1 Introduction

Zeolites of crystalline aluminosilicate have attracted tremendous attentions for their prominent characters of large surface areas, uniform and abundant porosities, tunable acidity and excellent thermal/hydrothermal stability, which endow them wide application in the field of catalysis, adsorption and separation, ion exchange, energy transformation and so on [1,2,3,4,5,6,7,8]. Among those hundreds of reported zeolites, the MFI-type ZSM-5 zeolite is undoubtedly one of the most popular members because of its special three-dimensional channel system with 10-rings channel window, which can be used as catalysts and catalyst supports in oil refining, petrochemical and fine chemicals processing and exhibit outstanding performance [9,10,11,12,13,14,15,16,17,18,19,20,21]. Heavy use of zeolite materials has led to creased demand of reasonable zeolite preparation. However, the conventional preparation method of ZSM-5, as well as some other important zeolites, is hydrothermal process that consuming organic structural directing agents (OSDA) and plenty of water as solvent, usually existing the cost and environment problems [22,23,24,25,26,27]. For example, the consumption of expensive organic templates will result in high cost of synthesis process. Both the template and the decomposition products of template (NOx, CO2) are almost noxious. In addition, large amount of water in hydrothermal system not only produce lots of waste liquid, but the dissolution of Si- and Al- nutrients in solvent lead to the loss of zeolite product yield. Therefore, the development of sustainable economic and green synthesis routes without organic template and solvent for important zeolites is of significance and much-anticipated [28,29,30,31,32,33].

In general, the functions of templates in the zeolite synthesis involve structure-directing, channel-filling and charge-balance [34]. So, other alternatives with similar roles are necessary in OSDA-free zeolite synthesis system. Fortunately, the ZSM-5 zeolite can be synthesized by various templating routes. Except the most commonly used tetrapropyl ammonium templates (TPAOH and TPABr), other organic amines such as n-butylamine and ethanediamine, alcohols and alkali metal ions can also act as templates for successful preparation of ZSM-5 zeolite [25,26,27, 35,36,37,38,39]. To pursue the goal of OSDA-free synthesis, the utilization of alkali metal ions and alcohols are seemed to be more promising. The most typical is sodium ions, which can induce the synthesis of ZSM-5 in template-free hydrothermal system and the obtained products possessing open channel even without calcination treatment. But the drawback of low yield still exists for the use of large amount of water solvent in synthetic system [40,41,42,43,44].

Until Xiao and coworkers pronounced sustainable synthesis strategy for ZSM-5 and some other zeolites by conflating organotemplate-free and solvent-free routes, the green synthesis of zeolites had made further substantial progress [29, 45,46,47,48,49]. The solid raw materials are first mixed and ground well and then suffered crystallization treatment, the crystalline zeolite can be prepared. Because there is no use water solvent, the crystal water in raw materials plays a crucial role in formation of zeolite products. The raw materials of this green OSDA-free synthesis process can be expanded from conventional commercial reagent to natural mineral, such as kaolin and illite, which further reduce the synthesis cost [50,51,52,53]. However, the metal impurities in natural materials have significant effect on synthesis of zeolites, hampering the clarification of key synthetic conditions in synthesis process. In addition, it worth noting that most aforementioned green synthesis process are assisted by pre-synthesized target zeolites as seed. As Wu et al. had stated, same synthetic system will generate amorphous product without seed [47]. Therefore, it is obvious that the sustainable synthesis strategy for ZSM-5 zeolite can be further optimized.

Herein, a sustainable OSDA-, solvent- and seed-free synthesis for ZSM-5 with commercial silica gel or prepared Stöber-derived silica spheres as raw materials is researched. The influences of system composition and crystallization parameters on product are investigated to clarify the crucial synthetic conditions of this sustainable synthesis process. The synthesized ZSM-5 zeolites are characterized. In addition, this sustainable synthesis method can be applied to fabricate metal-zeolite bifunctional catalysts for hydroisomerization of linear alkane.

2 Experimental

2.1 Chemicals

Silica gel (specific surface area: 350–460 m2/g, pore size: 8–12 nm, SiO2: 99%.) was purchased from Qingdao Xinchanglai Silica Gel Ltd. Tetraethylorthosilicate (TEOS), polyvinylpyrrolidone (PVP K30, M.W. = 38,000) and dihydrogen hexachloroplatinate (H2PtCl6·6H2O) were purchased from Shanghai Aladdin Biochemical Technology Co. Sodium aluminate (NaAlO2), aluminium sulphate (Al2(SO4)3·18H2O), sodium hydroxide, sodium carbonate, ethanol, ammonium chloride and ammonia hydroxide (NH3·H2O, 25–28 wt %) were purchased from Tianjin Damao Chemical Co. The distilled water was homemade. All the reagents were used without further purification.

2.2 Synthesis of Pt nanoparticles

The Pt nanoparticles was synthesized by alcohol reduction process [54]. Firstly, 85.7 mg PVP and 1.08 mL H2PtCl6 aqueous solution (38.6 mM) were dissolved in 45 mL ethanol and 5 mL water. Then, the synthetic mixture was refluxed at 80 °C under stirring for 2 h. Finally, the generated black Pt nanoparticles were collected by centrifugation and were redispersed in ethanol for further use.

2.3 Synthesis of SiO2 spheres and Pt@SiO2

The SiO2 colloidal spheres were synthesized by a well-known Stӧber method with slight modification. In typical, 70 mL ethanol, 15 mL water and 4 mL NH3·H2O were mixed together, followed by the addition of 3.5 g TEOS. After stirring constantly at room temperature for 24 h, the SiO2 colloidal spheres were obtained by centrifugation and dried at 100 °C. The synthesis process of Pt@SiO2 is similar to SiO2 spheres except for the additional introduction of recommended amount of Pt nanoparticles (0.025 mmol) preceding the addition of TEOS.

2.4 Sustainable synthesis of ZSM-5 and derived catalysts

The synthesis of ZSM-5 zeolite was performed by reported solid-state conversion. In a typical run, 1.5 g of silica source (silica gel or Stӧber SiO2 sphere), 0.10 g of NaAlO2, 0.09 g of NaOH and 0.90 g of water were mixed by grinding for 10 min. Then the mixture was transferred into 25 mL Teflon-lined autoclave and heated at 170 °C for 24 h and the crystalline product can be obtained. For the synthesis of Pt@ZSM-5 catalyst, Pt@SiO2 was used as silica source and other conditions remained unchanged. The H-form zeolites were obtained by ion-exchange of as-synthesized Na-form products in 1 M NH4Cl aqueous solution at 80 °C for 8 h, followed by calcination process (550 °C, 4 h). This ion-exchange process needs to be repeated once.

For comparison, supported Pt/ZSM-5 catalyst was prepared with H-form ZSM-5 from Stӧber SiO2 sphere as carrier via incipient-wetness impregnation method.

2.5 Characterization

Powder X-ray diffraction (XRD) pattern was recorded in a Bruck D8 Advance powder X-ray diffractometer using Cu Kα radiation in 2θ range of 4–50o. Scanning electronic microscopy (SEM) images were taken on SU8010 field-emission scanning electronic microscope operating at 5 kV. Transmission electronic microscopy (TEM) and high resolution transmission electronic microscopy (HRTEM) images were taken on a JEM-2100 electronic microscope with an accelerating voltage of 200 kV. Nitrogen physical adsorption/desorption isotherms were measured on Quantachrome Autosorb-IQ2-MP physical adsorption apparatus. The specific surface areas were calculated using the BET method. Ammonia temperature programmed desorption (NH3-TPD) measurements were performed on Quantachrome TPD/TDR-Pulsar chemisorption analyzer in the range of 150–600 °C at a ramp rate of 20 °C min−1. The SiO2/Al2O3 ratio in the samples was calculated according to the analysis results on Bruker D8 Tiger X-ray fluorescence (XRF) spectrometer. The Pt content in metal-zeolite bifunctional catalysts were analyzed by inductively coupled plasma spectrometry (ICP) on Agilent ICP-OES 725 instrument. X-ray photoelectron spectroscopy (XPS) was measured on Thermo Fisher Scientific ESCALAB Xi+ spectrometer (Al Kα, hv = 1480 eV).

2.6 Catalytic tests

Catalytic performance of the prepared catalysts for hydroisomerization of n-heptane were operated on a fixed-bed stainless steel reactor at atmospheric pressure. Before reaction, the catalyst (0.5 g) was reduced by H2 flow at 400 °C for 2 h, and then was cooled to reaction temperature. The n-heptane was fed into reactor with HPLC pump at a weight hourly space velocity (WHSV) of 2.0 h−1 and the H2/n-heptane molar ratio was fixed at 10. The reaction products were detected on Techcomp GC7890 equipped with a TM-PONA capillary column (50 m × 0.2 mm × 0.5 μm) and FID detector.

3 Results and discussion

3.1 Sustainable synthesis for ZSM-5 zeolite

The absolute green synthesis for ZSM-5 zeolite without using organotemplate, solvent and zeolite seed is described in Fig. 1. The starting materials are mixed by grinding in the absence of adequate solvent and further crystallize into zeolite. Figure 2 shows the XRD patterns of prepared zeolites with various SiO2/Al2O3 molar ratio from commercial silica gel and sodium aluminate. It can be observed that the SiO2/Al2O3 ratio of starting materials has significant influence on the synthesis. Only suitable SiO2/Al2O3 ratio (30–40) can produce the MFI-type zeolite with highly crystallinity and the lower Al content of starting materials (SiO2/Al2O3 = 50–60) have generated the zeolite product accompanied by amorphous phase (Table S1), which confirmed by the broad reflection of XRD pattern in 2θ of 20–25o. Further decreasing the Al content of starting materials (SiO2/Al2O3 = 100) has led to the formation of tetragonal SiO2 impurity, which is dominant in the product from Al-free synthesis system (Fig. S1). Therefore, well crystalline ZSM-5 zeolite can be formed in narrow SiO2/Al2O3 ratio region and the SiO2/Al2O3 = 40 was used in follow-up research. The SEM images of products with different SiO2/Al2O3 ratio are showed in Fig. 2. It can be found that the synthesized ZSM-5 zeolite possesses uneven morphology with micrometer size. Except the hexagonal crystal main body, most samples contain some needle-like particles that might be the associated impurity (Fig. 3).

Fig. 1
figure 1

Schematic representation for the sustainable synthesis of ZSM-5

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Fig. 2
figure 2

XRD patterns of as-synthesized products prepared from the starting materials with SiO2/Al2O3 ratio at (a) 30, (b) 40, (c) 50, (d) 60 and (e) 100. Synthesis condition: Na2O/SiO2 = 0.072, H2O/SiO2 = 2.0, 170 °C, 24 h

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Fig. 3
figure 3

SEM images of as-synthesized products prepared from the starting materials with SiO2/Al2O3 ratio at (a) 30, (b) 40, (c) 50 and (d) 60. Synthesis condition: Na2O/SiO2 = 0.072, H2O/SiO2 = 2.0, 170 °C, 24 h

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It is well known that the sodium ions not only work as charge balancing in aluminosilicate zeolites, but also play a role of templating in OSDA-free synthesis process [48, 51, 52]. So, the Na2O/SiO2 ratio is another important factor affecting our OSDA-free synthesis of ZSM-5. Figure 4 shows the XRD patterns of samples synthesized with different Na2O/SiO2 ratio. Compared to the well crystalline product synthesized with Na2O/SiO2 ratio at 0.072 (Fig. 2), whether increasing or decreasing the sodium amount are unfavorable to the crystallization of product. At the Na2O/SiO2 ratio of 0.108, the product also exhibits well-defined MFI-type diffraction peaks with lower relative crystallinity (54%, Table S1), which will be further decreased by increasing the sodium amount. When the Na2O/SiO2 ratio was decreased to 0.036, no appreciable diffraction peaks can be found in the product. So, the Na2O/SiO2 ratio of 0.072–0.108 is suitable for this sustainable synthesis of ZSM-5. It should be noted that the sodium in our synthesis system is provided by NaOH and alumina source (NaAlO2). If the NaOH is replaced by NaHCO3 or the NaAlO2 is replaced by aluminum sulfate while remaining the total Na2O/SiO2 ratio at 0.072, no MFI zeolite can be formed (Fig. S2), which may be due to the variation of the basicity of synthesis system. It follows that under premise of suitable Na2O/SiO2 ratio, adequate basicity is necessary to successful synthesis of MFI zeolite.

Fig. 4
figure 4

XRD patterns of as-synthesized products prepared from the starting materials with Na2O/SiO2 ratio at (a) 0.036, (b) 0.108 and (c) 0.144. Synthesis condition: SiO2/Al2O3 = 40, H2O/SiO2 = 2.0, 170 °C, 24 h

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Even in the so-called solvent-free synthesis of zeolites, the water is significant and necessary. In previous reports, the crystal water of raw materials usually plays a crucial role in solvent-free synthesis of zeolites without consuming additional water, which lead to the ambiguity of the lowest level of water content for successful synthesis of zeolite [29, 48]. Herein, the employed solid silica gel contains only tiny amount of water (Fig. S3) and needs assistance of additional water with specific dosage to ensure the successful synthesis of zeolite. As a result, the dose threshold of water for successful synthesis of zeolite might be ascertained. Figure 5 indicates the effect of water amount on the synthesis of ZSM-5. As expected, MFI zeolite cannot be prepared in the water-free system. When a small amount of water was added into the synthesis system (H2O/SiO2 = 1.0), MFI crystal appeared in the product, but the relative crystallinity is lower (48%) and the amorphous phase also exists. By comparison, the H2O/SiO2 ratio at 2.0 generated highly crystalline ZSM-5 zeolite (Fig. 2) and the products remain well MFI structure with further increasement of water. These results confirmed the importance of water for zeolite crystallization. However, it seems that the crucial role of water is not irreplaceable. When water is replaced with an equal amount of ethanol, ZSM-5 zeolite with well crystallinity can also be synthesized (Fig. S4), which confirms the availability of organic solvent in this OSDA and solvent-free zeolite synthesis process.

Fig. 5
figure 5

XRD patterns of as-synthesized products prepared from the starting materials with H2O/SiO2 ratio at (a) 0, (b) 1.0, (c) 3.0 and (d) 4.0. Synthesis condition: SiO2/Al2O3 = 40, Na2O/SiO2 = 0.072, 170 °C, 24 h

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Similar to the conventional hydrothermal synthesis, this sustainable synthesis process uses the common crystallization temperature at 170 °C for more than 18 h (Fig. 6). When the crystallization time is less than 12 h, or the crystallization temperature is lower (150 °C), only amorphous products can be obtained. After crystallized at 170 °C for 18 h, the product displays typical MFI structure with relative crystallinity of 61%. Further increasing the crystallization time to 24 h, the XRD of product shows the strongest peaks in intensity (Fig. 2, Table S1). When the crystallization time reaches to 48 h, the relative crystallinity of sample slightly decreased (88%). Moreover, the SEM analysis (Fig. S5) also shows that the 48h-crystallized sample contain some needle-like impurity, which cannot be found in the sample crystallized for 18 h, indicating that long crystallization time is not necessary for this sustainable synthesis for MFI zeolite. In addition, it had been reported that the pre-reactions of raw materials °Ccurred during grinding are referential to the solvent-free synthesis of zeolite products [55]. In current sustainable synthesis, no identifiable crystallized products are formed in grinding stage and suitable thermal treatment is necessary to the formation of zeolite crystal, which indicates that the mechanochemical reaction in solvent-free synthesis process is associated with raw materials.

Fig. 6
figure 6

XRD patterns of as-synthesized products crystalized at (a) 150 °C for 24 h, (b) 170 °C for 12 h, (c) 170 °C for 18 h and (d) 170 °C for 48 h. Synthesis condition: SiO2/Al2O3 = 40, Na2O/SiO2 = 0.072, H2O/SiO2 = 2.0

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The above experiments were performed with solid silica gel as silica source. In fact, the SiO2 spheres prepared by Stöber method are also available in this sustainable synthesis for ZSM-5. As Fig. 7 has shown, XRD pattern of the sample synthesized from Stöber SiO2 spheres displays highly crystalline MFI structure. The SEM image exhibits irregular crystals with micrometer size regardless of the starting raw materials with uniform diameter (Fig. S6), which indicates that the Stöber SiO2 spheres aggregate and crystallize into micro-size zeolite particles at the expense of losing original morphology. The micro scale particles and crystalline character of prepared zeolite are also confirmed by TEM and HRTEM images, respectively (Fig. S7).

Fig. 7
figure 7

XRD pattern (a) and SEM image (b) of as-synthesized samples from Stöber SiO2 spheres. Synthesis condition: SiO2/Al2O3 = 40, Na2O/SiO2 = 0.072, H2O/SiO2 = 2.0, 170 °C, 24 h

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Figure 8 presents the N2 physical adsorption-desorption isotherms of the prepared ZSM-5 zeolites and corresponding textural properties are listed in Table 1. Owing to the template-free synthesis route, the as-synthesized samples without any thermal treatment possess open porous structure. It can be found that both the zeolites from solid silica gel and Stöber SiO2 sphere exhibit adsorption at low P/P0 ( < 0.01), confirming the presence of open micropores in the samples. Compared to the type-I isotherm of silica gel-derived sample, the zeolite from Stöber SiO2 sphere shows a hysteresis loop at high relative pressure (0.5–0.9), which indicates that the sample contains some larger pores and leading to higher total pore volume (Table 1). In addition, the total surface areas of sustainable synthesized zeolites herein are less than that of sample from conventional hydrothermal synthesis, which may be related to the blocked effect of sodium ions and other impurities in the channel of zeolites and is in accordance with previous report [52].

Fig. 8
figure 8

N2 physical adsorption-desorption isotherms of as-synthesized samples from (a) solid silica gel and (b) Stöber SiO2 sphere. Synthesis condition: SiO2/Al2O3 = 40, Na2O/SiO2 = 0.072, H2O/SiO2 = 2.0, 170 °C, 24 h

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Table 1 Textural properties of prepared samples
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The as-synthesized Na-type zeolites had been converted into H-type ones through NH4+- exchanging process to obtain acid properties. Figure 9 shows the NH3-TPD profiles of the obtained H-type zeolites synthesized from solid silica gel and Stöber SiO2 sphere. Both the samples display profiles with two intensive desorption peaks at temperature of 200–220 and 425–435 °C, indicating the weak and strong acid sites in the samples, respectively. Compared to the zeolite from solid silica gel, the SiO2 sphere-derived sample shows lower acid amounts, which might be associated with the difference on actual Al2O3 content in two samples (Table 1).

Fig. 9
figure 9

NH3-TPD profiles of H-type zeolites synthesized from (a) Solid silica gel and (b) Stöber SiO2 sphere

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3.2 Fabrication of Pt-ZSM-5 catalysts and catalytic performance

The availability of Stöber SiO2 spheres has expanded this sustainable synthesis route into fabrication of metal-zeolite composites, which benefit from the universality of Stöber process in constructing metal@SiO2 hybrids [56, 57]. Herein, PVP-stabilized Pt nanoparticles from alcohol reduction process were incorporated into Stöber system, and then Pt@SiO2 core-shell hybrid had been obtained. By utilizing this Pt@SiO2 as silica source, the Pt@ZSM-5 composite can be synthesized through aforementioned sustainable synthesis approach. For comparison, the supported Pt/ZSM-5 was prepared via incipient-wetness impregnation with solid silica gel-derived ZSM-5 as carrier. The fabrications of Pt-zeolite catalysts are schematically described in Fig. 10.

Fig. 10
figure 10

Schematic representation for the synthesis of Pt-zeolite bifunctional catalysts

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The TEM analysis shows that the original Pt nanoparticles and Pt@SiO2 hybrid possessing the metallic particles with size of 2 ~ 3 nm (Fig. 11a, b). By contrast, the prepared Pt@ZSM-5 (H-form) exhibits that slightly increased metallic nanoparticles (~5 nm) embedded in ZSM-5 matrix (Fig. 11c), which can be attributed to the thermal treatment during ion-exchange procedure. After all, serious aggregation of metal particles was avoided for the zeolite confinement effect. However, the supported Pt/ZSM-5 exhibits uneven sized metal particles with some individuals exceed 10 nm (Fig. 11d), indicating that the agglomeration of Pt nanoparticles occurred in calcination process. Even so, the XRD pattern of both Pt@ZSM-5 and Pt/ZSM-5 only displays MFI diffraction peaks with high crystallinity and no characteristic peaks associated with Pt or PtOx phase are visible (Fig. 11e), which maybe due to the low Pt content in the samples (0.36% for Pt@ZSM-5 and 0.57% for Pt/ZSM-5). Highly crystallinity of Pt@ZSM-5 suggests that Pt nanoparticles embedded in colloid SiO2 spheres did not hamper the formation of zeolite. When the Pt nanoparticles were directly mixed with SiO2 spheres and other raw materials, nevertheless, a poor crystalline product had been obtained (Fig. S7). This result indicates that the OSDA and solvent-free synthesis process is susceptible to other external factors. The Pt species in the prepared catalysts were identified by XPS and is showed in Fig. 11f. Because of different synthesis approaches for the two Pt-zeolite catalysts, their Pt state is different. The XPS spectrum of Pt@ZSM-5 synthesized from Pt nanoparticles exhibits doublet signals at 71.3 eV and 74.6 eV, which are assigned to metallic Pt 4 f7/2 and Pt 4 f5/2, respectively [58]. The crystallization of zeolite and subsequent treatment did not change the oxidative state of Pt species. However, the Pt species in Pt/ZSM-5 prepared by impregnation method are mainly oxide (PtO). It should be noted that the Al 2p signal is also comprised in Pt 4f XPS spectra.

Fig. 11
figure 11

TEM images of Pt nanoparticles (a), Pt@SiO2 (b), Pt@ZSM-5 (c) and Pt/ZSM-5 (d), XRD pattern (e) and Pt 4f XPS (f) of Pt@ZSM-5 and Pt@ZSM-5

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The catalytic performance of prepared Pt-ZSM-5 catalysts had been investigated in hydroisomerization of n-heptane, which is a typical reaction catalyzed by bifunctional metal-acid catalysts [59,60,61]. As Fig. 12a has shown, both Pt@ZSM-5 and Pt/ZSM-5 show low n-heptane conversion (<5%) at lower reaction temperature (<220 °C). With the reaction temperature increasing, the n-heptane conversion for two catalysts also increases and is over 90% at 300 °C. However, the selectivity to isomers for both catalysts decline with the increase of temperature (Fig. 12b). For the Pt@ZSM-5, whose isomer selectivity is more than 50% when reaction temperature is below 260 °C and cracked products are predominant under higher temperature. Detailed products distribution listed in Table S2 indicate that mono-branched 2-/3-methylhexanes and propane/iso-butane dominate isomerization and cracking products, respectively, which are accordance with previous report [62]. It is generally acknowledged that acid and redox metal sites of catalysts governing the activity and product selectivity respectively for the hydroisomerization of alkane. Especially, the acid site-catalyzed isomerization reaction is considered as rate-determining step [63, 64]. Compared to the Pt/ZSM-5 catalyst, however, Pt@ZSM-5 exhibits superior n-heptane conversion and isomer selectivity at main investigated temperature range, regardless of its lower acid amount and Pt content. Therefore, suitable synergistic effect between acid and metal sites of bifunctional catalysts is probably more significant for hydroisomerization of alkane [65]. In addition, the different spatial organization two catalysts may be one of critical factor for different catalytic performance. The encapsulation structure of Pt@ZSM-5 may provide better metal-acid sites intimacy and excellent thermal stability [60], which is favorable to catalytic reaction. These results are instructive for preparation of highly effective bifunctional catalysts for hydroisomerization of n-heptane. For comparison, Pt-zeolite catalyst with conventional hydrothermal synthesized ZSM-5 nanocrystal as acid support was also prepared and tested in this reaction (Fig. S9). It can be found that the catalytic performances of both Pt@ZSM-5 and Pt/ZSM-5 are inferior to the conventional synthesized one, which may be due to the excellent accessibility of latter. Therefore, sustainable synthesis of zeolites with controllable particle size will be significant and promising.

Fig. 12
figure 12

Conversion (a) and products selectivity (b) of prepared Pt-ZSM-5 catalysts in hydroisomerization of n-heptane

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4 Conclusion

In summary, the sustainable synthesis for ZSM-5 zeolites without organotemplates, solvents or seeds have been further developed. By using highly purity of silica sources (commercial silica gel and Stӧber colloidal silica), the crucial influencing factors to the successful synthesis such as raw materials composition, crystallization temperature and time, have been obtained unambiguously. In addition, this green sustainable synthesis can also be extended to the construction of Pt-ZSM-5 bifunctional catalysts for hydroisomerization of n-heptane. It is revealed that the encapsulated Pt@ZSM-5 catalyst possessing excellent thermal stability and metal-acid sites intimacy, which is instructive for fabrication of effective catalysts for hydroisomerization of n-alkane. Despite the problems of limited Al content, relative lower surface areas and sensitive synthetic conditions, the merits on cost and environment of green synthesis determine its good prospect of application in heterogeneous catalysis.