(79j) Structural Factors Determining the Thermal Decomposition Temperature of Phosphonium-Type Ionic Liquids On Metal-Oxide Supports

Structural Factors Determining the Thermal
Decomposition Temperature of Phosphonium-type Ionic Liquids on Metal-oxide Supports

Volkan Balci,
Asli Akcay, Alper Uzun*

Department of
Chemical and Biological Engineering, Koç University, 34450 Sarýyer, Istanbul,
Turkey
*Corresponding Author: auzun@ku.edu.tr

Ionic
liquids (ILs) are new-generation green solvents and potential alternatives for their
volatile organic counterparts. They have received tremendous attention during the
last decade because of their unique physical and chemical properties such as non-volatility,
non-flammability, high chemical and thermal stability, high solvating ability
and tunable miscibility. These physicochemical properties can be tailored by incorporating
almost infinite combinations of different anions and cations; hence they are often
called as ?designer? or ?task-specific? solvents. Such structural
diversity and functional flexibility has led to a growing interest towards
application in numerous research fields such as synthesis, electrochemistry,
separation, and catalysis.

As one of
the emergent application areas, two primary concepts have been pioneered to
exploit tunable features of ILs in catalysis -supported ionic liquid
phase (SILP) and solid catalysts with an ionic liquid layer (SCILL).
For SILP-type catalyst, a thin layer of ionic liquid containing a homogeneous
catalyst is applied to the internal surface of a porous support material. Similarly,
in SCILL concept, heterogeneous catalyst or catalytically active materials are
coated with a thin film of IL. These promising catalytic concepts lead to
enhanced selectivity, product distribution, yields, and selective solubility
for intermediates and products because of promoting interactions between active
sites, supports and IL.  However, application of ILs in these
supported-catalyst concepts is limited with their thermal stability on
corresponding support materials. Though most of the ILs are thermally stable at
elevated temperatures (> 300 ^°C) in their pure state, they become
less stable when coated on metal-oxide supports because of the interactions
between IL and metal-oxide.  Here we report the results of a systematic
investigation on the elucidation of structural factors determining the thermal
stability limits of 12 different phosphonium type ILs on three of the most
commonly used catalyst supports, SiO_2,g-Al₂O₃, and MgO.

Thermogravimetric
analysis (TGA) and IR spectroscopy based results illustrate that thermal
stability of phosphonium type ILs on metal-oxide supports mainly depends on the
point of zero charges of metal-oxides and the structure of ILs.  In general,
the thermal decomposition temperature of phosphonium-type ILs decreases
significantly as the acidity level of support decreases from SiO₂ to
g-Al₂O₃
and to MgO.  For instance, derivative onset of the decomposition
temperatures for tetrabutylphosphonium methanesulfonate decreases from
372 °C to 212 °C as the metal-oxide changes from SiO₂ to MgO.  

Moreover,
data further illustrate that the thermal decomposition temperature is strongly
influenced by the physicochemical properties of ILs determined by their structure. 
Figure 1 summarizes these variations in thermal decomposition temperature with varying
phosphonium-type IL structure on different metal-oxide surfaces. Results show
that thermal stability of ILs on the same metal-oxide decreases significantly with
increasing anion size. For example, for trihexyltetradecylphosphonium
cation, the thermal stability of ionic liquid with bis(2,4,4-trimethylpentyl)phosphinate
anion decreases compared to those with smaller anions such as bromide. 
When compared with these halide anions, a switch from Br^- to more
electronegative Cl^- anion further decreases the decomposition
temperature by approximately 20 °C on all supports.  On the other hand, thermal
stability of ILs on metal oxide supports increases when an aromatic group is
inserted into a methansulfonate group. For instance, on g-Al₂O_3, thermal
stability increases from 259 °C to 291 °C when the methanesulfonate anion
is substituted with p-toluenesulfonate.

Results
presented here reveal the structural factors determining the thermal stability
limits of phosphonium-type ILs on three of the most commonly used metal-oxide
catalyst supports. These stability limits serve as the basis for the selection
of suitable phosphonium-type ILs according to the application conditions of
IL-assisted supported catalysts. Thus, they add to the gap in IL literature
focusing on the properties of ILs on metal-oxide supports.