catalysts

Article One-Pot Synthesis of N,N0-dialkylureas via Carbonylation of Amines with CO2 Applying Y0.08Zr0.92O1.96 Mixed Oxide (YSZ-8) as a Heterogeneous Catalyst

Dalei Sun *, Kaihong Xie, Yanxiong Fang and Xianghua Yang

Department of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; [email protected] (K.X.); [email protected] (Y.F.); [email protected] (X.Y.) * Correspondence: [email protected]; Tel./Fax: +86-20-3932-2237



Received: 17 April 2018; Accepted: 26 April 2018; Published: 2 May 2018


Abstract: One-pot synthesis of N,N0-dialkylureas were successfully achieved from catalytic carbonylation of aliphatic primary amines with CO2 as the carbon source and Y0.08Zr0.92O1.96 mixed oxide (Yttria-stabilized zirconia, YSZ-8) as the heterogeneous catalyst. The yield of the target product was obtained up to 80.60% from a 48 h reaction with an aliphatic primary amine and 3.0 MPa CO2 in N-methyl-2-pirrolidinone at 160 ◦C. A multi-pronged mechanistic study was carried out where factors that might influence the reaction efficiency were studied, including catalyst structure, substrates basicity, CO2 pressure, solvent polarity and reaction time. The presence of oxygen vacancies in YSZ-8 was found to be essential for the carbonylation process by creating additional reduction potential for the activation of CO2 which would lead to the key intermediate species.

0 Keywords: Y0.08Zr0.92O1.96 mixed oxide; N,N -dialkylurea; carbonylation; CO2; heterogeneous catalysis

1. Introduction N,N0-dialkylureas are widely used in the chemical industry, especially in the manufacture of pesticides, herbicides, pharmaceuticals, resin precursors and plant growth regulators [1]. Conventionally, they are prepared from highly toxic reagents depending on methods such as phosgene or isocyanates [2]. In the past few years, a number of safer and more environmentally benign synthetic routes were invented [3], including carbonylation of amines with various carbonyl derivatives such as carbonyldiamide [4], acetoacetanilide [5], carbonate [6], urea [7], carbamate [8] etc.; or with gaseous C1 sources such as CO [9,10] or CO2 [11]. Amongst them, direct carbonylation of amines by CO2 is far more attractive from the standpoint of either environmental consideration or resource utilization [3]. 0 For the preparation of N,N -dialkylureas from catalytic carbonylation of amines with CO2, the following steps are generally involved (Scheme1):

Catalysts 2018, 8, 188; doi:10.3390/catal8050188 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 188 2 of 11 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 11

0 Scheme 1. Reaction steps involved in the synthesis ofof N,,NN′-dialkylureas from aminesamines andand COCO22.

It is well-known that CO2 readily combines with amines at room temperature and ambient It is well-known that CO2 readily combines with amines at room temperature and ambient pressure, forming the corresponding carbamic acids [[12].12]. Thus, the key step in thethe aforementionedaforementioned procedure isis the the equilibrium-limited equilibrium-limited dehydration dehydration of carbamicof carbamic acids acids to ureas/or to ureas/or isocyanates isocyanates [13]. Either [13]. Either harsh conditions◦ (200 °C, and CO2 pressures higher than 10 MPa) or the addition of harsh conditions (200 C, and CO2 pressures higher than 10 MPa) or the addition of stoichiometric stoichiometric amounts of dehydrating agents (such as dicyclohexylcarbodiimide, PCl5, POCl3, and amounts of dehydrating agents (such as dicyclohexylcarbodiimide, PCl5, POCl3, and so on) are usuallyso on) are needed usually to needed shift the to equilibriumshift the equilibriu towardsm towards the desired the productdesired product (Scheme (Scheme1)[ 14,15 1)], [14,15], which wouldwhich would inevitably inevitably lead tolead a seriesto a series of side of side products. products. However, However, the the adoption adoption of of a a properproper catalystcatalyst could be an ideal solution, such as Y2(C2O4)3 [9], K3PO4 [16], 1,5-diazabicyclo[4.3.0]non-5-ene (DBU) could be an ideal solution, such as Y2(C2O4)3 [9], K3PO4 [16], 1,5-diazabicyclo[4.3.0]non-5-ene [17], guanidine [18], Cs2CO3 [19], CsOH/[bmim] OH [20], polymer-immobilized nanogold (DBU) [17], guanidine [18], Cs2CO3 [19], CsOH/[bmim] OH [20], polymer-immobilized nanogold (Au@polymer) [21], KOH/polyethylene glycol [22], TBA2[WO4] (TBA = [(n-C4H9)4N]+) [23], and (Au@polymer) [21], KOH/polyethylene glycol [22], TBA2[WO4] (TBA = [(n-C4H9)4N] )[23], and cesium benzotriazolide [24] [24].. It It is is of of particular interest that the employment of Au@polymerAu@polymer as a heterogeneous catalystcatalyst resultedresulted in in an an appreciable appreciable yield yield of ofN ,NN,0N-dialkylureas.′-dialkylureas. However, However, the the stability stability of Au@polymerof Au@polymer catalyst catalyst is notis not promising, promising, due due to the to relativelythe relatively weak weak chemical chemical bonding bonding between between gold nanoparticlesgold nanoparticles and the and self-instable the self-instable polymer polyme support,r support, particularly particularly under extreme under reaction extreme conditions reaction conditions (5.0 MPa ◦CO2, 180 °C, 20 h，and alkaline reaction environment in the presence of amine (5.0 MPa CO2, 180 C, 20 h, and alkaline reaction environment in the presence of amine as raw materials).as raw materials). Obviously, Obviously, the exploration the exploration of more efficientof more catalysts,efficient catalysts, especially especially heterogeneous heterogeneous catalysts catalysts that can be easily recycled, is still highly desired for the catalytic carbonylation of amines that can be easily recycled, is still highly desired for the catalytic carbonylation of amines by CO2. by COAs2. a ceramic material with good thermal and chemical stability, Yttria-stabilized zirconia (YSZ)As is a also ceramic one material of the most with widely good thermal used heterogeneous and chemical catalysts/catalyststability, Yttria-stabilized supports zirconia in the (YSZ) field ofis also catalytic one of synthesis the most [25 widely]. It was used reported heterogeneous to catalyze catalysts/catalyst reactions such as supports oxidation in of the hydrocarbons field of catalytic [26], synthesis [25]. It was reported to catalyze reactions such as oxidation of hydrocarbons [26], hydrogenation of CO and CO2 [27,28], electrochemically assisted NOX storage-reduction [29], and hydrogenation of CO and CO2 [27,28], electrochemically assisted NOX storage-reduction [29], and so so on. In this study, yttria-stabilized ZrO2 (8 mol % Y, YSZ-8) was employed as a heterogeneous on. In this study, yttria-stabilized0 ZrO2 (8 mol % Y, YSZ-8) was employed as a heterogeneous catalyst for the production of N,N -dialkylureas via carbonylation of amines directly with CO2. Factors influencingcatalyst for thethe reactionproduction efficiency of N, wereN′-dialkylureas studied systematically, via carbonylation where aof possible amines reactiondirectly mechanism with CO2. wasFactors also influencing proposed. the reaction efficiency were studied systematically, where a possible reaction mechanism was also proposed. 2. Experimental 2. Experimental 2.1. Synthesis of the YSZ-8 Catalyst 2.1. Synthesis of the YSZ-8 Catalyst The YSZ-8 catalyst was synthesized using the co-precipitation method at room temperature as describedThe YSZ-8 in the catalyst literature was [30 synthesized]. The typical using procedure the co-p isrecipitation as follows: method firstly, ZrOClat room2· 10Htemperature2O (>98.5% as pure,described Aladdin in the Chemicals) literature and[30]. Y(NO The 3typical)3·6H2 Oprocedure (>99.5% pure,is as follows: Aladdin firstly, Chemicals) ZrOCl were2·10H dissolved2O (>98.5% in de-ionizedpure, Aladdin water Chemicals) and mixed and according Y(NO3)3·6H to a2O molar (>99.5% ratio pure, of Zr/Y Aladdin = 97:3 Chemicals) in a 250 mL were three-neck dissolved flask in equippedde-ionized with water a magneticand mixed stirrer according at room to a temperature molar ratio toof formZr/Y a= precursor97:3 in a 250 solution. mL three-neck Then, the flask pH valueequipped of the with precursor a magnetic solution stirrer was at adjusted room temperat to 8 withure 0.1 to M form NH3 solutiona precursor (25 wt.solution. % in water, Then, Aladdin the pH Chemicals),value of the resulting precursor in asolution cream-white was adjusted suspension. to 8 The with white 0.1 precipitateM NH3 solution was recovered (25 wt. % by in filtration, water, washedAladdin withChemicals), ethanol, resulting dried at 100in a◦ cream-whiteC overnight, suspension. and calcinated The in white vacuum precipitate at 600 ◦ Cwas for recovered 4 h. by filtration,As a washed comparison, with ethanol, pure Y2 Odried3 and at ZrO100 2°Ccatalysts overnight, were and prepared calcinated following in vacuum the at same 600 °C procedure for 4 h. as was usedAs a forcomparison, YSZ-8, without pure Y zirconium2O3 and ZrO or2yttrium catalysts additives, were prepared respectively. following the same procedure as was used for YSZ-8, without zirconium or yttrium additives, respectively.

Catalysts 2018, 8, 188 3 of 11

2.2. Characterization of Catalyst X-ray diffraction (XRD) spectroscopy was performed on a Brucker D8 Advance diffractometer (Bruckstr., Beijing, China) with Cu Kα1 radiation. Scans were made over a range of 2θ = 6–80◦, at a rate of 1◦/min. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific/k-Alpha (Thermo Fisher Scientific, Shanghai, China) with Al Kα radiation (E = 1486.6 eV). A C1s binding energy (BE) of 284.6 eV was used as a reference.

2.3. Carbonylation of Amine with CO2

The carbonylation of amine with CO2 (>99.9% pure, Jun Duo Gas Co., Guangzhou, China.) was performed using a 50 mL Teflon-lined stainless-steel autoclave equipped with a magnetic stirrer. In a typical procedure, 5 × 10−2 mol of amine, 30 mL of solvent and 4.5 × 10−4 mol of catalyst were firstly charged into the autoclave. Then, a flow of nitrogen was introduced to drive out the air contained in the autoclave, and a mixed gas of CO2/N2 (2/1 in mole ratio) was introduced into the reactor at room temperature to meet a total pressure of 2.5 MPa, and then the gas inlet was shut off. After that, the temperature was slowly increased to 160 ◦C (caution: this would usually elevate the pressure to ca. 6.5 to 7.0 MPa), where the reaction mixture was remained for 6–48 h with stirring. When the reaction completed, the reactor was opened at room temperature, where the catalyst can be recovered simply by filtration, washed with deionized water and calcined under vacuum at 600 ◦C for 4 h. All these products were qualitatively and quantitatively analyzed over a QP2010 plus GC-MS instrument (Shimadzu, Shanghai, China) equipped with a RXiTM-5MS capillary column (30 m × 0.25 mm). Both the conversion of amines and yields of various products were determined by comparing to an external standard and calculated following the equation Wsp = Wst·Asp/Ast × 100%, where sp and st refer respectively to specimen and standard. The reaction selectivity was calculated via area normalization method, based on the converted amine. As for the isolation of N,N0-dialkylurea, 100 mL of 1.0 mol % sodium carbonate aqueous solution was added to the previous filtrate, precipitating N,N0-dialkylurea as a white solid, which was then collected by filtration and dried at 80 ◦C for 24–48 h, and the filtrate was distillated to remove water for recovering the organic solvent. The solid was identified to be N,N0-dialkylurea via mass and IR spectroscopies. Based on the weight of solid, the isolated yield of N,N0-dialkylurea was obtained. All the chemicals used above for the synthesis of N,N0-dialkylureas were purchased from Aladdin Chemical Co. (Shanghai, China), and dried prior to use to remove water.

3. Results and Discussion

3.1. Structural Characterization of the Catalysts

The XRD pattern of the YSZ-8 (Figure1a) is totally consistent with that of the Y 0.08Zr0.92O1.96 mixed oxide (JCPDS No. 48-0224); no other crystalline nor amorphous phase was detected. It can be concluded that the YSZ-8 (Figure1a) catalyst prepared in this work possesses exactly the structure of Y0.08Zr0.92O1.96 mixed oxide. Nevertheless, the position of a few diffraction peaks at 2-theta angles of 30.2◦, 55.3◦, and 58.6◦ for YSZ-8 show a slight shift to high angle compared to the standard diffraction XRD card of Y0.08Zr0.92O1.96 (JCPDS No. 48-0224). This could be ascribed to a homogeneous insertion of yttrium to the zirconia structure, which could result in sub-lattice zirconium defects in YSZ-8. When comparing the XRD pattern of pure Y2O3 catalyst (Figure1c) with that of YSZ-8 (Figure1a), no distinguishable peak could be indexed to Y2O3, further demonstrating the incorporation of Y into the zirconia framework. Meanwhile, the XRD intensity of YSZ-8 increased dramatically comparing to that of pure ZrO2 (Figure1b), indicating that the insertion of yttrium to the zirconia would result in a more perfect crystalline form. CatalystsCatalysts 20182018,, 88,, 188x FOR PEER REVIEW 4 of 11 Catalysts 2018, 8, x FOR PEER REVIEW 4 of 11

◦ Figure 1.1. X-rayX-ray diffraction diffraction patterns patterns of of catalysts catalysts after after thermal thermal treatment treatment at at 600 °C.C. ( a)) 8 mol % Figure 1. X-ray diffraction patterns of catalysts after thermal treatment at 600 °C. (a) 8 mol % yttria-stabilizedyttria-stabilized zirconiazirconia (YSZ-8);(YSZ-8); ((bb)) purepure ZrOZrO22;(; (cc)) purepure YY22O3.. yttria-stabilized zirconia (YSZ-8); (b) pure ZrO2; (c) pure Y2O3.

The chemicalchemical states of Y, Zr,Zr, andand OO atomsatoms inin YSZ-8YSZ-8 werewere investigatedinvestigated byby XPS.XPS. AsAs shownshown inin FigureFigure2 2,, thethe XPSXPS spectra spectra ofof Zr3d, Zr3d, Y3dY3d andand O1s O1s in in YSZ-8 YSZ-8 powder powder areare consistent consistent withwith thethe reported reported characteristicscharacteristics of yttriumyttrium andand zirconiumzirconium atomsatoms inin yttria-stabilizedyttria-stabilized zirconiazirconia [[3131–−3535].]. TheThe elementalelemental ratioratio ofof Y/ZrY/Zr onon thethe surface surface of of YSZ-8 YSZ-8 obtained obtained by by XPS XPS is 0.09:1,is 0.09:1, which which is in is good in good agreement agreement with XRDwith XRD analysis. The BE of Zr3d5/2 in YSZ-8 was found to be 181.9 eV (Figure 2a)—slightly lower than analysis.XRD analysis. The BE The of BE Zr3d of5/2 Zr3din5/2 YSZ-8 in YSZ-8 was foundwas found to be to 181.9 be 181.9 eV (Figure eV (Figure2a)—slightly 2a)—slightly lower lower than pure than pure ZrO2 (182.5 eV [32])—while that of Y3d5/2 was 156.9 eV (Figure 2b)—slightly higher than pure ZrOpure2 ZrO(182.52 (182.5 eV [32 eV])—while [32])—while that of that Y3d of5/2 Y3dwas5/2 156.9was 156.9 eV (Figure eV (Figure2b)—slightly 2b)—slightly higher higher than pure than Y pure2O3 Y2O3 (156.2 eV [33]). The significant chemical shift observed on both Zr3d and Y3d core level peaks (156.2Y2O3 (156.2 eV [33 eV]). [33]). The significant The significant chemical chemical shift observed shift observed on both on Zr3d both and Zr3d Y3d and core Y3d level core peaks level mightpeaks probablymight probably be due be to due the insertionto the insertion of yttrium of yttrium into the into zirconia the zirconia framework, framework, leading leading to highly to mobilehighly oxygenmobile vacanciesoxygen vacancies in the catalyst, in the ascatalyst, observed as fromobserved the XRD from pattern the XRD of thepattern YSZ-8 of catalyst. the YSZ-8 Afterwards, catalyst. electronsAfterwards, from electrons yttrium from could yttrium be further could transferred be furthe tor thesetransferred vacancies, to these where vacancies, a similar where charge atransfer similar fromcharge the transfer vacancies from to zirconiumthe vacancies could to also zirconium happen, co ultimatelyuld also happen, leading to ultimately peaks of Zr3d leading shifting to peaks toward of lowerZr3d shifting BE and toward those of lower Y3d toward BE and higher those of BE. Y3d toward higher BE.

Figure 2. Zr3d (a), Y3d (b) and O1s (c)) ofof thethe synthesizedsynthesized YSZ-8YSZ-8 catalyst.catalyst. Figure 2. Zr3d (a), Y3d (b) and O1s (c) of the synthesized YSZ-8 catalyst.

The former hypothesis hypothesis is is further further supported supported by by the the O1s O1s spectra spectra (Figure (Figure 2c),2c), which which revealed revealed the I I II II theexistence existence of three of three oxygen oxygen ions: lattice ions: latticeoxygen oxygen OI (BE = O 529.4(BE eV), = 529.4 chemisorbed eV), chemisorbed oxygen OIIoxygen (BE = 531.1 O III III (eV),BE = and 531.1 oxygen eV), and related oxygen to related vacancy to vacancysites OIII sites (BE O = (BE532.4 = 532.4eV). This eV). Thisperfectly perfectly matched matched that that of ofyttria-stabilized yttria-stabilized zirconia zirconia as as reported reported in in the the literature literature [31,34,35], [31,34,35], except except that that the the relative relative intensities of thethe threethree oxygen oxygen components components in thein synthesizedthe synthesi YSZ-8zed YSZ-8 changed changed comparing comparing with the with YSZ-8 the reported YSZ-8 inreportedRef. 31 in. More Ref. 31 specifically,. More specifically, the ratio ofthe their ratio relative of their intensities relative intensities for the synthesized for the synthesized and reported and I II III I II III III III YSZ-8reported are YSZ-8 O :O :Oare O=I:O 56:24:20II:OIII = and 56:24:20 74:20:6, and respectively, 74:20:6, respectively, displaying displaying an increase an in theincrease O peak in the of theOIII III III synthesizedpeak of the YSZ-8. synthesized The increase YSZ-8. of theThe relative increa intensityse of the of relative the O peakintensity in the of synthesized the OIII peak YSZ-8 in could the synthesized YSZ-8 could be resulted from an increase of oxygen vacancy sites and/or an increase of the sub-lattice zirconium defects sites.

Catalysts 2018, 8, 188 5 of 11 be resulted from an increase of oxygen vacancy sites and/or an increase of the sub-lattice zirconium defects sites. Catalysts 2018, 8, x FOR PEER REVIEW 5 of 11 CatalystsCatalysts 2018 2018, 8, x, 8FOR, x FOR PEER PEER REVIEW REVIEW 5 of 511 of 11 3.2. Carbonylation of 1-Butanamine by CO2 3.2. Carbonylation of 1-Butanamine by CO2 3.2. Carbonylation of 1-Butanamine by CO2 3.2.Various Carbonylation catalysts of and 1-Butanamine solvents were by CO tested2 for the carbonylation of 1-butanamine (1a) by CO2 and Various catalysts and solvents were tested for the carbonylation of 1-butanamine (1a) by CO2 the resultsVarious were catalysts listed inand Table solvents1. In thewere absence tested offor a the catalyst carbonylation (entry 1), theof 1-butanamine conversion is (1a) only by 4.17% CO2 and the Variousresults were catalysts listed and in Tablesolvents 1. Inwere the testedabsenc fore of the a catalystcarbonylation (entry of1) , 1-butanaminethe conversion (1a) is onlyby CO 2 and the selectivity in N,N0-dibutylurea (2a) is 94.03% after 24 h. Unsurprisingly, the employment of 4.17%andand the and theresults resultsthe wereselectivity were listed listed inin TableinN ,TableN′ -dibutylurea1. In1. theIn theabsenc absenc(2a)e ofise aof94.03% catalyst a catalyst after (entry (entry 24 1) ,h. 1)the ,Unsurprisingly, theconversion conversion is only isthe only a catalyst resulted in a large increase in conversion (entries 2~11) and, in most cases, an increase in employment4.17%4.17% and and the of thea selectivitycatalyst selectivity resulted in inN, NinN′-dibutylurea ,aN large′-dibutylurea increase (2a) in(2a) isconversion 94.03%is 94.03% after(entries after 24 2~11)24h. h.Unsurprisingly, and, Unsurprisingly, in most cases, the the selectivity of 2a. anemployment increaseemployment in of selectivity a of catalyst a catalyst of resulted 2a. resulted in a in large a large increase increase in conversion in conversion (entries (entries 2~11) 2~11) and, and, in most in most cases, cases, an increasean increase in selectivity in selectivity of 2a. of 2a. Table 1. Results for carbonylation of 1-butanamine (1a) to the corresponding N,N0-dibutylurea (2a) Table 1. Resultsa for carbonylation of 1-butanamine (1a) to the corresponding N,N′-dibutylurea (2a) withTable CO 1.2 Results. for carbonylation of 1-butanamine (1a) to the corresponding N,N′-dibutylurea (2a) withTable CO2 a.1. Results for carbonylation of 1-butanamine (1a) to the corresponding N,N′-dibutylurea (2a) 2 a withwith CO CO. 2 a.

Sel c. (%)c Selc . (%) Sel . (%)c d b MainSel by-Products . (%) Yields Entry Catalyst Solvent Con . (%) d Main by-Products Yields d b b 2a Main by-Products Yields(%)Yields EntryEntryEntry CatalystCatalystCatalyst SolventSolvent Solvent ConCon . (%) b.. (%) (%) Main by-Products d 2a 2a (%)(%) (%)

1 [11] - NMP 4.17 94.03 - 5.97 3.92

1 [11]2 1 [11] Y2-O 3 - NMPNMP 44.254.174.17 96.1494.0394.03 - - 3.865.97 5.97 42.54 3.92 3.92 1 [11] - NMP 4.17 94.03 - 5.97 3.92 2 Y2O3 NMP 44.25 96.14 - 3.86 42.54 3 22 ZrO YY2 O2O3 NMPNMPNMP 35.79 44.25 95.2596.14 - - - 4.753.86 3.86 42.54 34.09 42.54 2 3 43 3 YSZ-8ZrOZrO 2 NMPNMP 64.32 35.79 35.79 90.1395.2595.25 7.96 - - 2.01 4.75 4.75 57.97 34.09 34.09 3 ZrO2 NMP 35.79 95.25 - 4.75 34.09 4e YSZ-8 NMP 64.32 90.13 7.96 2.01 57.97 5 44 YSZ-8 YSZ-8YSZ-8 NMP NMPNMP 72.32 64.32 90.7890.13 7.63 7.96 7.96 1.592.01 2.01 57.97 65.65 57.97 e 65 e,f5 5e e YSZ-8YSZ-8YSZ-8YSZ-8 NMP NMP NMP NMP 83.95 72.32 72.32 91.01 90.78 90.78 7.74 7.63 7.63 7.63 1.251.59 1.59 65.65 76.40 65.65 65.65 e,f 67 6g e6, fe,f YSZ-8YSZ-8YSZ-8YSZ-8 NMP NMP NMP NMP 63.92 83.95 83.95 89.68 91.01 91.01 7.80 7.74 7.74 7.74 2.521.25 1.25 76.40 57.32 76.40 76.40 g 78 7 7g g YSZ-8YSZ-8YSZ-8 NMPDMI NMP NMP 60.17 63.92 63.92 89.59 89.68 89.68 8.43 7.80 7.80 7.80 1.982.52 2.52 57.3253.91 57.32 57.32 98 88 YSZ-8 YSZ-8YSZ-8 DMADMI DMIDMI 50.0260.17 60.17 90.4189.5989.59 7.388.43 8.43 8.43 2.211.98 1.98 53.9145.22 53.91 53.91

109 99 YSZ-8 YSZ-8YSZ-8 DEDMDMA DMADMA 35.1950.02 50.02 90.3390.4190.41 7.527.38 7.38 7.38 2.152.21 2.21 45.2231.7945.2245.22

1110 1010 YSZ-8 YSZ-8YSZ-8 DEDM BuOH DEDMDEDM 35.19 8.95 35.19 69.7190.33 90.33 12.09 7.52 7.52 7.52 0.56 2.15+ 17.64 2.15 h 31.7931.796.2431.79 h 1111 YSZ-8 YSZ-8 BuOH BuOH 8.95 8.95 69.71 69.71 12.09 12.09 0.56 0.56 ++ 17.64 h 6.246.24 a Unless11 otherwiseYSZ-8 mentioned, BuOH all reactions 8.95 were carried 69.71 out under 12.09 conditions 0.56 as + follows: 17.64 5 × 106.24−2 aa −2 −2 UnlessUnlessa otherwiseotherwise mentioned, mentioned, all all reactions reactions were−3 were carried carried out under out under conditions conditions as follows: as follows: 5 × 10 5 mol× 10 of −2 mol Unlessof amine, otherwise 30 mL ofmentioned, solvent,−3 2.5 all ×reactions 10 mol wereof catalyst, carried 2.5 out MPa under total conditions pressure asof follows:a mixed 5gas × 10 amine, 30 mL of solvent, 2.5 × 10 mol of catalyst,−3 2.5 MPa total pressure of a mixed gas CO2/N2 (2/1 in mol of amine,◦ 30 mL◦ of solvent, 2.5 × 10 mol−3 of catalyst, 2.5 MPa total pressure of a mixed gas mole)CO2mol/N at2 25of(2/1 amine,C, in 160 mole) 30C, mL 24at h; 25ofb ,csolvent,°C,Conversion 160 2.5°C, × of24 10 amineh; molb,c andConversion of selectivitycatalyst, of 2.5 to aminevarious MPa andto products,tal selectivitypressure obtained of to a various viamixed area gas b,c CO2/N2 (2/1 in mole) at 25 °C, 160 °C, 24 h;d Conversionb,c of aminee and selectivity to various normalizationproducts,CO2/N 2obtained (2/1 method in basedmole)via area on at GC 25normalizatio determined °C, 160 data;°C,n 24methodGC h;yield Conversionbased (isolated on yield); GC of determinedamine0.5 g 4 A and zeolite selectivitydata; as the d dehydratingGC to yield various f g d agentproducts, was added;obtainedReaction via area time normalizatio = 48 h; Reactionn method was conducted based on by GC using determined the recovered data; catalyst GC afterd yield the (isolatedproducts, yield); obtained e 0.5 g 4via A areazeolite normalizatio as the dehydratingn method agent basedh was on added; GC determined f Reaction timedata; = 48GC h; yieldg reaction in entry 4.e This catalyst was reused for the tenth time; Selectivity to n-butylf isocyanate was 0.56%g andReaction(isolated(isolated the rest wasyield); number yield); conducted 0.5 fore 0.5g other4 bygA 4 byproducts,usingzeoliteA zeolite the as recoveredtheas including thedehydrating dehydrating cataly mainly stagentN after-butyl agent wasthe carbamate, was reactionadded; added; butyl inReaction entryf Reaction butylate 4. time This andtime = catalyst dibutyl48 = h;48 h; g 0 carbonate.wasReactionReaction reused was NMP: for was conducted theN conducted-methyl-2-pirrolidinone; tenth time;by usingby h Selectivityusing the therecovered DMI: recoveredto n-butyl dimethylimidazolidinone; cataly catalyisocyanatest afterst after the was thereaction 0.56% reaction DMA: andinN , entryNin the-dimethylacetamide; entry rest4. This 4.number This catalyst catalyst for DEDM: 2-methoxyethyl ether. h otherwaswas reused byproducts, reused for forthe including thetenth tenth time; mainlytime;Selectivity h Selectivity N-butyl to carbamate, n-butyl to n-butyl isocyanate butyl isocyanate butylate was was 0.56% and 0.56% dibutyl and and the carbonate. therest rest number number NMP: for for Nother-methyl-2-pirrolidinone;other byproducts, byproducts, including including DMI: mainly mainlydimethylimidazolidinone; N-butyl N-butyl carbamate, carbamate, butyl DMA:butyl butylate butylate N,N’ and-dimethylacetamide; and dibutyl dibutyl carbonate. carbonate. DEDM: NMP: NMP: The conversion of 1a increased most when catalyzed by YSZ-8, where conversions above 64.32% 2-methoxyethylN-methyl-2-pirrolidinone;N-methyl-2-pirrolidinone; ether. DMI: DMI: dimethylimidazolidinone; dimethylimidazolidinone; DMA: DMA: N,N’ N-dimethylacetamide;,N’-dimethylacetamide; DEDM: DEDM: (entries2-methoxyethyl 4–7)2-methoxyethyl and selectivity ether. ether. up to 90.13% were achieved. It can be easily seen that both the conversion of 1aThe and conversion the yield of of 2a 1a for increased the catalysts most involved when catalyzed in this work by YSZ-8, have an where order ofconversions YSZ-8 > Yabove2O3 > ZrO64.32%2The. Undoubtedly,(entriesThe conversion conversion 4−7) and YSZ-8of 1aofselectivity 1aincreased possesses increased up most theto most90.13% best when catalyticwhen were catalyzed catalyzed achiev activity ed.by amongstbyItYSZ-8, canYSZ-8, be where alleasily where these conversionsseen testedconversions that catalysts.both above theabove 64.32% (entries 4−7) and selectivity up to 90.13% were achieved. It can be easily seen that both the Basedconversion64.32% on the (entriesof structural1a and 4− 7)the and characterizations yield selectivity of 2a for upthe basedto catalysts 90.13% on XRD involvedwere and achiev XPSin thised. patterns Itwork can havebe of easily YSZ-8 an order seen in Section ofthat YSZ-8 both 3.1 > , the conversion of 1a and the yield of 2a for the catalysts involved in this work have an order of YSZ-8 > suchY2Oconversion3 an> ZrO activity2. Undoubtedly,of can 1a and be ascribed the yieldYSZ-8 to of the possesses2a interfacialfor the catalyststhe structure best involvedcatalytic of YSZ-8 inactivity this catalyst work amongst which have allan would orderthese be oftested more YSZ-8 > Y2O3 > ZrO2. Undoubtedly, YSZ-8 possesses the best catalytic activity amongst all these tested favorablecatalysts.Y2O3 > forBased ZrO adsorption2 . onUndoubtedly, the andstructural activation YSZ-8 characteriza of possesses CO2 duetions tothe anbased best increase oncatalytic XRD in oxygen andactivity XPS vacancies amongst patterns with allof the YSZ-8these yttrium testedin insertionSectioncatalysts.catalysts. 3.1, intoBased suchBased zirconia. on an onthe activity This thestructural canstructural can be befurthercharacteriza ascribedcharacteriza validated totions thetions from basedinterfacial based the on literature: onXRD structure XRD and as and wasXPS of XPS YSZ-8 pointedpatterns patterns catalyst out of byYSZ-8of Ge,whichYSZ-8 dein in Section 3.1, such an activity can be ascribed to the interfacial structure of YSZ-8 catalyst which LeitenburgwouldSection be andmore3.1, co-workerssuch favorable an activity [36for], COadsorption can2 preferred be ascribed and to adsorbactivation to the on interfacial negatively of CO2 duestructure charged to an defectsof increase YSZ-8 in the catalystin latticeoxygen which of would be more favorable for adsorption and activation of CO2 due to an increase in oxygen catalystvacancieswould through withbe more the a bent yttrium favorable configuration, insertion for adsorption into resulting zirconia. inand the This activation can be further ofof theCO adsorbed 2validated due to COan from 2;increase while, the literature: based in oxygen on vacancies with the yttrium insertion into zirconia. This can be further validated from the literature: aas comparison vacancieswas pointed with of YSZ-8,out the byyttrium Y 2Ge,O3, anddeinsertion Leitenburg ZrO2 intoupon zirconia. COand2 adsorptionco-workers This can bybe[36], Pennerfurther CO2 andvalidated preferred co-workers from to adsorbthe [37 ],literature: their on as was pointed out by Ge, de Leitenburg and co-workers [36], CO2 preferred to adsorb on COnegatively2asadsorptive was chargedpointed capacity defectsout displayedby in Ge, the delattice an Leitenburg order of ofcatalyst YSZ-8 and >through Yco-workers2O3 > a ZrO bent 2[36],, andconfiguration, theCO2 relative preferred resulting fraction to adsorb ofin CO the2 on negatively charged defects in the lattice of catalyst through a bent configuration, resulting in the adsorptionactivationnegatively of on the thecharged adsorbed oxygen defects vacancies CO 2;in while, the sites lattice based with of on YSZ-8 catalyst a comparison is much through higher of a YSZ-8, bent than configuration, withY2O3 the, and others. ZrO resulting2 upon CO in2 the activation of the adsorbed CO2; while, based on a comparison of YSZ-8, Y2O3, and ZrO2 upon CO2 adsorptionactivation by of Penner the adsorbed and co-workers CO2; while, [37], based their on CO a comparison2 adsorptive of capacity YSZ-8, Ydisplayed2O3, and ZrOan order2 upon of CO 2 adsorption by Penner and co-workers [37], their CO2 adsorptive capacity displayed an order of YSZ-8adsorption > Y2O3 > byZrO Penner2, and theand relative co-workers fraction [37], of theirCO2 adsorptionCO2 adsorptive on the capacity oxygen vacanciesdisplayed sites an orderwith of YSZ-8 > Y2O3 > ZrO2, and the relative fraction of CO2 adsorption on the oxygen vacancies sites with YSZ-8YSZ-8 is much > Y2O higher3 > ZrO than2, and with the the relative others. fraction of CO2 adsorption on the oxygen vacancies sites with YSZ-8YSZ-8 is much is much higher higher than than with with the theothers. others.

Catalysts 2018, 8, 188 6 of 11

Notably, when a recovered YSZ-8 catalyst was employed under the same conditions of entry 4, almost the same result as that of entry 4 was obtained (entry 7); clearly stating the reusability of the 0 YSZ-8 catalyst in the carbonylation of amines by CO2 to the corresponding N,N -dialkylureas. Meanwhile, the employment of a dehydrating agent (4A zeolite, entry 5), or prolonging of the reaction time (from 24 h to 48 h, entry 6), based on the YSZ-8 catalyst, led to the formation 2a, especially in the latter case, and a selectivity as high as 91.01% along with 76.40% GC yield was achieved (entry 6). Solvents were also found to affect the results obviously (cf. entries 2, 8–11): the conversion of 1a in a protic solvent such as BuOH was practically low (entry 11), while higher conversions were acquired in aprotic polar solvents including N-methyl-2-pirrolidinone (NMP), 1,3-dimethylimidazolidinone (DMI), N,N0-dimethylacetamide (DMA), and 2-methoxyethyl ether (DEDM), where the conversion increased with the polarity increase of the aprotic solvent. Two reasons can be ascribed here: Firstly, the basicity of amines would be reduced in protic solvents with respect to aprotic ones [38]. As a consequence, the carbonylation process tends to be retarded in BuOH but not in aprotic solvents such as NMP, DMI, DMA, and DEDM. Secondly, the stimulation of the conversion by aprotic solvents displayed an order of NMP > DMI > DMA > DEDM, coinciding with the order of their polarity increase [39]. This result could be ascribed to the stronger inductive effect on CO2 with aprotic solvents of higher polarity. For instance, NMP is an excellent choice for such a combined physical and chemical trapping of CO2, as it not only absorbs CO2, but also stabilizes the ionic intermediates of the chemisorptions [40]. What is also notable is that N,N0-dibutyloxamide (3a) was identified as one of the main byproducts 0 in the synthesis of N,N -dialkylurea only when catalyzed by YSZ-8 rather than by ZrO2 and/or Y2O3. This closely correlated to the presence of oxygen vacancy sites and/or sub-lattice zirconium defects sites in YSZ-8, which does not exist in ZrO2 and/or Y2O3 catalysts. In fact, it is reported that the presence of oxygen vacancies in the Rh/CeO2 and CeO2-promoted Rh/SiO2 catalysts could also lead to the reduction of CO2 by creating additional reduction potential, on the basis of adsorption and 0 activation of CO2 to surface carbonaceous species [41]. Also reported is that N,N -dialkyloxamides can be prepared from direct oxidative double-carbonylation of amines with CO and O2 catalyzed either by homogeneous Pd or Ni complexes or by heterogeneous supported gold nanoparticles (Au NPs) [9]. Furthermore, it must be noted that butyl butylate formed from oxidation of n-butanol was also detected from the reaction of 1-butanamine and CO2 catalyzed by YSZ-8 in n-butanol, which further evidenced that CO2 could be partially dissociated into CO and oxygen in the presence of YSZ-8 catalyst. Summarizing, these results clearly revealed that the couple of YSZ-8 catalyst/NMP solvent possessed a unique and significantly superior activity for the carbonylation of amines to the corresponding ureas with CO2. Under the optimized conditions (Table1, entry 5: with YSZ-8 as the catalyst, NMP as the solvent and 4A zeolite as the dehydrating agent), the substrate scope of amines was investigated (Table2). Aliphatic primary amines were converted to the corresponding N,N0-dialkylureas with moderate to high yields (entries 1–7). Specifically, for linear amines, the selectivity of 2 slightly changed with the increasing of C number or chain length of the attached alkyl, while the total conversion of the substrate amine increased rapidly. This may be due to that, on the one hand, starting amines with longer chains possess higher basicity which are more favored by the reaction, as in good agree with the reactivity difference between aprotic solvents and protic ones; on the other hand, the high steric hindrance had a positive effect on the formation of the carbamoyl species from which 2 was generated via reductive elimination. This was further confirmed by a highly hindered branched amine, iso-butylamine 1d, with an 81.38% conversion of 1d and a 96.89% GC selectivity of 2d at 160 ◦C in 24 h (entry 6). This works perfectly with cyclic aliphatic primary amines such as cyclohexylamine 1e, N,N0-dicyclohexylureas (2e), which was obtained with excellent yield (72.11% isolated) and quite high conversion (up to 85.89%) under the same condition (entry 7). Besides the more favored high steric hindrance, the particularly high reactivity of 1e could be ascribed to the greater nucleophilicity and basicity of the N atom compared with the rest amines including 1a/b/c/d/f. Catalysts 2018 8 Catalysts 2018,, 8,, 188x FOR PEER REVIEW 7 of 11 Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 CatalystsTable 2018 2., 8Results, x FOR PEER for carbonylation REVIEW of various amines (1) to the corresponding N,N0-dialkylureas (2)7 of 11 Table 2. Results for carbonylation of various amines (1) to the corresponding N,N’-dialkylureas (2) Catalystswith 2018 CO, 8, ax. FOR PEER REVIEW 7 of 11 Table 2. Results2 a for carbonylation of various amines (1) to the corresponding N,N’-dialkylureas (2) CatalystsTablewith 2018 2. CO Results, 82, x. FOR for PEER carbonylation REVIEW of various amines (1) to the corresponding N,N’-dialkylureas (2) 7 of 11 Table 2.a Results for carbonylation of various amines (1) to the corresponding N,N’-dialkylureas (2) Catalystswith CO 20182 a,. 8, x FOR PEER REVIEW 7 of 11 Tablewith CO 2. 2Results a. for carbonylation of various amines (1) to the corresponding N,N’-dialkylureas (2) Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 Tablewith CO 2. 2Results a. for carbonylation of various amines (1) to the corresponding N,N’-dialkylureas (2) CatalystswithTable 2018 CO 2., 8 2,Results ax. FOR PEER for carbonylation REVIEW of various amines (1) to the corresponding N,N’-dialkylureas (2)7 of 11 CatalystswithTable 2018 CO ,2. 82, Resultsax. FOR PEER for carbonylationREVIEW of various amines (1) to the corresponding N ,N’-dialkylureas (27) of 11 Catalysts 2018, 8, xa FOR PEER REVIEW b 7 cof 11 Tablewith CO 2. 2Results . for carbonylation of various amines (1) to SeltheSel corresponding. (%)b. (%) N,N’ -dialkylureasYields of (2 2) b cc a b Sel . (%) Yields of 2 c Tablewith CO 2. 2Results . for carbonylation of variousCon amines. (1) to the correspondingb By-Products N ,N’-dialkylureas(%) (2) c Entry 1 Time (h) b Sel . (%) Yields of 2 Con b. By-ProductsBy-Products (%) Tablewith CO 2. 2Results a. for carbonylation of variousCon(%) . amines (21 ) to the correspondingbBy-Products N,N’-dialkylureas(%) (2) c EntryEntry 11 TimeTime (h) (h) Con Conb. (%) b. Sel . (%)By-Products Yields(%) of 2 Table 2. Resultsa for carbonylation of various(%) amines22 (1) to the bcorresponding N,N’-dialkylureas (2) c Entrywith CO2 . 1 Time (h) b Sel . (%) Yields (%) of 2 Con(%) . 2 By-Products Entrywith1 CO2 a. 1 Time12 (h) Con50.12 b . 94.10 Sel b. 2.81(%)By-Products 3.09 Yields 47.16(%) of 2 c Entry 1 1a Time (h) (%) 2 1 12 50.12Con b. 94.10 Sel2.81 b. (%) By-Products 3.09 Yields 47.16(%) of 2 c 1 2 1a 12 24 50.1272.32(%) 94.1090.782 2.817.63 3.09 1.59 47.16 65.65 Entry1 1 1a Time12 (h) 50.12b 94.10 b 2.81 3.09 47.16 c 2 1 1224 72.32 50.12Con(%) . 90.78 94.102 Sel 2.817.63 . (%) By-Products 3.091.59 Yields 65.6547.16(%) of 2 2Entry d 1 1a Time24 (h) 72.32 90.78 7.63 1.59 65.65 31 1a 1a 2412 80.2350.12b 95.8494.10 b 2.892.81 1.273.09 76.8947.16 c 2 1a 24 Con72.32(%) . 90.782 Sel . 7.63(%)By-Products 1.59 Yields 65.65(%) of 2 3Entry d 1 1 1a 1a Time24 12 (h) 80.2350.12 95.8494.10 2.892.81 1.27 3.09 76.89 47.16 42 d 1a 1b 1a 24 71.2372.32b 94.8790.78 b 3.057.63 2.081.59 67.5865.65 c 31 2 1a 242412 72.32Con80.2350.12(%) . 90.7895.8494.102 Sel 7.63 . 2.892.81(%)By-Products 1.59 1.273.09 Yields 65.65 76.8947.16(%) of 2 4Entry 2 1 1b 1a Time24 24 (h) 71.2372.32 94.8790.78 3.05b 7.63 2.08 1.59 67.58 65.65 c 4 35 d 1a 1b 1a1c 24 24 71.23Con82.7480.23(%) b . 94.8796.2795.842 Sel3.05 . 2.412.89(%) By-Products 2.08 1.321.27 Yields 67.58 79.6576.89(%) of 2 Entry421 1 1b1a1a Time2412 (h) 71.2372.3250.12 94.8790.7894.10 3.057.632.81 2.081.59 3.09 67.5865.65 47.16 5 3 d 1c 1a 24 24 82.74Con80.23 b . 96.2795.84 2.412.89 By-Products 1.32 1.27 79.65 76.89(%) 641 3 d 1c 1d 1b1a 242412 80.2381.3871.2350.12(%) 95.8496.8994.8794.102 2.892.033.052.81 1.27 1.082.083.09 76.89 78.8567.5847.16 Entry52d 1 1a1c Time2424 (h) 82.7472.32 96.2790.78 2.417.63 1.32 1.59 79.65 65.65 6 34 1a 1a 24 24 81.3871.2380.23(%) 96.8994.8795.842 2.033.052.89 1.08 2.081.27 78.85 67.5876.89 6 15 1d 1b 1a1c 24 1224 81.3850.1282.74 96.8994.1096.27 2.032.812.41 1.08 3.091.32 78.85 47.1679.65 7362 d 1d1a 2424 85.8981.3872.3280.23 93.8496.8990.7895.84 2.152.037.632.89 4.011.081.59 1.27 80.6078.8565.65 76.89 154 1e 1b1a 1224 50.1282.7471.23 94.1096.2794.87 2.812.413.05 3.091.322.08 47.1679.6567.58 7 2 d 4 1a1c 2424 24 85.89 71.2372.32 93.84 94.8790.78 3.052.157.63 2.084.01 1.59 80.6067.58 65.65 7 36 1b 1e 1d1a 24 24 85.8981.3880.23 93.8496.8995.84 2.152.032.89 4.01 1.081.27 80.60 78.8576.89 7154 1e 1a1b1c 241224 85.8950.1282.7471.23 93.8494.1096.2794.87 2.152.812.413.05 4.013.091.32 2.08 80.6047.1679.65 67.58 826d 1e1d 24 72.3281.383.07 90.7896.89- 7.632.03 - 1.591.08 65.6578.85 - 34 1f1b1a 24 80.2371.23 95.8494.87 2.893.05 1.272.08 76.8967.58 8 7265 1e 1c 24 2424 3.0785.8972.3281.3882.74 -93.8490.7896.89 96.27 - 2.157.632.032.41 4.011.591.08 1.32 - 80.6065.6578.85 79.65 d 5 1f 1d1a 24 82.74 96.27 2.41 1.32 79.65 394785 1c 1b 1a1c 24 80.2371.2385.8982.743.07- 95.8494.8793.8496.27- 2.893.052.152.41- 1.272.084.011.32 76.8967.5880.6079.65 - 6d 1e1f1g1d 24 81.38 96.89 2.03 1.08 78.85 9 3487 1b1a 24 24 -71.2380.2385.89 3.07 -94.8795.8493.84 - - 3.052.892.15 - 2.081.274.01 - 67.5876.8980.60 - 9 a5 1g 1e1f 1c 24 24 -82.74 -96.27 - 2.41 1.32 - 79.65 896Unless 6 otherwise noted, 1d all24 reactions24 were 81.3881.383.07 carried- 96.89out96.89- under the 2.03 2.03following - conditions: 1.08 1.08 YSZ-8 78.85 78.85 (0.313 - a 47 1f1b1g 2424 71.2385.89 94.8793.84 3.052.15 2.08 4.01 67.58 80.60 a Unless569 otherwise1d noted, 1d1e 1c all reactions24 were carried82.7481.38- out 96.2796.89under- the following2.412.03- conditions: 1.321.08 YSZ-8 (0.313 79.6578.85 - g),a8 4 A zeolite (0.500 1g g), amine 24(0.05 mol), 3.07NMP (30 mL),- total pressure - of a 2/1 mixture of CO - 2/N2 57Unless otherwise noted, 1e1f 1c all reactions24 were82.7485.89 carried out96.2793.84 under the 2.412.15following conditions: 1.324.01 YSZ-8 79.6580.60 (0.313 g), 6948 A zeolite (0.500 g),1d amine b (0.052424 mol), NMP81.383.07- (30 mL),96.89- - total pressure2.03- - of a 2/1 1.08mixture of CO 78.852/N - - 2 (2.5a7 Unless MPa otherwise at 25 °C), noted, 1601g1f °C; all reactionsDetermined24 were by85.89 carried GC using out93.84 anunder extern theal2.15 following standard conditions: method 4.01 and YSZ-8 calculated 80.60 (0.313 g),69 74 A zeolite (0.500 1e g), amineb 24 24(0.05 mol), 85.8981.38 NMP- (30 93.84 96.89mL),- total 2.15 pressure2.03- of a 4.01 2/1 1.08 mixture of 80.60 CO 78.85 - 2/N 2 (2.5usinga8 UnlessMPa the at otherwise 25area °C), normalization 160 noted, 1d 1g°C; ball Determined reactions method;24 cwere byGC GC 3.07yield carried using based out an - onunder extern 1; d Thetheal standard followingreaction - method was conditions: carried and outcalculatedYSZ-8 under (0.313 - 30 g),(2.57 4 MPa A zeolite at 251e (0.500°C), 1e1601f g), °C; amine b Determined24 (0.05 mol), by85.89 NMP GC using(30 93.84mL), an totalextern pressureal2.15 standard of a method2/1 4.01 mixture and calculatedof 80.60CO2/N 2 a 9 1g 24 c - - d - - using87Unless the area otherwise normalization noted, all method; reactions24 2GC were2 yield85.893.07 carried based out 93.84on- under1; The the reaction 2.15following - was conditions: carried 4.01 out YSZ-8 under 80.60 (0.313 -30 atmg),(2.5 4 (atMPa A 25zeolite at °C) 25 of (0.500°C), a 2/1 1601e1f g),mixture °C; amine b Determined of (0.05 CO /Nmol),c . by NMP GC using(30 mL), an totalexternd pressureal standard of a method2/1 mixture and ofcalculated CO2/N2 usinga9 the area normalization 1g method;24 GC yield- based -on 1; The reaction- was carried out under - 30 atmg),8 Unless(at 4 25A zeolite°C) otherwise of a (0.500 2/1 noted,mixture g), amine all bof reactions CO24 (0.05 2/N 2mol),. were3.07 NMP carried (30 out mL),- under total the pressure following - of a conditions: 2/1 mixture YSZ-8 of CO (0.313 - 2/N2 atm(2.5using (at8 MPa 25 the °C) atarea of25 a normalization°C), 2/1 mixture1601f °C; 24 of Determinedmethod; CO /N . c GC 3.07 by yield GC basedusing - onan 1extern; d The -al reaction standard was method carried and out calculated under - 30 atma98 (at 25 °C) of a 2/1 1g mixture of24 CO 2/N2. 3.07- - - - (2.5g),Unless 4MPa A zeoliteotherwise at 251f °C),(0.500 noted, 1601f g), °C; amineall b reactionsDetermined (0.05 mol),cwere by NMPcarried GC using(30 out mL), anunder externtotald the pressureal following standard of conditions: amethod 2/1 mixture and YSZ-8 calculatedof CO (0.3132/N2 Consistentlyusing the area withnormalization the results method; reported2 GC2 yieldin the based literature on 1; [19],The reaction aromatic was primary carried out amines under such 30 as aatm9 (at 25 °C) of a 2/1 mixture of24 CO /N . - - - - Consistentlyg),(2.5Unless 4 MPaA zeoliteotherwise at with25 (0.500°C), thenoted, 1601g g),results °C; amineall breactions Determined reported (0.05 mol), werec in by NMPcarriedthe GC literature (30using out mL), underan total extern[19],d the pressure aromaticalfollowing standard of primaryaconditions: method2/1 mixture aminesand YSZ-8 ofcalculated CO (0.313such2/N2 as anilineConsistentlyatmusing9 1f (at the showed25 area°C) with of normalization a very the2/1 mixtureresults low method; reactivityofreported24 CO 2/N GC2 . intowards yieldthe- literaturebased the -on carbonylation 1 [19],; The aromatic reaction- reaction, wasprimary carried withamines out underonly such - 303.07% as aConsistently with 1gthe results reported in the literature [19], aromatic primary amines such as Unless9 otherwise noted, all24b reactions werec - carried out - underd the - following conditions: YSZ-8 - (0.3132 2 aniline g),atm(2.5using1f 4 MPa(atshowedA the zeolite25 at area°C) 25 of (0.500verynormalization°C), a 2/1 160 g),lowmixture °C; amine reactivity Determined method;of (0.05 CO2 /Nmol), towards2GC. by NMPyield GC using(30basedthe mL), carbonylation anon totalextern1; The pressureal reactionstandard reaction, of wasa method2/1 carried mixture with and out onlyofcalculated under CO 3.07%/N 30 conversionConsistentlya and 1ga with negligible the results GC reportedyield of in2f, the where literature quiazoline [19], aromatic and 1-isocyanatobenzene primary amines such were as anilineUnless 1f showed otherwise very noted, low allb reactionsreactivity werec towards carried outthe under carbonylationd the following reaction, conditions: with YSZ-8 only (0.313 3.07% conversiong),(2.5using 4 MPaA andthe zeolite at areaa 25 negligible normalization(0.500°C), 160 g), °C; amine GC Determinedmethod; yield(0.05 2mol), ofGC2 by2f, NMPyield GCwhere basedusing(30 mL),quiazoline anon totalextern1; The pressureal reactionstandardand 1-isocyanatobenzeneof wasa method2/1 carried mixture and out calculatedof under CO 2/Nwere 302 conversiondetectedaConsistentlyatm as(atand the25 °C)a main negligible withof a 2/1by-products the mixture resultsGC ofyield reported(entryCO /Nof .8). 2f, in This wherethe couldliterature quiazoline be rooted [19], andaromatic in the1-isocyanatobenzene weak primary nucleophilicity amines suchwere and as anilineconversiong),Unless 41f A otherwiseshowedzeoliteand a (0.500 noted,negligiblevery g), all lowamine reactions b GCreactivity (0.05 wereyield mol), carriedc oftowards NMP2f, out underwhere (30 themL), the quiazoline following carbonylationtotald pressure conditions: and of reaction, 1-isocyanatobenzenea YSZ-8 2/1 mixture (0.313 with g), of 4 AonlyCO zeolite2/N 3.07%2were detected(2.5usingConsistentlyatm as MPa(at thethe 25 atarea main°C) 25 of normalizationwith°C), by-productsa 2/1 160 the mixture °C; results Determinedmethod; of(entry COreported2/N 8).GC2. byThis yieldin GC the could basedusing literature beonan 1externrooted; The[19],al reaction in standardaromatic the weak was method primary carried nucleophilicity and out amines ◦calculated under such ◦ 30and as detectedbasicityaniline(0.500 as 1fof g),the anilinesshowed amine main (0.05 by-products[39].very mol), Nevertheless,low NMP (30reactivity(entry mL), total8).N -dibutylamine towardsThis pressure could of the a 2/1be carbonylation1g, mixturerooted as a ofin secondary CO the2/N weakreaction,2 (2.5 MPaprimarynucleophilicity atwith 25 C),amine, only 160 C;and3.07%could conversion and a negligible b GC yieldc of 2f, where quiazolined and 1-isocyanatobenzene were detectedbusing(2.5 MPaas the the atarea 25main normalization°C), by-products160 °C; Determinedmethod; (entry2 GC2 8).by yield ThisGC basedusing could onan be1extern; rooted Theal reaction standard in the was weakmethod carried nucleophilicity and out calculated underc 30 and basicityconversionanilineatm ConsistentlyDeterminedof 1f(atanilines showed25and °C) by aof [39]. GC with negligibleavery using2/1 Nevertheless,the mixture anlow results external GC reactivityof CO standardreportedyield N/N-dibutylamine . of methodtowards in2f, thewhere and literaturethe calculated 1g, quiazolinecarbonylation as using[19],a secondary thearomatic and area reaction, normalization1-isocyanatobenzene primary primary with method;amine,amines only could GC such 3.07%were as hardlydetectedyieldusing be basedas theconverted the area on main1; normalizationd The toby-products reaction the corresponding was method; carried (entry c out GC 8). under yield 1,3-di-This 30 based atmcouldN,N (at on-dibutyl-2-urea 25be 1◦; C)drooted The of a reaction 2/1 in mixture the via was weak ofcarbonylation carried CO nucleophilicity/N out. under with 30 COand2 basicityanilineatmConsistently of(at1f 25anilinesshowed °C) ofwith a [39]. very2/1 the mixture Nevertheless, lowresults ofreactivity COreported2/N N2. -dibutylaminetowards in the literature the 1g,carbonylation [19],as a aromaticsecondary reaction, primary primary2 with2 amines amine, only such could3.07% as hardly(entrydetectedconversion be 9). convertedas Apparently, theand maina tonegligible by-productsthe such corresponding a GCstriking (entryyield differenc of8).1,3-di- 2f,This whereN ecould, Nin-dibutyl-2-urea reactivity quiazolinebe rooted between inand via the 1-isocyanatobenzenecarbonylation weakprimary nucleophilicity and with secondary CO were and2 basicityhardlyConsistentlyatm be of(at converted anilines25 °C) ofwith a[39]. 2/1to the mixturetheNevertheless, results corresponding of COreported2/N N2. -dibutylamine in1,3-di- the literatureN,N-dibutyl-2-urea 1g, [19],as a aromaticsecondary via carbonylationprimary primary amines amine, with such could CO as2 (entrydetectedanilineconversion 9). 1fApparently, as showedtheand main a negligible very suchby-products lowa striking GCreactivity (entryyield differenc 8).oftowards This2f,e where incould thereactivity quiazolinebecarbonylation rooted between inand the reaction,primary 1-isocyanatobenzeneweak nucleophilicity withand secondaryonly 3.07% wereand aminesbasicityhardly becannotof convertedanilines be simply [39]. to theNevertheless, attributed corresponding to Nbasicity,-dibutylamine 1,3-di- nucleophilicy,N,N-dibutyl-2-urea 1g, as aor secondary steric via hindrance carbonylation primary between amine, with them.could CO2 aniline(entryconversiondetectedConsistently 9).1f as Apparently, showed andthe main a with negligiblevery by-products thesuch low results a GC reactivitystriking reportedyield(entry differenc of8).towards in 2f,This the wheree literaturecould inthe reactivity quiazoline carbonylationbe rooted [[19],19], aromaticbetween inand the reaction,1-isocyanatobenzene weak primary nucleophilicitywith aminesand only secondary such 3.07%were and as aminesStill,hardlybasicity itcannot isbe of notable convertedanilines be simply that [39]. to N attributed, theNNevertheless,′-dialkylureas corresponding to basicity, N were-dibutylamine 1,3-di- nucleophilicy,obtainedN,N-dibutyl-2-urea as1g, the as or maina steric secondary byproduct viahindrance carbonylation primary in between the amine, synthesis with them. could CO of2 anilineconversion(entryaminesdetectedConsistently 1f9).cannot1f as showed Apparently, showed andthe be maina very with simply negligiblevery by-productslow thesuch low attributed reactivityresults a GC reactivitystriking reported(entryyield towardsto basicity,differenc of8).towards in 2f,This the the where carbonylationnucleophilicy, e could literatureinthe reactivity quiazoline carbonylationbe rooted [19], reaction, or betweenaromatic instericand the reaction, with1-isocyanatobenzene hindranceweak primary only nucleophilicity with 3.07% aminesandbetween only conversionsecondary such 3.07%them.were and as Still,(entryhardlybasicity it is 9).notablebe of Apparently,converted anilines that N[39]. to, Nsuch ′the -dialkylureasNevertheless, correspondinga striking were Ndifferenc-dibutylamine 1,3-di-obtainede Nin,N asreactivity-dibutyl-2-urea the1g, mainas a between secondarybyproduct via primarycarbonylation primaryin the andsynthesis amine, secondarywith could ofCO 2 isocyanatesconversionaminesStill,aniline it iscannot1f notable showedandfrom be a monoamides that simply negligiblevery N,N attributedlow′-dialkylureas GC ofreactivity oxalicyield to basicity, acidwereoftowards 2f, with obtainedwhere nucleophilicy, peroxydisulfatethe quiazoline carbonylationas the mainor stericand [42]. byproduct reaction,1-isocyanatobenzene Inhindrance fact, insmallwith the between synthesisamountsonly 3.07%them.were of isocyanatesanddetected(entrybasicityhardly a negligible 9). be of as from Apparently, convertedanilinesthe monoamides GCmain yield[39]. by-productsto such the ofNevertheless, 2f, correspondingofa where strikingoxalic (entry quiazoline acid Ndifferenc 8).-dibutylamine with1,3-di-This and peroxydisulfateecould Nin 1-isocyanatobenzene,N reactivity-dibutyl-2-urea be1g, rooted as a betweensecondary[42]. in the Invia wereweakfact, primarycarbonylation primary detectedsmallnucleophilicity and amountsamine, assecondary with the could main ofandCO 2 isocyanatesiscocyanatesaminesStill,conversion it iscannot fromnotable and were bemonoamides a that alsosimplynegligible Ndetected,N attributed′-dialkylureas ofGC byoxalic yieldGC-MS to acid basicity, wereof in with2f, the obtained where nucleophilicy,peroxydisulfatereaction quiazolineas mixtures the mainor [42]. stericandof byproduct primaryIn 1-isocyanatobenzene hindrancefact, aminessmall in the between amounts withsynthesis CO them. wereof2, inof by-productsdetectedbasicityisocyanates ofas anilinesthefrom (entry main monoamides 8).[39]. by-products This Nevertheless, could of be oxalic(entry rooted N acid-dibutylamine8). in This thewith weakcould peroxydisulfate nucleophilicitybe1g, rooted as a secondary in [42]. the and Inweak basicityfact,primary nucleophilicity small of amine,anilines amounts could [and39 of].2 iscocyanatesamineshardly(entry 9).becannot wereconvertedApparently, bealso simply detected to suchthe attributed corresponding bya striking GC-MS to basicity, indifferenc the1,3-di- reaction nucleophilicy,eN in,N -dibutyl-2-ureareactivity mixtures or ofbetween steric primary via hindrance carbonylationprimary amines betweenwithand secondaryCOwith 2them., inCO iscocyanatesespecialStill,isocyanatesdetected it is at asnotable wereearly thefrom mainalsostages. monoamidesthat detected by-productsN Therefore,,N′-dialkylureas by of GC-MS oxalicit(entry is clear inwereacid8). the thatThis with obtainedreaction the could peroxydisulfate reactivity mixturesbeas therooted differencemain of in primary[42]. byproductthe Inweakbetween fact,amines nucleophilicityin small thethe with two synthesisamounts CO types2, inand ofof Nevertheless,basicityhardlyiscocyanates(entryamines 9). becannotof Apparently,convertedanilines wereN -dibutylaminebe also simply[39]. to detected such theNevertheless, attributed correspondinga 1g, strikingby as GC-MS ato secondary Nbasicity,differenc-dibutylamine in1,3-di- the enucleophilicy,reaction primary Nin,N -dibutyl-2-ureareactivity 1g, mixtures amine, as a or betweensecondary could stericof primaryvia hardly hindrancecarbonylationprimary primary amines be converted andbetween amine, with secondarywith CO tocouldthem. CO2 the, in2 especialaminesisocyanatesStill, it at is dependsearly notable from stages. monoamidesonthat Therefore,their N,N ′-dialkylureasability of it oxalicisto clear generate wereacid that withtheobtained an reactivity peroxydisulfateisocyanate, as the difference mainwhich [42]. byproduct betweenis In formed fact, thein small thein two situ synthesisamounts types via ofthe of hardlyiscocyanatesespecialbasicity be atof convertedearlyanilines were stages. also [39]. to detected Therefore, theNevertheless, corresponding by itGC-MS is clearN-dibutylamine in 1,3-di-that the thereactionN, Nreactivity-dibutyl-2-urea 1g, mixtures as differencea secondary of primaryvia betweencarbonylation primary amines the amine, twowith with types CO could CO2, ofin2 aminescorresponding(entryisocyanatesaminesStill, it depends9). iscannot notableApparently, from 1,3-di-beon monoamides thatsimplytheirN ,NsuchN ,Nability-dibutyl-2-urea attributed′-dialkylureas a strikingof to oxalic generate to basicity, differenc acid viawere carbonylation anwith obtained nucleophilicy,eisocyanate, inperoxydisulfate reactivity as withthe which main or CObetween steric [42]. 2 isbyproduct(entry formedhindranceIn primary fact, 9). Apparently, insmall theandbetweensitu synthesisamounts secondaryvia such them.the ofof a aminesdehydrolysisiscocyanates depends wereof on carbamic alsotheir detected ability acids byto originated GC-MSgenerate in theanfrom reactionisocyanate, CO2 andmixtures whichprimary of is primary formedamines amines in(Scheme situ with via 1).CO the Not2, in striking(entryaminesespecialhardly 9). be cannotdifferencedependsat Apparently,convertedearly be stages. on insimply reactivitytotheir such Therefore,the attributed ability correspondinga betweenstriking itto isto generateclear primarybasicity,differenc 1,3-di-that an andnucleophilicy,thee N inisocyanate, , secondaryNreactivity reactivity-dibutyl-2-urea difference amines orwhichbetween steric via cannotis hindrance between primarycarbonylationformed be simply the inandbetween twositu secondary attributedwith typesvia them. COthe of2 dehydrolysisiscocyanatesStill,isocyanates it is notable of fromwere carbamic thatmonoamidesalso Ndetected ,Nacids′-dialkylureas ofbyoriginated oxalicGC-MS wereacid infrom the withobtained reactionCO peroxydisulfate2 andas mixtures the primary main of [42].byproduct primaryamines In fact, amines(Scheme in small the with synthesis amounts1). CONot2, inofof dehydrolysissurprisingly,especial(entry 9). at Apparently,earlyof the carbamic stages.reaction suchTherefore, runsacids a faster strikingoriginated it atis higherclear differenc fromthat reaction theeCO in reactivity 2 pressurereactivityand primary difference(compare between amines with betweenprimary entries(Scheme the and0 2 two and secondary1). types 3)Not of toaminesStill,dehydrolysis basicity, it iscannotdepends notable nucleophilicy, of be carbamicthatonsimply theirN,N or attributed ′-dialkylureas stericabilityacids hindrance originatedto to generate basicity, were between fromobtained annucleophilicy, isocyanate,CO them. 2as and the Still, mainprimaryorwhich it issteric notablebyproduct is amineshindrance formed that inN(Scheme ,the Ninbetween -dialkylureas situsynthesis 1).via them. Notthe of surprisingly,especialisocyanatesiscocyanates at theearly from were reaction stages. monoamides also runs detectedTherefore, faster of by at oxalicit GC-MS higheris clear acid reactionin that thewith the reaction peroxydisulfatepressure reactivity mixtures (compare difference of[42]. primarywith Inbetween entriesfact, amines small the 2 and two withamounts 3) types CO2, ofin aminesdehydrolysisIt is cannotdepends well-known of be carbamic onsimply theirthat attributed COabilityacids2 easily originatedto to combinesgenerate basicity, from with annucleophilicy, isocyanate, COamines2 and to primaryaffordorwhich steric the is amineshindrance correspondingformed (Scheme inbetween situ carbamic 1).via them. Notthe wereStill,isocyanatessurprisingly, obtainedit is notable from the as thereactionmonoamidesthat main N, Nruns byproduct′-dialkylureas faster of oxalic at in higher the wereacid synthesis reactionwithobtained peroxydisulfate of pressureisocyanates as the main(compare from[42]. byproduct monoamides Inwith fact, entries in small the 2 of synthesis amountsand oxalic 3) acid2 of aminesiscocyanatesespecialIt is well-known dependsat early were stages. onalso that their detected COTherefore, 2ability easily by GC-MSittocombines isgenerate clear in that withthe an reactionthe aminesisocyanate, reactivity mixturesto afford differencewhich of the primary iscorresponding betweenformed amines thein situwith twocarbamic typesviaCO ,the inof acidsdehydrolysissurprisingly,Still,It is itat well-knownis room notable the oftemperature reaction carbamicthat that N CO, Nruns ′-dialkylureasunder2acids easily faster atmosphericoriginated combines at higher were withreaction from obtainedpres amines sureCO pressure 2as [12].and tothe affordHowever, mainprimary(compare thebyproduct correspondingfurther amineswith entries transformationsin(Scheme the 2 synthesisandcarbamic 1). 3) Not of withisocyanatesiscocyanatesespecialaminesIt peroxydisulfate is at dependswell-known early from were stages. monoamidesalsoon [that42 theirdetected Therefore,]. CO In ability fact,2 easilyofby oxalic smallitGC-MSto is combines generateclear amountsacid in that thewith with anthereaction of peroxydisulfate isocyanate, iscocyanatesreactivityamines mixtures to difference affordwhich were of[42]. primary the also is Inbetween correspondingformed fact, detected amines small thein by twowithsituamounts GC-MS carbamic types COvia 2, the inofinof acidscarbamicsurprisingly,dehydrolysis at room acids temperature theof toreactioncarbamic ureas under runswere acids faster atmosphericmuch originated at highermore presreactionfromdifficulsure COt, pressure[12]. 2except and However, primary(comparein the further presence amineswith transformationsentries (Schemeof a2 andcatalyst 1).3) ofNot or iscocyanatesacidsisocyanatesIt at is room well-known fromwere temperature monoamidesalso that detected CO under2 easilyofby atmosphericoxalicGC-MS combines acid in thewith preswith reaction peroxydisulfatesure amines [12]. mixtures toHowever, afford of[42]. primary the furtherIn corresponding fact, amines transformations small with amounts carbamic CO2, inof theespecialaminesdehydrolysis reaction dependsat early mixtures of stages. oncarbamic oftheir Therefore, primary abilityacids amines itoriginatedto is generateclear with that COfrom anthe2, in isocyanate,reactivityCO especial2 and difference atprimarywhich early stages.is amines betweenformed Therefore, (Schemethein twositu it typesvia1). is clear Notthe of carbamicsurprisingly, acids theto reactionureas were runs 2fastermuch atmore higher difficul reactiont, exceptpressure in (compare the presence with entriesof a catalyst2 and 3) or stoichiometricacidsiscocyanatesIt at is room well-known were temperatureamounts also that detected of CObases under easily by[13], atmospheric GC-MS whichcombines in is the also preswith reaction insure amines accordance [12]. mixtures toHowever, afford with of primary theour further corresponding experimental amines transformations with resultscarbamic CO2, asofin thatespecialaminescarbamic the reactivitydependsat acidsearly stages. toon difference ureas their Therefore, wereability between much itto is the generate clearmore two that typesdifficul an the isocyanate, ofreactivityt, amines 2except depends differenceinwhich the is onpresence betweenformed their ability ofthein a twositu tocatalyst generatetypesvia the ofor stoichiometricdehydrolysissurprisingly,It is well-known amounts theof reactioncarbamic ofthat bases runs CO acids 2 [13], fastereasily originated which at combines higher is also reactionfrom with in accordanceCO amines pressure and to primarywith(compareafford our the amines experimentalwith corresponding entries (Scheme 2 results and carbamic1). 3) asNot stoichiometricshownacids at in room Table amounts temperature 1. Besides, of bases no under reaction[13], atmospheric which was isobserved also pres in sure accordanceto occur [12]. betweenHowever, with our secondary further experimental transformations amine results and CO as2 inof aminescarbamicstoichiometricespecial dependsat acids early amounts stages.toon ureas their Therefore,of werebasesability [13],much itto is which generateclearmore thatis difficul alsoan the inisocyanate, reactivity t,accordance 2except differenceinwhich withthe ourispresence betweenformed experimental ofthein a situtwo catalyst results typesvia the orasof andehydrolysissurprisingly, isocyanate,It is well-known theof which reactioncarbamic is that formed runs COacids faster2 ineasily situoriginated at combinesviahigher the reaction dehydrolysisfrom with CO amines pressure and of to carbamic primary(compare afford the acidsamines with correspondingoriginated entries (Scheme 2 and from carbamic1). 3) CONot2 shownthecarbamicacids presence in at Table room acids of1. temperature Besides,catalystto ureas YSZ-8no were reactionunder (see much atmospheric Tablewas more observed 2, entry difficul pres 9).tosure occurMechanistict, except[12]. between However, in studies thesecondary presencefurther in literature amine transformations of aand showed catalyst CO2 inthat orof dehydrolysisstoichiometricshownamines independs Table ofamounts 1. carbamiconBesides, their of nobasesabilityacids reaction [13], originatedto whichwasgenerate observed is from also an in isocyanate,COto accordance occur2 and between primarywhich with secondary ourisamines formed experimental amine(Scheme in situ and results 1).via CO Not2the asin theandsurprisingly,carbamicacids presence primaryIt atis roomwell-known acidsof amines thecatalyst temperature reactionto ureas (Scheme YSZ-8that runs COwere under1(see). 2faster easily Not muchTable atmospheric surprisingly,at combines higher2,more entry reactiondifficul 9).preswith the Mechanisticsure reaction amines t,pressure except [12]. runsto However, (comparestudiesaffordin faster the thein atpresence furtherwith literature highercorresponding entries transformations reactionof showed a2 andcatalyst pressurecarbamic 3) that orof thethestoichiometric presence presence of ofcatalyst amounts oxygen YSZ-8 vacanciesof bases (see [13], Tablein catalysts which 2, entry is(such also 9). Mechanisticasin ceriaaccordance2 oxide studies and with rhodium inour literature experimental oxide) showed played results that a key2 as surprisingly,shownthedehydrolysis presence in Table theofof 1.catalyst reaction carbamicBesides, YSZ-8 runs noacids reaction2faster (see originated Table at washigher 2, observed entry reactionfrom 9). CO toMechanistic pressure occur and between (compareprimary studies secondary amines within literature entries amine(Scheme 2 showed andand 1).3) CO thatNot in the(comparestoichiometricacidscarbamic presenceIt at is roomwell-known with ofacids oxygen entriestemperatureamounts to ureas vacancies 2that andof CObases wereunder 3). ineasily [13], much catalystsatmospheric combineswhich more (such is difficulalso preswith as ceriainsure amines t,accordance [12].exceptoxide toHowever, and affordin with rhodiumthe the our presencefurther corresponding experimental oxide) transformations of played a catalyst resultscarbamic a key asofor theroleshown presence in thein Table adsorptionof oxygen 1. Besides, vacancies and activationno reaction in catalysts of was CO 2(suchobserved on the as catalyst ceria to occur oxide surface between and by rhodium creatingsecondary oxide) additional amine played and reduction a CO key2 in acidsthesurprisingly, presenceIt at is room well-known oftheof temperature oxygencatalyst reaction that vacanciesYSZ-8 runs CO under 2 faster (seeeasily inatmospheric Table catalystsat combines higher 2, entry (such reaction preswith 9). assure amines Mechanistic ceriapressure [12]. oxide toHowever, (compareafford andstudies rhodium the further with incorresponding literature entries oxide) transformations 2playedshowed and carbamic 3) a thatkey of roleshowncarbamicstoichiometric in the in adsorption Tableacids amounts 1.to Besides, andureas activationof nowerebases reaction much[13], of CO whichwas more2 on observed theis difficul also catalyst intot, accordance occur exceptsurface between inby with creatingthe secondary ourpresence additionalexperimental amineof a reduction andcatalyst results CO 2 orinas rolepotentialthe in presence the adsorption for ofthe catalyst transformation and activationYSZ-8 (see of ofTable COCO2 on22, to entrythe CO catalyst 9). and/or Mechanistic surface surface by studies creatingcarbonaceous in additionalliterature species showedreduction [41,43]. that the presenceIt is well-known of oxygen that vacancies CO2 easily in catalysts combines2 (such with as aminesceria oxide to afford and rhodium the corresponding oxide) played carbamic a key acidscarbamicrolestoichiometric in at the room acidsadsorption temperatureamounts to ureas and of activation werebasesunder [13],much atmospheric of which COmore onis difficul alsothepres catalyst sureint, accordance except[12]. surface However, in withbythe creating ourpresencefurther experimental additional transformations of a catalyst reductionresults 2 orofas potentialtheshown presence infor Table the of catalyst1.transformation Besides, YSZ-8 no reaction(see of TableCO was2 to2, observedentryCO and/or9). toMechanistic occur surface between studiescarbonaceous secondary in literature speciesamine showed and[41,43]. CO that in Meanwhile,theroleacids presence in at the room adsorption XRD of temperature oxygen and XPSand vacancies activationanalysis under in atmospheric revealed catalysts of CO2 onthat (such thepres oxygen catalystassure ceria vacancy[12]. surfaceoxide However, andsites by rhodium creatingand/or further sub-lattice additional oxide) transformations played zirconium reduction a key of carbamicstoichiometricpotentialshownthe presence in Tableacidsfor of theamounts 1.catalystto Besides, transformationureas ofYSZ-8 werebasesno reaction (see [13],much of Table which COwas more 22, observedto isentry difficulalsoCO 9). inand/or to t,Mechanisticaccordance occurexcept surface between in withstudies thecarbonaceous secondary ourpresence in experimental literature amineof species a showed catalystand results [41,43].CO that2 orasin Meanwhile,the presence XRD of andoxygen XPS vacancies analysis revealed in catalysts that2 (such oxygen as ceriavacancy oxide sites and and/or rhodium sub-lattice oxide) playedzirconium a key rolepotentialcarbamic in the foracidsadsorption the to transformationureas and activationwere much of of CO COmore2 toon difficultheCO catalystand/ort, except surfacesurface in by thecarbonaceous creating presence additional ofspecies a catalyst reduction [41,43]. or stoichiometricshownMeanwhile, in Table XRD amounts 1. andBesides, XPS of nobasesanalysis reaction [13], revealed which was observed thatis also oxygen in to accordance occur vacancy between siteswith secondaryand/or our experimental sub-lattice amine and zirconiumresults CO2 asin rolethethe presencepresencein the adsorption ofof catalystoxygen and vacanciesYSZ-8 activation (see in Table catalystsof CO 2,2 onentry (such the 9). catalyst as Mechanistic ceria surface oxide andstudies by creatingrhodium in literature additional oxide) showedplayed reduction a that key potentialMeanwhile,stoichiometric for XRD theamounts andtransformation XPS of analysisbases [13], ofrevealed whichCO2 to thatis alsoCO oxygen inand/or accordance vacancy surface siteswith carbonaceous and/or our experimental sub-lattice species zirconium results [41,43]. as shownthe presence in Table of oxygen1.catalyst Besides, vacanciesYSZ-8 no reaction (see in Table catalysts was 2, observed entry (such 9). as toMechanistic ceria occur oxide between andstudies rhodiumsecondary in literature oxide) amine playedshowed and CO a thatkey2 in Meanwhile,potentialrole in the for adsorption XRD the andtransformation XPSand analysisactivation ofrevealed ofCO CO2 2 tothaton COthe oxygen catalystand/or vacancy surfacesurface sites bycarbonaceous creatingand/or sub-lattice additional species zirconium reduction [41,43]. the shown presence in Table of catalyst1. Besides, YSZ-8 no reaction (see Table was 2, observed entry 9). toMechanistic occur between studies secondary in literature amine showed and CO that2 in theMeanwhile,rolepotential presence in the foradsorption XRD of the oxygen and transformation andXPS vacancies activationanalysis in revealed ofcatalysts of COCO22 ontothat (such theCO oxygen catalyst asand/or ceria vacancy surfaceoxidesurface andsites by carbonaceous rhodiumcreating and/or sub-lattice additional oxide) species played zirconiumreduction [41,43]. a key the presence of catalyst YSZ-8 (see Table 2, entry 9). Mechanistic studies in literature showed that therole potentialMeanwhile, presence in the foradsorption XRDof the oxygen andtransformation andXPS vacancies activation analysis in of revealedcatalysts of CO CO2 2 toon that(such theCO oxygen catalystasand/or ceria vacancy surfaceoxidesurface and sites by carbonaceous rhodiumcreating and/or sub-latticeadditional oxide) species played zirconiumreduction [41,43]. a key the presence of oxygen vacancies in catalysts (such as ceria oxide and rhodium oxide) played a key rolepotential Meanwhile, in the foradsorption XRD the andtransformation andXPS activationanalysis ofrevealed of CO CO2 2 to onthat theCO oxygen catalystand/or vacancy surfacesurface sites by carbonaceous creating and/or sub-latticeadditional species zirconiumreduction [41,43]. potentialMeanwhile,role in the for adsorption XRD the andtransformation XPSand activationanalysis ofrevealed of CO CO2 2 to onthat COthe oxygen catalystand/or vacancy surfacesurface sites by carbonaceous creatingand/or sub-lattice additional species zirconium reduction [41,43]. Meanwhile,potential for XRD the andtransformation XPS analysis ofrevealed CO2 tothat CO oxygen and/or vacancy surface sites carbonaceous and/or sub-lattice species zirconium [41,43]. Meanwhile, XRD and XPS analysis revealed that oxygen vacancy sites and/or sub-lattice zirconium

Catalysts 2018, 8, 188 8 of 11

It is well-known that CO2 easily combines with amines to afford the corresponding carbamic acids at room temperature under atmospheric pressure [12]. However, further transformations of carbamic acids to ureas were much more difficult, except in the presence of a catalyst or stoichiometric amounts of bases [13], which is also in accordance with our experimental results as shown in Table1. Besides, no reaction was observed to occur between secondary amine and CO2 in the presence of catalyst YSZ-8 (see Table2, entry 9). Mechanistic studies in literature showed that the presence of oxygen vacancies in catalysts (such as ceria oxide and rhodium oxide) played a key role in the Catalysts 2018, 8, x FOR PEER REVIEW 8 of 11 adsorption and activation of CO2 on the catalyst surface by creating additional reduction potential for the transformation of CO2 to CO and/or surface carbonaceous species [41,43]. Meanwhile, XRD defects sites were formed on the surface of our YSZ-8 catalyst, as discussed in Section 3.1. Based on and XPS analysis revealed that oxygen vacancy sites and/or sub-lattice zirconium defects sites were the combined experimental and literature study, a plausible reaction mechanism is proposed formed on the surface of our YSZ-8 catalyst, as discussed in Section 3.1. Based on the combined (Scheme 2): initially, a CO2 molecule is adsorbed to an oxygen vacancy site on YSZ-8, generating experimental and literature study, a plausible reaction mechanism is proposed (Scheme2): initially, complex A which would result in the activation of a carbon-oxygen double bond in the CO2 a CO2 molecule is adsorbed to an oxygen vacancy site on YSZ-8, generating complex A which would molecule. Then, the carbon atom of CO2 undergoes a nucleophilic attack by an amine, forming result in the activation of a carbon-oxygen double bond in the CO2 molecule. Then, the carbon complex B as the key active intermediate species. A subsequent proton transfer from amine to the atom of CO2 undergoes a nucleophilic attack by an amine, forming complex B as the key active carbonyl oxygen in complex B would result in complex C, which is then nucleophilically attacked intermediate species. A subsequent proton transfer from amine to the carbonyl oxygen in complex by another amine, forming the desired product N,N′-dialkylurea (2) with H2O as a co-product with B would result in complex C, which is then nucleophilically attacked by another amine, forming the the catalyst being regenerated0 (Route I1). Elimination of complex C could also afford the byproduct desired product N,N -dialkylurea (2) with H2O as a co-product with the catalyst being regenerated isocyanate (Route I2). (Route I1). Elimination of complex C could also afford the byproduct isocyanate (Route I2).

0 Scheme 2. Plausible mechanism for the preparationpreparation ofof NN,,NN′-dialkylurea from amineamine andand COCO22 over

YSZ-8 catalyst.

Meanwhile, an electron transfer from the oxygen vacancy site of YSZ-8YSZ-8 toto thethe adsorbedadsorbed COCO22 in complex AA could could result result in thein formationthe formation of both of COboth species CO species (D) and (D) O− byand creating O− by additionalcreating additional reduction potentialreduction [potential22,43]. Afterwards, [22,43]. Afterwar a nucleophilicds, a nucleophilic attack of attack the freshly-formed of the freshly-formed CO by anCO amine by an occurs,amine generatingoccurs, generating H+ and complexH+ and complex E. Subsequent E. Subsequent C-C coupling C-C coupling of complex of Ecomplex would yieldE would the byproduct yield the Nbyproduct,N0-dialkyloxamide N,N′-dialkyloxamide (3), accompanied (3), accompanied by the catalyst by regenerationthe catalyst regeneration (Route II). In general,(Route II). the In reduction general, ofthe CO reduction2 to CO andof CO oxygen2 to CO is much and moreoxygen difficult is much than more that of difficult CO2 with than amines. that of Consequently, CO2 with amines. route I Consequently, route I is relatively easier to process than route II, which is consistent with our experimental results where the selectivity of the byproduct N,N′-dialkyloxamide (3) is much lower than that of the desired product N,N′-dialkylurea (2) (see Tables 1 and 2).

4. Conclusions The YSZ-8 material prepared in this work was identified as being totally consistent with the compositions of nanosized Y0.08Zr0.92O1.96. XRD and XPS patterns of YSZ-8 indicated that the yttrium moiety was incorporated into the zirconia framework, resulting in an increase of oxygen vacancy sites. As a heterogeneous catalyst, YSZ-8 exhibits very high activity and selectivity toward the carbonylation of aliphatic primary amines with CO2, forming N,N′-dialkylurea under relatively

Catalysts 2018, 8, 188 9 of 11 is relatively easier to process than route II, which is consistent with our experimental results where the selectivity of the byproduct N,N0-dialkyloxamide (3) is much lower than that of the desired product N,N0-dialkylurea (2) (see Tables1 and2).

4. Conclusions The YSZ-8 material prepared in this work was identified as being totally consistent with the compositions of nanosized Y0.08Zr0.92O1.96. XRD and XPS patterns of YSZ-8 indicated that the yttrium moiety was incorporated into the zirconia framework, resulting in an increase of oxygen vacancy sites. As a heterogeneous catalyst, YSZ-8 exhibits very high activity and selectivity toward the carbonylation 0 of aliphatic primary amines with CO2, forming N,N -dialkylurea under relatively mild conditions. The yield of N,N0-dialkylurea was obtained up to 80.60% after 48 h at 160 ◦C under 3.0 MPa, with YSZ-8 as the catalyst, N-methyl-2-pirrolidinone as the solvent and 4 A zeolite as the dehydrant. Mechanistic studies revealed that the presence of oxygen vacancies in the YSZ-8 catalyst played a key role in the adsorption and activation of CO2 by creating additional reduction potential for the reduction of CO2 to CO and/or surface carbonaceous species. This would result in the coordination of a carbonyl oxygen of CO2 to an oxygen vacancy site, which is the key intermediate species to the activation of CO2. Then, a reaction intermediate formed from a nucleophilic attack of the amino group towards the carbonyl carbon, which would end up in the final carbonylation product.

Author Contributions: Dalei Sun and Xianghua Yang conceived and designed the experiments; Kaihong Xie performed the experiments; Dalei Sun and Yanxiong Fang analyzed the data; Yanxiong Fang contributed reagents/materials/analysis tools; Dalei Sun wrote the paper. Acknowledgments: We are grateful for the Science and Technology Program of Guangzhou, China (No. 201607010166), the National Natural Science Foundation of China (No. 21206019), and Natural Science Foundation of Guangdong Province (No. 2014A030310449) for financial support. Conflicts of Interest: The authors declare no conflict of interest.

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