Characterization of a brazilian smectite by solid state NMR and X-ray diffraction techniques

Journal of the Brazilian Chemical Society

Univ. Federal do Rio de Janeiro¹  Menezes, Sonia M.C.¹  Guarino, Alcides Wagner Serpa¹  Univ. Federal do Rio de Janeiro, Rio de Janeiro, Brazil¹  Polivanov, Helena¹  Centro de Pesquisas da Petrobras¹  Gil, Rosane A. S. San¹ 

关键词: smectite;    solid-state NMR;    X-ray diffraction;   

DOI  :  10.1590/S0103-50531997000600004

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Natural clays were among the earliest solid acid catalysts used in the oil industry to promote cracking and isomerization reactions. The development of new catalysts with high activity and good thermal stability has lead to the study of new materials from natural clays1. Owing to growing environmental concerns over the disposal of depleted acid catalysts, there is interest in replacing traditional catalysts, such as aluminum chloride and hydrofluoric acid, by recyclable solid acids. Since the early 1950s many attempts have been made to develop artificial methods of introducing interlayers into expandable silicates2. In the midst of natural clays, smectites are important as the interlayer ions distance can be modified, by changing the ions interlayer with other cations and complexes, which allows the end-product to acquire advantageous chemical properties. Smectite is often a dominant clay in soils formed under alkaline weathering conditions3. Natural clays contain varying percentages of clay-grade materials: non-clay and clay mineral components. In general, fine-grained materials (< 2 mm)4 have been called clay so long as they had distinct plasticity and small amounts of coarse material, to warrant the appellations silt (2-50 mm) and sand (> 50 mm)5. High resolution solid-state NMR spectroscopy is recognised as an important and sometimes indispensable complementary technique to traditional diffraction methods6. NMR is sensitive to short-range ordering, local geometries, and local symmetry, whereas X-ray diffraction studies average much longer range periodicities and ordering. 29Si and 27Al solid state NMR experiments are of fundamental interest because they can provide complementary structural information to that determined by diffraction techniques. Magic Angle Spinning (MAS) is one of the line-narrowing techniques available, which significantly improves the resolution of peaks in the NMR spectrum of a solid, and allows direct determination of the isotropic chemical shift. 29Si and 27Al MAS NMR have been extensively used to analyse aluminosilicates such as clays. Since silicon has a low chemical shift anisotropy (CSA), the line broadening can be removed at easily achievable spinning rates, whereas 27Al requires high field and high spinning rates to overcome the quadrupolar interactions7. Of particular relevance and importance is the direct determination of the amount of non-clay material present in the Brazilian clay and the isomorphic substitution of Si by Al at the tetrahedral sites of the aluminosilicates. The aim of this work was to use X-ray diffraction (XRD), differential thermal analysis (DTA) and solid state nuclear magnetic resonance of 29Si and 27Al (MAS-NMR) techniques to monitor the fractionation steps of a raw material from Campina Grande, Brazil. The NMR data (e.g., number of peaks, individual peak intensities and chemical shifts) were used to characterise the different fractions obtained.   ExperimentalFractionation of the raw materialThe previously crushed raw material (sample 1) from Campina Grande (PB, Brazil) was passed through a 270 mesh sieve. There are several types of soil treatment mentioned in literature3,8,9,10, used to characterise the samples by X-ray analysis. We followed the procedure suggested by Math3. The sand fraction (sample 2) was separated by sieving. The silt (sample 3) and clay (sample 4) fractions were separated by decantation. The clay fraction was subjected to a treatment which involved dissolution of soluble salts and carbonates with sodium acetate buffered at pH 5, followed by oxidation of organic matter with 30 per cent hydrogen peroxide. Iron oxides were removed by sodium dithionite-citrate-bicarbonate treatment4 (sample 5). This fraction was then saturated with Mg2+ (sample 6) and K+ (sample 7); The sample 6 was glycolated and the sample 7 was heated to 823 K for 1 h before XRD analysis4.Characterization and measurementsXRD analysis of oriented fractions were made using a Rigaku-Geigerflex 2013 diffractometer equipped with a proportional counter and pulse height analyser using Ni filtered CuKa radiation, produced under conditions of 40 kV and 30 mA. Diffraction patterns were collected at a scanning rate of 4º(2q)/min in the interval of 2° and 70°. Thermal analysis (DTA) was performed using a Rigaku-TG termoflex 8110-TAS100 thermal analyser. The chemical analysis was determined by atomic absorption spectrophotometry11, using a VARIAN AA6 spectrophotometer. The cation exchange capacity (CEC) was determined by flame emission spectrophotometry12 as 74 meq/ 100 g clay: 1 g of material (natural clay) was weighed and exchanged (through centrifugation) with a K+ solution. The suspension was washed with ethanol 95%, and the aqueous phase was discarded. Then, the K+ was exchanged by NH4+ ion five times and the resulting solutions were collected in a volumetric flask. The amount of K+ ion in this solution was determined by flame spectrometry, using a flame photometer MICRONAL B262. The chemical analysis of the raw material11 was: SiO2, 61.60%; Al2O3, 15.30%; Fe2O3, 6.40%; MgO, 2.60%; Na2O, 1.40%; TiO2, 1.10 %; CaO, 1.00% K2O, 0.54%; H2O, 10.05%; the P, Cr, Zn, Mn and Sr elements were present in less than 300 ppm. Solid state NMR measurements were carried out on a Varian VXR-300 spectrometer operating at a B0 of 7.05 Tesla (corresponding to a Larmor frequency of 59.6 MHz for silicon and 78.2 Mhz for aluminum). A single pulse sequence was used with quadrature detection for both nuclei. The magic angle was accurately adjusted prior to data acquisition using KBr. Samples were spun in zirconia rotors equipped with Kel-F caps, at speeds of 3 kHz for 29Si, and 7 kHz for 27Al. A 2.5 ms (30°) pulse was used for silicon, with pulse delay of 120 s, for quantitative analysis13. For aluminum experiments a short pulse of 0.7 ms (p/12) was chosen, so that the results could be quantitatively exploited (flip angle  p/2l+1). The recycle time was 0.2 s. About 5000 and 500 scans were needed to obtain 27Al and 29Si spectra of the samples. The 29Si signal positions were referenced using a secondary standard of caulim (d = -91.5 ppm), in relation to TMS, while the 27Al chemical shifts were referenced using a solid sample of AlCl3.6H2O (d = 0 ppm).   Results and DiscussionDTA analysisThe differential thermal analysis (DTA) of the raw material (Fig. 1) showed an intense endothermic peak at 110 °C. This peak could be due to water molecules adsorbed in the interlayer or coordinated with exchangeable cations. The form and position of this peak depend on the nature of the adsorbed cation and on the smectite clay mineral. In this case, these cations are probably Na+ and K+. The peak observed at 550 °C is a characteristic one, and would be due to loss of structural hydroxyls in iron rich smectites. Santos14 pointed out that the crystalline structure of the smectite is preserved after the loss of the hydroxyl groups until 800 °C. Above 800 °C, a double peak, endo-exothermic, appeared at 890 °C and 920 °C, respectively. The endothermic peak represents the destruction of the crystalline framework, while the exothermic peak represents the formation of a or b mulite quartz.   XRD analysis Figure 2 shows the XRD patterns of the samples 1 to 7. Table 1 shows the reflections for the oriented samples (a diffractometer pattern from a strongly oriented clay specimen may show only the 00l series of basal reflections with small or no evidence of hkl reflections)15.     The sample 1 showed a peak at 14.9 Ã… (2q = 5.9°). The Mg2+ and K+ saturations, following treatment with ethylene glycol and heating at 823 K, respectively, confirmed the presence of a clay mineral of the smectite group, through the 001 peak for 17.0 Ã… (2q = 5.2°) (sample 6, Fig. 2) and breakdown to 10.0 Ã… (2q = 8.8°) (sample 7, Fig. 2)4. The peaks at 4.26 Ã… (2q = 20.9°); 3.33 Ã… (2q = 26.7°) and 3.25 Ã… (2q = 27.4°) suggest the presence of quartz and feldspar, respectively, in the raw material. The sample 2 showed a strong reflection corresponding to 3.33 Ã… and a minor one at 3.25 Ã…, suggesting that quartz was a predominant mineral in this fraction. Quartz was also a predominant mineral in sample 3, as a strong peak appeared at 3.33 Ã…. This sample also showed a peak at 14.9 Ã… which indicated the presence of smectite. This could be justified as a result of pre-crushing of the original material and the difficulty to separate the fractions by decantation. The clay mineral corresponding peak of a smectite group in sample 5 was more pronounced than in sample 4, due to the CBD treatment. The precision of the peak intensities for values below 2q = 5° is relatively small; the clay mineral of the smectite group presented in this work was identified by the increase and decrease of the 001 peak, as observed in Fig. 2, samples 6 and 7. The sample 6 showed an expanded peak corresponding to 17.0 Ã… while sample 7 showed a peak at 10.0 Ã…, indicating that smectite is the dominant mineral 2:1. It suggests the predominance of hydroxyl interlayered smectite in the fine clay fractions3. These results are similar to those of Volzone17,18, for argentine bentonite, although our sample is not a bentonite. 27Al NMR spectraFor 27Al, which has spin I = 5/2, the m= 1/2 m=-1/2 transition is independent of first order quadrupolar interaction but is affected by second-order quadrupolar effects. Large variations of the quadrupole coupling constant provoked distortion of the peak shape because of the increasing influence of second-order terms. The sideband pattern is also modified. The quadrupole coupling constant being the product of the quadrupole moment by the electrical field gradient at the nucleus, the peak shape is very sensitive to a change in the symmetry of the coordination shell and in the distribution of electrical charges. Here also in first approximation, the weaker the shielding, the more sensitive the shift. The highest available field, namely 7.05 T, was used for 27Al in order to decrease as much as possible the effect on the shift of the second-order quadrupolar effect19. The 27Al MAS-NMR spectra of the samples are shown in Fig. 3. The spectra consist of one or two principal components and a series of sidebands associated to the spinning of the samples. The line around 0 ppm must be assigned to octahedral Al sites and the line at 56 ppm to tetrahedral Al ions. These assignments are in agreement with the structural compositions of the samples.   The amounts of AlIV and AlVI for each sample were indicated in Table 2.   In the raw material (sample 1) both AlIV and AlVI were present. The tetrahedral aluminum sites would be present both in the structure of feldspar and smectite, in the

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