Performance of Ethane Dehydrogenation over PtSn Loaded onto a Calcined Mg ( Al ) O LDH with Three Mg : Al Molar Ratios Using a Novel Method

Layered double hydroxide (LDH) is a layered solid containing positively charged layers with negatively charged anions as an interchangeable interlayer. In this research, Mg(Al)O supports were synthesized with three different Mg:Al molar ratios, and bimetallic PtSn catalysts were loaded onto the supports via the anion exchange method. The properties of ethane dehydrogenation of the PtSn/Mg(Al)O catalysts were investigated. The results show that the structure and properties of the PtSn/Mg(Al)O catalysts were influenced by the Mg:Al molar ratio of the hydrotalcites, which consequently influenced the ethane dehydrogenation performance. When the Mg:Al ratio was 5:1, the ethane dehydrogenation performance was optimal, relative to the Mg:Al ratios of 2:1 and 10:1.


Introduction
The production of light alkenes is an indicator of the developmental degree of the national petrochemical industry [1,2].Traditionally, light alkenes are produced through petroleum cracking.However, a shortage of oil resources and refining capacity severely limit the supply of light alkenes.Therefore, it is urgent to develop other means of producing light alkenes [3,4].With the rapid development of shell gas and natural gas, the catalytic dehydrogenation of light alkanes can be a more economical and environmentally friendly method of production when compared with traditional methods [1][2][3][4].
Platinum is widely studied for its use in the dehydrogenation of light alkanes.However, when only Pt is loaded onto the supports, there exists low olefin selectivity, and the rapid deactivation of catalysts occurs [5][6][7][8][9].It was found that the addition of Sn can suppress the coking reaction, as well as decrease the sintering of the metal, further inhibiting the deactivation of catalysts [10][11][12][13].
The catalytic behavior of the alkane dehydrogenation reaction is very dependent on the method of PtSn preparation [14][15][16].For example, Vertes et.al compared the alkane dehydrogenation performances of PtSn/γ-Al 2 O 3 catalysts prepared via two different methods.They found that different catalysts synthesized via two different methods had different states of tin (one was the ionic state, and the other was the superficial PtSn x alloy state).Furthermore, different alkane dehydrogenation performances were observed [15].Baronetti et al. found that co-impregnation or successive impregnation can also influence the final properties of PtSn [16].Preparation techniques play a significant role in controlling interactions that occur between tin and platinum, as well as those between these metals and the support, thus influencing catalytic performance [14,17].
The support is also crucial for ethane dehydrogenation.Good supports must be thermally stable and possess an appropriate level of acidity, a large surface area, and uniform pore-size distribution for the effective dehydrogenation of alkanes [2].Recent studies showed that the calcined layered double hydroxide is a great support for the light alkane dehydrogenation reaction.Mg(Al)O is an especially attractive support because it is moderately basic and exhibits high thermal stability.Moreover, the Al cations at the support surface help stabilize the dispersed metal particles against sintering [18][19][20][21].
A layered double hydroxide (LDH) is a layered solid containing positively charged layers and an exchangeable interlayered negative charge.The main structure of hydrotalcite is formed by a positively charged layer and an anion interlayer.The interlayered anions may be inorganic anions, organic anions, or heteropoly anions, and these interlayered anions have exchangeability properties [22,23].Therefore, the exchangeability of hydrotalcites can be used for the synthesis of catalysts via the anion exchange method [24].However, few papers report on the use of a PtSn/LDH catalyst synthesized via anion exchange for light alkane dehydrogenation.In a study by Bell et al., Pt, PtIr, and PtSn nanoparticles were synthesized in organic solvents and dispersed in toluene, before being added to a suspension of Mg(Al)O in anhydrous toluene.The mixture was stirred overnight, and the resulting product was precipitated by tripling the volume with the addition of anhydrous ethanol, before being separated by centrifugation.The solid material was heated at 723 K for 1 h in air to remove the surface capping agents, and was then reduced in an atmosphere of H 2 in Ar [13].The other method involved the introduction of Pt onto calcined Mg(Al)O via the incipient wetness impregnation of Pt(acetylacetonate) 2 dissolved in toluene [18].In Virnovskaia's work, active metals in the high-loading catalysts, Pt/Mg(Al)O, Sn/Mg-(Al)O, and PtSn/Mg(Al)O, were also deposited onto uncalcined hydrotalcite via the impregnation method [21].In our previous work, we used the anion exchange method to load the PtSn metals; however, that work aimed to determine the promotion effect of Ga for ethane dehydrogenation [20].Thus, the synthesis of a PtSn metal via the anion exchange method using the properties of LDHs is a novel method for alkane dehydrogenation.
In this study, Mg(Al)O LDHs were synthesized as supports with three different Mg:Al molar ratios, and the PtSn metal particles were prepared via the anion exchange method.The influences of loading PtSn via the anion exchange method were investigated, as was the effect of various Mg:Al molar ratios in the hydrotalcite supports on the performance of ethane dehydrogenation.

Synthesis of Mg(Al)O LDH with Different Mg:Al Ratios
The X-ray diffraction (XRD) patterns of Mg(Al)O LDH with different Mg:Al molar ratios are shown in Figure 1.In all cases, characteristic diffraction peaks of magnesium aluminum hydrotalcite materials were detected.Major peaks were detected for (003) at 11.7 • , (006) at 23.6 • , (110) at 60.9 • , and (113) at 62.3 • (JSPDS No. 035-0964).The intensity of the XRD increased with an increase in the Mg:Al molar ratio.This indicates that the structure of the Mg(Al)O LDHs improved with higher Mg:Al molar ratios.The XRD characterization also showed that the variation in the Mg:Al molar ratio influenced the crystal structure of the hydrotalcite carrier space.The basal spacing of (003) was calculated to be approximately 0.76 nm, 0.79 nm, and 0.91 nm when the Mg:Al molar ratios were 2:1, 5:1, and 10:1, respectively.This is due to the Al 3+ ionic radius being smaller than the Mg 2+ ionic radius; thus, an increase in the Mg:Al molar ratio results in an increase in the average distance of the crystal cell parameters with respect to the adjacent cation, and the charge density of the layer plane decreases, resulting in a reduction in both the subject layer plane and the electrostatic force of interlayer anions, subsequently increasing the spacing.All the TEM images show characteristics typical of magnesium aluminum hydrotalcite.The images reveal that when the Mg:Al ratio was 2:1, small pieces with a size of about 100 nm appeared (Figure 2a,b).When the Mg:Al ratio increased to 5:1, the small pieces became bigger and thinner (Figure 2c,d).When the Mg:Al ratio increased to 10:1, the pieces became much larger, appearing almost bulk in structure (Figure 2e,f).The TEM images are consistent with the XRD results.

Synthesis of PtSn/Mg(Al)O via the Anion Exchange Method
Figure 3 shows the schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.Hydrotalcite-like layered double hydroxides are a host-guest material consisting of positively charged metal-hydroxide sheets with intercalated anions and water molecules [25][26][27].Anions are capable of being intercalated into the interlayers of LDH [28][29][30].Figure 2 shows TEM images of the Mg(Al)O LDHs with the three different Mg:Al molar ratios.All the TEM images show characteristics typical of magnesium aluminum hydrotalcite.The images reveal that when the Mg:Al ratio was 2:1, small pieces with a size of about 100 nm appeared (Figure 2a,b).When the Mg:Al ratio increased to 5:1, the small pieces became bigger and thinner (Figure 2c,d).When the Mg:Al ratio increased to 10:1, the pieces became much larger, appearing almost bulk in structure (Figure 2e,f).The TEM images are consistent with the XRD results.All the TEM images show characteristics typical of magnesium aluminum hydrotalcite.The images reveal that when the Mg:Al ratio was 2:1, small pieces with a size of about 100 nm appeared (Figure 2a,b).When the Mg:Al ratio increased to 5:1, the small pieces became bigger and thinner (Figure 2c,d).When the Mg:Al ratio increased to 10:1, the pieces became much larger, appearing almost bulk in structure (Figure 2e,f).The TEM images are consistent with the XRD results.

Synthesis of PtSn/Mg(Al)O via the Anion Exchange Method
Figure 3 shows the schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.Hydrotalcite-like layered double hydroxides are a host-guest material consisting of positively charged metal-hydroxide sheets with intercalated anions and water molecules [25][26][27].Anions are capable of being intercalated into the interlayers of LDH [28][29][30].

Synthesis of PtSn/Mg(Al)O via the Anion Exchange Method
Figure 3 shows the schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.Hydrotalcite-like layered double hydroxides are a host-guest material consisting of positively charged metal-hydroxide sheets with intercalated anions and water molecules [25][26][27].Anions are capable of being intercalated into the interlayers of LDH [28][29][30].Therefore, we chose PtCl  Images of the PtSn/Mg(Al)O catalysts with different Mg:Al ratios were captured by TEM (Figure 4).All the images show that the calcined hydrotalcites and the metal particles were dispersed uniformly on the surface of the support.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding particle-size distributions of PtSn/Mg(Al)O with different Mg:Al ratios are shown in Figure 5. HAADF-STEM images provide further evidence of the average distribution and small PtSn particles on the supports.The mean diameters of the dispersed PtSn particles were 1.50 ± 0.3 nm, 2.26 ± 0.4 nm, and 2.57 ± 0.4 nm when the Mg:Al ratios were 2:1, 5:1, and 10:1, respectively (Figure 5).The results indicate that the particle sizes were less than 3 nm for all tested ratios, and a lower proportion of added Mg resulted in a better metallic particle size in this range.Images of the PtSn/Mg(Al)O catalysts with different Mg:Al ratios were captured by TEM (Figure 4).All the images show that the calcined hydrotalcites and the metal particles were dispersed uniformly on the surface of the support.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding particle-size distributions of PtSn/Mg(Al)O with different Mg:Al ratios are shown in Figure 5. HAADF-STEM images provide further evidence of the average distribution and small PtSn particles on the supports.The mean diameters of the dispersed PtSn particles were 1.50 ± 0.3 nm, 2.26 ± 0.4 nm, and 2.57 ± 0.4 nm when the Mg:Al ratios were 2:1, 5:1, and 10:1, respectively (Figure 5).The results indicate that the particle sizes were less than 3 nm for all tested ratios, and a lower proportion of added Mg resulted in a better metallic particle size in this range.Images of the PtSn/Mg(Al)O catalysts with different Mg:Al ratios were captured by TEM (Figure 4).All the images show that the calcined hydrotalcites and the metal particles were dispersed uniformly on the surface of the support.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding particle-size distributions of PtSn/Mg(Al)O with different Mg:Al ratios are shown in Figure 5. HAADF-STEM images provide further evidence of the average distribution and small PtSn particles on the supports.The mean diameters of the dispersed PtSn particles were 1.50 ± 0.3 nm, 2.26 ± 0.4 nm, and 2.57 ± 0.4 nm when the Mg:Al ratios were 2:1, 5:1, and 10:1, respectively (Figure 5).The results indicate that the particle sizes were less than 3 nm for all tested ratios, and a lower proportion of added Mg resulted in a better metallic particle size in this range.The actual percentages of the Pt and Sn to be ion-exchanged into the LDH for each sample were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) analyses.The actual amounts of Pt exchanged onto Mg(Al)O with different Mg:Al ratios were quite similar, with values of 0.34 wt%, 0.35 wt%, and 0.33 wt% (Table 1).The actual amounts of Sn exchanged into all LDHs were also quite similar, with values of 0.08 wt%, 0.09 wt%, and 0.08 wt% (Table 1).The SEM images of PtSn/Mg(Al)O with different Mg:Al molar ratios are displayed in Figure 6.The SEM images clearly indicate a highly sintered material, which is characteristic of solids consisting of aggregates.With increasing Mg:Al ratio, the particle sizes of the aggregates became increasingly large.The actual percentages of the Pt and Sn to be ion-exchanged into the LDH for each sample were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) analyses.The actual amounts of Pt exchanged onto Mg(Al)O with different Mg:Al ratios were quite similar, with values of 0.34 wt%, 0.35 wt%, and 0.33 wt% (Table 1).The actual amounts of Sn exchanged into all LDHs were also quite similar, with values of 0.08 wt%, 0.09 wt%, and 0.08 wt% (Table 1).The SEM images of PtSn/Mg(Al)O with different Mg:Al molar ratios are displayed in Figure 6.The SEM images clearly indicate a highly sintered material, which is characteristic of solids consisting of aggregates.With increasing Mg:Al ratio, the particle sizes of the aggregates became increasingly large.Ethylene selectivity was strongly related to the Mg:Al ratio.Figure 7b reveals that the PtSn/MgAlO-2 selectivity was 92.9% in the initial stage, before increasing rapidly to 98.8% at 8 min, and then slowly stabilizing for 120 min.For PtSn/MgAlO-5, the selectivity was 97.7% in the initial stage, before increasing rapidly to 99.3% at 8 min, and then slowly stabilizing for 120 min (>99%).For PtSn/MgAlO-10, the selectivity was 98.2% in the initial stage, before increasing rapidly to 98.7% at 8 min, and then slowly stabilizing for 120 min (>99%).Ethylene selectivity was strongly related to the Mg:Al ratio.Figure 7b reveals that the PtSn/MgAlO-2 selectivity was 92.9% in the initial stage, before increasing rapidly to 98.8% at 8 min, and then slowly stabilizing for 120 min.For PtSn/MgAlO-5, the selectivity was 97.7% in the initial stage, before increasing rapidly to 99.3% at 8 min, and then slowly stabilizing for 120 min (>99%).For PtSn/MgAlO-10, the selectivity was 98.2% in the initial stage, before increasing rapidly to 98.7% at 8 min, and then slowly stabilizing for 120 min (>99%).Ethylene selectivity was strongly related to the Mg:Al ratio.Figure 7b reveals that the PtSn/MgAlO-2 selectivity was 92.9% in the initial stage, before increasing rapidly to 98.8% at 8 min, and then slowly stabilizing for 120 min.For PtSn/MgAlO-5, the selectivity was 97.7% in the initial stage, before increasing rapidly to 99.3% at 8 min, and then slowly stabilizing for 120 min (>99%).

Catalytic Performances of PtSn/Mg(Al)O with Different Mg:Al Ratios in Ethane Dehydrogenation
For PtSn/MgAlO-10, the selectivity was 98.2% in the initial stage, before increasing rapidly to 98.7% at 8 min, and then slowly stabilizing for 120 min (>99%).
The selectivity of PtSn/Mg(Al)O-10 was the highest; however, its conversion was the lowest.The conversion of PtSn/Mg(Al)O-2 was intermediate; however, its selectivity was the lowest.The PtSn/Mg(Al)O-5 catalyst had the highest catalytic activity and a relatively high selectivity.Thus, the moderate Mg:Al ratio of the PtSn/Mg(Al)O catalyst is appropriate for the ethane dehydrogenation reaction.[31].With an in the Mg:Al molar ratio, the intensity of the diffraction peaks increased.Moreover, there is no signal from the Pt or Sn species because of their low concentrations.
Figure 8b shows the formation of the crystalline phase-which occurred during the calcination of Mg(Al)O LDHs-at various temperatures between 50 • C and 650 • C. The calcined products of LDHs are very reactive mixed oxides, called layered double oxides (LDOs) [32].The in situ XRD patterns show that the whole process was divided into three temperature regions: 50-200 • C, 200-300 • C, and 300-650 • C. For the first region of calcination, from 50-200 • C, the main diffraction peaks were attributed to the structure of LDHs.In this stage, the main process was the removal of physically absorbed water, which did not change the structure of LDHs [32].For the second region, with temperatures between 250 and 300 • C, the peak at 13 • -corresponding to the plane (003) of LDHs-disappeared, and the diffraction peaks were slightly broader; however, the layer structure still existed.As the temperature continued increasing, the diffraction peaks attributed to the LDHs totally disappeared, indicating that the layer structure collapsed completely at this point.When the calcination temperature reached 350 • C, two broad diffraction peaks appeared, which were roughly attributed to a periclase MgO-like mixed oxide structure.As the calcination temperature increased to 650 • C, there was no obvious change in the diffraction peak positions.Meanwhile, the intensity of the diffraction peaks increased.However, this does mean that crystallinity increased with increasing temperature.
The selectivity of PtSn/Mg(Al)O-10 was the highest; however, its conversion was the lowest.The conversion of PtSn/Mg(Al)O-2 was intermediate; however, its selectivity was the lowest.The PtSn/Mg(Al)O-5 catalyst had the highest catalytic activity and a relatively high selectivity.Thus, the moderate Mg:Al ratio of the PtSn/Mg(Al)O catalyst is appropriate for the ethane dehydrogenation reaction.

Discussion of PtSn/Mg(Al)O with Different Mg:Al Ratios in Ethane Dehydrogenation
Figure 8a presents the XRD patterns of Mg(Al)O LDHs loaded with PtSn via the anion exchange method after calcination, resulting in the characteristic structure of periclase (MgO).The diffraction peaks of hydrotalcite disappeared.Previous works showed that the calcination products of magnesium aluminum hydrotalcite have a MgO-like structure, where Al and Ga ions are dissolved in the lattice, forming a solid solution [31].With an increase in the Mg:Al molar ratio, the intensity of the diffraction peaks increased.Moreover, there is no signal from the Pt or Sn species because of their low concentrations.
Figure 8b shows the formation of the crystalline phase-which occurred during the calcination of Mg(Al)O LDHs-at various temperatures between 50 °C and 650 °C.The calcined products of LDHs are very reactive mixed oxides, called layered double oxides (LDOs) [32].The in situ XRD patterns show that the whole process was divided into three temperature regions: 50-200 °C, 200-300 °C, and 300-650 °C.For the first region of calcination, from 50-200 °C, the main diffraction peaks were attributed to the structure of LDHs.In this stage, the main process was the removal of physically absorbed water, which did not change the structure of LDHs [32].For the second region, with temperatures between 250 and 300 °C, the peak at 13°-corresponding to the plane (003) of LDHsdisappeared, and the diffraction peaks were slightly broader; however, the layer structure still existed.As the temperature continued increasing, the diffraction peaks attributed to the LDHs totally disappeared, indicating that the layer structure collapsed completely at this point.When the calcination temperature reached 350 °C, two broad diffraction peaks appeared, which were roughly attributed to a periclase MgO-like mixed oxide structure.As the calcination temperature increased to 650 °C, there was no obvious change in the diffraction peak positions.Meanwhile, the intensity of the diffraction peaks increased.However, this does mean that crystallinity increased with increasing temperature.To evaluate the textural properties of the series of PtSn/Mg(Al)O catalysts, a low-temperature N2 adsorption-desorption isotherm experiment was carried out, and the results are shown in Figure 9.All samples revealed typical V-type adsorption isotherms, indicating a typical mesoporous structure.Table 1 shows the low-temperature N2 adsorption-desorption data of the catalysts.The SBET values were 188 m 2 /g, 113 m 2 /g, and 85 m 2 /g when the Mg:Al ratios were 2:1, 5:1, and 10:1, respectively.The results mean that the SBET value is inversely correlated with the Mg:Al ratio.To evaluate the textural properties of the series of PtSn/Mg(Al)O catalysts, a low-temperature N 2 adsorption-desorption isotherm experiment was carried out, and the results are shown in Figure 9.All samples revealed typical V-type adsorption isotherms, indicating a typical mesoporous structure.
Table 1 shows the low-temperature N 2 adsorption-desorption data of the catalysts.The S BET values were 188 m 2 /g, 113 m 2 /g, and 85 m 2 /g when the Mg:Al ratios were 2:1, 5:1, and 10:1, respectively.The results mean that the S BET value is inversely correlated with the Mg:Al ratio.Furthermore, the pore volume has a tendency to be positively correlated with the Mg:Al ratio, with values of 0.30 cm 3 /g, 0.65 cm 3 /g, and 0.54 cm 3 /g, respectively.The results indicate that the structure of PtSn/Mg(Al)O changed to some extent.According to the textural parameters in Table 2, the PtSn/Mg(Al)O-5 sample had a moderate S BET value and the highest pore volume.The surface structure and pore volume have a great influence on the activity and selectivity of alkane dehydrogenation.A relatively large surface area and uniform pore-size distribution can lead to an optimal distribution of the PtSn particle structure, thus resulting in a higher ethane dehydrogenation activity.Furthermore, the pore volume has a tendency to be positively correlated with the Mg:Al ratio, with values of 0.30 cm 3 /g, 0.65 cm 3 /g, and 0.54 cm 3 /g, respectively.The results indicate that the structure of PtSn/Mg(Al)O changed to some extent.According to the textural parameters in Table 2, the PtSn/Mg(Al)O-5 sample had a moderate SBET value and the highest pore volume.The surface structure and pore volume have a great influence on the activity and selectivity of alkane dehydrogenation.A relatively large surface area and uniform pore-size distribution can lead to an optimal distribution of the PtSn particle structure, thus resulting in a higher ethane dehydrogenation activity.It is well known that optimal acidity and basicity of the supports are needed to avoid undesirable side reactions.Mg(Al)O is particularly attractive because it has moderate acid-base properties, and it exhibits high thermal stability to steam and reduction-oxidation cycling [2].Therefore, the ethane dehydrogenation activity is also related to the support's acidity and basicity [2].The acidic properties were investigated using the temperature-programmed desorption of NH3 (NH3-TPD) method, and the results are shown in Figure 10.For the PtSn/Mg(Al)O-2 and PtSn/Mg(Al)O-5 samples, the results show three peaks around 217 °C, 511 °C, and 590 °C, attributed to weak, medium, and strong acid sites, respectively [17,[33][34][35].The pulse results show that total acidity was 1242.2 mol/g and 945.0 mol/g, respectively.For PtSn/Mg(Al)O-10, the peak ascribed to moderate acid sites disappeared, and the total acidity was 655.0 mol/g.It can be seen that the total acidity of the catalysts decreased with an increase in Mg:Al molar ratio.This is due to the presence of a greater proportion of Al, which creates more acid sites.The increase in the number of acid sites in the catalyst can improve the dehydrogenation performance; however, it can also lead to deep cracking of dehydrogenation products, which facilitates carbon inactivation of the catalysts.It is well known that optimal acidity and basicity of the supports are needed to avoid undesirable side reactions.Mg(Al)O is particularly attractive because it has moderate acid-base properties, and it exhibits high thermal stability to steam and reduction-oxidation cycling [2].Therefore, the ethane dehydrogenation activity is also related to the support's acidity and basicity [2].The acidic properties were investigated using the temperature-programmed desorption of NH 3 (NH 3 -TPD) method, and the results are shown in Figure 10.For the PtSn/Mg(Al)O-2 and PtSn/Mg(Al)O-5 samples, the results show three peaks around 217 • C, 511 • C, and 590 • C, attributed to weak, medium, and strong acid sites, respectively [17,[33][34][35].The pulse results show that total acidity was 1242.2 µmol/g and 945.0 µmol/g, respectively.For PtSn/Mg(Al)O-10, the peak ascribed to moderate acid sites disappeared, and the total acidity was 655.0 µmol/g.It can be seen that the total acidity of the catalysts decreased with an increase in Mg:Al molar ratio.This is due to the presence of a greater proportion of Al, which creates more acid sites.The increase in the number of acid sites in the catalyst can improve the dehydrogenation performance; however, it can also lead to deep cracking of dehydrogenation products, which facilitates carbon inactivation of the catalysts.The temperature-programmed desorption of CO2 (CO2-TPD) was used to detect the surface basicity of the catalyst series.Figure 11 reveals three CO2 desorption peaks for all samples, two of which were around 200 °C and 290 °C, attributed to weak and moderate basicity sites; the third peak was around 500 °C, and was attributed to strong basicity [33][34][35].As the Mg:Al ratio increased, the desorption peaks ascribed to weak and medium basicity shifted to higher temperatures, and the intensity increased.The results in Figures 10 and 11 prove that the acidity and basicity of the catalysts are markedly influenced by the Mg:Al ratio, which influence the properties of the ethane dehydrogenation reaction.Based on previous studies, the increased acidity of catalysts can improve the activity of ethane dehydrogenation, and too much acidity may lead to cracking of products and the deactivation of catalysts due to coking [36].All these results suggest that a moderate Mg:Al ratio affords the catalyst with the appropriate acidity and basicity, which are beneficial to ethane dehydrogenation reaction.
Results of the catalytic stability test are given in Figure 12.A 700-min ethane dehydrogenation reaction was processed for the PtSn/Mg(Al)O-5 catalyst.From Figure 12a, the initial value of ethane conversion was 21.5%, and the ethane conversion was ultimately maintained above 14% after 700 min.In Galvita's work, PtSn/Mg(Al)O was used for ethane dehydrogenation, and the catalyst exhibited a rapid loss in conversion activity during the first 40 min.The conversion of ethane to products decreased from approximately 10% to 2.6% for PtSn/Mg(Al)O [37].In Zhang's work, the The temperature-programmed desorption of CO 2 (CO 2 -TPD) was used to detect the surface basicity of the catalyst series.Figure 11 reveals three CO 2 desorption peaks for all samples, two of which were around 200 • C and 290 • C, attributed to weak and moderate basicity sites; the third peak was around 500 • C, and was attributed to strong basicity [33][34][35].As the Mg:Al ratio increased, the desorption peaks ascribed to weak and medium basicity shifted to higher temperatures, and the intensity increased.The temperature-programmed desorption of CO2 (CO2-TPD) was used to detect the surface basicity of the catalyst series.Figure 11 reveals three CO2 desorption peaks for all samples, two of which were around 200 °C and 290 °C, attributed to weak and moderate basicity sites; the third peak was around 500 °C, and was attributed to strong basicity [33][34][35].As the Mg:Al ratio increased, the desorption peaks ascribed to weak and medium basicity shifted to higher temperatures, and the intensity increased.The results in Figures 10 and 11 prove that the acidity and basicity of the catalysts are markedly influenced by the Mg:Al ratio, which influence the properties of the ethane dehydrogenation reaction.Based on previous studies, the increased acidity of catalysts can improve the activity of ethane dehydrogenation, and too much acidity may lead to cracking of products and the deactivation of catalysts due to coking [36].All these results suggest that a moderate Mg:Al ratio affords the catalyst with the appropriate acidity and basicity, which are beneficial to ethane dehydrogenation reaction.
Results of the catalytic stability test are given in Figure 12.A 700-min ethane dehydrogenation reaction was processed for the PtSn/Mg(Al)O-5 catalyst.From Figure 12a, the initial value of ethane conversion was 21.5%, and the ethane conversion was ultimately maintained above 14% after 700 min.In Galvita's work, PtSn/Mg(Al)O was used for ethane dehydrogenation, and the catalyst exhibited a rapid loss in conversion activity during the first 40 min.The conversion of ethane to products decreased from approximately 10% to 2.6% for PtSn/Mg(Al)O [37].In Zhang's work, the The results in Figures 10 and 11 prove that the acidity and basicity of the catalysts are markedly influenced by the Mg:Al ratio, which influence the properties of the ethane dehydrogenation reaction.Based on previous studies, the increased acidity of catalysts can improve the activity of ethane dehydrogenation, and too much acidity may lead to cracking of products and the deactivation of catalysts due to coking [36].All these results suggest that a moderate Mg:Al ratio affords the catalyst with the appropriate acidity and basicity, which are beneficial to ethane dehydrogenation reaction.
Results of the catalytic stability test are given in Figure 12.A 700-min ethane dehydrogenation reaction was processed for the PtSn/Mg(Al)O-5 catalyst.From Figure 12a, the initial value of ethane conversion was 21.5%, and the ethane conversion was ultimately maintained above 14% after 700 min.In Galvita's work, PtSn/Mg(Al)O was used for ethane dehydrogenation, and the catalyst exhibited a rapid loss in conversion activity during the first 40 min.The conversion of ethane to products decreased from approximately 10% to 2.6% for PtSn/Mg(Al)O [37].In Zhang's work, the ethane conversion of PtSn/Mg(Al)O catalysts decreased from 25% to 8.2%.The percentage of decrease was 66.8% during a 14-h reaction [20].For the ethane dehydrogenation process in the current study, the results indicate that the catalyst possesses good catalytic stability compared with other works in the literature.The PtSn/Mg(Al)O-5 catalyst possessed a relatively high ethylene selectivity (>98.5%)during the initial stage, and possessed nearly 100% selectivity after the entire 700-min dehydrogenation reaction (Figure 12b).
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 14 ethane conversion of PtSn/Mg(Al)O catalysts decreased from 25% to 8.2%.The percentage of decrease was 66.8% during a 14-h reaction [20].For the ethane dehydrogenation process in the current study, the results indicate that the catalyst possesses good catalytic stability compared with other works in the literature.The PtSn/Mg(Al)O-5 catalyst possessed a relatively high ethylene selectivity (>98.5%)during the initial stage, and possessed nearly 100% selectivity after the entire 700-min dehydrogenation reaction (Figure 12b).13f shows that a large quantity of coke carbon was generated after C2H6 dehydrogenation.The thickness of the generated amorphous carbon layer was nearly 40 nm. Figure 13g,h provide energy dispersive X-ray (EDX) element mapping images of PtSn/Mg(Al)O-5 before and after C2H6 dehydrogenation.The distribution of magnesium and aluminum were uniform on the support before and after ethane dehydrogenation.The distributions of platinum and tin appear dispersive, indicating a uniform distribution of the metal particles.Furthermore, it can be seen from the mapping images in Figure 12g,h that the formation of carbon obviously increased after the reaction.The main reason for the deactivation of the catalyst is carbon deposition onto the catalyst surface.13f shows that a large quantity of coke carbon was generated after C 2 H 6 dehydrogenation.The thickness of the generated amorphous carbon layer was nearly 40 nm. Figure 13g,h provide energy dispersive X-ray (EDX) element mapping images of PtSn/Mg(Al)O-5 before and after C 2 H 6 dehydrogenation.The distribution of magnesium and aluminum were uniform on the support before and after ethane dehydrogenation.The distributions of platinum and tin appear dispersive, indicating a uniform distribution of the metal particles.Furthermore, it can be seen from the mapping images in Figure 12g,h that the formation of carbon obviously increased after the reaction.The main reason for the deactivation of the catalyst is carbon deposition onto the catalyst surface.
To identify the loss of carbon from the gas stream due to coke formation, the carbon balance was calculated from the ratio of the overall hydrocarbon amounts after the reaction to the initial ethane amount.High carbon balances (96.1%, 96.5%, and 98.8%) were obtained over the PtSn/Mg(Al)O-2, PtSn/Mg(Al)O-5, and PtSn/Mg(Al)O-10 catalysts, respectively, at 120 min, indicating that the majority of ethane was dehydrogenated during the reaction, with some loss of carbon due to coke formation.To identify the loss of carbon from the gas stream due to coke formation, the carbon balance was calculated from the ratio of the overall hydrocarbon amounts after the reaction to the initial ethane amount.High carbon balances (96.1%, 96.5%, and 98.8%) were obtained over the PtSn/Mg(Al)O-2, PtSn/Mg(Al)O-5, and PtSn/Mg(Al)O-10 catalysts, respectively, at 120 min, indicating that the majority of ethane was dehydrogenated during the reaction, with some loss of carbon due to coke formation.
It was reported that coke formation is related to the behavior of ethylene absorbed onto the active sites of the catalyst, and that strong acidity can promote carbon deposition [1,2].Our results show that the total acidity of the catalysts decreased with an increase in Mg:Al molar ratio.This is due to the presence a greater proportion of Al, thus generating more acid sites.The increase in the number of acid sites on the catalyst can improve the dehydrogenation performance; however, it can also lead to deep cracking of dehydrogenation products, which enables carbon inactivation of the catalysts.Thus, a moderate Mg:Al ratio is best suited for ethane dehydrogenation.

Materials and Methods
Synthesis of Mg(Al)O LDH: Hydrotalcites were synthesized using the co-precipitation method.Different molar ratios of Mg(NO3)2•6H2O (Macklin Co., AR., Shanghai, China) (3.33 g, 4.17 g, and 8.33 g) and Al(NO3)3•9H2O (Macklin Co., AR., Shanghai, China) (2.57g) were dissolved in 70 mL of deionized water.NaOH (Sinopharm Chemical Reagent Co., AR., Shanghai, China) and Na2CO3 (Sinopharm Chemical Reagent Co., AR., Shanghai, China) were also dissolved in deionized water in another beaker, before adding the two solutions dropwise to the third beaker with some deionized water.The solution was stirred vigorously at room temperature.The resulting suspension was aged overnight at 100 °C, before being filtered and washed with deionized water.It was then dried at 100 °C for 24 h.Synthesis of PtSn/Mg(Al)O catalysts via the anion exchange method: PtSn/Mg(Al)O was prepared via the anion exchange method.Using a typical procedure, the hydrotalcite (LDH) was exchanged in a mixed solution of K2PtCl6 (Aladdin Reagents (Shanghai) Co., Ltd., Shanghai, China) It was reported that coke formation is related to the behavior of ethylene absorbed onto the active sites of the catalyst, and that strong acidity can promote carbon deposition [1,2].Our results show that the total acidity of the catalysts decreased with an increase in Mg:Al molar ratio.This is due to the presence a greater proportion of Al, thus generating more acid sites.The increase in the number of acid sites on the catalyst can improve the dehydrogenation performance; however, it can also lead to deep cracking of dehydrogenation products, which enables carbon inactivation of the catalysts.Thus, a moderate Mg:Al ratio is best suited for ethane dehydrogenation.

Materials and Methods
Synthesis of Mg(Al)O LDH: Hydrotalcites were synthesized using the co-precipitation method.Different molar ratios of Mg(NO 3 ) 2 •6H 2 O (Macklin Co., AR., Shanghai, China) (3.33 g, 4.17 g, and 8.33 g) and Al(NO 3 ) 3 •9H 2 O (Macklin Co., AR., Shanghai, China) (2.57g) were dissolved in 70 mL of deionized water.NaOH (Sinopharm Chemical Reagent Co., AR., Shanghai, China) and Na 2 CO 3 (Sinopharm Chemical Reagent Co., AR., Shanghai, China) were also dissolved in deionized water in another beaker, before adding the two solutions dropwise to the third beaker with some deionized water.The solution was stirred vigorously at room temperature.The resulting suspension was aged overnight at 100 • C, before being filtered and washed with deionized water.It was then dried at 100 • C for 24 h.Synthesis of PtSn/Mg(Al)O catalysts via the anion exchange method: PtSn/Mg(Al)O was prepared via the anion exchange method.Using a typical procedure, the hydrotalcite (LDH) was exchanged in a mixed solution of K 2 PtCl 6 (Aladdin Reagents (Shanghai) Co., Ltd., Shanghai, China) and Na 2 SnO 3 (Aladdin Reagents (Shanghai) Co., Ltd., Shanghai, China) at 70 • C for 24 h with moderate stirring.The resulting solution was filtered and washed with deionized water, and was then dried at 100 • C overnight, before being reduced in H 2 /N 2 (10 vol.%) at 873 K for 3 h.
Ethane dehydrogenation: Ethane dehydrogenation was carried out in a fixed-bed quartz reactor with an inner diameter of 8 mm under atmospheric pressure.The catalyst (0.1 g of PtSn/Mg(Al)O after anion exchange) was diluted with 0.5 g of quartz particles, with sizes ranging from 0.25-0.5 mm.The mixed catalysts were put in the center of the quartz tube, before being reduced in a 10% H 2 /N 2 atmosphere at 873 K for 3 h.The ethane dehydrogenation reaction was performed in a 10% C 2 H 6 /N 2 atmosphere at 823 K.The reaction products were analyzed using gas chromatography (GC; Agilent 7890 B, Agilent, Santa Clara, CA, USA) equipped with two thermal conductivity detectors (TCDs) and a flame ionization detector (FID).The FID was used to measure the concentrations of all organic compounds, including CH 4 , C 2 H 6 , and C 2 H 4 .One TCD was used to measure the concentrations of N 2 , and the other was used to quantify the concentrations of H 2 .
Ethane conversion and ethane selectivity were defined as follows: C Catalyst characterizations: The SEM images were obtained using a field-emission scanning electron microscope (SEM; SU-70, Hitachi Corporation, Tokyo, Japan) operated at 5 kV.The TEM images were obtained using a transmission electron microscope (TEM; JEM-2010F, JEOL, Tokyo, Japan) operated at 120 kV.The XRD analysis was characterized by an X'Pert Pro MPD diffractometer (PW 3040/60, PANalytical, Almelo, Netherlands) with a scan rate of 5 • /min.It was operated at 40 kV, and the applied current was 100 mA.The Brunauer-Emmett-Teller (BET, Quantachrome, Boynton Beach, FL, USA) surface area characterizations were measured via N 2 adsorption-desorption using an ASIQMO002-2 analyzer (Quantachrome, Boynton Beach, FL, USA).The average pore diameters were calculated using the Barrett-Joyner-Halenda (BJH) pore-size model.Temperature-programmed desorption experiments (NH 3 -TPD and CO 2 -TPD) were performed with a CPB-1 analyzer (Quantachrome, Boynton Beach, FL, USA).The ICP-AES analyses were carried out with an inductively coupled plasma atomic spectrometer (ICPS-8100, Shimadzu, Japan).

Figure 2
Figure 2 shows TEM images of the Mg(Al)O LDHs with the three different Mg:Al molar ratios.All the TEM images show characteristics typical of magnesium aluminum hydrotalcite.The images reveal that when the Mg:Al ratio was 2:1, small pieces with a size of about 100 nm appeared (Figure2a,b).When the Mg:Al ratio increased to 5:1, the small pieces became bigger and thinner (Figure2c,d).When the Mg:Al ratio increased to 10:1, the pieces became much larger, appearing almost bulk in structure (Figure2e,f).The TEM images are consistent with the XRD results.

Figure 2
Figure 2 shows TEM images of the Mg(Al)O LDHs with the three different Mg:Al molar ratios.All the TEM images show characteristics typical of magnesium aluminum hydrotalcite.The images reveal that when the Mg:Al ratio was 2:1, small pieces with a size of about 100 nm appeared (Figure2a,b).When the Mg:Al ratio increased to 5:1, the small pieces became bigger and thinner (Figure2c,d).When the Mg:Al ratio increased to 10:1, the pieces became much larger, appearing almost bulk in structure (Figure2e,f).The TEM images are consistent with the XRD results.

Figure 3 .
Figure 3. Schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.

Figure 3 .
Figure 3. Schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.

Figure 3 .
Figure 3. Schematic illustration of loading Pt and Sn precursors onto Mg(Al)O LDHs through the anion exchange method.

Figure 7
Figure 7 shows the ethane dehydrogenation reaction performances over PtSn/Mg(Al)O catalysts with different Mg:Al ratios.In Figure 7a, for the sample of PtSn/Mg(Al)O-5, the initial ethane conversion was 18.7%, and the ethane conversion was nearly 20% throughout the 120-min reaction stage.The same phenomenon occurred for PtSn/Mg(Al)O-2.The ethane conversion was consistently almost 18% for the entire stage.For the PtSn/Mg(Al)O-10 sample, the ethane conversion was 18.1% in the initial stage, before decreasing quickly during the first 20 min, and ultimately decreasing to 11.1% after 120 min.The results demonstrate that, when the Mg:Al ratio was 5:1, ethane conversion exhibited the highest catalytic activity.

2. 3 .
Catalytic Performances of PtSn/Mg(Al)O with Different Mg:Al Ratios in Ethane Dehydrogenation

Figure 7
Figure 7 shows the ethane dehydrogenation reaction performances over PtSn/Mg(Al)O catalysts with different Mg:Al ratios.In Figure 7a, for the sample of PtSn/Mg(Al)O-5, the initial ethane conversion was 18.7%, and the ethane conversion was nearly 20% throughout the 120-min reaction stage.The same phenomenon occurred for PtSn/Mg(Al)O-2.The ethane conversion was consistently almost 18% for the entire stage.For the PtSn/Mg(Al)O-10 sample, the ethane conversion was 18.1% in the initial stage, before decreasing quickly during the first 20 min, and ultimately decreasing to 11.1% after 120 min.The results demonstrate that, when the Mg:Al ratio was 5:1, ethane conversion exhibited the highest catalytic activity.

Figure 7
Figure 7 shows the ethane dehydrogenation reaction performances over PtSn/Mg(Al)O catalysts with different Mg:Al ratios.In Figure 7a, for the sample of PtSn/Mg(Al)O-5, the initial ethane conversion was 18.7%, and the ethane conversion was nearly 20% throughout the 120-min reaction stage.The same phenomenon occurred for PtSn/Mg(Al)O-2.The ethane conversion was consistently almost 18% for the entire stage.For the PtSn/Mg(Al)O-10 sample, the ethane conversion was 18.1% in the initial stage, before decreasing quickly during the first 20 min, and ultimately decreasing to 11.1% after 120 min.The results demonstrate that, when the Mg:Al ratio was 5:1, ethane conversion exhibited the highest catalytic activity.

2. 4 .
Figure8apresents the XRD patterns of Mg(Al)O LDHs loaded with PtSn via the anion exchange method after calcination, resulting in the characteristic structure of periclase (MgO).The diffraction peaks of hydrotalcite disappeared.Previous works showed that the calcination products of magnesium aluminum hydrotalcite have a MgO-like structure, where Al and Ga ions are dissolved in the lattice, forming a solid solution[31].With an in the Mg:Al molar ratio, the intensity of the diffraction peaks increased.Moreover, there is no signal from the Pt or Sn species because of their low concentrations.Figure8bshows the formation of the crystalline phase-which occurred during the calcination of Mg(Al)O LDHs-at various temperatures between 50 • C and 650 • C. The calcined products of LDHs are very reactive mixed oxides, called layered double oxides (LDOs)[32].The in situ XRD patterns show that the whole process was divided into three temperature regions: 50-200 • C, 200-300 • C, and 300-650 • C. For the first region of calcination, from 50-200 • C, the main diffraction peaks were attributed to the structure of LDHs.In this stage, the main process was the removal of physically absorbed water, which did not change the structure of LDHs[32].For the second region, with temperatures between 250 and 300 • C, the peak at 13 • -corresponding to the plane (003) of LDHs-disappeared, and the diffraction peaks were slightly broader; however, the layer structure still existed.As the temperature continued increasing, the diffraction peaks attributed to the LDHs totally disappeared, indicating that the layer structure collapsed completely at this point.When the calcination temperature reached 350 • C, two broad diffraction peaks appeared, which were roughly attributed to a periclase MgO-like mixed oxide structure.As the calcination temperature increased to 650 • C, there was no obvious change in the diffraction peak positions.Meanwhile, the intensity of the diffraction peaks increased.However, this does mean that crystallinity increased with increasing temperature.

Figure 13
Figure 13 displays the PtSn/Mg(Al)O-5 catalyst before and after the C2H6 dehydrogenation reaction.Figure 13a,b show the SEM images of PtSn/Mg(Al)O-5 before C2H6 dehydrogenation reaction, and Figure 13d,e show the SEM images of PtSn/Mg(Al)O-5 after C2H6 dehydrogenation.The morphology indicates that clusters of aggregate crystal particles formed from the flake structure after calcination.Figure 13c,f show the TEM images of PtSn/Mg(Al)O-5 before and after the C2H6 dehydrogenation reaction.The PtSn metal particles were dispersed uniformly on the surface of the support before the reaction.The TEM image in Figure13fshows that a large quantity of coke carbon was generated after C2H6 dehydrogenation.The thickness of the generated amorphous carbon layer was nearly 40 nm.Figure13g,h provide energy dispersive X-ray (EDX) element mapping images of PtSn/Mg(Al)O-5 before and after C2H6 dehydrogenation.The distribution of magnesium and aluminum were uniform on the support before and after ethane dehydrogenation.The distributions of platinum and tin appear dispersive, indicating a uniform distribution of the metal particles.Furthermore, it can be seen from the mapping images in Figure12g,h that the formation of carbon obviously increased after the reaction.The main reason for the deactivation of the catalyst is carbon deposition onto the catalyst surface.

Figure 13
Figure 13 displays the PtSn/Mg(Al)O-5 catalyst before and after the C 2 H 6 dehydrogenation reaction.Figure 13a,b show the SEM images of PtSn/Mg(Al)O-5 before C 2 H 6 dehydrogenation reaction, and Figure 13d,e show the SEM images of PtSn/Mg(Al)O-5 after C 2 H 6 dehydrogenation.The morphology indicates that clusters of aggregate crystal particles formed from the flake structure after calcination.Figure 13c,f show the TEM images of PtSn/Mg(Al)O-5 before and after the C 2 H 6 dehydrogenation reaction.The PtSn metal particles were dispersed uniformly on the surface of the support before the reaction.The TEM image in Figure13fshows that a large quantity of coke carbon was generated after C 2 H 6 dehydrogenation.The thickness of the generated amorphous carbon layer was nearly 40 nm.Figure13g,h provide energy dispersive X-ray (EDX) element mapping images of PtSn/Mg(Al)O-5 before and after C 2 H 6 dehydrogenation.The distribution of magnesium and aluminum were uniform on the support before and after ethane dehydrogenation.The distributions of platinum and tin appear dispersive, indicating a uniform distribution of the metal particles.Furthermore, it can be seen from the mapping images in Figure12g,h that the formation of carbon obviously increased after the reaction.The main reason for the deactivation of the catalyst is carbon deposition onto the catalyst surface.To identify the loss of carbon from the gas stream due to coke formation, the carbon balance was calculated from the ratio of the overall hydrocarbon amounts after the reaction to the initial ethane amount.High carbon balances (96.1%, 96.5%, and 98.8%) were obtained over the PtSn/Mg(Al)O-2, PtSn/Mg(Al)O-5, and PtSn/Mg(Al)O-10 catalysts, respectively, at 120 min, indicating that the majority of ethane was dehydrogenated during the reaction, with some loss of carbon due to coke formation.

Table 1 .
The inductively coupled plasma atomic emission spectrometry (ICP-AES) elemental analysis results revealing the amounts of Pt and Sn to be ion-exchanged into the layered double hydroxide (LDH) layer.

Table 1 .
The inductively coupled plasma atomic emission spectrometry (ICP-AES) elemental analysis results revealing the amounts of Pt and Sn to be ion-exchanged into the layered double hydroxide (LDH) layer.
2 H 6 conversion = C 2 H 6 in − C 2 H 6 out C 2 H 6 in H 6 in and C 2 H 6 out are the ethane content in the feed and exit gases, respectively, and C 2 H 4 out is the ethylene content in the exit gas.The carbon balance was calculated as follows: Carbon balance = C x H y (out)/C 2 H 6 (in) × 100%.