Catalytic Activity of Cobalt-Molybdenum in Gas-Phase Ketonisation of Pentanoic Acid

Ketonisation of pentanoic acid was carried out over cobalt-molybdenum and its supported catalysts in the gasphase using fixed-bed reactor at 320-420°C and atmospheric pressure. Bulk Co-Mo and its supported catalysts were active in the ketonisation of pentanoic acid. 20%Co-Mo/Al2O3 showed the best catalytic performance in comparison to other supported catalysts. It gave 95% 5-nonanoe selectivity at 91% of pentanoic acid conversion at 308°C and ambient pressure for 5h time on stream (TOS). 20%Co-Mo/Al2O3 catalyst was stable for 15h TOS with small catalytic deactivation. All catalysts used in this study were characterized by different techniques such as TGA analysis, BET surface area and porosity, the catalysts acidity was measured in this work by using FTIR spectroscopy.


Introduction
Ketonisation reaction achieved large part of biorenewable fuels and several applications of catalytic reactions that observed receiving gain interest for it [1][2][3][4][5]. The heat of biomass in the absence of oxygen to create of bio-oil from small chain of carboxylic acids as good materials for the application of ketonisation knows as pyrolysis method. In this case, several carboxylic acids have been used for this purpose. Ketonisation reaction needs high temperature and that leads to make this reaction impractical. Therefore, ketonisation reaction was developed by using different kind of catalysts. Up to now, catalytic reaction mechanisms and suitable catalyst properties have not exactly known, but the active site as Lewis acid might be important in this reaction. It has been reported that Lewis acid sites on the surface of catalysts are require to form carboxylate as an intermediate species in the ketonisation reaction mechanisms [5,6].
Ketonisation of carboxylic acids catalyzed by many metal oxides and mixed metal oxides as well as other catalysts such as heteropoly acids [7] and zeolites [8] in the gas phase has been investigated in the temperature range of 200-500°C [9][10][11][12][13][14][15][16][17]. Ketonisation mechanisms and the nature of catalytic active sites are not clear so far even many significant researches has been done during over three decades since the first ketonisation of acetic acid carried out using calcium acetate to produce acetone [8].
Glinski et al. [18] tested MnO 2 , CeO 2 and ZrO 2 supported on Al 2 O 3 , SiO 2 and TiO 2 catalysts in the ketonisation of heptanoic acid in the gas phase in the temperature range 300-450°C. It has been found that, at low temperature MnO 2 catalyst showed the best catalysts performance comparing to CeO 2 and ZrO 2 . Regarding to supported catalysts, all catalysts were active in the ketonisation reaction, but MnO 2 /Al 2 O 3 was the most active one at 400°C, it gave 95% of 7-tridecanone yield. For CeO 2 /Al 2 O 3 and ZrO 2 /Al 2 O 3 they gave 82% and 24% of 7-tridecance yields respectively. In another work, ketonisation of propionic acid to form 3pentanone over CeO 2 and CeO 2 -based composite oxides in the gas phase were investigated in the temperature range of 300-425°C for 5h TOS. It has been reported that, the conversion of acid increased with increasing the temperature, but ketone selectivity was decreased [19]. CeO 2 which has 40.1 m 2 /g specific surface area was active catalyst in the ketonisation of propionic acid; it gave 93.8% of 3-pentanone selectivity at 51.0% of acid conversion. The mixed oxide of CeO 2 -Mn 2 O 3 which has 38.4 m 2 /g specific surface area showed better catalytic activity in the ketonisation reaction. It gave 97.4% of 3-pentanone selectivity at 73.9% of propionic acid [14].
Fresh catalyst of CeO 2 which calcined at low temperature has been tested in ketonisation of acetic acid at low temperature of 230°C, with constant mass of catalyst. This temperature was lower than the typical temperatures that used in the ketonisation of carboxylic acids over many different catalysts structures and properties which have been previously reported. The results showed that greater acetone yields achieved by using CeO 2 catalyst that calcined at low temperature [5].
Steam reforming of acetic acid to form acetone in the presence of steam using 1-bed steam reforming catalyst and 2-bed catalytic system has been investigated. In the single bed hydrogen was produced, however, 2-bed catalytic system, ketonisation of acetic acid was taken place. Co-based system reforming catalyst under steam atmosphere was also tested in the steam reforming reaction. MgAl 2 O 4 , ZnO, CeO 2 and activated carbon were used as catalyst without adding in the reforming reaction. Co-supported catalyst showed stable catalytic performance with even less coke deposition. The catalytic stability was improved by using 2-bed catalytic steam reforming of acetic acid to acetone and the coke deposition was decreased in the comparison to 1-bed steam reforming catalysts [20].
The ketonisation of carboxylic acids, acetic and propionic has been studied using different catalysts. In this case, zinc and chromium mixed catalysts as bulk and supported tested in ketonisation acetic, propionic and pentaoic acid and these catalysts were active in the ketonisation reactions [21]. For the ketonisation of pentanoic acid, series of Zn/Cr atomic ratio 1:30 to 30:1, ZnO and Cr 2 O 3 tested in the ketonisation of pentanoic acid. All catalyst showed good catalytic performance but Zn-Cr (10:1) was the most active catalyst at 380°C and ambient pressure in the comparison with ZnO and Cr 2 O 3 . It gave 82% of 5-nonanone selectivity at 86% of acid conversion. For more investigations, Zn-Cr (1:10) studied at different temperature from 300-400°C. The study showed that, the activity increased with increasing the temperature over 380°C, that because might be some coke deposited on the surface of catalyst and some of active sites might be blocked by coke [19][20][21][22].
Cobalt and molybdenum bulk and supported catalysts were tested in the deoxygenation of propionic acid in the gasphase and atmospheric of N 2 and H 2 at 200-400°C. Co-Mo Supported catalyst on alumina showed the best catalytic activity under N 2 to form 3-pentanone, this catalyst gave 67% of 3-pentanone selectivity at 65% of propionic acid conversion (44% of ketone yields) in comparison with the same reaction conditions under H 2 which gave 16% 3pentanone at 100% of propionic acid conversion (16% of ketone yields). For further testing, the catalyst stability was tested for 15 h time on stream using nitrogen atmosphere and the catalyst showed stable performance with small decrease in the catalytic activity after 10h time on stream, it might some coke has been deposited on the catalyst surface [22].
Recently, Co-Mo bulk and supported catalysts on Al 2 O 3 , TiO 2 and SiO 2 were tested in the ketonisation of acetic acid in the gas phase from 200-400°C using 0.2g of catalyst, 1 bar pressure, 2 vol% of acid and 20 mLmin -1 of N 2 flow rate. Both of bulk and supported catalysts were active in the reaction. Co-Mo bulk catalyst showed durable catalytic activity for 4h TOS. The catalyst gave 91% acetone selectivity at 86% of acetic acid conversion at 380°C. However, at the same reaction condition 20%Co-Mo/Al 2 O 3 was the best one in the ketonisation of acetic acid reaction in comparison with bulk and other supported catalysts. It gave 95% of acetone selectivity at 96% of acetic acid conversion (91% of acetone yields). In the case of catalytic stability, both bulk and 20%Co-Mo/Al 2 O 3 supported catalysts were more stable than the bulk one at 380°C for 12h TOS with no any deactivation noted during the reaction [23].
In the current work, bulk Co-Mo and 20%Co-Mo impregnated on Al 2 O 3 , TiO 2 and SiO 2 were tested in gas phase ketonisation of pentanoic acid to form 5-nonanone in the temperature range 320-420°C, 20 mLmin -1 , 2 vol% of acid and 0.2g of catalyst. The catalysts were characterized using different techniques such as BET surface area and porosity, TGA and FTIR-pyridine adsorption for measuring both Brɵnsted and Lewis acid sites on the surface of bulk Co-Mo catalyst.

Chemicals and Materials
In this study, all chemical were purchased from Sigma Aldrich and used without any further purification. Acetic acid was ≥ 99.5% pure. Aerosil 300, Titanoxid P25 and Aluminiumoxid C were from Degussa. The distilled water was used for preparation and washing the catalysts.

Catalyst Preparation
The amount of Cobalt hydrates acetate (Co (CH 3 CO 2 ) 2 .4H 2 O) and amount of molybdenum phosphoric acid (H 3 PMo 12 O 40 .13H 2 O) were dissolved separately in a minimum of distilled water and then mixed together in one beaker. The mixture were evaporated by a rotary evaporation at 65°C, and dried at 110°C overnight. For Co-Mo supported catalysts preparation, the catalysts were prepared by impregnation method. 20% of Co-Mo was impregnated on Al 2 O 3 , SiO 2 and TiO 2 . In this case, the same materials that used in the preparation of bulk catalyst were dissolved in minimum of distilled water and added to Al 2 O 3 , SiO 2 or TiO2. After that, the mixture was stirred for 3h, evaporated, dried over night at 110°C in the oven. Finally, the catalysts were calcined at 400°C for 2h under N 2 atmosphere [22,23].

Catalyst Characterisation
In this work, catalysts surface area and porosity were measured by the BET method using Micromeritics ASAP 2010 instrument. The diffuse reflectance infrared Fourier transform spectra (DRIFTS) has been used for measuring the catalytic acidity. Thermogravimetric analysis (TGA) was performed using a Perkin Elmer TGA 7 instrument under nitrogen flow for measuring the amount of water in the catalysts.

Catalyst Testing
The reaction was carried out in a quartz glass fixed-bed reactor which was located in a vertical tubular furnace and fed from the top. To control reaction temperature, the Eurothern controller using a thermocouple placed in the center of catalyst bed. 0.2 g of powder catalyst was loaded with the reactor and pretreated at the reaction temperature under 20 ml/min of N 2 or H 2 flow for 1h. The reaction was conducted at 320-400°C, and ambient pressure, 20ml/min N 2 flow rate, 4.0 g h mol -1 space time and 2 vol % pentanoic acid concentrations. At regular time intervals, the downstream gas flow was analyzed by the on-line GC ((Varian 3800 instrument with a 30 m × 0.32 mm × 0.5 µm Zebron ZB-WAX capillary column and a flame ionization detector (FID)) which was calibrated for each component. The conversion of acid and selectivity of each product are calculated from equations (1) and (2)

Catalyst Characterisation
The nature of acid sites and texture of surface are, pore volume and pore diameter have been investigated previously [22]. Here, it can be seen that 20%Co-Mo/Al 2 O 3 showed the best catalytic performance in the ketonisation of pentanoic acid has 97 m 2 /g surface area with 0.12 cm 3 /g pore volume and 49 Å as an average pore diameter and that similar to results reported previously [22,23]. Figure 1 shows that there are three bands, the first one at 25-180 attributed to loss of physisorbed water contents. The second band is at 180-490°C is assigned to the structure water contents. The third band is at 490-700°C is assigned to the decomposition of catalyst's Keggin structure [22][23][24][25][26]. Bulk Co-Mo and catalyst were characterized by Thermogravimetric analysis (TGA) and FTIR using pyridine adsorption as previously reported by Hossein Bayahia [22,23]

Bulk Co-Mo Catalyst
The results observed that gas-phase ketonisation of pentanoic acid over the bulk Co-Mo in the range of temperature of 320-420°C are presented in the table 1. From these results it can be seen that the bulk catalyst was active in the ketonisation reaction and the catalytic activity depends significantly on the temperature. Catalytic activity increased by increasing the reaction temperature. At 380°C the catalyst showed the best catalytic activity, it gave 91% of 5-nonanone selectivity at 80% of acid conversion. The hydrocarbons products were formed as by-products and their amounts were increased by increasing the temperature. It can be noted that, there are some unknown products that slightly increased with increasing the temperature. The unknown products might be isopropanol or propanol and propanal. Carbon monoxide and carbon dioxide were not monitored.

Supported Co-Mo Catalysts
Catalytic activity was improved by Supporting 20%Co-Mo on γ-Al 2 O 3 , SiO 2 and TiO 2 . Both Al 2 O 3 and TiO 2 showed better catalyst performance in comparison with SiO 2 in the ketonisation of pentanoic aid in the gas-phase to form 5nonanone (Table 2). It can be seen that, Co-Mo supported catalysts possessing significant amount of Brɵnsted and Lewis acid sites that can be the reason of their activity in addition to themselves activity in the ketonisation reaction [22,23]. However, SiO 2 has been found inactive at lower temperature than 400°C (Table 2), but it showed that it was active at higher temperature as previously reported [23]. Interestingly, 20%Co-Mo/Al 2 O 3 showed the best catalytic activity at 380°C, it gave 95% of ketone selectivity at 91% of acid conversion.
For further catalytic investigations, 20%Co-Mo/Al 2 O 3 was tested for long term of ketonisation of pentanoic reaction; the catalyst shows that stable catalytic performance for 15 h TOS with a small drift of catalyst conversion from 91% at the first hour of reaction to 83% at the constant ketone selectivity ( Figure 4). Similar results has been reported for ketonisation of acetic and propionic acids in the gas-phase using the same catalyst of Co-Mo bulk and its supported catalysts that prepared by Hossein Bayahia [22,23]. In the case of catalytic deactivation, there are several reasons such as poisoning, fouling, thermal degradation and so on [23,27]. The deactivation of 20%Co-Mo/Al 2 O 3 might be caused by the coke that can be deposited on the surface of catalyst and blocked the active sites [23]. The amount of coke measured by C, H analysis and it has indeed found in the catalyst and amounted 2.7wt%.

Conclusion
Bulk Co-Mo has been demonstrated as an active catalyst and durable catalyst for the ketonisation of pentanoic acid in the gsa-phase at 320-400°C and atmospheric pressure to form 5-nonanone. For improving catalytic performance, the bulk catalyst of Co-Mo was supported on SiO 2 , γ-Al 2 O 3 and TiO 2 . Among of these catalysts, 20%Co-Mo on γ-Al 2 O 3 presented the best catalytic performance in the ketonisation of pentanoic acid to form 5-nonanone. This catalyst gave 95% of 5-nonanone selectivity at 91% pentanoic acid conversion at 380°C for 5 h TOS. 20%Co-Mo/Al 2 O 3 was stable for 15 h TOS with a small deactivation in the catalytic activity. Bulk and supported catalysts were characterized by different techniques such as BET surface area and porosity, TGA analysis and DRIFTS of pyridine adsorption for measuring catalysts acidity.

Conflicts of Interest
The author declares that he has no competing interests.