Tianlei Wang,Zhikang Xu,Yuanyuan Yue,Tinghai Wang,Minggui Lin,Haibo Zhu,2,

1 National Engineering Research Center of Chemical Fertilizer Catalyst,School of Chemical Engineering,Fuzhou University,Fuzhou 350002,China

2 Qingyuan Innovation Laboratory,Quanzhou 362801,China

3 State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China

Keywords:Hierarchical ZSM-5 Propane dehydrogenation Desilication PtSn particles

ABSTRACT A series of PtSn/hierarchical ZSM-5 catalysts were developed for propane dehydrogenation,in which the PtSn bimetallic particles are confined in the mesopores of hierarchical ZSM-5 zeolite.The synthesis of PtSn/hierarchical ZSM-5 catalysts was achieved via the loading of Pt and Sn species onto the hierarchical ZSM-5 catalysts that are obtained through a desilication of conventional ZSM-5.The PtSn/hierarchical ZSM-5 catalysts were fully characterized by XRD,N2 adsorption,STEM,XPS,and CO-IR techniques,which reveals that highly dispersed PtSn bimetallic nanoparticles are enclosed into mesopores of hierarchical ZSM-5.The catalytic performance of PtSn/hierarchical ZSM-5 is greatly affected by the concentrations of alkali solution in the desilication process and Sn/Pt ratios in PtSn bimetallic particles.The PtSn1.00/ZSM-5(0.8)catalyst shows the highest efficiency in propane dehydrogenation,which gives an initial conversion of 46%and selectivity of 98%at 570°C.The high efficiency in these PtSn/hierarchical ZSM-5 catalysts for propane dehydrogenation is mainly ascribed to the confinement of PtSn particles in the mesopores of hierarchical ZSM-5 zeolite.

1.Introduction

Propylene is an important basic chemical for the production of a wide range of chemicals,such as polypropene,acrolein,polyacrylonitrile,and acrylic acid,and the market demand for propylene increases in recent years [1].The propane dehydrogenation technology has received great attentions,because it provides an alternative way for the production of propylene [2,3].

The Pt based catalyst has been commercialized in propane dehydrogenation for the production of propylene [4].However,there are still some problems in Pt-based catalysts [5–7].The propane dehydrogenation reaction is an endothermic process,which requires a high temperature of above 550 °C to achieve a feasible propane conversion.Under such harsh condition,the growth of Pt particle always takes place,which leads to both a rapid deactivation and a significant decrease in selectivity [8].Therefore,how to stabilize Pt particles on the oxide substrate under the reaction condition remains a substantial challenge.

The stability of the Pt particles at the surface of supports can be greatly enhanced by the manipulation of metal-support interaction[9,10].The Al2O3with coordinatively unsaturated pentacoordinate Al3+(Al3+penta)sites can stabilize the atomically dispersed Pt specie via the formation of Al3+penta-O-Pt bond,which results in anchoring Pt particles at the surface of Al2O3support.The Pt particles can be tightly stabilized on the heteroatoms in the lattice of oxide substrate [11].Small Pt particles stabilized in the latticeconfined Sn(IV/II)produces a highly selective and stable Pt-Sn catalyst for propane dehydrogenation[12].Pt particles confined at the Sn single-site in zeolite framework show high catalytic performance in the propane dehydrogenation,and the enhanced performance of Pt/Sn-Beta catalysts is mainly attributed to the strong interaction between the Pt particles and the framework of Snzeolite [13].

Recently,the encapsulation of Pt clusters into the micropore of zeolite becomes an attractive approach for the synthesis of sintering-resistant Pt@zeolite.The fabrication of Pt@zeolite is achieved via one-pot synthesis [14–17].The Pt-amine complex is localized into the zeolite crystal during zeolite crystallization process,and the subsequent reduction treatment leads to a formation of Pt clusters encapsulated into the micropore of zeolite.These Pt@zeolite catalysts show very high stability,selectivity and activity in the propane dehydrogenation reaction to form propylene[18].Although Pt@zeolite shows high performance in propane dehydrogenation,the straightforward synthesis of Pt@zeolite remains a difficult task.The hydrothermal zeolite crystallization is carried out in alkaline condition at high temperature,and the precipitation of Pt species always takes place which leads to a phase separation between Pt particles and zeolite crystal.In such a case,the encapsulation efficiency is not high,and only a portion of Pt species are trapped into the micropore of zeolite crystal.

Herein,a series of PtSnx/hierarchical ZSM-5 were developed via the loading of Pt and Sn species into mesopores of hierarchical ZSM-5.The hierarchical ZSM-5 was synthesized by a treatment of ZSM-5 in the NaOH solution under a mild condition [19].The PtSn bimetallic particles are confined in the mesopores,which allow them to become a stable catalyst in propane dehydrogenation at high temperature.

2.Experimental

2.1.Catalyst preparation

H-ZSM-5 (SiO2/Al2O3=25) was commercially available from Tianjin Nankai University catalyst Co.,Ltd.The desilication of ZSM-5 was carried out according to following procedure:the NaOH solutions with different concentrations (0.2 mol·L-1,0.4 mol·L-1,0.6 mol·L-1,0.8 mol·L-1and 1.0 mol·L-1NaOH solution) were obtained by dissolving a certain amount of NaOH in 20 ml of deionized water.Then,commercial H-ZSM-5 zeolite was added into the above solution (20 ml·g-1zeolite) at 65 °C,and the resulting suspension was kept on stirring for 2 h.Finally,the mixture was filtered,washed thoroughly with deionized water several times to remove residual NaOH and finally dried at 80 °C overnight.The final powder product was labeled as ZSM-5(y) (y represents the concentration of NaOH solution).

The loading of Pt and Sn species into hierarchical ZSM-5 follows the procedure in our previous report [20,21].Typically,the ZSM-5 zeolite was dehydroxylated under the vacuum at 300°C overnight.The resulted zeolite was added into a hexane solution of tributyltin hydride(Bu3SnH),and the obtained mixture was stirred gently for 24 hours.After that,the isolated solid product was transferred into a solution of Pt(COD)Me2in toluene,and the obtained mixture was kept on stirring for 24 hours.The solid powder was filtered and washed several times to obtain the complexes product labeled as Pt(COD)Me/SnBu3/ZSM-5.Finally,the organometallic complex was dried and then treated in a flow of hydrogen at 150 °C for 4 h to get the final catalyst.The obtained catalysts were named as PtSnx/hierarchical ZSM-5(y),in which the Pt loading is kept at around 0.40 % (mass) and × represents the nominal Sn/Pt mole ratio (x varies with 0,1/3,1/2,1,2 and 3).The Sn and Pt loadings in the PtSn/hierarchical ZSM-5 were measured by ICP-OES technique,and the obtained results are shown in Table 1.

Table 1 Pt and Sn loadings in PtSnx/ZSM-5(0.8) obtained by ICP-OES technique

2.2.Catalyst characterization

The crystallinity of ZSM-5 zeolite catalysts was analyzed by the powder X-ray diffraction (XRD) on a Rigaku Ultimate III diffractometer using Cu Kα (γ=0.15406 nm) radiation in the 2θ range of 5° to 90°.N2adsorption–desorption isotherms of the catalysts were determined by Micromeritics ASAP 2046M nitrogen adsorption instrument at -196 °C.Prior to the measurement,all the catalysts were degassed in vacuum at 300 °C for 7 h.The crystal size and morphology of the samples were performed on a scanning electron microscope (SEM,Phenom ProX,Netherlands) at an accelerating voltage of 15 kV.Before measurement,the sample was ground to powder in agate and then evenly dispersed on the conductive adhesive to be gold-plated and vacuumed for 60 s.XPS spectra studies were carried out in a Thermo Scientific ESCALAB 250 (USA),and the experiment data was collected by a monochromatized Al Kα X-ray source (150 W).The binding energies were calibrated with respect to the carbon 1 s core level energy centered at 284.6 eV.The HAADF-STEM images of the sizes of PtSn particles were recorded on an aberration-corrected FEI Titan instrument.Samples were pretreated on 200 mesh carbon-coated copper by using a drop of an analytical alcohol dispersion of a catalyst and evaporating at room temperature.The Fourier Transform Infrared (FT-IR) spectra of chemisorbed CO samples were performed on a IS 50 spectrometer equipped with a cooling and evacuation system.Typically,a self-supporting wafer was transferred and reduced in a flow of 10% (volume) H2/Ar at 300 °C for 30 min.Straight after,the samples were flushed with Ar to collect the background spectrum at room temperature and then contacted with a flow of CO for 10 min.The IR spectra obtained by purging with Ar for 30 min and compared with before CO chemisorption.

2.3.Propane dehydrogenation reaction

The propane dehydrogenation over PtSnx/hierarchical ZSM-5 catalysts was carried out under atmospheric pressure in a fixedbed quartz reactor with an inner diameter of 6 mm.100 mg of catalyst was introduced into the quartz reactor with quartz wool as support,and then reduced under 10% (volume) H2at 570 °C for 1 h.The propane dehydrogenation was performed under 10 (volume) C3H8and 10 (volume) H2in N2with a weight hourly space velocity(WHSV)of 7.07 h-1for propane.A thermocouple was kept at the catalyst bed layer in order to accurately control the reaction temperature.The products were analyzed using an on-line gas chromatograph(GC,Pannuo-A91)with a flame ionization detector(FID).The carbon balance(above 98%)was calculated from the following Eq.(1),

and the flow rate of CH4,C2H6,C2H4,C3H6,and C3H8was estimated by using the flow rate of N2as a reference.Propane conversion and selectivity of the propylene products were calculated using following equations:

3.Results and Discussion

Scheme 1 shows the general procedure for the synthesis of Pt-Sn/hierarchical ZSM-5 catalysts,which consists of the creation of mesopore in ZSM-5 zeolite and loading Pt-Sn into hierarchical zeolite.In the first step,ZSM-5 zeolite was treated with different concentrations of NaOH solution for the synthesis of hierarchical ZSM-5 zeolite [22–26].Secondly,the Sn and Pt precursors were introduced into hierarchical ZSM-5 zeolite followed by a reduction process,which leads to the formation of Pt-Sn/hierarchical ZSM-5.

Scheme 1.Synthesis procedure for PtSnx/ZSM-5(y) catalysts with different Sn/Pt ratios.

The hierarchical ZSM-5 with microporosity and mesoporosity was prepared by a typical desilication process,which is carried out by a treatment of ZSM-5 in the NaOH solution under a mildcondition[25].The structural changes of desilicated ZSM-5 zeolites were investigated by X-ray powder diffraction,and the XRD pattern of these samples were shown in Fig.1.All ZSM-5 zeolite exhibit feature diffraction peaks located at 2θ=7.92°,8.80°,14.78°,23.10°,and 23.90°,which are attributed to the characteristic reflection peaks of the MFI structure(JCPDS No.42–0024)[27].However,the crystallinity of desilicated ZSM-5 zeolites decreases with an increase of alkali concentration.Therefore,it can be inferred that the desilication of ZSM-5 with NaOH solution from 0.2 to 1.0 mol·L-1leads to a decrease of crystallinity,but the topology of MFI is still well kept.

Fig.1.XRD patterns of hierarchical ZSM-5 obtained via a desilication treatment.

The pore structure of these desilicated ZSM-5 zeolite was studied by a nitrogen adsorption measurement.Fig.2 shows N2adsorption–desorption isotherms and the pore-size distribution of ZSM-5 samples.All the ZSM-5 zeolites exhibit a significant nitrogen absorption at relatively low pressure(P/P0＜0.1),which is assigned to the nitrogen filling in the micropore ZSM-5 zeolite.In comparison to the parent ZSM-5 zeolite,the desilicated ZSM-5 zeolites have an evident adsorption at P/P0above 0.5,which suggests that there is a certain level of mesopores in these zeolites[28].The texture properties of these ZSM-5 zeolites analyzed from nitrogen adsorption are shown in Table 2.A general trend can be observed that the alkali treatment has a great impact on the texture properties of desilicated ZSM-5 zeolites,the surface area and total pore volume is positively relevant to the concentration of NaOH solution [29].The parent ZSM-5 zeolite has a surface area of 257 m2·g-1and pore volume of 0.15 cm3·g-1.The treatment of ZSM-5 zeolite with NaOH solution at a low concentration of 0.2 mol·L-1leads to an increase of surface area to 294 m2·g-1and total pore volume to 0.17 cm3·g-1.The highest surface area of 406 m2·g-1and pore volume of 0.41 cm3·g-1are obtained in the desilicated ZSM-5 zeolite treated with 1.0 mol·L-1NaOH solution.

Table 2 Textural properties of ZSM-5 and hierarchical ZSM-5 samples

Fig.2.Nitrogen adsorption isotherms and pore size distribution (inset) of hierarchical ZSM-5 samples.

Fig.3.SEM images of H-ZSM-5,ZSM-5(0.2),ZSM-5(0.4),ZSM-5(0.6),ZSM-5(0.8) and ZSM-5(1.0) samples,respectively.

Fig.4.Propane conversion and propylene selectivity over different PtSn1.00/hierarchical ZSM-5(y)catalysts and ZSM-5(0.8)sample in the propane dehydrogenation reaction at 570 °C.

The morphology of the alkali-treated ZSM-5 zeolites can be visually reflected from scanning electronic microscopy characterization.Fig.3 represents the SEM images of parent ZSM-5 and desilicated ZSM-5 samples.The parent ZSM-5 has a hexahedron morphology with crystal size of 2–3 μm [30].As expected,the alkali treatment can partially destroy the structure of ZSM-5 zeolite,which leads to a change of its morphology.Evidently,some voids can be clearly seen at the surface of desilicated ZSM-5 crystals,which results from the removal of Si atoms in the alkali treatment.It should be noted that when ZSM-5 zeolites are treated with the alkali solution below 1.0 mol·L-1,their morphologies are still kept.However,when the ZSM-5 zeolite is treated with the alkali solution of 1.0 mol·L-1,the zeolite crystals are significantly destroyed which leads to formation of lots of small particles.

Pt and Sn with Sn/Pt ratio of 1.00 were loaded into the hierarchical ZSM-5 obtained from NaOH treatment with different concentrations,and the obtained Pt-Sn1.00/hierarchical ZSM-5 were studied in propane dehydrogenation at 570 °C.As shown in Fig.4,the NaOH concentration in the desilication process has a great impact on the catalytic performances of Pt-Sn/hierarchical ZSM-5 in propane dehydrogenation.The PtSn/ZSM-5 shows a low catalytic performance,which delivers an initial propane conversion of 28%and propylene selectivity of 65%.The conversion in this catalyst decreases to 17% and selectivity increase to 93% after 15 hours.In contrast,the Pt-Sn/hierarchical ZSM-5 catalysts obtained from the NaOH treatment show an enhanced catalytic performance.Even a low concentration (0.2 mol·L-1) of alkali treatment in the synthesis can promote the propane conversion to 35% and propylene selectivity to 98%.Significantly,when the NaOH concentration is increased to 0.8 mol·L-1,the obtained PtSn1.00/hierarchical ZSM-5(0.8) shows excellent catalytic performance with a high initial propane conversion of 47% and propylene selectivity of 97%.However,the ZSM-5(0.8) sample without loading Pt and Sn species delivers a propane conversion less than 1%.A further increase of the NaOH concentration to 1.0 mol·L-1results in a slight decrease in propane conversion and propylene selectivity.Therefore,it can be concluded that the optimized NaOH concentration for the synthesis of PtSn/hierarchical ZSM-5 catalysts should be around 0.8 mol·L-1,and the PtSn1.00/hierarchical ZSM-5(0.8) catalyst delivers the highest efficiency for the production of propylene from propane dehydrogenation.

Fig.5.XRD patterns of PtSnx/hierarchical ZSM-5(0.8) samples with different Sn/Pt ratios.

Fig.6.Nitrogen adsorption isotherms and pore size distribution (inset) of PtSnx/hierarchical ZSM-5(0.8) catalysts with different Sn/Pt ratios.

As discussed above,the concentration of NaOH in the desilication process has a great impact on the catalytic performance of PtSn1.00/hierarchical ZSM-5,because the texture property of hierarchical ZSM-5 is dependent on the desilication condition.It is found that the desilication carried out with 0.8 mol·L-1NaOH gives the best catalysts for propane dehydrogenation.Therefore,the NaOH concentration in the desilication process is kept at 0.8 mol·L-1in the following investigation.The Sn/Pt ratio is another important factor in PtSn/hierarchical ZSM-5 for the propane dehydrogenation reaction,and therefore the effect of Sn/Pt ratios from 0.33 to 3 on the catalytic performance of PtSn/hierarchical ZSM-5 is systematically studied in the following study.

Fig.5 shows the XRD patterns of the PtSnx/hierarchical ZSM-5(0.8) samples with different Sn/Pt ratios.All samples show similar diffraction patterns of typical MFI topology,indicating that the loading of Pt and Sn species does not affect the crystalline structure of the ZSM-5 zeolite framework [29].Moreover,no characteristic diffraction peaks related to metallic Pt and SnO2phases are observed,which suggests that Pt and Sn species are highly dispersed in the structure of hierarchical ZSM-5 zeolite.

The nitrogen adsorption isotherms of all the samples are shown in Fig.6.All the samples show a typical adsorption behavior of hierarchical zeolite with classic IV-type isotherm and H3-type hysteresis loop [31].The adsorption isotherms consist of a micropore filling under P/P0＜0.1 and a mesopore absorption with the P/P0above 0.8.The surface area and pore volume estimated from the nitrogen adsorption isotherm are shown in Table 3.All the PtSnx/hierarchical ZSM-5(0.8)samples show high pore volumes of about 0.35 cm3·g-1and large surface areas of about 350–400 m2·g-1,which are similar to the hierarchical ZSM-5(0.8)zeolite.Therefore,the introduction of Pt and Sn species into hierarchical ZSM-5(0.8)has no effect on its texture property.

Table 3 Textural properties of PtSnx/hierarchical ZSM-5(0.8) catalysts with different Sn/Pt ratios

Fig.7 shows STEM images of the representative PtSn1.00/hierarchical ZSM-5(0.8)sample.The STEM shows that all bimetallic Pt-Sn particles are uniformly distributed at the surface of ZSM-5,and no evident aggregation is observable [32].The average size of these PtSn particles is about 1.33 nm.It should be noted that the PtSn particle is bigger than the micropore of ZSM-5 zeolite,and it is found that these PtSn particles are trapped in the mesopores of hierarchical ZSM-5 zeolite(red area in the STEM).Elemental mapping in STEM was also performed on these bimetallic samples.The result shows that the two elements are uniformly distributed in the structure of hierarchical ZSM-5 zeolite,and the overlap of these two metal components confirms the formation of well-alloyed PtSn bimetallic particles [21].

Fig.7.STEM of and elemental mapping of a typical PtSn1.00/hierarchicalZSM-5(0.8) sample.

The local electronic environment of Pt-Sn bimetallic nanoparticles was studied by the FTIR spectroscopy with CO adsorption,and this characterization tool can be used to identify the formation of bimetallic Pt-Sn nanoparticles.The IR spectra of the CO adsorption on PtSnx/hierarchical ZSM-5(0.8) samples with different Sn/Pt ratios are shown in Fig.8.Generally,two types of CO adsorption on Pt particles can be observed:the linear CO adsorption on Pt nanoparticles located at around 2070 cm-1and the bridge CO adsorption on the adjacent Pt atoms located at around 1840 cm-1.The strong adsorptions at 2067 cm-1are observable in all PtSnx/hierarchical ZSM-5(0.8) samples.On the contrary,the bridge CO adsorption is only detectable in the Pt/hierarchical ZSM-5(0.8)sample[20,33].The doping of Pt with Sn leads to a disappearance of the bridge CO adsorption,which indicates that Pt atoms are isolated by Sn atoms for the formation of PtSn bimetallic particles.

Fig.8.IR spectra of the CO adsorption on PtSnx/hierarchical ZSM-5(0.8) catalysts with different Sn/Pt ratios.

The XPS measurement on Sn 3d band energy was carried out on the PtSnx/hierarchical ZSM-5 catalysts with different Sn/Pt ratios,in order to know the chemical state of Sn element [34].There is still a debate on the analysis of Sn 3d XPS spectra,but it is generally accepted that the Sn 3d XPS spectra can be fitted into three types of bands,which are assigned to the metallic Sn,Sn(II)and Sn(IV)species,respectively.As shown in Fig.9,these three types of Sn species are detectable in the PtSnx/hierarchical ZSM-5 catalysts.As we discussed in the previous report[13],these three Sn species play different roles in the PtSn/zeolite structure.Sn(IV)atoms are probably bonded into zeolite framework through four Si-O-Sn linkages,and therefore these Sn atoms are incorporated into the structure of ZSM-5 zeolite.The Sn(II)atoms are partially incorporated into zeolite framework,and they remain at the surface of ZSM-5 zeolite for the stabilization of Pt particles.The metallic Sn atoms are alloyed into Pt particles for the formation of Pt-Sn bimetallic alloy.

Fig.9.XPS of Sn3d core level of PtSnx/hierarchical ZSM-5(0.8) samples with different Sn/Pt ratios.

The catalytic performances of PtSnx/hierarchical ZSM-5(0.8)samples with different Sn/Pt ratios in propane dehydrogenation are shown in Fig.10.It is well-known that the Sn loadings in the PtSn bimetallic catalysts have a significant effect on the propane dehydrogenation reaction.The catalytic performance of the Pt/hierarchical ZSM-5(0.8) sample without Sn doping keeps at a low level.The initial propylene selectivity in this catalyst is only about 88%,and it increases to 94%after 24 hours.The propane conversion in Pt/hierarchical ZSM-5(0.8) decreases from 37% to 10% during 24 h reaction.An introduction of Sn species to the Pt/hierarchical ZSM-5(0.8) catalysts does not only improve the propane conversion,but also greatly increase the propylene selectivity.The propane conversion is positively linked to the Sn loading when the Sn/Pt ratios increase from 0.33 to 1.00.The most efficient catalyst is achieved in PtSn1.00/hierarchical ZSM-5(0.8) sample,which delivers an initial propane conversion of 45% and propylene selectivity of 98%.A slow drop of propane conversion is observed in this catalyst,but the propane conversion still keeps at 36% after 24 hours.An increase of Sn/Pt ratio in PtSn/hierarchical ZSM-5(0.8)from 1 to 3 leads to a significant decrease of propane conversion,and no evident drop of propylene selectivity is observed.Therefore,it can be concluded that the optimized Sn/Pt ratio in PtSn/hierarchical ZSM-5(0.8)should be kept at around 1,and achieving a high weight hourly space velocity (WHSV=7.07 h-1) for propane.

Fig.10.Propane conversion and propylene selectivity over different PtSnx/hierarchical ZSM-5(0.8) catalysts at 570 °C.

The optimal PtSn1.00/hierarchical ZSM-5(0.8) catalyst was tested for a long-time reaction of 72 hours.The propane conversion in the PtSn1.00/hierarchical ZSM-5(0.8) still keeps at about 20%after 70 hours (Fig.11).Also,the regenerability of the deactivated catalyst was investigated.The deactivated catalyst was regenerated by a simple calcination in a flow of air at 500 oC.As shown in Fig.12,the propane conversion over the regenerated catalyst is almost the same as that in the fresh catalyst.Therefore,the PtSn1.00/hierarchical ZSM-5(0.8)catalyst shows a long-time stability and good regenerability in the propane dehydrogenation reaction.

Fig.11.Catalytic performance of PtSn1.00/hierarchical ZSM-5(0.8) catalyst for 70 h time on stream reaction.

Fig.12.Regeneration of PtSn1.00/hierarchical ZSM-5(0.8) catalyst for propane dehydrogenation.

The high performance of these PtSn/hierarchical ZSM-5 in propane dehydrogenation is mainly attributed to the presence of mesopore in hierarchical ZSM-5 zeolite.The mesopores play two positive roles in the PtSn/hierarchical ZSM-5 for propane dehydrogenation.Firstly,the PtSn particles can be confined in the mesopore of hierarchical ZSM-5,and the confinement effect of mesopores enable to stabilize the PtSn particles in hierarchical ZSM-5 structure.Therefore,these PtSn particles can keep stable in the propane dehydrogenation at high temperature.Secondly,the presence of mesopores is helpful to accelerate the diffusion rate.The propane can easily access to the PtSn particles for dehydrogenation reaction,which is beneficial for enhancing propane conversion.Also,the diffusion of propylene out of zeolite crystal becomes fast,which leads to a decrease of cracking reaction and coking formation.Therefore,high catalytic performance is achieved in the PtSn/hierarchical ZSM-5 for propane dehydrogenation.

4.Conclusions

In summary,a series of PtSn/hierarchical ZSM-5 catalysts were developed for propane dehydrogenation.The syntheses of Pt-Sn/hierarchical ZSM-5 catalysts consist of the creation of mesopore in ZSM-5 zeolite and loading Pt-Sn into hierarchical zeolite.The hierarchical ZSM-5 zeolite was obtained via a treatment of ZSM-5 zeolites with alkali solution under a mild condition.The introduction of Sn and Pt species into hierarchical ZSM-5 zeolite leads to the formation of Pt-Sn/hierarchical ZSM-5.The effects of the concentrations of alkali solution and Sn/Pt ratios on the catalytic performance of PtSn/hierarchical ZSM-5 were systematically studied,and it is found that the PtSn1.00/ZSM-5(0.8) catalyst shows the highest efficiency in propane dehydrogenation,which gives an initial conversion of 46% and selectivity of 98% at 570 oC.The high efficiency of these PtSn/hierarchical ZSM-5 catalysts for propane dehydrogenation is mainly attributed to the confinement of PtSn particles in the mesopores of hierarchical ZSM-5 zeolite.

CRediT authorship contribution statement

Tianlei Wang:Methodology,Formal analysis,Data curation,Writing–review &editing.Zhikang Xu:Investigation,Formal analysis,Writing–original draft.Yuanyuan Yue:Project administration,Investigation.Tinghai Wang:Data analysis,Writing review &editing.Minggui Lin:Investigation,Methodology.Haibo Zhu:Conceived the concept,Funding acquisition,Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial supports of the National Natural Science Foundation of China (21878050,22178062),Foundation of State Key Laboratory of Coal Conversion (J21-22-620) and Green Petrochemical Engineering Base of Intelligence Introduction for Innovation (111 Project D17005) are gratefully acknowledged.