TiO
2
-TON zeolite synthesis using an ionic liquid
as a structure-directing agent
Christian W. Lopes
a, Pedro Henrick Finger
b, Marcelo L. Mignoni
b, Daniel J. Emmerich
b,
Fabiana Magalh~aes Teixeira Mendes
c, Stefany Amorim
c, Sibele B.C. Pergher
a,*aLaboratory of Molecular Sievese LABPEMOL, Institute of Chemistry, Federal University of Rio Grande do Norte e UFRN, Av. Senador Salgado Filho, 3000,
Lagoa Nova, University Campus, Natal, RN 59072-970, Brazil
bChemistry Department, Universidade Regional Integrada do Alto Uruguai e das Miss~oes, Av. Sete de Setembro, 1621, Erechim Campus, Erechim, RS
99700-000, Brazil
cNational Institute of Technology (INT), Ministry of Science and Technology, Catalysis and Chemical Process Division (DCAP), Av. Venezuela 82, Centro, Rio
de Janeiro, RJ CEP: 21081-312, Brazil
a r t i c l e i n f o
Article history:
Received 24 February 2015 Received in revised form 9 April 2015
Accepted 10 April 2015 Available online 20 April 2015
Keywords: Ionic liquid TON zeolite Synthesis TiO2
a b s t r a c t
The 1-butyl-3-methylimidazolium chloride ionic liquid was employed as a structure-directing agent for the preparation of TiO2-TON zeolites under different crystallization conditions. By varying the crystal-lization parameters (i.e., time, structure-directing agent, and TiO2content), the ionic liquid was deter-mined to be a powerful structure-directing agent for the TON zeolite, which was confirmed by13C NMR. In both stirring and static modes, the TON zeolites crystallize. The analysis of the X-ray diffraction data indicated that the synthesized materials are highly crystalline. X-ray photoelectron spectroscopy ana-lyses indicated that titanium elements were present in an octahedral (anatase) and tetrahedral (bridging the zeolitic structure) environment. Electron probe microanalysis indicated a good distribution of tita-nium in the crystals, possessing rice grain morphology that is characteristic of this zeolitic structure. The ionic liquid was modified during hydrothermal synthesis, and further investigation regarding its possible reuse as a structure-directing agent will be conducted.
© 2015 Elsevier Inc. All rights reserved.
1. Introduction
Since the introduction of zeolites as catalysts by Mobil in the early 1960s, the use of this important group of microporous ma-terials has grown, mainly in the petrochemical and refining in-dustries [1]. The most challenging step in the synthesis of these materials is the synthesis of a microporous solid with a specific structure and a specific catalytic application[2]. In heterogeneous photocatalysis, for example, several studies have focused on developing zeolites with TiO2supported as the active species for
the reaction[3e5]. There has been considerable academic and in-dustrial interest in this class of materials.
The use of structure-directing agents in zeolite synthesis is widespread, and ionic liquids have been recently used as
templates/structure-directing agents in ionothermal[6,7]and hy-drothermal zeolite syntheses[8e10]. The two advantages of ionic liquids compared to traditional structure-directing agents are their negligible vapor pressure and high thermal stability. Syntheses using ionic liquids as templates/structure-directing agents have been reported for zeolites[8,9,11,12], MCM-41 mesoporous mate-rials[13], and other materials[14,15]. The potential to modify the size, shape andflexibility of the ionic liquid molecule makes these compounds interesting for zeolite synthesis in addition to their environmental appeal. However, to the best of our knowledge, studies addressing the post-synthesis behavior of ionic liquids have not been previously reported. This behavior is important due to potential reuse of these compounds.
TON zeolites, which require the use of a structure-directing agent, are medium pore zeolites (0.55 0.45 nm) that are rich in silicon and characterized by a system of one-dimensional channels. Their structure has 5-, 6- and 10-membered rings. However, only the 10-membered rings result in the pore system because the smaller channels do not allow access for molecular diffusion and * Corresponding author. Tel.: þ55 84 94135418.
E-mail address:[email protected](S.B.C. Pergher).
Contents lists available atScienceDirect
Microporous and Mesoporous Materials
j o u r n a l h o m e p a g e : w w w .e l se v i e r. co m/ lo ca t e / m i c r o m e s ohttp://dx.doi.org/10.1016/j.micromeso.2015.04.014
are not considered to be pores[16]. The traditional synthesis of the ZSM-22 zeolite (TON isostructure) uses 1,8-diaminoctane[17]as a structure-directing agent. The pure silica ZSM-22 and Al-ZSM-22 were synthesized with the 1-butyl-3-methylimidazolium ionic liquid in an interesting approach[18].
To the best of our knowledge, although many studies on microporous and mesoporous materials containing titanium have been reported[19e21], zeolites with a TON structure have not been studied. In this study, TiO2-TON zeolites were synthesized using
1-butyl-3-methylimidazolium chloride (Fig. 1), which is a versatile ionic liquid that has been used in interesting synthesis approaches
[10]. Several factors related to the synthesis are described including the character of the ionic liquid after synthesis as well as possible applications.
2. Experimental section 2.1. BMI.Cl synthesis
1-Butyl-3-methylimidazolium chloride (BMI.Cl) was synthe-sized according to a published procedure[22]. In a typical reaction, 1-methylimidazole (1 mol) and 1-chlorobutane (1.1 mol) were dissolved in acetonitrile and subjected to stirring under reflux for 48 h at 80C. Ionic liquid crystallization was performed by adding the BMI.Cl solution to a balloon containing ethyl acetate followed by storage of the solution in a freezer for 24 h. The resulting solid was dried under reduced pressure until a constant weight was achieved. The reaction yield was 75%. The ionic liquid obtained was characterized by 1H and 13C liquid NMR, and the spectra were collected on a Varian Inova instrument operating at 300 MHz.
1H NMR (300 MHz, TMS)
d
/ppm: 0.96 (t, 3H, J¼ 7.3 Hz), 1.38 (m, 2H), 1.91 (m, 2H), 4.14 (s, 3H), 4.34 (t, 2H, J¼ 7.4 Hz), 7.57 (s, 1H), 7.74 (s, 1H) and 10.6 (s, 1H).13C NMR (300 MHz, CDCl3)d
/ppm:12.78, 18.93, 31.64, 36.14, 49.26, 121.8, 123.44 and 136.97. These results are consistent with the data in the literature for this ionic liquid[23].
2.2. TiO2-TON zeolite synthesis
The zeolite synthesis was performed by mixing SiO2 (Aerosil
200, Degussa), BMI.Cl or 1,8-diaminoctane as OSDA, TiO2
(P25-Degussa), NaOH (Aldrich) and H2O. The mixture was stirred for
15 min, and then, three equal volumes of the gel were poured into 60 mL Teflon-lined stainless steel autoclaves, which were main-tained in an oven at 160C for 24 h to 7 days without stirring or with stirring. After the reaction period, the solids werefiltered and washed with water and acetone, and the samples were dried for 24 h at 100C. The corresponding molar composition of the gels is as follows: 1 SiO2: 0.15 OSDA: x TiO2(x for Si/Ti¼ 10, 25, 50, 100,
500 and∞): 0.2 NaOH: 24 H2Oe SeeTable 1.
2.3. Characterization of the obtained zeolites
The characterizations were carried out using calcined materials. The solids were calcined at 600C for 5 h in a muffle furnace at a heating rate of 5C min1. Only13C NMR analysis was developed using without calcining samples.
The structural parameters were evaluated by X-ray diffraction on a Bruker D2 Phaser instrument using CuK
a
(l
¼ 0.154 nm) ra-diation with a Nifilter, a step size of 0.02, a current of 10 mA, avoltage of 30 kV, and a Lynxeye detector.
The integrity of the occluded organic cation in the zeolite pores was verified by NMR using a 500 MHz Agilent Spectrometer (model DD2) operating at 125.7 MHz.
Thermogravimetric analyses were performed under a N2flow
with a TA Instruments device (SDT Q600) using a heating rate of 10C min1to 600C for organic composts using an alumina pan. Diffuse reflectance spectra in the UVeVis region for the obtained zeolites were recorded on a UV-Cary-100 apparatus. The attached DRA-CA-301 (Labsphere) was used to analyze the samples in the reflectance mode.
X-ray photoelectron spectra (XPS) were recorded using a hemispherical spectrometer (PHOIBOS 150 - SPECS) equipped with an X-ray Gun (XR-50) and an Al K
a
source (soft X-ray source at 1486.6 Ev, which is non-monochromatic). The anode was operated at 10 W (10 kV, 10 mA), and the analyzer was operated at a constant pass energy of 50 eV for the survey spectra and 20 eV for the selected regions. The binding energy shifts due to surface charging were corrected using the C 1 s level at 284.6 eV, as an internal standard. The spectra were Shirley background-subtracted across the energy region andfitted using CasaXPS Version 2.3.16. The base pressure in the analysis chamber was maintained at 5 1010to1 109mbar.
Textural characterization of the sample obtained after 1 day of stirring (Si/Ti¼ 25) was performed on an Autosorb-iQ (Quantach-rome Instruments) instrument. The sample was degassed at 250C for 10 h and analyzed with N2at196C. The specific area was
determined using the BET (BrunauereEmmetteTeller) method at relative pressures (p/p0) ranging from 0.001 to 0.01. The micropore
volume (V
m
p) was determined using the Dubinin-Radushkevich (DR) equation for a p/p0range of 5.3 106to 2.5 104for N2adsorption. The total pore volume was determined using the Gur-vich rule at p/p0of 0.95.
The morphologies and chemical compositions of the samples were evaluated using an EPMA Shimadzu Model 1720H microprobe operating at an accelerating voltage of 15 kV and 50 nA BC to 0.01 nA. For this analysis, the samples were dispersed in a carbon ribbon and metallized with gold.
3. Results and discussion 3.1. TiO2-TON synthesis
Table 1shows the synthesis results using an OSDA (normally an ionic liquid) under different crystallization conditions. In synthesis results shown inTable 1, the TiO2content does not affect zeolite
crystallization, which allows for synthesis of materials with a desired TiO2content for specific reactions. Therefore, a pure silica
zeolite was obtained (synthesis 7). The role of ionic liquids as an OSDA was observed in synthesis 4, where 1,8-diamineoctane Fig. 1. 1-Butyl-3-methylimidazolium chloride (BMI.Cl).
Table 1
Syntheses conditions.
Synthesis Si/Ti Time (days) Stirring/Static OSDA Zeolitic phase 1 10 1e3 Just stirring IL TON 2 25 1e 3 e 7 Both IL TON 3 50 1e 3 e 7 Both IL TON 4 50 1e 3 e 7 Both DAO Amorphous 5 100 1e 3 e 7 Both IL TON 6 500 1e 3 Just stirring IL TON
(traditional OSDA for TON zeolite) was used under the same con-ditions as that of synthesis 3, which resulted in no crystalline products. Synthesis 2 was repeated in a shorter time period (hours), and zeolitic products were observed in 16 h (with amorphous content,Fig. 2), which confirmed the good structure-directing ef-fect of the 1-butyl-3-methylimidazolium in this system. TON zeo-lites typically require two days to crystallize under stirring[17]. The molecules may have rotational centers leading the molecule to exhibit different forms in the zeolitic structure[24,25]. The TON zeolite exhibits a unidimensional pore system, and the ionic liquid may be in its linear form inside the pores, which is consistent with the results reported by Wen et al.[18]. Wen et al. noted that 1-butyl-3-methylimidazolium occupies the zeolitic channel via end-by-end mode. In addition, the influence of the alkyl chain length on the structure-directing effect was confirmed, and the size of 1-butyl-3-methylimidazolium (1 nm along alkyl chain direction) is consistent with the channel size of TON[18]. These results are in agreement with our TGA results, where one cation per unit cell of the synthesized pure silica TON in afluoride media was observed (These results are the subject of a different paper).
1-Butyl-3-methylimidazolium may also direct other zeolitic phases (i.e., BEA, MFI, and IM-20) by employing different synthesis conditions (i.e., mineralizing agent, silicon source, and heteroatom substitution)[8,9,26]. In addition to the structure-directing effect of this OSDA, an interesting second template effect was observed by Mignoni et al.[8], where a spherical ZSM-5 with a regular particle size was obtained.
The solids obtained in synthesis 2 (Si/Ti¼ 25, seeTable 1) via both the stirring mode and static mode at all of the crystallization times were initially characterized by X-ray diffraction, and the re-sults are shown inFig. 3.
Based on the results inFig. 3, the presence of crystalline mate-rials was evident from the existence of well-defined reflections in the diffractograms. By comparing the obtained results with data in the literature[17], the zeolites produced possess a TON topology. The characteristic reflections of the TON phase are located at 2
q
angles of 8.17, 10.15, 12.8, 16.3, 19.4, 20.3, 24.2, 24.6 and 25.7. After one day of synthesis, the TON phase was obtained without the presence of concurring zeolite phases, and this phase remained until the seventh day of the synthesis without altering
the diffraction standards in the stirring mode or static mode. As mentioned above, 48 h of stirring is necessary to produce the ZSM-22 (TON topology) zeolite. The presence of a reflection (shoulder) at 25corresponds to the anatase phase (Fig. S1, Supplementary data), which is one of the possible crystalline phases of TiO2(CIF 9852e
ICSD). This result may indicate the presence of extraframework material. This reflection was observed for only the Si/Ti ratio with the highest quantity of titanium in the synthesis gel. The presence of a reflection at 21.5was observed in the diffractogram of sample
(f), and this peak is characteristic of the cristobalite phase (marked with a C). The appearance of cristobalite in the obtained zeolites only occurred in samples obtained at longer crystallization times (3 and 7 days), which is in agreement with the metastable behavior of zeolites.
The integrity of the 1-butyl-3-methylimidazolium molecule was confirmed by solid state13C NMR. A sample obtained after 3 days
under stirring (not calcined, synthesis 2) was analyzed, and its spectrum is shown inFig. 4. The same signals were observed in the two NMR analyses, suggesting that the molecule trapped in the zeolite pores was intact confirming the role of BMI.Cl as a structure-directing agent.
A sample obtained after 1 day of stirring (synthesis 2) was analyzed by N2adsorption/desorption, and its isotherm is shown in Fig. 5.
As shown in Fig. 5, the analyzed sample exhibited a type I isotherm[27], which is characteristic of microporous materials due to the adsorbed volume significantly increasing at low pressures and later reaching a saturation plateau. The specific area was Fig. 2. X-ray diffractograms of materials obtained in synthesis 2 with reaction times of
4, 8 and 16 h.
Fig. 3. X-ray diffractograms of zeolites obtained in synthesis 2 with reaction times of (a) 1 day, (b) 3 days, (c) 7 days with stirring, and (d) 1 day, (e) 3 days, and (f) 7 days in static mode.
calculated to be 290 m2.g1using the BET method, which is in agreement with previously reported results[28]. The micropore volume and total pore volume, which were determined using the Dubinin-Radushkevich (DR) equation and the Gurvich rule, respectively, were 0.1 cm3.g1 and 0.12 cm3.g1, respectively. Therefore, based on these data, the micropores contribute nearly all of the material porosity.
As observed by X-ray diffraction, the samples from synthesis 2 exhibited phases of titanium oxide in its X-ray diffractogram. However, to verify the coordination state of titanium in the zeolites, diffuse reflectance spectroscopic analyses in the UVeVis region was performed for a Si/Ti ratio of 25 (Fig. 6).
As shown inFig. 6, the presence of two bands can be observed in each of the zeolite spectra (i.e., one peak at 190e210 nm and the second one at 250e350 nm). The first band could be assigned to the electronic transition of the connecting charge (O2) for species isolated from the tetracoordinated Ti (IV), and the second band indicates the presence of extraframework titanium (TiO2e anatase)
[29,30]. Prakash and collaborators [31] suggested that TiO2 can
produce a band between 250 and 300 nm, which encompasses a large range of bands in the spectrum. The spectrum of the P25 used as TiO2source is present inFig. S2for comparison. The same band
between 250 and 300 nm can be observed in both zeolites and P25 spectra.
The simplest way to examine the formation of TieOeSi bonds is to use infrared spectroscopy. The infrared antisymmetric stretching vibration band observed at 910960 cm1is widely accepted as the characteristic vibration corresponding to the formation of TieOeSi
[32]. Our infrared spectra exhibited a little shoulder in this region (Fig. S3), which is inconclusive and error-prone. The weak intensity of this band in the FTIR spectra suggests that the titania species may be on the external surface, which does not affect the bulk zeolite framework[5].
X-ray photoelectron spectroscopy (XPS) analysis was employed to obtain detailed information on the oxidation states of all of the Ti elements in the prepared solid. As expected, the survey spectra exhibited the presence of O1s, Si2p, Ti2p, and C1s photoelectron lines with a surface titanium amount close to 1.4% atomic. Fig. 7
shows the high-resolution spectra of the Ti2p (Fig. 7a) and O1s (Fig. 7b) photoelectron lines for the prepared sample (sample ce synthesis 2). The peak model was used as implemented in the CASAXPS tools along with the typically constraints, such asfixing the ratio between the 2p doublet area (Ti2p3/2/Ti2p1/2¼ 0.5). The broad Ti 2p core level peak suggests the presence of titanium with lower oxidation states. In fact, binding energies at 458.8, 459.1 and 459.6 eV were confirmed. The photoelectron peak at 458.8 eV was assigned to the presence of Ti (IV) in an octahedral coordination (extraframework, anatase), which is in agreement with the previ-ously observed band in the UVeVis spectra. According to the literature[33], the high binding energy contribution observed at 459.1 eV may be due to the presence of terminal Ti (IV) associated with silica (TieOeSi)[33]. The last contribution, which occurs at a higher binding energy side (459.6 eV), can also be assigned to the TieOeSi bond [34]. However, the presence of possible domains corresponding to TiO2bridged to the zeolitic structure cannot be
ruled out. According to previous studies, these species are related to photocatalytic activity on SiO2eTiO2 systems [35]. These results
suggest that the prepared sample was not a pure silica material but a TiO2bridged TON zeolite. The O1s core level binding energies are
Fig. 4.13C NMR spectra of BMI.Cl (bottom) and as-prepared zeolite (XRD sample b)
(top).
Fig. 5. N2Adsorption/desorption isotherm at196C. (synthesis 2, reaction time of
1 day with stirring).
Fig. 6. Diffuse reflectance spectra of TiO2-TON zeolites obtained in the presence of
BMI.Cl: (a) 1 day, (b) 3 days, (c) 7 days under a static condition, and (d) 7 days, (e) 3 days, and (f) 1 day with stirring at a Si/Ti ratio of 25.
in the 530.9e536.2 eV regions. The binding energy at 530.9 eV and 533.7 eV can be assigned to TiO2and SiO2, respectively. The peak at
536.2 eV corresponded to the OeH or NaeOH bonds. By spectros-copy analyses used to characterize the Ti in the zeolites, we cannot confirm the presence of Ti in tetrahedral coordination, just evi-dence it.
To investigate the morphology of the synthesized materials, a scanning electron microscopy analysis with elemental mapping by electron probe microanalysis (EPMA) was performed.Fig. 8shows a scanning electron microscopy image and chemical maps (Si, Ti and O) for a sample obtained after 1 day of stirring for a Si/Ti ratio of 25. Based on the images inFig. 8, a homogenous morphology in the shape of rods (similar to grains of rice) was observed with a particle size in the 5e30
m
m range. This morphology is characteristic of the TON[37]phase. From the chemical mapping results, a good dis-tribution of titanium was observed, and no metal bits were apparent. Because the synthesized material was primarily composed of Si and O, a good color density, which indicated the presence of these elements, was observed.The presence of the anatase phase in the zeolites allows for their application to hydrogen peroxide decomposition [38]. The
synthesized zeolites can also be applied to photocatalytic reactions where anatase is the active phase[39]. One advantage over com-mercial titanium oxide (P25), which is commonly used in photo-catalytic reactions, is that P25 is a nanoscale material that is difficult to recover. In contrast, the zeolites obtained in this study consist of micron-sized particles that can be recovered byfiltration. This interesting synthesis approach can be considered a“one-pot” method where thefinal zeolite contains both TiO2and the ionic
liquid. The typical methods require sequential steps to support TiO2
in the zeolites.
3.2. Mother liquor studies
At the end of the syntheses, the mother liquor and the resulting solids had a yellowish color, suggesting either modification of the ionic liquid during the synthesis or its degradation. In previous studies, the hydroxide 1-butyl-3-methylimidazolium was “syn-thesized” by reacting 1-butyl-3-methylimidazolium chloride with sodium hydroxide in equimolar proportions, yielding a viscous brown compound. However, in Yuen's[40]research, this reaction was contested, and the author indicated that it was not possible to Fig. 7. a) Ti2p and b) O1s XPS spectra of sample c from synthesis 2.
determine whether the compound was indeed hydroxide 1-butyl-3-methylimidazolium or a heterocyclic carbene.
In our attempt to recover the chloride from 1-butyl-3-methylimidazolium from the mother liquor, a viscous brown compound was obtained, indicating that a change occurred in the compound during synthesis. This behavior was observed in all of the syntheses regardless of the time elapsed. Zones described a similar result for zeolite synthesis using imidazolium cations in the form of iodide and attributed this behavior to the decomposition of an organocation that was not incorporated into the zeolite[41].
A13C NMR analysis of the compound obtained from the mother liquor was performed to confirm the structural change in 1-butyl-3-methylimidazolium after synthesis. The thermal behavior of the post-synthesis compound was also investigated by thermogravi-metric analysis and compared to the ionic liquid. These results are shown inFig. 9.
Based on the thermogravimetric analysis (Fig. 9a), the com-pound obtained from the mother liquor (post-synthesis) still exhibited high thermal stability (i.e., approximately 310C) similar to its precursor (i.e., 1-butyl-3-methylimidazolium chloride). In addition, the spectra shown inFig. 9b indicate that most of the signals observed for the compound obtained from the mother li-quor correspond to BMI.Cl signals, except for the signal centered at
d
¼ 170 ppm. It is believed that the displacement of this signal (from 140 ppm in the cation spectrum to 170 ppm in the post-synthesis compound spectrum) indicates the formation of a carbonyl compound, which shifts the signal to the left (lowerfield). Future studies are necessary to determine whether it is possible to obtain zeolite structures using the molecule recovered from the mother liquor.4. Conclusions
The synthesis of TiO2-TON zeolites using
1-butyl-3-methyl-imidazolium chloride ionic liquid as a structure-directing agent was very efficient. In addition, this ionic liquid is a powerful structure-directing agent for TON zeolites. TiO2was synthesized in
both stirring and static modes and with different titanium dioxide amounts. The obtained materials exhibited good distribution of titanium in the zeolite particles and the titanium was present in octahedral and possibly tetrahedral coordination. The presence of SieOeTi bonds was confirmed by spectroscopic analyses.13C NMR
was employed to confirm that the post-synthesis compound recovered at the end of the synthesis was a modified ionic liquid molecule. The TiO2 containing zeolites can be employed for
hydrogen peroxide decomposition in addition to photocatalysis, where the anatase is the active phase. Our“one-pot” synthesis
methodology avoids successive metal oxide impregnation steps, and this methodology can be extended to other metal oxides. Acknowledgments
The authors gratefully acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnologico (CNPq) for financial support, the Federal University of Rio Grande do Norte (UFRN) for the physical structure and the EPMA analyses, the Federal Univer-sity of Rio Grande do Sul (UFRGS) for the13C NMR and DRS analysis and Prof. Karim Sapag of the UNSL (San Luis e Argentina) for assistance with the N2adsorption/desorption study.
Appendix A. Supplementary data
Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.micromeso.2015.04.014.
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