Synergy between shape selective and non-shape selective bifunctional zeolites modelled via the Single-Event MicroKinetic (SEMK) methodology

Chemical Engineering Science 65 (2010) 174 -- 178

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Synergy between shape selective and non-shape selective bifunctional zeolites modelled via the Single-Event MicroKinetic (SEMK) methodology I. Roy Choudhury a , J.W. Thybaut a, ∗ , P. Balasubramanian a,1 , J.F.M. Denayer b , J.A. Martens c , G.B. Marin a a b c

Laboratory for Chemical Technology, Ghent University, Krijgslaan 281–S5, Gent B-9000, Belgium Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, Brussels B-1050, Belgium Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, Leuven B-3001, Belgium

A R T I C L E

I N F O

Article history: Received 7 July 2008 Received in revised form 30 June 2009 Accepted 15 August 2009 Available online 21 August 2009 Keywords: Zeolites Simulation Kinetics Hydroconversion Catalyst selectivity Synergy Single-Event MicroKinetics (SEMK)

A B S T R A C T

In decane hydroisomerization the synergy effect between a non-shape selective Pt/NaHY and a shape selective Pt/H-ZSM-22 catalyst leads to enhanced total isomer yields. In a physical mixture of the two catalysts, monobranched isomers are formed through hydroconversion on Pt/H-ZSM-22. The latter undergo further isomerization on Pt/NaH-Y catalyst with mild activity to produce multibranched isomers while simultaneously suppressing cracked product formation. A Single-Event MicroKinetic (SEMK) model, obtained through addition of the models for the individual catalysts, is used to simulate the synergy effect. With an estimation of only the “catalyst descriptor” values for the individual catalysts using the data on the pure catalysts, the conversion and selectivity associated with the synergy effect can be well simulated without further adjustment of any other model parameters. The Pt/H-ZSM-22 catalyst used in this work is more active than Pt/NaH-Y for n-decane hydroconversion which is reflected in the protonation enthalpies for secondary carbenium ion formation, i.e., −62.9 kJ mol−1 on ZSM-22 compared to −54.3 on Y. According to the model, approximately 50% of the n-decane is transformed into 2-methyl nonane on Pt/H-ZSM-22 before conversion to multibranched isomers starts taking place on Pt/NaH-Y. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Hydroisomerization and hydrocracking are important refinery processes for the production of high quality fuels employing zeolite based catalysts. The stringent fuel quality specifications in today's petroleum industry drive refineries towards the development of more selective catalysts to maximize the desired product yields (Van Veen, 2002). In the hydroconversion of linear alkanes on a large pore zeolite such as Pt/H-USY, the reaction mechanism follows “free carbenium ion chemistry” and no particular product selectivity occurs (Denayer et al., 2000; Martens et al., 1991). On the other hand, bifunctional catalysts based on 10-membered ring (10-MR) mediumpore zeolites, exhibit molecular shape selectivity in the hydroconversion of n-alkanes (Ernst et al., 1989; Martens et al., 1991, 2001). A typical product pattern of a medium pore zeolite such as Pt/H-ZSM22 shows primarily monobranched isomers as products where the formation of di- and tri-branched isomers is sterically suppressed resulting in less cracked products and overall enhanced isomer

∗ Corresponding author. Tel.: +32 9 264 4519; fax: +39 9 264 4999. E-mail address: [email protected] (J.W. Thybaut). 1 Current address: Indian Institute of Technology Guwahati, Guwahati-781 039, India. 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.08.020

yields. Remarkably, hydroconversion of n-paraffins on an intimate physical mixture of these two types of catalysts produces a higher isomer yield particularly of multibranched isomers than the individual catalysts and is termed as synergy effect (Parton et al., 1991; Martens et al., 1993). The conversion remains in between that of the individual catalysts. Such behaviour has been related to the excellent monobranching capacity of Pt/H-ZSM-22 followed by further isomerization on Pt/NaH-Y. Other examples of the increase in isomer yield for n-alkane hydroconversion over a physical mixture of two catalyst components compared to that of pure catalysts have also been reported in literature (Dzikh et al., 1999; Elangovan and Hartmann, 2003; Kinger et al., 2002). Single-Event MicroKinetic (SEMK) models have been developed for n-paraffin hydroconversion on Pt/H-USY (Baltanas et al., 1989; Vynckier and Froment, 1991; Svoboda et al., 1995; Martens et al., 2000) considering the widely accepted bifunctional reaction mechanism for ideal hydrocracking (Weisz, 1962). A sequence of quasiequilibrated physisorption, dehydrogenation and deprotonation elementary steps occur between the gas phase species and the carbenium ions on the acid sites. The rate-determining step in a catalytic cycle is the acid catalysed conversion such as methyl shift, protonated cyclopropane (PCP) branching or -scission. Similarly, a SEMK model accounting for the shape selective effects induced by the ZSM-22 has been constructed (Laxmi Narasimhan et al., 2003b,

I. Roy Choudhury et al. / Chemical Engineering Science 65 (2010) 174 -- 178

2004). Both models are discussed in somewhat more detail in Section 3. The current work aims at the fundamental kinetic modelling of the synergy effect using the previously developed SEMK models for the individual catalysts. To that purpose, catalyst descriptor values such as protonation enthalpies are obtained from regression of the data on the pure samples and the synergy effect is assessed by predicting the behaviour on the catalyst mixtures.

175

Table 1 Physico-chemical properties of the reference catalysts Pt/H-ZSM-22 and Pt/NaHY used in this work. Catalysts

Si/Al

Pt (wt%)

BrBnsted acid sites Ct (mol kgcat −1 )

BrBnsted acid sites at pore mouth Ctpm (mol kgcat −1 )

Pt/H-ZSM-22 Pt/NaHY

45 2.6

0.5 0.5

0.36 3.22a

0.23×10−03 –

a

Corresponding to Si/AlF which is 70% of the total Al concentration.

2. Procedures 2.1. Catalyst synthesis

between the observed and the calculated molar outlet flow rates:

Two bifunctional catalysts, Pt/NaH-Y and Pt/H-ZSM-22, are used in this work as reference catalysts for the hydroconversion of the n-decane. The ZSM-22 zeolite was synthesized according to a recipe described elsewhere (Ernst et al., 1989). The zeolite crystal was then calcined, exchanged with ammonium cations and impregnated with an aqueous solution of Pt(NH3 )4 Cl2 to obtain a Pt loading of 0.5 wt% (Martens et al., 1991). The Si/AlF ratio in the ZSM-22 framework was determined by quantitative 27 Al NMR (MAS-NMR), recorded by using a Bruker Avance DRX400 spectrometer (9.4 T) (Huybrechts et al., 2006; Hayasaka et al., 2007). The bulk Al content was determined from the absolute intensity of the 27 Al NMR spectrum followed by comparison with a reference ZSM-22 sample of known composition and the Si/Al ratio amounted to 45. The pore mouth and micropore acid site concentration were estimated from crystal size and morphology through SEM pictures and are given in Table 1. NaH-Y zeolite was prepared from Y zeolite by two successive ion exchanges with ammonium and sodium cations. The presence of Na is required to moderate the acidity of the large pore zeolite, which facilitates the synergy effect (Parton et al., 1991). A platinum loading of 0.5 wt% was obtained by impregnation of the NaH-Y sample with a solution of Pt(NH3 )4 Cl2 . The aluminum content expressed as the silica–alumina ratio for NaH-Y was 2.6, see Table 1. Intimate mixtures of the Pt/NaH-Y and Pt/H-ZSM-22 catalyst powders in different proportions were carried out by ball-milling. Both the catalysts retain the metal components on them. Different combinations of the reference catalysts are denoted in the present work in the form of (%Pt/H-ZSM-22:%Pt/NaH-Y). Accordingly, the pure Pt/H-ZSM-22 and Pt/NaH-Y catalysts are denoted by (100:0) and (0:100).

SSQ =

nob nresp  

b wPj (FPj ,k − Fˆ Pj ,k )2 −→ Min

(1)

k=1 j=1

where SSQ is the objective function to be minimized, wPj corresponds to the diagonal element in the inverse error covariance matrix, FPj ,k and Fˆ are the experimental and model calculated outlet flow rates, Pj ,k

b is the model parameter vector, which is expected to approach the real parameter vector  when the optimum is reached. nresp is the number of responses which are the flow rates for feed components, branched isomers and cracked products. nob is the number of observations measured at various temperature levels with fixed spacetime. In the present work, parameter estimation was done using the explicit orthogonal distance regression option of the ODRPACKpackage version 2.01 (Boggs et al., 1992). Some additional source code was added in order to retrieve additional statistical information. For the calculation of model responses, a pseudo-homogeneous one dimensional isothermal and isobaric reactor model was used for the laboratory packed bed reactor (Froment and Bischoff, 1990): dFˆ Pj dW

= RPj

(2)

In case of the catalyst mixtures, the net production rate of a species i, RPi is calculated as an average of the net production rates obtained using the rate equations for the individual catalysts weighted according to the catalyst mixture composition. LSODA (http://www.netlib.org) was used for the integration of the set of ODEs. 3. Determination of catalyst descriptors on reference catalysts

2.2. Experimental set-up

3.1. SEMK model for Pt/NaHY

The catalyst testing was done with n-decane in a high-throughput reactor set-up at vapour phase conditions (Huybrechts et al., 2003).

The reaction rates of the rate-determining steps are expressed as follows:

Oi,j ˜ pi k (m , m )K˜ (O ; O )K˜ (m )K (P ; O )C sat K   AS/PCP/ 1 2 isom i,j r prot 1 deh i i,j i L,i pH2  rAS/PCP/  (m1 , m2 ) =  npar  nole  i Oi,j ˜  pi sat ˜ Kisom (Oi,j ; Or )Kprot (m1 )Kdeh (Pi ; Oi,j )Ci KL,i 1 + KL,i pi + pH2 i i=1 j=1 R+ Ct

(3)

i,k

The experiments were performed with 40 mg catalyst samples at a fixed space time, W/F0 of 2020 kg s mol−1 and varying temperatures between 180 and 240 ◦ C. The n-decane and hydrogen partial pressures at the reactor inlet were 0.9 kPa and 0.35 MPa. The n-decane conversion ranges from 2% to 98%. 2.3. Parameter estimation Parameter values are estimated using the Levenberg–Marquardt method (Marquardt, 1963) for minimization of the following objective function which is the weighed sum of the squared differences

where Oi,j , R+ and   are the global symmetry numbers of the i,k reactants and the transition state. K˜ isom (Oi,j ; Or ) is the single-event isomerization equilibrium coefficient between the alkene Oi,j and the reference alkene Or and Kdeh (Pi ; Oi,j ) is the equilibrium coefficient for (de)-hydrogenation reaction between Pi and Oi,j calculated from thermodynamic data (Benson et al., 1969). The Langmuir physisorption coefficient, KL,i , for an alkane has been determined from separate physisorption measurements. The adjustable parameters in the model are K˜ prot (m1 ) and k˜ AS/PCP/  (m1 , m2 ), i.e., two single-event equilibrium coefficients for protonation of the reference alkene to a carbenium ion of type m1 , either secondary or tertiary and the singleevent rate coefficients which depend only on the reaction family and

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the type of alkylcarbenium ions involved. The former coefficients are denoted as `catalyst descriptors' whereas the latter are the `kinetic descriptors'. The net rate of formation of alkane i is calculated from the rates of formation and disappearance of the corresponding alkylcarbenium ions, see Eq. (3), and the net rate of formation of alkenes directly formed via -scission reactions. In the present work, only catalyst descriptors are estimated, while the activation energies and preexponential factors are taken from the literature for a similar catalyst (Martens et al., 2000; Thybaut et al., 2004). The n-decane hydroconversion data on Pt/NaH-Y catalyst are used to estimate the standard protonation enthalpies, as reported in Table 2 along with the 95% confidence interval values. 3.2. SEMK model for Pt/H-ZSM-22 In contrast to USY, where all acid sites are topologically equivalent, in ZSM-22 framework, acid sites are available at different locations, i.e., at pore mouths, bridges between the latter and micropores (see Fig. 1). The shape selective effects exhibited by Pt/HZSM-22 are site specific and occur in every step of the bifunctional reaction mechanism (Laxmi Narasimhan et al., 2003b, 2004; Laxmi Narasimhan, 2004). A contribution method has been developed for describing the physisorption of branched alkanes at the pore mouths and bridge sites (Laxmi Narasimhan et al., 2003a). As branched alkenes cannot enter the pore beyond the pore mouth, the micropore acid sites, located inside the pores (Souverijns et al., 1998) are only accessible to the linear alkenes. The corresponding branched alkenes can only be protonated either at pore mouth acid sites or at bridge acid sites. Unlike on a Y zeolite, the standard protonation enthalpy at pore mouths and micropores in ZSM-22 has a certain dependency on the number of carbon atoms along the straight chain inside the pore. Table 2 Estimated values of SEMK model parameters for individual catalysts, Pt/H-ZSM-22 and Pt/NaHY, in kJ mol−1 along with the 95% confidence intervals obtained from regression using n-decane hydroconversion data at operating conditions described in Section 2.2. Pt/NaHY o Hprot s

Pt/H-ZSM-22 o Hprot t

Hprotpm/mp

Hprotbs

prot

hin

E(s,p)

−54.3 ± 2.4 −83.7 ± 6.4 −62.9 ± 1.8 −53.1 ± 3.0 −1.45 ± 0.4 158.82 ± 1.0

Differences are assumed to level out after six carbon atoms (Laxmi Narasimhan, 2004): prot o Hprot = Hprotpm/mp + hin CNin,pm CNin,pm = 6 ∀CNin,pm ⱖ 6 (4) pm/mp

In the above equations, Hprotpm/mp corresponds to a measure of the intrinsic acid site strength as described by Thybaut et al. (2001) for prot Pt/H-USY catalyst and hin CNin,pm is the carbon number dependent zeolite stabilization effect on the carbenium ions. A similar carbon number dependency is observed for the standard protonation enthalpy at bridge acid sites, however, no distinction is made between the protonation enthalpy leading to formation of secondary or tertiary carbenium ions because of sterical reasons (Laxmi Narasimhan, 2004). At a pore mouth site only s–s PCP branching, s–s and s–p -scissions can occur, because the ionic centre of the carbenium ion should not move too far away from the active site. -scission of secondary carbenium ions with the formation of primary product carbenium ions is the only reaction which is assumed to occur inside the micropores (Martens et al., 1991, 2001). The full reaction network is accounted for at the bridge acid sites. Because of the equal protonation enthalpies for secondary and tertiary carbenium ions, reactions starting from tertiary carbenium ions contribute much less to the overall conversion than reactions starting from secondary carbenium ions. Three catalyst descriptors need to be estimated for Pt/H-ZSM22 catalyst (Laxmi Narasimhan, 2004), i.e., the standard protonation enthalpies at the pore mouth/micropore sites Hprotpm/mp and at the prot

bridge sites Hprotbs and hin , the incremental standard protonation enthalpy contribution per carbon atom inside the pore. The same kinetic descriptors as in Section 3.1 are used and are complemented with the one for s-p -scission reaction, E(s,p) as this reaction is negligible for hydroconversion on Pt/H-USY. The hydroconversion data on reference Pt/H-ZSM-22 catalyst are used to estimate these model parameters (see Table 2). 4. Simulation of the synergy effect Using the estimated values of the catalyst descriptors for the individual catalysts, see Table 2, the additive model predicts synergy effects for different catalyst mixtures, see Fig. 2. While the n-decane conversion for a physical mixture of Pt/NaH-Y and Pt/HZSM-22 is intermediate to that of the individual catalysts, see

bridge site pore mouths (easy accessible acid sites)

micropores

pore mouth site external surface (Pt-clusters) bridge site X Y Z

Fig. 1. Schematic of ZSM-22 framework topology showing location of different types of acid sites (source: Atlas of zeolite frame work types).

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177

80 Cracked Products

80

Product yields / [%]

n-decane conversion /[%]

100

60 40 20 0 430

Isomer yield / [%]

100

450

470 490 Temperature/ [K]

510

530

70

60

50 60

40

20

0 0

20

40

60

80

100

Fig. 3. Prediction of SEMK model for Pt/NaHY: yield of multibranched isomers and cracked products versus conversion for hydroconversion of (1) n-decane, —; (2) mixture of 60% n-decane and 40% 2-methyl nonane, - - - - - - - -; (3) mixture of 40% n-decane and 60% 2-methyl nonane, – · – · – · at operating conditions described in Section 2.2 using the estimated model parameters given in Table 2 and the activation energies taken from Thybaut et al. (2004). For uniformity, the conversion is calculated based on 100% n-decane as feed.

80

100

40

20

0 0

Multibranched Isomers

Conversion based on 100% n-decane / [%]

90

80

60

20

40 60 n-decane conversion / [%]

80

100

Fig. 2. Comparison of experimental data with prediction of additive SEMK model for individual catalysts and different physical mixtures: (a) conversion versus temperature and (b) total isomer yield versus conversion for n-decane hydroconversion at operating conditions described in Section 2.2 using the estimated model parameters given in Table 2 and the activation energies taken from Thybaut et al. (2004): symbols: experimental data; 䊏: Pt/H-ZSM-22, : Pt/NaHY, : 80:20, 䊉: 60:40, +: 40:60, ∗: 20:80, (%ZSM-22:%NaHY); solid lines: model prediction.

Fig. 2b. Note that Pt/H-ZSM-22 is the more active catalyst in the mixture, since higher conversions are obtained on this zeolite at identical operating conditions. This higher activity is reflected in the estimated values of the catalyst descriptors (see Table 2). The eso is −54.3 kJ mol−1 for Pt/NaH-Y while for timated value of Hprot s o Pt/H-ZSM-22, Hprotpm/mp calculated by Eq. (4), varies between −64.3

and −71.6 kJ mol−1 . The most significant synergy effect is observed when two catalysts are mixed in similar proportions, i.e. (40:60) or (60:40). The maximum isomer yield obtained by the SEMK model is 75.5% corresponding to the mixture composition of (60:40). The simulation of the synergy effect is somewhat less pronounced compared to the experimentally obtained synergy effect. However, taking into account that the only adjustable parameters were the catalyst descriptors that have been estimated from the pure catalyst data, the simulations can be considered adequate. The origin of the synergy effect can be traced back to the difference in hydroconversion patterns between Pt/NaH-Y and Pt/HZSM-22 catalysts. The hydroconversion of n-decane on Pt/H-ZSM-22 gives high isomerization yields, particularly of monobranched isomers. Because of its shape-selective properties, further isomerization and cracking is limited on Pt/H-ZSM-22. However, at higher temperatures and, hence, higher conversions, slow (s,p)- scission

reactions become more important. The maximum isomer yield on Pt/NaH-Y is lower than that on Pt/H-ZSM-22, e.g., 63.5% compared to 70%, because the secondary isomerization and cracking reactions that are inhibited by the ZSM-22 topology, do occur on a Y zeolite. As a result, a substantial amount of multibranched isomers is found by secondary isomerization as well as cracked products by subsequent -scission reactions. In a catalyst mixture with a Pt/HZSM-22 sample that is more active than a Pt/NaH-Y sample, the formation of monobranched isomers, particularly 2-methyl nonane, occurs on the Pt/H-ZSM-22 catalyst. These monobranched alkanes undergo further isomerization on Pt/NaH-Y yielding multibranched alkanes. The activity of the Pt/NaH-Y should be moderate in order to avoid subsequent cracking. Using a catalyst mixture allows to combine the desired behaviour of both catalysts: the Pt/NaH-Y allows to achieve higher multibranched isomer yields and the Pt/H-ZSM-22 provides the monobranched isomers to this Pt/NaH-Y. If the latter catalyst were also to provide the monobranching function, its activity would have to be increased to have similar n-decane conversions and, as a result, cracking would be unavoidable. Simulations for hydroisomerization on Pt/NaH-Y catalyst with feed mixtures containing 40 and 60 mol% of 2-methyl nonane in n-decane illustrate the above explanation of the synergy effect for the multibranched isomers. The experimentally observed increase in multibranched isomer yield amounts to about 5%. Fig. 3 shows that for a feedstock containing 40 or 60 mol% of 2-methyl nonane in ndecane, the yield of multibranched isomers increases around 3–7%. A comparison of these simulation results with the experimental observations indicates that about 50% of the n-decane is isomerized into 2-methyl nonane on Pt/H-ZSM-22 prior to any conversion on Pt/NaH-Y. Compared to the behaviour on 100% Pt/NaH-Y, the formation of monobranched isomers is facilitated on the catalyst mixtures in such a way that further isomerization on Pt/NaH-Y can occur without extensive cracking. On the other hand, compared to the behaviour on 100% Pt/H-ZSM-22, the catalyst mixture allows extending the isomerization conversion beyond the constraints imposed by the shape selectivity of the ZSM-22 topology. 5. Conclusions A Single-Event MicroKinetic model obtained by proportionally adding the reaction rates on the individual catalysts in a physical

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mixture of Pt/NaH-Y and Pt/H-ZSM-22 allows assessing the synergy effect observed in n-decane hydroconversion. The higher isomer yield obtained using such a physical mixture is the result from primary monobranching on Pt/H-ZSM-22 and secondary multibranching on Pt/NaH-Y. The good monobranching capacity of Pt/H-ZSM-22 makes that the activity of Pt/NaH-Y can be tuned towards multibranching and, hence, avoiding excessive cracking. The synergy effect depends on the relative activity of the two catalysts. For an optimal synergy effect, the protonation enthalpy for secondary carbenium ion formation at the pore mouth acid sites of ZSM-22 should be at least 10 kJ mol−1 more negative than at the acid sites of NaHY. Acknowledgement This research has been carried out in the framework of the Interuniversity Attraction Poles Program funded by the Belgian Science Policy. References Atlas of zeolite frame work types, http://www.iza-structure.org/databases/. Baltanas, M.A., Van Raemdonck, K.K., Froment, G.F., Mohedas, S.R., 1989. Fundamental kinetic modeling of hydroisomerization and hydrocracking on noble-metalloaded faujasites. 1. Rate parameters for hydroisomerization. Industrial & Engineering Chemistry Research 28, 899–910. Benson, S.W., Cruickshank, F.R., Golden, D.M., Haugen, G.R., O'Neal, H.E., Rodgers, A.S., Shaw, R., Walsch, R., 1969. Additivity rules for the estimation of thermochemical properties. Chemical Reviews 69, 279–324. Boggs, P.T., Byrd, R.H., Rogers, J.E., Schnabel, R.B., 1992. User's reference guide for ODRPACK version 2.01 software for weighted orthogonal distance regression, National Institute of Standards and Technology (NISTIR), 4834. Denayer, J.F., Baron, G.V., Vanbutsele, G., Jacobs, P.A., Martens, J.A., 2000. Evidence for alkylcarbenium ion reaction intermediates from intrinsic reaction kinetics of C6 –C9 n-alkane hydroisomerization and hydrocracking on Pt/H-Y and Pt/USY zeolites. Journal of Catalysis 190, 469–473. Dzikh, I.P., Lopes, J.M., Lemos, F., Ramôa Ribeiro, F., 1999. Mixing effect of USHY+HZSM-5 for different catalyst ratios on the n-heptane transformation. Applied Catalysis A: General 176, 239–250. Elangovan, S.P., Hartmann, M., 2003. Evaluation of Pt/MCM-41//MgAPO-n composite catalysts for isomerization and hydrocracking of n-decane. Journal of Catalysis 217, 388–395. Ernst, S., Weitkamp, J., Martens, J.A., Jacobs, P.A., 1989. Synthesis and shape-selective properties of ZSM-22. Applied Catalysis 48, 137–148. Froment, G.F., Bischoff, K.B., 1990. Chemical Reactor Analysis and Design. second ed. Wiley, New York. Hayasaka, K., Liang, D., Huybrechts, W., De Waele, B.R., Houthoofd, K.J., Eloy, P., Gaigneaux, E.M., van Tendeloo, G., Thybaut, J.W., Marin, G.B., Denayer, J.F.M., Baron, G.V., Jacobs, P.A., Kirschhock, C.E.A., Martens, J.A., 2007. Formation of ZSM-22 zeolite catalytic particles by fusion of elementary nanorods. Chemistry A European Journal 13, 10070–10077. Huybrechts, W., Mijoin, J., Jacobs, P.A., Martens, J.A., 2003. Development of a fixed-bed continuous-flow high-throughput reactor for long-chain n-alkane hydroconversion. Applied Catalysis A: General 243, 1–13. Huybrechts, W., Thybaut, J.W., De Waele, B.R., Vanbutsele, G., Houthoofd, K.J., Bertinchamps, F., Denayer, J.F.M., Gaigneaux, E.M., Marin, G.B., Baron, G.V.,

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