Microporous and Mesoporous Materials 146 (2011) 69–75
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Simple modification of macroporous alumina supports for the fabrication of dense NaA zeolite coatings: Interplay of electrostatic and chemical interactions Sonia Aguado a,b,⇑, Jorge Gascon a, David Farrusseng b, Jacobus C. Jansen a, Freek Kapteijn a a b
Catalysis Engineering, ChemE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR5256, CNRS/Université Claude Bernard, Lyon 1, 2, Av. A. Einstein, F-69626 Villeurbanne, France
a r t i c l e
i n f o
Article history: Received 27 January 2011 Received in revised form 10 March 2011 Accepted 22 March 2011 Available online 14 May 2011 Keywords: Zeolite NaA Film Modified support Polymer
a b s t r a c t Continuous thin zeolite NaA films have been synthesized on macroporous a-alumina supports subjected to a simple pretreatment and without seeding. By modifying the supports with a cationic polymer and even with glucose or polyethylene glycol and subsequent calcination at 673 K, a substantial improvement in terms of layer continuity and crystal intergrowth is observed compared to coatings prepared on unmodified supports. We attribute this positive effect to a fine interplay between the presence of a monolayer formed by carbon–oxygen species, mainly of the carboxylic type, on top of the calcined supports and to the negative net charge of the surface under synthesis conditions. The resulting double layer of solvated Na+ ions on the support would act as nucleation promoter and formation of the NaA thin film. This new method presents several advantages and may form the basis to scale up membrane synthesis in view of the simple and cheap pretreatment together with the elimination of pre-seeding steps. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Gas, vapor and liquid membrane separation processes [1], membrane reactors [2,3], chemical sensors [4], dielectric thin films [5] and many other devices used in nanotechnology rely on the fabrication of thin films and coatings with a well-defined porosity. Zeolites, polymers, metal oxides, activated carbons and metal organic frameworks [6–10] are normally used for this purpose. In spite of the great advances in thin film technology, especially when related to polymeric materials, serendipity still rules the formation of films out of nano-structured solids such as zeolites. Among the different synthetic strategies used for manufacturing zeolite films, hydrothermal synthesis onto porous supports, often with a preliminary seeding procedure, has been thoroughly studied. A variety of seeding techniques exists including rubbing with zeolite crystals over the outer side of a tubular side [11] or on the inner side by brushing [12], laser ablation [13], cross-flow filtration of a suspension of crystals [14], spin-coating [15], dipcoating of a colloidal mixture of nanocrystals [16] and electrophoretic deposition of nanocrystals [17]. In the subsequent hydrothermal synthesis process, the uncovered or seeded substrates are
⇑ Corresponding author. Present address: Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR5256, CNRS/Université Claude Bernard, Lyon 1, 2, Av. A. Einstein, F-69626 Villeurbanne, France. Tel.: +33 (0) 472445384; fax: +33 (0) 472445436. E-mail addresses:
[email protected],
[email protected] (S. Aguado). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.03.044
exposed to the synthesis liquid or vapor at elevated temperature. The batchwise hydrothermal synthesis process is difficult to scale up, although the use of microwave heating has improved the process considerably [18]. We have been investigating alternative synthesis methods that eliminate pre-seeding steps, next to a continuous hydrothermal synthesis. Other concepts to reduce the price for the preparation of comprise e.g. the template-free synthesis, a lower cost of the support and a continuous flow system synthesis [19,20]. One of the challenges in in situ synthesis is to ensure a high nucleation site density on the support, while avoiding the formation of undesired thick films. Different treatments of the supporting material in order to increase the number of surface ‘‘anchoring’’ groups by means of Van der Waals interactions, covalent bonding and H-bonding have been investigated. Kim et al. reported treatments with trimethylchlorosilane to create Si–Cl bonds on the surface of alumina supports in order to enhance adhesion of a mesoporous layer [21]. Pretreatment of the support surface by impregnation or spin-coating with the appropriate template was also shown to direct the formation of crystallites on the support. Chau et al. found that a coating of a metal oxide (e.g. Fe2O3) on the support surface provided a simple technique to control the number and type of nucleation sites available on the support surface [22]. Their results indicated that there was a direct correlation between the number of nucleation sites and the amount of iron(III)oxide present on the surface of the support and allowed the synthesis of a more complete and uniform zeolite film. Van den Berg et al. used UV-radiation to increase the number
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of defect sites and hydroxyl groups to promote the hydrophilic properties of a TiO2-coated support [23]. The authors reported an increased nucleation and the resultant formation of a uniform, intergrown zeolite A film, well attached to the support, with good gas separation properties [24]. This is however limited to UVsensitive support materials. More recently, Caro’s group has introduced several covalent linking groups that enhance tremendously the adhesion of zeolites and MOFs, resulting in thinner, defect free, coatings [10,25,26]. On the other hand, many endeavors have been carried out on adapting the layer-by-layer (LbL) technique to zeolite synthesis, which can easily control the film composition, structure and thickness by alternately electrostatic adsorbing zeolite nanocrystals and polyelectrolytes [27]. By this convenient and versatile approach, zeolite coatings on latex spheres [28] and carbon fibers [29] have been successfully prepared. This method of assembly of zeolite crystals was also used to prepare coatings of silicalite-1, TS-1, ZSM-5 and zeolite A on stainless steel [30] and ZSM-5 and zeolite A on glass [31]. Liu et al. reported the preparations of a SBA-15 film modified electrode [32]. Chai et al. have carried out MCM-22/silica composite films by this assembly technique [33]. In the case of membranes and dense coatings, after the attachment of the zeolite coating on the substrate, generally a secondary growth step is employed. This method has been used to create zeolite A films on silicon wafers [34] and on stainless steel multi-channel plates [35]. Also ZSM-5 films have been prepared on modified gold surfaces [36]. The major issue in closing the gaps between crystals during synthesis is the identical electrostatic charge on the growing crystal surfaces and the grow nutrients in the solution, repelling each other. Noack et al. therefore applied ‘intergrowth supporting substances’ (ISS) in between subsequent synthesis steps to modify the surface charge of the growing zeolite films to improve their membrane quality [37]. The surface charge under synthesis conditions and the local chemical environment for nucleation and growth appear the most essential elements in the creation of surface coatings. Here, we report the one-step synthesis of a zeolite NaA film directly on a macroporous alumina support, only a few times reported in the literature [19,38], in which these elements are present. This technique consists of the modification of the support surface with an organic modifier followed by a calcination that provides an improved attachment and intergrowth of zeolite crystals, resulting in a well-intergrown zeolite NaA film. The origin of this improvement is investigated.
2. Experimental 2.1. Materials and synthesis procedures
a-Alumina tubular supports (10 mm outer diameter, 7 mm inner diameter, 15 cm length, 45% porosity), with a top layer on the inner surface containing pores of 200 nm, were supplied by Inocermic GmbH. TiO2-coated a-alumina disc supports (25 mm diameter, 30% porosity, Pervatech) consist of a thin smooth layer of TiO2 with an average pore size of 0.03 lm on top of a porous a-alumina support. a-Alumina particles were used for an instrumental analysis of the effect of surface coating, for which the support was less suitable (Alfa Aesar, particle size 0.35–0.39 lm, BET surface area 9 m2 g 1). A surface modification of the support with a cationic polymer, poly(diallyldimethylammonium chloride) PDDA (Aldrich, average Mw 100,000–200,000), was carried out to create a positive support surface and to promote adhesion of the zeolite particles or precursors, which are negatively charged in the synthesis mixture. First of all, a modification solution was prepared by dissolving amounts of
PDDA solution (20 wt.%) ranging between 0.001 and 1 g mL 1 in a 0.5 M NaCl (Sigma–Aldrich, 99.5%) solution in distilled water, and applying an ultrasonic bath for 5 min to homogenize the mixture. The supports were immersed in the liquid for 20 min, followed by rinsing with distilled water to remove excess polymer and salt. This procedure was thought to improve the electrostatic adherence of zeolite A seeds by dip-coating of alumina supports in case of a seeding procedure [27], just like in application of ISS [37] or electrophoretic deposition [17]. Since in this work we are not using a seeding step, and normally a calcination step must be carried out to remove the sacrificial organic phase, also a calcination step is carried out after the immersion of the support in the organic modifier. So, a calcination at 673 K for 3 h in air using a heating and cooling rate of 0.5 °C min 1 was applied. Furthermore, the use of an alternative, non-polyelectrolytic organic component, viz. glucose instead of PDDA, for the surface modification was explored, using D-(+)-Glucose 99.5% supplied by Aldrich, carrying out the procedure in the same way. Also a non-ionic polymer with lower molecular weight than PDDA, poly(ethylene glycol) PEG (Aldrich, Mw 20,000), was used for that purpose. The clear solution used for the synthesis of NaA zeolite membrane had a molar composition of 12 Al2O3: 53 SiO2: 503 Na2O: 10,000 H2O, adapted from the work of Zhu et al. [24]. Two reactant solutions were prepared by dissolving sodium metasilicate pentahydrate (97%, Sigma) and sodium aluminate anhydrous (Riedel-de Haen) in freshly made sodium hydroxide solutions. The total amount of sodium was distributed in a ratio of 1:1.38 between the corresponding silicate and aluminate solutions while the water was divided evenly. The silicate and aluminate solutions were aged for approximately 1 h at room temperature; then the aluminate solution was added to the silicate solution under continued stirring. The synthesis solution was further aged for 1 h at room temperature. After the preparation of the synthesis mixture, it was poured into a Teflon-lined stainless steel autoclave in which a support was placed vertically. The autoclave was then put in an oven. The hydrothermal in situ synthesis was carried out in one stage at 353 K for 4 h. The autoclave was rotated at 60 rpm during synthesis to prevent the incorporation of suspended crystals into the zeolite layer. 2.2. Characterization The surface and cross-section morphology of the as-synthesized membranes was examined by scanning electron microscopy (SEM) using a Philips XL20 microscope. The existence of remaining organic modifier on the alumina support was determined by a thermogravimetric analysis (TGA), in a Thermogravimetric Analyzer type Mettler Toledo, TGA/SDTA851. For these experiments the a-alumina powder was modified in the same way as the tubular supports. This test was performed with three temperature steps. First of all, the sample was heated up to 473 K in air with a rate of 1 K min 1 and held for 2 h at this reference temperature. After that, it was heated up to 673 K and 773 K in two steps, with the same heating rate and the same hold time. During the test, the air flow over the sample carries away the released compounds. For the identification of the possible created groups on the surface of the support, IR and Raman experiments were carried out. IR experiments were performed in a diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy accessory (Thermo Nicolet, Nexus) using KBr as background. Zeta potential data were collected in a Malvern Nano Sizer with a 633 nm laser. XPS analysis was carried out in a KRATOS AXIS Ultra DLD spectrometer using a hemispherical analyzer, working at a vacuum lower than 10 9 mbar. All the data were acquired using monochromated Al Ka X-rays (1486.6 eV, 150 W), a pass energy of 20 eV, and a hybrid lens mode. The area analyzed is
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700 lm 300 lm. Charge neutralization was required for all samples. The peaks were referenced to the C–(C,H) components of the C1s band at 284.6 eV. Shirley background subtraction and peak fitting with Gaussian–Lorentzian product peak were performed using an XPS processing program (vision 2.2.6 KRATOS) to determine the different binding energies and the elemental surface region composition of these samples.
3. Results 3.1. Effect of calcination Table 1 lists the modification conditions applied to the a-alumina tubular support, including type of organic modifier, post-calcination step and concentration of organic modifier. Data of the samples synthesized on a non-modified support, as well as for samples synthesized on a-alumina TiO2-coated disk supports irradiated with UV are given. Fig. 1 shows SEM micrographs of samples S12, S01, S02, S03, S06 and S07. Sample S12 is synthesized on an a-alumina TiO2coated disk support irradiated with UV. Previous studies showed that UV-irradiation of TiO2 increases the hydrophilic properties of the surface of that wafer by increasing the number of hydroxyl groups [23]. This boost in hydroxyl groups causes a substantial increase in nucleation of zeolite A. Due to the increased number of zeolite NaA precursor nuclei, the resulting zeolite A membrane on an irradiated TiO2 support is more dense, more homogeneous in thickness and better attached to the support (Fig. 1a). This effect is lost when the synthesis is carried out with an unmodified a-alumina tubular support (Fig. 1b, sample S01). However, modification of the alumina tubular support with a mixture of PDDA and NaCl solution enhanced the density, homogeneity, intergrowth and attachment to the support of the NaA layer (Fig. 1c and d). Sample S02 modified with a mixture of 1 g mL 1 of PDDA reveals a significant improvement in comparison with S01. This enhancement is attributed to the expected positively charged support surface under synthesis conditions that would promote adhesion of zeolite nanoparticles or precursor nuclei. Although continuity and thickness of the layer is comparable to sample S03, submitted to a calcination step, a poorer intergrowth of crystals is observed in the case of the uncalcined polymer. Apparently, better coatings are ob-
Table 1 Pretreatment conditions of the employed support samples prior to the synthesis of zeolite NaA and Zeta potential results of a-alumina powder measured at pH = 7 and room temperature. Sample*
*
Organic modifier concentration (g mL
1
)
Organic modifier
Calcination step at 673 K
S01 S02 S03 S04 S05 S06 S07
no pretreatment 1 1 0.1 0.01 0.001 0.001
PDDA PDDA PDDA PDDA PDDA PDDA
no yes yes yes no yes
S08 S09 S10 S11
0.1 0.1 0.01 0.001
Glucose Glucose Glucose Glucose
no yes yes no
S13 S14
1 1
PEG PEG
no yes
S12
UV irradiation
Zeta potential (mV) 21.1 49.7 18.4
5.04 12.5
All samples synthesized on a a-alumina tubular support, except S12 synthesized on a a-alumina TiO2-coated disk support.
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tained when a calcination step at 673 K is applied on supports coated with the PDDA/NaCl mixture. In order to prove the additional favorable effect of the calcination step, syntheses were carried out on a modified support with 0.001 g mL 1 of PDDA, with and without calcination step. It is obvious that the calcination step enhances the attachment of the zeolitic layer to the macroporous support (Fig. 1e and f). Evaluating samples S02, S06 and S07 two different effects are observed: (a) The presence of PDDA enhances the adhesion of crystals due to the positive charge of the surface, when higher concentrations of PDDA are used (1 over 0.001 g mL 1), a better intergrowth is observed. (b) The quality of the coatings improves dramatically when the coated support is calcined prior to hydrothermal synthesis. 3.2. Effect of amount of organic modifier SEM micrographs of samples S03, S04, S05 and S07, synthesized on a modified support treated with a solution in the range of 0.001–1 g mL 1 of PDDA and subjected to a post-calcination in air at 673 K. An almost constant thickness layer of 4 lm is revealed in all cases, but an increase in crystal size is observed. Average crystal sizes of 1.4, 1.5, 1.8 and 3.3 lm were found for samples based on supports modified with solutions of 1, 0.1, 0.01 and 0.001 g mL 1 of PDDA, respectively (Table 2). The results indicate a direct correlation between the number of nucleation sites for zeolite deposition and the amount of PDDA used. As a consequence of the larger number of nucleation sites the crystal size decreases with increasing the amount of polymer used. This effect is in good agreement with previous observations by Chau et al. in the formation of ZSM-5 films by adsorption of organic surfactants [22]. The effect of the type of the organic modifier was also studied: a mixture glucose/NaCl was used in samples S08–S11, while for samples S13–S14 a similar solution using polyethylene glycol (PEG) as organic modifier was utilized. Fig. 2a shows a big difference in intergrowth and surface coverage when a mixture of 0.001 g mL 1 of glucose is employed for the modification of the support, compared to that of PDDA samples synthesized following a similar procedure, with the better results for glucose. PEG yielded results similar to those of PDDA. This difference disappears when the modified support is calcined. Surprisingly, in every case calcination of the organic precursor results in an excellent coating quality. Fig. 2a and b reveal a similar crystal size, continuity, intergrowth and thickness layer for ex-PDDA and ex-glucose samples. 3.3. Characterization of the surface modification Several techniques have been applied in order to shed light on the surface modification in both calcined and uncalcined supports and, if relevant, elucidate the nature of the new ‘‘anchoring’’ sites generated. To this purpose samples of a-alumina powders were subjected to similar pretreatments as the supports. Powder form was chosen as it is better accessible for different characterization techniques like XPS. The thermal gravimetric (TGA) curves of alumina powder under a stream of air are shown in Fig. 3, presented as weight loss normalized to the initial mass of each sample. The results follow an expected trend according to the amount of organic modifier used and to the calcination. This weight loss decreases in the order 1 g mL 1 PDDA > 1 g mL 1 PDDA calcined > 0.1 g mL 1 glucose > 0.001 g mL 1 PDDA > 0.001 g mL 1 PDDA calcined > untreated. The largest differences are observed for the samples treated with 1 g mL 1 of PDDA. Evidently, the calcined sample has a lower initial weight. Both samples show a first weight loss up to 473 K due to the release of water, and a second weight loss
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Fig. 1. SEM micrographs of zeolite NaA films synthesized on a modified support compared with an unmodified support and a UV pretreated TiO2 coated supports (synthesis at 353 K for 4 h with rotation at 60 rpm). Left: top view. Right: cross sectional view. Key from top to bottom: (a) S12 on a UV treated TiO2 coated support; (b) S01 on a nonmodified tubular support; (c) S02 on a modified support with 1 g mL 1 of PDDA; (d) S03 on a modified support with 1 g mL 1 of PDDA and a post-calcination step, (e) S06 on a modified support with 0.001 g mL 1 of PDDA; (f) S07 on a modified support with 0.001 g mL 1 of PDDA and a post-calcination step.
up to 673 K due to the decomposition and oxidation of PDDA and its residues, the loss being higher in the case of the non-calcined sample. The Zeta potential provides an indication about the magnitude of the interaction between two surfaces in a liquid environment. Its measurement brings detailed insight in the dispersion mechanism and is the key to electrostatic dispersion control. For instance the layer-by-layer method relies on electrostatic interactions between the support and the coating materials: in general negatively
charged particles are deposited on the positively charged surface of the support. In order to determine if the improved coatings were due to changes in surface charge of the support thanks to the organic modifier treatment, the zeta potential of a-alumina powders was determined. Table 1 shows zeta potential values, in mV of samples of a-alumina powders exposed to different pretreatments at a constant pH. The measured zeta potential values are in good agreement with the treatment performed to the samples. The unmodified alumina gives a value of 21.1 mV, inferring a net neg-
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S. Aguado et al. / Microporous and Mesoporous Materials 146 (2011) 69–75 0.020
Table 2 Coating features for samples represented in Figs. 1 and 2.
a
Coverage
Crystal size (lm)
S01 S02 S03 S06 S07
5.0 2.7 2.7 1.7 3.7
poor medium good poor good
poor good good poor good
– 4.6 1.7 1.9 3.3
S10 S11
2.7 3.0
good good
good good
1.9 1.7
S12
3.0
good
good
2.1
700
600 0.010 500 0.005
0.000
ative charge on the alumina surface at neutral pH and therefore a point of zero charge (PZC) below 7. Samples treated with PDDA solution result in a positive zeta potential value, and therefore possess a PZC higher than 7. Therefore, at the basic pH of the synthesis, the treated supports may still present a positively charged surface, enhancing their affinity for the negatively charged zeolite primary units formed in solution and therefore increasing the quality of the coatings, as already observed by other authors [22]. In contrast, for the calcined samples already at pH 7 negative zeta potential values are found, although less negative than for the untreated particles, as a result of the decomposition and partial burn-off of the organic modifier. These results suggest that the formation of the best coatings is not directly related to a change in the electrostatic properties of the supports but to a change in their chemical properties. To address this counter intuitive behavior of the calcined supports, and to identify the nature of the surface species formed upon calcination, the relative molar concentrations of O, N, C and Cl for the samples S02, S03, S06 and S07, immersed in 1 or 0.001 g mL 1 of PDDA and with/without post-calcination step were determined by means of XPS (Table 4 and Fig. 4). The deconvolution of the C1s binding energies is included for the ex-PDDA sample in Fig. 4. Four major carbon species are considered, viz. graphitic or aliphatic carbon (284.6–285.1 eV), alcoholic or ether carbon or C–N in amine (C–O–, C–N at 285.5– 287 eV), carbonylic carbon (C = O at 287–288.5 eV) and carboxylic or ester carbon (O = C–O– at 288.5–290 eV) [39–41]. After the oxidative treatment the carboxylic carbon appears to represent the largest fraction on the surface of the alumina particles. In the PDDA samples S02 and S06 a relative high amount of the C–N species in the ionic polymer is detected.
800
0.015
i
Crystal intergrowth
Δm/m [-]
Thickness (lm)
b
400 c d e f
0
Temperature [K]
Sample
300 200
400
600
800
Time [min] Fig. 3. TGA analysis of a-alumina powder (a) with 1 g mL 1 of PDDA; (b) with 1 g mL 1 of PDDA with a post-calcination step; (c) with 0.1 g mL 1 of glucose; (d) with 0.001 g mL 1 of PDDA; (e) with 0.001 g mL 1 of PDDA and a post-calcination step; (f) untreated alumina.
4. Discussion The use of cationic polymers as support modifiers has been reported in the literature as a method to attach monolayers of colloidal seed crystals (zeolites, metal organic frameworks) that are grown afterwards under hydrothermal conditions. In this work we have shown that, for zeolite LTA, this seeding step is not necessary, since the direct use of the organic modifier coated support already results in dense coatings in one synthetic step. Moreover, the use of inexpensive glucose as surface modifier results in high quality coatings too, suggesting that the better quality of the coatings is more related to the presence of organic species on the interface of the support than with its net surface charge. More surprisingly, the mild calcination of the pre-coated supports enhances quantitatively the quality of the coating. From the XPS results (Table 3 and Fig. 4), similar surface oxygen concentrations are found in all the (oxide) samples. As expected, higher carbon concentrations are present in samples S02 and S03 prepared with 1 g mL 1 of PDDA in solution. The disappearance of nitrogen in samples S03 and S07 after the calcination step is due to the decomposition of dimethylamine at the calcination temperature. The C1s XPS spectra, in Fig. 4, deconvoluted into four peaks (Table 4) indicates the strong presence of carbon–oxygen
Fig. 2. SEM micrographs of zeolite NaA films synthesized on a modified support with glucose (synthesis at 353 K for 4 h with rotation at 60 rpm). Left: top view. Right: cross sectional view. (a) S11 on a modified support with 0.001 g mL 1 and (b) S10 on a modified support with 0.01 g mL 1 and a post-calcination step.
S. Aguado et al. / Microporous and Mesoporous Materials 146 (2011) 69–75 1000
1000
900
900
800
800
Intensity [CPS]
Intensity [CPS]
74
700 600 500
700 600 500
400
400
a b
300
300
c
200
292
288
290
284
286
282
280
278
Binding Energy [eV]
200
292
290
288
286
284
282
280
278
Binding Energy [eV]
Fig. 4. XPS analysis of a-alumina powder samples after different treatments. (a) S03 with 1 g mL 1 of PDDA and a post-calcination step; (b) S07 with 0.001 g mL and a post-calcination step; (c) S02 with 1 g mL 1 of PDDA. Right: Deconvolution of the C1s binding energies of sample S03.
Table 3 Relative concentrations of O, N and C determined by XPS. Sample*
Relative concentrations (atomic%) O
N
C
S02 S03 S06 S07
PDDA
58.0 60.9 65.7 64.9
1.4 traces 0.2 traces
12.5 12.1 6.1 6.3
S8 S9
Glucose
59.4 59.5
– –
8.9 7.8
S13 S14
PEG
54.6 54.5
– –
21.7 5.9
*
Samples of a-alumina powder treated in a similar way as the corresponding supports.
Table 4 Deconvolution of the C1s peak of the treated a-alumina samples. Sample
C–C/C–H
C–O–/C–N
C=O
O = C–O–
S02
284.6 eV 44% 284.6 eV 76% 284.6 eV 84% 284.2 eV 82%
285.4 eV 45% –
286.7 eV 11% 286.7 eV 6% 286.6 eV 6% –
–
284.5 eV 60% 284.5 eV 51%
–
–
–
–
285.1 eV 16% 284.5 eV 62%
–
–
–
–
–
288.9 eV 23%
S03 S06 S07 S8 S9 S13 S14
285.6 eV 10% 286.0 eV 9%
289.1 eV 18% – 288.5 eV 9% 288.3 eV 27% 288.2 eV 36%
species only for the calcined samples, showing the highest intensity in sample S03 immersed in 1 g mL 1 of PDDA solution. From the TGA results with the treated alumina powder follows that a 0.2–0.5 wt.% is left after the mild calcination procedure, depending on the initial loading of modifier used. Considering an external surface area of 9 m2 g 1 and assuming the residual weight is due to the presence of (C–O) species, this would correspond to an average density of 4–11 (C–O) species per nm2. This is as high as the hydroxyl group concentration in c-alumina and silica [42], and demonstrates that full surface coverage of the a-alumina particles
1
of PDDA
is achieved in the experiments performed at the highest loadings [42]. The major part of these carbon–oxygen species are carboxylic in nature as follows from XPS. The net result is a moderation of the surface acidity: less negative zeta potential values at pH = 7 are found for the modified and calcined supports than for the untreated alumina. The latter may seem surprising, as a positive zeta potential is expected for aluminas based on their PZC of 8–9 [43]. However, the zeta potential of sapphire (high purity a-alumina) is even lower ( 40 to 60 mV) and these differences are attributed to either the purity of the material or to contaminations.[43] In any case, whatever the surface status of the alumina particles or the supports is, the coating with a virtual monolayer of carbon–oxygen species will result in a comparable net charge during the membrane synthesis. Coming back to the traditional cationic modification method, several authors reported on the calcination of the intermediate polymeric layer after attachment of the seed crystals prior to the secondary growth: Mintova et al. reported a strong attachment of ZSM-5 nanoseeds after the calcination process, notwithstanding that the nature of the bonding between seed crystals and substrate surface is not entirely clear [36]. Boudreau et al., following a similar approach, studied the effect of the calcination of the intermediate polymeric layer after attachment of the seed crystals and did not find any effect [34]. It is well known that the aluminum content on zeolites highly influences the first steps of the synthesis and the final topology of the framework. Indeed, high-aluminum contents drive the synthesis towards low framework density zeolites. This has been attributed to the necessity of large pore volume to accommodate enough counter-cations that compensate a negative charge of the zeolite framework. Similar effects are found for fluoride anions that tend to be placed at the smallest interstices available in the zeolite structure, yielding negatively charged fluoro-silicates that need to be compensated by the occluded organic [44]. We suggest a somehow similar effect of the observed carboxyl species on top of the calcined supports and the negative net charge of the surface [39]. The high density of these carboxyl species could generate a layer of solvated Na+ counter ions on top of the support as an electric double layer [43]. This layer can act as template for the nucleation and condensation of the silico-aluminate on the support, just like happens in the formation of NaA crystals. In fact, the concentration of all charged species in solution might be significantly different close to the surface than in the bulk of the solution. As a result, depending on the PZC of the support, it might be easier or more difficult to achieve the supersaturation required for nucleation close to the surface. However, the magnitude of these interactions and
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of the double layer thickness depends on the ionic strength of the solution, which in the case of LTA synthesis is quite high. As result this double layer should be extremely thin (of the order of few nm maximum). If true, this method would only work for zeolites with high aluminum contents: indeed, several attempts to translate this synthetic approach to other zeolites with higher Si/Al ratios like ZSM-5 or DD3R did not result in dense coatings. These results demonstrate that still much work is necessary in order to understand the first steps of zeolite synthesis and on how to transfer this knowledge to the synthesis and adherence of coatings and membranes. Both electrostatic and chemical interactions at zeolite-support interface are of primary importance, and the right optimum has to be thoroughly investigated. Regarding the specific case of zeolite NaA, the method reported in this work may form the basis to scale up membrane synthesis in view of the simple and cheap pretreatment and the elimination of pre-seeding steps. 5. Conclusions A preparation of continuous thin zeolite NaA films on macroporous a-alumina supports has been presented in this work. The films are prepared by simple modification of alumina support followed by an in situ hydrothermal synthesis. The modification of the support is carried out with a cationic polymer, glucose or PEG and a subsequent mild calcination at 673 K. This procedure allows the preparation zeolite NaA films on macroporous supports without seeding. A substantial improvement in terms of layer continuity and crystal intergrowth is observed when comparing with coatings prepared on non-modified supports. We attribute this positive effect to the presence of carboxyl species at the surface of the calcined supports and to the negative net charge of the surface. The surface charge will be compensated by a solvated Na+ double layer on top of the support, acting as template for the formation of the NaA thin film. The improved film fabrication procedure may form the basis to scale up membrane synthesis in view of the low pressure drop because in asymmetric supports together with the elimination of pre-seeding steps. Acknowledgments
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
Shell Global Solutions International B.V., Amsterdam is gratefully acknowledged for financial support. The authors thank Laurence Massin (IRCELYON) for her expert assistance with the XPS measurements and for fruitful discussions on XPS spectra. J.G. gratefully acknowledges The Netherlands National Science Foundation (NWO) for his personal VENI grant. References [1] Y.S. Lin, I. Kumakiri, B.N. Nair, H. Alsyouri, Sep. Purif. Methods 31 (2002) 229.
[38] [39] [40] [41] [42] [43] [44]
75
J. Coronas, Chem. Eng. J. 156 (2010) 236. J. Coronas, J. Santamaria, Top. Catal. 29 (2004) 29. T. Bein, Chem. Mater. 8 (1996) 1636. Z.B. Wang, H.T. Wang, A. Mitra, L.M. Huang, Y.S. Yan, Adv. Mater. 13 (2001) 746. S. Aguado, C.H. Nicolas, V. Moizan-Baslé, C. Nieto, H. Amrouche, N. Bats, N. Audebrand, D. Farrusseng, New J. Chem. 35 (2011) 41. H. Bux, F. Liang, Y. Li, J. Cravillon, M. Wiebcke, J. Caro, J. Am. Chem. Soc. 131 (2009) 16000. J. Gascon, S. Aguado, F. Kapteijn, Micropor. Mesopor. Mater. 113 (2008) 132. J. Gascon, F. Kapteijn, Angew. Chem. Int. Ed. 49 (2010) 1530. Y. Li, F. Liang, H. Bux, A. Feldhoff, W. Yang, J. Caro, Angew. Chem. Int. Ed. 49 (2010) 548. M.P. Pina, M. Arruebo, A. Felipe, F. Fleta, M.P. Bernal, J. Coronas, M. Menendez, J. Santamaria, J. Membr. Sci. 244 (2004) 141. F. Tiscareno-Lechuga, C. Tellez, M. Menendez, J. Santamaria, J. Membr. Sci. 212 (2003) 135. K.J. Balkus, A.S. Scott, Chem. Mater. 11 (1999) 189. M. Pera-Titus, R. Mallada, J. Llorens, F. Cunill, J. Santamaria, J. Membr. Sci. 278 (2006) 401. M. Tsapatsis, G.R. Gavalas, MRS Bull. 24 (1999) 30. X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Micropor. Mesopor. Mater. 43 (2001) 299. A. Berenguer-Murcia, L. Gora, W.D. Zhu, J.C. Jansen, F. Kapteijn, D. CazorlaAmoros, A. Linares-Solano, Ind. Eng. Chem. Res. 46 (2007) 3997. Y.S. Li, H.L. Chen, J. Liu, W.S. Yang, J. Membr. Sci. 277 (2006) 230. S. Aguado, J. Gascon, J.C. Jansen, F. Kapteijn, Micropor. Mesopor. Mater. 120 (2009) 170. J. Caro, M. Noack, P. Kolsch, Adsorption 11 (2005) 215. Y.S. Kim, K. Kusakabe, S.M. Yang, Chem. Mater. 15 (2003) 612. J.L.H. Chau, C. Tellez, K.L. Yeung, K.C. Ho, J. Membr. Sci. 164 (2000) 257. A.W.C. Van den Berg, L. Gora, J.C. Jansen, M. Makkee, T. Maschmeyer, J. Membr. Sci. 224 (2003) 29. W. Zhu, L. Gora, A.W.C. Van den Berg, F. Kapteijn, J.C. Jansen, J.A. Moulijn, J. Membr. Sci. 253 (2005) 57. A.S. Huang, J. Caro, Chem. Mater. 22 (2010) 4353. A.S. Huang, F.Y. Liang, F. Steinbach, J. Caro, J. Membr. Sci. 350 (2010) 5. S.M. Lai, C.P. Ng, R. Martin-Aranda, K.L. Yeung, Micropor. Mesopor. Mater. 66 (2003) 239. X.D. Wang, W.L. Yang, Y. Tang, Y.J. Wang, S.K. Fu, Z. Gao, Chem. Commun. (2000) 2161. Y.J. Wang, Y. Tang, X.D. Wang, W.L. Yang, Z. Gao, Chem. Lett. (2000) 1344. Y.J. Wang, Y. Tang, X.D. Wang, W. Shan, C. Ke, Z. Gao, J.H. Hu, W.L. Yang, J. Mater. Sci. Lett. 20 (2001) 2091. G.S. Lee, Y.J. Lee, K.B. Yoon, J. Am. Chem. Soc. 123 (2001) 9769. Y. Liu, Y.Y. Yu, Q.Y. Yang, Y.H. Qu, Y.M. Liu, G.Y. Shi, L.T. Jin, Sens. Actuators B 131 (2008) 432. J.Y. Choi, Z.P. Lai, S. Ghosh, D.E. Beving, Y.S. Yan, M. Tsapatsis, Ind. Eng. Chem. Res. 46 (2007) 7096. L.C. Boudreau, J.A. Kuck, M. Tsapatsis, J. Membr. Sci. 152 (1999) 41. G.H. Yang, X.F. Zhang, S.Q. Liu, K.L. Yeung, J.Q. Wang, J. Phys. Chem. Solids 68 (2007) 26. S. Mintova, J. Hedlund, B. Schoeman, V. Valtchev, J. Sterte, Chem. Commun. (1997) 15. M. Noack, P. Kolsch, A. Dittmar, M. Stohr, G. Georgi, M. Schneider, U. Dingerdissen, A. Feldhoff, J. Caro, Micropor. Mesopor. Mater. 102 (2007) 1. Z.B. Wang, Q.Q. Ge, J. Shao, Y.S. Yan, J. Am. Chem. Soc. 131 (2009) 6910. S. Biniak, G. Szymanski, J. Siedlewski, A. Swiatkowski, Carbon 35 (1997) 1799. W.H. Lee, J.G. Lee, P.J. Reucroft, Appl. Surf. Sci. 171 (2001) 136. H.Y. Su, C.T. Hsieh, J.M. Chen, H.C. Shih, Nanosci. Technol. 121–123 (2007) 407. K.W. Goyne, A.R. Zimmerman, B.L. Newalkar, S. Komarneni, S.L. Brantley, J. Chorover, J. Por. Mater. 9 (2002) 243. G.V. Franks, L. Meagher, Colloids Surf. A 214 (2003) 99. B. Marques, S. Leiva, A. Cantin, J.L. Jorda, M.J. Sabater, A. Corma, S. Valencia, F. Rey, Stud. Surf. Sci. Catal. 174 (2008) 249.