Synthesis of ZSM-23 Zeolite by Two-Stage Temperature-Varied Crystallization and Its Isomerization Performance

Several ZSM-23 zeolites with different acid distributions are synthesized by two-stage temperature-varied crystallization and changing the species of aluminum source via conventional hydrothermal synthesis. The crystallinity, micropores, external specific surface area and the Si/Al ratios are measured by XRD, BET, ICP and XPS, indicating that both the body phase and the surface of the zeolite synthesized by two-stage temperature-varied crystallization have higher Si/Al ratio, and the zeolite synthesized with aluminum nitrate as the aluminum source exhibit the largest specific surface area. The properties of acidity and Pt obtained by NH3-TPD, TEM, Py-IR and H2-TPR show that the suitable B-acid distribution leads to high Pt dispersion over the zeolite. Applying these catalysts to the isomerization of n-dodecane, the zeolite synthesized with aluminum sulfate as aluminum source by two-step temperature-varied crystallization shows the best isomerization performance, that the selectivity of i-dodecane reaches 81.2% at 90.7% conversion. Therefore, the matching of acidity, external specific surface area and Pt dispersion of the zeolites is the key to improve the isomerization performance of long-chain alkanes.


Introduction
With the increasingly strict requirements of environmental protection laws and regulations and the rapid development of the automobile industry, the demand for new mechanical equipment for high-quality lubricating oil is increasing gradually, leading to higher quality requirements for oil products [1]. As a result, the development of low sulfur, low ash, high oxidation resistance, low volatility and good viscosity is imperative. Therefore, isomerization dewaxing technology is known as an important technological progress in the field of petroleum refining in recent years [2], and the core technology is the hydroisomerization catalyst.
The hydroisomerization of long-chain n-alkanes is mainly carried out on bifunctional catalysts. The isomerization performance of bifunctional catalysts is determined by the properties of acid carrier, metal properties and the synergistic of metal and acid carrier. The acid sites of zeolites provide isomerization function and metal sites provide hydrogenation/dehydrogenase functions [3,4]. As a metal center, Pt has good hydrogenation/dehydrogenation performance, which can rapidly saturate olefins and avoid coking and capping acid centers. Guo [5] et al. Studied the performance of Pt supported SAPO series and ZSM-5, ZSM-22 and ZSM-23 catalysts in n-octane hydroisomerization. The results showed that ZSM-23, one-dimensional mesoporous zeolite, had high selectivity for isomers

Characterization
The phase purity and crystallinity of the samples were characterized using powder X-ray diffraction by a PANalytical X'Pert 3 diffractometer loaded with Cu-K α radiation (40 kV, 40 mA, λ = 1.5418 Å). The morphology of the samples was characterized with a JSM-7001F field emission scanning electron microscope (SEM). Aggregation state and particle size of Pt were measured transmission electron microscope (TEM) images using a Tecnai G2 F20 S-Twin instrument. Elemental composition of the samples was obtained using ICP-OES apparatus (Thermo Scientific iCAP 7000 Series), after dissolving the ZSM-23 zeolites in strong acidic solution. XPS is widely used for the determination of the surface chemical composition, which was carried out on a Thermo ESCALAB 250XI spectrometer, using an aluminum K Alpha (hv = 1486.6 eV) as the X-ray source, operating at 150 W. The external surface area, microporous volume, and microporous area were determined by the t-plot method. Basic properties of the different samples were carried out by N 2 physisorption measurements on a Micromeritics ASAP 2460 at −196 • C. The coordination structure of the Al was detected by NMR to determine whether the Al entered the zeolite framework. 27 Al MAS NMR were recorded by 4 mm probe, with a rotational speed of 13 kHz and resonance frequency of 156.4 MHz. Pt dispersion was measured by CO pulse adsorption on a TP-5080 chemical adsorption instrument. The average size of Pt particle was calculated by assuming that one CO molecule was adsorbed by a Pt atom. H 2 -TPR was measured by the TP-5080, with TCD as detector for H 2 . The acid strength of zeolites were measured by NH 3 -TPD. The quantities of Brönsted and Lewis acid sites were estimated by pyridine adsorption Fourier-transform infrared (Py-FTIR) in the range of 1400-1600 cm −1 , and B acid on the outer surface of zeolites was measured by dTBPy on a Thermo NICOLET 6700.

Catalytic Performance Test
The n-dodecane isomerization was used as a model reaction in a fixed-bed reactor at a total pressure of 4 MPa, with liquid hourly space velocity (LHSV) of 1.2 h −1 and a volumetric H 2 /n-dodecane ratio of 750. Firstly, the bifunctional catalyst was reduced in situ for 4 h at 400 • C in a flow of H 2 , then the reaction temperature was reduced to the initial reaction temperature, finally the n-dodecane was fed into the reactor under a certain flow rate. In the reaction test of each catalyst, 5 or 6 temperature points were taken for analysis. The products were collected every 24 h and the liquid products were analyzed using gas chromatography (Agilent HP-1).

Structure and Textural of ZSM-23 Zeolites
The XRD patterns of the different zeolites synthesized under different hydrothermal synthesis conditions are showed in Figure 1. All the samples present characteristic diffraction peaks of ZSM-23 [21], and no impurity phase is formed. The crystallinity of ZSM-23 zeolites synthesized with aluminum sulfate as silicon source is higher than that with aluminum nitrate as silicon source (Table 1). In addition, the ZS-1 has the highest crystallinity due to the low temperature pre-crystallization and SO 4 2− anions, which introduces more silicon into the framework of zeolite.

Structure and Textural of ZSM-23 Zeolites
The XRD patterns of the different zeolites synthesized under different hydrothermal synthesis conditions are showed in Figure 1. All the samples present characteristic diffraction peaks of ZSM-23 [21], and no impurity phase is formed. The crystallinity of ZSM-23 zeolites synthesized with aluminum sulfate as silicon source is higher than that with aluminum nitrate as silicon source (Table  1). In addition, the ZS-1 has the highest crystallinity due to the low temperature pre-crystallization and SO4 2− anions, which introduces more silicon into the framework of zeolite. It is well known that the use of different raw materials and crystallization methods in zeolite synthesis will result in products with significantly different crystal sizes and shapes. The SEM and TEM images of three ZSM-23 zeolites are shown in Figure 2. The ZS-1 is formed by sheet cross polymerization with a length of 600-900 nm. This is due to the formation of a lot of crystal nucleus during aging and then aggregation. Sample ZS-2 presents the typical needle-shaped morphology of MTT zeolites [21] with a size of about 60 nm in diameter and above 1 µ m in length. However, the sample ZN-1 shows the jujube core morphology, which is polymerized by lots of irregular flakes with length of about 40 nm. Si/Al represent the ratios of Si/Al. 1 Obtained by feed ratio of initial solutions. 2 Calculated using ICP-OES. 3 The value of Si/Al on the surface is obtained by XPS. 4 Obtained by chemical adsorption of CO. The average size of Pt particle was assumed to be uniform semispherical Pt particle. 5 Obtained by pulse adsorption of CO. 6 Obtained by using ZS-1(100%) as a standard.  Si/Al represent the ratios of Si/Al. 1 Obtained by feed ratio of initial solutions. 2 Calculated using ICP-OES. 3 The value of Si/Al on the surface is obtained by XPS. 4 Obtained by chemical adsorption of CO. The average size of Pt particle was assumed to be uniform semispherical Pt particle. 5 Obtained by pulse adsorption of CO. 6 Obtained by using ZS-1 (100%) as a standard.
It is well known that the use of different raw materials and crystallization methods in zeolite synthesis will result in products with significantly different crystal sizes and shapes. The SEM and TEM images of three ZSM-23 zeolites are shown in Figure 2. The ZS-1 is formed by sheet cross polymerization with a length of 600-900 nm. This is due to the formation of a lot of crystal nucleus during aging and then aggregation. Sample ZS-2 presents the typical needle-shaped morphology of MTT zeolites [21] with a size of about 60 nm in diameter and above 1 µm in length. However, the sample ZN-1 shows the jujube core morphology, which is polymerized by lots of irregular flakes with length of about 40 nm.  The textural properties of the zeolites obtained are shown in Table 1. Different samples have the same feed ratio of Si/Al, while all samples have higher surface aluminum content than body phase. However, the Si/Al ratio of body phase and the surface of the zeolite prepared from using aluminum sulfate as aluminum source are higher than that prepared from using aluminum nitrate as aluminum source. This may be due to the fact that the presence of the SO4 2-inhibited the incorporation of Al [15] into the ZSM-23 framework. And the value of Si/Al on the surface of zeolite was determined by XPS.
From the 27 Al MAS NMR spectra (Figure 3), it can be seen that an intense signal appears at 54 ppm in all the samples, which is assigned to the four-coordinating Al, and no Al signal belonging to the extra-framework is detected at 0 ppm. It is indicated that the Al atoms of ZSM-23 obtained by the three schemes are almost in the framework of zeolite. The textural properties of the zeolites obtained are shown in Table 1. Different samples have the same feed ratio of Si/Al, while all samples have higher surface aluminum content than body phase. However, the Si/Al ratio of body phase and the surface of the zeolite prepared from using aluminum sulfate as aluminum source are higher than that prepared from using aluminum nitrate as aluminum source. This may be due to the fact that the presence of the SO 4 2− inhibited the incorporation of Al [15] into the ZSM-23 framework. And the value of Si/Al on the surface of zeolite was determined by XPS. From the 27 Al MAS NMR spectra (Figure 3), it can be seen that an intense signal appears at 54 ppm in all the samples, which is assigned to the four-coordinating Al, and no Al signal belonging to the extra-framework is detected at 0 ppm. It is indicated that the Al atoms of ZSM-23 obtained by the three schemes are almost in the framework of zeolite. Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 16 At P/P0 < 0.01, the adsorption amount of N2 molecules increase dramatically, and exhibit a type IV N2 isotherm, which show typical microporous structures ( Figure 4). The increase of the curve at higher relative pressures is caused by the multi-layer adsorption of N2 on the surface of the zeolite. Compared with the other two samples, the hysteretic ring of H-ZN-1 is larger, which is due to the increase of mesoporous in zeolite composed of small flake crystals, consistent with the SEM and TEM images.
The basic properties of respective zeolite are shown in Table 2. The external surface area of H-ZS-2 is about 45.9 m 2 /g, which is lower than that of the other two samples as well as the lowest microporous volume, which is due to the regular needle-shaped morphology. The H-ZN-1 has a large mesoporous volume of about 0.29 cm 3 /g, due to the aggregation of small flakes, which is consistent with the adsorption and desorption curves of nitrogen. The H-ZS-1 and H-ZN-1 have large microporous surface area, which is caused by relatively small crystal size and rough outer surface of the samples. As shown in the Figure S1 and Table S1, the N2 adsorption for before protonation, after Pt loading and after catalytic reaction of the samples also was performed. The BET surface area and micropore surface area of the same carriers under different conditions did not change significantly, which indicated that the texture properties of zeolites were mainly depend on the properties of zeolites.  At P/P0 < 0.01, the adsorption amount of N 2 molecules increase dramatically, and exhibit a type IV N 2 isotherm, which show typical microporous structures ( Figure 4). The increase of the curve at higher relative pressures is caused by the multi-layer adsorption of N 2 on the surface of the zeolite. Compared with the other two samples, the hysteretic ring of H-ZN-1 is larger, which is due to the increase of mesoporous in zeolite composed of small flake crystals, consistent with the SEM and TEM images.

Acidity
The acid strength of different catalysts is determined by NH3-TPD. It can be seen from Figure 5, all the zeolites have two main peaks around 190 and 400 °C, presenting typical TPD curves, which are attributed to the desorption of NH3 from the weak and strong acid centers, respectively. Generally, NH3 has a large absorption force at the strong acid adsorption site, which requires high temperature for desorption. Herein, different samples have almost same desorption temperature for NH3 molecular. Then, we tested the samples after Pt impregnation for NH3-TPD ( Figure S2), and the results showed that the NH3 desorption temperature was basically the same as the samples in the form of protonation. The basic properties of respective zeolite are shown in Table 2. The external surface area of H-ZS-2 is about 45.9 m 2 /g, which is lower than that of the other two samples as well as the lowest microporous volume, which is due to the regular needle-shaped morphology. The H-ZN-1 has a large mesoporous volume of about 0.29 cm 3 /g, due to the aggregation of small flakes, which is consistent with the adsorption and desorption curves of nitrogen. The H-ZS-1 and H-ZN-1 have large microporous surface area, which is caused by relatively small crystal size and rough outer surface of the samples. As shown in the Figure S1 and Table S1, the N 2 adsorption for before protonation, after Pt loading and after catalytic reaction of the samples also was performed. The BET surface area and micropore surface area of the same carriers under different conditions did not change significantly, which indicated that the texture properties of zeolites were mainly depend on the properties of zeolites.

Acidity
The acid strength of different catalysts is determined by NH 3 -TPD. It can be seen from Figure 5, all the zeolites have two main peaks around 190 and 400 • C, presenting typical TPD curves, which are attributed to the desorption of NH 3 from the weak and strong acid centers, respectively. Generally, NH 3 has a large absorption force at the strong acid adsorption site, which requires high temperature for desorption. Herein, different samples have almost same desorption temperature for NH 3 molecular. Then, we tested the samples after Pt impregnation for NH 3 -TPD ( Figure S2), and the results showed that the NH 3 desorption temperature was basically the same as the samples in the form of protonation.

Acidity
The acid strength of different catalysts is determined by NH3-TPD. It can be seen from Figure 5, all the zeolites have two main peaks around 190 and 400 °C, presenting typical TPD curves, which are attributed to the desorption of NH3 from the weak and strong acid centers, respectively. Generally, NH3 has a large absorption force at the strong acid adsorption site, which requires high temperature for desorption. Herein, different samples have almost same desorption temperature for NH3 molecular. Then, we tested the samples after Pt impregnation for NH3-TPD ( Figure S2), and the results showed that the NH3 desorption temperature was basically the same as the samples in the form of protonation.  The surface acidity of the catalyst has an important influence on the hydroisomerization of alkane. Figure 6 shows the hydroxyl properties of ZSM-23 zeolites by FT-IR spectra. The vibration region of infrared hydroxyl group is 3800-3300 cm −1 , and the infrared peak at 3745 cm −1 belongs to the isolated silicon hydroxyl group on the outer surface of the sample [22]. The infrared peak at 3735 cm −1 belongs to the silicon hydroxyl group in the ZSM-23 micropores [22,23]. The broad peak at the center of 3500 cm −1 belongs to the nested silica hydroxyl group, and the hydroxyl group bridged by aluminum silicate appears at 3600 cm −1 [23], which is B acid. Usually the B acid sites of silica-aluminum zeolites Appl. Sci. 2020, 10, 7546 8 of 16 are supplied by hydroxyl groups bridging (Si-OH-Al) between silica and aluminum, and all samples have the same type of B acid, which is consistent with the same NH 3 -TPD desorption temperature of the three zeolites described above.
The surface acidity of the catalyst has an important influence on the hydroisomerization of alkane. Figure 6 shows the hydroxyl properties of ZSM-23 zeolites by FT-IR spectra. The vibration region of infrared hydroxyl group is 3800-3300 cm −1 , and the infrared peak at 3745 cm −1 belongs to the isolated silicon hydroxyl group on the outer surface of the sample [22]. The infrared peak at 3735 cm −1 belongs to the silicon hydroxyl group in the ZSM-23 micropores [22,23]. The broad peak at the center of 3500 cm −1 belongs to the nested silica hydroxyl group, and the hydroxyl group bridged by aluminum silicate appears at 3600 cm −1 [23], which is B acid. Usually the B acid sites of silicaaluminum zeolites are supplied by hydroxyl groups bridging (Si-OH-Al) between silica and aluminum, and all samples have the same type of B acid, which is consistent with the same NH3-TPD desorption temperature of the three zeolites described above. The IR spectrum of chemisorption of basic molecules has been widely used to determine the acidity of porous materials. The amount of B acid and Lewis acid (L acid) of the catalyst can be further measured by Py-IR. The total acid content of zeolites was calculated by pyridine desorption at 150 °C, and the strong acid content of the zeolites was calculated by desorption of pyridine at 350 °C ( Figure 7 and Table 3). The bands at 1445-1460 and 1540-1548 cm −1 are related to adsorption peaks of pyridine on L acid and B acid sites, respectively, while the adsorption peak of 1490 cm −1 is caused by the interaction between B acid and L acid [24]. The H-ZS-2 contains the least L acid. While, the sample H-ZN-1 has the highest amount of B acid, which is due to its large microporous surface area higher aluminum content on the outer surface. To further clarify the catalytic mechanism, we performed Py-IR characterization of the impregnated catalyst. From Figure S3 and Table S2, we can see the content of the strong acid and weak acid for B acid and L acid was almost the same as the samples in the form of protonation.  The IR spectrum of chemisorption of basic molecules has been widely used to determine the acidity of porous materials. The amount of B acid and Lewis acid (L acid) of the catalyst can be further measured by Py-IR. The total acid content of zeolites was calculated by pyridine desorption at 150 • C, and the strong acid content of the zeolites was calculated by desorption of pyridine at 350 • C ( Figure 7 and Table 3). The bands at 1445-1460 and 1540-1548 cm −1 are related to adsorption peaks of pyridine on L acid and B acid sites, respectively, while the adsorption peak of 1490 cm −1 is caused by the interaction between B acid and L acid [24]. The H-ZS-2 contains the least L acid. While, the sample H-ZN-1 has the highest amount of B acid, which is due to its large microporous surface area higher aluminum content on the outer surface. To further clarify the catalytic mechanism, we performed Py-IR characterization of the impregnated catalyst. From Figure S3 and Table S2, we can see the content of the strong acid and weak acid for B acid and L acid was almost the same as the samples in the form of protonation.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 16 Due to the small pore size of ZSM-23 zeolite, the isomerization reaction of long-chain alkanes is mainly carried out on the outer surface and pore mouth of ZSM-23 zeolite. Therefore, the surface acid properties of ZSM-23 zeolite have an important effect on its catalytic performance. The acid sites on the external surface can be determined by selecting the alkaline molecules with a molecular diameter larger than the pore size of zeolite. The kinetic diameter of 2,6-di-tert-butylpyridined (TBPy) is 1.05 nm [25], and it cannot enter into the channels of the ZSM-23 (0.45 nm × 0.52 nm) zeolite, so the acidic sites on the zeolite surface can be studied by IR spectra of dTBPy. Figure 8 shows the infrared results of dTBPy desorption on different zeolites at 150 °C. The infrared peak at 1615 cm −1 is due to the interaction between dTBPy and B acid located on the outer surface of the zeolites. The B acid on the outer surface of the samples is in the order of H-ZN-1 > H-ZS-1 > H-ZS-2, indicating that large external area exposes more acidic sites.   Due to the small pore size of ZSM-23 zeolite, the isomerization reaction of long-chain alkanes is mainly carried out on the outer surface and pore mouth of ZSM-23 zeolite. Therefore, the surface acid properties of ZSM-23 zeolite have an important effect on its catalytic performance. The acid sites on the external surface can be determined by selecting the alkaline molecules with a molecular diameter larger than the pore size of zeolite. The kinetic diameter of 2,6-di-tert-butylpyridined (TBPy) is 1.05 nm [25], and it cannot enter into the channels of the ZSM-23 (0.45 nm × 0.52 nm) zeolite, so the acidic sites on the zeolite surface can be studied by IR spectra of dTBPy. Figure 8 shows the infrared results of dTBPy desorption on different zeolites at 150 • C. The infrared peak at 1615 cm −1 is due to the interaction between dTBPy and B acid located on the outer surface of the zeolites. The B acid on the outer surface of the samples is in the order of H-ZN-1 > H-ZS-1 > H-ZS-2, indicating that large external area exposes more acidic sites.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 16 Due to the small pore size of ZSM-23 zeolite, the isomerization reaction of long-chain alkanes is mainly carried out on the outer surface and pore mouth of ZSM-23 zeolite. Therefore, the surface acid properties of ZSM-23 zeolite have an important effect on its catalytic performance. The acid sites on the external surface can be determined by selecting the alkaline molecules with a molecular diameter larger than the pore size of zeolite. The kinetic diameter of 2,6-di-tert-butylpyridined (TBPy) is 1.05 nm [25], and it cannot enter into the channels of the ZSM-23 (0.45 nm × 0.52 nm) zeolite, so the acidic sites on the zeolite surface can be studied by IR spectra of dTBPy. Figure 8 shows the infrared results of dTBPy desorption on different zeolites at 150 °C. The infrared peak at 1615 cm −1 is due to the interaction between dTBPy and B acid located on the outer surface of the zeolites. The B acid on the outer surface of the samples is in the order of H-ZN-1 > H-ZS-1 > H-ZS-2, indicating that large external area exposes more acidic sites.

Properties of Platinum
In addition to study the acid properties of the carrier, the noble metal Pt has an important effect on the hydroisomerization of the alkane. Pt with a content of 0.5 wt. % was loaded on all the samples. Figure 9 exhibits the TEM images of Pt dispersed on ZSM-23 zeolites, the Pt sizes shown here represent the size of most Pt particles over different samples, respectively. Pt/H-ZS-2 shows a maximum Pt particle size of about 2.5 nm, which may be due to its minimum external area and smooth external surface (Table 2). While the Pt particle size of about 1.9nm of the Pt/H-ZN-1 can be

Properties of Platinum
In addition to study the acid properties of the carrier, the noble metal Pt has an important effect on the hydroisomerization of the alkane. Pt with a content of 0.5 wt. % was loaded on all the samples. Figure 9 exhibits the TEM images of Pt dispersed on ZSM-23 zeolites, the Pt sizes shown here represent the size of most Pt particles over different samples, respectively. Pt/H-ZS-2 shows a maximum Pt particle size of about 2.5 nm, which may be due to its minimum external area and smooth external surface ( Table 2). While the Pt particle size of about 1.9nm of the Pt/H-ZN-1 can be attributed to the high B acid content and the rough outer surface of the sample ( Table 1). The average size of Pt particle calculated by CO pulse adsorption is shown in Table 1. We can see that the particle size of Pt calculated is close to that observed by TEM. The order of B acid content on the outer surface of the zeolites is Pt/H-ZN-1 > Pt/H-ZS-1 > Pt/H-ZS-2, and the Pt particle size decreases in the order of Pt/H-ZN-1 > Pt/H-ZS-1 > Pt/H-ZS-2. Therefore, the dispersion and the size of Pt particles mainly depend on the B acid amount on the outer surface of the zeolites.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 16 attributed to the high B acid content and the rough outer surface of the sample ( Table 1). The average size of Pt particle calculated by CO pulse adsorption is shown in Table 1. We can see that the particle size of Pt calculated is close to that observed by TEM. The order of B acid content on the outer surface of the zeolites is Pt/H-ZN-1 > Pt/H-ZS-1 > Pt/H-ZS-2, and the Pt particle size decreases in the order of Pt/H-ZN-1 > Pt/H-ZS-1> Pt/H-ZS-2. Therefore, the dispersion and the size of Pt particles mainly depend on the B acid amount on the outer surface of the zeolites. In order to further study the effect of the surface properties of different zeolites on the properties of Pt particle, the reducibility of Pt over different catalysts was characterized by H2-TPR. As shown in Figure 10, all the catalysts have two H2 reduction peaks in the temperature range of 150 to 350 °C and one broad peak at 400-500 °C. The hydrogen reduction peaks at 200 °C of the catalysts are attributed to the reduction of PtO and PtO2 species coordinated with the outer surface of zeolites [26]. And the broad peak at 400-500 °C is generally considered to be the Pt species that is difficult to be reduced over the catalyst because of the strong metal-support interaction [27]. Pt/H-ZS-1 and Pt/H-ZS-2 have similar hydrogen reduction peaks of Pt. However, the Pt/H-ZN-1 has a large reduction peak at a higher temperature of 275 °C due to its higher acid density and acid strength. The results show that the catalyst with aluminum sulfate is easy to be reduced. The results show that the catalyst with aluminum sulfate as aluminum source is easier to be reduced due to the weakening interaction between the metal and the zeolite.  In order to further study the effect of the surface properties of different zeolites on the properties of Pt particle, the reducibility of Pt over different catalysts was characterized by H 2 -TPR. As shown in Figure 10, all the catalysts have two H 2 reduction peaks in the temperature range of 150 to 350 • C and one broad peak at 400-500 • C. The hydrogen reduction peaks at 200 • C of the catalysts are attributed to the reduction of PtO and PtO 2 species coordinated with the outer surface of zeolites [26]. And the broad peak at 400-500 • C is generally considered to be the Pt species that is difficult to be reduced over the catalyst because of the strong metal-support interaction [27]. Pt/H-ZS-1 and Pt/H-ZS-2 have similar hydrogen reduction peaks of Pt. However, the Pt/H-ZN-1 has a large reduction peak at a higher temperature of 275 • C due to its higher acid density and acid strength. The results show that the catalyst with aluminum sulfate is easy to be reduced. The results show that the catalyst with aluminum sulfate as aluminum source is easier to be reduced due to the weakening interaction between the metal and the zeolite.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 16 attributed to the high B acid content and the rough outer surface of the sample ( Table 1). The average size of Pt particle calculated by CO pulse adsorption is shown in Table 1. We can see that the particle size of Pt calculated is close to that observed by TEM. The order of B acid content on the outer surface of the zeolites is Pt/H-ZN-1 > Pt/H-ZS-1 > Pt/H-ZS-2, and the Pt particle size decreases in the order of Pt/H-ZN-1 > Pt/H-ZS-1> Pt/H-ZS-2. Therefore, the dispersion and the size of Pt particles mainly depend on the B acid amount on the outer surface of the zeolites. In order to further study the effect of the surface properties of different zeolites on the properties of Pt particle, the reducibility of Pt over different catalysts was characterized by H2-TPR. As shown in Figure 10, all the catalysts have two H2 reduction peaks in the temperature range of 150 to 350 °C and one broad peak at 400-500 °C. The hydrogen reduction peaks at 200 °C of the catalysts are attributed to the reduction of PtO and PtO2 species coordinated with the outer surface of zeolites [26]. And the broad peak at 400-500 °C is generally considered to be the Pt species that is difficult to be reduced over the catalyst because of the strong metal-support interaction [27]. Pt/H-ZS-1 and Pt/H-ZS-2 have similar hydrogen reduction peaks of Pt. However, the Pt/H-ZN-1 has a large reduction peak at a higher temperature of 275 °C due to its higher acid density and acid strength. The results show that the catalyst with aluminum sulfate is easy to be reduced. The results show that the catalyst with aluminum sulfate as aluminum source is easier to be reduced due to the weakening interaction between the metal and the zeolite.

Catalytic Performance
At present, there is still much debate on the effective acidic sites in long-chain alkanes hydrogen isomerization. However, many studies have shown that [28] the branched chains formed in the hydroisomerization of long-chain alkanes cannot be carried out in the pores due to the diffusion limitation. Martens [29] and others further studied that the isomerization of long-chain alkanes mainly occurred on the surface of zeolite and the acidic sites near the pore, but could not occur in the zeolite pore. They also proposed the concept of "pore and key-lock" catalysis in alkane isomerization of microporous zeolites.
For the metal-acid bifunctional catalysts, it is recognized that alkanes are first adsorbed at the metal sites and dehydrogenated to form olefins, then the olefins diffuse to the acid sites for isomerization, and finally the iso-olefins intermediates diffuse to the metal site for hydrogenation reaction [30]. Thus, the balance between metals and acids is crucial to isomerization performance of alkanes [31].
In this paper, we use n-dodecane isomerization as the probe reaction to study the catalytic performance of zeolites. The results of the isomerization reaction of n-dodecane on different samples are shown in Figure 11. From these results, we can see that the conversion of all the catalysts increases gradually with the increase of reaction temperature (Figure 11a). Pt/H-ZN-1 with the strongest B acid sites showed the highest conversion of n-dodecane. In contrast, Pt/H-ZS-2 with the fewest B acid sites needs higher reaction temperature to reach the same conversion as Pt/H-ZS-1 and Pt/H-ZN-1 catalysts. The variation trend of i-dodecane selectivity on the catalysts with the reaction conversion is plotted in Figure 11b. With the increase of n-dodecane conversion, the i-dodecane selectivity of all catalysts decreases gradually, which is due to the fact that high temperature is favorable for cracking side reaction. When the reaction temperature is 271.5 • C, the n-dodecane conversion on the Pt/H-ZN-1 is 91.4% and the selectivity of i-dodecane is only 72.2%. This may be due to the H-ZN-1 carrier contains a large number of strong L acid and B acid, which enhance the cracking reaction. When the reaction temperature is 245.5 • C, the n-dodecane conversion on Pt/H-ZS-2 is 36.8%, and the selectivity of the i-dodecane is 89.7%, which is due to its Low B acid strength and low L acid content, and L acids can cause cracking of alkanes. When the reaction temperature rises to 275.8 • C, the conversion of n-dodecane is 90.4%, while the selectivity of i-dodecane is decreased to 79.5%. This may be because the low dispersion and large particle size of Pt on the Pt/H-ZS-2, providing fewer metal sites, which makes the de/hydrogenation and isomerization reactions mismatched [32]. Under the condition of high conversion, the olefin intermediates cannot be hydrogenated at the metal center in time, resulting in a decrease of catalyst activity and isomer selectivity. However, at the reaction temperature of 274.4 • C, the n-dodecane conversion of Pt/H-ZS-1 is 90.7%, and the i-dodecane selectivity is 81.2%. Obviously, Pt/H-ZS-1 exhibits higher stability and i-dodecane selectivity than the other two catalysts under high conversion conditions, which attributes to the suitable B acid and high Pt dispersion.

Catalytic Performance
At present, there is still much debate on the effective acidic sites in long-chain alkanes hydrogen isomerization. However, many studies have shown that [28] the branched chains formed in the hydroisomerization of long-chain alkanes cannot be carried out in the pores due to the diffusion limitation. Martens [29] and others further studied that the isomerization of long-chain alkanes mainly occurred on the surface of zeolite and the acidic sites near the pore, but could not occur in the zeolite pore. They also proposed the concept of "pore and key-lock" catalysis in alkane isomerization of microporous zeolites.
For the metal-acid bifunctional catalysts, it is recognized that alkanes are first adsorbed at the metal sites and dehydrogenated to form olefins, then the olefins diffuse to the acid sites for isomerization, and finally the iso-olefins intermediates diffuse to the metal site for hydrogenation reaction [30]. Thus, the balance between metals and acids is crucial to isomerization performance of alkanes [31].
In this paper, we use n-dodecane isomerization as the probe reaction to study the catalytic performance of zeolites. The results of the isomerization reaction of n-dodecane on different samples are shown in Figure 11. From these results, we can see that the conversion of all the catalysts increases gradually with the increase of reaction temperature (Figure 11a). Pt/H-ZN-1 with the strongest B acid sites showed the highest conversion of n-dodecane. In contrast, Pt/H-ZS-2 with the fewest B acid sites needs higher reaction temperature to reach the same conversion as Pt/H-ZS-1 and Pt/H-ZN-1 catalysts. The variation trend of i-dodecane selectivity on the catalysts with the reaction conversion is plotted in Figure 11b. With the increase of n-dodecane conversion, the i-dodecane selectivity of all catalysts decreases gradually, which is due to the fact that high temperature is favorable for cracking side reaction. When the reaction temperature is 271.5 °C, the n-dodecane conversion on the Pt/H-ZN-1 is 91.4% and the selectivity of i-dodecane is only 72.2%. This may be due to the H-ZN-1 carrier contains a large number of strong L acid and B acid, which enhance the cracking reaction. When the reaction temperature is 245.5 °C, the n-dodecane conversion on Pt/H-ZS-2 is 36.8%, and the selectivity of the i-dodecane is 89.7%, which is due to its Low B acid strength and low L acid content, and L acids can cause cracking of alkanes. When the reaction temperature rises to 275.8 °C, the conversion of ndodecane is 90.4%, while the selectivity of i-dodecane is decreased to 79.5%. This may be because the low dispersion and large particle size of Pt on the Pt/H-ZS-2, providing fewer metal sites, which makes the de/hydrogenation and isomerization reactions mismatched [32]. Under the condition of high conversion, the olefin intermediates cannot be hydrogenated at the metal center in time, resulting in a decrease of catalyst activity and isomer selectivity. However, at the reaction temperature of 274.4 °C, the n-dodecane conversion of Pt/H-ZS-1 is 90.7%, and the i-dodecane selectivity is 81.2%. Obviously, Pt/H-ZS-1 exhibits higher stability and i-dodecane selectivity than the other two catalysts under high conversion conditions, which attributes to the suitable B acid and high Pt dispersion.  Martens et al. [33,34] showed that the shape-selective effect of zeolites results in linear alkanes being adsorbed in micropores ("pore mouth" adsorption) or both ends in adjacent micropores ("key-lock" adsorption). Claude et al. [34] further pointed out that the adsorption type of n-alkane on the microporous zeolites gradually changed from "pore mouth" adsorption to " key-lock" adsorption with the increase of reaction temperature. Accompanied by an increase in the n-dodecane conversion, the selectivity of the mono-branched isomers decreased gradually ( Figure 12a); however, the selectivity of the multi-branched isomers increased gradually (Figure 12b). The selectivity of the mono-branched isomers on the Pt/H-ZS-2 decreases significantly following the increase of n-dodecane conversion, and when the conversion is greater than 81%, the selectivity of multi-branched isomers is higher than that of the other two catalysts. Due to the weak interaction between metal and the carrier and low Pt dispersion, the residence time of isomeric intermediates at the acid sites increase inevitably, which increases the possibility of further branching isomers. At the same time, the ratio of selectivity for mono-branched isomers to multi-branched isomers on different samples is depicted in Figure 12c. Which showed that there is the highest proportion of multi-branched isomers in i-dodecane products on Pt/H-ZS-1 under the same conversion. It is worth noting that the proportion of multi-branched products is relatively low at lower n-dodecane conversion on Pt/H-ZS-2, but when the n-dodecane conversion is higher than 85%, the proportion of multi-branched products is higher than on the Pt/H-ZS-1. This is largely because part of the olefin intermediates cannot be hydrogenated to alkanes at the metal center in time due to the low Pt dispersion on Pt/H-ZS-2 under the high conversion, and further were isomerized to multi-branched i-dodecane.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 16 Martens et al. [33,34] showed that the shape-selective effect of zeolites results in linear alkanes being adsorbed in micropores ("pore mouth" adsorption) or both ends in adjacent micropores ("keylock" adsorption). Claude et al. [34] further pointed out that the adsorption type of n-alkane on the microporous zeolites gradually changed from "pore mouth" adsorption to " key-lock" adsorption with the increase of reaction temperature. Accompanied by an increase in the n-dodecane conversion, the selectivity of the mono-branched isomers decreased gradually ( Figure 12a); however, the selectivity of the multi-branched isomers increased gradually (Figure 12b). The selectivity of the mono-branched isomers on the Pt/H-ZS-2 decreases significantly following the increase of ndodecane conversion, and when the conversion is greater than 81%, the selectivity of multi-branched isomers is higher than that of the other two catalysts. Due to the weak interaction between metal and the carrier and low Pt dispersion, the residence time of isomeric intermediates at the acid sites increase inevitably, which increases the possibility of further branching isomers. At the same time, the ratio of selectivity for mono-branched isomers to multi-branched isomers on different samples is depicted in Figure 12c. Which showed that there is the highest proportion of multi-branched isomers in i-dodecane products on Pt/H-ZS-1 under the same conversion. It is worth noting that the proportion of multi-branched products is relatively low at lower n-dodecane conversion on Pt/H-ZS-2, but when the n-dodecane conversion is higher than 85%, the proportion of multi-branched products is higher than on the Pt/H-ZS-1. This is largely because part of the olefin intermediates cannot be hydrogenated to alkanes at the metal center in time due to the low Pt dispersion on Pt/H-ZS-2 under the high conversion, and further were isomerized to multi-branched i-dodecane. In order to study the influence of different acid distribution on the hydroisomerization of long-chain alkanes, the selectivity of cracked by-products and the distribution of several different isomers were analyzed under the condition of n-dodecane conversion of about 90%. The selectivity of cracked products against the carbon numbers distribution is plotted in Figure 13. On different catalysts, most of the cracked products account for a large proportion of linear alkanes, and the proportion of cracked isomers in the cracked products of the same carbon number increases with the increase of carbon number, which follow the type C β-cracking [35]. The distribution of cracked by-products is similar on Pt/H-ZS-1 and Pt/H-ZS-2 because of their similar acidic distribution, while the Pt/H-ZN-1 shows the highest cracking selectivity due to the more B acidity and L acid content of the carrier. Under the condition that the conversion rate of n-dodecane is about 90%, the distribution of several different i-dodecanes is analyzed, as shown in Table 4, the main branches of the mono-branched isomers are methyl. An increase in the reaction temperature corresponds to weak B acid sites and less Pt sites of the Pt/H-ZS-2 leads to the enhancement of key-lock adsorption. Further, the selectivity of the intermediate branched isomers was improved and the second cracked reaction was inhibited. As a result, Pt/H-ZS-2 exhibits the highest selectivity for 4~6-Methylundecane and 2,4~9-Dimethylnonane. The secondary cracking reaction of Pt/H-ZN-1 is intensified by its strong and high B acid content, which shows the lowest selectivity of the multi-branched i-dodecanes.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 16 In order to study the influence of different acid distribution on the hydroisomerization of longchain alkanes, the selectivity of cracked by-products and the distribution of several different isomers were analyzed under the condition of n-dodecane conversion of about 90%. The selectivity of cracked products against the carbon numbers distribution is plotted in Figure 13. On different catalysts, most of the cracked products account for a large proportion of linear alkanes, and the proportion of cracked isomers in the cracked products of the same carbon number increases with the increase of carbon number, which follow the type C β-cracking [35]. The distribution of cracked by-products is similar on Pt/H-ZS-1 and Pt/H-ZS-2 because of their similar acidic distribution, while the Pt/H-ZN-1 shows the highest cracking selectivity due to the more B acidity and L acid content of the carrier. Under the condition that the conversion rate of n-dodecane is about 90%, the distribution of several different idodecanes is analyzed, as shown in Table 4, the main branches of the mono-branched isomers are methyl. An increase in the reaction temperature corresponds to weak B acid sites and less Pt sites of the Pt/H-ZS-2 leads to the enhancement of key-lock adsorption. Further, the selectivity of the intermediate branched isomers was improved and the second cracked reaction was inhibited. As a result, Pt/H-ZS-2 exhibits the highest selectivity for 4~6-Methylundecane and 2,4~9-Dimethylnonane. The secondary cracking reaction of Pt/H-ZN-1 is intensified by its strong and high B acid content, which shows the lowest selectivity of the multi-branched i-dodecanes.

Summary
ZSM-23 zeolites as-synthesized are considerable difference in acid distribution and Pt loading status over the zeolites. In the hydroisomerization of n-dodecane, the Pt/H-ZS-2 has the minimum external specific surface area and Pt dispersion, and Pt/H-ZN-1 has the maximum B acid amount and the strongest acid strength, both of which exhibit lower bi-branched isomerism selectivity and the highest cracking selectivity. The results indicate that the strong acidity of zeolite and the longer residence time of isomerization intermediates due to the low Pt dispersion are the important factors for the cracked reaction, which follow the type C β-cracking mechanism. The Pt/H-ZS-1 with suitable B-acidity and enough Pt sites, resulting in a good match between the acidic position and the metal position, exhibited the best isomerization performance for long-chain alkanes. The experimental results show that SO 4 2− can promote the formation of zeolite crystals and improve the crystallinity of zeolites, while NO 3 − facilitates the entry of Al into zeolite framework.

Conflicts of Interest:
The authors declare no conflict of interest.