Synthesis of bimetallic Ni-Pt/SAPO-11 composite and the catalytic application in n-heptane hydroisomerization

The monometallic Pt/SAPO-11 and bimetallic Ni-Pt/SAPO-11 catalysts were synthesized with Instant Exactness Synthesis (IES) method and characterized by XRD, nitrogen adsorption–desorption, SEM, TEM, FT-IR, Py-IR, NH3-TPD and XPS, and their properties were evaluated for the n-heptane hydroisomerization. The catalysts had spherical structure and moderate acid strength, and exhibited good n-heptane conversion and catalytic selectivity toward n-heptane isomers. Typically, Ni-Pt/SAPO-11 exhibited an isomer selectivity of 80.9% with a conversion of 69.9% at 310 °C, which was obviously better than Pt/SAPO-11 (isomer selectivity of 75.1% with a conversion of 60.6% at 310 °C). Moreover, the Ni-Pt/SAPO-11 catalyst displayed higher percentage of i-heptane (79%) and lower percentage of n-heptane (6%) than Pt/SAPO-11. The continuous test of 360 min indicated Ni-Pt/SAPO-11 catalyst had good catalytic stability. Bimetallic Ni-Pt alloy loaded on SAPO-11 had significantly effect on catalytic properties of n-heptane hydroisomerization.


Introduction
Catalysts containing previous metal (Pt, Pd) are excellent catalysts in many important catalytic fields [1].Significantly improving the catalytic efficiency of precious metals while maintaining a low dosage has always been a goal pursued by people, but it remains a challenge [2].The n-alkanes hydroisomerization is of great significance for improving the low-temperature flowability and utilization efficiency of fuel, and is expected to alleviate energy shortages [3].Designing and developing efficient and stable catalysts is the key to hydroisomerization reactions.The precious metal Pt catalyst supported on molecular sieve carriers is still a widely used catalytic system in the field of hydroisomerization [4].Pt catalysts prepared by traditional impregnation often require a certain amount of Pt loading (>0.5 wt%) to meet the metal functional requirements of catalytic reactions as large size and poor dispersibility of Pt [5].The relatively high Pt loading limits the widespread industrial application.Therefore, while maintaining low Pt usage and good catalytic stability, significantly improving its catalytic efficiency remains a challenge [6].
Nickel (Ni) is a typical post transition metal element and a potential non-precious metal catalyst, as it is in the same main group and has similar electronic structure with the noble metals Pt and Pd [7].In hydroisomerization area, catalysts containing single metal Ni often exhibit relatively poor catalytic activity and severe cracking reactions [8].The catalysts loading Ni tend to migrate and aggregate into large nanoparticles as Ni has low polarizability and weak interaction with the molecular sieve carrier, leading to poor metal function [9].In addition, due to electronic structure characteristics, metal Ni exhibits relatively strong Ni-H interaction, which is not conducive to the desorption of H species at the Ni site [10].Zou [11] reported that the electron transfer between different components could induce the redistribution of electrons at the interface between the two components, regulate the surface charge state, and thus promote catalytic ability.Therefore, Therefore, it is feasible to combine other elements to regulate the electronic structure of Ni, and then regulate its de-/ hydrogenation activity [12].
In this study, SAPO-11 was synthesized by IES method, and metal Ni was introduced into Pt/SAPO-11 to form bimetallic Ni-Pt/SAPO-11 catalyst.The as-prepared monometallic and bimetallic catalysts were studied by XRD, SEM, TEM, FT-IR, Py-IR, NH 3 -TPD, and XPS, and the influence of Ni-Pt alloy on catalytic properties on n-heptane hydroisomerization was further investigated.O) and heptane were purchased commercially from Aladdin Co., Ltd., and used without further purification.H 2 was generated by hydrogen generator (SGH-300).N 2 was purchased from Daqing Xue Long Gas Co., Ltd.

Preparation SAPO-11.
The molar ratio of each raw material was about Al 2 O 3 : P 2 O 5 : template: SiO 2 = 1:1:1:0.5;The aluminum source originated from pseudo boehmite powder, the phosphorus source came from phosphoric acid, the template agent was dipropylamine or diisopropylamine, and the silicon source was a solution of ethyl orthosilicate or silica gel.Pseudo boehmite powder was placed in a mortar, phosphoric acid, dipropylamine, and ethyl orthosilicate were added in sequence.Carefully grinded the mixture to a uniform paste, and then transferred it into a crystallization reactor, heated at 200 °C for 1.5 h.After the crystallization time was reached, took it out and cooled to room temperature, and then washed the crystallized product with deionized water.When the pH of lotion was 7, the product was filtrated, and then placed in a vacuum drying oven, dried at 80 °C for 12 h.The dried product underwent a one-step roasting at 600 °C for 4 h to obtain the final product SAPO-11 molecular sieve.
Pt/SAPO-11.Pt loading catalysts Pt/SAPO-11 were synthesized by impregnation method (Pt loading was 0.5%).A certain amount of SAPO-11 added into deionized water of chloroplatinic acid with continuously stirring.After standing for 5 h and drying at 80 °C for 12 h, light yellow solid was obtained.The product was set at crucible and transferred into a muffle furnace, which was programmed heating to 400 °C and kept for 6 h.The as-prepared product was labeled as Pt/SAPO-11.The mass proportion of Pt in SAPO-11 was 0.5%.
Ni-Pt/SAPO-11.Ni-Pt bimetallic loading catalysts were synthesized by impregnation method.A certain amount of as-prepared Pt/SAPO-11 added into the deionized water solution of nickel acetate, stirring for 2 h.The suspension dried with oven at 80 °C for 12 h, and then transferred into crucible, and calcined at 400 °C for 4 h.The as-prepared product was labeled as Ni-Pt/SAPO-11.The mass proportion of Ni and Pt in SAPO-11 was separately 5.0% and 0.5%.

Characterization
X-ray diffraction (XRD) was performed on a SmartLat SE diffractometer using Cu Kα radiation (λ = 0.154 nm).Scanning electron microscope (SEM) spectra were observed by a Zeiss ΣIGMA.Transmission electron microscope (TEM) spectra was performed by FEI Talos F200X.Nitrogen adsorption-desorption measurements were carried out at −196 °C by an ASAP 2460 instrument.Fourier transform infrared spectroscopy (FT-IR) spectra were collected on a Tensor II infrared spectrometer.Fourier transform infrared spectroscopy spectra of pyridine (Py-IR) were recorded on a Germany Brook Tensor 27 infrared spectrometer.Ammonia of temperature programmed desorption (NH 3 -TPD) curves were tested using Autochem II 2920 instrument.X-ray photoelectron spectroscopy (XPS) was determined on a EscaLab 250Xi spectrometer using Al Kα radiation.

Hydroisomerization
Asbestos, support rod, asbestos, quartz sand, as-prepared catalysts (60-80 mesh), and quartz sand were in sequence filled into reaction tube from top to bottom.The catalysts filled in the middle of the reactor were activated in the H 2 gas flow at a heating rate of 2 °C min −1 , and the thermocouple placed in the center of catalyst bed was used to measure the reaction temperature.Upon the activation of catalysts, n-heptane was pumped into the reactor with dual plunger micro pump at a predetermined flow rate to start the catalytic reaction.The product mixture was collected and analyzed by a gas chromatograph (GC 7980A), equipped with a hydrogen flame ionization detector.

Results and discussion
3.1.Phase structure XRD patterns of samples are shown in figure 1.The as-prepared SAPO-11 sample showed typical peaks at the 2θ values of 7.9, 9.6, 12.6, 13.3, 15.9, 19.3, 19.8, 21.4, 21.6, 22.5, 23.1, 25.6, 29.6, 38.4,which were attributed to typical SAPO-11 topology structure (JCPDS 00-047-0614), and no additional peaks are observed.The XRD patterns of Ni-Pt/SAPO-11 showed same peaks with that of Pt/SAPO-11 and SAPO-11, indicating that the structure of SAPO-11 was not significantly affected by mixing metal Pt and Ni.Moreover, the diffraction peaks of SAPO-11 were sharp and narrow, indicating the zeolite had good crystalline, but diffraction peaks of catalysts, such as peaks at the 2θ values of 12.6, 13.3, 15.9, 22.5, obviously became weak after Ni, Pt metal component was introduced into SAPO-11, this might be due to the decrease of crystallinity through rehydration [13].In addition, Ni, Pt species entered the pores of SAPO-11, which might have a negative impact on pore structure, and caused a decreased in the intensity of peaks.The diffraction of Pt in Pt/SAPO-11 and Ni in Ni-Pt/SAPO-11 were not detectable with XRD, being relative with low loading amount and high dispersion of Pt and Ni over the support [2,14,15].

Morphology
The SEM and TEM experiments were performed to observed the morphological features of the catalysts, as depicted in figures 2 and 3. SAPO-11 had typical spherical crystals assembled in layers, with about particle size of 2 μm (figure 2(a)).The morphologies of catalysts (figures 2(b)-(d)) were similar with that of SAPO-11.The particle size of Pt/SAPO-11 and Ni-Pt/SAPO-11 was slightly larger than that of SAPO-11, of which were about 3 μm (figures 2(b) and 8 μm (figure 2(c)), respectively.The increase of particle size of catalysts might be explained by metal loading and particle agglomeration [16].Compared with SAPO-11, the surface of Ni-Pt/SAPO-11 particle (figure 2

Textural properties
The N 2 adsorption-desorption isotherms of the samples are shown in figure 4(a).It can be seen from figure 4(a) that the N 2 adsorption-desorption curves of the samples were all belonged to type IV.As for SAPO-11, an adsorption peak appeared at 0.05 ∼ 0.1 P/P 0 , meaning SAPO-11 had microporous structure; a hysteresis loop  It can be seen from table 1 that the values of specific surface area (121 m 2 g −1 ) and pore volume (0.101 cm 3 g −1 ) of SAPO-11 decreased after metal loading, indicating metal species would load in pores of SAPO-11, and result in the blockage of pores.

Acid properties
FT-IR spectra of samples are displayed in figure 5.The peaks at 3610 cm −1 were due to the stretching vibrations of Si-OH-Al species, which corresponded to Brønsted acid centers, and the intensity depended on Si sources [19].The bands located at about 3455 cm −1 and 710 cm −1 separately represented stretching vibrations of Al-OH and Al-O.The asymmetric vibration peak and bending vibration peak of Si-O-Si corresponded to the bands around 1130 cm −1 and 470 cm −1 , respectively.The bands at 520 cm −1 and 1650 cm −1 could attributed to bending vibration of O-P-O and surface bending vibration of molecular sieve, respectively.In addition, compared with SAPO-11, some bands of catalysts, such as stretching vibrations of Si-OH and Al-OH occurred slight blue shift, as shown in figure 5(b).The intensity of some peaks, such as stretching vibrations of Si-O-Si, became weak, but some peaks, such as stretching vibrations of zeolite-OH, became strong, it might caused by the incorporation of Pt and Ni in zeolitic framework [20].
Py-IR spectra were used to test the types of acid site of catalysts, as shown in figure 6.The bands located around 1560 cm −1 and 1455 cm −1 were separately assigned to Brønsted acid sites and Lewis acid sites.While the bands around 1500 cm −1 were attributed to the interaction between Lewis and Brønsted acid sites.It was worth noting that, the peak intensity of Brønsted acid sites and Lewis acid sites became weak and peak location of Brønsted acid sites and Lewis acid sites underwent red shift after Ni species loaded into Pt/SAPO-11, indicating that the acid amount of Brønsted and Lewis acid decreased after Ni loading.The density of acid site played an important role in catalytic performance, which influenced product distribution.NH 3 -TPD were carried out to test the density of Lewis and Brønsted acid sites of catalysts, as shown in figure 7. NH 3 desorption peaks, in the range of 150 °C-225 °C (T1), 225 °C-300 °C (T2) and 300 °C-500 °C (T3), were detected, which assigned to the  appearance of the peaks of Pt 4d located at 310 eV indicated the exist of Pt 2+ species, and confirmed the exist of Pt°and Pt 4+ species as well (figures 8(c) and (d)).It was worth noting that the banding energy of Pt 4f and Pt 4d shifted to a lower value over Ni-Pt/SAPO-11 in comparison of Pt/SAPO-11, probably suggested the interaction between Ni and SAPO-11 [24,25].

Catalytic properties
The hydroisomerization of n-heptane was performed on Pt/SAPO-11 and Ni-Pt/SAPO-11 catalysts, and experimental results are presented in figure 9. Figure 9(a) presents the curves of the conversion of the catalysts as a function of reaction temperature in the whole reaction temperature range.With the gradual increase in reaction temperature, the n-heptane conversion of catalysts increased, and the catalytic conversion of Ni-Pt/ SAPO-11 was higher than Pt/SAPO-11.The selectivity to isomers as a function of conversion over catalysts is presented in figure 9(b).The isomer selectivity over the Ni-Pt/SAPO-11 significantly surpassed that over the Pt/ SAPO-11 catalyst in the whole range, typically, Ni-Pt/SAPO-11 shows an isomer selectivity of 80.9% with a      The good performance of the Ni-Pt/SAPO-11 catalyst can be explained by the fact that the strong and medium Brønsted acid sites of the SAPO-11 based catalyst are the active sites for skeletal isomerization of olefin intermediates [26][27][28][29], which had been demonstrated by the above measurements of NH 3 -TPD.Compared with Pt/SAPO-11, the Brønsted acid of Ni-Pt-SAPO-11 could be obtained much more easily from the stable crystalline structure.Therefore, the catalytic conversion of Ni-Pt/SAPO-11 with larger number of medium and strong B acid sites was higher than Pt/SAPO-11.In addition, the available pore structure of catalysts observed from BET results, made it possible that the as-prepared catalysts exhibited good catalytic properties for n-heptane hydroisomerization.
Nickel loading could form Ni-Pt alloy to improve the conversion and selectivity in the reaction process [28], the small size and surface electronic modulation of Pt particles by Ni species benefited from the strong interaction between Ni and Pt species, which can promote the metal function of Ni catalyst for timely hydrogenation of alkene intermediates [6].According to the TEM results, the dispersibility of metal particles in the Ni-Pt/SAPO-11 catalyst was higher than Pt/SAPO-11.The aggregation of Pt species of the catalyst led to insufficient hydrogenation activity of the metal sites, which made the hydrogenation ability of metal sites not match the isomerization ability of acid sites, and affected the isomerization selectivity.The introduction of second metal species could not only improve the dispersion of active species, but also made the metal sites and acid sites achieve synergistic catalysis.The reason for the alloy compounds significantly improving the isomerization selectivity of catalysts could be explained by the change of electronic properties of metal phases, as alloy compounds changed the adsorption and desorption of recants and products on metal sites [29].Ni-Pt/ SAPO-11 has been compared with other catalysts reported in the literature, and it exhibited good catalytic properties, as shown in table 3. It was worth noting that the n-heptane catalytic conversion of Ni-Pt/SAPO-11  ≈86% isomer selectivity at 82% conversion [35] was higher than that of Ni/SAPO-11 synthesized with same method, indicating that catalytic properties of Ni-Pt bimetallic was better than monometallic catalyst Ni/SAPO-11 and Pt/SAPO-11.
The yield of the products over catalysts exhibited a similar trend, as shown in figure 9(c).Hydroisomerization was the dominant reaction, the cracking products of n-heptane included C1, C2, C3, C4, C5 and C6 fractions, which might be due to the weak metal functionality and unsatisfactory synergistic effect between metal sites and acidic sites, resulting in the cracking of multibranched olefin intermediates [33,34].Compared with the Pt/SAPO-11 catalyst, the Ni-Pt/SAPO-11 catalyst displayed higher percentage of i-heptane (79%) and lower percentage of n-heptane (6%).Besides a small amount of C3 components (about 7%), only a little amount of C1 (1%), C2 (1%), C4 (3%), C5 (2%) components were in the products.According to the above results, Ni-Pt/SAPO-11 was an effective catalyst for the isomerization of n-heptane, indicating that bimetallic Ni-Pt alloy significantly influenced the catalytic activity of catalysts in the hydroisomerization of n-heptane.A continuous test of Ni-Pt/SAPO-11 was conducted to test the catalytic stability, as shown in figure 9(d).The catalyst still exhibited good activity and selectivity upon 360 min of continuous testing, indicating that the Ni-Pt/SAPO-11 catalyst had good catalytic stability.

Catalytic mechanism
The n-heptane hydroisomerization over the monometallic Pt/SAPO-11 catalyst following the bifunctional metal-acid mechanism [35][36][37].The n-heptane molecules dehydrogenated on Pt sites to generate n-heptene intermediates, which were protonated and rearrange at acid sites.The rearranged intermediates diffused to and adsorbed on the Pt sites, and underwent hydrogenolysis to form corresponding heptane isomers.The reaction scheme of n-heptane hydroisomerization over Ni-Pt /SAPO-11 is shown in figure 10.
Compared with Pt/SAPO-11, the hydrogenolysis of bimetallic Ni-Pt/SAPO-11 catalyst was effectively restrained, when the n-heptane molecules adsorbed on the Ni-Pt alloy.Moreover, the synergistic effect between Ni and Pt could alter the metal properties of Pt, including reducing the assembly size of Pt, modulating the electronic properties of Ni which detected by XPS, thereby inhibiting the hydrogenolysis of Pt and improving the selectivity toward n-heptane isomers [28].The isomerization mechanism of n-alkanes can reasonably explain the catalytic performance of Ni-Pt/SAPO-11 for n-heptane hydroisomerization.The introduction of metal Ni could improve the conversion of n-heptane and selectivity toward n-heptane isomers.TEM, XPS showed that the active components Ni and Pt were uniformly dispersed on the surface of carrier SAPO-11.The FT-IR, Py-IR, NH 3 -TPD characterization analysis suggested that the Ni-Pt/SAPO-11 catalyst contained more strong and medium acid sites, resulting in enhanced reduction property.The isomer selectivity over the Ni-Pt/SAPO-11 (80.9%) was significantly better than that of Pt/SAPO-11 (75.1%) at 310 °C.Bimetallic Ni-Pt/SAPO-11 catalyst effectively restrained the hydrogenolysis process, when the n-heptane molecules adsorbed on the Ni-Pt alloy.Moreover, the introduction of Ni metal species improved the dispersion of active species, and made the metal sites and acid sites achieve synergistic catalysis.Ni-Pt alloy improved the conversion of n-heptane and selectivity toward n-heptane isomers in the reaction process.

Conclusion
(d)) was more uneven and the agglomeration phenomenon was more obvious.The TEM of Pt/ SAPO-11 are shown in figures 3(a), (b), and Pt clusters (≈5 nm) composed of Pt atoms with well dispersion on SAPO-11 support could be clearly observed.The formation of Pt clusters could be attributed to the adsorption of [PtCl 6 ] 2− ions on SAPO-11 and interaction with supports [17].As for Ni-Pt/SAPO-11, the lattice spacing of 2.28 and 2.03 Å stemmed from (111) crystal plane of Pt and (011) crystal plane of Ni, respectively (figures 3(c), (d)).TEM results indicated that Pt and Ni successfully loaded and distributed uniformly on SAPO-11 support.

Figure 4 (
b) displayed the pore size distributions of the samples.It was clearly observed that micropores and mesopores existed in the samples.The calculated Brunauer-Emmett-Teller (BET) results were listed in table 1.

A
bimetallic Ni-Pt catalyst was successfully synthesized by introducing metal Ni into the Ni-Pt/SAPO-11 catalyst and it presented good catalytic performance of n-hexpane hydroisomerization.The characterization of

Table 1 .
Parameter of samples determined by BET.

Table 2 .
Acidity properties of samples determined by NH 3 -TPD.
conversion of 69.9% at 310 °C, obviously higher than Pt/SAPO-11 (isomers selectivity of 75.1% with a conversion of 60.6% at 310 °C), indicating that Ni loading had a significant impact on selectivity.

Table 3 .
Comparison of catalysts reported in literature.