称号及び氏名

博士（工学）

学位授与の日付

2007 年 3 月 31 日

論

「Sub-Critical Water Conversion of D-fructose as Model for文

名Feridoun Salak Asghari

Carbohydrates and Biomass: Formation of Bio-based
Building Block Chemicals 」(糖質及びバイオマスのモデル物質として用いたD-フルクトースの亜臨界水転化－バイオ由来の基礎物質の生成－)
論文審査委員

主査

吉田浩之

副査

小西康裕

副査

坂東博

論文要旨

Fossil fuels are indispensable resources in the world. However, it is obvious that fossil fuels
are a finite resources and their availability is limited. On the other hand, it is estimated that about
170 billion tons of biomass are biosynthesized a year. Today, the uses of biomass as cheap andsustainable alternative resources, for the supply of primary energy and as raw materials insynthesis of organic chemistry represent an important facet of present and future technologies.
The transition to a sustainable raw material base will be one of the most important challenges
facing society in the 21st century. As part of this endeavor, substantial increases in the use ofbiomass-derived feedstock in the chemical industry are expected over the next years. On the other
hand, in recent years supercritical water and particularly sub-critical water have received more
attention as a novel and clean medium for chemical processes of environmental and industrial
importance. The use of sub- and supercritical water for many kinds of chemical reactions has
been gaining interests due to its potential as green solvent. More recently, growing attention has
been devoted to the hydrolysis and decomposition of biomass in sub-critical water for thesynthesis of industrial chemicals and related materials. Our aim is using this clean technique toconversion of biomass and carbohydrates to valuable chemicals in order to find direct
replacements for fossil derived chemicals.

The present work is aimed at developing sub-critical water technique to produce biobased
chemical compounds using fructose and/or other monosaccharides as models of renewablebiomass and carbohydrates. In the experimental study both batch and flow-type sub-critical water
reaction systems were used. The effect of the temperature, pressure, and other operating
parameter were investigated in detail.

It is well known that, in the dehydration of saccharides,

acids are necessary in normal pressure as catalyst. In the present work, different homogeneousand heterogeneous acids were used in the sub-critical water conditions. The major and importantresults of the present work are summarized as follows:
In chapter one, the general introduction and importance of the present work as well as
review of previous works were presented.
In chapter two, the optimization of conversion of fructose to 5-hydroxymethylfurfural(hereafter HMF) was an essential aim of this investigation. A batch-type sub-critical water
reaction system was used to perform the decomposition of fructose over a temperature range of473-593 K. Generally two series of experiments; reaction in absence and presence of acidcatalysts were carried out. Although, under the normal temperature and pressure conditions, thepresence of acid catalysts is necessary for the production of HMF, the results of sub-critical water
experiments showed that the dehydration of fructose to HMF can be conducted even in theabsence of acids. However lower amount of HMF was obtained in the absence of acids at 533 K.
Later, the catalytic effects of various organic and inorganic acids with pH values in the range of
1.5-5 were evaluated. In this study, hydrochloric, sulphuric, phosphoric, oxalic, citric, maleic,and p-toluenesulfonic acids that have previously been used to decompose the saccharides undernormal temperature and pressure conditions, were selected as the common homogeneous acids,
for evaluation of their applicability to the dehydration of fructose in sub-critical water. Other
pertaining parameters such as concentration of initial fructose, sub-critical water temperature andreaction time, which influenced the production yield of HMF and of by-products, wereinvestigated too.
It was found that not only the pH but also the nature of the acids had great influence on
the decomposition pathway. At lower pH, a rehydration of HMF to levulinic and formic acids
occurred, whereas at higher pH, polymerization reactions occurred. Phosphoric acid (at pH 2)
was determined to give the best balance between activity, yield, and by product amounts in thehomogenous acid catalysts used in this study. The optimum condition for the best yield of HMF(65%) was achieved at a temperature of 513 K for a residence time of 120 s.
The decomposition reaction of some other monosaccharides and disaccharides, whichwere sorbose, mannose, galactose, glucose, sucrose, and cellobiose, were also investigated. It was
found that the ketohexoses gave higher yield of HMF than aldohexoses. However, all saccharides
showed the possibility of the conversion to HMF in sub-critical water. The UV-Vis absorbanceshowed the formation of soluble polymers from oligomerization and polymerization ofsaccharides and HMF. These soluble polymers can not be quantitated; however it could bedetected by absorbance at 350-450 nm. One of the main by-products of decomposition ofsaccharides was the formation of these soluble polymers. In sub-critical water with phosphoricacid catalyst, since the higher yield of HMF was obtained, the formation of these polymers wasfound to be much smaller than other cases.
Another reason for causing the decrease of HMF yield is decomposition of HMF into
other compounds. The decomposition of fructose is consecutive-type reaction, by which formed
HMF can rehydrate to levulinic and formic acids. It is worth noting that good catalytic
activity of hydrochloric acid was observed at lower pH among all homogeneous acids used inthis study to rehydration of HMF into levulinic and formic acids.
In chapter three, possibility of applying solid acid catalysts in batch-type sub-criticalwater dehydration of fructose and or glucose, which were used as models of carbohydrate, wereinvestigated. Eighteen laboratory-made zirconium phosphate solid acids were prepared. Widerange of catalysts from amorphous to high crystalline forms was obtained by refluxing inconcentrate phosphoric acid and/or by calcination in two temperatures (i.e. 573 and 873 K).
Physicochemical properties of the prepared solid acids were studied. The experimental results
showed that the surface area decreased almost linearly by refluxing time and/or by heat
treatment. By increasing the crystallinity, the surface area decreased. Therefore, with respect tothe data of surface areas, it can be realized that both refluxing and calcination may increase thecrystallinity of solids toward higher crystal forms. On the other hand, results showed thatcalcining have a more noticeable effect on the amorphous forms than on higher crystallineforms.
Based on IUPAC classification on adsorption isotherms, it was found that the preparedsamples have nonporous or macroporous structures. Other experimental results showed that theamorphous and crystalline forms zirconium phosphates had different pore-size and its
distribution. Crystalline form had narrow range of pore distribution compared to the amorphousone. Specific pore volume decreased with increasing crystallinity; however it was found that inall solid samples their volumes of macroporous were small.
As application of zirconium phosphates, dehydration of fructose and glucose to HMF inbatch type sub-critical water was performed in the presence of different kinds of these solid
acids at 513 K. In the presence of amorphous form of zirconium phosphate, about 80% offructose was decomposed in sub-critical water at after 120 s, and the selectivity of thedehydration reaction of fructose to HMF rose to 61%. No rehydration products were identified.
Soluble polymers and 2-furfuraldehyde were the only major and minor side-products found,respectively. As result, it was realized that zirconium phosphate solid acids were stable in
sub-critical water and they can easily be recovered without changing the catalytic properties. Atthe experimental conditions, no corrosion of stainless steal reactors was observed when using
solid acids as catalyst.
In chapter four, home made flow-type sub-critical water system was used fordecomposition of fructose and formation of HMF, levulinic and formic acids simultaneously.
Previous results showed that in presence of hydrochloric acid, produced HMF can be
rehydrating to equimolar amounts of levulinic and formic acids. The experimental conditions
were 483 to 543 K, 4 to 15 MPa, and residence time of 0.5 to 300 s. The pH of initial feedsolution was adjusted at 1.8. The pressure of sub-critical water did not affect the decomposition
reactions; this differed from other operating variables.
The main products were found to be HMF, 2-furfuraldehyde, levulinic acid, formic acid,and soluble polymers. The HMF, levulinic and formic acids were also individually subjected fordecomposition at the same condition. From the decomposition of HMF, the products wereobtained as levulinic acid, formic acid, and soluble polymers; from decomposition of levulinic
and/or formic acids, we did not identified any decomposition products, however it was realizedthat soluble polymers can not be produced from levulinic and or formic acids. 2-furfuraldehyde
was found to be produced only from fructose and not from HMF.
A simplified reaction pathway was proposed. Based on the experimental data obtained
from decomposition products, a kinetic model, in which all identified pathways were taken into
consideration, was developed. This model distinguished between two different soluble polymers
based on the source of production. Theoretical lines matched well with the experimental
decomposition and formation curves of fructose, 2-furfuraldehdye, HMF, levulinic and formicacids. The best-fit parameters were determined by nonlinear regression analysis. The proposed
kinetic model can accurately predict the rate constant of fructose as well as other decomposition
products, especially of two kinds of soluble polymers, which can not be quantified
experimentally.

These results showed that the rate constants increase with the reaction temperatureobeying the Arrhenius equation. The activation energies of the reactions with rate constants
regarding to the kind of substrate along with the average activation energies (without concerning
the kind of substrate) were calculated for the reactions.
In chapter five, general conclusions of the present work were given.
From the results obtained in this thesis, it is concluded that conversion of not onlymonosaccharides, but also any kind of biomass-derived feedstock (and particularly various wastebiomasses) to valuable bio-based chemical compounds using sub-critical water as simple, clean,and environmentally friendly method is a feasible process. Also the author believed that thepotentially ease of scale-up of this operationally simple method are attractive for large-scale
production of valuable chemical compounds in order to replace fossil derive chemicals.

審査結果の要旨

本論文は、木質系バイオマスから化学物質を作るための基礎研究として、その単位化学構造であるフルクトースや他の単糖類を出発物質とし、各種ポリマーの原料に使われている5-hydroxymethylfurfural (以下ＨＭＦと略)を合成することを試みたもので、以下の成果を得ている。
（１）現在、ＨＭＦは単糖類を原料として有機溶媒中で合成されているが、この方法では大量生産が困難であり、また環境問題から脱有機溶媒反応が強く求められている。本研究では、臨界点以下の高温高圧水である亜臨界水反応の適用を試み、収率及び大量生産の可能性も含め極めて有効であることを示した。
（２）バッチ型亜臨界水反応器を用いて反応温度 473～593K の範囲でフルクトースの分解実験を行った。常温常圧においてはフルクトースの分解は酸触媒無しでは起こらない。しかし、亜臨界水中では、酸触媒無しでもＨＭＦが収率 21％で生成することを明らかになった。さらに水に溶解する種々の酸触媒について検討した結果、リン酸触媒（pH 2）を用いた場合、触媒効果が最も大きく、513K、反応時間 120 秒で収率 65％を得た。
（３）上記の反応は極めて複雑であるが、生成物の詳細な分析結果を基に、反応パスウェイを提案した。
（４）固体酸触媒の適応の可能性について検討を加えた。18 種類のリン酸ジリコニウムを合成した。これらは、アモルファス型から結晶質まで広範囲に内部構造を変化させたものである。アモルファス型の方が表面積は大きく細孔分布も広範囲に及んでいた。これらを亜臨界水反応に用いたところ、触媒粒子の表面積や細孔分布に無関係にほぼ同程度の触媒効果を示し、513K、反応時間 120 秒でＨＭＦの収率が 61％という大きな値を示した。
（５）連続亜臨界水処理装置を用いてフルクトースを分解しＨＭＦを生成する亜臨界水反応を塩酸触媒下で、温度、圧力、pH 及び反応時間を様々に変化させて実験した。反応は非常に複雑であるがそれらを簡略化した反応パスウェイとそのパスウエイに基づく反応動力学モデルを提案し、汎用性のある解析解を求めた。この理論解析結果は各種反応生成物の経時変化の実測値と良好な一致を示した。

本委員会は本論文の審査及び最終試験の結果から、博士（工学）の学位を授与することを適当と認める。