If you have learned chemistry in high school or higher educational opportunity, you should know Friedel-Crafts reaction. It is a very useful reaction which introduces a side chain onto an aromatic ring to form a framework of organic compound. Ordinal textbooks state that the catalyst for this reaction is AlCl₃. For example, the reaction is used to produce cumene [isopropylbenzene or (1-methylethyl)benzene in IUPAC rule] from benzene and propylene (propene). This reaction is so-called cumene method for environmental benign way to produce phenol. However, there is a problem. In practice, the liquid reactants are mixed into a solvent, and AlCl₃ is dissolved. In place of it, H₂SO₄ can be used. During the reaction, the catalyst is dissolved to form a homogeneous mixture with the reactant, and therefore it is called "homogeneous catalyst".

There is a serious problem when such a reaction is carried out in an industrial scale. After the reaction, the desired product and catalyst are mixed into the solution, and the separation is difficult. Distillation is effective for separation of organic compounds, but in this case, the catalyst causes side reactions at a high temperature and so that the distillation is difficult. NaOH aq. is added into such a solution to form oil and water layers, where the product, unreacted raw material and solvent are contained by the oil layer, while the salt formed by neutralization of catalyst by NaOH is contained by the water layer. They are easily separated. In the water layer, theoretically, only NaCl, NaAlO₂ and Na₂SO₄ are contained, just like sea water or hot spring. However, waste of such water should cause environmental pollution due to the by-products. It needs a post treatment for decomposition of harmful materials, but it takes energy to increase CO₂. In addition, new catalyst as well as NaOH is necessary for each run of the reaction. Obtaining such resources consumes energy and causes environmental pollution additionally. In other words, a soluble catalyst has a large impact on the environment. Nowadays, it becomes difficult to use the homogeneous catalysts.

Here, let's imagine a solid with acid sites which play a role of acid on the surface. A solid with the surface acid sites is called a solid acid, or a solid acid catalyst when it is utilized as a catalyst. If you pack the solid acid catalyst into a tube, and feed the reactants (for example, benzene and propylene) from the inlet of the tube, you will obtain only the desired product (for example, cumene)!!! Solid catalysts can realize completely harmless chemical plants from a view of environmental protection. Cumene production has been carried out with a solid acid catalyst (zeolite), and replacement of homogeneous catalyst by solid catalyst is strongly demanded in various fields. Suzuki-Miyaura reaction, famous of recent Nobel Prize, requires homogeneous organic Pd complex. Residual of harmful catalyst is a problem in this case. Also this should be solved by using a solid catalyst.

　高校以上で化学を学んだ人は，Friedel-Crafts反応というのを聞いたことがあるでしょう．ベンゼン環に側鎖を導入し，有機化合物の骨格構造を作ることができる，極めて有用な反応です．ふつうの教科書には，この反応の触媒はAlCl₃だと書いてあります．たとえばベンゼンとプロピレン(IUPAC名プロペン)からクメン[イソプロピルベンゼン，または(1-メチルエチル)ベンゼン]をつくるときに使われます．高校の教科書に出てくるフェノール製造のための「クメン法」で，環境に優しい方法だと習いましたね・・・・ところが，問題があります．実際の反応では，液体の原料を溶媒に混ぜ，AlCl₃を溶かして反応させます．H₂SO₄が使われることもあります．反応するときに触媒が溶けており，反応物と均一な系内にあるので，そのような状態にある触媒を「均一系触媒」と呼びます．

　このような反応を大規模に実施するには問題があります．反応が終わった後，溶媒の中に目的生成物と触媒が溶けており，簡単には分離できません．有機化合物同士はたいてい蒸留で分離できるのですが，触媒が入っていると加熱によって副反応が起き，望ましくない生成物ができてしまいます．このような混合物にNaOH水溶液を加えると油層と水層が分かれ，油層に生成物(と，普通は溶媒)が，水層に触媒とNaOHが中和した廃塩が含まれるので，分離できます．水層には，理論的にはNaCl, NaAlO₂，Na₂SO₄など塩類が含まれるのみで，海水や温泉のような成分しか含まれないことになりますから，捨ててもよいですね・・・・ちょっと待ってください．有害な成分が多少なりとも含まれますから，捨てたら環境汚染，無害化処理をすればエネルギーの浪費でCO₂増加と，いずれにしても環境に有害です．それだけでなく，反応のたびに新しい触媒を消費し，NaOHまで消費します．このような資源を得るためにさらにエネルギーが消費され，環境も汚されます．つまるところ，溶ける触媒は環境負荷が大きいのです．昔は見逃されてきた均一系触媒反応も，しだいに実施が困難になってきています．

　さて，もし固体の表面に酸のはたらきをする酸点があったら・・・・酸点を表面に持つ固体を固体酸といい，触媒として用いられる場合には固体酸触媒といいます．パイプの中に固体酸触媒を詰め，入口から反応物(例えばプロピレンとベンゼン)を入れると出口からクメンのみが出てきます．固体触媒を使うと，手品のような，完全に無害な工場が実現できます．クメンの製造は既に固体酸触媒(ゼオライト)を使って行われており，均一系触媒の固体触媒での置き換えはいろいろな分野で強く求められています．ノーベル賞で話題になったSuzuki-Miyaura反応では，均一系の有機Pd錯体が触媒として用いられています．この場合には有害な触媒成分が製品に残存するのが問題となっています．このような問題も固体触媒によって解決できます．