Hierarchical Mordenite Dedicated to the Fluid Catalytic Cracking ...

Article pubs.acs.org/JPCC

Hierarchical Mordenite Dedicated to the Fluid Catalytic Cracking Process: Catalytic Performance Regarding Textural and Acidic Properties Kinga Góra-Marek,*,† Karolina Tarach,† Justyna Tekla,† Zbigniew Olejniczak,‡ Piotr Kuśtrowski,† Lichen Liu,§ Joaquin Martinez-Triguero,*,§ and Fernando Rey§ †

Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland H. Niewodniczański Institute of Nuclear Physics of PAN, Radzikowskiego 152, 31-342 Kraków, Poland § Instituto de Tecnología Química, Universidad Politécnica de Valencia, Camino de Vera s.n., 46022 Valencia, Spain ‡

ABSTRACT: This work was attempted to show that the sequential dealumination and desilication with the use of tetraalkylammonium cations as pore directing agents (PDA) are an effective procedure for the fabrication of hierarchical mordenite zeolites with preserved crystallinity and uniform intracrystalline mesoporosity. Additionally it was demonstrated that desilication performed in the presence of PDAs offered greater mesoporosity development when compared to pure NaOH treatment. IR studies employing ammonia and pyridine as probes exhibited considerably Brønsted acidity of the 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 a 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, 1,3,5-tri-isopropylbenzene, and vacuum gas oil.

1. INTRODUCTION One of the most effective additives for fluid catalytic cracking (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 micro-microporous composites. Many reports have concerned micro-mesoporous composites such as TUD-C and ZSM-5/MCM-41,3−5 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 has 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 the C3−C4 gas fraction. Also, zeolite Beta has been considered as an alternative to ZSM-5 as a potential additive for the USY-based FCC catalyst.11−13 However, the commercial use of zeolite beta as an 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 one of the greatest challenges in materials science dedicated to FCC technology. © 2014 American Chemical Society

The zeolite mordenite (MOR) consists of parallel 12-MR channels with dimensions of 0.65 × 0.70 nm connected via 8MR side pockets of 0.26 × 0.57 nm. Because of 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 the MOR zeolite structure is generally regarded as one dimensional. 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 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 the Si/Al ratio on Received: October 8, 2014 Revised: November 7, 2014 Published: November 7, 2014 28043

dx.doi.org/10.1021/jp510155d | J. Phys. Chem. C 2014, 118, 28043−28054

The Journal of Physical Chemistry C

Article

Table 1. List of Samples Used in the Study sample name native dealuminated with HNO3 desilicated with NaOH desilicated with NaOH and TBAOH

sample symbol

preparation of sample

MOR DeAl/MOR

3-fold ion exchange with 0.5 M NH4NO3 at 60 °C for 1 h The 100 mL of 3 M HNO3 was contacted with 6.0 g of NH4-form of zeolite at 80 °C for 1 h

DeSi_NaOH/DeAl/MOR

The 100 mL of 0.2 M NaOH solution was contacted with 3.0 g of dealuminated zeolite at 80 °C for 30 min The 100 mL of 0.2 M NaOH and TBAOH solution was contacted with 3.0 g of dealuminated zeolite at 80 °C for 30 min. The 0.2 M mixture TBAOH/(NaOH + TBAOH) ratio was 0.4

DeSi_NaOH&TBAOH/DeAl/MOR

pH. Next a 4-fold ion-exchange with 0.5 M NH4NO3 was performed at 60 °C 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). 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 Xray diffraction (XRD) measurements were carried out using a PANalytical Cubix diffractometer, with Cu Kα 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 22.5° and 25.0°. The N2 sorption processes at −196 °C were studied on an ASAP 2420 Micromeritics after activation in a vacuum at 400 °C 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 the 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 scanning transmission electron microscopy (STEM) was performed in a 200 kV field emission electron microscope JEOL 2100F equipped with a STEM camera. 2.2.3. 29Si MAS NMR and 27Al MAS NMR. Solid state MAS NMR spectra were acquired on an APOLLO console (Tecmag) at a magnetic field of 7.05 T (Magnex). For the 29Si 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 NMR measurements the samples were kept in 75% relative humidity for 48 h. 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 pretreated in situ in homemade quartz IR cell at 550

desilication processes and the amorphization of high aluminum zeolites under alkaline treatment have been widely discussed.17 It has been recognized that AlO4− tetrahedra protect neighboring Si atoms against hydroxide ion attack.18 Thus, zeolites of low Si/Al, thus, of lesser ability to desilication, have been alkaline treated in the presence of pore directing agents (PDAs) that ensure the crystallinity preservation but lower extent 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 improve desilication yield, i.e., the more effective fabrication of mesopores, is also the removal of Al atoms from the 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 compared to 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 the PDA. 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 improving both the activity and selectivity of these zeolites, when molecules with a large kinetic diameter were processed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The zeolites investigated in this study were modified via dealumination followed by a desilication procedure. Dealumination the native MOR zeolite (Si/Al = 7.5, Zeolyst International, CBV 10A) was carried out in solutions of 3 M HNO3 at 80 °C for 1 h. After dealumination the suspension was cooled down in an ice-bath, filtered, and washed with water until it had a neutral pH. The resulting dealuminated zeolite was denoted hereafter as DeAl/MOR. Desilication of dealuminated zeolite DeAl/MOR was carried out in solutions of 0.2 M NaOH and 0.2 M mixtures of NaOH and tetrabutylammonium hydroxide (TBAOH) at 80 °C for half an hour. For the 0.2 M mixture, the TBAOH/(NaOH + TBAOH) ratio was 0.4. The 100 mL of solution was added to 3.0 g of zeolite. Again, after desilication the suspension was cooled down in an ice-bath and filtered. The hierarchically structured zeolites were washed with water until it had a neutral 28044

dx.doi.org/10.1021/jp510155d | J. Phys. Chem. C 2014, 118, 28043−28054

The Journal of Physical Chemistry C

Article

°C under vacuum conditions for 1 h. 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 neutralize all acid sites, was adsorbed at 130 °C, 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 °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 cm2 μmol−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 cm μmol−1 for the NH3L band was obtained in experiments in which ammonia was adsorbed in dehydroxylated mordenite HMOR (pretreated at 800 °C), 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 adducts was calculated as the difference between the amount of ammonia adsorbed and the small amount of ammonia reacting with protonic sites, which survived pretreatment at 800 °C. Quantitative approach of the sorption of 2,6-di-tertbutylpyridine (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 °C were taken as a measure of the acid strength of the sites. The sorption of CO (Linde Gas Poland, 99.95%) as the probe molecule was performed at −130 °C. 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 micro activity test (MAT) unit described previously.25,26 Pellets of zeolites were crushed and sieved; a 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 catto-oil ratios of 1,3,5-tri-isopropylbenzene (TIPB) were performed at 500 °C and for 60 s time on stream (TOS), with 200 mg of catalyst. For n-decane cracking at 500 °C 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 the 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 a reaction temperature of 520 °C and with

TOS of 30 s. Gases were analyzed by gas chromathography in a Rapid Refinery gas analyzer from Bruker (450-GC) and simulated distillation of liquids in a Bruker SIMDIS. 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−1 TOS)−1 [ε X + (1 + ε) ln(1 − X)]

(1)

K = −(cat oil−1 TOS)−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 the use of vacuum gas oil as the feed of the composition listed in Table 2. A detailed description of the catalytic tests is presented in Table 3. Table 2. Reference VGO Feedstock Properties parameters

values

density (15 °C) aniline point (°C) sulfur (%) N2 (ppm) Na (ppm) Cu (ppm) Fe (ppm) Ni (ppb) V (ppb) ASTM D-1160 (°C) 5% 10% 30% 50% 70% 90%

0.9172 g/cm3 79.2 1.65 1261 0.18