阅读说明：本技术 一种酸性温和的超细纳米zsm-5分子筛的合成方法 (Synthesis method of superfine nano ZSM-5 molecular sieve with mild acidity ) 是由 吴伟 冯超群 苏晓芳 王巍 白雪峰 于 2020-12-01 设计创作，主要内容包括：一种酸性温和的超细纳米ZSM-5分子筛的合成方法,它属于沸石分子筛催化剂的制备领域,它要解决现有合成纳米ZSM-5沸石分子筛存在晶化温度较高、晶化时间长,且晶粒尺寸较大,结晶度低、酸强度过高的问题。合成方法：一、称取原料；二、将异丙醇铝、四丙基氢氧化铵水溶液、去离子水和过硫酸钠混合搅拌,加入正硅酸乙酯,制备混合凝胶；三、搅拌混合凝胶；四、在75～85℃下晶化。本发明在初始凝胶过程中加入过硫酸钠作为羟基自由基引发剂,促进分子筛晶化过程晶核的形成。本发明的制备工艺简单、能耗低、分子筛的结晶度高、酸性温和、晶粒尺寸小,单个纳米晶的尺寸仅为5～10nm。(A method for synthesizing an ultrafine nano ZSM-5 molecular sieve with mild acidity belongs to the field of preparation of zeolite molecular sieve catalysts and aims to solve the problems of high crystallization temperature, long crystallization time, large grain size, low crystallinity and overhigh acid strength of the existing synthesized nano ZSM-5 zeolite molecular sieve. The synthesis method comprises the following steps: firstly, weighing raw materials; mixing and stirring aluminum isopropoxide, tetrapropyl ammonium hydroxide aqueous solution, deionized water and sodium persulfate, and adding tetraethoxysilane to prepare mixed gel; thirdly, stirring and mixing the gel; fourthly, crystallizing at 75-85 ℃. In the invention, sodium persulfate is added as a hydroxyl radical initiator in the initial gelling process to promote the formation of crystal nucleus in the crystallization process of the molecular sieve. The preparation method disclosed by the invention is simple in preparation process, low in energy consumption, high in crystallinity of the molecular sieve, mild in acidity and small in grain size, and the size of a single nanocrystal is only 5-10 nm.)

1. A synthesis method of a mild-acidity superfine nano ZSM-5 molecular sieve is characterized by comprising the following steps:

weighing 1.0 part of aluminum isopropoxide, 46.9-312.5 parts of deionized water, 31.2-207.8 parts of ethyl orthosilicate, 19.4-129.5 parts of tetrapropyl ammonium hydroxide aqueous solution and 0.2-1.6 parts of sodium persulfate as raw materials in parts by weight, wherein the mass fraction of the tetrapropyl ammonium hydroxide in the tetrapropyl ammonium hydroxide aqueous solution is 50-55%;

mixing and stirring the aluminum isopropoxide weighed in the step one, a tetrapropyl ammonium hydroxide aqueous solution, deionized water and sodium persulfate to obtain a clear solution, and then adding tetraethoxysilane into the clear solution to obtain mixed gel;

thirdly, stirring and mixing the gel for 3-8 hours to obtain stirred gel;

and fourthly, adding the stirred gel obtained in the third step into a stainless steel closed reaction kettle with a polytetrafluoroethylene inner liner, crystallizing at 70-85 ℃ for 12-120 hours, cooling, sequentially performing centrifugal washing, filtering, drying and roasting to obtain the superfine nano ZSM-5 molecular sieve.

2. The method for synthesizing the ultra-fine nano ZSM-5 molecular sieve with mild acidity according to claim 1, wherein the mass fraction of the tetrapropylammonium hydroxide in the tetrapropylammonium hydroxide aqueous solution in the first step is 54.78%.

3. The synthesis method of the ultra-fine nanometer ZSM-5 molecular sieve with mild acidity according to claim 1, wherein in the step one, 1.0 part by weight of aluminum isopropoxide, 60 to 180 parts by weight of deionized water, 35 to 120 parts by weight of ethyl orthosilicate, 25 to 80 parts by weight of tetrapropyl ammonium hydroxide aqueous solution and 0.2 to 1.0 part by weight of sodium persulfate are weighed as raw materials.

4. The synthesis method of the ultrafine nanometer ZSM-5 molecular sieve with mild acidity according to claim 1, wherein the mass ratio of sodium persulfate to tetraethoxysilane in the first step is 0.005-0.01: 1.

5. The method for synthesizing the superfine nano ZSM-5 molecular sieve with mild acidity according to claim 1, wherein the atomic ratio of silicon to aluminum in the mixed gel in the second step is 50-100: 1.

6. The method for synthesizing the superfine nano ZSM-5 molecular sieve with mild acidity according to claim 1, wherein in the second step, tetraethoxysilane is added into the clear solution under the stirring condition of the rotating speed of 260-310 r/min.

7. The synthesis method of the ultra-fine nanometer ZSM-5 molecular sieve with mild acidity according to claim 1, wherein the gel is stirred and mixed in the third step under the stirring condition of the rotation speed of 280-360 r/min.

8. The method for synthesizing the ultrafine nano ZSM-5 molecular sieve with mild acidity according to claim 1, wherein the crystallization is performed at 80-85 ℃ for 24-48 hours in the fourth step.

9. The method for synthesizing the ultra-fine nano ZSM-5 molecular sieve as claimed in claim 1, wherein the calcination in step four is at 550 ℃ for 3 hours.

10. The method for synthesizing the ultra-fine nano ZSM-5 molecular sieve of claim 1, wherein the ultra-fine nano ZSM-5 molecular sieve product has a particle size of 5-10 nm.

Technical Field

The invention belongs to the field of preparation of zeolite molecular sieve catalysts, and particularly relates to a synthesis method of a superfine nano ZSM-5 molecular sieve with mild acidity.

Background

The ZSM-5 molecular sieve has wide application in the fields of petroleum refining and fine chemical industry as an environment-friendly catalyst due to the unique two-dimensional cross pore channel structure and good hydrothermal stability. Compared with the traditional micron ZSM-5 molecular sieve, the nanometer ZSM-5 molecular sieve has a shorter pore channel structure, can obviously improve the diffusion performance, inhibits carbon deposition inactivation, has larger external surface area, can expose more accessible acid sites, further improves the utilization rate of acid centers, and shows higher catalytic activity and stability. The low-temperature crystallization can promote the generation of crystal nucleus and inhibit the growth of crystal grains, and is a common means for preparing the nano-crystal. In the molecular sieve crystallization process, a free radical initiator is added to generate hydroxyl free radicals (. OH), which can promote the breakage of Si-O-Si bonds and generate silicon oxygen free radicals with high polycondensation capacity, remarkably improve the nucleation rate and greatly shorten the crystallization time, thus being an environment-friendly molecular sieve synthesis method.

The Chinese patent publication No. CN 107963639A discloses a rapid synthesis method of a ZSM-5 molecular sieve with uniform nanometer particle size, which comprises the steps of firstly mixing deionized water, a silicon source and an organic template agent, stirring for 1-6 hours at 50-100 ℃, then mixing the deionized water, an alkali source and an aluminum source, stirring for 1-6 hours at 50-100 ℃, then standing for 10-24 hours at 20-50 ℃ to obtain a crystallization precursor solution of the ZSM-5 molecular sieve, and performing oil bath crystallization for 3-30 min at 140-200 ℃ in a metal tube reactor; the ZSM-5 molecular sieve with the grain diameter of 100nm is prepared. The preparation process of the method is complex, and the requirement on a crystallization reactor is high.

The Chinese patent publication No. CN 104192859A discloses a rapid synthesis method of a small-grain ZSM-5 molecular sieve, which comprises the steps of firstly nucleating the prepared silicon-aluminum mixed gel at a low temperature of 60-120 ℃ for 1-3 hours, and then carrying out crystallization growth at a high temperature of 150-170 ℃ for 1-3 hours to prepare the ZSM-5 molecular sieve with the grain size of 270-450 nm. In order to obtain a product with higher crystallinity, the method still needs secondary crystallization at high temperature, and the grain size of the synthesized molecular sieve is larger.

Chinese patent publication No. CN 102874843A discloses a rapid synthesis method of a nano-scale ZSM-5 molecular sieve, which comprises the steps of mixing a silicon source, an aluminum source and a template agent, aging for 2-6 hours to obtain sol gel, drying at 60-90 ℃ to prepare dry glue, adding the dry glue serving as a seed crystal into the mixed gel containing the silicon source and the aluminum source, and statically crystallizing at 150-200 ℃ for 12-40 hours to prepare the nano ZSM-5 molecular sieve with the grain size of 60-120 nm. The preparation process of the method is complex, dry glue needs to be prepared firstly as seed crystal, the crystallization temperature is high, and the grain size of the synthesized molecular sieve is large.

Chinese patent publication No. CN 104876238A discloses a method for synthesizing molecular sieve by ultraviolet radiation assistance, which comprises mixing and stirring a silicon source, a template agent and water according to a certain proportion for 12-24 hours to obtain pure silicon Silicalite-1 initial gel, transferring the initial gel into a quartz vessel, and performing ultrasonic treatment at a power density of 20-80 w/m²The crystallization is carried out for 36 to 90 hours under the ultraviolet radiation, and the crystallization temperature is 60 to 70 ℃.

In summary, the disclosed method for synthesizing the nano ZSM-5 zeolite molecular sieve still has the problems of long crystallization time, large grain size, easy carbon deposition and deactivation due to strong acidity, and the like, and thus the industrial application thereof is limited to a great extent. Therefore, the development of a novel method for efficiently preparing the ultrafine nano ZSM-5 molecular sieve with mild acidity has important research value and application prospect.

Disclosure of Invention

The invention aims to solve the problems of longer crystallization time, larger particle size of the synthesized molecular sieve, lower crystallinity, carbon deposition inactivation caused by high acid strength and the like in the process of synthesizing the nano ZSM-5 molecular sieve by the conventional hydrothermal method, and synthesizes the ultrafine nano ZSM-5 molecular sieve with high crystallinity and mild acid in a shorter crystallization time at a lower crystallization temperature by adding a certain amount of sodium persulfate serving as a free radical initiator into an initial gel system.

The synthesis method of the mild-acidity superfine nano ZSM-5 molecular sieve is realized by the following steps:

weighing 1.0 part of aluminum isopropoxide, 46.9-312.5 parts of deionized water, 31.2-207.8 parts of ethyl orthosilicate, 19.4-129.5 parts of tetrapropyl ammonium hydroxide aqueous solution and 0.2-1.6 parts of sodium persulfate as raw materials in parts by weight, wherein the mass fraction of the tetrapropyl ammonium hydroxide in the tetrapropyl ammonium hydroxide aqueous solution is 50-55%;

mixing and stirring the aluminum isopropoxide weighed in the step one, a tetrapropyl ammonium hydroxide aqueous solution, deionized water and sodium persulfate to obtain a clear solution, and then adding tetraethoxysilane into the clear solution to obtain mixed gel;

thirdly, stirring and mixing the gel for 3-8 hours to obtain stirred gel;

and fourthly, adding the stirred gel obtained in the third step into a stainless steel closed reaction kettle with a polytetrafluoroethylene inner liner, crystallizing at 70-85 ℃ for 12-120 hours, cooling, sequentially performing centrifugal washing, filtering, drying and roasting to obtain the superfine nano ZSM-5 molecular sieve.

Compared with the traditional hydrothermal synthesis method for preparing the ZSM-5 molecular sieve, the method has the advantages that the sodium persulfate is added in the process of preparing the initial gel, and the sodium persulfate can generate hydroxyl radicals (. OH) under the heating condition, so that the crystallization of the molecular sieve can be accelerated at a lower temperature, and the synthesized ZSM-5 molecular sieve has higher crystallinity, smaller grain size, larger external surface area and milder acidity, and is a novel molecular sieve synthesis method with simple process and low energy consumption.

The method for synthesizing the superfine nano ZSM-5 molecular sieve has the following beneficial effects:

1. the invention provides a novel method for synthesizing an ultrafine nano ZSM-5 molecular sieve with mild acidity by a hydrothermal method. Sodium persulfate is added in the process of preparing the initial gel, can generate hydroxyl radicals (. OH) under the heating condition, can promote depolymerization and polycondensation of a silicon source and an aluminum source under the mild condition, completes crystallization at a lower temperature, shortens the crystallization time, reduces the energy consumption in the synthesis process of the molecular sieve, and is a novel method for efficiently preparing the ultrafine nano ZSM-5 molecular sieve.

2. The method provided by the invention is simple and easy to implement, the superfine nano ZSM-5 molecular sieve can be synthesized at a lower crystallization temperature, the obtained nanocrystals have uniform sizes and spherical aggregates in appearance, the size of a single crystal grain is only 5-10nm, the mass transfer performance of the molecular sieve as a catalyst can be obviously improved, the acidity of the synthesized ZSM-5 molecular sieve is milder, the carbon deposition inactivation can be effectively inhibited, and the service life of the catalyst can be prolonged.

Drawings

FIG. 1 is an X-ray diffraction spectrum of a nano ZSM-5 molecular sieve prepared by the example;

FIG. 2 is a scanning electron micrograph of a nano ZSM-5 molecular sieve prepared in the first example;

FIG. 3 is an X-ray diffraction spectrum of the nano ZSM-5 molecular sieve prepared in example two;

FIG. 4 is a scanning electron micrograph of the nano ZSM-5 molecular sieve prepared in example two;

FIG. 5 is an X-ray diffraction spectrum of the nano ZSM-5 molecular sieve prepared in example three;

FIG. 6 is a scanning electron micrograph of the nano ZSM-5 molecular sieve prepared in example III;

FIG. 7 is an X-ray diffraction spectrum of the nano ZSM-5 molecular sieve prepared in example four;

FIG. 8 is a TEM photograph of the nano ZSM-5 molecular sieve prepared in example IV;

FIG. 9 is a TEM photograph of the nano ZSM-5 molecular sieve prepared in example IV;

FIG. 10 shows NH of the nano ZSM-5 molecular sieve prepared in the first example₃-a TPD profile;

FIG. 11 shows NH of nano ZSM-5 molecular sieve prepared in example two₃-a TPD profile;

FIG. 12 is NH of nano ZSM-5 molecular sieve prepared in example three₃-a TPD profile;

FIG. 13 is NH of nano ZSM-5 molecular sieve prepared in example four₃-TPD profile.

Detailed Description

The first embodiment is as follows: the synthesis method of the superfine nano ZSM-5 molecular sieve with mild acidity in the embodiment is implemented according to the following steps:

weighing 1.0 part of aluminum isopropoxide, 46.9-312.5 parts of deionized water, 31.2-207.8 parts of ethyl orthosilicate, 19.4-129.5 parts of tetrapropyl ammonium hydroxide aqueous solution and 0.2-1.6 parts of sodium persulfate as raw materials in parts by weight, wherein the mass fraction of the tetrapropyl ammonium hydroxide in the tetrapropyl ammonium hydroxide aqueous solution is 50-55%;

mixing and stirring the aluminum isopropoxide weighed in the step one, a tetrapropyl ammonium hydroxide aqueous solution, deionized water and sodium persulfate to obtain a clear solution, and then adding tetraethoxysilane into the clear solution to obtain mixed gel;

thirdly, stirring and mixing the gel for 3-8 hours to obtain stirred gel;

and fourthly, adding the stirred gel obtained in the third step into a stainless steel closed reaction kettle with a polytetrafluoroethylene inner liner, crystallizing at 70-85 ℃ for 12-120 hours, cooling, sequentially performing centrifugal washing, filtering, drying and roasting to obtain the superfine nano ZSM-5 molecular sieve.

The second embodiment is as follows: the present embodiment is different from the first embodiment in that the mass fraction of tetrapropylammonium hydroxide in the tetrapropylammonium hydroxide aqueous solution in the first step is 54.78%.

The third concrete implementation mode: the difference between the first embodiment and the second embodiment is that in the first step, 1.0 part by weight of aluminum isopropoxide, 60-180 parts by weight of deionized water, 35-120 parts by weight of ethyl orthosilicate, 25-80 parts by weight of tetrapropyl ammonium hydroxide aqueous solution and 0.2-1.0 part by weight of sodium persulfate are weighed as raw materials.

The fourth concrete implementation mode: the difference between the first embodiment and the third embodiment is that the mass ratio of the sodium persulfate to the tetraethoxysilane in the first step is 0.005-0.01: 1.

The fifth concrete implementation mode: the difference between this embodiment and one of the first to fourth embodiments is that the ratio of silicon to aluminum (atomic ratio) in the mixed gel in the second step is 50 to 100: 1.

The sixth specific implementation mode: the difference between the embodiment and one of the first to fifth embodiments is that in the second step, tetraethoxysilane is added into the clear solution under the stirring condition of the rotating speed of 260-310 r/min.

The seventh embodiment: the difference between this embodiment and the first to the sixth embodiment is that the gel is stirred and mixed in the third step under the stirring condition with the rotation speed of 280 to 360 r/min.

The specific implementation mode is eight: the difference between this embodiment and the first to seventh embodiments is that the crystallization is performed at 80-85 ℃ for 24-48 hours in the fourth step.

The specific implementation method nine: this embodiment is different from the first to eighth embodiments in that the calcination described in the fourth step is calcination at 550 ℃ for 3 hours.

The detailed implementation mode is ten: the difference between this embodiment and one of the first to ninth embodiments is that the particle size of the ultra-fine nano ZSM-5 molecular sieve product is 5-10nm (range value).

The first embodiment is as follows: the synthesis method of the mild-acidity ultrafine nano ZSM-5 molecular sieve of the embodiment is implemented according to the following steps:

firstly, 0.2790g of aluminum isopropoxide, 22.01g of deionized water, 14.519g of ethyl orthosilicate, 9.127g of tetrapropyl ammonium hydroxide aqueous solution and 0.1124g of sodium persulfate are weighed as raw materials in parts by weight, wherein the mass fraction of the tetrapropyl ammonium hydroxide in the tetrapropyl ammonium hydroxide aqueous solution is 54.78%;

mixing and stirring the aluminum isopropoxide weighed in the step one, the tetrapropylammonium hydroxide aqueous solution, the deionized water and the sodium persulfate to obtain a clear solution, and adding the tetraethoxysilane into the clear solution under the stirring condition of the rotating speed of 260rpm to obtain mixed gel;

thirdly, stirring and mixing the gel for 5 hours under the stirring condition with the rotating speed of 350r/min to obtain stirred gel;

fourthly, adding the stirred gel obtained in the third step into a stainless steel closed reaction kettle with a polytetrafluoroethylene inner liner, crystallizing for 24 hours at 85 ℃, cooling to 25 ℃, sequentially carrying out centrifugal washing for three times, filtering, finally drying and roasting at 550 ℃ to obtain the superfine nano ZSM-5 molecular sieve, which is marked as NZ5-1, the SiO of the sample₂/Al₂O₃Equal to 100.

The centrifugal washing in step four of this example is to separate the solid product by centrifuging for 10min at 12500r/min after each washing.

The XRD spectrum of the ultra-fine nano ZSM-5 molecular sieve synthesized in this example is shown in fig. 1, and as can be seen from fig. 1, characteristic diffraction peaks corresponding to MFI topology appear at 2 θ of 7.92 °, 8.80 °, 14.78 °, 23.10 °, 23.90 ° and 24.40 ° of the prepared nano ZSM-5 molecular sieve, and no diffraction peaks of other heterocrystals appear, indicating that a pure-phase ZSM-5 molecular sieve is synthesized. As can be seen from the SEM photograph of FIG. 2, the prepared ultrafine nano ZSM-5 molecular sieve sample exists in the form of spherical aggregates formed by stacking nanocrystals, and the size of the aggregates is about 100 nm.

Example two: the synthesis method of the mild-acidity ultrafine nano ZSM-5 molecular sieve of the embodiment is implemented according to the following steps:

firstly, 0.1395g of aluminum isopropoxide, 21.973g of deionized water, 14.519g of ethyl orthosilicate, 9.127g of tetrapropyl ammonium hydroxide aqueous solution and 0.1124g of sodium persulfate are weighed as raw materials in parts by weight, wherein the mass fraction of the tetrapropyl ammonium hydroxide in the tetrapropyl ammonium hydroxide aqueous solution is 54.78%;

mixing and stirring the aluminum isopropoxide weighed in the step one, the tetrapropylammonium hydroxide aqueous solution, the deionized water and the sodium persulfate to obtain a clear solution, and adding the tetraethoxysilane into the clear solution under the stirring condition of the rotating speed of 260rpm to obtain mixed gel;

thirdly, stirring and mixing the gel for 5 hours under the stirring condition with the rotating speed of 350r/min to obtain stirred gel;

fourthly, adding the stirred gel obtained in the third step into a stainless steel closed reaction kettle with a polytetrafluoroethylene inner liner, crystallizing for 24 hours at 85 ℃, cooling to 25 ℃, sequentially carrying out centrifugal washing for three times, filtering, finally drying and roasting at 550 ℃, and obtaining the superfine nano ZSM-5 molecular sieve, which is marked as NZ5-2, the SiO of the sample₂/Al₂O₃Equal to 200.

The XRD spectrum of the ultra-fine nano ZSM-5 molecular sieve synthesized in this example is shown in fig. 3, and as can be seen from fig. 3, characteristic diffraction peaks corresponding to MFI topology appear at 2 θ of 7.92 °, 8.80 °, 14.78 °, 23.10 °, 23.90 ° and 24.40 ° of the prepared nano ZSM-5 molecular sieve, and no diffraction peaks of other heterocrystals appear, indicating that a pure-phase ZSM-5 molecular sieve is synthesized. As can be seen from the SEM photograph of FIG. 4, the prepared ultrafine nano ZSM-5 molecular sieve sample exists in the form of spherical aggregates formed by stacking nanocrystals, and the size of the aggregates is about 100 nm.

Example three: the difference between this example and the first example is that in the fourth step, the crystallization is carried out at 85 ℃ for 48 hours.

The ultrafine nano ZSM-5 molecular sieve obtained in this example is marked as NZ 5-3.

The XRD spectrum of the ultra-fine nano ZSM-5 molecular sieve synthesized in this example is shown in fig. 5, and as can be seen from fig. 5, characteristic diffraction peaks corresponding to MFI topology appear at 2 θ of 7.92 °, 8.80 °, 14.78 °, 23.10 °, 23.90 ° and 24.40 ° of the prepared nano ZSM-5 molecular sieve, and no diffraction peaks of other heterocrystals appear, indicating that a pure-phase ZSM-5 molecular sieve is synthesized. As can be seen from the SEM photograph of FIG. 6, the prepared ultrafine nano ZSM-5 molecular sieve sample exists in the form of spherical aggregates formed by stacking nanocrystals, and the size of the aggregates is about 100 nm.

Example four: the difference between this example and the first example is that in step four, the crystallization is carried out at 85 ℃ for 72 hours.

The ultrafine nano ZSM-5 molecular sieve obtained in this example is marked as NZ 5-4.

The XRD spectrum of the ultra-fine nano ZSM-5 molecular sieve synthesized in this example is shown in fig. 5, and as can be seen from fig. 5, characteristic diffraction peaks corresponding to MFI topology appear at 2 θ of 7.92 °, 8.80 °, 14.78 °, 23.10 °, 23.90 ° and 24.40 ° of the prepared nano ZSM-5 molecular sieve, and no diffraction peaks of other heterocrystals appear, indicating that a pure-phase ZSM-5 molecular sieve is synthesized. As can be seen from the SEM photograph of FIG. 8 and the TEM photograph of FIG. 9, the prepared ultrafine nano ZSM-5 molecular sieve sample exists in the form of spherical aggregates formed by stacking nanocrystals, the aggregate size is about 100nm, and the single crystal grain size is about 5-10 nm.

The physicochemical properties of the ultrafine nano ZSM-5 molecular sieves prepared in the first to fourth examples were characterized, and the results are shown in tables 1 and 2. As can be seen from the data in Table 1, the ultrafine nano ZSM-5 molecular sieve prepared by the invention has very large external surface area and mesoporous pore volume due to small crystal grain size, the proportion of the external surface area to the BET total surface area is as high as 49.9-62.1%, and the proportion of the mesoporous pore volume to the total pore volume is as high as 84.4-85.7%. Because the superfine nano ZSM-5 molecular sieve synthesized by the method has rich intercrystalline mesopores, when the molecular sieve is used as a catalyst, the molecular sieve is beneficial to improving the mass transfer performance of a pore channel, improving the diffusion performance of reactants and products and further improving the catalytic reaction performance. As can be seen from the data in Table 2, the acid strength of the strong acid sites of the ultrafine nano ZSM-5 molecular sieve prepared by the method is milder (the peak temperature corresponding to the strong acid sites is lower) than that of the disclosed micron ZSM-5 molecular sieve synthesized by the traditional hydrothermal method.

TABLE 1 specific surface area and pore volume of the samples

TABLE 2 acid strength and acid amount of the samples