Effect of activation temperature on surface basicity of natural aluminosilicates

Montmorillonite and nontronite are layered aluminosilicates of smectite group minerals widely demanded in many industries owing to their unique physical-chemical and other properties. By thermal activation of raw clays there are variations in their porosity, surface area and physical-chemical properties, including formation and redistribution of surface active site of acid-base or redox character. The aim of present studies included investigation of the effect of thermal activation on the character of distribution and a number of basic sites on the surface of natural layered aluminosilicates by means of the new method of inverse thermoprogrammed desorption of СО2. 
Samples of natural aluminosilicates rich in montmorillonite (Montmorillonite 67%, illite 5%, quartz 5%, feldspars 21%) and nontronite (nontronite 70%, illite 10%, kaolinite 5%, quartz 10%, feldspars 8%) were characterized by XRD, XRF, BET N2 adsorption techniques. To probe surface basicity and determine the number of basic sites a new iTPD-CO2 was used. Prior the iTPD-CO2 measurement 100 mg of a sample was activated at 200, 300, 400oC, then cooled down and loaded with CO2 (3ml/min flow rate of CO2 for30 min). Next, the reactor was flushed by 5 ml/min N2-flow to desorb weakly sorbed CO2. The iTPD-CO2 profiles were recorded within 20-800oC at a 20oC/min heating rate and treated using ChemStation software. 
The experimental profiles of CO2 desorption for Mt and Nt samples observed two temperature regions. Low temperature peaks evolved around 80-90oC for Mt and between 110-127oC for Nt were most likely related to the weak basic sites, whereas high temperature peaks around 510 and 620oC for Mt and above 320oC for Nt testified to stronger ones. The reasoning of the obtained iTPD-profiles was done considering thermal behavior of layered aluminosilicates. 
The total basicity of Nt and Mt samples was 359.2 and 209.9 mmol/g respectively. The 1.6 times higher basicity of Nt was, obviously, caused by its phase and chemical composition and developed surface area and porosity. At higher activation temperatures the number of weak basic sites related to hydroxyl groups of adsorbed water molecules gradually decreased, namely, by 21 times for Mt and by 2.8 times for Nt. 
Dehydroxylation of structural Al-OH, Fe-OH, Mg-OH above 200oC, which becomes irreversible above 300oC, provided formation of residual oxygen atoms and their contribution to population of stronger basic sites. In accordance with thermal behavior of dioctahedral smectites, is assumed that strong basic sites of Mt are trans- and cis-vacant Al-OH groups dehydroxylating correspondingly at ~550 and 650oC. Fe-rich sample of Nt rapidly lost hydroxyls at rather lower temperatures that resulted in more heterogeneous distribution of strong basic sites of varying strength. At higher activation temperatures the ratio of stronger sites number to weak sites increased from 23 to 200 for Mt, whereas for Nt this ratio varied between 54-67 times. In general, total basicity of Mt and Nt decreased by 2.2-2.3 times as a result of their dehydration and dehydroxylation by thermal activation. The normalized values of basicity per unit surface area (BΣ/S, mmol/m2) were 1,5 times higher for Mt surface, testifying to higher occupancy and density of active sites for Mt than that of Nt.


Introduction
Montmorillonite and nontronite are layered aluminosilicates belonging to smectite group minerals formed during weathering of igneous rocks in an alkaline media and constituting soil loams and sedimentary rocks [1][2][3]. The structure of monemotillonite and nontronite are represented by three-layer packages (T-O-T) of silicaoxygen tetrahedral and alumina-oxygen octahedra. Different extent of isomorphic substitutions of Si 4+ for Al 3+ , and Al 3+ for Fe 2+ , Fe 3+ or Mg 2+ in tetrahedral or octahedral sheets, respectively, leads to origin of 0.2-0.6 negative layer charge in smectites [1]. The chemical composition of octahedral sheet is the major difference of montmorillonite from nontronite that is revealed in their general chemical formulas [1,3]: Montmorillonite (M + y . H2O)(Al 3+ 2-yMg 2+ y)Si 4+ 4O10(OH)2 (1) Nontronite (M + y . H2O)Fe 3+ 2(Si 4+ 4-xAl 3+ x)O10(OH)2 (2) Smectites are widely demanded in most of industrial productions owing to their unique physical, chemical, mechanical, rheological and other properties, as well as their rich and spread natural deposits making them environmentally compatible and safe [4][5][6][7][8]. For operation with gases or volatile compounds [9,10] and, currently, by cement and concrete productions [11] to be used as a source of cementitious materials, raw clays need to be activated. Most often thermal activation is applied, as a result of which molecules of СО2, H2O and other substances leave the surface providing an increase in porosity and specific surface of a sorbent [9][10][11][12]. However, the exposure of high temperature can be destructive for aluminosilicates irreversibly changing their layered structure [1,4,12]. Processes of dehydroxylation at t>300 o C and amorphization at t>500 o C of layered aluminosilicates are the reasons for a decline of their internal surface, loss of ability to swell, ionexchange and adsorb [10,12,13]. In addition, thermal exposure changes physicalchemical properties and, most often, increases free energy of the surface due to formation, redistribution or enhancing the accessibility of surface active site of acidbase or redox character [14,15].
At ambient conditions, acid-base equilibrium of surface active site of natural clays is shifted towards their higher acidity [14][15][16]. The contribution of basic sites of the surface (structural hydroxyls, adsorbed water, edge ОН-groups) becomes evident by the change of external conditions (pH, temperature), chemical [17][18][19][20] or physical [21-23] modification of aluminosilicates. In a number of processes basicity of catalyst or sorbent surface plays a decisive role [24,25] that makes studies on basic properties of the surface under activation or modification of materials of current interest.
The aim of present studies included investigation of the effect of thermal activation on the character of distribution and a number of basic sites on the surface of natural layered aluminosilicates by means of the new method of inverse thermoprogrammed desorption of СО2.

Materials and methods
Samples of natural aluminosilicates rich with montmorillonite (Mt-0) from Industrial Minerals GmbH, Germany, and nontronite, Nt-0 from Voronezh deposit "Podgornoe" were chosen as objects of investigation. Phase and chemical compositions of investigated natural aluminosilicates samples were determined by XRD and XRF techniques [19] and represented in Table 1.
Specific surface area of the samples was calculated from the N2 adsorptiondesorption isotherms obtained at -196°C using a TRISTAR 300 gas adsorption. Outgassing conditions included 2 h at 90°C followed by 8 h at 250°C. BET-method was applied for calculation of specific surface area (SSA); BJH method for mesopores volume, and t-plot analysis were used for assessment of micropores volume.
Inverse thermal programmed desorption of CO2 (iTPD-CO2) was used to probe surface basicity and determine the number of basic sites. The new iTPD-technique [26] TPD-is based on the FID-detection of CH4 formed at an intermediate step of methanization of CO2 desorbed from the surface of investigated sample by a GC. A specially designed sample loop (SL) of a fixed volume (250 µL) allows calibration of the FIDsignal of GC received from the measurement and thus quantitatively determine volume of desorbed CO2. For the iTPD-CO2 measurement 100 mg of a sample was placed in to quartz U-tube reactor and acti-vated at different temperatures (200, 300, 400 o C) in the N2-flow for 8 hours. After that samples were loaded with CO2 in the 3ml/min flow of CO2 for 30 min at ambient temperature. Next, the reactor was flushed by 5 ml/min N2-flow to desorb weakly sorbed CO2. At further step, the profile of iTPD-CO2 was recorded within the 20-800 o C temperature range at a heating rate of 20 o C/min with a total duration of TPD of 90 min. The iTPD-CO2 profiles were treated using ChemStation software.

Results and discussion
iTPD-CO2 profiles of montmorillonite and nontronite samples treated at different temperatures are given in Fig.1.
As follows from the Fig.1, there were two temperature regions of the CO2 desorption in the profiles of Mt and Nt samples. Low temperature peaks evolved around 80-90 o C for Mt and between 110-127 o C for Nt were most likely related to the weak basic sites [27], whereas high temperature peaks around 510 and 620 o C for Mt and above 320 o C for Nt testified to stronger ones. To reason the obtained iTPD-profiles one should consider thermal behavior of layered aluminosilicates [28] as dehydration state and products will determine surface properties. It is well confirmed by thermal analysis that smectites and dioctahedral 2:1 layered silicates loose water in three temperature ranges: i) dehydration at <220 o C involving loss of H2O from the interlayer space of expandable clays; ii) OH groups bound to the surface between 220-350 o C, and iii) dehydroxylation between 350-1000 o C resulting from the reaction between the octahedral hydroxyls. Due to this, gradual decrease in the intensities of the low temperature peaks at higher activation temperatures testified to a decrease in the weak basic sites related to hydroxyl groups of adsorbed water molecules. As shown for Na-Mt [27] 23% of basic sites were weak and caused desorption between 20 and 110°C, whereas at 140-200°C CO2 desorption was caused by medium strength basic sites accounting for 42%. Elevation of activation temperature from 200 to 400 o C caused further dehydration of the samples and a decrease in the number of weak surface sites. Moreover, at temperatures above 200 o C there is a process of dehydroxylation of structural hydroxyls Al-OH, Fe-OH, Mg-OH, which becomes irreversible above 300 o C due to damage of the 2:1 layered structure of aluminosilicates. As shown in [29], mainly octahedral hydroxyls dehydroxylate according to a scheme 2 (OH -)=H2O+O 2-, providing formation of residual oxygen atoms in the structure and their contribution to population of stronger basic sites along with octahedral hydroxyls. As dehydroxylation of dioctahedral smectites typically has a duplet on DTA curve due to earlier dehydroxylation of transvacant (tv) Al-OH groups (~550 o C), while that for cis-vacant (cv) Al-OH appears at higher temperature (~650 o C) [30], it is similarly assumed that strong basic sites of Mt sample are represented by correspond-ingly trans-and cis-vacant Al-OH groups. The distance between the adjacent OH-OHgroups is shorter in tv-(2.45 A [30]) then in cv-(2.85 A [30]) dioctahedral 2:1 layer minerals that facilitates dehydroxylation of tv-Al-OH groups and, hence, determines variations in the strength of basic sites formed. Redistribution of A1-O and Al-OH bond strengths due to migration of Al cations to empty five-coordinated sites testifies to heterogeneity of aluminous cv-layers dehydroxylation [30,31].
In case of Nt sample, the distribution of strong basic sites is more heterogeneous as follows from the shoulder and overlapped peaks at 350-650 o C. Variation of chemical composition of Nt from Mt, mainly, in the octahedral sheet strongly affects their surface sites distribution, thermal behavior and CO2 sorption ability. It was confirmed for Fe-rich smectites (nontronites) and micas (glauconites, celadonites) consisting of tv 2:l layers that they rapidly loose hydroxyls at significantly lower temperature as compared to Al-rich samples [30,32]. Indeed, iTPD-CO2 profiles of Nt samples (Fig. 1b), rich in Fe, differs from those of Mt samples by the broad and insufficiently resolved high temperature region. Apart from week basic sites, formed around 100 o C, formation of stronger basic sites started above 300 o C and resulted in heterogeneous distribution of various strength basic sites. One may distinguish three different basic sites attributed to i) shoulders around 315-320 o C (Nt-200, Nt-300), ii) maxima at 480 o C (Nt-200, Nt-300) and iii) maxima at 700 o C (Nt- To compare the basicity of unit area of the minerals surface, one may normalize the obtained basicity value BΣ, mol/g, per unit surface area and produce values of BΣ/S, mol/m 2 , given in Table 2. The values of BΣ/S testified that basicity of the unit area of Mt surface was 1.5 times higher than that of Nt surface obviously due to a higher occupancy and density of active sites for Mt.

Conclusion
Applying the iTPD-CO2 technique it was shown that surface of natural and thermally treated layered aluminosilicates of smectite group possesses one type of weak basic sites and a spectrum of strong basic sites. The number of weak basic sites for both Mt and Nt strongly depended on activation temperature applied and fast decreased due to the loss of loosely bound water molecules at elevating temperature. The distribution of strong basic sites was bimodal for Mt surface and, presumably, was primarily caused by presence of cis-vacant and transvacant Al-OH groups in octahedral sheet of the mineral. The Nt sample observed heterogeneous distribution of stronger basic sites represented by Fe 3+ -OH-Al 3+ and Fe 3+ -OH-Fe 3+ in the octahedral sheets proving thermal sensitivity of Fe-rich smectites. The total basicity and total volume of CO2 adsorbed by Nt samples exceeds that of Mt in 1.6-1.7 times. Nevertheless, the determined value of specific basicity of unit area of Mt surface was 1.5 times higher than that of Nt obviously due to variations in chemical and phase composition of samples that resulted in a higher density of active sites of Mt containing sample. The rise of activation temperature from 200 to 400 o C resulted in a 2.2-2.3 times drop of total basicity of both Mt and Nt samples as a consequence of their dehydration and dehydroxylation. Монтмориллонит и нонтронит представляют слоистые алюмосиликаты группы смектита, которые широко востребованы во многих промышленных производствах благодаря своим физикохимическим и многим другим свойствам. Термическая активация глинистого сырья изменяет пористость, удельную поверхность и физико-химические свойства поверхности, включая образование и перераспределение поверхностных активных центров кислотно-основного или окислительновосстановительного характера. Целью данного исследования явилось изучение эффекта термической активации на характер распределения и количество основных центров поверхности природных слоистых алюмосиликатов с помощью усовершенствованного метода обращенной термопрограммированной десорбции СО 2 .