US3480687A - Promotion of olefin cracking - Google Patents

Description

United States Patent 3,480,687 PROMOTION OF OLEFIN CRACKING Kenneth J. Frech, Tallmadge, Ohio, assignor to The Goodyear Tire & Rubber Company, Akron, Ohio, a corporation of Ohio No Drawing. Continuation-impart of application Ser. No.

449,278, Apr. 19, 1965. This application Mar. 3, 1966, Ser. No. 531,353

The portion of the term of the patent subsequent to May 31, 1983, has been disclaimed Int. Cl. C07c 3/32, 11/12 US. Cl. 260-680 12 Claims ABSTRACT OF THE DISCLOSURE The decomposition of 2-methylpentene-2 and 3-methylpentene-Z to isoprene is enhanced by cracking in the presence of hydrogen sulfide. The pyrolysis of various other olefins which contain in their molecule a carbon-tocarbon single bond in a position beta to the double bond likewise is disclosed as being enhanced if pyrolyzed in the presence of hydrogen sulfide.

This application is a continuation-in-part of application S.N. 449,278, filed Apr. 19, 1965, now abandoned, which in turn was a continuation of application S.N. 61,818, filed Oct. 11, 1960, now abandoned.

This invention relates to the cracking of certain olefins. More specifically, it relates to methods of improving the cracking of certain olefins. Most specifically, it relates to methods of improving the efficiency of cracking of olefins to form specific diolefins and paraflinic hydrocarbons or to form certain other specific olefins.

It is known that certain olefins may be thermally decomposed or cracked by subjecting them to relatively high temperatures. By the terms cracking, decomposing, decomposed, or cracked, as employed throughout this application and appended claims, is meant that the olefin molecule splits into two or more fragments, these fragments themselves form molecules of other hydrocarbons as explained later in greater detail. The olefins with which this invention is concerned are those olefins which will crack upon the application of heat. The olefins of this invention, if they are to crack at all, must contain in their molecules a double bond which is 2 carbon atoms removed from another carbon-to-carbon bond. This is true because it has been found that olefins always crack at the carbon-carbon bond which is in the beta position to the double of the olefin.

This thermal decomposition or cracking of olefins is usually conducted within a closed zone or reactor in the absence of oxygen. Temperatures employed in cracking of olefins usually range from about 300 C. to about 1000 C. Usually olefins are cracked While in a gaseous state and may be fed to the cracking zone either relatively pure, as mixtures of olefins, in mixture with other hydrocarbons usually saturated, i.e., mixed feed streams of pentane-pentene, or in mixture with diluents such as nitrogen, steam and the like. The thermal decomposition of olefins usually results in the formation of two lower molecular weight materials. There is usually a predominance of a diolefin and a paraflinic hydrocarbon and/ or in the formation of a predominance of two other olefins formed in this cracking operation. The particular materials which predominate when olefins are cracked depend largely upon the configuration of the olefin which is cracked. By configuration is meant the position of the double bond and the position of the side chains, if any. For instance, an olefin containing six carbons with a side chain, i.e., a methyl group attached to the second carbon atom of the main or straight chain portion of the compound and the double bond in the 2 position, such an olefin is Z-methyl pentene-Z, when subjected to cracking will upon decomposition produce as the predominant products 2-methy1 butadiene-l,3 or isoprene, a diolefin, and methane, a paraffin. On the other hand, another 6 carbon olefin having a methyl group attached to the second carbon of the straight chain and the double bond in the 1 position, such an olefin is 2-methyl pentene-l, when cracked will produce two other olefins, isobutylene and ethylene. When still another isomer such as 4-methyl pentene-l is cracked two mols of propylene are produced. These differences in products obtained when different isomeric forms of methyl-pentene are cracked are due to the fact that olefins crack at the position beta to the double bond. That is, the scission occurs at the bond that is in the position beta to the double bond or that the split occurs between the two carbon atoms that are second and third removed from the carbon atom which is attached to the double bond. Further, the sum of the carbon atoms of the main or predominant products of the cracking is usually equal to the number of carbon atoms contained in the olefin which was subjected to the cracking. In Z-methyl pentene-Z the cracking removes only one carbon atom from the 6 carbon olefin, in 2-methyl pentene-1 two carbon atoms are removed, in 4- methyl pentene-l three carbon atoms are removed because of the location of the beta position, resulting in the products mentioned above. Thus, the particular olefin employed usually designates the main or predominant products which result from the cracking of olefins. Another significant feature to remember is that since olefins crack at the bond beta to the double bond, an olefin if it is to crack at all must have in its molecule a double bond which is two carbon atoms removed from another carbon-tocarbon bond. Examples of olefins which do not have this molecular make-up are ethylene, propylene, butene-2, isobutene, Z-methyl butene-Z and 2,3-dimethyl butene-Z. These aforementioned olefins, since they do not crack, are not within the scope of the olefins which are to be cracked in accordance with this invention.

Employing the most favorable conditions conducive to cracking olefins to form these desired products, it has been found that olefins decompose at a very low rate per pass through the cracking zone. These conditions effecting the cracking are temperature, residence time in the zone, and the ratio of the olefin to the gaseous diluent, if any, employed. It is usually the practice to increase the yield of decomposition of olefins and of the desired end products by separating the unreacted or undecomposed olefin from the products resulting from the cracking and returning or recycling the unreacted olefin to the cracking zone. Usually, however, regardless of how many recycles are carried out the ultimate yield or ultimate decomposition of the olefin is not greater than about 50 mol percent of the olefin being converted or decomposed to the desired products, the remaining 50 mol percent being converted to undesirable products as the result of side reactions caused by high temperature or long residence times in the cracking zone.

It is, therefore, the main object of this invention to provide a method whereby the yield of the desired products produced by cracking olefins is increased. Another object is to provide a method whereby the cracking of olefins to desired products per pass is increased. Another object is to increase the ultimate yield or ultimate decomposition of desired products from the cracking olefins. Another object is to provide a method whereby the residence time of cracking of olefins may be decreased. Another object is to provide a method whereby the promotion of undesirable side reactions during the cracking of olefins is decreased. Another object is to provide a method whereby olefins may be cracked at lower temperatures. Another object is to provide a promotor for the cracking of olefins to the desired products. Still another object is to provide a method whereby the size of equipment needed to crack a certain volume of olefins is reduced. Still another object is to reduce the amount of material required to be recycled. Still other objects will become apparent as the description proceeds.

The objects of this invention are accomplished by subjecting olefins which have in their molecular structure a double bond which is two carbon atoms removed from another carbon-to-carbon bond to cracking conditions in the presence of an SH radical.

In general, the cracking of olefins in accordance with the practice of this invention may be carried out in any conventional manner usually employed in the art of cracking olefins.

Generally, the conditions of cracking which may be employed in this invention may be widely varied, depending upon the particular olefin to be cracked and the products desired. For instance, the cracking temperature may be varied from about 300 C. to about 1000 C. However, it is usually preferred to crack olefins at temperatures ranging between 500 and 800 C. and it is generally most preferred to employ temperatures ranging between 625 and 725 C.

The time that the olefins are in the cracking zone during the practice of this invention may range broadly from about 0.001 to about 3 seconds. However, it is preferable that times varying from 0.05 to 1.0 second and most preferably 0.1 to 0.5 be employed. These times are referred to usually as residence times and are usually defined as the time required for one mol of incoming gas, be it pure olefin or mixtures with other olefins or diluents, to pass through the cracking zone.

Generally, the olefins are fed to the cracking reactor either as pure olefins or in mixture with other olefins or in mixture with some inert diluent. It is usually desirable to employ a diluent such as steam, carbon dioxide, hydrogen, or paraffins such as methane, ethane, propane, butanes, pentanes, and olefins such a ethylene and propylene and butene-2 and the like. These hydrocarbons do not crack at the temperatures employed to crack the olefins in the practice of this invention. Of these, steam is preferred because of economy. Propane and pentane may in some cases be preferred in place of steam.

The ratio of diluent to olefin employed in the practice of this invention may be widely varied from about 0.5/1 to about 15/ 1 or more mols of diluent per mol of olefin. However, if more than about a 15/1 ratio is employed the process is no longer economical. It is preferred to use a diluent to olefin ratio ranging from about 2.0/1 to 4.0/1. The olefins may also be cracked without diluent.

The pressures employed in the cracking zone while cracking olefins may be varied from about millimeters of mercury to 500 pounds per square inch gauge. However, it is preferred that the pressure range from about atmospheric to about 100 pounds per square inch gauge with about 1 to about 2 atmospheres being most preferred.

The SH radical which is employed to increase the efficiency obtained when olefins are cracked in accordance with the practice of this invention may be provided by various means. Hydrogen sulfide will under the conditions employed in the practice of this invention dissociate to form hydrogen and an SH radical. Low molecular weight mercaptans such as ethyl, normal propyl, isopropyl, normal butyl, isobutyl and the amyl and hexyl mercaptans will also form an SH radical at the temperatures employed in the cracking operation. It is believed that any aliphatic mercaptan will decompose at the operating conditions to give hydrogen sulfide and an olefin with the same number of carbon atoms as originally contained in the aliphatic mercaptan. This hydrogen sulfide then provides the SH radical as indicated above. Thus, in the practice of this invention not only can hydrogen sulfide gas be employed but any other material may be employed which under the operating conditions decomposes to provide hydrogen sulfide. Of these methods which may be employed to supply SH radi cals in the practice of this invention hydrogen sulfide is the preferred material.

The SH radical employed in the practice of this invention may be used in amounts varying from about 0.5 to about 50 mol percent of SH radical based on the total mols of the olefin to be cracked. It has been found, however, that excellent results have been obtained by employing from about 5 to about 10 mol percent of the SH radical. It should be noted that one mol of the SH radical is produced for each mol of the SH radical producing compound employed. Since one mol of hydrogen sulfide provides one mol of SH radical these amounts are applicable to the amount of hydrogen sulfide employed as well.

The olefins listed below are in groups which will crack to produce predominantly the desired diolefins or the desired olefins. These lists of olefins are by no means intended to be limiting to the scope of this invention and are only listed as representative examples. As was stated before, this invention is limited in scope only to those olefins which can be decomposed by a true thermal decomposition.

Representative of the olefins that will decompose to form predominantly Z-methyl pentadiene-l,3 and 4- methyl pentadiene-1,3 when cracked in the presence of SH radicals are: 2-methyl heXene-3; 2,4-dimethyl pentene-2; 2-methyl heptene-3; 4,4-dimethyl hexene-2; 2- propyl pentene-2; 2 methyl 3 ethyl pentene-1; 2,6-dimethyl heptene-3 and 2-propyl heXene-l.

Representative of the olefins which will decompose to form predominantly 3-methyl pentadiene-1,3 are 3-methyl heptene-3; 3,4 dimethyl hexene 2 and 3,6 dimethyl heptene-3.

Representative of the olefins which decompose to form predominantly 2,3-dimethy1 butadiene-1,3 are 2,3-dimethyl pentene 2; 3 methyl 2 ethyl butene 1; 2,3,3- trimethyl butene-1; 2-isopropyl pentene-1; 2,3,3-trimethy1 entene-l and 2,3-dimethyl heptene-2.

Representative of the olefins which decompose to form predominantly 2-ethyl butadiene-1,3 are 2-ethyl pentene- 2; 3-ethy1 pentene-2; 3-ethyl hexene-Z; 3-methyl-2-ethyl pentene-l.

Representative of the olefins which will decompose to form predominantly butadiene-1,3 are pentene-2; hexene- 2; 3-methyl pentene-l; cyclohexene; 3-methyl butene-1; Z-heptene; 3-methyl hexene-l; S-methyl hexene-2; 2- octene; S-methyl heptene-Z; 3,5-dimethyl hexene-l; 6- methyl heptene-2; nonene-2 and 3-methyl octene-l.

Representative of the olefins which will decompose to form predominantly isoprene are: 2-methyl pentene-2; 3-methyl pentene-2; 2-ethyl butene-1; 3,3-dimethyl butene-1; 2,3-dimethyl butene-1; 2-methyl hexene-2; 3-methyl hexene-2; 2-ethyl pentene-1; 2,3-dimethyl pentene-l; 3,3-dimethyl pentene-1; Z-methyl heptene-2; 3-methyl heptene-Z; Z-ethyl hexene-l; 3,3-dimethyl hexene-l; 2,5-dimethyl hexene-Z; 3,5-dimethyl hexene-Z;

to form ethylene as a major product are: pentene-l;

Z-methyl pentene-l. I

Representative of the olefins which Will decompose to form propylene as a major product are: pentene-l; hexene-l; 4-methyl pentene-l; heptene-l; 2-methyl hexene-l; 4-methyl hexene-l; S-methyl hexene-1; 2,4-dimethyl pentene-l; 4,4-dimethyl pentene-l; octene-l; 4-methyl heptene-l; 5'-methyl heptene-l; 6-methy1 heptene-l; and 4-ethyl hexene-l.

Representative of the olefins which will decompose to form isobutylene as a major product are: Z-methyl pentenel; Z-methyl hexene-l; 5-m ethyl hexene-l', 2,4-dimethyl pentene-l; and 4,4-dimeth'yl pentene-l.

Representative of the olefins which will decompose to form butene-1 and/ or butene-2 as major products are: heptene-l; 4-methyl hexene-l; Z-methyl heptene-l; and 2,4-dimethyl hexene-l.

Representative of the olefins which will decompose to form 2-methyl butene-I and/or 3-methyl butene-1 and/or Z-methyl butene-2 as major products are: S-methyl heptene-l; 6-methy1 heptene-l; 4,4-dimethyl hexene-l; 4,5-dimethyl hexene-l and 2,6-dimethyl heptene-l.

Representative of the olefins which will decompose to form pentene-l and/or pentene-Z as major products are: 4-methyl heptene-l; 4ethyl hexene-l and Z-methyl octene-l.

Representative of the olefins which will decompose to form 2,3-dimethyl butene-l and/or 2,3-dimethyl butene-2 as major products are: 4,4,5-trimethyl hexene-l and 2,5,6-trimethyl heptene-l or -2.

Of these olefins it is particularly desired to form isoprene by the practice of this invention by cracking Z-methyl pentene-2; 3-methyl pente'ne-Z', 2-ethyl butene-l; 3,3-dimethyl butene-l and 2,3-dimethyl butene-l while employing a compound which will decompose to form SH radical under the cracking conditions.

The practice of this invention is illustrated by the following experiments which are to be interpreted as representative rather than restrictive of the scope of this invention. The results and conditions of the cracking experiments are reported in table form.

All of the cracking experiments were performed in a reactor assembly consisting of a hairpin coil prepared from At-inch OD 316 stainless steel tubing. This coil reactor was immersed in a bed of fluidized heat transfer powder which was microspheroidal silica-alumina cracking catalyst. The heat transfer powder was heated both by electrical resistance heaters and by combusting a natural gas flame in the fluidized powder bed. The temperature gradient from top to bottom of the bed was never more than 5 to 6 C. and the gradient from the fluidized bed to the tube walls was about 56 C. The temperatures within the fluidized .bed were measured by conventional thermocouple techniques as were the temperatures within the cracking zone. The procedure em ployed was to bring the heat transfer powder up to about 500 C. employing the electrical resistance heaters while fluidizing the heat transfer powder with air. Then the natural gas burner was employed to bring the heat transfer powder up to the desired cracking or operating temperature. The SH radical was supplied in the form of an SH radical-liberating compound which at the cracking temperatures dissociated to produce the SH radical, a calculated amount of the compound was dissolved in either the olefins or the water, depending on the particular compound employed, to give the desired amount of SH radical producing compound to produce the SH radical required in each experiment. In these particular examples the n-propyl mercaptan was added to the olefin. The water and the olefin were pumped at the proper rates necessary to produce the H o/hydrocarbon ratio desired and to give the desired residence time of the materials in the cracking zone or cracking reactor. When all variables had been adjusted to give the desired operating conditions the products resulting from the cracking operation were collected by means of cooled receivers, if liquid, and were metered at atmospheric and room temperature conditions, if gas. The products were analyzed for content and yields by conventional analytical methods. Conventional recycle techniques were employed to obtain the ultimate yield.

The results of each of the experiments in the following examples as well as the operating conditions are reported in tables below. Column 1 is the experiment number; column 2 is the residence time in seconds; column 3 is the temperature employed in the cracking operation in degrees centigrade; column 4 is the material employed to supply the SH radical and the amount employed in mol percent based on the olefin to be cracked, if any SH radical was employed (where no SH radical was employed this column lists none); column 5 is the mol percent yield of isoprene per pass based on the 2-methy1 pentene-Z charged; column 6 is the reaction efficiency or ultimate yield and is the amount of isoprene obtained based on the Z-methyl pentene-2 charged using conventional recycle techniques.

EXAMPLE I Decomposition of Z-methyl pentene-2 to isoprene Isoprene Yield,

percent Residence Temper- Time, ature, Seconds Reaction Promoter and E percent (Based on HC) 650.5 n-Propylmercap- 651. 0 None EXAMPLE II Decomposition of 2-methy1 pentene-2 to isoprene Expercieney, iment percent In Experiment 1 n-propyl mercaptan was employed to supply the SH radical. Experiment 2 has no promoter and is a control. Steam was the diluent employed at a mol ratio of about 3/1 of H O/hydrocarbon in both experiments.

Isoprene Reaction Yield, Etfi- Mol eiency, percent percent Resideuce Time, Seconds Temperature,

Promoter and ercent Exper- D (B as ed on H C) iment 673. 2 n-Propylmercap- None EXAMPLE III Decomposition of Z-methyl pentene-2 to isoprene In Experiment 1 n-propyl mercaptan was employed to supply the SH radical. In Experiment 2 no promoter was employed and it is a true thermal cracking and is considered to be the control Steam was used as a diluent in both experiments at 21 mol ratio of H O/hydrocarbon of about 3/1.

Decomposition of Z-methyl pentene-Z to isoprene In an experiment similar to Example I except that the reaction conditions and the promoter employed were as set forth in the table below, Z-methyl pentene-2 was pyrolyzed to isoprene.

Iso- Resiprene Reaction dence Temper- Promoter and Yield, Elfi- Exper- Time, ature, percent M01 ciency, iment Seconds 0. (Based on HC) percent percent 1 0. 675 H28, 3% 20.1 54. 4 2 0.15 675 None 21. 1 47. 7

As can be observed from the above examples, in Example I more than a 70% increase in yield per pass is obtained by the practice of this invention when Z-methyl pentene-2 is cracked to isoprene. At the same time under the conditions employed in this example more than a 5% increase in reaction cfificiency is obtained. This means that by employing this invention in a production operation at these particular cracking conditions smaller equipment could be utilized. In Example II, at the conditions employed in this example, more than a increase in isoprene yield per pass is obtained which is also accompanied by more than 17% increase in reaction efficiency. This means that the employment of an SH radical in the cracking of olefins, particularly cracking 2-methyl pentene-2 to isoprene, is vastly superior to the cracking of Z-methyl pentene-Z to isoprene through thermal processes alone. The same is true for Example III wherein more than 45% increase in yield per pass was obtained. In Example IV a like improvement is observed, where more than a 20% improvement in yield and more than 14% improvement in efficiency is obtained. Thus, it can be readily seen that by the practice of this invention a superior and more efficient process can be developed in the cracking of olefins by the expedient of supplying as a cracking promoter from material which forms SH radicals at the cracking conditions employed.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

1. A process useful to produce isoprene which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge at least one olefin selected from the group of 2- methyl pentene-Z; 3-methyl pentene-Z; 2-ethyl butene-l; 3,3-dimethyl butene-l; 2,3-dimethyl butene-l; 2-methy1 hexene-2; 3-methy1 hexene-2 and 3,3-dimethyl pentene-l, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce isoprene and recovering said isoprene.

2. The method according to claim 1 in which the olefin is 2-methyl pentene-2.

3. The method according to claim 1 in which the olefin is 3-methyl pentene-2.

4. The method according to claim 1 in which the olefin is 2,3-dimethyl butene-l.

5. A process useful to produce 2-ethyl butadiene-1,3

which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding pounds per square inch gauge, at least one olefin selected from the group of 3-ethyl pentene-Z; 2-ethyl pentene-Z; and 3-ethyl hexene-Z, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce Z-ethyl butadiene-1,3 and recovering said 2-ethyl butadiene-1,3.

6. The method according to claim 5 in which the olefin is 2-ethy1 pentene-Z.

7. The method according to claim 5 in which the olefin is 3-ethyl pentene-2.

8. A process useful to produce butadiene-1,3 which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge, at least one olefin selected from the group of hexene-Z and pentene-2, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce butadiene-l,3 and recovering said butadiene-1,3.

9. A process useful to produce piperylene which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge at least one olefin selected from the group of hexene-3; 4-methyl pentene-Z; heptene-3; and 4-methyl hexene-Z, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce piperylenc and recovering said piperylene.

10. A process useful to produce 2,3-dimethyl butadiene-1,3 which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge at least one olefin selected from the group of 2,3-dimethyl pentene-Z; 3- methyl-Z-ethyl butene-l; 2,3,3-trimethyl butene-l; 2,3,3- trimethyl pentene-l; and 2,3-dimethyl heptene-Z, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce 2,3-dimethyl butadiene-1,3 and recovering said 2,3-dimethyl butadiene-1,3.

11. A process useful to produce 3-methyl pentadiene- 1,3 which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge at least one olefin selected from the group of 3-methyl heptene-3; and 3,4-dimethyl hexene-Z, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbon-to-carbon single bond which is in the position beta to the double bond of said olefin to produce 3-methyl pentadiene-1,3 and recovering said 3-methyl pentadiene-1,3.

12. A process useful to produce 2-methyl pentadiene- 1,3 and 4-methyl pentadiene-1,3 which comprises subjecting to temperatures ranging between 500 C. and 800 C. for times from about 0.05 to about 1 second, at pressures not exceeding 100 pounds per square inch gauge at least one olefin selected from the group of 2,4-dimethyl pentene-Z; Z-methyl heptene-3; 4,4-dimethyl hexene-2; and 2-propyl pentene-Z, in the presence of from about 5 to about 10 mole percent calculated on the total moles of said olefin of hydrogen sulfide, cleaving the carbonto-carbon single bond which is in the position beta to the 9 double bond of said olefin to produce Z-methyl penta- 2,450,686 dime-1,3 and 4-methyl pentadiene-1,3 and recovering 3,238,270 said Z-methyl pentadiene-1,3 and 4-rnethyl pentadiene-1,3. 3,254,136 3,284,532

References Cited UNITED STATES PATENTS 8/1938 Rosen 260680 2/1947 Folkins et a1. 260--683 Rasmussen et a1 260-683 Turnquest 260680 Frech 260 -680 Frech 260-680 5 PAUL M. COUGHLAN, JR., Primary Examiner US. Cl. X.R.