MODELING AND OPTIMIZATION FOR PRODUCTION OF STAINLESS STEEL CORROSION INHIBITORS FORMULATED WITH 1,3-DIPHENYL-3-PHENYLSULFANYL-PROPAN-1-ONE: A CLEAN PROCESS CATALYSED BY FLUORAPATITE

The optimization for process production of stainless steel corrosion inhibitor formulated with 1.3-Diphenyl-3-phenylsulfanyl-propan-1-one was studied using a 2 block central composite design including 3 factors (weight of catalyst, reaction time, and quantity of solvent). This process catalyzed by Fluorapatite coupled with their ease of use and reduced environmental problems makes them attractive alternatives to homogeneous basic reagents.


INTRODUCTION
Corrosion problems have received a considerable amount of attention because of their attack on materials, like the failure of some stainless steel resulting from pitting corrosion which is considered a technological problem .Consequently, much effort has been expended in attempting to understand and overcome the pitting corrosion; therefore, the use of inhibitors is one of the most practical methods for protection against corrosion.In this case several works have studied the influence of organic compounds on the corrosion of steel in acidic media .
Recently, the Moroccan authors are showed that the electrochemical study for 1.3-Diphenyl-3-phenylsulfanylpropan-1-one (DPSP), it's revealed a good corrosion inhibition of stainless steel in phosphoric acid .The conjugate addition of thiophenol to chalcone is a convenient route for synthesis of this sulfanyl organic compound.In classic methods , this reaction catalyzed by strong bases such as alkali metal alkoxides, hydroxides, transition metal complexes and piperidine.The employment of these strong bases in these reactions, however, leads to two main problems affecting the environment; the necessity to dispose of huge amounts of organic waste due to formation of undesirable side products resulting from polymerization, bis-addition and self condensation, and total dissolved salts formed following the neutralization of soluble bases with acids.The replacement of liquid basic catalysts by solid bases in the synthesis of fine and intermediate organic chemicals allows one to avoid corrosion and environmental problems .
In view of this, the aim of the present study is investigate the application of experiments Design for modeling and optimization of the production of DPSP stainless steel corrosion inhibitor catalyzed by Fluorapatite 7 .

Chemicals and instrumentations
All commercial reagents and solvents were used without further purification.X-ray diffraction (XRD) patterns of the catalysts were obtained on a Philips 1710 diffractometer using Cu-K radiation.Surface areas were determined at 77 K using a Coulter SA 31000 instrument with an automated gas volumetric method employing nitrogen as the adsorbate.NMR spectra were recorded on a Bruker ARX 300 spectrometer.Mass spectra were recorded on a VG Autospec spectrometer.FTIR spectra were recorded on an ATI Mattson-Genesis Series spectrophotometer using the KBr disc method.

Preparation and characterisation of catalyst
The Fluorapatite was prepared by the co-precipitation method using salts of phosphate and the calcium: 250 mL of a solution containing 7.92 g of diammonium hydrogen phosphate and 1g of ammonium fluoride, maintained at pH greater than 12 by addition of ammonium hydroxide (15-20 mL), were dropped under constant stirring into 150 mL of a solution containing 23.6 g calcium nitrate (Ca(NO 3 ) 2 ,4H 2 O).The suspension was refluxed for 4h.Doubly distilled water was used to prepare the solutions.The Fluorapatite crystallites were filtered, washed and dried overnight at 80 °C and calcined in air at 700 °C for 30 min before use.The structure of Fluorapatite is identified by X-ray diffraction (space group hexagonal system; a 9.364 Å, c 6.893 Å), infrared spectra IR and chemical analysis (Ca = 38.29 %, P = 17.78 %, Ca/P = 1.66).The BET specific surface area was found to be S = 15.4 m 2 /g.The total pore volume was calculated by the BJH method at P/P 0 = 0.98 (V t = 0.0576 cm 3 /g).

General procedure
The general procedure for production of DPSP stainless steel corrosion inhibitor is as follows: To a flask containing an equimolar mixture (1 mmol) of chalcone and thiophenol in methanol, the Fluorapatite was added and the mixture was stirred at room temperature.The experimental setup is shown in Figure 1.The catalyst was filtered, washed with dichloromethane and the filtrate was concentrated under reduced pressure.The crude product of sulfanyl corrosion inhibitor was purified by recrystallization and identified by 1 H, 13 C, NMR and IR spectroscopy.

RESULTS AND DISCUSSION
In this study, we have investigated the optimization of the production of DPSP stainless steel corrosion inhibitor using central composite rotatable design 8 .The factors are the experimental parameters considered above: weight of catalyst, reaction time and quantity of methanol.The values used in this design and the levels X i of the 3 factors are indicated in Table 1.The software STATGRAPHICS 9 was used for the experimental design, data analysis, model building and graph plotting.In this experimental design, the equation of estimated responses ( ) can be written: The 10 coefficients of this design are easily calculated by the least squares method.The significance of effects can be estimated by comparing the F distribution of the experimental values to a critical value (F 0.05 (1.6) = 5.99).According to the results showed in Table 3.In this case, the linear terms (X 1 , X 2 and X 3 ), the squared terms (X 1 2 , X 22 and X 3

2
) and the interaction terms (X 1 X 2 , X 1 X 3 and X 2 X 3 ) were significant model terms.It appears that all effects are significant.So, the estimated response for production of DPSP stainless steel corrosion inhibitor ( %DPSP ) can be written: From this equation, it is possible to compute estimated values and the corresponding residuals.In the other hand, the estimate of the variance for experimental error was showed in Table 4. From this analysis of variance, the model F-value of 539.9 implied that the model was significant.Values of Prob.> F less than 0.01 indicated that the model terms were significant.
Figure 2 shows the three-dimensional response surfaces which were constructed to show the effects of the weight of catalyst and the quantity of solvent at constant value of the reaction time (X 2 = 0) on the production of DPSP stainless steel corrosion inhibitor.As can be seen from Figure 3, at constant value of the reaction time (X 2 = 0.5) when the weight of catalyst (X 1 = -to 0.8) and the quantity of solvent (X 3 = -to 0.4) increase together or when the weight of catalyst increases and the quantity of solvent remaining unchanged, then the yield of DPSP increases up to 98 %.The investigation of equation ( 2) showed that, if X 1 = 0.25, X 2 = 0.50 and X 3 = -0.67; the value predict from the results using response surface model is 98 %.The experimental checking in this point, i.e. under the optimum reaction conditions such as: weight of catalyst = 0.35 g, reaction time = 30 min, and quantity of MeOH = 2 mL with high yield of stainless steel corrosion inhibitor 96 %, confirms this result.
Solid catalysts become particularly interesting when they can be regenerated.Indeed, in our case, the Fluorapatite was recovered quantitatively by simple filtration and regenerated by calcination for 15 min at 700 °C.The recovered catalysts were reused several times without loss of activity, even after the seventh cycle product of DPSP stainless steel corrosion inhibitor was obtained with the same yield.

CONCLUSIONS
In summary, the process synthesis of stainless steel corrosion inhibitor catalyzed by Fluorapatite has been studied using central composite design.The model equation for the optimization of the reaction conditions for this process was established.From this equation it was possible to forecast the optimal reaction conditions for production of sulfanyl corrosion inhibitor in high yield.
This process bring advantages such as high catalytic activity and selectivity under mild reaction conditions, easy separation of the catalyst by simple filtration, possible recycling of the catalyst, use of non-toxic and inexpensive catalysts and especially, elimination of salts and by-product pollutants.This new solid base catalyst becomes then a practical alternative to soluble bases.In continuation of our ongoing program to develop clean and economical processes for the production of fine chemicals, other applications will be reported elsewhere.

Fig. 2 .
Fig. 2. Response surface plot for production of stainless steel corrosion inhibitor at constant value of the reaction time (X 2 = 0).

Fig. 3 .
Fig. 3. Contours of estimated response surface of yields in pure product of stainless steel corrosion inhibitor at constant value of the reaction time (X 2 = 0.5).

Table 1 .
Study field and coded factors.However, yields in pure product of DPSP were chosen as response for this design.The 8 experiments were done in the 2 following blocks: the first block with a complete factorial design 2 3 with 2 center points, the second block according to axial design with the distance to center equal to 1.68179 and with 2 center points.The values of the factors used in this design and the experimental response (% DPSP) are reported in Table2.

Table 3 .
Estimated coefficients of the model and their significances.

Table 4 .
Regression variance analysis for the model.