Capítulo 4 – Resultados e Discussão
6. SUGESTÕES PARA FUTUROS TRABALHOS
6.1 SUGESTÕES PARA FUTUROS TRABALHOS
No decorrer do desenvolvimento deste trabalho percebeu-se que alguns aspectos do material em estudo necessitavam de melhor investigação. Os completos esclarecimentos destes pontos permitem conhecimento mais detalhado destes materiais de forma a destiná- los a um maior aproveitamento industrial. As seguintes sugestões visam completar e expandir o conhecimento sobre esses catalisadores recomendando-se caracterizar estes catalisadores por outras técnicas como Redução a Temperatura Programada (RTP), Fluorescência de Raios-X (FRX), Ressonância Magnética Nuclear de alumínio e silício (RMN), Microscopia Eletrônica de Transmissão (MET) e Espectroscopia Fotoelétrica de Raios-X (EFX).
• Utilizar outras técnicas de medida de acidez, como adsorção de piridina ou amônia seguida de espectroscopia de infravermelho, para estudar a natureza dos sítios ácidos (Bronsted ou Lewis), presentes na superfície dos catalisadores;
• Testar os catalisadores em outras reações modelo para avaliar a sua eficiência em alguns processos industriais tais como: isomerização do xileno, hidroisomerização, etc. variando a velocidade espacial e temperatura de reação;
• Estudar as propriedades de troca iônica do SAPO-11, para avaliar a possível introdução de metais neste processo de modo a gerar catalisadores para utilização em reações bifuncionais;
• Estudo mais detalhado da regeneração destes materiais utilizando-se outras técnicas; • Realizar estudos de estabilidade térmica e hidrotérmica visando melhorar esta estabilidade frente a condições agressivas de reação sob presença de vapor de água,
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Synthesis and characterization of SAPO-11 molecular sieve synthesized in hydrothermal media
Thiago Chellappaa,*, Rosane B. Oliveiraa, Edjane Buritib, Valter J. Fernandes Jr.b, Dulce Melob, Antonio S.Araujob
a
Department of Materials Science and Engineering, Federal University of Rio Grande do Norte, P.O. Box 1662 59078-970, Natal/RN, Brazil.
b
Department of Chemistry, Federal University of Rio Grande do Norte, 59078-970, Natal/RN, Brazil.
Abstract
A microporous SAPO-11 Molecular sieve was synthesized by the hydrothermal method, using a single agent, as an organic template: di-isopropylamine. The obtained solid was calcined at 550° C for three hours, after which the flow of nitrogen was exchanged for that of synthetic air and submitted for another ten hours of calcination, so as to remove the single agent: di-isopropylamine, which after the removal of the template could be observed by the high crystallization of the sample, Furthermore the molecular sieve was characterized by XRD, FT-IR, SEM and N2.Adsorption desorption (BET analysis). The obtained
catalyst proved to have a high potential catalytic activity and selectivity, through the obtained characterization results.
Keywords: Catalysts; Characterization methods; Microporous materials; Molecular sieves; SAPO-11.
* Corresponding Author. Tel/fax: + 55 84 3202 3004
1. Introduction
Silicoaluminophosphate molecular sieves denoted as SAPO were synthesized by Lok et al. [1] and composed of strictly alternating AlO4, PO4and SiO4tetrahedra. Among these SAPO materials, SAPO-11
has the AlPO4-11 (AEL) topology, comprising of unidirectional, non-intersecting, 10-membered ring channels. With elliptical pore apertures of 0.39 nm_0.63 nm. [2]
Silicoaluminophosphates are an important class of adsorbents and catalytic materials generated by the introduction of silicon into its respective aluminophosphate phase framework [3-7]. This
isomorphic substitution can occur by a replacement of one aluminum by one silicon (SM1), replacement of one phosphorous by one silicon (SM2), or replacement of aluminum-phosphorous pairs by two silicon (SM3) [7]. The catalytic activity and medium acid sites can be generated in SAPO-11 by isomorphic substitution of silicon or transition metals for aluminum and phosphorous on its surface [8-12].
SAPO-11 molecular sieve is usually synthesized through traditional static hydrothermal crystallization at 160–220° C using a single agent, such as, di-isopropylamine as structure-directing template, H3PO4as source of P, pseudoboehmite as source of Al and silica sol or tetraethylorthosilicate
(TEOS) as source of Si. The crystal morphology of SAPO-11 synthesized by the traditional hydrothermal method often exhibits pseudospherical or orthorhombic aggregates of cubic plates ranging from 3 to 10 ȝm owing to the rapid congregativeness of crystal nuclei [13–15].
In this present work, the SAPO-11 molecular sieve after the hydrothermal method of synthesis was washed with distilled water, dried and calcined with the intention of easing the removal of the template agent from the micrpores of the catalyst, and furthermore submitted to characterization through: XRD, SEM, FT-IR and N2.Adsorption desorption (BET analysis). The aim of this present work was to
obtain results that indicate that the SAPO-11 molecular sieve has a high potential in relation to its catalytic activity and selectivity.
2. Experimental
2.1 Preparation of the SAPO-11 molecular sieve
The SAPO-11 molecular sieve sample was synthesized by the hydrothermal method and prepared as follows. First, pseudoboehmite (70 wt% Al2O3) was dissolved in the distilled water and
remained under magnetic stirring during a period of 30 min, afterwards mixed with orthophosphoric acid (85 wt% H3PO4) which was added dropwise in the remaining distilled water, and kept a
Silicoaluminophosphate (SAPO-11) was synthesized by the hydrothermal method, starting from inorganic sources of precipitated silica, pseudobohemite (Catapal), 85% orthophosphoric acid (Merck), and water. Di-isopropylamine (Riedel) was used as the organic template. The as-synthesized products were washed, centrifuged, filtered, dried at 120°for 3 h, and then calcined at 550°C for another 10 h in air in order to remove the template.
2.2 Characterization
Powder X-ray diffraction patterns (XRD) was recorded on a SHIMADZU-6000 diffractometer, using the CuKa (k = 1.5404 A) radiation at 40 kV and 30 mA with a scanning rate of 2 °/min.
The morphology of the products was examined by a Cambridge S-360 scanning electron microscope (SEM). The composition of the final material was determined by sequential X-ray Fluorescence Spectrometer (SHIMADZU, XRF-1800).
The characterization of surface area and pore volume of the SAPO-11 molecular sieve was performed by using N2adsorption-desorption at -196 °C, in a NOVA 2000 Quantachrome Instruments
surface are & pore size analyzer automatic adsorption apparatus. Samples were outgassed at 350°C for 3 h under a vacuum of 1.33 x 10-3Pa prior to N2physisorption. The BET surface areas of the samples
(ABET) were calculated by applying the BET equation [16] to N2adsorption data in relative vapor pressure
(p/p0) range of 0.05–0.30. The cross-sectional area of N2molecule was taken as 14.0 Å. The pore volume
(Vp) of the catalysts were obtained by extrapolating the upper parts of the desorption branches of the N2
model. The pore size distributions in the pore diameter range of 1.5–100 nm of the samples were obtained by applying the expanded BJH equations [18] to the N2desorption branches of the hysteresis loops. Size
distribution of microspores was determined by the HK method, and the external surface areas of the samples were calculated by the t-plot method [19].
The IR spectrum of the synthesized molecular sieves was recorded on a Nicolet Magna-IR 560 E.S.P. spectrometer in rang of 4000–400 cm-1using KBr as reference. To avoid the effect of water, the samples were treated at 120°C for a period of 2 h.
3. Results and discussion
3.1 Microstructural Properties
3.1.1 XRD
The X-ray powder diffraction patterns of the sample synthesized by hydrothermal media, SAPO- 11 are shown in Fig.2. The characteristic peaks of the SAPO-11 phase (i.e. 2u = 7.4°, 8.1°, 9.5°, 13.1°, 15.6°, 19.8°, 21.0°, 22.1°–23.2°) were observed for all samples, and were very similar to those reported for SAPO-11 in Ref.[20]. Among other peaks that were observed, one in 7.4° was reported, indicating that the samples were almost entirely free from phase impurities. The high intensity of XRD peaks indicated that the sample was highly crystalline. A result that was furthermore corroborated when the cristallinity of the sample was found to be 95%. The X-ray powder diffraction patterns identified that the sample synthesized from aqueous media possessed the AEL structure.
5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 In tens it y (a.u. ) (1 1 0 ) (2 0 0 ) (2 1 1 ) ( 1 3 0 ) (0 0 2 ) ( 3 2 1 ) ( 2 0 2 )
3.1.2 FT-IR Spectra
In agreement with the XRD data, the infrared spectra displayed the gradual formation of SAPO- 11 molecular sieves. In figure 4, the bands at 1636 and 1649 cm-1detected during the aging period are assigned to the C–H bending modes of the template. At the initial period of aging, i. e. aged for 0 min, some characteristic bands of amorphous phase were observed such as 540, 636 and 730 cm-1. With the aging time increasing, the bands of amorphous phase weakened. When the nuclei of SAPO-11 molecular sieves were formed, the characterization absorbed bands of SAPO-11 sharpened as the aging process further proceeded, implying that the SAPO-11 crystals were growing up with time.
4000 3500 3000 2500 2000 1500 1000 500 540 636 730 1095 1633 2988 3610 T% W avenum bers (cm-1 )
Fig 2. FT-IR spectrum of a SAPO-11 molecular sieve synthesized by the hydrothermal method.
Morphological Properties
3.1.3 SEM
The morphology of the sample synthesized by hydrothermal media was characterized by the scanning electron micrographs. As shown inFig. 3, there was little difference in the crystal morphology between SAPO-11 (c) and SAPO-11 (nc). Both of them exhibited pseudo-spherical aggregates ranging from 7 to 10 mm assembled from cubic plate small crystallites. Another interesting observation that could be made is that an orthorhombic geometrical pattern was observed in some of the micrographs, which
Fig 3. SEM images of a SAPO-11 molecular sieve synthesized by the hydrothermal method.
3.1.4 Surface Area
The specific surface area (BET), microporous surface area and external surface area of the samples are listed together with the total microporous volume, mean pore diameter, pore size and relative cristallinity of the samples that were measured. During crystallization, Si was released slowly from the silica sol and the organic phase to the aqueous phase, so that the Si content of SAPO-11 was low. During the synthesis of SAPO-11, the framework of SAPO-11 is identical to that of AlPO-11 with AEL type structure. The cavity volume consists of nonintersecting elliptical 10 membered ring pores of 0.39 nm _ 0.63 nm. The N2adsorption–desorption isotherm curves showed that the samples synthesized from
hydrothermal media had a well-defined adsorption–desorption hysteresis loop above the relative vapor pressure of 0.3, indicating the existence of some mesoporous materials that originated from the secondary pores. The N2adsorption–desorption hysteresis loop of this material belongs to type E of de Boer’s
Table 1. Physicochemical properties of SAPO-11 molecular sieve
ABET(m2g-1)
Sample
Stotala Smicrob Sextc
Microporous Volume (mL/g) Pore size (Å) Pore diameter / (nm) Relative crystallinity (%) SAPO-11 239 173,11 65,9 0,088 14 1,4 85,9 a
total surface area;bexternal surface area;cmicroporous surface area.
Conclusions
SAPO-11 molecular sieves were obtained successfully by synthesis through the hydrothermal media, afterwards were submitted to characterizations by: XRD, SEM, N2Adsorption Desorption and FT-
IR under specific conditions such as, passing through calcination to remove the di-isopropylamine template from the micropores of the catalyst. The obtained results confirm that the sample has the AEL framework indicating a typical silicolauminophosphate structure, has a high cristallinity, viewed through the typical peaks in XRD diffraction patterns, the morphological aspect shows a microporous structure due to its orthorhombic geometry mingled with some pseudo-spherical aggregates and a high specific surface area with a majority of internal surface area, showing the adequacy of the synthesis route used. Consequently, SAPO-11 molecular sieves would be of great interest for potential application in catalytic reactions due to their relatively high surface area.
Acknowledgments
This work was financially supported by grants from Brazilian agencies CAPES, ANP, CNPq and the PPgCEM of the Federal University of Rio Grande do Norte/Brazil, thus the authors wish to
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