Studies On The Catalysts For Catalytic Pyrolysis Of Paraffin To Light Olefins

Naphtha catalytic cracking to produce light olefns becomes a potentially alternativeprocess of naphtha thermal cracking, because of its lower reaction temperature, higherselectivity of high-value products and adjustable P/E ratio. Paraffin, as one of the mainconstituent of naphtha, is very difficult to crack under mild condition of catalytic pyrolysis,thus paraffin pyrolysis becomes an obstacle to hinder further development of naphthacatalytic pyrolysis.This article used n-heptane as a model compound and the feasibility of dehydrogenationcracking path was firstly confirmed by adding DH-cat to cracking catalyst. Then, weintroduced reducible metal dehydrogenation active sites into the HZSM-5zeolite by zeolitemodification, prepared series of bi-functional catalysts with dehydrogenation and crackingactivity, studied these catalytic performances in fixed bed reactor, and chose one bestcatalyst–Ag-P/HZSM-5. Combined with XRD, N2adsorption-desorption, NH3-TPD andH2-TPR results, discuss the action mechanisms of silver and phosphorus on high activity, andthe optimum amount of silver and phosphorus was also obtained. In addition, the catalyticperformance was also investigated in a circulating fluidized bed reactor to obtain the optimumoperating conditions.It was found that dehydrogenation-cracking of paraffin was a promising way to producelight olefins. The optimum preparation metrod for dehydrogenation and cracking bi-functionalcatalyst is metal element and phosphorus co-modification, firstly impregnating metal elementand then impregnating phosphorus over HZSM-5zeolite. Transition metal elementsmodification HZSM-5zeolite catalysts showed very high light olefin selectivity andpropylene/ethylene ratio. Among them, Ag-P/HZSM-5catalyst exhibited the best performance, and showed very good regeneration ability. The introduction of silver intoHZSM-5zeolite provided the dehydrogenation active sites. The more silver content, thestronger dehydrogenation ability, but the higher hydrogen transfer activity. The introductionof P could increase the acidity of catalyst and dehydrogenation activity also could be furtherincrease by phosphorous doping after silver modification. The optimum adding amount ofsilver and phosphorus was1.5wt%and4.0wt%, respectively. New acid sites were be formedwith Ag+reduction during cracking reaction, and further promoted the cracking of n-heptane.The interaction of dehydrogenation and cracking active sites makes it to be a promisingcatalyst with highest activity. At600oC, over1.5Ag-4P/HZSM-5catalyst, the conversion ofn-heptane cracking reached58.04%and the yield of ethylene, propylene, and butylene was13.46%,27.02%, and13.28%, respectively, in the fixed bed reactor. By comparison tounmodified HZSM-5catalyst, conversion increased by36percentages, and ethylene andpropylene also enhanced by about2wt%and3wt%, separately. Experimental results in thecirculating fluidized bed reactor showed that high temperature and short residence timecontributed to better reaction results.45.34wt%conversion was achieved and the yield ofethylene, propylene, and butylene was9.26%,22.53%, and13.55%, respectively, under thecondition of590oC reaction temperature and1s residence time.