Zeolites are an important class of materials that are widely used in industrial processes that include catalysis and adsorption, owing to their selective nature and tailorable acidic property. Zeolites are usually available as a very fine powder that can cause significant decline in pressure and diffusion issues when employed in catalysis and adsorption industrial processes [1]. To address this issue, they are generally diluted with binders and shaped as pellets. However, this type of modification often alters important zeolite properties, occasionally restricting molecular diffusion, and altering the acidity and micropore size and volume [1, 2]. Hence, in an attempt to maintain zeolite properties, improve diffusion, and reduce pressure drop, supporting zeolites on macroscopic structures were examined recently and subsequently tested in various applications [3, 4]. However, with a few exceptions [5], the literature studies typically investigate macroscopic structural supports such as cordierite and β-SiC over which the zeolite is either grown or deposited [6, 7]. This method affords low zeolite loading amounts and requires multiple hydrothermal reactions with seeding.
According to the IUPAC convention [8], pores are classified based on their width; namely, micropores (< 2 nm), mesopores (2-50 nm), and macropores (> 50 nm). Zeolitic materials are mainly microporous [9]. To improve access of molecules to the adsorption/catalytic sites, hierarchically porous zeolites, i.e., materials that feature multiple types of pores mentioned above, are preferred. Post-synthesis treatments can generate hierarchically porous zeolites through selective leaching of aluminum (dealumination) or silicon (desilication) from the framework. Dealumination has been carried out to improve the acid strength and increase the intracrystalline mesoporosity by removing the tetrahedrally coordinated aluminum by leaching, using either steam or acid [10]. The major disadvantage of such a process is the partial reduction of the crystalline nature of the zeolite, consequently reducing the catalytic activity. A more recent process adopted to create hierarchical porosity in zeolites is desilication. In this method, leaching of silicon is conducted in an aqueous alkaline solution [11]. Desilication is highly suitable for introducing intracrystalline mesopores in the zeolite crystals, leading to an improved molecular diffusion and thus providing easier access to the micropores via the mesopores. However, if the Si/Al ratio is excessively low, the excess aluminum will inhibit the removal of silicon and subsequent formation of mesoporosity. Other methods, such as the use of secondary mesoporous templates, termed as hard templating, have also been examined; carbon, aerogel, and mesoporous silica spheres have been investigated as templates. However, this process is usually expensive and time-consuming [12]. The synthesis of monolithic ZSM-5 zeolites was achieved using a dry-gel conversion method [13] and mesoporous glass [1, 14]. Cationic polymers have also been successfully employed to produce hierarchically meso-microporous zeolites [15]. Various other strategies have been appl ied to obtain zeolites with a hierarchical porosity, as described in various reviews [11, 16]. Nevertheless, few strategies address the simultaneous creation of hierarchical porosity and the monolithic structure.
To overcome some of the drawbacks mentioned above, we synthesized a self-supporting zeolite monolith with a hierarchical porosity i.e., a monolithic-shaped material that has both micro- and mesopores, via a bottom-up approach using a hard-templating procedure. Relatively cheap polyurethane foams were used as a structural template during the hydrothermal synthesis. The zeolite that was built on the foam was shaped according to the shape of the foam. Final removal of the polyurethane template foam that is partially decomposed during the zeolite synthesis was achieved by calcination, thereby affording the monolithic shape of the zeolite. The zeolite has inherent micropores; the template is responsible for the formation of meso- and macropores in the monolithic zeolites, and these larger pores facilitate diffusion of gases and reduce the extent of pressure drops [12, 17].
In the present work, we discussed the synthesis of self-supporting ZSM-5 monolith, its adsorption capabilities for trichloroethylene (TCE), and catalytic activity towards the decomposition of TCE. According to a recent study on the quality of air in Europe, air pollution and the various components lead to a reduced life expectancy of 8.6 months per person. The associated losses in productivity and medical costs in 2009 were estimated between EUR 102 and 169 billion (Enviromental European Agency, Every breath we take Improving air quality in Europe, Luxembourg, 2013). TCE was selected as an example of a chlorinated volatile organic compound (VOC) that is among the most widespread toxic pollutants. VOCs are typically used as organic solvents in industrial dry cleaning and degreasing.
The foam template was prepared according to a method previously described [18]. Briefly, polymer 4,4-methylene bisphenyl diisocyanate (MDI, BASF) was added to a mixture of polyol (F-5521, Repsol YPF), water, silicone oil surfactant (B8232, Evonik Industries AG), and catalysts 1,4- diazabicyclo[2.2.2]octane (DABCO 33LV, Air Products) and dibutyltin dilaurate (Merck, > 97%). The resulting mixture was stirred for 20 s, after which the homogeneous gel was transferred to a container and allowed to stand for 10 min for polymerization (growth of PUF) to take place. The PUF was placed in an oven at 70 °C for 30 min to ensure complete polymerization. The obtained PUF was cut into cylindrical shapes with a diameter of 2.5 cm and height of 5 cm, and used as a shape-directing template.
ZSM-5 zeolite was prepared using a hydrothermal synthesis method. A precursor gel was prepared by vigorous mixing of 29.9 mL water, 6.1 mL zeolite pore structure-directing template tetrapropylammonium hydroxide (TPAOH), 11.03 mL tetraethyl orthosilicate as silica source, and 1.95 mL sodium aluminate (1 mol/L) as aluminum source. The resulting mixture was stirred further for 4 h. The homogeneous gel was transferred to a Teflon-coated autoclave containing the monolith shape-directing template PUF. The PUF was squeezed several times to remove air trapped in the foam to allow the gel solution to impregnate the foam. The autoclave was sealed tightly and placed inside an oven at 120 °C for 48 h under air flow. The monolithic zeolite ZSM-5 was subsequently obtained. The lightweight monolith was washed repeatedly until the pH of the filtrate was neutral. The pale yellow monolith was then calcined at 550 °C for 4 h at a heating ramp of 10 °C/min to produce a pure white crystalline ZSM-5 monolith.
Powder X-ray diffractograms (XRD) were recorded on a Philips PX 1820 X-ray diffractometer using Cu Kα source with a wavelength of 1.54 Å, operating at 20 mA and 50 kV. Scanning electron microscopy (SEM) images were obtained on a scanning electron microscope (Hitachi S2400) equipped with a standard energy-dispersive X-ray detector (EDS, Rontec). The open-cell volume and density of the PUF template and ZSM-5 monolith were measured with an automatic gas pycnometer (AccuPyc1330, Micrometrics) using dry nitrogen. The instrument chamber was purged 10 times at 170 kPa, then 10 expansion measurements were conducted at 170 kPa with an equilibrium criterion of 0.0345 kPa/min. The open-cell content was calculated from the external geometrical volume of a cylinder (Ve) (either polyurethane or ZSM-5 monolith) and the volume of the cylinder measured by gas pycnometry (Vg). The percentage fraction of the open volume, also known as the open-cell content, was calculated as follows: (Ve−Vg)/Ve × 100%. N2 adsorption isotherms at −196 °C were measured on a Quantachrome (NOVA 2200e); ~0.05 g of the crushed monolith was degassed at 300 °C for 3 h prior to measurement.
Adsorption of trichloroethylene (Merck, for analysis) was studied using a microbalance (C.I. Electronics), which allowed a precision of 10 µg. The pressure readings were obtained from a pressure transducer (Pfeiffer Vacuum CMR 262). Prior to the measurements, the samples were outgassed for 3 h at 300 °C under high vacuum. The adsorption temperature was maintained by a thermostatic water bath (Grant Instrument GD-120) at 25 ± 0.1 °C. TCE was purified by conducting freezing-vacuum-thawing cycles. To allow direct comparison with the sorption amounts obtained using the standard nitrogen adsorption method, the adsorbed amounts of TCE are reported in liquid volume per gram of adsorbent material. These values, expressed in cm3/g, were obtained by converting the adsorbed amounts in mmol/g into the respective millimolar volumes at varying adsorption temperatures.
ZSM-5 was investigated as a catalyst in the vapor-phase oxidation of TCE without any modification. Catalytic oxidation was carried out between 200 and 650 °C at atmospheric pressure, in a dry air flow. A typical vapor-phase down-flow reactor set-up was used. The outlet was connected to a gas chromatograph for periodic analysis of the gaseous products. The internal diameter of the reactor was 1.2 cm and the dimension of the monolithic zeolite was 1.8 cm × 1 cm. The activity of ZSM-5 was compared with that of commercial ZSM-5 (Si/Al = 25) from Zeolyst International.
The powder XRD pattern of ZSM-5 monolith displayed peaks that were consistent with the simulated data (obtained from the International Zeolite Association website) (Fig. 1). This showed that the crystalline structure of the ZSM-5 zeolite was obtained. SEM image in Fig. 2 shows that the open-cell structure of the polyurethane foam was maintained in the ZSM-5 zeolite monolith. The foam acted as a template, providing sites for the seeding of the crystals, onto which nucleation preferentially occurred than in the bulk solution. As previously shown, the foam decomposed during zeolite synthesis [18, 19]. The open-cell content of the ZSM-5 monolith was similar to that of the PUF (i.e., 95%-96%), as measured by gas pycnometry (the calculations are provided in the Experimental section).
N2 adsorption-desorption isotherm of ZSM-5 (Fig. 3) featured a mixed type I and type IV, according to the IUPAC classification [8]. The material was primarily microporous, as expected for a ZSM-5 zeolite. A microporous volume of 0.137 cm3/g was obtained, as quantified from the t-plot method [8]. The specific surface area (ABET) of the material was 319 m2/g. The ZSM-5 monolith also featured mesoporosity, as indicated by the hysteresis loop in Fig. 3. A mesoporous volume of 0.044 cm3/g was determined from the difference between the total porous volume (0.181 cm3/g) and microporous volume. The mesopore size distribution of the monolith is shown in the inset of Fig. 3. As observed, the monolithic material has mesopores with widths between 5 and 25 nm, with a maximum pore size distribution at 10 nm.
The TCE adsorption isotherm of the monolith measured at 25 °C (Fig. 4) features a very steep adsorption at low relative pressures, indicating that adsorption predominantly occurs in the micropores of the zeolite. The N2 adsorption data (at −196 °C) also displayed the same profile, whereby the amount of the adsorbate is expressed in liquid volume (cm3/g). It showed that TCE and N2 adsorption overlapped mostly, owing to the presence of microporosity that greatly determined the accessibility of the small N2 molecules and TCE molecules to the pores.
The conversion of TCE over the monolithic catalyst as a function of the temperature is shown in Fig. 5. The activity was compared with that of commercial ZSM-5; similar results to those previously reported in the literature were obtained [20]. However, the temperatures at which 50% and 90% conversion levels (T50 and T90) were achieved were lower by 45 and 50 °C, respectively, when compared with the conversion temperatures of the commercial powder catalyst. The reduction in the conversion temperature can be attributed to the improved diffusion of TCE to the active sites when compared with that in a powder catalyst. The monolithic zeolite withstood a flow of 500 mL/min and was stable throughout the reaction. No significant pressure drops within the reactor over the monolithic ZSM-5 were noted. Thus, monolithic zeolites can improve the catalytic activity, and the advantages associated with the structure can be successfully exploited in various organic transformations such as the decomposition of noxious volatile organic compounds.
Monolithic ZSM-5 was synthesized by coupling a traditional pore structure-directing template TPAOH with a structure- directing template polyurethane foam under hydrothermal processes. Monolithic ZSM-5 was crystalline and featured a hierarchically porous structure comprising micro-, meso-, and macropores. The presence of meso- and macropores facilitated the diffusion of reactants to the micropores and also reduced the extent of pressure drop associated with the use of conventional powder and pellet catalysts. The high activity of the synthesized material towards the removal of TCE via adsorption and catalytic oxidation was sustained in both processes. Thus, the prepared monolithic zeolite displays high catalytic activity and the advantages associated with the structure can be successfully exploited in various organic transformations such as the treatment of noxious volatile organic compounds.
The authors are grateful to Prof. Juan R. González Velasco and Dr. Manuel Romero-Saéz for their support. Divakar Duraiswami acknowledges FCT for the post-doc grant SFRH/BPD/ 72721/2010 and Ana C. Fernandes for the grant SFRH/BD/ 72058/2010
2014, Vol. 35 Issue (9): 1492-1496
PDF (1492KB)
2014, Vol. 35 Issue (9): 1492-1496
PDF (1492KB)