Sajo P. Naik
Naperville, Illinois, USA
spnaik@gmail.com
“The synthetic chemist rarely borrows accessories for the laboratory from the vast stockroom of nature. We resist
scooping some dust into a reaction flask. Yet elegant chemistry can be performed if clays are used as supports or
catalysts.”
-Pierre Laszlo
Over the years clay minerals have generated a lot of interest as catalysts in organic reactions. In fact, before the 1960s —and prior to the advent of synthetic zeolites— pure and modified clay minerals were the workhorses of petrochemical industry as catalysts in many organic reactions1. Nevertheless, even today clays are being still used in processing of heavier oil fraction and in certain alkylation reactions involving petrochemical feed stocks or raw materials. In fact, vegetable bleaching oil processing utilizes several hundreds of tons of natural clay globally as an adsorbent/catalyst to breakdown various pigments, odor causing organics and other undesirable organics from raw vegetable oil in a refining process called bleaching-deodorization of refined vegetable oil. Clarion, BASF, EP minerals, Tyco, Ashapura (India) are some of the clay companies who mine and process clays for bleaching, catalysis, adsorption, and many other applications.
Before we proceed an important distinction needs to be made between the terms ‘clays’ and ‘clay minerals’. Generally, clays are defined as a natural material with plastic properties and containing particles of very fine size, customarily those defined as particles smaller than two micrometers (7.9 × 10−5 inch). As such, technically, even non-clay minerals may be defined as clays if they exhibit above properties. On the other hand, clay minerals can be defined as hydrous and crystalline phyllosilicates or sheet-like materials containing a sprinkling of other cations in their structure. Clay minerals have distinct and repeating units of silica and alumina/magnesia. Because of isomorphous framework substitutions, negative charge is introduced in the clay structure which is often balanced by a cation that is present in the clay interlayers. This cation can be easily exchanged with other cations or even positively charged organic species to modify or functionalize the clay to suit the intended application. Therefore, from the catalysis standpoint, here I mean clay minerals when referred as catalysts.
Clays are abundant in nature and are often called the ‘dirt basket’ of nature as they assimilate other minerals, cations, and residual organics from the environment. Clays are endowed with adsorption and catalytic properties as they have a porous structure (except for 1:1 clays such as kaolinite), high surface area, presence of framework substituted cations, and high negative charge on the surface. These unique features lead to accumulation of large number of active sites (Lewis and Bronsted acid sites) distributed on the structure. Although hundreds of clay minerals belonging to several different clay group exist in nature, not all of them are porous or have appreciable catalytic activity. However, many clay minerals due to their unique structure and properties find niche applications in many industrial processes. For example, kaolinite in paper making, vermiculite in packaging, montmorillonite as a toxin binder in animal feed, etc.
As mentioned above, the catalytic activity of clays is dependent on its structure, and the number and type of available active sites. From a practical perspective, one cannot just pick some clay from the ground or from under the top soil and use it directly in a chemical or catalytic process. Usually, the freshly mined clay contains a lot of water (up to 30%), organics, grit and other impurities which must be separated through processing and purification stages.
Very often 1:2 type (structural feature) clays belonging to groups such as ‘smectite’ or ‘palygorskite’ are desired as catalysts as they have rather open structures and a much higher degree of porosity. In fact, the relationship between some clay structures and their activity has been quite well established. Seemingly, clays like montmorillonite (named after Montmorillon, a village in France where it was first discovered) which has a smectite structure are preferred for classical acid activation. Such acid modified clays have higher Bronsted acidity and are currently used as catalysts in many petrochemical reactions.
From a historical perspective—and a subject that could be rather controversial to some—it has been reported very extensively that clays appear to have played a role in catalyzing the starting of the life on planet earth.2 In fact, it is known that the montmorillonite clay catalyzes the polymerization of RNA from activated ribonucleotides. Some clays are known to accelerate the spontaneous conversion of fatty acid micelles into vesicles. Certain nano-or micron sized clay particles often get encapsulated in such vesicles, thus providing a pathway for the prebiotic encapsulation of catalytically active surfaces within membrane vesicles. In addition, RNA adsorbed on clay can also be encapsulated within vesicles. Once formed, such vesicles can grow by incorporating fatty acid supplied as micelles and can divide without dilution of their contents by extrusion through small pores thus propagating formation of molecules essential to the evolution of life. Clays minerals are known to exist for over millions of years on the surface of the earth, they have formed through chemical transformation and metamorphosis brought about by changing environmental conditions and weathering on the surface of the earth. In fact, NASA has discovered the presence of clays on the planet mars leading to speculations on the existence of some form of life on planet mars.4
Anyway, as mentioned previously, clays have interesting catalytic properties and following is a list of some organic reactions in which the catalytic activity of clay minerals is demonstrated.5-7
1. Fischer indole cyclisation
2. Friedel-Crafts acylation and alkylation
3. Friedlander synthesis
4. Heck vinylation
5. Hosomi-Sakurai (HS) reaction
6. Knoevenagel condensation
7. Markovnikov addition
8. Michael addition
9. Pechmann condensation
10. Pinacol coupling reaction
11. Beckmann rearrangement
12. Synthesis of heterocylic compounds
13. Ether formation
14. Acetalization
15. Synthesis of carboxylic acids
16. Anhydride formation
17. Synthesis of α, β-unsaturated aldehydes
18. Synthesis of polyaromatic nitro compounds
19. Synthesis of Mannich bases
20. Formation of dioxolane
21. Synthesis of (Thio)barbituric Acid
22. Synthesis of α-acetamido ketones
23. Porphyrin synthesis
24. Methylenedioxyprecocene
25. Synthesis of γ-Lactones via the Ene reaction
27. Synthesis of imines and enamines
28. Asymmetric synthesis of arylhydroxycyclic amines and silanols
29. Synthesis of bismaleimides and bisphthalimides
30. Synthesis of poly-(ε-caprolactone-co-tetrahydrofuran)
31. Synthesis of symmetric diimides and 3a, 4, 7, 7a-tetrahydroisoindole-1,3-dione
32. Formylation of phenols
33. Oxidation of sulfides to sulfoxides
34. Reductive alkylation of amines
35. Hydroamination of alkynes
36. Coupling of amines to imines
37. Methylthiolation of thiophenes
38. Conversion of aldehydes into the nitriles
39. Hydroformylation
40. Hydrogen peroxide oxygenation
41. Allylsilylation
42. Polymerization
43. Deoximation
44. Redox reaction
45. Pyrolytic elimination
Natural Na-bentonite (montmorillonite/smectite) is the most studied clay as a catalyst in various organic reactions. However, care must be exercised in selecting the appropriate clay as a catalyst in the desired reaction. Na-montmorillonite is a swelling clay in that it has a lot of affinity for water which hydrates sodium cation in the interlayer leading to an increase of the interlayer spacing. Alternatively, clays belonging to Palygroskite and Sepiolite families have interesting structures and higher porosities than montmorillonite and are known to be selective and active for catalytic disintegration and bleaching of natural pigments, etc. from raw vegetable oils. If higher Bronsted acidity is desired, then classically acid activated clay will be a much better choice. Such acid-activated clays displaying a range of acidity are commercially available.
Also, when deciding to use natural clay mineral in a reaction, one should also consider the purity of clay as very often natural clays contain other mineral impurities such as silica, quartz, iron oxide, etc. Beside the clay should be pretreated to charge the active sites and enhance their performance. As clays are layered mineral containing interstitial water molecules they have limited thermal stability and the pre-treatment temperature should be maintained below the interlayer collapse temperature.
To conclude, clay minerals due to their natural occurrence, catalytic nature and high porosity are great options for greener chemical transformations. Clays are ecofriendly in nature, non-toxic, non-corrosive, economical and recyclable, and thus can be efficiently used as heterogeneous catalysts in a variety of organic reactions. Furthermore, handling of clays is easy since the clay does not dissolve in the reaction medium or solvent and can be simply be filtered away after the completion of the reaction. In addition, clays have a lot of scope for further modifications through ion exchange, vapor deposition and chemical treatments. Various cations including organocations can be exchanged with the intrinsic cation present in the interlayer of clay, such rationally chosen cations can impart new functionalities to the clay structure.
References
1. S. Chitnis, Reactive & Functional Polymers 32, 1997, 93.
2. P. Lasjlo, Science, 25, 235, 1987, 1473.
3. J. Ballentine Clay Minerals, 18,1983, 347.
4. https://mars.nasa.gov/resources/21453/clays-of-ladon-basin/
5.N. Kaur et al. J. Chem. Pharm. Res., 4(2), 2012, 991.
6. D. Ortego et al. Chemosphere, 122, 8, 1991, 769.
7. G. Nagendrappa, Resonance 2002, 64-77.
