(Adds U.S. Supreme court vacating appeals court stay)
KANSAS CITY, Mo., Jan 29 (Reuters) - The U.S. Supreme Court on Wednesday lifted a temporary stay of execution for a Missouri man convicted of killing a jewelry store owner during a 1991 robbery, denying last-minute appeals that in part challenged the drug to be used.
The Supreme Court late Wednesday also vacated a stay from the Eighth Circuit U.S. Court of appeals that had prevented the execution of Herbert Smulls, 56.
Lawyers for Smulls filed another request with the U.S. Supreme Court for a stay late on Wednesday, and it was not immediately clear whether Missouri would be allowed to carry out his execution before the state's death warrant expires at midnight.
Smulls was convicted of shooting jewelry-store owner Stephen Honickman to death while robbing his store in July 1991. Honickman's wife, Florence Honickman, was also shot during the attack and sustained permanent injuries.
Lawyers for Smulls have sought to block his execution on multiple grounds, arguing in part that the compounded pentobarbital drug that Missouri plans to use may not be as pure and as potent as it should be, which could cause undue suffering.
Missouri and several other states have turned to compounding pharmacies, which are not regulated by the U.S. Food and Drug Administration, to acquire drugs for executions after an increasing number of pharmaceutical manufacturers objected to their drugs being used in capital punishment.
The increasing use of compounded drugs and untested drug mixes has brought renewed debate over the death penalty in the United States.
In Oklahoma, an inmate said he felt burning through his body when the lethal drugs were injected during an execution in early January. Later in the month, an Ohio man gasped and convulsed during his execution with a two-drug mix never before used in the United States.
In the Smulls case, the Eighth Circuit found on Friday that his lawyers did not propose a feasible or more humane alternative than the compounded pentobarbital or show that Missouri sought to cause him unnecessary pain by using the drug.
The Eighth Circuit had separately granted a stay until the U.S. Supreme Court decided whether to hear the case.
The Supreme Court granted Smulls the temporary stay late Tuesday, hours before his execution was to be carried out, to consider his lawyer's arguments that prosecutors had improperly eliminated a black woman as a possible juror, leaving him with an all-white jury at trial.
On Wednesday afternoon, the Supreme Court vacated the temporary stay and denied the request for a stay or to hear the appeal on the jury selection issue.
If the execution is carried out, Smulls would be the sixth person executed in the United States in 2014 and the third in Missouri since November. (Reporting by Carey Gillam and Kevin Murphy in Kansas City, Lawrence Hurley in Washington and Heide Brandes in Oklahoma City; Writing by David Bailey; Editing by Cynthia Johnston, Prudence Crowther, Mohammad Zargham and Ken Wills)