Manufacture Porous Materials by Dabco-Based Lonic Liquid

The long-chain ionic liquid (IL) hexadecyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide was used as a template to prepare the hexagonally ordered siliceous mesoporous molecular sieve MCM-41 as well as the disordered mesoporous molecular sieve designated as KIT-1. The synthesized products were studied via X-ray diffraction (XRD), Fourier transform infrared (FTIR) and N 2 adsorption-desorption analysis. The surface area (BET), pore volume, and pore diameter (BJH) are determined. These kind of ILs that have DABCO (1,4-diazabicyclo[2.2.2]octane) in their structures, were prepared with an easy method. When the two templates, cetyltrimethylammonium bromide (CTAB) and IL, have the similar structures, MCM-41 mesoporous molecular sieve produced with more ordered, uniform mesoporous channel and high surface area in comparison to without IL. When just IL used instead of CTAB, the KIT-1 with non-uniform mesoporous was obtained. Here we prepared the KIT-1 mesoporous molecular sieve with IL without CTAB but it seems, using dual template with changing IL structures in fabricated of molecular sieve provide new opportunity to design targeted adsorbents and catalysis.


Introduction
Over the last two decades, the preparation of high surface area mesoporous materials has captured great interest within the scienti c community. Much attention has been given lately to various mesoporous silica materials, such as M41S, KIT-1, and SBA-15 [1][2][3]. These structurally well-de ned materials have promising utilizations in catalysis, separation, adsorption, sensing, and delivery of drugs [4][5]. In 1992, mesoporous molecular sieves M41S were discovered by Mobil staff. This outstanding nding has provoked substantial research in this area. One of the most noteworthy mesoporous molecular sieves to mention is MCM-41 which has a hexagonal array of uniform mesoporous [1]. Some innovative templates were used in our former work in pursuance of enhancing the preparatory process [6].
KIT-1 is a noncrystalline molecular sieve that displays short wormlike mesoporous channels. The channels are organized in a disordered three-dimensional system concluded abundant seams. The channel widths, however, are as uniform as the well-ordered mesoporous molecular sieves, MCM-41 [2].
The hydrothermal stability of KIT-I is higher than that of the ordered MCM-41 [7]. This kind of molecular sieve reduces probability of channel blockage in their catalytic activity.
There has been much research in recent years into the synthesis methods of MCM-41, [8][9][10][11] but only a few studies concerning the synthesis methods of KIT-1. One of these studies was conducted in 1996 by Ryoo et al., and it successfully synthesized KIT-I, for the rst time, by an electrostatic templating route by means of ethylenediaminetetraacetic acid tetrasodium salt (EDTANa 4 ), HTAC1, and sodium silicate [2].
Another study was done in 1997 by Ryoo et al. which established another approach to the synthesis of KIT-1 [12]. They synthesis of KIT-1 has been achieved, in the presence of different polyacids, by hydrothermal polymerization of silicate anions surrounding the molecular organization of hexadecyltrimethylammonium chloride [12].
Ionic liquids (ILs), organic salts with low melting points, have been extensively examined and have exceptional characteristics such as extreme stability, high ionic conductivity, non-volatility, adjustable polarity, and recyclability. [13][14] Lately, ILs have been the focus of noticeable consideration not only in chemistry but also in materials science [13]. Speci c self-assembled structures can be created with the opportunity of adjustment the amphiphilicity of ILs by modifying the polarizability of the anion and cation or the alkyl chain length. Some researchers in the domain of sustainable green synthesis have paid considerable attention to ILs during the last few decades [15]. ILs were used as template or solvent, in green chemistry, for nanomaterial synthesis [10,13].
There has been no work in the publications that reports the use of long chain IL based DABCO for mesoporous system synthesis. In the present work, the new IL, hexyldecyl-4-aza-1azaniabicyclo[2.2.2]octane was synthesized. Structure of used IL is shown in Scheme 1. This structure in compared with CTAB structure is the same long chain and the anion (Scheme 1). The question arises what effect has similarity between CTAB and IL structures used on the mesoporous structure? No work in the literature describes the effect of this IL in mesoporous structure, as far as we know.
In connection with our interest in in uence of different ILs as co-template in micro-mesoporous MCM-41 structure and disordered mesoporous material KIT-1, we report here the results of a study on the new IL, hexyldecyl-4-aza-1-azaniabicyclo[2.2.2]octane (IL-C 16 , Scheme 1) with similar structure with CTAB as dual template (M-1) and absence of CTAB (K-1) [6,16]. Also, the results were compared with M-2 as blank sample and K-2 materials from our previous article [16]. In preparation of M-2 was used only CTAB as template and K-2 was used dual template CTAB and dodecyl-4-aza-1-azaniabicyclo[2.2.2]octane (IL-C 12 ).

Characterization
All the reagents were obtained from Merck or Aldrich and were used without further puri cations. 1 H NMR spectra were recorded on either a Bruker DRX-250 (250 MHz) spectrometer in CDCl 3 . Carbon nuclear magnetic resonance ( 13 C NMR) spectra were recorded on either a Bruker DRX-250 (62.9 MHz). The crystalline structures of the resultant samples were characterized by X-ray diffractometer (XRD, X'Pert Pro MPD, PANalytical) with Cu Kα radiation (40 kV and 40 mA). The FT-IR spectra were recorded using a Perkin-Elmer BX-II IR spectrometer. N 2 adsorption/desorption isotherms of samples were made at 77 K (BELSORP-mini II, MicrotracBEL, Corp.). The speci c surface area and mesopore size distribution were determined by BET and BJH (Barrett-Joyner-Halenda) method, respectively [ To 17.8 mmol DABCO (98% Pure) in 50 mL AcOEt, 18.7 mmol 1-bromohexadecan was added, and the mixture was stirred at room temperature (r.t.) for 24 h. The product was ltered and then it was suspended into diethyl ether (30 cc) and stirred for one hour at r.t. The product was ltered and dried in vacuum to give the title compound (6.7 g, 90%). 1  TEOS was added, the solution was mixed-up rmly for two hours at r.t. The mixture was then moved into a stainless-steel autoclave and exposed to a high temperature at 373 K for 48 hours. The resultant product was washed with EtOH one time and then two times with deionized water. Then it was left to dry at a high heat of 373 K to get the product. The as-synthesized sample was then left in the air to get calcined for six hours at a temperature of 823 K and a heating rate of 1 K min − 1 .

Preparation of K-1
K-1 was synthesized utilizing IL-C 16 as template solution, and TEOS as a silica source. EA was used to regulate the pH of this solution. In a typical synthesis, Ethylamine was used to control the pH value of the aqueous solution. The needed quantity of TEOS was added drop-wise to the solution. The mixed gel had the following molar constitutions: (SiO 2 :IL:EA:H 2 O) = (1:0.4:0.6:100). After the TEOS was added, the solution was mixed-up rmly for two hours at room temperature. The mixture was then moved into a stainless-steel autoclave and exposed to a high temperature at 373 K for 48 hours. The resultant product was washed with EtOH a single time and then two times with deionized water. Then it was left to dry at a high heat of 373 K to get as-synthesized sample. The as-synthesized sample was then left in air to get calcined for six hours at a temperature of 823 K and a heating rate of 1 K min − 1 .

Preparation of M-2 and K-2
M-2 and K-2 were prepared based on our previous work [16]. The small-angle XRD patterns of the products are shown in Fig. 2. M-1 sample is related to the dual templates of CTAB and IL-C 16 , and M-2 sample is related to the only CTAB as template, K-1 sample means only template IL-C 16 , K-2 sample used for the dual templates of CTAB and IL-C 12 . XRD patterns of M-1 indicated an intense sharp (100) peak, followed by minor (110) and (200) peaks, matching to a highly ordered hexagonal structure of MCM-41. XRD patterns con rm that by using dual templates, CTAB and IL, the crystallization of M-1 increased in comparing with M-2. The (100) peak of M-1 shift toward the slightly bigger angle, which shows the lattice space d 100 of the pores (d 100 = λ/2sinθ) became smaller. On the other hand, the low intensity and peak broadening detected in the XRD pattern of K-1 and K-2 indicate that these materials are not as well ordered as MCM-41 and the structure array of their channels is disorder [18]. Two wide peaks which correspond to (100) and (200) diffraction in XRD pattern for K-1 showed a more disordered structure than K-2. As the Fig. 2 showed, using the dual templates to produce M-1 and K-2 ( Nitrogen adsorption/desorption isotherms of samples show the typical type IV isotherm according to the IUPAC nomenclature for mesoporous structure (Fig. 3). The sharp in ection at P/P 0 = 0.3-0.35 in the isotherms of M-1 sample indicates the narrow uniform mesoporous distribution. The existence of hysteresis loops in the M-1 sample illustrates interaction network of porous structure which may be correlated to the existence of micropores, but K-1 shows bigger hysteresis loop corresponds to the threedimensional interconnection structure [19]. The sharp in ection p/p 0 in the isotherm K-2 indicates the narrow uniform mesoporous distribution in comparing with K-1 (Fig. SD. 3) [16]. Table 1 summarizes surface area and the pore volume of the characteristic MCM-41 and KIT-1 samples.

Results And Discussion
The BET surface area of M-1 is 1221 m 2 /g, which is bigger than the value of 1028 m 2 /g for M-2 but BET surface area of K-2 is 1012 m 2 /g, which is bigger than about twice the value of 524 m 2 /g for K-1. The speci c surface area of the K-2 (obtained by dual template) is upper 1000 m 2 g − 1 and is also near to the ordered MCM-41.
The pore volume of M-1 is 0.68 cm 3 /g, which is smaller than the value of 1.03 cm 3 /g for M-2 and the pore volume of K-1 is 0.55 cm 3 /g, which is smaller than about half the value of 1.12 cm 3 /g for K-2.
The pore size distribution of the mesopore and micropore of the products was obtained from desorption data using the BJH method (Fig. 4). K-1 Sample shows non-uniform channels with 4-25 nm radiuses in comparison to M-1. Fig. SD. 4 shows the TEM micrograph of K-2 sample. It presents that the pore structure is the disorder network of short wormlike channel of mesoporous. These channels are connecting in a three dimensional with fairly uniform pore size [26]. Also the results con rm the ordered structure of M-2 in comparing with the M-1 and the K-2 in comparing with the K-1. It seems using IL-C 16 role is ordered micelles formation with CTAB. Only using IL-C 16 with steric hindrance in cationic center lead to micelles formation that doesn't have good interaction with silicate polyanion in the polymerization process.
Wang et al. reported to investigation of 1-hexadecyl-3-methylimidazolium chloride (C 16 mimCl) as template on synthesis of mesoporous silica in which the molar compositions of the mixed gel were: 1 TEOS: 0.543 C 16 mimCl: 0.512 NaOH: 56 H 2 O led to MCM-48 structure [20]. As we mentioned using only [C 16 Dabco]Br without CTAB provide the disordered mesoporous system as KIT-1.
In continuing our previous researches, [6,16] three cationic ILs based on DABCO with different chain length were applied as co-templates along with CTAB to produce micro-mesoporous silica materials. The using IL-C 4 as co-template, showed good impact to prepare MCM-41 with higher interconnection network than using just CTAB [6]. The IL-C 12 with the same molecular weight with CTAB in causes to form the KIT-1 micro-mesoporous system [16]. The IL-C 16 with the similar to CTAB structure in the length of the alkyl side chain led to producing the micro-mesoporous MCM-41 with more order structure than comparison with only using CTAB (the present work). All of these synthesized mesoporous material has high surface area (Scheme 2).

Conclusion
In conclusion, using co-template in based on DABCO depend on length chain has different advantages like increasing interconnections, preparation with the disordered channel (KIT-1) and more ordered structure of mesoporous. Using only ILs led to KIT-1 with less order in comparing with the dual template. This work has completed the results of previous works about the effect of DABCO based ionic liquid in manufacture porous materials.

Declarations Supplementary Data
Supplementary data associated with this article can be found, in the online supplementary content.