Effects of acidity and mesoporosity development in ZSM-5 on biomass-derived ethylene oligomerization for liquid fuel synthesis

Transcription

1 Journal of Chongqing University (English Edition) [ISSN ] Vol. 14 No. 2 June 2015 doi: /j.issn To cite this article: WANG Bao-lin. Effects of acidity and mesoporosity development in ZSM-5 on biomass-derived ethylene oligomerization for liquid fuel synthesis [J]. J Chongqing Univ: Eng Ed [ISSN ], 2015, 14(2): Effects of acidity and mesoporosity development in ZSM-5 on biomass-derived ethylene oligomerization for liquid fuel synthesis WANG Bao-lin Research Institute of Sinopec Sichuan Vinylon Works, Changshou District, Chongqing , P. R. China Received 15 September 2014; received in revised form 9 October 2014 Abstract: The acidities of different SiO 2 /Al 2 O 3 ratio ZSM-5 zeolites, CBV3024E, CBV5524G and CBV8014 were investigated with temperature-programmed desorption of ammonia and diffuse reflectance infrared Fourier transform spectroscopy, and their catalytic performances were evaluated to screen the optimal CBV8014 catalyst for ethylene oligomerization. The mesoporosity development in CBV8014 zeolite was conducted by desilication in alkaline medium. The porous characteristics, structural properties and acidic properties of parent and alkali-treated CBV8014 zeolites were studied, and their catalytic performances were evaluated, indicating that CBV8014 treated by 0.2 mol/l NaOH solution has an appropriate mesoporosity development, well preservation of catalyst acidity and crystallinity, good catalytic activity and stability, and high liquid fuel yield for ethylene oligomerization. The effect of reaction pressure on ethylene oligomerization over 0.2HZ catalyst was also investigated, and JP-8 likely hydrocarbon jet fuel was obtained by using 0.2HZ catalyst at MPa with a high catalyst stability and high liquid yield. Keywords: ZSM-5; acidity; desilication; mesoporosity development; ethylene oligomerization CLC number: O69 Document code: A 1 Introduction a ZSM-5 zeolites are crystalline microporous aluminosilicates featuring the properties of high thermal stability, high surface area, uniform micropore structure, shape selectivity and strong intrinsic acidity. These properties make ZSM-5 zeolites particularly useful as heterogeneous acid catalysts for olefin oligomerization into liquid fuel [1-4]. Mobile company used ZSM-5 as a catalyst to develop the MOGD WANG Bao-lin ( 汪宝林 ): wblin88@hotmail.com; Fax: ; Tel: (mobile olefin to gasoline and distillate) process which converts light olefins to higher molecular weight gasoline and diesel fuel [5]. ZSM-5 catalysts show good activity in olefin oligomerization [6-7], but they also are deactivated fast, possibly due to the diffusional limitations of ZSM-5 micropore structure restricting transport of molecules to and from the active sites and strong intrinsic acidity leading to coke formation on the active sites and pore blockage [8]. The acidity plays a significant role in coke formation. The stronger the active sites the faster the reactions, and a higher density of acid sites will lead to a higher coke content [9-12]. It is therefore of great importance to increase molecular transport to and from active sites especially when bulky 39

2 molecules are involved and to tailor the acidity of ZSM-5 zeolites for a more efficient use of ZSM-5 catalysts in olefin oligomerization. An effective approach to enhance the accessibility and molecular transport in microporous zeolites is mesoporosity development by incorporation of supplementary carbon sources [13-14], namely carbon-templating [15-17], or posttreatment of the calcined zeolite to induce dealumination by steam or acid-leaching treatment [18] or desilication by alkaline treatment [19-22]. In carbon-templating, porous carbon, carbon nanotubes or carbon nanofibers are included in the synthesis gel during hydrothermal synthesis, thereby leaving pores in the zeolite matrix after hightemperature combustion of the carbon-zeolite composite. Although substantial and tunable mesoporosity can be obtained in this way, the crystallinity of the final product is often a major issue. Moreover, this method is difficult to use for zeolite production on a large scale due to the high cost of carbon nanotubes or carbon nanofibers [16,21]. Dealuminization by conventional steaming and acid leaching methods can lead to selective removal of Al from the framework and unavoidable modification of the acidic properties of the resultant zeolite material, but these methods for the intracrystalline mesoporosity development are primarily effective only for the zeolites with a relatively high Al content (low Si/Al 2 ratios), such as zeolite Y and mordenite [19,23-24]. Dealuminization of more siliceous zeolites such as ZSM-5, generally induces very limited mesoporosity development. However, another post-treatment method, namely desilication (framework Si extraction in alkaline medium) has been recently introduced as an effective and promising approach for mesoporosity development in various zeolites among which MFItype structure zeolites with high Si content (high Si/Al 2 ratios) appear to be the most suitable, and has attracted significant research interest [21,25]. Significant extraporosity can be created by preferential extraction of framework Si due to hydrolysis in the presence of OH - ions, basically preserving Al environment and the related acidic properties without impacting the acidity [19,26]. Xu et al. reported that a better stability of alkali-treated ZSM-5 was obtained for isomerization and aromatization due to the introduction of mesopores, which facilitate diffusion of reactant/product and reduce the channel blockage [27-28]. A study by Svelle et al. demonstrated that the lifetime of ZSM-5 was improved for methanol conversion to gasoline [29]. Recently, investigation and development of liquid fuel from biomass have gained significant attention due to the high price of crude oil and environmental issues; particularly catalytic oligomerization of light olefins (C = 2 -C = 4 ) derived from steam cracking of biomass over ZSM-5 acidic catalysts is a promising approach for liquid fuel synthesis [30-34]. Zecchina et al. [35] proposed a three-step mechanism of ethylene oligomerization over ZSM-5: 1) formation of short-lived hydrogenbonded precursor, 2) a protonation step, and 3) a chaingrowth and elimination step. The oligomerization reactions are often accompanied by various parallel reactions, such as disproportionation, isomerization, cracking, cyclization, methyl and hydrogen transfer and aromatization, and a wide range of product spectrum can be observed in the products, including olefins, paraffins, cycloalkanes and aromatics in addition to dimmers, trimers, tetramers, and even higher oligomers [33,36]. The conversion, liquid fuel yield, selectivity and product spectrum of ethylene oligomerization are influenced by both the nature of the ZSM-5 catalysts and reaction conditions. In this work, the effects of acidity of commercial ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratios upon the conversion and selectivity of ethylene oligomerization was evaluated by using ethylene as the model gas of light olefins (about 80% ethylene) derived from steam cracking of biomass, and the mesoporosity development of CBV8014 zeolites was conducted by alkaline treatment with different concentrations of 40 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

3 NaOH solutions. The effects of mesoporosity development of CBV8014 and the reaction pressure upon ethylene oligomerization reactions were also studied. The aim of this work was to study the effects of the acidity, mesoporosity development by alkaline treatment and reaction pressure upon ethylene oligomerization, and to improve the stability and selectivity of ethylene oligomerization over modified H-ZSM-5 catalysts. 2 Materials and methods 2.1 Parent zeolites and treatment Parent zeolites Three commercial NH 4 -ZSM-5 zeolites CBV3024E, CBV5524G and CBV8014 with different Si/Al 2 ratios were supplied by zeolyst company in ammonium form (SiO 2 /Al 2 O 3 mole ratios of 30, 50 and 80 respectively). The as-received samples were calcined in a muffle furnace at 550 C for 5 h at a heating rate of 2 C/min to obtain H-form ZSM-5 ready for use and further investigation Alkaline treatment Alkaline treatment of the parent zeolites were carried out by a desilication process. Briefly, 10 g of calcined CBV8014 (HZ) samples were added to 80 ml NaOH solution with different concentrations (0.1 mol/l, 0.2 mol/l, 0.5 mol/l and 1.0 mol/l, respectively) and stirred at 70 C for 60 min. After filtration, being washed with demineralized water, and dried overnight at 110 C, the solid products were converted into alkali-treated NH 4 -form ZSM-5 samples by 2 h exchange in a 0.1 mol/l NH 4 NO 3 solution at 60 C, subsequent filtration, washing and drying overnight. Then the alkali-treated NH 4 -form ZSM-5 samples were calcined in a muffle furnace at 550 C for 5 h to obtain alkali-treated H-form ZSM-5 zeolites 0.1HZ, 0.2HZ, 0.5HZ and 1.0HZ, respectively. 2.2 Characterization Temperature-programmed desorption of ammonia (NH 3 -TPD) was performed to analyze the acidic properties of the samples. For the NH 3 -TPD experiment, 0.1 g sample was loaded into a U-shaped tube and cleaned by passing helium (He) (25 ml/min) and pretreated at 500 C for 0.5 h. The sample was then cooled to 120 C and saturated with anhydrous ammonia (10% in He). Helium was then flushed through the sample for 2 h to remove excess ammonia and any physisorbed ammonia. After that, the temperature was raised from 120 C to 800 C at a heating rate of 10 C/min. The NH 3 -TPD profile was recorded by measuring the amount of ammonia desorbed between 120 C to 800 C in the He flow using a mass spectrometer. Nitrogen gas (N 2 ) adsorption-desorption isotherms of the parent and alkali-treated ZSM-5 at 196 C was performed in a Micrometrics ASAP 2020 surface area and porosity analyzer. Prior to the adsorption measurement, the samples were degassed in vacuum at 200 C for 8 h. The BET (Brunauer Emmett Teller) surface area was measured from physical adsorption of N 2 using the BET equation. The total pore volume (V total ) was calculated on the basis of the absorbed N 2 after finishing pore condensation at a relative pressure of P/P 0 = The micropore area (S micro ) and volume (V micro ) were determined by the t-plot method. The mesopore area (S meso ) was calculated by subtracting the micropore area from the BET surface area, and the mesopore volume (V meso ) was obtained by subtracting the micropore volume from the total volume. The pore size distribution was calculated according to the density functional theory (DFT) method. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded at J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

4 300 C in an N 2 flow on a Nicolet Nexus 470 Fourier transform spectrometer using a Spectratech diffuse reflectance accessory, equipped with a hightemperature cell. X-ray diffraction (XRD) patterns were collected on a Panalytical Xpert PRO X-ray diffractometer using Cu Kα radiation (λ= nm, 45 kv, 40 ma) by continuous scanning from 5 to 70, with a 2θ step size of and a time per step of 100 s. The temperature-programmed-oxidation (TPO) experiments were performed to determine the amount of carbon deposited on the catalysts after reaction by using a LECO RC-612 Carbon Analyzer. About 90 mg to 110 mg of the spent catalyst was loaded into the sample holder and put in the heating zone. During the analysis, the temperature of the heating zone was increased from 100 C to 950 C at a heating rate of 30 C/min under a 750 ml/min O 2 flow. The CO 2 produced was monitored by an IR detector, and the amount of carbon deposited on the sample was then quantified, based on the calibration with the standard sample. 2.3 Catalyst evaluation Ethylene oligomerization reaction was conducted in a fixed-bed flow reactor with electric furnance for heating. Prior to each experiment, the 0.5 g catalyst was pretreated at 450 C for 30 min under 200 ml/min nitrogen stream at atmospheric pressure to activate the catalyst. After that, the flow reactor was cooled down to the reaction temperature, and the feed gas containing 78.4% C 2 H 4 and 22.6% N 2 in terms of volume fraction was introduced into the flow reactor, and the oligomerization reaction was carried out at specified pressure. The effluent gas was analyzed by using a TCD gas chromatograph to calculate the ethylene conversion based on the total ethylene consumption. The liquid products were collected and weighed to calculate the liquid yield. 3 Results and discussion 3.1 Effect of ZSM-5 acidity on ethylene oligomerization The acid strength and acid density of ZSM-5 play a significant role in the catalyst activity, stability, coke formation and catalyst lifetime of olefin oligomerization. The acidic properties of three ZSM-5 samples, and CBV3024E, CBV5524G and CBV8014 were studied by NH 3 -TPD and DRIFTS. Fig. 1 shows the NH 3 -TPD profiles of ZSM-5 samples. Each data set on the graph has been normalized to distinguish the relative peak areas of weak acid and strong acid. For CBV3024E, CBV5524G and CBV8014, the NH 3 -TPD peaks appeared around 220 C and around 430 C, which may be attributed to the desorption of two types of ammonia species adsorbed on weak acid sites (mostly Lewis acid) and strong acid sites (mostly Brønsted acid) respectively. The Brønsted acid of ZSM-5 samples features the typical stretching vibrational band of the bridging OH groups bonded to framework aluminium at cm 1 as shown in Fig. 2, which can donate the proton for ethylene protonation to initiate oligomerization, the band at cm 1 is assigned to terminal silanol groups attached to the framework, and the band at cm 1 is assigned to hydroxyl groups located in non-framework Al sites. The concentration of acid sites in the samples CBV3024E, CBV5524G and CBV8014 was quantified and presented in Table 1. As summarized in Table 1, the weak, strong and total acidities of ZSM-5 samples increase with the decrease of silica/alumina ratio, and the total amount of acid sites increases from mmol/g to mmol/g for CBV8014, CBV5524G, CBV3024E. In this study, three ZSM-5 catalysts, CBV3024E, CBV5524G and CBV8014, with similar surface areas from 405 m 2 /g to 425 m 2 /g were tested to investigate the effect of ZSM-5 acidity on ethylene oligomerization in a flow reactor at 573 K and 42 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

5 WHSV(weight hourly space velocity) of 12 h 1 under atmospheric pressure. The experimental results of ethylene conversion and liquid yield as a function of time on stream (TOS) over these catalysts are shown in Fig. 3. For CBV8014 catalyst, the ethylene conversion over 6 h TOS slightly decreased from initial conversion 95.7% to ultimate conversion 93.3% compared with CBV5524G catalyst from 98.7% to 87.9%, CBV3024E catalyst from 99.5% to 75.2%, indicating that CBV8014 features more catalytic stability than CBV5524G and CBV3024E catalysts, and that CBV3024E catalyst features higher initial catalyst activity and faster deactivation. Also, CBV8014 catalyst exhibited the higher and more stable yield of liquid products over 6 h TOS compared with CBV5524G and CBV3024E, as shown in Fig. 3. Fig. 1 NH3-TPD desorption curves of ZSM-5 samples Fig. 2 DRIFTS spectra of ZSM-5 samples Catalysts SiO 2 /Al 2 O 3 ratios Table 1 The concentration of acid sites of ZSM-5 samples Surface area /(m 2 g 1 ) Weak acidity /(mmol g 1 ) Strong acidity /(mmol g 1 ) Total acidity /(mmol g 1 ) CBV3024E CBV5524G CBV Fig. 3 Conversion and liquid yield vs. time-on-stream over ZSM-5 catalysts J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

6 The role of the acid sites in ethylene oligomerization mechanism is not completely elucidated yet, but it is reasonably believed that the zeolite-catalyzed ethylene oligomerization is of a carbenium ion character, initiated by the Brønsted acid sites. So increasing the initiator concentration, i.e. increasing the concentration of acid sites could increase the initial activity of a catalyst, which is expected from the standard polymerization mechanism. The lower SiO 2 /Al 2 O 3 ratio and stronger acidity of ZSM-5 lead to a higher catalytic activity, which is in good agreement with the above experimental results presented in Fig. 3. Moreover, variation in the acidity of ZSM-5 also influences the formation of coke deposits which is an important parameter responsible for catalyst deactivation. A higher acidity leads to a higher coke content and faster deactivation. Fig. 4 shows the TPO-IR spectra of CBV8014, CBV5524G and CBV3024E spent catalysts (300 C, under atmospheric pressure, WHSV 12 h 1 and 6 h on stream). Two broad CO 2 peaks located in the range of 160 C to 320 C and 400 C to 600 C can be observed. The 160 C to 320 C peak resulted from the oxidation of heavy products coke precursors trapped inside the pore channels which could be removed from the catalyst sample simply through volatilization in inert nitrogen. The 400 C to 600 C peak was attributed to the oxidation of hard coke which remained on the catalyst even at a high temperature (600 C) and was removed by burning. The contents of coke precursors, hard coke and total coke are listed in Table 2. CBV8014 catalyst with a lower acidity features a lower coke content compared with CBV5524G and CBV3024E. A higher density of acid sites leads to a higher risk of reactants for converting into coke because of more chances for the reactants to encounter active sites while diffusing through zeolite catalysts; in addition, some bimolecular reactions require more than one acid site, e.g. hydrogen transfer, which is part of coke formation. Therefore, a higher acidity, i.e. higher number, strength and density of acid sites, leads to a higher coke content and faster deactivation. Fig. 4 TPO-IR results of the spent catalysts after 6 h TOS of C 2 H 4 oligomerization Table 2 Coke content of ZSM-5 spent catalysts after 6 h TOS Spent catalysts Coke content/ % Coke precursors Hard coke Total coke CBV3024E CBV5524G CBV Besides the important influence of zeolite acidity on the activity, liquid yield and coke formation of ethylene oligomerization, the catalyst nature also plays an important role in oligomers and other side products distributions. In addition to the primary oligomerization reaction, there are a lot of other side reaction, such as isomerization, disproportionation, cracking, hydrogen transfer and aromatization. As a result, a broad spectrum of hydrocarbons was obtained in the liquid fuel products, including olefins, paraffins, cycloalkanes and aromatics. Also, gaseous products of methane, propane, propylene and isobutane were identified in this study from reactor effluent gas by qualitative analysis of FID-GC due to side reactions, such as cracking and hydrogen transfer reaction. The SimDis distillation curves of ethylene oligomerization at 3 h of TOS and 573 K over ZSM-5 44 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

7 catalysts are shown in Fig. 5 with JP-8 for reference, the shapes of all the curves are of subtle sigmoid type that one would expect for a highly complex fluid with many components, distributed over a large range of relative molecular mass. The distillation curves of Fig. 5 show that liquid fuel product from 3024E is the most volatile of the four fluids and the curve of 8014 is closer to the curve of JP-8. The SimDis cut points of liquid fuel products are summarized in Table 3. The data in Table 3 show that the liquid fuel product from CBV8014 catalyst has a higher boiling point for the same cut-off value (distillation fraction) and becomes richer in heavier components compared with the products from CBV5524G and CBV3024E catalysts. 3.2 Effect of mesoporosity development of CBV8014 by alkaline treatment on ethylene oligomerization Porous characteristics N 2 -adsorption-and-desorption isotherm for parent CBV8014 and alkaline treated CBV8014 samples are shown in Fig. 6. The N 2 isotherm is transformed from type I, typical for a microporous material with insignificant mesoporosity, to a combined type of types I and IV with a pronounced hysteresis loop at a higher relative pressure, indicating the presence of both micropores and mesopores in the zeolite structure after the samples were treated by NaOH solutions. The pore size distribution curves in Fig. 7 were plotted based on the DFT method, showing that the alkali-modified samples have broad mesopore size distributions. The mesoporosity formation is highly dependent on the NaOH concentration during desilication. A higher NaOH concentration leads to a higher mesopore volume, larger pores and a broader pore size distribution. The changes of textural properties of CVB8014 treated by different NaOH concentrations are summarized in Table 4. With the increase of NaOH concentration, the mesopore surface area and mesopore volume increase while the micropore volume slightly decreases compared with the untreated CBV8014 sample. For the case of 0.5 mol/l NaOH solution, the mesopore volume increases to cm 3 /g, 2.71 times higher than in the case of untreated CBV8014, and the maximum related to the highest fraction of mesopores shifts from 2.2 nm of parent sample to 11.7 nm of 0.5HZ, as shown in Fig. 7 due to substantial Si extraction from the zeolite framework at a high alkaline concentration, which creates even macropore. So, the increase of alkaline concentration results in not only the increase of the mesoporous volumes but also the formation of larger pores and a broader pore size distribution, and the hierarchical extraporosity development can be tuned by desilication with different NaOH concentrations. Fig. 5 SimDis distillation curves of liquid products at 3 h of TOS, where t b is the boiling, and x is the mass fraction of cut off J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

8 Table 3 SimDis cut points of liquid products at 3 h of TOS, where x is the mass fraction of cut off, and t b is the boiling point CBV 3024E CBV 5524G CBV 8014 JP-8 t b / C x/% t b / C x/% t b / C x/% t b / C x/% 40.2 to to to to to to to to to to to to to to to to Fig. 6 Nitrogen isotherms of parent and alkali-treated CBV8014 catalysts, where Vad is the volatile (air-dried) conent, and p/p 0 is the relative pressure Fig. 7 DFT pore size distribution of parent and alkali-treated CBV8014, where V is the pore volume, and D is the pore diameter 46 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

9 Table 4 Textural properties of parent CBV8014 (HZ) and that treated with NaOH solution of different concentration (C) Sample C S meso S BET V total V micro V meso Pore Size /(mol L 1 ) /(m 2 g 1 ) (m 2 g 1 ) /(cm 3 g 1 ) /(cm 3 g 1 ) /(cm 3 g 1 ) /nm HZ HZ HZ HZ HZ Structural properties of alkali-treatment CBV Acidic properties X-ray diffraction was carried out to investigate possible structural changes in CBV8014 upon alkalitreatment. The XRD patterns of parent CBV8014 and the samples treated with 0.2 mol/l, 0.5 mol/l, 1.0 mol/l NaOH solutions are shown in Fig. 8. As it can be seen, the long-range crystal ordering is maintained upon NaOH treatment and the characteristic diffraction lines of MFI crystalline are preserved since no distinct changes in peak positions are observed, but their intensities of the characteristic diffraction lines decrease distinctly with the increase of NaOH concentrations. To quantify this structural modification, the values of the degree of crystallinity, C XRD, of alkalitreated samples were evaluated by the ratio between the areas of all the diffraction peaks of the alkali-treated samples and the area of parent CBV8014 sample, with the latter chosen as reference. Peak integration was made using software. The results are summarized in Table 5. As shown in Table 5, the crystallinity of the alkali-treated sample was largely preserved when treated with 0.2 mol/lnaoh solution. However, with the increase of NaOH concentration to 1.0M, the crystallinity significantly decreased to 41.9%, indicating the severe destruction of ZSM-5 lattice by high NaOH concentration treatment. It can be concluded that an optimal NaOH concentration for mesoporosity development by desilication in term of well preservation of crystallinity and increase of mesopore volume was 0.2 mol/l concentration. The NH 3 -TPD was conducted to investigate the changes of acidity for CBV8014 treated by 0.1 mol/l to 1.0 mol/l NaOH solutions. The quantitive results of the acidity of HZ, 0.1HZ, 0.2HZ, 0.5HZ and 1.0HZ are listed in Table 6. The total acidities of CBV8014 samples treated by NaOH concentrations slightly decreased from mmol/g of CBV8014 to mmol/g of 0.5 HZ, and the alkaline desilication treatment for mesoporosity development slightly changes the acidic properties of CBV 8014 because of its preferential extraction of Si in alkaline medium and well preservation of Al sites. The NH 3 -TPD results are also supported by the DRIFTS spectra Fig. 9. Fig. 9 indicates that the band of Brønsted acid sites (3 602 cm 1 ) in the alkali-treated CBV8014 samples is well preserved. Apparently the mesoporosity development obtained by Si extraction slightly impacts the acidic properties of CBV8014 zeolite under present experimental conditions. However, the band at cm 1, representing isolated silanols, becomes intenser (Fig. 10), probably due to the increased outer surface of the zeolite crystals as a result of the intracrystalline mesopore formation Catalytic performances of alkali-treatment CBV8014 As mentioned above, alkali-treated CBV8014 could lead to mesoporosity development, preservation of J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

10 catalyst acidity and destruction of zeolite lattice which could influence the catalytic performance of catalysts. The parent HZ and alkali-treated 0.1HZ, 0.2HZ, 0.5HZ and 1.0HZ catalysts were evaluated for ethylene oligomerization at 300 C and WHSV of 12 h 1 for liquid fuel synthesis under atmospheric pressure. decreased over time during 6 h on stream. But the 0.2HZ catalyst exhibited more stable performances and higher liquid yield. The ethylene conversion over 0.2HZ only decreased slightly from 94.5% to 93.5% during 6 h time on stream, the average liquid yield over 0.2HZ reached 32.2%, higher than those of other catalysts, and 0.2HZ catalyst exhibits good activity and stability compared with parent and other alkali-treated catalysts. Although 1.0HZ catalyst had a significant number of larger mesopores, it still exhibited poor activity with an initial ethylene conversion of 89.1% and poor stability, possibly attributable to substantial extraction of Si and severe destruction of zeolite lattice. Fig. 8 XRD patterns of parent and alkali-treated CBV8014 Table 5 Degree of crystallinity of parent and alkalitreated CBV8014, C XRD Catalysts HZ 0.2HZ 0.5HZ 1.0HZ C XRD /% HZ 0.5HZ 0.2HZ 0.1HZ HZ Figs. 10 and 11 show the ethylene conversion and liquid yield as a function of time on stream (TOS) over these catalysts respectively. For these catalysts, their ethylene conversion and yield of liquid fuel products Fig. 9 DRIFTS spectra of parent and alkali-treated CBV8014 Catalysts Table 6 The concentration of acid site of parent and alkali-treated CBV8014 Weak acidity /(mmol/g) Strong acidity /(mmol/g) HZ HZ HZ HZ HZ Total acidity /(mmol/g) 48 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

11 Fig. 10 Ethylene conversion vs. time on stream over parent and alkali-treated CBV8014 Fig. 11 Liquid yield vs. time on stream over parent and alkali-treated CBV Effects of reaction pressure on ethylene oligomerization The effects of reaction pressure on ethylene oligomerization were investigated using 0.2HZ catalysts at 300 C and WHSV of 12 h 1. The experimental results are shown in Fig. 12. When the reaction pressure was raised from atmospheric pressure to MPa, it is observed that the catalytic activity was improved, the initial ethylene conversion increased from 95.00% to 99.98%, and the ethylene conversion decreased slightly from 99.98% to 88.30% during 6 h TOS; the average yield of liquid fuel products for 6 h TOS dramatically increased from 30% to 56.44% as expected. This observation could be interpreted by that the increase of reaction pressure improves the physisorption and chemisorption of ethylene at active sites and favors the oligomerization reaction. In addition, the reaction pressure has an important influence on oligomers distributions as shown in Fig. 13. The SimDis distillation curve at MPa closely approaches the distillation curve of JP-8 jet fuel, and these two curves are almost the same. It might be tempting to conclude that the liquid fuel product from MPa has a similar chemical composition to JP-8. JP-8 likely hydrocarbon fuel was obtained by using 0.2HZ catalyst at MPa. For the MPa case, the ethylene conversion decreased from 99.98% to 72.40% during 6 h TOS, and the average liquid yield for 6 h TOS was 52.03%. Similarly for MPa case, the ethylene conversion decreased from 100% to 64.67% during 6 h J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

12 TOS, and the average liquid yield was 51.03%. The activity of catalysts at MPa and MPa decreased faster than that at MPa. Moreover, the distillation curves in Fig. 13 show that liquid fuel products from MPa and MPa are more volatile than JP-8 fuel and these two curves also deviate distinctly from the curve of JP-8, which can possibly be attributed to the higher reaction temperature rises due to highly exothermic ethylene oligomerization at MPa and MPa and higher diffusion resistance for heavier products under high pressure resulting in coke deposits. Fig. 12 Conversion and liquid yield vs. time on stream under different pressure Fig.13 SimDis distillation curves of liquid products under different pressure at 3h of TOS 50 J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2): 39-53

13 4 Conclusions The study for the effects of acidities of different SiO 2 /Al 2 O 3 ratio ZSM-5, CBV3024E, CBV5524G and CBV8014 upon catalytic activity, liquid yield, coke formation and oligomer distributions of ethylene oligomerization indicated that CBV8014 with a high Si/Al 2 ratio of 80 had higher catalytic stability and liquid yield, lower coke content deposited on catalyst, and more heavy components in liquid fuel product compared with CBV5524G and CBV3024E catalysts. The hierarchical extraporosity development was achieved by desilication with different NaOH concentrations in this work, and the mesopore size and volume, preservation of catalyst acidity and destruction of zeolite lattice can be controlled to a certain extent by varying NaOH concentrations. CBV8014 treated by 0.2 mol/l NaOH solution for mesoporosity development showed that zeolite lattice and catalyst acidity were preserved well. The catalytic performance of parent and alkali-treated catalysts indicated that 0.2HZ catalyst exhibited a better catalytic activity and stability compared with parent and other alkali-treated catalysts most probably due to the improvement in the diffusivities of bulky hydrocarbon molecules and preservation of zeolite lattice and catalyst acidity. In addition, reaction pressure plays an important and complicated role in ethylene oligomerization. High pressure can increase the initial activity and liquid yield, but also lead to higher coke deposits and fast deactivation. The 0.2HZ catalyst testing showed that JP-8 likely hydrocarbon fuel was obtained with a high catalytic stability and high liquid yield by using 0.2HZ catalyst at MPa under 300 C and WHSV of 12 h 1. References [1] O'Connor CT, Kojima M. Alkene oligomerization [J]. Catalysis Today, 1990, 6(3): [2] Knifton JF, Sanderson JR. Olefin oligomerization via zeolite catalysis [J]. Catalysis Letters, 1994, 28: [3] Baker J, Bessell S, Seddon D. Conversion of propene into gasoline and middle distillate using alkalised ZSM-5 zeolite catalysts [J]. Applied Catalysis, 1988, 45: Ll-L9. [4] Heveling J, Van Der Beek A, De Pender M. Oligomerization of ethylene over nickel-exchanged zeolite Y into a Diesel-range product [J]. Applied Catalysis, 1988, 42: [5] Tabak SA, Krambeck FJ, Garwood WE. Conversion of propylene and butylene over ZSM-5 catalyst [J]. AIChE Journal, 1986, 32(9): [6] Occelli ML, Hsu JT, Galya LG. Propylene oligomerization over molecular sieves Part I. Zeolite effects on reactivity and liquid product selectivities [J]. Journal of Molecular Catalysis, 1985, 32: [7] Henry M, Bulut M, Vermandel W, et al. Low temperature conversion of linear C4 olefins with acid ZSM-5 zeolites of homogeneous composition [J]. Applied Catalysis A: General, 2012, : [8] Lallemand M, Finiels A, Fajula F, et al. Catalytic oligomerization of ethylene over Ni-containing dealuminated Y zeolites [J]. Applied Catalysis A: General, 2006, 301(2): [9] Guisnet M. Model reactions for characterizing the acidity of solid catalysts [J]. Accounts of Chemical Research, 1990, 23: [10] Guisnet M, Magnoux P. Coking and deactivation of zeolites: influence of the pore structure [J]. Applied Catalysis, 1989,54: [11] Guisnet M, Magnoux P. Roles of acidity and pore structure in the deactivation of zeolites by carbonaceous deposits [J]. Studies in Surface Science and Catalysis, 1997, 111: [12] Babitz SM, Kuehne MA, Kung HH, et al. Role of Lewis acidity in the deactivation of USY zeolites during 2-methylpentane cracking [J]. Industrial & Engineering Chemistry Research, 1997, 36: J. Chongqing Univ. Eng. Ed. [ISSN ], 2015, 14(2):

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