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Molecular Sieving Separation of Hexane Isomers within Nanocomposite (B)-MFI-Alumina Hollow Fiber Membranes: A Modeling Study Z. Deng,†,‡ M. Pera-Titus,*,‡ Y. Guo,*,† and A. Giroir-Fendler‡ Key Laboratory for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, 200237 Shanghai, P. R. China, and UniVersite´ de Lyon, Institut de Recherches sur la Catalyse et l’EnVironnement de Lyon (IRCELYON), UMR 5256 CNRSsUniVersite´ Lyon 1, 2 AV. A. Einstein, 69626 Villeurbanne cedex, France
High quality nanocomposite B-MFI hollow fibers were successfully prepared by pore-plugging hydrothermal synthesis. The incorporation of boron into the MFI structure modifies the n-hexane/2,2-dimethylbutane vapor separation properties of the membranes, with the materials improving the n-hexane/2,2-dimethylbutane intrinsic separation factors of Al-MFI by a factor of 2 (up to a value of 200) at comparable conditions. The permeation performance of the membranes is essentially governed by a molecular sieving mechanism driven by preferential diffusion of n-C6 and configurational diffusion effects favoring n-hexane adsorption from n-hexane/2,2dimethylbutane mixtures. A strong confinement scenario for surface diffusion of both n-hexane and 2,2dimethylbutane seems to prevail. 1. Introduction Tertiary ethers such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) have been traditionally used as blending oxygenates for reformulated gasolines to reduce evaporative and tailpipe emissions, promote complete burning, and boost octane numbers. The future use of these ethers is however under discussion due to their growing demand, particularly for MTBE, the dependence of MTBE and TAME on methanol, and the stricter environmental standards on unburned hydrocarbons and aromatics, carbon monoxide, NOx, and particulates (MTBE has been classified by the U.S. EPA as drinking water contaminant candidate).1 These limitations motivate the need for alternative solutions promoting the octane number of reformulated gasolines while facing a more stringent emission scenario. Four technological possibilities emerge for gasoline upgrading: (i) survey of less volatile oxygenates such as iso-propyl tert-butyl ether (IPTBE)2 or sustainable substitutes produced from renewable sources alternative to bioethanol such as acetals and ketals from glycerol,3,4 (ii) upstream conversion of volatile compounds (e.g., dimerization of photoreactive isoamylenes to diisoamylenes5), (iii) comixing of the gasoline with synthesis gas in the combustion chamber,6,7 and (iv) hydroisomerization of linear paraffins to branched alkanes, offering improved burning efficiency and enhanced octane numbers. Among the different alternatives mentioned above, hydroisomerization of C5 and C6 alkanes appears to be a serious option. This process is carried out industrially using a catalyst based on platinum over mordenite (Pt/H-MOR), β-zeolite (Pt/ H-BEA), or chlorinated alumina (Pt/Al2O3-Cl) minimizing simultaneous hydrocracking.8 Because of equilibrium limitations, linear alkanes cannot be fully converted into more valuable dibranched alkanes, providing the highest octane numbers. For comparison, n-hexane (n-C6) has a reseach octane number (RON) of 30, while the dibranched isomers, 2,2-dimethybutane (2,2-DMB) and 2,3-dimethybutane (2,3-DMB), have RONs of * To whom correspondence should be addressed. E-mail:
[email protected] (M.P.-T.);
[email protected] (Y.G.). † East China University of Science and Technology. ‡ Universite´ de Lyon.
94 and 105, respectively. A separation unit of nonconverted linear and monobranched alkanes is therefore necessary, this usually consisting of a pressure-swing adsorption column filled with a zeolite 5A adsorbent,9 the separated isomers being further recirculated to the reaction unit. The large recirculation flow rates involved in the process makes the integration of the reaction and the separation processes in the same unit economically advantageous. MFI-type (i.e., silicalite-1 and ZSM-5) zeolite membranes have been largely advocated as promising candidates for the separation of linear from branched alkanes, showing promising potentials for n-C6 hydroisomerization in catalytic membrane reactors.10,11 The unit cell of MFI zeolites contains 96 tetrahedral SiO2 units giving rise to four straight channel sections and four sinusoidal channels with four intersections. The straight channels are elliptical with openings of 5.4 × 5.6 Å2, whereas the sinusoidal channels display an almost circular cross section with a diameter about 5.4 Å.12 Since the kinetic diameters of n-C6 and 2,2-DMB are about 4.3 and 6.3 Å,13 respectively, the MFI channel sizes should make, in principle, possible the separation of 2,2-DMB from n-C6 by molecular sieving or diffusion differences. As a matter of fact, the diffusivity of 2,2-DMB has been reported to be 3 orders of magnitude lower than that of n-C6 in silicalite-1 at 423 K.14 On the basis of this principle, Vroon et al.15 have measured n-C6/2,2-DMB mixture separation factors >2000 at 473 K on disk-shaped silicalite membranes. Flanders et al.16 prepared HZSM-5 membranes on stainless steel and R-alumina tubes both showing n-C6/2,2-DMB separation factors about 1000 at 373 K, but the selectivity decreased dramatically to 10 as the temperature increased to 573 K. Also, Giroir-Fendler et al.17 reported the preparation of MFI-alumina tubular membranes displaying n-C6/2,2-DMB separation factors about 240 at 473 K for a 45/55 feed molar ratio and high pure permeation ratios. This picture is however incomplete at high loadings. It is well-known that MFI channels are not completely rigid and can expand to a certain extent upon adsorption, especially at elevated temperatures.18,19 This property promotes 2,2-DMB accommodation in MFI channel intersections to a maximum level of 4 molec/uc. This value is lower than the maximum adsorption level of n-C6 (8 molec/uc), since n-C6 can “pack” efficiently
10.1021/ie101566q 2010 American Chemical Society Published on Web 10/18/2010
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both in channel intersections and inside sinusoidal channels due to sorbate-sorbate interactions driven by configurational entropy effects.20-23 This effect is a consequence of the so-called “confinement” or “nest” effect proposed by Derouane and recently reviewed by Corma.24 Configurational entropy effects cause in its turn preferential permeation of n-C6 from n-C6/ 2,2-DMB mixtures in MFI membranes at high vapor pressures and reduced temperatures, while nonzeolitic pore pathways are responsible for 2,2-DMB permeation. For instance, Gump et al.25 have reported n-C6/2,2-DMB separation factors of 400 for equimolar mixtures at 398 K and 8 kPa n-C6 feed vapor pressure. Coronas et al.26 have reported the preparation of tubular ZSM-5 membranes with n-C6/DMB separation factors as high as 2600 at 374 K, but the selectivity decreased dramatically with temperature, so that no separation was obtained at 410 K. Because the n-C6/DMB permselectivity was only 2.1 at 374 K, these authors concluded that preferential adsorption rather than molecular sieving mainly drove separation. Comparable results were reported by Funke et al.27 for a series of binary mixtures of n-C6 with different branched isomers. Matsufuji et al.28 have reported n-C6/2,3-DMB separation factors up to 270 using pervaporation separation, the process being governed by the high n-C6 loading in the MFI zeolite. In addition to preferential n-C6 adsorption, Yu et al.29 have also evoked a possible role of nanometer-sized grain boundary sealing in MFI membranes, this being attributed to MFI crystal expansion upon n-C6 adsorption. Taking the earlier study of Giroir-Fendler et al.17 as a starting point, we provide here a detailed study on the molecular sieving performance of MFI-alumina hollow fibers with nanocomposite architecture for n-C6 separation from 2,2-DMB. For comparison, separation using B-MFI-alumina hollow fibers has also been surveyed, since these materials have already proven enhanced selectivity for p/m-xylene and p/o-xylene separation by size exclusion.30 Indeed, Gade et al.31 have reported on the improved selectivity of B-MFI membranes for n-C6 separation from its isomers. A modeling study for n-C6/2,2-DMB mixture separation relying on the IAS theory for mixture adsorption and the Maxwell-Stefan formalism for zeolite diffusion is also provided, stressing the crucial role of configurational entropy effects and preferential diffusion on n-C6 separation. 2. Experimental Section 2.1. Membrane Preparation. The membranes used in this study consisted of Al-MFI and B-MFI zeolite membranes grown as nanocomposites in the cavities of alumina hollow fibers (1.67 mm o.d., 1.15 mm i.d., 250 mm length) by in situ hydrothermal synthesis, showing an effective MFI thickness about 1 µm in both cases. The Si/Al ratio of both hollow fibers was about 30, while the Si/B ratio of the B-MFI sample was 88. The details dealing with the membrane synthesis and mounting for testing can be found in refs 30, 32. The quality of the Al-MFI and B-MFI hollow fibers was first inferred from their low viscous contribution to mass transfer (50). Furthermore, both materials offered high p-xylene/ m-xylene separation performance, showing separation factors >100 at 473 K for 0.7 kPa p-xylene, 0.8 kPa m-xylene, and 0.8 kPa o-xylene feeds. 2.2. Single Vapor Permeation and Mixture Separation. Before the vapor permeation and separation tests, the membranes were pretreated at high temperature (673 K for 6 h with a 20 mL(STP)/min N2 flow at both sides of the membrane) to remove adsorbed moisture and organic species.
The hexane vapor separation tests were conducted by saturating a 10 mL(STP)/min N2 carrier stream with a single or binary hexane isomer mixture at room temperature and ambient pressure using two saturators combined in series. The total vapor pressure was further adjusted using another dry N2 stream at a flow rate in the range 0-90 mL(STP)/min depending on the desired vapor pressure. The permeate side of the membrane was swept with a countercurrent 40 mL(STP)/min N2 stream. In all cases, the temperature of the membrane system was varied from 323 to 573 K by increments of 50 K. To avoid any vapor condensation during the experiments, all the lines heated and maintained at 323 K with electrical heating tapes. In all the separations, gas flows and feed compositions were controlled by mass flow controllers (Brooks, types 5850TR and 5850E). The compositions of the feed, retentate, and permeate streams were analyzed online using a GC (Shimadzu GC-14A) equipped with Solgel-Wax capillary column and flame ionization detector (FID). The mixture permeance of a target species was defined as the permeation flux divided by its corresponding log-mean differential partial pressure. The separation factor was defined as the enrichment factor of one component to another in the permeate as compared to the feed composition ratio, while the membrane permselectivity (or ideal selectivity) was defined as the quotient of pure vapor permeances. 3. Modeling 3.1. Generalized Maxwell-Stefan (GMS) Equations for Micropore Diffusion. The GMS equations applied to surface diffusion in microporous materials relate the isothermal chemical potential gradient, -∇Tµi, as the true driving force with the flux of each species i as follows -εFp
qi ∇ µ ) RT T i
C
∑
j)1
(qjNSi - qiNSj ) qM,jÐSij
+
NSi ÐSi
i ) 1,..., C
j*i
(1) where Ði is the MS surface diffusivity of species i and Ðij is the counterexchange MS surface diffusivity between species i and j. Equation 1 constitutes the fundamental expression used in this study to describe mass transfer within zeolite membranes combined with a convenient equilibrium adsorption model to relate sorbate loading with partial pressure for each species. We distinguish two different particular cases of eq 1 for modeling both pure and mixture vapor (or gas) permeation. 3.1.1. Pure n-C6 and 2,2-DMB Vapor Permeation. In the case of pure vapor permeation, eq 1 can be simplified to the following expression: NS ) -εFp
q S Ð (q)∇Tµ RT
(2)
Rewriting the surface potential as a function of pressure, eq 2 transforms into q NS ) -εFpÐS(q) ∇Tp p
(3)
The resolution of eq 3 requires a suitable isotherm model relating the sorbate loading with pressure, as well as a convenient expression of the MS surface diffusivity as a function of the sorbate loading. On the guidance of the studies published by Song and Rees33 and Vlugt et al.34 on silicalite-1, let us
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Table 1. Expressions Obtained for Pure Permeation Flux as a Function of Isotherm Models Chosen and the Level of Confinement of the Sorbatea isotherm model
diffusion model
equation
(
)
SL
WCF
NS )
1 + Kpfeed εFpqMÐ (0) ln lMFI 1 + Kpperm
SL
SCF
NS )
(Pfeed - Pperm) εFpqMÐS(0)K lMFI (1 + Kpfeed)(1 + Kpperm)
DSL
WCF
NS )
1 + K1pfeed 1 + K2pfeed εFpÐS(0) qM,1 ln + qM,2 ln lMFI 1 + K1pperm 1 + K2pperm
DSL
SCF
NS )
εFpÐS(0) qM,1qM,2 lMFI qM,1 + qM,2
S
[ ( (
(9)
)
[
){
ln
(10)
(
)]
(11)
[
(1 + K1pfeed)(1 + K2pfeed) (1 + K2pfeed) (1 + K1pfeed) 2 - K2 ln K ln (1 + K1pperm)(1 + K2pperm) (Ki,1 - Ki,2) 1 (1 + K2pperm) (1 + K1pperm)
]
2 2 εFpÐS(0) qM,2 qM,1 K1 K2 + (p - pperm) lMFI(qM,1 + qM,2) (1 + K1pfeed)(1 + K1pperm) (1 + K2pfeed)(1 + K2pperm) feed
a
]}
-
(12)
Nomenclature: SL, single-site Langmuir isotherm; DSL, dual-site Langmuir isotherm; WCF, weak confinement; SCF, strong confinement.
consider first the single-site Langmuir (SL) and dual-site Langmuir (DSL) isotherms for modeling, respectively, 2,2-DMB and n-C6 adsorption in MFI zeolites
NSi ) -εFp
qMKp SL: q ) 1 + Kp DSL: q )
situation, the first term on the right-hand in eq 1 vanishes and this equation transforms into a similar form of eq 2
(4)
qM,2K2p qM,1K1p + 1 + K 1p 1 + K2p
(5)
In addition, following the experimental results reported by Ferreira et al.35 for n-C6 adsorption on HZSM-5 (Si/Al ) 50), the truly active material of our membranes, the SL isotherm has also been considered for modeling n-C6 adsorption. The MS surface diffusivities are regarded here as a function of the sorbate loading using the general expression ÐS(q) ) ÐS(0)f(q)
(6)
where the following expressions for function f(q) have been chosen to account for the classical weak and strong confinement scenarios weak confinement (WCF): f(q) ) 1
(
strong confinement (SCF): f(q) ) 1 -
(7) q qM
)
(8)
Introducing eqs 4, 5, 7, and 8 into eq 3 and assuming that mass transfer only takes place along the z-direction perpendicular to the membrane surface, a series of expressions can be obtained for the permeation flux after integration. Table 1 collects the set of expressions obtained (eqs 9-13). 3.1.2. Binary n-C6 and 2,2-DMB Vapor Permeation. The permeation performance of n-C6/2,2-DMB mixtures can be described by eq 1 taking the symplifing assumption of a fast exchange between n-C6 and 2,2-DMB in the pore intersections. As a matter of fact, for zeolite topologies with high connectivities, the counterexchange coefficients, ÐijS, are expected to be high (i.e., ÐijS f ∞), implying a lack of correlation effects between the mobility of adsorbed n-C6 and 2,2-DMB. In this
qi S T Ð (q )∇Tµi RT i
i ) 1,2
(13)
where n-C6 and 2,2-DMB have been denoted, respectively, as 1 and 2. The chemical potential gradients can be transformed into molar loading gradients introducing the thermodynamic factors, Γij qi ∇µ ) RT i
N
qi,M
∑Γ q ij
∇qj
i,j ) 1,2
(14)
j,M
j)1
( )
Γij ≡ qi
qj,M ∂ ln(pi) qi,M ∂qj
(15)
According to the study of Krishna and Baur,36 eq 10 can be rewritten as a function of the partial pressure gradient of each species i as follows qi ∇µ ) RT i
N
qi ∂pi
∑ p ∂q ∇q
j
j)1
i
)
j
qi ∇p pi i
i ) 1,2
(16)
Introducing eq 12 into eq 9, the surface flux of each species i can be rewritten as a sole function of its partial pressure and loading in a similar manner as eq 3 qi NSi ) -FpÐSi (qT) ∇pi pi
i ) 1,2
(17)
where now the MS surface diffusivity is a function of the total loading, i.e., qT ) q1 + q2. For convenience, let us rewrite eq 13 in dimensionless form as follows Ri ) -
qi f(qT) δpi qM,i pi δη
i ) 1,2
(18)
where η ) z/lMFI is the dimensionless effective zeolite layer thickness and Ri is the dimensionless flux of species i defined by eq 15
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Ri )
NSi lMFI εFSqM,iÐSi (0)
i ) 1,2
(19)
On the guidance of the earlier study of Krishna and Paschek,37 the classical ideal adsorbed solution theory (IAST) of Myers and Prausnitz38 has been used as framework to infer thermodynamically consistent mixture n-C6/2,2-DMB adsorption isotherms from pure adsorption data. This formulation relies on the assumption that mixture adsorption occurs at constant surface potential, Φ (or spreading pressure in the original formulation), which can be computed from the pure adsorption isotherm as follows Φ ) RT
∫
Po(Φ)
0
qoi (pi) δpi g 0 pi
i ) 1,2
(20)
In analogy with Raoult’s Law, the molar fractions of the gas and adsorbed phases are related by eq 17 pi ) xiγi(T, Φ)Poi (T, Φ)
i ) 1,2
(21)
where γi(T,Φ) is the activity coefficient of species i in the sorbate, which tends to 1 in the IAS theory. However, unlike Raoult’s Law, there is a subtle difference in the meaning of Pio(T,Φ). In the case of vapor-liquid equilibrium, Pio is the saturation vapor pressure of species i corresponding to the solution and only depends on temperature, while in the case of a sorbate it corresponds to the adsorptive saturation pressure for a given temperature and surface potential. The IAS formulation requires the resolution of eqs 20 and 21 for all the species in the mixture. This allows the representation of a y-x equilibrium diagram. In addition, a loading diagram is also necessary, which links the total adsorbed amount, qi, with the vapor phase mole fraction of each species x2 x1 1 + o ) o T q q1(Φ) q2(Φ)
(22)
q1 ) x1qT; q2 ) x2qT
(23)
3.3. Calculation Details. The set of eqs 9-12 listed in Table 1 combined with eq 23 were used to fit experimental pure n-C6 and 2,2-DMB permeation data within Al-MFI and B-MFI membranes. In the fitting process, the adsorption constants and MS surface diffusivities at zero loading were expressed as a function of temperature using, respectively, Van’t Hoff and Arrhenius-type equations
[ (
K(T) ) K(Tref)exp -
1 ∆HS 1 R T Tref
[ (
Ð(0)(T) ) Ð(0)(Tref)exp -
)]
1 ES 1 R T Tref
)]
(25)
(26)
where Tref is the mean temperature of the series (485.65 K for n-C6 and 445.15 K for 2,2-DMB). The relevant parameters in eqs 25 and 26 were fitted using the Levenberg-Marquardt algorithm by comparison of predicted and experimental n-C6 and 2,2-DMB fluxes. The quality of the fittings was assessed from the values of the regression coefficients and from the confidence intervals and correlation matrix of the fitted parameters. In the case of mixture n-C6/2,2-DMB permeation, eq 14 was solved numerically for each species through discretization of derivatives. The partial pressures at each position were calculated iteratively slabwise according to the methodology proposed in a previous study.41 After calculation of dimensionless fluxes, a least-squares nonlinear optimization method also based on the Levenberg-Marquardt algorithm was used to fit the MS surface diffusivities at zero loading by comparison of predicted and experimental n-C6 and 2,2-DMB mixture fluxes.
where q1o(Φ) and q2o(Φ) are the loadings of pure species 1 and 2, respectively, at surface potential Φ, while q1 and q2 correspond to the loadings of species 1 and 2 for the given mixture. 3.2. Contribution of the Support. Although the resistance of the support to mass transfer is expected to be much lower than that ascribed to the zeolite layer, the support might cause a certain drop in the partial pressure gradient due to its large thickness (about 3 mm), as has been highlighted by van de Graaf et al.39 Taking into account the nature of our experiments (Wicke-Kallenbach permeation mode), mass transfer within the each support layer can be regarded to mainly take place by molecular diffusion. The partial pressure gradient for each layer can be then computed using the Maxwell-Stefan formalism by an expression similar to eq 9, but including explicitly the molecular diffusivities of species i and j, Dmij -
εS ∇p ≈ RT i
C
∑
j)1
(yjNSi - yiNSj ) Dm ij
i, j ) 1, 2, carrier
(24)
j*i
In these calculations, the molecular diffusivities have been estimated by the classical Chapman-Enskog model.40
Figure 1. Pure n-C6 and 2,2-DMB VP fluxes and n-C6/2,2-DMB permselectivity as a function of temperature for an Al-MFI (top) and a B-MFI (bottom) hollow fiber. The lines represent the fittings to MS+SL equations for strong confinement (eq 10).
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Figure 2. Pure n-C6 and 2,2-DMB VP fluxes and n-C6/2,2-DMB permselectivity as a function of the total feed vapor pressure for an Al-MFI (top) and a B-MFI (bottom) hollow fiber. The lines represent the fittings to the MS equations based on the SL isotherm formalism for a strong confinement scenario (eq 10).
4. Results 4.1. Pure n-C6 and 2,2-DMB Vapor Permeation. Figure 1 plots the pure n-C6 and 2,2-DMB vapor permeation performance as a function of temperature at reduced vapor pressures (
