Silica and alumina based functional materials: Substructures, adsorption and gas chromatographic properties

Accepted Manuscript Silica and alumina based functional materials: substructures, adsorption and gas chromatographic properties V.I. Zheivot, E.V. Parkhomchuk, K.A. Sashkina, E.A. Melgunova, V.I. Zaikovskii, E.M. Moroz PII: DOI: Reference:

S1387-1811(14)00530-7 http://dx.doi.org/10.1016/j.micromeso.2014.09.027 MICMAT 6772

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Microporous and Mesoporous Materials

Received Date: Revised Date: Accepted Date:

9 August 2014 4 September 2014 9 September 2014

Please cite this article as: V.I. Zheivot, E.V. Parkhomchuk, K.A. Sashkina, E.A. Melgunova, V.I. Zaikovskii, E.M. Moroz, Silica and alumina based functional materials: substructures, adsorption and gas chromatographic properties, Microporous and Mesoporous Materials (2014), doi: http://dx.doi.org/10.1016/j.micromeso.2014.09.027

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Silica and alumina based functional materials: substructures, adsorption and gas chromatographic properties V.I. Zheivota, E.V. Parkhomchuka,b,c,*, K.A. Sashkinaa,c, E.A. Melgunovaa, V.I. Zaikovskiia,b, E.M. Moroza a – Boreskov Institute of Catalysis SB RAS, 5 Lavrentieva st., Novosibirsk 630090, Russia b – Novosibirsk State University, 2 Pirogova st., Novosibirsk 630090, Russia c – Research and Education Center, NSU, 2 Pirogova st., Novosibirsk 630090, Russia * – corresponding author, [email protected] Abstract Modification of nano- and adsorption textures of alumina and silica as well as study of their chemical surface properties have been made. The use of specific templates as structure directing agents allowed varying their textural characteristics and obtaining macroporous and mesoporous functional materials with preferred influence of the substructures on the chromatographic properties of the materials. As packings of chromatographic columns structured materials gave an opportunity to carry out gas chromatographic analysis of mixtures faster than their unstructured analogues. In some cases, the efficiency of columns packed by such materials was just over than those packed by nontemplated counterparts. Modification of silica and alumina by carbonization allowed varying the chemical nature of initial surface and expanding the number of functional materials based on them. It was shown that adsorption potential of dispersion (non-specific) interaction of n-paraffins with carbonized silica and alumina was much higher compared to that of the noncarbonized materials. Some examples of successful application of the studied materials in practical gas chromatography are presented. Keywords: silica, alumina, ordered meso/macroporosity, carbonized materials, gas chromatography 1. Introduction Silica and alumina have found a well-defined, but rather limited use in molecular gas chromatography [1]. This is primarily due to their large specific surface area defining a limited number of substances, as a rule, light hydrocarbons, being analyzed [1,2]. Besides, the existence of geometrical nonuniformity, often caused by the method

of material preparation, leads to the elution of chromatographic peaks in the band tailing form. Thermochemical activation and hydrothermal treatment used sometimes don’t lead generally to significant improvement of the oxides texture A final materials have pores with uncontrollable size and often a broad size distribution [2,3]. Modern technologies of synthesis of porous adsorbents and supports, using primarily templates as structure directing agents and varying the preparation conditions, allow obtaining controllable structural and adsorption textures of silica and alumina with predominant contribution of meso- [4-6] or macroporosity [7-10]. Surfactants are used as templates for mesostructure producing and latex – for macroporosity obtaining. Regular texture in both cases is obtained by filling the space between template particles by the precursor solution, which subsequently forms a solid skeleton as a template replica. Among the materials with controllable adsorption texture mesoporous mesophase material should be mentioned. They represent a specific class of materials with a solid state, defined as the mesophase, representing an intermediate state between the amorphous and crystalline phases [4-6]. Their adsorption texture is typical for mesoporous materials. Many materials, including silica, alumina, zeolites, have been synthesized which have in addition to the typical texture an additional system of regular transport macropores [7-10]. Interest to the study of the templated silica and alumina, in particular by molecular gas chromatography, is based on the assumption of availability of improved adsorption and chromatographic properties, required in practical gas chromatography, associated with their well-organized substructure compared with the unstructured counterparts. In addition, ideas existing in chromatography about a predominant influence of the alumina surface hydroxyl groups on to the interaction with adsorbates, in particular, unsaturated hydrocarbons [11,12], turned out to be at least incomplete. Moreover the statement, that the alumina has less polarizing effect than silica has [11], is suspected to be also disputable. In this regard the present work is devoted to the study of the following aspects: a) an intermolecular interactions between adsorbate and adsorbent, where the silica and alumina were used as adsorbents in the conventional forms and modified by different ways including template method and carbonization; b) the texture, morphology and phase properties of these functional materials using different techniques with the view of their influence on the chromatographic properties; and c) the extended possibility of modified silica and alumina application in practical gas chromatography.

2. Experimental Templated mesoporous mesophase silicate and macroporous silicas and alumina as welll as their unstructured counterparts have been synthesized. Synthesis methods are given below. The spectrum of silica- and alumina-containing materials was broad, in particular silica materials had a wide range of specific surface area beginning from 28.3 m2·g-1 for macroporous silica (SiO2–M) up to 1086 m2·g-1 for aerogel (Table 1). The last one was synthesized by drying of the precursor gel in the supercritical ethanol according with [13]. Also silipor -200 (Czechoslovakia) with a particle size of 0,25 - 0.5 mm was investigated. Reference samples of γ-Al2O3 and δAl2O3 were used to compare them with synthesized templated alumina samples. Moreover alumina type A-2, produced by the domestic industry, was also used which didn’t contain the structure of macropores [12]. Additionally carbonized samples of silipor-200, aerogel as well as alumina A-2 were obtained. 2.1. Preparation of mesophase mesoporous silicate materials (SBA-15) Mesoporous mesophase silicate material (SBA-15) was prepared as described in [14] by precipitation of sodium silicate as a source of silica in the presence of a structuredirecting agent Pluronic P123, which has a linear structure of the block copolymer (EO)20 (PO)70 (EO)20. The mixture was subjected to hydrothermal treatment, then filtered, rinsed with distilled water, dried in air and calcined at 550 °C. Surface area of mesopores for this sample was 865 m2 g-1. 2.2. Preparation of templated silica and alumina The first stage of templated inorganic material synthesis involves the preparation of polystyrene (PS) spheres as a template. The PS spheres were synthesized by emulsion polymerization, as described previously [15,16]. Polymer suspension was centrifuged for 12 hours at a relative centrifugal acceleration of 160 g. The precipitate was dried at room temperature to produce a template. For further syntheses templates consisting of PS spheres with a size of 250 nm were used. Templated amorphous silica (sample SiO2-M) was prepared by mixing 5 g of silica gel with a particle size of 20 nm, 0.05 g of ethylene glycol, 5 g of the PS template, and 10 mL of 1 M NaOH. The paste obtained was molded in the form of small beads, which were dried in air at room temperature. Polymeric template then was removed by calcination of beads for 6 hours in air at 550 °C. Excess of sodium hydroxide in the past were removed by rinsing the product with distilled water and then beads were dried in air at room temperature. The beads were then crushed and sieved to obtain a fraction of

0.18-0.45 mm. Reference sample of nontemplated silica (sample SiO2-NM) of the same composition was prepared by the same way described above but in the absence of the PS template. Samples of amorphous silica originated from silipor-200 were prepared by mixing 10 g of silipor-200, 0.1 g of ethylene glycol, 15 mL of 1 M NaOH in the absence (sample silipor-200-NM) and in the presence of 5 g of PS template (sample silipor-200-M) . Further synthesis procedure was similar to that described above. Templated alumina (sample Al2O3-M) was prepared in the presence of PS template by mixing 10 g of dry pseudoboehmite powder with 7.5 g of PS template, adding to the mixture of 3 ml of distilled water, acidified with a few drops of HNO3, and kneading the paste for 30 minutes. A reference sample of alumina (sample Al2O3-NM) was prepared in the absence of PS template under the similar conditions. The mixtures were extruded to yield cylindrical pellets 2 mm × 5 mm, which were dried in air overnight and calcined in air at 900 °C for 4 h at a heating rate of 100 °C/h The pellets then were crushed and sieved to obtain a fraction 0.25-0.50 mm. 2.3. Preparation of carbonized aerogel, silica and alumina Silica silipor-200 and industrial γ-alumina type A-2 were modified by a layer of carbon deposition during the catalytic decomposition of propane-butane mixture. To ensure a uniform deposition of carbon on the surface, the reaction was performed in the kinetic region when the rate of decomposition was determined by the rate of the proper reaction and not by the rate of hydrocarbon delivery to the surface [17]. The amount of carbon in the prepared samples was determined by the weight loss in the combustion process at 923 K in a stream of oxygen diluted with argon. Two samples C/silipor-200 and C/alumina A-2 were prepared with 39.3 wt. % C and 31.5 wt. % C, respectively. The carbonized aerogel was obtained by decomposition of propane-butane mixture in a nitrogen atmosphere. Adsorption properties of silipor-200 and alumina A-2, initial and carbonized, were compared also with those of the nonporous graphitized thermal carbon black (GTCB) and microporous carbon molecular sieve (Carbosphere) obtained according to [18]. 2.4. Physico-chemical characterization of prepared materials Low-temperature nitrogen adsorption/desorption measurements were conducted with an automated ASAP-2400 instrument (Micromeritics, USA). Specific surface area of the sample (SBET) was determined by the BET-equation. The total volume of pores (Vtotal) was calculated from their maximum filling point. The volume of the micropores (Vµ) was determined by comparative method [19]. Mesopore size distributions was estimated

from nitrogen adsorption isotherms by BJH method. Mercury porosimetry for alumina pellets before crushing was carried out on an AutoPore IV 9500 porosimeter (Micromeritics). Scanning electron microscopy (SEM) images were taken with a JEOL JSM6460LV microscope at an operating voltage of 15–20 kV. Transmission electron microscopy (TEM) images were obtained by a JEOL JEM-2010 microscope operating at 200 kV with resolution of 0.14 nm. Powder X-ray diffraction (XRD) patterns were recorded by ARL X’TRA diffractometer equipped with a Cu Kα radiation (λ = 0.154 nm) at scanning range 10° - 60° and 10° - 75° for silica and alumina, respectively, scanning step 0.1° and storage time 5 s. 2.5. Gas chromatographic experiments Gas chromatography properties of the materials taken were studied using Tsvet chromatograph equipped with catarometer (TCD) and flame ionization detector (FID). The carrier gas was helium. To improve its purity filters were used to remove water and oxygen traces. The carrier gas flow rate was 30 cm3 min-1. Each of adsorbents (0.25-0.5 mm fraction) was packed into stainless steel chromatographic column. It was conditioned in a special thermostat during 8 hours in the argon stream. Light hydrocarbons, saturated and unsaturated, were taken as an adsorbate molecules. The basis for this choice was connected with peculiarities of materials studied and mainly with its high specific surface areas. In addition, the elution order of such adsorbates often allows making a conclusion about the chemical surface nature of adsorbent studied. The retention data of adsorbates, retention time (tR) and specific retention volume (Vg), were evaluated according to the following relationship [1]: Vg = (t R − t M ) F0 (TC / T0 ) j / W ,

(1)

where tR – retention time of the investigated sorbate (min); tM – retention time of an unretained sorbate (air for TCD and methane for FID) (min); F0 – volumetric flow rate of the carrier gas (helium) at column outlet and ambient temperature (cm3 min-1); TC – column temperature (K); T0 – ambient temperature (K); W – mass of the adsorbent in the chromatographic column (g); j - compressibility correction factor, given by the following equation: 2

3 ( Pi / Po ) − 1 , j= 2 ( Pi / Po )3 − 1

where Pi and P0 – carrier gas pressure at the inlet and outlet of the chromatographic column, respectively (mm Hg). Zero samples were taken to work at the conditions when interactions between molecules of adsorbate could be neglected and where the retention time depends only on the presence of active centers on the material surface and consequently interactions of adsorbed substance and adsorbent mainly occur. In order to detect such small samples a FID was used. Moreover, temperatures were chosen to obtain symmetrical chromatographic peaks of adsorbates for which their retention times were practically independent of the sample size. In such conditions, retention values, evaluated practically at a zero-sized of the sample, equal to adsorption equilibrium constant, K a / c , from Henry's isotherm: Vg = K a / c [1], where K a / c = a / c ; a – the proportion of the adsorbed component relative to 1 g of the adsorbent studied; c – the proportion of a component in equilibrium gaseous phase relative to the unit of it volume. In addition, the measured temperature intervals were changed in the narrow-range whose variations depended on the adsorbent taken: silipor-200 was investigated at the temperatures varying from 100 °C to 150 °C, alumina – at the 135–165 °C temperature interval, silipor-200 and alumina, both modified by carbon, – at the 190–260 °C and 220–270 °C, respectively. The retention data of adsorbates were individually measured 5-7 times at each temperature (4 - 5 temperatures). At these conditions the corresponding differential change in internal molar energy, ∆U1 , equal to an initial differential adsorption heat (at zero surface filling), was determined from the dependence of lg Vg on 1 / T [1]: ∆ U1 = R

d ln Vg d (1/ T )

,

where R = 8.314 J mol-1 K-1 – Boltzmann constant. 3. Results and discussion 3.1. Textural and phase properties of silica and alumina materials according to SEM, TEM and XRD analyses The use of templates as structure-directing agents leads not only to the formation of meso- or macroporous structures depending on the template size during the material synthesis but also to the ordering of their substructures. Electron micrographs obtained by SEM (Fig.1a) and TEM (Fig.1b) of the sample SBA-15, produced with the use of Pluronic P123, are an evidence of a highly ordered substructure, practically whole

sample being regularly mesoporous. The material consists of particles having different shapes penetrated by channels with diameter of 10 nm (Fig.1b). Shapes of spheres and bent plates are predominant, polygonal shape can be observed in some cases. The channels are ordered in cubic substructure. Small disordering of the channels structure is also observed for some particles. According to low-temperature nitrogen adsorption data mesopore surface area of this sample is 85 % of the specific surface area determined by the BET method. The micropores are practically absent in this sample, their volume is 0.08 cm3 g-1 (Table 1). Investigations of low-temperature nitrogen adsorption on aluminas show that both samples, obtained in the presence and absence of PS template, are typical mesoporous adsorbents of type IV nitrogen adsorption isotherms with a characteristic H3-type hysteresis loop (Fig. 2) [19]. Meanwhile, the average mesopore diameter of Al2O3-NM is 10.4 nm and that of templated sample Al2O3-M is 12 nm (Fig. 2). No more differences in the textures can be observed by nitrogen adsorption but additional macropores with an average size of 113 nm are detected for Al2O3-M by mercury porosimetry, the contribution of macropores being 35 % to the total pore volume (Table 1). Mercury porosimetry data of these two samples are presented in more details in [20]. Micropores are absent in the both alumina samples. In addition, images of SEM shown in Fig.4 demonstrate a fairly uniform substructure formed in the sample Al2O3-M when PS template was used compared with nontemplated sample. Moreover the first sample has spongy morphology with a lot of macropores on its surface, diameters of whose are about 1 µm, while the last one has a relatively smooth surface (Fig. 3). TEM analyses of the sample Al2O3-M show that spongy morphology of the sample is formed by particle aggregates (Fig.4). The last ones are organized into solid or broken spherical shells, 200 nm and more in diameter and 20-50 nm thick. Closings between spheres are hardened and 100 nm in size. At the same time a primary structure of aggregates consists of spliced laminar nanoparticles or “needles” of alumina with the sizes 30-50 x 5-10 x 10-20 nm. The size of mesopores is determined by the needles packing and varied from 1 nm to 10 nm according to TEM, which coincides with nitrogen adsorption data. As for phase composition of aluminas, the structure of low-temperature γ- Al2O3 is close to spinel-type one [21]. Its elementary cell is formed by cubic closest packing of 32 oxygen atoms, in which metal atoms occupy 16 octahedral and 8 tetrahedral positions. According to an electroneutrality, part of these positions is vacant with their statistical distribution. Moreover part of oxygen atoms displaced with hydroxyl groups.

As one can see from X-ray patterns, placed in Fig. 5a, the phase composition of two synthesized aluminas are very similar to each other and close to γ- Al2O3. At the same time, there are some elements of δ-Al2O3 structure in these samples of aluminas. Comparative patterns of aluminas, Al2O3-NM and Al2O3-M as well as γ-Al2O3 and δAl2O3, give evidence that synthesized samples have intermediate state between γ-Al2O3 and δ-Al2O3 structures or represent a mixture of these two phases. Investigations of silica samples, initial and templated, using the XRD method show both samples to be similar to each other and to be X-ray amorphous with the size of coherent scattering region less than 2.5 nm (Fig. 5). Similar to the macroporous alumina PS template allows preparing silica samples (SiO2-M and silipor-200-M) with the more looser structure, as one can see from SEM (Fig.6a,7a), than that of nontemplated counterparts presented in Fig. 6b, 7b. According to high-resolution electron microscopy the sample SiO2-M is macroporous. Morphology of the sample is formed under the strong influence of the PS template. A presence of spherical surfaces of 50 nm in radius (Fig.8a) is recorded. One can reasonably suggest that its porous structure is formed as a result of SiO2 aggregating in free space between contacting of PS spheres. Silica material replicates geometric forms of these vacancies (tetrahedrons and octahedrons) after the template removal, producing macropores in the sample SiO2-M. The presence of nanosized pores close to 5 nm are also observed in both SiO2-M and SiO2-NM samples. Contrary to the sample SiO2-M, aggregated isometric nanoparticles (20-30 nm) are observed in the sample of silipor-200-M. They are amorphous and accretions between them are 10-20 nm thick. Compared with the sample SiO2-M mesopores in silipor-200-M have approximately the same dimensions. As for the quantitative characteristics of the silipor-200 adsorption textures, they don’t practically change according to the low temperature N2 adsorption data (Table 1). 3.2. Gas chromatographic properties of silicas and aluminas with different textures Fig. 9 – 13 show separations of light hydrocarbons, also liquid ones, cyclic (saturated and unsaturated) and aromatic, obtained on the materials synthesized using the templates, as well as their nontemplated counterparts. The comparison of two chromatograms presented in Fig.9 shows that hydrocarbons C1 – C4 are eluted from the column packed with Al2O 3-M by the more symmetrical peaks than from that packed with the Al2O3-NM, while these both samples are characterized by practically the same pores distribution upon their size (Fig. 2). But

the presence of transport macropores in the case of templated alumina, discussed above, makes probably easier the adsorption – desorption processes of adsorbed molecules there providing the elution of the adsorbed molecules from the column with Al2O3 - M in the form of practically symmetrical peaks. This results in the column packed with macroporous alumina to have a separation capability, for example, of a propylene – butane pair, as shown in Fig..9b, well over than that of the column with an untemplated alumina presented in Fig..9a. It should also be noted that the gas chromatographic analysis of light hydrocarbons on a column of macroporous alumina takes approximately 2-fold less time than nontemplated analogue does. Almost symmetrical chromatographic peaks were obtained, as expected for mesoporous mesophase silicate material SBA-15 (Fig. 10). So, the tailing factor of the propylene peak, evaluated as a ratio between frontal width of this peak at 0.1 hight and tail width of peak at o.1 hight is 0.9. The width of peak is measured by extrapolating the relative straight sides to the baseline. A tailing factor is 1.0 for symmetrical peak [21]. Macroporosity of templated silica, for example, SiO2 - M, packed in a column 2 m × 2 mm, characterized by low value of its specific surface area enables to carry out a fast (in less than 1 min)

chromatographic analysis

of benzene, cyclohexene and

cyclohexane which are the catalytic reaction products of benzene hydrogenation (or dehydrogenation of cyclohexane) (Fig. 11). The greater uniformity of substructures of macroporous materials results in some cases in the more efficient chromatographic columns, having the more separation capability compared to that of the column with nontemplated samples. For instance, the efficiency (the number of effective plates) of the chromatographic column with templated silipor-200-M, estimated according to [11] for the peak of n-butane, is 870 t.p./m (Fig.12b), while the column with its nontemplated analogue silipor-200-NM has an efficiency equals to 750 t.p./ m (Fig. 12a). The following Fig. 13 shows the comparative chromatograms of separation of saturated and unsaturated hydrocarbons C1 – C4, which are the product of many catalytic reactions, obtained on the initial and modified silipor-200. The greater separation power of the column with templated silipor-200-M, which allows separating not only the pairs of ethylene/propane and n-butane/propylene, but also cis-and trans-butene one should be noted. In addition, this separation proceeded in an isothermal mode of the column and during the shorter time than the separation on the nontemplated silipor-200, which was carried out by the temperature column programmed from 25°C with a 10-min delay to 120 °C (Fig. 13).

Table 2 presents the data on the retention of light hydrocarbons on the surface of silicas and aluminas, including templated ones. These data indicate that the materials taken are, as was expected, the adsorbents of II-type. They are characterized by the well-defined elution order, for example, of saturated and unsaturated hydrocarbons with the same number of the carbon atoms in the molecule, associated with the presence of an excess positive charge on their surface [1]. Thus, unsaturated hydrocarbons whose molecules have an excess electron density are eluted from such sorbents after the saturated ones. The higher the unsaturation in the sorbate molecule the stronger molecules are retained on the sorbents. So, ethylene is retained on all silicas and aluminas stronger than ethane is, and acetylene - significantly stronger than ethylene is. However it should be noted that for all silicas used (silica, SBA-15 and aerogel), despite the fact that their specific surface areas differ by the order, the relative (with respect to n-propane) retention of acetylene is about the same and is close to unity. This points to the same properties of the surface unit and indicates the uniformity of the intermolecular forces of the sorbates and sorbents interaction. They are mainly determined by the specific adsorption centers of silica surface related, as it is known, with the presence of protonized hydroxyl (silanol) groups [1,11,12]. The observed differences in the relative retention of acetylene within the class of silica materials (silica, aerogels and mesoporous materials) are probably due to a different content of silanol groups on the surface. The greater quantity of the groups is located, probably, on the surface of the conventional silipor-200, for which the value of the relative acetylene retention is maximum and equals to 2.5. Meanwhile, there is a contradictory opinion of some authors in chromatography about the nature of intermolecular interactions in the system sorbate - alumina [1,11,12], associated, in particular, with the approval of the predominance of the influence of the surface hydroxyl (thermal) Al-OH groups on these interactions [11,12]. The statement that alumina has less polarizing effect than silica has [11] is suspected to be also controversial. In fact, on all silicas acetylene is eluted before propylene is, while on both aluminas, initial and templated samples, it is eluted after propylene and even after nbutane. Relative retention data of acetylene on aluminas of different textures, listed in the Table 2, being approximately the same value, is order of magnitude higher than those on silicas, and suggest a different origin of these positive charges influenced on the intermolecular interactions of unsaturated hydrocarbons with the surface of these oxides. Along with the surface Al-OH hydroxyl groups, a presence of Al-aprotic acid centers, characterized by their electronic unsaturation, predominantly influence on the

intermolecular interactions, in particular, of unsaturated hydrocarbons with the surface of the aluminas [1]. The presence of these two types of specific interaction centers does provide the stronger retention of sorbates, in particular, of unsaturated hydrocarbons, compared to their retention on the surface of silicas, composed mainly of weaker silanol groups. Note, the temperature enhancement of the column, containing silica, leads to a weakening of the intermolecular interaction between the partially protonized surface hydroxyl groups and molecules that possess an excess electron density [2]. This results in retention decrease, in particular, of acetylene on a column with silipor-200 in a greater extent than that of propane and consequently to a reduction of the relative acetylene retention at high column temperature. As the data of Table 3 show, Vgacet / V gpropane = 2.0 at the column temperature of 150 °C, while Vgacet / Vgpropane = 2.5 for the

same column, kept at a temperature of 50 °C (Table 2). This dependence is even more pronounced for Al2O3-M (Tables 2, 3) due to the more significant reduction of the acetylene retention compared to propane retention on the alumina adsorption centers, described above. 3.3. Adsorption and gas chromatographic properties of original and carbonized silicas and aluminas One of the ways to change the chemical nature of the surface of silicas and aluminas is forming of carbon layer on the surface [17, 22-25]. It should be noted that the surface of carbon materials is uncharged, and all of the sorbate molecules interact with it through the dispersion (non-specific) interaction. In turn, the dispersion interaction is associated with the electronic polarizability (α) of the sorbate molecules, and for hydrocarbons, for example, it depends on the number of hydrogen atoms in their molecule [1]. This interaction, and thus the retention of sorbates is the stronger the more hydrogen atoms in the hydrocarbon molecule. So, for carbon materials, unlike silicas and aluminas, retention of saturated hydrocarbons are stronger than that of unsaturated ones with the same number of carbon atoms. For example, acetylene is eluted from them earlier than ethylene and ethane [1,18,26,27]. Generalizing conclusions from facts obtained now and earlier [18], a conclusion may be arrived that the chemical nature of carbon surface is a factor combining all of the carbon-containing materials. In this connection, there is of the special interest to compare adsorption and gas chromatography properties of silica and alumina carbonized with those of initial ones and real carbons according to their capacity for non-specific interactions with adsorbates. Thus, the data of Table 3 show that acetylene, which is retained on the silipor-200 substantially stronger than

ethylene, is eluted from carbonized silipor-200 eairlier than ethylene, as it is typical of all the carbon graphite-like materials. But any texture, including microporous or non-porous ones, does not practically important in this case. We took carbosieve (Carbosphere) and GTCB as real carbons to compare its carbon properties with those of carbonized alumina and silica. Indeed, its elution order on silipor-200 modified by the high carbon content (39.3 wt.%) is the same as, for example, both on microporous carbpsphere and non-porous carbon such as graphitized thermal carbon black (GTCB). This indicates that the surface of silipor-200 is, apparently, fairly full covered by carbon. At the same time, due to incomplete carbonization of alumina and aerogel surfaces (acetylene is eluted from them after ethylene) their carbon properties are performed in the less extent. The electron microscopy analyses of alumina coated by 31.5 wt % C showed that the surface of the alumina “needles” (5 nm x 20 nm) were coated by the carbon layer of thickness 1.5 – 2.0 nm. Moreover, there were loose carbon deposits of some oversize dimensions (up to 5.0 nm) and alumina sites uncoated by the carbon. The presence of the last ones together with the carbon coating determined the combination properties of these both materials. Tables 4 – 7 show the values of the specific retention volumes (Vg) of saturated C4 - C7 hydrocarbons obtained at various temperatures and values of the differential molar changes in internal energy, - ∆Ū1, at their adsorption onto initial and carbonized silicas and aluminas. The dependences of lgVg on 1/T are linear for taken paraffins in the above temperature range, with a correlation coefficient R = 0.999 – 0.996. Figure 14 shows the dependence of - ∆Ū1 values on the number of carbon atoms in n-paraffins molecules when adsorbed on the initial (curves 1 and 2) and carbonized (lines 4 and 5) samples of silica and alumina. For comparison, a similar dependences obtained earlier [18] for the graphite-like materials, such as nonporous graphitized thermal carbon black (curve 3), and microporous carbon molecular sieves (line 6) are shown. It is evident that adsorption potential of the dispersion (nonspecific) interaction of the carbon surface, including carbonized silica and alumina, is significantly higher compared with that for unmodified silica and alumina. Within the class of carbon adsorbents, it changes in accordance with their nano and adsorption characteristics and varied over a wide range. Among carbon adsorbents, carbon materials, the surface of which consists of the basal faces of the graphite crystal structure, exhibits the lowest adsorption potential of nonspecific interactions and the lowest retention values of adsorbates. Microporous carbon-containing adsorbents exhibit the highest values. Carbonized silica and alumina are intermediate in them.

4. Conclusions X-ray diffraction and electron microscopy, adsorption techniques and gas chromatography were used to study silicas and aluminas, initial and modified by different methods, including templated and carbonized. The chemical nature of the surface of all materials characterized by concentrated positive charge integrates them as a class of II-type adsorbents. In this case, the elution order, for example, of saturated and unsaturated hydrocarbons is determined not only by the presence of this charge but also its origin. Among the adsorbents studied the highest retention of unsaturated hydrocarbons is observed on alumina due to aprotic acidic Al-centers along with the Al-OH surface hydroxyl groups. Templates

as

structure

directing

agents

allow

obtaining

a

sufficient

homogeneous texture of silica and alumina with controllable substructures. The more ordering of textures of such materials, as well as the formation of additional macropores lead to facilitation of adsorption – desorption processes. This results in the elution of substances from the column, packed with these materials, in the form of practically symmetrical peaks. Gas chromatographic analysis using templated macroporous materials based on silica and alumina can be performed faster than that using nontemplated analogues. Chromatographic columns based on templated macroporous materials have in some cases just over efficiency compared with those packed with nontemplated counterparts. Templated macroporous materials based on silica and alumina as the sorbents of II-type can be used in gas chromatography analysis not only of light hydrocarbons but also of liquid ones. Chemical nature of silica and alumina modified by the carbon showed the properties of carbon or combination of both. The adsorption potential of nonspecific (dispersion) interactions of silica and alumina surfaces modified by the carbon is much higher than that of unmodified ones. Within the class of carbon adsorbents, it changes and varied over a wide range. The lowest adsorption potential of nonspecific interactions is observed for GTCB, whose surface consists of the basal faces of the graphite crystal structure. Microporous carbon-containing adsorbents exhibit the highest values. Carbonized silica and alumina are intermediate among them. Acknowledgements

The authors thank N.A. Rudina and D.A. Zyuzin for their help in the adsorbent characterization, A.F. Danilyuk for aerogel synthesis, and V.N. Krivoruchko for assistance in gas chromatographic analyses. The work was performed in the framework of the joint Research and Educational Center for Energy-efficient Catalysis (Novosibirsk State University, Boreskov Institute of Catalysis). The work was partially financed by RFBR grant 12-03-93116_a. References 1.

A.V. Kiselev, Ya.I.

Yashin, Adsorption Chromatography, Plenum Press, New

York, 1969. 2.

A.V. Kiselev. Intermolecular interactions in the adsorption and chromatography, Vysshaya shkola, Moskow, 1986.

3.

A.V. Kiselev, D.P. Poshkus, Ya.I. Yashin, The molecular basis of adsorption chromatography, Khimiya, Moscow, 1986.

4.

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck. Nature 359 (1992) 710–712

5.

D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky. Journal of the American Chemical Society, 120, 24 (1998) 6024-6036

6.

D. Gu, F. Schüth. Chemistry Society Reviews 43 (2014) 313-344.

7.

B.T. Holland, C.F. Blanford, A. Stein. Science, 281 (1998) pp. 538-540.

8.

P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B.F. Chmelka, G.M. Whitesides, G.D. Stucky. Science. 282 (1998) 2244-2246.

9.

. Y. Xia, B. Gates, Y.Yin, Y. Lu. Advanced Materials. 12, 10 (2000) 693-713.

10. A.Stein. Microporous and Mesoporous Materials, 44-45 (2001) 227-239. 11. D.A. Vyahirev, A.Ph. Shushunova. Manual for gas chromatography. M.: Vysshaya shkola, 1987, p. 170. 12. K.A. Gol'bert, M.S. Vigdergauz. Introduction to the gas chromatography. M.: Khimiya, 1990, p. 117. 13. A.F. Danilyuk, E.A. Kravchenko, A.G. Okunev, A.P. Onuchin, S.A. Shaurman. Nuclear Instruments and Methods in Physics Research A 433 (1999) 406-407 14. E.A. Melgunova. The synthesis of mesoporous materials using surfactant Pluronic P123 and the study of their texture. PhD thesis, 2010, Novosibirsk, Russia. 15. D. Zou, L. Sun, J.J. Aklonis, R. Solovey. Journal of Polymer Science: Part A; Polymer Chemistry 30 (1992) pp. 1463-1475. 16. E.V. Parkhomchuk, K.A. Sashkina, N.A. Rudina, N.A. Kulikovskaya, V.N. Parmon. Catalysis in Industry, 5/1 (2013) pp. 80-89.

17. M.E. Shalaeva, V.I. Zheivot, N.A. Prokudina, V.V. Chesnokov, V.V. Malakhov. Journal of Analytical Chemistry 51/6 (1996) pp. 560-564. 18. V.I. Zheivot. Journal of Analytical Chemistry. 61/9 (2006) pp. 832-852. 19. S.J.Gregg, K.S.W. Sing, Adsorption, surface area, and porosity. Academic Press, London. 1982. 20. E.V. Parkhomchuk, A.I. Lysikov, A.G. Okunev, P.D. Parunin, V.S. Semeikina, A.B. Ayupov, V.A. Trunova, V.N. Parmon. Industrial & Engineering Chemistry Research, 52, 48 (2013) 17117–17125. 21. J.C.P. Breckhoff, B.G. Linsen in B.G. Linsen (ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Chap. 1, Academic Press, New York, 1970. 22. V.I. Zheivot, E.M. Moroz, V.I. Zaikovskii, V.V. Chesnokov, V.Yu. Gavrilov. Journal of Chromatography, A. 786 (1997) 117-124. 23. A. Gierak, R. Leboda. Journal of Chromatography, A. 483 (1989) pp. 197-207. 24. V.Ya. Davydov, T.M. Roshchina, G.N. Filatova, N.M. Khrustaleva. Vestn MGU. Ser.2. Khimia 35 (1994) 411. 25. V.I. Zheivot, V.V. Chesnokov. Chromatographia 69 (2009) pp. 701-708. 26. V.I. Zheivot, V.V. Molchanov, V.I. Zaikovskii, V.N. Krivoruchko, N.A. Zaitseva, M.N. Shchuchkin. Microporous and Mesoporous materials, 130 (2010) pp. 7-13. 27. M.E. Shalaeva, V.I. Zheivot, V.B. Fenelonov, L. Friedman, V.V. Malakhov. Journal of Analytical Chemistry, 48/10 (1993) pp. 1608-1614.

Fig. 1. SEM (a) and TEM (b) images of the sample SBA-15. Fig. 2. Nitrogen adsorption/desorption isotherms (a) and mesopore size distribution estimated by BJH method (b) for silica and alumina samples. Fig. 3. SEM images of alumina samples: Al2O3-M (a) and Al2O3-NM (b), prepared in the presence and in the absence of PS template, respectively. Fig. 4. TEM images of templated alumina sample Al2O3-M. Fig. 5. XRD patterns of silica and alumina samples. Fig. 6. SEM images of silica samples: SiO2-M (a) and SiO2-NM (b), prepared in the presence and in the absence of PS template, respectively. Fig.7. SEM images of silica samples: silipor-200-M (a) and silipor-200-NM (b), prepared in the presence and in the absence of PS template, respectively. Fig. 8. TEM images of templated silica samples: SiO2-M (a) and Silipor-200-M (b). Fig. 9. Chromatograms of light hydrocarbons obtained at 70 oC on the 2 m x 2 mm columns packed with alumina samples: Al2O3-NM (a) and Al2O3-M (b). Fig. 10. Chromatograms of light hydrocarbons obtained at 50 oC on the 2 m x 2 mm columns packed with the sample SBA-15. Fig. 11. Chromatograms of the mixture of cyclohexane, cyclohexene and benzene, obtained at 140 oC on the 2 m x 2 mm columns packed with silica samples: SiO2-NM (a) and SiO2-M (b). Fig. 12. Chromatograms of light hydrocarbons obtained at 50 oC on the 2 m x 2 mm columns packed with Silipor-200 samples: Silipor-200-NM (a) and Silipor-200-M (b). Fig. 13. Chromatograms of saturated and unsaturated light hydrocarbons obtained: a) on the 2 m x 3 mm column packed with Silipor-200 at temperature column programmed from room temperature (10 min) to 120 oC and b) on the 2 m x 2 mm column packed with Silipor-200-M at a constant T = 50 oC. Fig. 14. Dependence of differential internal molar energy of n-paraffines adsorption on silica and alumina, unmodified and carbonized by the large amount of carbon, on the number of carbon atoms in a paraffine molecule.

a)

b)

Fig. 1. SEM (a) and TEM (b) images of the sample SBA-15.

3500 3000

300

SBA-15 Aerogel

250

-1

4000

b) Al2O3-M

dV/dD, cm3 g-1 nm

Volume adsorbed, cm3 g-1 STP

a)

Al2O3-NM

200

2500 2000

150

1500

100

1000 50

500 0 0,0 600 500

0,2

0,4

0,6

0,8

1,0

0,2

0,4

0,6

0,8

1,0

Silipor-200-M Silipor-200-NM

400

0,04

SBA-15 Aerogel

0,5

Al2O3-M Al2O3-NM

0,03 0,4 0,3

0,02

0,2 0,01 0,1

0 0,0 600

SiO2-M SiO2-NM

0,6

0,0

0,00 0

10

20

30

40

50

70

80

SiO2-M SiO2-NM

0,04

400

60

0

10

20

30

40

50

Silipor-200-M Silipor-200-NM 0,02

0,03

300

0,02 200

200

0,01 0,01

100 0 0,0

0,2

0,4

0,6

0,8

1,0

0 0,0

Relative pressure, p/po

0,2

0,4

0,6

0,8

1,0

0,00

0,00 0

10

20

30

40

50

60

70

0

20 40 60 80 100 120 140 160 180 200 220 240 260

Average pore diameter, nm

Fig. 2. Nitrogen adsorption/desorption isotherms (a) and mesopore size distribution estimated by BJH method (b) for silica and alumina samples.

a)

b)

Fig. 3. SEM images of alumina samples: Al2O3-M (a) and Al2O3-NM (b), prepared in the presence and in the absence of PS template, respectively.

Fig. 4. TEM images of templated alumina sample Al2O3-M.

Intensity, cps

Al 2 O3-M Al 2 O3-NM δ -A l2 O3 γ -A l 2O 3

SiO2-M SiO2-NM

500

400

300

200

100

0

10

15

20

25

30

35

40

45

50

55

60

65

70

75

10

2 θ , degrees

Fig. 5. XRD patterns of silica and alumina samples.

20

30

40

50

60

2θ, degrees

a)

b)

Fig. 6. SEM images of silica samples: SiO2-M (a) and SiO2-NM (b), prepared in the presence and in the absence of PS template, respectively.

a)

b)

Fig.7. SEM images of silica samples: silipor-200-M (a) and silipor-200-NM (b), prepared in the presence and in the absence of PS template, respectively.

a)

b)

Fig. 8. TEM images of templated silica samples: SiO2-M (a) and Silipor-200-M (b).

Fig. 9. Chromatograms of light hydrocarbons obtained at 70 oC on the 2 m x 2 mm columns packed with alumina samples: Al2O3-NM (a) and Al2O3-M (b).

Fig. 10. Chromatograms of light hydrocarbons obtained at 50 oC on the 2 m x 2 mm columns packed with the sample SBA-15.

26

Fig. 11. Chromatograms of the mixture of cyclohexane, cyclohexene and benzene, obtained at 140 oC on the 2 m x 2 mm columns packed with silica samples: SiO2-NM (a) and SiO2-M (b).

27

Fig. 12. Chromatograms of light hydrocarbons obtained at 50 oC on the 2 m x 2 mm columns packed with Silipor-200 samples: Silipor-200-NM (a) and Silipor-200-M (b).

28

Fig. 13. Chromatograms of saturated and unsaturated light hydrocarbons obtained: a) on the 2 m x 3 mm column packed with Silipor-200 at temperature column programmed from room temperature (10 min) to 120 oC and b) on the 2 m x 2 mm column packed with Silipor-200-M at a constant T = 50 oC.

29

Fig. 14. Dependence of differential internal molar energy of n-paraffines adsorption on silica and alumina, unmodified and carbonized by the large amount of carbon, on the number of carbon atoms in a paraffine molecule.

30

Table 1. Textural characteristics of silica- and alumina based materials, initial and modified, as well as carbons according to data of low-temperature N2 adsorption and Hg porosimetry.

Sample SBET, m2 g-1 Vtotal., cm3 g-1 Vµ, cм3 g-1

Al2O3NM

Al2O3-М

SiO2NМ

SiO2-М

Silipor-200NM

Silipor-200-M

Silipor-200

SBA-15

Aerogel

GTCB

Carbosphere*

136 0.36

187 0.55 (N2) 0.84 (Hg) 0

171 0.88

28.3 0.25

135 0.86

126 0.73

136 0.79

1017 1.19

1086 5.67

10 -

890 0.34

0

0

0

0

0

0.08

0.22

0

0.34

0

* – according to [27];

Table 2. Specific retention volume (cm3 g-1) of light hydrocarbons and retention volume of acetylene relative to n-propane obtained at 50 ° C for different silica and alumina materials

Aluminas Adsorbate Air Ethane Ethylene Acetylene Propane Propylene n-Butane Vgacetylene / Vgpropane

Al2O3-NM 1.55 3.69 67.2 6.73 24.1 28.2 10.0

Silicas

Al2O3-M 1.82 4.54 81.1 7.97 31.0 34.0 10.2

SiO2-NM

SiO2-M

Silipor-200-NM

Silipor-200-M

Silipor-200

Aerogel

1.27 1.75 4.22 4.22 7.88 12.9 1.0

1.09 1.22 3.66 3.82 7.44 11.1 1.0

0.93 1.23 2.06 3.49 5.53 10.3 0.6

0.96 1.39 2.29 3.39 6.40 10.1 0.7

0.97 2.49 8.63 3.52 13.5 2.5

7.05 8.88 34.5 29.3 52.7 1.2

SBA-15 12.7 23.5 51.9 66.9 191 0.8

Table 3. Specific retention volume (cm3 g-1) of light hydrocarbons and retention volume of acetylene relative to n-propane obtained for original and carbon-containing materials

Adsorbate Methane Ethane Ethylene Propane Acetylene Propylene Vgacetylene / Vgpropane

Silipor-200, 150 oC 0.22 0.39 0.55 1.10 1.12 2.0

* – according to [27];

C/Silipor-200, 150 oC 1.24 1.01 6.68 0.76 7.11 0.1

Aerogel, 50 oC 7.66 13.6 29.3 34.5 84.2 1.2

C/Aerogel, 50 oC 7.05 8.88 30.5 16.5 52.7 0.5

Al2O3-M, 70 oC 1.07 2.34 4.59 29.6 12.8 6.5

Alumina A-2, 100 oC 1.85 3.53 10.4 48.6 24.7 4.7

C/Alumina A-2, 100 oC 3.30 2.87 25.6 4.69 28.4 0.2

Carbosphere*, 100 oC 12.0 142 85.0 2200 47.6 1400 0.02

GTCB 86.2 oC 0.66 21.9 11.9 473 5.5 0.01

Table 4. Specific retention volume Vg of paraffins obtained at different temperatures and differential molar changes of internal energy, - Ū1, for the sample Silipor-200 Adsorbate n-Pentane n-Hexane n-Heptane

100 °С 5.41 11.2 22.6

Vg , cm3 g-1 108 °С 118 °С 128 °С 4.18 3.39 2.81 8.22 6.39 5.12 15.6 11.9 9.15

137 °С 2.37 4.21 7.28

- Ū1, kJ mole-1 27.9 33.0 38.1

Table 5. Specific retention volume Vg of paraffins obtained at different temperatures and differential molar changes of internal energy, - Ū1, for the carbonized sample C/Silipor-200 Adsorbate n-Butane n-Pentane n-Hexane

194 °С 11.2 -

212 °С 7.15 -

230 °С 4.96 18.2 64.8

Vg , cm3 g-1 239 °С 240 °С 4.29 13.6 49.1 -

249 °С 11.2 38.8

257 °С 9.61 29.8

- Ū1, kJ mole-1 42.5 53.4 64.3

Table 6. Specific retention volume Vg of paraffins obtained at different temperatures and differential molar changes of internal energy, - Ū1, for the sample alumina A-2 Adsorbate n-Pentane n-Hexane n-Heptane

135 °С 18.6 52.6 147

Vg , cm3 g-1 150 °С 155 °С 13.5 12.8 37.2 34.7 95.8 87.9

165 °С 10.9 27.7 71.1

- Ū1, kJ mole-1 26.3 31.2 35.9

Table 7. Specific retention volume Vg of paraffins obtained at different temperatures and differential molar changes of internal energy, - Ū1, on their adsorption on the sample C/alumina A-2 Adsorbate n-Butane n-Pentane n-Hexane

221°С 25.1

231°С 5.40 20.2

Vg , cm3· g-1 249°С 253°С 258°С 3.89 3.56 3.32 13.5 10.8 52.2 39.3

262°С 38.7

264°С 36.4

- Ū1, kJ mole-1 41.2 50.9 60.6

Highlights • Ordered porosity of adsorbents results in hydrocarbons elution as symmetrical peaks • Gas chromatographic analysis using templated macroporous SiO2 and Al2O3 is fast • Efficiency of columns on templated macroporous SiO2 and Al2O3 is high • Templated SiO2 and Al2O3 can be used in gas chromatography for liquid hydrocarbons • The adsorption potential of nonspecific interactions of C/SiO2 and C/Al2O3 is high

37