Ethylene Oligomerization over Ni-HÎ² Heterogeneous Catalysts

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Kinetics, Catalysis, and Reaction Engineering

Ethylene Oligomerization over Ni-H# Heterogeneous Catalysts Oliver Jan, Kunlin Song, Anthony B. Dichiara, and Fernando Luis P. Resende Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01902 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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Industrial & Engineering Chemistry Research

Ethylene Oligomerization over Ni-Hβ Heterogeneous Catalysts Oliver Jan, Kunlin Song, Anthony Dichiara, Fernando L.P. Resende* University of Washington, School of Environmental and Forest Sciences, Seattle, WA 98195 *corresponding author: [email protected]

Abstract We report results for the oligomerization of ethylene in a continuous packed bed reactor loaded with Ni-Hβ. We performed a parameterized study of the effects of temperature (50 ºC-190 ºC), ethylene partial pressure (8.5-25.6 bar), and weight hourly space velocity (WHSV, 2.0-5.5 h-1) on the ethylene conversion and product selectivity. The steady-state ethylene conversion increased from 38 to 57 % as the pressure increased from 8.5 to 25.6 bar, due to increased concentration of the reactant ethylene and lower linear velocities at higher pressures. Higher temperatures led to the formation of larger oligomers and coke, but the effect of temperature on the ethylene conversion was small. The WHSV played an important role on ethylene conversion and product selectivity. An increase in the WHSV from 2.0 to 5.5 h-1 resulted in a 13 % decrease in conversion. At a low space velocity (2.00 h-1), we observed 57% conversion, whereas a high space velocity (5.50 h-1) resulted in 44 % conversion and higher selectivities to butene (74.9 wt.%). The parametric analysis provided the basis for a 78 hours-on-stream study to examine the deactivation of Ni-Hβ. We conducted this study at 19.0 bar partial pressure of ethylene, 120 ºC, and 3.1 h-1 WHSV. Catalyst deactivation took place only during the initial startup period largely due to coke formation. However, negligible coke formation occurred after the initial 8 hours, and

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the conversion remained steady at 47% for the duration of the experiment. Our results indicate that Ni-Hβ is a viable catalyst for ethylene oligomerization. Introduction Ethylene is the simplest alkene observed in nature. This simple organic chemical is also a vital reactant for several chemical reactions for the formation of high value products. Most of the ethylene available today is produced by steam cracking of ethane or heavier hydrocarbons extracted from crude oil in the petroleum industry (such as naphtha)[1], [2]. However, crude oil and other fossil fuels are nonrenewable resources. In contrast, renewable routes for ethylene production have been proposed[1]. Bioethanol derived from natural lignocellulosic resources, such as biomass, can be a starting point for ethylene production, through a dehydration reaction to produce ethylene and water, with oligomerization of the produced ethylene to form the desired olefins. Ethylene oligomerization forms higher-order olefins (primarily alpha-olefins) that can be further upgraded to produce a myriad of commercial products, such as acids, detergents, plasticizers, oil additives, and transportation fuels[1], [3], [4]. This process has been performed commercially typically in liquid-phase with a homogeneous catalyst in an organic solvent[1]. For instance, organometallic catalysts (Ni, Co, Cr, Fe) bearing alkyl ligands have demonstrated high activity and high selectivity towards targeted olefins[3]. Despite these advantages, the general drawbacks associated with homogeneous catalysts include low catalyst reusability, high sensitivity to impurities in the reactant stream, as well as the poor separation of the catalysts from the product stream[1], [5]. In addition, homogeneous-phase reactions suffer from high solvent usage and are not considered as environmentally-friendly processes.


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Due to the challenges and environmental drawbacks associated with homogeneous catalysts, expanding research into developing processes implementing heterogeneous catalysts for ethylene oligomerization has been of considerable academic and industrial interest. Much of the previous work pertaining to heterogeneous ethylene oligomerization has employed threephase slurry reactors that operate in semi-batch mode[1], [4]–[8]. In this system, the solid catalyst is suspended in a solvent, such as heptane, contained in a sealed vessel, and ethylene is used to pressurize the reactor. The ethylene solubility in the solvent, which strongly depends on pressure, is an important property for ethylene oligomerization. Other types of reactor configurations, such as a packed bed reactor, can be used to promote gas-solid interactions without the need for organic solvents, minimizing separation requirements. In this work, we carried out oligomerization of ethylene in a packed bed reactor using a mesoporous solid, metalloaded silica-alumina catalyst. The activity of metal-loaded inorganic substrates consisting of silica-alumina has been studied previously for ethylene oligomerization, with Earth abundant nickel metal on silicaalumina frameworks showing the most promise. Several major parameters of these solid materials, namely the pore size and the silicon to aluminum ratio, had profound effects on the extent of the reaction[2], [6], [7], [9]–[11]. The pore size was found to have a proportional relationship with catalyst activity, with larger pore sizes resulting in higher oligomerization activity. When microporous materials, such as Ni-MCM-22 and nickel-loaded zeolites, were used, only small amounts of oligomers were detected[6], [9]. Mesoporous materials, such as NiMCM-41, demonstrated markedly high catalyst activity[7], [12], [13]. It was theorized that the mesopores in the framework can more readily facilitate the diffusion of heavier oligomeric products, which are sterically hindered in microporous materials due to their narrower pore sizes.



Furthermore, rapid catalyst deactivation was observed in microporous materials due to a pore plugging effect, while mesoporous catalysts can prevent product accumulation, allowing prolonged oligomerization activity. Besides porosity, the silicon to aluminum ratio was found to play a significant role in the overall catalytic activity. The presence of silicon in the framework (4+ charge) neutralizes the charges of the surrounding oxygen atoms. When aluminum (3+ charge) replaces the silicon, an unbalanced negative charge is created in the silica-alumina framework. This resultant negative charge can only be compensated with a complementary cation, such as acidic protons and metal ions, including nickel. Martínez and coworkers observed that the relationship between acidic protons and nickel in their Ni-Hβ catalysts had a profound effect not only on the textural properties of the material, but also on the catalyst activity and the resulting oligomerization products. For Ni-Hβ catalysts, an increase in the nickel loading above 1.8 wt.% resulted in a decrease of the available surface area along with a decrease in the concentration of Brønsted acid sites. The presence of both nickel and acidic protons throughout the catalyst, however, was found to be critical, as nickel was theorized to be solely responsible for the initial activation of ethylene and primary oligomerization pathways, whereas acidic protons were critical to carry out secondary isomerization, oligomerization, and cracking reactions[3]. The oligomerization of ethylene over Ni-Hβ catalysts has been reported mostly on a few recent studies[3], [14]–[17], which have detailed the effects and nature of the nickel active sites[15], [18], support properties[14], and the interplay between the two[17]. While the effect of catalyst structure and surface ions have been reported, the current literature lacks information on the influence of reaction conditions. Further insight onto the performance of this catalyst for oligomerization, particularly detailing the effects of process parameters, would be beneficial for


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understanding its potential. In this work, we adopt a Reaction Engineering approach and discuss the roles of pressure, temperature, and weight hourly space velocity (WHSV) in a continuous packed bed reactor loaded with Ni-Hβ. To the best of our knowledge, this is the first systematic study reporting the effect of these operating parameters for ethylene oligomerization over Ni-Hβ. In addition, we evaluate the long-term activity of Ni-Hβ catalyst over more than 72 hours on stream (previous deactivation studies were limited to 12 hours at most[3]). Experimental Methods Catalyst Synthesis We purchased the commercial beta zeolite from Zeolyst (CP814E, SiO2/Al2O3=25, ammonium form) and used it as is without prior treatment. To introduce nickel on the support, we slowly dripped a 0.34 M solution of nickel nitrate (Sigma Aldrich, purity 99.9%) onto the zeolite powder for over 5 hours at room temperature to achieve a liquid-to-solid ratio of 5 cm3/g. This slow exchange resulted in a dark grey slurry, which we labeled as Ni-NH4-β. After 5 hours of mixing, we washed the resulting dark grey slurry with deionized water and vacuum filtered it to obtain a grey paste. We then dried the paste at 110 ºC for 12 hours, which resulted in a pale green powder. Immediately after drying, we calcined the powder at 550 ºC for 5 hours in an air oven to remove ammonia from any residual ammonium sites, which produced a light grey powder, labeled henceforth as Ni-Hβ catalyst. We then crushed the catalyst using a mortar and pestle and used it as is without any further treatment.

Catalyst Characterization To determine the nickel loading in the material, we used ICP-OES (inductively coupled plasma-optical emission spectroscopy) according to EPA protocol 200.7, where we digested the



solid material with ultra-high purity grade nitric acid and hydrochloric acid. However, this protocol is unable to accurately measure the silicon and aluminum content, and we assume that the supplier provided SiO2/Al2O3 ratio of 25 is correct. We measured nitrogen adsorption and desorption isotherms at -193 ˚C (77 K) with a Micromeritics TriStar II Plus instrument (Norcross, GA, USA). We outgassed all samples in vacuum at 300 ˚C for 4 h to remove any possible adsorbed impurities prior to data collection. We used the Brunauer-Emmett-Teller (BET) method for the specific surface area (SSA) calculations, and determined the volume of micro-pores (Vm) and the total pore volume (Vt) from the N2 adsorption isotherms as per the IUPAC standards. We obtained the overall pore-size distribution by fitting the N2 isotherms in the Horváth-Kawazoe (HK) model for cylindrical pores. To determine the nanocrystalline structure of the catalyst, we imaged the materials using transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. We sonicated the catalyst powder in ethanol to create a fine dispersion, and then dripped the solution onto a 250 m copper grid. We then loaded the copper grid into the microscope using a single-tilt holder into the sample compartment of a Tecnai G2 F20 Supertwin TEM. In addition to TEM, we obtained the X-ray diffraction (XRD) profiles of the materials using a Bruker D8 Discover with a high-power rotating Cu anode X-ray source. We also collected scanning electron microscopy (SEM) images with a FEI Sirion XL30 microscope after sputter coating the samples with a thin layer (2-5 nm) of gold to mitigate distortion and image hot spots due to electron charging.

Catalytic Reactor


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We performed ethylene oligomerization experiments in a fixed-bed reactor constructed with 1” Schedule 80 SS316 tubing, as shown in Figure 1. For the purposes of our experimentation, we designed the system to be operated in a continuous manner with downstream, online analysis. We loaded the reactor with 8.0 g of fresh catalyst in-between two beds of quartz wool, which created a bed approximately 1.25’’ in height. The temperature of the bed was continuously measured with a thermocouple inserted at the center of the pipe and halfway through the length of the bed. We note that this approach does not ensure isothermal operation of the bed. However, we assume that the temperature measured at the center of the bed is the best representation of the system temperature profile. Prior to each experiment, we leak-checked and pressure-tested the reactor with nitrogen gas. After ensuring that the system was free of leaks and could maintain the desired reaction pressure, we pretreated the catalyst at 300 ºC in N2 (Praxair, 99.999%) at 200 mL/min for 16 hours under atmospheric pressure. This pretreatment step was intended to purge any air or moisture from the system, as moisture has been previously observed to severely hinder the catalyst performance. We monitored the gas composition online with a downstream Shimadzu GC-2014 equipped with a HP-PLOT-Q capillary column (0.53mm x 30m x 40µm) coupled with a flame ionization detector (FID) and thermal conductivity detector (TCD). After pretreatment, we lowered the temperature of the reactor to the desired reaction temperature. We then pressurized the reactor using nitrogen gas (Praxair, 99.999%). The pressure was controlled and maintained by an Equilibar dome-loaded backpressure regulator. We note that, at all conditions, the pressure drop throughout the system was measured, but was negligible compared to the total pressure or the partial pressure of ethylene. For this reason, the pressure drop is not considered in the discussion of results.



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After stabilizing the set point temperature and pressures, we initiated the reactions by flowing the ethylene gas (Praxair, 99.5%, 350 mL/min) with nitrogen (314 mL/min). A heated transfer line downstream of the reactor was kept at 120 ºC to prevent product condensation. We measured product gases periodically through on-line GC-FID-TCD, and determined the coke yield from the difference in the weight of the reactor contents before and after the reaction. We constructed calibration curves for ethylene, propylene, and 1-butene using gas standards, and used liquid injection of solutions of various concentrations for the higher olefins (1-hexene, 1-octene, 1decene, 1-dodecene). The mass balances, excluding the unreacted ethylene for all reported experiments, were closed above 90%. The reproducibility was verified by replicating three experiments at the reaction conditions: 120 ºC, 3.1 h-1, and 19.0 bar, and a total time-on-stream (TOS) of 6 hours. Error bars are reported for subsequent experiments using the standard deviations obtained from these replicates at each measured time interval.

The selectivity, , the conversion, , and the coke yield have been defined as in Equations 1-3: . % =

. % =

× 100

! " "#$ % ! " "#$ "

&'( )*( + . % =

" "#$

(1)

× 100

"$" ,%" "$" , " "#$∗"


× 100

(2)

(3)

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In reporting the selectivities, we group the hydrocarbons by the number of carbon atoms in the alpha-olefins (C4, C5, C6, etc.), which were determined based on their retention time from the calibrations conducted previously.

Online Analysis GC-TCD

Figure 1. Process flow diagram of ethylene oligomerization system

Results and Discussion Catalyst Characterization The X-ray diffractogram for both the parent ammonium-β and metal-loaded Ni-Hβ are shown in Figure 2. The XRD diffraction pattern of pure Hβ zeolite before nickel impregnation contains typical Bragg reflections characteristic of BEA-type molecular sieves[19], as shown in Figure 2. The narrow peak at 7.7 º and 22.5º are also visible at the same positions in the Ni-Hβ patterns, indicating that the well-ordered zeolite structure was maintained after nickel impregnation. Compared with the pure Hβ zeolite, four distinct diffraction peaks at 37.2º, 43.3º, 62.9º, and 76º are observed on the sample after nickel impregnation. These peaks correspond to the (111), (200), (220), and (311) plane Miller indices of face-centered cubic NiO crystallites, which is consistent with other studies reported in the literature[3]. The XRD analysis provide



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reasonable evidence for the presence of NiO clusters on the zeolite material after nickel impregnation and calcination.

Figure 2. Diffraction profile of the parent β zeolite before and after nickel impregnation From the data in Figure 2, the relative nanoparticle size (d) can be determined using the DebyeScherrer equation, as follows: + =

./ 0 123 4

(4)

Where 5 (0.9) is defined as the shape factor, 6 (1.54 7) is the wavelength of the X-ray, 8 is the full width half max (FWHM) of the indicative peak, and 9 is the angular position of the peak[20]. In our calculations, we chose the peak corresponding to the (111) plane at 37.2º 29. Based on these calculations, the average crystallite size of the NiO particles was determined to be 24 nm. The presence of the NiO crystallites is also apparent in the transmission electron microscopy image, shown in Figure 3a. The darker spots on the TEM image represent the NiO


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clusters on the external surface of the material. Using Image J, the average crystallite size was found to be in the range of 20-25 nm, which is consistent with the size calculated using the Debye-Scherrer equation. The lack of uniformity of the crystallite size, however, is noted. Martínez and coworkers have previously reported that NiO clusters can appear at nickel loadings above 1.8 wt.%[3]. The nickel content of the material used in the present work was measured to be 3.4 wt.% by ICP-OES. Therefore, the presence of NiO clusters is expected in accordance to the prior literature. Exchange sites that were once occupied by the ammonium in the parent material can initially be occupied with divalent nickel ions or residual protons resulting from the removal of ammonia from non-exchanged ammonium. After saturation of the exchange positions inside the pores with nickel ions, the additional nickel is deposited on the outside of the pores in the form of NiO clusters[3]. Though the nature of the active nickel sites is still a subject of debate, it is generally accepted that the metallic, exchanged nickel is the active form of the catalyst[3]. It is assumed that the NiO clusters do not have catalytic activity for ethylene oligomerization[14]. As noted by Martinez et al., the textural and physical properties of the parent material do not appear to change drastically upon impregnation of the nickel metal[3]. In addition, the authors observed that the surface area and pore diameter/volume were largely unaffected throughout the course of the reaction. We also collected images of the catalyst at the microscale using scanning electron microscopy, as shown in Figure 3b and 3c. The granules of NiHβ, shown in Figure 3c, appear to be similar to those shown with the parent ammonium-β imaging in Figure 3b. This suggests that the parent ammonium-β material is unaffected by the incorporation of nickel, which is consistent with the XRD data.



Figure 3. a) Representative TEM and (b) SEM images of ammonium-β before and (c) after nickel impregnation

The adsorption isotherms for Ni-Hβ before and after reaction and corresponding pore size distributions analyzed by the Horváth-Kawazoe (HK) model are shown in Figure 4. Both fresh and spent catalysts exhibit high micropore filling at low relative pressure, as indicated by the steep increase at very low relative pressure (Figure 4A). The fresh and spent Ni-Hβ also show a small hysteresis loop at relative pressures higher than 0.5 due to mesopore condensation. The dominated micropore structure of the catalysts with a small portion of mesopores was also confirmed by the pore size distribution (Figure 4B), showing strong peaks below 1 nm for micropores and weak peaks above 10 nm for mesopores. Compared with the fresh catalyst, the spent catalyst presents a less steep (or more flat) rising trend at low relative pressure and decrease in height of the pore size distribution peaks, especially for the micropores, indicating


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some micropores disappeared after the reaction. Similar phenomenon of the disappeared pores of catalysts after reaction was also observed elsewhere[21].

Figure 4. a) N2 adsorption/desorption isotherms and b) pore size distribution plots of the catalysts at 77K for Ni-Hβ before and after reaction (120 ºC, 25.6 bar ethylene partial pressure)

The detailed textural properties of the fresh and spent catalysts are listed in Table 1. The specific surface area, S, and average pore volume, V, of the fresh Ni-Hβ are 677 m2/g and 0.849 cm3/g, respectively. For the spent catalyst, there is a drastic decrease in specific surface area and pore volume, which are reduced to 247 m2/g and 0.564 cm3/g, respectively. We note that the reduced pore volume is mainly a consequence of the decrease in micropore volume from 0.206 cm3/g of the fresh catalyst to 0.043 cm3/g of the spent catalyst. These variations are possibly caused by the formation of coking substances blocking micropores during the reaction. In addition, there is a significant fivefold decrease in microporous surface area Sm for the spent NiHβ compared to the fresh catalyst. This reveals that the accumulation of coke substances during the oligomerization of ethylene fills the microporous regions more rapidly since they are



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inherently narrower. This result is consistent with previous works reporting the accumulation of various products in microporous materials[6], [9].

Table 1. Textural and porosimetry properties for Ni-Hβ. S, Sm, Sm/S, V, Vm, and Vm/V are specific surface area, micropore surface area, ratio of micropore surface area to total surface area, pore volume, micropore volume, and ratio of micropore volume to total pore volume. *indicates post reaction conditions of material at 120 ºC, 25.6 bar ethylene partial pressure.

Ni-Hβ Ni-Hβ*

S (m2/g) 677 247

Sm (m2/g) 521 105

Sm/S (%) 77 43

V (cm3/g) 0.849 0.564

Vm (cm3/g) 0.206 0.043

Vm/ V (%) 24 8

Another characteristic of the catalyst that could potentially change because of the reaction is the nickel loading. To verify this possibility, we analyzed a sample of the spent catalyst (i.e. after reaction) using ICP-OES. Results showed that the nickel content is 3.2%, which is close to 3.4% for the fresh catalyst (i.e. before reaction).

Parameterization Experiments We conducted a systematic evaluation of the effect of process conditions with Ni-Hβ in the catalytic reactor. We evaluated the effects of pressure, temperature, and weight hourly space velocity (WHSV). The base case condition was arbitrarily selected as: 19.0 bar, 120 ºC, and 3.1 h-1. To isolate the effect of each variable, we changed one variable at a time, while keeping the others constant. The ethylene to nitrogen volumetric flow ratio was held constant at 1.12 in all experiments. We connected the experimental points in all conversion plots with straight lines as a visual aid.


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One of the factors that could potentially affect the results is the dispersion in the packed bed. Dispersion is a complex parameter related to bed characteristics (L, D), gas properties (velocity v, density ρ, viscosity µ, flow rate Q), catalyst morphology (particle size d, shape, porosity), and bed packing (porosity and uniformity)[22]. To accurately assess the dispersion in our system, a careful study based on numerical models would have to be conducted, but this is out of the scope of the present research. Nevertheless, we estimated the ethylene gas dispersion in the fix bed reactor described in this study to be relatively small, based on the large ratio of reactor diameter/catalyst particle size (> 15), and the low Reynolds number (12.2-79.8) compared to other packed bed systems previously reported in the literature[22]. For simplification purposes, the following assumptions were made: the particle size distribution is relatively uniform, the shape of the catalyst particles is spherical, and the gas flow is laminar.



Effect of Ethylene Partial Pressure

100 90 80 70 60 50 40 30 20 10 0

a

Selectivity, wt.%

8.50 bar 19.0 bar 25.6 bar

C4

100 Conversion, mol.% of inlet ethylene

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

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C5

C6

C7 C8 C9 Carbon Products

C10 C10+ Coke

b

80 60

25.6 bar

40

19.0 bar 8.50 bar

20 0 0

100

200 Time, minutes

300

400

Figure 5. a) Steady-state selectivities of carbon products from ethylene oligomerization and b) ethylene conversion as a function of ethylene partial pressure. Reaction conditions, T: 120 ºC, WHSV: 3.1 h-1


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In a preliminary set of experiments, we condensed and collected the hydrocarbons and analyzed their composition via GC-MS-FID. We found that, typically, 80 % of the products are alpha-olefins, 17 % are paraffins, 2.2 % are naphthenes, and 0.8 % are aromatics. In addition, we observed that the C4 products are all linear, and the C8 products are all branched. The C6 products have a mixed composition (69 % linear and 31 % branched). For the results reported here, we only distinguish products by the number of carbons, since the analysis was performed with a GC-FID. The effect of ethylene partial pressure on product selectivity is shown in Figure 5a. At 8.5 bar, the primary product was C4 at 84.1 wt.% selectivity, which decreases as the pressure increases. We observed the highest selectivities of C6 (9.6 wt.%), C8 (11.0 wt.%) and C10 (7.3 wt.%) oligomers at 19.0 bar. At 25.6 bar, the selectivity of the coke products increases, resulting in an overall decrease of the selectivity of the other pertinent oligomeric products. The coke selectivity appears to increase linearly with ethylene partial pressure, increasing from 1.4 wt.% at 8.50 bar to 8.7 wt.% at 19.0 bar, and to 14.7 wt.% at 25.6 bar. The time-on-stream conversion, depicted in Figure 5b, shows that the steady state ethylene conversion increases with the ethylene partial pressure. In the range of conditions tested, the highest steady-state conversion (55 %) was achieved at an ethylene partial pressure of 25.6 bar. The increase in conversion may be attributed to a large amount of ethylene flowing through the reactor at high pressure and low velocity. At a constant mass flow rate, higher pressures compress the gas and reduce its volumetric flow rate, therefore increasing residence times of molecules in the gas phase. Due to the lower velocity of the stream and the large concentration of ethylene molecules, ethylene can more readily adsorb on the Ni and acidic active sites, allowing for an overall higher rate of ethylene consumption,



producing butene and increasing conversion. On the contrary, lower pressures/ high velocities could increase the likelihood that ethylene molecules flow through the system completely bypassing the active sites, or even bouncing off the active sites due to high kinetic energy. This reduction in velocity also affects the selectivities shown in Fig. 5a, which can be explained based on a sequence of reaction pathway steps that are promoted as the pressure increases. As reaction products desorb from the catalyst active sites, low velocities would allow such species to re-adsorb on other sites and continue reacting. Therefore, the residence time/pressure of the ethylene affects how long all species remain absorbed on the surface of the catalyst. Therefore, as the pressure increases from 8.50 to 19.0 bar, C4 species have more time to react and generate higher oligomers, such as C6, C8, and C10. However, as the pressure continues to increase from 19.0 to 25.6 bar, those species may re-adsorb on the catalyst, reacting to form even larger molecules (such as coke). This is consistent with the trends in Figure 5a, where intermediate sized oligomers go through a maximum with the increase in pressure. For this reason, the selectivity to C4 species decreases from 8.50 to 19.0 bar. However, if the pressure increases further, more of the larger molecular species (C6+) re-adsorb, slightly increasing the C4 selectivity.

All the reactions were conducted at the solid-fluid interface in a packed bed reactor. This configuration contrasts with much of the previous literature on heterogeneous oligomerization of ethylene, which occurred in the liquid-phase by pressurizing the gas into a solvent, such as heptane, which held the solid catalyst. In this three-phase system, ethylene pressure was necessary to solubilize ethylene in the solvent to allow for increased contact with the entrained solid catalyst. Accordingly, the oligomerization rate in these slurry-bed studies increased


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proportionally with pressure. In packed bed reactors, some studies involving Ni-HY (Ni deposited on a faujasite zeolite HY) and nickel on silica-alumina substrates have observed an overall increase in ethylene conversion as a function of pressure[11], [23], [24]. The results presented in this section are consistent with these previous works, in which butenes were observed to be the primary product at lower pressures.



Effect of Temperature

a 50ºC 120ºC 190ºC

Selectivity, wt.%

100 90 80 70 60 50 40 30 20 10 0

C4

C5

C6

C7 C8 C9 C10 C10+ Coke Carbon Products

120 Conversion, mol.% inlet ethylene

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

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b

100

190 ºC

80 120 ºC 60 40

50 ºC

20 0 0

100

200 TOS, minutes

300

400

Figure 6. a) Steady-state selectivities of carbon products from ethylene oligomerization and b) ethylene conversion as a function of temperature. Reaction conditions, PC2H4: 19.0 bar, WHSV: 3.1 h-1


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We investigated the effect of the reaction temperature on ethylene oligomerization over Ni-Hβ by varying the temperature from 50 ºC to 190 ºC, with all other conditions held constant. As shown in Figure 6a, butenes were the major product at 50 ºC, with over 83% selectivity to C4 and roughly 6% selectivity towards C6. As the temperature increased to 120 ºC, the product distribution varied drastically, with increased selectivities towards higher oligomeric products, such as C6, C8, and C10. Interestingly, the formation of odd-numbered carbon products did not noticeably increase with an increase in temperature in the range of conditions tested, suggesting that either: 1) cracking reactions that form C5, C7 and C9 are not promoted to a high extent at higher temperature, or 2) the mentioned products react away. The coke selectivity increased with the reaction temperature. Conversion generally increases as temperature increases due to higher rates of reaction. However, as shown in Figure 6b, the effect of temperature on ethylene conversion with Ni-Hβ was small, with little to none statistically significant increase in conversion in the range 50 ºC to 190 ºC for both the transient and steady-state portions of the experiment. This suggests that the dimerization of ethylene to form butenes may not be sensitive to the reaction temperature. It is important to mention that condensation of high-molecular weight products may be a factor especially at the lower temperatures, such as 50°C. It is possible that a liquid film forms on the catalyst surface. For instance, the decene boiling point is 172°C, which implies the possibility of a liquid film at both 50 and 120°C. This liquid film likely increases mass transfer resistance for the reactants and products. The selectivity results are in agreement with previously published literature. At near room temperature, ethylene oligomerization has been previously observed to favor the formation of primarily butenes[4], [8], [25], [26]. In prior studies with other nickel/silica-alumina species



conducted in the range of 120 ºC to 300 ºC, an increase in the reaction temperature has generally been connected to the increase in the rate of acid-catalyzed reactions, namely secondary-cracking and isomerization reactions, which are responsible for the formation of both even and odd carbon products. Increasing the reaction temperature led to further oligomerization, producing longer chain oligomers, and cracking reactions that led to reactive species, which in turn combine to produce coke. The upward trend in the selectivity of coke with reaction temperature could be a direct result of the interaction of the oligomers with the acid sites of the catalyst, leading to the formation of larger oligomers that were unable to diffuse out of the catalyst.


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Selectivity, wt.%

Effect of WHSV

90 80 70 60 50 40 30 20 10 0

a -1

2.00 h-1 2.00/hr 3.10 h-1 3.10/hr 5.50 h 5.50/hr

C4

100

Conversion, mol% of inlet ethylene

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


C5

C6

C7 C8 C9 Carbon Products

C10 C10+ Coke

b

80 2.00 h-1

60

3.10 h-1 40

5.50 h-1

20 0 0

50

100

150 200 250 Time, minutes

300

350

400

Figure 7. a) Steady-state selectivities of carbon products from ethylene oligomerization and b) ethylene conversion as a function of WHSV. Reaction conditions, PC2H4: 19.0 bar, T: 120ºC

We examined the effect of the WHSV (h-1) by increasing the ethylene flow rate over a constant mass of Ni-Hβ catalyst in the bed. The selectivity and conversion data are shown in Figure 7. As



the WHSV increases, the selectivities of C4 and coke go through a minimum, whereas the selectivity of C6, C8, and C10 go through a maximum. In Figure 7b, as the WHSV increased from 2.0 h-1 to 5.5 h-1, the steady-state ethylene conversion decreased from 57% to 44%. At lower space velocities, the ethylene has more time adsorb on the catalyst surface and can more readily undergo various types of surface reactions, such as oligomerization and isomerization. An increase in the ethylene mass flow rate lowers the amount of time for ethylene to interact with active sites on the catalyst. As shown in Figure 7a, at the highest WHSV (5.50 h-1), the most prevalent product formed was the C4 product at 73 wt.% selectivity. Decreasing the WHSV to 3.10 h-1 led to the formation of heavier oligomers (C6, C8, and C10). The trend changed, however, when the WHSV decreased to 2.0 h-1, as there was an increase in the selectivity towards butenes (70 wt.%) and towards coke (14.2 wt.%). It is likely that heavier oligomers in the form of catalyst coke were formed due to the longer time for reactions at the catalyst at the lower WHSV. In addition, as the WHSV increases beyond 3.1 h-1, the lighter desorbed products will not re-adsorb and therefore cannotform long oligomers. This, in turn, prevents the C4 molecules from reacting further to C6, C8, and C10. It is also possible that the existing C6, C8, and C10 molecules react further to make additional coke at 5.5 h-1. As a result, the “coke” that is measured would be higher molecular weight molecules as these coke products are likely the result of large, bulky oligomers that were unable to thoroughly desorb from the bed. This is consistent with the reduction in microporous surface area observed in the N2 sorption isotherms of the spent catalysts. Interestingly, the conversion profiles at 3.1 h-1 and 5.5 h-1 are largely similar, yet the product distribution between the two experiments are markedly different. At 3.1 h-1, a myriad of oligomers is produced, ranging from C4 (59 wt.%) to C10 (7.3 wt.%), with coke accounting for 8.7 wt.% of the product distribution. However, at 5.5 h-1, C4s are the major


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product, with a selectivity of 70.2 wt.% and coke at 12.3 wt.%. Apparently, changing the WHSV from 3.10 to 5.50 h-1 does not significantly affect the transport rate of ethylene from the bulk of the fluid to the catalyst surface., However, it does affect the conversion of butene into higher oligomers. This could indicate that the mass transfer step is not the limiting factor under these conditions. In order to confirm our hypothesis, we performed calculations based on the conversion results and the particle characteristics to confirm the presence of external and internal mass transfer limitations. The Weisz-Prater criterion[27] establishes that internal diffusion limitations are significant if the parameter Cwp is much larger than unity, where Cwp is defined as: &! =

%; ? @A B;C

(4)

In which rA is the rate of reaction (kmol/kg cat.s), ρC is the density of the catalyst (kg/m3), R is the catalyst particle radius (m), De is the effective diffusivity (m2/s), and CAS is the concentration of ethylene at the surface of the catalyst. Here, we approximated the density of the catalyst by the bulk density, and assumed the tortuosity of the particles to be equal to one. External diffusion limitations can be evaluated by the Mear’s criterion[27], which indicate that external diffusions can be neglected if: %; EF B;G

Ethylene Oligomerization over Ni-HÎ² Heterogeneous Catalysts

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