Chromium-Based Catalysts

1 - 24 . (canceled)

25 . A process for preparing a supported chromium-based catalyst for the production of polyethylene comprising:

providing a silica-based support having a specific surface area of at least 250 m 2 /g and of less than 400 m 2 /g and comprising a chromium compound deposited thereon, the ratio of the specific surface area of the support to chromium content being at least 50000 m 2 /g Cr;

dehydrating the silica-based support; and

titanating the dehydrated silica-based support in an atmosphere of dry and inert gas containing at least one vaporised titanium compound of the general formula selected from R n Ti(OR′) m and (RO) n Ti(OR′) m , wherein R and R′ are the same or different hydrocarbyl groups containing from 1 to 12 carbon atoms, and wherein n is 0 to 3; m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a ratio of specific surface area of the support to titanium content of the titanated catalyst ranging from 5000 to 20000 m 2 /g Ti.

26 . The process of claim 25 , wherein a ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 20000 m 2 /g Ti when the support has a specific surface area of from at least 2501m 2 /g to less than 380 m 2 /g and the ratio of specific surface area of the support to titanium content of the titanated catalyst ranges from 5000 to 8000 m 2 /g Ti when the support has specific surface area of from at least 380 m 2 /g to less than 400 m 2 /g.

27 . The process of claim 25 , wherein the dehydration is carried out at a temperature of at least 220° C. in an atmosphere of dry and inert gas.

28 . The process of claim 25 , wherein titanation is carried out at a temperature of at least 220° C.

29 . The process of claim 25 , wherein titanation is carried out at a temperature of at least 250° C.

30 . The process of claim 25 , wherein titanation is carried out a temperature of at least 270° C.

31 . The process of claim 25 , wherein the support has a specific surface area of from 280 to 380 m 2 /g.

32 . The process of claim 31 , wherein the support has a specific surface area of from 280 to 350 m 2 /g.

33 . The process of claim 25 , wherein the at least one titanium compound is selected from the group consisting of tetraalkoxides of titanium having the general formula Ti(OR′) 4 wherein each R′ is the same or different and can be an alkyl or cycloalkyl group each having from 3 to 5 carbon atoms, and mixtures thereof.

34 . The process of claim 25 , wherein a ratio of the specific surface area of the support to titanium content of the titanated catalyst is from 6500 to 15000 m 2 /g Ti.

35 . The process of claim 25 , wherein a ratio of the specific surface area of the support to chromium content ranges from 50000 to 200000 m 2 /g Cr.

36 . The process of claim 25 further comprising:

activating the titanated chromium-based catalyst at a temperature of from 500 to 850° C.

37 . The process of claim 25 further comprising:

activating the titanated chromium-based catalyst at a temperature of from 500 to 700° C.

38 . A method for preparing polyethylene comprising:

polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms in the presence of the activated chromium-based catalyst of claim 25 .

39 . Polyethylene formed by the method of claim 38 comprising a semi-high molecular weight polyethylene, with an HLMI ranging from 5 to 12 g/10 min.

40 . An article formed by the polyethylene of claim 39 , wherein the article is selected from blow molded articles, films and pipes.

41 . A method for polymerising ethylene comprising:

injecting an activated catalyst into a gas-phase polymerisation reactor;

injecting ethylene and any optional alpha-olefinic comonomer into said reactor, allowing said ethylene and any optional comonomer to (co)polymerise and recovering a polyethylene powder, characterised in that the activated catalyst is manufactured by a process comprising:

providing a support with a chromium compound deposited thereon;

dehydrating the support to form a dehydrated support;

titanating the dehydrated support in an atmosphere of dry and inert gas containing at least one vaporised titanium alkoxide compound to form a titanated support; and

activating the titanated support at a temperature of at least 500° C.

42 . The method of claim 41 , wherein the support is a silica-based support.

43 . The method of claim 41 , wherein the support is titanated at a temperature of at least 250° C. in an atmosphere of dry and inert gas.

44 . The method of claim 41 , wherein the activating is carried out at a temperature of from 500 to 850° C. in an oxidising atmosphere.

45 . The method of claim 41 , wherein the support has a specific surface area of at least 250 m 2 /g and of less than 600 m 2 /g.

46 . The method of claim 41 , wherein a chromium concentration is at least 0.1 wt-% and at most 1.0 wt-%, based on the weight of the titanated chromium-based catalyst.

47 . The method of claim 41 , wherein the at least one titanium alkoxide compound is selected from R n Ti(OR′) m , (RO) n Ti(OR′) m and mixtures thereof, wherein R and R′ are the same or different hydrocarbyl groups containing from 1 to 12 carbon atoms, and wherein n is 0 to 3, m is 1 to 4 and m+n equals 4.

48 . The method of claim 47 , wherein the at least one titanium alkoxide compound is selected from the group consisting of tetraalkoxides of titanium having the general formula Ti(OR′) 4 wherein each R′ is the same or different and can be an alkyl or cycloalkyl group each having from 3 to 5 carbon atoms, and mixtures thereof.

49 . The method of claim 48 , wherein a concentration of deposited titanium is from 1.0 wt-% up to 5.0 wt-% based on the weight of the titanated chromium-based catalyst.