Chlorine has many applications as disinfectant, purifier, and a raw material for manufacturing important monomers (toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and vinyl chloride), solvents, agrochemicals, pharmaceuticals, and so on. In the processes for manufacturing these products, HCl is formed as a byproduct. Moreover, in the manufacture of TDI or MDI, the byproduct of HCl is four times the molar amounts of TDI and MDI. The demands for TDI and MDI are increasing sharply, which will result in the inevitable production of more HCl than its market demand, and the serious disposal problem of a toxic waste and not meeting increasingly stringent environmental regulations. Therefore, it is necessary and urgent to recycle chlorine in chlorine-related industries [1, 2].
Although the catalytic oxidation of HCl to chlorine by air or O2 (so-called Deacon process) has been known for over 140 years [3], the Deacon process is limited by general disadvantages such as fast deactivation of the catalyst due to the volatilization of the catalytic metals in the form of chlorides, and severe corrosion issues caused by unreacted HCl and H2O product [1]. Therefore, the Deacon process has been largely replaced by the electrolysis of HCl, but this has the disadvantage of high electricity consumption. In recent years, some breakthroughs in chlorine production from the catalytic oxidation of HCl have been achieved over Ru-based catalysts, namely, RuO2/rutile-TiO2 catalyst by Sumitomo [4] and RuO2/SnO2- Al2O3 catalyst by Bayer [5, 6]. However, Ru is a noble metal and its market price is expensive and highly fluctuating (http://www.platinum.matthey.com/prices/price-charts) [2], which will limit the large scale industrialization of Ru-based catalysts. The Deacon process has attracted increasing attention because of its relative ease of operation and lower energy consumption and thermal requirements. Therefore more efforts have focused on developing efficient Cu-based catalysts [7, 8, 9] and other effective catalysts with lower costs to replace the expensive Ru-based catalysts [10, 11, 12, 13, 14].
In our previous work, an efficient Cu-K-La/γ-Al2O3 catalyst was developed for the catalytic oxidation of HCl to chlorine [15]. The conversion of HCl was 75%-82% and the average conversion of HCl was 78% for a reaction of over 9600 h under the reaction conditions of 0.1 MPa, 340 °C, space velocity of 450 L/(kg-cat·h), and HCl/O2 molar ratio = 2:1. A demonstration plant of 1000 ton/year of chlorine from the catalytic oxidation of HCl is under construction. The aims of this work are to investigate the effect of KCl on the catalytic performance of the Cu-K-La/γ-Al2O3 catalyst for HCl oxidation and explore the promotion mechanism of KCl.
The Cu-K-La/γ-Al2O3 catalysts were prepared by the incipient wetness impregnation method in our previous work [15], in which the loadings of CuCl2 and LaCl3 were 15 and 10 wt%, respectively. The mass ratios of KCl to CuCl2 were 0, 1/6, 1/3, 1/2, 2/3, and 1, respectively.
Powder X-ray diffraction patterns were recorded on a Bruker AXS D8 Focus diffractometer operated at 40 kV, 40 mA (Cu Kα radiation, λ = 0.15406 nm). The diffraction patterns were measured in the range of 10° < 2θ < 80° at a scanning rate of 6°/min. Elemental analysis was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a TJA IRIS ADVANTAG 1000 instrument. H2 temperature- programmed reduction experiments were carried out in a flow system equipment with a TCD detector. Sample (100 mg) was loaded into the quartz tube reactor which then was heated from 100 to 800 °C at the rate of 10 °C/min in an atmosphere of 5% H2/N2 at 45 mL/min.
The catalytic oxidation of HCl over the Cu-K-La/γ-Al2O3 catalysts was investigated in a quartz fixed-bed reactor under the reaction conditions of 30 g catalyst, 0.1 MPa, space velocity of 450 L/(kg-cat·h), and HCl/O2 = 2:1 (molar ratio). The flow rates of HCl and O2 were 150 and 75 mL/min, respectively, which were fed into the reactor by mass flow controllers. The product of chlorine was analyzed by the iodometry method and the unreacted HCl was quantified by a standard NaOH solution. There was no byproduct in the catalytic oxidation of HCl, hence the selectivity for chlorine was 100% and the conversion of HCl was equal to that of the yield of chlorine.
Figure 1 shows the effect of the mass ratio of KCl/CuCl2 on the conversion of HCl over the Cu-K-La/γ-Al2O3 catalysts for HCl oxidation. The mass ratio of KCl/CuCl2 had a significant influence on the conversion of HCl. The conversion of HCl increased significantly with increasing mass ratio of KCl/CuCl2 up to 1/3, and the conversions of HCl were almost the same over Cu-K-La/γ-Al2O3 catalysts with the mass ratio of KCl/CuCl2 of 1/6 and 1/3. When the reaction temperature was higher than 360 °C, the conversions of HCl were similar over the Cu-K-La/ γ-Al2O3 catalysts with the mass ratios of KCl/CuCl2 of 1/6 and 1/3. The Deacon process is an exothermic reaction, and a lower reaction temperature is favorable for increasing the equilibrium conversion. The Deacon process is a kinetics-controlled reaction at lower temperatures and a thermodynamics- controlled reaction at higher temperatures [16]. However, with further increasing of the mass ratio of KCl/CuCl2, the active sites of Cu2+ species were partly covered by K species, which resulted in an obvious decrease in the conversion of HCl to be even lower than that of the catalyst without the addition of KCl.
Figure 2 shows the effect of the space velocity of the feed gas on the conversion of HCl over the Cu-K-La/γ-Al2O3 catalyst with the mass ratio of KCl/CuCl2 of 1/3. The decline rate of the conversion of HCl became slower with increasing space velocity of the feed gas. When the space velocity of the feed gas was increased from 150 to 450 L/(kg-cat·h), the conversion of HCl was decreased from 94.5% to 86.6%. However, on further increasing the space velocity of the feed gas from 450 to 750 L/(kg-cat·h), the conversion of HCl only decreased from 86.6% to 83.9%. An increase in the space velocity of the feed gas decreased the residence time of HCl on the catalyst surface and hence resulted in a decrease in the conversion of HCl. Therefore the Cu-K-La/γ-Al2O3 catalyst showed good catalytic activity over a wide range of the space velocity of the feed gas.
Figure 3 shows the stability of the Cu-K-La/γ-Al2O3 and Cu-La/γ-Al2O3 catalysts with 5 wt% KCl loading for HCl oxidation for 100 h. It was found that the conversion of HCl was kept above 85% and remained little change for 100 h. After reaction for different times, the water product was collected for 5 h and then ICP-AES was used for the evaluation of the loss of Cu from the Cu-K-La/γ-Al2O3 catalyst. The loss of Cu at the sampling points of reaction times for 55 and 100 h were 5.5 × 10-7 and 3.5 × 10-7 g/h, respectively, which indicated that the loss of Cu was negligible. However, over the Cu-La/γ-Al2O3 catalyst, the initial conversion of HCl was 80% and it declined gradually after reaction for 62 h. Therefore the Cu-K-La/γ-Al2O3 catalyst showed better activity and stability due to the promotion by KCl.
Figure 4 shows the XRD patterns of the Cu-K-La/γ-Al2O3 catalysts with different mass ratios of KCl/CuCl2. When the mass ratios of KCl/CuCl2 were lower than 2/3, there were only the broad characteristic diffraction peaks of the γ-Al2O3 (JCPDS No. 10-0425) support at 2θ = 37.6°, 45.9°, and 67.1°, and the characteristic diffraction peaks of Cu, K, and La species were not observed, which indicated that they were highly dispersed on the surface of the γ-Al2O3 support. With an increase in the mass ratio of KCl/CuCl2, the intensities of the diffraction peaks of the γ-Al2O3 support decreased gradually. With increasing of the mass ratio of KCl/CuCl2 to 2/3 above, the characteristic diffraction peaks of KCl were observed at 2θ = 28.3°, 40.6°, and 50.8°, which indicated that a small amount of K species existed in the form of KCl nanocrystals.
Figure 5 shows the H2-TPR profiles of the Cu-K-La/γ-Al2O3 catalysts with different mass ratios of KCl/CuCl2. There were two obvious reduction peaks over all the Cu-based catalysts, which were attributed to the step reduction of Cu2+ → Cu+ → Cu0. With increasing the mass ratio of KCl/CuCl2 from 1/6 to 1, the peak temperature of the reduction peak of Cu2+ to Cu+ increased from 308 to 352 °C.
Combining with the results in Fig. 1, the trend of the mass ratio effect on the conversion of HCl was the same as on the peak temperature of the reduction peak of Cu2+ to Cu+, in which the lower temperature of the reduction of Cu2+ to Cu+ was favorable for increasing the catalytic activity of the active sites of Cu2+ species. Therefore, the addition of KCl favored the reduction of Cu2+ to Cu+ and thus improved the catalytic activity for HCl oxidation. These findings are similar to those reported by Rouco et al. [17] for low temperature ethylene oxyhydrochlorination over CuCl2-based catalysts.
The addition of KCl improved the activity and stability of Cu-K-La/γ-Al2O3 catalyst for HCl oxidation. The conversion of HCl was kept above 85% and nearly unchanged for 100 h over Cu-K-La/γ-Al2O3 catalyst with 5 wt% KCl loadings under the reaction conditions of 0.1 MPa, 360 °C, space velocity of 450 L/(kg-cat·h), and HCl/O2 molar ratio = 2:1. The results showed that Cu, K, and La species were highly dispersed on the support surface; the addition of KCl promoted the reduction of Cu2+ to Cu+ and thus improved the activity of the active sites of Cu2+ species for HCl oxidation.
2014, Vol. 35 Issue (8): 1359-1363
PDF (520KB)
2014, Vol. 35 Issue (8): 1359-1363
PDF (520KB)