Research Article pubs.acs.org/acscatalysis
Selective Formation of Linear Alkanes from n‑Hexadecane Primary Hydrocracking in Shape-Selective MFI Zeolites by Competitive Adsorption of Water Roald Brosius,* Patricia J. Kooyman, and Jack C. Q. Fletcher Institute for Catalysis Research, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa S Supporting Information *
ABSTRACT: MFI type zeolites have been known for decades for their strong tendency toward gas formation in hydrocracking. Our strategy for selectivity control in hydrocracking turns strong adsorption in micropores, responsible for secondary cracking, to an advantage. Enhancing diesel yield is a challenging goal in hydrocracking catalysis. Additionally, linear alkanes increase the diesel fuel cetane number and, consequently, effect a dramatic reduction in exhaust emissions. This study demonstrates for a set of Pt on mesoporous MFI zeolite catalysts that the gains in activity and selectivity attributed to enhanced mass transport are but modest by comparison to the effects of the competitive adsorption of water. Water suppresses secondary cracking, and primary cracking is now reported over MFI zeolites. Furthermore, competitive adsorption of water in shape-selective MFI zeolites facilitates desorption of the primary n-C16 cracking products and suppresses subsequent isomerization resulting in high yields of linear alkanes. The selectivity for linear alkanes from hydrocracking of n-hexadecane over Pt/MFI nanosheets reaches 80% at 80% conversion. KEYWORDS: hydrocracking, zeolite nanosheets, competitive adsorption, heterogeneous catalysis, platinum nanoparticles several model and real feeds.5−8 Pure primary cracking, the breaking of only one carbon−carbon bond in the reactant, has been observed since the earliest description of n-hexadecane (nC16) hydrocracking over Pt/ASA9 and subsequently in the ideal hydrocracking of n-C16 over the more active large-pore Pt/Ca-Y zeolite.10 In ideal hydrocracking a strong (de)hydrogenation activity maintains a steady-state concentration of n-alkenes high enough to displace the alkylcarbenium ions from the acid sites through competitive adsorption. Moreover, the absence of shape selectivity allows for the formation of multibranched carbenium ions and offers efficient desorption of the primary products which consist almost entirely of isoalkanes.11 In shape-selective hydrocracking over bulk12 and nanosheet (NS)7 MFI zeolites, only monobranched alkylcarbenium ions are formed. Isomerization of the primary linear cracking products is controlled by competitive adsorption.12 The occurrence of secondary cracking in bulk and NS MFI zeolites has been attributed to hindered diffusion of the largest fragments.7,12 Water was shown to suppress the activity of a Pt/HY catalyst but compensated for the loss of acid sites in Pd/REX by hydrating the rare-earth cations, resulting in a slight increase in
1. INTRODUCTION Hydrocracking remains the key technology for the production of middle distillates such as diesel and aviation fuel, either from crude oil or from Fischer−Tropsch waxes, the latter of which may be derived from alternative sources such as natural gas or renewables. Pollution from diesel-powered internal combustion engine emissions can be alleviated using either exhaust gas aftertreatment or via the use of clean-burning fuels. This study considers the latter. Dramatic reductions in emissions of unburnt hydrocarbons and CO have been achieved via diesel with a high cetane number (CN).1 This clean diesel also allows large exhaust gas recirculation, reducing NOx and soot emissions.1 Currently, diesel quality is improved by hydrotreating the molecules with the lowest CN, such as polyaromatics and naphthalenes, in light cycle oil from fluid catalytic cracking (FCC).2 In addition to the quality, also the yield needs to be maximized. Utilizing the lower boiling point of the (shorter) cracked products to separate them out of a reaction mixture, process conditions can be found to minimize secondary reactions that lead to the formation of gasoline and gas,3 but this is effective only at low wax conversions over Pt on amorphous silica−alumina (ASA).4 Recent advances in the control over noble-metal location5 as well as in the preparation of mesoporous zeolites6,7 have demonstrated that catalyst improvement can lower the degree of secondary cracking for © XXXX American Chemical Society
Received: August 4, 2016 Revised: September 30, 2016
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DOI: 10.1021/acscatal.6b02223 ACS Catal. 2016, 6, 7710−7715
Research Article
ACS Catalysis
stream of 50 cm3 min−1 for 30 min. Subsequently the carrier gas was switched to flowing He for 2 h to remove physisorbed NH3. Ammonia temperature-programmed desorption proceeded at 5 °C/min to 600 °C. The molar quantity of NH3 desorbed per gram of catalyst was calculated using the signal intensity at complete adsorption of NH3 from the He stream. Prior to CO chemisorption at 35 °C, in a MicroMeritics ASAP2020 instrument, a 0.3 g sample of Pt/H-MFI was subjected to both the same high (1 °C/min) and low (0.3 °C/ min, 0.4 °C/min) heating rate pretreatments as was the case prior to catalytic testing. 2.3. Catalyst Preparation and Pretreatment. H-MFI-90 zeolite with MFI type framework topology in powder form (SiO2/Al2O3 = 90) was supplied by Süd-Chemie AG. A reference sample H-Y zeolite with FAU type framework topology (CBV760, SiO2/Al2O3 = 60) was obtained from Zeolyst. Catalyst samples coded Pd/MFI, Pt/HY, Pt-1/MFI, Pt-6/MFI, Pt-1/NS-8, and Pt-1/NS-2 were prepared by incipient wetness impregnation. The incipient wetness point was determined at 0.70 mL/g MFI, 1.63 mL/g HY, 1.70 mL/g NS-8, and 2.10 mL/g NS-2 with water. Stock solutions of 6.41 wt % Pd(NH3)4(NO3)2 and 2.79 wt % Pd(NH3)4(NO3)2 were diluted with water and mixed with zeolite powder to give metal loadings of 0.9 wt % Pd and 0.9 wt % Pt. Catalysts were subsequently dried at room temperature overnight. Prior to reaction, catalysts were calcined at 350 °C (heating rate 0.3 °C min−1) in flowing medical grade oxygen (120 cm3min−1) and then cooled to 50 °C under nitrogen. Then reduction was performed in flowing hydrogen (120 cm3 min−1) at 225 °C (heating rate 0.4 °C min−1) for 8 h. For comparison, the method was altered and a heating rate of 1 °C min−1 was applied. The reference catalyst, Pt/HY, was oxidized and reduced at a heating rate of 0.2 °C min−1, as was reported to be optimal for high Pt dispersion in HY zeolites.20 2.4. Catalyst Testing. A comprehensive description of the laboratory-scale reactor construction, the testing procedure, and the definitions of conversion and selectivity are given in the Supporting Information. The reaction rates reported in this paper are all obtained with at the most 1 g of catalyst powder, in the absence of external diffusion limitations and with the reactor operated under an ideal plug flow regime (see Figure S8a,b in the Supporting Information).
activity and higher selectivity for isoalkanes.13 The cracking activity of the acid sites in a hybrid Co/SiO2-zeolite was depressed by water, the main byproduct of the Fischer− Tropsch (FT) synthesis, but an influence on selectivity was not reported.14 The fact that water can moderate zeolite acidity is mostly ignored in studies of acid zeolite supported FT catalysis.15 In the hydrocracking of anthracene over NiFeMFI, a combination of CO2 and water eliminated nitrogencontaining poisons and increased yields of desirable products.16 This study now demonstrates the selective formation of linear alkanes (with the highest CN) in n-C16 hydrocracking by preventing secondary cracking as well as secondary isomerization of the primary cracking products through competitive adsorption of water at the MFI acid sites.
2. EXPERIMENTAL SECTION 2.1. Zeolite NS Synthesis. The structure directing agents (SDA) for NS synthesis were diquaternary ammonium surfactant (C22H45−N+(CH3)2−C6H12−N+(CH3)2−C6H13) and tetraquaternary ammonium surfactant (C 22 H 45 − N + (CH 3 ) 2 −(C 6 H 12 −N + (CH 3 ) 2 ) 3 −C 6 H 13 ). The bromide forms of the SDAs were prepared via alkylation of tertiary amines by alkyl halides according to Choi et al.17 The zeolite preparation method was adopted from Machoke et al.18 and modified to allow for secondary nucleation assisted hydrothermal crystallization.19 Zeolite synthesis gels of composition 1 Al2O3/100 SiO2/a SDA(Br)x/21 H2SO4/30 Na2O/4000 H2O/ 400 EtOH were prepared, using the diquaternary SDA (a = 10, x = 2) and the tetraquaternary SDA (a = 5, x = 4) (further details are given in the Supporting Information). 2.2. Catalyst Characterization. Zeolite MFI topology of all synthesized nanosheet samples was confirmed using a Bruker D8 Advance diffractometer, equipped with a cobalt source (λKα1 = 0.178897 nm) and a position-sensitive detector. Individual nanosheet batches gave identical diffractograms, and the reflection intensities were within 5% from the average. Bright-field transmission electron microscopy images of the NS-2, NS-8, and H-MFI samples after calcination were acquired using an FEI Tecnai F20 equipped with a FEG and operated at 200 kV. BF-STEM and HAADF-STEM images were recorded using a double-aberration-corrected JEOL ARM200 F equipped with a FEG and operated at 200 kV. No amorphous material was found with TEM, confirming the validity of the correlation of XRD line widths with coherent crystalline domain size. Prior to nitrogen adsorption on our MicroMeritics TriStar II (at liquid nitrogen temperature), samples were evacuated at 350 °C overnight. Nitrogen adsorption isotherms of the reference MFI show strong uptake of N2 at low relative pressure due to adsorption in the micropores and some capillary condensation at a high relative pressure: 0.9 owing to some macropores in the agglomerates of crystalline domains. NS-2 and NS-8 also show strong N2 uptake in the zeolite micropores and adsorb more nitrogen than the reference MFI. The micropore volume and the external surface area were obtained from a t plot. The mesopore and macropore size distributions were calculated using the Barrett−Joyner− Halenda model from the desorption branch. All individual nanosheet synthesis batches gave nearly identical N2 adsorption and desorption isotherms. Zeolite acidity was quantified using a MicroMeritics Autochem 2950 instrument. A 0.3 g portion of zeolite was heated to 600 °C at 10 °C/min under flowing He, cooled to 100 °C, and saturated with NH3 from a 1 mol % NH3 in a He
3. RESULTS AND DISCUSSION To elucidate the effects of enhanced mass transport in shapeselective MFI zeolite, the activity and selectivity of n-C16 hydrocracking was investigated over MFI NS prepared by seeded crystallization with two different ammonium SDAs (NS2 and NS-8) and a commercial bulk MFI. XRD confirms the MFI topology for NS-2 and NS-8. The observed diffraction lines correspond to in-plane (a−c plane) reflections in the [h0k] crystallographic directions. Reflections with a b component are absent in the NS-2 sample but appear faintly as shoulders in the NS-8 sample and grow in intensity in the HMFI bulk crystal. Decreasing reflection width in the order NS-2 > NS-8 > MFI suggests that the size of the crystalline microporous domains increases from NS-2 to MFI (see Figure S1 in the Supporting Information). The bulk MFI is composed of 0.2−0.6 μm wide macroporous aggregates of 50 nm globular microporous crystallites (see Figure S2 in the Supporting Information). NS-2 and NS-8 display hysteresis loops in the N2 isotherms owing to capillary condensation in the mesopores (see Figure S3a in the Supporting Information), with a wide 7711
DOI: 10.1021/acscatal.6b02223 ACS Catal. 2016, 6, 7710−7715
Research Article
ACS Catalysis
in Figure 1a show that the smallest easily visible particles for the high heating rate pretreatment of Pt-6/MFI are larger than 5 nm, whereas for the low heating rate pretreatment of Pt-1/MFI only particles with a diameter
