WO2020018253A1

WO2020018253A1 - Preparation and purification of dimethylbiphenyl compounds

Info

Publication number: WO2020018253A1
Application number: PCT/US2019/039756
Authority: WO
Inventors: Catherine M. DORSI; Monica D. LOTZ; Thomas T. Sun
Original assignee: Exxonmobil Chemical Patents Inc.
Priority date: 2018-07-17
Filing date: 2019-06-28
Publication date: 2020-01-23

Abstract

In a process for separating a dimethylbiphenyl compound from a mixture thereof with a (methylcyclohexyl)toluene compound, the mixture is cooled to a temperature less than the melting point of the dimethylbiphenyl compound but above the thermal solidification transition temperature of the (methylcyclohexyl)toluene compound to produce (i) a crystallization product comprising at least part of the dimethylbiphenyl compound mixture and (ii) a mother liquor comprising the (methylcyclohexyl)toluene compound. The crystallization product is then recovered.

Description

PREPARATION AND PURIFICATION OF DIMETHYLBIPHENYL COMPOUNDS

INVENTOR(S): Catherine M. Dorsi, Monica D. Lotz, and Thomas T. Sun CROSS-REFERENCE OF RELATED APPLICATIONS

[0001] This application claims the benefit of Provisional Application No. 62/699,506, filed July 17, 2018, and European Application No. 18190750.2, filed August 24, 2018, the disclosures of which are incorporated herein by their reference.

FIELD

[0002] This disclosure relates to the preparation and purification (separating components of a mixture) of dimethylbiphenyl compounds.

BACKGROUND

[0003] Dimethylbiphenyl (DMBP) compounds are useful intermediates in the production of a variety of commercially valuable products, including polyesters and plasticizers for PVC and other polymer compositions. For example, DMBP can readily be converted to an ester plasticizer by a process comprising oxidation of the DMBP to produce the corresponding mono- or dicarboxylic acid followed by esterification with a long chain alcohol. For certain uses, it is important to maximize the level of the 3,4'-isomer and particularly the 4,4'-isomer in the product.

[0004] In addition, 4,4'-biphenyl-dicarboxylic acid, optionally together with biphenyl-3, 4'- dicarboxylic acid, is a potential precursor, either alone or as a modifier for polyethylene terephthalate (PET), in the production of polyester fibers, engineering plastics, liquid crystal polymers for electronic and mechanical devices, and films with high heat resistance and strength.

[0005] For example, homopolyesters of 4,4'-biphenyl dicarboxylic acid (BDA) and various aliphatic diols have been disclosed in the literature. For example, Ezard disclosed homopolyester between 4,4'-biphenyl dicarboxylic acid and ethylene glycol in the Journal of Polymer Science, 9, 35 (1952). In the British Polymer Journal, 13, 57 (1981), Meurisse et al. disclosed homopolyesters made from 4,4'-biphenyl dicarboxylic acid and a number of diols including ethylene glycol, 1 ,4-butanediol and 1 ,6-hexanediol. Homopolyesters of 4,4'-biphenyl dicarboxylic acid and ethylene glycol were also disclosed in U.S. Pat. Nos. 3,842,040 and 3,842,041.

[0006] Copolyesters of 4,4'-biphenyl dicarboxylic acid and mixtures of aliphatic diols are also disclosed in the literature, for example, in U.S. Pat. No. 2,976,266. Morris et al. disclosed copolyesters from 4,4'-biphenyl dicarboxylic acid, and the mixtures of 1,4- cyclohexanedimethanol and 1 ,6-hexanediol in U.S. Pat. No. 4,959,450. Copolyesters of 4,4'- biphenyl dicarboxylic acid and terephthalic acid, and certain aliphatic diols are disclosed in the literature, for example, in the Journal of Polymer Science, Polym. Letters, 20, 109 (1982) by Krigbaum et al. U.S. Pat. No. 5,138,022 disclosed copolyester of 3,4' biphenyl dicarboxylic acid and optionally 4,4'-biphenyl dicarboxylic acid, and certain aliphatic diols like ethylene glycol, l,4-butanediol, and 1 ,4-cyclohexanedimethanol.

[0007] As disclosed in US Patent Nos. 9,580,572 and 9,663,417, the entire disclosures of which are incorporated herein by reference in their entirety, DMBP compounds may be produced by hydroalkylation of toluene followed by dehydrogenation of the resulting (methylcyclohexyl)toluene (MCHT). Alternative routes to DMBP compounds via benzene are described in US Patent No. 9,085,669, again incorporated herein by reference, in which the benzene is initially converted to biphenyl, either by oxidative coupling or by hydroalkylation to cyclohexyl benzene (CHB) followed by dehydrogenation of the CHB, and then the biphenyl is alkylated with methanol. However, these processes inevitably yield a mixture of all six DMBP isomers, namely 2,2', 2,3', 2,4', 3,3', 3,4' and 4,4' DMBP, in which the 2,X' (where X' is 2', 3' or 4') and 3,3' DMBP isomer content may be 40% by weight or more of the total DMBP product. Thus, for acceptable process carbon efficiencies, viable commercial processes for the production of 3,4’- and 4,4’-DMBP from toluene may require utilization of the 3,3’- and 2,X'- isomer content, for example by isomerization in the presence of a solid acid catalyst as disclosed in US Patent Application Publication No. 2016/0176785, the entire disclosure of which is incorporated herein by reference in its entirety.

[0008] In addition to producing the less desirable 3,3’- and 2,X'-isomers, the production of DMBP from toluene via hydroalkylation and dehydrogenation typically results in a reaction product which contains significant quantities of unreacted MCHT and often unreacted toluene. The presence of MCHT in a DMBP stream is detrimental to some downstream DMBP processing steps. Thus, when the conversion of MCHT to DMBP is incomplete, the MCHT must be separated from the DMBP. In addition, any DMBP isomers which remain with the MCHT will not be isomerized in the downstream processing of MCHT and will build up in the recycle streams unless they are hydrogenated back into MCHT or can be separated from the MCHT. However, MCHT and some isomers of DMBP boil at similar temperatures and are thus difficult to separate via distillation.

[0009] An additional potential reference of interest is U.S. Provisional Application No. 62/619,966. SUMMARY

[0010] According to the present disclosure, it has now been found that at least some of the isomers of DMBP have higher melting temperatures than the thermal solidification transition temperatures of the MCHT isomers. Thus, crystallization can be used to separate at least some of the DMBP isomers from MCHT, particularly at least some of the DMBP isomers which co boil with MCHT in a distillation column, as well as other light by-products that are present in the stream.

[0011] Thus, in one aspect, the present disclosure provides a process for separating a dimethylbiphenyl compound from a mixture thereof with a (methylcyclohexyl)toluene compound, the process comprising:

(al) cooling the mixture to a temperature less than the melting point of the dimethylbiphenyl compound but above the thermal solidification transition temperature of the (methylcyclohexyl)toluene compound to produce (i) a crystallization product comprising at least part of the dimethylbiphenyl compound in the feed mixture and (ii) a mother liquor comprising the (methylcyclohexyl)toluene compound; and

(bl) recovering the crystallization product.

[0012] In another aspect, the present disclosure provides a process for producing 3,3’, 3,4’ and/or 4,4’ dimethylbiphenyl compounds, the process comprising:

(a2) contacting toluene with hydrogen in the presence of a hydroalkylation catalyst under conditions effective to produce a hydroalkylation product comprising (methylcyclohexyl)toluene;

(b2) dehydrogenating at least part of the hydroalkylation reaction product in the presence of a dehydrogenation catalyst under conditions effective to produce a dehydrogenation reaction product comprising a mixture of dimethyl-substituted biphenyl isomers and unreacted (methylcyclohexyl)toluene; and

(c2) cooling at least part of the dehydrogenation reaction product in a crystallizer to a temperature less than 6°C, and above -80°C to produce (i) a crystallization product comprising at least part of the dimethyl-substituted biphenyl isomers in the dehydrogenation reaction product and (ii) a mother liquor comprising unreacted (methylcyclohexyl)toluene.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a flow diagram of a process according to one aspect of the present disclosure for producing 4,4’ -dimethylbiphenyl from toluene. [0014] Figure 2 is a flow diagram of a process according to a further aspect of the present disclosure for producing 4,4’-dimethylbiphenyl from toluene.

[0015] Figure 3 is a graph of the predicted melting temperature of dimethylbiphenyl (DMBP) isomers versus mole fraction in an ideal solvent.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] Unless otherwise indicated, room temperature is 23 °C.

[0017] As used herein,“wt%” means percentage by weight,“vol%” means percentage by volume,“mol%” means percentage by mole,“ppm” means parts per million, and“ppm wt” and“wppm” are used interchangeably to mean parts per million on a weight basis. All“ppm” as used herein are ppm by weight unless specified otherwise. All concentrations herein are expressed on the basis of the total amount of the composition in question. Thus, the concentrations of the various components of the first mixture are expressed based on the total weight of the first mixture. All ranges expressed herein should include both end points as two specific embodiments unless specified or indicated to the contrary.

[0018] As used herein, the term“thermal solidification transition temperature” with respect to a compound refers to the glass transition temperature or the melting point temperature of the compound, whichever is higher. It should be recognized for purposes of this disclosure that a compound may exhibit a glass transition temperature but not a melting point, or alternatively may exhibit a melting point but not a glass transition temperature, or yet alternatively may exhibit both a melting point and a glass transition temperature.

[0019] Disclosed herein is a process for the separation of dimethylbiphenyl (DMBP) compound(s) from a mixture thereof with (methylcyclohexyl)toluene (MCHT) compound(s) and particularly, but not exclusively, a process for the separation of DMBP compounds from a mixture of DMBP and MCHT compounds produced by the dehydrogenation of the reaction product of the hydroalkylation of toluene. The process comprises cooling a mixture, generally a liquid mixture, of the DMBP and MCHT compounds to a temperature less than the melting point of the DMPB compound(s) and above the thermal solidification transition temperature of the MCHT compound(s) to produce (i) a crystallization product comprising at least part of the DMBP compound(s) in the feed mixture and (ii) a mother liquor comprising the MCHT compound(s). The crystallization product can then be recovered for further purification of the DMBP compound(s), while the mother liquor can be forwarded to one or more further processing steps to utilize the MCHT compound(s), for example, by recycle to the dehydrogenation step. [0020] In particular, it has now been found that, while many of the isomers of DMBP and MCHT boil at similar temperatures within a narrow temperature band of approximately 260- 280°C, at least the majority of the DMBP isomers have higher melting temperatures than the thermal solidification transition temperatures of the MCHT isomers. Thus the present process provides a potentially valuable tool in the separation DMBP and MCHT compounds, which is a critical step in the commercialization of toluene hydroalkylation as a route for producing polyesters and plasticizers.

Toluene Hydroalkylation

[0021] Toluene hydroalkylation is a two-stage catalytic reaction, in which toluene initially undergoes partial hydrogenation to methylcyclohexene which then alkylates additional toluene to produce a mixture of (methylcyclohexyl)toluene isomers. The overall reaction can be summarized as follows:

[0022] The catalyst employed in the hydroalkylation reaction is generally a bifunctional catalyst comprising a hydrogenation component and a solid acid alkylation component, typically a molecular sieve. The catalyst may also include a binder such as clay, alumina, silica, and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be used as a binder include those of the montmorillonite and kaolin families, which families include the subbentonites and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. Suitable metal oxide binders include silica, alumina, zirconia, titania, silica-alumina, silica-magnesia, silica- zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions such as silica-alumina-thoria, silica-alumina- zirconia, silica-alumina-magnesia and silica-magnesia- zirconia.

[0023] Any known hydrogenation metal or compound thereof can be employed as the hydrogenation component of the catalyst, although suitable metals include palladium, ruthenium, nickel, zinc, tin, and cobalt, with palladium being particularly advantageous. In certain embodiments, the amount of hydrogenation metal present in the catalyst is between 0.05 and 10 wt %, such as between 0.1 and 5 wt %, of the catalyst.

[0024] Often, the solid acid alkylation component comprises a large pore molecular sieve having a Constraint Index (as defined in U.S. Pat. No. 4,016,218) less than 2. Suitable large pore molecular sieves include zeolite beta, zeolite Y, Ultrastable Y (USY), Dealuminized Y (Deal Y), mordenite, ZSM-3, ZSM-4, ZSM-18, and ZSM-20. Zeolite ZSM-4 is described in U.S. Pat. No. 4,021,447. Zeolite ZSM-20 is described in U.S. Pat. No. 3,972,983. Zeolite Beta is described in U.S. Pat. No. 3,308,069, and Re. No. 28,341. Low sodium Ultrastable Y molecular sieve (USY) is described in U.S. Pat. Nos. 3,293,192 and 3,449,070. Dealuminized Y zeolite (Deal Y) may be prepared by the method found in U.S. Pat. No. 3,442,795. Zeolite UHP-Y is described in U.S. Pat. No. 4,401,556. Mordenite is a naturally occurring material but is also available in synthetic forms, such as TEA-mordenite (i.e., synthetic mordenite prepared from a reaction mixture comprising a tetraethylammonium directing agent). TEA-mordenite is disclosed in U.S. Pat. Nos. 3,766,093 and 3,894,104.

[0025] Alternatively, the solid acid alkylation component preferably comprises a molecular sieve of the MCM-22 family. The term“MCM-22 family material” (or“material of the MCM- 22 family” or“molecular sieve of the MCM-22 family”), as used herein, includes one or more of: molecular sieves made from a common first degree crystalline building block unit cell, which unit cell has the MWW framework topology. (A unit cell is a spatial arrangement of atoms which if tiled in three-dimensional space describes the crystal structure. Such crystal structures are discussed in the“Atlas of Zeolite Framework Types”, Fifth edition, 2001); molecular sieves made from a common second degree building block, being a 2-dimensional tiling of such MWW framework topology unit cells, forming a monolayer of one unit cell thickness, preferably one c-unit cell thickness; molecular sieves made from common second degree building blocks, being layers of one or more than one unit cell thickness, wherein the layer of more than one unit cell thickness is made from stacking, packing, or binding at least two monolayers of one unit cell thickness. The stacking of such second degree building blocks can be in a regular fashion, an irregular fashion, a random fashion, or any combination thereof; and molecular sieves made by any regular or random 2-dimensional or 3 -dimensional combination of unit cells having the MWW framework topology.

[0026] Molecular sieves of MCM-22 family generally have an X-ray diffraction pattern including d-spacing maxima at 12.4+0.25, 6.9+0.15, 3.57+0.07 and 3.42+0.07 Angstrom. The X-ray diffraction data used to characterize the material are obtained by standard techniques using the K- alpha doublet of copper as the incident radiation and a diffractometer equipped with a scintillation counter and associated computer as the collection system. Molecular sieves of MCM-22 family include MCM-22 (described in U.S. Pat. No. 4,954,325), PSH-3 (described in U.S. Pat. No. 4,439,409), SSZ-25 (described in U.S. Pat. No. 4,826,667), ERB-l (described in European Patent No. 0293032), ITQ-l (described in U.S. Pat. No. 6,077,498), ITQ-2 (described in International Patent Publication No. WO 97/17290), MCM-36 (described in U.S. Pat. No. 5,250,277), MCM-49 (described in U.S. Pat. No. 5,236,575), MCM-56 (described in U.S. Pat. No. 5,362,697) and mixtures thereof.

[0027] In addition to the toluene and hydrogen, a diluent, which is substantially inert under hydroalkylation conditions, may also be included in the hydroalkylation feed. In certain embodiments, the diluent is a hydrocarbon, in which the desired cycloalkylaromatic product is soluble, such as a straight chain paraffinic hydrocarbon, a branched chain paraffinic hydrocarbon, and/or a cyclic paraffinic hydrocarbon. Examples of suitable diluents are decane and cyclohexane. Although the amount of diluent is not narrowly defined, desirably the diluent is added in an amount such that the weight ratio of the diluent to the aromatic compound is at least 1:100; for example at least 1:10, but no more than 10:1, desirably no more than 4: 1.

[0028] The hydroalkylation reaction can be conducted in a wide range of reactor configurations including fixed bed, slurry reactors, and/or catalytic distillation towers. In addition, the hydroalkylation reaction can be conducted in a single reaction zone or in a plurality of reaction zones, in which at least the hydrogen is introduced to the reaction in stages. Suitable reaction temperatures are between l00°C and 400°C, such as between l25°C and 250°C, while suitable reaction pressures are between 100 and 7,000 kPa, such as between 500 and 5,000 kPa. The molar ratio of hydrogen to aromatic feed is typically from 0.15:1 to 15:1.

[0029] In the present process, it is found that MCM-22 family molecular sieves are particularly active and stable catalysts for the hydroalkylation of toluene. In addition, catalysts containing MCM-22 family molecular sieves exhibit improved selectivity to the 3,3'-MCHT, the 3,4'-MCHT, the 4,3'-MCHT and the 4,4'-MCHT isomers in the hydroalkylation product, while at the same time reducing the formation of fully saturated and heavy by-products. For example, using an MCM-22 family molecular sieve with a toluene feed, it is found that the hydroalkylation reaction product may comprise: at least 60 wt %, such as at least 70 wt %, for example at least 80 wt % of the 3,3', 3,4', 4,3' and 4,4'-isomers of (methylcyclohexyl)toluene based on the total weight of all the (methylcyclohexyl)toluene isomers; less than 40 wt %, such as less than 30 wt %, for example from 15 to 25 wt % of the 2,2', 2,3', and 2,4'-isomers of (methylcyclohexyl)toluene based on the total weight of all the (methylcyclohexyl)toluene isomers; less than 30 wt % of methylcyclohexane and less than 2% of dimethylbicyclohexane compounds; and generally less than 1 wt % of compounds containing in excess of 14 carbon atoms, such as di(methylcyclohexyl)toluene. When the methyl group is located in the 1- position (quaternary carbon) on the cyclohexyl ring of the (methylcyclohexyl)toluene, ring isomerization can occur forming (dimethylcyclopentyl)toluene and (ethylcyclopentyl)toluene which, on dehydrogenation, will generate diene by-products which are difficult to separate from the desired product. Accordingly, the formation of I,C' (where X' is 2', 3' or 4')-MCHT isomers in the hydroalkylation product is preferably minimized.

[0030] The hydroalkylation reaction product may also contain significant amounts of residual toluene, for example up to 50 wt %, such as up to 90 wt %, typically from 50 to 80 wt % of residual toluene based on the total weight of the hydroalkylation reaction product. The residual toluene can optionally be removed from the reaction effluent by, for example, distillation. The residual toluene can then be recycled to the hydroalkylation reactor, together with some or all of any unreacted hydrogen. In some embodiments, it may be desirable to remove some of the C15+ reaction products, such as di(methylcyclohexyl)toluene, for example, by distillation.

Dehydrogenation of (Methylcvclohexyl)toluene

[0031] After optional separation of unreacted toluene, the remainder of the hydroalkylation reaction effluent, comprising (methylcyclohexyl)toluenes, is then dehydrogenated to convert the (methylcyclohexyl)toluenes to the corresponding methyl-substituted biphenyl compounds. The dehydrogenation is conveniently conducted at a temperature from 200°C to 600°C and a pressure from 100 kPa to 3550 kPa (atmospheric to 500 psig) in the presence of dehydrogenation catalyst. A suitable dehydrogenation catalyst comprises one or more elements or compounds thereof selected from Group 10 of the Periodic Table of Elements, for example platinum, on a support, such as silica, alumina or carbon nanotubes. In one embodiment, the Group 10 element is present in an amount from 0.1 to 5 wt % of the catalyst. In some cases, the dehydrogenation catalyst may also include tin or a tin compound to improve the selectivity to the desired methyl-substituted biphenyl product. In one embodiment, the tin is present in an amount from 0.05 to 2.5 wt % of the catalyst.

[0032] Particularly using an MCM-22 family-based catalyst for the upstream hydroalkylation reaction, the product of the dehydrogenation step comprises dimethylbiphenyl compounds in which the concentration of the 3,3'-, 3,4'- and 4,4' isomers is at least 50 wt %, such as at least 60 wt %, for example at least 70 wt % based on the total weight of dimethylbiphenyl compounds. Typically, the concentration of the 2, X' -dimethylbiphenyl isomers in the dehydrogenation product is less than 50 wt %, such as less than 30 wt %, for example from 5 to 25 wt % based on the total weight of dimethylbiphenyl compounds.

Treatment of Dehydrogenation Product

[0033] In addition to a mixture of dimethylbiphenyl (DMBP) isomers, the dehydrogenation product contains unreacted (methylcyclohexyl)toluene (MCHT) and often residual toluene as well as by-products including hydrogen, methylcyclohexane dimethylcyclohexylbenzene, and C15₊ heavy hydrocarbons. Thus, often, prior to any separation of the dimethylbiphenyl isomers, the raw product of the MCHT dehydrogenation is subjected to one or more initial separation steps to remove at least part of the residues and by-products with significantly different boiling points from the desired dimethylbiphenyl isomers.

[0034] For example, the hydrogen by-product can be removed in a vapor/liquid separator and recycled to the hydroalkylation and/or MCHT dehydrogenation steps and/or elsewhere within the process. The remaining liquid product can then be fed to one or more distillation columns to remove residual toluene and methylcyclohexane by-product, as well as effect initial separation of some of the lower boiling DMBP isomers. Table 1 below depicts normal boiling points, melting points, and heat of fusion values of various DMBP isomers.

Table 1

[0035] The melting point data depicted in Table 1 of the DMBP isomers are taken from the arithmetic average of phase transition data compiled in the NIST Standard Reference Database Number 69. available from U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology (NIST), compiled under the Standard Reference Data Program. The reported melting point of 2,2’-DMBP was calculated excluding two outlier measurement values. The heat of fusion data depicted in Table 1 of the 2,2’, 3,4^", and 4,4’-DMBP isomers were collected using Differential Scanning Calorimetry (DSC) using commercially available equipment such as a Discovery series DSC available from TA Instruments, using the following procedure. Between 4 to 7 mg of the sample, that had been stored at room temperature for at least 48 hours, was sealed in an aluminum pan and loaded into the instrument at room temperature. The sample was equilibrated at 25 °C, and a first heat- cool cycle was performed by heating the sample at a heating rate of lO°C/min to l50°C, holding the sample at l50°C for 5 minutes, cooling the sample at a cooling rate of lO°C/min to -l00°C, and holding the sample at -l00°C for 5 minutes. A second heat-cool cycle was then performed by heating the sample from -l00°C at a heating rate of lO°C/min to l50°C, holding at l50°C for 5 minutes, and cooling the sample at a cooling rate of l0°C/min to 30°C. The endothermic melting transition, if present, was analyzed for onset of transition and peak temperature. Heat of fusion values reported herein were taken from the second heat-cool cycle. The thermal output was recorded as the area under the melting peak of the sample and was measured in Joules as a measure of the heat of fusion. The reported heat of fusion of the 3,3’-DMBP isomer depicted in Table 1 was estimated from the reported melting point for this isomer combined with a predicted entropy of fusion calculated from the heat of fusion and melting point data of the 2,2’, 3,4’, and 4,4’-DMBP isomers on the assumption that the entropy of fusion is the same for all DMBP isomers.

[0036] Table 2 below depicts normal boiling points and thermal solidification transition temperatures of various MCHT isomers.

Table 2

[0037] The thermal solidification transition temperature data depicted in Table 2 were collected using DSC using commercially available equipment such as a Discovery series DSC available from TA Instruments, using the following procedure. Between 5 to 10 mg of the sample, that had been stored at room temperature for at least 48 hours, was sealed in an aluminum pan and loaded into the instrument at room temperature. The sample was equilibrated at 25 °C, then cooled at a cooling rate of l0°C/min to -90°C, and then heated at a heating rate of l0°C/min to 200°C. For samples displaying multiple peaks, the melting point (or melting temperature) is defined to be the peak melting temperature (i.e., associated with the largest endothermic calorimetric response in that range of temperatures) from the DSC melting trace. From the DSC analysis, it was found that l,4’-MCHT exhibited a melting point at -6°C. It is expected that the other l,X’-MCHT isomers would also exhibit a melting point at or below -6°C. The other MCHT isomers analyzed via DSC, i.e., 3,3’-MCHT and 3,2’-MCHT, did not exhibit either a melting point or a glass transition temperature over the tested temperature range. Likewise, it is expected that the remaining untested MCHT isomers (i.e., 2,2’ -MCHT, 2,3’-MCHT, 2,4’-MCHT, 3,4’-MCHT, 4,2’-MCHT, 4,3’-MCHT, and 4,4’- MCHT) would also not exhibit either a melting point or a glass transition temperature above - 80°C.

[0038] Tables 1 and 2 illustrate the narrow range of normal boiling points for several isomers of DMBP and MCHT. There is a l5°C difference in boiling point between 2,4’-DMBP and 3,3’-DMBP, which allows 3,3’-DMBP, 3,4’-DMBP, and 4,4’-DMBP to be separated from the other isomers of DMBP and MCHT, if desired. However, all of the remaining isomers of both DMBP and MCHT have < lO°C in separation from another isomer. By contrast, all of the reported melting points of the DMBP isomers are significantly greater than the reported thermal solidification transition temperatures of the MCHT isomers, allowing for separation via crystallization. Particularly, it is anticipated that no MCHT isomers exhibit a solidification transition temperature above -6°C, which is >lO°C lower than the lowest reported melting point of any DMBP isomer. Moreover, it is expected that the majority of MCHT isomers do not exhibit a solidification transition temperature when cooled to -80°C. Accordingly, at least some of the DMBP isomers from a mixture of MCHT and DMBP may be recovered by crystallization. In some cases, the crystallization can be conducted in multiple stages to allow initial separation of the higher melting point DMBP isomers, especially the 4,4’ isomer, before separation of remaining DMBP isomers from the MCHT.

[0039] It will also be seen that all of the reported melting points of the DMBP isomers are above the atmospheric boiling point of C3 hydrocarbon refrigerants, which makes crystallization possible with such a refrigerant, or one with similar atmospheric boiling point, if the concentration of the lowest melting point DMBP isomer(s) are sufficiently high. If the DMBP isomers with lower melting points are in the mixture of MCHT/DMBP at a low concentration, a C₂ hydrocarbon, or similar, refrigerant can be used to recover at least a portion of all of the DMBP isomers at a lower temperature.

[0040] Thus, in the present process, after optional distillation as described above, at least part of the dehydrogenation reaction product is cooled in one or more crystallizers to a temperature less than the melting temperature of the lowest melting DMBP isomer, but above the thermal solidification transition temperature of all or some of the MCHT isomers, to produce (i) a crystallization product comprising at least part of the DMBP isomers in the dehydrogenation reaction product and (ii) a mother liquor comprising unreacted MCHT.

[0041] Figure 3 depicts the predicted melting point of the 2,2’-DMBP, 3,3’-DMBP, 3,4’- DMBP and 4,4’ -DMBP isomers versus mole fraction in an ideal solvent. With respect to Figure 3 (and throughout), melting point predictions are based on an expression which predicts the melting temperature of a solid in a solvent based on the solid properties (melting temperature, melting enthalpy, etc.):

[0042] The melting temperature in Figure 3 were calculated through the expression above assuming an ideal solvent (g=l) and DCp=0. [0043] As can be seen from Figure 3, it is predicted that each of these isomers would crystallize out of solution at a temperature of -70°C or lower (i.e., l0°C higher than the maximum expected thermal solidification transition temperature of most MCHT isomers) over the entire mole fraction range. Additionally, it can be seen from Figure 3 that at least the 2,2’- DMBP and 4,4’-DMBP isomers would be expected to crystallize out of solution at a temperature of -6°C or lower (i.e., the maximum expected thermal solidification transition temperature of l,X’-MCHT isomers) at a mole fraction of 0.2 or greater. Accordingly, in most cases, the crystallization to separate the DMBP from the MCHT is conducted at temperature less than 6°C but greater than -l00°C, such as from 6°C to -80°C, or from 6°C to -6°C (particularly if l,X’-MCHT isomers are present in the dehydrogenation product).

[0044] As will be described below, by a combination of distillation and one or more stages of crystallization, a variety of separations between the DMBP and the MCHT isomers and between certain of the DMBP isomers can be achieved.

[0045] Thus, in one example, some DMBP isomers such as 4,4’-DMBP, 3,4’-DMBP, and/or 3,3’-DMBP are separated from the stream containing mixed MCHT and DMBP isomers via distillation. Components lighter than MCHT can also be separated from the MCHT and DMBP via the same or a different distillation tower. A crystallizer can then be utilized to separate some or all of the remaining DMBP isomers, mainly 2,X' (where X' is 2', 3' or 4')- DMBP isomers, particularly 2,2’ -DMBP, from the MCHT in one or several steps. Such an example is shown in Figure 1 which is discussed in more detail below.

[0046] In another example, the distillation of DMBP and MCHT can be replaced entirely by use of crystallization. Due to the wide range of DMBP isomer melting temperatures (l20.8°C to 6.7°C), this method can also be used to provide at least a part of the pure product stream for certain isomers such as 4,4’-DMBP. Multiple stages of crystallization, possibly at different temperatures, can be used to improve recovery of the low concentration or low melting point DMBP isomers, or to provide multiple pure isomer streams. Pure DMBP extract which is not a desired isomer or has a mixture of isomers can be routed directly to a DMBP isomerization unit or to another crystallizer unit for further 4,4’ -DMBP recovery as it is free of MCHT and other components which might be detrimental to the performance of the isomerization unit.

[0047] In order to shift the solid-liquid equilibrium between the DMBP isomers and MCHT, a mixed MCHT/DMBP stream can be separated via crystallization before removing high boiling point DMBP isomers such as 4,4’-DMBP, 3,4’-DMBP, and 3,3’-DMBP. However, to further improve the recovery of these high boiling point isomers, a distillation step can be added to the MCHT raffinate stream. Distillation of an MCHT free, mixed DMBP isomer crystallizer extract can further purify the product stream because some of the DMBP isomers with similar melting points have enough difference in boiling points to be separated.

[0048] A further example would use a stream rich in the lower boiling point DMBP isomers, such as 2,2’-DMBP, 2,3’-DMBP, and 2,4’-DMBP to mix with the MCHT/DMBP stream to shift the solid-liquid equilibrium, aiding the separation of DMBP from MCHT. One source of low boiling point DMBP isomers is to use distillation to separate a DMBP isomerization product into higher and lower boiling point isomer streams.

[0049] In any of the above examples, part or all the MCHT-containing mother liquor remaining after the crystallization process described herein can be recycled to the hydroalkylation reactor. Additionally or alternatively, part or all the MCHT-containing mother liquor can be supplied to a separate transalkylation reactor to make more of the desired DMBP isomers.

[0050] In any embodiment, some DMBP isomers may remain in the MCHT-containing mother liquor. However, by employing the crystallization process described herein, the amount of these DMBP isomers is advantageously reduced to a level such that a subsequent hydrogenation step may either be eliminated entirely, or alternatively, the hydrogenation unit used to perform the hydrogenation may be of a smaller scale than would otherwise be required, resulting in improved process efficiency. Accordingly, in some aspects, a portion of the mother liquor may be hydrogenated in a hydrogenation unit prior to being supplied to the transalkylation reactor or recycled to the hydroalkylation reactor. Alternatively, any remaining DMBP isomers can be allowed to remain without hydrogenation. In such aspects, for example, any DMBP isomers remaining in the MCHT stream sent to transalkylation will generally eventually be recycled to extinction.

[0051] The transalkylation reaction can be conducted over a wide range of conditions but in most embodiments is effected at a temperature from 75 to 250°C, such as from 100 to 200°C, for example, 125 to l80°C and a pressure from 100 to 3550 kPa-absolute, such as from 1000 to 1500 kPa-absolute. The reaction is normally conducted in the presence of a solid acid catalyst, such as a molecular sieve and in particular a molecular sieve having a large pore molecular sieve having a Constraint Index (as defined in U.S. Pat. No. 4,016,218) less than 2. Suitable large pore molecular sieves include zeolite beta, zeolite Y, Ultrastable Y (USY), Dealuminized Y (Deal Y), mordenite, ZSM-3, ZSM-4, ZSM-18, ZSM-20, and mixtures thereof. Other suitable molecular sieves include molecular sieves of the MCM-22 family, including MCM-22 (described in U.S. Pat. No. 4,954,325), PSH-3 (described in U.S. Pat. No. 4,439,409), SSZ-25 (described in U.S. Pat. No. 4,826,667), ERB-l (described in EP 0293032), ITQ-l (described in U.S. Pat. No. 6,077,498), ITQ-2 (described in WO 97/17290), MCM-36 (described in U.S. Pat. No. 5,250,277), MCM-49 (described in U.S. Pat. No. 5,236,575), MCM- 56 (described in U.S. Pat. No. 5,362,697) and mixtures thereof. In particular, it is found that, as a result of steric issues, transalkylation favors the conversion of l,X’- (if present) and 2,X’- MCHT compounds to 3,Y’ and 4,Y’ (where Y’ is 3’ or 4’)-MCHT compounds. Notably, the

1,X’-MCHT compounds (if present) will not be converted to any extent in the dehydrogenation to form DMBPs, and if not chemically converted, will continue to build up in the stream and require a purge for removal.

[0052] At least part of the transalkylation product is then catalytically dehydrogenated to produce a mixture of DMBP isomers including the 3,3’-, 3,4’- and 4,4’-isomers. The catalyst employed in the dehydrogenation process is not critical but, often, comprises (i) an element or compound thereof from Group 10 of the Periodic Table of Elements, for example platinum, and (ii) tin or a compound of tin, both mounted on a refractory support, such as silica, alumina or carbon nanotubes. Suitable catalysts comprise a Group 10 element in an amount from 0.1 to 5 wt % of the catalyst and tin in an amount from 0.05 to 2.5 wt % of the catalyst. The dehydrogenation is conveniently conducted at a temperature from 200 to 600°C and a pressure from 100 kPa-absolute to 3550 kPa-absolute (atmospheric to 500 psig). Optionally, the dehydrogenation is conducted in the same reactor as that used to dehydrogenate the product of the initial toluene hydroalkylation reaction.

[0053] In any of the above examples, part or all the undesirable DMBP isomers remaining after separation of the desired DMBP isomer(s), especially the 4,4’ isomer, can be subjected to isomerization so that the yield of the desired isomer(s) can be maximized. Any acid catalyst, especially a heterogeneous solid acid catalyst, such as a metal oxide, a clay or, more preferably, a molecular sieve can be used to effect DMBP isomerization. Particularly suitable catalysts are molecular sieves having a Constraint Index (as defined in U.S. Pat. No. 4,016,218) less than

2, especially molecular sieves selected from the group consisting of BEA, FAU and MOR structure type molecular sieves and mixtures thereof. The conditions required to effect isomerization of a DMBP-containing feed are not closely controlled, but suitably include a temperature from 100 to 450°C, such as 100 to 250°C, a pressure from 2 to 7,000 kPa-a, such as from 100 to 2000 kPa-a, and a WHSV from 0.2 to 20 hr¹. In certain aspects, it may be desirable to select the temperature and pressure such as to maintain the DMBP components of the feed substantially in the liquid phase since this may reduce carbon losses resulting from cracking. More details of a DMBP isomerization process can be found in U.S. Patent Application Publication No. 2016/176785.

[0054] In any of the above examples, pure 4,4’-DMBP can be separated by crystallization from the mixture of DMBP isomers resulting from (a) initial low temperature cooling of the hydroalkylation/dehydrogenation product, (b) isomerization of a 4,4’ -depleted DMBP stream and/or (c) the product of the transalkylation/dehydrogenation sequence described above. Such separation is conveniently achieved in one or more crystallizers operating at a temperature from -30 to 40°C to separate 4,4’ -DMBP as a solid fraction from DMBP isomer mixture.

[0055] Referring now to the drawings, one aspect of a process for producing 4,4’- dimethylbiphenyl according to the present disclosure is shown in Figure 1. In this embodiment, fresh and recycled toluene and recycled hydrogen are supplied by line 11 to a hydroalkylation reactor 12, in which the toluene undergoes hydroalkylation in the presence of a bifunctional catalyst as described above. The hydroalkylation reaction product is removed from the reactor 12 via line 13 and fed to a condenser 14, where the product is divided into a gaseous fraction comprising a mixture of MCHT isomers together with unreacted hydrogen and toluene and a liquid fraction comprising any C15+ by-products. The gaseous fraction of the hydroalkylation reaction product is collected in line 15, while the liquid fraction is removed from the condenser 14 via line 16.

[0056] The gaseous fraction from the condenser 14 is fed by line 15 to a dehydrogenation reactor 17 where at least part of the MCHT in the light fraction is converted to the corresponding DMBP isomers. The dehydrogenation reaction product is collected in line 18 and combined with the liquid fraction of the hydroalkylation reaction product in line 16 before the mixture is supplied to a gas/liquid separator 19, where hydrogen is removed via line 21 for recycle to the hydroalkylation reactor 12 and/or elsewhere in the process or larger chemical plant.

[0057] The liquid fraction exiting the gas/liquid separator 19 is a mixed stream comprising DMBP isomers, residual MCHT, unreacted toluene and some by-products and is fed by line 22 to a MCHT distillation tower 23. The tower 23 is operated to separate the mixed stream in line 22 into (i) an overhead stream containing at least a portion of the unreacted toluene and any C7- by-products, (ii) an intermediate stream containing a portion of the unreacted toluene, the residual (methylcyclohexyl)toluenes and most of the lower boiling point dimethylbiphenyl isomers, namely the 2, X’ -DMBP isomers, and (iii) a bottoms stream containing most of the higher boiling point dimethylbiphenyl isomers, namely 3,3’, 3,4’ and 4,4’-DMBP, and the higher boiling point by-products. [0058] The overhead stream from the MCHT distillation tower 23 is fed by line 24 to a toluene condenser 25, where any excess hydrogen and light hydrocarbons are removed and purged from the system, before the remaining toluene is recycled via line 26 to the hydroalkylation reactor 12.

[0059] The intermediate stream from the MCHT distillation tower 23 is fed by line 27 to a first crystallizer 28 where the stream is cooled to a temperature from less than 6°C but above - 80°C to precipitate out DMBP isomers in the stream and leave an MCHT-rich mother liquor. The DMBP isomers are collected in line 29 while the MCHT-rich mother liquor is fed by line 31 to a transalkylation reactor 32, where the l,X’-MCHT (if present) and 2, X’ -MCHT compounds in the mother liquor are selectively converted to 3,X’- and 4, X’ -MCHT compounds. The transalkylation product is then recycled by line 33 to the condenser 14. In an alternative embodiment, shown by the dotted line 34, part or all of the mother liquor is recycled directly to the hydroalkylation reactor 12. In any embodiment, optionally, a portion of the mother liquor may be hydrogenated in a hydrogenation unit (not shown) prior to being fed to the transalkylation reactor 32 or recycled to the hydroalkylation reactor 12.

[0060] The bottoms stream from the MCHT distillation tower 23 is fed by line 35 to a heavies distillation tower 36 where the Ci₄₊ by-products are removed and purged from the system and an overhead stream rich in 3,3’, 3,4’ and 4,4’-DMBP is collected in line 37. The overhead stream is supplied by line 37 to a further crystallizer 38 operating at a temperature from -30 to 40°C to separate the stream into a solid fraction comprising 4,4’ -DMBP and liquid fraction comprising at least 3,3’ and 3,4’-DMBP and deficient in 4,4’-DMBP. The solid fraction is recovered via line 39 for further processing and liquid fraction is supplied by line 41 to an isomerization reactor 42 where the liquid fraction is returned to an equilibrium concentration of DMBP isomers. The effluent from the isomerization reactor 42 therefore has a higher concentration of 4,4’ -DMBP than the liquid fraction in line 41 and is recycled via line 43 to the crystallizer 38 for recovery of additional 4,4’ -DMBP. Optionally, a slip stream 44 can be removed from line 43 and fed to the separator 19 to allow eventual purging of any unwanted by-products generated by the isomerization reaction.

[0061] Figure 2 illustrates another aspect of a process for producing 4,4’ -dimethylbiphenyl according to the present disclosure. The process of Figure 2 is similar to that shown in Figure 1 and hence the same reference numerals are used to indicate the same components in both drawings. In the process of Figure 2, the heavies distillation tower 36 is omitted and the MCHT distillation tower 23 is operated to separate the mixed stream in line 22 into (i) an overhead stream containing at least a portion of the unreacted toluene, (ii) an intermediate stream containing a portion of the unreacted toluene, the residual MCHT and most of the DMBP compounds and (iii) a bottoms stream comprising the C15₊ by-products. In other words, there is no deliberate separation of DMBP isomers in the MCHT distillation tower 23.

[0062] The overhead stream from the MCHT distillation tower 23 in the process of Figure 2 is again fed by line 24 to a toluene condenser 25, where any excess hydrogen and light hydrocarbons are removed and purged from the system, before the remaining toluene is recycled via line 26 to the hydroalkylation reactor 12. Optionally, some or all of the toluene in line 26 can be recycled to the transalkylation reactor 32.

[0063] Similarly, the intermediate stream from the MCHT distillation tower 23 in the process of Figure 2 is fed by line 27 to a first crystallizer 28 where the stream is cooled to precipitate out DMBP isomers in the stream and leave a MCHT-rich mother liquor. In some cases, the cooling in the crystallizer 28 is conducted so as to remove a single stream which is a mixture of some or all of the DMBP isomers which are then fed by line 45 to a further crystallizer 38 operating at a temperature from -30 to 40°C to precipitate out the 4,4’ -DMBP via line 39. The remaining liquid fraction, which is deficient in 4,4’-DMBP, is the supplied by line 41 to an isomerization reactor 42 where the liquid fraction is returned to an equilibrium concentration of DMBP isomers. The effluent from the isomerization reactor 42 therefore has a higher concentration of 4,4’ -DMBP than the liquid fraction in line 41 and is recycled via line 43 to the crystallizer 38 for recovery of additional 4,4’ -DMBP. In other cases, given the wide range of reported DMBP isomer melting temperatures (120.8°C to 6.7°C), the process of Figure 2 can also be used to provide at least a part of the pure product stream for certain isomers such as 4,4’ -DMBP (dotted line 46). Multiple stages of crystallization, possibly at different temperatures, can be used to improve recovery of the low concentration or low melting point DMBP isomers, or to provide multiple pure isomer streams. Pure DMBP extract which is not a desired isomer or has a mixture of isomers can be routed directly to the DMBP isomerization reactor 42 (as shown by the dotted line 47) or to another crystallizer unit for further 4,4’ -DMBP recovery as it is free of MCHT and other components which might be detrimental to the performance of the isomerization unit. Again, a slip stream 44 can be removed from line 43 and fed to the separator 19 to allow eventual purging of any unwanted by-products generated by the isomerization reaction.

[0064] As in the process of Figure 1, the MCHT-rich mother liquor remaining after removal of the DMBP component in crystallizer 28 is fed by line 31 to a transalkylation reactor 32, where the l,X’-MCHT (if present) and 2, X’ -MCHT compounds in the mother liquor are selectively converted to 3,X’- and 4,X’-MCHT compounds. The transalkylation product is then recycled by line 33 to the condenser 14. In an alternative embodiment, shown by the dotted line 34, part or all of the mother liquor is recycled directly to the hydroalkylation reactor 12. In any embodiment, optionally, a portion of the mother liquor may be hydrogenated in a hydrogenation unit (not shown) prior to being fed to the transalkylation reactor 32 or recycled to the hydroalkylation reactor 12. The bottoms stream from the MCHT distillation tower 23 in the process of Figure 2 is purged from the system via line 49.

[0065] While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention. All documents described herein are incorporated by reference herein, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text.

[0066] The term“comprising” is considered synonymous with the term“including,” and whenever a composition, an element or a group of elements is preceded with the transitional phrase“comprising”, it is understood that we also contemplate the same composition or group of elements with transitional phrases“consisting essentially of,”“consisting of’,“selected from the group of consisting of,” or“is” preceding the recitation of the composition, element, or elements and vice versa.

Claims

1. A process for separating a dimethylbiphenyl compound from a mixture thereof with a (methylcyclohexyl)toluene compound, the process comprising:

(al) cooling the mixture to a temperature less than the melting point of the dimethylbiphenyl compound but above the thermal solidification transition temperature of the (methylcyclohexyl)toluene compound to produce (i) a crystallization product comprising at least part of the dimethylbiphenyl compound and (ii) a mother liquor comprising the (methylcyclohexyl)toluene compound; and

(bl) recovering the crystallization product.

2. The process of claim 1, wherein the mixture is cooled to a temperature less than the melting point of the dimethylbiphenyl compound but above -l00°C.

3. The process of claim 1 or claim 2, wherein the mixture is cooled to a temperature from less than 6°C but above -80°C.

4. The process of claim 3, wherein the mixture is cooled to a temperature from less than 6°C but above -6°C.

5. The process of any one of the preceding claims, wherein the mixture and the crystallization product comprise at least one 2,X' (where X' is 2', 3' or 4')- dimethylbiphenyl compound.

6. The process of any one of the preceding claims, wherein the mixture and the mother liquor comprise at least one 3,2’- and/or 3,3’-methylcyclohexyltoluene compound.

7. The process of any one of the preceding claims, wherein the mixture and the crystallization product comprise at least 2,2'-dimethylbiphenyl.

8. The process of any one of the preceding claims, wherein the mixture and the crystallization product comprise at least one 3,3’-, 3,4’- and/or 4,4’-dimethylbiphenyl compound.

9. A process for producing 3,3’, 3,4’ and/or 4,4’ dimethylbiphenyl compounds, the process comprising:

(b2) dehydrogenating at least part of the hydroalkylation reaction product in the presence of a dehydrogenation catalyst under conditions effective to produce a dehydrogenation reaction product comprising a mixture of dimethyl- substituted biphenyl isomers and unreacted (methylcyclohexyl)toluene; and (c2) cooling at least part of the dehydrogenation reaction product in a crystallizer to a temperature less than 6°C and above -80°C to produce (i) a crystallization product comprising at least part of the dimethyl-substituted biphenyl isomers in the dehydrogenation reaction product and (ii) a mother liquor comprising unreacted (methylcyclohexyl)toluene.

10. The process of claim 9 and further comprising:

(d2) distilling at least part of the dehydrogenation reaction product to produce (i) a light fraction comprising 2,X' (where X' is 2', 3' or 4')-dimethylbiphenyl compounds and unreacted (methylcyclohexyl)toluene and (ii) a heavy fraction comprising 3,3’, 3,4’ and/or 4,4’ dimethylbiphenyl compounds; and

(e2) supplying at least part of the light fraction to step (c2).

11. The process of claim 9 or claim 10 and further comprising:

(f2) supplying at least part of the crystallization product from (c2) to a crystallizer operating at a temperature from -30 to 40°C to separate the crystallization product into a solid fraction comprising 4,4’ -dimethylbiphenyl and a liquid fraction comprising at least 3,3’ and 3,4’ -dimethylbiphenyl and depleted in 4,4’- dimethylbiphenyl as compared to the crystallization product.

12. The process of claim 11 and further comprising:

(g2) contacting at least part of the liquid fraction with an isomerization catalyst under conditions effective to produce an isomerized product comprising an increased 4,4’-dimethylbiphenyl content as compared to the liquid fraction.

13. The process of claim 12 and further comprising:

(h2) supplying the isomerized product from (g2) to a crystallizer operating at a temperature from -30 to 40°C to separate a solid fraction comprising 4,4’- dimethylbiphenyl from the isomerized product from (g2).

14. The process of any one of claims 9 to 13, wherein the dehydrogenation reaction product and the crystallization product of (c2) comprise at least 2,2'-dimethylbiphenyl.

15. The process of any one of claims 9 to 14, wherein the dehydrogenation reaction product and the mother liquor comprise at least one 3,2’- and/or 3,3’-methylcyclohexyltoluene compound.

16. The process of any one of claims 9 to 15, wherein the dehydrogenation reaction product of (c2) is cooled to a temperature less than 6°C and above -6°C.

17. The process of any one of claims 9 to 16 and further comprising:

(i2) contacting at least part of the crystallization product from (c2) with an isomerization catalyst under conditions effective to produce an isomerized product comprising an increased 4,4’-dimethylbiphenyl content as compared to the crystallization product.

18. The process of claim 17 and further comprising:

(j2) supplying the isomerized product from (i2) to a crystallizer operating at a temperature from -30 to 40°C to separate a solid fraction comprising 4,4’- dimethylbiphenyl from the isomerized product from (i2).

19. The process of any one of claims 9 to 18 and further comprising:

(k2) recycling at least part of the mother liquor from (c2) to the contacting (a2).

20. The process of any one of claims 9 to 19 and further comprising:

(12) trans alkylating at least part of the mother liquor from (c2) under conditions effective to produce a transalkylation product comprising a different mixture of (methylcyclohexyl)toluene isomers than the mother liquor; (m2) dehydrogenating at least part of the transalkylation product to produce a dehydrogenation product comprising dimethylbiphenyl isomers; and

(n2) supplying the transalkylation product to the cooling step (c2).

21. The process of claim 20 wherein the dehydrogenating (b2) and the dehydrogenating

(m2) are conducted in the same reaction zone.

22. The process of any one of claims 9 to 21 and further comprising:

(o2) hydrogenating at least part of the mother liquor from (c2) under conditions effective to produce a hydrogenation product comprising an increased

(methylcyclohexyl)toluene content as compared to the mother liquor.

23. A process for producing 3,3’, 3,4’ and/or 4,4’ dimethylbiphenyl compounds, the process comprising:

(a3) contacting toluene with hydrogen in the presence of a hydroalkylation catalyst under conditions effective to produce a hydroalkylation product comprising (methylcyclohexyl)toluene;

(b3) dehydrogenating at least part of the hydroalkylation reaction product in the presence of a dehydrogenation catalyst under conditions effective to produce a dehydrogenation reaction product comprising a mixture of dimethyl- substituted biphenyl isomers and unreacted (methylcyclohexyl)toluene;

(c3) distilling at least part of the dehydrogenation reaction product to produce (i) a light fraction comprising 2,X' (where X' is 2', 3' or 4')-dimethylbiphenyl compounds and unreacted (methylcyclohexyl)toluene and (ii) a heavy fraction comprising 3,3’, 3,4’ and/or 4,4’ dimethylbiphenyl compounds; and

(d3) cooling at least part of the light fraction in a crystallizer to a temperature less than 6°C and above -80°C to produce (i) a crystallization product comprising at least part of the dimethyl-substituted biphenyl isomers in the light fraction and (ii) a mother liquor comprising unreacted (methylcyclohexyl)toluene.