Hybrid Nanoarchitectonics with Cr, Fe-MOF/ Graphene Nanocomposite for Removal of Organic Sulfur Compounds from Diesel Fuel

Metal–organic frameworks (Cr-MOF and Fe-MOF) and their graphene hybride nano-composites were prepared via green solvo-themal method. The prepared samples were characterized by XRD, FTIR spectroscopy, N2 adsorption–desorption isotherm and XPS. The composites were used for the adsorption of thiophenic sulfur compound (thiophene, dibenzothiophene, 4,6-dimethyldibenzothiophene) in a model fuel oil. It was found that, graphene in the MOF composite has positive effect on sulfur removal. The removal efficiency increase from 62% to % 95.6 using Fe-MOF and Fe-MOF/Gr (9:1), respectively. This enhancement effect is attributed to a greater number of coordinatively unsaturated sites (CUS) in the composites. The results indicated that the adsorption reach to 96.6% for DBT adsorption from model diesel oil and 62% for diesel fuel on using Cr-MOF/Gr composite.


Introduction
Deep desulfurization of liquid hydrocarbon fuels has become an increasingly important subject worldwide. The sulfur content in the transportation fuels is a very serious environmental concern. Thus, a number of countries have initiated and applied a set of strict environmental regulations and laws to reduce the sulfur contents in diesel oil to ultralow levels (given as 10-15 ppm). The main goal of applying such strict regulations is to minimize the harmful exhaust emissions, improve air quality and limit air pollution.
Elimination of sulfur compound in transportation fuel can be achieved either by utilizing different desulfurization processes for the fuel products, which is commonly carried out using the catalytic hydrogen processing approaches, or by removing sulfur compounds contained in stack gases. Reducing the sulfur content in diesel fuel to less than 15 ppm is a complicated process [1].
HDS is the conventional process used for the removal of sulfur compounds contained in diesel oil, this process has considered a real challenge. This is due to the fact that some sulfur species contained in diesel oil have low reactivity and complicated removal mechanisms. Researchers and experts are currently enhancing these processes by synthesis of new HDS catalysts, improved reactors and optimized operating conditions were tested for the removal of certain sulfur compounds that are characterized by low reactivity [1].
Deep hydrodesulfurization process may result in changing the properties of the diesel oil, such as reducing the lubricity. This may occur due to the elimination of some compounds that are responsible for lubricity of the diesel fuel. In addition, HDS may result in reducing the density of the produced diesel oil which is directly related to a reduction in the energy content by around 1%. Moreover, producing diesel oil with ultra-low sulfur content (ULSD) (15 ppm) has higher costs compared to producing low-sulfur content diesel oil (500 ppm). However, a significant reduction in the cost of producing diesel oil with sulfur content that is less than 15 ppm can be achieved by integrating the HDS units with a non-hydrogenation process, such as adsorptive desulfurization units [1].Various adsorbents, such as reduced metals, metal oxides, activated charcoal, alumina, metal sulfides, zeolites and silica, are utilized in this process.
Metal organic frameworks (MOFs), as a class of materials constructed by metal ions and organic ligands, have been widely studied in fields of fuel storage, capture of gas, and catalyst application [2]. MOFs are promising materials for 1 3 liquid phase adsorption applications, because of the ultrahigh specific surface area and the easy tunability of their pore size and shape [3]. MOFs based novel adsorbents have shown the very promising results in removal of aromatic sulfur compounds from liquid fuel in recent years [4]. Graphene oxide (GO), two-dimensional (2D) carbon-based nanomaterial with oxygen-containing functional groups, has received tremendous interest in recent years. The interlayer distance of GO is usually between 6 and 12 ˚A [5]. Owing to its unique structure, GO has been used to build various composites materials with MOFs. MOF-GO, having covalent nature of interactions between the MOFs and GO applying the prepared hybrid nanocomposites as adsorbent.
In this paper, Fe-MOF and Cr-MOF and its graphene composites were prepared using simple, facile and green solvothermal method, then characterized using XRD, FTIR, N2 adsorption/desorption isotherms analysis. The performances of the adsorptive property of the prepared materials are investigated for the first time towards the adsorption of thiophenic compounds from the model diesel oils. Liquid phase batch adsorption experiments are performed at room temperature and under atmospheric pressure.

Preparation of (MOFs)
The MOFs, used in this study were prepared by mixed Fe(NO3)2·3H2O (1.73 g, 0.004 mmol in water) with benzene-1,3,5-tricarboxylic acid (H3BTC, 0.5 g, 0.0024 mmol, in ethanol) in tightly sealed Teflon reactor. The reactor was heated in an oven at 110 °C for 24 h, then cooled to room temperature. Brown solid products of MOF were collected and washed several times with ethanol, then dried at 70 °C for 12 h. For synthesis of Cr-MOF the metal source is Cr(NO3)3·9H2O under the same steps of the previous one.

Synthesis of M-BTC/ Graphene (MOF/Gr) Nanocomposites
The MOF/Gr nanocomposites used in this study were prepared as follow: (1)

X-ray Powder Diffraction Analysis (XRD)
X-ray Powder Diffraction Analysis (XRD) was used to ensure the various changes in the crystalline structure and the different phases accompanied the preparation process. Analysis was carried out using a Shimadzu XD-1 diffractometer using Cu K α radiation (λ = 0.1542 nm) at a beam voltage of 40 kV and 40 mA beam current. The intensity data was collected at 25 °C in a 2θ range of 4-80° with a scan rate of 0.7 • s −1 .

Fourier Transformer Infrared Spectroscopy (FTIR)
Fourier transformer infrared spectroscopy (FTIR) was used to investigate the function groups of the prepared samples. Analysis was carried out using ATI Mattson 1001 in the wave number region of 400-4000 cm −1 .

The Textural Properties
The textural properties were determined from the N 2 adsorption/desorption isotherms measured at liquid nitrogen temperature (-196 °C) using a NOVA 3200 Unit, USA apparatus.

Adsorption Experiment
The liquid phase adsorption properties were assessed through batch adsorption experiments to compare the performance of MOF with MOF/Gr hybride nanocomposites towards the desulfurization of the model diesel fuel. A stock solution of di-benzothiophene (DBT) as a model diesel oil was prepared (1000 ppm, in dodecane). For all experiments, an exact amount of the prepared adsorbents (0.05-0.2 g) was put in 10 mL of DBT stock solutions. The mixture was then shaked using a flask shaker oscillating at 400 oscillations/min for a fixed period of time (0.5-5 h) at an ambient temperature. The adsorption equilibrium isotherms measured in a DBT concentration ranged between 200 and 1000 ppm at 25 °C for 5 h. For analysis, the solution was separated from the adsorbents by filtration and the DBT concentration was measured using GC-FPD. The adsorption capacity and desulfurization rate were calculated by the following: where q i is the adsorption capacity of sulfur adsorbed on the adsorbent (mg S g −1 ), m is the mass of model oil (g), M is the mass of the MOF (g), C 0 and C i are the initial and final S concentrations in the model oil, respectively, C e is the equilibrium S-concentrations in the model oil.

X-ray Diffraction Analysis (XRD)
X-ray diffraction analysis (XRD)of the prepared Fe-MOF samples ( Fig. 1) with different Fe: BTC molar ratio (1:1, 2:1, and 3:1) emphasized that, all MOF samples displayed the same peaks (2θ° on 10.05°, 19.01°, and 24.4°) as the simulated XRD pattern which was calculated using single crystal data and published by [1], which confirming the successful synthesis of Fe-MOF. Moreover, for the samples with molar ratio 3:1 (Fe: BTC) there is another characteristic peak to Fe 2 O 3 (JCPDS NO.84-0311) which is a side product due to the high Fe concentration. While, the low Fe concentration (1:1) avoids the Fe 3+ ions reduction, and the intensity of the main diffraction peaks of Fe-MOF is lower than that of the prepared sample with molar ratio (2:1). Thus, the sample with molar ratio (2:1) is selected to complete the preparation of the Fe-MOF/Gr hybrid nano-composite sample. The XRD pattern of the Fe-MOF/Gr (9:1) sample is mainly consistent with that Fe-MOF (Fig. 2). No discernible diffraction peaks belonging to GO (normally at about 9.3°) be detected in the pattern of Fe-MOF/Gr. This result established that the incorporation of GO does not disturb the formation of Fe-MOF and the composites preserve the structure of Fe-MOF. Also, the absence of the characteristic peaks of GO could be ascribed to the low GO content (~ 10 wt%) and/ or the exfoliation and reduction of GO in ethanol via the sonication during the synthesis procedure.

FTIR Spectra of the Prepared Fe-MOF Samples
FTIR spectra of the prepared Fe-MOF samples with the different Fe: BTC molar ratio (Fig. 3) show different characteristic bands, confirming the successful synthesis of MOF structure [6]: (1)  On the other hand, the decreasing the Fe: BTC molar ratio, from 2 to 1, lead to disappear of all the vibrational bands. Also, the increase of Fe: BTC molar ratio from 2 to 3 lead to a decrease of all the vibrational bands which attributed to the inhibition of MOF structure formation, in agreement with XRD data.

FT-IR Spectra of the Fe-MOF/Gr Hybrid Nano-Composite
FT-IR spectra of the Fe-MOF/Gr hybrid nano-composite sample with a MOF: Gr weight ratio (9:1) ( Fig. 4) is similar to the spectrum of the parent Fe-MOF. On the other hand, the increase in the intensity of the peak at 729 cm −1 attributed to the Fe-O group formed in GO-Fe(III) [8], and the increase in the intensity of the peak at 1700 cm −1 be attributed to the partial replacement of the carboxylic groups of BTC in the MOF structure. This creates a coordinate between the oxygen groups in GO and the metal center in the MOF structure, which established that the MOF and GO are well composited to form the MOF/Gr hybrid nanocomposite material.

Texture Properties
Texture properties of the prepared materials were investigated, using N 2 adsorption/desorption isotherms. Both Fe-MOF and Fe-MOF /Gr samples showed type-I isotherm according to Brunauer's classification which is typical for microporous materials with a high nitrogen adsorption amount (Fig. 5a).The steep at the initial region is due to strong adsorption and micro-pore filling, as indicated by v-t plot and PSD (1.5-4 nm), (Fig. 5b, c). The overlaps of adsorption and desorption curves indicated that the adsorption-desorption reaction is fully reversible. The BET surface area, micropore surface area, pore volume, micropore volume and pore radius obtained from N 2 adsorption isotherms at low P/Po ranges are included in Table 1.
From data, the incorporation of GO in the prepared Fe-MOF sample lead to a decrease in the surface area in parallel with the increase in total pore volume (Table1). Moreover, BJH calculation shows unimodal pore size distribution with an obvious porous distribution around 2 nm (Fig. 5b). Accordingly, the slightly decrease in surface area and increase of pore structure may be attributed to the incorporated GO (low concentration) into MOF in agreement wih Yujie Li data [9].
In the X-ray photoelectron spectroscopy spectrum of Fe 2p for Fe-MOF (Fig. 30), there are two main peaks at 711.5 and 727 eV and a satellite peak at 718.5 eV, which matches well with the Fe-MOF [10]. All the spectrum of Fe 2p for Fe-MOF/Gr show four peaks at 714, 719.2,725.6, and 728 eV, indicating that the chemical environment of Fe(III) had not changed and still bonded with benzene rings in as-prepared Fe-MOF (Fig. 6).
According to Fig. 7, the main diffraction lines of the prepared Cr-MOF samples with different Cr: BTC molar ratio (1:1, 2:1, and 3:1) are in agreement with the simulated XRD pattern which was calculated using single crystal data of MIL-100 (Cr) with the main 2θ° (10.37°, 15.12°, and 25.27°) [11], which confirming the successful synthesis of Cr-MOF.
Moreover, for the sample with molar ratio 1:1 (Cr: BTC) the intensity of the main diffraction peaks of Cr-MOF is lower than the sample of (2:1) molar ratio, while the sample with molar ratio 3:1 (Cr: BTC) there is a quite disappear for

The XRD Pattern of the Cr-MOF/Gr
The XRD pattern of the Cr-MOF/Gr is mainly consistent with that Cr-MOF (Fig. 8). Also, No discernible diffraction peaks belonging to GO (normally at about 9.3°) has been detected in the pattern of Cr-MOF/Gr. The absence of GO characteristic peaks could be ascribed to the low GO content (10 wt%) and /or the exfoliation and reduction of GO in ethanol by sonication during the synthesis procedure as described before.

FTIR Spectra of the Prepared Cr-MOF Samples
FTIR spectra of the prepared Cr-MOF samples with the different Cr: BTC molar ratio (Fig. 9) show different characteristic bands, confirming the successful synthesis of MOF structure On the other hand, the decrease of Cr: BTC molar ratio from 2 to 1, lead to a slight decrease in the intensity of the bands. Also, the increase of Cr: BTC molar ratio from 2 to 3 lead to a slight decrease of all the vibrational bands that attributed to the inhibition of MOF structure formation which in agreement with XRD data. FT-IR spectra of Cr-MOF & Cr-MOF/Gr wt. ratio (9:1) samples (Fig. 10). Represent that the spectrum of Cr-MOF/ Gr hybrid nanocomposite is quite similar to the spectrum The texture properties of the prepared Cr-MOF and Cr-MOF/Gr hybrid nano-composites were studied and the data are graphically illustrated in Fig. 11. The GO sample have no isotherm since it exhibits no porosity.
Both Cr-MOF and Cr-MOF/Gr hybrid nanocomposites show type-I isotherm, according to Brunauer's classification which characterize the predominant of micro pore structure without hysteresis loops. On the other hand, the prepared samples show a lower deviation of the v-t plot (Fig. 11) indicating a micro pore size distribution.
Also, the incorporation of GO in the prepared Cr-MOF samples lead to slightly decrease in surface area in parallel with the increase in pore. Moreover, BJH calculation shows unimodal pore size distribution with obvious porous distribution around 2 nm (Fig. 11).
Accordingly, the slightly decrease in surface area and increase of pore structure may be attributed to the incorporation of the low concentration of GO into MOF, in agreement wih Yujie Li data [9], as in the synthesis of nanocomposites, the epoxy groups of GO act as H 2 O molecules, which usually coordinate with the central metal ions of the MOFs. During the linking procedure, delamination of the GO layers might occur, and the formed Gr layers didn't significantly disturb the crystallization/porosity of the MOF (Fig. 12). The X-ray photoelectron spectroscopy spectrum for Cr 2p For Cr-MOF represents two peaks at 587.02 eV and 576.63 eV (Fig. 37b). Meanwhile, for Cr-MOF/Gr, these two peaks are slightly shifted to 587.5 and 578.13 eV corresponded to typical binding energies for Cr 3+. Also, the slight shift in the binding energies points out an increment in the electron density around the Cr atom due the attractions between Cr 3+ center and lone pair electrons of oxygen in the graphite oxide. No other oxidation states of chromium were observed, which means the stability of the chromium trimers during the functionalization of the linker and anchoring of graphite oxide.

Desulfurization Process
To compare the effectiveness of the prepared GO, MOF and MOF/Gr hybrid nanocomposite for adsorption of DBT from n-dodecane as model oil, DBT was chosen as a representative model of aromatic compounds containing-sulfur as it was the most common refractory compound in fuel after the HDS process it is found that the removal efficiency of the prepared GO is only 26 mgS/gm. Figure 13 shows the effect of the Fe-MOF and Fe-MOF/ Gr composite on the desulfurization process, the DBT adsorption experiment was conducted at room temperature for 300 min using an initial concentration of 1000 ppm (adsorbate) and the adsorbent weight was 20 g/L (optimum condition) from the previous published paper [12]. Actually, presence of graphene in the MOF composite has positive effect on sulfur removal. The removal efficiency increases from 62% to 95.6% using Fe-MOF and Fe-MOF/Gr (9:1) weight ratio.
The same effect was found when using Cr-MOF and Cr-MOF/Gr hybrid nanocomposite (Fig. 13). While, Cr-MOF/ Gr shows best removal efficiency that reach 96.9% when using 20 g/L adsorbent, which confirm the vital effect of Gr in the performance of MOF/Gr as adsorbent for desulfurization process.

Diesel Fuel-Desulfurization process
As known, the commercial diesel fuel usually contains certain refractory sulfur compounds thiophene (TH),
From the previous result it can be concluded that, either the prepared Fe-MOF/Gr and/or Cr-MOF/Gr (9:1) weight ratio have relatively higher adsorption efficiency towards the removal of DBT than Cu-MOF/Gr [12], So the Cr-MOF/Gr adsorbent was selected to study the removal of more than sulfur compound that consist the model diesel oil.
The optimum adsorption conditions from our published paper [12] are chosen to carry out the desulfurization experimental processes of the model diesel oil either contain two and/or three organic sulfur compounds (DBT and TH), (DBT and 4,6-DMDBT), and (DBT, TH and 4,6-DMDBT).The adsorption experiments were conducted at room temperature for 300 min, an initial concentration of 1000 ppm model diesel oil (adsorbate) and on using 0.2 g of the selected Cr-MOF/Gr adsorbent one.
Data in Fig. 14, clarified that the removal of the aromatic sulfur compounds (thiophene (TH), dibenzothiophene (DBT) and 4.6 dimethyldibenzothiophene (4.6-DMDBT)) follows the order: This behavior may be related to the double adsorption mechanism, i.e., physisorption and chemisorption. Taking thiophene for example, if sole weak physisorption is active, the adsorption capacity for thiophene should be higher than for dibenzothiophene due to the more steric restrictions of the dibenzothiophene molecule. However, when both physisorption and chemisorption occur in the adsorption process, and even chemisorption is dominant, the adsorption capacity for thiophene should be lower than for dibenzothiophene.In fact, the presence of additional aromatic ring in dibenzothiophene (as comparing with thiophene) increases the π-electron density, enhancing the probability of π-complexation to the exposed metal sites [14]. Moreover, the electron densities on the sulfur atoms of dibenzothiophene, and thiophene were 5.758, and 5.696 [15] respectively which probably enhances the direct interaction between the metal sites and the S-molecules. Despite of 4,6-dimethyldibenzothiophen have approximately less more electron density on sulfur atom (5.760) than dibenzothiophene the activity of dibenzothiophene is higher than 4,6-dimethyldibenzothiophene. This is may be due to the bonds between M and sulfur atoms was hindered by the alkyl groups that presented at 4 and 6 positions in 4,6-dimethyldibenzothiophene.
Accordingly, the desulfurization experiments for the diesel fuel fraction (from Cairo Oil Refining company, with boiling point (175-320 °C), and 600 ppm sulfur content) were carried out at reaction temperature range from 25 to 60ºC on using the selective active adsorbent Cr-MOF/Gr ranging from 0.2 to 2 gm. 10 ml of the feed (fuel diesel) with the required amount of the adsorbent was shacked for 300 min at the desired reaction temperature.
From the results (Fig. 15), it was found that, with the increase in the adsorbent amount from 0.2 to 1 gm the % removal of sulfur compound gradually increased, and followed by a marginal change with the increase in the adsorbent amount to 2 gm.
The increase in the adsorbent amount provides accessibility to large surface area, more adsorption sites, and active functional groups, thereby at the initial stage the adsorption uptakes gradually increases, followed by a marginal increment at the later stage. this is due to the In other words, this phenomenon is known as overcrowding of particles or solid concentration effect in the adsorption process. Accordingly, one-gram adsorbent amount is the most selected one for sulfur removal. Figure 16 represents the effect of the reaction temperatures (range from 25 to 60 °C) on the desulfurization of the diesel oil at 300 min and the amount of adsorbent 1gm/10 ml. The data clarifies that, the removal % desulfurization at room temperature reaches 57.7%, whereas at 40 and 60 ºC decrease to 55 and 49% respectively. i.e.) the highest desulfurization is obtained at room temperature.
Concurrently, the exothermic nature of the adsorption process is in parallel with the negative value of ∆H° (Table 2), verifying the more availability of adsorption at lower temperature. And the positive value of ∆S° suggests the randomness at the solid/liquid interface in the adsorption system. Also, the negative values of ΔG at reaction adsorption temperature 298, 313, and 333 K confirm the feasibility of the process and the spontaneous nature of adsorption. The increase of the negative value of ∆G° with the decrease in temperature established that the adsorption process becomes more favorable at lower temperature. Similar result is also reported by Waqas Ahmed, et al. [16], they found that with the increase in the temperature from room temperature to 60 ºC the % desulfurization decreases from 63 to 58% on using Zn-Montmorollonite for adsorptive process.
From another point of view, Aromatics compounds in the diesel fuel have an effect on the combustion quality of the fuel and an increase in the amount of aromatics can have a negative impact on vehicle emissions. Moreover, fluorescent indicator adsorption method (FIA) was used to determine the three major type of hydrocarbons in the diesel fuel (before and after adsorption) which are saturates, olefins, and aromatics. Data represented that, the aromatic compounds were decreased from 6 to zero, olefins from 12 to 8.52 and saturates relatively increase from 82 to 91.48% after adsorption. The aniline point (AP) was also measured and the temperature increases from 75.5 (the mother diesel fuel) to 77.5 °C (the treated one). So, we can conclude that, using Cr-MOF/ Gr for diesel desulfurization (optimum conditions: 300 min, 1gm of adsorbent, room temperature) not only decrease sulfur content but also decrease the aromatic content which lead to increase in the ignition quality, improves cold start performance and also reduce PM emission.
By applying the following Eqs. (3,4): where DI is the Diesel index, AP is the aniline point (Fº), API is American petroleum institute gravity, CN is the cetane number. Taken into consideration, the aniline point and the aromatic content of the diesel fuel before and after treatment, it was found that the diesel index (DI) was increased from 66.8  (the mother diesel fuel) to 68.3 (the treated one) and also the cetane number from 58.096 (the mother diesel fuel) to 59 (the treated one) which lead to an increase in the ignition quality, improves cold start performance and also reduce PM emission [17].

Conclusion
In this work, Fe-MOF & Cr-MOF and its graphene composites are prepared by using simple green solvothermal method and characterized by XRD, FT-IR, N 2 adsorption desorption isotherm and XPS. The characterization results indicate that the prepared MOF and MOF-Graphene adsorbent materials have the features of a metal-organic framework material. From the experimental results, both MOF, and both MOF-graphene show high adsorption abilities toward DBT at ambient temperatures. The Cr-MOF-Graphene adsorbent has achieved the highest adsorption removal of 96.6% at (300 min. room temperature, 20 g/l adsorbent), it was found that the adsorption removal for diesel fuel contain 600 ppm using Cr-MOF/Gr as an adsorbent at room temperature not only remove sulfur compounds but also decrease the aromatic content which has a positive effect on the diesel fuel characterization such as: aniline point from 75.5 (the mother diesel fuel) to 77.5 °C (the treated one), Diesel index from 66.8 (the mother diesel fuel) to 68.3 (the treated one) and also cetane number from 58.096 (the mother diesel fuel) to 59 (the treated one), which lead to increase in the ignition quality, improves cold start performance and also reduce PM emission.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).
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