Adsorption on Amine-Grafted SBA-15 - American Chemical Society

ARTICLE pubs.acs.org/JPCC

FTIR Study of CO2 Adsorption on Amine-Grafted SBA-15: Elucidation of Adsorbed Species Alon Danon, Peter C. Stair,* and Eric Weitz* Institute for Catalysis in Energy Processes and Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

bS Supporting Information ABSTRACT: The interaction between amines and CO2 offers a possible route for the catalytic activation of CO2. In situ infrared spectroscopy was used to study the interaction of CO2 with aminegrafted SBA-15. We employed three different types of amine-grafted SBA-15 surfaces to quantify the effect distinct tethered amine moieties have on the chemistry of CO2 interacting with amine-grafted SBA-15. When the SBA-15 surface has a low density of amines and is “capped” to mitigate against interactions with surface-bound moieties, no new chemical species are observed on exposure to carbon dioxide. An ionic carbamate and a surface-bound carbamate are observed on the other SBA-15 surfaces on exposure to CO2. The formation of carbamates decreases the bond order of the carbon oxygen bond of the carbon dioxide molecule. The role of the different amine moieties and the surface silanol groups in the formation of the carbamates is discussed. Our results suggest that controlling the local environment around surface-grafted amines, which could be achieved by the use of suitably engineered surface environments, could facilitate the adsorption and activation of CO2.

I. INTRODUCTION The need to better understand chemical and physical processes involving CO2 is of the utmost importance since activation and conversion of CO2 into useful products has important industrial and societal benefits.1,2 The incorporation of CO2 in industrial processes involving heterogeneous catalysts remains limited to only a handful of catalytic systems.1 The difficulty in incorporating CO2 in catalytic systems stems from its stability and low energy content. Over metal surfaces, adsorption of CO2 leads to the formation of chemisorbed species only under very limited conditions.1 In contrast, on metal oxide surfaces, CO2 typically adsorbs to form stable and nonreactive carbonate and bicarbonate species.3,4 Consequently, it is important to find systems that are able to activate CO2, yet where the species formed do not have a large negative free energy of formation. The use of amines to activate CO2 has gained considerable attention.57 In nature, initial activation and incorporation of CO2 in the Calvin cycle is performed by the primary amine moiety of a lysine amino acid.8 Recent work utilizing amines to activate CO2 in catalytic systems demonstrates new potential routes for catalytic CO2 activation.5,6,9 For heterogeneous catalytic systems, immobilized amines on silica surfaces have been shown to activate CO2 in the cycloaddition of CO2 to epoxides for formation of cyclic carbonates.6,9 Amines have been shown to interact with CO2 in a variety of environments.5,6,10 Yet, one of the most interesting environments r 2011 American Chemical Society

is amine-grafted silicas owing to their potential in heterogeneous catalysis. Initial studies using immobilized amines on silica surfaces were mainly performed to seek replacements for energy intensive amine scrubbers.1113 These inorganic/organic hybrid systems are typically synthesized either by co-condensation of 3-aminopropyl silyl (APS) molecules and a silica precursor or by grafting of the amine silanes onto a porous or nonporous silica framework.11,14 In situ infrared (IR) spectroscopy has been the main spectroscopic tool used to probe the adsorption of CO2 over APS-grafted materials owing to the strong IR absorptions displayed by adsorbed CO2 species. Adsorption of CO2 on these materials, and more specifically the chemical identity of species formed by interaction of CO2 with the grafted amine sites, remains a widely debated topic in the literature.1113,15 The disagreements that exist may be partially a result of differences in synthetic procedures. Yet, the difficulty in assigning vibrations to adsorbed species because of the overlapping of bands in the carbon fingerprint region (18001100 cm1) likely plays a major role as well. For example, in one of the first FTIR studies on amine-grafted silicas, performed by Leal et al., vibrational bands were assigned to both the carbamate and bicarbonate species when CO2 was adsorbed on an APS-grafted silica gel.11 Chang Received: January 27, 2011 Revised: April 21, 2011 Published: May 19, 2011 11540

dx.doi.org/10.1021/jp200914v | J. Phys. Chem. C 2011, 115, 11540–11549

The Journal of Physical Chemistry C Scheme 1. Synthetically Engineered Amine-Grafted SBA-15 Materials

et al., using diffuse reflectance infrared spectroscopy (DRIFTS), assigned absorptions to the formation of monodentate and bidentate carbonate as well as monodentate and bidentate bicarbonate subsequent to CO2 adsorption on an APS-grafted SBA-15 support. The formation of carbamic acid was also reported.12 In more recent work, Knofel et al. studied CO2 adsorption over two different APS-grafted metal oxide supports: APS-grafted mesoporous silica and APS-grafted mesoporous titania.15 In addition, Bacsik et al. studied CO2 adsorption over extracted and postsynthetically altered mesocaged silicas.15,16 In the Knofel et al. study, comparison of the amine-grafted silica and titania led to a better understanding of the chemical identity of adsorbed CO2 on these materials.15 On the APS-grafted silicas both Bacsik et al. and Knofel et al. reported the formation of carbamate and carbamic acid species.15 A bidentate bicarbonate species was reported to form upon heating in the study by Knofel et al. In all of the studies presented above, the suggested chemical identity of the chemisorbed CO2 species was evaluated by attempting to deconvolute overlapping bands in the carbon fingerprint region. In the present study we synthesized three different materials with the objective of preparing materials capable of exhibiting specific interactions between CO2 and moieties tethered to the surface of SBA-15 (Scheme 1). The fact that different interactions are dominant for different moieties, along with the use of 13 CO2, greatly facilitated the assignment of IR absorption bands of molecules interacting with specific surface-grafted species (Scheme 1). It will be shown that nearest neighbor amine interactions and the SBA-15 surface play a significant role in the adsorption of CO2.

ARTICLE

The first material we worked with is a densely loaded APS (DAPS)-grafted SBA-15. The dense loading of amines on the material maximizes nearest neighbor interactions between grafted surface species through amineamine hydrogen bonding.17,18 The grafted amines can also interact with the surface as a result of hydrogen bonding between the amines and surface hydroxyls.17 The next two materials were prepared to minimize the interactions between grafted nearest neighbor amines. This is accomplished by using a benzylimine spacer (capped APS) to restrict hydrogen bonding during the grafting process, as well as to introduce a sterically bulky group.19 The first of these latter two materials is prepared by employing the capped APS reagent but leaving the surface uncapped before hydrolysis of the protecting agent. The resulting material is referred to as CUAPS for the initially capped APS uncapped (silanol) surface and is used to probe the interaction of isolated amines with CO2 in the presence of surface hydroxyl groups. The second of these latter two material is synthesized by grafting the capped APS followed by a subsequent capping of the surface using hexamethyldisilazane. This material is referred to as CCAPS for the initial capped APS capped surface before hydrolysis. The CCAPS synthesis was developed by Hicks et al.19 for the preparation of isolated amines with limited interaction between the grafted amines and surface hydroxyls.17 Characterization of the relevant materials is followed by a description of CO2 adsorption studies on the materials using both 12 CO2 and isotopically labeled 13CO2. Evidence will be presented for the formation of two types of species. The first is a weakly adsorbed alkylammonium carbamate stabilized by a nearest neighbor amine. This ionic carbamate is only observed on the densely loaded (DAPS) sample. The second is a surface-bound carbamate directly bonded to the silica surface. The formation of the bound carbamate is observed on both the DAPS and the CUAPS samples. Interestingly, on the CCAPS sample no adsorbed species associated with CO2 are observed. These results imply that interactions with surface-bound species can change the bonding modality of CO2 and in turn lead to a decrease in the bond order of bound CO2, potentially making it easier to activate CO2.

II. EXPERIMENTAL METHODS A. Materials. The following chemicals were used as received: anhydrous benzaldehyde (Aldrich), anhydrous toluene (Aldrich), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO20:PO70:EO20) pluronic 123 (Aldrich), tetraethylorthosilicate (TEOS) (Aldrich), HCl (EMD), aminopropyl-triethoxy silane (Aldrich), 1,1,1,3,3,3-hexamethyldisilazane (HMDS) (Aldrich), anhydrous pentane (Aldrich, repurified over an alumina column), ninhydrin (Aldrich), and methanol (Aldrich). A conventional Schlenk line was used for the moisture sensitive synthesis. For gas adsorption studies, UHP 12CO2 (Air Gas, 99.999%) and 13CO2 (Aldrich, 99%) were used. B. Synthesis of SBA-15. SBA-15 was prepared according to the reported procedure using TEOS as a silica source.18 Briefly, 4 g of pluronic 123 was used as a template and dissolved in 150 mL of 2 M distilled aqueous HCl solution. TEOS (9.08 mL) was added, and the gel was allowed to age overnight. Repeated washing, using distilled H2O under vacuum filtration, gave the intermediate product. The as-prepared material was calcined using the following temperature program: (1) increasing the temperature (5 °C/min) to 200 °C, (2) heating at 200 °C for 7 h, (3) increasing at 5 °C/min to 600 °C, and (4) holding at 600 °C for 8 h. 11541

dx.doi.org/10.1021/jp200914v |J. Phys. Chem. C 2011, 115, 11540–11549

The Journal of Physical Chemistry C C. Synthesis of Densely Loaded Amine-Modified SBA-15 (DAPS). SBA-15 (0.5 g) was pretreated by drying the material at

200 °C under vacuum and allowing SBA-15 to cool to room temperature. Anhydrous toluene (50 mL) was then added, followed by 0.63 mL of APTES, under flowing nitrogen. The solution was then heated to 100 °C under nitrogen for 20 h. After cooling, the functionalized material was washed with toluene and vacuum filtered. The material was then dried overnight at 90 °C and stored in a drybag. D. Synthesis of Well-Dispersed Amines. The benzyl amine spacer was synthesized following a procedure reported elsewhere.17 Briefly, 1 mL of APTES and 0.5 mL of benzaldehyde were refluxed in 40 mL of anhydrous toluene using a Dean Stark trap to drive the reaction. The toluene was removed in vacuo, followed by subsequent removal of excess APTES by heating overnight at 100 °C. NMR data (500 MHz, CDCl3) revealed the disappearance of the aldehyde proton (∼10 ppm, 1H) with the formation of the imine (∼8.1 ppm, 1H). Benzylimine (0.3 g) was subsequently grafted onto 0.5 g of pretreated SBA-15 in 40 mL of toluene at 100 °C. The SBA-15 grafted benzylimine was then divided to make two samples. The first of the two samples, capped APS uncapped surface (CUAPS), was uncapped (APS uncapping) using a 45 mL solution of 1:1:1 H2O:methanol:HCl(12.1 N) to expose the amine functionalities. The CUAPS was subsequently washed with DI water and methanol. The second of the two samples, CCAPS, was surface capped using excess HMDS in 60 mL of anhydrous pentane. The sample was dried, followed by a subsequent hydrolysis step, similar to the one used for exposure of the CUAPS amine functionalities. A second step was used to cap the hydroxyl groups formed by hydrolysis of the ethoxy ligands. The second step involved capping the exposed hydroxyls using 0.4 mL of HMDS in 60 mL of anhydrous pentane followed by washing with anhydrous pentane and toluene. CUAPS and CCAPS were both dried overnight at 80 °C and stored in a drybag. (See Scheme 1) E. Characterization of the Materials. Thermogravimetric (TGA) measurements were performed on a thermogravimetric analyzer TA Q50 (TA Instruments). Samples were heated under a dry, high purity He flow (20 mL/min) to 150 °C and kept at this temperature for 10 min to remove residual water. This step was followed by the introduction of O2 (80 mL/min) and heating of the samples from 150 to 600 °C at a rate of 10 °C/min. The samples were kept at 600 °C for 1 h. The TGA curves were used to calculate the APS and capped APS loading (Supporting Information). XPS measurements were performed using an Omicrometer electron spectroscopy for chemical analysis (ESCA) probe with a monochromatic Al KR X-ray source, and data was collected using Omicron EIS software. Samples were pressed onto an indium foil held on the sample holder by copper tape and loaded into the vacuum chamber (1 1010 torr) for outgassing overnight. Calculated individual peak areas for Si2s and N1s were divided by their respective atomic sensitivity factors, and binding energies were calibrated to the alkyl carbon response at 284.5 eV. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements were performed on a Thermo Nicolet Nexus 870 with an MCT detector. Samples were compared to a background spectrum of SBA-15. Spectra were taken at ambient temperature at 2 cm1 resolution between 600 and 4000 cm1. Nitrogen adsorptiondesorption experiments were performed on an ASAP 2010 Micromeritics apparatus, at 77 K. Samples were allowed to outgas under vacuum, overnight at 373 K, to remove all physisorbed species. The BET surface area and

ARTICLE

BJH adsorption average pore diameter (Å) were determined. Adsorption isotherms were plotted to characterize adsorbateadsorbent behavior. Colorimetric experiments were performed using ninhydrin to detect the presence of primary amines. Solutions of 0.2 g ninhydrin dissolved in 8 mL of DI water were prepared. Approximately 8 mg of each sample was added to each solution. Ninhydrin interacts with primary amines changing color to a deep purple known as Ruhemann’s purple (Supporting Information).20 The reaction of ninhydrin with secondary amines is discernible by a reddishyellow color. F. Adsorption of CO2 Using in Situ Transmission Infrared Spectroscopy. In situ FTIR spectra were recorded with a BioRad Excalibur FTS-3000 infrared spectrometer equipped with a MCT detector. Each spectrum was obtained by averaging 40 scans at a resolution of 2 cm1. The homemade reactor, which has been described previously,21,22 consists of a stainless steel cube with two CaF2 windows that can be differentially pumped. For this study samples were pressed on a photoetched tungsten grid held between two nickel jaws. The grid is resistively heated to a temperature measured by a chromelalumel thermocouple attached to the center of the grid. The cell can be pumped down to a base pressure of 1 107 torr. Samples were pretreated by heating to 100 °C for 6 h, under vacuum. Unless stated otherwise, background spectra are of the samples cooled to ambient temperature, under vacuum, after pretreatment. A Baratron capacitance manometer was used to monitor the pressure of CO2. The CO2 absorption bands were deconvoluted using Igor Pro’s multipeak fit package.

III. RESULTS AND DISCUSSION A. Characterization of Materials. For simplicity and to avoid redundancy, the results displayed for the characterization procedures are for the DAPS and the CCAPS samples as the CUAPS sample was prepared from the same initial batch of the grafted benzylimine spacer used for the preparation of CCAPS. The CUAPS sample showed similar results to the CCAPS sample. The results of XPS measurements for DAPS, CCAPS, and SBA15 are displayed in Figure 1. The XPS measurements were taken at different times so the total X-ray count was not constant between samples. The C1s response of the DAPS sample is observed at 284.4 eV and is assigned to a combination of adventitious carbon on the surface as well as the grafted propyl chains.12 Responses due to Si2p and Si2s are observed and are attributable to the SBA-15 silica support. A distinct feature due to N1s is observed at 398 eV and is attributed to the APTS-grafted amines.12 The CCAPS sample displays a larger C1s response relative to the N1s response when compared to the DAPS sample. The large response of C1s from the CCAPS sample is attributed to a combination of propyl chain linkages in APTS and methyl groups attached to the surface through HMDS grafting, as well as to adventitious carbon. The N1s (398 eV) response compared to the support responses of the CCAPS sample is of lower intensity than the response of N1s (398 eV) and the support peaks on the DAPS sample. The relative ratios between the Si2s:N1s responses on the DAPS and CCAPS samples are 8.2 and 24.9, respectively, indicating a lower grafting density of amines for the CCAPS sample. The lower density of amines was confirmed by subsequent TGA and colorimetric experiments. The blank SBA-15 support displays a C1s response indicative of adventitious carbon and framework responses assigned to Si2p and Si2s. 11542


The Journal of Physical Chemistry C

ARTICLE

Figure 1. XPS of amine-loaded samples. Responses were normalized to the Si2s response. (Y offset: CCAPS þ0.4, DAPS þ0.2.)

Figure 2. Nitrogen sorption isotherms at 77 K for the DAPS, CCAPS, and SBA-15 samples. Samples all display type 4 isotherms with hysteresis.

Table 1. Summary of Nitrogen Sorption Results Table 2. Assignment of Bands from DRIFTS nitrogen sorption

wavenumber (cm1)

BJH adsorption BET surface 2

1

samples

area (m g )

diameter (Å)

SBA-15 DAPS

528 256

74 68

CCAPS

206

69

δ CH2 3 3 3 N

1412

average pore

δ CH2

1450

ref 25 25

∼1597

bending NH

15

∼1643

bending NH (no H bonding)

25

CH stretch CH stretch

15, 25 25

2870, 2935 2958 33303400

The nitrogen sorption results are summarized in Table 1. The isotherms for nitrogen sorption at 77 K on the SBA-15, DAPS, and CCAPS samples are displayed in Figure 2. All of these data display a type IV isotherm, where the N2 saturation level is reached at a pressure below the saturation vapor pressure.15 This is explained by initial N2 sorption in the mesopores of SBA-15. It is apparent from the type IV isotherms exhibited by both the DAPS and CCAPS samples that the mesoporous structure is intact postsynthesis. A large loss in BET surface area is observed for both the DAPS and CCAPS samples. One explanation for the large decrease in surface area is the loss in microporosity of the samples as the grafted species sterically hinder access to the SBA15 micropores. The reduction in the average pore diameter (Å) obtained from BJH adsorption data is consistent with the postsynthesis functionalization of the inner pore walls of the mesoporous structure.16 Colorimetric experiments using ninhydrin as an indicating agent for the presence of primary amines on DAPS, CUAPS, and CCAPS qualitatively displayed a larger number of absorbing centers for the DAPS sample in comparison with the CUAPS and CCAPS samples. A strong purple color was observed for the DAPS sample while the CUAPS and CCAPS samples exhibited a faint purple, indicating significantly less absorbing centers (Supporting Information). Moreover, results from the XPS data indicated a higher loading of amines for the DAPS sample in comparison with the CCAPS sample, ∼ 3:1 loading of amines.

assignment

3650∼3740 3748

NH stretch

15, 25

hydrogen-bonded hydroxyls

24

isolated hydroxyls

24

B. DRIFTS Spectra. The DRIFTS results for the samples were obtained by subtracting the SBA-15 background and are summarized in Table 2. The silica framework of SBA-15 has a strong, broad IR absorption band at 1110 cm1 with a broad shoulder at 1180 cm1 as a result of transverse (TO) and longitudinal (LO) optical components of the network motions (not shown).23 The OH region (Figure 3a) of the grafted samples displays a broad loss band between 3750 and 3650 cm1, with a large loss peak at 3748 cm1. The broad loss band between 3750 and 3650 cm1 is in the region characteristic of hydrogen bonding between surface silanols.24 The loss peak at 3748 cm1 results from the loss of isolated silanols on the surface in comparison with the SBA-15 background. These negative peaks are attributed to the successful grafting of APTS with SBA-15 surface silanols. The CH stretching region of CCAPS is dominated by a band at 2962 cm1 due to the asymmetric stretch of methyl groups on the methyl-silylated surface. On the DAPS sample, bands at 2935 and 2869 cm1 are assigned to the asymmetric and symmetric modes, respectively, of the methylene groups on APS.12 The symmetric stretching mode of the CH3 groups on the CCAPS surface is overlapped with the asymmetric stretching mode of the grafted APS methylene groups. CH3 and CH2 deformation bands are observed in the 1400 cm1 region.25 11543



ARTICLE

Table 3. Adsorbed Species Observed over DAPS, CUAPS, and CCAPS Samples wavenumber (cm1)

species

assignment

ref

13351430

ionic carbamate

sym CO2 2 bands

14851550

ionic carbamate

sym NH3þ deformation (m)

∼1510

bound carbamate

CHN group (s)

∼1564

ionic carbamate

asym CO2 stretch (s)

16, 25, 34

∼1625

ionic carbamate

NH3þ deformation (m)

15, 33

∼1714

bound carbamate

CdO stretch

25

∼2500

ionic carbamate

NH3þ symmetric stretch (w-m) (br)

25, 39

3380 3450

ionic carbamate ionic carbamate

loss band NH stretch NH stretch

15, 34 15

Figure 3. DRIFTS spetra of DAPS vs CCAPS: (a) CH, NH, and OH stretching region, 38002500 cm1; (b) deformation bands of CH and NH between 1700 and 1300 cm1 region.

NH stretches are typically observed between 3600 and 3300 cm1.25 For the DAPS sample, bands are observed in this region at 3305 and 3370 cm1. Only one band is observed on the CCAPS sample at 3305 cm1. This may be due to the lower loading on the CCAPS sample since the 3305 cm1 band on the CCAPS sample is difficult to discern because of its weak intensity. This is also the case with the NH deformation mode region as

15, 25, 33 25, 33, 34 25

well. A band at 1597 cm1 is easily discerned for DAPS, yet is not as pronounced for the lower APS-loaded CCAPS sample. In the CCAPS sample, a band at 1643 cm1 is evident and may be due to the deformation of isolated amines (no hydrogen bonding).25 C. Adsorption Studies Using in Situ Transmission Infrared Spectroscopy. We emphasize that the background IR spectrum for results presented in this section is of the grafted sample, taken minutes before the introduction of CO2. Thus, the spectra presented are effectively the difference between spectra taken before and after CO2 interacts with the grafted materials. As with the DRIFTS spectra discussed above, “negative absorptions” are observed if the interaction of the sample with CO2 shifts or leads to removal of species associated with the specific vibrational bands in the background. Positive bands result from the formation of new species or from shifts in absorptions present in the background. Isotopic labeling was used to verify whether an absorption was due to the interaction of CO2 with the grafted materials or to vibrational bands not associated with the labeled carbon containing species that form as a result of the interaction of grafted amine with CO2. Additionally, blanks were taken of SBA-15 in the presence of CO2 and of DAPS in the presence of He. For the former system, the asymmetric stretching mode of gas phase CO2 is observed at 2349 cm1 and 2ν2 þ ν3 and ν1 þ ν3 combination bands are observed at 3613 and 3716 cm1 respectively.26 There have been many studies of the interaction of metal oxides with carbon dioxide that report the formation of carbonates and bicarbonates with a variety of bonding modalities that absorb in the region between 1800 to 1100 cm1.1,4,27 Nevertheless, the literature does not provide evidence that carbon dioxide chemisorbs on mesoporous silica materials, and thus carbonates and/or bicarbonates are not expected to form and are not observed on these materials.28 However, some degree of interaction between CO2 and SBA-15 is suggested by a sharp infrared absorption that is observed at 2340 cm1, which is slightly red-shifted from the gas phase value, and has been reported to be due to linearly physisorbed carbon dioxide.4,16,28 It was suggested that since the enthalpy of adsorption of CO2 on the silica surface is small, this band may arise from the interaction of CO2 with the walls of the SBA-15 cavities.16 As expected, in these studies, no absorptions are observed in the carbon fingerprint region, 11001800 cm1, on any of the blank experiments. Analogous results have been reported for similar systems.15,16 D. Adsorption of 12CO2 and 13CO2 on DAPS. In contrast to observations with the unmodified SBA-15, with the DAPS sample, the introduction of 20 torr of 12CO2 (Figure 4a and 5a) leads to the appearance of multiple overlapped absorptions in the carbon fingerprint region (18001100 cm1). 11544



Figure 4. (a) DAPS sample during CO2 exposure; (b) evacuated DAPS sample after CO2 exposure. Tightly bound chemisorbed species are discerned.

Similar absorption profiles have been reported for a number of systems where amines have been grafted on silica frameworks.11,12,15,16 However, the relative intensity of the absorptions and the assignment of species attributed to these absorptions vary. Based on the position of the absorptions observed in this study, their shift on 13C substitution, and their behavior on evacuation, we will present evidence that these absorptions are due to an alkylammoniumcarbamate and a surfacebound carbamate formed as a result of the interaction of carbon dioxide with amine-grafted SBA-15. We have no evidence for the production of carbonates or bicarbonates in this system. On exposure of the DAPS sample to CO2 (Figure 4), a broad band is observed at ∼2500 cm1, as is an intense absorption between 1550 and 1485 cm1, along with bands between 3200 and 3000 cm1 and a weak broad band at ∼1625 cm1. On evacuation, the large majority of the amplitude of these bands disappear indicating they are due to a weakly bound species. However, a small fraction of the amplitude does remain, indicating that evacuation at room temperature is not sufficient to remove all of these species. Bands near 2500 cm1 are unusual. However, a strong absorption in this region was previously observed for solid amino acids and assigned to an NH3þ absorption.29 Absorptions falling between 1550 and 1485 cm1 were also reported in the study of solid amino acids as well as in the previously mentioned study by Knofel et al.15 These observations and assignments suggest the formation of a weakly bound alkylammonium carbamate that is stabilized by nearest neighbor amine interactions. It is postulated that the formation of the NH3þ carbamate counterion involves the transfer of a proton from the primary amine forming the carbamate to a nearest neighbor amine. In other studies of analogous systems, the transfer of a proton resulted in the appearance of a negative peak at ∼3380 cm1 along with a new absorption at ∼3435 cm1 in the region typical of NH stretching modes.15,16 We observe a negative peak at ∼3380 cm1 along with the appearance of a new absorption at ∼3450 cm1 (Figure 5a). A broad feature suggesting NH hydrogen bonding is observed at ∼33003000 cm1. Since the evacuation is performed at RT, residual alkylammonium carbamate is observed in Figure 5b. These features provide evidence for the formation of an

ARTICLE

Figure 5. FTIR spectra of DAPS during CO2 exposure. (a) Shift is observed from 3380 to 3450 cm1 indicating a change in the NH stretching region. (b) FTIR spectra of DAPS after CO2 exposure. Intensity loss in bands associated with NH shift from 3380 to 3450 cm1 upon evacuation is observed.

alkylammonium moiety which results from the transformation of NH2 in the initially grafted amine to NH(COO) subsequent to interaction with CO2. The transfer of a proton would then result in the formation of the NH3þ counterion.15 Consistent with this assignment, absorptions for the carbamate CO2 moiety have been previously observed between ∼1420 and 1335 cm1 and are typically associated with two peaks due to symmetric stretching along with an absorption in the mid-1500 cm1 region due to the asymmetric stretching mode.16,25 We observe an intense absorption at ∼1564 cm1 that is consistent with the expected position of the asymmetric stretching mode of CO2,15,25 along with two peaks at 1390 and 1430 cm1 that are consistent with absorptions due to the symmetric stretching mode of CO2.16 On 13C substitution it is clear that the bands due to the symmetric stretching mode shift modestly (∼67 cm1 for the absorption at 1390 cm1 in Figure 6). However, due to the overlap of absorptions in this region, the exact magnitude of the shift of the symmetric stretching mode is difficult to quantify. The absorption centered at 1564 cm1 also clearly shifts to a lower frequency. This shift results in this absorption being extensively convoluted with the symmetric NH3þ deformation, which is not expected to shift, and the band centered at 1564 cm1 now appears with a high frequency shoulder due to the shift of the asymmetric stretch on 13C substitution. Due to the breath of this absorption and the lack of distinct features on the high frequency shoulder, it is not possible to determine the exact isotope shift. The evidence presented so far is clearly consistent with one of the species formed being an ionic carbamate with an NH3þ counterion. This species has been depicted differently in various sketches in the literature.11,12,16 In our work we represent this species as a resonant hybrid of the carbamate moiety. Our depiction is based on the fact that the observed CO frequencies suggest a force constant that falls between CO and CdO. On evacuation of a cell containing a DAPS sample that has been exposed to CO2, the bands attributable to the ionic carbamate disappear and bands attributable to another species 11545



Figure 6. FTIR spectra of 12CO2 vs 13CO2 adsorption over DAPS sample. Spectra a and c are of DAPS sample during exposure to 12CO2 and 13CO2, respectively. Bands of the evacuated samples b and d show a clear shift due to isotopic labeling. The most obvious is the shift in the band assigned to the carbonyl stretching mode at 1714 cm1.

remain. This result indicates that the ionic carbamate is weakly bound. The band centered at ∼1701 cm1 blue shifts due to the disappearance of an underlying absorption resulting from NH3þ. The remaining absorption feature is now centered at 1714 cm1, which is in the region where carbonyls typically absorb.16 In addition, the NH stretching region displays a shift back to the original background spectrum revealed by the loss of the negative peak at 3380 cm1 and the positive peak at 3450 cm1 in Figure 5b. This correlates with the desorption of CO2 and a proton transfer step which leads to the loss of the NH3þ counterion and reformation of the primary amine. Bands attributable to another species besides an alkylammonium carbamate remain. As stated previously, the remaining absorption feature centered at 1714 cm1 is in a frequency region where carbonyls typically absorb. Carbonyl vibrations can be found anywhere from 1850 to 1550 cm1, and their position is dependent on a number of factors. First, the more electronegative the atom directly attached to the carbonyl, the higher the observed carbonyl frequency. In addition, hydrogen bonding, which may occur with surface hydroxyl groups on silica surfaces, can decrease the carbonyl vibrational frequency by as much as 60 cm1.25 Interestingly, the most common assignment for a carbonyl stretching motion has been to the formation of an alkylcarbamic acid. For example, Knofel et al. assigned a band at 1680 cm1 to the carbonyl vibrational mode of an alkylcarbamic acid.15 In addition, Bacsik et al., in their study of CO2 adsorption over grafted and incorporated mesocaged silicas, observed a band at 1701 cm1 which they also assigned to the carbonyl stretching motion of an alkylcarbamic acid.16 However, the formation of an alkylcarbamic acid is unlikely as thermodynamic considerations30 have shown that similar carbamic acid species are unstable in the temperature regime of the experiments performed in the referenced previous studies.15,16 Calculations for gaseous carbamic acid NH2CO2H have shown that this molecule is unstable with respect to both the formation of an ammonium carbamate, NH3þCO2 (ΔG0r = 219 kJ mol1), and to NH3(g) þ CO2(g) (ΔG0r = 93 kJ mol1).30 The instability of carbamic acid was

ARTICLE

also reported elsewhere.31,32 Furthermore, important spectral features such as the appearance of an absorption due to the hydroxyl moiety of carbamic acid were not observed or reported in the studies performed by Knofel et al. and Bacsik et al.15,16 It is also important to note that the evidence for the formation of carbamic acid at ambient temperature (the temperature regime in the above papers) relied on assignments of bands due to carbamic acid formation at substantially lower temperatures. These FTIR studies were performed for cold deposited ammonia with CO2, as well as methylamine with CO2, and did not reach ambient temperature.33,34 As stated previously, upon evacuation the most noticeable absorption is a strong band at 1714 cm1 (Figure 4b). This band is assigned to the CdO carbonyl stretching motion of a surface-bound carbamate.25 When 13CO2 is used this absorption (Figure 6d) is red-shifted, Δ, to 1680 cm1, where Δ = 35 cm1 (Figure 6b f 6d). This is consistent with the displacement of the carbon atom in the carbonyl stretching motion, and similar shifts have been observed in other studies of isotopically labeled carbonyls.29 A band is observable at 1510 cm1 after evacuation and is assigned to the CHN group of the bound carbamate. Absorptions of cyclical urethanes have been observed in this region.25 The CHN band shifts to 1503 cm1, Δ = ∼7 cm1, when 13CO2 is used (Figure 6b f 6d). The isotopic shift confirms that this band is associated with chemisorbed CO2. A shoulder near 1580 cm1 has also been observed following CO2 desorption.16 A comparison between the 1580 cm1 absorption in Figure 6b and 6d demonstrates that the shoulder feature also shifts when the sample is exposed to 13CO2 indicating this band arises from a labeled carbon species. As expected, the band for linear physisorbed CO2, observed at 2340 cm1 for 12CO2, red shifts to 2275 cm1, Δ = 65 cm1, when 13CO2 is employed (not shown). E. Adsorption of 12CO2 on CUAPS. The CUAPS sample was prepared using a bulky steric group to limit nearest neighbor interactions. Unlike the CCAPS sample, the surface was left uncapped to allow aminehydroxyl interactions, before hydrolysis of the capping agent. Figure 7a displays a spectrum of the CUAPS sample in the presence of 20 torr of 12CO2. The bands that were associated with an alkylammonium carbamate on the DAPS sample are not observed on the CUAPS sample. These differences suggest that the formation of an alkylammonium carbamate requires the presence of a nearest neighbor amine, which, in general is expected to be lacking on the CUAPS sample. The presence of bands on the DAPS sample that are assigned to an alkylammonium carbamate and their absence on the CUAPS sample also assist in ruling out the assignment of these bands to carbonate and bicarbonate structures. Formation of carbonate or bicarbonate structures is not dependent on nearest neighbor amines.35 Thus, if carbonates and/or bicarbonates do not form on the CUAPS sample, there is no reason to expect them to form on the DAPS sample, and, as previously stated, we see no evidence for their formation.30,35 The low frequency component of the v3 asymmetric CO stretching mode of both monodentate and bidenate carbonates is typically below 1500 cm1. We do not see an absorption in this region on the CUAPS sample. Bicarbonates exhibit absorptions in the 12001450 cm1 region: a region in which we do not see an absorption for the CUAPS surface. In addition, bicarbonates show a broad OH band which we do not observe. Rather, we see depletion of surface OH which is attributed to the formation of the surfacebound carbamate.27,30,35 Thus, we conclude that on the DAPS sample two species are observed, a surface-bound carbamate and 11546



ARTICLE

Scheme 2. CO2 Adsorption on Amine-Grafted DAPS, CUAPS, and CCAPS As Suggested by Infrared Absorptions

Figure 7. (a) FTIR spectra of CUAPS sample during CO2 exposure. Bands associated with the formation of an ionic carbamate involving two amine species are not observed. Only bands associated with a cyclic carbamate are observed. (b) Evacuated CUAPS sample after introduction of 20 torr of CO2. The proposed bound carbamate is stable at ambient temperature under vacuum.

an alkylammonium carbamate: Both are inconsistent with absorption profiles associated with the formation of carbonates and bicarbonates. Other groups have also reported that carbonates and bicarbonates do not form under conditions similar to ours; however, in some cases this conclusion is based on the stipulation that water is absent from their experimental apparatus. Though our apparatus is similar to what has been used in the referenced studies, we do not have to make this assumption to reach the same conclusion.15,16 In the presence of 20 torr of 12CO2 (Figure 7a), a strong absorption at ∼1714 cm1 is observed on the CUAPS sample. This band is observed on the DAPS sample in the presence of (before blue shifting) and after the evacuation of CO2 and was assigned to a bound carbamate (vide supra, Scheme 2). A band at ∼1510 cm1 is also observed. This band which is masked by the absorptions of the alkylammonium carbamate for CO2 adsorption on the DAPS sample was assigned to the NHC deformation mode of the surface-bound carbamate.25 Observation of this band on the CUAPS sample suggests that nearest neighbor amines are not the stabilizing factor for the formation of this bound species. As expected, evacuation of CO2 does not change the spectrum (Figure 7b) as the carbamate is tightly bound to the surface at ambient temperatures. Figure 8 provides evidence for the driving force behind formation of the surface-bound carbamate. In Figure 8 spectra of the CUAPS sample are shown in the presence of 20 torr of CO2 and after evacuation, between 4000 and 3400 cm1, the region typical of hydroxyl vibrational modes.24 In Figure 8a the hydroxyl region of the SBA-15 support is overlaid with absorptions previously assigned to the combination bands of gas phase CO2. Upon evacuation (Figure 8b) of CO2, the gas phase combination bands are removed allowing the observation of negative bands between 3750 and 3500 cm1 which are consistent with the loss of surface hydroxyls.24 These negative peaks are also observed on the DAPS sample after CO2 adsorption. This suggests that formation of the surfacebound carbamate involves a reaction between an adsorbed CO2

and a surface hydroxyl to produce the dehydration products: a surface-bound carbamate and thermodynamically stable water (ΔH0f = 285 kJ mol1).3 As stated previously, formation of a carbamic acid species is unlikely from a thermodynamic perspective. In addition, the OH region in previous studies would have been overlapped with CO2 combination bands making it difficult to observe the loss of OH absorptions observed in our study.11,12,15 Analogous cyclical carbon bonded carbamates display similar frequencies for the carbonyl stretch and CHN groups.36,37 Thus, spatial dispersion of amines on the CUAPS sample, while leaving the surface uncapped, led to the formation of only one observable species, a surface-bound carbamate. We also note the appearance of a small band at 2077 cm1 in Figure 7a. This small band was observed in multiple experiments using CUAPS and is not an artifact. Performing a NIST species data search for vibrational absorptions at this frequency reveals that the only plausible assignment of this band is to a CN stretching motion.38 This band disappears on evacuation of CO2. The only CN containing species that seems plausible for this 11547



ARTICLE

primary amines but to provide data in support of their accessibility to CO2. Ninhydrin is used in a variety of fields to detect primary amines.20 Both CUAPS and CCAPS gave similar colorimetric results after introduction of ninhydrin, displaying a light Ruhemann’s purple color indicative of primary amines. DAPS displayed a much darker Ruhemann’s purple color consistent with a dense loading of APTS (Supporting Information). Thus, primary amines are present and are accessible to ninhydrin on all samples. Since access of ninhydrin to the amines should be much more sterically challenging than access of CO2 to amines, it is difficult to see how ninhydrin could access the surface-bound amines and CO2 would not.

Figure 8. (a) CUAPS sample in the presence of 20 torr of CO2. CO2 combination bands overlay the silanol region of SBA-15. (b) Evacuated CUAPS sample after introduction of 20 torr of CO2. Negative bands are observed suggesting the disappearance of OH groups.

system is SiCN; however, the mechanism by which a CN containing species forms is not clear to us and is beyond the scope of the current work. F. Adsorption of 12CO2 and 13CO2 on CCAPS. The CCAPS sample is not expected to have significant amineamine interactions or aminehydroxyl interactions since the surface is capped before hydrolysis of the imine. This material has been used in the past to study amine dispersion on SBA-15 by Hicks et al.17 Using fluorescence spectroscopy they reported that the grafting of benzyl-capped amines leads to a spatial separation of the grafted species. This was inferred as a result of the inhibition of excimer emission. Excimer formation involving adsorbed species does occur on more densely loaded samples where the amines are not spatially separated. FTIR difference spectra were taken of CCAPS after exposure to 12CO2, after evacuation of 12CO2, and after exposure of CCAPS to 13CO2 (spectra not shown). No bands associated with chemisorbed CO2 or its reaction products are observed. These observations are consistent with our prior conclusion that the formation of a surface-bound carbamate involves a reaction with surface hydroxyls. These results suggest that if access to surface hydroxyls is inhibited the formation of this surface-bound carbamate is also significantly inhibited. The results are also consistent with observations and explanations for the formation of products on both the CUAPS and DAPS surfaces. Formation of an alkylammonium carbamate requires the presence of nearest neighbor amines. Since the CCAPS sample should display a similar amine loading to the CUAPS sample, formation of an alkylammonium carbamate species is not expected. One concern of ours was that the multiple synthesis steps possibly altered the primary amines on the CCAPS sample. In addition to the steps taken by Hicks et al. to establish this synthesis of CCAPS as a reproducible procedure for the synthesis of isolated primary amines without surface interactions, we used DRIFTS and ninhydrin to verify successful grafting of primary amines.17 Ninhydrin was used to verify not only the presence of

IV. CONCLUSIONS We have presented evidence in support of the formation of two different surface species on densely loaded amine silica samples (Summarized in Table 3). This evidence was acquired by using three different synthetically engineered materials, where spatial and steric hindrances were used to direct bond formation of desired species. Spatial and steric hindrances were generated by the employment of capped APTS on the CUAPS sample, as well as HMDS grafting on the CCAPS sample. Both an ionic carbamate, stabilized by interaction with NH3þ, and a proposed surface-bound carbamate, were observed on the densely loaded DAPS sample. Upon evacuation of CO2 from the cell at ambient temperature, bands associated with alkyl ammonium carbamate disappear and only bands associated with the bound carbamate are left intact, indicating a larger adsorption enthalpy for the surface-bound carbamate. When CO2 was introduced over CUAPS, only the surface-bound carbamate was observed. This difference in behavior suggests that the formation of the ionic carbamate involves at least two nearest neighbor amines, while formation of the surface carbamate does not involve a binary amine system. Over CCAPS (the sample material with a high spatial dispersion of amines with a capped surface to hinder surface interactions), no species associated with chemisorbed CO2 were observed (Scheme 2). Isotopic labeling (13CO2) was used to facilitate assignments for the carbonyl stretching mode on the DAPS sample. Over the CUAPS sample a negative peak was observed in the OH region assigned to the reaction between an intermediate carbamic acid or carbamate with a surface silanol. This work provides evidence for the formation of an alkylammonium carbamate resulting from CO2 adsorption on aminegrafted SBA-15. Evidence is put forth for the formation of a new species not previously described for such systems. This species is assigned as a surface-bound carbamate formed by reaction of an intermediate carbamate or carbamic acid with a surface silanol to produce water and the bound species.39 Carbamic acid, though suggested in other studies, is not believed to form under our experimental conditions, as this species is unstable at room temperature. These results demonstrate that surfaces can be modified to favor the formation of a desired species. Our results show that separation of tethered amine moieties followed by adsorption of CO2 leads to the exclusive formation of surface-bound carbamates. These results suggest that grafting a diamine compound followed by capping of the surface for CO2 adsorption could lead to the exclusive formation of an alkylammonium carbamate, as we have shown evidence that a binary amine system is needed for formation of this species. Both the surface-bound carbamate and 11548


The Journal of Physical Chemistry C alkylammonium carbamate displayed a reduced bond order for the CdO moiety of CO2 indicating partial activation of the initially adsorbed carbon dioxide. This suggests that the species may be used to activate CO2 in catalytic processes. Further work involves engineering single adsorption sites on silica surfaces to better understand the adsorption enthalpies and activation abilities of each site toward CO2.

’ ASSOCIATED CONTENT

bS

Supporting Information. Ninhydrin colorimetric results, summary of nitrogen sorption and thermogravimetric analysis, and curve fitting on evacuated DAPS samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Award No. DE-FG02-03-ER15457). The DRIFTS and XPS work was performed in the Keck-II facility of NUANCE Center at Northwestern University. NUANCE Center is supported by NSFNSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University. The authors are grateful to Dr. Rabuffeti and the Poeppelmeier lab from Northwestern for use of their TGA and Dr. Junling Lu for his collaboration and for obtaining the TEM images used in the TOC. We also thank Dr. Peter Zapol and Dr. Haiying He for discussions of their theoretical results relating to the stability of carbamates. ’ REFERENCES (1) Freund, H. J. Surf. Sci. Rep. 1996, 25, 225. (2) Whitesides, G. M.; Crabtree, G. W. Science 2007, 315, 796. (3) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11, 1. (4) Ramis, G.; Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1991, 29, 425. (5) Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. J. Am. Chem. Soc. 2008, 130, 6342. (6) Srinivas, D.; Ratnasamy, P. Microporous Mesoporous Mater. 2007, 105, 170. (7) Chu, D.; Qin, G. X.; Yuan, X. M.; Xu, M.; Zheng, P.; Lu, J. Chemsuschem 2008, 1, 205. (8) Lodish, H. F. Molecular Cell Biology, 5th ed.; W. H. Freeman and Company: New York, 2004. (9) Srivastava, R.; Srinivas, D.; Ratnasamy, P. Microporous Mesoporous Mater. 2006, 90, 314. (10) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939. (11) Leal, O.; Bolivar, C.; Ovalles, C.; Garcia, J. J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183. (12) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468. (13) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Ind. Eng. Chem. Res. 2003, 42, 2427. (14) Bass, J. D.; Katz, A. Chem. Mater. 2003, 15, 2757. (15) Knofel, C.; Martin, C.; Hornebecq, V.; Llewellyn, P. L. J. Phys. Chem. C 2009, 113, 21726. (16) Bacsik, Z.; Atluri, R.; Garcia-Bennett, A. E.; Hedin, N. Langmuir 2010, 26, 10013. (17) Hicks, J. C.; Jones, C. W. Langmuir 2006, 22, 2676.

ARTICLE

(18) Hicks, J. C.; Dabestani, R.; Buchanan, A. C.; Jones, C. W. Chem. Mater. 2006, 18, 5022. (19) McKittrick, M.; Jones, C. W. Che. Mater. 2003, 15, 1132. (20) Friedman, M. J. Agric. Food Chem. 2004, 52, 385. (21) Yeom, Y. H.; Wen, B.; Sachtler, W. M. H.; Weitz, E. J. Phys. Chem. B 2004, 108, 5386. (22) Basu, P.; Ballinger, T. H.; Yates, J. T. Rev. Sci. Instrum. 1988, 59, 1321. (23) Borodko, Y.; Ager, J. W.; Marti, G. E.; Song, H.; Niesz, K.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 17386. (24) Gallas, J. P.; Goupil, J. M.; Vimont, A.; Lavalley, J. C.; Gil, B.; Gilson, J. P.; Miserque, O. Langmuir 2009, 25, 5825. (25) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; Wiley: Chichester, New York, 2001. (26) Herzberg, G. Infrared and Raman Spectra; Van Nostrand: New York, 1945. (27) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (28) Roque-Malherbe, R.; Polanco-Estrella, R.; Marquez-Linares, F. J. Phys. Chem. C 2010, 114, 17773. (29) Steger, E.; Turcu, A.; Macovei, V. Spectrochim. Acta 1963, 19, 293. (30) Dell’Amico, D. B.; Calderazzo, F.; Labella, L.; Marchetti, F.; Pampaloni, G. Chem. Rev. 2003, 103, 3857. (31) Remko, M.; Rode, B. M. J. Mol. Struct—Theochem. 1995, 339, 125. (32) Wen, N. P.; Brooker, M. H. J. Phys. Chem. 1995, 99, 359. (33) Bossa, J. B.; Theule, P.; Duvernay, F.; Borget, F.; Chiavassa, T. Astron. Astrophys. 2008, 492, 719. (34) Bossa, J. B.; Borget, F.; Duvernay, F.; Theule, P.; Chiavassa, T. J. Phys. Chem. A 2008, 112, 5113. (35) Masuda, K.; Ito, Y.; Horiguchi, M.; Fujita, H. Tetrahedron 2005, 61, 213. (36) Lee, J. K.; Suh, Y. W.; Kung, M. C.; Downing, C. M.; Kung, H. H. Tetrahedron Lett. 2007, 48, 4919. (37) Kirilin, A. D.; Belova, L. O.; Gavrilova, A. V.; Korobova, E. A. Russ. J. Gen. Chem. 2009, 79, 2137. (38) Maier, G.; Reisenauer, H. P.; Egenolf, H.; Glatthaar, J. Eur. J. Org. Chem. 1998, 1307. (39) Emsley, J. Chem. Soc. Rev. 1980, 9, 91.

11549


Adsorption on Amine-Grafted SBA-15 - American Chemical Society

Recommend Documents