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Hydrocarbon Reactions
Role of Water in Reactions Under Supercritical Conditions – Hydrocarbons
a report by
Tadafumi Adschiri,
1
Masaru Watanabe
2
and Takafumi Sato
3
1. WPI Advanced Institute for Materials Research, Tohoku University; 2. Research Centre of Supercritical Fluid Technology, Tohoku University;
3. Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University
Water becomes supercritical above its critical temperature and pressure Friedel-Crafts reaction to form a polymer. The suppression of this
(374°C and 22.1MPa, respectively). Above the critical point, further polymerisation of lignin in SCW is an area of interest. We propose a
condensation of water upon compression does not occur, and around method for recycling the phenolic compounds produced in the reaction
the critical point the density of water changes drastically with small system.
1,2
In fact, by adding p-cresol to the reaction system, it is possible
changes in temperature and/or pressure. Because of this significant to completely suppress the formation of solid residue. Surprisingly, the
change in density, most properties of water change dramatically. The product obtained by this method was mainly 2-(4-hydroxy-benzyl)-4-
dielectric constant of water, which is a controlling factor in the solubility methyl-phenol (BMP). As shown in Figure 1, the greatest yield of BMP was
of the material dissolved in the water, the reaction rate and the reaction as high as 80%. This method can be used to recover BMP from wood.
2
equilibrium vary significantly around the critical point; this suggests that
tunable reaction atmosphere can be realised by controlling the Pyrolysis of Hexadecane and Polyethylene
temperature and pressure of water near its critical point. Another This section covers the solvent effect of water on pyrolysis of
important feature of supercritical water (SCW) is that its dielectric hydrocarbons. We conducted a detailed mechanistic and kinetic study
constant is in the range of 2–10, which is comparable to that of polar on hydrocarbon pyrolysis,
3,4
and concluded that the effect of water on
organic solvent. the elemental reactions of pyrolysis is insignificant.
3
However, when
pyrolysis was carried out on the long-chain hydrocarbon PE, we
Owing to the significant change in the properties of water at the critical observed a very interesting result: the reaction rate increased with
point, phase behaviour also changes drastically around this point. For a increasing water density and the resulting product distribution changed
two-component system of gas and water, a homogeneous phase is significantly.
3,4
We explained this result on the basis of phase behaviour,
formed at any composition. Since SCW exists in the form of compressible as shown in Figure 2. In Ar atmosphere, the main reactions take place
high-density steam, it is miscible with light gases. Taking into in the molten PE phase. However, when SCW is introduced, it is
consideration that the dielectric constant of SCW is similar to that of dissolved into the molten PE phase and extracts the relatively light
polar organic solvents, it can be concluded that oil and water become components formed by pyrolysis. Thus, the pyrolysis reaction takes place
miscible under supercritical conditions. in both phases. Here, it is important to know the effect of concentration
A noteworthy effect of water in reactions is the so-called ‘solvent effect’.
Tadafumi Adschiri is a Professor of Chemical Engineering
Owing to the drastic change in the properties of water around the critical
at the WPI Advanced Institute for Materials Research at
point, the solvation (local density/dielectric constant) of the reactant, the Tohoku University. He is the inventor of supercritical
intermediate material formed and the product obtained may change
hydrothermal synthesis methods for nanoparticles, which
have led to novel fabrication strategies for the
significantly with small changes in temperature and pressure. The
generation of inorganic/polymer hybrid materials with
solvated water molecules on the reactant or the intermediate material
novel controlled properties. Professor Adschiri has served
as a project leader on several national Japanese projects
can serve as the acid or base catalyst; thus, under supercritical conditions,
related to supercritical fluid technology and has received
even in the absence of a catalyst, an acid- or base-catalysed reaction can research awards from the Japanese Energy Society, the Chemistry Society of Japan and the
efficiently occur. These remarkable changes of the dielectric constant and
Chemical Engineering Society of Japan.
solvation with small changes in temperature and pressure may cause the E: ajiri@tagen.tohoku.ac.jp
reaction rate or reaction equilibrium to vary, and can sometimes result in
Masaru Watanabe is an Associate Professor at Tohoku
the formation of a unique reaction atmosphere.
University focusing on supercritical fluid technology.
Previously, he was a Research Associate in the
Thus, water can be used as a solvent for organic reactions, where the
Department of Chemical Engineering at Tohoku
University. He graduated in applied chemistry from
water molecules function as the catalyst and the solvent effect on the
Meiji University and conducted his PhD under the
reaction is tunable by varying the temperature and pressure. In this
direction of Professor Kunio Arai.
article, we limit our discussion to the role of water in hydrocarbon
reactions with lignin, hexadecane, polyethylene (PE) and bitumen.
Takafumi Sato is a Research Associate in the
Department of Applied Chemistry at Utsunomiya
Selective Lignin Conversion University. Previously, he was a Research Fellow at the
Lignin has a complicated chemical structure; however, studies have
Supercritical Fluid Research Centre of the Japanese
National Institute of Advanced Industrial Science and
revealed that this structure is composed of repeating units of guiacol and
Technology. He graduated from Tohoku University in
glycerol. When dissolved in SCW, this structure is easily hydrolysed to give
chemical engineering, and has also been a Visiting
Scholar to the University of Waterloo, Canada.
phenolic compounds and aldehydes. However, immediately after
hydrolysation the aldehyde combines with the phenolic compound via a
© TOUCH BRIEFINGS 2009
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