FORM 2 THE PATENTS ACT, 1970 (39 of 1970) As amended by the Patents (Amendment) Act, 2005 & The Patents Rules, 2003 As amended by the Patents (Amendment) Rules, 2005 COMPLETE SPECIFICATION (See section 10 and rule 13) TITLE OF THE INVENTION A process of recovery of Pt/C catalyst in the preparation of Azithromycin dihydrate INVENTORS Name: Acharya Rakesh Nationality: Indian National Address : Tambet Bhavan, Behind Vireshwar Temple, Mahad, Dist Raigad 402301, Maharashtra, India Name: Mhamunkar Deepak Bhaurao Nationality: Indian Address: Samarth Nagar, At/Post - Utekhol, Tal - Mangaon, Dist Raigad, Maharashtra, India Name: Jadhav Santosh Bhau Nationality: Indian National Address: C/o Arun Deshmukh, Opp Vireshwar Temple, Tal Mahad, Dist Raigad 402301, Maharashtra, India Names: Gupte Rajan Vitthal, Hire Chandrabhan Madhav and Sabade Kishore Balaji Nationality : all Indian Nationals Address : Kopran Research Laboratories Limited, D-28/2, TTC Industrial Area, MIDC, Turbhe, Navi Mumbai - 400705, Maharashtra, India APPLICANTS Name : KOPRAN RESEARCH LABORATORIES LTD Nationality: Indian Company Address : Parijat House, 1076 Dr E Moses Road, Worli, Mumbai 400018, Maharashtra, India PREAMBLE TO THE DESCRIPTION The following specification particularly descr1Bes the nature of this invention and the manner In which it is to be performed Field of Invention The invention relates to a process of recovery of Pt/C catalyst from the hydrogenation of 6,9-Imino ether of formula (II) Formula (II) in the preparation of Azithromycin dihydrate of formula (I) H3C Formula (I) Background of the invention Azithromycin is a semi-synthetic macrolide ant1Biotic chemically related to Erythromycin. Azithromycin is a broad-spectrum bactericide and effective against a wide variety of microorganisms, such as Hemophilus influenzae, Streptococcus pneumoniae, Mycoplasma pneumoniae, Staphylococcus aureus, and Mycobacterium avium, and many others. 2 The transformation of Erythromycin A into Azithromycin comprises the conversion of Erythromycin into its oxime; Beckmann's rearrangement of the Erythromycin oxime into 6,9-Imino ether; hydrogenation of the 6,9-Imino ether to 9-deoxo-9a-aza-9a-homoerythromycin A and further reductive methylation to obtain Azithromycin. The hydrogenation of 6,9-Imino ether to a secondary amine, 9-Deoxo-9a-aza-9a-homoerythromycin A, is carried out by using Sodium borohydride in Methanol which is disclosed in J. Chem. Soc. Perkin Trans. 1, 1986, 1881; J. Org. Chem. 1997, 62, 7479-7481; US 4,328,334; US 5,869,629; International publications WO 01/00640; WO 03/082889 and WO 03/102009; or by catalytic hydrogenation in the presence of platinum dioxide and acetic acid as solvent, which is disclosed in Tetrahedron lett. 1994, 35, 3025; International publications WO 94/26758 and WO 03/102009; or by catalytic hydrogenation in the presence of platinum over carbon (Pt /C) or rhodium over carbon (Rh /C) at 3-10 atm in solvent consisting of water-acetic acid-methanol mixture, which is disclosed in US 4,328,334; US 5,869,629; and EP 0,879,823. International publication WO 03/102009 descr1Bes hydrogenation of 6,9-Imino ether with platinum / carbon in water as solvent to which acid is added to adjust the pH till 4 followed by crystallization to obtain 9-Deoxo-9a-aza-9a-homoerythromycin A in crystalline form. Further EP 0,879,823 discloses preparation of Azithromycin from 6,9-Imino ether by carrying out the hydrogenation and reductive methylation sequentially with the noble catalyst and hydrogen in the presence of formaldehyde. In this patent, the preferred catalyst is 5 % rhodium over carbon but platinum, palladium or rhuthenium, can also be used. It is stated at page 3 line 20 of this patent that if so desired, the catalyst can be recycled and reused several times thus rendering the process more economic. It does not, however, give any procedure for recycling and reusing the catalyst. WO 96/02323 discloses a catalyst containing platinum metal in at least two bond energy states deposited on titanium or vanadium silicalite for liquid phase olefin hydro oxidation where the oxidation catalyst is regenerated by burning off carbon coatings in a controlled manner at temperature from 350° to 650° C, followed by reduction with hydrogen. WO 01/41926 descr1Bes the activation and regeneration of a hydro oxidation catalyst where the catalyst comprises of at least one metal selected from gold, silver, platinum group metal. The 3 activation or regeneration process involves contacting the catalyst with ozone. The regeneration stream contains ozone, oxygen, water, and helium; temperature for regeneration is 140° C and the processing time is 3 to 6 hours. Chemical abstract 1976, 85, 37659C (Akimoto, Yumi Kuroda; Yaso, Kobayashi; Mitsubishi metal corporation) discloses the regeneration of spent catalyst Pt, Pd or Pu supported by various carriers such as SiO2, Al2O3, C by treating the spent catalyst in chlorine atmosphere at 350° C to 750° C and subsequently treating in an inert gas containing 5%, heating at 150° C to 850° C. All the above descr1Bed activation and regeneration methods consume valuable time to heat up the catalyst to activation or regeneration temperature and cooling down to operating temperature. Further, high input of heat energy is required to effect the activation or regeneration process. Further, the reported activation and regeneration methods require treatment in which catalyst is heated in presence of ozone or chlorine atmosphere or inert gas, which needs special instrument. The reported activation or regeneration methods will be time consuming, need special instrument and expensive. Further the activation or regeneration at high temperature may deteriorate or degrade the properties of Pt metal as catalyst, thereby affecting the catalyst lifetime. Objects The main object of the invention is to provide a process of recovery of spent Pt/C catalyst by reactivation and recycling of the reactivated Pt/C catalyst in the hydrogenation of 6,9-Imino ether to 9-deoxo-9a-aza-9a-homoerythromycin A in the preparation of Azithromycin dihydrate. Another object of the invention is to provide the process of recovery of Pt/C catalyst where reactivation of spent catalyst is carried out at room temperature and atmospheric pressure, which minimizes deterioration of the Pt/C catalyst, thereby prolonging the catalyst's lifetime. Yet another object of the invention is to provide the process of recovery of spent Pt/C catalyst where reactivation of spent Pt/C catalyst can be performed in a short period of time. 4 Yet another object of the invention is to provide the reactivated Pt/C catalyst, which is used in hydrogenation of 6,9-Imino ether in the preparation of Azithromycin without affecting yield and purity. Yet another object of the invention is to provide the process for recycling of reactivated Pt/C where the recycling of reactivated catalyst can be done for atleast ten cycles of hydrogenation of 6,9-Imino ether. Yet another object of the invention is to provide the recovery process of the spent Pt/C catalyst which is a simple and economical. Yet another object of the invention is to provide the process for recycling of Pt/C where the reactivation of spent Pt/C catalyst is carried out without need of regeneration of spent Pt/C catalyst for at least ten cycles of hydrogenation of 6,9-Imino ether. Detailed description of invention The present invention discloses a process of recovery of Pt/C catalyst from the hydrogenation of the 6,9-Imino ether in preparation of Azithromycin dihydrate comprising isolating spent Pt/C catalyst from the hydrogenation of 6,9-Imino ether by filtration; reactivating the wet spent Pt/C catalyst by treating the same with strong acid till pH 3-6 while stirring; washing the treated catalyst with aqueous sodium carbonate solution followed by washing with hot water (55° C) till the pH of washing is neutral, to obtain reactivated catalyst; and recycling the reactivated catalyst along with fresh catalyst in the weight ratio of 98: 2 in the hydrogenation of 6,9-Imino ether. The strong acid is selected from the group consisting of perchloric acid; halogenated acid like hydrochloric acid, hydroiodic acid, or hydrobromic acid; percarboxylic acid like peracetic acid or perbenzoic acid; or nitric acid. The preferred strong acid is perchloric acid. Particularly, the perchloric acid used in reactivation of the spent Pt/C catalyst is 50 % perchloric acid. The aqueous sodium carbonate used in the reactivation of spent Pt/C catalyst is 10 % aqueous sodium carbonate. 5 The reactivation of catalyst (Pt/C) is carried out at room temperature, particularly 25° to 35° C and atmospheric pressure. In the present invention, the spent Pt/C catalyst is treated with acid like perchloric acid or any above mentioned acid or with any other suitable reagents to oxidize the impurities present on the catalyst; washed with aqueous sodium carbonate solution to remove the oxidized impurities which goes into the filtrate; further washed with water to remove sodium carbonate and thus obtained is the reactivated Pt/C catalyst with the free active sites, ready to use in new cycle of hydrogenation. In the present invention the 6,9-Imino ether of formula (II) ,^H3 u r 0H J-_ Formula II is hydrogenated by dissolving the 6,9-Imino ether in methanol, cooling the reaction mixture at temperature 5°-6° C; adjusting the pH of the reaction mixture to 5.5 with 70 % perchloric acid; hydrogenating the reaction mixture with 20 % Pt/C catalyst with respect to 6,9-Imino ether, at temperature 30°-50° C and pressure 10-14 kg/cm ; filtering the reaction mass to isolate wet spent catalyst; distilling out the methanol from the reaction mass to obtain residue, adding water to the residue, adjusting the pH of the reaction mass by adding 5 % aqueous sodium hydroxide solution; and filtering out the 9-deoxo-9a-aza-9a-homoerythromycin A, followed by washing 9-deoxo-9a-aza-9a-homoerythromycin A with water and drying at 65° C. The isolated wet spent catalyst is reactivated according to the invention and the reactivated Pt/C catalyst is recycled along with the fresh catalyst in the weight ratio of 98:2 in the hydrogenation of 6,9-Imino ether 6 The reactivated Pt/C catalyst is recycled in the hydrogenation of 6,9-Imino ether for atleast ten times with subsequent reactivation by the above mentioned reactivation process at the end of the each cycle of hydrogenation. The use of reactivated Pt/C catalyst or recyclization of the reactivated catalyst did not find any adverse affect on yield and purity of Azithromycin dihydrate. The spent Pt/C catalyst is given out for regeneration only after the recycling the reactivated Pt/C catalyst for at least ten cycle of the hydrogenation of 6,9-Imino ether. Further, 9-deoxo-9a-aza-9a-homoerythromycin A obtained from hydrogenation of 6,9-Imino ether is converted to Azithromycin dihydrate of formula (I) By a) methylating the 9-deoxo-9a-aza-9a-homoerythromycin A of formula (III) u ^ OH I Formula III 7 with formaldehyde and formic acid in presence of lower chain alcohol /acetone to obtain Azithromycin dihydrate of formula (I), HoCL 3L ••i.l .2H,0 Formula (I) b) isolating the Azithromycin dihydrate of formula (I) H3C,,-..l CH3 Formula (I) by separating acetone/ alcohol layer from the methylation of 9-deoxo-9a-aza-9a-homoerythromycin A mixture followed by adding water to the acetone / alcohol layer within 12 hour while stirring, further stirring the mixture at 20° C for 12 hours; filtering the Azithromycin dihydrate of formula (I) 8 CH3 H,C. °,H1. 3 ■'..l/^OH I CH3l, HJ 5