Chemical conversion of biomass via gasification and pyrolysis has attracted significant scientific attention, because of the potential for energy production. The largest factor deterring further investment in biomass conversion is the presence of impurities present in the producer gas, namely tars. Tars are a complex mixture of low and high molecular weight Poly-Aromatic Hydrocarbons (PAHs). PAHs deactivate catalysts, cause blockages in transfer lines, and damage downstream units such as compressors, turbines, and fuel cells. Additionally, PAHs are carcinogenic in nature and result in problems with disposal. Thus, a process to convert PAHs into potentially useful products is extremely attractive.The composition of PAHs in the producer gas can be affected by operating parameters during the gasification process such as reactor temperature, pressure, residence time, biomass type, and gasifying medium. Entrained flow gasifiers produce almost no PAHs, but require flexible designs and high costs. Reduction of PAHs can also be accomplished via downstream thermal cracking, however reaction temperatures in excess of 1000 �C are required. Catalytic transformation of tars may be one of the most promising options for PAH reduction. This process has the potential to convert PAHs into lighter hydrocarbons, which can, in turn, be reformed to produce valuable synthesis gas. Both metallic (nickel) and non-metallic (dolomite and olivine) catalysts have been extensively studied for PAH reduction activity. Unfortunately, rapid deactivation due to H2S and coke formation has been observed. As a result, the lifetime of the aforementioned catalysts remains a challenge.[1]Zeolites may be promising materials for PAH transformation. Zeolites exhibit unique properties (such as aciditiy, microporosity, and shape selectivity), which make them ideal for catalytic applications. Fluid catalytic cracking (FCC) units employ zeolites to crack heavy hydrocarbons into lighter valuable products, such as gasoline and diesel fuel. However, low diffusion rates of large molecules through the highly microporous structure can result in coke formation, while due to their shape selectivity bulky molecules can be totally excluded from the pore structure. The main objective of this study is to test the overall effectiveness of zeolite catalysts with respect to PAH transformation. ZSM-5 zeolite has been tested, using benzene as a model compound. The following parameters have been tested: 1) The ZSM-5 pore structure; 2) the catalyst and feed residence time in the reactor; 3) the presence of Ni on the ZSM-5; and 4) the co-feeding of CO2. The pore structure of ZSM-5 has been modified by top down techniques for the incorporation of mesoporosity. Feed and catalyst residence time have been varied by changing the amount of benzene injected in the reactor and the feed flow rate. Nickel has been deposited on the catalyst by dry impregnation. The overall goal of the study is to gain a fundamental understanding on the reaction mechanism of benzene over the zeolites.   ExperimentalModel CompoundsThe present study has focused on studying the transformation of benzene as a model compound. Benzene is the simplest aromatic hydrocarbon and representative of the tars mixture derived from biomass gasification. Future studies will be conducted with heavier model compounds: naphthalene, fluoranthrene and coronene. Experimental Set up                Benzene is tested as a model compound using the set-up depicted in Figure (2). Different initial amounts (0.8 gm to 6.5 gm) are injected through the heated vessel (625 �C). The vaporized benzene is carried by nitrogen through the heated (625 �C and 700 �C) quartz tube reactor (7 mm ID) loaded with 0.4 gm of catalyst. The nitrogen flow rate is controlled and varied between 25 ml/hr and 100 ml/hr to decrease the residence time of benzene in the reactor. Liquid effluent is collected in methanol by an impinger/ice water bath system. Vapor bubbled through the system is collected in 5 L gas bags. Liquid samples are taken from the impinger and analyzed by gas chromatography equipped with mass spectroscopy (GC/MS) detector. The vapor samples are analyzed by a GC equipped with a thermal conductivity detector (TCD). Coked catalyst is collected and studied with thermogravimetric analysis (TGA). Additionally, CO2 was tested as a carrier gas.  Materials and characterizationZeolite ZSM-5 was obtained from Zeolyst International (CBV 8014-Si/Al 40). Unmodified zeolite was used after calcination at 550�C ZSM-5. The ZSM-5 pore structure was modified by a top �down, desilication[2] method as follows: Zeolite is first calcined at 550 �C. Then it is treated with NaOH at 65 �C for 35 min. The concentration of the NaOH solution controls the extent of the introduced mesoporosity. The suspension is quenched, filtered, washed until a neutral pH is observed, and dried. The powder is then treated with HCl at 65 �C for 6 hrs. The resulting mixture is filtered, washed and dried. Next, the powder is ion-exchanged three times with NH4NO3 for a period of 12 hrs after each wash. Finally, the material is filtered, washed, dried and calcined at 550 �C.Nickel is also deposited on ZSM-5 via dry impregnation, using Ni(NO3)6H2O crystals. The final material is calcined at 550�C and then reduced using 5% H2.The materials were characterized using X-ray diffraction (XRD), N2 adsorption BET, scanning electron microscopy (SEM) and Thermal Gravimetric Analysis (TGA). Results and DiscussionMaterial CharacterizationN2 isotherms of the various tailored mesoporous ZSM-5 are displayed in Figure 1. The severity of the alkaline treatment plays a key role in the formation of mesoporosity. The mesopore volume of the modified ZSM-5 increases as the alkaline treatment becomes more severe. The average pore volume increases from 6.1 to 10.7 nm as the NaOH concentration increases from 0 to 0.8 M.  Remarkably, the alkaline treatment does not diminish the microporosity of the ZSM-5. The acid treatment step helps to remove any alumina debris generated by the alkaline treatment, which might cause blocking of the micropores.    Figure 1 . N2 adsorption of ZSM-5 and meso ZSM-5 XRD analysis of the parent and the mesoporous samples revealed some crystal destruction with increasing the severity of alkaline treatment. The mesoporous ZSM-5 treated with 0.1 M NaOH was used for the benzene conversion experiments, mainly because of the decreased crystal destruction observed after the alkaline treatment. The Ni deposited on the ZSM-5 zeolite was measured by TGA and was found 3.7 wt%. Benzene conversion resultsa)       Effect of feed residence timeIncreasing the feed flow rate from 25 to 100 ml/min does not significantly affect the overall benzene conversion. However, increasing the benzene feed concentration from 0.8 to 6.5 ml substantially reduces the conversion due to catalyst deactivation.b)       Effect of mesopore structure of ZSM-5The conversion of benzene over ZSM-5 results in the production of liquid hydrocarbons, mainly toluene. Introduction of mesoporosity in ZSM-5 increases the production of liquids (as shown in Figure 2), mainly because of the production of heavier polyaromatic hydrocarbons such as naphthalene and biphenyl. In addition, the more open structure of the mesoporous ZSM-5 resulted in reduced coke production. The above phenomena may be attributed to reduced diffusion limitation in the mesoporous structure.Figure 2.Effect of mesoporosity in ZSM-5 on the conversion of benzene to liquid productsd)       Effect of Ni in ZSM-5   Figure 3.Effect of Ni in ZSM-5 on the conversion of benzene to coke Figure 3 depicts that the presence of Ni in the ZSM-5 increases conversion of benzene to coke. Additionally, a higher H2 yield has been observed due to the presence of Ni.    Figure 4.Liquid production distribution using ZSM-5, mesoporous ZSM-5 and Ni-ZSM-5 Toluene was the main product when the reaction was carried out over the unmodified ZSM-5 catalyst. Once mesoporosity was introduced, heavier compounds (such as naphthalene and biphenyl) were observed in the liquid effluent. The nickel embedded catalyst exhibited a far greater selectivity towards biphenyl than any other compound. Figure 4 displays the aforementioned product distribution over the three catalysts.e)     Effect of temperatureIncreasing the reaction temperature from 625 �C to 700 �C, increases the overall conversion of benzene. Conclusion and Future WorkBenzene transformation was studied over ZSM-5, mesoporous ZSM-5 and Ni-ZSM-5 catalyst. The more open structure of mesoporous ZSM-5 results in the production of heavier hydrocarbon and less coke compared to microporus ZSM-5, indicating less diffusion limitation. Nickel deposited on ZSM-5 exhibited greater selectivity towards larger molecules, but higher coke yields. Future experiments will be conducted with naphthalene, fluoranthrene and coronene as model compounds. These benzene trials will be used as a baseline to which the trials with larger compounds will be compared.  Acknowledgement.  The study is funded by the National Science Foundation Award CBET-1236738. References[1] Radwan A.M.; Kyotani T.; Tomita, Appl. Catal. A.,2000, 192, 43. [2] Groen J.C.; Moulijn J. A.; and J. P�rez-Ram�rez, Microporous     Mesoporous Mater., 2005, 87(2), 153.

Catalytic reaction of aromatic hydrocarbons using hierarchical pore structure zeolites

247th National Meeting (2014)

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