(290e) Synthesis of Carbon Nanofibers from Light Liquid Paraffins Using the Liquid Pulse Injection Technique

  Carbon nanofibers (CNFs) show many unique properties, most of them common with carbon nanotubes (CNTs).  As the mass production of CNFs is thought to be much easier than CNTs, the demand of CNFs is growing at a quicker pace.  However, the production cost of CNFs is still high, therefore applications in which the usage of them is thought to be feasible is still quite limited.

  Previously, we introduced a new method to efficiently produce CNFs, the Liquid Pulse Injection (LPI) technique.  The main feature of this method is the introduction of the fiber source into the reactor as liquid pulses.  Liquid pulse introduction leads to the generation of extremely active catalyst particles in a highly dense state.  Therefore, CNFs can be easily obtained at high carbon yields, up to 85%, through this method.  We also experimentally showed that CNFs can be obtained at high carbon yields even from an inexpensive mixture of benzene, toluene and xylene.

  Aromatics are generally used as the carbon source for CNF production.  However, such hydrocarbons are highly toxic, and many regulations exist for the usage of them.  As the LPI technique allows the generation of extremely active catalyst particles, CNFs are thought to be attainable at high yields even from hydrocarbons which were thought not to be suitable for CNF production.  Up till now, we have shown that CNFs can be efficiently produced from various alcohols.  In this work we verified the possibility of efficiently producing CNFs from another category of inexpensive and abundant hydrocarbons, light liquid paraffins.

EXPERIMENTAL

  An apparatus equipped with a tube reactor, an electric furnace and two mass flow controller systems, one to control the flow rate of hydrogen, and the other to control the flow rate of nitrogen was used for CNF production.  Hydrogen was used as the carrier gas, and a mixture of ferrocene, the carbon source and thiophene (catalyst promoter) was used as the starting material.  Hexane was selected to represent light liquid paraffins, and was used as the carbon source in production experiments.  First the reactor was thoroughly purged with nitrogen, and then hydrogen was introduced to the reactor.  Next the temperature of the reactor was raised to 1,473 K using the electric furnace.  After the temperature distribution in the reactor reached a steady state, 20 liquid pulses of the starting material were intermittently introduced into it at an interval of 30 s or 60 s.  In some experiments, this production sequence was modified in order to collect CNFs at different growth stages.  After the starting material was injected into the reactor, the generated CNFs were quickly blown out from the hot temperature reaction zone by rapidly increasing the flow rate of the carrier gas when they reached the position where they were intended to be sampled.  Through this sequence, fibers representing the CNFs at this position can be collected.  By varying the blowing out timing of the CNFs, a series of fibers can be obtained, and the growth behavior of the fibers can be clarified by analysing the collected fibers.  During CNF production, the composition of the reactor outlet gas was also monitored using a micro GC.  After 60 s from the injection of the final liquid pulse, the reactor was quickly purged with nitrogen, and the electric furnace was turned off.  The reactor was cooled down to room temperature, and the produced CNFs　located at the bottom of the reactor were collected and weighed.  The carbon yield was calculated as the ratio between the amount of carbon collected as CNFs, and the amount of carbon introduced into the reactor.  The amount of pyrolytic carbon formed on the reactor wall was evaluated by oxidizing it using air, and measuring the amount of CO₂formation.  Finally, the carbon balance was calculated.

RESULTS AND DISCUSSION

  Although lower than the values achievable when aromatics are used as the carbon source, CNFs could be obtained at moderate carbon yields even when hexane was used as the carbon source.  The obtained CNFs were extremely long, a feature common to typical CNFs produced using the LPI technique.

  Through the observation of CNFs collected at various growth stages, it was found that when hexane is used as the carbon source, fiber growth initiates when the catalyst particles reach the region in the reactor where the temperature is around 1073 K.  This temperature is about 200 K lower than that observed when benzene is used as the carbon source, and is thought to be due to the lower thermal stability of hexane.  Interestingly, fiber growth was found to terminate when the growing fibers reach and reside in the region in the reactor where the temperature is in the range of about 1273 K to 1373 K and then growth resumes when the fibers reach the region where the temperature increases to about 1423 K.  Gas analysis revealed that fiber growth termination occurs where methane dominates the gas phase and resumes when ethylene starts to appear in the gas phase through the coupling of methane.

  Next we attempted to increase the productivity of CNFs by adjusting parameters of the LPI technique.  As expected, decreasing the carrier gas flow rate to increase the residence time of the growing CNFs within the reactor led to the increase of carbon yield, but was accompanied by the decrease in the purity of the obtained CNFs.  So next we increased the volume of the liquid pulses and shortened the pulse intervals.  When benzene is used as the carbon source, such modifications lead to a significant decrease in product purity, but this was not the case when hexane was used as the carbon source.  Such results can be explained by the fact that non-catalytic carbon formation in the gas phase, an undesirable reaction which leads to the generation of impurities during CNF production, proceeds much easier when benzene is used as the carbon source.  By this modification of process conditions, a high productivity of 2.5 kg(m³-reaction zone volume)^-1h^-1 could be achieved even when hexane was used as the carbon source.  This value is higher than the productivity achieved when CNFs are produced from benzene using standard process conditions.