Article pubs.acs.org/JPCC
Effects of Structure of Ionic Liquids and Phosphoric Acid on Structure of Aluminum Isopropoxide Xin Sun,† Dewey H. Barich,‡ and Jennifer L. Anthony*,† †
Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66502, United States University of Kansas Solid State NMR Facility, Lawrence, Kansas 66047, United States
‡
ABSTRACT: Ionic liquids (ILs) have recently been used as the solvent to synthesize aluminophosphates, but the effect of IL on structures of precursors and the effect of one precursor on the other precursors are not well understood. In this work, the influence of IL and phosphoric acid on the coordination environment of Al atoms in aluminum isopropoxide is investigated by 27Al solid state NMR. It is found that with the increase of alkyl chain length on the imidazolium ring in ILs, the ratio of four-coordinated aluminum atoms (Al(IV)) to six-coordinated aluminum atoms (Al(VI)) increases; with the addition of phosphoric acid in the mixture, this change is remarkable regardless of the IL. Higher temperature also aids in the increase of ratio of Al(IV) to Al(VI). These results provide useful insight regarding the early stages of the formation of porous aluminophosphates in ionothermal synthesis.
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INTRODUCTION Ionic liquids (ILs) are chemicals that are commonly composed of large organic cations and inorganic anions. Unlike traditional inorganic salts, they generally have melting points lower than 100 °C due to the low lattice energies between cations and anions. ILs recently have been used in many applications, including new chemical and extractive process,1,2 fuel cells,3 catalysis,4,5 and lubricants6 due to their unique properties as compared to common organic solvents. These advantageous properties include a wide liquid range, negligible vapor pressure for aprotic ILs, and the sheer number of ILs that can be synthesized by combining the numerous options for different cations and anions.7−9 Because of many ILs’ favorable properties, especially the low vapor pressure, they are employed as solvent and sometimes a simultaneous structure directing agent (SDA) in the synthesis of microporous materials, such as silica-based zeolites and aluminophosphates.10−12 This method is called ionothermal synthesis as compared to hydrothermal synthesis where water is used as the solvent. Since most ILs have negligible vapor pressure, synthesis of molecular sieves could be conducted in an open container. The use of ILs also could simplify the synthesis process by eliminating the need for additional SDA since ILs often have structures similar to common SDA used in hydrothermal synthesis. The ILs most commonly reported in ionothermal synthesis of silica-based zeolites and aluminophosphates are the 1-alkyl-3-methylimidazolium bromides ([Cnmim][Br]). Much research has focused on trying to understand the mechanism of molecular sieve formation in hydrothermal synthesis,13−15 but less is known about synthesizing molecular sieves in ILs. Researchers have found heating source (conventional oven vs microwave heating), water content, addition of © 2013 American Chemical Society
structure directing agent (or acid), and other factors could influence the frameworks synthesized.16−19 There are several studies on the conformation of Al atoms in the final frameworks of ionothermally synthesized aluminophosphates as determined by solid state NMR. Fayad and coworkers found that without addition of SDA in the reaction gel, the final frameworks contains four-coordinated Al atoms (Al atoms in the framework, δ = 32.5 ppm), five-coordinated Al atoms (Al atoms in the framework with a fifth ligand, probably F− ions, δ = 21 ppm), and six-coordinated Al atoms (tentatively assigned to water coordinated Al in the cages, δ = −13 ppm).18 Wang and co-workers obtained similar solid state NMR patterns for magnesium-containing aluminophosphate molecular sieves. They found four-coordinated Al has an intense signal in the δ = 40.5 and 39.6 ppm region, five-coordinated Al around δ = 28 ppm, and a broad peak at δ = −13 ppm corresponding to six-coordinated Al,20 but the coordination of Al atoms in the solution prior to the synthesis is unknown. The four-, five-, and six-coordinated forms of aluminum can also be seen when aluminum isopropoxide is in solution in solvents such as toluene.21−23 The generally accepted chemical structures for the aluminum coordination environments of interest are shown in Figure 1.21−23 Researchers report that the tetrameric form is the preferred coordination of aluminum isopropoxide in the solid form or dissolved in apolar solvents; however, the concentration, temperature, and solvent environment can help shift the equilibrium toward the other forms, which can play a role in the eventual chemistry of the aluminum isopropoxide.21−23 The Received: July 10, 2013 Revised: October 25, 2013 Published: October 25, 2013 25615
dx.doi.org/10.1021/jp4068125 | J. Phys. Chem. C 2013, 117, 25615−25621
The Journal of Physical Chemistry C
Article
Figure 1. Chemical structures of aluminum isopropoxide in the (a) tetrameric or 6-coordinated form, the (b) trimeric or 5-coordinated form where R is an isopropyl group, and the (c) tetrahedral form of aluminum.
Table 1. Molar Composition of Samples and Their Mixing Temperature
a
sample ID
Al(OiPr)3
1 2 3 4 5 6 7
1 1 1 1 1 1 1
ionic liquid 0 [C2mim][Br] [C4mim][Br] [C6mim][Br] [C2mim][Br] [C4mim][Br] [C6mim][Br]
H3PO4 0 0 0 0 2.55 2.55 2.55
10 10 10 10 10 10
H2Oa
