Modulation of the Acidity of a Vermiculite and its Potential Use as a Catalytic Support

1 Modulation of the acidity of a vermiculite and its potential use as a catalytic support Jahaziel Amayaa, Luis Bobadillab, Lola Azancotb, Miguel Centenob, Sonia Morenoa, and Rafael Molinaa*. a Estado Sólido y Catálisis Ambiental (ESCA) Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Kra 30 N°45-03 Bogotá Colombia. b Instituto de Ciencia de Materiales de Sevilla, Centro mixto Universidad de Sevilla-CSIC, 49 Av. Américo Vespucio, 41092 Sevilla, España *E-mail: [email protected] Abstract The modulation and characterization of the acidity of a vermiculite was carried out, which was modified by delamination by means of hydrothermal and acid treatments with the subsequent incorporation of AlZr and AlCe species to modulate the acidity. The effect of these species was evaluated regarding the structural (XRD, XPS and IR), textural (N2 sortometry), and acidity properties (NH3-TPD, NH3-DRIFTS and CO adsorption at low temperature). The catalytic performance was studied in the dehydration-dehydrogenation reactions of 2-propanol and the hydro-conversion of decane, which generate important information about the acidity properties such as the type, number and strength of acidic sites. The correlation between the number, type and acid strength with the catalytic behavior allowed to establish the important effect regarding the nature of the mineral, its method of delamination and the nature of the incorporated cation, thus generating tools for controlled processes for the potentiation of the acidity of new supports from raw vermiculite. Keywords: Delaminated vermiculite; acid modulator AlZr and AlCe; characterization of acidic properties; probe molecules; decane hydroconversion. Introduction Acid catalysts are used in different industrial processes such as: isomerization of olefins, dehydration of alcohols, Beckmann transitions, isomerization of paraffins, alkylation, cracking, dehydration, isomerization and alkylation [1] among many others, demonstrating the importance of this type of catalysis on an industrial level [2]. In this 2 context, the oil refining process has stimulated the search for solids with acid surface properties which are widely used in catalytic cracking, hydrocracking and hydroisomerization processes [3]. The systematic study of clay minerals has been carried out for several decades given its natural origin, abundance, low cost and the processes of modification and potentiation of its environmentally friendly intrinsic acidity. Among the many uses for which it has been proposed is as an alternative for the development of acid supports or as catalysts for various applications, including use as an alternative to replace zeolites in catalytic processes [4]. Vermiculite in particular has attracted attention in acid catalysis due to its high interlayer charge attributed to the greater number of isomorphic substitutions of Si4+ for Al3+ in the tetrahedral layers compared with those present in montmorillonites [5]. This decompensation of layer charge generated by the isomorphic substitution described, promotes the formation of sites with an acid character, either because the deprotection of positive charges allows for the acceptance of electronic pairs (Lewis acidity) or because it can capture protons and then yield them (Brönsted acidity) [6]. However, the substitution of Si4+ ions for Al3+ ions leads to an excess of negative charge in the structure, which is compensated by Mg2+ or Ca2+, which results in a poor swelling capacity [7], thus making it difficult to be modified in the interlayer. For the reduction of said excess layer charge, various processes of the natural mineral have been reported, such as HydroThermal Treatment (HTT) [8], the use of plasma [9], ultrasound [10], cationic or anionic surfactants, peroxide [9] or acid [11] or a combination of physical and chemical treatments, such as the use of peroxide with ultrasound [12]. These processes carried out on the raw vermiculite facilitate its subsequent structural modification. In this way, the mineral has been intercalated with organic surfactants [13], the sheets have been 3 exfoliated or opened (delamination) [11, 14] and, after employing HTT, the vermiculites have been modified with Zr and Ti species by a method of sulphation, generating more acidic sites in the material [15], or pillared with mixed oxides of Al-Zr, Al-Ce and Al-Hf, obtaining a catalytic performance similar or higher to catalysts based on USY zeolites in the decane reaction [6]. On the other hand, the use of probe molecules is a central tool in the determination of acidity in the solid state since it allows for obtaining specific information such as the number, type, strength and distribution of acid sites [16]. Accordingly, a large variety of probe molecules have been used, such as pyridine, trimethylphosphine oxide, carbon monoxide, quinoline and ammonia, among others, in order to determine and discriminate acid sites in the solid state [17]. Among these, ammonia is probably the most widely used in acidity measurements due to its strongly basic character, high thermal stability and its easy access to acidic sites in pores larger than 4 Å (3.7 x 3.9 x 3.1 Å) [18]. On the contrary, the adsorption of CO at a low temperature (-196 ºC) by FTIR has been less used in the characterization of clay minerals. It allows for obtaining useful information concerning the strength of the acid sites of the solids [19] and has not yet been reported to evaluate the acidity in vermiculites. On the other hand, catalytic reactions such as the dehydration of an alcohol and the hydro-conversion of decane generate important information not only regarding the catalytic performance but also concerning the acidic characteristics. In the first case, dehydration occurs in the acidic sites producing the corresponding alkene while acetone will be produced in the basic sites [20]. Decane hydroconversion, meanwhile, allows for evaluating the type, strength and distribution of the acid sites in the solids [21]. Although, as already mentioned, pillarized vermiculites have shown their great potential in hydroconversion reactions, it is striking that there are still no reports of delaminated vermiculites for this type of reactions, which could favor the traffic of bulky molecules 4 for adsorption and catalysis processes as well as providing a greater availability of acidity than that offered in the pillared mineral. In this context, this work has focused on the study of the evaluation of the main physicochemical properties generated by the delamination and subsequent incorporation of AlZr and AlCe species in a raw vermiculite. With special emphasis on the evolution of acidic properties, to our knowledge, there are still no reports in the literature regarding delaminated vermiculite using HTT for the reduction of the layer charge, an acid treatment to achieve the exfoliation of the mineral sheets, nor of its modified acid potential with AlZr or AlCe species, nor of the characterization of acidity using CO-FTIR and dehydration dehydrogenation of propanol. This contribution allows for generating tools for the use of these materials in future applications. Experimental Starting material The starting mineral used was a vermiculite from the Santa Marta region, Colombia, which has been the subject of other studies and part of its characteristics have been previously reported [8, 22]. Its cation exchange capacity is 1.1 meg·g-1 and has the following structural formula: [(Si3.04Al0.92Ti0.04)(Al0.11Fe3+0.35Fe2+0.07Mg2.41Mn0.003) O10(OH)2]Ca0.21K0.05Na0.10. Modification of the mineral. Delamination process. The vermiculite was subjected to a HydroThermal Treatment (HTT) previously reported [8, 22], which allows for the reduction of the mineral layer charge by eliminating part of the aluminum located in the tetrahedral sheet, thus producing a layer charge reduction facilitating the following modifications. Subsequently, a 10% dispersion of the vermiculite in distilled water was left in contact with a solution of 2 M nitric acid (10 mL HNO3·g-1 vermiculite), for 4 h in constant agitation and between 90-95 ºC to generate the 5 opening and/or exfoliation of the mineral sheets. The solid obtained was washed with distilled water by centrifugation, dried at 60 °C, and calcined at 400 °C for 4 h, increasing the temperature at a rate of 10 °C·min-1 [8, 22]. Synthesis and incorporation of the AlZr or AlCe phase. The incorporation of the AlZr and AlCe phases in the delaminated supports was carried out using Al(NO3)3, ZrOCl2 and Ce(NO)3 [6] as precursor compounds. A 90:10 ratio of aluminum-metal [6, 23] was handled in a 0.1 molar solution which was subsequently hydrolyzed with NaOH, maintaining an OH/metal ratio=2 [24]; this solution was stirred for 1 h at 300 rpm and subsequently brought to pH=9 using 1 M ammonium hydroxide and generated the formation of a solid which was washed to a conductivity close to that of deionized water [25]. This solid was dissolved in acid with an H+/metal ratio = 0.27 and taken to ultrasound for 10 min. Subsequently, impregnation was carried out on the clay at 30%, and it was stirred for 2 h, followed by drying overnight at 60 °C. Finally, the solids were calcined at 400 °C for 4 h with a temperature ramp of 10 °C·min-1 [25]. The solids obtained were denominated VN for the raw vermiculite, VD for the process of delamination, and VD- AlZr, VD-AlCe, for the delaminated minerals subsequently modified with AlZr and AlCe respectively. Characterization The X-ray diffraction patterns were performed on a PANalytical X'PertPRO MPD equipment with Cu copper anode (λ = 1.54056 Å), using a speed of 0.02θ with 2s. Prior to analysis, solids were screened in a 100 ASTM mesh, diffractograms were obtained at room temperature [25–27]. The XPS measurements were performed on a X(NAP-XPS) Specs with a PHOIBOS 150 1D-DLD analyzer, equipped with a monochromatic source Al-Kα (1486.7 eV, 13 kV, 100 6 W) with a step energy of 30 eV and step of 0.1 eV. 30 measurement cycles were performed and charge compensation of 2 2 eV and 20 µA was used. The specific surface area was obtained from the nitrogen adsorption isotherm at -196 °C with a Micromeritics ASAP 2020 instrument. For the determination of the specific surface area the BET model was used, the external and micropore area was calculated using the Harkins-Jura equation and the pore size distribution with the BJH model, based on the Kelvin equation and applied in the desorption branch of the isotherms [25, 27, 28]. Total acidity analyzes (NH3-TPD) were carried out with a Micromeritics AutoChem 2920 device. 200 mg of the solid were used, which were degassed in He for 30 min at 350 °C, with a temperature ramp of 10 °C·min-1 and a flow of 30 mL·min-1. Subsequently, it cooled to 50 °C and ammonia was adsorbed for 30 min (5% NH3/He). The assembly was then purged with He for 1 h and finally brought to 700 °C with a ramp of 10 °C·min-1 [25, 29]. Diffuse reflectance infrared Fourier-transform spectra (DRIFTS) were measured on an FT-IR spectrometer (IR Tracer-100) equipped with a Harrick Praying Mantis accessory and a high-temperature reaction chamber for diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies of catalysts, which is carried out according to previous reports [25, 27, 30, 31]: i) Pre-treatment of the solid at 400 °C with an N2 flow of 30 mL·min-1 for 1 h, ii) Adsorption NH3 (g) for 15 min at room temperature at 30 mL·min-1, iii) Desorption for 30 min with a N2 flow at 30 mL·min-1 at room temperature experiments in a Praying Mantis chamber. All spectra were averaged from 150 scans collected at a resolution of 4 cm−1 and were converted to an absorption spectrum using the Kubelka–Munk function. For the adsorption of CO at low temperature analyzed by FTIR, the solids were pressed on self-supporting wafers of approx. 12 mg cm-2 and activated in a quartz cell placed in 7 the IR beam, at 550 °C in a vacuum for 1 h. After pretreatment, the solids were exposed to CO doses until saturation at -196 ºC by using a trap cooled by liquid nitrogen and then evacuated. The transmittance spectra were recorded with a THERMO NICOLET Avatar 380 FTIR Spectrophotometer, equipped with a DTGS/KBr detector and accumulating 128 scans at a spectral resolution of 4 cm-1. The spectra were processed using Nicolet OMNICTM software [32]. For the dehydration-dehydrogenation analysis of 2-propanol, a pretreatment was carried out at 50 mL·min-1 of He for 1 h at 400 °C, then 50 mL·min-1 of He was passed through a saturator of 2-propanol at 0 °C. The saturated mixture was fed into a fixed bed reactor loaded with 100 mg of solid increasing the temperature from 100 to 400 °C at 10 °C·min- 1. The reaction was followed by mass spectrometry, using a Balzers Thermostar controlled by Balzer Quadstar 422 software with capabilities for quantitative analysis. Signals from 1 to 200 m/z were recorded. Among them, 45, 43, 41, 18 and 2 m/z were assigned to 2-propanol and the main reaction products: acetone, propene, water and hydrogen, respectively [33]. Decane hydroconversion was performed using 300 mg of the solid (particle size between 125 and 150 µm) in a U reactor (with internal diameter of 3 mm, length of 26 cm, with a bulb of 5 cm with internal diameter of 1 cm, the bubble was to the first 4 cm of the reactor. The reactor is filled with SiC and the catalyst is located inside the bulb) at atmospheric pressure. The pretreatment of the solid was carried out at 400 °C at a heating rate of 10 °C·min-1 and was maintained in a flow for 2 h 50 mL·min-1 of dry air. Then, a cleaning with N2 for 15 min and a flow of H2 was passed for 2 h in order to activate the catalyst. The catalytic activity was carried out with a H2/hydrocarbon ratio of 17, with a total flow of 16 mL·min-1 and the WHSV = 1.16 h-1, in a temperature range of 150 °C to 400 °C, making measurements at 25 °C intervals. The reaction products were analyzed online by 8 gas chromatography using a Shimadzu GC-17 chromatograph, with a ZB-1 column (60m x 0.53mm x 0.32 μm) and a FID detector. The catalytic activity was expressed in terms of conversion, yield to isomerization (monobranched: methylnonanes, dibranched: ethyloctanes, and tribranched: propylheptanes) and cracking (all products with less than 10 carbon atoms), as previously reported, the error is ±3% in all cases [25, 27, 31, 34]. Results and discussion Structure and Texture Fig. 1. registers the characterization by XRD of i), the raw mineral, ii) following the HTT, (iii) following the delamination process and iv) with the incorporation of AlZr and AlCe. As can be seen, the reflection clearly defined for VN at 6.40 °2θ, indicates that the starting vermiculite is of high purity and with a high degree of crystallinity [22]. This can be attributed to the highly localized negative charge exhibiting a high three-dimensional order [22]. In addition, the characteristic reflections for vermiculite (9.8, 12.38, and 18.60, °2θ) are observed according to the JCPDS records: 01-076-0847 and 00-016-0613 [11, 35]. Figure 1 Diffractograms of raw vermiculite (VN), following hydrothermal treatment (V-HTT), delaminated (VD) and the different modified solids. v = vermiculite. v v d(001) v 9 The reflection found at 6.40 °2θ corresponds to an interlayer space of 14.9 Å. After the HTT process, a shift towards higher º2θ values is observed, which indicates a decrease of interlayer space between 12 and 13 Å, this effect is attributed to a closing of the sheets due to the extraction of Al from the tetrahedra of the clay showing that only the HTT process does not have the capacity to delaminate the material. After the acid treatment, the disappearance of said reflection (6.40 °2θ) is observed, which indicates an effective delamination of the vermiculite, indicating the obtaining of a faceface, face-edge and edge-edge order in the mineral [36], evidencing in this case the beneficial effect of this process on the structural properties of the mineral. Finally, the absence of crystalline phases for Zr (17.43, 28.2, 31.45, 34.25, °2θ) or Ce (28.54, 33.11, °2θ) oxides for the materials is observed, which can be attributed to the fact that Zr or Ce are highly disordered on the surface [37] or, also, to the low percentage of these oxides used with a high dispersion located below the detection limit of the technique [38]. In contrast, the results by XPS (Fig. 2.) suggest the existence of interaction between Zr or Ce cations with the mineral. In the case of Zr, the 3d signal registers a doublet located at 182.8 eV with a separation of 2.4 eV (Fig. 2a.), corresponding to the species Zr4+ in whose first coordination sphere oxygen is found [39]. These values indicate the absence of ZrO2, which presents a signal at 182.2 eV (corroborating what is described by XRD). The shift in signals with respect to pure ZrO2 suggests the formation of Zr4+ species linked to more electronegative species [39]. Therefore, is likely that the Zr is interacting with the mineral structure formed by the species: i) AlZrOX-, ii) ZrSiO-(H), iii) OH groups associated with Zr, as reported for pillared clays or iv) Zr-Si whose presence is evidenced by the shift of Si 2p in the XPS verifying that the Zr favors both the existence of species 16 For the AlCe system, a trend similar to that discussed for AlZr is observed, obtaining greater BET areas and especially mesopores, compared to pillarized minerals (BP-AlCe; BET = 55, Smeso = 25, VP-AlCe; BET = 30 , Smeso = 12) [6, 23], favoring the greater distribution of AlCe species on the surface (consistent with the XPS results), improving acidic properties, even greater than the delaminated bentonite and the incorporation of the same species [25]. In the case of the maximum ammonia desorption temperatures, following the delamination process an increase in the number of acid sites with greater strength for VD is observed, going from VN=3 μmol of NH3·g-1 (97 ºC) to VD = 257 μmol of NH3·g-1 (121 ° C). On the other hand, the incorporation of the AlZr and AlCe species slightly increases the desorption temperatures by 16 and 6 °C respectively, attributed to the fact that the use of Zr for the modification of clays increases the acid strength [53], which is mainly associated with Lewis type acidity [54]. In the case of Ce, this increase is related to the stabilization of Brönsted type acid centers [55]. These results corroborate that the incorporation of these species in this delaminated mineral not only increases the number of sites (from 257 to 392 μmol) but also the strength of these (from 121 to 137 ºC) as has been reported [56]. To have a better understanding of the type and strength of the acid sites obtained, the use of NH3-DRIFTS and the adsorption of CO at -196 ºC were used as complementary techniques to discriminate between the Brönsted or Lewis types and to understand the role of the M+3, M+4 cations present in the solids. Fig. 4. illustrates the infrared spectra (in black) corresponding to the different materials obtained after degassing the samples at 400 °C in a N2 atmosphere for 1 h. In all cases, the signals characteristic of the presence of OH groups are observed in the wave number range of 3400 to 3650 cm-1, characteristic of clay minerals. On the other hand, the signals 17 can also be observed at 3600 cm-1 and 3650 cm-1 which are attributed to SiOH groups close to aluminum in tetrahedral coordination and to AlOH [57, 58] species, respectively. The bands around 3620-3630 cm-1 correspond to the characteristic vibrations of the structural OH groups; The shoulder around 3270 cm-1 can be attributed to a water overtone linked with a vibration visible around 1650 cm-1. The band at 1200 cm-1 is due to the vibration of the silicon-oxygen bond, within which are included the vibrations of the apical silicon-oxygen units which are perpendicular to the sheets (oxygen atoms shared by tetrahedral and octahedral sheets) and the stretching vibrations of this bond within the tetrahedral sheet. A broad band around 1200 cm-1 corresponds to quartz O-Si- O [59]. Figure 4. DRIFTS spectra at room temperature, then 1 h at 400 °C in N2 of a) VN, b) VD, c) VD-AlZr, and d) VD- AlCe, before (black) and after (red) NH3 adsorption. * Brönsted acid sites, ** Lewis acid sites and *** physiosorbed ammonia. The band at 610 cm-1 is related to the perpendicular vibration of the octahedral cations (R-O-Si), where R can be Mg or Al [60]. The partial substitution of magnesium by Al and Fe is reflected in the variety of complementary signals in this same region. The groups AlOH a b c d Si-O SiOH AlOH OH Si-O O-Si-O R-O-Si ** * *** * ** * *** *** ** 18 AlOH, AlFeOH and AlMgOH are related to vibrations at 925, 880 and 750 cm-1 respectively [60]. On the other hand, the main differences of the spectra between the different solids are found in the hydroxyl region located between 3000 and 3600 cm-1 (Fig. 5.). There, raw vermiculite has a broad band and poorly defined signals, which can be attributed to a greater amount of water associated with its structure [61] compared to the mineral after the delamination process which presents a clear signal at 3750 cm-1; these signals are assigned to bridged hydroxyl groups through pairs AlVI-AlVI and AlIV-AlVI of the delaminated minerals [62]. The spectra for the VD-AlZr and VD-AlCe solids are like that of the non-delaminated material. However, there is an increase in the width and relative intensity of the band close to 3680 cm-1 that can be associated with "external" silanol groups which, following the process of incorporation of Al-M species, are more exposed on the surface. On the other hand, when the surface charge is concentrated in the oxygens involved in the Si-O-Si bonds, the hydrogen bound to the hydroxyl groups of these oxygens absorbs in the region near 3440 cm-1, overlapping the region of absorption associated with water-water connections and increasing the amplitude of the band [63, 64]. In addition, a change in the region of 3000 to 3800 cm-1 is observed for the solids VD-AlZr and VD-AlCe with respect to the delaminated material, with a wide shoulder in the region of 3620-3300 cm-1, suggesting a possible change in the structure of the clay during the process of incorporation of the metals originated by the presence of small strongly polarized groups such as Al3+, due to the Zr or Ce located in the clay sheet [64]. The small signals observed at 3746, 3735 and 3675 cm-1 for the materials after the incorporation of AlZr or AlCe are characteristic of hydroxyl groups coordinated to two or three M4+ cations. On the other hand, the bandwidth at 3550 cm-1 can be attributed to the presence of the amorphous phase of zirconia or ceria [65] which correlates with the 19 XRD results obtained, where it was concluded that the incorporated species are highly disordered. This characteristic is highlighted in the spectrum region around 3000 cm-1, where the bands attributed to the stretches of OH groups, both free and structural [60], are greater in the materials with Ce than in those with Zr. In addition to the identification of the typical functional groups of a clay, the evaluation of the acidic properties of the solids was carried out using DRIFT and the NH3 as a probe molecule. In this way, the interaction between the acid support and the probe molecule was studied, which generates mainly two vibration groups: i) at 1425-1460 cm-1 corresponding to the presence of protonated ammonia associated with the interaction with Brönsted sites and ii) 1600-1640 cm-1 of ammonia coordinated to Lewis acid sites [31, 34]. Figure 5 DRIFTS spectra in the OH region, of the different solids, after degassing at 400 °C for 1 h in N2. As illustrated in Fig. 4(a), the VN before and after the adsorption of ammonia does not present significant differences, despite the potentially acidic sites generated by the high interlayer charge density that characterizes this material [48], which is consistent with the results found by NH3-TPD. This apparent contradiction is explained by the structural position of the acid sites in the mineral that hinder access and interaction with the probe 3750 cm-1 3680 cm-1 3300 cm-1 3440 cm-1 3550 cm-1 20 molecule. After the delamination process an increase in the acid sites was observed, reflected in the growth of the bands at 1625 and 1450 cm-1 after the adsorption of ammonia (Fig. 4(b)), confirming the presence of the acid sites belonging to the material and now available, in coherence with the delamination process as a first step to potentiate the raw acidity of the mineral. On the other hand, it was corroborated that the incursion of the AlZr and AlCe species also generates an increase in acidity, as recorded in Fig. 4(c-d), confirming that the use of these cations allows for the controlled modulation of the acidic properties of the materials, improving the extrinsic acidity of the same. To semi-quantify the increase in the acid properties of the starting minerals and following each of the modifications, the proportion and strength of the acid sites during desorption of NH3 from ambient temperature to 400 °C was determined. For this, the bands corresponding to the Brönsted and Lewis sites were integrated in the spectra at different temperatures, assuming that the concentration is proportional to the area under the curve [31]. Fig. 6. records, by way of illustration, the spectra obtained for VD after degassing and the interaction with NH3 at different temperatures, evidencing the decrease of NH3 adsorbed in the modified mineral as a function of the temperature. Figure 6. DRIFTS spectra of delaminated vermiculite, after degassing, and desorption of NH3 at each of the different temperatures. 21 In this way, the type, number and strength of the acid sites for the raw mineral, delaminated and following the incorporation of the AlZr and AlCe species were determined. Fig. 7. describes the trends found in the NH3 desorption as a function of the temperature between 25 ºC and 400 ºC. These results corroborate the difficulty of accessing the VN acid sites and the effect of the delamination process in exposing the intrinsic acid sites of the mineral, which present a great strength, if compared to a bentonite under the same treatment [25], as indicated by the slopes of the desorption curve. Figure 7. Area under the curve of the bands at 1450 cm-1 Brönsted acidity, (a) and 1650 cm-1 Lewis acidity (b), as a function of the degassing temperature after adsorption with ammonia. On the other hand, the beneficial effect on the acidic properties generated by the incursion of AlZr and AlCe metals is clearly observed, which is mainly associated with the acidic character of the metal oxides incorporated on the surface of delaminated minerals and discussed previously. It was also determined that it is the solids with presence of Ce that present the highest acid values determined by both NH3-TPD and NH3-DRIFTS as shown in Fig. 8. a b 22 Figure 8. Correlation between total acidity determined by NH3-TPD and NH3-DRFITS at 25ºC. In general, the solids with Ce exhibit mainly Brönsted acidity, which is attributed to Ce's ability to stabilize said acid centers [55]. In this sense, the ratio of Brönsted acid sites to Lewis acid sites (B/L) is higher for AlCe modified solids compared to those modified with AlZr (Table 1), highlighting a greater formation of Lewis acid sites when the AlZr species are incorporated, consistent with literature reports [66]. It is important to note that for solids modified with AlZr and AlCe both types of acidity (Brönsted and Lewis) are still maintained at high temperatures (400 ºC), indicating their stability. Finally, it is highlighted that the solids with Ce in their structure have a higher acid strength, exhibiting a greater number of protonic acid sites. To obtain information regarding the resistance and number of Lewis acid sites and to relate this to the observed differences in the behavior of the acidic properties the CO adsorption was carried out at low temperature (-196 ºC) followed by FTIR measurements. The IR spectra of the solids were recorded after 1 h of activation at 550 °C in a vacuum, in the stretching regions for increasing amounts of adsorbed CO (with the spectrum of the activated sample subtracted as a reference) for the different materials (Fig. 9). This adsorption of CO generates coordination bonds with sites of acidic Lewis cations, causing a disturbance of the hydroxyl group vibration on the surface that results in an upward displacement of the CO stretch mode at 2143 cm-1 compared to the one registered for the 23 solid without adsorption. The extent of these changes provides useful information concerning the Lewis strength of acid sites in solids [67]. Figure 9. FTIR difference spectra after the adsorption of CO in the ν region (CO) for the different solids. It is observed that the CO adsorption results in the appearance of a single band in the ν region (CO) at between 2140 and 2170 cm-1, and the increase in CO pressure leads to an increase in its intensity. In this sense, the number and strength of the acid Lewis sites are compared to the different solids (Fig. 10). It’s observed that the raw mineral does not have adsorption of CO, which is attributed to the few acid sites to which this material has access to. Moreover, after the delamination process, an increase in the adsorption of CO is observed, associated with the Lewis acid sites to which it now has access, this effect agrees with what is observed by the NH3-TPD and NH3-DRIFTS of ammonia. On the other hand, after the incorporation of AlZr and AlCe, there was a pronounced increase in the number of Lewis acidic sites in the materials, being more significant for AlCe in comparison with AlZr (Fig. 10 (a)). In this way, to give a practical interpretation of CO 24 adsorbed in cations of Lewis sites, a correlation has been proposed between the CO interaction energy and the wavenumber change illustrated in equation 1 [68]. ∆𝐻( 𝑘𝐽 𝑚𝑜𝑙)=10,5+ 0,5 ∆𝜈 (𝐶𝑂) 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1 For the case of raw and delaminated mineral, signals were found at 2135 and 2158 cm-1. The band at 2135 cm-1 appears very close to the CO gas and can be attributed to the weakly adhering CO in the pores of the clay [68], in addition, the band at 2158 cm-1 could be associated with the interaction of the silanol or aluminol groups with the CO [68], generating corresponding energies to an ΔH of -8 and 13 kJmol-1 respectively, (Fig. 10(b)). Figure 10. a) Relationship of the area with the amount of CO adsorbed. b) FTIR for the different solids after the adsorption of the maximum of CO in the ν region (CO). The inset shows the estimated adsorption of heat for the adsorption of CO in the different sites of Lewis acid cations. While, following the incorporation of the AlZr and AlCe species, there is a notable increase in Lewis type acid strength (the bands observed at higher frequencies indicate greater interaction when Ce and Zr cations were added). The AlZr-modified solid presents a slight shift in the signal of adsorption of CO (2140 and 2170 cm-1; ΔH= 24.5 kJmol-1) compared to the AlCe (2137 and 2165 cm-1; ΔH= 21.5 kJmol-1), which can be associated with a slight increase in the Lewis type acid strength (discussed previously). This is associated to the different chemical species with which the CO interacts. In this way, they correspond to 2140, 2170, 2137 and 2165 cm-1 and are attributed to the Zr or Ce cations, a b 25 coordinated with CO respectively [68, 69], which, as observed by XPS are more likely to correspond to Zr4+ and Ce3+. However, absorption near 2140 can also be attributed to species from pseudo-liquid CO species observed after CO saturation as reported [70]. Considering the Δν changes of the ν band (CO) for the different Zr and Ce cations, it can be concluded that the presence of small amounts of these cations increases the acidity of the materials. These results were previously discussed by NH3-DRIFTS and NH3-TPD, which indicates that, in effect, the solids incorporated with AlCe present the highest number of acid sites, while with Zr there is a slightly greater strength of acidic Lewis sites. Catalytic activity Fig. 11. illustrates the evolution of the intensities of the signals of 2-propanol, acetone, propylene, water and hydrogen respectively, during the dehydration-dehydrogenation of 2-propanol. Figure 11. Monitoring of different species as a function of temperature and time during the dehydrationdehydrogenation of 2-propanol. 32 de Antioquia, especially to Andres Moreno and Natalia Suarez from the QUIREMA Group, for their valuable support in carrying out the NH3-TPD analyzes. Conflicts of interest There are no conflicts to declare. 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