Preparation of Carbon-Alumina (C/Al2O3) aerogel nanocomposite for benzene adsorption from flow gas in fixed bed reactor

Graphical abstract


Method details
Background Volatile Organic Compounds (VOCs) with high toxicity and carcinogenetic characteristics are well known as compounds with great of concern among different air pollutants [1]. The VOCs are emitted from various industrial activities such as automobile exhausts, petrol and gasoline evaporation, solvent paints, and organic solvents [2,3]. Amongs the different compounds of VOCs group; Benzene is classified as a carcinogenic compound by the international agency for research on cancer (IARC) [4]. Long-term to Benzene compounds leads to a range of severe adverse health effects such as hematologic cancers, destructive effects on the immune system, and aplastic anemia [5]. Therefore, on-site emission control and removal of benzene before release to the atmosphere are so importantin view point of health. The adsorption process is an acceptable method for removal of VOCs from polluted air. Several adsorbents such as bagasse ash [6], zeolite [7], and activated carbon have been applied as adsorbents for the removal of VOCs from the air [2,[8][9][10]. However, the efficiency of these adsorbents is nearly low due to pore blocking, low specific surface area, and decrease in adsorption capacity after regeneration [11]. Aerogels with high specific surface areas, low density, high selectivity and high porosities show promise for removal VOCs componets from polluted air flow [11]. Also, the adsorption capacity of aerogels does not change even after a long period and using for many adsorption-desorption cycles [12]. However, it is also responsible for the relatively poor mechanical behavior of this material which is typically expressed as fragility and brittleness [13]. Therefore, enhancing the mechanical properties of aerogel is essential to broaden the range of its application. For this purpose, in recent years, various materials, including polymers organic, carbon nanotube, and activated carbon have been used to enhance the mechanical strength of inorganic aerogel [14][15][16][17]. Resorcinol-formaldehyde (RF) can promote the mechanical structure of aerogel; it is converted to carbon through the pyrolysis process at the temperature range of 750-1500 C. In this sense, several composite aerogels, including Al 1 -RF 2 , Nb 2 O 5 3 -RF, and TiO 2 -RF, SiO 2 -RF have been prepared and in addition, the enhanced catalytic and adsorption efficiency have been reported [18][19][20][21]. Studies indicated that the addition of RF to alumina aerogel structure increased the specific surface area and total pore volume [13,22]. However, these compounds are toxic, dangerous and costly.
To overcome thesedrawbacks, a group of researchers reported that the Novolac is a low-price commercial resin with high char-yield efficiency and pyrolysis of Novolac-containing aerogels leads to the formation of carbon aerogels with significant properties [23]. As regards, the use of Novalac polymer has not been investigated to increase mechanical strength alumina aerogel. In the present research, the Novolac/alumina aerogel (N/Al 2 O 3 ) was prepared using the sol-gel technique and ambient drying. Finally, the structures and properties of carbon-alumina aerogel as adsorbents were characterized to fill the knowledge gap. Therefore, the present study aimed to evaluate the removal efficiency of composite aerogel (C/Al 2 O 3 )of removal of benzene from contaminated air.

Chemical reagents
Aluminum chloride hexahydrate (AlCl 36 H 2 O) and Novolac resin (made in Iran with 9% wt of hexamethylenetetramine (HMTA)) were applied as an alumina source and organic phase precursor, respectively. Other chemical materials such as 2-propanol, deionizer water, and propylene oxide (PO) were used as solvent, hydrolysis agent and crosslinking, respectively. All the chemical materials and solutions were of analytical grade and used without further purification.

Preparation of composite aerogel (C/Al 2 O 3 )
A schematic diagram of the synthesis composite aerogel (C/Al 2 O 3 ) is illustrated in Fig. 1. The sol-gel method was applied to synthesis of C/Al 2 O 3 according to the following steps ( , before converting to gels, were mixed thoroughly and exposed to ultrasound for 30 min. It should be noted that the molar ratio of Novolac to Al 2 O 3 was fixed with a ratio of 4:1. 4 The consistent uniform of solutions was poured into a falcon tube with the interlayer distance of 3 cm. 5 The solution was placed inside a high-pressure autoclave containing a low amount of 2-propanol and treated at 105 C for five hours and then cooled down to room temperature. 6 The resulting gel was dried at ambient temperature for 48 h and subsequently dessicated at 120 C for 48 h. 7 Subsequently, the prepared aerogel nanocomposite was carbonized at 500 C for two h under nitrogen gas flow rate of 10 ml/min. In this stage, composite aerogel preparation was grained and sieved with 20-40 mesh. 8 Granulation of N/Al 2 O 3 : The N/Al 2 O 3 composite aerogel was converted to C/Al 2 O 3 at 1000 C for 3 h under nitrogen gas flow with a rate of 10 ml/min [13,23].

Characterization
The C/Al 2 O 3 composite aerogel was placed in cylinder cast, and the specific area of the aerogel (C/Al 2 O 3 ) was characterized through measurement of adsorption nitrogen gas at À196 C by Belsorb mini II device, and the elements existing in the synthesized sample were determined with Energydispersive X-ray spectroscopy (EDX) analysis (ARL X 0 TRA diffract meter (Rigaku)) with Cu-Ka radiation (30 kV, 30 mA). The morphology of the structure sample was characterized using an LEO-1530VP field emission scanning electron microscope (SEM) operating at 10 keV. Joyner-Halenda (BJH) method was applied to measure the pore size distribution (PSD) of aerogel composite. Table 1 presents the information on the total specific pore volume, surface area, average pore diameter, external surface area of the aerogel composite. The C/Al 2 O 3 has a high specific surface area (762.97 m 2 /g) with a total pore volume of 0.59 cm 3 /g and 3.29 nm pores in diameter. Fig. 2(A) shows the adsorption/desorption isotherms of the composite aerogel (C/Al 2 O 3 ). The results indicated that the adsorption-desorption isotherm is fitted with type IV with H1type hysteresis loop, indicating a porous structure with cylindrical pores. Fig. 2(B) shows the pore size distributions of the composite aerogel (C/Al 2 O 3 ). The pore size distribution is estimated using the Barrett-Joyner-Halendar method. As observed in this figure, single pores with the maximum pore size in the range of 2-12.17 nm. Therefore, according to the IUPAC pore size are mesoporous. Also, Fig. 2(C) shows the presence of chemical elements (Al, C, and O) in the building of the prepared aerogel composite. These results indicate the existence of C (65.7%), O (25.5%) and Al (7.9%) elements in the composites. The FESEM analysis (Fig. 2(D)) shows that the spherical solids are distributed uniformly over the surface. It is suggested that the existence of formation cylindrical pores in the structure of aerogel composite, explaining of suitable porosity for sorption of contaminants.  Fig. 3 indicates the schematic diagram of experimental set-up used in the present study. In this study, an air pump was employed to provide air flow rate and remove impurities transformed from an activated carbon column. The pure air was transported into mixing chamber to which liquid benzene was injected by a syringe pump (2 ml h À1 ). The air flow rate of entering to mixing chamber was adjusted at 7 l min À1 to obtain the air containing benzene with a concentration of 100 ppm. All experiments runs were performed in a fixed-bed continuous-flow reactor with internal diameter of 1.5 cm and a height of 30 cm. A glass gauge filter was installed in the bottom of the column to make sure of uniform distribution of the flow air into reactor. Then, 1 g of fine particles of composite aerogel was loaded inside the column in each step of experiments to determine the adsorption capacity of aerogel composite. The sampling from designated locations was performed by a gas tight syringe for determination of the concentrations of benzene. Then, the amount of 100 mL was injected into the gas chromatography (Varian CP-3800),  equipped with a detector (FID) and column (0.25 m inner diameter and 15 m length), to determine the concentration levels of benzene. The temperature program of the column was set from 50 to 150 C with an increasing ramp of 2 C min. Injection temperature of 240 and the injection volume was set to split with a ratio of 100:5. In addition, a temperature of 250 C was set for the detector. At the end, the following equation (Eq. (1)) was applied to compute the quantity of adsorption capacity in the breakthrough point:

Experiments of adsorption in dynamic conditions
Where, BC: Adsorption capacity of benzene on a gram of adsorbent when it reaches a breakpoint (mg/g) C in : Input concentration of benzene (mg/m 3 ) T bk : breakthrough time (h) Q: Air flow rate (m 3 ) M: The amount of applied adsorbent (g) Adsorption process study The effect of flow rate on benzene removal The breakthrough time of composite aerogel for removal of benzene was studied at different flow rate. The findings in Fig. 4 indicate that, with increasing flow rate from 0.2 l/min to 0.5 and 0.8 l/min, the breakthrough time of the benzene onto composite aerogel diminished from 13 h to 7 and 3 h, respectively. The decrease of breakthrough time with the increase of flow rate can be attributed to low expose time between benzene molecules with active sites on the surface of the composite aerogel [24]. Additionally, shorter breakthrough time can be due to transporting more mass of benzene inside the bed. Hence, the active sites of the composite aerogel quickly are saturated and breakthrough time occurs in relatively slower time. According to similar studies, the breakthrough time with the increase of flow rate was decreased [25,26]. These results are in agreement with the other studies results [24].

Effect of benzene concentration
The effect of benzene concentrations on the breakthrough time were studied at different inlet concentrations. The Fig. 5 indicates that the breakthrough time decreased from 420 min to 180 and 60 min with increasing of the Benzene concentration from 300 ppmv to 600 and 900 ppmv, respectively. Besides, Eq. (1) was employed to calculate removal capacity in the breakthrough time by C/Al 2 O 3 aerogel composite. The findings also showed that the amount of adsorption capacity on C/Al 2 O 3 decreased from 126.73, 84.5 to 38 mg g À1 by increasing the inlet Benzene concentration. It can be explained by the fact that more mass was transferred to the adsorbent surfaces [25,27,28].

Reusability aerogel composite
To the adsorption properties of aerogel composite repeated adsorpion/desorption cycles of benzene on aerogel composite was performed. The regeneration of composite aerogel performed by extraction of adsorbed benzene and drying of sample at 250 C under vacuum conditions. Fig. 6 shows the breakthrough time curves of C/Al2O3for benzene vapors. The findings indicate that the differences between breakthrough time in the first and five series are negligible. It can be related to the stability of pours due to the entry and exist of benzene molecules into it. Also, it can be concluded that adsorption of benzene molecules on the aerogel composite was poor and adsorbents molecules can be separated by thermal purification [29]. Hong Sui has reported that the breakthrough time for silica gel and activated carbon commercial were different when the recycling time is increased from the first cycle to five one This difference is due to the remain of toluene molecules on the adsorbent after the desorption step [29,30].