Upgrading of oxygenated compounds present in aqueous biomass-derived feedstocks over NbOx-based catalysts

A. Fernández-Arroyo , D. Delgado , M. E. Domine * and J. M. López-Nieto *
Instituto de Tecnología Química (UPV-CSIC), Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain. E-mail: mdomine@itq.upv.es; jmlopez@itq.upv.es

Received 19th May 2017 , Accepted 24th September 2017

First published on 25th September 2017


The influence of synthesis and post-synthesis procedures of different niobium oxides on their catalytic performance in the aqueous phase condensation of oxygenated compounds is studied. Hydrothermally synthesized niobium oxide with a pseudo-crystalline structure shows enhanced acid properties, surface area and consequently better catalytic activity than Nb2O5 prepared by other synthesis methods. The optimized NbOx-based catalyst also demonstrates higher stability after several reuses compared to the Ce–Zr mixed oxide reference catalyst.


In the past few years, lignocellulosic biomass valorisation has become a sustainable alternative to the use of fossil sources for the production of fuels and chemicals.1,2 In this context, after primary treatment of biomass via fast pyrolysis processes, bio-oils are predominantly obtained.3,4 These bio-oils are complex mixtures containing water and oxygenated organic compounds in varying concentrations, which are characterized by their high reactivity. This hinders their storage and direct use as liquid fuels. Therefore, an upgrading approach is needed to improve their fuel properties (O content reduction, water elimination, etc.). The most common upgrading process includes bio-oil hydrotreatment with NiMoS/Al2O3 and CoMoS/Al2O3 industrial catalysts at high H2 pressures (>60–70 bar) and temperatures (350–450 °C).5,6 The resulting oil possesses better fuel quality but H2 and energy consumption during the process is quite high, while liquid productivities are low mainly because the highly abundant C1–C4 oxygenated compounds are converted into gases during the process. Alternatively, a liquid–liquid separation process by water addition to the pyrolytic bio-oils is also feasible.7 Then, the separated organic fractions are further processed for their application as liquid fuels, whereas the aqueous fractions containing C1–C4 acids (i.e. acetic acid), aldehydes, ketones, alcohols and low amounts of heavier water-soluble compounds constitute nowadays waste effluents at bio-refineries.7,8

Aligned with the new bio-economy concept,9 the transformation of these low-value water-soluble compounds would be of great interest. Indeed, they can be transformed into a mixture of hydrocarbons and aromatics useful for blending with automotive fuels. In the first step, this can be achieved by performing “one pot” C–C bond formation reactions, such as aldol condensation and ketonization, among others.10 The importance of ketonization lies in its capacity for reducing the carboxylic acid content whilst condensation reactions allow consecutive C–C bond formation. Both reaction mechanisms have been widely studied by using probe molecules as reactants,11,12 and it was concluded that a combination of Lewis acid and base sites is necessary to assist in the formation of intermediates on the surface of the catalyst. In this context, catalysts based on Ce–Zr–O mixed oxides have been extensively considered12 for the gas-phase conversion of small aldehydes in the presence of acids and water at high temperatures (>300 °C). In addition, CeMOx (M: Ce, Mn, Fe, and Al) catalysts have been reported to show poor structural stability in the ketonization of acetic acid in the condensed phase.13 In general, the activity of these catalysts is based on their acid/base bifunctional character, although their low stability under more realistic operating conditions (mainly due to Ce leaching) is a critical factor to be further improved.12,13 Moreover, TiO2[thin space (1/6-em)]14,15 and ZrO2[thin space (1/6-em)]12,16 materials possessing mostly Lewis and Brønsted acid sites have been studied as catalysts for condensation reactions using acetic acid or propanal as probe molecules. In both cases, the loss of catalytic activity at high reaction temperatures (300–450 °C) and their performance in the presence of water become their main drawbacks.

Based on these considerations, the stability and acid properties of niobium oxides at high reaction temperatures (even in the presence of water) make these materials attractive for industrial use as heterogeneous catalysts:17 i) they show great stability against acids; ii) most of their Lewis acid sites remain active even in aqueous environments when tested in different reactions (i.e. acetalization, isomerization).18,19 Moreover, their crystalline structure, surface area and acid properties can be modified depending on the synthesis procedure and/or the post-synthesis thermal treatment.20,21 Specifically, we have found that both the Brønsted/Lewis ratio and the total number of acid sites in niobium-containing materials can be tuned by an appropriate combination of hydrothermal synthesis and post-synthesis treatments, thus improving both the acid characteristics and the catalytic performance of NbOx-based catalysts.22

In this work, we show the influence of both the synthesis procedure and the heat-treatment conditions on the catalytic performance of different niobium oxides in the valorisation of aqueous effluents obtained by liquid phase separation of pyrolytic bio-oils. The catalytic behaviour of NbOx-based catalysts is investigated by using an aqueous mixture of representative C2–C3 oxygenated compounds, which is closer to the real conditions at the industry and differing from the usual probe molecule studies performed even in the absence of water in most of the cases. The catalytic results are compared with those of Ce–Zr–O mixed oxide, which is the reference material in the literature.14

Nb oxide-based catalysts were prepared by hydrothermal synthesis22 (HT-series, ESI). For comparative purposes, niobium oxides prepared by conventional precipitation (PR-series) and commercial Nb2O5 from Sigma-Aldrich (C-series) have also been tested (see the ESI). All the niobium-based samples were heat-treated in air or N2 to obtain the final catalysts. The catalysts are named HT-xy, PR-xy and C-xy (for hydrothermal, precipitation and commercial materials, respectively), where x is the temperature and y is A or N, depending on the atmosphere (air or N2, respectively) used during heat treatment. In addition, Ce0.5Zr0.5O2 mixed oxide (CeZrO) was prepared via co-precipitation following the recipe in ref. 23 (see the ESI).

The catalysts were characterized by XRD, TEM, TG, EA, FT-IR of adsorbed pyridine, and N2 adsorption. Important differences in the crystallinity and particle size of the materials have been observed depending on the preparation procedure and heat treatment (Fig. 1; see also Fig. S3 in the ESI for the rest of the materials). In this sense, HT-series and PR-series materials heat-treated in N2 tend to form a pseudo-crystalline (Fig. 1a) or an amorphous phase (Fig. 1b), respectively, while C-series samples give rise to a well-ordered T-Nb2O5 phase (JCPDS: 00-027-1313) (Fig. 1c). In addition, oxides prepared by the hydrothermal method show the smallest particle size (ca.10–20 nm) (Fig. 1a).


image file: c7cy00916j-f1.tif
Fig. 1 XRD patterns and TEM images of representative NbOx-based catalysts heat-treated in N2 at 450 °C: (a) HT-450N, (b) PR-450N and (c) C-450N.

Pseudo-crystalline materials obtained through the hydrothermal procedure present Bragg signals corresponding to the 001 and 002 planes in the crystal structure ordered along the c-axis (c = 3.9 Å), which is related to the ReO3-type structure (Fig. S3 and S5, ESI), as has been observed in other NbOx-based materials.24,25 It is noteworthy to mention that heat treatments in air promote the crystallization, at least partially, of the orthorhombic T-Nb2O5 phase (JCPDS: 00-027-1313) (Fig. S3, ESI). On the other hand, the C-Nb2O5 samples show a well-ordered crystal structure regardless of the heat treatment atmosphere (Fig. S3 and S5, ESI).

Catalytic results of the selected catalysts tested in the transformation of oxygenated compounds present in an aqueous model mixture including acetic acid, ethanol, propanal and acetol (the liquid phase system in an autoclave reactor at 200 °C and PN2 = 13 bar over 7 h, see the ESI) are shown in Table 1. Results expressed in terms of total organic yield and yield of the main reaction products are calculated by considering that the maximum attainable value of total organic products for the composition of the aqueous mixture studied is ≈30% (see the ESI). As a reference, the catalytic performance of CeZrO has also been included.

Table 1 Physicochemical and catalytic characteristics of representative catalysts
Samples Heat-treatment (°C) Surface areaa (m2 g−1) Acid sitesb Density of acid sitesc (μmol m−2) Total organic yieldd (%) Main product yield (%) Carbon balancef (%) Initial reaction rateg (mmol min−1 g−1)
LAS BAS C5–C8e C9–C10
a Calculated values from N2 adsorption isotherms (BET method). b Values were calculated by pyridine adsorption FT-IR measurements. c Density of total acid sites (BAS + LAS). d Total organic product yield. e Sum of 2-methyl-2-pentenal and other C5–C8 products. f Carbon balance calculated from GC-FID. g Initial reaction rate values were calculated at 1 h of reaction (see the ESI). Reaction conditions: 3.0 g of aqueous model mixture, 0.15 g of catalyst at 200 °C and PN2 = 13 bar over 7 h.
HT-series 300 (N2) 163 135.3 41.5 1.08 59.0 43.0 16.0 90 1.88
400 (N2) 152 54.7 57.4 0.74 69.7 51.7 18.0 98
450 (N2) 108 65.4 33.5 0.92 69.3 52.0 17.3 97 1.67
550 (N2) 70 20.6 16.1 0.52 65.3 46.0 19.3 96 1.58
C-series 450 (N2) 13 4.5 3.0 0.58 61.0 39.3 21.7 97 1.05
PR-series 450 (N2) 52.0 37.7 14.3 97
CeZrO 450 (air) 112 87.6 17.4 0.94 68.7 43.4 25.3 93


Additionally, the effect of the atmosphere (air or N2) used during heat treatment on the catalytic activity has also been studied (Table S2, ESI). In all cases, higher organic product yields are observed when the heat treatment is carried out in a N2 atmosphere (Table S2, ESI).

More importantly, hydrothermally synthesized niobium oxides (HT-series) reached a higher propanal conversion (>90%) and 2-methyl-2-pentenal (2M2P) yield (Table S2, ESI), thus achieving the highest total organic product yields (Table 1).

A deeper understanding of the HT-series catalytic activity can be achieved by individually studying the behaviour of reactant conversion present in the aqueous model mixture (Fig. 2a). Complete conversion of acetol and high conversion of propanal are reached in short times, while ethanol and acetic acid exhibit lower conversions (≈50% and 15%, respectively).


image file: c7cy00916j-f2.tif
Fig. 2 (a) Conversion of acetic acid, ethanol, propanal and acetol and (b) evolution of product yields vs. reaction time over the HT-450N catalyst [reaction conditions: 3.0 g of aqueous model mixture, 0.15 g of catalyst at 200 °C and PN2 = 13 bar over 7 h].

Note that a low acetic acid conversion is expected under the moderate reaction conditions used here, mainly due to the strong competition for the active sites of more reactive molecules, such as propanal and acetol, present in the mixture. Thus, the study of catalyst performance by using complex mixtures is essential for the development of future industrial applications. In addition, the identification of the main reaction products and their distribution profile observed by GC and GC-MS (Fig. S1, ESI) allow us to draw the evolution of product yield with reaction time when the HT-450N sample is used (Fig. 2b).

Accordingly, a reaction network is proposed (Fig. 3, see also the ESI) in which the main intermediate (C5–C8) and final (C9–C10) products as well as ethyl acetate (formed by the esterification reaction of acetic acid with ethanol) and acetone (as an acetic acid ketonization product) have been highlighted.


image file: c7cy00916j-f3.tif
Fig. 3 Proposed reaction network. The initial mixture (orange), intermediate C5–C8 products (red) and final C9–C10 products (blue) are outlined.

As can be seen in Fig. 2b, high product yields are reached within 3 hours of time on stream due to fast condensation reactions between propanal and acetol, whereas ethyl acetate is produced from acetic acid and ethanol esterification. At longer reaction times, the products from the second condensation stage (C5–C8 intermediates, mainly formed by cross-condensation between acetol and propanal) are formed, which can continue to react with propanal or other intermediates (acetone, 3-pentanone, etc.) at long reaction times to generate C9–C10 products (Fig. 2b and S8). However, the ethyl acetate yield remains constant over time, suggesting that this reaction probably only takes place at the beginning of the experiment.

In general, commercial Nb2O5 catalysts (with well-ordered structure) and PR-samples (presenting an amorphous nature) present the lowest catalytic activity (see Fig. S3 and Table S2), whereas samples prepared hydrothermally (HT-series, with a pseudocrystalline structure) present the highest catalytic activity in both ketonization and aldol condensation reactions. The better catalytic performance of pseudocrystalline HT-series samples can be explained in terms of their improved acid and textural properties. Thus, they show an increase in total acidity (Fig. S6, ESI), surface areas (>70 m2 g−1) and pore volumes (which probably favours a higher accessibility of reactants to active sites) (see Table S1b, ESI). The different crystalline structures of the materials have only minor effects on the catalytic activity; the acid and textural properties are more decisive in this case, thus giving rise to higher initial reaction rates (Table 1). If the total organic product yield at 1 h of reaction is normalized to the density of acid sites, the differences among all the materials can easily be noticed. Thus, the Nb-based catalysts synthesized hydrothermally (HT-series) display a better catalytic performance than the C- and PR-series samples (see Table S3, ESI).

In the HT-series samples, the density of acid sites increases when the heat-treatment temperature is decreased despite the increment of the surface area observed (Table 1). Indeed, the higher amount of total acid sites and their proper distribution on the catalyst surface lead to an increase in the organic product yields during the process (Fig. 4). The optimal combination of these parameters has been found for the HT-400N and HT-450N samples, which leads to a better catalytic behaviour in terms of total organic yield than that of the CeZrO reference catalyst (Table 1 and Fig. 4).


image file: c7cy00916j-f4.tif
Fig. 4 Variation of total organic product yield (TOP, black), intermediates (INTs, red), and C9–C10 product (blue) yields with the density of acid sites for all catalysts: HT-series (filled symbols); PR-series and C-series (empty symbols). Results for the CeZrO sample (star symbols) are also included.

Interestingly, in spite of the higher density of acid sites and initial reaction rate observed for the sample heat-treated at the lowest temperature (i.e. sample HT-300N), its higher acidity could also favour undesired polymerization reactions during the process, thus lowering the amount of the final total organic products found in the liquid reaction mixture (Table 1 and Fig. S7, ESI). This effect can also be observed by the decrease of the carbon balance when HT-300N was used (Table 1).

Finally, recycling experiments were performed by testing the selected catalysts during three consecutive reuses to corroborate the resistance of Nb oxides under the reaction conditions (see the ESI), and the results in terms of total organic yield attained are given in Fig. 5. On the one hand, it was observed that the HT-450N catalyst maintained its catalytic performance as it was practically invariable after three reuses due to its structure preservation, as confirmed by X-ray diffraction and Raman spectroscopy measurements. On the other hand, the C-450N and PR-450N samples exhibited high stability (low carbon deposition), although the catalytic activity of C-450N decreased probably due to surface reconstruction/change processes after being used in aqueous solutions (Fig. S9, ESI), while a very low activity in the PR-type sample was observed even with the fresh catalyst (R0). In both Nb-based catalysts, no leaching of Nb was observed. Remarkably, the hydrothermally prepared catalysts still showed a much better stability than the CeZrO mixed oxide, which suffered a bigger loss of activity due to carbon deposition as measured by TG and EA (see the ESI), but mostly due to the significant leaching of Ce (>30 wt% lost from the initial content in solid) clearly detected in the liquids after the reaction (see the ESI).


image file: c7cy00916j-f5.tif
Fig. 5 Effect of the reuses (R0, R1 and R2) of different catalysts on the total organic yield (%) attained during the condensation of oxygenated compounds in an aqueous model mixture (at 200 °C and PN2 =13 bar over 7 h).

Although further studies must be performed, these results are very promising and open up new possibilities for the application of Nb-based catalysts for the condensation of oxygenated compounds present in complex aqueous mixtures.

Conclusions

In summary, niobium oxide-based materials prepared by hydrothermal synthesis with a pseudo-crystalline structure (ordered just along the c-axis) and showing tunable acid properties have been shown to be active and selective catalysts for the valorisation of oxygenated compounds in aqueous effluents obtained by phase separation of pyrolytic bio-oils. These optimized NbOx materials present higher stability (after several reuses) and catalytic activity than those previously reported by other authors, mainly because they are highly water-resistant catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support by the Spanish Government (CTQ-2015-67592, CTQ-2015-68951-C3-1, and SEV-2012-0267) is gratefully acknowledged. A. F.-A. and D. D. thank the “La Caixa-Severo Ochoa” Foundation and the Severo Ochoa Excellence Program (SVP-2014-068669), respectively, for their fellowships. Authors also thank the Electron Microscopy Service of Universitat Politècnica de València for their support.

Notes and references

  1. G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044 CrossRef CAS PubMed .
  2. C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and M. Poliakoff, Science, 2012, 337, 695 CrossRef CAS PubMed .
  3. G. W. Huber and A. Corma, Angew. Chem., Int. Ed., 2007, 46(38), 7184 CrossRef CAS PubMed .
  4. D. M. Alonso, J. Q. Bond and J. A. Dumesic, Green Chem., 2010, 12(9), 1493 RSC .
  5. A. Pinheiro, D. Hudebine, N. Dupassieux and C. Geantet, Energy Fuels, 2009, 23(2), 1007 CrossRef CAS .
  6. V. N. Bui, G. Toussaint, D. Laurenti, C. Mirodatos and C. Geantet, Catal. Today, 2009, 143(1–2), 172 CrossRef CAS .
  7. D. Radlein and A. Quignard, US Pat., 2014/0288338, 2014 Search PubMed .
  8. M. Asadieraghi, W. M. A. Wan Daud and H. F. Abbas, Renewable Sustainable Energy Rev., 2014, 36, 286 CrossRef CAS .
  9. F. Cherubini, G. Jungmeier, M. Wellisch, T. Willke, I. Skiadas, R. van Ree and E. de Jong, Biofuels, Bioprod. Biorefin., 2009, 3(5), 534 CrossRef CAS .
  10. C. A. Gaertner, J. C. Serrano Ruiz and J. A. Dumesic, J. Catal., 2009, 266(1), 71 CrossRef CAS .
  11. T. Pham, T. Sooknoi and D. E. Resasco, J. Catal., 2012, 295, 169 CrossRef CAS .
  12. A. Gangadharan, M. Shen, T. Sooknoi, D. E. Resasco and R. G. Mallinson, Appl. Catal., A, 2010, 385(1–2), 80 CrossRef CAS .
  13. R. W. Snell and B. H. Shanks, ACS Catal., 2014, 4, 512–518 CrossRef CAS .
  14. S. Wang, K. Goulas and E. Iglesia, J. Catal., 2016, 340, 302 CrossRef CAS .
  15. S. Wang and E. Iglesia, J. Catal., 2017, 345, 183 CrossRef CAS .
  16. K. Wu, M. Yang, Y. Chen, W. Pu, H. Hu and Y. Wu, AIChE J., 2017, 63(7), 2958–2967 CrossRef CAS .
  17. K. Nakajima, Y. Baba, R. Noma, M. Kitano, J. N. Kondo, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2011, 133(12), 4224 CrossRef CAS PubMed .
  18. R. Rodriguez, D. Mandelli, N. S. Gonçalves, P. P. Pescarmona and W. Carvalho, J. Mol. Catal. A: Chem., 2016, 422, 122 CrossRef .
  19. H. T. Kreissl, K. Nakagawa, Y.-K. Peng, Y. Koito, J. Zheng and S. C. E. Tsang, J. Catal., 2016, 338, 329 CrossRef CAS .
  20. M. Paulis, M. Martín, D. B. Soria, A. Díaz, J. A. Odriozola and M. Montes, Appl. Catal., A, 1999, 180(1–2), 411 CrossRef CAS .
  21. G. S. Foo, D. Wei, D. S. Sholl and C. Sievers, ACS Catal., 2014, 4(9), 3180 CrossRef CAS .
  22. M. E. Domine, J. M. López-Nieto, D. Delgado and A. Férnandez-Arroyo, ES Pat., P201630339, 2016 Search PubMed .
  23. J. C. Serrano-Ruiz, J. Luettich, A. Sepúlveda-Escribano and F. Rodríguez-Reinoso, J. Catal., 2006, 241(1), 45 CrossRef CAS .
  24. K. Omata, K. Matsumoto, T. Murayama and W. Ueda, Catal. Today, 2016, 259(1), 205 CrossRef CAS .
  25. T. Murayama, J. Chen, J. Hirata, K. Matsumoto and W. Ueda, Catal. Sci. Technol., 2014, 4(12), 4250 CAS .

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cy00916j

This journal is © The Royal Society of Chemistry 2017