Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation

https://doi.org/10.1016/j.ijhydene.2018.02.158Get rights and content

Highlights

  • Microwave cracking of toluene over Fe/SiC catalysts was efficient.

  • The composition of cracking gas of toluene was mainly H2.

  • The highest cracking efficiency (86.3%) and H2 production yield (84.5%) were achieved.

  • Deposited carbon were spherical and filamentous with carbon deposition yield of 2.3%.

Abstract

In this work, toluene as a model compound of biomass tar and inexpensive Fe supported on SiC powder as catalysts were used to investigate the influence of specific microwave power levels, Fe content, and space velocity on the microwave cracking of toluene. The results revealed that the microwave-aided cracking of toluene was efficient and that the composition of cracking gas was mainly H2. During the reaction, the instantaneous cracking efficiency and the instantaneous hydrogen yield increased initially and then decreased, with the highest values being recorded as 94.4% and 89.8% at 10–12 min of the reaction. For the whole reaction, the overall cracking efficiency and the overall hydrogen yield reached maximum (86.3% and 84.5%, respectively) when the tests were conducted under the optimum conditions. The catalysts before and after reaction were analyzed by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), thermogravimetry (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). The results confirmed that the deposited carbon included spherical and filamentous carbon and carbon deposition yield could reach 2.3%, which had a negative effect on the cracking of toluene.

Introduction

In recent times, the industrial demand for energy has increased substantially but the supply falls far short of the demand. Therefore, finding an abundant renewable energy source to meet this demand has become very important worldwide [1], [2]. Biomass has attracted much attention from researchers because it is available from a wide range of sources and is renewable and environmentally friendly [3]. The problem of energy shortage will be alleviated greatly if the biomass energy can be utilized efficiently [4]. Among the biomass utilization methods, biomass gasification technology is preferred as it can be used on a small scale and has a high energy utilization rate. Moreover, decomposition gases produced by biomass gasification, such as H2 and CO [5], are easy to transport and control [6], [7]. Along with the production of decomposition gases and solid carbon, biomass tar is also generated during biomass gasification [8], [9]. Biomass tar (hereinafter referred to as tar) is a dark brown mixture with a high aromatic content, mainly toluene, naphthalene, phenol, benzene, and other organic substances [10]. Tar has many disadvantages as it blocks and corrodes the equipment pipeline. Furthermore, the abundant energy in tar cannot be used effectively if the tar is removed completely [11]. Therefore, remove and reuse of tar have become hot topics of research across the globe.

The methods for the disposal of tar mainly include wet purification, dry purification, thermal cracking, and catalytic cracking [12], [13]. Among these methods, catalytic cracking is the most advanced because of low temperature requirements and high efficiency [14]. Catalysts for cracking of tar include natural ore, alkali metal, and nickel-based catalysts [15], [16], but their shortcomings are deactivation under some conditions and high cost for purchase and operation [17].

Fe-based catalyst is an attractive metal catalyst which is economical and environmentally friendly [18], [19], [20]. Azhar et al. [21] used several kinds of iron oxide catalysts produced under different atmospheres for the cracking of tar and found that the composition of the produced gas differed in each case and this difference had the notable influence on the yield of H2. Nordgreen et al. [22] used metallic Fe and iron oxide as catalysts to crack tar and found that metallic Fe was more effective. Zou et al. [23]cracked toluene (model compounds of tar) with different types of 3Fe8Ni/PG catalysts obtained by changing the calcination temperature and calcination time during the preparation. It was found that the 3Fe8Ni/PG catalysts calcined at 700 °C for 2 h had the highest activity.

However, in an earlier study, catalytic cracking of tar in the presence of Fe-based compounds under conventional heating was not efficient. This could be due to the following two reasons. First, compared with the expensive metal catalysts, Fe-based catalysts had inherently lower activities [24]. Second, uneven heating by conventional methods hindered heat transfer between the tar and the catalyst. In comparison with the conventional heating method, microwave heating technology plays an increasingly important role in energy utilization because of its fast and uniform heating characteristics [25], [26], [27]. Moreover, when a metal with shape irregularity or sharp edges is subjected to microwave irradiation, discharge phenomenon may take place; the effects of hot spots and plasma accompanied by the metal discharge can accelerate the rate of toluene cracking and compensate for the low activity by inexpensive metal catalysts and make up for the low activity inexpensive metal catalysts [28].

Nevertheless, there are only a few studies on the catalytic cracking of tar with microwave assistance, and most of the research focus has been on the cracking rate of toluene and gas production rate under different working conditions [29]. However, as the evolution behavior of the toluene cracking rate during the entire reaction is not completely clear, it is crucial to understand the cracking mechanism of toluene and design an optimum reactor for cracking toluene.

In this article, we investigated the effects of microwave power, Fe content, and space velocity on the toluene cracking rate (X), and the hydrogen production rate (φ) on a laboratory-scale fixed bed reactor using Fe-based catalysts. SiC powder was used as carriers in view of its splendid microwave absorption properties. Considering the absolute microwave parameter only applicable to the facility employed in the specific research work, instead, we use the power per 1 g sample as a new criterion, i.e., the specific microwave power (SMP) [30]. Moreover, the performance of cracking of toluene during the reaction was investigated experimentally and the key factors, i.e., temperature variation of the samples at different reaction stages and carbon deposition rate (Y) of the reaction, were analyzed to elucidate their effects on the catalyst activity. Subsequently, the catalysts were characterized by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), thermogravimetric (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). This is expected to confirm the evolution of carbon deposition during the reaction and help in understanding the cracking mechanism of toluene under microwave irradiation using inexpensive Fe-based catalysts.

Section snippets

Catalyst and preparation method

Fe/SiC catalysts were prepared by an incipient impregnation method [31], in which SiC was used as the support and varying Fe contents (2%, 4%, 6%, 8%, 10%, and 12%) were loaded on SiC as the active ingredient. First, Fe(NO)9H2O was chosen as the source of Fe, and SiC powder with a mesh size of 80 was dissolved in an aqueous solution of Fe(NO)9H2O.The mixed solution was impregnated overnight with continuous stirring, followed by drying at 160 °C for 2 h. Then, the catalysts were calcined at

Results and discussion

Many chemical reactions may occur during the reaction of toluene cracking. On the whole, toluene was cracked into macromolecular hydrocarbon and H2 first (Eq. (4)), and some of the macromolecular hydrocarbon continued cracking into carbon and H2 (Eq. (5)) [15], [23], as shown below:nC7H8 → mCxHy + pH2CxHy → xC + y/2H2

Conclusions

  • (1)

    Low space velocity and a high Fe content could improve toluene cracking efficiency (X) and carbon deposition yield (φ); improving specific microwave power could enable X and φ to increase and then drop. The highest overall X (86.3%) and φ (84.5%) were obtained when the SMP was 70 W/g, space velocity was 637 h−1, and Fe content was 10%.

  • (2)

    For the entire reaction period, the instantaneous X and φ initially exhibited an increase and followed by a decrease trend. The highest values could reach 94.4%

Acknowledgements

This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 51576118 and 51506116) and Young Scholars Program of Shandong University (Grant No. 2016WLJH37).

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