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Modified activated carbons for esterification of acetic acid with ethanol
Journal Pre-proof Modified activated carbons for esterification of acetic acid with ethanol
Beata Krzyżyńska, Anna Malaika, Karolina Ptaszyńska, Agnieszka Tolińska, Piotr Kirszensztejn, Mieczysław Kozłowski PII:
S0925-9635(19)30454-6
DOI:
https://doi.org/10.1016/j.diamond.2019.107608
Reference:
DIAMAT 107608
To appear in:
Diamond & Related Materials
Received date:
5 July 2019
Revised date:
5 October 2019
Accepted date:
2 November 2019
Please cite this article as: B. Krzyżyńska, A. Malaika, K. Ptaszyńska, et al., Modified activated carbons for esterification of acetic acid with ethanol, Diamond & Related Materials (2018), https://doi.org/10.1016/j.diamond.2019.107608
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© 2018 Published by Elsevier.
Journal Pre-proof
Modified activated carbons for esterification of acetic acid with ethanol Beata Krzyżyńska, Anna Malaika*, Karolina Ptaszyńska, Agnieszka Tolińska, Piotr Kirszensztejn, Mieczysław Kozłowski Faculty of Chemistry, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 8 61-614 Poznań, Poland
Abstract
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Activated carbons from brown coal and pinewood sawdust were prepared and applied in
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heterogeneous catalytic esterification of acetic acid with ethanol. The precursors were
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chemically activated with potassium hydroxide and the samples obtained were treated with
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different agents (HNO3, CH3COOOH, H2O2, (NH4)2S2O8, air, and ammonia) in order to produce new or alter the existing surface functional groups. The materials synthesised were
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then subjected to elemental and textural analyses, temperature programmed decomposition
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(TPD) and potentiometric titration. The effect of chemical modifications on the catalytic activity of the samples in the esterification reaction was investigated. It was found that the
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type of carbon modification has a significant impact on the catalytic performance of the materials obtained. The samples treated with ammonia almost did not catalyse the esterification of acetic acid with ethanol, while the oxidised carbons showed significant activity in the reaction. This phenomenon was a simple consequence of acidic character of the oxidised carbons’ surface. Moreover, it was established that the yield of the product depends not only on the acid-base properties of the materials obtained but also on their textural parameters. Keywords: Activated carbons; Carbon modification; Esterification; Acetic acid; Ethanol *
Corresponding author. Tel.: +48 61 829 1718; fax: +48 61 829 1555.
E-mail address: [email protected]
1
Journal Pre-proof 1. Introduction Esters of carboxylic acids and alcohols or phenols are a group of organic compounds very important for industrial applications. As the ester functionality is responsible for odour (it is the so-called osmophore), esters are widely applied in cosmetic industry (e.g. for perfume production) and food industry (as aroma components). They are also used in pharmaceutical, chemical and petrochemical sectors (as polymer plasticisers, components of solvents and
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paints, or fuels) [1]. Recently, the interest in some esters of low molecular weight has been increased significantly because of their potential application as biodiesel additives [2].
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Esters can be synthetically obtained in different reactions [3]. From among the methods
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described in literature, one of the most often applied and the simplest is the direct reaction of
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carboxylic acid with alcohol in the presence of an acidic catalyst (the so-called Fischer
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esterification). On an industrial scale, the process of esterification is realised in the liquid phase with the use of strong mineral acids, like e.g. sulphuric acid. The method is effective as
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ensures high yields of esters, however, the use of a homogeneous catalyst of strongly corrosive properties implies the need of additional steps such as neutralisation and separation
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of the catalyst, which increases the cost and generates impurities [4]. Moreover, the conventional homogeneous acid-catalysed esterification systems suffer from problems associated with the side reactions (e.g. etherification, dehydration) [5]. Furthermore, catalysts of this type often cannot be used more than once. Besides the homogeneous catalysts, also heterogeneous ones can be applied in esterification reactions, e.g. ion-exchange resins, zeolites or superacids [6-9]. An interesting option is the use of activated carbons in the process. These materials have been proved to show good catalytic properties in many reactions, e.g. industrial production of phosgene, removal of nitrogen and sulphur oxides from exhaust gases [10], methane decomposition [11], ethylbenzene dehydrogenation [12] and other processes realised in laboratory scale. The catalytic activity of activated carbon stems
2
Journal Pre-proof from the chemical character of their surfaces related to the presence of active centres of different types. Because of relatively low cost of activated carbon production, availability of substrates for the preparation of carbons, facility of surface modification, their thermal stability, resistance to poisoning and simple utilisation, these materials seem to be an interesting alternative to the conventional catalysts. So far, only a few authors have been interested in carrying out esterification with the use of activated carbon catalysts [13-18] and to the best of our knowledge, only few papers have
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been devoted to esterification of acetic acid with ethanol in the presence of activated carbon.
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Liu and co-workers [18] have compared the catalytic activity of sulphonated activated carbon
-p
(AC-SO3H) and two commercial materials, i.e. Nafion NR50 and Amberlyst 15, in the
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esterification of selected carboxylic acids (acetic, hexanoic and decanoic acids) with ethanol. In the esterification of acetic acid with ethanol, the best results have been obtained for the
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ion-exchange resin Amberlyst 15. The authors have concluded that the high catalytic activity
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of this material follows from a high content of sulphonic groups in this resin. However, in the esterification of carboxylic acids of longer chains, the catalyst AC-SO3H has been shown to
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have much higher catalytic activity than the commercial catalysts tested. This fact has been related to the texture of the activated carbon used, its large surface area and mesoporous structure. The greatest drawback of the sulphonated carbon was the washing out of SO3H groups from the material structure during the reaction. The advantages of using heterogeneous catalysts in esterification reactions, including simple product isolation, ease in recovery and reuse of the catalysts, reduction in the generation of the wasteful products etc. [19], have prompted us to check the performance of modified activated carbons in esterification of acetic acid with ethanol. According to literature data, the catalytic properties of different materials in the esterification process are strongly related to their acidity. Because of that, the activated carbons were subjected to modifications
3
Journal Pre-proof in oxidising conditions to endow their surface with acidic character. For the sake of comparison, the activated carbons of basic character (obtained by modification with ammonia) were also tested. The activated carbons prepared were thoroughly characterised and correlations between their catalytic performance and chemical properties of their surfaces as well as textural parameters were checked. 2. Experimental
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2.1. Preparation of catalysts
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Activated carbons were obtained from two precursors: brown coal (from Polish colliery
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"Konin") and pinewood sawdust. The substrates were crumbled (the brown coal was
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additionally demineralised by the Radmacher and Mohrhauer method [20]), and subjected to chemical activation by potassium hydroxide in solid state in argon atmosphere at 1073 K for
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45 minutes. The weight ratio of the precursor to KOH was 1:1. The activated carbons
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obtained were washed with a 5% solution of hydrochloric acid and then with distilled water, next they were dried overnight at 383 K. The initial activated carbon samples (labelled
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according to the type of carbon precursor used: Konin coal after demineralisation – KD, and sawdust – S), were further modified either by oxidation or by ammonia treatment in order to change the amount and type of functional groups on their surfaces. The oxidation was realised with the liquid agents such as concentrated nitric acid, peroxyacetic acid, diammonium peroxydisulphate, hydrogen peroxide, or gas agent (air). The catalyst samples labelling, conditions of modifications and yields of carbon catalysts obtaining with respect to the mass of the initial activated carbon are given in Table 1. Characterisation of the materials and details of their synthesis have been given in our previous paper [21].
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Journal Pre-proof 2.2. Characterisation of catalysts All catalyst samples tested were subjected to elemental analysis performed with the use of an Elemental Analyser Vario EL III. The porous structure of activated carbon samples was determined by a Micromeritics Sorptometr ASAP 2010 with nitrogen as adsorbate at 77 K. The apparent total specific surface areas (SBET) were calculated using the BET equation, whereas the t-plot method was applied to calculate the micropore volumes (Vmicro) and
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"external" (mesopores and macropores) surface areas (Sext) [22]. The total pore volumes (Vtot) were obtained from the N2 amount adsorbed at a relative pressure close to unity. In order to
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define the character of the oxygen functional groups, the temperature programmed
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decomposition (TPD) method was applied. In a typical TPD run, the sample (0.1 g) was
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heated up to 1323 K in Ar flow (10 cm3/min) at a heating rate of 10 K/min. The amounts of
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CO and CO2 evolved were measured using a Gas Analyser ThermoStar GSD 301 T2 (Pfeiffer Vacuum) equipped with a mass spectrometer. The total acidity of samples was determined by
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potentiometric titration. In each experiment, 0.1 g of a dry carbon sample (0.05 g in the case of Amberlyst 15) was contacted with 50 cm3 of a 0.01 M NaOH solution and then shaken at
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room temperature for 20 h. After that time, the suspension was filtered and titrated with a 0.05 M HCl solution.
2.3. Catalytic measurements
The esterification of acetic acid with ethanol was performed at 348 K for 20 h, using activated carbons prepared as catalysts and a mixture of ethanol and acetic acid at a molar ratio of 10:1. The content of a carbon catalyst was 2% of the reagents' mass. The products obtained (collected after 1.5, 3, 4.5, 6 and 20 h) were chromatographically analysed on a Porapak QS column of 1 m in length. The esterification reaction of acetic acid with ethanol in the presence of H2SO4 (commercial homogeneous catalyst), Amberlyst 15 (commonly applied heterogeneous catalyst) and the blank test were also carried out. To assess the activity
5
Journal Pre-proof of samples in subsequent reaction cycles, a selected carbon was re-used in the process (after being filtered and then washed with hot distilled water). After the 3rd reaction, the additional regeneration step with 5% HCl was performed. 3. Results and discussion 3.1. Characterisation of samples The raw brown coal was characterised by a high content of ash (22.6%), so it was
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subjected to demineralisation by the Radmacher and Mohrhauer method, after which the
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content of ash was reduced to 0.9%. Thanks to a low content of mineral components in
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pinewood sawdust, its demineralisation was not needed. The precursors were chemically activated by potassium hydroxide in solid state at 1073 K for 45 min. The initial activated
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carbon samples obtained in this way (labelled as KD and S, respectively) were subsequently
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modified with different agents to get materials of different chemical properties. Analysis of
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the yields of the modification products (Table 1) proved that all the processes of oxidation resulted in insignificant mass loss of the treated carbons (the yield of modification products
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was above 90%), which means that deep oxidation took place to a small degree. The yields of activated carbons obtained via modification of initial samples with ammonia were lower (82-85%), which could be explained by hydrogasification of carbon under the conditions applied (decomposition of ammonia at high temperatures generates small amounts of hydrogen [23]). The most important data on elemental and textural analyses of the catalysts obtained are presented in Table 2. As expected, the treatment of initial samples with oxidising agents (both in liquid or gas forms) resulted in an increase in the molar ratio of oxygen to carbon in the materials after modification. The increase in the O/C parameter depended on the type of oxidising agent and conditions (temperature and duration) of modification. The effectiveness of oxidation of the initial activated carbons KD and S with the applied agents was found to 6
Journal Pre-proof decrease in the series: APS > HNO3 > H2O2 > PAA > AIR. The amounts of oxygen in the activated carbons after modification correspond to the total acidity of the samples – the greater the content of oxygen, the higher the acidity, see Table 2. The concentration of acidic groups was only slightly dependent on the type of a precursor used. Table 1 Scheme of modification of initial activated carbons (KD and S) and yields of products Reaction Reaction Sample code Modifying agent Yield (%) time (h) temperature (K) 4
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S-PAA-4h-333 K
96.3 CH3COOOH
KD-PAA-8h-333 K
KD-H2O2-24h-333 K S-H2O2-24h-333 K KD-HNO3-4h-333 K S-HNO3-4h-333 K
-p 24
333
4
333
conc. HNO3
8
333
(NH4)2S2O8
24
303
(NH4)2S2O8
24
333
air
4
573
air
8
573
conc. HNO3
S-HNO3-8h-333 K
KD-AIR-8h-573 K
S-NH3-4h-923 K
99.7 96.3 98.6 95.3 97.1 92.4
S-AIR-8h-573 K KD-NH3-4h-923 K
99.7 99.1
KD-AIR-4h-573 K S-AIR-4h-573 K
90.7 99.8
KD-APS-24h-333 K S-APS-24h-333 K
96.7 91.7
KD-APS-24h-303 K S-APS-24h-303 K
98.1 96.3
303
H2O2
98.3 95.0
333
24
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KD-HNO3-8h-333 K
333
re
S-H2O2-24h-303 K
H2O2
8
lP
KD-H2O2-24h-303 K
CH3COOOH
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S-PAA-8h-333 K
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KD-PAA-4h-333 K
93.0 91.0 92.6
NH3
4
923
82.3 84.5
7
Journal Pre-proof The treatment of initial activated carbons with ammonia at a high temperature resulted in the removal of considerable amount of oxygen from them. It is a consequence of thermal decomposition of oxygen functional groups as well as their reduction by hydrogen, generated in the process of partial thermal decomposition of ammonia [23]. The treatment applied also led to an increased content of nitrogen in the materials modified (results not shown), which can be a consequence of incorporation of this element in the form of pyridinium and pyrrole groups or quaternary systems [24] into the carbon structure.
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The initial activated carbons (KD and S) show relatively high apparent surface areas of
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over 900 m2/g and 1150 m2/g, respectively (Table 2). The processes of modification of the
-p
initial activated carbon samples had various effects on the apparent surface areas of modified
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materials, but in general the changes were rather small. Quite different results were obtained for KD and S samples modified with diammonium peroxydisulphate at 333 K. The modified
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samples had drastically smaller apparent surface areas. Most probably, it is related to
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destruction of the porous structure of the initial activated carbons upon their intense oxidation and/or with the obstruction of the access to pores by large volume oxygen functional groups
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(e.g. carboxylic ones) [25] introduced upon modification (high oxygen content in the APS-modified samples, see Table 2). Further analysis of the results presented in Table 2 proves that in all activated carbon samples the dominant is microporous structure as the contributions of meso- and macropore areas to the total surface areas are low, while the contributions of micropore volumes to the total pore volumes are high (Sext/SBET*100% and Vmicro/Vtot*100% parameters, respectively). Detail data characterising the physicochemical properties of the activated carbons obtained are given in our previous paper [21].
8
Journal Pre-proof
Sext/SBET *100%
Vmicro/Vtot *100%
O/C
Total acidity (mmol/g)
KD
953
2.31
91.7
0.037
0.78
KD-PAA-4h-333 K
1041
2.11
94.2
0.093
1.01
KD-PAA-8h-333 K
980
2.14
93.9
0.081
1.02
KD-H2O2-24h-303 K
938
1.81
93.8
0.096
1.09
KD-H2O2-24h-333 K
958
2.09
92.0
0.125
1.49
KD-HNO3-4h-333 K
856
2.10
93.0
0.167
1.99
KD-HNO3-8h-333 K
881
2.27
93.2
0.197
2.32
KD-APS-24h-303 K
744
2.28
92.1
0.272
3.09
KD-APS-24h-333 K
415
3.37
86.4
0.317
3.72
KD-AIR-4h-573 K
999
2.20
92.0
0.047
0.80
KD-AIR-8h-573 K
1100
2.45
92.7
0.077
0.92
KD-NH3-4h-923 K
1041
2.21
92.3
0.011
0.15
S
1157
1.99
94.7
0.045
0.58
S-PAA-4h-333 K
1178
1.87
93.2
0.094
1.08
S-PAA-8h-333 K
1154
2.08
93.1
0.073
1.09
1131
1.68
93.1
0.120
1.27
1057
2.18
91.2
0.136
1.63
S-HNO3-4h-333 K
1067
1.97
94.3
0.199
2.17
S-HNO3-8h-333 K
1058
2.08
94.2
0.178
2.52
S-APS-24h-303 K
951
2.63
91.7
0.296
3.36
S-APS-24h-333 K
462
3.46
87.5
0.394
3.81
S-AIR-4h-573 K
1217
2.38
93.4
0.072
0.96
S-AIR-8h-573 K
1159
1.73
93.1
0.076
1.07
S-NH3-4h-923 K
1197
1.92
93.3
0.000
0.08
S-H2O2-24h-303 K
ro
-p
lP
Jo ur
S-H2O2-24h-333 K
na
Sample
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SBET (m2/g)
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Table 2 Physicochemical properties of investigated samples
The type of oxidising agent determines not only the amount of oxygen introduced to the carbon scaffold (as shown in Table 2) but also the type of oxygen structures formed [26]. Since the oxygen functionalities govern the surface chemistry of carbons, determining their
9
Journal Pre-proof nature and type is crucial, especially for catalysis. Thus, in order to know the surface chemistry of carbons, the prepared materials were previously tested in selected model reactions, i.e. decomposition of isopropanol, cyclisation of acetonylacetone and cumene decomposition. Details of these processes and some important results were shown in our previous paper [21]. Briefly, it was found that the carbon samples oxidised with diammonium peroxydisulfate or concentrated nitric acid showed almost fully acidic character and almost only Brönsted acid centers were observed. The other oxidised carbons had not only acidic
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sites but also some basic centers on their surfaces (similarly as KD). The sample with
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a particularly high ratio of basic to acidic centers was carbon modified with H2O2. On the
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mainly in the form of basic structures.
-p
other hand, activated carbons treated with H2 or NH3 showed lower amount of active sites,
To identify more precisely the form of oxygen present on the surfaces of the samples, the
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obtained activated carbons were further subjected to temperature-programmed decomposition
na
(TPD). The usefulness of this technique for carbon characterisation has been well documented by many researchers [27,28]. Generally, the TPD method involves heating a sample of carbon
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in a stream of inert gas while increasing the temperature. The resulting gaseous products are then analysed usually using a mass spectrometer. The nature of oxygen groups on the carbon surface can be assessed by the type of gas (CO and/or CO2) and the temperature at which it is released from the sample. There is a rich literature concerning the TPD technique and the temperature ranges specific to the decomposition of various oxygen groups [27-29]. However, it is well known that the assignment of TPD signals (corresponding to release of CO and CO2) to the specific functional groups might be a bit ambiguous. It follows from the fact that the thermal decomposition of oxygen functional groups can depend on the porous structure of the material, rate of its heating or on the type of measuring instrument used [27]. Nevertheless, certain general tendencies in behaviour of particular oxygen groups upon heating have been
10
Journal Pre-proof established and presented by Figueiredo [27] and Szymański [29]. According to the results presented by the aforementioned authors, the least stable are carboxylic groups that undergo decomposition in the range 373-673 K with release of carbon dioxide. The same product is formed as a result of decomposition of lactones, taking place in the range 463-940 K. The phenolic and hydroquinone groups (873-1110 K), carbonyl and quinone groups (973-1253 K) and ether groups (823-1100 K) decompose in similar temperatures with liberation of CO. Carboxylic anhydrides (623-1173 K) and pyrone structures (1173-1473 K) decompose with
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release of both CO2 and CO. The interpretation of TPD profiles presented in our paper was
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performed taking into account the above data. As the TPD results obtained for individual
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series of samples were similar, in this paper we present only the results obtained for activated
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carbons prepared from brown coal, oxidised under the more "drastic" conditions, that is in the higher of the temperatures specified above or for the longer time of the process. For
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shown.
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comparison, also the TPD profiles of the initial carbon and that modified with ammonia are
The CO2 profiles of the oxidised carbon samples show a characteristic band in the range
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473-673 K (Fig. 1), which can be assigned to the carboxylic groups present in the samples [27,29]. As follows from these profiles, the content of carboxylic groups in the oxidised activated carbons decreases in the sequence APS > HNO3 > H2O2 > PAA > AIR, and well correlates with the total acidity of the samples (Table 2). Similar relations have also been obtained by our group for activated carbons prepared from peach stones [26]. The carbon sample oxidised with air has fewer carboxylic structures than the initial carbon catalyst (KD), which is a consequence of a relatively high temperature of the modification (573 K). Under such conditions, both the initially present and newly formed carboxylic groups undergo decomposition. The CO2 profiles show also a broad peak above the decomposition temperature of carboxylic groups. This signal can be assigned to the presence of carboxylic
11
Journal Pre-proof anhydrides and lactone groups in the activated carbons studied [26,30]. These groups can be originally present in the samples or can be generated during TPD measurements [26]. As known from literature, the neighbouring carboxylic groups can undergo dehydration to anhydrides, moreover, carboxylic groups can react with phenolic groups with formation of lactones [30]. The low intensity of the signal appearing above 1173 K means that pyrone structures can be present in the samples studied only in small amounts. According to our data, the profile of sample KD-NH3-4h-923 K shows the CO2 signal of much lower intensity than
of
that of the initial activated carbon KD. It means that the treatment with ammonia leads to
ro
removal of the majority of surface oxygen groups, which was also indicated by elemental
-p
analysis (see Table 2). -10
3.5x10
-10
-10
2.0x10
-10
-10
-11
5.0x10
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
na
2.5x10
Jo ur
Ion Current [pA]
-10
1.0x10
KD-H2O2-24h-333 K
lP
3.0x10
1.5x10
KD KD-APS-24h-333 K KD-HNO3-8h-333 K
re
CO2
0.0 200
400
600
800
1000
1200
1400
Temperature [K]
Fig. 1. CO2 release profiles of selected samples. The main part of CO profiles of all carbon samples investigated are broad bands in the range 700-1250 K (Fig. 2). They are probably related to decomposition of ether, phenol and carbonyl (quinone) functionalities or anhydrous groups. According to literature data, ether
12
Journal Pre-proof groups can be formed upon TPD measurements as a result of a combination of phenolic groups [30]. As mentioned earlier, carboxylic anhydrides undergo decomposition with liberation of CO2 and CO. As the CO2 profiles of the samples oxidised in solutions indicate the presence of anhydrides (Fig. 1), it can be assumed that a significant part of the signal from the range 700-1250 K on the CO release curve also comes from these groups (some of them are generated from carboxylic groups). -10
6x10
KD KD-APS-24h-333 K KD-HNO3-8h-333 K
of
-10
5x10
CO
-10
3x10
ro -p
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
-10
4x10
re
Ion Current [pA]
KD-H2O2-24h-333 K
-10
lP
2x10
-10
400
Jo ur
0 200
na
1x10
600
800
1000
1200
1400
Temperature [K]
Fig. 2. CO release profiles of selected samples.
Air treatment is very different from liquid modification. The sample oxidised with air contains a small amount of anhydride structures as evidenced by a low intensity of the band assigned to CO2 in the above mentioned temperature range (Fig. 1). So, the peak of CO release in the range 700-1250 K observed for this sample must come from the phenolic, carbonyl (quinone) and ether groups (Fig. 2). Thus, the results obtained in this study confirm the literature suggestion that the oxidation in solution of oxidisers leads mainly to formation of surface carboxyl groups, while oxidation with air leads to formation of both carbonyl and
13
Journal Pre-proof hydroxyl (phenolic) groups [27,29]. The profiles of CO release also show a weak signal at about 550 K (Fig. 2). This signal is probably related to the thermal decomposition of carbonyl groups in α- substituted ketones or aldehydes [29,30]. The curve of CO release for the sample treated with ammonia is flat and shows some increase in the intensity of the signal in high temperatures only, which proves that the sample studied contains small amounts of oxygen groups, mainly those particularly resistant to high temperature, i.e. pyrone and carbonyl ones.
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3.2. Catalytic activity of carbons
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The activated carbons prepared were tested as catalysts for esterification of acetic acid
-p
with ethanol for 20 h operation period. The performance of the samples was discussed in terms of the yield of ethyl acetate formed. Figs 3 and 4 present the time dependence of
re
catalytic activity of selected samples, that is the activated carbons oxidised under more
lP
"drastic" conditions (at the higher temperature or for a longer time), the samples treated with ammonia and the initial activated carbons. For the sake of comparison, Fig. 3 presents also the
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results obtained in the reaction carried out in the presence of the conventional homogeneous
test.
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catalyst (H2SO4), a commonly used heterogeneous catalyst Amberlyst 15, and for the blank
It is well known that esterification reaction is a slow and equilibrium-limited process. Typical esterification reactions require several days to attain the equilibrium in the absence of catalyst [31]. As follows from Fig. 3, the equilibrium is reached very fast for the reaction with H2SO4 used as a catalyst and the yield of ethyl acetate is close to 95%. Only slightly worse catalytic performance is shown by Amberlyst 15. The activity of carbon materials applied is lower and the yield of the product increases almost linearly with the time of the process. Obviously, this is due to the lower acidity of the carbon samples (see Table 2) compared to that of sulphuric acid or Amberlyst 15 (4.6 mmol/g). Moreover, some authors suggest that heterogeneous catalysts exhibit some limitations on catalysing esterification reaction related
14
Journal Pre-proof to mass transfer resistance or loss of the active acid sites in the presence of a polar medium [32,33]. 100 90 KD KD-APS-24h-333 K KD-HNO3-8h-333 K
70
KD-H2O2-24h-333 K
50
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
of
60
blank test Amberlyst 15 conc. H2SO4
40 30
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Yield of ethyl acetate [%]
80
-p
20
0 0
3
6
re
10
9
12
15
18
21
lP
Time of reaction [h]
Fig. 3. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction
na
obtained over selected activated carbon catalysts prepared from brown coal.
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In the absence of catalyst, the yield of the product is of about 9% after 20 h of reaction (Fig. 3). In this case, the acetic acid acts as a mild autocatalyst for esterification of ethanol, which has been also reported by other authors [19,34]. The initial activated carbon catalyst shows rather small yields of ethyl acetate, which are only slightly higher than those obtained in the blank test. This is due to the low oxygen content detected for this sample and its low acidity (see Table 2 and Figs 1 and 2). The poorest results were obtained for the NH3-modified sample. Ammonia treatment removes oxygen groups from the catalyst surface and leads to the reduction of the total acidity (see Table 2). On the other hand, a high content of nitrogen was detected in NH3-modified carbon, and the character of the sample surface assessed applying model reactions was basic (see our previous paper [21]). According to Fig. 3, the yield of the product is even slightly lower for this sample than that obtained in the
15
Journal Pre-proof blank test. It might suggest that the basic groups partially catalyse the hydrolysis of ester giving back the reactants. All oxidised activated carbons are more active in the esterification process studied than the initial activated carbon sample. This means that the modification methods applied were effective. Similarly, as when the catalysts are the activated carbon samples based on brown coal (KD), also for the catalysts obtained from pine sawdust (S), an increase in the yield of ethyl acetate was observed with increasing time of the esterification process, but the catalytic activity of the pine sawdust based samples was in general lower
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(Fig. 4).
S-H2O2-24h-333 K
40 35
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30 25 20 15
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10
0
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S-PAA-8h-333 K S-AIR-8h-573 K S-NH3-4h-923 K
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Yield of ethyl acetate [%]
45
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S S-APS-24h-333 K S-HNO3-8h-333 K
50
5
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55
0
3
6
9 12 Time of reaction [h]
15
18
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Fig. 4. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction obtained over selected activated carbon catalysts prepared from pinewood sawdust. The oxidation of the initial activated carbon samples was realised either under the "mild" conditions (lower temperature or shorter time of oxidation) or under "drastic" conditions (higher temperature or longer time of oxidation), see Section 2.1. Fig. 5 present the catalytic activities of all activated carbon samples prepared, measured after 20 h of the process. As can be seen, the yields of ethyl acetate show rather considerable differences for different oxidised
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Journal Pre-proof samples and varies from ~19 to 76% for the activated carbons obtained from brown coal (Fig. 5 a) and from ~15 to 53% for the samples obtained from pine sawdust (Fig. 5 b). Generally, this behaviour should be related to the amount of acidic oxygen-containing groups (like carboxylic, phenolic etc.), formed during the oxidative treatments (see Figs 1 and 2), which is manifested by an increase in the total acidity of the materials tested (Table 2). Surprisingly, from among the oxidised activated carbons, the product formation is found to be the lowest for the samples modified with APS for 24 h at 333 K, which show the highest
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total acidity (3.72 and 3.81 mmol/g, respectively). However, these samples also have the
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lowest apparent surface area from among the catalysts studied (415 and 462 m2/g), which can
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significantly affect the yield of the ester formed because of limited access of reagents to the
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active centres of the catalysts. The highest catalytic activity in the esterification of acetic acid with ethanol was found for the carbon samples modified with APS at the lower temperature
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(303 K) – both in the case of materials prepared from brown coals, as well as obtained from
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pinewood sawdust. These samples show both high total acidity and large apparent surface area (Table 2). As resulted from the TPD analysis, on the surfaces of such samples carboxylic
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groups dominate. On the other hand, the treatment with liquid APS generates also lactones, anhydrides and phenolic groups in noticeable amount (see the discussion on Figs 1 and 2). Carboxylic and phenolic structures are probably responsible for the activity of these carbons in the esterification reaction, as these groups act as proton donors to the carboxylic acid. For the other samples, the activity changes in a wide range. The carbon samples oxidised with hydrogen peroxide show poorer catalytic performance than the activated carbons oxidised with PAA or air, although they show higher total acidity and the apparent surface area comparable to those materials (Table 2).
17
2
O
S-
24 h
-3
3
33
3
33
3
33
30
h-
h-
-8
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-3
-3
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S-
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AP
AP
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K
K
K
K
K
K
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-8
AA
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03 K
K
-8
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33
h33
-4 h
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K
K
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-3
K
K
K
PS 3 K -2 4h KD -3 -A 03 PS K -2 4h -3 33 K
KD -A
4h
3
3
h33
h33
-2 4
N O
2
O
2
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3
-5 7
h57
-4
AA
N O
KD -H
KD -H
2
KD -H
2
KD -H
KD -P
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K
10
-4
-2
-2
H N O
2
H
2
O
8h -
-3
3
KD -P
3
KD
-9 2
-4 h
IR -8
KD -A
-4 h
R
KD -A I
3
H
KD -N
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S-
S-
2
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S-
A-
PA
S-
4h
57
50
A-
K
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PA
3
K
S
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S-
h-
57
h-
-8
R
AI
92
40
S-
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AI
h-
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-4
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N H
S-
Yield of ethyl acetate [%]
Yield of ethyl acetate [%]
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a)
70
60
50
40
30
20
10
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Fig. 5. The yield of ethyl acetate after 20 h of the reaction performed over the catalysts
obtained from brown coal (a) and pinewood sawdust (b).
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Journal Pre-proof Probable explanation of this behaviour might be related to the formation of new basic sites in addition to acidic groups during the H2O2 treatment, or other possibility – H2O2 modification could favour the stabilization of existing basic groups. According to Santiago et al. [35], H2O2, because of its gentle oxidant character, is able to render some basic groups recognised as quinones, chromenes or pyrenes. The results of acetonylacetone transformation (a model reaction applied for the determination of acid-base properties of materials) with the use of the carbons obtained from brown coal and pinewood sawdust presented in our previous paper
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seems to confirm this supposition [21]. According to these studies, carbons modified with
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H2O2 showed the highest ratio of basic sites to acidic ones among the samples subjected to
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oxidation. Thus, a low level of esterification with the samples modified with hydrogen
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peroxide could be due to the existence of the above basic sites which might catalyse the hydrolysis of ester to give back the reactants. All of the above relations prove that the yield of
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ester is controlled not only by the chemical but also by the textural properties of the catalysts.
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This conclusion is additionally confirmed by the results obtained for carbons oxidised with nitric acid. These samples show both high acidities, as well as high surface areas (Table 2),
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and that is why they are characterised by high yield of ethyl acetate. Activated carbons modified with air show quite a high yield of ethyl acetate (catalytic activities of these samples are higher than those of the carbons modified with PAA or H2O2, Fig. 5). This is unexpected because of the relatively low total acidities of these samples postulated by oxygen contents and results of titration (Table 2). However, according to our previous work concerning cumene decomposition, the activated carbons modified with air have stronger acid sites of Brönsted type than PAA-modified and H2O2-modified carbons [21]. It could be the reason for the higher activity of these materials. Further analysis of the data obtained indicates that the type of carbon precursor also has influence on the yield of the esterification product. From among the two series of carbons
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Journal Pre-proof tested, much better results were achieved for the catalysts obtained from brown coal as a precursor. For the catalysts oxidised with APS at 303 K for 24 hours, the difference between the ester yields achieved with the catalyst based on brown coal and pine sawdust reached about 23% after 20 h of the reaction. Fig. 6 presents the data on the yield of ethyl acetate formed in the esterification catalysed
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samples prepared from brown coal samples prepared from sawdust
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60
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50
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40
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30 20 10 0
1
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0
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Yield of ethyl acetate [%]
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by the prepared carbons versus the sample total acidities.
2
3
4
Total acidity [mmol/g]
Fig. 6. Yield of ethyl acetate obtained in the esterification reactions versus the total acidity of the carbons studied.
As can be seen, the activity of the catalysts studied in the above reaction shows a tendency to increase with increasing of their acidities, although some clearly marked deviations from this tendency are noted. For example, the samples showing the highest total acidity in both carbon series, but at the same time the lowest SBET parameters (KD-APS-24h-333 K and S-APS24h-333 K), produce only small amounts of the ester. This indicates that besides the carbon acidity also catalyst texture affect the catalytic performance of the activated carbons. The results of performed model reactions (see our previous paper [21]) also well correlate with the
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Journal Pre-proof data of the esterification process (Figs 1S and 2S, Supplementary Material), which further proves the relationship between the activity of the samples and their acidic sites. Fig. 7 shows the results of reusability tests obtained for a selected sample (KD-HNO3-8h333 K). As can be seen, there are some changes in the catalytic activity of the prepared carbon
in subsequent cycles. The highest decrease in ethyl acetate yield is observed after the first process (about 40%). Further changes are not so significant. Overall, the sample performance are still quite high, and the ethyl acetate yields in the next experiments are higher than that
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obtained in the blank run. On the basis of the obtained results, it can be concluded that active
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centers are quite strongly attached to the carbon surface and they are leached during the
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process only to a small extent. The regeneration step with a 5% solution of HCl (after the 3rd
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cycle) did not restore the sample activity. This confirms that the active centers are rather leached than blocked.
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100 90
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70 60 50 40 30
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Yield of ethyl acetate [%]
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20 10 0
I
II
III
IV
Reaction cycle
Fig. 7. Results of reusability tests performed for a selected catalyst (KD-HNO3-8h-333 K).
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Journal Pre-proof 4. Conclusions On the basis of the results of this study, it can be concluded that the activated carbon samples subjected to oxidation show catalytic activity in the reaction of esterification and the yield of the product increases with the time of the process which is related to the time of reaching equilibrium of the reaction system. The catalytic activities of the samples studied differed depending on the type of the oxidising agent and kind of the carbon precursor
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applied. It was found that the catalytic activity of activated carbons in the esterification reaction was correlated not only to the amount and the type of the acidic oxygen-containing
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groups, but also to the amount of the basic functionalities and additionally to the textural
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properties of the samples. The carbon samples modified with ammonia were shown to inhibit
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the reaction of esterification of acetic acid with ethanol.
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Acknowledgement
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This work was supported by National Science Centre (grant no. N N204 179140).
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[16] M. Hara, T. Yoshida, A. Takagaki, T. Takata, J.N. Kondo, S. Hayashi, K. Domen, A carbon material as a strong protonic acid, Angew. Chem. Int. Ed. 43 (2004) 2955–2958.
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[23] H.P. Boehm, G. Mair, T. Stoehr, A.R. de Rincόn, B. Tereczki, Carbon as a catalyst in
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[33] N. Calvar, B. González, A. Dominguez, Esterification of acetic acid with ethanol: Reaction kinetics and operation in a packed bed reactive distillation column, Chem.
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[34] Y. Ma, Q.L. Wang, H. Yan, X. Ji, Q. Qiu, Zeolite-catalyzed esterification I. Synthesis of acetates, benzoates and phthalates, Appl. Catal. A 139 (1996) 51–57.
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[35] M. Santiago, F. Stüber, A. Fortuny, A. Fabregat, J. Font, Modified activated carbons for catalytic wet air oxidation of phenol, Carbon 43 (2005) 2134–2145.
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Captions for figures: Fig. 1. CO2 release profiles of selected samples. Fig. 2. CO release profiles of selected samples. Fig. 3. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction obtained over selected activated carbon catalysts prepared from brown coal. Fig. 4. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction
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obtained over selected activated carbon catalysts prepared from pinewood sawdust.
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Fig. 5. The yield of ethyl acetate after 20 h of the reaction performed over the catalysts
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obtained from brown coal (a) and pinewood sawdust (b).
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Fig. 6. Yield of ethyl acetate obtained in the esterification reactions versus the total acidity of the carbons studied.
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Fig. 7. Results of reusability tests performed for a selected catalyst (KD-HNO3-8h-333 K).
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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical Abstract
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Highlights Activated carbons were modified using various reagents The impact of carbon surface chemistry on the esterification process was studied A correlation between sample acidity and the yield of ethyl ester was found The yield of product was also dependent on the textural parameters of samples
30
Beata Krzyżyńska, Anna Malaika, Karolina Ptaszyńska, Agnieszka Tolińska, Piotr Kirszensztejn, Mieczysław Kozłowski PII:
S0925-9635(19)30454-6
DOI:
https://doi.org/10.1016/j.diamond.2019.107608
Reference:
DIAMAT 107608
To appear in:
Diamond & Related Materials
Received date:
5 July 2019
Revised date:
5 October 2019
Accepted date:
2 November 2019
Please cite this article as: B. Krzyżyńska, A. Malaika, K. Ptaszyńska, et al., Modified activated carbons for esterification of acetic acid with ethanol, Diamond & Related Materials (2018), https://doi.org/10.1016/j.diamond.2019.107608
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© 2018 Published by Elsevier.
Journal Pre-proof
Modified activated carbons for esterification of acetic acid with ethanol Beata Krzyżyńska, Anna Malaika*, Karolina Ptaszyńska, Agnieszka Tolińska, Piotr Kirszensztejn, Mieczysław Kozłowski Faculty of Chemistry, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 8 61-614 Poznań, Poland
Abstract
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Activated carbons from brown coal and pinewood sawdust were prepared and applied in
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heterogeneous catalytic esterification of acetic acid with ethanol. The precursors were
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chemically activated with potassium hydroxide and the samples obtained were treated with
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different agents (HNO3, CH3COOOH, H2O2, (NH4)2S2O8, air, and ammonia) in order to produce new or alter the existing surface functional groups. The materials synthesised were
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then subjected to elemental and textural analyses, temperature programmed decomposition
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(TPD) and potentiometric titration. The effect of chemical modifications on the catalytic activity of the samples in the esterification reaction was investigated. It was found that the
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type of carbon modification has a significant impact on the catalytic performance of the materials obtained. The samples treated with ammonia almost did not catalyse the esterification of acetic acid with ethanol, while the oxidised carbons showed significant activity in the reaction. This phenomenon was a simple consequence of acidic character of the oxidised carbons’ surface. Moreover, it was established that the yield of the product depends not only on the acid-base properties of the materials obtained but also on their textural parameters. Keywords: Activated carbons; Carbon modification; Esterification; Acetic acid; Ethanol *
Corresponding author. Tel.: +48 61 829 1718; fax: +48 61 829 1555.
E-mail address: [email protected]
1
Journal Pre-proof 1. Introduction Esters of carboxylic acids and alcohols or phenols are a group of organic compounds very important for industrial applications. As the ester functionality is responsible for odour (it is the so-called osmophore), esters are widely applied in cosmetic industry (e.g. for perfume production) and food industry (as aroma components). They are also used in pharmaceutical, chemical and petrochemical sectors (as polymer plasticisers, components of solvents and
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paints, or fuels) [1]. Recently, the interest in some esters of low molecular weight has been increased significantly because of their potential application as biodiesel additives [2].
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Esters can be synthetically obtained in different reactions [3]. From among the methods
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described in literature, one of the most often applied and the simplest is the direct reaction of
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carboxylic acid with alcohol in the presence of an acidic catalyst (the so-called Fischer
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esterification). On an industrial scale, the process of esterification is realised in the liquid phase with the use of strong mineral acids, like e.g. sulphuric acid. The method is effective as
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ensures high yields of esters, however, the use of a homogeneous catalyst of strongly corrosive properties implies the need of additional steps such as neutralisation and separation
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of the catalyst, which increases the cost and generates impurities [4]. Moreover, the conventional homogeneous acid-catalysed esterification systems suffer from problems associated with the side reactions (e.g. etherification, dehydration) [5]. Furthermore, catalysts of this type often cannot be used more than once. Besides the homogeneous catalysts, also heterogeneous ones can be applied in esterification reactions, e.g. ion-exchange resins, zeolites or superacids [6-9]. An interesting option is the use of activated carbons in the process. These materials have been proved to show good catalytic properties in many reactions, e.g. industrial production of phosgene, removal of nitrogen and sulphur oxides from exhaust gases [10], methane decomposition [11], ethylbenzene dehydrogenation [12] and other processes realised in laboratory scale. The catalytic activity of activated carbon stems
2
Journal Pre-proof from the chemical character of their surfaces related to the presence of active centres of different types. Because of relatively low cost of activated carbon production, availability of substrates for the preparation of carbons, facility of surface modification, their thermal stability, resistance to poisoning and simple utilisation, these materials seem to be an interesting alternative to the conventional catalysts. So far, only a few authors have been interested in carrying out esterification with the use of activated carbon catalysts [13-18] and to the best of our knowledge, only few papers have
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been devoted to esterification of acetic acid with ethanol in the presence of activated carbon.
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Liu and co-workers [18] have compared the catalytic activity of sulphonated activated carbon
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(AC-SO3H) and two commercial materials, i.e. Nafion NR50 and Amberlyst 15, in the
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esterification of selected carboxylic acids (acetic, hexanoic and decanoic acids) with ethanol. In the esterification of acetic acid with ethanol, the best results have been obtained for the
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ion-exchange resin Amberlyst 15. The authors have concluded that the high catalytic activity
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of this material follows from a high content of sulphonic groups in this resin. However, in the esterification of carboxylic acids of longer chains, the catalyst AC-SO3H has been shown to
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have much higher catalytic activity than the commercial catalysts tested. This fact has been related to the texture of the activated carbon used, its large surface area and mesoporous structure. The greatest drawback of the sulphonated carbon was the washing out of SO3H groups from the material structure during the reaction. The advantages of using heterogeneous catalysts in esterification reactions, including simple product isolation, ease in recovery and reuse of the catalysts, reduction in the generation of the wasteful products etc. [19], have prompted us to check the performance of modified activated carbons in esterification of acetic acid with ethanol. According to literature data, the catalytic properties of different materials in the esterification process are strongly related to their acidity. Because of that, the activated carbons were subjected to modifications
3
Journal Pre-proof in oxidising conditions to endow their surface with acidic character. For the sake of comparison, the activated carbons of basic character (obtained by modification with ammonia) were also tested. The activated carbons prepared were thoroughly characterised and correlations between their catalytic performance and chemical properties of their surfaces as well as textural parameters were checked. 2. Experimental
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2.1. Preparation of catalysts
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Activated carbons were obtained from two precursors: brown coal (from Polish colliery
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"Konin") and pinewood sawdust. The substrates were crumbled (the brown coal was
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additionally demineralised by the Radmacher and Mohrhauer method [20]), and subjected to chemical activation by potassium hydroxide in solid state in argon atmosphere at 1073 K for
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45 minutes. The weight ratio of the precursor to KOH was 1:1. The activated carbons
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obtained were washed with a 5% solution of hydrochloric acid and then with distilled water, next they were dried overnight at 383 K. The initial activated carbon samples (labelled
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according to the type of carbon precursor used: Konin coal after demineralisation – KD, and sawdust – S), were further modified either by oxidation or by ammonia treatment in order to change the amount and type of functional groups on their surfaces. The oxidation was realised with the liquid agents such as concentrated nitric acid, peroxyacetic acid, diammonium peroxydisulphate, hydrogen peroxide, or gas agent (air). The catalyst samples labelling, conditions of modifications and yields of carbon catalysts obtaining with respect to the mass of the initial activated carbon are given in Table 1. Characterisation of the materials and details of their synthesis have been given in our previous paper [21].
4
Journal Pre-proof 2.2. Characterisation of catalysts All catalyst samples tested were subjected to elemental analysis performed with the use of an Elemental Analyser Vario EL III. The porous structure of activated carbon samples was determined by a Micromeritics Sorptometr ASAP 2010 with nitrogen as adsorbate at 77 K. The apparent total specific surface areas (SBET) were calculated using the BET equation, whereas the t-plot method was applied to calculate the micropore volumes (Vmicro) and
of
"external" (mesopores and macropores) surface areas (Sext) [22]. The total pore volumes (Vtot) were obtained from the N2 amount adsorbed at a relative pressure close to unity. In order to
ro
define the character of the oxygen functional groups, the temperature programmed
-p
decomposition (TPD) method was applied. In a typical TPD run, the sample (0.1 g) was
re
heated up to 1323 K in Ar flow (10 cm3/min) at a heating rate of 10 K/min. The amounts of
lP
CO and CO2 evolved were measured using a Gas Analyser ThermoStar GSD 301 T2 (Pfeiffer Vacuum) equipped with a mass spectrometer. The total acidity of samples was determined by
na
potentiometric titration. In each experiment, 0.1 g of a dry carbon sample (0.05 g in the case of Amberlyst 15) was contacted with 50 cm3 of a 0.01 M NaOH solution and then shaken at
Jo ur
room temperature for 20 h. After that time, the suspension was filtered and titrated with a 0.05 M HCl solution.
2.3. Catalytic measurements
The esterification of acetic acid with ethanol was performed at 348 K for 20 h, using activated carbons prepared as catalysts and a mixture of ethanol and acetic acid at a molar ratio of 10:1. The content of a carbon catalyst was 2% of the reagents' mass. The products obtained (collected after 1.5, 3, 4.5, 6 and 20 h) were chromatographically analysed on a Porapak QS column of 1 m in length. The esterification reaction of acetic acid with ethanol in the presence of H2SO4 (commercial homogeneous catalyst), Amberlyst 15 (commonly applied heterogeneous catalyst) and the blank test were also carried out. To assess the activity
5
Journal Pre-proof of samples in subsequent reaction cycles, a selected carbon was re-used in the process (after being filtered and then washed with hot distilled water). After the 3rd reaction, the additional regeneration step with 5% HCl was performed. 3. Results and discussion 3.1. Characterisation of samples The raw brown coal was characterised by a high content of ash (22.6%), so it was
of
subjected to demineralisation by the Radmacher and Mohrhauer method, after which the
ro
content of ash was reduced to 0.9%. Thanks to a low content of mineral components in
-p
pinewood sawdust, its demineralisation was not needed. The precursors were chemically activated by potassium hydroxide in solid state at 1073 K for 45 min. The initial activated
re
carbon samples obtained in this way (labelled as KD and S, respectively) were subsequently
lP
modified with different agents to get materials of different chemical properties. Analysis of
na
the yields of the modification products (Table 1) proved that all the processes of oxidation resulted in insignificant mass loss of the treated carbons (the yield of modification products
Jo ur
was above 90%), which means that deep oxidation took place to a small degree. The yields of activated carbons obtained via modification of initial samples with ammonia were lower (82-85%), which could be explained by hydrogasification of carbon under the conditions applied (decomposition of ammonia at high temperatures generates small amounts of hydrogen [23]). The most important data on elemental and textural analyses of the catalysts obtained are presented in Table 2. As expected, the treatment of initial samples with oxidising agents (both in liquid or gas forms) resulted in an increase in the molar ratio of oxygen to carbon in the materials after modification. The increase in the O/C parameter depended on the type of oxidising agent and conditions (temperature and duration) of modification. The effectiveness of oxidation of the initial activated carbons KD and S with the applied agents was found to 6
Journal Pre-proof decrease in the series: APS > HNO3 > H2O2 > PAA > AIR. The amounts of oxygen in the activated carbons after modification correspond to the total acidity of the samples – the greater the content of oxygen, the higher the acidity, see Table 2. The concentration of acidic groups was only slightly dependent on the type of a precursor used. Table 1 Scheme of modification of initial activated carbons (KD and S) and yields of products Reaction Reaction Sample code Modifying agent Yield (%) time (h) temperature (K) 4
ro
S-PAA-4h-333 K
96.3 CH3COOOH
KD-PAA-8h-333 K
KD-H2O2-24h-333 K S-H2O2-24h-333 K KD-HNO3-4h-333 K S-HNO3-4h-333 K
-p 24
333
4
333
conc. HNO3
8
333
(NH4)2S2O8
24
303
(NH4)2S2O8
24
333
air
4
573
air
8
573
conc. HNO3
S-HNO3-8h-333 K
KD-AIR-8h-573 K
S-NH3-4h-923 K
99.7 96.3 98.6 95.3 97.1 92.4
S-AIR-8h-573 K KD-NH3-4h-923 K
99.7 99.1
KD-AIR-4h-573 K S-AIR-4h-573 K
90.7 99.8
KD-APS-24h-333 K S-APS-24h-333 K
96.7 91.7
KD-APS-24h-303 K S-APS-24h-303 K
98.1 96.3
303
H2O2
98.3 95.0
333
24
Jo ur
KD-HNO3-8h-333 K
333
re
S-H2O2-24h-303 K
H2O2
8
lP
KD-H2O2-24h-303 K
CH3COOOH
na
S-PAA-8h-333 K
of
KD-PAA-4h-333 K
93.0 91.0 92.6
NH3
4
923
82.3 84.5
7
Journal Pre-proof The treatment of initial activated carbons with ammonia at a high temperature resulted in the removal of considerable amount of oxygen from them. It is a consequence of thermal decomposition of oxygen functional groups as well as their reduction by hydrogen, generated in the process of partial thermal decomposition of ammonia [23]. The treatment applied also led to an increased content of nitrogen in the materials modified (results not shown), which can be a consequence of incorporation of this element in the form of pyridinium and pyrrole groups or quaternary systems [24] into the carbon structure.
of
The initial activated carbons (KD and S) show relatively high apparent surface areas of
ro
over 900 m2/g and 1150 m2/g, respectively (Table 2). The processes of modification of the
-p
initial activated carbon samples had various effects on the apparent surface areas of modified
re
materials, but in general the changes were rather small. Quite different results were obtained for KD and S samples modified with diammonium peroxydisulphate at 333 K. The modified
lP
samples had drastically smaller apparent surface areas. Most probably, it is related to
na
destruction of the porous structure of the initial activated carbons upon their intense oxidation and/or with the obstruction of the access to pores by large volume oxygen functional groups
Jo ur
(e.g. carboxylic ones) [25] introduced upon modification (high oxygen content in the APS-modified samples, see Table 2). Further analysis of the results presented in Table 2 proves that in all activated carbon samples the dominant is microporous structure as the contributions of meso- and macropore areas to the total surface areas are low, while the contributions of micropore volumes to the total pore volumes are high (Sext/SBET*100% and Vmicro/Vtot*100% parameters, respectively). Detail data characterising the physicochemical properties of the activated carbons obtained are given in our previous paper [21].
8
Journal Pre-proof
Sext/SBET *100%
Vmicro/Vtot *100%
O/C
Total acidity (mmol/g)
KD
953
2.31
91.7
0.037
0.78
KD-PAA-4h-333 K
1041
2.11
94.2
0.093
1.01
KD-PAA-8h-333 K
980
2.14
93.9
0.081
1.02
KD-H2O2-24h-303 K
938
1.81
93.8
0.096
1.09
KD-H2O2-24h-333 K
958
2.09
92.0
0.125
1.49
KD-HNO3-4h-333 K
856
2.10
93.0
0.167
1.99
KD-HNO3-8h-333 K
881
2.27
93.2
0.197
2.32
KD-APS-24h-303 K
744
2.28
92.1
0.272
3.09
KD-APS-24h-333 K
415
3.37
86.4
0.317
3.72
KD-AIR-4h-573 K
999
2.20
92.0
0.047
0.80
KD-AIR-8h-573 K
1100
2.45
92.7
0.077
0.92
KD-NH3-4h-923 K
1041
2.21
92.3
0.011
0.15
S
1157
1.99
94.7
0.045
0.58
S-PAA-4h-333 K
1178
1.87
93.2
0.094
1.08
S-PAA-8h-333 K
1154
2.08
93.1
0.073
1.09
1131
1.68
93.1
0.120
1.27
1057
2.18
91.2
0.136
1.63
S-HNO3-4h-333 K
1067
1.97
94.3
0.199
2.17
S-HNO3-8h-333 K
1058
2.08
94.2
0.178
2.52
S-APS-24h-303 K
951
2.63
91.7
0.296
3.36
S-APS-24h-333 K
462
3.46
87.5
0.394
3.81
S-AIR-4h-573 K
1217
2.38
93.4
0.072
0.96
S-AIR-8h-573 K
1159
1.73
93.1
0.076
1.07
S-NH3-4h-923 K
1197
1.92
93.3
0.000
0.08
S-H2O2-24h-303 K
ro
-p
lP
Jo ur
S-H2O2-24h-333 K
na
Sample
of
SBET (m2/g)
re
Table 2 Physicochemical properties of investigated samples
The type of oxidising agent determines not only the amount of oxygen introduced to the carbon scaffold (as shown in Table 2) but also the type of oxygen structures formed [26]. Since the oxygen functionalities govern the surface chemistry of carbons, determining their
9
Journal Pre-proof nature and type is crucial, especially for catalysis. Thus, in order to know the surface chemistry of carbons, the prepared materials were previously tested in selected model reactions, i.e. decomposition of isopropanol, cyclisation of acetonylacetone and cumene decomposition. Details of these processes and some important results were shown in our previous paper [21]. Briefly, it was found that the carbon samples oxidised with diammonium peroxydisulfate or concentrated nitric acid showed almost fully acidic character and almost only Brönsted acid centers were observed. The other oxidised carbons had not only acidic
of
sites but also some basic centers on their surfaces (similarly as KD). The sample with
ro
a particularly high ratio of basic to acidic centers was carbon modified with H2O2. On the
re
mainly in the form of basic structures.
-p
other hand, activated carbons treated with H2 or NH3 showed lower amount of active sites,
To identify more precisely the form of oxygen present on the surfaces of the samples, the
lP
obtained activated carbons were further subjected to temperature-programmed decomposition
na
(TPD). The usefulness of this technique for carbon characterisation has been well documented by many researchers [27,28]. Generally, the TPD method involves heating a sample of carbon
Jo ur
in a stream of inert gas while increasing the temperature. The resulting gaseous products are then analysed usually using a mass spectrometer. The nature of oxygen groups on the carbon surface can be assessed by the type of gas (CO and/or CO2) and the temperature at which it is released from the sample. There is a rich literature concerning the TPD technique and the temperature ranges specific to the decomposition of various oxygen groups [27-29]. However, it is well known that the assignment of TPD signals (corresponding to release of CO and CO2) to the specific functional groups might be a bit ambiguous. It follows from the fact that the thermal decomposition of oxygen functional groups can depend on the porous structure of the material, rate of its heating or on the type of measuring instrument used [27]. Nevertheless, certain general tendencies in behaviour of particular oxygen groups upon heating have been
10
Journal Pre-proof established and presented by Figueiredo [27] and Szymański [29]. According to the results presented by the aforementioned authors, the least stable are carboxylic groups that undergo decomposition in the range 373-673 K with release of carbon dioxide. The same product is formed as a result of decomposition of lactones, taking place in the range 463-940 K. The phenolic and hydroquinone groups (873-1110 K), carbonyl and quinone groups (973-1253 K) and ether groups (823-1100 K) decompose in similar temperatures with liberation of CO. Carboxylic anhydrides (623-1173 K) and pyrone structures (1173-1473 K) decompose with
of
release of both CO2 and CO. The interpretation of TPD profiles presented in our paper was
ro
performed taking into account the above data. As the TPD results obtained for individual
-p
series of samples were similar, in this paper we present only the results obtained for activated
re
carbons prepared from brown coal, oxidised under the more "drastic" conditions, that is in the higher of the temperatures specified above or for the longer time of the process. For
na
shown.
lP
comparison, also the TPD profiles of the initial carbon and that modified with ammonia are
The CO2 profiles of the oxidised carbon samples show a characteristic band in the range
Jo ur
473-673 K (Fig. 1), which can be assigned to the carboxylic groups present in the samples [27,29]. As follows from these profiles, the content of carboxylic groups in the oxidised activated carbons decreases in the sequence APS > HNO3 > H2O2 > PAA > AIR, and well correlates with the total acidity of the samples (Table 2). Similar relations have also been obtained by our group for activated carbons prepared from peach stones [26]. The carbon sample oxidised with air has fewer carboxylic structures than the initial carbon catalyst (KD), which is a consequence of a relatively high temperature of the modification (573 K). Under such conditions, both the initially present and newly formed carboxylic groups undergo decomposition. The CO2 profiles show also a broad peak above the decomposition temperature of carboxylic groups. This signal can be assigned to the presence of carboxylic
11
Journal Pre-proof anhydrides and lactone groups in the activated carbons studied [26,30]. These groups can be originally present in the samples or can be generated during TPD measurements [26]. As known from literature, the neighbouring carboxylic groups can undergo dehydration to anhydrides, moreover, carboxylic groups can react with phenolic groups with formation of lactones [30]. The low intensity of the signal appearing above 1173 K means that pyrone structures can be present in the samples studied only in small amounts. According to our data, the profile of sample KD-NH3-4h-923 K shows the CO2 signal of much lower intensity than
of
that of the initial activated carbon KD. It means that the treatment with ammonia leads to
ro
removal of the majority of surface oxygen groups, which was also indicated by elemental
-p
analysis (see Table 2). -10
3.5x10
-10
-10
2.0x10
-10
-10
-11
5.0x10
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
na
2.5x10
Jo ur
Ion Current [pA]
-10
1.0x10
KD-H2O2-24h-333 K
lP
3.0x10
1.5x10
KD KD-APS-24h-333 K KD-HNO3-8h-333 K
re
CO2
0.0 200
400
600
800
1000
1200
1400
Temperature [K]
Fig. 1. CO2 release profiles of selected samples. The main part of CO profiles of all carbon samples investigated are broad bands in the range 700-1250 K (Fig. 2). They are probably related to decomposition of ether, phenol and carbonyl (quinone) functionalities or anhydrous groups. According to literature data, ether
12
Journal Pre-proof groups can be formed upon TPD measurements as a result of a combination of phenolic groups [30]. As mentioned earlier, carboxylic anhydrides undergo decomposition with liberation of CO2 and CO. As the CO2 profiles of the samples oxidised in solutions indicate the presence of anhydrides (Fig. 1), it can be assumed that a significant part of the signal from the range 700-1250 K on the CO release curve also comes from these groups (some of them are generated from carboxylic groups). -10
6x10
KD KD-APS-24h-333 K KD-HNO3-8h-333 K
of
-10
5x10
CO
-10
3x10
ro -p
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
-10
4x10
re
Ion Current [pA]
KD-H2O2-24h-333 K
-10
lP
2x10
-10
400
Jo ur
0 200
na
1x10
600
800
1000
1200
1400
Temperature [K]
Fig. 2. CO release profiles of selected samples.
Air treatment is very different from liquid modification. The sample oxidised with air contains a small amount of anhydride structures as evidenced by a low intensity of the band assigned to CO2 in the above mentioned temperature range (Fig. 1). So, the peak of CO release in the range 700-1250 K observed for this sample must come from the phenolic, carbonyl (quinone) and ether groups (Fig. 2). Thus, the results obtained in this study confirm the literature suggestion that the oxidation in solution of oxidisers leads mainly to formation of surface carboxyl groups, while oxidation with air leads to formation of both carbonyl and
13
Journal Pre-proof hydroxyl (phenolic) groups [27,29]. The profiles of CO release also show a weak signal at about 550 K (Fig. 2). This signal is probably related to the thermal decomposition of carbonyl groups in α- substituted ketones or aldehydes [29,30]. The curve of CO release for the sample treated with ammonia is flat and shows some increase in the intensity of the signal in high temperatures only, which proves that the sample studied contains small amounts of oxygen groups, mainly those particularly resistant to high temperature, i.e. pyrone and carbonyl ones.
of
3.2. Catalytic activity of carbons
ro
The activated carbons prepared were tested as catalysts for esterification of acetic acid
-p
with ethanol for 20 h operation period. The performance of the samples was discussed in terms of the yield of ethyl acetate formed. Figs 3 and 4 present the time dependence of
re
catalytic activity of selected samples, that is the activated carbons oxidised under more
lP
"drastic" conditions (at the higher temperature or for a longer time), the samples treated with ammonia and the initial activated carbons. For the sake of comparison, Fig. 3 presents also the
na
results obtained in the reaction carried out in the presence of the conventional homogeneous
test.
Jo ur
catalyst (H2SO4), a commonly used heterogeneous catalyst Amberlyst 15, and for the blank
It is well known that esterification reaction is a slow and equilibrium-limited process. Typical esterification reactions require several days to attain the equilibrium in the absence of catalyst [31]. As follows from Fig. 3, the equilibrium is reached very fast for the reaction with H2SO4 used as a catalyst and the yield of ethyl acetate is close to 95%. Only slightly worse catalytic performance is shown by Amberlyst 15. The activity of carbon materials applied is lower and the yield of the product increases almost linearly with the time of the process. Obviously, this is due to the lower acidity of the carbon samples (see Table 2) compared to that of sulphuric acid or Amberlyst 15 (4.6 mmol/g). Moreover, some authors suggest that heterogeneous catalysts exhibit some limitations on catalysing esterification reaction related
14
Journal Pre-proof to mass transfer resistance or loss of the active acid sites in the presence of a polar medium [32,33]. 100 90 KD KD-APS-24h-333 K KD-HNO3-8h-333 K
70
KD-H2O2-24h-333 K
50
KD-PAA-8h-333 K KD-AIR-8h-573 K KD-NH3-4h-923 K
of
60
blank test Amberlyst 15 conc. H2SO4
40 30
ro
Yield of ethyl acetate [%]
80
-p
20
0 0
3
6
re
10
9
12
15
18
21
lP
Time of reaction [h]
Fig. 3. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction
na
obtained over selected activated carbon catalysts prepared from brown coal.
Jo ur
In the absence of catalyst, the yield of the product is of about 9% after 20 h of reaction (Fig. 3). In this case, the acetic acid acts as a mild autocatalyst for esterification of ethanol, which has been also reported by other authors [19,34]. The initial activated carbon catalyst shows rather small yields of ethyl acetate, which are only slightly higher than those obtained in the blank test. This is due to the low oxygen content detected for this sample and its low acidity (see Table 2 and Figs 1 and 2). The poorest results were obtained for the NH3-modified sample. Ammonia treatment removes oxygen groups from the catalyst surface and leads to the reduction of the total acidity (see Table 2). On the other hand, a high content of nitrogen was detected in NH3-modified carbon, and the character of the sample surface assessed applying model reactions was basic (see our previous paper [21]). According to Fig. 3, the yield of the product is even slightly lower for this sample than that obtained in the
15
Journal Pre-proof blank test. It might suggest that the basic groups partially catalyse the hydrolysis of ester giving back the reactants. All oxidised activated carbons are more active in the esterification process studied than the initial activated carbon sample. This means that the modification methods applied were effective. Similarly, as when the catalysts are the activated carbon samples based on brown coal (KD), also for the catalysts obtained from pine sawdust (S), an increase in the yield of ethyl acetate was observed with increasing time of the esterification process, but the catalytic activity of the pine sawdust based samples was in general lower
of
(Fig. 4).
S-H2O2-24h-333 K
40 35
lP
30 25 20 15
Jo ur
10
0
re
S-PAA-8h-333 K S-AIR-8h-573 K S-NH3-4h-923 K
na
Yield of ethyl acetate [%]
45
-p
S S-APS-24h-333 K S-HNO3-8h-333 K
50
5
ro
55
0
3
6
9 12 Time of reaction [h]
15
18
21
Fig. 4. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction obtained over selected activated carbon catalysts prepared from pinewood sawdust. The oxidation of the initial activated carbon samples was realised either under the "mild" conditions (lower temperature or shorter time of oxidation) or under "drastic" conditions (higher temperature or longer time of oxidation), see Section 2.1. Fig. 5 present the catalytic activities of all activated carbon samples prepared, measured after 20 h of the process. As can be seen, the yields of ethyl acetate show rather considerable differences for different oxidised
16
Journal Pre-proof samples and varies from ~19 to 76% for the activated carbons obtained from brown coal (Fig. 5 a) and from ~15 to 53% for the samples obtained from pine sawdust (Fig. 5 b). Generally, this behaviour should be related to the amount of acidic oxygen-containing groups (like carboxylic, phenolic etc.), formed during the oxidative treatments (see Figs 1 and 2), which is manifested by an increase in the total acidity of the materials tested (Table 2). Surprisingly, from among the oxidised activated carbons, the product formation is found to be the lowest for the samples modified with APS for 24 h at 333 K, which show the highest
of
total acidity (3.72 and 3.81 mmol/g, respectively). However, these samples also have the
ro
lowest apparent surface area from among the catalysts studied (415 and 462 m2/g), which can
-p
significantly affect the yield of the ester formed because of limited access of reagents to the
re
active centres of the catalysts. The highest catalytic activity in the esterification of acetic acid with ethanol was found for the carbon samples modified with APS at the lower temperature
lP
(303 K) – both in the case of materials prepared from brown coals, as well as obtained from
na
pinewood sawdust. These samples show both high total acidity and large apparent surface area (Table 2). As resulted from the TPD analysis, on the surfaces of such samples carboxylic
Jo ur
groups dominate. On the other hand, the treatment with liquid APS generates also lactones, anhydrides and phenolic groups in noticeable amount (see the discussion on Figs 1 and 2). Carboxylic and phenolic structures are probably responsible for the activity of these carbons in the esterification reaction, as these groups act as proton donors to the carboxylic acid. For the other samples, the activity changes in a wide range. The carbon samples oxidised with hydrogen peroxide show poorer catalytic performance than the activated carbons oxidised with PAA or air, although they show higher total acidity and the apparent surface area comparable to those materials (Table 2).
17
2
O
S-
24 h
-3
3
33
3
33
3
33
30
h-
h-
-8
3
03
33
-3
-3
h-
24
S-
S-
AP
AP
3
H N O
S-
S-
3
4h
4h
33
33
K
K
K
K
K
K
K
K
K
O
-2
-8
AA
3
3
03 K
K
-8
-3
3
33
h33
-4 h
h33
K
K
of
ro
-3
K
K
K
PS 3 K -2 4h KD -3 -A 03 PS K -2 4h -3 33 K
KD -A
4h
3
3
h33
h33
-2 4
N O
2
O
2
3
3
-5 7
h57
-4
AA
N O
KD -H
KD -H
2
KD -H
2
KD -H
KD -P
-p
re
lP
na
K
10
-4
-2
-2
H N O
2
H
2
O
8h -
-3
3
KD -P
3
KD
-9 2
-4 h
IR -8
KD -A
-4 h
R
KD -A I
3
H
KD -N
0
S-
S-
2
H
S-
A-
PA
S-
4h
57
50
A-
K
Jo ur b)
PA
3
K
S
3
60
S-
h-
57
h-
-8
R
AI
92
40
S-
R -4
AI
h-
20
S-
-4
30
3
N H
S-
Yield of ethyl acetate [%]
Yield of ethyl acetate [%]
Journal Pre-proof
80
a)
70
60
50
40
30
20
10
0
Fig. 5. The yield of ethyl acetate after 20 h of the reaction performed over the catalysts
obtained from brown coal (a) and pinewood sawdust (b).
18
Journal Pre-proof Probable explanation of this behaviour might be related to the formation of new basic sites in addition to acidic groups during the H2O2 treatment, or other possibility – H2O2 modification could favour the stabilization of existing basic groups. According to Santiago et al. [35], H2O2, because of its gentle oxidant character, is able to render some basic groups recognised as quinones, chromenes or pyrenes. The results of acetonylacetone transformation (a model reaction applied for the determination of acid-base properties of materials) with the use of the carbons obtained from brown coal and pinewood sawdust presented in our previous paper
of
seems to confirm this supposition [21]. According to these studies, carbons modified with
ro
H2O2 showed the highest ratio of basic sites to acidic ones among the samples subjected to
-p
oxidation. Thus, a low level of esterification with the samples modified with hydrogen
re
peroxide could be due to the existence of the above basic sites which might catalyse the hydrolysis of ester to give back the reactants. All of the above relations prove that the yield of
lP
ester is controlled not only by the chemical but also by the textural properties of the catalysts.
na
This conclusion is additionally confirmed by the results obtained for carbons oxidised with nitric acid. These samples show both high acidities, as well as high surface areas (Table 2),
Jo ur
and that is why they are characterised by high yield of ethyl acetate. Activated carbons modified with air show quite a high yield of ethyl acetate (catalytic activities of these samples are higher than those of the carbons modified with PAA or H2O2, Fig. 5). This is unexpected because of the relatively low total acidities of these samples postulated by oxygen contents and results of titration (Table 2). However, according to our previous work concerning cumene decomposition, the activated carbons modified with air have stronger acid sites of Brönsted type than PAA-modified and H2O2-modified carbons [21]. It could be the reason for the higher activity of these materials. Further analysis of the data obtained indicates that the type of carbon precursor also has influence on the yield of the esterification product. From among the two series of carbons
19
Journal Pre-proof tested, much better results were achieved for the catalysts obtained from brown coal as a precursor. For the catalysts oxidised with APS at 303 K for 24 hours, the difference between the ester yields achieved with the catalyst based on brown coal and pine sawdust reached about 23% after 20 h of the reaction. Fig. 6 presents the data on the yield of ethyl acetate formed in the esterification catalysed
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samples prepared from brown coal samples prepared from sawdust
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60
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50
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Yield of ethyl acetate [%]
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by the prepared carbons versus the sample total acidities.
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Total acidity [mmol/g]
Fig. 6. Yield of ethyl acetate obtained in the esterification reactions versus the total acidity of the carbons studied.
As can be seen, the activity of the catalysts studied in the above reaction shows a tendency to increase with increasing of their acidities, although some clearly marked deviations from this tendency are noted. For example, the samples showing the highest total acidity in both carbon series, but at the same time the lowest SBET parameters (KD-APS-24h-333 K and S-APS24h-333 K), produce only small amounts of the ester. This indicates that besides the carbon acidity also catalyst texture affect the catalytic performance of the activated carbons. The results of performed model reactions (see our previous paper [21]) also well correlate with the
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Journal Pre-proof data of the esterification process (Figs 1S and 2S, Supplementary Material), which further proves the relationship between the activity of the samples and their acidic sites. Fig. 7 shows the results of reusability tests obtained for a selected sample (KD-HNO3-8h333 K). As can be seen, there are some changes in the catalytic activity of the prepared carbon
in subsequent cycles. The highest decrease in ethyl acetate yield is observed after the first process (about 40%). Further changes are not so significant. Overall, the sample performance are still quite high, and the ethyl acetate yields in the next experiments are higher than that
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obtained in the blank run. On the basis of the obtained results, it can be concluded that active
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centers are quite strongly attached to the carbon surface and they are leached during the
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process only to a small extent. The regeneration step with a 5% solution of HCl (after the 3rd
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cycle) did not restore the sample activity. This confirms that the active centers are rather leached than blocked.
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Yield of ethyl acetate [%]
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II
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Fig. 7. Results of reusability tests performed for a selected catalyst (KD-HNO3-8h-333 K).
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Journal Pre-proof 4. Conclusions On the basis of the results of this study, it can be concluded that the activated carbon samples subjected to oxidation show catalytic activity in the reaction of esterification and the yield of the product increases with the time of the process which is related to the time of reaching equilibrium of the reaction system. The catalytic activities of the samples studied differed depending on the type of the oxidising agent and kind of the carbon precursor
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applied. It was found that the catalytic activity of activated carbons in the esterification reaction was correlated not only to the amount and the type of the acidic oxygen-containing
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groups, but also to the amount of the basic functionalities and additionally to the textural
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properties of the samples. The carbon samples modified with ammonia were shown to inhibit
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the reaction of esterification of acetic acid with ethanol.
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Acknowledgement
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This work was supported by National Science Centre (grant no. N N204 179140).
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Captions for figures: Fig. 1. CO2 release profiles of selected samples. Fig. 2. CO release profiles of selected samples. Fig. 3. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction obtained over selected activated carbon catalysts prepared from brown coal. Fig. 4. The degree of acetic acid conversion to ethyl acetate versus the time of the reaction
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obtained over selected activated carbon catalysts prepared from pinewood sawdust.
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Fig. 5. The yield of ethyl acetate after 20 h of the reaction performed over the catalysts
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obtained from brown coal (a) and pinewood sawdust (b).
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Fig. 6. Yield of ethyl acetate obtained in the esterification reactions versus the total acidity of the carbons studied.
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Fig. 7. Results of reusability tests performed for a selected catalyst (KD-HNO3-8h-333 K).
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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical Abstract
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Highlights Activated carbons were modified using various reagents The impact of carbon surface chemistry on the esterification process was studied A correlation between sample acidity and the yield of ethyl ester was found The yield of product was also dependent on the textural parameters of samples
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