Characterization of Sulfur Compounds in MTBE

A study is carried out on chemical constitution of sulfur compounds in MTBE and their formation mechanisms. These sulfur compounds are classified into three types: common sulfur compounds, newly formed sulfur compounds, and high boiling sulfur compounds. Common sulfur compounds which include mercaptans, low molecule sulfides and disulfides, are directly from C4, one of the stocks for production of MTBE. The newly formed sulfur compounds, with one sulfur atom and five or more total carbon atoms in one molecule, are mainly tert-butyl methyl sulfide and tert-butyl ethyl sulfide, thioetherification products of thiols with butenes. Many high boiling sulfur compounds, including polysulfides such as dimethyl trisulfide, multisulfur heterocyclic compounds such as 3,5-dimethyl-1,2,4-trithiolane, and oxygen-containing sulfur compounds such as 2-methoxy-3-methylthiobutane, are also found newly formed in the processes of LPG refining and succedent etherification reaction for producing MTBE. Polysulfides are additional products of elemental sulfur to disulfides, and other high boiling sulfur compounds may be formed by thiols reacting with dienes.


Introduction
Methyl tert-butyl ether, or MTBE for short, is used as high octane number component for gasoline, excellent solvent, and stock for cracking preparation of polymerization grade isobutene and so forth.It is approved as the largest group of nonhydrocarbons blending component in gasoline in China.Production capacity of MTBE in China was over 10MT in 2013.
Organic sulfur compounds are acknowledged to be harmful to all fuel oil products including gasoline.So in less than 30 years, limits to sulfur contents in various petroleum products have been greatly reduced.For example, the sulfur content allowance in gasoline which met EN228-1993 (European Standard: Automotive fuels-Unleaded petrol-Requirements and test methods) must be less than 0.10%, but sulfur limit in EN228-2009 must be less than 0.0010%, with a reduction of 99% in 15 years.Another case is automotive LPG, expressed in EN589 (European Standard: Automotive fuels-LPG-Requirements and test methods).In EN589-1993, total sulfur content is limited to less than 0.0200%, but in EN589-2008 the sulfur content limit is reduced to no more than 0.0050%.
In China, MTBE is the most important gasoline blending component; its volume in gasoline can be up to 14.9 (v)%.The sulfur content of "Gasoline for Vehicle in China" was allowanced no more than 0.20% before 2000, and it is now limited to no greater than 0.0050% and will be limited to less than 0.0010% before 2017 [1].But sulfur content in MTBE produced in China is usually 0.0040-0.1000%,some even over 0.2000%.Sulfur content is now the main bottleneck restricting MTBE as gasoline blending component.
MTBE is mainly synthesized industrially through the etherification reaction of methanol with isobutene existent in mixed C4 hydrocarbons under the catalysis of acidic catalysts, such as strongly acidic cation exchange resins.The mixed C4 hydrocarbons are usually obtained by distillating liquefied petroleum gas in the gas fractionation unit, containing isobutene, butane, 1-butene, and 2-butene besides isobutene.Experiences show that 80% or more of sulfur in LPG will be concentrated in MTBE.So usually sulfur contents in MTBE are 5 or more times as in refined LPG.
Some studies have been carried out on sulfur compounds that existed in LPG in China and abroad [2][3][4][5][6][7][8].But there have been only a few researches on how to remove sulfur compounds from MTBE [9,10].There are fewer investigations on sulfur compounds in MTBE.Therefore, it is necessary to make clear structures and forming mechanisms of sulfur compounds in MTBE.Understanding of these compounds will be beneficial for making practicable programs to reduce sulfur content of MTBE.

Experimental
2.1.Instruments and Methods.Gas Chromatography Agilent GC7890, equipped with a 50-meter-long PONA column, dual detectors of SCD (sulfur chemiluminescence detector), and FID (flame ionization detector), was applied to acquire sulfur signal or both sulfur and organic component signals simultaneously, operated at the following GC temperature program: initial temperature at 35 ∘ C, ramped at 3 ∘ C/min to 180 ∘ C at 2 ∘ C/min with a final hold of 5 min.
Gas chromatograph-mass spectrometer (GC-MS) is composed of Agilent GC7890B and 5977A MSD, used to determine chemical structures of most sulfur compounds in MTBE.Gas chromatograph-high resolution mass spectrometer (GC-HRMS), composed of Agilent GC7890 and JEOL AccuTOF GCV High-Resolution Mass Spectrometer, was used to confirm molecular formulae of some complicated sulfur-containing compounds in MTBE.
Total sulfur contents of all samples were obtained through TS3000 UV total sulfur analyzer, which is provided by Thermo Fisher Scientific Inc., according to the method of SH0689 or D5453-12 (Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Spark Ignition Engine Fuel, Diesel Engine Fuel, and Engine Oil by Ultraviolet Fluorescence).
Infrared Spectrometer, Nicolet 6700, also product of Thermo Fisher Scientific Inc., was used to get IR spectrums of some samples.NMR (nuclear magnetic resonance) spectrometer of Varian Company, named INOVA 500 MHz, was used to obtain both 13C NMR and 1H NMR of samples.
MTBE synthesis equipment is a device provided by the Scientific Equipment Center of Research Institute of Petroleum Processing and was used here to simulate the formation of some trace sulfur compounds while MTBE is being synthesized under normal conditions.

Experimentation
Preparations of Disulfides in Sulfur-Free Solvent.Methanethiol, ethanethiol, isopropanothiol, propenothiol, and tertbutanethiol were dissolved in an aqueous solution of 10% sodium hydroxide with sulfur concentration of about 100 mg/L each.Cobalt phthalocyanine sulfonates were added in the solution at 100 mg/L and stirred vigorously for 10 min.
Then a sulfur-free solvent (90-120 mineral ether) was added to the alkali solution with a volume identical to that of alkali solution and stirred vigorously for another 10 min.The mixture was then settled down for 30 min.The upper layer of mineral ether was the extracted layer and may contain corresponding disulfides which might occur in refined C4.
Simulation Synthesis of Sulfides in MTBE.Methanethiol and ethanethiol are dissolved in methanol at the concentration of 200 mgS/L.The solution was then mixed with isobutene in a N 2 -replacement jar at 0.4 Mpa, with the molar ratio of methanol : isobutene equal to 1.1 : 1.The mixture was then passed through a bed loaded with macroporous cation exchange resin D006 at 65 ∘ C, with a liquid hourly space velocity of 1.0 h −1 .The output was collected in a jar cooled below −10 ∘ C.After the reaction, the output was flashed to remove the surplus isobutene and methanol at atmosphere and ambient temperature.An MTBE sample was obtained, possibly containing simulated synthesized trace organic sulfur compounds which remained therein.
Preparations of Polysulfides in Sulfur-Free Solvent.An unregenerated alkali for refining LPG was got from a refinery.It contained sodium hydroxide of 12.5%, mercaptides of up to 22000 mgS/L of CH 3 S − and 5600 mgS/L of CH 3 CH 2 S − .7.3 g of sodium sulfide was dissolved in 50 mL of the alkali to get a concentration of S 2− about 60000 mgS/L, and then about 5 mg of cobalt phthalocyanine sulfonates was added and mixed vigorously while being exposed to the air for 30 min.At last, 50 mL of 90-120 mineral ether was used to extract possible newly formed fat-soluble substances in the alkali.It is expected that a series of polysulfides were contained in the extraction phase.
A calibration table of retention time (  ) in GC-SCD for individual sulfur compounds possibly existent in MTBE was obtained by using commercial standard substance or laboratory-synthesized sulfur compounds.

Discussion
MTBE is mainly manufactured from the etherification reaction of methanol with isobutene contained in mixed C4, which is usually bottom product in the gas fractionation device, with the refined LPG as the feed.Sulfur analysis for nearly 50 sources of MTBE shows that almost all sulfur compounds are originated from refined LPG, for sulfur contents of methanol used are usually less than 0.00005%.So it is important to investigate how sulfur compounds are transferred from refined LPG to MTBE.

Calibration of Common Sulfur Compounds.
Here common sulfur compounds are referred to as sulfur-containing compounds simply transferred from LPG or C4 to MTBE.These sulfur compounds may be directly from LPG stock or may be changed morphologies in LPG refining processes.
The GC-SCD/FID instrument is used to measure retention time of relevant sulfur compounds under the condition of 2.1.Standard sulfur compounds are valve-injected into the column cap of the GC.Retention time of these sulfur  shown in Figure 1 (MTBE05).Results for five of these samples (MTBE01, MTBE02, MTBE03, MTBE04, and MTBE05 from different refineries) are listed in Table 2. Figure 1(a) represents the whole GC-SCD for MTBE05 in Table 2, and Figure 1(b) represents its spectrogram 10 times amplified on the -axis.Figure 1(b) shows that there are 5 significant peaks, peak 5, 6, 7, 8, and 9, indicating the existence of 5 main types of sulfur compounds in the sample.In Figure 1(a), some more trace amount of sulfur compounds can be seen.Among these peaks, peak 1-4, 6, and 9 can be resolved as methanethiol, ethanethiol, dimethyl sulfide, propyl methyl sulfide, dimethyl disulfide, and ethyl methyl disulfide, respectively, referring to Table 1.Other types of sulfur compounds need further research.
Many MTBE samples were analyzed in this way and much more types of sulfur compounds, most of which were often ignored before, were found.
It can be found in Table 2 that sulfur content and sulfur distribution of various MTBE samples are very different from each other.Most of sulfur compounds are dimethyl disulfide, ethyl methyl disulfide, and diethyl disulfide.There are seldom higher molecular disulfides in MTBE.Sometimes there are little lighter sulfur compounds such as C1-C2 thiols and C2-C4 sulfides.
Disulfides have a formula as follows: where R 1 and R 2 are alkyl.Disulfides are the products of sweetening of thiols, forming in LPG refining process.In most cases, disulfides are the main forms of sulfur compounds in refined LPG or C4; therefore disulfides often account for main sulfur compounds in MTBE.However, In some extreme case, there are almost no common sulfur  2. That means sometimes the LPG refining process produces no disulfides.Some MTBE samples contain lighter sulfur components, such as methanethiol, ethanethiol, and dimethyl sulfide.These sulfur compounds can be controlled by improving alkali-washing condition (for reducing thiols), or adjusting stable adsorption condition (for reducing ethanethiol and sulfides), or optimizing operation conditions of MTBE catalysis distillation tower (for reducing all sulfur compounds with B.P. < 56 ∘ C, B.P. of MTBE).
Of particular note is that in Table 2 there are many more other forms of sulfur compounds except common ones.These forms of sulfur account for about 30%-50%, occasionally over 80%, of total sulfur as in MTBE01 and can never be ignored.They cannot be simply identified by deduction from sulfur compounds in refined LPG or C4.
When ultralow sulfur MTBE is requested, sulfur compounds with minor content in MTBE should also be investigated before making a scientific process to greatly reduce content of sulfur compounds in MTBE.It is necessary to investigate structures of other forms of sulfur compounds besides common sulfur compounds in MTBE listed in Table 2.

Research on Complicated Sulfur Compounds in MTBE.
In Table 2, there are many other forms of sulfur compounds in MTBE besides common sulfur compounds.Sometimes they account for the majority of sulfur as in MTBE01.Analyses show that these compounds are seldom found in refined LPG or C4.They could be second reaction products of common sulfur compounds with some components in C4 under the etherification conditions for producing MTBE.It is necessary to investigate structures of these sulfur compounds.

Light Complicated Sulfur Compounds and Formations.
These sulfur compounds cover   in GC-SCD from 15 min to 60 min.For the sake of research, they are classified into light complicated sulfur compounds and heavy complicated sulfur compounds to the limit of   = 32.0 min in GC-SCD.That means newly formed sulfur compounds with   < 32.0 min are light complicated sulfur compounds and those with   > 32.0 min are heavy complicated sulfur compounds.
In Figure 1, peak numbers 5, 7, and 8 often appear in GC-SCD.Under certain circumstances, peak number 5 as in Figure 1 is even the strongest peak (as MTBE01).
For sample MTBE03 in Table 2, the peak intensity corresponding to peak number 6 in Figure 1 is very strong.So GC-MS is directly applied to carry out a fine analysis.Result shows that the most matching structure is as follows: Its CAS registry number is 6163-64-0, nominated as 2methyl-2-(methylthio)-propane or tert-butyl methyl sulfide (MTBS).The structure is similar to MTBE except oxygen atom being displaced by sulfur atom: As is known to all, MTBE is synthesized industrially through etherification reaction as follows: Methanol and ethanol can react with isobutene to form MTBE and ETBE under acidic catalytic condition.It is deduced that methanethiol and ethanethiol may also react with isobutene to form MTBS (methyl tert-butyl sulfide) and ETBS (ethyl tert-butyl sulfide), respectively, as the same mode of formation of MTBE and ETBE. (2) For confirming this deduction, a simulation test was carried out.About 200 mg/kg (by S) of methanethiol and ethanethiol is, respectively, added to absolute methanol.The methanol containing thiols was then mixed with isobutene, with molar ratio of methanol/isobutene about 1.1 : 1. 40 mL of D006 macroporous cation exchange resin was packed in a tabular reactor prepared for the reaction.
The mixture was then pumped up-to-down through the reactor with LHSV 1.0 h −1 and temperature of 55-70 ∘ C. Product eluted from the reactor was collected in a cooled jar.The product was flashed at ambient temperature to remove unreacted isobutene and some possible thiols.The remained main substance was theoretically MTBE and is analyzed by GC-SCD to determine the distribution of sulfur compounds.GC-FID measurement showed that major component was MTBE.
Figure 2 is a comparison of sulfur compounds forms in methanol adding the thiols and in MTBE synthesized with the methanol and isobutene.
Obviously in the etherification reaction of methanol with isobutene to synthesize MTBE, thioetherification reactions of thiols with butene to form alkyl-butyl sulfides also concur.In other MTBE samples, structures as are also found by GC-MS (  at 19.20 min, 19.50 min, resp., at the GC condition), but usually with lesser concentration.These newly formed alkyl sulfides may also be produced through thioetherification reactions of thiols with butenes similar to the reaction (2).
Increasing water content in methanol or (and) lowering etherification reaction temperature can reduce the production of these sulfides.But the effects are very limited.An easy and good way to avoid newly formed alkyl sulfides is removing thiols thoroughly from C4 or LPG in the process of LPG refining.

Heavy Complicated Sulfur Compounds.
There are more or less heavy sulfur compounds in most of all MTBE samples under our research.For example, peak 11-24 in Figure 1 indicate existence of sulfur compounds with higher boiling point.Usually, their concentrations are often very low, and so close attention was not paid in the past.But it is still important to study their chemical structures for deep desulfurization of LPG and MTBE.
For about 50 MTBE samples analyzed by the GC-SCD, there are three circumstances about heavy sulfur compounds: 13 samples present heavy sulfur compounds as in Figure 3(a) and 17 samples present heavy sulfur compounds  2.
as in Figure 3(b).The other MTBE samples contain too few heavy sulfur compounds to have enough SCD signals for recognition.Figure 3 is GC-SCD of MTBE03 and MTBE04 in Table 2.The enlarged area clearly shows that some heavy sulfur compounds exist in some MTBE samples.
It is found in GC-SCD figures that most of MTBE samples have a peak of 32.70 min, as peak 10 in Figure 3(a).Furthermore, there is a smaller peak adjacent to the peak 32.70 min, with retention time of 32.50 min and intensity of about one-third of the former.
Sample MTBE03 has very high sulfur content.It was distilled (first at atmosphere and then at gradually reduced pressure to ensure the temperature of liquid phase less than 100 ∘ C) carefully to remove lighter fractions and leave heavier component.A GC-SCD analysis is carried for the heavier component.Figure 4 shows the comparison of signal obtained by SCD and FID for the heavier components in retention time between 20 and 40 min.It is apparently found in Figure 4 that almost every SCD signal corresponds to a FID signal.So it can be considered that the heavier component obtained is almost a mixture of concentrated sulfur compounds.
Because of small amount of the concentrated sample, it is difficult to separate the heavier components further into individual sulfur compound.Instrumental methods of IR (Infrared spectrum), HRMS (high resolution mass spectrum), and NMR (nuclear magnetic resonance) are applied to help characterizing the heavier components.
HRMS result shows that peak of   at 32.70 min has a relative molecular weight of 134.24.According to the origination of MTBE and sulfur content of the sample, elements composed of the heavy components are carbon,

Me
From the point of view of molecular configurations, the two stereochemistries should have a small difference in dipole moment.This may induce a tiny difference in retention time in GC analysis.The S,R relative stereochemistry should have a slightly larger dipole moment and so its chromatographic peak will elute a little earlier than S,S relative stereochemistry on the nonpolarity GC column.This may be the reason that there is always a pair of peaks at 32.50 min and 32.70 min for most sulfur-containing MTBE.structures are very different from those in LPG or C4.Most of these sulfur compounds may be formed in the process of etherifying isobutene with methanol.Generally, there is about 0.1-0.5% of 1,4-diene in stock C4.In etherification process, methanol is the other stock.If there is even trace methanethiol in stock C4, a reaction may occur to a certain degree: Reaction product is 2-methoxyl-3-methylthio-butane.Considering steric hindrance, relative stereochemistry S,R type may have more production than S,S type.
In stock C4, especially C4 originated from coking process, there is likely trace ethenethiol (CH 2 =CH-SH).In the refining process of LPG, it will be transferred to aqueous alkali phase and then in the regeneration unit easily converted to oil soluble divinyl disulfide under the oxygenation of trace oxygen, and part of that will get back to refined LPG when treating LPG with this regenerated alkali.Elemental sulfur S 0 can be produced by oxidation of S 2− in the alkali and often exists in refined LPG [11].Either oxygen or S 0 can oxidize thiols to disulfide and produce trace H 2 S.And H 2 S can react with divinyl disulfide to generate

Figure 3 :
Figure 3: GC-SCD of MTBE from different refineries.(a) Sulfur distribution of sample MTBE03 in Table 2. (b) Sulfur distribution of sample MTBE04 in Table2.

Figure 4 :CHCHCH 3 OCH 3 SCH 3
Figure 4: Comparison of SCD with FID for the heavy component.(a) GC-SCD for heavy component.(b) GC-FID for heavy component.

Table 1 :
Retention time of possible common sulfur compounds in MTBE.
impurities mainly sec-butyl alcohol, tert-butyl alcohol, tertamyl methyl ether (TAME), and trace C5-C12 hydrocarbons.That meant all MTBE samples are normal.All samples were analyzed through GC-SCD.Total sulfur contents were obtained by TS3000 sulfur analyzer.One of typical sulfur distributions in MTBE obtained by GC-SCD is

Table 2 :
Common sulfur compounds in MTBE.
compounds in MTBE, as MTBE01 in Table

Table
Complicated sulfur compounds found in MTBE.