Mild hydroisomerization of heavy naphtha on mono- and bi-metallic Pt and Ni catalysts supported on Beta zeolite

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Mild hydroisomerization of heavy naphtha on mono- and bi-metallic Pt and Ni catalysts supported on Beta zeolite Stamatia A. Karakouliaa, , Eleni Heracleousa,b, , Angelos A. Lappasa ⁎

⁎

a

Chemical Process and Energy Resources Institute (CPERI)/Centre for Research and Technology Hellas (CERTH), 6th km Charilaou-Thermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece b School of Science & Technology, International Hellenic University (IHU), 14th km Thessaloniki – Moudania, 57001 Thessaloniki, Greece

ARTICLE INFO

ABSTRACT

Keywords: Hydroisomerization Pt and Ni catalysts Beta zeolite n-Octane Refinery naphtha

Hydroisomerization of long chain paraffins has emerged as a promising process for the valorization of low value fuel cuts, such as heavy naphtha, from crude oil-, coal- and bio-based feedstocks. In this study, the hydroisomerization reaction is investigated on a series of mono- and bi-metallic Pt (0.2 wt%) and/or Ni (10 wt%) catalysts supported on Beta zeolite with SiO2/Al2O3 ratio 75. Catalysts exhibit similar reactivity in the hydroisomerization of n-octane model compound, despite the very different loading and the 40% lower Brönsted acidity of the Ni-containing materials. This suggests that hydrogenation/dehydrogenation is the rate limiting step of the reaction due to very low loading of Pt and the weak dehydrogenating capacity of Ni. The latter also leads to up to 34% lower selectivity to i-C8 for the monometallic Ni catalyst. The addition of Pt in the bimetallic Ni-Pt catalyst, not only restores, but enhances the isomerization selectivity, favoring the production of dibranched to mono-branched isomers. The characterization results show that Pt enhances the reducibility and improves the dispersion of Ni, leading to increased metallic/acid sites ratio. Hydroisomerization tests with refinery naphtha as feed highlight the promising performance of this catalyst, as Ni-Pt/H-Beta increases the octane number of naphtha by 13% at mild temperature and pressure (260 °C, 10 bar), improving significantly the quality of naphtha for gasoline applications.

1. Introduction Coal is one of the dominant energy sources, accounting for 27.6% of the primary energy production in 2017 [1]. It is also the most abundant fossil resource by global Reserves/Production (R/P) ratio [2], defined as the number of years reserves would last if use continued at the current rate, with coal resources being located more widely throughout the world than oil or natural gas reserves. Coal can be converted to liquids either directly or indirectly. The indirect coal liquefaction proceeds via gasification of coal to synthesis gas and subsequent conversion of syngas to fuels by Fischer-Tropsch. The process is commercial since the 1950’s by Sasol in South Africa [3]. Coal can also be converted directly into liquids through the Direct Coal Liquefaction (DCL) process at temperatures of 450–500 °C and pressures of 100–200 atm, typically under H2 atmosphere and in suitable solvents in the presence or absence of catalysts [4]. The process was first developed in the 1930’s and several commercial processes were developed during the oil crises in the 1970’s but were then abandoned due to the availability of cheap

crude oil. Today, the increasing demand for liquid transportation fuels, in combination with concerns surrounding crude oil depletion, price volatility and energy security, have renewed interest in the DCL process. The only major current commercial activity is the DCL plant in Inner Mongolia, China, operated by the Shenhua Group that began operating in 2008 [5]. The quality of the DCL products is variable as it depends on a number of parameters, such as type of the coal feedstock, process conditions, type of solvent and catalyst etc. In all cases, a distribution of products is obtained that includes light gases, distillate liquids, and non-distillable liquid which may be solid at room temperature [3]. A large portion of the distillate liquids is in the heavy naphtha range. This naphtha typically requires upgrading to reduce its naphthenic nature and boost the octane number without further increase in the aromatics content. An efficient upgrading strategy is the co-processing of DCL naphtha with fossil-based naphtha via hydroisomerization. Isomerization of normal to iso-paraffins is a key reaction for improving the quality of fuels. In the last years, hydroisomerization of heavier naphtha

⁎ Corresponding authors at: Chemical Process and Energy Resources Institute (CPERI)/Centre for Research and Technology Hellas (CERTH), 6th km CharilaouThermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece. E-mail addresses: [email protected] (S.A. Karakoulia), [email protected] (E. Heracleous).

https://doi.org/10.1016/j.cattod.2019.04.072 Received 4 December 2018; Received in revised form 13 February 2019; Accepted 19 April 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Stamatia A. Karakoulia, Eleni Heracleous and Angelos A. Lappas, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.04.072

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has been attracting the interest of both the refining and bio-refining industry for valorizing low value fuel cuts [6,7]. The hydroisomerization reaction is catalyzed by metal catalysts supported on acidic supports that provide the two necessary functionalities for the reaction to occur: metal sites for dehydrogenation/hydrogenation and acid sites for the skeletal re-arrangement of the carbon chain [8,9]. Optimum balance between the metal and the acidic function of the catalyst is required to minimize the cracking reactions that increase with the hydrocarbon chain length and subsequently the increasing number of possible scission reactions [6,10]. Catalysts with high hydrogenation activity and low acidity are typically required to maximize hydroisomerization versus hydrocracking [9,11]. The pore opening of the support plays an equally important role. Depending on the chain length, too small pores will favor direct hydrocracking due to space limitations, while very large pores will promote secondary cracking reactions of the produced isomers [12,13]. Most common hydroisomerization catalysts consist of noble metals, such as Pt, Pd, and bimetallic Ni-based systems (Ni/Co, Ni/W, Ni/Mo in sulfided form) supported on amorphous oxides (SiO2-Al2O3, ZrO2/SO4) [14,15], zeolites (ZSM-5, ZSM-22, ZSM-23, Beta, Mordenite, Y) [13,16], silicoaluminophosphates (SAPO-11, SAPO-31, SAPO-41) [17,18,19] and mesoporous materials (MCM-41, AlMCM-41, SBA-15) [11,20]. Most literature studies cited above use single model paraffins for the assessment of the catalytic performance. Without overlooking their importance in understanding the key catalyst properties and deriving information on the reaction mechanism, catalysts should also be evaluated with real heavy naphtha streams that typically contain different hydrocarbons (n-paraffins, i-paraffins, naphthenes, olefins, aromatics) of variable chain length. Such studies are scarcer in literature [21–25] due to the increased complexity of the involved reactions. Hydroisomerization of real naphtha with n-paraffins, isoparaffins, naphthenes and aromatics in the C5 - C9 carbon range on Pt/Beta (SiO2/Al2O3 ratio = 25) resulted in a significant increase of the research octane number (RON) of the feed, which increased from 44 to 80, rendering it suitable for gasoline applications [23]. In an effort to reduce the noble metal loading, bi-metallic Pt catalysts with Ni or Cr were also studied in the presence [24] and absence [25] of hydrogen. We previously showed that surrogate naphtha feeds with chain length up to 10 can be successfully isomerized over low loading (0.1 wt %) Pt catalysts supported on ZSM-5, Beta and Mordenite zeolites [26]. Pt/ZSM-5 was most active; however the extent of secondary cracking reactions was equally high due to the strength and density of the Brönsted acid sites in combination with space limitations in the microporous ZSM-5. The addition of mesoporosity via controlled desilication/dealumination increased the isomerization selectivity (as it probably alleviated diffusion limitations that increase the residence time of the reaction intermediates and thus favor over-cracking reactions), but decreased activity as the catalyst lost part of its acidity [27,28]. We also showed very promising results in hexadecane hydroisomerization, relevant for the upgrading of biofuels from vegetable/ algal oil or biomass-derived FT-waxes, over non-noble metal Ni-based catalysts on desilicated H-Beta [29]. Besides improved hydroisomerization activity, the desilication of H-Beta improved the stability with respect to carbon formation. In this work, we extent our investigation in the hydroisomerization of long chain paraffins over Pt and Ni monometallic and bimetallic catalysts supported on Beta zeolite with SiO2/Al2O3 ratio 75, aiming at moderate acidity while preventing side hydrocracking reactions. Low Pt (0.2 wt%) and high Ni (10 wt%) loading is employed to reduce the catalyst cost while achieving an appropriate metal/acid sites ratio for hydroisomerization. Tests with n-octane as model compounds are complemented with tests with complex heavy refinery naphtha to show the applicability of the developed catalysts in actual industrial processes.

2. Experimental 2.1. Preparation of catalysts Zeolite of Beta type with SiO2/Al2O3 (SAR) of 75 was used as support (CP811E-75 by Zeolysts Int.). This zeolite was initially in NH4+form and was converted to H+- form by calcination at 550 °C for 3 h under air flow. Mono- and bi-metallic Pt (0.2 wt%) and/or Ni (10 wt%) supported catalysts were prepared via dry impregnation with aqueous solutions of H2PtCl6 and/or Ni(NO3)2·6H2O respectively at appropriate amounts. Bi-metallic catalyst was prepared with stepwise impregnation: Ni (10 wt%) was introduced first on the zeolite surface and after an intermediate calcination step at 450 °C for 3 h in air, subsequent impregnation of Pt (0.2 wt%) was performed. After drying, all impregnated samples were calcined at 400 °C (for Pt) or 450 °C (for Ni) for 3 h under air flow. 2.2. Characterization of catalysts Inductive Coupled Plasma - Atomic Emission Spectroscopy (ICPAES) was used for determination of the metal chemical composition. N2 adsorption-desorption at −196 °C was performed on an Automatic Volumetric Sorption Analyzer (Autosorb-1, Quantachrome) for the determination of surface area (BET method), pore volume and pore size distribution (BJH method) of samples previously outgassed at 250 °C for 16 h under 1.33 × 10−1 Pa vacuum. Powder X-ray diffraction (XRD) experiments were performed on a Siemens D-500 X-ray diffractometer with Cu Kα radiation in the 2θ range of 5–85°. Acidic characteristics of the catalysts were studied with NH3-temperature programmed desorption (TPD-NH3). In a typical experiment, 0.1 g of the sample was loaded in a fixed bed quartz reactor and pretreated at 400 °C (Pt, Ni-Pt) or 450 °C (Ni) in He for one hour and then the catalyst was cooled down to 100 °C under He flow. Adsorption of ammonia was then performed with a flow of 5% NH3/He for 1 h at 100 °C. After flushing with pure helium at 100 °C for 12 h to remove the physiosorbed ammonia, TPD analysis was carried out from 100 to 800 °C at a heating rate of 10 °C/min in helium. The composition of the exit gas was monitored on line with a quadrupole mass analyser (Omnistar, Balzer). The m/z fragments registered were as follows: NH3 = 17, 16, 15; H2O = 18; N2 = 28; NO = 30; N2O = 44. Quantitative analysis of the desorbed ammonia was based on m/z = 15. The type and strength of acid sites were determined with pyridine adsorption monitored by IR. The IR spectra were collected using a Nicolet 5700 FTIR spectrometer (resolution 4 cm−1) by means of OMNIC software. Data processing was carried out via the GRAMS software. All catalyst samples were finely ground in a mortar and pressed in self-supporting wafers (˜15 mg/cm2). Wafers were placed then in a homemade stainless steel, vacuum cell, with CaF2 windows. High vacuum was reached by means of a turbomolecular pump and a diaphragm pump placed in line. Before pyridine adsorption and IR analysis, samples were heated at 450 °C under high vacuum (10−6 mbar) for 1 h in order to desorb any possible physiosorbed species (activation step). Spectra were collected at 150 °C in order to eliminate the possibility of pyridine condensation. Initially, the reference spectrum of the activated sample was collected. Then pyridine was added in pulses and adsorption of pyridine was realized at 1 mbar by equilibrating the catalyst wafer for 1 h. The desorption of pyridine was step wisely monitored by evacuating the sample for 30 min at 150, 250, 350 and 450 °C and cooling down to 150 °C after each step to record the corresponding spectrum. The calculation of the acid sites was based on the integrated Lambert-Beer law, while the Brönsted (band at ˜1540 cm−1 attributed to pyridinium ions) and Lewis (band at ˜1445 cm−1 attributed to coordinated pyridine) molar extinction coefficients of Emeis [30] were employed. Reducibility was studied by temperature-programmed reduction with H2 (TPR-H2). In a typical experiment, 0.1 g of the catalyst sample 2

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was loaded in a fixed bed quartz reactor and pre-treated at 400 °C (Pt, Ni-Pt) or 450 °C (Ni) under He for one hour. The catalyst was then cooled down to 40 °C and TPR-H2 analysis was carried out from 40 to 800 °C at a heating rate of 10 °C/min in 5% H2/He flow. The composition of the exit gas was monitored online with a quadrupole mass analyzer (Omnistar, Balzer). The m/z fragments registered were as follows: H2 = 2, H2O = 18 and He = 4. Quantitative analysis of the consumed H2 was based on m/z = 2. The nature of the active sites was probed with Diffuse Reflectance UV–vis spectroscopy (DRUV-vis). The measurement was performed at room temperature using a U-3010 spectrometer (Hitachi) with a scanning rate of 60 nm/min and scanning range of 900–190 cm−1. The sampler window was a fused silica. The dispersion of the metals on the catalyst surface was assessed with CO adsorption followed by temperature-programmed desorption in He (TPD-CO). These tests were performed in the same apparatus as the TPR-H2 measurements. In a typical experiment, the catalyst sample (0.1 g) was pre-reduced at 600 °C in 5% H2/He flow for 1 h. After cooling to 30 °C under He, the catalyst was exposed to 10% CO/He flow for 1 h at 30 °C, followed by flushing in He to remove any physiosorbed CO molecules. TPD analysis was carried out from 30 to 700 °C at a heating rate of 10 °C/min in 100% He flow. The composition of the exit gas was monitored online with a quadrupole mass analyzer (Omnistar, Balzer). The m/z fragments registered were as follows: CO = 28, H2 = 2, H2O = 18 and He = 4. Quantitative analysis of the desorbed CO was based on the integration of m/z 28 signal in the range of 50–600 °C. Metal dispersion was calculated by assuming metal (Pt, Ni):CO stoichiometry equal to 1:1. In order to obtain information on the amount and nature of coke deposited on the catalytic surface during reaction, Temperature Programmed Oxidation (TPO) was performed on STA 449 F5 Jupiter thermogravimetric analyzer. Typically, 20–30 mg of the used sample were loaded in an alumina crucible, and the temperature was raised from room temperature to 900 °C at a heating rate of 10 °C/min in synthetic air flow. The system was maintained isothermally at 900 °C for 30 min. Elemental composition of the catalysts after the reaction (carbon and hydrogen) was measured using a LECO-628 CHN elemental analyzer, according to ASTM D5291.

Table 1 Properties and composition of naphtha feedstock. Density, g/cm3 RON Composition, wt% C5 normal paraffin C6 normal paraffin C7 normal paraffin Total normal paraffins C5 iso paraffin C6 iso paraffin C7 iso paraffin C8 iso paraffin Total iso paraffins C5 saturated naphthene C6 saturated naphthene C7 saturated naphthene C8 saturated naphthene Total saturated naphthenes C6 aromatic C7 aromatic Total aromatics Total

0.7 55 1.91 18.90 11.01 31.82 0.58 13.16 17.43 1.17 32.34 0.67 16.69 14.98 0.48 32.82 1.31 1.71 3.02 100

on stream. Liquid samples were analyzed by GC to determine the amount of paraffin (P), isoparaffins (I), aromatics (A), naphthalene (N), and olefins (O). Based on the results of the PIANO analysis, the research octane number (RON) was automatically calculated with the software Carburan. The composition of the reaction gases was detected with a GC-FID. 3. Results and discussion 3.1. Catalyst characterization Chemical composition and measured SiO2/Al2O3 (SAR) molar ratio of the support and metal catalysts are presented in Table 2. With regards to SAR, the measured and nominal values coincide, suggesting no dealumination during the catalyst preparation phase. The metal content is also close to nominal for both Pt and Ni in all synthesized catalysts. XRD patterns present the typical structure of zeolites with BEA framework according to the International Zeolite Association (IZA) database [31,32] (Fig. 1). No peaks corresponding to Pt are identified, indicating that the Pt is well-dispersed on the surface of the zeolite in the form of small crystals below 5 nm. On the contrary, Ni-containing catalysts present five distinct diffraction peaks at 2θ = 37°, 43°, 63°, 75°, 79°, corresponding to cubic NiO [33,34]. The NiO crystal size, calculated with the Scherrer equation based on the reflection at 43°, is in both samples ˜ 17 nm. Calculation of the relevant crystallinity of the H-Beta zeolite based on the 23° diffraction (Table 2) shows that deposition of Pt at such small loadings has a minor effect in crystallinity. Introduction of Ni at higher quantities (10 wt%) reduces crystallinity by about 18%, but it is still retained at high levels. The shape of N2 adsorption/desorption isotherms is similar for all materials and typical of that of the Beta zeolite (Fig. 2) [31]. It is a combination of Type I and II isotherms (according to IUPAC classification [35]), characteristic of a material with both micro and mesoporosity, as verified also from the BJH pore size distribution (Fig. 2b) [36]. The textural properties of the Pt/H-Beta zeolite are very similar to those of the bare support, with negligible changes of less than 2%. Nickel deposition in both the mono- and bi-metallic catalyst reduces the surface area and pore volume by about 16% due to the much higher loading, indicating partial blockage of the pores by the NiO species (Table 3). This is in line with the size of the NiO crystallites, calculated from XRD, suggesting the deposition of NiO domains mainly on the external surface of the zeolite. Ammonia is a suitable probe molecule for acidity due to its small size and its basicity that allows interaction with the majority of the acid

2.3. Catalyst evaluation tests Catalysts performance in hydroisomerization was investigated in a high-pressure small scale unit equipped with an electronic feed control system for both gases and liquid feeds. The unit’s feeding system consists of three gas lines, each equipped with high accuracy mass flow controllers, and one liquid feed line, using a high precision pump. The stainless steel fixed bed-type reactor (ID: 9.3 mm) is externally heated with a three-zone furnace. The reaction temperature is monitored with a thermocouple inserted in catalytic bed. The exit stream of the reactor, after cooling using a heat exchanger, is sent to a liquid-gas separator for collection of liquid and separation from gaseous products. In a typical experiment, the required catalyst amount, previously crushed to particle size 250 μm–335 μm, dried overnight and diluted with equal amount of SiC, was introduced into the reactor. Prior to measurements, the catalyst was pre-reduced in-situ in H2 at 400 °C for 2 h. The reaction was conducted at temperature 260–300 °C, pressure 10–30 bar, weight hour space velocity (WHSV) 4–10 h−1 and H2/feed molar ratio 15. The entire set of experimental conditions was tested in one consecutive run. After exposing the catalyst to the different operating variables, the initial experimental condition was repeated to check for possible degradation in the catalyst performance. Initial screening tests were performed with n-octane as representative model compound. The optimum catalyst was also tested with refinery crudeoil based naphtha stream, with composition similar to that of DCL naphtha. The composition and main properties of naphtha are shown in Table 1. Steady-state activity data were determined after > 8 h of time 3

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Table 2 Chemical composition, crystalline properties and dispersion. Catalyst

ICP-AES SAR

H-Beta Pt/H-Beta Ni/H-Beta Ni-Pt/H-Beta

(a)

76 75 72 75

XRD

Pt (wt.%)

Ni (wt.%)

Al (wt.%)

– 0.19 – 0.21

– – 11.82 11.15

1.16 1.17 1.04 1.04

TPD-CO (b)

Crystallinity (%) 100 96 87 82

Desorbed CO (μmol/g)

Metal Dispersion (%)

– 4.4 14.4 25.1

– 100 7.2 13.2

(a)

SiO2/Al2O3 molar ratio and (b) relevant crystallinity measured from the 2θ peak of 23 degrees of each catalyst’s XRD pattern compared with the initial H-Beta zeolite before metal modification.

Fig. 1. X-ray Diffraction patterns.

sites. The amount of adsorbed ammonia is proportional to the number of acid centers, while the temperature of desorption is indicative of the acid site strength. During temperature-programmed desorption (TPDNH3), the pure zeolite support exhibits a large broad peak extending from 200 to 500 °C, with maximum at ˜400 °C and a tail up to > 800 °C (Fig. 3). This wide range suggests acid sites of different strength on the Beta zeolite. In the Pt/H-Beta catalyst, desorption starts at higher temperature (250 °C), with maximum at the same temperature, and a more well-defined shoulder at 550 °C. Quantification of the desorbed ammonia shows 12% decrease in the number of acid sites (Table 4), associated with the additional calcination step after Pt impregnation. The profiles of the Ni-containing catalysts are narrower and exhibit distinct differences. The desorption maximum shifts to lower values (˜350 °C), showing the presence of weaker acid sites attributed to NiO species. The decrease of total acidity is higher than on Pt/H-Beta (23%) due to the higher calcination temperature employed for the Ni catalysts and the partial blockage of the zeolite pores (evidenced with N2 adsorption/desorption), reducing accessibility to the acid sites. Chemisorption of pyridine followed by IR is useful to detect the number and strength of surface aprotic (Lewis) acid and protonic (Brönsted) acid sites on the catalyst surface [37]. Although a weaker basic molecule than ammonia and able to only titrate stronger acid sites, pyridine is able to interact via nitrogen lone-pair electrons with these acid sites giving rise to characteristic bands. IR spectra obtained after pyridine chemisorption at 150 °C and distribution of Brönsted acid sites strength are shown in Fig. 4a. According to previous studies [38,39], the band at about 1540 cm−1 is attributed to the vibrational modes of Brönsted coordinated pyridine, whereas the band at 1445 cm−1 corresponds to vibration of pyridine chemisorbed on Lewis acid sites. An additional band appears at 1490 cm−1 attributed to pyridine associated with both Brönsted and Lewis acid sites [40]. Total acidity, estimated with pyridine-FTIR and TPD-NH3, differs considerably due to the weaker basicity of pyridine that does not allow

Fig. 2. (a) N2 adsorption/ desorption isotherms and (b) BJH pore size distribution from adsorption data.

titration of very weak acid sites (Table 4). For the Pt/H-Beta catalyst, both techniques show a reduction in the total number of acid sites compared to the pure zeolite. Based on the pyridine-FTIR results, these sites are of Brönsted type, confirming that the reduction is associated with the calcination treatment of the catalyst. No significant change in the strength of these sites is however observed (Fig. 4b). In the Nicontaining catalysts, the acidity measured with pyridine is very similar to that of the parent zeolite, contrary to the NH3-derived acidity which is lower by 23%. This suggests that NiO species deposit on weak acid sites, not detectable by pyridine. Moreover, deposition of NiO decreases considerably the Brönsted sites (due to coverage and thermal treatments) and generates new acid sites of Lewis type. This leads to a much lower Brönsted/Lewis (B/L) acid sites ratio compared to the Pt/H-Beta and H-Beta samples (Table 4). Temperature-programmed reduction of Pt/H-Beta does not demonstrate any hydrogen consumption (Fig. 5). The corresponding profiles for the mono- and bi-metallic Ni-containing catalysts show multiple reduction peaks, indicative of the presence of different NiOx 4

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Table 3 Porosity and textural properties. Catalyst H-Beta Pt/H-Beta Ni/H-Beta Ni-Pt/H-Beta

Total surface area (m2/g)(a)

Micro-pore (m2/g)(b)

Total Pore volume (cm3/g)(c)

Meso-macro-pore volume (cm3/g)

Micro-pore volume (cm3/g)(b)

Textural Volume (cm3/g)(d)

679 671 570 579

443 437 369 376

0.94 0.93 0.87 0.98

0.37 0.36 0.30 0.31

0.18 0.17 0.15 0.15

0.40 0.40 0.42 0.52

Pore diameters (nm)(e) 12.0, 11.9, 11.9, 11.8,

42.6 45.2 44.0 24.5

(a)

From multi-point BET method, (b) from V-t analysis plot method, (c) calculated at P/Po = 0.99, (d) Textural volume = Total pore volume − volume calculated at P/ Po = 0.90 and (e) Average Meso- Macro-Pore diameter from BJH analysis using adsorption data.

Fig. 3. Ammonia desorption profiles during TPD-NH3. Table 4 Total acidity and type of acid sites. Catalyst

H-Beta Pt/H-Beta Ni/H-Beta Ni-Pt/H-Beta

TPD-NH3 (μmol/g)

344 298 264 269

FTIR pyridine (μmol/g) Total

Brönsted

Lewis

B/L ratio

214 201 227 220

133 138 84 83

81 63 143 136

1.6 2.2 0.6 0.6

species. The amount of consumed hydrogen corresponds to full reduction of NiO to metallic Ni on both catalysts. The Ni/H-Beta catalyst exhibits two broad reduction peaks, with maximum at ˜ 150 and 450 °C, in addition to a shoulder at ˜550 °C. The peak at 450 °C is assigned to the reduction of bulk NiO (nano)particles [41–43], while the shoulder corresponds to NiO species interacting strongly with the support and/or Ni2+ ions penetrating into the zeolite lattice [44,45]. The high temperature shoulder disappears in Ni-Pt/H-Beta, while the maximum of the main reduction peak shifts to ˜70 °C lower temperature, suggesting that even a very small amount of Pt (0.2 wt%) facilitates the reduction of the NiO species [46]. Pt metal sites act as active centers for H2 dissociation and nucleation, enabling the reduction of NiO at lower temperature [47]. DRUVS absorption spectra of the investigated catalysts are shown in Fig. 6. Corresponding spectra of bulk NiO and the quartz sample holder are also included for reference. As expected, the spectrum of Pt/H-Beta is almost flat, demonstrating no UV–vis absorbance. The monometallic Ni/H-Beta shows bands with maximum at 285 nm, 380 nm and 720 nm, typical of octahedrally coordinated NiOx species [51]. The bands between 285–329 nm and 409–430 nm are broader and red-shifted compared to bulk NiO [52]. This can be ascribed to the well-known quantum size effect, evidencing the presence of smaller NiO particles on the Beta zeolite compared to unsupported NiO [53]. The presence of Pt in the bi-metallic Ni-Pt/H-Beta catalyst eliminates the broad absorption

Fig. 4. (a) FTIR spectra after pyridine equilibrium at 150 °C and (b) distribution of Brönsted acid sites strength.

band in the region between 409–430 nm and in parallel reveals a weak absorption at 380 nm that suggests modification of the NiOx species on the catalyst. The number of active sites and metal dispersion was probed with CO chemisorption, followed by temperature programmed desorption in helium. The CO desorption profiles (Fig. 7) demonstrate two broad desorption peaks at 50–200 °C and 200–500 °C, corresponding to associative [48] and dissociative [49] CO adsorption on the metal sites. The monometallic Pt/H-Beta catalyst exhibits two equally sized CO desorption peaks centered on 135 °C and 300 °C. The low temperature peak becomes much smaller and shifts to 170 °C on Ni/H-Beta, while the population of CO species interacting more strongly with the surface distinctly increases. Promotion with Pt clearly influences the nature of the Ni sites, as Ni-Pt/H-Beta demonstrates only one major CO desorption peak with maximum at 170 °C; the high temperature peak is basically eliminated. It can thus be deduced that the presence of Pt modifies the electronic structure of the Ni metal sites [50] and significantly weakens interaction with CO molecules. 5

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Metal dispersion was calculated on the basis of the quantified CO desorption. The desorbed CO and the respective metal dispersions are presented in Table 2. As expected, the dispersion of Pt in the monometallic Pt/H-Beta catalyst is close to 100%, given the very low noble metal loading (0.2 wt%). The amount of desorbed CO is considerably higher in Ni/H-Beta, however considering the high Ni loading (10 wt%) this corresponds to lower dispersion, equal to 7.2%. In the bimetallic Ni-Pt/H-Beta catalyst, Pt not only weakens CO interaction with the surface as discussed above, but also improves dramatically the dispersion of Ni, which doubles to a value of 13.2%. 3.2. Evaluation of catalytic performance 3.2.1. Tests with n-octane model compound Table 5 shows the catalytic performance of the mono- (Pt or Ni) and bi-metallic (Ni-Pt) Beta-supported catalysts in n-octane hydroisomerization at different operating conditions. In all cases, the reaction leads to gaseous and liquid products, with the latter grouped into isooctane, cracked isomers i-C5-C7, cracked linear alkanes n-C5-C7 and cyclooctane c-C8. No production of other cycloalkanes or aromatics is observed at any of the investigated conditions. Variation of the temperature, pressure and space velocity of the reaction causes similar changes on all catalysts. At 260 °C, 10 bar and 10 h−1, cracking to gaseous alkanes is limited, the main product being the liquid isomerate comprising mainly of iso-octane and some cracked isomers with carbon number 5–7. Increase of the reaction temperature to 300 °C leads to almost complete conversion of n-octane, at the expense of the isomerization selectivity to i-C8. At this temperature, there is a large formation of dry gases and cracked alkanes, indicating the predominance of hydrocracking reactions. It is interesting to note that the increase of temperature leads to higher production of isomerized than linear cracked paraffins, indicating extensive secondary cracking of C8 isomers. The composition of the light gases at 300 °C is plotted in Fig. 8. The main gas is in all cases i-butane, followed by n-butane and propane. The high content in i-C4 suggests that isomerization does take place, but the produced isomers react further in secondary cracking reactions to smaller carbon chain length isomers. Production of methane is also detected only on Ni-containing catalysts, consistent with the known hydrogenolysis functionality of this metal [54,55]. Decreasing the space velocity from 10 h−1 to 4 h−1 at 260 °C and 10 bar leads, as expected, to increased n-octane conversion, without however concurrent formation of gases or cracked products. Selectivity to iso-octane remains at similar levels and even improves markedly in the case of the Ni/H-Beta catalyst. Finally, the pressure increase from 10 bar to 30 bar at 260 °C and 4 h−1 negatively affects conversion that decreases by 20–30 wt%, with minor improvements in the hydroisomerization selectivity. The lower conversion at high pressure can be clearly explained by the lower concentration of olefinic intermediates at high pressures and a higher degree of hydrogen occupation on the catalytically active sites with respect to the hydrocarbon molecules [56]. With regards to the metal functionality, comparison of the different materials shows similar reactivity on all catalysts, with n-octane conversion ranging from ˜25 to 90 wt% depending on the reaction conditions. However, in view of the considerably different metal loadings, Ni catalysts are much less active compared to monometallic Pt catalyst; fifty times more nickel (10 wt%) is needed to obtain the same activity with that attained with 0.2 wt% platinum. Jordao et al. [57] reported similar results when comparing Ni and Pt catalysts on HUSY zeolite for the hydrorisomerization of n-hexane. The acidic properties of the catalysts (type and amount of acid sites) do not appear to influence reactivity. Typically, a direct relation between catalyst activity and the number of Brönsted acid sites is witnessed [58]. This is not observed here; despite the substantially lower number of Brönsted acid sites in the Ni-containing catalysts (about 40% less than in the Pt catalyst, Table 4), similar conversion levels are recorded. Activity is thus

Fig. 5. TPR-H2 profiles of supported catalysts.

Fig. 6. UV–vis spectra of supported catalysts, NiO and quartz holder.

Fig. 7. TPD-CO profiles of supported catalysts.

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Table 5 Catalytic performance in n-octane hydroisomerization. Catalysts

Pt/H-Beta

Ni/H-Beta

Ni-Pt/H-Beta

Reaction conditions

Conversion, wt%

−1

T = 260 °C, P = 10 bar, WHSV = 10 h T = 300 °C, P = 10 bar, WHSV = 10 h−1 T=260 °C, P=10bar, WHSV=4h−1 T = 260 °C, P = 30 bar, WHSV = 4 h−1 T = 260 °C, P = 10 bar, WHSV = 10 h−1 T = 300 °C, P = 10 bar, WHSV = 10 h−1 T = 260 °C, P = 10 bar, WHSV = 4 h−1 T = 260 °C, P = 30 bar, WHSV = 4 h−1 T = 260 °C, P = 10 bar, WHSV = 10 h−1 T = 300 °C, P = 10 bar, WHSV = 10 h−1 T = 260 °C, P = 10 bar, WHSV = 4 h−1 T = 260 °C, P = 30 bar, WHSV = 4 h−1

Yield, wt%

26.7 88.6 52.0 40.0 30.7 91.4 51.8 35.2 24.1 92.1 53.7 34.3

Selectivity, wt%

Gas

Liquid

i-C8

n-C5-C7

i-C5-C7

c-C8

Gases

4.2 39.3 8.0 2.7 10.4 31.8 9.7 12.5 2.7 36.5 5.1 2.4

95.8 60.7 92.0 97.3 89.6 68.2 90.3 87.5 97.3 63.5 94.9 97.6

74.4 44.9 78.9 82.2 47.9 39.5 68.3 63.1 81.3 33.9 76.8 82.8

19.9 5.4 10.6 14.2 40.3 16.3 18.4 20.7 15.1 13.2 15.6 13.4

1.1 10.0 2.3 0.9 1.5 12.1 3.6 3.7 0.8 16.1 2.4 1.4

0.1 0.2 0.1 0.0 0.0 0.3 0.0 0.0 0.0 0.3 0.1 0.0

4.2 39.3 8.0 2.7 10.4 31.8 9.7 12.5 2.7 36.5 5.1 2.4

catalysts on HY zeolite for n-octane hydroisomerization [60]. The detailed product distribution at similar n-octane conversion, as well as the total ratio of isomerized/linear alkanes and multi/monobranched isomers, is illustrated in Fig. 9a–c. On all catalysts, the reaction yields mainly mono-branched isomers with a methyl group in the linear carbon chain, with extremely low production of ethyl-alkanes. The multi-branched products are di-methyl in nature and the degree of branching is highest in the C8 fraction. No tri-branched isomers are detected at any of the investigated conditions/catalysts. This is consistent with the thermodynamically-dictated reduction in the degree of branching with increasing temperature. The molar distribution of the cracked products is symmetrical, indicating a primary cracking reaction network [61]. Moreover, cracking to butanes is favored over the pathway to pentanes and propane. The higher occurrence of central βscission suggests more extensive branching of n-octane prior to the CeC bond cleavage [61], which is expected for the pore structure of the BEA zeolite. The i/n C4 ratio is in all cases higher than the equilibrium value of 1. This indicates that butanes are produced from the cracking of diand tri-branched isomers and/or that the catalysts are also active in the hydroisomerization of n-butane. The 2C4/C3+C5 ratio is higher on the mono-metallic Pt catalyst (1.8) compared to the Ni-containing materials (1.4). Overall, the i/n alkanes ratio is higher on the Pt catalyst, due to the stronger hydrogenation/dehydrogenation function of Pt compared to Ni. On the other hand, the multi/mono isomers ratio is higher for the Ni-containing catalysts. Yoshioka et al. [47] also reported an increase in the production of di-branched alkanes from n-hexane with increasing Ni content in bimetallic Ni-Pt catalysts supported on HUSY zeolite. The enhanced production of multi-branched products is very important considering the much higher octane number of these alkanes compared to mono-isomers. This, in combination with the enhanced iso-octane selectivity and the low dry gas formation, renders the bimetallic Ni-Pt/ H-Beta catalyst a promising catalyst for the hydroisomerization of heavy naphtha.

Fig. 8. Selectivity to the gaseous products in n-octane hydroisomerization (reaction conditions: T = 300 °C, P = 10 bar, WHSV = 10 h−1, H2/n-C8 molar ratio = 15).

governed by the concentration of the available metal surface sites rather than acid sites and hydrogenation/dehydrogenation is the rate limiting step of the reaction due to very low loading in the case of Pt and the weak dehydrogenating capacity of the metal in the Ni catalysts. Similar findings have previously been reported on low loading Pt/HBEA catalysts in n-pentane hydroisomerization [59]. Although reactivity is comparable, important differences exist in the product distribution over the investigated catalysts. The monometallic Pt catalyst demonstrates consistently much higher selectivity to isooctane, in the range of ca. 75–85 wt% at 260 °C, than the Ni catalyst, with respective values around 40–68 wt%. The low isomerization selectivity of Ni/H-Beta is a result of both the weak hydrogenating power capacity of Ni compared to Pt, which does not allow rapid hydrogenation of the carbenium ions and desorption as alkanes before they undergo cracking, and the low amount of Brönsted acid sites that limits skeletal rearrangements reactions. In the bimetallic catalyst, addition of Pt not only restores, but also enhances isomerization selectivity. At the reference conditions of 260 °C, 10 bar and 10 h−1, selectivity to isooctane increases from 75 wt% for Pt/H-Beta to 81 wt% for Ni-Pt/H-Beta (Table 5), suggesting a synergistic effect between Ni and Pt. As shown by the characterization results, the presence of Pt significantly enhances reducibility of Ni cations (see Fig. 5). This was also confirmed by the TPD-CO measurements. Pt not only greatly improves metal dispersion, but also modifies that nature of the metal sites and weakens the metalCO interaction. It is therefore deduced that in the bimetallic catalyst, the total number of active metallic sites is higher, leading to an increase of the metallic/acid sites ratio toward optimum value for isomerization reactions. Similar observations were reported for bimetallic Ni-Pd

3.2.2. Tests with refinery naphtha stream The hydroisomerization performance of the bimetallic Ni-Pt/H-Beta catalyst was further assessed with refinery naphtha to investigate the behavior under real, more complex feedstock. The properties and composition of the naphtha feed are shown in Table 1. Tests were conducted at the optimum conditions identified from the experiments with n-octane (P = 10 bar, H2/n-C8 = 15, WHSV = 4 h−1) at temperature of 220–300 °C. The distribution of the hydrocarbons in the feed and in the reaction product in terms of carbon number and hydrocarbon type is shown in Fig. 10. The naphtha feedstock consists of a C6 and C7 fraction, almost equally distributed to normal paraffins, isoparaffins and cycloalkanes. Over the bimetallic Ni-Pt catalyst these primary components are mainly converted to propane, butane, pentane, isobutane and isopentane, indicating the occurrence of both cracking and 7

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Fig. 9. Molar distribution of C1-C8 products in n-octane hydroisomerization over Pt/H-Beta (a), Ni/H-Beta (b) and Ni-Pt/H-Beta (c) (reaction conditions: T = 300 °C, P = 10 bar, WHSV = 10 h−1, H2/n-C8 molar ratio = 15).

isomerization reactions, the extent of which increases with reaction temperature. It should however be stressed that the formation of dry gases remains low at 220 °C and 260 °C and becomes significant only at 300 °C (18 wt% gas yield). At all temperatures, the formation of aromatics is negligible. The ratio of total i/n paraffins in the liquid product of the reaction, as well as the i/n ratio of individual components, is plotted in Fig. 11 as a function of reaction temperature. The corresponding values for naphtha feedstock are also included for comparison reasons. Overall, hydroisomerization increases the total concentration of isomers with maximum value attained at 260 °C. The same applies for the individual isoheptane/n-heptane ratio; on the other hand the isohexane/n-hexane ratio increases progressively with temperature. Looking at the cyclic components, the ratio of methyl-cyclopentane (MCP) to cyclohexane (CH) and dimethyl-cyclopentane (DMCP) to methy-cyclohexane (MCH) in the liquid product also increases with reaction temperature. It is thus apparent that the bimetallic catalyst is able to successfully isomerize not only the linear but also the cyclic hydrocarbons in more complex feedstocks at mild temperature and pressure conditions. The increased degree of branching in the cyclic hydrocarbons is very important, as multi-branched components have higher octane numbers. This is confirmed by the octane number of the liquid product that increases from 55 in the feed to ˜61 at 260 °C and 300 °C (Fig. 12). These results are very promising and indicate that the bimetallic Ni-Pt/H-Beta catalyst can, at relatively low temperature (260 °C) and pressure (10 bar), improve significantly the quality of naphtha for gasoline fuel applications with ˜13% boost in the octane number and very low formation of dry gases (< 4.5 wt%).

3.3. Catalyst stability and post-reaction characterization The n-octane hydroisomerization performance of the mono- (Pt and Ni) and bi-metallic (Ni-Pt) Beta-supported catalysts at different reaction conditions was evaluated in one consecutive run with duration of ˜24 h time-on-stream. The initial test condition was repeated at the end of the experiment to check for possible catalyst degradation issues. The experimental sequence and the measured n-octane conversion as a function of time-on-stream for the three catalysts is shown in Fig. 13. It is evident that all three catalysts maintain their initial activity as the conversion levels at the beginning and the end of the test, at the same experimental conditions, are at similar level and within the experimental error. Although the time frame of the exposure of the catalysts under reaction conditions is too short to draw definite conclusions, these first indications suggest that the catalysts do not easily deactivate. The used catalysts, after the end of the 24 h run, were characterized with elemental analysis and temperature programmed oxidation (TPO) in a TGA analyzer to check for possible coke deposits. The amount of carbon, calculated from the weight loss curves at T > 160 °C (to exclude weight loss from desorption of physically adsorbed water [62,63]) and with elemental analysis, is shown in Table 6. The carbon content is similar on all materials and varies between 5.5 to 6.9 wt%. With both methods of analysis, Ni/H-Beta appears to favor coke deposition to a higher extent than the Pt supported catalyst. The bimetallic Ni-Pt/H-Beta presents the lowest coke formation, suggesting that the increased metal content on this material favors the removal of the coke precursors from the catalyst surface. TGA profiles show that in all cases carbon can be easily removed with air at temperature up to 300 °C (Fig. 14). Thus, easily removable

Fig. 10. Product distribution as a function of temperature in naphtha hydroisomerization over the Ni-Pt/H-Beta catalyst (reaction conditions: P = 10 bar, WHSV = 4 h−1, H2/naphtha molar ratio = 15). 8

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Fig. 13. Conversion as a function of time-on-stream in n-octane hydroisomerization at different reaction conditions.

Fig. 11. Ratio of isomers to normal alkanes in the liquid product from naphtha hydroisomerization over the Ni-Pt/H-Beta catalyst (reaction conditions: P = 10 bar, WHSV = 4 h−1, H2/naphtha molar ratio = 15). MCP: Methyl-cyclopentane; CH: Cyclohexane; DMCP: Dimethyl-cyclopentane; MCH; Methylcyclohexane.

Table 6 Characterization of coke deposition on the used catalysts after n-octane hydroisomerization reaction. Catalyst

Pt/H-Beta/used Ni/H-Beta/used Ni-Pt/H-Beta/used (a)

Elemental analysis

TGA-TPO

C (wt%)

H (wt%)

Weight loss (%)(a)

5.6 6.9 5.5

1.8 1.6 1.5

5.9 6.2 5.5

Calculated from weight loss at T > 160 °C.

4. Conclusions Noble Pt (0.2 wt%), non-noble Ni (10 wt%) and bimetallic Ni-Pt (10–0.2 wt%) catalysts supported on moderate acidity Beta zeolite with SAR 75 were successfully tested in n-octane hydroisomerization at mild reaction conditions. The deposition of high amount of Ni decreases considerably the Brönsted sites due to partial blockage of the zeolite pores and generates new acid sites of Lewis type. However, due to the low Pt loading and the weak metal function of Ni, hydrogenation/dehydrogenation becomes the rate-limiting step, leading to similar n-C8 conversion (˜25–90 wt% depending on the reaction conditions) over all catalysts. Selectivity to iso-octane differs significantly, lying in the range of 75–82 wt% on Pt/H-Beta compared to 48–68 wt% on Ni/HBeta at 260 °C. Introduction of 0.2 wt% Pt in the bimetallic Ni-Pt catalyst enhances the isomerization selectivity, due to a synergetic effect

Fig. 12. Research Octane Number (RON) of the liquid product from naphtha hydroisomerization over the Ni-Pt/H-Beta catalyst (reaction conditions: P = 10 bar, WHSV = 4 h−1, H2/naphtha molar ratio = 15).

highly hydrogenated coke species (so called “white” or “soft” coke) are predominantly formed [56]. The differential weight loss (DTG) curves evidence the existence of different species on the mono-metallic Pt catalyst and the Ni-containing ones (both mono- and bi- metallic). Pt/ H-Beta demonstrates one sharp peak with maximum at 190 °C, indicative of easily accessible uniform carbon deposits. The Ni-containing catalysts have identical DTG curves, with a broad peak extending from 150 °C to 300 °C and maximum at 220 °C, revealing the presence of coke species with different reactivity and/or location [64]. It is possible that carbon is deposited more extensively on the metallic Ni active sites on the surface, as well as on Ni species located in the zeolite pores (as evidenced by TPR-H2) which are thus less accessible. In any case, formation of hard coke (combustion at T > 400 °C) is not observed on any of the investigated catalysts. This coincides with the constant catalytic behavior that the materials demonstrate under reaction conditions during the stability test. It is possible that the highly hydrogenated coke species observed are removed under the hydroisomerization conditions, thus leaving most of the active sites accessible for reaction. Overall, this is a positive outcome as it demonstrates that, even in the case of deactivation, the mono- and bi-metallic Ni and Pt catalysts on Beta can be easily regenerated with only a mild oxidation treatment.

Fig. 14. Weight loss and differential weight loss profiles during TGA-TPO for the used catalysts after 24 h under n-octane hydroisomerization reaction conditions.

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between the two metals that favors the reducibility of Ni and improves dispersion, thereby increasing the total metal/acid sites ratio. Moreover, higher degree of branching is observed in the isomerate product of the bimetallic material. The catalysts demonstrate stable performance under 24 h time-on-stream. Analysis of the spent catalysts shows the predominant formation of highly hydrogenated coke species during reaction that can be easily removed with a mild air oxidation treatment (300 °C). The bimetallic catalyst successfully hydroisomerized a complex refinery naphtha with n-paraffins, i-paraffins and cycloalkanes with six and seven carbon atoms at mild reaction conditions. These promising results indicate that the Ni-Pt/H-Beta catalyst can, at relatively low temperature (260 °C) and pressure (10 bar), improve significantly the quality of the naphtha for gasoline fuel applications with ˜13% boost in the octane number and very low formation of dry gases (< 4.5 wt%).

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