Trimetallic system (Ni, Cu, and Ag) supported on alumina was utilized for hydrogenation of nitrophenols. The catalytic active centers for hydrogenation were attributed only to the presence of Ni. However, the presence of bi- or trimetallic systems improves the catalytic activity via extra synergism. The catalytic activity was measured as the time for reaching 100% conversion. The function of synergism was fitted for both bimetallic systems (Ni:Ag; Ni:Cu) individually. Subsequently, three-dimensional function was fitted for trimetallic system (Ni:Cu:Ag) based on the linear combination of data for individual bimetallic system. After a complex calculation areal function was evaluated. An Excel program was written to simply evaluate the catalytic activity of trimetallic system with high accuracy. Characterization of catalysts was performed using EPR and pulsed chemisorption by hydrogen. These characterizations of samples enable us to evaluate particle size, metallic surface area, and degree of dispersion. These values were successfully correlated with the synergism function. The program written then could be capable of predicting these values for any trimetallic system.
Hydrogenation reactions are one of the most important reactions for preparation of many fine chemicals and important intermediates [
In order to fulfill this purpose, we choose a simple effective hydrogenation reaction. Thus hydrogenation of nitrophenol into amino phenol using supported metal and using hydrazine hydrate as a hydrogen source was proven to be 100% selective and finishes with 100% conversion in only few minutes [
NaOH (Merck), copper nitrate (Merck), nickel nitrate (Merck), silver(I) nitrate (Merck), hydrazine hydrate (99.999% Merck), alumina (FLUKA Typ 507c), P-nitrophenol (PNP) (Merck), o-nitrophenol (oNP) (Merck), m-nitrophenol (mNP) (Merck), P-aminophenol (PAP) (Merck), m-aminophenol (mAP) (Merck), and o-aminophenol (oAP) (Merck) were used as a standard materials.
Typically 10 g of alumina was impregnated with 5 wt% metals. Different mole ratios of mono-, di-, and trimetal are varied keeping 5 wt% constant. After drying at 100°C the catalyst is subjected to chemical reduction using hydrazine hydrate in alkaline solution of NaOH. After completion of reduction, the catalyst was rapidly filtered and dried and kept in a closed bottle.
Typically, a solution containing 0.125 g of nitrophenols with 20 mL of hydrazine hydrate was heated to 80–100°C in a three-necked flask connected to a condenser.
0.5 g of the catalyst was added to the above solution. The time was recorded just upon the addition. After reaction completion the color of the system is changed to grayish white indicating 100% conversion [
X-Ray diffractograms of various solids were collected using a Bruker D8 advance instrument with CuK
The EPR spectra were recorded on EMX Bruker instrument operated at
The H2 chemisorption experiments were carried out at 373 K using a pulse reactor to determine the Ni metal area and Ni metal particle size. In a typical experiment, approximately 150 mg of the catalyst sample was loaded in a micro quartz reactor (8 mm i.d., and 250 mm long) and the catalyst sample was first subject to hydrogen flow at 420 K for 1 h and kept at the same temperature for 1 h under He flow and then the reactor was cooled to 373 K under helium gas flow. The outlet of the reactor was connected to a microthermal conductivity detector (TCD) of GC-17A (Quantachrome ChemBET 3000, USA) through an automatic six-port valve after cooling of the sample to 303 K, pulses of gas (5% H2 balance He) (500 mL), until there is no further change in the intensity of TCD peak due to gas pulse.
Figure
XRD patterns of pure Ni, some trimetallic systems, and bare alumina.
EPR spectra of all samples are given in Figure
EPR spectra of different investigated samples.
In this research we aimed to study and evaluate trimetallic system of Ni, Cu, and Ag as an effective catalyst for hydrogenation of nitrophenols.
In order to study this system we make a series of bimetallic systems Ni:Cu and Ni:Ag with different molar ratios (Table
Examples of some measured bimetallic and trimetallic systems.
Catalyst (5 wt% loading on alumina) | Time to reach 100% conversion (sec) | ||
---|---|---|---|
PNP | MNP | ONP | |
Ni | 505 | 149 | 80 |
1Ni:1Cu | 426 | 67 | 125 |
2Ni:1Cu | 145 | 23 | 43 |
1Ni:2Cu | 669 | 106 | 197 |
1Ni:1Ag | 403 | 64 | 119 |
1Ni:2Ag | 603 | 96 | 178 |
2Ni:1Ag | 136 | 22 | 40 |
1Ni:1Cu:1Ag | 275 | 44 | 81 |
1Ni:2Cu:1Ag | 300 | 47 | 89 |
1Ni:1Cu:2Ag | 327 | 52 | 96 |
2Ni:1Cu:1Ag | 138 | 22 | 41 |
2Ni:1Cu:2Ag | 320 | 52 | 95 |
2Ni:2Cu:1Ag | 179 | 29 | 53 |
From experimental test it was found that neither Cu nor Ag alone gives any catalytic activity in these reactions; however in presence of Ni as a bimetallic system they could enhance the catalytic activity.
In order to study the effect of these metals on the activity of Ni, we first determined what is called the linear catalytic activity which is the catalytic activity of catalyst based on its % nickel theoretically calculated from the pure nickel catalyst. Based on these values we can check any presence of synergism occurring due to addition of either Cu or Ag as bimetallic system with nickel. A function of synergism was evaluated for bimetallic system individually (Ni:Cu and Ni:Ag), was given below, and was fitted with regression with
Fitted function of synergism of Ag.
Fitted function of synergism of Cu.
where
where
From Figures
In order to study the trimetallic system we make a linear addition of both functions of individual synergism. Figure
3D presentation of trimetallic system Ni:Cu:Ag. Cu:Ag against synergism per % Ni.
The above data was simulated as 3D function as given below: where
Figure
3D model for the trimetallic system of Ni:Cu:Ag.
Figures
This 3D figure could give us the ability to evaluate the four parameters in one time (% syn, % Ni, % Cu, and % Ag). However, subsequently, analysis of the real system should be compared which will be performed in the next section.
After applying the previous function on real trimetallic systems we observed that extra synergism occurs due to presence of both Cu and Ag together. In order to evaluate this extra synergism we correlate the difference in synergism with the % Ni (Figure
Difference between real and linear synergism against % Ni.
Thus, it was found that extra synergism is observed only in % Ni between 5 and 40%. In order to calculate it automatically we simulate this function as follows: where
After applying the above function on the three-dimensional linear synergism function we could obtain the real function of synergism (Figure
3D representation of calculated real synergism function.
Obtaining the previous function enables us to deduce the catalytic activity and consequently the time to reach 100% for trimetallic system simply by input of the mole ratio of Ni:Cu:Ag.
where
We correlate the particle size of different ratio of trimetallic catalysts with the function of real synergism Figure
Correlation between particle size and function of synergism over % Ni.
The function correlating the syn/Ni with particle size is as follows: where
By the above function we can deduce the particle size of any trimetallic system by the syn/Ni function.
In order to evaluate the secret of catalytic activity we make pulsed chemisorption to measure the metallic surface area and degree of dispersion and average crystallite size of Ni. This pulsed chemisorption was performed at reaction temperature (Table
Experimental data for some selected samples for dispersion and metallic surface area.
Catalyst | % Ni | Dispersion | Metal surface area |
---|---|---|---|
1Ni:1Ag:1Cu | 25.65 | 30.41 | 10.13 |
2Ni:1Cu | 64.9 | 55.27 | 18.42 |
1Ni:1Ag | 35.43 | 32.51 | 10.83 |
Ni | 100 | 4.12 | 1.37 |
2Ni:1Ag | 52.32 | 3.63 | 1.21 |
1Ni:1Cu | 48.04 | 2.71 | 0.9 |
Correlating surface area per unite % Ni with the synergism function we can obtain a good correlation as in Figure
Metallic surface area per % Ni against synergism per % Ni.
We can simulate the previous function as follows: where
By the above correlation we can simulate the metallic surface area to the synergism function easily to fully simulate the trimetallic system. The program written in Excel could simulate easily with a good precision the whole system for the all isomers of nitrophenol (p-, m-, and o-).
Figure
Image of Excel program.
A comparison between real and calculated results of some random samples combined with % error.
Catalyst (5 wt% loading on alumina) | Time to reach 100% conversion (sec) | ||||||||
---|---|---|---|---|---|---|---|---|---|
PNP | MNP | ONP | |||||||
Real | Calculated | % error | Real | Calculated | % error | Real | Calculated | % error | |
Ni | 505 | 505.1 |
|
149 | 148.7 |
|
80 | 79.86 |
|
1Ni:1Cu | 426 | 425.5 |
|
67 | 67.27 |
|
125 | 125.3 |
|
2Ni:1Cu | 145 | 144.9 |
|
23 | 22.9 |
|
43 | 42.66 |
|
1Ni:2Cu | 669 | 668.3 |
|
106 | 105.7 |
|
197 | 196.8 |
|
1Ni:1Ag | 403 | 403.1 |
|
64 | 63.72 |
|
119 | 118.7 |
|
1Ni:2Ag | 603 | 603.3 |
|
96 | 95.39 |
|
178 | 177.7 |
|
2Ni:1Ag | 136 | 135.7 |
|
22 | 21.45 |
|
40 | 39.95 |
|
1Ni:1Cu:1Ag | 275 | 274.7 |
|
44 | 43.42 |
|
81 | 80.88 |
|
1Ni:2Cu:1Ag | 300 | 299.1 |
|
47 | 47.29 |
|
89 | 88.08 |
|
1Ni:1Cu:2Ag | 327 | 327.2 |
|
52 | 51.73 |
|
96 | 96.35 |
|
2Ni:1Cu:1Ag | 138 | 137.3 |
|
22 | 21.71 |
|
41 | 40.43 |
|
2Ni:1Cu:2Ag | 320 | 319.6 |
|
52 | 50.53 |
|
95 | 94.11 |
|
2Ni:2Cu:1Ag | 179 | 178.3 |
|
29 | 28.19 |
|
53 | 52.51 |
|
4Ni:1Cu:2Ag | 140 | 138.2 |
|
23 | 21.84 |
|
42 | 40.68 |
|
1Ni:2Cu:3Ag | 1128 | 1122 |
|
180 | 177.3 |
|
333 | 330.3 |
|
3Ni:1Cu:1Ni | 104 | 105 |
|
18 | 16.6 |
|
32 | 30.92 |
|
1Ni:3Cu:2Ag | 718 | 716.8 |
|
115 | 113.3 |
|
213 | 211.1 |
|
2Ni:1Cu:2Ag | 322 | 319.6 |
|
52 | 50.53 |
|
96 | 94.11 |
|
|
|
|
|
From Table
Kinetic curve of reaction of nitrophenols into aminophenol under our reaction conditions.
A durability study for some trimetallic samples for reduction of PNP.
Sample | Time to reach 100% for PNP (sec) | ||||
---|---|---|---|---|---|
1st use | 2nd use | 3rd use | 4th use | 5th use | |
1Ni:1Cu:1Ag | 275 | 280 | 290 | 300 | 330 |
1Ni:2Cu:1Ag | 300 | 310 | 310 | 330 | 350 |
1Ni:1Cu:2Ag | 327 | 328 | 330 | 335 | 340 |
2Ni:1Cu:1Ag | 138 | 140 | 142 | 146 | 150 |
2Ni:1Cu:2Ag | 320 | 322 | 324 | 330 | 340 |
2Ni:2Cu:1Ag | 179 | 182 | 183 | 185 | 190 |
The trimetallic system (Ni, Cu, and Ag) was proved to be very effective for reduction of nitrophenols using hydrazine hydrate. Thus, nickel metal was found to be the only active center in this reaction. However, the presence of bimetallic and trimetallic alloy with nickel exhibits extra synergism. A program with high precision was successfully written to simulate the synergism function and the catalytic activity of the trimetallic system. The method of calculation used will open new aspect to deal with similar system in the future. Some physical properties such as particle size and metallic surface area could be simulated successfully for whole trimetallic system.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant no. 392/130/1436. The authors, therefore, acknowledge with thanks DSR technical and financial support. The authors would like also to acknowledge King Fahd Medical Center for the support in measuring the ESR samples.