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Adsorption and Separation of C1C8 Alcohols on SAPO-34 Tom Remy,† Julien Cousin Saint Remi,† Ranjeet Singh,‡ Paul A. Webley,‡ Gino V. Baron,† and Joeri F. M. Denayer*,† † ‡
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Department of Chemical Engineering, Monash University, P.O. Box 36, Clayton Victoria, 3800, Australia
bS Supporting Information ABSTRACT: Adsorption and separation of 1-alcohols were studied on the SAPO-34 molecular sieve, which is the catalyst of choice for the methanol-toolefins (MTO) process. Vapor phase adsorption isotherms of methanol and ethanol were measured at 343 K using the gravimetric technique. Liquid phase isotherms of pure 1-alcohols and mixtures were obtained by batch adsorption measurements at room temperature. These experiments highlighted the occurrence of a window effect, giving rise to a strongly chain length-dependent adsorption and diffusion. Lower alcohols such as methanol and ethanol were able to fill the entire pore volume. Alcohols larger than 1-butanol only adsorb in very small amounts after 3 h, as their adsorption in the SAPO-34 pores is sterically hindered, which gives rise to diffusional limitations. Binary batch experiments showed preferential adsorption of short chain alcohols (i.e., ethanol) from longer chain molecules. Breakthrough separation experiments with several alcohol mixtures were performed at 298473 K and varying flow rates from 0.1 to 4.0 mL/min. Separation of ethanol from hexanol could be achieved at room temperature. SAPO-34 is also able to selectively remove ethanol from 1-propanol at room temperature. An increase in temperature to 348 K, however, improves the separation as a result of the improved ethanol diffusivity. The observed effect can be used to separate short chain molecules from longer ones.
’ INTRODUCTION Recently, the interest in the production of bioethanol and other biofuels or biochemicals such as biobutanol has greatly increased as a result of increasing fossil fuel prices and associated environmental issues.1 If the desired oxygenates are produced via fermentation, their separation remains a significant challenge since they only constitute a minor fraction of the fermentation broth. The final butanol concentration is typically at most 20 g/L due to product inhibition.2 An energy intensive and expensive distillation step would therefore be required to separate the alcohol from the solution. Several processes including pervaporation,3,4 liquidliquid extraction,5 gas stripping,6 and adsorption onto zeolites have been proposed as other potential recovery techniques for the produced alcohols.7,8 Qureshi mentions adsorption on zeolitic materials as the most energy-efficient technique to recover butanol from the fermentation broth.9 Positive features of zeolites include high temperature and pressure stability, selectivity, low heat capacity, and homogeneity. Most of the adsorption studies related to the separation of the desired alcohol from the fermentation broth have focused on the alcohol/water separation, while only few investigated the competitive adsorption effects due to other oxygenated byproducts (mainly ethanol and acetone in biobutanol production). Bowen et al. showed that acetic acid could significantly reduce ethanol adsorption from a ternary ethanol/water/acetic acid mixture.8 Oudshoorn reported the competitive adsorption of 1-butanol, acetone, and ethanol on three structurally different and commercially available high-silica zeolites.10 r 2011 American Chemical Society
The MFI type CBV 28014 (Si/Al = 280) was clearly able to discriminate between the different C2, C3, and C4 components. In a recent paper, Daems et al. reported the results of a study concerning the liquid phase adsorption of C1C8 alcohols on K-chabazite.11 Liquid phase batch adsorption experiments showed that molecules larger than 1-butanol were almost fully excluded from the pores of the adsorbent. Contrarily, methanol and ethanol were able to fill the complete pore volume, thus giving rise to a chain length-dependent adsorption. The question whether this effect could be used to separate the different C2, C3, and C4 alcohols under continuous conditions, however, remained unanswered. Chabazite (CHA) is a cage and window type molecular sieve built up of ellipsoidal-shaped cages of 6.7 10 Å2 interconnected via 8-membered ring windows with pore apertures of 3.8 3.8 Å2. The adsorption onto cage and window type molecular sieves has been the subject of debate around the occurrence of a so-called window effect, that is, a drastic decrease in molecular diffusivity once the adsorbate becomes just too large to fit within one zeolite cage, which leads to a chain length-dependent adsorption.12 Although the correctness of the early measurements by Gorring12 is still a matter of debate,1315 several experimental and modeling studies have shown that peculiar adsorption phenomena occur once the size of the Received: December 7, 2010 Revised: February 15, 2011 Published: April 05, 2011 8117
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The Journal of Physical Chemistry C adsorbate becomes (almost) equal to or larger than the size of the windows of the adsorbent's cages.1625 Molecular simulations of the adsorption of n-alkanes on zeolites consisting of cages connected via small windows by Dubbeldam et al. revealed a distinct decrease in the Henry adsorption constant starting from a certain alkane chain length of comparable size to the zeolite's cage.26 Denayer et al. studied the low coverage adsorption properties of n-alkanes (C1C12) on SAPO-34 via pulse chromatographic experiments.16 SAPO-34 is a silicon-, aluminum-, and phosphorus-based molecular sieve with the CHA topology. The obtained Henry constants decreased when the alkane chain length became just larger than the zeolite cage, which is the case for alkanes in the C6C8 range. Krishna and van Baten performed configurational-bias Monte Carlo (CBMC) simulations on the adsorption of n-alkanes on the pure silica forms of CHA, ERI, AFX, RHO, and KFI. In the vapor phase, the longer chain is preferentially adsorbed in the zeolite pores, while in the liquid phase, a selectivity reversal occurs with a higher adsorption affinity for the shorter alkane as it packs more easily within the zeolite cages.27 As with its isostructural CHA analogue, experimental adsorption studies up to now on SAPO-34 have focused on small (gaseous) molecules.2830 Agarwal et al. found SAPO-34 to be a potential candidate for the kinetic separation of propylene from propane.31 Maple and Williams investigated the separation of nitrogen from methane.32 Takeguchi et al. reported selective adsorption of CO2 from N2 onto Ni incorporated SAPO-34.33 The separation of CO2 from CH4 and H2 on SAPO-34 membranes has extensively been studied.3436 Obtained selectivities in the separation of carbon dioxide from methane or hydrogen are high at room temperature but strongly decrease with increasing temperature. SAPO-34 is also widely known as the catalyst of choice for the MTO process.37 Its large olefin yield results from the combination of the diffusional constraints on one hand and its mild acidity on the other hand.38 A whole body of literature has already been devoted to the kinetics of the MTO process on SAPO-34 while only few studies have been undertaken to investigate its fundamental adsorption characteristics. As a result of the intriguing results from molecular simulations and experimental adsorption data on the isostructural K-CHA, it was decided to investigate the fundamental adsorption characteristics on SAPO-34. Static and dynamic adsorption properties of selected n-alcohols and n-alkanes were recorded to investigate the fundamental adsorption properties of SAPO-34 and to evaluate the possible occurrence of a window effect. At the same time, with the prospect of a changing fuel mix from hydrocarbons to oxygenates, the study of the adsorption characteristics of alcohols on adsorbents such as zeolites39 and the subsequent development of structure property correlations and/or adsorptive separation processes for these compounds become highly relevant.
’ EXPERIMENTAL METHODS Materials. Commercial SAPO-34 samples were obtained from Tianjin Chemist Scientific Ltd. (Tianjin, China). The unit cell formula is Si4.02Al18.32P14.58O72 as obtained via inductively coupled plasma atomic emission spectroscopy (ICP-AES). The pore volume is 0.30 mL/g, and the BET surface area is 590 m2/g as determined via N2 porosimetry (Autosorb AS-1, Quantachrome Instruments, United States). The sample was degassed by heating to 350 C under vacuum. This final temperature was
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kept for 20 h. The crystal size, as determined by scanning electron microscopy, ranged from 5 to 30 μm. Vapor Phase Adsorption. Vapor phase adsorption isotherms were measured using the gravimetric technique. A reservoir, filled with the liquid adsorbate, is held at constant temperature through Peltier elements. Helium (He) bubbling through the container entrains the organic vapor. This He-organic vapor stream continuously flows over the sample positioned in a sample holder connected to the microbalance. About 10 mg of the adsorbent powder was placed in a quartz sample holder and positioned in the microbalance system (VTI Corporation, United States). After activation by heating to 623 K at a heating rate of 1 K/ min under helium flow (325 mL/min), adsorption isotherms of methanol and ethanol were determined at 343 K by weighing the adsorbate uptake at different partial pressures of the adsorbate. The adsorbate partial pressure was altered by changing the temperature of the reservoir and/or by diluting the saturated flow. It also has been attempted to measure the adsorption isotherm of 1-propanol, but adsorption of this component is subject of severe diffusional limitations (vide infra), leading to experimental equilibration times of several days per isotherm point. Therefore, it was decided not to measure the complete adsorption isotherms of 1-propanol and the higher 1-alcohols. Batch Adsorption. The batch adsorption technique (static method or immersion technique) was chosen to determine the adsorption capacity of several n-alkanes and 1-alcohols and the competitive adsorption effects of binary mixtures of 1-alcohols in liquid phase, with the use of an inert, nonadsorbing solvent. For all adsorbates, except for methanol, iso-octane (g99.5% purity, BioSolve) was used as a solvent, as it is too large to enter the SAPO-34 pores. Because methanol does not dissolve into iso-octane, tert-butanol (g99% purity, Fluka) was used as a nonadsorbing solvent for methanol. All adsorbates studied (n-alkanes and 1-alcohols) were of analytical grade. Adsorbent samples (0.10.4 g) were put in 20 mL glass vials and slowly heated (1 K/min) in a ventilated oven until a maximum temperature of 623 K was reached, which was kept overnight. The applied temperature program allowed the removal of water and impurities present inside the adsorbent pores. After regeneration, the vials were immediately sealed with a cap with a septum to avoid water uptake from the air. After determination of the mass of the regenerated zeolite, the liquid mixture was injected through the septum. For isotherm determination of the pure components, the concentration of the adsorbate in the solvent ranged from 0.3 to 15 wt %. This also allowed us to verify whether the saturation capacity was affected by the adsorbate concentration. During the binary mixture experiments, the fractions of the adsorbing components in the solvent varied between 0 and 12.5 wt %. For every binary mixture, a certain amount (∼1.5 mL) was used as a blank. Afterward, the vials were stirred continuously at room temperature and atmospheric pressure. Liquid samples were taken after 3 h. All binary blanks and samples were analyzed using a gas chromatograph (GC) with flame ionization detector. An Agilent HP-5 column (5% phenyl methyl siloxane, 30 m 320 μm 0.25 μm film thickness) was used to determine the composition of the mixture. Calibration lines for the pure components in iso-octane were obtained from analysis of seven blank adsorbateiso-octane mixtures. For every binary mixture, a calibration line was obtained by analysis of the binary blanks (see above). The adsorbed amount of adsorbate Q (g/ g) was obtained by calculation of the mass balance: Q ¼ 8118
C0 ML, 0 Ceq ðML, 0 Vmicro Fsorbate Ms Þ 100Ms
ð1Þ
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where C0 and Ceq are the adsorbate concentrations before adsorption and at equilibrium (in wt%), ML,0 is the total mass (g) of external liquid phase before adsorption, Ms is the adsorbent mass (g) after regeneration, Vmicro is the micropore volume (mL/g) of the adsorbent, and Fsorbate is the density of the adsorbate at room temperature (g/mL). The number of adsorbate molecules per supercage of the SAPO-34 adsorbent (N) was calculated with the following formula: N ¼ Q sat 3
Na Nc
ð2Þ
where Qsat is the saturation capacity (in mol/g), Na is Avogadro's number (6.02 1023), and Nc is the number of supercages per gram SAPO-34 (2.73 1020 supercages/g). For the binary selectivity diagrams, the internal mole fraction Yi and the external mole fraction Xi have to be calculated. Converting the adsorbed amounts of both components in g/g, as obtained from formula 1, to molar amounts and taking the ratio of the molar adsorbed amount of component i to the total molar adsorbed amount yields the internal mole fraction Yi. Because the initial mass of adsorbate and the adsorbed amount (see formula 1) are known, the adsorbate mass in the fluid phase easily can be obtained. The ratio of the molar amount of component i in the external fluid phase to the total external molar amount is equal to the external molar fraction Xi. Within this paper, X will refer to the external mole fraction of the longest chain adsorbate. The internal mole fraction of component i (Yi) can be regarded as the selectivity of the adsorbent for that given adsorbate i. Breakthrough Experiments. Breakthrough experiments were conducted on a fully automated setup from ILS GmbH (Berlin, Germany). A detailed description and scheme of the setup can be found in the Supporting Information. A stainless steel column (15 cm length, 1.27 cm internal diameter) was packed with SAPO-34 powder and regenerated overnight under N2 flow (25 mL/min) at 473 K. Dried iso-octane was used as a solvent for the binary 1-alcohol mixtures. The ethanolhexanol mixture contained 5.6 wt % ethanol, 5.8 wt % 1-hexanol, and 88.6 wt % iso-octane. For the ethanol1propanol mixture, the composition was as follows: 10 wt % ethanol, 10 wt % 1-propanol, and 80 wt % iso-octane. A ternary methanol1-propanol1-butanol mixture consisting of 5.0 wt % methanol, 5.4 wt % 1-propanol, and 89.6 wt % 1-butanol was also investigated. The mixtures were pumped over the column with a flow rate ranging from 0.1 to 4.0 mL/min. Experiments were performed in the temperature range 298473 K. Liquid samples were taken every 60 s during at least 50 min and analyzed online by a TRACE-GC (Interscience). The amount of component i adsorbed is calculated from the following formula:40 qi ¼
νi 3 τ i ε Ci 1 1 ε Fp L
ð3Þ
where vi is the interstitial velocity (cm/s), τi is the mean breakthrough time (s), L is the column length (cm), ε is the porosity of the bed (), Ci is the initial concentration of adsorbate i (g/mL), and Fp is the density of the adsorbent (g/mL).
Figure 1. Vapor phase adsorption isotherm of methanol on SAPO-34 at 343 K. Inset: Uptake kinetics for four selected points in the adsorption isotherm.
Figure 2. Vapor phase adsorption isotherm of ethanol on SAPO-34 at 343 K. Inset: Uptake kinetics for three selected points in the adsorption isotherm.
The mean breakthrough time is obtained from eq 4: ! Z ¥ xi, t 1 dt τi ¼ xi, 0 0
ð4Þ
where xi,t is the mole fraction of component i at time t and xi,0 is the initial mole fraction of component i. In the resulting Figures 913, the normalized flow rate F/F0 (the exit flow rate divided by the feed flow rate) has been plotted as a function of the eluted volume V (in mL).
’ RESULTS AND DISCUSSION Vapor Phase Adsorption of Alcohols. Vapor phase adsorption isotherms of methanol and ethanol on SAPO-34 are given in Figures 1 and 2, respectively. Both isotherms are of type I with almost equal saturation capacities for both alcohols: 18 wt % for methanol and 16 wt % for ethanol. The maximum adsorbed amount of methanol is in very good agreement with previously reported data by Chen et al., who obtained a methanol capacity of 17 wt %, corresponding to 6.2 methanol molecules per SAPO-34 cage.41 To highlight the kinetics of the adsorption process, Figures 1 and 2 also show the adsorbed amount versus the time needed to attain equilibrium for at least three selected points in the adsorption isotherms of methanol and ethanol. The zero level for these curves in Figures 1 and 2 always corresponds to the 8119
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Figure 4. Adsorbed amounts after 3 h of equilibration time for C1C8 1-alcohols on SAPO-34 at 298 K. Figure 3. Vapor phase adsorption of 1-propanol on SAPO-34 at 343 K as a function of time after a step in partial 1-propanol pressure from 0 to 1.2 mbar.
adsorbed amount of the previous recorded isotherm point. Equilibration times are relatively small for both alcohols, that is, always less than 30 min. Because the slope of the isotherms becomes smaller at higher adsorbate pressures, uptake curves will also show decreasing initial slopes at higher partial pressures of the alcohol. Therefore, the equilibration time slightly increases when the maximum capacity for the alcohol is approached. Uptake kinetics of 1-propanol differ significantly from those of methanol and ethanol. Figure 3 shows the uptake of 1-propanol as a function of time after a step in partial pressure from 0 to 1.2 mbar. The equilibration time is not in the order of minutes now but in the order of days. It took about 350 h (almost 15 days) to reach an equilibrium adsorbed amount of 11 wt %. The full adsorbed amount of propanol could be desorbed by heating the material to 473 K at a rate of 1 K/min under helium flow. Therefore, it can be stated that the very slow increase in amount adsorbed at large times (time >100 h) is not a result of coking but clearly a result of diffusional limitations. Similar diffusional limitations are expected for longer alcohols. At the given adsorbate partial pressure of 1.2 mbar, the equilibrium adsorbed amount of 1-propanol (11 wt %) is almost equal to the equilibrium values obtained with methanol and ethanol (9.8 and 10 wt %, respectively). However, at an equilibration time of 2 h, methanol and ethanol will both reach their equilibrium values (∼10 wt %), whereas only 1.3 wt % of 1-propanol will be adsorbed. Liquid Batch Adsorption—Single Component. To check whether the chain length-dependent adsorption, as perceived in vapor phase conditions, also appears in liquid phase, liquid phase batch experiments were carried out with C1C8 alcohols. Given the very low diffusivity of 1-propanol vapor into SAPO34 (see Figure 3), we prefer to specify adsorbed amounts of 1-propanol and higher alcohols after a few hours (3 h in this study), rather than to mention adsorption isotherms, which represent equilibrium values, since equilibrium in the strict sense cannot be reached in a couple of hours. In addition, the chosen time frame is relevant for industrial separations where the time scale for the adsorptive separation in liquid phase is maximum a few hours. Moreover, evaporative losses of adsorbate are minimized in this way.
Experiments have shown that the adsorbed amounts of 1-propanol and higher alcohols after 24 h differ by a maximum 1.5 wt % from those after 3 h (Tables S1 and S2 in the Supporting Information). Measurements after 96 h revealed a small increase in capacity, that is, an approximate increase of about 3.0 wt % for propanol and butanol (Tables S1 and S2 in the Supporting Information). After 144 h, the increase in capacity for propanol and butanol is, respectively, 3.5 and 4 wt % (Tables S1 and S2 in the Supporting Information). The above observations again point out the very low diffusivity of 1-propanol and higher alcohols. Figure 4 shows the adsorbed amounts of the different C1C8 1-alcohols in the liquid phase on SAPO-34 at room temperature after 3 h. Methanol and ethanol are adsorbed in far larger quantities than propanol and the other longer 1-alcohols. The largest saturation capacity is reached for ethanol, with about 23 wt % slightly exceeding the capacity as observed in vapor phase at 343 K (Figure 2). Methanol is adsorbed up to 22 wt %. On the other hand, 1-propanol is adsorbed in much smaller amounts after 3 h (
