La-Modified SBA-15 Prepared by Direct Synthesis: Importance of Determining Actual Composition

Abstract

Lanthanum (La) integration (at various nominal contents) in SBA-15 prepared under acidic medium was intended from corresponding direct nitrate addition during mesoporous silica formation. Materials were impregnated with Pt (1.5 wt%) and studied through several textural (N₂ physisorption), structural (XRD, TG-DTG), and surface (FTIR, STEM-HAADF, SEM-EDS, NH₃, and CO₂ TPD) instrumental techniques. Pt-impregnated solids were tested in phenol hydrodeoxygenation (HDO, T = 250 °C, 3.2 MPa, batch reactor, n-decane as solvent). Catalytic activity (in pseudo-first-order kinetic constant, k_HDO basis) was not directly related to Pt dispersion, which was not determined by nominal rare earth content. Determining the actual composition of modified SBA-15 materials is crucial in reaching sound conclusions regarding their physicochemical properties, especially when La modifier is directly added during mesoporous matrix formation, where efficient interaction among constituents could be difficult to get. Otherwise, results from some characterization techniques (N₂ physisorption and FTIR, for instance) could be misleading and even contradictory. Indeed, extant modifier precursors, when under SBA-15 synthesis conditions, could affect the properties of prepared materials even though they were absent in obtained formulations. Performing simple compositional analysis could eliminate uncertainties regarding the role of various modifiers on characteristics of final catalysts. However, several groups have failed in doing so.

1. Introduction

SBA-15 materials possess interesting textural characteristics related to high surface area, tunable pore size, and large pore volume. Also, thick walls (3–6 nm) could impart significant hydrothermal stability. Those Si-based mesostructured solids have been used as supports of various catalyst types due to their uniform porous network that could enhance reactant/product diffusion/counter diffusion to/from active sites. However, SBA-15 surface is not particularly suitable for dispersing impregnated phases, mainly due to its inertness (very low acidity [1]). Significant efforts have been aimed at functionalizing those rather inert surfaces by introducing diverse inorganic/organic species that could be utilized to tailor silica surfaces to specific applications [2]. For instance, titania was efficiently introduced in SBA-15 matrices, resulting in improved Pt dispersion [3]. However, the method used to modify SBA-15 solids could be crucial in determining the textural, structural, and surface properties of the final doped materials. Regarding binary catalysts containing lanthanum, our group reported on the properties in hydrodeoxygenation (HDO) reactions of Pt (1 wt%) supported on La-modified alumina where rare earth at various contents was introduced by incipient wetness impregnation [4]. Beneficial effect on guaiacol HDO (as to material not modified by rare earth addition) conversion for catalyst at 1 wt% La was observed. However, selectivity to deoxygenated compounds decreased, those effects being related to surface basicity imparted by La domains. Thus, investigation on noble metal (NM) with La-SBA-15 support was envisaged. Efficiently incorporating lanthanum into structured silica matrices could be difficult. For instance, it was reported [5] that instead of obtaining intended Si/La ratios (20 and 60 in SBA-15 solids), actual ones were much bigger where rare earth salt precursor was directly introduced during silica matrix synthesis. The authors claimed that those tiny quantities of incorporated La were due to the high solubility of rare earth nitrate in the low pH reaction media. Thus, determining the actual amount of integrated modifier becomes crucial in relating the physicochemical properties of catalysts to their performance in various reactions. However, in several cases, concentration of components was assumed without analytically determining the real ones. For instance, Roy et al. [6] prepared V-modified SBA-15 (Si/V = 20, from NH₄VO₃) directly added during SBA-15 hydrothermal synthesis under acidic medium (by HCl addition) used as support of Pt-Pd solids tested in toluene hydrogenation (HYD). The authors reported improved HYD over the V-modified material, attributing that to enhanced noble metals (NM) dispersion. Although the difficulty of heteroatom integration in siliceous matrix prepared under acidic medium (due to V-O-Si linkage dissociation at low pH) was mentioned by the authors, prepared materials were characterized through some textural and surface techniques, but their composition was not actually determined. Thus, efficient vanadium integration in the SiO₂ matrix was just assumed. Probably, that approach could be better applied in the case of catalysts obtained by post-synthesis modifier impregnation on already-preformed SBA-15, where heteroatom deposition could be more controllable. As it could be seen from results reported in the present article, analysis through physicochemical characterization techniques could be misleading if materials compositional analysis is not included.

In the present contribution, La integration (at various nominal contents) in SBA-15 matrices prepared through a well-known protocol [7] was intended from corresponding nitrate addition during mesoporous silica formation (acidic medium). Materials were impregnated with Pt (1.5 wt%) and studied through several textural, structural, and surface instrumental techniques. Catalysts were tested in phenol (P, model species representing oxygenated aromatics in depolymerized lignin [8]) hydrodeoxygenation (HDO). Catalytic activity was not directly related to NM dispersion, which was not determined by the nominal rare earth content in the supports.

2. Results and Discussion

2.1. Materials Textural Properties

SBA-15 pristine matrix and corresponding La-modified materials had type IV N₂ adsorption isotherms (Figure 1a, nomenclature defined in Section 3. Experimental) with H1 hysteresis, typical profiles of mesostructured solids with cylindrical pores [9].

No significant differences in isotherm shapes were observed at increased nominal La concentration in reaction media, as opposed to the case of Ti-modified SBA-15 at various compositions [3], suggesting low rare earth-silica network interaction. However, important diminutions in N₂ adsorbed volume at saturation were registered for samples at various nominal rare earth contents. Changes in pore volume (V_p) did not follow a definite trend with lanthanum concentration, although those of nominally doped solids were all minor to that of pristine SBA-15. Textural properties of studied carriers did not show (

Table 1. Textural properties of SBA-15 and modified materials at several nominal La contents.

Sample	SBET (m2g−1)	Vp (cm3g−1)	aDp Peak 1	(nm) Peak 2	bSBET (m2g−1)	SBET/bSBET
SBA-15	793	0.98	3.98	6.6	-
La1	706	0.78	4.09	5.10	785	0.90
La2	582	0.61	3.93	5.66	777	0.75
La4	674	0.75	3.89	5.60	761	0.89
La8	685	0.76	3.86	5.58	730	0.94

^a Pore diameter from the BJH plot and desorption branch data. ^b Theoretical value considering well-dispersed non-porous La₂O₃ phase component.

) correspondence with nominal amounts of lanthanum modifier used. As a reference, the theoretical surface area of binary solids considering incorporation of non-porous La₂O₃ components in SBA-15 matrices (^bS_BET) was included (sixth column, Table 1).

Ratio of actual surface area (S_BET) respecting theoretical value for solids at given La₂O₃ content (^bS_BET) did not show good concordance (excepting La8) for solids at several nominal La concentrations. SBA-15 pore size (D_p) distribution (PSD, from the Barret–Joyner–Halenda method, BJH, desorption data branch) was alike to those previously reported (maxima at ~6 nm accompanied by a minor peak at ~4 nm) when studying corresponding Ti-modified solids prepared by the alike technique (Figure 1b) [3]. Although shift to lower diameter (just for major peak) was registered for nominally La-modified samples, in general PSD was alike to that of pristine SBA-15. Again, that suggested weak La-SBA-15 interaction, if any. It is worth mentioning that considering the P kinetic diameter (~0.6 nm) PSD of prepared materials seems to be adequate for the intended application. Differences found mainly in materials surface area and pore volume pointed out to some structural modifications in mesoporous SiO₂ matrices by nominal rare earth addition.

After Pt loading, surface area and pore volume of studied materials suffered significant losses respecting those of corresponding supports, in spite of low NM loading (Figure S1 and Table S1). Conversely, with PLa2 as the sole exception, corresponding PSD profiles remained essentially unaltered after Pt deposition (Figure S1b). That suggested NM particles at low dispersion located outside a porous network, being big enough to partially plug carriers porosity.

2.2. X-ray Diffraction

Wide signals in the 15–35° 2θ range in diffractograms of studied samples corresponded to amorphous silica constituting SBA-15 walls (Figure 2) [10]. Peaks at 39.85, 46.3, and 67.6° (2θ values) of NM-containing formulations belonged to (111), (200), and (220), which are face-centered cubic metallic platinum facets.

No PtO_x species were identified, although studied solids were analyzed by XRD after calcining (500 °C, 6 h, under air), with no reduction treatment being carried out. NM autoreduction of tetraamine platinum species after high-temperature calcination has been reported in the past [11]. Also, our group evidenced NM autoreduction when using H₂PtCl₆ as precursor salt deposited on SBA-15 modified matrices [3]. More studies on the effect of different Pt precursors on final catalyst properties are obviously needed [12]. Pt crystal size (from Scherrer formula, averaged from (111), (200), and (220) reflections,

Table 2. Pt crystal size of various NM-impregnated materials prepared, as determined by Scherrer formula using Pt (111), (200), and (220) reflections (peaks shown in Figure 2).

Catalyst	Crystal Size (nm)
PSBA-15	22.3
PLa1	23.6
PLa2	27.9
PLa4	32.8
PLa8	21.2

) augmented in PLa4 (see the most intense Pt (111) signal; Figure 2) as to the rest of samples. In any case, all impregnated solids had well-defined peaks, pointing out sintering due to weak interaction with carriers. Determined crystal sizes (Table 2) were rather similar to those observed over catalysts supported on SBA-15 modified by various Ga contents and at similar NM loading [13]. Pt autoreduction could lead to crystal agglomeration in samples in weak interaction with the carrier [11]. Low NM dispersion could justify significant S_BET loss of impregnated samples much larger than those expected considering Pt loading (column 6, Table S1) due to crystals big enough to partially plug pores of used carriers.

2.3. FTIR Characterization

Infrared signals observed in all solids (Figure 3) corresponded to those well-known for SBA-15 materials [3]. Shifts to higher wavenumbers in the absorption band at 958–960 cm⁻¹ in SBA-15 (corresponding to Si-OH asymmetric stretching vibrations [14]) have been considered evidence of La intrusion in silica networks that hypsochromic displacement is a function of rare earth concentration [15].

In our case, blue-shifted signals were observed for nominally La-containing samples (Figure 3 and

Table 3. Hypsochromic shifts in the 955 cm⁻¹ band of SBA-15 and NM-impregnated solids at several La nominal contents.

Sample	Peak Position (cm−1)
SBA-15	955
PSBA-15	959
PLa1	961
PLa2	964
PLa4	968
PLa8	971

), suggesting La incorporation in the SiO₂ network. However, those shifts have also been reported in Rh (1 wt%, deposited by incipient wetness impregnation) over SBA-15 due to the effect of NM impregnation on Si-O- vibration [16]. The signal observed at 2361 cm⁻¹ (more notable in the PSBA-15 spectrum) accompanied by small humps at approximately 1633, 1441, and 1256 cm⁻¹ could be related to physisorbed non-dissociated CO₂ [17]. Also, contributions from linearly adsorbed carbon dioxide could be related to absorptions at ~2320 cm⁻¹ [18]. However, no defined carbonate–bicarbonate signals (1800–1200 cm⁻¹ region) were clearly identified, suggesting no significant amount of basic sites on analyzed materials. It is worth mentioning that, expectedly, La addition could contribute to enhancing basic properties of corresponding rare earth-containing surfaces [19].

2.4. STEM (HAADF) Studies

In HAADF-STEM, a finely focused probe of high-energy electrons from STEM is scanned through a thin sample. Part of the electronic beam could be scattered at larger angles to be collected by an annular detector, then used to create Z-contrast images. Considering that cross-section of this type of scattering depends upon squared atomic number (Z) of analyzed atoms (i.e., Z²), corresponding micrographs enable chemical analysis due to image contrast where heavier atoms (NM ones) look brighter than those from silica carriers [20]. Thus, Pt crystals (around ~10 nm) were clearly identified on PLa4 (the sample containing the highest proportion of the smallest particles, as it will be further shown) (Figure 4a).

Meanwhile, honeycomb-like mesoporous structure (uniform channels along pores axes) [3] could be observed in the background in the mentioned micrograph. No segregated La domains were observed. Also, bigger sinterized particles were identified for that solid (Figure 4b). From particle size measurements, corresponding histograms were obtained (Figure 5). According to them, PLa4 had enhanced amounts of smaller NM crystals.

In contrast, PSBA-15 and PLa1 showed a larger proportion of bigger Pt particles (Figure 4c,d). The NM crystal size of various solids as determined from XRD (Table 2) could not coincide with those measured by electron microscopy, as particles below 3–4 nm could not be detected by the former technique. Differently from the beneficial influence of titanium content on Pt dispersion over SBA-15 [3], no positive influence of nominal La concentration in studied solids was evidenced, pointing out weak (if any) interaction between those species (see large NM particle in Figure 4b,d).

2.5. HDO Reaction Test

Pt over various supports has been studied in HDO reactions of several feedstocks, while SBA-15 has been used as a corresponding carrier [21]. Solids containing Pt over SBA-15 have been studied in m-cresol HDO [22] as well. Also, the effect of rather basic dopants (niobium in this case) on properties of SBA-15 Ni catalyst carriers was also studied in anisole HDO [23]. The effect of various metals (Nb, Ru, Pd, and Pt) supported on SBA-16 was tested in phenol HDO [24] as well.

Prepared materials performance in P HDO was compared on a k_HDO basis (Equation (1), Figure S2, and

Table 4. Pseudo-first-order kinetic constants (phenol HDO) over various supported NM catalysts prepared. T = 250 °C, P = 3.2 MPa, n-decane as solvent, batch reactor, ∼107 rad s⁻¹ (~1000 rpm) mixing speed.

Sample	kHDO (×10−5) (s−1)
PSBA-15	0.3
PLa1	2.0
PLa2	2.0
PLa4	2.0
PLa8	0.7

).

The least active solids were PSBA-15 and PLa8, in agreement with their lower NM dispersion (Figure 5). In contrast, PLa1, PLa2, and PLa4 promoted improved conversion. However, no direct correlation with corresponding particle size in Figure 5 was found, probably due to the high proportion of large NM crystals (Table 2) not included in histograms over all studied samples. Otherwise, an inverse correlation between metallic crystallite dimensions and P transformation capabilities could be expected. Specific activity (per gram of Pt) inversely proportional to NM particle size was observed in anisole HDO over Pt/SBA-15 [25]. Interestingly, production of deoxygenated compounds also requires acid sites [25,26]. Considering that cyclohexane (CH, see Scheme S1) was produced in our case and that silica surface lacks acidity [1], it could be possible that some oxidized PtO_x species (our catalysts were not submitted to reduction prior to the HDO reaction test) could provide the needed acid sites as proposed by others, where it was also considered that water produced under HDO conditions could partially oxidize supported metals, generating acid centers [27]. That is a point that deserves further study. No definitive trend on either P HDO activity or selectivity due to the effect of nominal rare earth content in carriers was disclosed (Table 4). Thus, surface characterization (acidity and basicity) of various NM-impregnated materials was devised to try to elucidate the reason for that unexpected behavior.

2.6. NH₃ and CO₂ Temperature-Programmed Desorption (TPD)

SBA-15 is well-known for essentially lacking either Lewis or protonic acidity [1]. Surprisingly, NH₃ TPD profiles for all studied materials were identical (Figure S3), displaying just a small amount of weak acid sites (Table S2). That was in full agreement with that reported for Pt/SBA-15 solids [28], where catalysts were tested in cellulose, hemicellulose, and lignin catalytic pyrolysis. Very limited deoxygenation was observed over solids of low acidity, in concordance with that previously commented [4]. Then, CO₂ TPD was carried out to analyze La contribution on basic site creation [19,29]. Again, corresponding TPD profiles of studied samples were very similar, pointing out no basic sites attributable to lanthanum nominal content (Figure S4, Table S3). CO₂ adsorption has been directly related to surface rare earth coverage over La-alumina supports [30]. Thus, in our case, observed data suggested absence of surface La domains. The small hump around 90 °C (Figure S4) has been related to very weakly adsorbed (physisorbed indeed) CO₂ [31,32]. Otherwise, SBA-15 modification with basic agents could be reflected in the creation of surface sites able to retain CO₂ at high temperatures (by formation of carbamates [32] or carbonates/bicarbonates [19], for instance).

2.7. Thermal Analysis (TG-DTG)

La8 (and corresponding Pt-impregnated solid, PLa8), the sample of highest nominal La content, and parent SBA-15 (as reference) calcined materials were studied by TG-DTG to determine carbonates/bicarbonates (from atmospheric CO₂ adsorption on basic sites, if the case) presence. No additional signals as to that from mesoporous silica were observed (Figure S5, due to humidity loss), pointing out, again, rare earth absence on La8. Otherwise, carbonates decomposition could have been identified [4].

2.8. SEM-EDS Analysis

SBA-15 morphology and those of samples at various nominal La contents were similar one to another and to those already reported [3] exhibiting rounded faceted particles of around 3–4 μm. Regarding compositional analysis, EDS has been considered a very reliable technique, as compared to the more complicated and expensive inductively coupled plasma (ICP) one [33], for instance. Unexpectedly, no lanthanum was identified in any support sample (

Table 5. EDS analyses of prepared supports at various nominal rare earth contents.

Sample	O (wt%)	Si (wt%)	La (wt%)
SBA-15	52.94	47.06	0.00
La1	49.49	50.50	0.02
La2	49.15	50.85	0.00
La4	46.63	53.27	0.10
La8	47.52	52.39	0.10

).

These findings meant that La integration into the silica network was not possible under acidic SBA-15 synthesis medium [5]. One important point to consider was that if actual material composition has not been determined, results from some characterization techniques (textural and IR studies, for instance) could have been misleading. Thus, wrong conclusions could have been reached by relating trends found in catalytic activity in P HDO to those data. It seems those textural properties changes due to nominal La content in various supports (Table 1) could have been provoked by the presence of La salt during SBA-15 synthesis. Those domains, however, were evidently lixiviated under acidic media used [3,4]. It is worth nothing that those textural modifications could indeed affect NM dispersion (Figure 4 and Figure 5), which could in turn influence HDO performance of various tested solids (Table 4), without any direct effect of no extant rare earth domains. In the same direction, hypsochromic shifts on the 955 cm⁻¹ band of SBA-15 solids at several nominal La concentrations (Table 3) could originate misinterpretations in the absence of compositional data (Table 5). As already mentioned, in our case those shifts (Table 3) seem to have originated by Si-O-Pt interactions [16]. Preparation of V-modified SBA-15 as NM phase carrier has been reported [6], but the actual composition was not determined, making it difficult to assess the real effect of vanadium integration during mesoporous silica synthesis. A similar situation was reported by Han et al. [15] when studying La-modified MCM-41 as support of CO methanation Ni catalysts. In this case, again, no actual rare earth content in samples were determined in solids where La salt was directly added during silica network preparation (under alkaline conditions). Our group has reported [29] on La incorporation at different contents (through corresponding nitrates) during sol–gel alumina synthesis of those binary materials being used as hydrotreating catalysts (Co-Mo-P) supports. During active phases precursor impregnation through one-pot acidic (pH~1) solutions La was lixiviated, provoking strong structural collapse reflected in significant textural losses. Conversely to that found in present communication, lixiviated La remained in catalyst formulations (determined by EDS) as one-pot deposition of Co-Mo-P was carried out by incipient wetness followed by drying and calcining. That was also proven by carbonates/bicarbonates formation through corresponding CO₂ adsorption IR analyses [29].

Summarizing, determining the actual composition of modified SBA-15 materials is crucial to reaching sound conclusions regarding their physicochemical properties. Although it could be reasonable considering well-integrated components when added by impregnation or deposition techniques are used [5], getting efficient interaction among constituents could be more difficult when corresponding modifications are tried during pristine matrices direct synthesis (under acidic medium).

Last but not least, research on the actual effect of La at various contents (loaded by pore-filling impregnation) on the catalytic properties of Pt/SBA-15 solids in phenol HDO is in due course and will be the subject of upcoming reports.

3. Experimental

3.1. Material Synthesis

3.1.1. La-Modified SBA-15

The preparation of the SBA-15 protocol based on that proposed in [7] was carried out. Reactants and their amounts used were as detailed in [3]. Corresponding rare earth-modified materials (at 1, 2, 4, and 8 wt% La₂O₃, Lac, and c lanthanum oxide nominal content) were synthesized similarly as Ti-containing ones described in [3], just utilizing required amounts of La(NO₃)₃•6H₂O (Sigma-Aldrich, 99.999%, St. Louis, MO, USA) instead of titanium precursor.

3.1.2. Pt-Containing La-Modified SBA-15

Regarding Pt-impregnated (1.5 wt%) carriers, they were obtained by incipient wetness using tetraammine platinum (II) nitrate (Pt(NH₃)₄(NO₃)₂, Aldrich, 99,9%, St. Louis, MO, USA). The rest of the preparation steps of NM-containing solids were entirely similar to those used during Pt-loaded materials synthesized in [3]. Totality of Pt impregnation on supports was assumed (due to pore-filling technique used). Then, platinum loading refers to a nominal value. Key PLac was used to identify NM-loaded materials at various rare earth nominal concentrations. A detailed flow diagram describing material synthesis is shown in Scheme S2.

3.2. Materials Characterization

The texture (surface area, pore size distribution (PSD), and pore volume) and structural order of various studied solids were studied by N₂ physisorption and X-ray diffraction, respectively. Further details can be found elsewhere [3]. Pt crystal size of various NM-impregnated materials prepared was determined by the Scherrer formula using Pt (111), (200), and (220) reflections through XPert HighScore Plus software (version 5.1). Fourier transform infrared spectra (FTIR) of prepared materials were obtained by Fourier transform infrared spectroscopy using Perkin Elmer Frontier equipment. Surface acidity and basicity of various materials were determined by temperature-programmed desorption (TPD) of NH₃ and CO₂, respectively, by utilizing BELCAT-B (BEgJapan Inc., Kyoto, Japan) equipment with thermal conductivity (TCD) detector. Typically, 500 mg samples were degassed under He (550 °C, 1 h). Then, for total acidity measurements, solids were contacted with NH₃ (7% NH₃ balance He), 50 mL min⁻¹ flow (30 min.). Then, the system was cooled down to 40 °C under mixture flow (20 mL min⁻¹, 1 h). Weakly adsorbed NH₃ was eliminated through He flush (20 mL min⁻¹, 1 h). Finally, the temperature was increased to 550 °C (10 °C min⁻¹) to determine the TPD profile. Regarding surface basicity studies, solids were firstly degassed under He flow (50 mL min⁻¹, 300 °C, 1 h). Then, samples were cooled down to 50 °C under He flow (50 mL min⁻¹, 30 min). CO₂ adsorption was carried out (20 mL min⁻¹, 1 h). The He flow was applied (20 mL min⁻¹, 50 °C, 30 min) to desorb weakly retained (physisorbed) CO₂. Finally, the CO₂ TPD profile was recorded in the 50–550 °C range (10 °C min⁻¹, heating ramp). Thermal profiles (TG-DTG) of some calcined materials were obtained by thermogravimetrical (TG) analysis. Moreover, 15–50 mg samples were analyzed through TA instruments Q2000 equipment in the room temperature-550 °C range (heating ramp of 10 °C min⁻¹) under static air atmosphere. Differential thermogravimetric (DTG) profiles were obtained through derivatives of corresponding TG curves. NM particle morphology was studied through scanning electron microscopy (SEM), JEOL JSM-6010LA (Jeol, Tokyo, Japan) operating at 20 kV accelerating voltage, high vacuum, and at various magnifications. Compositional analyses were carried out by attached EDS (energy-dispersive X-ray spectroscopy) apparatus. Obtained micrographs were processed by InTouchScopeTM software 3.10a. The materials were characterized by high-resolution transmission electron microscopy (HR-TEM) in a Titan 80–300 microscope with a Schottky-type field emission gun operating at 300 kV. The point resolution and the information limit were better than 0.085 nm. HR-TEM digital images were obtained through a CCD camera and analyzed by Digital Micrograph (GATAN^TM, version 3.x) software. Powdered materials were ultrasonically dispersed (ethanol) and supported on Lacey carbon-coated copper grids. Prepared solids were also studied by scanning transmission electron microscopy (STEM), with micrographs being obtained in a JEOL JEM-2200FS with Schottky-type field emission gun operating at 200 kV. A high-angle annular dark field (HAADF) detector was used to acquire the images. To prepare the materials for observation, the powder samples were dispersed in ethanol and supported on Lacey Formvar-carbon-coated Cu grid.

3.3. Phenol HDO Reaction Test

P HDO tests for various catalysts were carried out in a batch system (Parr 4842), stainless steel, 100 mL vessel, operating at 250 °C, 3.2 MPa (initial H₂ pressure), 4 h. To discard external diffusion control vigorous stirring was used (>1000 rpm). The reaction mixture was composed of 0.2 g of catalyst to be tested, 0.3 g P in 100 mL n-decane (Sigma-Aldrich, ≥95%), P molar conc. [0.032], and 694 ppm O. Tested catalysts were ground and sieved (U.S. mesh 80–100, ~165 µm particle size) to discard reaction control by internal diffusion phenomena [1]. NM-loaded solids were not submitted to reduction to avoid excessive active phase sintering as metallic Pt was identified in calcined samples (see Section 2.2, XRD Diffraction). Liquid samples taken periodically were analyzed through gas chromatography (GC) in a Varian Star 3400CX apparatus equipped with a flame ionization detector and a DB-5 non-polar capillary column ((5%-phenyl)-methylpolysiloxane) 30 m, 0.53 mm, and 1.50 µm in film thickness). At HDO conditions, P could be initially transformed by two main reaction pathways, HYD and direct deoxygenation (DDO), Scheme S1 [8]. The first one comprises aromatic ring saturation to cyclohexanone (HYD1 in Scheme S1) that could be transformed by subsequent saturation (HYD2 in Scheme S1) to cyclohexanol, whose dehydration leads to cyclohexane (CH, through cyclohexene intermediate) as the final product. On the other hand, DDO could take place from C_sp2-O bond breaking conducting to benzene (B), followed by ring saturation (HYD4 in Scheme S1) to CH. In our case and according to retention times, the only peak detected was assigned to CH. However, as the non-polar GC column used separated species by boiling point (bp), some amounts of B in the detected peak (attributed to CH) could not be ruled out due to the very similar bp of those species. P conversion was calculated through the determination of areas of corresponding chromatographic peak. Properties of various tested materials were compared on a kinetic constant (k_HDO) basis considering pseudo-first-order reactions (regarding P concentration and H₂ excess considered constant).

k_HDO = −ln(1 − x)/t

(1)

where:

• x = P conversion at time t;
• t = reaction time (s).

4. Conclusions

Regarding SBA-15 modified by La addition, determining the actual materials composition is crucial, especially if the rare earth dopant is directly added during mesoporous silica network formation (under low pH conditions). Otherwise, results from some characterization techniques (N₂ physisorption and FTIR, for instance) could be misleading and even contradictory. Indeed, extant modifier precursors, when under SBA-15 synthesis conditions, could affect the properties of prepared materials even though they were absent in obtained formulations. A simple compositional analysis could eliminate uncertainties regarding the role of various modifiers on the characteristics of final catalysts.