Acid-Base Bifunctional Hf Nanohybrids-Enabled High Selectivity in Catalytic Conversion of Ethyl Levulinate to γ-Valerolactone

1 State Key Laboratory Breeding Base of Green Pesticide & Agricultural Bioengineering, Key Laboratory of Green Pesticide & Agricultural Bioengineering, Ministry of Education, State-Local Joint Laboratory for Comprehensive Utilization of Biomass, Center for Research & Development of Fine Chemicals, Guizhou University, Guiyang, Guizhou 550025, China; weibo_wu@foxmail.com (W.W.); 15761698544@163.com (Y.L.); hli13@gzu.edu.cn (H.L.); 18275623845@163.com (W.Z.); jhzx.msm@gmail.com (S.Y.)


Introduction
Nowadays, with the continuous development of human society, the demand for energy and fine chemicals is increasing [1].However, the use and gradual depletion of fossil resource needs people to face and solve a series of problems, such as environmental pollution, greenhouse effect, and energy crisis [2].The development of renewable energy is thus highly demanded, where biomass source is the only sustainable carbon feedstock, showing great potential for practical applications [3].Lignocellulose is abundant in nature, which has been demonstrated to be capable of being efficiently converted to a series of high valuable chemicals and biofuels through chemo-catalytic reactions coupled with well-designed processes [4,5].
As one of biomass-derived versatile platform molecules, γ-valerolactone (GVL) can be used as a green solvent for biomass conversion and organic transformations [6].In addition, it is able to be employed for producing biofuels and fuel additives (e.g., 2-methyltetrahydrofuran) [7] and as a key intermediate in the synthesis of fine chemicals (e.g., pentenoic acid, α-methylene-γ-valerolactone (MeGVL)), as shown in Scheme 1 [8,9].Typically, GVL can be prepared from lignocellulose following sequential catalytic pathways involving different reactions such as hydrolysis, isomerization, dehydration, etherification, esterification, hydrogenation and lactonization [10][11][12][13][14][15][16].Amongst these conversion processes, cascade hydrogenation and cyclization are deemed as the key step for catalytic upgrading of levulinic acid and its esters to GVL in recent several years [17][18].Scheme 1. Synthesis and application of GVL Catalytic transfer hydrogenation (CTH) is widely used in the reduction of carbonyl compounds, where H2 gas is replaced by other liquid molecules (e.g., formic acid and alcohols) as hydrogen-donors [19][20][21].It is worth noting that there is a certain requirement for the quality of the used reactor and the catalyst stability to tolerate acidic reaction conditions [22,23].In this regard, alcohols seem to be a better candidate for CTH.
In the past decade, Zr-based catalysts have been reported to be efficient for CTH of levulinic acid and its esters to GVL, and their reactivity is highly dependent on the catalyst compositions.For instance, ZrO2 [27,28], Zr(OH)4 [29], ZrFeOx [30], Al-Zr [31], Zr-B [32], and ZrOCl2 [33] need relatively harsh conditions (ca.200 ºC or > 6 h) to achieve moderate yields of GVL.The stability and catalytic performance can be improved by incorporation of Zr species into solid supports like SBA-15, while it generally involves complicated preparation procedures and relatively high production cost due to the use of template agents [34].Therefore, it is necessary to overcome these shortcomings by improving the catalyst stability with facile preparation method.
Organic phosphonates as ligands combined with metal ions can fast take place under liquid-phase conditions to afford corresponding inorganic-organic metal phosphonates with enhanced chemical and thermal stability [35][36][37][38][39][40][41].Through the coordination of metal ions with organic phosphoric acid, additional micro-and mesopores are introduced into the resulting catalysts, thus greatly rendering increased specific surface areas [42,43], and the catalyst functionalities can be simply tuned by changing the organic ligand or preparation method [44].In the present study, a stable inorganic-organic metal phosphonate catalyst (PPOA-Hf) was prepared from phenylphosphonic acid (PPOA) and hafnium (Hf, in the same group as Zr on the periodic table) chloride by using a simple assembly method.This acid-base bifunctional solid catalyst was able to efficiently promote EL being converted to GVL under mild reaction conditions, and systematic studies were thereby conducted.

Catalytic Activity Measurements
The reactions for conversion of EL to GVL were all carried out in a 10 mL Teflon-lined autoclave heated by an oil bath.In a general procedure, 1 mmol EL (144 mg), 72 mg catalyst and 5 mL 2-propanol were added into the Teflon-lined reactor.The sealed reaction kettle was then put into the oil bath at a prefixed temperature, and the reaction time was accordingly recorded.After reaction, the solid catalyst was removed by centrifugation, and the liquid mixture passed through filter membrane (0.22 μm) prior to GC and GC-MS analysis.

Product Analysis
Liquid products were identified by Agilent 6890N GC-MS with 5973MS mass spectrometer.For quantitative analysis of EL and GVL, an Agilent GC7890B equipped with a HP-5 19091J-413 column (30 m × 0.32 mm × 0.25 mm) and a FID detector was used Internal standard method was adopted for quantitative calculation based on standard curves (R 2 > 0.99) of EL and GVL, and naphthalene was used as the internal standard substance.Substrate conversion (X, %) and product yield (Y, %) were calculated using below equations: After each cycle of reactions, the catalyst was separated by centrifugation from the reaction mixture, successively washed with 10 mL DMF, ethanol and methanol, and then dried at 45 ºC for 12 h.The resulting solid catalyst was directly used for the next run.

Catalyst Characterization
Prior to conducting catalytic reactions, the PPOA-Hf catalyst was characterized by BET, STEM, TEM, TG, XRD and PY-IR.PPOA-Hf-1:1.5 was examined to be mesoporous (average pore diameter 3.4 nm) with surface area of 215 m 3 /g and pore volume of 0.16 cm 3 /g by N2 adsorption-desorption isotherms (Fig. 1A).From PY-IR spectrum in Fig. 1B, it can be seen a peak at 1450 cm -1 , which is indicative of Lewis acid sites in PPOA-Hf.
Moreover, there are small peaks at 1490 and 1520 cm -1 , showing the presence of weak Brønsted acid sites in PPOA-Hf.TG analysis was conducted to study the thermal stability of PPOA-Hf (Fig. 1C).It is interesting to note that less than 10% catalyst weight was lost till 400 ºC and increased weight loss was observed when reaching 550 ºC.This result indicates that the PPOA-Hf catalyst is thermally stable and can be a good candidate for chemical reaction under thermal conditions.XRD analysis shows that PPOA-Hf does not have high crystalline (Fig. 1D), with some wide bands belonging to tetragonal (t) and monoclinic (m) phases, while the peak at 2θ = 6º is possibly resulted from the interlayer clearance of the phosphate [45].TEM images further confirm that PPOA-Hf-1:1.5 (Fig. 2A) is noncrystalline as compared with HfO2 (Fig. 2B), and the detected lamellar structure is consistent with the result (2θ = 6º) clarified by XRD (Fig. 1A).Gratifyingly, the particle size of PPOA-Hf-1:1.5 seems to be smaller than that of HfO2 (Fig. 2), due to its amorphous structure.Further, STEM elemental mappings indicate the even and well-connected combination of organic ligand and Hf (Fig. 3).The base and acid properties of PPOA-Hf-x catalysts in different molar ratios of PPOA/Hf were determined by CO2-TPD and NH3-TPD, respectively.As shown in Fig. 4, it can be clearly observed that PPOA-Hf is an acid-base bifunctional catalyst, and the contents of acid and base sites increase with the increase of Hf relative to PPOA.In addition, the amount of acid-base sites remained nearly unchanged after recycling for five times, indicating that PPOA-Hf catalyst was stable and reusable.The strength of acid and base sites was investigated by XPS analysis, and the results are provided in Fig. 5. Typically, low and high binding energy of O 1s and Hf 4f are indicative of high strength of both base and acid sites [46].It is not difficult to see that the strength of acidic and basic sites in the PPOA-Hf-x catalysts is negatively correlated with the increase of PPOA/Hf ratio (Fig. 5).In this respect, both content and strength of acid-base sites in PPOA-Hf-x can be controlled by adjusting the molar ratio of PPOA/Hf.  is clearly visible in the spark line of HfO2.On the other hand, the peaks at 600 cm -1 (corresponding to aromatic ring) and 700 cm -1 (out-of-plane bending vibration of C-H bond) decrease sharply with the increase of Hf content, implying the tight combination of hafnium and organic ligand.Interestingly, with the combination of PPOA and Hf, the position of the O-H (2400-3200cm -1 ) bond and Ph-H (1900-2300 cm -1 ) shifted to 3200-3700 cm 1 and 3000-3100 cm 1 .In addition, the characteristic peaks of P-O (1000 cm -1 ), C-P (1150-1200 cm -1 ) and C=C (1400-1700 cm -1 ) are all present, indicating the well connection of organic ligand and Hf and the possible origin of acid and base sites (e.g., Brønsted acid sites: -OH; Lewis acid-base sites: -Hf-O-P-).Fig. 6 FT-IR patterns of PPOA-Hf-x with different PPOA/Hf molar ratios

Catalytic Performance of PPOA-Hf-x
The catalytic activity of PPOA-Hf-x with different PPOA/Hf molar ratios in CTH reaction of EL to GVL was investigated at first, and the results are shown in Fig. 7.It is obvious to observe that as the ratio of PPOA/Hf ranges from 2:1 to 1:1.5, the conversion rate of EL and the yield of GVL increase from 72% and 55% to 100% and 85% respectively.This tendency is approximately consistent with the content and strength of acid-base sites (Fig. 4 and Fig. 5B).But further rise of the PPOA/Hf ratio to 1:2 did not increase the yield of GVL, showing the acidity and basicity of PPOA-Hf-1:1.5 is appropriate for GVL synthesis from EL.Therefore, PPOA-Hf-1:1.5 is considered to be the optimal catalyst for subsequent studies.

Effect of Reaction Time and Temperature
Reaction temperature and time are important parameters for reactivity control in the chemical reactions.The effect of reaction temperature (120-180 ºC) on the conversion of EL to GVL over PPOA-Hf-1:1.5 in 6 h was studied, and the results are shown in Fig. 8.
Relatively high yield of GVL (ca.85%) can be obtained in 6 h at 160 ºC and 4 h at 180 ºC respectively.Isopropyl levulinate (IPL) was detected as the dominant byproduct, which reduces the selectivity of GVL.From an economic point of view, the optimal reaction conditions should be 160 ºC and 6 h.In addition, the kinetics of EL-to-GVL conversion was studied by assumption of pseud 1 st order reaction, where four reaction temperatures of 120 ºC (393 K), 140 ºC (413 K), 160 ºC (433 K) and 180 ºC (453 K) were selected.Well-fitted linear curves by plotting -ln(1-X) (X = FUR conversion) with time were obtained for different reaction temperatures, as shown in Fig. 9A.The apparent activation energy (Ea) was calculated to be 53kJ/mol according to the Arrhenius equation, after the obtained reaction rate constant (lnk) being plotted with temperature (1/T; Fig. 9B).This value is much lower than other previously reported catalytic systems, such as Shvo-Ru (69 kJ/mol) [47], Ru tris(m-sulfonatophenyl)phosphine (61 kJ/mol) [48], and Ti-Beta (69 kJ/mol) [49], indicating that PPOA-Hf-1:1.5 is an more effective and promising solid catalyst for catalytic conversion of EL to GVL.

Effect of Catalyst Dosage and Reactivity Comparison with Different Catalysts
In this part, the influence of catalyst dosage was examined (Fig. 10).The absence of catalyst leads to almost no GVL formed, indicating that the reaction requires the participation of the catalyst.When the catalyst dosage increased, the yield of GVL rose accordingly.At catalyst dosage of 72 mg, both GVL yield and EL conversion reach maximum.There is no further increase of GVL yield when more than 72 mg PPOA-Hf-1:1.5 was used, indicating that 72 mg (mass ratio 2:1) is the best catalyst dosage.

Catalyst Leaching Experiments and Recycling Study
Based on the results of TG analysis, the catalyst has good thermal stability.To examine the chemical stability of the solid catalyst (PPOA-Hf-1:1.5),the hot filtration experiment was carried out.In the catalytic conversion of EL to GVL, PPOA-Hf-1:1.5 was filtered out after 3 h as soon as the reaction system cooled to ca. 80 ºC.Then the GVL yield was measured per hour in the next three hours.No significant increase in GVL yield was observed (Fig. 11), and ICP analysis shows that there is almost no Hf leaching (<0.01 ppm Hf detected).This result demonstrates that PPOA-Hf-1:1.5 is a heterogeneous catalyst, and chemically stable in the reaction system.
Furthermore, the reusability of the solid catalyst was investigated (Fig. 12), and the PPOA-Hf catalyst could be reused for five consecutive times without a significant reduction in GVL yield and EL conversion.The IR spectra in Fig. 7  catalyst keeps the intact structure after recycling for five times.In addition, the acid and base activity sites were remained in the reused catalyst, as illustrated by NH3-and CO2-TPD (Fig. 4).These results prove the good reusability of the solid catalyst PPOA-Hf.
Techniques BET (Brunauer-Emmett-Teller) surface areas of the porous materials were determined from nitrogen physisorption measurements at liquid nitrogen temperature on a Micromeritics ASAP 2460 instrument.FT-IR (Fourier transform infrared spectrometer) spectra were measured by Thermo Fisher Nicolet iS50 spectrometer in wavenumber range of 400-4000 cm -1 .The Lewis and Brønsted acid sites of PPOA-Hf was determined by vacuum adsorption surface reaction infrared in situ characterization analysis system (Dalian Institute of Chemical Physics, Chinese Academy of Sciences) with Thermo Fisher Nicolet iS50 AEM in wavenumber range of 1400-1600 cm -1 .Content of Hf and P species in reaction system were determined by inductively coupled plasma-optical emission spectrometer (ICP-OES) on a PerkinElmer Optima 5300 DV.STEM (scanning transmission electron microscopy) and TEM (Transmission electron microscope) test was measured by JEOL 2100 TEM/STEM.Thermogravimetry (TG) analysis was carried out by a NETZSCH STA 449 F3 Jupiter thermal gravimetric analyzer.The acidity and basicity of catalysts were measured by NH3-TPD and CO2-TPD (temperature programmed desorption) using a Micromeritics AutoChem 2920 chemisorption analyzer.

Table 1 .
Activity comparison of different solid catalysts in conversion EL to GVL a a Reaction conditions: EL: 1 mmol, cat: 72 mg, 2-propanol: 5 mL, T= 160 ºC t=6 h b Average rate is defined as (mol of formed GVL) / (catalyst weight ×time)