mesoporous materials based on H-ZSM-22 zeolite

Journal of Catalysis 335 (2016) 11–23

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Synthesis, characterization and isomerization performance of micro/mesoporous materials based on H-ZSM-22 zeolite Suyao Liu a,c, Jie Ren a,b,⇑, Huaike Zhang a,b, Enjing Lv b, Yong Yang a,b, Yong-Wang Li a,b,⇑ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China National Energy Research Center for Clean Fuels, Synfuels China Co., Ltd, Beijing 101400, PR China c University of Chinese Academy of Sciences, Beijing 100049, PR China b

a r t i c l e

i n f o

Article history: Received 3 August 2015 Revised 9 November 2015 Accepted 14 December 2015

Keywords: Micro/mesoporous Recrystallization H-ZSM-22 Isomerization Multi-branch selectivity

a b s t r a c t Micro/mesoporous materials with different mesoporosities were prepared through recrystallization of HZSM-22 zeolite in alkaline solution with cetyltrimethylammonium bromide template (CTAB). The structure, morphology, pore properties, acidity and isomerization performance of the catalysts by using the resulting materials were characterized and assessed. The dissolution and recrystallization procedure introduced the well-developed mesoporous structure of MCM-41 type with the meso-scale channels of about 3 nm in size on the outer surfaces of the microporous H-ZSM-22 zeolites, forming the micro/ mesoporous materials, which possessed increased weak B acid sites at the pore mouths and a reduced amount of total acid sites. It is shown that the presence of well-developed mesopores could remarkably improve the selectivity to multi-branched products and suppress the side cracking reactions in n-dodecane isomerization. The micro/mesoporous Pt/ZSM-22/MCM-41 bifunctional catalyst with suitable recrystallization degree exhibits high isomerization selectivity under high conversion in longchain n-alkane isomerization compared to the original microporous Pt/H-ZSM-22 catalyst. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Skeletal isomerization of long-chain n-alkanes to their branched isomers nowadays is one of the most demanded processes that have been widely used to improve the cold-flow properties of diesel and lube base oil [1,2], especially in the industrial processes of coal/natural gas to liquids (CTL/GTL) via Fischer–Tropsch synthesis [3–7]. For the isomerization of synthetic middle distillates, developing catalysts with high selectivity to various isomers and low to cracking products is the important subject of many investigations [8–10]. For the isomerization of the synthetic middle distillates composing mainly n-alkanes free from contaminate (sulfur and nitrogen), bifunctional catalysts with platinum on zeolite supports have been evaluated [11,12] as potential candidates for industrial applications. It has been found that the platinum on ZSM-22 molecular sieve with one-dimensional 10-membered ring channels of 0.45  0.54 nm is one of the successful n-alkane isomerization catalysts owing to the unique shape-selectivity [13,14].

⇑ Corresponding authors at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China. E-mail addresses: [email protected] (J. Ren), [email protected] (Y.-W. Li). http://dx.doi.org/10.1016/j.jcat.2015.12.009 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

It has been proposed that both the narrow one directional tubular micropores and the strong acid sites at the pore mouth of ZSM-22 lead to the lengthy diffusion pathways between the acidic sites and the metal sites seem to be the major reason that the isomerization products are predominantly of mono-branched components, while the multi-branched isomers are of essential importance for desired properties of middle distillate isomerization [15,16]. However, it is difficult to find the direct evidence on this assumption. Nevertheless, introducing mesoporous structure on the basis of the microporous zeolite crystals has been expected to be a useful way to overcome the inherent diffusion limitations [17,18] and optimize the acid site distribution [19]. Several approaches have successfully been developed for the preparation of hierarchical porous zeolites, namely the posttreatment method [20,21], the hard-templating method and the soft-templating method [22]. Ivanova’s group demonstrated a controllable approach, including simultaneous dissolution and redeposition of the dissolved species on the surface of the parent materials in the presence of long-chain alkylammonium surfactants [23,24], which would permit a better control of mesopore size [25]. Their work has highlighted the new perspectives to formulate the controllable micro/mesoporous materials [26,27]. Inspired by this synthesis concept, a few micro/mesoporous materials have been prepared with remarkable performance proved for

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several catalytic reactions [24]. For example, micro/mesoporous MOR/MCM-41 materials with well-distributed mesopores were prepared and proved to be of high performance in cumene disproportionation [28] and both the short n-alkane (n-C6 and n-C8) and the long-chain n-alkane (n-C16) isomerization [29] due to the improved accessibility of active sites and the transportation of bulky molecules. Similarly, the recrystallized ferrierite materials showed improved catalytic performance in 1-butene isomerization, demonstrating that the enhancing effect of coating mesopore layers on suppressing the catalyst deactivation [30]. However, the synthesis of micro/mesoporous ZSM-22 materials with improved selectivity to multi-branched isomers in long-chain n-alkane isomerization has not seen much success. Moreover, the straightforward explanation for the effect of the uniform mesopores on long-chain n-alkane isomerization is still not available. In the present work, series of micro/mesoporous materials with a varying degree of mesoporosity were prepared by the recrystallization of the H-ZSM-22 zeolite in an alkaline solution with the assistance of CTAB. The introduced mesoporous phase and the optimal recrystallization degree are addressed on the basis of the correlations between the structure, texture, acidity and the catalytic performance. Isomerization of n-dodecane over these catalysts is chosen as a model reaction for long-chain n-alkanes isomerization to explore the role of the mesopores, for promoting the distribution of B acid sites and the catalytic selectivity to different isomers. Moreover, for comparison, the conventional microporous and desilicated ZSM-22 materials are prepared and characterized, and the isomerization performance of the corresponding catalysts is discussed. 2. Experimental 2.1. Reagents The reactants used in the present study are aluminum sulfate, 1,6-diaminohexane (DAH), cetyltrimethylammonium bromide (CTAB) (Sinopharm Chemical Reagent Co., Ltd.), potassium hydroxide, sodium hydroxide, ammonium nitrate (Beijing Chemical Co., Ltd.), silica sol (40 wt.% Qingdao Haiyang Chemical Co., Ltd.), and de-ionized water. 2.2. Synthesis of H-ZSM-22 The ZSM-22 zeolite was synthesized through hydrothermal treatment of aluminosilicate solution with the chemical composition of 1.0SiO2:0.12K2O:0.014Al2O3:0.3DAH:40H2O. The synthesis processes were performed at 160 °C for 48 h in an autoclave under stirring with the self-generated pressure. The solid zeolite products were recovered by filtration, washed extensively with de-ionized water and dried at 120 °C. Finally, the template in the zeolites was removed by calcination in air at 550 °C, and the final product was obtained as K-ZSM-22. The K-ZSM-22 zeolites were converted to protonic form by ion-exchange with NH4NO3 at 80 °C. The ionexchanged samples were dried at 120 °C overnight, and calcined at 550 °C for 4 h. These ion-exchange treatment steps were repeated for 3 times, and the final H-ZSM-22 product was formed. 2.3. Synthesis of micro/mesoporous ZSM-22 The micro/mesoporous ZSM-22 materials were prepared by the following procedures. The H-ZSM-22 samples were mixed with a NaOH aqueous solution, and the mixtures were hydrothermally treated in the presence of CTAB at 100 °C for 24 h. After this step, the recrystallization was performed by adjusting the pH value of the suspension solution to right levels at 120 °C for 24 h, leading

to the assembling of the dissolved species into mesoporous phase around the parent zeolite structures. The degree of destruction of the parent H-ZSM-22 was adjusted by varying the NaOH concentration in the aqueous solution from 0.15, 0.30, 0.45 to 0.90 M, and the prepared materials were denoted as RZEO-1, RZEO-2, RZEO-3, and RZEO-4, respectively. The products were washed with de-ionized water to neutral and dried at 120 °C for 12 h. All samples were calcined to remove the surfactant at 550 °C for 8 h. The micro/mesoporous ZSM-22 materials were subjected to ionexchange with aqueous solution of NH4NO3. Then, the materials were washed, dried and calcined in the air flow at 550 °C for 4 h. The ion-exchange steps were repeated for 3 times. For comparison, the desilicated samples were prepared by stirring the K-ZSM-22 zeolites in the aqueous solution of 0.3 M NaOH at 80 °C for 0.5 h without CTAB, and then this solid was filtered, dried and washed in a solution of 0.1 M HCl at 80 °C for 8 h. The samples were filtered, washed to neutral with de-ionized water and dried overnight at 120 °C. The alkali metal was entirely removed from the products by exchanging with 0.5 M NH4NO3 for 3 times. Finally, the samples were dried and calcined in air flow in a furnace at 550 °C for 8 h. 2.4. Preparation of catalysts To obtain the catalysts for isomerization tests, the H-ZSM-22, desilicated sample (DeZEO) and recrystallized materials (samples RZEO-1, RZEO-2, RZEO-3, and RZEO-4) were extruded by adding quasi-boehmite (15 wt.%) as the binder, crushed, sieved and impregnated in a solution of H2PtCl6
6H2O (1.0 wt.%, pH = 2.3). Then, the suspension was slowly evaporated to remove moisture at 80 °C. Finally, the resulting solid was dried at 120 °C for 12 h and calcined in air at 450 °C for 4 h with a temperature ramp rate of 1 °C/min. The content of loading Pt species was 0.5 wt.%. The obtained catalysts were denoted as Pt/H-ZSM-22, Pt/DeZEO, Pt/ RZEO-1, Pt/RZEO-2, Pt/RZEO-3, and Pt/RZEO-4, respectively. 2.5. Characterization The X-ray diffraction (XRD) patterns were obtained with a Bruker D8 Advance diffractometer with Cu Ka (c = 1.5418 Å, 40 kV, 40 mA) radiation in the scan range of 1°–50° with a step size of 0.02°. The morphology of different samples was determined by methods of field emission scanning electron microscopy (FE-SEM) recorded using an FEI QUANTA 400. Transmission electron microscopy (TEM) images were obtained using an FEI Tecnai G2 F30 operating at 300 kV for high resolution measurements. The bulk chemical composition was analyzed by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The XPS analyses were carried out on a Physical Electronics Company Quantum-2000 Scanning ESCA Microprobe equipped with Al Ka source and a multichannel detector to investigate the surface Si/ Al ratio. The binding energies were calculated with respect to the C–(C,H) component of the C 1s peak of adventitious carbon fixed at 284.8 eV. The spectra of different samples were conducted using Shirley background subtraction and Gaussian/Lorentzian (70/30) product function. Molar fractions were calculated using the normalized peak areas on the basis of acquisition parameters and sensitivity factors (Sisf = 0.82, Alsf = 0.54) provided by manufacturer and the transmission function. Solid-state 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were performed on a Bruker Advance 600 NMR spectrometer. Kaolin and Al(NO3)3 were used as the chemical shift references at 91.5 and 0 ppm, respectively. Low temperature N2-adsorption was measured at 196 °C using Micromeritics ASAP 2020 analyzer and 2420 analyzer that

S. Liu et al. / Journal of Catalysis 335 (2016) 11–23

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were used to obtain the microporous and mesoporous porosities, respectively. The total surface area was determined according to the BET method, and the most probable pore sizes in the ranges of 0–2 nm and 2–50 nm were calculated by the Horvath–Kawazoe (HK) method and the Barrett–Joyner–Halenda (BJH) method, respectively. To characterize the acidity, the temperature programmed desorption of ammonia (NH3-TPD) was carried out at a dynamic chemisorption analyzer (Micromeritics ASAP 2920). Prior to NH3adsorption at 100 °C, the samples were pretreated in the flowing helium at 550 °C for 1 h. The physically adsorbed NH3 was removed in a flow of helium for 1 h, and then the adsorbed samples were heated with a rate of 10 °C/min to 600 °C under helium (50 mL/min). The signals were recorded by monitoring the desorbed ammonia with a TCD detector and a Pfeiffer Omnistar Quadrupole mass spectrometer, simultaneously. Pyridine- and 2,6-dimethylpyridine-adsorbed infrared (Py-IR and DMPy-IR) spectra were used to discriminate acid type of samples on a EQUINOX 70 (Bruker, Germany). Wafers of compressed zeolite samples were mounted in the FTIR cell and degassed in vacuum of 10 2 Pa at 400 °C for 2 h. Samples were saturated with pyridine or 2,6-dimethylpyridine vapor at 30 °C for 10 min. After equilibration, the samples were evacuated at 200 °C and 350 °C to remove the excess of probe molecules, respectively, and the IR spectra were recorded in the range of 1400–1580 cm 1 and 1550–1700 cm 1, correspondingly. CO-chemisorption was performed in the same apparatus as NH3-TPD. Prior to the test, catalyst samples were reduced in situ in flow of H2 (50 mL/min) at 400 °C for 4 h and then purged with He for 0.5 h at 450 °C. After being cooled to 50 °C, several pulses of 5% CO/95% He were injected at regular intervals till the sample was saturated. The average Pt particle size was calculated by assuming an adsorption of one CO molecule per Pt atom. 2.6. Catalytic experiments The isomerization of n-dodecane as the model reaction was carried out in a continuous flow fixed-bed reactor under the conditions of 2 MPa, the volumetric H2/n-alkane ratio of 600 and a liquid hourly space velocity (LHSV) of 2.0 h 1. In each run, 5 mL of the catalysts was loaded into the reactor. Prior to reaction, the catalyst was reduced with hydrogen at 400 °C for 4 h and then the temperature was lowered to the reaction temperature. Subsequently, the feedstock of n-dodecane was fed to the reactor by a microscale pump at a certain flow rate. For the reaction test of each catalyst sample, 5–6 temperature points were sampled with the interval of 3 h for each sampling period, in which the temperature was kept at a constant value. During a sampling period (3 h), the liquid products were collected and analyzed by an offline gas chromatograph (Agilent 7890N) while the gas products by an on-line gas chromatograph. After a sampling period, the temperature was increased at a higher value for the next test till the maximum conversion was reached, which led to the reaction tests on stream about 20 h for each catalyst sample. 3. Results and discussion 3.1. Structure The wide angle range XRD patterns of the parent H-ZSM-22, the desilicated sample (DeZEO) and the different recrystallized materials (samples RZEO-1, RZEO-2, RZEO-3, and RZEO-4) are plotted in Fig. 1a. The calculated crystallinity is listed in Table 1. As shown in Table 1, the as-synthesized H-ZSM-22 zeolite exhibits high crystallinity [31] free from the impurities. The much sharper diffraction

Fig. 1. Wide (a) and small (b) angle XRD patterns of different samples.

peaks of the RZEO-1 sample indicate that the mild hydrothermal treatment leads to the enhancement of crystallinity. This result could be attributed to the dissolution of some amorphous phase containing in the as-synthesized zeolite crystal structure during the mild alkaline hydrothermal step. However, the intensity of diffraction peaks weakens gradually from RZEO-1, RZEO-2 to RZEO-3 after the hydrothermal treatment at increasing alkalinity. Further increasing alkalinity leads to the disappearance of H-ZSM-22 diffraction peaks (RZEO-4) due to the complete dissolution of the microporous phase. The crystallinity of the desilicated DeZEO sample decreases obviously with very short treatment in alkaline solutions, showing a typical result of framework dissolution. The small angle range XRD patterns are shown in Fig. 1b. The absence of the diffraction peaks in the region of 1° < 2h < 6° reveals that the uniform mesoporous phase is absent in the parent zeolite (H-ZSM-22) and the desilicated sample (DeZEO). A single diffraction peak assigned to the (1 0 0) reflection of mesoporous material starts appearing at about 2h = 2.35° from RZEO-1 sample, and the intensity of this peak increases with the increasing recrystallization degree for RZEO-2, RZEO-3, and RZEO-4, illustrating the increasing content of the mesoporous phase in these materials, for which the appearance of peaks at 2h = 4.04° and 4.66° attributed to the (1 1 0) and (2 0 0) reflections further indicates the presence of hexagonal pore symmetry in composites [32]. The diffraction peak intensity of RZEO-4 indicates that this recrystallized material is of the MCM-41 type material with high

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Table 1 Characteristic of different samples.

a b

Samples

CNaOH (M)

Si/Al (Chem. anal.)

Si/Al (XPS)

Phase

Crysta (%)

H-ZSM-22 DeZEO RZEO-1 RZEO-2 RZEO-3 RZEO-4

– 0.30 0.15 0.30 0.45 0.90

31.76 25.14 31.84 32.18 32.35 33.07

25.09 20.14 44.07 51.23 52.40 52.89

TON TON TON TON/MCM-41 TON/MCM-41 MCM-41

100 86 108 88 76 –

BETb Smic

Sex

165.17 96.58 135.35 114.51 78.35 –

61.72 189.87 152.25 179.82 355.01 946.67

Relative crystallinity degree of calculated from XRD (normalized intensity of TON (1 0 0) peak). Obtained by the N2-adsorption at 196° using Micromeritic ASAP 2020.

crystallinity with complete disappearance of the parent ZSM-22 structure. Consequently, the XRD results illustrate that the recrystallized micro/mesoporous samples (REZO-1, RZEO-2, and RZEO-3) consist of both the H-ZSM-22 zeolitic phase and the MCM-41 mesoporous phase formed by the recrystallization treatment. 3.2. Morphology and elements distribution The SEM images are presented in Fig. 2. The as-synthesized HZSM-22 zeolite displays the needle-shaped crystals, showing characteristic morphology of TON-type zeolites [33]. After the hydrothermal treatment in a mild alkali solution with CTAB, the crystal of the RZEO-1 sample becomes rough probably due to the formation of a small amount of mesoporous phase on the outer surfaces of the original well crystallized H-ZSM-22. With the increasing alkalinity, the mesoporous crystals become thicker in the order of RZEO-1, RZEO-2 and RZEO-3. The morphology of RZEO-4 sample shows lamellar particles of mesoporous material, suggesting completely dissolving of the original H-ZSM-22 crystals and the reorganization of structures into the MCM-41 type phase with the help of CTAB. However, the sample DeZEO after desilication procedure almost keeps the original needle-like morphology. More detailed information concerning the structure of different samples is shown in TEM images (Fig. 3). The TEM image of HZSM-22 shows that the crystal size is uniform with the dimensional size of 700–900 nm in length and 30–50 nm in diameter and in the absence of any secondary phase. On the contrary, the image of RZEO-1 displays a thin film of worm-like mesoporous solid on the outer surface of the H-ZSM-22 crystals. In the case of the RZEO-2 and RZEO-3 samples, the mesoporous phase grows and develops into obvious layers of the mesoporous phase, which fully covers the original crystal rods. These observations provide the direct evidences that the dissolving of H-ZSM-22 from outer surfaces and assembling of these dissolved species into mesoporous phases with the help of CTAB are the major separate processes occurring on the outer surfaces of the original needle-like nanorods of H-ZSM-22. The recrystallized RZEO-4 shows complete change of the needle-like crystals into the lamellar ones with existence of highly ordered hexagonal array. In the case of DeZEO sample, the external surface of individual nanorods roughens, while the crystals are slimmer. The roughened external surface of the DeZEO sample is main source of the intra-crystalline mesopores [34] formed from alkaline treatment, which are completely different from the uniform mesoporous structure of the recrystallized samples. Since the external surface of the recrystallized materials is affected by alkaline treatment and coated by the new mesoporous phase as confirmed by SEM and TEM, a large number of Si species with a small amount of Al simultaneously in the framework have been dissolved in the alkaline solution and then reassembled with help of CTAB. The XPS and ICP-AES techniques are used to determine the surface and bulk Si/Al ratios (Table 1), respectively. From the

difference between bulk and surface Si/Al ratios, the Al distribution in the parent zeolite is uneven, revealing that the aluminum is concentrated in the rim, which is in line with those obtained by Hayasaka et al. [35]. After alkaline treatment, the bulk and surface Si/Al ratios of the DeZEO sample drop from 32.76 and 25.09 to 25.14 and 20.14, respectively, indicating that the alkaline treatment mainly dissolves silica from the silicon rich framework of H-ZSM-22. The recrystallization leads to the slightly increase of bulk Si/Al ratios from original 31.76 to 31.84 for RZEO-1, 32.18 for RZEO-2, 32.25 for RZEO-3, and 33.07 for RZEO-4, respectively, and the surface Si/Al ratios increase remarkably from original 25.09 to 44.07 for RZEO-1, 51.23 for RZEO-2, 52.40 for RZEO-3, and 52.89 for RZEO4, respectively, indicating that the most of silicon dissolved by alkaline treatment can be reassembled in the recrystallized process and the dissolved Al, which is in smaller amount than that contained in the dissolved structure of original zeolites, is also incorporated into the meso-pore structures. Combined with the TEM images and the XPS analysis (with depth of the order of a few nanometers [36]), it is verified that the mesoporous material has been formed at the outer-surface of the alkaline treated H-ZSM22 structure by the dissolved Si and very small amount of Al from the parent zeolite during the recrystallization step. 3.3. Coordination properties To confirm the coordination environment for the aluminum, the Al MAS NMR spectra are performed (Fig. 4a). In the spectra of the parent H-ZSM-22 and recrystallized micro/mesoporous materials, there are two signals with chemical shifts at 54.5 and 0 ppm corresponding to the tetrahedral Al in the framework and the octahedral form of the extra-framework Al, respectively [37]. The high temperature calcination steps account for the presence of extraframework aluminum sites [38]. The decreased intensity of signal at 0 ppm with the increasing recrystallization degree is the result of the partial dissolution of tetrahedral and extra-framework Al. Only one signal at 54.5 ppm is observed in the spectra of RZEO-4 material, revealing that the microporous zeolites were completely dissolved and then these species reassembled into the high crystallinity mesoporous structure during recrystallization steps with extreme dissolution conditions. For the RZEO-1, RZEO-2, and RZEO-3 samples, apart from the clear signals for the tetrahedral Al in the framework (54.5 ppm), the signals at 0 ppm for the octahedral form of the extra-framework Al are weaker than the original H-ZSM-22 sample, indicating that the non-framework Al may be partially dissolved and reorganized in the recrystallization treatment. Furthermore, in the case of DeZEO, the peak at 0 ppm becomes much weaker due to the partial removal of the extraframework Al in the acid-washing treatment [20]. The results of 29Si MAS NMR are shown in Fig. 4b. The resonance peaks at 114.5 and 105 ppm are assigned to Si(0Al) and Si(1Al) respectively [39], suggesting that the parent H-ZSM-22 crystals are defect-free [40]. The spectra of DeZEO sample are similar to those 27

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Fig. 2. SEM images of parent H-ZSM-22, desilicated sample and recrystallized materials.

of parent zeolite, implying no obvious destruction of the chemical surroundings of framework Si during the desilication process. For the recrystallized samples, the resonances broaden gradually from RZEO-1 to RZEO-4. The peaks at around 100.5 ppm for RZEO-3 and at 100.5 ppm and 108.7 ppm for RZEO-4 can be assigned to the Si(0Al) and Si(1Al) of MCM-41 mesoporous materials [41,42]. 3.4. Pore properties The N2-adsorption isotherms of different samples are depicted in Fig. 5. Obviously, all samples exhibit steep raise at P/P0 < 0.01, typical for the microporous structure. The H-ZSM-22 zeolite exhibits a typical isotherm of pure microporous material, while the desilicated DeZEO sample has a larger hysteresis loop due to the

presence of intra-crystalline mesopores. For the recrystallized samples, the shift of desorption branch to a lower relative pressure and the increased hysteresis loop suggest a partial loss of micro-structural organization and the formation of mesopores. Nevertheless, the RZEO-x samples (x = 1, 2, 3, 4) exhibit a rather sharp isotherm at about P/P0  0.35, indicating the existence of uniform mesopores with diameter of 3–4 nm [27]. The pore distribution for all samples is shown in Fig. 6. From the pore distributions, it can be seen that the well distributed micropores (about 5.0 Å) are presented in the parent H-ZSM-22, the alkaline treated DeZEO, and the partially dissolved/recrystallized samples (RZEO-1 to RZEO-3), while the absence of micropores in REZO-4 is due to complete destruction of the microporous structure of H-ZSM-22 and the establishment of new mesoporous phases in the recrystallization process. For the mesopores, the

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Fig. 3. TEM images of parent H-ZSM-22, desilicated sample and recrystallized materials.

parent H-ZSM-22 structure shows no mesopore distribution, implying that the lateral surfaces of the rod shape H-ZSM-22 are probably inert [43,44] for catalysis interests. The DeZEO from alkaline treatment shows broad mesopore distribution in the range of 2–30 nm with the maximum at about 7.5 nm, indicating that the framework was dissolved irregularly due to the absence of the template [45] and the alkaline treatment creates significant defects (irregular holes), which are active for adsorption with probably catalysis potentials [16], on the lateral surfaces of the rod structure. The recrystallized samples show sharp mesopore distributions of increasing narrowing at about 3 nm with increased intensity in the order of RZEO-1, RZEO-2, RZEO-3, and RZEO-4 in agreement of conditions used in the post treatment, showing the obvious difference of the mesopores from those formed in DeZEO sample. This difference may be due to the absence of the template [45] during the preparation for DeZEO sample and also the recrystallization of dissolved species, which may start building up around the defects created in the dissolution procedure. The results obtained from the textural analysis of all samples are presented in Table 1. On the basis of the as-synthesized HZSM-22 zeolite with only micropores, the introduction of intracrystalline mesoporosity by desilication (DeZEO) leads to the increase of external surface area and the decrease of microporous area after alkaline treatment [20]. The RZEO-x samples display much higher BET surface area, especially the external surface area, suggesting the formation of the uniform mesoporous structure with the relative high microporous area kept by the proper control of recrystallization conditions.

3.5. Acidity The NH3-TPD, FTIR spectra of pyridine adsorption (Py-FTIR), FTIR spectra of 2,6-dimethylpyridine (DMPy-FTIR) adsorption were used for characterizing the acidity of all samples. The NH3-TPD curves are shown in Fig. 7, and all FTIR spectra and the corresponding quantitative calculation method can be found in Supplementary Information (SI) (Figs. S1 and S2). The results of total acidity distributions from these characterizations are listed in Table 2. The parent H-ZSM-22 has clearly strong and weak adsorption peaks, and the DeZEO sample exhibits a slight decrease in the both peaks due to the partial withdrawal of silica species from the framework as shown in Fig. 7. The NH3-TPD results also show that the recrystallization at changing conditions (RZEO-1 to RZEO-4) leads to an enhancing trend of the decrease in the amount and strength of acid sites with the high temperature desorption peak shifting to lower temperature and greatly weakening (Fig. 7). This can well be explained by the dissolving of the parent H-ZSM-22 structure and the reassembling of the new mesoporous structure with weak acidity. The results of weak and strong acid sites obtained from NH3TPD tests (Table 2) all indicate an obvious trend of decreasing acidity in H-ZSM-22 > DeZEO > RZEO-1 > RZEO-2 > RZEO-3 > RZEO-4. Further discrimination tests with Py-FTIR and DMPy-FTIR are summarized in Table 2 for the distributions of the different acid sites [46,47]. The Py-FTIR results reveal that the B acid sites follow a clear decreasing trend in H-ZSM-22 > DeZEO > RZEO-1 > RZEO-2 > RZEO-3 > RZEO-4, and the L acid sites follow increasing trend in

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Fig. 4.

27

Al (a) and

17

29

Si (b) MAS NMR spectra of different samples.

Fig. 6. Micropore (a) and mesopore (b) size distribution.

Fig. 5. N2-adsorption isotherms of parent H-ZSM-22, desilicated sample and recrystallized materials. Fig. 7. NH3-TPD profiles of different samples.

H-ZSM-22 < RZEO-1 < RZEO-2 < RZEO-3 < RZEO-4 < DeZEO. The results indicate that both the destruction of microporous structures in the H-ZSM-22 framework and the formation of mesoporous structure cause the decrease of B acidity and the increase of L acidity [23,48,49].

The DMPy-IR results show the variation of the B acidity at outside of micropores (at micropore pore mouths and in mesopores) [50–52]. These quantitative results in Table 2 show that the strong B acid sites (measured at 350 °C) follow a clear decreasing trend in

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Table 2 The acidity of different samples determined by NH3-TPD, Py-FTIR and DMPy-FTIR spectra. Aciditya (lmol/g)

Samples

Awb

H-ZSM-22 DeZEO RZEO-1 RZEO-2 RZEO-3 RZEO-4 a b c d e f

201 175 188 180 165 88

Am

Acid typee (lmol/g) c

– – – – – 65

Asd

B

184 179 168 146 95 –

L

200 °C

350 °C

200 °C

350 °C

200 °C

350 °C

61 52 50 48 47 35

59 49 31 26 23 12

60 80 68 70 71 72

46 58 50 53 51 52

29 33 32 38 38 26

30 28 19 19 18 10

Obtained from the NH3-TPD spectra. Weak acid sites estimated from the relative area of the deconvoluted peak. Medium acid sites estimated from the relative area of the deconvoluted peak. Strong acid sites estimated from the relative area of the deconvoluted peak. Acidity calculated from the peak areas of FT-IR spectra by pyridine desorption at different temperatures. Brönsted acidity calculated from the peak areas of FT-IR spectra by 2,6-dimethlypyridine desorption at different temperatures.

H-ZSM-22 > DeZEO > RZEO-1 > RZEO-2 > RZEO-3 > RZEO-4, indicating the suppressing effect of the dissolving and recrystallization treatments on the strong B sites [53,37]. However, for mild B sites (200 °C), samples RZEO-2 and RZEO-3 show the highest adsorption amount while the RZEO-4 shows the lowest adsorption amount among all the samples evaluated, implying that the proper control of the recrystallization conditions can enhance the mild B acidity, some of which are on the mouths of micropores and some are in the mesopores. 3.6. Metallic properties The balance between the (de)hydrogenation function and the acidity function plays a significant role in a bifunctional isomerization catalyst. The Pt dispersion and the average platinum particle size are determined by CO-chemisorption, and the results are listed in Table 3. The catalyst samples are designed by loading 0.5 wt.% Pt on the synthesized support samples, namely Pt/H-ZSM-22, Pt/ DeZEO, Pt/RZEO-1, Pt/RZEO-2, Pt/RZEO-3, and Pt/RZEO-4. From the CO-chemisorption results, it can be found that the metal dispersions for all the preliminary samples are in the increasing order of Pt/H-ZSM-22Pt/DeZEO < Pt/RZEO-1 < Pt/RZEO-2 < Pt/RZEO3 < Pt/RZEO-4 and the particle sizes are in the opposite order, indicating that the recrystallization treatment for the supports can enhance the metal dispersion under the same impregnation conditions due probably to the enriched surfaces from the formation of mesoporous structures. It should be noted that the formation of the mesoporous structure may be the essential factor for the higher metal dispersion observed in this study. The formed pores on the mesopore layer with typical sizes of 3 nm will provide more ideal space for the small Pt particles 1–2 nm being confined in the mesoporous structure than the outer surface of the H-ZSM-22, which is rather inert for catalysis purposes indicated by N2-adsorption analysis. TEM images of the reduced catalysts are shown in Fig. 8. For the Pt/H-ZSM-22 catalyst, it is shown that the Pt particles are around

Table 3 Result of CO-chemisorption of different catalysts.

a

B acid sitesf (lmol/g)

Catalysts

Pt loading (wt.%)

Pt dispersion (%)

Pt crystal sizea (nm)

Pt/H-ZSM-22 Pt/DeZEO Pt/RZEO-1 Pt/RZEO-2 Pt/RZEO-3 Pt/RZEO-4

0.5

47.29 46.12 54.14 59.61 68.95 71.95

2.1 2.1 2.1 2.0 1.4 1.3

Average of Pt particle sizes, assuming homogeneous semispherical Pt particle.

2.20 nm in size, which are too large to penetrate into the microporous channels and thus locate on the external surface of H-ZSM22 nanorods and the binders randomly. The introduction of intra-crystalline mesoporosity by desilication treatment (DeZEO) hardly has a significant influence on the location and crystal size of the Pt particles. However, for the reduced Pt/RZEO-2 catalysts, the Pt particles tend to locate on the uniform mesoporous layers, and the Pt crystal size decreases obviously, which is in line with the CO-chemisorption results. The Pt/RZEO-4 catalyst with broad channels and high surface areas has the smallest metal particles (<1.50 nm) confined within the mesopores. It should be noted that the current work is focusing on the influences of different structures of acidic supports on the isomerization reaction and more efforts for improving the balance and contact between acidic sites and metal sites should be made in further studies

3.7. Catalytic performance The n-dodecane isomerization is tested over the catalysts, Pt/HZSM-22, Pt/DeZEO, Pt/RZEO-1, Pt/RZEO-2, Pt/RZEO-3, and Pt/RZEO4, which are prepared with the same procedure for all the support samples simply by impregnation in a solution of H2PtCl6
6H2O. The conversion and isomerization selectivity results of the n-dodecane reactant under different temperatures are shown in Fig. 9. From the conversion results, it is clearly shown that the activities of all the catalyst samples in Fig. 9a, for n-dodecane isomerization/cracking follow the same trends as the B acid site variation, suggesting that the stronger B acidity accounts for the higher activity. The recrystallized materials are obviously less active than the purely microporous H-ZSM-22 zeolites, and require higher temperature to achieve an equivalent conversion level. The reaction temperature of Pt/RZEO-1 catalyst is 5 °C higher than that of Pt/H-ZSM22 for the same conversion level, and increases with the enhancement of recrystallization degree from Pt/RZEO-1 to Pt/RZEO-4. The lower catalytic activity of the recrystallized materials is in agreement with the decrease of the amount of strong B acid sites at pore mouths, which has been believed to be the active sites for long chain n-alkane isomerization/cracking [54–56]. It should be noted that the catalyst Pt/RZEO-4 with completely dissolved and recrystallized support (mesoporous material), on which no strong B site available, needs very high temperature (360–380 °C) to reach the high conversion (about 90%). At the high temperature, ndodecane isomerization/cracking may have both catalytic and thermal behaviors, in which the thermal reactions of n-dodecane indicated by our check test (as shown in Table S1) are mainly cracking into lighter hydrocarbons. The thermal reactions of the current feed mainly occur at high temperature above 380 °C with

S. Liu et al. / Journal of Catalysis 335 (2016) 11–23

19

Fig. 8. TEM images and particle sizes of four representative catalysts after reduction.

detectible conversions from 1.1% at 380 °C to 5.3% at 420 °C, indicating that in most cases investigated, thermal reactions will not bias the isomerization results in this study. Compared with the Pt/RZEO-x catalysts, the catalytic activity of Pt/DeZEO is high, keeping the same level as the catalysts with the original H-ZSM-22 support, especially above 280 °C. This phenomenon may be explained by the canceling effect of the suppression of B acidity and creation of defects (holes) on the lateral surface of the H-ZSM-22 rods by dissolving of ZSM-22 framework, leading to no obvious loss of the activity. The total isomerization selectivity results in Fig. 9b show that the selectivity to isomerization decreases with the conversion increase, indicating that higher conversion leads to higher cracking rate. It is notable that the sample Pt/RZEO-2 shows the highest total selectivity to isomerization in a very slow decrease trends with the conversion increase, showing a potential candidate for an isomerization catalyst. The other samples show a significant decrease of the total isomerization selectivity with the conversion increase. In all, the total isomerization selectivity of all samples follows the order of Pt/RZEO-2 > Pt/RZEO-3 > Pt/RZEO-1 > Pt/ DeZEO > Pt/RZEO-4 > Pt/H-ZSM-22. In addition, the further results of the selectivities to monobranched isomers and to the multi-branched isomers are shown in Fig. 10a and b, respectively. It is clear that the selectivity to the mono-branched isomers follows the same order as that of the total selectivity for the samples Pt/RZEO-2, Pt/RZEO-3, Pt/ RZEO-1, and Pt/DeZEO, while the complete recrystallized sample Pt/RZEO-4 has the lowest mono-branched isomer selectivity and the Pt/H-ZSM-22 catalyst has higher mono-branched isomer selectivity than the Pt/RZEO-4. For the multi-branched isomer selectivity, the order becomes Pt/RZEO-4 > Pt/RZEO-3 > Pt/RZEO-2 > Pt/ RZEO-1Pt/DeZEO > Pt/H-ZSM-22.

The selectivity behaviors of the series of samples are in general related to the properties of catalysts used in this study. The first type of catalyst is the Pt/H-ZSM-22 with the highest cracking activity and low isomerization selectivity due to the strong B acidity nature of the microporous H-ZSM-22 support. The second type is the Pt/DeZEO catalyst with improved isomerization selectivity and without loss of activity due to suppression to the original strong B acidity, probably formation of the defects and the irregular mesoporous deposition on the outer (lateral) surfaces of the original H-ZSM-22 support by the dissolution treatment. The third type of catalysts is the Pt/RZEO-x (x = 1,2,3) with greatly improved isomerization selectivity and loss of activity due to the greatly suppression of strong B acidity, formation of the defects, the welldeveloped mesoporous layers on the outer (lateral) surfaces of the original H-ZSM-22 rod shaped support by the dissolution and the recrystallization treatment. The last type of catalyst is the Pt/RZEO-4 with slightly improved isomerization selectivity due to the complete loss of strong B acidity, leaving mild B sites only, the complete destruction of the original H-ZSM-22 structure and the formation of the mesoporous structure by the dissolution and the recrystallization treatment. 3.8. Structure effect on isomerization performance Modification of the H-ZSM-22 structure can greatly improve the isomerization performance of Pt/H-ZSM-22 type catalysts [57], and this has been revealed systematically in this study. However, it is desirable that the isomerization catalyst should have both good activity and appropriate selectivity, not only to the monobranched isomers but also to the multi-branched isomers. According to classical bifunctional mechanism of hydro-isomerization/ cracking of hydrocarbon chains, the mono- and multi-branched

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S. Liu et al. / Journal of Catalysis 335 (2016) 11–23

Fig. 9. Conversion (a) and selectivity (b) of n-dodecane isomerization over different bifunctional catalysts (Reaction condition: H2/n-dodecane = 600, LHSV = 2 h 1, P = 2 MPa).

Fig. 10. Selectivity of mono- (a) and multi-branched (b) isomers versus conversion over different catalysts (Reaction condition: H2/n-dodecane = 600, LHSV = 2 h 1, P = 2 MPa).

Table 4 Results of n-dodecane isomerization over different catalysts. Catalysts

Pt/H-ZSM-22

Pt/H-DeZEO

Pt/H-RZEO-2

Pt/H-RZEO-4

Temperature (°C) Conversion (%) ST (%) SMoB (%) SMuB (%) SMoB/SMuB

290 90.26 47.09 42.77 4.32 9.90

290 90.24 60.70 52.52 8.18 6.42

300 91.32 91.70 73.43 18.27 4.02

370 84.71 50.29 19.75 30.54 0.65

S (%) 2-Methylundecane 3-Methylundecane 4-Methylundecane 5-Methylundecane 6-Methylundecane 2,4-Dimethyldecane 2,5-Dimethyldecane 2,6-Dimethyldecane 2,7-Dimethyldecane 2,8-Dimethyldecane 2,9-Dimethyldecane 3,x-Dimethyldecane Methylethyl nonane Other isomers

15.52 10.84 6.42 7.05 2.94 0.25 0.37 0.38 0.59 0.84 1.48 0.28 0.12 0.01

17.70 14.14 8.30 8.01 4.37 0.69 0.68 0.79 0.92 1.75 1.89 0.59 0.82 0.05

26.63 18.69 10.72 10.04 7.35 1.09 1.56 2.16 2.25 3.87 4.27 0.95 2.01 0.11

5.32 4.59 3.75 3.40 2.69 3.36 3.38 2.91 2.97 3.82 3.94 4.14 4.12 1.90

ST, SMoB, SMuB and S corresponded to the selectivity of total isomers, mono-branded isomers, multi-branched isomers and different main products, respectively, at about 90% of conversion.

S. Liu et al. / Journal of Catalysis 335 (2016) 11–23

isomers are formed successively along with the cracking of carbon chains via carbenium ion mechanism [58], in which the formation of carbenium ions on typically B acid sites is essentially important for the catalytic activity for both isomerization and cracking reactions. Apart from the B acid sites, the metal sites are of importance in the hydro-isomerization/cracking catalysis for an instant hydrogenation function for fast hydrogenating the unsaturated intermediates formed via carbenium ion mechanism over B sites, avoiding secondary (side) reactions. It is generally understood that the balance and the intimate contact between metal sites and B acid sites are essentially important for practical catalysts for the isomerization of hydrocarbon chains [59]. In this study, the metal sites are provided by Pt dispersed on the acidic supports with a same impregnation procedure for all support samples to reflect the differences of catalytic behaviors of the catalysts using different supports, and the optimization of the balance/contact between acidic sites and metal sites may need to develop different preparation procedures and will be the important direction for future studies. The n-dodecane isomerization products for four representative catalyst samples, Pt/H-ZSM-22, Pt/DeZEO, Pt/RZEO-2, and Pt/ RZEO-4, are exhaustively analyzed and shown in Table 4. The Pt/ H-ZSM-22, Pt/DeZEO, and Pt/RZEO-2 catalysts exhibit higher selectivity to mono-branched isomers than that of multi-branches, while the Pt/RZEO-4 catalyst is in the opposite order. The 2-methylundecane and 3-methylundecane are the main monobranched isomers over Pt/H-ZSM-22, Pt/DeZEO, and Pt/RZEO-2 catalysts. The methyl branching preference decreases toward the center of the chain. This result is in good agreement with the ‘‘pore mouth” shape selectivity model, which favors branching at 2-C and/or 3-C positions and is less favorable at more central positions along the carbon chain due to the partially n-alkane penetration into one single pore opening [13,14], suggesting the monobranched isomers are formed in the pore mouth of zeolite micropores. For the selectivity of di-branched isomers, the 2,x-methyl-nonane and 3,x-methyl-nonane isomers are the most abundantly formed species over these three catalysts, which implies that the formation of di-branched isomers may be through a reaction step consecutive to the mono-branched products according to the bifunctional mechanism. Additionally, the selectivity to 2,x-methyl-nonane isomers deceases in the order of 2,9-dimethylnonane > 2,8-dimethylnonane > 2,7-dimethylnonane > 2,6-dimethylnonane > 2,5-dimethylnonane > 2,4-dimethylnonane, illustrating that the preferred position for the second branching is located at the symmetric carbon atoms at the two side of a carbon chain as explained by the ‘‘key-lock” shape selectivity model, further inferring that the formation of methyl isomer mainly contributed by the acid sites of the micropores. The Pt/RZEO-4 catalyst exhibits entirely different distribution of methyl-undecane or di-methyl-decane positional isomers with respect to that of the other catalysts, due to the effect of broad channels, resulting in the probability of formation of methyl-undecane positional isomers is equivalent. The Pt/H-ZSM-22 catalyst with the metal dispersed on the microporous H-ZSM-22 support of strong B acidity shows the highest conversion of n-dodecane, the lowest total selectivity and multi-branch selectivity, implying that the strong B acidity is responsible for the high cracking activity and thus low isomerization selectivity. The higher mono-branch selectivity of the Pt/HZSM-22 catalyst than that of the complete mesoporous supported Pt catalyst Pt/RZEO-4 with only mild B acid sites may reflect the contribution of the microporous structure to the mono-branched isomers with shape selectivity [60,61], while the contribution of the mesoporous structure in Pt/RZEO-4 to the higher multibranch shape selectivity [62,63]. It should be noted that mesoporous Pt/RZEO-4 catalyst shows too low activity due to its too weak B acidity.

21

Fig. 11. Schematic representation of the structure change from original H-ZSM-22 by dissolution and/or recrystallization treatment, forming the treated samples in this study: DeZEO, REZO-1, RZEO-2, RZEO-3, and RZEO-4.

It is evident that the defects created by dissolution of the H-ZSM-22 structure in the supports DeZEO and RZEO-x (x = 1,2,3) with varying suppression of B acidity can improve the monobranch selectivity and the formation of the mesoporous structure on the outer surfaces of the H-ZSM-22 support can promote the formation of the multi-branch isomers. At this point, it is necessary to discuss the structure change along with the variation of B acidity of all the supports. Combining the XRD (Fig. 1), TEM (Fig. 3), MAS NMR (Fig. 4), and N2-adsorption (Fig. 6) results, these changes can be represented schematically by the following diagram shown in Fig. 11. This model is also partially supported by the results of Martens et al. [58]. The original H-ZSM22 of rod shape (about 30–50 nm in diameter, and 700–900 nm in length) with only one-dimensional tubular micropores of about 0.5 nm has been known to have B acid sites distributed in the micropores and on the pore mouths [64,65], forming main active surfaces, on which Pt is dispersed as metal sites. In such environment, hydrocarbon chains have limited interaction with active sites, the end parts of the chains may easily insert into the micropores [66], and the point of the chain on the pore mouth may access the both the B acid and metal sites, and thus easily undergo isomerization/cracking reactions, leading to the formation of

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S. Liu et al. / Journal of Catalysis 335 (2016) 11–23

mainly mono-branch isomers or cracking products. The cracking selectivity is high because of the strong B acidity [57]. The dissolution sample DeZEO with the same rod shape as the original zeolite crystals and with both the similar original micropores and the mesopores, which are mainly the defects or holes formed by alkaline treatment on the originally inert lateral surfaces of the crystal rods, has slightly less strong B acidity, greatly improving the monobranch selectivity due to lowered cracking activity of the reduced B acid strength as compared with the original Pt/H-ZSM-22 catalyst. Due to the existence of the mesoscale holes created by alkaline treatment on the external/lateral surfaces of the DeZEO rods, the multi-branch selectivity is improved compared to that of Pt/H-ZSM-22. The RZEO-1, RZEO-2 and RZEO-3 with both the micropores in the cores of the rods and the well-established mesoporous layer (mesopore of 3 nm in diameter) on the outer surfaces of the microporous core rods have gradually lowered B acidity located on the micropore mouths and in the micropores and probably on the mesopore surfaces (mild B acid sites), forming the major active sites. In such micro/mesoporous environment with reduced strong B acidity, the hydrocarbon chains will interact with not only the microporous structure but also the mesoporous structure. The lowered B acidity suppresses the activity of the ndodecane conversion by mainly reducing the cracking possibility and greatly improves the isomerization selectivity, and, however, reduction of microporous structure may cause the decrease in mono-branch selectivity, leading to the existence of an optimal recrystallization degree (RZEO-2 in this study) for high isomerization selectivity. Furthermore, it is interesting that the multibranched isomers selectivity is significantly improved by the enhanced portion of mesoporous structure. One reason is that the outer layered mesoporous structure would put constrains on the hydrocarbon reacting with the acid site at the microporous structure, making the longer residence time of the reactants in the interconnectivity between mesoporous layers and the new opening pore mouths of the zeolites external surface and thus increasing the secondary isomerization opportunity on the microporous pore mouths [67]. Nevertheless, the dispersed Pt sites are very important for the reactivity of the hydrocarbon conversion. It is suspected that the formation of the mesoporous layer on the outer surfaces of the micro-porous structure could provide better environment for the Pt dispersion, which would improve the contact between Pt sites and acid sites. This should be further addressed by both theoretical and experimental investigation, for example, the question on where the Pt particles should be located with respect to the B acid sites.

4. Conclusions A series of micro/mesoporous materials with different mesoporous layers are successfully synthesized by the hydrothermal recrystallization of the H-ZSM-22 zeolite in different alkalinity with template, and their long-chain n-alkanes isomerization performances are studied. The dissolution and recrystallization procedures introduce the well-established mesoporous layer with pores of about 3 nm in diameter on the outer surfaces of the microporous H-ZSM-22 zeolites, forming the micro/mesoporous materials, which have both increased weak B acid sites and reduced amount of total acid sites locating on the micropore pore mouths and in the micropores and probably on the mesopore surfaces (mild B acid sites), forming the major active sites. Consequently, these catalysts exhibit low cracking activity and rather high isomerization selectivity, while decrease of microporous structure may cause the decrease of mono-branched isomers selectivity, leading to the existence of an optimal recrystallization degree (RZEO-2) for high isomerization selectivity at even high conversion. The presence of

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