Direct use of humic acid mixtures to construct efficient Zr-containing catalysts for Meerwein–Ponndorf–Verley reactions

Yufei Sha , Zhenhuan Xiao , Huacong Zhou *, Keli Yang , Yinmin Song , Na Li , Runxia He , Keduan Zhi and Quansheng Liu *
College of Chemical Engineering, Inner Mongolia University of Technology, Inner Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank Carbon Resources, Huhhot 010051, Inner Mongolia, China. E-mail: hczhou@imut.edu.cn; liuqs@imut.edu.cn

Received 27th June 2017 , Accepted 15th August 2017

First published on 18th August 2017


With the increasing demands for energy and carbon resources, exploration of novel utilization approaches of fossil resources and promotion of the conversion of sustainable resources become critical issues facing human society. In this study, we used humic acids (HAs), which are important derivatives from the low-rank coal, and the transition metal zirconium (Zr) to construct novel Zr-containing catalysts (Zr-HAs) for Meerwein–Ponndorf–Verley (MPV) reactions of biomass-derived platforms. Both commercial HAs and HAs extracted directly from lignite without any purification were used as raw materials to prepare the catalysts. The results showed that Zr-HAs catalysts were highly efficient for the conversion of furfural, with a high furfuryl alcohol yield of up to 97%, and also effective for the conversion of other carbonyl compounds with different structures under mild conditions. This novel strategy to construct catalysts using HAs as raw materials may be beneficial for both value-added utilization of low-rank coal and the conversion of biomass resources.


Introduction

With the high-speed development of human society and the fast depletion of fossil resources (such as coal) on earth, two critical contradictions are gradually exposed: the contradiction between the increasing demands for resources of human society and the limited reserves of fossil resources and the contradiction between the urgency of environmental protection and the serious pollutions partially resulting from the excessive exploitation and use of fossil resources.1–3 To remit or even overcome the abovementioned contradictions, on the one hand, it is essential for human beings to improve the utilizing efficiency of fossil resources, and on the other hand, it is more logical to explore sustainable carbon resources in the long run.2,4–6

Coal, as an important representative of fossil resources, has played a critical role during the development of human society and will still be the main energy and material source in the foreseeable future.2 Although the coal reserve on earth is abundant, it is estimated that ca. 40% of the total coal deposits are low-rank lignite.7 Several obvious disadvantages, including low energy density, high content of volatiles and moisture, and bad stability to air, make it difficult to use lignite as cleanly and efficiently as high-rank coal.7,8 These drawbacks severely limit the direct use of low-rank coal as a primary energy source. On the other hand, lignite possesses different specific properties, including the high content of volatiles and oxygen-functional groups (OFGs), high reactivity, and low pollution-forming impurities such as sulfur, nitrogen, and heavy metals, as compared to high-rank coal; thus, lignite is also a potential carbon resource.8 In the past few decades, extensive processes including drying,9 pyrolysis,10 combustion,11 gasification,12 and liquefaction13 have been developed for the clean and efficient utilization of lignite. Although these processes are commonly applied and efficient for the utilization of lignite, undeniably, they also suffer from several disadvantages, such as intensive energy input, high release of pollutants, and/or loss of the inherent structures and functional groups, to some extent.2,14 Therefore, exploration of more green and value-added approaches to utilize lignite, especially its specific properties, is still an urgent and critical issue during the utilization of low-rank coal.

Depolymerization of lignite and further conversion of the obtained fractions into useful products is a potential approach for the efficient and clean utilization of lignite.15,16 Humic acids (HAs), an important and common class of organic components in lignite, are composed of various aromatic ring-based organic acids and are rich in acidic OFGs. HAs exhibit wide applications in various fields such as in agriculture techniques,17,18 environmental protection,19,20 and material sciences.21 The abundant OFGs in HAs and lignite could also be used to prepare value-added chemicals such as benzene poly(carboxylic acid)s,22–24 small-molecule fatty acids,22,25 and other value-added chemicals.14,25 Therefore, preparation of valuable materials from HAs or even directly from lignite and improvement of the added values of the derived products have become a potential way for the utilization of lignite.

In addition to improving the utilizing efficiency of the coal resources, searching sustainable carbon resources is an inevitable choice for human beings in the long run.26 Biomass is a well-known, green, and sustainable carbon resource on earth. Compared to the fossil resources, biomass possesses several obvious advantages such as being renewable and having large reserves, broad distribution, and abundance in varieties.5,6,27 Useful molecules with relatively high quality could be obtained from biomass raw materials via suitable processes.28–30 Through catalytic processes, these molecules can be converted into various platform compounds, which can be further transformed into various target products.31–36 Among the biomass-derived chemicals, carbonyl compounds such as furfural,37,38 levulinic acid and its esters,39,40 and ketone compounds41 are important platforms because they can be further converted into valuable chemicals. Extensive studies have been reported on how to obtain these platforms such as levulinic acid from biomass, and the reaction mechanisms have also been well-investigated.42,43 Meerwein–Ponndorf–Verley (MPV) reaction is commonly used for the conversion of carbonyl compounds via selective hydrogenation of the carbonyl bonds, leaving the C[double bond, length as m-dash]C double bonds intact.44,45 During the MPV reaction, the C[double bond, length as m-dash]O bonds are chemoselectively reduced with secondary alcohols as hydrogen sources under the catalysis of MPV catalysts.46,47 Zr-Based catalysts including ZrO2,48 zirconium alkoxides,49 Zr-containing zeolites,50 Zr(OH)4,51etc. are commonly applied in the MPV reaction. Recently, construction of efficient Zr catalysts using novel and low-cost raw materials has drawn significant attention. Xue et al.52 designed a novel Zr-based coordination polymer using cyanuric acid, a common chemical block in industry, as the building block. The obtained material was used as a catalyst in the MPV reaction of levulinic acid to gamma-valerolactone (GVL), which was proved to be highly efficient. Song et al.53 and Do-Yong Hong et al.54 constructed Zr-based MPV catalysts using aromatic ring-based acidic compounds, including 4-hydroxybenzoic acid dipotassium salt, benzenedicarboxylic acids, and benzenetricarboxylic acids, as raw materials, and most of the obtained catalysts were proved to be efficient for the MPV reaction of ethyl levulinate into GVL. In our recent study,45 a more abundant and cheap material, natural phytic acid containing six phosphate acid groups, was used to construct new MPV catalysts, and high efficiency was achieved for the MPV reactions of various carbonyl compounds with different structures. These reports indicate that construction of MPV catalysts using the acid groups in chemicals, especially from natural sources, has huge potential for practical applications due to the cost advantages, and it is still highly desirable and meaningful to explore more raw materials to synthesize MPV catalysts in the field of biomass conversion.

In this study, we constructed novel and highly efficient Zr-based MPV catalysts (termed as Zr-HAs hereinafter) using low-rank lignite-derived HAs as the building blocks (Scheme 1). Both commercial HAs and HAs extracted directly from lignite without purification were used as raw materials to prepare the catalysts, and the obtained catalysts were applied in the selective hydrogenation conversion of biomass-derived furfural (FF) into furfuryl alcohol (FA), a key reaction step during the reaction chains of biomass conversion. The structures of the prepared catalysts were characterized in detail. The results showed that Zr-HAs catalysts based on both commercial and extracted HAs showed high efficiency and excellent stability for the MPV reaction of FF into FA. It is believed that the proposed approach to construct MPV catalysts using HAs as the raw materials is beneficial for both the promotion of value-added utilization of low-rank lignite and the conversion of biomass resources.


image file: c7gc01925d-s1.tif
Scheme 1 Schematic of the direct use of HA mixtures from low-rank coal to prepare a Zr-HA catalyst and its application in the conversion of biomass-derived carboxyl compounds via the MPV reaction.

Experimental

Materials

Furfural (FF, 99%), furfuryl alcohol (FA, 98%), ethyl levulinate (EL, 98%), gamma-valerolactone (GVL, 98%), and ZrOCl2·8H2O (AR) were provided by J&K Scientific Ltd. Commercial humic acids were purchased from Aladdin Industrial Corporation. Isopropanol (AR), ethanol (AR), KOH (AR), decane (AR), and other chemicals were obtained from the Beijing Institute of Chemical Reagents. The raw lignite sample was obtained from Shengli coalfield in Inner Mongolia, China. The as-received lignite sample was crushed and ground to fine powder with the average sizes ca. 38–75 μm, followed by drying at 105 °C in air for 4 h.

Catalyst preparation

Preparation of the catalyst Zr-HAsC. Commercial HAs were first used as building blocks to explore the optimal preparation conditions of the catalysts. Typical procedures were as follows: 2 g HAs and 0.34 g KOH were separately dissolved in 80 mL distilled water. The KOH solution was mixed with the HA solution to promote the dispersion of HAs, and the mixtures were continuously stirred for 5 h at room temperature. The pH value of the neutralized HA solution was tuned by HCl to ca. pH = 1; then, the mixed solution was slowly added dropwise to the solution of Zr precursor containing 3.76 g ZrOCl2·8H2O, and the obtained mixture was further stirred for 10 h at room temperature. Finally, a brown slurry was obtained by centrifugation, washed for at least 5 times with distilled water and 2 times with ethanol until no Cl was detected by AgNO3, dried in vacuum at 80 °C for 12 h, and then ground into powders for use (denoted as Zr-HAsC). The ratios of KOH and zirconium precursor to HAs were varied to optimize the preparation conditions of the catalysts.
Preparation of the catalyst Zr-HAsE. HAs extracted directly from raw lignite were also utilized to prepare the catalyst. The extraction process was conducted in a laboratory scale-up batch reactor with an inner volume of 500 mL (Scheme 2). Typically, 2 g lignite powders (with an average particle size of ca. 75 μm) were dispersed in a 350 mL 1 M NaOH solution, and then, the reactor was sealed. The mixture was mechanically agitated at 100 °C for 2 h. Then, the slurry was filtered, and the obtained filtrate was tuned with HCl until pH < 1. HAs were precipitated due to their poor solubility under strongly acidic conditions. The precipitated HAs were obtained by filtration, dried under vacuum at 80 °C, and ground into fine powders for further use. The procedure of the catalysts preparation using the extracted HAs was similar to that of the commercial HAs, as stated above. Herein, 2 g of extracted HAs was first dissolved in a dilute NaOH solution; the HCl solution was added to change the pH to acidic, and then, the HA solution was added dropwise into the Zr salt solution containing 8 g ZrOCl2·8H2O. The mixture was stirred at 80 °C for 5 h. The treatment procedures of the catalysts were the same as those of the commercial HAs. The prepared catalysts were denoted as Zr-HAsE.
image file: c7gc01925d-s2.tif
Scheme 2 Schematic of the batch reactor for the extraction of HAs from lignite.

Catalyst characterization

Scanning electron microscopy (SEM) measurements were performed using a Hitachi S-3400N scanning electron microscope operated at 20 kV with an energy dispersive spectrometer (EDS) apparatus. Transmission electron microscopy (TEM) images were obtained using a TEM JEOL-1011 with an accelerating voltage of 120 kV. Fourier transform-infrared spectra (FTIR) were obtained using a PerkinElmer spectrometer. X-ray diffraction (XRD) was carried out via an XD8 Advance-Bruker AXS X-ray diffractometer using Cu-Kα radiation (λ = 532 nm) and Ni filter scanning at 2° per minute ranging from 3° to 80°. The tube voltage was 40 kV, and the current was 40 mA. The thermogravimetric (TG) analysis of Zr-FA was performed using a thermogravimetric analysis system (Diamond TG/DTA6300, PerkinElmer Instruments) under an Ar atmosphere at the heating rate of 10 °C min−1. The surface area and porosity properties were determined via the nitrogen adsorption–desorption method using a Micromeritics ASAP 2020 V3.04 G system. The XPS measurements were carried out via an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific) at a pressure of ∼3 × 10−9 mbar using Al Kα as the excitation source ( = 1486.6 eV) and operating at 15 kV and 150 W.

Reaction

The activity of the prepared catalysts was detected via the reaction of the catalytic transfer hydrogenation of furfural. The reaction was performed in a 10 mL Teflon lined stainless steel autoclave equipped with a magnetic stirrer. Typically, furfural (1 mmol), isopropanol (5 mL), and 0.2 g catalyst were introduced into the reactor. After sealing, the reaction mixture was stirred and allowed to react at suitable temperatures for a desired time. After the reaction, the reactor was cooled down in cold water to quench the reaction, and the reaction solution was transferred and diluted with isopropanol. The samples were quantitatively analyzed by gas chromatography (TECHCOMP GC7900) with a flame ionization detector using decane as the internal standard. Identification of the products and the reactant was conducted using a GC-MS (SHIMADZU-QP 2010) as well as by comparing the retention times with the respective standards in GC traces.

To check the heterogeneity of the catalysts, the solid catalysts were removed from the reaction mixture after reaction for a short time, and the supernatant was allowed to react to determine if the product yield further increased in the absence of the solid catalysts. In the reusability experiments, the catalyst was separated by centrifugation, washed three times with fresh isopropanol, and then reused for the next run without further treatment.

Results and discussion

Optimization and characterization of the Zr-HAsC catalysts

The commercial HAs (HAsC) were first used to prepare the Zr-HAsC catalysts. The preparation conditions, including the type of alkali and the ratio of the Zr precursor (ZrOCl2·8H2O in this case) to HAsC, were investigated; in the experiments, HAs were found to be hard to directly react with Zr4+ to form the expected Zr-HAsC precipitates at room temperature. This might be because the dispersion of HAsC in water was poor and/or the acidity of carboxylic and phenolic hydroxyl groups in HAs was not strong enough; this led to a weak interaction between the acidic groups and Zr4+, especially at low temperatures. The dispersion of the neutralized HAs improved, and the solution was allowed to react with the Zr4+ precursor. As expected, the Zr-HAsC precipitate was successfully formed, and the solid catalysts were obtained. The amounts of alkali were first varied to study the effects of alkali on the activity of the catalysts (entries 1–4, Table 1). The results showed that the activity of the catalysts first increased and then decreased with an increase in the alkali ratios and reached the highest value at a HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH mole ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1. The lesser the amount of alkali used, the lower the content of active Zr4+ species in as-prepared catalysts, leading to a lower activity. When excessive alkali was used, more or even all the acidic groups in HAsC were neutralized; this resulted in a decrease in the number of acidic sites in the final catalysts. According to our previous studies, the residual acidic sites were favourable for improving the activity.45 Thus, the HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 was optimal to achieve higher activity. Moreover, the ratio of Zr precursor to HAsC is another parameter during the preparation of the catalysts, and its effects on the activity of the catalysts were studied via fixing the ratio of HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH at 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (entries 2, 5 and 6, Table 1). The results showed that increase in the usage of Zr precursor could significantly enhance the activity of the catalysts, with the highest activity at a HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH[thin space (1/6-em)]:[thin space (1/6-em)]Zr ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2. Further increase in the addition of the Zr precursor could not improve the activity of the catalysts. Therefore, the condition of the HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH[thin space (1/6-em)]:[thin space (1/6-em)]Zr mole ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 was chosen to prepare the catalysts in the subsequent studies.
Table 1 Investigation of the preparation conditions of the catalystsa
Entry Mole ratio of HAsC[thin space (1/6-em)]:[thin space (1/6-em)]KOH[thin space (1/6-em)]:[thin space (1/6-em)]Zr

image file: c7gc01925d-u1.tif

Conv./% Yield/% Sel./%
a Reaction conditions: FF 1 mmol, isopropanol 5 mL, Zr-HAsC 200 mg, reaction temperature 50 °C, and reaction time 3 h.
1 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]2 46.6 27.5 59.0
2 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 63.0 51.9 82.4
3 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]2 29.5 25.6 86.8
4 1[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]2 31.1 23.7 76.3
5 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 34.8 24.0 68.9
6 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3 46.4 33.3 71.8


The performance of the prepared catalyst was compared with that of other analogous catalysts focusing on the chemoselective conversion of furfural to FA reported in the literatures. Table S1 shows the results of commonly used processes for the abovementioned conversion, including catalytic transfer hydrogenation (CTH) using secondary alcohols as hydrogen sources and liquid or vapor phase hydrogenation (LPH or VPH) using H2 as hydrogen sources. For the CTH processes (entries 1–9, Table S1), there is no need to use high pressure H2, and the reaction temperatures are often not very high, especially for the Zr-based CTH catalysts. For the LPH process, both noble and noble-free metal catalysts were reported to be efficient for the conversion of furfural. Undoubtedly, the noble catalysts (entries 10–15, Table S1) performed well under much milder conditions (at lower temperatures and under H2 pressure) than the noble-free metal catalysts (entries 16–22, Table S1), with similar level conversions and product yields. Compared with other CTH catalysts, the prepared Zr-HAsC catalysts in this study could provide comparative conversion and yield (both higher than 97%) at lower temperatures (entry 2, Table S1). From the viewpoint of the reaction conditions and the cost of raw materials for catalyst preparation, the Zr-HAsC catalyst had obvious advantages as compared to the noble and other transition metal catalysts (entries 1, 2 and 10–15, Table S1).

To well-understand the structures of the catalysts, detailed characterizations were conducted. SEM showed that the Zr-HAsC catalyst was composed of particles with no uniform shapes (Fig. 1a). EDS shows a strong Zr signal for the catalyst after the interaction of Zr4+ with HAsC (Fig. 1b). Powder XRD of Zr-HAsC was conducted to gain an insight into the bulk phase of the prepared catalyst (Fig. 1c). It was found that the catalyst had no obvious X-ray crystal structure. In the structure of HAs, there are relatively strong carboxylate groups and weaker phenate groups besides the main structures comprising aromatic cores and aliphatic side chains.55,56 Less strong coordination groups resulted in a lower degree of crystallinity of the Zr-HAsC complex. The XRD pattern showed one broad diffraction peak around 27°, indicating that the as-prepared catalyst was amorphous.53 As shown in Fig. 1d, the FTIR spectra of HAsC and the as-prepared catalyst displayed the characteristic asymmetric (HAsC, 1572 cm−1; Zr-HAsC, 1579 cm−1) and symmetric vibrations (HAsC, 1375 cm−1; Zr-HAsC, 1395 cm−1) of the carboxylate groups. Compared with the FTIR spectrum of HAsC, the wavenumber difference of the asymmetric and symmetric vibrations of carboxylate anions in Zr-HAsC was narrowed from 197 cm−1 to 184 cm−1; this indicated that carboxylate groups were coordinated to Zr4+ ions.53,57 N2 adsorption–desorption characterization showed that the catalyst was porous with mesopores centered around 4 nm, and the surface area and pore volume were 123 m2 g−1 and 0.17 cm3 g−1, respectively (Fig. 1e). The local environment of Zr species in Zr-HAsC was detected by the Zr 3d XPS. Compared with ZrO2, the binding energy of Zr in Zr-HAsC moved to a higher level (Fig. 1f). The higher binding energy of Zr 3d indicates a higher positive charge on the Zr atoms, resulting in a stronger Lewis acidity of Zr.58 The higher acidity of Zr species could improve the activity of the Zr-HAsC catalyst.45 TG analysis showed that the prepared catalyst had a low weight loss of 18% at 170 °C resulting from the desorption of water and ethanol adsorbed on the catalyst57 and had excellent stability at reaction temperatures (below 100 °C) (Fig. S1).


image file: c7gc01925d-f1.tif
Fig. 1 Characterization of the as-prepared Zr-HAsC catalyst by SEM-EDS (a and b) (EDS data were obtained from the square area), powder XRD pattern (c), FTIR spectra (d), N2 adsorption–desorption isotherm and pore size distribution (e), and Zr 3d XPS spectra (f).

The effect of the catalyst dosage

The effect of catalyst dosage on the conversion of FF into FA was studied (Fig. 2). It was observed that both the conversion and the product yield significantly increased with an increase in the catalyst dosage in the range of 50–150 mg under experimental conditions. The increasing trend of conversion became slower, and the yield slightly decreased when the catalyst dosage was up to 250 mg. In our experiment, we found that the reaction system exhibited a slurry state, and the catalyst particles had a poor dispersion at larger catalyst dosage, which had a negative effect on mass transportation during the reaction. The slight decrease of FA yield at higher catalyst dosage could also be caused by product adsorption on the catalyst during sample treatment after the reaction due to the large volume of the catalyst. A suitable catalyst dosage of 200 mg in this study was used in subsequent studies.
image file: c7gc01925d-f2.tif
Fig. 2 Effect of the Zr-HAsC dosage on the conversion of FF into FA. Reaction conditions: FA 1 mmol, isopropanol 5 mL, reaction temperature 50 °C, and reaction time 3 h.

The effect of the reaction temperature

The effect of reaction temperature on the performance of Zr-HAsC catalyst was studied, and the results are shown in Fig. 3. Reaction temperature had a dramatic influence on the performance of the catalyst, and the FA yield significantly increased with an increase in temperature, especially before 70 °C. Although further increase in temperatures could promote the conversion of the reactant, the selectivity slightly decreased; this led to no obvious change in the yield. During the experiments, some unknown side-products were observed during GC detection when the temperature was higher than 70 °C. Thus, it was reasonable to conduct the reaction at temperatures not higher than 70 °C. Considering both the reaction rate and selectivity, a medium temperature of 70 °C was chosen as the optimal reaction temperature.
image file: c7gc01925d-f3.tif
Fig. 3 Effect of the temperature on the conversion of FF into FA. Reaction conditions: FF 1 mmol, isopropanol 5 mL, Zr-HAsC 200 mg, and reaction time 3 h.

The effect of the reaction time

To obtain a higher FA yield, the influence of reaction time was investigated (Fig. 4). The results showed that the reaction proceeded rapidly within the first 3 h, and the FA yield increased to 75% at 70 °C (Fig. 4a). The FA yield increased at a slower rate upon further prolonging the reaction time, and the yield reached 80% at 5 h with 91% furfuryl conversion. The conversion could reach up to 98% when the reaction time was prolonged to 9 h, whereas the yield maintained the same level as that at 5 h; this resulted in a slight decrease in selectivity. During GC detection, it was noticed that the intensities of the unknown side-products increased with the prolongation of reaction time. To decrease the formation of side-products, the reaction was proceeded at a lower temperature (50 °C, Fig. 4b). The reaction rate sharply decreased when the reaction temperature was lowered. Interestingly, both the conversion and yield increased with the prolongation of reaction time, different from those obtained at 70 °C. The conversion and yield were ca. 97% with high selectivity of ca. 99% when the reaction time was increased to 15 h. Therefore, high temperatures could obviously increase the reaction rate, whereas lower temperatures were favourable for enhancing the selectivity.
image file: c7gc01925d-f4.tif
Fig. 4 Effect of the reaction time on the conversion of furfuryl into FA at different temperatures: (a) 70 °C and (b) 50 °C. Reaction conditions: Furfuryl 1 mmol, isopropanol 5 mL, and Zr-HA 200 mg.

Reusability and heterogeneity of Zr-HAsC

To check the stability of the Zr-HAsC catalyst during recycling process, reusability and heterogeneity were investigated. In each cycle, the catalyst was recovered by centrifugation, washed with fresh isopropanol (5 mL, 3 times), and then used for the next run. The results showed that there was no considerable decrease in the conversion, yield, and selectivity after nine cycles as compared to those after the first use; this indicated that the catalyst was very stable (Fig. 5a). The leaching of the catalytic species and heterogeneity of the catalyst were also studied (Fig. 5b). The reaction was stopped via removing the catalyst from the reaction mixture after the reaction proceeded for 3 h, and the solution was allowed to react under the same reaction conditions to see if the FA yield further increased without the solid catalyst. It was observed that there was no further increase in the FA yield after the solid catalyst was removed. This result confirmed that the active sites in Zr-HAsC did not leach into the reaction mixture, and it was a heterogeneously catalytic process for Zr-HAsC catalyst. The structures of the Zr-HAsC catalyst recovered after nine cycles were characterized by SEM, powder XRD, and FTIR, and compared with that of the freshly prepared catalysts (Fig. S2). It can be seen that the structures of the catalyst have no obvious change after use. These results further proved that the catalyst had an excellent stability under the studied reaction conditions.
image file: c7gc01925d-f5.tif
Fig. 5 Reusability (a) and heterogeneity (b) of the Zr-HAsC catalyst. Reaction conditions: (a) Furfuryl 1 mmol, isopropanol 5 mL, Zr-HAsC 200 mg, reaction temperature 70 °C, and reaction time 3 h. (b) Reaction temperature 50 °C; other conditions were similar as abovementioned. The solid catalyst was removed after 3 h.

Performances of the Zr-HAsE

To check the universality and feasibility of the proposed approach to synthesize a Zr-based catalyst, HAs extracted from low-rank lignite were directly used as raw materials to prepare the catalyst (Zr-HAsE) without further separation or treatment. The extracted HAs and corresponding Zr-HAsE catalyst were characterized by SEM-EDS, FTIR, and XRD (Fig. S3). The results showed that no obvious inherent metallic mineral residues, except for the trace amount of sulfuric species, were observed in the extracted HAsE by EDS analysis; this may be attributed to the HCl precipitation process during extraction. SEM-EDS and FTIR proved that Zr4+ ions were successfully coordinated with the acidic groups in HAsE. The obtained Zr-HAsE catalyst had no obvious X-ray diffraction peaks; this indicated that the as-prepared catalyst was amorphous.

The performances of the Zr-HAsE catalyst for the conversion of furfuryl are shown in Fig. 6. It could be seen that the FA yield significantly increased with the increase in temperatures before 70 °C, and further increase in the temperatures had no obvious improvement in the yields (Fig. 6a). The yield slightly increased from 87.7% to 92.1% via prolonging the reaction time from 5 h to 9 h at 70 °C (Fig. 6b). Compared with the Zr-HAsC catalyst, Zr-HAsE also had a good stability during recycling, which could be reused for at least 7 times without an obvious change in performance (Fig. 6c). Heterogeneity studies showed that the Zr-HAsE catalyst also catalyzed the reaction via the heterogeneous form (Fig. 6d). The structure comparison of the freshly prepared and recycled catalysts was conducted (Fig. 7). SEM-EDS showed that the amorphous morphology of the catalyst had no obvious change after recycling, and the content (wt%) of Zr in the fresh and recycled catalysts was estimated to be 28.9% and 28.0%, respectively, indicating that there was no significant leaching of Zr species (Fig. 7a–d). FTIR and XRD patterns proved that the microstructures of the recycled catalyst had no obvious change as compared to that of the fresh catalyst, indicating an excellent stability during recycling (Fig. 7e and f).


image file: c7gc01925d-f6.tif
Fig. 6 Performances of the Zr-HAsE catalyst prepared via direct use of HAs extracted from lignite. Typical reaction conditions except otherwise stated in figures: furfuryl 1 mmol, isopropanol 5 mL, Zr-HAsE 200 mg. (a) Influences of reaction temperatures: 5 h. (b) Influences of reaction time: 70 °C. (c) Reusability, 3 h and 70 °C. (d) Heterogeneity, 70 °C.

image file: c7gc01925d-f7.tif
Fig. 7 Comparison of the freshly prepared and recycled Zr-HAsE catalysts after seven reuses. SEM-EDS of the fresh (a and b) and recycled catalyst (c and d), FTIR spectra (e), and XRD patterns (f).

Conversion of different carbonyl compounds

Encouraged by the excellent performance of the prepared catalysts for the conversion of furfuryl, we explored the possibility of the MPV reactions of other carbonyl compounds with different structures using the Zr-HAsC catalyst (Table 2). The results showed that Zr-HAsC showed good activity for the examined compounds as well. Generally, ketones, especially with long aliphatic chains, needed a longer reaction time or harsh conditions to achieve satisfied conversions and yields as compared to aldehydes (Table 2, entries 1–6); this might be caused by steric hindrance due to the larger structures.45 Interestingly, Zr-HAsC exhibited moderate activity for the diketone compounds (2,5-hexanedione in this case, entry 7), giving a conversion of 81.0% and a 2,5-hexanediol (2,5-HDO) yield of 42.8% under not yet optimized conditions, and a yield of 8.9% for 2,5-dimethyltetrahydrofuran (intramolecular dehydration product from 2,5-HDO) was also observed. These results showed that the Zr-HAsC catalyst was efficient for the conversion of different carbonyl compounds with various structures.
Table 2 MPV reaction of carbonyl compounds over the Zr-HAsC catalysta
Entry Reactant Product T (°C) t (h) Conv. (%) Yield (%)
a Reaction conditions: Substrate 1 mmol, isopropanol 5 mL, and Zr-HAsC 200 mg.
1 image file: c7gc01925d-u2.tif image file: c7gc01925d-u3.tif 70 9 98.0 80.0
2 image file: c7gc01925d-u4.tif image file: c7gc01925d-u5.tif 50 15 97.4 96.9
3 image file: c7gc01925d-u6.tif image file: c7gc01925d-u7.tif 100 3 95.5 82.5
4 image file: c7gc01925d-u8.tif image file: c7gc01925d-u9.tif 150 15 >99 85.0
5 image file: c7gc01925d-u10.tif image file: c7gc01925d-u11.tif 100 5 98.1 92.9
6 image file: c7gc01925d-u12.tif image file: c7gc01925d-u13.tif 150 9 92.9 92.8
7 image file: c7gc01925d-u14.tif image file: c7gc01925d-u15.tif 120 5 81.0 42.8
image file: c7gc01925d-u16.tif 8.9


Mechanism analysis

Based on the characterization results and previous reports,45,53 the possible mechanism of the Zr-HAs-catalyzed MPV reaction of carbonyl compounds was proposed (Scheme 3). It was reported that acidic and basic sites in the catalyst were essential for the MPV reaction.45,46 In Zr-containing MPV catalysts, Zr4+ and O2− (carboxylate and phenate groups) formed the acid and basic sites, respectively.59,60 First, isopropanol was adsorbed on the catalyst and interacted with the acid–base sites (Zr4+–O2−) on Zr-HAs; this resulted in its dissociation to the corresponding alkoxide. Moreover, the carbonyl groups in the substrate molecules were adsorbed on the adjacent sites on the catalyst. Then, hydrogen transfer took place between the dissociated alcohol and the activated substrate molecules via a concerted process involving a six-link intermediate to form the corresponding products. Isopropanol was converted into acetone in this process after the loss of two hydrogen atoms.
image file: c7gc01925d-s3.tif
Scheme 3 Possible mechanism for the Zr-HAs-catalyzed MPV reactions of carboxyl compounds.

Conclusion

In summary, a novel strategy to prepare Zr-containing catalysts using low-rank coal-derived humic acids (HAs) as raw materials was proposed. Both commercial HAs and HAs extracted directly from low-rank lignite were used to synthesize the Zr-HAs catalysts. The catalyst obtained from commercial HAs (Zr-HAsC) was proved to be highly efficient for the MPV reactions of various carboxyl compounds with different structures. HAs extracted from lignite could be directly applied in the preparation of the catalyst with no need of further separation or treatment, and the obtained catalyst (Zr-HAsE) was identified to be efficient for the conversion of furfuryl into FA with excellent stability. Use of HAs as raw materials to synthesize catalysts is believed to be an effective approach for both value-added utilization of lignite derivatives and the conversion of biomass resources. The novel catalyst may find potential applications in catalytic fields with the advantages of abundance of HAs, low-cost of Zr metal, and excellent performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21606134, 21676149, 21566029, 21566028, and 21563022), the Natural Science Foundation of Inner Mongolia (2016BS0204), the Science and Research Projects of Inner Mongolia University of Technology (IMUT) (ZD201603), the Incentive Fund for the Scientific and Technology Innovation Program of Inner Mongolia, the Major Basic Research Open Programs of Inner Mongolia, and the Startup Fund for New Teachers of (IMUT).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/C7GC01925D
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2017