Hydroreforming of the LDPE Thermal Cracking Oil ... - ACS Publications

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Hydroreforming of the LDPE Thermal Cracking Oil over Hierarchical Ni/Beta Catalysts with Different Ni Particle Size Distributions José M. Escola,*,† David P. Serrano,†,‡ José Aguado,† and Laura Briones† †

Group of Chemical and Environmental Engineering, ESCET, Universidad Rey Juan Carlos, Tulipán s/n, 28933 Móstoles, Madrid, Spain ‡ IMDEA Energía, Ramón de la Sagra 3, 28935 Móstoles, Madrid, Spain ABSTRACT: A number of catalysts made of Ni particles of different size distributions supported on a hierarchical beta zeolite (Ni/h-beta) were prepared and tested in the hydroreforming of low-density polyethylene (LDPE) thermal cracking oils. The materials were prepared by identical impregnation method but using different nickel precursors: chloride, nitrate, acetylacetonate, and tris(ethylenediamine) nickel(II) chloride (TEDAN). A variation in the Ni particles size distribution was obtained depending on the metal precursor, whereas no significant differences in the acidic properties were detected. Ni nitrate and acetylacetonate resulted in smaller and more dispersed particles, thus closer to the acid sites, driving to higher hydrogenation and hydrocracking activities. In contrast, Ni TEDAN and chloride led toward larger nickel particles which prevented intermediate species to be rapidly hydrogenated and so favored the production of aromatic and branched hydrocarbons. Hence, the catalyst coming from Ni nitrate produced 25 wt % iso-paraffins and 71% light products (C1−C12), while the material prepared with Ni chloride produced 35% iso-paraffins and 60% light products.

1. INTRODUCTION Plastic wastes generation is an issue of main concern in western societies because of the continuously increasing amount of these wastes produced every year. Nowadays, a huge amount of plastic wastes is still landfilled (about 38% in Europe in 2013).1 In order to reduce this rate, novel processes, such as chemical treatments, have been investigated in the last years.2−4 Fuels and chemical production by chemical treatments is one of the currently most interesting routes because of the added value of the products.5−9 A two-stage process combining thermal cracking of LDPE followed by hydroreforming over bifunctional catalysts of the obtained liquids was proposed as an effective way to produce gasoline and diesel-like mixtures, useful for the formulation of high-quality automotive fuels.10−12 Catalytic processes under inert atmosphere usually lead to high amounts of olefins that can cause gums formation in the injection systems of automobiles.13,14 Hence, catalytic processes in the presence of hydrogen are required in order to saturate the olefins and to enhance the properties as fuels of the obtained hydrocarbon mixtures. Herein, the term hydroreforming is used to define the catalytic process under hydrogen atmosphere which converts highly olefinic and lineal hydrocarbon mixtures into high-quality fuels by means of different reactions: saturation of CC bonds (hydrogenation); cleavage of C−C bonds (hydrocracking); production of branched hydrocarbons (hydroisomerization); and aromatic compounds (aromatization). Hydroreforming requires a bifunctional catalyst that combines a hydro/dehydrogenating function, usually provided by metal particles, and an acidic function coming from a porous acid solid. This kind of procedure can also be applied to other feedstocks, such as fatty acids or vegetable oils, to produce high-quality diesel fuels.15 Different metal/support combinations have been investigated in hydroreforming reactions.16−19 It is generally accepted that a proper balance between the two components is © XXXX American Chemical Society

required. Thus, an adequate dispersion of the metallic phase, along with a high surface area support and a convenient interaction between the metal and acid sites, are critical parameters affecting the overall activity and selectivity of the catalysts. Impregnation is the most extensively studied method used to incorporate the metal on the support due to its simplicity. One way to tune the metal particles size on the support is an appropriate choice of the metal precursor. Although inorganic salts with high solubilities in water, such as nitrates or chlorides, are usually employed, the resulting metal particles are often larger than desired. Metal chelates were soon employed with the aim of reducing the metal particle size. Hence, Pt-EDTA chelates resulted in platinum particles smaller than 5 nm on Al2O3.20 The purpose of the chelate is multiple: on one hand, it increases significantly the viscosity of the solution, preventing its movement out during the drying stage;21 on the other hand, it promotes stronger support-metal-counterion interactions.22 More recently, other ligands, such as multiamino22−26 and citrate21,26−29 complexes were used yielding smaller and more dispersed metal particles than the widely used nitrates and chlorides. Other chelates such as acetates and acetylacetonates also led to smaller and more dispersed metal particles than inorganic salts.25,30−32 Nevertheless, inorganic salts are still usually preferred because their high solubility in water and lower cost. Among the different inorganic precursors, such as nitrate, chloride, or sulfate, nitrate usually leads to the smallest particles and chloride to the largest ones.33 Received: March 28, 2015 Revised: June 4, 2015 Accepted: June 14, 2015

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DOI: 10.1021/acs.iecr.5b01160 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

solid products are separated by centrifugation, rinsed with distilled water, dried overnight at 110 °C and calcined in stagnant air at 550 °C for 5 h. The obtained material shows a bimodal micro/mesoporous structure due to the voids generated by the removal of both SDA and SSA during the calcination. 2.2. Nickel Incorporation and Activation. Nickel has been incorporated on the support by impregnation using nickel nitrate hexahydrate (Aldrich, > 98.5%), nickel(II) acetylacetonate (Aldrich, 95%), tris(ethylenediamine) nickel(II) chloride hydrate (TEDAN, Aldrich, 99.999%) or nickel chloride (Aldrich, 98%) as nickel precursors. Impregnations have been carried out using a solution volume to pore volume ratio of 2. The solvent was distilled water, and the concentration of these solutions was modified so the amount of Ni in the final catalysts was 7 wt %. Exceptionally, due to the low solubility of acetylacetonate in water, chloroform was used as the solvent. In this case, a solution volume to pore volume ratio of 10 was needed to reach a significant metal incorporation. In spite of these modifications, only a 4 wt % of Ni was achieved in this sample. Before impregnation, the solid support was outgassed in a rotavapor under vacuum. Then, the samples were impregnated and subjected again to rotation and vacuum for mixing and drying. The dry materials were calcined under static air at 550 °C for 5 h using a heating rate of 20 °C min−1. Subsequently, the catalysts were activated by hydrogen reduction (30 N mL min−1) in a Micromeritics Microactivity Pro fixed bed reactor with a heating rate of 2 °C min−1 up to 550 °C. Then, the reactor was cooled down to room temperature by passing N2 flow. 2.3. Catalysts Characterization. The calcined catalysts were subjected to powder X-ray diffraction (XRD) in a Phillips X’PERT MPD diffractometer using Cu Kα radiation. XRD patterns within the 5−70° range were recorded using a step size and a counting time of 0.1° and 10 s, respectively. The reflections corresponding to NiO have been used to calculate the mean NiO particle size of each sample according to the Scherrer equation. The silicon, aluminum, and nickel contents of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a VARIAN Vista AX Axial CCD Simultaneous ICP-AES apparatus. Previously, the samples were digested by acid treatment with H2SO4 and HF. The reducibility of NiO for each sample was inferred from H2TPR experiments carried out in a Micromeritics AutoChem 2910 system. Initially, the samples were outgassed under Ar flow (35 N mL min−1) heating up to 80 °C with a rate of 15 °C min−1 and holding for 30 min. The analyses were performed under a 35 N mL min−1 flow of 10% H2 in Ar and heating up to 800 °C with a rate of 5 °C min−1, holding this temperature for 10 min. Prior to reaching the detector, the effluent gases were cooled using a liquid nitrogen/isopropanol mixture at −80 °C. After finishing the tests, the samples were cooled down to room temperature under Ar flow. The general aspect of the catalysts, as well as the shape, size, and distribution of the Ni particles were derived from the transmission electron micrographs (TEM) collected on a Phillips TECNAI 20 microscope equipped with a LaB6 filament under an accelerating voltage of 200 kV. Prior to the observation, the samples were dispersed in acetone, sonicated and deposited on a carbon-coated copper grid. Nitrogen adsorption−desorption isotherms at 77 K were obtained in a Micromeritics Tristar 3000 device, whereas Argon adsorption−desorption isotherms at 87 K were acquired using a Quantachrome Autosorb 1 MP automated gas sorption system. BET surface area analyses were determined using the relative

Although some authors have reported that smaller metal particles enhance hydro/dehydrogenation activities,34−37 other works have proven different results. Some reactions are only sizesensitive when the metal particle size is very small. For example, Molina and Poncelet32 determined that benzene hydrogenation over Ni/Al2O3 catalysts was size-sensitive to Ni sizes below 4 nm and insensitive to sizes above it. Nazimek and Ryczkowski also reported that hydrogenolysis and isomerization of n-butane38 or ethane and propane39 over Pt/Al2O3 varied with Pt particles size up to 5 and 3 nm, respectively, being constant for a range of larger sizes. For some catalysts, a drop in activity when decreasing the metal particles size is observed, which appears to be due to a very strong metal−support interaction and the formation of the corresponding aluminates or silicates.30 Finally, it is also possible that, even if the smallest particles are indeed more active, this effect becomes overcompensated when the total amount of metal in the catalysts is high enough.25 Hence, the effect of the metal particles size must be evaluated for each reaction and each catalyst. The aim of this work is to determine if different nickel precursors, showing varying molecular sizes and diverse interactions with the support, can lead to differently sized Ni particles upon the impregnation of a hierarchical beta zeolite (hbeta). A hierarchical zeolite was chosen because of its bimodal micro/mesoporous structure, which is expected to enhance the accessibility to the active sites, as well as the dispersion of the metal phase compared to a traditional microporous zeolite. Ni supported on h-beta has shown a remarkable activity in the hydroreforming of the oil coming from LDPE thermal cracking, reaching more than 80 wt % joint percentage of automotive fuels (gasoline + diesel), with a very low content of olefins (bromine index of just 2.5 g Br2/100 g sample).10−12 In this work, it is intended to check if smaller Ni particles originate higher activities in any of the reactions (hydrogenation, hydrocracking, hydroisomerization, and aromatization) involved in the hydroreforming of LDPE thermal cracking oils. Thus, four different nickel precursors (nitrate, acetylacetonate, chloride, and tris(ethylenediamine)nickel chloride) were used to impregnate the h-beta support, and the obtained Ni/h-beta catalysts were tested in the hydroreforming of the mentioned feed.

2. EXPERIMENTAL SECTION 2.1. Hierarchical Beta synthesis. Hierarchical beta zeolite (h-beta) was prepared according to a protozeolitic-unitssilanization method published elsewhere.40 The procedure comprises three stages: precrystallization, silanization, and crystallization. Initially, a solution containing aluminum (aluminum flakes, Aldrich, 99.9%) and silicon (fumed silica, Degussa) precursors, tetraethylammonium hydroxide (TEAOH, Alfa, 35% aqueous solution), as structure directing agent, (SDA) and distilled water (molar composition: Al2O3:60 SiO2:15.5 TEAOH:1000 H2O) is aged under stirring at room temperature for 20 h and precrystallized in Teflon-lined steel autoclave reactors at 135 °C for 3 days in order to obtain a gel containing protozeolitic nanounits. Subsequently, 8% mol (with regard to the silicon content) of a seed silanization agent (SSA) is added to the synthesis medium and left reacting at 90 °C for 6 h under reflux. The SSA employed has been phenylaminopropyltrimethoxysilane [PHAPTMS, (C6H5)NH(CH2)3Si(OCH3)3, Aldrich, > 97%]. The SSA reacts with the surface hydroxyls forming an organic moiety that partially prevents the nanounits to aggregate into bigger crystals. Finally, the samples are crystallized in autoclave reactors at 135 °C for 7 days. The B


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methyl silicone capillary column. From GC results, the product distributions by carbon atom number were obtained and then lumped into four groups: gases (C1−C4), gasoline (C5−C12), light diesel (C13−C18), and heavy diesel (C19−C40). No heavier products than C40 were ever detected.

pressure range of 0.05−0.17 in the nitrogen adsorption isotherm as a range of linearity. Pore sizes distributions were obtained from Ar adsorption isotherms by applying the nonlocal density functional theory (NLDFT), assuming Ar adsorption in cylindrical pores. The acid properties of the catalysts were determined by ammonia temperature-programmed desorption (NH3-TPD) in the Micromeritics AutoChem 2910 system using He as carrier gas. Previously, the samples were outgassed by heating under helium flow up to 550 °C and keeping this temperature for 30 min. After cooling to 180 °C, an ammonia flow was passed through the sample for 30 min and, subsequently, the physisorbed ammonia was removed by flowing helium at 180 °C for 90 min. The chemisorbed ammonia was determined by increasing the temperature with a heating rate of 15 °C min−1 up to 550 °C and keeping constant this temperature for 30 min. The ammonia concentration in the effluent helium stream was monitored with a thermal conductivity detector (TCD). 2.4. Hydroreforming Tests. Reactions were carried out in a stirred stainless-steel autoclave reactor provided with gases inlet and outlet, ceramic oven, and electronic controller. Initially, virgin LDPE pellets (MW = 416000, REPSOL) were subjected to thermal cracking at 400 °C and 90 min under 1.5 bar of nitrogen atmosphere. The obtained gases were vented out through an ice-cooled trap. The obtained liquid products or oils were collected and used as feed for the hydroreforming reactions. In order to get homogeneous feed for all the catalytic tests, thermal cracking was performed several times, the oils mixed, and then characterized as one. These oils are a mixture of hydrocarbons within the C4−C40 range, the main components being n-paraffins and 1-olefins. To avoid oxidation and oligomerization of the feed components, the oils were stored below −20 °C until needed for reaction. The hydroreforming tests were performed in the same reactor. The oils and the corresponding amount of catalyst, added as powder, oil to catalysts mass ratio of 30, were loaded into the reactor vessel, and hydrogen was cold charged up to 20 bar. The hydroreforming reactions were performed at 310 °C for 45 min under stirring (700 rpm). After cooling down to room temperature, gases were vented out and collected in a gases bag. The liquid products were filtered under vacuum to separate and recover the solid catalyst. 2.5. Analysis of the Hydroreforming Products. The liquid products were subjected to HPLC analyses in a HP 1100 device with a ZorbaxNH2 packed column of 25 cm length and 4.6 mm × 5 μm internal diameter in order to calculate the total aromatics content according to the ISO 12916:2006 standard. The olefin content was determined by calculating the bromine index according to the ASTM D2710 standard with a Methrom 836 apparatus after diluting the samples in an appropriate solvent. Gaseous and liquid products were analyzed by gas chromatography. In order to perform accurate analyses, the liquid products were previously separated by distillation into a light fraction (C5−C16) and a heavy fraction (C10−C40). The gases and the light liquid fraction were analyzed in a Varian 3800 GC apparatus using a 100 m length × 0.25 mm i.d. Chrompack capillary column. PIONA (paraffins, iso-paraffins, olefins, naphtenes, aromatics) analyses were obtained from the Varian Star Detailed Hydrocarbon Analysis 5.0 software (Varian DHA) and used to determine the iso-paraffin content in the light fraction. The heavy fraction was dissolved in carbon disulfide and analyzed in a Varian 3900 GC equipped with a 25 m length

3. RESULTS AND DISCUSSION 3.1. Characterization of the Ni/h-beta Catalyst Prepared with Different Ni Precursors. X-ray diffraction was used to bear out the average size of the NiO particles in the calcined catalysts. Figure 1 illustrates the XRD patterns of these

Figure 1. XRD patterns of the NiO/h-beta catalysts impregnated with different nickel precursors. * NiO reflections.

materials. In the calcined samples, Ni is in the form of NiO which crystallizes in the cubic system. Its main reflections, placed at 2θ ∼ 37.2, 43.2, and 62.8°, are marked with asterisks. The most intense one, at 2θ ∼ 43.2°, was used to calculate the mean particles sizes according to the Scherrer equation. The obtained values are included in Figure 1, where significant differences can be observed. TEDAN led to the largest NiO particles size (240 nm). This result might be ascribed to the bulky nature of the TEDAN complex, which is not likely to be accessible to zeolite micropores. Thus, after impregnation, it is externally located on the support and the subsequent calcination treatment enhances its aggregation. Very large NiO particles were also obtained when using NiCl2, about 180 nm large. In this case, the most likely cause of this size is not the bulky nature of this compound but its weak interaction with the h-beta support, which eases the aggregation of the metal particles during the calcination treatment. On the other hand, acetylacetonate and nitrate are accessible to all the pores of the h-beta and show a stronger interaction with the support. Therefore, after calcination, the mean NiO particles sizes are 35 and 15 nm for the acetylacetonate- and the nitrate-impregnated sample, respectively. Different precursors lead to nickel oxide particles with different interactions with the zeolite support, as the H2-TPR profiles in Figure 2 reveal. The actual Ni contents of the samples, obtained by ICP-AES, are included in the graph. During impregnation, the metal precursors interact with the silanols of the support. On heating, they decompose and migrate over the surface at different rates depending on their viscosity and molecular size. The metal−support interaction is affected by the Ni environment during impregnation (solvent, pH, and precursor) as well as by the calcination conditions. Besides, the Lewis acidity of the zeolite may cause variations on the electronic C


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heating leads to smaller and more interacting nickel oxide particles. TEM images of the activated catalysts, Ni present as Ni0, are illustrated in Figure 3. They confirm that Ni particles of different size were present on these catalysts. The nickel nitrateimpregnated catalyst (Figure 3, panels a and b) contained metal particles quite similar in size, around 15−20 nm, and quite homogeneously distributed on the support. The nickel acetylacetonate-impregnated sample (Figure 3, panels c and d) presented a considerable number of very small particles, below 10 nm. Nevertheless, a second population of medium-sized particles is clearly appreciated, some of them larger than 40 nm. The chloride-impregnated catalyst (Figure 3, panels e and f) presented a nonuniform appearance, with medium-sized Ni particles, upon 20 nm, as well as larger ones, about 100−200 nm. Besides, in this material some very large metal particles were present that may be considered as unsupported. Finally, the nickel TEDAN-impregnated sample (Figure 3, panels g and h) presented very large particles, above 200 nm, as well as very small ones. Thus, in Figure 3h, some nickel particles even below 6 nm can be observed. More clarifying data can be extracted from the histogram shown in Figure 4, obtained by direct measurement of more than 100 particles in a collection of TEM images. The graph shows the relative frequency of different nickel particles size ranges ( 30 nm) detected for each sample. Accordingly, the nickel chloride-impregnated sample contained no particles in the smallest size interval (30 nm). The location of the metal particles on the support has been inferred from the pore size distributions of the four catalysts (Figure 5, black lines) and the pristine zeolite (gray lines), obtained from the NLDFT method applied to the adsorption branch of the Ar isotherms at 87 K. This figure is accompanied by the pore volumes measured for different pore size intervals that are summarized in Table 1, which also displays BET surface area values extracted from N2 physisorption analyses at 77 K. All four catalysts showed a decrease in the total pore volumes due to the nonporous nature of nickel and also to pore blocking. This porosity decrease was especially relevant in the pores range corresponding to zeolitic micropores (0−0.8 nm), likely due to their partial blockage. The pore volume loss in this interval was especially relevant for nickel nitrate- and nickel TEDANimpregnated samples, where it reached 35%. In these cases, the presence of very small Ni particles inside the pores is assumed, for such a high loss cannot be explained only by external blockage, given the three-dimensional porous structure of the Beta zeolite. These ultrasmall particles inside the pores would be formed by thermal fracturing and movement inward of the Ni particles upon calcination and hydrogen reduction, as it has been observed in previous works.12,45 The lesser decreases in micropores volume detected for nickel acetylacetonate- and nickel chloride-impregnated catalysts were due to the lower nickel content in the former and to partially unsupported nickel particles in the later. Between 0.8 and 20 nm, the profile of the acetylacetonate-impregnated catalyst was very similar to that corresponding to the pristine h-beta support, confirming that the majority of nickel was located on the external surface. For the

Figure 2. H2-TPR profiles of the NiO/h-beta catalysts impregnated with different nickel precursors. *Actual Ni contents measured by ICP-AES.

structure of the metal species,41 leading to different nickel oxides that are reduced in a wide range of temperatures. The presence of aluminum may also conduct to the formation of nickel aluminate spinels which are not reduced but at very high temperatures (above 600 °C).42 Finally, during impregnation, ion-exchange of the protons of the zeolite by nickel cations can occur. This leads upon subsequent calcination to very small and dispersed nickel species (oxides and silicates) that are also reduced at very high temperatures.43,44 The presence of these spinels or exchanged Ni was not evidenced for any of the catalysts here prepared. In light of the obtained results, Ni chloride species seem to migrate faster to the external surface than nitrate and acetylacetonate precursors. For the case of TEDAN, the situation is different since this precursor is too bulky to enter the zeolite micropores. The sample coming from Ni chloride was almost completely reduced below 350 °C, pointing out the presence of NiO particles very weakly anchored to the support or even unsupported. Only a very small fraction of the oxide was reduced at higher temperatures, upon 420 °C. It is known that bulk NiO shows only one reduction peak centered at 300−350 °C. Likewise, TEDAN complex led toward weakly interacting supported particles, which were reduced in a single step around 351 °C with a small shoulder at 420 °C. Thereby, NiO particles from TEDAN require higher temperatures to be reduced than those observed on the Ni chloride sample. In contrast, the profiles corresponding to the samples impregnated with Ni acetylacetonate and Ni nitrate showed two main reduction stages at high temperatures, 420−470 and 530 °C, respectively, although reduction occurs in a very wide range of temperatures, especially for Ni acetylacetonate, indicating that a great variation of nickel species and particles exist. In both cases, these peaks at high temperature are assigned to nickel oxides species harder to reduce, due to their smaller size and also to their location inside the porous structure of the zeolite. Besides, these high temperatures suggest an additional interaction of these species with the support, possibly with the acid sites of the zeolite. Only for the nickel nitrate-impregnated material, the NiO population reduced at higher temperature was predominant. The H2-TPR profile presented in this work for the sample impregnated with nickel nitrate differs from the one previously reported using this same precursor because it was calcined with a different heating rate (20 °C min−1 instead of 1.8 °C min−1).10 In this sense, it is clear that the heating rate influences the stabilization of the metal particles upon calcination. For the case of Ni nitrate, a faster D


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Figure 3. TEM micrographs of the Ni/h-beta catalysts impregnated with different nickel precursors.

nickel nitrate-impregnated catalysts, a considerable decrease in the small mesopores volume (0.8−10 nm) was appreciated (38%), bearing out the presence of nickel inside them. Nickel chloride- and TEDAN-impregnated catalysts also showed a meaningful decrease (around 24%) in the volume of these pores. In the case of larger mesopores (10−20 nm), the main drop was observed for nickel chloride- and nickel nitrate-impregnated catalysts. Nickel loading also decreased slightly the BET surface area compared to that of the zeolitic support (see Table 1),

except for nickel TEDAN impregnated sample, wherein this decrease was more pronounced, from 654 to 587 m2 g−1. The acidic properties of the catalysts and the h-beta support are included in Table 2. The total acidity of the Ni-containing catalyst is lower than expected for a 7% nickel, suggesting again that some pore blocking is happening. Notwithstanding, differences between the catalysts are minor. A slight interaction of the Ni particles with the acid sites turns into a scarcely lower temperature of maximum ammonia desorption for all the catalysts. Only the material prepared with acetylacetonate E


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Industrial & Engineering Chemistry Research Table 2. Acidic Properties of the Ni/h-beta Catalysts Impregnated with Different Ni Precursors total acidity (meq NH3 g−1) temperature of max desorption (°C)

h-beta

nitrate

chloride

TEDAN

0.425

0.374

0.361

acetyl

0.407

0.363

339

310

319/426

300

326

distributions. In order to determine how these parameters affect the overall activity of the catalysts, several hydroreforming tests have been carried out using as feed the liquid product coming from virgin LDPE thermal cracking at 400 °C for 90 min. It should be reminded that this feed is a hydrocarbon mixture in the range of C4−C40, made up mainly of n-paraffins and 1-olefins. Reaction conditions, chosen from previous works,11 were T = 310 °C (low enough to avoid thermal cracking); t = 45 min; P = 20 bar H2; catalyst/feed mass ratio = 1/30; and N = 700 rpm. As stated above, the term hydroreforming encompasses several reactions occurring in the reaction medium such as hydroisomerization and hydrocracking that proceed through carbocationic intermediates formed in the acid sites while the role of the metal sites is to hydrogenate/dehydrogenate the hydrocarbons.46 Nevertheless, nickel centers may catalyze other reactions. Hydrogenolysis is the reductive elimination of a substituted group, it is, the cleavage of a C−C or C−heteroatom bond by hydrogen addition, giving two smaller molecules. It is sensitive to the kind and position of the substituted groups and is a reaction fully catalyzed by the metal sites. The cleavage of the terminal C−C bond is highly selective on the Ni sites,47 thus, the possibility of hydrogenolysis was checked out considering the presence or not of methane in the gases obtained after reaction. The obtained product distributions by carbon atom number are shown in Figure 6. The feed is a mixture in which no clear peaks are observed. Notwithstanding, the higher percentage correspond to C9 and C10, both above 7%. The product distributions for nickel nitrate-, acetylacetonate-, and TEDANimpregnated catalysts led to similar profiles, with a maximum at C5 about 8−9%, a valley for C6 hydrocarbons and an upturn from C7 to C10 with values upon 8−10%. The use of the nickel chloride-impregnated catalyst resulted in a slightly different profile being the first maximum shifted from C5 to C4 with a value of 7%. Looking at Figure 6 from the bottom up, it can be appreciated that shares between C3 and C8 were increased in the order chloride < TEDAN < acetylacetonate < nitrate. A decrease in all compounds above C15 was observed following the same order. Besides, over the nickel chloride-impregnated catalyst, the contribution of C24+ hydrocarbons was clearly higher than in the feed. This is in agreement with the oligomerization activity that has been identified over pristine h-beta and Ni/h-beta with large Ni particles.12 It is also important that no methane was detected in the hydroreforming gases over any catalyst, ruling out that hydrogenolysis was happening, although almost 1% C 2

Figure 4. Ni particles size distributions for the Ni/h-beta catalysts impregnated with different nickel precursors.

Figure 5. Pore sizes distributions for the Ni/h-beta catalysts impregnated with different nickel precursors.

shows a strong interaction between nickel and acid sites, displaying a second maximum at high temperatures (426 °C). 3.2. Hydroreforming Experiments. The impregnation with different Ni precursors has proved to produce Ni/h-beta catalyst with Ni particles of different sizes, locations, and

Table 1. Textural Properties of the Ni/h-beta Catalysts Impregnated with Different Ni Precursors BET surface area (m2 g−1) zeolitic micropores volume 0−0.8 nm (cm3 g−1) small mesopores volume 0.8−10 nm (cm3 g−1) other mesopores 10−20 nm (cm3g−1) total pores volume 0−20 nm (cm3g−1)

h-beta

nitrate

acetyl.

chloride

TEDAN

654 0.249 0.177 0.021 0.447

629 0.164 0.109 0.013 0.286

649 0.207 0.180 0.019 0.406

621 0.206 0.139 0.012 0.357

587 0.161 0.133 0.018 0.312

F


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experimented by light products (gases + gasolines) compared to the feed may be considered as an indirect hint of the hydrocracking reactions extent. Thereby, the following order for the catalysts can be set up for the different catalysts: nickel TEDAN (50%) < nickel chloride (60%) < nickel acetylacetonate (65%) < nickel nitrate (71%). These two fractions, gases and gasoline, are formed from both diesel fractions, light and heavy. Thus, nickel nitrate achieves the lower share of light and heavy diesel, 21% and 8%, respectively. The hydrocracking activity cannot be ascribed, then, to the acidity of the catalysts, since no significant variations have been detected by ammonia TPD except for the sample coming from acetylacetonate, as mentioned above. The origin of these differences should be related to the dispersion of the metal phase on the support. The hydrocracking of n-alkanes is a process that involves dehydrogenation of the paraffin in the metal site and subsequent protonation of the formed olefin in an acid site to obtain a carbenium ion that can then undergo skeletal isomerization or cracking. The formed ions are then deprotonated and hydrogenated again in the metallic site to produce normal and branched paraffins.48−50 This mechanism requires that metallic and acid sites are in a close proximity. Therefore, as inferred from Figure 7, the catalysts in which Ni particles are smaller, more disperse, and more strongly attached to the support (nickel acetylacetonate and nickel nitrate) and thus closer to the acid sites, enhance the extent of the hydrocraking reactions. Additionally, it has to be mentioned that the combined share of transportation fuels (gasoline + light diesel) is close to 80% for the feed and all reaction products except that of the acetylacetonate-impregnated catalysts, with a value of 73%. This means that the loss of potential valuable products upon hydroreforming is minimal. The properties of the liquid products obtained in the hydroreforming reactions are shown in Table 3. Total aromatics

Figure 6. Hydroreforming products distribution by carbon atom number obtained over the Ni/h-beta catalysts impregnated with different nickel precursors.

Table 3. Properties of the Hydroreforming Liquid Products Obtained over Ni/h-beta Catalysts Impregnated with Different Ni Precursors

compounds (mainly ethylene) were detected using the nickel acetylacetonate-impregnated catalyst. The reaction results are presented in Figure 7 lumped by fractions, gases C1−C4, gasoline C5−C12, light diesel C13−C18, and heavy diesel C19−C40. The feed presents 46% gasoline, 34% light diesel, and 19% heavy diesel, gases being below 1%. The rise

bromine index (g Br2/100 g sample) total aromatic content (wt %)a iso-paraffins in gasoline (wt %)b a

feed

nitrate

acetyl.

chloride

TEDAN

54.1

1.4

4.4

4.4

5.6

1.7

13.5

15.1

18.2

16.1

13.0

24.7

28.3

34.9

30.6

Measured by HPLC. bCalculated by PIONA analysis.

and iso-paraffins in gasoline have been measured by HPLC and PIONA analyses, respectively, and taken as indicators of the aromatization and hydroisomerization activities of the catalysts. The bromine index, on its part, is a measurement of the olefinic double bonds content. The mechanism conducting to isoparaffins formation has been mentioned above. The origin of new olefins and aromatics is different. New olefins may come from dehydrogenation on a nickel site under an abated hydrogen pressure. In this sense, it has been confirmed by the pressure drop along the reaction that hydrogenation of olefinic double bonds starts during the heating stage of the reaction. In a typical reaction, pressure rises from 20 to ∼24 bar from room temperature to approximately 90 °C. At this temperature hydrogenation starts and a pressure drop is observed until temperature reaches ∼260 °C and the rest of the reaction takes place under a lowered hydrogen pressure. The pressure drop depends on the extent of hydrogenation over each catalyst. Then,

Figure 7. Hydroreforming products distribution by fraction obtained over the Ni/h-beta catalysts impregnated with different nickel precursors. G



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pressure rises again as the temperature increases up to 310 °C and gases are formed by cracking. Olefins can be also produced by hydride abstraction in a Lewis acid site, creating a carbocation that undergoes β-scission, giving a paraffin and an olefin.51 Cyclization and aromatization of light alkenes are also catalyzed by the acid sites via carbenium ions.52 As observed in Table 3, the trend of both iso-paraffins and aromatics formation is the opposite of that previously observed for hydrocracking: nickel chloride > nickel TEDAN > nickel acetylacetonate > nickel nitrate, pointing out that both reactions were favored over the catalysts with large metal particles. The differences were slight for aromatics, from 18% using the nickel chloride-impregnated catalysts to 13% over the Ni nitrateimpregnated one (starting from 1.7% in the feed) and more pronounced for iso-paraffins, ranging from 35% for chloride to 25% for nitrate (starting from 13%). A high selectivity of mesoporous zeolites to branched hydrocarbons has been reported for other reaction systems, such as Fischer−Tropsch synthesis.53 Isomerization and aromatization share the common feature of requiring bulky transition states, which is not the case of cracking. In this regard, a plausible explanation for the obtained iso-paraffins and aromatics contents is that smaller and more dispersed Ni particles, closer to the acid sites, resulted in either steric hindrances for the production of aromatics and branched hydrocarbons in the proximity of the acid sites or, in a faster hydrogenation of the intermediate carbocationic species, reducing the extent of the secondary reactions of hydroisomerization and aromatization. Other authors have also reported that isomerization is favored for larger metal particles due to a lesser presence of corner and edge sites.54 Other major point of interest is the degree of hydrogenation provided by each catalyst. Hence, the bromine index of the liquids was measured as indicator of the olefin content. As a result of the thermal cracking reactions, producing α-olefins, the feed presents a high bromine index (54 g Br2/100 g sample). This value is easily reduced after hydroreforming at 20 hydrogen bar under the presence of all the catalysts, up to 1.4 for nickel nitrate-, 4.4 for nickel acetylacetonate- and nickel chloride-, and 5.6 for the TEDAN-impregnated sample. These results indicate an olefins saturation degree ranging from 90% to 97%, noticeably higher for the nitrate-impregnated catalysts, the one with smaller metal particles, due to the higher surface of nickel exposed.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This research has been funded through the Comunidad de Madrid and Rey Juan Carlos University project URJC-CM-2010CET-5450. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS The size and location of Ni particles on a hierarchical beta zeolite may be varied by choosing the appropriate nickel precursor, regarding to its molecular size and affinity with the support, used during the impregnation step. On the contrary, no significant differences in the acidic properties of the catalysts were evidenced for any of the catalysts. The results of the hydroreforming of LDPE thermal cracking oils indicate that a relationship between Ni particles size and distribution on the zeolite surface and the overall activity of the catalyst exists. Smaller and more strongly interacting Ni metal particles (the ones coming from nitrate and acetylacetonate) translates into an enhanced hydro/dehydrogenation ability for these catalysts. This fact originates less olefins and a more intense hydrocracking. In contrast, the presence of larger Ni particles in the catalysts coming from chloride and TEDAN favors the production of aromatics and isoparaffins because of the longer distance between the metal phase and the acid site. H


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