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STUDY OF NHC GOLD (I) COMPLEXES CATALYZED ETHANE SELECTIVE OXIDATION TO ACETIC ACID FOR THE FIRST TIME
Dr. Ajay Kumar
Department of Chemistry, N.A.S College, Meerut, India
Abstract - The unique utilisation of N-heterocycliccarbene (NHC) and heterocyclicoxo- carbene (NHOC) gold (I) catalysts under moderate circumstances allows for the environmentally friendly production of acetic acid (40 percent yield) straight from ethane.
Comparing this protocol to the two most commonly used methods, this one is a promising and selective method for producing acetic acid directly from ethane: I the three-step, expensive and energy-intensive process based on high-temperature methane conversion to acetic acid; (ii) the current industrial methanol carbonylation processes, based on iridium and expensive rhodium catalysts The novel ethane oxidation procedure's environmental advantages are highlighted through green metrics. This extraordinary technique is compared to previously reported published catalysts in order to highlight its unique qualities.
Keywords: NHOC Gold, NHC Gold, Catalysts.
1. INTRODUCTION
It is well acknowledged that N- heterocyclic carbene (NHC) ligands have played an increasingly important role in the creation of homogeneous catalysts in recent years. Chemical processes involving NHC metal complexes include alkene activation, alkyne hydration, hydroamination, hydrosilylation, and C–C coupling, to name a few examples. Alkane oxidations, and in particular the oxidation of the light gaseous alkanes, are among the most difficult chemical processes to understand. Because of their chemical inertness, alkanes have been unable to be used as feedstock for the synthesis of functionalized added-value goods, notably carboxylic acids, despite the fact that they are the most plentiful and relatively low- cost form of carbon.
Excessive reaction conditions (high temperatures, acidic medium, lengthy reaction durations, etc.), low product yields and/or selectivities, and a lack of efficient catalytic systems for the selective activation of C-H bonds are all issues that need to be addressed in order to overcome these limits. Acetic acid is a high-value commodity with a large tonnage market due to its widespread industrial application (the global acetic acid market is expected to reach a volume of 9.07 million tonnes in 2020). It is primarily produced through a three-step, capital- and energy-intensive process based on the high-temperature conversion of methane or coal to syn-gas, the conversion of syn-gas to methanol, and finally the carbonylation of methanol to
produce acetic acid The two most widely used commercial methanol carbonylation procedures are CATIVATM and MONSANTOTM, which are both dependent on pricey iridium and rhodium catalysts, respectively.
In addition to the positives, which include increased productivity, there are some downsides to consider, including environmental concerns and severe corrosion of equipment due to the requirement for an iodide co-catalyst, which must be used. Acetic acid can also be produced by carbonylating acetaldehyde over cobalt or manganese acetate catalysts at temperatures between 150 and 160 degrees Celsius and pressures between 80 and 100 bar, or by the halide-free carbonylation of dimethyl ether over zeolites.
The latter produces methyl acetate, which is hydrolyzed to generate methanol and acetic acid with a high degree of selectivity (>99 percent). The reaction rates are currently not meeting commercial goals, despite the fact that they are promising. As a result, being able to directly generate acetic acid by the oxidation of analkane would be considerably more pure, cheaper, and hence smarter, but it would also be extremely difficult. A promising alkane for such a procedure is ethane, however some recent developments in the synthesis of acetic acid from methane via carboxylation with CO have been described. However, because to the strong
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stability of the C–H bond in ethane, the development of a practice.
Al technique for the partial oxidation of ethane under moderate circumstances has been severely hampered in recent years. The fact that one must not only activate the inert alkane but also be able to control the oxidation of desired products into chemicals such as formic acid or carbon dioxide adds to the difficulty of the task.
Ethane processing has been considered using either direct (Figure1a) or indirect (Figure1b) methods. First, ethane must be converted to ethylene (in order to activate the alkane substrate), then a two-step ethylene-acetaldehyde-acetic acid process must be performed to produce the later product. Studies have demonstrated that direct oxidation of ethane can achieve economic parity with methanol carbonylation (at comparable levels of production scale), but studies have also indicated that indirect oxidation of ethane is preferred since it eliminates the necessity for the isolation of intermediary stages.
Figure 1 Direct and indirect ways to obtain acetic acid from ethane 2. LITERATURE REVIEW
Welton, 1999, Wilkes, 2002 states that Compounds composed of ions that exhibit low melting points (usually below 100 °C) have come to be known collectively as ionic liquids (ILs). These salts have extremely low vapor pressures, wide liquid range, good electrolytic properties with large electrochemical window, tunable polarity and are easy to recycle.
They therefore have received great attention as potential solvents to replace volatile organic solvents in a wide variety of chemical reactions, separation and
manufacturing processes to provide excellent protocols for clean and green ideology.
Wasserscheid and Keim, 2000 states that since ionic liquids (ILs) have quite unique properties and can be used as solvents in organic synthesis, electrolytes in electrochemistry and catalysts in catalysis chemistry, they recently gained a lot of attention from both chemists and physicists.
Jin et al., 2005, Chiou et al., 2006 states that many transition-metal complexes dissolve readily in ionic liquids, which enable their use as solvents for transition-metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by
“greener” alternatives in transition-metal catalysis and these new materials show additional intrinsic magnetic, spectroscopic or catalytic properties depending on the enclosed metal.
3. OBJECTIVES OF THE STUDY The major objectives of the study are as follows:
To know about the complexes catalyzed ethane.
To study on the major solvents and reagents that is useful for this study.
To know about the unique
utilization of N-
heterocycliccarbene (NHC) and heterocyclicoxo-carbene (NHOC) 4. MATERIALS AND METHODS
All of the solvents and reagents used in this study were purchased from commercial sources (Sigma-Aldrich, Munich, Germany) and were not further purified before use. The N-heterocyclic carbene (NHC) and N-heterocyclic oxo- carbene (NHOC) gold(I) complexes 1–4 were synthesised in accordance with the literature. The gold(I) complexes 1–4 were synthesised in accordance with the literature. According to Figure 2, the strategic synthesis includes itrile-based pathways, which are described in more detail. Ethane was subjected to the catalytic oxidation process to yield acetic acid in a 13.5-mL stainless-steel reactor
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with a capacity of 13.5mL. 6 mL of (1:1) H2O/CH3CN solution was added to the reactor, along with 4.33mmol of oxidant (K2S2O8) and 1.50 mol of catalyst (1–4).
(or 5.5mL of trifluoroacetic acid). When it was necessary, the radical trap (TEMPO) was implemented.
Figure 2 Gold (I) complexes 1–4 containing N-heterocyclic carbene (NHC) and N-heterocyclic oxo-carbene
(NHOC) R = cyclohexyl (1–3) or cyclododecyl (1–3) have been
synthesized.
The reactor was shut down and cleansed three times with ethane before being pressurised to 3 atmospheres (barometric pressure) (0.78 mmol). It was heated in an oil bath at 80 degrees Celsius for 20 hours (higher temperatures are not suggested for this reaction owing to a decrease in the solubility of the ethane as well as the breakdown of the oxidant), stirring occasionally. After finishing the reaction, the reactor was put in an ice bath to cool down to room temperature, after which it was degassed and opened for inspection. 1 mL of reaction was added to 5 mL of diethyl ether (to extract organic compounds and precipitate the catalyst) and 90 mL of de n-butyric acid (as an internal standard) and allowed to stir for 30 minutes at room temperature. The mixture was then filtered and subjected to gas chromatography (GC) analysis using the internal standard technique, which was performed after that.
The GC studies were performed using a FISONS Instruments GC 8000 (Tokyo, Japan) series gas chromatograph equipped with a FID detector and a capillary column (DB-WAX 0.32 mm
internal diameter and 30 m column length, respectively), with helium serving as the carrier gas. Specifically, the software used was the Jasco-Borwin v1.50 (Tokyo, Japan). The injection temperature was 240 degrees Celsius. The temperature is first set at 100oC for 1 minute, then climbs at a rate of 10oC/min until it reaches 180oC, where it is maintained for 1 minute. Using the average of runs with identical outcomes, calculate the value of Yield for each individual run.
5. RESULTS AND DISCUSSIONS 5.1 Direct Oxidation of Ethane to Acetic Acid
Several N-heterocyclic carbene (NHC) and N-heterocyclic oxo-carbene (NHOC) gold(I) chloride complexes, including saturated and unsaturated N-heterocyclic carbene (NHOC) complexes, were synthesised.
Several cyclic carbene complexes with the 2, 6-diisopropylphenyl group on one nitrogen atom and the cyclohexyl or the cyclododecyl group on the other nitrogen atom were chosen with the goal of investigating structure-reactivity connections (Figure 3). It is well known that the monodentate NHC electron rich σ-donor ligands attach to Au(I) in a "push- pull" process, resulting in a strong bond.
Figure 3 A symmetrical N-heterocyclic carbene (NHC) gold(I) chloride complex and an unsymmetrical N-heterocyclic
oxo-carbene (NHOC) gold(I) chloride complex (3).
An experiment was carried out at 80oC for 20 hours on the direct oxidation of ethane to acetic acid (Figure 1a), utilising potassium peroxodisulfate as an oxidant and Au(I) complexes 1–4 as catalysts in
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aqueous acetonitrile (optimized conditions). The influence of the N- structure, heterocycle's as well as the substitution pattern at the nitrogen
atoms, was investigated under the same reaction circumstances as the previous experiments.
Table 1 Direct oxidation of ethane by K2S2O8 in water/acetonitrile (1:1) mixture Entry Catalyst C2H6 Catalyst
mount Yield(%)a TONb
Acetic
acid Proplonic
Acid Acetic
acid Proplonic Acid (atm) (µmol)
1 1 3 1.5 39.8 210
2 2 3 1.5 35.5 187
3 3 3 1.5 34.3 181
4 4 3 1.5 33.9 178
5 3 2.2
6 c 1 3 1.5
7 d 1 3 1.5 13.4 67
Reaction conditions: water/acetonitrile (1:1) mixture as solvent (6 mL), K2S2O8 as oxidant, T = 80◦C, 20 h. a molar yield (%) based on C2H6, i.e., moles of acid per 100 mol of ethane. b TON (turnover number) = moles of product per mol of catalyst. c no oxidant. d in the presence of TEMPO.
Fig. 4 Acetic acid yields achieved by direct oxidation of ethane by K2S2O8, catalysed by complexes 1–4.
The observed trend in terms of the structure of the core cycle is consistent with the behaviour reported earlier for other catalytic reactions using these complexes, which was previously discovered. The higher electron density at the metal center of the saturated NHC ligand compared with the unsaturated one, in addition to a synergetic effect of π back-donation and σ donation of the saturated NHC ligand, leads to a more stable complex and this stability plays an important role in the catalytic properties of the gold(I) complexes, in particular for long reactions.
Blank experiments were performed in the presence of C2H6 and K2S2O8
(metal-free as analyzed by ICP) and confirmed (see entry 5 of Table 1 for catalyst 1) that almost no acetic acid
formation was detected unless the Au catalyst was used.
The traditional solvent for the oxidation of ethane, trifluoroacetic acid (TFA), was also used with the Au(I) complexes 1–4. As depicted in Figure 4, the performance of all pre-catalysts is much worse in TFA, leading to yield decreases of ca. 60% when compared with the results attained in the water/
acetonitrile (1:1) mixture.
5.2 Green Metrics
Table 2 contains numerous specific green analytical chemistry metrics that were produced with the goal of measuring the improvement brought about by our new ethane-oxidation technique in as quantitative a manner as feasible.
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Table 2 Au(I) complexes 1–4 are responsible for the oxidation of ethane to acetic acid, which is measured using green chemistry metrics.
Ent
ry Solvent Cata
lyst Theoretical E-factor E-
factor AE
(%) MI RME AU (%) f
1 H2O/ACN 1 0.2 0.5 20 36.5 66.5 100 107.1
2 TFA 2.7 90.3 26.9 696.8
3 H2O/ACN 2 0.2 0.7 20 41 59.4 100 120.1
4 TFA 1.9 71.6 34 552.6
5 H2O/ACN
3 0.2 0.8
20 42.4 57.1
100 124.3
6 TFA 4 120.2 20.1 927.2
7 H2O/ACN 4 0.2 0.8 20 42.9 55.3 100 125.8
8 TFA 2.9 91.6 25.9 705.7
It was estimated to reveal the waste created by the current process by using the Environmental Factor (E-factor), which is the total weight (in kg) of all trash generated in an industrial process per kilogramme of product. An E-factor number that is close to zero indicates that less waste is created, and as a result, the process is more environmentally sustainable.
In accordance with the results reported in Table 1, the use of the solvent mixture H2O/acetonitrile ACN resulted in consistently lower E-factor values than the values obtained under the identical reactional circumstances but using TFA as the solvent in all cases. Despite the fact that there is no direct assessment of the dangers or environmental concerns associated with the created waste, the drop in the values for the E-factor parameter illustrates the shift toward a more environmentally friendly solvent system.
Under the premise of complete reagent conversion and waste minimization, a theoretical E-factor was also calculated, and we can see from Table 3 that this process, in and of itself, is quite promising as a technical or industrial process with a low environmental effect. The difference between the measured and theoretical E- factor values is the smallest for catalyst 1 in water/ACN mixture, entry 1 of Table 3, which corresponds to the highest achieved acetic acid yield (Table 1 and Figure 4). It is also the smallest for pre- catalyst 1 in TFA mixture, entry 1 of Table 3, which corresponds to the highest achieved acetic acid yield (Table 1 and Figure 4). (Table 3). As might be predicted, a low quantity of created products implies a low-activity process with space for further development in the future.
Other factors were established using a multi-metrics technique, which was followed. When it comes to chemical process constraints, Atom Economy (AE) and Mass Intensity (MI) are significant indicators to look for. Atom Economy (AE) measures a chemical process's atomic efficiency while Mass Intensity (MI) measures its application (as quantifies reaction efficiency, stoichiometry, amount of solvents, all reagents and auxiliary substances). As a result, an increase in AE should result in an increase in MI. The atom economy is 20 percent in this piece of work (see Table 2). Although this figure appears to be modest, it is important to remember that oxidation reactions often require a large amount of oxidant (this new process uses a reagent: oxidant ratio of 1:6).
A comparison of our results, specifically for complex 1 (entry 1 of Table 2), with the previous complex resulting in the highest yield of acetic acid (40.9 percent), i.e., [ReClFN2C(O)Ph(Hpz)z
(PPh3)], reveals that complex 1 (entry 1 of Table 2) yielded the highest yield of acetic acid (40.9 percent). The fact that the value of MI doubles for our new procedure (Hpz = pyrazole) (entry 7 of Table S1, ESI) demonstrates this (as the initial amount of reagents in the method using the Re catalyst is almost the double). A reactive input's relevance in achieving the intended results might be demonstrated in this way.
Solvent and Catalyst
Environmental Impact Parameter (f) was developed to aid in the quantification of the overall "greenness" of the process.
This metric, which takes into account the real masses of materials used in the process (and whether they are recycled, recovered or eliminated), was developed to aid in the quantification of the overall
"greenness" of the process (Table 2). There are a variety of factors that can contribute
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to this increase, one of which is the improved selectivity (and, consequently, higher efficiency) of our Au(I) catalysts.
The amount of gold complexes utilised by us is also substantially less (1.5 vs. 20 mol, see Table S1, ESI), which indicates that the conversion happens in a more efficient manner per atom than that employed by other researchers. This appears to be the case for complex 1, which should be more stable than the rhenium catalyst as a result of the above (entry 7, Table S1, ESI).
6. CONCLUSION
The development of an effective and easy catalytic approach for the synthesis of acetic acid from ethane has resulted in the development of an acetic acid synthesis process. In light of the promising results obtained by using NHC gold(I) catalysts, which are also supported by the determined green metrics, further research into the mechanism involved, as well as optimization studies (e.g., catalyst recycling), is being conducted to aid in the design and development of a sustainable catalytic process for this pivotal transformation.
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