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196 A. Mosca et al. / Microporous and Mesoporous Materials 120 (2009) 195–205 surface of the cordierite support allows for a sufficiently high zeo- lite loading so that the adsorption capacity and thermal stability of adsorbed species can be investigated even for very thin films. Fur- thermore, such composites may be useful as selective NOx adsor- bents, especially in pressure swing processes (PSA) or may be used as a component in NSR systems. In the present work, discrete MFI crystals and cordierite mono- liths coated with thin MFI films are characterized by SEM, ICP-AES, XPS, N2 sorption, TG/MS, and the NO2 adsorption properties are evaluated by NO2 adsorption and temperature programmed desorption experiments. Different synthesis methods were used to produce films with varying Si/Al and Na/Al ratios and the influ- ence of the Si/Al and Na/Al ratios of the film on the NO2 adsorption properties is determined. 2. Experimental 2.1. Synthesis of the materials Cordierite monoliths (400 cpsi, Corning Inc.) with a diameter of 2 cm and a length of 10 cm were used as supports. MFI films were grown on the supports as described in detail earlier [15], and only a brief description of the preparation procedure is given below. The support was treated with a solution containing cationic polymer molecules followed by electrostatic adsorption of silicalite-1 seed crystals (60 nm) [16,17]. The seeded monoliths were hydrother- mally treated in a gel or in a clear synthesis mixture. MFI films were grown in six or 13 steps for 48 h or nine steps for 96 h and atmospheric pressure in a clear synthesis solution with a molar composition of 3TPAOH:25SiO2:0.25Al2O3:1Na2O:1600H2O:100E- tOH. The samples were rinsed with a 0.1 M aqueous ammonia solution and treated with ultrasound for 10 min in between each hydrothermal treatment. After the final hydrothermal treatment, the samples were rinsed in a 0.1 M aqueous ammonia solution for 4 days and treated with ultrasound for 1 h each day during this period. The film thickness was controlled by the number of hydro- thermal treatments [15]. Films were grown in several steps in or- der to reduce sedimentation during synthesis as discussed elsewhere [15]. An MFI film was also grown on a monolith using a template-free gel of molar composition 100SiO2:1Al2O3:30 Na2O:3996H2O. The synthesis was performed in one step in a Tef- lon-lined autoclave for 12 h and 180 °C. After synthesis, this sam- ple was treated with ultrasound for 2 min and rinsed thereafter in distilled water for six times without the use of ultrasonication. This sample was not rinsed in a 0.1 M ammonia solution in order to avoid ion exchange from sodium form to ammonium form. The ends of all monolith samples were polished until the total length of each monolith was 75 mm, to remove sedimented zeolite. The samples were finally calcined at 550 °C for 6 h with a heating and cooling rate of 1.75 °C/min. The sample codes for the samples grown in the clear solution start with the letter C followed by the synthesis duration (48 or 96 h) and the number of hydrothermal treatments. When using the gel, the sample code is given by the letter G. Sample codes and preparation procedures for monolith samples are summarized in Table 1. MFI films were also grown on polished quartz single crystals in the clear solution (6 steps of 48 h or 9 steps of 96 h) and in the gel (12 h), similar to the films grown on the monoliths. The quartz sup- ports were cleaned as described previously before synthesis [18]. The same rinsing procedure used for the MFI film samples grown on cordierite was performed after synthesis on the quartz sub- strates, but no ultrasonication was used as before [9]. The films were calcined at 400 °C as before [8,9]. These samples were used for XPS depth profiling, which requires a smooth sample on a non-contaminating substrate, such as quartz single crystals. Table 1 Sample characteristics. Sample Hydrothermal Film code treatment thickness (lm) Zeolite Micropore Mesopore Mesopore loading vol. (cm3/ vol. (cm3/ vol. (cm3/ (g/g) g sample) g sample) g zeol.) C48-6 48 h 6, 75 oC C48-13 48 h 13, 75 oC C96-9 96 h 9, 75 oC G 12 h, 180 oC 0.8 0.06 0.009 1.9 0.10 0.016 1.9 0.14 0.022 1.9 0.16 0.025 0.005 0.09 0.010 0.10 0.015 0.11 0.009 0.06 Table 2 Size and composition of the MFI crystals by ICP-AES. The sample codes for the MFI film samples grown on quartz sub- strates are the same as the ones for the films grown on cordierite, but followed by the letter Q. A reference MFI powder of discrete crystals [9] was prepared by hydrothermal treatment at 100 °C for 72 h in the clear synthesis solution with identical composition as the one used for film prep- aration (sample denoted DC72). No support was present. The size of the crystals in this reference powder was about 1 lm and the micropore volume was 0.157 cm3/g zeolite, as expected for MFI zeolite [23]. Since these crystals are relatively large, the mesopore volume per gram sample should be small and this sample was thus used as a reference for the N2 sorption measurements. For comparison with films, discrete MFI crystals were also grown at 75°C for 48h and 96h in one step by adding 10ppm seeds to the clear synthesis solution (without support) employed for film preparation. Discrete MFI crystals were also grown for 14 h and 180 °C by adding 10 ppm silicalite-1 seeds to the tem- plate-free gel. The crystals were purified by repeated centrifuga- tion (14000g, 1 h) and redispersion. These crystals were used for ICP analysis in order to indicate the composition of the film. The samples are labeled as DCG, DCC48 and DCC96. The first two letters are abbreviation for discrete crystals. These letters are followed by G (for gel), C (for clear solution), and 48 or 96 (synthesis time, hours). Sample codes and preparation procedures for discrete crys- tals are given in Table 2. 2.2. Characterization The morphology of the films and film thickness were deter- mined using a Philips XL30 scanning electron microscope (SEM) after deposition of a thin layer of gold on the samples by sputter- ing. The monoliths were cut with a knife in the channel direction to smaller pieces in order to obtain cross-sections and to be able to view the films inside the channels. The composition of the crystals grown by seeding the two syn- thesis mixtures was measured by inductively coupled plasma- atomic emission spectrometry (ICP-AES, Analytica, Sweden) by using an ARL-3560 instrument. The depth profiles (Si/Al and Na/ Al) of the films grown on the quartz crystals were measured by Sample code Hydrothermal treatment Size (nm) Si/Al Na/Al DCC48 Clear solution, 0.001wt% silicalite-1 240 seeds, 48 hours, 75 °C DCC96 Clear solution, 0.001wt% silicalite-1 450 seeds, 96 hours, 75 °C DC72 Clear solution, 72 h 100 °C 1000 DCG Gel, 0.01 wt.% silicalite-1 seeds,14 h, n.a. 180 °C autoclave 367 5.7 178 3.2 n.a. n.a. 14 0.95PDF Image | Structured Zeolite Adsorbents for PSA Applications
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