Chemingi­neer­ing

Chemical Industry Digest - - What’s In? -

Mechanochem­istry of­fers a green al­ter­na­tive to con­ven­tional chem­i­cal pro­cesses, and in fact this was one of the main trig­gers for its re­vival. The sec­ond, more ex­cit­ing, ben­e­fit of mechano-chem­istry is that it achieves re­ac­tions pre­vi­ously con­sid­ered im­pos­si­ble, and in the process yields unique mol­e­cules and com­pounds.

This month, our colum­nist takes a peep into the in­trigu­ing world of Mechanochem­istry, where chem­i­cal re­ac­tions are ini­ti­ated by me­chan­i­cal forces. Mechanochem­i­cal re­ac­tions are al­most sol­vent­free and yield novel ma­te­ri­als and mi­crostruc­tures. Their ex­act mech­a­nisms need to be thor­oughly un­der­stood be­fore they come into the main­stream in a big way.

To ini­ti­ate a chem­i­cal re­ac­tion, re­ac­tants usu­ally need heat; some­times light. But there are chem­i­cal re­ac­tions that can be trig­gered by me­chan­i­cal en­ergy. Wel­come to the fas­ci­nat­ing world of Mechanochem­istry. Mechanochem­istry is that branch of chem­istry which is con­cerned with chem­i­cal and physic­o­chem­i­cal changes of sub­stances due to the in­flu­ence of me­chan­i­cal en­ergy. Mechanochem­istry holds out much promise and has been gain­ing con­sid­er­able trac­tion in re­cent years.

His­tory

Greek philoso­pher Theo­pras­tus (circa 4th C.B.C.) crush­ing mercury sul­fide and vine­gar in a cop­per mor­tar and pes­tle to pro­duce el­e­men­tal mercury is prob­a­bly the first chron­i­cled ac­count of a mechanochem­i­cal re­ac­tion. How­ever, Carey Lea is widely re­garded as the Fa­ther of mechanochem­istry based on his work in the late 19th Cen­tury. Lea was the first to demon­strate that re­ac­tions ini­ti­ated by me­chan­i­cal en­ergy are dis­tinctly dif­fer­ent from those ini­ti­ated by heat. Lea’s work on the de­com­po­si­tion of sil­ver and mercury halides are fre­quently cited as his­toric mile­stones of mechanochem­istry. They are also the clear­est ex­am­ples of the unique­ness of mechanochem­i­cal re­ac­tions. In the 1960s, mechanochem­istry was widely used to make metal al­loys. Now there is a re­newed in­ter­est in mechanochem­istry as a sus­tain­able “green” route to syn­the­sis of chem­i­cal com­pounds.

Ad­van­tages

Mechanochem­istry has two no­table ad­van­tages. First one is en­vi­ron­men­tal. Un­like con­ven­tional chem­istry, mechanochem­i­cal re­ac­tions do not re­quire the re­ac­tants to be dis­solved in a sol­vent. Thus, mechanochem­istry of­fers a green al­ter­na­tive to con­ven­tional chem­i­cal pro­cesses, and in fact this was one of the main trig­gers for its re­vival.

The sec­ond, more ex­cit­ing ben­e­fit of mechanochem­istry is that it achieves re­ac­tions pre­vi­ously con­sid­ered im­pos­si­ble, and in the pro- cess, yields unique mol­e­cules and com­pounds. There are many re­ports of mechanochem­i­cal trans­for­ma­tions that ei­ther do not hap­pen or hap­pen only with great dif­fi­culty in con­ven­tional so­lu­tion-based chem­istry. Mechanochem­i­cal re­ac­tions pro­duce com­pounds and mi­crostruc­tures that are essen­tially dif­fer­ent from con­ven­tional chem­i­cal re­ac­tions. This has huge im­pli­ca­tions for phar­ma­ceu­ti­cal and so­lar en­ergy in­dus­tries.

Equip­ment

Forces used in mechano-chem­istry are com­pres­sion, shear and fric­tion. Mechanochem­i­cal re­ac­tions are car­ried out in ball mills. Shaker and Plan­e­tary mills are the most pre­ferred de­signs. In the shaker de­sign, the jars swing back and forth with a fre­quency that de­ter­mines the milling in­ten­sity. In the plan­e­tary de­sign, the jar ro­tates around a cen­tral axis while spinning around its own axis. Jars and

milling balls are made of stain

less steel, zir­co­nia, tung­sten

car­bide or even PTFE. Dif­fer­ent

types of me­chan­i­cal mo­tions have been found to yield dif­fer­ent types of prod­ucts with vary­ing ki­net­ics. En­ergy in­put is ad­justed by vary­ing milling

time and fre­quency and also

K Sa­has­rana­man In­de­pen­dent Con­sul­tant - Process Engi­neer­ing, En­ergy, Util­i­ties and Safety

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