Al2O3 for Propane Dehydrogenation

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Platinum-Modified ZnO/Al2O3 for Propane Dehydrogenation: Minimized Platinum Usage and Improved Catalytic Stability Gang Liu, Liang Zeng, Zhi-Jian Zhao, Hao Tian, Tengfang Wu, and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China S Supporting Information *

ABSTRACT: Compared to metallic platinum and chromium oxide, zinc oxide (ZnO) is an inexpensive and low-toxic alternative for the direct dehydrogenation of propane (PDH). However, besides the limited activity, conventional zinc-based catalysts suffer from serious deactivation, because of ZnO reduction and/or carbon deposition. Considering the high cost of platinum, reducing the amount of platinum in the catalyst is always desirable. This paper describes a catalyst comprising ZnO modified by trace platinum supported on Al2O3, where the Zn2+ species serve as active sites and platinum acts as a promoter. This catalyst contains less platinum than traditional platinum-based catalysts and is much more stable than conventional ZnO catalyst or commercial chromium-based systems during PDH. It is proposed that ZnO was promoted to a stronger Lewis acid by platinum; thus, easier C−H activation and accelerated H2 desorption were achieved. KEYWORDS: propane dehydrogenation, heterogeneous catalysis, platinum, ZnO catalyst, hydrogen desorption, Lewis acid

P

strongly reducing conditions, were supposed to be the culprit for deactivation.3,5,6 Although the catalytic performance is far from satisfactory, few methods have been reported to simultaneously inhibit the deactivation and improve the activity of ZnO. Platinum-based catalysts have shown high activity for many reactions, but noble metal platinum has a major problem in its limited availability. It is always attractive to reduce the usage of platinum while keeping considerable catalytic performance. Thus, using a small amount of platinum to modify other nonnoble active sites provides an alternative strategy, and many exciting results have been reported.7−10 This paper describes how the addition of trace amounts of platinum substantially improves the catalytic performance for PDH over ZnO supported on Al2O3. A series of catalyst materials, including ZnO/Al2O3 and PtZnO/Al2O3, have been prepared by an impregnation method (see section S1 in the Supporting Information), and their physicochemical properties and catalytic performance are illustrated in Table 1. ZnO/Al2O3 catalysts exhibit relatively low activity and rapid deactivation. After 4 h on stream, the rates of C3H6 formation decrease from 11.3 mmol/h/gcat to 5.6 mmol/h/gcat and from 15.5 mmol/h/gcat to 7.0 mmol/h/gcat, corresponding to the deactivation parameters of 50% and 55%, for the 10Zn and 15Zn catalysts (Table 1), respectively. With

ropylene is an important feedstock employed in the production of polypropylene, propylene oxide, acrylonitrile, etc. Growing attention has been focused on propane dehydrogenation, because of the rising demand for propylene. Commercial dehydrogenation of propane (PDH) processes operate at temperatures above 500 °C and generally use chromium oxide- or platinum-based catalysts. However, chromium oxide systems suffer from severe deactivation, because of carbon deposition and need for regeneration after every 12 min PDH step, and chromium oxide also causes heavy pollution. Although platinum shows excellent activity and stability, it is limited by its high expense.1 All of these factors restrict further development of the PDH industry, and the demand for new catalysts is urgent. Alternatively, among the various metal oxides that are actively used for PDH (vanadium oxide, gallium oxide, etc.),1 zinc oxide (ZnO) is a promising candidate, with both low cost and low toxicity. Despite that several studies have been reported about ZnO in the dehydrogenation of light alkanes, fast deactivation and limited activity are still challenging issues.2−5 Pidko et al. prepared a homogeneous dispersion of Zn cations on HZSM-5 via dimethylzinc chemical vapor deposition, which showed high activity for propane dehydroaromatization but rapid deactivation.5 Zn−Nb−O composites were also suggested as active catalysts and a synergetic effect between ZnO and ZnNb2O6 was proposed. But only 28.1% propylene yield was achieved at 580 °C and a weight hourly space velocity (WHSV) of propane of 0.5 h−1 with serious deactivation.3 Both coke deposition and ZnO reduction, under © XXXX American Chemical Society

Received: December 16, 2015 Revised: January 26, 2016

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ACS Catalysis Table 1. Physicochemical Properties and Catalytic Performance catalyst

a

10% Zn 15% Zn 10% Zn−0.1% Pt 15% Zn−0.1% Pt 0.1% Pt 0.5% Pt

code

BET specific surface area, SBET (m2/g)

C3H8 conversionb (%)

10Zn 15Zn 10Zn0.1Pt 15Zn0.1Pt 0.1Pt 0.5Pt

203 191 187 174 224 225

22 (12) 28 (16) 30 (27) 35 (31) 11 (8) 52 (40)

C3H6 rateb (mmol/h/gcat) 11.3 15.5 16.7 18.4 5.2 18.2

(5.6) (7.0) (14.1) (15.3) (3.1) (16.4)

C3H6 selectivityb (%) 95 94 95 94 96 57

(98) (97) (98) (97) (98) (77)

coke depositionc (g/gcat)

deactivation parameterd (%)

0.025 0.029 0.031 0.030 0.043 0.094

50 55 15 15 40 10

a Theoretical contents of elemental zinc and platinum in the catalyst. bThe values outside and inside the brackets are the data obtained in the initial period and at 4 h, respectively. Catalytic test conditions: T = 600 °C, atmospheric pressure, WHSV propane = 3 h−1, 500 mg of catalyst, C3H8/H2 = 1/1, with balance N2 for total flow rate of 50 mL/min. cThe amount of coke is calculated based on thermogravimetric analysis (TGA). dDeactivation parameter is calculated by eq 3 in the Supporting Information.

the addition of 0.1 wt % Pt, the initial rates of C3H6 formation were up to 16.7 and 18.4 mmol/h/gcat for the 10Zn0.1Pt and 15Zn0.1Pt catalysts, respectively, and the deactivation parameters were only 15% for both cases. All the catalysts exhibited outstanding selectivity toward C3H6 over 94% at the beginning of the reaction and over 97% at the end of the reaction. Evidently, after the addition of a small amount of platinum, the ZnO/Al2O3 achieved higher activity and much better stability. The best catalyst, 15Zn0.1Pt, exhibited activity comparable to that of the 0.5% Pt catalyst but with much higher propylene selectivity.11 Note that the former case only contains 20% platinum, compared to the platinum-only catalyst (see Figure 1).

Figure 2. (a) High-angle annular dark field−scanning transmission electron microscopy (HAADF-STEM) image of PtZnAl catalyst particle (left), along with an EDX linescan (right). (b) A proposed model for the PtZnAl catalyst.

linearly bonded to metallic Pt (2060 cm−1) disappeared over the 15Zn0.1Pt catalyst (Figure S3 in the Supporting Information). These results imply that the Pt clusters may be covered by oxide species (likely ZnO), which is consistent with the Pt−Sn(Ga) and Pd−Zn catalysts.14−18 X-ray photoelectron spectroscopy (XPS) measurements suggest that the majority of Zn was present in the 2+ valence state and was mainly on the surface of Al2O3 (Table S1 and Figure 3), which is similar to Figure 1. C3H6 rate and selectivity during 4 h PDH over the 15Zn, 15Zn0.1Pt, and 0.5Pt catalysts (T = 600 °C, atmospheric pressure, WHSV propane = 3 h−1, 500 mg of catalyst, C3H8/H2 = 1/1, with balance N2 for a total flow rate of 50 mL/min).

Figure 2 shows a HAADF-STEM image of the representative 15Zn0.1Pt catalyst particle, which exhibits the best catalytic performance. No Pt or ZnO crystallites were detected by analyzing the number of particles, indicating a high dispersion and small particle size. Energy-dispersive X-ray (EDX) further confirmed the uniform Pt−Zn element distribution over the Al2O3 support. This result was also supported by analysis of the X-ray diffraction (XRD) patterns (Figure S1 in the Supporting Information) and ultraviolet−visible (UV-vis) absorption spectra (Figure S2 in the Supporting Information). To further explore the surface model and the origin of the catalytic reactivity over PtZnAl catalysts, H2−O2 titration experiments were carried out to test the dispersion of Pt species.12,13 Pt dispersion of 0.1Pt, 10Zn0.1Pt and 15Zn0.1Pt catalysts were 8.70%, 0.09%, and 0.05%, respectively. An obvious decrease in platinum dispersion was observed when both zinc and platinum were present. Moreover, the typical vibrational features for CO

Figure 3. Zn L3M4.5M4.5 Auger peaks for (a) the fresh and spent 15Zn catalysts and (b) the fresh and spent 15Zn0.1Pt catalysts.

PtSn/Al2O3 catalysts.19 Thus, it is very likely that, over our PtZnAl catalyst, small ZnO and Pt clusters are homogeneously dispersed on the Al2O3 surface, while most of the platinum surface is covered by ZnO species. The feasible surface model is depicted in Figure 2b. Since almost no platinum surface was exposed, we consider that the PDH activity of PtZnAl is not mainly derived from the platinum. In order to further clarify the functionality of ZnO and Pt during propane dehydrogenation, additional catalysts with more platinum or less ZnO were 2159

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ACS Catalysis

sites and propane dehydrogenation activity was obtained on Ga2O3 and Al2O3−Ga2O3 solid solutions.22,23 The amount of total Lewis acid site on the 15Zn and 15Zn0.1Pt catalysts surface was estimated by the integral peak areas in the infrared spectrum of adsorbed pyridine (see Figure S5 in the Supporting Information). Clearly, the presence of platinum had a negligible influence on the total amount of acid site (i.e., 1:1 for the 15Zn and 15Zn0.1Pt catalysts). In addition, the presence of platinum had little effect on the distribution of zinc, and surface concentration of zinc only slightly decreased to 13.2% from 14.1% (see Table S1 in the Supporting Information). Therefore, higher activity for the 15Zn0.1Pt catalyst is not attributed to such a small change in the amount of active sites. In order to further understand the dehydrogenation mechanism of propane on ZnO catalysts, DFT calculation on ZnO(1010̅ ), which was one of the most dominant surfaces of the ZnO particle,24 was selected as the model ZnO catalyst. As shown in Figure 4a, the highest dehydrogenation barrier is 1.60

prepared. Decreasing the concentration of platinum to 0.05 wt % over the 10Zn0.05Pt sample caused a worse performance, compared to the 10Zn0.1Pt sample; meanwhile, the increase in platinum content to 0.15 wt % over the 15Zn0.15Pt and 10Zn0.15Pt catalysts resulted in activity similar to that of the 15Zn0.1Pt and 10Zn0.1Pt catalysts, respectively (Figure S4 in the Supporting Information). However, the decrease in ZnO content to 1% Zn dramatically decreased the propylene production rate on the 1Zn0.1Pt catalyst. It was even lower than the rate of pure 0.1Pt without any ZnO (Figure S4), which was consistent with earlier work that excessive metal promoter addition, such as Ga, and Cu, can decrease the dehydrogenation activity of platinum.11,14 Since a much larger amount of zinc was present on the 10Zn0.1Pt and 15Zn0.1Pt catalysts, platinum should exhibit negligible dehydrogenation activity over these catalysts. Consequently, it is reasonable to deduce that Zn2+ species functions as the active dehydrogenation sites, with platinum serving as a unique promoter over our PtZnAl catalysts. Coke formation is generally regarded as the major cause for catalyst deactivation in PDH.1 However, the enhanced stability of Pt-ZnO/Al2O3 catalysts is not related to the deposited coke, since a similar amount of coke was obtained, compared to the reference catalysts (Figure S6 in the Supporting Information). It has been reported that the decomposition of oxygenated zinc clusters or the formation of metallic zinc is a significant factor for ZnO deactivation.5,6 In this study, only very weak reduction of ZnO could be observed under hydrogen conditions (see Figure S7 in the Supporting Information). To examine whether metallic zinc was present on the surface of the 15Zn and 15Zn0.1Pt catalysts, XPS characterization of fresh and spent catalysts was employed. The Zn L3M4.5M4.5 Auger peak was chosen because the change in the valence state from Zn2+ to Zn0 was observed as a pronounced 3 eV downward shift of the binding energy.20 In Figure 3, both fresh 15Zn and 15Zn0.1Pt catalysts displayed only one obvious peak at 499 eV, which was assigned to Zn2+. After 4 h of PDH reaction, a pronounced shoulder feature appeared at ∼496 eV for the 15Zn catalyst, which was attributed to Zn0, while no similar feature was found for the 15Zn0.1Pt catalyst. These results suggest that part of Zn2+ on the fresh catalyst was reduced to Zn0 upon reaction on the 15Zn catalyst, the amount of Zn0 was ∼20%, as estimated by the fitting results of XPS curve (Figure S8 in the Supporting Information).21 However, no metallic zinc was detected on the spent 15Zn0.1Pt catalyst. In addition, considering the fact that zinc metal is volatile (mp 420 °C) under the reaction conditions, the zinc loading would decrease if metallic zinc was formed during dehydrogenation. Accordingly, inductively coupled plasma optical emission spectrometer results showed that the zinc loading for the spent 15Zn catalyst declined from 15.5% to 13.8%, while almost no change for the spent 15Zn0.1Pt catalyst was observed, from 14.4% to 14.1%. Furthermore, H2O was also detected during the reaction to investigate the reduction of zinc oxide, as shown in Figure S9 in the Supporting Information. The intensity of H2O for the 15Zn catalyst was much higher than that for the 15Zn0.1Pt catalyst, directly indicating that the 15Zn catalyst suffered from a more serious reduction of zinc oxide. Overall, the addition of platinum inhibits the reduction of zinc oxide; thus, more active sites are maintained and improved stability is achieved. Previous studies have revealed that Zn2+ catalyzes propane dehydrogenation as Lewis acid sites,4 similar to gallium oxide catalysts. In addition, linear correlation between surface acid

Figure 4. Energy profile for (a) the dehydrogenation of propane via π adsorbed propylene (black) or di-σ adsorbed propylene (red) on Zn(101̅0); (b) desorption of H2 or H2O. (See the Supporting Information for a detailed description of the elementary steps that are involved.)

eV, indicating that ZnO itself can catalyze the dehydrogenation of propane. (See the Supporting Information for a detailed description of the elementary steps that are involved.) Studies on gallium oxide have shown that Ga−H and Ga−OH species are difficult to regenerate, and the formation of H2 may be less favorable than water elimination, which limits the rate of dehydrogenation reaction and facilitates the reduction of gallium oxide.25−27 Similar behavior was observed on ZnO. The recombination of two H atoms on Zn−OH requires several hydrogen diffusion steps, to form an adjacent Zn−H and Zn−OH pair before H−H bond combination. Although the combination barrier is as low as 0.37 eV, the prior diffusions must overcome a barrier as high as 0.96 eV. On the other hand, the formation of H2O, with the diffusion of OH instead of H, has a rate-determining barrier of 0.46 eV, which is significantly lower than the rate-determining barrier for H2 formation. Nevertheless, both rate-determining barriers to form H2 and H2O are lower than the calculated dehydrogenation barrier, indicating fast formation of both H2 and H2O under reaction conditions on ZnO. Upon the addition of trace amount of platinum into ZnO/ Al2O3, a lesser degree of ZnO reduction and higher activity have been observed. This may be attributed to the electronic interactions between Pt and ZnO, which has been previously reported in the Pd−Ga 2 O 3 , Pt−SnO x , and Cu−ZnO systems.19,28,29 Principally, the zinc oxide has a higher conduction band energy than the Fermi level of metallic 2160

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ACS Catalysis platinum,30 which facilitates the electron transfer from the oxide to the metal, leading ZnO to be a stronger Lewis acid. This hypothesis was confirmed by the fact that a strong NH3 desorption peak at 450 °C, which could be regarded as strong acid site, was observed when platinum was present (Figure 5a).

platinum-based catalyst, this catalyst exhibited comparable activity with much less platinum.1 Moreover, it can remain stable over 20 h with ∼100% propylene selectivity at 550 °C (Figure S13 in the Supporting Information). Thus, the catalyst shows a greater advantage over industrial chromium-based catalysts, which suffer from serious deactivation and need regeneration within 12 min. Overall, the PtZnAl catalyst is much more affordable than the platinum-based PDH catalyst, and, meanwhile, it is more environmentally friendly and stable than chromium oxide-based catalysts. In addition, we have provided evidence indicating that Pt serves as a unique promoter to optimize the catalytic performance over ZnO/ Al2O3 catalyst. It is likely that ZnO was modified as a stronger Lewis acid by platinum because of the electron interaction, thus easier C−H breaking and accelerated H2 desorption were achieved; however, the specific reaction mechanism is still being explored. The results may give inspiration for other alkane activation systems.

Figure 5. (a) NH3-TPD and (b) H2-TPD profile over the fresh 15Zn and 15Zn0.1Pt catalysts.

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Therefore, the diffusion of OH− is more difficult than H+ on the ZnO surface with less electrons, so the formation of H2 will be faster than that of H2O over the modified catalyst. The result of H2-TPD proved this speculation (Figure 5b). Specifically, hydrogen adsorbed on the 15Zn0.1Pt catalyst desorbed at 460 °C, compared to 520 °C on the 15Zn catalyst, which implied easier H2 desorption on the 15Zn0.1Pt catalyst. In addition, breaking the C−H bond in C3H8 will be easier on ZnO with platinum, i.e., with a stronger Lewis acid, than on pure ZnO, because the H and C3H7 are Lewis bases.31 This can be demonstrated by the TPSR experiments (shown in Figure S12 in the Supporting Information), which showed that the 15Zn0.1Pt catalyst catalyzed propane dehydrogenation to propylene from 460 °C, compared to 560 °C on the 15Zn catalyst. Therefore, it is not surprising that the 15Zn0.1Pt catalyst exhibited a higher propylene rate than the 15Zn catalyst, although both catalysts exhibited a similar amount of active sites. Hence, the results well illustrate that both H2 desorption and C−H activation are promoted by platinum. The possible reaction mechanism is depicted in Scheme 1.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02878. Catalytic performance results, supplementary analysis, and details of DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (Nos. 21406162, 21506149, and 21525626), the Program for New Century Excellent Talents in University (No. NCET-10-0611), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (MoE), and the Program of Introducing Talents of Discipline to Universities (No. B06006).

Scheme 1. Possible Reaction Mechanism over the PtZnAl Catalyst

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REFERENCES

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In summary, we have demonstrated that the incorporation of a trace amount of platinum (0.1 wt %) has a profound influence on the catalytic performance of ZnO/Al2O3 catalysts. At 600 °C and atmospheric pressure with WHSV = 3 h−1 propane, 18.4 mmol/h/gcat C3H6 (corresponding to 35% C3H8 conversion) and 97% C3H6 selectivity were achieved over the 15Zn0.1Pt catalyst and the activity only decreased 15% after 4 h of PDH reaction. Compared to the 0.5% Pt/Al2O3 catalyst, which contains a similar amount of platinum as a commercial 2161

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