Production of Highly Dispersed Ni within Nickel Silicate Materials with

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Production of Highly Dispersed Ni within Nickel Silicate Materials with the MFI Structure for the Selective Hydrogenation of Olefins Khaled O. Sebakhy,* Gerardo Vitale, and Pedro Pereira-Almao Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4

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S Supporting Information *

ABSTRACT: Aluminum-free Ni-incorporated MFI zeolite materials were synthesized in a batch reactor under mild reaction conditions using the hydrothermal method and were proven to exhibit highly active sites for the saturation of aromatics and olefinic molecules due to the high dispersion of the Ni species. The synthesized solid materials were characterized by various methods, and computational modeling aided in understanding the Ni dispersion in the MFI framework. The novelty of this study is corroborated to the unprecedented hydrogenation activity of the Ni-MFI materials when tested with a 1-octene model molecule at 0.48 MPa H2 pressure and moderate temperatures (423−473 K). Another important aspect of this work is the significant elimination of olefinic molecules from a sulfur-rich industrial cracked naphtha feedstock when using extrudates of the same material, which enforces that Ni-MFI could be commercially applied in industry for the selective hydrogenation of olefins present in light petroleum streams.

1. INTRODUCTION Hydrogenation is an industrially important reaction that could be carried out using various Ni-based zeolitic materials (such as Ni/ZSM-5).1−21 Furthermore, when the proportion of heteroatoms such as Al, Fe, Ga, and Ni in MFI frameworks is minimal, the thermal and hydrothermal stability is significantly enhanced in comparison with aluminosilicate zeolites. Despite Ni catalysts finding potential industrial applications, it is very important to pinpoint that their activity, selectivity, and stability will highly depend on the way Ni is incorporated on the carrier. Meanwhile the catalytic activity of those materials for any targeted reaction could be significantly enhanced by ensuring that the metallic sites are highly dispersed. Platinum and palladium noble metal catalysts on various supports are well known and have been extensively studied in hydrogenation reactions. As a more economic option, nickel has also been used; however, for the nickel case, usually large amounts of nickel are impregnated on the supports, compared to the ones deposited on Pd and Pt, being generally attributed to poor dispersion of Ni and lower intrinsic activity per site.5−15 Thus, important quantities of the metal are wasted as they are hidden from the reactants when large particles of metallic nickel are formed upon catalyst activation. Indeed high Ni dispersions can be successfully obtained in zeolites by several approaches, such as precipitation, impregnation, and ion exchange. Although the above-mentioned techniques are all well-known in the literature, the most frequently used techniques are impregnation and ion-exchange that modify the physical and chemical features of the final zeolite materials. © XXXX American Chemical Society

Despite the peculiarity of both methods in the synthesis of metallic modified zeolites, it is expected that embodiment of nickel species while the zeolite is produced can also efficiently yield a catalyst with high metallic dispersion in one step.22,23 Thus, our method of preparation (hydrothermal incorporation synthesis method) is proposed as a cheaper technique to prepare Ni-MFI materials, which might exclude impregnation and ion-exchange techniques that most of the time increase the number of unit operations used and thus reduce the final cost of the catalyst. Additional steps such as drying and calcination can increase the possibility of nickel sintering, which will alter the final activity and selectivity of the catalyst. In this study, we will also demonstrate that low amounts of Ni which are highly dispersed in the MFI framework are capable of obtaining high selectivity for olefin hydrogenation, compared to Pt and Pd catalysts which can migrate double bonds during olefin hydrogenation, producing olefinic isomers. This in addition to the lower price justifies the use of highly dispersed Ni in the hydrogenation of deleterious olefinic molecules. The fundamental goal of this study is to find alternative solids as a replacement to noble metal catalysts in hydrogenation reactions of olefinic and aromatic compounds. These materials are also thought to find high demand in the petroleum industry for the treatment of commodity feedReceived: Revised: Accepted: Published: A

December 3, 2018 April 23, 2019 May 6, 2019 May 6, 2019 DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research stocks.23 To replace noble metals by our suggested catalyst, we tested our prepared materials in the selective catalytic hydrogenation of olefins and aromatics. In this study, we first used model molecules with tuning the experimental conditions and adjusting the metallic loading for the targeted reactions. The produced Ni-MFIs were then utilized in the hydrogenation of deleterious olefins present in an industrial cracked naphtha containing high levels sulfur to avoid the creation of polymeric materials and deposits while transporting such feedstocks (e.g., cracked naphthas, gasolines, and pyrolysis gasoline). Additionally, minimal aromatics hydrogenation was targeted in order to maintain low hydrogen consumption and enhance the octane number of the naphtha fractions. The focus of this study is to prepare cost-effective MFI materials containing 1, 2, and 3 wt % nickel using the hydrothermal route, in addition to the detailed characterization of the NiMFI solids and applying those materials as selective hydrogenation catalysts for olefinic molecules. Furthermore, many light petroleum fractions contain deleterious olefinic molecules that tend to polymerize and form gums and deposits in pipelines during transportation; thus, it is of vital necessity to investigate new catalytic materials that can potentially eliminate those olefins, tolerate high sulfur levels, and compete with noble metals. This comprehensive work will first describe in detail the synthesis of the Ni-MFI materials and then will include all the necessary characterization to confirm the NiMFI structure, discuss the unprecedented hydrogenation activity for toluene and 1-octene molecules, test the most suitable Ni loading for the selective hydrogenation of 1-octene with minimal toluene hydrogenation, evaluate the selective hydrogenation of olefins present in an industrial naphtha feedstock containing high sulfur levels using extrudates of the Ni-MFI, and finally gain a deep understanding of Ni dispersion via computational modeling.

are not attractive to generate commercial quantities of these catalysts for the treatment of commodity feedstocks as those present in the oil industry. Another possibility is to take advantage of the fact that under strong acidic conditions not only the transition metals inserted do not form insoluble hydroxides but also sodium silicate (an inexpensive silica source) tends to form important quantities of monomeric silicate species. Thus, this strategy can be carried out to generate the desired bonding between the heteroatoms and the silicate framework in order to produce a stable metal silicate precursor. Under the selected preparation conditions of hydrothermal synthesis, the solution-mediated model mechanism will yield the final metal silicate material by using conventional raw materials, which then will be suitable for feasible commercial catalytic applications.28 This is the path presented in this work in order to produce a nickel silicate material with the MFI structure suitable for selective hydrogenation of olefins at low temperatures and pressure. The following reagents were acquired from Sigma-Aldrich without extra purification: sodium silicate (SiO2 26.5%, Na2O 10.6%), sulfuric acid (98%), sodium hydroxide (99%), tetrapropyl ammonium bromide (TPABr), and nickel nitrate hexahydrate Ni(NO3)2.·H2O (98%). In a conventional synthesis protocol, as depicted in Figure S1, an acidic solution of H2SO4 was prepared in deionized water and the targeted quantity of nickel salt was then added to it, and after the salt was completely dissolved, TPABr salt was carefully dissolved. Following this step and under vigorous mixing, sodium silicate was dissolved and the mixture was subsequently homogenized. After that, sodium hydroxide in the form of pellets was added during agitation. Finally homogenization was carried out for 15 min, and then the suspension was added to a Parr reactor having a volume of 300 mL to undergo crystallization for 40 h at 463 K and 300 rpm. The mole fractions of the reagents for all prepared MFI samples are included in Table S1. After completion of the hydrothermal synthesis, the product in the form of suspension was collected, filtered, and washed multiple times with deionized water. After that, the final solid was dried under vacuum for 18 h at 373 K to obtain a fluffy powder material. The dried product was then calcined at 10 K/min for 6 h to a final temperature of 823 K. This shall enable to access the nickel inside the MFI framework through the micropores of the material. The final samples used in the catalytic tests were given the following nomenclature: 1.0Ni-MFI, 2.0NiMFI, and 3.0Ni-MFI. According to the IUPAC Zeolite Commission and the Structural Commission of the International Zeolite Association (IZA), the MFI framework composition (described by the three letter code from the first material used to characterize the structure of the ZSM-5) in its aluminosilicate form can be described in the anhydrous form as follows: |Na + n | [AlnSi96‑nO192]-MFI where n defines the number of aluminum atoms substituting the silicon atoms in the MFI framework and as a matter of fact producing the negative charge in the framework that has to be compensated by a counter positive ion which is usually a Na+ cation that is present during the synthesis.29,30 In the current work, we produced nickel-silicate materials having nickel contents of 1, 2, or 3 wt % which are written in ideal and anhydrous forms as shown in the Supporting Information (Table S2). Monomeric species are produced mainly as a result of the acidification of the silicate material, where nickel ensures that those species are interacting mostly as monomeric species.

2. EXPERIMENTAL SECTION 2.1. Preparation of Nanocrystalline NiO Reference. For comparison and reference purposes, a sample of nanocrystalline NiO was synthesized similar to the method described previously in the literature.24 A brief explanation of the preparation method is included elsewhere.24 For details, see the Supporting Information (section S.1). 2.2. Synthesis of Ni-MFI Catalysts Having Various Nickel Contents for the Efficient Hydrogenation of Olefins. The incorporation of transition metallic species in the zeolite framework has been carried out by modifications of the experimental parameters in order to avoid the production of massive quantities of the insoluble hydroxides, which are not incorporated into the silicate framework, especially when organic structural directing agents (OSDA) are used. This is because the heteroatoms are not needed to form the pure silicate framework that is directed by the OSDA. In order to add heteroatoms to the desired silicate framework, most typically, it is required to use special silicate reagents like tetraethoxysilane better known as tetraethyl orthosilicate (TEOS), the use of fluoride gels or dealumination of the aluminosilicate zeolite and further treatment with vapors of the desired metal chloride.25−27 These protocols have allowed the successful production of metal silicates with structures similar to the ones using OSDA and producing aluminosilicates like, for instance, ZSM-5. Even though these methodologies have been proven successful to produce several metal-silicates, from the commercial feasibility stand point, however, these methods B

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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represent 2 and 3 wt % of Ni in the MFI structure, as shown in Table 1 below.

Increasing the pH produces oligomeric species from silica “capturing” the nickel atoms in the amorphous framework of the gel. Furthermore, when the pH reaches a value of 6−7 the silica solubility is significantly reduced, and then Ni is captured in the framework. After that, the crystallization of the MFI structure is induced, which is guided by the TPABr organic template at a pH value of 11. As evidence for this, when the gel was collected and dried before crystallization, no nickel hydroxide was detected by XRD, which confirms that there are strong bonds in the form of Ni−O−Si, and as a result of the gel homogeneity, a high Ni dispersion is maintained. Therefore, if the silicate in the acidic media is not used, increasing the pH to 11 will result in the precipitation of Ni(OH)2 as result of the nickel not being protected as it happened in the case of the produced Ni(OH)2 used to prepare the NiO nanoparticles for reference in this work.24 2.3. Ni-MFI Catalysts Pretreatment. 2.3.1. Ni-MFI Catalyst Activation and Reactivity Experiments for Toluene and 1-Octene Hydrogenation. Before any hydrogenation tests were carried out, the Ni-MFI catalysts (400 mg) were activated at atmospheric pressure through H2 pretreatment at 773 K for 5 h using a flow of 180 sccm. This procedure was performed to reduce some of the Ni2+ species in the material to Ni(0) which is the active site for hydrogenation. An exact setup and procedure was previously reported for catalyst activation and is cited here.31−33 2.4. Catalytic Activity Tests for Model Molecules. The activity tests took place in a continuous flow as discussed in the Supporting Information (section S.2) and is also cited elsewhere.31−33 2.5. Characterization of the Prepared Materials. The instruments and conditions of analysis for the characterization of the samples by XRD, textural properties, HRTEM, XPS, SEM-EDX, TGA, ICP, and FTIR were described previously in references in our previous works.31−33 For further insights on the pore structure of the prepared and catalytically tested 1NiMFI material the SAIEUS program (Solution of Adsorption Integral Equation Using Splines) was employed. SAIEUS calculates the pore size distribution of the material from the adsorption isotherm via NLDFT (Non-Local Density Functional Theory) models.34−36 For the present case, the zeolite model “Zeolite (Me-Form)-N2-77, NLDFT, Cylindrical model”, within the SAIEUS program, was employed to obtain the pore size distribution. 2.5.1. Characterization of Cracked Naphtha and Hydrogenated Products. A cracked naphtha provided by Nexen Ltd. Canada and its hydrogenated products after catalytic reaction were quantified using a group type analysis via a SFC equipment provided by Selerity Technologies.37−39 The detailed procedure of characterization was previously published in our previous work and is cited here.31−33 2.6. Computational Studies. As described in the experimental preparation of the Ni-MFI materials, compositions of 1, 2, and 3 wt % Ni on the MFI silicate framework were targeted. Calculations on composition contents of Ni on the silicate MFI framework indicate that in order to have a minimum of one metallic nickel cluster containing two nickel atoms [2Ni] in the MFI framework, it is required to produce a super cell of 1 × 1 × 2 of the MFI structure in order to account for the 1 wt % Ni; the same is true for two metallic [2Ni] clusters or one metallic [4Ni] cluster and for three metallic [2Ni] clusters, two metallic [3Ni] clusters, one metallic [2Ni] and one metallic [4Ni], or one metallic [6Ni] cluster that will

Table 1. Composite Materials with Nickel Clusters Having Two, Four, and Six Atoms to Account for the Intended Amount of Nickel in the Prepared Materials Ni (%)

zeolite (%)

1.0

99.0

2.0

98.0

3.0

97.0

composite material [Si192 O384] [Ni2] [Si192 O384] [Ni4] [Si192 O384] [Ni6]

possible metallic Ni clusters 1 [2Ni] 2 [2Ni] or 1 [4Ni] 3 [2Ni]; 2 [3Ni]; 1 [2Ni] + 1 [4Ni] or 1 [6Ni]

The type and shape of nickel clusters have been extensively reported in several theoretical and experimental studies.40−48 For our calculations, we have selected the small clusters that can fit inside the channels of the MFI silicate structure producing the targeted amount of nickel in the composite material which have been previously reported to be stable.40−48 They are all shown in Figure 1. In our study, the nickel clusters were geometrically optimized using BIOVIA Forcite as shown in Figure 1. Furthermore, BIOVIA Forcite was used to optimize the silicate MFI supercell (1 × 1 × 2) framework prior to their use on the location of the clusters within the MFI structure.49 For the other work, BIOVIA Sorption Locate module was utilized which allowed guest molecules to be located within microporous crystals in order to predict the locations of the nickel clusters within the MFI silicate framework.50−59 More computational insights are described in the Supporting Information (section S.3).

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The corresponding XRD patterns for the synthesized Ni-MFI solid powders after calcination are shown in Figure 2. All of the peaks showed that the catalyst prepared is crystalline with the Ni-MFI structure, and no additional peaks from impurities or beyond the MFI structure were detected. Figure 2 shows the diffractograms of all samples with different Ni contents after calcination at 823 K for 6 h. All samples showed a similar diffraction pattern, evidencing that the synthesized solids exhibit the Ni-MFI structure. Intensities in XRD rely on several parameters, and even though the nickel atoms are larger than silicon and oxygen but because they are uniformly dispersed at a low content in the framework, it becomes very difficult to detect them by XRD. As a comparative study, a mechanical mixture of 3 wt % Ni mixed with a pure silicate with the MFI structure (silicalite I) synthesized in the same way as the Ni-MFI materials but without the addition of nickel was prepared by using the synthesized 5 nm NiO. Figure 3 shows the region where NiO can be detected. The MFI topology has a complex structure presenting diffraction lines in the whole diffraction pattern; however, as depicted in the Figure 3, the 3 wt % Ni incorporated material shows a similar diffraction pattern to the silicalite I material. On the other hand, the XRD pattern of the mechanical mixture clearly indicates that if NiO is present at a concentration of 3 wt % Ni, it will be possible to see it by XRD. The fact that the 3 wt % incorporated material does not show the NiO features indicates the high dispersion of the nickel in C

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Metallic nickel clusters used in the computational studies.

Figure 2. XRD patterns of calcined MFI samples at 823 K with various Ni loadings.

mapping clearly showed that the Ni is well dispersed in the MFI structure. HRTEM pictures for the various calcined MFI samples are depicted in Figure S4. The thermal stability of the calcined Ni-MFI solids in air were high, where a total weight loss of 6% occurred at a final temperature of 973 K as shown in Figure S5. Additionally, thermal stability was the same for all MFI samples regardless of the Ni content. Furthermore, the Ni contents in the Ni-MFI samples were quantified using ICP and the wt % Ni was calculated as shown in Table 2. The values of the % Ni matches well from both methods and they agree with targeted Ni contents. The reduction profiles of the Ni-MFI samples were obtained via H2-TPR of the calcined samples as shown in Figure S6. It is shown that the Ni2+ species in the MFI materials starts to reduce from 573 K and finishes at 1273 K. The MFI samples used in the catalytic reactivity tests were reduced/activated at 723 K. As expected and depicted in Table 3, the 3.0Ni-MFI sample consumed almost three times the amount of H2 taken by the 1.0Ni-MFI material. The infrared spectrum of the NiO 5 nm reference together with the spectra of the 1.0Ni-MFI before and after calcination are shown in Figure 5. As would be expected, the infrared of the NiO nanoparticles shows an appreciable −OH stretching band because the important amount of hydroxyl groups on the surface of the material for being a nanoparticle. The vibrational absorption band occurring between 400 and 600 cm−1 in the framework is due to Ni−O stretching vibration mode. However, this peak was observed to be broad, which indicates

Figure 3. Comparison of the XRD diffraction patterns for Nimechanical mixture (MM-3Ni + silicalite), Ni-incorporated (3NiMFI), pure silicalite I, and the 5 nm NiO reference.

the prepared samples not only in the low Ni content but also in the higher Ni content samples. Figure 4 depicts the surface morphology of all Ni-MFI materials were they were composed of microcrystals. Also, increasing the amount of nickel from 1 to 3 wt % in the MFI structure facilitated the crystal growth. Furthermore, EDX chemical mapping was used as semiquantitative method to quantify various elements present in the 1.0 Ni-MFI sample, as shown in Figure S3. Moreover, elements like Ni, Na, and Si present in the solids were identified by EDX. The EDX D

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Images obtained by SEM for various Ni-MFI samples.

Table 2. Ni Contents Determined by ICP-EDX Analysis sample

% Nia

% Nib

1.0Ni-MFI 2.0Ni-MFI 3.0Ni-MFI

1.11 2.05 3.22

1.26 2.16 2.81

a

Determined by ICP (mean obtained from three measurements). Determined by EDX mapping.

b

Table 3. H2 Consumption by Ni-MFI Samples (μmol/g) during TPR Experiments % Ni ICP

equivalent % NiO

expected μmol H2/g

experimental μmol H2/g

1.11 2.05 3.23

1.41 2.61 4.11

189 349 550

232 374 578

Figure 5. Infrared spectra of the reference NiO nanomaterial and the 1Ni-MFI before and after calcination.

that the NiO sample is composed of nanocrystals.60 Furthermore, a clear broad band occurring at 1400 cm−1 is assigned to −O−H bending vibrations modes. Additionally, the 5 nm NiO nanoparticle has more −OH present than a bulk material will have because of important surface to volume ratio of nanoparticles. Furthermore, the −OH stretching region (centered around 3600 cm−1) is also very intense, indicating a NiO nanoparticle type material. The spectra of the 1Ni-MFI before and after calcination shown in Figure 5 indicate clearly the modifications of the vibrations by the disappearance of the TPA organic agent after calcination to free the channels and cavities of the zeolitic material (bands between 2880 and 2980 cm−1 and the band at 1472 cm−1). Also, the broad region between 1200 and 600 cm−1 corresponding to the silicate modes of vibration (Si−O− Si; O−Si−O; Si−O−Ni), which are not present in the NiO

material, is noticeable. There is an important −OH broad region for both the 1Ni-MFI before and after calcination, as expected, as this material is prepared under hydrothermal conditions producing also some defects. It is known that there are five types of distinguishable hydroxyl groups in zeolitic materials.60 These types of hydroxyls increase the bands occurring in the stretch region at about 3740, 3720, 3680, 3580−3520, and 3600−3650 cm−1, respectively. For the 1NiMFI materials it is clearly easy to observe the bands corresponding to the silanol and hydroxyl nest appearing at around 3750 cm−1 and the band at 3630 cm−1 corresponding probably to −OH groups attached to T-atom-containing species, respectively. As the spectra were taking on samples which were exposed to the environment, some water molecules must be absorbed in the channels, thus producing the large E

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 6. Comparison of the infrared spectra in the framework region of the materials before (a) and after (b) calcination.

Figure 7. XPS spectra for the Ni2p3/2 and O1s regions, respectively, for (a) and (e) NiO; (b) and (f) 1Ni-MFI as prepared; (c) and (g) 1Ni-MFI calcined; and (d) and (h) 1Ni-MFI after activation.

broadening of the −OH stretching and hiding any other distinguishable hydroxyl. Figure 6 shows the infrared spectra in the framework region (400−1600 cm−1) for the different materials before and after calcination. Before calcination (Figure 6a), the vibration of the MFI containing nickel and without nickel are almost the same except for the 3Ni-MFI sample. After calcination, there are modifications in the relative intensities of some signals,

especially the vibrations around 800, 975, 1050, 1100, and 1200 cm−1, when the three Ni-bearing materials are compared with the non-nickel material. Thus, the distribution of nickel in the three materials is different as would be expected because of the different Ni loading. XPS was performed on the 1.0Ni-MFI activated material as this solid showed to be a very promising catalyst for the selective hydrogenation of olefins with minimal aromatics F

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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two bands one around 531 eV and the most intense one around 533 eV. For the aluminosilicate ZSM-5 it has been shown that there is a dependence of the binding energy of O1s XPS peaks with the electronegativity of the exchanged cations.64 The Na form has a binding energy around 533.2 eV (similar to the intense one for the 1Ni-MFI materials), but if the Na+ ion is exchanged by Li+ or H+ there is a shift to higher binding energies of almost 1 eV (534.1 eV). On the contrary, if Cs+ is the ion incorporated there is a shift to lower binding energy (532.5 eV).64 The most intense signal in the prepared 1.0Ni-MFI materials is at 533 eV which is typical for Na-form MFI type materials as described above. The small signal around 531 eV may be ascribed to defects in the structure as the area of this signal is larger in the 1Ni-MFIactivated material where some nickel must be extracted from the structure during the reduction treatment with H2 and, thus, be reduced forming hydroxyl nest in the framework where the nickel was originally located which was also suggested by infrared. To further understand the prepared materials, the pore size distribution (PSD) of the 1Ni-MFI after different treatments was obtained from the N2-adsorption isotherm branch with the SAIEUS program. Figure 8 shows the PSD of the five samples which were tested. Figure 8a shows the whole PSD range from 0 to 300 Å for the silicalite-I (no nickel), 1Ni-MFI-fresh (calcined at 550 °C), the 1Ni-MFI material after activation, the 1Ni-MFI material after being used, and the 1Ni-MFI material after being regenerated. It is possible to distinguish three regions in the PSD, and insets in each region are presented in Figure 8b−d showing the pore width regions 2−16, 24−69, and 100−320 Å, respectively. The micropore region between 0 and 16 Å indicates a maximum around 5.7 Å for the silicalite I material and a maximum around 6.3 Å for the 1Ni-MFI-fresh (calcined at 550 °C), a maximum around 6.1 for the 1Ni-MFI activated, 4.8 Å for the spent 1Ni-MFI, and 6.3 Å for the regenerated 1Ni-MFI material. Fitting the obtained microporous distribution profiles with Gaussian curves helped to extract more information on the variation of the PSD for the 1Ni-MFI material after the different treatments. The fitting plots can be found in the Supporting Information (Figures S15−S19), and the sizes of the pores can be found in Table 6. As can be seen in Table 6 (and in the figures in the Supporting Information), most of the fitting required the use of four Gaussian curves except for the 1Ni-MFI-spent material that required only two curves as the large pore size having maxima at around 7 and 8 Å disappeared from the PSD for this particular sample. The MFI framework possesses two types of 10-ring channels one lineal and one sinusoidal that interconnect each other forming a larger cavity. The maximum diameter of a sphere that can be included within this framework was estimated to be 6.36 Å, and the maximum diameter of the largest-free-sphere that can diffuse along the “a”, “b”, and “c” axes were estimated to be 4.7, 4.46, and 4.46 Å, respectively.65,66 The analysis of the 1Ni-MFI material indicated that nickel slightly modified the PSD of the microporous MFI structure (if compared with the silicalite I material); however, after being used in the catalytic test, the bigger pore sizes (having maxima at around 7 and 8 Å) seemed to disappear giving some indication of the covering of the walls of the intersecting channels by possible coke material reducing in this way the largest micropore size that should be found within the intercepting channels. Regeneration by calcination

saturation, as will be shown later in the catalytic tests. For the present study, XPS spectra for the reference material NiO and the 1 wt % Ni-MFI as prepared after calcination and after activation were carried out. Figure 7 shows the Ni2P3/2 and O1s regions of the selected materials. The Si2p and Na1s regions of the 1Ni-MFI materials are not presented as they do not provide useful distinguishable features. The fitting of the Ni2p3/2 regions was carried out following the indications presented in ref 61. The spectra are shown in the top part of Figure 7 where (a) corresponds to the fitting carried out to the spectrum of the nanoparticles of NiO and the other three to the 1Ni-MFI material with different treatments. The NiO binding energies obtained in Table 4 agree quite well with Table 4. Binding Energies for the Signals of the Ni2p3/2 Region of the Tested Materials binding energy of signals of Ni2p3/2 (eV) sample

1

2

3

4

5

NiO (5 nm) ref 1Ni-MFI-RT 1Ni-MFI-calcined

853.2 853.9 853.5

854.8 856.4 857.0

860.2 858.0 858.6

863.5 862.0 862.6

866.0 863.4 863.5

those reported for NiO in reference.62 The Ni2p3/2 region in the XPS spectra of the 1Ni-MFI materials are very noisy, especially the one obtained for the H2 activated material; thus, the fitting was more complicated than in the NiO reference. For the activated material, it was not possible to get a reliable fitting, and this may be because of the low amount of metallic nickel present in this material and the high dispersity of it. The depth of analysis in the XPS technique is around 5−10 nm, and thus, the amount of detectable metallic nickel and of course the nonreduced nickel being both in very low quantities and very well dispersed in this material may explain the noisy spectrum obtained for this sample. The binding energies values obtained from XPS for the 1.0Ni-MFI as prepared and after calcination show that nickel is mainly abundant in the 2+ oxide form; however, the environment is different in the silicates. The bottom images of Figure 7 corroborate to the O1s region of the MFI catalysts. Figure 7e shows the O 1s region of the NiO reference material, and as can be seen in the spectrum there are two oxygen signals, one around 529 eV which was ascribed to bulk oxygen in the NiO. The second one around 531 eV was assigned to defects in the structure, which include the surface hydroxyls that in the tested sample contains an important amount of them for being nanoparticles (this is correlated easily with the intense and broad infrared −OH stretching band observed in the infrared of this material in Figure 5).63 As shown in Table 5, the area of this signal corresponds to 29% of the total, indicating the important amount of oxygen-type defects for the 5 nm NiO nanoparticles. The O1s signals for the 1Ni-MFI materials also show Table 5. Binding Energies for the Signals of the O1s Region of the Tested Materials O2−

O defects sample

BE (eV)

area (%)

BE (eV)

area (%)

NiO 5 nm ref 1Ni-MFI-RT 1Ni-MFI-calcined 1Ni-MFI-activated

530.8 530.7 530.8 530.5

29.1 4.1 2.3 6.1

528.8 532.7 533.0 532.6

70.9 95.9 97.7 93.9 G

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 8. Pore size distribution of the 1Ni-MFI material after different treatments and its comparison with the silicalite I material (no added nickel) obtained from the N2-adsorption isotherm branch with the SAIEUS program: (a) PSD range from 0 to 300 Å; (b) PSD range from 2 to 16 Å; (c) PSD range from 24 to 69 Å; and (d) PSD range from 100 to 320 Å.

There seem to be four main micropore sizes within the prepared materials (including the silicalite I); for the 1Ni-MFI they are one around 4.5 Å, one around 5.5 Å, one around 6.8 Å, and one around 8.8 Å. When compared to the silicalite I material, it is obvious that the incorporation of nickel modified the sizes of these pores making them slightly bigger than the material without nickel. For the particular case of the 1Ni-MFISpent material it seems that the micropore mouth around 4.6 and 5.4 Å are still accessible for the N2 molecule to go inside and measure them, but the larger pores at around 7 and 8 Å are blocked or reduced in size. The prepared 1Ni-MFI solid, with different treatments, showed hysteresis loops indicating mesoporosity (Figure S7). The PSD profile obtained with the SAEIUS program indicated two main mesoporous maxima: one at around 40 Å and one around 200 Å. When the spent material is compared with the other ones, it is seen that the mesoporous around 40 Å is not

Table 6. Extracted Pore Size Values by Fitting with Gaussian Curves the PSD Profiles Obtained with the SAIEUS Program on the 1Ni-MFI Material after Different Treatments pore size values after fitting with Gaussian curves the PSD profiles obtained with the SAIEUS program sample

pore 1 (Å)

pore 2 (Å)

pore 3 (Å)

pore 4 (Å)

silicalite I 1Ni-MFI-fresh 1Ni-MFI-activated 1Ni-MFI-spent 1Ni-MFI-regenerated

4.5 4.7 4.7 4.6 4.8

5.2 5.6 5.5 5.4 5.7

6.2 6.9 6.8

7.7 8.8 8.5

7.0

8.9

of the material recovers the larger pores near 7 and 8 Å as can be seen in Table 6 and Figure 8b.

Figure 9. Catalytic reactions performed for toluene (experimental conditions: pressure = 0.4 MPa, W8 hly space velocity = 0.43 h−1, H2 flow rate = 120 sccm, and time on stream = 24 h): (a) 2.0Ni-MFI and (b) 3.0Ni-MFI. H

DOI: 10.1021/acs.iecr.8b05991 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

hydrogenation.68 The catalysts also showed a high stability for a duration of 96 h. More interesting, when the H2 flow rate was decreased from 120 to 33 sccm the catalytic activity of both MFI catalysts in 1octene hydrogenation was not altered. This mentioned, it would be of vital neccessity to prove that the catalyst has a potential hydrogenating power for a mixture of olefins and aromatics where the main target is to hygrogenate olefins without changing the aromatics concentration. 3.2.3. Catalytic Hydrogenation of a 50 wt % 1-Octene/ Toluene Mixture Using 1.0Ni-MFI Catalyst. To prove that MFI catalysts could be a potential substitute to noble metal catalysts in the selective hydrogenation of olefins without touching aromatic molecules, 1.0Ni-MFI catalyst was tested. The hydrogenation reaction carried out at 413 K at 0.4 MPa and was shown to selective for 1-octene hydrogenation, where the conversion of 1-octene to n-octane was 49% with minimal toluene hydrogenation (