Coking of Catalysts in Catalytic Glycerol Dehydration to Acrolein

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Coking of Catalysts in Catalytic Glycerol Dehydration to Acrolein Xuechao Jiang, Chunhui Zhou, Riccardo Tesser, Martino Di Serio, Dongshen Tong, and Junrui Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01776 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Industrial & Engineering Chemistry Research

Coking of Catalysts in Catalytic Glycerol Dehydration to Acrolein Xue Chao Jianga, Chun Hui Zhoua,b,c,∗, Riccardo Tesserd, Martino Di Seriod, Dong Shen Tonga, Jun Rui Zhanga a Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China b

Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province, China National Bamboo Research Center,

Hangzhou 310012, People’s Republic of China c Centre for Future Materials, University of Southern Queensland, Toowoomba, Queensland 4350, Australia d Department of Chemical Sciences – University of Naples “Federico II” – Via Cintia 21 – Complesso Monte S. Angelo - 80126 Naples, Italy

Correspondence to: Prof. CH Zhou E-mail: [email protected]; [email protected]

Abstract: Catalytic glycerol dehydration provides a sustainable route to produce acrolein as glycerol is a bio-available platform chemical. However, in this process catalysts are rapidly deactivated due to coking. This paper examines and discusses recent insights into coking in catalytic glycerol dehydration. The nature and location of coke and the rate of coking depend on feedstock, operating conditions, and the acidity and the pore structure of the solid catalysts. Several methods have been suggested for inhibiting the coking and slowing the deactivation of catalyst, including: (1) co-feeding of oxygen; (2) tuning of the pore size of the solid acid catalysts; (3) doping noble metals (Ru, Pt, Pd) into the solid acid catalysts ; and (4) designing new reactors. The present methods for inhibiting coking are still unsatisfactory. The deactivated catalysts can be regenerated by removing coke. Nevertheless, the rapid deactivation of the regenerated catalyst remains problematic. The literature survey indicates that the exact compositions of the coke in glycerol dehydration remain elusive. The thermodynamics, kinetics and the formation mechanism of coking need to be probed so as to advance the development of a catalyst with high activity, selectivity and resistance to coking to put the catalytic glycerol dehydration into practice. Key words: coking; glycerol dehydration; acrolein; solid acid catalyst; regeneration

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1. Introduction Owing to environmental and ecological concerns and dwindling of fossil fuel resources, the demand for green and sustainable chemical processes has dramatically increased.1,2 Accordingly, using bio-based feedstock to produce fine chemicals in a cleaner approach has received increasing attention over the past decade.3 Glycerol is one of bio-based platform chemicals. In industry, it is a by-product in the hydrolysis of fat and plant oils to fatty acid,4 and in the saponification of triglycerides to soap,5and in the transesterification of fats and plant oils to biodiesel.6,7,8 Moreover, glycerol can be also produced in the microbial metabolism9 and by the fermentation of sugar.10 Physically, glycerol is a clear, colorless, odorless, hygroscopic, viscous and sweet tasting liquid;11,12 chemically, a glycerol molecule , with two primary and one secondary hydroxyl groups, is much reactive.11 Such characteristics of glycerol enable it to undergo many reactions,13 for example oxidation,14,15 hydrogenolysis,16,17 etherification,18,19 reforming20 and dehydration.21,22 Among those reactions, catalytic glycerol dehydration has attracted considerable attention as the reaction can produce ethers,23 acetals,24 ethylene glycol25 and acrolein. 26 Acrolein, in particular, can be converted into many value-added chemicals and polymers (Fig 1a).

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Fig 1. a) Categorical statistics showing acrolein to value-added chemicals and polymers (by percentage) according to peer-reviewed scientific papers (published from 2006 to 2016). b) Annual number of peer-reviewed papers (published from 2006 to 2016) relevant to the topics of glycerol dehydration and coking. (Data from Web of ScienceTM Core Collection. Search terms contained in title: glycerol dehydration and coking).

Catalytic glycerol dehydration can be conducted either in the gas phase27,28 or in the liquid phase.29,30 In 1928, Freund31 reported that acrolein was able to be produced from glycerol at 180 °C over a diatomaceous silica catalyst. In 1936, Groll and Hearne32 gained a 49% acrolein yield by heating aqueous glycerol solution (6.3 wt.%) at 190 °C in the presence of sulfuric acid (8 wt.%). In 1949, Heinemann et al.33 conducted gas phase glycerol dehydration over an activated bauxite catalyst at 430 °C, achieving a 42% acrolein yield. In 1993, Degussa et al.34 reported a process to produce acrolein by the dehydration of glycerol in the liquid phase (180-340 °C) or gas phase (250-340 °C) over alumina-supported phosphoric acid catalysts. Catalytic glycerol dehydration can also conducted in sub- and supercritical water (250-390 °C and 25-35 MPa) with H2SO4,35 ZnSO4,36 WO3/TiO237 or NbOx/TiO238 as the catalysts. Typically, the gas phase glycerol dehydration over sloid catalysts has advantages over the liquid phase process in the separation of products. In addition, recent studies have shown that in many cases, the acrolein yields in the gas phase process appeared higher than those in the liquid phase one.39 Therefore, over the past few decades, most studies on the catalytic gas phase glycerol dehydration focus on the development of solid acidic catalysts. The reported catalysts such as metal phosphates,40,41 metal sulphates,42 metal oxides,43,44 heteropolyacids (HPA),45,46 zeolites,47,48,49 clay minerals50,51 and zeolite-supported HPA catalysts.52,53 The temperature is usually between 270 and 350 °C. Over those 3



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solid catalysts, high initial activity and acrolein yield can be readily obtained. However, studies revealed that the lifetime of such active catalysts are short because rapid coking occurs on the surface of the catalysts.54,55,56 Namely, the rapid deactivation of the catalysts are problematic. Inherently, glycerol dehydration is a multi-step process involving many reactions, leading to many active intermediates, which can further react to form coke. Besides those intermediates, glycerol and acrolein molecules are also reactive and have also high tendency to polymerize and then to form large carbonaceous deposits and coke.

Such systems make the coking much sophisticated and hence to

disclose the mechanism become much difficult. In practice, coking will stop the chemical process, leading to much low productivity and significant economic losses. Clearly, such a problem of coking must be well tackled and become a major challenge in pushing the catalytic glycerol dehydration into practice.38,57,58

2. Formation of coke Coking is primarily a chemical process, involving many types of reactions.59,60 During catalytic glycerol dehydration to acrolein, the large carbonaceous precursors of coke are generally formed through the polymerization of glycerol, intermediates, and acrolein.61,62 For example, in the presence of solid catalysts and under the conditions for catalytic glycerol dehydration, glycerol molecules can also polymerize to form polyglycols and the acrolein can polymerize to form polyaromatics.28,63 Furthermore, many intermediates may produce in the reactors from side reactions including etherification, acetalization, esterification and condensation.64 These reactions lead to many kinds of intermediates or by-products such as glycerol ether, glycerol formal, glycerol carbonate, acetol, acetone and acetaldehyde (Fig 2). All these products contribute more or less to coking as they can form carbonaceous macromolecules and then coke by cyclisation, condensation and polymerization.59,65 Most possibly, the coking reactions start with intramolecular cyclization reactions and the intermolecular condensation reactions of reactants, intermediates and products.60 Examples include the polycondensation

of

phenol

with

formaldehyde

or

acetaldehyde

to

form

phenol

formaldehyde/acetaldehyde resins, the cyclization of acetone with glycerol to form solketal, aldol condensation of acrolein, acetaldehyde and acetol.28,39,63,66

4


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Fig 2. Possible reactions and intermediates in the system of catalytic glycerol dehydration into acrolein.58,64,67,68

Besides the compounds, the surface and the pores of the catalysts also play a critical role in coking. Supposed that glycerol, intermediates and acrolein are strongly adsorbed, thus the coking is certainly felicitated. In other words, coking are involved not only in much complicated chemical reactions, but also in the texture and structure of the catalyst.69 Though rapid coking is observed in all solid acid catalysts, there are only a few studies measure the coke. Vieira et al.70 and Rodrigues et al.71 performed catalytic glycerol dehydration over MFI zeolites and MWW-series zeolites catalysts in the gas phase at 300-320 °C under atmospheric pressure in a fixed-bed reactor and measured the carbon deposit. A gradual decrease in glycerol conversion with time-on-stream was observed for all the zeolite catalysts. It was also revealed that in addition to the pore size of the catalysts, the size of the crystal of the catalysts influenced the coking. After 10 h, , the amount of carbon deposited on MCM-22, MCM-36, ITQ-2, Al-L and Ga-L (corresponding to Al-MFI and Ga-MFI with large zeolite crystals, respectively), Al-S and Ga-S (corresponding to Al-MFI and Ga-MFI with small zeolite crystals, respectively) catalyst is 24.6, 20.4, 37.0, 9.0, 8.0, 4.0 and 3.0 wt.%, respectively (Table 1). The quantities of coke on the Al-L catalyst and the Ga-L catalyst were greater 5



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than those on the Al-S catalyst and the Ga-S catalyst. Larger zeolite crystals and particles possessed longer channels, resulting in the polymerization of glycerol and acrolein. Accordingly, the probability of the coking remarkably increased. In the case of microporous zeolites (e.g. MCM-22), the coke is generally deposited within external micropores and it is more damage to catalytic performance, even for low coke content, due to pore blockage and obstruction of the access of glycerol to narrow channels located deep in the crystals. For mesoporous zeolites (e.g. MCM-36 and ITQ-2), the coke is preferentially located between the sheets, facilitating the retention of precursors of coke and the occurrence of coking.

Table 1. Typical catalytic and coking behaviors and of some solid acid catalysts during catalytic glycerol dehydrationa. Reaction

Deactivation

Glycerol

Acrolein

condition

time

conversion

yield

(T/°C,

(h)

(%)

(%)

SBET (m2/g)

Pore size (Å)

Coke

Catalyst

Ref fresh

used

fresh

used

Content wt.%

P/atm) 77(45)

35(20)

556

167

-

-

10.27

0.25Nb0.75WAl

8Nb/SiZr5

325, 1

8

100

70.4

79

73

178

149

-

0.5Nb0.5WAl

100

71.9

81

82

170

136

-

100

69.3

101

81

148

129

-

98.8

64.4

96

49

101

109

-

0.75Nb0.25WAl 305, 1

3

0.25Nb0.75WTi

78

72

0.5Nb0.5WTi

99.3

59.1

92

38

93

111

-

0.75Nb0.25WTi

99.3

54.9

108

37

93

122

-

γ-Al2O3

71.3(53.6)

25.9(19.7)

213

185

-

-

8.6

H-ferrierite(20)

42.0(19.8)

17.1(7.4)

390

53

-

-

9.9

H-β(25)

76.4(28.9)

34.9(13.4)

508

113

-

-

21.5

H-ZSM-5(23)

36.3(20.6)

16.6(8.6)

572

46

-

-

8.7

H-Y(5.1)

29.7(13.2)

8.8(3.9)

631

51

-

-

15.4

H-mordenite(20)

49.5(23.7)

20.1(8.1)

424

62

-

-

9.9

Al-L

85.6(70.4)

26.1(22.1)

-

-

-

-

8.3

57 H-zeolites

Ga-L

74.8(47.6)

16.6(10.3)

-

-

-

-

7.4

Al-S

90.2(53.7)

28.4(14.8)

-

-

-

-

3.8

Ga-S

82.3(43.3)

17.3(10.3)

-

-

-

-

2.9

MCM-22

99.8(20.2)

49.9(4.3)

565

-

-

-

24.6

89.0(10.5)

6.7(0.1)

603

-

-

-

20.4

58.1(17.6)

44.4(7.2)

852

-

MFI zeolites

MWW zeolites

300, 1

MCM-36

320, 1

6

10

ITQ-2

70

37.0

a The data in the brackets obtained after deactivation. 8Nb/SiZr5：8 wt.% niobium oxide was supported on a mesoporous zirconium doped silica (Si/Zr molar ratio: 5). xNbyWAl and: Al2O3 supported Nb2O5 and WO3, x,y represent the mass fraction of Nb2O5 and WO3 loading, respectively. xNbyWTi: TiO2 supported Nb2O5 and WO3, x,y represent the mass fraction of Nb2O5 and WO3 loading, respectively. Al-L and Ga-L: Al-MFI and Ga-MFI with large zeolite crystals.

6


71

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Al-S and Ga-S: Al-MFI and Ga-MFI with small zeolite crystals.

A third issue is that because the carbonaceous macromolecular deposits and coke are produced from many types of reactions, they consist of many components. Theoretically, the carbonaceous depoitsare composed of soluble compounds and insoluble one in a given organic solvent. The distribution of soluble and insoluble fractions depends on the catalysts and on the reaction conditions: ratio of glycerol to water, gas, temperature, time, pressure and reactors.60 So far there have been few studies on the solubility of the components in the carbonaceous deposits and coke. Nevertheless,

13

C

NMR spectra of carbonaceous compounds adsorbed on the spent catalysts have clearly indicated the existence of polyaromatics and polyglycols on the surface of the catalysts (Fig 3).70,71 Typically, in the 13

C MAS-NMR spectra of the spent zeolites, the peak at σ = 127-130 ppm is attributed to carbon atoms

in polyaromatic compounds, peaks at σ = 13-35 ppm is associated with saturated carbon atoms of terminal chains bounded to the oligomeric compounds that have not yet cyclized, and the peaks at σ = 63-77 ppm are ascribed to polyglycol compounds. 73,74 ,75,76 ,77 In addition, the thermogravimetric analysis revealed that carbonaceous compounds deposited on the spent MFI zeolites are polyglycols and polyaromatics formed during catalytic glycerol dehydration.70 Besides, the thermal analysis of the coke on spent 8Nb/ ZrSi5 (8 wt.% niobium oxide supported on a zirconium doped silica with a Si/Zr molar ratio of 5) catalyst by García-Sancho et al.78 suggested that the carbonaceous deposits comprised aliphatic and aromatic compounds. It is worth noting that, for porous catalysts, the formation of carbonaceous deposits appears at different sites, for example on the external surface of crystallites, in the 10 MR channels, pores or cages. Accordingly, the components of the carbonaceous deposits vary and are elusive.79,80

7



Chemical shift 13C (ppm) Fig 3. Typical 13C NMR spectra of coking from the spent zeolite catalysts. (A) MCM-22, MCM-36, ITQ-2, (B) Al-L, Al-S, Ga-L, and Ga-S. 70,71(Reprinted from Ref 70. Copyright (2015), with permission from Elsevier. Reprinted from Ref 71. Copyright (2015) with permission from Elsevier.)

The reaction pathway to polyglycols and polyaromatics over zeolite catalyst has been proposed (Fig 4). The formation of polyglycols involves the activation of glycerol molecules on adjacent acid sites of the catalysts, followed by the successive coupling of hydroxyl groups of glycerol.71,81 Polyglycols are mainly deposited on the outer surface of the zeolite catalysts. By contrast, polyaromatics were more abundant inside the pores of zeolites. Compared with the formation of polyglycols, the formation of polyaromatics is much complicated. Firstly, it involves more reactants or intermediates. Secondly, it is thought a multi-step process. In other words, the identification of highly active monomeric species, which are produced during glycerol dehydration, is not an easy task. The diffusion, adsorption-desorption of those species and oligomerization inside pores are certainly much sophisticated.76,82

8


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Fig 4. The formation of (A) polyglycols on the internal surface of catalyst and (B) polyaromatics on the external surface of catalyst during catalytic glycerol dehydration.68,70,71,

Carbonaceous deposits or coke can be strongly chemisorbed to form a monolayer on the surface of catalysts, following by physisorption in multilayers. Such processes finally encapsulate active sites completely and block pores, thereby preventing reactants from entering these pores.70,76,83,84 For example, Massa et al.72 performed catalytic glycerol dehydration over Al2O3- and TiO2-supported Nband W-oxide catalysts under atmospheric pressure and at 305 °C in a vertical stainless-steel reactor. For the Al2O3-supported samples, the changes in textural properties including the decreasing pore size imply that heavy compounds and coke molecules are deposited on the pore walls (site blocking). On the contrary, the changes occurring for the titania-supported samples showing some increase in the measured pore size indicated that smaller pores are totally blocked (pore blocking). The first monolayer by chemical adsorption is formed by major chemical binding through certain groups of organic species to the surface of the solid catalysts. Such interaction might play a leading role in coking. In addition, such adsorption can lead to the remarkable carbon imbalance between the reactant into the reactor and the product off the reactor. In the above-mentioned study by Massa et al.72, after 3 h, carbon balances of all samples were incomplete (ranging from 48.6 to 92.3 wt.%) due to coke deposits on the catalyst. Coke can change the physical properties of catalysts. Commonly, the formation of the coke on 9



solid acid catalysts can be readily observed with eyes because the coke turns the color of the catalysts into black.85,86,87 The specific surface area, pore size and pore volume of solid acid catalysts will decrease when coke forms on the external or internal surface of the catalysts(Table 1). García-Sancho et al.78 performed catalytic glycerol dehydration over a 8Nb/SiZr5 catalyst under atmospheric pressure and at 325 °C in a fixed-bed continuous-flow stainless steel reactor. Glycerol conversion was 77% in the first 2 h, while decreased to 45% after 8 h due to coke deposited on the external surface of the catalyst and clogging its pores. The color of the catalyst changed from white to black during the reaction. The specific surface area and the pore volume of the spent catalyst decreased from 556 to 167 m2/g and 0.42 to 0.17 cm3/g, respectively. Yong et al.57 found the surface area of H-zeolite and mesoporous γ-Al2O3 catalyst decreased rapidly after 2 h in catalytic glycerol dehydration due to coke deposited on acid sites (Table 1).

3. Conditions affecting coking 3.1 Influence of feedstock Both glycerol and water in the feed affect coking on the surface of catalysts in the process of catalytic glycerol dehydration. At high glycerol concentrations, glycerol molecules tend more to polymerize and form polyglycols, one of coke precursors.71,68, 88 , 89 Furthermore, when glycerol concentration is increased, there are more side reactions including etherification, esterification, oxidation, hydrogenolysis. 90 These side reactions lead to ethers, monoglycides, hydroxyacetone, acetaldehyde, propanediol, etc. All these compounds may result in coking, such as aldol condensation, Diels-Alder cyclization and polymerization.55 However, these reactions during the catalytic glycerol dehydration are complex and no detailed information is available at present. Martinuzzi et al.61 conducted glycerol dehydration using various glycerol concentrations over an industrial heteropolyacid catalyst (ZR24) at 270 °C. When the glycerol concentration increased from 2 to 4%, the glycerol conversion decreased from 100 to 95% and the selectivity of cyclic compounds increased from 2.5 to 5.4%. Such a decrease of the glycerol conversion was due to the decrease of available acid sites primarily resulting from large cyclic compounds deposited on the catalyst. The cyclic compounds consisted mostly of polyglycols and polyaromatics. Polyglycols result from the 10


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polymerization of glycerol via the intermolecular reaction. Polyaromatics are generated by the multiple polymerization of acrolein. The precedent reactions include hydrid transfer, cracking, isomerization and oligomerization, and so forth70,71 Coking is also related to the amount of water in the feed. Specifically, the amount and the state of water molecules in the reactor influence the coking in several aspects.61 Firstly, the presence of water molecules can tune the chemisorption of glycerol (Fig 5).26 Particularly, the water chemisorbed on acid sites competed with the adsorption of glycerol.26 In addition, bimolecular condensation and subsequent polymerization of glycerol molecules can be suppressed when the water content increases.57 Consequently, the formation of polyglycols as coke precursors can be suppressed. Furthermore, water vapor can inhibit secondary reactions of acrolein (Fig 5).92 Under supercritical conditions, the roles of water in the coking could be more multiple, but there are no such detailed studies on this issue.

Fig 5. The possible mechanism for explaining the influence of water on coking.

11



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The correlation between water content in the feed and coke on the catalyst has been studied by Yong et al. who conducted catalytic glycerol dehydration under atmosphere pressure and 315 °C in a fixed-bed reactor over H-zeolites57,92 and silica–alumina catalysts.26 When the molar ratio of water to glycerol increased from 15.7 to 91.7% in the feed, there was no significant difference in glycerol conversion, while acrolein selectivity increased. Nevertheless, the extent of the influence on coke formation appears to be dependent on the catalysts (Table 2). Over some zeolite catalysts, only small changes in the mass of coke have been observed, but it has been confirmed that with the increase in water content, coke can be reduced. Interestingly, the changes of water content seem not to change the components of coke remarkably, as shown in the list of the H/C ratios of the coke on the catalyst surface. The H/C ratios provide some clues on which type of compounds as coke deposit might form on the surface of the solid acid catalysts.91 It has been reported that the H/C ratio of the coke on the surface of zeolite catalyst slightly increased with increasing SiO2/Al2O3 ratio. However, the correlation between H/C ratio and coke has not been discussed detailed so far during catalytic glycerol dehydration.

Table 2. Correlation between water content and the coke content over various catalysts during glycerol dehydration. Catalyst

H-β (25)

H-ferrierite (55)

H-ZSM-5(30)

H-ZSM-5(150)

Si0.8Al0.2Oz

Si0.2Al0.8Oz

η-Al2O3

Water content

Glycerol

Acrolein

Deactivation

Coke

(mol%)

conversion (%)

yield (%)

time (h)

(wt.%)

H/C

15.7

74.9(26.5)

8.8 (1.5)

21.5

51.9

68.1(26.6)

21.6 (5.8)

21.5

1.1

91.7

74.1(29.8)

44.5 (14.5)

19.9

1.1

15.7

40.0 (15.2)

3.7 (0.9)

7.9

1.6

51.9

31.0 (14.6)

10.5 (3.3)

7.9

1.6

91.7

51.5 (18.1)

39.5 (10.6)

7.7

1.7

12

1.1

15.7

48.5(26.3)

3.3(1.3)

10.8

1.7

51.9

51.1(25.6)

12.5(4.2)

10.1

1.5

91.7

43.6(29.0)

25.2(16.6)

9.1

1.6

12

15.7

69.0(16.8)

9.3(0.8)

9.6

1.4

51.9

65.9(15.4)

22.6(4.6)

9.7

1.5

91.7

72.7(28.5)

53.2(23.2)

8.9

1.4

15.7

54.0(14.1)

4.5(0.5)

15.3

1.7

91.7

42.9(18.3)

21.4(6.8)

12.0

1.9

15.7

36.1(23.1)

1.8(0.7)

15.1

2.0

11.5

2.3

12.9(6.9)

12

91.7

38.7(19.6)

15.7

36.1(13.3)

0.9(0.3)

8.9

2.2

91.7

10.9(8.9)

5.4(4.0)

8.1

2.4

SixAlyOz: mixed silica and alumina oxides, x,y represent the mass fraction of silica and alumina, respectively. The data in the brackets correspond to the products obtained after deactivation

12


Ref

57

92

26

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3.2 Influence of temperature and contact time The catalytic glycerol dehydration is favored at high reaction temperatures because it is an endothermic process.93 Studies by Yong et al.57,92 Suprun et al.63 Herbon et al.100 and Lauriol-Garbay et al.94 revealed that glycerol conversion significantly increased with increasing reaction temperature, but severe coking was observed at high temperatures (Table 3). Recently, Costa et al.95 performed catalytic glycerol dehydration at various temperatures over sulfonic functionalized SBA-15 mesoporous silica catalyst. The initial conversion of glycerol (at 0.5 h TOS) was 98% at 350 °C, while at 275 °C it only reached 62%. Furthermore, the coking occurred more rapidly rate at 350 °C than that at 300 °C. Accordingly, at different temperatures, both the amount and the composition of coke vary. As shown by the comparison the mass of the coke and the H/C ratio of coke over the different catalysts (Table 3) The inherent reason of the influence of temperature on coking can be partly explained by the different reactions and the resulting compounds at the temperatures below and beyond 300 °C (Fig 6). There are also suggestions that coke can be classified into low (350 °C) coke.60,96 At low reaction temperature, coking chiefly involves the polymerization of glycerol or arolein70,68 and aldol condensation of acrolein, acetaldehyde and acetol.28,39 Accordingly, coke formed at low temperature mainly consists of products with high boiling points (glycerol oligomers, acrolein oligomers and aldol condensation products).60,63,96 In addition, coke formed at low temperature is found to be primarily located either on the external surface97 or within the internal surface of catalysts. 98 By contrast, at high reaction temperatures (above 350 °C), in addition to polymerization and condensation, oxygenates may undergo further dehydration and condensation to form coke.60,96 Under such circumstances, the coke is made up of unsaturated and aromatic compounds.60,63,96 At high temperature, coke generally formed and deposited into the micropores of the catalyst.99 Moreover, as discussed above, high temperature will yield more coke on the catalyst. The influences of reaction temperature on coking during catalytic glycerol dehydration over H-ZSM-5(150),92 H-β(15) and H-ferrierite(55),57 ZrNbO-12,94 Al2O3-/TiO2-supported phosphate and SAPO (silica-alumina-phosphate)63 and silica-supported phosphotungstic acid catalysts are summarized in Table 3.100 Generally, the amount of carbon deposited on the catalysts at high temperatures is higher 13



than that at low temperatures. It is believed that at higher temperature acrolein can undergo more secondary reactions which yield acetol, propanal, acetaldehyde, and many other non-identified products.95 A clear proof is that the selectivity of acetol, propanal and acetaldehyde usually increased with increasing reaction temperatures. In addition, it is possible that acrolein could be converted to many other aldehydes, olefins, heterocyclic and aromatic compounds with high molecular weight by hydrid transfer, cracking, isomerization, reductive coupling, aldol condensation, Diels-Alder reaction, dehydrogenation and hydrogenation.55 The carbon-carbon bonds can be formed by the reactions: 1) aldol condensation of acrolein and acetaldehyde to form 3-hydroxy-1-valeraldehyde and 3-hydroxybutyraldehyde; 2) reductive coupling of aldehydes to form polyolefins; and 3) condensation of two acrolein molecules to give a C6H8O product (2-methyl-2,4-pentadienal). 101 For example, heterocyclic compounds can be formed from allyl alcohol which is in situ produced by a secondary hydrogenation reaction of acrolein.29, 102 The products are cyclic unsaturated and oxygenated compounds like phenol, dihydrofuran, cyclopentenone and cyclohexanone. In the system of catalytic glycerol dehydration, all those compounds could be responsible for coking.

Fig 6. Possible compounds formed at different temperatures in the system of catalytic glycerol dehydration.63 (Reprinted from Ref 63. Copyright (2009), permission from Elsevier.)

14


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Table 3. Influence of reaction temperature on coke over various catalysts. Glycerol

Acrolein

Deactivation

temperature (°C)

conversion (%)

yield (%)

time (h)

265

21.2

3.4

290

49.8

25.1

340

93.7

53.8

10.1

1.6

290

60.1

29.2

18.4

1.2

340

95.2

44.7

23.6

1.0

290

40.2

28.7

7.1

1.7

340

70.9

54.6

8.1

1.5

Catalyst

H-ZSM-5(150)

H-β (15)

H-ferrierite (55)

ZrNbO-12

Al2O3–PO4

TiO2–PO4

SAPO-11

SAPO-34

HPW/SiO2

Coke

Reaction

12

12

12

(wt.%)

H/C

8.6

1.9

9.1

1.6

280

64.0

45.4

24

4.0

290

93.0

66.0

28

3.0

300

100

70.0

32

0

280

100

42

300

-

-

10

-

2.4

0.54

3.9

0.52

320

-

-

5.8

0.49

280

98.0

36.3

3.1

0.55

300

-

-

6.8

0.50

320

-

-

8.9

0.48

280

88.0

65

4.6

0.68

300

-

-

7.4

0.55

320

-

-

9.5

0.40

280

59.0

42

9.6

0.62

300

-

-

12.7

0.54

10

10

10

320

-

-

16.2

0.47

280

95.0

42.0

6.4

2.4

300

98.0

40.0

7.9

2.1

330

99.0

30.0

14.9

1.9

10

Ref

92

57

94

63

100

ZrNbO-12: mixed zirconium and niobium oxides; Al2O3-PO4 and TiO2-PO4: Al2O3 and TiO2 supported phosphate; SAPO: silica-alumina-phosphate; HPW: H3PW12O40·6H2O; HPW/SiO2: silica-supported phosphotungstic acid catalysts

Catalytic dehydration under laboratory conditions is mostly conducted on a plug-flow micro-reactor.94 In general, coking is also related to the flow rate of the feed, including the flow rate of carrier gas. Generally, the glycerol conversion decreases when the flow rate of carrier gas increases. This is because that increasing the flow rate of carrier gas shortens the residence time of glycerol on the surface of catalyst. Accordingly, the probability of the intramolecular cyclization of reactant and the 15



intermolecular condensation reactions decrease when the flow rates of carrier gas increase. In addition, in view of acrolein, a higher flow rate of carrier gas also leads to short residence time of acrolein in the reactor and quicker desorption of acrolein from the surface of the catalyst. As a result, it is usually observed that the selectivity to acrolein increases with increasing the flow rate of carrier gas. Inherently, it is also helpful to reduce the formation of coke precursors, for example olefins, acetol and acetaldehyde. The decrease contact time delayed coking by reducing the contact time of coke precursors in the reactor. Nevertheless, it is worth noting that there is no a precise rule on the influence of the contact time on the catalytic performance. Some viewpoints might be contradictory primarily because of the different catalysts used by different researchers. For example, over a catalyst of zirconium-niobium mixed oxides at 300 °C, when the nitrogen flow rate increased from 75 to 200 ml/min and accordingly the contact time of glycerol decreased from 1.9 to 1.0 s, no remarkable influence on the selectivity to acrolein was observed.94 Nevertheless, the deactivation was slowed down rate. Yet, over Pd-H3PW12O40/Zr-MCM-41 103 and phosphotungstic acid supported on Cs+ modified SBA-15 catalyst,104 it have been found that the selectivity to acrolein increased with decreasing contact time. One possible reason is that under such conditions, consecutive reactions of acrolein to small molecules such as CO and acetaldehyde occurred less.105 However, the conversion of glycerol and the selectivity to acrolein decreased with a further decrease of contact time. The decrease of the conversion of glycerol is due to shorter residence time for glycerol. The further reaction between more unconverted glycerol and acrolein to form oligomers led to the decreases of the selectivity to acrolein and instead gives rise to more coking. In the engineering, tedious tests are still need to determine the optimal contact time.

3.3 Influence of acidity and pore size of solid acid catalysts In view of acidic sites, solid acid catalysts influence catalytic performances in four aspects: the type; the amount; the strength; and the availability of acid sites. The former three is heavily dependent on the chemical composition, the latter one are usually decided by the pore size and shape of the solid catalyst. Most studies indicated the Brönsted acid sites are active center for catalyzing the glycerol dehydration. Yet such sites are also thought to be responsible for coking. For example, studies from 16


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Suprun et al.63 and Erfle et al.54 suggested that over silica–alumina mixed oxides supported H3PMo12O40 (HPMo) or H4PVMo11O40 (HPVMo) and Al2O3 or TiO2 supported phosphate catalysts, the coking took place preferentially on the Brönsted acid sites during catalytic glycerol dehydration. Increasing the number of Brönsted acid sites significantly increased the amount of coke on the catalyst surface.106 Nevertheless, from those findings, it is improper to draw a conclusion that other acid sites fail to facilitate coking. As discussed above, many intermediates and reaction networks are involved in coking. Various reactions to coke reasonably need different acid sites as catalytic active center. This is partly reflected, for instance, by the fact that the H/C ratios of the carbonaceous deposits or coke also appear to correlate with the total acidity. The acidity of catalyst can affect coking rate during catalytic glycerol dehydration (Fig 7). The higher density of acid sites means that the sites are closer to each other. As a result, polycondensation of glycerol and acrolein and other side reactions, increase to form coke precursors, hence accelerating the coking.95 Nevertheless, this can also be partly explained by the proposition that due to the high density of acid sites, the intermolecular dehydration of glycerol activated on adjacent acid sites accelerate, followed by the successive coupling of hydroxyl groups of glycerol and then formation of carbonaceous deposits or coke.108 For instance, in 2009, Suprun et al.63 performed catalytic glycerol dehydration under atmospheric pressure and at 280 °C in a fixed-bed reactor, feeding the catalytic bed a 5 wt.% of aqueous glycerol solution over SAPO-11, SAPO-34, Al2O3-PO4 and TiO2-PO4 catalysts (total acidity: 1.33, 0.5, 0.3 and 0.3 mmol/g⋅cat, respectively). After 10 h, the amount of the resultant coke was found in the order: SAPO-34 (>9%) > SAPO-11 (4.6%)> TiO2-PO4 (3.1%) >Al2O3-PO4 (2.4%). The H/C ratio of carbonaceous deposits formed on TiO2-PO4 and Al2O3-PO4 was 0.54 to 0.55, respectively. Whereas the H/C ratio of carbonaceous deposits formed on SAPO samples were considerably higher (0.62-0.66).

17



Acid strength

Rate of reaction

Acid site density

Number of successive reaction step

Coke

along the diffusion path

Retention of coke precursors and coke molecules

Bimolecular reactions

Coking rate

Fig 7. Influences of the acidity of catalyst on coking.60 (Reprinted from Ref 60. Copyright (2009), with permission from Elsevier.)

The strength of acid sites also has a significant effect on coking during catalytic glycerol dehydration. Basically, a stronger acid sites can strongly adsorb glycerol, intermediates and acrolein, thereby promoting side reactions and then the faster coking.107,108,109 Talebian-Kiakalaieh et al.108 recently studied the effect of the density of acid sites and strength of acidic sites on coking during catalytic glycerol dehydration over a series of silicotungstic acid/zirconia catalysts (HSiW: H4SiW12O40·14H2O; HSiW loading: 10, 20, 30, and 40 wt.%). Generally, the acid site strength increased with the increase of HSiW loading. After 3 h, the coke on the spent HSiW and ZrO2 catalyst was 0.21 and 0.89 wt.%, respectively. By contrast, increasing the HSiW loading on/zirconia from 10 to 40 wt.% lead to the amount of the coke from 1.01 to 2.10 wt.%. Particularly, the total coke amount on the 30 wt.% HSiW/zirconia catalyst was the highest (2.10 wt.%). This catalyst possessed strongest acidic sites compared with the other ones. Nevertheless, to tune the strength of acid sites to be optimal, so far there is no general law available and it remains a challenge in the field of solid acid catalysis. Pore size and the distance of internal channels of the catalysts significantly influence the diffusion and the adsorption-desorption of reactive species in the system of catalytic glycerol dehydration. Subsequently, they decide the formation and the retention of the coke precursors and coke.106,110 If the pore sizes of a catalyst are so small that it slows the diffusion of glycerol, acrolein and by-products (ethers, hydroxyacetone, mono- and diglycerides, acetol, acetone and acetaldehyde), the formation of bulky molecules by polycondensation and cyclization could increase, hence resulting in more coking.64,89 Long channels of a porous catalyst lead to longer residence of glycerol, acrolein and 18


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by-products. The occurrence of the reactions to form coke precursors, for example polymerization, is also enhanced.48 In the case of microporous zeolites, coke deposits within external micropores can quickly block the openings.111 Thus, the acid sites inside the pores become unavailable. As a result, the catalytic activity immediately decreases. It has been found that even when the amount of coke is less, the pore openings can severely be blocked, thus hindering reactants from diffuse into pores, including narrow channels and cages.71,85,112 In the aspect, enlarging the pore openings and channel can enable glycerol and acrolein diffused quickly, thereby mitigating coking to some extent.73 As schematically summarized in Fig. 8, possible ways of carbonaceous deposits onto zeolite catalysts can be categorized as follows: (a) reversible adsorption on acid sites, (b) irreversible adsorption on sites with partial blocking of pore intersections, (c) partial steric blocking of pores and (d) extensive steric blocking of pores by exterior deposits.113 In model (a) and model (b), the reactants to the active sites in a cage or a channel intersection is partly blocked, due to chemical and/or steric reasons. The coke molecules are reversibly or irreversibly adsorbed on the acid sites (site poisoning or site coverage). Only the diffusion of reactant molecules through the cavity or the channel intersection is partly limited or blocked. The two types of coke have less negative effect on the catalytic performances since only some sites located in the cavity or at the channel intersection are deactivated. In the cases of model (c) and model (d), the blockage is due only to steric reasons; the diffusion of reactants to the acid sites inside pores is severely blocked. This blockage poison coke very highly and even completely as a large number of active sites are generally located in the inner pores.

19



(a)

Page 20 of 39

(b) Catalyst

Coke

Channel

(c)

(d)

Coke

(

Representative material can diffuse through the channel;

Representative material can’t diffuse through the channel.)

Fig 8. Schematic drawings of the four possible modes of carbonaceous deposits onto zeolites: (a) reversible adsorption on acid sites, (b) irreversible adsorption on sites with partial blocking of pore intersections, (c) partial steric blocking of pores, (d) extensive steric blocking of pores by exterior deposits.60,83 (Reprinted from Ref 60. Copyright (2009), with permission from Elsevier. Reprinted from Ref 83. Copyright (2001), with permission from Elsevier.)

4. Coking inhibition and catalyst regeneration Three methods have thus far been proposed for mitigating coking: (1) co-feeding of the oxygen molecule;114 (2) using mesoporous catalyst;115 and (3) doping solid acid catalysts with platinum group metals (Ru, Pd and Pt).116 Nevertheless, these methods can merely slow down coking. The catalyst discussed above will be eventually deactivated. The regeneration of the spent catalyst is thought to be an alternative to reducing the cost of the catalyst. The critical issue in regeneration is primarily on the removal of coke by oxygen-aided combustion. In this context, major concerns are about the choice of feeding oxygen and the regenerating reactor. The current approaches reported in the literature are: (1)

in situ regeneration by injection of oxygen into the reactor;117,118 (2) cyclic regeneration under air or oxygen;119,120 and (3) design of reactors for easy catalysts regeneration.121 Studies indicated that after such regeneration, the activity of the catalyst can be restored to some extent, but catalytic performances of the regenerated catalysts is not as good as the original catalyst and the catalysts still suffers from coking.84 20


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4.1 Co-feeding oxygen or air Co-feeding of oxygen or air can oxidize coke precursors into CO and CO2, thus inhibiting coking. When a mixture oxygen, glycerol, and steam were co-fed into the reactor, the catalyst deactivation can be inhibited and the amount of coke decreases considerably.93105,122 Interesting, the selectivity to acrolein in the presence of O2 can be slightly higher than that in the absence of O2. Meanwhile, the formation of CO and CO2 significantly increased due to the oxidation of carbonaceous compounds.41 For example, Liu et al.123 revealed that over cesium phosphotungstate/niobium oxide (CsPW-Nb) catalyst at 320 °C in a fixed bed reactor, when the O2/N2 ratio increased from 0/18 to 3/15 (L/L), the amount of coke on the CsPW-Nb500 catalyst after 9-10 h decreased from 0.023 to 0.01 g/g. Meanwhile, glycerol conversion increased from 89.0 to 93.8% and selectivity to acrolein increased from 55.0 to 76.5 %. Further, the selectivity of CO and CO2 also increased when the O2/N2 ratio increased. Similar phenomena have been found over many other catalysts when oxygen is co-fed. So far, the oxidization of glycerol and coke precursors into CO, CO2 and H2O is considered as a main reason for inhibiting coking. Yet, recent studies have shown that co-feeding can also decrease the selectivity of by-products such as acetol, acetaldehyde and formaldehyde, whereas the amount of organic acids, like acrylic acid, acetic acid and formic acid increased due to the subsequent oxidation of aldehydes.90 Another proposition is that because the presence of oxygen can also greatly decrease the adsorbed carbon species, the lifetime of the catalyst is extended.41 The co-feeding of oxygen has been used up to now for evaluating many catalysts, including vanadium pyrophosphate (VPO-800),41 cesium salts of phosphotungstic acid (CsPW), 124 tungsten oxide on zirconia (WO3/ZrO2), 125 , 126 iron phosphates (Fex(PO4)y), 127 boron phosphates (BPO), 128 phosphotungstic acid supported on Cs+ modified SBA-15 (HPW/Cs-SBA-15)104 and heteropolyacid.61 Under different circumstances, the findings and theoretic hypothesis or explanations are sometimes inconsistency. For example, co-feeding oxygen had little influence on acrolein selectivity and increased the carbon balance over vanadium pyrophosphate (VPO-800) and tungsten oxide on zirconia (WO3/ ZrO2) catalysts. However, over boron phosphate (BPO) and iron phosphate (Fex(PO4)y), the acrolein selectivity dramatically decreased by 16% and 29% due to further oxidation of acrolein to carbon oxides (CO and CO2). In addition, over cesium salts of phosphotungstic acid (CsPW) and phosphotungstic acid/ Cs+-SBA-15 (HPW/Cs-SBA-15) catalyst, co-feeding oxygen has a positive influence on acrolein selectivity and 21



catalyst stability. Nonetheless, the oxidation is exothermic and hence lead to hot-spots in the reactor and on the bed of the catalyst. Thus, it is not suitable for temperature-sensitive catalysts.129,130 Besides, high oxygen fraction (＞7%) may lead to explosion.

4.2 Using mesoporous catalysts A microporous zeolite catalyst, with high specific surface area and Brönsted acidity, exhibit good thermal stability and activity in catalytic glycerol dehydration.131,132,133 The porosity can endow the catalyst with shape-selectivity by controlling the diffusion of reactants, transient states, intermediates and product molecules.29,85 But these merits are offset by easy and severe coking. Enlarging the pore size of microporous zeolites or creating hierarchical-structured zeolites are considered an effective way to inhibit coking. A strategy is to fabricate micro-mesoporous zeolites as the catalysts. In general, the hierarchical-structured zeolite combines the advantages of both microporous and mesoporous materials134. Molecular adsorption, diffusion in such materials and accessibility of acid sites will all be improved theoretically135. For other catalysts, rather than zeolites, to create large pore also appeared to be help to mitigating coking. For instance, Znaiguia et al.73 found that a catalysts prepared from incorporating W into hydrous zirconia (ZrO2·xH2O) by anionic exchange (WSi/ZrO2-E catalysts) had pore diameter larger than 7 nm and can keep 100% glycerol conversion up to 70 h in the catalytic glycerol dehydration . Such large mesopores (>7 nm) enabled glycerol and acrolein diffuse in them quickly, thus hindering carbon deposition. By contrast, the catalyst with lower pore diameter deactivated quickly, resulting from small mesopores (4.2 nm) which were quickly clogged most possibly by glycerol condensation. In order to create hierarchical porosity into zeolitic materials, a bottom-up route and a top-down route have been successfully developed.136 The bottom-up route is usually referred as hard-templating or soft-templating methods to create mesopores in zeolites. The so-called hard-templates such as carbon blacks, carbon nanofibers, and carbon nanotubes, have been earlier employed during the synthesis of zeolite to create hierarchical ZSM-5 zeolite with intracrystal mesopores.137 Recently, the soft-template, including organic molecules, polymers, and surfactants, has been drawing increasing attention.138 The top-down process to produce mesoporosity in microporous zeolites involves the removal of aluminum or silicon components of zeolites. 139,140 Dealumination of zeolites can be 22


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conducted by such treatments as steaming141,142,143 and acid leaching.144 Delamination and assembly is another top-down route to produce hierarchically porous zeolite materials. The treatment of zeolites with alkaline can remove Si from the zeolite framework (desilication).145,146 Nevertheless, it is worth noting that for aluminum-rich zeolite crystal, the desilication process does not necessarily increase the density of local acid sites.139 Rather, dissolution of silica or desilication is favored at the aluminum-poor regions, thereby increasing the mean aluminum content in the residual material.147,148 However, these top-down routes can be regard as a destructive way and may bring fully damage to the original zeolite if the treatment is not well-controlled.149 Interesting, in this way, though the resistance to deactivation have been enhanced, the amount of coke at a given time might be increased. For instance, Decolatti et al.110 used an aqueous NaOH solutionto treat H-ZSM5 zeolite (Si/Al:15) catalyst to increase mesoporosity by extracting Si from the zeolite framework. Such a treatment led to the mesoporous H-ZSM5 with the surface area increased from 254 (before treatment) to 325 m2/g (after treatment). The micro-mesoporous H-ZSM5 catalyst, appeared more stable than the conventional H-ZSM5 catalyst. Typically, after 5 h, the amount of coke on the micro-mesoporous H-ZSM5, under the two space velocities, i.e. WHSV = 0.75 and 3 h-1, were 14.76% and 17.63%, respectively; by contrast, the amount of coke on the conventional H-ZSM5 catalyst, were 12.85% and 13.01% respectively. The strange phenomena were then explained by the proposition that the greatest amount of coke deposited on the micro-mesoporous catalyst was associated with better utilization of the inner surface and lowering the pore mouth blockage of the catalyst. Blockage of the pore mouths occurs on micropores much faster than on the mesopores. In addition, higher density acid sites (1.238 mmol/g) were obtained in the micro-mesoporous zeolite as compared to the original zeolite (0.592 mmol/g). Although the total amount of cake become higher on the micro-mesoporous zeolite. Yet, the amount of coke deposits on per number of acid sites on the micro-mesoporous zeolite was lower than that on the original zeolite catalyst. More detailed influences of zeolite catalysts after delamination or desilication on the coking at the atomic level of the active sites are untouched.

4.3 Noble metal-doped catalysts Although heteropolyacid catalysts have high activity in catalytic glycerol dehydration, they are 23



easily deactivated from coking.86, 150 In addition, to regenerate heteropolyacid catalysts by coke combustion seems impractical because of the relatively low thermal stability of heteropolyacids.151 It has been earlier revealed that doping a noble metal (Ru, Pd and Pt) into solid acid catalyst can effectively inhibit coking in the presence of H2.152,153 Following such methodology, doping Ru, Pd or Pt into heteropolyacid catalysts and co-feeding hydrogen for inhibiting coking in glycerol dehydration have also been investigated by some researchers.103,116,154 Doping the catalyst with Ru, Pd and Pt and co-feeding hydrogen can indeed reduce the amount of coke by hydrogenating coke precursors.155 For instance, Alhanash et al.116 performed catalytic glycerol dehydration over Cs2.5H0.5PW12O40 (CsPW) catalyst in a vertical fixed bed reactor at 275 °C and under atmospheric pressure. The CsPW catalyst exhibited 100% conversion of glycerol and 98% selectivity to acrolein in the first 1 h. However, the conversion decreased to 40% after 6 h, without impairing acrolein selectivity. A large amount of coke (8.9 wt.%), a mixture of aliphatic and polyaromatic carbonaceous compounds, was deposited on the catalyst surface during the process.156 When the CsPW doped with platinum group metals (Ru, Pd and Pt) was used with the feed of external hydrogen into the reactor, The 0.5%Pd/CsPW catalyst gave a 96% selectivity to acrolein at a 79% conversion of glycerol (ca.76% yield) at 275 °C and 5 h time on stream. After 6 h, the amount of coke on Ru, Pt and Pd-doped CsPW catalysts was 6.8, 8.0 and 4.5 wt.% respectively. In addition, adding hydrogen in the reactor can not only enhance the H/C ratio of coke precursors, but also reduce the unsaturation and polarity of coke precursors by the hydrogenation. As a result, the adsorption of coke precursors on the acid sites of Ru, Pt and Pd-doped CsPW catalysts s was significantly weakened. Accordingly, desorption of coke precursors from the modified catalysts became easier. Consequently, the amount of coke deposited on the active sites was significantly decreased. A recent study implies that co-feeding hydrogen is not always necessary to mitigate coking over noble metals-doped catalysts. Instead, coking can be inhibited by capturing coke precursors in a Pd lattice.154 For instance, Ma et al.103 found that for catalytic glycerol dehydration over HPW/Zr-MCM-41 and Pd-HPW/Zr-MCM-41 catalyst (HPW: H3PW12O4) in a vertical fixed bed reactor under atmospheric pressure at 320 °C. For HPW/Zr-MCM-41 catalyst, after 50 h, the glycerol conversion decreased from 100% to 55%. By contrast, for Pd-HPW/Zr-MCM-41 catalyst, the glycerol conversion decreased slightly from 97% to 87%. Moreover, the amount of coke of on the HPW/Zr-MCM-41 was 6.9 wt.%, whereas the amount of coke on the Pd-HPW/Zr-MCM-41 catalyst was only 2.8 wt.%. It has been revealed that because the loosening of the palladium lattice allowed coke precursors go into the Pd 24


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lattice of the catalyst, forming the so-called “metastable Pd6C phase”.103 In addition, platinum has a high affinity to olefins resulting in the migration of olefin from the acid sites to the Pd metal sites.157 Therefore, the coking is significantly reduced. Such findings and mechanistic explanation provide new ways to inhibiting coking.

4.4 Regeneration Catalysts deactivated by coke are typically regenerated by burning off coke in air or oxygen atmosphere (Fig 9).158 The removal of coke on the catalyst by combustion in a flow of air or oxygen can be carried out in the original reactor (in situ regeneration)56,109 or in specially designed regenerator (periodic regeneration).159,160

4.4.1 Coke combustion Carbonaceous compounds can be converted into small molecules such as CO, CO2 and H2O by in situ regeneration in the presence of oxygen molecules in the original reactor. The oxygen molecules can also provide in the form of a mixture of gases containing oxygen molecules. Atia et al.86 demonstrated that the spent H3PW12O40·xH2O/aluminosilicate catalyst can be regenerated in situ. Nitrogen flow was replaced by a mixture of 99% of N2 and 1% of O2 and temperature was raised to 325 °C for 24 h. Coke was then burned into small molecules such as CO, CO2 and H2O. After switching back to reaction conditions for catalytic glycerol dehydration, glycerol conversion recovered to 100%. Though recovery of initial catalyst activity is possible by in situ oxidative regeneration, the regenerated catalysts also got deactivation due to coking and in many cases the coking will be much faster than the fresh catalysts. For example, for Pd-HPW/Zr-MCM-41 in the catalytic glycerol dehydration, after it was used for 60 h, the used Pd-HPW/Zr-MCM-41 was heated to 450 °C under an oxygen flow (10 mL/min) for 4 h.103 The amount of carbon species on the regenerated Pd-HPW/Zr-MCM-41 catalyst became less than 0.1 wt.%. The regenerated Pd-HPW/Zr-MCM-41 catalyst exhibited a 95% conversion of glycerol and 84% selectivity to acrolein. The regenerated Pd-HPW/Zr-MCM-41 catalyst exhibited almost the same glycerol conversion and acrolein selectivity for about 35 h as the fresh catalyst. However, faster deactivation was observed after 40 h, indicating that the regenerated catalysts had lower stability than fresh catalysts. 25



Fig 9. In situ regeneration of the catalyst through coke combustion under air or oxygen.

Coke on the spent catalyst can also be burned off under a flow of air. In view of engineering, the operations can be adjusted or optimized. For example, Katryniok et al. 161 regenerated 20 wt.% silicotungstic acid/SBA-15 catalyst by continuously co-feeding dry or wet air periodically at 275 °C for 6 h. Both dry and wet air co-feeding could significantly improve the catalytic activity. The regenerated 20 wt.% silicotungstic acid/SBA-15 catalyst by such a periodic regeneration method could exhibit 76% and 74% acrolein yield even after 24-25 h and 96-97 h reaction time, respectively. However, the regenerated catalysts did not recover its initial catalytic activity and they deactivated quickly. Similar phenomena have been observed for many other regenerated catalysts. For instance, Viswanadham et al.84 found that 30 wt.% phosphotungstic acid (PTA) /niobia catalyst exhibited 99.8% conversion of glycerol with 92 % selectivity to acrolein during 1-5 h. The conversion and selectivity was stable until 9-10 h and then decreased rapidly, due to coking. After 20 h, there was about 1.23% carbon deposited on the catalyst. The catalyst was able to be regenerated by simple oxidative treatment under airflow at 400 °C for 4 h to remove the coke on the surface of the catalyst. Although the Keggin ion of PTA remained intact after regeneration of the catalyst, the intensity in Brönsted acidity of the catalyst decreased significantly. The regenerated sample shows only 80% conversion and 62% acrolein selectivity which suggested the regenerated catalyst did not recover the initial performance. Sergey et al.162 discovered that a thermal treatment at 375 °C under an air flow of 50 ml·min−1 was able to 26


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remove the coke formed on Pt/γ-Al2O3 catalyst. The catalytic activity was significantly improved and there was a similar initial period of selectivity increase. One possible reason is that after coke combustion, more acid sites were additional created. Such a positive role could depend on the components in the coke, which interact with the surface of the catalysts. During combustion, they decompose into different active species or organic groups, including acidic ones. But at present no detailed experiments to probe this issue and to evidence such assumption.

4.4.2 Innovative reactor design The process of regenerating spent catalyst requires high energy input as high temperature is essential to for burning off the coking.163,164,165 Further, the main challenge of regeneration under air or oxygen is that high an oxygen fraction (7%) may lead to explosion.166 More importantly, such regeneration process should be judiciously controlled so that the catalytic active sites or centers should be recovered as much as possible. Innovative design for the reactor is necessary and is regarded as an effective approach to regenerate spent catalyst.117 Nevertheless, such studies are scarcely reported in the scientific papers. Typically, for a circulating bed reactor patented by Dubois et al121 and adapted by Corma et al167 and O'Connor et al,159 preheated catalysts are co-fed with the reactants into the reactor continuously at a very short residence time (e.g. 0.3-2 s). In addition, there was a separator for storing the spent catalyst and burning off coke (Fig 10). It was shown that using such a reactor, the process could be carried out in an autothermic way. Namely, the heat from coke combustion can be used to evaporate the aqueous glycerol solution. The catalyst was continuously separated from the reaction products and the spent catalyst was regenerated.

27



Hydrocarbon products

Stripper CO2.N2

T=290-530℃

Stripper injection Riser reactor T=650-750℃

T=290-550℃

Riser secondary injection point, high throughput T=600-680℃

Riser first injection point, high throughput T=650-750 °C

Mixing pot injection Air or O2

Mixing

Fig 10. Circulating bed reactor showing the injection of glycerol into FCC (fluid catalytic cracking)-type reactor and catalyst regeneration.39 (Reprinted from Ref 39. Copyright (2008), with permission from Elsevier.)

5. Concluding remarks and perspectives Many solid acid catalysts, including sulfates, metal phosphates, metal oxides, heteropolyacids and zeolites have proved to be active in glycerol dehydration. These solid acid catalysts suffer from coking and resultant catalyst deactivation. Now this problem represents a major challenge in the course of commercialization of catalytic glycerol dehydration. Coke exists on the surface and inside the pores of catalysts. Aromatic macromolecules and polyglycols have been identified as main coke precursors and components in the coke on the spent catalyst. However, the exact compositions and the detailed processes of coking in glycerol dehydration remain unclear. Because of the coke and the related carbonaceous deposits consist of a large number of compounds, thus, to get an accurate analysis is not an easy task. Nevertheless, recent progress has provided some information or possible ways to solve this issue. For example, further work can be conducted for more detailed analysis on the soluble and 28


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insoluble fractions of coke, and the ratio of H/C. Meanwhile, the thermodynamics, kinetics and the formation mechanism of carbon deposition can be probed. Thus the real networks for the formation of coke could be well disclosed. In addition, studies have clearly revealed that coking is relevant to both the catalyst and reaction conditions including glycerol concentration, water content, reaction temperature, contact time, catalyst acidity and pore size. It has been concluded that when higher concentrations of glycerol are fed into the reactor, typically polymerization of glycerol to polyglycols is more likely to occur and accordingly carbon deposits are more easily formed. Certain intermediates or products, which depend heavily on reaction temperatures and catalysts, may promote the formation of carbonaceous compounds. Furthermore, contact time and water content could affect the interaction between reactants, products and active sites of catalysts. Although pore size is believed critical in coking, it is currently difficult to measure quantitatively the impacts of acidity and pore structure on coking. These findings imply that major direction for future work should lay emphases o tuning the surface acidity and the pore structures and meanwhile the relevant reaction conditions need optimization. As surveyed and discussed above, coking can be, to some extent, mitigated by co-feeding oxygen molecules, using mesoporous catalyst, and doping noble metals into solid acid catalysts. However, the resultant inhibition for coking cannot last for long time and the solid acid catalysts still deactivate due to coking. The spent catalyst can be regenerated in situ and ex situ at high temperatures by oxidative treatment under O2 or air. However, the lifetime of the catalyst after regeneration is still short. Regeneration also brings new problems such as tedious operations and low productivity. Hence, designing a new catalyst with both high activity and good coking-resistance represents a critical issue and it needs addressing so that the catalytic glycerol dehydration can be put into practice.

Acknowledgements:

The authors wish to acknowledge the financial support from the National Natural Scientific Foundation of China (41672033; 21373185); the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology (GCTKF2014006); Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province (2016); and the State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, China (CRE-2016-C-303). The authors also thank all of the anonymous reviewers for their invaluable comments and suggestions that help to improve very much the quality of the work.

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Coking of Catalysts in Catalytic Glycerol Dehydration to Acrolein

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