Toward a Delaminated Organotalc: The Use of Polyamidoamine

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Towards a Delaminated Organotalc: The Use of Polyamidoamine Dendrons Marcos Antonio Santana Andrade, and Heloise O. Pastore ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09963 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 13, 2016

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ACS Applied Materials & Interfaces

Towards a Delaminated Organotalc: The Use of Polyamidoamine Dendrons Marcos A. S. Andrade Jr., Heloise O. Pastore* Micro and Mesoporous Molecular Sieves Group, Institute of Chemistry, University of Campinas, 270, Monteiro Lobato St., Cidade Universitária Zeferino Vaz, CEP 13083-861, Campinas-SP, Brazil. *[email protected]

ABSTRACT

A sequence of generations of polyamideamine dendron, PAMAM-talc-Gn (n=17), was constructed on the surfaces of ethylenediaminepropyl-functionalized magnesium phyllosilicate lamellas by using a modified microwave assisted synthesis. The successful functionalization of the inorganic layers by the organic dendrimer was confirmed by FTIR and 13C-NMR spectroscopies, elemental analyses and TGA. The solid materials presented an increase in their interlamellar space and disorganization of lamella packing with the growth of the dendrons. Thermal programmed desorption analysis showed that the lower dendron generation, PAMAM-talc-G1, adsorbed 1.30 mmol CO2/g sorbent at 30 °C. PAMAM-talc-G5 adsorbed the double of PAMAM-talc-G3, probably due to the higher amount of primary amine group; however, PAMAM-talc-G5 adsorbed more CO2 than PAMAM-talc-G7 probably because in the delaminated 7th generation, intradendron N-H

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interactions were more prevalent than in the 5th generation and blocked CO2 interaction sites.

Keywords: Organotalc, dendrimer, PAMAM, delamination, CO2 adsorption 1. Introduction Talc is a 2:1 layered magnesium phyllosilicate mineral with the chemical formula Mg3Si4O10(OH)2 1. The elementary sheet of talc is composed of octahedral magnesium oxide hydroxide structures, the brucite layers, sandwiched between sheets of tetrahedral silica in which the components are linked by covalent bonds. The silica-brucite-silica packs of layers in talc are bonded together by weak Van der Waals forces between surfaces 2,3

. Magnesium phyllosilicates are used in several scientific and industrial applications due

to their availability in nature, and the fact that the structure can be modified both chemically and physically

4–6

. It is generally used as a fine powder in the industries of

paper, cosmetics, paints, polymers, ceramics, and refractories, pharmaceuticals, among others, due to its chemical inertness, softness, whiteness, high thermal and low electrical conductivity, and adsorption properties. It has been intensely studied by many researchers due to the high degree of anisotropy of its crystal structure 1,7. Talc has a perfect basal cleavage, the lamella are slightly flexible. Crucial for many of these properties is the high aspect ratio of the layers, which will be made more profitable by the delamination of this material. Previous works reported procedures to delaminate talc essentially using grinding, intercalation and sonication. Ishimori reported that the addition of titania sol to talc particles changed their aspect ratio to more than double during wet mechanical delamination in a ball mill 6. Shao and co-workers related that intercalation and delamination have been realized by solid state shear compounding using a pan mill 5. Jamil and Palaniandy investigated the delamination of the talc by 2 ACS Paragon Plus Environment

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sonication in water and in acidic media 8. The sonicated talc exhibited severe delamination and reduction of the plate size in the lateral dimension. Delamination diminishes the diffraction corresponding to (00l) planes, causing preferential amorphization along this plane 10 . Organotalcs behave differently from purely inorganic talcs. Mann and co-workers have shown that the delamination of the organotalc could be achieved by a simple ultrasound treatment in water since the RNH2 groups were protoned in these conditions and created a number of sites for ion exchange within the interlayer space that, upon delamination, became essentially surface groups on the lamella. However, although very simple, this procedure did not yield a satisfactory level of delamination 10. Recently, Jeng et al.11 reported an increase up to 126 Å of the montmorillonite interlayer space intercalating dendrons via an ion-exchange process. The chromophorecontaining dendrons in this layered silicate were capable of undergoing a critical conformation change into an ordered structure. At the same time, this group investigated the intercalation of phenyl end-groups dendrons with different molar ratios, revealing the d spacing was influenced by the contents of dendrons as well as dendron conformation into interlayer space 12. Dendrimers are highly symmetrical and structurally defined macromolecules, characterized by a combination of high end-groups functionality and compact molecular structure. PAMAM represent a class of macromolecular architecture called “dense star” polymers which have a high degree of molecular weight distribution, specific size and shape characteristics, besides the presence of highly functionalized terminal amino groups 13–15

. The structural precision of PAMAM dendrimers has motivated numerous studies in

the areas of biomedical applications, CO2 and heavy metal adsorption 16, drug carriers and DNA delivery 17.



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Talcs have already been intercalated with polymers for diverse purposes

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18

. Such

organoclays combine physicochemical features arising from both components the polymers and the talc. Dendron-modified organotalc can be used as adsorbents: in heavy metals and other emergent pollutant removal processes in aqueous effluents or for removal of acid gases from the atmosphere. In the literature, dendrimer-modified molecular sieves have been presented as promising materials mainly for the adsorption of mercury ions19,20 and CO2

21–24

, solid phase in separation methods25,26 and drug delivery27. In this sense,

some improvements in this kind of hybrid organic-inorganic materials can be achieved due to the possibility of modulation and expansion of the interlamellar space in layered materials28,29. This work intends to show the chemical construction of PAMAM dendrons in the interlayer spaces of talc, its delamination by propagation of polyamideamine (PAMAM) generations on the surface and the variation of CO2 adsorption capacity depending on the generations of PAMAM construction. 2. Experimental 2.1. Chemicals The synthesis of the organotalc utilized the following chemical reagents: magnesium nitrate hexahydrate (Mg(NO3)2.6H2O 99%), N-[3-(triethoxysilyl)propyl] ethylenediamine (MSPEA 97%) and tetraethyl orthosilicate (TEOS 98%) from SigmaAldrich and sodium hydroxide (NaOH 99%) from Merck. The functionalization of the talc with several PAMAM generations dendrimers was performed by using the following chemicals: methyl acrylate (MA 99%), ethylenediamine (EDA 99%) from Sigma-Aldrich, and methanol (MeOH 100%) from J.T.Baker. 2.2. Microwave synthesis of organotalc


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The ethylenediaminepropyl organotalc (EDA-Talc) was prepared by sol-gel 30

synthesis as described by Ferreira et al. in 2008

by using microwave irradiation in

accordance with the method reported by Moura et al 31,32. Briefly, Mg(NO3)2.6H2O (0.012 mol) was dissolved in 100 mL of distilled water. Then, the silicon sources, TEOS (0.014 mol) and MSPEA (0.002 mol) were added and formed a white suspension. To this mixture, 48 mL of 0.5 mol L-1 NaOH aqueous solution was slowly added under magnetic stirring. The obtained suspension was aged for 4h at 50 °C. Afterwards, the suspension was submitted to hydrothermal treatment under microwave irradiation for 2 h under maximum potency of 300 W. The product was centrifuged, washed with distilled water and dried at room temperature. The light yellow product formed was ground and sieved at 0.075 mm to produce a powdered material. 2.3. Functionalization of organotalc by PAMAM dendron The PAMAM-talc-Gn materials (n refers to the generation of PAMAM) were obtained in two steps: (1) Michael addition of MA to amino groups followed by (2) amidation of the resulting esters with EDA

13,33

. The grafting procedure is shown in

Scheme 1. The first generation (PAMAM-talc-G1) was synthesized by mixing 1.0 g EDA-talc, 4.0 mL methanol and 1.0 mL MA in an 80 mL all-polymer autoclave lined with Teflon. A Provecto Analitica DGT100 Plus microwave reactor was used for this part of the reaction, at 40 W for 40 min. The product of the Michael addition (PAMAM-talc-G0.5) was centrifuged and washed three times with 15 ml portions of methanol. Then, the product of the above reaction, PAMAM-talc-G0.5 together with 6 mL methanol and 4 mL EDA were mixed in the same container as described above; the Gabriel synthesis was carried out at 40 W for 40 min. The resulting first generation (PAMAM-talc-G1) was also centrifuged and



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washed three times with 15 ml portions of methanol. Further generations of dendrons (up to PAMAM-talc-G7) were prepared in a similar process by repeating the required steps. 2.4. Characterization of organotalcs X-ray diffraction analyses were performed with Cu Kα radiation (40 kV, 30 mA) on a Shimadzu model XRD 7000 diffractometer at room temperature using 0.5°, 0.5°, and 0.3 mm slits for entrance, scattering, and exit. Elemental analysis (C, H and N) was carried out by Perkin Elmer CHNS/O Analyzer 2400. The amount of primary amines was quantified using an adapted Kaiser method. It was used a calibration curve following the procedure described by Poli et al.34. The Kaiser test on samples was performed adding 10 mg of the sample in a test tube followed by 1 mL of ninhydrine solution (2.0 mmol L-1), and absolute ethanol up to a total volume of 6 mL. A blank contained 10 mg of talc without amine pendants. The tubes were sealed, heated up to 100 °C under stirring for 90 min and cooled down to room temperature. The mixtures were centrifuged at 3000 rpm for 10 min to sediment the organotalc. The UV-vis spectra of supernatants were measured between 200 and 800 nm (λmax = 586 nm) using a 4 mL quartz cell with a path length of 1 cm on a Cary 50 Varian UV-Vis-NIR-spectrophotometer. Thermogravimetric analysis was performed in a Setaram Instrument model SETSYS 16/18 Evolution TGA with an alumina pan under O2 atmosphere in a temperature range 20-1000 °C at a heating rate of 10 °C min-1 using approximately 10 mg of sample. Infrared spectra were acquired on a Nicolet model 6700 FTIR spectrophotometer using 0.05 wt % KBr pressed samples. 128 scans at 4 cm-1 resolution were accumulated. Nuclear magnetic resonance spectra of the solid materials were obtained on a Bruker Avance II+ 400 at room temperature. The measurements were made at a resonance frequency of 79.5 MHz for

29

Si and 100.6 MHz for

13

C. For the


29

Si{1H} spectra the

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HPDEC technique with a pulse repetition time of 60 s and a pulse angle of 90° was employed. The CP-MAS 13C spectra were measured with a pulse repetition time of 3 s and a contact time of 0.003 s. Transmission electron microscopy (TEM) was performed on JEOL-JEM 3010 URP working with an acceleration voltage of 300 kV. Thermoprogrammed desorption of carbon dioxide (CO2-TPD) was used to assess the interactions occurring between carbon dioxide and the PAMAM-talc materials. The latter were introduced in a quartz cell and the measurements were performed between 30 and 160 °C. Prior to TPD, each sample was dried under 30 ml min-1 helium at 160 °C for 3 h and then cooled to 30 °C. At this temperature, a 5% CO2 in helium was introduced at 20 mL min-1. The non-adsorbed CO2 excess was eliminated by 30 mL min-1 helium until no CO2 was detected. The amine efficiency, or amount of CO2 captured divided by the amount of amine groups present for a given weight of adsorbent, was used as a tool to compare the adsorbents with different generations. Amine efficiency is defined as the number of moles CO2 captured/ number of moles of N 35,36.

3. Results and discussion 3.1. The effect of PAMAM on talc long range order The X-ray powder diffraction patterns of the precursor EDA-talc and the PAMAM-functionalized talc (PAMAM-talc-Gn, n=1-7) are shown in Fig. 1. Five main diffractions occur at 5.1, 21.3, 26.2, 34.5 and 59.7° 2θ corresponding to distances of 1.44, 0.42, 0.34, 0.26, and 0.15 nm. These peaks are broader than those of the natural talc (ICDD card No. 19-770)

37,38

indicating a lower general organization of the

structure due to presence of the organic chains 39. Moreover, the interlayer d001 spacings for 7 ACS Paragon Plus Environment


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the organically modified phyllosilicates were larger than the basal spacing of naturally occurring talc (~0.96 nm), and increased with the increase of PAMAM dendrimer generations. According to general indexation of organotalcs diffraction patterns, these peaks correspond to (001), (020,110), (004), (130,220), and (060) reflections

22,23

. The

position of the (060) peak observed at 0.15 nm indicates the presence of a trioctahedral 2:1 phyllosilicate structure (in the octahedral sheets each O atom or OH group is surrounded by three divalent cations)

41

and remained unchanged among the various PAMAM

dendrimers organotalcs synthesized. This confirms that the layered inorganic framework can accommodate different generations of the dendron in the interlayer space without losing the tetrahedral-octahedral-tetrahedral structure 9,10. Table 1 shows the values of the basal spaces calculated by the (001) diffraction in comparison with the molecular diameter of the PAMAM dendrimer by Nanjwade et al.

44

and with the PAMAM dendron calculated by PM6 semiempirical method (for generations 1 up to 4). Note that with the increase of the size of the pending group, the interlayer distance increases. In addition, in this table is observed that increasing the size of the PAMAM from G6 to G7 is not as significant as the increase between previous generations, but it is enough to cause a disorganization resulting in the delamination of talc. Fig. 2 pictorially represents the sequence of events occurring in the interlayer space caused by the increasing number of generations of the PAMAM dendron according with the molecular size of the PAMAM in Table 1 24. As observed by X-ray diffraction, with the growth of PAMAM samples in generations G6 and G7 the (001) and the (004) diffraction peaks disappear, indicating that the increase in the PAMAM generation causes such a disorganization of the layers in these materials as to yield a delaminated organotalc. The delamination of PAMAM-talc-G7 is observed on the TEM images in Fig. 3. PAMAM-talc-G1 presents a pack of layers typical of talc 7. As a matter of fact, the


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micrograph in Figure 3, A, shows the layers deposited one above the others. Separated and curved lamella are observed on the transmission micrograph of PAMAM-talc-G7 (Figure 3, B). As already pointed out, talc lamella are flexible, therefore the behavior observed is not totally unexpected. 3.2. The success of the organic reactions on talc-grafted reactants Evidence for dendron growth on talc was obtained by elemental analysis, FTIR and NMR spectroscopies. Based on elemental analysis of nitrogen, carbon and hydrogen (Table 2), it can be observed that nitrogen content increases with increasing dendron generation. The nitrogen content increased from 1.04 mmol g-1 for EDA-talc, reflecting the initial amine loading on the organotalc, to a maximum of 8.47 mmol g-1 for the hybrid material, PAMAM-talc-G7. Experimental content of N and C are in accordance with the calculated values. It could be confirmed that different generations of PAMAM dendron were properly synthesized onto the interlamelar space of the talc. However, the C/N molar ratios (Table 2) presented slight differences compared to the calculated values. Such differences were the first evidence that the materials adsorb CO2 from the air. Additionally, it can be attributed to incomplete branch extensions or cross-linked structure with the growth of the dendron23. This assumption can be clearly observed based on the TGA data and the quantification of primary amines. The thermogravimetric curves of all dendron-modified materials (Fig. 4) present an initial weight loss at 105 °C due to the desorption of water and CO2 followed by dendrons decomposition above 190 °C. The residual weight gradually decreased as the PAMAM generation of the sample increased. From the relationship between the experimental and calculated weight loss (Table 2) it was possible to obtain the overall yield of the dendron synthesis. The overall yield decreases with the increase of PAMAM generations as a consequence of the defects increased concentration. The same trend was observed with the



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density of primary amines in the materials. The experimental density of primary amines, Table 2, was lower than the calculated values for PAMAM-talc generations higher than 3. This suggests that, due to the steric hindrance into the interlamelar space bearing the larger dendrons, both amine groups in a portion of ethylenediamine may react with neighbor ester groups producing two secondary amines, decreasing the density of primary amines in the surface of these dendrons. Similar decreased of PAMAM-functionalized MCM-41 overall yields have been reported in literature; the authors showed the same trend of decreasing with the growth of the dendron14. In the present work the values obtained were larger than 65 %, which confirms that the method of growing PAMAM dendron onto the talc surface is viable. The FTIR spectra of the organotalc and that of the PAMAM-modified derivatives in the CH3OOC-terminal (the half generations, scheme 2-IE) and in the NH2-terminal (in complete generations, scheme 2-IIB) are shown in Fig. 5. The spectra exhibited typical bands of magnesium phyllosilicates in the region of 1186 and 1000 cm-1 with the most intense one at 1013 cm-1 assigned to Si-O-Si antisymmetric stretching. The Si-O-Si bending is found at 457 cm-1 and the Mg-O stretching frequency at 550 cm-1 30,45. Vibration bands were observed between 3000 and 2750 cm-1 corresponding to symmetric and antisymmetric stretching and deformations of −CH2− groups and at 1450 cm-1 from bending vibration of −CH2− of the organic moieties pending in the interlayer space 46. For EDA-talc, the N-H stretching and deformation vibrations of primary and secondary amine groups from ethylenediaminepropyl group are found in the literature at 3294 and 777cm-1, respectively. In spectrum (a) of Figure 5 they are hardly visible due to the low concentration, but they are clear in the spectra from the further PAMAM dendron generations. The band at 1648 cm-1 is assigned to the C-O stretching and –OH bending vibration of absorbed CO2 and H2O 47.


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For PAMAM-talcs (Figure 5 b-k) the characteristic absorption bands of −NH2, −NH−, −CH2− and –C=O may be clearly observed in the spectra. Two strong bands at 1648 cm-1 and 1550 cm-1 are attributed to the C-O stretching and N-H deformation vibrations, respectively 16. Half generations PAMAM-talcs present a C=O stretching band at 1730 cm-1 (Figure 5 b, d, f, h, j) indicating the existence of an ester terminal group (Scheme 2-IE) in these materials

48

. The absence of this band in the materials for entire

generations (Figure 5 c, e, g, i, k) confirms the success of the reaction with ethylenediamine and the presence of –NH2 terminal groups (scheme 2-IIB). The successful preparation of the organotalc by microwave assisted synthesis and the dendrimer functionalization from the organic core bound to the layered surfaces were also confirmed by 13C- and 29Si-solid state NMR spectroscopies. The 29Si{1H} NMR spectra of EDA-talc (Fig.6 a) exhibited signals at -78, -86 and 92 ppm corresponding to the Q0, Q1 and Q2 types of tetrahedral silicon atoms represented as: MgOSi(OH)3, MgOSi(OSi)(OH)2, and MgOSi(OSi)2(OH), respectively

18,19

. The peak

between -50 and -54 ppm are assigned to the silicon atoms of the type T1 and T2, respectively represented by RSi(OH)2(OMg) and RSi(OSi)(OH)(OMg). Integration of signals corresponding to T and Q silicon sites allows the calculation of the T/(T+Q) ratio that corresponds to the percentage of the silicon atoms that are bound to an organic group, i.e., the percentage of pending groups in organotalc 1,8. The synthesis was performed to yield a material where 12.5 % of the total silicon atoms were substituted with a pending group. The calculation of such percentage by 29Si{1H} NMR for EDA-talc yields a value of 9%, very close to the value in the synthesis mixture; for PAMAM-talc-G7 (Fig. 6 b), the percentage of silicon atoms bearing a pending group is 10%. The essentially equal values indicate that the growth of PAMAM dendrons do not disturb the primary bonding of the organic moieties to the inorganic lamella.



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13

C-CP-MAS NMR was used in the study of the anchored compounds, the spectra

are shown in Fig. 7. The EDA-talc spectrum (Fig. 7a) exhibits signals at 12.4, 22.1, 40.7 and 50.5 ppm, corresponding to the methylene groups of the ethylenediaminopropyl chain (see Figure 7 I), respectively

19,20

. The presence of peaks at 16 and 58 ppm, marked with

asterisks, indicate the residual ethoxy group bound to the silicon atom, which could be due either to the incomplete condensation of MSPEA or to the surface reaction of silanol groups with ethanol during extraction 20-22. In addition, a peak appears that is not correlated with the organic chain: the signal at 165 ppm (6) corresponds to carbamate, due to CO2 adsorption from ambient air 23. The PAMAM-talc spectra (Figure 7, b-h) present two broad peaks at 42 and 52 ppm, which correspond to the contribution of the carbon atoms neighboring the amide group and the ones attached directly to the nitrogen of the amine terminal (C3 and C4), respectively. The broadness of these multi-peaks may come from the overlap of the signals of those carbons which have similar atomic environments 16. A peak observed at 175 ppm is related to the carbonyl carbon atoms of amide PAMAM (C5). The contribution of the two remaining carbon atoms (C1 e C2) from the ethylenediaminepropyl decreased in comparison with the significant concentration of carbon atoms from the dendron backbone 14

. The materials with different generations of PAMAM adsorb CO2 from air forming

carbamate in the same manner as EDA-talc and as discussed in the elemental analysis results, causing the appearance of the signal at 165 ppm (C6). To confirm that the peak at 165 ppm is due to formation of carbamate, the spectra of PAMAM-talc-G1 heated up to 120 °C for 20 h under vacuum (to desorb water and traces of CO2 adsorbed from air) and the same material after adsorption of CO2 (Fig.7), were compared. It could be observed that heat-treated PAMAM-talc-G1 displays a


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decreased peak at 165 ppm. The adsorption of CO2 causes the reappearance of the carbamate peak. 3.3. The reactivity of NH2 sites in the pending groups The CO2 adsorption by PAMAM-talc materials was investigated by thermal programmed desorption of carbon dioxide. Fig. 8-I shows the CO2-TPD curves for the PAMAM-talc-Gn (n=1, 3, 5 and 7). As seen, a broad desorption event began at approximately 45 °C and lasted until ca.160 °C for all samples. Table 3 showed that the the maximum capacity of CO2 adsorption was 1.30 mmol de CO2 per gram of PAMAMtalc-G1 and the amount of CO2 adsorbed into the materials decreased with the increase to higher dendron generation. The increase from PAMAM-talc-G3 to PAMAM-talc-G5 doubled the concentration of CO2 adsorbed (0.17 and 0.34 mmol of CO2/g of sorbent). PAMAM-talc-G7 adsorbed 0.07 mmol of CO2/g of sorbent, a smaller amount of CO2 than materials with lower generation, which contrasts with the higher amount of primary amines of PAMAM-talc-G7 (192 –NH2 groups per pending group). This smaller amount of adsorbed CO2 can be explained by the studies from Maiti et al. in 2004

55

. These

researchers carried out a series of fully atomistic simulations and constructed the 3D structures of PAMAM generations 1 through 11. Based on these studies, the authors concluded that the rheological properties and surface reactivities of the PAMAM dendrons depend strongly on the location of the terminal groups and their distribution within the molecule. However, they found a substantial degree of folding back of the end groups inside the dendrimer molecule

55

. This indicates that the end groups of different outer

generations of a given dendron are sufficiently flexible to interpenetrate nearly the whole molecule. In particular, the end groups of the higher generations even come close to the core of the molecule. From the sixth generation one, a larger number of primary amines are penetrating into the core and the outer tertiary amines are crowding into it 56.



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These explanations are also supported by the efficiency values on CO2 adsorption, as measured by the CO2/N molar ratios (Table 3). Under dry conditions, the maximum efficiency of the primary amines is 0.5 mol of CO2 per mol of N and for secondary amide it is 1.0 mol of CO2 per mol of N. The values described in Table 3 show that, although the N sites are completely accessed by CO2 in PAMAM-talc-G1, the same is not observed to the further generations of PAMAM dendron onto the talc surface, the N sites are poorly accessed by CO2. As a matter of fact, the second higher efficiency is attained at PAMAM-talc-G5 where this value is 16 times smaller than the maximum calculated by the stoichiometry of the carbamate reaction, for example. This suggests that either the system displays diffusion problems or that the supposed active sites are already involved in other intramolecular interactions as indicated by the literature 39. As shown in Fig. 8-II, five distinct peaks can be observed in the CO2-TPD profiles of PAMAM-talc samples. The temperature of a desorption peak generally indicates the strength of bonding between the sorbate and sorbent, therefore, desorption peaks may suggest distinct types of sorption sites with different activation energies for desorption 57. It should also be noticed, though, that multiple peaks in a TPD spectrum do not always mean that there is a presence of multiple sorption sites 24. Interactions between CO2 molecules or the penetration of CO2 from surface into the dendrons scaffolding can also produce extra peaks. Two larger intensity peaks are presented in the curves and must be related with the two possible adsorption sites of the PAMAM dendron: primary amines and secondary amides. The CO2 interacts with primary amines producing carbamates and react with amides behaving as Lewis base

58

. Due to their high basicity (pKa=10.7), the terminal

primary amine groups within PAMAM structure interact more strongly with CO2 molecules and desorb at higher temperatures than amide groups. Moreover, the bond


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lengths between carbon dioxide and both adsorptive sites are larger for the CO2-amide complex. In addition, primary amines are more available to interact with CO2 because the amide groups are further inside the PAMAM dendron

22,59

. TPD-CO2 curve from

PAMAM-talc-G1 shows an extra desorption peak at 146 °C. In this lower generation, CO2 interacts also with free silanol groups from lamellae surface forming silylpropyl carbamate32. This peak is not observed to the higher dendron generations because, possibly, they are covering the lamellae surface. PAMAM-talc-G1 adsorbs a higher amount of CO2 than the other adsorbents reported in the literature using the same dendron in superior generations (PAMAM dendrimer = ~0.1 mmol g-1 22; impregnated PAMAM in SBA-15 = 0.1 mmol g-1

22

and PAMAM modified-SBA-15 = 0.8 mmol g

-1 23

) and even,

higher than solids functionalized by other kind of dendrimer, for example tris(2aminoethyl)amine-modified SBA-15 (~0.8 mmol g-1 (impregnated PEI in SBA-15 = 4.35 mmol g mmol g-1

36

21

).

Although other solids

-1 24

; impregnated PEI in magadiite = 6.11

; triamine-modified talc = 3.6 mmol g-1

32

) have presented higher CO2

adsorption capacities than the present material, the adsorption processes were carried out at temperatures higher than 75 °C, whilst in the present work the adsorption of CO2 was performed at 30 °C. PAMAM-talc-G1 can be an alternative to the very expensive current technology applied in the industry based on liquid sorbents (monoethanolamine and diethanolamine) that requires a large amount of solvent. Hence, solid sorbents can prevent corrosion problems and loss of amines by evaporation or decomposition in power plants36.

4. Conclusions A novel organotalc functionalized with PAMAM dendrimers was synthesized by a divergent method and presented different surface properties in relation to the lamellar



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

organotalc intermediate in PAMAM synthesis. The progress in PAMAM generations caused delamination of the organotalc by the resulting increase in the size of the polymer. Despite the consequent increase in the concentration of amine groups on the surface of the organotalc as generation evolved, the expected increase in CO2 adsorption was not observed, probably due to an also increased steric hindrance to active sites and NH2 intramolecular interactions.

The adsorptive sites interact with CO2 by different

mechanisms and with different bond energy causing desorption in distinct temperature ranges. The efficiency is low due to the conformation of the dendrons in higher generation where the positions of the terminal primary amines are close to the core of the pending group and unavailable for adsorption. Therefore, PAMAM-talc-G1 presented a capacity of absorption of CO2 higher than similar PAMAM-modified materials reported in literature, showing its potential as an alternative to the current technology used in the industry. Acknowledgements The authors are indebted to Fundação de Amparo à Pesquisa no Estado de São Paulo,

FAPESP

(2013/05911-1),

for

financial

support,

to

Coordenadoria

de

Aperfeiçoamento de Pessoal de Nível Superior, CAPES, and Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPQ, for fellowships and to Laboratório Nacional de Nanomateriais, LNANO, for TEM images.

(1)

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(2)

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Figure captions Scheme 1. Synthesis pathway of the 7th generation PAMAM dendron organotalc. Scheme 2. I-Michael addition and II-amidation mechanisms. Fig. 1. X-ray diffractograms of a) EDA-talc and PAMAM-talc-G b) 1, c) 2, d) 3 e) 4 f) 5, g) 6 and h) 7. Fig. 2. Schematic comparison of the organotalcs with different dendron generations and the delaminated structure of PAMAM organotalc. Fig. 3. TEM images of A) PAMAM-talc-G1 and B) PAMAM-talc-G7. Fig. 4. Thermogravimetric curves of PAMAM-talc-G a) 1, b) 2, c) 3, d) 5 and e) 7 and the precursor EDA-talc insert. Fig. 5. Infrared spectra from the PAMAM functionalized materials a) EDA-talc and PAMAM-talc-G b)0.5, c)1, d) 1.5, e) 2, f) 2.5, g) 3, h) 3.5, i) 4, j) 4.5, and k) 7. Fig. 6. 29Si{1H} solid-state nuclear magnetic resonance spectra from the materials a) EDAtalc and b) PAMAM-talc-G7. 22 ACS Paragon Plus Environment

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Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 7.

13

C solid-state CPMAS nuclear magnetic resonance spectra a) EDA-talc and

PAMAM-talc-G b) 1, c) 2, d) 3 e) 4 f) 5, g) 6 and h) 7. Fig. 8. I- CO2 thermal programmed desorption curves from PAMAM-talc-Gn (n=1, 3, 5 and 7). II- CO2-TPD profiles of PAMAM-talc-Gn [n= a) 1, b)3, c)5 and d) 7] with peak deconvolution. CO2-TPD conditions: sample weight, 100 mg; gas, 5% vol. CO2/He; temperature of adsorption 30 °C; time of adsorption, 5 h; flow rate adsorption/desorption, 20 mL min-1.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

1

Scheme 1.

2 3

Scheme 1. Synthesis pathway of the 7th generation PAMAM dendron organotalc.

4

24

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Page 25 of 37

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Scheme 2.

Scheme 2. I-Michael addition and II-amidation mechanisms.


25


Fig. 1

(001)

500 cps

(004) (110,020)

(200,130)

(060,330)

a)

Intensity/cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

b) c) d) e) f) g) h) i)

0

10

20

30

40

50

60

70

80

2θ/ θ/ degrees Fig.1. X-ray diffractograms of a) EDA-talc,PAMAM-talc-G b) 1, c) 2, d) 3 e) 4 f) 5, g) 6 h) 7 and i) natural talc.


26

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig.2

Fig.2. Schematic comparison of the organotalcs with dendrons of different generations and the delaminated structure of PAMAM organotalc.


27


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Page 28 of 37

Fig.3

Fig. 3. TEM images of A) PAMAM-talc-G1 and B) PAMAM-talc-G7.


28

Page 29 of 37

Fig. 4

100

80

Weight loss / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

a) b) c)

100

90

d) 40

80

e)

70

f) 20

60

0

100

200

200

400

600

300

400

800

1000

500

600

700

800

900

1000

Temperature / °C Fig. 4. Thermogravimetric curves of PAMAM-talc-G a) 1, b) 2, c) 3, d) 5 and e) 7 and the precursor EDA-talc insert.


29


550 457

777

895

1013

1190

1730 1648 1550 1446 1365

3295 3084 2940 2845

3687

Fig. 5.

a)

Transmitance/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

b) c) d) e) f) g) h) i) j) k) 50 %

4000

3000

1500

1000

Wavenumber /cm-1

500

Fig. 5. Infrared spectra from the PAMAM functionalized materials a) EDA-talc and PAMAMtalc-G b)0.5, c)1, d) 1.5, e) 2, f) 2.5, g) 3, h) 3.5, i) 4, j) 4.5, and k) 7.


30

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig.6

2

Q 1

Q

0

Q 1

T T2

a)

b)

0

-50

-100

-150

-200

δ/ppm Fig. 6.

29

Si{1H} solid-state nuclear magnetic resonance spectra from a) EDA-talc and b)

PAMAM-talc-G7.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

Fig. 7.

4

5 6

3

2 1

a)

Carbamate O

b)

3

3

1 Si

O

NH

2

NH +3 4

O

c)

I

d)

O

O Si

O

e)

3

1 2

-

6 3 NH

O

CO2 adsorbed

NH 4

O

NH *

f)

O

120°C/ 20 h under vaccumm

6

*

O

NH

O

g)

-

+

NH 3 O

h)

II 175

150

125

100

δ/ppm

75

50

25

0

2

3

NH

N

N

200 190 180 170 160 150 140 130 120

δ / ppm

3 3

O

200

3

1 Si

O

O

4 5

*

O

NH

*

3

4 NH 2

Fig. 7. 13C solid-state CP-MAS nuclear magnetic resonance spectra from the materials a) EDAtalc and PAMAM-talc-G b) 1, c) 2, d) 3 e) 4 f) 5, g) 6 and h) 7. Inserted are the 13C solid-state CP-MAS nuclear magnetic resonance spectra of dehydrated PAMAM-talc-G1 and of the product of CO2 adsorption.


32

Page 33 of 37

Fig.8.

PAMAM-talc-G1 PAMAM-talc-G3 PAMAM-talc-G5 PAMAM-talc-G7

Signal TCD / a.u.

25

20

160 140 120 100

15

80 10

60 40

5

Temperature / °C

20 0

I

0 0

200

400

600

800

1000

1200

1400

4

b)

2

111 °C

86.5 °C

5

65.4 °C

r =0.998 46.9 °C

0 10

2

r =0.998

115 °C

64.7 °C

84.5 °C

a)

c)

2

r = 0.998

0

200

117 °C

97.6 °C

85.7 °C

46.7 °C

d)

0

67.8 °C

0 6 3

II

85 °C

146 °C

66.9 °C

30 20 10 0 8

118 °C

Time / s

Signal TCD / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

r =0.997 400

600

800

1000

1200

1400

Time / s Fig. 8. I- CO2 thermal programmed desorption curves from PAMAM-talc-Gn (n=1, 3, 5 and 7). II- CO2-TPD profiles of PAMAM-talc-Gn [n= a) 1, b)3, c)5 and d) 7] with peak deconvolution. CO2-TPD conditions: sample weight, 100 mg; gas, 5% vol. CO2/He; temperature of adsorption 30 °C; time of adsorption, 5 h; flow rate adsorption/desorption, 20 mL min-1.


33


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Page 34 of 37

Table 1. Calculated interlayer distances by the (001) diffraction and comparative values with the PAMAM dendron diameter. Materials

(001) 2θ θ/°

d001 / nm

Molecular dimension

Molecular

/nma

dimension /nmb

EDA-talc-25

6.10

1.44

---

0.98

PAMAM-talc-G1

4.70

1.87

0.76

2.2

PAMAM-talc-G2

3.90

2.26

0.94

2.9

PAMAM-talc-G3

2.96

2.98

1.23

3.6

PAMAM-talc-G4

2.25

3.92

1.43

4.5

PAMAM-talc-G5

1.90

4.64

---

5.4

PAMAM-talc-G6

1.63

5.41

---

6.7

PAMAM-talc-G7

---

---

---

8.1

-a -b

Molecular dimensions calculated by PM6 semiempirical method. Molecular dimensions determined by size-exclusion chromatography determined by

Nanjwade et al. 44. --- non obtained values.


34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Nexp / mmol g-1 1.36 ± 0.07 2.94 ± 0.07 4.73 ± 0.46 6.47 ± 0.12 7.59 ± 0.05 8.02 ± 0.09 8.23 ± 0.07 9.43 ± 0.06

Ncalc/ mmol g-1

1.04

3.08

5.02

6.50

7.46

8.02

8.31

8.47

Samples

EDA-talc

PAMAM-talc-G1

PAMAM-talc-G2

PAMAM-talc-G3

PAMAM-talc-G4

PAMAM-talc-G5

PAMAM-talc-G6


PAMAM-talc-G7

21.72

20.77

20.03

18.64

16.24

12.54

7.70

2.60

Ccalc/ mmol g-1

26.30 ± 0.07

25.54 ± 0.22

21.95 ± 0.10

21.22 ± 0.04

17.78 ± 0.02

12.67 ± 0.01

8.56 ± 0.12

4.35 ± 0.08

Cexp / Mmol g-1

2.6

2.5

2.5

2.5

2.5

2.5

2.5

2.5

C/Ncalc

2.8

3.1

2.7

2.8

2.8

2.7

2.9

3.2

C/Nexp

98.1

96.2

92.7

86.1

74.6

56.9

33.9

10.6

Weight losscalc /%

67

63

56

56

54

48

30

10

Weight lossexp /%

68

65

60

65

72

84

88

94

Overal yield / %

3.75

3.30

2.64

2.42

2.05

1.51

1.16

0.52

[NH2]calc / mmol g-1

3.23 ± 0.02

3.03 ± 0.04

2.43 ± 0.07

2.07 ± 0.01

1.79 ± 0.01

1.51 ± 0.03

1.04 ± 0.01

0.68 ± 0.01

[NH2]exp / mmol g-1

Table 2. Elemental analysis, comparison between experimental and calculated TGA data and density of primary amines for the organotalcs.

Page 35 of 37 ACS Applied Materials & Interfaces

35


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Page 36 of 37

Table 3. Efficiency values of CO2 adsorption in PAMAM-talc-Gn (n=1, 3, 5 and 7).

Samples

a

CO2 adsorbed mmol/g adsorbent

mmol N/ g adsorbent a

Efficiency CO2/N (mol/mol)

PAMAM-talc-G1

1.30

2.31

0.56

PAMAM-talc-G3

0.17

4.85

0.04

PAMAM-talc-G5

0.34

6.01

0.06

PAMAM-talc-G7

0.07

7.07

0.01

Primary amine and secondary amide amount calculated from elemental analysis results.


36

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TOC


37

Toward a Delaminated Organotalc: The Use of Polyamidoamine

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