2006
Ind. Eng. Chem. Res. 2007, 46, 2006-2012
Crystallization Field and Rate Study for the Synthesis of Ferrierite Zuhal Go1 gˇ ebakan, Hayrettin Yu1cel, and Ali C¸ ulfaz* Department of Chemical Engineering, Middle East Technical UniVersity, Ankara 06531, Turkey
A partial crystallization field and rate study was performed for the synthesis of ferrierite from gels of variable composition: 1.85Na2O‚Al2O3‚xSiO2‚592H2O‚19.7R, where R ) pyrrolidine or ethylenediamine. The variables of syntheses were the SiO2/Al2O3 molar ratio of the gel (10 < x < 25), temperature (150 °C < T < 225 °C), and crystallization time (t < 480 h). The crystallization rate was determined to be strongly dependent on temperature, whereas the SiO2/Al2O3 ratio and template type had no significant influence. Ferrierite was the only crystalline phase observed throughout the entire crystallization field studied when ethylenediamine was the templating agent. For some batches, mordenite appeared as a crystalline impurity when pyrrolidine was the template. Thermal stability tests indicated that ferrierites retained their crystallinities up to 1050 °C. The methanol sorption capacity of highly crystalline ferrierite samples approached the theoretical sorption volume, indicating that it can be used as a measure of purity for single-phase ferrierite samples. 1. Introduction The zeolite ferrierite (FER) is known both as a mineral and as a synthetic material. Natural ferrierite belongs to the mordenite group of minerals and can be found, although rare, as small crystalline incrustations in volcanic rocks, or as extensive sedimentary deposits. Ferrierite is one of the most siliceous natural zeolites, having SiO2/Al2O3 molar ratios in the range of x ) 6.4-12.4.1 The crystal structure of ferrierite has been determined by Vaughan2 and by Kerr3 on natural ferrierite samples, and, later, it has been verified and refined using synthetic analogs.4 In the ferrierite structure, one channel system parallel to the c-axis is defined by 10-membered oxygen rings of roughly elliptical apertures with dimensions of 0.42 nm × 0.54 nm. The second channel system, which is oriented parallel to the b-axis, is bounded by 8-membered rings with dimensions of 0.37 nm × 0.47 nm. In natural ferrierites, these cavities are usually occupied by hydrated Mg cations, which seem to be difficult to remove via acid treatment or via exchange by other cations. Ferrierites have good acid and thermal stabilities, because of their relatively high SiO2/Al2O3 ratio. Ferrierite, similar to MFItype zeolites with 10-membered windows, is intermediate between shape-selective small-pore (8-membered ring) zeolites and surface-selective large-pore (12-membered ring) zeolites; therefore, it can serve as shape-selective adsorbents and catalysts in various forms, including membranes. As acidic catalysts, 10membered window zeolites are being used or are candidates for industrial catalytic processes such as the conversion of methanol to dimethyl ether and then to olefins and hydrocarbons in the gasoline range5 and isomerization of xylenes to p-xylene,6 as well as for selective cracking of n-paraffins in a hydrocarbon mixture.7 Ferrierite is also a promising catalyst for the isomerization of n-butene to isobutene.8,9 Ferrierite can be also be used for NO reduction by methane in catalytic converters in automobiles.10 Some of the synthesis methods for ferrierite involve alkaline aluminosilicate gels that contain single or mixed salts or bases involving sodium, potassium, calcium, strontium, or barium under autogenous conditions.11-13 These methods yield products with SiO2/Al2O3 ratios within a range of x ) 7.4-14, imitating * To whom correspondence should be addressed. Tel.: +90 312 210 2611. Fax: +90 312 210 2600. E-mail address:
[email protected].
natural products. Kibby et al. were the first to use an organic agent (tetramethylammonium hydroxide) in ferrierite synthesis.7 Some of their products contained analcite and/or mordenite impurities and SiO2/Al2O3 molar ratios were again within the range of x ) 7.4-14. Following general trends in zeolite synthesis, most of the studies on ferrierite synthesis used aqueous aluminosilicate solutions that involved organic templates.14 However, nonaqueous environments can also be utilized.15,16 The synthesis of ferrierites via the transformation of solid precursors by suitable vapor templates was also reported.17 Ferrierite samples synthesized using these methods had SiO2/ Al2O3 molar ratios that were significantly higher than those of natural ferrierites. Aluminum-free end-member ferrierites were also reported.18 Many patented zeolite species, such as ZSM35, FU-9, ISI-6, and Nu-23, were reported to have a ferrierite topology.14-19 This study involves a partial crystallization field and rate study of ferrierite synthesis. Under the light of some synthesis studies20-24 involving different templates (including pyrrolidine and ethylenediamine), an aqueous gel of composition 1.85Na2O‚ Al2O3‚xSiO2‚592H2O‚19.7R was chosen. The investigation of the crystallization field is significant in regard to delineating the compositional range within which ferrierite could be synthesized as a pure phase and observing when and which competing phases start to appear. The synthesis variables are the template type, R, the SiO2/Al2O3 molar ratio (10 < x < 25), temperature (150 °C < T < 225 °C), and crystallization time (t < 480 h). Crystallization rate curves are of interest in regard to clarifying how fast ferrierite can be obtained as a pure phase and also in regard to checking the stability of the ferrierite formed in the synthesis medium at extended times. Products were identified and investigated using several techniques, including X-ray diffractometry (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and energydispersive X-ray analysis (EDAX). Thermal stability tests and methanol sorption measurements also were performed on selected samples. 2. Experimental Methods 2.1. Synthesis. The raw materials used in the synthesis were colloidal silica (LUDOX AS-30, 30 wt % SiO2, Aldrich Chemical Company, Lot No. 02302DO), sodium hydroxide (98% pellets, Merck, Lot No. B102362802), aluminum hydrox-
10.1021/ie061013+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2007
ide (pure powder, Aldrich Chemical Company, Lot No. K24826491817), pyrrolidine (99%, Aldrich Chemical Company, Lot No. S05094-061) and ethylenediamine (99.5%, Aldrich Chemical Company, Lot No. 01746DI-081). The synthesis gel was prepared by mixing a sodium aluminate solution (solution A) with a mixture composed of colloidal silica and template (solution B), which were prepared separately. The preparation of a typical batch of a predetermined composition (1.85Na2O‚Al2O3‚20SiO2‚592H2O‚19.7Py) is given below. First, solution A was prepared by adding 1.16 g of Al(OH)3 powder to a NaOH solution, which is formed by mixing 1.12 g of NaOH pellets and 57.65 g of distilled water. This mixture was heated on a hot plate and stirred for ∼1 h to dissolve alumina particles until a clear sodium aluminate solution was obtained. The loss by evaporation was corrected by the precise quantitative addition of makeup water. Solution B was prepared by adding 29.68 g of colloidal silica solution to 10.21 g of pyrrolidine (Py) with constant stirring with a magnetic stirrer. Next, solution B was added to solution A and the mixture was stirred further for ∼0.5 h at room temperature. Gelation occurred immediately upon mixing. At this stage, the reaction mixture had a molar composition of 1.85Na2O‚Al2O3‚20SiO2‚592H2O‚ 19.7Py. The gel was transferred to several stainless steel autoclaves with polytetrafluoroethylene (PTFE) inserts with a capacity of ∼35 mL to a fill level of ∼80%. The autoclaves were then heated in an oven to the desired temperature in the range of 150-225 °C and kept there for predetermined periods of time (up to 480 h) under autogenous pressure and without stirring. To stop the crystallization process, the autoclaves were removed from the oven at different intervals of time and quenched immediately with tap water. The resultant solid product was quantitatively recovered by filtering and washed thoroughly with distilled water until pH 8 was attained. Finally, the products were dried at 100 °C overnight and then kept in a desiccator type vessel that contained saturated potassium sulfate solution to provide a constant relative humidity environment. Both the gel that was placed into the autoclaves and the weight of dry product were recorded for yield calculations. 2.2. Characterization Methods. 2.2.1. X-ray Diffraction. Phase identification of the synthesized zeolites was made by Philips model PW1729 X-ray diffractometer with nickel-filtered Cu KR radiation at 30 kV and 24 mA. To quantify the crystallinity index of the products obtained at different periods of crystallization time, a reference pattern with 20 of its most intense diffraction peaks was selected, based on the XRD data reported for natural ferrierites25,26 and synthetic ferrierites or ZSM-35.7,12,13 These peaks are listed in Table 1, together with the averaged normalized intensities for the nine highly crystalline ferrierite samples synthesized in this study. The crystallinity index of the samples, as defined in eq 1, was calculated using the intensity summation method: 20
crystallinity index )
∑In,s n)1 20
(1)
∑In,0
n)1
In eq 1, In,s represents the peak intensity of the synthesized sample and In,0 represents the peak intensity of the reference peak intensity data. Reference peak intensity data were formed by averaging the intensity summations of 20 characteristic peaks
Table 1. Reference Peak Intensity Data for High-Crystallinity Ferrierite peak
d-spacing (Å)
relative intensity, I/I0 × 100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
9.60 7.07 6.61 5.76 4.95 3.98 3.93 3.85 3.77 3.66 3.53 3.47 3.39 3.31 3.13 3.04 2.95 2.92 2.64 2.47
53 ( 5 22 ( 2 17 ( 2 9(2 9(2 72 ( 8 62 ( 4 29 ( 2 54 ( 5 31 ( 3 100 89 ( 5 20 ( 3 22 ( 2 33 ( 3 19 ( 2 12 ( 3 10 ( 2 10 ( 2 9(2
of nine ferrierite samples of high crystallinity. All XRD measurements were conducted on as-synthesized samples without template removal by calcination. To determine the crystalllinity indices of the samples that have mordenite as a competing phase, five of the ferrierite peaks that do not overlap with the mordenite peaks were used. 2.2.2. SEM Micrographs. The morphologies and crystal sizes of the synthesized samples were observed using a LEO435VP Zeiss-Leica SEM system in the magnification range of 1000× to 10000×. The initial observations were also made via optical microscopy. During the SEM investigations, EDAX was also performed, to determine the SiO2/Al2O3 molar ratio of the product crystals. 2.2.3. TGA/DTGA Analysis. The as-synthesized ferrierite crystals contain, in addition to a small amount of water, pyrrolidine or ethylenediamine templates, as occluded in their frameworks. The dehydration and desorption/decomposition behavior of templates were investigated by TGA, using the Perkin-Elmer Pyris 1 TGA system. Nitrogen or air was used as purge/oxidant gases at flow rates of 100 mL/min. Heating rate was chosen as 10 K/min. 2.2.4. Thermal Stability Tests. Thermal stability testing was applied to obtain the structure breakdown temperature of the as-synthesized ferrierite samples. For this purpose, ∼0.6 g of sample was placed in crucibles as a thin layer of solids and exposed to heat treatment in a furnace for 0.5, 1, and 2 h at different temperatures in the range of 200-1100 °C. Samples were heated to the final heat-treatment temperature at a rate of 20 K/min. The loss of the crystallinity index were determined with smear mount scans by XRD of these 0.6 g heat-treated samples. 2.2.5. Methanol Sorption Capacity. Sorption capacities of some high-crystallinity ferrierite samples for sorbate methanol at 0 and 25 °C were measured in a conventional gravimetric adsorption system, which has been described in detail elsewhere.27 This system consists of an electronic balance enclosed in a vacuum chamber, a high-vacuum pump unit, and a doser chamber. Prior to sorption measurements, based on TGA data, the as-synthesized samples were calcined at 650 °C for 8 h in a temperature-programmable furnace to remove the templates. The calcined samples then were kept over saturated potassium sulfate solution overnight to rehydrate the sample in a constant relative humidity environment. Thirty to forty milligrams of the
2008
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007
Table 2. Crystallization Conditions, Identities, and Yields of Products sample
SiO2/Al2O3 molar ratio, x, in batch
templatea
temperature, T (°C)
time, t (h)
phase(s)
yield (g /100 g batch)
crystallinity index for ferrierite (%)
conversion to ferrierite (%)
1-a1 1-a2 1-b1 1-b2 1-c1 1-c2 1-d1 1-d2 2-a 2-b 2-c 3-a1 3-a2 3-b 3-c1 3-c2 3-d
10 10 15.2 15.2 20 20 25 25 20 20 20 20 20 20 20 20 20
Py Py Py Py Py Py Py Py Py Py Py ED ED ED ED ED ED
177 177 177 177 177 177 177 177 150 200 225 150 150 177 200 200 225
36 168 48 288 72 192 36 168 240 48 10 240 480 120 24 200 8
FER FER + MOR FER + MOR FER + MOR FER FER FER FER FER FER FER FER FER FER FER FER FER
4.90 5.55 7.69 7.64 8.16 9.53 9.73 10.12 8.12 9.02 8.81 8.50 9.45 7.33 8.47 8.44 8.32
36 57 92 96 90 100 100 100 89 100 100 90 98 96 100 100 100
26 46 88 91 62 81 84 88 61 82 75 64 77 59 70 70 70
a
Py ) pyrrolidine; ED ) ethylenediamine.
hydrated sample was placed in an aluminum sample pan and, to remove water from zeolite structure fully, it was treated at 673 K under a vacuum of
