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New Brønsted Ionic Liquids: Synthesis, Thermodinamics and Catalytic Activity in Aldol Condensation Reactions

Written By

I. Cota, R. Gonzalez-Olmos, M. Iglesias and F. Medina

Submitted: 25 June 2012 Published: 23 January 2013

DOI: 10.5772/51163

From the Edited Volume

Ionic Liquids - New Aspects for the Future

Edited by Jun-ichi Kadokawa

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1. Introduction

It is a continuous challenge to find new catalysts able to perform with good activities and selectivity condensation reactions for the synthesis of pharmaceutical and fine chemicals. In the last years room temperature ionic liquids (ILs) have received a lot of interest as environmental friendly or “green” alternatives to conventional molecular solvents. They differ from molecular solvents by their unique ionic character and their “structure and organization” which can lead to specific effects [1].

Room-temperature ILs have been used as clean solvents and catalysts for green chemistry, stabilizing agents for the catalysts or intermediates, electrolytes for batteries, in photochemistry and electrosynthesis etc [2-6]. Their success as environmental benign solvents or catalysts is described in numerous reactions [7-11], such as Diels-Alder reactions [12, 13], the Friedel-Crafts reaction [14-17], esterification [18-20], cracking rections [21], and so on. The link between ionic ILs and green chemistry is related to the solvent properties of ILs. Some of the properties that make ILs attractive media for catalysis are: they have no significant vapour pressure and thus create no volatile organic pollution during manipulation; ILs have good chemical and thermal stability, most ILs having liquid ranges for more than 3000C; they are immiscible with some organic solvents and therefore can be used in two-phase systems; ILs polarity can be adjusted by a suitable choice of cation/anion; they are able to dissolve a wide range of organic, inorganic and organometallic compounds; ILs are often composed of weakly coordinating anions and therefore have the potential to be highly polar.

The number of ILs has increased exponentially in the recent years. Many of them are based on the imidazolium cation and in a lesser proportion, alkyl pyridiniums and trialkylamines (Scheme 1). By changing the anion or the alkyl chain of the cation, a wide variety of ILs may be designed for specific applications. They can be of hydrophobic or hydrophilic nature depending on the chemical structures involved.

Scheme 1.

Main cations and anions described in literature [1].

ILs can be divided into two broad categories: aprotic ionic liquids (AILs) and protic ionic liquids (PILs).

AILs largely dominate the open literature due to their relative inertness to organometallic compounds and their potential of applications, particularly in catalysis. They are synthesized by transferring an alkyl group to the basic nitrogen site through SN2 reactions [1].

PILs are formed through proton transfer from a Brønsted acid to a Brønsted base. Recently there has been an increasing interest in PILs due to their greater potential as environmental friendly solvents and promising applications. Moreover, they present the advantage of being cost-effective and easily prepared as their formation does not involve the formation of residual by-products. A specific feature of the PILs is that they are capable of developing a certain hydrogen bonding potency, including proton acceptance and proton donation and they are highly tolerant to hydroxylic media [22-23].

The application of new policies on terms of environment, health and safety deals towards minimizing or substituting organic volatile solvents by green alternatives, placing a renewed emphasis on research and development of lesser harmful compounds as ILs. On the other hand, recently the interest in the use of PILs to tailor the water properties for cleaning applications in processes of minimization of CO2/SO2 emissions has increased [24-26].

In the last years numerous studies report the use of ILs as selective catalysts for different reactions, like aldol condensation reactions where several ILs have been successfully applied as homogeneous and heterogeneous catalysts [27-30]. Abelló et al. [28] described the use of choline hydroxide as basic catalyst for aldol condensation reactions between several ketones and aldehydes. Better conversions and selectivities were obtained when compared to other well-known catalysts, such as rehydrated hydrotalcites, MgO and NaOH. In addition, higher performance was obtained when choline was immobilized on MgO.

Zhu et al. [27] described the use of 1,1,3,3-tetramethylguanidine lactate ([TMG] [Lac]) as recyclable catalyst for direct aldol condensation reactions at room temperature without any solvent. It was demonstrated that for each reaction only the aldol adduct was produced when the molar ratio of the IL and substrate was smaller than 1. Moreover, after the reaction the IL was easily recovered and recycled without considerably decrease of activity.

Kryshtal et al. [29] described the application of tetraalkylammonium and 1,3-dialkylimidazolium perfluoro-borates and perfluoro-phosphates as recoverable phase-transfer catalysts in multiphase reactions of CH-acids, in particular in solid base-promoted cross-aldol condensations. The catalysts retained their catalytic activity over several reaction cycles.

In the study of Lombardo et al. [30] two onium ion-tagged prolines, imidazolium bis (trifluoromethylsulfonyl)imide-substituted proline and butyldimethylammonium bis (trifluoromethylsulfonyl) imide-substituted proline, were synthesized and their catalytic activity in the direct asymmetric aldol condensation was studied. The catalytic protocol developed by this group makes use of a 6-fold lower amount of catalyst with respect to the preceding reports [31, 32] and affords greater chemical yields and higher enantioselectivity.

The main objective of this chapter is to develop and study the applications of a new family of ILs based on substituted amine cations of the form RNH3+ combined with organic anions of the form R’COO- (being of different nature R and R’). The variations in the anion alkyl chain, in conjunction with the cations, lead to a large matrix of materials.

This kind of compounds show interesting properties for industrial use of ILs: low cost of preparation, simple synthesis and purification methods. Moreover, the very low toxicity and the degradability of this kind of ILs have been verified. Thus, sustainable processes can be originated from their use.

Recently, many studies dealing with the application of ILs in organic synthesis and catalysis have been published, pointing out the vast interest in this type of compounds [33-36]. With these facts in mind, we studied their catalytic potential for two condensation reactions of carbonyl compounds. The products obtained from these reactions are applied in pharmacological, flavor and fragrance industry.

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2. Experimental

2.1. Preparation of ILs and supported ILs

The ILs synthesized in this work are: 2-hydroxy ethylammonium formate (2-HEAF), 2-hydroxy ethylammonium acetate (2-HEAA), 2-hydroxy ethylammonium propionate (2-HEAP), 2-hydroxi ethylammonium butanoate (2-HEAB), 2-hydroxi ethylammonium isobutanoate (2-HEAiB) and 2-hydroxi ethylammonium pentanoate (2-HEAPE).

The amine (Merck Synthesis, better than 99%) was placed in a three necked flask all-made-in-glass equipped with a reflux condenser, a PT-100 temperature sensor for controlling temperature and a dropping funnel. The flask was mounted in a thermal bath. A slight heating is necessary for increasing miscibility between reactants and then allow reaction. The organic acid (Merck Synthesis, better than 99%) was added drop wise to the flask under stirring with a magnetic bar. Stirring was continued for 24 h at laboratory temperature, in order to obtain a final viscous liquid. Lower viscosity was observed in the final product by decreasing molecular weight of reactants. No solid crystals or precipitation was noticed when the liquid sample was purified or stored at freeze temperature for a few months after synthesis. The reaction is a simple acid–base neutralization creating the formiate, acetate, propionate, butanoate, isobutanoate or pentanoate salt of ethanolamine that in a general form should be expressed as follows:

( H O C H 2 C H 2 ) N H 2 +   H O O C R ( H O C H 2 C H 2 ) N H 3 + ( O - O C R ) E1

For example, when formic acid is used this equation shows the chemical reaction for the reactants ethanolamine + formic acid, with 2-HEAF as neutralization product.

Because these chemical reactions are highly exothermic, an adequate control of temperature is essential throughout the chemical reaction; otherwise heat evolution may produce the dehydration of the salt to the corresponding amide, as in the case for nylon salts (salts of diamines with dicarboxy acids).

As observed in our laboratory during IL synthesis, dehydration begins around 423.15 K for the lightest ILs. The color varied in each case from transparent to dark yellow when the reaction process and purification (strong agitation and slight heating for the vaporization of residual non-reacted acid for at least for 24 h) were completed.

There was no detectable decomposition for the ILs studied here when left for over 12 months at laboratory temperature. Less than 1% amide was detected after this period of time. On the basis of these results it appears obvious that the probability of amide formation is low for this kind of structures.

In order to obtain the supported ILs, 1 g of IL was dissolved in 7 ml of ethanol and after stirring at room temperature for 30 min, 1 g of alanine (Fluka, better than 99%) was added. The mixture was stirred for 2 h and then heated at 348 K under vacuum to remove ethanol. The supported ILs thus obtained were labelled hereafter as a-ILs.

2.2. Spectroscopy test

FT-IR spectrum was taken by a Jasco FT/IR 680 plus model IR spectrometer, using a NaCl disk.

2.3. Physical properties equipment

During the course of the experiments, the purity of ILs was monitored by different physical properties measurements. The pure ILs were stored in sun light protected form, constant humidity and low temperature. Usual manipulation and purification in our experimental work was applied [22].

The densities and ultrasonic velocities of pure components were measured with an Anton Paar DSA-5000 vibrational tube densimeter and sound analyzer, with a resolution of 10−5 g cm−3 and 1 m s−1. Apparatus calibration was performed periodically in accordance with provider’s instructions using a double reference (millipore quality water and ambient air at each temperature). Accuracy in the temperature of measurement was better than ±10−2 K by means of a temperature control device that apply the Peltier principle to maintain isothermal conditions during the measurements.

The ion conductivity was measured by a Jenway Model 4150 Conductivity/TDS Meter with resolution of 0.01µS to 1 mS and accuracy of ±0.5% at the range temperature. The accuracy of temperature into the measurement cell was ±0.5 C.

2.4. Catalytic studies

The studied reactions were the condensation between citral and acetone and between benzaldehyde and acetone. The reactions were performed in liquid phase using a 100 mL batch reactor equipped with a condenser system. To a stirred solution of substrate and ketone (molar ratio ketone/substrate = 4.4) was added 1 g of IL, and the flask was maintained at 333 K using an oil bath. Samples were taken at regular time periods and analyzed by gas chromatography using a flame ionization detector and an AG Ultra 2 column (15 m x 0.32 mm x 0.25 µm). Tetradecane was used as the internal standard. Reagents were purchase from Aldrich and used without further purification.

In order to separate the ILs from the reaction mixture, at the end of the reaction 6 mL of H2O were added. The mixture was stirred for 2 h and then left 15 h to repose. Two phases were separated: the organic phase which contains the reaction products and the aqueous phase which contains the IL. In order to separate the IL, the aqueous phase was heated up to 393 K under vacuum.

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3. Results and discussion

As Figure 1 shows, the broad band in the 3500-2400 cm-1 range exhibits characteristic ammonium structure for all the neutralization products. The OH stretching vibration is embedded in this band. The broad band centered at 1600 cm-1 is a combined band of the carbonyl stretching and N-H plane bending vibrations. FT-IR results clearly demonstrate the IL characteristics of compounds synthesized in this work.

Due to space considerations, we will present the thermodynamic properties only for two of the studied ILs: 2-HEAF and 2-HEAPE.

The molar mass and experimental results at standard condition for 2-HEAF and 2-HEAPE are shown in Table 1.

Figure 1.

FT-IR spectrum for 2-HEAPE.

IL Molecular Weight
(g∙mol-1)
Exp. Density
(g∙cm-3)
Exp. Ultrasonic Velocity
(ms-1)
Exp. Conductivity
(μS∙cm-1)
2-HEAF 107.11 1.176489 1709.00 4197.6
2-HEAPE 163.21 1.045479 1591.59 239.6

Table 1.

Experimental data for pure ionic liquids at 298.15 K and other relevant informationa

aOther experimental data for comparison are not available from the literature.

The densities, ultrasonic velocities and isobaric expansibility of 2-HEAF and 2-HEAPE are given in Table 2, and the ionic conductivities are given in Table 3. From the results obtained it can be observed that an increase in temperature diminishes the interaction among ions, lower values of density and ultrasonic velocity being gathered for rising temperatures in each case.

2-hydroxy ethylammonium formate (2-HEAF)
T
(K)
ρ
(gcm-3)
u
(ms-1)
κS
(TPa-1)
103 · α
(K-1)
T
(K)
ρ
(gcm-3)
u
(ms-1)
κS
(TPa-1)
103 · α
(K-1)
338.15 1.148091 1613.59 334.53 0.6188 327.16 1.155890 1639.38 321.90 0.6148
337.90 1.148254 1614.14 334.26 0.6187 326.91 1.156069 1639.97 321.62 0.6147
337.66 1.148433 1614.71 333.97 0.6186 326.66 1.156247 1640.57 321.34 0.6146
337.40 1.148608 1615.30 333.67 0.6185 326.41 1.156426 1641.16 321.06 0.6145
337.15 1.148785 1615.87 333.39 0.6184 326.16 1.156603 1641.75 320.78 0.6144
336.91 1.148963 1616.46 333.09 0.6183 325.91 1.156780 1642.34 320.50 0.6143
336.66 1.149139 1617.04 332.80 0.6182 325.65 1.156957 1642.94 320.21 0.6142
336.41 1.149316 1617.63 332.51 0.6182 325.40 1.157136 1643.53 319.93 0.6141
336.16 1.149494 1618.22 332.21 0.6181 325.16 1.157314 1644.12 319.66 0.6140
335.90 1.149669 1618.81 331.92 0.6180 324.90 1.157490 1644.72 319.37 0.6139
335.65 1.149848 1619.38 331.64 0.6179 324.65 1.157669 1645.32 319.09 0.6138
335.40 1.150027 1619.96 331.35 0.6178 324.40 1.157846 1645.91 318.81 0.6137
335.16 1.150205 1620.55 331.05 0.6177 324.15 1.158023 1646.50 318.54 0.6136
334.90 1.150384 1621.13 330.77 0.6176 323.90 1.158201 1647.09 318.26 0.6135
334.66 1.150560 1621.71 330.48 0.6175 323.65 1.158378 1647.68 317.98 0.6134
334.40 1.150740 1622.30 330.19 0.6174 323.40 1.158556 1648.28 317.70 0.6133
334.16 1.150916 1622.89 329.90 0.6173 323.15 1.158734 1648.90 317.42 0.6132
333.90 1.151094 1623.48 329.61 0.6173 322.90 1.158910 1649.47 317.15 0.6131
333.65 1.151271 1624.06 329.32 0.6172 322.66 1.159088 1650.06 316.87 0.6130
333.41 1.151449 1624.64 329.03 0.6171 322.41 1.159265 1650.66 316.59 0.6129
333.16 1.151625 1625.23 328.75 0.6170 322.16 1.159442 1651.25 316.32 0.6128
332.90 1.151804 1625.82 328.46 0.6169 321.91 1.159620 1651.85 316.04 0.6127
332.65 1.151981 1626.41 328.17 0.6168 321.65 1.159797 1652.43 315.77 0.6126
332.41 1.152159 1626.99 327.88 0.6167 321.40 1.159976 1653.03 315.49 0.6125
332.15 1.152338 1627.58 327.59 0.6166 321.15 1.160154 1653.63 315.22 0.6124
331.90 1.152514 1628.16 327.31 0.6165 320.91 1.160330 1654.22 314.94 0.6124
331.65 1.152694 1628.75 327.02 0.6164 320.66 1.160509 1654.81 314.67 0.6123
331.40 1.152871 1629.34 326.74 0.6163 320.40 1.160688 1655.41 314.39 0.6122
331.16 1.153048 1629.93 326.45 0.6162 320.15 1.160863 1656.01 314.12 0.6121
330.90 1.153225 1630.52 326.16 0.6162 319.90 1.161042 1656.60 313.85 0.6120
330.65 1.153405 1631.11 325.88 0.6161 319.65 1.161218 1657.19 313.58 0.6119
330.41 1.153582 1631.69 325.59 0.6160 319.40 1.161398 1657.79 313.30 0.6118
330.15 1.153761 1632.29 325.30 0.6159 319.15 1.161574 1658.39 313.03 0.6117
329.90 1.153939 1632.88 325.02 0.6158 318.91 1.161750 1658.98 312.76 0.6116
329.65 1.154114 1633.47 324.73 0.6157 318.65 1.161930 1659.58 312.48 0.6115
329.41 1.154294 1634.06 324.45 0.6156 318.40 1.162110 1660.18 312.21 0.6114
329.15 1.154469 1634.65 324.17 0.6155 318.16 1.162286 1660.78 311.93 0.6113
328.91 1.154648 1635.24 323.88 0.6154 317.90 1.162462 1661.37 311.67 0.6112
328.65 1.154826 1635.84 323.59 0.6153 317.65 1.162643 1661.97 311.39 0.6111
328.40 1.155003 1636.43 323.31 0.6152 317.41 1.162820 1662.56 311.12 0.6110
328.15 1.155181 1637.02 323.03 0.6151 317.15 1.162998 1663.16 310.85 0.6109
327.90 1.155360 1637.61 322.75 0.6150 316.91 1.163174 1663.75 310.58 0.6108
327.66 1.155535 1638.20 322.47 0.6149 316.65 1.163352 1664.35 310.31 0.6107
316.15 1.163706 1665.55 309.77 0.6105 303.90 1.172408 1695.01 296.88 0.6054
315.90 1.163885 1666.15 309.50 0.6104 303.65 1.172587 1695.62 296.62 0.6053
315.65 1.164062 1666.74 309.23 0.6103 303.40 1.172764 1696.23 296.36 0.6052
315.40 1.164240 1667.34 308.96 0.6102 303.15 1.172937 1696.81 296.11 0.6051
315.15 1.164417 1667.94 308.70 0.6101 302.90 1.173120 1697.43 295.85 0.6050
314.90 1.164597 1668.54 308.43 0.6100 302.65 1.173295 1698.04 295.59 0.6049
314.65 1.164774 1669.14 308.16 0.6099 302.40 1.173473 1698.64 295.34 0.6048
314.40 1.164951 1669.73 307.89 0.6098 302.15 1.173648 1699.25 295.09 0.6047
314.15 1.165128 1670.33 307.63 0.6097 301.90 1.173826 1699.86 294.83 0.6046
313.90 1.165305 1670.94 307.35 0.6096 301.65 1.174003 1700.47 294.57 0.6045
313.65 1.165485 1671.54 307.09 0.6095 301.40 1.174180 1701.07 294.32 0.6043
313.40 1.165661 1672.13 306.82 0.6094 301.15 1.174361 1701.68 294.06 0.6042
313.15 1.165839 1672.72 306.56 0.6093 300.90 1.174535 1702.29 293.81 0.6041
312.90 1.166018 1673.34 306.29 0.6092 300.65 1.174714 1702.90 293.56 0.6040
312.65 1.166194 1673.94 306.02 0.6091 300.40 1.174891 1703.50 293.30 0.6039
312.40 1.166372 1674.54 305.75 0.6090 300.15 1.175070 1704.12 293.05 0.6038
312.15 1.166549 1675.14 305.49 0.6089 299.90 1.175247 1704.73 292.79 0.6037
311.90 1.166726 1675.74 305.22 0.6088 299.65 1.175425 1705.33 292.54 0.6036
311.65 1.166903 1676.34 304.96 0.6086 299.40 1.175602 1705.95 292.29 0.6035
311.40 1.167085 1676.95 304.69 0.6085 299.15 1.175780 1706.55 292.04 0.6034
311.15 1.167260 1677.55 304.43 0.6084 298.90 1.175955 1707.16 291.78 0.6033
310.90 1.167437 1678.14 304.17 0.6083 298.65 1.176133 1707.77 291.53 0.6032
310.65 1.167617 1678.74 303.90 0.6082 298.40 1.176311 1708.39 291.28 0.6030
310.40 1.167794 1679.35 303.63 0.6081 298.15 1.176489 1709.00 291.02 0.6029
310.15 1.167970 1679.94 303.38 0.6080 297.90 1.176666 1709.61 290.77 0.6028
309.90 1.168149 1680.55 303.11 0.6079 297.65 1.176842 1710.22 290.52 0.6027
309.65 1.168325 1681.15 302.85 0.6078 297.40 1.177019 1710.84 290.27 0.6026
309.40 1.168502 1681.75 302.59 0.6077 297.15 1.177201 1711.45 290.02 0.6025
309.15 1.168680 1682.35 302.32 0.6076 296.90 1.177373 1712.06 289.77 0.6024
308.90 1.168859 1682.96 302.06 0.6075 296.65 1.177553 1712.67 289.52 0.6023
308.65 1.169036 1683.55 301.80 0.6074 296.40 1.177729 1713.28 289.27 0.6022
308.40 1.169213 1684.16 301.54 0.6073 296.15 1.177905 1713.90 289.01 0.6021
308.15 1.169391 1684.76 301.28 0.6072 295.90 1.178085 1714.52 288.76 0.6019
307.90 1.169567 1685.36 301.02 0.6071 295.65 1.178265 1715.13 288.51 0.6018
307.65 1.169742 1685.96 300.76 0.6070 295.40 1.178438 1715.75 288.26 0.6017
307.40 1.169922 1686.56 300.50 0.6069 295.15 1.178617 1716.36 288.01 0.6016
307.15 1.170102 1687.17 300.23 0.6068 294.90 1.178798 1716.97 287.76 0.6015
306.90 1.170276 1687.77 299.98 0.6067 294.65 1.178971 1717.58 287.52 0.6014
306.65 1.170454 1688.37 299.72 0.6066 294.40 1.179148 1718.20 287.27 0.6013
306.40 1.170632 1688.98 299.45 0.6065 294.15 1.179325 1718.81 287.02 0.6012
306.15 1.170810 1689.58 299.20 0.6064 293.90 1.179505 1719.42 286.77 0.6011
305.90 1.170986 1690.18 298.94 0.6063 293.65 1.179682 1720.04 286.52 0.6009
305.65 1.171165 1690.79 298.68 0.6062 293.40 1.179858 1720.66 286.27 0.6008
305.40 1.171343 1691.39 298.42 0.6060 293.15 1.180037 1721.27 286.03 0.6007
305.15 1.171518 1691.99 298.16 0.6059 292.90 1.180210 1721.88 285.78 0.6006
304.90 1.171699 1692.60 297.90 0.6058 292.65 1.180390 1722.50 285.53 0.6005
304.40 1.172053 1693.80 297.39 0.6056 292.15 1.180744 1723.72 285.04 0.6003
304.15 1.172230 1694.41 297.13 0.6055 291.90 1.180923 1724.34 284.80 0.6002
291.65 1.181104 1724.95 284.55 0.6000 279.40 1.189760 1755.38 272.77 0.5944
291.40 1.181278 1725.57 284.30 0.5999 279.15 1.189935 1756.03 272.53 0.5943
291.15 1.181453 1726.18 284.06 0.5998 278.90 1.190108 1756.62 272.31 0.5941
290.90 1.181631 1726.80 283.81 0.5997 278.65 1.190288 1757.23 272.08 0.5940
290.65 1.181809 1727.43 283.56 0.5996 278.40 1.190464 1757.88 271.83 0.5939
290.40 1.181990 1728.05 283.32 0.5995 278.15 1.190632 1758.50 271.60 0.5938
290.15 1.182162 1728.67 283.07 0.5994
289.90 1.182339 1729.29 282.83 0.5993
289.65 1.182515 1729.91 282.58 0.5991
289.39 1.182700 1730.84 282.24 0.5990
289.15 1.182877 1731.59 281.95 0.5989
288.89 1.183052 1732.13 281.73 0.5988
288.64 1.183228 1732.78 281.48 0.5987
288.39 1.183407 1733.34 281.25 0.5986
288.15 1.183574 1733.91 281.03 0.5985
287.90 1.183753 1734.51 280.79 0.5983
287.64 1.183941 1735.04 280.58 0.5982
287.40 1.184107 1735.67 280.33 0.5981
287.15 1.184289 1736.27 280.10 0.5980
286.90 1.184462 1736.82 279.88 0.5979
286.65 1.184637 1737.45 279.63 0.5978
286.40 1.184815 1738.07 279.39 0.5977
286.15 1.184986 1738.68 279.16 0.5975
285.90 1.185168 1739.24 278.93 0.5974
285.65 1.185344 1739.86 278.69 0.5973
285.40 1.185519 1740.47 278.46 0.5972
285.15 1.185700 1741.08 278.22 0.5971
284.90 1.185886 1741.82 277.94 0.5970
284.64 1.186059 1742.42 277.71 0.5968
284.40 1.186228 1742.99 277.49 0.5967
284.15 1.186403 1743.61 277.25 0.5966
283.90 1.186582 1744.21 277.02 0.5965
283.65 1.186756 1744.84 276.78 0.5964
283.40 1.186933 1745.46 276.54 0.5963
283.15 1.187110 1746.08 276.30 0.5961
282.90 1.187288 1746.70 276.06 0.5960
282.65 1.187467 1747.32 275.82 0.5959
282.40 1.187641 1747.95 275.59 0.5958
282.15 1.187817 1748.57 275.35 0.5957
281.90 1.187991 1749.20 275.11 0.5956
281.65 1.188172 1749.83 274.87 0.5954
281.40 1.188344 1750.39 274.66 0.5953
281.15 1.188523 1751.00 274.42 0.5952
280.90 1.188699 1751.60 274.19 0.5951
280.40 1.189050 1752.86 273.72 0.5948
280.15 1.189231 1753.49 273.48 0.5947
279.90 1.189407 1754.12 273.24 0.5946
279.65 1.189580 1754.75 273.01 0.5945
2-hydroxy ethylammonium pentanoate (2-HEAPE)
T
(K)
ρ
(gcm-3)
u
(ms-1)
κS
(TPa-1)
103 · α
(K-1)
T
(K)
ρ
(gcm-3)
u
(ms-1)
κS
(TPa-1)
103 · α
(K-1)
338.15 1.020672 1468.15 454.54 -3.6736 307.90 1.039467 1558.18 396.24 -3.8607
337.90 1.020820 1468.77 454.09 -3.6729 307.65 1.039618 1558.99 395.77 -3.8646
337.65 1.020969 1469.46 453.60 -3.6723 307.40 1.039772 1559.78 395.31 -3.8684
337.40 1.021126 1470.18 453.08 -3.6716 307.15 1.039925 1560.61 394.83 -3.8723
337.15 1.021280 1470.87 452.59 -3.6710 306.90 1.040077 1561.44 394.35 -3.8763
336.90 1.021436 1471.58 452.09 -3.6705 306.65 1.040230 1562.25 393.89 -3.8803
336.65 1.021593 1472.29 451.58 -3.6700 306.40 1.040384 1563.08 393.41 -3.8843
336.40 1.021745 1473.00 451.08 -3.6695 306.15 1.040533 1563.89 392.94 -3.8883
336.15 1.021898 1473.73 450.56 -3.6690 305.90 1.040687 1564.73 392.47 -3.8924
335.65 1.022205 1475.12 449.58 -3.6683 305.40 1.040991 1566.38 391.52 -3.9007
335.40 1.022364 1475.83 449.08 -3.6679 305.15 1.041143 1567.19 391.06 -3.9050
335.15 1.022520 1476.54 448.58 -3.6677 304.90 1.041297 1568.03 390.59 -3.9092
334.90 1.022671 1477.24 448.09 -3.6674 304.65 1.041450 1568.87 390.11 -3.9135
334.65 1.022828 1477.95 447.59 -3.6672 304.40 1.041602 1569.72 389.63 -3.9178
334.40 1.022986 1478.66 447.09 -3.6670 304.15 1.041753 1570.56 389.16 -3.9222
334.15 1.023146 1479.37 446.59 -3.6669 303.90 1.041907 1571.39 388.69 -3.9266
333.90 1.023305 1480.07 446.10 -3.6668 303.65 1.042059 1572.25 388.21 -3.9310
333.65 1.023463 1480.78 445.60 -3.6667 303.40 1.042209 1573.09 387.74 -3.9355
333.40 1.023622 1481.49 445.11 -3.6667 303.15 1.042363 1573.94 387.26 -3.9400
333.15 1.023780 1482.20 444.61 -3.6667 302.90 1.042516 1574.79 386.79 -3.9445
332.90 1.023940 1482.92 444.11 -3.6667 302.65 1.042668 1575.65 386.31 -3.9491
332.65 1.024100 1483.63 443.62 -3.6668 302.40 1.042820 1576.51 385.83 -3.9537
332.40 1.024257 1484.34 443.12 -3.6669 302.15 1.042972 1577.39 385.34 -3.9584
332.15 1.024414 1485.06 442.63 -3.6671 301.90 1.043124 1578.23 384.88 -3.9631
331.90 1.024574 1485.77 442.13 -3.6673 301.65 1.043277 1579.11 384.39 -3.9678
331.65 1.024732 1486.48 441.64 -3.6675 301.40 1.043429 1579.97 383.92 -3.9726
331.40 1.024890 1487.19 441.15 -3.6678 301.15 1.043579 1580.82 383.45 -3.9774
331.15 1.025050 1487.90 440.66 -3.6681 300.90 1.043732 1581.71 382.96 -3.9822
330.90 1.025207 1488.62 440.17 -3.6684 300.65 1.043883 1582.58 382.49 -3.9871
330.65 1.025363 1489.35 439.67 -3.6688 300.40 1.044037 1583.48 382.00 -3.9920
330.40 1.025523 1490.05 439.19 -3.6692 300.15 1.044188 1584.38 381.51 -3.9970
330.15 1.025679 1490.79 438.69 -3.6697 299.90 1.044340 1585.27 381.02 -4.0020
329.90 1.025838 1491.51 438.20 -3.6702 299.65 1.044492 1586.16 380.54 -4.0070
329.65 1.025997 1492.23 437.71 -3.6707 299.40 1.044644 1587.08 380.04 -4.0121
329.15 1.026310 1493.70 436.71 -3.6719 298.90 1.044973 1588.87 379.07 -4.0224
328.90 1.026467 1494.41 436.23 -3.6726 298.65 1.045148 1589.78 378.57 -4.0275
328.65 1.026627 1495.14 435.74 -3.6732 298.40 1.045311 1590.70 378.07 -4.0328
327.90 1.027097 1497.32 434.27 -3.6755 297.65 1.045807 1593.44 376.60 -4.0487
327.65 1.027255 1498.06 433.77 -3.6764 297.40 1.045975 1594.39 376.09 -4.0540
327.40 1.027411 1498.78 433.29 -3.6772 297.15 1.046142 1595.32 375.59 -4.0594
327.15 1.027568 1499.51 432.80 -3.6781 296.90 1.046304 1596.24 375.10 -4.0649
326.90 1.027725 1500.24 432.32 -3.6791 296.65 1.046470 1597.18 374.60 -4.0704
326.65 1.027883 1500.98 431.82 -3.6801 296.40 1.046642 1598.12 374.10 -4.0759
326.40 1.028039 1501.70 431.34 -3.6811 296.15 1.046804 1599.08 373.59 -4.0814
326.15 1.028194 1502.44 430.85 -3.6821 295.90 1.046975 1600.00 373.10 -4.0870
325.90 1.028352 1503.16 430.38 -3.6832 295.65 1.047135 1600.95 372.60 -4.0927
325.65 1.028508 1503.88 429.90 -3.6844 295.40 1.047303 1601.93 372.08 -4.0983
325.40 1.028665 1504.64 429.40 -3.6855 295.15 1.047465 1602.89 371.58 -4.1041
325.15 1.028822 1505.36 428.92 -3.6868 294.90 1.047628 1603.86 371.07 -4.1098
324.90 1.028976 1506.11 428.43 -3.6880 294.65 1.047795 1604.81 370.58 -4.1156
324.65 1.029135 1506.84 427.95 -3.6893 294.40 1.047960 1605.78 370.07 -4.1214
324.40 1.029289 1507.58 427.47 -3.6906 294.15 1.048125 1606.77 369.56 -4.1273
324.15 1.029445 1508.32 426.98 -3.6920 293.90 1.048288 1607.74 369.05 -4.1332
323.90 1.029602 1509.05 426.50 -3.6934 293.65 1.048451 1608.73 368.54 -4.1391
323.65 1.029757 1509.79 426.02 -3.6948 293.40 1.048614 1609.75 368.02 -4.1451
323.15 1.030071 1511.28 425.05 -3.6978 292.90 1.048944 1611.75 366.99 -4.1571
322.90 1.030226 1512.02 424.57 -3.6993 292.65 1.049105 1612.77 366.47 -4.1632
322.65 1.030381 1512.75 424.10 -3.7009 292.40 1.049271 1613.76 365.96 -4.1693
322.40 1.030537 1513.50 423.62 -3.7025 292.15 1.049433 1614.77 365.45 -4.1755
322.15 1.030693 1514.23 423.14 -3.7042 291.90 1.049593 1615.76 364.94 -4.1817
321.90 1.030846 1514.98 422.66 -3.7059 291.65 1.049759 1616.79 364.42 -4.1879
321.65 1.031002 1515.72 422.18 -3.7076 291.40 1.049921 1617.83 363.90 -4.1942
321.40 1.031159 1516.46 421.71 -3.7094 291.15 1.050082 1618.87 363.37 -4.2005
321.15 1.031314 1517.21 421.23 -3.7112 290.90 1.050244 1619.95 362.83 -4.2068
320.90 1.031468 1517.96 420.75 -3.7130 290.65 1.050407 1620.99 362.31 -4.2132
320.65 1.031625 1518.71 420.27 -3.7149 290.40 1.050566 1622.02 361.80 -4.2196
320.40 1.031780 1519.46 419.79 -3.7168 290.15 1.050730 1623.16 361.23 -4.2261
320.15 1.031934 1520.22 419.31 -3.7188 289.90 1.050889 1624.19 360.72 -4.2326
319.90 1.032088 1520.97 418.83 -3.7208 289.65 1.051050 1625.29 360.18 -4.2391
319.65 1.032243 1521.73 418.35 -3.7228 289.40 1.051211 1626.38 359.64 -4.2457
319.40 1.032399 1522.49 417.87 -3.7249 289.15 1.051372 1627.47 359.10 -4.2523
319.15 1.032553 1523.24 417.40 -3.7270 288.90 1.051531 1628.60 358.55 -4.2590
318.90 1.032709 1524.00 416.92 -3.7292 288.65 1.051691 1629.70 358.01 -4.2656
318.65 1.032862 1524.77 416.44 -3.7313 288.40 1.051853 1630.82 357.46 -4.2724
318.40 1.033016 1525.53 415.96 -3.7336 288.15 1.052010 1631.92 356.93 -4.2791
318.15 1.033171 1526.28 415.49 -3.7358 287.90 1.052170 1633.05 356.38 -4.2859
317.90 1.033327 1527.05 415.01 -3.7381 287.65 1.052330 1634.18 355.83 -4.2928
317.40 1.033635 1528.57 414.06 -3.7428 287.15 1.052647 1636.52 354.71 -4.3065
317.15 1.033790 1529.33 413.59 -3.7452 286.90 1.052803 1637.66 354.16 -4.3135
316.65 1.034098 1530.86 412.64 -3.7502 286.40 1.053121 1639.97 353.06 -4.3275
316.40 1.034253 1531.63 412.16 -3.7527 286.15 1.053282 1641.17 352.49 -4.3345
316.15 1.034406 1532.39 411.69 -3.7552 285.90 1.053440 1642.36 351.93 -4.3416
315.90 1.034559 1533.16 411.22 -3.7578 285.65 1.053595 1643.59 351.35 -4.3488
315.40 1.034867 1534.71 410.26 -3.7631 285.15 1.053914 1645.91 350.25 -4.3632
315.15 1.035022 1535.47 409.80 -3.7659 284.90 1.054069 1647.20 349.65 -4.3704
314.90 1.035175 1536.22 409.34 -3.7686 284.65 1.054227 1648.38 349.10 -4.3777
314.65 1.035330 1536.99 408.86 -3.7714 284.40 1.054384 1649.68 348.50 -4.3850
314.40 1.035483 1537.77 408.39 -3.7742 284.15 1.054542 1650.96 347.91 -4.3924
314.15 1.035638 1538.53 407.92 -3.7771 283.90 1.054697 1652.23 347.32 -4.3998
313.90 1.035792 1539.30 407.46 -3.7800 283.65 1.054853 1653.49 346.74 -4.4072
313.65 1.035945 1540.06 406.99 -3.7829 283.40 1.055012 1654.78 346.15 -4.4147
313.40 1.036100 1540.83 406.53 -3.7859 283.15 1.055166 1656.17 345.52 -4.4222
313.15 1.036252 1541.60 406.06 -3.7889 282.90 1.055325 1657.46 344.93 -4.4297
312.90 1.036406 1542.37 405.60 -3.7919 282.65 1.055479 1658.73 344.35 -4.4373
312.65 1.036558 1543.14 405.13 -3.7950 282.40 1.055637 1660.17 343.70 -4.4449
312.40 1.036711 1543.91 404.67 -3.7981 282.15 1.055795 1661.49 343.10 -4.4526
312.15 1.036865 1544.69 404.20 -3.8013 281.90 1.055948 1662.83 342.50 -4.4603
311.90 1.037019 1545.47 403.73 -3.8045 281.65 1.056104 1664.24 341.87 -4.4680
311.65 1.037171 1546.25 403.26 -3.8077 281.40 1.056260 1665.61 341.26 -4.4758
311.40 1.037325 1547.02 402.80 -3.8110 281.15 1.056416 1667.01 340.63 -4.4836
311.15 1.037479 1547.82 402.33 -3.8143 280.90 1.056572 1668.41 340.01 -4.4914
310.65 1.037785 1549.39 401.39 -3.8210 280.40 1.056883 1671.29 338.74 -4.5072
310.40 1.037938 1550.17 400.93 -3.8244 280.15 1.057038 1672.76 338.10 -4.5152
310.15 1.038089 1550.96 400.46 -3.8279 279.90 1.057192 1674.21 337.46 -4.5232
309.90 1.038244 1551.75 400.00 -3.8314 279.65 1.057349 1675.59 336.86 -4.5312
309.65 1.038396 1552.56 399.52 -3.8349 279.40 1.057504 1677.18 336.17 -4.5393
309.40 1.038550 1553.36 399.05 -3.8385 279.15 1.057659 1678.69 335.52 -4.5474
309.15 1.038704 1554.16 398.58 -3.8421 278.90 1.057816 1680.20 334.86 -4.5556
308.90 1.038856 1554.95 398.12 -3.8458 278.65 1.057971 1681.62 334.25 -4.5637
308.65 1.039008 1555.77 397.64 -3.8494 278.40 1.058124 1683.11 333.61 -4.5720
308.40 1.039161 1556.55 397.18 -3.8532 278.15 1.058279 1684.75 332.91 -4.5802
308.15 1.039313 1557.36 396.71 -3.8569

Table 2.

Densities (ρ), ultrasonic velocity (u), isentropic compressibilities (κS), isobaric expansibilities (α), 278.15-338.15K

The contrary effect is observed for conductivity. At the same temperature, higher viscosity was observed when the salt was of higher molecular weight. The effect of the temperature is similar for all salts.

A frequently applied derived property for industrial mixtures is the isobaric expansibility or thermal expansion coefficient (α), expressed as the temperature dependence of density. Thermal expansion coefficients are calculated by means of ( Δ ρ / ρ ) as a function of temperature and assuming that α remains constant in any thermal range. As in the case of pure chemicals it can be computed by way of the expression:

α =   ( ln ρ T ) P , x E2

taking into account the temperature dependence of density. The results gathered in Table 2 showed that a minimum of isobaric expansibility is obtained (in terms of negative values) at approximately the same temperature for all ILs. The smaller the size of the cation (monoethylene cation), the lower the value of isobaric expansibility was obtained.

Temperature (K) 2-HEAF 2-HEAPE
278.15 2158.20 83.6
288.15 3069.00 143.3
298.15 4197.60 239.6
308.15 5623.20 453.4
318.15 6959.70 632.6
328.15 8563.50 910.8
338.15 10404.90 1202.9

Table 3.

Values of ionic conductivity (µS∙cm-1) of the 2-HEAF and 2-HEAPE in the range 278.15 – 338.15 K

The values of ionic conductivity are gathered in Table 3. These results show an increasing trend for higher temperatures in each case. This fact may be ascribed to the increasing mobility of the ions for increased temperatures. At the same time, the ionic conductivity values decrease when molecular weight increases, thus 2-HEAPE has a lower ionic conductivity than 2-HEAF, the shortest member of this IL family [23].

The factor studied in this work is the chain length of the anion. The influence of anion residue is higher in terms of steric hindrance, due to its longer structure [2, 23]. This factor produces a higher disturbation on ion package. This fact may be observed in terms of higher values of densities and ultrasonic velocities for those salts of the lighter anion [37].

The ILs studied in this work showed interesting properties for industrial use: low cost of preparation, simple synthesis and purification methods. Moreover, the very low toxicity and the degradability have been verified [38]. Thus, sustainable processes can be originated from their use.

With this in mind, we decided to test their catalytic potential for several aldol condensation reactions with interest for fine chemicals synthesis. At industrial level aldol condensations are catalyzed by homogeneous alkaline bases (KOH or NaOH) [39,40] but with this kind of catalysts numerous disadvantages arise such as loss of catalysts due to separation difficulties, corrosion problems in the equipment and generation of large amounts of residual effluents which must be subsequently treated to minimize their environmental impact. Consequently, new technological solutions have to be developed in order to generate new and more environmental friendly processes.

The condensation reaction between citral and acetone leads to the formation of pseudoionones which are precursors in the commercial production of vitamin A. In the last years, the aldol condensation between citral and acetone has been studied by several groups employing different types of catalysts: rehydrated hydrotalcites [41], mixed oxides derived from hydrotalcites [42, 43], organic molecules [44], ionic liquids [28] etc.

Using the mixed oxides derived from hydrotalcites Climent et al. [42, 43] obtained a conversion of 83% and selectivity to pseudoionones of 82% in 1 h. Abello et al. obtained a citral conversion of 81% in only 5 min employing rehydrated hydrotalcites as catalysts [41] highlighting that Brønsted basic sites are more active than Lewis sites for aldol condensation reactions. In the study of Cota et al. [44] it was shown that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) which has Lewis basic properties, is inactive for aldol condensation reactions; however when it reacts with equimolar amounts of water, this molecule transforms towards a complex that shows Brønsted basic properties and becomes active giving a conversion of 89.17% and a selectivity of 89.6% in 6 h. When choline hydroxide (ionic liquid) was used as catalyst a citral conversion of 93% and selectivity of 98.2% were obtained in 1 h [28].

Among the ILs studied in this work, for citral and acetone condensation (entry 1, Table 4) the most active IL is 2-HEAA, which gives a conversion of 52%, the less active is 2-HEAiB which gives a conversion of 10%. The selectivity obtained in this reaction ranges between 49-83%. No traces of diacetone alcohol derived from the self-condensation of acetone were found but other secondary products coming from the self-condensation of citral and oligomers derived from citral are detected in small quantities in the reaction mixture.

Entry Substrate Ketone Product Catalyst Time Conversion Selectivity
(h) (%) (%)
1 2-HEAF
2-HEAP
2-HEAA
2-HEAB
2-HEAiB
2-HEAPE



7

35
40
52
33
10
38
83
63
74
60
53
49
2 2-HEAF
2-HEAP
2-HEAA
2-HEAB
2-HEAiB
2-HEAPE
4
3
4
2
2
2
94
100
99
99
93
98
82
86
85
85
85
77

Table 4.

Condensation reactions catalyzed by the studied ILs.

For the production of benzylideneacetone from the aldol condensation between acetone and benzaldehyde, Cota et al. [44] obtained a conversion of 99.9% and 93.97 selectivity in 2 h. When choline hydroxide was employed as catalyst [28] the total conversion was obtained in 0.1 hours but due to the production of dibenzylidenacetone the selectivity to benzylidenacetone decreased around 77%.

When ILs presented in this study were employed for this reaction (entry 2, Table 4), in 2 h of reaction, a conversion of 99% and a selectivity of 85% are obtained when using 2-HEAB as catalyst. Good conversion was also obtained with 2-HEAiB (93%) and 2-HEAPE (98%) with selectivity of 85% and 77% respectively. The decrease in the selectivity to benzylidenacetone is due to the formation of secondary products which include products of aldolisation of benzylidenacetone, like dibenzylidenacetone and other oligomers. The other studied ILs reached the maximum conversion in 3h (2-HEAP) and 4h (2-HEAF and 2-HEAA) and provided high selectivities between 82-86%.

For the repeated runs experiments, we used 2-HEAB in the condensation reaction between acetone and benzaldehyde. The catalyst was recycled 3 times, and in all runs a very good conversion was obtained. The results are presented in Figure 2.

Figure 2.

Repeated runs experiments using 2-HEAB in benzylideneacetone synthesis.

The loss of activity noticed in the second and third run can be attributed, on one hand to the loss of IL during the separation process and on the other hand due to the absorption of reaction products on the active sites of the catalyst. IL is partially soluble in the reaction product therefore during the separation procedure small quantities of IL can be dissolved in the organic phase and therefore lost during the separation process. This hypothesis is sustained by the evolution of the specific bands of the ILs which appear in the range 3500-2400 cm-1, almost disappearing in the re-used sample as Figure 3 shows.

A weak band around 1591 cm-1 is present in the re-used sample accounting for the carbonyl stretching and N-H plane bending vibrations. On the other hand, deactivation of the catalyst, moreover exhibiting a dark yellow color, is probably due to the adsorption of oligomers and other secondary products on the surface of the catalyst during the reaction. This hypothesis is supported by the appearance of new bands in the re-used IL spectrum. The bands detected in the 1700-1200 cm-1 region corresponding to the symmetric and stretching vibrations of CH modes can be assigned to oligomeric species adsorbed on the surface. On the other hand in the 1260-700 cm-1 region bands which are normally weak appear and can be assigned to the C-C skeletal vibrations.

Figure 3.

FT-IR spectra for (a) 2-HEAB before reaction, (b) 2-HEAB after reaction (3 consecutive runs).

In order to facilitate the recovery and re-use of the ILs we decided to immobilize them on a solid support. Immobilization and supporting of ILs can be achieved by simple impregnation, covalent linking of the cation or the anion, polymerization etc [45-47]. Compared to pure ILs, immobilized ILs facilitate the recovery and re-use of the catalyst. Previous reports describe the immobilization of ILs by adsorption or grafting onto silica surface and their use as catalysts for reactions like Friedel-Crafts acylation [45], hydrogenation [48] and hydroformilation [49]. Organic polymers [30], natural polymers [50] and zeolites [51] have been also used as supports for ILs.

For this purpose, the ILs were supported on alanine, a cheap readily available aminoacid. Their catalytic activity was tested in the same reactions as the pure ILs.

The catalytic activity results of the a-ILs for the citral-acetone condensation are presented in Table 5. After 6 h of reaction, the two isomers of citral can be converted into the corresponding pseudoionone with conversion between 30-56% except for a-HEAiB for which a conversion of 9% was obtained. The most active IL for this reaction is a-2-HEAA which provides a conversion of 56%. The selectivity obtained in this reaction ranges between 48-80%. No traces of diacetone alcohol derived from the self-condensation of acetone were found, but other secondary products coming from the self condensation of citral and oligomers derived from citral are detected in the reaction mixture. The support (entry 1) is not catalytically active.

In the condensation reaction of benzaldehyde and acetone the first step is the deprotonation of an acetone molecule to give the enolate anion whose nucleophilic attack on the C=O group of benzaldehyde leads to the β-aldol. This latter is easily dehydrated on weak acid sites and benzylidenacetone is obtained.

Entry Catalyst Conversion Selectivity
    (%) (%)
1 alanine 0 0
2 a-2-HEAF 30 61
3 a-2-HEAA 56 74
4 a-2-HEAP 49 80
5 a-2-HEAB 35 63
6 a-2-HEAiB 9 52
7 a-2-HEAPE 33 48

Table 5.

Conversion at 6 h for citral-acetone condensation catalyzed by a-ILs

Entry Catalyst Conversion Selectivity
    (%) (%)
1 alanine 0 0
2 a-2-HEAF 99 83
3 a-2-HEAA 99 82
4 a-2-HEAP 99 85
5 a-2-HEAB 99 84
6 a-2-HEAiB 78 82
7 a-2-HEAPE 98 80

Table 6.

Conversion at 2 h for benzaldehyde-acetone condensation catalyzed by a-ILs

In 2 hours of reaction a conversion of 98-99% is achieved for the majority of a-ILs, while a lower conversion (78%) is obtained for a-2-HEAiB (Table 6). The selectivity toward benzylidenacetone is around 80-86% due to the formation of dibenzylidenacetone as secondary product. The support, alanine (entry 1) is not active for citral acetone condensation.

It is noteworthy that, for both studied reactions, the conversions obtained with the a-ILs are in the same range as the ones obtained with free ILs (Figure 4 and 5).

The a-ILs are easily separated from the reaction mixture and reused. For the consecutive runs experiments we chose condensation between benzaldehyde and acetone as model reaction. The catalysts were recycled for 3 consecutive runs and in all runs a very good conversion was obtained. The results are presented in Figure 6.

Figure 4.

Conversion at 6 h for citral-acetone condensation for free ILs and a-ILs.

Figure 5.

Conversion at 2 h for benzaldehyde-acetone condensation for free ILs and a-ILs.

Figure 6.

Consecutive runs experiments in benzaldehyde acetone condensation.

In the case of each IL, only a negligible loss of activity is detected in the second and third run which can be attributed to the possible adsorption of reactants or reaction products to the active sites of the catalyst.

From the comparison made with the aforementioned basic catalysts employed for these two aldol condensation reactions we can conclude that the ILs presented in this study are not the most active catalysts for these reactions but due to their green character and easy separation from the reaction media represent a convenient and environmental friendly alternative for the traditional homogeneous catalysts.

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4. Conclusions

In this work, we present a simple and efficient synthesis protocol for protic ionic liquids and the experimental data for density, ultrasonic velocity and ionic conductivity of these liquid salts. It was found that increased temperature diminishes the interaction among ions and therefore lower values of density, ultrasonic velocity, viscosity, surface tension and refractive index are obtained for increased temperatures in each case. The contrary effect is observed for conductivity.

The influence of chain length of the anion on the physicochemical properties of the ILs has been also studied. The effect of the anion residue is higher in terms of steric hindrance, due to its longer structure. This factor produces a higher disturbation on ion package. The physicochemical data of ILs are important for both, designing cleaner technological processes and understanding the interactions in this kind of compounds

The catalytic potential of these new ILs was tested for two aldol condensation reactions with interest for fine chemistry industry. Conversions ranging from 35 to 52% and selectivities up to 83% are obtained for the condensation of citral with acetone. In the synthesis of benzilidenacetone, conversions above 93% with selectivities around 85% are obtained. We also studied the optimization of the recovery process of the ILs and their reuse in repeated runs of experiments. The catalysts can be recycled and reused for three consecutive cycles without significant loss of activity.

In addition, in order to improve the recovery process, the ILs were immobilized on alanine, a cheap readily available aminoacid. The catalytic activity of the alanine supported ILs was tested for citral-acetone and benzaldyde-acetone condensations. It is noteworthy that, for both studied reactions, the conversions obtained with the a-ILs are in the same range as the ones obtained with free ILs; moreover the catalysts can be recycled and reused for three consecutive cycles without significant loss of activity.

The ILs studied in this work showed interesting properties for industrial use: low cost of preparation, simple synthesis and purification methods. Moreover, the very low toxicity and the degradability have been verified. Thus, sustainable processes can be originated from their use.

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Acknowledgments

This work has been financed by the MEC of Spain and the Generalitat of Catalunya (ICREA ACADEMIA AWARD).

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Written By

I. Cota, R. Gonzalez-Olmos, M. Iglesias and F. Medina

Submitted: 25 June 2012 Published: 23 January 2013