Synthesis of graphene oxide-supported meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride as a heterogeneous nanocatalyst for the mercaptan removal from the gas stream
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Sulfur compounds are one of the major problems and undesirable contaminants in the oil and gas industries. To address this issue, mercaptan removal from the gas stream in a fixed bed reactor under nanocatalyst was investigated. In this work, meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride-supported graphene oxide [GO-FeTCPP (Cl)] nanocatalyst was synthesized and adsorption of mercaptan on nanocatalyst was studied. Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), BET, and Raman spectroscopy analysis were used to characterize the nanocatalyst. This experiment investigated how temperature and Gas Hour Space Velocity (GHSV) parameters affect the mercaptan removal in presence of nanocatalyst. The research results confirmed that the reaction rate improves with increasing temperature and decreasing GHSV. According to the results, at 100 °C and GHSV of 1000 h−1, the maximum conversion (~ 96%) of reaction was reached.
KeywordsNanocatalyst Mercaptan removal Porphyrin Fixed bed reactor Gas hour space velocity (GHSV)
Natural gas includes heavy hydrocarbon and sulfur compounds. Mercaptans among sulfur compounds are corrosive and malodorous. Thus, the removal of sulfur compounds and mercaptans is a critical issue in refining industries . Various methods have been served for mercaptan removal, but heterogeneous nanocatalysts proved the most effective among catalysts for removal of mercaptan and other sulfur compounds in the gas stream . Among catalysts as a support, nanocarbon structures play a productive role in improving the performance of catalysts. Graphene, as one of the carbon structures, due to it's unique characteristics such as large specific surface area, chemical and thermal stability, and high adsorption capacity is extensively applied as an effective support for heterogeneous nanocatalysts and can enhance the activity and selectivity of catalysts [3, 4, 5]. Metal phthalocyanines (MPcs) display a number of unparalleled features thanks to their large conjugated molecular structure which has sturdy π–π and also the covalent interactions between aromatic rings . As a tetrapyrrolic aromatic macrocycle, porphyrin is characterized by a coordination site for diverse transition metals at different oxidation positions. Therefore, the electronic properties of porphyrins are dependent on both the coordinated metals and their substituents . In addition, porphyrins play a crucial to catalytic oxidation or reduction reactions as well as oxygen transport. Porphyrins have predictable firm compositions and photochemical electron-tractor ability [8, 9, 10]. Graphene oxide (GO) through its surface functionalities such as carboxyl, epoxy functional groups, and covalent interaction is highly activated to create more active sites on nanoparticles, and also increases physical and chemical adsorption [11, 12]. Metalloporphyrins for formation of complexes can be gathered onto the surfaces of GO by covalent interactions. GO, as a support for porphyrin complexes and metal nanoparticle, results in increased electrocatalytic activity and stability of the nanocatalyst. The incorporation of organic molecules into supported graphene sheets makes nanoscale building blocks for potential applications in organocatalysis. The reaction between strong acidic groups and the carboxylic groups of GO bring about high activity and stability of the GO-based nanocatalyst [13, 14]. In the previous studies, carbon nanotubes (CNTs) were considered as supports for porphyrin derivatives. As high conversion of mercaptan was achieved using 10% cobalt phthalocyanine/MWCNT and 20% w (Fe) TCPP-MWCNT in the variant operating condition, we developed this method for GO as a support [15, 16]. In this paper, we intend to use another derivative of nanocarbon structure like GO as a support for porphyrin complex. For this purpose, GO-supported meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride nanocatalyst was synthesized for the removal of mercaptan from gas streams. Also, the impact of the kinetic and operating parameters on the removal of mercaptan was investigated.
Materials and methods
Chemicals and reagents
All reagents including N,N-dimethylformamide (DMF), sulfuric acid (H2SO4) 90%, phosphoric acid10% (H2PO4), potassium permanganate (KMnO4), hydrochloric acid(HCI), hydrogen peroxide(H2O2), iron(III) chloride (FeCI3), sodium hydroxide (NaOH), 4-carboxy-benzaldehyde (HO2CC6H4CHO), pyrrole (C4H5N), propionic acid (CH3CH2COOH) and potassium hydroxide (KOH) were provided by Merck Company. Graphene oxide (GO) (size: 1–5 μm; thickness: 0.6–1.2 nm) was obtained from purified natural graphite using a modified Hummers method.
The morphology of GO and FeTCPP (Cl) and FeTCPP (Cl)-GO nanoparticles was observed using a SEM. The chemical structures of GO and FeTCPP (Cl) (before and after linking) were characterized using FT-IR and Raman spectroscopy. To determine the crystallinity of GO, FeTCPP (Cl) and FeTCPP (Cl)/GO composite, XRD was utilized. BET and BJH methods were employed to perform the specific surface area analysis, using a Micromeritics ACAP 2020 analyzer. All the samples were outgassed at 200 °C for 5 h before the measurements.
Synthesis of graphene oxide nanoparticles
To synthesize graphene oxide, modified Hummers’ method was used. Graphite was added to a solution containing 90% sulfuric acid and 10% phosphoric acid under magnetic stirring. During stirring in the following step, potassium permanganate was slowly added to the mixture. The produced mixture was stirred for 3 days. Subsequently, the mixture was gently decanted into a vessel containing ice and hydrogen peroxide. The graphene oxide solution was washed with deionized water several times and hydrochloric acid was removed in a laboratory centrifuge. Finally, the graphene oxide was dehydrated at 80 C˚ for 24 h under vacuum conditions.
Synthesis of Meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride
Synthesis of graphene oxide-supported meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride nanocatalyst
Synthesis of FeTCPP (Cl) complex supported on GO was carried out in reflux conditions. Then, graphene oxide was added to deionized water and received an ultrasonic treatment for a period of 5 min. also, (Fe) TCPP was added to the reaction mixture and was subsequently left for another 15 min. The resulting mixture was placed under reflux for 24 h at 100 °C. Then the mixture was washed with deionized water and finally was dried for 6 h at 70 °C (Scheme 1b).
Results and discussion
X-ray diffraction spectroscopy
The FT–IR spectra of graphane (G), graphane oxide (GO), meso-tetrakis (4-carboxyphenyl) porphyrinatoiron (III) chloride FeTCPP (Cl) complex and [GO-FeTCPP (Cl)] nanocatalyst are shown in Fig. 1b. The peaks at 1071, 1220 cm−1, 1616 cm−1, and 1721 cm−1 are accounted for by the C–O–C (epoxy), C–OH, C=C, and C=O (carbonyl and carboxylic) groups present in graphene oxide, respectively, while a stretching band at 3435 cm−1 is linked to the O–H stretching. Approximately at 2923 cm−1 and 2855 cm−1, the absorption intensity indicated the aromatic stretching vibrations of C–H bonds of GO. Once the functionalization is done with FeTCPP (Cl), a new peak emerges at 1570 cm−1 matching the C=C vibration of porphyrins while the peak of the C–O stretching vibration moves to 1105 cm−1. The presence of peaks at 1695 cm−1 and 1008 cm−1 is explained by the bending vibration of the C=N of the porphyrin ring and Fe–N stretching. All these points are visibly consistent with the presence of porphyrins grafted onto the GO sheets. At around 1726 cm−1, the FeTCPP (Cl) (Fig. 1b) shows a broad C=O stretching bond and at 995 cm−1, the C–H stretch of pyrrole ring appeared. The shift in C=O stretch from 1726 cm−1 to 1705 cm−1 in GO-FeTCPP (Cl) confirmed the graft between FeTCPP (Cl) and GO [20, 21, 22, 23, 24].
Raman spectroscopy characterization
According to Fig. 1c, the Raman spectrum of graphite shows vibrational peaks near 1337 and 1580 cm−1, probably because of the D and G bands, respectively. Moreover, the GO Raman spectrum shows D and G bands at 1340 cm−1 and 1591 cm−1, respectively. Rapid increases at D and G bands are explained by the extended graphite oxidation. Degree of disorder and defect in graphite is usually estimated using the intensity ratio (ID/IG) of D and G bands. The higher ratio (ID/IG) of GO (0.79) compared with just GO is owing to additional carboxyl groups, and also GO was more disturbed than G. The band at 1557 cm −1 of porphinato complex can be attributed to the C–C stretch as well as the band at 1366 cm−1 was assigned to C–C bonds of the pyrrole rings. After the grafting of FeTCPP (Cl), the ratio of D band to 1345 cm−1 and G-band to 1601 cm−1 was increased, which could be attributed to the interruption of the conjugated structure of GO-induced by perpendicularly grafted FeTCPP (Cl) molecules. Enhancement in the D band intensity is often linked to successful attachment of FeTCPP (Cl) complex onto the GO surface. Here, the intensity ratio (ID/IG) increased up to 0.98 for GO-FeTCPP (Cl), a finding that is consistent with the introduction of sp3 defects after the functionalization and a partial recovery of the graphene structure [20, 21, 25, 26, 27].
Scanning electron microscopy analysis (SEM)
Nitrogen adsorption–desorption analysis of GO-FeTCPP (Cl) nanocatalyst
BET surface area of GO and FeTCPP (Cl)-GO nanocatalyst
BET surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore diameter (nm)
Test system and method for removing mercaptan
Figure 5b shows the effect of GHSV on removal of mercaptan in 1000–3000 h−1 range at 100 °C. As can be seen, by increasing GHSV, the speed of the reactant rises, while the residence time decreases; thus, lower adsorption happens on the catalyst surface.
The process of regeneration of the FeTCPP (Cl)-GO nanocatalyst
The most valid mechanism for the mercaptan removal process is listed below: the general reaction:
In this investigation, a heterogeneous nanocatalyst was developed for removal of mercaptan in a fixed bed reactor. For this purpose, FeTCPP (Cl)-GO nanocatalyst was prepared by synthesis of FeTCPP (Cl) complex within GO as a support. Kinetic parameters related to catalytic reaction for FeTCPP (Cl)-GO nanocatalysts were obtained. The Raman spectrum of nanocatalyst displayed an enhancement of the ratio (ID/IG) as a result of successful attachment of FeTCPP (Cl) complex onto the GO surface. In addition, the FT–IR spectra for FeTCPP (Cl) indicated a new peak at 1570 cm−1 while the peak of the C–O stretching vibration moves to 1105 cm−1. The GO-FeTCPP (Cl) nanocatalyst showed a (001) peak at 2θ = 9°, a figure smaller than that of GO. According to SEM images, the average size of nanoparticles was approximately 20–50 nm and the isotherm of GO-FeTCPP (Cl) nanocatalyst was classified as Type I. Regarding regeneration process, catalyst can be recovered and the highest rate of mercaptan conversion was obtained at 400 °C. The results of the study show temperature as a major factor in controlling the conversion. On the other hand, the conversion rate of mercaptan under nanocatalyst increased by the increase of temperature value and the decrease of GHSV. Finally, the maximum conversion of reaction in the best condition was obtained at GHSV of 1000 h−1, T of 100 °C, and conversion of 96%.
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