One-Pot Synthesis of Methyl-Substituted Benzenes and Methyl

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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One-Pot Synthesis of Methyl-Substituted Benzenes and MethylSubstituted Naphthalenes from Acetone and Calcium Carbide Dong Wang, Zhenyu Liu, and Qingya Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

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S Supporting Information *

ABSTRACT: Methyl-substituted benzenes and methyl-substituted naphthalenes are important starting chemicals for organic synthesis and polymer production and superior additives of high-density fuels. This work indicates for the first time that they can be synthesized from acetone and calcium carbide (CaC2) in one pot at 200−350 °C. As reported in the literature on acetone conversion, 3,5-xylenol is simultaneously formed with the methyl-substituted benzenes. At the optimized conditions, the total carbon yield of methylsubstituted benzenes and methyl-substituted naphthalenes reaches 48% and the carbon yield of 3,5-xylenol is about 20%. The role of CaC2 and the synthetic pathway of methylsubstituted benzenes and methyl-substituted naphthalenes are investigated through identification of intermediates, control experiments, and quantum chemical calculation by density functional theory (DFT). It is found that methyl-substituted benzenes and methyl-substituted naphthalenes are formed via tandem reactions involving acetone condensation and intermediates aromatization. CaC2 promotes acetone condensation and its alkynyl moiety directly participates in aromatization reaction to form methyl-substituted naphthalenes, which are attributed to the stronger nucleophilicity of the alkynyl moiety. This protocol enables the direct synthesis of methyl-substituted benzenes and methyl-substituted naphthalenes from acetone and CaC2, and opens up a safe and efficient route for CaC2 application. reported so far is no more than 27%20−22 and the long and complex synthesis procedures hinder the overall synthesis efficiency. Much effort, therefore, is still made in finding efficient protocols for bio-acetone to aromatic hydrocarbons and 3,5-xylenol. It is recognized that IP is a product of acetone trimerization catalyzed by MgO,23 Mg−Al mixed oxides,24 TiO2 or alkalinedoped TiO2,25 or Mo2N.26 The reactions involve consecutive condensation/dehydration and then cyclization into IP as shown in Scheme 1.27−29 The dehydration reactions were reported to be equilibrium-controlled,30 and the synthesis efficiency should be enhanced if the H2O is removed timely. IP is then converted into aromatic hydrocarbons and 3,5-xylenol via aromatization (Scheme 1), which are also catalyzed by basic catalystsCr2O3 for 3,5-xylenol31,32 and MoOx for aromatic hydrocarbons,18 for example. If the acetone condensation and IP aromatization could be coupled in one pot using a suitable catalyst, the steps for IP synthesis and isolation would be eliminated and the synthesis efficiency of aromatic hydrocarbons and 3,5-xylenol would be significantly

1. INTRODUCTION With the increasing depletion of nonrenewable resources, conversion of abundant biomass to fuels and chemicals has attracted much interest because of its sustainability. Fermentation of lignocellulose, the main component of agriculture wastes and forest residues, has been industrialized to produce versatile platform chemicals: acetone−butanol−ethanol (ABE).1−3 Efficient conversion of ABE into valuable chemicals or building blocks is a potential path to supplement traditional petrochemical and coal-chemical processes. Aromatic compounds are vital starting chemicals for organic synthesis or polymer production, solvents of paint and dyes, and important additives of high-density fuels.4,5 To date, their production still heavily depends on fossil fuel chemical technology,6 such as synthesis of alkyl-substituted benzenes from benzene or toluene over zeolites.7−10 Conversion of ABE into aromatic compounds has become an attractive topic in the research field of biomass utilization especially during the past decade. Mesitylene and 3,5-xylenol are two of the most reported products.11−13 Mesitylene has been synthesized directly from acetone over Mg−Zr oxides, MgO, or zeolite catalysts.14−17 In many other protocols, isophorone (IP) was synthesized first from acetone, which is followed by different chemical processes for the production of aromatic hydrocarbons18 and 3,5-xylenol.19 Unfortunately, the IP yield © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 10, 2019 March 25, 2019 March 28, 2019 March 28, 2019 DOI: 10.1021/acs.iecr.9b00175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Reaction Pathway of Acetone to 3,5-Xylenol and Mesitylene27−29

Vg =

PV 1 P0

(1)

The composition of gas product was determined with an Agilent GC-7890B gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and two packed columns of Agilent Porapak Q (6 ft × 1/8 × 2.0 mm) and Mol Sieve 5A (6 ft × 1/8 × 2.0 mm). Helium was used as the carrier gas at a flow rate of 30 mL min−1. The temperatures of the injection port, column, and detector were 100, 70, and 200 °C, respectively. The average density of gas product was estimated according to the composition, while the mass of gas product was estimated according to the average density. (Details can be found in the Supporting Information.) The solid product was dried at 120 °C under a vacuum for 12 h and then weighed. The X-ray diffraction (XRD) pattern of the solid product was recorded on a D8FOCUS powder diffractometer using Cu Kα radiation (λ = 1.5432 Å) at 40 kV and 40 mA. The scan range, step length, and rate were 5−80°, 0.1°, and 0.1° s−1, respectively. The content of organic carbon in solid products was determined by a total organic carbon analyzer (SSM-5000A, Shimadzu). The mass of liquid product was determined by subtracting the mass of gas and that of solid products from the total mass of reactants. Qualitative analysis of the liquid product was performed on an Agilent GC−MS (mass spectrometer) equipped with an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) and a quadrupole analyzer system (5977B) with a high efficiency ion source (GC/MSD) operated at 300 °C. Helium was used as the carrier gas at a flow rate of 1.0 mL min−1, the sample size was 1.0 μL, and the split ratio was 50:1. The injection temperature was 250 °C, while the temperature program of the column was 35 °C for 5 min, 10 °C min−1 to 230 °C, and 230 °C for 1 min. The scan m/z of the MS ranged from 30 to 350. Quantitative analysis of the liquid product was performed on an Agilent GC-7890B equipped with an HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID). 2-Pentanol was used as the internal standard, and the mass ratio of 2-pentanol to sample was 1:10. N2 was the carrier gas at a flow rate of 1.0 mL min−1, and the detector temperature was 250 °C. Other conditions are the same as those in GC−MS. Based on the quantitative results, acetone conversion (Xacetone), carbon conversion (Xcarbon), carbon yields of liquid product (Yliquid), gas product (Ygas), and solid products (Ysolid), as well as carbon balance (%C) were estimated with eqs 2, 3, 4, 5, 6, and 7, respectively.

improved. Calcium carbide (CaC2) comes to our attention due to its good dewatering ability and potential catalytic activity because of its basicity.33 Furthermore, it has been reported that CaC2 can be produced from biochar at temperatures lower than that required in current technology,34,35 which renders the above idea more attractive because of its sustainability. Based on the above analysis, CaC2-promoted bio-acetone conversion to aromatic hydrocarbons and 3,5-xylenol in one pot was proposed. Much to our surprise, the preliminary results not only verify our supposition but also indicate the formation of high-value-added methyl-substituted naphthalenes besides methyl-substituted benzenes and 3,5-xylenol. This finding is promising to open up a simple and safe application of CaC2 in organic synthesis and an efficient and green synthetic route for the production of downstream products of acetone. Inspired by the preliminary results, the effects of reaction temperature and time on acetone conversion and the product carbon yield were studied in detail, and the role of CaC2 in various reactions and the synthetic pathway of methyl-substituted naphthalenes were investigated via control experiments and density functional theory (DFT) simulation.

2. EXPERIMENTAL SECTION 2.1. Materials. Acetone with a purity of 99.5% (Beijing Chemical Works) was dehydrated with 3A molecular sieves. CaC2 with a purity of >97% (Acros Organics) was ground and screened to 80−100 mesh in a glovebox under N2. The commercial reagents of isophorone (97%), calcium hydroxide, calcium oxide, magnesium oxide, titanium dioxide, and hydrotalcites (Mg/Al = 3:1) were used as received. 2.2. Reaction Experiments. Typically, acetone (10.0 g, 172 mmol) and CaC2 (1.5 g, 23 mmol) were added into a quartz liner in a stirred autoclave reactor (Parr 4597). The reactor was then sealed, purged with Ar to replace air, and heated to a designated temperature under stirring at 200 rpm. After a specified time, the reactor was immersed into an ice− water bath to quench the reaction. The gauge pressure after the reaction was recorded as P1. The gas product was collected into a gas bag, and the solid and liquid products were transferred to a centrifuge tube for separation. 2.3. Analysis of Reaction Products. The volume of gas product at the atmospheric pressure (termed Vg) was estimated with eq 1, where V is the void volume of the reactor under reaction condition and P0 is the atmospheric pressure.

total carbon in acetone in liquid product zy i zz Xacetone = jjj1 − total carbon in acetone fed into reactor { k

(2)

× 100% Xcarbon = total carbon in converted acetone and calcium carbide total carbon fed into reactor

(3)

× 100% Yliquid =

total carbon in a specific liquid product × 100% total carbon in fed into reactor (4)

B

DOI: 10.1021/acs.iecr.9b00175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Ygas =

total carbon in gas products × 100% total carbon fed into reactor total carbon in solid products × 100% total carbon fed into reactor

Ysolid = %C =

than 10 obvious peaks and some minor ones. MS results shown in Table S1 indicate that, in addition to the five anticipated products, IP, mesityl oxide (MO), diacetone alcohol (DAA), mesitylene, and 3,5-xylenol, seven aromatic compounds are also identified: p-xylene (PX), 2-ethyl-1,4-dimethylbenzene (2E-1,4-DMB), 2,3,5-trimethylphenol (2,3,5-TMP), 2-methylnaphthalene (2-MN), 2,6-dimethylnaphthalene (2,6-DMN), 2,3,6-trimethylnaphthalene (2,3,6-TMN), and 3,4-dihydro3,3,6,8-tetramethylnaphthalen-1(2H)-one (3,3,6,8-TMNO). The formation of methyl-substituted naphthalenes is surprising and encouraging because they have never been reported in previous studies and are valuable platform chemicals for polymer synthesis.37−40 For example, 2,6-DMN is an important precursor of 2,6-naphthalenedicarboxylic acid (a monomer for polyethylene naphthalate (PEN)).39,40 Generation of methyl-substituted benzenes, methyl-substituted naphthalenes, and 3,5-xylenol confirm that coupling of acetone condensation and subsequent aromatization in one pot is realized in the presence of CaC2. 3.2. Effects of Reaction Temperature and Time on Product Carbon Yield. The positive results are elaborated by quantifying acetone conversion and the carbon yields of methyl-substituted benzenes (mesitylene, PX, and 2-E-1,4DMB), methyl-substituted naphthalenes (2-MN, 2,6-DMN, and 2,3,6-TMN) and 3,5-xylenol at different temperatures. Carbon balance excluding the minor compounds in Figure 1 are higher than 89% in all the runs (Table S2), suggesting the reliability of experimental methods in this work. Herein, the methyl-substituted benzenes are discussed as a group and the methyl-substituted naphthalenes are discussed as another group. 2,3,5-TMP is ignored because its carbon yield is far less than that of 3,5-xylenol in all the runs. The effect of reaction temperature at a reaction time of 5 h is shown in entries 1−5 in Table 1. The acetone conversion increases slightly from 51.7% at 150 °C to 67.2% at 350 °C. The carbon yields of desired products are quite low at 150 °C: 3.4% for methyl-substituted benzenes, 8.9% for methylsubstituted naphthalenes, and zero for 3,5-xylenol. Elevating the reaction temperature effectively promotes the formation of desired products, and the carbon yields of methyl-substituted benzenes and methyl-substituted naphthalenes reach 24.7 and 19.8% at 350 °C, respectively. 3,5-Xylenol shows the maximum

(5)

(6)

total carbon identified in products × 100% total carbon fed into reactor

(7)

2.4. Computational Method. Quantum chemical calculation was performed with DFT using the Gaussian 09 software.36 Full geometry of all the molecules concerned, such as reactants, transition states, intermediates, and products, was optimized using the correlation functional B3LYP with the 6311G(d,p) basis set. The correctness of obtained transition states was verified by the intrinsic reaction coordinates (IRCs). The energy of each configuration was calculated under the same theory level. The calculation formula of the energy barrier corresponding to the elementary reaction is shown in eq 8, where ΔE, ETS, and Er denote the energy barrier, the energy of the transition state, and the energy of the reactant, respectively. ΔE = E TS − Er

(8)

3. RESULTS AND DISCUSSION 3.1. Determination of Liquid Product. Taking the reaction at 250 °C for 5 h as an example, the composition of liquid product was determined by GC−MS. The total ion chromatogram is presented in Figure 1, which shows more

Figure 1. Total ion chromatogram of the liquid product of acetone reaction in the presence of CaC2 at 250 °C for 5 h.

Table 1. Acetone Conversion and Carbon Yield of Liquid Products under Different Conditionsa carbon yield (%) entry

temp (°C)

time (h)

acetone conv (%)

1 2 3 4 5 6 7 8 9 10 11 12

150 200 250 300 350 250 250 250 250 250 250 250

5 5 5 5 5 1 3 5 7 10 12 15

51.7 56.7 59.6 64.6 67.2 52.4 56.2 59.6 67.3 71.4 74.9 76.9

3,5-xylenol

methyl- substituted benzenesb

methyl- substituted naphthalenesc

MO

IP

3,3,6,8-TMNO

org carbon in solid residual (mmol)

0.0 1.5 8.6 10.7 6.5 2.2 5.7 8.6 10.9 14.2 16.6 17.5

3.4 5.4 12.4 23.2 24.7 3.6 8.0 12.4 15.6 21.8 25.3 24.8

8.9 15.4 14.2 16.4 19.8 7.8 11.4 14.2 15.1 15.7 15.3 15.6

3.1 1.3 1.1 0.6 0.5 0.9 0.9 1.1 1.0 0.7 0.6 0.7

32.0 20.1 8.2 1.3 1.0 11.2 9.8 8.2 7.3 5.7 3.3 2.4

3.5 6.1 8.0 0.0 0.0 22.1 14.0 8.0 2.6 0.0 0.0 0.0

4.5 3.7 2.8 1.1 0.5 13.5 8.4 2.8 1.1 0.6 0.6 0.0

a The amount of acetone is 172 mmol; that of CaC2 is 23 mmol. bMethyl-substituted benzenes include PX, mesitylene, and 2-E-1,4-DMB. cMethylsubstituted naphthalenes include 2-MN, 2,6-DMN, and 2,3,6-TMN.

C


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Industrial & Engineering Chemistry Research Table 2. Comparison of Various Catalysts during Acetone Conversion at 250 °C for 5 ha carbon yield (%) entry

catalyst

acetone conv (%)

3,5-xylenol

mesitylene

PX and 2-E-1,4-DMB

methyl- substituted naphthalenes

MO

IP

1 2 3 4 5 6 7

none CaC2 Mg−Al oxides MgO TiO2 CaO Ca(OH)2

4.3 59.6 23.4 25.7 21.7 27.4 20.3

0.0 8.6 3.7 3.3 1.9 3.7 3.1

0.0 6.9 3.1 3.3 4.0 5.1 2.4

0.0 5.5 0.0 0.0 0.0 0.0 0.0

0.0 14.2 0.0 0.0 0.0 0.0 0.0

0.0 1.1 1.1 3.1 2.5 3.2 2.9

0.0 8.2 1.4 4.5 2.3 3.6 1.6

a

The amount of metals in each run is 23 mmol.

Table 3. Distribution of Liquid Products Generated by Reaction of IP and CaC2a carbon yield (%) entry

temp (°C)

time (h)

3,5-xylenol

mesitylene

PX

2-E-1,4-DMB

2-MN

2,6-DMN

2,3,6-TMN

3,3,6,8-TMNO

1 2 3 4

250 250 300 300

5 10 5 10

6.0 6.3 9.6 9.2

12.0 12.9 16.2 15.5

1.7 2.4 2.9 3.5

1.0 1.5 2.2 3.0

2.5 3.0 4.4 5.0

1.5 2.3 2.6 2.9

1.7 2.3 2.6 2.9

4.9 3.9 3.5 2.8

a

The amounts of IP and CaC2 are 57 and 23 mmol, respectively.

carbon yield at 300 °C, suggesting its degradation at a higher temperature. Accordingly, the total selectivity of target products increases gradually from 22.1% at 150 °C to 73.0% at 350 °C (entries 1−5 in Table S2). The effect of reaction time was evaluated at 250 °C, and the results are summarized in entries 6−12 in Table 1. The acetone conversion reaches 52.4% in 1 h and increases to 76.9% in 15 h. The carbon yields of desired products are quite low in 1 h: 3.6% for methyl-substituted benzenes, 7.8% for methyl-substituted naphthalenes, and 2.2% for 3,5-xylenol. They increase gradually over time in 7 h and follow different trends after then. Longer reaction time increases the carbon yields of 3,5-xylenol and methyl-substituted benzenes but has a slight effect on that of methyl-substituted naphthalenes. The maximum carbon yields of 3,5-xylenol and methyl-substituted benzenes are around 17 and 25% in 12−15 h, respectively, while that of methyl-substituted naphthalenes is about 15− 16% in 7−15 h. The total selectivity of desired products increases greatly from 24.2% in 1 h to 74.4% in 12 h and then changes little over time (entries 6−12 in Table S2). The slight increase in acetone conversion versus the great increase in selectivity over time suggests that the desired products are formed via tandem reactions and the last step is possibly the controlling one. The catalysts used for acetone conversion in the literature, Mg−Al mixed oxides, MgO, and TiO2, were evaluated at 250 °C in our reactor and compared with CaC2. The results shown in Table 2 indicate that the acetone conversion is only 4.3% without a catalyst, far lower than that with a catalyst, and the acetone conversion with CaC2 is the largest. More importantly, PX, 2-E-1,4-DMB, and methyl-substituted naphthalenes are observed over CaC2 but not over other catalysts. These results indicate the superior promoting effect of CaC2 on acetone conversion and the decisive role of CaC2 (probably the alkynyl moiety) in the formation of PX, 2-E-1,4-DMB, and methylsubstituted naphthalenes. Furthermore, formation of PX, formation of 2-E-1,4-DMB, and formation of methylsubstituted naphthalenes are concurrent. Previous studies on direct conversion of acetone were mostly performed in the gas phase with fixed-bed flow reactors.

Their liquid products were mainly the dehydrated compounds, MO, IP, or mesitylene, and the aromatization product (3,5xylenol) observed in this work was absent (Table S3). This difference suggests that aromatization of IP to 3,5-xylenol is slow and requires a long reaction time, and the requirement is not easy to meet by fixed-bed flow reactors. It is true that most tandem reactions were realized in a batch reactor by reacting a large amount of reactants at the same time. It is noted that the literature work on stepwise conversion of acetone to aromatics, with IP as the intermediate, produced mainly C8−C9 methyl-substituted benzenes and 3,5-xylenol. The maximum carbon yield of IP from acetone was 26.7% at 240 °C,20 the carbon yield of C8−C9 methyl-substituted benzenes from IP was about 80% at 450 °C,18 and that of 3,5xylenol was 48% at 520 °C.32 That is, the carbon yields of C8− C9 methyl-substituted benzenes and 3,5-xylenol are about 22 and 13% on the basis of acetone, respectively. The optimum results of our work (300 °C for 5 h, for example) are close to the literature results, but the one-pot synthesis method developed in this work is simpler, requires a lower temperature, and produces methyl-substituted naphthalenes simultaneously. 3.3. Intermediates of Tandem Reactions. It is recognized that the formation of MO is the first step of acetone conversion and IP is one of the important intermediates of 3,5-xylenol and mesitylene as indicated in Scheme 1. To confirm the intermediates in the presence of CaC2, the carbon yields of MO and IP in Table 1 are discussed in detail. It is seen that at 250 °C (entries 6−12 in Table 1) the carbon yield of MO varies between 0.6 and 1.1% while that of IP decreases gradually from 11.2 to 2.4% over time, indicating that MO conversion is quick while IP conversion is relatively slow. It is unanticipated that 3,3,6,8-TMNO is formed and its carbon yield decreases from 22.1% to 0 from 1 to 10 h at 250 °C, suggesting that it is also an intermediate. Since 3,3,6,8TMNO is a two-ring compound, it may be an intermediate of methyl-substituted naphthalenes. 3,3,6,8-TMNO is similar to IP in structure and therefore may originate from IP. To verify this assumption, reaction of CaC2 with IP was carried out at 250 and 300 °C. The results in Table 3 indicate that 3,3,6,8-TMNO is formed and its carbon D


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Industrial & Engineering Chemistry Research

CaO and Ca(OH)2 were carried out at 250 °C for 5 h and the results are shown in the bottom two rows of Table 2. The acetone conversions are 27.4 and 20.3% with the addition of CaO and Ca(OH)2, respectively, and the products include MO, IP, 3,5-xylenol, and mesitylene. 3,3,6,8-TMNO and methyl-substituted naphthalenes are not observed in the two cases due to the absence of the alkynyl moiety as addressed in section 3.2. Clearly, the catalytic activities of CaO and Ca(OH)2 are similar to those of MgO and Mg−Al oxides reported in the literature and much lower than that of CaC2, indicating the important role of the alkynyl moiety in enhancing the catalytic activity of Ca2+. The catalytic activity of CaC2 takes effect at 150 °C as evidenced by the high acetone conversion and IP carbon yield (entry 1 in Table 1). According to the reaction stoichiometry, three acetone molecules would yield one IP and two H2O molecules,11,25 and the amount of H2O produced in 5 h at 150 °C would be at least 59.3 mmol (determined according to the acetone conversion). Since the H2O would quickly react with CaC2 to yield the same amount of C2H2, the small amount of C2H2 observed (Table S4) suggests that C2H2 is consumed quickly or the acetone condensation to IP over CaC2 does not experience the H2O formation. Since CaC2 is gradually consumed in the reaction, its initial quantity would influence the product yield. The effect of CaC2 loading was evaluated at 300 °C for 10 h, and the results are shown in Figure 3. It is seen that, with increasing the CaC2

yield decreases over time. Methyl-substituted benzenes and methyl-substituted naphthalenes are also observed in the product and the carbon yield of methyl-substituted naphthalenes increases over time, even exceeding that of 3,3,6,8TMNO at 300 °C in 10 h. These observations indicate that IP and 3,3,6,8-TMNO are possibly the intermediates for methylsubstituted naphthalenes, PX and 2-E-1,4-DMB. Mesitylene and 3,5-xylenol are obviously more than other products, indicating that CaC2 significantly promotes dehydration and demethanation of IP. 3.4. Role of CaC2 in Tandem Reactions. The discussion made so far confirms that CaC2 is a promoter for the acetone conversion and also a reactant for the production of methylsubstituted naphthalenes. To better understand the role of CaC2 in the reactions, the solid residuals were analyzed by XRD and TOC, and the gas products were analyzed by GC. It is seen in Figure 2 that the XRD diffraction peaks of CaC2 are

Figure 2. XRD patterns of CaC2 raw material and solid residuals of acetone + CaC2 reaction at 250 °C.

observable at 250 °C in 1 h but disappear in 5 and 10 h. The diffraction peaks of Ca(OH)2 are visible in 1 h and become stronger over time but other calcium-containing compounds are not observed, indicating that CaC2 is mainly converted into Ca(OH)2. The contents of organic carbon in solid residuals are listed in Table 1. Supposing all the carbon in the residuals was from the unreacted CaC2, the amounts of CaC2 converted in 1, 3, 5, and 7 h would be 16.3, 18.8, 21.6, and 22.5 mmol, respectively, corresponding to CaC2 conversions of 71, 82, 94, and 98%, respectively. These results confirm that almost all the CaC2 is converted at 250 °C in 5 and 10 h. These CaC2 conversions however are much higher than the amounts of C2H2 in the gas product, 1.1 mmol at 250 °C in 1 h (entry 1 in Table S4), for example, which indicates that most of the alkynyl moiety in CaC2 directly or indirectly participates in aromatization reactions to form two-ring compounds such as 3,3,6,8-TMNO intermediate and methyl-substituted naphthalenes. This inference is consistent with the total amounts of 3,3,6,8-TMNO and methyl-substituted naphthalenes formed, 12.5 mmol in 1 h at 250 °C (determined according to their carbon yields shown in Table 1), for example. To evaluate the role of the alkynyl moiety and Ca2+ of CaC2 in promoting the acetone conversion, control experiments with

Figure 3. Effect of CaC2 loading on acetone conversion and product carbon yield at 300 °C for 10 h.

loading from 23 to 46 mmol, the carbon yields of 3,5-xylenol, methyl-substituted benzenes, and methyl-substituted naphthalenes increase from 20.0, 30.3, and 17.7% to 23.8, 34.1, and 18.6%, respectively, corresponding to an 8.5% increase in total carbon yield and a 0.9% increase in methyl-substituted naphthalenes. The carbon contents of the residuals indicate that CaC2 conversions approximate 99% at the CaC2 loadings of 23−40 mmol and 94% at a CaC2 loading of 46 mmol (Table S5). If all the alkynyl moiety in the converted CaC2 was transformed into methyl-substituted naphthalenes, the increase in carbon yield of methyl-substituted naphthalenes should be 6.1%, rather than the observed value of 0.9%. These results suggest that the initial CaC2 loading has a minor effect on enhancing the acetone conversion and methyl-substituted naphthalene formation. E


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Industrial & Engineering Chemistry Research Scheme 2. Plausible Promoting Effect of CaC2 in Acetone Conversion to IP

Scheme 3. Plausible Synthesis Pathways of 3,3,6,8-TMNO from IP

3.5. Synthetic Pathway of Methyl-Substituted Naphthalenes and PX. The above discussion indicates that IP and 3,3,6,8-TMNO are the important intermediates for aromatic hydrocarbons. Acetone condensation to IP under the promoting effect of CaC2 does not release H2O, and a CaC2-derived intermediate may be generated in the reactions. Based on this information and the superior nucleophilicity of CaC2, the promoting effect of CaC2 on acetone condensation to IP is proposed and shown in Scheme 2. First, the α-H atom in acetone is activated by CaC2 to form a carbon anion and a hydrogen cation, and the carbon anion reacts with the carbonyl

in another acetone via nucleophilic addition to form a diacetone alcohol anion. Then, the alcohol anion couples with the hydrogen cation to form a diacetone alcohol adsorbed on CaC2. The diacetone alcohol subsequently transforms to MO along with calcium acetylide (A) formation, and MO finally reacts with another acetone to form phorone, which is activated by CaC2 (route a in Scheme 2). It is also possible that the α-H atom in diacetone alcohol is activated in situ by CaC2 to form a hydrogen cation and calcium acetylide (A), followed by addition of acetone to form phorone and conversion of calcium acetylide (A) to Ca(OH)2 and C2H2 F


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It shows that the energy barriers of routes a and b are close, 320−325 kJ mol−1, and are obviously lower than that of route c, 447 kJ mol−1. These observations are consistent with the small amount of H2 in the products and confirm the reliability of the above speculation. The transformation route of 3,3,6,8-TMNO to methylsubstituted naphthalenes is proposed according to the gas product distribution and literature reports and is shown in Scheme 4. It starts from decomposition of 3,3,6,8-TMNO to form 2,2,7-trimethyl-1,2,3,4-tetrahydronaphthalene (termed 2,2,7-TM-THN) and CO through ring opening, transalkylation, and cyclization, and is followed by the transformation of 2,2,7-TM-THN to 2-MN via demethanation, a route similar to the aromatization of IP to 3,5-xylenol. The 2MN reacts with mesitylene to form 2,6-DMN and PX by transalkylation as reported in the literature.41 Similarly, 2,6DMN reacts with mesitylene to form 2,3,6-TMN and PX. This reaction pathway indicates that PX, 2,6-DMN, and 2,3,6-TMN should be formed simultaneously, which is consistent with the results in Table 2. This reaction pathway, along with demethanation of IP to 3,5-xylenol, indicates that the amount of CH4 formed should be similar to the sum of 3,5-xylenol and 2(2-MN + 2,6-DMN + 2,3,6-TMN), which is supported by the experimental data of longer reaction time or higher temperatures in Table S4. DFT simulation was also performed to verify the conversion of 3,3,6,8-TMNO to 2,2,7-TM-THN. The optimal configurations of each substance involved in the reactions are shown in Figure S4. The energies of each substance configuration were determined and are shown in Figure 5. It is seen that the energy barrier of 3,3,6,8-TMNO to TS1 is 332 kJ mol−1 and that of IM2 to TS2 is 283 kJ mol−1, indicating that the ringopening reaction is the rate-determining step in decomposition of 3,3,6,8-TMNO. The lower energy of 3,3,6,8-TMNO in comparison with the sum of 2,2,7-TM-THN and CO indicates that 3,3,6,8-TMNO decomposition is endothermic. In this sense, a higher temperature is beneficial for the generation of 2,2,7-TM-THN, which is consistent with the experimental phenomenon that the carbon yield of 3,3,6,8-TMNO decreases while that of methyl-substituted naphthalenes increases with elevating the reaction temperature from 250 to 350 °C (entries 3−5 in Table 1).

(route b in Scheme 2). Since the amount of C2H2 formed is small, route b may not be the main one if it occurs. Phorone is further cyclized to generate IP as widely reported in the literature.11,21,25 As discussed earlier, IP could be transferred to 3,3,6,8TMNO with the participation of the alkynyl moiety in CaC2. This transformation may start with the addition of acetone to IP according to the literature,31,32 where acetone or IP is activated by CaC2 or calcium acetylide (A). Following this rationale and aided by the elemental balance and the solid and gas product distribution, one plausible pathway for IP conversion to 3,3,6,8-TMNO is formulated as shown in Scheme 3. IP is first activated by calcium acetylide (A) and converted to C14 intermediate with the participation of the alkynyl moiety and acetone, which is accompanied by the formation of Ca(OH)2. The C14 intermediate is converted to 3,3,6,8-TMNO via three possible routes: (a) reacting with C2H2 with simultaneous formation of C2H4; (b) reacting with C2H4 with simultaneous formation of C2H6; (c) decomposition with simultaneous formation of H2. The dominant routes are probably (a) and (b) as evidenced by the formation of C2H4 and a large amount of C2H6 (Table S4). To verify the conversion of C14 intermediate to 3,3,6,8-TMNO, DFT simulation was performed. The optimal configurations of each substance involved in the reactions are shown in Figures S1−S3 in the Supporting Information, and the potential energy curves of the three reaction pathways are presented in Figure 4.

4. CONCLUSIONS It is found for the first time that methyl-substituted benzenes and methyl-substituted naphthalenes can be synthesized from acetone and CaC2 in one pot via tandem reactions. At the optimized conditions, the carbon yield of methyl-substituted benzenes and methyl-substituted naphthalenes reaches 48% along with 20% 3,5-xylenol. In this protocol, isophorone (IP) and 3,4-dihydro-3,3,6,8-tetramethylnaphthalen-1(2H)-one (3,3,6,8-TMNO) are the important observable intermediates. IP transforms to mesitylene and 3,5-xylenol while 3,3,6,8TMNO transforms to 2-methylnaphthalene (2-MN) via 2,2,7trimethyl-1,2,3,4-tetrahydronaphthalene (2,2,7-TM-THN), which is evidenced by DFT simulation. 2-MN further reacts with mesitylene to form 2,6-dimethylnaphthalene (2,6-DMN) and 2,3,6-trimethylnaphthalene (2,3,6-TMN) along with pxylene (PX). In this process, CaC2 plays two roles: promoting acetone condensation by its superior nucleophilicity and participating in aromatization reaction to form methylsubstituted naphthalenes via calcium acetylide. In comparison with the known multistep acetone conversion route, the one-

Figure 4. Potential energy curves of C14 intermediate to 3,3,6,8TMNO via three pathways in Scheme 3. Route a, reacting with C2H2; route b, reacting with C2H4; route c, decomposition. G


Article

Industrial & Engineering Chemistry Research Scheme 4. Plausible Synthesis Pathway of Methyl-Substituted Naphthalenes from 3,3,6,8-TMNO

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the Major State Basic Research Project (No. 2011CB201306) and Prof. Z. Conrad Zhang for valuable suggestions.

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Figure 5. Potential energy curve of 3,3,6,8-TMNO decomposition to 2,2,7-TM-THN and CO.

pot protocol developed in this work is simpler, the temperature is lower, and methyl-substituted naphthalenes are additionally produced. This protocol also opens up a new safe route for in situ application of CaC2 in organic synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00175. Quantification calculation of gas products; mass spectra of various liquid products; carbon balance and product selectivity in the reaction of acetone and CaC2; results of acetone conversion in flow reactors reported in the literature; amount of gas product produced at different reaction conditions and CaC2 loading; optimal configuration of each substance involved in the three reaction pathways of C14 intermediate to 3,3,6,8-TMNO (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenyu Liu: 0000-0002-3525-273X Qingya Liu: 0000-0003-0354-9026 H


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One-Pot Synthesis of Methyl-Substituted Benzenes and Methyl

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