Oligomerization of Biomass-Derived Light Olefins to Liquid Fuel: Effect

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Oligomerization of Biomass-Derived Light Olefins to Liquid Fuel: Effect of Alkali Treatment of HZSM-5 Catalyst Xiaoxing Wang, Xiaoyan Hu, Chunshan Song, Kenneth W Lux, Mehdi Namazian, and Tahmina Imam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02316 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Industrial & Engineering Chemistry Research

Oligomerization of Biomass-Derived Light Olefins to Liquid Fuel: Effect of Alkali Treatment of HZSM-5 Catalyst

Xiaoxing Wang*1, Xiaoyan Hu1, Chunshan Song*1,2 , Kenneth W. Lux3, Mehdi Namazian3, Tahmina Imam3

1

Clean Fuels and Catalysis Program, PSU-DUT Joint Center for Energy Research, EMS Energy

Institute and Department of Energy & Mineral Engineering, the Pennsylvania State University, 209 Academic Projects Building, University Park, PA, 16802, USA. 2

Department of Chemical Engineering, the Pennsylvania State University, University Park, PA, 16802, USA.

3

Altex Technologies Corporation, 244 Sobrante Way, Sunnyvale, CA 94086, USA

*E-mail: [email protected] (CS), Tel: 814-863-4466, Fax: 814-865-3573; [email protected] (XW)

1 ACS Paragon Plus Environment


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Abstract As a part of a new approach to convert biomass to liquid fuels, we investigated the effects of alkali treatment on the property and performance of HZSM-5 for oligomerization of biomassderived ethylene under atmospheric pressure. The characterization results showed that alkali treatment led to the increase in the total and mesopore volumes, but decrease in the surface area and micropore volume. When NaOH concentration was low (< 0.5 M), the ZSM-5 structure was largely preserved with the increase in the mesopores and acidity, while higher NaOH concentration can severely destroy the zeolite structure, resulting in a significant reduction in the micropores and acidity. The ethylene oligomerization results showed that not only the ethylene conversion and the liquid yield increased, but also the catalyst stability was improved after proper NaOH treatment. The relationship between the structure and performance has then been discussed. Keywords: Biomass, Mesoporous HZSM-5, Zeolite, Alkali treatment, Ethylene oligomerization, Catalysis


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1. Introduction Biomass is receiving significant attention worldwide as a renewable feedstock for the sustainable production of liquid fuels and chemicals, which is one of the promising ways to address energy and environmental issues caused by our dependence on fossil fuels.1-4 It can significantly reduce or even eliminate life-cycle CO2 emission for liquid fuels.1,

5

Different

processes for obtaining liquid fuels from biomass are currently under investigation and development. Pennsylvania State University (PSU) and Altex Technologies Corporation (Altex) have been collaborating to develop a new process for conversion of carbonaceous materials to liquid fuels6-8 which is outlined in Figure 1. The carbonaceous feedstocks (e.g., biomass, waste) is first converted to a gaseous component (at reaction temperature) and a solid component including char and ash. The gaseous component is fed to a second reactor and converted to light olefins, predominantly ethylene and propylene. The light olefins are converted in the third reactor to produce a targeted synthetic fuel like liquid transportation fuels through oligomerization. For this new process, the development of inexpensive oligomerization catalysts with improved catalytic performance is essential to enable the production of synthetic fuels from biomass at a relatively low cost, which is the focus of the present research. Oligomerization of light olefins is of considerable academic and industrial interest because it is one of the major processes for production of higher olefins, which are feed stocks for plastics, plasticizers, lubricants, detergents, oil additives, fatty acids, etc.9-10 Olefin oligomerization has also attracted more and more attention for the production of environmentally friendly and sulfur-free transportation liquid fuels including gasoline and diesel fuels11-13, especially when light olefins can be produced from clean and renewable sources including



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biomass through the processes such as conversion of bio-ethanol14 and the above-mentioned biomass conversion process8. The catalytic oligomerization of ethylene can be carried out using homogeneous or heterogeneous catalysts. Although homogeneous catalysts including transition metal complexes such as nickel complexes and organoaluminum compounds are commonly used in the industrial oligomerization processes due to their high activity and selectivity, the homogeneous catalysts suffer from several drawbacks such as high sensitivities to impurities, difficulties in handling, recycling and regeneration, among others.15 Due to easy separation of catalyst and the products, a great deal of efforts have been devoted to developing heterogeneous oligomerization processes in the past decades.9, 13, 15-24 Among them, zeolites including H-ZSM-5, H-Beta and zeolites X and Y have been examined either as the catalysts or supports for transition metals. The commercial processes such as the “Conversion of Olefins to Distillate” (COD) process25-26 and the “Mobil Olefins to Gasoline and Distillates” process (MOGD)21, 27 are based on H-ZSM-5 catalyst as it shows good activity for olefin oligomerization20, 28. Although progress has been made in the development of zeolite-based catalysts, some problems remain to be solved. One key issue is the transport limitation due to the microporous characteristic of zeolites, causing the micropore blockage with heavy products, thus resulting in a severe deactivation.9, 19, 29-30 In recent years, hierarchical zeolites which contain both micropores and mesopores have attracted much attention in catalysis as it possesses the advantages of both crystalline zeolites (high

stability and

acidity)

and

amorphous

mesoporous

materials

(efficient

mass

transportation).31-34 The hierarchical zeolites can be prepared by post-treatments such as alkali treatment on the zeolitic material.35 With the alkali treatment, an increase in the mesopore volume and external surface area of ZSM-5 has been reported.36-37 After alkali treatment, the


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diffusion in ZSM-5 can be enhanced as demonstrated by Groen et al.38. The alkali treated ZSM5 also exhibited a better stability in butane aromatization39, aromatization and isomerization of 1hexene40 and the reaction of methanol to gasoline41, which could be attributed to the improved diffusion of reactants/products and the reduced pore blockage with the induced mesopores. However, little has been reported so far on the utilization of mesoporous ZSM-5 zeolite for ethylene oligomerization. The purpose of the present work is thus to investigate the influence of alkali treatment of ZSM-5 in terms of variation in the pore structure and acidity of mesoporous zeolite on the performance of ethylene oligomerization under atmospheric pressure. The mesoporous ZSM-5 was prepared through post-treatment with NaOH aqueous solution. The effect of NaOH concentration on the pore structure and acidity of the mesoporous ZSM-5 has been examined. The catalytic performance of the NaOH-treated ZSM-5 for ethylene oligomerization has been studied in a fixed-bed flow system at atmospheric pressure. The relationship between the structure and the catalytic performance of the NaOH-treated ZSM-5 catalysts has been discussed as well.

2. Experimental 2.1 Catalyst preparation A commercial NH4ZSM-5 zeolite was obtained from Zeolyst Company (CBV 5524G, SiO2/Al2O3 ratio = 50). The as-received sample was calcined in muffle furnace at 550 ºC for 5 h to obtain the H-form ZSM-5, which is termed as HZ. Mesoporous H-ZSM-5 samples were prepared by post-treatment of HZ with aqueous solutions of NaOH at different concentrations according to the procedure reported in the literature36-37,

42

, followed by ion exchange to



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transform the Na-form into H-form materials. Typically, 10 g of HZ sample was added to 80 mL NaOH solution with different concentrations (0.1, 0.3, 0.5, 1.0, 1.5, 2.0 M) under stirring at 70 ºC for 60 min. Subsequently, the sample was recovered by filtration and washed with deionized water thoroughly. The obtained solid was then dried at 110 ºC overnight and calcined at 550 ºC for 5 h. The ion exchange was performed over the NaOH-treated sample (5 g) with 100 mL NH4NO3 aqueous solution (1.0 M) at 50 ºC for 2 h with ultrasonic aid. The slurry was again filtered and washed with deionized water, dried at 110 ºC overnight, followed by calcination at 550 ºC for 5 h, to convert into H-form material. The proton-exchanged samples are denoted as xHZ, where x is the concentration of aqueous NaOH solution used for post-treatment.

2.2 Catalyst characterization X-ray diffraction (XRD) patterns were recorded on a PANalytical X’pert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å, 45 KV, 40 mA). N2 physisorption was carried out at liquid nitrogen temperature (-196 °C) on a Micromeritics ASAP 2020 surface area and porosity analyzer to examine the porous property and the surface area of each sample before and after NaOH treatment. Prior to analysis, each sample was evacuated at 200 °C in vacuum for 8 h. The Brunauer-Emmett-Teller (BET) surface area (SBET) was obtained from physical adsorption of N2 using the BET equation. The total pore volume (Vtotal) was calculated on the basis of the adsorbed N2 after finishing pore condensation at a relative pressure of P/P0 = 0.995. The micropore surface area (Smicro) and micropore volume (Vmicro) were calculated by the t-plot method. The mesopore surface area (Smeso) was obtained by subtracting the micropore area from the total BET surface area, and the mesopore volume (Vmeso) was calculated by subtracting the micropore volume from the total volume. The pore size distribution was determined by the


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density functional theory (DFT) method. The field emission scanning electron microscopy (FESEM) images were obtained on a Zeiss Sigma FESEM instrument operating at 2 kV. Transmission electron microscopy (TEM) micrographs were taken on FEI Talos F200X transmission electron microscope operated at an accelerated voltage of 200 kV. Samples for TEM measurements were suspended in ethanol and dispersed ultrasonically. A drop of suspension was then applied on the lacey carbon copper grids to drop-cast the samples. To obtain the SiO2/Al2O3 ratio before and after NaOH treatment, ICP-AES analysis was performed on the Perkin-Elmer Optima 5300, Inductively Coupled Plasma Emission Spectrometry (ICP-AES). The acid property of each sample was analyzed by ammonia temperature-programmed desorption method (NH3-TPD) over a Micromeritics AutoChem 2910 TPR-TPD instrument. Typically, 0.1 g sample was loaded into a U-shaped quartz tube and pretreated at 500 ºC in flowing He (25 ml/min) for 30 min. The adsorption of NH3 was then performed at 120 ºC in an NH3-He mixture (10 v% NH3 in He) for 60 min, and then the remaining or weakly adsorbed NH3 was purged by ultra-high purity (UHP) He for 2 h. NH3-TPD was carried out in the He flow by raising the temperature to 850 ºC at a ramp rate of 10 ºC/min. The desorbed ammonia in the exit was detected continuously by a mass spectrometer (MS) using the fragment with m/e = 16 as the parent peak of m/e = 17 is usually affected by the desorbed water. The amount of coke formed on the spent catalysts after ethylene oligomerization was determined by temperature-programmed oxidation (TPO) method using a LECO RC-612 multiphase carbon analyzer equipped with infrared (IR) detector. About 70-80 mg of the spent catalyst was loaded into the sample holder and put into the heating zone. During the analysis, the temperature of the heating zone was increased from 100 to 950 ºC at a rate of 30 °C/min under



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UHP O2 flow (750 mL/min) and the signal was recorded. The amount of carbon on the sample was then quantified on the basis of the calibration with the standard.

2.3 Catalyst evaluation The catalytic performance of the catalysts for ethylene oligomerization to liquid hydrocarbons was examined in a fixed-bed flow reactor operated at atmospheric pressure. In a typical run, approximately 0.5 g of the catalyst was packed in the reactor and pretreated at 450 ºC for 30 min under the flow of UHP N2 (100 mL/min). After the temperature was decreased to the desired reaction temperature, the feed gas containing 78.4 v% C2H4 – 22.6 v% N2 was introduced to start the reaction. An on-line GC-TCD (SRI 8610C) equipped with a molecular sieve 5A column and a silica-gel column was used for the analysis of gas products including N2, C2H4, and other light hydrocarbons if present. The liquid products were periodically collected by a condenser operated at room temperature. The reaction was terminated by switching off the ethylene flow and N2 was kept flowing while the reactor was cooling down. The spent catalyst was then recovered and stored at room temperature for further analysis. The ethylene conversion and liquid yield were calculated via the following equations: Conversion (mol%) =

C2 H 4 ninC2 H 4 − nout × 100% ninC2 H 4

liquid mout Liquid Yield (wt%) = C2 H 4 × 100% min

2.4 Analysis of oligomerization liquid product The obtained liquid from ethylene oligomerization was analyzed by GC-FID for the hydrocarbon distribution and by a Simulated Distillation GC (SimDis-GC) to identify the boiling 8 ACS Paragon Plus Environment

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point range of the liquid product. The GC-FID analysis was conducted using Varian CP 3800 gas chromatograph with a Rxi®-5Sil MS column coupled with a flame ionization detector. The column temperature was programmed at 40 ºC for 10 min, then ramped from 40 to 250 ºC at a rate of 4 ºC/min, and maintained at 250 ºC for 5 min. SimDis-GC analysis was performed using a Hewlett Packard Model 5890 II Plus gas chromatograph with a MXT®-2887 column. The column temperature was programmed from 40 to 350 ºC at a rate of 15 ºC/min, and maintained at 350 ºC for 10 min.

3. Results and discussion 3.1 Physical properties The XRD patterns of parent and NaOH-treated ZSM-5 samples are shown in Figure 2. All the characteristic diffraction peaks belonging to MFI crystalline structure can be observed over the NaOH-treated ZSM-5 samples. When the NaOH concentration used for post-treatment was no more than 0.5 M, the intensities of these peaks were similar to those of the parent ZSM-5, indicating that the crystalline structure of ZSM-5 was largely preserved. When the NaOH concentration exceeded 0.5 M, however, a significant decrease in the intensities of the diffraction peaks was obtained, especially when the concentration of NaOH used for post-treatment was higher than 1.0 M, showing that the regularity of the long-range order of ZSM-5 decreased with NaOH treatment. In particular, at the NaOH concentration of 2.0 M, the relative MFI crystallinity of the 0.2HZ sample, which was calculated by comparing the area of diffraction peak at 2θ of 23.2º with respect to the parent H-ZSM-543, was only 25% of the parent ZSM-5, implying that the destruction of the zeolite crystal occurred in the NaOH post-treatment. Similar trend was also reported by Cheng et al.42



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Figure 3 shows nitrogen adsorption-desorption isotherms of the parent and NaOH-treated HZSM-5 samples. The N2 adsorption-desorption isotherms show that the parent HZ sample exhibits an IUPAC type-I isotherm with a plateau at high relative pressure which is a characteristic for microporous materials. A small hysteresis loop at the P/P0 of 0.45-1.0 is observed over the HZ sample as well, indicating that the commercial ZSM-5 zeolite itself contains some mesoporosity similar to the observation by Groen et al.37. After treatment with 0.1M NaOH aqueous solution, along with the type-I isotherm characteristic, a slight change in the hysteresis loop at the P/P0 of 0.45-1.0 and an increase in the N2 uptake can be detected. Increasing NaOH concentration for post-treatment to 0.3 and 0.5 M, the hysteresis loop becomes larger and more noticeable. And the position of the hysteresis loop also slightly shifts to the P/P0 of 0.50-1.0. It implies that more mesopores with larger pore diameter are generated during the NaOH treatment at the concentration of 0.3-0.5M. Further increasing the NaOH concentration up to 2.0 M, the N2 uptake increases dramatically and the hysteresis loop becomes more remarkable compared to the parent HZ sample. The position of the hysteresis loop even shifts to the P/P0 of 0.80-1.0, suggesting the pore diameter becomes much larger. The gradual change in the isotherms from type-I to the type-I & type-IV mixture with more obvious hysteresis loop at higher relative P/P0 for the NaOH-treated H-ZSM-5 samples indicates 1) the microporous structure is basically preserved after NaOH treatment, which is consistent with the XRD results; and 2) new mesopores with large pore diameters are generated. Based on the adsorption-desorption isotherms, the BET surface area (SBET), micropore surface area (Smicro), mesopore surface area (Smeso), total pore volume (Vtotal), micropore volume (Vmicro), and mesopore volume (Vmeso) were calculated and are summarized in Table 1. The parent HZ sample had a surface area of 416 m2/g and a total pore volume of 0.31 cm3/g along


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with the mesopore surface area of 142 m2/g and mesopore volume of 0.16 cm3/g, as indicated by the N2 adsorption-desorption isotherm (Figure 3a). After NaOH treatment, a gradual decrease in the BET surface area and micropore surface area along with an increase in the mesopore surface area, total pore volume and mesopore volume was observed with the increase of NaOH concentration for the post-treatment. It should be pointed out that when the NaOH concentration was more than 0.5 M, the changes in the micropore surface area, micropore volume, mesopore surface area and mesopore volume became more significant. For the 0.1HZ, 0.3HZ and 0.5HZ samples, the micropore surface area and the micropore volume were ranged around 242-233 m2/g and 0.12-0.11 cm3/g, while the mesopore surface area and mesopore volume increased to 149-154 m2/g and 0.20-0.36 cm3/g, respectively. However, over the 1.0HZ, 1.5HZ and 2.0HZ samples, the micropore surface area and micropore volume decreased dramatically to 176, 114 and 56 m2/g, and 0.10, 0.06 and 0.03 cm3/g, respectively. It further proves that although new mesopores are generated with the treatment of NaOH aqueous solution, the micropores had been little impacted when the concentration of NaOH solution used for the treatment was no more than 0.5M, while high NaOH concentration (≥ 1.0 M) could significantly reduce the micropores, as also observed by XRD. The pore size distribution curves were calculated with the DFT method, and the results are presented in Figure 4. The HZ sample showed a strong peak centered at ca. 0.56 nm, which is corresponding to the typical pore diameter of ZSM-5. Additionally, a small peak is attained at pore diameter ranged from 1 to 3 nm. After post-treatment with 0.1 M NaOH solution, bare change in the pore size distribution was observed over the 0.1HZ sample. Further increasing the NaOH concentration for the treatment, the intensity of the peak at 0.56 nm decreased gradually. Meanwhile, a new peak at 4-20 nm was emerged over the 0.3HZ. That peak was centered at 20



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nm over the 0.5HZ. Over the 1.0HZ, 1.5HZ and 2.0HZ samples, however, multi-peaks located at 20-110 nm were found, suggesting a random mesopore system generated over these samples. Furthermore, the pore size distribution peak at 0.56 nm was totally disappeared over the 2.0HZ sample, which proves a significant destruction or collapse of the micropore structure of ZSM-5. It is consistent with XRD results showing only 25% zeolite crystallinity was preserved over the 2.0HZ sample. The porous structure and the morphology of the parent and alkali treated HZ samples was analyzed by FESEM and TEM techniques. Figure 5 shows the FESEM and TEM images for the parent HZ and alkali-treated HZ samples at different NaOH concentration. Based on the FESEM images, the morphology of 0.1HZ sample was similar to HZ sample, while more changes in the particles were observed over the 0.5HZ sample. As displayed by the TEM images, the generation of mesopores in the crystalline ZSM-5 zeolite was clearly observed after NaOH treatment. The mesopores became more significant over the 0.5HZ sample. Moreover, the collapse of the zeolite crystal was also observed over 0.5HZ from the TEM image. The results are consistent with the observation from XRD and N2-physisorption analysis.

3.2 Acid property The acid property of the parent and NaOH-treated H-ZSM-5 samples has been investigated by ammonia temperature-programmed desorption (NH3-TPD) technique, and the obtained profiles are shown in Figure 6. The parent HZ sample exhbited two distinct NH3 desorption peaks centered at around 220 and 410 oC, which is the characterisitic of MFI zeolite.44 The low- and high-temperature desorption peaks are normally ascribed to hydrogen-bonded NH3 molecules and the NH3 moelcules chemisorbed on Bronsted acid sites, i.e., the weak and strong


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acid sites, respectively.45 After NaOH treatment, the two NH3 desorption peaks were preserved over the samples when the NaOH concentration for the treatment was not higher than 0.5 M. When the NaOH concentration was higher than 1.0 M, the high-temperature desorption peak faded away and the low-temperature desorption peak shifted to a slightly higher temperature, from 210 ºC to 230 ºC, suggesting a significant change in the acid property of HZSM-5 after treatment with NaOH solution at high concentration. Since the peak area is proportional to the number of the acid sites, the amount of acid sites was thus calculated via peak deconvolution of the NH3-TPD profiles (see Figure S1 in the Supporting Information) and is listed in Table 1. The parent HZ sample showed a total acid amount of 99.1 µmol/g with 78.7 µmol/g of strong acid sites and 20.4 µmol/g of weak acid sites. After treatment with NaOH solution of 0.1-0.5 M, a gradual increase in the amount of the weak acid sites and a slight increase in the amount of the strong acid sites were observed with the increase of NaOH concentration, which resulted in an increase in the total acid amount. The highest total acid amount (117.5 µmol/g) with 86.3 µmol/g of strong acid sites and 31.2 µmol/g of weak acid sites was obtained over the 0.5HZ sample. Such a change could be attributed to the result of the desilication during the NaOH-treatment of the H-ZSM-5 zeolite. Further increasing NaOH concentration to 1.0 M, a drop in the amount of the strong scid sites to 80.6 µmol/g was obtained while the amount of weak acid sites also dropped to 24.9 µmol/g. As a result, it showed the total acid amount of 105.5 µmol/g, lower than that of 0.5HZ sample, but still higher than that of the original HZ sample. When the NaOH concentration was increased to 1.5 and 2.0 M, the strong acid sites further dropped greatly to 51.3 and 22.9 µmol/g, while on the contrary, the weak acid sites increased much to 36.6 and 39.5 µmol/g, respectively. The great change of both the strong and weak acid sites over the 1.5HZ and 2.0HZ samples 13 ACS Paragon Plus Environment


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reveals that the NaOH solution with higher concentrations could significantly influence the chemcial structure of zeolite, possibly because not only desilication, but also dealunimination occurred during the treatment with high NaOH concentration. To prove our projection of the dealumination, we have conducted ICP-AES analysis on the HZ and 0.1HZ samples. Considering that the alumina from dealumination may exist over the external surface, the directly measured SiO2/Al2O3 ratio represents its bulk value in the sample. In order to obtain a more accurate value for the framework SiO2/Al2O3 ratio, the sample was fruther treated with 1M HCl solution at room temperature under untrasonic aid for 30 min to remove the external alumina. As shown in Table S1 (please see the Supporting Information), the bulk SiO2/Al2O3 ratio for HZ and 0.1HZ was 55.8 and 55.5, respectively, showing a slight decrease. It confirms that silicon atoms have been removed from the framework of ZSM-5 zeolite during NaOH treatment. After HCl treatment, however, the SiO2/Al2O3 ratio of 0.1HZ was 57.4, slightly higher than that of HZ (56.6). It reflects that the framework SiO2/Al2O3 ratio increased slightly after NaOH treatment. In other words, the dealumination actually also occurred during the NaOH treatment even at low concentration (0.1M). Consequently the increased weak acid sites after NaOH treatment can also be ascribed to the formed alumina from the dealumination on the external surface, which becomes more with the increase of NaOH concentration.

3.3 Catalytic performance The parent and alkali-treated HZ catalysts were evaluated for ethylene oligomerization at 300 ºC and WHSV of 12 h-1 for liquid fuel production under atmospheric pressure. Figure 7 displays the ethylene conversion and liquid yield as a function of time on stream (TOS) over


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these catalysts. For the parent HZ catalyst, the ethylene conversion decreased gradually from 93% to 73% within 5 h of TOS, as shown in Figure 7A. After treatment with NaOH solution at the concentrations of 0.1-1.0 M, the catalysts exhibited more stable performances for ethylene conversion. The ethylene conversion over 0.1HZ, 0.3HZ and 0.5HZ only decreased from 97% to 93% within 5 h on stream, and it decreased from 97% to 84% over 1.0HZ. For the 1.5HZ catalyst, however, the conversion decreased even more significantly than that of the HZ, from 78% to 19% after 5 h of reaction. Only 1% of ethylene conversion was found over the 2.0HZ catalyst. The yield of liquid product as a function of TOS is shown in Figure 7B. It usually increased in the first hour and then decreased over all the parent and NaOH-treated HZ catalysts. Compared with the parent HZ catalyst, the 0.1HZ, 0.3HZ, 0.5HZ and 1.0HZ catalysts led to a higher and more stable liquid yield, but the 1.5HZ catalyst gave lower liquid yield due likely to its lower activity. No liquid product was obtained over the 2.0HZ catalyst. The obtained liquid fuel was analyzed by GC-FID and the results are displayed in Figure 8A. It showed that the liquids obtained over the parent and NaOH-treated HZ catalysts had similar composition, and were mainly comprised of C6-C10 hydrocarbons. The zeolite-catalyzed oligomerization is of carbenium ion character initiated by the Brønsted acid sites17, 46, probably via three steps: 1) formation of short-lived hydrogen-bonded precursor, 2) a protonation step and 3) a chain-growth step47. In addition, the protonic acid sites can also catalyze a series of other reactions, such as disproportionation, isomerization, cracking, hydrogen transfer and aromatization.16-17,

21, 27

As a result, the obtained liquid could be complexed with olefins,

paraffins, cycloalkanes and aromatics, as shown in Figure 8A. Compared to other catalysts, the content of C6 hydrocarbons in the liquid obtained over the 1.5HZ catalyst was higher, while the



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C7-C10 hydrocarbons were lower. It can be attributed to the lower activity of the 1.5HZ catalyst (Figure 7A), resulting in a less reaction in the chain-growth step. Figure 8B shows the distillation curves of the oligomerization liquid products from the parent HZ and NaOH-treated 0.1HZ catalysts on the basis of Simulated Distillation GC data. It can be seen that about 85% of hydrocarbon product from ethylene oligomerization over HZ catalyst have the boiling point in the range of gasoline (30-200 °C). After NaOH treatment, this portion increased to 92% over the 0.1HZ catalyst. Compared to the HZ catalyst, the 0.1HZ catalyst showed a higher mesoporous volume (Table 1) and slightly increased mesopore channels (Figure 4), which can benefit the diffusion of formed liquid hydrocarbons out of the pores of ZSM-5 zeolite and thus reduce the consequent chain-growth reaction. As a result, more hydrocarbons with boiling point less than 200 °C was obtained over the 0.1HZ catalyst than that over the parent HZ catalyst.

4. Discussion As described above, the characterizations with XRD, N2 physisorption and NH3-TPD tend to suggest that the NaOH treatment could significantly alter both the acidity and the pore properties of ZSM-5. The variation is considerably influenced by the NaOH concentration for the treatment, which further influence its catalytic performances in ethylene oligomerization. The reaction results can in turn enable us to discuss the roles of the acidity and the created mesopores in the catalytic behaviors of the HZSM-5 zeolite catalyst. As shown in Table 1, alkali treatment by NaOH with 0.1-0.5 M increased both the strong and weak acid sites of ZSM-5 due mainly to the dissolution of silica in the zeolite. However, high concentration of NaOH (1.5 and 2.0 M) led to significant destruction and collapse of a large


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part of zeolite structure, according to the XRD results (Figure 2), which should account for the decrease in the total acid amount and the disappearance of strong acid sites on the 1.5 HZ and 2.0HZ, although a slight increase in the weak acid sites was still observed (Table 1). The conversions for ethylene oligomerization were increased in comparison with that of HZ catalyst and were similar over the 0.1HZ, 0.3HZ, 0.5HZ and 1.0HZ catalysts, but much lower over the 1.5 HZ and 2.0HZ catalysts. It suggests that the strong acid sites played a much more important role in ethylene conversion than the weak acid sites. Therefore, the reaction conversion and liquid yield as a function of the amount of acid sites including total acid, strong and weak acid, respectively, over the alkali-treated HZSM-5 catalysts for ethylene oligomerization is plotted and shown in Figure 9. It is clear that the weak acid has a poor correlation with ethylene conversion and liquid yield (R2 = 0.5988 and 0.5979, respectively); however, the total acid (R2 = 0.9179 and 0.8898, respectively), especially the strong acid (R2 = 0.9528 and 0.9301, respectively) has a good correlation with ethylene conversion and liquid yield for ethylene oligomerization. With the increase in the strong acid sites, both the ethylene conversion and liquid fuels yield increases. It confirms that the strong acid sites are more favorable for the ethylene oligomerization, which is consistent with the observation reported by other researchers17, 46. It is also found that compared to the parent HZ catalyst, not only the ethylene conversion and liquid yield in ethylene oligomerization were increased, but also the stability was improved with the extension of TOS over the alkali-treated HZSM-5 catalysts, as presented in Figure 7. The increase in the strong acid sties could promote the oligomerization, but also cause side reactions such as cracking or aromatization reactions. Generally, the zeolites with high acid site density are more prone to fast deactivation due to higher coke formation induced by the side reactions and pore blockage caused by diffusion limitation and coke formation.48-49 Thus, the



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better stability of the alkali-treated HZSM-5 catalysts than the parent HZ catalyst cannot be explained by the increment of acidity. As a consequent, the increase in the strong acid sites density resulting from the alkali treatment mainly contributes to the increased reaction conversion and oligomerization liquid yield of the HZSM-5 zeolite, while the improved catalyst stability should be mainly due to the changes of zeolite pore structure. The alkali treatment can induce the formation of mesopore within HZSM-5 zeolite. With the increase of NaOH concentration for the treatment, the mesopore volume increased greatly while the micropore volume decreased gradually (Table 1). With the preservation of the HZSM5 structure, the catalyst stability in ethylene oligomerization was obviously improved with the development of mesopores, as observed over the 0.1HZ, 0.3HZ, 0.5HZ and 1.0HZ catalysts. To understand the catalyst deactivation, the carbon deposit and texture properties of the spent catalysts after 5 hrs of ethylene oligomerization were analyzed by temperatureprogrammed oxidation (TPO) and N2 physisorption, respectively. Figure 10 shows the TPO profiles of the spent catalysts. The corresponding amount of carbon deposit is calculated and listed in Table 2. Two broad and distinct peaks centered at 270 and 550 °C can be observed. The first peak corresponds to the oxidation of extractable organics trapped inside the pore channels, which was confirmed by the disappearance of the peak after heat-treatment under vacuum at 200 °C for 24 h. The second peak at higher temperature can be attributed to the oxidation of carbon species deposited inside pores50. It can be seen that the deposit carbon content decreased gradually, from 9.43% over HZ catalyst to 9.02, 8.75, 8.26, 7.01 and 3.08% over the 0.1HZ, 0.3HZ, 0.5HZ, 1.0HZ and 1.5HZ catalysts, respectively. Such a trend indicates that the formation of mesopores could alleviate the carbon deposition over HZSM-5 catalyst in ethylene oligomerization. More interestingly, the content of solid carbon species deposited on the spent


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0.1 HZ, 0.3HZ, 0.5HZ, and 1.0HZ catalyst was similar, but the amount of the trapped extractable organics decreased gradually over these catalysts (Table 2). It suggests that the trapped extractable organics were easier to diffuse out of the catalyst with larger pore size. The spent HZ catalyst showed a lower amount of trapped extractable organics due likely to the further transformation of the trapped organics to solid carbon inside micropore channels, resulting in the highest content of deposit carbon. The texture properties of the used catalysts are summarized in Table 2. After 5 hrs of reaction, the loss in the surface area and total pore volume of the parent HZSM-5 catalyst reached 298 m2/g and 0.187 cm3/g, respectively. The decrease in the pore volume is mainly contributed by the loss of micropore volume (0.131 cm3/g), suggesting that the pore blockage by coke formation was more pronounced in the micropores rather than that in the mesopores. After alkali treatment, the loss in the surface area and pore volume became smaller, e.g., over the 0.1HZ, the loss was 220 m2/g and 0.138 cm3/g for the surface area and total pore volume, respectively, demonstrating the blockage of micropores caused by carbon deposition is reduced via the formation of mesopores through the alkali treatment. To illustrate the roles of micropore and mesopore in the deactivation of HZSM-5 catalyst, the total carbon deposit as a function of the micropore and mesopore volumes of the treated zeolites are plotted in Figure 11. Both the micropore volume and mesopore volume show a good correlation with the total carbon deposition (R2 = 0.9797 and 0.9768, respectively), but at an opposite direction. With the increase of the micropore volume, the total deposit carbon content increases; while it decreases as increasing the mesopore volume. Such a correlation shows that the carbon deposition mainly occurs inside micropores due likely to its limitation in the diffusion of formed liquid products, exhibiting a fast decrease in the ethylene conversion and liquid yield.



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The presence of mesopores could benefit the products diffusion within the pore channels, resulting in a higher conversion and liquid yield, and less carbon deposition.

5. Conclusion As one step of the new approach for biomass conversion to liquid fuels developed by Pennsylvania State University (PSU) and Altex Technologies Corporation (Altex), modified HZSM-5 catalysts were prepared via alkali treatment, characterized and studied for ethylene oligomerization at atmospheric pressure. It was found that both the zeolite structure and the catalytic performance can be significantly influenced by the concentration of NaOH in the treatment. The alkali treatment led to the increase in the total pore volume and mesopore volume, but the decrease in surface area and micropore volume. At low NaOH concentration (≤ 0.5 M), the HZSM-5 structure was largely preserved with great increase in the mesopores and acid sites. While higher NaOH concentration (≥ 1.0 M) can significantly reduce the micropores, severely destroy the zeolite structure and dramatically reduce the total amount of acid sites, especially the strong acid sites. The ethylene oligomerization results showed that not only the ethylene conversion and the liquid yield increased, but also the catalyst stability was significantly improved after proper NaOH treatment, which can be attributed to the increased strong acid sites and the newly formed mesopores, respectively.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acs.iecr.xxxxxxx.


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The peak deconvolution of the NH3-TPD profiles over HZ and alkali-treated HZ samples, and ICP-AES results of HZ and 0.1HZ samples before and after 1M HCl treatment. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.S.); [email protected] (X.W.) ORCID Chunshan Song: 0000-0003-2344-9911 Xiaoxing Wang: 0000-0002-1561-3016

Notes The authors declare no competing financial interest.

Acknowledgement This study was supported in part by the U.S. Department of Energy, National Energy Technology Laboratory through DOE Grants DE-SC0006466, DE-FE0010427, and DEFE0023663, and by the U.S. Department of Defense through the DOD-SBIR Grant W911SR-11C-0018 via subcontracts to Altex.

References (1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, 4044-4098.



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Table 1. Texture and acid properties of parent and NaOH-treated HZSM-5 samples. NaOH

SBET

Smicro

Smeso

Vtotal

Vmicro

Vmeso

Acid amount (µmol/g)

Conc. (M)

(m2 g−1)

(m2 g−1)

(m2 g−1)

(cm3 g−1)

(cm3 g−1)

(cm3 g−1)

Total

Strong

weak

Sample

HZ

̶

416

274

142

0.31

0.15

0.16

99.1

78.7

20.4

0.1HZ

0.1

391

242

149

0.32

0.12

0.20

105.5

80.0

25.5

0.3HZ

0.3

382

229

153

0.36

0.11

0.25

103.9

75.5

28.4

0.5HZ

0.5

387

233

154

0.47

0.11

0.36

117.5

86.3

31.2

1.0HZ

1.0

334

176

158

0.72

0.10

0.62

105.5

80.6

24.9

1.5HZ

1.5

305

114

191

0.90

0.06

0.84

87.9

51.3

36.6

2.0HZ

2.0

247

56

191

0.91

0.03

0.88

62.4

22.9

39.5


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Table 2. Texture properties and coke content of the spent catalysts Coke content (%) SBET

∆SBET

Vtotal

Vmicro

Vmeso

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

Sample

Extractable

Carbon

organics

deposit

Total

HZ

118

298

0.12

0.016

0.106

9.43

2.65

6.78

0.1HZ

171

220

0.18

0.027

0.151

9.02

3.36

5.66

0.3HZ

161

221

0.22

0.049

0.171

8.75

3.14

5.61

0.5HZ

144

243

0.33

0.004

0.327

8.26

2.60

5.66

1.0HZ

155

179

0.49

0.009

0.477

7.01

1.33

5.68

1.5HZ

226

80

0.80

0.037

0.766

3.08

0.27

2.81



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Figure Captions Figure 1. Schematic representation of the new biomass-to-fuel process developed by PSU-Altex. Figure 2. XRD patterns of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ. Figure 3. Nitrogen isotherms of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ. Figure 4. DFT pore size distribution curves of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ. Figure 5. FESEM images of (a) HZ, (b) 0.1HZ, and (c) 0.5HZ; and TEM micrographs of (d) HZ, (e) 0.1HZ and (f) 0.5HZ. Figure 6. NH3-TPD profiles of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ. Figure 7. (A) The ethylene conversion and (B) Liquid yield as a function of the time on stream (TOS) for ethylene oligomerization over the parent and NaOH-treated HZ catalysts at 300 ºC and atmospheric pressure. Figure 8. (A) GC-FID profiles and (B) Simulated Distillation GC-profiles of liquid products obtained from ethylene oligomerization over the parent HZ and NaOH-treated HZ catalysts at 300 ºC and atmospheric pressure. Figure 9. The ethylene conversion and the liquid yield at the TOS of 2 h as a function of the amount of acid sites including total acid, strong and weak acid, respectively, over the alkalitreated ZSM-5 catalysts for ethylene oligomerization. Figure 10. Temperature-programmed oxidation (TPO) profiles of the spent HZ and NaOHtreated HZ catalysts.


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Figure 11. The total carbon content deposited on the spent catalyst as a function of the pore volume of micropores and mesopores.



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Figure 1. Schematic representation of the new biomass-to-fuel process developed by PSU-Altex.


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(a)

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) (c) (d) (e) (f) (g)

5 10 15 20 25 30 35 40 45 50 55 60 2θ, degree Figure 2. XRD patterns of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume Adsorbed, cm3-STP/g


Page 34 of 43

(g) (f) (e) (d) (c) (b) (a) 0

0.2

0.4

0.6

0.8

1

P/P0 Figure 3. Nitrogen isotherms of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ.


Page 35 of 43

(g) 20 nm 0.56 nm

1.5 nm

dV/dw, cm3/g-Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(f)

(e) 2 nm

(d) (c) (b) (a)

1

10

100 Pore diameter, Å

1000

Figure 4. DFT pore size distribution curves of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

(b)

(a)

(c)

(d)

(e)

(f)

Figure 5. FESEM images of (a) HZ, (b) 0.1HZ, and (c) 0.5HZ; and TEM micrographs of (d) HZ, (e) 0.1HZ and (f) 0.5HZ.


Page 37 of 43

220 ºC

9.5

900

410 ºC

7.5

(a)

700

(b)

600

(c)

500

(d) (e) (f)

400

5.5

3.5

1.5

300 200

(g)

-0.5

0

10

20

30

40

50

60

70

Temp., oC

800

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

80

Time, min Figure 6. NH3-TPD profiles of (a) parent HZSM-5, (b) 0.1HZ, (c) 0.3HZ, (d) 0.5HZ, (e) 1.0HZ, (f) 1.5HZ and (g) 2.0HZ.



C2H4 conversion, %

100 80 60

HZ 0.1HZ 0.3HZ 0.5HZ 1.0HZ 1.5HZ 2.0HZ

40 20

(A)

0 0

100

200

300

TOS, min 40 (B)

Liquid yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

30 20 10 0 0

100

200

300

TOS, min Figure 7. (A) The ethylene conversion and (B) Liquid yield as a function of the time on stream (TOS) for ethylene oligomerization over the parent and NaOH-treated HZ catalysts at 300 ºC and atmospheric pressure.


Page 39 of 43

100 a b

C8

d ce

C9

(A)

C10

g f h mn k

92% 85%

1.5HZ

80

%Recovered

C6 C7

Relative intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0HZ 0.5HZ 0.3HZ 0.1HZ

(B)

60

40 HZ 0.1HZ

20

HZ

0

0

10

20

30

40

50

60

0

50

Retention Time (min)

100 150 200 250 300

Temp., oC

Figure 8. (A) GC-FID profiles and (B) Simulated Distillation GC-profiles of liquid products obtained from ethylene oligomerization over the parent HZ and NaOH-treated HZ catalysts at 300 ºC and atmospheric pressure.



Ethylene Conv. , %

100 80

R² = 0.5988 R² = 0.9179

60 R² = 0.9528

(A)

40

Total strong weak

20 0 0

20

40

60

80

100

120

Acid amount, µmol/g 50

Total strong Weak

(B)

Liquid Yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

40 30 20

R² = 0.9301

R² = 0.5979

R² = 0.8898

10 0 0

20

40

60

80

100

120

Acid amount, µmol/g Figure 9. The ethylene conversion (A) and liquid yield (B) at the TOS of 2 h as a function of the amount of acid sites including total acid, strong and weak acid, respectively, over the alkalitreated ZSM-5 catalysts for ethylene oligomerization.


Page 41 of 43

270

oC

550 oC

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HZ 0.1HZ 0.3HZ 0.5HZ 1.0HZ 1.5HZ

100 200 300 400 500 600 700 800 900

Temp., oC Figure 10. Temperature-programmed oxidation (TPO) profiles of the spent HZ and NaOHtreated HZ catalysts.


Page 42 of 43

10

(A) 8 R² = 0.9797

6 4 2 0 0

0.05

0.1

0.15

0.2

Micropore volume, cm3/g Total carbon deposit, wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Total carbon deposit, wt%


10

(B) 8 6 R² = 0.9768

4 2 0 0

0.2

0.4

0.6

0.8

1

Mesopore volume, cm3/g Figure 11. The total carbon content deposited on the spent catalyst as a function of the pore volume of micropores (A) and mesopores (B).


Page 43 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents (TOC) Graphic


Oligomerization of Biomass-Derived Light Olefins to Liquid Fuel: Effect

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