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  Zeolitic molecular sieves with uniform microporous structures exhibit unique shape selectivity. The crystalline feature and ion-exchangeable properties endow them high hydrothermal stability and adjustable acid property. Therefore,they show robust adsorptive and catalytic properties,and widely used in petrochemical industry^[1-5]. The selectivity of zeolite-based catalysts can be controlled by choosing appropriate reactants,transition states,and/or reaction products^{[6, 7]}.

  Metal species or metal oxide particles encapsulated in zeolites becomes selectively accessible to reactant molecules. In addition,it exhibits high dispersion and anti-poisoning ability compared to that encapsulated in less restrictive mesoporous zeolites^[8-12]. Nonetheless,hydrogen molecules can penetrate into the inner pore,decompose into hydrogen atoms,and spill from metal clusters to outside aromatic compounds,resulting in the occurrence of hydrogenation reaction. This phenomenon could be very useful for the processing of fuel oils with high concentration of aromatics^[13-21]. In catalytic hydrogenation of aromatic compounds,sulfides possibly transform into H₂S,which is the smallest sulfur-containing molecules and has a kinetic diameter of 0.36 nm. Small-pore supports should be selected to encapsulate and protect noble metal particles against catalyst poisoning.

  Pt/SOD and Pt/GIS have been directly synthesized by introducing the metal precursor [Pt(NH₃)₄](NO₃)₂ in the SOD and GIS synthesis gels^[22]. Pt/ANA was prepared through recrystallization of Pt/GIS in order to avoid precipitation of metal precursors. Zhen et al^[23] in situ synthesized Pt/SOD with Pt(NH₃)₄Cl₂-H₂O precursor. Pt/KA prepared by ion-exchanging Pt/NaA with aqueous KCl solution and chemical vapor depositing tetraethyl orthosilicates^[24].

  In this work,sodalite with a pore opening of 0.28nm was used to prepare Pt/SOD through crystal transformation. The prepared sample was characterized with powder X-ray diffraction (XRD) and H₂ temperature-programmed desorption (H₂-TPD) techniques,and its catalytic properties for benzene hydrogenation and sulfur-tolerant ability were investigated.

  1   Experimental

  1.1   Synthesis of Pt/NaA

  All the chemicals,including sodium aluminate (NaAlO₂,Tianjin Kermel Chem. Reagent Co. Ltd.),sodium metasilicate (Na₂SiO₃·9H₂O,Tianjin Kermel Chem. Reagent Co. Ltd.),Pt(NH₃)₄Cl₂ (Xi’An Catalyst Chem. Co. Ltd.),distilled H₂O,benzene (C₆H₆,Tianjin Dengfeng Chem. Reagent Co. Ltd.),and hendecane (C₁₁H₂₄,Shanghai Energy Chem.),were used as received without further purification. The homogeneous synthesis gel has a molar composition of 0.04 Pt(NH₃)₄Cl₂∶ 2.17 Na₂O∶ 1.17 SiO₂∶ Al₂O₃∶ 208 H₂O. It was crystallized at 100℃ for 12h in a 100mL Teflon-lined stainless steel autoclave. The product (designated as Pt/NaA) was filtered,thoroughly washed with distilled water,dried at 120℃ for 2h and calcined at 500℃ for 4h. Pt/CaA was obtained by repeatedly ion-exchanging Pt/NaA with 0.1mol/L CaCl₂ solution for three times.

  1.2   Preparation of Pt/SOD

  The slurry of prepared Pt/NaA was further crystallized at M℃ for Nh,filtered,thoroughly washed with distilled water and dried at M℃ for Nh. Then,it was calcined at 500℃ for 4h,and designated as Pt/NaA-M-N. The Pt/NaA samples hydrothermally synthesized at 120℃ for 144h,130℃ for 96h,140℃ for 60h,150℃ for 42h,and 160℃ for 30h were designated as Pt/SOD-120,Pt/SOD-130,Pt/SOD-140,Pt/SOD-150 and Pt/SOD-160,respectively.

  1.3   Benzene hydrogenation

  1% benzene in hendecane was added into a 50mL Teflon-lined stainless steel autoclave. The catalyst (60-100mesh) was prepared by grinding Pt/NaA or Pt/NaA-M-N and HZSM-5 mixture for 30min,and pretreated at 400℃ for 2h in argon. The reaction was carried out at 100℃ for 6h in hydrogen atmosphere (3MPa) under stirring conditions (300r/min). The product was analyzed by a gas chromatograph equipped with a flame ionization detector.

  1.4   Sulfur-tolerant test

  The prepared Pt/SOD catalyst (60-100mesh) was treated with 5.02% H₂S/H₂ at 230℃ for 2h for rapid and complete poisoning. The poisoned sample was then treated at 400℃ for 2h in nitrogen atmosphere. After that,it was cooled to room temperature in nitrogen flow. The resultant poisoned Pt/SOD-120,Pt/SOD-130,Pt/SOD-140,Pt/SOD-150 and Pt/SOD-160 were designated as Pt/SOD-120-SP,Pt/SOD-130-SP,Pt/SOD-140-SP,Pt/SOD-150-SP and Pt/SOD-160-SP,respectively.

  1.5   Characterization method

  XRD pattern was recorded on a Shimadzu LabX XRD-6000 X-ray diffractometer equipped with Cu Kα radiation. The related parameters were set as follows: λ of 0.15418nm,step length of 0.02°,scanning rate of 8(°)/min,voltage of 40kV,and current of 30mA. H₂-TPD profile was measured on a FINESORB-3010 chemisorption analyzer (FINETEC INSTRUMENTS) equipped with a thermal conductivity detector (TCD). The sample was first pretreated at 400℃ for 2h. Then,it adsorbed hydrogen for 40min at room temperature. Finally,it was heated from room temperature to 700℃ at a rate of 20(°)/min,and the desorbed H₂ was monitored with a TCD. Transmission electron microscopy (TEM) images were measured on a Philips FEI Tecnai G2 F20 s-twin microscope at an acceleration voltage of 200kV. Before the measurement,the sample was ultrasonically dispersed in alcohol and dropped on a carbon-coated copper grid.

  2   Results and discussion

  2.1   XRD

  Figure 1 shows the powder XRD patterns of NaA,Pt/NaA and Pt/CaA. Pt/NaA and Pt/CaA show the typical diffraction lines of LTA-type zeolite,indicating that addition of [Pt(NH₃)₄](NO₃)₂ did not induce formation of impure materials. A decrease in the intensity of diffraction lines may be due to formation of NaA zeolite having low crystallinity^[25].

  Figure 1.  Powder XRD patterns of NaA,Pt/NaA and Pt/CaA

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  The XRD patterns of Pt/NaA-M-N samples synthesized by further crystallizing Pt/NaA at 120,130,140,150 or 160℃ for a certain time are shown in Figure 2. After crystallization at 120℃ for 96h,the obtained products contained both LTA and SOD crystalline phases. When the crystallization time was increased to 144h,the diffraction lines characteristic of LTA disappeared,and sodalite was the sole crystalline phase. No obvious Pt diffraction lines were observed. However,a further increase in the crystallization time to 192h contrarily greatly decreased the crystallinity of sodalite. This shows that pure Pt/SOD can be obtained by further crystallizing the sample at 120℃ for 144h. This phenomenon is similar to those observed for Pt/NaA-130-N,Pt/NaA-140-N,Pt/NaA-150-N and Pt/NaA-160-N. Thus,Pt/SOD can be successfully synthesized through further crystallization of Pt/NaA at 130℃ for 96h,140℃ for 60h,150℃ for 42h or 160℃ for 30h. A further extension of crystallization time at different temperatures resulted in the aggregation of Pt particles (Pt/NaA-140-84,Pt/NaA-150-48,and Pt/NaA-160-36),decreasing the Pt dispersion,as confirmed by observing Pt(111) and Pt(200) diffraction lines. This is because Pt clusters easily aggregated and extruded from the β-cages of sodalite zeolite at high crystallization temperatures.

  Figure 2.  XRD patterns of Pt/NaA-M-N samples (M and N represent the crystallization temperature and time respectively)

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  2.2   TEM image

  Figure 3 shows the TEM image of Pt/SOD sample. It is clear that uniform Pt clusters have a diameter of ca. 1nm,which is similar to the β-cage diameter of sodalite (window of 0.28nm). H₂ has a kinetic diameter of 0.28nm,thus can be activated by the Pt clusters encapsulated in SOD. However,H₂S with a kinetic diameter of 0.36nm cannot enter into the β-cages of SOD. Therefore,Pt clusters encapsulated in the β-cages of SOD are protected against H₂S.

  Figure 3.  TEM image of Pt/SOD sample

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  2.3   Hydrogenation activity and sulfur-tolerant ability

  Table 1 shows the benzene conversions obtained over different catalysts in the benzene hydrogenation reaction. Pt/NaA is inactive because it cannot effectively absorb benzene molecules to react with dissociated hydrogen atoms as a result of Pt encapsulated in NaA with a pore opening of 0.40nm. In contrast,the Pt/CaA gave a benzene conversion of 14.81%,implying that Pt/CaA with a pore diameter of approximately 0.5nm can absorb benzene molecules having a diameter of 0.59nm. Regardless of this,the physical mixture of Pt/NaA and HZSM-5 showed a benzene conversion of 58.01%. This is due to the contribution of HZSM-5,which can serve as a spillover hydrogen acceptor and absorb benzene molecules^{[26, 27]}. However,after being poisoned with H₂S,it became inactive because H₂S with a kinetic diameter of 0.36nm can diffuse into the cages of NaA and poison Pt clusters. This shows that Pt/NaA cannot tolerate small H₂S molecules. It should be pointed out that conventional Pt catalysts supported on open porous supports,e.g. Pt-NaA,exhibit appreciable catalytic activity in benzene hydrogenation^{[26, 27]}. This indicates that Pt was successfully encapsulated in NaA zeolites for the Pt/NaA.

  Table 1.  Benzene conversions obtained over different catalysts

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  EntrySampleConversion x/%^a1Pt/NaA04Pt/NaA-SP+HZSM-50

  2	Pt/CaA	14.81
  3	Pt/NaA+HZSM-5	58.01
  a: determined by GC using the area normalization method

  Table 1.  Benzene conversions obtained over different catalysts

  Table 2 compares the benzene conversions obtained over Pt/SOD-M and its physical mixture with HZSM-5 in benzene hydrogenation. Like Pt/NaA,Pt/SOD is also inactive,but its physical mixture with HZSM-5 shows high catalytic activity as HZSM-5 acts as a spillover hydrogen acceptor. Hence,it can be deduced that Pt particles were successfully encapsulated in the SOD. Pt clusters trapped in the SOD cannot adsorb benzene molecules,which,hence,cannot react with dissociated hydrogen atoms. The benzene conversion of Pt/SOD-160 was 3.74%. This may be due to the too high crystallization temperature to stabilize the crystallization environment and sufficiently encapsulate Pt nanoparticles.

  Table 2.  Benzene conversions obtained over Pt/SOD-M and its physical mixture with HZSM-5 (M represents the crystallization temperature)

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  SampleConversion x/%^awithout HZSM-5mixed with HZSM-5mixed with HZSM-5 after being poisoned by sulfurPt/SOD-120026.6915.29Pt/SOD-16003.743.51

  Pt/SOD-130	0	35.65	30.90
  Pt/SOD-140	0	40.81	32.33
  Pt/SOD-150	0	45.17	43.28
  a: determined by GC using the area normalization method

  Table 2.  Benzene conversions obtained over Pt/SOD-M and its physical mixture with HZSM-5 (M represents the crystallization temperature)

  After being poisoned with sulfur,all the physical mixtures of Pt/SOD and HZSM-5 still show appreciable catalytic activities despite that a decline was observed depending on the samples. This shows that Pt nanoparticles have been indeed trapped in the β-cages of SOD,and can effectively tolerate sulfur compounds.

  2.4   Hydrogen spillover study

  Figure 4 shows the H₂-TPD profiles of Pt/NaA,Pt/CaA,physical mixture of Pt/NaA and HZSM-5 and its sulfur-poisoned sample. Compared with Pt/NaA,the sharp peak shifted to high temperature and increased in intensity probably owing to the larger surface area. A significant shift was also observed for the physical mixture of Pt/NaA and HZSM-5. In particular,the peak area greatly increased. This indicates that the hydrogen spillover occurs,and a large number of hydrogen atoms dissociated on Pt/NaA spill to HZSM-5. After being poisoned with H₂S,it still showed a very intense peak although the peak area decreased. Consequently,a high catalytic activity was still detected.

  Figure 4.  H₂-TPD profiles of Pt/NaA ,Pt/CaA,Pt/NaA+HZSM-5 and Pt/NaA-SP+HZSM-5

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  Figure 5 compares the H₂-TPD profiles of Pt/SOD and its physical mixture with HZSM-5. It is clear that the peak area of Pt/SOD significantly increased after physically mixing with HZSM-5 due to the hydrogen spillover effect. This is in agreement with the results shown in Figure 3 and Table 2. This shows that Pt was indeed encapsulated in the cages of SOD,and the hydrogen spillover effect of the physical mixture of Pt/POD and HZSM-5 led to its high catalytic activity. The catalyst of Pt/SOD-150+HZSM-5 gave the highest benzene conversion. After poisoning Pt/SOD with sulfur,the hydrogen amount adsorbed on the physical mixture decreased,but the degree was not significant. This is because H₂S with a kinetic diameter of 0.36nm cannot enter into the SOD cages,while hydrogen molecules can diffuse into the cages,dissociate into hydrogen atoms on Pt nanoparticles,and spilled to HZSM-5. Therefore,Pt/SOD is an effective catalyst resistant to H₂S poisoning.

  Figure 5.  H₂-TPD profiles of Pt/SOD-M,Pt/SOD-M+HZSM-5 and Pt/SOD-M-SP+HZSM-5

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  3   Conclusions

  Platinum nanoparticles have been successfully encapsulated in sodalites by a two-step crystallization process,viz. first,crystallized at 100℃ for 12h; secondly,crystallized at 120℃ for 144h,130℃ for 96h,140℃ for 60h,150℃ for 42h,or 160℃ for 30h. The crystallization temperature higher than 160℃ in the second step is unfavorable for encapsulation of Pt in sodalite as agglomeration of Pt nanoparticles occurs,and thus,resulting in a low dispersion. The obtained Pt/SOD shows high catalytic activity for hydrogenation of benzene when mixing with hydrogen acceptor of HZSM-5. This is due to the great hydrogen spillover effect. Encapsulation of Pt clusters in sodalite isolates them from contacting with H₂S. Therefore,Pt/SOD shows excellent sulphur-resistant ability. This work will contribute to future researches on the preparation of anti-poisoning catalysts.

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  Figure 1  Powder XRD patterns of NaA,Pt/NaA and Pt/CaA

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  Figure 2  XRD patterns of Pt/NaA-M-N samples (M and N represent the crystallization temperature and time respectively)

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  Figure 3  TEM image of Pt/SOD sample

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  Figure 4  H₂-TPD profiles of Pt/NaA ,Pt/CaA,Pt/NaA+HZSM-5 and Pt/NaA-SP+HZSM-5

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  Figure 5  H₂-TPD profiles of Pt/SOD-M,Pt/SOD-M+HZSM-5 and Pt/SOD-M-SP+HZSM-5

  (M represents the crystallization temperature)

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  Table 1.  Benzene conversions obtained over different catalysts

  EntrySampleConversion x/%^a1Pt/NaA04Pt/NaA-SP+HZSM-50

  2	Pt/CaA	14.81
  3	Pt/NaA+HZSM-5	58.01
  a: determined by GC using the area normalization method

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  Table 2.  Benzene conversions obtained over Pt/SOD-M and its physical mixture with HZSM-5 (M represents the crystallization temperature)

  SampleConversion x/%^awithout HZSM-5mixed with HZSM-5mixed with HZSM-5 after being poisoned by sulfurPt/SOD-120026.6915.29Pt/SOD-16003.743.51

  Pt/SOD-130	0	35.65	30.90
  Pt/SOD-140	0	40.81	32.33
  Pt/SOD-150	0	45.17	43.28
  a: determined by GC using the area normalization method

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