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Characterization of zeolite Beta grown in microgravity
Microporous and Mesoporous Materials 71 (2004) 1–9 www.elsevier.com/locate/micromeso
Characterization of zeolite Beta grown in microgravity Burcu Akata *, Bilge Yilmaz, Siricharn S. Jirapongphan, Juliusz Warzywoda, Albert Sacco Jr. * Center for Advanced Microgravity Materials Processing, Department of Chemical Engineering, Northeastern University, 360 Huntington Avenue, 147 Snell Engineering Center, Boston, MA 02115, USA Received 19 December 2003; received in revised form 3 March 2004; accepted 10 March 2004 Available online 17 April 2004
Abstract Zeolite Beta was grown in the microgravity environment (103 –105 g) of the International Space Station from the precursor solutions held unmixed until activation on orbit. The space-grown product had the same spheroidal and truncated square bipyramidal particle morphology, and close to identical ‘‘surface’’ and framework Si/Al ratio, and the same unit cell dimensions as the terrestrial/control product. However, the flight particles were 10% larger on average. The less intense terminal silanol infrared band acquired for the flight particles was consistent with their larger average size, but may also indicate a more uniform or ‘‘smoother’’ surface. The Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone with 2-propanol performed using the heat-treated flight samples showed lower catalyst activity and higher tr-4-tert-butylcyclohexanol selectivity when compared with the terrestrial/controls. This suggests smaller amounts of aluminum partially coordinated to the framework (as characterized by the 3670 cm1 infrared band), and more space being available in the straight channels of the flight zeolite Beta for hydrogen transfer (i.e., no steric hindrances). This is consistent with a higher degree of perfection and order in the space-grown zeolite Beta framework, and higher degree of thermal stability of the flight product. 2004 Elsevier Inc. All rights reserved. Keywords: Characterization; Hydrothermal synthesis; Meerwein–Ponndorf–Verley reaction; Microgravity; Zeolite Beta
1. Introduction Zeolites find extensive use as commercial ion exchangers, adsorbents, and catalysts. Promising new areas of zeolite/zeotype application include selective membranes, chemical sensors, molecular electronics, and stereoselective catalysis. Better utilization of zeolites in the traditional and emerging applications necessitates improved control over their crystal and chemical properties. Insufficient understanding of nucleation and growth of zeolite and zeotype materials usually precludes independent control of important characteristics such as size, morphology, composition, and defect concentration. The objective of this investigation was to explore the low gravity environment of the International
*
Corresponding authors. Tel.: +1-617-373-8702; fax: +1-617-3732209 (B. Akata), Tel.: +1-617-373-7910; fax: +1-617-373-8148 (A. Sacco Jr.). E-mail addresses: [email protected] (B. Akata), asacco@ coe.neu.edu (A. Sacco Jr.). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.03.012
Space Station as a possible means of producing zeolite Beta with a high degree of crystalline perfection. Reduced gravitational forces might influence the zeolite crystallization by minimizing density-driven natural convection and sedimentation, as well as enhancing the role of surface tension [1,2] and in so doing better control nucleation. Thus, the microgravity environment of low earth orbit might promote the growth of crystals with fewer defects. 2. Experimental 2.1. Zeolite synthesis Zeolite Beta was grown at 403 K in the free fall environment (103 –105 g) of the International Space Station (ISS) in a three-zoned zeolite crystal growth (ZCG) furnace. The flight and the terrestrial/control zeolite precursor solutions were prepared from a common batch. These solutions were loaded into specially designed ZCG autoclaves and were held unmixed until
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activation on orbit, or loaded later and activated as controls (typically 2–4 days after launch). The ZCG autoclaves consisted of aluminum (18 units) or titanium (1 unit) cylindrical housing containing two separate Teflon-lined sample chambers and a mixing mechanism. On orbit, the ZCG autoclaves were inserted into annular, furnace tubes with axial and radial temperatures controlled to ±1 and ±0.1 K, respectively. Precursor solutions were mixed by initially injecting the solutions, one into the other, followed by moving the solutions from one sample chamber to another. During this ‘‘activation’’ process a perforated paddle was continuously rotating. Then the furnace was activated and allowed to operate for a predetermined duration, typically 15 days, virtually unattended, with computer control and data acquisition. Terrestrial/control samples were identically processed on earth in the ZCG autoclaves in a ground furnace identical to the flight unit. The ground unit was covered with an insulating blanket to reduce convective losses to match ISS heat-up and cool-down rates. The zeolite Beta formulation used (2.2Na2 O:Al2 O3 : 30SiO2 :4.6(TEA)2 O:444H2 O, where TEA ” tetraethylammonium) was prepared from two precursor solutions. A sodium aluminate precursor solution was prepared by dissolving sodium aluminate (technical, EM Science) in a hot solution of sodium hydroxide (NaOH) made from NaOH pellets (97+%, Aldrich) and deionized water (resistivity > 18 MX cm). After cooling to room temperature, the sodium aluminate solution was filtered through 0.1 lm polysulfone filter membrane (Gelman) and tertaethylammonium hydroxide (35% aqueous solution, Alfa Aesar) was added. The silica containing precursor solution was prepared by mixing Dupont Ludox HS-40 colloidal silica (40% suspension in water, Aldrich) with deionized water. 2.2. Materials characterization X-ray powder diffraction (XRD) data were collected on a Bruker D5005 diffractometer (CuKa radiation) equipped with a curved graphite crystal diffracted beam monochromator and a NaI scintillation detector. Refinement of the lattice parameters was accomplished by the least-squares method. The peak positions were corrected by applying a calibration curve generated by fitting a second order polynomial to the known peak positions of a lanthanum hexaboride powder (NIST SRM 660a) used as the external standard. Field emission scanning electron microscopy (FE-SEM) of uncoated samples was performed on a Hitachi S-4700 FE-SEM in the secondary electron imaging mode using the upper detector, 3 mm working distance, 2 kV accelerating voltage, and 10 lA emission current. The particle size distributions (PSDs) were measured on an API Aerosizer LD equipped with an API Aero-Disperser dry powder dispersion system (TSI, Inc.) that
enabled a sample of dry powder to be dispersed in the air stream prior to sizing. For these measurements the density of zeolite Beta was assumed to be 1610 kg/m3 [3]. The measurement for each sample was performed three times and the results were reproducible to less than 2% for all samples. The energy dispersive X-ray spectroscopy (EDX) analysis was carried out utilizing a Phoenix EDAX X-ray analyzer equipped with sapphire super ultra thin window detector attached to the Hitachi S-4700 FE-SEM (5 kV accelerating voltage, 10 lA emission current). Thermogravimetric (TG) analysis was carried out on a Mettler TG50 thermobalance. Samples were heated from 308 to 1173 K at a rate 5 K/min under dry air flow (200 ml/min, NTP) in 150 ll alumina crucibles. Infrared (IR) studies were conducted on a Nicolet Magna-IR 560 spectrometer equipped with a DTGS KBr detector, using a DRIFT chamber with KBr windows (high temperature/vacuum chamber, SpectraTech Corp). The chamber was configured so that dry nitrogen (99.9%, moisture content < 10 ppm, Med-Tech Gases) was purged continuously over the sample at 30 ml/min (NTP) as it was heated and cooled. Fine powder of each sample (0.02 g) was loosely packed into a ceramic sample cup located inside the chamber and flattened using a glass microscope slide. A thermocouple was located in the sample cup and temperature was controlled to ±1 K (Eurotherm). Potassium bromide (KBr, 99+%, infrared grade, Acros) was used as the background, and the samples were analyzed neat in the OH stretching region (4000–3200 cm1 ) at room temperature (293 K) and after cooling from the appropriate heat treatment temperature to 373 K. Analysis in the framework vibration region (1250–450 cm1 ) was accomplished at room temperature using untreated and heat treated samples mixed with KBr (1/200 weight ratio). Before collecting any spectrum, nitrogen was purged through the beam path at 14 l/min (NTP). The spectra were collected using 128 scans and resolution 2 cm1 . Ammonia desorption analysis was performed using a conventional TCD-TPD equipment. The samples (0.02 g) were activated using the appropriate heat treatment procedure (Section 2.3, samples F/T-Act, Table 1) inside the TPD reactor. This was followed by adsorption of ammonia at 423 K in order to diminish the extend of physical adsorption [4]. Before desorption experiments started, further precautions were taken by purging the samples with adsorbed ammonia overnight with ultra high purity helium (Med-Tech Gases) flowing at a rate 33 ml/min (STP). The adsorbed ammonia was desorbed by heating the sample from 423 to 873 K, at a rate 10 K/min under helium flow (33 ml/min, STP). The concentration of acid sites was determined by deconvoluting the experimental TPD profiles according to the method described by Niwa et al. [5,6].
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Table 1 Heat treatment procedures applied to as synthesized flight and terrestrial/control samples Sample
F/T-Act F/T-mild F/T-harsh a b
Pretreatment (Furnace)a
Activation (Reactor/IR chamber)b
Final temperature (K)
Heating rate (K/min)
Duration at final temperature (h)
Final temperature (K)
Heating rate (K/min)
Duration at final temperature (h)
– 773 973
– 1 10
– 10 10
823 973 773
1 1 1
6 6 6
Performed under stagnant ambient air. Performed under dry nitrogen (flow rate 33 cc/min, STP).
2.3. Sample pretreatment/activation procedure Portions of the flight (F) and the terrestrial/control (T) sample in the as-synthesized form were subjected to three different heat treatment procedures (pretreatment and/or activation, Table 1). Details on these pretreatment and ‘‘activation’’ procedures are discussed elsewhere [7]. One heat treatment procedure consisted of heating as-synthesized F and T samples under nitrogen flow (33 cc/min, STP) to 823 K at a rate 1 K/min, maintaining 823 K for 6 h, and then cooling to 373 K (MPV reaction temperature) at a rate 2 K/min both in the IR spectrometer and in the reactor (samples F/TAct, Table 1). The other two heat treatment procedures were based on a recent study conducted using H-Beta samples that resulted in dissociation of intensities of the 3780 and 3670 cm1 IR band [7]. These procedures resulted in two sample pairs (F/T-mild and F/T-harsh, Table 1). 2.4. Catalytic experiments The Lewis acid catalyzed Meerwein–Ponndorf–Verley (MPV) reduction of 4-tert-butylcyclohexanone (4-TBCH) with 2-propanol to a mixture of cis- and tr-4tert-butylcyclohexanols (4-TBCHLs) has been suggested as an ideal test reaction to study the changes in the zeolite Beta framework induced by heat treatment because the zeolite activity in this reaction is highly dependent on the sample activation procedure [8,9]. It has been recently proposed that Lewis acidity responsible for the catalytic activity of zeolite Beta in the MPV reaction is associated with the presence of octahedrally coordinated Al atoms, which are still partially attached to the framework [10] and stem from the reconfiguration of some tetrahedral (framework) Al atoms during zeolite heat treatment [10,11]. Octahedral symmetry of these Al atoms was postulated to arise by their coordination with water molecules [12,13]. The MPV reaction occurs inside the pores of zeolite Beta [9]. It is believed that the transition states for the formation of cis-4-TBCHL and tr-4-TBCHL differ substantially in size and the tr-alcohol requires bulkier transition state [9]. It was also hypothesized that the higher cis- than tr-alcohol selectivity in the zeolite Beta-catalyzed MPV reaction is due
to the fact that the transition state for cis-alcohol can be easily accommodated within the straight channels of zeolite Beta, whereas the transition state for tr-alcohol cannot [14]. Tr-alcohol production is believed to be inhibited either by the pore dimensions or the blockage of the pores caused by structural distortions such as dealumination of the catalyst [9]. Therefore, the MPV reaction is a convenient method to assess and compare the ease of dealumination, pore structure, and thermal stability of the flight and the terrestrial/control zeolite Beta crystals. The MPV reactions were carried out in a vertical, down flow, fixed bed stainless steel reactor at 373 K under atmospheric pressure. The reactor consisted of a 12.7 mm i.d. stainless steel tube placed inside a threezone furnace (vertical split-tube furnace, Thermcraft Inc.) where the isothermal zone temperature was controlled to ±1 K. The reactants were 2-propanol (IPA, HPLC grade, Acros) and 4-tert-butylcyclohexanone (4TBCH, 99%, Acros). Prior to reaction 4-TBCH was purified by dissolving the 4-TBCH crystals in IPA, recrystallizing them by evaporation, and filtering (qualitative filter paper, grade 1 (11 lm), Whatman). This procedure increased the purity of 4-TBCH from 99.0% to 99.9%. The IPA/4-TBCH mixture (60 g of IPA per 1 g of 4-TBCH) was introduced with a controlled rate of 10 ml/h using a syringe pump (kdScientific, model 100, ±1% accuracy). The catalyst sample (0.02 g) was mixed with quartz sand (3 g, 3Q-Rock, US Silica) and then loaded into the reactor. 4-TBCH was delivered to the reactor using dry nitrogen (99.9%, moisture content < 10 ppm, Med-Tech Gases) as a carrier gas (110 ml/min, STP). Reaction products were analyzed using an on-line Agilent 6890 gas chromatograph equipped with an FID detector, and a 30-m, 5% phenyl methyl siloxane capillary column. Before the catalytic studies a ‘‘blank’’ experiment was performed using the identical temperature, feed composition, and nitrogen flow conditions but over only quartz sand. This resulted in no cis-4-TBCHL in the product and no more than 3% tr-4-TBCHL selectivity (activity < 1%). All catalytic data described were collected after steady-state for conversion and selectivity was reached (i.e., between 6 and 10 h). Initial tests were performed to determine whether external mass transfer limitations existed under the conditions of
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B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9 1
dNumber/dlog(Diameter)
- Flight 0.8
- Terrestrial/ control
0.6 0.4 0.2 0 0.1
1
10
Geometric Diameter (µm) Fig. 2. PSDs of flight and terrestrial/control zeolite Beta samples.
Fig. 1. FE-SEM images of (a) flight and (b) terrestrial/control zeolite Beta samples.
this study. The conversion as well as the cis- and tr-4TBCHL selectivities did not change after 6 ml/h, when the flow rate of the reactants was varied between 2 and 10 ml/h at constant space velocity. Therefore, a flow rate of 10 ml/h was used in all the catalytic experiments. The activity of zeolite Beta in the MPV reaction depends on the catalyst particle size i.e., internal mass transfer limitations [9,10]. The critical diameter of a zeolite Beta catalyst particle, which would result in the internal diffusion limitations under the experimental conditions used, was calculated to be 230 lm. Since particles in both the flight and the terrestrial/control samples were much smaller than 230 lm (i.e., 0.3–1.5 lm; Figs. 1 and 2), it is unlikely that any observed difference in the activity is due to internal mass transfer limitations.
3. Results and discussion The XRD patterns of as-synthesized F and T samples matched the literature XRD patterns of as-synthesized zeolite Beta [15,16], and thus indicated that both the flight and the terrestrial/control products were pure materials. Table 2 illustrates the tetragonal unit cell parameters determined for as-synthesized F and T
samples using six sharp reflections on the XRD pattern in the 5–35 2h range [3], and shows that the unit cell parameters of both flight and terrestrial/control zeolite Beta were identical within experimental error. The EDX analysis yielded the identical ‘‘surface’ Si/Al ratio for assynthesized F and T samples (Table 2). The FE-SEM analysis (Fig. 1) indicated the same morphology (spheroidal and truncated square bipyramidal particles) of F and T samples. As illustrated in Figs. 1 and 2, particles in both products were predominantly in the 0.3–1.5 lm size range, however, the PSD analysis (Fig. 2, Table 2) suggested that the flight particles are slightly larger on average (10%) that the terrestrial/control particles. The differential thermogravimetric (DTG) analysis (Fig. 3b) revealed four distinct zones of weight loss for as-synthesized F and T samples, in agreement with literature [16,17]. Both samples were characterized by the identical, within sensitivity of the instrument, temperature range of each zone (DTG analysis, Fig. 3b) and by the identical weight loss in each zone (TG analysis, Fig. 3a). The sum of weight losses in zones III and IV is a reasonable estimation of the amount of TEAþ compensating the negative charge of the zeolite Beta framework [16,17]. Calculations using this concept showed 5 TEAþ per unit cell for both samples. The identical ‘‘surface’’ Si/Al ratio of 11.0 measured for assynthesized F and T samples (Table 2) gives 5.3 Al atoms per unit cell of each material. Thus, it appears that 94% of the negative charge is counter-balanced by TEAþ in as-synthesized flight and terrestrial/control zeolite Beta samples. This leaves 6% of the framework charge compensated by Naþ . This is a reasonable result because for samples exhibiting less than 6 Al atoms per unit cell, less than 5% of the negative charge is counterbalanced by Naþ [17]. These results suggest the same framework Si, Al composition of as-synthesized flight and terrestrial/control zeolite Beta. Fig. 4 illustrates the room temperature OH stretching region IR spectra of as-synthesized F and T samples, which were analyzed without any prior heat treatment.
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
5
Table 2 Tetragonal unit cell parameters of as synthesized and heat-treated flight and terrestrial/control samples, and Si/Al ratios and average particle size of as synthesized flight and terrestrial/control samples Sample F T F-harsh T-harsh
Unit cell parameters c a (A) ESD (A)
c (A)
ESD (A)
12.4751 12.4749 12.4711 12.4619
26.5909 26.5817 26.3318 26.3305
0.0244 0.0244 0.0239 0.0239
0.0102 0.0102 0.0102 0.0102
Si/Ala
APS (lm)b
11.0 1.0 11.0 1.0 – –
0.84 0.01 0.76 0.01 – –
c
a
EDX analysis. Average particle size. c Estimated standard deviation.
(a)
›
30
Flight
20
Terrestrial/ control
Absorbance
Weight Loss (% dry weight)
b
10
Flight Terrestrial/ control
0 4000
(b)
3900
3800
3700
3600
3500
3400
dm/dT (a. u.)
Wavenumbers (cm-1) Fig. 4. IR spectra of as synthesized flight and terrestrial/control zeolite Beta samples examined in the 4000–3400 cm1 range at 293 K without any prior heat treatment.
zone II zone IV Flight
zone III
Terrestrial/ control
zone I 350
450
550
650
750
850
950 1050 1150
Temperature ( K) Fig. 3. (a) TG and (b) DTG patterns of as synthesized flight and terrestrial/control zeolite Beta samples.
Only one IR band was observed in this region with maximum at 3734 cm1 . This band is assigned to terminal silanol groups on the outer crystal surface [15,18]. As illustrated in Fig. 4, the flight sample exhibited a less intense terminal silanol IR band compared to the terrestrial/control sample. This may indicate a lower roughness of the flight crystals. This is consistent with what we have reported for zeolites A and X [19] and for ZSM-5 [20], and is in agreement with the work of Coker et al. [21]. An alternate explanation could be that this may indicate smaller cumulative external surface area of the flight crystals [22]. This is consistent with their larger average particle size (Fig. 2, Table 2). Since we were unable to image these crystals using AFM the exact explanation is not known. Table 3 illustrates the wave number of the T–O stretching vibration observed for samples examined in
Table 3 The wave number of T–O stretching vibration observed in as synthesized and heat-treated flight and terrestrial/control samples, and the corresponding framework Si/Al ratio calculated using the correlation from Ref. [23] Sample
Wave number (cm1 )
Si/Al
F T F-mild T-mild F-harsh T-harsh
1084 1084 1097 1097 1101a 1103a
10.8 10.8 148.1 148.1 – –
a
Value outside the range for which the correlation from Ref. [23] is applicable.
the framework vibration region and the corresponding framework Si/Al ratio calculated using the linear correlation between the wave number of the T–O stretching vibration and the number of framework Al atoms per unit cell [23]. As shown in Table 3, as-synthesized F and T samples had identical T–O stretching vibrations. This vibration corresponded to a framework Si/Al ratio of 10.8, which matches well the ‘‘surface’’ Si/Al ratio measured by EDX (Table 2). After the ‘‘mild’’ heattreatment, the frequency of the T–O stretching vibration increased to the same wave number for both samples, implying that the framework of both samples was
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
Absorbance
›
dealuminated to the same extent (Table 3). The ‘‘harsh’’ heat treatment resulted in a higher T–O stretching vibration (further dealumination), which appeared to be slightly lower for the flight sample (Table 3). Although within experimental error, directionally this is consistent with flight crystals being more thermally stable. The values of unit cell parameters for the ‘‘harshly’’ heat treated samples (Table 2) were qualitatively consistent with the conclusion drawn from the analysis of the wave number of T–O stretching vibration in these samples. The results illustrated in Table 3 are consistent with other results published in literature [23–25] and indicate a progressively higher dealumination of zeolite Beta upon harsher heat treatment conditions. The IR spectra of F/T-Act samples in the OH stretching region are illustrated in Fig. 5, and show three bands at 3745, 3670, and 3610 cm1 . The classical vibrations at 3610 and 3745 cm1 are due to the framework bridging acidic OH and terminal silanol groups, respectively [15,18]. The intensity of the IR band at 3670 cm1 was very low and virtually identical for F/ T-Act samples (Fig. 5). The true nature of this band is still a matter of debate; it was assigned to OH groups associated with either octahedral extraframework (EFAl) [15,23] or partially framework octahedral (PFAl) Al species [12,13,26]. Our recent study [7] indicated that the species associated with the 3670 cm1 IR band are likely responsible for the cis-alcohol selectivity in the zeolite Beta catalyzed MPV reduction of 4TBCH with IPA. The IR ban at 3780 cm1 was not observed in F/T-Act samples (Fig. 5). This band was correlated with the activity of zeolite Beta in the liquid phase MPV reaction [8,9]. It is assigned to the OH groups attached to tricoordinated Al atoms connected to the framework by two oxygen bonds [15,23,27]. The absence of the 3780 cm1 band and the very low intensity of the 3670 cm1 band in F/T-Act samples is likely due to the fact that the heat treatment used to prepare these samples (Table 1) was not severe enough
F-Act T-Act
4000
3900
3800
3700
3600
3500
3400
Wavenumbers (cm-1) Fig. 5. IR spectra of F/T-Act samples (Table 1) examined in the 4000– 3400 cm1 range at 373 K.
60
Conversion/Selectivity (%)
6
Conversion tr -4-TBCHL cis -4-TBCHL
50 40 30 20 10 0 F-Act
Sample
T-Act
Fig. 6. Selectivity and conversion in the MPV reduction of 4-TBCH to cis- and tr-4-TBCHL over F/T-Act samples (Table 1).
to create Lewis sites in amounts sufficient to give rise to these IR bands. The steady-state results of the catalytic reduction of 4TBCH with IPA to cis- and tr-4-TBCHL using F/T-Act samples are shown in Fig. 6. As illustrated in Fig. 6, the F-Act samples resulted in a 1% conversion of 4-TBCH compared to 6% for the T-Act samples. At steadystate, the terrestrial/control sample was more active for the gas phase MPV reaction than the flight sample. It has been hypothesized that the activity of zeolite Beta in the MPV reaction is related to the aluminum that is only partially attached to the framework [8–10]. Since the only difference in the catalytic experiments performed using F/T-Act samples was where the catalyst sample was synthesized, the higher activity of the terrestrial/ control sample can be attributed to more aluminum atoms that are only partially attached to the zeolite Beta framework. Since the Si/Al ratio in both as synthesized samples (Tables 2 and 3) and the mass of both samples used in the MPV reactions were identical, this indicates a higher extent of partial destruction of the framework in the terrestrial/control sample. Therefore, the flight sample appears to be more thermally stable, having fewer defects and better crystallographic order. This conclusion is further supported by the selectivity results. As shown in Fig. 6, the cis-4-TBCHL selectivity was higher than the tr-4-TBCHL selectivity in the MPV reactions utilizing samples F/T-Act as catalysts. This is consistent with the literature data [8,9,25,28]. Higher cisthan tr-alcohol selectivity is due to the fact that the cistransition state can be easily accommodated within the straight channels of zeolite Beta, whereas the bulkier trtransition state cannot [8,9]. The bulkier tr-transition state may fit in the intersections of channels of zeolite Beta, however, the presence of catalytically active site at these positions has been questioned [9]. The results also indicate (Fig. 6) that the cis-4-TCBHL selectivity of the MPV reaction performed over sample T-Act was slightly higher than selectivity recorded for F-Act sample, but was within the error limits. This is supported by little or
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
0.06
mmol NH3 min-1 g-1
Flight 0.05
Table 4 Concentration ðA0 Þ of Lewis (low-T ) and Brønsted (high-T ) acid sites for F-Act and T-Act samples measured by TPD of ammonia A0 (mmol g1 )
Sample F-Act T-Act
Low-T
High-T
0.12 0.17
0.57 0.62
(a) F-mild
F-harsh
Absorbance
no difference of the intensity of the 3670 cm1 band associated with this selectivity [7]. On the other hand, the tr-4-TBCHL selectivity was higher when performed over the flight sample (Fig. 6, F-Act). Since the tralcohol requires bulkier transition state [8,9,29], the higher tr-alcohol selectivity of F-Act sample implies more space to accommodate this bulkier transition state. This is consistent with the flight crystals being less active (Fig. 6), due to the lower extent of partial destruction of their framework (i.e., partially connected octahedral Al atoms coordinated with water molecules [8,10], could be expected to restrict the available pore volume). The results of ammonia TPD carried out on F/T-act samples are shown in Fig. 7. The deconvolution of the experimental TPD profiles for ammonia desorption in the 423–673 K temperature range for F/T-Act samples resulted in two peaks, consistent with the results of Camiloti et al. [4] and Hedge et al. [30]. These authors showed that the amount of ammonia desorbed in the lower temperature range (low-T peak) characterizes structural defects, which were suggested to be the potential Lewis acid sites. The amount of ammonia desorbed in the higher temperature range (high-T peak) was shown to quantify the concentration of Brønsted acid sites in zeolite Beta [4,30]. The analysis of the low-T peak indicated that the concentration of Lewis acid sites is higher for sample T-Act than for sample F-Act (Table 4). These results are consistent with the observed higher activity (6% versus 1% conversion) of T-Act samples (Fig. 6). The results shown in Table 4 for the high-T peak, indicating higher concentration of Brønsted acid sites for sample T-Act, are consistent with the IR spectra of F/T-Act samples illustrated in Fig. 5. The IR spectra of samples F/T-mild and F/T-harsh (Fig. 8) showed four bands (3780, 3745, 3670, and 3610 cm1 ) in the OH stretching region. However, the effect of the ‘‘mild’’ and the ‘‘harsh’’ heat treatment was somewhat different for the flight and the terrestrial/ control sample. As shown in Fig. 8a and Table 5, the
7
(b) T- mild
T-harsh
3900
3800
3700
3600
3500
Wavenumbers (cm ) -1
Fig. 8. IR spectra of (a) F-mild and F-harsh (b) T-mild and T-harsh samples (Table 1) examined in the 3900–3500 cm1 range at 373 K.
Table 5 Relative intensity of the 3780 and 3670 cm1 bands in heat-treated flight and terrestrial/control samples Sample
Relative intensity of the 3670 cm1 band
Relative intensity of the 3780 cm1 band
F-mild T-mild F-harsh T-harsh
0.033 0.040 0.025 0.028
0.059 0.071 0.060 0.135
Terrestrial/ control
0.04 0.03 0.02 0.01 0
400 450 500 550 600 650 700 750 800 850 900
Temperature ( K) Fig. 7. TPD profiles for ammonia desorption for F/T-Act samples (Table 1).
harsher heat treatment of sample F (F-mild versus Fharsh) did not lead to a significant change in the relative intensities of the 3780 cm1 band (2%), and resulted only in a very slight decrease of the intensity of the 3670 cm1 band (24%). The harsher heat treatment of sample T (T-mild versus T-harsh) led to a larger increase in the relative intensity of the 3780 cm1 band (47 %) and a slightly greater decrease in the relative intensity of the 3670 cm1 band (30%), as shown in Fig. 8b and Table 5. These results support the idea that the terrestrial/control sample can be more easily
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
influenced by heat-treatment than the flight sample, particularly with respect to the Al species characterized by the IR band at 3780 cm1 . The trend of the decreasing relative intensity of 3670 cm1 band and the increasing relative intensity of 3780 cm1 band upon harsher heat treatment of the terrestrial/control sample is consistent with what was observed in our previous study [7] utilizing H-Beta samples as starting materials. The larger shift of position of the IR band in the T–O stretching region (Table 3) as well as an increase in the relative intensity of the 3780 cm1 band observed in sample T (Table 5) together indicate higher thermal stability of the flight samples. The MPV reaction was used to compare the changes in the zeolite Beta framework induced by ‘‘mild’’ and ‘‘harsh’’ heat treatment of both flight and terrestrial/ control samples. As shown in Fig. 9, the conversion of 4TBCH over both F-mild and F-harsh samples was identical (13.5%) and lower than the conversion over T-mild and T-harsh samples (20%). The amount of cis4-TBCHL produced over the same amount of catalyst was found to be the same for all samples except for the catalyst T-mild, which resulted in the largest amount of cis-alcohol (Fig. 10). Comparison of the results shown in Table 5 and Fig. 8 indicated that sample T-mild had also the highest relative intensity of the 3670 cm1 band with the remaining samples characterized by lower, virtualy identical relative intensity of this band. This is consistent with our previous study, which showed the correlation between the catalyst activity and the relative intensity of the 3670 cm1 band [7]. However, the maximum amount of tr-4-TBCHL was produced over sample F-mild, other samples resulting in the lower and nearly identical amount of tr-4-TBCHL (Fig. 10). This shows that sample F heat treated mildly had more space in the pores available for the formation of transition state for tr -alcohol. After the harsh heat treatment of sample F, the formation of more extraframework Al (EFAl) species (higher dealumination of sample F-harsh than sample F-mild, Table 3) might have blocked the
25
Conversion (%)
20 15 10 5 0 F-mild
T-mild
F-harsh
T-harsh
Sample Fig. 9. Effect of heat treatment on the conversion of 4-TBCH over F/Tmild and F/T-harsh samples (Table 1).
80
Amount of 4-TBCHLx10 3 (mmol g-1 h-1)
8
cis -4-TBCHL tr -4-TBCHL
60
40
20
0 F-mild
T-mild
F-harsh
T-harsh
Sample Fig. 10. Amount of cis- and tr-4-TBCHL produced over F/T-mild and F/T-harsh samples (Table 1) at steady-state.
pores to decrease the space available for the transition state for tr-alcohol being produced over sample F-harsh. The lower and constant tr-alcohol production over samples T-mild and T-harsh, can be due to the already blocked pores of sample T even after the mild heat treatment, since the dealumination of the terrestrial/ control sample even after the mild heat treatment was enough to limit the production of tr-alcohol (Fig. 10). These results (Figs. 9 and 10) are again consistent with the hypothesis that the crystals synthesized in space are more ordered and therefore have higher thermal stability. 4. Conclusions Space-grown pure zeolite Beta had the same spheroidal and truncated square bipyramidal particle morphology, identical ‘‘surface’’ and framework Si/Al ratio and unit cell dimensions as the terrestrially grown pure zeolite Beta. However, the average particle size was 10% larger for the flight than for the terrestrial/control product. The flight particles were characterized by the less intense terminal silanol IR band compared to the terrestrial/control particles. This is consistent with other zeolites grown in space and is likely the result of reduced surface roughness, but could also be due to the slightly smaller particle size of the terrestrial/control crystals. The heat-treated flight zeolite Beta exhibited lower activity in the MPV probe reaction than identically heattreated terrestrial/control zeolite Beta. These results, hypothesized to be due to the smaller amount of Al atoms partially coordinated to the framework and characterized by the 3670 cm1 band, indicate the higher thermal stability of the space-grown zeolite framework. The higher tr-alcohol selectivity in the MPV reaction was obtained using the flight zeolite Beta. This indicates more space available in the flight zeolite Beta framework, suggesting a smaller extent of blockage of the pores after heat treatment of the flight product. Taken together the results of the catalytic experiments
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
suggest a higher degree of perfection and order in the space-grown material. Acknowledgements The authors would like to thank NASA for financial support.
References [1] L.B. Sand, A. Sacco Jr., R.W. Thompson, A.G. Dixon, Zeolites 7 (1987) 387. [2] A. Sacco Jr., Proc. Soc. Photo-opt. Instrum. Eng. 1557 (1991) 6. [3] J.B. Higgins, R.B. La Pierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr, W.J. Rohrbaugh, Zeolites 8 (1988) 446. [4] A.M. Camiloti, S.L. Jahn, N.D. Velasco, L.F. Moura, D. Cardoso, Appl. Catal. A 182 (1999) 107. [5] M. Niwa, N. Katada, Catal. Surv. Jpn. 1 (1997) 215. [6] Y. Miyamoto, N. Katada, M. Niwa, Micropor. Mesopor. Mater. 40 (2000) 271. [7] B. Akata, J. Warzywoda, A. Sacco Jr., J. Catal. 222 (2004) 397. [8] P.J. Kunkeler, B.J. Zuurdeeg, J.C. van der Waal, J.A. van Bokhoven, D.C. Koningsberger, H. van Bekkum, J. Catal. 180 (1998) 234. [9] E.J. Creyghton, S.D. Ganeshie, R.S. Downing, H. van Bekkum, J. Mol. Catal. A 115 (1997) 457. [10] J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. van Bekkum, J. Catal. 211 (2002) 540. [11] G.H. Kuehl, H.K.C. Timken, Micropor. Mesopor. Mater. 35–36 (2000) 521. [12] E. Bourgeat-Lami, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula, T. des Courieres, Appl. Catal. 72 (1991) 139. [13] C. Jia, C.P. Massiani, D. Barthomeuf, J. Chem. Soc. Faraday Trans. 89 (1993) 3659.
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[14] E.J. Creyghton, S.D. Ganeshie, R.S. Downing, H. van Bekkum, J. Chem. Soc. Chem. Commun. (1995) 1859. [15] I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker Jr., R. Millini, C. Perego, G. Bellussi, J. Phys. Chem. 98 (1994) 4627. [16] J. Perez-Pariente, J.A. Martens, P.A. Jacobs, Appl. Catal. 31 (1987) 35. [17] F. Vaudry, F. diRenzo, F. Fajula, P. Schulz, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. H€ olderich (Eds.), Zeolites and Related Microporous Materials: State of the Art 1994, Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994, p. 163. [18] A. Jentys, J.A. Lercher, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, vol. 137, Elsevier, Amsterdam, 2001, p. 364. [19] J. Warzywoda, M. Valcheva-Traykova, G.A. Rossetti Jr., N. Bac, R. Joesten, S.L. Suib, A. Sacco, J. Cryst. Growth 220 (2000) 150. [20] J. Warzywoda, N. Bac, G.A. Rossetti Jr., N. van der Puil, J.C. Jansen, H. van Bekkum, A. Sacco, Micropor. Mesopor. Mater. 38 (2000) 423. [21] E.N. Coker, J.C. Jansen, F. Di Renzo, F. Fajula, J.A. Martens, P.A. Jacobs, A. Sacco Jr., Micropor. Mesopor. Mater. 46 (2001) 223. [22] S.-B. Pu, T. Inui, Zeolites 19 (1997) 452. [23] C. Yang, Q. Xu, Zeolites 19 (1997) 404. [24] M. Maache, A. Janin, J.C. Lavalley, J.F. Joly, E. Benazzi, Zeolites 13 (1993) 419. [25] O. Bortnovsky, Z. Sobalik, B. Wichterlova, Z. Bastl, J. Catal. 210 (2002) 171. [26] E. Loeffler, U. Lohse, C. Peuker, G. Oehlmann, L.M. Kustov, V.L. Zholobenko, V.B. Kazansky, Zeolites 10 (1990) 266. [27] A. Vimont, F. Thibault-Starzyk, J.C. Lavalley, J. Phys. Chem. B 104 (2000) 286. [28] J.C. VanderWaal, P.J. Kunkeler, H. vanBekkum, 3rd World Congress on Oxidation Catalysis, 1997, p. 1015. [29] A. Corma, M.E. Domine, S. Valencia, J. Catal. 215 (2003) 294. [30] S.G. Hedge, R. Kumar, R.N. Bhat, P. Ratnasamy, Zeolites 9 (1989) 231.
Characterization of zeolite Beta grown in microgravity Burcu Akata *, Bilge Yilmaz, Siricharn S. Jirapongphan, Juliusz Warzywoda, Albert Sacco Jr. * Center for Advanced Microgravity Materials Processing, Department of Chemical Engineering, Northeastern University, 360 Huntington Avenue, 147 Snell Engineering Center, Boston, MA 02115, USA Received 19 December 2003; received in revised form 3 March 2004; accepted 10 March 2004 Available online 17 April 2004
Abstract Zeolite Beta was grown in the microgravity environment (103 –105 g) of the International Space Station from the precursor solutions held unmixed until activation on orbit. The space-grown product had the same spheroidal and truncated square bipyramidal particle morphology, and close to identical ‘‘surface’’ and framework Si/Al ratio, and the same unit cell dimensions as the terrestrial/control product. However, the flight particles were 10% larger on average. The less intense terminal silanol infrared band acquired for the flight particles was consistent with their larger average size, but may also indicate a more uniform or ‘‘smoother’’ surface. The Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone with 2-propanol performed using the heat-treated flight samples showed lower catalyst activity and higher tr-4-tert-butylcyclohexanol selectivity when compared with the terrestrial/controls. This suggests smaller amounts of aluminum partially coordinated to the framework (as characterized by the 3670 cm1 infrared band), and more space being available in the straight channels of the flight zeolite Beta for hydrogen transfer (i.e., no steric hindrances). This is consistent with a higher degree of perfection and order in the space-grown zeolite Beta framework, and higher degree of thermal stability of the flight product. 2004 Elsevier Inc. All rights reserved. Keywords: Characterization; Hydrothermal synthesis; Meerwein–Ponndorf–Verley reaction; Microgravity; Zeolite Beta
1. Introduction Zeolites find extensive use as commercial ion exchangers, adsorbents, and catalysts. Promising new areas of zeolite/zeotype application include selective membranes, chemical sensors, molecular electronics, and stereoselective catalysis. Better utilization of zeolites in the traditional and emerging applications necessitates improved control over their crystal and chemical properties. Insufficient understanding of nucleation and growth of zeolite and zeotype materials usually precludes independent control of important characteristics such as size, morphology, composition, and defect concentration. The objective of this investigation was to explore the low gravity environment of the International
*
Corresponding authors. Tel.: +1-617-373-8702; fax: +1-617-3732209 (B. Akata), Tel.: +1-617-373-7910; fax: +1-617-373-8148 (A. Sacco Jr.). E-mail addresses: [email protected] (B. Akata), asacco@ coe.neu.edu (A. Sacco Jr.). 1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.03.012
Space Station as a possible means of producing zeolite Beta with a high degree of crystalline perfection. Reduced gravitational forces might influence the zeolite crystallization by minimizing density-driven natural convection and sedimentation, as well as enhancing the role of surface tension [1,2] and in so doing better control nucleation. Thus, the microgravity environment of low earth orbit might promote the growth of crystals with fewer defects. 2. Experimental 2.1. Zeolite synthesis Zeolite Beta was grown at 403 K in the free fall environment (103 –105 g) of the International Space Station (ISS) in a three-zoned zeolite crystal growth (ZCG) furnace. The flight and the terrestrial/control zeolite precursor solutions were prepared from a common batch. These solutions were loaded into specially designed ZCG autoclaves and were held unmixed until
2
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
activation on orbit, or loaded later and activated as controls (typically 2–4 days after launch). The ZCG autoclaves consisted of aluminum (18 units) or titanium (1 unit) cylindrical housing containing two separate Teflon-lined sample chambers and a mixing mechanism. On orbit, the ZCG autoclaves were inserted into annular, furnace tubes with axial and radial temperatures controlled to ±1 and ±0.1 K, respectively. Precursor solutions were mixed by initially injecting the solutions, one into the other, followed by moving the solutions from one sample chamber to another. During this ‘‘activation’’ process a perforated paddle was continuously rotating. Then the furnace was activated and allowed to operate for a predetermined duration, typically 15 days, virtually unattended, with computer control and data acquisition. Terrestrial/control samples were identically processed on earth in the ZCG autoclaves in a ground furnace identical to the flight unit. The ground unit was covered with an insulating blanket to reduce convective losses to match ISS heat-up and cool-down rates. The zeolite Beta formulation used (2.2Na2 O:Al2 O3 : 30SiO2 :4.6(TEA)2 O:444H2 O, where TEA ” tetraethylammonium) was prepared from two precursor solutions. A sodium aluminate precursor solution was prepared by dissolving sodium aluminate (technical, EM Science) in a hot solution of sodium hydroxide (NaOH) made from NaOH pellets (97+%, Aldrich) and deionized water (resistivity > 18 MX cm). After cooling to room temperature, the sodium aluminate solution was filtered through 0.1 lm polysulfone filter membrane (Gelman) and tertaethylammonium hydroxide (35% aqueous solution, Alfa Aesar) was added. The silica containing precursor solution was prepared by mixing Dupont Ludox HS-40 colloidal silica (40% suspension in water, Aldrich) with deionized water. 2.2. Materials characterization X-ray powder diffraction (XRD) data were collected on a Bruker D5005 diffractometer (CuKa radiation) equipped with a curved graphite crystal diffracted beam monochromator and a NaI scintillation detector. Refinement of the lattice parameters was accomplished by the least-squares method. The peak positions were corrected by applying a calibration curve generated by fitting a second order polynomial to the known peak positions of a lanthanum hexaboride powder (NIST SRM 660a) used as the external standard. Field emission scanning electron microscopy (FE-SEM) of uncoated samples was performed on a Hitachi S-4700 FE-SEM in the secondary electron imaging mode using the upper detector, 3 mm working distance, 2 kV accelerating voltage, and 10 lA emission current. The particle size distributions (PSDs) were measured on an API Aerosizer LD equipped with an API Aero-Disperser dry powder dispersion system (TSI, Inc.) that
enabled a sample of dry powder to be dispersed in the air stream prior to sizing. For these measurements the density of zeolite Beta was assumed to be 1610 kg/m3 [3]. The measurement for each sample was performed three times and the results were reproducible to less than 2% for all samples. The energy dispersive X-ray spectroscopy (EDX) analysis was carried out utilizing a Phoenix EDAX X-ray analyzer equipped with sapphire super ultra thin window detector attached to the Hitachi S-4700 FE-SEM (5 kV accelerating voltage, 10 lA emission current). Thermogravimetric (TG) analysis was carried out on a Mettler TG50 thermobalance. Samples were heated from 308 to 1173 K at a rate 5 K/min under dry air flow (200 ml/min, NTP) in 150 ll alumina crucibles. Infrared (IR) studies were conducted on a Nicolet Magna-IR 560 spectrometer equipped with a DTGS KBr detector, using a DRIFT chamber with KBr windows (high temperature/vacuum chamber, SpectraTech Corp). The chamber was configured so that dry nitrogen (99.9%, moisture content < 10 ppm, Med-Tech Gases) was purged continuously over the sample at 30 ml/min (NTP) as it was heated and cooled. Fine powder of each sample (0.02 g) was loosely packed into a ceramic sample cup located inside the chamber and flattened using a glass microscope slide. A thermocouple was located in the sample cup and temperature was controlled to ±1 K (Eurotherm). Potassium bromide (KBr, 99+%, infrared grade, Acros) was used as the background, and the samples were analyzed neat in the OH stretching region (4000–3200 cm1 ) at room temperature (293 K) and after cooling from the appropriate heat treatment temperature to 373 K. Analysis in the framework vibration region (1250–450 cm1 ) was accomplished at room temperature using untreated and heat treated samples mixed with KBr (1/200 weight ratio). Before collecting any spectrum, nitrogen was purged through the beam path at 14 l/min (NTP). The spectra were collected using 128 scans and resolution 2 cm1 . Ammonia desorption analysis was performed using a conventional TCD-TPD equipment. The samples (0.02 g) were activated using the appropriate heat treatment procedure (Section 2.3, samples F/T-Act, Table 1) inside the TPD reactor. This was followed by adsorption of ammonia at 423 K in order to diminish the extend of physical adsorption [4]. Before desorption experiments started, further precautions were taken by purging the samples with adsorbed ammonia overnight with ultra high purity helium (Med-Tech Gases) flowing at a rate 33 ml/min (STP). The adsorbed ammonia was desorbed by heating the sample from 423 to 873 K, at a rate 10 K/min under helium flow (33 ml/min, STP). The concentration of acid sites was determined by deconvoluting the experimental TPD profiles according to the method described by Niwa et al. [5,6].
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3
Table 1 Heat treatment procedures applied to as synthesized flight and terrestrial/control samples Sample
F/T-Act F/T-mild F/T-harsh a b
Pretreatment (Furnace)a
Activation (Reactor/IR chamber)b
Final temperature (K)
Heating rate (K/min)
Duration at final temperature (h)
Final temperature (K)
Heating rate (K/min)
Duration at final temperature (h)
– 773 973
– 1 10
– 10 10
823 973 773
1 1 1
6 6 6
Performed under stagnant ambient air. Performed under dry nitrogen (flow rate 33 cc/min, STP).
2.3. Sample pretreatment/activation procedure Portions of the flight (F) and the terrestrial/control (T) sample in the as-synthesized form were subjected to three different heat treatment procedures (pretreatment and/or activation, Table 1). Details on these pretreatment and ‘‘activation’’ procedures are discussed elsewhere [7]. One heat treatment procedure consisted of heating as-synthesized F and T samples under nitrogen flow (33 cc/min, STP) to 823 K at a rate 1 K/min, maintaining 823 K for 6 h, and then cooling to 373 K (MPV reaction temperature) at a rate 2 K/min both in the IR spectrometer and in the reactor (samples F/TAct, Table 1). The other two heat treatment procedures were based on a recent study conducted using H-Beta samples that resulted in dissociation of intensities of the 3780 and 3670 cm1 IR band [7]. These procedures resulted in two sample pairs (F/T-mild and F/T-harsh, Table 1). 2.4. Catalytic experiments The Lewis acid catalyzed Meerwein–Ponndorf–Verley (MPV) reduction of 4-tert-butylcyclohexanone (4-TBCH) with 2-propanol to a mixture of cis- and tr-4tert-butylcyclohexanols (4-TBCHLs) has been suggested as an ideal test reaction to study the changes in the zeolite Beta framework induced by heat treatment because the zeolite activity in this reaction is highly dependent on the sample activation procedure [8,9]. It has been recently proposed that Lewis acidity responsible for the catalytic activity of zeolite Beta in the MPV reaction is associated with the presence of octahedrally coordinated Al atoms, which are still partially attached to the framework [10] and stem from the reconfiguration of some tetrahedral (framework) Al atoms during zeolite heat treatment [10,11]. Octahedral symmetry of these Al atoms was postulated to arise by their coordination with water molecules [12,13]. The MPV reaction occurs inside the pores of zeolite Beta [9]. It is believed that the transition states for the formation of cis-4-TBCHL and tr-4-TBCHL differ substantially in size and the tr-alcohol requires bulkier transition state [9]. It was also hypothesized that the higher cis- than tr-alcohol selectivity in the zeolite Beta-catalyzed MPV reaction is due
to the fact that the transition state for cis-alcohol can be easily accommodated within the straight channels of zeolite Beta, whereas the transition state for tr-alcohol cannot [14]. Tr-alcohol production is believed to be inhibited either by the pore dimensions or the blockage of the pores caused by structural distortions such as dealumination of the catalyst [9]. Therefore, the MPV reaction is a convenient method to assess and compare the ease of dealumination, pore structure, and thermal stability of the flight and the terrestrial/control zeolite Beta crystals. The MPV reactions were carried out in a vertical, down flow, fixed bed stainless steel reactor at 373 K under atmospheric pressure. The reactor consisted of a 12.7 mm i.d. stainless steel tube placed inside a threezone furnace (vertical split-tube furnace, Thermcraft Inc.) where the isothermal zone temperature was controlled to ±1 K. The reactants were 2-propanol (IPA, HPLC grade, Acros) and 4-tert-butylcyclohexanone (4TBCH, 99%, Acros). Prior to reaction 4-TBCH was purified by dissolving the 4-TBCH crystals in IPA, recrystallizing them by evaporation, and filtering (qualitative filter paper, grade 1 (11 lm), Whatman). This procedure increased the purity of 4-TBCH from 99.0% to 99.9%. The IPA/4-TBCH mixture (60 g of IPA per 1 g of 4-TBCH) was introduced with a controlled rate of 10 ml/h using a syringe pump (kdScientific, model 100, ±1% accuracy). The catalyst sample (0.02 g) was mixed with quartz sand (3 g, 3Q-Rock, US Silica) and then loaded into the reactor. 4-TBCH was delivered to the reactor using dry nitrogen (99.9%, moisture content < 10 ppm, Med-Tech Gases) as a carrier gas (110 ml/min, STP). Reaction products were analyzed using an on-line Agilent 6890 gas chromatograph equipped with an FID detector, and a 30-m, 5% phenyl methyl siloxane capillary column. Before the catalytic studies a ‘‘blank’’ experiment was performed using the identical temperature, feed composition, and nitrogen flow conditions but over only quartz sand. This resulted in no cis-4-TBCHL in the product and no more than 3% tr-4-TBCHL selectivity (activity < 1%). All catalytic data described were collected after steady-state for conversion and selectivity was reached (i.e., between 6 and 10 h). Initial tests were performed to determine whether external mass transfer limitations existed under the conditions of
4
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9 1
dNumber/dlog(Diameter)
- Flight 0.8
- Terrestrial/ control
0.6 0.4 0.2 0 0.1
1
10
Geometric Diameter (µm) Fig. 2. PSDs of flight and terrestrial/control zeolite Beta samples.
Fig. 1. FE-SEM images of (a) flight and (b) terrestrial/control zeolite Beta samples.
this study. The conversion as well as the cis- and tr-4TBCHL selectivities did not change after 6 ml/h, when the flow rate of the reactants was varied between 2 and 10 ml/h at constant space velocity. Therefore, a flow rate of 10 ml/h was used in all the catalytic experiments. The activity of zeolite Beta in the MPV reaction depends on the catalyst particle size i.e., internal mass transfer limitations [9,10]. The critical diameter of a zeolite Beta catalyst particle, which would result in the internal diffusion limitations under the experimental conditions used, was calculated to be 230 lm. Since particles in both the flight and the terrestrial/control samples were much smaller than 230 lm (i.e., 0.3–1.5 lm; Figs. 1 and 2), it is unlikely that any observed difference in the activity is due to internal mass transfer limitations.
3. Results and discussion The XRD patterns of as-synthesized F and T samples matched the literature XRD patterns of as-synthesized zeolite Beta [15,16], and thus indicated that both the flight and the terrestrial/control products were pure materials. Table 2 illustrates the tetragonal unit cell parameters determined for as-synthesized F and T
samples using six sharp reflections on the XRD pattern in the 5–35 2h range [3], and shows that the unit cell parameters of both flight and terrestrial/control zeolite Beta were identical within experimental error. The EDX analysis yielded the identical ‘‘surface’ Si/Al ratio for assynthesized F and T samples (Table 2). The FE-SEM analysis (Fig. 1) indicated the same morphology (spheroidal and truncated square bipyramidal particles) of F and T samples. As illustrated in Figs. 1 and 2, particles in both products were predominantly in the 0.3–1.5 lm size range, however, the PSD analysis (Fig. 2, Table 2) suggested that the flight particles are slightly larger on average (10%) that the terrestrial/control particles. The differential thermogravimetric (DTG) analysis (Fig. 3b) revealed four distinct zones of weight loss for as-synthesized F and T samples, in agreement with literature [16,17]. Both samples were characterized by the identical, within sensitivity of the instrument, temperature range of each zone (DTG analysis, Fig. 3b) and by the identical weight loss in each zone (TG analysis, Fig. 3a). The sum of weight losses in zones III and IV is a reasonable estimation of the amount of TEAþ compensating the negative charge of the zeolite Beta framework [16,17]. Calculations using this concept showed 5 TEAþ per unit cell for both samples. The identical ‘‘surface’’ Si/Al ratio of 11.0 measured for assynthesized F and T samples (Table 2) gives 5.3 Al atoms per unit cell of each material. Thus, it appears that 94% of the negative charge is counter-balanced by TEAþ in as-synthesized flight and terrestrial/control zeolite Beta samples. This leaves 6% of the framework charge compensated by Naþ . This is a reasonable result because for samples exhibiting less than 6 Al atoms per unit cell, less than 5% of the negative charge is counterbalanced by Naþ [17]. These results suggest the same framework Si, Al composition of as-synthesized flight and terrestrial/control zeolite Beta. Fig. 4 illustrates the room temperature OH stretching region IR spectra of as-synthesized F and T samples, which were analyzed without any prior heat treatment.
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
5
Table 2 Tetragonal unit cell parameters of as synthesized and heat-treated flight and terrestrial/control samples, and Si/Al ratios and average particle size of as synthesized flight and terrestrial/control samples Sample F T F-harsh T-harsh
Unit cell parameters c a (A) ESD (A)
c (A)
ESD (A)
12.4751 12.4749 12.4711 12.4619
26.5909 26.5817 26.3318 26.3305
0.0244 0.0244 0.0239 0.0239
0.0102 0.0102 0.0102 0.0102
Si/Ala
APS (lm)b
11.0 1.0 11.0 1.0 – –
0.84 0.01 0.76 0.01 – –
c
a
EDX analysis. Average particle size. c Estimated standard deviation.
(a)
›
30
Flight
20
Terrestrial/ control
Absorbance
Weight Loss (% dry weight)
b
10
Flight Terrestrial/ control
0 4000
(b)
3900
3800
3700
3600
3500
3400
dm/dT (a. u.)
Wavenumbers (cm-1) Fig. 4. IR spectra of as synthesized flight and terrestrial/control zeolite Beta samples examined in the 4000–3400 cm1 range at 293 K without any prior heat treatment.
zone II zone IV Flight
zone III
Terrestrial/ control
zone I 350
450
550
650
750
850
950 1050 1150
Temperature ( K) Fig. 3. (a) TG and (b) DTG patterns of as synthesized flight and terrestrial/control zeolite Beta samples.
Only one IR band was observed in this region with maximum at 3734 cm1 . This band is assigned to terminal silanol groups on the outer crystal surface [15,18]. As illustrated in Fig. 4, the flight sample exhibited a less intense terminal silanol IR band compared to the terrestrial/control sample. This may indicate a lower roughness of the flight crystals. This is consistent with what we have reported for zeolites A and X [19] and for ZSM-5 [20], and is in agreement with the work of Coker et al. [21]. An alternate explanation could be that this may indicate smaller cumulative external surface area of the flight crystals [22]. This is consistent with their larger average particle size (Fig. 2, Table 2). Since we were unable to image these crystals using AFM the exact explanation is not known. Table 3 illustrates the wave number of the T–O stretching vibration observed for samples examined in
Table 3 The wave number of T–O stretching vibration observed in as synthesized and heat-treated flight and terrestrial/control samples, and the corresponding framework Si/Al ratio calculated using the correlation from Ref. [23] Sample
Wave number (cm1 )
Si/Al
F T F-mild T-mild F-harsh T-harsh
1084 1084 1097 1097 1101a 1103a
10.8 10.8 148.1 148.1 – –
a
Value outside the range for which the correlation from Ref. [23] is applicable.
the framework vibration region and the corresponding framework Si/Al ratio calculated using the linear correlation between the wave number of the T–O stretching vibration and the number of framework Al atoms per unit cell [23]. As shown in Table 3, as-synthesized F and T samples had identical T–O stretching vibrations. This vibration corresponded to a framework Si/Al ratio of 10.8, which matches well the ‘‘surface’’ Si/Al ratio measured by EDX (Table 2). After the ‘‘mild’’ heattreatment, the frequency of the T–O stretching vibration increased to the same wave number for both samples, implying that the framework of both samples was
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
Absorbance
›
dealuminated to the same extent (Table 3). The ‘‘harsh’’ heat treatment resulted in a higher T–O stretching vibration (further dealumination), which appeared to be slightly lower for the flight sample (Table 3). Although within experimental error, directionally this is consistent with flight crystals being more thermally stable. The values of unit cell parameters for the ‘‘harshly’’ heat treated samples (Table 2) were qualitatively consistent with the conclusion drawn from the analysis of the wave number of T–O stretching vibration in these samples. The results illustrated in Table 3 are consistent with other results published in literature [23–25] and indicate a progressively higher dealumination of zeolite Beta upon harsher heat treatment conditions. The IR spectra of F/T-Act samples in the OH stretching region are illustrated in Fig. 5, and show three bands at 3745, 3670, and 3610 cm1 . The classical vibrations at 3610 and 3745 cm1 are due to the framework bridging acidic OH and terminal silanol groups, respectively [15,18]. The intensity of the IR band at 3670 cm1 was very low and virtually identical for F/ T-Act samples (Fig. 5). The true nature of this band is still a matter of debate; it was assigned to OH groups associated with either octahedral extraframework (EFAl) [15,23] or partially framework octahedral (PFAl) Al species [12,13,26]. Our recent study [7] indicated that the species associated with the 3670 cm1 IR band are likely responsible for the cis-alcohol selectivity in the zeolite Beta catalyzed MPV reduction of 4TBCH with IPA. The IR ban at 3780 cm1 was not observed in F/T-Act samples (Fig. 5). This band was correlated with the activity of zeolite Beta in the liquid phase MPV reaction [8,9]. It is assigned to the OH groups attached to tricoordinated Al atoms connected to the framework by two oxygen bonds [15,23,27]. The absence of the 3780 cm1 band and the very low intensity of the 3670 cm1 band in F/T-Act samples is likely due to the fact that the heat treatment used to prepare these samples (Table 1) was not severe enough
F-Act T-Act
4000
3900
3800
3700
3600
3500
3400
Wavenumbers (cm-1) Fig. 5. IR spectra of F/T-Act samples (Table 1) examined in the 4000– 3400 cm1 range at 373 K.
60
Conversion/Selectivity (%)
6
Conversion tr -4-TBCHL cis -4-TBCHL
50 40 30 20 10 0 F-Act
Sample
T-Act
Fig. 6. Selectivity and conversion in the MPV reduction of 4-TBCH to cis- and tr-4-TBCHL over F/T-Act samples (Table 1).
to create Lewis sites in amounts sufficient to give rise to these IR bands. The steady-state results of the catalytic reduction of 4TBCH with IPA to cis- and tr-4-TBCHL using F/T-Act samples are shown in Fig. 6. As illustrated in Fig. 6, the F-Act samples resulted in a 1% conversion of 4-TBCH compared to 6% for the T-Act samples. At steadystate, the terrestrial/control sample was more active for the gas phase MPV reaction than the flight sample. It has been hypothesized that the activity of zeolite Beta in the MPV reaction is related to the aluminum that is only partially attached to the framework [8–10]. Since the only difference in the catalytic experiments performed using F/T-Act samples was where the catalyst sample was synthesized, the higher activity of the terrestrial/ control sample can be attributed to more aluminum atoms that are only partially attached to the zeolite Beta framework. Since the Si/Al ratio in both as synthesized samples (Tables 2 and 3) and the mass of both samples used in the MPV reactions were identical, this indicates a higher extent of partial destruction of the framework in the terrestrial/control sample. Therefore, the flight sample appears to be more thermally stable, having fewer defects and better crystallographic order. This conclusion is further supported by the selectivity results. As shown in Fig. 6, the cis-4-TBCHL selectivity was higher than the tr-4-TBCHL selectivity in the MPV reactions utilizing samples F/T-Act as catalysts. This is consistent with the literature data [8,9,25,28]. Higher cisthan tr-alcohol selectivity is due to the fact that the cistransition state can be easily accommodated within the straight channels of zeolite Beta, whereas the bulkier trtransition state cannot [8,9]. The bulkier tr-transition state may fit in the intersections of channels of zeolite Beta, however, the presence of catalytically active site at these positions has been questioned [9]. The results also indicate (Fig. 6) that the cis-4-TCBHL selectivity of the MPV reaction performed over sample T-Act was slightly higher than selectivity recorded for F-Act sample, but was within the error limits. This is supported by little or
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
0.06
mmol NH3 min-1 g-1
Flight 0.05
Table 4 Concentration ðA0 Þ of Lewis (low-T ) and Brønsted (high-T ) acid sites for F-Act and T-Act samples measured by TPD of ammonia A0 (mmol g1 )
Sample F-Act T-Act
Low-T
High-T
0.12 0.17
0.57 0.62
(a) F-mild
F-harsh
Absorbance
no difference of the intensity of the 3670 cm1 band associated with this selectivity [7]. On the other hand, the tr-4-TBCHL selectivity was higher when performed over the flight sample (Fig. 6, F-Act). Since the tralcohol requires bulkier transition state [8,9,29], the higher tr-alcohol selectivity of F-Act sample implies more space to accommodate this bulkier transition state. This is consistent with the flight crystals being less active (Fig. 6), due to the lower extent of partial destruction of their framework (i.e., partially connected octahedral Al atoms coordinated with water molecules [8,10], could be expected to restrict the available pore volume). The results of ammonia TPD carried out on F/T-act samples are shown in Fig. 7. The deconvolution of the experimental TPD profiles for ammonia desorption in the 423–673 K temperature range for F/T-Act samples resulted in two peaks, consistent with the results of Camiloti et al. [4] and Hedge et al. [30]. These authors showed that the amount of ammonia desorbed in the lower temperature range (low-T peak) characterizes structural defects, which were suggested to be the potential Lewis acid sites. The amount of ammonia desorbed in the higher temperature range (high-T peak) was shown to quantify the concentration of Brønsted acid sites in zeolite Beta [4,30]. The analysis of the low-T peak indicated that the concentration of Lewis acid sites is higher for sample T-Act than for sample F-Act (Table 4). These results are consistent with the observed higher activity (6% versus 1% conversion) of T-Act samples (Fig. 6). The results shown in Table 4 for the high-T peak, indicating higher concentration of Brønsted acid sites for sample T-Act, are consistent with the IR spectra of F/T-Act samples illustrated in Fig. 5. The IR spectra of samples F/T-mild and F/T-harsh (Fig. 8) showed four bands (3780, 3745, 3670, and 3610 cm1 ) in the OH stretching region. However, the effect of the ‘‘mild’’ and the ‘‘harsh’’ heat treatment was somewhat different for the flight and the terrestrial/ control sample. As shown in Fig. 8a and Table 5, the
7
(b) T- mild
T-harsh
3900
3800
3700
3600
3500
Wavenumbers (cm ) -1
Fig. 8. IR spectra of (a) F-mild and F-harsh (b) T-mild and T-harsh samples (Table 1) examined in the 3900–3500 cm1 range at 373 K.
Table 5 Relative intensity of the 3780 and 3670 cm1 bands in heat-treated flight and terrestrial/control samples Sample
Relative intensity of the 3670 cm1 band
Relative intensity of the 3780 cm1 band
F-mild T-mild F-harsh T-harsh
0.033 0.040 0.025 0.028
0.059 0.071 0.060 0.135
Terrestrial/ control
0.04 0.03 0.02 0.01 0
400 450 500 550 600 650 700 750 800 850 900
Temperature ( K) Fig. 7. TPD profiles for ammonia desorption for F/T-Act samples (Table 1).
harsher heat treatment of sample F (F-mild versus Fharsh) did not lead to a significant change in the relative intensities of the 3780 cm1 band (2%), and resulted only in a very slight decrease of the intensity of the 3670 cm1 band (24%). The harsher heat treatment of sample T (T-mild versus T-harsh) led to a larger increase in the relative intensity of the 3780 cm1 band (47 %) and a slightly greater decrease in the relative intensity of the 3670 cm1 band (30%), as shown in Fig. 8b and Table 5. These results support the idea that the terrestrial/control sample can be more easily
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
influenced by heat-treatment than the flight sample, particularly with respect to the Al species characterized by the IR band at 3780 cm1 . The trend of the decreasing relative intensity of 3670 cm1 band and the increasing relative intensity of 3780 cm1 band upon harsher heat treatment of the terrestrial/control sample is consistent with what was observed in our previous study [7] utilizing H-Beta samples as starting materials. The larger shift of position of the IR band in the T–O stretching region (Table 3) as well as an increase in the relative intensity of the 3780 cm1 band observed in sample T (Table 5) together indicate higher thermal stability of the flight samples. The MPV reaction was used to compare the changes in the zeolite Beta framework induced by ‘‘mild’’ and ‘‘harsh’’ heat treatment of both flight and terrestrial/ control samples. As shown in Fig. 9, the conversion of 4TBCH over both F-mild and F-harsh samples was identical (13.5%) and lower than the conversion over T-mild and T-harsh samples (20%). The amount of cis4-TBCHL produced over the same amount of catalyst was found to be the same for all samples except for the catalyst T-mild, which resulted in the largest amount of cis-alcohol (Fig. 10). Comparison of the results shown in Table 5 and Fig. 8 indicated that sample T-mild had also the highest relative intensity of the 3670 cm1 band with the remaining samples characterized by lower, virtualy identical relative intensity of this band. This is consistent with our previous study, which showed the correlation between the catalyst activity and the relative intensity of the 3670 cm1 band [7]. However, the maximum amount of tr-4-TBCHL was produced over sample F-mild, other samples resulting in the lower and nearly identical amount of tr-4-TBCHL (Fig. 10). This shows that sample F heat treated mildly had more space in the pores available for the formation of transition state for tr -alcohol. After the harsh heat treatment of sample F, the formation of more extraframework Al (EFAl) species (higher dealumination of sample F-harsh than sample F-mild, Table 3) might have blocked the
25
Conversion (%)
20 15 10 5 0 F-mild
T-mild
F-harsh
T-harsh
Sample Fig. 9. Effect of heat treatment on the conversion of 4-TBCH over F/Tmild and F/T-harsh samples (Table 1).
80
Amount of 4-TBCHLx10 3 (mmol g-1 h-1)
8
cis -4-TBCHL tr -4-TBCHL
60
40
20
0 F-mild
T-mild
F-harsh
T-harsh
Sample Fig. 10. Amount of cis- and tr-4-TBCHL produced over F/T-mild and F/T-harsh samples (Table 1) at steady-state.
pores to decrease the space available for the transition state for tr-alcohol being produced over sample F-harsh. The lower and constant tr-alcohol production over samples T-mild and T-harsh, can be due to the already blocked pores of sample T even after the mild heat treatment, since the dealumination of the terrestrial/ control sample even after the mild heat treatment was enough to limit the production of tr-alcohol (Fig. 10). These results (Figs. 9 and 10) are again consistent with the hypothesis that the crystals synthesized in space are more ordered and therefore have higher thermal stability. 4. Conclusions Space-grown pure zeolite Beta had the same spheroidal and truncated square bipyramidal particle morphology, identical ‘‘surface’’ and framework Si/Al ratio and unit cell dimensions as the terrestrially grown pure zeolite Beta. However, the average particle size was 10% larger for the flight than for the terrestrial/control product. The flight particles were characterized by the less intense terminal silanol IR band compared to the terrestrial/control particles. This is consistent with other zeolites grown in space and is likely the result of reduced surface roughness, but could also be due to the slightly smaller particle size of the terrestrial/control crystals. The heat-treated flight zeolite Beta exhibited lower activity in the MPV probe reaction than identically heattreated terrestrial/control zeolite Beta. These results, hypothesized to be due to the smaller amount of Al atoms partially coordinated to the framework and characterized by the 3670 cm1 band, indicate the higher thermal stability of the space-grown zeolite framework. The higher tr-alcohol selectivity in the MPV reaction was obtained using the flight zeolite Beta. This indicates more space available in the flight zeolite Beta framework, suggesting a smaller extent of blockage of the pores after heat treatment of the flight product. Taken together the results of the catalytic experiments
B. Akata et al. / Microporous and Mesoporous Materials 71 (2004) 1–9
suggest a higher degree of perfection and order in the space-grown material. Acknowledgements The authors would like to thank NASA for financial support.
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