SUPPORTED CORE-SHELL BIMETALLIC CATALYST WITH HIGH SELECTIVITY FOR PROPANE DEHYDROGENATION

Abstract

A supported core-shell bimetallic catalyst with high selectivity for propane dehydrogenation, containing platinum (Pt) as active species, 3d transition metals (Fe, Co and Ni) as promoters and SBA-15 as support. The addition of 3d metals and the formation of Pt3d alloys in subsurface result in a core-shell bimetallic catalyst which promotes the propene selectivity by decreasing the d-band center of surface Pt atoms and facilitating the desorption of propene on Pt. In another aspect, the reduced usage of Pt is achieved with the addition of 3d transition metals as well as the increased utilization of Pt atoms. The catalyst can be effectively used as a catalyst for the preparation of propene by propane dehydrogenation and 85% of propene selectivity can be achieved in propane dehydrogenation.

Description

TECHNICAL FIELD

The present invention relates to a catalyst with high selectivity for propane dehydrogenation and a preparation method thereof. In certain embodiments, the present disclosure relates to a supported core-shell bimetallic catalyst with Pt-skin surface and a core of Pt and 3d transition metals (Fe, Co and Ni) and its application in improving the selectivity to propene during propane dehydrogenation.

BACKGROUND

Owing to their superior activity, better structural stability, and higher selectivity, bimetallic catalysts are widely used in a variety of heterogeneous catalytic processes. In general, the addition of another component can not only reduce noble metal consumption but also often tune the catalytic properties of catalysts. For example, Pt-based bimetallic catalysts are widely applied among these noble metal catalysts. Nonetheless, the high cost and scarcity of Pt have driven efforts to diminish Pt usage, such as the addition of 3d transition metals (3d TM) to nanoparticles' subsurface. With the considerable surface free energy gap, well-defined Pt-skin catalysts modified by 3dTM in subsurface regions are produced. The activity of Pt sites is thus modified via. electronic effect and geometric effect which are manifested in the d-band center downshift and enlarging utilization of Pt atoms respectively.

Propene, as a valuable and versatile chemical feedstock, has been employed in the production of polypropylene, propylene oxide and propionaldehyde. Traditionally, a major source of propene is naphtha cracking or refining processes which suffer from high energy consumption, poor selectivity as well as scarce oil resources. Recently, propane dehydrogenation (PDH) has become more popular because it is highly efficient and more economical than traditional industrial processes. About 5 million tons of propene per year is produced via PDH processes. In these commercial processes, Pt-based catalysts have been widely employed due to its excellent reactivity. Nonetheless, the high cost and scarcity of Pt have given rise to these researches attempting to increase the atom utilization of Pt. Moreover, the PDH process (C₃H₈→C₃H₆+H₂ΔH_298K=124.3 kJ/mol) is endothermic and is limited by thermodynamic equilibrium. High temperatures can result in not only desirable propene yields but also coke deposits and decreased selectivity to propene. Hence, the design of special catalysts to weaken the propene adsorption on Pt is carried out to improve the selectivity to propene.

SUMMARY

In accordance with the objectives of the invention, this disclosure relates to address the shortcomings of present technologies and improve the selectivity to propene over Pt-based catalysts. The core-shell bimetallic catalyst Pt3d@Pt/SBA-15 is prepared by a coimpregnation-reduction method combined with post-acid leaching treatments. 3d transition metal atoms in subsurface regions weaken the interaction between propene and Pt-skin surface. The coke deposition caused by deep dehydrogenation is thus inhibited while the catalytic performance of Pt, such as the selectivity to propene, is improved.

The supported core-shell catalyst with high selectivity is SBA-15 supported Pt and 3d transition metals (Fe, Co and Ni). Preferably, the content of platinum is 0.5 wt. % to 1 wt. % of the total weight of the catalyst and more preferably 0.75%-0.8% of the total weight of the catalyst. Preferably, the mole ratio of Pt to 3d metals is about (3-5):(0.75-1) and more preferably about 3:(0.75-0.85). Core-shell bimetallic nanoparticles with a Pt surface layer and core of Pt and 3d metals form over SBA-15. The distribution of Pt species is in the gradient of decreasing from surface to inner core while the distribution of 3d metals (Fe, Co and Ni) is in the gradient of increasing from surface to inner core.

The support is commercial SBA-15.

The 3d transition metals contain Fe, Co and Ni.

The preparation method of a supported core-shell bimetallic catalysts with high selectivity for propane dehydrogenation according to an example of the present invention comprises the following steps:

Step 1: adding the support to the liquid solution and then stirring the compound until the solvent evaporates thoroughly to load platinum and 3d metal.

The support is commercial SBA-15 in step 1.

The 3d metal can be Fe, Co or Ni in step 1.

The compound of solution and support is stirred by mechanical or ultrasonic agitator for 20-24 hours in step 1, with a speed of 200-300 revolutions per minute at 20-25° C.

The support is impregnated in the solution of deionization water, ethanol and metal precursors of Pt and 3d metals, wherein the volume ratio of deionization water to ethanol preferably is (1-2):(1-3) and more preferably is 1:1, while the mole ratio of Pt and 3d metals preferably is (3-5):(1-1.5) and more preferably is 3:(1-1.5) or (3-5):1 based on the mass of the support in step 1.

Step 2: the compound in step 1 is dried and calcined in air to form metal oxides at 300-350° C. for 2-4 hours with a ramp rate of 2-5° C. per minute.

The compound is dried in a stove at 50-70° C. for 10-12 hours and followed by grinding in step 2.

Step 3: the compound in step 2 is reduced in the flowing of hydrogen and argon (H₂/Ar) at 400-450° C. for 4-6 hours with a ramp rate of 2-5° C./min. The volume percentage of hydrogen in the flowing is 5-10%.

During the reduction at high temperature in step 3, regarding to the different thermodynamics and surface free energy between Pt and 3d metals (Fe, Co and Ni), Pt atoms with lower surface free energy tends to enrich at surface whereas 3d metals (Fe, Co and Ni) tends to enrich in core. In this way, the decreasing fraction of Pt and increasing fraction of 3d metals (Fe, Co and Ni) from surface to core is formed.

Step 4: the reduced compound in step 3 is washed by acid several times to remove the 3d metals (Fe, Co and Ni) at surface and obtain a core-shell structure with Pt at surface and 3d metal at inner core.

In step 4, the acid used can dissolve 3d metals instead of platinum, such as 5×10⁻⁴ of dilute nitric acid solution. The acid leach process is performed at 20-25° C. and lasts 1-20 min, 10-20 min preferably. 30 mg of reduced catalysts was leached in 15 ml of dilute acid solution and followed by ultrasonic shaking for 30 seconds. The solid in acid solution is segregated by centrifugation, washed 3-5 times by deionized water and dried at 60-80° C. for 10-12 hours.

The application of the supported core-shell bimetallic catalyst with high selectivity in propane dehydrogenation.

The catalyst is used after pelleting to a 20-40 mesh size distribution.

The catalyst is loaded into the quartz tubular reactor and pretreated at 600-620° C. with a ramp rate of 3-5° C./min, under 10-15 volume % H₂/N₂ for more than 0.5 hour; more preferably for 1-2 hours. After the pretreatment, dehydrogenation is performed at 550-650° C. under the flowing of the mixture of propane, nitrogen and hydrogen (propane: H₂:N₂=7:7:11, WHSV=3-10 h⁻¹).

The supported bimetallic core-shell catalyst, with SBA-15 as support, Pt as active components and 3d transition metals as cocatalysts, is a well-defined Pt-skin catalyst modified by 3d transition metal atoms in subsurface regions. In one aspect, the 3d transition metals in subsurface regions lead to a decrease of d-band center of platinum and significantly promote the C₃H₆ selectivity. In another aspect, the high utilization rate of platinum is increased by the addition of 3d transition metals. The supported bimetallic core-shell catalyst is applicable to direct propane dehydrogenation and exhibits excellent reactivity and 85% of total selectivity to propene.

BRIEF DESCRIPTION OF THE DRAWINGS

The application of the preferred embodiments in the present invention is best understood with reference to the accompanying figures, wherein:

FIG. 1 shows a reasonable design of the supported core-shell bimetallic catalyst for alkane dehydrogenation; where Gray ball represents Pt atom, and black ball represents the 3dTM atom.

FIG. 2 shows a the XRD patterns of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

FIG. 3 shows a the enlarged XRD patterns of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

FIG. 4 shows an XPS Fe2p peaks from unleached PtFe/SBA-15 and leached PtFe@Pt/SBA-15 catalysts.

FIG. 5 shows a C3H6-TPD of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

FIG. 6 shows a CO-FTIR of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

FIG. 7 shows a C3H8 conversion of PtFe@Pt/SBA-15-t and Pt/SBA-15-t (acid leaching time t=5, 10, 20 min) as a function of time on stream.

FIG. 8 shows a C3H6 selectivity of PtFe@Pt/SBA-15-t and Pt/SBA-15-t (acid leaching time t=5, 10, 20 min) as a function of time on stream.

FIG. 9 shows different values of C3H6 selectivity between Pt-3d@Pt/SBA-15 and Pt/SBA-15 as a function of acid leaching time.

FIG. 10 shows a concentration of Fe and Pt in acid solution as a function of leaching time and the values from the unreduced sample are included as a reference.

FIG. 11 shows an EDS line profiles of leached PtFe@Pt/SBA-15 (inset of the nanoparticle).a.u., arbitrary units.

FIG. 12 shows a XANES Fe K-edge structures from PtFe/SBA-15 and PtFe@Pt/SBA-15 catalysts after air exposure at room temperature; where standard Fe foil and Fe2O3 samples are included as references; where NP represents nanoparticle.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The preferred embodiments of the present invention are described with the attached drawings. The core-shell catalyst composed of Pt and 3d TMs was supported on SBA-15 and named as Pt3d@Pt/SBA-15 (3d═Fe, Co, Ni),

EXAMPLE 1

Step 1: 5 mL of deionized water and 6 mL of ethyl alcohol were stirred in a beaker. 0.75 mL of the H₂PtCl₆ precursor solution (0.010 g/mL) was dropped in the stirring beaker and the Fe(NO₃)₃ precursor solution (molar ratio of Pt/Fe, 3:1) was added. 1 g of SBA-15 was dispersed in the prepared precursor solution, followed by stirring at room temperature for 24 h. Water was removed by evaporation at 60° C.

Step 2: the catalyst was completely dried in an oven at 80° C. for 12 hours.

Step 3: after grinding, the powder was calcined in a muffle furnace at 300° C. for 2 hours at a ramp of 2° C./min.

Step 4: the calcined material was obtained by a further reduction in tube furnace under the flowing of 5% H₂/Ar (30 ml/min) at 400° C. for 4 hours at a ramp of 2° C./min.

Step 5: 0.3 g of the reduced material was dispersed in 15 ml of dilute acid solution (HNO₃ concentration, 5×10⁻⁴ mol/L) and ultrasonically shaked for 30 s. After left for 10 min, the catalyst was separated by centrifuge process and washed three times. The obtained material was dried at 60° C. for 12 h and named as leached PtFe@Pt/SBA-15-10 min.

Step 6: the leached PtFe@Pt/SBA-15-10 min catalyst was pelleted to a 20 to 40 mesh size distribution.

Step 7: 0.24 mg of the pelleted catalyst was loaded into the fixed reactor and exposed to 10 volume % H₂/N₂ at 600° C. for 1 hour.

Step 8: after the reduction, propane dehydrogenation was performed at 600° C. The weight hourly space velocity of propane was 10 h⁻¹ while the mole ratio of propane to hydrogen was 1:1 with nitrogen balanced. The volume ratio of propane, hydrogen and nitrogen was 7:7:11.

Propane conversion, propene selectivity and propene yield were determined by equations as follows:

propane conversion:

$\mathrm{Conv}\left(\%\right)=\frac{{\left[F_{C_3H_8}\right]}_{\mathrm{in}}-{\left[F_{C_3H_8}\right]}_{\mathrm{out}}}{{\left[F_{C_3H_8}\right]}_{\mathrm{in}}}\times 100$

propene selectivity:

$\mathrm{Sel}\left(\%\right)=\frac{3\times {\left[F_{C_3H_6}\right]}_{\mathrm{out}}}{3\times {\left[F_{C_3H_6}\right]}_{\mathrm{out}}+2\times {\left[F_{C_2H_4}\right]}_{\mathrm{out}}+2\times {\left[F_{C_2H_6}\right]}_{\mathrm{out}}+{\left[F_{{\mathrm{CH}}_4}\right]}_{\mathrm{out}}}\times 100$

propene yield:

$\mathrm{Yield}\left(\%\right)=\frac{\mathrm{Conv}\left(\%\right)\times \mathrm{Sel}\left(\%\right)}{100}$

The product gas was analyzed by an online gas chromatograph. The initial propene selectivity of Pt/SBA-15 and leached PtFe@Pt/SBA-15-10 min are shown in TABLE 1 to figure out the influence of Fe atoms at subsurface on selectivity.

TABLE
the influence of Fe atoms at subsurface on propene
selectivity during propane dehydrogenation
	Catalysts	Pt/SBA-15	PtFe@Pt/SBA-15
	Initial propene selectivity	69	84
	(%)

Pt/SBA-15 was prepared by the same manner in Example 1 except the addition of Fe and PtFe@Pt/SBA-15 is the catalyst of this embodiment. As shown in TABLE 1, the selectivity of the PtFe@Pt/SBA-15 catalyst is ˜15% higher than that of the Pt/SBA-15 catalyst. The improved propene selectivity over PtFe@Pt/SBA-15 is attributed to the promotion of Fe in subsurface regions. In contrast to Pt/SBA-15, a decrease of d-band center of platinum significantly deriving from electronic effect and strain effect promotes the propene selectivity over PtFe@Pt/SBA-15. In another aspect, the high utilization rate of platinum is increased by the addition of 3d transition metals.

X-ray diffraction (XRD) patterns of Pt/SBA-15 and PtFe@Pt/SBA-15 are shown in FIG. 2. The formation of PtFe alloy phase in subsurface was determined by the shift of 20 peak of Pt/SBA-15 at 39.7° to 39.9° of PtFe@Pt/SBA-15. X-ray photoelectron spectroscopy (XPS) is used to demonstrate the removal of surface Fe species. FIG. 4 shows the XPS of Fe 2p. The surface Fe becomes oxidized when exposed to air, while the Fe in subsurface regions keeps a metallic state in air due to the kinetic limit of outward diffusion of Fe. The remaining Fe in subsurface regions does not change its chemical state when exposed to air at room temperature. As a result, surface Fe can be removed completely after acid leaching. XPS was measured by PerkinElmer PHI 1600 ESCA using X-ray irradiation Al Kα(hv=1486.7 eV).

Concentration of Fe and Pt in acid solution as a function of leaching time is measured by inductively coupled plasma atomic emission spectrometry (ICP-MS, 7700x, Agilent), As the leaching time increases to 10 min, ˜25% of Fe was leached away, and this percentage remains almost constant over an extended period of leaching time. Accordingly, surface Fe atoms should be removed completely after 10 min of leaching treatment, leaving ˜75% of Fe in subsurface regions. The downshift of the d-band center after the addition of Fe is confirmed by diffuse reflectance infrared Fourier transform spectroscopy of chemisorbed CO (CO-DRIFTS) and temperature-programmed desorption of propene (C₃H₆-TPD, the temperature was ramped from 100° C. to 800° C. with a heating rate of 10° C./min). The red-shifted band of CO linearly adsorbed on PtFe@Pt/SBA-15 and lower desorption temperature of propene, compared with Pt/SBA-15 demonstrate the downshift of d-band center of Pt and the weakened adsorption of propene over Pt.

A platinum surface layer and a core of Pt and 3d metals (Fe, Co and Ni) of PtFe@Pt/SBA-15 is further determined by X-ray absorption near-edge structure (XANES). The Fe K-edge XANES study results in FIG. 12 demonstrate that the chemical state of Fe changes from an oxidized state to a metallic state after acid leaching when exposed to air, further supporting the formation of Pt-skin structure with inner metallic Fe, which is consistent with the ICP and XPS results. From the energy-dispersive spectroscopy (EDS) line profiles of PtFe@Pt catalyst, the thickness of the Pt shell is to be ˜1.3 nm, containing 3-5 atomic layers of Pt.

EXAMPLE 2

PtFe@Pt/SBA-15-5 min was prepared in the same manner as described in Example 1, except that the catalyst in Step I was left in dilute acid solution for 5 min instead of 10 min.

EXAMPLE 3

PtFe@Pt/SBA-15-20 min was prepared in the same manner as described in Example 1, except that the catalyst in Step 1 was left in dilute acid solution for 20 min instead of 10 min.

EXAMPLE 4

PtFe@Pt/SBA-15-60 min was prepared in the same manner as described in Example 1, except that the catalyst in Step 1 was left in dilute acid solution for 60 min instead of 10 min.

EXAMPLE 5

PtFe@Pt/SBA-15-0 min was prepared in the same manner as described in Example 1, except that the catalyst in Step 1 wasn't left in dilute acid solution.

EXAMPLE 6

PtCo@Pt/SBA-15-10 min was prepared in the same manner as described in Example 1, except that the 3d transition metal precursor solution in Step 1 was Co(NO₃)₂.

EXAMPLE 7

PtCo@Pt/SBA-15-5 min was prepared in the same manner as described in Example 6, except that the PtCo/SBA15 was left in dilute acid solution for 5 min in step 2.

EXAMPLE 8

PtCo@Pt/SBA-15-20 min was prepared in the same manner as described in Example 6, except that the PtCo/SBA-15 was left in dilute acid solution for 20 min in step 2.

EXAMPLE 9

PtCo@Pt/SBA-15-60 min was prepared in the same manner as described in Example 6, except that the PtCo/SBA-15 was left in dilute acid solution for 60 min in step 2.

EXAMPLE 10

PtNi@Pt/SBA-15-10 min was prepared in the same manner as described in Example 1, except that the 3d transition metal precursor solution in Step 1 was Ni(NO₃)₂.

EXAMPLE 11

PtNi@Pt/SBA-15-5 min was prepared in the same manner as described in Example 10, except that the PtNi/SBA-15 was left in dilute acid solution for 5 min in step 2.

EXAMPLE 12

PtNi@Pt/SBA-15-20 min was prepared in the same manner as described in Example 10, except that the PtNi/SBA-15 was left in dilute acid solution for 20 min in step 2.

EXAMPLE 13

PtNi@Pt/SBA-15-60 min was prepared in the same manner as described in Example 10, except that the PtNi/SBA-15 was left in dilute acid solution for 60 min in step 2.

Propane dehydrogenation was performed using the catalysts prepared in Examples 2-13. The influence of a series of 3d transition metals and acid leach time on propene selectivity was investigated. The results are shown in TABLE 2.

TABLE 2
the influence of acid leaching time and 3 d transition
metals at subsurface on propene selectivity
	Selectivity differential between Pt3d@Pt/SBA-15 and
	Pt/SBA-15
Acid leaching	PtFe@Pt/SBA-	PtCo@Pt/SBA-	PtNi@Pt/SBA-
time (min)	15	15	15
5	4%	−20%	−35%
10	15%	10%	1%
20	14%	9%	0.3%
60	13%	6%	−0.7%

With the increase of acid leaching time, the propene selectivity improves gradually at first and keeps constant thereafter. After acid leaching treatment for 10 min, the selectivity deferential reaches the largest and remains relatively constant even with acid treatment for much longer time, which demonstrates the role of 3d transition metals in the improvement of selectivity.

With different 3d transition metals, the improved selectivity to propene is distinguishable. The propane selectivity increases markedly in the row Pt/SBA-15 PtNi@Pt/SBA-15<PtCo@Pt/SBA-15<PtFe@Pt/SBA-15 which means the addition of different 3d transition metals leads to different d-band center of surface Pt as well as different propene selectivity. Moreover, because the core-shell structure hasn't formed in the leached PtCo/SBA-15 and leached PtNi/SBA-15 after acid leaching for 5 min, C—C bonds in propane is broken easily over Co and Ni atoms at surface, resulting in poor propene selectivity.

The catalyst of the present invention can be prepared by parameter variations of the formulation and preparation process, and the similar performance with the examples can be obtained. The present invention has been described in detail above. It should be noted that any simple modifications, alterations, or other equivalents of those skilled in the art without departing from the scope of the present invention fall within the scope of the present invention.

Claims

1. A supported core-shell bimetallic catalyst with high selectivity for propane dehydrogenation, comprising: Pt and a plurality of 3d transition metals; wherein, an amount of Pt is 0.5-1 weight % of a total amount of a catalyst composition and a mole ratio of Pt to the plurality of 3d transition metals is (3-5):(1-1.5); wherein a concentration of the Pt gradiently decreases from a surface to an inner core of the supported core-shell bimetallic catalyst and a concentration of the plurality of 3d transition metals gradiently increases from the surface to the inner core of the supported core-shell bimetallic catalystis; the plurality of 3d transition metals include Fe, Co and Ni.

2. The supported core-shell bimetallic catalyst with high selectivity for propane dehydrogenation of claim 1, wherein a weight content of Pt is in a range of 0.75-0.8% and the mole ratio of Pt to the plurality of 3d transition metals is 3:(0.75-0.85).

3. The supported core-shell bimetallic catalyst with high selectivity for propane dehydrogenation of claim 1, wherein a support is commercial mesoporous silicon dioxide SBA-15.

4. A preparation method of a supported core-shell bimetallic catalyst with high selectivity for propane dehydrogenation comprising the following steps: step 1: a support is impregnated in a solution of deionization water, ethanol and metal precursors of Pt and the plurality of 3d transition metals; stirring a compound until a solvent evaporates thoroughly forming a powder; wherein, a volume ratio of the deionization water and the ethanol is (1-2):(1-3) and a mole ratio of the Pt and the plurality of 3d transition metals is (3-5):(1-1.5); a weight content of the Pt is 0.5%-1% of mass of the support;step 2: calcinating the powder at 300-350° C. for 2-4 h at a ramp rate of 2-5° C./min after drying;step 3: reducing a calcined catalyst at 400-450° C. for 4-6 h at the ramp rate of 2-5° C./min in a gas mixture of H2 and Ar, wherein H2 is 5-10% by volume;step 4: leaching the reduced catalyst by an acid to remove the plurality of 3d transition metals at surface and preparing a core-shell structure of the supported core-shell bimetallic catalyst having Pt at a surface and 3d metal at an inner core.

5. The preparation method of claim 4, wherein the support used in the step 1 is commercial SBA-15; the stirring is a mechanical or a sonication stirring and the stirring process is performed with 200-300 rpm at 20-25° C. for 20-24 hours.

6. The preparation method of claim 4, wherein the volume ratio of deionization water to ethanol is 1:1, and an atomic ratio of Pt to the plurality of 3d transition metals in the precursor solution is 3:(1-1.5) or (3-5):1; the weight content of Pt is 0.75-0.8% of the weight of support.

7. The preparation method of claim 4, wherein the acid selected to leach dissolves the plurality of 3d transition metals and not react with Pt in the step 4.

8. The preparation method of claim 4, wherein the acid leaching takes 1-20 min at RT and an optimum leaching time is 10-20 min in the step 4.

9. The preparation method of claim 4, wherein the selected acid is nitric acid in concentration of 5×10−4 mol/L in the step 4.

10. A method of preparing propene, comprising a dehydrogenation of propene by using the supported core-shell bimetallic catalyst of claim 1.

11. The method of preparing propene of claim 10, wherein particle size of the supported core-shell bimetallic catalyst is in a range of 20-40 mesh size.

12. The method of preparing propene of claim 10, wherein the supported core-shell bimetallic catalyst is pre-reduced in a reactor at 600-620° C. at a ramp rate of 3-5° C./min in a gas mixture of H2 and N2; the propane dehydrogenation reaction is tested at 550-650° C. at a mass hour space velocity of 3-10 h−1 based on the propane in the reaction gas mixture of propane, H2 and N2 in a volume ratio of 7:7:11.

13. The method of preparing propene of claim 12, wherein a reduction time is at least for 0.5 h and the volume of H2 in reduction gas mixture is 10-15%.

14. The method of preparing propene of claim 10, wherein the reduction time is 1-2 h.