# Separation of CO2 from Small Gas Molecules Using Deca-Dodecasil 3 Rhombohedral (DDR3) Membrane Synthesized via Ultrasonically Assisted Hydrothermal Growth Method

Authors: Muhammad Mubashir; Yeong Yin Fong; Sidra Saqib; Ahmad Mukhtar; Sikander Rafiq; Muhammad Bilal K. Niazi; Muhammad Babar; Sami Ullah; Abdullah G. Al-Sehemi; Farrukh Jamil
Journal: Advances in Polymer Technology
 (2020)

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## Abstract

Deca-dodecasil 3 rhombohedral (DDR3) membrane has received much attention in CO2 separation from small gas molecules because of its molecular sieving property and stable characteristics. Therefore, the present work is focusing on the utilization of previously fabricated membrane (synthesized in 3 days as reported in our previous work) to study the effect of hydrocarbons and its durability at the previously optimized conditions. Subsequently, gas permeation study was conducted on the DDR3 membrane in CO2 separation from small gas molecules and it was found that the permeance of H2, CO2, N2, and CH4 decreased in the order of H2 > CO2 > N2 > CH4, according to the increase in kinetic diameter of these gas molecules. Besides, it was observed that the ideal selectivities of the gas pairs decreased in the sequence of CO2/CH4 > CO2/N2 > H2/CO2. On the other hand, it was found that the presence of hydrocarbon impurities in the gas mixture containing CO2 and CH4 has directly affected the performance of DDR3 membrane and contributed to the losses of CO2 permeability, CH4 permeability, and CO2/CH4 selectivity of 39.1%, 14.8%, and 4.2%, respectively. Consequently, from the stability test, the performance of DDR3 membrane remained stable for 96 h, even after the separation testing using CO2 and CH4 gas mixture containing hydrocarbon impurities.


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## Body

## 1. Introduction

Over the last few decades, much research has been done on the CO2 separation especially in syngas production (mainly H2) and natural gas purification. The presence of CO2 in the syngas reduces the yield of hydrogen and methane recovery during the Fischer-Tropsch (FT) process [1, 2]. Similarly, the presence of CO2 decreases the heating value of natural gas and caused equipment corrosion in the existence of water [3, 4]. Meanwhile, membrane separation technology has received much attention in CO2 separation mainly due to its advantages compared to the conventional separation technologies [5–11]. Zeolite membrane is favored in the CO2 separation among the membrane materials over polymeric and mixed matrix membranes due to its characteristics, including well-specified pores, the molecular filtering property, and strong CO2 permeation [12, 13]. Table 1 shows the comparison of different zeolite membranes reported in the literature for CO2 separation from CH4.Table 1
Comparison of different zeolite membranes used for CO2 separation from CH4.
MembranesH2/CO2 selectivityCO2/CH4 selectivityCO2/N2 selectivityCO2 permeance (mol/m2sPa)Ref.MFI membrane70——0.80×10−7[14]AlPO-18 membrane—220456.30×10−7[15]SAPO-34 membrane0.44258.301.60×10−7[13]DD3R membrane3.70——1.10×10−7[17]DD3R membrane——2003.0×10−7[18]DD3R membrane——6.18.70×10−7[19]Referring to Table1, Li et al. [13] reported CO2 separation from light gases through the SAPO-34 membrane and CO2 permeance of 1.60×10−7 mol/m2sPa was achieved. Meanwhile, H2/CO2, CO2/CH4, and CO2/N2 selectivities of 0.44, 25, and 8.30 were obtained, respectively. Subsequently, Wang and his coworkers [14] reported on the CO2 removal from small gas molecules at a high temperature of 500°C using the MFI membrane. They found that H2 and CO2 permeances as well as H2/CO2 selectivity of 1.20×10−7 mol/m2sPa, 0.80×10−7 mol/m2sPa, and 70, respectively, were achieved. Along with the MFI membrane, they also studied the permeation performance of AlPO-18 membrane for CO2 separation from small gas molecules at room temperature. H2, CO2, and CH4 permeances of 6.50×10−7 mol/m2sPa, 6.30×10−7 mol/m2sPa, and 1.0×10−7 mol/m2sPa were reported, respectively. On the other hand, CO2/N2 and CO2/CH4 selectivities of 45 and 220 were attained [15, 16].Recently, the DDR3 zeolite membrane has attracted various researchers due to its aperture size of0.36nm×0.44nm which resulted in a remarkable performance in the separation of CO2 from CH4 [17–21]. In 27 to 31 days, many scientists used a secondary hydrothermal growth process and evaluated membrane efficiency in CO2 separation from CH4. DDR3 membrane is synthesized by different researchers. Tomita et al. documented CO2 permeance over a membrane of DDR3 of 3.0×10−7 mol/m2sPa and selectivity CO2/CH4 of 200. CO2 permeance was then obtained at 7.80×10−9 mol/m2sPa and selectivity at CO2/CH4 at 6.1 obtained by Nakayama et al. [19]. Recently, Bose et al. [17] investigated the fabrication of DDR3 membrane in 7 days and they found that CO2 permeance of 1.1×10−7 mol/m2sPa and H2/CO2 selectivity of 3.70 was achieved at ambient conditions.Although several studies have reported the gas permeation of DDR3 membrane in CO2 separation from CH4, researchers rarely reported on the membrane performance in CO2 separation from other small gas molecules including H2 and N2. Therefore, in the present work, we focus on the performance of the DDR3 membrane synthesized in our previous work [22] for CO2 separation from small gas molecules including H2, CH4, and N2. This research also explores the impact of the impurities of hydrocarbons on the efficiency of CO2 distinguishing the DDR3 membrane from CH4.


## 2. Materials and Methods

Chemicals and the procedure used for the synthesis of DDR3 membrane have been reported in detail in our previous work [10, 22]. Firstly, the mixture solution was prepared using 1-adamantaneamine, tetramethoxy silane, ethylenediamine, and water. Subsequently, the mixture solution was undergoing ultrasonic-irradiation pretreatment for 1 h prior to hydrothermal growth for 1 day. Then, the seeds were recovered, washed with deionized water, and attached on α-alumina support via a vacuum-assisted seeding method. In the typical synthesis of DDR3 membrane, the seeded support was placed in an autoclaved filled with solution mixture prepared using a similar recipe and procedure as described in our previous work [10]. After that, the membrane was grown in 2 days under hydrothermal heating. Consequently, the resultant membrane was preceded for calcination and characterization. The crystallinity and morphology of the membrane were characterized using an X-ray diffractometer (Bruker D8 Advance) and field emission scanning electron microscopy (Zeiss Supra 55 VP).


### 2.1. Permeation Study

#### 2.1.1. Pure Gas Permeability

The performance of the resultant DDR3 membrane was investigated using the permeation test rig as shown in Figure1. The membrane was sealed in the permeation cell, and the flow rate of the targeted gases including, H2, CO2, N2, and CH4 was fixed at 200 ml/min while the temperature (MKS instrument) and feed pressure were maintained at room temperature and 2 bar, respectively. Temperature and feed pressure were controlled with an error of +/-0.1°C and 0.1 bar using MKS instrument temperature and feed pressure controllers, respectively. The gas permeation was calculated using equation (1), while ideal selectivity was calculated using equation (2) as follows [22, 23].
(1)Pi=JiΔP,(2)αi,j=PiPj,where Ji is the flux (mol/m2s) and ΔP is the pressure drop (Pa) while αi,j is the gas pair selectivity.Figure 1
Schematic of permeation test rig used in the present work.


#### 2.1.2. Effect of Hydrocarbon Impurities and Stability Test on DDR3 Membrane

The effect of hydrocarbon impurities and stability test over the resultant membrane in CO2 separation from CH4 was also conducted using the same permeation test rig. The experiments were conducted in three consecutive steps in which the first and third steps were used to investigate the stability of DDR3 membrane. Meanwhile, the second step of the experiment was carried out to study the effect of hydrocarbon impurities on the performance of the resultant membrane in CO2 separation from CH4.During the first step of the experiment, temperature, inlet pressure, and CO2 inlet composition were maintained at 30.1°C, 1 bar, and 10 vol% (90 vol% of CH4), respectively, because this experimental condition was obtained as the optimum CO2/CH4 separation condition as reported in our previous work [10]. After 24 h, the second step of the testing was conducted by feeding the CO2 and CH4 gas mixture containing hydrocarbon impurities with the compositions of 35.84% CO2, 44.50% CH4, 6.27% C3H8, 3.44% C4H10, and 9.94% C2H6 into the system. This composition resembled the natural gas composition presence in North Sumatra Basin, Indonesia. After 48 h of testing, the feed gas was cut off and the experiment was continued using the feed gas composition applied in the first step of the testing. After each step, prior to the measurement of gas permeances, the system was allowed to stabilize for 30 min.Gas compositions were calculated using gas chromatographs in feed, permeate, and retention streams (PerkinElmer Clarus 580). The CO2 and CH4 permeations were determined with equation (1) with the amount of pressure decrease obtained by equation (3) [24]. The selectivity of the composition of CO2/CH4 was determined subsequently from the mole fractions as defined in equation (4).
(3)ΔPi=Pf−Pp,where Pf and Pp are partial pressures in feed and permeate, respectively.
(4)Compositionselectivity=Xi/XjPXi/XjF,where Xi and Xj are mole fractions of the i and j elements of the binary mixture. P and F subscriptions reflect both permeate and feed.


## 2.1. Permeation Study

### 2.1.1. Pure Gas Permeability

The performance of the resultant DDR3 membrane was investigated using the permeation test rig as shown in Figure1. The membrane was sealed in the permeation cell, and the flow rate of the targeted gases including, H2, CO2, N2, and CH4 was fixed at 200 ml/min while the temperature (MKS instrument) and feed pressure were maintained at room temperature and 2 bar, respectively. Temperature and feed pressure were controlled with an error of +/-0.1°C and 0.1 bar using MKS instrument temperature and feed pressure controllers, respectively. The gas permeation was calculated using equation (1), while ideal selectivity was calculated using equation (2) as follows [22, 23].
(1)Pi=JiΔP,(2)αi,j=PiPj,where Ji is the flux (mol/m2s) and ΔP is the pressure drop (Pa) while αi,j is the gas pair selectivity.Figure 1
Schematic of permeation test rig used in the present work.


### 2.1.2. Effect of Hydrocarbon Impurities and Stability Test on DDR3 Membrane

The effect of hydrocarbon impurities and stability test over the resultant membrane in CO2 separation from CH4 was also conducted using the same permeation test rig. The experiments were conducted in three consecutive steps in which the first and third steps were used to investigate the stability of DDR3 membrane. Meanwhile, the second step of the experiment was carried out to study the effect of hydrocarbon impurities on the performance of the resultant membrane in CO2 separation from CH4.During the first step of the experiment, temperature, inlet pressure, and CO2 inlet composition were maintained at 30.1°C, 1 bar, and 10 vol% (90 vol% of CH4), respectively, because this experimental condition was obtained as the optimum CO2/CH4 separation condition as reported in our previous work [10]. After 24 h, the second step of the testing was conducted by feeding the CO2 and CH4 gas mixture containing hydrocarbon impurities with the compositions of 35.84% CO2, 44.50% CH4, 6.27% C3H8, 3.44% C4H10, and 9.94% C2H6 into the system. This composition resembled the natural gas composition presence in North Sumatra Basin, Indonesia. After 48 h of testing, the feed gas was cut off and the experiment was continued using the feed gas composition applied in the first step of the testing. After each step, prior to the measurement of gas permeances, the system was allowed to stabilize for 30 min.Gas compositions were calculated using gas chromatographs in feed, permeate, and retention streams (PerkinElmer Clarus 580). The CO2 and CH4 permeations were determined with equation (1) with the amount of pressure decrease obtained by equation (3) [24]. The selectivity of the composition of CO2/CH4 was determined subsequently from the mole fractions as defined in equation (4).
(3)ΔPi=Pf−Pp,where Pf and Pp are partial pressures in feed and permeate, respectively.
(4)Compositionselectivity=Xi/XjPXi/XjF,where Xi and Xj are mole fractions of the i and j elements of the binary mixture. P and F subscriptions reflect both permeate and feed.


## 2.1.1. Pure Gas Permeability

The performance of the resultant DDR3 membrane was investigated using the permeation test rig as shown in Figure1. The membrane was sealed in the permeation cell, and the flow rate of the targeted gases including, H2, CO2, N2, and CH4 was fixed at 200 ml/min while the temperature (MKS instrument) and feed pressure were maintained at room temperature and 2 bar, respectively. Temperature and feed pressure were controlled with an error of +/-0.1°C and 0.1 bar using MKS instrument temperature and feed pressure controllers, respectively. The gas permeation was calculated using equation (1), while ideal selectivity was calculated using equation (2) as follows [22, 23].
(1)Pi=JiΔP,(2)αi,j=PiPj,where Ji is the flux (mol/m2s) and ΔP is the pressure drop (Pa) while αi,j is the gas pair selectivity.Figure 1
Schematic of permeation test rig used in the present work.


## 2.1.2. Effect of Hydrocarbon Impurities and Stability Test on DDR3 Membrane

The effect of hydrocarbon impurities and stability test over the resultant membrane in CO2 separation from CH4 was also conducted using the same permeation test rig. The experiments were conducted in three consecutive steps in which the first and third steps were used to investigate the stability of DDR3 membrane. Meanwhile, the second step of the experiment was carried out to study the effect of hydrocarbon impurities on the performance of the resultant membrane in CO2 separation from CH4.During the first step of the experiment, temperature, inlet pressure, and CO2 inlet composition were maintained at 30.1°C, 1 bar, and 10 vol% (90 vol% of CH4), respectively, because this experimental condition was obtained as the optimum CO2/CH4 separation condition as reported in our previous work [10]. After 24 h, the second step of the testing was conducted by feeding the CO2 and CH4 gas mixture containing hydrocarbon impurities with the compositions of 35.84% CO2, 44.50% CH4, 6.27% C3H8, 3.44% C4H10, and 9.94% C2H6 into the system. This composition resembled the natural gas composition presence in North Sumatra Basin, Indonesia. After 48 h of testing, the feed gas was cut off and the experiment was continued using the feed gas composition applied in the first step of the testing. After each step, prior to the measurement of gas permeances, the system was allowed to stabilize for 30 min.Gas compositions were calculated using gas chromatographs in feed, permeate, and retention streams (PerkinElmer Clarus 580). The CO2 and CH4 permeations were determined with equation (1) with the amount of pressure decrease obtained by equation (3) [24]. The selectivity of the composition of CO2/CH4 was determined subsequently from the mole fractions as defined in equation (4).
(3)ΔPi=Pf−Pp,where Pf and Pp are partial pressures in feed and permeate, respectively.
(4)Compositionselectivity=Xi/XjPXi/XjF,where Xi and Xj are mole fractions of the i and j elements of the binary mixture. P and F subscriptions reflect both permeate and feed.


## 3. Results and Discussion

### 3.1. Characterization

The details on XRD, FESEM, FTIR, and EDX analysis on DDR3 membrane have been described in our previous work [10]. The significant XRD peaks and FESEM images obtained from the resultant membrane showed comparable crystallinity and hexagonal morphology which are analogous with those findings documented in the researches for DDR3 membrane [9, 10, 25, 26]. The surface and cross-sectional view of the membrane showed that a uniform, flat, and fully intergrown membrane layer was formed.


### 3.2. Gas Permeation

Figure2 shows the performance of the membrane at ambient temperature and feed pressure of 2 bar. Referring to Figure 2, H2, CO2, N2, and CH4 permeances of 4.10×10−7 mol/m2sPa, 2.46×10−7 mol/m2sPa, 9.11×10−8 mol/m2sPa, and 4.73×10−8 mol/m2sPa are obtained, respectively. It can be observed from Figure 2 that H2 permeance is higher than that of those values obtained for CO2, N2, and CH4 gases. This could be due to the significant difference in the Knudsen diffusion value of the gases through the membrane. H2 with a molecular diameter of 0.29 nm is smaller than of the molecular diameter of CO2, N2, and CH4, as well as the DDR3 pore channel. Meanwhile, the molecular diameter of CO2, N2, and CH4 of 0.33 nm, 0.36 nm, and 0.38 nm, respectively, is closer to the DDR3 pore channel. Thus, H2 can diffuse faster compared to CO2, N2, and CH4 gases.Figure 2
Single gas permeation results of small molecular gas over DDR3 membrane at room temperature and feed pressure of 2 bar (inset: ideal selectivities for H2/CO2, CO2/N2, and CO2/CH4).From Figure2, it is found that the gas permeances are decreased in the order of H2 > CO2 > N2 > CH4. This decreasing trend is mainly attributed to the molecular sieving behaviour of DDR3 membrane and Knudsen diffusion mechanism of the gases [25–28]. The diffusion rate of these gases through the DDR3 membrane decreases when the kinetic diameter of the gases increases, and thus, decreasing order of gas permeance has been observed [17]. Moreover, H2, CO2, N2, and CH4 permeances obtained from the membrane were comparable with those results reported in the literature [17, 20, 25].As shown in Figure2 also, the ideal selectivities of CO2/CH4, CO2/N2, and H2/CO2 of 5.22, 2.27, and 1.66, respectively, are obtained. It is interesting to note that the ideal selectivities of the gas pairs decreased in the sequence of CO2/CH4 > CO2/N2 > H2/CO2. This result is attributed to the CH4 permeance, which is much smaller than the permeance of H2 and N2 as shown in Figure 3. Overall, although the gas permeance and ideal selectivity results obtained in the present work are comparable with those results reported in the literature [25], the relatively low performance of DDR3 membrane could be due to the presence of minor defects or intercrystalline pores which have reduced the performance of the membrane. Thus, it is suggested that the membrane should be modified with a TEOS solution to minimized intercrystalline defects to enhance its performance in gas separation [24]. TEOS could penetrate into intercrystalline defects and seal it through the formation of hydrogen bonding between membrane zeolite materials and TEOS. In addition, it also slows down the CH4 permeation by providing extra resistance due to its higher kinetic diameter [25].Figure 3
Mechanism of small molecular gas transport over DDR3 membrane.


### 3.3. Effect of Hydrocarbon Impurities on the Performance of DDR3 Membrane

Figure4 shows the stability test and effect of hydrocarbon impurities on the performance of the membrane in CO2 separation from CH4 for the testing duration of 96 h. Sections І and III shown in Figure 4 display the stability results of DDR3 membrane in CO2 separation from a binary gas mixture of CO2 and CH4 at a temperature of 30.1°C, inlet pressure of 1 bar, and CO2 inlet composition of 10 vol% because this experimental condition was obtained as the optimum CO2/CH4 separation condition which provided higher permeation performance of DDR3 membrane as reported in our previous work [10].Figure 4
Effect of hydrocarbon impurities on the performance of DDR3 membrane and its durability test (separation results points C, E, and G were determined at a temperature of 30.1°C, feed pressure of 1 bar, with the CO2 composition of 35.84% (CH4 composition of 64.16%)).Meanwhile, section II in Figure4 exhibits the effect of hydrocarbon impurities on the performance of DDR3 membrane in CO2 separation from CH4 at a temperature of 30.1°C and feed pressure of 1 bar. Referring to Figure 4, at points C and E, CO2 and CH4 permeances of 3.13×10−8 mol/m2sPa and 2.81×10−8 mol/m2sPa are obtained, respectively. Meanwhile, CO2/CH4 selectivity of 2.59 was obtained at point G. Results at points C∗, E∗, and G∗ were obtained when the feed gas was changed from a pure binary mixture of CO2/CH4 to CO2/CH4 gas mixture containing hydrocarbon impurities. At points C∗ and E∗, CO2 and CH4 permeances of 3.11×10−8 mol/m2sPa and 2.83×10−8 mol/m2sPa are obtained, respectively. On the other hand, CO2/CH4 selectivity of 2.59 was obtained at point G∗.It can be seen from Figure4 that the performance of the membrane slightly decreases when the gas mixture containing hydrocarbon impurities is fed into the system. Besides, from Figure 4, it is found that CO2 and CH4 permeances decrease from point E∗ to F and C∗ to D, respectively, after testing duration of 40 h (from 32 h to 72 h). Meanwhile, CO2 and CH4 permeances of 1.91×10−8 mol/m2sPa and 1.65×10−8 mol/m2sPa were obtained at point F and point D, respectively. This result shows that CO2 permeance is gradually dropped by about 39.1% with respect to the value of CO2 permeance obtained at point E∗. Similarly, CH4 permeance decreased by about 14.8% with respect to the initial value of CH4 permeance obtained at point C∗. The decrease in CO2 and CH4 permeances could be due to the adsorption of hydrocarbon impurities in DDR3 pores which inhibited the diffusion of CO2 and CH4 molecules [3]. Adsorption of hydrocarbon impurities into the pores of DDR3 pore reduced its capacity for the diffusion of CO2 and CH4 molecules. Subsequently, adsorption of hydrocarbon impurities may also cause saturation for adsorption of CO2 and CH4 molecules which may also resulted to drop the performance of the membrane. In addition, a competitive sorption effect has been reported previously that mixed gas performance is always lower than pure gas due to a competitive sorption mechanism [4, 16]. However, CO2/CH4 selectivity of 2.54 was found at point H, which is slightly lower than the CO2/CH4 selectivity obtained at point G∗. It is found that the loss of CO2/CH4 selectivity at point H was only about 4.2% compared to the CO2/CH4 selectivity which was initially obtained at point G∗.Furthermore, it can be seen from Figure4 (section I) that, when the hydrocarbon impurities are not presented in the feed gas, the performance of DDR3 membrane remains stable. After 24 h, nearly constant CO2 and CH4 permeances of 7.23×10−8 mol/m2sPa and 4.51×10−8 mol/m2sPa were obtained, respectively, while CO2/CH4 selectivity of 3.66 was achieved (line A). These results are similar to those values obtained for CO2 permeance, CH4 permeance, and CO2/CH4 selectivity when the feed gas was changed to the binary gas mixture of CO2 and CH4 at the testing duration from 80 h to 96 h. In section III, CO2 and CH4 permeances of 7.08×10−8 mol/m2sPa and 4.41×10−8 mol/m2sPa were obtained (line B), while CO2/CH4 selectivity of 3.69 was achieved.Therefore, it can be concluded that the presence of hydrocarbon impurities in CO2 and CH4 gas mixture has affected the performance of DDR3 membrane and caused a slight reduction of CO2 and CH4 gas over a testing duration of 40 h (from 32 h to 72 h). Consequently, CO2/CH4 selectivity of DDR3 membrane remained stable at testing condition of temperature of 30.1°C and feed pressure of 1 bar and CO2 feed composition of 10 vol%.


### 3.4. Stability of DDR3 Membrane

Figure4 also shows the stability of DDR3 membrane in CO2 separation from CH4. It can be seen from Figure 4 that CO2 permeability, CH4 permeability, and CO2/CH4 selectivity (7.06×10−8 mol/m2sPa, 4.38×10−8 mol/m2sPa, and 3.66, respectively) obtained over DDR3 membrane after 24 h are comparable with those values achieved after 96 h (CO2permeance=7.08×10−8 mol/m2sPa, CH4permeance=4.41×10−8 mol/m2sPa, and CO2/CH4selectivity=3.69). Overall, after 96 h of testing duration, CO2 permeability, CH4 permeability, and CO2/CH4 selectivity were dropped by about 0.28%, 0.68%, and 0.81% compared to the original values obtained at line A shown in Figure 4. From these results, it has been found that membrane performance was dropped in small quantity and thus, it was decided that the membrane was stable in the presence of hydrocarbons.In conclusion, the permeation performance of DDR3 membrane in the separation of CO2/CH4 remained steady up to 96 h with small losses of CO2 permeability, CH4 permeability, and CO2/CH4 selectivity. Thus, the separation performance of DDR3 membrane is durable and stable in the presence of hydrocarbon impurities.The findings seen in the literature under the same temperature and pressure condition assess the efficiency of DDR3 membrane in CO2/CH4, H2/CO2, and CO2/N2 separations. With respect to Figure 5(a), the ideal CO2/CH4 selectivity obtained in this analysis is comparable to the published findings by Yajima and Nakayama [21]. However, this value is below the published results by Himeno et al. [18] and van den Bergh et al. [25]. Relatively low CO2/CH4 ideal selectivity of the DDR3 membrane obtained during the present research may be triggered by the existence of slight membrane defects which may influence CO2 or CH4 transport pathways via the membrane.Figure 5
Comparison of gas pair ideal selectivities with reported literature ((a) CO2/CH4, (b) CO2/N2, and (c) H2/CO2).
(a)(b)(c)On the other hand, the performance of DDR3 membrane in CO2/N2 and H2/CO2 separation is shown in Figures 5(b) and 5(c). From Figure 5(b), it is found that the CO2/N2 selectivity obtained in the present work is lower than those results reported by Nakayama et al. [19]. Referring to Figure 5(c), the ideal selectivity of H2/CO2 obtained in the present study is slightly higher than those results reported by Tomita et al. [20] and Zheng et al. [26]. However, these values were lower than those results reported by Bose et al. [17]. Low ideal selectivity values of CO2/N2 and H2/CO2 obtained in the present work could be because of the similar reason as described previously for the performance of the membrane in CO2/CH4 separation. The presence of defects on the membrane affected the transport mechanisms of N2, H2, and CO2 through the membrane and lowered the selectivity values of CO2/N2 and H2/CO2.


## 3.1. Characterization

The details on XRD, FESEM, FTIR, and EDX analysis on DDR3 membrane have been described in our previous work [10]. The significant XRD peaks and FESEM images obtained from the resultant membrane showed comparable crystallinity and hexagonal morphology which are analogous with those findings documented in the researches for DDR3 membrane [9, 10, 25, 26]. The surface and cross-sectional view of the membrane showed that a uniform, flat, and fully intergrown membrane layer was formed.


## 3.2. Gas Permeation

Figure2 shows the performance of the membrane at ambient temperature and feed pressure of 2 bar. Referring to Figure 2, H2, CO2, N2, and CH4 permeances of 4.10×10−7 mol/m2sPa, 2.46×10−7 mol/m2sPa, 9.11×10−8 mol/m2sPa, and 4.73×10−8 mol/m2sPa are obtained, respectively. It can be observed from Figure 2 that H2 permeance is higher than that of those values obtained for CO2, N2, and CH4 gases. This could be due to the significant difference in the Knudsen diffusion value of the gases through the membrane. H2 with a molecular diameter of 0.29 nm is smaller than of the molecular diameter of CO2, N2, and CH4, as well as the DDR3 pore channel. Meanwhile, the molecular diameter of CO2, N2, and CH4 of 0.33 nm, 0.36 nm, and 0.38 nm, respectively, is closer to the DDR3 pore channel. Thus, H2 can diffuse faster compared to CO2, N2, and CH4 gases.Figure 2
Single gas permeation results of small molecular gas over DDR3 membrane at room temperature and feed pressure of 2 bar (inset: ideal selectivities for H2/CO2, CO2/N2, and CO2/CH4).From Figure2, it is found that the gas permeances are decreased in the order of H2 > CO2 > N2 > CH4. This decreasing trend is mainly attributed to the molecular sieving behaviour of DDR3 membrane and Knudsen diffusion mechanism of the gases [25–28]. The diffusion rate of these gases through the DDR3 membrane decreases when the kinetic diameter of the gases increases, and thus, decreasing order of gas permeance has been observed [17]. Moreover, H2, CO2, N2, and CH4 permeances obtained from the membrane were comparable with those results reported in the literature [17, 20, 25].As shown in Figure2 also, the ideal selectivities of CO2/CH4, CO2/N2, and H2/CO2 of 5.22, 2.27, and 1.66, respectively, are obtained. It is interesting to note that the ideal selectivities of the gas pairs decreased in the sequence of CO2/CH4 > CO2/N2 > H2/CO2. This result is attributed to the CH4 permeance, which is much smaller than the permeance of H2 and N2 as shown in Figure 3. Overall, although the gas permeance and ideal selectivity results obtained in the present work are comparable with those results reported in the literature [25], the relatively low performance of DDR3 membrane could be due to the presence of minor defects or intercrystalline pores which have reduced the performance of the membrane. Thus, it is suggested that the membrane should be modified with a TEOS solution to minimized intercrystalline defects to enhance its performance in gas separation [24]. TEOS could penetrate into intercrystalline defects and seal it through the formation of hydrogen bonding between membrane zeolite materials and TEOS. In addition, it also slows down the CH4 permeation by providing extra resistance due to its higher kinetic diameter [25].Figure 3
Mechanism of small molecular gas transport over DDR3 membrane.


## 3.3. Effect of Hydrocarbon Impurities on the Performance of DDR3 Membrane

Figure4 shows the stability test and effect of hydrocarbon impurities on the performance of the membrane in CO2 separation from CH4 for the testing duration of 96 h. Sections І and III shown in Figure 4 display the stability results of DDR3 membrane in CO2 separation from a binary gas mixture of CO2 and CH4 at a temperature of 30.1°C, inlet pressure of 1 bar, and CO2 inlet composition of 10 vol% because this experimental condition was obtained as the optimum CO2/CH4 separation condition which provided higher permeation performance of DDR3 membrane as reported in our previous work [10].Figure 4
Effect of hydrocarbon impurities on the performance of DDR3 membrane and its durability test (separation results points C, E, and G were determined at a temperature of 30.1°C, feed pressure of 1 bar, with the CO2 composition of 35.84% (CH4 composition of 64.16%)).Meanwhile, section II in Figure4 exhibits the effect of hydrocarbon impurities on the performance of DDR3 membrane in CO2 separation from CH4 at a temperature of 30.1°C and feed pressure of 1 bar. Referring to Figure 4, at points C and E, CO2 and CH4 permeances of 3.13×10−8 mol/m2sPa and 2.81×10−8 mol/m2sPa are obtained, respectively. Meanwhile, CO2/CH4 selectivity of 2.59 was obtained at point G. Results at points C∗, E∗, and G∗ were obtained when the feed gas was changed from a pure binary mixture of CO2/CH4 to CO2/CH4 gas mixture containing hydrocarbon impurities. At points C∗ and E∗, CO2 and CH4 permeances of 3.11×10−8 mol/m2sPa and 2.83×10−8 mol/m2sPa are obtained, respectively. On the other hand, CO2/CH4 selectivity of 2.59 was obtained at point G∗.It can be seen from Figure4 that the performance of the membrane slightly decreases when the gas mixture containing hydrocarbon impurities is fed into the system. Besides, from Figure 4, it is found that CO2 and CH4 permeances decrease from point E∗ to F and C∗ to D, respectively, after testing duration of 40 h (from 32 h to 72 h). Meanwhile, CO2 and CH4 permeances of 1.91×10−8 mol/m2sPa and 1.65×10−8 mol/m2sPa were obtained at point F and point D, respectively. This result shows that CO2 permeance is gradually dropped by about 39.1% with respect to the value of CO2 permeance obtained at point E∗. Similarly, CH4 permeance decreased by about 14.8% with respect to the initial value of CH4 permeance obtained at point C∗. The decrease in CO2 and CH4 permeances could be due to the adsorption of hydrocarbon impurities in DDR3 pores which inhibited the diffusion of CO2 and CH4 molecules [3]. Adsorption of hydrocarbon impurities into the pores of DDR3 pore reduced its capacity for the diffusion of CO2 and CH4 molecules. Subsequently, adsorption of hydrocarbon impurities may also cause saturation for adsorption of CO2 and CH4 molecules which may also resulted to drop the performance of the membrane. In addition, a competitive sorption effect has been reported previously that mixed gas performance is always lower than pure gas due to a competitive sorption mechanism [4, 16]. However, CO2/CH4 selectivity of 2.54 was found at point H, which is slightly lower than the CO2/CH4 selectivity obtained at point G∗. It is found that the loss of CO2/CH4 selectivity at point H was only about 4.2% compared to the CO2/CH4 selectivity which was initially obtained at point G∗.Furthermore, it can be seen from Figure4 (section I) that, when the hydrocarbon impurities are not presented in the feed gas, the performance of DDR3 membrane remains stable. After 24 h, nearly constant CO2 and CH4 permeances of 7.23×10−8 mol/m2sPa and 4.51×10−8 mol/m2sPa were obtained, respectively, while CO2/CH4 selectivity of 3.66 was achieved (line A). These results are similar to those values obtained for CO2 permeance, CH4 permeance, and CO2/CH4 selectivity when the feed gas was changed to the binary gas mixture of CO2 and CH4 at the testing duration from 80 h to 96 h. In section III, CO2 and CH4 permeances of 7.08×10−8 mol/m2sPa and 4.41×10−8 mol/m2sPa were obtained (line B), while CO2/CH4 selectivity of 3.69 was achieved.Therefore, it can be concluded that the presence of hydrocarbon impurities in CO2 and CH4 gas mixture has affected the performance of DDR3 membrane and caused a slight reduction of CO2 and CH4 gas over a testing duration of 40 h (from 32 h to 72 h). Consequently, CO2/CH4 selectivity of DDR3 membrane remained stable at testing condition of temperature of 30.1°C and feed pressure of 1 bar and CO2 feed composition of 10 vol%.


## 3.4. Stability of DDR3 Membrane

Figure4 also shows the stability of DDR3 membrane in CO2 separation from CH4. It can be seen from Figure 4 that CO2 permeability, CH4 permeability, and CO2/CH4 selectivity (7.06×10−8 mol/m2sPa, 4.38×10−8 mol/m2sPa, and 3.66, respectively) obtained over DDR3 membrane after 24 h are comparable with those values achieved after 96 h (CO2permeance=7.08×10−8 mol/m2sPa, CH4permeance=4.41×10−8 mol/m2sPa, and CO2/CH4selectivity=3.69). Overall, after 96 h of testing duration, CO2 permeability, CH4 permeability, and CO2/CH4 selectivity were dropped by about 0.28%, 0.68%, and 0.81% compared to the original values obtained at line A shown in Figure 4. From these results, it has been found that membrane performance was dropped in small quantity and thus, it was decided that the membrane was stable in the presence of hydrocarbons.In conclusion, the permeation performance of DDR3 membrane in the separation of CO2/CH4 remained steady up to 96 h with small losses of CO2 permeability, CH4 permeability, and CO2/CH4 selectivity. Thus, the separation performance of DDR3 membrane is durable and stable in the presence of hydrocarbon impurities.The findings seen in the literature under the same temperature and pressure condition assess the efficiency of DDR3 membrane in CO2/CH4, H2/CO2, and CO2/N2 separations. With respect to Figure 5(a), the ideal CO2/CH4 selectivity obtained in this analysis is comparable to the published findings by Yajima and Nakayama [21]. However, this value is below the published results by Himeno et al. [18] and van den Bergh et al. [25]. Relatively low CO2/CH4 ideal selectivity of the DDR3 membrane obtained during the present research may be triggered by the existence of slight membrane defects which may influence CO2 or CH4 transport pathways via the membrane.Figure 5
Comparison of gas pair ideal selectivities with reported literature ((a) CO2/CH4, (b) CO2/N2, and (c) H2/CO2).
(a)(b)(c)On the other hand, the performance of DDR3 membrane in CO2/N2 and H2/CO2 separation is shown in Figures 5(b) and 5(c). From Figure 5(b), it is found that the CO2/N2 selectivity obtained in the present work is lower than those results reported by Nakayama et al. [19]. Referring to Figure 5(c), the ideal selectivity of H2/CO2 obtained in the present study is slightly higher than those results reported by Tomita et al. [20] and Zheng et al. [26]. However, these values were lower than those results reported by Bose et al. [17]. Low ideal selectivity values of CO2/N2 and H2/CO2 obtained in the present work could be because of the similar reason as described previously for the performance of the membrane in CO2/CH4 separation. The presence of defects on the membrane affected the transport mechanisms of N2, H2, and CO2 through the membrane and lowered the selectivity values of CO2/N2 and H2/CO2.


## 4. Conclusions

In the present work, the performance of DDR3 membrane for the CO2 separation from gas molecules exhibiting comparatively smaller kinetic diameters, i.e., N2 and CH4, has been successfully conducted. The permeances of the gases obtained over the membrane decreased in the order of H2 > CO2 > N2 > CH4. Subsequently, it was also found that the DDR3 membrane demonstrated an ideal selectivity of 5.22 for CO2/CH4 which was higher than the ideal selectivity of H2/CO2 and CO2/N2 gas pairs. Besides, ideal selectivities obtained from DDR3 membrane decreased in the sequence of CO2/CH4 > CO2/N2 > H2/CO2. Furthermore, it was observed that the presence of hydrocarbon impurities in the CO2 and CH4 gas mixture directly affects the performance of DDR3 membrane and contributed to the losses of CO2 permeability, CH4 permeability, and CO2/CH4 selectivity of 39.1%, 14.8%, and 4.2%, respectively. Consequently, from the stability test, it can be suggested that the permeation performance of DDR3 membrane remained stable for 96 h, even after the separation testing of a gas mixture containing hydrocarbon impurities, with losses of CO2 permeability, CH4 permeability, and CO2/CH4 selectivity of 2.35%, 4.43%, and 0.55%, respectively. From the comparison study, it was concluded that the permeation performance of DDR3 membrane in CO2 separation from H2, CH4, and N2 still needs to be improved mainly due to the presence of defects in the membrane.



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*Source: 1097309-2020-07-20.xml*