Preparation and Studying the Optimum Performance for Both CuO and CeO2 as A Metal Oxide Nanoparticles Catalyst for Synthesis of Glycerol Carbonate from Reaction of Glycerol with Carbon Dioxide Gas

Two important types of metal oxide nanoparticle catalysts Copper (II) oxide (CuO) and Cerium oxide (CeO2) are prepared by a suitable method which was traditional precipitation (PT) method at calcination temperature of 400oC for 5h and used for the synthesis of glycerol carbonate GC (C4H6O4) from the direct reaction by the carbonylation of Glycerol GL (C3H8O3) with Carbone Dioxide. The precipitation (PT) was an important route for the preparation of nanoparticles catalyst. The effects of performance of (CuO and CeO2) nanoparticle catalysts on the conversion of glycerol GL, yield of glycerol carbonate GC, selectivity of glycerol carbonate are researched. XRD, XPS, BET, FT-IR, CO2-TPD, H2-TPR are used for the characterization of the prepared catalysts. Comparing the optimal performance between them under reaction conditions were 150 oC, 4MPa (40 bar.), 5h, and both CuO and CeO2 catalyst amount 37.6 % (based on ratio of glycerol weight) by using 2-pyridinecarbonitrate (C6H4N2) as dehydrating agent and dimethylformamide (DMF), (C3H7NO) as solvent. The glycerol conversion (XGL), glycerol carbonate yield (YGC) and glycerol carbonate selectivity (SGC) over 0.7g CuO are 57.151%, 47.524%, and 83.156%, respectively, and glycerol carbonate yield over 0.7 CeO2 is 36.2185% or 35.076%, and the yield of GC could reach as high as 78.234% over 1.73g CeO2, the both catalysts could be easily regenerated by washing with methanol and water after a reaction and then dried at 60 oC overnight after that calcination at 400◦C for 5h without loss of activity after five recycling times, In addition to, the (ICPMS) results confirmed that the leaching of CuO and CeO2 was below the detection limit.


Introduction
Glycerol carbonate (GC) was a high value-added derivatives but Glycerol (GL) was byproduct of biodiesel manufacture, is available in a great quantity, it is predicted that the global production capacity of biodiesel will reach 50 million tons a year in 2020. Because of rapidly increasing production of global biodiesel in a great quantity, it becomes a research and study focus to transform GL to value-added chemicals. One of the derivatives of GL is the glycerol carbonate (GC), GC has a number of science and industrials applications such as a polar high boiling solvent, chemical intermediates, a surfactant component, carrier in batteries, lubricating oils, monomer for polymers and as components for gas separation membranes. [1][2][3][4][5] GL can be converted to GC by several routes, indirect and direct routes. For the direct route (GL converted to GC by carbonylation with CO2 by using suitable catalyst), Among this method, the most suitable industrial process for producing GC is the carboxylation of GL with CO2 due to the non-toxic raw material, mild operation condition, high selectivity of GC and simple purification of GC. [6][7][8] The first work attempt was carried out by Vieville et al. [9] using GL and CO2 gas under supercritical conditions as reactants in the presence of zeolites and basic ion-exchange resins as catalyst, when adding the co-reactant materials such as ethylene carbonate, could GC be formed.
Even though the yield of GC could reach 32%, there was no evidence about the direct insertion of CO2.Also the metal-impregnated zeolite [10] and Tin complexes [11] were reportedly for the carboxylation of glycerol with CO2, but the conversion of glycerol was not high came to only 2.5% (180 O C, 5 MPa, 6 h) and 5.8% (180 O C, 10 MPa, 3 h), respectively. Thermodynamic calculations showed that the low conversion of GL to GC was because of the number of equilibrium limitations [12], so dehydrate should be used to change the thermodynamic limit.
13X type of zeolite and acetonitrile were employed for this purpose with both Cu/La2O3 [13], Bu2SnO [14] and achieved to a good result. Despite of all these improvements, the conversion of GL is still relatively low and it is a challenge to improve and develop new effective catalytic system.
Currently, the broad and suitable availability of glycerol at low prices, together with the need of new and good economic synthetic routes for chemicals starting from non-petrochemical sources, have created a huge interest in glycerol molecule as a building block, mainly because of the very broad spectrum of its valuable derivatives [7].
Nano metal oxide heterogeneous catalysts are a technologically very important as acid-base and unique redox properties such as (La2O3, CeO2, NiO, CuO and Co3O4 and others). [20] As the heterogeneous catalysts, Nano metal oxide catalysts performance showed excellent in some catalytic reactions, such as CeO2 in reduction of carboxylic acid [21], dehydration of alcohols [22] and in alkylation of aromatic compounds [23], Honda (4)(5) h.
In our present work, we prepared and employed two type of metal oxide nanoparticle (CuO and CeO2) as the catalyst for the synthesis of GC from GL and CO2 in the presence of (2pyridinecarbonitrate) which was used as a dehydration agent to pull water from the middle of the chemical reaction as side product and shift the chemical equilibrium to the GC production side and solvent of CO2 (Dimethylformamide (DMF)). The important objective of this work was to compare the optimal performance among them (CeO2 and CuO) as a best of optimal performance, and develop a new effective catalytic system (carbonylation system) to increase the reaction rate and selectivity of the carbonylation of GL. The stability and activity of the suitable catalysts were studied in detail. From our knowledge, this is the first work of the application of (CuO-PT) prepared (Nano particles metal oxide)-based catalyst for using in the GL carbonylation for GC production.  H2 temperature programmed reduction (H2-TPR) measurements were carried out on the CHEMBET 3000 TPR/TPD instrument. Before the reduction, a sample was preheated in a gas (30 mL/min) at 400 o C for 30 min to remove surface contaminants. After the sample cooled down to 50 o C, a mixture of 5.01% H2/Ar was flowing through the reactor and the temperature was increased from 30 o C to 920 o C. The hydrogen consumption was monitored by a TCD detector.
Inductively coupled plasma mass spectrometry (ICP-MS): Is a type of mass spectrometer capable of detecting minerals and many minerals that are not present at low concentrations such as one part in 10 15 (part of quadrillion, ppq) on non-overlapping low-contrast isotopes. This is achieved by ionizing the sample using a coupled plasma and then using the mass spectrometer to separate and quantify these ions. For most clinical methods using ICP-MS, there is a relatively simple and rapid preparation process for preparation by acid digestion using HNO3/ H2O2 mixture to identify metal oxide in sediments using the (ICP-MS) (Varian company production) acid Digestion.

Reaction procedure.
Glycerol carbonate (GC) was obtained from the carbonylation of glycerol (GL) with CO2 over nanoparticles catalysts. As shown in scheme 1.
Scheme1. Carboxylation of glycerol with CO2 over nanoparticles metal oxide catalysts.
The tests of the catalytic activities of the nanoparticles metal oxide catalysts were carried out in a stainless-steel autoclave reactor system with an inner volume of 200ml and it has thermostat with an electric heating jacket, pressure gauge and agitator, the autoclave reactor was one of the most important chemical engineering equipment and its operation is not easy, it requires attention and caution when operating, because it works under conditions of high temperature and high pressure. After ascertaining the validity of the autoclave system ( fig.1.), the typical procedure is as follows: 40mmol glycerol (GL), 37mmol% Cat./GL, 16 g of Dimethylformamide (DMF) 6 g of 2-pyridinecarbonitrate, were added into the autoclave together, and then the reactor was sealed, purged with N2 or CO2 for 3 times and then pressurized with CO2 to 4 MPa.
Subsequently, the autoclave was heated to the reaction temperature (150 o C) and maintained for certain reaction time (5h) under vigorous stirring. After reaction, the reactor was cooled to room temperature and depressurized, the product mixture was taken out from the autoclave reactor to centrifugal filtration 5000 rpm for 6 min to separation the solid catalyst and liquid products, after that take all liquid product to analyzing. The conversion of GL, XGL, and the yield of GC, YGC, and selectivity of GC, SGC were calculated according to the following equations: Where .

the number of initial moles of GL,
. is the number of moles of GL output (unreacted), . is the number of moles of GC product.

Result and discussion.
The synthesis of GC from GL and CO2 by carbonylation reaction over metal oxide nanoparticles catalyst in the presence of (2-pyridincarbonitrate), which was used as a dehydration agent to pull water from the middle of the chemical reaction as byproduct to produced 2-picolinamide (C6H6N2O) and shift the chemical equilibrium to the GC production side and solvent of CO2 Dimethylformamide (DMF). As shown in mechanism of carboxylation reaction in scheme.2. The conditions of reaction were (150 o C temperature, 5h time, and 4MPa initial pressure of CO2 and 500 rpm of mixing. The Gas chromatography (G.Ch.) analysis of the liquid product mixture out of autoclave reactor is given in Fig.2 it can be found a good peak separation is achieved for all components.

Effect of reaction conditions
The effect of reaction conditions over (CuO-PT-400) nanoparticle catalyst as reaction temperatures, CO2 pressure, time of reaction and amount of catalyst all had investigated in our research and were shown the results in Fig.3 (a-d) and the optimal performances of reaction conditions had shown in Table.2. the optimal performances of reaction conditions had shown in Table.3.

Catalyst characterization.
3.3.1 XRD.  fig.6. It means that the effectiveness of the CeO2-PT-400 was a little and less than CuO-PT-400, which is accordant with the order of the catalytic activity for these catalysts (Table 1, except with CuO-PT-400), meaning that the crystal face (111) for CuO-PT-400 nanoparticles catalyst may be have more active site for the carbonylation of GL with CO2. The predominantly exposed planes were the most stable (111) plane, whereas the CeO2 and CuO nanoparticles catalyst predominantly exposed the well-defined and less stable (200) and (220) planes. Since the energy required to create oxygen vacancies on the plane has strong relevance with their stabilities, the difference of exposed plane might have affection the catalytic performance of CuO-PT-400 and CeO2-PT-400 nanoparticles catalyst.

CO2-TPD
The basicity of CuO nanoparticle catalysts is characterized by CO2-TPD and the profiles are shown in Fig. 7. In the TPD profiles of these samples, the peaks at the temperature range of 50~277 o C, 277~490 o C, and > 490 o C are attributed to desorption of CO2 from weak, medium, and strong basic sites, respectively. By integrating these peak areas shows that except the medium basic sites, the amounts of weak and strong basic sites increase with the increase of calcination temperature and the total amounts of the basic sites as well as. The results mean that the quantitative distribution of different strength basic sites and total amount of desorbed CO2 are dramatically influenced by the calcination temperature.   [25] By integrating these peak areas, the amounts of basic sites can be evaluated and the results are presented in Table 4.
In general, a more basicity is beneficial for the carbonylation of GL with CO2. Compared with CuO-PT-400 and CeO2-PT-400 has higher amount of basic sites, so it also has higher GC yield. It is also observed that among these samples, though CeO2-PT-400 has the highest basicity.

H2-TPR
The H2-TPR was used to determine the redox ability and oxygen vacancy density of CuO-PT-400 nanoparticle catalysts in our present work compared with CeO2-PT-400 nanoparticle catalysts in the J. Liu, et al. 2016 [25]. The H2-TPR profiles of CuO and CeO2 are shown in Fig.   8 and Fig. 9; the data of H2 consumption at 400 o C are listed in the sixth column in Table 3. It is found that all of the samples have only one strong and sharp reduction peak, indicating that there may be a type of CuO and CeO2 species in these samples. Meanwhile, in these H2-TPR profiles, the temperature of H2 consumption maximum is different and it was 277 o C for CuO-PT-400. In these samples, CuO-PT-400 has the lowest reduction temperature, meaning this sample can be easily reduced.
The H2-TPR characterization can be engaged to determine the redox ability and oxygen vacancy density of CeO2. The H2-TPR profiles of CeO2 are depicted in Fig. 9 and the data of H2 consumption below 600•C is listed in Table 3. Two obvious peaks could be observed from the reduction profiles: the low-temperature peak at about 540•C and the high-temperature peak at about 890•C. The peak below 600•C is generally interpreted as the surface shell reduction, including the reduction of the surface Ce from Ce 4+ to Ce 3+ and the formation of bridging OH 1groups, and the peak above 600•C is corresponding to the bulk reduction [25].   [25] These differences may be due to the difference of the particle size, surface area and morphology with various exposed crystal planes. The H2 consumption can be a glancing representative of oxygen vacancy density and decreases with the increase of the calcination temperature (see the sixth column in Table 3), suggesting that CuO-PT-400 may have the highest oxygen storage/release capacity and CeO2-PT-400 has the lowest. In the present work, we have found that the catalytic activity of CuO nanoparticle catalyst is connected to not only its amount of basic sites and surface area, but also the redox ability and oxygen vacancy density. CuO-PT-400 with the best redox ability and a higher oxygen vacancy gives the highest yield of GC. In contrast, CeO2-PT-400 produces the lowest GC yield by using the same amount of catalyst (0.7g) because of the least oxygen vacancy and the weakest redox ability. On the basis of these understanding, it is not unreasonable to predict that the best catalyst for the synthesis of GC from carbonylation of GL with CO2 should have not only high amount of basic sites and surface area, but also high redox ability and oxygen vacancy.   [25]. The O (1s) spectra was composed of two overlapping peaks. The main peak with lower binding energy of 529.1eV roots in the lattice oxygen with Ce 4+ ions [28]. The lower intense peak at 531.2 eV was assigned to different attributions: CO3 2− contamination [29], hydroxyl contamination [30], highly polarized oxygen around the defect site [31] and oxygen vacancies in metal oxides [28].

Nitrogen adsorption-desorption isotherms analysis (BET)
The catalysts showed large BET surface areas: 58.54 m 2 /g and 59.40 m 2 /g for nanoparticles catalyst, (CuO-PT-400) and (CeO2-PT-400), respectively, as listed in Table 5. Compared with the other two type of nanoparticles catalyst, (CuO-PT-400) and (CeO2-PT-400) and in the Jiaxiong Liu, et al. 2016 results [25] showed larger surface area and average pore diameter, which was in favor of the activation and diffusion of the reactants. Typically, metal oxide grains grew into nanoparticles catalyst, the dissolution and recrystallization at the crystal-solution interface under hydrothermal condition, which was beneficial of the homogenous morphology and crystallite size.

Stability of the CuO-PT-400 and CeO2-PT-400 nanoparticle catalyst.
Stability of CuO-PT-400 nanoparticles catalyst is very important to complete all the functions of using the catalyst and one of these functions is recyclability of catalyst several times at least five times, the used catalysts were recovered. The stability of CuO-PT-400 was also researched and the result is shown in Fig. 13. It is found that at the fourth recycling, the activity of CuO-PT-400 hardly decreases and the GL conversion and GC yield can also reach 46.09% and 37.71%, respectively. At the fifth recycling, the GL conversion and GC yield reach 46.10% and 35.86%, respectively, indicating that the activity of CuO-PT-400 slightly decreases. In order to ascertain the reason of the decrease of the catalytic activity for the CuO-PT-400 catalyst, the recovered CuO-PT-400 in the fifth recycling was also characterized by XRD and FT-IR. Fig.14 (a) shows that the crystalline structure of recovered CuO-PT-400 is changed, and it has a strong cubic Cu phase (2θ= 43.5 o , 50.65 o , see PDF 00-001-1242). Fig.15 (a) shows that in the FT-IR spectra of recovered CuO-PT-400, the characteristic peaks attributed to Cu-O stretching mode (at 517 and 598 cm -1 ) are vanished. These results imply that generation of Cu phase is responsible for the deactivation of the CuO-PT-400 catalyst. Interestingly, when the recovered CuO-PT-400 is calcined at 400 o C, its main phase can be converted back into the monoclinic CuO again (Fig. 14 (b), Fig. 15 (b)).   One possible reason might be that the produced amide, such as benzamide, was adsorbed on the CeO2 surface and poisoned the active sites of CeO2. In order to further investigate the used catalysts, XRD and FT-IR characterizations were conducted. XRD profiles were unchanged for the catalysts before and after the reaction with 2-cyanopyridine as dehydrating agent, suggesting that the nanostructure of CeO2 was stable under the reaction conditions. The IR spectra of the used catalysts showed no obvious sign of adsorption of amides with respect to that of the fresh one, but the peak of hydroxyl became noticeably larger.
For the recycle use of CeO2, the regeneration of spent catalysts was performed by calcination at 400 o C for 5 h. The recyclability of CeO2 nanoparticles was verified as shown in Fig.16. The yield of glycerol carbonate and 2-picolinamide over regenerated CeO2 stayed practically the same as that of the fresh one even after recirculation for five times [25]. ICP results confirmed that the leaching of CeO2 was below the detection limit, indicating that the microstructure of the catalyst was stable and the active sites were easily regenerated by a simple calcination procedure [25].

Comparison the Optimal performance for both CuO and CeO2
Comparison the Optimal performance for both CuO-PT-400 and CeO2-PT-400 as a metal oxide nanoparticles Catalyst in the carbonylation the Glycerol with Carbon Dioxide to produce  -TPD Catalyst characterizations for both CuO-PT-400 and CeO2-PT-400 were most   stable 111 face, high basic sites respectively, but the Catalyst characterizations H2-TPR and XPS for CuO-PT-400 were (high redox ability and high O2 vacancy) and (high polarized O2 with impurity C) respectively, and for CeO2-PT-400 were (low redox ability and low O2 vacancy) and (high polarized O2) respectively, also the BET Surface area (m 2 /g) for CuO-PT-400 was 58.54 m 2 /g, and for CeO2-PT-400 was 59.40 m 2 /g. Moreover the recyclability for CuO-PT-400 and CeO2-PT-400 were five times.
Through the above mentioned and according to our knowledge, the optimal performance of CuO-PT-400 and CeO2-PT-400 are both high and higher than metal oxide nanoparticles Catalyst else in the carbonylation the Glycerol with Carbon Dioxide to produce Glycerol carbonate, but when comparing the overall optimum performance of both CuO-PT-400 and CeO2-PT-400 (Table.6,7) were very close and when looking closely, the optimal performance of CuO-PT-400 is somewhat better than CeO2-PT-400 because the catalyst/GL was very suitable less than 20% but over CeO2-PT-400 good yield of GC with high amount of catalyst/GL was more than 188%.

Conclusion.
CuO and CeO2 nanoparticle were synthesized by precipitation method (PT). It showed the best two and excellent catalytic optimal performance among nanoparticles metal oxide catalyst in catalyst not only has higher surface area, but also higher mechanical strength, and is suitable for the industrial reactor. The stability research for CuO and CeO2 nanoparticle shows that the catalyst can be reused five times with little loss of activity and can be easily regenerated by calcination at 400 o C after washing in methanol or ethanol three times. In our future work, the suitable support for this catalyst and mixed this catalyst with another suitable nanoparticles metal oxide catalyst will be investigated and reported in due course and when Comparison the Optimal performance for CuO, CeO2 in this our work and CeO2 from J. Liu, et al. 2016 [25] article work found the Optimal performance were very close in low mass of catalyst less than 1g and the Optimal performance of CuO-PT-400 more than CeO2-PT-400 but at high mass of catalyst more than 1g the Optimal performance of CeO2-PT-400 more than CuO-PT-400. On the other hand, in chemical engineering and chemical industries, the amount of catalyst must be less than the reactants as the mass ratio catalyst/GL = 0.1 -0.3, but in J. Liu, et al. 2016 [25] article the amount of catalyst as the mass ratio CeO2 /GL =1.88 ( cat./GL=188%) this very high, in our work the amount of catalyst as the mass ratio CuO /GL = 0.19 ( cat./GL=19%) ( Table 6, 7).
Economically and industrially: in our work and comparison with results of ref. [25], the optimal performance of CuO-PT-400 more than CeO2-PT-400 because the CuO-PT-400 was an economic, efficient and new catalyst for synthesis of glycerol carbonate from glycerol and Carbon Dioxide.