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Acidity characterization of a titanium and sulfate modified vermiculite
Materials Research Bulletin 43 (2008) 1630–1640 www.elsevier.com/locate/matresbu
Acidity characterization of a titanium and sulfate modified vermiculite W.Y. Hernández a, M.A. Centeno b, J.A. Odriozola b, S. Moreno a, R. Molina a,* a
Estado Sólido y Catálisis Ambiental, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No. 45-03, Bogota, Colombia b Instituto de Materiales de Sevilla y Departamento de Química Inorgánica, Centro Mixto CSIC-Universidad de Sevilla, Avda. Americo Vespuccio 49, 41092 Seville, Spain Received 17 July 2007; received in revised form 27 September 2007; accepted 19 October 2007 Available online 30 October 2007
Abstract A natural vermiculite has been modified with titanium and sulfated by the intercalation and impregnation method in order to optimize the acidity of the clay mineral, and characterization of samples were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption isotherms, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption with ammonia (TPD-NH3). All the modified solids have a significantly higher number of acidic sites with respect to the parent material and in all of these, Brönsted as well as Lewis acidity are identified. The presence of sulfate appears not to increase the number of acidic centers in the modified clay. For the materials sulfated with the intercalation method, it is observed that the strength of the acidic sites found in the material increases with the nominal sulfate/metal ratio. Nevertheless, when elevated quantities of sulfur are deposited, diffusion problems in the heptane reaction appear. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; B. Intercalation reactions; C. X-ray diffraction; C. Infrared spectroscopy; D. Catalytic properties
1. Introduction In the past few years, a wide range of solids with strong acidic properties have been developed and applied in catalytic reactions. Particularly, the promotion of oxides such as ZrO2, TiO2 and Fe2O3 with sulfate species has led to the production of materials whose catalytic properties have come to be considered as super acidic solids [1,2]. On the other hand, the synthesis of pillared clays by intercalation of polycationic species in smectites has allowed us to obtain materials which combine acidic properties with a variably porosity and relatively high thermal stability. All these factors depend on the chemical nature of the mixed species, the type of clay mineral and the synthesis procedure used [3–5]. As a result of the combination of knowledge that has been acquired in the synthesis procedures of these two types of materials, pillared sulfated clays (of the smectite type) have been suggested as new materials characterized by a higher acidic strength than non-sulfated clays. Their physical–chemical properties may be modulated in function of the type of oxide that makes up the pillar, the method of sulfating and the sulfate/metal ratio, among other variables [6– 10]. For instance, it has been reported that the pillarization of smectites by intercalation of titanium species results in
* Corresponding author. Tel.: +57 1 3165000x14473. E-mail address: [email protected] (R. Molina). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.018
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an increase in the acidity of the sample, generating both Brönsted and Lewis acid sites [11]. This acidity induced by titanium is higher than the one obtained if any other metal intercalating species is used. In addition, the acidic strength of the system is enhanced when sulfate anions, mainly those associated with the pillar, are incorporated. This effect is due to the ability of S O in sulfate complexes to accommodate electrons from basic molecule, this effect being a strong function of the environment of sulfate ions [12]. Besides this, the typology and properties of the acidic sites found in pillared clays depend on the nature of the parent clay. In the case of vermiculite, this mineral may be considered as swelling trioctahedral micas with Al for Si substitutions in the tetrahedral layers, and Fe and Al for Mg substitutions in the octahedral layers. They are intermediate minerals in the natural weathering sequence of micas to smectites, with a negative layer charge density between that of micas and smectites, and where the charge neutralizing K ions of the parent mica have been replaced by exchangeable hydrated cations, most often Mg2+ and Ca2+, providing partial swelling properties [13]. Unfortunately, the high density of the interlaminate charge present in the vermiculite hinders the intercalation process of the metallic species. Recently, efforts have been applied to diminish the charge density and, consequently, to make intercalation easier [13,14]. This work explores for the first time, the modification of a vermiculite with an intercalating titanium solution and the application of two methods of sulfating in order to study the changes in the acidic properties of the material produced by the presence of titanium and the oxo-anion sulfate. The methods of sulfating evaluated were: (i) intercalation (the sulfate comes into contact with the intercalated solution) and (ii) impregnation over the modified and calcined solids. The type and strength of the acid centers present in the solids obtained were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) experiments, employing ammonia as a probe molecule. On the other hand, the catalytic potential of the acidic sites generated in the solids was evaluated in the hydroconversion of heptane, considering that this reaction is catalyzed by materials of acid nature, including pillared clays [14–16] and whose selectivity towards isomerization or hydrocracking products, is strongly affected by the strength of the acid centers, and the accessibility towards it of the adsorbed species. 2. Experimental 2.1. Starting materials The initial clay mineral is a natural vermiculite of high interlaminar charge coming from Santa Marta (Colombia). The high purity and crystallinity of the material, as well as its high interlaminate Mg2+ cationic content has been recently reported [14,17,18]. The natural vermiculite was crushed and sifted, and the fractions smaller than 150 mm was subjected to hydrothermal treatment (THT, 2.0 L h 1 of 50% water vapour pressure in nitrogen, for 6 h at 400 8C) in order to diminish the interlaminate charge of the material [14,18]. After that, the sample was washed with nitric acid 0.5 M (10 mL of acid per gram of clay) at 80 8C for 1 h under stirring, with the purpose of removing the extra framework species retained [14]. The solid was washed by centrifugation and dried at 60 8C. Finally, in order to approach the clay to a homoionic form and to facilitate the forthcoming exchange process with the polyhidroxication, it was homoionized with NaCl 3 M solution at 80 8C for 1 h, repeating this process three times, renewing NaCl solution every time. Finally washing and drying were carried out as above. The resulting material is called ‘‘vermiculite THT’’. 2.2. Preparation of modified solids Vermiculite THT was modified by intercalation with titanium and titanium sulfate species, employing sulfate/metal molar ratios of 0, 0.25 and 0.35. The titanium precursor solution was prepared according to Valverde et al. [19]. The required volume of a titanium ethoxide in excess of ethanol (Sigma–Aldrich, 20% Ti) solution was slowly added to HCl 5 M, in sufficient quantity to establish a ratio of hydrolysis H+/Ti = 2.2 and to provide a quantity of 10 mmol of metal per gram of clay to be modified (solution 1). For the incorporation of the sulfate into the intercalating solution, the method described by Del Castillo et al. was adopted [9]. A (NH4)2SO4 0.05 M solution was used as precursor of the oxo-anion, mixed with solution 1 and aged 3 h at room temperature under continuous stirring. After that, it was slowly added to a suspension of the vermiculite in water (0.15%, w/w), under constant stirring. Once the addition was
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completed, the suspension obtained was aged under the same conditions for 12 h, then washed by centrifugation and dried for 12 h at 60 8C. Finally, the samples were calcined at 500 8C for 2 h. The solids obtained were called VT, VT (0.25) and VT(0.35), considering the sulfate/Ti molar ratio employed for the modification. Additionally, and with the aim of establishing the effect of the method of sulfating on the modified vermiculites, the solid VT was impregnated at RT with a 0.1 M ammonium sulfate solution for an hour under constant stirring, at a nominal sulfate/metal ratio of 0.35. The excess of water was evaporated through a drying process at 110 8C for 12 h. The solid obtained was calcined at 650 8C for 2 h. This solid was called VT(0.35i) (where i refers to sulfating through impregnation). 2.3. Physical–chemical characterization techniques The textural properties were studied by N2 adsorption measurements at liquid nitrogen temperature. The experiments were carried out in a Micromeritics ASAP 2010 equipment. Before analysis, the samples were degassed for 2 h at 150 8C in vacuum. X-ray diffraction (XRD) analysis was performed on a Siemens D-500 diffractometer. Diffraction patterns were recorded with Cu Ka radiation (40 mA, 40 kV) and a position-sensitive detector using a step size of 0.058 and a step time of 1 s. The titanium content of the samples was determined by X-ray fluorescence spectrometry (XRF) in a Siemens SRS 3000 sequential spectrophotometer with a rhodium tube as the source of radiation. XRF measurements were performed onto pressed pellets (sample included in 10 wt% of wax). Sulfur was analyzed by using an elemental analyzer LECO CS200 apparatus. Sulfur was thermally decomposed at high temperature in presence of oxygen flow, and measured like SO2 by infrared absorption. 2.4. Characterization of the acidic properties of the modified solids 2.4.1. DRIFTS-NH3 DRIFTS spectra were obtained using a Nexus Thermo Nicolet Infrared Spectrometer, with KBr optics and type B MCT detector (Mercury cadmium telluride) cooled off with N2 liquid. A controlled environment reflectance cell (Spectra-Tech 0030–101) equipped with ZnSe windows was coupled to the spectrometer. Before introduction of the adsorbate (NH3), the solid was heated at 400 8C for 1 h in 50 mL/min of nitrogen, cooled down to room temperature (RT) at the same atmosphere and the reflectance spectrum collected. This spectrum was taken as reference for the adsorption experiment. Then, a flow of 35 mL/min of NH3 (2000 ppm in He, Abelló Oxígeno Linde) was passed through the solid for 1 h at room temperature. Later, spectra were taken after outgassing the sample under nitrogen flow (50 mL/min) at room temperature and after heating for 1 h at 100, 150, 200, 300 and 400 8C. Detailed descriptions of similar procedures to study the acidity of samples by using ammonia have been reported elsewhere [12,20]. Spectra were obtained by coadding 100 scans at 4 cm 1 of resolution. They are presented in Kubelka–Munk mode without any other manipulation. In this way, qualitative and quantitative analyses of both gas phase and adsorbed species are possible. 2.4.2. TPD-NH3 TPD-NH3 analyses were carried out by in a Quantachrome TPD/TPR/TPO Chembet 3000 analyzer fitted with a thermal conductivity detector (TCD). Approximately 400 mg of the sample were placed in a quartz reactor and pretreated under nitrogen flow while the temperature was increased at a speed of 20 8C/min up to 400 8C. After 1 h at this temperature, the sample was cooled at room temperature and saturated with a mixture of NH3 in He at 5% (v/v) for 1 h. The surplus ammonia not interacting strongly with the solid was swept out under nitrogen flow until its presence was not detected at the exit of adsorbate gas. Finally, ammonia desorption was carried out by ramping the temperature until 600 8C at 10 8C/min. The detector signal and the temperature were recorded simultaneously. 2.5. Heptane hydroconversion Heptane hydroconversion was carried out in a fixed-bed microreactor under constant flow, operated at atmospheric pressure. The liquid hydrocarbon was saturated with hydrogen by having it go through a saturator at 27 8C, which keeps a
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vapour pressure of heptane of 56.9 mmHg. The catalytic activity was assessed at a WHSV of 1.2 g of Heptane/g of catalyst per hour, at a temperature ranging between 150 and 400 8C. The analysis of the reaction products was performed by gas chromatography with a Shimadzu GC-17 apparatus, equipped with a FID detector and using a ZB-1 column (60 m 0.53 mm 3.00 mm). The details of the reaction conditions have been previously reported [14,15,17]. 3. Results and discussion Fig. 1 shows the XRD diagrams of the considered solids. The d0 0 1 plane of the vermiculite THT is observed at 1180 pm. Besides this, the THT treatment induces a little collapse of the layers, as deduced from the little signal observed at 930 pm. The success of the modification of the solid with the titanium and titanium sulfate intercalation solution is confirmed by the shift of the d0 0 1 plane towards 1380 pm. This shift is low and suggests a low size of the species incorporated, probably due to the intercalation of oligomeric species of low polymerization degree [13,21,22]. Besides this, the calcination process at 500 8C produces a higher collapse of the layers not intercalated, which is observed by the growth of the signal at 930 pm, corresponding with a dehydrated phase of the mineral [23]. Finally, the solid sulfated by impregnation presents the lowest degree of intercalation and the highest degree of collapse of the d0 0 1 plane, as expected due to the higher calcination temperature used (650 8C). In addition, new diffraction peaks corresponding probably to SO3 or MxSyOz are found (XRD taken from 108 to 908 2u not shown). Table 1 shows the specific surface area and titanium and sulfur content of the studied solids. After the hydrothermal treatment (THT), the specific surface area of the material increases significantly, as expected from the extraction of part of the structural aluminum of the material [14,18]. The modification of the vermiculite THT with titanium induces a little decrease in the SBET area. This area loss is higher in the case of the solid modified by intercalation with titanium sulfate species, in such a way that, higher is the sulfate/titanium ratio used, lower is the BET area of the resulting solid. This behavior has been described early for smectite-type clay minerals pillared and sulfated by this method, and explained by blocking of the material pores due to large polymeric species formed in the intercalating solution by the presence of sulfate, which will not form inside of interlayer space, but on the surface of the layers [9]. On the other hand, it is interesting to note that the titanium-to-sulfur ratio in the obtained material is constant, whatever the sulfate/ titanium ratio used in the intercalating solution. This point to the fixation of the sulfur is associated with the incorporation of titanium. The solid sulfated by impregnation presents a much lower SBET area, which can be related with the very high sulfur content of the material, the higher calcination temperature used, and the higher collapse of the d0 0 1 plane detected by XRD. 3.1. DRIFTS-NH3 Fig. 2 shows the DRIFTS spectra obtained after ammonia adsorption at room temperature for the vermiculite THT and for the solid modified with the titanium simple system (VT), as example of the modified solid. Some general
Fig. 1. XRD diagrams of the studied solids.
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Table 1 BET specific surface area and titanium and sulfur content of the studied solids Solid
Ti content (% w/w)
S content (% w/w)
Ti/S atomic ratio
SBET (m2 g 1)
Natural vermiculite Vermiculite THT VT VT(0.25) VT(0.35) VT(0.35i)
0.72 0.72 3.59 4.70 5.29 5.08
– – – 0.044 0.050 4.343
– – – 71.4 70.8 0.78
4 66 59 48 39 14
features can be detected in the spectra. The intensity of these bands is clearly lower, almost absent, in the case of vermiculite THT, indicating that this solid adsorbs a very small quantity of ammonia, and pointing out its very low acidity or, in any case, the non-accessibility of the its acid centers to ammonia molecules [14,17,18]. In the 2850– 3600 cm 1 region, several bands corresponding to N–H stretching vibrations of adsorbed ammonia species are observed [24]. The position and intensity of these bands do not change as function of the sulfate/titanium ratio or the sulfating method used. A negative band at 3737 cm 1 is also detected. This band is characteristic of the structural OH groups (Mg–OH or Al–OH) of the clay, and its negative intensity is indicative of their interaction with ammonia molecules [13]. Finally, strong positive bands at around 1683, 1647, 1440 and 1216 cm 1 are observed. The position of these bands remains unaltered from one sample to another, Fig. 3, even in the spectra obtained at higher temperatures (not shown). The thermal evolution of adsorbed NH3 over the considered solids is not shown since the qualitative DRIFTS spectra is similar to that described at RT. The main difference among the spectra of the modified solids after ammonia adsorption resides in the intensity of these four bands, which can be related with the changes in the acidity of the samples. Bands at 1440 and 1683 cm 1 are ascribed to asymmetric and symmetric bending modes of adsorbed NH4+ species, respectively [20,25], which undoubtedly indicate the presence of Brönsted acid sites on the catalyst surface, associated to the hydroxyl groups responsible of the negative band observed at 3737 cm 1. Bands at 1647 and 1216 cm 1 are ascribed to asymmetric and symmetric bending modes of adsorbed NH3 species on Lewis acid sites of the solids [25]. These acid sites can be associated to titanium atoms of the pillar, since they are present in the VT sample, without sulfate. However, the S O bond of sulfates has been described also as Lewis acid sites capable of adsorbing ammonia [12]. Fig. 3 shows the spectra of the solids of the VT series obtained after ammonia adsorption at RT in the region under 2000 cm 1. It is clear from the figure that the intensity of the bands decrease when the sulfate to metal ratio increases. The lower intensity is observed for the sample sulfated by impregnation. This behavior is better observed in Table 2, where the area of the 1440 and 1216 cm 1 bands, taken as representative of the Brönsted (B) and Lewis (L) acidity, respectively, are showed. On the other hand, the Brönsted to Lewis (B/L) ratio increases with the sulfate content of the
Fig. 2. DRIFT spectra after NH3 adsorption on the vermiculite THT and the VT solid.
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Fig. 3. DRIFT spectra after NH3 adsorption on VT, VT(0.25), VT(0.35) and VT(0.35i) solids.
sample, pointing to the preferential Lewis acidity decay induced by the sulfate. This observation agrees with most reports existing in the literature for titanium pillared sulfated clays, in which Brönsted type acidity prevails [6–9]. The lower acidity showed for by solid sulfated by impregnation (VT(0.35i)), reflects that, although this sulfating methodology results in the fixation of a high amount of sulfur (as compared with the intercalation method), probably the blocking of acid sites, mainly Lewis, is favored by the presence of sulfur that does not directly interact with them. It is worth highlighting that by means of the DRIFT technique, the study of metal–sulfur type interactions, which are in the spectrum region below 1400 cm 1 is not facilitated [26,27], as here. Problems associated to diffuse specular reflection do not allow for the appropriate resolution of the curves [28]. In addition, in this region of the spectrum, M– O–M type flexions should also be found, being M = Si4+ and/or Al3+ [28], which are characteristic of the clay matrix and would prevail over any other type of superficial interactions. In fact, all spectra are dominated by a very intense band at 1226 cm 1, which shifts towards higher wave numbers with the sulfate content, Fig. 4. This band results from the overlapping of the framework M–O–M vibrations, more ore less perturbed by the presence of sulfate, and the bands of sulfate species. In addition, the solid sulfated by impregnation presents new clear bands at 1290, 1154 and 1062 cm 1 characteristics of sulfate/bisulfate species [29], pointing out the formation of a new bulk sulfate phase, in good agreement with XRD data. On the other hand, the presence of water adsorbed, which is characterized by a deformation H–O–H band at 1620 cm 1, makes the analysis of the bands complicated due to Lewis and Brönsted ammonia adsorbed species at 1625 and 1647 cm 1. For that reason, the analysis of the bands appearing after ammonia adsorption is not so easy. The situation is even more complicated for the study at high temperatures, where thermal effects are added. For instance, the intense band at 1226 cm 1, shifts towards higher wave numbers with temperature. Similar effects have been described early for sulfated titania and zirconia [30,31] and ascribed to the presence of water which coordinates with S O bonds. Because of all this problems, only a semi-quantitative study on the stability of Brönsted type acid sites as a function of the desorption temperature, has been carried out, following the band at about 1440 cm 1, Fig. 5. As expected, the Table 2 Brönsted and Lewis acidity of the studied samples, calculated from the area of the 1440 and 1216 cm
1
DRIFTS bands
Solid
Brönsted acidity (a.u.)
Lewis acidity (a.u.)
Brönsted/Lewis ratio
VT VT(0.25) VT(0.35) VT(0.35i)
5.74 1.90 0.79 0.62
3.50 0.50 0.20 0.006
1.64 3.8 3.95 103
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Fig. 4. DRIFT spectra at room temperature of the considered solids.
Brönsted acidity decreases with temperature until disappear in all solids at 400 8C. On the other hand, although the sulfation process, whether by intercalation or impregnation, significantly affects the number of Brönsted acids, the presence of the sulfate anion increases their thermal stability in the structure of the material, since lower is the nominal sulfate/metal ratio, sharper is the slope of the curve characterizing the loss of acidity as a function of temperature. This result provides evidence that, as has been reported for sulfated metal oxides [1,2] and sulfated pillared clays [6–10], the presence of the sulfate anion associated with this type of systems increases the strength of the acid centers found in the material. In the case of solids modified with titanium species, the strength of Brönsted type acid sites is mainly favored, possibly due to the generation of bisulfate species (HSO4 ), where OH groups are bound through the hydrogen atom to a superficial oxygen of the oxide [32,33]. 3.2. Temperature programmed desorption (TPD-NH3) Fig. 6 shows the desorption profiles of ammonia in function of the temperature for the modified materials sulfated by intercalation and impregnation. For all solids there is an ammonia desorption signal around 200 8C, which has been previously reported for clays modified via intercalation pillarization [9,34,35]. This agrees with the observation by DRIFTS of the almost total degassing of the adsorbed ammonia at 300 8C in all solids. In addition, a notorious effect is observed on the maximum temperature for ammonia desorption due to the presence of sulfate, generating an increase
Fig. 5. Brönsted acid quantification results of the considered solids as function of temperature.
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Fig. 6. Ammonia desorption profiles of modified solids.
of around 30 8C in this value for solid VT(0.35), with respect to the non-sulfated material. The above suggests that the presence of sulfate promotes the strength of the acid sites found in these materials, corroborating the results obtained by the DRIFTS analysis. When sulfation is performed by the impregnation method (VT(0.35i)), the desorption profile does not show an intense signal in the range of temperatures observed for the solids sulfated by intercalation, that which can be related with the semi-quantitative results obtained by DRIFTS (Fig. 5). However, for this solid, a very intense desorption signal is observed, focused around 900 8C (Fig. 7) corresponding to the exit of the sulfur found in the material, most certainly as gaseous SO2, which can also be differentiated by the thermal conductivity detector [36]. The absence of a signal with significant intensity attributable to the desorption of ammonia from some type of acid site provides evidence on the possible blockage of acid centers found in these solids as a result of the large amount of sulfur fixed and of its disposition on the material’s surface, showing that not only does the sulfur content affect the strength of the acid sites, but also the type of interaction this can generate in the material. 3.3. Heptane hydroconversion The catalytic activity of solids was assessed in heptane hydroconversion, taking into account the variation in the conversion percentage in function of the temperature, as well as the parameter called T10ISO, whose value corresponds to the temperature at which 10% conversion in isomerization products is reached [14–16].
Fig. 7. Ammonia desorption profiles of VT(0.35i).
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Fig. 8. Heptane total conversion curves for modified solids.
Fig. 8 shows the follow up on the total conversion of heptane in function of the temperature for all modified solids. In all cases, an increase in the catalytic activity of materials modified and sulfated by intercalation is evident, as compared with the vermiculite THT, in spite of the marked effect generated by the incorporation of sulfate groups by the two sulfation methodologies applied on the catalytic activity of solids. In fact, when sulfation is performed by the intercalation method, regardless of the nominal sulfate/metal nominal relation used or the amount of sulfur fixed, the conversion falls in an almost identical form, provoking a reduction of around 22% in the value of the total conversion at the maximum isomerization temperature displayed by the material without sulfating. For the material sulfated through the impregnation methodology (VT(0.35i)), this value gets lower, until it is equivalent to the behavior of the vermiculite used to start out. These results show that for the solids sulfated by intercalation and in which the largest amount of sulfur has been incorporated, visible diffusion problems are generated which cause a marked drop in the activity of catalysis [37,38]. This has also been established for these materials through techniques like nitrogen sortometry and XRD [36]. Besides, as previously discussed, although the presence of sulfate in the solids analyzed promotes an increase in the strength of the acid sites present, it also reduces their quantity as a result of a probable blockage caused by the presence of the sulfate groups. Table 3 shows the parameters for catalytic evaluation obtained for these materials: T10ISO (temperature at which the catalyst reaches 10% isomerization), Tmax (maximum isomerization temperature), conv. t (total conversion to Tmax), YISO (conversion to isomers at Tmax), YCRA (conversion to cracking products at Tmax), SelISO (selectivity to isomers at Tmax), % mono and % di (percentages related to mono and biramified isomers at Tmax). The data recorded in this table clearly show that the system displaying better properties for obtaining ramified derivates as a result of the heptane hydroconversion reaction corresponds to solid VT, for which one of the largest isomerization percentages is achieved (40.2%) at the lowest T10ISO for this group of materials (236 8C), with a noticeable selectivity towards isomerization products (91%). The behavior described may indicate that a larger content of Brönsted type acid sites promotes a larger selectivity for isomerization products in solid VT, taking into account that in some bibliographical reports on the bifunctional mechanism of the transformation of the heptane molecule, Brönsted acid sites are presented as preferential centers for Table 3 Catalytic results in heptane hydroconversion of the considered solids Solid
T10ISO (8C)
Tmax (8C)
conv. t (%)
YISO (%)
YCRA (%)
SelISO
mono (%)
di (%)
Natural vermiculite VT VT(0.25) VT(0.35) VT(0.35i)
– 236 275 270 -
350 300 350 350 375
7.0 44.2 22.5 20.6 5.7
2.6 40.2 18.4 17.0 4.9
4.4 4.0 3.7 3.4 0.6
36.7 91.0 81.7 82.4 86.7
93.3 94.5 94.3 94.6 96.5
6.7 5.5 5.7 5.4 3.5
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the formation of carbocations [38]. Besides, a possible better distribution of such sites may be equally implicit, which favors accessibility of the heptane molecule to active centers at lower temperatures. Likewise, it is observed that within the isomerization products recorded there is a higher selectivity to monoramified isomers, suggesting that the reaction possibly occurs through monomolecular and bimolecular mechanisms through the reordering of alkylcarbenium ions and ß-separation [39], which requires acid sites with intermediate strength. 4. Conclusions Through the acidity study carried out, by means of DRIFTS and TPD-NH3 techniques and heptane hydroconversion, it was possible to establish the existence, on the solids prepared, of both types of acid centers, Brönsted and Lewis type, at a quantity above that present in the parent clay. In addition, it was found that on the materials of the VT series, Brönsted acidity prevails over Lewis acidity, being more stable facing the thermal treatments applied. Likewise, the beneficial effect generated by the presence of oxo-anion sulfate on the strength of the acid centers found in the material was proved, which is not reflected in the selectivity of these materials regarding the heptane hydroconversion reaction, possibly due to the blockage of the acid sites by the high presence of sulfate. Results indicate that the sulfur incorporated by the intercalation method favors specific superficial interactions while the impregnation method provokes the blockage of the acid sites found in the solid, bringing about diffusion problems put in evidence during the evaluation of these materials in the heptane hydroconversion reaction. Finally, it is worth noticing that all materials display high selectivity towards isomerization products. Acknowledgements Our gratitude to joint project CSIC-COLCIENCIAS-2006CO0015 (Spain-Colombia) and the financial support of VRI-DIB at Universidad Nacional de Colombia (QUIPU 20201007579) and Junta de Andalucia (PAI, TEP106). WYH wishes to thank Universidad Nacional de Colombia for the financial support through the ‘‘Outstanding Postgraduate Students’’ scholarship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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[25] A.A. Davidov, Infrared Spectroscopy of Adsorbed Species on the Surface of Transitions Metal Oxides, John Wiley & Sons (Ed.), England, 1990, p. 33. [26] X. Li, K. Nagaoka, J.A. Lercher, J. Catal. 227 (2004) 130. [27] B. Klose, F. Jentoft, R. Schlögl, J. Catal. 233 (2005) 68. [28] J. Madejová, P. Komadel, Clay Clay Miner. 49 (2001) 410. [29] J.R. Sohn, S.H. Lee, P.W. Cheon, H.W. Kim, Bull. Korean Chem. Soc. 25 (2004) 657. [30] E. Ortíz-Islas, T. Lopez, J. Navarrete, X. Bokhimi, R. Gomez, J. Mol. Catal. A: Chem. 2228 (2005) 345. [31] C. Morterra, G. Cerraato, C. Enmanuel, V. Bolis, J. Catal. 142 (1993) 132. [32] L.M. Kustov, V.B. Kazansky, F. Figueras, D. Tichit, J. Catal. 150 (1994) 143. [33] V. Adeeva, J.W. de Haan, J. Janchen, G.D. Lei, V. Schunemann, L.J.M. Van de Ven, W.M.H. Sachtler, R.A. Van Santen, J. Catal. 151 (1995) 364. [34] T. Mishra, K.M. Parida, J. Colloid Interf. Sci. 301 (2006) 554. [35] K. Shimizu, Y. Nakamuro, R. Yamanaka, T. Hatamachi, T. Kodama, Micropor. Mesopor. Mater. 95 (2006) 135. [36] W.Y. Hernández, S. Moreno, R. Molina, Rev. Colombiana Química 36 (1) (2007) 73. [37] J.W. Johnson, J.F. Brody, S.L. Soled, W.E. Gates, J.L. Robbins, E. Marucchi-Soos, J. Mol. Catal. A: Chem. 107 (1996) 67. [38] N. Bouchenafa-Saib, R. Issaadi, P. Grange, Appl. Catal. A 259 (2004) 9. [39] E. Blomsma, J.A. Martens, P.A. Jacobs, J. Catal. 159 (1996) 323.
Acidity characterization of a titanium and sulfate modified vermiculite W.Y. Hernández a, M.A. Centeno b, J.A. Odriozola b, S. Moreno a, R. Molina a,* a
Estado Sólido y Catálisis Ambiental, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No. 45-03, Bogota, Colombia b Instituto de Materiales de Sevilla y Departamento de Química Inorgánica, Centro Mixto CSIC-Universidad de Sevilla, Avda. Americo Vespuccio 49, 41092 Seville, Spain Received 17 July 2007; received in revised form 27 September 2007; accepted 19 October 2007 Available online 30 October 2007
Abstract A natural vermiculite has been modified with titanium and sulfated by the intercalation and impregnation method in order to optimize the acidity of the clay mineral, and characterization of samples were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption isotherms, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption with ammonia (TPD-NH3). All the modified solids have a significantly higher number of acidic sites with respect to the parent material and in all of these, Brönsted as well as Lewis acidity are identified. The presence of sulfate appears not to increase the number of acidic centers in the modified clay. For the materials sulfated with the intercalation method, it is observed that the strength of the acidic sites found in the material increases with the nominal sulfate/metal ratio. Nevertheless, when elevated quantities of sulfur are deposited, diffusion problems in the heptane reaction appear. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Layered compounds; B. Intercalation reactions; C. X-ray diffraction; C. Infrared spectroscopy; D. Catalytic properties
1. Introduction In the past few years, a wide range of solids with strong acidic properties have been developed and applied in catalytic reactions. Particularly, the promotion of oxides such as ZrO2, TiO2 and Fe2O3 with sulfate species has led to the production of materials whose catalytic properties have come to be considered as super acidic solids [1,2]. On the other hand, the synthesis of pillared clays by intercalation of polycationic species in smectites has allowed us to obtain materials which combine acidic properties with a variably porosity and relatively high thermal stability. All these factors depend on the chemical nature of the mixed species, the type of clay mineral and the synthesis procedure used [3–5]. As a result of the combination of knowledge that has been acquired in the synthesis procedures of these two types of materials, pillared sulfated clays (of the smectite type) have been suggested as new materials characterized by a higher acidic strength than non-sulfated clays. Their physical–chemical properties may be modulated in function of the type of oxide that makes up the pillar, the method of sulfating and the sulfate/metal ratio, among other variables [6– 10]. For instance, it has been reported that the pillarization of smectites by intercalation of titanium species results in
* Corresponding author. Tel.: +57 1 3165000x14473. E-mail address: [email protected] (R. Molina). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.018
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an increase in the acidity of the sample, generating both Brönsted and Lewis acid sites [11]. This acidity induced by titanium is higher than the one obtained if any other metal intercalating species is used. In addition, the acidic strength of the system is enhanced when sulfate anions, mainly those associated with the pillar, are incorporated. This effect is due to the ability of S O in sulfate complexes to accommodate electrons from basic molecule, this effect being a strong function of the environment of sulfate ions [12]. Besides this, the typology and properties of the acidic sites found in pillared clays depend on the nature of the parent clay. In the case of vermiculite, this mineral may be considered as swelling trioctahedral micas with Al for Si substitutions in the tetrahedral layers, and Fe and Al for Mg substitutions in the octahedral layers. They are intermediate minerals in the natural weathering sequence of micas to smectites, with a negative layer charge density between that of micas and smectites, and where the charge neutralizing K ions of the parent mica have been replaced by exchangeable hydrated cations, most often Mg2+ and Ca2+, providing partial swelling properties [13]. Unfortunately, the high density of the interlaminate charge present in the vermiculite hinders the intercalation process of the metallic species. Recently, efforts have been applied to diminish the charge density and, consequently, to make intercalation easier [13,14]. This work explores for the first time, the modification of a vermiculite with an intercalating titanium solution and the application of two methods of sulfating in order to study the changes in the acidic properties of the material produced by the presence of titanium and the oxo-anion sulfate. The methods of sulfating evaluated were: (i) intercalation (the sulfate comes into contact with the intercalated solution) and (ii) impregnation over the modified and calcined solids. The type and strength of the acid centers present in the solids obtained were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) experiments, employing ammonia as a probe molecule. On the other hand, the catalytic potential of the acidic sites generated in the solids was evaluated in the hydroconversion of heptane, considering that this reaction is catalyzed by materials of acid nature, including pillared clays [14–16] and whose selectivity towards isomerization or hydrocracking products, is strongly affected by the strength of the acid centers, and the accessibility towards it of the adsorbed species. 2. Experimental 2.1. Starting materials The initial clay mineral is a natural vermiculite of high interlaminar charge coming from Santa Marta (Colombia). The high purity and crystallinity of the material, as well as its high interlaminate Mg2+ cationic content has been recently reported [14,17,18]. The natural vermiculite was crushed and sifted, and the fractions smaller than 150 mm was subjected to hydrothermal treatment (THT, 2.0 L h 1 of 50% water vapour pressure in nitrogen, for 6 h at 400 8C) in order to diminish the interlaminate charge of the material [14,18]. After that, the sample was washed with nitric acid 0.5 M (10 mL of acid per gram of clay) at 80 8C for 1 h under stirring, with the purpose of removing the extra framework species retained [14]. The solid was washed by centrifugation and dried at 60 8C. Finally, in order to approach the clay to a homoionic form and to facilitate the forthcoming exchange process with the polyhidroxication, it was homoionized with NaCl 3 M solution at 80 8C for 1 h, repeating this process three times, renewing NaCl solution every time. Finally washing and drying were carried out as above. The resulting material is called ‘‘vermiculite THT’’. 2.2. Preparation of modified solids Vermiculite THT was modified by intercalation with titanium and titanium sulfate species, employing sulfate/metal molar ratios of 0, 0.25 and 0.35. The titanium precursor solution was prepared according to Valverde et al. [19]. The required volume of a titanium ethoxide in excess of ethanol (Sigma–Aldrich, 20% Ti) solution was slowly added to HCl 5 M, in sufficient quantity to establish a ratio of hydrolysis H+/Ti = 2.2 and to provide a quantity of 10 mmol of metal per gram of clay to be modified (solution 1). For the incorporation of the sulfate into the intercalating solution, the method described by Del Castillo et al. was adopted [9]. A (NH4)2SO4 0.05 M solution was used as precursor of the oxo-anion, mixed with solution 1 and aged 3 h at room temperature under continuous stirring. After that, it was slowly added to a suspension of the vermiculite in water (0.15%, w/w), under constant stirring. Once the addition was
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completed, the suspension obtained was aged under the same conditions for 12 h, then washed by centrifugation and dried for 12 h at 60 8C. Finally, the samples were calcined at 500 8C for 2 h. The solids obtained were called VT, VT (0.25) and VT(0.35), considering the sulfate/Ti molar ratio employed for the modification. Additionally, and with the aim of establishing the effect of the method of sulfating on the modified vermiculites, the solid VT was impregnated at RT with a 0.1 M ammonium sulfate solution for an hour under constant stirring, at a nominal sulfate/metal ratio of 0.35. The excess of water was evaporated through a drying process at 110 8C for 12 h. The solid obtained was calcined at 650 8C for 2 h. This solid was called VT(0.35i) (where i refers to sulfating through impregnation). 2.3. Physical–chemical characterization techniques The textural properties were studied by N2 adsorption measurements at liquid nitrogen temperature. The experiments were carried out in a Micromeritics ASAP 2010 equipment. Before analysis, the samples were degassed for 2 h at 150 8C in vacuum. X-ray diffraction (XRD) analysis was performed on a Siemens D-500 diffractometer. Diffraction patterns were recorded with Cu Ka radiation (40 mA, 40 kV) and a position-sensitive detector using a step size of 0.058 and a step time of 1 s. The titanium content of the samples was determined by X-ray fluorescence spectrometry (XRF) in a Siemens SRS 3000 sequential spectrophotometer with a rhodium tube as the source of radiation. XRF measurements were performed onto pressed pellets (sample included in 10 wt% of wax). Sulfur was analyzed by using an elemental analyzer LECO CS200 apparatus. Sulfur was thermally decomposed at high temperature in presence of oxygen flow, and measured like SO2 by infrared absorption. 2.4. Characterization of the acidic properties of the modified solids 2.4.1. DRIFTS-NH3 DRIFTS spectra were obtained using a Nexus Thermo Nicolet Infrared Spectrometer, with KBr optics and type B MCT detector (Mercury cadmium telluride) cooled off with N2 liquid. A controlled environment reflectance cell (Spectra-Tech 0030–101) equipped with ZnSe windows was coupled to the spectrometer. Before introduction of the adsorbate (NH3), the solid was heated at 400 8C for 1 h in 50 mL/min of nitrogen, cooled down to room temperature (RT) at the same atmosphere and the reflectance spectrum collected. This spectrum was taken as reference for the adsorption experiment. Then, a flow of 35 mL/min of NH3 (2000 ppm in He, Abelló Oxígeno Linde) was passed through the solid for 1 h at room temperature. Later, spectra were taken after outgassing the sample under nitrogen flow (50 mL/min) at room temperature and after heating for 1 h at 100, 150, 200, 300 and 400 8C. Detailed descriptions of similar procedures to study the acidity of samples by using ammonia have been reported elsewhere [12,20]. Spectra were obtained by coadding 100 scans at 4 cm 1 of resolution. They are presented in Kubelka–Munk mode without any other manipulation. In this way, qualitative and quantitative analyses of both gas phase and adsorbed species are possible. 2.4.2. TPD-NH3 TPD-NH3 analyses were carried out by in a Quantachrome TPD/TPR/TPO Chembet 3000 analyzer fitted with a thermal conductivity detector (TCD). Approximately 400 mg of the sample were placed in a quartz reactor and pretreated under nitrogen flow while the temperature was increased at a speed of 20 8C/min up to 400 8C. After 1 h at this temperature, the sample was cooled at room temperature and saturated with a mixture of NH3 in He at 5% (v/v) for 1 h. The surplus ammonia not interacting strongly with the solid was swept out under nitrogen flow until its presence was not detected at the exit of adsorbate gas. Finally, ammonia desorption was carried out by ramping the temperature until 600 8C at 10 8C/min. The detector signal and the temperature were recorded simultaneously. 2.5. Heptane hydroconversion Heptane hydroconversion was carried out in a fixed-bed microreactor under constant flow, operated at atmospheric pressure. The liquid hydrocarbon was saturated with hydrogen by having it go through a saturator at 27 8C, which keeps a
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vapour pressure of heptane of 56.9 mmHg. The catalytic activity was assessed at a WHSV of 1.2 g of Heptane/g of catalyst per hour, at a temperature ranging between 150 and 400 8C. The analysis of the reaction products was performed by gas chromatography with a Shimadzu GC-17 apparatus, equipped with a FID detector and using a ZB-1 column (60 m 0.53 mm 3.00 mm). The details of the reaction conditions have been previously reported [14,15,17]. 3. Results and discussion Fig. 1 shows the XRD diagrams of the considered solids. The d0 0 1 plane of the vermiculite THT is observed at 1180 pm. Besides this, the THT treatment induces a little collapse of the layers, as deduced from the little signal observed at 930 pm. The success of the modification of the solid with the titanium and titanium sulfate intercalation solution is confirmed by the shift of the d0 0 1 plane towards 1380 pm. This shift is low and suggests a low size of the species incorporated, probably due to the intercalation of oligomeric species of low polymerization degree [13,21,22]. Besides this, the calcination process at 500 8C produces a higher collapse of the layers not intercalated, which is observed by the growth of the signal at 930 pm, corresponding with a dehydrated phase of the mineral [23]. Finally, the solid sulfated by impregnation presents the lowest degree of intercalation and the highest degree of collapse of the d0 0 1 plane, as expected due to the higher calcination temperature used (650 8C). In addition, new diffraction peaks corresponding probably to SO3 or MxSyOz are found (XRD taken from 108 to 908 2u not shown). Table 1 shows the specific surface area and titanium and sulfur content of the studied solids. After the hydrothermal treatment (THT), the specific surface area of the material increases significantly, as expected from the extraction of part of the structural aluminum of the material [14,18]. The modification of the vermiculite THT with titanium induces a little decrease in the SBET area. This area loss is higher in the case of the solid modified by intercalation with titanium sulfate species, in such a way that, higher is the sulfate/titanium ratio used, lower is the BET area of the resulting solid. This behavior has been described early for smectite-type clay minerals pillared and sulfated by this method, and explained by blocking of the material pores due to large polymeric species formed in the intercalating solution by the presence of sulfate, which will not form inside of interlayer space, but on the surface of the layers [9]. On the other hand, it is interesting to note that the titanium-to-sulfur ratio in the obtained material is constant, whatever the sulfate/ titanium ratio used in the intercalating solution. This point to the fixation of the sulfur is associated with the incorporation of titanium. The solid sulfated by impregnation presents a much lower SBET area, which can be related with the very high sulfur content of the material, the higher calcination temperature used, and the higher collapse of the d0 0 1 plane detected by XRD. 3.1. DRIFTS-NH3 Fig. 2 shows the DRIFTS spectra obtained after ammonia adsorption at room temperature for the vermiculite THT and for the solid modified with the titanium simple system (VT), as example of the modified solid. Some general
Fig. 1. XRD diagrams of the studied solids.
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Table 1 BET specific surface area and titanium and sulfur content of the studied solids Solid
Ti content (% w/w)
S content (% w/w)
Ti/S atomic ratio
SBET (m2 g 1)
Natural vermiculite Vermiculite THT VT VT(0.25) VT(0.35) VT(0.35i)
0.72 0.72 3.59 4.70 5.29 5.08
– – – 0.044 0.050 4.343
– – – 71.4 70.8 0.78
4 66 59 48 39 14
features can be detected in the spectra. The intensity of these bands is clearly lower, almost absent, in the case of vermiculite THT, indicating that this solid adsorbs a very small quantity of ammonia, and pointing out its very low acidity or, in any case, the non-accessibility of the its acid centers to ammonia molecules [14,17,18]. In the 2850– 3600 cm 1 region, several bands corresponding to N–H stretching vibrations of adsorbed ammonia species are observed [24]. The position and intensity of these bands do not change as function of the sulfate/titanium ratio or the sulfating method used. A negative band at 3737 cm 1 is also detected. This band is characteristic of the structural OH groups (Mg–OH or Al–OH) of the clay, and its negative intensity is indicative of their interaction with ammonia molecules [13]. Finally, strong positive bands at around 1683, 1647, 1440 and 1216 cm 1 are observed. The position of these bands remains unaltered from one sample to another, Fig. 3, even in the spectra obtained at higher temperatures (not shown). The thermal evolution of adsorbed NH3 over the considered solids is not shown since the qualitative DRIFTS spectra is similar to that described at RT. The main difference among the spectra of the modified solids after ammonia adsorption resides in the intensity of these four bands, which can be related with the changes in the acidity of the samples. Bands at 1440 and 1683 cm 1 are ascribed to asymmetric and symmetric bending modes of adsorbed NH4+ species, respectively [20,25], which undoubtedly indicate the presence of Brönsted acid sites on the catalyst surface, associated to the hydroxyl groups responsible of the negative band observed at 3737 cm 1. Bands at 1647 and 1216 cm 1 are ascribed to asymmetric and symmetric bending modes of adsorbed NH3 species on Lewis acid sites of the solids [25]. These acid sites can be associated to titanium atoms of the pillar, since they are present in the VT sample, without sulfate. However, the S O bond of sulfates has been described also as Lewis acid sites capable of adsorbing ammonia [12]. Fig. 3 shows the spectra of the solids of the VT series obtained after ammonia adsorption at RT in the region under 2000 cm 1. It is clear from the figure that the intensity of the bands decrease when the sulfate to metal ratio increases. The lower intensity is observed for the sample sulfated by impregnation. This behavior is better observed in Table 2, where the area of the 1440 and 1216 cm 1 bands, taken as representative of the Brönsted (B) and Lewis (L) acidity, respectively, are showed. On the other hand, the Brönsted to Lewis (B/L) ratio increases with the sulfate content of the
Fig. 2. DRIFT spectra after NH3 adsorption on the vermiculite THT and the VT solid.
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Fig. 3. DRIFT spectra after NH3 adsorption on VT, VT(0.25), VT(0.35) and VT(0.35i) solids.
sample, pointing to the preferential Lewis acidity decay induced by the sulfate. This observation agrees with most reports existing in the literature for titanium pillared sulfated clays, in which Brönsted type acidity prevails [6–9]. The lower acidity showed for by solid sulfated by impregnation (VT(0.35i)), reflects that, although this sulfating methodology results in the fixation of a high amount of sulfur (as compared with the intercalation method), probably the blocking of acid sites, mainly Lewis, is favored by the presence of sulfur that does not directly interact with them. It is worth highlighting that by means of the DRIFT technique, the study of metal–sulfur type interactions, which are in the spectrum region below 1400 cm 1 is not facilitated [26,27], as here. Problems associated to diffuse specular reflection do not allow for the appropriate resolution of the curves [28]. In addition, in this region of the spectrum, M– O–M type flexions should also be found, being M = Si4+ and/or Al3+ [28], which are characteristic of the clay matrix and would prevail over any other type of superficial interactions. In fact, all spectra are dominated by a very intense band at 1226 cm 1, which shifts towards higher wave numbers with the sulfate content, Fig. 4. This band results from the overlapping of the framework M–O–M vibrations, more ore less perturbed by the presence of sulfate, and the bands of sulfate species. In addition, the solid sulfated by impregnation presents new clear bands at 1290, 1154 and 1062 cm 1 characteristics of sulfate/bisulfate species [29], pointing out the formation of a new bulk sulfate phase, in good agreement with XRD data. On the other hand, the presence of water adsorbed, which is characterized by a deformation H–O–H band at 1620 cm 1, makes the analysis of the bands complicated due to Lewis and Brönsted ammonia adsorbed species at 1625 and 1647 cm 1. For that reason, the analysis of the bands appearing after ammonia adsorption is not so easy. The situation is even more complicated for the study at high temperatures, where thermal effects are added. For instance, the intense band at 1226 cm 1, shifts towards higher wave numbers with temperature. Similar effects have been described early for sulfated titania and zirconia [30,31] and ascribed to the presence of water which coordinates with S O bonds. Because of all this problems, only a semi-quantitative study on the stability of Brönsted type acid sites as a function of the desorption temperature, has been carried out, following the band at about 1440 cm 1, Fig. 5. As expected, the Table 2 Brönsted and Lewis acidity of the studied samples, calculated from the area of the 1440 and 1216 cm
1
DRIFTS bands
Solid
Brönsted acidity (a.u.)
Lewis acidity (a.u.)
Brönsted/Lewis ratio
VT VT(0.25) VT(0.35) VT(0.35i)
5.74 1.90 0.79 0.62
3.50 0.50 0.20 0.006
1.64 3.8 3.95 103
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Fig. 4. DRIFT spectra at room temperature of the considered solids.
Brönsted acidity decreases with temperature until disappear in all solids at 400 8C. On the other hand, although the sulfation process, whether by intercalation or impregnation, significantly affects the number of Brönsted acids, the presence of the sulfate anion increases their thermal stability in the structure of the material, since lower is the nominal sulfate/metal ratio, sharper is the slope of the curve characterizing the loss of acidity as a function of temperature. This result provides evidence that, as has been reported for sulfated metal oxides [1,2] and sulfated pillared clays [6–10], the presence of the sulfate anion associated with this type of systems increases the strength of the acid centers found in the material. In the case of solids modified with titanium species, the strength of Brönsted type acid sites is mainly favored, possibly due to the generation of bisulfate species (HSO4 ), where OH groups are bound through the hydrogen atom to a superficial oxygen of the oxide [32,33]. 3.2. Temperature programmed desorption (TPD-NH3) Fig. 6 shows the desorption profiles of ammonia in function of the temperature for the modified materials sulfated by intercalation and impregnation. For all solids there is an ammonia desorption signal around 200 8C, which has been previously reported for clays modified via intercalation pillarization [9,34,35]. This agrees with the observation by DRIFTS of the almost total degassing of the adsorbed ammonia at 300 8C in all solids. In addition, a notorious effect is observed on the maximum temperature for ammonia desorption due to the presence of sulfate, generating an increase
Fig. 5. Brönsted acid quantification results of the considered solids as function of temperature.
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Fig. 6. Ammonia desorption profiles of modified solids.
of around 30 8C in this value for solid VT(0.35), with respect to the non-sulfated material. The above suggests that the presence of sulfate promotes the strength of the acid sites found in these materials, corroborating the results obtained by the DRIFTS analysis. When sulfation is performed by the impregnation method (VT(0.35i)), the desorption profile does not show an intense signal in the range of temperatures observed for the solids sulfated by intercalation, that which can be related with the semi-quantitative results obtained by DRIFTS (Fig. 5). However, for this solid, a very intense desorption signal is observed, focused around 900 8C (Fig. 7) corresponding to the exit of the sulfur found in the material, most certainly as gaseous SO2, which can also be differentiated by the thermal conductivity detector [36]. The absence of a signal with significant intensity attributable to the desorption of ammonia from some type of acid site provides evidence on the possible blockage of acid centers found in these solids as a result of the large amount of sulfur fixed and of its disposition on the material’s surface, showing that not only does the sulfur content affect the strength of the acid sites, but also the type of interaction this can generate in the material. 3.3. Heptane hydroconversion The catalytic activity of solids was assessed in heptane hydroconversion, taking into account the variation in the conversion percentage in function of the temperature, as well as the parameter called T10ISO, whose value corresponds to the temperature at which 10% conversion in isomerization products is reached [14–16].
Fig. 7. Ammonia desorption profiles of VT(0.35i).
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Fig. 8. Heptane total conversion curves for modified solids.
Fig. 8 shows the follow up on the total conversion of heptane in function of the temperature for all modified solids. In all cases, an increase in the catalytic activity of materials modified and sulfated by intercalation is evident, as compared with the vermiculite THT, in spite of the marked effect generated by the incorporation of sulfate groups by the two sulfation methodologies applied on the catalytic activity of solids. In fact, when sulfation is performed by the intercalation method, regardless of the nominal sulfate/metal nominal relation used or the amount of sulfur fixed, the conversion falls in an almost identical form, provoking a reduction of around 22% in the value of the total conversion at the maximum isomerization temperature displayed by the material without sulfating. For the material sulfated through the impregnation methodology (VT(0.35i)), this value gets lower, until it is equivalent to the behavior of the vermiculite used to start out. These results show that for the solids sulfated by intercalation and in which the largest amount of sulfur has been incorporated, visible diffusion problems are generated which cause a marked drop in the activity of catalysis [37,38]. This has also been established for these materials through techniques like nitrogen sortometry and XRD [36]. Besides, as previously discussed, although the presence of sulfate in the solids analyzed promotes an increase in the strength of the acid sites present, it also reduces their quantity as a result of a probable blockage caused by the presence of the sulfate groups. Table 3 shows the parameters for catalytic evaluation obtained for these materials: T10ISO (temperature at which the catalyst reaches 10% isomerization), Tmax (maximum isomerization temperature), conv. t (total conversion to Tmax), YISO (conversion to isomers at Tmax), YCRA (conversion to cracking products at Tmax), SelISO (selectivity to isomers at Tmax), % mono and % di (percentages related to mono and biramified isomers at Tmax). The data recorded in this table clearly show that the system displaying better properties for obtaining ramified derivates as a result of the heptane hydroconversion reaction corresponds to solid VT, for which one of the largest isomerization percentages is achieved (40.2%) at the lowest T10ISO for this group of materials (236 8C), with a noticeable selectivity towards isomerization products (91%). The behavior described may indicate that a larger content of Brönsted type acid sites promotes a larger selectivity for isomerization products in solid VT, taking into account that in some bibliographical reports on the bifunctional mechanism of the transformation of the heptane molecule, Brönsted acid sites are presented as preferential centers for Table 3 Catalytic results in heptane hydroconversion of the considered solids Solid
T10ISO (8C)
Tmax (8C)
conv. t (%)
YISO (%)
YCRA (%)
SelISO
mono (%)
di (%)
Natural vermiculite VT VT(0.25) VT(0.35) VT(0.35i)
– 236 275 270 -
350 300 350 350 375
7.0 44.2 22.5 20.6 5.7
2.6 40.2 18.4 17.0 4.9
4.4 4.0 3.7 3.4 0.6
36.7 91.0 81.7 82.4 86.7
93.3 94.5 94.3 94.6 96.5
6.7 5.5 5.7 5.4 3.5
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the formation of carbocations [38]. Besides, a possible better distribution of such sites may be equally implicit, which favors accessibility of the heptane molecule to active centers at lower temperatures. Likewise, it is observed that within the isomerization products recorded there is a higher selectivity to monoramified isomers, suggesting that the reaction possibly occurs through monomolecular and bimolecular mechanisms through the reordering of alkylcarbenium ions and ß-separation [39], which requires acid sites with intermediate strength. 4. Conclusions Through the acidity study carried out, by means of DRIFTS and TPD-NH3 techniques and heptane hydroconversion, it was possible to establish the existence, on the solids prepared, of both types of acid centers, Brönsted and Lewis type, at a quantity above that present in the parent clay. In addition, it was found that on the materials of the VT series, Brönsted acidity prevails over Lewis acidity, being more stable facing the thermal treatments applied. Likewise, the beneficial effect generated by the presence of oxo-anion sulfate on the strength of the acid centers found in the material was proved, which is not reflected in the selectivity of these materials regarding the heptane hydroconversion reaction, possibly due to the blockage of the acid sites by the high presence of sulfate. Results indicate that the sulfur incorporated by the intercalation method favors specific superficial interactions while the impregnation method provokes the blockage of the acid sites found in the solid, bringing about diffusion problems put in evidence during the evaluation of these materials in the heptane hydroconversion reaction. Finally, it is worth noticing that all materials display high selectivity towards isomerization products. Acknowledgements Our gratitude to joint project CSIC-COLCIENCIAS-2006CO0015 (Spain-Colombia) and the financial support of VRI-DIB at Universidad Nacional de Colombia (QUIPU 20201007579) and Junta de Andalucia (PAI, TEP106). WYH wishes to thank Universidad Nacional de Colombia for the financial support through the ‘‘Outstanding Postgraduate Students’’ scholarship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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