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.

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 H.sub.2 and Ar, wherein H.sub.2 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 510.sup.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 H.sub.2 and N.sub.2; the propane dehydrogenation reaction is tested at 550-650 C. at a mass hour space velocity of 3-10 h.sup.1 based on the propane in the reaction gas mixture of propane, H.sub.2 and N.sub.2 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 H.sub.2 in reduction gas mixture is 10-15%.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The application of the preferred embodiments in the present invention is best understood with reference to the accompanying figures, wherein:

[0026] 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.

[0027] FIG. 2 shows a the XRD patterns of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

[0028] 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.

[0029] FIG. 4 shows an XPS Fe2p peaks from unleached PtFe/SBA-15 and leached PtFe@Pt/SBA-15 catalysts.

[0030] FIG. 5 shows a C3H6-TPD of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

[0031] FIG. 6 shows a CO-FTIR of PtFe@Pt/SBA-15 catalysts with line 1 and Pt/SBA-15 catalysts with line 2.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] FIG. 11 shows an EDS line profiles of leached PtFe@Pt/SBA-15 (inset of the nanoparticle).a.u., arbitrary units.

[0037] 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

[0038] 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 (3dFe, Co, Ni),

EXAMPLE 1

[0039] Step 1: 5 mL of deionized water and 6 mL of ethyl alcohol were stirred in a beaker. 0.75 mL of the H.sub.2PtCl.sub.6 precursor solution (0.010 g/mL) was dropped in the stirring beaker and the Fe(NO.sub.3).sub.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.

[0040] Step 2: the catalyst was completely dried in an oven at 80 C. for 12 hours.

[0041] 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.

[0042] Step 4: the calcined material was obtained by a further reduction in tube furnace under the flowing of 5% H.sub.2/Ar (30 ml/min) at 400 C. for 4 hours at a ramp of 2 C./min.

[0043] Step 5: 0.3 g of the reduced material was dispersed in 15 ml of dilute acid solution (HNO.sub.3 concentration, 510.sup.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.

[0044] Step 6: the leached PtFe@Pt/SBA-15-10 min catalyst was pelleted to a 20 to 40 mesh size distribution.

[0045] Step 7: 0.24 mg of the pelleted catalyst was loaded into the fixed reactor and exposed to 10 volume % H.sub.2/N.sub.2 at 600 C. for 1 hour.

[0046] Step 8: after the reduction, propane dehydrogenation was performed at 600 C. The weight hourly space velocity of propane was 10 h.sup.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:

[0047] propane conversion:

[00001]$\mathrm{Conv}.Math..Math.\left(\%\right)=\frac{{\left[F_{C_3.Math.H_8}\right]}_{\mathrm{in}}-{\left[F_{C_3.Math.H_8}\right]}_{\mathrm{out}}}{{\left[F_{C_3.Math.H_8}\right]}_{\mathrm{in}}}100$

[0048] propene selectivity:

[00002]$\mathrm{Sel}.Math..Math.\left(\%\right)=\frac{3{\left[F_{C_3.Math.H_6}\right]}_{\mathrm{out}}}{3{\left[F_{C_3.Math.H_6}\right]}_{\mathrm{out}}+2{\left[F_{C_2.Math.H_4}\right]}_{\mathrm{out}}+2{\left[F_{C_2.Math.H_6}\right]}_{\mathrm{out}}+{\left[F_{{\mathrm{CH}}_4}\right]}_{\mathrm{out}}}100$

[0049] propene yield:

[00003]$\mathrm{Yield}.Math..Math.\left(\%\right)=\frac{\mathrm{Conv}.Math..Math.\left(\%\right)\mathrm{Sel}.Math..Math.\left(\%\right)}{100}$

[0050] 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-US-00001 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 (%)

[0051] 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.

[0052] 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 K(hv=1486.7 eV).

[0053] 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 (C.sub.3H.sub.6-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.

[0054] 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

[0055] 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

[0056] 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

[0057] 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

[0058] 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

[0059] 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(NO.sub.3).sub.2.

EXAMPLE 7

[0060] 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

[0061] 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

[0062] 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

[0063] 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(NO.sub.3).sub.2.

EXAMPLE 11

[0064] 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

[0065] 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

[0066] 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.

[0067] 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-US-00002 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%

[0068] 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.

[0069] 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, CC bonds in propane is broken easily over Co and Ni atoms at surface, resulting in poor propene selectivity.

[0070] 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.