SUPPORTED CORE-SHELL STRUCTURED ZnO CATALYST, AND PREPARATION METHOD AND USE THEREOF

Abstract

The present invention belongs to the technical field of supported catalysts, and discloses a supported core-shell structured ZnO catalyst, and a preparation method and use thereof. With Al.sub.2O.sub.3 as a support and ZnO as active sites, the catalyst is characteristic of a NiZn@ZnO core-shell structure, which consists of a NiZn alloy core and a ZnO shell The preparation method comprises firstly dissolving Ni(NO.sub.3).sub.3.6H.sub.2O and Zn(NO.sub.3).sub.2.6H.sub.2O in deionized water; then impregnating Al.sub.2O.sub.3 with the solution described above, followed by uniform ultrasonic dispersion and complete drying; and finally the obtained solid is calcinated and reduced to obtain the target catalyst, which exhibits high activity, selectivity and stability. The catalyst can be used for the dehydrogenation of light alkanes to alkenes, especially in dehydrogenation of propane to propylene.

Claims

1. A supported core-shell structured ZnO catalyst, wherein the catalyst is composed of Al.sub.2O.sub.3 as a support and ZnO as active sites; a NiZn@ZnO core-shell structure, which consists of a NiZn alloy core and a ZnO shell, is supported on the Al.sub.2O.sub.3, denoted as NixZny/Al.sub.2O.sub.3, wherein x:y=(1:1)-(1:4), representing the molar ration of Ni/Zn.

2. The supported core-shell structured ZnO catalyst according to claim 1, wherein the catalyst contains 1%-3% of Ni based on the mass of the Al.sub.2O.sub.3 support.

3. The supported core-shell structured ZnO catalyst according to claim 2, wherein the catalyst contains 0.5%-6% of Ni based on the mass of the Al.sub.2O.sub.3 support.

4. The supported core-shell structured ZnO catalyst according to claim 1, wherein x:y=1:3.

5. A method for preparing the supported core-shell structured ZnO catalyst according to claim 1, wherein the method comprises the following steps: (1) dissolving Ni(NO.sub.3).sub.3.6H.sub.2O and Zn(NO.sub.3).sub.2.6H.sub.2O in deionized water; (2) impregnating Al.sub.2O.sub.3 with the solution obtained in step (1), followed by uniform ultrasonic dispersion and complete drying; and (3) the solid obtained in step (2) being calcinated at 500-600° C. for 2-4 h, followed by the reduction treatment to obtain a core-shell structured NiZn@ZnO catalyst supported on Al.sub.2O.sub.3.

6. The method for preparing the supported core-shell structured ZnO catalyst according to claim 5, wherein the drying process in step (2) involves drying the mixture naturally at room temperature followed by drying at 80-100° C. until the mixture is completely dried

7. The method for preparing the supported core-shell structured ZnO catalyst according to claim 5, wherein the reduction treatment in step (3) is performed at 500-700° C. for 1-2 h.

8. A use of the supported core-shell structured ZnO catalyst according to claim 1 in dehydrogenation of light alkanes to alkenes.

9. The use of the supported core-shell structured ZnO catalyst in dehydrogenation of light alkanes to alkenes according to claim 8, wherein the light alkane is propane, and the alkene is propylene.

10. The method of claim 5, wherein the catalyst contains 1%-3% of Ni based on the mass of the Al.sub.2O.sub.3 support.

11. The method of claim 10, wherein the catalyst contains 0.5%-6% of Ni based on the mass of the Al.sub.2O.sub.3 support.

12. The method of claim 5, wherein x:y=1:3.

13. The use of the supported core-shell structured ZnO catalyst of claim 8, wherein the catalyst contains 1%-3% of Ni based on the mass of the Al.sub.2O.sub.3 support.

14. The use of the supported core-shell structured ZnO catalyst of claim 13, wherein the catalyst contains 0.5%-6% of Ni based on the mass of the Al.sub.2O.sub.3 support.

15. The use of the supported core-shell structured ZnO catalyst of claim 8, wherein x:y=1:3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 illustrates catalytic performance of catalysts prepared in Embodiments 1 to 6; where (a), and (b) show the conversion of C.sub.3H.sub.8 and selectivity of C.sub.3H.sub.6 as a function of time on stream over various NixZny/Al.sub.2O.sub.3, respectively, and (c) shows the comparison of deactivation rate constant over different catalysts.

[0025] FIG. 2 shows catalytic performance of catalysts prepared in Embodiments 1, 7, 8 and 9.

[0026] FIG. 3 shows catalytic performance of Ni1Zn3/Al.sub.2O.sub.3 catalysts prepared in Embodiments 1, 14 and 15.

[0027] FIG. 4 is a graph showing the result of a regeneration stability test at 550° C. over the Ni1Zn3/Al.sub.2O.sub.3 catalyst prepared in Embodiment 1.

[0028] FIG. 5 shows XRD patterns of the catalysts prepared in Embodiments 1, 2, 4 and 5, where I, II, III and IV correspond to Embodiments 5, 4, 2 and 1, respectively.

[0029] FIG. 6 shows the EDS-mapping image of the Ni1Zn3/Al.sub.2O.sub.3 catalyst prepared in Embodiment 1.

[0030] FIG. 7 shows the TEM image of the Ni1Zn3/Al.sub.2O.sub.3 catalyst prepared in Embodiment 1.

[0031] FIG. 8 shows the DRIFTS spectra of catalysts prepared in Embodiments 1, 5 and 6 after CO chemisorption at 25° C. and subsequently purging with He for 30 min; where (a), (b) and (c) correspond to the catalysts prepared in Embodiment 5, Embodiment 1 and Embodiment 6, respectively.

[0032] FIG. 9 shows the catalytic activity as a function of the metallic surface area of Ni over the catalysts prepared in Embodiments 1, 2, 4, and 5; where (a) shows the metallic surface area of Ni of the catalysts prepared in Embodiments 1, 2, 4 and 5; and (b) shows conversion of propane as a function of the metallic surface area of Ni.

[0033] FIG. 10 shows the H.sub.2-TPD profiles of the catalysts prepared in Embodiments 1 and 6; where (a) and (b) correspond to the catalysts prepared in Embodiment 6 and Embodiment 1, respectively.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0034] The present invention is described in further detail by specific embodiments which enables those skilled in this field to fully understand the invention without limiting it in any way.

Embodiment 1

[0035] (1) 0.15 parts by mass of Ni(NO.sub.3).sub.3.6H.sub.2O and 0.45 parts by mass of Zn(NO.sub.3).sub.2.6H.sub.2O were dissolved in 1 mL of deionized water;

[0036] (2) 1 part by mass of Al.sub.2O.sub.3 was impregnated in the solution described above, the obtained system was subject to ultrasonic treatment for 0.5-1 h and naturally dried at room temperature for 12 h, and then completely dried at 80-100° C.;

[0037] (3) the solid obtained in step (2) was calcinated in air atmosphere at 600° C. for 3 h, and then it was reduced at 600° C. for 1 h to obtain the NiZn@ZnO core-shell structured catalyst supported on Al.sub.2O.sub.3, which contained 3% of Ni based on the mass of the support, named as Ni1Zn3/Al.sub.2O.sub.3;

[0038] (4) the prepared catalyst was ground and sieved to a certain size (20-40 mesh); and

[0039] (5) the prepared catalyst was loaded into a fixed-bed reactor, and the reaction was operated under a mixture of C.sub.3H.sub.8 and H.sub.2 (molar ration: 1:1) within N.sub.2 as a balance gas. The weight hourly space velocity (WHSV) of propane was 4 h.sup.−1.

Embodiment 2

[0040] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.45 parts by mass of Zn(NO.sub.3).sub.2.6H.sub.2O in step (1) being replaced by 0.15 parts by mass; and the catalyst contained 3% of Ni based on the mass of the support, named as Ni1Zn1/Al.sub.2O.sub.3.

Embodiment 3

[0041] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.45 parts by mass of Zn(NO.sub.3).sub.2.6H.sub.2O in step (1) being replaced by 0.6 parts by mass; and the catalyst contained 3% of Ni based on the mass of the support, named as Ni1Zn4/Al.sub.2O.sub.3.

Embodiment 4

[0042] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.45 parts by mass of Zn(NO.sub.3).sub.2.6H.sub.2O in step (1) being replaced by 0.05 parts by mass; and the catalyst contained 3% of Ni based on the mass of the support, named as Ni3Zn1/Al.sub.2O.sub.3.

Embodiment 5

[0043] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.45 parts by mass of Zn(NO.sub.3).sub.2.6H.sub.2O in step (1) being replaced by 0 parts by mass; and the catalyst contained 3% of Ni based on the mass of the support, named as Ni/Al.sub.2O.sub.3.

Embodiment 6

[0044] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.15 parts by mass of Ni(NO.sub.3).sub.3.6H.sub.2O in step (1) being replaced by 0 parts by mass, and the catalyst contained 10% of Zn based on the mass of the support, named as ZnO/Al.sub.2O.sub.3.

Embodiment 7

[0045] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.15 parts by mass of Ni(NO.sub.3).sub.3.6H.sub.2O in step (1) being replaced by 0.025 parts by mass, and the catalyst contained 0.5% of Ni based on the mass of the support, named as Ni1Zn3/Al.sub.2O.sub.3.

Embodiment 8

[0046] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.15 parts by mass of Ni(NO.sub.3).sub.3.6H.sub.2O in step (1) being replaced by 0.05 parts by mass, and the catalyst contained 1% of Ni based on the mass of the support, named as Ni1Zn3/Al.sub.2O.sub.3.

Embodiment 9

[0047] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction with 0.15 parts by mass of Ni(NO.sub.3).sub.3.6H.sub.2O in step (1) being replaced by 0.3 parts by mass, and the catalyst contained 6% of Ni based on the mass of the support, named as Ni1Zn3/Al.sub.2O.sub.3.

Embodiment 10

[0048] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the calcination temperature was 400° C.

Embodiment 11

[0049] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the calcination temperature was 500° C.

Embodiment 12

[0050] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the calcination time was 2 h.

Embodiment 13

[0051] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the calcination time was 4 h.

Embodiment 14

[0052] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the reduction temperature was 500° C.

Embodiment 15

[0053] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the reduction temperature was 700° C.

Embodiment 16

[0054] This embodiment was carried out using the method described in Embodiment 1 for preparation and reaction, and only differed in that in step (3), the reduction time was 2 h.

[0055] The catalysts prepared in the above embodiments were tested for catalytic performance in the propane dehydrogenation reaction, and the catalyst activity was expressed in terms of conversion of propane, selectivity of propylene, and deactivation rate, which will be discussed below in combination with the calculation results:

[0056] The catalysts of Embodiments 1 to 6 corresponding to different Ni/Zn ratios were tested for catalytic performance in propane dehydrogenation, and their catalytic performances were shown in FIG. 1, where (a), and (b) show the conversion of C.sub.3H.sub.8 and selectivity of C.sub.3H.sub.6 as a function of time on stream over various NixZny/Al.sub.2O.sub.3, respectively, and (c) shows the comparison of deactivation rate constant over different catalysts. As can be seen from FIG. 1, the NixZny/Al.sub.2O.sub.3 catalysts corresponding to Embodiments 1 to 3 performed well in catalytic stability; whereas pure Ni/Al.sub.2O.sub.3 of Embodiment 5 showed high initial activity, but poor selectivity to propylene and underwent an induction period during which rapid deactivation occurred due to the fast coke deposition covering the highly reactive sites, thereafter becoming relatively stable and low-active. The ZnO/Al.sub.2O.sub.3 of Embodiment 6 exhibited consistent high selectivity towards propylene but low activity with a propensity for continuous rapid deactivation with a deactivation rate constant (k.sub.d) higher than 0.37 h.sup.−1, indicating a poor stability during the propane dehydrogenation reaction. In addition, as can be seen from FIG. 1, with the increase of Zn addition, the catalytic behavior of NixZny/Al.sub.2O.sub.3 tended to transform from Ni-like to ZnO-like which may imply the transformation of active sites. For Ni1Zn3/Al.sub.2O.sub.3, the deactivation tendency was significantly suppressed and higher activity together with similar selectivity was achieved when compared with ZnO/Al.sub.2O.sub.3, with an initial conversion of propane of 37%, and a selectivity of propene of more than 90%.

[0057] Embodiments 1, 7, 8 and 9 provide catalysts prepared with different Ni loadings (based on the mass of the support) and their catalytic performance in propane dehydrogenation. It can be seen from FIG. 2 that the conversion of propane increased gradually with the increase of Ni loading. But the selectivity to propylene dropped dramatically as the Ni loading increased to 6 wt %, which can be attributed to the partial exposure of Ni sites resulting from the higher Ni content. The catalytic performance was optimal when the content of Ni was 3 wt %.

[0058] Embodiments 1, 14 and 15 provide catalysts prepared at different reduction temperatures and their catalytic performance in propane dehydrogenation. It can be seen from FIG. 3 that there was no significant change in the catalytic performance when the reduction temperature was between 500° C. and 600° C., but the conversion of propane decreased significantly when the reduction temperature was increased to 700° C., which can be explained by the deep reduction of ZnO as an active species, forming metallic Zn with a lower melting point (420° C.) and no activity for propane dehydrogenation, resulting in the decrease in activity.

[0059] The catalyst prepared in Embodiment 1 was further subject to a long-term regeneration stability test at 550° C., and the result is shown in FIG. 4. While maintaining a stable selectivity of more than 90%, the deactivation rate constant (k.sub.d) of the Ni1Zn3/Al.sub.2O.sub.3 catalyst was as low as 0.017 h.sup.−1, indicating excellent long-term stability, breaking the limitation of rapid deactivation of ZnO-based catalysts.

[0060] XRD analysis was performed over the catalysts of Embodiments 1, 2, 4 and 5 with different Ni/Zn ratios, and the resulting patterns are shown in FIG. 5, where I, II, III and IV correspond to Embodiments 5, 4, 2 and 1, respectively. the transformation trend from Ni(111) to NiZn(101) can be clearly observed with the increase of Zn addition, indicating Zn incorporating into the bulk phase of Ni and the formation of NiZn alloy.

[0061] EDS-mapping analysis was performed over the Ni1Zn3/Al.sub.2O.sub.3 catalyst prepared in Embodiment 1, and the images are shown in FIG. 6. The uniform Ni—Zn element distribution over Al.sub.2O.sub.3 support excluded the possibility of phase separation, implying the surface segregation of certain form of Zn species during the formation of NiZn alloy nanoparticles.

[0062] Also referring to FIG. 7, the Ni1Zn3/Al.sub.2O.sub.3 catalyst prepared in Embodiment 1 was analyzed by high-resolution TEM. The existence of homogenous ZnO overlayers on the surface of bulk NiZn alloy nanoparticles was discovered through the identification of lattice fringes, establishing a NiZn@ZnO core-shell nanostructure.

[0063] Surface-sensitive DRIFTS measurements using CO adsorption as a probe were performed on the catalysts prepared in Embodiments 1, 5 and 6, and the results are shown in FIG. 8, where (a), (b) and (c) correspond to the catalysts prepared in Embodiments 5, 1 and 6, respectively. It was found that the CO adsorption peak on Ni at 2055 cm.sup.−1 disappeared over Ni1Zn3, while a CO linear adsorption peak on ZnO appeared at 2198 cm.sup.−1, together with adsorption peaks of some carbonate species on ZnO at 1696 and 1522 cm.sup.−1, validating the reverse encapsulation of ZnO on Ni induced by strong metal-oxide interaction.

[0064] Furthermore, H2 pulse chemisorption experiments were performed over the catalysts prepared in Embodiments 1, 2, 4 and 5 to measure the active metallic Ni surface area H.sub.2 pulse chemisorption experiments were performed over the catalysts prepared in Embodiments 1, 2, 4 and 5 to measure the active metallic Ni surface area, as shown in FIG. 9, where (a) displays the metallic surface area of Ni; and (b) displays conversion of propane as a function of the metallic surface area of Ni. The active metallic Ni surface area firstly increased and then gradually decreased to near zero with the increase of Zn addition. The increase could be due to the initial formation of NiZn alloy, which improved the dispersion of Ni. However, further addition of Zn gave rise to ZnO overlayers forming on the surface of NiZn alloy, leading to the reduction of metallic surface area of Ni. The near zero value of metallic Ni surface area for Ni1Zn3/Al.sub.2O.sub.3 and the simultaneous reaching of the highest propane conversion confirmed the successful construction of the NiZn@ZnO core-shell structure without Ni exposure on the surface. This result excluded the assumption of Ni sites directly participating in the reaction, which meant Ni exclusively acted as a promoter while ZnO overlayers functioned as the active sites for propane dehydrogenation.

[0065] H.sub.2-TPD tests were performed on the catalysts prepared in Embodiments 1 and 6, and the results are shown in FIG. 10, where (a) and (b) correspond to the catalysts prepared in Embodiments 6 and 1, respectively. These results can explain the inhibited deactivation of the ZnO species over NiZn@ZnO. The core-shell structure induced by strong metal-oxide interaction changes the geometric environment of Zn and O sites and concomitant electron transfer from the ZnO shell to the alloy core reduces the electron density of O sites, which weakens O—H binding and thus facilitates the dissociation of O—H bond in preference to dissociation of Zn—OH bond over surface ZnO, accelerating H.sub.2 desorption and therefore retarding the reduction of ZnO during reaction.

[0066] Although the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the embodiments described above, which are intended to be illustrative and not restrictive. enlightened by the present invention, those skilled in this field can make many specific changes without departing from the purpose of the present invention and the protection scope of the claims, and these all fall within the protection scope of the present invention.