Thermodynamic Insights into Valorization of Biomass-Derived

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Thermodynamic Insights into Valorization of Biomass-Derived Oxygenates and Reconciliation with Experimental Study Gul Afreen,†,# Tanmoy Patra,†,‡,# and Sreedevi Upadhyayula*,† †

Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India Department of Chemistry, University of Delhi, Delhi 110007, India

‡

S Supporting Information *

ABSTRACT: Thermodynamic equilibria for the promising model alkylation reaction between iso-propanol and m-cresol were investigated using the Gibbs free energy minimization method based upon different product distribution models and physical-property measurements. The simulations were performed in the temperature range of 200−700 °C under atmospheric pressure and a reactant molar ratio range of 0.5:1 to 5:1. The optimum reaction temperature to obtain the maximum yield of the desired product thymol was found to be 250−275 °C at 5:1 iso-propanol/m-cresol molar ratio. Thermodynamic results were found to be in good agreement with the catalytic activity study in the temperature range of 210−300 °C using different nanocrystalline zinc aluminate spinel (ZnAl2O4, ZAL) catalysts. Thymol was obtained as the major product of the reaction with 79.5% selectivity at the highest m-cresol conversion of 85.7% at 270 °C for ZAL-III catalyst. The theoretically obtained thermodynamic limitations in correlation with the experimental studies were used to minimize the side products under optimized reaction conditions. These investigations provide new insights into the alkylation of biomass-derived oxygenates obtained as a major fraction in bio-oils by catalytic C−C coupling reactions. Hence, selective production of C10−C13 range fuel precursors and reduction of carbon fraction loss during hydrodeoxygenation can be achieved. efficient production of C10-C13 range fuel precursors.14,15 The desired product is mainly 2-iso-propyl-5-methyl phenol (IMP), also known as thymol, used widely in fuels, food packaging, pharmaceuticals, rubber, and the paint industry.16,17 Several side products such as benzene ring side-chain isomers of thymol, ether, dialkylated products, iso-butylated products, and thermally isomerized as well as dehydrated products from alcohol are also formed in significant amount in this reaction under different reaction conditions. Several experimental studies have been reported for the efficient alkylation over different functionalized catalysts to optimize the catalytic activity with 1:1 to 5:1 iso-propanol to mcresol molar ratio in the temperature range of 200−400 °C under atmospheric pressure by minimizing the side products for selective production of thymol.11,12,16−21 The results showed that the reported conversion of m-cresol varied depending on the surface acidity, porosity, texture, and other physicochemical properties of the catalysts such as Fecontaining Cr, Si, and K-oxides (17%), Al-MCM-41 (80%), and Mg−Al hydrotalcites (40%), respectively. The earlier literature reports are mostly concerned with the tuning of the

1. INTRODUCTION Because of the rapid depletion of fossil fuels, the worldwide supply of energy has been considered a major industrial concern in last few decades.1 However, alternative renewable energy resources are limited and the technology for their largescale applications is still under development.1−4 As a promising prospect, biomass conversion to biofuels and value added chemicals has evolved immensely in the past decade. Pyrolysis of the biomass results in the formation of bio-oils which are composed of phenolic compounds and small oxygenates, among other compounds.4−7 Prior to the high temperature and high pressure hydrodeoxygenation step, the valorization of these components to C10−C13 range fuel precursors helps to preserve the carbon fractions by suppressing the gasification of the small oxygenates.8 Moreover, the valorization process is carbon-waste free and also has tremendous potential to produce, exclusively, fuel range components without any industrial waste. In this context, the C−C alkylation reaction between phenolic compounds and small oxygenates such as alcohols has become a highly significant industrial reaction.9,10 Furthermore, the alkylated phenolic compounds obtained in this process are used in the paint, rubber, pharmaceutical, and cosmetics industry as fine chemicals.11−13 Alkylation of mcresol with iso-propanol has been considered one such industrially important model reaction to demonstrate the © XXXX American Chemical Society

Received: March 2, 2018 Accepted: May 11, 2018

A

DOI: 10.1021/acs.jced.8b00171 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

catalyst functionalities and detailed kinetic analysis of this reaction.22,23 Grabowska et al.,11 Kong et al.,24 and Chen et al.25 demonstrated the use of spinels as potential heterogeneous catalysts for the alkylation reaction due to their high thermal and chemical stability, high mechanical strength, tunable surface area and pore size, and adjustable surface acidity. Both Zn and Al are reported to enhance Lewis acid site density of solid oxide catalysts and have been reported as useful catalysts in this process.15 However, the catalytic efficiency of the reported ZnAl2O4 spinels (ZAL) were obtained to be low with 78.2% mcresol conversion, and can be further enhanced by optimizing the Lewis acid site density and porosity. However, none of these studies provide an overall thermodynamic perspective of the alkylation reactions under commercially viable reaction conditions. In particular, to achieve industrially acceptable production efficiency in alkylation reactions, the effect of process parameters such as reaction temperature and reactant molar ratios on the conversion and product distribution must be optimized. Therefore, thermodynamic analysis of the model alkylation reaction between m-cresol and iso-propanol is helpful in terms of understanding the detailed thermodynamic behavior of a similar class of biomass-derived phenolic compounds. In this study, the thermodynamic limitations of the model reaction between m-cresol (as lignin-derived phenolics) and isopropanol (as small oxygenates) were investigated in detail under different reaction conditions. Equilibrium chemical compositions as a function of temperature at atmospheric pressure and different reactant molar ratios were estimated using the Gibbs free energy minimization method. All the possible product distribution models were considered to optimize the reaction conditions for minimized side product formation resulting from side reactions such as oligomerization, isomerization, transalkylation, dehydration, dealkylation, etc. Furthermore, selective formation of the desired product thymol over different nanocrystalline ZAL catalysts in this reaction was exemplified under the optimized reaction conditions obtained from the thermodynamic analysis to check the integrity of the predictions. All these catalysts were chosen based on their varied acid site densities and surface properties owing to different precursors used in their preparative methods as well as varying ZnO/Al2O3 molar ratio, to investigate their overall effect on the Friedel-Craft alkylation of m-cresol with isopropanol. The combined theoretical thermodynamic analysis and experimental catalytic investigation correlating structural, acidic, morphological, and thermal properties of the synthesized catalysts are essential for industrial scale process development for the manufacture of C10-C13 range fuel precursors.

The thermodynamic quantities obtained from these group contribution methods were compared with the experimental data values for the components available in the NIST database. All these thermodynamic quantities were obtained with the structural information data using JOBACK, JOBACK and GANI second order (fragmented products only), LYDERSEN, AMBROSE, FEDORS, BENSON R8, and JOBACK group contribution methods, respectively. Furthermore, the best method for prediction of a particular thermodynamic quantity was chosen based on the least deviation from the experimentally determined data bank values for the available components as shown in Supporting Information (Table S1− S11). Ideal gas heat capacities (Cp) were estimated using the JOBACK group contribution method following eq 1: Cp (J mol−1 °C−1) =

∑ ai − 37.93 + [∑ bi + 0.210]T + [∑ ci − 3.91 × 10−4]T 2 + [∑ di + 2.06 × 10−7] T3

(1)

The normal boiling temperature (Tb) was estimated using the JOBACK group contribution method following eq 2: Tb (°C) = −75.15 +

∑ nitb,i

(2)

i

The critical temperature (Tc) was estimated using the LYDERSEN group contribution method following eq 3: Tc (°C) =

(Tb + 273.15) 0.567 + ∑i tc, i − (∑i tc, i)2

+ 273.15 (3)

The critical pressure (Pc) was estimated using AMBROSE group contribution method following eq 4: Pc (atm) =

M (0.34 + ∑i nipc , i )2

(4)

The critical volume (Vc) was estimated using FEDORS group contribution method following eq 5: Vc (m 3kmol−1) =

26.6 + ∑i nivc, i (5)

1000

The standard enthalpy of formation (ΔHf,25 °C) was estimated using the BENSON R8 group contribution method following eq 6:

2. MATERIALS AND METHODS 2.1. Computational Thermodynamic Analysis. 2.1.1. Estimation of Thermodynamic Quantities. For calculation of the thermodynamic equilibrium conversion and product distribution under different reaction conditions, ASPEN Plus software simulation package (Aspen Technology, Inc. 2014) was used. Group contribution methods such as BENSON R8, JOBACK, LYDERSEN, AMBROSE, FEDORS, GANI (both first order and second order) available in the ASPEN Plus software package were used for the estimation of the standard thermodynamic quantities (ideal gas heat capacities, normal boiling temperature, critical temperature, critical pressure, critical volume, standard enthalpy of formation, and standard Gibbs free energy of formation) of all the pure components of this reaction to explore the thermodynamic limitations.26−30

ΔHf25°C (kJmol−1) =

∑ nihf,i

(6)

i

The standard Gibbs free energy of formation (ΔGf,25 °C) was estimated using the JOBACK group contribution method following eq 7: ΔG f25°C (kJmol−1) = 53.88 +

∑ nigf,i i

(7)

where M is molar mass; tb,i, tc,i, pc,i, vc,i, hf,i, and gf,i are the group contributions of type-i for the estimation of Tb, Tc, Pc, Vc, ΔHf, and ΔGf, respectively; and ni is the number of groups of type-i in the compound. The Joback method uses a four parameter polynomial to describe the temperature dependency of the ideal gas heat capacity. B



Article

2.1.2. Selection of Property Method. Equilibrium conversion predictions for the two-phase (liquid−gas) system was taken into account as the species considered in this thermodynamic study were present in the liquid phase for a considerable range of reaction temperatures, which have not been studied before. The binary interaction property in ASPEN Plus library was utilized to estimate the vapor and liquid mole fractions of iso-propanol in m-cresol using different property methods at different temperatures under atmospheric pressure.31 The estimated values were compared with the experimental data of vapor liquid equilibrium (VLE) available for the iso-propanol/m-cresol system in the ASPEN database and are summarized in Table 1. The VLE data obtained from the UNIFAC property method was found to be the one with the least deviation from the experimental VLE data at different temperatures. Hence, the UNIFAC group contribution based

activity coefficient method was selected to carry out further thermodynamic study. 2.1.3. Reactions Involved in m-Cresol Alkylation. The reaction of m-cresol is associated with a number of side reactions such as dehydration of iso-propanol, oligomerization of the dehydrated products, isomerization of the m-cresol, transalkylation, and dealkylation of the formed alkylated products, etc. The possible reaction routes of m-cresol alkylation are presented in Table 2. The literature reports suggest that the dehydration of iso-propanol results in the formation of propene which in turn acts as the main alkylating agent in this reaction.32 All these reactions may occur in parallel with the desired C10 range product formation and finally contribute to the overall conversion of m-cresol under different reaction conditions. In total 34 species including iso-propylated m-cresols, di-iso-propylated m-cresols, triiso-propylated mcresols, methylated cresols, butylated m-cresols, ethylated phenols, butene isomers, residual alkylating agents such as propene and di-iso-propyl ether (DIIPE), and water were included in the simulation as provided in Table S1 of the Supporting Information. 2.1.4. Estimation of Heat and Gibbs Free Energies of Possible Reactions. ASPEN plus simulation package was used to estimate the enthalpy of formation and Gibbs free energy of formation values at different temperature for all the components. Further, the values of heat and Gibbs free energy of the possible reactions at different temperature were calculated using eq 8 and eq 9, respectively. The heat of reaction and Gibbs free energy values of the fragmentation reactions and multialkylation reactions were not considered as the well-defined stoichiometric correlations cannot be predicted.

Table 1. Comparison of VLE Data Estimated Using Different Property Methods with the Experimental Value for isoPropanol/m-Cresol Systema iso-propanol in liquid phase T (°C)

methods

xipa

201

experiment NRTL PR IDEAL UNIFAC WILSON experiment NRTL PR IDEAL UNIFAC WILSON experiment NRTL PR IDEAL UNIFAC WILSON experiment NRTL PR IDEAL UNIFAC WILSON experiment NRTL PR IDEAL UNIFAC WILSON experiment NRTL PR IDEAL UNIFAC WILSON

0 0 0 0 0 0 0.2672 0.0609 0.0595 0.0600 0.1200 0.0852 0.4507 0.1211 0.1000 0.1200 0.2000 0.1623 0.7005 0.2620 0.2200 0.2600 0.3200 0.2956 0.9112 0.6082 0.4800 0.6000 0.7009 0.6452 1 1 1 1 1 1

161

143

120

95

82

deviation 0 0 0 0 0 −0.2063 −0.2077 −0.2072 −0.1472 −0.1820 −0.3296 −0.3507 −0.3307 −0.2507 −0.2844 −0.4385 −0.4805 −0.4405 −0.3805 −0.4049 −0.3030 −0.4312 −0.3112 −0.2103 −0.2660 0 0 0 0 0

iso-propanol in vapor phase yipa 0 0 0 0 0 0 0.7815 0.7059 0.7054 0.7080 0.7222 0.7102 0.9239 0.8592 0.8335 0.8644 0.8899 0.8712 0.9917 0.9529 0.9459 0.9560 0.9648 0.9587 1 0.9901 0.9872 0.9923 0.9943 0.9930 1 1 1 1 1 1

deviation 0 0 0 0 0 −0.0756 −0.0761 −0.0735 −0.0593 −0.0713

ΔH =

∑ ΔHf

ΔG =

∑ ΔG f

product

product

−

∑ ΔHf

−

∑ ΔG f

reactant

(8)

reactant

(9)

2.1.5. Simulation Operating Conditions. Gibbs free energy minimization method was employed to determine the equilibrium composition of the reaction as implemented in the RGibbs reactor model in the ASPEN Plus software package.31 The method was used to estimate the equilibrium conversion and the product distribution of m-cresol alkylation reaction in the temperature range of 200 to 700 °C under atmospheric pressure conditions with an iso-propanol to mcresol molar ratio in the range of 0.5:1 to 5:1. The following equations were used to evaluate the equilibrium conversion of m-cresol (Xe,m‑cresol) as well as the selectivity to product i (Si):

−0.0647 −0.0904 −0.0596 −0.0340 −0.0527 −0.0388 −0.0458 −0.0357 −0.0269 −0.0330 −0.0099 −0.0128 −0.0077 −0.0057 −0.0070

⎡ Fο ⎤ − Fm ‐ cresol Xe, m ‐ cresol % = ⎢ m ‐ cresolο × 100⎥ Fm ‐ cresol ⎣ ⎦

(10)

⎡ F ⎤ Si% = ⎢ i × 100⎥ ⎣ ∑ Fi ⎦

(11)

where F m‑cresol and Fm‑cresol are the molar flow rates of m-cresol at the inlet and the outlet, and Fi is the molar flow rate of species i at the outlet. On the basis of the thermodynamic quantities, different product distribution models were adopted to obtain the equilibrium conversion data. The effect of different product composition, varying feed ratio, and side reactions such as dehydration of iso-propanol on the equilibrium conversion of o

0 0 0 0 0

a

xipa = mole fraction of iso-propanol in liquid phase, yipa = mole fraction of iso-propanol in vapour phase. C



Article

Table 2. Key Reactions Involved in Alkylation of m-Cresol with Iso-propanola reaction no. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. a

equation

reactions

IPA → propene + water

dehydration of isopropanol to propene m − cresol + propene → thymol thymol production m − cresol + propene → IPMCE mono-O-alkylation 4m − cresol +4propene → thymol + 2I − 3MP + 4I − 3MP + 3I − 5MP mono-C-alkylation (iso − propylated m − cresol)1 ↔ (iso − propylated m − cresol)2 isomerization of isopropylated m-cresols 2IPA → DIIPE + water dehydration of isopropanol to DIIPE 10m − cresol +20propene →2,4DI−3MP + 2,5DI − 3MP + 2,6DI− 3MP + 2,3DI − 5MP + 2,4DI − 5MP + 3,4DI − 5MP + dialkylation IP − 2I − 3MPE + IP − 4I − 3MPE + IP − 2I − 5MPE + IP − 3I − 5MPE m−cresol + 3Propene → 2,4,6TI − 3MP trialkylation Iso − propylated m − cresol ↔ Di − iso − propylated m − cresol alkylation of isopropylated m-cresols Iso − propylated m − cresol + di − iso − propylated m − cresol ↔ fragmented intermediate side chains fragmentation reactions methylated cresols ↔ m − cresol + propene butylated m − cresols ↔ m − cresol + butene isomers ethylated phenols ↔ m − cresol

Iso-propanol is denoted by IPA in the equations.

m-cresol was investigated in detail. To ensure the accuracy in the numerical algorithm of the ASPEN Plus simulation results for this iso-propylation reaction (alkylation of m-cresol with isopropanol), the results of ASPEN Plus were verified with the results obtained for this reaction from the first principle. 2.2. Experimental Study of m-Cresol Alkylation. The thermodynamic predictions obtained from ASPEN Plus simulations were verified and correlated with experimental study by performing the iso-propylation reaction of m-cresol with iso-propanol using different ZAL catalysts in a fixed-bed reactor. 2.2.1. Materials. Iso-propanol (99%), m-cresol (98%), and zinc acetate dihydrate (99%) were purchased from SigmaAldrich Chemicals Pvt. Ltd., India. Aluminum metal and aluminum chloride (99%) were purchased from Sisco Research Laboratory Pvt. Ltd., India. Aluminum iso-propoxide was purchased from Spectrochem Pvt. Ltd., India. All chemicals were of analytical reagent (A.R.) grade and were used as received without any further purification unless mentioned otherwise. 2.2.2. Preparation of Catalysts. ZAL catalysts were synthesized using hydrothermal methods with different aluminum precursors and different molar ratio of ZnO/Al2O3. In general, four different sets of solutions were prepared with different ZnO/Al2O3 molar ratio and aluminum precursors. For the preparation of solution (1), basic aluminum chloride with the empirical formula Al2(OH)6−xClx, where x was close to 1, was used as the aluminum precursor.11 The Al2(OH)6−xClx species was obtained by hydrolysis of aluminum metal powder in aqueous solution of aluminum chloride at high temperatures (around 353 °C) for 4 days. To prepare the synthesis mixture, zinc acetate dihydrate was added to an aqueous solution of basic aluminum chloride maintaining the molar ratio of ZnO/ Al2O3 at 1:1. A clear solution was obtained when the mixture was stirred for 20 min at room temperature. For the preparation of solution (2), (3), and (4), aluminum hydroxide was used as the aluminum precursor, formed by the hydrolysis of aluminum iso-propoxide in excess of water at 70 °C, and was continuously stirred to form a slurry of AlOOH. To this slurry, an aqueous solution of zinc acetate dihydrate was added maintaining the molar ratio of ZnO/Al2O3 at 1:1, 1:2,

and 2:1 in the case of solution (2), (3), and (4), respectively. The mixture was kept at 70 °C with constant stirring for another 20 min. The solutions (1), (2), (3), and (4) were poured, separately, into a Teflon-lined stainless-steel autoclave and heated under autogenous pressure at 162 °C for 3 h and then quenched in cold water. The resulting slurry of precipitated powders was washed, and the gels were extruded, dried in oven, and calcined at 600 °C for 6 h. The resulting solid catalysts were named as ZAL-I, ZAL-II, ZAL-III, and ZAL-IV, respectively. 2.2.3. Catalyst Characterization. The crystallite structure, phase composition, and mean crystallite size of the synthesized catalysts were determined by the powder X-ray diffractometer instrument (Rigaku MiniFlex-300) using Cu Kα source (1.5429 Å) and the 2θ range of 10−80° with 0.8° min−1 scan rate. The average crystallite size (d) of the catalysts was estimated following the Debye−Scherrer formula as shown in eq 12: d=

kλ β cos θ

(12)

where, k = 0.9 is the shape factor, λ is the X-ray wavelength of Cu Kα radiation (1.5429 Å), θ is the Bragg diffraction angle, and β is the full width at half-maximum height (fwhm) of the diffraction peak (311) at ∼36°. The lattice parameters were also calculated using the peaks at a hkl value of (311) of the X-ray diffraction patterns in the formula as shown in eq 13: a = dhkl(h2 + k 2 + l 2)1/2

(13)

where a is the lattice parameter (Å), d is the interplanar distance (Å), hkl is the Miller indices of the peak (311).33 The BET surface area, micropore volume, and pore-size distribution of the catalysts were determined by physisorption of N2 at −196 °C in a Micromeritics ASAP 2010 accelerated surface area and porosity analyzer. The equivalent particle size was calculated based on the BET surface area using the formula as shown in eq 14: DBET = D

6000 ρSBET

(14) DOI: 10.1021/acs.jced.8b00171 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Scheme 1. Reaction Network of the Possible Reactions Involved in m-Cresol Iso-propylationa

a

The nos. 1−13 denote the reaction no. as described in Table 2 and Table 3.

during the reaction. Maximum temperature fluctuation during the whole set of reactions was found to be in between ±2 °C. Prior to the reaction, the catalysts were treated in situ at 500 °C for 3 h in N2 flow. Further, the reaction mixture consisting of iso-propanol and m-cresol at a molar ratio of 5:1 was fed into the reactor using a syringe pump. The catalytic reactions were performed at atmospheric pressure, and N2 was used as an inert carrier gas. A coiled condenser was connected to the bottom of the reactor to condense the gaseous products. After steady state had set in, the liquid products were collected at chosen temperatures and analyzed using a NUCON-GC supplied by AIMIL India Ltd. equipped with a CHROMSORB-WHP (2 m × 3.175 mm × 2 mm) column and flame ionization detector (FID). The percent conversion of m-cresol (Xm‑cresol) and percent selectivity to product i (Si; Sthymol for thymol) was calculated using the following equations (eqs 15 and 16):

where DBET is the equivalent particle size (nm), ρ is the theoretical density of ZnAl2O4 (4.62 g/cm3), and SBET is the BET surface area (m2/g).34 Acidity type and acidity strength distribution of the investigated catalysts were determined by temperature-programmed desorption (TPD) of NH3 in a Micromeritics ChemiSorb 2720 pulse chemisorption instrument. The samples were treated at 500 °C in He gas for 2 h, then cooled down to 90 °C, and finally exposed to a 10% NH3−90% He mixture to eliminate the physisorbed NH3 by flushing with He for 90 min. Then the temperature was increased at a rate of 10° min−1 in He flow of 60 cm3 min−1 and desorbed NH3 was analyzed by Thermal Conductivity Detector (TCD). 2.2.4. Activity Testing. The catalytic activity study of gasphase alkylation reactions of m-cresol with iso-propanol was carried out in a fixed bed continuous flow reactor with length to diameter (L/D) ratio of ∼50. About 1 g of each catalyst was placed between thin layers of quartz wool inside the reactor to prevent the movement of the catalyst bed during the reaction and then filled with ceramic beads. The heating furnace was set as a function of increasing temperature and controlled using a k-type thermocouple sensor. Another k-type thermocouple was placed inside the thermowell of the quartz reactor tube below the catalyst bed to recheck the possible temperature gradient E

⎡Yο ⎤ − Ym ‐ cresol × 100⎥ X m ‐ cresol % = ⎢ m ‐ cresolο Y m ‐ cresol ⎣ ⎦

(15)

⎡ Y ⎤ Si % = ⎢ i × 100⎥ ⎣ ∑ Yi ⎦

(16) DOI: 10.1021/acs.jced.8b00171 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 3. Estimation of Enthalpy and Gibbs Free Energy of Possible Reactions at 200 °C reaction no. 1. 2. 3. 4. 5. 6

reactions dehydration of iso-propanol to propene thymol production mono-O-alkylation mono-C-alkylation isomerization of iso-propylated mcresols dehydration of iso-propanol to DIIPE

ΔH (kJ mol−1)

at 300 °C

ΔG (kJ mol−1)

ΔH (kJ mol−1)

at 500 °C

ΔG (kJ mol−1)

ΔH (kJ mol−1)

ΔG (kJ mol−1)

−170.27

49.21

−216.65

49.43

−263.05

49.77

−309.49

−91.83 −65.66 −372.37 −26.18

−23.60 39.41 −91.58 −63.01

−91.23 −65.63 −369.94 −25.60

−9.24 61.62 −32.47 −70.86

−90.60 −65.41 −367.40 −25.19

5.01 83.80 26.20 −78.79

−89.94 −64.94 −364.71 −24.99

19.17 105.94 84.49 −86.77

−17.13

−130.50

−16.97

−154.48

−16.53

−178.50

−15.74

−202.61

Scheme 2. Reaction of m-Cresol with Propene to Form Thymol

3. RESULTS AND DISCUSSION 3.1. Computational Thermodynamic Analysis. 3.1.1. Estimation of Heat and Gibbs Free Energies of Possible Reactions. The detailed reaction network of all the possible reactions involved in m-cresol iso-propylation as listed in Table 2 is presented in Scheme 1. The heat and Gibbs free energy of these possible reactions at different temperatures is summarized in Table 3. It is evident from Table 3 that the dehydration reactions of iso-propanol (R-1 and R-6) are spontaneous reactions at the reaction temperatures considered in this study. The Gibbs free energy of the reactions suggests that the formation of propene is always thermodynamically preferred over DIIPE and hence, propene acts as the main alkylating agent in this reaction. O-Alkylation reactions leading to the formation of the ether product, iso-propyl-3-methylphenyl ether (IPMCE) (R-3), has a positive Gibbs free energy at all temperatures and hence, is not thermodynamically favored, whereas monoalkylation reaction (R-4) leading to isopropylated m-cresols occurs spontaneously in the temperature range of 200−300 °C and is thermodynamically preferred to Oalkylation. Interestingly, thymol production reaction is spontaneous until 300 °C, and hence, an active reaction in the temperature ranges of 200−300 °C (R-2). Isomerization of iso-propylated m-cresols (R-5) is spontaneous at all the reaction temperatures considered in this analysis. These observations are in line with the observations reported by Grabowska et al. and Yadav et al.11,17 3.1.2. Model Verification. The results obtained from the simulation using RGibbs reactor model in ASPEN Plus was verified by comparing with the equilibrium conversion results of the m-cresol alkylation reaction to form thymol (reaction no. 2, as shown in Scheme 2) obtained from the first principle calculations. A correlation between equilibrium constant (Keq) and temperature (T in °C) was developed using the pure component thermodynamic properties as provided in the NIST database as shown in eq 17. This correlation is valid in the entire temperature range of our thermodynamic analysis. 11178.89 − 18.17 (T + 273.15)

ΔH (kJ mol−1)

49.06

where, Y0m‑cresol and Ym‑cresol are the molar fraction of m-cresol in the feed and in the product, and Yi is the molar fraction of products formed from m-cresol. The carbon balance was obtained to be around 98.1% and ring mass balance in each run was more than 98.9%. The percentage of liquid products composition reported was based on iso-propanol-free m-cresol.

ln Keq =

at 400 °C

ΔG (kJ mol−1)

Alternatively, Keq for reaction no. 2 can also be expressed in terms of fractional conversion as shown in eq 18: Keq =

xm ‐ cresol(2 − xm ‐ cresol) (1 − xm ‐ cresol)2

(18)

where xm‑cresol is the fractional conversion of m-cresol at equilibrium. Keq was calculated from eq 17 in the temperature range of 200 to 700 °C at intervals of 10 °C under atmospheric pressure. The values of Keq were substituted in eq 18 to calculate the equilibrium conversions of m-cresol corresponding to respective temperatures. The solution of quadratic eq 18 had one real root less than unity and was taken into account in the calculations. The obtained trends in ln Keq vs 1000/T as well as equilibrium conversion vs temperature as obtained from first principle analysis and ASPEN RGibbs reactor model are summarized in Figure 1. As evidenced from Figure 1, both the trends in ln Keq as well as the equilibrium conversions with temperature obtained from both the approaches matched well in the entire range of temperature, thereby, ensuring the accuracy and validity of the RGibbs reactor model in ASPEN Plus for this reaction. 3.1.3. Effect of Operating Conditions on Equilibrium Conversion of m-Cresol. The equilibrium conversion of mcresol with different product distribution models for the reaction was estimated using ASPEN Plus software simulation package. The models were based on the most probable products formed in this alkylation reaction along with the occurrence of side reactions such as dehydration, oligomerization, transalkylation, fragmentation, dealkylation, etc. In general, the probable products of the reaction between mcresol (C) and iso-propanol (P) were classified as water (W), thymol (Thymol), iso-propylated isomers (M), di-iso-propylated m-cresols (D), tri-iso-propylated m-cresols (T), intermediate fragmented side-chain products (I), and butylated mcresol and butene isomers (B). These products were distributed to develop different models (CPWThymol, CPWM, CPWMD, CPWMDT, CPWMIB, CPWMDTIB) to correlate the change in equilibrium conversion of m-cresol with formation of different products under various reaction conditions. CPWThy-

(17) F



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Figure 1. Comparison of the equilibrium data obtained from first principle calculations and ASPEN RGibbs reactor model.

Figure 2. Equilibrium conversion of m-cresol as a function of temperature and iso-propanol to m-cresol molar ratio for different product distribution models: (a) CPWThymol, (b) CPWM, (c) CPWMD, (d) CPWMDT, (e) CPWMIB, and (f) CPWMDTIB. Product distribution models include the following terms, (C) m-cresol, (P) iso-propanol, (W) water, (M) iso-propylated m-cresols, (D) di-iso-propylated m-cresols, (T) tri-iso-propylated mcresols, (I) intermediate fragmented side-chain products, (B) butylated m-cresols and butene isomers.

CPWM, CPWMD, and CPWMDT product distribution models, the equilibrium conversion of m-cresol increased significantly from ∼44% to ∼90% at 200 °C as the reactant molar ratio increased from 0.5 to 2 and beyond. However, with an increase in reaction temperature to 700 °C at all reactant molar ratios, the equilibrium conversion of m-cresol almost diminished for all these four product distribution models. The equilibrium conversion of m-cresol was observed to be significantly higher for the production of thymol isomers (CPWM model) as compared to selective thymol production (CPWThymol model) at temperatures above 270 °C. In general, with an increase in temperature the stability of isopropylated m-cresols decreases leading to dealkylation reactions, thereby resulting in lower equilibrium conversion of m-

mol, CPWM, CPWMD, and CPWMDT models were chosen to observe the effect of alkylation reaction products, namely, thymol, iso-propylated m-cresols, di-iso-propylated m-cresols, and tri-iso-propylated m-cresols, respectively, on the equilibrium conversion of m-cresol. The effect of the products formed from different side reactions on equilibrium conversion were studied with the help of models CPWMIB and CPWMDTIB. The equilibrium conversion of m-cresol as a function of temperature (200−700 °C) and feed ratio (0.5 to 5) for different product distribution models is shown in Figure 2. The equilibrium conversion of m-cresol varied from ∼92−99% at 200 °C to ∼47−54% at 700 °C for CPWMIB and CPWMDTIB product distribution models for the entire reactant molar ratio considered in this study. For CPWThymol, G



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cresol for both CPWThymol and CPWM models. However, in the presence of thymol isomers (CPWM model), the isomerization between the iso-propylated m-cresols predominated over the dealkylation reaction and hence, higher equilibrium conversion was obtained in the case of the CPWM model as compared to CPWThymol model. In general, with increase in reaction temperature, the equilibrium conversion of m-cresol decreased with a sharp change in the temperature range of 270−350 °C, which is similar for all the product distribution models. This change in equilibrium conversion of m-cresol can be attributed to the dealkylation of the alkylated products in the temperature range of 270−350 °C. In the case of the CPWMIB and CPWMDTIB product distribution models as shown in Figure 2e,f, the least change in equilibrium conversion was observed from ∼92−97% at 270 °C to ∼84−89% at 350 °C at all reactant molar ratios. The dealkylation resulted in fragmentation of the alkylated products which is evident from the increased selectivity to the methylated cresols, ethylated phenols, and butene isomers which in turn increased the selectivity to iso-butylated mcresols. In the presence of these intermediate fragmented sidechain products (as considered in CPWMDTIB and CPWMIB models), the equilibrium still resided more toward the product side, and hence, the smallest change in equilibrium conversion was observed in these cases, whereas, in the case of CPWMDT, CPWMD, and CPWM product distribution models, the equilibrium conversion changed moderately from ∼98−99.5% at 270 °C to ∼59−66%, and at 350 °C with a reactant molar ratio of 1 and higher. The significant drop in equilibrium conversion from ∼93% at 270 °C to ∼37% at 350 °C with a reactant molar ratio of 1 and higher was observed for selective production of thymol as evident for the CPWThymol model shown in Figure 2a. Since the stability of the monoalkylated products decreases with temperature, the CPWM and CPWThymol models showed the maximum change in equilibrium conversion. Moreover, in the absence of intermediate fragmented side chains, the dealkylation shifted the equilibrium toward the reactant side. Iso-propanol gets primarily converted to propene and DIIPE which themselves act as the alkylating agents. In the presence of multiple alkylating agents, m-cresol gets converted to multiple side products and hence, the equilibrium conversion varied accordingly. In the presence of the intermediate fragmented side-chain products (CPWMIB and CPWMDTIB models), the equilibrium conversion of m-cresol was significantly higher in the temperature range below 270 °C for iso-propanol to mcresol molar ratio of 0.5 and 1. This confirms thermodynamic feasibility of the fragmented side chain product formation even at lower temperature in the presence of the higher amount of m-cresol leading to the formation of methylated cresols and ethylated phenols. These results confirm that the maximum equilibrium conversion of m-cresol can be obtained below 300 °C with reactant molar ratio in the range of 2−5. At a temperature of 270 °C, the variation in equilibrium conversion of m-cresol for all the product distribution models under different reactant molar ratios is shown in Figure 3. It is evident from Figure 3 that the equilibrium conversion of mcresol increased significantly in the iso-propanol to m-cresol molar ratio range of 0.5:1 to 3:1, and reaches maximum at 5:1. The increase in m-cresol conversion can be attributed to the formation of iso-propylated m-cresols in the presence of higher amount of iso-propanol, that is, higher ratio of alkylating agent to m-cresol. At a temperature of 270 °C, pressure of 1 atm, and

Figure 3. Equilibrium conversion of m-cresol as a function of isopropanol to m-cresol molar ratio at 270 °C; Product distribution models include the following terms, (C) m-cresol, (P) iso-propanol, (W) water, (M) iso-propylated m-cresols, (D) di-iso-propylated mcresols, (T) tri-iso-propylated m-cresols, (I) intermediate fragmented side-chain products, (B) butylated m-cresols and butene isomers.

iso-propanol to m-cresol molar ratio of 5:1, for exclusive thymol production, the conversion reached around 89%. In the presence of thymol isomers, the conversion increased to 96.3%. With the formation of di- and tri-iso-propylated mcresols, the conversion increased further to 99.1%. However, with the formation of the fragmented intermediate side chain products such as butylated m-cresols and butene isomers, the conversion decreased to 95.8%. 3.1.4. Effect of Operating Conditions on Product Distribution. The trends in product distribution for this alkylation reaction with the variation of temperature in the range of 200−700 °C and reactant (iso-propanol/m-cresol) molar ratio in the range of 0.5:1 to 5:1 are summarized in Figure 4. The iso-propylated m-cresols were obtained to be the major product with the highest selectivity reaching ∼40−60% for the reactant molar ratio beyond 2:1 and in the temperature range of 200−350 °C. As the temperature increased beyond 350 °C, the formation of propene becomes predominant due to the dealkylation reactions with the selectivity increasing from ∼12−18% at 350 °C to ∼45−57% at 700 °C. Moreover, with an increase in the reactant molar ratio beyond 1:1, the di- and tri-iso-propylated m-cresols which were present in significant amount (∼20−50% and ∼30−56%, respectively) in the temperature range of 200−250 °C, showed a gradual decrease with further increase in temperature due to their dealkylation to iso-propylated m-cresols.15,35,36 It is evident from the product distribution of the iso-propanol dehydration reaction (Supporting Information Figure S1) that butene isomers were predominant as compared to propene at temperatures below 280 °C. However, the higher selectivity to iso-propylated mcresols indicated that the alkylation of m-cresol with propene was thermodynamically more favored as compared to butene isomers. Hence, the selectivity to butylated m-cresols was always found to be lower than the iso-propylated m-cresols. The amount of methylated cresols decreased gradually from ∼67% at 0.5:1 to 17% at 5:1 with maxima in the temperature range of 270−300 °C. In the presence of lower alcohol concentration, the thermodynamically highly feasible fragmented intermediate H



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Figure 4. Product distribution as a function of temperature at iso-propanol to m-cresol molar ratio: (a) 0.5:1, (b) 1:1, (c) 2:1, (d) 3:1, (e) 4:1, and (f) 5:1.

of 5:1 were found to be the optimum reaction parameters to obtain maximum m-cresol equilibrium conversion (97.3%) with minimized formation of butylated m-cresols and other side products. Henceforth, to investigate the thermodynamic limitations, experiments were carried out in a continuous flow fixed bed reactor with laboratory synthesized thermally stable ZAL catalysts in the whole temperature range of 210−300 °C which covers the optimum 250−275 °C temperature and 5:1 reactant molar ratio for the selective production of industrially important product thymol. 3.2. Experimental Study of m-Cresol Alkylation. 3.2.1. Catalyst Characterization. The X-ray diffraction patterns of the ZAL catalysts synthesized by the hydrothermal process showed the characteristic peaks in the 2θ range of 10 to 80° as shown in Figure 5. It is evident from the peaks of the XRD patterns that all the samples were present in a single cubic spinel ZnAl2O4 phase (JCPDS Card 05-0669).34 The characteristic peaks of the synthesized catalysts at 2θ around 31, 36, 45, 49, 55, 59, 65, 74, and 77° corresponded to (220), (311), (400), (331), (422), (511), (440), (628), and (533) diffraction planes, respectively, and are in agreement with the characteristic peaks reported by Ianos et al. (JCPDS Card No. 05-0669).34 Peaks of any other phase were not detected in the XRD pattern of the spinel samples. The peak broadening observed for the spinel samples indicates dispersion in the crystallite shape.38 The average crystallite size calculated for pure nanocrystalline ZAL-I, ZAL-II, ZAL-III, and ZAL-IV was 6.52, 4.36, 4.27, and 6.24 nm, respectively. The lattice parameter (d311) calculated for ZAL-I, ZAL-II, ZAL-III, and ZAL-IV catalysts was 8.187 Å, 8.026 Å, 8.079 Å, and 8.052 Å, respectively, which are similar to the theoretical value of gahnite (8.084 Å).34 The textural properties of the hydrothermally synthesized ZAL catalysts seemed to depend on different aluminum

side chain product formation predominated as evidenced by higher equilibrium conversion of m-cresol in the temperature range of 200−300 °C. The amount of ethylated phenols increased with an increase in temperature up to 700 °C while gradually decreasing from ∼29% at 0.5:1 to ∼6% at 5:1 confirming the higher amount of fragmentation reaction at higher temperature due to dealkylation reactions. Hence, to obtain maximum iso-propylated m-cresols and in particular, thymol, the reaction temperature should be in the range of 250−350 °C to prevent the formation of propene as well as di- and tri-iso-propylated m-cresols. Also, the isopropanol concentration should be higher than m-cresol concentration to prevent fragmented intermediate side chain product formation. 3.1.5. Effect of Operating Conditions on Side Reactions. A number of side reactions also occur such as dehydration of isopropanol, transalkylation, and isomerization of m-cresol to other cresylic isomers which affect the overall equilibrium conversion of m-cresol. Almost 100% dehydration of isopropanol led to the formation of propene, DIIPE, and butene isomers, and water under all the reaction temperatures for all the reactant molar ratios (as shown in Figure S1 in the Supporting Information). The selectivity to propene predominated over the other dehydrated products (butene isomers and DIIPE) above 250 °C. Hence, propene acts as the main alkylating agent as also evidenced in Table 3. Furthermore, the isomerization reaction of m-cresol to other cresol isomers predominated beyond 270−275 °C.37 The isomerization reactions led to lower concentration of m-cresol in the feed and hence, the equilibrium conversion also decreased beyond 275 °C. Overall, from the thermodynamic analysis of this reaction, the temperature ranges of 250−275 °C and reactant molar ratio I



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Figure 6. N2 adsorption−desorption isotherm for catalyst ZAL-III. Inset: pore size distribution based on the BJH desorption method.

Figure 5. Powder XRD patterns of synthesized ZAL catalysts.

The density and strength of the surface acidic sites of the prepared ZAL catalysts were determined by TPD of NH3 preadsorbed samples as the temperature increased from 100 to 600 °C. The NH3 desorption profile of different catalysts as a function of temperature is shown in Figure 7. The acid sites

precursors and different ZnO:Al2O3 ratios used in the synthesis. The surface and pore properties of the ZAL catalysts obtained after outgassing the catalysts at 200 °C in vacuum of 10−3 Torr are listed in Table 4. The average pore diameter (>2 nm and 2 nm and 500 °C.42 At weaker acidic sites, NH3 gets desorbed at lower temperature due to weaker interaction between NH3 and catalyst active sites, whereas high temperature is required for NH3 desorption due to strong interaction

Table 4. Textural and Acidity Properties of Synthesized ZAL Catalysts catalyst

ZnO:Al2O3 molar ratios

specific surface area, SBET (m2 g−1)

total pore volume (P/Po = 0.99) (cm3 g−1)

micropore volume (cm3 g−1)

average pore diameter (nm)

acid site density from NH3TPD (μmol g−1)

ZAL-I ZAL-II ZAL-III ZAL-IV

1:1 1:1 1:2 2:1

75.68 141.84 219.67 130.53

0.2315 0.3555 0.4101 0.2743

0.0160 0.0089 0.0079 0.0176

12.24 10.03 7.47 8.41

819 1122 1416 655

J



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Figure 8. Conversion of m-cresol (Xm‑cresol) and selectivity (Si) to alkylated products as a function of temperature for catalysts: (a) ZAL-I, (b) ZAL-II, (c) ZAL-III, and (d) ZAL-IV. Reaction conditions: PT = 1 atm; WHSV = 0.5 h−1; iso-propanol to m-cresol molar ratio = 5:1.

between NH3 and strong catalyst acidic sites.43 The NH3desorption peak at lower temperature corresponding to weak Lewis or Bronsted acidic sites appeared in the range of 160− 230 °C for all the ZAL samples. ZAL-I exhibited a sharp peak corresponding to weak acidity at 160 °C along with a shoulder at 400 °C corresponding to strong acidity. Whereas, broad peaks at 421 and 405 °C corresponding to strong acidic sites were observed for ZAL-II and ZAL-III, respectively. However, ZAL-IV showed a very broad peak at 306 °C corresponding to medium acid sites and a distinct peak at 552 °C corresponding to very strong acid sites. Furthermore, the acid site densities were obtained by deconvolution and integration of NH3-TPD profiles and are presented in Table 4 (Figure S6 in Supporting Information). On a weight basis, ZAL-III exhibited highest density of surface acid sites (1416 μmol/g), followed by ZAL-II (1122 μmol/g), ZAL-I (819 μmol/g), and ZAL-IV (655 μmol/ g). The acid site density difference between ZAL-I and ZAL-II which consisted of the same ZnO:Al2O3 molar ratio can be attributed to the different aluminum precursors used in their hydrothermal synthesis. While the difference in chemical composition (i.e., ZnO:Al2O3 molar ratio) for ZAL-II, ZALIII, and ZAL-IV leads to their different acid site densities. The highest acid site density obtained in ZAL-III was the result of

the higher aluminum content.44 On the other hand, acid site density of the samples can be influenced by the presence of free zinc ions on the surface of ZAL spinel; for instance, the Lewis acid site density decreased progressively with increasing zinc content from ZAL-II to ZAL-IV as zinc is inherently a weaker Lewis acid than aluminum.45 3.2.2. Catalytic Activity toward m-Cresol Alkylation. The influence of reaction temperature on the catalytic activity and product distribution for all the thermally stable nanocrystalline ZAL catalysts (TG-DTA data provided as Figure S5 in Supporting Information to confirm thermal stability) was tested in gas phase iso-propylation of m-cresol with a catalyst loading of 0.5 h−1. Catalytic reactions were performed at temperatures 210, 225, 240, 255, 270, 285, and 300 °C. The conversion of m-cresol (Xm‑cresol) and selectivity (Si) to the alkylated products with respect to temperature for ZAL spinel catalysts are shown in Figure 8 (Tables S12−S15 in the Supporting Information). The maximum conversion of mcresol for catalysts ZAL-I, ZAL-II, ZAL-III, and ZAL-IV was obtained to be 77.5%, 78.1%, 85.7%, and 66.1%, respectively, at 270 °C which falls in the optimum temperature range as prescribed by thermodynamic study. This further confirmed that the isomerization of m-cresol becomes a major reaction K



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sites through its −OH group and the iso-propyl cation. Nonetheless, only the sixth positioned carbon of benzene ring of m-cresol readily reacts with the iso-propyl cation due to the least steric hindrance caused by the methyl group present at the m-position.48 Thymol further formed a C−C bond with another iso-propyl cation at higher temperatures and increased the selectivity toward 2,6DI-3MP. Selectivity to 2,4DI-5MP increased with increase in temperature which may be at the cost of 4I-3MP and 3I-5MP. Nonetheless, 4I-3MP, 3I-5MP, and 2,4DI-5MP were obtained as secondary products due to their initial lower concentration as well as the presence of a very low amount (

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