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Supported Flat-Sheet and Tubular Carbon Membranes

In contrast to unsupported CMS film, which is fragile, supported CMS membranes manufactured along a porous support (e.g., ceramics or metal) exhibit significantly improved mechanical strength. Different options such as dip-coating [15,16], spin-coating [17-20], spray-coating [21], and chemical vapor deposition (CVD) [22] have been employed to coat the support with polymeric precursor. These methods can reduce the thickness of the membrane selective layer, which usually provides a higher gas permeance than a self-standing film. Representative supported carbon membranes have been reviewed by Lie et al. [23], and the most recently developed membranes are included in Table 2.1.

Support Modification

The properties of porous supports can directly affect the structure of deposited CMS membranes and, consequently, membrane separation performance. For instance, commercial ceramic porous supports are commonly macropo- rous (pore size -200 nm), which may lead to interfacial defects when forming the selective layer [24]. Thus, multilayer substrates are widely used for CMS membrane fabrication to improve the interface. A porous intermediate layer (normally 1-10 nm pore size) between the macroporous support and the CMS selective layer is often incorporated during the membrane development process. For example, to avoid large pores in an a-ALO, tube support that might cause pinholes in the CMS membrane, it is possible to deposit a thin y-AL03 layer with mean pore size of -4 nm [15], which is generally formed by multi dip-coating of a boehmite (y-AlOOH) sol and followed with a calcination procedure [15, 25, 26].

The separation performance of CMS membranes can be improved by tuning the properties of the intermediate layer. Tseng et al. [27] reported enhanced H2/C02 separation performance by modifying the A1203 support with а ТЮ2 intermediate layer. They suggested that the intermediate layer could provide a networking interlocking pattern with CMS membranes, which is beneficial

TABLE 2.1

Examples of supported CMS membrane preparation conditions and gas permeation results.

Supported support / geometry

Pore size (pm)

Precursor

Method*

Pyrolysis

Gas test temperature

Permeance (10-9 nr2 s-i pa-')**

mol

Ideal selectivity (separation factor)

Reference

H2 (He)

co2

o2

h2/n2

o2/n2

со2/сн4

Carbon/disk Ф 35 mm

Matrimid

S

700 °C - 2 h (vacuum)

25 °C

(27)

36.6

8.1

-

4?

8.1 (23)

[42]

Carbon/disk Ф 35 mm

Phenolic resin

S

700 °C (vacuum)

25 °C

(8.2)

2.0

1.2

265

87(165)

[43]

u-AUO,/tube (o.d. 2.3 mm)

0.14

Phenolic resin + sulfonated phenolic resin

D

500 °C -1 h (N2)

35 °C

  • 56
  • 134
  • 10
  • 40
  • 2.3
  • 10

160

  • 10.8
  • 12
  • 170
  • 54

[44]

a-ALOj/tube (o.d. 10 mm, i.d. 7 mm)

0.2

Novolac resin + bohemite

D

550 °C - 2h (N2)

Room

temperature1

  • 145
  • (79.5)

-

3.0

725

15

-

[45]

a-Al20,/Tube (o.d. 0.9 mm, i.d. 0.6 mm)

  • 0.005
  • 0.0035

Furfuryl

alcohol

V

600 °C - 1 h 600 °C - 1 h

25 °C 25 °C

  • 25.5
  • 6.04
  • 5.82
  • 2.67
  • 0.775
  • 0.845
  • 347
  • 91
  • 10.6
  • 12.7
  • 92
  • 82

[46]

u-Al,0,/o.d. 2.25 mm, i.d. 1.8 mm

Lignocresol

D

600 °C - 1 h (N2)

35 °C 105 °C

56 (29) 82 (43)

  • 17
  • 2.3
  • 2.7
  • 8.2
  • 167
  • 44
  • 8.0
  • 4.5
  • 87
  • 17

[47]

Anodic alumina/ 4 cm2

0.020

Graphene

oxide

V

-

20 °C

100

0.03?

-

-900

-

-

[48]

Coal disk/Ф 40 mm 2 mm thick

  • 0.71
  • (largest

size)

PMDA-ODA

S

800 °C - 2 h

54.55

8.80

7.45

76.3

10.4

[49]

a-ALOj / o.d. 13 mm i.d. 8 mm

3.0

Polyfurfuryl

alcohol

D

700 °C - 4 h (Ar)

35

10

7.5

58

12.5

[50]

' D: dip-coating, S: spin-coating, V: vapor phase deposition. ” Values were read from figures.

' After air exposure.

11 After H20 activation.

to both gas permeability and selectivity. A proposed interlocking network for the CMS layer and support is illustrated in Figure 2.3. When the polymeric precursor was deposited on the A1203 support, the polymer chain penetrated the porous support and formed a dense phase on the surface, as shown in Figure 2.3a. Flowever, the induced intermediate Ti02 layer could provide interconnected channels with the polymer solution, which reduces the connected depth compared with the A1203 support, resulting in a thinner and larger d-spaced CMS membrane layer, as shown in Figure 2.3b. Similarly, Wey et al. [20] investigated the interface between the CMS layer and а ТЮ2/ A1203 composite support. Owing to the mechanical interlocking effects of ТЮ2, the adhesion between the CMS layer and the A1203 support is improved and thus permeability and selectivity are increased.

 
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