Kinetic Modeling of Quinoline Hydrodenitrogenation over a NiMo(P

Article pubs.acs.org/IECR

Kinetic Modeling of Quinoline Hydrodenitrogenation over a NiMo(P)/ Al2O3 Catalyst in a Batch Reactor Minh-Tuan Nguyen,†,‡ Melaz Tayakout-Fayolle,*,† Gerhard D. Pirngruber,‡ Fabien Chainet,‡ and Christophe Geantet† †

Institut de recherches sur la catalyse et l’environnement de Lyon, IRCELYON, UMR 5256-CNRS, 2 avenue Albert Einstein, F-69626 Villeurbanne, France ‡ IFP-Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France S Supporting Information *

ABSTRACT: A kinetic study of the hydrodenitrogenation of quinoline is performed in a batch reactor, over a NiMo(P)/γ-Al2O3 sulfide catalyst, in the range of temperature of 340−360 °C and concentration of 1−2 wt % of quinoline. Liquid−vapor mass transfer is considered in the reactor model, and the kinetic expression using Langmuir−Hinshelwood model considers competitive adsorption of reactants, products, and solvents. The activation energies of every elementary reaction and adsorption enthalpies of nitrogen compounds are calculated. The kinetic modeling shows that the hydrogenation of 1,2,3,4tetrahydroquinoline into decahydroquinoline is the rate-determining step of the principal reaction pathway. The self-inhibition effect due to competitive adsorption of nitrogen-containing compounds is confirmed. The adsorption constants of nitrogen compounds decrease in the order saturated amines > NH3 > aromatic amines, showing that their adsorption strength is related to the basicity of molecules. Moreover, the kinetic model is validated by an additional experiment using ammonia as an inhibitor.

1. INTRODUCTION The refining industry observes an increasing demand for upgrading heavy feedstocks such as vacuum gas oil (VGO) or vacuum residues from heavy crude oils. These heavy crudes are rich in nitrogen- and sulfur-containing aromatic compounds, which make the hydrotreatment of such feedstocks difficult. Nitrogen-containing organic compounds that were not removed by hydrotreating show many negative effects on downstream oil refining processes and on the specification of final products: (i) poisoning acid catalysts in upgrading processes such as hydrocracking (HCK) and catalytic cracking (FCC) processes (even below 50 ppm);1−3 (ii) inhibiting effect on deep hydrodesulphurization (HDS) and hydrodearomatization (HDA) due to their competitive adsorption;4−6 and (iii) causing instability and color degradation to final products. To achieve a more efficient removal of nitrogen compounds in heavy feedstocks, refiners can choose severe reaction conditions for hydrotreatment (higher temperature and pressure), but this option is very energy-consuming. A better solution is to improve hydrodenitrogenation (HDN) catalysts. The development of more active HDN catalysts is achieved by a better understanding of the reaction pathways and role of reaction sites. Real feedstocks like VGO are very complex mixtures of many different molecules, and it is very difficult to carry out comprehensive mechanistic studies with real feedstocks. A better option is to use well-defined model molecules where the reaction network can be studied in detail. A small molecule like quinoline already has a rather complex reaction network, with numerous elementary steps and parallel competing pathways (Figure 1). A detailed kinetic analysis of the reaction network of quinoline allows the evaluation of the rate constants of each step of hydrogenation of aromatic rings, ring opening, or C−N bond cleavage. Inhibition effects by © XXXX American Chemical Society

Figure 1. Reaction scheme of quinoline hydrodenitrogenation.

adsorption of nitrogen-containing intermediates or products can also be assessed. Because of the complexity of the reaction network, most past works on modeling of quinoline HDN have been investigated with assumptions and simplifications of the reaction scheme. In the study of the quinoline hydrodenitrogenation over NiMo/ Al2O3, Satterfield and Yang7 assembled propyl-cyclohexylamine (PCHA), propylcyclohexene (PCHE), propylbenzene (PB), and propylcyclohexane (PCH) into a lumped compound. They further defined three groups of compounds, which have the same adsorption constant: (i) the aromatic amines including quinoline, 1,2,3,4-tetrahydroquinoline (14THQ), 5,6,7,8-tetrahydroquinoline (58THQ), and ortho-propyl-aniline (OPA); (ii) the secondary amines, decahydroquinoline (DHQ); and Received: June 16, 2015 Revised: September 12, 2015 Accepted: September 14, 2015

A

DOI: 10.1021/acs.iecr.5b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

quinoline. The initial concentration of quinoline for kinetic studies was set as 1, 1.5, or 2 wt %. After closing the system, the reactor was purged two times by 0.6 MPa of nitrogen and two times by 1 MPa of hydrogen to avoid any air contamination and then charged by 3.2−3.3 MPa of hydrogen (approximately) at ambient temperature. The reactor was heated to the desired temperature (340, 350, or 360 °C), and the total pressure increased to 7 MPa. The speed of agitation (800 rpm) and the size of catalyst particles (80−125 μm) were selected so that diffusional limitation was absent. To validate the kinetic model, an additional experiment was also carried out (350 °C, 7 MPa, and 1 wt % of quinoline). In this test, 3 g of dodecylamine were added to the reaction mixture. Dodecylamine is very reactive and quickly releases NH3, which is expected to represent an inhibiting effect on quinoline HDN. For each test, when the pressure and temperature of the reaction mixture arrive at 7 MPa and desired temperature (340, 350, or 360 °C), a liquid sampling was carried out; this moment was denoted as the initial point of reaction (to). The liquid samplings were carried out every 15 min in the first hour, every 30 min in the second hour, and subsequently every 1 h. To compensate the hydrogen consumption (about 0.1 MPa during 1 h) in the reaction, the inlet flow of H2 is utilized to maintain the reaction pressure at 7 MPa. The reaction products were analyzed by gas chromatography (model HP) equipped with the silicone capillary column Agilent 190955-023E HP5 (5% Phenyl Methyl Siloxane, 30 m × 530 μm × 0.88 μm film thickness) and flame ionization detector. The column temperature increased at 5 °C·min−1 from 40 to 70 °C, 1.5 °C·min−1 from 70 to 170 °C, and 15 °C· min−1 from 170 to 300 °C (hold for 8 min). The gas vector (nitrogen) flow entering in the GC column was 5.1 mL·min−1. Hexadecane was used as internal standard. The identification of products was performed by using GCxGC/MS analysis, with 2 columns (column 1, ZB1 30 m × 0.25 μm × 0.25 μm; column 2, ZB50 10 m × 0.1 mm × 0.1 μm), and the temperature increased at 2 °C·min−1 from 50 to 310 °C. Several heavy condensed products were observed in trace amounts by GCxGC/MS. Because of the complexity in the identification of these products, the exact structure of the molecules is unknown. Although the quantification was not performed, it is likely that the formation of these molecules was favored by the increase of temperature and the initial concentration of quinoline. Satterfield and Yang11 also found that the appearance of higher molecular weight products containing nitrogen was enhanced with increasing temperature. These compounds, representing less than 5% of the products, are formed by the condensation reaction and isomerization. These reactions were not taken into account in our kinetic model. After the reaction was stopped by cooling, the used catalyst was recovered and then washed in hot n-heptane (about 95 °C) for 24 h by Soxhlet extraction to remove traces of solvent and adsorbed products. Finally, the used catalyst was dried at 95 °C under vacuum before measuring its nitrogen adsorption isotherm and its carbon and sulfur content (by CHNS analysis). The investigation of textural properties of used NiMo(P)/ Al2O3 catalyst showed that the average pore diameter did not significantly change after reaction. However, the values of total pore volume (TPV) and BET surface area decreased by about 10% after catalytic tests because of the coke deposition and the

(iii) ammonia (NH3), which was separately treated. The authors also studied the influence of H2S partial pressure on the HDN activity of a NiMo/Al2O3 catalyst. The kinetic analysis allowed them to demonstrate that a low partial pressure of H2S promotes the C−N bond-breaking steps, while it slightly inhibits hydrogenation steps. Surprisingly, further kinetic works often did not consider the inhibition of ammonia, leading to incorrect evaluations of kinetic parameters. In a study of Jian and Prins,8 not only quinoline, but also the intermediates THQ, DHQ, and OPA were used as reactants to obtain a more reliable estimation of the kinetic parameters. A kinetic analysis of the reaction network of quinoline on NiMo/Al2O3 catalysts with or without phosphorus was performed. The kinetic model involved three types of catalytic sites: the site responsible for (de)hydrogenation of an aromatic heterocyclic ring, the second site for hydrogenation of an olefin and the phenyl ring, and the last for C−N cleavage. Their results of modeling indicated that the hydrogenation of 14THQ to DHQ and the ring-opening from DHQ to PCHA are the rate-limiting steps. In the parallel reaction pathway via OPA, the conversion of OPA either by hydrogenation to PCHA or by C−N cleavage to PB are ratedetermining. Concerning the role of phosphorus in the catalyst, the kinetic modeling allowed circumstantiating an inhibiting effect of phosphorus on C−N bond breaking (although the overall activity is increased) that had been intuited from a qualitative analysis of the data.9,10 These examples from the literature demonstrate that a detailed kinetic analysis is very useful for explaining catalytic behavior and analyzing the role of additives and promoters. In the present work, we revisit the reaction network of quinoline and present a very thorough kinetic study that allowed us to determine not only precise kinetic constants for all elementary steps (and their activation energy), but also adsorption constants for all nitrogen-containing compounds (including NH3) and the adsorption enthalpies. The methodology that is outlined here will allow us to compare and explain in detail the catalytic behavior of different HDN catalysts. These results will be reported in a separate paper.

2. EXPERIMENTAL AND KINETIC MODELING 2.1. Catalyst. The catalyst used in this study is a NiMo(P)/ γ-Al2O3 catalyst synthesized by incipient wetness impregnation of an γ-alumina support (extrudates), followed by calcination at 450 °C. It contains 12.5 wt % Mo, 2.8 wt % Ni, and 1.9 wt % P (according to inductively coupled plasma analysis). The metal loading is high compared to that of the NiMo/Al2O3 catalysts used in the previous studies and is more representative of current industrial catalyst. The catalyst has a BET surface area of 201 m2·g−1 and a total pore volume of 0.467 mL·g−1 according to nitrogen isotherm adsorption. 2.2. Catalytic Test. Prior to the catalytic test, the NiMo(P)/Al2O3 catalyst was crushed, sieved (80−125 μm), and then sulfided ex-situ in a continuous-flow reactor operating at atmospheric pressure, with the mixture of 15 vol % H2S in H2, flow rate of 4 L/h. Sulfidation was carried out with the temperature increase of 10 °C/min, until 400 °C, and then the temperature was maintained for 4 h. The quinoline HDN tests were performed in a batch reactor (model Parr 4842) with a volume of 300 mL. For each catalytic test, the reactor was loaded with 100 g of a mixture of squalane and m-xylene (50/ 50 wt %), 0.75 g of sulfide catalyst, 20 μL of dimethyldisulfide (leading to a pressure of H2S of 13.2 KPa in the batch), and B


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Figure 2. Liquid−vapor mass-transfer scheme in batch reactor.

Assuming a competitive adsorption on the same catalytic sites between H2, nitrogen compounds and the solvents, we used the generalized Langmuir− Hinshelwood formalism to express the fractional occupation of molecule i on the catalytic sites (θi):

adsorption of heavy poly condensed products (see Supporting Information Table S3). The content of carbon deposed on used catalysts varied in the range of 1−2 wt %, corresponding to 7− 15 mol % of initial concentration of quinoline. We noted, however, that the used NiMo(P)/Al2O3 catalyst after a test without quinoline (at 350 °C, for 6 h of reaction) showed also carbon content of 0.6 wt % and a decrease of TPV and BET surface area. This indicated that the conversion of solvent (mxylene and squalane) contributed also to the formation of coke and heavier products. The pore size distribution of catalyst was not modified by the deposit of carbon on the catalyst’s surface. When the batch reactor is used for evaluating catalyst activity, the formation of coke on very active sites of the fresh catalyst is one of the inconveniences. The evolution of conversion and yields with time may be influenced by this transient behavior of the catalyst. However, because the carbon content (1−2%) in our used catalysts is not high, we assume that this deactivation can be neglected. 2.3. Kinetic Modeling. In contrast to Prins and coworkers,10,12 we chose to not to distinguish different catalytic sites in the kinetic modeling. Apart from the fact that the exact structure of the sites for hydrogenation and C−N cleavage reactions is still under debate, the notion of fixed, well-defined catalytic sites may not be appropriate for sulfide catalysts. During the catalytic cycle, the MoS2 edge undergoes constant transformation. The creation or consumption of sulfur vacancies and the dissociative adsorption or release of H2 may transform hydrogenation into hydrogenolysis sites and vice versa. Not distinguishing hydrogenation and C−N bond cleavage sites in the kinetic model means that the adsorption on both sites is assumed to be the same. Other simplifying assumptions in the kinetic model are (i) The adsorption constants of H2 and H2S are much smaller than those of nitrogen compounds, so they are neglected in the kinetic model. (ii) Volume of liquid and vapor is constant during reaction. (iii) The liquid and gas phases are perfectly mixed. (iv) There is no internal and external diffusion limitation in catalyst particles. (v) Reactions are first-order. (vi) The concentration of H2 in liquid phase is constant and equal to the equilibrium concentration. (vii) Liquid−vapor mass transfer is represented by a linear driving force. (viii) Surface reactions are the rate-limiting steps of reaction.

θi =

K iintCi n 1 + K sintCs + ∑ j = 1 K jintCj

(1)

int int In the above equation, Kint i , Kj , and Ks are intrinsic adsorption constants of component i, j, and solvents, respectively; n is the total number of components in the system, except for the solvents. For the system at (or near) saturation,13 we have 1 + n int int n int Kint s Cs + ∑j = 1Kj Cj ≈ Ks Cs + ∑j = 1Kj Cj Therefore, eq 1 can be rewritten as follows:

θi = = =

K iintCi n K sintCs + ∑ j = 1 K jintCj K iintCi /K sintCs n + ∑ j = 1 (K jintCj/K sintCs)

K sintCs/K sintCs

K iCi n 1 + ∑ j = 1 KjCj

(2)

where Ki, and Kj are equilibrium adsorption constants of components i and j, relatively compared to the adsorption of solvents (L·mmol−1). The adsorption constants of solvents (squalane and m-xylene) were not estimated in kinetic equations. In fact, the apparent adsorption constant (Ki) of reactant and products contains the contribution of the adsorption of solvents, as explained by eqs 1 and 2. The global volumetric reaction rate of component i on a catalytic site in the reaction i + H2 → j is hence defined as rsolid v,i = kc,i·θi·θH2, where θH2 and θi are the fractional occupation of H2 and component i on the catalytic sites. As the hydrogen concentration is assumed constant and equal to the concentration at the equilibrium, the volumetric reaction rate of component i is hence written as rv,solid i =

kc, i·K iCiK H 2. C H*2,liq n

(1 + ∑ j = 1 KjCj)2

=

kiK iCi n

(1 + ∑ j = 1 KjCj)2

(3)

In eq 3, ki is the apparent rate constant (mmol·L−1·s−1), which depends on the temperature, pressure, and nature of solvents. In the kinetic model, ki and Ki were calculated via C


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condensation of heavy components in gas sampling tube. Therefore, the concentration of components in the gas phase was calculated by eq 9. In this equation, Ci,liq * is the equilibrium concentration of component i in liquid phase and is calculated by the simulation of liquid−vapor equilibrium with ProSim software (version 3.3). Using the Grayson−Streed thermodynamic model,15 the simulation of the flash in ProSim allows us to calculate the equilibrium constant of every component and the equilibrium concentration of all components in the liquid phase. The effluent entering in the flash contains representative compounds (quinoline, propylcyclohexane, and propylbenzene), solvents (m-xylene and squalane), H2, H2S, and NH3. Due to the lack of data for several compounds in ProSim, quinoline was used to represent other nitrogen compounds in the mixture (quinoline, 14THQ, 58THQ, OPA, and PCHA); PCH was used to represent PCH and PCHE. The experimental concentrations of all components obtained under “cold” conditions (25 °C, 3.2 MPa) for different reaction times (with molar balance errors lower than 5%) were used as ProSim input data. The simulation results showed that values of the equilibrium constant of quinoline, PB, PCH, and NH3 at a given temperature and pressure do not change with their composition. The values of equilibrium constants at reaction conditions are given in Table S2 inSupporting Information. From the liquid−vapor equilibrium constant of component i (μi) and the total concentration of solvents in the liquid phase, and considering that there is no mass-transfer limitation on the gas phase, we calculated the C*i,liq by eq 10.

Arrhenius law (eq 4) and Van’t Hoff equation (eq 5), respectively: ki = A ·exp( −Ea /RT )

(4)

K i = B ·exp( −ΔHads/RT )

(5)

When the composition of the liquid sample at reaction conditions (“hot” conditions) is compared to that after cooling the reactor to ambient temperature (“cold” conditions) at different reaction times (Table S1 in Supporting Information), a considerable evaporation of compounds was found at reaction conditions. Because of the existence of liquid−vapor equilibrium at reaction conditions, a liquid−vapor mass-transfer term was added in the kinetic model for all components, except for hydrogen, to take into account the vaporized mass fractions of reactants and products (Figure 2). The molar balance of component i in the batch reactor can be written in liquid phase (eq 6) and in vapor phase (eq 7) as follows: Vliq ·

Vg ·

dCi ,liq dt

dCi ,g dt

= NiexchangeL−V ·Sexchange − rv,solid i · Vsolid

= −NiexchangeL−V ·Sexchange

(6)

(7)

Replacing = kL·(C*i,liq − Ci,liq), and a = Sexchange/ Vliq in the two above equations of molar balance, eqs 6 and 7 become NexchangeL−V i

dCi ,liq dt dCi ,g dt

= kLa ·(Ci*,liq − Ci ,liq) − rvsolid ,i · = −kLa ·(Ci*,liq − Ci ,liq).

Vsolid Vliq

Ci*,liq = Ci ,g . (8)

(10)

where T and Pare the temperature and total pressure of reactions, respectively; Ctotal liq is the total concentration of all components in the liquid phase (including also solvent and H2 concentrations), and μi is the liquid−vapor equilibrium constant of component i.

Vliq Vg

RT 1 total . . C liq P μi

(9)

The coefficient of L−V transfer, kLa, is a physical parameter, which depends on the physical properties of the system such as viscosity, surface tension, rate of agitation, and configuration of reactor. To summarize, the data used for kinetic modeling is 981 experimental points corresponding to the concentrations of components in liquid samples (measured by gas chromatography) at different reaction times. The kinetic model will fit concentrations of component in liquid phase (calculated from equation set) with their experimental concentrations. A set of 20 molar balance equations of all components (except for H2) is used to estimate 39 parameters. The 39 parameters estimated by the kinetic model include • Pre-exponential factors for kinetic parameters (A) • Activation energies (Ea) • Pre-exponential factors for adsorption parameters (B) • Adsorption enthalpies (ΔHads) • Liquid−vapor mass-transfer coefficient (kLa) The software used is Matlab (version 2013b), and the Levenberg-Marquard minimization method provides parameter estimation.14 The parameters related to liquid−vapor equilibrium are calculated, such as equilibrium concentration of components in liquid phase (C*i,liq) and liquid−vapor equilibrium constants of components (μi), as described in the following paragraph. The concentration of components in gas phase cannot be measured because of the difficulty of the gas sampling and the

3. RESULTS AND DISCUSSION 3.1. Effect of Temperature and Initial Concentration on Quinoline HDN. The reaction scheme (Figure 1) clearly indicated that DHQ and PCHE were intermediates products whereas PB and PCH were final products. We found that PB which was formed by Csp2−N bond cleavage of OPA accounted for about 6−10 wt % of the total conversion at the end of reaction, depending on the reaction temperature. As compared to the reaction scheme published previously,9 we have neglected the dehydrogenation pathway PCHE to PB because, if this reaction is considered or not, the kinetic parameters of the overall scheme are similar for most of the reactions (except k11 and k14) with no impact on the accuracy of the results. More data obtained from this model with different catalysts confirm this hypothesis (to be published). A very small concentration of PCHA was detected because of its high rate of disappearance. To compare the reaction rates at different temperatures and different initial concentrations of quinoline, the HYD conversion (hydrogenation conversion) and HDN conversion (hydrodenitrogenation conversion) were calculated by eqs 11 and 12, respectively. ⎛ nQ + n14THQ + n58THQ ⎞ ⎟⎟ · 100% HYD Conv = ⎜⎜1 − nQo ⎝ ⎠ (11) D


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Figure 3. Product yield of Quinoline + 14THQ + 58THQ (left vertical axis) (with C = [Q] + [14THQ] + [58THQ]) and HDN conversion (right vertical axis) versus reaction time plot obtained from three tests at 350 °C, at 1, 1.5, and 2 wt % of quinoline; Co is the initial concentration of quinoline (Symbols: △, ratio C/Co; ○, HDN conversion. Colors: blue, 1 wt % quinoline; red, 1.5 wt % quinoline; green, 2 wt % quinoline).

⎛n + n ⎞ PCHE + nPCH ⎟ HDN Conv = ⎜⎜ PB ⎟ ·100% nQo ⎝ ⎠

of reactant, intermediates, and products. This inhibition is interpreted through a Langmuir−Hinshelwood kinetic model, as indicated in eq 1. 3.2. Results of Kinetic Modeling. From the kinetic model described in section 2.3 and the experimental data, the adsorption parameters (B, ΔHads), kinetic parameters (A, Ea) and mass-transfer coefficient (kLa) were estimated. As the results of kinetic modeling, we found that kLa values varied in the range of 0.08−0.23 s−1 (Table S6 in Supporting Information). The difference between kLa values at different temperatures is small. The accuracy of estimated values of kLa is quite good (3−13%). The values of kLa fit with estimation from empirical parameters16 (empirical kLa = 0.18−0.23 s−1 at 350 °C). The results of the estimation including the values of kinetic and adsorption parameters and their confidence are given in Tables 1−3. Figures 4−6 show that the evolution of the concentrations of quinoline, intermediates, and final

(12)

noQ

where is the initial molar quantity of quinoline; nQ, n14THQ, n58THQ, nPB, nPCHE and nPCH are total molar quantities of quinoline, 14THQ, 58THQ, PB, PCHE, and PCH at each reaction time, respectively. The total molar quantity of these compounds was obtained by taking into account their evaporation under reaction conditions. In the calculation of the HYD conversion, quinoline, 14THQ and 58THQ were lumped together because the proportions of these three compounds rapidly approach thermodynamic equilibrium. The effects of temperature on quinoline HDN were investigated by catalytic tests at 340, 350, and 360 °C. HDN activity increased with the increase of temperature. Moreover, temperature played an important role in the selectivity to HDN products. Concretely, an increase of temperature from 340 to 360 °C increased the yield of PB by about 6 mol %, at full HYD conversion. However, although a high temperature resulted in an increase in PB yield, PCH remained the main product. At the same reaction conditions of temperature and pressure, the yield distribution versus HYD conversion of three tests (1, 1.5, and 2 wt % of quinoline) were not much different (see Figure S1 in Supporting Information). This result showed that the initial concentration of reactant did not change the relative rates of different reaction pathways. However, the increase of the initial concentration of quinoline decreased the disappearance rate of mixture (quinoline + 14THQ + 58THQ) and the HDN conversion (Figure 3). Therefore, the decrease of reaction rates with the increase of initial concentration could be explained by the inhibiting effect of competitive adsorption

Table 1. Values of ΔHads, B, and Adsorption Constants of Nitrogen Compound Familiesa nitrogen compounds Q and 58THQ 14THQ and OPA DHQ and PCHA NH3 a

E

B 5.45 × 10−7 (±13%) 4.91 × 10−6 (±11%) 1.86 × 10−6 (±11%) 3.40 × 10−6 (±10%)

ΔHads (kJ/mol)

adsorption constant at 350 °C (L/mol)

−48.2 (±1.0%) −36.5 (±0.7%) −48.6 (±0.5%) −44.7 (±0.6%)

6.0 5.2 22.1 19.0

The number in parentheses is the accuracy of estimated parameter. DOI: 10.1021/acs.iecr.5b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Table 2. Values of Ea, ln A, Apparent Rate Constants (ki), and Effective Rate Constants (ki·Ki) of (De)hydrogenation and RingOpening Reactions reaction

ln A

Q → 14THQ 14THQ → Q Q → 58THQ 58THQ → DHQ DHQ → 58THQ 14THQ →DHQ 14THQ → OPA DHQ → PCHA OPA → PCHA PCHA → OPA

± ± ± ± ± ± ± ± ± ±

11.8 17.5 29.2 6.2 18.6 20.0 29.6 34.6 18.1 17.0

Ea (kJ/mol) 0.1 0.1 0.2 0.0 0.1 0.1 0.1 0.2 0.1 0.1

29.0 69.8 130.7 16.0 95.6 90.6 145.3 167.4 83.7 90.3

± ± ± ± ± ± ± ± ± ±

0.2 0.4 1.0 0.1 0.7 0.6 0.7 1.0 0.6 0.5

ki at 350 °C (mmol·L−1·s−1)

ki·Ki at 350 °C (s−1)

× × × × × × × × × ×

2.87 0.28 0.31 0.14 0.03 0.07 0.03 0.21 0.04 0.02

0.48 0.53 0.52 0.23 1.19 0.13 4.97 9.48 7.00 0.66

3

10 102 102 102 100 102 100 100 100 100

Table 3. Values of Ea, ln A, Apparent Rate Constants (ki), and Effective Rate Constants (ki·Ki) of Denitrogenation Reactions (without Ring Opening) and (De)hydrogenation of PCHE reaction

ln A

OPA → PB PCHA → PCHE PCHA → PCH PCHE → PCH

± ± ± ±

36.7 34.5 28.9 9.6

Ea (kJ/mol) 0.2 0.2 0.2 0.1

178.6 158.1 133.4 56.6

± ± ± ±

ki at 350 °C (mmol·L−1·s−1)

ki·Ki at 350 °C (s−1)

8.96 × 10 0.53 × 102 0.23 × 102 −

0.046 1.187 0.506 0.266

0

1.1 1.0 0.6 0.4

charge of nitrogen atoms, but also on the hindrance effect and the molecule size. The mechanism of the adsorption of nitrogen compounds on coordinatively unsaturated sites has been discussed. Two principal adsorption modes of nitrogen compounds on catalytically active sites were proposed: end-on (through the nitrogen atom only) or side-on (through the heterocycle or aromatic ring or π-bond).18 The density functional theory (DFT) calculations of Sun et al.19 showed that on the well-defined NiMoS hydrotreating catalyst edge surface, the basic nitrogen-containing molecules such as DHQ, 58THQ, and quinoline are preferably adsorbed through the lone pair electrons of the nitrogen atom. This end-on adsorption mode produces relatively high adsorption energies, as we found. The nonbasic nitrogen-containing molecules interact with the NiMoS catalyst edge surface through the πelectrons of the carbon atoms (side-on mode). It should be noted that the values of adsorption enthalpies in our case are the relative values obtained under the competitive coverage of hydrogen and the solvents on the catalytic sites. Therefore, their absolute values are not comparable with the values calculated by DFT, but the ranking is similar to the values obtained by DFT or kinetic studies. A higher negative charge on the nitrogen atom of the saturated nitrogen compounds (DHQ and PCHA) leads to a stronger adsorption on catalyst surface, as compared to aromatic amines and NH3. Our results showed that the adsorption constants of saturated amines are 3−4 times greater than those of aromatic amines, which is in agreement with values estimated by Satterfield and Yang.11 Although the adsorption enthalpy of NH3 is lower than that of quinoline, we found that the adsorption constant of NH3 is greater because of a higher pre-exponential factor and the correction made from L−V equilibrium. This might be attributed by a smaller size of NH3 molecule and hence a higher accessibility of NH3 to the catalytic sites as compared to other nitrogen molecules. According to the DFT calculations of Sun et al.,20 the adsorption constant of NH3 is higher than that of pyridine and aniline on the NiMoS edge. The two last molecules (pyridine and aniline) exhibit electronic structure of the

products are well-represented by the chosen kinetic model. The difference between experimental data and calculated concentrations increases with temperature, probably due to the formation of byproducts and coke, which were not accounted for in the model. 3.2.1. Adsorption Parameters of Nitrogen Compounds. As mentioned before, the decrease of reaction rates with the increase of initial concentration of quinoline was explained by the inhibiting effect of competitive adsorption of reactant, intermediates, and products. Studies in the literature showed that the competitive adsorption of nitrogen compounds induces the self-inhibition of quinoline HDN and also the inhibition of other hydrotreating reactions (HDS, HDA), as described by the Langmuir−Hinshelwood model. This has been investigated by Koltai et al.,6 Beltramone et al.,5 Bhinde,17 and Ho et al.4 In our kinetic model, on the basis of the difference in electronic structure of the nitrogen atom in molecules, we distinguished four groups of nitrogen compounds. The aromatic amines include quinoline and 58THQ, in which the free electron pair of nitrogen atom does not conjugate with the aromatic ring. The saturated amines include DHQ and PCHA. The third group includes 14THQ and OPA, in which the electron pair on nitrogen atom conjugates with the aromatic ring. The ammonia is grouped by itself. Each group shows the same values of adsorption parameters (ΔHads, B, Ki). The adsorption parameters including pre-exponential factors (B), adsorption enthalpies (ΔHads), and adsorption constants (Ki) are shown in the Table 1. The values of adsorption enthalpies varied in the range of 35−50 kJ mol−1, which was in agreement with the adsorption enthalpy of o-methyl-aniline over NiMo/Al2O3 obtained by Prins and co-workers.12 We found that the difference between adsorption enthalpies of nitrogen compounds was not much. However, it is interesting to remark that the values of adsorption enthalpies decreased in this order: DHQ + PCHA > Q + 58THQ > NH3 > 14THQ + OPA. This result gives an estimation of the relative strength of chemical bonds formed by the adsorption of nitrogen compounds on the catalytic sites. The adsorption of nitrogen compounds depends not only on their basicity or the negative F


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Figure 5. Comparison of simulation results (continuous line) and experimental data (points) of three tests at 350 °C at 1 wt % (a), 1.5 wt % (b), and 2 wt % (c) of quinoline.


good correlation with proton affinity was observed by La Vopa and Satterfield,21 with an adsorption constant of ammonia 20 times lower than that of quinoline. These two last studies were both carried out in the gas phase, whereas our study is performed in the liquid phase. Our adsorption constants have a fairly good correlation with the relative order of pKa22,23 (Figure 7). The ranking of adsorption constants is close to the

nitrogen atom similar to that of quinoline and OPA. In the investigation of the inhibiting effect of nitrogen compounds on the HDS of 4,6-dimethyldibenzothiophene, Beltramone et al.5 also found that the ammonia adsorption affinity was 6 times stronger than that of quinoline. Their results were correlated with Mulliken charge on the nitrogen atom. On the contrary, a G


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Figure 7. Comparison of calculated adsorption constant (left vertical axis) and pKa values (right vertical axis) of nitrogen compounds.

With the preliminary results of modeling, we found that the rate constants of backward reactions from 58THQ to quinoline and from DHQ to 14THQ were much smaller than those of the respective forward reactions. To decrease the number of parameters in the kinetic model, we neglected the backward reactions. Forward and backward reactions were accounted for in the interconversion of quinoline and 14THQ, 58THQ and DHQ, and OPA and PCHA. From the ratio of the apparent rate constants of forward and backward reaction which are given in Table 2, equilibrium constants can be estimated (Table 4). Table 4. Equilibrium Constants of Equilibrium Reactions (Keq) reaction

Keq

at 340 °C

at 350 °C

at 360 °C

Q ↔ 14THQ 58THQ ↔ DHQ OPA ↔ PCHA

k1/k2 k4/k5 k9/k10

10.27 24.79 10.85

9.03 19.30 10.63

7.97 15.14 10.42

We first focus on the hydrogenation of quinoline to THQ and DHQ. The hydrogenation of quinoline to 14THQ was very rapid, and thermodynamic equilibrium was rapidly established. The value of equilibrium constant of the Q−14THQ system is comparable with the value obtained by Satterfield and Yang,11 at the same temperature and pressure. The hydrogenation of the benzenic ring of quinoline (to 58THQ) was significantly slower and had higher activation energy. The same trend is observed in the hydrogenation of THQ to DHQ. The hydrogenation of 58THQ is faster than that of 14THQ and has much lower activation energy. In both cases, the hydrogenation of the nitrogen-containing ring is much faster than the hydrogenation of the benzenic ring. The presence of nitrogen in the aromatic ring significantly decreases its resonance energy, thereby making it more reactive.22 The observation that the rate constant of hydrogenation of naphthalene into tetralin is much smaller than that of quinoline hydrogenation24 is in line with this interpretation. Overall, DHQ is formed by two pathways: the hydrogenation of 14THQ with the reaction rate r6 and the hydrogenation of 58THQ with reaction rate r4. The last reaction is in equilibrium with the dehydrogenation of DHQ (r5). Table 5 shows the variation of the ratio r6/(r4 − r5) as a function of reaction time,


one observed by Beltramone et al.5 and opposite to the one related to proton affinity.21 3.2.2. Kinetics of Hydrogenation Reactions. The reaction network of quinoline HDN comprises several hydrogenation and dehydrogenation reactions in thermodynamic equilibrium. H


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Table 5. Variation of Ratios r6/(r4 − r5) and r8/(r9 − r10) during the Quinoline HDN at 350°C, 7 MPa, and 1 wt % of Quinoline time (h)

0h

0.25 h

0.5 h

0.75 h

1h

1.5 h

2h

2.5 h

3h

4h

5h

r6/(r4 − r5) r8/(r9 − r10)

75.2 69.2

10.4 43.1

5.3 23.0

4.0 13.6

3.2 11.7

2.3 8.8

1.9 6.8

1.7 5.4

1.4 3.3

1.3 2.1

1.2 0.8

calculated from the catalytic test at 350 °C and 1 wt % of quinoline. This variation indicates that DHQ is mainly produced by the hydrogenation of 14THQ before 2 h of reaction because of the high concentration of 14THQ, but the two pathways are subsequently equivalent. 3.2.3. Comparison of the Two Reaction Pathways of Quinoline HDN. The HDN of quinoline takes place via two reaction pathways,8,10,25,26 as given in Figure 1. Pathway I proceeds by the formation of DHQ and then Csp3−N bond cleavage (ring opening) to form PCHA, finally by C−N bond breaking of PCHA to form PCH and PCHE. Pathway II proceeds via direct hydrogenolysis of 14THQ to form OPA and then the hydrogenation of OPA to PCHA or the direct cleavage of Csp2−N bond of OPA to form PB. Our kinetic model allows obtaining the rate-determining step of each reaction pathway. The kinetic parameters of each elementary reaction such as activation energies, pre-exponential factors, apparent rate constants, and effective rate constants at 350 °C are given in Tables 2 and 3. For pathway I, the two slowest reaction steps are the hydrogenation of 14THQ to DHQ and the subsequent ring-opening of DHQ to PCHA. The values of effective rate constants showed that the hydrogenation step is the rate-limiting step of pathway I. Both observations are in good agreement with earlier kinetic studies of quinoline HDN over sulfide catalysts. This result is different from the evaluation of reaction kinetic on noble catalysts. In contrast to NiMo/Al2O3, the rate-limiting step of quinoline HDN over noble metal catalysts is the C−N bond scission because of a high hydrogenation activity of noble metal catalysts.27 Concerning reaction pathway II, OPA reacts further by hydrogenation to PCHA or by Csp2−N bond cleavage to PB. Earlier studies have shown that the pathway via PCHA is faster.10,12,28 Our kinetic analysis also confirms this result. In fact, the value of k9 is much higher than that of k11. PCHA is formed in two ways: the ring opening of DHQ with reaction rate r8 and the hydrogenation of OPA with reaction rate r9. The last reaction is in equilibrium with the dehydrogenation of PCHA (r10). The variation of ratio r8/(r9 − r10) during reaction, given in Table 5, showed that PCHA is mainly produced from DHQ. The rate of the hydrogenolysis of 14THQ into OPA (k7) is about 3 times slower than the rate of the hydrogenation of 14THQ (k6), and PCHA is mainly formed from the ring opening of DHQ, as mentioned before. Therefore, pathway I passing through the hydrogenation of 14THQ was privileged under our hydrotreating conditions. In pathway II, the Csp2−N bond cleavage of OPA to produce PB is much slower than the Csp3−N bond breaking of 14THQ. In fact, the Csp2−N bond in OPA is much stronger than the Csp3−N bond in 14THQ because of the conjugation of free electron pair on the nitrogen atom with the aromatic ring in OPA (Csp2−N 614 kJ mol−1, Csp3−N 305 kJ mol−1); hence, it is more difficult to break. This explains the low selectivity to PB.29 Regarding the estimated Arrhenius parameters including ln A and Ea in Tables 2 and 3, we found that the hydrogenation reactions have lower activation energies as compared to Csp2−N and Csp3−N bond breaking. The hydrogenation of nitrogen

rings in quinoline and 58THQ has very low values of activation energies, whereas the hydrogenation of aromatic rings shows higher ones. The C−N bond cleavage reactions need very high activation energies, especially the Csp2−N bond breaking of OPA to produce PB. NiMo(P)/γ-Al2O3 presents high activation energies for the β-elimination reaction of PCHA (to produce PCHE) and the hydrogenolysis of PCHA (to produce PCH), but it has a high activity for these reactions, i.e., high rate constants, because the values of pre-exponential factors (A) in the Arrhenius equation are quite high. 3.3. Validation of Kinetic Model. The kinetic and adsorption parameters were estimated from nine tests of quinoline HDN at three different temperatures and three initial quinoline concentrations. An additional experiment was carried out to validate the kinetic model, with dodecylamine as a saturated paraffinic amine to produce rapidly NH3 in the quinoline HDN reaction medium. The dodecylamine was fully converted after 1.5 h of reaction time (Figure 8). The

Figure 8. Conversion of dodecylamine in the presence of quinoline at 350 °C and 7 MPa.

comparison between the experimental data and the simulation was performed from 1.5 h, at the time when the dodecylamine was totally converted and the quinoline HDN was only further inhibited by extra ammonia. The values of kinetic parameters, adsorption parameters, and liquid−vapor mass-transfer coefficient at 350 °C obtained by the kinetic modeling were used to calculate the evolutions of every component concentration as a function of reaction time. These simulation curves showed good agreement with experimental data (Figure 9). This allowed the validation of the kinetic model, or the kinetic and adsorption parameters estimated from the kinetic modeling were applicable within our reaction conditions range. Moreover, the inhibiting effect of NH3 on the quinoline HDN was confirmed by this validation.

4. CONCLUSIONS Kinetic studies remain a unique tool to access the understanding of reaction mechanism and active site performances. A new kinetic approach of analyzing the HDN reaction, involving I


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distribution of components in gas phase; and equation set of mass balance of all components in kinetic model (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +33 (0) 472445426. E-mail: melaz.tayakout@ircelyon. univ-lyon1.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for support from IFP Energies Nouvelles and CNRS (Centre National de la Recherche Scientifique).

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Figure 9. Comparison of simulation results (continuous line) and experimental data (points) of additional tests for the validation of the kinetic model.

L−V equilibrium, was proposed. The kinetic model of quinoline HDN developed and validated in our study allowed the kinetic parameters and adsorption parameters to be estimated. From that, the kinetic aspect of the reaction was investigated in more detail. Our kinetic modeling results confirmed that the HDN of quinoline occurred via the hydrogenation of aromatic rings followed by Csp3−N bond breaking as a main reaction pathway. Over NiMo sulfide catalyst, the hydrogenation of 14THQ into DHQ is the slowest step of this pathway. The self-inhibition effect due to competitive adsorption of nitrogen compounds, especially the saturated amines and NH3, was confirmed. The difference in adsorption constants of nitrogen compounds was found to be in an agreement with their pKa values or their aqueous basicity, which directly related to the acidity of the active phase. The saturated amines such as DHQ and PCHA showed the highest adsorption capacity; in addition, the adsorption constant of a small molecule such as NH3 was also high. Because of high confidence of parameters and the validation of the kinetic model by an additional experiment, the kinetic model was able to predict for HDN of our model molecule under different reaction conditions, as long as the formation of heavy poly condensed products and coke was negligible. For further investigations, the HDN of different model molecules with different basicity will be studied. It will provide useful knowledge in kinetics of HDN reactions that will later be transposed to the investigation of HDN of heavy feedstock, such as vacuum gas oil.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02175. Composition of liquid sample at reaction conditions and after cooling the reactor; liquid−vapor equilibrium constants of components; standard deviations of experiments; textural characterizations and carbon content of used and fresh NiMo(P)/Al2O3 catalyst; comparison of the product yield as a function of HYD conversion of catalytic tests at different initial quinoline concentration; J

NOMENCLATURE a = exchange surface density (dm−1) A, B = pre-exponential factors Ci,liq * = molar concentration of component i in liquid phase at equilibrium (mmol·L−1) Ci,liq = molar concentration of component i in liquid phase (mmol·L−1) Ci,g = molar concentration of component i in gas phase (mmol·L−1) Ctotal liq = total concentration of all components in liquid phase (mmol·L−1) Co = initial concentration of quinoline (mmol·L−1) Cs = concentration of solvents (mmol·L−1) DFT = density functional theory ki = apparent rate constant (mmol·L−1·s−1) Ki = equilibrium adsorption constant of component i, relatively as compared to the adsorption of solvents (L· mmol−1) Keq = equilibrium constant of equilibrium reaction int Kint i , Ks = intrinsic adsorption constant of component i and solvents, respectively kL = mass-transfer coefficient from gas−liquid interface to bulk liquid (dm·s−1) kLa = coefficient of L−V transfer (s−1) NexchangeL−V = molar quantity of component i exchanged by i L−V surface (mmol·s−1.dm−2) ni = molar quantity of component i (mmol) noQ = initial molar quantity of quinoline (mmol) P = pressure of reaction (kPa) pKa = logarithmic acid dissociation constant r4 = reaction rate of the hydrogenation of 58THQ into DHQ (mmol·L−1·s−1) r5 = reaction rate of the dehydrogenation of DHQ into 58THQ (mmol·L−1·s−1) r6 = reaction rate of the hydrogenation of 14THQ into DHQ (mmol·L−1·s−1) r8 = reaction rate of the ring opening of DHQ into PCHA (mmol·L−1·s−1) r9 = reaction rate of the hydrogenation of OPA into PCHA (mmol·L−1·s−1) r10 = reaction rate of the dehydrogenation of PCHA into OPA (mmol·L−1·s−1) rsolid i,v = volumetric rate of formation of component i on the catalytic sites (mmol·L−1·s−1) Sexchange = surface through which the L−V transfer takes place (dm2) DOI: 10.1021/acs.iecr.5b02175 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research SBET = specific surface area (m2·g−1) to = initial point of reaction T = temperature (Kelvin) TPV = total pore volume (ml·g−1) Vliq, Vg = liquid volume and gas volume of reactor, respectively (L) Vsolid = solid volume of catalyst (L)

(12) Qu, L.; Flechsenhar, M.; Prins, R. Kinetics of the hydrodenitrogenation of o-toluidine over fluorinated NiMoS/Al2O3 and NiMoS/ASA catalysts. J. Catal. 2003, 217, 284. (13) Lettat, K.; Jolimaitre, E.; Tayakout, M.; Tondeur, D. Liquid phase diffusion of branched alkanes in silicalite. AIChE J. 2011, 57, 319. (14) Couenne, F.; Jallut, C.; Tayakout-Fayolle, M. On minimal representation of heterogeneous mass transfer for simulation and parameter estimation: Application to breakthrough curves exploitation. Comput. Chem. Eng. 2005, 30, 42. (15) Chávez, L. M.; Alonso, F.; Ancheyta, J. Vapor−liquid equilibrium of hydrogen-hydrocarbon systems and its effects on hydroprocessing reactors. Fuel 2014, 138, 156. (16) Mitrovic, M. Etudes des transferts de matière dans un réacteur triphasique gaz-liquide-solide, d’investigation cinétique (réacteur MahoneyRobinson). Thesis, Université Claude Bernard Lyon 1, Lyon, France, 2001. (17) Bhinde, M. V. Ph.D. Thesis, University of Delaware, Newark, DE, 1979. (18) Delgado, S. Catalysis by Metal Complexes, Organometallic Modeling of the Hydrodesulfurization and Hydrodenitrogenation Reactions; Springer: Netherlands, 2002. (19) Sun, M.; Nelson, A. E.; Adjaye, J. First principles study of heavy oil organonitrogen adsorption on NiMoS hydrotreating catalysts. Catal. Today 2005, 109, 49. (20) Sun, M.; Nelson, A. E.; Adjaye, J. Adsorption thermodynamics of sulfur and nitrogen-containing molecules on NiMoS: A DFT Study. Catal. Lett. 2006, 109, 133. (21) La Vopa, V.; Satterfield, C. N. Poisoning of thiophene hydrodesulfurization by nitrogen compounds. J. Catal. 1988, 110, 375. (22) Ho, T. C. Hydrodenitrogenation catalysis. Catal. Rev.: Sci. Eng. 1988, 30, 117. (23) Katritzky, A. R. Handbook of heterocyclic chemistry; Elsevier: Oxford, 2010. (24) Sundaram, K. M.; Katzer, J. R.; Bischoff, K. B. Modeling of hydroprocessing reactions. Chem. Eng. Commun. 1988, 71, 53. (25) Prins, R. Catalytic hydrodenitrogenation. Adv. Catal. 2001, 46, 399. (26) Luan, Y.; Zhang, Q.; He, D.; Guan, J.; Liang, C. Hydrodenitrogenation of quinoline and its intermediates over sulfided NiW/ γ-Al2O3 in the absence and presence of H2S. Asia-Pac. J. Chem. Eng. 2009, 4, 704. (27) Peeters, E.; Geantet, C.; Zotin, J. L.; Breysse, M.; Vrinat, M. Deep hydrodenitrogenation on Pt supported catalysts in the presence of H2S, comparison with NiMo sulfide catalyst. Stud. Surf. Sci. Catal. 2000, 130, 2837. (28) Hrabar, A.; Hein, J.; Gutiérrez, O. Y.; Lercher, J. A. Selective poisoning of the direct denitrogenation route in o-propylaniline HDN by DBT on Mo and NiMo/γ-Al2O3 sulfide catalysts. J. Catal. 2011, 281, 325. (29) Katzer, J. R.; Sivasubramanian, R. Process and catalyst needs for hydrodenitrogenation. Catal. Rev.: Sci. Eng. 1979, 20, 155.

Reactant, Intermediates,and Products

14THQ = 1,2,3,4-tetrahydroquinoline 58THQ = 5,6,7,8-tetrahydroquinoline DHQ = decahydroquinoline OPA = o-propylaniline PB = propylbenzene PCH = propylcyclohexane PCHA = propyl-cyclohexylamine PCHE = propylcyclohexene Q = quinoline Greek Letters

θi = fractional occupation of molecule i on the catalytic sites μi = liquid−vapor equilibrium constant of component i Ea = activation energy (kJ mol−1) ΔHads = adsorption enthalpy (kJ mol−1) Subscripts and Superscripts

g = gas phase i = counter of component int = intrinsic parameter liq = liquid phase solid = solid phase

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REFERENCES

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Kinetic Modeling of Quinoline Hydrodenitrogenation over a NiMo(P

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