INTRODUCTION:

Alkyl and aromatic mercaptans present in the petroleum products are highly undesirable due to their foul odor and highly corrosive nature. There are many commercially viable processes known for the removal of mercaptans from petroleum products. The most common practice is to oxidize the mercaptans present to disulphides with air in the presence of a catalyst.

Merox process is one such catalytic process used worldwide for the chemical treatment of petroleum fractions for the removal of sulphur present in the form of mercaptans [Merox Extraction].

In the Merox process, mercaptans are oxidized to disulfides in alkaline medium in the presence of a metal phthalocyaninc catalyst¹. The metal may be either cobalt², iron³, manganese^4,5, molybdenum⁶, and vanadium^7,6. Phthalocyanines of cobalt and vanadium are the preferred catalyst⁸.

As the metal phthalocyanines are not very soluble in aqueous medium, for improved catalytic activity there sulphonated ,carboxylated and amide derivatives may be also used in the Merox process. It has been found that the amide derivatives of cobalt phthalocyanines have a greater solubility of organic compounds in aqueous solution⁹. This may achieved by either sulphonation or carboxylation of phthalocyanines. The degree of sulphonation or carboxylation may be 1,2 or 4.

Cobalt phthalocyanine monosulfonate is used as the principle component in Merox sweetening catalyst and there are a large number of examples indicating its significance^10,11. The monosulfonated derivatives are preferred because the higher sulphonated derivatives are more soluble in water, which causes their leaching away from the catalyst bed. However, the use of cobalt phthalocyanine disulphonate¹², tetrasulphonate¹³, and a mixture of mono and disulphonate¹⁴ has also been reported.

In our present research work we have developed carboxylamide derivatives of cobalt phthalocyanine which could be employed for extractive sweetening of LPG. The catalysts synthesized are given in Table 1.

Further we have described the characterization and studied the catalytic activity of the synthesized compounds.

Table1: List of the compounds synt hesized

CoPc	Cobalt phthalocyanine
CoPc(CONH₂)₄	Cobalt phthalocyanine tetracarboxylamide
CoPc(CONH₂)₂	Cobalt phthalocyanine dicarboxylamide

EXPERIMENTAL METHOD:

All the chemicals used were of LR grade and were obtained from LOBA Cheme, Laboratory Reagents, & fine chemicals, Mumbai. Infrared (IR) spectra of the samples synthesized were recorded on Perkin Elmer 1760X FTIR spectrophotometer, in KBr pellets qualitatively.

Synthesis of cobalt phthalocyanine

Cobalt phthalocyanine was prepared the Urea Process. Standard conditions of this process involve heating a mixture of phthalic anhydride (4moles), urea (16moles), metal salt (1mole), and a catalytic amount of ammonium molybdate in a high boiling solvent such as kerosene, nitrobenzene, or trichlorobenzene.

The reaction occurs as given below.

C₆H₄-(CO)₂O + CO(NH₂)₂+ MX₂ ¾^CATALYST¾® MPc

Here, Pc represents the phthalocyanine anion (C₃₂H₁₆N₃^2-) , M²⁺represents the metal ion and MX₂ represents the metal salt.

Into a 500 ml three neck flask made of glass equipped with necessary equipment such as a stirrer, reflux condenser, and the like were charged 17.64 g of phthalic anhydride, 18 g urea, 1 g of ammonium molybdate, 8.9 g of cobalt chloride tetra hydrate and 300 ml of trichlorobenzene. The resulting mixture was stirred for 1 hour at temperature of 130°C, and subsequently stirred for 6 hrs at temperature of 200°C. The mixture was cooled to about 100°C, 400 ml of hot water of about 80°C was then gradually added to the reaction mixture, and further stirred for 2 to 3 hrs with refluxing, after filtration at high temperature, the mixture was washed with 3L of hot water.

Fig. 1: Chemical Structure of CoPc

Synthesis of cobalt phthalocyanine-tetra-carboxamide

A mixture of trimellitic anhydride (22.39 g, 116.6 mmol), Urea (120 g, 2.0 mol), cobalt chloride (8.9 g, 37.6 mmol), ammonium molybdate (1.0 g) was heated to 120°C over 3 hours at which time a green color appeared, the temperature was increased to 170°C over next 6 hrs. Heating was continued for a total of 20 hrs to a final temperature of 200°C. The product was scratched after cooling to normal temperature and then crushed the softened product to powder and powdered cake was stirred in 500 ml of 10% aqueous hydrochloric acid overnight, and then filtered with water washing to a damp cake. The cake was stirred in 1000 ml DMF at 100°C for two days, suction filtered with DMF and acetone washing, and dried under vacuum at 125°C to constant mass.

Fig. 2: Chemical Structure of CoPc(CONH₂)₄

Synthesis of cobalt phthalocyanine-di-carboxamide

A mixture of trimellitic anhydride (11.19 g, 58.3 mmol), phthalic anhydride (8.64 g, 58.3 mmol), Urea (120 g, 2.0 mol), cobalt chloride (8.9 g, 37.6 mmol), ammonium molybdate (1.0 g) was heated to 120°C over 3 hrs at which time a green color appeared, the temperature was increased to 170°C over next 6 hrs. Heating was continued for a total of 20 hrs to a final temperature of 200°C. The product was scratched after cooling to normal temperature and then crushed the softened product to powder and powdered cake was stirred in 500 ml of 10% aqueous hydrochloric acid overnight, and then filtered with water washing to a damp cake. The cake was stirred in 1000 ml DMF at 100°C for two days, suction filtered with DMF and acetone washing, and dried under vacuum at 125°C to constant mass.

Fig. 3: Chemical Structure of CoPc(CONH₂)₂

RESULTS AND DISCUSSION:

Sweetening of petroleum products involves extraction of the mercaptan present in it by aqueous alkaline solution followed by their oxidation to disulphide with air in the presence of metal chelate like cobalt phthalocyanine (Co²⁺) as catalyst. The reaction mechanism as proposed by Wallace et al.¹⁵ is shown below.

RSH +OH^- ® RS^- + H₂O

2Co²⁺ + O₂® 2Co³⁺ + O₂^2-

2RS^-+ Co³⁺ ® Co²⁺+ RS^*

2RS^* ® RSSR

O₂^2- + H₂O ® 2OH^- + ¹/₂O₂

The overall reaction:

2RS + ¹/₂O₂ ® RSSR + H₂O

In commercial sweetening process the catalyst is injected to 12- 14% aqueous sodium hydroxide solution, which is kept circulating in the system consisting of extractor, oxidizer and disulphide separator.

As the metal phthalocyanines are normally insoluble in aqueous alkaline solution, their carboxylated or sulphonated derivatives are used as catalysts. For sweetening of LPG, whereby the presence of carboxylamide group not only makes cobalt phthalocyanine more soluble in aqueous sodium hydroxide but also increases its catalytic activity probably due to change of redox potential of cobalt

In the synthesis of CoPc 11.3 g of an intended compound was obtained at a yield of 52.9 %. The result of the elemental analysis is C = 67.21, H = 2.80, N = 19.51, Co = 10.05 (Calcd. for C₃₂H₁₆N₈Co, C = 67.25, H = 2.80, N = 19.61, Co = 10.33); Mass (FD), Mol. Ion peak at 571; UV.

In the synthesis of CoP(CONH₂)₄a dark greenish solid was obtained andthe yield was 21.5 g (77 %). The result of the elemental analysis shows C = 58.32, H = 2.85, N = 22.25, Co = 7.58 (Calcd. for C₃₆H₂₀N₁₂O₄Co, C = 58.14, H = 2.69, N = 22.61, Co = 7.94); Mass (FD), Mol. Ion peak at 743; IR, 3339.3 cm^-1 (N-H str), 1577.2 cm^-1 (N-H def), 1656 cm^-1 (>C=O str).

In the synthesis of CoPc(CONH₂)₂ a dark greenish solid was obtained at a yield of 13.7 g (71.5 %). The result of the elemental analysis shows C = 58.32, H = 2.85, N = 22.25, Co = 8.98 (Calcd. for C₃₄H₁₈N₁₀O₂Co, C = 62.22, H = 2.74, N = 21.30, Co = 7.94); Mass (FD), Mol. Ion peak at 657.3; IR, 3340.1 cm^-1 (N-H str), 1579.3 cm^-1 (N-H def), 1657.1 cm^-1 (>C=O str).

CONCLUSION:

It was found that the catalyst, cobalt phthalocyanine tetra-carboxylamide and cobalt phthalocyanine di-carboxylamide prepared by carboxylation of cobalt phthalocyanine shows greater solubility of organic compounds in aqueous alkaline solutions hence could be further studied for its commercial utility.

REFERENCE:

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Received on 19.01.2010 Accepted on 11.02.2010

Research J. Engineering and Tech. 1(1): Jan.-Mar. 2010 page 24-26