Theory and Practice of Alkyl Aromatic Hydrocarbon Synthesis. 1

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Theory and practice of alkyl aromatic hydrocarbons synthesis I. Branched alkylbenzenes Pavel Naumkin, Tatyana Nesterova, Igor Nesterov, Alexander Toikka, and Vladimir Shakun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02021 • Publication Date (Web): 19 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015

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Industrial & Engineering Chemistry Research

Theory and practice of alkyl aromatic hydrocarbons synthesis I.

Branched alkylbenzenes

a

Pavel V. Naumkin * , Tatyana N. Nesterova b, Igor A. Nesterov b, Alexander M. Toikka a, Vladimir A. Shakun b a

Saint Petersburg State University, Universitetsky Prospect 26, Petrodvoretz, Saint-Petersburg

198504, Russia b

Samara State Technical University, Molodogvardeyskaya, 244, Samara 443100, Russia

* Corresponding author. Tel.: +7 812 4284052. E-mail: [email protected]

Abstract The alkylation of benzene, toluene, ortho-xylene and biphenyl was carried out over aluminum halides and ion-exchange resin Amberlyst 36 Dry with use of branched C5 alkylating agents. Tertiary product selectivity is related to a catalytic system of aluminum halides with reduced activity or ion-exchange resin Amberlyst 36 Dry. High selectivity of branched secondary products is provided by an equilibrium condition. The equilibrium in a liquid phase of positional and structural isomerization, and transalkylation at 303 – 363 K was studied. The prediction method of an isomerization equilibrium was recommended for C5 branched alkylbenzenes. Keywords: chemical equilibrium; isomerization, pentylbenzenes

Introduction Many alkyl aromatic hydrocarbons (AAH) are large-scale products of organic and petrochemical synthesis. Despite a relatively high level of the reaction study, an improvement of AAH production process is far away from depletion. Currently the alkylation of aromatic hydrocarbons gained a kind of the second birth by the reason of a new and effective catalytic systems development. However, there are some questions emerged that demand additional empirical information on a chemical equilibrium condition and a potential of modern catalysts usage in processes of a production of individual AAH or preferred mixtures. The present article is concerned with answers to these significant questions. We purposefully selected branched alkylbenzenes (BAB) as the first model which is suitable for a generic issues study of the AAH production. Structural features of BAB molecules have sufficient variety for that, and a distinction in physiochemical properties does not make a problem 1 ACS Paragon Plus Environment


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with an analysis (by GLC) of their mixtures. Actually BAB and to a lesser degree branched butyl-, hexyl- and higher alkylbenzenes were the fundamental model for the answer to complicated problems of a reaction mechanism development for benzenes alkylation with branched alkenes, alcohols and haloalkanes,1–6 a structural isomerization of alkyl substituent,1,3–9 a transalkylation and positional isomerization in aromatic ring of AAH.10 Experimental data had been accumulated within 30 years (from 1940 to 1970). Actually the following information was discussed. Mainly (1,1-dimethylpropyl)benzene (1,1-diMPB) is formed by a benzene alkylation over the following catalysts in liquid phase: • H2SO4: with 2-methyl-2-butene,6,7 3-methyl-1-butene,4 any of three branched pentenes at 0°C,2 neopentyl alcohol;1 • HF with 3-methyl-1-butene at 35°C;7 • BF3 with 2-methyl-2-butene at 28°C;7 • FeCl3 with 2-chloro-2-methylbutane;5 • AlCl3+CH3NO2 (0.1 mole + 1 mole) at 25°C over 0.25 h with 1-chloro-2-methylbutane or 2chloro-3-methylbutane;9 • AlCl3+HCl at -40°C with 2-methyl-2-butene or 3-methyl-1-butene at -40°C and -60°C;7 • AlCl3 (3.5 g with 5 mole of benzene and 1 mole of alkylating agent) at 24°C with 2chloro-2-methylbutane.7 Mainly

(1,2-dimethylpropyl)benzene

(1,2-diMPB)

which

is

secondary

branched

pentylbenzene is formed by benzene alkylation in liquid phase: • over

AlCl3:

with

3-methyl-1-butene

5

at

21°C,4,7

2-methyl-2-butene,6

2-chloro-2-

1

methylbutane, and 1-chloro-2,2-dimethylpropane at 0°C.

Mainly (2,2-dimethylpropyl)benzene (2,2-diMPB) is formed by benzene alkylation in liquid phase: • over AlCl3 with neopentyl alcohol at 0°C.1 In other words, the discussed information shows that individual 1,1-diMPB or 1,2-diMPB can be synthesized by the benzene alkylation with any branched C5-alkylating agents (isopentenes isomers or branched chloropentanes and even neopentyl alcohol) practically. It is important to support these conversions with an appropriate catalysis and/or respective conditions. In that time period investigators were focused on a mechanism development for alkylation and isomerization. Varieties of hypotheses were considered, new studies were carried out to prove or refute them. 2 ACS Paragon Plus Environment

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Schmerling suggested5 the bimolecular nucleophilic substitution mechanism (SN2) to describe the alkylation, especially over aluminum chloride. It could have explained the formation of 1,2diMPB in case of the benzene alkylation over AlCl3 with tert-chloropentane, while pure 1,1-diMPB is formed over ferric chloride. The further study of the benzene alkylation over AlCl3 or ZrCl4 with 2-chloro-2,3dimethylbutane is followed by a rearrangement with the primary formation of secondary isomer – 2,2-dimethyl-3-phenylbutane that was shown by Schmerling and West in work.3 The rearrangement did not run over FeCl3 or mixture of AlCl3 and nitromethane, and in this case 2,3-dimethyl-2phenylbutane was formed. A treatment of the last product over AlCl3 leads to the formation of the secondary isomer – 2,2-dimethyl-3-phenylbutane. The authors admit that the benzene alkylation over AlCl3 has a tendency to form the secondary alkylbenzene (sec-AB) instead of tert-alkylbenzene (tert-AB) as the main product, contrary to popular opinion. Collected data allows the investigators made the assumption that if haloalkane contains primary and secondary carbon atoms, then only secAB are formed during the benzene alkylation. The authors discussed the alternative alkylation mechanism and they preferred SE rather than SN2. Although it is hard to explain the prevalence of sec-AB within the reaction mechanism with an carbocations involvement, the hypothesis were developed which the path to them laid through tert-AB, then the resonance-stabilized tertiary benzylic carbocation and, finally, sec-AB. Therefore Schmerling and West came to the conclusion that sec-AB are preferred products of the benzene alkylation with branched reactants over AlCl3 or the other Friedel-Crafts catalysts. Friedman and Morritz7 showed that the concentration ratio of 1,1-diMPB and 1,2- diMPB depends on catalysts and temperature during the benzene alkylation with isoamylenes and 2-chloro2-methylbutane over AlCl3+HCl, AlBr3+HBr, HF and BF3. Concentration of 1,1-diMPB decreases with a temperature rise of alkylation and 1,2-diMPB formation. Reaction mixtures with 100 % concentration of 1,1-diMPB were obtained at -40 and -60°C over AlCl3+HCl, at 24°C over BF3, and 35°C over HF. The mixture containing only 1,2-diMPB was prepared at 21°C over AlCl3+HCl. Friedman and Morritz7 showed that the concentration ratio of 1,1-diMPB and 1,2-diMPB changes toward the last one at an temperature or time increase during the benzene alkylation with isoamylenes and 2-chloro-2-methylbutane over AlCl3+HCl, AlBr3+HBr, HF and BF3. The reaction mixtures with 100 % concentration of the each isomer were produced at conditions selection. Nevertheless, the authors came to the conclusion, that the obtained results and literature data did not

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allow to made the choice between two mechanisms (SN2 and SE) suggested by Schmerling and West. Isomerization of pentylbenzenes over AlCl3 was studied by Roberts and Han intentionally.8 It was found that 1,1-diMPB isomerize into 1,2-diMPB rapidly, and the last product transforms into 2,2-diMPB slowly. 1,1-diMPB and 1,2-diMPB at 82 : 18 ratio are formed during the benzene alkylation with 1chloro-2-methylbutane and 2-chloro-3-methylbutane over AlCl3 at 25°C that was shown by Roberts and McGuire.9 The main product at 0°C was 1,1-diMPB, but the concentration of 1,2-diMPB increased up to the same ratio of 82 : 18 as time passed. Roberts and McGuire’s work 9 is the first one where isomerization equilibrium of 1,1-diMPB into 1,2-diMPB was discussed. The authors made the conclusion so there were two possible explanation of the following isomerization of 1,1-diMPB into 1,2-diMPB. The first was defined by the mechanism of proceeding conversions. The quick initial formation of 1,1-diMPB can be the example of a kinetic control as well as the formation of 1,1-diMPB as the singular product. The second belonged to the case when 1,2-diMPB is the main product, in other words there is an equilibrium control of the reaction. In our opinion these observations represent the highest value of work,9 but they are virtually unnoticed. Along with the considered questions revealing the essence of the structural isomerization of alkyl substituents in benzene, the process description in general also require data, which belong to positional isomerization in aromatic ring or transalkylation. Next experimental fact would be important. Khalaf and Roberts

10

found that transalkylation of 1,1-diMPB with toluene and the

counter reaction proceed without the isomerization of carbon skeleton of the substituent if the catalyst is a mixture of AlCl3+CH3NO2. In this case there is an equilibrium attainment between meta-(1,1-diMP)- and para-(1,1-diMP)-toluenes, their concentration are 67 – 70 % and 30-33 % mass correspondingly. tert-Pentylbenzenes and tert-pentyltoluenes dramatically isomerize into the corresponding secondary structures when the catalytic system is more active. Transalkylation of secpentylbenzene proceeds slower than tert-pentylbenzene has. The paper 10 strongly demonstrates, that the rearrangement of 1,1-diMPB into 1,2-diMPB is carried out with a chain reaction mechanism, when the exchange of hydride ion is performed with a participation of phenyl and a formation of phenonium intermediate. Obviously, the different transalkylation mechanism is implemented over AlCl3+CH3NO2, which does not provide the isomerization of the carbon skeleton of alkyl substituent. In recent paper 4 ACS Paragon Plus Environment

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Verevkin et al

11,12

show that the transalkylation and the positional isomerization also proceed with

an adequate selectivity (without carbon skeleton change of pentyl) up to the equilibrium attainment in the system of benzene + toluene + (1,1-diMP)benzene + meta-di(1,1-diMP)benzene + paradi(1,1-diMP)benzene + meta-(1,1-diMP)toluene + para-(1,1-diMP)toluene in the presence of chloroaluminate ionic liquids (1-methyl-3-butyl-imidazolium chloride and aluminum chloride mixture in varied molar ratios – [BMIM][Cl]:[AlCl3]), FeCl3 or ion-exchange resin Amberlyst 15 as the catalysts. Hence the catalysts variety that provides the selective production of tert-pentyl substituted hydrocarbons (and potentially not only hydrocarbons) should be wide enough. The question about the maximum selectivity in alkylaromatic hydrocarbon processes and possibly not only hydrocarbons is still unsolved. That sort of questions are solved with aid of thermodynamic and focused testing of catalysts’ efficiency as applied to alkylation, isomerization and transalkylation, and our paper is dedicated to those problems on the example of pentylbenzenes. Experimental section Alkylation. The purification of benzene, toluene, ortho-xylene and biphenyl was performed by distillation. According to gas chromatography analysis (GC) the purity of the substances is more than 99 %. Isopentenes (2-methyl-2-butene, 75 % mass; 2-methyl-1-butene, 20% mass; 3-methyl-1butene, 5 % mass), which were obtained from catalytic decomposition of tert-amyl methyl ether (TAME) over sulfonic ion-exchange resin Amberlyst 36 Dry (A-36) and following distillation, 1chloro-3-methylbutane and 2-chloro-2-methylbutane (Reakhim, Russia) with purity 99 % mass (GC) are used as alkylating agents. Industrial grade TAME (98 %) was obtained from Sanors, Russia. The alkylation catalysts are sulfonic ion-exchange resin Amberlyst 36 Dry (Dow chemical) and anhydrous AlCl3 (Sanors, Russia) or AlBr3 (Reakhim, Russia) with no less than 99 % mass purity. Before the reaction the sulfonic ion-exchange resins were dried to a constant weight in an air bath at 105°C. Thermo gravimetric analysis (Mettler Toledo HR83 moisture analyzer) showed the residual moisture content in the resins of 1.2 ± 0.1 % mass. The alkylation was performed in sealed batch reactors (molybdenum glass, volume 4 – 5 ml, diameter 0.6 cm). The process was carried out at isothermal conditions (± 1°C) and the temperature range from 60 to 180°C. The obtained reaction mixtures did not require special treatment before an analysis. The alkylation of benzene, toluene, ortho-xylene and biphenyl with 2-chloro-2-methylbutane or 1-chloro-3-methylbutane over AlCl3 and AlBr3 was carried out in a reactor with a steer and a 5 ACS Paragon Plus Environment


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jacket at 10°C. Temperature stability was provided by circulating heat carrier through the jacket. Isomeric conversions of all kinds were virtually excluded due to the process being run in the alkylating agent environment. Meanwhile the catalytic complex, which was formed during the alkylation, was completely dissolved and it did not have sufficient activity to carry out isomerization. The alkylation was stopped after the catalytic complex started to drop out from solution. A derived product was separated from the complex by a decantation. The catalytic complex represents compound of aluminum halide and corresponding hydrogen halide with several molecules of alkylaromatic hydrocarbon.

Study of pentylbenzene isomerization equilibrium. The reaction feed composition which was used for the study of equilibrium was varied. The changes of conversion level and conversion rate of isomerization achieved by running the alkylation process under different conditions of catalytic system. In general a high degree of isomerization conversion and an equilibrium attainment in a system usually possible when the reaction system consists of two immiscible phases: dissolved catalyst (upper phase) and catalytic complex (lower phase). The stable heterogeneous catalytic system in the considered hydrocarbons was obtained with substrate to alkylating agent molar ratio of 1.0 : ≤0.2 and with aluminum halide to haloalkane molar ratio of 1.0 : >2.5. Alkyl to aryl ratio represents molar concentration ratio for reactants which contain alkyl substituents in aromatic ring and total quantity of compounds with aromatic ring. Aromatic hydrocarbons like benzene, toluene, xylenes, pentylbenzenes are used as the substrate and at same time the maximum alkyl to aryl ratio is restricted by only a molecular structure of the substrate and the alkylating agent. The condition of the system when the catalytic complex is completely diluted practically suppresses the isomerization conversion at substrate to alkylating agent molar ratio of 1.0 : 1.0. At that moment the catalytic complex with aluminum halide represents a homogeneous system. Therefore, there are two kinds of the catalytic systems which were used for the next purposes of the study: 1) the alkylation over the catalytic complex that was completely dissolved in the reactive mixture; 2) isomerization and equilibrium attainment over the heterophase catalytic complex. The catalytic systems were in liquid state in both cases. The equilibrium study was carried out in a heterophase system at a minimum possible quantity of catalytic complex. The initial compounds are either individual hydrocarbons (reaction system, which is essentially enriched with an individual product) or products of their isomerization. In the 6 ACS Paragon Plus Environment

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latter case the alkylation and isomerization procedures were combined in the following manner. The reaction mixture after alkylation is decanted from the catalytic complex. The upper phase contains alkylation products and a slight amount of the diluted catalytic complex with insufficient concentration for the isomerization until equilibrium attainment. The reaction mixture was loaded into the reactor with the stirrer and the jacket, it was brought up to the investigation temperature then the catalyst was added (from 3.3 to 11 % mass) at constant mixing. Since this moment we begin counting the experiment time for chemical equilibrium investigation for reactions 1–9 (figure 1). Samples, which were taken every 2 – 30 min from the upper phase, were instantly treated by water to decompose the catalytic complex. Additional experiments show that there is no distortion in observed compositions at the suggested sampling method. The isomerization and transalkylation equilibrium for pentylbenzenes were studied both as individual system and in combination with pentyltoluenes and pentyl-ortho-xylenes. The catalytic complex is more active in the presence of toluene, ortho-xylene and their pentyl derivatives13 than in medium of benzene and pentylbenzenes. The catalytic complex enriched with polyaromatic hydrocarbons like biphenyl, decreases own activity.13 Considering that, the chemical equilibrium study in pentylbiphenyl system (reaction 5) was conducted with catalytic system containing toluene. This approach allows all considered reactions to attain the equilibrium state without an additional intervention at a simultaneous catalyst feed. Structural isomerization of pentyl substituent

Reaction 1

Reaction 2

Reaction 3

Reaction 5 Reaction 4 Positional isomerization in aromatic ring and transalkylation



Reaction 6

Reaction 7

Reaction 8

Reaction 9 Figure 1. Studied isomerization and transalkylation reactions for pentylbenzenes, pentyltoluenes, pentyl-ortho-xylenes and pentylbiphenyls The time of equilibrium attainment depends on the process parameters and was from 0.5 to 24 hours. The criteria for equilibrium constants selection is no less than two time excess of the isomerization time at concentration ration consistency over the time that precede this state. There is an example of reaction 1 equilibrium attainment on figure 2. 6 Concentration ratio of 1,2diMPB on 1,1-diMPB

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5

Kx

4 3 2 1 0 0

100

200 300 Isomerization time, min

400

500

Figure 2. Chemical equilibrium attainment of reaction 1 (343 K, AlCl3) Analysis and identification. The analysis of the reaction mixtures were carried out with Chromatec-Crystal 2000 M gas chromatograph equipped with a flame ionization detector (FID) and a capillary column SE-30 (stationary phase cross-linked 100 % dimethylpolysiloxane) 50 m × 250 µm × 0.25 µm (a column length × an internal diameter × a film thickness). The carrier gas is helium. An efficiency of chromatographic separation and a temperature profile for a column oven are demonstrated in figure 3. Temperature of injection is 270°C, temperature of FID is 200°C. The identification of structural isomers in reaction products was conducted with Finnigan Trace DSQ mass spectrometer. Obtained mass-spectrum and their interpretation are presented in work.14 8 ACS Paragon Plus Environment

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Identification of positional isomers in the aromatic ring for pentyltoluenes, pentyl-orthoxylenes and pentylbiphenyls was performed by means of chemical experiment and application of the results of

15,16

, which show that for most substances at the chemical equilibrium state there is a

higher concentration of meta-alkylbenzenes over para-alkylbenzenes, with retention of their structural identity.

Figure 3. Typical chromatogram for pentylbenzenes, pentyltoluenes, pentyl-ortho-xylenes isomerization. 1. Benzene. 2. Toluene. 3. ortho-Xylene. 4. (2,2-Dimethylpropyl)benzene. 5. (1,2Dimethylpropyl)benzene. 6. (1,1-Dimethylpropyl)benzene. 7. 3-(2,2-Dimethylpropyl)toluene. 9. 4(2,2-Dimethylpropyl)toluene. 9. 3-(1,2-Dimethylpropyl)toluene. 10. 3-(1,1-Dimethylpropyl)toluene. 11. 4-(1,2-Dimethylpropyl)toluene. 12. 4-(1,1-Dimethylpropyl)toluene. 13. 4-(2,2-Dimethelpropyl)ortho-xylene. 14. 4-(1,2-Dimethelpropyl)-ortho-xylene. 15. 4-(1,1-Dimethelpropyl)-ortho-xylene.

Sample volume is 0.2–0.4 ml. The volume ratio of reaction mixture to the catalyst complex by the end of the chemical equilibrium study run was not less than 5 : 1. If the ratio is decreased during the sampling, the catalytic complex is separated from the reaction mixture and a new portion of aluminum halide is added to the mixture. A conversion of side reactions (dealkylation and condensation) were evaluated by internal standard in GLC at the equilibrium study in liquid phase. Total mass of non-eluated components does not exceed 5 % after 3 hours of isomerization over AlCl3. That indicates a low fraction of molecule condensation processes at the study conditions. It is evident that high temperature of the 9 ACS Paragon Plus Environment


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study (:J?EFKC

where •

,

∆HL@M I

∆DL@M G

,

(6)

is the contribution to entropy from vibrational motion of a molecule;

rotation of separate groups in the molecule: @AB

9:; :J?L@M KC

where

∆H@F I

∆D@F G

,

(7)

is the contribution to entropy from inner rotation of groups. The symmetry values

accepted in calculation of

∆H@F I

corresponds to geometric symmetry of the rotating groups.



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Figure 12. Chart of equilibrium data analysis.

Calculation of the contributions to entropy

∆HEF ∆HL@M I

,

I

and

∆H@F I

were performed by the following

methods: 1.

Molecular geometry is optimized with Gaussian 09 revision D.09

29

series of programs.

B3LYP functional and 6-311++G(2d,2p) basis set were used. 2.

Vibrational frequencies sets were calculated as the same level of theory as the geometry

optimization. 3.

The potential curves of rotation barriers were evaluated with the MMX force-field molecular

mechanics method (Allinger MM2 modernized field). The values of equilibrium constants obtained after the exclusion of all contributions to entropy give valuable information about the entropic constituent of intermolecular interactions energy of substituents in reacting molecules. In the case of structural isomerization this information is important for development of equilibrium prediction methods for substances including branched alkylbenzenes. Every step of the sequential exclusion of contributions to entropy from the equilibrium constants for reactions 1, 2, 3, 6, and 7 is shown in table 4. 22 ACS Paragon Plus Environment

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9:; After the exclusion of all considered contributions the final level of O a 9:; 9:; 9:; 9:;

Theory and Practice of Alkyl Aromatic Hydrocarbon Synthesis. 1

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