By-product formation in the nitrobenzene hydrogenation reaction with an 1

industrial nickel catalyst 2

3

aJoão D. Sousa, bClemente P. Nunes, aPaulo Araújo, cNuno M.C. Oliveira, dFilipe G. 4

Freire 5

6

aCUF – Químicos Industriais, S.A., Quinta da Indústria, 3860-680 Estarreja, Portugal 7

8

bChemical Process Centre, Chemical Engineering Department, Instituto Superior Técnico, 9

Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 10

11

cChemical Engineering Department, Faculty of Science and Technology, University of 12

Coimbra, Rua Sílvio Lima, 3030-790 Coimbra, Portugal 13

14

dIBB-Institute for Biotechnology and Bioengineering, CERENA, Instituto Superior Técnico, 15

Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 16

17

18

Corresponding author: Prof., Filipe Gama Freire; qgfreire@ist.utl.pt; IBB, Instituto Superior 19

Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 20

21

Received [Dates will be filled in by the Editorial office] 22

23

24

Although aniline synthesis via nitrobenzene hydrogenation is well documented, 25

research about simultaneous by-product formation is still scarce. 26

The understanding of undesired side reactions has significant implications for process 27

design, including both the reactor and separation units, and also for tackling important 28

environmental issues. To achieve optimum aniline selectivity, a kinetic mechanism and 29

model, able to predict by-product formation during nitrobenzene hydrogenation at an 30

industrial scale, are fundamental tools. 31

The formation of by-products in the nitrobenzene hydrogenation to aniline was studied 32

in a CSTR during transient operation, using a Ni/SiO2 catalyst. Introducing different 33

2

quantities of the detected by-products in the feed stream, and observing their time evolution 34

during the transient operation, the interactions between the different species were determined. 35

The proposed new mechanism that is now presented, considers all detected species, 36

and the experimental results were simultaneously adjusted to a relative error of approx. 10%. 37

The new mechanism is able to explain the formation of cyclohexanone, and its 38

subsequent hydrogenation to cyclohexanol, and also provides new insights on the inter-39

conversion between cyclohexanone and N-cyclohexylideneaniline. 40

41

Keywords: Aniline, by-products, mechanism, CSTR kinetics, hydrogenation 42

43

44

Introduction 45

46

With more than 300 different end products, aniline (ANL) is currently used as a raw 47

material for p,p-methylene diphenyl diisocyanate (MDI) production (Kirk-Othmer, 2007). 48

MDI, discovered by Bayer in 1930 (Wegener et al., 2001), is one of the main isocyanates that 49

reacts with alcohols (such as polyols and polyetherols) to produce polyurethanes. 50

Applications for MDI based polyurethanes are mainly rigid and semi-rigid foams, elastomers 51

and coating resins for construction, insulation, furniture and the automotive industries 52

(Ullmann’s, 2003). Over 85% of commercial ANL is synthetized by nitrobenzene (MNB) 53

hydrogenation in the presence of a metal catalyst (Fig. 1). The reaction can take place in 54

stirred (CSTR) or tubular reactors. Supported Pd and Pt catalysts are suitable for tubular 55

reactors, while Ni, Rh and Ru catalysts are the most common in CSTR designs (Aramendia et 56

al., 1982; Galvano et al, 1987; Lee et al., 2000; Gottlinger et al., 2004; Gelder, 2005; Meng et 57

al., 2009; Raj et al., 2012; Knauf et al., 2013). For support, silica, alumina or activated carbon 58

are usually chosen. 59

60

61

Fig. 1. Hydrogenation of nitrobenzene to aniline. 62

63

3

The MNB hydrogenation reaction can occur in biphasic or in triphasic reactors (Raj et 64

al., 2012; Gelder, 2005; Relvas et al., 2008a; Lynch & Ryan, 2012). In triphasic 65

configurations, slurry type CSTR reactors are normally used. Solid/liquid separation, catalyst 66

homogeneity and mass/heat transfer limitations become the main challenges in this 67

technology. Small catalyst particles, with circa 50 μm of diameter, associated with milling 68

effects and low sedimentation rates, difficult the catalyst recovery from the reaction effluent. 69

The MNB hydrogenation is carried out in slurry reactors at 353-523 K and 0.1-0.6 70

MPa (Neves, 2007). MNB conversion is normally expected to be 98-99% or higher. Water 71

and ANL form immiscible liquid phases, with 8% (w/w) water in ANL, at 298.15 K 72

(Downing et al., 1997); consequently, MNB is usually mixed in the input stream with an 73

organic solvent to reduce the water mass fraction, thus avoiding liquid phase separation. For 74

economical purposes, the best solvent is often ANL itself. This reaction is also well known 75

for its high exothermicity (ΔrH=-537 kJ mol-1) (Machado 1994; Wegener et al., 2001; Klemm 76

et al., 2001). 77

Nowadays ANL synthesis by MNB hydrogenation is a well documented and patented 78

process (Zhener 2005; Bocquenet 2008; Mahata et al., 2008; Králik et al., 2012; Qin et al., 79

2014). The reaction has been thoroughly studied and several mechanisms and kinetic models 80

have been presented through the years. Langmuir-Hinshelwood based models are usually 81

considered to describe this phenomenon (Turek et al., 1986; Frikha et al., 2006; Relvas et al., 82

2008a). Nevertheless, studies of by-product formation studies in ANL plants are as yet scarce 83

when compared to the main reaction. Few authors have described all of the species involved, 84

and even fewer have presented kinetic models. This is an important subject for economic 85

reasons, since a large fraction of the present operating costs in ANL manufacture is incurred 86

in the subsequent separation of the by-products originated during the hydrogenation step. 87

Hence the significance of more studies, like the present one, which describe the formation of 88

by-products during the MNB hydrogenation to ANL in industrial plants. 89

90

By-products of the MNB hydrogenation 91

92

In 1986, Turek et al. studied the liquid phase MNB hydrogenation in the presence of 93

Ni/Al2O3 in ethanol. The following by-products were detected by gaseous chromatography 94

(GC): cyclohexanol (CHOL), N-ethylaniline, toluidine, cyclohexylamine (CHA) and 95

diaminobenzene. The authors only constructed a kinetic model for CHOL and N-ethylaniline, 96

which were the main by-products. Since ethanol was used as a solvent, these two by-products 97

4

are expected to reach higher concentrations, as a result of the reaction between ethanol and 98

ANL molecules. 99

Graham (1980) and Weidig (1989) patented methods for pure ANL storage during 100

large periods, i.e., to avoid discoloration and oxidation. Their reaction effluent consisted 101

mainly of ANL and water, but Graham noticed that, of all impurities, cyclohexanone 102

(CHONA) was the critical one because of its similar volatility to ANL, thus hindering the 103

purification of this product by distillation. The authors also pointed out that CHONA reacts 104

reversibly with ANL to form N-cyclohexylideneaniline (CHENO) and water (Fig. 2); this 105

reaction is fast even at room temperature. Finally, according to these authors, one can displace 106

the equilibrium to the right by adding phosphoric acid or a derivative thereof. For this 107

purpose, the concentration of phosphoric acid should not exceed 5% (w/w). 108

109

110

Fig. 2. Cyclohexanone / N-cyclohexylideneaniline equilibrium. 111

112

Recently, Shimizu et al. (2011) studied the selective hydrogenation of 113

nitrocyclohexane to cyclohexanone oxime, an important reactant in the nylon-6 synthesis, by 114

alumina-supported gold cluster catalysts. Cyclohexanone oxime can be also hydrogenated into 115

CHONA and CHA. They react between them to form cyclohexyl-cyclohexylidene amine and 116

dicyclohexylamine (DICHA) appeared as by-product. 117

Narayanan et al. (1995; 1997) studied the products formation in the ANL 118

hydrogenation over Co/Al2O3 and Ni/Al2O3. The products identified were: cyclohexylamine 119

(CHA), N-cyclohexylaniline (CHANIL), dicyclohexylamine (DICHA) and, through CHA 120

deamination, cyclohexane and ammonia (Fig. 3). For short residence times, only CHANIL 121

was observed; for medium residence times CHA and DICHA start to appear and, finally, 122

cyclohexane and ammonia were also detected. The authors suggested that ANL is first 123

hydrogenated to CHA and then reacts with another ANL molecule to form CHANIL. This last 124

species can then be hydrogenated to DICHA; this hypothesis was supported by the fact that 125

no CHANIL and DICHA were formed without CHA in the reaction bulk. Comparing 126

Co/Al2O3 and Ni/Al2O3, in the presence of Co/Al2O3, the authors observed only the formation 127

5

of CHA and CHANIL, while in the presence of nickel DICHA was also detected. Residual 128

cyclohexane and ammonia were also detected, but only for temperatures higher than 523 K. 129

130

131

132

Fig. 3. Mechanism of by-product formation during the MNB hydrogenation, by Narayanan et 133

al. (1997). 134

135

In their work on the MNB hydrogenation in trickle-bed reactors, using aluminum 136

supported palladium catalysts, Mitchell et al. (2010) not only observed CHA, CHANIL and 137

DICHA, CHONA and CHENO, but also ciclohexanol (CHOL), benzene and diphenylamine. 138

Mahata et al. (2008), considered the MNB hydrogenation over nickel nanoparticles 139

stabilized by filamentous carbon, and detected the existence of parallel reactions where 140

impurities in MNB act as reactants. The authors identified, for example, the hydrogenation of 141

p-nitrophenol to p-aminophenol. Nitrophenols can be found as residual impurities in 142

commercial MNB (>50 ppm), since they are by-products in the synthesis of MNB through the 143

benzene nitration process. However, this phenomenon can be better observed in lab scale 144

experiments while in industrial MNB plants it is more rarely detected. On the other hand, 145

several authors (Neves 2007; Relvas 2008) mentioned the existence of toluidine in 146

commercial ANL. Toluidine is a nitrotoluene hydrogenation product. Nitrotoluene is an 147

impurity in the MNB feed flow (>200 ppm) formed by the toluene nitration (which is an 148

impurity of benzene). 149

In 1997, Nagata et al. proposed a mechanism for by-product formation in the MNB 150

hydrogenation. The authors identified CHA, CHANIL, CHONA, CHENO and DICHA. Fig. 4 151

presents this mechanism for the MNB hydrogenation in the gas phase, over a Pd/Pt catalyst. 152

6

ANL is first hydrogenated to form an imine, a hypothetical intermediate (not detected by gas 153

chromatography analysis), in a rate-limiting step. This imine is considered to be extremely 154

reactive and originates CHA, CHONA or CHANIL. Like Graham et al., (1980) Nagata et al. 155

(1997) indicate that CHONA reacts with ANL to form CHENO and water. CHENO can be 156

also hydrogenated to CHANIL and further hydrogenated to DICHA. Apparently these last 157

authors did not verify any relation between the presence of CHA and CHANIL, like 158

Narayanan et al. (1997) did. 159

160

Fig. 4. Mechanism for by-product formation during the MNB hydrogenation, by Nagata et al. 161

(1997). 162

163

Cirtiu et al. (2007) indicates that CHONA can be hydrogenated to CHOL and Petrov et al. 164

(1989) mentioned also the presence of benzene, ammonia, diphenyl and methane. 165

With a 2% Ru/Al2O3 catalyst, Debdut and Raghunath (2005) identified aminocyclohexene 166

by GC analysis, as the intermediate of the aniline hydrogenation to cyclohexylamine and 167

proposed a simple mechanism for CHA formation. 168

Relvas (2008; et al., 2008a), in his work on MNB hydrogenation with supported nickel 169

catalysts, analysed the by-product formation in industrial reactors. The author verified the 170

influence of temperature, pressure, MNB concentration and catalyst mass in CSTR 171

experiments, and proposed also a mechanism for by-product formation. Of the already 172

7

mentioned species, CHA, CHANIL, DICHA, CHONA, CHOL and CHENO were observed. 173

The kinetic model was based on the Nagata et al. (1997) mechanism, with the CHONA 174

hydrogenation step (suggested by Cirtiu et al. in 2007) also included (Fig. 5). Although 175

Relvas (2008; et al., 2008) includes the imine formation, this species was not experimentally 176

detected. The type of catalyst and its catalytic activity seemed to have an important role in the 177

extension of the reactions. For example, the author verified that the CHONA concentration 178

increased as the catalyst became deactivated. A problematic aspect was describing the 179

CHENO formation, since this model presented relative deviations higher than 20%, which 180

only occurred with this chemical species. 181

182

183

Fig. 5. Mechanism for by-product formation during MNB hydrogenation, by Relvas (2008; et 184

al., 2008). 185

186

More recently, Králik et al. (2012) suggested a complex reaction mechanism for the 187

hydrogenation of ANL to CHA in the presence of a commercial powder Ru catalyst, directed 188

towards the CHA production. CHA is an important intermediate for the production of CBS 189

8

(N-cyclohexyl-2-benzothiazolesulphenamide), an accelerator for rubber vulcanization and 190

water treatment, among other uses. Yields of CHA of up to 95% were achieved, whereas 191

DICHA was formed as the main side product. The authors claim that the ANL conversion to 192

CHA, which requires the total hydrogenation of the aromatic ring, is a complicated procedure 193

in the liquid phase. It requires a wide number of side reactions as shown in Fig. 6, involving 194

several unstable intermediate species to form CHA and/or DICHA. These occur in several 195

reversible steps, although many of these intermediate species may be assumptions of plausible 196

intermediates that are not readily detected experimentally. However, as the ANL 197

hydrogenation occurs in the absence of oxygen molecules, i.e., in the absence of water and 198

MNB molecules, previously detected species such as CHONA and CHOL are not included in 199

this reaction mechanism. 200

Furthermore, Králik et al. (2012) also addressed the MNB hydrogenation reaction in 201

the liquid phase (Fig. 7). The authors notice that the thermal energy of the output streams in 202

the industrial MNB hydrogenation is also important, due to the high exothermicity of this 203

reaction. In this scenario, although the highest reaction temperature is preferred for maximum 204

heat recovery, the by-product selectivity and lifetime of the catalyst need also to be 205

considered, which constitute potentially conflicting objectives. This compromise has 206

motivated inventors Pasek & Petrisko (2012) to patent a multi-stage reactor, consisting of two 207

continuous reactors in cascade. The first reactor operates at a higher temperature (up to 208

250ºC) with conversion of MNB up to circa 99%, while the second reactor operates at a lower 209

temperature (≤100ºC), to hydrogenate the non-converted MNB. This enables to production of 210

high-pressure steam in the first reactor and ANL with high selectivity in the second reactor, 211

resulting in virtually 100% conversion of MNB. 212

213

9

214

Fig. 6. Reaction routes of aniline hydrogenation, by Králik et al. (2012). 215

216

217

218

Fig. 7. Reaction routes of MNB hydrogenation, by Králik et al. (2012). 219

220

10

221

This overview indicates that the information about by-product formation during ANL 222

production is still little understood. The amount of by-products, the possible mechanism 223

paths, and the possibility of having reversible, parallel and/or side reactions create a very 224

large number of plausible mechanisms that could be considered good initial candidates to 225

quantitatively describe the formation of by-products. However, despite the previous efforts by 226

several authors and to the best of our knowledge, no single mechanism published in the open 227

literature seems to adequately describe the complete formation of by-products resulting from 228

the liquid phase hydrogenation of MNB to ANL, in conditions of industrial interest. 229

The availability of an established mechanism, and a reliable kinetic model for by-230

product formation during the hydrogenation phase, is a key factor for thorough ANL plant 231

optimization, since they can provide a systematic basis for improved reactor designs that 232

allow an increased heat recovery in this phase, while simultaneously leading to significant 233

reductions of the heat loads necessary in the final distillation steps. 234

The main goal of this work is therefore to contribute to a better understanding of the 235

mechanism of by-product formation during the ANL production by liquid phase 236

hydrogenation of MNB. Due to the complexity of the mechanisms previously proposed, an 237

innovative aspect of this study is the use of transient data from various laboratory runs, where 238

some of the by-products are selectively included in the input stream, in various experiments. 239

These combinations of experimental conditions provide independent data that allow a better 240

elucidation of the existing interactions between different by-products during the several 241

reaction steps. 242

243

244

Experimental 245

246

To study the by-product formation, several hydrogenation tests in the presence of a 247

commercial Ni/SiO2 supported catalyst were carried out in a 1 L Parr continuous stirred lab-248

scale reactor (model 4520), for 8 hours each. The diagram of the experimental installation 249

used is presented in Fig. 8. To promote a homogeneous solution with effective gas dispersion, 250

the reactor has an air-impelled stirrer. In each experiment, the reactor was first loaded with 251

catalyst and ANL (≥99.8%) and then pressurized with H2. The H2 flow was monitored with a 252

Brooks flow meter and a pressure regulator is used to control the reactor pressure. The 253

reaction bulk was heated with an electric resistance, while a water-cooling coil removed the 254

11

excess reaction heat. All tests were operated under near isothermal and isobaric conditions, 255

with a stirring speed above 1000 rpm to ensure a chemical regime (Frikha et al., 2006; 256

Machado 1994). A stainless steel filter guaranteed solid-liquid separation of the output flow. 257

The input stream was promoted with a HPLC pump. 258

The set of operating conditions is presented in Table 1. Due to confidentiality reasons, 259

and notwithstanding the main motivation of completely understanding the kinetic mechanism 260

for by-product formation in MNB hydrogenation, some of the operating conditions used are 261

indicated only as reference values, while some of the concentrations are reported in 262

adimensional units (a.u.). 263

264

265

Fig. 8. Schematic representation of the experimental setup. 266

267

268

Table 1. Experimental reactor conditions 269

270

Type CSTR

Temperature (K) T

Pressure (MPa) P

Liquid volume (L) 0.5

Residence time (min) 100

Catalyst concentration (kgcat/kgbulk) Ccat

Catalyst particle diameter (μm) 1-1000

Stirring speed (rpm) >1000

271

Reactor

PI

H2

Sample

Pump

Input stream

Cooling water

Flowmeter

PI

Compressed air

TT

FT

N2

Heat exchanger

Filter

12

272

All experiments were performed at the same temperature, pressure, and residence 273

time, using the same catalyst concentration. On the other hand, in each experiment the input 274

stream had a different composition as shown in Table 2. Five experiments were chosen: in 275

experiment EANL, pure ANL (≥99.8% w/w) was introduced in the reactor; experiments 276

E0.08CHA, E1CHA and EH2O consisted in CHA (0.08% w/w), CHA (1% w/w) and H2O (4% w/w) 277

diluted in the ANL input stream, respectively. These three experiments were then compared 278

with the MNB hydrogenation by feeding the reactor with an ANL/MNB mixture (70/30% 279

w/w). 280

281

282

Table 2. Hydrogenation reaction experiments. 283

284

Experiment Input stream composition

(% w/w)

EANL ANL (≥99.8)

E0.08CHA ANL + CHA (0.08%)

E1CHA ANL + CHA (1%)

EH2O ANL + H2O (4%)

EMNB ANL + MNB (30%)

285

286

Organic compounds were quantified by GC, using an Agilent 6890A chromatograph, 287

equipped with two parallel FID detectors using two columns: a HP-5 column (5% diphenyl e 288

95% dimethylpolysiloxane, 50m × 320µm × 1,05µm) and a wide bore HP-1 column (100% 289

dimethylpolysiloxane, 30m × 320µm × 4µm). The calibration method was done by an external 290

standard. 291

Besides ANL and MNB, the species identified in the output flow were CHA, CHOL, 292

CHONA, CHENO, CHANIL, DICHA and some benzene. Residual o-toluidine was also 293

detected. Species like cyclohexane and phenol were not detected. ANL was the major 294

compound in every sample analysis (≥95% w/w). As ANL behaves in the case as a solvent, its 295

concentration was determined by mass balance closure. For modeling purposes, ANL was 296

assumed to be in excess and its concentration was assumed to remain constant throughout the 297

experiment. 298

13

299

Mathematical modeling 300

301

MNB hydrogenation kinetic rate 302

303

Several authors claim that the MNB hydrogenation reaction can be considered 304

irreversible. Gelder (2005) states that the nitro group is one of the most easily transformed of 305

all functional hydrogenations groups (losing only to acetylene hydrogenation). The estimated 306

value of the equilibrium constant for the MNB hydrogenation reaction is 1.53×1084 at 307

standard temperature and pressure, which indicates that the reaction is essentially irreversible 308

for our practical purposes. The kinetic term for MNB hydrogenation is well described by 309

Langmuir-Hinshelwood based models like the ones shown in expressions Eq. (1), claimed by 310

Relvas et al. (2008a), or Eq. (2), claimed by Turek et al. (1986), respectively: 311

312

(1) 313

314

(2) 315

316

Here r1 is the rate of MNB hydrogenation per mass of catalyst, CMNB and CH2 are the 317

concentrations of MNB and H2, k1 is the rate constant, KMNB and KH2 are adsorption constants 318

for MNB and H2, respectively. Since in this work the total pressure was maintained constant 319

throughout the experiments, the influence of the H2 partial pressure is not quantified. 320

Moreover, as the temperature and residence time also remained constant throughout the tests 321

in this CSTR apparatus, the effect of each adsorption term on the kinetic rate is difficult to be 322

discriminated. However, for MNB concentrations inferior to 50 ppm in the reactor, the 323

reaction can be approximated by a first order model relative to the reactant (Relvas et al., 324

2008a). According to the experimental data, the MNB content was practically residual and 325

therefore, Eq. (1) and Eq. (2) can be simplified to Eq. (3) 326

327

(3) 328

329

)1( 22

2211

HHMNBMNB

HMNBHMNB

CKCKCCKKk

r++

=

)1)(1( 22

2211

HHMNBMNB

HMNBHMNB

CKCKCCKKk

r++

=

MNBCkr '11 )( =-

14

where k’1= k1KMNBKH2CH2 is the apparent kinetic rate constant for the MNB 330

hydrogenation, per mass of catalyst. 331

332

By-product kinetic rates 333

334

All reactions that lead to the formation of by-products were initially assumed as 335

reversible steps, in various possible reaction networks. The setup mass balances can be 336

generically written for species as eq. 4. 337

338

(4) 339

340

where Cj,in and Cj are the concentrations of species j in the input stream and output 341

stream, respectively; τ is residence time, mcat is the catalyst mass, rForm and rCons are the rates 342

of formation/consumption that occur on the surface of the catalyst (and thus are dependent on 343

the catalyst mass), and αl,j are the stoichiometric coefficients of the species j on reaction l. 344

Given the previous assumptions, each reaction rate was considered elementary and so 345

expressed as a product of a kinetic constant and the concentrations of the reactant species j 346

and l as in eq. 5: 347

348

(5) 349

350

Non-linear regression was applied to estimate the kinetic parameters for different 351

hypothetical reaction mechanisms and experimental data. For this purpose, an objective 352

function (eq. 6) based on the weighted sum of squares was minimized, 353

354

(6) 355

356

where ωi,j are the weights for species j in experiment i, which in the present case are 357

given by eq. 7: 358

359

catlConslForml

jljinjj mrrCC

dtdC

úû

ùêë

é-+-= å )()(1 ,,,, a

t

jill CCkr =

( )2

, 0, )()()( å å

=

-=ji

t

ti

Modji

Expjjii tCtCkE v

15

(7) 360

361

E(ki) is the objective function to be minimized; and are the 362

concentrations of the species j, at time t in experiment i, given by the experimental data and 363

corresponding model, is the average concentration of species j registered in experiment 364

i. A nonlinear programming (NLP) formulation was constructed, corresponding to the 365

minimization of this objective function subject to the set of previous mass balances, their 366

kinetic models and parameter constraints, as shown in Eq. (8). In this minimization all 367

parameters and concentrations have non-negative values. 368

Given the nature of the kinetic mechanisms described previously, the existence of 369

intermediate species such as imines (not detected experimentally, though) is very likely. 370

When the mechanisms considered include the presence of this type of species, an additional 371

constraint that limits the maximum concentration of these intermediates to the GC threshold 372

of detection (GC LD) is also added to the formulation. 373

374

(8) 375

376

Approximate standard errors (ASE), sometimes also referred to as an asymptotic 377

standard error, were calculated for each parameter estimate. ASE can be used for computing 378

confidence intervals in place of exact standard errors (Graybill, 1994). 379

Considering the previous kinetic models, proposed by other authors, and the 380

qualitative analysis of the experimental results obtained, an initial mechanistic scheme was 381

considered (Fig. 9). The parameters ki in this model were initially determined, and they were 382

compared with their respective ASE(ki) values. When large ASE(ki) values were detected, 383

various simplifications in the kinetic terms were considered, including suppressing these 384

kinetic steps. 385

386

ϖ i, j =1!CjExp

l

CjExp(t)

lCj

Mod (t)l

!CjExp

l

Min E(ki )s.t. Eqs. (4 − 5)

ki ≥ 0, ∀ik ≡ ki,k−i{ }Cj ≥ 0, ∀iCintermediates ≤GC LD

16

387

388

Fig. 9. Initially proposed mechanism for by-production formation during MNB 389

hydrogenation, in the presence of a supported nickel catalyst. 390

391

392

All of the reaction schemes previously described were used as starting points for this 393

purpose, and modifications were introduced when necessary. Both Narayanan’s and Nagata’s 394

models were not sufficient by themselves to describe all detected species, and thus needed to 395

be combined with additional reaction paths to describe the complete system. Between 396

Relvas’s and Králik’s models, the later could produce better agreement with experimental 397

data, but at the cost of a greatly increased model complexity. 398

The parameter estimation and ASE calculations were performed by COPASI, a 399

Complex Pathway Simulator (Hoops et al., 2006). 400

401

402

Results and discussion 403

404

17

Preliminary experiments, at room temperature, confirmed the equilibrium between 405

CHONA and CHENO, as shown before in Fig. 2. By adding CHONA to a pure ANL sample, 406

the formation of CHENO was immediately detected. Although this experimental data was not 407

modeled, it helped to reach a final reaction mechanism (especially relatively to step 12 in Fig. 408

9). 409

Since the initial scheme has many interdependencies between the by-products giving 410

low precision to some of the parameters, ki, we chose an experimental design that was able to 411

“break” some of these correlations, especially between the intermediate species, by perturbing 412

the eventually reversible steps that involve these species and the reaction steps. 413

Using this procedure, we were able to simplify the initially proposed mechanism to the 414

simplified mechanism presented in Fig. 10. 415

416

417

418

Fig. 10. Final mechanism for by-production formation during the MNB hydrogenation in the 419

presence of a Ni supported catalyst, based on the kinetic modeling results shown in Table 3. 420

421

422

NH2

NH2

ANL

CHA

HN

HN

CHANILDICHA

N

CHENO

O OH

ANL+NH3

CHONA CHOL

NO2

MNB

H2O

2H2O

NH3ANL

NH3ANL

r1

r3

r2

r13

r12

r9

r5

18

Experimental profiles and modeling results achieved with this new proposed 423

mechanism are shown in Figs. 11 to 16, for each case, and duly analysed. 424

As can be observed, the effect of water creates two very different scenarios. In its 425

absence, compounds like CHA, CHANIL, CHENO and DICHA are favored, while no 426

significant amounts of CHONA and CHOL are observed. In fact, in experiments with ANL 427

and ANL+CHA, the amounts of CHONA and CHOL are proportional to the residual humidity 428

in the ANL input stream (<0.01%). In the second scenario, when water exists in the medium, 429

whether introduced in the reactor or appearing as a by-product of the MNB hydrogenation, 430

not only the concentrations of CHONA and CHOL increase considerably, but also the amount 431

of the other compounds is reduced. This phenomenon explains why Narayanan (see Fig. 3) 432

did not observe any CHONA/CHOL. An important fact is that the CHENO formation, 433

although in a way not totally clear yet, plays a major role in the reactor. Most of the reactions 434

steps are influenced by this species and thus its comprehension is crucial to better 435

understanding the formation and behavior of other by-products. 436

Other species like CHA, DICHA and CHOL are molecules with saturated carbon 437

cycles. These come most likely from hydrogenation reactions with ANL, CHANIL and 438

CHONA molecules, respectively, where activated hydrogen at the catalyst surface reacts with 439

the unsaturated bonds in the aromatic rings. The existence of a small quantity of benzene 440

suggests that hydrogenolysis reactions take place in the reactor, by withdrawing the nitro or 441

the amine group from MNB or ANL molecules, respectively. However no cyclohexane was 442

detected. Therefore no hydrogenolysis reactions with molecules with saturated carbon cycles 443

like CHA, CHOL, CHONA and DICHA seemed to take place. 444

A comparison of the reaction profiles in each experiment shows a distinct behavior for 445

CHENO and for CHONA. The initial rate of CHENO formation is the highest among them 446

(Fig. 12). After a peak, the CHENO concentration drops before reaching stationary state. On 447

the other hand, CHONA (Fig. 13) only appears if water exists in the solution. This 448

observation is in total disagreement with the Nagata’s model (Fig. 4) as CHONA is not 449

present in experiments ANL and ANL+CHA. As such, CHENO must come from another 450

source rather than the reaction between ANL and CHONA (step 12, Fig. 9), which is probably 451

from an imine (step 4, Fig. 9). These statements are supported by the fact that the CHENO 452

concentration diminishes as CHONA increases. Nevertheless, additional modeling results 453

show that these two steps are not sufficient to explain the CHONA and CHENO behaviors, 454

and consequently step 11 cannot be ignored. According to Fig. 14, CHOL was formed only in 455

the presence of CHONA (step 13); this step is expected and intuitive. 456

19

457

458

459

Fig. 11. Comparison of experimental data (symbols) and fitted CHA concentrations (lines) for 460

all experiments. 461

462

463

464

Fig. 12. Comparison of experimental data (symbols) and fitted CHENO concentrations (lines) 465

for all experiments. 466

467

468

20

469

Fig. 13. Comparison of experimental data (symbols) and fitted CHONA concentrations (lines) 470

for all experiments. 471

472

473

Fig. 14. Comparison of experimental data (symbols) and fitted CHOL concentrations (lines) 474

for all experiments. 475

476

477

478

21

Fig. 15. Comparison of experimental data (symbols) and fitted CHANIL concentrations 479

(lines) for all experiments. 480

481

482

483

Fig. 16. Comparison of experimental data (symbols) and fitted DICHA concentrations (lines) 484

for all experiments. 485

486

487

Kinetic constants, ASE and final modeling errors are shown in Table 3 and Table 4, 488

respectively. 489

These suggest that the CHA, CHONA, CHANIL, DICHA and CHOL formation 490

occurs in essentially irreversible reactions. This behavior was to be expected, since it seems 491

unlikely that stable saturated species (like CHA, DICHA or CHOL) are able to dehydrogenate 492

back to their unsaturated forms, and also reinforces the consistency of the kinetic scheme. 493

Nevertheless, this interpretation cannot be extended to all of the steps considered. For 494

instance, in step 2 the results indicate that the reverse constant is orders of magnitude higher 495

than the direct constant. However, as kinetic rates are also proportional to the concentration of 496

the reactants, if we multiply the direct rate constant by the ANL concentration and the reverse 497

constant by the CHENO and NH3 concentrations, we see that step 2 occurs most probably in 498

equilibrium. On the other way, the CHONA formation, known as reversible was achieved as 499

an irreversible reaction. These can be explained as the CSTR experimental conditions are fare 500

from the equilibrium. 501

502

503

22

Table 3. Kinetic rate constants for forward (ki) and reverse (k-i) reactions based on the 504

reaction mechanism considered in Fig. 10. 505

506

Step ki Asymptotic Std.

Error ASE(ki)

r1 4.0×102 - r2 4.1×10-4 0.2×10-4 r-2 1.8×103 0.2×103 r3 2.6×10-3 0.08×10-3 r5 1.70×10-3 0.07×10-3 r9 4.3 0.15 r12 19.2 0.8 r13 0.44 0.01 507

508

Table 4. Kinetic model errors. 509

510

Experiment Regression error E(ki) (×107)

EANL 4.1 E0.08CHA 2.3 E1CHA 0.87 EH2O 1.0 EMNB 2.4 Objective Value 10.7×10-7 511

512

Also according to Table 4, the final values of the objective function obtained for each 513

of these experiments, is of the same order of magnitude. These results indicate that the final 514

model is able to explain equally well each of the experimental tests considered, without a bias 515

towards a particular test. 516

Parity plots for all species and experimental runs are shown below, in Fig. 17. The 517

region inside the dotted lines represents deviations within a relative range of 10%. As can be 518

observed, the model is able in general to interpret reasonably well the experimental data, with 519

some exceptions, such as the CHONA and DICHA species. 520

521

522

23

523

524

525 Fig. 17. Parity plots of experimental concentrations (Exp.) and the corresponding model 526

values (Mod.) for all species and experiments. Dotted lines represent the ±10% error area. 527

528

529

Given the incremental procedure followed in the construction of this reaction scheme, 530

and the many alternative candidates evaluated, we believe that these results are representative 531

of the level of accuracy that can be obtained at this level of model complexity. 532

Based on the NLP results, the final scheme proposed is presented in Fig. 10, 533

constitutes a new and better insight of the overall mechanism for the formation of by-products 534

in the nitrobenzene hydrogenation reaction. 535

536

Conclusions 537

538

24

This work studied the by-product formation during the nitrobenzene hydrogenation 539

with a supported metal catalyst. This is a problem with significant industrial relevance, due to 540

the high costs incurred during the purification phase of aniline manufacture. A literature 541

review indicated that while this subject was considered in several previous works, its present 542

understanding is currently incomplete, and hinders the optimization of the overall industrial 543

process units where this reaction takes place. 544

Using experimental data obtained from a laboratory scale CSTR, various tests were 545

performed, using streams with varying compositions, to elucidate the interdependences of the 546

different by-product species. 547

Based on these results and partial information relative to possible reaction mechanisms 548

resulting from previous studies, a simple new reaction scheme was proposed in Fig. 10, that is 549

a major advance in the overall interpretation of the proposed reaction. 550

A major contribution of this work is that the corresponding kinetic model is able to 551

interpret, for the first time, the concentration profiles obtained in the experimental tests, 552

within a reasonable experimental error. The final reaction scheme illustrates the close 553

dependence of the formation of these secondary species. 554

The results obtained show that the effect of water in the reactor has a decisive impact. 555

In its presence, by-product formation moves towards cyclic carbon molecules with oxygen, 556

like cyclohexanone and cyclohexanol. On the other hand, in its absence molecules like 557

cyclohexylamine, dicyclohexylamine and N-cyclohexylaniline are obtained as the main by-558

products. 559

It was also found that it is most likely for dicyclohexylamine to come mainly from a 560

reaction between cyclohexylamine and aniline molecules and not from the hydrogenation of 561

N-cyclohexylaniline. According to the now proposed new model, for cyclohexanone to exist 562

the bulk reaction mixture must have water, while, on the other hand, N-563

cyclohexylideneaniline is always detected, even without the presence of water. It is also 564

important to underline that in the absence of cyclohexanone, no cyclohexanol is detected. 565

566

567

568

Symbols 569

570

ANL aniline 571

ASE approximate standard error 572

25

CHA cyclohexylamine 573

CHANIL N-cyclohexylaniline 574

CHENO N-cyclohexylideneaniline 575

CHOL cyclohexanol 576

CHONA cyclohexanone 577

CSTR continuous stirred tank reactor 578

DICHA dicyclohexylamine 579

GC LD gas chromatography threshold detection 580

MDI p,p-methylene diphenyl diisocyanate 581

MNB nitrobenzene 582

NLP non-linear programing 583

584

585

Acknowledgements 586

The authors acknowledge financial support from the Fundação para a Ciência e 587

Tecnologia through the Ph.D. Grant SFRH/33920/2009, and technological support from CUF-588

QUI, S.A., for the development of this work. 589

590

591

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