INTRODUCTION:

Mechanochemistry¹ is the coupling of the mechanical and the chemical on a molecular scale and includes mechanical breakage, chemical behaviour of mechanically-stressed solids tribology, polymer degradation under shear, cavitation-related phenomena shockwave chemistry and physics, and even the burgeoning field of molecular machines. Mechanochemistry can be seen as an interface between chemistry and mechanical engineering. At the beginning of this century, W. Nernst classified the different fields of chemistry according to the type of energy supplied to the system: thermochemistry, electrochemistry, photochemistry, etc. The name mechanochemistry was applied to the field of reactions caused by mechanical energy. A narrower field tribochemistry was used for reactions generated by friction during the milling of solid reagents.

The term mechanochemistry is also sometimes used in molecular nanotechnology as a synonym for mechanosynthesis, the hypothetical process by which molecular assemblers would operate.

Also called "positional synthesis" or "positional assembly", it is a technique for forming chemical bonds by direct computer control of the position of molecules.^2-3

Mechanochemical phenomena have been utilized since time immemorial, for example in making fire. The oldest method of making fire is to rub pieces of wood against each other, creating friction and hence heat, allowing the wood to undergo combustion at a high temperature. Another method involves the use of flint and steel, during which a spark (a small particle of pyrophoric metal) spontaneously combusts in air, starting fire instantaneously.⁴

An interface is in continuous interaction with its environment. The interaction leads to continuous changes in local composition and local surface structure. By mechanical force action, the defect amount on the surface increases. Point defects are known to migrate within the lattice by changing place with other parts of rubbing surfaces. Because of the motion of the point defects, so called vacancy lattice is established. Energy is preferably stored by it.²

A variety of processes takes place on mechanical grinding of solids such as:⁵

(a) Conminution of the particles to a very small size.

(b) Generation of large new surfaces.

(d) Phase transformations in polymorphic materials.

(e) Chemical reactions: decomposition, ionic exchange, oxidation-reduction, complex and adduct formation, etc.

The occurrence of these reactions was attributed to the heat generated in the milling process, favored by the large area of contact between the solids. However, since the end of the last century, Carey Lee noticed that mechanochemical processes were different from thermal processes. For example, heating of AgCl and HgCl leads to melting and subliming of these solids, while milling them produces their decomposition into Cl₂ gas and metal. The role played by mechanical defects as high energy structures and its importance in chemical transformations was recognized later.^6-9

THEORY:^10-14

At present, mechanochemistry appears to be a science with a sound theoretical foundation which exhibits a wide range of potential application. Amongst the potential viable processes we can mention the modification of properties of building materials, the new method of fertilizer production, the activity enhancement and regeneration of catalysts, the new method of modification of solubility and bioavailability of drugs in pharmacy, the control of reaction kinetics in the chemical technology and last but not least the recent achievements in the synthesis of advanced materials. Its benefits include lower reaction temperatures, increased reaction rate, increased dissolution and the formation of water soluble compounds. As a consequence, further processing can be performed in simpler and less expensive reactors during shorter reaction times.^1-3 The key factor in mechanochemistry is the application of suitable mill which can work in different working regimes. In fact, there are various factors affecting the operation of milling process in Mechanochemistry and different types of mills are used (Fig. 1).

Fig. 1. Types of mills applied on mechanochemistry; planetary mill and rolling mill.

MECHANOCHEMICAL REACTORS:

Milling can be carried out in a variety of ways. The simplest is the laboratory mortar and pestle. [15] This hand milling processes can provoke a large number of mechanochemical reactions which do not require surmounting a high energy barrier. Ball mills are used when higher energy is required and when the milling time involves hours or even days. Laboratory vibrators of the Wiggle-Bug type are very efficient in milling small samples. Very high energy vibrators such as high speed attritors or stainless steel ball mills of high impact are used for prolonged high energy milling as in mechanical alloying or amorphization of hardcrystalline solids Ultrasonic can also be used mechanochemically.

MECHANOCHEMICAL PROCESSES:^16-19

The best way to study mechanochemical transformations is to analyze in-situ the milled mixture using appropriate spectroscopic methods, since chemical handling can obscure the true nature of the initial products. Most common tools are IR and XRD techniques which normally allow the identification of the products. In organic reactions, solid state NMR can be very useful and with Fe and Sn compounds, Mossbauer spectroscopy is most valuable. Other techniques like HREM, EXFAX and X-ray cyclotron resonance have been used to study the new surfaces. In sophisticated experiments like reactions produced during crack formation, MS enables the determination of the gaseous products as in the decomposition of nitrates and bromates.

PHASE TRANSITIONS: Polymorphic materials have two or more phases which are stable in certain temperature and pressure range. Examples are collected. These phase changes are the simplest transformations that can be studied tribochemically. They have importance in geochemical studies as they give information on the geological processes of rocks. They also allow the estimation of the pressures obtained during a milling process, which is very difficult to assess by other means. Due to the observed phase transitions, the pressure range reached by hand milling in a mortar can be estimated to be as high as 15 000 bars. It is possible to transform the stable low pressure form to the high pressure one by milling, if sufficient mechanical energy is available to overcome the activation barrier. Milling the high pressure form also leads to the appearance of the low pressure one, since milling provides a whole spectrum of pressures.

MECHANOCHEMICAL REACTIONS IN KBR DISKS: IR spectra of solids are normally run, in pressed KBr disks. Also used are KCl, CsBr and CsI salts as matrices. During the grinding and pressing processes, the analyte can undergo mechanochemical changes such as polymorphic transitions, ion exchange processes and complex formation and oxidation-reduction reactions. The chemists are not always aware of these transformations and many pitfalls have been reported in the literature concerning IR spectra of solids. We will now illustrate several of these reactions.

FORMATION OF SOLID SOLUTIONS: When the analyte and KBr have similar structures, the milling can lead to the formation of solid solutions. The presence of moisture in the KBr or its absorption from the atmosphere during milling and pressing increases the diffusion of ions and facilitates the process. Other alkali halide matrices present the same phenomena.

IONIC EXCHANGE: Exchange of anions and cations with KBr leading to formation of new products can take place in the KBr disk when the DF of the reaction is negative as discussed by Milne. A simple case is the formation of insoluble bromides as occur with Ag, Hg and Pb salts.

COMPLEX FORMATION: KBr can form complexes with organic substances such as adducts with sugars, thiourea Succinimide and pyridine oxide. However, the nature of many of these complexes is not well established. With SnBr₄, KBr forms the salt K₂SnBr₆ .We are at present studying the reactions of alkali halides with sugars, the most promising salt being KF which forms crystalline adducts with mono and disaccharides, whose nature is now under study.

OXIDATION-REDUCTION REACTIONS: Since the Br anion is a reducing agent, many analytes which are oxidants can be reduced during milling to obtain the pressed KBr disk. For example, all ferricyanides are reduced to ferrocyanides. The velocity of the reaction depends strongly on the outer cation of the ferricyamide, for example. K<<Zn<Mn, Co, Ni<Cu. This is related to the path of the reducing electron from the Br to the central Fe cation: formation of solid solutions in alkali halide matrices Salt Matrix LiCl NaCl, CsCl NaBr, CsI NaBr, NaBr CsBr, NaCl CsCl. When the Br^- is bound to the outer transition metal cation, the bridge is established for electron transfer. With its possibility of taking the electron to form Cu⁺¹ also speeds the process which ends with the electronic transmition through the CN bond to Fe. An interesting case occurs with Co⁺² ferricyanide. The reduction to ferrocyanide goes through a stable intermediate, a mixed valence ferri-ferrocyanide complex.

ACID-BASE REACTIONS: The reactions of organic acids with amines in the solid state have been studied in our laboratory. Strong acids and bases react rapidly on milling to form ammonium salt Weak acids and bases can form stable hydrogen bonded complexes some of which are not stable in water solutions. We could not obtain amides even under the strongest mechanochemical conditions. An interesting acid-base reaction takes place when acidic substances are milled with KF. This salt tends to form the acid fluoride anion HF₂^- and abstracts protons from salts such as KHSO₄, ammonium salts and organic acids.

AMORPHIZATION OF POLYMERS: Macromolecules, natural or synthetic can exhibit crystalline regions due to association of the chains in short range order as in cellulose. This secondary structure is very important in the biological properties of the polymers. Milling in a mortar can disrupt the hydrogen bond interactions and amorphizise the polymer. This happens with starch and polypeptides. The milling of enzymes destroys their biological properties. Strong milling leas to the accumulation of mechanical energy in the long chains and strong bonds such as C–C, C–O and C–N can be ruptured. The phenomena can be detected by ESR due to the formation of free radicals. S–S bonds in proteins are easily broken. In synthetic polymers, milling can be used to create these broken bonds and forms the product with other chemicals for grafting and modifying their physical properties.

INDUSTRIAL APPLICATIONS:^20-27

Mechanosynthesis, Organicmechano synthesis is in its infancy but the possibility of carrying out reactions in the absence of solvent is a very attractive propositions. The processes are not only simpler and economical but also ecologically advantageous, eliminating the contamination of the surroundings with obnoxious chemicals. The implanting of pharmaceuticals on polymeric matrices for its transport in the organism is also a promising pharmaceutical field. In the inorganic field there are experiences in the syntheses of alloys, cermets, spinels, semiconductors and superconductors, catalysts, fertilizer, ceramics construction materials, etc.

WASTE MANAGEMENT²¹: Mechanochemistry is defined to describe the chemical and physicochemical transformation of substances during the aggregation caused by the mechanical energy. Mechanochemical technology has several advantages, such as simple process, ecological safety and the possibility of obtaining a product in the metastable state. It potentially has a prospective application in pollution remediation and waste management. The modification of fly ash and asbestos containing wastes (ACWs) can be achieved by mechanochemical technology. Waste metal oxides can be transformed into easily recyclable sulfide by mechanochemical sulfidization. Besides, the waste plastics and rubbers, which are usually very difficult to be recycled, can also be recycled by mechanochemical technology.

COLD ALLOYING: The milling at room temperature of two metals can lead to the production of alloys. The milling distorts the metallic particles to laminae and on further milling, diffusion leads to a homogenous alloy. Cold alloying is a rapidly expanding field

CERMETS: Milling a soft metal with a hard ceramic material leads to the formation of valuable composites with new important properties: the cermets. On milling, the hard ceramic particle becomes imbedded in a metal host matrix. On further milling, the hard particles are micronized and the metal covers the fine ceramic units leading to spherical particles, the cermets.

MECHANICAL ACTIVATION OF MINERALS: Milling certain minerals alters their properties in ways that can be used in industry: Silica, alumina and alumino-silicates can be activated to be valuable catalyst, with the incorporation of small amount of metals. Bauxite can be activated by milling, allowing its extraction of Al(OH)3 by Conc. NaOH solutions at lower temperatures. This allows a purer Al(OH)3 be obtained for Al production. Phosphate rock, the main source of P in the fertilizer industry, can be amorphized under high mechanical impact, leading to a material than can be attacked by the plant roots. This new fertilizer, tribophos, is cheaper than those obtained by treating the mineral with sulfuric or phosphoric acid. If instead of using sand and gravel in the concrete industry, the freshly ground rock is mixed with the cement and water, a very hard concrete results, superconcrete, with the cement phase strongly bound to the hard rock and sand phase.

MECHANOCHEMISTRY WITHOUT MACHINERY:

A more familiar form of mechanochemistry works by pounding materials in a ball mill. The substances to be reacted (relatively soft solids) are placed in a chamber together with hard balls (e.g., steel) which are then vibrated or tumbled. Flakes of solid that are unfortunate enough to be piled on a spot where two balls collide are smashed together enormous pressure, binding, flattening, and fragmenting them. The resulting laminated flakes undergo this again and again. To understand the general nature of the process, imagine stacking, flattening, breaking apart, and re-stacking what are initially two pieces of different material: Each cycle doubles the number of layers, and a few dozen cycles will thin the layers to less than an atomic diameter -which means that there are, in fact, no longer any layers, because the materials have been mixed at the molecular level. Ball-mill processing has long been used to make alloy powders of incompatible metals, and to mix compounds without a solvent. Mixing, impact heating, and mechanical force itself can activate or drive reactions. This sort of mechanochemical process can be used to perform mechanosynthesis.¹⁶

MECHANOCHEMISTRY WITH FORCEFUL MACHINES:

Querying Google Scholar about “mechanochemistry” returns results dominated by papers discussing mechanical forces in biological molecular machines. An outstanding example is ATP synthase, which consists of two coupled motors, one driven by the breakdown of ATP, the other driven by the flow of protons across a membrane. The motors are coupled in a way that always forces one of them to run in reverse, and as the name suggests, the normal mode of operation uses the proton-driven machine to drive the ATP-driven machine in reverse. This results in the synthesis of ATP, and thereby enables you to move, think, and so on, from moment to moment. (For more on this remarkable device, see Reverse engineering a protein: the mechanochemistry of ATP syntheses.²⁶

MECHANOCHEMISTRY WITH (SOMETIMES) FORCE-FREE MACHINES:^28-29

The above forms of mechanochemistry and mechanosynthesis all employ force, while only the molecular biological instances employ both force and machines. The forms of mechanosynthesis that I discussed in the preceding post are closer to the biological examples, in that they employ machines, but the crucial aspect in this instance is the use of machines and mechanical constraints to guide reactive encounters on a molecular scale. As we’ve seen this does not necessarily require the use of mechanical force (or motive power of any sort), although guidance and applied force can be a potent combination.

Thus, the terms “mechanochemistry” and “mechanosynthesis” embrace processes that involve enormous force, but no machines; others that involve machines, but no force; and yet others that involve both. The entire spectrum offers lessons for the future of nanomechanical mechanochemistry. The first, perhaps, is the unity of chemistry and mechanics, and the danger of trying to divide that unity with sharp definitions where there are no sharp lines to be drawn.

MECHANOCHEMICAL SYNTHESIS:

Mechanochemical processing is a powder metallurgy process, in which the application of mechanical energy induces chemical reactions and phase transformations. Mechanical alloying is a powder-processing technique involving deformation, cold welding, fracturing, and rewelding of powder particles in a ball mill.^30-31 Apart from the basic ideas, these authors also present actualindustrial application of the two processes, such asoxide-dispersion strengthened (ODS) alloys and physicalvapor deposition (PVD) targets. A substance class whose mechanochemistry is particularly well-understood are spinel ferrites, MeFe, where Me represents a divalent transitionmetal cation.^32-33 High-energy millingin a stainless-steel vial reduces the average crystallitesize of MgFe to the nanometer range.Prolonged mechanical milling leads to chemicalreduction and formation of a solid solution of FeO and MgO, with metallic iron as a byproduct, as evidenced both by Mo¨ssbauer spectroscopy and X-ray powder diffraction. Mechanochemistry is also frequently investigated in the area of hydrogen chemistry and hydrogen storage in metal-containing solids.^34-35 Ball milling for the synthesis of doped sodium alanate was introduced and is now widely used. However, ball milling in turn may lead to catalytic decomposition of NaAlH4. Only prolonged milling leads to H₂ elimination from LiAlH4. They attribute this to the catalytic effect of iron, which is introduced as a contaminant during the mechanical treatment. H₂ forms through the reaction between surface water molecules and mechanoradicals created by the rupture of Si-O or Al-O-Si bonds. The H₂ concentration increases as long as the grinding continues, which suggests that mechanoradicals are also continuously formed. Even more intriguing is the formation of methane and ethane in the mechanical treatment of NiZrHx, ZrHx, NiZr, Zr, Ni, or Zr + Ni in the presence of CO + H₂, CO, or graphite. In the mixture of hydrides with carbon, 100% of the hydride may be converted into methane. The opposite behavior is observed upon ball milling of the aromatic hydrocarbons biphenyl, naphthalene, anthracene, and phenanthrene, which are converted to graphite. Mechanochemistry is also potentially important in atmospheric chemistry, i.e., for the concentration of trace gases in the atmosphere.³⁶ Upon grinding in a ring-roller mill, calcium carbonate in the form of calcite loses crystallinity and an abundant release of CO₂ is observed. The authors suggest that this mechanochemical route could play an important role in the natural release of CO₂ into the earth’s atmosphere.³⁷ Reactions within or between molecular crystals, which are activated by mechanical methods. We can conclude that solvent-free mechanical methods, such as cogrinding, milling, and kneading, represent promising routes for the preparation of novel molecular and supramolecular solids.³⁸ Hybrid organic organometallic materials are obtained by manual grinding of an organometallic complex with a number of solid bases in a solvent-free reaction, which involves molecular diffusion through the lattice, breaking and reassembling of hydrogen-bonded networks, as well as proton transfer. Mechanochemical synthesis of tris(pyrazolylborate) complexes of Mn, Co, and Ni. Interestingly, and also the formation of a substituted pyrazole ligand, which results from hydrolysis of the corresponding tris(pyrazolylborate). Hydrolysis seems to be a frequent reaction in mechanochemistry. Single-walled carbon nanotubes and cyclodextrins mixed by high-speed vibration milling are watersoluble, which was attributed to the formation of nanotube-cyclodextrin complexes and the debundling of the nanotubes. Strained single-walled carbon nanotubes experience oxidative acid attack, and leading to the etching of the kinked sites of the nanotube.³⁹ This may be an example of the theoretically proposed effect that mechanical strain increases the proton affinity of binding sites. The strength and breaking mechanisms under high tensile load of films and ropes of single-wall carbon nanotubes, as well as multiwalled carbon nanotubes, have recently been investigated. Coal can be activated by milling or grinding, where mechanical rupture causes the formation of radicals, which react with oxygen or water to basic or acidic groups.⁴⁰ The process promises to yield activated coal powders at a competitive price and higher quality than thermal activation procedures.

Following examples considered deal with utilization of mechanical energy as a driving force for chemical reactions. Here, the generation of local high-pressure spots is presumed to activate local reaction sites. Furthermore, the absence of any solvent molecule brings the reacting species into the closest contact without any solvation. Such a reaction system causes a novel chemical reaction to occur. Of course, the main targets of our attention were the reactions leading to new desired products. Continuing research to establish the suitability of mechanochemical synthesis to different reaction types may lead to the development of novel green chemistry. Of course, solvent-free mechanochemistry is an energy-intensive technique compared with conventional solvent-based chemical synthesis. However, the energy needed to produce, deliver, collect, and dispose of the solvents and restore the environment is considerably higher.⁴¹ Therefore, the advantages of the mechanochemical approach are noteworthy.

Olefines can be mechanochemically oxidized to carbonic acids with potassium permanganate in a solvent-free environment. The presence of water was found to enhance the product yield. Again, the influence of water indicates that hydrolysis plays a role in this process. Grinding of crystalline organic acids and amines leads to proton transfer with ammonium salt formation or to hydrogen-bonded complexes. Mechanochemical reactions may also be induced by milling and pressing of analytes with KBr to form a disk for IR spectroscopy, resulting in a change of the spectrum of the analyte. This effect has been known for long, and the literature was recently reviewed by Fernandez-Bertran. Research in this area is ongoing.⁴²

Room temperature grinding of a mixture of calcium hydroxide and silicagel with different water contents was conducted in a planetary ball mill. Afwillite was synthesized mechanochemically by controlling the water weight rate at about 0.23-0.30. Two-hours grinding of the mixture containing with a water weight rate of more than 0.38 enabled the synthesis of other different types of calcium silicate hydrates. On the other hand, tobermorite was synthesized by three-hours grinding of the mixture having a Ca/Si molar ratio of about one and a water rate at about 0.8, while calcium silicate hydrates-(B) were mechanochemically synthesized by grinding during about 1.5 h. The water amount in the mixture plays a significant role to achieve the mechanochemical synthesis of these compounds.⁴³ Synthesis of schiff base of p-toludine and vaniline was carried using mortar-pastel is a recent example of mechanichemical synthesis.¹⁵

DISCUSSION:

Mechanochemistry is a branch of solid state chemistry which inquires into processes which proceed in solids due to the application of mechanical energy. At present, mechanochemistry appears to be a science with a sound theoretical foundation which exhibits a wide range of potential application. Amongst the potential viable processes we can mention the modification of properties of building materials, the new method of fertilizer production, the activity enhancement and regeneration of catalysts, the new method of modification of solubility and bioavailability of drugs in pharmacy, the control of reaction kinetics in the chemical technology and last but not least the recent achievements in the synthesis of advanced materials. Almost all the information is discussed in introduction along with its applications and uses. Therefore the information of this technique is must for researchers in Pharmaceutical synthesis field to potentiate their research work.

CONCLUSION:

The name mechanochemistry was applied to the field of reactions caused by mechanical energy. There are several applications of it among them, Mechanical alloying is a powder-processing technique involving deformation, cold welding, fracturing, and rewelding of powder particles. Hydrolysis seems to be a frequent reaction in mechanochemistry.

Several organic and inorganic compounds are reported previously, which are synthesized using mechanochemistry. By virtue of its pharmaceutical compounds can also be synthesized. It may be the new tool in green chemistry for synthesis of drugs and their intermediates. It is pollution free, rate of reaction may be less than that of traditional route of synthesis and it is inexpensive.

Therefore mechanochemistry may be the new tool for young researchers who worked on synthesis techniques.

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Received on 21.03.2011 Modified on 02.04.2011

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