The invention relates to a process for the production of a vanadium/phosphorus mixed oxide catalyst precursor, its transformation into the active catalyst and a process for the production of maleic anhydride using this catalyst
Maleic anhydride is a well known and versatile intermediate for manufacturing unsaturated polyester resins, pharmaceuticals or agrochemicals. It is usually produced by catalytic partial oxidation of aromatic (e. g., benzene) or non-aromatic (e. g., n-butane) hydrocarbons.
The main component of the active catalyst in the oxidation of non-aromatic hydrocarbons like n-butane to maleic anhydride is vanadyl pyrophosphate, (VO)2P2O7, which as a rule is obtained by thermal treatment of vanadyl acid orthophosphate hemihydrate of the formula (VO)HPO4xe2x80xa20.5H2O, acting as catalyst precursor. Both vanadyl pyrophosphate and vanadyl acid orthophosphate hemihydrate may, if desired, be accompanied by a promoter element selected from the groups IA, IB, IIA, IIB, IIIA, IIIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table of elements, or mixtures of such elements.
Methods for preparing the precursor compound conventionally involve reducing a pentavalent vanadium compound under conditions which will provide vanadium in a tetravalent state (average oxidation number +IV).
Prior art knows a great many different procedures, which however in general involve the use of vanadium pentoxide (V2O5) as a source of pentavalent vanadium and orthophosphoric acid (H3PO4) as the phosphorus source (see e.g. U.S. Pat. No. 5,137,860 or EP-A-0 804 963).
As a reducing agent in principle any inorganic or organic compound containing elements which are able to act as a redox couple possessing an oxidation potential suitable for this kind of reaction may be suitably applied.
The most common reducing agent is hydrogen chloride in aqueous solution.
Also favourably applied are organic media like primary or secondary aliphatic alcohols or aromatic alcohols such as benzyl alcohol as these compounds seem to at least in part dissolve the reactants and thereby facilitate the redox reaction.
The most preferred organic reducing agent is isobutyl alcohol as isobutyl alcohol combines optimal characteristics such as (i) a boiling point of 108xc2x0 C. at atmospheric pressure, (ii) dissolution of the vanadium alcoholates formed from V2O5, thus favouring a complete redox reaction in the liquid phase and (iii) achieving a redox potential for the couples isobutyl alcohol/isobutyraldehyde and isobutyl alcoholrisobutyric acid suitable to let the alcohol act as reducing agent. The tetravalent vanadium reacts with phosphoric acid (H3PO4) and leads to precipitation of the precursor vanadyl acid orthophosphate hernihydrate of the formula (VO)HPO4xe2x80xa20.5H2O. The precipitate is usually washed with isobutyl alcohol and subsequently dried.
A major disadvantage of the conventional method as described above is that even after drying the precursor contains some percent of organic compounds from the organic reaction medium, compounds which are supposedly either (i) strongly adsorbed at the solid surface, and therefore not easily removable by the washing and drying treatment, or (ii) physically trapped in between the crystals of the precursor, or (iii) physically or chemically trapped a (xe2x80x9cintercalatedxe2x80x9d) in the crystalline structure of the precursor.
It has been found that this percentage of organic compound which remains trapped in the precursor is a fundamental parameter which can adversely affect the performance characteristics of the active catalyst obtained after the thermal treatment.
The object of the present invention therefore was to provide a method for controlling the carbon content in a vanadium/phosphorus mixed oxide catalyst precursor and accordingly to provide a superior catalyst precursor which, when activated, leads to superior results in the conversion of a non-aromatic hydrocarbon to maleic anhydride.
It was found that the objectives could be achieved with a new process for the preparation of a vanadium/phosphorus mixed oxide catalyst precursor according to claim 1.
The invention comprises reducing a source of vanadium in the presence of a phosphorus source in an organic medium which comprises
(a) isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcohol and
(b) a polyol
in the weight ratio of 99:1 to 5:95.
In a mixture of isobutyl alcohol and benzyl alcohol the benzyl alcohol content is as a rule between 5 and 50 wt %.
As a source of vanadium a tetravalent or pentavalent vanadium compound may be applied. Representative examples, although not limiting, are vanadium tetrachloride (VCl4), vanadium oxytribromide (VOBr3), vanadium pentoxide (V2O3), vanadyl phosphate (VOPO4xe2x80xa2nH2O) and vanadium tetraoxide (V2O4). Vanadium pentoxide is the preferred vanadium source.
As mentioned above, the vanadium source may, if desired, be accompanied by promoter elements selected from the groups IA, IB, IIA, IIB, IIIA, IIIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table of elements, or mixtures thereof.
Preferred promoter elements are selected from the group consisting of zirconium, bismuth, lithium, molybdenum, boron, zinc, titanium, iron and nickel.
Orthophosphoric acid (H3PO4) is the preferred phosphorus source.
Isobutyl alcohol is the preferred component (a).
Polyols which can be used as the component (b) are expediently C2-6 aliphatic polyols, preferably C2-6-alkanediols such as 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol, 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 2,3-hexanediol, 2,4-hexanediol, 2,5-hexanediol and 3,4-hexanediol.
Most preferred polyols are the C2-4-alkanediols 1,2-ethanediol, 1,3-propanediol and 1,4-butanediol.
The preferred mixture of alcohols contains 5 to 30 mol % of polyol with respect to isobutyl alcohol.
As a rule the vanadium source together with the phosphorus source is suspended in the organic medium and the mixture is kept under agitation at a temperature of expediently 90xc2x0 C. to 200xc2x0 C., preferably 100xc2x0 C. to 150xc2x0 C. over a period of 1 h to 24 h.
The ratio of vanadium source to phosphorus source is conveniently such that the P/V atomic ratio is in the range of 1:1 to 1.3:1, preferably 1.1:1 to 1.2:1.
As a rule after the reduction the precursor vanadyl acid orthophosphate hemihydrate of the formula (VO)HPO4.0.5H2O is formed which is filtered, washed and subsequently dried at a temperature of expediently 120xc2x0 C. to 200xc2x0 C.
Due to the reduction treatment according to the invention the carbon content of the precursor can be controlled in the range of 0.7 wt. % to 15.0 wt. %, preferably in the range of 0.7 wt. % to 4 wt. %.
It has been found that best results are obtained with catalyst precursors which, after an additional thermal treatment at about 300xc2x0 C. for about 3 hours in air have a residual carbon content of 0.7 wt. % to 3 wt. %, most preferably 0.8 wt. % to 1.5 wt. %.
Once prepared the precursor can in view of its further activation treatment be formed into defined structures with defined properties. Such procedures may include wet grinding to a specific particle size, the addition of additives to improve attrition resistance, and the a formation of a convenient shape.
A spherical shape for instance is most suitable for the application of the catalyst in a fluidized bed.
The further transformation of the so formed precursor into the active catalyst can be performed following a great number of activation processes known in the art, but in general include a heat treatment applying temperatures of up to 600xc2x0 C. More in detail, these processes may involve:
(a) an initial heating of the precursor to a temperature not to exceed 250xc2x0 C.
(b) a further heat treatment from about 200xc2x0 C. to at least 380xc2x0 C. to 600xc2x0 C. at the maximum
(c) maintaining the temperature of stage (b) over a certain time and
(d) cooling the activated catalyst, thereby maintaining an individually controlled atmosphere in all steps.
In a preferred embodiment, the activation of the precursor is accomplished using the procedure described in EP-A-0 804 963.
In a more preferred embodiment, the activation comprises the steps of
(a) heating the catalyst precursor from room temperature to a precalcination temperature of about 300xc2x0 C. in air or oxygen-depleted air
(b) keeping at said pracalcination temperature,
(c) further heating the precalcined catalyst precursor in nitrogen up to a calcination temperature of about 550xc2x0 C. and
(d) keeping at said calcination temperature.
After the transformation into the active catalyst the catalyst is ready to be applied for the conversion of non-aromatic hydrocarbons to maleic anhydride.
Such processes are well known in the art, e. g. from U.S. Pat. No. 4,594,433, U.S. Pat. No. 5,137,860 or U.S. Pat. No. 4,668,652.
In general the non-aromatic hydrocarbon is converted with oxygen or an oxygen containing gas at a temperature from about 320xc2x0 C. to 500xc2x0 C. to maleic anhydride. The non-aromatic hydrocarbon is expediently selected from aliphatic C4-10 hydrocarbons, preferably nbutane. The conversion can take place in a fixed bed or a fluidized bed reactor but preferably in a fluidized bed reactor. The following examples are given by way of illustration only and arc not construed as to in any way limit the invention.