Effects of the Preparation Variables on the Synthesis of Nanocatalyst

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Influence of the preparation variables on the synthesis of nanocatalyst for in-situ upgrading applications Erika Maria Scheele-Ferreira, Carlos E. Scott, Maria Josefina Perez-Zurita, and Pedro Rafael Pereira-Almao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01107 • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Industrial & Engineering Chemistry Research

Influence of the preparation variables on the synthesis of nanocatalyst for in-situ upgrading applications Erika Scheele-Ferreira1*, Carlos E. Scott1, M. Josefina Perez-Zurita1 and Pedro Pereira-Almao1 *[email protected]

1. Schulich School of Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada. Abstract In-situ upgrading technology (ISUT) is a patented process based on the use of hydroprocessing with ultra dispersed catalyst (UDC) for in reservoir upgrading of heavy oil and bitumen to reach transportability specifications. UDC properties play a crucial role on the performance of the process. The focus of this work was to study the influence of different operational variables during the catalyst synthesis (such as type of mixer, stirring speed, sulfiding agent and catalyst formulation) on the particle size in a customize experimental set up built for this purpose. Additionally, the effect of the sulfiding agents on the composition of the active phases present in the catalyst surface was investigated. It was possible to synthesize catalyst with nanometric dimensions, and the variables with significant influence on the particle size were the type of mixer and sulfiding agent. Nanometric scale was reached using ammonium sulfide and high shear mixing. Moreover, advantages on the use of thiourea as sulfur source during the catalyst sulfiding stage were observed. Finally, NiMoS phase was observed for preparations with both (ammonium sulfide and thiourea) sulfiding agents.

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1

Introduction

In the last decade, the exploitation and consumption of heavy and extra-heavy hydrocarbons has been progressively increasing, positioning them as a crucial factor within the global hydrocarbon market nowadays, especially in countries like Canada or Venezuela where there are enormous reserves of this kind of crude oils.1 The development of appropriate technologies for handling and processing heavy oils has been a key factor to make access to these hydrocarbon reserves viable, not only technically, but also economically. In particular, Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology that has made bitumen production possible, however, it is energy and carbon intensive and uses large quantities of fresh water. These characteristics of SAGD have triggered increasing environmental concerns which, along with the certain implementation of a carbon taxes, makes it imperative to develop new oil extraction technologies that consumes less energy and water. In order to seek possible solutions, the catalysis for fuels and energy research group at the University of Calgary, Canada, has been performing during the last decade high level investigations in the development of innovative technologies for hydroprocessing of heavy oils based on the use of ultradispersed nanocatalysts with applications for surface processes and in-situ upgrading (upgrading within the reservoir). In situ upgrading of heavy oil (ISUT) is a novel technology developed and patented.2,3 It is a pioneer technology in the incorporation of catalytic functions in a thermal process of oil extraction using ultradispersed nanocatalysts. ISUT integrates hot fluid injection (such as vacuum gasoil or vacuum residue) and in reservoir catalytic upgrading of heavy oil. In this technology, the heaviest fraction of the oil is separated at the surface and is used as carrier for the incorporation of the ultradispersed nanocatalyst into the reservoir, along with any other hot fluid and hydrogen. The nanocatalyst is contained in the reservoir and remains active for months to a few years. Hydrogen and catalyst promote the hydroprocessing reactions and inhibit coke formation, representing important advantages over the use of merely thermal processing. In order to properly implement the in-situ upgrading process, it is necessary to guarantee an adequate nanoparticles dispersion along the reservoir porous matrix. Bell, J. et. al.4


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analyzed sand samples impregnated with bitumen of the Athabasca oil sands region to determine their pore size distribution. The results using a 1.3μm resolution showed that interparticle pores diameters ranged from 15 to 41μm. Thus, it is important to ensure a particle diameter within nanometric range when using ISUT to allow the nanoparticles to mobilize and penetrate the porous media.5,6 Moreover, the catalyst available surface area per mass unit is substantially increased when working at this scale which enhances the catalytic activity of the particles. Catalytic down-hole upgrading has been under investigation since the late 1990’s,7-9 however, the proper introduction of the catalyst into the reservoir has been a major issue for any commercial application. Dispersion of catalysts in a hydrocarbon feed (ultradispersed catalyst) is one of the most promising ways of introducing the catalyst inside the reservoir. Nonetheless, it is important to highlight that even though the advantages of using ultradispersed nanocatalysts are notorious, its continuous incorporation in oil at nanometric scale as needed for ISUT has presented challenges for the implementation of this technology. Three principal methods are known for the preparation of ultradispersed catalyst in oil: a) dispersion of fine powder in the hydrocarbon media,10 b) water in oil emulsion where oil soluble salts of the precursors are dissolved in the aqueous phase, and c) solubilisation of lipophilic metal compounds in the feed.11 A method for the continuous flow preparation of dispersions of nanoparticles in oil has also been developed 12. The procedure implies the decomposition of “transient emulsion” (TE), which are those sustained by the energy supplied to form them (no surfactant added), but when the energy source is cut off, the aqueous droplets coalesce and the oil and water phases separate. Once the TE is obtained it is rapidly directed to a thermal decomposition chamber where the water flashes out and the solid particles are formed suspended in the oil. The diameter of the ultradispersed particle is a function of: the original size of the aqueous solutions droplets and their disintegration caused by micro explosions produced by the intense evaporations of the water.13 In this method, soluble salts of the catalytic metals (such as nickel, tungsten, molybdenum, etc.) are dissolved in the aqueous phase and used as precursors. These solutions are incorporated to the hydrocarbon media through a high efficiency mixing system as high shear mixers or static mixers connected in series. The objective is to



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generate enough mixing energy to form an average droplet diameter of submicron dimensions, preferably nanometric in size. Later, the emulsion is submitted to the salts decomposition temperatures usually above 300 ºC depending on the type of salt and its ease to decompose towards oxides or metal sulfides. In this manner, active metallic species performing catalytic functions are obtained. The time lapse between the formation of the emulsion and its decomposition should be as short as possible to avoid droplets coalescence when surfactants are not used. The final suspension of particles in the heavy hydrocarbon is what is used in ISUT to incorporate the catalysts in the reservoir11, and a unit to produce the catalytic suspension for the commercial process has been designed and patented.14 Vasquez15 carried out catalytic preparations via water-in-oil emulsions, where the emulsions were first prepared (using surfactant) and then injected to a decomposition unit in which the metal reacts to form oxides and sulfides. In his work, a high shear mixer was used and Ni-W and Co-Mo mixtures were prepare aiming to establish the best formulation in terms of activity for thiophene hydrodesulfurization (HDS) and toluene hydrogenation (HYD). When comparing with supported catalysts, ultradispersed catalyst showed a higher activity even though it was not possible to synthesize particles with nanometric dimensions in order to maximize their performance. Thompson et. al.16, using a similar preparation procedure, studied the influence of the decomposition temperature on the particle size. It was found that as temperature increases so does the particle size, possibly due to the increase on the kinetic energy leading to higher probability of particles collisions that triggers particles coalescence. Contreras et. al.12, using TE and a lubricant base oil, assessed the influence of water percentage in emulsions and stirring speed on the particle size for ultradispersed catalyst preparations composed by Mo or Ni in a base oil media obtaining particles of approximately 400 nm for all the tested stirring speeds. However, the primary particle size observed by Transmission Electron Microscopy (TEM) was around 35 nm, indicating that sizes obtained by Dynamic Light Scattering (DLS) is the result of agglomeration of smaller particles. Later on, Galarraga and Pereira-Almao17 tested the synthesis of ultradispersed catalysts through direct decomposition of water-in-bitumen emulsions formulated with Ni, Mo, W and surfactants where submicron catalytic particles were successfully produce by decomposing water-in-bitumen emulsions at moderate temperatures.


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Even though several of these works assessed some preparation variables that can have significant impact on important properties such as particle dimension of the catalyst, the size of the particles as produced, suspended in the heavy hydrocarbon, were not determined due to the lack of an appropriate technique for this type of suspensions. In those work the particle size determinations were done by DLS using lighter and clearer oil or by Electron Microscopy of samples that had been previously calcined in order to remove the bitumen or heavy hydrocarbon, which could cause changes in the sizes of the particles. In the present work the diameters of the particles obtained, suspended in Athabasca Bitumen, were determined using nanoparticle tracking analysis (NTA) as previously reported.18 The effect of different preparation variables, like type of mixer, particle composition and addition of sulfiding agents, on the average diameters and surface composition of the particles obtained was studied. These properties are of paramount importance for an appropriate performance of the catalytic functions. 2 2.1

Experimental Methods

Ultradispersed Nanocatalyst Preparation

In the first step of the preparation, bitumen from the Athabasca region in Alberta, Canada was diluted using 10% wt. of toluene (99.8% Aldrich) to reduce its viscosity and facilitate its pumping at room conditions. Pure bitumen properties are presented in Table 1. Next, aqueous solutions of metal precursor salts were prepared in such a way that the total amount of water incorporated would produce a 2% water-in-oil emulsion. Four metals (Mo, Ni, W and Fe) were used in the different preparations and the amount of metallic salts added was such that the concentrations of metals in the obtained suspension would be as shown in Table 2. The metallic salts used during the preparation, depending on the metal to incorporate were: nickel acetate tetrahydrate (98% Aldrich), ammonium metatungstate hydrate (99% Aldrich), ammonium molybdate tetrahydrate (99% Aldrich) and iron acetate (II) (99% Aldrich). Ammonium sulfide (Aldrich) in 40-44%wt aqueous solution and thiourea (99% Aldrich) were used as sulfiding agents, always ensuring that an excess of the stoichiometric sulfur required to form the corresponding metal sulfides was used in the preparation. In



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this way, the formation of the metal sulfides needed to promote the hydroprocessing catalytic effect is maximized. Since nickel and iron salts tend to precipitate either when contacting ammonium sulfide (AmS) or at high concentrations of molybdenum and/or tungsten salts, two separate solutions were prepared for the bimetallic and trimetallic formulations in order to avoid precipitation issues. The first solution was composed by nickel or iron salts and the second one by molybdenum and/or tungsten salts depending on the targeted catalyst composition. For the preparation using thiourea (Thio) as sulfiding agent, a portion of it was separately added in each solution (the one with Ni or Fe and the one with Mo and/or W), while for the preparations with ammonium sulfide, the whole amount was added in a single step to the solutions composed by molybdenum and/or tungsten salts. For the preparation using ammonium hydroxide (AmOH), this compound was added in form of a 28 %wt. aqueous solution (Aldrich), which was mixed together with the thiourea and incorporated to the corresponding precursor solution (only Mo for this case). The calculation of the amount of AmOH added to the preparation using Thio was based on the concentration of ammonium in the AmS sample, in order to evaluate the effect of the ammonium in the particle size. The measured pH of the solution with AmOH was between 7 and 8. The influence of the following variables on the particle size and surface composition of the UDC was studied: sulfiding agent (AmS or Thio), metal (Mo, Ni, Fe and W), addition of ammonium hydroxide for activation of natural surfactants present in the bitumen, type of mixer (static and high shear mixer) and stirring speed (6500 and 24000 rpm) for the high shear mixer. 2.2

Experimental setup

The experimental setup used for this work is equipped with a High Shear Mixer (HSM) by IKA model Ultraturrax T25, a stainless steel mixing tank with Swagelok NPT type connections, a high temperature metering gear pump Zenith H 9000, a tubular decomposition reactor of 3.8 cc with a temperature sensing point for monitoring purposes, a Swagelok 75 cc separator with a counter current nitrogen bubbling injection and finally a liquid trap submerged in a water bath. For the heating of the system, a six channels Omega


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temperature controller coupled with heating tapes was used. A diagram of the experimental setup is presented in Figure 1. For the setup operation, the first step was to charge the mixing vessel with approximately 110 g of bitumen diluted with 10 %v/v of toluene. Then, the prepared metal solutions were added. For the bimetallic and trimetallic emulsions, it was necessary to add first the nickel or iron solutions and then the molybdenum and/or tungsten solutions, having a certain period of mixing in between to assure that the emulsion with the first solution was already formed before the second solution was added, also, to avoid the precipitation of nickel or iron sulfides or molybdates if both solutions were in contact before the mixing stage. High shear mixing was immediately initiated at 6500 rpm or 24000 rpm stirring speeds. Once the metering gear pump was started, the recirculation of the mixture was initiated for a period of 10 min or 20 min in order to enhance the homogeneity of the mixture. Since the minimum flow rate of the pump was out of range for appropriate residence time in the decomposition reactor, it was necessary to install a set of valves to regulate the flow rate going through the reactor and achieve in this way the required residence time of 1 to 2 min. Once the recirculation time was completed, the emulsified mixture was passed through the decomposition reactor and subsequently to a purge line until reaching a constant temperature of 300 °C inside the reactor. Once the temperature requirement was fulfilled, the stream was sent to a hot separator operated at 150 °C where water and toluene were separated. Gases from the hot separator composed by water, toluene and the gases generated in the decomposition were sent to a lights trap submerged in a water bath in order to condense and recover toluene and water. The gas stream exiting the lights trap was passed through a sodium hydroxide trap to neutralize any hydrogen sulfide formed during the decomposition of the emulsion and the remaining gases were vented. In order to evaluate the variation of particle size with the type of mixer, the original set up was modified. The high shear mixer was replaced by one or two 3/16" diameter and 21 mixing elements static mixers supplied by Koflo Corporation. The first step in the operation was to add 110 g of feed to the mixing tank. Then, the precursors solutions were added at a flow rate of 0.21 cc/min using a graduated burette and the pump was immediately initiated allowing a recirculation time of 20 minutes. The subsequent procedure was the same as the original setup previously explained.



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2.3

Catalyst characterization

2.3.1 Nanoparticle Tracking Analysis (NTA) The determination of the nanocatalyst particle size was carried out using a NanoSight NS3000 instrument provided by Malvern Instruments Ltd., and the nanoparticle tracking analysis (NTA) methodology developed by Rodriguez-DeVecchis, et. al. 18. This technique is based on a combination of measurements of particles individually and as bulk. NTA starts with the identification of the suspended particles visualized within the viewing range of the equipment. This step is performed by the detection of the scattered light, product of the irradiation of particles with a laser, viewed through a charged couple device camera. Next, the equipment follows the movement of each particle and calculates the particle size using a relationship between the distance traveled by each particle in a certain period of time. In this way, NTA technique makes an individual particle size measurement and repeats the procedure for all the visualized particles. In this work, the particle size reported corresponds to the mode of the measurements. This technique allows the measurement of particles as small as 9-15 nm of diameter as long as the particles scatter enough light to produce a signal. Dilution with toluene is the only pretreatment needed in order to obtain an appropriate particle concentration for the NTA. 2.3.2 Inductively Coupled Plasma (ICP) ICP was selected for the determination of the nanocatalyst concentration in the bitumen samples following the ASTM D7691-05 norm using a Thermoelemental IRIS Intrepid II spectrometer. Samples were prepared for analysis using microwave-assisted digestion following the ASTM D7455 norm. First, 0.25 to 0.3 g of sample were added to a Teflon digestion cell along with 10.5 mL of a mixture of nitric acid (70% Aldrich) and 1 ml of 85% wt. phosphoric acid solution. The latter is used to enhance the dissolution of tungsten contained in the hydrocarbon matrix. Then, the Teflon cells were placed in a microwave digester CEM model MARS 6 and were submitted to a heating ramp of 10 °C/min until reaching a temperature of 210 °C which was held for 20 min. After cooling down, the content of the cell was transferred to a 25 ml volumetric flask that was filled with deionized water. Sample was then ready for ICP analysis.


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2.3.3 X-ray Photoelectron Spectroscopy (XPS) XPS was used for the determination of the composition of the metal species present in the surface of the nanocatalyst. Since catalyst particles are dispersed in the bitumen, an isolation stage was required in order to recover a given amount of particles that would allow performing the XPS analysis. This was accomplished by doing a series of toluene dilutions and centrifugations as follows: 20 g of bitumen with UDC and 40 ml of toluene were put in a Teflon test tube which was centrifuged for 30 min with a rotation speed of 8000 rpm in an Eppendorf Centrifuge 5804. After the first centrifugation, the supernatant liquid was carefully removed always ensuring that the solid particles would remain in the bottom of the tube for further redilution with toluene. This process was repeated several times until visually the organic matrix was completely removed. Finally, the solid particles were dried at room temperature for 24 h and in a vacuum oven at 80 °C overnight. Even though every effort was made to remove the entire hydrocarbon present in the solids, some may have remained after the aforementioned procedure. The next step was to perform the XPS analysis. It was carried out with a PHI VersaProbe 5000 spectrometer. Approximately 5 mg of nanocatalyst sample were placed in the sample holder that was subsequently introduced into the ultra vacuum chamber of the spectrometer operating at 10-8 Pa. Sputtering was done to remove the first layer of the sample and minimize the error due to possible oxidation during sampling manipulation. A broad sample survey was performed to identify all elements present in the sample. Then, a narrower energy range (10-30 eV) was carried out to determine the chemical environment of the present elements. XPS spectra were obtained and analyzed through deconvolution techniques to separate and assign the signals corresponding to the chemical forms of the elements of interest present in the solid catalyst sample. 3 3.1

Results and Discussion

Metal content

Due to the importance of knowing the quantity of catalyst incorporated to the oil, the metal content was measured for all the preparations. Thus, metal contents obtained for the monometallic (Mo), bimetallic and trimetallic samples are presented in Table 3 and Table 4 respectively. It can be appreciated that for all samples prepared with high shear mixing



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(HSM) the experimental concentration of Mo, Ni, W and Fe was close to the expected quantity. The experimental error was less than or equal to 15% for those preparations. For the preparations with the static mixers, the experimental concentration of Mo was above the expected values (more than 15% error), possibly caused by problems with the precursor solution dosage with the burette used. Based on the obtained results, it can be affirmed that the incorporation of the metallic precursor solutions was correct for the majority of the evaluated cases. Furthermore, most of the experimental values are close to the expected ones indicating the reliability of the setup. 3.2 3.2.1

Particle size Effect of sulfiding agent

The sulfiding agent added to the catalyst preparations can vary depending on the application and the composition of the active phase needed. Commonly, ammonium sulfide has been used in the nanocatalyst preparation, but for this work the use of thiourea was also explored. Since it comes in a solid presentation, it is simpler and practical to handle. Additionally, it does not stink and its manipulation is safer due to lower risk of forming hydrogen sulfide during its processing. Figure 2 shows a bar graphic that compares the particle size for monometallic (Mo) and trimetallic (Mo, Ni and W) formulations prepared using high shear mixing at 6500 rpm for the two sulfiding agents: AmS and Thio. In both metallic formulations the catalysts prepared with ammonium sulfide presented lower particle size than the ones prepared with thiourea. For the monometallic catalysts, the difference on the particle size obtained using AmS or Thio was approximately 100 nm, whereas for the trimetallics the difference was approximately 50 nm. This is possibly related to the fact that the recirculation time for the trimetallic formulation was 20 minutes due to the consecutive addition of two different solutions with stirring times of 10 minutes each. However, the available data does not allow to affirm that increasing mixing time can significantly reduce the particle size. A possible explanation for the lower particle size obtained with ammonium sulfide is that in aqueous solution, it generates ammonium hydroxide and hydrogen sulfide. While


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the hydrogen sulfide reacts with the metal in solution to form the respective metal sulfides, ammonia, which is a basic compound, can react with the naphthenic acids of the oil forming naphthenic salts that are known to have natural surfactants properties.19 This can help to produce and maintain a stable microemulsion. To evaluate this hypothesis, preparations with thiourea and ammonium hydroxide were performed in order to add an ammonium source in a simple way (solution form). Figure 2 shows that, in fact, when comparing the obtained results with both monometallic formulations prepared with thiourea, the one with ammonium hydroxide generated a catalyst with a particle size 50 nm lower than the one that contains only thiourea. However, it does not reach to nanometric dimensions required as in the case of the preparation with ammonium sulfide. Therefore, it is possible that the presence of ammonia has a positive effect on the reduction of the particle size, but some other factors may be also paying a role in the final size of the particles when thiourea (instead of ammonium sulfide) is used in the preparations. Nevertheless, ammonium hydroxide is stronger as a base compared with ammonium sulfide due to the presence of the hydroxyl ion, and then it should be a more effective activator of the natural surfactants than the ammonium sulfide. 3.2.2

Effect of promoting metal

Since nanocatalyst can be manufactured with mixtures of different metals, it is important to verify that the particle size does not change when using different promoting agents. In Figure 3, the particle size obtained for the preparations using high shear mixer at 6500 rpm and ammonium sulfide as sulfiding agent is compared for different promoting metals. It is observed that the promoting metal only has a little effect on the particle size because for all the monometallic, bimetallic and trimetallic preparations with the different metallic salts evaluated the particle size was always in the nanometric scale (84-100 nm). 3.2.3

Effect of type of mixer

When comparing the particle size obtained using high shear mixer vs. static mixers, as presented in Figure 4, it is observed that in the first case the particles are about 50 nm smaller than the catalyst prepared using static mixers. This result corroborates the better efficiency of the high shear mixing.



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As shown in Figure 4, the preparations using one or two static mixers produced similar results (150 nm), higher than expected target (

Effects of the Preparation Variables on the Synthesis of Nanocatalyst

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