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The Effect of Fe and Cr Impregnated ZSM-5 on Olefin Selectivity

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The Effect of Fe and Cr Impregnated ZSM-5 on Olefin Selectivity Applied Petrochemical Research (2018) 8:119–129 https://doi.org/10.1007/s13203-018-0200-2 RESEARCH NOTE Catalytic cracking of naphtha: The efect of Fe and Cr impregnated ZSM‑5 on olefn selectivity Ebrahim Mohiuddin1 · Masikana M. Mdleleni1 · David Key1 Received: 5 December 2017 / Accepted: 25 April 2018 / Published online: 4 May 2018 © The Author(s) 2018 Abstract This study focuses on the modifcation of ZSM-5 in order to enhance the catalytic cracking of refnery naphtha to produce light olefns. ZSM-5 was metal modifed using diferent loadings (0.5–5 wt%) of Fe and Cr via the impregnation method. The metal modifed ZSM-5 samples are compared and the efect of metal loading on the physicochemical properties and catalytic performance is investigated. Fe and Cr modifcation had an efect on both the physicochemical properties of the catalysts as well as catalytic activity and selectivity. Metal loading caused a decrease in the specifc surface area which decreased further with increased metal loading. Fe had a greater efect on the total acidity in particular strong acid sites when compared to Cr. The optimum Fe loading was established which promoted selectivity to olefns, in particular propylene. Fe also had a dominant efect on the P/E ratio of which a remarkable ratio of fve was achieved as well as enhanced the stability of the catalyst. Cr was found to be a good promoter for selectivity to BTX products with a two-fold increase observed when compared to Fe-modifed catalysts. Keywords ZSM-5 · Metal-modifed · Catalytic cracking · Naphtha · Olefns Introduction Catalytic cracking of various types of hydrocarbons including naphtha range hydrocarbons has been investi- Production of light olefns such as ethylene and propylene gated over ZSM-5 zeolites to produce light olefns. The vital to many petrochemical processes has been achieved reaction occurs at much lower temperatures of 550–600 °C through steam cracking of hydrocarbons for almost half a compared to steam cracking and the yields of ethylene and century [1]. However, it is a high energy consuming process propylene are high enough to compete with steam cracking. that requires temperatures in excess of 800 °C. Together with Additionally, higher P/E ratios is excess of one are attainable other disadvantages such as high CO­ 2 emissions, limited compared to 0.4–0.7 for steam cracking [3]. This is highly control over the propylene/ethylene (P/E) ratio and increas- advantageous as the demand or propylene continues to grow. ing propylene demand, has led researchers to identify more Zeolites, in particular ZSM-5 are active catalysts and/or efcient routes of light olefn production to overcome these supports for a range of reactions such as cracking, alkyla- disadvantages. Many catalytic processes focusing on “on tion, aromatization and isomerization of hydrocarbons, due purpose” light olefn production have been developed to to their activity, shape selectivity [4, 5], ion-exchanging supplement the demand such as propane dehydrogenation, properties, special three dimensional micropore struc- methanol to olefns, olefn metathesis and catalytic cracking ture and large specifc surface area in ZSM-5 [6, 7]. They of naphtha [2]. play a signifcant part in the olefn industry and in current processes that are under development and modifcation in order to commercialize these technologies. For instance P/E ratios can be adjusted to meet global demands by control- * Masikana M. Mdleleni ling the acid type, strength and distribution of Bronsted/ [email protected] Lewis acid sites of the ZSM-5 catalyst as well as the operat- 1 PetroSA Synthetic Fuels Innovation Center, South African ing conditions of temperature, pressure and WHSV. Other Institute for Advanced Materials Chemistry, University modifcations of the ZSM-5 catalyst include alterations of of the Western Cape, Robert Sobukwe Road, Bellville 7535, the acidic properties using transition metals [1]. Previous South Africa Vol.:(0123456789)1 3 120 Applied Petrochemical Research (2018) 8:119–129 studies conducted by Lu et al. [8] have shown that Fe and isotherms at − 196 °C using the Micromeritics Tristar 3000 Cr promoted ZSM-5 have good selectivity to C 2 and C3 analyzer. Prior to N2 physisorption samples were degassed at olefns in the cracking of Isobutane at 625 °C. This may 150 °C for 4–5 h using helium. NH3-TPD was used to deter- be due to metals such as Fe and Cr being able to enhance mine the amount and strength of acid sites in the ZSM-5 the dehydrogenative cracking of Isobutane. Isobutane when zeolites. TPD profles were obtained using an Autochem dehydrogenated to isobutene can then be easily cracked to II Micromeritics chemisorption analyser. Typically 0.1 g lighter olefns. However, the efect of Fe and Cr metal load- of catalyst is loaded into the reactor. This is then activated ing and Cr 6+/Cr3+ ion ratio on altering the acidity as well as by heating rapidly to 500 °C and holding for 20 min under electronic promoter efects to enhance selectivity to olefns helium fow at 30 ml/min. The catalyst is then cooled to is not clearly understood. Therefore, Fe and Cr were seen as 120 °C under fowing helium. The gas was then switched a suitable choices as a metal promoters for the cracking of to 5% NH3 in balance helium and the NH3 was adsorbed refnery naphtha. Moreover, due to the diference in reactiv- at 120 °C for 30 min at a fow rate of 15 ml/min. Helium ity of the various compounds present in naphtha, selectivity was once again allowed to fow over the catalyst to remove to valuable olefns may be afected. Further investigations any physisorbed NH3. Under fowing helium NH3 desorp- using complex refnery naphthas are required to understand tion occurred. The fow rate was kept constant at 25 ml/min the process under industrial conditions. while the temperature was ramped up to 700 °C at a rate of In this study we examined the use of Fe and Cr modi- 10 °C/min. The signal was detected using a thermal conduc- fed ZSM-5 as catalysts for the catalytic cracking of naptha tivity detector (TCD) and the outputs recorded on a com- obtained from the Chevron refnery to enhance light olefn puter. H2-TPR was performed to determine the reducibility selectivity. Catalysts with loadings of 0.5–5 wt% of Fe and of the metal impregnated samples as well as to confrm the Cr have been prepared and its efects of the catalyst prop- metal oxide phase. Typically 0.1 g of catalyst is loaded into erties such as porosity and acidity have been studied. The the reactor. This is then purged with Argon while the cold catalytic performance has also been evaluated and its efect trap is being prepared. 10% H 2 balance Argon was used in on activity and selectivity to olefns is investigated. the experiment. The 10% H2/Ar was allowed to fow over the sample at 50 ml/min as the temperature was ramped to 900 °C at 10 °C/min. Scanning electron microscopy (SEM) Experimental micrographs were obtained using a high resolution–SEM EHT 5.00 kV. All samples were carbon coated before imag- Catalyst preparation ing. The HRSEM was also equipped with an EDS spectrom- eter with the AZTEC EDS system by Oxford Instruments for Catalysts with diferent Fe and Cr loadings were prepared elemental analysis of zeolites. using the impregnation method. Fe and Cr (0.5, 2 and 5 wt%) were loaded onto to Zeolyst ZSM-5 (SiO 2/Al2O3 = 30) using Catalytic testing Fe(NO3)3·9H2O and Cr(NO3)3·9H2O salts. The total mass of impregnated zeolite was 5 g. The appropriate amount of the The catalytic performance was evaluated using a stainless steel nitrate salts were dissolved in excess distilled water (25 ml) fxed bed reactor. Typically 0.25 g of catalyst was loaded and and stirred until the salt completely dissolved. For example, supported using quartz beads. The reaction conditions were −1 approximately 0.192 g of Cr(NO3)3·9H2O was added to the as follows T = 550 °C, P = atmospheric, WHSV = ~ 16 h . aqueous solution to obtain a 0.5 wt% metal loading. ZSM-5 The feed fow rate was kept at 0.1 ml/min while nitrogen was was then added to the solution slowly and stirred at 80 °C passed through at 74.8 ml/min to obtain a N 2/Feed ratio of 4:1. until all the water had evaporated. It was then left to air dry Products were analysed by on a Bruker 450-GC. Gas products and was then further dried in an oven at 110 °C overnight. were analysed on a BR—Alumina–Na2SO4 column while liq- The catalysts were then calcined at 500 °C for 6 h to convert uid products were analysed on a RTX-100-DHA 100 m sta- the metals to the oxide form. The metal modifed ZSM-5 tionary phase column. The method used to determine product samples were characterised using XRD, N2 physisorption, distribution was the ASTM D6729 method. Hydrocarbons NH3-TPD, H2 TPR, HRSEM and EDS. in the C1–C14 range were analysed and identifed as paraf- fns, olefns, aromatics and others using PONA analysis. A Characterisation signifcant amount of products which could not be identifed was grouped as others and was not included in the selectivity XRD powder patterns were recorded using a Bruker AXS to BTEX products. Naphtha conversion was calculated based D8 Advance (Cu-Kα radiation λKα1 = 1.5406 Å) 40 kV in on the product distribution results obtained from the detailed the 2θ range 5°–90°. Surface area measurements and poros- hydrocarbon analysis (DHA).
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