Cuiting Yang,Bowen Wu,Zewei Liu,Guang Miao,Qibin Xia,Zhong Li,Michael J.Janik,Guoqing Li,Jing Xiao,*

1 School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,China

2 Department of Chemical Engineering,Pennsylvania State University,PA 16802,USA

Keywords:Adsorption Catalysis Selectivity Ultra-deep desulfurization Transformer oil

ABSTRACT Ultra-deep desulfurization of transformer oil is of great demand among power industry.In this work,the effective and deep removal of various types of organosulfurs,including mercaptan,sulfide and disulfide via catalytic adsorptive desulfurization (CADS) using bifunctional Ti-based adsorbent is reported.Compared to adsorptive desulfurization (ADS),dramatically improvement of the organosulfur uptakes were achieved under CADS process.The equilibrium adsorption capacity at 5 μg·g-1 S reached up to 15.7,33.4,11.6 and 11.9 mg·g-1 for propyl mercaptan(n-PM),dimethyl sulfide(DMS),di-t-butyl disulfide(DTBDS) and dibenzyl disulfide (DBDS),which was 262,477,97 and 128 times to that of ADS process,respectively,and was the highest among the reported desulfurization adsorbents.Moreover,it achieved superior breakthrough capacity of 2050,530 and 210 ml F·(g A)-1 at the breakthrough S concentration of 1 μg·g-1 of the commercial transformer oil S containing 10,50 and 150 μg·g-1,respectively.The effectiveness of CADS is associated to the transformation of sulfur species to higher polar sulfonic species with the assistance of mild oxidant,which can be readily captured by silanol groups on SiO2 through Hbonding interaction.The excellent recyclability of the adsorbent can be realized through solvent washing or oxidative air treatment.This work provides an effective and economic approach for the elimination of trace amount of mercaptan,sulfide and disulfide from transformer oil.

1.Introduction

Transformer oil (or electrical insulating oil) which functions as electrical insulation and heat transfer fluid have been widely used in the transformers of electric power delivery system around the world nowadays [1].According to the ‘‘13th Five-Year Plan for Electric Power Development”,the market size of transformer oil is estimated to be 6.2 billion yuan in 2020,with an requirement for transformer oil about 0.63 million tons,which is about 13%increase compared with 2017 [2].However,the corrosive sulfur including mercaptan,sulfide and disulfide present in the transformer oil are commonly regarded as precursors for the corrosion of copper parts in the transformer and leading to the formation of highly conductive copper sulfide,which is culprit for deterioration of transformer oil and the failure of transformers and reactors[3-6].To ensure the reliable operation of transformers,regulatory agencies have imposed stringent legislation on the maximum level of corrosive sulfur content in transformer oil,i.e.,the International Electrotechnical Commission (IEC) limiting the content of leading corrosive sulfur DBDS in transformer oil to less than 5 μg·g-1.To meet the rigorous standards,there is a great need to devise effective approach for ultra-deep desulfurization of transformer oil.

Currently,addition of metal surface passivators and liquid-liquid extraction have been employed as mitigation techniques to reduce the impact of corrosive sulfur,but both of them come with some undesired collateral effects [1,7-9].For instant,the consumption of large amount of passivating agents lead to an increase in the production cost of transformer oil[8,10].In terms of extraction,the residual trace extractant in transformer oil would have adverse effect on the quality of oil,and further measures need to be taken to remove the small portion of extractant,which complex the overall desulfurization processes.In comparison,adsorptive desulfurization (ADS) has attracted considerable interest due to its merits of mild operating conditions of pressure and temperature,cost-efficiency and ease of regeneration [11-15].Various types of adsorbents have been discovered and intensively applied for the elimination of organosulfur compounds through different bonding mechanism,including activated carbon,metal organic framework,transition metal-based adsorbent and boron nitride,etc[16-19].However,the inadequacy in ultra-deep desulfurization which is associated to the poor adsorptive selectivity limits the practical application of ADS process.Qianet al.[6] reported Ag+-based Al2O3for the removal of DBDS from transformer oil,and reported a moderate adsorption uptake of 15.2 mg·g-1,but the precious Ag+loading was up to 11% (mass) and its regenerability was unclear.Therefore,development of highly selective and regenerable desulfurization process is highly demanded for the realization of ultra-deep desulfurization.

As a complement,the integration of oxidation and adsorption process has been reported as an effective approach for the selective ADS of thiophenic compounds,which involves the oxidation of low-polar thiophenic compounds to their respective high-polar sulfones,followed by the selective adsorption of high-polar sulfones over the same adsorbent.Zhanget al.[17] successfully synthesized a core-shell microsphere catalyst,which consists of a SiO2core and a mesoporous shell made up of C-dots and quaternary ammonium phosphotungstate,for the CADS of dibenzothiophene (DBT).Even with a reduced ratio of n(H2O2)/(DBT)compared to the stoichiometric amount,the calcined catalyst showed remarkable capacity for the DBT removal and achieved 98.08% CADS efficiency.Wanget al.[20] reported a hierarchically mesoporous titanosilicate nanospheres adsorbent which can completely convert DBT to DBTO2within 5 minutes under 25 °C and the products was adsorbed onto the silanol groups of silica.The fast catalytic kinetics could be attributed to the mesoporous structure which promoted the diffusion rate of reactants.Our group reported the TiO2-supported adsorbent for the CADS removal of thiophenic compounds,and further revealed the potential of TiO2-supported adsorbents as well as its extrudates for the ultradeep desulfurization of low sulfur-content diesel fuel [21-23].The CADS approach demonstrates to be effective but only limited to the thiophenic sulfur compounds up to date.

With the stubborn organosulfur species,including mercaptan,sulfide and disulfide present in transformer oil,the objective of this work is to explore the effectiveness of CADS for the selective and deep removal of various types of sulfur species under ambient conditions.In this work,propyl mercaptan (n-PM),dimethyl sulfide(DMS),di-t-butyl disulfide (DTBDS) and dibenzyl disulfide (DBDS)were chosen as representative corrosive sulfur.The CADS performance of the prepared adsorbent was evaluated in a batch system and a fixed-bed flow sorption system.The CADS sulfur chemistry for the capture of mercaptan,sulfide and disulfide on the Tibased adsorbent was discussed based on the detailed analysis of the sulfur compounds in the treated fuel.In addition,the regeneration experiments in multiple cycles were carried outviatesting different regeneration methods.

2.Materials and Methods

2.1.Adsorbent syntheses

All of the materials were commercially available and used directly without further purification.The powdered 2.5% (mass)TiO2/SiO2was synthesized by the incipient wetness impregnation method.First,a mixed solution using 1 g of SiO2(Shanghai Xianfeng Chemicals Co.)and 46 ml of ethanol(EtOH,AR,Guangdong Chemicals Co.)was stirred fleetly,and then certain amount of tetrabutyl titanate (AR,Shanghai Lingfeng Chemicals Co.) was dissolved in 4 ml of ethanol,and then it was added into the mixed solution dropwise under vigorous stirring at room temperature.Subsequently,the mixture was stirred at room conditions for 2 h,and then the ethanol was removed using a vacuum rotary evaporator at 55°C for 30 min.After this,the mixture was dried in oven at 100°C for 4 h,and then calcined in muffle under air flow at 550°C for 4 h ramped at 5 °C·min-1.The as-prepared samples were denoted as Ti/SiO2.The as-prepared adsorbents were stored in the sample bottles and dried at 100 °C for 2 h before testing.

2.2.Model oil

The model oils with sulfur concentration of 100-1000 μg·g-1-S were prepared by dissolving certain amount of n-propyl mercaptan(n-PM,99%,Aladdin),dimethyl sulfide(DMS,99%,Aladdin),di-tertbutyl disulfide (DTBDS,98%,Aladdin),and dibenzyl disulfide(DBDS,99%,Aladdin),ethanesulfonic acid(95%,energy chemical),dimethyl sulfoxide(99%,Gaungdong Guanghua Sci-Tech.Co.Ltd),S-methyl methanethiolsulfonate(97%,Shanghai Ailan Chemical Technology Co.,Ltd.) and S-phenyl benzenethiosulfonate(98%,energy chemical)in dodecane(99%,Guangdong Guanghua Chemicals Co.),respectively.The commercial transformer oil with 10,50 and 150 μg·g-1-S-containing were prepared respectively by dissolving DBDS in 0 μg·g-1-S-containing commercial transformer oil (Guangdong LOSH Lubricating Oil Co.)

2.3.Static desulfurization experiments

The static adsorption experiments which integrate oxidation and adsorption steps were carried out in a batch reactor setup at 30 °C.About 0.0375 g of adsorbent was well mixed with TAHPadded model oil until it reached adsorption equilibrium within 2 h,where the fuel-to-adsorbent ratio (mass) and O/S (TAHP/organosulfur compound in fuel) molar ratio were 100 and 5,respectively.The sulfur concentration in initial and desulfurized fuel were tested using a HPLC equipped with a UV-vis detector at 230 nm and TS-5000S analyzer with a sulfur detection limit of 0.2 μg·g-1.For comparison,the adsorption ofn-PM,DMS,DTBDS,DBDS,ethanesulfonic acid,dimethyl sulfoxide,S-methyl methanethiolsulfonate and S-phenyl benzenethiosulfonate over SiO2and 2.5 %Ti/SiO2without TAHP were tested,where dodecane was used as solvent.

2.4.Dynamic desulfurization experiments

The dynamic desulfurization performance of adsorbents was evaluated in a fixed-bed flow adsorption system at 30 °C.Before each test,about 0.5-1.5 g (depending on the volume of stainless steel column,WHSV,and packing density) of the dried Ti/SiO2adsorbent was packed into a stainless steel column of a suitable size (φ10 mm × 100 mm).Asbestos meshes and glass beads on both sides of the adsorbents were filled in the fixed bed to ensure that the adsorbents were packed tightly.The commercial transformer oil mixed with TAHP (O/S molar ratio was 5) was fed into the column from the bottom through a high-performance liquid chromatography (HPLC) pump.The organosulfur concentration of the treated commercial fuel was analyzed by a TS-5000S analyzer.The dynamic desulfurization experiment of SiO2in the commercial fuel without TAHP was also conducted for the purpose of comparison.

2.5.Regeneration

In the regeneration process,methanol (AR,Guangdong Chemicals Co.),ethanol,deionized water(H2O),acetonitrile(AR,Shanghai Lingfeng Chemicals Co.Ltd) and acetone (AR,Guangdong Chemicals Co.) were used as eluents for the regeneration of Ti/SiO2.The spent Ti/SiO2adsorbents were washed with 20 ml of above solvent for 1 h,respectively.After that,the adsorbents were filtered and dried at 100 °C for 4 h before testing.Oxidative air treatment was also employed as alternative for the recovery of spent Ti/SiO2.The saturated adsorbent was dried and then calcined at 550 °C for 4 h ramped at 5 °C·min-1.After that,the sample was stored in a desiccator before the next desulfurization cycle.

2.6.Characterization

The sulfur species in treated fuel was identified by GC-MS and LC-MS.The GC-MS method was initially set at:held at 50 °C for 1 min,and then increased to 250 °C at the ramp of 20 °C·min-1,then increased to 280 °C at the ramp of 20 °C·min-1and kept for 7 min.For LC-MS,the column for separation was Agilent SB C-18 and the detected wavelength was set at 210 nm.The oven temperature was set at 30 °C.The mobile phases were acetonitrile and water with volume ratio of 80:20.The mass spectrometer was operated using a 3 kV ESI spray voltage.XRF was performed on Axios Pw4400 (PANalytical B.V.) to analyze the composition of SiO2.

2.7.Computational calculation

Core-valence bifurcation(CVB)index has been used to describe the strength of hydrogen bonding based on the idea of topological analysis on electron-location function [24-27].In this work,Si(OH)4molecule refers to the original SiO2structure.The Si(OH)4-adsorbate system consists of one adsorbate molecule pointing to one of -OH groups from the optimized and fixed Si(OH)4.Both Si(OH)4and Si(OH)4-adsorbate system were optimized using Gaussian 09 with B3LYP/6-311+g(d,p) basis.Also,CVB index for each Si(OH)4-adsorbate system was all calculated using the same method as optimization.

3.Results and Discussion

3.1.Adsorption isotherms of various organosulfur compounds

To verify the effectiveness of the CADS process for the organosulfur uptake,the CADS isotherm ofn-PM,DMS,DTBDS,DBDS over Ti/SiO2adsorbent referred to that under sole ADS were compared in Fig.1.As it can be seen,the adsorptive desulfurization capacity for the four sulfur compounds can only reached less than 5 mg·g-1in the absent of oxidant TAHP.Conversely,pronounced desulfurization capacity was observed after the introduction of oxidant.At the low S equilibrium concentration(Ce) of 15 μg·g-1,the CDAS capacity of Ti/SiO2forn-PM,DMS,DTBDS and DBDS reached as high as 19.7,62.8,17.7 and 18.1 mg·g-1,while only 0.18,0.22,0.34 and 0.25 mg·g-1was achieved under ADS,respectively.In terms ofCeof 5 μg·g-1,the CADS performance reached 15.7,33.4,11.6 and 11.5 mg·g-1forn-PM,DMS,DTBDS and DBDS,which were 262,477,97 and 128 times of that when oxidant was absent,respectively.As evidenced by the above results,CADS process with the introduction of TAHP can be regarded as an efficient strategy to enhance the desulfurization performance of various types of organosulfur compounds in oil.

The experimental data was further fitted to the Langmuir isotherm (Fig.1),which can be expressed as follows:

WhereCeis the equilibrium concentration,μg·g-1;kis the adsorption equilibrium constant which is relative to the adsorption heat;Qmis the maximum adsorption capacity for sulfur compounds,mg·g-1.The fitting parameters of Langmuir model are shown in Table 1.The high correlation coefficients for all of the organosulfur species under CADS and ADS process indicate that the isotherm data can be well fitted by the Langmuir adsorption model.The maximum adsorption capacities(Qm) for CADS process were found to be 22.6 mg·g-1forn-PM,112 mg·g-1for DMS,24.1 mg·g-1for DTBDS,and 25.2 mg·g-1for DBDS,which is much greater than ADS process,suggesting the favorable adsorption of organosulfur species after the introduction of TAHP.

Fig.2 compares the adsorption capacity ofn-PM,DMS and DBDS in this work with those reported in literatures.As shown,then-PM adsorption capacities in several kinds of carbon materials reported by previous studies range from 1.2 to 4.4 mg·g-1[28,29],while the ADS performance of Ti/SiO2reached 9.16 mg·g-1in present work.Furthermore,the CADS process was 2.4 times higher adsorption capacity than the ADS process.Modified carbon materials,such as MOF-5/AC and Ag/WSAC,have also been investigated as adsorbents for the elimination of DMS and found to display saturated capacity of 3.84 and 16 mg·g-1,respectively,while 102 mg·g-1was achieved in this work [28,30].In terms of DBDS,various low-cost adsorbents were reported to exhibit poor adsorption uptakes below 2 mg·g-1[1,31],while a recent work mentioned a supreme saturated adsorption capacity of 15.2 mg·g-1for Ag/γ-Al2O3[6].However,the aromatic hydrocarbon present in transformer oil may compete with the DBDS toward Ag+active sites through π-complexation and hinder the practical application of Ag/γ-Al2O3.In comparison,Ti/SiO2displayed a higher adsorption capacity of 24.4 mg·g-1for DBDS under CADS process.Additionally,the discrepancy of adsorption interaction between aromatic compounds and oxidized DBDS products provides the possibility to capture DBDS with high selectivity.

On the other hand,kvalues of Ti/SiO2in the presence of oxidant are both around two orders of magnitude higher than that in the absence of oxidant,suggesting the dramatic enhancement in adsorbent-adsorbate interaction after the introduction of oxidant.Notably,Ti/SiO2showed a higherkvalue for the CADS removal ofn-PM,which was around 5 times of that for DMS and 2.4-2.7 times for DTBDS and DBDS,implying the better adsorption affinity of the CADS product of propyl mercaptan towards Ti/SiO2.This could attributed to the different mechanisms of CADS process for mercaptan,sulfide and disulfide removal while using TAHP as oxidant.Improved desulfurization performance of aromatic sulfur compounds under CDAS process were reported previously and the enhanced adsorption capacity could be attributed to oxidative transformation of aromatic sulfur compounds to high polar sulfone,which can be easily adsorbed over the silanol of supports[21,32].However,the CADS mechanism for the ultra-deep desulfurization of mercaptan,sulfide and disulfide is still blurry.The underlying detailed mechanism will be discussed in the later section.

3.2.Dynamic breakthrough desulfurization of various organosulfur compounds

To evaluate the dynamic CADS performance for sulfur compounds in the present of competitive compounds,which is of significant importance for the industrial application,the dynamic CADS of commercial transformer oil with different original S concentration (10,50,150 μg·g-1) onto Ti/SiO2in the fixed bed was performed.DBDS was chosen as representative sulfur species adding to the 0 μg·g-1-S-containing commercial transformer oil,since it was reported as refractory corrosive sulfur present in real fuel.Fig.3 represents the breakthrough curves of Ti/SiO2for real transformer oil with different initial S content and the relative desulfurization conditions,the dynamic CADS capacity as well as fitting YN model parameters are listed in Table 2.

Table 1Fitted Langmuir parameters of the desulfurization isotherm

Table 2Dynamic desulfurization conditions,dynamic CADS capacities of Ti/SiO2 and fitted Y-N model parameters

Fig.1.ADS and CADS isotherms of n-PM(a),DMS(b),DTBDS(c) and DBDS(d) over Ti/SiO2 at 30 °C.(O/S mole ratio was 5,fuel-to-adsorbent mass ratio was 100).

The dynamic adsorption capacity was calculated with the following equation:

Where ρ is the density of transformer fuel,g·ml-1;Wis the amount of adsorbent,g;Vis the flow rate of fuel,ml·min-1.

Since the desulfurization process is the integration of catalytic oxidation of initial sulfur species and the adsorption of oxidative product,the breakthrough curves were fitted to the two-stage model composed of the following two equations:

Whent≤tB,the adsorption sites are sufficient enough for the capture of high polar CADS product and the rate-determining step falls to the catalytic oxidation of DBDS.We consider this case as stage A,where the outlet concentration of S is constant and can be fitted to Eq.(3);On the other hand,whent＞tB,the adsorption sites are almost fully loaded by CADS product over time and the overall process performance is determined by the adsorption step.This case is denoted as stage B and the relative breakthrough curve can be well fitted with Y-N model (Eq.(4)).Eq.(4) can be transformed into:

where τ is the time for 50%sulfur compound breakthrough,min;Kfis the adsorption rate constant.

Fig.2.Comparison of n-PM,DMS and DBDS adsorption capacity of the Ti/SiO2 adsorbent and those adsorbents reported in other literatures.

Fig.3.Breakthrough curves of different S-content.

As shown in Table 2,the dynamic sulfur equilibrium concentration of 10,50 and 150 μg·g-1breakthrough curves in stage A are both below 0.2 μg·g-1,suggesting the excellent desulfurization performance in the early stage which result from the high catalytic efficiency of Ti/SiO2with the aid of TAHP.It should be noted that the breakthrough S-capacity at 1 μg·g-1for the 10 μg·g-1CADS breakthrough curve reached as high as 16.3 mg·g-1or 2050 ml F·(g A)-1,implying the industrial potential of Ti/SiO2for the ultra-deep de-disulfide from transformer oil.Meanwhile,the breakthrough S-capacity at 10 μg·g-1for 50 and 150 μg·g-1CADS breakthrough curves were achieved 20.8 and 24.7 mg·g-1,correspond to 530 and 210 ml F·(g A)-1,respectively.Conversely,without the aid of TAHP,SiO2showed poor adsorptive desulfurization performance for 50 μg·g-1-S-containing DBDS,which was evidenced by the low breakthrough S-capacity at 10 μg·g-1(2.2 mg·g-1).In the CADS process,theKfincreased significantly when the initial S concentration in the real fuel increased from 10 μg·g-1to 50 μg·g-1,and remained stable when S concentration raised from 50 μg·g-1to 150 μg·g-1(Table 2 and Fig.S1,in Supplementary Material).These phenomena indicated that the introduction of sulfur compound to the solution with relative high S content have no apparent impact on the dynamic adsorption rate of Ti/MCM.The highKffor the ADS process of SiO2can be rationalized by the physical adsorption for DBDS featuring the fast adsorption and low selectivity.

Table 3ADS performance of SiO2 and Ti/SiO2 to four kinds of commercial sulfonic acid sulfoxide and thiosulfonate species

3.3.Regeneration

The reusability of adsorbent is economically important for the industry application,which is relative to the overall efficiency and operating cost of the adsorption process.Different polar solvents were first used to regenerate the spent Ti/SiO2after the CADS ofn-PM.The recovery efficiency at one adsorption-regeneration cycle by solvent extraction was displayed in Fig.4(a).It is obvious that the CADS performance of Ti/SiO2dropped up to half of the original desulfurization capacity in one regeneration run by simple solvent washing.This results indicated that the adsorbed CADS product ofn-PM is too resistant to be removed by simple solvent washing,which coincides with the adsorption isotherm results.Thermal regeneration was further performed to remove the stubbornly adsorbed CADS product from spent Ti/SiO2and the reusability for propyl mercaptan adsorption on Ti/SiO2under consecutive adsorption-regeneration cycles are shown in Fig.4(b).As expected,there was almost no loss of desulfurization performance at 5 consecutive desulfurization-regeneration cycles,indicating the active sites can be fully recovered after regeneration.

In contrast to the difficulty of desorbing CADS product ofn-PM,the oxidized products of sulfide and disulfide were found to be easily removed by simple solvent washing.Fig.5 displays the CADS capacity of Ti/SiO2for DMS,DTBDS and DBDS,respectively,after regeneration by ethanol washing.Results show that the samples display comparable desulfurization performance in first four cycles.The weak interaction of Ti/SiO2towards oxidized products of DMS,DTBDS and DBDS may contribute to their full regenerability.

3.4.Sulfur chemistry on Ti/SiO2

On the basis of Like Dissolves Like Theory,polar sulfur species adsorbed over the adsorbent can be released into solvents with similar polarity.Herein,to further understand the sulfur chemistry involved in CADS process of mercaptan,sulfide and disulfide,we washed the spent Ti/SiO2with sulfur-free methanol and detected the eluent with LC-MS or GC-MS.As shown in Fig.6,rather than the initial sulfur species,relative oxidative products were found to be adsorbed over Ti/SiO2.Interestingly,there is a distinction between the oxidant products.Propanesulfonic acid was observed as the CADS product ofn-PM,while sulfone compounds were identified as the CADS product of DMS,thiosulfonate compounds were detected as the CADS product of DTBDS and DBDS.It should be mentioned that only initial organosulfur species were detected in the spent Ti/SiO2when TAHP was absent during the ADS process.The variety of sulfur species on the spent adsorbent implied the introduction of TAHP have changed the sulfur chemistry occurred during desulfurization.With the aid of TAHP,n-PM,DMS,DTBDS,DBDS were chemically transformed to their relative oxidative products.It should be noted that,without the addition Ti/SiO2,negligible conversion of the sulfur species in the TAHP-added fuel was found,indicating the catalytic role of Ti/SiO2for the conversion ofn-PM,DMS,DTBDS,DBDS (Fig.7).

Fig.4.Recovery capacities of Ti/SiO2 under different solvent-washing (a) and CADS capacities of Ti/SiO2 for n-PM capture at five consecutive CADS-thermal regeneration cycles(b).

Fig.5.CADS capacities of Ti/SiO2 for DMS(a),DTBDS(b) and DBDS(c) removal at four consecutive CADS-regeneration cycles.

Fig.6.MS pattern of sulfur species in the methanol eluent of spent Ti/SiO2 used for the removal of n-PM(a),DMS(b),DBDS (c);DTBDS (d).

Fig.7.Conversion of n-PM(a),DMS(b),DTBDS(c) and DBDS(d) onto different material in the TAHP-added fuel.

To further determine the effect of each component on the catalytic role of Ti/SiO2,a series of batch experiments of organosulfur conversion were conducted using various adsorbents at room temperature (Fig.7).Compared to the commercial TiO2and support SiO2with ＜50% conversion forn-PM,DTBDS,DBDS,Ti/SiO2exhibited dramatically improved catalytic performance and can completely convert these compounds to their relative oxidative products,suggesting that synergistic effect between TiO2and SiO2plays an important role for the catalytic reaction(Fig.7(a),(c),(d)).Our previous work has shown that high dispersion of TiO2over SiO2-based adsorbent would result in high proportion of tetrahedral-coordinated Ti4+,which contributes to the high reactivity for the catalytic conversion of organosulfur species.Interestingly,100%conversion of DMS can be achieved in the TAHP-added fuel with the addition of SiO2,indicating SiO2served as catalyst rather than TiO2for the oxidative transformation of DMS (Fig.7(b)).This result is dissimilar with previous reports,where TiO2is responsible for the catalytic transformation of aromatic sulfur compounds while SiO2-based support only acts as adsorbent and provides adsorption sites for the CADS products [21].

To investigate the reason causing the supreme catalytic performance of SiO2to DMS conversion,XFR was employed to figure out the composition of SiO2.As shown in Table S1,SiO2is mainly composed of SiO2with trace amounts of Fe2O3.Previous studies indicated that highly active oxidizing complex would be formed between oxidant and Fe2O3,which is capable of oxidizing DMS to DMSO2[33].Fe2O3present on SiO2may contribute to the catalytic performance of the adsorbent,but further research is necessary to elucidate this phenomenon.

On the other hand,only minute sulfur compounds were detected in the liquid phase after desulfurization over Ti/SiO2in the presence of TAHP,signifying that transformed sulfur species were adsorbed over Ti/SiO2rather than desorbing from the surface.That is to say,Ti/SiO2also served as adsorbent.Silanol groups on the mesoporous molecular sieve are commonly considered as adsorption site for the uptake of high polarity organosulfur species,such as sulfoxide and sulfone,during the CADS process.In order to investigate the role of SiO2to the adsorption sites in our desulfurization system,static adsorption experiments of commercial sulfonic acid,sulfoxide and thiosulfonate species were carried out onto SiO2and Ti/SiO2.As evidenced by the similar adsorption capacity of SiO2and Ti/SiO2for the commercial ethanesulfonic acid(90.3%versus93.1%),dimethyl sulfoxide(97.7%versus98.0%),Smethyl methanethiolsulfonate(97.2%versus97.0%),and S-phenyl bezenethiosulphonate (94.4%versus94.1%),as shown in Table 3,SiO2is confirmed to play a critical role for the CADS products adsorption,which most likely captures sulfonic acid and sulfone compound though silanol groupsviahydrogen bonding[21].Combined with the above results,the CADS process of Ti/SiO2can be considered to comprise two steps:the oxidation of the mercaptan,sulfide and disulfide to the oxidized sulfonic acid compound and sulfone compounds,respectively,followed by the adsorption of the CADS products over Ti/SiO2.

To elucidate the stronger adsorption affinity of propanesulfonic acid onto Ti/SiO2compared to other oxidant products,the hydrogen bond strength between CADS products and Si-OH were investigated using computational calculation.Fig.S2 displays the optimized adsorption configuration between Si(OH)4and CADS products.The relative CVB index of the four Si(OH)4-CADS products systems which provide the information of hydrogen bond strength is shown in Table 4.It is evident that the Si(OH)4-propanesulfonic acid system shows the most negative value of CVB index,while the Si(OH)4-CADS products systems of DMS,DBDS and DTBDS show the more positive CVB values.These results indicate that propanesulfonic acid shows stronger interaction toward Si-OH through hydrogen bond compared with the other three CADS products.The simulation results are consistent with the adsorption isother-mal experiment and can rationalize the regeneration phenomenon.This could ascribe to the abundance of hydrogen bonds and the less steric hindrance of propanesulfonic acid.Overall,the superior CADS performance and good regenerability indicated the potential of Ti/SiO2for the practical application of desulfurization (Fig.8).

Table 4CVB index for four Si(OH)4-CADS product systems

Fig.8.The CADS process of n-PM.DMS,DTBDS,DBDS over Ti/SiO2 adsorbent.

4.Conclusions

Catalytic adsorptive desulfurization of transformer oil using Tibased adsorbent under mild conditions was investigated comprehensively in this work.The excellent CADS performance of Tibased adsorbent to various types of organosulfur was revealed by the ultra-deep desulfurization experiments,exemplified by the high saturated desulfurization uptakes of 22.6,112.4,24.1 and 25.2 mg·g-1forn-PM,DMS,DTDBS and DBDS,respectively,which were significantly higher than that under ADS process,and was the highest among the reported desulfurization adsorbents.Furthermore,it was able to remove the refractory DBDS to ＜1 μg·g-1from real transformer oil and showed superior breakthrough capacities of 2050,530 and 210 ml F·(g A)-1for 10,50,150 μg·g-1-Scontaining breakthrough curves,separately,indicating the applicability of Ti-based adsorbent in the industrial-trace corrosive sulfur capture.The dramatic promoting effect of CADS can be ascribed to the transformation ofn-PM,DMS,DTDBS and DBDS to higher polar sulfur species with the aid of mild oxidant,which can be facilely captured by silanol groups on SiO2through hydrogen bonds.Different oxidative products ofn-PM,DMS,DTDBS and DBDS during the CADS process were obtained,in whichn-PM was transformed to propanesulfonic acid and the others were converted to their relative sulfone and thiosulfonate species.Stronger adsorption interaction between propanesulfonic acid and Ti-based adsorbent was observed compared to that of the other CADS products,which may be attributed to rich hydrogen bonds and less steric hindrance.The adsorbent retained excellent recyclability after multicycle regeneration.In summary,with the advantages of excellent static and dynamic desulfurization capacity,facile reproducibility,mild operation conditions and low-cost adsorbent synthesis,Tibased adsorbent under CADS can be envisioned of great potential for the practical applications involving transformer oil processing.

Declaration of Competing Interest

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.

Acknowledgements

The authors gratefully acknowledge the research grants provided by the National Natural Science Foundation of China(22022806,21776097),and Natural Science Foundation of Guangdong Province (2017A030312005).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.031.