Surface chemistry analysis of acidified and nickel impregnated carbon for desulfurization of fuels

Emission norms tend to reduce sulfur content in transport fuels to 10ppm. Considering this new method like adsorptive desulfurization are widely researched. The main motive in adsorptive desulfurization is to selectively adsorb Dibenzothiophene (DBT) and 4,6-DimethylDBT. Considering this adsorbent with nickel impregnation and acidic surface functionalities were developed. Topographical and surface chemical features were studied of developed carbon adsorbents. Presence of carboxylic surface functionalities and reduced solid nickel was confirmed on adsorbents surfaces. In addition to this formation of nickel complexes due to neutralization of nickel ion and carboxylic groups was confirmed. This type of adsorbents can act as potential adsorbents for adsorption of DBT and 4,6-dimethyl DBT.


Introduction
Due to stringent environmental regulations around the world, sulfur concentration has become an important parameter in transportation fuels.High amounts of sulfur in fuels after combustion result in formation of SO which further result into acid rain as well as poisoning of catalytic converters.Considering this, the European union has implemented Euro VI emission norms since 2014 and the Indian government has implemented BS 6 emission norms since April 2020 which mandate the sulfur content in transport fuel to be below 10 ppm.Diesel is also seen as potential to produce hydrogen for fuel cell application.The organic sulfur compounds during production of hydrogen also acts as a poison to catalytic converters in fuel cells thus for hydrogen production in fuel cells sulfur content less than 1 ppm is desirable [1][2][3][4].This limitation paves the way for the use of suitable selective adsorbent for organo-sulfur compounds.
The conventional hydrodesulfurization (HDS) process has been into use at refineries and can reduce sulfur content up to 500 ppm very effectively [5].To achieve sulfur content up to 10 ppm or 1 ppm by ultra-hydrodesulfurization high investments and operational costs are required.Commercial fuels like diesel/ Petrol/ Jet fuel contain 4,6-dimethyl dibenzothiophene (4,6-DMDBT) in high proportion after HDS [5,6].Due to steric hindrance by methyl groups in 4,6-DMDBT the catalyst action fails resulting in very high portions of 4,6-DMDBT in diesel.This motivated the need for specific adsorbents.Considering high costs researchers are trying to develop desulfurization technologies based on oxidative 1291 (2023) 012001 IOP Publishing doi:10.1088/1757-899X/1291/1/012001 2 desulfurization [7], adsorptive desulfurization [8,9] etc. Carbon is widely investigated due to its high surface area, porosity & pore volume.Apart from topographical features of carbon the surface chemistry has greatly impressed and drawn the attention of several investigators.Researchers like [3,9] have also proved carbon to be best adsorbent on model diesel fuels compared to alumina, alumina silicates.Suryawanshi et.al. [10] have also proved 100 % adsorption of DBT from model fuels on metal impregnated activated carbon.
In this study we focus on development and characterization of carbon that can selectively adsorb dibenzothiophene (DBT) and 4,6 dimethyl dibenzothiophene (4,6-DMDBT).It has been observed by Kim et.al. [3] that DBT specially adsorbs on nickel due to direct coordination between sulfur atom and metallic nickel on adsorbent surface, contrary to this 4,6-DMDBT with extra methyl groups close to sulfur atom adsorbs towards activated carbon with carboxylic surface functionalities and form an electron-donor mechanism with sulfur and the π electrons on the compound interact with graphene by π-π dispersive interaction with graphene layer in carbon [5].The extra methyl groups also enhance the π electrons on 4,6-DMDBT compared to DBT; it has been observed that 4,6-DMDBT competes with DBT for the same adsorption site [3].Taking this into consideration we have developed three types of activated carbon with a combination of developed acidic surface functionalities and/ or Nickle impregnation that can act as a potential absorber for DBT as well as 4,6-DMDBT.
In present study, surface characteristics, features of acidic surface functionalities, quantitative analysis of the amount of Nickel impregnated on activated carbon are studied.And possible chemistry at the carbon surface of newly developed carbons are discussed.For quantification of acidic surface functionalities Boehm titration was carried out.Effectiveness of the modified activated carbon against removal of DBT and 4,6-DimethylDBT from crude diesel with the help of GCMS analysis is also discussed.

Materials
Carbon for this study was purchased from LOBA chemie (Catalogue number-0269300500) and had particle size of 150 -200 µm.Nickel nitrate, NaOH, Na2CO3, NaHCO3, KHP (Potassium Hydrogen Phthalate), HNO3 and HCl were also purchased from LOBA chemie and were used without further purification.Crude diesel sample was taken from Hindustan Petroleum Corporation Limited, Mumbai which was not desulfurized by any process.

Preparation of Modified Activated Carbon
Carbon was washed with distilled water till all the black was removed.Four different types of carbon were prepared.Details of the modification procedure are mentioned below.

Normal Carbon (N)
A portion of washed carbon was oven dried at 110°C and ambient humidity for 24 hours.

Acid Modified Carbon (A)
Similar portion of carbon (Sect.2.2.1) was soaked in 0.1M HNO3 in a ratio of 0.45 gm mL-1 .This carbon was soaked for 24 hours inclusive 6 hours of stirring under ambient temperature and humidity.After soaking, the carbon sample was oven dried at 110°C for 24 hours.

Ni2+ Impregnated Carbon (I)
Similar portion of carbon (Sect.1.2.1) was soaked in 0.1M solution of Ni(NO3)2.6H2O in a ratio of 0.3 gm.mL-1 for 24 hours inclusive 6 hours of stirring under ambient temperature and humidity.The carbon samples were dried at similar conditions as mentioned in (Sect.2.2.2).The nickel ion solutions were further stored for quantification of Ni+2 ion uptake.

Acid and Ni+2 Impregnated Carbon (A+I)
Similar portion of carbon (Sect.2.2.1) was soaked in 0.1M HNO3 in a ratio of 0.45 gm.mL-1 for 24 hours inclusive 6 hours stirring was followed by drying at 110°C for 24 hours.Carbon was then soaked in 0.1M Ni(NO3)2 solution in a ratio of 0.3 gm.mL-1 for 24 hours inclusive 6 hours of stirring and dried at conditions mentioned in (Sect.2.2.2).The nickel ion solutions were further stored for quantification of Ni+2 ion uptake.
All prepared variants were activated by calcination at 300°C before performing the experiment.These acidification procedures were studied from [11].and the nickel impregnation procedures were studied from [12].

Boehm Titration
Determination of carbon surface functionalities (CSF's) was carried out by Boehm titration.0.1 M solutions of NaOH, NaHCO3 and Na2CO3 were prepared.0.3 g of activated carbon (AC) of N, A, A+I and I grade were soaked in 13mL of each base.All the carbons were soaked for 24 hours and were stirred for half hour before titration.10mL of soaked base aliquots were back titrated with 20mL standardized HCl.20mL of standardized HCL was used to back titrate 5mL soaked Na2CO3 solution.Excess volume of HCl was used considering diprotic nature of Na2CO3 base.Blank base solutions were used to exempt variation due to heterogeneity by carbon particles.Manual colorimetric titration was carried out to quantify base adsorption by acidic surface functionalities on carbon.Only two drops of phenolphthalein were added to skip deviation in neutralization point.NaOH solutions were standardized by KHP and the HCL solution was standardized by standardized NaOH.Titrants were standardized every time before titration.0.1M bases 4 were used to minimize the concentration of CO2 in solution as suggested by Goertzen et.al. [13].Triplicate measurements were taken to minimize error.Followed procedures were studied from [13][14][15][16].The number of acidic sites were calculated under the assumption that NaHCO3 neutralizes carboxylic groups, Na2CO3 neutralize carboxylic and lactonic and NaOH neutralizes carboxylic, lactonic and phenolic groups [14,15].The formula for calculation of moles of acidic sites neutralized by bases is mentioned in Eq. (1).
Where, VB, VA and VaB are volume of titrant base, titrant acid and aliquot base solution, mB, mA and maB are equivalent ions of titrant base, titrant acid and aliquot base and WC is weight of carbon soaked in VaB aliquot base.The experimental procedure, derivation of Eq. ( 1), calculation of acidic sites from obtained burette readings and standardization burette readings are mentioned in supporting information (Sect.S1).

Carbon Surface Characterization
The surface area of the modified carbon was studied by N2 adsorption at 77.35 K on an automated apparatus (Quanta Chrome autosorb iQ2).Before N2 adsorption the carbon samples were degassed at 353 K.The surface area was predicted by Brunauer-Emmet-Teller method and pore volume was determined by volume of nitrogen adsorbed on relative pressure of 0.998.

Absorptive Desulfurization Test
To test the ability of the modified activated carbon samples' to adsorb DBT and 4,6-DimethylDBT from the crude diesel, 4 gm of activated carbon (AC) of N, A, A+I and I grade were added to 20 mL of crude diesel and four different test samples each with a different type of modified (AC) as adsorbent were prepared.Crude diesel used was first filtered by filter paper and then a 10 ml sample was stored as a reference for quantitative analysis.The GCMS-MS Analysis Technique was used for characterization of these four samples and one reference diesel sample.The parameters for the GCMS-MS Tests were kept the same for all the five samples and are listed below.

Surface Acidic Functionalities
The calculated carbon surface functionalities i.e. carboxylic, lactonic and phenolic groups are mentioned in Tab. 1.The carboxylic and lactonic groups have increased when the carbon N was treated with oxidizing media HNO3 to synthesize A. This treatment has increased carboxylic, lactonic groups and phenolic groups.The credit of increase in phenolic groups in carbon A given to the treatment of carbon at 110°C twice during the development process once after washing carbon and then after acid soaking.This increase mainly occurs in presence of water vapors even at ambient conditions and is called 'aging' [15].The changes in surface functionalities in I grade is quite higher as compared to that in A. The increase in phenolic groups can be attributed to the heat treatment at 110°C but major differences can be seen in the quantity of carboxylic and lactonic groups.It has been observed by many authors that the adsorbed transition metal ions like Pd, Ag, Au etc. on to the carbon reduce to solid metals, metal oxides or neutralize to metal complexes inside the pores of carbon surfaces [18][19][20][21].It has been observed that the complexes form only in presence of a large number of carboxylic surface functionalities thus the formation of metal complexes can be neglected in I.It is possible that metallic or metal oxide structures are formed in the carbon pores of I.The reduced metal surfaces exhibit acidic nature [18,22] at the nano scale and thus an increase in carboxylic and lactonic groups have been recorded.The modification of A+I carbon was done in a specific manner (Acid treatment and Nickel impregnation) after considering the conclusion by kim et.al. [3].As reported by Kim and co-worker's adsorption of 4,6-DMDBT over nickel sites is hindered due to presence of methyl groups near Sulfur atom.And thus, 4,6-DMDBT tends to adsorb more on the graphene sheets in carbon with pool of π electrons by π-π dispersive interactions.Contradictory to this behavior DBT compound were seen to adsorb more on adsorbent with solid nickel metals which bond by electron donated by Sulfur atom.Thus, while developing A+I action of both DBT and 4,6-DMDBT were taken into consideration i.e. presence of π electrons and ample of acidic sites for adsorption of 4,6-DMDBT's and metallic nickel for adsorption of DBT's.
The carboxylic surface functionalities at the carbon surface of A+I are recorded to be lower than on I (Tab.1).As observed by Silva et.al., Wang and Lu, and Biniak et.al. [19,21,23] the nickel ions would have neutralized with the carboxylic surface functionalities to form metal complexes with strength equivalent to lactonic group.A set of model reactions Eq. ( 2) and Eq.(3) of Nickel and surface functionalities were proposed by Noh and Schwarz [20].Due to this there has been less reduction of metallic nickel ion at the pores of the carbon which show lower carboxylic acid character.Reduction in amount of phenolic surface functionalities is also observed.The change in phenolic groups was not prominent during synthesis of I which have been synthesized like A+I. (2)

Nickel Impregnation
The quantification of nickel adsorbed on carbon are reported in Tab. 3. The I grade has adsorbed slightly greater number of nickel ions compared to A+I.The reduction in the nickel adsorption also supports the results obtained in Tab. 1 where there is drop in carboxylic surface functionalities.It can be said that formation of complexes acquires higher volumes which block nickel reduction at carbon surface.But silva et.al. [19] have reported contrast results that, higher the surface functionalities at carbon surface greater is the adsorption of Ni +2 ions.The results obtained in the Tab. 3 could be so because smaller pore volumes and average pore radius of carbon used in this study.(Tab.1).

Conclusion
Analysis of topographical and surface chemistry features of developed adsorbents for selective adsorption of DBT and 4,6-DMDBT was carried out.The developed adsorbents were modified by nitric acid treatment and/or nickel metal impregnation.In acid modified activated carbon A increase in carboxylic, lactonic and phenolic surface functionalities was reported while in I increase in carboxylic and lactonic surface functionalities was greater than in A. The reason for this increase could be reduced nickel metal at carbon pores.Increase in phenolic groups were also observed in both I and A+I AC.In A+I there was a drop in carboxylic surface functionalities and nickel ion uptake.This has happened due to formation of nickel complexes with carboxylic surface functionalities which would have occupied are sourced from[25,26,27,28]

Figure 1 .
Figure 1.N2 adsorption isotherm at 77 K of Normal and Modified activated carbon.
Nickel impregnation UV-Visible spectroscopy of the carbon-soaked nickel ion solutions were carried out to quantify the nickel ion adsorbed in I and A+I.The nickel ions were observed to absorb wavelength of 394.1 nm.

Table 2 .
[24]15,24]face functionalities on normal and modified AC surfaces.Surface Characteristics Surface features of N and modified A+I carbon are mentioned in Tab. 2 and the N2 adsorption isotherms are presented in Fig.1.The A+I carbon has double surface area compared to N carbon.The increase in surface area is due to reduced metal nickel at surface as mentioned in Sect.3.1.The formation of metallic surface can also be supported by difference in volume adsorbed at the relative pressure 0.1(P/Po) in Fig.1there is an immediate increase at the low relative pressure this can be due to newly developed nickel metallic surface at carbon pores.These observations can defend the hypothesis of formation of only metal complexes at carbon surfaces.Considering the metal complex formation on A+I as mentioned in Sect.3.1 a hypothesis can be made that the I grade would have higher surface area compared to A+I as a part of adsorbed Ni +2 ions have been neutralized to metal complex where as in I higher reduction of Ni +2 ions to metallic nickel is possible as there are very less carboxylic surface functionalities in N from which I grade is synthesised .The Complex formed may also result in high volume acquisition at micro pores of AC.N2 isotherm of A+I attains plateau at high pressure this proves formation of micropores at A+I surface.Similar observations have been made by suryawanshi et.al., H. P. Boehm, Wibonw et.al.[10,15,24]that the treatment of oxidizing agents like HNO3 on carbon result in increase of micropore volume due formation of CO2 at the micropore.Although very slight increase in surface area has been recorded due to only acid treatment by Wibowo et.al.[24].

Table 3 .
Surface characteristics of normal and modified AC

Table 4 .
Nickel ions adsorbed on modified carbon.