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New Nb-P-Si ternary oxide materials and their use in heterogeneous acid catalysis
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New Nb-P-Si ternary oxide materials and their use in heterogeneous acid catalysis Antonella Gervasini a , Paolo Carniti a , Filippo Bossola b , Claudio Imparato c , Pasquale Pernice c , Nigel J. Clayden d , Antonio Aronne c,∗ a
Università degli Studi di Milano, Dipartimento di Chimica, via Golgi 19, 20133 Milano, Italy CNR-Istituto di Scienze e Tecnologie Molecolari, via Camillo Golgi 19, 20133 Milano, Italy c Università di Napoli Federico II, Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, P.le Tecchio 80, 80125 Napoli, Italy d School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK b
a r t i c l e
i n f o
Article history: Received 28 July 2017 Received in revised form 11 September 2017 Accepted 3 October 2017 Available online xxx Keywords: Solid acids Intrinsic acidity Effective acidity Aqueous heterogeneous catalysis Inulin hydrolysis
a b s t r a c t The surface and catalytic properties of oxide ternary materials consisting of niobium-phosphorus-silicon, Nb-P-Si, with Nb/P = 1 and general formula xNb2 O5 ·xP2 O5 ·(100–2x)·SiO2 , with x = 2.5, 5, 7.5, and 10, and Si content ranging from 95 to 80 mol% are presented. Amorphous gel-derived samples were obtained, characterized by a very high degree of silicon cross-linking with presence of Si O Nb bridges, allowing phosphorus to be stably anchored to the matrix through Nb O P bonds. The morphology (surface area and porosity), the intrinsic acidity (density and average strength of the surface acid sites, by PEA-TPD in a TGA-DSC coupled instrument), and the nature of the acid sites were determined. The LAS/BAS ratio has been evaluated by FTIR of adsorbed pyridine in absence/presence of water to discover the differences between the intrinsic and effective nature of the acid sites. The amount of acid sites decreased with the Nb-content in the samples as well as the surface average acid strength. Both LAS and BAS are maintained in the presence of water, even if BAS are predominant in any case. The catalytic performances of the Nb-P-Si samples in the reaction of inulin hydrolysis were investigated in aqueous solution under mild conditions (below 100 ◦ C). © 2017 Elsevier B.V. All rights reserved.
1. Introduction The search for new catalysts plays a key role in the development of risk-free chemical processes and more generally for green chemistry. In particular, it is important to study new solid acid catalysts with tailored properties in terms of both type and density of acid sites and water tolerance, as in bio-refinery reactions are carried out in presence of water as vapor or even in liquid state. Among the catalysts currently used in the conversion of renewable biomass to fuels or valuable chemicals, niobium oxophosphate (NbOPO4 , NbP) has been widely used thanks to its interesting characteristics such as the high ratio of Brønsted to Lewis acid sites [1–4]. The preparation of new catalytic systems based on Nb and P has often required the addition of a third component, Si, allowing to modulate the distribution of acid sites and density at the surface of the material [5,6]. In recent time, amorphous niobium–phosphorus–silicon mixed oxide materials (Nb-P-Si),
∗ Corresponding author. E-mail address: [email protected] (A. Aronne).
with a molar ratio P/Nb = 1 and a Si content ranging from 95 to 80 mol%, have been prepared by some of us using an innovative hydrolytic sol-gel route distinguished by the easy manipulation of precursors and wholly performed at room temperature [7]. These solids are characterized by both a very high degree of silicon crosslinking in which Si-O-Nb are formed allowing phosphorus to be anchored through Nb O P bonds within the framework, and a high content of OH groups mainly linked to phosphorus even after heating at temperatures higher than 500 ◦ C [7], making them strong acid solids that can work effectively as a water tolerant catalyst, as was shown for the Nb-P-Si sample with the highest SiO2 content [8]. Here we report the morphological and surface acid-base characterization of the other samples of the Nb-P-Si series, i.e. the solids containing a lower SiO2 content: 90, 85 and 80 mol%. Moreover, the catalytic performances of these Nb-P-Si solids in the hydrolysis of inulin were also investigated and compared with those of the sample with the highest SiO2 content (95 mol%) and niobium oxophosphate (NbOPO4 , NbP), which is considered one of the best water-tolerant acid catalysts. The inulin is a polysaccharide of fructose, which merits to be valorised for the possibility to give rise to syrups with high fructose
http://dx.doi.org/10.1016/j.mcat.2017.10.006 2468-8231/© 2017 Elsevier B.V. All rights reserved.
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content by its hydrolysis. Catalytic tests have been performed in aqueous solution under mild conditions (below 100 ◦ C). For all the studied catalysts relationships between acidity and activity properties were searched with the aim of tailoring the acid density and acid strength of the surfaces for this reaction. 2. Experimental 2.1. Material preparation Niobium-phosphorus-silicon mixed oxide materials, whose nominal molar compositions can be expressed by the formula xNb2 O5 ·xP2 O5 ·(100-2x)·SiO2 , with x = 2.5, 5, 7.5 and 10, were prepared by a sol-gel procedure previously described [7]. Phosphoryl chloride, POCl3 (99%, Aldrich Chemical), niobium chloride, NbCl5 (99%, Gelest), and tetraethoxysilane, Si(OC2 H5 )4 (99%, Gelest), were used as starting materials. For all samples, transparent gels were obtained that were fully dried in air at 110 ◦ C and, subsequently, were annealed at 500 ◦ C for 1 h. Hereafter these samples will be indicated as 2.5NbP, 5NbP, 7.5NbP and 10NbP. A commercial catalyst with NbOPO4 composition (Companhia Brasileira de Metalurgia e Minerac¸ão CBMM, labelled as NbP) has been used for comparative purpose. 2.2. Characterization of the materials 2.2.1.
31 P
solid state NMR spectra were acquired by Direct Polarization (DP) Magic Angle Spinning (MAS) technique with a Bruker AVANCE 300 (Bruker Biospin, Milan, Italy) magnet equipped with a 4 mm widebore MAS probe as described in [7]. 31 P-NMR
2.2.2. UV–vis DRS spectroscopy Ultraviolet and visible light diffuse reflectance (UV–vis DRS) spectra were recorded in the range of 190–800 nm on a double beam Jasco spectrophotometer, using barium sulfate as reflectance standard. 2.2.3. N2 adsorption-desorption isotherms Morphologic characteristics of Nb-P-Si samples were studied by N2 adsorption and desorption isotherms measured at liquid nitrogen temperature with an automatic surface area analyzer (Sorptomatic 1900 instrument). Prior to the measurements, 0.2–0.4 g of each sample was thermal activated at 150 ◦ C for 16 h (overnight) under vacuum. The surface area was calculated using the BET equation (N2 molecular area of 16.2 Å2 ) and the Dubinin–Radushkevich equation allowed measuring the sample microporosity. 2.2.4. Intrinsic acidity determination by PEA-TPD in a TGA-DSC instrument A coupled TGA-DSC 3+ (Mettler-Toledo, STARe System) was used for the analyses; a temperature programmed desorption of PEA probe (PEA-TPD) was performed. Each sample was pretreated and saturated with PEA (2-phenylethylamine), chosen as base probe molecule, as reported in the follow. The dried sample (120 ◦ C) was put in a tubular cell (ca. 0.10 g) and treated under air flowing (8 mL min−1 ) at 150 ◦ C for 4 h; then, the powder was evacuated at the same temperature overnight (ca. 16 h). The activated sample was then transferred in a glass cell equipped with connections for vacuum/gas line and liquid PEA (purity > 99% from Fluka) was introduced in the cell up to complete covering of the powder. The slurry rested at room temperature under N2 flowing for ca. 1 h (saturation step). Then, the excess of non-adsorbed PEA was removed by filtration and the saturated sample was dried under N2 flowing for ca. 1 h at RT. The
PEA-saturated sample was thermally treated at 150 ◦ C overnight to remove the physically adsorbed PEA. The obtained sample was loaded on the alumina pan of the TGA-DSC (ca. 15–20 mg) and the PEA-TPD analysis was carried out from RT to 500 ◦ C at a rate of 10 ◦ C min−1 under flowing N2 at 50 mL min−1 . From TGA profile and its derivative (DTGA) as a function of time/temperature, the amount of mass of water or PEA desorbed at different temperature intervals could be quantified. From DSC, the heat flow profile (mW or W g−1 ) as a function of time/temperature with peaks associated with water or PEA desorption (endothermic peak) was obtained. Integration of the endothermic peaks gave the enthalpy of water/PEA desorption (des Hwater or des HPEA , in Joule). By coupling the TGA and DSC results, it was possible to determine the value of the average enthalpy of desorption of PEA from the acid sites of the samples (des HPEA , in Joule (molPEA )−1 ), corresponding to the average acid strength of the sample surface. 2.2.5. Intrinsic and effective nature of the acid sites by FT-IR adsorption of pyridine Lewis (LAS) and Brønsted (BAS) acid sites as well as their tolerance to water, were investigated by Fourier Transform Infrared Spectroscopy (FTIR) (Biorad FTS-60A) using pyridine as probe molecule both in vapor phase and aqueous solution [9]. All the samples were pressed into self-supporting disks (0.65 cm2 geometrical area) and before each analysis they were calcined for 2 h at 150 ◦ C in air. After outgassing for 30 min in high vacuum the samples were contacted with pyridine vapors at room temperature for 10 min or, alternatively, a 0.10 M pyridine aqueous solution was dropped on self-supporting disks under argon flow, always at room temperature. After pyridine adsorption, the samples were outgassed for 30 min in high vacuum at different temperatures (i.e., RT, 50, 100 ◦ C). BAS and LAS concentrations, expressed as microequivalents of adsorbed pyridine per gram of catalyst (eqPy gcat −1 ), were determined by integrating the peaks at 1540 and 1448 cm−1 , respectively, of the spectra collected after outgassing at 100 ◦ C, according to the procedure reported by Emeis [10]. 2.3. Catalytic test of hydrolysis of inulin The catalytic hydrolysis of inulin to fructose (and in very lower extent to glucose) was performed in water in a glass slurry batch reactor (Syrris, Atlas, UK) under increasing temperature from 50 to 90 ◦ C (0.12 ◦ C min−1 , 6 h of reaction). Inulin, in powder form, was obtained from Carlo Erba (pre reagent, RPE) with an average number of polymerization (DPn) of 25. In a typical catalytic test, the weighted and dried (120 ◦ C for ca. 16 h) catalyst sample (ca. 0.3 g sieved between 20 and 80 mesh) was put in the reactor filled with water (150 mL) at RT under gentle stirring, then, a weighted amount of inulin (1.5 g) was added to obtain an inulin concentration of ca. 50 mM. Then, the reaction started, while the temperature was allowed to increase (0.12 ◦ C min−1 ). The yield to reducing sugars (corresponding to the extent of reaction and called inulin conversion herein after below) was evaluated as the ratio between the reducing sugars produced and the reducing sugars at complete hydrolysis of substrate. The selectivity to fructose and glucose was determined from the ratio between fructose or glucose formed and the reducing sugars produced. The reducing sugars were evaluated by the classical colorimetric Nelson-Somogyi method by using copper(II) sulfate pentahydrate (Cu(SO4 )·5H2 O, Carlo Erba, RPE) and ammonium molybdate ((NH4 )6 Mo7 O24 ·4H2 O, Carlo Erba, RPE). The blue complex formed was analyzed and quantified at 520 nm. Moreover, the amount of glucose and fructose was determined by a Boehringer enzymatic assay technique.
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Table 1 Catalyst composition and morphological properties: BET surface area, total pore volume, micropore volume (determined by Dubinin–Radushkevich equation) and mean pore radius. Sample
2.5NbP
5NbP
7.5NbP
10NbP
Oxide
Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2
Composition
Surface Area
Total Pore Volume
Micropore Volume
Mean Pore Radius
Molar (%)
Weight (%)
(m2 g−1 )
(cm3 g−1 )
(cm3 g−1 )
(nm)
2.5 2.5 95 5 5 90 7.5 7.5 85 10 10 80
9.88 5.27 84.85 17.85 9.53 72.62 24.41 13.04 62.55 29.92 15.98 54.10
466
0.30
0.11
1.3
139
0.56
0.038
8.0
59
0.27
0.010
9.0
41
0.30
0.0064
14
The kinetic coefficients at the average temperature Tm between two samplings (kTm ) were calculated by Eq. (1): kTm = (C/t)/Cm
(1)
where t is the time variation between two samplings (t = 30 min and T = 5 ◦ C); C is the variation of bond concentration between two samplings; Cm is the average bond concentration between two samplings. The catalyst samples recovered after reaction by filtration and drying for ca. 16 h in air, were analyzed by TGA analysis to determine the amount of organic residues present on their surfaces. Analysis was performed from 50 to 800 ◦ C (10 ◦ C min−1 ) under air flowing. Following the experimental approach of Sahoo et al. [11], the thermogravimetric profile of each sample was divided in three subintervals of temperature relevant to volatile organic compounds removal (VOC, 50–180 ◦ C), soft-coke (180–330 ◦ C), and hard-coke (330–800 ◦ C).
Fig. 1. Distribution of phosphate units for the studied catalysts in terms of Q’N notation; results obtained from 31 P NMR spectra [7].
3. Results and discussion 3.1. Structural and morphological characterization All the studied materials are ternary Nb-P-Si oxides with different ratio among components and their composition is listed in Table 1. In the samples, Nb and P are always in 1:1 molar ratio, as Nb and P content increase, SiO2 decreases from 95 to 80% molar (corresponding to 85–54 wt.%). All the samples have been fully characterized by XRD, 29 Si and 31 P NMR, Raman and FTIR spectroscopy in a previous work [7]. XRD analysis has shown that all dried gels as well as the gels annealed at 500 ◦ C are amorphous. The analysis of solid state NMR data has shown that the siloxane matrix is highly cross linked already in the dried gels and that after heating to 500 ◦ C the Q4 units (SiO4 tetrahedra with 4 bridging oxygen) increase only slightly up to about 80%. This high value of Q4 can be reasonably explained by the formation of Si O Nb bonds mostly as the Q4 fraction increases with Nb/P content. Here the cross linking in the 31 P NMR spectra is denoted by the Q’N notation, where Q’N stands for OP(OX)N (OH)3-N , with X = P or Nb. 31 P NMR data indicated that in all the materials heated at 500 ◦ C, phosphorus is present mainly as cross linked PO4 units (Q’1 , Q’2 , Q’3 ) with only very small amount of Q’0 , see Fig. 1 and Table S1. In previous papers on binary SiO2 -P2 O5 materials it was demonstrated that phosphorus was present only as Q 0 units, being the Si O P bonds hydrolysed also by ambient humidity [12,13]. The NMR data are therefore consistent with the preferential reaction between silicon and niobium on the one hand and niobium and phosphorus on the other with the niobium serving to anchor phosphorus into the silicon network with a less hydrolytically labile
Fig. 2. UV–vis diffuse reflectance spectra of the Nb-P-Si materials. The measured intensity is normalized and expressed as the Kubelka–Munk function F(R).
linkage. In other words, phosphorus is anchored through niobium bridges into the P-O-Nb-O-Si network. To get insight about the surface distribution of niobium structural units, UV–vis DRS spectra of all studied samples were recorded (Fig. 2). Features at about 200, 225 and 255 nm are recognized, related to charge transfer transitions associated with isolated NbO4 tetrahedra, low distorted NbO4 tetrahedra and high distorted NbO6 octahedra, respectively [8]. A diffuse absorption in the range 300–400 nm is also observed indicating the presence of NbO6 octahedra with lower distortion [8]. It could be expected that raising the
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ARTICLE IN PRESS A. Gervasini et al. / Molecular Catalysis xxx (2017) xxx–xxx Table 2 Intrinsic acidic properties of the catalyst surfaces determined from PEA-TPD.a Sample
2.5NbP 5NbP 7.5NbP 10NbP NbP a
Density of acid sites
PEA-desorption enthalpy
(eq g−1 )
(eq m−2 )
des HPEA (kJ mol−1 )
241 181 150 120 420
0.52 1.30 2.54 2.93 3.65
73.9 44.2 31.6 28.4 37.8
Determined in the temperature interval from 200 to 500 ◦ C.
3.2. Surface acidity determination
Fig. 3. Comparison of the adsorption and desorption N2 isotherms (at −196 ◦ C) between 2.5NbP and 5NbP. The isotherms for the other samples can be found in S.I. (Fig. S1).
content of Nb and P would cause at least a partial phase separation and segregation of Nb clusters in the solids, resulting in an increase of the amount of NbO6 octahedra at the expense of NbO4 tetrahedra and a corresponding shift of the absorption band to higher wavelengths. Actually, only minor changes are seen in the UV-DRS profiles, attesting that the niobium units maintain their distribution and good dispersion as the Nb content ranges from 2.5 to 10 mol%. The small rise of the absorption tail over 350 nm can be due to the conversion of some distorted NbO6 octahedra into more ordered ones. The band gap energy value obtained by Tauc plot elaboration (not reported) results almost the same in all samples (about 3.9 eV for direct transitions), confirming their similarity. A comparison of the UV–vis DRS spectrum of 2.5NbP with commercial NbOPO4 and Nb2 O5 was reported in [8]. Sample morphology was studied by N2 adsorption and desorption isotherms to determine surface area and porosity. Table 1 lists the surface area and the microporosity values of each sample. 2.5NbP has very high surface area and some amount of micropores. The Nb and P increase causes a prominent decrease of surface area with notable loss of microporosity, as observed in particular for the 5NbP and 7.5NbP samples. The marked change of morphology observed comparing the 2.5NbP and 5NbP samples is clearly displayed in Fig. 3 (the adsorption-desorption isotherms for 7.5NbP and 10NbP are shown in Fig. S1). 2.5NbP has an adsorption isotherm of type I (following IUPAC classification) typical of microporous materials with also presence of a little hysteresis indicating some amount of mesopores (the existence of micropores in 2.5NbP and 5NbP was comparatively illustrated by the DubininRadushkevich representation in Fig. S2). In contrast, 5NbP has an adsorption isotherm typical of non-porous materials (type II following IUPAC classification) as well as the other samples (7.5NbP and 10NbP, Fig. S1). The trend of surface area values observed by increasing the niobium and phosphorus content was similar to that already observed for binary SiO2 -P2 O5 [13] and SiO2 -Nb2 O5 materials [14]. Finally, the sample morphology seems closely related to the trend of cross-linking degree between the oxide components. SiO2 weight content decreases significantly from 2.5NbP to 10NbP (see Table 1), so the porous siloxane matrix is reduced and more cross-linked Nb and P units concentrate on the surface, favouring a decrease of specific area and available micropore volume. The pore size distribution varies accordingly, increasing linearly with Nb and P content, while the total pore volume does not follow a clear trend, suggesting that 7.5NbP and 10NbP possess less pores but larger ones.
The acid properties of the Nb-P-Si samples have been studied by thermal desorption of a basic probe (PEA) previously adsorbed over the sample surface to obtain complete saturation of all the acid sites of the samples. The experiments have been performed in a coupled TGA-DSC instrument that allowed obtaining the amount of PEA desorbed (thermogravimetric signal), from which it was possible to determine the acid site density of the surfaces, and the enthalpy of desorption of PEA (ads HPEA , calorimetric signal), which corresponds to the average acid strength of the surfaces. PEA was chosen as probe for its high molecular weight (121.18 g mol−1 ) that allows to quantitatively monitor the desorption, following the mass decrease. At first, the amount of PEA in moles, computed from the mass loss of PEA determined by the TGA profile, corresponds to the moles of acid sites titrated. In general, PEA was desorbed in the temperature range from 200 to 500 ◦ C. A typical thermogravimetric profile obtained from the PEA-TPD experiments with the TGA and relevant derivative (DTGA) curves as a function of temperature is displayed in Fig. S3. At lower temperatures (50–150 ◦ C), a mass loss related to water desorption is observed for all the samples. The results obtained in terms of density of acid sites normalized per mass or surface unit are listed Table 2. The amount of specific acid sites density (expressed in eq g−1 ) regularly decreased with Nb (and P) increase in the sample, while the amount of intrinsic acid sites density (expressed in eq m−2 ) increase regularly with the Nb (and P) content. Likely, the titrated acid sites comprise those associated to Si (weak acidity) and to Nb and P species (medium-strong acidity). These singular results are due to the important differences of specific surface area among the samples (Table 1). Second, from the calorimetric profile of PEA desorption, determined by the DSC curve as a function of temperature, the average strength of surface acidity could be determined for each sample. Fig. 4 displays a typical calorimetric profile obtained for a representative sample. In the interval 50–150 ◦ C, there are two endothermic peaks associated to the desorption of physisorbed and chemisorbed water from the surface. In general, more than one endothermic peaks related to PEA desorption were observed in the temperature interval from 200 to 500 ◦ C, in the same range where the mass loss of PEA occurs. The total amount of heat absorbed is the enthalpy of PEA desorption (ads HPEA ). Two distinct endothermic peaks are seen in the 200–500 ◦ C range, that are relevant to PEA desorption from the acid sites. These observations indicate the heterogeneous distributions in strength of the acid sites, comprising silanol groups and acid species associated to Nb and P. Table 2 reports also the average enthalpy of PEA desorption for each sample; values are decreasing with the Nb (and P) increase as already observed for the specific density of acid sites. 2.5NbP possesses high surface acid strength, 73.9 kJ mol−1 , much higher than the values of the other samples (in the range 32–44 kJ mol−1 ). This trend well agrees with the structural evolution occurring with the increase of Nb (P) producing the increase of the crosslinking degree of both Nb-O-Si and
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Fig. 4. DSC profile of the PEA-TPD analysis on a chosen representative catalyst. The enthalpy of PEA desorption (des HPEA , in kJ mol−1 , see Table 2) has been calculated by summing up the two endothermic peaks of PEA desorption (qPEA , in J g−1 ) and considering the sample mass used in the analysis and the amount of PEA desorbed (in moles) in the same temperature interval in the coupled TGA analysis.
Nb-O-P [7]. Consequently, the fraction of isolated NbO4 tetrahedra, with which a very strong Lewis acidity is associated, decreases slightly, as can be observed in UV–vis DRS spectra (Fig. 2). The nature of the acid sites is another information of great importance besides the amount of sites and acid strength, to know in full the acid properties of any surface. FTIR spectroscopic experiments with basic probe molecules allow the knowledge of the LAS (Lewis acid sites) or BAS (Brønsted acid sites) nature of surface acid sites by studying the molecular complexes formed following adsorption of the probe. Pyridine is a typical probe used for these studies; distinction between the pyridinium ion (typical absorption at 1540 cm−1 ) and coordinated pyridine (typical absorption at 1448 cm−1 ) to the acid sites is possible as well as
5
the obtainment of LAS and BAS concentrations and relative ratios by peak integration and application of suitable extinction coefficients. Fig. 5 shows the FTIR desorption spectra collected in the range 1350–1700 cm−1 after contact of pyridine both in vapor phase (intrinsic nature of the acid sites) and aqueous solution (effective nature of the acid sites) with the acid surfaces of the 2.5NbP, 5NbP, and 7.5NbP samples. All the recorded spectra exhibited band patterns typical of BAS and LAS sites but showing different intensities. The sharp bands corresponding to the interaction of pyridine molecules with LAS at 1448 cm−1 (19b mode) and 1610 cm−1 (8a mode) are clearly observed, while broad bands at 1540 cm−1 (19b mode) and 1637 cm−1 (8a mode) are typical of BAS presence. Low–coordinated Nb sites in tetrahedral coordination could be responsible for the Lewis acidity, as proposed for niobic acid (Nb2 O5 ·nH2 O) and H3 PO4 –treated niobic acid [15], while Brønsted acidity comes, for the most part, from the silanol, P-OH and Nb-OH species. The quantitative determination of Lewis and Brønsted acid sites has been made on the basis of the bands located at 1448 cm−1 and 1540 cm−1 , according to the procedure reported by Emeis [10]. Pyridine in aqueous solution was employed to investigate the acidic properties of the surfaces mimicking the real catalyst working conditions (i.e. in aqueous solutions). The evolution of the band relative to the scissoring mode of adsorbed water molecule at 1620 cm−1 was followed in order to confirm the co-adsorption of water and pyridine also during desorption experiments [16]. In Table 3 are summarized the results of the determination of the acid sites for all the samples. With pyridine in vapor phase, the LAS/BAS ratios are very high, close to 1 in any case. According to the analysis of UV–vis DRS data the distribution of Nb structural units exhibits both isolated and low distorted NbO4 tetrahedra accounting for this value of LAS/BAS ratio. In the presence of water only the LAS concentration decreased strongly, while the BAS concentration remained almost unchanged; therefore, the LAS/BAS ratios decreased down to ca. 0.4–0.5. This observed behaviour is somehow unexpected because in the presence of water the acid strength should decrease as has been previously observed for NbP catalyst [17]. In this work, the LAS to BAS ratio for NbP drastically
Fig. 5. FT-IR desorption spectra at 100 ◦ C of pyridine on 2.5NbP (a), 5NbP (b), and 7.5NbP (c) with pyridine adsorbed in vapor phase (Vap.) and aqueous phase (Aq.). Bands due to Brønsted (B) and Lewis (L) acid sites are indicated.
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Table 3 Determination of the nature of the acid sites by FT-IR after pyridine adsorption in absence and in presence of water.a Sample
Py(vap) b LAS
2.5NbP 5NbP 7.5NbP NbP a b c d e
d
82 0.18e 72d 0.52e 35d 0.59e 81d 0.70e
Py(aq) c BAS d
96 0.21e 95d 0.68e 33d 0.56e 39d 0.34e
LAS/BAS 0.85 0.76 1.05 2.08
LAS d
26 0.06e 24d 0.17e 16d 0.27e 61d 0.53e
BAS d
70 0.15e 61d 0.44e 33d 0.56e 36d 0.31e
LAS/BAS 0.37 0.39 0.48 1.69
Values reported after pyridine desorption at 100 ◦ C. Pyridine contacted in vapor phase. Pyridine contacted in aqueous solution. Expressed in eq g−1 . Expressed in eq m−2 .
decreases going from 2.08, measured in pyridine vapor, to 1.69, when measured in water. For Nb-P-Si ternary samples, the strong LAS reduction may be related to the substantial hydrolysis of NbO4 isolated tetrahedra, producing terminal Nb OH groups [8]. This phenomenon partially counteracts the missed contribution of the Si-OH groups giving the materials strong Brønsted acid properties characterized by LAS/BAS 4 times lower than NbP. Increasing the amount of Nb (and P) does not seem to provide a higher number of strong acid sites, probably because Nb and P units become more cross-linked. The larger differences of LAS and BAS populations have been observed between the 2.5NbP and 5NbP samples due to the very important difference of morphology (in particular, specific surface area). The acid properties of the Nb-P-Si materials suggest their ability to work mainly as Brønsted catalysts in reactions requiring watertolerant protonic sites. 3.3. Catalytic test of inulin hydrolysis Inulin is a natural polysaccharide of fructose consisting of linear chains of fructose units linked by -1,2 bonds and terminated with a sucrose residue; it acts as energy reserve in various plants (e.g., chicory and Jerusalem artichoke). The exploitation of such cultivations and the resulting inulin extraction make possible the production of syrups with higher fructose content than from other traditional processes, like glucose isomerization [18–20]. NbOPO4 catalyst has been recently confirmed as an interesting catalyst for this reaction [8] thanks to its Brønsted acidity; the same reaction was studied over the ternary Nb-P-Si samples. The time course of the inulin hydrolysis was studied as a function of time/temperature in the 50–90 ◦ C interval by adding the dry catalyst sample to the inulin aqueous solution. Fig. 6 shows the increasing trends of inulin conversion (calculated by analyzing the total reducing sugars formed) for all the samples as a function of temperature, which was continuously increasing with time. Inulin hydrolysis started at ca. 70 ◦ C and reached more than 80% at the highest reaction temperature (90 ◦ C) over 2.5NbP. Over all the other catalyst samples, similar increasing trends were observed, but the conversion curves are shifted at (little) higher temperatures. The total reducing sugar analyzed corresponded to concentration of hydrolyzed bonds, the increasing inulin conversion led to increasing selectivity towards the monosaccharides (fructose + glucose). Fig. 7 shows the trends of the curves of selectivity to fructose as a function of inulin conversion. Unique trends can be guessed with obtainment of selectivity values higher than 80% for all the catalysts. This suggests that the nature of the sites are similar and they give similar selectivity to fructose in the inulin hydrolysis reaction. Over 2.5NbP, the higher amount of acid sites (expressed in eq g−1 ,
Fig. 6. Conversion of inulin as a function of temperature evaluated as the ratio between the reducing sugars produced and the reducing sugars at complete hydrolysis of substrate.
Fig. 7. Selectivity to fructose as a function of inulin conversion on the Nb-P-Si catalysts; selectivity was evaluated from the ratio between fructose formed and the reducing sugars produced. Table 4 Kinetic coefficients for the reaction of inulin hydrolysis evaluated at three representative temperatures and Arrhenius parameters. Sample
2.5NbP 5NbP 7.5NbP 10NbP NbP a
kT (meq g−1 min−1 ) T = 60 ◦ C
T = 70 ◦ C
T = 80 ◦ C
0.0160 0.0040 0.0055 0.0050 0.0124
0.0712 0.0229 0.0242 0.0239 0.0564
0.291 0.120 0.098 0.105 0.236
Ea (kJ mol−1 )
Ln Aa
142 ± 8 167 ± 7 140 ± 9 149 ± 9 144 ± 11
47 ± 3 55 ± 2 46 ± 3 48 ± 3 48 ± 4
A expressed in meq g−1 min−1 .
Table 2) justifies the higher conversion observed than for the other catalyst samples. Activation parameters for the catalysts have been determined from Arrhenius plots using the rate coefficients evaluated at different Tm (average temperature on 30 min of reaction). Table 4 reports the results of the kinetic coefficients evaluated at three representative reaction temperatures (60, 70 and 80 ◦ C) and the activation parameters. The reaction rate over 2.5NbP compared with the other ternary oxides is four times higher at 60 ◦ C, three times higher at 70 ◦ C and two and a half times higher at 80 ◦ C. Even commercial NbP shows a lower reaction rate than 2.5NbP in the whole temperature range. This behaviour confirms the higher activity of 2.5NbP respect to the other catalysts thanks to a larger strength and amount of acid sites (Table 2), in particular Brønsted sites requested for inulin hydrolysis. The calculated apparent activation energy values did not differ significantly for the five catalyst samples (Ea around 140–160 kJ mol−1 and ln A around 45–55). The used catalysts after filtration and drying have been analyzed by TGA to determine the amount of carbon deposited on
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Acknowledgements The authors thank Dr. Pierluigi Mazzei and the Centro Interdipartimentale di Ricerca sulla Risonanza Magnetica Nucleare per l’Ambiente, l’Agro-Alimentare ed i Nuovi Materiali (Università di Napoli Federico II) for MAS NMR measurements. The use of instrumentation purchased through the SmartMatLab Project (Cariplo Foundation project 2013-1776) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.10. 006. Fig. 8. Results of TGA analysis of the used Nb-P-Si catalysts after reaction: quantitative determination of insoluble organic residues from mass loss at given temperature intervals [11].
the surface, in view of a possible recycle of the catalysts. In general, coke samples exhibit three distinct temperature regions [11]: region I (T < 180 ◦ C) can be ascribed to the loss of water and volatile species in the samples; region II (180 ◦ C ≤ T ≤ 330 ◦ C) and III (330 ◦ C ≤ T ≤ 750 ◦ C) can be ascribed to desorption of coke as CO or CO2 . The coke desorbed in region II are more mobile carbonaceous residues or physisorbed products or side products (altogether can be termed as ‘soft coke’) whereas in region III, it is more bulky carbonaceous compounds (termed as ‘hard coke’). All the three types of coke were present on the used samples analyzed (Fig. 8). Hard coke shows a clear decreasing concentration trend with the acidity strength of the samples. Even if acid site density (in eq m−2 ) of the samples increased with the increase of Nb content, the average surface acid strength decreased (see Table 2) and consequently low amount of hard-coke was formed. This represents an advantage for the long stability of catalysts activity. This also shows that some of the carbon insoluble residues, which are highly condensed, might be deposited on the most acidic sites of the catalyst surface causing its deactivation. 4. Conclusions The Nb-P-Si ternary oxides studied can be considered interesting strong acid solids, in which stable phosphate units are crosslinked through niobium bridges in the silicate matrix. The morphological and the surface properties of these solids are strongly influenced by their composition: surface area, porosity and acid strength decrease increasing the niobium and phosphorus content. Pyridine adsorption experiments indicate a high concentration of both Lewis and Brønsted acid sites in vapor phase, while in water the BAS concentration prevails keeping about the same value of the vapor phase. All studied materials exhibited an excellent catalytic activity in the hydrolysis of inulin in aqueous solution at low temperature, even if the sample with the lowest Nb and P content shows the best results in terms of inulin conversion and yield to fructose, likely due to its higher intrinsic Brønsted acidity lively-active in water. The Nb-P-Si ternary oxides are promising catalysts for aqueous reactions requiring stable Brønsted acid sites.
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New Nb-P-Si ternary oxide materials and their use in heterogeneous acid catalysis Antonella Gervasini a , Paolo Carniti a , Filippo Bossola b , Claudio Imparato c , Pasquale Pernice c , Nigel J. Clayden d , Antonio Aronne c,∗ a
Università degli Studi di Milano, Dipartimento di Chimica, via Golgi 19, 20133 Milano, Italy CNR-Istituto di Scienze e Tecnologie Molecolari, via Camillo Golgi 19, 20133 Milano, Italy c Università di Napoli Federico II, Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, P.le Tecchio 80, 80125 Napoli, Italy d School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK b
a r t i c l e
i n f o
Article history: Received 28 July 2017 Received in revised form 11 September 2017 Accepted 3 October 2017 Available online xxx Keywords: Solid acids Intrinsic acidity Effective acidity Aqueous heterogeneous catalysis Inulin hydrolysis
a b s t r a c t The surface and catalytic properties of oxide ternary materials consisting of niobium-phosphorus-silicon, Nb-P-Si, with Nb/P = 1 and general formula xNb2 O5 ·xP2 O5 ·(100–2x)·SiO2 , with x = 2.5, 5, 7.5, and 10, and Si content ranging from 95 to 80 mol% are presented. Amorphous gel-derived samples were obtained, characterized by a very high degree of silicon cross-linking with presence of Si O Nb bridges, allowing phosphorus to be stably anchored to the matrix through Nb O P bonds. The morphology (surface area and porosity), the intrinsic acidity (density and average strength of the surface acid sites, by PEA-TPD in a TGA-DSC coupled instrument), and the nature of the acid sites were determined. The LAS/BAS ratio has been evaluated by FTIR of adsorbed pyridine in absence/presence of water to discover the differences between the intrinsic and effective nature of the acid sites. The amount of acid sites decreased with the Nb-content in the samples as well as the surface average acid strength. Both LAS and BAS are maintained in the presence of water, even if BAS are predominant in any case. The catalytic performances of the Nb-P-Si samples in the reaction of inulin hydrolysis were investigated in aqueous solution under mild conditions (below 100 ◦ C). © 2017 Elsevier B.V. All rights reserved.
1. Introduction The search for new catalysts plays a key role in the development of risk-free chemical processes and more generally for green chemistry. In particular, it is important to study new solid acid catalysts with tailored properties in terms of both type and density of acid sites and water tolerance, as in bio-refinery reactions are carried out in presence of water as vapor or even in liquid state. Among the catalysts currently used in the conversion of renewable biomass to fuels or valuable chemicals, niobium oxophosphate (NbOPO4 , NbP) has been widely used thanks to its interesting characteristics such as the high ratio of Brønsted to Lewis acid sites [1–4]. The preparation of new catalytic systems based on Nb and P has often required the addition of a third component, Si, allowing to modulate the distribution of acid sites and density at the surface of the material [5,6]. In recent time, amorphous niobium–phosphorus–silicon mixed oxide materials (Nb-P-Si),
∗ Corresponding author. E-mail address: [email protected] (A. Aronne).
with a molar ratio P/Nb = 1 and a Si content ranging from 95 to 80 mol%, have been prepared by some of us using an innovative hydrolytic sol-gel route distinguished by the easy manipulation of precursors and wholly performed at room temperature [7]. These solids are characterized by both a very high degree of silicon crosslinking in which Si-O-Nb are formed allowing phosphorus to be anchored through Nb O P bonds within the framework, and a high content of OH groups mainly linked to phosphorus even after heating at temperatures higher than 500 ◦ C [7], making them strong acid solids that can work effectively as a water tolerant catalyst, as was shown for the Nb-P-Si sample with the highest SiO2 content [8]. Here we report the morphological and surface acid-base characterization of the other samples of the Nb-P-Si series, i.e. the solids containing a lower SiO2 content: 90, 85 and 80 mol%. Moreover, the catalytic performances of these Nb-P-Si solids in the hydrolysis of inulin were also investigated and compared with those of the sample with the highest SiO2 content (95 mol%) and niobium oxophosphate (NbOPO4 , NbP), which is considered one of the best water-tolerant acid catalysts. The inulin is a polysaccharide of fructose, which merits to be valorised for the possibility to give rise to syrups with high fructose
http://dx.doi.org/10.1016/j.mcat.2017.10.006 2468-8231/© 2017 Elsevier B.V. All rights reserved.
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content by its hydrolysis. Catalytic tests have been performed in aqueous solution under mild conditions (below 100 ◦ C). For all the studied catalysts relationships between acidity and activity properties were searched with the aim of tailoring the acid density and acid strength of the surfaces for this reaction. 2. Experimental 2.1. Material preparation Niobium-phosphorus-silicon mixed oxide materials, whose nominal molar compositions can be expressed by the formula xNb2 O5 ·xP2 O5 ·(100-2x)·SiO2 , with x = 2.5, 5, 7.5 and 10, were prepared by a sol-gel procedure previously described [7]. Phosphoryl chloride, POCl3 (99%, Aldrich Chemical), niobium chloride, NbCl5 (99%, Gelest), and tetraethoxysilane, Si(OC2 H5 )4 (99%, Gelest), were used as starting materials. For all samples, transparent gels were obtained that were fully dried in air at 110 ◦ C and, subsequently, were annealed at 500 ◦ C for 1 h. Hereafter these samples will be indicated as 2.5NbP, 5NbP, 7.5NbP and 10NbP. A commercial catalyst with NbOPO4 composition (Companhia Brasileira de Metalurgia e Minerac¸ão CBMM, labelled as NbP) has been used for comparative purpose. 2.2. Characterization of the materials 2.2.1.
31 P
solid state NMR spectra were acquired by Direct Polarization (DP) Magic Angle Spinning (MAS) technique with a Bruker AVANCE 300 (Bruker Biospin, Milan, Italy) magnet equipped with a 4 mm widebore MAS probe as described in [7]. 31 P-NMR
2.2.2. UV–vis DRS spectroscopy Ultraviolet and visible light diffuse reflectance (UV–vis DRS) spectra were recorded in the range of 190–800 nm on a double beam Jasco spectrophotometer, using barium sulfate as reflectance standard. 2.2.3. N2 adsorption-desorption isotherms Morphologic characteristics of Nb-P-Si samples were studied by N2 adsorption and desorption isotherms measured at liquid nitrogen temperature with an automatic surface area analyzer (Sorptomatic 1900 instrument). Prior to the measurements, 0.2–0.4 g of each sample was thermal activated at 150 ◦ C for 16 h (overnight) under vacuum. The surface area was calculated using the BET equation (N2 molecular area of 16.2 Å2 ) and the Dubinin–Radushkevich equation allowed measuring the sample microporosity. 2.2.4. Intrinsic acidity determination by PEA-TPD in a TGA-DSC instrument A coupled TGA-DSC 3+ (Mettler-Toledo, STARe System) was used for the analyses; a temperature programmed desorption of PEA probe (PEA-TPD) was performed. Each sample was pretreated and saturated with PEA (2-phenylethylamine), chosen as base probe molecule, as reported in the follow. The dried sample (120 ◦ C) was put in a tubular cell (ca. 0.10 g) and treated under air flowing (8 mL min−1 ) at 150 ◦ C for 4 h; then, the powder was evacuated at the same temperature overnight (ca. 16 h). The activated sample was then transferred in a glass cell equipped with connections for vacuum/gas line and liquid PEA (purity > 99% from Fluka) was introduced in the cell up to complete covering of the powder. The slurry rested at room temperature under N2 flowing for ca. 1 h (saturation step). Then, the excess of non-adsorbed PEA was removed by filtration and the saturated sample was dried under N2 flowing for ca. 1 h at RT. The
PEA-saturated sample was thermally treated at 150 ◦ C overnight to remove the physically adsorbed PEA. The obtained sample was loaded on the alumina pan of the TGA-DSC (ca. 15–20 mg) and the PEA-TPD analysis was carried out from RT to 500 ◦ C at a rate of 10 ◦ C min−1 under flowing N2 at 50 mL min−1 . From TGA profile and its derivative (DTGA) as a function of time/temperature, the amount of mass of water or PEA desorbed at different temperature intervals could be quantified. From DSC, the heat flow profile (mW or W g−1 ) as a function of time/temperature with peaks associated with water or PEA desorption (endothermic peak) was obtained. Integration of the endothermic peaks gave the enthalpy of water/PEA desorption (des Hwater or des HPEA , in Joule). By coupling the TGA and DSC results, it was possible to determine the value of the average enthalpy of desorption of PEA from the acid sites of the samples (des HPEA , in Joule (molPEA )−1 ), corresponding to the average acid strength of the sample surface. 2.2.5. Intrinsic and effective nature of the acid sites by FT-IR adsorption of pyridine Lewis (LAS) and Brønsted (BAS) acid sites as well as their tolerance to water, were investigated by Fourier Transform Infrared Spectroscopy (FTIR) (Biorad FTS-60A) using pyridine as probe molecule both in vapor phase and aqueous solution [9]. All the samples were pressed into self-supporting disks (0.65 cm2 geometrical area) and before each analysis they were calcined for 2 h at 150 ◦ C in air. After outgassing for 30 min in high vacuum the samples were contacted with pyridine vapors at room temperature for 10 min or, alternatively, a 0.10 M pyridine aqueous solution was dropped on self-supporting disks under argon flow, always at room temperature. After pyridine adsorption, the samples were outgassed for 30 min in high vacuum at different temperatures (i.e., RT, 50, 100 ◦ C). BAS and LAS concentrations, expressed as microequivalents of adsorbed pyridine per gram of catalyst (eqPy gcat −1 ), were determined by integrating the peaks at 1540 and 1448 cm−1 , respectively, of the spectra collected after outgassing at 100 ◦ C, according to the procedure reported by Emeis [10]. 2.3. Catalytic test of hydrolysis of inulin The catalytic hydrolysis of inulin to fructose (and in very lower extent to glucose) was performed in water in a glass slurry batch reactor (Syrris, Atlas, UK) under increasing temperature from 50 to 90 ◦ C (0.12 ◦ C min−1 , 6 h of reaction). Inulin, in powder form, was obtained from Carlo Erba (pre reagent, RPE) with an average number of polymerization (DPn) of 25. In a typical catalytic test, the weighted and dried (120 ◦ C for ca. 16 h) catalyst sample (ca. 0.3 g sieved between 20 and 80 mesh) was put in the reactor filled with water (150 mL) at RT under gentle stirring, then, a weighted amount of inulin (1.5 g) was added to obtain an inulin concentration of ca. 50 mM. Then, the reaction started, while the temperature was allowed to increase (0.12 ◦ C min−1 ). The yield to reducing sugars (corresponding to the extent of reaction and called inulin conversion herein after below) was evaluated as the ratio between the reducing sugars produced and the reducing sugars at complete hydrolysis of substrate. The selectivity to fructose and glucose was determined from the ratio between fructose or glucose formed and the reducing sugars produced. The reducing sugars were evaluated by the classical colorimetric Nelson-Somogyi method by using copper(II) sulfate pentahydrate (Cu(SO4 )·5H2 O, Carlo Erba, RPE) and ammonium molybdate ((NH4 )6 Mo7 O24 ·4H2 O, Carlo Erba, RPE). The blue complex formed was analyzed and quantified at 520 nm. Moreover, the amount of glucose and fructose was determined by a Boehringer enzymatic assay technique.
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Table 1 Catalyst composition and morphological properties: BET surface area, total pore volume, micropore volume (determined by Dubinin–Radushkevich equation) and mean pore radius. Sample
2.5NbP
5NbP
7.5NbP
10NbP
Oxide
Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2 Nb2 O5 P2 O5 SiO2
Composition
Surface Area
Total Pore Volume
Micropore Volume
Mean Pore Radius
Molar (%)
Weight (%)
(m2 g−1 )
(cm3 g−1 )
(cm3 g−1 )
(nm)
2.5 2.5 95 5 5 90 7.5 7.5 85 10 10 80
9.88 5.27 84.85 17.85 9.53 72.62 24.41 13.04 62.55 29.92 15.98 54.10
466
0.30
0.11
1.3
139
0.56
0.038
8.0
59
0.27
0.010
9.0
41
0.30
0.0064
14
The kinetic coefficients at the average temperature Tm between two samplings (kTm ) were calculated by Eq. (1): kTm = (C/t)/Cm
(1)
where t is the time variation between two samplings (t = 30 min and T = 5 ◦ C); C is the variation of bond concentration between two samplings; Cm is the average bond concentration between two samplings. The catalyst samples recovered after reaction by filtration and drying for ca. 16 h in air, were analyzed by TGA analysis to determine the amount of organic residues present on their surfaces. Analysis was performed from 50 to 800 ◦ C (10 ◦ C min−1 ) under air flowing. Following the experimental approach of Sahoo et al. [11], the thermogravimetric profile of each sample was divided in three subintervals of temperature relevant to volatile organic compounds removal (VOC, 50–180 ◦ C), soft-coke (180–330 ◦ C), and hard-coke (330–800 ◦ C).
Fig. 1. Distribution of phosphate units for the studied catalysts in terms of Q’N notation; results obtained from 31 P NMR spectra [7].
3. Results and discussion 3.1. Structural and morphological characterization All the studied materials are ternary Nb-P-Si oxides with different ratio among components and their composition is listed in Table 1. In the samples, Nb and P are always in 1:1 molar ratio, as Nb and P content increase, SiO2 decreases from 95 to 80% molar (corresponding to 85–54 wt.%). All the samples have been fully characterized by XRD, 29 Si and 31 P NMR, Raman and FTIR spectroscopy in a previous work [7]. XRD analysis has shown that all dried gels as well as the gels annealed at 500 ◦ C are amorphous. The analysis of solid state NMR data has shown that the siloxane matrix is highly cross linked already in the dried gels and that after heating to 500 ◦ C the Q4 units (SiO4 tetrahedra with 4 bridging oxygen) increase only slightly up to about 80%. This high value of Q4 can be reasonably explained by the formation of Si O Nb bonds mostly as the Q4 fraction increases with Nb/P content. Here the cross linking in the 31 P NMR spectra is denoted by the Q’N notation, where Q’N stands for OP(OX)N (OH)3-N , with X = P or Nb. 31 P NMR data indicated that in all the materials heated at 500 ◦ C, phosphorus is present mainly as cross linked PO4 units (Q’1 , Q’2 , Q’3 ) with only very small amount of Q’0 , see Fig. 1 and Table S1. In previous papers on binary SiO2 -P2 O5 materials it was demonstrated that phosphorus was present only as Q 0 units, being the Si O P bonds hydrolysed also by ambient humidity [12,13]. The NMR data are therefore consistent with the preferential reaction between silicon and niobium on the one hand and niobium and phosphorus on the other with the niobium serving to anchor phosphorus into the silicon network with a less hydrolytically labile
Fig. 2. UV–vis diffuse reflectance spectra of the Nb-P-Si materials. The measured intensity is normalized and expressed as the Kubelka–Munk function F(R).
linkage. In other words, phosphorus is anchored through niobium bridges into the P-O-Nb-O-Si network. To get insight about the surface distribution of niobium structural units, UV–vis DRS spectra of all studied samples were recorded (Fig. 2). Features at about 200, 225 and 255 nm are recognized, related to charge transfer transitions associated with isolated NbO4 tetrahedra, low distorted NbO4 tetrahedra and high distorted NbO6 octahedra, respectively [8]. A diffuse absorption in the range 300–400 nm is also observed indicating the presence of NbO6 octahedra with lower distortion [8]. It could be expected that raising the
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2.5NbP 5NbP 7.5NbP 10NbP NbP a
Density of acid sites
PEA-desorption enthalpy
(eq g−1 )
(eq m−2 )
des HPEA (kJ mol−1 )
241 181 150 120 420
0.52 1.30 2.54 2.93 3.65
73.9 44.2 31.6 28.4 37.8
Determined in the temperature interval from 200 to 500 ◦ C.
3.2. Surface acidity determination
Fig. 3. Comparison of the adsorption and desorption N2 isotherms (at −196 ◦ C) between 2.5NbP and 5NbP. The isotherms for the other samples can be found in S.I. (Fig. S1).
content of Nb and P would cause at least a partial phase separation and segregation of Nb clusters in the solids, resulting in an increase of the amount of NbO6 octahedra at the expense of NbO4 tetrahedra and a corresponding shift of the absorption band to higher wavelengths. Actually, only minor changes are seen in the UV-DRS profiles, attesting that the niobium units maintain their distribution and good dispersion as the Nb content ranges from 2.5 to 10 mol%. The small rise of the absorption tail over 350 nm can be due to the conversion of some distorted NbO6 octahedra into more ordered ones. The band gap energy value obtained by Tauc plot elaboration (not reported) results almost the same in all samples (about 3.9 eV for direct transitions), confirming their similarity. A comparison of the UV–vis DRS spectrum of 2.5NbP with commercial NbOPO4 and Nb2 O5 was reported in [8]. Sample morphology was studied by N2 adsorption and desorption isotherms to determine surface area and porosity. Table 1 lists the surface area and the microporosity values of each sample. 2.5NbP has very high surface area and some amount of micropores. The Nb and P increase causes a prominent decrease of surface area with notable loss of microporosity, as observed in particular for the 5NbP and 7.5NbP samples. The marked change of morphology observed comparing the 2.5NbP and 5NbP samples is clearly displayed in Fig. 3 (the adsorption-desorption isotherms for 7.5NbP and 10NbP are shown in Fig. S1). 2.5NbP has an adsorption isotherm of type I (following IUPAC classification) typical of microporous materials with also presence of a little hysteresis indicating some amount of mesopores (the existence of micropores in 2.5NbP and 5NbP was comparatively illustrated by the DubininRadushkevich representation in Fig. S2). In contrast, 5NbP has an adsorption isotherm typical of non-porous materials (type II following IUPAC classification) as well as the other samples (7.5NbP and 10NbP, Fig. S1). The trend of surface area values observed by increasing the niobium and phosphorus content was similar to that already observed for binary SiO2 -P2 O5 [13] and SiO2 -Nb2 O5 materials [14]. Finally, the sample morphology seems closely related to the trend of cross-linking degree between the oxide components. SiO2 weight content decreases significantly from 2.5NbP to 10NbP (see Table 1), so the porous siloxane matrix is reduced and more cross-linked Nb and P units concentrate on the surface, favouring a decrease of specific area and available micropore volume. The pore size distribution varies accordingly, increasing linearly with Nb and P content, while the total pore volume does not follow a clear trend, suggesting that 7.5NbP and 10NbP possess less pores but larger ones.
The acid properties of the Nb-P-Si samples have been studied by thermal desorption of a basic probe (PEA) previously adsorbed over the sample surface to obtain complete saturation of all the acid sites of the samples. The experiments have been performed in a coupled TGA-DSC instrument that allowed obtaining the amount of PEA desorbed (thermogravimetric signal), from which it was possible to determine the acid site density of the surfaces, and the enthalpy of desorption of PEA (ads HPEA , calorimetric signal), which corresponds to the average acid strength of the surfaces. PEA was chosen as probe for its high molecular weight (121.18 g mol−1 ) that allows to quantitatively monitor the desorption, following the mass decrease. At first, the amount of PEA in moles, computed from the mass loss of PEA determined by the TGA profile, corresponds to the moles of acid sites titrated. In general, PEA was desorbed in the temperature range from 200 to 500 ◦ C. A typical thermogravimetric profile obtained from the PEA-TPD experiments with the TGA and relevant derivative (DTGA) curves as a function of temperature is displayed in Fig. S3. At lower temperatures (50–150 ◦ C), a mass loss related to water desorption is observed for all the samples. The results obtained in terms of density of acid sites normalized per mass or surface unit are listed Table 2. The amount of specific acid sites density (expressed in eq g−1 ) regularly decreased with Nb (and P) increase in the sample, while the amount of intrinsic acid sites density (expressed in eq m−2 ) increase regularly with the Nb (and P) content. Likely, the titrated acid sites comprise those associated to Si (weak acidity) and to Nb and P species (medium-strong acidity). These singular results are due to the important differences of specific surface area among the samples (Table 1). Second, from the calorimetric profile of PEA desorption, determined by the DSC curve as a function of temperature, the average strength of surface acidity could be determined for each sample. Fig. 4 displays a typical calorimetric profile obtained for a representative sample. In the interval 50–150 ◦ C, there are two endothermic peaks associated to the desorption of physisorbed and chemisorbed water from the surface. In general, more than one endothermic peaks related to PEA desorption were observed in the temperature interval from 200 to 500 ◦ C, in the same range where the mass loss of PEA occurs. The total amount of heat absorbed is the enthalpy of PEA desorption (ads HPEA ). Two distinct endothermic peaks are seen in the 200–500 ◦ C range, that are relevant to PEA desorption from the acid sites. These observations indicate the heterogeneous distributions in strength of the acid sites, comprising silanol groups and acid species associated to Nb and P. Table 2 reports also the average enthalpy of PEA desorption for each sample; values are decreasing with the Nb (and P) increase as already observed for the specific density of acid sites. 2.5NbP possesses high surface acid strength, 73.9 kJ mol−1 , much higher than the values of the other samples (in the range 32–44 kJ mol−1 ). This trend well agrees with the structural evolution occurring with the increase of Nb (P) producing the increase of the crosslinking degree of both Nb-O-Si and
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Fig. 4. DSC profile of the PEA-TPD analysis on a chosen representative catalyst. The enthalpy of PEA desorption (des HPEA , in kJ mol−1 , see Table 2) has been calculated by summing up the two endothermic peaks of PEA desorption (qPEA , in J g−1 ) and considering the sample mass used in the analysis and the amount of PEA desorbed (in moles) in the same temperature interval in the coupled TGA analysis.
Nb-O-P [7]. Consequently, the fraction of isolated NbO4 tetrahedra, with which a very strong Lewis acidity is associated, decreases slightly, as can be observed in UV–vis DRS spectra (Fig. 2). The nature of the acid sites is another information of great importance besides the amount of sites and acid strength, to know in full the acid properties of any surface. FTIR spectroscopic experiments with basic probe molecules allow the knowledge of the LAS (Lewis acid sites) or BAS (Brønsted acid sites) nature of surface acid sites by studying the molecular complexes formed following adsorption of the probe. Pyridine is a typical probe used for these studies; distinction between the pyridinium ion (typical absorption at 1540 cm−1 ) and coordinated pyridine (typical absorption at 1448 cm−1 ) to the acid sites is possible as well as
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the obtainment of LAS and BAS concentrations and relative ratios by peak integration and application of suitable extinction coefficients. Fig. 5 shows the FTIR desorption spectra collected in the range 1350–1700 cm−1 after contact of pyridine both in vapor phase (intrinsic nature of the acid sites) and aqueous solution (effective nature of the acid sites) with the acid surfaces of the 2.5NbP, 5NbP, and 7.5NbP samples. All the recorded spectra exhibited band patterns typical of BAS and LAS sites but showing different intensities. The sharp bands corresponding to the interaction of pyridine molecules with LAS at 1448 cm−1 (19b mode) and 1610 cm−1 (8a mode) are clearly observed, while broad bands at 1540 cm−1 (19b mode) and 1637 cm−1 (8a mode) are typical of BAS presence. Low–coordinated Nb sites in tetrahedral coordination could be responsible for the Lewis acidity, as proposed for niobic acid (Nb2 O5 ·nH2 O) and H3 PO4 –treated niobic acid [15], while Brønsted acidity comes, for the most part, from the silanol, P-OH and Nb-OH species. The quantitative determination of Lewis and Brønsted acid sites has been made on the basis of the bands located at 1448 cm−1 and 1540 cm−1 , according to the procedure reported by Emeis [10]. Pyridine in aqueous solution was employed to investigate the acidic properties of the surfaces mimicking the real catalyst working conditions (i.e. in aqueous solutions). The evolution of the band relative to the scissoring mode of adsorbed water molecule at 1620 cm−1 was followed in order to confirm the co-adsorption of water and pyridine also during desorption experiments [16]. In Table 3 are summarized the results of the determination of the acid sites for all the samples. With pyridine in vapor phase, the LAS/BAS ratios are very high, close to 1 in any case. According to the analysis of UV–vis DRS data the distribution of Nb structural units exhibits both isolated and low distorted NbO4 tetrahedra accounting for this value of LAS/BAS ratio. In the presence of water only the LAS concentration decreased strongly, while the BAS concentration remained almost unchanged; therefore, the LAS/BAS ratios decreased down to ca. 0.4–0.5. This observed behaviour is somehow unexpected because in the presence of water the acid strength should decrease as has been previously observed for NbP catalyst [17]. In this work, the LAS to BAS ratio for NbP drastically
Fig. 5. FT-IR desorption spectra at 100 ◦ C of pyridine on 2.5NbP (a), 5NbP (b), and 7.5NbP (c) with pyridine adsorbed in vapor phase (Vap.) and aqueous phase (Aq.). Bands due to Brønsted (B) and Lewis (L) acid sites are indicated.
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Table 3 Determination of the nature of the acid sites by FT-IR after pyridine adsorption in absence and in presence of water.a Sample
Py(vap) b LAS
2.5NbP 5NbP 7.5NbP NbP a b c d e
d
82 0.18e 72d 0.52e 35d 0.59e 81d 0.70e
Py(aq) c BAS d
96 0.21e 95d 0.68e 33d 0.56e 39d 0.34e
LAS/BAS 0.85 0.76 1.05 2.08
LAS d
26 0.06e 24d 0.17e 16d 0.27e 61d 0.53e
BAS d
70 0.15e 61d 0.44e 33d 0.56e 36d 0.31e
LAS/BAS 0.37 0.39 0.48 1.69
Values reported after pyridine desorption at 100 ◦ C. Pyridine contacted in vapor phase. Pyridine contacted in aqueous solution. Expressed in eq g−1 . Expressed in eq m−2 .
decreases going from 2.08, measured in pyridine vapor, to 1.69, when measured in water. For Nb-P-Si ternary samples, the strong LAS reduction may be related to the substantial hydrolysis of NbO4 isolated tetrahedra, producing terminal Nb OH groups [8]. This phenomenon partially counteracts the missed contribution of the Si-OH groups giving the materials strong Brønsted acid properties characterized by LAS/BAS 4 times lower than NbP. Increasing the amount of Nb (and P) does not seem to provide a higher number of strong acid sites, probably because Nb and P units become more cross-linked. The larger differences of LAS and BAS populations have been observed between the 2.5NbP and 5NbP samples due to the very important difference of morphology (in particular, specific surface area). The acid properties of the Nb-P-Si materials suggest their ability to work mainly as Brønsted catalysts in reactions requiring watertolerant protonic sites. 3.3. Catalytic test of inulin hydrolysis Inulin is a natural polysaccharide of fructose consisting of linear chains of fructose units linked by -1,2 bonds and terminated with a sucrose residue; it acts as energy reserve in various plants (e.g., chicory and Jerusalem artichoke). The exploitation of such cultivations and the resulting inulin extraction make possible the production of syrups with higher fructose content than from other traditional processes, like glucose isomerization [18–20]. NbOPO4 catalyst has been recently confirmed as an interesting catalyst for this reaction [8] thanks to its Brønsted acidity; the same reaction was studied over the ternary Nb-P-Si samples. The time course of the inulin hydrolysis was studied as a function of time/temperature in the 50–90 ◦ C interval by adding the dry catalyst sample to the inulin aqueous solution. Fig. 6 shows the increasing trends of inulin conversion (calculated by analyzing the total reducing sugars formed) for all the samples as a function of temperature, which was continuously increasing with time. Inulin hydrolysis started at ca. 70 ◦ C and reached more than 80% at the highest reaction temperature (90 ◦ C) over 2.5NbP. Over all the other catalyst samples, similar increasing trends were observed, but the conversion curves are shifted at (little) higher temperatures. The total reducing sugar analyzed corresponded to concentration of hydrolyzed bonds, the increasing inulin conversion led to increasing selectivity towards the monosaccharides (fructose + glucose). Fig. 7 shows the trends of the curves of selectivity to fructose as a function of inulin conversion. Unique trends can be guessed with obtainment of selectivity values higher than 80% for all the catalysts. This suggests that the nature of the sites are similar and they give similar selectivity to fructose in the inulin hydrolysis reaction. Over 2.5NbP, the higher amount of acid sites (expressed in eq g−1 ,
Fig. 6. Conversion of inulin as a function of temperature evaluated as the ratio between the reducing sugars produced and the reducing sugars at complete hydrolysis of substrate.
Fig. 7. Selectivity to fructose as a function of inulin conversion on the Nb-P-Si catalysts; selectivity was evaluated from the ratio between fructose formed and the reducing sugars produced. Table 4 Kinetic coefficients for the reaction of inulin hydrolysis evaluated at three representative temperatures and Arrhenius parameters. Sample
2.5NbP 5NbP 7.5NbP 10NbP NbP a
kT (meq g−1 min−1 ) T = 60 ◦ C
T = 70 ◦ C
T = 80 ◦ C
0.0160 0.0040 0.0055 0.0050 0.0124
0.0712 0.0229 0.0242 0.0239 0.0564
0.291 0.120 0.098 0.105 0.236
Ea (kJ mol−1 )
Ln Aa
142 ± 8 167 ± 7 140 ± 9 149 ± 9 144 ± 11
47 ± 3 55 ± 2 46 ± 3 48 ± 3 48 ± 4
A expressed in meq g−1 min−1 .
Table 2) justifies the higher conversion observed than for the other catalyst samples. Activation parameters for the catalysts have been determined from Arrhenius plots using the rate coefficients evaluated at different Tm (average temperature on 30 min of reaction). Table 4 reports the results of the kinetic coefficients evaluated at three representative reaction temperatures (60, 70 and 80 ◦ C) and the activation parameters. The reaction rate over 2.5NbP compared with the other ternary oxides is four times higher at 60 ◦ C, three times higher at 70 ◦ C and two and a half times higher at 80 ◦ C. Even commercial NbP shows a lower reaction rate than 2.5NbP in the whole temperature range. This behaviour confirms the higher activity of 2.5NbP respect to the other catalysts thanks to a larger strength and amount of acid sites (Table 2), in particular Brønsted sites requested for inulin hydrolysis. The calculated apparent activation energy values did not differ significantly for the five catalyst samples (Ea around 140–160 kJ mol−1 and ln A around 45–55). The used catalysts after filtration and drying have been analyzed by TGA to determine the amount of carbon deposited on
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Acknowledgements The authors thank Dr. Pierluigi Mazzei and the Centro Interdipartimentale di Ricerca sulla Risonanza Magnetica Nucleare per l’Ambiente, l’Agro-Alimentare ed i Nuovi Materiali (Università di Napoli Federico II) for MAS NMR measurements. The use of instrumentation purchased through the SmartMatLab Project (Cariplo Foundation project 2013-1776) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.10. 006. Fig. 8. Results of TGA analysis of the used Nb-P-Si catalysts after reaction: quantitative determination of insoluble organic residues from mass loss at given temperature intervals [11].
the surface, in view of a possible recycle of the catalysts. In general, coke samples exhibit three distinct temperature regions [11]: region I (T < 180 ◦ C) can be ascribed to the loss of water and volatile species in the samples; region II (180 ◦ C ≤ T ≤ 330 ◦ C) and III (330 ◦ C ≤ T ≤ 750 ◦ C) can be ascribed to desorption of coke as CO or CO2 . The coke desorbed in region II are more mobile carbonaceous residues or physisorbed products or side products (altogether can be termed as ‘soft coke’) whereas in region III, it is more bulky carbonaceous compounds (termed as ‘hard coke’). All the three types of coke were present on the used samples analyzed (Fig. 8). Hard coke shows a clear decreasing concentration trend with the acidity strength of the samples. Even if acid site density (in eq m−2 ) of the samples increased with the increase of Nb content, the average surface acid strength decreased (see Table 2) and consequently low amount of hard-coke was formed. This represents an advantage for the long stability of catalysts activity. This also shows that some of the carbon insoluble residues, which are highly condensed, might be deposited on the most acidic sites of the catalyst surface causing its deactivation. 4. Conclusions The Nb-P-Si ternary oxides studied can be considered interesting strong acid solids, in which stable phosphate units are crosslinked through niobium bridges in the silicate matrix. The morphological and the surface properties of these solids are strongly influenced by their composition: surface area, porosity and acid strength decrease increasing the niobium and phosphorus content. Pyridine adsorption experiments indicate a high concentration of both Lewis and Brønsted acid sites in vapor phase, while in water the BAS concentration prevails keeping about the same value of the vapor phase. All studied materials exhibited an excellent catalytic activity in the hydrolysis of inulin in aqueous solution at low temperature, even if the sample with the lowest Nb and P content shows the best results in terms of inulin conversion and yield to fructose, likely due to its higher intrinsic Brønsted acidity lively-active in water. The Nb-P-Si ternary oxides are promising catalysts for aqueous reactions requiring stable Brønsted acid sites.
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