Impacts of Binder-Zeolite Interactions on the Structure and Surface

Article pubs.acs.org/IECR

Impacts of Binder-Zeolite Interactions on the Structure and Surface Properties of NaY−SiO2 Extrudates Nan-Yu Chen,† Ming-Chun Liu,† Shih-Chieh Yang,† Hwo-Shuen Sheu,*,‡ and Jen-Ray Chang*,† †

Department of Chemical Engineering, National Chung Cheng University, Chia-Yi, Taiwan National Synchrotron Radiation Research Center, Hsinchu, Taiwan

‡

Downloaded by TEXAS A&M INTL UNIV on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.iecr.5b01369

S Supporting Information *

ABSTRACT: NaY−SiO2 extrudate was prepared by blending NaY powder with silica gel, followed with kneading, extruding, and calcination. The impact of silica binder on the structure and surface properties of NaY−SiO2 extrudate was investigated. The nitrogen adsorption/desorption isotherm suggests the formation of cylindrical mesopores for NaY−SiO2. X-ray powder diffraction, transmission electron microscopy, and Fourier transform infrared suggest that in kneading, dealumination and cleavage of the intracrystalline Si−O−Al bond via acid hydrolysis occur. This acid hydrolysis causes shrinkage of the zeolite lattice along the zeolite channel due to dislodgement of Al3+ cations. The dealumination increases the silica/alumina ratio of NaY concomitantly with the removal of Na+, leading to an increase in the hydrophobicity of NaY, while reaction of the silica binder with NaY debris and/or aluminum residue forms new Brønsted acid sites. Moreover, backfilling the vacancies created in the dealumination by mobile Si(OH)4 makes the zeolite more thermally stable, which lends the extrudate itself to industrial applications.

1. INTRODUCTION As synthesized, synthetic zeolite is in fine powder form with the particle size in the range of 1−5 μm. Use of such powder in the fixed bed reactor or adsorption column will cause a high pressure drop, leading to reactor plugging and impossible operation; for example, a cylinder fixed bed (D = 1 m, L = 3 m) used for treating 2 m3/min VOCs containing air effluent, the pressure drops through the bed as estimated from the Ergun equation are about 0.14 and 64.7 atm, respectively, for adsorbent with a particle diameter of 2 mm and 5 μm, respectively.1 Hence, for commercial operation, the zeolite powder has to be pelletized with a binder to form a sphere, pellet, or cylindrical extrudate. In the pelletization process to form extrudate, the mixture of the zeolite powder and refractory oxide binder (e.g., alumina and silica) and clay (e.g., kaolin) are thoroughly mixed, wetted with water, and then extruded to form extrudates. The shaped material is then dried and then calcined at high temperatures to achieve necessary mechanical strength and attrition resistance.2 In this extrudate preparation process involving high temperature and pressure, significant interactions between the zeolites and the binder take place. The nature of the binder greatly influences pore structure3−5 and surface acidity via zeolite-binder interactions, leading to changes in catalytic properties, for example, the interaction between alumina binder and zeolite leading to an increase in acid sites and metal−support interactions;6−9 the reaction of solid-state ion exchange between the zeolite proton and clay sodium resulting in a decrease in acidity;10−15 the chemical interaction between phosphorus and alumina-bound zeolite to form crystalline aluminophosphate (AlPO) leading to decreases in meso-porosity and surface acidity of the zeolite extrudate.15 Silica binder is specifically of interest in this study because (1) silica-bound zeolite with low binder acidity makes it useful as a catalyst for hydrocarbon processing, for © XXXX American Chemical Society

example, alkylation, catalytic cracking, dewaxing, aromatization, isomerization, and toluene disproportion;17,18 (2) silica-bound zeolite can be used as an adsorbent for the separation of air into oxygen-rich and nitrogen gases,19 and for the removal of VOCs (volatile organic compounds) from industrial waste gases;20,21 (3) the catalytic and adsorption properties of silica-bound zeolites can be improved by converting silica present in the silicabound zeolite into zeolite;22 and (4) different silica precursors such as fibrous silica colloid can be used to prepare tailor-made catalysts.23 The preparation of zeolite bound by an MFI structure-type zeolite have been investigated by Verduijn et al.22 This catalyst comprises zeolite core crystal bound by a binder which is converted subsequently into different types of zeolites. Since both core and binder zeolite can induce separate reactions, the catalyst is particularly useful for the toluene disproportion process to enhance the conversion of toluene to xylene and to maximize the product selectivity toward the production of paraxylene.22 Recently, Khare disclosed a method to prepare elongated silica-bound zeolite and indicated that the substitution of spherical silica with elongated silica increased the zeolite-tobinder ratio, resulting in an increase in active sites of the catalysts.23 Notwithstanding publication of a number of patents for pelletization of silica-bound zeolite and papers for binder effects,3,8,17,20,24,25 the influence of silica binder on the zeolite structure, including the zeolite framework, unit cell, locations of cation and water molecules, and crystalline-to-amorphous ratio, have rarely been studied. Hence, our research goals are to Received: April 12, 2015 Revised: August 2, 2015 Accepted: August 4, 2015

A

DOI: 10.1021/acs.iecr.5b01369 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

to remove water. The predried extrudates were then calcined in the air using a Carbolite furnace at a heating rate of 10 °C/min to 450 °C and held for 2 h. This sample was noted as NaY−SiO2. The NaY−SiO2 and NaY calcined at different temperatures were noted as NaY-x°C and NaY−SiO2-x°C, respectively, where x stands for treatment temperature. To investigate the thermal stability of NaY and NaY−SiO2 in a moisture laden air environment, 0.5 g of sample was loaded into a quartz cell, and moisture-saturated air (flowing at 20 mL/min) was introduced into the cell. Air was saturated in a water bath at 25 °C. The tests then were conducted by heating the samples at a rate of 10 °C/ min to 450, 600, and 800 °C, respectively, and maintaining the final temperature for 2 h. The tested samples were noted as NaY−x°C(w) and NaY−SiO2−x°C(w), respectively, for NaY and NaY−SiO2,where x stand for temperature. The morphology of amorphous silica binder and the geometry of the resulting NaY−SiO2 extrudate were characterized by scanning electron microscopy (Hitachi S-4800). Pore size distribution of the NaY powder and NaY−SiO2 extrudates was characterized with a Micromeritics Tristar 3000 analyzer. The water content of the samples was measured with a thermal gravity analyzer (TA Instruments 2050 TGA). The TGA measurement was carried out with 10 mg samples heated at a constant rate of 10 °C/min in an atmosphere of N2 at a constant purge rate of 10 mL/min. The TGA spectra were recorded between 20 and 800 °C. The collapsed of the NaY crystal in pelletization was examined by TEM. For TEM measurement, NaY powder and carefully pulverized NaY−SiO2 samples were dispersed in ethanol, fetched on Cu grids, and then dried for later TEM analysis. The TEM (Philips, TECNAI 20) is typically operated at 200 keV. The changes of Si/Al ratio and the distribution of aluminum distribution of the NaY framework during pelletization were investigated by comparing 29Si and 27Al MAS NMR spectra characterizing NaY+SiO2 (physical mixture of 1/3 silica powder and 2/3 NaY by weight) and NaY−SiO2. The NMR measurements were performed on a Bruker Advance 400 spectrometer. 29 Si MAS NMR spectra were recorded at 79.48 MHz using a 3.0 μs pulse, 60 s repetition time, and 880 scans and using a 4 mm MAS NMR rotor with a sample spinning rate of 5.0 kHz. 27Al MAS NMR spectra were recorded at 104.26 MHz using a 0.8 μs pulse, 2.0 s repetition time, and 680 scans and using a 4 mm MAS NMR rotor with a sample spinning rate of 5.0 kHz. The NMR spectra were deconvoluted into several Gaussian peaks, and the relative intensities of these peaks were calculated to estimate the Si/Al ratio. 2.2. Synchrotron XRPD. X-ray powder diffraction (XRPD) was performed at the BL01C2 beamline of the National Synchrotron Radiation Research Center (NSRRC), with a wavelength of 0.9537 Å (13.0 keV). The ring of NSRRC was operated at an energy of 1.5 GeV with a typical current of 360 mA with top-up injection mode. The synchrotron X-ray was produced from a 5.0 T superconducting wavelength shift magnet. Beyond the prefocusing mirror, a double crystal monochromator, which used the Si(111) plane to yield the monochromatic beam, was followed by a refocusing toroidal mirror. The NaY−SiO2 extrudate samples were pulverized into powder. For NaY, NaY+SiO2, and NaY−SiO2 samples, 0.01 g powders were pressed and loaded into an XRPD cell for the measurement. Amorphous silica of 0.0034 g prepared from silica colloid was also measured for comparison. During the X-ray exposure, the sample was kept at a fast spin in order to increase the orientations of crystalline powders, which could provide

examine surface properties and structures of NaY and NaY−SiO2 extrudate, thereby developing fundamental understandings of zeolite−silica binder interactions. The information would be useful for catalyst and adsorbent design and preparation. To implement the research goals, complementary characterization techniques, namely, high-resolution radiation XRPD (X-ray powder diffraction), diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy, N2 adsorption−desorption isothermal, transmission electron microscopy (TEM), and scan electron microscopy (SEM) were used. High-resolution synchrotron radiation XRPD has proven to be very useful in investigation of the zeolite structure. The advantage of synchrotron radiation is its high intensity with low dispersion. Hence, by Rietveld analysis in combination with a difference electron density map, structural parameters and the positions of Al3+, Na+, and water molecules can be extracted from XRPD patterns.26,27 By comparing the structural parameters of NaY powder and those of NaY−SiO2 extrudates, the role of binder in affecting the NaY structure can be rationalized. The Si-to-Al ratio in this study was estimated from cell parameters, and the accuracy was verified by FT-IR spectroscopy and magic angle spinning (MAS) NMR. IR spectra characterizing Si−O and Al−O stretching bands are sensitive to Si−Al composition of the framework. The shift of the stretching bands allows us to explore atom rearrangements in the zeolite framework occurring in the pelletization process.28 Besides T−O stretching frequencies characterizing zeolite lattice observed in the range between 600 and 1300 cm−1,29 results obtained from the IR spectra in the range of O−H stretching between 3000 and 3800 cm−1 can be used to determine Brønsted sites, terminal Si−OH groups, and hydrogen-bonded interactions between adsorbed water and zeolitic lattice. Since the interaction of NH3 with Lewis acid sites is distinctly different from that with Brønsted sites, in situ FT-IR spectra of NH3 chemisorption has also been used to characterize the nature of acid sites for the extrudate and powder samples.

2. EXPERIMENTAL SECTION 2.1. Preparation of NaY−SiO2 Extrudate. NaY−SiO2 extrudate was prepared from NaY powder (Na2O:Al2O3:5.0SiO2:xH2O, GRACE Davison) and silica colloid. The silica colloid was prepared by mixing 68 g of silica powder (T600, PPG Industries) with 120 mL of 3 N nitric acid solution. To measure its acidity, the silica colloid was centrifuged at a speed of 1000 rpm for 5 min, and the pH value of the water extracted from the colloid was 2.6. A 132 g sample of NaY powder was kneaded in a bowl, while the silica colloid was added to make the consistency of the mixture suitable for extrusion. After being kneaded for 20 min and then cured overnight, the workable paste was passed manually into the inlet hopper of the extruder. The extrusion apparatus included a three phase induction motor (Sumi-tomo 0.75 KW), an inverter (AGE Electric Machinery), a single screw extruder operating at a maximum speed of 32 rpm, dies, a solid plastic bar for feeding the paste into the chamber of the extruder, and a support tray for supporting the emerging extrudate. The paste was moved forward from the inlet hopper to the outlet die by the screw. The screw built up a pressure caused by friction force arising from the interaction of the paste with the surface of the screw and the surface of the barrel. The pressure in the paste was used to overcome the resistance of extrusion, allowing paste to flow through the die. The cylindrical extrudates of 2 mm diameter were cut to about 6 mm in length and predried at 120 °C for 8 h B


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introduced into the IR cell and maintained for about 20 min for equilibrium. The cell was then evacuated to a pressure of approximately 10−3 Torr, and IR spectra for NH3 adsorption were recorded. The procedure for FT-IR characterizing ethanol-adsorbed NaY, NaY−SiO2, and SiO2 is similar to that for NH3 adsorbed samples. After the pretreatments, the samples were cooled to 35 °C, and water containing ethanol (99%) was introduced into the cell and maintained for about 20 min. After the ethanol adsorption, the cell was evacuated for 20 min and IR spectra for ethanol adsorption were recorded.

homogeneous powder diffraction rings. Two dimensional diffraction patterns were recorded with a Mar345 imaging plate system, with a sample-to-detector distance of about 300 mm. The diffraction angle was calibrated with silver Behenate and Si powders (NBS640b) standards according to their Bragg positions. One dimensional XRD profiles were integrated from selected fan-like areas of the symmetrical 2-D powder rings using the Fit2D program.30 Crystal structure parameters were refined with the Rietveld method31 using the graphical interface package EXPGUI32 for the GSAS program,33 and the crystalline grain sizes were obtained from the commonly used Scherrer’s equation,34 t = kλ/B cos θ, with the crystal grain size t, shape correction constant k = 0.95 for the spherical particle, and fwhm of the related Bragg peaks B. In addition, by combining difference Fourier maps analyses, the atomic structure can be fully characterized.27,35 The framework parameters of NaY (space group Fd3m) reported in the literature with chemical constraints (Si/Al = 5 and occupancy of lattice oxygen = 1) were used as the starting structure model for refinement.35,36 The calculated diffraction profiles were refined based on the Pseudo−Voigt (Gaussian plus Lorentzian) profile function, and the broad background was fitted with a 22 parameter shifted-Chebyschev polynomial function. Since the occupancy parameters and the thermal parameters are highly correlated with each other, parameters of these two factors were refined alternatively with positional parameters. In the refinement, the program calculates structure factors F(calc) from the refinable structural parameters and extracts F(obs) from XRPD patterns. Given the quantities F(calc) and F(obs), the difference Fourier maps were calculated, and then the positions and occupancies of Al3+, Na+, and water molecules were located from a series of difference Fourier maps and subsequent refinement; the difference Fourier map is defined as Δρxyz =

1 V

3. RESULTS AND DISCUSSION 3.1. XRPD Data Analysis. XRPD patterns of NaY, NaY− SiO2, and SiO2 were shown in Figure 1. For NaY and NaY−SiO2,

Figure 1. X-ray powder diffraction patterns of (a) NaY (black line), (b) NaY−SiO2 (red line), (c) NaY+SiO2 (green line), and SiO2 (blue line).

h =+∞ k =+∞ l =+∞

∑ ∑ ∑

(F(obs) − F(calc))

h =−∞ k =−∞ l =−∞

both sharp Bragg peaks produced by the crystalline phase and broad background produced by amorphous matter were presented, while only broad backgrounds were observed for SiO2. The loss of NaY crystallinity in pelletization was roughly estimated by Bragg intensity. The comparison of XRPD patterns for NaY+SiO2 and NaY−SiO2 (Figure 1) showed that Bragg intensity for NaY−SiO2 is about 82.3% of that for NaY SiO2. The results suggested that about 18% of NaY was transformed from the crystalline into the amorphous phase in the process. Unit cell changes induced in pelletization can be determined qualitatively by the analysis of the XRPD peak location. As shown in Figure 1, the pelletization process shifts the Bragg peaks of XRPD patterns to higher 2θ, suggesting that the pelletization process causes a slight reduction in the unit cell. Crystallinity and unit cell parameters can be further quantified using Rietveld refinement. In addition, by combining difference Fourier map analyses, atomic structure can be fully characterized.27,35 3.2. Rietveld Refinement Results. On the basis of Monte Carlo simulation, sites that Na+ can occupy are as follows: sites I lie at the center of the hexagonal prism; sites I′ are inside the sodalite cages facing sites I; sites II are in the supercages near the single six-ring; and sites III are also in the supercages, rear fourrings of the sodalite cages.37 Since the potential energy of sites III is higher than those of sites I, I′, and II,37 the sites are normally vacant. After framework refinement, the difference Fourier maps showed that the main peaks are located in sites I′ and II for both

exp[− 2πi(hx + ky + lz)]

The map can be used to visualize the missing atoms in a structural model.26 2.3. FT-IR Spectroscopy. Diffuse reflectance infrared Fourier transform spectra (DRIFT) of NaY, SiO2, and NaY− SiO2 samples were recorded with a Shimadzu FT-IR IR Prestige21 instrument, having a spectral resolution of 2 cm−1. For the characterization of zeolitic lattice vibration and surface hydroxyl groups, the powder samples were diluted with KBr and loaded into a heatable in situ IR cell. The cell was connected to a vacuum system and evacuated to obtain a vacuum better than 0.001 Torr. The temperature was increased at about 10 °C/min to 200 °C at an interval of 25 to 200 °C. For each interval, the treatment temperature was maintained for about 20 min and then cooled to 50 °C for recording IR spectra. To characterize the framework hydroxyl group, transmission IR spectra of NaY and NaY−SiO2 were obtained using selfsupporting wafers in a conventional glass cell connected to a vacuum/heating apparatus. All samples were treated under a vacuum (10−4 Torr) of 450 °C for 4 h. After cooling to 40 °C, IR spectra were recorded. FT-IR characterizing NH3 adsorbed NaY, NaY−SiO2, and SiO2 samples were loaded into a DRIFT cell, purged with dry N2 at room temperature for 1 h, and then heated under a vacuum to 200 °C. After taking the background IR spectra, NH3 was C


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Table 1. Positional Parameters, Occupancy, and Isotropic Temperature Factors for NaY Powder atom

x

y

z

occupancy

Uiso

Si Al O1 O2 O3 O4 Ow1 Ow2 Ale NaI′ NaII

0.1220(2) 0.1254(4) 0.1076(4) 0.2466(6) 0.1756(1) 0.1815(8) 0.4290(9) 0.9183(2) 0.1020(7) 0.09(3) 0.2905(2)

0.9493(2) 0.949(1) 0.8923(6) 0.2466(6) 0.1756(1) 0.1815(8) 0.0165(8) 0.3316(8) 0.1020(7) 0.09(3) 0.2905(2)

0.0378(4) 0.0378(6) 0 0.1395(4) 0.9730(4) 0.3223(2) 0.0165(8) 0.0457(6) 0.1020(7) 0.09(3) 0.2905(2)

0.7 0.22(4) 1 1 1 1 0.2791(6) 0.3067(3) 0.4809(6) 0.1979(9) 0.4584(5)

0.0410(6) 0.0482(8) 0.1601(7) 0.1274(4) 0.0158(5) 0.0528(6) 0.3436(7) 0.1404(2) 0.7160(5) 0.3348(6) 0.7513(2)

NaY and NaY−SiO2. The results are consistent with the Monte Carlo simulation results. However, maybe because of the rather short distance between sites I and I′, the strong repulsive force does not allow both sites to be occupied in the same cage. Since I′ sites have relatively low potential energy, it is preferred to be occupied first. These sites (I′ and II) are mainly occupied by cation and water molecules. Inferred from the XRPD characterizing dealumination results reported by Agostiniet al.,35 sites I′ were assigned to Al3+ and Na+, and sites II to Na+ only. The atomic parameters resulting from Rietveld refinement for NaY are shown in Table 1, while the goodness of fit is shown in Figure 2; those for NaY

Table 2. Crystal Data of Rietveld Results for (a) NaY Powder, (b) NaY+SiO2, and (c) NaY−SiO2 Pellet (a) NaY Powder sample

NaY powder

space group a, Å volume, Å3 wRp, [(Σiwi|Yiobs − Yical|)/(Σi(Yiobs))]a,b,c Rp, [(Σ|yi(obs) − yi(cal)|)/(Σ|yi(obs))] χ2, [Σwi((yi(obs) − yi(cal))2)/(N − P)]d,e RF2, [((Σ|IK(obs))1/2 − (IK(cal)1/2)|)/(Σ(IK(obs))1/2))2]f crystal weight fraction, wt % crystalline size, nm (b) NaY+SiO2

Fd3m ̅ 24.5783(11) 14863.9(19) 0.0725 0.0498 2.413 0.0913 75.07(6) 66.2

sample

NaY+SiO2

space group a, Å volume, Å3 wRp Rp χ2 RF2 crystal weight fraction, wt % crystalline size, nm (c) NaY−SiO2

Figure 2. Rietveld refinement profiles for NaY: observed data (crosses), calculated (red curve), and difference between experimental and simulated value (blue line).

+SiO2 and NaY−SiO2 are shown in the Supporting Information (Figure S1 and Table S1, S2). The refined structural parameters for NaY, NaY+SiO2, and NaY−SiO2 are shown in Table 2. The schematic view of the NaY framework with the location of Na+ and water for NaY is shown in Figure 3. In this study, the Na(1)−O(3) bond distance of 2.92 Å is slightly shorter than 2.94 Å as reported by Kirschhock et al.,38 whereas it is longer than 2.77 Å as reported by Eulenberger et al.39 The discrepancy may be caused by the interactions between Na+ and adsorbed water, since ours and the samples of Kirschhock et al. are hydrated samples while the samples of Eulenberger et al. are dehydrated. The Na+−H2O interactions may weaken the

Fd3m ̅ 24.5765(9) 14844.4(9) 0.0490 0.0355 1.064 0.0768 52.3(1) 66.0

sample

NaY−SiO2 pellet

space group a, Å volume, Å3 wRp Rp χ2 RF2 crystal weight fraction, wt % crystalline size, nm

Fd3m ̅ 24.5018(3) 14786.94(2) 0.0332 0.0226 0.9476 0.1137 44.71(7) 62.8

a

wi: 1/yI. byobs: observed intensity at the ith step. cycal: calculated intensity at the ith step. dN: the number of observations (e.g., the number of yi’s used). eP: the number of parameters adjusted. fIK: Intensity assigned to the Kth Bragg reflection at the end of the refinement cycles.

bonding between Na+ and framework oxygen, leading to an increase in Na(1)−O(3) bond distance. The water adsorption process is started with adsorption on the hydrophilic sites and followed with the formation of a monolayer on the walls of the supercages by hydrogen bonding between D


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Figure 4. TGA characterizing weight loss for (a) NaY, (b) NaY−SiO2, and (c) SiO2.

crystallinity loss estimated directly from Bragg intensity is 18 wt %. The loss of crystallinity should be due to acid leaching and is associated with dealumination. It is not surprising that the acidic (pH = 2.6) silica colloid removes framework aluminum, contributing to a partial loss of crystallinity and a shrinkage of crystallite size (Table 2). The dealumination of NaY that took place in the pelletization was further confirmed by comparing 29Si and 27Al MAS NMR spectra obtained from NaY−SiO2 extrudate and NaY+SiO2 (Figure 5). Since the Si chemical shift in zeolite is associated with the number of neighboring aluminum atoms, the types of Si environments can be identified by the deconvolution of 29Si NMR spectra. Besides silica binder Q4b(0Al) peaking at −112 ppm, there are four types of silicon atoms that have been observed (Figure 5a), namely, Q4(0Al) at −106 ppm, Q4(1Al) at −100 ppm, Q4(2Al) at −96 ppm, and Q4(3Al) at −92 ppm, where Q4(0Al) represents a silicon bonded through an oxygen bridge to other silicon atoms and Q4(nAl)n=1−3 to n aluminum atoms. In a comparison of the spectra of NaY+SiO2, NaY−SiO2 presents higher Q4(0Al) intensity but lower Q4(1Al), Q4(2Al), and Q4(3Al) intensity. These results confirmed that aluminum atoms have been removed from the framework of NaY during pelletization. Assuming that NMR intensity is proportional to the number of associated silicon atoms and the total number of aluminum atoms in the NaY structure will be one-fourth of the total Si−Al bonds, the Si/Al ratio, R, can then be calculated by the following equation:

Figure 3. Illustration of the location of Al3+, Na+, and water in NaY.

water molecules and framework oxygen.40 The refined results show that ordered water molecules are preferentially located in II sites (supercage) only (Figure 3). The Na+ ions are surrounded by six water molecules at an average bond distance of 2.93 Å (NaII−Ow1, 3.31 Å; NaII−Ow2, 2.55 Å), and the distance between the two water molecules is 3.02 Å. The water molecules are also bonded to framework oxygen with a bond distance of about 2.9 Å (Figure 3). Since the distance between water molecules of 3.00 Å is characteristic of stable hydrogen bonds,40 the results suggest that water molecules adsorbed on NaY are stabilized by mutual hydrogen bonding. Similar water locations were observed for both NaY and NaY−SiO2, whereas the total number of Ow occupancy (Table 1) for NaY is about 1.4 times that for NaY− SiO2. As shown in Figure 4, the TGA results indicated that the water contained in NaY, NaY−SiO2, and SiO2 is 22, 12, and 6 wt %, respectively. Since NaY−SiO2 contains one-third of SiO2, without interactions between the NaY and SiO2 binder, the water content of NaY−SiO2 should be 16.7 wt %. The experimental results thus suggest the increase of NaY hydrophobicity in pelletization, as expected. The Rietveld refinement results show that unit cell dimensions for NaY and NaY−SiO2 are 24.5783 and 24.5018 Å, respectively; suggesting dealumination took place in the pelletization process. Since Na+ balances the negative charge of tetrahedron AlO4 in the framework, the removal of aluminum from the zeolite framework was accompanied by the removal of Na+ and the formation of defect sites. The dealumination of NaY, concomitant with the removal of Na+, increases NaY−SiO2 hydrophobicity. The dealumination is associated with a partial loss in crystallinity. Rietveld refinement results show that the crystallinities are 75.1, 52.3, and 44.7% for NaY, NaY+SiO2, and NaY−SiO2, respectively.26,41 On the basis of Rietveld refinement results, about 14.5 wt % of zeolite crystal was converted into the amorphous phase during pelletization; this

R=

Si = Al

4

4

∑ IQ4(n Al)/∑ 0.25nIQ4(n Al) 0

0

Using the equation, the Si/Al ratios for NaY+SiO2 and NaY− SiO2 are determined to be 3.1 and 4.2, respectively. The results show that a significant dealumination took place in the pelletization. On the basis of the estimated Si/Al ratios, the number of Al atoms per unit cell for NaY+SiO2 and NaY−SiO2 are 46 and 36, respectively. The 27Al NMR spectra of NaY+SiO2 and NaY−SiO2 are shown in Figure 5b. After deconvolution, there are two major coordination states for Al that have been observed: tetrahedral Al involving Al(IV)a, Al(IV)b, and Al(IV,V)c (distorted tetrahedral Al species or 5-ccordianted Al) with peaks appearing at 60, 55, E


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Figure 5. (a) 29Si MAS NMR spectra with deconvoluted peaks of NaY−SiO2 and NaY+SiO2. (b) 27Al NMR spectra with deconvoluted peaks of NaY−SiO2 and NaY+SiO2.

macroporosity interconnected to pores inside zeolite particles.1 As shown in Figure 6, channels of width larger than 500 Å (macropore) were formed in between NaY particles and amorphous silica binder. The channels allow reactants or adsorbates to diffuse directly inside NaY particles. As shown in Figure 7, the micropores with a radius less than 10 Å for NaY−SiO2 are mainly inherent in NaY raw material, while the mesopores with a radius in between 20 Å and 500 Å might be contributed from (1) the dehydration of silica gel itself (precursor of silica binder), (2) the reaction between silica gel with extra-framework aluminum or NaY debris, and (3) the interconnected pores in NaY crystals caused by the collapse of zeolite frameworks.20,45,46 Since pores formed from silica binder are much larger than pores in zeolite, the BET surface area of NaY−SiO2 extrudate decreases with increasing binder content. In addition, the surface area of extrudate could be further decreased due to the loss of crystallinity as well as the pore mouth plugging. Specifically, in this study, the total BET surface area of NaY powder is 624 m2/g, in which 580 m2/g is from micropores, while 44 m2/g is from mesopores. After pelletization, the surface area contributed from micropores decreases to 297 m2/g, whereas that contributed from mesopores increases to 56 m2/g. Since NaY−SiO2 extrudate contained two-thirds NaY, if there were no surface area loss in the pelletization, surface area contributed from

and 42 ppm, respectively, and octahedral Al involving Al(VI)a and Al(VI)c with peaks appearing at 0 and about −10 ppm. Inferred from the MAS NMR peak assignment for zeolite HY reported by van Bokhoven and Altwasser et al.,42,43 the peaks Al(IV)a and Al(IV)b are assigned as aluminum species located in the framework and in the vicinity of framework defects, respectively, while the peaks Al(VI)a and Al(VI)c are assigned as extra-framework aluminum species. Since Al(VI)c can be washed out, these species could react with silica binder to form amorphous SiO2−Al2O3. Moreover, Rietveld refinement results indicate that some of the Al atoms could be lodged in sites I′ (inside the sodalite cages; Table 1, Figure 3). Inferred from the works of Agotini et al.,44 Al(IV,V)c could thus be assigned as the sodalite-cage trapped Al species. As shown in Figure 5b, after pelletization, peak intensity for Al(IV)a decreases concomitantly with the increase of peak intensities for Al(IV)b, Al(VI)a, and Al(VI)c. These results not only confirm the dealumination of NaY in the pelletization but also suggest the transfer of framework Al to the extra framework and to the vicinity of framework defects. 3.3. Pore Structures of NaY and NaY−SiO2 Extrudate. Pore plugging NaY due to binder will increase diffusion resistance, leading to a sharp reduction in adsorption capacity and catalyst activity. To minimize the increase in diffusion resistance, the binder component must maintain a high F


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Figure 8. Nitrogen adsorption isotherm at 77 K for (a) NaY, (b )NaY− SiO2, and (c) SiO2.

to the shape of the mesopores. In contrast to the loop of upward curvature at relative pressure (P/P0) between 0.7 and 0.98 for SiO2, indicating a cylindrical type of the pore, the H4 hysteresis loop of NaY indicates the presence of inkbottle type mesopores. The hysteresis loop for NaY−SiO2 at P/P0 between 0.7 and 0.98 is similar to that for SiO2, while still preserving the H4 type in P/ P0:0.4−0.7. The result suggests that relatively larger cylindricaltype pores in silica binder interconnect with relatively smaller inkbottle-type pores in NaY crystals. Calculating from the desorption branch with the BJH method, the radius of the mesopores are in the range of 30−100, 30−500, and 30−600 Å for NaY, NaY−SiO2, and SiO2, respectively, (Figure 7); it is noted that the peak at 18 Å could be an artifact peak of the BJH method.41 The mesopore volume for NaY estimated from the PSD diagram is 0.064 mL/g, while that for NaY−SiO2 is 0.362 mL/g. Since the mesopore for silica prepared from the calcination of silica gel at 450 °C is 0.652 mL/g, some extra mesopores in NaY−SiO2 could be formed in pelletization; the average mesopores of the mixture of NaY and SiO2 would be 0.263 (1/3 × 0.652 + 2/3 × 0.064) rather than 0.362 mL/g as found. Three zones of different NaY structures, namely, (1) NaY lattice, (2) zeolite debris (amorphous NaY), and (3) mesopores formed from crystal damages, can clearly be differentiated from the TEM images of NaY and NaY−SiO2 (Figure 9a,b). For NaY−SiO2, silica binder near the amorphous NaY is noted as zone 4 (Figure 9b). Extra material deposited to or removed from NaY crystallites or silica binder should lead to a potential change and contrast difference of the TEM image.45 The darker spots in the TEM images shown in zone 1 are most likely due to the deposition of the dislodged aluminum species,45,49,50 while the darker spots in zone 4 could also be due to the deposition of the dislodged alumina and/or NaY debris. The connected bright dots shown in the zone 3 of the NaY image are suggested to be the distorted pores (cavities) formed from the collapse of the NaY lattice due to the hydrolysis of Al−O−Si bonds, followed by the removal of aluminum atoms in the post-treatment (such as steaming) of NaY. In the kneading step of the pelletization process, framework aluminum reacted with acid in the silica binder to form Al3+ cations, which are then driven and dislodged from the zeolitic framework by water, resulting in a formation of crystal defects, which can progress to amorphous NaY. This amorphous NaY

Figure 6. SEM images for (a) NaY powder and (b) NaY−SiO2.

Figure 7. Pore size distribution for (a) NaY, (b) NaY−SiO2, and (c) SiO2.

micropores of NaY should be 387 m2/g. On the basis of Rietveld refinement results, 11% of the surface area loss was suggested to be caused by crystal destruction; the other 12% surface area loss (46 m2/g) could be caused by pore mouth plugging. The effects of silica binder on the formation of the mesopore structure of NaY−SiO2 were shown in the comparison of nitrogen adsorption and desorption isotherms for NaY, SiO2, and NaY−SiO2 (Figure 8). For NaY, the hysteresis loop is similar to the IV type isotherm with type H4 hysteresis loop, according to an empirical classification of hysteresis loops given by IUPAC, whereas the loop of the IV type isotherm with type H1 was observed for SiO2.47,48 The shape of the hysteresis loop is related G


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then connects with the interior cavities to form mesopores. The dislodgement of aluminum from NaY framework shown in TEM images is also appearing in XRPD results; the appearance of a small shoulder in the (111) reflection (Figure 10(I)) for uncalcined NaY−SiO2 suggests that the acid leaching causes a shrinkage of the zeolite lattice along the channel due to the dislodgement of the Al3+ cations. In calcination and in the annealing process, the vacancies created in the dealumination may be backfilled by mobile Si(OH)4 formed in the hydrolysis of silica and/or the extracted Al3+ cations. The comparison of XRPD patterns for uncalcined NaY−SiO2 (Figure 10c) and NaY−SiO2-450 °C (Figure 10d) show that Bragg intensity lost in the extrusion was regained (Figure 10(II)) and the small shoulder disappeared in the calcination (Figure 10(I)) possibly by backfilling. The backfilling process might increase the thermal stability of NaY. As shown in the comparison of XRPD for NaY−SiO2-450 °C (Figure 10d) and NaY−SiO2-800 °C (Figure 10e), when air calcination was elevated to 800 °C, the Bragg intensity of zeolite does not change significantly (Figure 10(II)), indicating that the calcined NaY− SiO2 is rather stable. In contrast, as shown in Figure 10(III) (the comparison of XRPD patterns for NaY (Figure 10a) and NaY800 °C (Figure 10b)), a significant decrease of the peak intensity was observed. Since VOCs (volatile organic compounds)-containing effluents from industrial processes often contain high levels of moisture, adsorbents for VOC abatement should have a high thermal stability in a moisture-containing environment. The effects of pelletization on the thermal stability of NaY in the moisture-laden air environment were examined by comparing XRPD patterns characterizing NaY−SiO2-x°C(w) with those of NaY-x°C(w) (Figure 11). For NaY, the thermal treatment leads to a collapse of zeolite frameworks accompanied by dealumination, as evidenced by the decrease in peak intensities and the shift of Bragg peaks to higher 2θ with increasing treatment temperatures (Figure 11). In contrast, no significant loss of peak intensities and only a slight shift of Bragg peaks were observed for NaY−SiO2. The comparison confirmed that the thermal stability of NaY in a moisture-laden environment was enhanced after the pelletization with silica gel as a binder. 3.4. Characterizing Structure and Surface Properties of NaY and NaY−SiO2 by Use of FT-IR. The number Ow

Figure 9. TEM images for (a) NaY and (b) NaY−SiO2.

Figure 10. X-ray powder diffraction patterns of (a) NaY, (b) NaY-800 °C, (c) uncalcined NaY−SiO2, (d) NaY−SiO2-450 °C, (e) NaY−SiO2-800 °C, and (f) NaY+SiO2. H


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stretching band of water molecule from 3450 to 3400 cm−1.51,52 The shoulders at about 3250 and 3050 cm−1 were assigned as the asymmetric OH stretching vibration of the water molecules coordinated to H+ and Na+ cations, respectively, via lone pair electron of the oxygen center.51−53 Since the bending band of molecular free water is at 1600 cm−1, the peak found at 1620 cm−1 was assigned as the characteristic peak of the adsorbed water.52 The assignments are consistent with water adsorbed on a monovalent cation resulting in a shift of water bending mode to higher frequency, while that of stretching mode to lower frequencies was reported by Jacob et al.54 In comparison with the IR spectrum of SiO2+NaY, that of NaY−SiO2 presents lower intensity for the characteristic peaks of the adsorbed waters (Figure 12). The results are consistent with the increase of NaY hydrophobicity after pelletization suggested by TGA. In addition, as suggested by Rietveld refinements of XRPD data, Na+ ions were surrounded by water molecules. Dealumination from framework in pelletization removes the balanced Na+ and the water molecules bound to Na+. The IR characteristic peaks for the hydroxyl groups of the dehydrated NaY and NaY−SiO2 are shown in Figure 13a. The assignments of the IR peaks are based on the shift of frequency caused by π-bonding interactions between an unshared electron on the oxygen atom and an empty d orbital of the atom (Al or Si) attached to a hydroxyl group.52 The frequency of hydroxyl groups may decrease with increasing π-bonding interactions. Hence, the absorption band at 3720 cm−1 for NaY may be attributed to terminal hydroxyl groups bonded to Si (SiOH) of the lattice defect remaining after dealumination or amorphous SiO2 containing material. The band at 3675 cm−1 with the shoulder at 3650 cm−1 was attributed to hydroxyl groups linked to Al species present on extra-framework NaY. The band at 3595 cm−1 was assigned to bridging zeolitic hydroxyls located in supercages.28,55−57 Inferred from the partial collapse of the NaY framework in pelletization evidenced by XRPD, the appearance of a much higher peak intensity at 3720 cm−1 for NaY−SiO2 as opposed to NaY (Figure 13a) was suggested to be caused by the hydrolysis of Al−O−Si of NaY taking place in pelletization while the appearance of absorption band at 3625 cm−1 was due to the formation of a hydroxyl group bonded to an aluminum deficient NaY framework from dealumination.58 There are no significant peaks being observed at about 3565 cm−1, suggesting that no bridging zeolitic OH was formed in sodalite cages. These results are consistent with the Rietveld refinement results; sites I′ were bound to Na+, (Alδ−−ONa+− Si), rather than H+ (Table 1, Figure 3). Complementary to the synchrotron XRPD with Rietveld refinement, FT-IR can be used to confirm the acid-leaching dealumination during pelletization by examining the IR frequency of the Si(Al)−O stretching band in the zeolite framework.28 As shown in Figure 13b, the shoulders found at about 1170 and 740 cm−1 for NaY are assigned as the asymmetric and symmetric vibration bands, respectively, of the primary (internal) TO4 tetrahedral unit of zeolite framework, and the peaks found at about 1060 and 810 cm−1 are the asymmetric and symmetric vibration bands of external TO4 linkage structure.28,38 The band appearing at 585 cm−1 is the characteristic band of double ring vibration, which is normally used to calculate the Sito-Al ratio. The internal vibrations of the TO4 tetrahedron are not sensitive to other structural vibrations, and no obvious shift in IR characteristic peaks has been observed. In contrast, external asymmetric vibration bands, ωDR, are relatively sensitive to the Si-

Figure 11. X-ray powder diffraction patterns of (a) NaY, (b) NaY-450 °C(w), (c) NaY-800 °C(w), (d)NaY−SiO2-450 °C(w), (e) NaY−SiO2-600 °C(w), and (f) NaY−SiO2-800 °C(w).

occupancy in Rietveld refinement results from XRPD patterns may be useful to rationalize the change of NaY hydrophobicity in pelletization. However, it is not possible to estimate O w occupancy accurately by Rietveld refinement because of the strong coupling between occupancy parameter and Debye− Waller factor. In addition, since XRPD data can only reflect the order from water, water adsorbed on amorphous NaY or silica cannot be detected by XRPD. Hence, the affinity of water for NaY and NaY−SiO2 are further characterized by the dehydration of moisture saturated samples. The dehydration process was monitored by FT-IR, and the absorbance spectra of NaY, NaY− SiO2, and SiO2 taken at ambient temperature to 200 °C under a vacuum (about 10−4 Torr) are shown in the Supporting Information (Figure S2). The comparison of the difference spectra in the OH stretching region before and after dehydration is shown in Figure 12. In the difference spectra (Figure 12), the “positive absorbance” represents the absorption bands of the molecular-adsorbed water, while the “negative absorbance” reveals the surface hydroxyl groups. Hence, the broad band at 3400 cm−1 shown in Figure 12 was attributed to the adsorbed water molecules, which stabilized each other by hydrogen bond. The mutual hydrogen-bond interactions cause the shift of OH

Figure 12. Difference spectra obtained from the difference between the spectra recorded before and after dehydration for (a) NaY, (b) NaY +SiO2, (c) NaY−SiO2, and (d) SiO2. I


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spectra and from the XRPD pattern for NaY−SiO2 could be due to the contribution of IR bands from the SiO2 binder that we do not take into account in calculation. The effects of pelletization on the surface acidity were explored by FT-IR spectra characterizing NH3 adsorbed on NaY, NaY− SiO2, and SiO2, respectively (Figure 14). The spectrum of NH3

Figure 14. FT-IR of NH3 adsorbed on (a) NaY−SiO2, (b) NaY, and (c)SiO2.

interacting with Lewis acid is distinctly different from that interacting with a Brønsted acid. Ammonium cations (NH4+) formed from the interactions between NH3 and Brønsted acid sites are characterized by the FT-IR band at 1450 cm−1, while the NH3 molecules are coordinated to Lewis acid sites by the bands at 1620 and 1300 cm−1.56,57 The comparison of IR spectra of NH3 adsorbed on SiO2, NaY, and NaY−SiO2 samples (Figure 14) indicates that both Brønsted and Lewis acid sites were present in these samples. Among them, SiO2 exhibits much weaker peak intensity, suggesting that the acid sites contributed from SiO2 itself is insignificant. Hence, the appearance of a shoulder at 1430 cm−1 for a NH3-adsorbed NaY−SiO2 sample (Figure 14) indicates additional Brønsted acid sites formed in the pelletization process. Inferring from Hensen et al.60 that the isolated aluminum grafted onto the silica surface brings about the Brønsted acidity, we suggest that SiO2 is not just an innocent binder; silica interacts with aluminum in the zeolite debris, leading to the formation of Brønsted acid sites. 3.5. Impacts of Binder−Zeolite Interactions on Adsorption. Zeolites are frequently used to remove volatile organic compounds (VOCs) contained in the effluents of industrial processes. In commercial operation, the zeolite powder has to be pelletized with binder for use in a fixed bed adsorption unit. When silica is used as a binder, interconnecting macro-, meso-, and micropores in NaY−SiO2 are formed; hence, VOCs of different molecular sizes can be removed effectively from the effluent by adsorption without strong diffusional hindrance. However, the pelletization process may influence the adsorption properties of NaY due to NaY-silica binder interactions; hence, these effects are investigated by the comparison of FT-IR spectra characterizing ethanol chemically adsorbed on NaY (EtOHv/ NaY), SiO2 (EtOHv/SiO2), NaY−SiO2 (EtOHv/NaY−SiO2), and physically adsorbed on KBr (EtOH·KBr; Figure 15). Since effluent streams often contain moisture, water containing ethanol was used as a model adsorbate in this study. As shown in line b of Figure 15, a big-broad-negative band at 3730 with a shoulder at 3670 cm−1 was observed for NaY−SiO2. The negative adsorption bands at 3730 cm−1 indicate the loss of terminal hydroxyl groups bonded to Si (SiOH) upon

Figure 13. (a) FT-IR spectra of a sample structure at high frequency for (a) NaY−SiO2, (b) NaY, and (c) SiO2. (b) FT-IR spectra of a sample structure at low frequency for (a) NaY−SiO2, (b) NaY, and (c) SiO2.

to-Al (Si/Al) ratio and shift to higher frequency with decreasing number of tetrahedral aluminum atoms.28 Moreover, different from NaY with a rather symmetric peak appearing at 585 cm−1 for ωDR (Figure 13b), an asymmetric peak was observed for NaY−SiO2 (Figure 13b). The Si/Al ratio in the framework for NaY−SiO2 then was calculated by the curve deconvolution of the ωDR band. Shown in Figure 13b, for NaY−SiO2, the asymmetric peak was deconvoluted to two peaks appearing at 585 and 604 cm−1. By the use of the empirical relation given by the equation, x = 3.857 − 0.00619ωDR (cm−1) with Si/Al ratio R = (1 − x)/x.59 The calculated Si/Al ratio corresponding to the deconvoluted band at 604 cm−1 is 7.7 (noted as high Si/Al) and that at 585 cm−1 is 3.2 (low Si/Al). Calculated from the peak area by assuming the same extinguish coefficient for both peaks, Si/Al ratio for NaY−SiO2 is 4.7. In contrast, based on a0 (cell parameter) estimated from Rietveld refinement of XRPD patterns, the Si/Al ratio calculated from the equation, x = 5.348a0 − 12.898, is 4.36 for NaY−SiO2, while that for NaY is 3.31.28 The discrepancy of the Si/Al ratio calculated from FT-IR J


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debris or on amorphous silica−alumina, which are formed from the hydrolysis and the cleavage of the intracrystalline Si−O−Al bond followed by the reaction with silica binder. The NaY and binder interactions make the zeolite more hydrothermally stable which lends the extrudates itself to industrial applications, such as a high temperature regenerative VOC removal process. However, the change of acid sites from Lewis type to Brønsted because of the pelletization could impact its catalytic performances.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01369. Figures and tables containing Rietveld refinement output (PDF)


Figure 15. FT-IR spectra of (a) ETOHv/NaY, (b) EtOHv/NaY-SiO2, (c) EtOHv/SiO2, and (d) EtOH·KBr.

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adsorption, which could be attributed to the adsorption of ethanol and/or water on the silanol groups via the hydrogenbridging bond, while the peak at 3670 cm−1 is attributed to the formation of protonated ethanol (water) via the proton donated from the [(HO)x−Al−(O−Si)3‑x] fragment of zeolite debris, amorphous silica−alumina, or bridging zeolitic hydroxyl groups. The relatively small C−H stretch band intensity for EtOHv/NaY, as opposed to EtOHv/NaY−SiO2, suggests that a lesser amount of ethanol is adsorbed on NaY. The appearance of a broad band at about 1310 cm−1 for EtOHv/NaY and EtOHv/NaY−SiO2 was assigned as a bending vibration of the ethanol hydroxyl group. This upward shift compared to C−O−H bending vibration of free ethanol (1253 and 1225 cm−1) was caused by the interaction between ethanol and NaY or NaY−SiO2 adsorbent.61 This upward shift is consistent with the coordination of ethanol to Lewis acid sites reported by Hussein et al.62 Therefore, besides surface hydroxyl groups, ethanol molecules are also adsorbed on Lewis acid sites, such as Al3+ or Na+ of NaY, via the lone pair electron of its oxygen atom for both samples. The appearance of bands at 1665 and 920 cm−1 for EtOHv/ NaY−SiO2 and EtOHv/NaY may be assigned as the C−O−H2 bending vibration and CO stretching vibration of the protonated ethanol (C2H5OH2δ+), respectively. The bands appeared at about 1420 cm−1 for NaY and NaY−SiO2 could be assigned as CH2 and CH3 bending vibration bands. Since the inductive effect caused by ethanol-adsorbent interactions on the bending vibration is minor, the band shift being observed is relatively small in comparison with the C−O−H2 bending vibration.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The supports of the Ministry of Economic Affairs, R.O.C. (Contract No. 98-EC-17-A-10-S1-113), National Science Council (Contract No. NSC-101-2221-E-194-052), and National Synchrotron Radiation Research Center (NSRRC) are acknowledged.

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REFERENCES

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4. CONCLUSION A NaY extrudate suitable for use as adsorbents in a fixed bed operation has been prepared by binding NaY with silica to form cylindrical extrudates. In the preparation, NaY powder was blended with silica gel, kneaded, and then extruded and calcined. It has been discovered that dealumination of framework aluminum combined with the interactions between NaY and binder took place in the preparation procedure, leading to a decrease in crystal size and crystallinity, while there is an increase in Si/Al ratio, hydrophobicity, and thermal stability. The increase in Si/Al ratio does not cause the decrease in the number of acid sites on NaY−SiO2, however. On the contrary, IR characterizing NH3 adsorbed on NaY and NaY−SiO2 indicated that pelletization causes an increase in Brønsted acid sites. Apparently, the new acid sites are formed from a hydroxyl group attached on NaY K


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