Synthesis and Characterization of Iron-Substituted ZSM-23 Zeolite

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Kinetics, Catalysis, and Reaction Engineering

Synthesis and characterization of iron-substituted ZSM-23 zeolite catalysts with highly selective hydroisomerization of n-hexadecane Yujing Chen, Chuang Li, Xiao Chen, Yan Liu, Chi-Wing Tsang, and Changhai Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03806 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Industrial & Engineering Chemistry Research

Synthesis and characterization of iron-substituted ZSM-23 zeolite catalysts with highly selective hydroisomerization of n-hexadecane Yujing Chen, Chuang Li, Xiao Chen, Yan Liu, Chi-Wing Tsang and Changhai Liang* Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China *Corresponding author. Tel: +86-0411-84986353; Fax: +86-0411-84986353; E-mail: [email protected]. Homepage: http://amce.dlut.edu.cn

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Abstract A series of Fe-substituted ZSM-23 samples with different Fe/(Fe+Al) ratios were synthesized. Combined characterization by FT-IR, ESR and XPS methods confirmed the presence of Fe in the ZSM-23 framework. The crystal size decreased with increasing Fe/(Fe+Al) ratio, and the Fe-substituted ZSM-23 samples exhibited large specific areas and pore volumes. The introduction of Fe in the framework reduced the acid strength compared with the Fe-free counterparts. Compared with the Pt/Fe-free ZSM-23 catalyst, the Pt/Fe-substituted catalysts showed lower activity to the hydroisomerization of n-hexadecane while higher isomerization selectivity. This is attributed to their higher ratios of weak and strong Brønsted acidic sites and smaller crystal size, which improved the product diffusion and suppressed the further product cracking. More 5M-C15 isomers were obtained over the Pt/Fe-substituted catalysts, which was different from the product distribution of Pt/Fe-free ZSM-23 where 2M-C15 were the major isomer products. Keywords: iron-substituted ZSM-23; hydroisomerization; Pt/Fe-ZSM-23; n-hexadecane.

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1. Introduction Reducing fossil fuel dependency and growing environmental concerns have led to the continuing development of biofuel researches.1 The production of green bio-diesel obtained from the hydro-refining processes of vegetable oils or animal fat is one example that is currently following this trend.2-4 As diesel is the desired product, hydroisomerization technology aiming at raising the cold flow plugging point and cetane number have been applied to increasing the isomer content in the diesel, and thus improving the oil quality in order to meet the specifications. Hydroisomerization is typically carried out using bifunctional catalysts containing both metallic sites and acidic sites for dehydrogenation-hydrogenation and carbon-carbon bond rearrangement respectively. Platinum (Pt) or palladium (Pd) over acidic supports have been documented to be useful for the hydroisomerization of n-paraffins.5-8 The balance between density and strength of acidic sites have crucial effect on both the reactivity and selectivity of the reactions, and the catalysts with high hydrogenation ability and appropriate acidic properties

are

ideal

for

n-paraffins

hydroisomerization.9

Medium

pore-sized

aluminosilicate zeolites have been found to exhibit high activity and isomerization selectivity for long-chain n-paraffins.10, 11 However, excess acidity of the zeolites could lead to the cracking reactions, and much effort has been dedicated to modify the acidity via improved synthetic method or post-treatment, such as changing the acid site distribution,12, 13 using mixed zeolitic catalysts,

14, 15

and so on.

16-19

Complete or partial

substitution for aluminum in the zeolite lattice by other trivalent heteroatoms have been extensively used to fine-tune the acidity of various zeolites.20 Metallosilicate zeolites can change the environment of the Brønsted acidic protons in the micropores, thus further improve activity and selectivity. In the early days, ZSM-5 zeolite was tested for the hydroisomerization of long-chain n-paraffins, while lots of cracking products rather than isomers could be obtained because of the strong acidity.5, 21 Thus, Chu et.al and Bemdt et. al investigated the nature, strength and concentration of the acidic sites of ZSM-5 which 3



was substituted by Ga, In, B and Fe, and found that the acid strength follows the order of Al-ZSM-5 > Ga-ZSM-5 > In-ZSM-5 > B-ZSM-5.22, 23 The modified ZSM-5 zeolites have been widely used in various catalytic reactions, such as oxidation, olefin cracking, xylene isomerization, and light olefin isomerizations, and their potentially unique properties are examined.24-28 ZSM-22 is another type of zeolite which has also been reported as an effective hydrosiomerization catalyst, and there are many reports focused on the modified metallosilicates of ZSM-22. Byggningsbacka and co-worker confirmed that Fe-ZSM-22 exhibited higher selectivity to isobutene than Al-ZSM-22, but caused slight formation of coke.29 Suyao Liu etc. also synthesized a type of Fe-substituted ZSM-22 zeolite catalyst without using any organic structure directing agents and applied them to n-dodecane isomerization. Noticeable results were obtained that compared with the Fe-free Pt/ZSM-22 catalyst, Pt/[Al,Fe]-ZSM-22 exhibited superior selectivity to mono-branched isomers due to more balanced bifunctionality between acid sites and metal sites that suppressed the cracking and facilitated deeper isomerization.30 Al-ZSM-23, a medium porous zeolite with MTT topological structure, has also been frequently reported and displayed excellent activity in the hydroisomerization reactions. 16, 31, 32

Isomorphous substitution for Al in ZSM-23 zeolite was also employed to further

improve its catalytic performance. Fe-substituted ZSM-23 zeolite was firstly reported by Rajiv Kumar et.al and significant ion exchange capacity and shape selectivity in the meta-xylene isomerization were informed in the study.33 Paul Mériaudeau and co-worker reported the hydrothemal synthesis of the Al- and Fe-ZSM-23 zeolites and tested their isomerization activity for n-butene.34 It was revealed that the substitution of Al with Fe significantly decreased the strength of Brønsted acid sites which led to a lower activity and higher selectivity, which was similar to the cases of other types of modified zeolites.30 To further investigate the effect of the Fe-substituted content on the acidity and the shape selectivity of ZSM-23 zeolite, a series of highly crystallized ZSM-23 zeolites were 4


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synthesized with entirely or partially Fe substitution. Various characterization techniques were employed to provide evidence of the presence of framework iron atoms and characterize the physical and acidity properties of samples prepared. n-Hexadecane hydroisomerization performances over the bifunctional catalysts of Pt-loaded [Al, Fe]-ZSM-23 zeolites were investigated. Special attention was paid to the influence of framework SiO2/Fe2O3 or Al2O3/Fe2O3 ratio of the catalysts on the catalytic activity and isomer selectivity. 2. Experimental The raw materials used in the synthesis were fumed silica (SiO2, Wacker Chemie AG), pyrrolidine (PY) ((CH2)4NH, Aladdin Industrial Corporation), aluminum sulfate (Al2(SO4)3•18H2O, Sinopharm Chemical Reagent Co., Ltd), sodium hydroxide (NaOH) and ferric nitrate (Fe(NO3)3•9H2O) obtained from Tianjin Kermel Chemical Reagents Development Center. 2.1 Synthesis of Fe-substituted and conventional ZSM-23 zeolites Fe-substituted ZSM-23 zeolites were synthesized under hydrothermal conditions at 180 °C with a molar composition of 1SiO2 : xAl2O3 : yFe2O3 : 0.083NaOH : 0.45PY : 45H2O, where x+y=0.01 and x/y=3/7, 5/5, 7/3 and 0/1. The products were denoted as AI-0.7, AI-0.5, AI-0.3 and AI-1, respectively, where A and I represented aluminum and iron atom. When y=0, the conventional ZSM-23 sample was obtained denoted as AI-0. As a typical synthesis process, NaOH, fumed silica and pyrrolidine were added into the deionized water subsequently as mixture A. Solution B was prepared with Al2(SO4)3• 18H2O or Fe(NO3)3•9H2O dissolved in the deionized water. Adding solution B into mixture A and after a sufficient stirring for 2 h, a homogeneous gel was obtained. It was then transferred into a 100 mL telfon-lined stainless steel autoclave and the mixtures were heated at 180 °C for 6 d. After filtration, washing and drying, the product was calcinated 5



at 550 °C for 3 h under static air atmosphere to remove the structure-directing agent. The zeolite powder was submitted to the ammonium ion exchange in a 0.5 M NH4NO3 aqueous solution at 80 °C for 1 h with a liquid-to-solid ratio of 30 mL/g, and the procedure was repeated for 3 times. After sample calcinated at 550 °C for 3 h, the as-synthesized zeolite samples were converted into proton form ZSM-23. 2.2 Preparation of catalysts Bifunctional catalysts were prepared with platinum supported on the proton form zeolite samples by deposition precipitation method using H2PtCl6·6H2O as the platinum precursor and urea as the precipitating base. The details can be found in our previous work.19, 35 2.3 Characterization The powder X-ray diffraction (XRD) was used to determine the purity and phase composition of samples on a Rigaku D/MAX2400 diffractometer using Cukα radiation (λ=0.154178 nm) at 40 mV and 100 mA. Scanning electron microscopy (SEM) coupled with an energy-dispersive spectrometry (EDS) device and transmission electron microscopy (TEM) images were recorded on an FEI QUANTA 450 and an FEI TECNAI F30, respectively. X-ray fluorescence (XRF) on a Bruker SRS-3400 and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to analyze the elemental contents. Ultraviolet-visible (UV-vis) spectra were recorded with a spectrometer system in a range of 200-800 nm built by Dalian Institute of Chemical Physics. Fourier transform infrared (FT-IR) spectra were collected on a Thermo Fisher iN10 spectrometer with a 0.4 cm-1 resolution. The samples were dried and then diluted with KBr. The textual properties of samples were determined by N2 physical adsorption measurement at -196 °C with a Quantachrome Autosorb IQ apparatus. All calcined samples were evacuated at 300 °C for 12 h before the analysis. The specific areas were 6


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calculated by Brunauer-Emmett-Teller (BET) method, and the individual contributions from microporosity and the external portion were determined by t-plot method. Pore volumes were calculated from the N2 adsorption isotherm at P/P0=0.99. The acidity properties of samples were characterized by temperature-programmed desorption of ammonia (NH3-TPD) and Fourier transform infrared measurement using pyridine as the probe molecule (Py-IR). NH3-TPD was carried out in a CHEMBET-3000 analyzer. The samples (~100 mg) were pretreated at 500 °C for 90 min in a He flow of 40 mL/min. After cooled to 120 °C, the samples were exposed to a mixed flow of NH3/He for 30 min and then were purged with He for 1 h. The adsorbed samples were finally heated to 700 °C with a rate of 10 °C/min in a He flow of 40 mL/min. The Py-IR spectra were collected on a EQUINOX55 apparatus. A self-supporting wafer prepared from ~10 mg zeolite powder was placed in an in situ cell with KBr windows, and then were activated at 450 °C under vacuum for 2 h. After cooling to 40 °C, the background spectra of the pretreated samples were recorded. Pyridine was adsorbed on the samples for 8 min, followed by evacuating the cell at 150, 300 and 450 °C. The FT-IR spectra were collected when the samples were cooled to 40 °C after each desorption. 2.4 Catalytic performance Hydroisomerization of n-hexadecane was carried out in a conventional fixed-bed reactor with at a total pressure of 4.0 MPa and a hydrogen-to-hydrocarbon volume ratio of 600. 100 mg catalyst mixed well with 5.0 mL quartz sands (60-80 mesh) was placed into the center of the reactor and was reduced in situ with a H2 flow of 40 mL/min at 400 °C for 3 h. After the reduction, the reactor was adjusted to the test temperature and the feedstock was fed into the reaction system by a high-pressure pump at a certain flow rate. The contact time, WCatalyst/Ffeed, was varied by altering the flow rate (Ffeed (g/min)) of 7



at the entrance of the reactor. The products during a sampling period were collected and analyzed by a gas chromatograph (Agilent 7890A) and a gas chromatography-mass spectrometry (Agilent 7890B-5977A). 3 Results and discussion 3.1 Synthesis of Fe-substituted ZSM-23 zeolites The XRD patterns of all synthesized samples are shown in Figure 1 (a). All samples displayed the typical peaks of MTT zeolite at 2θ=11.2-11.5°, 19.5-19.9°, 20.7-21.0° and 22.8-23.1°, consistent with those in the reported literature.34 No obvious impurities were detected, indicating that the framework structure of ZSM-23 was well retained after the Fe incorporation. Nevertheless, the XRD intensities of Fe-substituted ZSM-23 zeolites were slightly lower than the pure Al-ZSM-23 zeolite, which may be due to the decrease in crystal size, as reported in the work by Liu et al..30 Generally, the diffraction peaks of the zeolites will shift to lower angles when larger atoms substitute for the Al atoms.30, 36 The magnified peaks at ~11.3°, one of the most prominent diffraction peaks of MTT zeolites, were shown in Figure 1(b). A gradually obvious shift can be seen as the Fe content increasing in the products. Meanwhile, ZSM-23 zeolite could hardly be synthesized when Fe(NO3)3·9H2O and Al2(SO4)3·18H2O were both absent in the initial gel even though the other synthetic conditions were identical. Therefore, it can be inferred that Fe atoms were successfully incorporated into the structural framework of the Fe-substituted ZSM-23 zeolites. To further verify the incorporation of Fe atoms in the zeolite framework instead of just existing in the products as iron oxide phases, FT-IR was studied and the spectra in the framework vibration region of AI-0, AI-0.3, AI-0.5, AI-0.7 and AI-1 were shown in Figure 2. The spectra of Fe-substituted ZSM-23 were nearly identical with that of the conventional ZSM-23 sample AI-0 as shown in Figure 2 (a) except some changes in the 8


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width of the bands. The adsorption bands originating from iron oxides and iron hydroxides were not observed in the spectra of Fe-substituted samples. Peaks at ~1235 cm-1 and ~1099 cm-1 of AI-0 were resulted from asymmetric stretching of the T-O atoms in the ZSM-23 zeolite framework.37 The substitution of Fe for Al in the zeolite framework is generally accompanied by a shift of the bands to lower wavenumbers because the Fe-O bond is longer than the Al-O bond.38 With the Fe2O3/(Fe2O3+Al2O3) ratio increasing, as expected, the bands at 1235 cm-1 were incrementally shifted to lower wavenumbers. When enlarging the band between 900-1250 cm-1 in Figure 2 (b), it is clear that the peak assigned to the internal asymmetric stretching vibration of Si-O-T were also moved to lower wavenumbers and the sharpness of this band were increased slightly with increasing Fe content. The v(OH) region of FTIR spectra were obtained after the ZSM-23 samples were degassed at 380 oC. All samples exhibited a hydroxyl stretch at ~3724 cm-1 as shown in Figure 3, corresponding to the terminal SiOH. A band between 3588-3618 cm-1 could attributed to the acidic bridging OH, namely the Brønsted acidic sites. The peak at ~3588 cm-1 and ~3618 cm-1 were associated with the Al-OH and Fe-OH, respectively. The center of the band shifted to higher wavenumbers with the increase of the Fe content in the zeolites. It indicated a weakening trend of the acidic sites.39-41 This fact can be taken as a direct evidence for the isomorphous substitution of Fe3+ cations into the ZSM-23 framework structure. ESR spectroscopy in the X-band (ν≈9.5 GHz) is considered to be a good approach to characterizing the Fe sites in zeolites42-44 since ferric ion is paramagnetic in high-spin electronic configurations when it coordinates with some weak ligands such as hydroxide and framework oxygen. It is commonly accepted that the three signals at g≈4.3, g≈2.2-2.3, and g≈2.0 in the EPR spectrum of the Fe containing zeolite were assigned to the framework iron, iron in interstitial oxide or hydroxide phases and iron in cation-exchange sites, respectively. The EPR spectra of sample AI-0.7, AI-0.5 and AI-1 were shown in Figure 4. The signals at g=4.3 for all samples are assigned to Fe in distorted tetrahedral 9



positions in the zeolite lattice. The incorporation of iron into the ZSM-23 framework can also be evidenced by employing XPS spectroscopy and the spectra were shown in Figure 5. The Al2p peaks of AI-1 around 74.7 eV were absent and the signal of AI-0.7 was extremely weak due to the low Al content of the samples in Figure 5 (a). Similarly, the Fe2p peaks of AI-0 were absent and the signal of AI-0.3 was too weak to be identified in Figure 5 (b). Two peaks around 712.3 and 725.0 eV assigned to Fe2p3/2 and Fe2p1/2 can be found in the case of AI-1, AI-0.7 and AI-0.5, according to the binding energy database of NIST. The binding energy was different from that of the pure iron oxide at about 710.9 and 724.0 eV, indicating that the Fe atoms were in the zeolite framework instead of existing as FexOy. The spectra of Si2p for AI-0 exhibited a single peak around 103.2 eV in Figure 5 (c), which could be ascribed to the framework Si species45. The peak position of Si2p shifted to lower binding energy in the Fe-substituted samples, and the Si migration was enhanced with increasing iron content. The O1s core level spectra of AI-0 also showed a sing peak at 532.9 eV in Figure 5 (d), ascribed to the oxygen atoms belonging to the Si-O bond in the zeolite lattice. No peak at 530.1 eV assigned to the oxygen associated with the extra-lattice iron species could be observed, further indicating that all Fe atoms were incorporated in the zeolite framework.38 The binding energy of O1s also gradually shifted to lower values from 532.9 to 532.7 eV with the increase of the iron content, suggesting that the Fe atom replaced Al adjacent to the Si-O groups, due to the much higher electronegativity of Fe than that of Al. The SEM images of sample AI-0, AI-0.3, AI-0.5, AI-0.3 and AI-1 were shown in Figure 6. It can be seen that the morphology of AI-0 sample was peanut-like which were about 15-20 µm assembled with small sheets. With the increasing Fe content in the zeolite, small change in the shape of the crystal morphology can be observed as it deviated from peanut shape and gradually became an aggregation accumulated with uniform slices. The length of the zeolite crystals observably decreased in the order of 10


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AI-0.3 (10-12 µm) > AI-0.5 (8-10 µm) > AI-0.7 > (6.5-8 µm) > AI-1 (5-6 µm). It is also suggested that this trend was due to the role of Fe played as nuclei of crystal growth in the synthesis of high-silica Fe-substituted zeolites.46 3.2 Textual properties The N2-adsorption and desorption isotherms of samples with different Fe content are shown in Figure 7. All sample exhibited characteristics of a typical type I isotherm, which was usually observed for microporous materials. The hysteresis loops existing may result from the mesoporosity of the crystal sheets47 and was found in decreasing order of AI-0 > AI-0.3 > AI > 0.5 > AI-0.7 ≥ AI-1, which were consistent with the results of SEM images that the slices of the aggregations became increasingly uniform. The textual properties of the samples are listed in Table 1. The specific areas and the pore volume of the Fe-substituted samples were slightly enhanced with increasing Fe content, while the Smicro exhibited an opposite trend. The reduced Smicro and Vmicro may be due to the fact that Fe might occupy more internal space than Al in the micropores, and eventually block the micropores during the post treatments. The values detected by ICP-AES and XPS were in good agreement with the molar ratio of Fe/(Fe+Al) in the samples in Table 1, except for AI-0.7 where the bulk Fe content was higher than that on the surface. It indicated the possible existence of the extra-framework Fe species. The increased external surface area contributed mostly to the specific surface areas which was probably due to some extra-framework Fe species being imperceptible in the XRD patterns. 3.3 Acidity properties The acidity of the samples is determined by NH3-TPD combined with FT-IR pyridine adsorption measurements. The NH3-TPD profiles of the Fe-free and Fe-substituted ZSM-23 materials were presented in Figure 8. The shape of the TPD signal shows that the acidic sites of all samples featured similar distribution of strength, with a remarkable desorption rate at a low temperature, which were characteristic of sites of weak strength, and a tail up to a higher temperature which was typical for strong acidity. The temperature of the peaks attributed to the weak acidic sites were similar for all samples 11



with small shift occurring progressively with increasing Fe content in the samples. However, the peaks at high temperature ascribed to the strong acidic sites gradually shifted to lower temperature dramatically. It indicates the decrease of the acid strength, as the acid strength of the acidic sites–bridge hydroxyl groups of Si-OH-Al are stronger than that of Si-OH-Fe in the zeolite framework. In order to get some insights into the relative proportion of the respective acidic sites, the NH3-TPD curves were deconvoluted into two populations and the result was listed in Table 2. Compared with AI-0, the amounts of total acidic sites of the Fe substituted samples slightly increased, which was mainly due to the increase of the strong acidic sites. It suggests that the introduction of Fe into the ZSM-23 zeolite may lead to more strong acidic sites albeit slightly weaker than that of AI-0 sample. More acidity information can be supplied by Pyridine-FTIR technique as shown in Figure 9. Typically, two bands at about 1540 cm-1 and 1450 cm-1 were ascribed to the C-N stretching of pyridinium irons to Brønsted acidic sites and the vibration of adsorbed pyridine to Lewis acidic sites, respectively. All samples possessed more Brønsted acidic sites than Lewis acidic sites. Further confirmation of quantifying the acidic sites is carried out by estimating the peak areas at 1450 and 1540 cm-1,48 and the results were displayed in Table 2. As the kinetic diameter of pyridine is much larger than that of the ammonia molecule, the amount of the acidic sites quantified from the Py-IR test were less than those obtained from the NH3-TPD result, considering that some acidic sites in the micropores are inaccessible to pyridine molecules. If defining the acidic sites coordinated with pyridine at 150 and 450 °C as the total and the strong acidic sites, the Fe-substituted samples possessed less strong Brønsted acidic sites and larger CBweak/CBstrong ratio on the pore mouths compared with AI-0 sample. This parameter is significantly relevant to the performance in the catalytic test described next. 3.4 Hydroisomerization of n-Hexadecane Hydroisomerization of n-hexadecane was studied on the bifunctional catalysts prepared from the H-form zeolite samples by loading with 0.5 wt% platinum. Pt/AI-0, 12


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Pt/AI-0.3, Pt/AI-0.5 and Pt/AI-1 with different Fe/(Fe+Al) molar ratio were chosen to investigate the effect of framework Fe on the hydroisomerization performance. The n-hexadecane conversion and the selectivity of iso-hexadecane over different catalysts are plotted against the contact time at 300 °C in Figure 10. The conversion of n-hexadecane increased with an increase of contact time in all catalysts as shown in Figure 10 (a). Higher conversions were obtained with AI-0 than the Fe-substituted catalysts at all contact time due to presence of more strong Brønsted acidic sites associated with Al3+. Meanwhile, the conversion of n-hexadecane decreased with increasing Fe content in the catalysts, which were consistent with the variation of the strong acidity in AI-0, AI-0.3, AI-0.5 and AI-1. Figure 10 (b) indicates that the iso-hexadecane (i-C16) selectivity with contact time over different catalysts varies. The iso-hexadecane selectivity kept decreasing from 28 to 7 wt% over Pt/AI-0 with the contact time increasing from 0.3 to 2.1 min. Over Pt/AI-0.3, Pt/AI-0.5 and Pt/AI-1, the selectivity of iso-hexadecane all increased from zero initially, and then tented to be constant. Pt/AI-0.3 displayed an increasing selectivity toward iso-hexadecane with a lower rate than Pt/AI-0.5 and Pt/AI-1. The isohexadecane selectivity increased in the order of Pt/AI-1 > Pt/AI-0.5 > Pt/AI-0.3 when the contact time was about 0.9 min and longer. It is generally accepted that hydroisomerization and hydrocracking occur simultaneously or/and competitively in the hydroconversion of long-chain n-alkanes. The selectivity for hydroisomerization reaction depends on the ratio of metal to acidic sites in the bifunctional catalysts which affects the rate of hydroisomerization and hydrocracking reactions.49, 50 The acidic sites here is mainly Brønsted acidic sites, and weak Brønsted acidic sites is thought to be benefiting to hydroisomerization while strong Brønsted acidic sites facilitates hydrocracking. 9, 41, 51 As the Pt contents in this work were 0.5 wt% for all catalysts, the acidity and the textual property was the main parameters impacting the activity and selectivity result. AI-0 possessed more and stronger Brønsted acidic sites than the other catalysts known from Table 2. Relatively higher selectivity was resulted 13



when the contact time was short, while more cracking could be observed with extending catalyst contact time. In the case of Pt/Fe-substituted, when the contact time was too short, only a little of n-hexadecane was converted and no isohexadecane was obtained. When the contact time increased, some n-hexadecane started to convert to isohexadecane and secondary hydrocracking may happen to the isohexadecane as the amount of strong Brønsted acidic sites was higher than that of the weak Brønsted acidic sites. When the contact time reached 1.2 min and longer, a balance was achieved between the cracking activity and the isomerization activity, due to a higher percent of weak Brønsted acidic sites in the Fe-substituted samples. Besides, the crystal sizes of the Fe-substituted samples were smaller than AI-0, which could enhance the diffusion of intermediates and prevent the products from further cracking. The conversion and selectivity of the catalysts were monitored at different reaction temperatures with a contact time of 1.2 min. Increase in the n-hexadecane conversion with the temperature were observed for all catalysts in Figure 11 (a), but the slope of Pt/Al-0 was a little bigger than those of other catalysts. The n-hexadecane conversion increased in the order of Pt/AI-0 > Pt/AI-0.3 > Pt/AI-0.5 > Pt/AI-1, consistent with the acidity change in the catalysts. The product selectivities over various catalysts were quite different in Figure 11 (b). The iso-hexadecane selectivity over Pt/Al-0 was kept decreasing with the temperature increasing as high temperature intensifies the cracking. However, the selectivities of iso-hexadecane over the Fe-substituted catalysts were increased at various degrees. In the test temperature range, a sharp increase of the selectivity occurred over Pt/AI-0.5 and Pt/AI-1 initially but the selectivity slowed down when the temperature was higher than 300 °C. The selectivity of iso-hexadecane increased with an almost constant rate over Pt/AI-0.3. When the temperature was low, a little n-hexadecane was conversed and no isohexadecane was found over the Pt/Fe-substituted catalysts. It is because more strong Brønsted acidic sites, which favors the cracking, existed than the weak Brønsted acidic sites. With the temperature increasing, 14


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both cracking and isomerization activity were improved. Considering that the strong Bronsted acidic sites of Fe-contained samples were weaker than those of Pt/AI-0 (referring to the NH3-TPD result), less secondary cracking happened. As a result, the selectivity of isohexadecane increased with the temperature increasing. It indicates that the Pt/Fe-substituted ZSM-23 were eligible catalysts for the hydroisomerization of n-hexadecane with good selectivity. It can be corroborated in the product distribution as follows. The distribution of i-C16, isoparaffins and n-paraffins over various catalysts at 300 °C with a n-hexadecane conversion of ~20 wt% was depicted in Figure 12. Both n-paraffin (n≤4) and n-paraffin (n=5–15) selectivities decreased, and the selectivity of isoparaffins and i-C16 increased with the increase of Fe content in the catalysts. i-C16 was the primary product in the isoparaffins for all catalysts, and it was especially obvious over Pt/AI-0.5 and Pt/AI-1. It further indicates that secondary cracking can be inhibited when Fe cations were introduced in the catalysts. More n-paraffins than isoparaffins could be obtained over Pt/AI-0 due to its lower Bweak/Bstrong. The isoparaffins selectivity over Pt/AI-0.3, Pt/AI-0.5 and Pt/AI-1 were much higher than that over Pt/AI-0, and the value of Pt/AI-1 could even reach to 98 wt%. The detailed product distributions over various catalysts at 300 °C and ~20 wt% n-hexadecane conversion were displayed in Table 3. Branching index is a measure of mono/multi branched ratio and is commonly used to evaluate hydroisomerization reactions.52 In this work, monobranching and hydrocracking principally occurred during the reaction. Methyl pentadecane was the only iso-hexadecane detected in the products over all catalysts. The code of the product was assigned as xM-C15, where M denotes the presence of methyl group and x denotes the position of methyl group. 2M-, 3M-, 4M- and 5M- were the main isomers with quite different distribution over various catalysts. The selectivity of each isomers over Pt/AI-0 showed little difference in the products. However, 5M-C15 was the major isomerized product over the Pt/Fe-substituted catalysts, and the 15



5M-C15 selectivity increased with the Fe content increasing in the catalysts. Considering that “pore mouth” shape selectivity model favors branching at 2-C and/or 3-C position, it was speculated that the introduction of Fe in the ZSM-23 zeolite framework increased the occurrence of another adsorption mode – “key lock” type which favors methyl branching at more central positions. It involves a penetration of the two tails in adjacent pore openings, with consideration of the larger external specific areas of the Fe-substituted samples. But no reference can be found and further investigation is needed. Potentially, other positional iso-hexadecanes could be generated from 2M-C15 according to the pore mouth mode through the progressive position optimization.53 As the methyl-alkanes with the methyl group at a central position along the chain have a lower pour point compared with the isomers branched near the end of the chain, the use of these types of Pt/Fe-substituted catalysts can be advantageous in the application of dewaxing process. Finally, the performance of the best catalyst prepared in this work was compared to those previously reported in the literature for the hydroisomerization of n-hexadecane. The comparison was based on the analysis of the n-C16 conversion and the selectivity to i-C16, and the result is listed in Table 4. Overall, it can be seen that Pt/AI-0.5 shows comparative catalytic performance when compared to other catalysts. This confirms that Fe-substituted ZSM-23 zeolites could be considered as available supports for n-hexadecane hydroisomerization. However, further study of optimizing the reaction conditions and the life cycle analysis would be desirable to establish the utility of these catalysts. 4

Conclusions A series of Fe-substituted ZSM-23 samples with different Fe/(Fe+Al) ratios and

crystal sizes were synthesized using pyrrolidine as the structure directing agent. The incorporation of Fe in the ZSM-23 framework was confirmed by comparing the peak position change from the IR and XPS spectra of the Fe-free and Fe-substituted samples. 16


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Compared with the Fe-free ZSM-23 sample. The introduction of Fe into the ZSM-23 framework can lead to larger amount of strong acidic sites which were slightly weaker than that of Fe-free sample and contribute no significant changes to the total number of weak acidic sites. However, the Fe-substituted ZSM-23 samples possessed higher CBweak/CBstrong ratios due to the lower polarizability of Fe compared with Al in the framework. Compared with Pt/Fe-free ZSM-23 catalyst, the Pt/Fe-substituted catalysts showed lower activity during the hydroisomerization of n-hexadecane while higher isomerization selectivity can be obtained benefiting from their higher CBweak/CBstrong ratios and smaller crystal size. More 5M-C15 isomers were obtained over the Pt/Fe-substituted catalysts and the product distribution was quite different from that of the Pt/Fe-free ZSM-23 where 2M-C15 were the main isomer products. The current catalytic technology can be applied to lower the pour point during dewaxing process. There was a considerable gap of the n-hexadecane conversion between the Pt/Fe-substituted catalysts and Pt/AI-0, and more efforts are needed to further improve the activity of the Pt/Fe-substituted catalysts. The findings here can also provide useful perspectives to the development of the hydroisomerization catalysts with better selectivity. Acknowledgements This work was supported by the National Key Research & Development Program of China (2016YFB0600305).

17



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23



Page 24 of 40

Table 1. Textual properties of as-synthesized samples. Sample

SiO2/(Al2O3+ Fe/(Fe+ SBETc Smicroc Sexc Vd b Fe/(Fe+Al) Fe2O3)a (m2/g) (m2/g) (m2/g) (cm3/g) Al)a

Vmicrod (cm3/g)

AI-0

59

~0

~0

203

87

116

0.25

0.04

AI-0.3

62

0.27

0.28

263

83

180

0.34

0.04

AI-0.5

60

0.42

0.41

257

84

173

0.30

0.04

AI-0.7

56

0.51

0.30

225

53

172

0.39

0.02

AI-1

63

~1

~1

208

46

162

0.41

0.02

a

Molar ratio, measured by ICP-AES.

b

Molar ratio, measured by XPS.

c

SBET and Smicro were calculated by BET and t-plot method. Sex= SBET-Smicro.

d

V was measured at P/P0=0.99 with pore size smaller than 40.0 nm and Vmicro is determined in the

t-plot micropore analysis.

24


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Table 2. Acidity properties of as-synthesized samples. Acidity types (mmol/g)b Acidity (NH3 mmol/g)a CB

CL

Sample Weak

AI-0

Strong

0.037

0.030

(236c)

(438c)

0.035

0.031

(231)

(409)

0.036

0.040

(230)

(405)

0.033

0.037

(228)

(404)

0.032

0.044

(226)

(382)

AI-0.3

AI-0.5

AI-0.7

AI-1

150

300

450

150

300

450

CBweak/

°C

°C

°C

°C

°C

°C

CBstrong

0.067

0.059

0.063

0.052

0.009

0.003 0.004

0.13

0.066

0.046

0.039

0.033

0.004

0.008 0.003

0.39

0.076

0.034

0.029

0.025

0.003

0.005 0.002

0.44

0.070

0.028

0.027

0.021

0.006

0.003 0.003

0.33

0.076

0.026

0.027

0.019

0.004

0.003 0.002

0.37

Total

a Acidity calculated from the deconvoluted peak areas of the NH3-TPD result. b Acidity distribution estimated from the areas of Py-IR spectra at various temperatures using the extinction coefficient reported by Emeis.48 c The temperature corresponded to the center of the deconvoluted peaks.

25



Page 26 of 40

Table 3. Product distribution after n-hexadecane hydroisomerization over various catalysts. Distribution (wt%)a Carbon Number

Product Pt/AI-0

Pt/AI-0.3

Pt/AI-0.5

Pt/AI-1

≤4

≤C4

16.2

9.6

8.4

0

5

2M-C4

0.8

0.9

1.2

0.6

n-C5

9.1

0.5

2.8

0.9

2M-C5

6.3

2.2

0.2

0.5

3M-C5

2.1

0.4

0.3

0.2

n-C6

2.6

0.3

0.3

0.4

2M-C6

1.4

0.8

0.4

0.5

3M-C6

1.4

1.1

0.5

0.4

n-C7

3.5

2.0

1.8

0

2M-C7

0.5

0.7

0.3

0.2

3M-C7

1.1

0

0

0

n-C8

3.5

0

0

0

9

2M-C8

0.7

0

0.5

0

10

n-C9

7.2

3.1

0

0

n-C10

5.7

4.2

1.5

0

11

2M-C10

6.0

30.2

16.0

23.6

12

n-C12

4.4

2.7

0.3

0

16

2M-C15

5.0

5.5

8.9

11.7

3M-C15

6.7

8.8

10.7

13.5

4M-C15

3.5

9.0

13.3

9.8

5M-C15

12.3

18.1

32.5

37.5

6

7

8

26


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Table 4. Comparison of catalysts for n-hexadecane hydroisomerization. Pt

n-C16

i-C16

Conversion

Selectivity/Yield

(wt%)

(wt %)

290

~61.3

~87.9 (S)

3

360

~65

~70 (S)

0.3

1.3

290

36.5

35.5 (Y)

PtMo/AlSBA-1556

1.0

3.5

320

48.4

73.8 (S)

Pt/ZSM-2257

0.5

1

270

31.7

71.66 (S)

Pt/HZSM-558

1.0

2

220

~80

~5 (S)

Pt/HMCM-2258

1.0

2

220

~85

~5 (S)

Pt/ZSM-2349

0.3

1

260

~60

~87 (S)

Pt/AI-0.5

0.5

0.7

300

~60

~62(S)

WHSV

T

(h-1)

(°C)

0.5

1.1

0.5

Pt/alumina-Beta55

Catalyst

content (wt %)

Pt/H-ZSM-2213 Pt/Al-mesoporous silica 54

27



Figure 1. XRD patterns of the as-synthesized Fe-substituted ZSM-23 zeolites.

28


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Figure 2. FT-IR spectra of the as-synthesized Fe-substituted ZSM-23 zeolites.

29



Figure 3. Hydroxyl stretching region of FRIR spectra of ZSM-23 samples.

30


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Figure 4. EPR spectra of Fe-substituted ZSM-23 samples.

31



Figure 5. XPS spectra of (a) Al, (b) Fe, (c) Si and (d) O for the as-synthesized Fe-substituted ZSM-23 samples.

32


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Figure 6. SEM images of the as-synthesized Fe-substituted ZSM-23 zeolite AI-0 (a, b), AI-0.3 (c, d), AI-0.5 (e, f), AI-0.7 (g, h) and AI-1 (i, j).

33



Figure 7. N2 adsorption and desorption isotherms of samples.

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Figure 8. NH3-TPD profiles of as-synthesized samples.

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Figure 9. Py-IR spectra of samples at different desorbed temperature of 150 (blue), 300 (orange) and 450 °C (olivine).

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Figure 10. Comparison of n-hexadecane conversion (a) and iso-hexadecane selectivity (b) as a function of contact time over various catalysts.

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Figure 11. Comparison of n-hexadecane conversion (a) and iso-hexadecane selectivity (b) as a function of temperature over various catalysts.

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Figure 12. Distribution of i-C16, iso-paraffin and n-paraffin over various catalysts at ~20 wt% n-hexadecane conversion.

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Table of Contents (TOC) Graphic

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Synthesis and Characterization of Iron-Substituted ZSM-23 Zeolite

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