Molecular Layer Deposition-Modified 5A Zeolite for Highly Efficient

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Molecular Layer Deposition (MLD) Modified 5A Zeolite for Highly Efficient CO2 Capture Zhuonan Song, Qiaobei Dong, Weiwei L. Xu, Fanglei Zhou, Xinhua Liang, and Miao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16574 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Molecular Layer Deposition (MLD) Modified 5A Zeolite for Highly Efficient CO2 Capture Zhuonan Songa, Qiaobei Dongb, Weiwei L Xua, Fanglei Zhoub, Xinhua Liangc and Miao Yub* a.

Department of Chemical Engineering, University of South Carolina, SC, 29208

b.

Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy,

NY 12180, USA. c.

Department of Chemical and Biochemical Engineering, Missouri University of Science and

Technology, Rolla, MO 65409, USA KEYWORDS: Molecular layer deposition; Pore misalignment; CO2 capture; CO2/N2 adsorption selectivity; CO2 adsorption rate

ABSTRACT: Effective pore mouth size of 5A zeolite was engineered by depositing an ultrathin layer of microporous TiO2 on its external surface and appropriate pore misalignment at the interface. As a result, slightly bigger N2 molecule (kinetic diameter: 0.364 nm) was effectively excluded, whereas CO2 (kinetic diameter: 0.33 nm) adsorption was only influenced slightly. The prepared composite zeolite sorbents showed ideal CO2/N2 adsorption selectivity as high as ~70; a four-fold increase over uncoated zeolite sorbents, while maintaining high CO2 adsorption capacity (1.62 mmol/g at 0.5 bar and 25 oC) and fast CO2 adsorption rate.

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1. INTRODUCTION Carbon dioxide is considered as one of the major greenhouse gases that contributes to the global warming, ocean acidification, and other environmental concerns.1-3 The concentration of CO2 in the atmosphere has increased by 40% from 280 ppm to 400 ppm over the past century.4 In 2013, Intergovernmental Panel on Climate Change (IPCC) predicted that by 2100, the global temperature will increase another 1.9 °C.4 The rapid increasing trend of CO2 emission, resulting from our heavy use of fossil fuels,5 may not be alleviated within next several decades if appropriate CO2 capture strategies are not adopted. To reduce CO2 emission, CO2 can be captured depending on different processes: post-combustion capture where CO2 is separated from the other components of the flue gas, pre-combustion capture with removal of CO2 from the fuel prior to combustion, and oxy-combustion where the fuel is burned in an oxygen stream.6 Among them, post-combustion CO2 capture from flue gas (predominantly CO2/N2 separation) has been identified as an attractive and immediate route for mitigating the escalating concentration of atmospheric CO2, and may also be readily retrofitted to existing power plants.3, 7-9

However, the biggest challenge at present is the large energy and cost penalty.2 Thus, cost-

effective, large scale separation technologies play crucial role for CO2 mitigation to become viable.10 Absorption processes using aqueous alkanolamine solutions (amine scrubbers) have been extensively studied and widely employed in conventional CO2 removal. The amine scrubbing captures CO2 with a high selectivity, but intensive energy input is required to break the C-N bond formed between CO2 and the amine functionality for regeneration.9, 11 Additionally, the amine solutions are also severely corrosive toward the vessels in which they are contained.12


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Membrane-based separation processes have emerged as an effective alternative in postcombustion CO2 capture.13 Membrane performance, however, still needs to be improved to overcome the trade-off between permeability and selectivity.13 Meanwhile, the low pressure of CO2 in the feed stream and thus low driving force for its permeation may limit the application of membranes in post-combustion CO2 capture.2 Relative to above mentioned methods, selective CO2 adsorption using porous adsorbents has demonstrated great potential for reducing energy and cost for CO2 capture.14-15 In the adsorption processes, the separation is usually based on the size and shape of the molecules to be separated, and/or their interaction with the adsorbent.16 With regard to post-combustion CO2 capture, various materials have been prepared and studied as adsorbents. For instance, amine-grafted silica was found to exhibit high affinity toward CO2 and good stability.17-18 However, these materials are typically prepared as powder that can be difficult to implement at a viable industrial scale for CO2 capture owing to the high pressure drops and poor heat and mass transport.19 In recent years, considerable efforts have been expended on the synthesis of metal organic frameworks (MOFs) for use as CO2 adsorbents.20 MOFs, emerging class of hybrid materials with porous structure, combine the connectivity of metal centers with the bridging organic ligands. Conceptually, MOFs can be synthesized to possess tunable pore sizes, large surface areas and specific adsorption affinities based on how building blocks bond together.21 For example, a series of CO2-selective MOF adsorbents were prepared by the research groups of Matzger and Eddaoudi, showing excellent CO2 selectivity and uptake at low pressure and ambient temperature.22-24 High cost of the organic ligands may limit the large scale applications of MOFs. The retail prices of the recent four commercialized MOFs from BASF are $10,000-15,000/kg.25 As a significantly cheaper porous material ($0.030.12/kg),26 zeolites have drawn lots of attention as adsorbents for CO2 capture for decades.


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Among them, small pore zeolites, such as CHA and NaA, have showed decent CO2/N2 separation selectivity (15-60) for a hypothetical flue gas stream, but the slow diffusion of gases throughout the materials may limit their practical application.27 Medium-sized pore zeolites, especially 5A and 13X, are considered as the promising adsorbents due to high CO2 adsorption capacity at low pressure, fast kinetics, and excellent thermal, chemical and mechanical stability.28-31 Their adsorption selectivity of CO2 over N2, however, is still low, typically in the range of 15-22 at 50 kPa and 25 oC.14,

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In this study, we are interested in optimizing the

adsorptive separation performance of medium-sized pore 5A zeolite by improving the CO2/N2 selectivity through pore mouth engineering, while maintaining a high CO2 capacity and fast uptake kinetics. In previous work, our group has shown a new method to improve adsorptive selectivity of zeolites using molecular layer deposition (MLD) technique.33-34 MLD, a gas phase deposition method, is a subset of atomic layer deposition (ALD), capable of depositing ultrathin conformal organic-inorganic hybrid coatings on various substrates. The obtained hybrid coatings can be converted into porous coatings by removing the organic compounds.35-36 The improved adsorptive separation performance of MLD coated zeolites was attributed to the narrowed zeolite pore mouth, resulting from the misalignment between MLD coating pores and zeolite pores, as shown schematically in Fig. 1a. The pore misalignment originates from the thermal interfacial shear stress during calcination, resulting from the different thermal expansion coefficients between zeolite and the MLD coating. We have shown that the extent of pore misalignment (or relative displacement) at the interface can be controlled by the MLD coating thickness, when calcination condition was fixed. This is consistent with Nassar’s analytical modelling study for composite materials.37 However, when the resistance, namely chemical bonding between the


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MLD coating and zeolite surface, balances the thermal shear force, the relative movement between the MLD coating and zeolite surface stops, and thus no further relative displacement was observed.34 In Pan’s recent study,38 it was found that the interfacial displacement of composite materials can be further influenced by varying calcination conditions. In this work, we selected 5A zeolite as a promising CO2 capture sorbent, and explored the potential of MLD coating on fine-tuning the pore entrance of 5A zeolite by controlling the calcination temperature and calcination residence time and the adsorptive separation performance of the resulting MLD modified composite 5A sorbent.

2. Experimental Section 2.1 Materials Ethylene glycol (99%, HO(CH2)2OH;) was obtained from Alfa Aesar. Titanium tetrachloride (99.9%, TiCl4) was obtained from Sigma Aldrich. 5A zeolites were obtained from W.R. Grace & Co.-Conn. 2.2 Characterization Field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus) was used to observe the structure of 5A zeolite crystals. Elementary composition of zeolites before and after MLD was analyzed by energy dispersive spectroscopy (EDS) using FESEM coupled with an EDAXS detector and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD instrument equipped with a monochromated Al Ka X-ray source and hemispherical analyzer capable of an energy resolution of 0.5 eV). The infrared (IR) spectra were taken on a Bruker equinox 55 in a diffuse reflection mode. A Praying Mantis


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diffuse reflection compartment was used to allow the IR beam to be reflected on powder samples. For each spectrum, 64 scans were collected to ensure high signal to noise ratio. The IR spectra were scanned in the range of 4000-1000 cm-1 with resolution of 4 cm-1. Pore size distribution of MLD coated zeolite adsorbents were calculated using Ar adsorption branch of the isotherms measured at -196 ºC using a Micrometeritcs ASAP 2020 unit. Prior to adsorption measurement, samples were degassed in situ at 170 °C overnight. 2.3 Titanium Alkoxide MLD Coating Zeolites were firstly outgassed at 200°C for 4 h for removing the adsorbed water. The titanium alkoxide MLD coatings (-Ti-O-CH2-CH2-O-Ti-) were prepared by using TiCl4 and EG as precursors. Each MLD cycle started with 240 s vacuum. TiCl4 was then introduced into the reactor until a pressure of 150 mTorr and settled for 120 s, and then 240 s vacuum was followed to evacuate extra unreacted TiCl4. Ultrahigh purity N2 (Airgas) was used to further clean the reactor with a flow rate of 20 sccm for 30 sec controlled by a mass-flow controller. Then 240 s vacuum was applied to evacuate N2. After that, EG was diffused into the reactor until a pressure of 50 mTorr and then settled for 120 s, followed by 240 s vacuum to evacuate extra unreacted EG. Ultrahigh purity N2 was used as the purge gas again. Then 240 s vacuum was applied to evacuate N2. This whole process finishes one MLD cycle, totally 60 cycles of titanium alkoxide MLD coatings were deposited on the zeolites at 100 °C. Then the coated samples were heated in air from room temperature to different elevated temperature (200 °C, 250 °C, and 350 °C) at a rate of 1°C/min, kept at elevated temperature for different residence time (1 min, 2 h, 4 h, and 8 h) and then cooled to room temperature at the same rate. 2.4 CO2 and N2 adsorption isotherms measurement


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Ultra-high purity CO2 (99.999%), and N2 (99.999%) were purchased from Airgas. Gas adsorption isotherms were measured by a volumetric method using a home-built adsorption system. Sorbent was firstly outgassed at 200 oC for 2 h. Helium was then used to calibrate the volume of adsorption cell with sorbent at 25 oC. After vacuum to remove residue gasses in the adsorption system, interested gases were introduced at 25 oC. The pressure change was collected in real time using a Swagelok E model transducer and LabVIEW 2012 software. 2.5 CO2 adsorption kinetics measurement 5A and 5A-MLD-250-2h (~0.065g) samples were placed in the adsorption system. CO2 was introduced at an initial pressure of 0.78 and 0.74 bar, respectively. The final pressure was ~0.5 bar after CO2 reached adsorption equilibrium. The system pressure change due to CO2 adsorption was collected in every 0.3 second using a Swagelok E model transducer and LabVIEW 2012 software. 2.6 Obtaining propylene transport discursivities from uptake data The propylene diffusivity was estimated by fitting the initial linear uptake data with an approximate analytical simplified solution given by Kaerger and Ruthven for short times: 6

= √

Where D is the Fickian diffusivity, and r is the edge length of the cubic crystal (~2 µm), is the mass adsorbed at time t, and is the mass adsorbed at equilibrium. Propylene (>99%) was

purchased from Sigma Aldrich. 5A, 5A-MLD-250-2h, 5A-MLD-250-8h and 5A-MLD-350-2h (~0.065g) samples were placed in the adsorption system. Propylene was introduced at an initial


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pressure of ~1 bar. The system pressure change was collected in every 0.3 second using a Swagelok E model transducer and LabVIEW 2012 software.

3. RESULTS AND DISCUSSION Field emission scanning electron microscopy (FESEM) image shows that 5A zeolite crystals are cubic with an average size of 2 µm (Fig. S1). A conformal hybrid MLD coating with a thickness of approximately 70 nm was deposited on 5A zeolite crystals by 60 cycles of MLD prior to calcination (Fig. 1b), which will be converted to a thinner porous coating upon removal of organic compound. The optical image of MLD coated 5A zeolite pellets is shown as the inset of Fig. 1b. No apparent difference can be seen between the uncoated 5A and MLD coated 5A by naked eyes due to the thin thickness of MLD coating. The elemental composition of MLD coated 5A zeolite was analysed by energy dispersive X-ray spectroscopy (EDS). For the uncoated 5A zeolite, O, Na, Al, Si and Ca signals were present (Fig. S2). In contrast, for the MLD coated 5A zeolite, the presence of Ti peak was clearly seen at around 4.5 keV (Fig. 1c). MLD coating on 5A zeolite was also confirmed by analyzing the surface composition of 5A zeolites before and after MLD using X-ray photoelectron spectroscopy (XPS). Spectrum of the uncoated 5A zeolite (Fig. S3) showed a strong O1s photoelectron peak at 531 eV, and NaAuger, Ca2s, Ca2p, C1s, Si2s, Al2s, Si2p, Al2p peaks at 500, 440, 349, 285, 151, 138, 99, and 73 eV, respectively. This is consistent with the EDS results (Fig. S2) and typical 5A zeolite chemical composition.39 XPS is a surface characterization technique, with a typical detection depth limit of around 10 nm.40 Therefore it is understandable that after MLD, peaks of Na, Ca, Si, and Al disappeared (Fig. 1d). Instead, O, Ti and C signals were observed at 531 eV (O1s), 460 eV (Ti2p), 33 eV (Ti3p), and 285 eV (C1s), suggesting the entire zeolite surface has been conformally covered by MLD coating.


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Figure 1. Illustration of pore misalignment at the interface between MLD coating and zeolite surface (a), TEM image of MLD coated 5A zeolite (the inset image shows the optical photograph of MLD coated 5A zeolite pellets) (b), EDS spectrum of MLD coated 5A zeolite (c) and XPS spectrum of MLD coated 5A zeolite(d).

Successful removal of the organic compound from the hybrid MLD coating by calcination was examined by IR (Fig. S4). Absorbance corresponding to the -CH2 asymmetric and symmetric stretching vibrations,41-42 in the hybrid MLD coating (-Ti-O-CH2-CH2-O-Ti-) was observed in the range of 2890-2950 cm-1 only for MLD coated 5A zeolite without calcination (5A-MLD-Uncal). After calcination at 250 oC for different times (1 min, 2 h and 8 h), -CH2 stretching vibrations disappeared. Furthermore, a small shoulder appeared at around 1580 cm-1, which is the characteristic band for –OH on TiO2,43 suggesting that the organic compound in the MLD coating was removed and the hybrid titanium alkoxide was converted to TiO2.


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Adsorption isotherms of CO2 (kinetic diameter: 0.33 nm)44 and N2 (kinetic diameter: 0.364 nm)44 were measured on 5A zeolite and MLD coated 5A zeolite up to 3 bars, as shown in Fig. S5 and S6. Figure S5a showed the adsorption isotherms on uncoated 5A zeolite, which are consistent with literature data reported by Wang.45 For MLD coated 5A zeolite, calcination was firstly carried out at different residence times (1 min, 2 h, 4 h and 8 h), while maintaining the calcination temperature at 250 oC. These samples are labelled as 5A-MLD-250-1min, 5A-MLD250-2h, 5A-MLD-250-4h, and 5A-MLD-250-8h. Corresponding abbreviations were used in the following discussion. Figure S5b-d showed adsorption loadings of CO2 and N2 decreased gradually for MLD coated 5A zeolites as the calcination residence time increased from 1 min to 4 h, although their adsorbed amounts decreased at different rates. This led to a complex adsorption selectivity change with calcination residence time (see discussion below). However, adsorption loadings of CO2 and N2 increased again after 8 h calcination at 250 oC (Fig. S2e). We then investigated the effect of calcination temperature (200 and 350 oC) while maintaining the residence time at 2 h. We found after calcination at 200 oC for 2 h, the adsorbed amounts of CO2 and N2 were still very low (90% of uncoated 5A) on 5A-MLD-250-1min (Fig. 2a) suggested that the organic compound in the MLD coatings started to decompose at a temperature between 200 and 250 oC. This is also consistent with the IR results (Fig. S4). The adsorbed amounts at 0.5 bar for 5A, 5A-MLD-250-1min, 5A-MLD-250-2h, 5A-MLD-250-4h, 5A-MLD250-8h, 5A-MLD-350-2h, and 5A-MLD-200-2h were summarized in Fig. 2a for CO2 (1.88, 1.79, 1.62, 1.04, 1.65, 1.52 and 0.021 mmol/g, respectively) and Fig. S7 for N2 (0.10, 0.090,


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0.022, 0.019, 0.074, 0.035 and 0.005 mmol/g, respectively). Apparently, different calcination conditions influenced drastically the adsorptive properties of MLD coated 5A composite sorbents. Since partial pressure of CO2 in flue gas is approximately 0.15 bar and N2 about 0.75 bar, the corresponding adsorbed amounts of CO2 and N2 and ideal selectivity were calculated and listed in Table S1. At lower CO2 pressure, the optimized 5A zeolite sorbent (5A-MLD-250-2h) showed slightly lower CO2 adsorption capacity than 5A but significantly improved CO2/N2 selectivity, demonstrating its great potential for highly selective CO2 capture from flue gas.

Figure 2. CO2 (left y-axis; black column) adsorptive capacity and CO2/N2 adsorptive selectivity (right y-axis; red column) at 0.5 bar and 25 oC on 5A and MLD coated 5A zeolite calcined at different conditions (a), and CO2 adsorption kinetic uptake curve on 5A zeolite (black symbol) and 5A-MLD-250-2h (red symbol) (b).

For MLD coated 5A zeolite with 2 and 4 h residence times at 250 oC and 2 h residence time at 350 oC, reduction of the adsorbed capacity was much larger for N2 (78%, 81% and 65%) than for


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CO2 (14%, 44% and 19%). As a result, the ideal CO2/N2 selectivity (the adsorbed capacity ratio of CO2 to N2 at 50 kPa) increased significantly from 19 (uncoated 5A) to 43 (5A-MLD-350-2h), 55 (5A-MLD-250-4h), and 74 (5A-MLD-250-2h) (Fig. 2a). Although the pore mouth size decreased, as suggested by the more effective exclusion of N2 and thus higher CO2/N2 adsorption selectivity, CO2 uptake rate did not decrease too much. As shown in Fig. 2b, for sorbent 5AMLD-250-2h, CO2 adsorbed amount reach 90% of its equilibrium amount at 0.5 bar after 1 min adsorption, only about 20 s slower than that of the 5A zeolite. This uptake time is still much shorter than that on activated carbon (~40 min),46 on HKUST-1 (~12 min),47 and on multi-walled carbon nanotubes (~9 min),48 demonstrating favourable uptake kinetics of MLD modifies 5A zeolite.

To understand the adsorption selectivity increase, we also studied the pore size

distribution of 5A zeolite and MLD coated 5A composite sorbents after calcination under different conditions by argon sorption measurements. The argon sorption isotherms and BET plots for 5A, 5A-MLD-250-2h, 5A-MLD-250-8h and 5A-MLD-350-2h were presented in Fig. S8 and Fig. S9. The isotherms showed sharp argon uptake at low relative pressure, which was expected for the microporous zeolitic materials. MLD coated zeolite sorbents showed similar surface area (Fig. S9). The pore size distribution was calculated by HK (Horvath-Kawazoe) method and shown in Fig. 3a; the sharp peak at around 0.53 nm was assigned to 5A zeolite, and the pore diameter of MLD coatings after removing organic compound was estimated to be around 0.80 nm. Moreover, the pore diameter of MLD coating did not change obviously upon changing the calcination condition. Therefore, we speculate the improved CO2/N2 selectivity should be attributed to the different extent of pore misalignment occurred at the interface between MLD coating and 5A zeolite surface under different calcination strategies; preferred extent of pore misalignment is expected to result in a smaller effective pore mouth size and thus


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increase the CO2/N2 adsorptive selectivity (Fig. S10). The decreased CO2/N2 selectivity after long time calcination (8 h at 250 oC) might be due to the unfavourable pore misalignment configuration generated at the interface and thus larger pores at the interface. To prove this assumption, we selected propylene (kinetic diameter: 0.40 nm),49 a tight fit molecule for 5A zeolite, and used its adsorption uptake kinetics to explore the small change of pore misalignment at the interface (Fig. S11). Compared with 5A zeolite, the propylene diffusivity decreased almost two orders of magnitude for MLD coated samples (Fig. 3b), suggesting the effective pore mouth size of 5A zeolite was effectively reduced by pore misalignment. For MLD coated sorbents, different propylene diffusivities resulted when calcination conditions were different with the lowest propylene diffusivity of sorbent 5A-MLD-250-2h. This indicates the extent of pore misalignment at the interface between MLD coating and zeolite surface can be tuned by controlling the calcination conditions.

Figure 3. Pore size distributions (a) for 5A zeolite, 5A-MLD-250-2h, 5A-MLD-250-8h, and 5AMLD-350-2h and propylene diffusivity on 5A zeolite, 5A-MLD-250-2h, 5A-MLD-250-8h, and


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5A-MLD-350-2h (b). Pore size distributions were shifted upward 1, 2 and 3 cm3/(g·nm) for 5AMLD-250-2h, 5A-MLD-250-8h, and 5A-MLD-350-2h to better distinguish them in (a).

We made a comparison of our MLD coated zeolite sorbents with other representative porous materials, including zeolites/molecular sieves with medium-sized pores, metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs), for CO2/N2 adsorptive separation, as shown in Fig. 4. We did not compare the narrow pore zeolites, such as Li-ZK-5 and AlPO-53, due to their relatively high resistance for CO2 diffusion. We found that our MLD modified 5A shows a much higher CO2/N2 selectivity, whereas the CO2 adsorption capacity is still comparable and/or superior to most of the porous sorbents.

(10)

80

CO2/N2 ideal selectivity at 0.5 bar

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No. (1)

70 60 (8)

50 40 (1)

30

(3)

20 (2)

10

(4)

(5)

(9)

(7)

(6)

0 0

1 2 3 CO2 capacity at 0.5 bar, mmol/g

Material

Symbol

Reference

Amine modified β-zeolite

(50)

(2)

ZIF-8

(3)

AlPO4-18

(52)

(4)

HZSM-5

(53)

(5)

Carbon molecular sieve

(54)

(6)

(51)

Activated carbon

(55)

(7)

HKUST-1

(51)

(8)

Bio-MOF-11

(56)

(9)

5A zeolite 5A-MLD-250-2h

(10)

(this work) (this work)

Figure 4. Comparison of MLD coated zeolite composite sorbents with porous adsorbents for CO2/N2 separation: CO2/N2 selectivity versus CO2 adsorption capacity at 0.5 bar. Blue squares (1-8) represent porous adsorbents from the literatures;50-56 black squares (9) are uncoated zeolite sorbents; red square (10) indicate MLD coated zeolite composite sorbents from this study.

4. CONCLUSION In conclusion, we prepared microporous TiO2 coated zeolite composite sorbents and optimized their pore sizes by optimizing the calcination conditions. The resulting MLD coated


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zeolites showed greatly improved CO2 capture performance. Specifically, our sorbents showed CO2/N2 selectivity as high as ~70 and CO2 adsorption capacity of 1.62 mmol/g at 0.5 bar and room temperature. Considering their high adsorptive selectivity and adsorption capacity, MLD modified zeolites are superior to most of the reported sorbents for CO2 capture.

ASSOCIATE CONTENT Supporting Information FESEM, EDS, XPS, IR, adsorption isotherms of CO2 and N2 at 25 oC, Adsorption of Ar at -196 o

C for BET surface area and pore size distribution characterization, and adsorption kinetics of

propylene at 25 oC. AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] Tel. : +1-518-276-6808 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources National Science Foundation (NSF) NO. CBET-1402772. ACKNOWLEDGMENT


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Molecular Layer Deposition-Modified 5A Zeolite for Highly Efficient

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