WO2014199349A2

WO2014199349A2 - Metal impregnated amorphous silicates for the selective conversion of ethanol to butadiene

Info

Publication number: WO2014199349A2
Application number: PCT/IB2014/062203
Authority: WO
Inventors: Mathias Feyen; Kirsten SPANNHOFF; Ulrich Müller; Xinhe Bao; Weiping Zhang; Dirk De Vos; Hermann Gies; Takashi Tatsumi; Feng-Shou Xiao; Yokoi TOSHIYUKI; Yilmaz BILGE
Original assignee: BASF China Co Ltd; BASF SE; Tokyo Institute of Technology NUC
Current assignee: BASF China Co Ltd; BASF SE; Tokyo Institute of Technology NUC
Priority date: 2013-06-13
Filing date: 2014-06-13
Publication date: 2014-12-18
Anticipated expiration: 2015-12-13
Also published as: WO2014199349A3

Abstract

The present invention relates to a catalyst for the preparation of butadiene comprising Hf and two or more further catalytically active metals M1 and M2, wherein the two or more fur- ther catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M1 is different from M2; as well as to a process for the preparation of butadiene comprising (i) providing a gas stream G-1 comprising ethanol; (ii) contacting the gas stream G-1 provided in (i) with theinventive catalyst, thereby obtaining a gas stream G-2 comprising butadiene; wherein the catalyst comprises Hf and two or more further catalytically active metals M and M2, wherein the twoor morefurther catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M1 is different from M2, as well as to the use of the catalyst comprises Hf and two or more further catalytically active 1 metals M1 and M2, wherein the twoor morefurther catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M1 is different from M2.

Description

Metal impregnated amorphous silicates for the selective conversion of ethanol to butadiene The present invention relates to a process for the preparation of butadiene using a catalyst comprises Hf and two or more further catalytically active metals M1 and M2, wherein the two or more two further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M1 is different from M2. The present invention further relates to a catalyst comprising Hf and two or more further catalytically active metals M1 and M2 as such, and to its use as a catalytically active material for the preparation of butadiene, preferably from a gas stream comprising ethanol and optionally acetaldehyde

INTRODUCTION

Butadiene is widely used in the chemical industry, for example as monomer and/or co- monomer for the polymerization of elastomers. Currently, butadiene is almost entirely produced as a by-product of ethylene stem cracking of naphtha or gas oil feedstock. Due to increasing prices of oil, alternative methods for producing butadiene are of major interest.

In Catal. Sci. Technol. 201 1 , 1 , 267-272 a variety of silica impregnated bi- and trimetallic catalysts for the conversion of ethanol into 1 ,3-butadiene is described. The highest selectivity observed was 67% at 45% conversion using a Cu, Zr, Zn, Si02 system. Further, Catal. Sci. Technol. 201 1 , 1 , 267-272 discloses the use of catalysts impregnated with Hf and Zn, wherein low selectivities to butadiene in the range of from 4.9 to 6.7 % and conversions in the range of from 15 to 26 % were achieved. Furthermore, all catalysts tended to show a reduced conversion rate over a period of 3 h.

In GB 331482 a process for the preparation of butadiene is described, wherein ethanol is contacted with aluminum oxide mixed with zinc oxide. However this process leads to a low yield of butadiene of 18 %. In Ind. Eng. Chem. 41 (1949), pages 1012-1017, the preparation of butadiene by a two step process is described. In the first step ethanol is dehydrogenated to acetaldehyde. In the second step, the obtained acetaldehyde is mixed with ethanol and converted to butadiene by use of impregnated catalysts. By use of the most efficient catalyst which comprises 2.3 weight % tantalum oxide on amorphous silica, a yield of butadiene of up to 69 % and a con- version of the starting material of 34 % were achieved for 8 h on stream. However, due to the price of tantalum, the catalyst is relatively expensive. Furthermore, US 2421361 discloses a process for the preparation of butadiene which comprises passing an acyclic mono-olefinic aldehyde like crotonaldehyde or acetaldehyde and a monohydric alcohol like ethanol over a catalyst of the group of zirconium oxide, tantalum oxide, columbium oxide and combinations of these oxides with amorphous silica. By use of the catalyst containing 2 weight-% of zirconium oxide, a 47 % single-pass yield of the butadiene fraction was obtained which contained about 93 weight-% butadiene.

In WO 2012/015340 A1 a process for the preparation of butadiene is disclosed by use of a solid catalyst containing metals chosen from the group of silver, gold or copper, and metal oxides, chosen from the group of magnesium, titanium, zirconium, tantalum or niobium oxide. However, only low conversion rates in the range of from 6 to 64 % were achieved in this process, wherein these values were determined during only 3 h time on stream.

DETAILED DESCRIPTION

Thus, it was an object of the present invention to provide a process for the preparation of butadiene which does not exhibit the disadvantages of the methods according to the prior art and wherein a high conversion of the starting material as well as a high selectivity to butadiene is achieved. Furthermore, it was an object of the present invention to improve the long term activity of the catalyst used.

Surprisingly, it was found that by a process for the preparation of butadiene in the presence of a catalyst comprising Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2, an unexpectedly high conversion of the starting material and at the same time also high selectivity towards butadiene is achieved. Further, it was surprisingly found that the catalysts used in the process of the present invention show an improved long time activity compared to the catalysts used in the prior art.

Therefore, the present invention relates to a catalyst for the preparation of butadiene comprising Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2.

According to the present invention, it is preferred that Hf and the two or more further catalytically active metals comprised in the catalyst are disposed on a support. Therefore, according to a preferred embodiment of the present invention, Hf and the two or more further catalytically active metals are disposed on a support. As regards the two or more further catalytically active metals M 1 and M2, no specific restrictions exist concerning the combinations of M 1 and M2 selected from the group consisting of Zr, Zn, Cu, and combinations of two or more thereof. Therefore, according to the present invention, all combinations of Hf, Zr, Zn and Cu are conceivable. Thus, according to the present invention, the catalyst may comprise Hf, Zr, Zn and Cu, or the catalyst may comprise Hf, Zr and Zn, or the catalyst may comprise Hf, Zr and Cu, or the catalyst comprise Hf, Zn and Cu. According to a particularly preferred embodiment of the present invention, the catalyst comprises Hf, Zn and Cu, which are disposed on the on a support. As regards the support on which Hf and the two or more further catalytically active metals are disposed, no specific restrictions exist concerning the type of support used. Therefore, all conceivable supports may be used. According to the present invention, the support comprises one or more metal oxides, preferably one or more metal oxides selected from the group consisting of alumina, silica, titania, titania-alumina, zirconia, zirconia-alumina, titania- zirconia, and mixtures of two or more thereof, more preferably from the group consisting of alumina, silica, titania-alumina, zirconia-alumina, and mixtures of two or more thereof, wherein more preferably the support comprises silica and/or alumina, preferably silica.

Therefore, according to a preferred embodiment of the present invention, the catalyst com- prises Hf, Zr, Zn and Cu disposed on a silica support or the catalyst comprises Hf, Zr and Zn disposed on a silica support or the catalyst comprises Hf, Zr and Cu disposed on a silica support or the catalyst comprises Hf, Zn and Cu disposed on a silica support. According to a particular preferred embodiment of the present invention, the catalyst comprises Hf, Zn and Cu disposed on a silica support.

As regards the molar ratio of Hf : M 1 : M2, no specific restrictions exist. By way of example, the molar ratio Hf : M 1 : M2, calculated as the respective element may be anywhere in the range of from 1 : (0.002 - 20) : (0.0015 - 15), preferably from (0.0015 - 15), more preferably from 1 : (0.02 - 10) : (0.01 - 8). According to the present invention, it is preferred that the molar ratio Hf : M 1 : M2, calculated as the respective element is in the range of from 1 : (0.2 - 3.0) : (0.1 - 2.0), preferably from 1 : (0.5 - 2.5) : (0.3 - 1.5), more preferably from 1 : (0.9 - 1.9) : (0.5 - 1). Therefore, according to a preferred embodiment of the present invention, the molar ratio Hf : M1 : M2, calculated as the respective element is in the range of from 1 : (0.002 - 20) : (0.0015 - 15), preferably from 1 : (0.02 - 10) : (0.01 - 8), preferably 1 : (0.2 - 3.0) : (0.1 - 2.0), more preferably from 1 : (0.5 - 2.5) : (0.3 - 1.5), more preferably from 1 : (0.9 -1.9) : (0.5 - 1).

As regards the content of Hf in the catalyst, no specific restrictions exist. Therefore, according to a preferred embodiment of the present invention, the content of Hf in the catalyst is in the range of from 0.05 to 30.0 weight-%, preferably in the range of from 0.1 to 15.0 weighted, more preferably in the range of from 0.3 to 10.0 weight-%, more preferably in the range of from 0.5 to 5.0 weight-%, more preferably in the range of from 1 to 4.0 weight-%, more preferably in the range of from 1.4 to 3.2 weight-%, wherein Hf is calculated as the element and based on the total weight of the catalyst. The same applies to the content of M 1 in the catalyst. According to the present invention, it is preferred that the content of M 1 in the catalyst is in the range of from 0.02 to 20.0 weight- %, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.1 to 5.0 weight-%, more preferably in the range of from 0.3 to 3.0 weight-%, preferably in the range of from 0.6 to 1.8 weight-%, more preferably in the range of from 0.8 to 1.2 weight-%, wherein M 1 is calculated as the element and based on the total weight of the catalyst.

Further, the same applies to the content of M2 in the catalyst. According to a preferred embodiment of the present invention, the content of M2 in the catalyst is in the range of from 0.01 to 15.0 weight-%, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.08 to 5.0 weight-%, more preferably in the range of from 0.1 to 2.0 weight-%, preferably in the range of from 0.2 to 1.2 weight-%, more preferably in the range of from 0.4 to 0.7 weight-%, wherein M2 is calculated as the element and based on the total weight of the catalyst.

According to a preferred embodiment of the present invention, M 1 , which is selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, comprises Cu and M2, which is selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, comprises Zn. Further, it is particularly preferred according to the present invention that M 1 is Cu and M2 is Zn.

Therefore, according to a particularly preferred embodiment of the present invention, M 1 comprises Cu and M2 comprises Zn, wherein preferably M 1 is Cu and M2 is Zn. According to a preferred embodiment of the present invention, the catalyst comprising Hf and two or more further catalytically active metals further comprises Ba in an amount of from 0.1 to 15 weight-%, preferably from 0.5 to 10 weight-%, more preferably from 1 to 5 weight-%, based on the total weight of the catalyst Further, according to a particularly preferred embodiment of the present invention, the catalyst comprising Hf and two or more further catalytically active metals, does not contain Ba.

According to a preferred embodiment of the present invention, wherein M 1 is Cu, the content of Cu in the catalyst is in the range of from 0.02 to 20.0 weight-%, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.1 to 5.0 weight- %, more preferably in the range of from 0.3 to 3.0 weight-%, preferably in the range of from 0.6 to 1.8 weight-%, more preferably in the range of from 0.8 to 1.2 weight-%, wherein Cu is calculated as the element and based on the total weight of the catalyst.

According to a preferred embodiment of the present invention, wherein M2 is Zn, the con- tent of Zn in the catalyst is in the range of from 0.01 to 15.0 weight-%, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.08 to 5.0 weight-%, more preferably in the range of from 0.1 to 2.0 weight-%, preferably in the range of from 0.2 to 1.2 weight-%, more preferably in the range of from 0.4 to 0.7 weight-%, wherein Zn is calculated as the element and based on the total weight of the catalyst.

Further, according to a particularly preferred embodiment of the present invention, wherein M 1 is Cu and M2 is Zn, the molar ratio Hf : Cu : Zn, calculated as the respective element is in the range of from 1 : (0.002 - 20) : (0.0015 - 15), preferably from 1 : (0.02 - 10) : (0.01 - 8), preferably 1 : (0.2 - 3.0) : (0.1 - 2.0), more preferably from 1 : (0.5 - 2.5) : (0.3 - 1.5), more preferably from 1 : (0.9 -1.9) : (0.5 - 1 ).

As regards Hf and the two or more further catalytically active metals comprised in the catalyst, no specific restrictions exist concerning the method by which Hf and the two or more further catalytically active metals are disposed on the support. Therefore, it may be con- ceivable to dispose Hf and the two or more further catalytically active metals on the support by impregnation, ion-exchange incipient wetness impregnation and/or by dry impregnation. According to a preferred embodiment of the present invention Hf and the two or more further catalytically active metals are disposed on support by incipient wetness impregnation, dry impregnation and/or by ion exchange. According to the present invention, it is particular- ly preferred to dispose Hf and the two or more further catalytically active metals on the support by incipient wetness impregnation.

As regards the incipient wetness impregnation, no particular restriction exists regarding the number of times said step is repeated. The incipient wetness impregnation is conducted with the aid of a solvent or solvent mixture in which Hf and the two or more further catalytically active metals to be disposed on the support are suitably dissolved. With respect to the type of solvent which may be used, there is again no particular restriction in this respect, provided that Hf and the two or more further catalytically active metals to be disposed on the support are may be solvated therein. Thus, by way of example, the solvent or mixture of solvents which may be used include water and alcohols, and in particular short chain alcohols selected among Ci-C₄, and preferably C1-C3 alcohols, in particular methanol, ethanol or propanol, including mixtures of two or more thereof. Examples of mixtures are mixtures of two or more alcohols, such as methanol and ethanol or methanol and propanol or ethanol and propanol or methanol and ethanol and propanol, or mixtures of water and at least one alcohol such as water and methanol or water and ethanol or water and propanol or water and methanol and ethanol or water and methanol and propanol or water and ethanol and propanol or water and methanol and ethanol and propanol. According to a preferred embodiment of the present invention, however, water or a mixture of water and one or more alcohols is preferred, wherein a mixture of water and ethanol is further preferred, deionized water being particularly preferred as the solvent for the one or more ion-exchange proce- dures.

As regards the amount of the one or more solvents preferably used in the incipient wetness impregnation in order to dispose Hf and the two or more further catalytically active metals on the support, there is again no particular restriction, provided that Hf and the two or more further catalytically active metals are effectively disposed on the crystalline material. According to a preferred embodiment of the present invention, incipient wetness impregnation may be achieved with a volume of solvent or a solvent mixture which slightly exceeds or approximately corresponds to or is slightly inferior to the porous volume of the crystalline material such that Hf and the two or more further catalytically active metals M1 and M2 are solvatized in the solvent or solvent mixture enters the porous system of the support by capillary action.

It is preferred that the impregnation is conducted by use of a liquid to solid weight ratio ranging anywhere from 0.1 to 50. According to said preferred embodiments of the present invention, however, it is preferred that the liquid to solid weight ratio being the weight ratio of the solvent or solvent mixture to the support, is comprised in the range of from 1 to 45, more preferably of from 5 to 40, more preferably of from 10 to 35, more preferably of from 12 to 30, and even more preferably of from 15 to 25. According to particularly preferred embodiments of the present invention, the liquid to solid weight ratio employed in the impreg- nation is comprised in the range of from 18 to 22.

According to a preferred embodiment of the present invention, Hf and the two or more further catalytically active metals are disposed on the support by impregnation, preferably by incipient wetness.

Therefore, according to a particularly preferred embodiment of the present invention, Hf, Cu and Zn are disposed on the support by impregnation, preferably by incipient wetness.

As regards the impregnation, no particular restrictions exist concerning the compounds comprising Hf and the two or more further catalytically active metals which are used to dispose Hf and the two or more further catalytically active metals on the support by impregnation, preferably by incipient wetness impregnation. According to the present invention, it is preferred to carry out the impregnation by use of one or more inorganic or organic salts as sources for one or more of the catalytically active metals. In general, no specific restrictions exist concerning the one or more inorganic or organic salts used in the impregnation procedure, provided that Hf and the two or more further catalytically active metals comprised in the one or more inorganic or organic salts are effectively disposed on the support. It is preferred according to the present invention that the one or more inorganic or organic salts are selected from the group consisting of halides, nitrates, sulfates, phosphates, hydroxides, carbonates, carboxylates, alcoholates, and combinations of two or more thereof, more pref- erably selected from the group consisting of chlorides, nitrates, acetates, and combinations of two or more thereof.

Thus, according to a preferred embodiment of the present invention, impregnation is carried out using one or more inorganic or organic salts of one or more of the catalytically active metals, wherein these salts are preferably selected from the group consisting of halides, nitrates, sulfates, phosphates, hydroxides, carbonates, carboxylates, alcoholates, and combinations of two or more thereof, more preferably selected from the group consisting of chlorides, nitrates, acetates, and combinations of two or more thereof. According to a preferred embodiment of the present invention, wherein one or more inorganic or organic salts as sources for one or more of the catalytically active metals are used, the Hf source is preferably selected from the group consisting of, hafnium halides, hafnium hydroxides, hafnium nitrates, hafnium alkoxides, and mixtures of two or more thereof, more preferably from the group consisting of hafnium bromide, chloride, hafnium nitrate, C1 -C4 alkoxides of Hf, and mixtures of two or more thereof, more preferably from the group consisting of hafnium chloride, bromide, hafnium nitrate, C2-C3 alkoxides of Hf, more preferably selected from the group consisting of hafnium chloride, bromide, hafnium nitrate and mixtures of two or more thereof, wherein more preferably the Hf source is hafni- um(IV)chloride

According to a preferred embodiment of the present invention, wherein one or more inorganic or organic salts as sources for one or more of the catalytically active metals are used, the Zn source is preferably selected from the group consisting of zinc halides, zinc hydroxides, zinc nitrates, zinc alkoxides, and mixtures of two or more thereof, more preferably from the group consisting of zinc bromide, chloride, fluoride, zinc nitrate, C1 -C4 alkoxides of Zn, and mixtures of two or more thereof, more preferably from the group consisting of zinc chloride, fluoride, zinc nitrate, C2-C3 alkoxides of Zn, more preferably selected from the group consisting of zinc chloride, fluoride, zinc nitrate and mixtures of two or more thereof, wherein more preferably the Zn source is zinc(ll)nitrate.

According to a preferred embodiment of the present invention, wherein one or more inorganic or organic salts as sources for one or more of the catalytically active metals are used, the Cu source is preferably selected from the group consisting of copper halides, copper hydroxides, copper nitrates, copper alkoxides, and mixtures of two or more thereof, more preferably from the group consisting of copper bromide, chloride, fluoride, copper nitrate, C1 -C4 alkoxides of Cu, and mixtures of two or more thereof, more preferably from the group consisting of copper chloride, copper nitrate, C2-C3 alkoxides of Cu, more preferably selected from the group consisting of copper chloride, copper nitrate, copper(ll)acetate, and mixtures of two or more thereof, wherein more preferably the Cu source is copper(ll)acetate According to a preferred embodiment of the present invention, wherein one or more inor- ganic or organic salts as sources for one or more of the catalytically active metals are used, the Zr source is preferably selected from the group consisting of zirconium and zirconyl hal- ides, zirconium hydroxide, zirconyl nitrate, zirconium alkoxides, and mixtures of two or more thereof, more preferably from the group consisting of zirconium and zirconyl bromide, chloride, fluoride, zirconyl nitrate, Zr(IV)oxynitrate, C1 -C4 alkoxides of Zr, and mixtures of two or more thereof, more preferably from the group consisting of zirconium and zirconyl chloride, fluoride, zirconyl nitrate, Zr(IV)oxynitrate, C2-C3 alkoxides of Zr, and mixtures of two or more thereof, more preferably from the group consisting of zirconium and zirconyl chloride, zirconyl nitrate, Zr(IV)oxynitrate, C3 alkoxides of Zr, and mixtures of two or more thereof, more preferably from the group consisting of zirconyl chloride, zirconyl nitrate, Zr(IV)oxynitrate, Zn-n-propoxide, and mixtures of two or more thereof, wherein more preferably the Zr source is Zr(IV)oxynitrate.

According to a particularly preferred embodiment of the present invention, wherein the catalyst comprises Hf, Cu and Zn, impregnation is carried out by use of hafnium(IV)chloride, copper(ll)acetate, and zinc(ll)nitrate.

According to a preferred embodiment of the present invention, the catalyst comprising Hf and two or more further catalytically active metals M 1 and M2 selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, wherein M 1 is different from M2, is provided by a impregnation process comprising

(a) preparing an aqueous synthesis mixture comprising one or more Hf sources, one or more M 1 sources, one or more M2 sources and a support;

(b) evaporating the aqueous synthesis mixture obtained in (a) to dryness, obtaining a catalyst comprising Hf and two or more further catalytically active metals M 1 and M2 se- lected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, wherein M 1 is different from M2.

According to a preferred embodiment of the present invention, the support used in (a) comprises one or more metal oxides, preferably one or more metal oxides selected from the group consisting of alumina, silica, titania, titania-alumina, zirconia, zirconia-alumina, titania- zirconia, and mixtures of two or more thereof, more preferably from the group consisting of alumina, silica, titania-alumina, zirconia-alumina, and mixtures of two or more thereof, wherein more preferably the support comprises silica and/or alumina, preferably silica.

According to a preferred embodiment of the present invention, the one or more Hf sources, one or more M1 sources and one or more M2 sources used in (a) are inorganic or organic salts wherein these salts are preferably selected from the group consisting of halides, nitrates, sulfates, phosphates, hydroxides, carbonates, carboxylates, alcoholates, and combinations of two or more thereof, more preferably selected from the group consisting of chlorides, nitrates, acetates, and combinations of two or more thereof.

According to a particularly preferred embodiment of the present invention, wherein M 1 is Cu and M2 is Zn, the Hf source is hafnium(IV)chloride, the Cu source is copper(ll)acetate and the Zn source is zinc(ll)nitrate and the support is silica. Further, the present invention relates to a process for the preparation of butadiene comprising

(i) providing a gas stream G-1 comprising ethanol;

(ii) contacting the gas stream G-1 provided in (i) with the catalyst for the preparation of butadiene according to the present invention, thereby obtaining a gas stream G-2 comprising butadiene;

wherein the catalyst comprises Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2.

As regards the catalyst for the preparation of butadiene used in the inventive process, the inventive catalyst according to any of the particular and preferred embodiments as described in the present application may be employed therein. According to preferred embodiments of the inventive process, the catalyst for the preparation of butadiene according to preferred and particularly preferred embodiments of the inventive catalyst as described in the present application are preferably used.

In the inventive process, a gas stream G-1 comprising ethanol is provided in step (i) and subsequently contacted with a catalyst in step (ii). According to a preferred embodiment of the present invention, the gas stream provided in step (i) additionally comprises acetaldehyde.

Concerning the gas stream G-1 provided in step (i), no particular restriction applies according to the present invention relative to the composition of the gas stream G-1 regarding eth- anol and optional acetaldehyde contained therein, provided that after contacting the gas stream G-1 with a catalyst in step (ii), a gas stream G-2 comprising butadiene is obtained. Thus, in general, no specific restrictions exist concerning the molar ratio of ethanol to acetaldehyde in the gas stream G-1. According to a preferred embodiment of the present invention, the molar ratio of ethanol to acetaldehyde in the gas stream G-1 is in the range of from 1 : 1 to 6 : 1 , preferably from 2 : 1 to 3.5 : 1 , and more preferably from 2.5 : 1 to 2.9 : 1. Concerning the composition of the gas stream G-1 , prior to contacting with the catalyst, no specific restrictions exist regarding the amount of ethanol or the mixture of ethanol and ac- etaldehyde comprised in the gas stream G-1. Thus, according to a preferred embodiment of the present invention, 70 vol.-% or more, preferably 75 vol.-% or more, more preferably 80 vol.-% or more of the gas stream G-1 comprises ethanol or a mixture of ethanol and acetal- dehyde. It is further preferred that prior to contacting with the catalyst, 85 vol.-% or more, preferably 90 vol.-% or more, more preferably 95 vol.-% or more of the gas stream G-1 comprises ethanol or a mixture of ethanol and acetaldehyde. Therefore, according to a preferred embodiment of the present invention, 80 vol.-% or more of the gas stream G-1 com- prises ethanol or of a mixture of ethanol and acetaldehyde, wherein preferably 90 vol.-% or more, more preferably 95 vol.-% or more of the gas stream G-1 comprises ethanol or of a mixture of ethanol and acetaldehyde.

As regards the particular conditions under which the gas stream G-1 is contacted with a catalyst according to the present invention in step (ii), no particular restrictions exist provided that a gas stream G-2 comprising butadiene is obtained. This, for example, applies to the temperature at which the contacting in step (ii) takes place. Accordingly, said contacting of the gas stream in step (ii) may be conducted according to the inventive process at a tem- perature in the range of from 200 to 500 °C, preferably from 220 to 470 °C, more preferably from 250 to 450 °C, more preferably from 270 to 420 °C, more preferably from 300 to 400 °C. According to a particular preferred embodiment of the present invention, contacting the gas stream G-1 with the catalyst is carried out at a temperature in the range of from 200 to 500 °C, preferably from 250 to 450 °C, more preferably from 300 to 400 °C.

Same applies accordingly to the pressure under which the gas stream G-1 is contacted with a catalyst according to the present invention in step (ii) of the inventive process. Thus, in principle, said contacting may be conducted at any conceivable pressure, provided that a gas stream G-2 comprising butadiene is obtained. According to a preferred embodiment of the present invention, contacting of the gas stream G-1 with the catalyst is carried out at a pressure in the range of from 1 to 5 bar, preferably from 1 to 2 bar.

According to a particularly preferred embodiment of the present invention, the gas stream G-1 is contacted with a catalyst according to the present invention in step (ii) at a tempera- ture in the range of from 200 to 500 °C, preferably from 250 to 450 °C, more preferably from 300 to 400 °C at a pressure in the range of from 1 to 5 bar, preferably from 1 to 2 bar.

Furthermore, no particular restriction applies relative to the manner in which the inventive process for the preparation of butadiene is conducted, such that both a non-continuous mode as well as a continuous mode may be applied to the inventive process, wherein the non-continuous process may for example be conducted as a batch-process. According to a preferred embodiment of the present invention, contacting gas stream G-1 with the catalyst is carried out in a continuous mode. No specific restrictions exist concerning the set-up of the continuous process. Preferred continuous process set-ups include the use of one or more fixed-bed reactor. Thus, according to a preferred embodiment of the present invention, contacting the gas stream G-1 with the catalyst is carried out in one or more reactors, wherein preferably the one or more reactors contain the catalyst in the form of a fixed bed.

Further, according to the present invention, it is preferred that prior to contacting the gas stream G-1 with the catalyst, the gas stream is heated. According to a preferred embodiment, the heating of the gas stream G-1 prior to contacting with the catalyst may be conducted at a temperature in the range of from 50 to 300 °C, preferably from 100 to 250 °C, and more preferably from 120 to 180 °C. Thus, according to a preferred embodiment of the present invention, prior to contacting the gas stream G-1 with the catalyst, the gas stream G-1 is heated, preferably to a temperature in the range of from 100 to 250 °C, more preferably from 120 to 180°C. Further, according to a preferred embodiment of the present invention an activation of the catalyst takes place prior to contacting the gas stream G-1 with the catalyst, wherein, for example, the activation may be conducted by heating of the catalyst. Thus, according to a preferred embodiment of the present invention, prior to contacting the gas stream G-1 with the catalyst, the catalyst is activated, preferably by heating.

As regards the specific conditions under which the catalyst is activated, no particular restrictions exist, provided that by use of the activated catalyst a gas stream G-2 is obtained. Accordingly, said activation of the catalyst prior to contacting with the gas stream G-1 may be conducted according a preferred embodiment of the inventive process at a temperature in the range of from 250 to 700 °C, preferably from 350 to 600 °C, more preferably from 440 to 510 °C. The same applies to the duration of the activation. Thus, according to a preferred embodiment of the present invention, the activation prior to contacting with the gas stream G-1 is conducted for a period in the range of from 0.5 to 10 h, more preferably from 1 to 7 h, more preferably from 2 to 5 h. Thus, according to a preferred embodiment of the present invention, the catalyst is activated by heating to a temperature in the range of from 250 to 700 °C, preferably from 350 to 600 °C, more preferably from 440 to 510 °C, preferably for a period in the range of from 0.5 to 10 h, more preferably from 1 to 7 h, more preferably from 2 to 5 h. According to a preferred embodiment of the present invention, a heating ramp is used for reaching the temperature of activation, wherein the heating rate preferably ranges from 0.5 to 10 K/min, preferably 1 to 5 K/min, preferably from 1 to 3 K/min. Thus, according to a preferred embodiment of the present invention, the catalyst is heated with a temperature ramp in the range of from 0.5 to 10 K/min, preferably 1 to 5 K/min, more preferably from 1 to 3 K/min.

Generally, no specific restrictions exist concerning the set-up in which the activation is conducted. According to a particularly preferred embodiment of the present invention, the catalyst is activated in the one or more reactors. It is preferred that during heating the catalyst is flushed with an inert gas. As to the chemical nature of the inert gas, no particular restrictions exist. According to a particularly preferred embodiment of the present invention, preferably the catalyst is flushed during heating with an inert gas, more preferably with an inert gas selected from the group consisting of helium, nitrogen, argon, and mixtures of two or more thereof, wherein the inert gas is more prefera- bly nitrogen.

As to the amount of butadiene comprised in the gas stream G-2 obtained from the contacting of the gas stream G-1 with the catalyst in step (ii) of the present invention, no particular restrictions exist. However, it was surprisingly found that by a process for the preparation of butadiene according to the inventive process, a gas stream G-2 is obtained containing butadiene in an amount of from 10 to 90 vol-%, preferably from 20 to 80 vol-%, more preferably from 30 to 70 vol-%, based on the total volume of the gas stream G-2. Therefore, embodiments of the present invention are preferred wherein the gas stream G-2 contains butadiene in an amount of from 10 to 90 vol-%, preferably from 20 to 80 vol-%, more prefera- bly from 30 to 70 vol-%, based on the total volume of the gas stream G-2.

According to preferred embodiments of the present invention, the process for the preparation of butadiene further comprises a separation of butadiene from the gas stream G-2 obtained from step (ii) of the present invention, wherein a purified gas stream G-3 comprising butadiene is obtained. Generally, there are no restrictions concerning the method for the separation of butadiene from the gas stream G-2, provided that a purified gas stream G-3 comprising butadiene is obtained. Such methods may include thermal separation. Preferably, the separation of butadiene from the gas stream G-2 is achieved by thermal separation, more preferably by distillation.

Thus, embodiments of the present invention are preferred, wherein the process for the preparation of butadiene further comprises

(iii) separating butadiene from the gas stream G-2, thereby obtaining a purified gas stream G-3 containing butadiene, wherein the separation is preferably achieved by thermal separation, more preferably by distillation. According to a preferred embodiment of the present invention, the gas stream G-2 comprising butadiene obtained from step (ii) of the present invention may comprise further compounds resulting from contacting the gas stream G-1 with the catalyst. Thus, according to preferred embodiments of the invention the gas stream G-2 comprising butadiene further comprises diethyl ether. If the gas stream G-2 comprising butadiene further comprises diethyl ether, it is preferred that the diethyl ether is separated from the gas stream G-2 comprising butadiene. It is further preferred that the separation is carried out by thermal separation, preferably by distillation. Further, it was found that the separated diethyl ether may be recycled to the process for the preparation of butadiene according to the present invention, wherein it is preferred to recycle the separated diethyl ether as a component of the gas stream G-1 which is contacted with a catalyst in step (ii). Therefore, according to a preferred embodiment of the present invention, the separated diethyl ether is recycled to the process for the preparation of buta- diene according to the present invention, wherein the separated diethyl ether is preferably recycled as a component of the gas stream G-1 which is contacted with the catalyst in step (ii) to obtain butadiene. Thus, according to a preferred embodiment of the present invention, the gas stream G-2 further comprises diethyl ether, and wherein the diethyl ether is separated from the gas stream G-2, preferably by thermal separation, more preferably by distilla- tion, and recycling the separated diethyl ether to the gas-phase process for the preparation of butadiene, preferably as component of the gas stream G-1.

According to preferred embodiments of the present invention, wherein G-2 further comprises diethyl ether which is preferably separated from G-2 and recycled preferably as compo- nent of the gas stream G-1 , it is preferred that the gas stream G-2 prior to separating the diethyl ether contains diethyl ether in an amount of from 1 to 65 vol.-%, preferably from 2 to 35 vol.-%, more preferably from 5 to 30 vol.-%, based on the total volume of the gas stream G-2. Thus, according to a particularly preferred embodiment of the present invention, the gas stream G-2 contains the diethyl ether in an amount of from 1 to 65 vol.-%, preferably from 2 to 35 vol.-%, more preferably from 5 to 30 vol.-%, based on the total weight of the gas stream G-2.

As regards the separated diethyl ether from the gas stream G-1 , according to a preferred embodiment of the present invention, at least a portion of the separated diethyl ether is hy- drolyzed to ethanol prior to its recycling to the gas-phase process for the preparation of butadiene, preferably as component of the gas stream G-1. Thus, according to a preferred embodiment of the present invention, the process for the preparation of butadiene further comprises hydrolyzing at least a portion of the separated diethyl ether to ethanol prior to its recycling to the gas-phase process for the preparation of butadiene, preferably as compo- nent of the gas stream G-1. As to the conditions under which hydrolyzation is conducted, according to a preferred embodiment of the present invention, the separated diethyl ether is hydrolyzed under acidic conditions, more preferably in the presence of a solid catalyst. According to a preferred embodiment of the present invention, wherein the gas stream G-2 further comprises diethyl ether, the gas stream G-2 may further comprise crotonaldehyde. Thus, according to a preferred embodiment of the present invention, the gas mixture G-2 further comprises crotonaldehyde. According to a preferred embodiment of the embodiment of the present invention, wherein the gas stream G-2 further comprises crotonaldehyde, it is preferred that the gas stream G- 2 contains crotonaldehyde in an amount of from 0.1 to 15 vol.-%, preferably from 0.5 to 10 vol.-%, more preferably from 1 to 5 vol.-%, based on the total weight of the gas stream G-2. According to a particularly preferred embodiment of the present invention, the gas stream G-2 comprising butadiene is free of crotonaldehyde or essentially free of crotonaldehyde, i.e. contains crotonaldehyde only in traces. Therefore, according to a particularly preferred embodiment of the present invention, the gas stream G-2 comprising butadiene contains 0.1 vol.-% or less crotonaldehyde, preferably contains butadiene in the range of from 0.0001 to 0.1 vol.-%, more preferably in the range of from 0.0005 to 0.05 vol.-%, more preferably in the range of from 0.001 to 0.01 vol.-%.

According to a preferred embodiment of the present invention, the catalyst is subjected to regeneration, wherein regeneration may be conducted by any suitable method. Conceivable methods are, for example to regenerate the catalyst by thermal treatment, preferably in the presence of oxygen. Further, there are no particular restrictions concerning the temperature under which the regeneration is conducted. According to a preferred embodiment of the present invention, the thermal treatment is conducted, for example, at a temperature in the range of from 100 to 700 °C, preferably from 350 to 600 °C, more preferably from 450 to 570 °C.

Therefore, according to a preferred embodiment of the present invention the process for the preparation of butadiene according to the present invention further comprises regenerating the catalyst, preferably by thermal treatment in the presence of oxygen, wherein the thermal treatment is preferably performed at a temperature in the range of from 100 to 700 °C, preferably from 350 to 600 °C, more preferably from 450 to 570 °C.

It was surprisingly found that the catalyst comprising Hf and two or more further active metals M 1 and M2 selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, wherein M 1 is different from M2, obtained or obtainable according to the present invention can be used as such for any suitable purpose and in particular as a catalyti- cally active material, such as a catalytically active material in a process for the preparation of butadiene according to the present invention.

Thus, the present invention also relates to the use of a catalyst comprising Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2, as a catalytically active material in a process for the preparation of butadiene, preferably from a gas stream comprising ethanol and optionally acetaldehyde.

According to a preferred embodiment of the present invention, the catalyst is used as a catalytically active material in a process for the preparation of butadiene, preferably from a gas stream comprising ethanol and optionally acetaldehyde: According to a particularly preferred embodiment of the present invention, the catalyst used as a catalytically active mate- rial in a process for the preparation of butadiene, preferably from a gas stream comprising ethanol and optionally acetaldehyde, is a catalyst comprising Hf and two or more further catalytically active metals M 1 and M2 as defined according to any of the particular and preferred embodiments of the present invention. Therefore, according to a particularly preferred embodiment of the present invention, the catalysts according to the present invention are used in a process for the preparation of butadiene according to the present invention, wherein the selectivity of the process relative to butadiene is at least 10 %, preferably in the range of from 10 to 90 %, more preferably from 20 to 80 %, more preferably from 30 to 70 %. Within the meaning of the present inven- tion, the selectivity of the process relative to butadiene generally designates any suitable process for the preparation of butadiene, and accordingly the selectivity relative to butadiene obtained by such a process. It is, however, preferred according to the present invention, that the selectivity relative to butadiene designates a selectivity as obtained according to any of the particular and preferred embodiments of the process for the preparation of butadiene according to present invention as defined in the present application.

The present invention includes the following embodiments, wherein these include the specific combinations of embodiments as indicated by the respective interdependencies defined therein:

1. A catalyst for the preparation of butadiene, the catalyst comprising Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2. The catalyst of embodiment 1 , wherein Hf and the two or more further catalytically active metals are disposed on a support.

The catalyst of embodiment 2, wherein the support comprises one or more metal oxides, preferably one or more metal oxides selected from the group consisting of alumina, silica, titania, titania-alumina, zirconia, zirconia-alumina, titania-zirconia, and mixtures of two or more thereof, more preferably from the group consisting of alumina, silica, titania-alumina, zirconia-alumina, and mixtures of two or more thereof, wherein more preferably the support comprises silica and/or alumina, preferably silica.

The catalyst of any of embodiments 1 to 3, wherein the molar ratio Hf : M 1 : M2, calculated as the respective element is in the range of from 1 : (0.002 - 20) : (0.0015 - 15), preferably 1 : (0.02 - 10) : (0.01 - 8), preferably from 1 : (0.2 - 3.0) : (0.1 - 2.0), more preferably from 1 : (0.5 - 2.5) : (0.3 - 1.5), more preferably from 1 : (0.9 -1.9) : (0.5 - 1).

The catalyst of any of embodiments 1 to 4, wherein the content of Hf in the catalyst is in the range of from 0.05 to 30.0 weight-%, preferably in the range of from 0.1 to 15.0 weight-%, more preferably in the range of from 0.3 to 10.0 weight-%, more preferably in the range of from 0.5 to 5.0 weight-%, more preferably in the range of from 1 to 4.0 weight-%, more preferably in the range of from 1.4 to 3.2 weight-%, wherein Hf is calculated as the element and based on the total weight of the catalyst.

The catalyst of any of embodiments 1 to 5, wherein the content of M 1 in the catalyst is in the range of from 0.02 to 20.0 weight-%, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.1 to 5.0 weight-%, more preferably in the range of from 0.3 to 3.0 weight-%, preferably in the range of from 0.6 to 1.8 weight-%, more preferably in the range of from 0.8 to 1.2 weight-%, wherein M 1 is calculated as the element and based on the total weight of the catalyst.

The catalyst of any of embodiments 1 to 6, wherein the content of M2 in the catalyst is in the range of from 0.01 to 15.0 weight-%, preferably in the range of from 0.05 to 10.0 weight-%, more preferably in the range of from 0.08 to 5.0 weight-%, more preferably in the range of from 0.1 to 2.0 weight-%, preferably in the range of from 0.2 to 1.2 weight-%, more preferably in the range of from 0.4 to 0.7 weight-%, wherein M2 is calculated as the element and based on the total weight of the catalyst.

The catalyst of any of embodiments 1 to 7, wherein M 1 comprises Cu and M2 comprises Zn, wherein preferably M 1 is Cu and M2 is Zn. The catalyst of any of embodiments 2 to 8, wherein Hf and the two or more further catalytically active metals are disposed on the support by impregnation, preferably by incipient wetness.

The catalyst of embodiment 9, wherein impregnation is carried out using one or more inorganic or organic salts, wherein these salts are preferably selected from the group consisting of halides, nitrates, sulfates, phosphates, hydroxides, carbonates, carbox- ylates, alcoholates, and combinations of two or more thereof, more preferably selected from the group consisting of chlorides, nitrates, acetates, and combinations of two or more thereof.

A process for the preparation of butadiene comprising

(i) providing a gas stream G-1 comprising ethanol;

(ii) contacting the gas stream G-1 provided in (i) with a catalyst according to any of embodiments 1 to 1 1 , thereby obtaining a gas stream G-2 comprising butadiene; wherein the catalyst comprises Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2.

The process of embodiment 1 1 , wherein the gas stream G-1 further comprises acetal- dehyde.

The process of embodiment 12, wherein the molar ratio of ethanol to acetaldehyde in the gas stream G-1 is in the range of from 1 : 1 to 6 : 1 , preferably from 2 : 1 to 3.5 : 1 , more preferably from 2.5 : 1 to 2.9 : 1.

The process of embodiment 12 or 13, wherein 80 vol.-% or more of the gas stream G- 1 consists of ethanol or of a mixture of ethanol and acetaldehyde, wherein preferably 90 vol.-% or more, more preferably 95 vol.-% or more of the gas stream G-1 consists of ethanol or of a mixture of ethanol and acetaldehyde.

The process of any of embodiments 1 1 to 14, wherein contacting of the gas stream G- 1 with the catalyst is carried out at a temperature in the range of from 200 to 500 °C, preferably from 250 to 450 °C, more preferably from 300 to 400 °C.

The process of any of embodiments 1 1 to 15, wherein contacting of the gas stream G 1 with the catalyst is carried out at a pressure in the range of from 1 to 5 bar, prefera bly from 1 to 2 bar. 17. The process of any of embodiments 1 1 to 16, wherein contacting the gas stream G-1 with the catalyst is carried out in a continuous mode.

18. The process of any of embodiments 1 1 to 17, wherein contacting the gas stream G-1 with the catalyst is carried out in one or more reactors, wherein preferably the one or more reactors contain the catalyst in the form of a fixed bed.

19. The process of any of embodiments 1 1 to 18, wherein prior to contacting the gas stream G-1 with the catalyst, the gas stream G-1 is heated, preferably to a tempera- ture in the range of from 100 to 250 °C, more preferably from 120 to 180°C.

20. The process of any of embodiments 1 1 to 19, wherein prior to contacting the gas stream G-1 with the catalyst, the catalyst is activated, preferably by heating. 21. The process of embodiment 20, wherein the catalyst is activated by heating to a temperature in the range of from 250 to 700 °C, preferably from 350 to 600 °C, more preferably from 440 to 510 °C, preferably for a period in the range of from 0.5 to 10 h, more preferably from 1 to 7 h, more preferably from 2 to 5 h. 22. The process of embodiment 20 or 21 , wherein the catalyst is heated with a temperature ramp in the range of from 0.5 to 10 K/min, preferably from 1 to 3 K/min.

23. The process of any of embodiments 20 to 22, wherein during heating the catalyst is flushed with an inert gas, preferably with an inert gas selected from the group consist- ing of helium, nitrogen, argon, and mixtures of two or more thereof, wherein the inert gas is more preferably nitrogen.

24. The process of any of embodiments 1 1 to 23, wherein the gas stream G-2 contains butadiene in an amount of from 10 to 90 vol.-%, preferably from 20 to 80 vol.-%, more preferably from 30 to 70 vol.-%, based on the total weight of the mixture G-2.

25. The process of any of embodiments 1 1 to 24, further comprising

(iii) separating butadiene from the gas stream G-2, thereby obtaining a purified gas stream G-3 containing butadiene, wherein the separation is preferably achieved by thermal separation, more preferably by distillation.

26. The process of any of embodiments 1 1 to 25, wherein the gas stream G-2 further comprises diethyl ether, and wherein the diethyl ether is separated from the gas stream G-2, preferably by thermal separation, more preferably by distillation, and re- cycling the separated diethyl ether to the gas-phase process for the preparation of butadiene, preferably as component of the gas stream G-1. 27. The process of embodiment 26, wherein the gas stream G-2 contains the diethyl ether in an amount of from 1 to 65 vol-%, preferably from 2 to 35 vol-%, more preferably from 5 to 30 vol-%, based on the total weight of the gas stream G-2.

28. The process of embodiment 26 or 27, further comprising hydrolyzing at least a portion of the separated diethyl ether to ethanol prior to its recycling to the gas-phase process for the preparation of butadiene, preferably as component of the gas stream G-1. 29. The process of embodiment 28, wherein the separated diethyl ether is hydrolyzed, preferably under acidic conditions, more preferably in the presence of a solid catalyst.

30. The process of any of embodiment 1 1 to 29, wherein the gas stream G-2 further comprises crotonaldehyde.

31. The process of embodiment 30, wherein the gas stream G-2 contains the crotonaldehyde in an amount of from 0.1 to 15 weight-%, preferably from 0.5 to 10 weight-%, more preferably from 1 to 5 weight-%, based on the total weight of the gas stream G- 2.

32. The process of any of embodiments 1 1 to 31 , further comprising regenerating the catalyst, preferably by thermal treatment in the presence of oxygen, wherein the thermal treatment is preferably performed at a temperature in the range of from 100 to 700 °C, preferably from 350 to 600 °C, more preferably from 450 to 570 °C. .

33. Use of a catalyst according to any of embodiments 1 to 10 as a catalytically active material in a process for the preparation of butadiene, preferably from a gas stream comprising ethanol and optionally acetaldehyde. 34. The use of embodiment 33, wherein the selectivity to butadiene is in the range of from 10 to 90 %, preferably from 20 to 80 %, more preferably from 30 to 70 %.

DESCRIPTION OF THE FIGURES

Figure 1 : shows the product formation and ethanol conversion indicated by "♦" in the

graph as function of time by use of the catalyst having a hafnium loading of 0.75 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-% on a Cab-O-Sil M5 support obtained from Example 1 , according to the present in- vention. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadiene, other C4 compounds, diethylether, and oxygen-containing C4 compounds, and on the y axis, the time in minutes is indicated.

Figure 2: shows the product formation and ethanol conversion indicated by "*"in the graph as function of time by use of the catalyst having a hafnium loading of 1.5 weight-

%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-% on a Cab- O-Sil M5 support obtained from Example 2, according to the present invention. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadi- ene, other C4 compounds, diethylether, and oxygen-containing C4 compounds, and on the y axis, the time in minutes is indicated.

Figure 3: shows the product formation and ethanol conversion indicated by "*"in the graph as function of time by use of the catalyst having a hafnium loading of 3.0 weight- %, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-% on a Cab-

O-Sil M5 support obtained from Example 3, according to the present invention. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadiene, other C4 compounds, diethylether, and oxygen-containing C4 compounds, and on the y axis, the time in minutes is indicated.

Figure 4: shows the product formation and ethanol conversion indicated by "*"in the graph as function of time by use of the catalyst having a galium loading of 1.5 weight- %, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-% on a Cab- O-Sil M5 support obtained from Comparative Example 1. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadiene, other C4 compounds, diethylether, and oxygen-containing C4 compounds, and on the y axis, the time in minutes is indicated.

Figure 5: shows the product formation and ethanol conversion indicated by "*"in the graph as function of time by use of the catalyst having a hafnium loading of 3 weight- %, a barium loading of 0.5 weight-% and a copper loading of 1 weight-% on a Cab-O-Sil M5 support obtained from Comparative Example 2. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadiene, other C4 compounds, diethylether, and oxygen-containing C4 compounds, and on the y axis, the time in minutes is indicated. Figure 6: shows the product formation and ethanol conversion indicated by "*"in the graph as function of time by use of the catalyst having a hafnium loading of 3.0 weight- % and a copper loading of 1 weight-% on a Cab-O-Sil M5 support obtained from Comparative Example 3. On the x axis, the percent values are shown for the conversion of ethanol, as well as for the selectivity relative to ethylene, propylene, acetaldehyde, butadiene, other C4 compounds, diethylether, and oxygen- containing C4 compounds, and on the y axis, the time in minutes is indicated.

EXAMPLES Reference Example 1 : General method for the conversion of ethanol to butadiene

Experiments were conducted in a fixed bed reactor. Ethanol was evaporated and the thus obtained feed stream was converted over the catalyst at an ethanol feed rate of 0.065 g ethanol/h. The gaseous product mixture was analyzed by gas chromatography. It is noted that the rele- vant results are obtained after a first start-up phase lasting about 200 min. At the beginning of the start-up phase, the reaction was carried out at 300 °C, wherein after 200 min time on stream the temperature was increased to 360 °C.

Example 1: Preparation of a catalyst loaded with 0.75 weight-% Hf, 0.5 weight-% Zn and 1 weight-% Cu

0.01346 g hafnium (IV) chloride 0.02274 g zinc nitrate hexahydrate and 0.03142 g cop- per(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was cal- cined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined material had a hafnium loading of 0.75 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%.

Example 2: Preparation of a catalyst loaded with 1.5 weight-% Hf, 0.5 weight-% Zn and 1 weight-% Cu

0.02692 g hafnium (IV) chloride, 0.02274 g zinc nitrate hexahydrate and 0.03142 g cop- per(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was cal- cined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined material had a hafnium loading of 1.5 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%.

Example 3: Preparation of a catalyst loaded with 3 weight-% Hf, 0.5 weight-% Zn and 1 weight-% Cu 0.05384 g hafnium (IV) chloride, 0.02274 g zinc nitrate hexahydrate and 0.03142 g cop- per(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was calcined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined material had a hafnium loading of 3 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%.

Comparative Example 1 : Preparation of a catalyst with 1.5 weight-% Ga, 0.5 weight-% Zn and 1 weight-% Cu

0.3788 g galium (III) nitrate monohydrate, 0.02274 g zinc nitrate hexahydrate and 0.03142 g copper(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was calcined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined mate- rial had a galium loading of 1.5 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%.

Comparative Example 2: Preparation of a catalyst with 3 weight-% Hf, 0.5 weight-% Ba and 1 weight-% Cu

0.05384 g hafnium (IV) chloride, 0.00758 g barium (II) chloride dihydrate and 0.03142 g copper(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was calcined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined mate- rial had a hafnium loading of 3 weight-%, a barium loading of 0.5 weight-% and a copper loading of 1 weight-%.

Comparative Example 3: Preparation of a catalyst with 3 weight-% Hf and 1 weight-% Cu 0.05384 g hafnium (IV) chloride and 0.03142 g copper(ll) acetate monohydrate were dissolved in 20 mL deionized water. 1 g of Cab-O-Sil M5 was added and water was slowly evaporated under stirring. The obtained material was calcined in air at 500 °C for 6 h with a heating rate of 1 K/min. The obtained calcined material had a hafnium loading of 3 weight- % and a copper loading of 1 weight-%.

Example 4:

The process according to the present invention was carried out according to Reference Example 1 by use of the catalyst obtained from Example 1 , i.e. a catalyst having a hafnium loading of 0.75 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%, wherein Cab-O-Sil M5 is used as support. The result of this experiment is shown in figure 1. Example 5:

The process according to the present invention was carried out according to Reference Example 1 by use of the catalyst obtained from Example 2, i.e. a catalyst having a hafnium loading of 1.5 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%, wherein Cab-O-Sil M5 is used as support. The result of this experiment is shown in figure 2.

Example 6: The process according to the present invention was carried out according to Reference Example 1 by use of the catalyst obtained from Example 3, i.e. a catalyst having a hafnium loading of 3 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%, wherein Cab-O-Sil M5 is used as support. The result of this experiment is shown in figure 3. Comparative Example 4:

The process for the preparation of butadiene was carried out according to Reference Example 1 by use of the catalyst obtained according to Comparative Example 1 , i.e. a catalyst having a galium loading of 1.5 weight-%, a zinc loading of 0.5 weight-% and a copper loading of 1 weight-%. The result of this experiment is shown in figure 4.

Comparative Example 5:

The process for the preparation of butadiene was carried out according to Reference Example 1 by use of the catalyst obtained according to Comparative Example 2, i.e. a catalyst having a hafnium loading of 3 weight-%, a barium loading of 0.5 weight-% and a copper loading of 1 weight-%. The result of this experiment is shown in figure 5.

Comparative Example 6:

The process for the preparation of butadiene was carried out according to Reference Example 1 by use of the catalyst obtained according to Comparative Example 3, i.e. a catalyst having a hafnium loading of 3 weight-% and a copper loading of 1 weight-%. The result of this experiment is shown in figure 6.

Summary and comparison of the results of Examples 4 to 6 and Comparative Examples 4 to 6

As may be taken from figures 1 , 2 and 3, which display the results of Examples 4, 5 and 6 carried out according to the invention by use of a catalyst comprising Hf, Zn and Cu, unexpected high conversions of ethanol and at the same time high selectivities to butadiene are achieved. Further, as may be taken from these figures, after a first start-up phase lasting about 200 min employed in all experiments, the conversions of ethanol and the selectivities to butadiene remain approximately constant over the whole tested temporal range. By use of the catalyst having a hafnium loading of 3 weight-%, a zinc loading of 0.5 weight- % and a copper loading of 1 weight-% an unexpected high selectivity to butadiene of 72 % was achieved, and at the same time the conversion of ethanol increases up to 99.8 %.

In contrast thereto, by use of the catalysts obtained according to Comparative Examples 1 to 3, lower conversions of ethanol and lower selectivities to butadiene are achieved compared to the Experiments carried out by use of catalysts according to the invention.

Figure 4 displays the result of Comparative Example 4, which was carried out by use of a catalyst having a galium loading of 1.5 weight-%, a zinc loading of 0.5 weight-% and a cop- per loading of 1 weight-%, wherein a conversion of ethanol of less than 90 weight-% and a selectivity to butadiene of less than 10 weight-% are achieved. This means that the conversion of ethanol is more than 10 % lower and the selectivity to butadiene is more than 86 % lower when using a catalyst in which Hf is replaced by the same amount of Ga compared to the results achieved by use of catalysts according to the present invention, i.e. catalysts comprising 1.5 weight-% Hf, 0.5 weight-% Zn and 1 weight-% Cu.

Further, figure 5 displays the result of Comparative Example 5, which was carried out by use of a catalyst having a hafnium loading of 3 weight-%, a barium loading of 0.5 weight-% and a copper loading of 1 weight-%. As may be taken from this figure, the conversion of ethanol as well as the selectivity to butadiene decreases with time. After 300 minutes on stream the conversion of ethanol is below 50 % and decreases below a value of 40 % after 1000 minutes on stream, wherein after 300 minutes on stream the selectivity to butadiene is approximately 30 % and decreases to a value of below 20 % after 1000 minutes on stream. Thus, this catalyst does not exhibit long time stability.

Furthermore, figure 6 displays the result of Comparative Example 6, which was carried out by use of a catalyst comprising Hf and one further catalytically active metal, i.e. a catalyst having a hafnium loading of 3 weight-% and a copper loading of 1 weight-%. As may be taken from this figure, the conversion of ethanol as well as the selectivity to butadiene sig- nificantly decreases with time. Thus, this catalyst does not exhibit long time stability.

Comparison of the results of Examples 4 to 6, which are carried out according to the present invention with the results of Comparative Examples 4 to 6, which are carried out either by use of catalysts in which one of the catalytically active elements comprised in the cata- lyst according to the present invention is replaced by another metal or by a catalyst which comprises Hf and only one further catalytically active metal, shows that the use of catalysts according to the present invention leads to a highly advantageous process for the preparation of butadiene. In particular, it was unexpectedly found that by use of a catalyst comprising three or more catalytically active metals, wherein one of these catalytically active metals is mandatorily Hf and the two or more further catalytically active metals are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, in a process for the preparation of butadiene, both an exceptionally high conversion of ethanol as well as a particularly high selectivity to butadiene may be achieved. Furthermore, comparison of Examples 4 to 6 with Comparative Examples 4 to 6 shows that by use of the catalysts according to the present invention the conversions of ethanol and the selectivities to butadiene remain approximately constant over the whole tested temporal range, wherein in Comparative examples 4 to 6 the conversions of ethanol and the selectivities to butadiene decreases with time. Thus, surprisingly it was found that the catalysts according to present invention have an unexpected high activity which is sustained for a long time compared to the catalysts used in Comparative Examples 4 to 6.

Cited Prior Art

- Catal. Sci. Technol. 201 1 , 1 , 267-272

- GB 331482

- US 2421361

- WO 2012/015340 A1

- Ind. Eng. Chem. 41 (1949), pages 1012-1017

Claims

1. A catalyst for the preparation of butadiene comprising Hf and two or more further cata- lytically active metals M 1 and M2, wherein the two or more further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2.

2. The catalyst of claim 1 , wherein Hf and the two or more further catalytically active metals are disposed on a support.

3. The catalyst of claim 2, wherein the support comprises one or more metal oxides.

4. The catalyst of any of claims 1 to 3, wherein the molar ratio Hf : M 1 : M2, calculated as the respective element is in the range of from 1 : (0.002 - 20) : (0.0015 - 15).

5. The catalyst of any of claims 1 to 4, wherein the content of Hf in the catalyst is in the range of from 0.05 to 30.0 weight-%, wherein Hf is calculated as the element and based on the total weight of the catalyst.

6. The catalyst of any of claims 1 to 5, wherein the content of M 1 in the catalyst is in the range of from 0.02 to 20.0 weight-%, wherein M 1 is calculated as the element and based on the total weight of the catalyst.

7. The catalyst of any of claims 1 to 6, wherein the content of M2 in the catalyst is in the range of from 0.01 to 15.0 weight-%, wherein M2 is calculated as the element and based on the total weight of the catalyst.

8. The catalyst of any of claims 1 to 7, wherein M 1 comprises Cu and M2 comprises Zn.

9. The catalyst of any of claims 2 to 8, wherein Hf and the two or more further catalytically active metals are disposed on the support by impregnation.

10. The catalyst of claim 9, wherein impregnation is carried out using one or more inor- ganic or organic salts.

1 1. A process for the preparation of butadiene comprising

(i) providing a gas stream G-1 comprising ethanol;

(ii) contacting the gas stream G-1 provided in (i) with a catalyst according to any of claims 1 to 1 1 , thereby obtaining a gas stream G-2 comprising butadiene;

wherein the catalyst comprises Hf and two or more further catalytically active metals M 1 and M2, wherein the two or more further catalytically active metals M 1 and M2 are selected from the group consisting of Zr, Zn, Cu and combinations of two or more thereof, and wherein M 1 is different from M2.

12. The process of claim 1 1 , wherein the gas stream G-1 further comprises acetaldehyde.

13. The process of claim 12, wherein the molar ratio of ethanol to acetaldehyde in the gas stream G-1 is in the range of from 1 : 1 to 6 : 1.

14. The process of claim 12 or 13, wherein 80 vol.-% or more of the gas stream G-1 con- sists of ethanol or of a mixture of ethanol and acetaldehyde.

15. The process of any of claims 1 1 to 14, wherein contacting of the gas stream G-1 with the catalyst is carried out at a temperature in the range of from 200 to 500 °C.

16. The process of any of claims 1 1 to 15, wherein contacting of the gas stream G-1 with the catalyst is carried out at a pressure in the range of from 1 to 5 bar.

17. The process of any of claims 1 1 to 16, wherein contacting the gas stream G-1 with the catalyst is carried out in a continuous mode.

18. The process of any of claims 1 1 to 17, wherein contacting the gas stream G-1 with the catalyst is carried out in one or more reactors.

19. The process of any of claims 1 1 to 18, wherein prior to contacting the gas stream G-1 with the catalyst, the gas stream G-1 is heated.

20. The process of any of claims 1 1 to 19, wherein prior to contacting the gas stream G-1 with the catalyst, the catalyst is activated.

21. The process of claim 20, wherein the catalyst is activated by heating to a temperature in the range of from 250 to 700 °C.

22. The process of claim 20 or 21 , wherein the catalyst is heated with a temperature ramp in the range of from 0.5 to 10 K/min.

23. The process of any of claims 20 to 22, wherein during heating the catalyst is flushed with an inert gas.

24. The process of any of claims 1 1 to 23, wherein the gas stream G-2 contains butadiene in an amount of from 10 to 90 vol.-%, based on the total weight of the mixture G-2.

25. The process of any of claims 1 1 to 24, further comprising

(iii) separating butadiene from the gas stream G-2, thereby obtaining a purified gas stream G-3 containing butadiene.

26. The process of any of claims 1 1 to 25, wherein the gas stream G-2 further comprises diethyl ether, and wherein the diethyl ether is separated from the gas stream G-2.

27. The process of claim 26, wherein the gas stream G-2 contains the diethyl ether in an amount of from 1 to 65 vol.-%, based on the total weight of the gas stream G-2.

28. The process of claim 26 or 27, further comprising hydrolyzing at least a portion of the separated diethyl ether to ethanol prior to its recycling to the gas-phase process for the preparation of butadiene.

29. The process of claim 28, wherein the separated diethyl ether is hydrolyzed.

30. The process of any of claims 1 1 to 29, wherein the gas stream G-2 further comprises crotonaldehyde.

31. The process of claim 30, wherein the gas stream G-2 contains the crotonaldehyde in an amount of from 0.1 to 15 weight-%, based on the total weight of the gas stream G- 2.

32. The process of any of claims 1 1 to 31 , further comprising regenerating the catalyst.

33. Use of a catalyst according to any of claims 1 to 10 as a catalytically active material in a process for the preparation of butadiene.

34. The use of claim 33, wherein the selectivity to butadiene is in the range of from 10 to 90 %.