Method for producing cyclopentanone and cyclopentene-1-carboxylic acid and their esters

A process for preparing cyclopentanone and cyclopentene-1-carboxylic acid or an ester thereof of the formula I ##STR1## where R is hydrogen or an aliphatic radical having 1-6 carbon atoms or a cycloaliphatic, araliphatic or aromatic radical having 6-12 carbon atoms comprises heating a compound of the formula II EQU X--(CH.sub.2).sub.4 --COOR II where X is formyl or hydroxymethyl and R is defined as above, and/or a compound which is converted into a compound of the formula II by reaction with water or alcohols ROH under the reaction conditions to from 200 to 450.degree. C. in the gas or liquid phase in the presence of a heterogeneous oxidic catalyst.

The present invention relates to a process for preparing cyclopentanone and
 cyclopentene-1-carboxylic acid and an ester thereof by reacting
 5-formylvaleric acid and an ester thereof and/or 6-hydroxycaproic acid and
 an ester thereof and/or a compound which is converted into
 6-hydroxycaproic acid or an ester thereof by reaction of water and
 alcohols under the reaction conditions, alone or as a mixture with adipic
 esters, over oxidic catalysts at from 200 to 450.degree. C. in the gas or
 liquid phase.
 EP-A-251 111 discloses the preparation of cyclopentanone by reacting adipic
 diesters over oxidic catalysts at an elevated temperature in the gas or
 liquid phase. Furthermore, EP-A-266 687 discloses the use of zeolitic
 catalysts or phosphate catalysts for this reaction.
 It is an object of the present invention to prepare cyclopentanone from
 starting materials which are even more easily obtainable than adipic
 diesters (readily obtainable by esterification of adipic acid), even at
 the cost of the coproduction of a further product of value.
 This product of value is cyclopentene-1-carboxylic acid or its esters,
 which have previously been prepared in a rather complicated way by
 reduction of cyclopentanone-2-carboxylic esters to give
 cyclopentanol-2-carboxylic esters and subsequent elimination of water
 (Heterocycles 47 (1996), 423-425.
 We have found that this object is achieved according to the invention by a
 process for preparing cyclopentanone and cyclopentene-1-carboxylic acid or
 an ester thereof of the formula I
 ##STR2##
 where R is hydrogen or an aliphatic radical having 1-6 carbon atoms or a
 cycloaliphatic, araliphatic or aromatic radical having 6-12 carbon atoms,
 which comprises heating a compound of the formula II
EQU X--(CH.sub.2).sub.4 --COOR II
 where X is formyl or hydroxymethyl and R is defined as above, and/or a
 compound which is converted into a compound of the formula II by reaction
 with water or alcohols ROH under the reaction conditions to from 200 to
 450.degree. C. in the gas or liquid phase in the presence of a
 heterogeneous oxidic catalyst.
 In a particular embodiment of the process, a mixture of a compound of the
 formula II and an adipic diester of the formula III
EQU ROCO--(CH.sub.2).sub.4 --COOR III,
 where R is defined as above, is reacted, in particular a mixture as
 obtained by the process according to DE-A 19 607 954.
 The reaction according to the invention can be represented, for example for
 the conversion of methyl 5-formylvalerate to cyclopentanone and methyl
 cyclopentene-1-carboxylate, by the following reaction equation.
 ##STR3##
 When 6-hydroxycaproic acid or an ester thereof or a compound which is
 converted into 6-hydroxycaproic acid or an ester thereof, e.g.
 .epsilon.-caprolactone, an additional simultaneous catalytic
 dehydrogenation is required.
 In all cases it was surprising that this reaction proceeded in high yields,
 selectivities and space time yields.
 Starting compounds of formula II are 5-formylvaleric acid and
 6-hydroxycaproic acid and esters thereof, alone or as a mixture with
 adipic diesters, in which case the esters may contain aliphatic radicals
 having 1-6 carbon atoms or cycloaliphatic, aromatic radicals or
 araliphatic radicals having 5-12, preferably 6-8, carbon atoms. Examples
 of radicals R are methyl, ethyl, propyl, isopropyl, butyl, isobutyl,
 tert-butyl, hexyl, cyclopentyl, cyclohexyl, phenyl or benzyl radicals.
 Examples of compounds of the formula II which may be used as starting
 materials include: 5-formylvaleric acid, 6-hydroxycaproic acid, methyl
 5-formylvalerate, ethyl 5-formylvalerate, isobutyl 5-formylvalerate,
 cyclohexyl 5-formylvalerate, benzyl 5-formylvalerate, phenyl
 5-formylvalerate, 6-hydroxycaproic acid, methyl 6-hydroxycaproate, propyl
 6-hydroxycaproate, n-butyl 6-hydroxycaproate, cyclopentyl
 6-hydroxycaproate, phenyl 6-hydroxycaproate, alone or as a mixture with
 dimethyl adipate, diethyl adipate or di-n-butyl adipate.
 It is also possible to use mixtures of compounds of the formula II
 featuring both formyl and hydroxymethyl groups as starting compounds.
 Further possible starting compounds are compounds which are converted into
 compounds of the formula II under the reaction conditions. For example,
 mixtures of caprolactone and water or alcohols may be used instead of
 6-hydroxycaproic acid or 6-hydroxycaproic esters. If, for example in the
 reaction of 6-hydroxycaproic esters according to the invention,
 caprolactone is byproduced, it can be separated off and recycled.
 5-Formylvaleric acid to be used as a starting compound may be obtained by
 hydroformylation of 3- and 4-pentenoic acid, for example as described in
 WO 97/08127. 5-Formylvaleric esters may be obtained by hydroformylation of
 3- and 4-pentenoic esters, for example as described in EP-A 556 681.
 6-Hydroxycaproic acid and 6-hydroxycaproic esters are formed, for example,
 by hydrolysis or alcoholysis of caprolactone.
 In a particular embodiment, mixtures of 6-hydroxycaproic esters and adipic
 diesters are used as obtained, for example, by the processes described in
 DE-A 19 607 954, in which case further compounds may be present in
 addition to 6-hydroxycaproic esters and adipic diesters, such as
 caprolactone, 6-alkoxycaproic esters, glutaric diesters, 5-hydroxyvaleric
 esters, 2-oxocaproic esters, 1,2-cyclohexanediols, valerolactone,
 unsaturated adipic diesters, for example dihydromuconic diesters,
 3-hydroxypentanoic esters, 4-oxopentanoic esters and 5-oxohexanoic esters.
 These compounds generally neither adversely affect the reaction according
 to the invention nor, surprisingly, give rise to a deterioration in
 product quality after purification by distillation.
 The proportion of adipic diester in the mixture to be reacted is typically
 up to 95, preferably up to 90, % by weight.
 Suitable catalysts are acidic or basic catalysts, but also catalysts having
 both acidic and basic properties. When 6-hydroxycaproic acid or an ester
 thereof is used as starting compound, the catalysts must also have
 dehydrogenating properties.
 For the purposes of the present invention, oxidic catalysts are not only
 oxides in the narrow sense but also complex oxygen-containing compounds
 which have intrinsic acidic or basic properties or may be doped
 accordingly. Hence it is also possible to use heteropolyacids, for example
 applied to a carrier, zeolites, which are present in the H-form for acidic
 activity and which are doped with alkali for basic activity, metal
 phosphates or compounds such as carbonates or hydroxides which can be
 converted into oxides.
 Examples of oxidic catalysts are oxides of elements of groups 1-14 of the
 Periodic Table of the Elements or rare earth metal oxides or mixtures
 thereof. For example, use may be made of alkali metal oxides such as
 sodium oxide, alkaline earth metal oxides, such as magnesium oxide,
 calcium oxide, barium oxide, furthermore boron trioxide, aluminum oxide,
 silicon dioxide, for example in the form of silica gel, fused silica,
 silicates or quartz, furthermore tin dioxide, bismuth oxide, copper oxide,
 zinc oxide, lanthanum oxide, titanium dioxide, zirconium dioxide, vanadium
 oxides, chromium oxides, molybdenum oxides, tungsten oxides, manganese
 oxides, iron oxides, cerium oxides, neodymium oxides or mixtures thereof.
 The catalysts may also be modified by applying additives, such as acids
 (for example phosphoric acids) or bases (for example sodium hydroxide).
 Specific examples are La.sub.2 O.sub.3, ZrO.sub.2' Cr.sub.2 O.sub.3
 /ZrO.sub.2, CaO/ZnO, MgO/ZnO, K.sub.2 O/TiO.sub.2, La.sub.2 O.sub.3
 /Al.sub.2 O.sub.3 and ZrO.sub.2 --SO.sub.4.
 The heteropolyacids to be used according to the invention contain, as
 essential element, tungsten or preferably molybdenum, which may be
 partially replaced by vanadium. If vanadium is used, V:Mo atomic ratios of
 1:6-1:12 are preferred. Examples of central atoms are phosphorus, silicon,
 arsenic, germanium, boron, titanium, cerium, thorium, manganese, nickel,
 tellurium, iodine, cobalt, chromium, iron, gallium, vanadium, platinum,
 beryllium and zinc. Phosphorus and silicon are preferred. A preferred
 ratio of molybdenum or tungsten atoms to the respective central atom is
 2.5:1-12:1, preferably 11:1-12:1.
 Specific examples of molybdenum-containing heteropolyacids are the
 following compounds:
 dodecamolybdophosphoric acid (H.sub.3 PMO.sub.12 O.sub.40 *n H.sub.2 O),
 dodecamolybdosilicic acid (H.sub.4 SiMo.sub.12 O.sub.40 *n H.sub.2 O),
 dodecamolybdoceric(IV) acid (H.sub.8 CeMo.sub.12 O.sub.42 *n H.sub.2 O),
 dodecamolybdoarsenic(V) acid (H.sub.3 AsMo.sub.12 O.sub.42 *n H.sub.2 O),
 hexamolybdochromic(III) acid (H.sub.3 CrMo.sub.6 O.sub.24 H.sub.6 *n
 H.sub.2 O),
 hexamolybdonickelic(II) acid (H.sub.4 NiMo.sub.6 O.sub.24 H.sub.6 *5
 H.sub.2 O),
 hexamolybdoiodic acid (H.sub.5 JMo.sub.6 O.sub.24 *n H.sub.2 O),
 octadecamolybdodiphosphoric acid (H.sub.6 P.sub.2 Mo.sub.18 O.sub.62 *11
 H.sub.2 O),
 octadecamolybdodiarsenic(V) acid (H.sub.6 As.sub.2 Mo.sub.18 O.sub.62 *25
 H.sub.2 O),
 nonamolybdomanganic(IV) acid (H.sub.6 MnMo.sub.9 O.sub.32 *n H.sub.2 O),
 undecamolybdovanadophosphoric acid (H.sub.4 PMo.sub.11 VO.sub.40 *n H.sub.2
 O),
 decamolybdodivanadophosphoric acid (H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 *n
 H.sub.2 O),
 hexamolybdohexatungstophosphoric acid (H.sub.3 PMo.sub.6 W.sub.6 O.sub.40
 *n H.sub.2 O).
 It is of course also possible to use mixtures of heteropolyacids.
 Preference is given to using dodecamolybdophosphoric acid and
 dodecamolybdosilicic acid.
 As well as free heteropolyacids, it is also possible, however, to employ
 their salts, in particular their alkali metal and alkaline earth metal
 salts, as catalysts. Preference is given to cesium salts. As with the free
 acids, corresponding mixtures of their salts may be used.
 The heteropolyacids and their salts are known compounds and can be prepared
 by known methods, for example by the methods described in Brauer (Editor):
 Handbuch der Praparativen Anorganischen Chemie, Volume III, Enke,
 Stuttgart, 1981 or by the methods described in Top. Curr. Chem. 76 (1978),
 1. Particular preference is given to preparation methods in which no
 organic solvent is used and which are carried out in aqueous solution
 instead.
 The heteropolyacids prepared in this manner are generally in hydrated form
 and are free from coordinatively bound water present therein prior to use.
 This dehydration can advantageously be carried out thermally, for example
 by the process described in Makromol. Chem. 190 (1989) 929. Depending on
 the heteropolyacid used, another possible method of dehydration is to
 dissolve the heteropolyacid in an organic solvent, for example in a
 dialkyl ether or alcohol, displace the water with the organic solvent from
 its coordinate bond with the heteropolyacid and remove the water
 azeotropically with the solvent.
 Typically, anhydrous heteropolyacids prepared by these methods are
 subsequently calcined at from 250 to 500.degree. C., preferably from 280
 to 400.degree. C. Depending on the temperature and pressure selected, the
 heteropolyacids are typically calcined for from 1 hour to 24 hours. The
 catalysts obtained in this manner can be used directly in the process of
 the invention.
 The heteropolyacid catalysts are preferably applied to a support. To this
 end, the heteropolyacid is applied to a support material such as active
 carbon, silicon dioxide, titanium dioxide or zirconium dioxide by methods
 known per se, for example by impregnating the relevant support material
 with a solution of the heteropolyacid in a solvent, preferably water, and
 subsequently drying under reduced pressure at from 100 to 250.degree. C.,
 preferably from 130 to 250.degree. C., until water can no longer be
 detected in the catalyst. Anhydrous heteropolyacids prepared by these
 methods are subsequently calcined at temperatures of from 250 to
 500.degree. C., preferably from 280 to 400.degree. C.
 Suitable zeolites include any zeolites having basic or acidic centers.
 In the case of zeolites having basic properties, zeolites containing alkali
 metals or alkaline earth metals are used, for example; in the case of
 zeolites having acidic properties, zeolites in the acidic H-form are used,
 in which the alkali metal ions are replaced by hydrogen ions.
 Preference is given to 12-ring zeolites of the structure type BETA, Y, EMT
 and Mordenite, and 10-ring zeolites of the Pentasil type. As well as the
 elements aluminum and silicon, zeolites can also contain boron, gallium,
 iron or titanium in their framework. Furthermore, they can also be
 partially ion-exchanged with elements of the groups 3 and 8 to 13 and the
 lanthanide elements.
 Zeolites to be used as catalyst include zeolites of the structure type MFT,
 MEL, BOG, BEA, EMT; MOR, FAU, MTW, LTL, NES, CON or MCM-22 according to
 the structure classification given in W. M. Meier, D. H. Olson, Ch.
 Baerlocher, Atlas of Zeolite Structure Types, Elsevier, 4.sup.th ed.,
 1996.
 Particular examples are the zeolites ZBM-20, Fe--H-ZSM5, Sn-beta zeolite,
 beta zeolite, Zr-beta zeolite, H-beta zeolite, H-mordenite, USY, Ce--V
 zeolite, H--Y zeolite, Ti/B-beta zeolite, B-beta zeolite or ZB-10.
 To obtain very high selectivity, high conversions and long times on stream,
 it is advantageous to modify the zeolites. A suitable method of modifying
 the catalysts comprises, for example, doping the shaped or unshaped
 zeolites with metal salts by ion exchange or impregnation. The metals used
 are alkali metals such as Li, Cs, K, alkaline earth metals such as Mg, Ca,
 Sr. Metals of main groups III, IV and V, such as Al, Ga, Ge, Sn, Pb or Bi,
 transition metals of subgroups IV-VIII such as Ti, Zr, V, Nb, Cr, Mo, W,
 Mn, Re, Fe, Ru, Os, Co, Rh, Sr, Ni, Pd, Pt, transition metals of subgroups
 I and II such as Cu, Ag or Zn, and rare earth metals such as La, Ce, Pr,
 Nd, Er, Yb and U.
 Doping is advantageously carried out by introducing the shaped zeolite into
 a riser pipe and passing an aqueous or ammoniacal solution of a halide or
 nitrate of the abovementioned metals over it at from 20 to 100.degree. C.
 Such an ion exchange can take place with the hydrogen, ammonium or alkali
 metal form of the zeolite. Another way of applying metal to the zeolite
 comprises impregnating the zeolitic material, for example with a halide,
 nitrate or oxide of one of the abovementioned metals in aqueous, alcoholic
 or ammoniacal solution. Both ion exchange and impregnation are followed at
 least by a drying operation or alternatively by another calcination.
 A possible embodiment comprises for example dissolving
 Cu(NO.sub.3).sub.2.times.3 H.sub.2 O or Ni(No.sub.3).sub.2.times.6 H.sub.2
 O or Ce(NO.sub.3).sub.3.times.6 H La(NO.sub.3).sub.2.times.6 H.sub.2 O or
 Cs.sub.2 CO.sub.3 in water and impregnating the shaped or unshaped zeolite
 with this solution for a certain period of time, for example 30 minutes.
 Water is removed from any supernatant solution in a rotary evaporator. The
 impregnated zeolite is then dried at about 150.degree. C. and calcined at
 about 550.degree. C. This impregnating step can be carried out several
 times in succession until the desired metal content is obtained.
 It is also possible to prepare an aqueous Ni(NO.sub.3).sub.2 solution or an
 ammoniacal Pd(NO.sub.3).sub.2 solution and to suspend the pure pulverulent
 zeolite therein at from 40 to 100.degree. C. by stirring for about 24
 hours. After filtration, drying at about 150.degree. C. and calcination at
 about 500.degree. C., the zeolitic material thus obtained can be further
 processed with or without a binder into extrudates, pellets or fluidizable
 material.
 An ion exchange of the zeolite present in the H-form or ammonium form or
 alkali metal form can be carried out by introducing the zeolite in the
 form of extrudates or pellets into a column and passing, for example, an
 aqueous Ni(NO.sub.3).sub.2 solution or ammoniacal Pd(NO.sub.3).sub.2
 solution over it in a recycle loop at a slightly elevated temperature of
 from 30 to 80.degree. C. for from 15 to 20 hours. This is followed by
 washing out with water, drying at about 150.degree. C. and calcination at
 about 550.degree. C. With some metal-doped zeolites, for example Pd-, Cu-
 or Ni-doped zeolites, an aftertreatment with hydrogen is advantageous.
 A further method of modifying the zeolite comprises treating the shaped or
 unshaped zeolitic material with an acid such as hydrochloric acid,
 hydrofluoric acid or phosphoric acid and/or steam, advantageously, for
 example, by treating the zeolite in pulverulent form with 1N phosphoric
 acid at 80.degree. C. for 1 hour. The treatment is followed by washing
 with water, drying at 110.degree. C./16 h and calcining at 500.degree.
 C./20 h. Alternatively, before or after being shaped with a binder,
 zeolites are treated for example at from 60 to 80.degree. C. with from 3
 to 25% strength by weight, in particular from 12 to 20% strength by
 weight, aqueous hydrochloric acid for from 1 to 3 hours. Afterwards, the
 zeolite thus treated is washed with water, dried and calcined at from 400
 to 500.degree. C.
 Further catalysts for preparing cyclopentanone are phosphates, in
 particular aluminum phosphates, silicon aluminum phosphates, iron aluminum
 phosphates, cerium phosphate, zirconium phosphates, boron phosphate, iron
 phosphate, calcium phosphate or mixtures thereof.
 Suitable aluminum phosphate catalysts for the process according to the
 invention are in particular aluminum phosphates synthesized under
 hydrothermal conditions. Examples of suitable aluminum phosphate include
 APO-5, APO-9, APO-11, APO-12, APO-14, APO-21, APO-25, APO-31 and APO-33.
 AlPO.sub.4 -5(APO-5) is synthesized, for example, by homogeneously mixing
 orthophosphoric acid with pseudoboehmite (Catapal SB.RTM.) in water,
 adding tetrapropylammonium hydroxide to this mixture, and then reacting in
 an autoclave under autogenous pressure at about 150.degree. C. for from 20
 to 60 h. The AlPO.sub.4 is filtered off, dried at from 100 to 160.degree.
 C. and calcined at from 450 to 550.degree. C. AlPO.sub.4 -9 (APO-9) is
 likewise synthesized from orthophosphoric acid and pseudoboehmite, but in
 aqueous DABCO solution (1,4-diazabicyclo-(2,2,2)-octane) at about
 200.degree. C. under autogenous pressure in the course of from 200 to 400
 h. If ethylenediamine is used in place of DABCO solution, APO-12 is
 obtained.
 AlPO.sub.4 -21 (APO-21) is synthesized from orthophosphoric acid and pseudo
 boehmite in aqueous pyrrolidine solution at from 150 to 200.degree. C.
 under autogenous pressure in the course of from 50 to 200 h.
 The process according to the invention can also be carried out with known
 silicon aluminum phosphates such as SAPO-5, SAPO-11, SAPO-31 and SAPO-34.
 These compounds are prepared by crystallization from aqueous mixture at
 from 100 to 250.degree. C. and under autogenous pressure in the course of
 from 2 hours to 2 weeks, the reaction mixture, comprising a silicon, an
 aluminum and a phosphorus component, being converted in an aqueous
 solution comprising amine.
 SAPO-5 is obtained, for example, by mixing a suspension of SiO.sub.2 in an
 aqueous tetrapropylammonium hydroxide solution with an aqueous suspension
 of pseudoboehmite and orthophosphoric acid and then reacting at from 150
 to 200.degree. C. under autogenous pressure in a stirred autoclave for
 from 20 to 200 h. The powder is filtered off, dried at from 110 to
 160.degree. C. and calcined at from 450 to 550.degree. C. Suitable silicon
 aluminum phosphates also include ZYT-5, ZYT-6, ZYT-7, ZYT-9, ZYT-11 and
 ZYT-12. A precipitated aluminum phosphate can also be used in the process
 as a phosphate catalyst.
 For example, such an aluminum phosphate is prepared by dissolving 92 g of
 diammonium hydrogen phosphate in 700 ml of water. 260 g of
 Al(NO.sub.3).sub.3.times.9 H.sub.2 O in 700 ml of water are added dropwise
 to this solution in the course of 2 h, during which pH 8 is maintained by
 adding 25% strength NH.sub.3 solution at the same time. The resulting
 precipitate is subsequently stirred for 12 hours and then filtered off
 with suction and washed. It is dried at 60.degree. C./16 h.
 A boron phosphate catalyst for use in the process according to the
 invention can be prepared, for example, by mixing and kneading
 concentrated boric acid and phosphoric acid and subsequently drying and
 calcining in an inert gas, air or steam atmosphere at from 250 to
 650.degree. C., preferably at from 300 to 500.degree. C.
 CePO.sub.4 is obtained by precipitating from 52 g of
 Ce(NO.sub.3).sub.3.times.6 H.sub.2 O and 56 g of NaH.sub.2
 PO.sub.4.times.2 H.sub.2 O. The material is filtered off and shaped into
 extrudates, which are dried at 120.degree. C. and calcined at 450.degree.
 C. Suitable phosphates for the process according to the invention also
 include SrHPO.sub.4, FePO.sub.4 and Zr.sub.3 (Po.sub.4).sub.4.
 The catalysts described here can be used, for example, in the form of from
 2 to 4 mm extrudates or as tablets having a diameter of, for example, from
 3 to 5 mm, or as granules having particle sizes of, for example, from 0.1
 to 0.5 mm, or in a fluidizable form.
 When hydroxycaproic acid is used, additional metals such as copper or
 silver on oxidic carriers such as metal oxides are typically used to
 provide the catalysts with dehydrogenation activity.
 It has been found that the use of basic catalysts leads to increased
 formation of cyclopentanone, whereas the use of catalysts having acidic
 properties leads to increased formation of cyclopentene-1-carboxylic
 esters I.
 The reaction according to the invention may be carried out without water.
 The addition of water leads to an increased selectivity and time on
 stream. The molar ratio of starting compound II to water is advantageously
 1:0-1:20, in particular 1:0.1-1:5.
 The reaction can be carried out in the gas or liquid phase with or without
 diluents. Examples of suitable diluents are solvents which are completely
 or substantially inert under the reaction conditions, for example ether
 such as dioxane or tetrahydrofuran and alcohols such as methanol and
 ethanol. A gas stage procedure is preferred, provided that easily
 volatilizible starting materials are used.
 The reaction can be carried out batchwise or continuously as a fixed bed
 reaction with a fixed bed catalyst, for example in an upflow or downflow
 process in the liquid or gas phase, or as a fluidized bed reaction with
 the catalyst in the fluidized state in the gas phase, or with a catalyst
 suspended in the liquid phase.
 The reaction is carried out at from 200 to 450.degree. C., preferably from
 250 to 390.degree. C., in particular from 300 to 380.degree. C. The
 reaction is generally carried out under atmospheric pressure. However, it
 is also possible to use slightly reduced or slightly elevated pressure,
 for example up to 20 bar. The space velocity is generally in the range
 from 0.01 to 40, preferably from 0.1 to 20, g of compound of the formula
 II per gram of catalyst per hour.
 The liquid-phase reaction is carried out, for example, by heating a mixture
 of the starting compound II with or without water to the desired reaction
 temperature in the presence of a suspended fixed-bed catalyst. After the
 required reaction time, the reaction mixture is cooled down and the
 catalyst removed, for example by filtration. The reaction mixture is then
 subjected to a fractional distillation to recover the products of value
 and the unconverted ester. The reaction products formed in the course of
 the reaction can also be continuously removed from the reaction mixture by
 distillation.
 In a preferred embodiment of the process according to the invention in the
 gas phase, a mixture of starting compound II with or without water is
 initially vaporized and then passed, with or without hydrogen or an inert
 gas, such as nitrogen, carbon dioxide or argon, in gaseous form into a
 fluidized catalyst bed at the desired reaction temperature.
 In another preferred embodiment of the process according to the invention
 in the gas phase, for example, a mixture of the starting compound II with
 or without water is initially vaporized and then passed, with or without
 an inert gas, such as nitrogen, carbon dioxide or argon, in gaseous form
 over a fixed catalyst bed in an upflow or downflow process at the desired
 reaction temperature.
 The reaction effluent is condensed by means of suitable cooling devices and
 then worked up by fractional distillation. Unconverted starting compounds
 may be recycled.
 Cyclopentanone obtained by the process of the present invention is a useful
 intermediate. For instance, reductive amination gives cyclopentylamine
 which is of interest for the synthesis of crop protection agents and
 pharmaceuticals.
 Cyclopentene-1-carboxylic esters are useful building blocks for the
 synthesis of intermediates.

EXAMPLE
 The percentages indicated to characterize the catalysts are by weight.
 a) Catalyst Preparation: