Article pubs.acs.org/EF
Hydroisomerization of Long Chain n‑Paraffins over Pt/ZSM-22: Influence of Si/Al Ratio Snehalkumar Parmar,† Kamal K. Pant,*,† Mathew John,‡ Kishore Kumar,‡ Shivanand M. Pai,‡ and Bharat L. Newalkar*,‡ †
Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110 016, India Corporate R&D Centre, Bharat Petroleum Corporation Limited, Plot 2A, Udyog Kendra, Greater Noida-201 306, India
‡
ABSTRACT: Hydroisomerization of n-hexadecane has been investigated over ZSM-22 framework by varying bulk molar Si/Al ratio in the range from 30 to 90 at constant platinum loading of about 0.45 wt %. Thus, ZSM-22 samples have been synthesized and characterized with respect to their crystallinity, textural parameters, and total acidity by means of X-ray diffraction, nitrogen adsorption desorption isotherm measurement at 77 K, and temperature-programmed desorption of ammonia technique, respectively. The effect of hydroisomerization reaction parameters, namely, temperature and weight hourly space velocity (WHSV), on n-hexadecane isomer selectivity and yield has been investigated. The optimal results over prepared catalysts demonstrated the best possible performance over ZSM-22 framework with bulk molar Si/Al ratio of 45, wherein the C16 product with isomer selectivity and yield (82.7 and 74.8%, respectively) and pour point of 252 K have been achieved at a conversion level of about 90% at 578 K. Based on the obtained results, the possible advantages of using ZSM-22 (bulk molar Si/Al ∼ 45) framework cold flow property improvement of long chain n-paraffins can be envisaged.
1. INTRODUCTION Today, with the advent of hydroprocessing routes in oil refining processes, hydroisomerization of long chain n-paraffin is the most preferred route to produce lube oil base stocks (LOBS) having excellent cold flow properties to meet desired lube oil specifications with high viscosity index (VI) and winter grade diesel.1 Lately, stringent emission norms and environmental concerns worldwide have added impetus for implementation of a biofuel program.2 This has led to the emergence of green diesel processes wherein vegetable oil is hydroprocessed to produce a diesel fraction having excellent cetane number (over 60) but with poor cold flow plugging point (CFPP) due to the presence of straight-chain C12−C18 n-paraffins.3,4 Thus, such a diesel stream demands hydroisomerization of long chain nparaffins to improve CFPP with minimum cetane loss. It may be noted that n-paraffins have a higher cetane number than do corresponding branched isomers. For example, the cetane number for n-hexadecane is 100, whereas its corresponding isomers such as 2,2,4,4,6,8,8-heptamethylnonane, 7,8-dimethyltetradecane, and 5-butyldodecane have cetane numbers of 15, 40, and 45, respectively.5 Typically, formed multibranched isomers during hydroisomerization are prone to cracking, which would not only reduce the yield but also lead to a significant drop in cetane number. Thus, it is desired to have formation of mono/dibranched isomers over multibranched ones during hydroisomerization of long chain n-paraffins.6 In view of this, it is imperative to optimize degree of branching to avoid cetane loss while achieving improvement in CFPP for green diesel with yield maximization, from commercial process point of view, at the best possible conversion level of n-paraffins to their corresponding isoparaffins. Thus, it is of utmost importance to tailor the hydroisomerization catalyst with respect to optimized branching. © 2015 American Chemical Society
Typically, hydroisomerization catalysts are bifunctional in nature, having an optimum balance of metal and acid functions (Brønsted acidity).7 In this context, the literature reports the use of a noble metal such as platinum (Pt) for the dehydrogenation/hydrogenation function over acidic support such as zeolites to perform isomerization reaction of the olefinic intermediates formed over the metal sites during hydroisomerization of long chain n-paraffin.6 In view of this, selection of appropriate acidic support with optimum acid site strength (Brønsted acidity)7 is of prime importance to achieve yield maximization and isomer selectivity under mild reaction conditions, thereby avoiding cracking reactions. For this purpose, the use of medium pore zeolites, namely, ZSM-22, ZSM-23, and ZSM-48, has been well documented in the literature, as they offer flexibility in tailoring of framework acidity by varying Si/Al ratio while achieving successful hydroisomerization of long chain n-paraffins via the pore mouth-key lock (PMKL) catalysis mechanism.7−9 The presence of PMKL catalysis mechanism is reported to favor formation of mono- and dibranched isomers.8,9 Such a mechanism is reported to be more pronounced during hydroisomerization of long chain n-paraffin, and textural parameters, namely, external surface area and pore mouth acidity, of the aforementioned medium pore zeolites are reported to influence such a mechanism.10 Accordingly, efforts have been made to tailor the zeolite framework acidity and literature reports various methodologies for tuning the acidity viz. post synthesis modification by ion-exchange, dealumination, and formation of a siliceous rim over the mouth of the zeolite pore for the isomerization of n-paraffins.11−14 Ion-exchange methodology Received: June 5, 2014 Revised: January 28, 2015 Published: January 28, 2015 1066
DOI: 10.1021/ef502591q Energy Fuels 2015, 29, 1066−1075
Article
Energy & Fuels
%) was mixed with 50 wt % low acidity alumina (SCFA 140L3, BET surface area = 140 m2/g, pore volume = 0.5 cc/g, pore diameter = 16 nm, supplied by M/s Sasol, Germany) and extruded in the shape of cylindrical extrudates of 1.6 mm diameter and having a length of ∼3 mm. Extrudates were calcined at 673 K under continuous flow of pure oxygen for 5 h. In all, four catalyst recipes viz. CAT-1 (Pt/H-ZSM-22, bulk molar Si/Al = 30), CAT-2 (Pt/H-ZSM-22, bulk molar Si/Al = 45), CAT-3 (Pt/H-ZSM-22, bulk molar Si/Al = 60), and CAT-4 (Pt/ H-ZSM-22, bulk molar Si/Al = 90) were prepared. 2.3. Characterization. X-ray diffraction patterns were recorded using a Philips X’pert powder diffractometer system using a Cu Kα radiation with a 0.02° step size and 0.4 s step time in the range 5° < 2θ < 40°. Platinum (Pt) content over prepared catalyst samples was estimated by means of inductively coupled plasma (ICP) technique (PerkinElmer, Model: Optima 2000). The morphology and crystallite size of the samples were investigated using scanning electron microscopy (SEM, Vega IITSU, TESCAN, U.S.A.). The average particle size was estimated by Dynamic Light Scattering (DLS) technique (Malvern ZEN 3690, U.K.). The solid-state MAS NMR spectra were gained using 500 MHz MAS NMR Bruker Advance Spectrometer (Bruker, U.S.A.) as per the method described elsewhere.17 29Si MAS NMR spectra were recorded using a 3l s radiofrequency (rf) pulse (p/2 flipping angle) and 4 kHz spinning speed with 256 scans and a delay of 40 s. The 27Al spectra were recorded using a 21 s rf pulse (p/6 flipping angle), 8 kHz spinning speed, and 1000 scans with acquisition delay 1 s. The frequency scales in ppm were referenced to tetramethylsilane (TMS) for 29Si and to 1 M solution of Al(NO3)3 for the 27Al spectra. The framework acidity of zeolite samples was estimated by means of ammonia temperature-programmed desorption (TPD) technique (AMI-200, Altamira instruments, U.S.A.). Typically, the samples were activated under helium flow at 573 K for 1 h, followed by adsorption of ammonia at 373 K using a gas mixture of 6% ammonia in helium at a constant flow rate of 25 mL/min. A higher adsorption temperature was selected in order to minimize the extent of physical adsorption. Furthermore, to ensure removal of any physically adsorbed ammonia, the sample was purged at 423 K for 2 h under a flow rate of 50 mL/min of high purity helium (99.99%). TPD of ammonia was performed by heating the sample in the temperature range 423−1173 K, at a rate of 10 K/min and under a helium flow rate of 25 mL/min. The amount of ammonia desorbed from the sample was determined using a thermal conductivity detector (TCD) and was used to quantify acidity in terms of micromoles of ammonia desorbed per gram of zeolite sample. Fourier transform infrared (FT-IR) spectra were recorded in the range 4000−3000 cm−1 for all the samples to verify the presence of the silanol (Si−OH) group by employing a thin wafer of uniform size (1 cm diameter and 0.2 mm thickness) of all samples; in situ drying of each wafer was performed at 393 K under vacuum
