Cur­rent Sta­tus

Chemical Industry Digest - - Chemingineering - Read­ers’ re­sponses may be sent to k.sa­has­rana­[email protected] or che­mindi­[email protected]

through choice of the milling me­dia. Milling balls made from denser ma­te­ri­als im­part greater ki­netic en­ergy dur­ing the milling process. An al­ter­na­tive ap­proach to con­trol­ling the mechanochem­i­cal re­ac­tions is through the use of small con­trolled amounts of liq­uid or solid ad­di­tives. Called Liq­uid As­sisted Grind­ing (LAG), this of­fers ad­van­tages like shorter re­ac­tion time and bet­ter prod­uct se­lec­tiv­ity.

Mechanochem­i­cal re­ac­tions are not limited to solids. Vig­or­ous mix­ing or son­i­ca­tion of liq­uids can re­sult in molec­u­lar forces that strain and break the bonds.

Ap­pli­ca­tions

Mechanochem­istry is no longer a lab­o­ra­tory cu­rios­ity. It is used to pro­duce use­ful prod­ucts with unique prop­er­ties.

Pho­to­voltaic cells us­ing Perovskites pro­duced by mechanochem­i­cal re­ac­tion have shown sig­nif­i­cant im­prove­ments in ef­fi­ciency. Named in hon­our of Leo Perovski, the Rus­sian ge­ol­o­gist, Perovskites are a large group of ma­te­ri­als like Cal­cium Ti­tanate, find­ing ap­pli­ca­tion in pho­to­voltaic cells. Sol­vent-based process for pro­duc­tion of Perovskites leave be­hind residues af­fect­ing its crys­talline struc­ture. Mechanochem­i­cally syn­the­sised Perovskites elim­i­nate th­ese struc­tural de­fects lead­ing to the higher ef­fi­ciency of pho­to­voltaic cells.

Pro­duc­tion of noble metal salts, of­ten used as cat­a­lysts and in elec­tron­ics, re­quire ag­gres­sive con­di­tions. Their chlo­ride salts, for ex­am­ple, are made by re­act­ing the met­als with aqua re­gia. Mechanochem­istry of­fers a con­ve­nient and be­nign al­ter­na­tive. Vig­or­ous shak­ing of the noble metal pow­ders in a zir­co­nia jar with sim­ple halides like potas­sium chlo- ride or am­mo­nium chlo­ride re­sults in the pro­duc­tion of noble metal halides. By adding lig­ands to the mill, var­i­ous gold and pal­la­dium com­plexes can also be pro­duced.

Mechanochem­istry holds out good promise in waste man­age­ment. Dif­fi­cult to re­cy­cle wastes like plas­tics and rub­ber have been suc­cess­fully treated by mechanochem­i­cal pro­cesses. Lead from spent Cath­ode Ray Tubes has been re­cov­ered by co-grind­ing the glass with el­e­men­tal sul­phur to yield lead sul­phide. Sim­i­larly, mechanochem­i­cal meth­ods have been suc­cess­ful in im­mo­bil­is­ing heavy met­als from fly ash.

Mechanochem­istry is used for syn­the­sis of phar­ma­ceu­ti­cal cocrys­tals, which are emerg­ing as an al­ter­na­tive solid drug form with tai­lored physic­o­chem­i­cal prop­er­ties. Some cocrys­tals, in fact, are pos­si­ble only through the mechanochem­i­cal route and not through the sol­vent process.

Sol­vent-free Chem­istry

Many con­ven­tional chem­i­cal re­ac­tions are car­ried out in sol­vents, which are of­ten haz­ardous or toxic. But sol­vents can some­times sup­press or slow down the re­ac­tions. Also, sol­vent-based chem­istry pre­sup­poses good sol­u­bil­ity of the re­ac­tants, which can some­times act as a de­ter­rent in the choice of raw ma­te­ri­als. A case in point is the syn­the­sis of nanographenes. As­sem­bling large or­ganic aro­matic struc­tures re­quire ei­ther harsh con­di­tions or mod­i­fi­ca­tion of the start­ing ma­te­ri­als to make them more sol­u­ble in sol­vents. Mechanochem­istry of­fers a sol­vent-free re­ac­tion en­vi­ron­ment which makes hith­erto chal­leng­ing re­ac­tions sim­pler and more ac­ces­si­ble. Sci­en­tists in Germany have re­cently syn­the­sized nanographenes and large poly­cyclic ar- omatic hy­dro­car­bons in ball mills. Mechanochem­istry thus over­comes the hin­drance of sol­vents in chem­i­cal syn­the­sis.

En­ergy Ef­fi­ciency

Mechanochem­i­cal re­ac­tions are also en­ergy ef­fi­cient. Ball mills are en­ergy in­ten­sive when used for com­minu­tion be­cause the high lat­tice en­er­gies have to be over­come. But mechanochem­i­cal re­ac­tion do not re­quire par­ti­cle size re­duc­tion to nanome­ter scale as the re­ac­tion de­pends on par­ti­cle mix­ing and sur­face ac­ti­va­tion. En­ergy con­sid­er­a­tions of mechanochem­i­cal pro­cesses are yet to be fully un­der­stood. The cur­rent un­der­stand­ing is that mechanochem­i­cal re­ac­tions con­sume less en­ergy than equiv­a­lent sol­vent chem­istry.

Mechanochem­istry is rapidly emerg­ing not only as a cleaner al­ter­na­tive to con­ven­tional chem­i­cal trans­for­ma­tions, but also a novel tool to make unique mol­e­cules and ma­te­ri­als. The main draw­back of mechanochem­istry is that we know very lit­tle about it cur­rently. Even as the li­brary of mechanochem­i­cal re­ac­tions is rapidly ex­pand­ing, the knowl­edge base of this chem­istry is still in its in­fancy. The atomic and molec­u­lar-level mech­a­nisms un­der­ly­ing mechanochem­i­cal re­ac­tions are yet to be fully un­der­stood. In com­par­i­son, con­ven­tional chem­istry has a head start of 200 years. The qual­i­ta­tive ben­e­fits of Mechanochem­istry have been fully demon­strated. Sys­tem­atic stud­ies and pre­cise the­o­ret­i­cal mod­els are re­quired to ad­vance this sci­ence to the next level.

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