Shape Selectivity in Hydroisomerization of Hexadecane over Pt

Article pubs.acs.org/IECR

Shape Selectivity in Hydroisomerization of Hexadecane over Pt Supported on 10-Ring Zeolites: ZSM-22, ZSM-23, ZSM-35, and ZSM48 Miao Zhang,† Yujing Chen,† Lei Wang,† Qiumin Zhang,‡ Chi-Wing Tsang,† and Changhai Liang*,† †

Laboratory of Advanced Materials and Catalytic Engineering, and ‡Institute of Coal Chemical Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: Hydroisomerization of long chain n-alkanes has been playing an important role in the petroleum industry, in which heavy distillate and residue are converted into value-added products such as gasoline, jet fuel, other middle distillates and lubricant oils. Herein, 10ring zeolites, including ZSM-22, ZSM-23, ZSM-35, and ZSM-48 were studied for the process. ZSM-35 and ZSM-48 are relatively less studied zeolites as hydroisomerization reaction catalysts though they are expected to display interesting shape selective properties. A higher conversion was obtained over Pt-ZSM-35 and a higher selectivity was obtained over PtZSM-23 at low temperature and short contact time, and a higher selectivity was obtained over Pt-ZSM-48 at high temperature and long contact time, while the mixed catalysts displayed interesting conversion and selectivity. Small differences in the channel system have a notable influence on the product distribution of hexadecane hydroisomerization over 10-ring zeolites. A combination of the evidence of no absence of multibranched products and the length of hexadecane molecular could infer that hexadecane hydroisomerization includes a pore mouth reaction mechanism.

1. INTRODUCTION The increasing demand for fuel and petrochemical products has led to the depletion of large number of oil resources over the past decades. Converting low-value hydrocarbon feedstocks into value-added products is thus considered to be one reasonable way of alleviating pressing energy problem. Hydrocracking and hydroisomerization of long chain n-alkanes have played an important role in the petroleum industry, in which heavy distillate and residue are converted into value-added products such as gasoline, jet fuel, other middle distillates, and lubricant oils.1 The paraffin base oil prepared by hydrocracking and hydroisomerization of wax decreases product cost and increases the quality of the base oil compared with the synthetic oil derived from the polymerization of α-olefins and solvent dewaxing.2,3 The branching of n-alkanes through hydroisomerization is one strategy to improve the octane number of gasoline and to enhance the performance of diesel or lubricating oils at low temperature.4,5 Properties such as pour point, freezing point, viscosity, and viscosity index are significantly improved by introducing side chains along the linear carbon chains. Bifunctional catalysts, which contain both metallic sites for (de)hydrogenation of the paraffin and acidity sites for forming the carbenium rearrangement, can be used for the hydroisomerization of n-alkanes.6−8 The reaction pathways of hydroisomerization of n-alkanes are shown in Scheme 1. The (de)hydrogenation process in steps (1), (3), (7), and (8) occur at the metallic sites, while the carbenium rearrangement process in steps (2), (4), (5), and (6) occur at the acidic sites. This © 2016 American Chemical Society

catalytic reaction mechanism has already been widely studied by different research groups.6,9 A suitable density of the acidic and metallic sites, and a right balance between them are critical elements to efficient hydrocracking and hydroisomerization.10 In particular, the chain length of n-alkanes affects the hydroisomerization activity. Generally, the reactivity increases with increasing chain length of n-alkanes.10−12 Hence, the longer the chain of the n-alkanes the much easier for the reaction of hydrocracking or/and hydroisomerization to occur. Most of the catalytic hydroisomerization of n-alkanes research is focused on molecular sieves, and such a reaction could be affected by many factors. Zeolites with 12member-ring channels can host monobranched, dibranched, and tribranched hydrocarbons, while multibranched hydrocarbons could easily lead to a cracking reaction.13,14 The effective hydroisomerization catalysts should increase the yield of isomerized products and decrease the yield of cracking products. Therefore, zeolites with 10-member-ring channels were considered to be the catalyst of choice, since the monobranched hydrocarbons can diffuse freely but the multibranched hydrocarbons through shape selection are suppressed leading to higher isomerization selectivity.15−18 Meanwhile, zeolites with 10member-ring channels can reduce coke formation and increase the catalyst life. Received: March 25, 2016 Accepted: May 10, 2016 Published: May 17, 2016 6069

DOI: 10.1021/acs.iecr.6b01163 Ind. Eng. Chem. Res. 2016, 55, 6069−6078

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Industrial & Engineering Chemistry Research Scheme 1. Reaction Pathways for Hydroisomerization of n-Alkanes

ZSM-23 was synthesized following the synthetic procedure described in the literature.30 Typically, 0.5 g of Al2(SO4)3·18H2O was added into the mixture containing 60.75 g of distilled water, 0.26 g of NaOH and 2.40 g of pyrrolidine (PY) under stirring, then 4.5 g of fumed silica was added. After stirring for 2 h, the resulting gel, with a composition of 100 SiO2/1 Al2O3/45 PY/ 4500 H2O/4.3 Na2O, was transferred into a Teflon-lined stainless steel autoclave and crystallized at 160 °C for 72 h. ZSM-35 was synthesized following the synthetic procedure described in the literature.26,31 Typically, 1.25 g of Al2(SO4)3· 18H2O was added into the mixture containing 13.32 g of distilled water, 0.18 g of NaOH and 1.65 g of pyrrolidine (PY) under stirring, then 4.5 g of fumed silica was added. After stirring for 2 h, the resulting gel, with a composition of 40 SiO2/1 Al2O3/12.4 PY/394.8 H2O/1.2 Na2O, was transferred into a Teflon-lined stainless steel autoclave and crystallized at 160 °C for 72 h. ZSM-48 was synthesized with preprepared hexamethonium bromide (HMBr2) as the structure agent following the synthetic procedure described in the literature.29,32 Typically, 0.25 g of Al2(SO4)3·18H2O was added into the mixture containing 67.5 g distilled water, 0.7 g NaOH and 2.265 g HMBr2 under stirring, then 4.5 g fumed silica was added. After stirring for 2 h, the resulting gel, with a composition of 200 SiO2/ 1 Al2O3/ 16.7 HMBr2/ 10000 H2O/23.3 Na2O, was transferred into a Teflonlined stainless steel autoclave and crystallized at 200 °C for 72 h. After the hydrothermal process treatment, the obtained products were filtered, washed, and dried at 85 °C overnight and then calcined at 550 °C for 3 h. Ion exchange was carried out with 5 wt % NH4NO3 solution at 85 °C for 8 h for three times. The dried NH4-type zeolites were calcined at 550 °C for 3 h to obtain H-type zeolites. The as-prepared four molecular sieves supported Pt catalysts were prepared using H2PtCl6 as the precursors of platinum by deposition precipitation with urea.33 Typically, 1.0 g of molecular sieves was suspended in 50 mL of aqueous solution of H2PtCl6 (1.2 × 10−3 g/mL) and urea (2.4 × 10−2 g/mL). The mixture was heated to 90 °C and maintained at this temperature for 4 h under vigorous stirring. The precipitate was filtered and washed until it was free from chloride ions. Finally, the sample was dried at 100 °C for 12 h and calcined at 400 °C for 3 h under 20% O2 in Ar at a flow rate of 100 mL/min.

The widely used molecular sieves for lube dewaxing by hydroisomerization of long chain paraffin include SAPO-11, SAPO-31, SAPO-41, ZSM-22, and ZSM-23.19 Mechanistic studies for ZSM-22 zeolite which contains one-dimensional 10member rings channel show that reaction occurs at the pore mouth.20 The skeletal rearrangement takes place in the pore mouth where there is less steric hindrance (pore mouth catalysis).21 ZSM-48 has been reported as the acidic support for hydroisomerization of n-alkanes,22,23 consisting of onedimensional 10-member-ring channels with a pore diameter of 0.53 nm × 0.56 nm.24 The transition state shape selectivity explained the isomerization selectivity of octane over bifunctional ZSM-48.19,25 ZSM-35 zeolite with FER structure has been reported in the dimethyl ether carbonylation reaction and nbutene skeletal isomerization to isobutylene reaction.26,27 ZSM35 consists of a one-dimensional channel of an eight-memberring, and one-dimensional channel of a 10-member-ring can be applied in the isomerization reaction because a 10-member-ring channel can suppress multibranched hydrocarbons diffusion, thus improving the selectivity of monobranched hydrocarbons. In this work, we extend our previous work on ZSM-48 with alkali treatment to investigate the influence of slight variations in channel size and shape of 10-member-ring zeolites on the product distribution. ZSM-22, ZSM-23, ZSM-35, and ZSM-48 have been synthesized and characterized. The catalytic performance on the hydroisomerization of n-hexadecane of four molecular sieves, which differ by pore structure, pore size, and acidity, are compared. The aim was to explore the collaborative effect on reactivity and selectivity through mixing the different molecular sieves.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. ZSM-22 was synthesized following the synthetic procedure described in the literature.28,29 Typically, 0.5 g of Al2(SO4)3·18H2O was added into the mixture containing 54 g of distilled water, 0.7 g of NaOH, and 2.61 g of hexamethylenediamine (HDA) under stirring, then 4.5 g of fumed silica was added. After stirring for 2 h, the resulting gel, with a composition of 100 SiO2/1 Al2O3/30 HDA/4000 H2O/ 11.6 Na2O, was transferred into a Teflon-lined stainless steel autoclave and crystallized at 160 °C for 72 h. 6070


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Industrial & Engineering Chemistry Research

Figure 1. XRD patterns of ZSM-22, ZSM-23, ZSM-35, and ZSM-48.

2.2. Catalyst Characterization. X-ray diffraction (XRD) analysis of the samples were carried out using a Rigaku D/MaxRB diffractometer with Cu Kα monochromatized radiation source (λ = 1.54178 Å), operated at 40 kV and 100 mA. Nitrogen adsorption−desorption isotherms were measured with Quantachrome autosorb iQ automated gas sorption analyzer. Prior to the measurements, all calcined samples were degassed at 300 °C for 3 h. The surface area was calculated by BET (Brunauer−Emmett−Teller). Pore volumes were calculated from the volume of liquid nitrogen at p/p0 = 0.99. Pore size distribution was calculated from the absorption branch of N2isotherm according to the method of nonlocal density functional theory (NLDFT). Elemental analysis was performed on a PerkinElmer Optima 2000DV inductively coupled plasma atomic emission spectroscopy (ICP-AES). Pyridine adsorption-IR was carried out on an EQUIOX-55 Fourier transform infrared spectrometer (Bruker Corp.). Selfsupporting wafers (15 mm diameter) were made from ca.10 mg of zeolites. The IR cell can hold the wafers of H-type zeolites. The pretreatment of the fresh samples were conducted as follows: the cell containing the zeolite wafer was evacuated while slowly raising the temperature to 450 °C. The sample was finally evacuated at 450 °C for 1 h and then cooled to room temperature to record the background spectra. Infrared spectra were measured at 4 cm−1 resolution. After that, the sample was saturated with pyridine and evacuated at 150 °C for 20 min. Subsequently, IR spectra were measured (after cooling to room temperature). The adsorption of pyridine at 300 °C is similar to that at 150 °C. In the case of NH3-TPD experiments, the reduced catalysts were outgassed in He at 350 °C for 60 min, and finally saturated at 100 °C in a 10% NH3/He stream (50 mL/min) for 1 h. After removing the more weakly physisorbed NH3 by flowing He (50 mL/min) for 30 min, the chemisorbed ammonia was determined by using TCD heating at 10 °C/min up to 700 °C under the same flow of He.

The morphology of different molecular sieves was characterized and analyzed by field emission scanning electron microscopy (SEM, Nova NanoSEM 450 from FEI Co.). 2.3. Catalytic reaction. The n-hexadecane hydroisomerization was carried out at 4 MPa in a fixed-bed microreactor, loaded with 0.1 g of the catalyst powder. The n-hexadecane conversion was monitored by varying the contact time and temperature. The volume ratio H2/n-C16 was chosen to be 600. The reaction products were analyzed using a GC with a FID detector equipped with a HP-5 capillary column. The code of the products was n-Ci and yM-Ci, in which n-Ci denotes the straight-chain paraffin, i denotes the number of carbon, and yM-Ci denotes the isomer product for which M denotes the methyl and y denotes the position of the methyl group. ZSM-23/35 denotes a mixture of 0.05g Pt/ZSM-23 and 0.05g Pt/ZSM-35.

3. RESULTS AND DISCUSSION XRD patterns of the synthesized ZSM-22, ZSM-23, ZSM-35, and ZSM-48 zeolite are shown in Figure 1. Only diffraction peaks attached to each kind of zeolite were detected in a series of samples with four zeolites compared with the standard patterns of each kind of zeolite. It can be concluded that the synthesized ZSM-22, ZSM-23, ZSM-35, and ZSM-48 zeolites are wellcrystallized and pure. The surface area and pore volume of ZSM-22, ZSM-23, ZSM35, and ZSM-48 zeolite were determined by N2-adsorption. The results are summarized in Table 1. The surface area measurements revealed that the ZSM-22, ZSM-23, ZSM-35, and ZSM-48 zeolite possessed a surface area of 209, 307, 404, and 219 m2/g, respectively. The surface area of ZSM-23 zeolite was larger than that in the literature,29 but the surface area of ZSM-48 zeolite was smaller than that in the literature.29 The pore volume measurements revealed that the ZSM-22, ZSM-23, ZSM-35, and ZSM-48 zeolite had pore volumes of 0.21, 0.64, 0.37, and 0.25 cm3/g, respectively. SEM images of ZSM-22, ZSM-23, ZSM-35 and ZSM-48 are shown in Figure 2. ZSM-22 zeolite displays needle shape crystals, and the crystals are about 2 μm in length. ZSM-23 zeolite displays 6071


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temperature, but Lewis acid sites of ZSM-22 decreased with increasing temperature. The number of Brönsted and Lewis acid sites of ZSM-23, ZSM-35, and ZSM-48 decreased with increasing temperature. The change in the number of Brönsted acid sites of ZSM-35 was smaller compared with that of ZSM-23 at higher temperature. The number of Brönsted acid sites is in the order of ZSM-23 > ZSM-35 > ZSM-48 > ZSM-22 at 150 °C. However, the number of Brönsted acid sites is in the order of ZSM-35 > ZSM-23 > ZSM-48 > ZSM-22 at 450 °C. The strength of the Brönsted acid site is different through comparing the Brönsted acid sites at different temperature. The strength of Brönsted acid sites of ZSM-35 is stronger than that of ZSM-23. The strength of acid sites was measured by temperatureprogrammed desorption of ammonia from the acid sites.36 ZSM22, ZSM-23, ZSM-35, and ZSM-48 zeolite displayed typical low temperature and high temperature desorption peaks. Two distinct NH3 desorption peaks at low temperature and high temperature corresponding to the weak and the strong acid sites respectively are shown in Figure 4. The weak acid sites of ZSM23, ZSM-35, and ZSM-48 displayed very broad desorption peaks centered at around 190 °C compared with ZSM-22 centered at around 210 °C. The strong acid sites reaching the maximum at about 450 °C for ZSM-35 and ZSM-48 displayed very broad desorption peaks comparing with ZSM-22 and ZSM-23 at about 420 °C. The acid site strength can be calculated by comparing with the temperature of maximum desorption. The weak acid site strength was ranked as ZSM-48 ≈ ZSM-35 > ZSM-23 > ZSM-22. The strength of the strong acid site was similar to that of the weak acid site. However, the acid site strength may be strongly influenced by other parameters, such as crystal size and acid site densities.29,36 There may be some discrepancies in the rank of acid site strength via theoretical calculation compared with experimental value.

Table 1. Textural Properties of the Synthesized Molecular Sieves

a

sample

topology

channel

size

SBETa (m2/g)

Vtotalb (cm3/g)

ZSM-22

TON

5.7 × 4.6 Å

209

0.21

ZSM-23

MTT

5.2 × 4.5 Å

307

0.64

ZSM-35

FER

0.37

MRE

3.5 × 4.8 Å 4.2 × 5.4 Å 5.6 × 5.3 Å

404

ZSM-48

one dimension 10-ring one dimension 10-ring two dimension 8ring and 10-ring one dimension 10-ring

219

0.25

BET method. bVtotal = Vads p/po = 0.99.

thin needle shape crystals of less than 1 μm in length. The morphology of ZSM-22 and ZSM-23 zeolite agrees with that in the previous report.29 ZSM-35 zeolite displays a fragment shape and the crystal is less than 1 μm in length. ZSM-48 zeolite displays rod shape crystals but not uniform crystals as reported in the previous work,29 and the crystals are about 2 μm in length. Figure 3 shows the IR spectra of pyridine adsorption at 150, 300, and 450 °C. The information about the relative changes in the Brönsted and Lewis acid sites can be obtained from the Py-IR spectra. The bands at ca. 1540 and 1455 cm−1 are assigned to Brönsted and Lewis acid sites, respectively.34 ZSM-22 zeolite and ZSM-48 zeolite consists of a large number of Lewis acid sites and a small number of Brönsted acid sites. ZSM-23 zeolite consists of a large number of Brönsted acid sites and a small number of Lewis acid sites. ZSM-35 zeolite almost consists of only Brönsted acid sites. The number of acid sites was calculated from FTIR results using the extinction coefficient reported by Emeis.35 The calculated results were listed in Table 2. The Brönsted acid sites of ZSM-22 zeolite remained unchanged with increasing

Figure 2. SEM images of ZSM-22, ZSM-23, ZSM-35, and ZSM-48. 6072


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Figure 3. Py-IR of ZSM-22, ZSM-23, ZSM-35, and ZSM-48 samples at the desorption temperature of 150, 300, and 450 °C.

Table 2. Brönsted and Lewis Acid Sites Distribution of the Four Samples CB (mmol/g)

CL (mmol/g)

CB/CL

sample

150 °C

300 °C

450 °C

150 °C

300 °C

450 °C

150 °C

300 °C

450 °C

ZSM-22 ZSM-23 ZSM-35 ZSM-48

0.0024 0.2182 0.1943 0.0129

0.0020 0.1982 0.1746 0.0101

0.0023 0.1296 0.1692 0.0063

0.0109 0.0297 0.0055 0.0260

0.0136 0.0187 0.0032 0.0166

0.0099 0.0185 0.0019 0.0162

0.22 7.14 35.32 0.49

0.15 11.11 54.56 0.61

4.30 0.14 89.05 0.39

Figure 4. NH3-TPD of ZSM-22, ZSM-23, ZSM-35, and ZSM-48 samples.

Figure 5 displays some representative TEM images of Pt/ ZSM-22, Pt/ZSM-23, Pt/ZSM-35, and Pt/ZSM-48. TEM study shows Pt particles distribution on the external surface of the four zeolites. TEM images of four catalysts show a more uniform size of the Pt particles about 10 nm and these Pt particles are well distributed over the four zeolites. The Pt loadings determined by ICP-AES for ZSM-22, ZSM-23, ZSM-35, and ZSM-48 were 0.90, 0.91, 0.95, and 0.93 wt %, respectively. Taking account into the Pt loading and size distribution, the affection of the metal Pt can be neglected. In other words, the difference of the supporters leads to the distinctions in catalytic performance.

Figure 5. TEM images of Pt/ZSM-22, Pt/ZSM-23, Pt/ZSM-35, and Pt/ ZSM-48.

The catalytic performance of the four different catalysts (Pt/ ZSM-22, Pt/ZSM-23, Pt/ZSM-35, and Pt/ZSM-48) in the conversion of hexadecane to iso-hexadecane was investigated as a 6073


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Figure 6. Conversion of C16 (a) and selectivity of i-C16 (b) at various contact times.

Figure 7. Conversion of C16 (a) and selectivity of i-C16 (b) at various temperatures.

the number of Brönsted acid sites.37,38 From the results of infrared spectroscopy of pyridine adsorption (as presented in Figure 3 and Table 2), the number of Brönsted acid sites of ZSM23 and ZSM-35 zeolite were higher than that of ZSM-22 and ZSM-48, so the selectivity of i-C16 over Pt/ZSM-23 and Pt/ZSM35 was higher than that over Pt/ZSM-22 and Pt/ZSM-48 at low contact time. At the high contact time, the product of i-C16 continued to react, cracking reaction occurred predominately, leading to the decrease in i-C16 selectivity. Figure 7 shows the effect of temperature on the conversion of C16 (a) and selectivity of i-C16 (b) at 1.4 min. The catalysts could also be divided into two groups: the broader range conversion of C16 was listed over the Pt/ZSM-23 and Pt/ZSM-35 catalysts at various contact times; however, the opposite change was seen over the Pt/ZSM-22 and Pt/ZSM-48 catalysts. It should be noted that the conversion of hexadecane increased with increasing temperature. The conversion of C16 over Pt/ZSM35 was the highest because the ZSM-35 has the highest number of Brönsted acid sites. The conversion of C16 over four different catalysts was in alignment with the number of the Brönsted acid sites of the four supports. The product selectivity of the catalysts as a function of temperature was examined at the contact time of 1.4 min. Over the Pt/ZSM-23 catalysts, the selectivity of i-C16 decreased sharply with increasing temperature, but the selectivity of i-C16 decreased slowly with increasing temperature over the Pt/ZSM35 catalysts. At high reaction temperature such as 300 °C, side reactions such as cracking lead to the low selectivity of i-C16. The conversion of C16 and selectivity of i-C16 were low at 260 °C over Pt/ZSM-22 and Pt/ZSM-48 because the number of Brönsted acid sites of ZSM-22 and ZSM-48 was less than that of ZSM-23

probe reaction. Product distribution and selectivity for the desired i-C16 were discussed at various reaction temperatures and contact times. Figure 6 shows the effect of contact time on the conversion of C16 (a) and selectivity of i-C16 (b) at 300 °C. The conversion of hexadecane increased with increasing contact time. The catalysts can be divided into two groups: the broader range conversion of C16 was listed over Pt/ZSM-23 and Pt/ZSM-35 catalysts at various contact time, however the opposite was observed over Pt/ZSM-22 and Pt/ZSM-48 catalysts. It should be noted that the conversion of C16 was about 98 wt % over the Pt/ ZSM-23 and Pt/ZSM-35 catalysts, whereas the conversion of C16 was about 50 and 30 wt % over the Pt/ZSM-48 and Pt/ZSM-22 catalysts, respectively. It can be concluded that the number of Brönsted acid sites leads to the differences. The product selectivity of the catalysts as a function of contact time was examined at 300 °C. Over the Pt/ZSM-35 catalysts, the selectivity of i-C16 decreased gradually with the increase in contact time from 60% at 0.24 min to 30% at 2.7 min. This observation can be explained by an increase in the rate of cracking reaction of hexadecane to light alkenes. However, the selectivity trend over Pt/ZSM-22, Pt/ZSM-23, and Pt/ZSM-48 was different with the Pt/ZSM-35 catalyst. The selectivity trend over Pt/ZSM-22, Pt/ZSM-23, and Pt/ZSM-48 increased first and then decreased with the increase in contact time. The selectivity of i-C16 over Pt/ZSM-22 and Pt/ZSM-48 appeared to be lower than that over Pt/ZSM-23 and Pt/ZSM-35 at low contact time, but the selectivity of i-C16 over Pt/ZSM-22 and Pt/ ZSM-48 was higher than that over Pt/ZSM-23 and Pt/ZSM-35 at high contact time. This observation can be explained by the difference in number of acid sites. The traditional concept was that the activity of hydroisomerization is usually proportional to 6074


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Figure 8. Conversion of C16 (a) and selectivity of i-C16 (b) at various contact times over the mixed catalysts.

Figure 9. Conversion of C16 (a) and selectivity of i-C16 (b) at various temperatures over the mixed catalysts.

Table 3. Distribution of Products from Hexadecane Hydroisomerization at 1.4 min and 300 °C yield distribution (wt %) carbon number

isomer

ZSM-22

ZSM-23

ZSM-35

ZSM-48

ZSM-23/35

ZSM-23/48

Shape Selectivity in Hydroisomerization of Hexadecane over Pt

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