Synthesis and Dehydration Activity of Novel Lewis Acidic Ordered

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Synthesis and Dehydration Activity of Novel Lewis Acidic Ordered Mesoporous Silicate: Zr-KIT‑6 Qing Pan,†,‡ Anand Ramanathan,† W. Kirk Snavely,† Raghunath V. Chaudhari,†,‡ and Bala Subramaniam*,†,‡ †

Center for Environmentally Beneficial Catalysis, ‡Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, Kansas 66045, United States S Supporting Information *

ABSTRACT: A zirconium-incorporated Ia3d cubic three-dimensional (3-D) mesoporous silicate, KIT-6, with different zirconium loadings, was synthesized via a one-step direct hydrothermal synthesis procedure employing Pluronic P123 triblock copolymer as the structure-directing agent under acidic conditions. Various characterization techniques, such as two-dimensional (2-D) small-angle X-ray scattering (SAXS), nitrogen sorption, temperature-programmed desorption of ammonia (NH3-TPD), and ultraviolet−visible-light (UV-Vis) spectra, showed that zirconium was incorporated as Zr4+ ions in the KIT-6 framework, which retained the structural integrity with a highly ordered pore structure. The Zr-KIT-6 materials exhibit very high surface areas (810−980 m2/g) and large pore volumes (1.07−1.65 cm3/g) that decrease with an increase in zirconium content. In contrast, the pore size distribution remains relatively unaffected by zirconium loading with an average pore diameter of ∼9.3 nm. The Zr-KIT-6 materials possess Lewis acidity that increases with zirconium loading. In the 180−300 °C range, the Zr-KIT-6 materials are shown to be highly active for the test reaction of isopropanol (IPA) dehydration to propylene (selectivity >98%). The activation energy for IPA dehydration, estimated from intrinsic rate constants normalized with respect to the Lewis acid sites, was ∼49 ± 1 kJ/mol and found to be lower than most other Brønsted or Lewis acidic heterogeneous catalysts reported in the literature for IPA dehydration. Furthermore, these catalysts showed nearly stable activity with very little deactivation during extended runs at 260 and 300 °C.

1. INTRODUCTION Aluminosilicates such as microporous zeolites and functionalized mesoporous molecular sieve (MCM-based) catalysts have received much attention as solid acid catalysts.1,2 However, pore diffusion limitations and/or deactivation hinder the performance of microporous solid acid catalysts.3,4 In addition, typical pore sizes of MCM-41 materials (∼2.5 nm) may pose steric hindrances to bulky and/or long-chain molecules often encountered when processing biomass-derived substrates.5 Therefore, the development of selective and stable solid acid catalysts that can accommodate a wide range of substrate sizes has received much interest in recent years.4 Dehydration reactions play a significant role in reducing the oxygen content of biomass-derived substrates. Catalytic dehydrations can be performed with catalysts containing both Lewis and Brønsted acid sites. For example, the conversion of bioethanol to ethylene proceeds smoothly over Brønsted acid sites present in the catalysts.6,7 However, Brønsted acid sites also catalyze secondary reactions such as cracking and oligomerization, leading to the deactivation of such catalysts.8 Recently, we reported that Zr-KIT-6 materials displayed Lewis acidity exclusively.9 Another potential advantage is that the large-pore Ia3d-type structure of Zr-KIT-6 materials with tunable pore size (tuned during hydrothermal treatment, yielding pores 4−12 nm in diameter) could provide better pore accessibility for bulkier substrates, compared to MCM-41type materials.10 The present work is focused on investigating the activity and selectivity of the Lewis acid sites of Zr-KIT-6 materials and on establishing the stability of such materials, © 2013 American Chemical Society

employing the catalytic dehydration of isopropyl alcohol as a test reaction. Even though isopropanol is not well-suited to test the absence of steric hindrances, its dehydration activity has been investigated extensively11−13 and, hence, serves as an ideal substrate to benchmark the activity of new materials, such as Zr-KIT-6, that display acidity. Clearly, establishing such activity is key to their potential use as solid acid catalysts, for even bulkier substrates.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Zr-KIT-6. Zr-KIT-6 with three different Si/Zr mole ratios of 100, 40, and 20 (designated as Zr-KIT6(x), where x is the Si/Zr molar ratio in the synthesis mixture) were synthesized following the procedure that has been described elsewhere.9 The main difference is that the materials used in the present work were synthesized using zirconyl chloride octahydrate (ZrOCl2·8H2O, 98% purity, Sigma− Aldrich) as the zirconium source, since zirconium(IV) propoxide solution (used as the zirconium source in our earlier work9) rapidly hydrolyzes under the acidic conditions used in KIT-6 synthesis, forming ZrO2 particles and resulting in Zr4+ yield loss. In contrast, zirconyl chloride octahydrate has a Special Issue: NASCRE 3 Received: Revised: Accepted: Published: 15481

June 20, 2013 August 19, 2013 September 3, 2013 September 3, 2013 dx.doi.org/10.1021/ie4019484 | Ind. Eng. Chem. Res. 2013, 52, 15481−15487

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samples at 120 °C and atmospheric pressure, and analyzing in the DRIFT mode using a TENSOR FTIR system. 2.3. Catalytic Dehydration of Isopropyl Alcohol (IPA). A schematic of the experimental unit is shown in Figure 1.

slower hydrolysis rate, compared to zirconium(IV) propoxide. Hence, as the silicate network forms, more of the Zr is incorporated as Zr4+ ions into the framework. In a typical synthesis, 5.0 g of the triblock copolymer Pluronic P123 (EO20−PO70−EO20, Aldrich) were dissolved in 180 mL of 0.5 M HCl solution at 35 °C. Then, 5.0 g of n-butanol were added, and the stirring was continued for another 1 h at the same temperature. Finally, 10.6 g of tetraethyl orthosilicate (TEOS, Aldrich) and required amounts of zirconyl chloride octahydrate were added to the mixture, and the stirring was continued for another 18 h. The synthesis mixture was then transferred to a Teflon-lined SS autoclave and heated at 100 °C for 24 h. The solid product was filtered with mild washing, dried at 100 °C overnight, and calcined in flowing air at 550 °C for 5 h. 2.2. Catalyst Characterization. Two-dimensional smallangle X-ray scattering (2-D SAXS) patterns were collected on a Rigaku system, using a S-MAX 3000 instrument with a Bede Scientific microfocus tube source operating at 45 kV and 0.66 mA. Patterns were rotationally averaged and presented as intensity versus scattering angle plots. A 10 cm × 10 cm wire detector was placed ∼150 cm from the sample position, and silver behenate was used to determine the exact pixel-toscattering angle conversion. Powder X-ray diffraction (XRD) patterns in the high angle range (2θ = 10°−80°) were collected on a Rigaku instrument with Cu Kα radiation. The textural properties (surface area, pore volume, and pore size distribution) were evaluated from nitrogen sorption isotherms at −196 °C using a Nova 2000e instrument. The Brunauer−Emmett−Teller (BET) equation was used to estimate the surface area based on adsorption data obtained at P/P0 values in the range of 0.01−0.25. The total pore volume was estimated from the amount of nitrogen adsorbed at P/P0 = 0.99, and the pore size distribution was determined by analyzing the adsorption branch of the N2 sorption isotherm using the Barrett−Joyner−Halenda (BJH) method. Elemental analysis was performed by digesting the samples in a mixture of HF and H2SO4 and analyzing them on an inductively coupled plasma−optical emission spectroscopy (ICP-OES) instrument. Transmission electron microscopy (TEM) micrographs were captured using a 2K × 2K CCD, each mounted on a 200 kV FEI Tecnai F20 G2 X-Twin fieldemission scanning/transmission electron microscope operating at 200 kV. The samples were dispersed in ethanol, and a drop of the suspension was placed on lacey carbon supported on 300 mesh copper grids. Diffuse-reflectance ultraviolet−visible-light (UV-vis) spectra were collected in the 200−800 nm range at room temperature, using Spectralon as the reference, with a Perkin−Elmer Model Lambda 850) spectrophotometer equipped with a diffuse-reflectance integrating sphere. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out with a Micromeritics Autochem 2910 instrument equipped with a thermal conductivity detector (TCD). Samples were heated from room temperature to 550 °C and subsequently cooled to 100 °C in flowing helium (30 sccm). [The unit sccm means standard cubic centimeters per minute.] Ammonia was adsorbed at this temperature for 30 min from a helium stream containing 9.98 vol % NH3 flowing at 30 sccm. Any physisorbed ammonia was removed via a helium flow (30 sccm) for another 30 min. Following this step, the temperature was increased from 100 °C to 550 °C at a ramp rate of 10 °C/min and the desorbed ammonia was recorded. Pyridine FTIR spectra of the samples were acquired by saturating the Zr-KIT-6 samples with pyridine, drying the

Figure 1. Schematic of experimental unit. [Legend: MFC, mass flow controller; BPR, back pressure regulator; PTC, profile thermocouple; and PT, pressure transducer.]

Conversion and selectivity experiments were carried out in a continuous fixed-bed reactor (1 cm ID, 30 cm long), made of Type 304 stainless steel. The reactor was heated by Cartridge Heaters (McMaster-Carr, Model 3618K193). Approximately 1.5 g (∼5 cm3) of the catalyst pellets (250−700 μm size range) were packed with two screens at each end as holders to ensure reproducible packing position. The entire reactor was covered with insulation (∼5 cm thickness) to minimize radial temperature gradients. The reactor temperature was measured with a profile thermocouple probe placed along the axis of the reactor with six measurement points, one of which was located at the center of the catalyst packing to measure the catalyst temperature. The measured temperature profiles (see the Supporting Information) suggest that the axial temperature gradient in the short (1 cm) catalyst bed was 98% in all cases and independent of conversion. The propylene selectivity is higher than the 39%−78% range reported on predominantly Lewis acidic catalysts such as Al2O3 and ZrO2.20 A 30-h extended run with Zr-KIT-6(20) at 260 °C (see Figure 10a) showed relatively little deactivation, with the IPA conversion dropping from 93.2% to 91.5%. Similarly, the Zr-KIT-6(40) and Zr-KIT6(100) samples were tested at 300 °C for 12 h. The IPA conversion decreased from 97% to 96% in the case of Zr-KIT6(40) and fluctuated around ∼94% for Zr-KIT-6(100) (see

Figure 6. Diffuse reflectance UV-Vis spectra of Zr-KIT-6(x).

display an absorption band in the 205−215 nm range.9,16−19 A blue shift of this main absorption peak was noticed in Zr-KIT-6 samples. This absorption band is attributed to ligand-to-metal charge transfer (LMCT) from an O2− to an isolated Zr4+ ion in a tetrahedral configuration. A distinct broad band of low intensity at ∼280 nm was observed in all the Zr-KIT-6 samples and is attributed to highly distributed ZrO2 nanoparticles.9,19 An absence of bulk ZrO2 in these samples is further evidenced from the lack of absorption peak in the vicinity of 230 nm. NH3-TPD of Zr-KIT-6 materials is shown in Figure 7. The total acidity of these samples estimated from the NH3-TPD profiles is listed in Table 1. The Zr-KIT-6 materials display a significant desorption of ammonia in the 100−400 °C region, because of the presence of acid sites of varying strengths. In general, the total acidity of Zr-KIT-6 materials increased with an increase in the Zr content. However, this increase was nonlinear, presumably because of the presence of ZrO2 nanoparticles at higher loadings (observed in diffuse-reflectance UV-Vis) that do not contribute to the acidity.9 For example, the total acidity of ZrO2 nanopowder (Sigma−Aldrich) was measured to be ∼0.12 NH3 mmol/g,9 which is lower than the measured values for the Zr-KIT-6 materials (0.19−0.49 NH3 mmol/g). 15484

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ke = −

νg Vc

ln(1 − X )

(1) −1

where ke is the effective rate constant (min ); vg the volumetric flow rate at reactor pressure P and temperature T (cm3/min); Vc is the packed volume of catalyst (cm3); and X is the observed IPA conversion at steady state. As shown in Figure 11, the effective rate constants increased at relatively low GHSV values and became invariant above

Figure 9. Effects of temperature on IPA conversion and propylene selectivity (conditions: IPA in feed, 5 mol % in N2; catalyst loading, 1.5 g; GHSV, 7200 h−1): (a) based on actual catalyst loading without normalizing, with respect to acid sites; and (b) IPA conversions normalized with respect to acid sites on loaded catalysts.

Figure 11. Dependence of effective rate constant (ke) on GHSV at 260 °C.

∼6000 h−1 at 260 °C. We attribute this to the elimination of external mass-transfer limitations at the higher space velocities. In addition, catalyst effectiveness factors (η), estimated using known correlations under conditions where external masstransfer limitations are eliminated (see the Supporting Information), are close to unity, which implies that porediffusion limitations are absent as well. In order to estimate the intrinsic activation energy, the reactions were conducted at temperatures between 200−260 °C employing a GHSV of 7200 h−1, wherein both external and internal mass-transfer limitations are avoided. As shown in Figure 12a, the Zr-KIT-6 samples with higher zirconium contents yielded higher rate constants when such rate constants are normalized with respect to the volume of the catalyst packing (eq 1). The activation energy in each case was ∼45−50 kJ/mol. However, when the rate constants were normalized with respect to the total acidity of the respective Zr-KIT-6

Figure 10. Plotted results of (a) a 30-h stability test on Zr-KIT-6(20) at 260 °C and (b) 12-h stability tests on Zr-KIT-6(40) and Zr- KIT6(100) at 300 °C. (Conditions: IPA in feed, 5 mol % in N2; Catalyst loading, 1.5 g; GHSV, 7200 h−1.)

Figure 10b). These results show that, compared to other reported catalysts, the tunable Lewis acidity of the Zr-KIT-6 materials favors high conversion and propylene selectivity with relatively low formation of the major byproduct (diisopropyl ether) and little deactivation. In order to compare the intrinsic activity of the various ZrKIT-6 materials, effective rate constants were estimated from the steady-state conversions measured in the 190−260 °C range, where conversions were much lower than 100%. Given the high propylene selectivity (>98.5%), only the dehydration reaction was considered. Furthermore, since N2 was the dominant component (>95 mol %), the volume change upon reaction is ignored as being insignificant. Based on these assumptions, an effective first-order rate constant (ke) based on a plug-flow reactor model is given by eq 1:

Figure 12. Estimation of activation energy from (a) intrinsic rate constants (ke) based on catalyst packing volume, and (b) intrinsic rate constants (ke′) based on catalyst acidity. 15485

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materials (eq 2), the rate constants at the various temperatures virtually overlapped for all the catalysts, suggesting that the dehydration reaction occurs on the Lewis acid sites (see Figure 12b). Consistent with this observation, the measured IPA conversions over the three catalysts also overlap when normalized with the acid sites on the catalyst (see Figure 9b): νg ke′ = − ln(1 − X ) Acwc (2)

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ref

bulk H3PW12O40(HPW) acid bulk CsnH3−nPW acid 15% HPW-ZrO2 15% HPW-TiO2 15% HPW-SiO2 15% HPW-Nb2O3 bulk H3PMo12O40 γ-Al2O3 ZrO2 Zr-KIT-6

104 68 59 90 86 43 117 173 133 49

21 21 21 21 21 21 20 20 20 this work

AUTHOR INFORMATION

Corresponding Author

*Tel.: 785-864-2903. Fax: 785-864-6051. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part with funds from the U.S. Department of Agriculture/National Institute of Food and Agriculture (USDA/NIFA), through Award No. 2011-1000630362. The authors thank Prof. Maheswari Rajamanickam (Anna University, Chennai, India) for performing the catalyst acidity measurements using pyridine adsorption and FTIR analysis. The authors also thank Prof. Brian Grady, University of Oklahoma, for his assistance with the SAXS analysis.

Table 2. Comparison of Activation Energies of Zr-KIT-6 Catalyst with Those Reported in the Literature E (kJ/mol)

ASSOCIATED CONTENT

S Supporting Information *

where ke′ is the intrinsic kinetic rate constant (min−1), wc the weight of catalyst used (g), and Ac the total acidity of catalyst ((cm3 NH3 (under standard conditions))/g catalyst). The intrinsic activation energy based on acidity-normalized rate constants was estimated as ∼49 ± 1 kJ/mol. As compared in Table 2, this value is generally lower than those reported for

catalyst

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REFERENCES

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IPA dehydration on solid acid catalysts that contain predominantly either Brønsted acid sites (40−120 kJ/mol on bulk and supported heteropolyacids) or Lewis acid sites (133 kJ/mol on γ-Al2O3 and 173 kJ/mol on ZrO2).20,21 This clearly shows that the Zr-KIT-6 materials are a superior and promising class of highly active, selective, and durable alcohol dehydration catalysts.

4. CONCLUSION Zirconium ions were successfully incorporated into the framework of KIT-6 silicates via a one-step synthesis procedure. Structural integrity and a high degree of pore ordering were revealed from complementary SAXS, N2 sorption, and TEM analyses. Diffuse reflectance UV-Vis and NH3-TPD results confirm Zr4+ ion incorporation in the framework. The total acidity of Zr-KIT-6 materials increased with zirconium content. For the dehydration of isopropanol to propene, high activity and selectivity (>98%) to propene were displayed in the 190− 300 °C temperature range. In sharp contrast, ZrO2 displayed little acidity or dehydration activity, confirming the enhanced Lewis acidity of the Zr-KIT-6 materials. Kinetic parameters obtained in the absence of mass-transfer limitations showed moderate activation energy (∼49 ± 1 kJ/mol) for all Zr-KIT-6 catalysts when normalized with the acid sites on the catalyst samples. The results presented here demonstrate that Zr-KIT-6 materials are superior Lewis acidic catalysts that display high activity, selectivity, and durability that could be potentially exploited in the dehydration of various substrates. The emerging biomass-based renewable chemicals industry could particularly benefit from the availability of such catalysts for dehydration of long-chain alcohols from biomass-based feedstock. 15486

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