H2 Selectivity Prediction of NaY, DD3R and Silicalite Zeolite

Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 11431−11438

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CO2/H2 Selectivity Prediction of NaY, DD3R, and Silicalite Zeolite Membranes Pasquale F. Zito,†,‡ Alessio Caravella,† Adele Brunetti,† Enrico Drioli,†,‡ and Giuseppe Barbieri*,† †

Ind. Eng. Chem. Res. 2018.57:11431-11438. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/14/18. For personal use only.

National Research Council − Institute on Membrane Technology (ITM-CNR), Via Pietro BUCCI, Cubo 17C, 87036 Rende CS Italy ‡ The University of Calabria − Dept. of Environment and Chemical Engineering, Via Pietro BUCCI, Cubo 44A, 87036 Rende CS Italy ABSTRACT: Using DD3R, silicalite and NaY zeolite membrane for gas separation is an attractive prospect for the development of a sustainable green process industry. The interest in CO2 and H2 lies in their relatively strong and weak adsorption in zeolite, respectively, causing a peculiar behavior of membrane selectivity, which is here evaluated in a wide range of temperature (273−573 K), feed pressure (100−500 kPa) and CO2 molar fraction (0.05−0.95). As a result, CO2/H2 selectivity of the considered zeolite is found to be high at a low temperature owing to the relatively strong CO2 adsorption with respect to the weak H2 one. Differently, selectivity is much lower at a higher temperature because of the reduced adsorption strength. The temperature strongly affects the membrane selectivity through surface diffusion closely correlated to gas adsorption, and CO2 adsorption results responsible for the high membrane selectivity in the lowtemperature range.

1. INTRODUCTION Separation of H2 from CO2 is a crucial step in various industrial processes, such as hydrogen production, methanol and ammonia production, and petroleum refining. For both species, this separation impacts in an important manner on the environment. In particular, H2 is well recognized as an alternative fuel with zero-emissions, while CO2 is today considered as one of the main greenhouse gases. Therefore, its separation and capture for an eventual further utilization is, today, one of the most important pillars to support and promote sustainability and to make the chemical industry competitive. The CO2/H2 mixture is conventionally separated through chemical and/or physical absorption. Membranes are a suitable green alternative to traditional methods, requiring lower energy and operational costs, avoiding the use of solvents, as well as the necessity for extreme operating conditions such as high pressures and/or low temperatures, thus making the separation more sustainable. In this field, zeolite membranes are quite interesting since they can be used in a wide range of temperature and pressure. CO2/H2 separation with zeolite membranes was extensively studied in the literature both as a single gas or as an equimolar mixture. Bakker et al.1 measured a CO2/H2 selectivity of 12 at 295 K and 1 bar, which decreases down to a reverse values of 2.4 at 623 K in the case of an equimolar mixture in silicalite-1 membrane. High-pressure performance of an MFI membrane were analyzed by Sandstrom et al.,2 achieving a maximum value of 53 for an equimolar mixture at 275 K and 1 MPa. Recently, Akhtar et al.3 prepared graded silicalite-1 substrates for CO2/H2 separation at 273 K, by varying feed pressure. The © 2018 American Chemical Society

authors achieved a maximum separation factor (i.e., the ratio of the molar fractions in the feed and permeate) of 12 at 5 bar. Other investigations were carried out at higher temperatures. In particular, Kusakabe et al.4 tested a large-pore NaY membrane, obtaining a selectivity of 28 ca. at 308 K. Interesting results were achieved at considerably low temperature by Hong et al.,5 using a small-pore SAPO-34 membrane. Specifically, the authors measured a CO2/H2 selectivity of 140 at 253 K. Other low temperature measurements were carried out by Korelskiy et al.,6 who tested a HZSM-5 membrane at high pressure (i.e., 9 bar) obtaining a selectivity of 210 at 235 K feeding an equimolar mixture. This value drops down to 17 at 310 K. In addition to the specific interest covered by CO2/H2 separation, the study of the separation of this two gases offers the possibility of a much clearer analysis about transport mechanisms involved since CO2 and H2 are strongly and weakly adsorbed in zeolite materials, respectively. Other gases such as, for instance, CH4, N2, CO, etc. have sorption properties between those of CO2 and H2. Permeation of light gases and hydrocarbons through zeolite membranes was typically attributed to surface diffusion,7,8 paired to gas translation diffusion (important at high temperatures)8 or Knudsen diffusion.9 Concerning the noble gases, such as Ne, Ar, and Kr, Liu et al.10 developed a Received: Revised: Accepted: Published: 11431

June 18, 2018 July 23, 2018 July 26, 2018 July 26, 2018 DOI: 10.1021/acs.iecr.8b02707 Ind. Eng. Chem. Res. 2018, 57, 11431−11438

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Industrial & Engineering Chemistry Research theoretical model that considers the average distance between molecule−molecule or molecule-wall collisions. In two previous papers11,12 a novel model pairing surface and effective Knudsen diffusion was developed and also validated with measures reported in the open literature for light gases separation through NaY and silicalite membranes. This model analysis foresees that surface diffusion, described by the Maxwell-Stefan equations, strongly depletes Knudsen diffusion through the zeolite channel. Specifically, the presence of an adsorbed phase on the zeolite surface hinders the “free” Knudsen permeation, which resulted drastically reduced. On the other hand, DD3R has pores of 0.4 nm and, thus, light gases should permeate through these membranes with the typical trend of the gas translation diffusion at a high temperature. However, some authors did not observe this regime and attributed the mass transport occurring only by surface diffusion,13 whereas some others found a minimum in the permeance-vs-temperature trend stating the presence of the gas-translation diffusion only at high temperatures.14 The present paper predicts and discusses CO2 and H2 permeance and CO2/H2 selectivity for NaY, DD3R and silicalite membranes. These zeolites have different pore size and Si/Al ratio. Specifically, NaY belongs to the FAU-type, having a Si/Al ratio between 1.5 and 3 and a pore size of 0.74 nm. DD3R is a small-pore DDR zeolite (pore size of ca. 0.4 nm), having a Si/Al ratio typically much greater than 1. Silicalite-1 is an MFI zeolite with no aluminum in its composition and channels of about 0.55 nm; on the contrary, ZSM-5, zeolite of the same MFI family, includes Al2O3. The experimental measures referred to in this paper are related to silicate-1.15 The Knudsen, rather than surface diffusion, contribution is quantified as a function of temperature, pressure and feed composition. Afterward, a comparison of the selectivity of the three zeolites highlights the nature of each zeolite and its interaction with CO2 rather than H2 and the fallout on membrane permeation. Permeance and selectivity of DD3R, silicalite and NaY zeolite membranes are simulated for several H2:CO2 mixture compositions by changing temperature (273−573 K), feed pressure (100−500 kPa). At low temperatures, the strong CO2 adsorption leads to interesting CO2/H2 selectivity and, therefore, CO2 rich permeate streams can be recovered and exploited as starting feed for green-chemistry-based processes. The retentate stream is indeed rich in H2, the greenest energy carrier by definition. The significant lower adsorption at a high temperature lowers CO2/H2 selectivity resulting in a reverse selectivity. In fact, the H2 diffusivity higher than the CO2 one and the really lower sorption assigns to zeolite membranes an H2 selectivity and the separation results in H2-rich permeate. The result discussion will focus on both selectivity and permeance of the three membranes because of both are significant for any membrane; however, we will emphasize the membrane selectivity and its variation with the operating conditions.

Table 1. Operating Conditions and Parameters of the Simulated System parameter

value

temperature, K feed pressure, kPa permeate pressure, kPa CO2 molar fraction membrane thickness, μm ε0/τ0

273−573 100−500 1 0.05−0.95 1 0.1

Finally, the temperature was changed from 273 to 573 K and CO2 molar composition from 5 to 95%. Permeation through DD3R pores was assumed to occur only by surface diffusion, as reported in,13 in which the gastranslation diffusion was not observed. Pre-exponential factor and activation energy for surface diffusion were taken from the single gas permeance of van den Bergh et al.,13 using the adsorption properties as calculated in a previous work.16 Vice versa, the Knudsen contribution was assumed to compete with the surface diffusion for zeolite with a larger pore size, in this specific case silicalite and NaY, for which the surface diffusion parameters were calculated in our previous works.11,12 This model was already validated in previous works for several mixtures.11,12 All the simulations were carried out considering a tubular zeolite membrane having thickness of 1 μm, with a nominal porosity/tortuosity ratio of 0.1 (e.g., ε0 = 0.15 and τ0 = 1.5). Here, the same value of 1.5 was used for τ0 as already estimated in a previous paper.12 The ratio ε0/τ0 of 0.1 is very close to the range 0.08−0.17 for shell silica sphere with both a larger porosity (0.68) and tortuosity ranging 4−9.17 Experimental values of both permeance13,14 and selectivity1,4,18 were taken from the open literature and then utilized in the comparisons with our simulated results. The operating conditions of these literature values are also indicated along the text where appropriate.

3. ADSORPTION PROPERTIES OF THE ZEOLITES CONSIDERED The separation performance of the investigated zeolites is affected by their adsorption capacity with regard to H2 and CO2. Figure 1 shows the saturation loadings of H2 and CO2 as functions of temperature for the three zeolites considered. The empirical expression of Do et al.19 (eq 1) was used to describe the temperature functionality of the saturation loading Cμs, in

2. DESCRIPTION OF THE SYSTEM AND HYPOTHESES The analyses were carried out focusing on the zeolite membrane layer. The porous support, which provides mechanical resistance to the membrane, was neglected by the present analysis since it is out of the scope of this work. Table 1 reports the main operating conditions and the parameters used in the simulations. The feed pressure was ranged 100−500 kPa and permeate pressure was set at 1 kPa.

Figure 1. H2 and CO2 saturation loading as a function of temperature in NaY, silicalite, and DD3R zeolites. 11432

DOI: 10.1021/acs.iecr.8b02707 Ind. Eng. Chem. Res. 2018, 57, 11431−11438

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Industrial & Engineering Chemistry Research which Cμs0 is the saturation loading at the reference temperature T0 and χ is an empirical parameter: Cμs = Cμs0e

[χ (1 − T )] T0

Table 3. Sips Heat of Adsorption of CO2 and H2 in DD3R, NaY and Silicalite Zeolites heat of adsorption, kJ mol−1

(1)

The Sips adsorption parameters available in16,20,21 were used for all the cases. Instead, the temperature dependence of H2 saturation loading in DD3R was described by the Langmuir model. In general, it can be observed that the saturation loading is inversely proportional to the Si/Al ratio: the larger the aluminum content, the higher adsorption capacity with regard to the species (Table 2 and Figure 1). This result is in line with what observed by Kosinov et al.,22 who pointed out that the zeolite framework becomes more polar.

zeolite

CO2

H2

DD3R16 NaY21 silicalite20

19.9 23.3 23.5

4.3 3.9 2.3

4. RESULTS AND DISCUSSION 4.1. Permeation Through DD3R Membranes. Figure 2 shows CO2 and H2 permeance as a function of CO2 molar

Table 2. CO2 and H2 Saturation Loadings at 273 and 373 K in DD3R, NaY and Silicalite saturation loading, mol kg−1 zeolite

DD3R

NaY

Si/Al ratio

≫1

1.5

silicalite ∞

dpore

0.40

0.74 nm

0.55 nm

CO2 H2 CO2/H2

3 0.97 3.1

9.4 1.9 4.9

3.4 0.46 7.4

CO2 H2 CO2/H2

2.3 0.49 4.7

7.8 0.8 9.7

2.8 0.25 11.2

273 K

373 K

Table 2 and Figure 1 present the adsorption strength of CO2 and H2. Furthermore, it was found that the temperature dependence of saturation loading is similar for the three zeolites in the case of CO2, the lines being almost parallel. On the other hand, the saturation loading of H2 on NaY has a more significant temperature dependence than that on the other zeolites. It is interesting to notice that DD3R shows a higher affinity than silicalite with regard to H2, whereas the same behavior was observed for CO2. Moreover, the saturation loading of H2 has a stronger temperature dependence than CO2, (steeper negative slope of H2 lines). Table 2 reports the values of saturation loading at 273 and 373 K, as well as their ratio, which provides a quantitative indication of the extent of the competitive adsorption between the two species. In particular, it can be observed that the most competitive adsorption was found in DD3R (CO2/H2 ratios of 3.1 and 4.7 at 273 and 373 K, respectively), whereas NaY and silicalite show a lower level of competition, CO2 adsorbing much more than H2. This aspect affects the permeation performance, as reported in the next section. Another important parameter providing an indication of the differences in the amount adsorbed between H2 and CO2 is the heat of adsorption (Table 3). The CO2 shows a much higher heat of adsorption than H2, for all the three zeolites, this reflecting in a much higher amount adsorbed, owing to the larger energy that the adsorbed molecules need to pass in the gas phase.19

Figure 2. CO2 and H2 permeance as a function of CO2 molar fraction at different temperatures in a DD3R membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa. ○ and ▽ experimental values of single gas measures of13 and,14 respectively.

fraction at different temperatures. The CO2 permeance was found to be dependent on molar fraction at the lower temperatures (273−363 K). In particular, a decreasing trend with CO2 composition was observed where adsorption is strong (i.e., 273 K), whereas permeance tends to be almost independent of molar fraction at the higher temperatures. On the contrary, H2 permeance decreases with increasing CO2 concentration in the whole temperature range considered, except at 573 K, where the effect owing to the presence of CO2 is quite weaker. This trend was attributed to the competitive adsorption favoring CO2 permeation with increasing molar fraction. Furthermore, CO2 shows a decrease of permeance with increasing temperature, owing to the reduction of the adsorbed phase (lower coverage). The permeance at 303 K is more than twice of that at 363 K, in agreement with the reduction observed by Himeno et al.23 from 298 to 373 K in single gas condition. Concerning H2, permeance increases with temperature owing to the reduction of CO2 adsorption. Nevertheless, this increment achieves a maximum followed by the decreasing trend of surface diffusion at high temperatures. The strong CO2 permeance reduction with increasing 11433


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Industrial & Engineering Chemistry Research temperatures predicted by our simulations up to 573 K was also observed by other authors in single gas conditions.13,14 Specifically, the open circles in Figure 2 are the experimental values of van den Bergh et al.13 between 303 and 673 K, using a pressure difference of 100 kPa for single gas and under pressure drop condition, with atmospheric pressure in the permeate side. The open triangle in Figure 2 are the results of Kanezashi et al.,14 in a temperature range comprised between 298 and 773 K, by feeding at 300 kPa and using sweep gas at atmospheric pressure on the permeate side. The different operating conditions (i.e., driving force and single gas and gas mixture) and zeolite thickness are the reason for the discrepancy in the permeance values. However, this discrepancy is more relevant in the case of Kanezashi et al.14 In fact, their experimental values were obtained for a 10 μm membrane thickness, whereas in this work thickness was chosen at 1 μm. Moreover, the higher feed pressure that they adopted (i.e., 300 kPa) causes a lower permeance. Therefore, the quite low permeance of Kanezashi et al. at a low temperature can be attributed to these two effects. On the contrary, the permeance of Kanezashi et al. becomes higher than that of this work at a high temperature, since the authors found the presence of gas translation diffusion that here was not taken into account. Figure 3 shows CO2/H2 selectivity as functions of CO2 concentration at different temperatures. Specifically, DD3R

Figure 4. CO2/H2 selectivity as functions of (left side) CO2 molar fraction and (right side) CO2 partial pressure at different feed pressures at 273 K.

selectivity changes from about 95 to 120 when feed pressure increases from 200 to 500 kPa, feeding an equimolar stream. 4.2. Permeation through Silicalite Membranes. Figure 5 shows CO2 and H2 permeance as functions of CO2 partial

Figure 3. CO2/H2 selectivity as functions of CO2 molar fraction at different temperatures in a DD3R membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa.

results to be selective toward carbon dioxide at low temperature (CO2/H2 of ca. 75 for an equimolar mixture at 273 K) owing to the competitive adsorption favoring the mostly adsorbing species (i.e., CO2) with respect to the weakly adsorbing one (i.e., H2). Moreover, the increment in selectivity with CO2 concentration is owing to the increment of the concentration of the mostly adsorbing species, further hindering permeation of H2. The relatively low adsorption at higher temperatures causes a reverse selectivity. Therefore, DD3R becomes selective toward H2, achieving selectivity of 7 ca. at 573 K, considering an equimolar mixture. Figure 4 shows the effect of feed pressure on the separation performance at 273 K; specifically, left and right sides describe CO2/H2 selectivity values as a function of CO2 molar fraction and feed partial pressure, respectively. For a fixed CO2 concentration, an increment of feed pressure increases the selectivity. In fact, CO2 permeance is slightly reduced by the higher interaction among the adsorbed molecules, whereas the H2 reduction is more pronounced. Therefore, CO2/H2

Figure 5. CO2 and H2 permeance as a function of CO2 molar fraction at different temperatures in a silicalite membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa.

pressure at different temperatures. Permeance of both the components was found to decrease with increasing CO2 composition at the lower temperatures (e.g., 273 K) for different reasons: CO2 permeance decreases since the increment in driving force becomes more important than that in molar flux,12 differently from the H2 one because of the higher blocking effect. On the other hand, a constant trend was observed at the higher temperatures (e.g., 573 K), stating the presence of the Knudsen diffusion contribution alone to permeation. Moreover, permeance of the two species has a different temperature dependence. Specifically, CO2 permeance continuously decreases with increasing temperature because of the loading reduction and the gradual change from surface to Knudsen diffusion domain. On the other hand, 11434


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Industrial & Engineering Chemistry Research H2 permeance at first increases with increasing temperature, stating the reduction of the blocking effect owing to the adsorbed CO2. Once reached a condition for which the adsorbed CO2 is quite low, H2 permeance starts to decrease with increasing temperature as in the Knudsen regime. Furthermore, by comparing the results of the two zeolites (Figure 2 and Figure 5), it can be observed that both H2 and CO2 permeance in DD3R is lower than that in silicalite. This large difference can be attributed to the higher diffusivity in silicalite, since the saturation loading of CO2 is similar (Figure 1), Figure 6 plots the selectivity as a function of CO2 feed molar fraction for different temperatures. Selectivity tends to remain

The effect of feed pressure on CO2/H2 selectivity is shown in Figure 7. Considering the same mixture composition, an


increment in feed pressure has a beneficial effect on the predicted selectivity, which can pass from 5 ca. (at 200 kPa) to 30 ca. (at 500 kPa) in the case of an equimolar mixture. This can be explained as the higher pressure amplifies the interaction among the molecules, causing a reduction of permeances. However, the H2 reduction is more pronounced with respect to CO2 and, therefore, selectivity increases. Specifically, CO2 permeance decreases because the saturation condition was approached, that is, high fractional coverage in both feed and permeate and, thus, low driving force to permeation.2,24 On the other hand, H2 permeance is not favored by feed pressure because CO2 adsorption increases, this further hindering the hydrogen permeation. This selectivity improvement with increasing feed pressure was also predicted by Wirawan et al.,25 who estimated separation factors of 3 and 8 at 273 K using a feed pressure of 2 and 5 times the permeate one, respectively. The selectivity values estimated in this work are higher (i.e., 6 ca. and 35 ca.) probably because defect-free membranes are taken into account in the simulations and the influence of the support is not considered. Furthermore, also Siöberg et al.26 obtained an increment of separation factor from 20 ca. to 30 ca. when feed pressure varies from 3 to 8 bar. 4.3. Permeation Through NaY Membranes. The same analysis was carried out for the zeolite NaY, for which the two species show a different behavior (Figure 8). In particular, CO2 permeance decreases with increasing molar fraction, which is in agreement with the results obtained for the other zeolites. It is interesting to notice that H2 permeance drastically drops with increasing CO2 concentration in the adsorption region (i.e., 273 and 303 K) up to achieving a minimum and then a slight increment for high CO2 concentration. This trend can be explained considering that the reduction of driving force becomes more important than that of the molar flux at relatively low H2 concentration in the feed stream. Concerning the temperature dependence of permeance, CO2 shows a different trend from that observed in DD3R and silicalite. In fact, its permeance initially increases with the temperature, since the maximum of surface diffusion is not achieved yet. This maximum in NaY is reached at a higher temperature than in silicalite and DD3R. An inverse temperature dependence was observed above 363 K, since both surface and Knudsen contributions are important for

Figure 6. CO2/H2 selectivity as a function of CO2 molar fraction at different temperatures in a silicalite membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa. O and Δexperimental values of 18 and 1, respectively.

constant in the whole CO2 feed molar fraction range considered, since both H2 and CO2 permeances decrease with increasing CO2 concentration. As expected, a significant reduction of its value was observed with increasing temperature, passing from 5 ca. (at 273 K) to 0.4 ca. (at 573 K). Furthermore, a reverse selectivity was observed above 363 K. These values were compared with some literature ones (Figure 6). Specifically, Bakker et al.1 tested an equimolar mixture, estimating selectivity values of ca. 8 and 0.47 at 303 and 573 K, respectively. It can be observed that these values (open green triangles) are higher than those estimated in this work. This discrepancy can be attributed to the differences in the zeolites: in fact, these simulations were carried out considering the surface diffusion parameters obtained in12 using the zeolite synthesized by Algieri et al.15 In addition, Kanezashi et al.18 tested an equimolar ternary mixture made up of CO2, CO, and H2, estimating CO2/H2 selectivity values of 5.8 ca. and 0.24 ca. at 298 and 573 K, respectively (open red circles). The selectivity at 298 K is similar to that obtained in this work at 273 K, whereas that at 573 K is equal to the Knudsen value (i.e., 0.21). In this work, the value at 573 K is slightly higher (ca. 0.4), stating a very small presence of surface diffusion even at a high temperature. Moreover, selectivity at 573 K slightly decreases with increasing CO2 composition, passing from 0.38 to 0.36. The two experimental values of the open literature, discussed above, are the one1 over and the other below18 than our calculated line showing a trend apparently opposite. Differences in the resulted membranes and operating conditions (e.g., feed and permeate pressure) might be the reasons of the discrepancy between the experimental measures and with the simulations. 11435


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Figure 9. CO2/H2 selectivity as functions of CO2 molar fraction at different temperatures in a NaY membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa. O experimental value of 4

therefore, the permeance continuously decreases (Figure 8). On the contrary, H2 permeance initially decreases (Figure 8) owing to the pore blocking and, after this maximum, slightly increases owing to the reduced H2 permeating driving force. On the other hand, selectivity was found to be independent of CO2 concentration at high temperature and an inversion of selectivity values was observed at around 423 K. Specifically, a value of 3 ca. was found at 573 K for H2/CO2. The quite high selectivity values predicted in the lowtemperature region are in a good agreement with the few literature data available. In fact, Kusakabe et al.4 measured a CO2/H2 selectivity of 27.8 for an equimolar mixture at 308 K (red circle in Figure 9) and the present simulation predicts a value of 40 ca. at 303 K. The specific behavior as discussed above for the H2 permeance reflects in the two orange and gray areas of Figure 9, which highlight a large difference in the range 273−303 K and 303−333 K and at lower and higher CO2 molar fraction, respectively. Figure 10 shows CO2/H2 selectivity values at 273 and 303 K as a function of CO2 feed molar fraction. An increasing or

Figure 8. CO2 and H2 permeance as functions of CO2 molar fraction at different temperatures in a NaY membrane. Feed pressure = 100 kPa, permeate pressure = 1 kPa.

permeation and both are inversely proportional with temperature. On the contrary, H2 permeation was found to follow mainly the Knudsen diffusion. Therefore, its permeance increases continuously with increasing temperature, because CO 2 blocking tends to disappear. This increment tends to become negligible at higher temperatures (453 and 573 K), where the blocking is not important and permeation approaches the Knudsen-type behavior. In fact, an inverse temperature dependence is expected above 573 K. A comparison with the results of the other zeolites (i.e., Figure 2 and Figure 5) shows that at low temperatures NaY has a CO2 permeability higher than that of DD3R but lower than that of silicalite. Despite the higher adsorption capacity and pore size of NaY with respect to silicalite, its lower permeability is owing to the lower diffusivity because of the blocking effect. On the other hand, NaY was found to be the more permeable at high temperatures (e.g., 573 K), since only Knudsen diffusion is present in these conditions and, thus, the pore size becomes important to the gas permeation. The important CO2 blocking effect in NaY has an significant impact on the selectivity values at low temperatures (Figure 9). In fact, selectivity increases with increasing CO2 molar fraction up to a maximum of 125 ca. at 273 K and 40 ca. at 303 K, corresponding to a CO2 partial pressure of 25 and 65 kPa, respectively. Once achieved the maximum, selectivity decreases with increasing CO2 concentration, since CO2 permeance decreases more than the H2 one. The CO2 coverage increases with CO2 feed molar fraction up to the maximum CO2 coverage possible of the pore surface, which happens around the maximum in CO2/H2 selectivity. The pore blocking is directly correlated to the coverage, thus, it reaches its maximum around selectivity maximum. After this point, any further increase in CO2 molar fraction and, hence, its partial pressure does not contribute to further increase CO2 flux;

Figure 10. CO2/H2 selectivity as a function of CO2 molar fraction at 273 and 303 K for a NaY membrane.

decreasing CO2/H2 selectivity is observed as a function of CO2 feed molar fraction at 303 or 273 K, respectively. This indicates as significant the temperature and composition are in the zeolite membrane separations. In addition, the selectivity at lower temperature is always higher. Figure 11 shows CO2/H2 selectivity as a function of CO2 concentration for different feed pressures. In this case, an 11436


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selectivity (the reverse of before) in both silicalite and NaY (3 ca.) is smaller than the Knudsen value (i.e., 4.7), stating that a small CO2 adsorption is still present even at a high temperature. On the other hand, a value slightly higher (7 ca.) was obtained for DD3R.

5. CONCLUSIONS In this work, CO2 and H2 permeation through zeolite membrane was investigated considering both surface and Knudsen diffusion. The relative contributions of such different diffusion mechanisms, whose extent strongly depends on temperature, was found to determine membrane permeance and selectivity. Specifically, we observed that H2 permeance is much lower than the CO2 one at a low temperature, whereas it increases with increasing temperature owing to the gradually weaker CO2 adsorption. Consequently, H2 permeation was observed to be faster than that of CO2, resulting in a very high CO2/H2 selectivity (>100) at 273 K. For instance, CO2/H2 selectivity at 273 K and at a feed pressure of 100 kPa was evaluated to be around 90, 75, and 5 for NaY, DD3R and silicalite, respectively, feeding an equimolecular mixture. These values decrease to 35, 20, and 2.5 at room temperature. An opposite trend of H2/CO2 selectivity−approaching the Knudsen value of 4.69 - was observed at a high temperature owing to the highly reduced CO2 sorption strength. The CO 2 molar fraction positively affected CO 2 /H2 selectivity in DD3R at room temperature, which shows an almost linear functionality, whereas the selectivity of silicalite was found to be almost independent of CO2 molar fraction. Concerning NaY, an increasing or decreasing CO2/H2 selectivity is observed at 303 or 273 K, respectively. Our systematic approach shows that CO2/H2 selectivity strongly depends on temperature, this being particularly evident for NaY membranes, which are characterized by the highest adsorption among the zeolites here proposed. Such a peculiarity indicates that NaY membranes are good candidates for an effective CO2/H2 separation for environmentally sustainable applications within an appropriate temperature range.


increment in feed pressure (and, hence, in driving force) does not affect the selectivity, except in the low CO2 concentration region. In these conditions, a change from 200 to 500 kPa causes a variation of selectivity from 70 to 200 for a mixture H2:CO2 = 95:5 at 273 K. Differently from the other zeolites, selectivity decreases with increasing CO2 concentration because CO2 permeance reduction is higher than that of H2, except at low pressure (i.e., 200 and 300 kPa), in which the opposite behavior was observed. 4.4. Zeolite Membranes Comparison. Figure 12 compares the separation performance of the three different

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Figure 12. Selectivity at (left side) 303 and (right side) 573 K for different H2:CO2 mixtures in (red) DD3R, (blue) silicalite and (green) NaY. Feed pressure = 100 kPa.

AUTHOR INFORMATION

Corresponding Author

*Phone: +39 0984 492029. Fax: +39 0984 402103. E-mail: g. [email protected].

zeolite membranes considering different mixture composition at 303 and 573 K. Zeolites NaY and DD3R provide the best selectivity at a low temperature for different reasons. NaY has the highest CO2 adsorption, whereas DD3R has an adsorption similar to silicalite but a smaller pore size, which magnifies the blocking effect on H2 permeation. This result was also confirmed by the molecular studies of Krishna and Van Baten,27 who analyzing several adsorbents, concluded that NaX and NaY are the best ones for CO 2 /H 2 separation. Furthermore, the selectivity toward CO2 tends to become higher with its increasing concentration, achieving a value of 38 ca. for a mixture CO2:H2 = 75:25 in NaY. A drastic reduction of selectivity down to 4 ca. was observed for a mixture CO2:H2 = 5:95, since CO2 composition is too small. The CO2/H2 selectivity in DD3R was 30 ca. for a CO2:H2 = 75:25 mixture. Selectivity values of the three materials are quite low at high temperatures (e.g., 573 K). It can be observed that H2/CO2

ORCID

Adele Brunetti: 0000-0003-4545-6637 Giuseppe Barbieri: 0000-0001-5583-8634 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.F.Z. gratefully acknowledges the “SIACE” Ph.D. Course held at the University of Calabria. The research project PON 01_02257 “FotoRiduCO2 − Photoconversion of CO2 to methanol fuel”, cofunded by MiUR (Ministry of University Research of Italy) with Decreto 930/RIC 09-11-2011 in the framework the PON “Ricerca e competitività 2007-2013″, is gratefully acknowledged. 11437


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(23) Himeno, S.; Tomita, T.; Suzuki, K.; Nakayama, K.; Yajima, K.; Yoshida, S. Synthesis and permeation properties of a DDR-type zeolite membrane for separation of CO2/CH4 gaseous mixture. Ind. Eng. Chem. Res. 2007, 46, 6989. (24) Golden, T. C.; Sircar, S. Gas Adsorption on Silicalite. J. Colloid Interface Sci. 1994, 162, 182. (25) Wirawan, S. K.; Creaser, D.; Lindmark, J.; Hedlund, J.; Bendiyasa, I. M.; Sediawan, W. B. H2/CO2 permeation through a silicalite-1 composite membrane. J. Membr. Sci. 2011, 375, 313. (26) Sjöberg, E.; Barnes, S.; Korelskiy, D.; Hedlund, J. MFI membranes for separation of carbon dioxide from synthesis gas at high pressures. J. Membr. Sci. 2015, 486, 132. (27) Krishna, R.; van Baten, J. M. In silico screening of zeolite membranes for CO2 capture. J. Membr. Sci. 2010, 360, 323.

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