Hierarchical Mordenite Dedicated to the Fluid Catalytic Cracking

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Hierarchical Mordenite Dedicated to FCC Process: Catalytic Performance Regarding Textural and Acidic Properties Kinga Góra-Marek, Karolina Tarach, Justyna Tekla, Zbigniew Olejniczak, Piotr Kustrowski, Li Cheng, Joaquin Martinez-Triguero , and Fernando Rey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510155d • Publication Date (Web): 07 Nov 2014 Downloaded from http://pubs.acs.org on November 12, 2014

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The Journal of Physical Chemistry

Hierarchical Mordenite Dedicated to FCC Process: Catalytic Performance Regarding Textural and Acidic Properties Kinga Góra-Mareka*, Karolina Taracha, Justyna Teklaa, Zbigniew Olejniczakb, Piotr Kuśtrowskia, Lichen Liuc, Joaquin Martinez-Trigueroc*, Fernando Reyc a

Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland

b

H. Niewodniczański Institute of Nuclear Physics of PAN, Radzikowskiego 152, 31-342 Kraków,

Poland c

Instituto de Tecnología Química, Universidad Politécnica de Valencia, Camino de Vera s.n., 46022

Valencia, Spain

ABSTRACT This work was attempted to evidence that the sequential dealumination and desilication with use of tetraalkylammonium cations as pore directing agents (PDA) is an effective procedure for the fabrication of hierarchical mordenite zeolites with preserved crystallinity and the uniform intracrystalline mesoporosity. Additionally it was demonstrated that desilication performed in the presence of PDAs offered a greater mesoporosity development when comparing with pure NaOH treatment. IR studies employing ammonia and pyridine as probes exhibited considerably Brønsted acidity of resulting materials. The strength of protonic sites was reduced upon the treatment, nevertheless their accessibility to hindered 2,6-di-tert-butylpyridine molecules became noticeably high owing to more open hierarchical structure. Concentration of the acid sites, their strength and accessibility were reflected in both catalytic activity and selectivity in the cracking of n-decane, 1,3,5tri-iso-propylbenzene and vacuum gas oil.

Keywords: hierarchical zeolites, MOR, dealumination, desilication, acidity, TIPB, n-decane, cracking, FCC

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1. INTRODUCTION One of the most effective additives for the FCC catalysts is zeolite ZSM-5 with a diameter of ca. 0.54 nm and two types of interconnected 10-MR channels. Nevertheless, the relatively narrow pores of zeolite ZSM-5 limit its application in the heavy oil catalytic cracking.1,2 To overcome this limitation many effective approaches have been made, e.g. the creation of micro-mesoporous and micromicroporous composites. Many reports have concerned micro-mesoporous composites such as TUD-C and ZSM-5/MCM-413-5 but these materials cannot adapt to steam conditions at high temperatures. The overgrowth of a continuous SAPO-5 polycrystalline shell around ZSM-56 has been successfully achieved ensuring a higher propylene yield and conversion of heavy oil. The application of desilicated hierarchical large pore zeolites for gasoil cracking have been recently studied for zeolite USY7, 8 and mordenite.9,10 It has been evidenced that the mesoporosity enhancement raises the yield of middle distillates, while preserving or even increasing overall catalytic activity and olefinicity in C3-C4 gas fraction. Also, zeolite Beta has been considered as an alternative to ZSM-5 as potential additive for the USY-based FCC catalyst.11-13 However, the commercial use of zeolite Beta as FCC additive is limited due to its faster deactivation when compared to ZSM-5. The fabrication of micro-mesoporous materials with a high hydrothermal stability and adjustable acidity and accessibility of sites remains to be one of the greatest challenges in materials science dedicated to FCC technology. The zeolite mordenite (MOR) consists of parallel 12-MR channels with dimensions of 0.65 x 0.70 nm connected via 8 MR side pockets of 0.26 x 0.57 nm. Due to the small size of the 8 MR channels the diffusion of most hydrocarbon molecules is hard. As mentioned above, mordenite is offered as a catalyst improving the octane quality of gasoline via hydroisomerization of linear alkanes to branched ones. Both adsorption of reagent molecules and desorption of products, namely the branched ones, often suffer from diffusion limitations and, thus MOR zeolite structure is generally regarded as onedimensional. For this reason the problem of the accessibility of the acid sites hidden inside the micropores is of very high importance. One of the most effective methods to eliminate the diffusional limitations is the fabrication of the secondary system of mesopores in the zeolite crystals by desilication with alkaline solutions.14-16 The


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success of desilication resulting in the versatility and simplicity is related to a number of parameters that may be tuned to obtain micro-mesoporous zeolites. The number of framework silicon atoms that could be removed without causing structural damage is governed by the zeolite features (Si/Al ratio and framework topology) as well as by the treatment of alkaline conditions (type and concentration of desilicating agent used). The influence of Si/Al ratio on desilication processes and the amorphisation of high aluminium zeolites under alkaline treatment have been widely discussed.17 It has been recognized that AlO4-tetrahedra protected neighbouring Si atoms against the hydroxide ion attack.18 Thus, zeolites of low Si/Al, thus, of lesser ability to desilication, have been alkaline treated in the presence of PDAs (pore directing agents) that assure the crystallinity preservation but lower extend of desilication. Similar results were reported by Verboekend et al.,19 who studied the effect of the addition of various organic cations to NaOH on porosity and structure of zeolites Beta and USY. Numerous works devoted to NaOH treatment of highly siliceous mordenite have been also reported.15,20-22 One of the methods to improvement of desilication yield, i.e. the more effective fabrication of mesopores, is also the removal of Al atoms from framework. Finally, dealumination enhanced the zeolites ability to desilication.21 In this work, by using a commercial MOR zeolite, we applied a combination procedure of dealumination with nitric acid and desilication with alkaline solution to produce a hierarchical porous structure. Additionally, desilication was performed in the presence of PDA which offered a greater mesoporosity development when comparing with pure NaOH treatment. According to our best knowledge it was the first attempt to optimize the mesoporosity in mordenite crystals with the use of tetraalkylammonium cations as pore directing agent. The impact of the sequential dealumination and desilication procedure on the structural, textural and acidic properties of mordenite zeolites with controlled mesoporosity was investigated. Furthermore, the mesopore-modified mordenite zeolites were tested as catalytic materials in the catalytic cracking of vacuum oil, in view of the generated mesopore system improved both the activity and selectivity of these zeolites, when molecules with large kinetic diameter were processed.



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2. EXPERIMENTAL SECTIONN 2.1. Catalyst Preparation The zeolites investigated in this study were modified via dealumination followed by desilication procedure. Dealumination the native MOR zeolite (Si/Al = 7.5, Zeolyst International, CBV 10A) was carried out in the solutions of 3M HNO3 at 80oC for 1 hour. After dealumination suspension was cooled down in ice-bath, filtered, and washed with water until neutral pH. The resulting dealuminated zeolite was denoted hereafter as DeAl/MOR. Desilication of dealuminated zeolite DeAl/MOR was carried out in the solutions of 0.2 M NaOH and the 0.2 M mixtures of NaOH and TBAOH (tetrabutylammonium hydroxide) at 80 oC for half an hour. For the 0.2 M mixture TBAOH/(NaOH+TBAOH) ratio was 0.4. The 100 ml of solution was added to 3.0 g of zeolite. Again, after desilication suspension was cooled down in ice-bath and filtered. The hierarchically structured zeolites were washed with water until neutral pH. Next a fourfold ionexchange with 0.5 M NH4NO3 was performed at 60 oC for 1 h. Finally, the zeolites were again filtrated, washed and dried at room temperature. The resulting materials were denoted hereafter as DeSi_NaOH/DeAl/MOR and DeSi_NaOH&TBAOH/DeAl/MOR (Table 1). Table 1. List of samples used in the study. Sample name

Sample symbol

Native

MOR

Dealuminated with HNO3

DeAl/MOR

Desilicated with NaOH

DeSi_NaOH/DeAl/MOR

Desilicated with NaOH&TBAOH

DeSi_NaOH&TBAOH/DeAl/MOR

Preparation of sample Three-fold ion exchange with 0.5 M NH4NO3 at 60 oC for 1 h. The 100 ml of 3M HNO3was contacted with 6.0 g of NH4-form of zeolite at 80 oC for 1 hour. The 100 ml of 0.2 M NaOH solution was contacted with 3.0 g of dealuminated zeolite at 80oC for 30 min.. The 100 ml of 0.2 M NaOH&TBAOH solution was contacted with 3.0 g of dealuminated zeolite at 80 oC for 30 min. The 0.2 M mixture TBAOH/(NaOH+TBAOH) ratio was 0.4.


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2.2. Characterization Methods 2.2.1.

Chemical Analysis

Si and Al concentrations in all zeolites investigated in this work were determined by the ICP OES method with an Optima 2100DV (PerkinElmer) spectrometer.

2.2.2.

Structural and Textural Parameters

The powder X-ray diffraction (XRD) measurements were carried out using a PANalytical Cubix diffractometer, with CuKα radiation, λ=1.5418 Å and a graphite monochromator in the 2θ angle range of 5-40°. X-ray powder patterns were used for structural identification of the relative crystallinity value (%Cryst) for all the zeolites. The determination of the relative crystallinity value was based on the intensity of the characteristic peaks in the range between 22.5o to 25.0o. The N2 sorption processes at -196 oC were studied on an ASAP 2420 Micromeritics after activation in vacuum at 400 oC for 12 h. Surface Area (SBET) and micropore volume (Vmicro) were determined by applying the BET and t-plot methods, respectively. Pore size distribution and volume of mesopores (Vmeso) were obtained by applying the BJH model to the adsorption branch of the isotherm. The mesopore surface area (Smeso) was calculated in the range between 2 and 30 nm with BJH model and it denotes external surface area. Transmission electron microscopy was done with using a Philips CM−10 microscope operating at 100 kV. The samples under investigation were ultrasonically dispersed in 2-propanol and then transferred to carbon coated copper grids. Dark field STEM (Scanning Transmission Electron Microscopy) have been performed in a 200 kV Field Emission Electron Microscope JEOL 2100F equipped with a STEM camera. 2.2.3.

29

Si MAS NMR and 27Al MAS NMR

The solid state MAS NMR spectra were acquired on an APOLLO console (Tecmag) at the magnetic field of 7.05 T (Magnex). For the

29

Si MAS-NMR spectra a 3 µsrf pulse (π/2 flipping angle) was

applied, 4 kHz spinning speed, and 256 scans with the delay of 40 s were acquired. Prior to 27Al MAS



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NMR measurements the samples were kept in 75% relative humidity for 48 hours. The 27Al spectra were recorded using the 2 µsrf pulse (π/6 flipping angle), 8 kHz spinning speed, and 1000 scans with acquisition delay 1 s. The frequency scales in ppm were referenced to TMS and to 1 M solution of Al(NO3)3, for the 29Si and 27Al spectra, respectively. Chemical shifts are reported in ppm relative to an external standard of 1 M aqueous Al(NO3)3 solution for 27Al and DSS for 29Si. The MAS NMR spectra were normalized to the same mass of sample. 2.2.4.

IR Studies

Prior to the FTIR study all samples were pressed into the form of self-supporting wafers (ca. 5-10 mg·cm-2) and pre-treated in situ in homemade quartz IR cell at 550 oC under vacuum conditions for 1 hour. The IR spectra were recorded with a Bruker Tensor 27 spectrometer equipped with a MCT detector. The spectral resolution was 2 cm−1. 2.2.4.1. Concentration of Acid Sites The concentration of Brønsted and Lewis acid sites was determined in quantitative IR studies of ammonia (PRAXAIR, 99.96%) adsorption.23 An excess of ammonia, sufficient to neutralise all acid sites, was adsorbed at 130 oC, followed by an evacuation at the same temperature to remove the gaseous and physisorbed ammonia. Subsequently, the IR spectrum was taken at a temperature of 130 o

C. The concentration of Brønsted and Lewis sites was calculated using respectively the integral

intensities of the 1450 cm−1 band of the ammonium ions (NH4+) and the 1620 cm−1 band of coordinatively bonded ammonia to Lewis sites (NH3L) by applying the respective extinction coefficients. The extinction coefficient of 13.5 cm2µmol-1 for the NH4+band was determined as the slope of the linear dependence of the intensity of the 1450 cm−1 band versus the amount of ammonia adsorbed in zeolite HMOR (containing protonic sites only). The extinction coefficient of 0.9 cmµmol-1 for the NH3L band was obtained in experiments in which ammonia was adsorbed in dehydroxylatedmordenite HMOR (pretreated at 800 oC), containing Lewis acid sites as the major species. Again, the value of the extinction coefficient for the NH3L adducts was calculated from the linear dependence of the 1620 cm−1 band versus the amount of ammonia bonded to Lewis sites only. The amount of ammonia in NH3L a adducts was calculated as the difference between the amount of


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ammonia adsorbed and the small amount of ammonia reacting with protonic sites, which survived pretreatment at 800 oC. Quantitative approach of the sorption of 2,6-di-tert-butylpyridine (SIGMA ALDRICH, 99.8%) was applied to determine the number of sites exposed on the mesopore surface, in line with the procedure given in ref.24 2.2.4.2. Acid Strength of Protonic Sites The acid strength has been determined based on ammonia thermodesorption and carbon monoxide adsorption studies. In the NH3-thermodesorption experiments, the conservation of the 1620 cm−1 (Lewis sites) and 1450 cm−1 (Brønsted sites) bands under the desorption procedure at 350 oC were taken as a measure of the acid strength of the sites. The sorption of CO (Linde Gas Poland, 99.95 %) as probe molecule was performed at -130 oC. The shift of IR band of the acidic hydroxyls (3200-3800 cm−1) due to its interaction with adsorbed CO molecules has been taken as a measure of the acid strength.

2.3. Catalytic Tests The cracking experiments were performed in a MAT (Micro Activity Test) unit described previously.25,26 Pellets of zeolites were crushed and sieved; fraction of the 0.59–0.84 mm was taken for cracking reactions. For each catalyst, catalytic experiments were carried out, preserving the amount of catalyst (cat) constant and varying feeds amounts (oil). Four cracking reactions with different cat-tooil ratios of 1,3,5-tri-iso-propylbenzene (TIPB) were performed at 500 oC and for 60 s time on stream (TOS), with 200 mg of catalyst. For n-decane cracking at 500 oC and for 60 s TOS, 300 mg of catalyst was diluted in 2.5 g of inert silica, and five experiments were performed. For first and last experiments the amount of feed was maintained in order to investigate the stability of catalysts. In case of gas oil cracking five experiments with different cat-to-oil ratio were also performed and 500 mg of catalyst was diluted in 2.5 g of inert silica; with reaction temperature of 520 oC and with TOS of 30 s. Gases were analysed by Gas Chromathography in a Rapid Refinery Gas Analyser from Bruker (450-GC) and simulated distillation of liquids in a Bruker SIMDIS.



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Kinetic rate constants (K) were calculated by fitting the conversions (X) to a first-order kinetic equation for a plug flow reactor (1) for n-decane and TIPB or to a second order kinetic equation for a plug flow reactor (2) for gas oil, assuming that the deactivation is enclosed in the kinetic constant and taking into account the volumetric expansion factor (3), K =−(cat oil−1TOS)−1[εX+(1+ε) ln(1−X)]

(1)

K =−(cat oil−1TOS)−1[X/(1−X)]

(2)

ε =(Σmolar selectivities of products) −1

(3)

These rate constants were used to compare the activities of the catalysts with their textural and acidic properties. The evaluation of hierarchical mordenites in cracking reactions was performed with use of vacuum gas oil as the feed of the composition listed in the Table 2. Detailed description of the catalytic tests is presented in Table 3. Table 2. Reference VGO feedstock properties. Parameters density (15 oC) aniline point (oC) sulphur (%) N2 (ppm) Na (ppm) Cu (ppm) Fe (ppm) Ni (ppb) V (ppb) ASTM D-1160 (◦C) 5% 10% 30% 50% 70% 90% Average molecular weight aromat. carb. (ndM%) naphten carb. (ndM%) paraff. carb. (ndM%) arom. rings./molec. (ndM) napht. rings./molec. (ndM)

Values 0.9172 g/cm3 79.2 1.65 1261 0.18

Hierarchical Mordenite Dedicated to the Fluid Catalytic Cracking

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