Hydrothermal Synthesis of High Crystalline Silicalite from Rice Husk Ash

The objective of this research work was to evaluate the hydrothermal synthesis of silicalite with high crystallinity within a small particle size. The current study focused on investigating the effects of silica sources such as rice husk ash (RHA) and silica gel (SG), crystallization time, and ratios of NaOH/SiO 2 , H 2 O/NaOH, and SiO 2 /TPABr.The crystallinity, particle size, and morphology were characterized by FT-IR, XRD, particle size analyser, and SEM.The conclusion of the main findings indicated that the XRD patterns of these samples clearly showed a pure phase of MFI structure corresponding to FT-IR spectra with vibration mode at 550 and 1223 cm. The highest crystallinity was obtained at reaction time only 6 hours with the mole ratios of NaOH/SiO 2 , H 2 O/NaOH and SiO 2 /TPABr as 0.24, 155, and 30, respectively. When SG was used as a silica source, it was found that the particle size was smaller than that from RHA.The morphologies of all silicalite samples were coffin and cubic-like shape.


Introduction
Silicalite or high silica ZSM-5 is MFI structure.It contains two intersecting channel systems composed of 10-membered ring straight and sinusoidal channels with a unique pore structure dimension of 0.54-0.56nm.According to a variety of its useful properties such as strong hydrophobicity, excellent shape selectivity, good catalytic activity, and high thermal stability, hence, it has been widely used in industrial applications for adsorption [1], catalysis [2], and gas and liquid separation [3,4].
Typically, silicalites are synthesized by the hydrothermal method from the gel compositions of silica, alkaline, and organic cation as a template.Several types of silica source were applied in the synthesis of silicalite such as fume silica in a molar composition of SiO 2 : 0.2 TPABr : 0.5 NaOH : 30 H 2 O at 180 ∘ C for 7 days [5], water glass in a reaction mixture of (1-5) Na 2 O : (50-60) SiO 2 : (1-2) TPABr : (600-770) H 2 O at 200 ∘ C for 72 hours [6], and TEOS as a silica source in a molar composition of SiO 2 : 0.12 TPAOH : 0.008 NaOH : 60 H 2 O at 95 ∘ C for 7 days [7].However, TEOS, fumed silica and colloidal silica are much more expensive compared to rice husk ash which is a potential silica source containing amorphous silica about 20% (w/w) [8] considered as a byproduct in form of an industrial waste abundance in agricultural countries.Moreover, another benefit of silica from rice husk is that it is highly reactive silica which can be simply extracted to be highly purifying silica (about 98%) by digesting with dilute acid and burning at 700 ∘ C for 4 hours [9].
As a result, the major aim of the present work focused on synthesizing high crystalline silicalite using rice husk ash considering the effect of silica sources, reaction times, and gel compositions by hydrothermal method.

Materials.
Silica from the rice husk from a local rice mill in the target area, that is, in Nakhon Ratchasima Province, Thailand, was used as the starting materials in the initial mixture for the silicalite synthesis together with pellets of sodium hydroxide (Merck), tetrapropyl ammonium bromide, TPABr (Sigma-Aldrich), and distilled water.Simultaneously hydrochloric acid 37% (Carlo-Erba) was used to prepare 1 M HCl.

Methods.
The preparation of the rice husk ash (RHA) extracted from rice husk and silica gel (SG) was in the following steps.
Firstly, the rice husk was thoroughly washed with deionized water and dried at 110 ∘ C overnight.Next, the dried rice husk was digested with dilute acid, by boiling with 1 M HCl for 3 hours, then repeatedly washed with water until it is neutral, and dried at 110 ∘ C overnight.After that the acid digested rice husk was burned at 700 ∘ C for 4 hours until it became clearly noticeable white ash.Finally, the white ash was ground till it became fine powder and later was sieved through a mesh with no.100.
Likewise, silica gel was prepared by dissolving RHA with 1 M NaOH through boiling it by covering its container to form sodium silicate solution.Then, the solution was filtered to remove carbon and silica residue.After the solution was cool, it was titrated with 1 M HCl until pH 4, and then it was left overnight to allow its aging in order to form gel. Later, the gel was crushed and repeatedly washed with distilled water until it was free from chloride ions and it was left overnight to dry at 120 ∘ C. Finally, the product was ground and sieved with the similar process as RHA.

Synthesis
Procedure.The procedure of silicalite synthesis was carried out from the system SiO 2 -NaOH-TPABr-H 2 O with the mole ratios of NaOH/SiO 2 , H 2 O/NaOH, and SiO 2 / TPABr within the ranges of 0.24-0.48,155-618, and 30-60, respectively.To begin, after the required amount of NaOH, TPABr, silica, and water was mixed together, it needed to be stirred continuously 5 minutes.Then, the following step was the hydrothermal treatment in a stainless steel bomb lined with PTFE under autogenous pressure at 187 ∘ C under various reaction times.Next, after reaching the desirable time, the reaction was stopped through being quenched with distilled water.After that, it was allowed to cool.Later, it was carefully filtered and thoroughly and repeatedly washed with distilled water.Lastly, it was dried overnight at 120 ∘ C. The synthesized silicalite samples under different conditions were denoted as SL1, SL2, SL3, SL4, SL5, SL6, and SL7 (see Table 2).

Characterization. Fourier transform infrared (FT-IR)
spectroscopy was carried out by Perkin Elmer Spectrum GX.The spectra were recorded within the range of 2000-400 cm −1 with 32 scans at a resolution of 4.0 cm −1 and using KBr pellet technique.X-ray powder diffraction (XRD) was collected on Siemens D5005 diffractometer using Cu K radiation.The percentage of the relative crystallinity was calculated from the ratios of the area of the highest intense reflection peak at 101, 200, 501, 151, and 133 as a reference and as the area of those peaks from the other samples.The chemical compositions of RHA and SG were determined by X-ray fluorescence spectrometer (Oxford DE200).The BET specific surface area and particle size analyzer were carried out by a Micrometric ASAP 2000 and Malvern instruments, respectively.The morphologies of the samples were detected   2). by SEM after gold coating with the operation of a JEOL instrument at 20 KV.

Results and Discussion
The chemical compositions and BET surface areas of silica sources are shown in Table 1.The purity of white RHA product is 98.78% with a little impurity of natural metal oxides.While the metal oxide and unburned carbon were removed during the procedure to prepare, SG could be clearly observed.Interestingly, the BET surface area was greatly increased from 246.91 to 790.33 m 2 /g.

Fourier Transform Infrared Spectroscopy.
The FT-IR spectra of all the samples are shown in Figure 1.All the synthesized silicalite samples (see Figure 1(c-f)) reflect the vibration band at 450, 796, and 1087 cm −1 corresponding to the typical Si-O-Si bending, Si-O-Si symmetric stretching (outer SiO 4 tetrahedron), and Si-O-Si asymmetric stretching (inner SiO 4 tetrahedron) within silica framework, respectively [10].The clearly observed vibration modes at 547 and 1219 cm −1 are attributed to double ring tetrahedral vibration and asymmetric stretching of Si tetrahedral in the zeolite framework, correspondingly resulting in MFI-structured zeolite [11,12], and it was not observed in amorphous silica.The bending vibration of adsorbed water appears at 1633 cm −1 .When comparing the spectra of RHA and SG (see Figure 1(a-b)), the additional band of Si-OH at 980 cm −1 is noticeable for SG.

X-Ray Diffraction.
Figure 2 shows the XRD patterns of all silicalite samples also demonstrate a pure phase of MFI structure.RHA and SG show amorphous phase with a broad hump at the 2 of around 20-22 ∘ (see Figure 2(a-b)).The typical characteristic patterns of silicalite zeolite indicate its indexable peaks as (101), ( 200), (501), (151), and (133) reflections [13,14].The crystallinity of silicalite is strongly influenced by the starting gel compositions as shown in Table 2.When RHA was used as a silica source with higher ratio of SiO 2 /TPABr (SL1-SL2), a crystallinity percentage sharply decreased from 90% to 43% due to a lower amount of template.Obviously, the mole ratio of SiO 2 /TPABr is 30 optimal for silicalite synthesis.Similarly, with a higher mole ratio of H 2 O/NaOH (SL2-SL4), the crystallinity percentage greatly dropped from 95% to 42%.With a higher mole ratio of NaOH/SiO 2 , the amount of TPA decreases due to a reaction with NaOH solution producing tripropylamine and propane [15].As a result, the percentage of crystallinity decreases.The optimal condition of the silicalite synthesis with RHA as a silica source indicated that the mole ratios of NaOH/SiO 2 , H 2 O/NaOH, and SiO 2 /TPABr became 0.24, 155, and 30, respectively, under only 6 hours of reaction time.When RHA was manipulated to improve some properties to become SG (see Table 1), this SG was then further applied as a silica source for silicalite synthesis under the same condition as RHA at the optimal condition.The highest crystallinity is the result.Moreover, a very small amount of template, TPABr, was used in the synthesis of silicalite in this work when compared to those as reported elsewhere [5,7,10,14].

Scanning Electron Microscope.
The scanning electron micrographs of silicalite (SL1-SL7) are shown in Figure 3.The SEM images reveal that all silicalite products emerge in   2).forms of two morphologies of coffin and cubic-like shape.In addition, it indicates amorphous nanoparticle on the crystal face of all silicalite samples when RHA is used as a silica source (SL1-SL6).On the opposite, if SG is used as a silica source (SL7), a clean surface clearly occurs.Figure 4 demonstrates the particle size distribution of two silicalite samples synthesized from RHA and SG under the condition of SL6 and SL7, accordingly.An average particle size of silicalite synthesized from RHA (∼10 m in length) is larger than that from SG (∼5 m).It can be evidently explained that a larger specific surface area of SG could depolymerize silica with NaOH more rapidly than that of RHA resulting in inducing faster nucleation.Additionally, with SG as a silica source, a more narrow size distribution was observed.

Conclusions
It is obvious that silica source and synthesis gel composition strongly influences crystallinity, particle size, and morphology of silicalite.An improvement of some properties of rice husk ash such as an increment of a specific surface area and a reduction of impurity can be successfully done by gel formation with a suggested procedure as an outcome of this study.The highest crystallinity, a narrow size distribution, and a smaller particle size of synthesized silicalite are efficiently achieved from silica gel as a silica source by hydrothermal method.

Table 1 :
The chemical compositions and BET surface areas of silica sources.

Table 2 :
Mole ratios of gel compositions for silicalite synthesis.