Catalytic systems and processes for producing bisphenol A

Abstract

Described is a catalyst system useful for the production of bisphenol A, which catalyst system comprises: (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto and having a pKa value of 3.5 or less; and (b) a catalyst promoter comprising at least one organic sulfur-containing compound.

Description

[Technical field]

[0001] The present invention relates to a catalyst system and process for producing bisphenol-A. [Background technology]

[0002] Bisphenol A (BPA), also known as 2,2-bis(4-hydroxyphenyl)propane or para,para-diphenylolpropane (p,p-BPA), is a commercially important precursor used to make polycarbonate, other engineering thermoplastics, and epoxy resins. Polycarbonate applications demand particularly high purity BPA due to stringent requirements for optical clarity and color in end product applications.

[0003] BPA is commercially produced by the condensation reaction of acetone with phenol, and in fact, the production of BPA is the largest consumer of phenol. The reaction may be carried out in the presence of a homogeneous strong acid, such as hydrochloric acid, sulfuric acid, or toluenesulfonic acid, or in the presence of a heterogeneous acid catalyst, such as a sulfonated ion exchange resin. In recent years, acid ion exchange resins (IERs) have become the overwhelming choice of catalyst for the condensation reaction in the production of bisphenols. Particularly useful IERs are sulfonated polystyrene ion exchange resins, in which sulfonic acid groups are chemically bonded to the polystyrene resin backbone. In some cases, organic mercapto groups, such as mercaptoalkylamines, are also chemically bonded to the polystyrene resin backbone as cocatalysts (see, for example, U.S. Patent 6,051,658). In other cases, organic mercaptans, such as methyl or ethyl mercaptans, or mercaptocarboxylic acids, such as 3-mercaptopropionic acid, circulate freely in the BPA reaction mixture away from the IER catalyst.

[0004] Despite its wide application in the production of bisphenols, catalysts based on IER have many drawbacks. For example, IER is operated at a limited temperature range to prevent desulfonation or catalyst degradation. It is also well known that cation exchange resins swell and shrink depending on the chemical environment. BPA processes are designed to accommodate the change in resin volume and its poor mechanical resistance, and therefore upflow bed reactors are usually used. The low structural integrity of IER is a result of the low crosslinking degree or low content of divinylbenzene in the styrene-divinylbenzene copolymer, which is required to control the pore size, active site accessibility and activity of IER. The low crosslinking level of IER leads to an increase in compressibility value, which can contribute to increasing the pressure drop through the catalyst bed in the reactor, which ultimately limits the production of BPA. IER not only exhibits low structural integrity, but also low chemical integrity due to the leaching of sulfonic acid groups. Since the presence of leached acid adversely affects the purity of the BPA product, excess acid is removed during start-up of the BPA reactor and acid concentrations are monitored during operation.

[0005] Therefore, there is great interest in developing improved catalyst systems and processes for producing bisphenol A. Summary of the Invention

[0006] According to the present invention, it has now been found that functionalized silica catalysts, in which organic sulfonic acid groups are attached to the silica backbone, provide higher conversion and higher selectivity to p,p-BPA than IER catalysts.Another advantage of functionalized silica compared to polymer-based materials is its rigid structure, which prevents thermal or chemical degradation.Functionalized silica catalysts exhibit a defined pore structure and constant volume, independent of chemical environment, temperature and pressure.This pore structure can be adjusted during the preparation of the catalyst to control properties such as surface area, pore size and volume, particle morphology and size, and chemical surface.

[0007] Thus, in one aspect, the invention is a catalyst system useful for the production of bisphenol A, comprising: (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto and having a pKa value of 3.5 or less; and (b) a catalyst promoter comprising at least one organic sulfur-containing compound.

[0008] In a further aspect, the present invention is a process for producing bisphenol A by the reaction of acetone with phenol in a reaction medium in the presence of a catalyst system comprising: (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto and having a pKa value of 3.5 or less; and (b) a catalyst promoter comprising at least one organic sulfur-containing compound. [Brief description of the drawings]

[0009] [Figure 1] 1 is a graph of p,p-BPA selectivity versus acetone conversion for a commercial ion exchange resin (Purolite™ CT-122), alkylsulfonic acid silica, and arylsulfonic acid silica in the condensation reaction of phenol with acetone according to the process of Example 1.

[0010] [Diagram 2] 1 is a graph of BPA selectivity versus temperature for Purolite™ CT-122 and arylsulfonic acid silica in the condensation reaction of phenol with acetone according to the process of Example 2.

[0011] [Diagram 3] 3(a) and 3(b) are graphs showing the results of the acid leaching tests of Example 3 for the ion exchange resin Purolite™ CT-122 and arylsulfonic acid silica.

[0012] [Figure 4]1 is a graph comparing the results of thermogravimetric analysis (TGA) testing of Example 4 for ion exchange resin Purolite™ CT-122 and arylsulfonic acid silica. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Described herein is a catalyst system comprising: (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto, the catalyst having a pKa value of 3.5 or less; and (b) a catalyst promoter comprising at least one organic sulfur-containing compound. Also described herein is the use of the catalyst system in condensation reactions, such as the condensation of carbonyl compounds with phenolic compounds to produce polyphenols, particularly the condensation of phenol with acetone to produce bisphenol A (BPA).

[0014] The pKa values ​​quoted herein are determined by titration using 0.15 g of catalyst dispersion in 40 grams of NaCl / water solution (2.5 wt%). The resulting slurry is left under stirring for a minimum of 4 hours. A titration is then carried out using NaOH solution (0.1-0.01 N). The pKa is determined at the titration point halfway along the titration curve. At this point the concentrations of base and acid are equal and therefore the pKa is equivalent to pH. At least three titrations are performed for each sample and the average of the three pKa values ​​is reported in milliequivalents / g. Acidic Heterogeneous Catalysts

[0015] The acidic heterogeneous catalyst used in the catalyst system comprises amorphous silica functionalized with an organic sulfonic acid group such that the catalyst has a pKa value of 3.5 or less. The amorphous silica comprises silica particles, conveniently in the form of a free running powder (i.e., in which the silica particles are physically separate and distinct) or in the form of a shaped body such as an extrudate in which the silica particles are composited together with or without the aid of a binder. In some embodiments, the amorphous silica is substantially free of zirconium, such that, for example, the amorphous silica comprises less than 1% by weight, such as less than 0.5% by weight, such as less than 0.05% by weight, preferably in an unmeasurable amount of zirconium.

[0016] As used herein, the term "amorphous" is used in its generally accepted sense to mean lacking long range order that would give rise to one or more intense peaks in an X-ray diffraction pattern.

[0017] Amorphous silica has the following formula: -R 1 The compound may be functionalized with one or more organic sulfonic acid groups having SO3H, where R 1 is chemically bonded to the silica and preferably comprises a substituted or unsubstituted alkyl, alkenyl, alkynyl group having up to 8 carbon atoms, or is a substituted or unsubstituted aryl group. In some embodiments, R 1 is an alkyl group, for example an alkyl group having 1 to 4 carbon atoms, and such functionalized silicas are also referred to herein as alkylsulfonate silicas. In other embodiments, R 1is an aryl group, such as an alkyl substituted phenyl group, in which the alkyl portion has 1 to 4 carbon atoms, such functionalized silicas are also referred to herein as arylsulfonate silicas. Non-limiting examples of suitable organic sulfonic acid functionalized silica compounds having the required pKa value of 3.5 or less include silica methanesulfonate, silica ethanesulfonate, silica propanesulfonate, silica butanesulfonate, silica benzenesulfonate, silica ethylbenzenesulfonate, silica vinylbenzenesulfonate, silica propylbenzenesulfonate, and silica butylbenzenesulfonate.

[0018] Many organic sulfonic acid functionalized silica compounds with pKa values ​​of 3.5 or less are commercially available. In addition, methods for the synthesis of organic sulfonic acid functionalized silica compounds with pKa values ​​of 3.5 or less are well known. Typical methods include (a) post-functionalization of existing silica supports by reaction of silanols on the silica support with alkoxysilanes containing thiol groups, such as 3-mercaptopropyltrimethoxysilane (MPTMS); and (b) co-condensation of alkoxysilanes containing thiol groups, such as MPTMS, with a silicon source, usually a siloxane precursor (e.g., tetraethyl orthosilicate, TEOS or tetramethyl orthosilicate, TMOS). The final step in the preparation of functionalized silica is the oxidation of thiol groups to sulfonic acids by using an oxidizing agent, such as H2O2. Organic Sulfur Accelerators

[0019] The catalyst system of the present invention also includes at least one organic sulfur-containing promoter, which generally includes at least one thiol, SH, group. Such promoters are ionically or covalently bound to the heterogeneous catalyst, or are added separately to the condensation reaction without being bound to the heterogeneous catalyst. Non-limiting examples of bound promoters include mercaptoalkylpyridines, mercaptoalkylamines, thiazolidines, and aminothiols. Non-limiting examples of unbound promoters include alkyl mercaptans, such as methyl mercaptan (MeSH) and ethyl mercaptan, mercaptocarboxylic acids, such as mercaptopropionic acid, and mercaptosulfonic acids.

[0020] The amount of organic sulfur-containing promoter used in the catalyst system will depend on the particular acidic heterogeneous catalyst used and the condensation process to be catalyzed, however, the organic sulfur-containing promoter is generally used in an amount of from 2 to 30 mol %, for example from 5 to 20 mol %, based on the sulfonic groups in the acid catalyst. Use of catalytic systems in condensation reactions

[0021] The above-described catalyst systems have been found to be active in condensation reactions, particularly between carbonyl compound reactants and phenolic compound reactants to produce polyphenol products. Examples of suitable carbonyl compounds are those represented by the following formula: [ka] is a compound represented by In this formula, R represents hydrogen or an aliphatic, alicyclic, aromatic, or heterocyclic group, including saturated or unsaturated hydrocarbon groups, such as alkyl, cycloalkyl, aryl, aralkyl, alkaryl; n is greater than 0, preferably 1 to 3, more preferably 1-2, and most preferably 1; when n is greater than 1, X is a bond or a polyvalent linking group having 1 to 14 carbon atoms, preferably 1 to 6 carbon atoms, and more preferably 1 to 4 carbon atoms; when n is 1, X represents hydrogen or an aliphatic, alicyclic, aromatic, or heterocyclic group, including saturated or unsaturated hydrocarbon groups, such as alkyl, cycloalkyl, aryl, aralkyl, alkaryl, except when X and R are both hydrogen.

[0022] Carbonyl compounds suitable for use in the present invention include aldehydes and ketones. These compounds generally contain 3 to 14 carbon atoms and are preferably aliphatic ketones. Examples of suitable carbonyl compounds include ketones such as acetone, methyl ethyl ketone, diethyl ketone, dibutyl ketone, isobutyl methyl ketone, acetophenone, methyl and amyl ketones, cyclohexanone, 3,3,5-trimethylcyclohexanone, cyclopentanone, 1,3-dichloroacetone, etc. Most preferred is acetone.

[0023] The carbonyl compound is reacted with a phenolic compound. A phenolic compound is an aromatic compound containing an aromatic nucleus to which at least one hydroxyl group is directly attached. Phenolic compounds suitable for use in the present invention include phenol and its homologs and substitution products that contain at least one replaceable hydrogen atom directly attached to the phenolic aromatic nucleus. Such groups replacing the hydrogen atom and directly attached to the aromatic nucleus include halogen groups, such as chloride and bromide, and hydrocarbon groups, such as alkyl, cycloalkyl, aryl, alkaryl and aralkyl groups. Suitable phenolic compounds include phenol, cresol, xylenol, carvacrol, cumenol, 2-methyl-6-ethylphenol, 2,4-dimethyl-3-ethylphenol, o-chlorophenol, m-chlorophenol, o-butylphenol, 2,5-xylenol, 2,5-di-t-butylphenol, o-phenylphenol, 4-ethylphenol, 2-ethyl-4-methylphenol, 2,3,6-trimethylphenol, 2-methyl-4-tert-butylphenol, 2-tert-butyl-4-methylphenol, 2,3,5,6-tetramethylphenol, 2,6-dimethylphenol, 2,6-ditert-butylphenol, 3,5-dimethylphenol, 2-methyl-3,5-diethylphenol, o-phenylphenol, p-phenylphenol, naphthol, phenanthrol, and the like. Most preferred are compositions containing phenol. Mixtures of any of the above may be used.

[0024] The above are not intended to limit the present invention, but rather to illustrate representative examples of carbonyl and phenolic compounds known in the art to produce desirable polyphenols, and for which one of skill in the art can substitute other similar reactants.

[0025] In the preparation of polyphenols, an excess of phenolic compound reactant relative to carbonyl compound reactant is usually desirable. In general, at least about 2 moles, preferably about 4 to about 25 moles, of phenolic compound per mole of carbonyl compound is desirable for high conversion of the carbonyl compound. Solvents or diluents are not required in the process of the present invention for the production of polyphenols except at low temperatures.

[0026] The polyphenolic compounds obtained in this process by the condensation reaction of a phenolic compound with a carbonyl compound are compounds in which at least two phenolic nuclei are directly linked by carbon-carbon bonds to the same single carbon atom in an alkyl group. Illustrative, non-limiting examples of polyphenolic compounds are represented by the following formula: [ka] In this formula, R1 and R2 each independently represent a monovalent organic group. Examples of such groups include hydrocarbon groups, such as aliphatic, alicyclic, aromatic or heterocyclic, more specifically, hydrocarbon groups, such as substituted or unsubstituted alkyl, cycloalkyl, aryl, aralkyl or alkaryl. Preferably, R1 and R2 each independently represent an alkyl group having 1 to 2 carbon atoms. Most preferably, the polyphenol compound comprises 2,2-bis(4-hydroxyphenyl)propane, i.e., bisphenol A (BPA).

[0027] The reaction conditions used to accomplish the above condensation reaction will vary depending on the type of phenolic compound, solvent, carbonyl compound, and condensation catalyst selected. Generally, the phenolic compound and the carbonyl compound are reacted in a reaction vessel in either a batch or continuous mode at a temperature ranging from about 20° C. to about 130° C., preferably from about 50° C. to about 90° C.

[0028] The pressure conditions are not particularly limited, and the reaction may be carried out at atmospheric, subatmospheric or superatmospheric pressure. However, it is preferred to carry out the reaction without introducing any external pressure, or at a pressure sufficient to force the reaction mixture across the catalyst bed, or to pump the reaction mixture up the upstream of a vertical reactor, or to maintain the contents of the reaction vessel in a liquid state if the reaction is carried out at a temperature above the boiling point of any of the components. The pressure and temperature should be set under conditions that keep the reactants in the liquid phase in the reaction zone. The temperature may exceed 130° C., but should not be so high as to degrade any of the components in the reaction vessel, nor so high as to degrade the reaction products or promote the synthesis of substantial amounts of undesirable by-products.

[0029] The reactants are introduced into the reaction zone under conditions that ensure a molar excess of the phenolic compound relative to the carbonyl compound. Preferably, the phenolic compound is reacted in a substantial molar excess relative to the carbonyl compound. For example, the molar ratio of phenolic compound to carbonyl compound is preferably at least about 2:1, more preferably at least about 4:1, and up to about 25:1.

[0030] When the unbound thiol promoter is methyl mercaptan and the carbonyl compound is acetone, 2,2-bis(methylthio)propane (BMTP) is produced in the presence of an acid catalyst. In the presence of a hydrolysis agent, BMTP dissociates in the reaction zone into methyl mercaptan and acetone, which then condenses with phenol to produce BPA. A convenient hydrolysis agent is water, which may be introduced in either the input feedstock, directly into the reaction zone, or produced in situ by a condensation reaction between the carbonyl compound and a phenolic compound. A molar ratio of water to BMTP catalyst promoter in the range of about 1:1 to about 5:1 is sufficient to adequately hydrolyze the BMTP catalyst promoter. This amount of water is produced in situ under typical reaction conditions. Thus, no additional water needs to be introduced into the reaction zone, although water may optionally be added if desired.

[0031] Any suitable reactor may be used as the reaction zone. The reaction can be carried out in a single reactor or in multiple reactors connected in series or parallel. The reactor can be a backmixed or plug flow reactor, the reaction can be carried out in continuous or batch mode, and the reactor can be oriented to produce upflow or downflow. In the case of a fixed bed flow system, the liquid hourly space velocity of the feed mixture fed to the reactor is typically between 0.2 and 50 h o. -1 In the case of a slurry bed batch system, the amount of the strong acid ion exchange resin used is usually 20-100% by weight based on the raw material mixture, although it may vary depending on the reaction temperature and pressure. The reaction time is usually 0.5-5 hours.

[0032] Any method known to those skilled in the art may be used to recover the polyphenol compounds. However, in general, the crude reaction mixture effluent from the reaction zone is fed to a separator, such as a distillation column. The polyphenol product, polyphenol isomers, unreacted phenol compounds and small amounts of various impurities are removed from the separator as a bottom product. This bottom product may be fed to a further separator. Although crystallization is the usual method of separating polyphenols, any known method of separating polyphenols from the mother liquor may be used depending on the desired purity of the polyphenol product. After separation, the mother liquor containing phenols and polyphenol isomers may be returned to the reaction zone as a reactant.

[0033] The invention will now be described in more detail with reference to the following non-limiting examples and the accompanying drawings. EXAMPLES

[0034] Separate samples of a mixture of 8 wt. % BPA, 85.5 wt. % phenol, 5 wt. % acetone, 1 wt. % sulfur accelerator, and 0.5 wt. % water were contacted with the following catalysts at 75° C.: (a) Commercially available ion exchange resin (IER), Purolite™ CT-122 MR8-711; (b) Silica propane sulfonate (also referred to herein as silica alkyl sulfonate); and (c) Ethylbenzenesulfonic acid silica (pKa=3.4) [also referred to herein as arylsulfonic acid silica].

[0035] Both the propane sulfonate silica and the ethylbenzene sulfonate silica were used as supplied by Silicycle.

[0036] In each test, the amount of catalyst used was equivalent to 5.17 milliequivalents of acid capacity.

[0037] The sulfur promoter was added as 2,2-bis(methylthio)propane, which was added in an amount sufficient to reach 1 wt % methanethiol (CH3SH) in the reaction mixture.

[0038] The selectivity of the catalysts to BPA production as a function of acetone conversion is shown in FIG. 1, which shows that Purolite™ CT-122 exhibited a selectivity for p,p-BPA of approximately 92.4% over the range of acetone conversion levels tested (50% to 100% conversion), while both organosulfonic acid silicas exhibited a selectivity for p,p-BPA of greater than 94% over a similar range of acetone conversion levels.

[0039] The relative reaction rates and selectivities of the functionalized catalyst (alkyl and aryl sulfonic acid silica) and the ion exchange resin after 0.75 hours are shown in Table 1. [Table 1]

[0040] From Table 1, it can be seen that the acetone reaction rate decreases in the following order: arylsulfonic acid > alkylsulfonic acid > IER. Considering that the reactor is loaded with the same number of acid sites, it can be seen that the arylsulfonic acid silica is 1.7 times more active than the ion exchange resin. Also, from Figure 1 and Table 1, it can be seen that the selectivity is dependent on the type of functional group, with alkylsulfonic acid silica showing greater selectivity for the production of p,p-BPA than arylsulfonic acid silica with respect to the type of functional group. A high p,p-BPA selectivity is desirable as it reduces the capital and operating costs of the downstream purification process. EXAMPLES

[0041] The process of Example 1 was repeated using ion exchange resin and arylsulfonic acid silica catalysts at various temperatures ranging from 70 to 95°C. The results are shown in Figure 2. As expected, activity increases and selectivity decreases as temperature increases. As shown in Figure 2, a 20°C increase in reaction temperature results in a 4.3% decrease in selectivity for the ion exchange resin, whereas for the same increase in temperature, the sulfonic acid silica shows only a 2.3% decrease in selectivity. This indicates that the sulfonic acid silica is less sensitive to changes in reaction temperature than the conventional resin catalyst. EXAMPLES

[0042] In this example, the resistance of ion exchange resin (Purolite™ CT-122) catalyst to aryl sulfonic acid silica (ethylbenzene sulfonic acid silica) catalyst to acid leaching was compared. The acid leaching test involved mixing the dry catalyst with 1 wt% water / 99 wt% phenol and holding at 85°C. At specific times, the supernatant was removed and titrated with 0.01N NaOH. The results are shown in Figures 3(a) and 3(b) and reveal that the ion exchange resin exhibits a high leaching rate, which reaches a steady rate after several leaching procedures. On the other hand, the water-phenol mixture shows a titration curve similar to that of the aryl sulfonic acid silica catalyst. From this, it was concluded that there is no acid to leach with the sulfonic acid silica catalyst. EXAMPLES

[0043] In this example, thermogravimetric analysis was used to determine the thermal stability of the arylsulfonic acid silica functionalized catalysts compared to ion exchange resins. In this test, each catalyst was first heated at 60°C under vacuum to remove adsorbed water, and then heated to 950°C at a ramp rate of 5°C / min while passing nitrogen over the catalyst at 20 mL / min. The TGA thermograms are shown in Figure 4. The sulfonic acid groups were decomposed in two stages, the first at a lower temperature and the second at a higher temperature, and the decomposition appears to depend on the type of sulfonic acid species and its interaction with the support. The decomposition of the sulfonic acid groups begins at 285°C for IER and 462°C for arylsulfonic acid silica. The higher onset decomposition temperature clearly indicates that the sulfonic acid silica is more stable than the ion exchange resin.

[0044] Although the present invention has been described and illustrated with reference to specific embodiments, those skilled in the art will recognize that the invention is susceptible to variations not necessarily illustrated herein, and therefore, reference should be made solely to the appended claims to determine the true scope of the present invention.

Claims

1. 1. A catalyst system useful for the production of bisphenol A, comprising: (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto and having a pKa value of 3.5 or less; and (b) a catalytic promoter comprising at least one organic sulfur-containing compound, the catalytic promoter not being bound to the heterogeneous catalyst; A catalyst system comprising:

2. 10. The catalyst system of claim 1, wherein the amorphous silica comprises discrete silica particles.

3. 10. The catalyst system of claim 1, wherein the amorphous silica comprises an extrudate comprising silica particles.

4. 4. The catalyst system of claim 1, wherein the amorphous silica is substantially free of zirconium.

5. The acidic heterogeneous catalyst has the following formula: １ SO ３ H-containing organic sulfonic acid groups are attached to the amorphous silica, １ 5. The catalyst system according to claim 1, wherein is a substituted or unsubstituted alkyl, alkenyl or alkynyl group having up to 8 carbon atoms, or is a substituted or unsubstituted aryl group.

6. R １ The catalyst system of claim 5, wherein is an alkyl group having 1 to 4 carbon atoms.

7. R １ The catalyst system of claim 5, wherein is an alkyl substituted phenyl group, wherein the alkyl portion has 1 to 4 carbon atoms.

8. 8. The catalyst system according to any one of claims 1 to 7, wherein the at least one organic sulfur-containing compound is selected from the group consisting of alkyl mercaptans, mercaptocarboxylic acids, mercaptosulfonic acids, mercaptoalkylpyridines, mercaptoalkylamines, thiazolidines and aminothiols.

9. A process for producing bisphenol A by the reaction of acetone with phenol in a reaction medium in the presence of a catalytic system, the catalytic system comprising (a) an acidic heterogeneous catalyst comprising amorphous silica having organic sulfonic acid groups chemically bonded thereto and having a pKa value of 3.5 or less; and (b) a catalytic promoter comprising at least one organic sulfur-containing compound, the catalytic promoter not being bound to the heterogeneous catalyst; The process includes:

10. 10. The process of claim 9, wherein the amorphous silica comprises discrete silica particles.

11. 10. The process of claim 9, wherein the amorphous silica comprises an extrudate comprising silica particles.

12. The process of any one of claims 9 to 11, wherein the amorphous silica is substantially free of zirconium.

13. The acidic heterogeneous catalyst has the following formula: １ SO ３ H-containing organic sulfonic acid groups are attached to the amorphous silica, １ The process of any one of claims 9 to 12, wherein is a substituted or unsubstituted alkyl, alkenyl, or alkynyl group having up to 8 carbon atoms, or is a substituted or unsubstituted aryl group.

14. R １ The process of claim 13, wherein is an alkyl group having 1 to 4 carbon atoms.

15. R １ The process of claim 13, wherein is an alkyl substituted phenyl group, wherein the alkyl portion has 1 to 4 carbon atoms.

16. 16. The process of any one of claims 9 to 15, wherein the at least one organic sulfur-containing compound is selected from the group consisting of alkyl mercaptans, mercaptocarboxylic acids, mercaptosulfonic acids, mercaptoalkylpyridines, mercaptoalkylamines, thiazolidines, and aminothiols.

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