Environmental-Friendly Synthesis of Alkyl Carbamates from Urea and Alcohols with Silica Gel Supported Catalysts

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Article Environmental-Friendly Synthesis of Alkyl Carbamates from Urea and Alcohols with Silica Gel Supported Catalysts

Yubo Ma 1,2 , Lei Wang 1,3,*, Xiaodong Yang 4,* and Ronghui Zhang 5 1 School of Chemical Engineering, Chongqing Chemical Industry Vocational College, Chongqing 401220, China; [email protected] 2 Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China 3 Department of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830011, China 4 Institute of Resources and Environment Science, Xinjiang University, Urumqi 830011, China 5 Guangdong Key Laboratory of Intelligent Transportation System, School of Engineering, Sun Yat-sen University, Guangzhou 510275, China; [email protected] * Correspondence: [email protected] (L.W.); [email protected] (X.Y.)

Received: 7 October 2018; Accepted: 19 November 2018; Published: 23 November 2018

Abstract: TiO2/SiO2, Cr2O3-NiO/SiO2, and TiO2-Cr2O3/SiO2 were prepared by the impregnation method for alkyl carbamate synthesis using urea as the carbonyl source. Up to 97.5% methyl carbamate yield, 97% ethyl carbamate yield, and 96% butyl carbamate yield could be achieved, respectively. The catalysts were characterized by ICP-AES, BET, XRD, XPS, NH3-TPD, and EPMA. Catalytic activity investigation revealed that TiO2/SiO2, Cr2O3-NiO/SiO2, and TiO2-Cr2O3/SiO2 were effective catalysts for methyl carbamate (MC), ethyl carbamate (EC), and butyl carbamate (BC), respectively. The recycling tests suggested that these silica gel supported catalyst system is active, stable, and reusable. A total of 96–97% alkyl carbamate (methyl, ethyl, and butyl) could be obtained in a 2 L autoclave, and these data suggested that our catalyst system is relatively easy to scale up.

Keywords: alkyl carbamate; TiO2/SiO2; TiO2-Cr2O3/SiO2; Cr2O3-NiO/SiO2; urea; alcohol

1. Introduction Alkyl carbamates are important intermediates and end products in the synthesis of a variety of organic compounds [1–3]. For example, it is extensively used for the synthesis of melamine derivatives, polyethylene amine, alkanediol dicarbamates, textile crease-proofing agents [4–8], and itself is also a promising “tranquilizing” drug [9–12]. In the meantime, alkyl carbamates could be used as feedstocks for the synthesis of corresponding organic carbonates such as dimethyl carbonate, diethyl carbonate, dibutyl carbonate, and so on. Significantly, the alkyl carbamate could be used as carbonyl source to replace phosgene for the synthesis of isocyanates [13–16]. Since more than 85% phosgene was consumed in isocyanate industry, this could be the promising usage of carbamates in the future [17]. Various systems have been reported for carbamate synthesis [18–28]. One of the typical methods was the reaction of alcohol and phosgene; however, large amounts of chloroformate ester would be formed during the reaction. Recently, the reaction of alcohol with urea has been more attractive because the ammonia emitted in this process could be easily recycled for synthesis of urea. As well known, urea production is the only way for large scale chemical fixation of carbon dioxide. Thus, carbon dioxide is indirectly used as carbonyl source in the whole process for carbamates synthesis with urea as the carbonyl sources. So, the green and cheap carbonyl source, carbon dioxide, could be successfully used for large-scale industrial process and the green-house gas emission could be reduced.

Catalysts 2018, 8, 579; doi:10.3390/catal8120579 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 Catalysts 2018, 8, 579 2 of 16 sources. So, the green and cheap carbonyl source, carbon dioxide, could be successfully used for large-scale industrial process and the green-house gas emission could be reduced. Since the end of the last century, different kinds of solid acidic and basic catalysts such such as as ZnO, MgO, CaO CaO,, and transition metal metal oxides oxides have have been been used used for for the the synthesis synthesis of of carbamates carbamates [4,5, [4,5 ,2121––2828]].. Unfortunately, the low yield towards the desireddesired products and the reusability of the catalyst have been signiﬁcantsignificant problems problem untils until now. now. In viewIn view of the of practical the practical application, application, the development the development of catalysts, of whichcatalyst shoulds, which be should simple, be active, simple, and active easily, reusable,and easily for reusable, the synthesis for the of synthesis alkyl carbamates of alkyl fromcarbamates urea is highlyfrom urea desirable. is highly desirable. Higher than 80% O O-substituted-substituted carbamate could be achieved M Moreover,oreover, a 10% difference of yield was observedobserved inin differentdifferent autoclaves autoclaves in in our our lab, lab, which which motivated motivated us us to studyto study the the possible possible reason reason for thisfor reaction;this reaction; the composites the composites of autoclave of autoclave were 1Cr9Ni18Ti, were 1Cr9Ni18Ti, which may which beplayed may be a key played for urea a key conversion for urea andconversion alkyl carbamate and alkyl yield. carbamate Therefore, yield. a series Therefore, of catalysts a series containing of catalysts Ti, Cr, containing Ni was prepared Ti, Cr, Ni by was the impregnationprepared by the method impregnatio and usedn for method synthesis and of used alkyl for carbamates. synthesis The of alkyl catalytic carbamates. performance The of catalytic catalyst forperformance MC synthesis of catalyst is little for effect, MC synthesis while an is obviously little effect, effect while were an observedobviously for effect BC were and ECobserved synthesis. for ThreeBC and different EC synthesis. catalysts Three were different selected catalysts by screening were selected the catalysts by screening containing the catalyst Ti, Cr,s and containing Ni. Three Ti, differentCr, and Ni. catalysts Three different were obtained catalysts for were MC, EC,obtained and BC for synthesis, MC, EC, respectively.and BC synthesis, respectively. More thanthan 98.7%98.7% MCMC yieldyield could could be be achieved achieved in in Ref. ref [27,27], EC EC and and BC BC was was not not studied; studied; the the catalyst catalyst of [2829] was a nano metal oxide and R3N, which was a mixed catalyst, and R3N was dissolved in the reaction system. Therefore, this kind kind of of reaction reaction was was studied studied in in this this work work over over supported supported catalysts catalysts containing containing Ti, Ti,Cr, Cr,and and Ni. Ni. Here, Here, we we report report our our results results about about the the preparation, preparation, characterization characterization and and usage usage of TiO22/SiO22, ,Cr Cr22OO3-3NiO/-NiO/SiOSiO2, 2and, and TiO TiO2-2Cr-Cr2O2O3/SiO3/SiO2 catalysts2 catalysts for for synthesis synthesis of of methyl methyl carbamate carbamate (MC), (MC), ethyl carbamate (EC),(EC), and butyl carbamate (BC) (Scheme(Scheme1 1).).

MeOH MeOH H2NCONH2 H2NCO2Me MeOCO2Me 1 2 EtOH EtOH H2NCONH2 H2NCOOEt EtOCO2Et 3 4 BuOH BuOH H2NCONH2 H2NCO2Bu BuOCO2Bu 5 6 SchemeScheme 1.1. Synthesis of methyl, ethyl,ethyl, butylbutyl carbamate/dimethyl,carbamate/dimethyl, diethyl diethyl,, and dibutyl carbonate carbonate..

2.2. Results and Discussion 2.1. Characterization of Catalysts 2.1. Characterization of Catalysts The catalysts were characterized by ICP-AES, BET, XPS, XRD, TPR, and EPMA. All the catalysts The catalysts were characterized by ICP-AES, BET, XPS, XRD, TPR, and EPMA. All the catalysts prepared and characterized in this work are shown in Table1. prepared and characterized in this work are shown in Table 1.

Table 1. Typical physicochemical properties of TiO2/SiO2, Cr2O3-NiO/SiO2, and TiO2-Cr2O3/SiO2 Table 1. Typical physicochemical properties of TiO2/SiO2, Cr2O3-NiO/SiO2, and TiO2-Cr2O3/SiO2 catalysts with different composition. catalysts with different composition. M/Si Surface Pore volume B. E. M/Si atm% Catalyst M/Si 2 Pore3 atom% Bulk Area/mSurface/g /cm /g B./eV E. M/SiSurface atm% Catalyst atom% volume SiO2 Area601/m2/g 0.74 /eV Surface 2.2 wt% TiO2/SiO2-500 Bulk /cm3/g 2.9 wt% TiO2/SiO2-500 Ti/Si 3.8 554 0.69 Ti 2p3/2 459.1 Ti/Si 3.0 2 3.6 wt% TiOSiO2/SiO 2-500 601 0.74 2.23.5 wt wt%% TiO Cr2O23/SiO-8.0 wt%2-500 NiO/SiO2-500 2.97.1 wt wt%% TiO Cr O2/SiO-6.3 wt%2-500 Cr/SiTi/Si 10 3.8 554 0.69 TiCr 2p 2p 3/2 459.1577.6 CrTi/Si /Si 12.6 3.0 2 3 370 0.47 3/2 3.6 wtNiO/SiO% TiO22/SiO-500 2-500 Ni/Si 7.9 Ni 2p3/2 856.6 Ni/Si 7.8 3.5 wt% Cr2O3-8.0 wt%

NiO/SiO2-500

Catalysts 2018, 8, x FOR PEER REVIEW 3 of 18

7.1 wt% Cr2O3-6.3 wt% Cr/Si 10 Cr 2p3/2 577.6 Cr /Si 12.6 370 0.47 NiO/SiO2-500 Ni/Si 7.9 Ni 2p3/2 856.6 Ni/Si 7.8 Catalysts 2018, 8, 579 3 of 16 9.0 wt% Cr2O3-3.6 wt%

NiO/SiO2-500 1.7 wt% TiO2-4.9 wt% Table 1. Cont.

2 3 2 Cr O /SiO -500 M/Si Surface Pore volume B. E. M/Si atm% Catalyst 1.4 wt% TiO2-6.1 wt% atom%Ti/Si Bulk 2.0 Area/m2/g /cm3/g Ti 2p/eV3/2 458.7 SurfaceTi/Si 2.4 434 0.49 9.0 wt%Cr2O Cr32/SiOO3-3.62-500 wt% Cr/Si 7.9 Cr 2p3/2 577.2 Cr/Si 8.6 NiO/SiO -500 1.0 wt% TiO2 2-8.5 wt% 1.7 wt% TiO2-4.9 wt% CrCr2O2O3/SiO3/SiO2-5002-500 1.4 wt% TiO -6.1 wt% Ti/Si 2.0 Ti 2p 458.7 Ti/Si 2.4 2 434 0.49 3/2 Cr2O3/SiO2-500 Cr/Si 7.9 Cr 2p3/2 577.2 Cr/Si 8.6 2 BET1.0 wt% surface TiO2-8.5 areas wt% and pore volumes of the catalysts are lower than that of pure SiO . The BET surfaceCr areas2O3/SiO decreased2-500 with the increasing of the loadings of oxides of transition metal and the surface area of the catalysts were in the range of 350–540 m2/g, Table 1.

BETThe surface XPS analysis areas and confirmed pore volumes that the of the bond catalysts valence are of lower titanium than that on the of pure surface SiO 2 of. The 2.9 BET wt% surfaceTiO2/SiO areas2-500 decreased was Ti4+ with (BE theof Ti increasing 2p3/2 = 459.1 of the eV) loadings (Figure of 1a oxides and Table of transition 1). The metalbinding and energies the surface of Ti area2p3/2 of and the catalystsCr 2p3/2 are were 458.7 in the eV rangeand 577.2 of 350–540 eV, respectively m2/g, Table, in1. 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 (FigureThe 1 XPSb and analysis Table 1 conﬁrmed). The binding that energies the bond of valence Cr 2p3/2 of and titanium Ni 2p3/2 on are the 577.6 surface eV and of 2.9856.6 wt% eV, 4+ TiOrespectively2/SiO2-500, in was 7.1 Tiwt%(BE Cr2 ofO3 Ti-6.3 2p wt%3/2 = NiO/SiO 459.1 eV)2-500 (Figure (Figure1a and 1c and Table Table1). The 1). Thus, binding the energies corresponding of Ti +4 3+ 2+ 2pchemical3/2 and Crstates 2p3/2 of arethese 458.7 surf eVace and Ti, Cr 577.2, and eV, Ni respectively, species are inTi 1.4, Cr wt%, and TiO Ni2-6.1. wt% Cr2O3/SiO2-500 (Figure1b and Table1). The binding energies of Cr 2p 3/2 and Ni 2p3/2 are 577.6 eV and 856.6 eV, respectively, in 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 (Figure1c and Table1). Thus, the corresponding chemical states of these surface Ti, Cr, and Ni species are Ti+4, Cr3+, and Ni2+.

2.6 Ti 2p3

2.4 a Ti 2p1 2.2

KCPS

2.0

1.8 475 470 465 460 455 450

Binding Energy (eV)

6.4 13.8 Cr2p3 Ni2p3 6.2 Cr2p1 13.6 b 6.0 Ni2p1 5.8 13.4

KCPS KCPS 5.6 13.2 5.4

5.2 13.0 595 590 585 580 575 570 900 890 880 870 860 850 Binding Energy (eV) Binding Energy (eV)

Figure 1. Cont.

Catalysts 2018, 8, x FOR PEER REVIEW 4 of 18

Catalysts 2018, 8, x FOR PEER REVIEW 4 of 18 Catalysts 2018, 8, 579 4 of 16 2.6 Ti2p3 6.8 Cr2p3

2.6 Cr2p1 6.6 Ti2p3 6.8 Cr2p3c 2.4 Ti2p1 6.4 Cr2p1 6.6 c 2.4 KCPS Ti2p1 KCPS 6.2 6.4

2.2 6.0

KCPS KCPS 6.2

2.2 5.8 470 465 460 455 450 6.0 595 590 585 580 575 570 Binding Energy (eV) Binding Energy (eV) 5.8 470 465 460 455 450 595 590 585 580 575 570 Binding Energy (eV) Binding Energy (eV) Figure 1. XPS patterns of (a) 2.9 wt%TiO2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; (c) 1.4

Figurewt% TiO 1. XPS2-6.1 patternswt% Cr2O of3/SiO (a) 2.92-500 wt%. TiO2/SiO2-500; (b) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500; Figure 1. XPS patterns of (a) 2.9 wt%TiO2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; (c) 1.4 (c) 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500. wt%XRD TiO showed2-6.1 wt% that Cr only2O3/SiO weak2-500 diffraction. peaks of body-centered tetragonal TiO2 (anatase) could be XRDobserved showed in 2.9 that wt% only TiO weak2/SiO diffraction2-500 (Figure peaks 2a), of which body-centered suggests tetragonalthat TiO2 in TiO the2 (anatase) 2.9 wt% couldTiO2/SiO be 2- observed500 isXRD poorly in showed 2.9 wt%crystallized. that TiO2 only/SiO Surprisingly, 2weak-500 (Figurediffraction 2althougha), whichpeaks the suggestsof bodyCr2O-3centered that content TiO 2tetragonalreachesin the 2.9 7.1 wt%TiO wt%,2 TiO (anatase) no2/SiO diffraction2 could-500 isbepeaks poorly observed of crystallized. Cr 2inO 32.9 are wt% observable Surprisingly, TiO2/SiO in2-500 7.1 although ( Figure wt% Cr 2 the2a),O3 -which6.3 Cr2 O wt%3 suggestscontent NiO/SiO that reaches2- 500TiO and2 7.1in the wt%, only 2.9 weak nowt% diffraction TiO diffraction2/SiO2- peaks500peaks is of poorlyof Cr faced2O3 crystallized.are-centered observable cubic Surprisingly, in NiO 7.1 wt%appeared although Cr2O (Figure3-6.3 the wt% 2Crb).2 NiO/SiOO In3 content reverse,2-500 reaches only and weak only7.1 wt%,diffraction weak no diffraction diffraction peaks of peakspeaksrhomb of of- faced-centeredcentered Cr2O3 are rhomboh observable cubicedral NiO inCr appeared7.12O3 was wt% observed Cr (Figure2O3-6.32 andb). wt% Inno reverse, NiO/SiOdiffraction only2-500 peaks weak and of only diffraction TiO 2weak appeared peaksdiffraction for of 1.4 rhomb-centeredpeakswt% TiO of faced2-6.1 wt%-centered rhombohedral Cr2O 3cubic/SiO2 -NiO Cr5002O (appearedFigure3 was observed2c). (Figure All these and 2b). results no In diffractionreverse, indicated only peaks that weak ofCr TiO2diffractionO3 2andappeared TiO peaks2 species for of 1.4rhombare wt% highly- TiOcentered 2dispersed.-6.1 wt%rhomboh Cr 2Oedral3/SiO Cr2-5002O3 was (Figure observed2c). All and these no diffraction results indicated peaks of that TiO Cr2 appeared2O3 and TiOfor 1.42 specieswt% TiO are2- highly6.1 wt% dispersed. Cr2O3/SiO2-500 (Figure 2c). All these results indicated that Cr2O3 and TiO2 species are highly dispersed. 4000 TiO d=3.53 3500 2 4000 3000 TiO d=3.53 a 3500 2 2500 3000 2000 a

2500 1500 TiO2 d=2.38 2000 TiO d=1.90 2 1000

Intensity (a.u)Intensity 1500 TiO d=2.38 500 2 TiO2 d=1.90 1000 Intensity (a.u)Intensity 0 500 10 20 30 40 50 60 70 80 90 0 Position [0 2Theta] 10 20 30 40 50 60 70 80 90

Position [0 2Theta] 4000 3500 4000 3000 b 3500 2500 NiO d=2.40 3000 NiO d=2.08 b 2000 NiO d=1.47

2500 NiO d=2.40 1500 NiO d=2.08 2000 NiO d=1.47 1000

Intensity (a.u)Intensity 1500 500 1000 Intensity (a.u)Intensity 0 500 10 20 30 40 50 60 70 80 90

0 Position [0 2Theta] 10 20 30 40 50 60 70 80 90 Figure 2. Cont. Position [0 2Theta]

Catalysts 2018, 8, x FOR PEER REVIEW 5 of 18 Catalysts 2018, 8, 579 5 of 16

Catalysts 2018, 8, x FOR PEER REVIEW 5 of 18

3000

25003000 c 2500 2000 c 2000 1500 Cr2O3 d=2.68 Cr O d=2.48 1500 Cr2O32 d=2.683 Cr O d=1.68 1000 Cr2O3 d=2.48 2 3 Cr O d=1.68 1000 2 3

Intensity (a.u)Intensity

500 (a.u)Intensity 500

0 0 1010 2020 3030 4040 5050 6060 70 7080 8090 90 PositionPosition [ 0[ 02Theta] 2Theta]

Figure 2. XPS patterns of (a) 2.9 wt% TiO2/SiO2-500; (b) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500; Figure 2. XPS patterns of (a) 2.9 wt%TiO2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; and (c) andFigure (c) 1.4 2. wt%XPS TiOpatterns-6.1 wt%of (a) Cr 2.9O wt%TiO/SiO -500.2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; and (c) 1.4 wt% TiO22-6.1 wt% Cr2O32/SiO32-500. 2 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500.

Pure silicaPure gel,silica 2.9 gel, wt% 2.9 wt% TiO TiO2/SiO2/SiO22--500,500, 1.4 1.4 wt% wt% TiO TiO2-6.12 wt%-6.1 Cr wt%2O3/SiO Cr2-O5003/SiO and 7.12-500 wt% and Cr2O 7.13- wt% Cr2O3-6.3Pure6.3 wt% wt%silica NiO/SiO NiO/SiO gel, 2.92- 500wt%2-500 were TiO werefurther2/SiO further 2characterized-500, 1.4 characterized wt% by TiOTPD2--NH6.1 by 3wt%. It TPD-NH indicated Cr2O3/SiO 3sign. Itificant2-500 indicated anddifference 7.1 signiﬁcantwt% in Cr2O3- acidities among the catalysts, Figure 3. Only one NH3 adsorption peak in the weak acid site region, difference6.3 wt% NiO/SiO in acidities2-500 among were further the catalysts, characterized Figure3 by. OnlyTPD- oneNH3 NH. It indicated3 adsorption significant peak in difference the weak in i.e., Tmax 423–523 K, appeared. The temperatures at which NH3 adsorption reached the maximum acidacidities site region, among i.e., theT maxcatalysts, 423–523 Figure K, appeared. 3. Only one The NH temperatures3 adsorption at peak which in NHthe 3weakadsorption acid site reached region, are 473K for 2.9 wt% TiO2/SiO2-500, 443K for SiO2, 453K for 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 and thei.e. maximum, Tmax 423 are–523 473K K, appeared. for 2.9 wt% The TiO temperatures2/SiO2-500, at 443K which for NH SiO32 ,adsorption 453K for 1.4 reached wt% TiO the2 -6.1maximum wt% 463K for 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500, respectively. Thus the 2.9 wt% TiO2/SiO2-500 exhibited Crare2O 473K3/SiOthe for 2strongest-500 2.9 andwt% acidic 463K TiO strength.2 for/SiO 1.42-500, The wt% TPD443K TiO analysis 2for-6.1 SiO wt% also2, 453K Crsuggested2O for3/SiO 1.4 that 2wt%-500, 500 TiO °C respectively. is2- a6.1 suitable wt% Crtemper Thus2O3/SiO theature 2.92- 500 wt% and ◦ TiO463K2/SiO forin2 1.4-500order wt%exhibited to achieve TiO2-6.1 more the wt% strongest acidic Cr2 sites,O3/SiO acidic Figures2-500, strength. 4 –respectively.6. The TPD Thus analysis the 2.9 also wt% suggested TiO2/SiO that2-500 500 exhibitedC is athe suitable strongest temperature acidic strength. in order The to achieve TPD analysis more acidic also sites,suggested Figures that4– 6500. °C is a suitable temperature

in order to achieve more acidic sites, Figures 4–6. 110 d

105 110 c d 100

105 b

Intensity (a.u) Intensity c 100 a

90 b 300 400 500 600 700 800 95

Intensity (a.u) Intensity Temperature (K) Catalysts 2018, 8, x FOR PEER REVIEW a 6 of 18

Figure 3. a ◦ b NH3-TPD proﬁles90 of ( ) SiO2 (pretreated at 600 C for 2 h); ( ) 2.9 wt% TiO2/SiO2-500; Figure 3. NH3-TPD profiles of (a) SiO2 (pretreated at 600 °C for 2 h); (b) 2.9 wt% TiO2/SiO2-500; (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500; and (d) 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500. (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO300 2-500400; and (d)500 1.4 wt% TiO600 2-6.1 wt%700 Cr2O3/SiO800 2-500. 110 Temperature (K) c 105 Figure 3. NH3-TPD profiles of (a) SiO2 (pretreated at 600 °C for 2 h); (b) 2.9 wt% TiO2/SiO2-500;

Intensity (a.u) Intensity a

400 500 600 700 800 Temperature (K)

Figure 4. NH3-TPD proﬁles of (a) 2.9 wt% TiO2/SiO2-400, (b) 2.9 wt% TiO2/SiO2-500; and (c) 2.9 wt% Figure 4. NH3-TPD profiles of (a) 2.9 wt% TiO2/SiO2-400, (b) 2.9 wt% TiO2/SiO2-500; and TiO2/SiO2-600. (c) 2.9 wt%TiO2/SiO2-600.

110 c

105

b 100

95 a

Intensity (a.u) Intensity

400 500 600 700 800 Temperature (K)

Figure 5. NH3-TPD profiles of (a) 7.1 wt% Cr2O3-6.3 wt%NiO/SiO2-400; (b) 7.1 wt% Cr2O3-6.3 wt%

NiO/SiO2-500; and (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-600.

110 c

105 b

100

a 95

Intensity (a.u) Intensity

400 500 600 700 800 Temperature (K)

Catalysts 2018, 8, x FOR PEER REVIEW 6 of 18

110

105 110 c 100 b 105

Intensity (a.u) Intensity 100 a b

90 95

Intensity (a.u) Intensity 400 500 600 700 a 800 Temperature (K) 90

400 500 600 700 800 Figure 4. NH3-TPD profiles of (a) 2.9 Temperature wt% TiO2/SiO (K)2-400, (b) 2.9 wt% TiO2/SiO2-500; and (c) 2.9 wt%TiO2/SiO2-600.

Figure 4. NH3-TPD profiles of (a) 2.9 wt% TiO2/SiO2-400, (b) 2.9 wt% TiO2/SiO2-500; and

Catalysts 2018, 8, 579 110 6 of 16 c

105 110 bc 100 105

95 b a

Intensity (a.u) Intensity 100

90 95 a Intensity (a.u) Intensity 400 500 600 700 800 Temperature (K) 90

400 500 600 700 800 Figure 5. NH3-TPD profiles of (a) 7.1 wt%Temperature Cr2O3-6.3 wt%NiO/SiO (K) 2-400; (b) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500; and (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-600.

Figure 5. NH3-TPD proﬁles of (a) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-400; (b) 7.1 wt% Cr2O3-6.3 wt% Figure 5. NH3-TPD profiles of (a) 7.1 wt% Cr2O3-6.3 wt%NiO/SiO2-400; (b) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500; and (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO 2-600. NiO/SiO2-500; and (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-600.

110 c

105 110 b c

100 105 ab 95

Intensity (a.u) Intensity 100

90 a 95

Intensity (a.u) Intensity 400 500 600 700 800 Temperature (K) 90

Figure 6.CatalystsNH3 -TPD2018, 8, x proﬁles FOR PEER ofREVIEW (a) 1.4 wt%400 TiO5002-6.1 wt%600 Cr2O3700/SiO2-400;800 (b) 1.4 wt% TiO7 of 182-6.1 wt%

Cr2O3/SiO2-500; and (c) 1.4 wt% TiO2-6.1 wt%Temperature Cr2O3/SiO (K)2-600. Figure 6. NH3-TPD profiles of (a) 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-400; (b) 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500; and (c) 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-600. The distributions of Ti, Cr, and Ni of 2.9 wt% TiO2/SiO2-500, 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500,The and distributions 7.1 wt% of CrTi, 2CrO, 3and-6.3 Ni wt%of 2.9 wt% NiO/SiO TiO2/SiO22-500-500, 1.4 inside wt% TiO the2-6.1 SiO wt%2 pelletCr2O3/SiO were2- analyzed with electron500 probe, and 7.1 microanalysis wt% Cr2O3-6.3 wt% (EPMA), NiO/SiO2 Figure-500 inside7. the It was SiO2 pellet very were clear analyzed that distributions with electron of Ti, Cr, probe microanalysis (EPMA), Figure 7. It was very clear that distributions of Ti, Cr, and Ni inside and Ni insideSiO SiO2 pellets2 pellets were relatively were uniform relatively in all three uniform catalysts in and all almost three synchronous catalysts distributions and almost of Cr synchronous- distributionsNi of and Cr- Ti Ni-Cr and were Ti-Cr formed were in 7.1 formed wt% Cr2O in3-6.3 7.1 wt% wt% NiO/SiO Cr2O23-500-6.3 and wt% 1.4 NiO/SiO wt% TiO2-6.12-500 wt% and 1.4 wt% Cr2O3/SiO2-500. This means that strong interaction between Cr and Ni or Ti and Cr may occur, and TiO2-6.1 wt% Cr2O3/SiO2-500. This means that strong interaction between Cr and Ni or Ti and Cr relatively uniform distributions of Ti, Cr, and Ni in support maybe favorable to get highly stable may occur, andcatalysts. relatively uniform distributions of Ti, Cr, and Ni in support maybe favorable to get highly stable catalysts.

The distribution of Cr The distribution of Ti in 2.0

in TiO2-Cr2O3 /SiO2 TiO2-Cr2O3 /SiO2

1.5

The distribution of Cr in The distribution of Ni in

1.0 Cr2O3-NiO/SiO2 Cr2O3-NiO/SiO2

Intensity 0.5

The distribution of Ti in TiO2/SiO2

0.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 length from edge to center of section plane of catalyst pellet (mm)

Figure 7. The distributions of Ti, Cr and Ni in the cross section of catalyst pellets: 2.9 wt% Figure 7. The distributions of Ti, Cr and Ni in the cross section of catalyst pellets: 2.9 wt% TiO2/SiO2- TiO2/SiO2-500, 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500, and 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 500, 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500, and 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 from edge to from edge tocenter. center.

2.2. Catalytic Activity Test

2.2.1. Effect of Active Metal Loading on Synthesis of Alkyl Carbamate As shown in Table 2, almost same urea conversion, 1–6 yields could be obtained in the absence of catalyst or SiO2 as the catalyst. The effect of amount of Ti loading on the synthesis of 1 and 2 was then examined. The results were shown in Table 2. The conversion of urea increases from 94% to 100% and remains unchanged with the increasing of Ti loading (entries 1–5). The highest yield, 97.5%, was obtained with 2.9 wt% of titanium loading.

Catalysts 2018, 8, 579 7 of 16

2.2. Catalytic Activity Test

2.2.1. Effect of Active Metal Loading on Synthesis of Alkyl Carbamate As shown in Table2, almost same urea conversion, 1–6 yields could be obtained in the absence of catalyst or SiO2 as the catalyst. The effect of amount of Ti loading on the synthesis of 1 and 2 was then examined. The results were shown in Table2. The conversion of urea increases from 94% to 100% and remains unchanged with the increasing of Ti loading (entries 1–5). The highest yield, 97.5%, was obtained with 2.9 wt% of titanium loading.

Table 2. Effect of catalyst loading on synthesis of alkyl carbamate *.

Entry Catalyst Con./% Y. Carbamate% Y. Dialkyl Carbonate/% 1 94 MC 89 DMC 0.3 2 SiO2 94 MC 90 DMC 0.4 3 2.2 wt% TiO2/SiO2-500 >99 MC 94 DMC 0.4 4 2.9 wt% TiO2/SiO2-500 >99 MC 97.5 DMC 0.6 5 3.6 wt% TiO2/SiO2-500 >99 MC 96 DMC 0.7 6 85 EC 80 DEC 0.2 7 SiO2 85 EC 80 DEC 0.2 8 3.5 wt% Cr2O3-8.5 wt% NiO/SiO2-500 >99 EC 94 DEC 0.3 9 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 >99 EC 97 DEC 0.5 10 9.0 wt% Cr2O3-3.6 wt% NiO/SiO2-500 >99 EC 96 DEC 0.55 11 92 BC 88 DBC 0.4 12 SiO2 92 BC 88 DBC 0.4 13 1.0 wt% TiO2-8.5 wt% Cr2O3/SiO2-500 >99 BC 95 DBC 0.2 14 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 >99 BC 96 DBC 0.5 15 1.7 wt% TiO2-4.9 wt% Cr2O3/SiO2-500 >99 BC 94 DBC 0.65 * Catalysts 0.1 g, urea 1 g, reaction temperature 170 ◦C. For MC synthesis, 13.5 mL methanol, 6 h. For EC synthesis, 15 mL ethanol, 4 h. For BC synthesis, 20 mL butanol, 4 h.

The effect of loadings of Cr and Ni, Ti, and Cr on the synthesis of EC, BC was also examined by the reaction of urea (1 g) with ethanol (15 mL) and butanol (20 mL) with catalyst (0.1 g) at 170 ◦C for 4 h, Table2. The yield of 3, 4, 5, and 6 is also found to be strongly dependent on the amount of transitional metal loadings. The yield of 3 is only 80% with pure SiO2 as catalyst and 97% yield was achieved in the presence of catalyst with 7.1 wt% Cr loading and 6.3 wt% Ni loading (entry 9). The yield of 5 is 88% with pure SiO2 as catalyst and 96% yield was obtained with catalyst containing 1.4 wt% Ti and 6.1 wt% Cr (entry 14).

2.2.2. Performances of Different Catalysts for Synthesis of MC, EC, and BC Interestingly, quite different catalytic activity exhibited by these three silica gel supported catalysts on synthesis of MC, EC, and BC. As shown in Table2, the catalyst 2.9 wt% TiO 2/SiO2-500, 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500, and 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 gave MC (1), EC (3), and BC (5) in high yields together with the corresponding lower yields of dialkyl carbonates as by-products, respectively. In the reaction of synthesizing alkyl carbamate, the yield for MC, EC, and BC were 97.5% 97%, and 96%, respectively. These indicate that supported transition metal oxides have good catalytic activity for alkyl carbamate synthesis from urea and alcohols.

2.2.3. Effect of Calcination Temperature The effect of calcined temperature on the yield of 1, 3 and 5 was shown in Table3. The yield of 1 increased from 95% to 97.5 % with the increasing of calcination temperature from 400 ◦C to 500 ◦C. A further increase in the calcination temperature led to the decrease of the yield of 1. Similar trends Catalysts 2018, 8, 579 8 of 16 were also observed in synthesis of 3 and 5. One of the possible reasons may be that, more acidic positions could be produced at such a temperature, as suggested by TPD-NH3 characterization results.

Catalysts 2018, 8Table, x FOR PEER 3. Effect REVIEW ofcalcinations temperature on the catalytic activity *. 9 of 18

EntryTable Catalyst 3. Effect of calcinations temperature Timeon the catalytic GC activity Yield *. of Alkyl Carbamate Entry Catalyst Time GC Yield of Alkyl Carbamate 1 2.9 wt% TiO2/SiO2-400 6 h MC 95% 21 2.9 wt%2.9 wt% TiO 2TiO/SiO2/SiO2-5002-400 6 6h hMC MC 95% 97.5% 32 2.9 wt%2.9 wt% TiO 2TiO/SiO2/SiO2-6002-500 6 6h hMC MC 97.5% 94% 43 7.1 wt% Cr22.9O3 -6.3wt% wt% TiO2 NiO/SiO/SiO2-600 2-4006 4h hMC EC 94% 95% 54 7.1 wt%7.1 wt% Cr2O Cr3-6.32O3- wt%6.3 wt% NiO/SiO NiO/SiO2-5002-400 4 4h hEC EC 95% 97% 65 7.1 wt%7.1 wt% Cr2O Cr3-6.32O3- wt%6.3 wt% NiO/SiO NiO/SiO2-6002-500 4 4h hEC EC 97% 96% 76 1.4 wt%7.1 wt% TiO2 Cr-6.12O wt%3-6.3 wt% Cr2O NiO/SiO3/SiO2-4002-600 4 4h hEC BC 96% 95% 87 1.4 wt%1.4 wt% TiO2 TiO-6.12- wt%6.1 wt% Cr2 OCr32/SiOO3/SiO2-5002-400 4 4h hBC BC 95% 96% 98 1.4 wt%1.4 wt% TiO2 TiO-6.12- wt%6.1 wt% Cr2 OCr32/SiOO3/SiO2-6002-500 4 4h hBC BC 96% 95% * Catalysts90.1 g, urea1.4 1 wt% g, reaction TiO2-6.1 temperature wt% Cr2O 1703/SiO◦C.2- For600 MC synthesis,4 h 13.5 mL methanol,BC 6 95% h. For EC synthesis, 15 mL ethanol,* Catalysts 4 h. For 0.1BC g, urea synthesis, 1 g, reaction 20 mL temperature butanol, 4 h. 170 °C. For MC synthesis, 13.5 mL methanol, 6 h. For EC synthesis, 15 mL ethanol, 4 h. For BC synthesis, 20 mL butanol, 4 h. 2.2.4. Effect of Alcohol/Urea Molar Ratio 2.2.4. Effect of Alcohol/Urea Molar Ratio The effectThe of effect the methanol/ureaof the methanol/urea molar molar ratio ratio on on thethe yields of of 1 1andand by by-product-product 2 is examined2 is examined by by changingchanging the amounts the amounts of methanol of methanol while while maintaining maintaining constantconstant initial initial amounts amounts of urea of urea(0.017 (0.017 mol) mol) and 2.9 wt%and 2.9 TiO wt%2/SiO TiO2/SiO2-5002-500 (0.1 (0.1 g). g). The results results were were shown shown in Figure in 8. Figure The conversion8. The conversion of urea remains of urea remainsunchanged and and it is it normally is normally 100%. 100%. The yields The of yields 1 and ofby-product1 and by-product2 are found to2 are be st foundrongly to be strongly dependentdependent on on the the amounts amounts of of methanol methanol (methanol/urea (methanol/urea molar molar ratio). ratio). The yield The of yield 1 reached of 1 reached a a maximum with a methanol/urea molar ratio of 20. Further increase of the amount of methanol caused maximum with a methanol/urea molar ratio of 20. Further increase of the amount of methanol caused the more production of 2. Thus, the reasonable methanol/urea molar ratio is ~20. the more production of 2. Thus, the reasonable methanol/urea molar ratio is ~20.

MC yield 95 MC selectivity

MC yield or selectivity %

14 15 16 17 18 19 20 21 22 23 Methanol/urea molar ratio

0.9

0.8

0.7

0.6 DMC yield 0.5

DMC yieldDMC %

0.4

14 15 16 17 18 19 20 21 22 23 Methanol/urea molar ratio

Figure 8. Effect of methanol/urea molar ratio on yield and selectivity of 1 and 2. Reaction conditions: ◦ 1 g urea, 0.1 g TiO2/SiO2, 170 C, and 6 h. Catalysts 2018, 8, x FOR PEER REVIEW 10 of 18 Catalysts 2018, 8, 579 9 of 16 Figure 8. Effect of methanol/urea molar ratio on yield and selectivity of 1 and 2. Reaction conditions: 1 g urea, 0.1 g TiO2/SiO2, 170 °C, and 6 h. In the synthesis of 3 and 5, the results with varied ethanol or butanol/urea molar ratio were In the synthesis of 3 and 5, the results with varied ethanol or butanol/urea molar ratio were shown inshown Table 4in. Table It was 4. foundIt was found that the that suitable the suitable ethanol/urea ethanol/urea molarmolar ratio ratio is is 15.7 15.7 and and the thebutanol/urea butanol/urea molar ratiomolar is 13ratio for is the13 for synthesis the synthesis of EC of andEC and BC, BC respectively., respectively.

Table 4. EffectTable 4 of. Effect molar of molar ratio ofratio ethanol/urea of ethanol/urea or or butanol/urea butanol/urea on on synthesis synthesis of EC of or EC BC or *. BC *. Urea Y. Y. Dialkyl CatalystCatalyst Alcohol/UreaAlcohol/Urea Urea Con./% Y. Carbamate/% Y. Dialkyl Carbonate/% Con./% Carbamate/% Carbonate/% 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 10.4 >99 EC 91 DEC 0.8 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 10.4 >99 EC 91 DEC 0.8 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 15.7 >99 EC 97 DEC 0.5 2 3 2 7.1 wt%7.1 Cr wt%2O3-6.3 Cr wt%O -6.3 NiO/SiO wt% NiO/SiO2-500-500 2115.7 >99>99 EC 93EC 97 DEC DEC 0.5 0.3 1.4 wt%7.1 TiO wt%2-6.1 Cr wt%2O3 Cr-6.32O wt%3/SiO NiO/SiO2-5002-500 9.721 >99>99 BC 92EC 93 DEC DBC 0.3 2.5 1.4 wt%1.4 TiO wt%2-6.1 TiO wt%2-6.1 Cr2 Owt%3/SiO Cr22-500O3/SiO2-500 139.7 >99>99 BC 96BC 92 DBC DBC 2.5 0.5 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 16.2 >99 BC 94 DBC 0.3 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 13 >99 BC 96 DBC 0.5 ◦ 1.4 wt% TiO2-6.1* Catalysts wt% Cr2 0.1O3/SiO g, urea2-500 1 g, reaction16.2 temperature 170>99 C, reactionBC time 94 4 h. DBC 0.3 * Catalysts 0.1 g, urea 1 g, reaction temperature 170 °C, reaction time 4 h. 2.2.5. Effect of Catalyst Loadings 2.2.5. Effect of Catalyst Loadings The effect of the loading of catalyst 2.9 wt% TiO2/SiO2-500 on the yields of 1 and by-product 2 The effect of the loading of catalyst 2.9 wt% TiO2/SiO2-500 on the yields of 1 and by-product 2 is is examined at 170 ◦C using a methanol/urea molar ratio of 20. The results were given in Figure9. examined at 170 °C using a methanol/urea molar ratio of 20. The results were given in Figure 9. The The conversionconversion of ureaof urea without without catalystcatalyst is is 94% 94% with with 90% 90% yield yield of 1 and of 10.4%and of 0.4% 2. The of conversion2. The conversion of urea of urea increasesincreases to moreto more than than 99% 99% when when 0.050.05 g catalyst was was used used and and the thecorresponding corresponding yield yieldwas 97.5%. was 97.5%. When theWhen catalyst the catalyst loading loading increased increased to 0.1 to g,0.1 more g, more byproduct byproduct2 2 wouldwould be be generated generated and and the theyield yield of of 1 decreased1 decreased to 95%. to 95%.

99 98 97 96 95

94 MC yield 93 urea conversion MC selectivity 92 91 90 Catalysts 2018, 8, x FOR PEER REVIEW89 11 of 18 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Amount of catalyst (g) Urea conversion/MC yield, selectivity %

0.55

0.50

0.45

0.40 DMC yield

DMC yieldDMC % 0.35

0.30

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Amount of catalyst (g)

Figure 9. FigureEffect 9. ofEffect amount of amount of 2.9 of 2.9 wt% wt%TiO TiO22/SiO2-5002-500 on on urea urea conversion, conversion, yields of yields 1 and of2, 1 andand 2, and selectivityselectivity for 1for. Urea 1. Urea 1 g,1 g, methanol methanol 13.5 mL, mL, 170 170 °C◦, C,and and 6 h. 6 h.

As aboveAs mentioned above mentioned in Table in2 Table, almost 2, almost same same urea urea conversion, conversion, 1–6 1– yields6 yields could could bebe obtained in in the the absence of catalyst or SiO2 as the catalyst. Therefore, there was no catalyst was used for blank test absence offor catalyst the effect or of SiO amount2 as theon synthesis catalyst. of Therefore, MC, EC, and there BC. was no catalyst was used for blank test for the effect of amountThe results on synthesisfor the optimization of MC, EC,of the and reaction BC. of urea with ethanol or butanol were shown in The resultsTable 5. The for theconversion optimization of urea without of the catalyst reaction is 85% of urea or 92%, with respectively, ethanol and or butanol the yield wereof 3 is shown80%, in Table5. The4 is conversion 0.2%, 5 is 88% of and urea 6 withoutis 0.4%. The catalyst conversion is 85% of or urea 92%, increases respectively, to more than and 99% the with yield the of 3 is employment of 5 wt% catalyst. At the same time, the yield of 3 or 5 increased to 97% and 96% if 10 wt% catalysts were applied. With further increase of the amount of catalyst, the yield of 3 or 5 will decrease to 94% or 90%, respectively.

Table 5. Effect of catalyst amount on synthesis of EC and BC *. Entry Catalyst/% Alcohol Con./% Y. Carbamate/% Y. Dialkyl Carbonate/% 1a 0 Ethanol 85 EC 80 DEC 0.2 2 a 5 Ethanol >99 EC 90 DEC 0.4 3 a 10 Ethanol >99 EC 97 DEC 0.5 4 a 15 Ethanol >99 EC 94 DEC 0.55 5 b 0 Butanol 92 BC 88 DBC 0.4 6 b 5 Butanol >99 BC 92 DBC 2.5 7 b 10 Butanol >99 BC 96 DBC 0.6 8 b 15 Butanol >99 BC 90 DBC 2.5 * ethanol 15 mL, butanol 20 mL, urea 1 g, reaction temperature 170 °C, reaction time 4 h. a: Catalyst,

7.1 wt%Cr2O3-6.3wt%NiO/SiO2-500; b: Catalyst, 1.4 wt%TiO2-6.1wt%Cr2O3/SiO2-500.

2.2.6. Effect of Reaction Temperature The effect of reaction temperature on reaction behavior is examined by the reaction of urea (1 g) with methanol (13.5 mL) at temperatures from 150 °C to 180 °C by using 2.9 wt% TiO2/SiO2-500 (0.1 g) as catalyst. The results are shown in Figure 10. The yield of 1 increased from 75 to 97.5% with the increase of reaction temperature from 150 °C to 170 °C. However, it decreases again if the reaction temperature is above 170 °C. Obviously, more production of 2, 1%, was obtained at 180 °C.

Catalysts 2018, 8, 579 10 of 16

80%, 4 is 0.2%, 5 is 88% and 6 is 0.4%. The conversion of urea increases to more than 99% with the employment of 5 wt% catalyst. At the same time, the yield of 3 or 5 increased to 97% and 96% if 10 wt% catalysts were applied. With further increase of the amount of catalyst, the yield of 3 or 5 will decrease to 94% or 90%, respectively.

Table 5. Effect of catalyst amount on synthesis of EC and BC *.

Entry Catalyst/% Alcohol Con./% Y. Carbamate/% Y. Dialkyl Carbonate/% 1 a 0 Ethanol 85 EC 80 DEC 0.2 2 a 5 Ethanol >99 EC 90 DEC 0.4 3 a 10 Ethanol >99 EC 97 DEC 0.5 4 a 15 Ethanol >99 EC 94 DEC 0.55 5 b 0 Butanol 92 BC 88 DBC 0.4 6 b 5 Butanol >99 BC 92 DBC 2.5 7 b 10 Butanol >99 BC 96 DBC 0.6 8 b 15 Butanol >99 BC 90 DBC 2.5 * Ethanol 15 mL, butanol 20 mL, urea 1 g, reaction temperature 170 ◦C, reaction time 4 h. a: Catalyst, 7.1 wt% Cr2O3-6.3wt% NiO/SiO2-500; b: Catalyst, 1.4 wt% TiO2-6.1wt% Cr2O3/SiO2-500.

2.2.6. Effect of Reaction Temperature The effect of reaction temperature on reaction behavior is examined by the reaction of urea (1 g) ◦ ◦ with methanol (13.5 mL) at temperatures from 150 C to 180 C by using 2.9 wt% TiO2/SiO2-500 (0.1 g) as catalyst. The results are shown in Figure 10. The yield of 1 increased from 75 to 97.5% with the increase of reaction temperature from 150 ◦C to 170 ◦C. However, it decreases again if the reaction temperature is above 170 ◦C. Obviously, more production of 2, 1%, was obtained at 180 ◦C. Catalysts 2018, 8, x FOR PEER REVIEW 12 of 18

100

85 MC yield Urea conversion 80 MC selectivity

150 155 160 165 170 175 180 Reaction temperature oC Urea conversion/ MC yield, selectivity %

1.0

0.8

0.6 DMC yield

0.4

0.2

DMC yieldDMC %

0.0

150 155 160 165 170 175 180 o Reaction temperature C Figure 10.FigureEffect 10 of. Effect reaction of reaction temperature temperature on on urea urea conversion; conversion; yields of of 1 1andand 2, and2, and selectivity selectivity for 1. for 1. Urea 1 g, methanol 13.5 mL, 0.1 g 2.9 wt% TiO2/SiO2-500, reaction time: 6 h. Urea 1 g, methanol 13.5 mL, 0.1 g 2.9 wt% TiO2/SiO2-500, reaction time: 6 h. In the reaction of urea with ethanol or butanol, it was also very sensitive to the reaction temperature, Table 6. At 150 °C, 3 and 5 are the only product with yields of 70% and 78%. Then the yields of 3 and 5 were sharply enhanced to 97% and 96% from 70% and 78% when the reaction temperature was increased to 170 °C from 150 °C. The formation of by-product 4 or 6 is observed at reaction temperatures higher than 160 °C. The yield of 4 and 6, 2% and 2.5%, was obtained at 180 °C. Therefore, the optimum reaction temperature is determined to be 170 °C.

Table 6. Effect of reaction temperature on synthesis of EC and BC *. Y. Y. Dialkyl Catalyst T/°C Con./% Carbamate/% Carbonate/% 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 150 72 EC 70 DEC 0 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 160 80 EC 78 DEC 0.3 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 170 >99 EC 97 DEC 0.5 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 180 >99 EC 94 DEC 2 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 150 80 BC 78 DBC 0 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 160 92 BC 90 DBC 0.4 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 170 >99 BC 96 DBC 0.6 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 180 >99 BC 92 DBC 2.5 * catalyst 0.1g, urea 1g, 4 h, 15 mL ethanol or 20 mL butanol.

Catalysts 2018, 8, 579 11 of 16

In the reaction of urea with ethanol or butanol, it was also very sensitive to the reaction temperature, Table6. At 150 ◦C, 3 and 5 are the only product with yields of 70% and 78%. Then the yields of 3 and 5 were sharply enhanced to 97% and 96% from 70% and 78% when the reaction temperature was increased to 170 ◦C from 150 ◦C. The formation of by-product 4 or 6 is observed at reaction temperatures higher than 160 ◦C. The yield of 4 and 6, 2% and 2.5%, was obtained at 180 ◦C. Therefore, the optimum reaction temperature is determined to be 170 ◦C.

Table 6. Effect of reaction temperature on synthesis of EC and BC *.

Catalyst T/◦C Con./% Y. Carbamate/% Y. Dialkyl Carbonate/%

7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 150 72 EC 70 DEC 0 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 160 80 EC 78 DEC 0.3 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 170 >99 EC 97 DEC 0.5 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 180 >99 EC 94 DEC 2 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 150 80 BC 78 DBC 0 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 160 92 BC 90 DBC 0.4 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 170 >99 BC 96 DBC 0.6 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 180 >99 BC 92 DBC 2.5 * Catalyst 0.1 g, urea 1 g, 4 h, 15 mL ethanol or 20 mL butanol. Catalysts 2018, 8, x FOR PEER REVIEW 13 of 18 2.2.7. Effect of Reaction Time 2.2.7. Effect of Reaction Time The resultsThe results for the for synthesis the synthesis of MC of MC with with varied varied reaction reaction time timewere were shown in in Figure Figure 11. 11 The. The yield yield of 1 increasedof 1 increased from 94.5 from to 94.5 97.5% to with97.5% the with increasing the increasing of reaction of reaction time time from from 4 h to 4 6h h.to However,6 h. However, it would it decreasewould if longer decrease reaction if longer time reac wastion applied. time was applied.

99.0 98.5 98.0 97.5 97.0 96.5 96.0 95.5 MC yield Urea conversion 95.0 MC selectivity 94.5 94.0 93.5 4 5 6 7 8 Reaction time (h)

Urea conversion/ MC yield, selectivity %

1.4

1.2

1.0

DMC yield 0.8

0.6

DMC yield DMC % 0.4

0.2 4 5 6 7 8 Reaction time (h) Figure 11.FigureEffect 11. ofEffect reaction of reaction time time on urea on urea conversion, conversion, yields yields of of1 1and and 22,, andand selectivity selectivity for for 1. 1Urea. Urea 1 g, 1 g, ◦ methanolmethanol 13.5 mL, 13.5 0.1 mL, g 2.90.1 wt%g 2.9 wt% TiO2 TiO/SiO2/SiO2-500,2-500, 170 170 C.°C.

In the reaction of urea and ethanol or butanol, the similar results were observed, Table 7. The yield of 3 and 5 increased from 93% to 97% and 94% to 96% if prolonged the reaction time to 4 h from 3 h. If the reaction time was longer than 4 hours, more by-product 4 or 6 would be produced.

Table 7. Effect of reaction time on synthesis of EC and BC *.

Y. Dialkyl Catalyst T/h Con./% Y. Carbamate/% Carbonate/%

7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 3 96 EC 93 DEC 0.2 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 4 >99 EC 97 DEC 0.5 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 5 >99 EC 94 DEC 1.3 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 3 97 BC 94 DBC 0.4 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 4 >99 BC 96 DBC 0.6 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 5 >99 BC 95 DBC 1 * catalyst 0.1 g, urea 1 g, 170 °C, 15 mL ethanol or 20 mL butanol.

Catalysts 2018, 8, 579 12 of 16

In the reaction of urea and ethanol or butanol, the similar results were observed, Table7. The yield of 3 and 5 increased from 93% to 97% and 94% to 96% if prolonged the reaction time to 4 h from 3 h. If the reaction time was longer than 4 hours, more by-product 4 or 6 would be produced.

Table 7. Effect of reaction time on synthesis of EC and BC *.

Catalyst T/h Con./% Y. Carbamate/% Y. Dialkyl Carbonate/%

7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 3 96 EC 93 DEC 0.2 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 4 >99 EC 97 DEC 0.5 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 5 >99 EC 94 DEC 1.3 Catalysts1.4 wt% 2018 TiO,2 8-6.1, x FOR wt% PEER Cr2O 3REVIEW/SiO2-500 3 97 BC 94 DBC 0.4 14 of 18 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 4 >99 BC 96 DBC 0.6 1.4 wt% TiO -6.1 wt% Cr O /SiO -500 5 >99 BC 95 DBC 1 2.2.8. Recyclability2 Test2 3 2 * Catalyst 0.1 g, urea 1 g, 170 ◦C, 15 mL ethanol or 20 mL butanol. As a potential reaction process in industry, the reusability of the catalyst some time was more 2.2.8.important Recyclability than the Test initial catalytic activity. The reusability performance for methyl carbamate synthesis was firstly performed at 170 °C for 6 h with 0.1 g of 2.9 wt% TiO2/SiO2-500 catalyst. As As a potential reaction process in industry, the reusability of the catalyst some time was more shown in Figure 12, MC, EC, and BC yields higher than 95.8%, 94%, and 94% were still remained important than the initial catalytic activity. The reusability performance for methyl carbamate synthesis even after 10 runs, 20 runs, and 10 runs for the three different catalysts, and MC, EC, and BC yield was ﬁrstly performed at 170 ◦C for 6 h with 0.1 g of 2.9 wt% TiO /SiO -500 catalyst. As shown in decreased 1.7%, 2%, and 2% after 10 runs. It is to say these silica2 gel 2supported catalyst system is Figure 12, MC, EC, and BC yields higher than 95.8%, 94%, and 94% were still remained even after active and reusable. 10 runs, 20 runs, and 10 runs for the three different catalysts, and MC, EC, and BC yield decreased 1.7%, But based on the blank test, there are about 10% MC, 18% EC and 8% BC yields due to the catalyst. 2%, and 2% after 10 runs. It is to say these silica gel supported catalyst system is active and reusable. MC, EC and BC yield decrease after 10 runs were actually corresponds to a 17%, 11%, and 25% for But based on the blank test, there are about 10% MC, 18% EC and 8% BC yields due to the catalyst. the catalytic performance, possible reason was that some mass loss of catalysts during the reaction MC, EC and BC yield decrease after 10 runs were actually corresponds to a 17%, 11%, and 25% for because of mechanical wear. There should be a lot of work need to be done for avoiding mechanical the catalytic performance, possible reason was that some mass loss of catalysts during the reaction wear in our future work. because of mechanical wear. There should be a lot of work need to be done for avoiding mechanical wear in our future work.

78 MC yield

Yield % 72 EC yield BC yield 66

60 0 2 4 6 8 10 12 14 16 18 20 22 Times of using catalysts

FigureFigure 12. 12. Reusability testing of of catalysts catalysts 2.9 2.9 wt% wt% TiO TiO2/SiO2/SiO2-5002-500 for forsynthesis synthesis of MC, of MC,7.1 wt%Cr 7.1 wt%2O3-

Cr6.32O wt%3-6.3 NiO/SiO wt% NiO/SiO2-500 for2-500 synthesis for synthesis of EC and of EC1.4 wt% and 1.4TiO wt%2-6.1 TiOwt%2-6.1 Cr2O wt%3/SiO Cr2-5002O3 /SiOfor synthesis2-500 for of synthesisBC. of BC. 2.2.9. Scaling Up in the 2 L Autoclave 2.2.9. Scaling Up in the 2 L Autoclave Based on the optimized reaction conditions obtained from the results in 90 mL autoclave, synthesis Based on the optimized reaction conditions obtained from the results in 90 mL autoclave, of MC, EC, and BC with urea and methanol (or ethanol and butanol) over 2.9 wt% TiO /SiO -500, synthesis of MC, EC, and BC with urea and methanol (or ethanol and butanol) over2 2.92 wt% 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500, and 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 as catalysts TiO2/SiO2-500, 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500, and 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 as were further tested in a 2 L autoclave. It was shown that 96–97% isolated yields were achieved. catalysts were further tested in a 2 L autoclave. It was shown that 96–97% isolated yields were In comparison with the results obtained from 90 mL autoclave, almost the same catalytic performance achieved. In comparison with the results obtained from 90 mL autoclave, almost the same catalytic was achieved in 2 L autoclave. That means our catalyst system is relatively easy to be scaled up. performance was achieved in 2 L autoclave. That means our catalyst system is relatively easy to be scaled up.

3. Experimental

3.1. Materials All the chemicals were of A.R. degree and were directly used without further treatment. Silica gel pellets: pore volume = 0.60–0.85 mL/g, pore diameter = 4.5–7 nm, BET surface area = 450–650 m2/g, average diameter of the beads = 3 mm).

3.2. Preparation of Catalysts

TiO2/SiO2: 10 mL 26–27 wt% HNO3 aqueous solution was added into a 100 mL beaker containing 3 mL tetrabutyl titanate (Ti(OC4H9)4) to get a homogeneous solution (pH value ~1–2). Then 10 g silica

Catalysts 2018, 8, 579 13 of 16

3. Experimental

3.2. Preparation of Catalysts

TiO2/SiO2: 10 mL 26–27 wt% HNO3 aqueous solution was added into a 100 mL beaker containing 3 mL tetrabutyl titanate (Ti(OC4H9)4) to get a homogeneous solution (pH value ~1–2). Then 10 g silica gel pellets was added after being calcined at 600 ◦C for 2 h and impregnated at room temperature for ~4 h. The silica gel pellets with complete absorption of the aqueous solution were dried at 90 ◦C for 4 h and then calcined at 500 ◦C for 4 h. The raw catalyst and 50 mL methanol were introduced into a 250 mL flask and refluxed at 80 ◦C for 8 h. Then it was filtrated and dried at 100 ◦C for 4 h in air. The resulted catalyst was denoted as 2.2 wt% TiO2/SiO2-500. Catalysts 2.9 wt% TiO2/SiO2-500 and 3.6 wt% TiO2/SiO2-500 were prepared with the same procedure. 2.9 wt% TiO2/SiO2-400 and 2.9 wt% TiO2/SiO2-600 were achieved by variation of calcination temperature. TiO2-Cr2O3/SiO2: 10 mL 26–27 wt% HNO3 aqueous solution was added into a 100 mL beaker containing 4 mL Ti(OC4H9)4 to get a homogeneous solution (pH value ~1–2). Then 3.85 g Cr(NO3)3·9H2O was further added (pH value ~2–3). Then 10 g silica gel pellets was added after being calcined at 600 ◦C for 2 h and impregnated at room temperature for ~4 h. The resulted catalyst precursor was dried at 90 ◦C for 4 h and then calcined at 500 ◦C for 4 h. The raw catalyst and 50 mL butanol were introduced into a 250 mL flask and refluxed at 120 ◦C for 8 h. Then it was filtrated and dried at 100 ◦C for 4 h in air. The resulted catalyst was denoted as 1.7 wt% TiO2-4.9 wt% Cr2O3/SiO2-500. Catalysts 1.4 wt% TiO2-6.3 wt% Cr2O3/SiO2-500 and 1.0 wt% TiO2-8.5 wt% Cr2O3/SiO2-500 were prepared by variation of the mass of Cr(NO3)3·9H2O with the same procedure. 1.4 wt% TiO2-6.3 wt% Cr2O3/SiO2-400 and 1.4 wt% TiO2-6.3 wt% Cr2O3/SiO2-600 were achieved by variation of calcination temperature. Cr2O3-NiO/SiO2: 7.69 g Cr(NO3)3·9H2O and 4.0 g Ni(NO3)2·6H2O were respectively added into a 100 mL beaker containing 10 mL H2O. After complete dissolution, the pH value of the solution was 3–3.5. Then 10 g silica gel pellets was added after being calcined at 600 ◦C for 2 h and then impregnated in the above solutions at room temperature for 4 h. The resulted catalyst precursor was dried at 90 ◦C for 4 h and then calcined at 500 ◦C for 4 h. The raw catalyst and 50 mL ethanol were introduced into a 250 mL flask and refluxed at 100 ◦C for 8 h. Then it was filtrated and dried at 100 ◦C for 4 h in air. The resulted catalyst was denoted as 3.5 wt% Cr2O3-8.0 wt% NiO/SiO2-500. Catalysts 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-500 and 9.0 wt% Cr2O3-3.6 wt% NiO/SiO2-500 were prepared by variation of the mass of Cr(NO3)3·9H2O with the same procedure. 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-400, and 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-600 were achieved by variation of calcinations temperature.

3.3. Characterization of Catalysts

3.3.1. Elemental Analysis (EA) The loadings of Ti, Cr, and Ni were determined on an inductively coupled plasma-atomic emission spectrometry (ICP-AES) (ThermoElemental Company Madison, WI, USA) by dissolving the samples in aqueous nitric acid.

3.3.2. BET Analysis The BET surface area, pore volume and average pore diameter of the catalysts were measured by physisorption of N2 at 76 K using a Micromeritics ASAP 2010 (San Francisco, CA, USA). Before the measurement, the samples were degassed at 200 ◦C for 12 h to remove adsorbed gases from the Catalysts 2018, 8, 579 14 of 16 catalyst surface. The isotherms were elaborated according to the BET method for surface area calculation, with the Horwarth-Kavazoe and BJH methods used for micropore and mesopore evaluation, respectively.

3.3.3. X-ray Photoelectron Spectroscopy (XPS) Measurement The surface composition was analyzed with a VG ESCALAB 210 instrument (Madison, WI, USA) with an Mg anode and a multi-channel detector. Charge referencing was measured against adventitious carbon (C 1s, 285.0 eV). The surface atomic-distributions were determined from the peak areas of the corresponding lines using a Shirley-type background and empirical cross-section factors for XPS.

3.3.4. X-ray Diffraction XRD Analysis X-ray diffraction (XRD) patterns were performed on a Siemens D/max-RB powder X-ray Diffractometer (Karlsruhe, Germany). Diffraction patterns recorded with Cu Kα radiation (40 mA, 40 kV) over a 2θ range of 15–70◦ in steps of 0.04◦ with a count time of 1 s.

3.3.5. Temperature-Programmed Desorption (TPD) Study

TPD analysis with NH3 was carried out on an improved GC112 instrument (Shanghai, China) to study the acidity of the catalyst surface. In a typical experiment, 250 mg of dried sample (dried at 393 K for 5 h) was taken in a U-shaped quartz sample tube. The sample was pretreated by passing argon over the catalysts at a flow of 50 mL/min at 773 K for 1 h. After pretreatment, the sample was cooled to ambient temperature and treated in a flow of 10% NH3-Ar mixture (75 mL/min) for 1 h at 313 K. After being flushed with pure argon (50 mL/min) at 373 K for 1 h to remove physisorbed NH3, TPD analysis was carried out from 373 K to 773 K with a heating rate of 10 ◦C/min in an argon flow of 50 mL/min.

3.3.6. EPMA Analysis Ti, Cr, and Ni distributions inside the silica gel pellets were analyzed with a JCXA-733 Super Probe Analyzer (Tokyo, Japan).

3.4. Reaction Conditions for Carbamates Synthesis

3.4.1. General Precedure for Synthesis of Alkyl Carbamate in a 90 mL Autoclave A total of 13.5 mL methanol (or 15 mL ethanol, or 20 mL butanol), 1 g urea, and 0.1 g of catalyst were successively introduced into a 90 mL autoclave inside a glass tube. After reacting at 170 ◦C for 4–6 h, the autoclave was cooled to room temperature for qualitative and quantitative analyses. In order to achieve higher yield for desired product, ammonia gas produced during the reaction was released 2–3 times. Ammonia produced in the MC (EC, BC) synthesis was vented as followed: ﬁrstly, the autoclave was heated to 170 ◦C within 20 min, after reacted 1.5 h (1 h, 1 h), the reaction was stopped and cooled to the room temperature by cool water; the pressure of the headspace was 0.2 MPa, the valve was opened. Then, the autoclave was heated to 170 ◦C within 20 min, after reacted 2.5 h (1.5 h, 1.5 h), the reaction was stopped again and cooled to the room temperature by cool water; the pressure of the headspace was 0.2 MPa, the valve was opened. Then, the autoclave was heated to 170 ◦C within 20 min, after reacted 1.5 h, the reaction was stopped again and cooled to the room temperature by cool water. After each reaction, the catalyst was separated by ﬁltration and washed by methanol (3 × 20 mL). After it was dried in air, it was directly reused for the next run without any other treatment. The reaction temperature was measured with a thermometer. Catalysts 2018, 8, 579 15 of 16

3.4.2. A Demonstration for the Scaling up of the Synthesis of Carbamates A total of 1000 mL methanol (or ethanol, or butanol), 50–70 g urea and 5–7 g catalysts were successively introduced into a 2 L autoclave with a mechanical stirrer and then sealed. After reacted at 170 ◦C for 4 h, the autoclave was cooled to room temperature for qualitative and quantitative analyses. During the reaction, the autoclave was opened 2–3 times to release the ammonia produced. After it was cooled to room temperature, the catalyst was removed by ﬁltration and the desired product, i.e., raw MC, EC, or BC, could be obtained after the alcohol was removed by distillation. The main impurity inside the product is unreacted urea. After dissolving the crude product in diethyl ether, the urea could be removed by ﬁltration easily. Then, white solid MC, EC, or BC with GC purity > 98% could be obtained. Qualitative analyses were conducted with a HP 6890/5973 GC-MS (Santa Clara, CA, USA) with a 30 m × 0.25 mm × 0.33 µm capillary column with chemstation containing a NIST mass spectral database. Quantitative analysis was conducted with an Agilent 6820 GC (Santa Clara, CA, USA) with a 30 m × 0.25 mm × 0.33 µm capillary column (FID detector) using an external-standard method.

3.5. Recyclability of the Catalyst The solid catalyst was recovered by simple ﬁltration, being washed with methanol and dried in air, and then directly reused for the next run without any other treatment.

4. Conclusions In conclusion, a series of silica gel supported catalysts were successfully prepared, characterized, and employed in the synthesis of alkyl carbamates using urea as the carbonyl source. Up to 96–98% GC yield was achieved for the synthesis of MC, EC, and BC. Reusability testing showed that these catalysts were enough stable and without obvious deactivation even after being reused for 10 times. Reaction investigation in 2 L autoclave suggested that this system is easily to be scaled up. Therefore, it should be helpful to develop industrially applicable catalysts for the synthesis of alkyl carbamate from urea and alcohol.

Author Contributions: Y.M., L.W., and R.Z. performed the experiments; Y.M. and X.Y. conceived the concept. All the authors contributed to the writing of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (21808192, U1703128, 21506246), and Chinese Government “Thousand Talent” Program (Y42H291501). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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