Zn Bimetallic Oxide Catalyst for Epoxidation of Styrene by Cumene

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Kinetics, Catalysis, and Reaction Engineering

La/Zn Bi-metallic Oxide Catalyst for Epoxidation of Styrene by Cumene Hydroperoxide: Kinetics and Reaction Engineering Aspects Sudip Das, Amanraj Gupta, Dheerendra Singh, and Sanjay M Mahajani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05538 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Industrial & Engineering Chemistry Research

La/Zn Bi-metallic Oxide Catalyst for Epoxidation of Styrene by Cumene Hydroperoxide: Kinetics and Reaction Engineering Aspects Sudip Das, Amanraj Gupta, Dheerendra Singh, Sanjay Mahajani* Chemical Engineering Department, Indian Institute of Technology Bombay, Mumbai, India 400076. ABSTRACT

Styrene oxide (STOX) is mostly produced by chlorohydrin process that generates large amount of waste. A greener alternative to chlorohydrin process is epoxidation of styrene induced by hydroperoxides. In this work, we have studied this reaction using cumene hydroperoxide (CHP) as oxygen inducing reagent under the influence of the prepared

La-Zn bi-metallic oxides, which offered reasonably high catalytic activity in terms of conversion and also high selectivity for styrene oxide. The side reactions can be suppressed by a proper choice of reaction conditions and mode of reaction. At a given reactant conversion, semibatch modes of operation exhibits higher styrene and CHP based product selectivity when compared with its batch counterpart. Parametric studies are performed to examine the effects of temperature (ambient temperature – 363 K), catalyst loading (0.25-1%), CHP-to-Styrene mole ratio (~1-3.5), presence of non

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reacting solvent and surface basicity of catalysts. Catalyst deactivation was studied in detail and a plausible way to regenerate the spent catalyst has been advocated. A suitable kinetic model is proposed that explains the trends in product yield, byproducts formation and catalyst deactivation. Parameters are estimated by non linear regression along with 95% confidence interval calculated by bootstrapping technique.

KEYWORDS: Styene oxide, epoxidation, cumene hydroperoxide, solid base catalyst, kinetics *Corresponding author: Tel.: +9122 2576 7246. Fax: +9122 2572 6895. E-mail: [email protected]

INTRODUCTION

Styrene oxide (STOX) is an industrially important intermediate with significant usage in pharmaceuticals, plastic additives, resins and majorly in flavors and fragrances [1, 2]. The chemistry of industrial production of STOX has changed over the years. First, it was manufactured by peracetic acid [3] route which involves a very costly reagent, acetic anhydride. It has been reported that approximately 8 kgs of acetic anhydride is needed to produce 1 kg of styrene oxide, leading to very low yields. Of late, chlorohydrin route [3-5] has become more profitable as it has better product yields but it involves corrosive compounds like HCl, Cl2 and produces a huge amount of chloride salt as waste. Use of


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hydrogen peroxide as epoxidizer [6] is a cleaner alternative but has its own disadvantages because hydrogen peroxide has a narrow operating range due to its thermal instability at higher temperature and less activity at lower temperature. Furthermore, the inevitable presence of water in the system leads to polyol formation thereby dwindling product selectivity. Strict enforcement of environmental laws in manufacturing hubs like China and India has necessitated the lookout for an alternative route to produce STOX which is clean, atom efficient, sustainable and financially feasible.

Organic hydroperoxide is a relatively unstable oxidized form of the corresponding alkane and acts as an efficient oxygen inducing reagents (OIRs) [7]. Shell’s SMPO process [8] and Sumitomo’s PO [9] process use aralkyl hydroperoxides of ethylbenzene (EBHP) and cumene (CHP), respectively, to epoxidize propylene. Tert-butyl hydroperoxide has also been tested as OIR for styrene to produce styrene oxide under the influence of various metal oxides as catalysts [10, 11]. Aralkyl hydroperoxides have also been used as OIRs of various alkenes like 1-octene [12], cyclohexene [13] etc. to produce corresponding epoxides. The route is clean, especially when the alcohol formed as a co-product can be converted back to a commercially important product.

Epoxidation of styrene by CHP in reasonably high yields gives rise to a new and green route towards the production of styrene oxide as shown in Figure 1. Cumene


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hydroperoxide is produced by free radical driven oxidation of cumene (Step 1). Also, the cumyl alcohol produced as a co-product of styrene oxide (desired product) in Step 2 can be converted back to cumene via hydrogenation. Cumene produced in Step 3 is mixed with the unconverted cumene in Step 2 before recycling back.

Likewise, the

unconverted styrene from Step 2 can be recycled back to its feed. The waste generation from this route is expected to be much lower than conventional chlorohydrin process. While the Step 1 and Step 3 are established industrial processes, there is no prior literature available on epoxidation of styrene using CHP as oxygen inducing agent.

Figure 1. Proposed green chemistry for styrene oxide production.


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It may be noted that there are several side reactions that include polymerization of styrene and thermal dissociation of CHP (Figure 2), which make this study interesting from the point of view of obtaining higher selectivity towards the desired product i.e. styrene oxide. Hence, proper decisions on the type of catalyst, reaction conditions and the type of reactor are important. Styrene tends to polymerize under the reaction conditions and deactivate the catalyst. Literature is replete on the studies in free radical based styrene polymerization [14]. CHP has been reported to be used as initiator for styrene polymerization in earlier works. On the other hand, the dissociation energy of the peroxy bond in CHP is more in the mother compound cumene (~31 kcal/mol) than in styrene (~20 kcal/mol) [15]. Furthermore, thermal decomposition of CHP happens even under ambient environment wherein, it dissociates into numerous products like cumyl alcohol, acetophenone (ACP), alpha methyl styrene (AMS) via free radical mechanism. Styrene polymerization and thermal decomposition of CHP have been reported to be induced by identical free radicals [16, 17]. Fundamentally, to obtain epoxide of styrene, it must be reacted with an OIR, which is CHP in this case. Hence understandably, there lies a symbiotic interaction between the reactants that would cause simultaneous side reactions. To induce the oxygen donation from hydroperoxides, basic catalysts like oxides of lanthanum (La) have been used in earlier works [18]. In our previous work, the oxide of La was reported to be combined with oxide of Zinc (Zn) to form La-Zn bimetallic oxide (LZ) that exhibits more surface area and improvement in the catalytic activity in trans-carbonation of glycerol with di-methyl carbonate [19]. It is reported in


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the literature, based on X-ray photoelectron spectroscopy analysis, that there exists metal−metal interaction between La and Zn that enhances the surface basicity [20].

Figure 2. Epoxidation of styrene and undesired parallel reactions

In this work, LZ catalysts of different La-to-Zn molar ratios are prepared and their catalytic activities are evaluated for styrene epoxidation reaction. The prepared catalysts are characterized by various standard characterization techniques to investigate surface properties and morphology. The epoxidation runs were performed in batch and semibatch modes to explore mitigation of undesired reactions. The catalysts were found to deactivate during the course of the reaction. The root cause of deactivation is identified and the catalyst is successfully regenerated. Furthermore, an intrinsic kinetic model has been proposed that explains the experimental results reasonably well.


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EXPERIMENTAL WORK

Materials Styrene (STY; 99.9% pure), CHP (80% pure, diluted by cumene), zinc nitrate (hexahydrate, 98% pure), 1-butanol (99.5% pure), acetic acid (99% pure), sodium thiosulphate (99% pure), potassium iodide (99.8% pure), triphenyl phosphine (TPP; 99% pure) and analytical grade sodium bicarbonate were procured from Merck Chemicals, India Ltd. Ethanol (99.9% pure), potassium iodide (99.8% pure) and analytical grade acetone were procured from SD Fine-Chem Ltd., and lanthanum nitrate (>99% pure) was obtained from KEM Light Laboratories Pvt. Ltd. Process water was deionized by Milli-Q deionizer. Reagent grade chloride of cobalt, nitrates of strontium and magnesium were obtained from SD Fine-Chem Ltd. and oxide of molybdenum was procured from Merck Chemicals, India Ltd.

Catalyst preparation All the oxide catalysts were prepared by precipitation method wherein, sodium bicarbonate (NaHCO3) is used as a precipitating agent. In a 1 M solution of metal precursor, a saturated solution of NaHCO3 was added at 333 K, and the mixture was stirred at high rpm (~1200). The precipitate thus obtained was filtered by porcelain made Buchner funnel under ~300 mmHg air pressure. Filtered wet powder was


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thoroughly washed by water and acetone, and dried at 373 K for 12 h. The catalyst was calcined at 823 K for 5 h. Zinc and lanthanum bimetallic oxide catalysts with molar ratios of 1:1 and 2:1 were prepared and labeled LZ1 and LZ2, respectively. Molybdenum oxide (Mo2O3) was used as purchased.

Catalyst characterization techniques Techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning

electron

microscopy

(SEM),

thermal-programmed

desorption/thermal-

programmed reduction (TPD/TPR), and N2 adsorption based on Brunauer-EmmettTeller

(BET)

theory,

were

used

for

characterization

of

the

catalysts.

N2

adsorption−desorption isotherms, measured on a 3Flex-3500 1MP Chemi instrument (Micromeritics, USA), provided the surface area. Approximately 0.2 g of powder was degasified at 673 K for 300 min and then used for analysis. For XRD, the catalyst powder was analyzed with an X’Pert diffractometer (PANalytical, UK). The data were collected over a 2θ range of 10−90° using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. A high-resolution field emission microscope (Tecnai G2-F30; Thermo Fisher Scientific, USA) provided TEM analysis using an accelerating voltage of 300 kV. A welldispersed solution was prepared by adding a small amount of catalyst powder to ethanol and sonicating it for 10−15 min followed by drop-casting on a TEM grid and drying under an IR lamp for 30 min. SEM analysis (JSM-7600F; JEOL, USA) was used to determine the surface morphology. The catalyst powder was drop-casted over


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aluminum foil and then pasted over the carbon tape, and images were taken under optimum operating conditions.

A TPDRO instrument (Thermo 1100, Thermo Scientific, USA) using 10% CO2 in helium as a probe molecule for basicity, was used for TPD analysis. The catalysts analyzed for TPD were pretreated with 5% H2/He at a flow rate of 20 mL/min and a temperature ramp rate of 20 K/min up to 623 K. The temperature was then raised to 673 K and maintained for 60 min under an argon flow rate of 20 mL/min. This pretreatment ensured that the metal oxide surface was free from impurities such as chlorides or nitrates. For TPD analysis, CO2 pulses were introduced at room temperature until saturation. The temperature was varied at a rate of 10 K/min over a range of 293−1023 K. To comprehend the nature of deactivation of the catalysts during the course of reactions, FTIR analysis of the spent catalyst was performed in Vertex 80 system coupled with 3000 Hyperion microscope (Bruker, Germany) wherein, samples were pelletized along with potassium bromide.

Reaction procedure and sample analysis The reactions were performed in batch and semi-batch modes as shown in Figure 3. Batch reactions (Mode M1) under different operating conditions were carried out in a 200 mL glass batch reactor coupled with a motor through magnetic drive. The reactor is kept in an electrically heated hot oil bath. For semi-batch operations (M2 and M3), a


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500 mL glass reactor was used in the same setup and liquid feed was introduced to the reactor by peristaltic pump (Masterflex, Cole-Parmer, USA). In M1 mode, both styrene and CHP were taken into the reactor at the beginning of the run. In M2 mode, styrene was kept in the reactor and CHP was added in the reactor continuously with time at a fixed rate. On the other hand, in M3 mode, both styrene and CHP were added continuously in a pool of solvent. A vertical overhead tap-water driven condenser was used for the purpose of reflux in all the experimental runs.

The samples from the reactor were withdrawn using syringe at regular time intervals, immediately stored at ~ 277K and later centrifuged to separate the solid catalyst. After each operation, remaining reaction mixture was discarded and the reactor was washed thoroughly by soap solution, tap water and acetone consecutively, dried in oven and made ready for next operation. Iodometric titration [21] was performed to determine the concentration of hydroperoxide. Moreover, all the liquid samples were treated with an excess of triphenylphosphine to ensure the conversion of hydroperoxides to alcohols [22]. The samples were then diluted with 1-butanol that was used as an external standard and analyzed on a gas chromatograph (Mak Analytica, India) using a DB5 capillary column (Agilent, 30 m × 0.25 mm × 25 μm film thickness). The oven temperature was varied from 323 to 503 K while maintaining both, injector and detector (FID), at 453 K. Furthermore, to have a closer look into the component distribution in samples, GC-MS, Agilent Technologies 7890 gas chromatograph coupled with a time-


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of-flight mass spectrometer (LECO Corporation, USA),

equipped with a 30m long

primary column (Rxi 5-MS, nonpolar in nature) and 1.2m long secondary column (Rxi 17Sil MS, polar in nature), was used. The samples were analyzed in the presence of cyclohexane external standard with identical temperature program used for GC analysis. Every run was repeated at least thrice to check the repeatability of data and the results were found to be consistent with an average standard deviation of Mo2O3>SrO>CoO>ZnO>MgO according to their activity in terms of product yield. The bimetallic LZ catalysts were found to be the best for epoxidation.

The catalytic activities of LZ catalysts were studied at 343K in M1 mode of operation with a STY-to-CHP feed molar ratio of 1.27. Table S3 shows that LZ1 exhibits better activity to styrene epoxidation. Styrene conversion was found to be increasing with an increase in strong basic site concentration of the catalysts. From the results it may be inferred that only strong basic sites are responsible for epoxidation.

Table 1: Comparison of uncatalyzed runs with catalyzed runs in different modes under the influence of 1 wt% catalysts. For batch operations feed molar ratio STY-to-CHP = 1.27, for necessary operations, volumetric flow rate of STY and CHP = 0.15 mL/min.

Catalyst

Mode

T

t

XSTY

XCHP

SSTOX-CHP


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(if any)

(K)

(min)

(%)

(%)

(%)

Uncatalyzed

M1

333

240

5.12

16.95

12.18

LZ1

M1

333

240

9.03

18.10

66.59

Uncatalyzed

M2

363

180

13.48

51.21

0

LZ2

M2

363

180

32.15

55.24

36.59

b. General course of reaction Figure 8 shows the general course of the reaction. Product analysis confirms formation of by-products like acetophenone (ACP) and alpha methyl styrene (AMS) along with cumyl alcohol (CA). The results show that the extents of formation of ACP and AMS were lesser than CA as there are two sources for CA, i.e., epoxidation of styrene and thermal dissociation of CHP. In the following sections, we have presented the effect of different parameters such as catalyst loading, temperature, mole ratio and solvent effect, on the batch reactions. Subsequently, we discuss the influence of mode of operation and catalyst deactivation.

c. Parametric study Effect of catalyst loading In order to understand the effect of catalyst loading on product selectivity, batch runs were performed at 353K at a feed molar ratio of STY-to-CHP of 3.34 at different catalyst


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loadings (0.25, 0.5 and 1% of the total weight of reactants on solvent free basis). Higher catalyst loading is found (Figure 9) to be aiding conversion of both styrene and CHP. Styrene and CHP based product selectivity in the batch reactor were found to be lying between similar brackets of ~15-20% (Figure 9b) and ~50-65% (Figure 9c), respectively. For all the catalyst loadings, CHP based product selectivity decreases slightly with increasing CHP conversion, which indicates that with time, the free-radical based undesired reactions dominate over the rate of desired epoxidation reaction. This is because the parallel side reaction of CHP decomposition takes place even in the absence of catalyst whereas main reaction is hampered due to catalyst deactivation at higher conversion. Hence, for lower catalyst loading the drop is significant compared to that at higher catalyst loading. Styrene based selectivity was also not very much dependent on catalyst loading.


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Figure 8. Product and reactant composition profiles in terms of mole fraction Temperature = 333K, feed molar ratio of STY-to-CHP = 1.27, rotor rpm = 1200; operated in batch mode, 1% LZ1 loading.

Effect of temperature Figure 10 shows the effect of temperature on the reaction. Experiments were performed at three different temperatures (333K, 343K and 353K) under otherwise identical conditions. At lower temperature (333K), product selectivity based on both styrene and

CHP were found to be higher (Figure 10a). However, at higher conversion of CHP, CHP based product selectivity decreases at all temperatures though the rate of decay of selectivity was found to be the most severe at 333K. Higher temperature has an adverse effect on selectivity as rates of both thermal dissociation of CHP (Figure 10b) and styrene polymerization (Figure 10c) are higher at higher temperature.


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Effect of solvent Interestingly, non-reacting solvent (e.g. toluene) was found to play an important role and its effect is shown in Figure 11. The reactant conversions were found to be less in the presence of solvent due to dilution effect. Moreover, styrene based product selectivity (Figure 11a) was found to be increasing unlike in the case of CHP as the di-mer, trimers or oligomers of styrene are mostly soluble in toluene. Experiments were performed with a reaction mixture in which toluene (TOL) was taken in a molar ratio of 1 and 3 with respect to styrene and it can be seen from the results that the product selectivity increased with increasing proportion of toluene. Since oligomerization is a higher order reaction, dilution effect enhances styrene based selectivity. CHP based selectivity also increases (Figure 11b) due to reduced rate of decomposition in the presence of inert.


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Figure 9. Effect of catalyst loading studied at operating temperature = 353K, Feed molar ratio of STY-to-CHP = 3.34, rotor rpm = 1200. (a) conversion of CHP vs time; (b) CHP based product selectivity vs conversion of CHP, and (c) styrene based product selectivity vs conversion of styrene


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Figure 10. Effect of temperature studied at operating Feed molar ratio of STY-to-CHP = 1.27, rotor rpm = 1200, under the influence of 1% LZ1 loading. (a) reactant conversion vs time; (b) CHP based product selectivity vs conversion of CHP, and (c) styrene based product selectivity vs conversion of styrene

Effect of mole ratio In order to study the effect of feed molar ratio (FMR) on epoxidation reaction performance, batch runs were performed at STY-to-CHP molar ratios of 1.27 and 3.34. From Figure 12a, it can be seen that at higher FMR, CHP acts like a limiting reagent


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and hence conversion of CHP increases, although CHP based product selectivity decreases with increasing styrene concentration (Figure 12b). As we know, the higher concentration of styrene promotes the decomposition of cumene. On the other hand, styrene based product selectivity (Figure 12c) decreases with increase in STY-to-CHP molar ratio as higher order side reactions dominate over epoxidation at higher styrene concentration.

Figure 11. Effect of use of non-reacting solvent toluene performed at 343K operating Feed molar ratio of STY-to-CHP = 1.27, rotor rpm = 1200, under the influence of 1% LZ1 loading. (a) styrene based product selectivity vs conversion of styrene, and (b) CHP based product selectivity vs conversion of CHP

d. Modes of operation In order to compare different modes of operation, representative runs were performed at 353 K in the presence of 1 % loading of LZ1 catalyst and toluene with TOL:STY = 1. In M1 mode, the representative run was operated at a feed molar ratio of STY-to-CHP of 1.27 for 6 hours. In M2


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mode, same amount of styrene as in M1 was taken and CHP was fed into the system at a rate of 0.15 mL/min till net CHP added in the system was equal to the moles of CHP taken in M1 mode. In M3 mode, same amount of CHP was taken and both the reactants were added into the reactor at a rate of 0.15 mL/min. Figure 13 compares the results of different modes of operation. From the results it is evident that in M1 mode, while the rate of reaction is relatively high, the styrene based product selectivity and the CHP based product selectivity lied only between 20-40% and 30-40%, respectively. To increase the product selectivity by decreasing concentration of one or both the reactant(s), semi-batch operations of M2 and M3 modes were introduced. In M2 mode, higher concentration of styrene and very high molar ratio of styrene to CHP resulted in a sharp drop in styrene based product selectivity (Figure 13c) as at this condition, CHP acted as an initiator to styrene polymerization. In M3, both the reactants are added in semibatch mode and hence the lower concentration of both the reactants, at any given time, resulted in lower reaction rate. As mentioned before, the presence of styrene in excess (in M1 and M2) decreases the bond dissociation energy of peroxy bond of CHP that leads to generation of free radicals, and thus the rate of undesired reactions increase. Hence, the CHP based selectivity was the highest for M3 among all the modes of operation. Interestingly, the CHP based product selectivity was found to be increasing with CHP conversion (Figure 13d) in M3 mode whereas it drops in the case of M1 and M2 modes of operation. Hence, it may be concluded that one can save both styrene and CHP from converting them to less valuable products at the cost of lower reaction rates in M3 mode of operation.


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Figure 12. Effect of feed molar ratio of STY-to-CHP studied at operating temperature = 333K, rotor rpm = 1200, under the influence of 1% LZ1 loading. (a) conversion of CHP vs time; (b) CHP based product selectivity vs conversion of CHP, and (c) styrene based product selectivity vs conversion of styrene

e. Catalyst reusability It was observed that the catalyst has a tendency to deactivate significantly even during the course of the reaction. To investigate this aspect, we performed the catalyst reusability test on LZ1 catalyst. The catalyst recovered from a batch run is dried for 6 h in an oven at 373 K and tested for its activity by contacting with the fresh reactants


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under the conditions of interest. Figure 14 shows the results for four such consecutive runs. Conversion of styrene and selectivity are found to decrease significantly after every run.

It is anticipated that the reduced activity of the reused catalyst results from the deposition of the products, especially the oligomers of styrene, on active sites on catalyst surface. Thorough toluene wash was found to be beneficial to increase the catalytic activity; it could only reach ~88% of the fresh activity in term of styrene conversion. On the contrary, re-calcining the spent catalyst at 773 K for 6 hrs could reclaim ~100% activity in terms of conversions of styrene and CHP, and the CHP based product selectivity. To understand the reason of deactivation, FTIR responses were examined for the spent and re-calcined LZ1 catalysts. The results are shown in Figure 15, which indicate the presence of alkene and cyclic alkyl C-H; vinylidene and monodistributed C=C. Furthermore, it can be inferred that the depositions on catalysts sites can be reclaimed by calcination.

REACTION KINETICS

A complex reaction scheme like the one studied in this work and the trends obtained have to be captured in a comprehensive kinetic model that explains all the trends and


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that can be used for optimization to achieve the conditions for the best possible yields. The governing equations for a batch/semibatch reactor may be written as follows,

𝑑𝐶𝑖 𝑑𝑡 𝑑𝑉 𝑑𝑡

= 𝑟𝑖 +

𝐹𝑖,𝑖𝑛 ― 𝐶𝑖𝑣𝑖 𝑉

= ∑𝑣𝑖

(8) (9)

Where, 𝐶𝑖 is the concentration (mol/L); 𝑡 is time (min); 𝑟𝑖 is the rate of production (mol/Lmin); 𝑉 is the reaction volume (L); 𝐹𝑖,𝑖𝑛 is the inlet molar flow rate (mol/min)– it is zero for the batch reactor. 𝑣𝑖 is the volumetric flow rate (L/min).

The reactions were performed at different speeds of agitation over a range ~800 to ~1300 rpm and it was found that, beyond 1200 rpm, there is no further effect of agitation on the rate of reaction showing no external mass-transfer limitations. In this reaction, the catalyst was taken in powder form with a particle size of ~200 nm, for which the value of the Weisz−Prater parameter is much less than 1, indicating that internal diffusion limitations can also be neglected. The


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Figure 13. Comparative study on different modes of operation. For all, temperature 353 K and 1% LZ1 loading, T:STY = 1; For M1, STY:CHP = 1.27; For M2, NSTY = 0.1747; vCHP = 0.15 mL/min; For M3, vSTY = vCHP = 0.15 mL/min.

(a) conversion of styrene vs time; (b) conversion of CHP vs time; (c) styrene based product selectivity vs conversion of styrene; and (d) CHP based product selectivity vs conversion of CHP.


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Figure 14. Reusability of the LZ1 catalyst at 343K with a catalyst loading of 1.0 wt % in 360 min and a feed molar ratio of STY-to-CHP of 1.27:1.


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Figure 15. FTIR responses of fresh, spent and re-calcined LZ1 catalysts.

reactions happening in the reactor are classified as styrene polymerization (eq. 10-12), styrene epoxidation (eq. 13-16) and thermal dissociation of CHP (eq. 17-27). Styrene polymerization is a free radical driven reaction that produces dimers, trimers and oligomers of styrene in the operating range of temperature [23]. The rate of styrene polymerization (𝑟1) can be expressed as,

(10)

𝑟1 = 𝑘1𝐶𝑛𝑆𝑇𝑌𝜉

(

𝐶𝐶𝐻𝑃𝜃

Where, 𝜉 = 𝑒𝑥𝑝 ― 𝐶𝑆𝑇𝑌 𝑇 𝑘1 = 𝑘0,1𝑒𝑥𝑝

(

― 𝐸𝐴,1 𝑅𝑇

)

(11)

)

(12)

𝜃 and 𝑛 being constants, the term 𝜉 has been incorporated into styrene polymerization rate to take care of the fact that when 𝐶𝐻𝑃 is used as initiator to styrene polymerization, the rate of polymerization decreases exponentially with increasing 𝐶𝐻𝑃 concentration, and gradient of rate of polymerization increases with increasing temperature [24]. Following the Langmuir-Hinshelwood theory, rate of epoxidation reaction (𝑟2) can be expressed as,

(

𝑟2 = 𝑘0,2𝑒𝑥𝑝

―𝐸𝐴,2 𝑅𝑇

)(

𝐶𝛼𝑆𝑇𝑌𝐶𝛽𝐶𝐻𝑃 1 + 𝐾𝑆𝑇𝑌𝐶𝑆𝑇𝑌 + 𝐾𝐶𝐻𝑃𝐶𝐶𝐻𝑃)𝛾

𝜑

(13)


32

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(

𝐾𝑖 = 𝐾0,𝑖𝑒𝑥𝑝

―𝛥𝐻𝑎𝑑,𝑖 𝑅𝑇

)

(14)

Where, 𝜑 is the fractional activity that has been incorporated into the rate expression to take care of deactivation of catalyst during the course of epoxidation. The different orders of deactivation were tried in otherwise identical modeling environment and second order deactivation was found to give the best fit. For second order deactivation, the fractional activity can be expressed as, 1

(15)

𝜑 = 1 + 𝑘𝑑𝑡 where, 𝑘𝑑 is the deactivation rate constant and follows Arrhenius equation, 𝑘𝑑 = 𝑘0,𝑑𝑒𝑥𝑝

―𝐸𝑑

( )

(16)

𝑅𝑇

Various possible steps of free radical driven thermal decomposition of cumene hydroperoxide [25] and dicumyl peroxide [26] take care of the formation of byproducts like CA, AMS and ACP. Table 2 gives the proposed mechanism of all the possible reactions in CHP decompostion, and corresponding rate expressions of each step that assumes all the steps to be elementary in nature.

Table 2: Rate expressions of the reaction steps involved in thermal dissociation of CHP

Reaction step

CHP  R1 + R2

Rate expression (𝒓𝒋, j=3 to 13) 𝑘0,3𝑒𝑥𝑝

(


).𝐶

𝐶𝐻𝑃

(17)


33


―𝐸𝐴,4

(

).𝐶

2CHP  R1 + R3 + H2O

𝑘0,4𝑒𝑥𝑝

R1 + CHP  CA + R3

𝑘0,5𝑒𝑥𝑝

(

―𝐸𝐴,5

R1 + CHP  CA + R4

𝑘0,6𝑒𝑥𝑝

(

―𝐸𝐴,6

R1  ACP + R5

𝑘0,7𝑒𝑥𝑝

(

―𝐸𝐴,7

R4  AMS + R6

𝑘0,8𝑒𝑥𝑝

(

―𝐸𝐴,8

CA + CHP  2R1 + H2O

𝑘0,9𝑒𝑥𝑝

(

2R5  C2H6

𝑘0,10𝑒𝑥𝑝

(

―𝐸𝐴,10

R3  AMS + R6


(

―𝐸𝐴,11

4R6  2W + 3O2


(

―𝐸𝐴,12

R2 + CHP  H2O + R3


𝑅𝑇

2 𝐶𝐻𝑃

(19)

).𝐶

(20)

).𝐶

(21)

).𝐶

(22)

𝑅𝑇

𝐶𝐻𝑃𝐶𝑅1


𝑅1

𝑅𝑇

𝑅4

𝑅𝑇

(18)

).𝐶

𝑅𝑇


).𝐶

𝑅𝑇

𝑅𝑇

(

Page 34 of 43

𝑅𝑇

𝐶𝐻𝑃𝐶𝐶𝐴

).𝐶

2 𝑅5

).𝐶

𝑅3

).𝐶

4 𝑅6


).𝐶


(23) (24) (25) (26) (27)

𝑅1 : 𝐶6𝐻5𝐶𝑂(𝐶𝐻3)2, 𝑅2 : 𝑂𝐻, 𝑅3 : 𝐶6𝐻5𝐶𝑂𝑂(𝐶𝐻3)2, 𝑅4 : 𝐶6𝐻5(𝐶𝐻3)(𝐶𝐻2)𝐶𝑂𝑂𝐻, 𝑅5 : 𝐶𝐻3, 𝑅6 : 𝑂2𝐻.

From the pseudo steady state assumption [27] on all the intermediate radicals, we can further express the rates of production of all components in reaction mixture as,

𝑟𝑆𝑇𝑌 = ― (𝑟1 + 𝑟2)

(28)

𝑟𝐶𝐻𝑃 = ― (𝑟1 + 𝑟2)

(29)

𝑟𝑆𝑇𝑂𝑋 = ( 𝑟2)

(30)

𝑟𝐶𝐴 = (𝑟2 + 𝑟5 + 𝑟6 ― 𝑟9)

(31)


34

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𝑟𝐴𝐶𝑃 = (𝑟7)

(32)

𝑟𝐴𝑀𝑆 = (𝑟8 + 𝑟11)

(33)

The set of ordinary differential equations (eq. 8-33) was solved simultaneously by Levenberg− Marquardt algorithm based non-linear regression analysis in MATLAB. The kinetic parameters are estimated by minimizing the sum of squares of error by comparing the calculated values of the moles of different components present in the system to experimentally obtained values and tabulated in Table 3. The confidence intervals (CI) of estimated parameters were calculated by bootstrapping technique [28] and were found to be well within the permissible limits.

Figure 16. Parity plot of experimental and calculated number of moles


35


Page 36 of 43

of components present in reaction mixture

The selectivity trends obtained in the experimental runs performed under different modes (M1, M2 and M3) can be seconded by the kinetic model and the estimated kinetic parameters. As the styrene polymerization and thermal decomposition of CHP are of higher order than the epoxidation reaction, lowering the concentration of reactants by operating in M3 mode, has rightfully aided the STOX selectivity (see section SI2 of Supporting Information).

Numbers of moles of all the constituents of liquid reaction mixture at different times were calculated using estimated parameters and the calculated values are compared with their experimentally generated counterparts. Figure 16 shows the parity plot of calculated and experimentally found numbers of moles indicating a sufficiently good fit. Here, the experimental data considered for the comparison is from all the modes of operation. Now that we have a comprehensive kinetic model that takes care of all the effects, one can use it in future for the optimization of yield in a given batch/semibatch/continuous reactor.

CONCLUSION


36

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La-Zn bimetallic oxide is a promising catalyst for styrene epoxidation with cumene hydroperoxide.

The catalyst with equal molar proportion of lanthanum and zinc (LZ1)

offers reasonably high conversion and selectivity. It appears that the activity for styrene oxide formation varies proportionally with the number of strong basic sites due to the presence of La while, Zn provides higher surface area. The performance of the LZ1 catalyst was evaluated in detail. It was reused three times, and the activity decreased during each cycle because of adsorption of the carbonaceous side products on the active sites. However, calcination reclaimed the activity of the catalyst. The rate of cumene hydroperoxide (CHP) conversion increased with temperature, catalyst loading and CHP-to-styrene (STY) molar ratio. Styrene based product selectivity for a given conversion (~10%) is found to be much higher in semi-batch operations (maximum selectivity ~80%) than the batch counterpart (maximum selectivity ~10%).

Moreover, CHP based product selectivity for a given conversion (~40%) is also found to be much higher in semi-batch operations (maximum selectivity ~60%) than the batch counterpart (maximum selectivity ~40%) as both the reactants undergo higher order side reactions. A kinetic model, which considers the free radical based side reactions along with main epoxidation reaction, catalyst deactivation of second order and most significantly, the symbiotic interaction between the reactants STY and CHP was proposed. The model is able to explain the experimental data of batch (M1) and semibatch modes of operation (M2 and M3).


37


Table 3. Estimated kinetic parameters and confidence interval (CI) for styrene epoxidation under the influence of LZ1 catalyst (i = 1-13)

0.091 ± 0.55(*)

1.376 ± 1.12

K0,STY x 10-10

0.131 ± 7.02

0.005 ± 0.001

0.714 ± 1.02

ΔHad,STY x 10-5 (J/mol)

0.986 ± 0.001

1.558 ± 0.02

0.883 ± 0.005

K0,CHP x 10-10

0.033 ± 1.02

0.016 ± 2.08

0.820 ± 0.15

ΔHad,CHP x 10-5 (J/mol)

0.998 ± 0.008

0.249 ± 1.02

0.999 ± 0.11

kd x 10-10

0.343 ± 1.02

0.252 ± 0.002

0.998 ± 2.09

Ed x 10-5 (J/mol)

1.083 ± 0.003

0.869 ± 5.01

n

2.98 ± 4.02

0.998 ±1.03

𝜃

0.99 ± 1.09

0.746 ± 0.0009

α

1.96 ± 0.02

0.854 ± 0.002

β

1.03 ± 1.22

0.031 ± 0.14

1.001 ± 0.001

γ

2.02 ± 0.01

0.419 ± 2.02

1.133 ±1.02

0.031 ± 8.32

0.998 ± 1.98

0.611 ± 0.0005 0.013 ± 6.32 0.008 ± 0.01 0.018 ± 0.87

EA,i x 10-5 (J/mol)

k0,i x 10-10 (**)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

*

Estimated parameter ± 0.95 CI (in

%) **

i = 1-13

ACKNOWLEDGEMENT

The authors gratefully acknowledge UAY initiative by Ministry of human resource and development, Govt. of India for financial support; SAIF, IIT Bombay, for their help in the catalyst characterization; Jainesh Jhaveri for his assistance in TPD analysis; Sumit Kamal and Mohammad Khan for their support in BET analysis.


38

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39


Page 40 of 43

NOMENCLATURE

𝐶𝑖

Concentration of i-th component [mol/L]

𝐸𝐴,𝑖

Activation energy of i-th reaction [J/mol]

𝐸𝑑

Decay activation energy [J/mol]

𝐹𝑖,𝑖𝑛

Molar flow rate of i-th component [mol/min]

𝛥𝐻𝑎𝑑,𝑖

Heat of adsorption of i-th component [J/mol]

𝐾0,𝑖 𝑘0,𝑖

Pre-exponential/frequency factor of i-th reaction

𝑁𝑖,𝑡

No of moles of i-th component at any time = t

𝑁𝑖,0

No of moles of i-th component at t = 0

𝑁𝑖,𝑛𝑒𝑡 𝑟𝑖 𝑟𝑘,𝑘 = 1,2,…

Total no of moles of i-th component Rate of production of i-th component [mol/L-min] Rate of k-th reaction [mol/L-min]

𝑅

Universal gas constant [=8.314 J/mol-K]

𝑆𝑗,𝑖

Selectivity of j-th component in reference with i-th component

𝑇

Temperature [K]

𝑉

Reaction volume [L]

𝑣𝑖

Volumetric flow rate of i-th component [L/min]

𝑋𝑖

Conversion of i-th component

𝑛,𝛼, 𝛽, 𝛾, 𝜃

Constants

ABBREVIATIONS


40

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SEM Scanning electron microscopy

ACP Acetophenone AMS Alpha methyl styrene

STOX Styrene oxide

BET Brunauer−Emmett−Teller

STY Styrene

CA Cumyl alcohol

TEM Transmittance electron microscopy

CHP Cumene hydroperoxide

TOL Toluene

FID Flame ionization detector

TPD Temperature-programmed desorption

FMR Feed molar ratio

TPP Triphenyl phosphine

OIR Oxygen inducing reagent

XRD X-ray diffraction

SUPPORTING INFORMATION Tables S1, S2 and S3 under section SI1 wherein surface analysis of LZ catalysts and comparison of catalytic activity of different catalysts have been tabulated. Section SI2 elucidates how the proposed kinetic model and estimated parameters are in line with experimentally obtained trends.

REFERENCES (1) Hibbert, H.; Burt, P. Styrene oxide. Org. Synth. 1928, 8, 102 103. (2) Clayton, G.D.; Clayton, F.E. Eds. Patty's Industrial Hygiene and Toxicology (3rd Ed). Wiley, New York. 1981. (3) Frisch, H.R.; Synthesis of styrene oxide. U.S. patent 2,582,114, January 8, 1952. (4) Handley, M.F.; Production of styrene oxide, U.S. patent 2,776,982, January 8, 1957. (5) Richey, W. F. Encyclopedia of Chemical Technology, (4th Ed); Wiley: New York, 1994. (6) Chiappe, C.; Sanzone, A.; Dyson, P.J. Styrene oxidation by hydrogen peroxide in ionic liquids: the role of the solvent on the competition between two Pd-catalyzed processes, oxidation and dimerization. Green Chem., 2011, 13, 1437 1441. (7) Buijink, J.K.F.; Lange, J.P.; Bos, A.N.R.; Horton, A.D.; Niele, F.G.M. Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis, Elsevier B.V., 2008.


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(8) Van Der Sluis, J.J. Process for the preparation of styrene and propylene oxide, U.S. patent 6,504,038, January 7, 2003. (9) Tsuji, J.; Uchida, K.; Oku, N.; Tamura, M. Ishino, M. Process for producing propylene oxide. U.S. patent 5,723,637, March 3, 1998. (10) Patil N. S.; Uphade B. S.; Jana P.; Bhargava S. K.; Choudhary V.R. Epoxidation of styrene by t-butyl hydroperoxide over gold supported on Yb2O3 and other rare earth oxides. Chem. Lett. 2004, 33, 400 401. (11) Patil, N. S.; Uphade, B. S.; Jana, P.; Choudhary, V.R.; Bharagava, S.K. Epoxidation of styrene by anhydrous t-butyl hydroperoxide over reusable gold supported on MgO and other alkaline earth oxides. J. Catal. 2004, 223, 236 239. (12) Barrio, L; Campos-Martín J.M.; Pilar de Frutos, M.; Fierro, J.L.G. Alkene epoxidation with ethylbenzene hydroperoxides using molybdenum heterogeneous catalysts. Ind. Engg. Chem. Res. 2008, 47, 8016 8024. (13) Harrod, J.F.; Hilaire M.S.; Knight, A.R.; Mcintyre, J.S. Catalyst for epoxidation process. US patent 3696052, October 3, 1972. (14) Khuong, K.S.; Jones, W.H.; Pryor, W.A.; Houk, K.N. The Mechanism of the Self-Initiated Thermal Polymerization of Styrene. Theoretical Solution of a Classic Problem. J. Am. Chem. Soc. 2005, 127, 1265 1277. (15) Fordham, J. W. L.; Williams, H.L. The thermal decomposition of cumene hydroperoxide in relation to certain aspects of emulsion copolymerization. Can. J. Res. 1949, 27, 943 960. (16) Walling, C; Chang, Y. Chain Transfer in the hydroperoxide initiated polymerization of styrene. J. Am. Chem. Soc. 1954, 76, 4878 4883. (17) Tobolsky, A.V.; Matlack, L.R. Cumene hydroperoxide‐initiated radical polymerization. J. Polym. Sci. 1961, 55, 49 56. (18) Choudhary, V.R.; Jha, R.; Jana, P. Epoxidation of styrene by TBHP to styrene oxide using barium oxide as a highly active/selective and reusable solid catalyst. Green Chem. 2006, 8, 689 690. (19) Singh, D.; Reddy, B.; Ganesh, A.; Mahajani, S. Zinc/Lanthanum mixed-oxide catalyst for the synthesis of glycerol carbonate by transesterification of glycerol. Ind. Eng. Chem. Res. 2014, 53, 18786 18795. (20) Yan, S.; Salley, S. O.; Ng, K. Y. S. Simultaneous transesterification and esterification of unrefined or waste oils over ZnO− La2O3 catalysts. Appl. Catal., A. 2009, 353, 203 212. (21) Li, J.; Shi, Y.; Xu, L .; Lu, G; Selective oxidation of cyclohexane over transition-metal-incorporated HMS in a solvent-free system. Ind. Eng. Chem. Res., 2010, 49, 5392 5399. (22) Hiatt, R.; Smythe, R. J.; McColeman, C; The reaction of hydroperoxides with triphenylphosphine. Can. J. Chem. 1971, 49, 1707 1711.


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(23) Mayo, F. R.; Chain transfer in the polymerization of styrene: The reaction of solvents with free radicals. J. Am. Chem. Soc. 1943, 65, 2324 2329. (24) Vanderhoff, B., M., E.; On the mechanism of emulsion polymerization of styrene. III. Polymerization initiated by oil‐soluble compounds. J. Polym. Sci., 1960, 48, 175 188. (25) Somma, I. D.; Andreozzi, R.; Canterino. M.; Caprio, V.; Sanchirico, R.;; Thermal decomposition of cumene hydroperoxide: Chemical and kinetic characterization. AIChE J. 2008, 54, 1579 1584. (26) Somma, I. D.; Marotta, R.; Andreozzi, R.; Caprio, V.; Dicumyl peroxide thermal decomposition in cumene: development of a kinetic model. Ind. Eng. Chem. Res., 2012, 51, 7493 7499. (27) Fogler, H. S.; Elements of chemical reaction engineering (4th Ed), Upper Saddle River, NJ : Prentice Hall PTR, 2006. (28) DiCiccio, T., J.; Efron, B.; Bootstrap Confidence Intervals, Stat. Sci. 1996, 11, 189 228.


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Zn Bimetallic Oxide Catalyst for Epoxidation of Styrene by Cumene

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