Mesoporous Alumina with Amidoxime Groups for CO2 Sorption at

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Mesoporous alumina with amidoxime groups for CO2 sorption at ambient and elevated temperatures Chamila Gunathilake, Mahinda E. Gangoda, and Mietek Jaroniec Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00674 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 3, 2016

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Industrial & Engineering Chemistry Research

Mesoporous alumina with amidoxime groups for CO2 sorption at ambient and elevated temperatures Chamila Gunathilake, Mahinda Gangoda, and Mietek Jaroniec∗ Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, USA.

RECEIVED DATE: Development of various mesostructures with introduced basic species such as amine groups represents a viable strategy for enhancing adsorption of acidic molecules such as CO2. To follow this strategy, mesoporous materials with incorporated alumina and amidoxime functionality were prepared by evaporation induced self-assembly of commercial boehmite nanoparticles as an alumina precursor, (3cyanopropyl)triethoxysilane as an organosilica precursor, and Pluronic P123 triblock copolymer as a soft template under acidic conditions. In the next synthesis step, the resulting mesoporous materials with cyanopropyl groups were subjected to hydrothermal reaction with hydroxylamine hydrochloride at slightly basic conditions and 80 oC to convert cyanopropyl groups to amidoxime functionalities. The latter sorbents showed fairly high CO2 uptake at ambient conditions (25 oC, 1.2 atm) and remarkably high sorption capacity (3.84 mmol/g) at 120 oC. Good thermal and chemical stabilities of these materials combined with high CO2 uptake at elevated temperatures make them of potential interest for sorption of acidic gaseous molecules such as CO2. KEYWORDS: Boehmite, cyanopropyl, amidoxime, temperature programmed desorption, CO2 sorption.

∗

Corresponding author: [email protected]

(Jaroniec)

Phone:

330-672-3790;

ACS Paragon Plus Environment

Fax:

330-672-3816;

E-mail:

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INTRODUCTION Carbon dioxide (CO2) is one of the main greenhouse gases causing global warming and climate change that results from human activities such as coal burning, vehicle emission, forest clearing, and industrial discharge. Thus, an adequate control of atmospheric CO2 concentration is an emergent demand to reduce the excessive CO2 concentration in atmosphere. The current technology for industrial capture of CO2 is mainly based on liquid scrubbers, which are used to separate CO2 from flue gas. The absorbed CO2 can be recovered through a regenerative process by heating and/or depressurization. Typical aqueous amine-based solutions of MEA (monoethanolamine), DEA (diethanolamine), diglycolamine (DGA), and their derivatives such as N-methyldiethanolamine (N-MDEA) are often used to capture CO2 in industrial power plants. A large deployment of this technology for carbon dioxide capture and sequestration (CCS) is somewhat troubled by potential degradation of amine groups, resulting in solvent loss, equipment corrosion, and relatively high cost of the process1,2 as well as by relatively low selectivity toward CO2 in the presence of other oxidizing and reducing gases (SO2, NO2, and NO).3 Thus, the development of a viable and cost effective process for CO2 capture from various sources including flue gas emissions of coal-based power plants is crucial to control the CO2 concentration in atmosphere. In contrast to the use of liquid scrubbers, the CO2 capture by solid sorbents seems to be an attractive process because of their potential advantages such as the well-developed porous structures featuring high microporosity and large specific surface area, high gas uptakes, and tunable selectivity in addition to economic feasibility and easiness in regeneration and handling. Physical or chemical solid sorbents can be used for CO2 capture. Typical physical adsorbents include activated carbons, carbon nanotubes, zeolites, and metal organic frameworks (MOF) and chemical sorbents include mainly alkali metal carbonates and amine-functionalized solid sorbents. Most of physisorbents display relatively low CO2 uptake and low selectivity as compared to chemisorbents. In general, introduction of basic species into


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porous solid chemisorbents is crucial for enhancing interactions with acidic CO2 to improve their sorption capacity and selectivity. Modification of solid sorbents such as porous silica and carbon materials with amine species by physical impregnation and chemical grafting for enhancing CO2 sorption has been studied for a long time. In particular, 3-aminopropyltriethoxysilane, ethylenediamine, (3-trimethoxysilylpropyl)diethylenetriamine, and some of their derivatives have been often used to synthesize amine-functionalized mesoporous organosilica materials for CO2 capture. For instance, Sayari and van der Voort studied periodic mesoporous organosilicas functionalized with various types of amines such as diaminobutane, diaminohexane, diaminododecane, diethylenetriamine, and tetraethylenepentamine to tune basic properties for CO2 sorption.4 Moreover, Sayari and co-workers investigated the stability of various amine groups introduced into mesoporous silica materials and demonstrated instability of many amine functionalities upon heating over 100 oC.5-7 The amidoximegrafted solid supports have been also explored for CO2 sorption. Previous thermogravimetric and pulse CO2 chemisorption studies of amidoxime-functionalized sorbents demonstrated their high affinity and selectivity toward acidic carbon dioxide and better thermal stability as compared to the amine-modified counterparts. Amidoxime functionalities are often incorporated into porous materials by a two-step process involving introduction of cyano groups (-R-C≡N; R = alkyl or aryl groups) and subsequent conversion of these groups to amidoxime (-R-C(NH2)=NOH) using hydroxylamine hydrochloride (NH2OH.HCl) solution. Amidoxime compounds and amidoxime-modified carbons and polymers were respectively studied for CO2 sorption at different temperatures by Zulfiqar et al.,10 Mahurin et al.,11 and Patel and co-workers.12 The latter group reported that the CO2 uptake at 25 oC increases with increasing percentage of amidoxime loading.12 The nucleophilic NH2 and hydroxyl groups present in amidoxime functionality can effectively bind carbon dioxide molecules. Therefore, numerous attempts have been made to prepare amidoxime-modified solid sorbents. Hiyoshi et al.13 and Prasetyanto et al.14 thoroughly investigated interactions between CO2 and amine groups (-NH2) and found that two nitrogen atoms at close proximity are essential for binding CO2 and form carbamate structure. Previous CO2 sorption


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studies at 60 and 120 oC reveal that mesoporous organosilicas with amidoxime groups show large sorption capacity and high affinity toward CO2.8 Also, incorporation of basic metal species into organic frameworks (MOF) and other porous materials can enhance their thermal and mechanical stability. Numerous inorganic species (alkaline and alkaline earth metal oxides) have been used to improve the basicity of porous sorbents, including alkaline (Li, Na, K),15-17 and alkaline earth metal species (Ca, Mg),15,18,19 transition metal (Zr, Ni) oxides,20,21 and alumina (Al).22-24 Among various metal oxides, alumina attracted a significant attention because of its high surface area, large pore volume, well-developed mesoporosity, high crystallinity, and high thermal and mechanical stability.25-28 For instance, mesoporous alumina was synthesized by Fulvio and coworkers using commercial boehmite in the presence of Pluronic P123.25 They showed that the boehmite-derived mesoporous alumina (BMA) exhibits better adsorption properties and higher thermal and chemical stability in comparison to the alkoxide-based ordered mesoporous alumina.25 Boehmite is one of major raw materials in industry, largely used for the preparation of alumina catalysts and supports.26-28 The BMA materials also exhibited larger amount of basic sites as evidenced by carbon dioxide (CO2) temperature programmed desorption (TPD). Thus, it would be interesting to develop composite sorbents based on the boehmite-derived mesopores alumina because alumina alone shows relatively high affinity toward CO2, which in combination with amine (amidoxime) grafting can enlarge CO2 uptake at ambient (1 atm, 25 oC) and flue gas (1 atm, 60-120 oC) conditions. Additional benefits of these materials are good thermal stability and high resistance towards corrosion under flue gas conditions. Here we report a simple synthesis of porous composite sorbents consisting of the boehmite-derived alumina in combination with amidoxime surface groups, and examine their surface area, porosity, and affinity toward nitrogen and CO2 adsorption. In this study, boehmite and (3-cyanopropyl)triethoxysilane (CPTS) were used as alumina and silica precursors, respectively, and inexpensive and biodegradable Pluronic P123 triblock copolymer was employed as a soft template, which at the final stage was


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removed under controlled thermal treatment.8 First, mesoporous organosilica with cyanopropyl groups was prepared by a one-pot soft-templating synthesis, and subsequently hydroxylamine hydrochloride (NH2OH.HCl) was employed to convert the aforementioned cyano groups into amidoxime ones under suitable reaction conditions (see Scheme S1 in supporting information). To the best of our knowledge, this study represents the first attempt toward preparation of mesoporous boehmite-derived aluminaorganosilica composites with cyanopropyl and amidoxime-groups for investigation of their sorption affinity toward CO2 at ambient and elevated temperatures. EXPERIMENTAL A detailed information including chemicals used in the synthesis and characterization of the composite alumina-organosilica samples using nitrogen adsorption, transmission electron microscopy (TEM), thermogravimetry (TG), CHNS analysis, solid state 27Al MAS-NMR, 1H-29Si NMR, and 1H-13C cross polarization NMR spectroscopy are included in the supporting information. Also, information about CO2 sorption measurements at room (25 oC) and elevated (120 oC) temperatures is also provided in the supporting information. Preparation of Mesoporous Composites Synthesis of the boehmite-derived alumina samples was analogous to that reported previously.25 Namely, 1.20 g of boehmite powder was dispersed in 20 cm3 of water acidified with 0.13 cm3 of concentrated nitric acid at 70 oC for 1 h. The resulting mixture was transferred to the polymer solution obtained by dissolving 2 g of Pluronic P123 in 30 cm3 of ethanol (200 proof) for 2 h at room temperature. The boehmite dispersion and polymer solution were mixed for additional 4 h and then the predetermined amount of CPTS (2, 5, 13 mmoles) was added to the mixture. The resultant mixture was further stirred for 24 h and aged at 100 oC for additional 24 h. The resulting solution was kept for another 6 h to facilitate the evaporation induced self-assembly (EISA). The resulting solid was collected and dried overnight in oven at 100 oC. The samples were heated in a horizontal quartz tube furnace at 370 oC for 2 h in flowing N2 with a heating rate of 2 oC / min. The amidoximation reaction (see Scheme S1 in Supporting information) was performed by employing the


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same recipe as that used previously for cyanopropyl-grafted mesoporous carbons and silica,8,9 which involves a simple treatment of the cyanopropyl-containing samples with 16.6 % hydroxylamine hydrochloride in a 50/50 H2O/MeOH solution at pH=8 (pH was adjusted with NaOH) and 80 oC for 24 h under reflux conditions followed by washing the product with methanol.29 The resulting alumina-organosilica samples are labeled as Bh-CPX where Bh and CP refer to boehmite and CPTS precursors used in the synthesis, respectively, X denotes the number of mmoles of the CPTS organosilane in the synthesis mixture. In the case of the as-synthesized samples asterisk * at the end of the sample code is added. The number of mmoles of CPTS in the initial reaction mixture was 2, 5 and 13. For instance, Bh-CP2 refers to the sample synthesized using 2 mmoles of CPTS and boehmite and subjected to the thermal treatment at 370 oC. All samples were calcined at 370 oC. For the purpose of comparison one boehmite-derived alumina sample was prepared using similar precursors and procedure without adding (3-cyanopropyl)triethoxysilane (CPTS). This sample (Bh$) was also heated in a horizontal quartz tube furnace at 370 oC for 2 h in flowing N2 with a heating rate of 2 oC / min. The Bh-CPX samples subjected to an additional post-synthesis modification to convert cyanopropyl groups to amidoxime (AO) groups (see Scheme S1 in supporting information) were denoted by Bh-AOX, where Bh and AO refer to the boehmite used in the synthesis and amidoxime groups, respectively, and X denotes the number of mmoles of CPTS in the synthesis mixture. For instance, Bh-AO2 refers to the sample obtained by post-synthesis modification of the Bh-CP2 sample with NH2OH.HCl to create amidoxime groups. All calcined Bh$, Bh-CPX, and Bh-AOX samples were also subjected to an additional pre-treatment (see supporting section) before CO2 temperature programmed desorption (TPD) analysis, which included CO2 pulse chemisorption. The samples subjected to the CO2 pulse chemisorption at 120 oC are labeled as Bh$-CO2, Bh-CPX-CO2, and Bh-AOX-CO2 (X=2, 5, 13), respectively.


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RESULTS AND DISCUSSION Properties of alumina-organosilica samples with cyanopropyl and amidoxime groups. Thermal stability of the Bh-CP2*, Bh-CP2, Bh-AO2, and Bh-AO2-CO2 samples was analyzed by using highresolution thermogravimetry (TG) and differential thermogravimetry (DTG) profiles recorded in flowing nitrogen. Removal of the template from as-synthesized samples with cyanopropyl groups, the conversion of these groups to amidoxime ones via reaction with NH2OH.HCl, and thermal stability of amidoxime-containing samples upon CO2 sorption were monitored by high resolution thermogravimetry.

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Figure 1. DTG curves for the as-synthesized (Bh-CP2*) and calcined (Bh-CP2) cyanopropyl-containing organosilica mesostructures, and for amidoxime-containing samples without (Bh-AO2) and with (BhAO2- CO2) chemisorbed CO2 (left panel), and DTG curves for the Bh$ and Bh-AOX (X=5,13) samples (right panel). Figures 1 and S1 (supporting information) display respectively the DTG and TG profiles obtained for the boehmite-organosilica composites studied. For the as-synthesized sample (Bh-CP2*) three important thermal events can be observed on the DTG profile at 25-110 oC, 250-340 oC, and 450500 oC (see the DTG curve for Bh-CP2* in Figure 1), which are analogous to those previously reported for cyanopropyl-containing organosilica (sample without boehmite).8,9 The first, second, ACS Paragon Plus Environment

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and third peaks are, respectively, responsible for evaporation of adsorbed water, thermodegradation of P123 block copolymer template, and decomposition of cyanopropyl groups.8,9 Note that a broad peak observed at 350-650 oC for the Bh$ sample synthesized only from boehmite, reflecting its dehydroxylation, can also contribute to the second weight lost (see the DTG curve for Bh$ in Figure 1 (right panel). A complete disappearance of the peak related to the removal of P123 template via calcination can be observed on the DTG profile recorded for Bh-CP2. This confirms the effective and complete removal of the polymer template upon calcination at 370 oC.8,9 In fact, it was shown in earlier studies that 370 oC is a reliable temperature to remove template without decomposition of cyanopropyl and amidoxime functional groups.8,9 The amidoxime-modified Bh-AO2 sample exhibits two DTG peaks at 100-230 oC and 400-460 oC (see Figure 1). These peaks are caused by desorption of adsorbed water and decomposition of amidoxime groups, respectively. The DTG curve for the Bh-AO2 sample with chemisorbed CO2 (Bh-AO2-CO2) is also displayed in Figure 1. A comparison of the DTG profiles recorded for Bh$, Bh-AO5, and Bh-AO13 samples is shown in Figure 1 (right panel). The Bh$ sample exhibits two peaks at 25-150 oC and 350-650 oC, respectively, reflecting the removal of water and dehydroxylation of the surface OH groups. The observed increase in the DTG peak intensity at ~490 oC for Bh-AO13 in comparison to the Bh-AO5 sample is due to the high loading of amidoxime groups.8,9 Note that dehydroxylation of the Bh-AO5 and Bh-AO13 samples, similar as in the case of Bh$, can also contribute to the observed thermal event (see TG profile in Figure S2 in supporting information). 1

H-13C CP/MAS NMR spectra of the selected samples, Bh-CP5 and Bh-AO5, are shown in Figure 2a. In

the case of the Bh-CP5 sample four resonance peaks at 13.0, 20.2, 38.8, and 120.5 ppm are observed. Peak at 13.0 ppm can be assigned to methylene carbon atom (-CH2-Si) directly connected to silicon. Additional peaks at 20.2 and 38.8 ppm can be attributed to methylene carbon (-C-CH2-C-) located in the middle position and to the methylene carbon (-C-CH2-CN) linked to cyano group, respectively. The


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Figure 2. a) 1H-13C-CP/MAS NMR and b) samples.

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Al-MAS NMR spectra of the Bh-CP5 and Bh-AO5

peak visible at 120.5 ppm indicates the presence of carbon in cyano group (-C≡N). Similarly to Bh-CP5, the 1H-13C CP/MAS NMR spectrum obtained for Bh-AO5 shows peaks at 13.4, 20.2, and 38.7 ppm that can be assigned to methylene carbon connected to silicon, methylene carbon located in the middle position, and methylene carbon linked to carbon in cyano group. The peak observed at 152.4 ppm confirms the conversion of cyanopropyl groups [-C≡N] present in Bh-CP5 to amidoxime groups [C(NH2)=N(OH)] in Bh-AO5. Note that a less intensive peak visible at 120.6 ppm indicates the presence of unreacted cyano groups.30-33 Note that the previously reported FT-IR analysis of amidoxime-modified organosilica materials 9 is fully applicable for the boehmite-organosilica composites studied in this work because in both cases the amidoxime functionality is attached to silicon atom via propyl linkage (see chemical structure in Figure 1a). 27

Al MAS NMR spectra were also recorded for selected Bh-CP5 and Bh-AO5 samples.

spectra reflect chemical environment around aluminum atoms. Figure 2b displays spectra of the Bh-CP5 and Bh-AO5 samples.

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Al NMR

Al MAS NMR

The spectrum recorded for Bh-CP5 shows two

resonance peaks at 6.7 and 68.8 ppm, respectively, related the aluminum species with octahedral and tetrahedral symmetries. Analogous spectrum was recorded for Bh-AO5. The NMR peaks visible at 7.3 and 70.2 ppm also correspond to octahedral and tetrahedral coordination of Al atoms, ACS Paragon Plus Environment

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respectively. Note that in both samples octahedral coordination dominates in comparison to the tetrahedral one.

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Al NMR spectra of both samples show similar peak intensities and positions. 29Si

MAS NMR spectra of the selected samples were recorded to better understand chemical environment around silicon atoms and stability of siloxane bonds during amidoximation reaction (Figure S3, supporting information). As can be seen from Figure S3 (supporting information), the 29Si MAS NMR spectrum of Bh-CP5 exhibits three resonance peaks at -67.5, -82.1, and -130.0 ppm attributed to T3 [RSi-(OSi)3], Q2 [Si-(OSi)2(OH)2], and Q4 [(Si(OSi)4], respectively. Similarly, the Bh-AO5 samples show peaks at -67.3, -89.2, and -131.2 ppm that can be assigned to T3, Q2, and Q4, respectively. No substantial deviation in the 29Si MAS NMR peak location and intensity observed on the spectrum of Bh-AO5 as compared to that of Bh-CP5 indicates the stability of siloxane bonds during amidoximation reaction.8,9,34 TEM and SEM images obtained for the Bh-CP5 and Bh-AO5 samples are also shown in Figures S4 and S5 (supporting information). TEM images reflect the disordered materials. The EDX spectra obtained for Bh-CP5 and Bh-AO5 also indicate the presence of aluminum in the composite mesostructures studied (Figure S6, supporting information). Table 1. Structural parameters of the Bh$, Bh-CPX, and Bh-AOX (X=2, 5, 13) samples studied. Content

Vsp Vmic SBET Wmax Vt (cm3/g) (cm3/g) (m2/g) (nm) (cm3/g) Bh$ 0.61 0.02 228 12.1 0.64 Bh-CP2 0.73 0.03 261 14.9 0.76 Bh-CP5 0.28 0.02 216 12.4 0.29 Bh-CP13 0.19 0.01 67 10.7 0.21 Bh-AO2 0.54 0.04 200 13.4 0.58 Bh-AO5 0.23 0.02 190 11.0 0.24 Bh-AO13 0.16 0.01 60 10.5 0.18 Vsp-single point pore volume calculated at the relative pressure of 0.98; Vmic–volume of fine pores (micropores and small mesopores below 3 nm) calculated by integration of the PSD curve up to 3 nm; SBET –specific surface area calculated from adsorption data in the relative pressure range of 0.05-0.20; Wmax-pore width calculated at the maximum of PSD using the improved KJS method; Vt-total pore volume calculated by integration of the PSD curve. Nitrogen adsorption isotherms measured at -196 oC for a series of cyanopropyl and amidoximecontaining samples calcined at 370 oC are shown in Figure 3 (left and right panels), respectively. The


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isotherm curves for the Bh$ and Bh-CP2 samples are type IV with distinct narrow H1 type hysteresis loops at relative pressures of about ~ 0.75-0.90 characteristic for mesoporous materials. The isotherms for the Bh-CP5 and Bh-CP13 samples are also type IV with broad H4 hysteresis loops. Nitrogen adsorption isotherms were used to obtain the basic parameters such as the specific surface area, total pore volume, and pore size, which are listed in Table 1. The nitrogen uptake obtained for the BhCP13 sample is smaller as compared with the values obtained for Bh-CP5 and Bh-CP2, indicating a decrease in the volume of mesopores. For instance, the Bh-CP2 sample exhibits the total pore volume and pore width of ~ 0.73 cm3/g and 14.9 nm, respectively, and these values are reduced to 0.19 cm3/g and 10.7 nm for Bh-CP13 sample. This reduction can be anticipated due to the geometrical constrictions combined with accommodation of a large amount of cyanopropyl groups in the mesostructure.

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Figure 3. Nitrogen adsorption isotherms for the boehmite-derived mesostructures without cyanopropyl (Bh$) and with cyanopropyl (Bh-CPX, X=2, 5, 13) groups (left panel; the isotherm curves 2, 3, 4 are shifted by 300, 700, and 900 cm3 STP/g, respectively, in relation to curve 1) and for the amidoximecontaining mesostructures (right panel; the isotherm curves 2 and 3 are shifted by 300 and 500 cm3 STP/g, respectively, in relation to curve 1).


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Similarly, all amidoxime-modified mesostructures exhibit type IV isotherms with H1/H4 hysteresis loops depending on the concentration of organic groups. Another exciting feature of amidoximemodified mesostructures is the absence of any significant alternation of the isotherm shape due to the modification used. For instance, the amidoxime-modified sample synthesized with smallest amount of CP (Bh-AO2) exhibits type IV isotherms with sharp capillary condensation-evaporation steps and distinct narrow H1 hysteresis loops starting at relative pressure of about ~ 0.75-0.85. Similarly to Bh-CP5 and Bh-CP13, the Bh-AO5 and Bh-AO13 samples feature type IV isotherms with H4 hysteresis loops. However, amidoximation causes a small decrease in the BET surface area and total pore volume as compared to the unmodified counterparts. For instance, the Bh-CP2 sample exhibits the BET specific surface area and the pore volume of ~ 261 m2/g and 0.76 cm3/g, respectively, and these values are respectively reduced to 200 m2/g and 0.58 cm3/g for Bh-AO2 after conversion of cyanopropyl groups to amidoxime groups (Table 1). CO2 Chemisorption. Temperature programmed desorption was used to evaluate the CO2 uptake for Bh$, Bh-CPX, and Bh-AOX (X=2, 5, 13) samples studied at 120 oC. Sorption properties of various acidic and basic gases have been extensively reported elsewhere.15,18,24,35-39 In our TPD experiments, the CO2 uptake on the Bh$, Bh-CPX, and Bh-AOX samples is used to identify their strength and the amount of basic sites available. Note that the thermal instability of many amines over 120 oC makes them in an unfavorable position for high temperature CO2 sorption (see introduction). In contrast, the thermal stability of amidoxime (AO) functional groups is better, which make them better suited for CO2 sorption at elevated temperatures (>60 oC). The CP and AO functionalities are thermally stable over 400 oC (see Figure 1), which could be expected mainly because of cyano (-C≡N) and amine/oxime [(NH2)/=NOH)] species are in -CH2-CH2-C≡N and CH2-CH2-C(NH2)=NOH groups in the alumina-organosilica mesostructures studied. The CO2 sorption experiments used consist of three steps including pretreatment, pulse CO2 chemisorption, and temperature programmed desorption. Pretreatment of the samples was carried out using inert He gas


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from room temperature to 370 oC to remove all impurities present on the surface of the materials prior to chemisorption. Materials were then subjected to pulse chemisorption followed by desorption by ramping temperature up to 370

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Figure 4. CO2 TPD (pulse at 120 oC) profiles recorded for the Bh$ and Bh-CPX (left panel) and BhAOX (right panel) samples. Table 2. Comparison of CO2 sorption values at 25 and 120 oC for the Bh$, Bh-CPX, and Bh-AOX samples studied. Sample

Max CO2 CO2 uptake at Micropore sorption at 25oC and 1.2 volume 120 oC atm (mmol/g) (cm3/g) (mmol/g) $ Bh 2.17 0.58 0.02 Bh-CP2 2.23 0.64 0.03 Bh-CP5 2.28 0.56 0.02 Bh-CP13 2.31 0.45 0.01 Bh-AO2 3.09 0.70 0.04 Bh-AO5 3.51 0.52 0.02 Bh-AO13 3.84 0.45 0.01 N (%), C (%), and H (%) obtained by CHNS analysis.

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C (%)

H (%)

1.09 2.58 5.46 1.25 2.83 6.12

4.44 9.58 15.62 23.93 11.30 13.67 23.72

1.80 2.12 2.66 3.36 2.52 2.62 3.61


13


The most of chemisorbed CO2 at 120 oC can be desorbed in the temperature range of 290-345 oC regardless of the samples studied (Figure 4). Figure 4 (left panel) exhibits the CO2 desorption profiles obtained for the

Bh$, Bh-CP2, Bh-CP5, and Bh-CP13 samples, which were used to

evaluate the CO2 uptake at 120 oC (Table 2). A broad peak observed on the CO2 desorption profiles of the aforementioned samples reflects the presence of basic sites with different strengths. The Bh$, Bh-CP2, Bh-CP5, and Bh-CP13 samples display distinct peaks at ~330, 331, 332, and 332 oC, respectively. As can be seen from Table 2, there is no significant increase in the CO2 uptake on BhCP2, Bh-CP5, and Bh-CP13 samples as compared with that of the Bh$ sample. For instance the Bh$ and Bh-CP13 samples show the lowest and highest CO2 sorption capacities of about 2.17 and 2.31 mmol/g at 120 oC, respectively (see Table 2, and Figure 5, left panel). o

120 C

o

120 C 25 C

2.5 2.0 1.5 1.0 0.5 0.0

0

50 60 30 40 20 10

%) CP (X

-CPX in Bh

ol/g) CO2 Uptake (mm

o

o

ol/g) CO2 Uptake (mm

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

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25 C

4 3 2 1 0

0

60 40 50 30 10 20 X h-AO

CP (X

% ) in

B

Figure 5. CO2 sorption capacity change with increasing amount of CPTS used in the synthesis of the Bh$ (X=0), Bh-CPX (X=2, 5, 13) (left panel) and Bh-AOX (X=2, 5, 13) (right panel) samples at 25 and 120 oC. A relatively small difference (~ 0.14 mmol/g) in the aforementioned sorption capacities revels that there is no significant sorption of CO2 on cyanopropyl groups (Table 2, and Figure 5, left panel). Thus, it can be concluded that the most of CO2 uptake measured for the Bh-CP2, Bh-CP5, and Bh-


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CP13 samples is due to the CO2 sorption on aluminum and hydroxyl species (Scheme S2 in supporting information). The presence of three different types of hydroxyl groups such as terminal, bi-bridged, and tribridged hydroxyl groups on the alumina surface up to 600 oC can be responsible for CO2 chemisorption at elevated temperature.35,36 Terminal OH groups are capable to form hydrogen carbonate upon CO2 immobilization as illustrated in Scheme S2 (top panel in supporting information).35,36,40,41 The acid-base pair sites present on the alumina surface (Al3+-O2-) can create bidentate carbonate complexes (see Scheme S2, bottom panel in supporting information). Note that acidic (Al3+) and basic (O2-) sites on the surface of the Bh$ and Bh-CPX (X=2,5,13) composites can effectively enhance the CO2 sorption capacity. Donation of electrons from electron rich oxygen atoms of CO2 can possibly reduce the Lewis acidity of Al3+ (so increase the Lewis basicity of Al3+) and thus create very favorable environment for CO2 chemisorption.35,36,40,41 (top and bottom panels in Scheme S2 in supporting information). On the other hand, the maximum CO2 desorption temperature is similar for all Bh-AO2, Bh-AO5, and Bh-AO13 samples (Figure 4 right panel). The CO2 desorption temperature for these samples is shifted to slightly lower temperature (319 oC) as compared with that of the corresponding samples with cyano groups. Moreover, the CO2 sorption capacity is significantly higher for the samples with amidoxime groups obtained by converting cyanopropyl groups. For instance, the CO2 uptake changes from 2.23 mmol/g for Bh-CP2 to 3.09 mmol/g for Bh-AO2 at 120 oC (see Table 2, Figure 5). The CO2 sorption capacity is enlarged by increasing the concentration of amidoxime groups in the composite samples. For instance, the CO2 uptake changes from 3.09 mmol/g for Bh-AO2 to 3.84 mmol/g for Bh-AO13 at 120 oC (see Table 2 and Figure 5 right panel). The overall sorption capacity increases in the following order: Bh-AO2< Bh-AO5< Bh-AO13 (Figure 5 right panel). Unlike in the samples with cyano groups, the area of the desorption peaks obtained for the corresponding samples with amidoxime groups increases with increasing concentration of these groups (compare Figures 4 and


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5). Note that the CO2 uptake at 120 oC measured for Bh-AO2, Bh-AO5, and Bh-AO13 is higher by 0.76, 1.23, and 1.53 mmol/g, respectively, than the corresponding values for the Bh-CP2, Bh-CP5, and Bh-CP13 samples. This finding indicates clearly the benefit of amidoxime sites for CO2 sorption as compared to cyanopropyl groups. In addition to the studies on the interaction of CO2 with alumina, there are numerous reports explaining the mechanism of CO2 sorption on the materials with amine groups.42,43 However, there is no report on the interaction between cyano (-C≡N) groups and CO2. Dankwerts and Caplow studied the interaction between different amines and CO2 and proposed a reasonable mechanism for CO2 sorption.42,43 It was shown that the stoichiometry between amine and CO2 depends on the absence or presence of water. In the presence of water, one mole of CO2 reacts with one mole of amine to form bicarbonate structure. Overall this reaction can be written as follows; R1R2NH+ + HCO3-

CO2 + R1R2NH + H2O

where R1 or R2 symbols represent H and aryl/alkyl groups, respectively, in the case of primary and secondary amines. However, under anhydrous conditions, one mole of CO2 reacts with two moles of amine groups. In this case, CO2 can react with primary and secondary amines to form carbamate structure as illustrated below. CO2 + 2R1R2NH

R1R2NH2+ + R1R2NCO2-

This overall reaction consists of two steps where CO2 react with one amine to form zwitterionic intermediate followed by deprotonation as indicated below. CO2 + R1R2NH

R1R2NH+CO2-

R1R2NH+CO2-+R1R2NH

R1R2NH2+ +R1R2NCO2-

However due to the lack of H required in the deprotonation step, there is no reaction between tertiary amine and CO2. In our case, amidoxime groups present in the samples can be considered as a primary amine, which can bind CO2 according the mechanism illustrated above. It is possible to


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break the N-H bond of amidoxime and trap proton by neighboring amidoxime group available in the samples studied.13,14 It is known that two amine groups in a close proximity create a very favorable environment for interaction with carbon dioxide to form carbamate structure in comparison to the isolated single amine (in this case amidoxime) group (Scheme S3 in supporting information).13,14,42 The amidoxime groups in modified alumina materials create a very favorable environment for CO2 sorption because of the presence amidoxime groups and the terminal, bi-bridged and tri-bridged hydroxyl groups. This observation is confirmed by high CO2 sorption capacities observed on the sorbents studied at elevated temperatures (see Table 2). Table 3. Comparison of the CO2 uptake values at elevated temperatures reported in literature for various sorbents with the data obtained in this work. Material Sorption Maximum CO2 Reference Temperature Uptake (oC) (mmol/g) MgO/Al2O3composites Al-supported metal (Ca,Mg,Ce,Cu,Cr) oxides Na oxide-based sorbents Zeolite-based sorbents Al incorporated organosilica PEI/MCM 41 Amidoxime (acetamidoxime/polyamidoxime) Amidoxime-modified mesoporous silica Amidoxime-modified mesoporous silica Alumina-zirconia silica composites Alumina-zirconia silica composites Alumina –silica with amidoxime groups

60 120 315 120 120 75 70 120 60 60 120 120

1.36 1.80 3.02 1.20 2.20 3.02 2.71 3.07 3.28 3.02 2.76 3.85

[18] [24] [15] [37] [44] [45] [10] [8] [8] [46] [46] This work

A comparison of the CO2 uptake values obtained in this work with previously reported data for amine impregnated/grafted materials, metal (Al, Mg, Ca, K) oxide composites, metal organic frameworks (MOF), and zeolite-based solid sorbents is provided in Table 3.8,15,18,24,37,44-46 For instance, Li and co-workers examined MgO/Al2O3 composite for CO2 sorption at 60 oC. The maximum CO2 uptake obtained for MgO/Al2O3 composites under hydrous and anhydrous conditions was 0.97 and 1.36 mmol/g, respectively.18 Cai and co-workers studied alumina and alumina-based metal (Ca,Mg,Ce,Cu,Cr) oxides for CO2 sorption at 120 oC and reported the CO2 uptakes in the range from 0.24 to 1.80 mmol/g depending on the combination and composition of metal oxides.24 ACS Paragon Plus Environment

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Sodium-containing sorbents investigated by Siriwardena and co-workers for CO2 sorption at 315 oC were able to capture up to 3 mmol/g of CO2.15 The same authors measured the CO2 uptake on the zeolite-based sorbents at 120 oC, which was of about 0.7 and 1.2 mmol/g at 1 and 20 bar, respectively.37 The CO2 sorption measured on mesoporous alumina-supported amines at 120 oC was of about 2.2 mmol/g.44 The PEI-impregnated MCM-41 with 75 wt % of PEI loading, studied by Xu and co-workers for CO2 sorption at 75 oC, captured of about 3.02 mmol/g of CO2.45 Amidoxime and its derivatives like acetamidoxime and polyamidoxime were applied for CO2 sorption by Zulfiqur and coworkers at 43 and 70 oC.10 The maximum CO2 uptakes of about 1.64 and 2.71 mmol/g at 43 and 70 oC, respectively, were reported.10 Recently, amidoxime-modified silica and related mesostructures were synthesized and tested for CO2 sorption at 120 oC, achieving the CO2 uptake of about 3.0 mmol/g.8,46 Thus, the above mentioned data indicate that the CO2 uptake of 3.84 mmol/g, reported in this work, is much higher than the aforementioned uptake values. Table 4. CO2 adsorption at 25 oC for the Bh$, Bh-CPX, and Bh-AOX samples studied. Samples

SBET CO2 uptake n*CO2 ASA 100ASA (m2/g) at 25oC and /SBET 1.2 atm nCO2 (mmol/g) (µmol/m2) (m2/g) (%) Bh$ 228 0.58 2.54 76 33 Bh-CP2 261 0.64 2.45 84 32 Bh-CP5 216 0.56 2.59 74 34 Bh-CP13 67 0.45 6.72 59 88 Bh-AO2 200 0.70 3.50 92 46 Bh-AO5 190 0.52 2.74 68 36 Bh-AO13 60 0.45 7.50 59 98 SBET -specific surface area calculated from adsorption data in relative pressure range 0.05-0.20; nCO2 number of moles of CO2 adsorbed per gram of the sample; n*CO2– number of moles of CO2 adsorbed per unit surface area of the sample; ASA – active surface area obtained from nCO2 by using 0.218 nm2/molecule (average value of the cross-sectional area of CO2 reported by McClellan and Harnsberger in J. Colloid Interface Sci. 1967, 23, 577-599); 100ASA/SBET – percentage of active surface area. CO2 Physisorption. Besides CO2 chemisorption at 120 oC, the CO2 adsorption at 25 oC and 1.2 atm on the boehmite-derived alumina-organosilica mesostructures was also investigated. Figure S7 (supporting information) display the CO2 uptake measured on the Bh$, Bh-CPX and Bh-AOX (X=2,


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5, 13) samples. Table 4 lists the CO2 uptake values for those samples measured at 25 oC. The CO2 uptake values were also used to calculate the number of moles of adsorbed CO2 per unit surface area, and to estimate the active surface area, and the percentage of active surface area (Table 4). For the purpose of comparison, Table 2 summarizes the micropore volume, N (%), and CO2 uptake values for the samples studied at 25 and 120 oC. The CO2 uptake at elevated temperatures (≥60 oC) occurs via chemisorption, whereas sorption at ambient conditions (≤ 25 oC) is governed through physisorption. As shown in Table 2, the CO2 uptake at 120 oC depends on the amidoxime content in the mesostructures studied. Higher CO2 uptakes at 120 oC are reported for the samples with higher concentration of amidoxime groups (Figure 5 right panel). For instance, the CO2 uptake increases in the following order: Bh-AO13> Bh-AO5> Bh-AO2. Note that N (%) increases from Bh-CPX to BhAOX for X=2, 5, and 13, which provides an additional evidence for conversion of cyano groups in amidoxime ones and shows the importance of nitrogen species in CO2 chemisorption. However, in the case of CO2 sorption at ambient conditions various basic sites such as amine and amidoxime are less important and microporosity becomes the dominating factor. For instance, the volume of micropores increases in the following order: Bh-AO2>Bh-AO5>Bh-AO13 (Table 2) and the same order is observed for CO2 uptake at ambient conditions. The highest CO2 uptake of about 0.70 mmol/g was observed for the Bh-AO2 sample with highest microporosity (0.04 cm3/g), while Bh-AO13 (microporosity 0.01 cm3/g) showed the lowest CO2 sorption capacity of about 0.45 mmol/g. Although, a similar CO2 uptake was obtained for the Bh-CP2 (2.23 mmol/g), Bh-CP5 (2.28 mmol/g), and Bh-CP13 (2.41 mmol/g) samples at 120 oC (Figure 5 left panel), the CO2 uptake measured at 25 oC increased in the order predicted by microporosity: Bh-CP2 >Bh-CP5>Bh-CP13 (Figure S7 left panel). The currently available CO2 capture technology is based on aqueous solutions of amines such as MEA (monoethanolamine), DEA (diethanolamine), diglycol amine (DGA) and some of their derivatives,47,48 which as indicated in introduction has some drawbacks. Thus, after several CO2 absorption-regeneration cycles, liquid amines need to be replaced, which makes the scrubbing ACS Paragon Plus Environment

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process expensive.47,48 Although the TPD-like regeneration process is somewhat energy intensive and costly, it is advantageous as compared with liquid amine scrubbers. For instance, aluminum species in the materials studied are corrosion resistant and thermally stable. In addition, cyanopropyl and amidoxime functionalities are stable up to 400 oC. Although the Bh-AOX materials show excellent CO2 uptakes at 120 oC, the sorption capacity is not only the factor to determine their suitability for commercial applications. Other factors such as the synthesis cost, cycle stability, heat of adsorption (adsorption enthalpy), and CO2 over N2 selectivity should be considered too. As discussed in introduction, triblock copolymer Pluronic P123 and boehmite are inexpensive, which can make this synthesis economically feasible. Note that the total cost of the current synthesis is about $ 8.7 per 1 kg, which is considered to be economically acceptable (< $10/kg).48 The stability of the selected Bh$, Bh-CP5, and Bh-AO5 samples was examined up to eleven cycles and only small reduction in the CO2 uptake (

Mesoporous Alumina with Amidoxime Groups for CO2 Sorption at

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