The dehydrogenation of n-butane is an attractive route to produce 1,3-butadiene (BD), because n-butane is a cheaper raw material  . Two possible routes are proposed by researchers in this field; one is the simple dehydrogenation (SDH) of n-butane, and another is the oxidative dehydrogenation (ODH) of n-butane. Although the SDH has the advantage in both equilibrium and a long catalyst lifetime, the formation of oxygenates as a by-product causes the decreasing selectivity for BD and the increasing separation cost. The SDH process afforded a relatively high selectivity to BD, but a relatively high reaction temperature is necessary to obtain a high BD yield. An alumina-supported Pt catalyst and its bimetallic catalysts modified with a second element, such as Sn, In, Ge, Ga, Cu, Zn, Y, Bi, or alkali metals, were widely investigated for the SDH of n-butane to improve the catalytic activity and selectivity to BD -. In this study, we report a new Pt-based bimetallic catalyst for the SDH prepared by modification of the Pt/Al2O3 catalyst with Ag.
2.1. Preparation of PtAg/Al2O3 Catalyst
Crushed Al2O3 (Neobead GB, Mizusawa Industrial Chemicals, Ltd., surface area = 190 m2∙g−1) having a 250 - 850 μm size was used for the catalyst preparation. After the sieved Al2O3 and an aqueous solution of H2PtCl4 were added to a 200-ml eggplant flask, they were mixed at 40˚C for 30 min. Water was removed from the flask using a rotary evaporator under reduced pressure, then the obtained pale-yellow granules were dried at 130˚C overnight and calcined at 550˚C for 3 h. Three types (Cat B-D) of PtAg/Al2O3 catalysts were prepared by the following different methods (Figure 1). During the preparation of all the PtAg/Al2O3 catalysts, residual Cl− derived from H2PtCl6 was removed by a steam treatment to avoid AgCl formation by the reaction of a Ag compound with Cl− before the following steps.
The Cl− removing step was performed as follows in Figure 1. The granules calcined after the Pt addition were place in the center of a stainless-steel tubular reactor (internal diameter = 10 mm), then the reactor was placed in a tubular muffle furnace. At 550˚C, helium (50 ml/min) and steam (37 ml/min) were introduced into the reactor, and this steam treatment (steaming) was performed for 5 h to remove the Cl− for the Cats B-D preparation (30 min for the Cat A preparation).
Figure 1. Outline of catalyst preparation.
Cat A: Pt/Al2O3 catalyst as a reference.
The calcined granules were reduced at 550˚C for 3 h to obtain the PtAg/Al2O3 catalyst.
Cat B: Cl− free catalyst.
The Cl− removing granules and an aqueous solution of AgNO3 were added to a 200-ml eggplant flask, then were mixed at 40˚C for 30 min. Water was removed from the flask using a rotary evaporator under reduced pressure, then the obtained pale-yellow granules were dried overnight at 130˚C, calcined at 550˚C for 3 h, and finally reduced at 550˚C for 3 h to obtain the PtAg/Al2O3 catalyst (Cat B).
Cat C: Cl-added catalyst.
To the Cl− removing granules, an aqueous solution of NH4Cl was added to control the residual amount of Cl−. After mixing them for 30 min at room temperature, the water was removed, and the obtained granules were calcined at 550˚C. After the Cl− addition, the PtAg/Al2O3 catalyst (Cat C) was prepared in the same manner as the Cat B preparation.
Cat D: Cl− added catalyst by modified method.
For this preparation, the H2PtCl4-supported on Al2O3 was calcined and pre-reduced before the steaming (the Cl− removal step). The pre-reduction promoted the formation of the Pt catalyst having a high dispersion. The Ag addition was performed by impregnation using the Cl− removing granules and an aqueous solution of AgNO3. Chloride ions were added to the Ag-added Pt/Al2O3 using an aqueous solution of NH4Cl by an impregnation method. The obtained precursor was calcined at 550˚C for 3 h, then reduced at the same temperature for 3 h (Cat D).
The prepared catalysts were characterized by a surface area analysis (Microtrac BEL, BELSORP-mini II), X-ray diffraction analysis (XRD, Brucker, D2 phaser), X-ray fluorescent analysis (XRF, PANalytical, PW 2400), thermogravimetry-differential thermal analysis (TG-DTA, Shimazu, DTG-60), scanning electron microscopy with an energy dispersive X-ray analyzer (SEM-EDX, Hitachi, S-3400), and CO chemisorption (Hemmi Slide Rule, BP-1).
2.2. Simple Dehydrogenation of n-Butane
Half a gram of the prepared catalyst was placed in the center of the tubular reactor, and the reactor was placed in a tubular muffle furnace. All the catalysts were reduced with a 40 vol% H2 (He balance) at the reaction temperature for 1 h just before the dehydrogenation. The dehydrogenation was performed at 550˚C for 6 h with hydrogen to suppress the coke formation. The composition of the reactant gas was n-butane: He:H2 = 1:6:1 (molar ratio) with a 99 ml/min total flow rate. The products distribution was quantitatively determined by a gas chromatograph equipped with a thermal conductivity detector (Shimazu, GC-8A). Two columns, VZ-7 and activated carbon, were used for the analysis.
3. Results and Discussion
3.1. Characterization of the PtAg/Al2O3 Catalysts
Figure 2 shows the XRD profiles of the prepared Cats A-D with the Ag/Al2O3 and Al2O3. In all the spectra, no peaks assigned to Pt and Pt-based bimetallic compounds were observed due to the low Pt content and the high dispersion of the Pt particles. Meanwhile, the major peak assigned to Ag was clearly observed at 2θ = 38.1˚, and its strong intensity indicated agglomeration of the Ag metal particles. Besides the peaks assigned to Ag, those assigned to AgCl were clearly observed at 2θ = 27.9 and 32.4˚, which was produced by the reaction of Ag and HCl during the catalyst preparation.
Table 1. Physicochemical properties of the prepared catalysts.
a. These data were the average of values obtained by two measurements.
Figure 2. XRD profiles of Al2O3-supported Pt, Ag, and PtAg catalysts.
3.2. Dehydrogenation of the Catalyst in the Presence of H2
The dehydrogenation was performed at 550˚C using Cats A-D for 6 h. The conversion of n-butane and selectivity of the butenes (1-butene, cis- and trans-2-butenes) and 1,3-butadiene are shown in Figure 3. The selectivity of the by-products, such as CO, CO2, CH4, C2H6, C3H6, and isobutane are summarized in Table 2.
The Pt/Al2O3 showed the lowest activity among the tested catalysts. The conversion gradually decreased from 28.5% to 14.8%. The selectivity of the butenes and 1,3-butadiene clearly increased with the reaction time (from 77% to 84%). The active sites, which caused a side-reaction, such as cracking, decreased during the initial stage of the reaction by the coke deposition with the decreasing activity. After the reaction, coke (4.0 wt% versus the catalyst weight) was formed on the catalyst.
When Ag was added to the Pt/Al2O3, the conversion significantly increased from 28.5 to 37.6% with the increasing selectivity of the desired products. As shown in Table 2, the addition of Ag suppressed the formation of CH4, C2H4, and C3H6. Although Cat B showed a high activity and selectivity of the butenes and 1,3-butadiene during the n-butane dehydrogenation, a large amount (about 30 wt% for the catalyst weight) of coke was formed after the reaction for 6 h. To confirm the reason for such an incredible coke formation, we performed the dehydrogenation using Ag/Al2O3. The Ag/Al2O3 catalyst showed a low activity for the n-butane dehydrogenation, whereas an extremely high activity for the coke formation was observed during the reaction of 1-butene. Therefore, we concluded that the free Ag particles on Cat B caused the formation of the large amount of coke.
To decrease the coke formation, we attempted modification of the catalyst with Cl− (Cat C). The addition of Cl− was considered to be able to convert the
Figure 3. Dehydrogenation of n-butane over Cat A-D.
Table 2. Distribution of the by-products during the dehydrogenation.
free Ag to AgCl which was inactive for the coke formation. In the XRD profile of Cat C, the diffraction peaks corresponding to AgCl were clearly observed. The conversion obtained using Cat C was higher than that obtained using Cat B. Additionally, the obvious effect of the Cl− addition was observed on the amount of coke, which dramatically decreased from about 30 to 4 wt%.
Meanwhile, the selectivity slightly decreased when Cl− was added to the Pt/Al2O3. From the detailed analysis of the products distribution, isobutane, which was an unpredictable by-product, was confirmed. Isobutane is considered to be formed by skeletal isomerization of n-butane on the strong acid sites of Al2O3. It is well known that the acid sites were induced by Cl− in the Al2O3-supported Pt catalysts used for the catalytic reforming of the crude oil.
As a known technology , during the re-generation of a Pt/Al2O3 reforming catalyst, agglomerated Pt metals were re-dispersed in the presence of a Cl-containing compound. We attempted the preparation of a highly-active catalyst based on the above concept. The highest activity was obtained by the dehydrogenation using Cat D, which was prepared through the pre-reduction, the dichlorination, the Ag addition, the Cl− addition, and the reduction. The increasing activity was due to the high dispersion of the Pt metals in spite of the lowest amount of CO chemisorption. This low amount of the CO chemisorption might be caused by the formation of PtAg bimetallic compounds.
The SEM photographs of the PtAg/Al2O3 catalyst (Cat D) and the mapping results of Ag and Cl obtained by the SEM-EDX analysis are shown in Figure 4. Many spherical particles (0.5 - 2 μm) were observed in the SEM photograph (A). The EDX mappings (B and C) of Ag and Cl indicated that the most of the particles included Ag, and some of them included Cl. The molar ratio of Ag to Cl was 4.5 in the observed area. This fact meant that these particles consisted of AgCl and metallic Ag particles. It was suggested that the added Cl formed AgCl by reacting with the small Ag particles. The melting point of AgCl (455˚C) is lower than the reduction temperature in the catalyst preparation, therefore, the formed AgCl melted and agglomerated during the catalyst preparation. Finally, we concluded that the dramatic decrease in the coke formation was due to the formation of AgCl.
(a) (b) (c) (d)
Figure 4. SEM-EDX photographs of PtAg/Al2O3 catalyst Cat D. (a) SEM photograph; (b) EDX mapping for Cl; (c) EDX mapping for Ag; and (d) EDX mapping for Pt.
We prepared PtAg bimetallic catalysts and evaluated these catalysts for the dehydrogenation of n-butane in the presence of H2. The catalyst prepared by Pt addition, reduction, Cl removal, Ag addition, Cl addition, and finally reduction showed a high activity for the dehydrogenation and produced butenes (1-butene, cis- and trans-2-butenes) with a high selectivity. The added Ag was used for modification of the Pt particles (bimetallic phase might be formed) to suppress the formation of the cracking products. The addition of Cl to the catalyst decreased the free Ag metal particles which formed a large amount of coke, resulting in the high activity and selectivity for the butenes and butadiene.