Environmental-Friendly Synthesis of Alkyl

catalysts 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 2 3 4 5

*

School of Chemical Engineering, Chongqing Chemical Industry Vocational College, Chongqing 401220, China; [email protected] Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Department of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830011, China Institute of Resources and Environment Science, Xinjiang University, Urumqi 830011, China 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 , Cr2 O3 -NiO/SiO2 , and TiO2 -Cr2 O3 /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 , Cr2 O3 -NiO/SiO2 , and TiO2 -Cr2 O3 /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 -Cr2 O3 /SiO2 ; Cr2 O3 -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

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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 catalysts such such as as ZnO, ZnO, MgO, CaO, and transition transition metal metal oxides oxides have have been beenused usedfor forthe thesynthesis synthesisof ofcarbamates carbamates[4,5, [4,5,21–28]. 21–28]. Unfortunately, the low yield towards the desired products and the reusability of the catalyst have been significant until now. In view of theofpractical application, the development of catalysts, significantproblems problems until now. In view the practical application, the development of which should be simple, active, and easily reusable, for the synthesis of alkyl carbamates from urea is catalysts, which should be simple, active, and easily reusable, for the synthesis of alkyl carbamates highly desirable. from urea is highly desirable. Higher than 80% O-substituted carbamate could be achieved Moreover, Moreover, a 10% difference of yield was observed our lab, which motivated us us to study the the possible reason for this observedin indifferent differentautoclaves autoclavesinin our lab, which motivated to study possible reason for reaction; the composites of autoclave were 1Cr9Ni18Ti, which may be played a key for urea conversion this reaction; the composites of autoclave were 1Cr9Ni18Ti, which may be played a key for urea and alkyl carbamate Therefore, a series of catalysts containing Ti, Cr, Ni was prepared bywas the conversion and alkylyield. carbamate yield. Therefore, a series of catalysts containing Ti, Cr, Ni impregnation method and used for synthesis alkylfor carbamates. catalytic performance catalyst prepared by the impregnation method andofused synthesisThe of alkyl carbamates. Theofcatalytic for MC synthesis is littlefor effect, while an is obviously effect were BC were and EC synthesis. performance of catalyst MC synthesis little effect, while an observed obviouslyfor effect observed for Three different catalysts were selected by screening the catalysts containing Ti, Cr, and Ni. Three BC and EC synthesis. Three different catalysts were selected by screening the catalysts containing Ti, different catalysts were obtained for were MC, EC, and BC Cr, and Ni. Three different catalysts obtained forsynthesis, MC, EC, respectively. and BC synthesis, respectively. More than 98.7% MC yield could be achieved in Ref. [27], EC andBC BCwas wasnot notstudied; studied; the the catalyst catalyst than 98.7% MC yield could be achieved in ref 27, EC and of [28] [29] was a nano metal oxide and R3N, which was a mixed catalyst, and R3N was dissolved in the reaction system. Therefore, this this kind kindof ofreaction reactionwas wasstudied studiedininthis this work over supported catalysts containing work over supported catalysts containing Ti, Ti, Here, report ourresults resultsabout aboutthe thepreparation, preparation, characterization characterization and and usage usage of Cr, Cr, andand Ni.Ni. Here, wewe report our TiO Cr22OO3-NiO/SiO , andTiO TiO2-Cr synthesisofofmethyl methylcarbamate carbamate (MC), (MC), TiO22/SiO /SiO22, ,Cr 2, 2and 2O 3/SiO 2 catalysts forfor synthesis 3 -NiO/SiO 2 -Cr 2O 3 /SiO 2 catalysts ethyl carbamate (EC), and butyl carbamate (BC) (Scheme (Scheme 1). 1). H2NCONH2

MeOH

H2NCO2Me

MeOH MeOCO2Me

1

2

EtOH

EtOH

H2NCOOEt

H2NCONH2

EtOCO2Et

3 H2NCONH2

BuOH

4 BuOH

H2NCO2Bu

BuOCO2Bu

5

6

Scheme 1. 1. Synthesis of methyl, ethyl, ethyl, butyl butyl carbamate/dimethyl, carbamate/dimethyl, diethyl, Scheme diethyl, and dibutyl carbonate.

2. Results and and Discussion Discussion 2. Results 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 Table 1. prepared and characterized in this work are shown in Table 1. Table 1. Typical physicochemical properties of TiO2 /SiO2 , Cr2 O3 -NiO/SiO2 , and TiO2 -Cr2 O3 /SiO2 Table 1. Typical physicochemical properties of TiO2/SiO2, Cr2O3-NiO/SiO2, and TiO2-Cr2O3/SiO2 catalysts with different composition. catalysts with different composition. Catalyst

Catalyst SiO2 2.2 wt% TiO2 /SiO2 -500 2.9 wt% TiO2 /SiO2 -500 SiO 2 3.6 wt% TiO 2 /SiO2 -500 wt% TiO Cr2 O23/SiO -8.0 wt% 2.23.5wt% 2-500 NiO/SiO2 -500 2.97.1wt% 2-500 wt% TiO Cr2 O23/SiO -6.3 wt% NiO/SiO -500 2-500 3.6 wt% TiO22/SiO

3.5 wt% Cr2O3-8.0 wt% NiO/SiO2-500

M/Si M/Si atom% Bulk

atom% Bulk

Ti/Si 3.8

Ti/Si103.8 Cr/Si Ni/Si 7.9

Surface Area/m2 /g

Surface 601 Area/m2/g

Pore volume Pore /cm3 /g

601

volume 0.74 /cm3/g 0.69 0.74

554

0.69

554

370

0.47

B. E. /eV B. E. /eV Ti 2p3/2 459.1

Ti 3/2 459.1 Cr2p 2p3/2 577.6 Ni 2p3/2 856.6

M/Si atm% Surface

M/Si atm% Surface Ti/Si 3.0

3.0 CrTi/Si /Si 12.6 Ni/Si 7.8


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7.1 wt% Cr2O3-6.3 wt% Cr/Si 10 NiO/SiO 2-500 Ni/Si 7.9 Catalysts 2018, 8, 579 9.0 wt% Cr2O3-3.6 wt% NiO/SiO2-500 1.7 wt% TiO2-4.9 wt% Cr2O3/SiO2-500 M/Si Catalyst 1.4 wt% TiO2-6.1 wt% atom% Ti/Si Bulk2.0 Cr2O 2-500 Cr/Si 7.9 9.0 wt% Cr32/SiO O3 -3.6 wt% NiO/SiO2 -500 1.0 wt% TiO2-8.5 wt% 1.7 wt% TiO2 -4.9 wt% Cr2 O 2O 3 2-500 Cr /SiO 3 /SiO 2 -500

370

Cr 2p3/2 577.6 Ni 2p3/2 856.6

0.47

Cr /Si 12.6 Ni/Si 7.8 3 of 16

Table 1. Cont. Surface Area/m2 /g

Pore volume /cm3 /g

434

1.4 wt% TiO2 -6.1 wt% Ti/Si 2.0 434 Cr2 O3 /SiO2 -500 Cr/Si 7.9 BET surface 1.0 wt% TiO2 -8.5areas wt% and pore volumes of the 2 O3 /SiO 2 -500 surfaceCrareas decreased with the increasing of

B. E.

0.49

Ti 2p 3/2 458.7 /eV Cr 2p3/2 577.2

0.49

Ti 2p3/2 458.7 Cr 2p3/2 577.2

M/Si atm% Ti/Si 2.4 Surface

Cr/Si 8.6

Ti/Si 2.4 Cr/Si 8.6

catalysts are lower than that of pure SiO 2. The BET 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. Thesurface XPS analysis confirmed that the bond valence titanium on the surface 2.9 BET wt% BET areas and pore volumes of the catalysts areof lower than that of pure SiO2of . The 4+ (BE of Ti 2p3/2 = 459.1 eV) (Figure 1a and Table 1). The binding energies of Ti TiO 2 /SiO 2 -500 was Ti surface areas decreased with the increasing of the loadings of oxides of transition metal and the surface 2p3/2ofand Cr 2p3/2 are 458.7 eVrange and 577.2 eV, respectively, area the catalysts were in the of 350–540 m2 /g, Tablein 1. 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500 (Figure Table 1). The binding Cr 2p3/2ofand Ni 2p3/2onarethe 577.6 eV and 856.6 eV, The 1b XPSand analysis confirmed that energies the bondofvalence titanium surface of 2.9 wt% 4+ respectively, in 7.1 wt% Cr 2 O 3 -6.3 wt% NiO/SiO 2 -500 (Figure 1c and Table 1). Thus, the corresponding TiO2 /SiO2 -500 was Ti (BE of Ti 2p3/2 = 459.1 eV) (Figure 1a and Table 1). The binding energies of Ti states of these surface Ti, Cr, and Ni species are Ti+4, Cr3+, and Ni2+. 2pchemical 3/2 and Cr 2p3/2 are 458.7 eV and 577.2 eV, respectively, in 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500

(Figure 1b and Table 1). The binding energies of Cr 2p3/2 and Ni 2p3/2 are 577.6 eV and 856.6 eV, respectively, in 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 (Figure 1c and Table 1). 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

KCPS

Ti 2p1 2.2

2.0

1.8 475

470

465

460

455

450

Binding Energy (eV) 13.8

6.4

Cr2p3 6.2

Ni2p3

Cr2p1

b

13.6

Ni2p1

KCPS

KCPS

6.0 5.8 5.6

13.4

13.2

5.4 5.2 595

590

585

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570

13.0 900

890

880

870

860

Binding Energy (eV)

Binding Energy (eV)

Figure 1. Cont.

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2.6

Ti2p3

Cr2p1

2.6

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KCPS KCPS

KCPS KCPS

2.4

Ti2p1

2.2

2.2

Cr2p3

6.8

470

465

460

455

450

Binding Energy (eV) 470

465

460

455

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Cr2p3 c Cr2p1

6.4 6.6

c

6.2 6.4 6.0 6.2 5.8 6.0 595

590

585

580

575

570

Binding Energy (eV)

5.8 595

590

Binding Energy (eV)

585

580

575

570

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 wt% TiO 2-6.1patterns wt% Cr2O 2-500. Figure 1. XPS of3/SiO (a) 2.9 wt% TiO2 /SiO2 -500; (b) 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500; 1. TiO XPS2 -6.1 patterns wt%TiO 2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; (c) 1.4 (c)Figure 1.4 wt% wt% of Cr2(a) O32.9 /SiO 2 -500. wt% 2-6.1 wt% Cronly 2O3/SiO 2-500. XRDTiO showed that weak diffraction peaks of body-centered tetragonal TiO2 (anatase) could

showed that only weak diffraction peaks ofwhich body-centered TiO be 2be XRD observed in 2.9 wt% TiO 2/SiO 2-500 (Figure 2a), suggeststetragonal that TiO2 in the 2.9 wt% could TiO2/SiO 2 (anatase) XRD showed that only weak diffraction peaks of body-centered tetragonal TiO 2 (anatase) could observed in 2.9 wt% TiO /SiO -500 (Figure 2a), which suggests that TiO in the 2.9 wt% TiO /SiO -500 500 is poorly crystallized. Surprisingly, although the Cr2O3 content2reaches 7.1 wt%, no 2 2 2 diffraction 2 observed TiO2/SiOin 2-500 (Figure which that TiOand 27.1 in the 2.9weak wt% TiO 2/SiO2isbe poorly Surprisingly, although the Cr2 O content reaches wt%, no diffraction peaks ofcrystallized. Cr2in O32.9 arewt% observable 7.1 wt% Cr2a), 2O3-6.3 wt% NiO/SiO 2-500 only diffraction 3suggests 500 isofpoorly Surprisingly, although Cr2NiO/SiO OIn 3 content reaches 7.1 wt%, diffraction peaks Crfaced-centered are observable inNiO 7.1 wt% Cr2 O(Figure wt% and only weaknodiffraction peaks of cubic appeared 2b). reverse, only weak diffraction peaks of 2 O3 crystallized. 3 -6.3the 2 -500 peaks of Cr 2 O 3 are observable in 7.1 wt% Cr 2 O 3 -6.3 wt% NiO/SiO 2 -500 and only weak diffraction peaks of faced-centered cubic NiO appeared (Figure 2b). In reverse, only weak diffraction peaks rhomb-centered rhombohedral Cr2O3 was observed and no diffraction peaks of TiO2 appeared forof1.4 peaks of faced-centered NiO appeared 2b).results Indiffraction reverse, only weak rhomb-centered rhombohedral Cr2 O(Figure and no peaks of appeared forof wt% TiO 2-6.1 wt% Cr2O3cubic /SiO2-500 2c).(Figure All these indicated that CrTiO 2diffraction O3 2and TiOpeaks 2 species 3 was observed rhomb-centered rhombohedral Cr 2O3 was observed and no diffraction peaks of TiO 2 appeared for 1.4 1.4 wt% TiO2dispersed. -6.1 wt% Cr2 O3 /SiO -500 (Figure 2c). All these results indicated that Cr O and TiO are highly 2 2 3 2 wt% TiO 2 -6.1 wt% Cr 2 O 3 /SiO 2 -500 (Figure 2c). All these results indicated that Cr 2 O 3 and TiO 2 species species are highly dispersed. are highly dispersed.

Intensity (a.u) (a.u) Intensity

4000 3500 4000 3000 3500 2500 3000 2000 2500 1500 2000 1000 1500 500 1000 0 500

TiO2 d=3.53 TiO2 d=3.53

a

TiO2 d=2.38 TiO2 d=1.90 TiO2 d=2.38 TiO2 d=1.90 10

20

0 10

20

Intensity (a.u) (a.u) Intensity

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4000 3500 4000 3000 3500 2500 3000 2000 2500 1500 2000 1000 1500 500 1000 0 500

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b NiO d=2.40 NiO d=2.08 b NiO d=1.47 NiO d=2.40 NiO d=2.08 NiO d=1.47

10

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Figure 2. Cont. 0

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3000

c

20002500

c

2000

Intensity (a.u)

Intensity (a.u)

25003000

Cr2O3 d=2.68 Cr2Cr O32O d=2.68 3 d=2.48 Cr2O3 d=1.68 Cr O d=2.48

1500

1500

2 3

1000

Cr2O3 d=1.68

1000

500

500

0 0

10 10 2020

30 30

40 40

5050 60 60 70 7080

8090

90

Position [0[02Theta] Position 2Theta]

Figure 2. XPS patterns of (a) 2.9 wt% TiO2 /SiO2 -500; (b) 7.1 wt% Cr2 O3 -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) Figure 2. wt% XPS TiO patterns of Cr (a)2Cr 2.9Owt%TiO 2/SiO2-500; (b) 7.1 wt%Cr2O3-6.3 wt% NiO/SiO2-500; and (c) and (c) 1.4 TiO -6.1wt% wt% 32/SiO 1.4wt% 22 -6.1 O32/SiO -500. 2 -500. 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500.

Pure silica TiO -500, 1.4 wt% TiO -6.1 wt% Cr2-500 O3 /SiO wt% 2 /SiO 2 -500 Puregel, silica2.9 gel,wt% 2.9 wt% TiO 2/SiO22-500, 1.4 wt% TiO2-6.12wt% Cr2O3/SiO2 and 7.1 wt%and Cr2O7.1 3Cr2 O3 -6.3 wt% NiO/SiO -500 were further characterized by TPD-NH . It indicated significant 6.3 wt% 2-500 further by TiO TPD-NH . It indicated difference in Cr2O32 were 3significant Pure silicaNiO/SiO gel, 2.9 wt% TiO 2/SiO2characterized -500, 1.4 wt% 2-6.13wt% Cr2O3/SiO 2-500 and 7.1 wt% acidities among the catalysts, 3. Only one NH adsorption the weak acid site region, difference in acidities among the Figure catalysts, Figure 3. 3 Only one3.peak NH peak indifference the weakin 3inadsorption 6.3 wt% NiO/SiO 2-500 were further characterized by TPD-NH It indicated significant Tmaxi.e., 423–523 K,423–523 appeared.K,The temperatures attemperatures which NH3 adsorption reachedadsorption the maximum acid site i.e., region, TheNH atpeak which reached acidities among theTmax catalysts, Figureappeared. 3. Only one 3 adsorption in NH the 3weak acid site 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 the maximum are 473K for 2.9 wt% TiO /SiO -500, 443K for SiO , 453K for 1.4 wt% TiO -6.1 wt% 2 2 i.e., Tmax 423–523 K, appeared. The temperatures at which NH 32 adsorption reached the2 maximum 463K for 1.4 wt% TiO2-6.1 wt% Cr2O3/SiO2-500, respectively. Thus the 2.9 wt% TiO2/SiO2-500 exhibited Crare -500 463K 1.4 wt%443K TiOanalysis -6.1SiO wt% Cr -500, respectively. Thus the 2.9 wt% 2 O473K 3 /SiO 2strongest 2for 2 Ofor 3 /SiO 2wt% 2.9and wt% TiOstrength. 2for /SiO 2-500, 2, 453K 1.4 wt% Cr 2O3/SiO 2-500 and thefor acidic The TPD also suggested that 500TiO °C is2-6.1 a suitable temperature ◦ C is TiO /SiO -500 exhibited the strongest acidic strength. The TPD analysis also suggested that 500 2 463K for wt% TiO2-6.1 wt% Cr2sites, O3/SiO 2-500, respectively. Thus the 2.9 wt% TiO2/SiO2-500 exhibited in2 1.4 order to achieve more acidic Figures 4–6. a the suitable temperature in order The to achieve more acidic Figures 4–6. strongest acidic strength. TPD analysis also sites, suggested that 500 °C is a suitable temperature in order to achieve more acidic sites, Figures 4–6. 110

d 105

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c 100

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Figure 3. NH3 -TPD profiles 90of (a) SiO2 (pretreated at 600 ◦ C for 2 h); (b) 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% Cr2 O3 -6.3 wt% NiO/SiO2 -500; and (d) 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500. (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO 2-500; 1.4 wt% TiO 300 400and (d)500 600 2-6.1 wt% 700 Cr2O3/SiO 800 2-500.

Temperature (K)

110

c Intensity (a.u)

105

Figure 3. NH3-TPD profiles of (a) SiO2 (pretreated at 600 °C for 2 h); (b) 2.9 wt% TiO2/SiO2-500; b wt% Cr2O3/SiO2-500. (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO 2-500; and (d) 1.4 wt% TiO2-6.1 100 95

a

90

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% 4. NH3-TPD profiles of (a) 2.9 wt% TiO2/SiO2-400, (b) 2.9 wt% TiO2/SiO2-500; and TiO2 /SiOFigure 2 -600. (c) 2.9 wt%TiO2/SiO2-600.

110

c

400

500

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800

Figure 4. NH3-TPD profiles of (a) 2.9 wt% TiO2/SiO2-400, (b) 2.9 wt% TiO2/SiO2-500; and Temperature (K) (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 (c) 2.9 wt%TiO2/SiO2-600. Catalysts 2018, 8, 579 110

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(a.u) IntensityIntensity (a.u)

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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% Temperature (K) 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% Cr2 O3 -6.3 wt% NiO/SiO2 -400; (b) 7.1 wt% Cr2 O3 -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% Cr2 O3 -6.3 wt% NiO/SiO2 -600. NiO/SiO2-500; and (c) 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2-600.

(a.u) IntensityIntensity (a.u)

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400 TiO500 600 Cr2 O3 700 800 (b) 1.4 wt% TiO Figure 6.Catalysts NH3 -TPD (a) 1.4 wt% /SiO2 -400; 2018, 8, xprofiles FOR PEER of REVIEW 7 of 18 2 -6.1 wt% 2 -6.1 wt% Cr2 O3 /SiO2 -500; and (c) 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO(K) Temperature 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% distributions Ti, 2Cr, of 2.9 wt% TiO2/SiO -500, 1.4 wt% TiO 2-6.1 wt% Cr2O3/SiO 2Cr2 O3 /SiO2 -500,The and 7.1 wt%of Cr O3and -6.3Niwt% NiO/SiO inside the SiO were analyzed 22-500 2 pellet and 7.1 wt% Cr2O3-6.3 wt% NiO/SiO2Figure -500 inside 2 pellet were analyzed with electron with electron500, probe microanalysis (EPMA), 7.the It SiO was very clear that distributions of Ti, Cr, probe microanalysis (EPMA), Figure 7. It was very clear that distributions of Ti, Cr, and Ni inside and Ni inside were uniform relatively uniform inand allalmost threesynchronous catalystsdistributions and almost 2 pellets SiOSiO 2 pellets were relatively in all three catalysts of Cr-synchronous wereTi-Cr formedwere in 7.1formed wt% Cr2O 1.4 NiO/SiO wt% TiO2-6.1 wt% and 1.4 wt% distributionsNiofand Cr-Ti-Cr Ni and in3-6.3 7.1wt% wt%NiO/SiO Cr2 O23-500 -6.3and wt% 2 -500 2O3/SiO2-500. This means that strong interaction between Cr and Ni or Ti and Cr may occur, and TiO2 -6.1 wt%CrCr O /SiO -500. This means that strong interaction between Cr and Ni or Ti and Cr 2 3 2 relatively uniform distributions of Ti, Cr, and Ni in support maybe favorable to get highly stable may occur, and relatively uniform distributions of Ti, Cr, and Ni in support maybe favorable to get catalysts. highly stable catalysts. 2.0

The distribution of Cr

The distribution of Ti in

in TiO2-Cr2O3 /SiO2

TiO2-Cr2O3 /SiO2

Intensity

1.5

1.0

The distribution of Cr in

The distribution of Ni in

Cr2O3-NiO/SiO2

Cr2O3-NiO/SiO2

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% TiO 2/SiO2TiO2 /SiO2 -500, 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500, and 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /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.


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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. Table 2. Effect of catalyst loading on synthesis of alkyl carbamate *. Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Catalyst SiO2 2.2 wt% TiO2 /SiO2 -500 2.9 wt% TiO2 /SiO2 -500 3.6 wt% TiO2 /SiO2 -500 SiO2 3.5 wt% Cr2 O3 -8.5 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 9.0 wt% Cr2 O3 -3.6 wt% NiO/SiO2 -500 SiO2 1.0 wt% TiO2 -8.5 wt% Cr2 O3 /SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500 1.7 wt% TiO2 -4.9 wt% Cr2 O3 /SiO2 -500

Con./%

Y. Carbamate%

Y. Dialkyl Carbonate/%

94 94 >99 >99 >99 85 85 >99 >99 >99 92 92 >99 >99 >99

MC 89 MC 90 MC 94 MC 97.5 MC 96 EC 80 EC 80 EC 94 EC 97 EC 96 BC 88 BC 88 BC 95 BC 96 BC 94

DMC 0.3 DMC 0.4 DMC 0.4 DMC 0.6 DMC 0.7 DEC 0.2 DEC 0.2 DEC 0.3 DEC 0.5 DEC 0.55 DBC 0.4 DBC 0.4 DBC 0.2 DBC 0.5 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, Table 2. 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 Table 2, the catalyst 2.9 wt% TiO2 /SiO2 -500, 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500, and 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /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 Table 3. 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


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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, 8, x FOR PEER REVIEW

Table 3. Effect of calcinations temperature on the catalytic activity *.

Entry

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Table 3. Effect of calcinations temperature on the catalytic activity *.

Catalyst

Time

GC Yield of Alkyl Carbamate

Catalyst Time GC Yield of Alkyl Carbamate 1 Entry 2.9 wt% TiO 6h MC 95% 2 /SiO2 -400 1 2.9 wt% 2/SiO 2-400 6 h MC 95% 2 2.9 wt% TiO2TiO /SiO -500 6 h MC 97.5% 2 2 2.9 wt% 2/SiO 2-500 6 6hh MCMC 97.5% 3 2.9 wt% TiO2TiO /SiO 94% 2 -600 3 7.1 wt% Cr22.9 wt%wt% TiO2NiO/SiO /SiO2-600 2 -400 6 4hh MCEC 94% 4 O3 -6.3 95% 4 7.1 wt% 7.1 wt% 2O3-6.3 wt% NiO/SiO 2-400 4 4hh ECEC 95% 5 Cr2 OCr wt% NiO/SiO 97% 3 -6.3 2 -500 6 Cr2 OCr wt% NiO/SiO 96% 5 7.1 wt% 7.1 wt% 2O3-6.3 wt% NiO/SiO 2-500 4 4hh ECEC 97% 3 -6.3 2 -600 7 TiO2Cr -6.1 Cr2 ONiO/SiO 95% 6 1.4 wt% 7.1 wt% 2Owt% 3-6.3 wt% 2-600 4 4hh ECBC 96% 3 /SiO2 -400 8 TiO2TiO -6.12-6.1 wt%wt% Cr2 O 96% 7 1.4 wt% 1.4 wt% Cr32/SiO O3/SiO 2-400 4 4hh BCBC 95% 2 -500 9 TiO2TiO -6.12-6.1 wt%wt% Cr2 O 95% 8 1.4 wt% 1.4 wt% Cr32/SiO O3/SiO 2-500 4 4hh BCBC 96% 2 -600 ◦ C.2-600 1.41 wt% TiO2-6.1 wt% Cr2O 3/SiO 4h BC695% * Catalysts90.1 g, urea g, reaction temperature 170 For MC synthesis, 13.5 mL methanol, h. For EC synthesis, 15 mL ethanol, 4 h. For synthesis, 20 mLtemperature butanol, 4 h.170 °C. For MC synthesis, 13.5 mL methanol, 6 h. For * Catalysts 0.1BC g, urea 1 g, reaction 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 effect ofeffect the methanol/urea molar ratio and by-product 2 is examined by The of the methanol/urea molar ratioon onthe the yields yields ofof1 1and by-product 2 is examined by changingchanging the amounts of methanol while maintaining initial amounts of urea the amounts of methanol while maintaining constant constant initial amounts of urea (0.017(0.017 mol) mol) and 2.9 wt% TiO2/SiO 2-500 (0.1g). g). The were shown in Figure The conversion of urea remains and 2.9 wt% TiO (0.1 Theresults results were shown in 8. Figure 8. The conversion of urea 2 /SiO 2 -500 and it isit normally 100%.100%. The yields 1 and of by-product 2 are found to be strongly remains unchanged unchanged and is normally Theofyields 1 and by-product 2 are found to be dependent on (methanol/urea molarmolar ratio).ratio). The yield 1 reached a strongly dependent onthe theamounts amountsofofmethanol methanol (methanol/urea The of yield of 1 reached 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 or selectivity %

99 98 97 96

MC yield MC selectivity

95 94 93 14

15

16

17

18

19

20

21

22

23

22

23

Methanol/urea molar ratio

0.9

DMC yield %

0.8 0.7 0.6

DMC yield

0.5 0.4 14

15

16

17

18

19

20

21

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.


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Catalysts 2018, 8, 579 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.

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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 in 4. Table It was found that the ethanol/urea molarratio ratiois is 15.7 the butanol/urea 4. It was found that suitable the suitable ethanol/urea molar 15.7 andand the butanol/urea molar ratio is 13 foristhe synthesis of EC BC,BC, respectively. molar ratio 13 for the synthesis of and EC and respectively. Table 4.ofEffect of molar ratio of ethanol/urea butanol/urea onon synthesis of EC BCor *. BC *. Table 4. Effect molar ratio of ethanol/urea ororbutanol/urea synthesis oforEC

Urea Y. Y. Dialkyl Y. Carbamate/% Y. Dialkyl Carbonate/% Con./% Carbamate/% Carbonate/% 7.1 wt% Cr2 O3 -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 10.4 >99 EC 91 DEC 0.8 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 >99 EC 97 DEC 0.5 7.1 wt% Cr 2 O 3 -6.3 wt% NiO/SiO 2 -500 15.7 >99 EC 97 DEC 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 21 >99 EC 93 DEC0.5 0.3 1.4 wt%7.1 TiO wt% >99 >99 BC 92 DBC0.3 2.5 wt% 2O3Cr -6.3 2-500 9.7 21 EC 93 DEC 2 -6.1Cr 2 Owt% 3 /SiONiO/SiO 2 -500 1.4 wt%1.4 TiO wt%2-6.1 Cr2 O >99 >99 BC 96 DBC2.5 0.5 2 -6.1TiO 3 /SiO wt% wt% Cr22-500 O3/SiO2-500 13 9.7 BC 92 DBC 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /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 ◦ * Catalysts g, urea 1 g, reaction temperature 170 time 1.4 wt% TiO2-6.1 wt% Cr20.1 O3/SiO 2-500 16.2 >99 C, reactionBC 94 4 h. DBC 0.3 CatalystCatalyst

Alcohol/Urea Urea Con./% Alcohol/Urea

* 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

Urea conversion/MC yield, selectivity %

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 Figure 9. examined at 170 °C using a methanol/urea molar ratio of 20. The results were given in Figure 9. The The conversion of urea without 94% with of 10.4% andof0.4% 2. The conversion of conversion of urea without catalyst catalyst isis94% with 90%90% yieldyield of 1 and 2. Theofconversion of urea urea increases to more than 99% when 0.05 g catalyst was used and the corresponding yield was 97.5%. increases to more than 99% when 0.05 g catalyst was used and the corresponding yield was 97.5%. the catalyst increased to g, 0.1more g, more byproduct22would would be andand the the yieldyield of of 1 When theWhen catalyst loadingloading increased to 0.1 byproduct begenerated generated decreased1 decreased to 95%. to 95%.

99 98 97 96 95

MC yield urea conversion MC selectivity

94 93 92 91 90

Catalysts 2018, 8, x FOR PEER REVIEW 89

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)

0.55

DMC yield %

0.50 0.45

DMC yield

0.40 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) Effect of amount of 2.9 wt%TiO /SiO2-500 on on ureaurea conversion, yields of 1 andof 2, 1 and Figure 9. Figure Effect9. of amount of 2.9 wt% TiO22/SiO conversion, yields and 2, 2 -500 ◦ selectivity for 1. Urea 1 g, methanol 13.5 mL, 170 °C, and 6 h. and selectivity for 1. Urea 1 g, methanol 13.5 mL, 170 C, and 6 h.

above mentioned in Table 2, almost same urea conversion, 1–6 yields could be obtained in As aboveAs mentioned in Table 2, almost same urea conversion, 1–6 yields could be obtained in the the absence of catalyst or SiO2 as the catalyst. Therefore, there was no catalyst was used for blank test absence offor catalyst or SiO2 as the catalyst. Therefore, there was no catalyst was used for blank test for the effect of amount on synthesis of MC, EC, and BC. the effect of amount on synthesis of MC, EC, and BC. of urea with ethanol or butanol were shown in The results for the optimization of the reaction The results for the optimization of the reaction of urea with ethanol and or butanol in Table 5. The conversion of urea without catalyst is 85% or 92%, respectively, the yield were of 3 is shown 80%, 4 is 0.2%, 5 is 88% and 6 is 0.4%. The conversion of urea increases to more than 99% with the Table 5. The conversion of urea without catalyst is 85% or 92%, respectively, and the yield 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/%



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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 a

1 2a 3a 4a 5b 6b 7b 8b

Catalyst/%

Alcohol

Con./%

Y. Carbamate/%


0 5 10 15 0 5 10 15

Ethanol Ethanol Ethanol Ethanol Butanol Butanol Butanol Butanol

85 >99 >99 >99 92 >99 >99 >99

EC 80 EC 90 EC 97 EC 94 BC 88 BC 92 BC 96 BC 90

DEC 0.2 DEC 0.4 DEC 0.5 DEC 0.55 DBC 0.4 DBC 2.5 DBC 0.6 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% Cr2 O3 -6.3wt% NiO/SiO2 -500; b: Catalyst, 1.4 wt% TiO2 -6.1wt% Cr2 O3 /SiO2 -500.

2.2.6. Effect of Reaction Temperature

Urea conversion/ MC yield, selectivity %

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.12 of 18 Catalysts 2018, 8, x FOR PEER REVIEW

100 95 90

MC yield Urea conversion MC selectivity

85 80 75 150

155

160

165

170

175

180

o

Reaction temperature C

1.0

DMC yield %

0.8 0.6

DMC yield

0.4 0.2 0.0 150

155

160

165

170

175

180

o

Reaction temperature C of reaction temperature on urea conversion; yields of 1 and 2, and selectivity for 1. Figure 10.Figure Effect10.ofEffect reaction temperature on urea conversion; yields of 1 and 2, and selectivity 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


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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 *. Catalyst

T/◦ C

Con./%

Y. Carbamate/%


7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500

150 160 170 180 150 160 170 180

72 80 >99 >99 80 92 >99 >99

EC 70 EC 78 EC 97 EC 94 BC 78 BC 90 BC 96 BC 92

DEC 0 DEC 0.3 DEC 0.5 DEC 2 DBC 0 DBC 0.4 DBC 0.6 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

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2.2.7. Effect of Reaction Time

2.2.7. Effect of Reaction Time

Urea conversion/ MC yield, selectivity %

The results for theforsynthesis of MC with varied reaction inFigure Figure11.11. The yield The results the synthesis of MC with varied reactiontime timewere were shown shown in The yield of 1 increased from 94.5 to 97.5% with the increasing of reaction time from 4 h to 6 h. However, it would of 1 increased from 94.5 to 97.5% with the increasing of reaction time from 4 h to 6 h. However, it decrease if longer reaction time was applied. would decrease if longer reaction time was applied.

99.0 98.5 98.0 97.5 97.0 96.5 96.0

MC yield Urea conversion MC selectivity

95.5 95.0 94.5 94.0 93.5 4

5

6

7

8

Reaction time (h)

1.4

DMC yield %

1.2 1.0

DMC yield

0.8 0.6 0.4 0.2 4

5

6

7

8

Reaction time (h) Figure 11. of Effect of reaction on urea conversion, yieldsofof11and and2, 2, and selectivity Urea 1 g,1 g, Figure 11. Effect reaction timetime on urea conversion, yields selectivityfor for1. 1. Urea ◦ C. methanol 13.50.1 mL, 0.1 wt% g 2.9 wt% 2/SiO 2-500, 170 °C. methanol 13.5 mL, g 2.9 TiO2TiO /SiO -500, 170 2

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 *.


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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 *. Catalyst 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 1.4 wt%2018, TiO28, -6.1 wt% PEER Cr2 O3REVIEW /SiO2 -500 Catalysts x FOR 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /SiO2 -500

2.2.8. Recyclability Test

T/h

Con./%

Y. Carbamate/%


3 4 5 3 4 5

96 >99 >99 97 >99 >99

EC 93 EC 97 EC 94 BC 94 BC 96 BC 95

DEC 0.2 DEC 0.5 DEC 1.3 DBC 0.4 DBC 0.6 DBC 1

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* 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 important than the 2.2.8. Recyclability Testinitial 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% TiO2 /SiO2 -500 catalyst. As shown in decreased 1.7%, 2%, and 2% after 10 runs. It is to say these silica gel supported 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. 96 90

Yield %

84 78

MC yield EC yield BC yield

72 66 60 0

2

4

6

8

10

12

14

16

18

20

22

Times of using catalysts Figure testing of ofcatalysts catalysts2.9 2.9wt% wt%TiO TiO synthesis of MC, 7.1 wt% Figure12. 12. Reusability Reusability testing 2/SiO 2-500 for for synthesis of MC, 7.1 wt%Cr 2O32 /SiO 2 -500 Cr6.3 wt% NiO/SiO for synthesis of EC and 1.4 wt% Cr 2-500 for synthesis of EC and 1.4 wt% TiOwt% 2-6.1 TiO wt%2 -6.1 Cr2O 3/SiO 2-500 for synthesis 2 Owt% 3 -6.3 NiO/SiO 2 -500 2 O3 /SiO 2 -500 forof synthesis of BC. 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% TiO2 /SiO2 -500, synthesis of MC, EC, and BC with urea and methanol (or ethanol and butanol) over 2.9 wt% 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500, and 1.4 wt% TiO2 -6.1 wt% Cr2 O3 /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


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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(OC4 H9 )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 -Cr2 O3 /SiO2 : 10 mL 26–27 wt% HNO3 aqueous solution was added into a 100 mL beaker containing 4 mL Ti(OC4 H9 )4 to get a homogeneous solution (pH value ~1–2). Then 3.85 g Cr(NO3 )3 ·9H2 O 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% Cr2 O3 /SiO2 -500. Catalysts 1.4 wt% TiO2 -6.3 wt% Cr2 O3 /SiO2 -500 and 1.0 wt% TiO2 -8.5 wt% Cr2 O3 /SiO2 -500 were prepared by variation of the mass of Cr(NO3 )3 ·9H2 O with the same procedure. 1.4 wt% TiO2 -6.3 wt% Cr2 O3 /SiO2 -400 and 1.4 wt% TiO2 -6.3 wt% Cr2 O3 /SiO2 -600 were achieved by variation of calcination temperature. Cr2 O3 -NiO/SiO2 : 7.69 g Cr(NO3 )3 ·9H2 O and 4.0 g Ni(NO3 )2 ·6H2 O were respectively added into a 100 mL beaker containing 10 mL H2 O. 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% Cr2 O3 -8.0 wt% NiO/SiO2 -500. Catalysts 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -500 and 9.0 wt% Cr2 O3 -3.6 wt% NiO/SiO2 -500 were prepared by variation of the mass of Cr(NO3 )3 ·9H2 O with the same procedure. 7.1 wt% Cr2 O3 -6.3 wt% NiO/SiO2 -400, and 7.1 wt% Cr2 O3 -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


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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.


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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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