Gas Separation Membranes

1 . A gas separation membrane comprising the following layers:

(i) optionally a support layer;

(ii) a discriminating layer;

(iii) a non gas separation selective further layer; and

(iv) a protective layer on top of the further layer (iii);

wherein:

(a) the further layer (iii) and the discriminating layer (ii) each independently comprise groups of Formula (1):

M-(O—) x   Formula (1)

wherein:

each M independently is a metal or metalloid atom;

O is an oxygen atom; and

each x independently has a value of at least 4;

(b) the further layer (iii) comprises 1.5 to 10 atomic % of M of Formula (1) groups, wherein M is as hereinbefore defined; and

(c) the discriminating layer (ii) comprises more than 10 atomic % of M of Formula (1) groups, wherein M is as hereinbefore defined.

2 . The gas separation membrane according to claim 1 wherein the further layer (iii) is formed by a plasma deposition process using precursors in the absence of oxygen.

3 . The gas separation membrane according to claim 1 wherein each M in Formula (1) in said discriminating layer or further layer (iii) independently is silicon, titanium, zirconium or aluminium.

4 . The gas separation membrane according to claim 1 wherein the further layer (iii) has a thickness in the range 1 nm to 1,500 nm.

5 . The gas separation membrane according to claim 1 wherein the further layer (iii) has a thickness in the range 5 nm to 1,000 nm.

6 . The gas separation membrane according to claim 1 wherein the further layer (iii) comprises an average value for the atomic % of M of Formula (1) groups which is substantially constant throughout its depth.

7 . The gas separation membrane according to claim 1 comprising on top of the further layer (iii) a polysiloxane protective layer (iv).

8 . A process for forming a gas separation membrane according to claim 1 which comprises the steps of:

(a) applying the discriminating layer (ii) to the support layer (i) by a plasma treatment process; and

(b) applying further layer (iii) to the discriminating layer (ii) (the DL) by a plasma treatment process; and optionally

(c) applying a protective layer (iv) to the further layer (iii).

9 . The process according to claim 8 wherein step (b) is performed by a plasma treatment process in the presence of one or more of the following precursor compounds: tetraethyl orthosilicate, hexamethyldisiloxane, tetramethyl orthosilicate, 1,3,5,7-tetramethylcyclotetrasiloxane, octamethyl cyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, bis(triethoxysilyl)ethane, tetrapropylorthotitanate, titanium ethoxide, titanium tetraisopropoxide or zirconium tetra-tert-butoxide.

10 . The process according to claim 8 wherein step (b) is performed in an atmosphere free from oxygen.

11 . The process according to claim 8 wherein the support layer (i) is or comprises a non-porous sheet and the process further comprises the step of removing the non-porous sheet from the gas separation membrane.

12 . A gas separation module comprising a gas separation membrane according to claim 1 .

13 . (canceled)

14 . The process according to claim 8 wherein step (a) comprises applying the discriminating layer (ii) to the support layer (i) by a plasma treatment process in an atmosphere comprising oxygen and step (b) comprises applying the further layer (iii) to the discriminating layer (ii) by a plasma treatment process in an atmosphere free from oxygen and in the presence of one or more of the following precursor compounds: tetraethyl orthosilicate, hexamethyldisiloxane, tetramethyl orthosilicate, 1,3,5,7-tetramethylcyclotetrasiloxane, octamethyl cyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, bis(triethoxysilyl)ethane, tetrapropylorthotitanate, titanium ethoxide, titanium tetraisopropoxide or zirconium tetra-tert-butoxide.