Yu Shiab,
Xianru Liab,
Xin Rongab,
Bin Guab,
Huangzhao Weia and
Chenglin Sun*a
aDalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, PR China. E-mail: clsun@dicp.ac.cn; Fax: +86 84699965; Tel: +86 84379133
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
First published on 4th April 2017
A study on the performance of Pt(0.3 wt%)Sn(0.2 wt%)K(0.5 wt%) catalysts supported on four different θ-Al2O3 for propane dehydrogenation is reported in this study. The θ-Al2O3 used as supports were prepared by four different methods as (a) calcination of the commercial γ-Al2O3 at 1223 K for 12 h, (b) synthesis by hydrochloric acid reflux method, (c) precipitation of Al(NO3)3·9H2O with ammonia solution and (d) extrusion use pseudo-boehmite powder, respectively. They were characterized by using XRD, BET, SEM, H2-TPR, NH3-TPD, CO-chemisorption, XPS and TG-DTA methods to study which characteristics of the carrier will affect the performance of the catalyst. The results show that high acidity and strong interactions of the Sn support can improve the propane dehydrogenation activity of the catalyst, large pore volume and large pore diameter can enhance the stability of the catalyst. Al2O3 synthesized by method (b) has the largest pore volume, pore diameter, relatively high surface acidity and strong interaction with Sn, which meant the catalyst with the support prepared by method (b) showed the highest propane conversion and superior selectivity. The average conversion is 38.6% and the average selectivity is 95.2% during the reaction time of 25 h.
The platinum–tin–potassium-supported catalysts have drawn intense attention in paraffin dehydrogenation reactions.8–12 Over the platinum-based catalysts, it is benefit to improve the stability and selectivity while suppressing hydrogenolysis reactions, cracking and coke formation process. The addition of alkali metal K can generally neutralize some acid sites of the support. The interaction between metal Pt and Sn, the valence of Sn, the properties of the support and the interactions between metal and support strongly influence the catalytic properties of PtSnK trimetallic catalysts. Sn0 may be a poison, while it could also acts as a promoter as Sn4+ or Sn2+.13,14 Moreover, suitable textural properties of the support and strong interactions between loaded metals and support are beneficial for the improvement of catalyst performance for propane dehydrogenation.15,16 Zhang et al.17 found uniform pore size distribution and a relatively large surface area of the support were favorable for the improvement of the dispersion of active metallic particles. Supports which with large pore size is beneficial for coke to cover the external surface of the support first instead of the metallic surface. There are many papers studied the influence of different supports on the alkane dehydrogenation process,17–22 the supports they studied include ZSM-5, SBA-15, SiO2, spinels (ZnAl2O4, MgAl2O4), γ-Al2O3, etc. But the effects of the preparation methods of θ-Al2O3 supports on the catalytic propane dehydrogenation performances have not yet been reported so far. In addition, θ-Al2O3 was selected as support by UOP Oleflex process in the industry. So, the study about the influence of different θ-Al2O3 support on the catalytic propane dehydrogenation is necessary.
In this paper, four different synthesis ways of θ-Al2O3 supports have been employed, and were applied as Pt based catalyst supports for propane dehydrogenation. The performance of these catalysts on propane dehydrogenation was systematically compared. Many physicochemical characterization methods were applied to demonstrate the influence of synthesize routes on the properties of θ-Al2O3 supports and further clarify the influence on PtSnK propane dehydrogenation catalysts, then the most suitable support was selected.
The textural properties of the alumina supports were calculated from N2 adsorption–desorption isotherms collected at liquid nitrogen temperature by a volumetric adsorption system (Quantachrome Autosorb-1, American). All the samples were previously degassed for 5 h under vacuum hood at 300 °C. The Brunauer–Emmett–Teller (BET) method was applied to calculate the specific surface areas of the samples and the desorption branch of the isotherms was used to calculate the average pore diameter by the Barrett–Joyner–Halenda (BJH) pore size model.
The SEM images were taken using a scanning electron microscope (JSM-7800F) operating at 20 kV.
The acidic of each samples were measured by NH3-TPD experiments on a Micromeritics Auto-Chem II 2910 (American) chemisorption analyzer. Samples (0.10 g) which pre-dried at 120 °C overnight were placed in a U-type quartz sample tube. NH3 was saturated at 100 °C after pretreatment at 600 °C for 1 h under helium stream (30 mL min−1). Subsequently, the desorption of ammonia was determined by a thermal conductivity detector at temperatures from 100 °C to 600 °C at a temperature ramp rate of 10 °C min−1.
Temperature-programmed reduction (TPR) method was used to measure the reducibility of the catalysts by a Micromeritics Auto-Chem II 2910 apparatus (American). Samples (0.20 g) which pre-dried at 120 °C overnight were placed in a U-shaped quartz reactor. The samples were pretreated in situ by flowing dry argon (99.99%, 30 mL min−1) at 300 °C for 2 h. After cooled to room temperature, 10% H2 in Ar was switched and the samples were heated to 700 °C at a ramp rate of 10 °C min−1. The hydrogen consumption was monitored by a TCD detector.
The XPS data of Pt–Sn–K/θ-Al2O3 catalysts were investigated on ESCALAB 250Xi using Al Kα radiation. All catalysts were previously reduced in a hydrogen flow at 600 °C for 2 h. The binding energies (BE) were calibrated using the C1s level at 284.6 eV with an uncertainty of ±0.2 eV.
The Pt dispersion of the catalysts were measured by pulse chemisorption of CO experiments (Micromeritics AutoChem II 2910, American). Samples (0.10 g) were pretreated under a He stream at 500 °C for 1 h to remove the moistures and other impurities. Then the samples were reduced under H2 atmosphere (99.99%, 30 mL min−1) at 600 °C for 1 h, and purged in He (99.99%, 30 mL min−1) at 600 °C for 1 h, CO was saturated after cooling to 50 °C in flowing He (30 mL min−1). The amount of the chemisorbed CO was determined by a TCD detector. A stoichiometry of CO:
Pt = 1 was assumed to estimate the dispersion of Pt.
The coke content was measured by a thermo gravimetric and differential thermo analysis (TG-DTA) apparatus (TA Q600, P R China) with a temperature increasing rate of 10 °C min−1 from room temperature to 800 °C in an air flow of 50 mL min−1.
The propane conversion (X%) and the propylene selectivity (S) were calculated according to the following formulate:
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Fig. 1 (a) Propane conversion and (b) propylene selectivity over the four different catalysts (reaction conditions: T = 600 °C, H2![]() ![]() ![]() ![]() |
The isotherms and the pore size distributions of four alumina supports are presented in Fig. 3. According to the IUPAC classification,27 all samples exhibit type IV N2 adsorption isotherm with H1-type hysteresis loop, which suggested the presence of meso-pores or macro-pores.28,29 When the size of meso-pores in the alumina support decrease (Fig. 3b), the capillary condensation steps will slightly shift to lower relative pressures (Fig. 3a), which is in accord with the literature.15
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Fig. 3 Nitrogen adsorption–desorption isotherms and pore size distributions of different alumina supports. |
The textural properties such as surface area, pore size and pore volume of the samples were calculated from the nitrogen adsorption–desorption study and are listed in Table 1. It is obvious that the four supports are quite different in surface area, pore size distributions and total pore volume. Al2O3-C presented the largest surface area of 104.6 m2 g−1 with abundant small pores,30 while Al2O3-D with less pores only shows the smallest surface area as 56.6 m2 g−1. On the other hand, Al2O3-B shows the largest pore volume (0.49 cm3 g−1), pore diameter (24.2 nm) and the widest pore size distribution.
It is well known, propane dehydrogenation catalysts lost the activity mainly due to the accumulation of coke.31–33 From Fig. 4, we can see that the deactivation parameter of the four catalysts is nearly negatively linear correlated with the average pore diameter, that is to say, the stability of the catalyst increased with the increase of the pore diameter. The deactivation parameter decreased with the increase of the total pore volume too (see Fig. S2 in the ESI†). Those results proved that the catalyst support which have large pore size and pore volume can more effectively overcome diffusion and mass transfer limitations, and could help the coke deposits migrate from the active sites to supports, improve the stability of the catalysts. For Al2O3-B synthesized by hydrochloric acid reflux method has the largest pore volume (0.49 cm3 g−1), pore diameter (24.2 nm), the widest pore size distribution, we can speculate that Cat-B ought to be more resistant to coking, show best stability during the reaction, reasonably, and it fits the experimental results well.
The surface structure of alumina synthesized by different methods was revealed by a SEM technique. Fig. 5 showed the representative SEM images of the four alumina samples. It can be seen that Al2O3-A and Al2O3-B show wider pore size distribution and larger pore size, which is in accordance with the results of nitrogen adsorption–desorption study. The pores with diameter of more than 20 nm could also be found in the images. For Al2O3-C and Al2O3-D, most pores of less than 20 nm in diameter implied that the mesoporous structure is exist.
Catalysts | Pt dispersiona (%) | Average particle sizea (nm) | Average conversionb (%) | Coke contentsc (%) | Deactivation parameterd (%) |
---|---|---|---|---|---|
a Calculated by CO-pulse chemisorption of fresh PtSnK catalysts.b The average conversion of propane during the 25 h reaction.c The coke contents of the deactivated catalysts by thermogravimetric experiments.d Deactivation parameter for the 25 h reaction. | |||||
Cat-A | 51.1 | 2.2 | 37.8 | 2.61 | 9.4 |
Cat-B | 55.1 | 2.0 | 38.6 | 2.99 | 4.2 |
Cat-C | 37.5 | 3.0 | 37.3 | 3.66 | 12.9 |
Cat-D | 47.6 | 2.4 | 34.8 | 0.99 | 17.5 |
NH3-TPD method was employed to determine the acidity of solid catalysts. It is well known that the acid properties of the catalyst markedly affect the dehydrogenation performance of the catalyst. The acidity of the four Pt–Sn–K/Al2O3 catalysts was examined by NH3-TPD profiles as depicted in Fig. 6.
Gaussian deconvolution method was used to the semi-quantitative analysis of the ammonia desorption peaks. The calculated results are compiled in Table 3. All samples exhibit three peaks. For Cat-A, Cat-B and Cat-C, the peak centered around 210 °C (peak I), 320 °C (peak II) and 430–450 °C (peak III) should be attributed to weak, medium and strong acid sites, respectively. For Cat-D, the first (peak I) and the second (peak II) peak can be ascribed to weak acid sites, and the third peak (peak III) can be assigned to medium and strong acid sites.35,36 According to the total desorption peak area, it can be inferred that the order of the total acid content of the four catalysts are as follows: Cat-B > Cat-A ≈ Cat-C > Cat-D. Note that a majority of acidic centers exhibited on Cat-A, Cat-B and Cat-C is weak or medium acid sites. But as for Cat-D, the medium or strong acidic centers are dominant.
Catalysts | TM (°C) | Total acidity (mmol NH3 per g) | Peak fraction (%) | Fitted parameters (R2) | ||||
---|---|---|---|---|---|---|---|---|
I | II | III | I | II | III | |||
Cat-A | 216.7 | 314.7 | 432.4 | 0.27 | 0.47 | 0.39 | 0.14 | 0.9823 |
Cat-B | 221.3 | 330.1 | 451.3 | 0.30 | 0.51 | 0.41 | 0.08 | 0.9878 |
Cat-C | 208.8 | 319.4 | 436.0 | 0.26 | 0.56 | 0.30 | 0.14 | 0.9819 |
Cat-D | 183.5 | 232.2 | 345.1 | 0.16 | 0.16 | 0.21 | 0.63 | 0.9923 |
The average conversion of propane versus the total acidity of the four catalysts are depicted in Fig. 7. Interestingly, the relationship between the average conversion of propane and the total acidity of the four catalysts is linear. The results indicate that the acidity of the catalyst not only affect the distribution of the products, but also influence the propane dehydrogenation activity of the catalyst in our experiment.
The influences of the four alumina supports on the reduction properties were characterized by H2-TPR as shown in Fig. 8. It is evident that all the catalysts show at least three reduction peaks at about 250 °C (peak I), 440 °C (peak II) and 600 °C (peak III), respectively. For the signal at 250 °C is belong to the reduction of Pt oxides,37 whereas the peaks at high-temperature can be ascribed to the partial reduction of Sn4+ to Sn2+ and Sn2+ to Sn0, respectively.38,39 Cat-A presents an evident reduction peak higher than 600 °C which corresponding to the reduction of Sn2+ to Sn0. For Cat-B, the reduction peak corresponding to Sn2+ to Sn0 is the smallest, and most Sn can keep in the oxidation state, which can act as a promoter, this implying strong interactions of the Sn support.
The XPS spectra and the detailed data of Sn3d5/2 of Pt–Sn–K/θ-Al2O3 catalysts are shown in Fig. 9 and Table 4, respectively. The Sn3d5/2 XPS spectra of the four catalysts were deconvoluted into three peaks at ∼485.3, ∼486.5 and ∼487.5 eV in Fig. 9, corresponding to different chemical states of Sn. Generally speaking, the component at low binding energy (∼485.3 eV) is assigned to the reduced tin phase, either in the metallic (Sn0) or in the alloyed (SnPtx) state; whereas the two others (∼486.5 and ∼487.5 eV) are ascribed to oxides tin with different types (Sn2+ or Sn4+).33,40 However, Sn2+ and Sn4+ can't be discriminate according to XPS spectra alone because their binding energy is very close.41–43
Catalysts | Binding energy (eV) | ||
---|---|---|---|
Sn3d5/2 | |||
Cat-A | 485.3(25.4%) | 486.5(47.4%) | 487.5(27.2%) |
Cat-B | 485.2(17.7%) | 486.3(33.7%) | 487.3(48.6%) |
Cat-C | 485.8(27.2%) | 486.6(19.9%) | 487.5(52.9%) |
Cat-D | 485.5(30.6%) | 486.5(31.9%) | 487.4(37.5%) |
According to the peak percentage given in parentheses in Table 4, the zero valent Sn percentage of the four catalysts are 25.4%, 17.7%, 27.2% and 30.6%, respectively. Tin species in oxidized form is dominant. Sn in zero valent is harmful for catalyst performance and Sn in the oxidation state is benefit for propane dehydrogenation. The presence of SnOx can increase the catalytic stability for they can keep the Pt sites clean from coke deposits.44 This implies that different synthesis routes of alumina can influence the properties of the support, and an appropriate alumina synthesis route could strengthen the Sn–Al2O3 interaction, stabilizing the oxidized tin species. For the zerovalent Sn percentage of Cat-B is slightly lower than that of the other three catalysts, which is in accordance with the TPR experiment, indicates that the alumina synthesized by oil-dropped method can interact stronger with Sn species and restrain the reduction of tin species. Cat-B which have the lowest Sn0 percentage shows the highest average propane conversion and the lowest deactivation parameter.
As mentioned before, the main reason for the catalyst deactivation in such studies is coking.45,46 After the reaction of 25 h, the coke amount deposited over the four catalysts was analyzed by TG measurements as depicted in Fig. 10. The weight losses above 300 °C are attributed to the combustion of coke deposited on the catalysts.47 Fig. 10 illustrates that the contents of coke deposited on the four catalysts are 2.61%, 2.99%, 3.66% and 0.99%, respectively, there is no obvious correlation between the amount of coke on the spent catalysts and the final catalytic activity. The highest amount of coke is observed over Cat-C may due to its lowest propene selectivity, and the coke on Cat-D with the lowest propane conversion is the least. Although the amount of coke over Cat-B is more than Cat-A, the activity of Cat-B in propane dehydrogenation experiment is better. As mentioned before that large pore size and large pore volume is benefit to overcome mass transfer and diffusion limitations, and could help the coke deposit migration from active sites to supports. With the addition of HMT, the pore-forming mechanism of Al2O3-B is very different from the other three alumina support, the decomposition of HMT during the calcination process can produce relatively large pores for Al2O3-B. Therefore, we can conclude that the coke in Cat-B is mainly located on the alumina support and does not block active sites, and therefore, though the coke content of Cat-B is not the least, it keeps the best final catalytic activity and the best stability.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra02141k |
This journal is © The Royal Society of Chemistry 2017 |