Influences of Mesoporosity Generation in ZSM-5 and Zeolite Beta on Catalytic Performance During n -Hexane Isomerization
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Influences of Mesoporosity Generation in ZSM-5 and Zeolite Beta on Catalytic Performance During n-Hexane Isomerization Bharat K. Modhera • Mousumi Chakraborty Hari C. Bajaj • Parimal A. Parikh
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Received: 31 January 2011 / Accepted: 14 April 2011 / Published online: 3 May 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Catalytic performance of Pt impregnated parent and desilicated nano-crystalline zeolites, ZSM-5 and Beta for n-hexane isomerization was studied. Difference in channel systems of the zeolites and absence/presence of mesopores therein were found to be reflected in product distributions. ZSM-5 was desilicated by NaOH and zeolite Beta with tetramethylammonium hydroxide (TMAOH.) Desilication was found to afford comparable catalytic performance to that of the parent counterpart at reaction temperature lower by 25 °C. Observed product distributions could be substantiated with characterizations such as ammonia TPD, surface area determination and SEM. Desilicated zeolite Beta was seen to be less prone to coking as deduced from the TGA study. Location of Pt with reference to proton sites within the channels and that inside the pores viz a viz external surface also have been discussed briefly. Keywords Zeolites ZSM-5 Zeolite Beta Isomerization Desilication Hierarchical zeolites Mesoporous zeolites
B. K. Modhera M. Chakraborty P. A. Parikh (&) Chemical Engineering Department, S.V. National Institute of Technology, Surat 395007, India e-mail: [email protected] H. C. Bajaj Discipline of Inorganic and Materials Catalysis, Central Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, Bhavnagar 364021, India Present Address: B. K. Modhera Chemical Engineering Department, M.A. National Institute of Technology, Bhopal 462 051, India
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1 Introduction Straight chain paraffin isomerization process comes to rescue to meet the cold flow requirements of a diesel fuel and that of octane enhancement of a gasoline fuel. There is an abundance of literature including review articles for e.g. [1] on isomerization. Its reaction mechanism on a bimetallic catalyst has also been widely reported [2]. To provide the acid function to the catalyst, many zeolites varying in their acid properties and channel systems have been attempted. For a metal function on the catalyst, Pt has been extensively used. Though zeolites do provide the necessary acidity for the reaction, they suffer from the drawback of the constrained space available for the formation of bulkier product molecules. These spatial constraints within the channels of the zeolite afford low diffusional mass transfer rates and also cause rapid blockage of the channels due to coking. This results in higher deactivation rates. To circumvent these channel configuration related problems, generation of mesoporosity in the zeolite crystals by way of controlled desilication using an alkali has now become a routine practice to the extent that host of chemical reactions have already been reported in the literature [3 and references therein].
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