Abstract
The catalytic dehydrogenation reaction of tail chlorine by Pd was studied using a fixed-bed reactor at low temperature from 30 to 100°C. Different catalyst supports such as SiO2 and Al2O3 were applied to prepare Pd catalysts by the incipient-wetness impregnation method. And the catalysts were characterized by XRD, FTIR, XPS, SEM, and N2 adsorption-desorption. The catalyst Pd loading on both SiO2 and Al2O3 had a catalytic effect on the dehydrogenation reaction, but the carrier Al2O3 was more superior. The hydrogen conversion and selectivity of hydrogen-oxygen reaction increased first and then decreased with Pd loading amount and temperature by using Pd/Al2O3 as catalysts, but the influence of temperature was limited when it was higher than 60°C. The hydrogen conversion was 97.38% and selectivity of hydrogen-oxygen reaction was 79% when the reaction temperature was at 60°C with 1 wt.% Pd/Al2O3.
1. Introduction
Chloralkali industry developed rapidly as a pillar industry in the inorganic chemical area. A large amount of chlorine-containing gas (tail chlorine), which contains hydrogen, nitrogen, oxygen, and other ingredients, is produced in the traditional production of chlorine in the electrolysis process. Considering the explosion risk of hydrogen, a part of the hydrogen must be removed to keep its content not higher than 4% by volume.
At the present stage, the typical treatment of tail chlorine is mainly by the combustion of a certain proportion of hydrogen and chlorine to synthesize the hydrochloric acid. However, the byproduct of hydrogen chloride would become excess and the cost of hydrogen chloride disposal by neutralization is high. Additionally, the shipment of this potentially hazardous waste is strictly limited. Thus, many researchers concentrated on the recovery of chlorine from hydrogen chloride by catalytic oxidation. Han et al. [1] made the conversion of hydrogen chloride to chlorine by catalytic oxidation in a two-zone circulating fluidized bed reactor at reaction temperature above 200°C. Feng et al. [2] prepared an efficient Cu-K-La/-Al2O3 catalyst for catalytic oxidation of chlorine from hydrogen chloride and the results showed a good catalytic performance and stability of conversion. Given that, the explosion problem of containing hydrogen in tail chlorine cannot be easily solved by combustion of hydrogen and chlorine to synthesize hydrochloric acid. The production of hydrogen chloride in the treatment of tail chlorine can also be reduced by other effective methods.
In 1957, Kulcsar and Kulcsar-Novakova developed a catalyst supported by activated carbon, which dehydrogenated the tail chlorine by catalyzing the reaction of hydrogen and chlorine to form hydrogen chloride [3]. Tanno [4] studied extensively the catalytic reaction conditions of hydrogen-chlorine reaction, which showed that higher temperature and lower gas flow rate made the performance of dehydrogenation better. Catalytic removal of hydrogen from electrolytic chlorine was studied by using charcoal, zeolite, and so forth at 20–300°C [5]. Pieters and Wenger [6] found that the concentration of hydrogen in a mixed gas could be reduced as low as ppm level by palladium catalyst loaded on Al2O3, or by catalysts LaCl3, KCl, and CuCl doped on SiO2. However, the way of removing hydrogen from tail chlorine by catalytic hydrogen-chlorine reaction has many disadvantages, such as high reaction temperature and short life of catalysts.
This paper aimed to research the catalytic performance of palladium on hydrogen-oxygen reaction, which could safely remove hydrogen from trail chlorine at low temperature and increase the economic value of tail chlorine.
2. Materials and Methods
2.1. Catalyst Preparation
The supported palladium catalysts were prepared by incipient-wetness impregnation method. 1.0 g supports as -Al2O3 or SiO2 powder were impregnated into the hydrochloric acid solution of palladium chloride at room temperature for 24 h and then dried in a drying oven for 6 h. Finally, the dried catalysts were calcined in muffle furnace at 500°C and activated in hydrogen atmosphere at 250°C for 4.5 h. The freshly prepared Pd/SiO2 and Pd/Al2O3 catalysts were stored in a dry and sealed plastic bag before use.
2.2. Catalytic Dehydrogenation Reaction of Tail Chlorine
The catalytic performance was evaluated in a fixed-bed reactor system as shown in Figure 1. A certain amount of catalysts was loaded in the fixed-bed reactor. The simulated gas mixture that was composed of Cl2, O2, H2, and N2 according to the composition of industrial tail chlorine was introduced into the reactor at a specific reaction temperature. The entire gas line was purged with nitrogen before introducing the simulated gas mixture. The products of catalytic dehydrogenation reaction were analyzed periodically. Water was absorbed by P2O5 stored in an adsorption device, which was weighed at set intervals. The weight increment of the water adsorption device was the water formed during that time interval. The produced hydrochloric acid was measured by titration in saturated NaCl solution.
2.3. Catalyst Characterization
Scanning electron microscopy (SEM) was performed on a Philips XL30 working at 20 kV. The specific surface area and distribution of pore size were measured by N2 adsorption-desorption method at −196°C with a Micromeritics (ASAP 2020) instrument. X-ray diffraction (XRD) patterns were recorded on a Rigaku-DMax (2500PC) with Cu Kα radiation in the 2θ range from 10° to 90° with 0.02°/min. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics (PHI 1600) spectrometer using Mg Kα radiation and photon energies of 1253.6 eV.
3. Results and Discussion
3.1. The Influence of Reaction Temperature on Catalytic Dehydrogenation Reaction
As shown in Figure 2, the hydrogen conversion increased with increasing temperature from 30 to 60°C, while the increase was not obvious when the temperature exceeded 60°C. The hydrogen conversion rose from 91.25% at 30°C to 97.38% at 60°C; with further increase of temperature to 70°C, it only reached 99.5%. In general, the selectivity of hydrogen-oxygen reaction increased with increasing temperature, while the variation trend could be separated into three stages. At low temperature from 30 to 50°C, the selectivity almost kept constant as 62%, and then it dramatically increased up to 79.19% at 60°C; after that, the selectivity became stable again at the high temperature range from 60 to 100°C. Thus, the reaction temperature of the catalytic dehydrogenation was 60°C, which provided high selectivity of the aimed reaction with low energy consumption.
3.2. The Influence of Support on the Performance of Catalytic Dehydrogenation Reaction
As shown in Figure 3, the dehydrogenation catalyzed by Pd/Al2O3 kept higher hydrogen conversion than that catalyzed by Pd/SiO2 during the whole reaction process, which was attributed to the different type of surface acid sites provided by Al2O3 and SiO2. It has been reported [7] that the L surface acid site was beneficial to hydrogen adsorption by Pd. The surface of Al2O3 had mainly L acid, but the surface of SiO2 had mostly B acid, and thus Pd/Al2O3 showed better catalytic performance in hydrogen conversion. The selectivity of hydrogen-oxygen reaction of Pd/SiO2 was slightly higher than that of Pd/Al2O3 as illustrated in Figure 4. Overall, the catalyst of Pd/Al2O3 had better performance for catalyzing the dehydrogenation.
3.3. The Influence of Pd Loading on Performance of Catalytic Dehydrogenation Reaction
The effect of Pd loading amount on the performance of Pd/Al2O3 in catalytic dehydrogenation reaction was assessed at 60°C as shown in Figures 5 and 6. Pd/Al2O3 with different Pd loading amount maintained the catalytic activity during the whole reaction process. The performance of Pd/Al2O3 was improved by increasing the Pd loading amount from 0.4 to 1.0% wt.; however, the continuous increase of Pd loading was detrimental. Thus, the optimal Pd loading amount was 1.0% wt. With the increasing concentration of in the impregnate solution, the active ingredient content of catalyst increased and the crystallinity of palladium increased as well, resulting in better performance of Pd/Al2O3. However, the too high Pd loading would cause the increase of grain and the decrease of particle dispersion, which reduced the effective contact area between the active ingredient and the reaction gas, resulting in poor Pd/Al2O3 performance. Dispersion of solid particles is a key factor affecting the optimal effects of the catalyst [8].
3.4. The Effect of Dehydrogenation Reaction on Pd/Al2O3 Catalyst
The characteristics of Pd/Al2O3 could be modified during the dehydrogenation reaction. The analyses of XRD, FTIR, XPS, and N2 adsorption-desorption were examined to investigate the modification of Pd/Al2O3 properties.
3.4.1. The XRD Patterns of Different Loadings of Pd/Al2O3
The XRD analysis of Pd/Al2O3 of different Pd loading amounts before reaction was illustrated in Figure 7. The characteristic peaks corresponding to metallic Pd presented at about 2θ = 40°, 45°, and 68°. The diffraction peaks of metallic Pd could not be observed with the loading amount from 0.4% to 0.6% due to the high dispersion of a small amount of Pd particles [9]. With increasing loading of Pd, the characteristic peaks of Pd became obvious and sharper at 1.2% wt., which indicated that the crystallization degree obviously increased. According to the previous studies [10], the high crystallization degree would cause the growth of grain and further enhance the degree of aggregation, which inhibited the activity of the catalyst. This was consistent with the experimental result in this study.
3.4.2. The XRD Patterns of Pd/Al2O3 before and after Reaction
Before reaction, the characteristic diffraction peaks of -Al2O3 were obtained at about 36°, 45°, and 68°, and the characteristic diffraction peaks of Pd were obtained at 40° and 47° as shown in Figure 8(a). After 240 min dehydrogenation reaction, the position and intensity of characteristic peaks of -Al2O3 and Pd did not change, as shown in Figure 8(b). This indicated that the catalyst of Pd/Al2O3 had good stability.
3.4.3. The FTIR Spectra of Pd/Al2O3
As shown in Figure 9, the absorption peak in 620 cm−1 was the six-coordinated characteristic absorption peak of Al3+ and the strong absorption peak in 880 cm−1 could be attributed to the four-coordinate of Al-O vibration. The relatively weak peak that appeared in 1650 cm−1 could be attributed to the bending vibration of H-OH bond which was relative to the existence of free water. The absorption peak in 3450 cm−1 might be caused by the stretching vibration of the water absorption by carriers or the absorption peak of –OH [11]. After catalytic dehydrogenation reaction, the characteristic absorption peaks in 400 cm−1~900 cm−1 still existed and the intensity of characteristic absorption peaks in 1650 cm−1 and 3450 cm−1 increased because of the absorption of the water generated in the reaction by -Al2O3.
3.4.4. The XPS Spectra of Pd/Al2O3
According to XPS analysis shown in Figure 10, the standard binding energies of Pd03d5/2 and Pd03d3/2 were at 335.2 eV and 340.5 eV, respectively. However, the binding energies of Pd03d5/2 and Pd03d3/2 were at 337.23 eV and 342.51 eV when Pd was loaded on -Al2O3, which indicated that the combining force of Pd and -Al2O3 was stronger, which was consistent with the conclusion obtained by Ihm et al. [12]. After the catalytic dehydrogenation reaction, the binding energies of Pd03d5/2 and Pd03d3/2 moved to 337.33 eV and 342.77 eV, whereas the intensity and area of peaks had a certain degree of decline. The result was attributed to the oxidation of Pd0 by O2 or Cl2 to Pd2+, leading to positive displacement of binding energy. Overall, Pd0 still had a high percentage after reaction and the catalyst characteristics remained relatively stable.
3.4.5. N2 Adsorption-Desorption and Pore Diameter of Pd/Al2O3
The N2 adsorption-desorption isotherms curves of Al2O3 and 1% Pd/Al2O3 were illustrated in Figure 11. The curves belonged to type IV isotherm [13] curve and the shape of the hysteresis loop was H3 type. The nitrogen adsorption curves became higher in high , corresponding to the existence of parallel tubular capillary and slit-shaped pores [14]. The shape of N2 adsorption-desorption isotherm of Al2O3 was not changed obviously by loading with Pd, which only had a slight decrease in the hysteresis loop area, which was the phenomenon of capillary condensation inside the carrier.
Figure 12 showed the pore diameter of Al2O3 and 1% Pd/Al2O3, which were analyzed by the BJH model. The pore distribution of alumina was mainly at about 5 nm before loading. However, there was a new peak at about 7 nm after loading by Pd, which was attributed to the minor damage of the pore structure by the immersion of acid. As could be seen from Table 1, the average pore diameters of Al2O3 and 1% Pd/Al2O3 were almost the same, but the specific surface area and total pore volume were reduced to a certain extent after loading, which indicated that the active ingredient of palladium was mainly formed at the external surface and the wall of pores close to the external surface of the support particle.
3.4.6. The SEM Analysis of the Influence of Pd Loading on Al2O3
Further evidence of the in-framework metal Pd loading on Al2O3 was provided by SEM analyses, as shown in Figures 13–15. There was a comparison of Al2O3 and Pd/Al2O3 in Figures 13 and 14, and Figure 15 shows the partial enlarged detail of Figure 14. Obviously, it could be seen from Figures 14-15 that metal Pd uniformly loaded on Al2O3 was noted in the SEM. These figures also showed that the skeleton structure of Al2O3 was not changed, which indicated that the catalyst Pd/Al2O3 structure was stable.
4. Conclusion
(1)Pd/Al2O3 and Pd/SiO2 both had a catalytic effect on the dehydrogenation reaction at low temperature.(2)With the increase of Pd loading and reaction temperature, the hydrogen conversion and selectivity of Pd/Al2O3 in catalytic dehydrogenation reaction increased firstly and then decreased, but the impact of reaction temperature on the catalytic performance is not obvious after 60°C.(3)When Pd loading is 1% and the reaction temperature is 60°C, hydrogen conversion and oxygen-hydrogen selectivity of Pd/Al2O3 are at 97.38% and 75%. The performance of Pd/Al2O3 is better than Pd/SiO2.
Competing Interests
The authors declare that they have no competing interests.