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EFFECT OF GEL VOLUME IN AUTOCLAVE ON
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HYDROTHERMAL SYNTHESIS OF MORDENITE
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FROM RICE HUSK SILICA
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Running head: Effect of gel volume on mordenite synthesis
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Suriyan Rakmae and Jatuporn Wittayakun*
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School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon
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Ratchasima 30000, Thailand. Tel.: +66 44 224 256; Fax: +66 44 224 185.
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Email: jatuporn@sut.ac.th
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*Corresponding author
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Abstract:
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Silica prepared by refluxing rice husk with HCl acid followed by calcination
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(refluxed RHS) and dissolution of rice husk ash followed by precipitation
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(precipitated RHS) was used as a silica source for a synthesis of mordenite in
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sodium form (NaMOR) by hydrothermal method. The gel volume in an
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autoclave was 30, 50 and 70%. The NaMOR products from both silica
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sources were analyzed by X-ray diffraction (XRD), X-ray fluorescence
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(XRF), N2 adsorption-desorption, ammonia temperature programmed
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desorption (NH3-TPD) and scanning electron microscopy (SEM). All samples
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showed characteristic XRD peaks of NaMOR. The properties of the products
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including Si/Al ratio, surface area, acidity and morphology were influenced
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by gel volume in the autoclave and the silica source.
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Keywords: Mordenite, hydrothermal synthesis, rice husk silica, reflux,
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precipitation
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Introduction
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In several countries including Thailand, rice husk (RH) is produced in a large
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quantity as a byproduct from rice milling. RH composes of about 20 wt% of
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inorganic compound in which silica is a main component (about 85-98% wt%)
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(Juliano et al., 1985). Several methods to obtain the silica from rice husk with
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high purity are acid treatment of RH before cacination (Liou, 2004, Panpa and
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Jinawath, 2009, Krishnarao et al., 2001) and dissolution RHA with alkali solution
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before precipitation with acidic solution (Kalapathy et al., 2000). Silica from RH
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is amorphous with a uniform dispersion by molecular unit, fine particle size, high
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purity and surface area (Liou et al., 2011). Moreover, when RH is burned, rice
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husk ash (RHA) with silica purity of 76-95 wt% was obtained depending on
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conditions (Kordatos et al., 2008, Naskar et al., 2012, Tepamat et al., 2013). Silica
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from leaching of RH and RHA can be used as a silica source for the synthesis of
42
silicon-based materials.
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With high purity, RHS can be used as a silica source for the production of
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porous inorganic materials such as zeolites LSX (Khemthong et al., 2007), NaY
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(Wittayakun et al., 2008; Khabuanchalad et al., 2008) NaBEA (Loiha et al.,
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2009), NaZSM-5 (Pimsuta et al., 2013) and MCM-41 mesoporous silica (Artkla et
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al., 2008). Moreover, rice husk charcoal was used as a silica source for the
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synthesis of zeolite NaA (Tepamat et al., 2013) and silicon carbide (SiC)
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(Makornpan et al., 2013). The interest of this work was to use silica from rice
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husk for the synthesis of zeolite mordenite (MOR).
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MOR is a zeolite with interconnected pore channels between 12-
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membered ring and 8-membered ring (Baerlocher and McCusker, www). The unit
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cell dimensions of NaMOR were first reported by Meier (1961). It has a pore
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channel between 12-membered ring in the c-axis and 8-membered ring in the b-
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axis. Si/Al ratio of MOR can be varied depending on composition of the synthetic
56
batch (Roland and Kleinschmit, 2003). MOR has been used as a solid catalyst for
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various industrial chemical reactions (Pantu et al., 2007).
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Kulawong et al. (2011) reported a synthesis of NaMOR in sodium form
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(NaMOR) using rice husk silica (RHS). They crystallized the NaMOR in a
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Teflon-lined autoclave with 150 mL capacity at various time and used the optimal
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condition for a synthesis with another autoclave with 250 mL capacity. However,
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the void volume was not reported for both vessels. Thus, it is an aim of this work
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to understand the influence of gel volume in an autoclave on physicochemical
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properties of MOR. Moreover, an effect of the silica source from acid leaching
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and from precipitation was studied.
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Materials and Methods
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Preparation of rice husk silica by reflux and calcination (RRHS)
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RH was obtained from a local mill near Suranaree University of
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Technology, Thailand. It was washed several times with tap water, soaked in DI
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water for 10 min and dried at 70 ºC overnight. Twenty five grams of the dried rice
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husk was filled in a 250-mL round bottom flask, added with 180 mL of 3 M HCl
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solution (prepared from 37% HCl, Merck) and refluxed at 100 ºC for 6 h. The
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refluxed rice husk was washed several times with tap water until pH of the wash
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solution equaled to that of the tap water, soaked in DI water for 10 min, dried at
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70 ºC overnight and calcined at 600 ºC for 6 h in a muffle furnace (CABOLITE)
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with a heating rate of 2 ºC/min.
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Preparation of rice husk silica by dissolution-precipitation (PRHS)
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First, rice husk ash (RHA) was prepared by calcination of the dried RH at
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600ºC in a muffle furnace with heating rate of 2 ºC/min and cooled to room
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temperature. Four grams of RHA was dissolved in 100 mL NaOH solution
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(Prepared from NaOH pellet (Merck) with RHA: NaOH weight ratio of 1:1.9) in
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polyethylene bottle under magnetic stirring with a speed of 300 rpm for 36 h at
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room temperature. Then, the solution was filtered through a filter paper (Whatman
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No.1) by an aspirator to a polyethylene beaker, added with a 6 N H2SO4 solution
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(Sigma Aldrich) dropwise from a burette until the pH was about 11. Then, the
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solution was heated to 90-95 ºC with continuous addition of sulfuric acid.
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Precipitation of silica began when the pH was about 10.5-10.6. Then, DI water
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(50 mL) was added and the gel was broken by magnetic stirring. The H2SO4
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solution was further added until the pH was 7 and the mixture was kept under
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stirring at this condition for 3 h. The precipitated silica was separated by
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centrifugation and washed repeatedly with warm DI water until the pH of washed
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water equal to that of DI water. Then, the precipitated silica powder was dried at
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120 ºC for 24 h in a hot air oven.
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Synthesis of mordenite (MOR)
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The NaMOR was synthesized with a procedure adapted from Kim and
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Ahn (1991). Silica (either RRHS or PRHS) was suspended in DI water to form
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slurry in a polypropylene bottle. Then, sodium aluminate (Riedel-de Haën®,
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41.383% Na2O, 58.604% Al2O3) in NaOH solution was added under a magnetic
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stirring (250 rpm) at room temperature to produce a gel with the following molar
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ratio: 6Na2O:Al2O3:29.2SiO2:780H2O. The mixture was transferred to a Teflon-
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lined stainless steel autoclave and crystallized at 170 ºC (with heating rate 2.5
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ºC/min) without agitation for 120 h. The autoclave volume was 245 cm3 and the
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percent of gel/autoclave volume was 30, 50 and 70%. The sample was named
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according to the silica type and gel volume. For example, R-MOR-30 was the
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product from refluxed RHS with volume 30% of the autoclave volume; P-MOR-
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70 was the product from precipitated RHS with volume 70% of the autoclave
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volume.
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washed thoroughly with DI water and dried at 100 ºC overnight.
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After crystallization, the product was separated by centrifugation,
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Characterization
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Phase of NaMOR was studied by an X-ray diffraction (XRD, Bruker AXS
116
D5005) with nickel-filtered Cu Kα radiation (wavelength = 1.54 Å) scanning in
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the 2θ range of 5–50º, at an increment of 0.02º and scan speed of 1 s/step).
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Elemental analysis was determined by X-ray fluorescence (XRF, Horiba
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Scientific XGT-5200). Morphologies of the silica were studied by scanning
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electron microscopy (SEM, JOEL/JSM-6010LV). All samples were coated with
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gold by sputtering (Neocoater/mp-19020NCTR).
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The surface area of obtained NaMOR was determined by N2 adsorption-
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desorption analysis (Micromeritics ASAP 2010) from the Brunauer-Emmet-Teller
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(BET) method in a relative pressure range of 0.01–0.2. Each sample was first
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degassed at 300 ºC and the analysis was carried out at liquid nitrogen temperature.
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The acidity of obtained NaMOR was determined by temperature-
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programmed desorption of ammonia (NH3-TPD) in a Belcat-B equipped with a
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thermal conductivity detector. A sample amount of 250 mg was packed in a
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tubular U-shaped quartz cell, preheated at 500 ºC with a heating rate of 10 ºC/min
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in a He flow (50 mL/min) for 110 min to eliminate physisorbed species and
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cooled to 70 ºC. Then a constant flow of 30 mL/min of 5 vol. % NH3/Ar gas
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mixture was introduced over the samples for 30 min to achieve saturation. After
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purging with He for 5 min to remove physisorbed NH3, then, the sample was
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heated to 130 ºC with a heating rate of 10 ºC/min and held for 3 h to remove
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physisorbed species. The NH3-TPD measurement was performed from 130 to 850
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ºC with a heating rate of 10 ºC/min with He as a carrier gas. The number of acid
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sites was calculated from the peak area compared to a reference material with a
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known number of acid sites (ZSM-5) that was analyzed with the same conditions.
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Results and discussion
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The XRD pattern of the obtained products from refluxed rice husk silica
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including R-MOR-30, R-MOR-50 and R-MOR-70 are illustrated in Figure 1. The
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pattern of all samples contained peaks with similar positions to those of NaMOR
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reported in literatures (Kulawong et al., 2011; Zhang et al., 2009, Mignoni et al.,
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2008). The order of relative peak intensity changed when the gel volume. From R-
146
MOR-30 the strongest peak was 9.8º followed by 25.7º corresponding to the plane
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200 and 202, respectively. From R-MOR-50 the order of relative intensity was
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similar but the second peak became stronger. From R-MOR-70 a switch in the
149
intensity order was observed. The growth of zeolite crystal depends on solubility
150
of the starting material and equilibrium within the autoclave during
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crystallization. The larger volume of the gel led to the higher autogeneous
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pressure in the autoclave and resulted in higher solubility of the starting material.
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The crystal growth in the 202 plane for from reflux RHS increased with the gel
154
volume. Kulawong et al. (2011) reported that NaMOR synthesized with a 150 mL
155
capacity autoclave showed the strongest XRD peak at 25.6º. When the synthesis
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was done with a 150 mL capacity autoclave, the peak at 9.8º became a dominant
157
one.
158
The XRD pattern of the obtained products from precipitated rice husk
159
silica including P-MOR-30, P-MOR-50 and P-MOR-70 are illustrated in Figure 2.
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The patterns of all samples also contain characteristic peaks of NaMOR
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(Kulawong et al., 2011; Zhang et al., 2009, M. L. Mignoni et al., 2008) with the
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strongest peak from the plan 202. The intensity of the plan 200 increased with the
163
gel volume. From P-MOR-50 and P-MOR-70, the intensity of the peak at 6.5º
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(corresponding to the plane 110) was significantly higher than that from P-MOR-
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50. The XRD pattern depends on the synthesis conditions. For example, it was
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reported by Structure Commission of the International Zeolite Association that
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intensity of the plan 200 was the highest in NaMOR with Si/Al ration of 5
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(Baerlocher and McCusker, www).
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The Si/Al ratios of all samples determined by XRF are listed in Table 1.
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The ratio in the product from both silica sources was higher than that in the gel.
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The Si/Al ratio of NaMOR from refluxed RHS increased with the gel volume.
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This result could be from the higher solubility of RHS for the sample with the
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higher gel volume. In case of NaMOR from precipitated RHS, the change in Si/Al
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ration could not be related to the gel volume. The lowest Si/Al ratio was from P-
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MOR-50. From observation the silica from precipitation could be dissolved more
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easily than the refluxed RHS.
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The N2 adsorption-desorption isotherm of the NaMOR are presented in
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Figure 3. The isotherm of all samples was type I which is the characteristic of
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microporous materials. The adsorption increased rapidly at low relative pressure
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corresponding to the adsorption in micropores and became nearly constant
181
afterward. The BET surface areas are included in Table 1. The surface area of
182
NaMOR synthesized with refluxed RHS increased with the gel volume. The
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surface area of R-MOR-30 was significantly lower than R-MOR-50 and R-MOR-
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70 implying that R-MOR-30 had lower crystallinity. This could be the effect from
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lower pressure in the autoclave which resulted in the lower solubility of RHS. In
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contrast, the surface areas of NaMOR prepared from precipitated RHS were
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similar. This result may due to the higher solubility of RRHS.
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The NH3-TPD profiles of the NaMOR synthesized with different volumes
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are shown in Figure 4. Normally, NaMOR exhibited only one peak at low
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temperature (Katada et al., 2007). All NaMOR from this work also exhibited an
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additional small peak at high temperature. Similar observation was reported by
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Kulawong et al. (2011). The peak at low and high temperature corresponded to
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ammonia desorbed from Brønsted and Lewis acid sites, respectively (Karge and
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Dondud, 1989; Ma et al., 2000). The intensity of desorption of NaMOR from both
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silica sources increased with the gel volume. The desorption temperature of R-
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MOR-70 and P-MOR-70 was lower than that of other samples corresponding to
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the weaker acid sites. Ma et al. (2000) proposed that the ammonia desorption from
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ZSM-5 at low temperature associated with the ammonia adsorbed either on Si–
199
OH or from non-zeolitic impurity. The peak desorbed at high temperature was
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related with the ammonia adsorbed on Si–OH–Al.
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SEM micrographs of the NaMOR synthesized from refluxed and
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precipitated RHS are illustrated in Figure 5 and 6, respectively. Particles of
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NaMOR synthesized from refluxed RHS (Figure 5) were not uniform in size and
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shape. Aggregated plate-like sheets and amorphous-like matter were observed.
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Particles of NaMOR synthesized from precipitated RHS (Figure 6) were more
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uniform with less amorphous-like matter. The particle size decreased with the gel
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volume in the autoclave. The larger crystal size related to small number of nuclei
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formation in the synthesis gel (Cubillas and Anderson, 2010; Cundy and Cox,
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2005). Moreover, the uniform of the zeolite crystal synthesized by using
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hydrothermal method crucially related to homogeneous distribution of the starting
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nuclei in the system (Mintova and Valtchev, 2002).
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Conclusions
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NaMOR was synthesized by hydrothermal method using refluxed and precipitated
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rice husk silica (RHS). All products gave characteristic XRD peaks of NaMOR
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with differences in relative intensity.
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refluxed RHS changed from plane 200 to 202 with the increase of starting gel in
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the autoclave. On the other hand, the strongest peak in NaMOR from precipitated
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RHS was from the plan 202. In NaMOR from refluxed RHS the Si/Al ratio,
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surface area and Brønsted acidity increased with the gel volume. In NaMOR from
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precipitated RHS the connection between either Si/Al ratio or surface area and the
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gel volume could not be made, however, the Brønsted acidity increased with the
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gel volume. The NaMOR from precipitated RHS had higher surface area and
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uniformity in particle shape and size more than that from refluxed RHS.
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The strongest peak in NaMOR from
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Acknowledgements
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Scholarship for S. Rakmae was from Royal Golden Jubilee Ph.D. program from
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Thailand Research Fund (TRF) and Suranaree University of Technology (Grant
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No. PHD/0173/2553).
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Table 1: Si/Al ratio and surface area of the synthesized MOR
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% Gel
volume in
autoclave
30
Gel
Si/Al
Product
Si/Al
BET surface
area (m2/g)
R-MOR-30
Method to
obtain rice
husk silica
Reflux
14.6
57.6
215
R-MOR-50
Reflux
50
14.6
62.2
357
R-MOR-70
Reflux
70
14.6
65.2
347
P-MOR-30
Precipitation
30
14.6
56.1
435
P-MOR-50
Precipitation
50
14.6
48.1
432
P-MOR-70
Precipitation
70
14.6
49.3
418
Sample
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325
326
327
328
Figure 1 XRD patterns of NaMOR synthesized from rice husk silica from reflux
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with volume 30, 50 and 70% of the autoclave
330
331
332
333
Figure 2 XRD patterns of NaMOR synthesized from precipitated rice husk silica
334
with volume 30, 50 and 70% of the autoclave
335
336
337
338
339
Figure 3. N2 adsorption–desorption isotherm of NaMOR synthesized with various
volumes by using (a) RRHS and (b) PRHS.
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
Figure 4 NH3-TPD spectra of NaMOR with different starting material volume;
361
(a) synthesized from R-RHS and (b) synthesized from P-RHS
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
Figure 5 SEM images of NaMOR zeolites synthesized from R-RHS (a) R-MOR-
381
30 (low magnification) (b) R-MOR-30 (high magnification) (c) R-
382
MOR-50 (low magnification) (d) R-MOR-50 (high magnification) (e)
383
R-MOR-70 (low magnification) (f) R-MOR-70 (high magnification)
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
Figure 6 SEM images of NaMOR zeolites synthesized from PRHS (a) P-MOR-
404
30 (low magnification) (b) P-MOR-30 (high magnification) (c) P-
405
MOR-50 (low magnification) (d) P-MOR-50 (high magnification) (e)
406
P-MOR-70 (low magnification) (f) P-MOR-70 (high magnification)