions (Fe(CN) 6 -, CH 3 COO - ) were carried out in the temperature range of 1- C. For the LDH-sample with ferrocyanide anion (Mg-Al-Fe1) (Table 17 and Figure a) at 2 C there was a significant decrease in the d 3. It observed that at 25 C all the cyanide ions get expelled out (as shown by decrease in d 3 ) but still the layered structure is retained with d 3 =.753 nm, i.e., for carbonate anion. That was because of the formation of LDH with mixed form of carbonate and ferrocyanide during synthesis and as the decarboxylation was taking place at higher temperature the layered structure is retained upto this temperature. The layered structure was completely destructed at C and finally form MgO (JCPDS-5-96) having a =.21 nm and characteristic d-spacing 2.9 (2 plane) and 1. (22 plane) at 3.1 and θ respectively. The LDH sample with acetate anion (Mg-Al-Ace1) (Table 17 and Figure b) at 1 C showed significant decrease in d 3 with no change in crystallinity. 9

47 Figure a. HT-XRD of Mg-Al-Fe1 Figure b. HT-XRD Mg-Al-Ace 1 95