- Email: [email protected]
Zeolite nanocomposites
Microporous and Mesoporous Materials 120 (2009) 351–358
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Design and synthesis of 3D ordered macroporous ZrO2/Zeolite nanocomposites Zhiyong Wang, Mohammed A. Al-Daous, Elizabeth R. Kiesel, Fan Li, Andreas Stein * Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 24 November 2008 Accepted 25 November 2008 Available online 6 December 2008 Keywords: Zeolite RuNaY nanocrystals Sulfated zirconia Tungstated zirconia Colloidal crystal templating Nanocomposite
a b s t r a c t Zeolite NaY nanocrystals (20 50 nm) were hydrothermally grown on polyelectrolyte-coated threedimensionally ordered macroporous (3DOM) activated zirconia. The zeolite nanocrystallites uniformly coated the macropore walls of 3DOM sulfated zirconia, but sulfate groups were lost during the hydrothermal reaction. 3DOM tungstated zirconia was found to be more stable in the strong basic environment of the zeolite precursor solution. Further, ion-exchange of zeolite NaY with ruthenium followed by hydrogen-reduction formed a composite of RuNaY and 3DOM activated zirconia. The presence of ruthenium was confirmed by X-ray photoelectron spectroscopy and the external shape of zeolite nanoparticles was maintained after ion-exchange, but the zeolite lattice fringes could not be observed by transmission electron microscopy after incorporation of reduced ruthenium. Although the catalytic activity of the materials has not yet been tested, these hierarchical nano- and microstructures bring multiple catalyst components (modified zeolite Y, a typical Fischer–Tropsch catalyst, and activated zirconia, an isomerization catalyst) together in intimate contact for potential multi-reaction processes. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are widely used as catalysts in the refining industry. For example, zeolite Y particles are good supports for Fischer–Tropsch synthesis (FTS) catalysts which are able to produce clean liquid fuels from synthesis gas (CO and H2) [1–3]. The shape selectivity of zeolites also maximizes production of gasoline-range hydrocarbons, overcoming the limitations of Anderson–Schulz–Flory polymerization kinetics [4,5]. However, zeolites are usually used as relatively large crystallites (with typical dimensions of several micrometers and above), for which diffusion of guest molecules can be limited to the outer shell, especially in fast-rate processes. Modern synthetic chemistry provides methods for preparing nanometer-sized zeolites [6,7], which can support FTS catalysts such as cobalt nanoparticles [8–11]. Nevertheless, such nanoparticle catalysts are in colloidal form and are therefore difficult to recycle after use. Moreover, these FTS catalysts produce mostly straight-chain hydrocarbons which yield low octane numbers [12]. Therefore, manipulation of product selectivity becomes an important objective for research in FTS processes. One approach is to combine the FTS catalyst with another component, which can transform FTS products to the desired compounds through a different reaction mechanism. Hydro-isomerization is an interesting process that can convert straight-chain alkanes to branched-chain alkanes which yield higher octane numbers.
* Corresponding author. Tel.: +1 612 624 1802; fax: +1 612 626 7541. E-mail address: [email protected] (A. Stein). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.11.024
Sulfated zirconia (SZ) was recently found to be highly active toward isomerization of linear alkanes, due to its inherent super-acid sites [13–20]. Conventional preparations of this catalyst lead to microporous materials with high catalytic activity for small molecules in the vapor phase, but these materials are less suited for liquid-phase reactions involving large molecules, such as long-chain alkanes [21,22]. In order to decrease production cost to meet industrial needs as well as increase surface areas and pore volumes, SZ particles with nanometer dimensions were synthesized and dispersed onto mesoporous substrates, such as silica and alumina [17,18,23–27]. Mesoporous SZ materials were also prepared directly by surfactant templating routes using appropriate zirconia precursors [28–33]. However, unmodified SZ suffers from low selectivity, rapid deactivation and sulfur loss during high-temperature reactions [34,35]. Tungstated zirconia (WZ) was reported to be more stable than SZ at high temperatures as well as in a reducing atmosphere [35,36], although its catalytic activity toward alkane isomerization is lower than SZ. Modification of WZ with promoters such as Pt, Fe, Al, Ga and Mn was found effective to improve its catalytic performance [37–41]. To fabricate a hybrid catalyst containing both FTS and isomerization functions, as well as producing hydrocarbons in the motor fuel range, many promising composites such as HY/Fe–Mn [42], promoted silicates/Fe–Mn–Ru [43], HY/Co [5], H–ZSM-5/Co [44– 46], H–ZSM-5-coated Co/SiO2 [47], H–ZSM-5/Fe [48,49], Pd–zeolite-b/Co–SiO2 [50] and Mo–Al2O3/Co [51] were designed. Addition of small amounts of platinum on SZ or simply mixing it with zeolite NaY particles that were partially ion exchanged with ruthenium (RuNaY, an FTS catalyst) improved the overall stability of
352
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
SZ toward deactivation while at the same time producing a hybrid catalyst for both isomerization and FTS [52]. However, this macroscopic blending cannot guarantee a homogeneous mixing of the two catalytic components, which may limit the efficiency of both catalysts due to diffusion of reagents into the bulk catalyst and reaction intermediates from one catalyst to the other. Mesoporous SZ catalysts facilitate diffusion of reactant molecules, but their narrow pores leave insufficient space for the loading of an additional FTS catalyst. Therefore, it is desirable to design a SZ support with large pores that can accommodate zeolite nanocatalysts as well as maintain a more catalytically active (toward n-alkane isomerization) tetragonal phase. Recently a catalyst with interconnected, three-dimensionally ordered macropores surrounded by tetragonal-phase SZ was synthesized by our group and its catalytic performance for n-butane isomerization was evaluated [53]. The central theme of this research involves the synthesis of a hybrid material with a biporous structure as a promising catalyst for FTS and isomerization. The material contains both nanometer pores for the small gas molecules involved in FTS and larger (submicrometer) pores for the accommodation of FTS catalyst as well as efficient transport of the bulkier hydrocarbon products. The catalyst is composed of two solid components mixed evenly on the nano-scale: (1) ruthenium-loaded microporous zeolite nanocrystals (RuNaY); (2) a macroporous sulfated zirconia (SZ) or platinumand iron-promoted support doped with tungsten oxide (WZ). In principle, these two components are intended to function as catalysts for FTS and isomerization, respectively. Several benefits were anticipated for this nanoporous system: (1) a high surface area originating from its hierarchical nanostructure; (2) a three-dimensionally interconnected porous network facilitating mass transport of reactant molecules (synthesis gas); (3) intimate contact between the zeolite and zirconia for improved catalyst stability; (4) the ability to control contact time between feed gas and catalyst surface so that catalyst efficiency (reactant conversion) may be optimized. With respect to the last point, in bulk mixtures a reduction in initial isobutene content was observed compared to separate catalyst layers and attributed to shorter average residence times of the primary FTS products on SZ [52]; the contact times may conceivably be decreased in this very open hybrid system [53]. To synthesize this hybrid catalyst, a three-dimensionally ordered macroporous (3DOM) zirconia network with either sulfate or promoted tungstate sites was synthesized by colloidal crystal templating. Multi-layer coatings of positively and negatively charged polyelectrolytes on the macropore wall surface facilitated further coating with negatively charged zeolite nanoparticles in a hydrothermal reaction. Ruthenium was then introduced into the zeolite Y nanoparticles by ion-exchange and reduction reactions to produce the final 3DOM zirconia–zeolite nanocomposite.
2. Experimental
were purchased from Mallinckrodt; ammonium metatungstate hydrate (99.9+%) and hexaammineruthenium(III) chloride (99%) were obtained from Strem Chemicals, Inc. All chemicals were used as received. PMMA colloidal crystals were prepared as described elsewhere [54]. Deionized (DI) water used in all synthesis was purified to a resistivity greater than 18 MX cm 1. 2.2. Synthesis of 3DOM promoted ZrO2/Zeolite nanocomposites
2.2.1. 3DOM sulfated zirconia (SZ) [53] 9.1 g concentrated sulfuric acid was added to 30 g of 35 wt% zirconyl nitrate (in dilute nitric acid) and stirred vigorously to form a homogeneous solution. Methanol was added to adjust the total volume of this solution to 30 ml. Next, 20 g of PMMA colloidal crystal pieces were soaked in this precursor solution for several minutes while blending by hand with a spatula until the PMMA was completely infiltrated. Excess liquid was removed by vacuum suction and the composite was further dried at room temperature (RT) for 24 h. The composite was calcined in a tube furnace with an airflow of 12 ml min 1. Typically the temperature was increased from RT with a rate of 2 °C min 1, held at 325 °C for 3 h, then ramped to 600 °C with a rate of 2 °C min 1, held for 2 h, and cooled down to RT naturally. This sample was denoted as 3DOM SZ. 2.2.2. 3DOM Pt and Fe-promoted tungstated zirconia (WZ) A precursor solution containing zirconium acetate, ammonium metatungstate and promoters such as FeSO4 and H2PtCl6 was first prepared according to previously developed methods [55]. In a typical synthesis, 2.55 g ammonium metatungstate powder was dissolved in 35.40 g zirconium acetate solution under stirring. 0.50 g FeSO4 7H2O was then added to this solution and the mixture was stirred to form a transparent orange solution. The final precursor solution was obtained by adding 10 ml of methanol solution containing 0.007 g H2PtCl6 dropwise under stirring. After infiltration of 10 g of this precursor solution into 10 g of the PMMA colloidal crystal template and drying, the composite was calcined in a static air atmosphere. The temperature was increased from RT to 325 °C with a rate of 2 °C min 1, held at 325 °C for 3 h, then ramped to 675 °C with a rate of 2 °C min 1, held for 2 h, and cooled down to RT naturally. This sample was denoted 3DOM WZ. 2.2.3. Polyelectrolyte coatings on 3DOM SZ and WZ 3DOM zirconia catalysts with a positively charged outer layer were prepared by depositing six layers of cationic PDDA and anionic PSS polyelectrolytes (each with a concentration of 4 mg ml 1 in water containing 0.1 M NaCl) on the surface of the 3DOM support in the order PSS/PDDA/PSS/PDDA/PSS/PDDA [56,57]. All of these adsorption steps were performed with an approximate volume ratio of 50:1 (polyelectrolyte solution to SZ or WZ powder) at RT for 20 min, and after each step the sample was washed and redispersed in DI water. This sample was denoted 3DOM SZ/P or 3DOM WZ/P.
2.1. Materials The chemicals used in this study were obtained from the following sources: zirconyl nitrate (35 wt% in dilute HNO3), zirconium acetate (solution in dilute acetic acid, containing 15–16% Zr), hydrogen hexachloroplatinate hydrate (H2PtCl6xH2O, 99.9%), poly(diallyl dimethylammonium chloride) (PDDA; 20 wt% in water, Mw 200,000–350,000), poly(4-styrenesulfonate sodium) salt (PSS; powder, Mw 70,000), aluminum isopropoxide (98+%), tetramethylammonium hydroxide (TMAOH; 25 wt% aqueous solution) and tetraethylorthosilicate (TEOS; 98%) were all purchased from Aldrich; sulfuric acid (96%), methanol, iron(II) sulfate (FeSO4 7H2O), sodium chloride and sodium hydroxide (pellets, 99%)
2.2.4. 3DOM SZ/NaY and 3DOM WZ/NaY nanocomposites A clear zeolite NaY precursor solution was prepared using a modification of a published synthesis [58]. Amounts of 0.028 g of 50 wt% NaOH solution, 8.8 g DI water, 1.0 g aluminum isopropoxide and 4.5 g of 25 wt% TMAOH were mixed and sonicated to promote dissolution. After vigorous stirring at RT for 1 h, 2.14 g TEOS was added and the mixture was stirred overnight. The molar ratio of components in the zeolite precursor solution was 0.07 Na: 2.5 TMAOH: 1.0 Al: 2.1 Si: 138 H2O: 3.0 iPrOH: 8.4 EtOH, the last two being hydrolysis products of the alkoxide precursors. This precursor solution was filtered through a syringe filter and added to the 3DOM SZ/P or 3DOM WZ/P powder in a TeflonÒ-lined, 23 ml
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
353
Fig. 1. SEM images of: (A) 3DOM SZ (B) 3DOM SZ/P (C) 3DOM SZ/NaY (D) 3DOM SZ/RuNaY. (E) TEM image of 3DOM SZ/NaY. (F) A high-resolution TEM image of 3DOM SZ/NaY showing ordered zeolite lattice fringes in some of the nanoparticles before Ru ion-exchange.
autoclave (Parr Instrument Company). The mixture was aged for 6 h at RT before being heated for 4 days at 100 °C. After the reaction, the sample was washed thoroughly with DI water and dried at RT for 24 h in air. The product was calcined in a quartz tube furnace at 550 °C with a heating rate of 1 °C min 1 for 10 h in static air. The calcined samples were denoted 3DOM SZ/NaY or 3DOM WZ/NaY. 2.3. Ion-exchange Ru was introduced into the supported zeolite NaY nanoparticles by ion-exchange [52]. 10 ml of an aqueous solution of 0.02 M Ru(NH3)6Cl3 was added dropwise to 0.05 g 3DOM SZ/NaY or 3DOM WZ/NaY powder. The mixture was then transferred to a glass flask with a black cover and gently rotated on a motor under ambient conditions for 3 days. After decanting the excess solution,
the sample was washed extensively until no Cl ion was detected in water by AgNO3(aq). The powder was then dried at 40 °C overnight. The [Ru(NH3)6]3+ cations in the 3DOM SZ/NaY composite were decomposed in a quartz tube under He flow. The temperature was ramped from RT to 420 °C with a rate of 2 °C min 1 and held constant for 4 h. Next, hydrogen gas was used to reduce Ru ions at 420 °C for 2 h. After the reaction, the sample was cooled down to RT under hydrogen flow. The final samples were denoted 3DOM SZ/RuNaY or 3DOM WZ/RuNaY. 2.4. Characterization To assure analyses that were representative of the bulk features (rather than external surface features), all samples were ground to a fine powder before being subjected to the following analyses. Powder X-ray diffraction (XRD) analysis was performed using a
354
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
Fig. 2. Powder X-ray diffraction patterns for: (A) 3DOM SZ (B) 3DOM SZ/P (C) 3DOM SZ/NaY (D) 3DOM SZ/RuNaY after H2 reduction. Crystalline phases for some peaks are marked with symbols ‘‘T” and ‘‘M” to indicate tetragonal and monoclinic phases for zirconia, respectively; arrows indicate zeolite NaY peaks.
Bruker AXS micro-diffractometer with a Cu X-ray source and a HiStar 2-D area detector. All diffraction patterns were acquired at a generator voltage of 45 kV, and a current of 40 mA. Crystallite sizes were calculated from Scherrer’s equation. Scanning electron microscopy (SEM) images were obtained with a JEOL 6500 scanning electron microscope equipped with an energy-dispersive Xray spectroscopy (EDS) accessory at an accelerating voltage of 15 kV. Samples were mounted on an aluminum stub with conductive carbon sticky tape and a thin (ca. 5 nm) coating of platinum was deposited on the sample prior to analysis. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai T-12 electron microscope that was operated at 120 kV. Samples for TEM measurements were drop-dried from ethanol onto a holey carbon film supported on a copper grid. Nitrogen-sorption measurements were performed on a Micromeritics ASAP 2000 gas sorptometer. Samples were degassed to 0.003 mmHg for 48 h at 120 °C. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and pore volumes were taken at the P/P0 = 0.985 single point. 3. Results and discussion 3.1. 3DOM SZ/RuNaY nanocomposites As-synthesized 3DOM SZ possessed a uniform macropore structure with fcc packing of interconnected voids and smooth macropore surfaces (Fig. 1A). Diffraction of light by this periodic structure resulted in a highly opalescent appearance of 3DOM SZ. The macropore walls in as-synthesized 3DOM SZ are composed of a pure tetragonal zirconia phase (Fig. 2A), which is known to exhibit high catalytic activity toward n-alkane isomerization [59,60]. The crystallite size of SZ was estimated by the Scherrer equation to be ca. 8.7 nm, which was only slightly higher than in a previous re-
Fig. 3. Nitrogen-sorption isotherms for 3DOM SZ, SZ/P, SZ/NaY before and after calcination.
port for 3DOM SZ [53] but significantly smaller than for nonsulfated 3DOM ZrO2 [61], indicating that sulfate groups effectively suppressed the grain growth of ZrO2 during calcination. After multi-layer polyelectrolyte coating, the 3DOM structure was maintained and the macropore wall surfaces became slightly rougher (Fig. 1B). Addition of the polyelectrolyte coating did not change the crystallographic phase but introduced background scattering in the region from 10 to 35° 2h of the powder XRD pattern (Fig. 2B). After the hydrothermal reaction with the zeolite precursor solution, a uniform nanoparticle coating was observed on the 3DOM SZ surface, composed of particles ca. 20 nm in average diameter (Fig. 1C and E). More interestingly, the zeolite lattice fringes could be directly observed at higher resolution (Fig. 1F). Some low-intensity peaks from diffractions of the zeolite lattice were observed in the XRD powder pattern (Fig. 2C). The low intensity is due to the small size of the NaY crystallites, line broadening effects and the low zeolite concentration in the sample. Ion-exchange with Ru(NH3)6Cl3 did not change the structure of 3DOM SZ/NaY and no significant particle growth of zeolite NaY was observed (Fig. 1D). Zeolite peaks could not be clearly observed for the H2-reduced sample (3DOM SZ/RuNaY; Fig. 2D), although studies on bulk NaY particles showed that the XRD peaks are retained even after the formation of Ru nanoclusters within zeolite frameworks [62,63]. Moreover, contributions from zirconia in the monoclinic phase appeared in the powder pattern, a phase which is less catalytically active (Fig. 2D). Nitrogen-sorption isotherms for 3DOM SZ, 3DOM SZ/P and 3DOM SZ/NaY all have similar shapes and they can be classified as type II with H3 hysteresis loops (Fig. 3), isotherms which correspond to the macroporous structure with interparticle pores formed by aggregates of the zeolite nanoparticles [64]. The zeolite loading on 3DOM SZ was ca. 10 wt% as estimated by measuring the weight difference between pristine 3DOM SZ and calcined 3DOM
Table 1 Nitrogen-sorption results: BET surface area, pore volume and micropore information. Sample
SBET (m2 g
SZ SZ/P SZ/Y SZ/Y (cal)
34 22 79 104
1
)
Total meso- and micropore volume (cm3/g)
Micropore volume (%)
0.04 0.039 0.11 0.13
4.8 2.5 14.5 12.3
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
355
Fig. 4. EDS spectra: (left) samples at different reaction stages from 3DOM SZ to calcined 3DOM SZ/NaY; (right) 3DOM SZ/RuNaY sample after hydrogen-reduction.
SZ/NaY. The increase in specific micropore volume of the 3DOM SZ/ NaY composite after incorporation of zeolite (Table 1) also supports a weight fraction of ca. 10% in the composite. Therefore, it is believed that interparticle mesopores formed by zeolite NaY aggregates contribute the most to the total pore volume determined by nitrogen-sorption analysis. As-synthesized 3DOM SZ has a BET surface area of 34 m2 g 1. Multi-layer coating with polyelectrolytes decreased the surface area due to blocking of surface micropores. After hydrothermal growth of zeolite nanoparticles, the surface area increased again to 79 m2 g 1. These changes are consistent with our previously reported results for 3DOM carbon/ TiO2 nanocomposites, where nanoparticle coatings contributed to the textural mesoporosity of the samples [57]. After calcination of the NaY-coated sample at 550 °C, the surface area increased further to 104 m2 g 1 due to the introduction of micropores inside the zeolite framework after removal of the structure-directing agent, TMAOH molecules. Moreover, from Table 1 we can see that the proportion of micropores increased after zeolite nanoparticle coating. In as-synthesized 3DOM SZ, Zr, O, and S were detected by EDS (Fig. 4). As expected, after multi-layer polyelectrolyte coatings, the inorganic composition did not change significantly. After hydrothermal reaction at 100 °C for 4 days, Na, Al and Si were detected, elements which were introduced by the zeolite coating; the O content also increased. However, the sulfur peak disappeared, which indicates the removal of sulfate groups from the 3DOM SZ surface. A likely reason for loss of sulfate is that during the hydrothermal reaction, the strong basic zeolite precursor solution neutralized surface acidic sites created by SO24 . Ru was detected in the 3DOM SZ/NaY sample after ion-exchange with Ru(NH3)6Cl3(aq) followed by hydrogen-reduction.
those grown on 3DOM SZ (Fig. 1C). The NaY coating on 3DOM WZ was not as uniform as that on 3DOM SZ (Fig. 1C), probably due to a weaker acidity of 3DOM WZ than 3DOM SZ which would result in less stable polyelectrolyte coatings. Moreover, zeolite particles were not as monodispere as those in 3DOM SZ/NaY. Diffraction peaks from NaY remained visible in the XRD pattern after ion-exchange with Ru(NH3)6Cl3(aq) solution (Fig. 5C). To acquire information about the elemental composition of 3DOM WZ/RuNaY, XPS was utilized instead of EDS (which had been adopted for 3DOM SZ/RuNaY), since the tungsten peaks in EDS spectra are very close to those of zirconium and silicon (from NaY). Besides Zr and O, other elements such as W, Fe (from surface groups) and Si, Al (from zeolite NaY) were detected (Fig. 7, left). This indicated that the catalytically active surface groups were not removed from 3DOM WZ surface after a series of reactions, unlike the loss of sulfate groups on 3DOM SZ after the hydrothermal reaction (Fig. 4, left). The presence of ruthenium was confirmed by high-resolution scans (Fig. 7, right). The carbon 1 s peak at 285.0 eV was used as an internal standard for the peaks of other elements.
3.2. 3DOM WZ/RuNaY nanocomposites Similar to 3DOM SZ, the as-synthesized 3DOM WZ contained zirconia in the tetragonal zirconia phase (Fig. 5A) and also exhibited smooth macropore wall surfaces (Fig. 6A). The average crystallite size was 9.6 nm, estimated from XRD line broadening, a value that is slightly higher than for 3DOM SZ. After the hydrothermal reaction to deposit NaY, this phase was maintained and a series of zeolite NaY peaks appeared (Fig. 5B). The macropore wall surface of 3DOM WZ was coated with crystalline particles which ranged from 25 to 50 nm in diameters (Fig. 6B and C), i.e., larger than
Fig. 5. XRD patterns of (A) as-synthesized 3DOM WZ, (B) 3DOM WZ/NaY, (C) 3DOM WZ/NaY after ion-exchange but before H2 reduction, and (D) 3DOM WZ/NaY after ion-exchange and H2 reduction (3DOM WZ/RuNaY). Here ‘‘t” and ‘‘Y” stand for tetragonal zirconia and zeolite NaY, respectively.
356
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
Another signal was detected at 280.1 eV, which could be assigned to the 3d5/2 peak of metallic Ru nanoclusters [65–67]. In addition, this value also suggested that the ruthenium in the 3DOM WZ/RuNaY sample is Ru(0), which is required for FTS. Zeolite peaks disappeared from 3DOM WZ/NaY after H2 reduction (Fig. 5D). In addition, it was difficult to observe zeolite lattice
fringes in TEM images (Fig. 6D). This is most likely the result of deterioration of the NaY framework in these small nanoparticles during reduction and Ru cluster growth. In contrast, in bulk NaY [63] or even in NaY nanoparticle samples with a significant fraction of particles 100 nm in diameter or larger (see Supplementary material), XRD peaks were preserved after Ru cluster formation.
Fig. 6. SEM images of (A) 3DOM WZ and (B) 3DOM WZ/NaY; TEM images of (C) 3DOM WZ/NaY and (D) 3DOM WZ/RuNaY after hydrogen-reduction.
Fig. 7. XPS data: (left) survey scan and (right) high-resolution scan of 3DOM WZ and 3DOM WZ/RuNaY after H2 reduction.
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
It is notable that the external shapes of zeolite nanoparticles in 3DOM WZ/RuNaY were maintained, and aggregates of Ru nanoparticles were neither visible inside nor on the external surfaces of NaY nanocrystals. The uniform contrast might indicate a relatively uniform distribution of ruthenium in clusters that were smaller than NaY supercages so that they were not visible in our TEM images [69]. Larger Ru nanoparticles (ca. 2 nm) are usually observable by TEM at appropriate magnifications when the zeolite framework remains intact [62,63,65,68,70]. Most ion-exchange experiments have been performed on zeolite particles with sizes around or larger than 100 nm [70–75], but reports of ion-exchange on even smaller NaY nanocrystals are quite rare [76]. In this study, Ru was loaded onto NaY nanocrystals (20 50 nm) supported on 3DOM zirconia via ion-exchange, and the zeolite lattice was preserved. After hydrogen-reduction, metallic Ru was detected by XPS, although the zeolite peaks disappeared from XRD and their zeolite fringes were no longer observed by TEM. The reduction might be too harsh so that the rapid growth of Ru nanoclusters or the release of gaseous decomposition products destroyed the frameworks of these relatively small zeolite particles. In the future, milder reduction processes may be able to prevent framework degradation.
4. Conclusions In this paper, we reported the design, synthesis and characterization of 3DOM ZrO2/NaY nanocomposites targeting a heterogeneous catalyst for clean fuel production via Fischer–Tropsch synthesis (FTS) and isomerization. The introduction of sulfate and tungstate groups into zirconia efficiently maintained 3DOM frameworks by suppressing grain growth during the high-temperature crystallization and stabilizing ZrO2 as the tetragonal phase which is also catalytically active toward n-alkane isomerization. Uniform coatings of NaY nanocrystals on 3DOM zirconia were achieved, especially on 3DOM SZ. The highly basic environment of the zeolite synthesis resulted in loss of sulfate groups on 3DOM SZ. In contrast, tungstate groups on 3DOM WZ survived the hydrothermal processing conditions. Ru was introduced into zeolite NaY via ion-exchange of the nanocomposites with a Ru complex solution. To the best of our knowledge, this is the first report of supported RuNaY nanoparticles smaller than 100 nm via ion-exchange and reduction. Large Ru nanoparticles were not observed after reduction, which might indicate an even distribution of Ru nanoclusters smaller than zeolite supercages through the NaY particles. 3DOM ZrO2 is a good support for zeolite nanocrystals, and these nanocomposites designs have the potential for shortening the diffusion pathlengths of reactants between FTS and isomerization catalysts. Improved hydrocarbon yield and selectivity are also expected for these hierarchical composite nano- and microstructures that bring multiple catalyst components together in intimate contact. Catalytic activities for FTS and isomerization processes can now be evaluated in future research.
Acknowledgments This research was supported by the Petroleum Research Foundation administered by the American Chemical Society (ACS-PRF Grant Number 42751-AC10) and the Office of Naval Research (Grant Number N00014-07-1-00608). Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from the NSF through the NNIN and MRSEC programs. Z.W. thanks the University of Minnesota Graduate School for a Doctoral Dissertation Fellowship.
357
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2008.11.024. References [1] [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] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63]
F. Fischer, H. Tropsch, Brennst. Chem. 4 (1923) 276. L. Guczi, I. Kiricsi, Appl. Catal. A 186 (1999) 375. M. Stoecker, Micropor. Mesopor. Mat. 82 (2005) 257. V.U.S. Rao, R.J. Gormley, Catal. Today 6 (1990) 207. S. Bessell, Appl. Catal. A 96 (1993) 253. S.C. Larsen, J. Phys. Chem. C 111 (2007) 18464. L. Tosheva, V.P. Valtchev, Chem. Mater. 17 (2005) 2494. A. Martinez, G. Prieto, J. Catal. 245 (2007) 470. Y. Wang, H. Wu, Q. Zhang, Q. Tang, Micropor. Mesopor. Mat. 86 (2005) 38. Q. Tang, Q. Zhang, P. Wang, Y. Wang, H. Wan, Chem. Mater. 16 (2004) 1967. Q. Tang, Y. Wang, Q. Zhang, H. Wan, Catal. Commun. 4 (2003) 253. H. Pichler, H. Schulz, D. Kuehne, Brennst. Chem. 49 (1968) 344. W. Stichert, F. Schuth, J. Catal. 174 (1998) 242. G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mat. 33 (1999) 1. X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. T. Yamaguchi, Appl. Catal. A 222 (2001) 237. R. Akkari, A. Ghorbel, N. Essayem, F. Figueras, Appl. Catal. A 328 (2007) 43. J.-H. Wang, C.-Y. Mou, Micropor. Mesopor. Mat. 110 (2008) 260. C.R. Vera, C.L. Pieck, K. Shimizu, J.M. Parera, Appl. Catal. A 230 (2002) 137. X. Li, K. Nagaoka, L.J. Simon, R. Olindo, J.A. Lercher, J. Catal. 232 (2005) 456. P.D.L. Mercera, J.G. van Ornmen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. Y.-Y. Huang, T.J. McCarthy, W.M.H. Sachtler, Appl. Catal. A. 148 (1996) 135. R. Akkari, A. Ghorbel, N. Essayem, F. Figueras, Micropor. Mesopor. Mat. 111 (2008) 62. X.-R. Chen, Y.-H. Ju, C.-Y. Mou, J. Phys. Chem. C 111 (2007) 18731. F. Li, F. Yu, Y. Li, R. Li, K. Xie, Micropor. Mesopor. Mat. 101 (2007) 250. M.A. Ecormier, A.F. Lee, K. Wilson, Micropor. Mesopor. Mat. 80 (2005) 301. Y. Sun, L. Zhu, H. Lu, R. Wang, S. Lin, D. Jiang, F.-S. Xiao, Appl. Catal. A 237 (2002) 21. U. Ciesla, M. Froeba, G. Stucky, F. Schueth, Chem. Mater. 11 (1999) 227. S.-Y. Chen, L.-Y. Jang, S. Cheng, J. Phys. Chem. B 110 (2006) 11761. M. Signoretto, A. Breda, F. Somma, F. Pinna, G. Cruciani, Micropor. Mesopor. Mat. 91 (2006) 23. E. Zhao, S.E. Hardcastle, G. Pacheco, A. Garcia, A.L. Blumenfeld, J.J. Fripiat, Micropor. Mesopor. Mat. 31 (1999) 9. D.M. Antonelli, Adv. Mater. 11 (1999) 487. Y. Sun, L. Yuan, W. Wang, C.-L. Chen, F.-S. Xiao, Catal. Lett. 87 (2003) 57. J.C. Yori, J.C. Luy, J.M. Parera, Catal. Today 5 (1989) 493. F.T.T. Ng, N. Horvat, Appl. Catal. A 123 (1995) L195. G. Larsen, E. Lotero, R.D. Parra, L.M. Petkovic, H.S. Silva, S. Raghavan, Appl. Catal. A 130 (1995) 213. G. Larsen, E. Lotero, S. Raghavan, R.D. Parra, C.A. Querini, Appl. Catal. A 139 (1996) 201. M. Scheithauer, R.E. Jentoft, B.C. Gates, H. Knozinger, J. Catal. 191 (2000) 271. S. Kuba, R.K. Grasselli, H. Knozinger, B.C. Gates, Chem. Commun. (2001) 321. X.-R. Chen, C.-L. Chen, N.-P. Xu, S. Han, C.-Y. Mou, Catal. Lett. 85 (2003) 177. S.-T. Wong, T. Li, S. Cheng, J.-F. Lee, C.-Y. Mou, J. Catal. 215 (2003) 45. N.O. Egiebor, W.C. Cooper, B.W. Wojciechowski, Appl. Catal. 55 (1989) 47. T. Inui, T. Kuroda, T. Takeguchi, A. Miyamoto, Appl. Catal. 61 (1990) 219. X. Li, K. Asami, M. Luo, K. Michiki, N. Tsubaki, K. Fujimoto, Catal. Today 84 (2003) 59. Y. Yoneyama, X. San, T. Iwai, N. Tsubaki, Energ. Fuel. 22 (2008) 2873. A. Martinez, S. Valencia, R. Murciano, H.S. Cerqueira, A.F. Costa, E.F.S. -Aguiar, Appl. Catal. A 346 (2008) 117. J. He, Y. Yoneyama, B. Xu, N. Nishiyama, N. Tsubaki, Langmuir 21 (2005) 1699. Y. Yoneyama, J. He, Y. Morii, S. Azuma, N. Tsubaki, Catal. Today 104 (2005) 37. A. Martinez, C. Lopez, Appl. Catal. A 294 (2005) 251. Z.-W. Liu, X. Li, K. Asami, K. Fujimoto, Catal. Commun. 6 (2005) 503. E. van Steen, E.L. Viljoen, J. van de Loosdrecht, M. Claeys, Appl. Catal. A 335 (2008) 56. X. Song, A. Sayari, Appl. Catal. A 110 (1994) 121. M.A. Al-Daous, A. Stein, Chem. Mater. 15 (2003) 2638. R.C. Schroden, M.A. Al-Daous, S. Sokolov, B.J. Melde, J.C. Lytle, A. Stein, M.C. Carbajo, J.T. Fernandez, E.E. Rodriguez, J. Mater. Chem. 12 (2002) 3261. S. Kuba, B.C. Gates, R.K. Grasselli, H. Knozinger, Chem. Commun. (2001) 321. F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. Z. Wang, N.S. Ergang, M.A. Al-Daous, A. Stein, Chem. Mater. 17 (2005) 6805. W. Song, V.H. Grassian, S.C. Larsen, Chem. Commun. (2005) 2951. W. Stichert, F. Schuth, S. Kuba, H. Knozinger, J. Catal. 198 (2001) 277. M. Hino, K. Arata, J. Chem. Soc. Chem. Comm. (1980) 851. B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538. S.M. Gheno, S. Damyanova, B.A. Riguetto, C.M.P. Marques, C.A.P. Leite, J.M.C. Bueno, J. Mol. Catal. A 198 (2003) 263. U.L. Portugal Jr., C.M.P. Marques, E.C.C. Araujo, E.V. Morales, M.V. Giotto, J.M.C. Bueno, Appl. Catal. A 193 (2000) 173.
358
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
[64] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology, Applications, Academic Press, New York, 1999. p. 20. [65] S.-P. Sheu, H.G. Karge, R. Schlogl, J. Catal. 168 (1997) 278. [66] B.L. Gustafson, J.H. Lunsford, J. Catal. 74 (1981) 393. [67] S.K. Das, P.K. Dutta, Micropor. Mesopor. Mat. 22 (1998) 475. [68] T.J. McCarthy, C.M.P. Marques, H. Trevino, W.M.H. Sachtler, Catal. Lett. 43 (1997) 11. [69] S.J. Cho, S.M. Jung, Y.G. Shul, R. Ryoo, J. Phys. Chem. 96 (1992) 9922. [70] L. Guczi, R. Sundararajan, Z. Koppany, Z. Zsoldos, Z. Schay, F. Mizukami, S. Niwa, J. Catal. 167 (1997) 482.
[71] R.A. Rakoczy, Y. Traa, Micropor. Mesopor. Mat. 60 (2003) 69. [72] N.B. Castagnola, P.K. Dutta, J. Phys. Chem. B 102 (1998) 1696. [73] P. Castillo-Villalon, J. Ramirez, M.-J. Peltre, C. Louis, P. Massiani, Phys. Chem. Chem. Phys. 6 (2004) 3739. [74] C. Frontaine, P. Castillo, J. Ramirez, M.J. Peltre, F. Warmont, C. Louis, P. Massiani, Stud. Surf. Sci. Catal. 154 (2004) 1784. [75] O. Hernandez-Ramirez, P.I. Hill, D.J. Doocey, S.M. Holmes, J. Mater. Chem. 17 (2007) 1804. [76] Y.-X. Jiang, D. Si, S.-P. Chen, S.-G. Sun, Electroanal. 18 (2006) 1173.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Design and synthesis of 3D ordered macroporous ZrO2/Zeolite nanocomposites Zhiyong Wang, Mohammed A. Al-Daous, Elizabeth R. Kiesel, Fan Li, Andreas Stein * Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 24 November 2008 Accepted 25 November 2008 Available online 6 December 2008 Keywords: Zeolite RuNaY nanocrystals Sulfated zirconia Tungstated zirconia Colloidal crystal templating Nanocomposite
a b s t r a c t Zeolite NaY nanocrystals (20 50 nm) were hydrothermally grown on polyelectrolyte-coated threedimensionally ordered macroporous (3DOM) activated zirconia. The zeolite nanocrystallites uniformly coated the macropore walls of 3DOM sulfated zirconia, but sulfate groups were lost during the hydrothermal reaction. 3DOM tungstated zirconia was found to be more stable in the strong basic environment of the zeolite precursor solution. Further, ion-exchange of zeolite NaY with ruthenium followed by hydrogen-reduction formed a composite of RuNaY and 3DOM activated zirconia. The presence of ruthenium was confirmed by X-ray photoelectron spectroscopy and the external shape of zeolite nanoparticles was maintained after ion-exchange, but the zeolite lattice fringes could not be observed by transmission electron microscopy after incorporation of reduced ruthenium. Although the catalytic activity of the materials has not yet been tested, these hierarchical nano- and microstructures bring multiple catalyst components (modified zeolite Y, a typical Fischer–Tropsch catalyst, and activated zirconia, an isomerization catalyst) together in intimate contact for potential multi-reaction processes. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are widely used as catalysts in the refining industry. For example, zeolite Y particles are good supports for Fischer–Tropsch synthesis (FTS) catalysts which are able to produce clean liquid fuels from synthesis gas (CO and H2) [1–3]. The shape selectivity of zeolites also maximizes production of gasoline-range hydrocarbons, overcoming the limitations of Anderson–Schulz–Flory polymerization kinetics [4,5]. However, zeolites are usually used as relatively large crystallites (with typical dimensions of several micrometers and above), for which diffusion of guest molecules can be limited to the outer shell, especially in fast-rate processes. Modern synthetic chemistry provides methods for preparing nanometer-sized zeolites [6,7], which can support FTS catalysts such as cobalt nanoparticles [8–11]. Nevertheless, such nanoparticle catalysts are in colloidal form and are therefore difficult to recycle after use. Moreover, these FTS catalysts produce mostly straight-chain hydrocarbons which yield low octane numbers [12]. Therefore, manipulation of product selectivity becomes an important objective for research in FTS processes. One approach is to combine the FTS catalyst with another component, which can transform FTS products to the desired compounds through a different reaction mechanism. Hydro-isomerization is an interesting process that can convert straight-chain alkanes to branched-chain alkanes which yield higher octane numbers.
* Corresponding author. Tel.: +1 612 624 1802; fax: +1 612 626 7541. E-mail address: [email protected] (A. Stein). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.11.024
Sulfated zirconia (SZ) was recently found to be highly active toward isomerization of linear alkanes, due to its inherent super-acid sites [13–20]. Conventional preparations of this catalyst lead to microporous materials with high catalytic activity for small molecules in the vapor phase, but these materials are less suited for liquid-phase reactions involving large molecules, such as long-chain alkanes [21,22]. In order to decrease production cost to meet industrial needs as well as increase surface areas and pore volumes, SZ particles with nanometer dimensions were synthesized and dispersed onto mesoporous substrates, such as silica and alumina [17,18,23–27]. Mesoporous SZ materials were also prepared directly by surfactant templating routes using appropriate zirconia precursors [28–33]. However, unmodified SZ suffers from low selectivity, rapid deactivation and sulfur loss during high-temperature reactions [34,35]. Tungstated zirconia (WZ) was reported to be more stable than SZ at high temperatures as well as in a reducing atmosphere [35,36], although its catalytic activity toward alkane isomerization is lower than SZ. Modification of WZ with promoters such as Pt, Fe, Al, Ga and Mn was found effective to improve its catalytic performance [37–41]. To fabricate a hybrid catalyst containing both FTS and isomerization functions, as well as producing hydrocarbons in the motor fuel range, many promising composites such as HY/Fe–Mn [42], promoted silicates/Fe–Mn–Ru [43], HY/Co [5], H–ZSM-5/Co [44– 46], H–ZSM-5-coated Co/SiO2 [47], H–ZSM-5/Fe [48,49], Pd–zeolite-b/Co–SiO2 [50] and Mo–Al2O3/Co [51] were designed. Addition of small amounts of platinum on SZ or simply mixing it with zeolite NaY particles that were partially ion exchanged with ruthenium (RuNaY, an FTS catalyst) improved the overall stability of
352
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
SZ toward deactivation while at the same time producing a hybrid catalyst for both isomerization and FTS [52]. However, this macroscopic blending cannot guarantee a homogeneous mixing of the two catalytic components, which may limit the efficiency of both catalysts due to diffusion of reagents into the bulk catalyst and reaction intermediates from one catalyst to the other. Mesoporous SZ catalysts facilitate diffusion of reactant molecules, but their narrow pores leave insufficient space for the loading of an additional FTS catalyst. Therefore, it is desirable to design a SZ support with large pores that can accommodate zeolite nanocatalysts as well as maintain a more catalytically active (toward n-alkane isomerization) tetragonal phase. Recently a catalyst with interconnected, three-dimensionally ordered macropores surrounded by tetragonal-phase SZ was synthesized by our group and its catalytic performance for n-butane isomerization was evaluated [53]. The central theme of this research involves the synthesis of a hybrid material with a biporous structure as a promising catalyst for FTS and isomerization. The material contains both nanometer pores for the small gas molecules involved in FTS and larger (submicrometer) pores for the accommodation of FTS catalyst as well as efficient transport of the bulkier hydrocarbon products. The catalyst is composed of two solid components mixed evenly on the nano-scale: (1) ruthenium-loaded microporous zeolite nanocrystals (RuNaY); (2) a macroporous sulfated zirconia (SZ) or platinumand iron-promoted support doped with tungsten oxide (WZ). In principle, these two components are intended to function as catalysts for FTS and isomerization, respectively. Several benefits were anticipated for this nanoporous system: (1) a high surface area originating from its hierarchical nanostructure; (2) a three-dimensionally interconnected porous network facilitating mass transport of reactant molecules (synthesis gas); (3) intimate contact between the zeolite and zirconia for improved catalyst stability; (4) the ability to control contact time between feed gas and catalyst surface so that catalyst efficiency (reactant conversion) may be optimized. With respect to the last point, in bulk mixtures a reduction in initial isobutene content was observed compared to separate catalyst layers and attributed to shorter average residence times of the primary FTS products on SZ [52]; the contact times may conceivably be decreased in this very open hybrid system [53]. To synthesize this hybrid catalyst, a three-dimensionally ordered macroporous (3DOM) zirconia network with either sulfate or promoted tungstate sites was synthesized by colloidal crystal templating. Multi-layer coatings of positively and negatively charged polyelectrolytes on the macropore wall surface facilitated further coating with negatively charged zeolite nanoparticles in a hydrothermal reaction. Ruthenium was then introduced into the zeolite Y nanoparticles by ion-exchange and reduction reactions to produce the final 3DOM zirconia–zeolite nanocomposite.
2. Experimental
were purchased from Mallinckrodt; ammonium metatungstate hydrate (99.9+%) and hexaammineruthenium(III) chloride (99%) were obtained from Strem Chemicals, Inc. All chemicals were used as received. PMMA colloidal crystals were prepared as described elsewhere [54]. Deionized (DI) water used in all synthesis was purified to a resistivity greater than 18 MX cm 1. 2.2. Synthesis of 3DOM promoted ZrO2/Zeolite nanocomposites
2.2.1. 3DOM sulfated zirconia (SZ) [53] 9.1 g concentrated sulfuric acid was added to 30 g of 35 wt% zirconyl nitrate (in dilute nitric acid) and stirred vigorously to form a homogeneous solution. Methanol was added to adjust the total volume of this solution to 30 ml. Next, 20 g of PMMA colloidal crystal pieces were soaked in this precursor solution for several minutes while blending by hand with a spatula until the PMMA was completely infiltrated. Excess liquid was removed by vacuum suction and the composite was further dried at room temperature (RT) for 24 h. The composite was calcined in a tube furnace with an airflow of 12 ml min 1. Typically the temperature was increased from RT with a rate of 2 °C min 1, held at 325 °C for 3 h, then ramped to 600 °C with a rate of 2 °C min 1, held for 2 h, and cooled down to RT naturally. This sample was denoted as 3DOM SZ. 2.2.2. 3DOM Pt and Fe-promoted tungstated zirconia (WZ) A precursor solution containing zirconium acetate, ammonium metatungstate and promoters such as FeSO4 and H2PtCl6 was first prepared according to previously developed methods [55]. In a typical synthesis, 2.55 g ammonium metatungstate powder was dissolved in 35.40 g zirconium acetate solution under stirring. 0.50 g FeSO4 7H2O was then added to this solution and the mixture was stirred to form a transparent orange solution. The final precursor solution was obtained by adding 10 ml of methanol solution containing 0.007 g H2PtCl6 dropwise under stirring. After infiltration of 10 g of this precursor solution into 10 g of the PMMA colloidal crystal template and drying, the composite was calcined in a static air atmosphere. The temperature was increased from RT to 325 °C with a rate of 2 °C min 1, held at 325 °C for 3 h, then ramped to 675 °C with a rate of 2 °C min 1, held for 2 h, and cooled down to RT naturally. This sample was denoted 3DOM WZ. 2.2.3. Polyelectrolyte coatings on 3DOM SZ and WZ 3DOM zirconia catalysts with a positively charged outer layer were prepared by depositing six layers of cationic PDDA and anionic PSS polyelectrolytes (each with a concentration of 4 mg ml 1 in water containing 0.1 M NaCl) on the surface of the 3DOM support in the order PSS/PDDA/PSS/PDDA/PSS/PDDA [56,57]. All of these adsorption steps were performed with an approximate volume ratio of 50:1 (polyelectrolyte solution to SZ or WZ powder) at RT for 20 min, and after each step the sample was washed and redispersed in DI water. This sample was denoted 3DOM SZ/P or 3DOM WZ/P.
2.1. Materials The chemicals used in this study were obtained from the following sources: zirconyl nitrate (35 wt% in dilute HNO3), zirconium acetate (solution in dilute acetic acid, containing 15–16% Zr), hydrogen hexachloroplatinate hydrate (H2PtCl6xH2O, 99.9%), poly(diallyl dimethylammonium chloride) (PDDA; 20 wt% in water, Mw 200,000–350,000), poly(4-styrenesulfonate sodium) salt (PSS; powder, Mw 70,000), aluminum isopropoxide (98+%), tetramethylammonium hydroxide (TMAOH; 25 wt% aqueous solution) and tetraethylorthosilicate (TEOS; 98%) were all purchased from Aldrich; sulfuric acid (96%), methanol, iron(II) sulfate (FeSO4 7H2O), sodium chloride and sodium hydroxide (pellets, 99%)
2.2.4. 3DOM SZ/NaY and 3DOM WZ/NaY nanocomposites A clear zeolite NaY precursor solution was prepared using a modification of a published synthesis [58]. Amounts of 0.028 g of 50 wt% NaOH solution, 8.8 g DI water, 1.0 g aluminum isopropoxide and 4.5 g of 25 wt% TMAOH were mixed and sonicated to promote dissolution. After vigorous stirring at RT for 1 h, 2.14 g TEOS was added and the mixture was stirred overnight. The molar ratio of components in the zeolite precursor solution was 0.07 Na: 2.5 TMAOH: 1.0 Al: 2.1 Si: 138 H2O: 3.0 iPrOH: 8.4 EtOH, the last two being hydrolysis products of the alkoxide precursors. This precursor solution was filtered through a syringe filter and added to the 3DOM SZ/P or 3DOM WZ/P powder in a TeflonÒ-lined, 23 ml
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
353
Fig. 1. SEM images of: (A) 3DOM SZ (B) 3DOM SZ/P (C) 3DOM SZ/NaY (D) 3DOM SZ/RuNaY. (E) TEM image of 3DOM SZ/NaY. (F) A high-resolution TEM image of 3DOM SZ/NaY showing ordered zeolite lattice fringes in some of the nanoparticles before Ru ion-exchange.
autoclave (Parr Instrument Company). The mixture was aged for 6 h at RT before being heated for 4 days at 100 °C. After the reaction, the sample was washed thoroughly with DI water and dried at RT for 24 h in air. The product was calcined in a quartz tube furnace at 550 °C with a heating rate of 1 °C min 1 for 10 h in static air. The calcined samples were denoted 3DOM SZ/NaY or 3DOM WZ/NaY. 2.3. Ion-exchange Ru was introduced into the supported zeolite NaY nanoparticles by ion-exchange [52]. 10 ml of an aqueous solution of 0.02 M Ru(NH3)6Cl3 was added dropwise to 0.05 g 3DOM SZ/NaY or 3DOM WZ/NaY powder. The mixture was then transferred to a glass flask with a black cover and gently rotated on a motor under ambient conditions for 3 days. After decanting the excess solution,
the sample was washed extensively until no Cl ion was detected in water by AgNO3(aq). The powder was then dried at 40 °C overnight. The [Ru(NH3)6]3+ cations in the 3DOM SZ/NaY composite were decomposed in a quartz tube under He flow. The temperature was ramped from RT to 420 °C with a rate of 2 °C min 1 and held constant for 4 h. Next, hydrogen gas was used to reduce Ru ions at 420 °C for 2 h. After the reaction, the sample was cooled down to RT under hydrogen flow. The final samples were denoted 3DOM SZ/RuNaY or 3DOM WZ/RuNaY. 2.4. Characterization To assure analyses that were representative of the bulk features (rather than external surface features), all samples were ground to a fine powder before being subjected to the following analyses. Powder X-ray diffraction (XRD) analysis was performed using a
354
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
Fig. 2. Powder X-ray diffraction patterns for: (A) 3DOM SZ (B) 3DOM SZ/P (C) 3DOM SZ/NaY (D) 3DOM SZ/RuNaY after H2 reduction. Crystalline phases for some peaks are marked with symbols ‘‘T” and ‘‘M” to indicate tetragonal and monoclinic phases for zirconia, respectively; arrows indicate zeolite NaY peaks.
Bruker AXS micro-diffractometer with a Cu X-ray source and a HiStar 2-D area detector. All diffraction patterns were acquired at a generator voltage of 45 kV, and a current of 40 mA. Crystallite sizes were calculated from Scherrer’s equation. Scanning electron microscopy (SEM) images were obtained with a JEOL 6500 scanning electron microscope equipped with an energy-dispersive Xray spectroscopy (EDS) accessory at an accelerating voltage of 15 kV. Samples were mounted on an aluminum stub with conductive carbon sticky tape and a thin (ca. 5 nm) coating of platinum was deposited on the sample prior to analysis. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai T-12 electron microscope that was operated at 120 kV. Samples for TEM measurements were drop-dried from ethanol onto a holey carbon film supported on a copper grid. Nitrogen-sorption measurements were performed on a Micromeritics ASAP 2000 gas sorptometer. Samples were degassed to 0.003 mmHg for 48 h at 120 °C. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and pore volumes were taken at the P/P0 = 0.985 single point. 3. Results and discussion 3.1. 3DOM SZ/RuNaY nanocomposites As-synthesized 3DOM SZ possessed a uniform macropore structure with fcc packing of interconnected voids and smooth macropore surfaces (Fig. 1A). Diffraction of light by this periodic structure resulted in a highly opalescent appearance of 3DOM SZ. The macropore walls in as-synthesized 3DOM SZ are composed of a pure tetragonal zirconia phase (Fig. 2A), which is known to exhibit high catalytic activity toward n-alkane isomerization [59,60]. The crystallite size of SZ was estimated by the Scherrer equation to be ca. 8.7 nm, which was only slightly higher than in a previous re-
Fig. 3. Nitrogen-sorption isotherms for 3DOM SZ, SZ/P, SZ/NaY before and after calcination.
port for 3DOM SZ [53] but significantly smaller than for nonsulfated 3DOM ZrO2 [61], indicating that sulfate groups effectively suppressed the grain growth of ZrO2 during calcination. After multi-layer polyelectrolyte coating, the 3DOM structure was maintained and the macropore wall surfaces became slightly rougher (Fig. 1B). Addition of the polyelectrolyte coating did not change the crystallographic phase but introduced background scattering in the region from 10 to 35° 2h of the powder XRD pattern (Fig. 2B). After the hydrothermal reaction with the zeolite precursor solution, a uniform nanoparticle coating was observed on the 3DOM SZ surface, composed of particles ca. 20 nm in average diameter (Fig. 1C and E). More interestingly, the zeolite lattice fringes could be directly observed at higher resolution (Fig. 1F). Some low-intensity peaks from diffractions of the zeolite lattice were observed in the XRD powder pattern (Fig. 2C). The low intensity is due to the small size of the NaY crystallites, line broadening effects and the low zeolite concentration in the sample. Ion-exchange with Ru(NH3)6Cl3 did not change the structure of 3DOM SZ/NaY and no significant particle growth of zeolite NaY was observed (Fig. 1D). Zeolite peaks could not be clearly observed for the H2-reduced sample (3DOM SZ/RuNaY; Fig. 2D), although studies on bulk NaY particles showed that the XRD peaks are retained even after the formation of Ru nanoclusters within zeolite frameworks [62,63]. Moreover, contributions from zirconia in the monoclinic phase appeared in the powder pattern, a phase which is less catalytically active (Fig. 2D). Nitrogen-sorption isotherms for 3DOM SZ, 3DOM SZ/P and 3DOM SZ/NaY all have similar shapes and they can be classified as type II with H3 hysteresis loops (Fig. 3), isotherms which correspond to the macroporous structure with interparticle pores formed by aggregates of the zeolite nanoparticles [64]. The zeolite loading on 3DOM SZ was ca. 10 wt% as estimated by measuring the weight difference between pristine 3DOM SZ and calcined 3DOM
Table 1 Nitrogen-sorption results: BET surface area, pore volume and micropore information. Sample
SBET (m2 g
SZ SZ/P SZ/Y SZ/Y (cal)
34 22 79 104
1
)
Total meso- and micropore volume (cm3/g)
Micropore volume (%)
0.04 0.039 0.11 0.13
4.8 2.5 14.5 12.3
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
355
Fig. 4. EDS spectra: (left) samples at different reaction stages from 3DOM SZ to calcined 3DOM SZ/NaY; (right) 3DOM SZ/RuNaY sample after hydrogen-reduction.
SZ/NaY. The increase in specific micropore volume of the 3DOM SZ/ NaY composite after incorporation of zeolite (Table 1) also supports a weight fraction of ca. 10% in the composite. Therefore, it is believed that interparticle mesopores formed by zeolite NaY aggregates contribute the most to the total pore volume determined by nitrogen-sorption analysis. As-synthesized 3DOM SZ has a BET surface area of 34 m2 g 1. Multi-layer coating with polyelectrolytes decreased the surface area due to blocking of surface micropores. After hydrothermal growth of zeolite nanoparticles, the surface area increased again to 79 m2 g 1. These changes are consistent with our previously reported results for 3DOM carbon/ TiO2 nanocomposites, where nanoparticle coatings contributed to the textural mesoporosity of the samples [57]. After calcination of the NaY-coated sample at 550 °C, the surface area increased further to 104 m2 g 1 due to the introduction of micropores inside the zeolite framework after removal of the structure-directing agent, TMAOH molecules. Moreover, from Table 1 we can see that the proportion of micropores increased after zeolite nanoparticle coating. In as-synthesized 3DOM SZ, Zr, O, and S were detected by EDS (Fig. 4). As expected, after multi-layer polyelectrolyte coatings, the inorganic composition did not change significantly. After hydrothermal reaction at 100 °C for 4 days, Na, Al and Si were detected, elements which were introduced by the zeolite coating; the O content also increased. However, the sulfur peak disappeared, which indicates the removal of sulfate groups from the 3DOM SZ surface. A likely reason for loss of sulfate is that during the hydrothermal reaction, the strong basic zeolite precursor solution neutralized surface acidic sites created by SO24 . Ru was detected in the 3DOM SZ/NaY sample after ion-exchange with Ru(NH3)6Cl3(aq) followed by hydrogen-reduction.
those grown on 3DOM SZ (Fig. 1C). The NaY coating on 3DOM WZ was not as uniform as that on 3DOM SZ (Fig. 1C), probably due to a weaker acidity of 3DOM WZ than 3DOM SZ which would result in less stable polyelectrolyte coatings. Moreover, zeolite particles were not as monodispere as those in 3DOM SZ/NaY. Diffraction peaks from NaY remained visible in the XRD pattern after ion-exchange with Ru(NH3)6Cl3(aq) solution (Fig. 5C). To acquire information about the elemental composition of 3DOM WZ/RuNaY, XPS was utilized instead of EDS (which had been adopted for 3DOM SZ/RuNaY), since the tungsten peaks in EDS spectra are very close to those of zirconium and silicon (from NaY). Besides Zr and O, other elements such as W, Fe (from surface groups) and Si, Al (from zeolite NaY) were detected (Fig. 7, left). This indicated that the catalytically active surface groups were not removed from 3DOM WZ surface after a series of reactions, unlike the loss of sulfate groups on 3DOM SZ after the hydrothermal reaction (Fig. 4, left). The presence of ruthenium was confirmed by high-resolution scans (Fig. 7, right). The carbon 1 s peak at 285.0 eV was used as an internal standard for the peaks of other elements.
3.2. 3DOM WZ/RuNaY nanocomposites Similar to 3DOM SZ, the as-synthesized 3DOM WZ contained zirconia in the tetragonal zirconia phase (Fig. 5A) and also exhibited smooth macropore wall surfaces (Fig. 6A). The average crystallite size was 9.6 nm, estimated from XRD line broadening, a value that is slightly higher than for 3DOM SZ. After the hydrothermal reaction to deposit NaY, this phase was maintained and a series of zeolite NaY peaks appeared (Fig. 5B). The macropore wall surface of 3DOM WZ was coated with crystalline particles which ranged from 25 to 50 nm in diameters (Fig. 6B and C), i.e., larger than
Fig. 5. XRD patterns of (A) as-synthesized 3DOM WZ, (B) 3DOM WZ/NaY, (C) 3DOM WZ/NaY after ion-exchange but before H2 reduction, and (D) 3DOM WZ/NaY after ion-exchange and H2 reduction (3DOM WZ/RuNaY). Here ‘‘t” and ‘‘Y” stand for tetragonal zirconia and zeolite NaY, respectively.
356
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
Another signal was detected at 280.1 eV, which could be assigned to the 3d5/2 peak of metallic Ru nanoclusters [65–67]. In addition, this value also suggested that the ruthenium in the 3DOM WZ/RuNaY sample is Ru(0), which is required for FTS. Zeolite peaks disappeared from 3DOM WZ/NaY after H2 reduction (Fig. 5D). In addition, it was difficult to observe zeolite lattice
fringes in TEM images (Fig. 6D). This is most likely the result of deterioration of the NaY framework in these small nanoparticles during reduction and Ru cluster growth. In contrast, in bulk NaY [63] or even in NaY nanoparticle samples with a significant fraction of particles 100 nm in diameter or larger (see Supplementary material), XRD peaks were preserved after Ru cluster formation.
Fig. 6. SEM images of (A) 3DOM WZ and (B) 3DOM WZ/NaY; TEM images of (C) 3DOM WZ/NaY and (D) 3DOM WZ/RuNaY after hydrogen-reduction.
Fig. 7. XPS data: (left) survey scan and (right) high-resolution scan of 3DOM WZ and 3DOM WZ/RuNaY after H2 reduction.
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
It is notable that the external shapes of zeolite nanoparticles in 3DOM WZ/RuNaY were maintained, and aggregates of Ru nanoparticles were neither visible inside nor on the external surfaces of NaY nanocrystals. The uniform contrast might indicate a relatively uniform distribution of ruthenium in clusters that were smaller than NaY supercages so that they were not visible in our TEM images [69]. Larger Ru nanoparticles (ca. 2 nm) are usually observable by TEM at appropriate magnifications when the zeolite framework remains intact [62,63,65,68,70]. Most ion-exchange experiments have been performed on zeolite particles with sizes around or larger than 100 nm [70–75], but reports of ion-exchange on even smaller NaY nanocrystals are quite rare [76]. In this study, Ru was loaded onto NaY nanocrystals (20 50 nm) supported on 3DOM zirconia via ion-exchange, and the zeolite lattice was preserved. After hydrogen-reduction, metallic Ru was detected by XPS, although the zeolite peaks disappeared from XRD and their zeolite fringes were no longer observed by TEM. The reduction might be too harsh so that the rapid growth of Ru nanoclusters or the release of gaseous decomposition products destroyed the frameworks of these relatively small zeolite particles. In the future, milder reduction processes may be able to prevent framework degradation.
4. Conclusions In this paper, we reported the design, synthesis and characterization of 3DOM ZrO2/NaY nanocomposites targeting a heterogeneous catalyst for clean fuel production via Fischer–Tropsch synthesis (FTS) and isomerization. The introduction of sulfate and tungstate groups into zirconia efficiently maintained 3DOM frameworks by suppressing grain growth during the high-temperature crystallization and stabilizing ZrO2 as the tetragonal phase which is also catalytically active toward n-alkane isomerization. Uniform coatings of NaY nanocrystals on 3DOM zirconia were achieved, especially on 3DOM SZ. The highly basic environment of the zeolite synthesis resulted in loss of sulfate groups on 3DOM SZ. In contrast, tungstate groups on 3DOM WZ survived the hydrothermal processing conditions. Ru was introduced into zeolite NaY via ion-exchange of the nanocomposites with a Ru complex solution. To the best of our knowledge, this is the first report of supported RuNaY nanoparticles smaller than 100 nm via ion-exchange and reduction. Large Ru nanoparticles were not observed after reduction, which might indicate an even distribution of Ru nanoclusters smaller than zeolite supercages through the NaY particles. 3DOM ZrO2 is a good support for zeolite nanocrystals, and these nanocomposites designs have the potential for shortening the diffusion pathlengths of reactants between FTS and isomerization catalysts. Improved hydrocarbon yield and selectivity are also expected for these hierarchical composite nano- and microstructures that bring multiple catalyst components together in intimate contact. Catalytic activities for FTS and isomerization processes can now be evaluated in future research.
Acknowledgments This research was supported by the Petroleum Research Foundation administered by the American Chemical Society (ACS-PRF Grant Number 42751-AC10) and the Office of Naval Research (Grant Number N00014-07-1-00608). Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from the NSF through the NNIN and MRSEC programs. Z.W. thanks the University of Minnesota Graduate School for a Doctoral Dissertation Fellowship.
357
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2008.11.024. References [1] [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] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63]
F. Fischer, H. Tropsch, Brennst. Chem. 4 (1923) 276. L. Guczi, I. Kiricsi, Appl. Catal. A 186 (1999) 375. M. Stoecker, Micropor. Mesopor. Mat. 82 (2005) 257. V.U.S. Rao, R.J. Gormley, Catal. Today 6 (1990) 207. S. Bessell, Appl. Catal. A 96 (1993) 253. S.C. Larsen, J. Phys. Chem. C 111 (2007) 18464. L. Tosheva, V.P. Valtchev, Chem. Mater. 17 (2005) 2494. A. Martinez, G. Prieto, J. Catal. 245 (2007) 470. Y. Wang, H. Wu, Q. Zhang, Q. Tang, Micropor. Mesopor. Mat. 86 (2005) 38. Q. Tang, Q. Zhang, P. Wang, Y. Wang, H. Wan, Chem. Mater. 16 (2004) 1967. Q. Tang, Y. Wang, Q. Zhang, H. Wan, Catal. Commun. 4 (2003) 253. H. Pichler, H. Schulz, D. Kuehne, Brennst. Chem. 49 (1968) 344. W. Stichert, F. Schuth, J. Catal. 174 (1998) 242. G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mat. 33 (1999) 1. X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. T. Yamaguchi, Appl. Catal. A 222 (2001) 237. R. Akkari, A. Ghorbel, N. Essayem, F. Figueras, Appl. Catal. A 328 (2007) 43. J.-H. Wang, C.-Y. Mou, Micropor. Mesopor. Mat. 110 (2008) 260. C.R. Vera, C.L. Pieck, K. Shimizu, J.M. Parera, Appl. Catal. A 230 (2002) 137. X. Li, K. Nagaoka, L.J. Simon, R. Olindo, J.A. Lercher, J. Catal. 232 (2005) 456. P.D.L. Mercera, J.G. van Ornmen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. Y.-Y. Huang, T.J. McCarthy, W.M.H. Sachtler, Appl. Catal. A. 148 (1996) 135. R. Akkari, A. Ghorbel, N. Essayem, F. Figueras, Micropor. Mesopor. Mat. 111 (2008) 62. X.-R. Chen, Y.-H. Ju, C.-Y. Mou, J. Phys. Chem. C 111 (2007) 18731. F. Li, F. Yu, Y. Li, R. Li, K. Xie, Micropor. Mesopor. Mat. 101 (2007) 250. M.A. Ecormier, A.F. Lee, K. Wilson, Micropor. Mesopor. Mat. 80 (2005) 301. Y. Sun, L. Zhu, H. Lu, R. Wang, S. Lin, D. Jiang, F.-S. Xiao, Appl. Catal. A 237 (2002) 21. U. Ciesla, M. Froeba, G. Stucky, F. Schueth, Chem. Mater. 11 (1999) 227. S.-Y. Chen, L.-Y. Jang, S. Cheng, J. Phys. Chem. B 110 (2006) 11761. M. Signoretto, A. Breda, F. Somma, F. Pinna, G. Cruciani, Micropor. Mesopor. Mat. 91 (2006) 23. E. Zhao, S.E. Hardcastle, G. Pacheco, A. Garcia, A.L. Blumenfeld, J.J. Fripiat, Micropor. Mesopor. Mat. 31 (1999) 9. D.M. Antonelli, Adv. Mater. 11 (1999) 487. Y. Sun, L. Yuan, W. Wang, C.-L. Chen, F.-S. Xiao, Catal. Lett. 87 (2003) 57. J.C. Yori, J.C. Luy, J.M. Parera, Catal. Today 5 (1989) 493. F.T.T. Ng, N. Horvat, Appl. Catal. A 123 (1995) L195. G. Larsen, E. Lotero, R.D. Parra, L.M. Petkovic, H.S. Silva, S. Raghavan, Appl. Catal. A 130 (1995) 213. G. Larsen, E. Lotero, S. Raghavan, R.D. Parra, C.A. Querini, Appl. Catal. A 139 (1996) 201. M. Scheithauer, R.E. Jentoft, B.C. Gates, H. Knozinger, J. Catal. 191 (2000) 271. S. Kuba, R.K. Grasselli, H. Knozinger, B.C. Gates, Chem. Commun. (2001) 321. X.-R. Chen, C.-L. Chen, N.-P. Xu, S. Han, C.-Y. Mou, Catal. Lett. 85 (2003) 177. S.-T. Wong, T. Li, S. Cheng, J.-F. Lee, C.-Y. Mou, J. Catal. 215 (2003) 45. N.O. Egiebor, W.C. Cooper, B.W. Wojciechowski, Appl. Catal. 55 (1989) 47. T. Inui, T. Kuroda, T. Takeguchi, A. Miyamoto, Appl. Catal. 61 (1990) 219. X. Li, K. Asami, M. Luo, K. Michiki, N. Tsubaki, K. Fujimoto, Catal. Today 84 (2003) 59. Y. Yoneyama, X. San, T. Iwai, N. Tsubaki, Energ. Fuel. 22 (2008) 2873. A. Martinez, S. Valencia, R. Murciano, H.S. Cerqueira, A.F. Costa, E.F.S. -Aguiar, Appl. Catal. A 346 (2008) 117. J. He, Y. Yoneyama, B. Xu, N. Nishiyama, N. Tsubaki, Langmuir 21 (2005) 1699. Y. Yoneyama, J. He, Y. Morii, S. Azuma, N. Tsubaki, Catal. Today 104 (2005) 37. A. Martinez, C. Lopez, Appl. Catal. A 294 (2005) 251. Z.-W. Liu, X. Li, K. Asami, K. Fujimoto, Catal. Commun. 6 (2005) 503. E. van Steen, E.L. Viljoen, J. van de Loosdrecht, M. Claeys, Appl. Catal. A 335 (2008) 56. X. Song, A. Sayari, Appl. Catal. A 110 (1994) 121. M.A. Al-Daous, A. Stein, Chem. Mater. 15 (2003) 2638. R.C. Schroden, M.A. Al-Daous, S. Sokolov, B.J. Melde, J.C. Lytle, A. Stein, M.C. Carbajo, J.T. Fernandez, E.E. Rodriguez, J. Mater. Chem. 12 (2002) 3261. S. Kuba, B.C. Gates, R.K. Grasselli, H. Knozinger, Chem. Commun. (2001) 321. F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. Z. Wang, N.S. Ergang, M.A. Al-Daous, A. Stein, Chem. Mater. 17 (2005) 6805. W. Song, V.H. Grassian, S.C. Larsen, Chem. Commun. (2005) 2951. W. Stichert, F. Schuth, S. Kuba, H. Knozinger, J. Catal. 198 (2001) 277. M. Hino, K. Arata, J. Chem. Soc. Chem. Comm. (1980) 851. B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538. S.M. Gheno, S. Damyanova, B.A. Riguetto, C.M.P. Marques, C.A.P. Leite, J.M.C. Bueno, J. Mol. Catal. A 198 (2003) 263. U.L. Portugal Jr., C.M.P. Marques, E.C.C. Araujo, E.V. Morales, M.V. Giotto, J.M.C. Bueno, Appl. Catal. A 193 (2000) 173.
358
Z. Wang et al. / Microporous and Mesoporous Materials 120 (2009) 351–358
[64] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology, Applications, Academic Press, New York, 1999. p. 20. [65] S.-P. Sheu, H.G. Karge, R. Schlogl, J. Catal. 168 (1997) 278. [66] B.L. Gustafson, J.H. Lunsford, J. Catal. 74 (1981) 393. [67] S.K. Das, P.K. Dutta, Micropor. Mesopor. Mat. 22 (1998) 475. [68] T.J. McCarthy, C.M.P. Marques, H. Trevino, W.M.H. Sachtler, Catal. Lett. 43 (1997) 11. [69] S.J. Cho, S.M. Jung, Y.G. Shul, R. Ryoo, J. Phys. Chem. 96 (1992) 9922. [70] L. Guczi, R. Sundararajan, Z. Koppany, Z. Zsoldos, Z. Schay, F. Mizukami, S. Niwa, J. Catal. 167 (1997) 482.
[71] R.A. Rakoczy, Y. Traa, Micropor. Mesopor. Mat. 60 (2003) 69. [72] N.B. Castagnola, P.K. Dutta, J. Phys. Chem. B 102 (1998) 1696. [73] P. Castillo-Villalon, J. Ramirez, M.-J. Peltre, C. Louis, P. Massiani, Phys. Chem. Chem. Phys. 6 (2004) 3739. [74] C. Frontaine, P. Castillo, J. Ramirez, M.J. Peltre, F. Warmont, C. Louis, P. Massiani, Stud. Surf. Sci. Catal. 154 (2004) 1784. [75] O. Hernandez-Ramirez, P.I. Hill, D.J. Doocey, S.M. Holmes, J. Mater. Chem. 17 (2007) 1804. [76] Y.-X. Jiang, D. Si, S.-P. Chen, S.-G. Sun, Electroanal. 18 (2006) 1173.