Synthesis and characterization of carbon cryogel / zeolite composites

A novel method for synthesis of carbon cryogel/zeolite composites was obtained. Method considers forming of carbon cryogel from the sol-gel polycondenzation of resorcinol and formaldehyde, followed by freeze drying, and subsequent pyrolysis in presence of different amount of zeolite. Characterization of composite materials by nitrogen adsorption shows that samples are microand mesoporous and that specific surface area decrease with increasing the amount of zeolite in samples. XRD method confirms amorphous structure of carbon cryogel and crystalline structure of zeolite, i.e. structure of zeolite has not been destroyed by carbonization process. SEM and EDX analyses reveal homogenous distribution of zeolite through out carbon cryogel and corresponding composition.


I. Introduction
Porous carbon, due to its high surface area and pore accessibility, excellent thermal and chemical stability, as well as, good electrical conductivity is extremely attractive and competitive material for application in separation technologies.Carbon aerogels and cryogels are special class of porous carbon materials [1−5] which are usually formed from the sol-gel polycondenzation of resorcinol and formaldehyde, followed by supercritical or freeze drying, and subsequent pyrolysis.These materials have high specific surface area and mesoporous structure and we consider that, in combination with zeolite, separation properties will be approved.
Carbon/zeolite composite is new material which posses properties of both zeolites and carbon.Surface of porous carbon materials is usually hydrophobic and surface of zeolites is hydrophilic and composite material obtained from these precursors combine these properties.Such new material is appropriate for adsorption of both organic substances and metallic ions from aqueous and gaseous phase.
Literature data show that carbon/zeolite composites usually consider dispersion of zeolites on surface of po-rous carbon [6−11].Only few studies report simultaneous synthesis of carbon/zeolite composites [12−14].Preparation of such composites is difficult due to lower stability of zeolites at higher temperatures than carbon.
In this paper we have presented a completely new method for preparation of carbon cryogel/zeolite composite.New composite materials were characterized by low-temperature nitrogen adsorption, X-ray diffraction and scanning electron microscopy.

Preparation of RF cryogel with zeolite
In the present work, resorcinol-formaldehyde (RF) gels with zeolite (Z) were synthesized by policondensation of resorcinol, (C 6 H 4 (OH) 2 ), with formaldehyde, (HCHO), according to the method proposed by Pekala et al. [1].Sodium carbonate, (Na 2 CO 3 ), was used as a basic catalyst.RF solutions were prepared from resorcinol, 99% purity (E.Merck) and formaldehyde, 36% methanol stabilized (Fluka Chemie), sodium carbonate, p.a. quality (E.Merck), and distilled and deionized water.The sodium form of synthetic zeolite LTA [15], manufactured by Union Carbide Co., was used as starting material.The cation exchange capacity (CEC) of this zeolite was 5.1 meq/g.Cation exchange operation was preformed as follows [16]: 15 g of the Na-LTA ze-olite was contacted overnight with 0.3 dm 3 of 0.21 M BaCl 2 solution at about 70°C.The solids were separated from liquids with filtration process and afterwards eight times contacted with the exchange solution.The results of atomic absorption spectrophotometer (ASS) by using a Perkin-Elmer 390 spectrophotometer show that the Na + content was less than 0.1 wt.%, so the Na + /Ba 2+ exchange can be considered as nearly complete.After exchange procedure the Ba-LTA zeolite was annealed up to 800°C for 1 h and its structural transformation to amorphous phase was confirmed by X-ray powder diffraction (XRD) analysis.
In all samples concentration of the starting solution was 20 wt.% and R/C molar ratio of resorcinol to catalyst was 100.To study the influence of the zeolite, the amount of zeolite was changed from 5 to 50 wt.%.The synthesis conditions are listed in Table 1.In samples notation zeolite content is used, for instance: CC + 5% Z, means carbon cryogel with 5 wt.% of zeolite.Suspensions with different amount of zeolite were decanted in glass tubes (inner diametar = 10 mm), sealed and placed 2 days at 25°C, 1 day at 50°C and 4 days at 85°C.RF cryogels with zeolite were prepared by freeze drying according to procedure of .RF gels were immersed in a 10-times volume of t-butanol, p.a. quality (Centrohem-Belgrade), for more than one day and rinsed to displace the liquid contained in the gels with t-butanol.The rinsing with t-butanol was repeated twice.The samples were prepared by freezedrying using Modulyo Freeze Dryer System Edwards, England, consisting of freeze dryer unit at High Vacuum Pump E 2 M 8 Edwards.Each of samples was pre-frozen in deep-freeze refrigerator at −30°C for 24 hours.After that, they were freeze dried in the acrylic chambers with shelves arrangements mounted directly on the top of the condenser of Freeze Dryer.The vacuum during twenty hours of freeze-drying was around 4 mbar.
Carbon cryogels/zeolite composites were prepared by carbonization of the cryogels in a conventional furnace, in nitrogen flow, at 800°C, and after pyrolysis, the furnace was cooled at room temperature.

Characterization of carbon cryogel/zeolite composites
Adsorption and desorption isotherms of N 2 were measured on carbon cryogels at −196°C using the gravimetric McBain method (laboratory set-up).The specific surface area, S BET , pore size distribution, mesopore including external surface area, S meso , and micropore volume, V mic , for the samples were calculated from the isotherms.Pore size distribution was estimated by applying BJH method [17] to the desorption branch of isotherms and mesopore surface and micropore volume were estimated using the high resolution α s plot method [18−20].Micropore surface area, S mic , was calculated by subtracting S meso from S BET .Crystalline phases were identified by X-ray diffraction (XRD) using filtered Cu Kα radiation (Siemens D5000).The microstructural study and energy dispersive analysis of X-rays (EDS) were performed on samples with Au coating by JEOL 6300F scanning electron microscope (SEM).

Adsorption isotherms -BET experiments
Nitrogen adsorption isotherms, as the amount of N 2 adsorbed as function of relative pressure at −196°C, are shown in Fig. 1.According to the IUPAC classification [21] isotherms are of type IV and with a hysteresis loop which is associated with mesoporous materials.Figure 1 shows samples with different wt.% of zeolite.Specific surface areas calculated by BET equation, S BET , are listed in Table 2. S BET values, for all samples, lie within 334−593 m 2 /g.It can be seen that S BET decrease with increasing the amount of zeolite in carbon cryogel sample.
Pore size distribution (PSD) of carbon cryogel samples with different amount of zeolite, shown in Fig. 2,  P r / P 0 confirm that samples are mesoporous with most of the pores radius between 2 and 5 nm.Samples with 10 and 15 wt.% of zeolite are not presented in Fig. 2 because PSD for these samples is very similar to PSD for CC + 5% Z sample.α s plots, obtained on the basis of the standard nitrogen adsorption isotherm, are shown in Fig. 3.The straight line in the medium α s region gives a mesoporous surface area including the contribution of external surface, S meso , determined by its slope, and micropore volume, V mic , is given by the intercept.Calculated porosity parameters (S meso , S mic , V mic ) are given in Table 2.For the samples with different weight ratio calculated parameters are the function of the amount of zeolite.Mesopore surface decreases with increasing the amount of zeolite.
From the data in Table 2, and by comparison with data for carbon cryogel without zeolite [22] it can be concluded that addition of zeolite (in lower wt.%) in carbon cryogel does not significantly change the specific surface area and porous structure of carbon cryogel.The higher amount of zeolite leads to decreasing of the specific surface area and the mesoporosity of the samples.
In samples with low zeolite content, the amount of new phase is too small for significant influence on structure of carbon cryogel.As the amount of zeolite increases (material with lower specific surface area), the overall specific surface area decreases.Also, with increasing the amount of zeolite in samples, its particles can penetrate into the structure of carbon cryogel at the moment when that structure is established and close certain number of pores.Decrease in specific surface area is mostly connected with the decrease in mesoporous surface area, Table 2, probably because zeolite easily penetrate in pores with larger dimensions.

XRD analysis of carbon cryogel/zeolite composites
Figure 4 show the XRD patterns of: a) initial ion exchange BA-LTA zeolite b) Ba-LTA zeolite after heating at 800°C. Figure 5 shows the XRD patterns of carbon cryogel samples with different amount of zeolite.Comparison of XRD patterns in Figs. 4 and 5 showed that characteristic peaks of the Ba exchanged zeolite exist in diffractograms for samples with higher amount of zeolite which confirm that zeolite structure was not destroyed during the carbonization process.
On the other hand, the XRD patterns show broadening of the diffraction lines in the regions at low 2θ which is associated with the amorphous structure of carbon material (carbon cryogel).The reduction in the peak intensity could be explained by the higher degree of pore filing of the zeolite with the carbon molecules.Increasing carbon/zeolite weight ratio enhances the tight packing of carbon cryogel molecule inside the pore channels of zeolite.

IV. Conclusions
A novel method for synthesis of carbon cryogel/ zeolite composites was demonstrated.Samples with different ratio of carbon cryogel and zeolite were obtained.Nitrogen adsorption showed that the structural properties such as specific surface area, mesoporosity and microporosity can be controlled by the amount of zeolite in the sample.XRD analysis confirms that zeolite structure was destroyed during the carbonization process in samples with lower amount of zeolite.With increasing the amount of zeolite in samples, structure of zeolite was kept and dimensions of crystallites increased.This confirms that, with corresponding ratio of carbon and zeolite, we can obtain samples with hydrophobic and hydrophilic properties.With SEM and EDX analyses we have shown that zeolite is uniformly distributed throughout the carbon cryogel and mass ratio between elements corresponds to the desired composition.with different ratio of carbon cryogel and zeolite were obtained.Nitrogen adsorption showed that the structural properties such as specific surface area, mesoporosity and microporosity can be controlled by the amount of zeolite in the sample.XRD analysis confirms that zeolite structure was destroyed during the carbonization process in samples with lower amount of zeolite.With increasing the amount of zeolite in samples, structure of zeolite was kept and dimensions of crystallites increased.This confirms that, with corresponding ratio of carbon and zeolite, we can obtain samples with hydrophobic and hydrophilic properties.With SEM and EDX analysis we have shown that zeolite is uniformly distributed through out the carbon cryogel and mass ratio between elements corresponds to the desired composition.

Figure 1 .
Figure 1.Nitrogen adsorption isotherms, as the amount of N 2 adsorbed as function of relative pressure for carbon cryogel samples with different amount of zeolite: a) 5, b) 10, c) 15, d) 20 and e) 50 wt.%(solid symbols -adsorption data, open symbols -desorption data)

Figure 3
Figure 2. Pore size distribution (PSD) for carbon cryogels with different amount of zeolite (5, 20 and 50 wt.%) /zeolite composite SEM image of CC + 20% Z carbon cryogel/zeolite composite is shown in Fig. 6.Typical image shows that zeolite is distributed uniformly through out the carbon cryogel.The EDS spectrum of CC + 20%Z sample is shown in Fig. 7.It was found that sample mainly consisted of C, O, Al, Si and Ba elements.The mass ratios of C : O : Al : Si : Ba were 84.52 : 12.63 : 0.46: 0.74 : 1.65.Indexing of the reflections on the EDS spectrum was compared with literature data [23] and JCPDS card No. 47-0001.