Concentrated colloidal solution system for preparation of uniform Zeolite-Y nanocrystals and their gas adsorption properties

Accepted Manuscript Concentrated colloidal solution system for preparation of uniform Zeolite-Y nanocrystals and their gas adsorption properties Darshana Nakrani, Mihir Belani, Hari C. Bajaj, Rajesh S. Somani, Puyam S. Singh PII:

S1387-1811(16)30598-4

DOI:

10.1016/j.micromeso.2016.12.039

Reference:

MICMAT 8068

To appear in:

Microporous and Mesoporous Materials

Received Date: 31 August 2016 Revised Date:

30 November 2016

Accepted Date: 31 December 2016

Please cite this article as: D. Nakrani, M. Belani, H.C. Bajaj, R.S. Somani, P.S. Singh, Concentrated colloidal solution system for preparation of uniform Zeolite-Y nanocrystals and their gas adsorption properties, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2016.12.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Quantity Adsorbed (cm /g STP)

Graphical Abstract

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1200

3

1000

600

Zeolite Y (Nanocrystalline)

400 200

Zeolite Y (Microcrystalline)

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0

0.0

0.2 0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

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Zeolite-Y of uniform sizes ca. 40 nm having excellent pore volume of ca. 0.91 cm3/g was prepared from concentrated colloidal solution system

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Concentrated colloidal solution system for preparation of uniform Zeolite-Y nanocrystals and their gas adsorption properties

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Darshana Nakrani1, Mihir Belani2, Hari C. Bajaj2, Rajesh S. Somani2 and Puyam S.

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Singh1*

RO Membrane Division, CSIR-Central Salt & Marine Chemicals Research Institute, G.B. Marg,

Bhavnagar 364021, India. 2

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Inorganic Material and Catalysis Division, CSIR-Central Salt and Marine Chemical Research

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Institute, Bhavnagar 364021, India

*Corresponding author. Tel: 91-278-2566511; Fax: 91-278-2567562; E-mail: [email protected] (P.S. Singh)

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ABSTRACT Nanocrystalline Zeolite-Y having large internal and external surface area due to its nanometer scale crystals are applied for catalysis and separation processes. In this study, a novel approach

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for the preparation of uniform size Zeolite-Y nanocrystals from concentrated colloidal solution system is explored. The synthesis composition systems studied were concentrated solutions with high amount of organic template to give a large number of nuclei resulting in formation of

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uniform size nanocrystalline Zeolite-Y. The nanocrystalline Zeolite-Y of uniform particle sizes ca. 40 nm was successfully synthesized. The nanocrystalline Zeolite-Y synthesized in present

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study had excellent pore volume of ca. 0.91 cm3/g which is about three times than the pore volume obtained by conventional microcrystalline Zeolite-Y. A decrease in the organic template content resulted into the Zeolite-Y crystals of bigger size up to 60 nm along with some presence of Zeolite-A as impurity. The gas adsorption study of O2, N2, CO2, and CH4 were performed on

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these nanocrystalline zeolites and their gas separation properties were analysed. A strong CO2 interaction with the nanocrystalline Zeolite-Y was observed according to the adsorption model expressed by Toth equation.

CO2/N2 selectivity and CO2/CH4 selectivity of ca. 300 were

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observed at lower relative pressure for the nanocrystalline Zeolite-Y which shows potential as promising material for application in CO2 separation of flue gas from thermal power plant and

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natural gas purification.

KEYWORDS: Concentrated synthesis, nanocrystalline Zeolite-Y, gas adsorption, separation

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1. Introduction Zeolite are crystalline microporous aluminosilicates with large surface areas and well defined pores of molecular dimensions and have wide industrial applications in catalysis, adsorption

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and membrane separation due to their unique properties such as molecular sieving, high thermal stability, adsorption capacities, shape selectivity, ion exchange capacities and acidity or basicity based on the type of cation present [1]. The sizes of typical industrial Zeolites are in the

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micrometre-length-scale2 and various synthetic efforts [3-10] have been paid to reduce the Zeolite size from micrometer to nanometer scale in view of unique properties of nanocrystalline

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Zeolites, such as decreased diffused path lengths, large external surface area, high surface reactivity, etc.

Nanocrystalline Zeolite-Y which exhibits a three dimensional pore structure where the basic structural units known as the sodalite cages are arranged in such a way to form the supercage of

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1.2 nm in diameter and large internal and external surface areas due to its nanometer scale crystals, had been applied in the process applications of catalytic cracking or separation process [11, 12]. Therefore, nanocrystalline Y Zeolite may be particularly useful as adsorbent in

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environmental applications by removing undesired gaseous molecules as well as for using as catalysts in the refinery and petrochemical industry [13].

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In this work, preparations of nanocrystalline Zeolite-Y from different colloidal synthesis solution are explored. The comparison of the present synthesis system with those of relevant literature data [4, 5, 7, 10, 13-15] is given in Table 1. Differently from the reported synthesis systems, the synthesis composition systems studied here were concentrated colloidal solutions containing initial silica particle average size of about 26 nm with high amount of

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tetramethylammonium cations. The concentrated solution with high amount of the organic template is to give a large number of nuclei resulting in to nano size crystals of Zeolite-Y. We used the organic template because of the reason that it is usually required to reduce the

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crystal size as reported in several reports [16-25]. We also thought that it will produce zeolite Y nanocrystals with monomodal size distribution. There are several reports on the synthesis of zeolite nanocrystals without template. However, methylcellulose is used to illustrate thermo-

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reversible polymer hydrogel-controlled synthesis of zeolite NaA and NaX nanocrystals [26]. A. Nouri et al. reported effects of hydrothermal parameters on the synthesis of Nanocrystalline

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zeolite NaY. They obtained zeolite NaY having the wide size distribution in the range 25 to 150 nm [27]. B. A. Holmberg et al. have synthesized High silica zeolite Y nanocrystals (30-40 nm) by dealumination and direct synthesis [28]. For the removal of TMA cations occluded in the frame work of zeolite Y they used ion exchange with 1 M sodium nitrate (NaNO3) or 1 M

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ammonium nitrate (NH4NO3) solution. The dealumination technique of Skeels and Breck [29] was used to de-aluminate zeolite nanocrystal suspensions. In this technique, the ammonium balanced form of zeolite Y (NH4–Y) is treated in a 80 °C, stirred, 1 wt.% zeolite aqueous slurry

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with ammonium hexafluorosilicate to extract and complex the framework aluminum, while at the same time replace the extracted Al with Si. A series of microsized NaY synthesis procedures

[30].

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without organic templates and under various conditions was also provided by Bo and Hongzhu

Furthermore, we had used colloidal silica of bigger size than those of the previously reported synthesis system, which increases crystallization time in order to transform perfect crystals. The gas adsorption experiments O2, N2, CO2, and CH4 were performed on these Zeolites and their gas separation properties were analysed.

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It is well-known that there is a strong adsorption affinity of CO2 in the framework of zeoliteY. Highly porous N-doped activated carbon monoliths are fabricated by carbonization and physical activation of mesoporous polyacrylonitrile monoliths in the presence of CO2. The

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monoliths exhibit exceptionally high CO2 uptake; 5.14 mmol g-1 at ambient pressure and temperature and 11.51 mmol g-1 at ambient pressure and 273 K [31]. Microporous carbon having BET surface area of 2186 m2 g−1 and micropore volume of 0.85 cm3 g−1 has been synthesized via

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KOH induced high temperature carbonization of a non-conjugated hypercrosslinked organic polymer. The resultant carbon material showed high uptake for CO2 (7.6 mmol g−1) and

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CH4 (2.4 mmol g−1) at 1 atm, 273 K together with very good selectivity for the CO2/N2 (30.2) separation. Furthermore, low pressure (1 atm) H2 (2.6 wt%, 77 K) and water uptake (57.4 wt%, 298 K) ability of this polymer derived porous activated carbon is noteworthy [32]. CO2 separation from power plant emissions (CO2/N2) and purification of natural gas (CO2/CH4) using

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various processes, including adsorption, absorption and membrane from adsorption have been of extensive research in the context of environmental impact and energy [31]. However, the CO2 adsorption can be influenced by operating conditions, such as temperature, pressure, moisture,

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concentration, and operational mode etc. [34–36] as a result of differences in sorption affinity and diffusivity (mobility) of the gases within the zeolite pore systems. The preferential

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interaction of CO2 than N2 over tetramethylammonium-zeolite-Y [37] and tetraethylammoniumzeolite-Beta [38] has recently been reported. The potential of the nanocrystalline zeolite-Y is explored for promising platform for application in flue gas (CO2/N2) separation and natural gas (CO2/CH4) purification.

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2. Experimental 2.1. Preparation of the nanocrystalline Zeolite-Y The preparation procedure of nanocrystalline Zeolite-Y as shown schematically in Figure 1 is

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described below. For a typical example of synthesis molar composition 0.03Na2O: 0.63Al2O3: 4.18SiO2: 2.38(TMA)2O: 100H2O, 8.85 g of tetramethylammonium hydroxide solution (TMAOH, 25% in water, Sigma-Aldrich), 1.31 g of aluminium iso propoxide (≥98% Sigma

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Aldrich) and 0.61 g of 0.5 M NaOH (SD Fine Chemicals, India) were mixed in the polypropylene bottle and the solution was stirred vigorously with mechanical device at 400 rpm

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for 2 h until the solution is clear. To the above solution, 3.20 g of Ludox AS-40 colloidal silica (40 wt. %, Sigma Aldrich) was added slowly and stirred at 400 rpm for another 1 h. Next, the bottle was set at preheated oil bath maintained at 100°Cand for 3 to 7 days. After the certain synthesis time, the product was separated from solution by centrifugation with 9200 rpm for 30

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minutes and washed with distilled water. The centrifugation and washing step was done for 3 or 4 times to purify the Nanocrystalline Zeolite-Y product. After that, the product was dried at 100°C for 4 h and then calcined at temperature of 500°C for 5 h. In order to investigate the effect

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of initial colloidal solution concentration of the synthesis system on the properties of nanocrystalline Zeolite-Y product, synthesis experiments were performed by varying water

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content while keeping the molar ratios of Na2O: Al2O3: SiO2: (TMA)2O constant. The synthesis molar compositions are given in Table 2.

2.2. Characterization of the nanocrystalline Zeolite-Y The synthesized Zeolites were characterized by several analytical techniques, including X-ray diffraction (XRD) on a PANalytical EMPYREAN over the 2θ range from 5 to 50° with a

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scanning rate of 0.039° min

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Scanning Electron Microscope (SEM) FE-SEM, JSM-7100F,

Transmission Electron Microscopy (TEM) JEOL, JEM 2100, Fourier transform infrared Spectroscopy (FT-IR) on PERKIN ELMER GX-FTIR, Dynamic light Scattering (DLS) on

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NaBiTec Spectro Size 300 light scattering apparatus (NaBiTec, Germany) with a He–Ne laser (633 nm, 4 mW), Thermogravimetric analysis (TGA) on TGA/SDTA 851e Mettler Toledo, Inductively Coupled Plasma Spectroscopy (ICP) on Perkin Elmer, Optima 2000 and adsorption

3. Results and discussion 3.1. XRD, SEM, TEM and DLS analysis

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analysis on ASAP 2010 (Micromeritics Inc. USA) apparatus.

XRD pattern of the nanocrystalline Zeolite-Y synthesized from the synthesis molar composition 0.029Na2O: 0.63Al2O3: 4.18SiO2: 2.38(TMA)2O: 100 H2O is shown in Figure 2.

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The XRD pattern of the sample is identical to that of the Microscale Zeolite-Y (supplied by Zeochem) indicating the FAU structure of the sample. As shown in Figure 2, the peak intensities are weaker with broader peak widths for the nanocrystalline Zeolite-Y which is expected because

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of the smaller size of crystals. The nanocrystalline Zeolite-Y obtained from different synthesis molar compositions (S1, S2, S3 and S4) by hydrothermal synthesis at 100°C for synthesis time

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varying from 3 to 7 days are shown in Figure 3. Sample S1 exhibited XRD reflections of pure Zeolite-Y crystals only while the formation of Zeolite-Y along with a small of Zeolite A (marked by an arrow) was evident for all the other samples (S2, S3 and S4). Based on the comparison of the (111) intensities, the amount of Zeolite A is decreased with increase in synthesis time (3 to 7 days) for the S2, S3 and S4 samples. Comparatively among the S2, S3 and S4 samples, the S2 sample after 7 days of synthesis time contained the least amount of Zeolite A, approximated as

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about 3 % based on difference of the intensities while the S3 and S4 after 7 days of synthesis time had about 6 and 15 % Zeolite A, respectively. This implied that the formation of Zeolite-A increased from the synthesis composition with decrease in TMAOH concentration. SEM images

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of the S1, S2, S3 and S4 samples are given in Figure 4 showing nanocrystalline cubical morphology.

For higher resolution of the nanocrystalline cubical crystals, TEM images in three

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maginfication scales for the samples were taken as shown in Figure 5 (a). The average size of the crystals were appeared to be smaller for the samples in the order of S4>S3>S2>S1. The average

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crystal size of the S1 sample was about 40 nm while it was about 60 nm for the S4 sample. The synthesis solution system is the most concentrated solution system among all the syntheses, therefore it is expected that a large number of nuclei resulting in to nano size crystals of smaller sizes are expected during the synthesis of the S1 sample.

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On the other hand, the diluted solution preparation system can have smaller number of nuclei resulting to larger crystal sizes of the Zeolite-Y as discussed based on the above phenomenon of solution concentration effect on the crystal sizes. In addition, as shown in the above XRD

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patterns, the samples obtained diluted solution preparations had contained some amount of Zeolite-A impurity, the amount of which is more in the samples obtained from less hydrothermal

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synthesis time. The TEM image of the sample S4 obtained from the synthesis time of 5 and 6 days indeed showed some presence of quite large particles (about 100 nm) presumably from the Zeolite A impurities. This is shown in Figure 5 (b). Nanocrystal size distributions of the samples were also obtained by DLS measurements as shown in Figure 6. The narrow size distribution was obtained for each case with increase in average size for the sample in the order of S1>S2>S3>S4. This was consistent with the size values obtained by TEM.

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3.2. FTIR and TGA studies The influence of TMA cations in the Zeolite structure formation from the different solution

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systems was probed using FT-IR studies. The FT-IR spectra collected on the Zeolite samples at various synthesis times from the most concentrated solution system (S1) or the most diluted solution system (S4) were shown in Figure 7. The IR spectra of S1 were quite similar to the

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spectra of S4 sample. All the samples exhibited infrared bands of similar intensities due to absorbed water at 1635 cm-1and 3400 cm-1; 1488 cm-1 due to CH3 asymmetric bending and 3013

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cm-1 due to CH3 asymmetric stretch. The IR band at about 1405 cm-1 is assigned to TMA cations present in the pores of Zeolite-Y. This peak is clearly distinctive in the IR spectra of the S1 and S4 samples after 7 days of synthesis time. At shorter synthesis time of the samples, a broad peak 1300-1450 cm-1 was observed, the intensity of which becomes smaller for the sample with

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increasing synthesis time and finally (after 7 days of synthesis time) to the sharper but small distinctive peaks at 1405 cm-1 (TMA cations) and 1422 cm-1 (CH3 umbrella bending). This implied that the TMA incorporation within the pores and cavities of the Zeolite was increased

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with the crystallization time. The Si/Al ratios of the samples analysed by ICP were found to be in the range of 1.69 – 1.81 as given in Table 3. The charge generated by Oxygen Bridge between

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each Si4+ and Al3+ is balanced by a cation giving a high fraction of cations (either Na or TMA) within the Zeolite pores. The Na/Al ratios of the samples were below 1 in the range of 0.54 – 0.60 which indicated TMA cations balancing the remaining charge of the Zeolite framework. It is known that the two IR peaks ~ 467cm-1 and ~516 cm-1 are associated with the Si/Al ratio of the zeolite. In IR spectra of the S1 and S4 [Fig. 7 (a) and 7(b)] two peaks at ~ 467 & ~ 516 cm-1 are not separately seen. Single peak at 470 cm-1 is observed in both the cases indicating that

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the Si/Al is very close for S1 & S4. This is supported by Si/Al ratio 1.77 in the S1, while it is 1.69 in the S4. Presence of zeolite–A of Si/Al ~1 in the S4 is not seen in the IR band at about 500 cm-1 which could be due to less quantity of zeolite-A in the S4.

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TGA plots of the samples are shown in Figure 8. From the plots, it can be seen two steps of weight losses for each sample. The first step of weight loss (ca. 15 %) is due to loss of water at temperature range of 25-200 °C while the second step of weight loss (ca. 10%) is due

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decomposition of organic template (TMA) at temperature range of 200-550 °C similar to the reported TGA plots of other Zeolite-Y [7]. The decomposition pattern of TMA was closely

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observed for each sample using Derivative Thermogravimetric Analysis (DTG) plots as given in Figure 8 (inset). It can be seen from the DTG plots of the S1, S2, S3 samples that TMA decomposes with maximum loss at 460 °C and onset of the decomposition temperature at 305 °C. In case of the S4 sample, the TMA decomposition started at 225 °C with some TMA loss at

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about 300 °C and a majority of TMA loss at 460 °C suggesting TMA in two different environments. Sample S4 contained significant amount of Zeolite-A among all the samples, therefore TMA might be present in different environments of Zeolite-A and Zeolite-Y crystals.

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On the other hand, only one environment of TMA is expected for the sample S1 since the sample S1 is of pure Zeolite-Y crystals. TGA and DTG plots (Figure 9) of S1 and S4 samples collected

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at different times (5, 6 and 7 days) during the syntheses were analysed further in order to confirm these differences between the sample S1 and S4 in terms of TMA environment. Indeed as shown in DTG plots, one TMA decomposition step ca. 300 - 550 °C in case of S1 and two TMA decomposition steps ca. 250 - 350°C and 350 - 550 °C in case of S4 sample were observed. The two distinct weight losses are attributed to the existence of TMA template free within the supercage and template bound to extra-framework cation sites on the walls of the supercage [28].

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Parameters like gel composition, water content (H2O/SiO2), alkalinity (H2O/Na2O), crystallization time and temperature, hydrothermal conditions of synthesis etc. determines the ratio of Si/Al. By increasing the TMABr/TMAOH (S) ratio in the synthesis mixture, synthesis of

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zeolite Y nanocrystals with a SiO2/Al2O3 of ratio of 5.84 was demonstrated by Brett A. Holmberg et al. [28]. Lots of efforts have been made in developing nontoxic and inexpensive organic structure directing agents (SDAs) to synthesize high silica zeolite Y, such as oxides),

inositol,

N-methyl

pyridinium

iodide

and

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poly(ethylene

1-ethyl(butyl)-3-

methylimidazolium bromide, which all have been successfully used to synthesize high silica

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zeolite Y with a Si/Al ratio of 3–3.5. Highly crystalline zeolite Y with a SiO2/Al2O3 ratio of higher than 7 is synthesized with TEAOH as an organic SDA. Pure FAU zeolite is synthesized within a narrow region of the TEA+/Na+ ratio and the OH-/Al ratio in the starting gel. Moreover, it is found that the TEA+/Na+ ratio of the starting gels has an important effect on the Si/Al ratio

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of the products. Compared with conventional zeolite Y synthesized without organic SDAs, Y zeolites synthesized with TEAOH as an SDA have remarkably higher Si/Al ratios and exhibit outstanding hydrothermal stability and thermal stability. The highest Si/Al ratio of the samples

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could reach 3.88 [39]. Preparation of nanocrystalline zeolite-Y of Si/Al >3 is currently explored. The pH of synthesis solution reflects the alkalinity/alkali concentration of the system. The

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increase in alkalinity (NaOH + TMAOH) of the synthesis system resulted increase in the size of initial silica colloidal particles as observed by small-angle X-ray scattering technique [40, 41]. A longer crystallization time of the zeolite was observed from the larger size of initial colloidal silica [40]. A high degree of aggregation or clustering of particles leading to bigger particle was clearly observed in the case where hydrolysis and condensation reactions were not better controlled whereas, in the case of a clear and highly alkaline solution system containing

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dissolved silicate anions and aluminate anions, the aluminosilicate polymerization reaction is slow and only loosely aggregated aluminosilicate structures are formed [41]. TMAOH is the most typical organic template and has excellent properties for controlling the zeolite size.

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Reduction of the particle size increases the ratio of the number of external to internal atoms and causes the zeolite nanoparticles to have large external surface areas and high surface activity.

and the water content [7, 14] 3.3 Nitrogen Sorption Isotherms Analysis

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The other variables affecting the crystal size of zeolite Y are temperature, aging time, alkalinity,

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The N2 adsorption-desorption isotherms of the nanocrystalline Zeolite-Y samples (S1, S2, S3 and S4) and commercial microcrystalline Zeolite-Y are shown in Figure 10. All the samples showed Type I sorption isotherm signifying the microporous nature of Zeolite-Y. A hysteresis adsorption-desorption curve at high relative pressure P/P0 of ca. 0.7-1 was observed in the

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sorption isotherms of the nanocrystalline Zeolite-Y samples. Such hysteresis sorption curve at P/P0 of ca. 0.7-1 was absent for the microcrystalline Zeolite-Y. Such hysteresis curve at high P/P0 is related with large mesopores of sizes well above 2 nm. This implied that all the

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nanocrystalline Zeolite-Y samples contain some amount of mesoporosity. The mesopores are derived from interparticle voids present within the aggregated nanocrystals. Furthermore, there

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was difference among the nanocrystalline Zeolite-Y samples in terms of size and onset P/P0 value of hysteresis curve. The size of hysteresis curve was in increasing order of S4S2>S3>S4 signifying that the amount of mesopores present was in increasing order of S4
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characteristics from analysis of the isotherms of the samples in terms of BET surface area, micropore volume, mesopore area and total pore volume are given in Table 4. A maximum pore volume (total) 0.91 cm3/g was observed for S1 sample (nanocrystalline Zeolite-Y) which is 3-

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times the pore volume of microcrystalline Zeolite-Y. All the nanocrystalline Zeolite-Y samples have higher pore volumes due to the presence of additional mesopores than that of

15-20% lower than that of the microcrystalline Zeolite-Y. 3.4. Gas adsorption properties

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microcrystalline Zeolite-Y. However, micropore volume of nanocrystalline Zeolite-Y was about

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CO2 adsorption isotherms for the nanocrystalline Zeolite-Y samples were measured at room temperature and pressure up to 850 mm Hg. The isotherms are shown in Figure 11 that can be expressed well with Toth isotherm equation [42], which is

ns KP [1 + ( KP ) m ]1 / m

........(1)

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n=

Where n is the adsorbed volume, ns is the monolayer capacity and m is a constant parameter. P is the pressure at equilibrium state. K is also a constant parameter related to the Henry’s law

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slope since the product of nsK is the Henry’s law slope, which is directly related to the CO2 interaction with the surface. Higher value of the Henry’s slope indicates a stronger CO2

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interaction with the surface. Similar slope with CO2 adsorbed volume of ca. 90 cm3/g was observed for all the samples. Similar CO2 adsorption isotherms were observed for other reported Zeolite-Y samples [43, 44]. However, the CO2 adsorption capacity may be influenced by the type of cation and moisture present within the Zeolite pores and cages [44, 45]. The nanocrystalline Zeolite-Y reported here had ca. 60% Na and 40% H as exchangeable cations.

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Figure 12 shows the N2, O2 and CH4 isotherms for all the samples. A linear increase in the absorbed gas as a function of increasing pressure was observed. This is exactly with the expression of Henry’s law which states that the amount of adsorbed gas is directly proportional

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to the partial pressure of the gas. The amount of CO2 adsorbed on the Zeolite was much higher than that of other adsorbed gases (N2, O2 and CH4) at similar temperature and pressure. The CO2/N2 and CO2/CH4 ratios are shown in Figure 13 showing high CO2 selectivity over the other

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gases. CO2/N2 selectivity of about 300 and CO2/CH4 selectivity of about 200 was observed at lower relative pressure for the samples. This is because of a relatively stronger interaction of CO2

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with the Zeolite-Y expressed by Toth equation of combined monolayer adsorption and pressure dependent adsorption of gases whereas the other isotherms follow Henry’s law of only pressure dependent adsorption of gases.

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4. Conclusions

Nanocrystalline Zeolite-Y of uniform particle size of 40 nm was synthesized by a hydrothermal method using a concentrated colloidal solution system with high amount of TMA

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cation. A decrease in the TMA content resulted to the Zeolite-Y crystals of bigger size up to 60 nm along with some presence of Zeolite-A impurity. A high amount of TMA cations resulting to

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nanocrystalline Zeolite-Y crystals might be due to generation of a large number of nuclei during the synthesis. The nanocrystalline Zeolite-Y had excellent pore volume of ca. 0.91 cm3/g which is about thrice the amount obtained by conventional microcrystalline Zeolite-Y. The high pore volume of the nanocrystalline Zeolite-Y is due to the additional presence of large mesopores. Adsorption isotherms of pure gases CO2, N2, O2, CH4 on the Zeolite-Y samples were measured at room temperature and pressures up to 850 mmHg. A strong CO2 sorption with the Zeolite-Y

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was observed at lower relative pressure as according to the model expressed by Toth equation whereas the adsorption isotherms of other gases followed a weak adsorption (pressure dependent) of gases according to the Henry’s law. CO2/N2 selectivity and CO2/CH4 selectivity of

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ca. 300 were observed for the nanocrystalline Zeolite-Y at lower relative pressure which shows potential for promising platform for application in flue gas (CO2/N2) separation and natural gas

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(CO2/CH4) purification.

Acknowledgements

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Financial assistance as research grants from the Council of Scientific & Industrial Research (CSIR network project CSC0104) Government of India as well as the instrumentation facility provided by Analytical Discipline & Centralized Instrument Facility, CSIR-CSMCRI,

References

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Bhavnagar, are gratefully acknowledged. CSIR-CSMCRI Registration No. 123/ 2016

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[17] S. Mintova, N.H.Olson, J. Senker, T. Bein, Angew. Chem., Int. Ed. 41 (2002) 2558. S. Mintova, N.H. Olson, V. Valtchev, T. Bein, Science 283 (1999) 958. [18] B. J. Schoeman, J.Sterte, J.E. Otterstedt, Zeolites 14 (1994) 110. [19] L.C. Boudreau, J.A. Kuck, M. Tsapatsis, J. Membr. Sci. 152 (1999) 41.

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[23] H.T. Wang, Z.B. Wang, Y.S. Yan, Chem. Commun. (2000) 2333. [24] H.T. Wang, L.M. Huang, B.A. Holmberg, Y.S. Yan, Chem. Commun. (2002) 1708. [25] H.T. Wang, B.A. Holmberg, Y.S. Yan, J. Mater. Chem. 12 (2002) 3640. [26] H, Wang, B.A. Holmberg, Y. Yan, J. Am. Chem. Soc. 125 (2003) 9928. [27] A. Nouri, M. Jafari, M. Kazemimoghadam, T. Mohammadi, Clays and Clay Minerals, 60 (2012) 610. [28] B.A. Holmberg, H. Wang, Y. Yan, Microporous Mesoporous Mater. 74 (2004) 189. [29] G.W. Skeels, D.W. Breck in D.H. Olson, A. Bisio (Eds.), Proceedings of 6th International Zeolite Conference, Butterworths, Guildford (1984)

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[42] J. Toth, in A. Myers, G. Belfort, (eds.) Fundamentals of Adsorption, Engineering Foundation, New York, 1984

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Table 1. Comparison of the present synthesis system with those of literature data Molar Composition

Synthesis time and temp.

Crystal Raw material source size (nm)

0.02Na2O:0.40Al2O3:1.75SiO2:0.96(TMA)2 O:0.96TMABr:100H2O

100°C, 4 days

120

0.01Na2O:0.27Al2O3:0.92SiO2:0.66(TMA)2 O:100H2O:3.68EtOH

100-130°C, 3-7 days

140

0.03Na2O:0.40Al2O3:1.75SiO2:0.96(TMA)2 O:100H2O

100°C, 2-3 days

30 - 50

Ludox HS-30, NaOH, TMAOH, Mintova Aluminium isopropoxide 1999 [7]

0.03NaCl:0.28Al2O3:1.13SiO2:0.77(TMA)2 O:100H2O

100°C, 14 days

80

Al powder, TEOS, TMAOH, NaCl

0.01Na2O:0.27Al2O3:0.92SiO2:0.66(TMA)2 O:100H2O

100°C, 6 days

~ 50

NaOH, TEOS, isopropoxide, TMAOH

0.01Na2O:0.30Al2O3:2.00SiO2:1.14(TMA)2 O:100H2O

100°C, 1 day

200

Ludox HS-30, TMAOH, NaOH, Larlus et al 2006 Aluminium isopropoxide [14]

0.03Na2O:0.38Al2O3:0.76SiO2:0.91(TMA)2 O:100H2O: 6.06EtOH:2.27PrOH

95°C, 3.5 days

~ 50

NaOH, TMAOH, TEOS, Aluminium Song et al. 2005 isopropoxide, isopropanol, ethanol [15]

~ 40

Ludox AS-40, NaOH, TMAOH, This work Aluminium isopropoxide

100°C, 7 days

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0.03Na2O:0.63Al2O3:4.18SiO2:2.38(TMA)2 O:100H2O

Reference

et

al.

Zhu et al. 1998 [10]

Aluminium Taufiqurrahmi et al. 2011 [13]

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Table 2. Synthesis molar composition for the initial colloidal solutions of the samples Synthesis molar composition Sample

Na2O

Al2O3

SiO2

SiO2/ Al2O3

(TMA)2O

S1

0.029

0.63

4.18

6.6

2.38

100

S2

0.019

0.41

2.71

6.6

1.54

100

S3

0.017

0.36

2.38

6.6

1.35

100

S4

0.014

0.30

2.00

6.6

1.14

100

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Table 3.

H2O

Si/Al and Na/Al ratios of the zeolite samples

Si/Al

Na/Al

S1

1.77

0.59

S2

1.81

0.54

1.80

0.55

1.69

0.60

S3

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Sample

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Table 4.

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Comparison of pore characteristics of nanocrystalline Zeolite Y (S1-S4) with conventional microcrystalline Zeolite Y Sample S1

S2

S3

BET Surface Area (m2/g)

590

582

656

BJH Adsorption Mesopore Area (m2/g)

241

169

BJH Desorption Mesopore Area (m2/g)

289

184

635

744

161

160

38

164

154

33

0.91

0.69

0.63

0.68

0.35

0.23

0.24

0.24

0.26

0.31

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t-Plot Micropore Volume (cm3/g)

Zeolite Y

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Total Pore Volume (cm3/g)

S4

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Characteristics

3

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TMAOH + Aluminium isopropoxide + 0.5M NaOH Soln + Distilled Water in Polypropylene Bottle

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Add Ludox AS-40

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Stirring at 400 rpm for 2 h

Stirring at 400 rpm for 1 h

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Colloidal Solution Bottle set in pre-heated oil bath at static condition 100°C, 7 days Centrifuge and dry

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Nanocrystalline Zeolite-Y

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Fig 1. Schematic representation for preparation procedure of nanocrystalline Zeolite-Y

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15000

Nanocrystalline Zeolite-Y

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intensity

10000

5000

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Microcrystalline Zeolite-Y

10

20

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40

50

2 theta degree

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Fig 2. XRD patterns of nanocrystalline and microcrystalline Zeolite-Y

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S4

S3

25000

7 days

40000

20000

intensity

intensity

7 days

6 days 15000

5 days

6 days

30000

5 days 20000

10000

4 days

4 days

3 days

3 days 0 10

20

30

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10000

5000

40

0

50

10

2 theta degree

20

30

40

50

2 theta degree

40000

S2

20000

S1

35000

7 days

intensity

6 days 10000

5 days

20000 15000

6 days

5 days

10000

4 days

5000

25000

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intensity

7 days

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30000

15000

4 days

5000

3 days

3 days

0

0 5

10

15

20

25

30

35

2 theta degree

40

45

50

10

20

30

40

50

2 theta degree

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Fig. 3. XRD patterns of nanocrystalline Zeolite-Y samples (S1, S2, S3 and S4) prepared from different synthesis molar compositions

S1

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S4

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S3

Fig. 4. SEM images of the S1, S2, S3 and S4 samples

10 nm

20 nm

10 nm

S1

5 nm

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5 nm

10 nm

S3

10 nm

10 nm

S4

10 nm

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20 nm

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S2

20 nm

Fig. 5 (a).TEM images of the S1, S2, S3 and S4 samples at 3 different magnifications

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S4 (5 days)

Bigger size

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S4 (6 days)

Fig. 5 (b). TEM images of the S4 sample obtained from the synthesis time of 5 and 6 days

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30

S1

20 15 S2

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10

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Intensity (a.u.)

25

S3

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S4

0 2

3

4 5 6 7

10

nm

2

100

3

4 5 6 7

1000

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Fig. 6. Particle size distribution of nanocrystalline Zeolite-Y samples as observed from DLS

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7 days 1405 1422

80

1488

6 days

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% T (a.u.)

60

5 days

40

4 days

3 days

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20

0 1300

1400

100

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1384

1500

1600

1700

1800

1900

7 days

3013

5 days 4 days

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60

40

3 days

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% T (a.u.)

6 days

TE D

80

20

0

500

1000

1500

2000

cm

2500

3000

3500

4000

-1

Fig. 7 (a). FT-IR spectra of the S1 sample obtained from the hydrothermal synthesis at 100 °C for various synthesis times

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100

S1 7 days 1405 1422

90

1488

S4 7 days 1405

70

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% T (a.u.)

80

1422

S4 6 days

60

1405

SC

50

S4 5 days

1365 1403

30 1200

1400

100

1600

1800

2000

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S1 7 days

80

S4 7 days

S4 6 days

S4 5 days

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% T (a.u.)

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40

40

20

0 500

1000

1500

2000

2500 -1

3000

3500

4000

cm Fig. 7 (b). FT-IR spectra of the S4 sample obtained from the hydrothermal synthesis at 100 °C for various synthesis times

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0.0 -0.2

-0.8 -1.0

S1 S2 S3 S4

300

-1.2

460

-1.4 100

200

90 85 80

S1

75 100 95 90

95 90 85 80 75 100

95

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90

S2

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80

S3

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Wt. %

85

100

400 600 °C

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95

75

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-0.6

SC

DTG (a.u.)

-0.4

85 80

S4

75

200

400

600

°C

Fig. 8. TGA and DTG plots of the S1, S2, S3 and S4 samples

800

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0.0

0.0

-0.2

-0.2 -0.4

DTG (a.u.)

-0.6 -0.8

S1 (7 days) S1 (6 days) S1 (5 days)

-1.0 -1.2

-1.0

-1.4 300

400

500 °C

600

700

200

300

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100

S4 (7 days) S4 (6 days) S4 (5 days)

500 °C

600

700

95

100

90

95

85

90 85 75

7 days

80

7 days

80

75

Wt. %

100

100 95 90 85

6 days

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80 75

S1

100 95 90 85

5 days

80

200

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75 400

600

95 90 85

6 days

80 75

100

S4

95 90 85

5 days

80 75

800

°C

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Wt. %

DTG (a.u.)

-0.4

Fig. 9. TGA and DTG plots of the S1 and S4 samples

200

400

°C

600

800

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S1 1000 S2

S3

600

SC

800

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Quantity Adsorbed (cm³/g STP)

1400

400

S4

200

0 0.0

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Zeolite Y (microcrystalline)

0.2 0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

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Fig. 10. N2 adsorption-desorption isotherms of the nanocrystalline (S1, S2, S3 & S4) and microcrystalline Zeolite-Y samples. Filled symbols: Adsorption; Empty symbols: Desorption

160 140 120 100 80 60 40 20 0

S1 S2

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S3

CO2

0

200

SC

S4

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Quantity Adsorbed(cm3/g)

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600

800

1000

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Absolute Pressure (mmHg)

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Fig. 11. CO2 adsorption-desorption isotherms for the zeolite-Y samples at room temperature. Filled symbols: Adsorption; Empty symbols: Desorption

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6 4 2

S2

4 2

S3

200 400 600 800 Absolute Pressure (mmHg)

0

1000

10 8

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CH4 N2 O2

8

Quantity Adsorbed (cm /g)

10

200 400 600 800 Absolute Pressure (mmHg)

6 4 2

S1

6 4 2

S4

0

0

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0

1000

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0

3

6

0

0

Quantity Adsorbed (cm /g)

8

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3

8

Quantity Adsorbed (cm /g)

3

Quantity Adsorbed (cm /g)

10

200 400 600 800 Absolute Pressure (mmHg)

1000

0

200 400 600 800 Absolute Pressure (mmHg)

1000

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Fig. 12. CH4 , N2 and O2 adsorption-desorption isotherms for the zeolite-Y samples at room temperature. Filled symbols: Adsorption; Empty symbols: Desorption

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1000

100

S1

S1 10 10

100

1000

1000

100

10

1000

100

100

1000

100

1000

100

1000

S3 10 1000

10

100

S4 10

10 10

10

100

1000

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S4

1

1000

Ratio, CO2/CH4

10

EP

S3

Ratio (CO2/N2)

100

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100

100

1000

10

10

1

100

S2

S2 1

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100

10

SC

1

10

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100

100

Absolute Pressure (mm Hg)

1000

10

Absolute pressure, mmHg

Fig. 13. CO2/N2 and CO2/CH4 adsorption selectivity for the zeolite-Y samples

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Highlights Concentrated colloidal solution synthesis approach for Zeolite-Y nanocrystals

•

Uniform size ca. 40 nm with excellent pore volume of ca. 0.91 cm3/g

•

CO2 selectivity over N2 and CH4 of ca. 300 at lower relative pressure

•

Promising material for CO2 separation of flue gas and natural gas purification

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